WO2014088518A1

WO2014088518A1 - Method for forming a nanoframe

Info

Publication number: WO2014088518A1
Application number: PCT/SG2013/000518
Authority: WO
Inventors: Mohammad Mehdi SHAHJAMALI; Can Xue; Yin Chiang Freddy Boey
Original assignee: Nanyang Technological University
Priority date: 2012-12-06
Filing date: 2013-12-06
Publication date: 2014-06-12

Abstract

The invention relates to a method for forming a nanoframe, and in particular, to a method of etching a core of a bimetallic core-shell nanostructure to obtain the nanoframe. The thus-obtained nanoframes possess specific local surface plasmon resonance (LSPR) bands in the visible range and near infra red (NIR) of the optical spectrum and can be used in different applications from catalytic to applications such as LSPR biosensing, bioimaging, and photothermal cancer therapy.

Description

METHOD FOR FORMING A NANOFRAME

Cross-Reference to Related Application

[0001] This application claims the benefit of priority of United States of America Provisional Patent Application No. 61/734,155, filed December 6, 2012, the contents of which being hereby incorporated by reference in its entirety for all purposes.

Technical Field

[0002] The invention relates to a method for forming a nano frame, and in particular, to a method of etching a core of a bimetallic core-shell nanostructure to obtain the nano frame. The thus-obtained nanoframes possess specific local surface plasmon resonance (LSPR) bands in the visible range and near infra red (NIR) of the optical spectrum and can be used in different applications from catalytic to applications such as LSPR biosensing, bioimaging, and photothermal cancer therapy.

Background

[0003] Hollow noble metal nano structures have attracted growing research interests owing to their tunable optical properties, large surface area and surface permeability, low density, and lower cost. Due to these unique physical and chemical characteristics, this class of materials offers potential applications in many fields including plasmonics and optical sensing, surface enhanced Raman spectroscopy (SERS), catalysis, and medical applications such as biomedical imaging, drug delivery, as well as photothermal cancer treatment.

[0004] In general, hollow nano structures are obtained based on different principles, such as galvanic replacement, chemical etching, Kirkendall effect, and self-assembly of nanoparticles to synthesize anisotropic nanobox, nanocages, and nanoframe structure of various shapes. A commonly adopted route toward fabricating hollow structures involves the growth of a shell of desired materials on removable templates by aforementioned methods. Among these synthesis routes, the one based on the galvanic replacement reaction is more commonly used in generating hollow nanostructures from different noble metals. Although the protocol based on the galvanic replacement reaction with chloroauric acid (HAuCl₄) is well-known and works good enough for silver (Ag) nanostructures, it has a negative aspect that limits the ability to achieve ultrathin structure with accurate control over thickness and uniformity. In the dealloying or etching process using gold (Au) salt, Au atoms are deposited on the Ag structure at the same time as the Au atoms oxidize the Ag structure, such that the redundant pairing between Ag etching and Au deposition makes it difficult to fine-tune the Au shells (alternatively termed as ridges) with ultrathin thickness, where the Au shells surround the Ag structure and eventually form the nanoframes after the Ag structure is etched.

[0005] Interests in such ultrathin nanostructures are further highlighted by latest progresses in ultrathin gold nanowires. Current preparation methods for ultrathin nanowires are mainly based on surfactant-mediated growth by using a ligand or mixtures of ligands on anisotropic nanostructures which allows anisotropically growth, or template-assisted synthesis which involves the use of a hard structure as directing agent to the growth of the nanowires. These templates can be zeolites or other mesoporous materials.

[0006] Although a significant amount of work has been done in developing synthesis methods for hollow spheres, cubes, and rods, little effort has been done on ultrathin structures of such morphologies.

[0007] Thus, there is a need to provide for a method of forming nanoframes that have ultrathin thicknesses, such as 6 nm or less. Summary

[0008] Present inventors have developed a method of site-selective gold coating and chemically etching a core of a bimetallic core-shell nanostructure to obtain a well-shaped ultrathin nanoframe. The thus-obtained nanoframes possess specific local surface plasmon resonance (LSPR) bands in the visible range and near infra red (NIR) of the optical spectrum. By adding a mild reducing agent, gold (Au) atoms havinga small mismatch lattice with a silver (Ag) core can be reduced on the edge of the core. In a next step, the Ag core may be oxidized or etched away to form the nanoframe. The ridge thickness of the nanoframe can be controlled by the amount of gold reduced and deposited on the Ag core, thereby tailoring the LSPR band of the nanoframe structure.

[0009] Thus, in one aspect, there is provided a method for forming a nanoframe. The method includes simultaneously adding a gold precursor and a reducing agent to a solution containing silver nanoparticles to form a bimetallic core-shell nanostructure. The core may include silver and the shell may include gold. The reducing agent may include hydroxylamine solution or a hydroxylamine salt. The method further includes etching the core of the bimetallic core-shell nanostructure with an oxidant to obtain the nanoframe.

[0010] The nanoframe thus-obtained by the present method can be used in applications such as catalysts, LSPR biosensing, bioimaging, and photothermal cancer therapy.

Brief Description of the Drawings

[0011] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings. [0012] Fig. 1 shows a scheme of present nanoframe synthesis method. In step A, silver nanoprisms edges are coated with gold through addition of aqueous HAuCU and hydro xylamine hydrochloride with a very controlled reducing way. This causes gold ions in solution to crystallize primarily on the exterior walls of the nanoprism and make a gold nanoframe or nanoring with triangular morphology. The product in this step may be called edge gold-coated nanoprism or Ag@Au- framed nanoprism. Subsequent addition of H₂0₂ +NH₄OH with appropriate concentration and rate (step B) causes silver metal particles to etch and this produces a very well-shaped triangular nanoframe with ultrathin (sub 2 nm) ridges and an interior triangular pore.

[0013] Fig. 2 shows that in reaction A, gold atoms are deposited on the Ag nanoprisms by HyA reaction with aqueous HAuCl₄ which leads to crystallization of gold on the edges of the nanoframes. In reaction B, Ag atoms are chemically etched away by H₂0₂+NH₄OH which can reduce the nanostructure to a well-defined nanoframe structure having conformal tunable ultrathin ridge thickness. The ridge thickness can be controlled from 1.8 (ultrathin) to 6 (thin) nm.

[0014] Fig. 3 shows a normalized UV-vis-NIR spectra of (A) Au nanoframe synthesized by H₂0₂+NH₄OH etching (al) the original triangular gold coated nanoprism with LSPR band at 755 nm. (a2-a7) Au nanoframes in different H₂0₂+NH₄OH amounts which is 50, 100, 150, 300, 500, 900 μΐ into 6 ml gold coated solution corresponding to a2-a6, respectively. (B) Au nanoframe synthesized by gold salt etching (bl) the original triangular gold coated nanoprism with LSPR band at 742 nm. (b2-b7) Au nanoframes in different HAuC14 amounts which is 50, 100, 150, 200, 220, 240 μΐ in 8 ml gold coated nanoprism solution corresponding to b2-b6, respectively.

[0015] Fig. 4 shows TEM images of flat-lying nanoframes in low magnification (A-B) Low magnification of gold nanoframe particles on fully carbon TEM grid. (C) Ultrathin gold nanoframe with 2.2 hm ridge thickness by using size A nanoprism. (D) Ultrathin gold nanoframe with 1.8 nm ridge thickness by using size B nanoprism (Size A: 37±9 nm and Size B: 58±10 nm), E) Electron diffraction pattern of the top of a single nanoframe of size B nanoframe. (F) HRTEM image of a nanoframe at one of its tips. The inset is the fast Fourier transform (FFT) pattern of the square part drawn in the image (G) HRTEM image of a nanoframe indicate the lattice parameter of 0.24 nm corresponding to (11 1) interplane distance of gold.

[0016] Fig. 5 shows TEM images of AuAg nanoframe etched by HAuCl₄ (A-B) the Au-Ag alloy nanoframes with size B silver nanoprism. The arrows illustrate the particles with silver core and full gold shell (Ag@Au) which did not etch away. (C) Triangular nanoframe structure with average ridge thickness of 10.2 nm. (D) Electron diffraction pattern of the top of a single nanoframe of part C. (E) Nanoframe of previous step (part C) reacted with higher amount of gold salt which leads to dendrite gold structure on the AuAg alloy nanoframe. (F) A single nanoparticle from part E which confirms the formation of dendrites and branches part attached to the ridges of the nanoframe.

[0017] Fig. 6 shows energy dispersive X-ray spectroscopy of gold nanoframe with 10% Ag/Au ratio which silver nanoprism dissolved with (A) H₂0₂+NH₄OH. (B) Gold salt. The table below each diagram corresponds to the EDX integrated peak areas of Au M and Ag L. Cu peaks is related to the copper supporting grid. (C) STEM image of a single nanoframe. (D) Energy dispersive X- ray mapping of part C correlate to Au M peaks in green color shows a frame structure. (E) Energy dispersive X-ray mapping correlate to Ag L peaks in red color.

[0018] Fig. 7 shows (a) UV-vis spectra showing the 4-nitrophenol before and after adding sodium borohydride in green and red spectra, respectively. The inset a-c shows 4-nitrophenol before and after adding sodium borohydride and final completely reduced solution, (b) UV-vis spectra showing the gradual reduction of 4-NP with Au nanoframe created by HAuC14 oxidation with ~ 10 nm ridge thickness (c) Plot of the absorbance versus time for the reduction of 4-NP of Au nano frame in part b. (D) UV-vis spectra confirms the creation of 4-aminophenol (4-AP) and omission of 4-NP (E) zoomed graph of part D on 4-aminophenol (4-AP) creation at 295 nm. (F) Plot of the absorbance versus time for the reduction of 4-NP of Au nano frame which shows a linear relation.

Description

[0019] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practised. These embodiments are described in sufficient detail to enable those skilled in the art to practise the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0020] Herein disclosed is a new templating approach to synthesising ultrathin metallic

nanoframes. According to present method for forming a nanoframe, a gold precursor and a reducing agent are simultaneously added to a solution containing silver nanoparticles to form abimetallic core-shell nanostructure. The core may include silver and the shell may include gold. The reducing agent may include hydroxylamine solution or a hydroxylamine salt. The method further includes etching the core of the bimetallic core-shell nanostructure with an oxidant to obtain the nanoframe.

[0021] By "ultrathin" is meant a dimension of a nanostructure of 2 nm or less, such as about 2 nm, 1.8 nm, 1.6 nm, 1.4 nm, or less. By "thin" is meant a dimension of a nanostructure of more than 2 nm but less than 8 nm, such as about 8 nm, 7 nm, 6 nm, 5 nm, 4 nm, 3 nm, or 2.5 nm. [0022] A nanostructure is a structure or object that can have any form and has dimensions typically ranging from 1 to a few hundred nm (nanometre). More specifically, a nanostructure has at least one dimension being less than 100 nm. Nanostructures can be classified, for example, into the following dimensional types: zero dimensional (0D) including, but not limited to, nanospherical particles (also called nanospheres); one dimensional (ID) including, but not limited to,nanorods, nanowires (also called nanofibers) and nanotubes; two dimensional (2D) including, but not limited to, nano flakes, nanodiscs, nanocubes and nano films; and three dimensional (3D).

[0023] In the present context, the nanostructures are metallic. Specifically, the nanostructure has a core-shell structure whereby the shell encapsulates the core. For example, the shell may encapsulate the core such that 95%, 96%, 97%, 98%, 99%, or more of the exterior surface of the core is coated with the shell. In various embodiments, the shell completely encapsulates the core. More particularly, the core-shell nanostructure is bimetallic whereby the core and shell each include a different metal. The present bimetallic core-shell nanostructure include a silver core and a gold shell encapsulating or surrounding the silver core.

[0024] The bimetallic core-shell nanostructure can have a plate-like configuration, whereby the longitudinal dimension is more than the height or thickness of the nanostructure. In various embodiments, the height (or thickness) of the bimetallic core-shell nanostructure may be about 2 to about 100 nm, such as 2 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm ,60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, or 100 nm, while the edge length of bimetallic core-shell nanostructure may be about 20 nm to about 200 nm, such as 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 1 10 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, or 200 nm. The thickness of the shell surrounding the core may be substantially uniform. Examples of plate-like configuration may include, but are not limited to, triangular shape, hexagonal shape, or circular shape. Accordingly, the core of the bimetallic core-shell nanostructure may include a triangular prism, hexagonal prism, or circular disc.

[0025] By "prism" or "nanoprism" is meant a metal composition that exhibits prismatic properties. In various embodiments, the bimetallic core-shell nanostructures exhibit prismatic properties. For brevity, the present core-shell nanostructures may sometimes be termed simply as nanoprism and the core as nanoprism core, for example. Prismatic properties can be detected using known techniques. Prismatic properties include, but are not limited to, characteristic resonances, such as surface plasmon dipole and quadrupole resonances. In cases where the nanoprism comprises two metals, such as a core metal and a shell metal, the surface plasmon resonances can be related to the thickness of the shell metal of the nanoprisms. Thus, nanoprisms disclosed herein can have plasmon resonances that have been tailored or controlled to specific wavelengths by controlling thickness of the gold shell.

[0026] For the purposes of illustration and brevity, the following discussion refers to triangular Ag nanoprisms and triangular Au nanoframes (see Fig. 1) but it is to be understood and appreciated that the scope of the invention is not limited as such.

[0027] A triangular nanoprism (or core) has two major opposing triangular surfaces and three adjoining edge surfaces. The edge surfaces may be preferentially coated with a metal. By

"preferentially coated" is meant that a selective deposition of the metal atoms on the edge surfaces (i.e. (110) facets) of the triangular nanoprism compared to the major surfaces (i.e. (I l l) facets). The metal coating on the three edge surfaces forms the shell or otherwise called frame or nanoframe of the nanoprism. [0028] A triangular nanoframe may thus be seen as three connected ultrathin nanowires. The aspect ratio of these 'nanowires' easily approaches from 10 to 100.

[0029] The thickness of the metallic gold coating, herein referred to as the ridge thickness, nanoframe thickness, or shell thickness, can be controlled by the amount of gold introduced or made available for the deposition step, which thickness in turn can tailor the LSPR band of the resultant nanoframe.

[0030] The nanoprisms which represent the starting materials for the method of the present invention can have an edge length of less than about 200 nm and preferably less than about 100 nm. More preferably, these nanoprisms have an edge length of less than about 80 nm and most preferably have an edge length of between about 70 nm and about 80 nm. These nanoprisms can have a thickness of less than about 100 nm and preferably less than about 25 nm. More preferably, these nanoprisms can have a thickness of less than about 20 nm and most preferably have a thickness of between about 5 nm and about 15 nm.

[0031] The gold precursor can be any gold salt or source of gold ions. The gold precursor is reduced to elemental gold by a suitable reducing agent. In various embodiments, the gold precursor may include chloroauric acid (HAuCl₄), gold (III) chloride (AuCl₃), gold (I) chloride (AuCl), and a mixture thereof. In one embodiment, the gold precursor may include HAuCl₄.

[0032] The reducing agent is one that reduces the gold precursor to elemental gold. The reducing agent may be sufficiently mild such that it only reduces the gold precursor and has minimal or no impact on the silver core. In particular, the reducing agent does not etch the silver core such that the silver core retains essentially its original structure or shape. The reducing agent may include hydroxylamine solution (HyA) or a hydroxylamine salt. Present reducing agent exhibits little or no etching of Ag and Au nanocrystals compared to other conventional reducing agents such as ascorbic acid. In various embodiments, 100 mM or less of the reducing agent may be used. For example, 100 mM, 95 mM, 90 mM, 85 mM, 80 mM, 75 mM, 70 mM, 65 mM, 60 mM, 55 mM, 50 mM, 45 mM, 40 mM, 35 mM, 30 mM, 25 mM, 20 mM, 15 mM, 10 mM, 5 mM, 1 mM or even less,, such as in the nM (nanomolar) range, of the reducing agent may be used.

[0033] In various embodiments, the oxidant or etchant may include hydrogen peroxide (H₂0₂), ammonia solution (NH₄OH), sulfuric acid (H₂S0₄), iron (III) nitrate (Fe(N0₃)3, and a mixture thereof. The oxidant may be added dropwise so as to control the rate of etching the core of the nanoprism. For example, in the instance of silver nanoframes etched with Η₂0₂+ΝΗ₄ΟΗ, successive additions of a hydroxylamine salt such as hydroxylamine hydrochloride and HAuCl₄ results in a gold nanoframe on the silver nanoprism core. The average edge length of the Ag@Au- framed nanoprism may be between about 30 nm and about 80 nm. Typically, the average edge length of the gold-silver alloy nanoprisms may be about 60 nm.

[0034] In various embodiments, after etching the core of the bimetallic core-shell nanostructure with the oxidant, a further amount of gold precursor and a further amount of reducing agent may be added to the solution containing the nanoframe. In this case, the gold precursor may be HAuCl₄ and the reducing agent may be hydroxylamine solution or a hydroxylamine salt such as hydroxylamine chloride. Doing so would increase the ridge thickness.

[0035] As mentioned above, the gold precursor and the reducing agent are added simultaneously (i.e. added at the same time) to the solution containing silver nanoparticles. Since the reducing power of HyA may be enhanced at higher pH, in various embodiments a basic solution may be added to the reducing agent and/or the further amount of reducing agent. By increasing the pH of the reducing agent HyA, the rate of deposition of gold onto the silver core may be enhanced. The basic solution may be added to the reducing agent prior to the simultaneous addition of the gold precursor and the reducing agent to the solution containing silver nanoparticles. Alternatively, the gold precursor, the reducing agent, and the basic solution are added simultaneously to the solution containing silver nanoparticles. For the example, the basic solution may include, but is not limited to, sodium hydroxide or potassium hydroxide.

[0036] The gold-coating reaction may be carried out at room temperature or lower. At lower temperatures, such as 20 °C, 15 °C, 10 °C, 5 °C, or 0 °C, better morphology of the resultant bimetallic core-shell nanostructure may be obtained. Thus, in various embodiments, the reaction mixture including the solution containing silver nanoparticles, the gold precursor, and the reducing agent (and also the basic solution, if present) may be placed in a container placed in an ice bath.

[0037] To further aid the deposition rate of gold, the reaction may be continuously stirred.

[0038] As mentioned above, it is desirable to ensure that the reducting of the gold precursor occurs on the surface of the silver nanoprism core while spontaneous nucleation of gold nanoparticles in the reaction mixture is avoided. Thus, in various embodiments, the reducing agent is added at a flow rate of about 1 to 3 ml/h. For example, the flow rate of adding the reducing agent may be about 1 ml/h, 1.5 ml/h, 2 ml/h, 2.5 ml/h, or 3 ml/h. The reducing agent may be added at a constant flow rate or at a variable flow rate.

[0039] Since the gold precursor is also added to the solution containing silver nanoparticles at the same time the reducing agent is added, the flow rate of the gold precursor may be adjusted accordingly and may or may not correspond directly to the flow rate of the reducing agent. In various embodiments, the gold precursor is added at a flow rate of about 1 to 3 ml/h. For example, the flow rate of adding the gold precursor may be about 1 ml/h, 1.5 ml/h, 2 ml/h, 2.5 ml/h, or 3 ml/h. The gold precuror may be added at a constant flow rate or at a variable flow rate. [0040] Advantageously, present method would be operative on nanoprisms formed by known methods and applicable in a face-selective manner, allowing the generation of ultrathin nano frame with sub-2 nm ridge thickness.

[0041] In order that the invention may be readily understood and put into practical effect, particular embodiments will now be described by way of the following non-limiting examples.

Examples

[0042] We demonstrate the selective gold deposition onto (110) facets of silver nanoprisms with thickness of 6±1 nm under controlled reducing conditions without using any surfactant. In the next step, silver is etched by a clean wet etchant. This offers a reliable path to create well-defined ultrathin single crystal Au nanoframes with tunable LSPR in visible and near infrared region in high yield.

[0043] It is an objective to find and optimize a facile approach to create a tunable ultrathin nanostructure with a proper integrity and uniformity, with minimum morphological changes with respect to the initial triangular Ag nanoprism template. The first attempts to site-selective gold coating of the nanoprism failed due to the vulnerability of silver nanoprism to etching since it merely contains a very thin planar structure (with just ~6 nm thickness) which can be easily etched by any oxidants such as HAuCl₄. We solved this problem by using mild reducing agent in a highly controlled reducing protocol. Silver nanoprism is selected for making nanoframe because of their interesting two-dimensional shape and their unique and great LSPR sensitivity. More importantly, each single nanoprism can contain two different main facet of (11 1) on the triangular plane and (1 10) on the edges as illustrated in Fig. 2. The diversity of the nanoprism facets and its broad edge length makes nanoprism a unique structure as a platform to create more complex plasmonic nanostructures. [0044] We used a facile seed-mediation approach that involves hydroxylamine hydrochloride (HyA) to reduce the gold salt on the Ag nanoprism as sacrificial seed. The reaction is designed to be quite mild, therefore the reduction of gold salts and deposition take place on the selective facets of Ag nanoprism seeds while avoiding spontaneous nucleation of gold nanospheres in the solution. In addition, it is notable to chose a reducing agent with less etching effect on the Ag template. In present method, HyA did not exhibit any etching effect on silver or gold nanostructures compared to other reducing agentssuch as ascorbic acid, which has been reported to show etching on both silver and gold nanostructure such as Ag nanoprisms and gold nanorods.

[0045] The gold coating process on the edges of nanoprism c.a. (110) facets was carried out by slowly and simultaneously adding HAuCl₄ and NH₂OH-HCl+NaOH to Ag nanoprism solution with vigorous stirring through two distinct tubes using a syringe pump with certain infusion rate. Since the relative surface energies of different facets of nanoprism are in the order of γπ^γιοο^πο for a face centered cubic (FCC) metal, to minimize surface energy, gold atoms take place on (110) since deposition on (1 11) facets of Ag nanoprism needs more energy. In our method, the reducing power of HyA is enhanced by increasing pH of its solution after 5 min of starting the reaction. We also introduced some NaOH into the HyA solution to increase the pH in order to prevent unwanted etching of silver by gold ions (reaction 3). Au atoms are deposited on silver nanoprism which serves as a template for the deposition via this proposed reaction:

Ag_x (s) + AuCLf (aq)→ Ag_xAu (s) + 4C1 (1 )

Ag_sAu_y(s) + AuCLf (aq)→ Ag_xAuy÷i (s) + 4C1^~ (2) xAu_y (s) + AuCLf (aq)→ Ag x_-3 Au _y+i (s) + 3Ag⁺ + 4C1 (3) [0046] Since the LSPR bands of gold and silver nanoparticles are highly sensitive to the changes of their size and shape, we could track and evaluate the structure evolution during both the gold coating and silver dissolution process based on their extinction spectra. By deposition of gold on the edges of two different sizes of nanoprism (Size A: 37±9 nm and Size B: 58±10 nm), the LSPR band shows red-shift with increasing intensity correlate with increase on edge-length of the final structure. As mentioned earlier, the epitaxial gold layer formation on the edges of the nanoprism is due to the high surface energy of (110) planes on prism edges so that gold atoms would prefer to deposit on these sites, making a frame-like structure made of Au on the silver nanoprism. We termed these edge-gold coated silver nanoprism as Ag@Au-framed nanoprism. Formation of the gold nano frame demonstrates a darker contrast at edges compare to interior of the structure under TEM as depicted in Fig. 1.

[0047] In next stage (i.e. nanoframe formation by H₂0₂+NH₄OH), dissolution (or etching) of the silver element from the as-synthesized Ag@Au-framed nanoprism was accomplished by mixture of hydrogen peroxide and ammonium hydroxide as a clean etchant. We surprisingly found that an etchant including H₂0₂+NH₄OH has a faster effect on the etching of Ag and even Ag₂0. At the same time, this etchant does not contaminate the metal particles by introduction of new ions such as Fe³⁺ or S^2" into the structure as other etchants do. Herein, in our basic medium experimental procedure (pH 10) H₂0₂ plays a role demonstrated in reaction 4 with standard redox potential of 0.87 V.

H₂0₂ +2e + OH-→30H- (4) [0048] So the silver part of the gold-coated nanoprism etches away according to the following chemical reaction with a mild difference standard redox potential (0.07 V) which control the rate of dissolving in a reasonable manner to have a smooth and uniform nanoframe structure:

2Ag(s) + H₂0₂ (aq) +OH"→ 2Ag⁺(aq) + 30H" (5)

[0049] For the aqueous ammonia reaction with Ag or Ag₂0, we have:

2Ag(s) + 4NH₄OH(aq) + ½0₂→ 2[Ag(NH₃)₂]⁺(aq) + 20H^"(aq) (6-1)

Ag₂0(s) + 4NH4OH→2[Ag(NH₃)₂]OH + 3¾0 (6-2)

[0050] By this method, the nanoframes were obtained in situ by oxidative etching of

H₂0₂+NH₄OH at room temperature. TEM analysis reveals that by introducing the wet etchant, only the silver part of nanoframe etches away which leads to the formation of gold triangular hollow nanoframe while the gold concentration approaches to 100% with ultrathin ridges (Fig. 6). EDX showed that the triangular nanoframes are composed of 91.7% gold, with the minor residual silver likely being in the form of silver chloride,oxide, or hydroxide trapped in the structure.

[0051] The UV-visible spectrum of the filled nanoframes can be monitored to review the progress of the pore creation or nanoframe creation (Fig. 3). In the case of gold nanoframes, the UV-visible spectrum is red-shifted and dampened with respect to the original Ag@Au-framed nanoprism (al and bl in Fig. 3).

[0052] We have selected the reduction of 4-nitrophenol (4-NP) by.NaBH₄ to 4-aminophenol (4- AP) as a model reaction to test the catalytic activity of the Au nanoframe as depicted in Fig. 7. This system can be used in water treatment of water polluted by phenol and phenolic compounds, for example. [0053] Aqueous solutions of 4-NP and NaBH₄ were added to deionized water in quartz cuvette under stirring. After adding (20 μί,) of Au nanoframe as catalyst, the bright yellow solution gradually fades as the reaction progresses. The final concentration of NaBH₄ became 7.2 mM and 4-NP became 0.1 mM. After adding Au nanoframe solution to the cuvette kinetics of 4-NP reduction in presence of these Au nanoframe particles, studieswere made in every 10 min interval in the range of 250-500 nm by UV-vis spectroscopy.In a typical measurement, a successive decrease at peak intensity at 400 nm (attributed to 4-nitrophenolate ions) with time is noticed.

[0054] Aqueous solution of 4-NP has an absorption maxima at 317 nm (Fig. 7). After the addition of freshly prepared NaBH₄ solution to the system, the peak shifted to 400 nm, indicating the formation of 4-nitrophenolate ions. This peak remains unaltered with time, which suggests that the reduction does not proceed in absence of a catalyst as reported by other literatures. After the addition of Au nanoframe particles, it was found that the peak height at 400 nm gradually decreases with time. With the gradual decrease in peak height at 400 nm, a new peak appeared at 298 nm, indicating the formation of 4-AP. This indicates that in the catalytic reduction the conversion of 4- NP gives only one product, 4-AP.

[0055] Since in our experiment the NaBH₄ concentration largely exceeds the 4-NP concentration, the reduction rate of 4-NP can be assumed to be independent of borohydride concentration. The plot of Absorbance vs. time for the reduction of 4-NP is shown in Fig. 7c and Fig. 7f. The conversion process of 4-NP to 4-AP can be directly related to the ratio of the respective absorbance versus time with a linear readout (R² =0.90).This linear dependence offers a simple method for reading out the concentration of 4-NP.

[0056] Synthesis of Ag@Au-framed nanoprism (edge gold-coated nanoprism). The as- prepared Ag nanoprism solution (-10 mL) was added into ~25 mL of Millipore water in a glass vial placed in an ice bath, (the optical density of the mixture should be in a way such that the diluted prism extinction is between 0.4 to 0.8. This ratio will control the nanoprism concentration in the solution), followed by infusion of about 1 to 100 mM, such as 11 mM HyA and 0.27 mM HAuCl₄ into the solution through two separate tubes on a mechanical syringe pump with vigorous stirring for 30 to 45 min, depending on the desired gold frame thickness. We use syringe pump (NE-4000 Double Syringe Pump, USA) with minimum infusion rate of 1.459 μΙ7ητ and accuracy of ± 1% . Herein the infusion rate was set as lmL.h^" Basic HyA solution was prepared by adding 250 ΐ. NaOH (0.5 M) into 5 mL as-prepared HyA solution.

[0057] Gold nanoframe formation by H₂O₂+NH₄OH. To 6 mL of as-synthesized Ag@Au- framed nanoprism in a 20 ml glass vial with maximum stirring rate, 0, 50, 100, 150, 300, 500 and 900 sL of a mixture of H₂0₂+NH₄OH is introduced respectively in a dropwise manner. The concentrations of H₂0₂+NH OH are adjusted as 50 mM in an equal volume ratio. After reaching each of these 6 values the solution is kept for 20 min to reach the stable condition and then UV-Vis spectroscopy reading of the solution is done.

[0058] Gold/Silver alloy nanoframe formation by gold salt etching. To 8 mL of as-synthesized Ag@Au- framed nanoprism in a 20 ml glass vial with maximum stirring rate, 0, 50, 100, 150, 300, 500 and 900 \iL of gold salt added dropwise. After reaching each of these 6 values the solution is kept for 20 min to reach the stable condition and then UV-Vis spectroscopy of the solution is done.

[0059] By "comprising" it is meant including, but not limited to, whatever follows the word "comprising". Thus, use of the term "comprising" indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present. [0060] By "consisting of is meant including, and limited to, whatever follows the phrase

"consisting of. Thus, the phrase "consisting of indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0061] The inventions illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including", "containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[0062] By "about" in relation to a given numerical value, such as for temperature and period of time, it is meant to include numerical values within 10% of the specified value.

[0063] The invention has been described broadly and genetically herein. Each of the narrower species and sub-generic groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0064] Other embodiments are within the following claims and non- limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

Claims

A method for forming a nanoframe, the method comprising:

simultaneously adding a gold precursor and a reducing agent to a solution containing silver nanoparticles to form a bimetallic core-shell nanostructure, wherein the core comprises silver and the shell comprises gold, wherein the reducing agent is hydroxylamine solution or a hydroxylamine salt;

etching the core of the bimetallic core-shell nanostructure with an oxidant to obtain the nanoframe.

The method of claim 1, wherein the oxidant is selected from the group consisting of hydrogen peroxide (H₂0₂), ammonia solution (NH₄OH), sulfuric acid (H₂S0₄), iron (III) nitrate (Fe(N0₃)₃, and a mixture thereof.

The method of claim 1 or 2, wherein the oxidant is added dropwise.

The method of any one of claims 1 to 3, wherein after etching the core of the bimetallic core- shell nanostructure with the oxidant, a further amount of gold precursor and a further amount of reducing agent are added to the solution containing the nanoframe.

The method of any one of claims 1 to 4, wherein prior to or during adding the reducing agent and/or the further amount of reducing agent, a basic solution is added to the reducing agent and/or the further amount of reducing agent.

The method of any one of claims 1 to 5, wherein the reaction mixture comprising the solution, the gold precursor, and the reducing agent is placed in a container placed in an ice bath.

The method of any one of claims 1 to 6, wherein the reaction mixture is continuously stirred. 8. The method of any one of claims 1 to 7, wherein the reducing agent is added at a flow rate of about 1 to 3 ml/h.

9. The method of claim 8, wherein the flow rate of the reducing agent is variable.

10. The method of any one of claims 1 to 9, wherein the gold precursor is added at a flow rate of about 1 to 3 ml/h.

1 1. The method of claim 10, wherein the flow rate of the gold precursor is variable.

12. The method of any one of claims 1 to 1 1, wherein the gold precursor is selected from the group consisting of chloroauric acid (HAuCl₄), gold (III) chloride (AuCl₃), gold (I) chloride (AuCl), and a mixture thereof.

13. The method of any one of claims 1 to 12, wherein concentration of the reducing agent is 100 mM or less.

14. The method of claim 13, wherein concentration of the reducing agent is 10 mM or less.

15. A nano frame formed by a method of any one of claims 1 to 14.

16. The nanoframe of claim 15, wherein the core comprises a triangular prism, hexagonal prism, or circular disc.