WO2021130501A1 - Biomass derived carbon quantum dots synthesized via a continuous hydrothermal flow process - Google Patents

Abstract

The invention discloses a continuous hydrothermal flow process comprising the synthesis of carbon rich, optionally doped, carbon quantum dots, typically of particle size less than 10nm, starting from a carbon precursor and optionally a doping precursor. The invention discloses corresponding carbon quantum dots which have excitation independent and/or excitation dependent luminescence. The invention also discloses a method of detecting Cr(VI), a sensor, a method of oil extraction and a composition for extracting oil.

Description

Biomass derived carbon quantum dots synthesized via a continuous hydrothermal flow process

The present invention relates to a continuous hydrothermal flow route and process for the synthesis of optionally doped carbon quantum dots starting from a carbon precursor and an optional doping precursor and the corresponding doped or undoped carbon quantum dots.

DESCRIPTION OF THE RELATED ART

Fluorescent carbon quantum dots (CQD) typically, with particle size <10 nm in diameter and quasi-spherical and discrete morphological structure have attracted increasing scientific and industrial interest over the last decade. In possessing either a core of graphite or an amorphous carbon framework (hybridised sp²/sp³), with a surface that can be richly coated with functional groups (e.g. oxygen moieties), polymers or other derivative species (all dependant on the synthetic route and the carbon source), CQDs offer a range of potential properties such as high photostability, good biocompatibility, and excellent optical properties, with low potential for environmental hazards. As such, CQDs are excellent candidates in a variety of applications including bio-imaging & bio-tagging, drug delivery, fluorescent ink, ion sensors, optoelectronics, photocatalysis, light-emitting devices, and solar cells.

SUMMARY OF THE INVENTION

This section is for the purpose of summarizing some aspects of the present invention and to briefly introduce some preferred embodiments. Simplifications or omissions may be made to avoid obscuring the purpose of the section. Such simplifications or omissions are not intended to limit the scope of the present invention. All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinence of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art.

It is acknowledged that the term 'comprise' may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term 'comprise' shall have an inclusive meaning— i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term 'comprised' or 'comprising' is used in relation to one or more steps in a method or process.

One aspect of the invention relates to a continuous hydrothermal flow route and process comprising the bottom-up synthesis of carbon rich, optionally doped, carbon quantum dots starting from a carbon precursor, which may be selected from citric acid, lignin, or a saccharide, such as glucose, fructose sucrose and cyclodextrin and an optional doping precursor which may be selected from a nitrogen, a sulphur,, a phosphorus or a boron precursor, wherein the nitrogen precursor may be selected from ethyldiamine, melamine, urea and ammonia; wherein the sulphur precursor may be selected from thiourea, sodium thiosulfate or a sulphonic acid agent, such as a polysulfonic acid agent, for example p- sulfonic acid calix[4]arene, p-sulfonic acid calix[6]arene and/or p-sulfonic acid calix[8]arene, typically, p-sulfonic acid calix[4]arene; wherein the phosphorus precursor may be selected from P-agents such as phosphoric acid, p-phosphonic acid calix[4]arene, p- phosphonic acid calix[6]arene and/or p-phosphonic acid calix[8]arene, typically, p- phosphonic acid calix[4]arene; and wherein the boron precursor maybe selected from boric acid, borax or sodium borate.

One aspect of the invention relates to a continuous hydrothermal flow route and process comprising the synthesis of carbon rich, and optionally nitrogen, phosphorus or boron doped, carbon quantum dots starting from a carbon precursor optionally selected from citric acid, glucose, fructose, sucrose and lignin and optionally a nitrogen precursor optionally selected from ethyldiamine, melamine, urea and ammonia, a phosphorus precursor optionally selected from P-agents such as phosphoric acid, p-phosphonic acid calix[4]arene, p-phosphonic acid calix[6]arene and/or p-phosphonic acid calix[8]arene, typically, p-phosphonic acid calix[4]arene or a boron precursor optionally selected from boric acid, borax or sodium borate.

In one aspect the invention is about a continuous hydrothermal flow route and process comprising; the synthesis of carbon rich, nitrogen-doped carbon quantum dots starting from citric acid and ammonia as precursors.

One aspect of the invention relates to N-doped carbon quantum dots having an excitation independent luminescence.

One aspect of the invention relates to N-doped carbon quantum dots obtainable by a process according to the claims.

One aspect of the invention relates to S-doped carbon quantum dots having an excitation independent and/or dependence luminescence.

One aspect of the invention relates to S-doped carbon quantum dots obtainable by a process according to the claims.

One aspect of the invention relates to P-doped or B-doped carbon quantum dots obtainable by a process according to the claims. One aspect of the invention relates to a method of detecting Cr(VI) comprising the steps of contacting the sample to be analysed for Cr(VI) with carbon quantum dots according to the claims, and measuring the change in emission intensity of the contacted sample.

One aspect of the invention relates to a method of detecting Cr(VI) wherein the Cr(VI) is in solution with carbon quantum dots according to the claims, typically, in an aqueous solution.

One aspect of the invention relates to a sensor that comprises carbon quantum dots according to the claims.

One aspect of the invention relates to a chemisensor for Cr(VI) detection using carbon quantum dots according to the claims.

One aspect of the invention relates to a method of extracting oil comprising contacting the oil containing substrate with carbon quantum dots in solution, wherein the carbon quantum dots are according to the claims.

One aspect of the invention relates to a composition for extracting oil comprising carbon quantum dots according to the claims, preferably in the form of an aqueous solution, more preferably a brine solution.

One aspect of the invention relates to the use of carbon quantum dots according to the claims as a Cr(VI) sensor.

One aspect of the invention relates to the use of carbon quantum dots according to the claims as an oil extractant additive.

One aspect of the invention relates to a solar cell, sensor, photocatalyst, optoelectronic device or biotag that comprises carbon quantum dots according to the claims.

One aspect of the invention relates to carbon quantum dots produced by a process according to the claims. Typically, N-doped carbon quantum dots and S-doped carbon quantum dots have excitation independent luminescence. The invention is therefore particularly relevant to the production of un-doped and doped carbon quantum dots, typically, N-doped carbon quantum dots and/or S-doped carbon quantum dots which have excitation independent or excitation dependent luminescence and their uses and methods of Cr (VI) sensing and/or oil extraction.

The un-doped and doped quantum dots prepared by the continuous hydrothermal flow route and process according to the present invention, typically, N-doped carbon quantum dots, may be suitable for detecting Cr(VI) comprising the step of contacting the Cr(VI) sample to N-doped carbon quantum dots having an excitation independent luminescence. Therefore, the doped carbon quantum dots reported herein may be suitable as chemisensors.

The optionally doped carbon quantum dots prepared by the continuous hydrothermal flow route and process according to the present invention, typically, S-doped carbon quantum dots, may be suitable for oil extraction by contacting the carbon quantum dots with oil in solution. The carbon quantum dots according to the present invention, in addition to their advantageous small particle size and optical properties, also display advantageous surfactant properties, in particular their application impact on oil-water interfacial tension reduction, wettability alteration and log-jamming properties. Therefore, the carbon quantum dots reported herein may be oil extractors that are proficient at very low concentrations.

Blue-luminescent N-doped carbon quantum dots (NCQDs) exhibiting rarely observed excitation independent optical properties are synthesised from citric acid in the presence of ammonia via a Continuous Hydrothermal Flow Synthesis (CHFS) approach. CHFS is an eco-friendly, rapid synthetic approach (within fractions of a second) facilitating ease of scale-up industrialization as well as offering materials with superior properties. The synthesised CQDs readily disperse in aqueous solution, have an average particle size of 3.3 +/- 0.7 nm, with highest emission intensity at 441 nm (and a narrow full width at half maximum, FWHM ~78 nm) under a 360 nm excitation wavelength. Carbon quantum dots, without any further modification, exhibited a high selectivity and sensitivity as a nano sensor for the highly toxic and carcinogenic chromium(VI) ions. The nano-chemo-sensor delivers significant advantages including simplicity of manufacturing via a continuous, cleaner technology (using targeted biomass precursor), high selectivity, sensitivity and fast response leading to potential applications in environmental industry as well photovoltaics, bio-tagging, bio-sensing and beyond.

Other features and advantages of the present invention will become apparent upon examining the following detailed description of an embodiment thereof, taken in conjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1: HRTEM images of N-doped carbon quantum dots at different magnification and scale: (a) 50 nm (b) 5 nm with (inset) showing particle size distribution histogram, average particle size of 3.3 ± 0.7 nm, (c) graphitic core lattice fringes (d) AFM image with inset showing particle size distribution histogram.

Figure 2: XPS survey scans of N-doped carbon quantum dots: (a) survey spectrum showing C(ls), N(ls) and O(ls) core levels, (b) - (d) fitted high resolution spectra of C(ls), N(ls) and O(ls) regions, respectively.

Figure 3: (a) Raman and (b) FTIR spectra of N-doped CQDs.

Figure 4: (a) UV-Vis absorption spectrum (black curve) and photoluminescence (PL) spectrum (red curve) of carbon quantum dots at 360 nm excitation wavelength (b) NCQDs excitation at wavelengths 320 - 380 nm gave emission spectra showing excitation independent optical behaviour, but excitation from 400 - 420 nm showed excitation dependent behaviour (inset) (c) pH influence over the emission intensity and (d) histogram of pH effect on the emission spectrum.

Figure 5: Selectivity of the N-doped CQDs based sensor over other ions and anions.

Figure 6: Cr(VI) ions influence on PL spectrum of N-doped CQDs in (a-b) reflecting the intensity changes in ppm concentration range with (inset) showing Stern-Volmer plot, log(Fo/F) versus concentration, and (c) photo of the effect of different Cr(VI) ions concentration on NCQDs emission under UV light.

Figure 7: The Inner Filter Effect of Chromate (CrOT ) representing the spectral overlap between the chromate normalised UV-Vis absorption band (red line), N-doped CQD's excitation spectrum (emission wavelength Em = 441 nm) (blue line), and the emission spectrum (excitation wavelength Ex_t = 370 nm) (black line).

Figure 8: Synthesis of N-doped carbon quantum dots (NCQDs) using a Continuous Hydrothermal Flow Synthesis (CHFS) process: (a) illustration of the CHFS synthesis process using citric acid as carbon source and ammonia as N-dopant, (b) simplified CHFS design.

Figure 9: Quantum yield determination via integrated fluorescence intensity vs absorbance plot method.

Figure 10: Cr(VI) ion influence on PL spectrum of NCQDs reflecting the intensity changes in ppb concentration range.

Figure 11: Stability analysis of the NCQDs in presence of Cr(VI) (50 ppm) were made by recording the fluorescence intensity at 441 nm emission wavelength of the mixture - (a) samples were initially exposed continuously for 5400 seconds (90 minutes) at 360 nm excitation, and (b) then at intervals of 2 hr, 4 hr, 24 hr and 48 hr.

Figure 12: Photoluminescence (PL) spectrum of N-doped carbon quantum dots at 360 nm excitation wavelength (a) showing the effect of CHFS reaction temperature (all other conditions were kept the same) and (b) comparison of the PL spectra of N-doped CQD (synthesised using citric acid and ammonia) and control reaction (CQDs) synthesised from citric acid only showing negligible photoluminescence. The synthesis reaction temperature in both cases was kept at 450 °G

Figure 13: Production process of S-doped carbon quantum dots (S-CQDs) using a Continuous Hydrothermal Flow Synthesis (CHFS) process: (a) illustraton of the CHFS synthesis process using glucose and p— sulfonic acid calix[4]arene (SCX4) (b) simplified CHFS design.

Figure 14: (a-b) HRTEM images of S-doped carbon quantum dots at different scales (a) 10 nm (b) 5 nm with inset showing particle size distribution histogram (c) AFM image of S- doped carbon quantum dots.

Figure 15: XPS survey scans of S-doped carbon quantum dots: (a) survey spectrum showing C(ls), S(2s) and O(ls) core levels, (b) fitted high-resolution spectra of C(ls), (c) O(ls) and (d) S(2p) regions.

Figure 16: (a) Raman spectrum of S-doped carbon quantum dots (b) FTIR spectra of -doped carbon quantum dots and SCX4.

Figure 17: (a) Photoluminescence spectra at the different excitation wavelengths (b) UV-vis spectrum with insert showing luminescence when irradiated with UV-light at 365 nm (c) Emission profiles of S-doped carbon quantum dots at various pH environments (d) histogram of pH effect on the emission spectrum.

Figure 18: UV-Vis measurements, absorbance vs time for stability studies of (a) S-doped carbon quantum dots and (b) g-CQDs. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

To provide an overall understanding of the invention, certain illustrative embodiments and examples will now be described. However, it will be understood by one of ordinary skill in the art that the same or equivalent functions and sequences may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the disclosure. The compositions, apparatuses, systems and/or methods described herein may be adapted and modified as is appropriate for the application being addressed and that those described herein may be employed in other suitable applications, and that such other additions and modifications will not depart from the scope hereof.

As used in the specification and claims, the singular forms "a", "an" and "the" include plural references unless the context clearly dictates otherwise. For example, the term "a transaction" may include a plurality of transaction unless the context clearly dictates otherwise. As used in the specification and claims, singular names or types referenced include variations within the family of said name unless the context clearly dictates otherwise.

Certain terminology is used in the following description for convenience only and is not limiting. The words "lower," "upper," "bottom," "top," "front," "back," "left," "right" and "sides" designate directions in the drawings to which reference is made, but are not limiting with respect to the orientation in which the modules or any assembly of them may be used.

This manuscript describes the first report for continuous hydrothermal flow synthesis (CHFS) process for the synthesis of carbon quantum dots (CQD). The CHFS reactor represents a green, short reaction time (residence time - fractions of a second) and single step synthetic approach delivering significant advantages over conventional methods, including batch hydrothermal reactors. Applying the CHFS methods to produce carbon-based materials from biomass precursors offers a new direction in achieving large-scale production of homogenous quality CQD, and potentially significantly reduced costs and lower environmental impact. The novelty of the approach and interest in carbon nanomaterials makes this submission of significant interest to a wide range of technologies and enhances the impact of the work.

This manuscript highlights a CHFS approach that directly obtains in a continuous manner luminescent carbon quantum dots (CQD) using targeted simple precursors: citric acid and ammonia, and expanded to other biomass derived precursors, glucose, sucrose, cyclodextrin, lignin and other N-precursors including ethyldiamine, melamine, urea, and so on.

The synthesis of CQDs via the CHFS system delivers the following advantages (a) from a series of synthetic stages to a single step approach (synthesis and simultaneous doping), (b) a significant reduction in reaction times from hours to seconds, thereby reducing energy consumption, (c) promoting the use of renewable precursors (citric acid, glucose) and solvent (water), (d) facilitating tunability and control over reaction parameters (e.g. temperature, pressure and flow rate) and hence particle properties (e) as well as easy industrial scalability. Applying CHFS methods to produce carbon-based materials from biomass derivatives offers a new direction in achieving large-scale production of homogenous quality and potentially significantly reduced costs and lower environmental impact.

It would be advantageous to develop bottom-up methodologies which utilize biomass resources as targeted precursors (e.g. citric acid, glucose) for conversion into nanosized carbon dots via intra- or inter-dehydration and/or decomposition processes. Contrary to the top- down route (derived from cutting larger dimensional pre-made parent precursors such as carbon nanotubes, graphite), the alternative bottom-up approaches can offer "one pot" surface functionalization via selective heteroatom doping (e.g. nitrogen) to give highly fluorescent materials without further need of a post-synthesis treatment (typical for top- down approaches). However, the current bottom-up approaches have their own challenges including lengthy manufacturing time, non-uniformity in CQD particle size distribution, inconsistent reproducibility and high energy costs. Consequently, applying the aforementioned synthetic approaches to an industrial scale within the current parameters would be limiting. In this regard, overcoming these challenges in providing a controllable, cleaner and rapid synthetic process with potential for industrial scale-up, as well delivering high-performance CQD is important.

Surprisingly, it has been found that Continuous Hydrothermal Flow Synthesis (CHFS), is may be used for the bottom-up approach offering significant synthetic advantages over traditional methods, delivering in a cleaner, rapid mode, high-quality nanosized materials. The CHFS process involves mixing a feed of supercritical water (374 °C, 22.1 MPa) with target precursor feed/s in the reaction zone, with reaction times approaching fractions of a second to give the desired product. The resulting treated reaction mixture passes through a cooler and the product is collected as an aqueous nanomaterial suspension. Compared with traditional hydrothermal methods, the CHFS process consumes less energy and time, delivering quality and reproducibility of a homogenous product, whilst offering in real time full control and tunability of the reaction parameters. The unique synthetic properties of CHFS method have previously been applied to the continuous hydrothermal flow synthesis of graphene quantum dots (first in this field) adding to the portfolio of CHFS materials delivered by our group. We have also reported the substantially lower environmental impact of CHFS when compared to equivalent batch hydrothermal processes used for the synthesis of graphene quantum dots. The current work adds further to the development of continuous flow technology of carbon related quantum dots nanomaterial. One of the unique and interesting features of heteroatom doped CQDs is their selectivity, sensitivity and response to ions and molecular compounds in solution that result in changes in optical behaviour (fluorescence enhancement or quenching). Their response is reported to be highly dependent on the structure of the carbon dots, in particular their surface structure. Herein, we will explore the sensing properties of the CQD for chromium (VI) detection. Cr(VI) contamination of soil and water is anthropogenic in origin and raises significant concern in both developed and developing countries as a hazardous pollutant to the environment and human life, due to its high toxicity, carcinogenicity and mutagenic behaviour compared to other valence states, such as Cr(0) and Cr(III). Thus, the detection/concentration determination of Cr(VI) in drinking water (rigorously regulated to a very low micromolar level) for example, is vital. The current techniques of analysis (atomic absorption spectrometry, chromatography) have common challenges including time consuming complex pre-treatment procedures and/ or use of costly equipment. Therefore, a low cost, highly sensitive and selective sensor for the detection and determination of Cr(VI) is needed, particularly in regions that are underdeveloped or lacking fiscal capability or access to such facilities.

Herein, a facile, green, one-step continuous hydrothermal flow synthesis route using inexpensive and simple precursors; for example citric acid as a carbon source and ammonia, for production of blue emission N-doped carbon quantum dots that can be utilized as chemisensors for Cr(VI) detection. The synthesis of CQDs such as N-doped CQDs (NCQD), via the CHFS system delivers the following advantages (a) from a series of synthetic stages to a single step approach (synthesis and simultaneous doping), (b) a significant reduction in reaction times from hours to seconds, thereby reducing energy consumption, (c) promoting the use of renewable precursors (citric acid) and solvent (water), (d) facilitating tunability and control over reaction parameters (e.g. temperature, pressure and flow rate) and hence particle properties (e) as well as easy industrial scalability. Applying CHFS methods to produce carbon-based materials from biomass derivatives offers a new direction in achieving large-scale production of homogenous quality NCQDs and potentially significantly reduced costs and lower environmental impact.

Surprisingly, it has been found that a continuous hydrothermal flow process is applicable to the production of nanomaterials using bottom-up synthesis.

Advantageously, the use of a continuous hydrothermal flow process according to the present invention results in the preparation of carbon quantum dots, typically doped carbon quantum dots, with excitation independent emission. Further, the carbon quantum dots have narrow single emission profiles which is a desirable feature of a sensor molecule, in addition to a large stokes shift suitable to avoid auto-luminescence thereby increasing their usability in sensor applications.

Advantageously, the carbon quantum dots, typically, doped carbon quantum dots, produced by continuous hydrothermal flow process, as described herein, have excellent homogeneity with a low variation of morphology and particle size. Accordingly, the continuous hydrothermal flow process can reliably produce large amounts of doped carbon quantum dots with little variation in their shape and size, and therefore, little variance in their properties making it a consistent and reliable synthesis process.

Advantageously, it has been found that the luminescence of carbon quantum dots, typically, doped carbon quantum dots, more typically N-doped carbon quantum dots, according to the present invention is quenched in the presence of Cr(VI) ions resulting the doped carbon quantum dots being effective as a small molecule sensor for chromium (VI) ions,

It has been surprisingly found that carbon quantum dots, typically, doped carbon quantum dots, more typically S-doped carbon quantum dots according to the present invention demonstrate excellent proficiency in providing high oil recovery (-17% for S-CQD and -15% for un-doped CQD) while using ultra-low concentrations of 0.01 wt%. Thereby, the carbon quantum dots according to the present invention present a novel method of extracting oil more efficiently and cost-effective compared to known nanoparticles. Further, the carbon quantum dots advantageously are highly stable in aqueous conditions, including in high salinity conditions.

Advantageously, carbon quantum dots, typically, doped carbon quantum dots, more typically, S-doped carbon quantum dots according to the present invention increase the contact angle on an oil drop on a limestone surface and therefore they are able to detach oil from the surface of stone and effectively modify the surface to be more water wet. This results in improved oil extraction because the carbon quantum dots effectively decrease the affinity of oil to the limestone so that it is more efficiently removed by the water.

Advantageously, the carbon quantum dots, typically, doped carbon quantum dots according to the present invention are stable at pH 5-12 which covers known ranges of environments for sensing or oil extraction. Therefore, the carbon quantum dots can be used without any degradation, or loss of efficacy.

The size of the carbon quantum dots described herein are measured using HRTEM (high resolution transition electron microscopy) and AFM (Atomic Force Microscopy). It is understood that the size relates to the diameter of the particles.

For the avoidance of doubt, the terms continuous hydrothermal flow route, continuous hydrothermal flow process, continuous hydrothermal flow synthesis, continuous hydrothermal flow synthesis process and continuous hydrothermal flow synthesis route are synonymous.

For the avoidance of doubt, the absorption and luminescence spectroscopy measurements of compounds using UV-Vis, and steady state spectrophotometry, occurred at 25 °C, in aqueous conditions at pH 7.0 unless stated otherwise. For the avoidance of doubt, the stokes shift is the difference between the band maxima of the absorption and emission of the same electronic transition. The stokes shift when used herein is reported as the difference in the wavelength, although it is understood to the skilled person that the stokes shift could also be represented as the difference in wavenumbers, energy, or frequency.

For the avoidance of doubt, the term "blue luminescence" relates to an emission with a maxima in the "blue" section of the visible spectrum. In other words, the emission maxima is between 400-485 nm.

For the avoidance of doubt, the term "carbon rich" relates to a carbon quantum dot that contains greater than 50% carbon.

EXPERIMENTAL SECTION

Chemicals: All the materials were purchased from commercial suppliers and used without further purification. Deionised water, 15 MW obtained from an ELGA Purelab system was used in all experiments. Anhydrous citric acid, ammonia (32%) and Cr(VI) (potassium chromate and potassium dichromate) were purchased from Fisher Chemicals (UK) and used as received. The solutions of metal ions were prepared from their nitrate, acetate or chloride salts. The chlorides of Na⁺, K⁺, acetate of Fe²⁺, Zn²⁺, Cu²⁺, nitrates of Ce³ , Co²⁺, Ni²⁺,

Ag⁺ and sodium: F , Cl , Br , I , NCh , SOU, and HCO₃ were all purchased from Sigma Aldrich (UK).

Anhydrous pure D-Glucose was purchased from Fisher Chemicals (U.K.) and used as received p-sulfonic acid calix[4]arene (SCX4) was synthesised via adaption of previously reported methods according to C.D. Gutsche, D. Dhawan, M. Leonis, D. Stewart, p-Tert- BUTYLCALIX[6]ARENE, Org. Synth. 68 (1990) 238, https://doi.org/10.15227/orgsyn.068.0238, R. Lamartine, J.B. Regnouf de Vains, P. Choquard, A. Marcillac, Process for the dealkylating sulfonation of p-alkyl calixarenes, US Patent No: 5,952,526 (US005952526A), 1999, and S. Shinkai, K. Araki, T. Tsubaki, T. Arimura, O. Manabe, New syntheses of calixarene-p-sulphonates and p-nitrocalixarenes, J. Chem. Soc. Perkin Trans. 1 (1) (1987) 2297-2299, https://doi.org/10.1039/pl9870002297, the contents of which are incorporated herein by reference.

The crude oil used as from Expro North Sea Ltd (U.K.). It was diluted with heptane to have an API (American Petroleum Institute) gravity of 23° (915.9 kg/m³) and dynamic viscosity of 72 cP (0.072 Pa s) at 25 °C. A 90,000 ppm NaCl solution was used as formation brine with a density of 1.087 g/mL. Indiana limestone cores with an absolute permeability and porosity of 214 mD (2.11 x 10¹³ m²) and 15% void volume, respectively, were purchased from Kocurek Industries INC., Hard Rock Division (USA). All cores were cleaned with toluene to remove crude oil, and acetone to remove NaCl, using a Soxhlet extraction, and were subsequently dried in an oven at 60 °C for 48 hrs before and after core flooding and centrifuge studies. Rock substrates for contact angle measurement were prepared from the same Indiana limestone cores and treated in crude oil, as stated earlier, at room temperature and atmospheric pressure for 60 days. The sand-pack was prepared with fine sand (grain size 150-210 pm) for measuring CQDs retention, where the sand-pack had a bulk volume (BV) of 75 mL, and pore volume (PV) of 24 mL.

Equipment:

Freeze-drying was performed using a Heto PowderDry PL 3000.

X-Ray Photoelectron Spectroscopy (XPS): XPS measurements were performed using a Kratos Axis Ultra DLD photoelectron spectrometer utilizing monochromatic Alka source operating at 144 W. Samples were mounted using conductive carbon tape. Survey and narrow scans were performed at constant pass energies of 160 and 40 eV, respectively. The base pressure of the system was ca. lxlO⁹ Torr rising to ca.4xl0⁹ Torr under the analysis of these samples. High Resolution Transmission Electron Microscopy (HRTEM):

(a) for N-doped carbon quantum dots - Double- corrected JEOL ARM200F, equipped with a cold field emission gun. For the investigation, the acceleration voltage has been set to 80 kV and the emission was set to 10 pA. The samples were prepared by depositing the aqueous solution of NCQDs onto a holey carbon coated Cu-grid (400 pm). The particle size of carbon quantum dots was measured from TEM images using ImageJ software.

(b) for S-doped carbon quantum dots - JEOL JEM2100 equipped with LaB6 filament was used for particle size analysis. For investigation, the acceleration voltage was set to 200 kV and the emission was set to 107 mA. The samples were prepared by depositing the aqueous solution of S-CQDs onto a holey carbon-coated Cu-grid. The particle size of CQDs was measure from TEM images using ImageJ software.

Fourier-Transform Infrared Spectroscopy (FT-IR): FT-IR spectra were recorded using a Nicolet Avatar 370DTGS spectrometer fitted with a Smart Orbit accessory (diamond 4000- 200 cm¹).

Raman Spectroscopy:

(a) The spectrum of the as-synthesised and dried N-doped carbon quantum dots was measured with a Horiba LabRAM HR Evolution spectrometer with radiation at 514 nm.

(b) The spectrum of the as-synthesised and dried S-doped CQDs was measured with a Renishaw Raman system using the 488 nm line of an Ah ion laser at a power of ~ 10 mW.

Atomic Force Microscopy (AFM) images were obtained (a) for N-doped carbon quantum dots via dynamic mode on a hpAFM with AFM Controller (NanoMagnetics Instruments, UK) using Nanosensor tapping mode probes. The micrographs were then processed with NMI Image Analyser (vl.4, NanoMagnetics Instruments), with plane correction and scar removal using the in-built functions, (b) for S-doped carbon quantum dots via immobilising CQDs on single-side-polished p-type Si wafers (average roughness 77 pm). Each solution was diluted with HPLC grade H20 (Fisher Scientific, UK) to 1% of its 'as received' concentration. For each CQDs type, a single 10 pL droplet was deposited on the Si wafer fragment and dried in an oven at 37 °C for 24 h. Upon visual inspection, a circular residue of solid material remained on the wafer. Upon AFM analysis, the outermost region of the circular residue was found to contain unaggregated CQDs at a surface density suitable for image acquisition.

Images of dimensions 2 pm ^c 2 pm were acquired using an Asylum Research MFP-3D AFM (Oxford Instruments, UK) operating in Intermittent Contact Mode at a temperature of 18 °C and a relative humidity of < 40%. Images were composed of 512 ^c 512 pixels and the scanning velocity was 2.5 pm/s. Rectangular pyramidal-tipped Si cantilevers (PPP-NCL, Windsor Scientific, UK) were employed; their nominal length, width, and tip diameter were 225 pm, 38 pm and <10 nm respectively. Images were analysed using Scanning Probe Image Processor software (Image Metrology, Denmark).

Steady-State Optical Characterization. After the sample was purified, it was optically characterized using absorption (UV-Vis spectrophotometry) and emission/excitation (photoluminescence spectrophotometry) techniques. The sample concentration ((a) N- doped carbon quantum dots 1.1 mg/mL (b) S-doped carbon quantum dots 1.5 mg/mL) was determined by freeze-drying 10 mL of the purified carbon dot sample.

UV-Vis spectrophotometry: Adsorption measurements were conducted using a Shimadzu UV-1800, in the range of 200-700 nm in a 10 mm quartz cuvette.

Photo luminescence spectroscopy (PL): The fluorescence spectra were recorded with Shimadzu RF-6000 Spectrofluorophotometer.

Quantum Yield (QY) determination: QY value of carbon quantum dots was calculated by measuring the integrated PL intensity in aqueous dispersion of the synthesised CQDs in comparison with the integrated PL of quinine sulphate in 0.1 M H₂SO₄ (standard) and it was plotted as integrated PL vs Absorbance (see Figure 9 for NCQDs) and from where were extracted the slopes (the gradient D).

Where; ©CQDs is the quantum yield of CQDs; 0S is the quantum yield of standard (quinine sulphate 54%); AQD_S is the slope of integrated PL of CQDs; As is the slope of integrated PL intensity of the standard; r)Q_Ds is the refractive index of water (1.33); r₎s is the refractive index of 0.1 M H₂SO₄ (1.33).

Zeta potential of carbon quantum dots was measured using a Particle Metrix Stabino®- NANOflex® System. The aqueous sample solution was added to a PTFE beaker fixed with an oscillating piston at the centre of the sample. The particles become immobilised between the beaker walls and piston and the oscillating piston created a fluid flow of mobile ions cloud around each particle. A streaming potential was consequently created and measured via in-situ electrodes. Running time for each sample was 60 s.

Images of oil pendant drop, oil bubble, and oil sessile drop were recorded using a goniometer/tensiometer comprising of a Leica Wild M3Z stereo microscope and a JVC TK- C1381 colour video camera. The shape of an oil drop/bubble was analysed by First Ten Angstroms Incorporated Drop Shape Analysis Software Version 2.0 to estimate the surface tension, interfacial tension (IFT), and contact angle.

Core flooding setup employed the following: three fluid accumulators which were filled with crude oil, brine, and CQDs nanofluid, accordingly; a Presearch Limited model 260D syringe pump with a Teledyne ISCO D-SERIES pump controller; and a Bronkhorst EL- PRESS pressure meter/digital controller to record the pressure during core flooding experiments. The oil production from core flooding experiments was measured using a Vinci video separator system which consists of a Sony FCB-EX980P camera, a 200 cm³ burette with 120 cm³ measurable volume, and 18 cm external diameter. The obtained measurements were analysed by using the Vinci Acquisition software. Each experiment was repeated three times, the mean value and standard deviation were reported. Prior to core flooding experiments, the core sample was firstly fully immerged in 90,000 ppm brine in a beaker and placed in a vacuum chamber. The vacuum was applied with a pump until there w ere as no visible air bubbles escaping from the core sample, the core sample was then considered fully saturated with brine.

For EOR studies, a stock concentration of CQDs nanofluid (1.5 mg/ml) was diluted with 90,000 ppm NaCl solution to prepare a series of solutions with concentrations of 0.001 wt%, 0.005 wt% and 0.01 wt% in accordance to previous studies according to S. Li, M. Genys, K. Wang, O. Torsater, Experimental study of wettability alteration during nanofluid enhanced oil recovery process and its effect on oil recovery, Soc- Pet. Eng. - SPE Reserv. Characterisation Simul. Conf. Exhib. RCSC 2015, Society of Petroleum Engineers, 2015, pp. 393-403, , https://doi.org/10.2118/175610-ms and D. Luo, F. Wang, J. Zhu, L.u. Tang, Z. Zhu, J. Bao, R.C. Willson, Z. Yang, Z. Ren, Secondary oil recovery using graphene-based amphiphilic janus nanosheet fluid at anultralow concentration, Ind. Eng. Chem. Res.56 (39) (2017) 11125-11132, the contents of which are incorporated herein by reference.

Wettability studies were evaluated via the U.S. Bureau of Mines (USBM) wettability index of core samples by measuring the water/oil displacement driven by the centrifugal force. This was determined by capillary pressure analysis during water-displacing oil (imbibition) and oil- displacing water (drainage) studies via a centrifuge method (VINCI Technologies refrigerated centrifuge model RC 4500). The capillary pressure was controlled by setting rotation speeds to increase from 1400 rpm to 3500 rpm according to a pre-test of the system. The selected parameter of 1400 rpm was the minimal rotation speed to exert pressure to have production, and 3500 rpm was the maximum rotation speed that avoids damaging the core samples studied. Data analysis was performed using CYDAREX CYDAR system.

Nanomaterial's retention on rock surfaces was measured using a KONTES 420830-1510 model Chromaflex glass column with 2.5 cm inner diameter and 15 cm length for sand- pack. Calibration curve method was employed using abovementioned UV-Vis spectrophotometry to determine CQDs concentration.

Fluorescence Experiments. Fluorescence sensitivity and selectivity experiments of carbon quantum dots with Cr(VI) were performed as follows: Cr(VI) stock solution (1000 ppm) was prepared by dissolving potassium chromate (iCCrC , 100 mg) in NCQD aqueous solution (100 mL) in a standard volumetric flask. Further, fresh NCQD solution (1.1 mg/mL) was placed into a 10 mL standard volumetric flask, followed by addition of required volume of 1000 ppm Cr(VI) stock solution to achieve concentrations of 500 ppm, 250 ppm, 100 ppm, 75 ppm, 50 ppm, 20 ppm, 10 ppm, and 5 ppm. Additional dilutions of Cr(VI) in the ppb concentration range were prepared from Cr(VI) 100 ppm solution giving the following concentrations: 5 ppb, 10 ppb, 20 ppb, 50 ppb, 100 ppb, and 500 ppb (Figure 10). After 3 minutes incubation and stirring at room temperature, the fluorescence spectra were measured for the quantitative analysis of Cr(VI). Each experiment was repeated in triplicate. The fluorescence emission spectra for ion- sensing experiments were recorded for excitation at 360 nm with the band-slits of both excitation and emission set as 5 nm. The sensitivity was fixed on high with a response time set at 0.5 s. The emission spectra were recorded from 300 to 650 nm, and the fluorescence intensity of NCQDs at 441 nm was used for quantitative analysis of both Cr(VI). For comparison purposes, a range of anions and various metal cations were tested for selectivity and sensitivity using 50 ppm as a standard concentration. Furthermore, stability analysis of the NCQDs in presence of Cr(VI) (50 ppm) were made by recording the fluorescence intensity of the mixture when initially exposed continuously for 5400 seconds (90 minutes) at 360 nm excitation, and then at intervals of 2 hr, 4 hr, 24 hr and 48 hr (Figure 11).

Synthetic Methodology:

Synthesis of NCQDs: The experimental procedure for NCQDs produced via the CHFS approach is depicted in Figure 8. Optimally for this study, CHFS consisted of three feeds: supercritical water feed (FI: flow rate of 20 mL/min) and two feeds of the precursors: citric acid (F2: with concentration of 70 mg/mL delivered at 10 mL/min in the mixing zone) and ammonia (F3: with concentration of 1 M pumped at 10 mL/min). The supercritical water feed was heated at 450°C (lower temperatures of 250°C and 350°C have also been explored) and the system pressure was kept constant at 24.8 MPa using a back-pressure regulator (labelled as BPR). The F2 and F3 were combined in a "T" junction prior to being delivered into the reaction zone (labelled as "Reactor") where it was mixed with the supercritical water feed (FI). The reaction mixture was then passed via "Cooler" to BPR and collected for further processing (see Schematic 1). The entire reaction mixture was filtered using 0.2 pm alumina membrane, and the filtrate was initially separated using 30 kD membrane in a tangential filtration unit, followed by 1 kD membrane. The resulting solution was concentrated to 1/5 of the initial volume and subjected to further analysis.

Synthesis of Carbon Quantum Dots (CQDs)

The experimental procedure for S - CQDs produced via Continuous Hydrothermal Flow Synthesis method is depicted in Figure 13, according to S. Kellici, J. Acord, J. Ball, H.S. Reehal, D. Morgan, B. Saha, A single rapid route for the synthesis of reduced graphene oxide with antibacterial activities, RSC Adv. 4 (29) (2014) 14858, https://doi.org/10.1039/c3ra47573e, S. Kellici, J. Acord, K.E. Moore, N.P. Power, V. Middelkoop, D.J. Morgan, T. Heil, P. Coppo, I.-A. Baragau, C.L. Raston, Continuous hydrothermal flow synthesis of graphene quantum dots, React. Chem. Eng. 3 (6) (2018) 949-958, and S. Kellici, J. Acord, N.P. Power, D.J. Morgan, P. Coppo, T. Heil, B. Saha, Rapid synthesis of graphene quantum dots using a continuous hydrothermal flow synthesis approach, RSC Adv. 7 (24) (2017) 14716-14720 the contents of which are incorporated herein by reference. The reaction system of Figure 13 is constructed with 316SS Swagelok stainless steel fittings and tubing. It consists of a water heater, three Gilson 307 HPLC pumps that deliver the aqueous solution of precursors and supercritical water to the reaction zone; a post reactor cooler; and a back pressure regulator (BPR) that maintains a constant pressure in the system. The flow rates used were 20:5:5 mL/min (P1:P2:P3) for Pump 1 (delivering DI water through heater), Pump 2 (pumping glucose, 70 mg/mL solution) and pump 3 (delivering p-sulfonic acid calix[4] arene (SCX4), 5 mg/mL solution), respectively. In a typical experiment, SCX4 (5 mg/mL) was delivered via Pump 3 to meet a flow of a glucose (70 mg/mL) solution at a T-junction. The resulting mixture was then combined with superheated water (450 °C, 24.8 MPa) inside a counter-current mixer, whereupon the product formation occurred in a continuous mode. This was then followed by a cooling step, where the reaction mixture was passed through a vertical cooler and collected for further purification. Two samples of CQDs were prepared, one with (S-CQD) and one without (g-CQD) the presence of SCX4; all other conditions were kept the same.

The purification process of the CHFS synthesized carbon quantum dots was divided into two steps: a) separation of the large particles according to size, and b) removal of small molecules by-products or/and precursors, followed by concentrating the materials to a stock volume. Initially, the larger particles were separated from the main solution via dead-end filtration through 0.2 pm pore size alumina membrane, followed by size separation via 30 kD membrane in a tangential filtration unit. The filtrate containing CQDs and other small molecules by-products and/or reaction precursors undergo further cleaning using a 1 kD membrane in a tangential filtration unit. The resulting solution is 20% of the initial volume and ready to be subjected to further experiments and analysis. RESULTS AND DISCUSSION

N-doped Carbon Quantum Dots

Carbon dots are known for their properties, specifically their intense fluorescence in the visible range. However, the materials often exhibit optical and structural heterogeneity as well as limited relevant synthetic approaches that do not readily facilitate large-scale production. We have explored and developed a rapid, synthesis approach that delivers in a continuous mode, carbon quantum dots (control) and blue luminescent N-doped carbon quantum dots by simply using citric acid (carbon source), in the absence and presence of ammonia (N-precursor) respectively, both in water under supercritical conditions (24.8 MPa and study reaction temperatures of 250 °C , 350 °C , and 450 °C). The optimal temperature was 450 °C for the NCQDs (see Figure 12) synthesised using Continuous Hydrothermal Flow Synthesis (a single step approach as shown in Figure 8) showing high homogeneity (narrow particle size distribution) and excellent optical properties (excitation independent fluorescence).

The optimal as-produced NCQDs were characterized using a variety of techniques including UV-Vis absorption and emission (PL) spectrophotometry to examine the optical properties, FT-IR and Raman spectroscopy to determine electronic properties and functionalities, X-ray Photoelectron Spectroscopy (XPS) to determine the composition and surface chemistry, High- Resolution Transmission Electron Microscopy (HRTEM) analysis and Atomic Force Microscopy (AFM) for particle size analysis and structural morphology.

HRTEM images (Figure 1) of the as-prepared NCQDs exhibit a quasi-spherical morphology with an average particle size of 3.3 ±0.7 nm from a sample population of 190 particles ranging between 1.9 nm to 4.7 nm in diameter (inset Figure lb). Each exhibited the same structural arrangement, indicating a consistency in homogeneity for the CHFS synthesized sample. The graphitic core arrangement of the carbon atoms (Figure lc) can be clearly identified with in- plane lattice spacing of 0.22 nm and is consistent with the reported literature data.

The atomic force microscopy (AFM) image (Figure Id) reveals the tomography of the as- synthesised NCQDs, distributed in the range from 1.0 to 5.2 nm, with an average value of 2.4 ± 1.0 nm, and is consistent (within experimental errors) with data from HRTEM.

X-ray photoelectron spectroscopy (XPS) measurements were performed for the surface characterization of NCQDs (Figure 2a) and reveal peaks typical for the presence of carbon (ca. 285 eV), nitrogen ( ca . 399 eV) and oxygen ( ca . 531 eV). The fitted Cls spectra (Figure 2b) peaks at 284.9 eV, 285.9 eV, and 288.4 eV can be assigned to the carbon atoms in the form of C=C bond (sp²), C-N (sp³), C=0 (sp²) and 0-C=0 (sp²), respectively, whilst the fitted N(ls) spectrum (Figure 2c) exhibits three peaks at 399.7 eV, 400.8 eV, and 401.6 eV, indicating that the nitrogen exists in pyrollic/amino N-H, protonated pyridinic N, and graphitic-N (sp³) forms respectively, signifying that the nitrogen atoms were efficiently doped into the structure. Elemental analysis (inset Figure 2a) shows that CHFS synthesised NCQDs contain 35.9 wt% oxygen, 10.8 wt% nitrogen and 53.3 wt% of carbon, concluding that NCQDs are nitrogen- doped and carbon-rich.

The Raman spectrum for the NCQDs (Figure 3a) displayed two broad peaks at 1392 and 1591 cm¹ which correspond to the D and G bands, respectively. The G band is attributed to an E_2g mode of vibration of sp² bonded carbon atoms associated with the graphitic core and is in good agreement with the HRTEM lattice spacing image described previously for the NCQDs (Figure lc). The smaller D band peak is due to the presence of a medium level of oxygen content (35.9 wt%) and the presence of sp³ carbon atoms, the results are complimentary with the XPS data (see Figure 2). The relative intensity ratio of the observed bands (ID/IG) gave a value of -0.76 for the NCQDs, typical of graphene oxide. The FT-IR spectroscopy (Figure 3b) further supports the XPS analysis. A broad absorption band (3450-2400 cm¹) can be ascribed to overlapping stretches that encompass those for O- H (R-OH,-COOH), amine and protonated amine (N-FF, N-FF⁺, N-FF⁺) stretches, and C-H stretching vibrations (3028 cm¹ and 2835 cm¹). The presence of protonated and deprotonated species is plausible with carboxylates and amine groups in close proximity on the NCQDs. Other stretches observed include pyridinic C=N at 1652 cm¹, a stretch which could also be attributed to an amino- vibration (-NH or -NFF). The amino vibrations could also be assigned for stretches at 864 cm¹ and 792 cm¹ as well as carbonyl (COO ) stretches at 1541 cm¹ and 1394 cm¹, asymmetric vibrations for C-NH-C at 1136 cm¹, and C-O and C-O-C vibrations may be assigned to stretches at 1202 cm¹ and 1035 cm¹, respectively.

The NCQDs were analysed with UV-Vis and the steady-state PF spectrophotometry. The characteristic specific absorption (black curve) and emission bands (red curve) recorded from aqueous solutions of NCQDs are shown in Figure 4. Figure 4a shows the strongest absorbance and emission band for NCQDs produced via CHFS. The UV-Vis spectrum displays two absorption bands that are characteristic of NCQDs, the first at -250 nm and the second peaking at 332 nm (broad absorption band from 300 nm tailing to 480 nm). The former band can be ascribed to p-p* transitions for aromatic sp² domains in the graphitic core and the latter to h-p* transitions for C=0 in the NCQDs. The absorption band displayed below 250 nm can be attributed to the C=C and the C-C bonds.

Photoluminescence (PF) was observed for both the as-prepared NCQDs and the CQDs (control) under UV excitation with wavelengths ranging from 300 - 420 nm. The control material, CQDs synthesised from citric acid only, showed negligible photoluminescence (Figure 12). The NCQDs on the other hand displayed an optimum excitation at 360 nm corresponding to a blue fluorescence emission at a consistent 441 nm for each excitation (Figure 4b), and a red shift lower intensity emission for lower energy excitation at 400 nm and 420 nm. The as-prepared NCQD material exhibited a rarely observed UV excitation independent emission behaviour. We attribute this zero-tunability (320 - 380 nm) behaviour to surface state defects of the NCQDs, which is also commonly associated with blue emission, a feature that has been reported for rGO, GQDs and CNDs. The red shift character due to the observed emission for lower energy excitations may be ascribed to the p-p* transitions (of isolated sp² clusters) within the graphitic carbon cores.⁴⁵ The proportion of surface defects are correlated with the degree of surface oxidation with the increasing presence of oxygen atoms in the make-up of the surface structure of CQDs which typically, leads to further reducing the band gap, i.e. a red shift of PL. However, given the excitation independent blue-luminescent for the as-prepared NCQDs, the process of nitrogen doping (CHFS) as compared to the control, has had a significant impact on the optical properties (Figure 12).

Since pH can play pivotal roles in various environmental and biological systems, the impact of pH on the NCQDs performance as a sensor is of significance. The pH- dependent behaviour of the NCQDs was explored (Figure 4c and 4d), where mildly acidic and alkali media have had a small to negligible impact on fluorescence stability showing the NCQDs to be stable over a broad pH range of 5-12. However, the fluorescence intensity of the NCQDs is significantly reduced in acidic media to 50% (against control intensity) at pH 4 and reduced further to just 13% at pH 3 and 3% at pH 1. A small but significant red shift occurs from pH 3 to pH 1. Similar observations have been made by Zhu et al. (for their excitation dependent N-doped CQDs hydrothermally synthesised from citric acid and ethylenediamine) and Dong et al. (for the N,S-CQDs) but neither adequately rationalize for the red shift. The diminishing intensity is a consequence of protonation of the nitrogen and carboxylate groups disrupting the surface charge and its associated emissivity thus, allowing emission from the graphitic core come to prominence over that of the surface as reflected by the red shift of the PL. The red-shift feature of the luminescence highlighted earlier is excitation dependent. At pH 13 there is also a reduction in emission intensity versus control. Furthermore, the NCQD material exhibited resistance to photo-bleaching with PL intensity remaining stable over a period of 6 months. The quantum yield value of our sample was measured to be 14.91 ± 0.24% (calibrated against quinine sulphate in 0.1 M H₂SO₄ as standard), comparable to many literature reports for CQDs.

Typically, excitation dependent emission is a common feature of N-doped CQDs, which tend to display complex emission spectra that are difficult to decipher in practical applications. Thus, single emission NCQDs are highly desirable. Our CHFS synthesised material uniquely exhibits the following: excitation independence with a narrow FWHM (~78 nm, where 100 nm is typical for CQDs) and a remoteness of the fluorescence emission (441 nm) from the UV excitation range (320 - 380 nm) - that usefully avoids auto luminescence. Each of which are desirable features for sensor applications and more so when combined. These characteristics will ultimately allow this material to be uniquely suitable in a range of practical applications, such as chromate anion (Cr(VI)) detection for example; a severe and highly toxic environmental pollutant even at trace (low mg L¹) levels.

Chromium (VI) ion-sensing:

Given the fluorescent properties of the NCQDs and their stability over a broad pH range (pH 5-12), investigations with respect to their interactions with and selectivity for, were undertaken for a range of environmentally relative anions and cations. These included the metal ions Na⁺, K⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ce³ , Ag⁺ and the anions: F , Cl , Br , I , NQfSO-f

, CH₃COO , HCO₃ , and the Cr ( VI) anions CrOT and CnOz^2- at concentrations of 50 ppm each in aqueous solutions. Sole selectivity for Cr(VI) detection was observed by significant fluorescence quenching of the NCQDs upon the addition of either Cr(VI) species (Figure 5), whilst the other ions exhibited limited/negligible quenching effect or fluorescence enhancement with minimal variation. This selectivity for Cr(VI) (CrOC / CnO²) by the NCQDs fluorescent probe was further investigated in regard to their sensitivity based on the PL spectra of NCQDs with a range of prepared concentrations of CrOA, each performed in triplicate. As shown in Figure 6 the fluorescence of NCQDs at around 441 nm is quenched after the addition of Cr(VI) with the emission intensity shown to be dependent on the concentration of Cr(VI) species. The fluorescence peaks of [CQDs-Cr(VI)] system was stable at 441 nm; no peak shifting was observed.

The fluorescence intensity of NCQDs decreased linearly with concentration of Cr(VI) (Figure 6). Stern-Volmer relation plot (inset Figure 6) showed a good correlation (R² = 0.998) giving a quenching constant Ksv value of 0.01113 (obtained from the slope of the line y = 0.01113x + 0.03897). The limit of detection (LOD) is calculated as follows: LOD = 3s/Ksv, where s is the standard error of the intercept. The LOD obtained value was 0.365 ppm and Limit of quantification-LOQ (=10s/ Ksv) was 1.218 ppm. Emission intensities at 441 nm for the pure NCQDs solution and [ NCQDs-CC ] (Figure 11) have shown negligible variation when the samples were continuously excited at 360 nm for 90 minutes. Further interval measurements of [NCQDs-Cr⁶⁺] revealed just an 8.3% diminishment in emission intensity over a 48 hour period (see Figure 11). This stability of the respective systems reflects the materials suitability as a sensor for Cr(VI) detection.

Given that there was no observed red-shift for the Cr(VI) studies with the NCQDs, contrary to that to the findings of the NCQDs pH study, and no observed absorption peak shifts for UV- Vis spectra of the Cr(VI) and NCQDs mixture compared to Cr(VI) and NCQDs controls, suggests a mechanism other than that involving Cr(VI)- NCQD surface interactions. The selectivity of the CHFS-synthesised NCQD towards Cr(VI) can be attributed to the Inner Filter Effect (IFE). The IFE occurs where there exists a spectral overlap between the absorption bands of the chromate (CrOA) and the excitation band and/or emission band of the NCQDs as shown in Figure 7.

The excitation spectrum for the NCQDs has two overlapping bands at 326 nm and 358 nm with its emission band centred at 441 nm (excitation at 370 nm), whereas the chromate (CrOA) has one of its absorption bands centred at 372 nm, with significant overlap of the maximum excitation band of the NCQDs. These factors generate an absorptive competition between anion units and NCQDs particles inside the solution, moreover, not only is the chromate effectively absorbing the radiation at 370 nm necessary for NCQDs to generate the transition to the excited state, but it can also absorb emitted light from the NCQDs, translating to a quenching of the NCQDs fluorescence. The quenching mechanism, IFE, has been previously reported as an effective on-off, rapid and enhanced sensitivity approach to chromium (VI) sensing.

S-doped carbon quantum dots

The S-CQDs were analysed using steady-state optical characterisation (UV-Vis and photoluminescence spectrophotometry) to determine their optical & electronic properties, Raman and FT-IR spectroscopy to determine their functionalities, X-ray photoelectron spectroscopy (XPS) to determine the surface chemistry and elemental composition, High Resolution Transmission Electron Microscopy (HRTEM) and Atomic Force Microscopy (AFM) to ascertain the dimensions of the particles.

Transmission Electron Microscopy (TEM) analysis showed round shaped morphology characteristic for CQDs with a mean particle size of 1.7 ± 0.7 nm, with a particle size range between 1.0 nm and 2.5 nm (Figure 14 a-b), with sample homogeneity observed from the particle size distribution histogram to be predominantly between 1.5 nm and 2.0 nm.

The atomic force microscopy (AFM) image (Figure 14c) reveals the topography of the as- synthesised S-CQD with nanoparticles exhibiting mean diameter of 1.5 ± 1.1 nm (and maximum diameter 4.9 nm) and is complimentary with values determined from TEM. A minimum of 150 particles were analysed.

X-ray photoelectron spectroscopy (XPS) studies (Figure. 15) reveal the presence of the carbon, oxygen and sulfur atoms on the surface of the S-CQDs. In the expanded XPS spectra, the Cls peaks at 284.6 eV, 286 eV, 287.4 eV, 288.8 eV, and 291 eV can be assigned to C-C (sp²), C-0/C-S(sp³), C=0 (sp²), 0-C=0 (sp²), and tt-tt* satellite respectively. The O (Is) peak has a maximum at 532.6 eV with an apparent asymmetry to lower binding energy. Due to the range of oxygen environments as inferred by the S(2p) and C(ls) core-level spectra, many of which will have similar binding energies, we have not attempted to fit this region and an S(2p^3/2) peak at 168.5 eV for C-SCEH. The atomic ratio C/O is 2.27, and the CHFS as produced S-CQDs contain 30 at% oxygen, 1 at% sulfur and 69 at% carbon.

Raman spectroscopy confirms the presence of sp² carbon (graphitic core) in the S-CQDs sample as shown in Figure 16a, two broad peaks at 13856 and 1598 cm¹ correspond to the D and G bands, respectively. It is well known that the G band is attributed to an E2_g mode of graphite associated with the vibration of sp² bonded carbon atoms, indicating the aromatic character of the carbon dot's core; D band corresponds to sp²3 molecular defects. The ratio of peak intensities of D and G bands, ID/IG for S-CQDs, which is indicative of disordered and aromatic domains, was determined as 0.7, revealing the graphitic core as the major component in the carbon dot's particle. The smaller D band peak is as a consequence of the oxygen content (30 at%) and presence of sp³ carbon atoms, the results are in agreement with XPS data (see Figure 15).

The FT-IR spectroscopy (Figure 16 b) lends further supports to the XPS and Raman analysis, indicating that S-CQDs exhibit a variety of oxygen functionalities (carbonyl, carboxyl or hydroxyl) on their surface. Also, this cocktail of oxygen functionalities lends excellent solubility in water of the S-carbon quantum dots. A broad peak (3500-2820 cm¹) can be ascribed to the vibration of O-H (R-OH, -COOH), and C-H stretching vibrations (2931 cm¹) for sp³ hybridized carbon atoms. The carbonyl vibrations (for carboxyl, COO) could also be assigned for stretches at 1708 cm¹, and C-O and C-O-C vibrations may be assigned to the stretch at 1045 cm¹, respectively. The IR vibrations characteristic for S]0 stretch (R-SO₃H) for SCX4 are located at 1164 cm¹ and 1047 cm¹; it's expected that these stretches would be significantly overlapped by carbon-oxygen functionalities of S-CQDs that absorb in the same domain given the low concentration of SCX4 used in synthesis (and is in agreement with XPS elemental analysis for S on the S-CQDs surface). The UV-Vis spectrum (Figure 17b) displays two absorption bands centred at 225 nm and 278 nm with a tail extending in the visible region. The Aabs = 225 nm band is generally assigned to the tt-tt* transitions of the aromatic C]C sp² domains typically, found in the carbon dots graphitic core, whereas the Aabs = 278 nm is due to the h-p* transitions of the functional groups located on the CQDs surface. Whilst irradiated by UV light, it was observed that the transparent brown S- CQD solutions gave a light green luminescence, which was in contrast to the light blue for glucose CQDs (g-CQDs).

The excitation independence (300-360 nm) observed can be assigned to surface state defects of the S-CQDs, a phenomenon previously reported for carbon based quantum dots (e.g. graphene oxide, doped carbon quantum dots). However, on excitation at longer wavelengths (380nm to 440nm) the emissions changed to excitation dependence with emission maximum shifting from blue to green (503 nm to 530 nm), a feature that may be attributed to excitation for the n-rt* transitions (for isolated sp² clusters) within the graphitic core. The pH-dependent behaviour of the S-CQDs can play an important role in their application in various systems; by varying the pH range between 1 and 13, the very nature of the S-CQDs surface structure can be modified as reflected by their emission performance, as explored in Figure 17 c-d. The S-CQDs fluorescence demonstrated stability over a broad pH range (pH 3-11), although curiously optimal emissions were achieved at pH 4 and pH 10, but only -80% of optimal emission intensity was achieved between pH 4 and pH 9. The lowest and highest pH values saw significant reduction (>40%) in emission intensity. The change in intensity is due to protonation and deprotonation of the various functionalities of the 0D structures surface (carboxylate, alcohol, sulfonate groups), thus disrupting the surface charge and its emissivity. The interesting behaviour observed between pH 3 and 11 may be attributed in part to the influence of SCX4 within the surface structure and the impact of intramolecular hydrogen bonding between its four phenolic protons (pKi = 3.26, pK2 = 12.3, pK₃ = 12.9, pK₄ = 13.6) and stabilisation between its varying anionic states. The quantum yield value of S-CQDs was measured to be 0.25% (calibrated against quinine sulfate in 0.1 M H₂SO₄ as standard).

Enhanced Oil Recovery

In applying CQDs in enhanced oil recovery (EOR), their particle colloidal stability would be a very important factor to investigate, as any sedimentation or aggregation of the CQDs would be detrimental to their performance in EOR. To evaluate particle stability in solution, analysis of the materials zeta potential was undertaken; absolute zeta potential values for the particles greater than 30 mV would indicate they are typically, stable. The zeta potential of the S-CQDs was recorded as -42.3 mV for a particle concentration of 0.5 mg/ml. The zeta potential of the control sample CQDs (g-CQD) was -25.8 mV for a concentration of 0.6 mg/ml, reflecting a lower stability compared to the S-CQDs. Solution of both CQDs sample types (S-CQD and g-CQD) were prepared at concentrations of 0.01 wt% to determine their colloidal stability in both purely aqueous and high salinity systems (formation brine at 90,000 ppm).⁵⁹

The solutions for each CQDs were stored at atmospheric pressure and 25 °C for 30 days and monitored by UV-Vis spectrophotometry to evaluate their degree of aggregation and deposition of the CQDs. UV-Vis spectrum analysis for the water and brine dilutions of the CQDs nanofluids (Figure 18) displayed good colloidal water stability for both CQDs sample types. However, the g-CQDs brine sample, Figure 18 b, revealed a significant reduction in UV absorbance reflecting aggregation/deposition of the particles from the solution. In contrast, the S-CQDs sample (Fig.18a) demonstrated excellent colloidal stability in brine.

Interfacial tension (IFT) between oil and water plays a very important role in oil recovery as water is the most common and convenient displacing medium in the industry. The surface tension of S-CQDs nanofluid in air, and IFT of crude oil drop in S-CQDs brine nanofluid, were measured using pendant drop/raising bubble photographical method. The surface tension and IFT of g-CQDs sample and p-sulfonic acid calix[4]arene (SCX4) aqueous solution were also measured for comparison. The concentration for all three samples were prepared at 0.01 wt%. The analysis of the results (as shown in Chemical Engineering Journal 405 (2021) 126631, https://doi.Org/10.1016/j.cej.2020.126631: page 8, Fig. 6, the contents of which are incorporated herein by reference) showed no significant change for either the surface tension or IFT.

In EOR, wettability plays a very important role, as water-wet rocks help improve water displacement of oil. The capillary driving force for water into a core's pores is stronger in a water-wet system. In this study, the wettability alteration was evaluated by contact angle measurement with oil sessile drop on a limestone substrate submerged in CQDs nanofluid (0.01 wt%). As shown in Chemical Engineering Journal 405 (2021) 126631, https://doi.Org/10.1016/j.cej.2020.126631, page 9, Fig. 7, the contents of which are incorporated herein by reference, it can be observed that S-CQDs increased the contact angle of an oil drop on a limestone surface from 32.79° (in brine) to 49.78°, hence strongly indicating the limestone surface has been modified to be more water-wet, and that the oil is more detached from the limestone surface. The g-CQDs also significantly increased the oil drop contact angle (42.63°) on the limestone surface but to a lesser extent to that of the S- CQDs.

The capillary pressure during water-displacing oil (imbibition) and oil-displacing water (drainage) processes was measured via the centrifuge method according to E.C. Donaldson, R.D. Thomas, P.B. Lorenz, Wettability determination and its effect on recovery efficiency, Soc. Pet. Eng. J. 9 (1969) 13-20, https://doi.org/10.2118/ 2338-pa, the contents of which are incorporated herein by reference. The wettability of samples of limestone core was estimated using brine and three different S-CQDs nanofluid concentrations (0.001 wt%, 0.005 wt%, and 0.01 wt%) were used. The drainage and imbibition curves of the capillary pressure versus brine saturation were plotted (as shown in Chemical Engineering Journal 405 (2021) 126631, https://doi.Org/10.1016/j.cej.2020.126631, page 9, Fig.8, the contents of which are incorporated herein by reference) to calculate the U.S. Bureau of Mines (USBM) wettability index estimate according to E.C. Donaldson, R.D. Thomas, P.B. Lorenz, Wettability determination and its effect on recovery efficiency, Soc. Pet. Eng. J. 9 (1969) 13- 20, https://doi.org/10.2118/ 2338-pa, the contents of which are incorporated herein by reference. The duration for one imbibition and drainage process was 14 days and thus considering the instability of g-CQD brine, this test explores the properties of S-CQDs only which showed excellent colloidal stability in brine (Figure 18).

The USBM wettability index was determined from the ratio of log of the area under drainage curve and log of the area under imbibition curve. For the water wet system, the USBM index is greater than zero; but for an oil wet system, it is less than zero. The USBM index trend for increasing S-CQDs nanofluid concentration (as shown in Chemical Engineering Journal 405 (2021) 126631 , https://doi.Org/10.1016/j.cej.2020.126631, page 10, Fig. 9, the contents of which are incorporated herein by reference) shows that the wettability shifted from oil-wet to more water-wet with increasing concentration of S-CQDs. As it is mentioned before, the disjoining pressure enables nanofluid to spread on the rock surface. Wasan and Nikolov stated that nanofluids tend to form thin wedges, and the wedges are driven by disjoining pressure according to D.T. Wasan, A.D. Nikolov, Spreading of nanofluids on solids, Nature 423 (2003) 156-159, https://doi.org/10.1038/nature01591 , the contents of which are incorporated herein by reference. The disjoining pressure force associated with one single nanoparticle is small. However, a large amount of nanoparticles present can result in much larger disjoining pressure force, which is enough to drive the thin wedge film of nanofluids in the contact region of the oil sessile drop and rock surface. As a result, the oil sessile drop is easier to be detached from the surface of rock. This reflects to a wettability alteration towards water-wet. The core flooding test was used to measure incremental oil production driven by S-CQDs nanofluid. It is a process that simulates oil recovery from a reservoir. Typically, dominant mechanisms such as wettability alteration and log-jamming are combined in this process, to recover oil from tight limestone under certain level of confining pressure (110 bar, 10 bar above oil injection pressure during drainage). Pressure drop versus oil production was recorded to determine the log-jamming mechanism.

As shown in Chemical Engineering Journal 405 (2021) 126631, https://doi.org/10.1016/jxej.2020.126631, page 10, Scheme 2, the contents of which are incorporated herein by reference, three vertical cylinders (accumulators) were filled with S- CQDs nanofluid, brine, and crude oil, accordingly; the pump connected to the accumulators injected the fluids into the pre-saturated core sample. The saturated core sample was then placed in the core holder (110 bar, 25 °C) and connected to the accumulators. The crude oil was injected at a rate of 1 mL/min through the core sample, using core flooding system for drainage process to create initial oil saturation according to S. Kiani, M. Mansouri Zadeh, S. Khodabakhshi, A. Rashidi, J. Moghadasi, Newly prepared nano gamma alumina and its application in enhanced oil recovery: an approach to low-salinity waterflooding, Energy Fuels 30 (5) (2016) 3791, https://doi.org/10.1021/acs.energyfuels.5b03008, the contents of which are incorporated herein by reference ² Then, brine was injected (1 mL/min) for secondary oil recovery, followed by S-CQDs nanofluid at concentrations of (a) 0.001 wt%, (b) 0.005 wt%, and (c) 0.01 wt% for injection for EOR (1 mL/min). Low flow rate of 1 mL/min was chosen to optimise the length of core flooding experiments and to avoid high backpressure, front distortion and fingering. The production fluids were collected in a U-shape burette system for separation and volume measurement. The changing of oil-water interface was recorded for every minute, and the oil production was calculated from recorded images.

From each test exercised there was an obvious pressure drop during S-CQDs nanofluid flooding, and a significant amount of oil was produced by the injection of S-CQDs nanofluid after the pressure drop occurred (As shown in Chemical Engineering Journal 405 (2021) 126631, https://doi.Org/10.1016/j.cej.2020.126631, page 11, Fig. 10, the contents of which will). The mechanism of this oil production can be explained by temporary log-jamming. Small sized particles tend to aggregate at the pores throats and block pathways, however, with continuous injection, the pressure on surrounding pores will build up due to this blockage, thus improving the sweeping efficiency. As more trapped oil is removed, the surrounding pressure decreases and the particle aggregates disassociate and disperse into the flooding fluid.

This can be associated to the observed pressure drop phenomenon as shown in Chemical Engineering Journal 405 (2021) 126631 , https://doi.Org/10.1016/j.cej.2020.126631, page 11, Fig. 11, the contents of which are incorporated herein by reference. As shown in Chemical Engineering Journal 405 (2021) 126631 , https://doi.Org/10.1016/j.cej.2020.126631, page 11, Fig. 11c, the contents of which are incorporated herein by reference, , the pressure was first increased followed by a sharp decrease. The increasing of pressure can be explained by temporary blockage of pores throats by the S-CQDs aggregation. The higher the concentration of S-CQDs nanofluid, the quicker and easier it is to have the log-jamming to occur, thereby increasing the additional oil recovery factor incrementally by 7 ± 0.1%, 13 ± 0.2% and 17 ± 0.2% respectively for S-CQDs Nanofluid concentrations of 0.001 wt%, 0.005 wt% and 0.01 wt%. For comparison, core flooding with 0.01 wt% (optimum concentration) g-CQDs was also conducted. This gave an additional Oil recovery of 15 ± 0.5%, whilst lower than that recorded for 0.01 wt% S-CQDs (possibly due to the lower colloidal stability and wettability alteration of the g-CQDs), the core flooding recovery obtained by using either material was higher than using conventional non-nano chemical surfactants or carbon derived NPs, as shown in Chemical Engineering Journal 405 (2021) 126631 , https://doi.Org/10.1016/j.cej.2020.126631, page 11, Table 1, the contents of which are incorporated herein by reference, according to A. Mohsenatabar Firozjaii, A. Derakhshan, S.R. Shadizadeh, An investigation into surfactant flooding and alkaline-surfactant-polymer flooding for enhancing oil recovery from carbonate reservoirs: Experimental study and simulation, Energy Sources Part A 40 (24) (2018) 2974-2985, https://doi.org/10.1080/15567036.2018.1514439, and D. Luo, F. Wang, J. Zhu, F. Cao, Y. Liu, X. Li, R.C. Willson, Z. Yang, C.-W. Chu, Z. Ren, Nanofluid of graphene-based amphiphilic Janus nanosheets for tertiary or enhanced oil recovery: High performance at low concentration, PNAS 113 (28) (2016) 7711-7716, the contents of which are incorporated herein by reference. It can also be seen that dodecyltrimethylammonium bromide achieved a higher oil recovery of 19%, but at a significantly higher surfactant concentration of 4 wt%, a four hundred fold difference in additive used. Both S-CQDs and g-CQDs proved to be more efficient and potentially, more economical.

The adsorption of the CQDs on to the rocks surface is a very important factor in EOR because it can alter the wettability and cause temporary log-jamming, however, if an overabundance of CQDs is adsorbed, It may reduce the oil recovery factor due to material loss and potential formation damage caused by blockage. S-CQDs had the best ability of enhancing oil recovery; therefore the S-CQDs retention was measured to evaluate its adsorption on to the rock surface by the mass loss of S-CQDs from solution. A quarter of pore volume (PV) of a series of S-CQDs nanofluid dilutions (0.001 wt%, 0.005 wt% and 0.01 wt%) was injected into a sand-pack, and the S-CQDs concentration in the effluent was assessed by UV-Vis spectrophotometry. The S-CQDs retention rate was determined from the mass ratio of the recovered S-CQDs in the effluent to that of the total injected S-CQDs. The retention rate of S-CQDs was estimated to be -20% with no significant change with increasing S-CQDs concentration as shown in Chemical Engineering Journal 405 (2021) 126631 , https://doi.Org/10.1016/j.cej.2020.126631, page 11, Fig. 11, the contents of which are incorporated herein by reference.

CONCLUSIONS

In conclusion, a continuous hydrothermal flow route was developed for the synthesis of carbon rich, nitrogen-doped carbon quantum dots starting from citric acid and ammonia as precursors. The photoluminescence studies for the NCQDs demonstrated excitation independent behaviour with the emission peak at 441 nm due to diverse functional groups surface coverage from N- doping of the CQDs. Furthermore, we provide the proof of principle application that in aqueous solutions the materials display a high selectivity and sensitivity for Cr(VI), rendering our materials suitable for environmental applications. The materials may also be applied in a spectrum of applications including photovoltaics, bio-tagging, energy storage and beyond.

Further, a continuous hydrothermal flow route (CHFS) was developed for the synthesis of doped carbon quantum dots, such as S-doped carbon quantum dots (S-CQDs) starting from glucose as a biomass precursor (carbon precursor) and p-sulfonic acid calix[4]arene as functionalising molecule (sulphur precursor). The photoluminescence studies for the S- doped carbon quantum dots exhibited an excitation independent behaviour (300-360 nm) with a maximum of emission peak at 433 nm and pH stability in the range 3-11. The CHFS produced S-CQDs and g-CQDs demonstrated excellent proficiency in providing high oil recovery of 17 ± 0.2% and 15 ± 0.5%, respectively using ultra-low concentrations of 0.01 wt%, which are more efficient and economically beneficial than using other nanoparticles. This can be attributed to their colloidal stability with the S-doped carbon quantum dots demonstrating greater stability over g-CQDs at high salinity conditions. The mechanisms proposed for S-CQDs in increasing oil sweeping efficiency involves altering rock wettability towards more water wet thus lowering retention on rock's surface, and creating temporary log-jamming, where the ultra-small particle size allows S-CQDs to recover oil trapped in tight reservoirs.

CONCLUSION

In concluding the detailed description, it should be noted that it would be obvious to those skilled in the art that many variations and modifications can be made to the preferred embodiment without substantially departing from the principles of the present invention. Also, such variations and modifications are intended to be included herein within the scope of the present invention as set forth in the appended claims. Further, in the claims hereafter, the structures, materials, acts and equivalents of all means or step- plus function elements are intended to include any structure, materials or acts for performing their cited functions.

It should be emphasized that the above-described embodiments of the present invention, particularly any "preferred embodiments" are merely possible examples of the implementations, merely set forth for a clear understanding of the principles of the invention. Any variations and modifications may be made to the above-described embodiments of the invention without departing substantially from the spirit of the principles of the invention. All such modifications and variations are intended to be included herein within the scope of the disclosure and present invention and protected by the following claims. The present invention has been described in sufficient detail with a certain degree of particularity. The utilities thereof are appreciated by those skilled in the art. It is understood to those skilled in the art that the present disclosure of embodiments has been made by way of examples only and that numerous changes in the arrangement and combination of parts may be resorted without departing from the spirit and scope of the invention as claimed. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description of embodiments.

Claims

Claims:

1. A continuous hydrothermal flow route and process comprising; the bottom-up synthesis of carbon rich, optionally doped, carbon quantum dots starting from a carbon precursor, which may be selected from citric acid, lignin, or a saccharide, such as, glucose, fructose, sucrose, and cyclodextrin, and an optional doping precursor which may be selected from a nitrogen, a sulphur, a phosphorus or a boron precursor; wherein the nitrogen precursor may be selected from ethyldiamine, melamine, urea and ammonia; wherein the sulphur precursor may be selected from thiourea, sodium thiosulfate or a sulphonic acid agent, such as a polysulfonic acid agent, for example p-sulfonic acid calix[4]arene, p-sulfonic acid calix[6]arene and/or p-sulfonic acid calix[8]arene, typically, p-sulfonic acid calix[4]arene; wherein the phosphorus precursor may be selected from P-agents such as phosphoric acid, p-phosphonic acid calix[4]arene, p-phosphonic acid calix[6]arene and/or p- phosphonic acid calix[8]arene, typically, p-phosphonic acid calix[4]arene; and wherein the boron precursor may be selected from boric acid, borax or sodium borate.

2. A continuous hydrothermal flow route and process comprising; the synthesis of carbon rich, optionally nitrogen, phosphorus or boron doped, carbon quantum dots starting from a carbon precursor optionally selected from citric acid, glucose, fructose, sucrose and lignin and optionally, a nitrogen precursor optionally selected from ethyldiamine, melamine, urea and ammonia, a phosphorus precursor optionally selected from P-agents such as phosphoric acid, p-phosphonic acid calix[4]arene, p-phosphonic acid calix[6]arene and/or p-phosphonic acid calix[8]arene, typically, p-phosphonic acid calix[4]arene, or boron precursor optionally selected from boric acid, borax or sodium borate.

3. A continuous hydrothermal flow route and process according to claim 2, wherein the synthesis is a bottom-up synthesis.

4. A continuous hydrothermal flow route and process according to any of claims 1-3, wherein the carbon precursor is citric acid and/or the nitrogen precursor is ammonia.

5. A continuous hydrothermal flow route and process according to claim 1, wherein the carbon precursor is glucose and/or the sulphur precursor is p-sulfonic acid calix[4]arene.

6. A continuous hydrothermal flow route and process according to any preceding claim, wherein the synthesis of the doped carbon quantum dots includes simultaneous doping thereof.

7. A continuous hydrothermal flow route and process according to any preceding claim, wherein the reaction time in the hydrothermal reaction zone is less than 1 hour, typically less than 5 minutes, more typically less than 10 seconds, 5 seconds or 2 seconds, most typically less than 1 second.

8. A continuous hydrothermal flow route and process according to any preceding claim, wherein the hydrothermal flow process comprises a water feed and a carbon precursor feed.

9. A continuous hydrothermal flow route and process according to any preceding claim, wherein the water for the hydrothermal reaction is supercritical, optionally, wherein the water is at 374 °C and at 22.1 MPa, or wherein the water is at about 450 °C and about 24.8 MPa.

10. A continuous hydrothermal flow route and process according to any preceding claim, wherein the carbon precursor and the doping precursor are combined prior to being delivered into the hydrothermal reaction zone where they are mixed with the supercritical water feed.

11. A continuous hydrothermal flow route and process according to any preceding claim, wherein the carbon precursor and doping precursor are introduced into the continuous reactor in a molar ratio of between 1:10 - 10:1, for example between 1:8 - 1:1, such as between 1:6 - 1:4, for example about 1:5 carbon precursor to doping precursor.

12. N-doped carbon quantum dots having an excitation independent or dependent luminescence, typically, independent.

13. N-doped carbon quantum dots obtainable by the process according to any of claims 1-11.

14. N-doped carbon quantum dots according to claims 12 or 13, wherein the luminescence is excitation independent in the UV light range, typically, in the excitation range of 300-380 nm, more typically, in the range of 320-360 nm.

15. N-doped carbon quantum dots according to any of claims 12-14, wherein the luminescence has a luminescence maxima at a wavelength from 400 to 480 nm, typically, from 420 to 460 nm, more typically, 425 to 450 nm such as 441 nm.

16. N-doped carbon quantum dots according to any of claims 12-15, wherein the luminescence is a single emission profile, typically, with a full width at half maximum (FWHM) of less than 100 nm, such as less than 90nm or 80nm, for example about 78 nm.

17. N-doped carbon quantum dots according to any of claims 12-16, wherein auto luminescence is avoided for the luminescence, typically, wherein the N-doped carbon quantum dots luminescence has a stokes shift value of at least 50 nm, typically, at least 75 nm, such as at least 85 nm or 100 nm, for example about 109 nm.

18. N-doped carbon quantum dots according to any of claims 12-17, wherein the average particle size is less than 10 nm, typically, less than 5 nm according to HRTEM.

19. N-doped carbon quantum dots according to any of claims 12-18 wherein the average particle size is between 1.5 nm to 5 nm, typically, between 1.9 nm to 4.7 nm, such as about 3.3+/- 0.7 nm, according to HRTEM.

20. S-doped carbon quantum dots having an excitation independent or excitation dependent luminescence, typically dependent.

21. S-doped carbon quantum dots obtainable by the process according to any of claims 1 and 5-11.

22. S-doped carbon quantum dots according to claims 20 or 21, wherein its luminescence is excitation independent or dependent in the UV light range, typically, in the excitation range of 300-380 nm, more typically, in the range of 300-360 nm, such as in the range of 320-380 nm, for example, in the range of 320-360 nm.

23. S-doped carbon quantum dots according to any of claims 20-22 wherein the luminescence has a luminescence maxima at a wavelength from 400 to 480nm, more typically, from 420 to 460nm, most typically, 425 to 450nm, such as about 433nm.

24. S-doped carbon quantum dots according to any of claims 20-23, wherein the luminescence is a single emission profile, typically, with a full width at half maximum (FWHM) of less than 100 nm, such as less than 90nm or 80nm.

25. S-doped carbon quantum dots according to any of claims 20-24, wherein auto luminescence is avoided for the luminescence, typically, wherein the S-doped carbon quantum dots excitation independent luminescence has a stokes shift value of at least 50 nm, typically, at least 75 nm, such as at least 100 nm or 125 nm, for example about 155 nm.

26. S-doped carbon quantum dots according to any of claims 20-25, wherein the average particle size is less than 10 nm, typically, less than 5 nm according to HRTEM.

27. S-doped carbon quantum dots according to any of claims 20-26, wherein the average particle size is between 0.1 nm and 5 nm, typically, between 0.25 nm and 3.5 nm, more typically, between 1.0 nm and 2.5nm, such as about 1.7+/- 0.7 nm, according to HRTEM.

28. P-doped or B-doped carbon quantum dots having an excitation independent or dependent luminescence, typically, independent.

29. P-doped or B-doped carbon quantum dots obtainable by the process according to any of claims 1-3 and 6-10.

30. P-doped or B-doped carbon quantum dots according to claims 28 or 29, wherein the luminescence is excitation independent in the UV light range.

31. P-doped or B-doped carbon quantum dots according to any of claims 28-30, wherein the luminescence is a single emission profile.

32. P-doped or B-doped carbon quantum dots according to any of claims 28-31, wherein auto-luminescence is avoided for the luminescence.

33. A method of detecting Cr(VI) comprising the steps of contacting the sample to be analysed for Cr(VI) with undoped carbon quantum dots obtainable by the process according to any of claims 1-5 or 7-9, or doped carbon quantum dots according to any of claims 12-32, and measuring the change in emission intensity of the contacted sample.

34. A method of detecting Cr(VI) according to claim 33, wherein the Cr(VI) is in solution with the carbon quantum dots, typically, in an aqueous solution.

35. A sensor that comprises undoped carbon quantum dots obtainable by the process according to any of claims 1-5 or 7-9, or doped carbon quantum dots to any of claims 12-32.

36. A chemisensor for Cr(VI) detection using undoped carbon quantum dots obtainable by the process according to any of claims 1-5 or 7-9, or doped carbon quantum dots according to any of claims 12-32.

37. A method of extracting oil comprising contacting the oil containing substrate with un-doped carbon quantum dots obtainable by the process according to any of claims 1-5 or 7-9 or doped carbon quantum dots according to claims 12-32 in solution, typically, brine solution.

38. A method of extracting oil according to claim 37, wherein the carbon quantum dots in solution are at a concentration of less than 1 wt%, typically, less than 0.5 wt%, more typically, less than 0.2 wt% and/or more than 0.0005 wt%, typically, more than 0.001 wt%, more typically, more than 0.005 wt%, for example between 0.005 wt% and 1 wt%, such as about 0.01 wt%.

39. A composition for extracting oil comprising undoped carbon quantum dots obtainable by the process according to any of claims 1-5 or 7-9, or doped carbon quantum dots according to any of claims 12-32, preferably in the form of an aqueous solution, more preferably a brine solution.

40. The use of undoped carbon quantum dots obtainable by the process according to any of claims 1-5 or 7-9, or doped carbon quantum dots according to any of claims 12-32 as a Cr(VI) sensor.

41. The use of undoped carbon quantum dots obtainable by the process according to any of claims 1-5 or 7-9, or doped carbon quantum dots according to claims 12-32 as an oil extractant.

42. A solar cell, sensor, photocatalyst, optoelectronic device or biotag that comprises undoped carbon quantum dots obtainable by the process according to any of claims 1-5 or 7-9, or doped carbon quantum dots according to any of claims 12-32.

43. Doped carbon quantum dots produced by a process according to any of claim 1-11.