WO2016079681A1 - Method for producing colloids comprising nanoparticles - Google Patents

Method for producing colloids comprising nanoparticles Download PDF

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Publication number
WO2016079681A1
WO2016079681A1 PCT/IB2015/058901 IB2015058901W WO2016079681A1 WO 2016079681 A1 WO2016079681 A1 WO 2016079681A1 IB 2015058901 W IB2015058901 W IB 2015058901W WO 2016079681 A1 WO2016079681 A1 WO 2016079681A1
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Prior art keywords
nanoparticles
target
absorbance
metal
colloid
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PCT/IB2015/058901
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French (fr)
Inventor
Chiara VERGARI
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Plasma Diagnostics And Technologies S.R.L.
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Publication of WO2016079681A1 publication Critical patent/WO2016079681A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J13/00—Colloid chemistry, e.g. the production of colloidal materials or their solutions, not otherwise provided for; Making microcapsules or microballoons
    • B01J13/0004—Preparation of sols
    • B01J13/0043—Preparation of sols containing elemental metal
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J13/00—Colloid chemistry, e.g. the production of colloidal materials or their solutions, not otherwise provided for; Making microcapsules or microballoons
    • B01J13/0086—Preparation of sols by physical processes

Definitions

  • the present invention relates to the generation of metal nanoparticles, of pure metal or metal oxide, by means of ablation of a starting metal material with pulsed laser in the presence of a liquid, to obtain colloids.
  • ions of metal salts are used as reducing agents, such as sodium citrate or boron hydride salts in water.
  • the anions that derive from the metal salt dissolved in water, and those that derive from the reducing agents, are active as surfactants, reaching the goal that the resulting nanoparticles take a same electric charge that prevents them from aggregation.
  • a drawback of such methods is that there are many by-products generated by chemical reaction, that are deposited on the surface of the nanoparticles, affecting the activity of the nanoparticles changing the degree of purity of the colloids .
  • stabilizers for example polyethylene glycol (PEG) , which works as "spacer” between the nanoparticles .
  • PEG polyethylene glycol
  • a cellular toxicity of the colloids can derive owing to the presence of the stabilizers.
  • a pulsed laser with very high repetition frequency (lOkHz-100 MHz), which preferably has a duration of the repetition preferably between 0.1-10 ps and a pulse energy preferably between 1- 10 micro-Joule.
  • Other examples use a continuous flow of water for cooling the illumination region and a prism movable above the target.
  • the statistical distribution of the diameters of the nanoparticles (Fig. 4) is very dispersed and include spherical nanoparticles of big diameter (>20 nm) and smaller nanoparticles ( ⁇ 5nm) clearly elliptical.
  • the generated nanoparticles have a low stability, and consequently a stabilizer is added, for example a certain an amount of citrate. There is however a toxicity from the by-products of chemical reductions generated by the use of stabilizers.
  • a Nd:YAG laser source is used, with 25 picosecond pulse duration, 40 mJ pulse energy, at the essential wavelength of 1064 nm, and a 1kHz repetition frequency in said range of energy, focusing the laser beam with a 20cm lens.
  • a Nd:YAG laser source is used, with 25 picosecond pulse duration, 40 mJ pulse energy, at the essential wavelength of 1064 nm, and a 1kHz repetition frequency in said range of energy, focusing the laser beam with a 20cm lens.
  • Nd:YAG laser source prearranging a Nd:YAG laser source, with pulse duration comprised between 15 and 40 picoseconds, pulse energy comprised between 2 and 100 mJ, at the essential wavelength of 1064 nm or at the second harmonic of 532 nm or at the third harmonic of 355 nm, and repetition frequency up to a maximum of 1 kHz at said pulse energy;
  • the optical unit irradiates the target with focus of the laser beam located beyond the target, so that the target is irradiated in an illumination region of a predetermined diameter not less than 0.5 mm, this forming, starting from this region, a colloid with desired dimension, statistical distribution and concentration.
  • the illumination region of predetermined diameter is comprised between 0.5 mm and 5 mm, preferably between 0.5 mm and 2 mm.
  • the adjustment of the diameter of said illumination region is provided through said optical unit, such that nanoparticles are obtained of different size responsive to the diameter of the illumination region.
  • Said liquid solvent can be selected from the group consisting of: water, acetone, 2-propanol, PAMAM (polyamidoamine) , PEG (polyethylene glycol), oil for microscopy, whereas the target can be made of a metal or metal oxide, selected from the group consisting of: Au, Ag, Ti, T1O2, Cu, Fe and Pd, pure or in the form of alloys of the above described metals, as well as other metals, metal alloys and metal oxides.
  • a metal or metal oxide selected from the group consisting of: Au, Ag, Ti, T1O2, Cu, Fe and Pd, pure or in the form of alloys of the above described metals, as well as other metals, metal alloys and metal oxides.
  • the liquid solvent is double distilled water.
  • the double distilled water is preliminarily treated keeping it at a predetermined conditioning temperature and then sterilized.
  • the conditioning temperature is about 100°C and the sterilization temperature is about 120°C. This way, a high reproducibility of the process for ablation is obtained and a much higher stability with time of the colloids obtained.
  • the liquid solvent in the workspace has a fixed flow rate less than 10 ml/min.
  • the lens fluid flow is sufficient for not requiring cooling of the liquid and for limiting at most the turbulence of the liquid solvent, reducing at most the collisions between metal nanoparticles capable of forming aggregations which would change the final quality of the colloid.
  • Such a lens speed allows a non-turbulent conveyance of the liquid solvent and such slowness requires several minutes for a complete replacement of the liquid surrounding the target, avoiding to affect the process of stabilization of the nanoparticles and allowing to obtain spherical nanoparticles .
  • a cuvette is arranged in which the target is put.
  • the cuvette can be for example a transparent container with an inlet and an outlet to allow the solvent to enter and the colloid to exit .
  • the workspace is arranged in such a way that the target rests on a plane support and is covered by a column of liquid of height comprised between 1-2 cm .
  • the target has a translation movement having a speed with respect to the laser beam less than 0.1 cm/sec.
  • Such low speed allows, on the one hand, not to move the optical unit, and on the other hand allows a gradual detachment of the nanoparticles by the target, avoiding turbulences of the liquid and allowing an homogeneous dispersion of the nanoparticles in the liquid solvent, avoiding aggregations between the particles.
  • the target is mounted in a cuvette with a motor that makes it possible a stepped motion in the directions x, y, to obtain a desired ablation and avoiding a re-deposit of the nanoparticles or the production of oxide on the surface of the target.
  • the optical unit comprises at least one lens and a prism with anti reflective coating. This way, losses for reflection are eliminated during the movement beam entrance and exit surfaces of the prism.
  • the optical unit comprises a lens and a deflection mirror. This way, the deflection mirror avoids reflection losses the cost of a prism with anti- reflective coating.
  • the optical unit has a converging lens with focal length of 20-30 cm.
  • the optical unit is arranged for deflecting the laser beam for hitting perpendicularly the target. This way, a non-homogeneity of the distribution of energy due to refraction of the liquid of the laser beam in the illumination region is avoided.
  • the laser beam is converted from the essential harmonic at the second harmonic, which has an energy efficiency about 50% less, the second harmonic being used both for remodulating the shape of the nanoparticles and for limiting the size less than 4 nm and very narrow statistical distribution.
  • the second harmonic is used for Au in acetone or for Pd in 2-propanol.
  • the third harmonic can be used for similar purposes.
  • a spectroscopic system is arranged that provides an absorbance spectrum in situ of the colloid in the workspace downstream of the illumination region, the laser beam and the optical unit being adjusted responsive to the absorbance spectrum so that the absorbance spectrum in situ corresponds to a predetermined range of metal nanoparticles, in particular with an average diameter comprised between 1 and 10 nm, spherical shape, crystalline structure and substantial absence of aggregations.
  • the absorbance spectrum in situ corresponds to a range of nanoparticles with an average diameter comprised between 1 and 5 nm for biomedical applications.
  • the spectroscopic system operates with UV-Vis spectroscopy, and the absorbance spectrum is sampled for each pulse, in the range UV-Vis from 200 to 1100 nanometres.
  • UV-Vis the absorbance spectrum
  • the shape, size and statistical distribution can be determined for each pulse, and this is very useful when the pulse energy is larger than a certain ablation threshold, which depends upon the material and the size of the illumination region of the target invested by the laser.
  • the measurement in situ of the UV- Vis absorbance of the colloid for controlling the stability provides an analysis of the spectral position of the plasmonic resonance and an analysis of the width of the plasmonic resonance.
  • the absorbance is determined by means of optical fibres, ending with a collimator, which collimates a light beam coming from a UV-Vis lamp and crossing the colloid in the volume, until the light beam has been collected by another collimator connected, through optical fibres, to a low resolution wideband spectrometer which operates between 200 and 1100 nm.
  • the spectral distribution of the absorbance is computed by a program for controlling the concentration of the colloid measured by a "Mie fit" equation starting from the absorbance.
  • the admissible threshold of absorbance is less or equal to 10, in order to ensure optimal stability to the colloid.
  • the measurement in situ of the absorbance is carried out for values of absorbance between 1 and 2, whereas for values of absorbance between 3 and 10 a step is provided of collecting the colloid, and diluting it in an auxiliary container, which is crossed by the collimated light beam coming from the UV-Vis lamp for determining the absorbance .
  • the target is displaced of one step after a predetermined number of laser pulses according to a predetermined function according to values of recorded absorbance.
  • the function is configured for adjusting the movement of the cuvette in at least one of the directions x, y responsive to the absorbance, so that, if an incipient decrease of the absorbance is observed, or a stop in the growth of the absorbance, the target is displaced for presenting not ablated portions of the target, until the absorbance starts growing again.
  • the function provides a linear growth of the absorbance.
  • the colloid has shape, average diameter, statistical distribution and optimal concentration of the nanoparticles , and it is possible to obtain a production of nanoparticles in the colloid substantially proportional to the number of pulses and/or to the power, with total reproducibility of the desired quality substantially independent from the production rate, in the range of power between 2-100 mJ and frequency less than 1 kHz.
  • the production of the colloid is carried out in the ablated cloud of nanoparticles, so-called "plume”, in a time of about one hundred microseconds, the repetition frequency of the pulses must not exceed 1 kHz for not to affect the features of the colloid produced in a single pulse.
  • the frequency range of pulses is increased between 50-100 Hz under a same energy for pulse.
  • the frequency of the production rate i.e. the amount of nanoparticles delivered per time unit, is greater with respect to what can be obtained by increasing the number of pulses under a same nominal power.
  • the liquid solvent is acetone and the laser emits the second harmonic at 532 nm with pulse energy about 15 mJ and diameter of the illuminated region 0.5-5 mm, in particular 0.5-2 mm, such that nanoparticles are obtained with an average diameter of 2.5 nm with dispersivity within 10% of the average diameter.
  • the nanoparticles are obtained from a target of silver, where to the liquid solvent a solute is added selected from the group consisting of: Sodium chloride, Lithium chloride.
  • the solute is added to the liquid solvent up to a 10 ⁇ 3 molar concentration.
  • the nanoparticles are obtained from a titanium target and the laser has a pulse energy selected from the group consisting of:
  • T1O2 which with pulse energy between 2 and 10 mJ or between 50 and 100 mJ is in the Rutile form and with pulse energy between 10 and 50mJ is in the Anatase form.
  • the nanoparticles are obtained from an iron target.
  • colloidal suspensions are obtained of nanoparticles of Fe that include iron oxides in the form of magnetite with strong magnetic characteristics and a reversible behaviour in an external magnetic field.
  • the nanoparticles are obtained from a target of Palladium with 2-propanol as liquid solvent, and the laser emits the second harmonic at 532 nm with pulse energy between 5-40 mJ and diameter of the illuminated region 0.5-5 mm, such that nanoparticles are obtained with an average diameter of 2 nm and dispersivity within 10% of the average diameter.
  • the pulse energy is between 50- 100 mJ, such that nanoparticles of Pd metal are obtained dominating with respect to palladium nanoparticles oxide, PdO. This way, palladium nanoparticles are obtained that can be used in catalysis and hydrogen storage applications .
  • an apparatus that carries out said method, in any of its embodiments.
  • Fig. 1 diagrammatically shows a possible implementation of the method for making nanoparticles by laser ablation in a liquid according to the invention
  • Fig. 1A diagrammatically shows a possible implementation of the method for making nanoparticles by laser ablation of a target in a cuvette with liquid solvent in motion;
  • Fig. 2 shows the cuvette of Fig. 1A where the nanoparticles are generated associated with a system for controlling, for each pulse, the growth of the concentration of the colloid and its characteristics;
  • Fig. 3 diagrammatically shows a step of maintenance of the flow of the fluid through the above described cuvette of Fig. 1A and an auxiliary system for measuring the absorbance, after dilution, when the absorbance in the cuvette is larger than 1-2;
  • Fig. 4 shows the growth of the absorbance of a colloid of Au nanoparticles in double distilled water, responsive to the number of laser pulses produced by the waves of said laser at 1064 nm, and the growth of the plasmonic resonance, and below it shows the linear growth of the plasmonic peak and, accordingly, of the concentration, with diameter of the illuminated region 1.4 mm and pulse energy 15 mJ;
  • Fig. 5 above shows the growth of the absorbance during the production of nanoparticles of Ag when the flow circulation system is operating, and below it shows the cyclical change of the liquid in the cuvette ;
  • HRTEM high resolution
  • Fig. 10A is the crystalline structure
  • Fig. 11 shows the stability of a colloid of nanoparticles of Ag generated with said method in double distilled water with waves at 1064 nm of said laser, with diameter of the illuminated region 1.4 mm and pulse energy 8 mJ, where a first line is the absorbance spectrum of the sample during the production and the other line is the absorbance spectrum of the same sample after 2 years;
  • FIG. 12A-12E show a succession of interactions of nanoparticles of magnetite with an external magnetic field, where: 12A - just produced sample, 12B - after applying a magnetic field, 12C - after one night, 12D
  • Fig. 13 shows a comparison of the absorbance spectrum of a sample of magnetite just produced and of the same sample after the succession of steps of Fig. 12.
  • a method and a relative apparatus for making colloids consisting of metal nanoparticles, of metal or metal oxide provide the production, by a source 15, of a Nd:YAG laser beam 1, with pulse duration comprised between 15 and 40 picoseconds, pulse energy comprised between 2 and 100 mJ, at the essential wavelength of 1064 nm and repetition frequency up to a maximum of 1 kHz in the range of energy for pulse.
  • the source 15 can provide switching the generation of the laser beam 1 even at the second harmonic at 532 nm or at the third harmonic at 355 nm.
  • a metal target 7 made of metal or metal oxide, for example Au, Ag, Ti, Cu, Fe or Pd, pure or in the form of alloys of the above described metals, is immersed in a liquid solvent 5, for example acetone, 2- propanol, PAMAM, PEG, oil for microscopy, water, preferably double distilled water.
  • a liquid solvent 5 for example acetone, 2- propanol, PAMAM, PEG, oil for microscopy, water, preferably double distilled water.
  • An optical unit 11 is configured for deflecting and concentrating on target 7 the laser beam 1 emitted by the laser source 10.
  • Target 7 is irradiated with the laser beam 1, thus producing a colloid 14 of the metal nanoparticles in the liquid solvent 5.
  • optical unit 11 focalizes the laser beam 1 that irradiates target 7 with focus 12 of the laser beam 1 located beyond target 7, so that target 7 is irradiated in an illumination region 13 of predetermined diameter not less than 0.5 mm, thus producing the colloid 14 about the illumination region 13, with desired dimension, statistical distribution and concentration of the nanoparticles for the respective uses.
  • the morphology of the nanoparticles is easily controllable, for example spherical shape, which for many applications is preferred.
  • the energy of the laser beam is distributed in illumination region 13 in a homogeneous way, with a mechanism of production and stabilization of nanoparticles that is "gentler" than other known methods.
  • the production of nanoparticles is carried out without that phenomena capable of generating undesirable shock waves, cavitation bubbles or intense transmission of plasma are present, due to the concentration in a single point of the laser energy, which can generate an abundant ablation, but a disordered production of nanoparticles, which brings quickly to aggregations, which prevent a control of the dimension, statistical size distribution and concentration of the colloid.
  • the control of the morphology of the particles is jeopardized. Therefore, the known methods require specific additives to limit the aggregation and increase the stability.
  • the colloid is formed starting from the illumination region 13 and slow colloidal streams 14 parallel are generated that rise towards the above with very slow convective and non-turbulent movement against the flow direction of the laser beam 1 that hits the target.
  • the slowness of the convective movement allows keeping steady the characteristics of the colloid and attract fresh pure liquid solvent 5 that is present in workspace 6.
  • the ablation is carried out for mutual interaction of many phenomena, i.e. evaporation, thermionic transmission and ionization heat assisted by photons.
  • the ablated material is mainly consisting of ions and atoms that are surrounded by the liquid in a thin thickness of evaporated liquid solvent. Such zone surrounding the laser beam above the target in which move the detached nanoparticles is called "plume”.
  • the production of nanoparticles is carried out in the plume during the expansion and, actually, ends when the pressure of the plume is equal to the pressure of the environment.
  • the ablated material is mainly made up of ions and atoms.
  • the atoms are produced by phases of the expansion.
  • a certain amount of clusters is also produced during the evaporation .
  • the ratio between said species depends upon the energy of the pulse and upon the illumination region, i.e. by the fluence of the laser energy that hits the target at each pulse, since the duration of each laser pulse is always comprised between 15-40 picoseconds.
  • the density of the species is large enough to permit an aggregation forming the nanoparticles.
  • the ions and the atoms are attracted by each other by Van-der- Waals forces, like dipole-dipole links, including permanent dipoles and rotors, and ione-dipole links (permanent or rotors) .
  • the stabilization of the nanoparticles is attained during the expansion of the plume after each laser pulse so that the final characteristics of the colloid, i.e. the size and their statistical distribution and concentration, is obtained from the sum of nanoparticles formed in a single pulse multiplied by the number of pulses. After several hundreds of microseconds, no further significant formation of nanoparticles is observed. This is another evidence that the nanoparticles are stabilized during the expansion of the plume.
  • the diameter of the illuminated region 13 is preferably adjustable by optical unit 11, and an increase of the nanoparticle size can be obtained responsive to an increase of the diameter of the illumination region 13.
  • a possible exemplary embodiment of an apparatus for making nanoparticles with laser ablation provides a picosecond pulsed laser beam 1 focused by optical unit 11 that can comprise a lens 3 with a prism 4, directed towards target 7.
  • optical unit 11 can comprise a lens 3 with a prism 4, directed towards target 7.
  • the prism preferably an anti-reflective prism, a mirror or a path of mirrors can be used.
  • the diameter of the illumination region is preferably comprised between 0.5 mm and 5 mm, preferably between 0.5 mm and 2 mm.
  • the focus of the laser beam is located under target 7, which is immersed in liquid volume 5, for example defined in a cuvette 6.
  • the height of the liquid of volume 5 above target 7 is preferably set between 1 and 2 cm.
  • the adjustment of the illuminated region on target 7 can be set by a micrometric screw 2.
  • the volume of liquid can be in motion, for example between an inlet 8 and an outlet 9.
  • the flow between the ducts 8 and 9 is selected in such a way that in the workspace is has a fixed flow rate less than 10 ml/min, allowing a full liquid substitution in the cuvette within several minutes, and then the substantial absence of turbulence .
  • Cuvette 6 can be located on a platform 10 connected to a step motor (not shown) , controlled by computer, which can move platform 10 and, accordingly, the target directions x, y.
  • the step motor has been chosen in order to cause platform 10 to move not continuously but in a stepped way. Each step can be triggered after a certain number of laser pulses 1, which depends upon the repetition frequency of the laser source 15 and upon the material of target 7.
  • the motor of platform 10 can cause a transmission screw (not shown) to transmit a stepped motion in the direction x or y of about 1-2 mm, so that the target is struck by the laser beam 1 in a not ablated region, and to stop again.
  • Fig. 2 shows a detail of the above described cuvette 6 of Fig. 1 with an apparatus 20 for measuring the absorbance for each pulse.
  • the light 24 emitted by the UV- Vis lamp 23 is conveyed via a fibre and a collimator (not shown) through the liquid 5 and is collected by another collimator (not shown) connected to the fibre 25 with a spectrometer 26 broadband that covers a spectral range of 200-1100 nm.
  • the collected spectrum is treated by a personal computer 27 that operates also the laser 15 similar to that of Fig. 1.
  • the UV-Vis absorbance computed with a so-called Mie fit algorithm, a control of quality of the generated nanoparticles is attained.
  • the absorbance can be determined in an auxiliary system in fluid communication with the cuvette (see Fig. 3) .
  • a value of 10 of the absorbance corresponds to the maximum concentration which ensures stability of the colloid.
  • the growth of the absorbance makes it possible to control the amount of ablated material.
  • the oxidation of the surface of target 7, during the production of nanoparticles of Ag can slow down the ablation of material so, causing a grow of absorbance in a way less than linear.
  • the motor moves the target exposing in a zone not yet ablated and it stops.
  • FIG. 3 shown a possible exemplary embodiment of a system with liquid circulation is diagrammatically shown.
  • a container 31 contains at first the liquid solvent 5.
  • a peristaltic pump 32 fills cuvette 6 until the height of the column of liquid 5 reaches typically 1-2 cm.
  • An inlet valve 33 is open whereas an exit valve 35 is closed.
  • the laser 1 starts the ablation to form the colloid 14 (Fig. 1 or 1A) and said system measures in situ the growth of the absorbance.
  • the valve 5 opens and the peristaltic pump 36 generates a non- turbulent a flow, so that in the cuvette a flow rate less than 10 ml/min is attained.
  • auxiliary peristaltic pump 27 injects a fraction of liquid in an auxiliary cuvette 28 where the absorbance is determined with the same apparatus of Fig. 2, or with a similar apparatus, after controlled dilution .
  • Fig. 4 shows an example of the absorbance responsive to the number of pulses during the ablation of Au (upper box) .
  • the lower box shows the growth of the peak value of plasmonic resonance that has a clearly linear behaviour.
  • Fig. 5 shows the operation of the system of Fig. 3 during the production of nanoparticles of Ag.
  • the sawtooth shape of the diagram in the lower box defines the operation of the flow conveyance system as described in Fig. 3.
  • Figs. 6-6A-6B, 7-7A-7B, 8-8A-8B show the characterisation of Au nanoparticles generated in double distilled water with waves at 1064 nm of a Nd:YAG laser with 25 ps duration and repetition frequency 10 Hz, diameter of the illuminated region 1.4 mm.
  • the average diameter decreases when the pulse energy increases as expected on the basis of the previous description of the production method.
  • the delivered energy grows, the fraction of ions in the ablated material grows as well.
  • the number of nanoparticle seeds increases but, at the same time, the amount of neutral atoms available that is responsible to the growth of a nanoparticle about the seed decreases.
  • the statistical distribution of the diameters narrows down.
  • the diameter of the illuminated region between 0.5 mm and 5 mm, preferably between 0.5 mm and 2 mm, permits to limit the fluctuations of the amount of ablated material, which would be caused by the roughness of the target. More precisely, the diameter of the illuminated region has to be sufficiently large to mediate such roughness in such a way that the amount of ablated material is about the same in different zones of the target. Furthermore, the illuminated region is selected sufficiently large to manufacture a sufficient amount of material for each pulse.
  • the nanoparticles of gold (Au) generated in double distilled water, at 1064 nm wavelength, are essentially spherical and their average diameter decreases by increasing the energy for each pulse. Typical average diameters are 3-8 nm. Larger diameters are obtained by changing the illuminated region on the target and/or the energy for each pulse.
  • the Au nanoparticles in acetone, generated with the second harmonic of said laser to 532 nm, have spherical shape with an average diameter of 2.5 nm and are monodispersed (Fig. 10-lOA-lOB) .
  • Said nanoparticles can be transferred in double distilled water maintaining the same characteristics and without aggregation.
  • the nanoparticles of silver (Ag) are generated in double distilled water, with the 1064 nm wavelength, mainly with spherical shape with an average diameter 1-5 nm with pulse energy between 8-30 mJ. Owing to the strong tendency to the oxidation, larger than Au, the plasmonic resonance is shifted towards the red of about 10-20 nm with respect to the calculated value of 385 nm expected for metal Ag.
  • the surface of the silver nanoparticles can be functionalized, without loss of stability, by adding Lithium chloride or Sodium chloride for increasing bactericide and anti-mould characteristics of the colloid. Up to a 10-3 Molar concentration of the salt, the main mechanism relates to the change of hydroxylic ions with chloride ions. This increases the negative charge, and then the stability, of said nanoparticles and increases the active surface. A similar mechanism of activation surface occurs in the formation of bimetal nanoparticles for galvanic exchange.
  • Ti02 titanium target
  • the method according to the invention applied to a metal titanium target (Ti) can produce directly Ti02 nanoparticles , with waves at 1064 nm.
  • the oxidation of the ablated material is very quick and only a few nanoparticles of metal Ti can be observed.
  • Ti02 (Titania) is considered an effective catalyst, in particular in the Anatase crystalline form.
  • Nanoparticles of Ti02 can be generated, with said method, in a prevailing crystalline form, either Rutile or Anatase, simply changing the pulse energy.
  • the average diameter is 5 nm.
  • particles larger than 10 nm and with hollow structure can be produced.
  • the above described characteristics can be modulated by changing the diameter of the illuminated region between 0.5-5 mm, in particular 0.5-2 mm.
  • the palladium nanoparticles (Pd) can be generated with said method, at the wavelength of 1064 nm, in bi- distilled water where they are strongly oxidized and polydispersed with diameters of about 5-15 nm. The oxidation reduces the active surface and prevent their use in catalysis and hydrogen storage.
  • the Pd nanoparticles generated in 2-propanol, with the second harmonic at 532 nm of said picosecond laser have an average diameter of 2 nm and low dispersivity .
  • the metal Pd nanoparticles become largely dominating with respect to the of oxide PdO nanoparticles .
  • such nanoparticles are ideal for catalysis and hydrogen storage for their degree of purity and the big surface-volume ratio.
  • the metal Pd nanoparticles are steady for at least 4 months.
  • the concentration of Pd in the colloid can be controlled and calculated, from the value of the UV-Vis absorbance, after calibration by ICP.
  • the concentration of nanoparticles of magnetite can be controlled and calculated, from the value of the UV-Vis absorbance, after calibration by ICP.
  • the applications of said colloidal solutions range from the pharmaceutical industry, where the NPs can be used as carrier for detection of drugs (AuNP) or for making AgNP colloidal solutions as disinfectant, to the food industry, for example using the NPs in the packaging field for protecting the freshness of products in the packages, and to the agricultural industry, for example using the AgNPs for increasing the duration the cut flowers or as antiparasitic aid.
  • AuNP drugs
  • the NPs can be used as carrier for detection of drugs (AuNP) or for making AgNP colloidal solutions as disinfectant
  • the food industry for example using the NPs in the packaging field for protecting the freshness of products in the packages
  • the agricultural industry for example using the AgNPs for increasing the duration the cut flowers or as antiparasitic aid.

Abstract

A method and an apparatus for making colloids consisting of metal nanoparticles, of metal or metal oxide. The method provides prearranging a source (15) of a Nd:YAG laser beam (1), with pulse duration comprised between 15 and 40 picoseconds, pulse energy comprised between 2 and 100 mJ, at the essential wavelength of 1064 nm or at the second harmonic of 532 nm or at the third harmonic of 355 nm, and repetition frequency up to a maximum of 1 kHz in the range of energy for pulse. Then, the method provides prearranging a metal target (7) made of metal or metal oxide in a workspace (6) where the target (7) is put immersed in a liquid solvent (5). An optical unit (11) is configured for deflecting and concentrating the laser beam (1) emitted by the laser source (10) on the target (7). The target (7) is irradiated with the laser beam (1) making a colloid (14) of the metal nanoparticles in the liquid solvent (5). The optical unit (11) irradiates the target (7) with focus (12) of the laser beam (1) located beyond the target (7), so that the target (7) is irradiated in an illumination region (13) of predetermined diameter not less than 0.5 mm, in order to produce the colloid (14) around the region (13) with dimension, statistical distribution and predetermined concentration of the nanoparticles.

Description

TITLE
METHOD FOR PRODUCING COLLOIDS COMPRISING NANOPARTICLES
DESCRIPTION
Field of the invention
[ 0001 ] The present invention relates to the generation of metal nanoparticles, of pure metal or metal oxide, by means of ablation of a starting metal material with pulsed laser in the presence of a liquid, to obtain colloids.
[ 0002 ] The present invention claims the priority of PI2014A000084 filed on 17/11/2014, in the name of the same applicant, and that here is incorporated by reference.
Description of the prior art
[ 0003 ] Methods are known for making nanoparticles of controlled size, shape and statistical distribution to obtain colloids of such nanoparticles in a liquid.
[ 0004 ] In a first type of methods, ions of metal salts are used as reducing agents, such as sodium citrate or boron hydride salts in water. The anions that derive from the metal salt dissolved in water, and those that derive from the reducing agents, are active as surfactants, reaching the goal that the resulting nanoparticles take a same electric charge that prevents them from aggregation. A drawback of such methods is that there are many by-products generated by chemical reaction, that are deposited on the surface of the nanoparticles, affecting the activity of the nanoparticles changing the degree of purity of the colloids .
[ 0005 ] For keeping the nanoparticles durably separated from each other, moreover, stabilizers are used, for example polyethylene glycol (PEG) , which works as "spacer" between the nanoparticles . However, in case of biomedical applications, a cellular toxicity of the colloids can derive owing to the presence of the stabilizers.
[0006] In US2012/0282134A1 is used a pulsed laser with very high repetition frequency (lOkHz-100 MHz), which preferably has a duration of the repetition preferably between 0.1-10 ps and a pulse energy preferably between 1- 10 micro-Joule. Other examples use a continuous flow of water for cooling the illumination region and a prism movable above the target. The statistical distribution of the diameters of the nanoparticles (Fig. 4) is very dispersed and include spherical nanoparticles of big diameter (>20 nm) and smaller nanoparticles (<5nm) clearly elliptical. The generated nanoparticles have a low stability, and consequently a stabilizer is added, for example a certain an amount of citrate. There is however a toxicity from the by-products of chemical reductions generated by the use of stabilizers.
[0007] In Anna Giusti et al . "Multiphoton fragmentation of PAMAM g5-capped gold nanoparticles induced by picosecond laser irradiation at 532 nm", Journal of Physical Chemistry C, vol. Ill, no. 41, 21 September 2007 (2007-09-21), pages 14984-14991, XP055206156, ISSN: 1932-7447, DOI : 10.1021/jp072611k * Figures 1-3; page 14984 -14985", a method is described to obtain high quality colloidal gold nanoparticles, by means of a pulsed laser. In particular, a Nd:YAG laser source is used, with 25 picosecond pulse duration, 40 mJ pulse energy, at the essential wavelength of 1064 nm, and a 1kHz repetition frequency in said range of energy, focusing the laser beam with a 20cm lens. [0008] It is desirable to increase further the stability of the nanoparticles in the colloid, which is obtained when the nanoparticles do not aggregate with each other at least after several months and up to one year, as it can be verified for example with UV-Vis spectroscopy tests. Once achieved the desired stability, the colloid can be kept or shipped without additional stabilizers and special precautions .
Summary of the invention [0009] It is then a feature of the present invention to provide a method for generating metal nanoparticles by means of ablation with pulsed laser of a starting material in the presence of a liquid to obtain colloids that remain steady with time, for example beyond a year, relatively to dimension, statistical distribution and concentration of the nanoparticles.
[0010] It is another feature of the present invention to provide such a method that makes it possible to obtain colloids so that a reliable evaluation of the delivered concentration can be made.
[0011] It is a further feature of the present invention to provide such a method that makes it possible to obtain colloids comprising nanoparticles with a big surface/volume ratio, low size dispersion and surface free from reaction by-products.
[0012] It is still a feature of the present invention to provide such a method so that the production rate is increased without affecting the above described features of the nanoparticles.
[0013] These and other objects are achieved by a method for making colloids consisting of metal nanoparticles , of metal or metal oxide, comprising the steps of:
- prearranging a Nd:YAG laser source, with pulse duration comprised between 15 and 40 picoseconds, pulse energy comprised between 2 and 100 mJ, at the essential wavelength of 1064 nm or at the second harmonic of 532 nm or at the third harmonic of 355 nm, and repetition frequency up to a maximum of 1 kHz at said pulse energy;
- prearranging a metal target made of metal or metal oxide ;
- prearranging a workspace where the target is put immersed in a liquid solvent;
- prearranging an optical unit configured for deflecting and concentrating a laser beam emitted by the laser source on the target,
- irradiating the target with the laser beam, forming a colloid of the metal nanoparticles in the liquid solvent .
According to the invention, the optical unit irradiates the target with focus of the laser beam located beyond the target, so that the target is irradiated in an illumination region of a predetermined diameter not less than 0.5 mm, this forming, starting from this region, a colloid with desired dimension, statistical distribution and concentration.
[0014] This way, by irradiating the target with the focus of the laser beam located beyond the target, there is the effect of adjusting precisely the energy that the laser beam delivers to the target illumination region with predetermined diameter, zeroing substantially the effects of turbulence in the liquid solvent, due to shock waves, to cavitation bubbles and to intense transmission of plasma owing to the excessive concentration energy in a single irradiation point. With respect to the prior art, in which the laser focus is located in the illumination region, causing local high turbulence of the liquid solvent, with the invention a high stability of the colloid is achieved, obtaining an extremely low aggregation of the nanoparticles, and reaching desired dimension, statistical distribution and concentration of the nanoparticles in the colloid.
[0015] In a possible implementation of the method, the illumination region of predetermined diameter is comprised between 0.5 mm and 5 mm, preferably between 0.5 mm and 2 mm. This way, an optimal balance is obtained between size of the illumination region and area of the so-called "plume", in which the colloid is formed and achieves an own stability, propagating progressively in the liquid solvent without phenomena of aggregation of the nanoparticles. In particular, the adjustment of the diameter of said illumination region is provided through said optical unit, such that nanoparticles are obtained of different size responsive to the diameter of the illumination region.
[0016] Said liquid solvent can be selected from the group consisting of: water, acetone, 2-propanol, PAMAM (polyamidoamine) , PEG (polyethylene glycol), oil for microscopy, whereas the target can be made of a metal or metal oxide, selected from the group consisting of: Au, Ag, Ti, T1O2, Cu, Fe and Pd, pure or in the form of alloys of the above described metals, as well as other metals, metal alloys and metal oxides.
[0017] Advantageously, the liquid solvent is double distilled water. In particular, the double distilled water is preliminarily treated keeping it at a predetermined conditioning temperature and then sterilized. In particular, the conditioning temperature is about 100°C and the sterilization temperature is about 120°C. This way, a high reproducibility of the process for ablation is obtained and a much higher stability with time of the colloids obtained.
[0018] In particular, the liquid solvent in the workspace has a fixed flow rate less than 10 ml/min. This way, the lens fluid flow is sufficient for not requiring cooling of the liquid and for limiting at most the turbulence of the liquid solvent, reducing at most the collisions between metal nanoparticles capable of forming aggregations which would change the final quality of the colloid. Such a lens speed allows a non-turbulent conveyance of the liquid solvent and such slowness requires several minutes for a complete replacement of the liquid surrounding the target, avoiding to affect the process of stabilization of the nanoparticles and allowing to obtain spherical nanoparticles .
[0019] Advantageously, in the workspace a cuvette is arranged in which the target is put. The cuvette can be for example a transparent container with an inlet and an outlet to allow the solvent to enter and the colloid to exit .
[0020] Preferably, the workspace is arranged in such a way that the target rests on a plane support and is covered by a column of liquid of height comprised between 1-2 cm .
[0021] Advantageously, the target has a translation movement having a speed with respect to the laser beam less than 0.1 cm/sec. Such low speed allows, on the one hand, not to move the optical unit, and on the other hand allows a gradual detachment of the nanoparticles by the target, avoiding turbulences of the liquid and allowing an homogeneous dispersion of the nanoparticles in the liquid solvent, avoiding aggregations between the particles.
[0022] In particular, the target is mounted in a cuvette with a motor that makes it possible a stepped motion in the directions x, y, to obtain a desired ablation and avoiding a re-deposit of the nanoparticles or the production of oxide on the surface of the target.
[0023] In a possible embodiment the optical unit comprises at least one lens and a prism with anti reflective coating. This way, losses for reflection are eliminated during the movement beam entrance and exit surfaces of the prism.
[0024] Alternatively, the optical unit comprises a lens and a deflection mirror. This way, the deflection mirror avoids reflection losses the cost of a prism with anti- reflective coating.
[0025] Advantageously, the optical unit has a converging lens with focal length of 20-30 cm. In a possible embodiment, the optical unit is arranged for deflecting the laser beam for hitting perpendicularly the target. This way, a non-homogeneity of the distribution of energy due to refraction of the liquid of the laser beam in the illumination region is avoided.
[0026] In a possible embodiment, the laser beam is converted from the essential harmonic at the second harmonic, which has an energy efficiency about 50% less, the second harmonic being used both for remodulating the shape of the nanoparticles and for limiting the size less than 4 nm and very narrow statistical distribution. In particular, the second harmonic is used for Au in acetone or for Pd in 2-propanol. Also the third harmonic can be used for similar purposes.
[0027] Advantageously, a spectroscopic system is arranged that provides an absorbance spectrum in situ of the colloid in the workspace downstream of the illumination region, the laser beam and the optical unit being adjusted responsive to the absorbance spectrum so that the absorbance spectrum in situ corresponds to a predetermined range of metal nanoparticles, in particular with an average diameter comprised between 1 and 10 nm, spherical shape, crystalline structure and substantial absence of aggregations. In particular, the absorbance spectrum in situ corresponds to a range of nanoparticles with an average diameter comprised between 1 and 5 nm for biomedical applications.
[0028] In particular, the spectroscopic system operates with UV-Vis spectroscopy, and the absorbance spectrum is sampled for each pulse, in the range UV-Vis from 200 to 1100 nanometres. This way, the shape, size and statistical distribution can be determined for each pulse, and this is very useful when the pulse energy is larger than a certain ablation threshold, which depends upon the material and the size of the illumination region of the target invested by the laser.
[0029] Advantageously, the measurement in situ of the UV- Vis absorbance of the colloid for controlling the stability provides an analysis of the spectral position of the plasmonic resonance and an analysis of the width of the plasmonic resonance.
This way, it is possible to control in real time the size and shape of the nanoparticles and their statistical distribution and their possibility of aggregation, by controlling the parameters of the laser beam selected among: wavelength, pulse energy and illumination region. After calibration with analysis ICP (Inductive Coupled Plasma) the UV-Vis spectral absorbance, makes it possible to control the concentration, the morphology of the nanoparticles and their statistical size distribution.
[0030] Advantageously, the absorbance is determined by means of optical fibres, ending with a collimator, which collimates a light beam coming from a UV-Vis lamp and crossing the colloid in the volume, until the light beam has been collected by another collimator connected, through optical fibres, to a low resolution wideband spectrometer which operates between 200 and 1100 nm.
[0031] Preferably, the spectral distribution of the absorbance is computed by a program for controlling the concentration of the colloid measured by a "Mie fit" equation starting from the absorbance.
[0032] Advantageously, the admissible threshold of absorbance is less or equal to 10, in order to ensure optimal stability to the colloid. In particular, the measurement in situ of the absorbance is carried out for values of absorbance between 1 and 2, whereas for values of absorbance between 3 and 10 a step is provided of collecting the colloid, and diluting it in an auxiliary container, which is crossed by the collimated light beam coming from the UV-Vis lamp for determining the absorbance .
[0033] In a possible exemplary embodiment, the target is displaced of one step after a predetermined number of laser pulses according to a predetermined function according to values of recorded absorbance. In particular, the function is configured for adjusting the movement of the cuvette in at least one of the directions x, y responsive to the absorbance, so that, if an incipient decrease of the absorbance is observed, or a stop in the growth of the absorbance, the target is displaced for presenting not ablated portions of the target, until the absorbance starts growing again.
[0034] Preferably, the function provides a linear growth of the absorbance. This way, the colloid has shape, average diameter, statistical distribution and optimal concentration of the nanoparticles , and it is possible to obtain a production of nanoparticles in the colloid substantially proportional to the number of pulses and/or to the power, with total reproducibility of the desired quality substantially independent from the production rate, in the range of power between 2-100 mJ and frequency less than 1 kHz. This way, since the production of the colloid is carried out in the ablated cloud of nanoparticles, so-called "plume", in a time of about one hundred microseconds, the repetition frequency of the pulses must not exceed 1 kHz for not to affect the features of the colloid produced in a single pulse.
[0035] Preferably, for increasing the production even if maintaining a maximum quality of the colloid, the frequency range of pulses is increased between 50-100 Hz under a same energy for pulse. This way, by increasing the frequency of the production rate, i.e. the amount of nanoparticles delivered per time unit, is greater with respect to what can be obtained by increasing the number of pulses under a same nominal power.
[0036] In particular, the liquid solvent is acetone and the laser emits the second harmonic at 532 nm with pulse energy about 15 mJ and diameter of the illuminated region 0.5-5 mm, in particular 0.5-2 mm, such that nanoparticles are obtained with an average diameter of 2.5 nm with dispersivity within 10% of the average diameter.
[0037] In a possible implementation of the method, the nanoparticles are obtained from a target of silver, where to the liquid solvent a solute is added selected from the group consisting of: Sodium chloride, Lithium chloride. In particular, the solute is added to the liquid solvent up to a 10~3 molar concentration. This way, a colloid is obtained whose stability and/or morphology is unchanged obtaining an effective anti-bacterial and anti-mould action starting from concentration 0.5-1 mg/1, and that can be used for example as disinfectant, in the treatment of wounds, and in the preservation of flowers and fruit.
[0038] In a possible implementation of the method, the nanoparticles are obtained from a titanium target and the laser has a pulse energy selected from the group consisting of:
2 and 10 mJ
- 10 and 50 mJ
- 50 and 100 mJ,
so that T1O2 is obtained, which with pulse energy between 2 and 10 mJ or between 50 and 100 mJ is in the Rutile form and with pulse energy between 10 and 50mJ is in the Anatase form.
[ 0039 ] In a possible implementation of the method, the nanoparticles are obtained from an iron target. This way, colloidal suspensions are obtained of nanoparticles of Fe that include iron oxides in the form of magnetite with strong magnetic characteristics and a reversible behaviour in an external magnetic field.
[ 0040 ] In a possible implementation of the method, the nanoparticles are obtained from a target of Palladium with 2-propanol as liquid solvent, and the laser emits the second harmonic at 532 nm with pulse energy between 5-40 mJ and diameter of the illuminated region 0.5-5 mm, such that nanoparticles are obtained with an average diameter of 2 nm and dispersivity within 10% of the average diameter. Alternatively, the pulse energy is between 50- 100 mJ, such that nanoparticles of Pd metal are obtained dominating with respect to palladium nanoparticles oxide, PdO. This way, palladium nanoparticles are obtained that can be used in catalysis and hydrogen storage applications .
[ 0041 ] According to another aspect of the invention, an apparatus is provided that carries out said method, in any of its embodiments.
Brief description of the drawings
[ 0042 ] The invention will be now shown with the following description of an exemplary embodiment thereof, exemplifying but not limitative, with reference to the attached drawings in which: — Fig. 1 diagrammatically shows a possible implementation of the method for making nanoparticles by laser ablation in a liquid according to the invention;
— Fig. 1A diagrammatically shows a possible implementation of the method for making nanoparticles by laser ablation of a target in a cuvette with liquid solvent in motion;
— Fig. 2 shows the cuvette of Fig. 1A where the nanoparticles are generated associated with a system for controlling, for each pulse, the growth of the concentration of the colloid and its characteristics;
— Fig. 3 diagrammatically shows a step of maintenance of the flow of the fluid through the above described cuvette of Fig. 1A and an auxiliary system for measuring the absorbance, after dilution, when the absorbance in the cuvette is larger than 1-2;
— Fig. 4 above shows the growth of the absorbance of a colloid of Au nanoparticles in double distilled water, responsive to the number of laser pulses produced by the waves of said laser at 1064 nm, and the growth of the plasmonic resonance, and below it shows the linear growth of the plasmonic peak and, accordingly, of the concentration, with diameter of the illuminated region 1.4 mm and pulse energy 15 mJ;
— Fig. 5 above shows the growth of the absorbance during the production of nanoparticles of Ag when the flow circulation system is operating, and below it shows the cyclical change of the liquid in the cuvette ;
— Figs. 6, 6A and 6B show typical characterization of a colloid of Au nanoparticles generated with said method with waves at 1064 nm of said laser in bi- distilled water, with diameter of the illuminated region 1.4 mm and pulse energy 15 mJ, where Fig. 6 shows the TEM micrography, Fig. 6A shows the statistical distribution of the diameters and Fig. 6B shows the absorbance spectrum, with nanoparticles having average diameter 4.0 nm, σ-=1.5 nm, σ+=2.0 nm, potential ζ =-50 mV;
— Fig. 7, 7A and 7B show a typical characterisation of a colloid of Au nanoparticles generated with said method with waves at 1064 nm of said laser in bi- distilled water, where the diameter of the illuminated region is 1.4 mm and the pulse energy 30 mJ, with Fig. 7 that shows the TEM micrography, Fig. 7A the statistical distribution of the diameters, and Fig. 7B the absorbance spectrum, where the average diameter of the nanoparticles is 3.2 nm, σ-=1.5 nm, σ+=2.8 nm, potential ζ =-45 mV;
— Figs. 8, 8A, 8B show a typical characterisation of a colloid of Au nanoparticles generated with said method with waves at 1064 nm of said laser in double distilled water, with diameter of the illuminated region 1.4 mm and pulse energy 40 mJ, where Fig. 8 shows the TEM micrography, Fig. 8A the statistical distribution of the diameters, and Fig. 8B the absorbance spectrum, where the average diameter of the nanoparticles is 3.0 nm, σ-=1.0 nm, σ+=2.0 nm, potential ζ =-44 mV;
— Figs. 9, 9A, 9B show a typical characterisation of a colloid of Au nanoparticles generated with said method with waves at 1064 nm of said laser in double distilled water, with diameter of the illuminated region 2.0 mm and pulse energy 15 mJ, where Fig. 9 is an electronic microscopy in transmission at a high resolution (HRTEM) , Fig. 9A is the crystalline structure and Fig. 9B the statistical distribution of the diameters, where the average diameter of the nanoparticles is 7.0 nm, σ-=3.0 nm, σ+=5.0 nm, potential ζ =-43 mV, outlining how a larger illuminated region produces, with the same energy for pulse, larger Au nanoparticles (see Fig. 6 as reference) ;
— Figs. 10, 10A, 10B show Au nanoparticles generated in acetone with said method with waves at 532 nm of said laser, with diameter of the illuminated region 1.4 mm and pulse energy 15 mJ, where Fig. 10 is an electronic microscopy in transmission at a high resolution (HRTEM), Fig. 10A is the crystalline structure and Fig. 10B is the statistical distribution of the diameters, where the average diameter of the nanoparticles is 2.5 nm, σ-<0.5 nm, σ+<0.5 nm, potential ζ =-30 mV after transfer in water;
— Fig. 11 shows the stability of a colloid of nanoparticles of Ag generated with said method in double distilled water with waves at 1064 nm of said laser, with diameter of the illuminated region 1.4 mm and pulse energy 8 mJ, where a first line is the absorbance spectrum of the sample during the production and the other line is the absorbance spectrum of the same sample after 2 years;
— Fig. 12A-12E show a succession of interactions of nanoparticles of magnetite with an external magnetic field, where: 12A - just produced sample, 12B - after applying a magnetic field, 12C - after one night, 12D
- after removal of the magnetic field, 12E - after stirring;
— Fig. 13 shows a comparison of the absorbance spectrum of a sample of magnetite just produced and of the same sample after the succession of steps of Fig. 12. Detailed description of some exemplary embodiments
[0043] With reference to Fig. 1, a method and a relative apparatus for making colloids consisting of metal nanoparticles, of metal or metal oxide provide the production, by a source 15, of a Nd:YAG laser beam 1, with pulse duration comprised between 15 and 40 picoseconds, pulse energy comprised between 2 and 100 mJ, at the essential wavelength of 1064 nm and repetition frequency up to a maximum of 1 kHz in the range of energy for pulse. The source 15 can provide switching the generation of the laser beam 1 even at the second harmonic at 532 nm or at the third harmonic at 355 nm.
[0044] In a workspace 6 a metal target 7, made of metal or metal oxide, for example Au, Ag, Ti, Cu, Fe or Pd, pure or in the form of alloys of the above described metals, is immersed in a liquid solvent 5, for example acetone, 2- propanol, PAMAM, PEG, oil for microscopy, water, preferably double distilled water.
[0045] An optical unit 11 is configured for deflecting and concentrating on target 7 the laser beam 1 emitted by the laser source 10. Target 7 is irradiated with the laser beam 1, thus producing a colloid 14 of the metal nanoparticles in the liquid solvent 5.
[0046] According to the invention, optical unit 11 focalizes the laser beam 1 that irradiates target 7 with focus 12 of the laser beam 1 located beyond target 7, so that target 7 is irradiated in an illumination region 13 of predetermined diameter not less than 0.5 mm, thus producing the colloid 14 about the illumination region 13, with desired dimension, statistical distribution and concentration of the nanoparticles for the respective uses. Furthermore, the morphology of the nanoparticles is easily controllable, for example spherical shape, which for many applications is preferred.
[0047] According to the invention, the energy of the laser beam is distributed in illumination region 13 in a homogeneous way, with a mechanism of production and stabilization of nanoparticles that is "gentler" than other known methods. In fact, according to the invention, the production of nanoparticles is carried out without that phenomena capable of generating undesirable shock waves, cavitation bubbles or intense transmission of plasma are present, due to the concentration in a single point of the laser energy, which can generate an abundant ablation, but a disordered production of nanoparticles, which brings quickly to aggregations, which prevent a control of the dimension, statistical size distribution and concentration of the colloid. Furthermore, also the control of the morphology of the particles is jeopardized. Therefore, the known methods require specific additives to limit the aggregation and increase the stability.
[0048] In particular, the colloid is formed starting from the illumination region 13 and slow colloidal streams 14 parallel are generated that rise towards the above with very slow convective and non-turbulent movement against the flow direction of the laser beam 1 that hits the target. The slowness of the convective movement allows keeping steady the characteristics of the colloid and attract fresh pure liquid solvent 5 that is present in workspace 6.
[0049] By using a 15-40 ps Nd:YAG laser, and keeping the diameter of illuminated region 13 between 0.5 mm and 5 mm, preferably between 0.5 mm and 2 mm, owing to the position of the focus 12 behind the target, and keeping the pulse energy between 2-100 mJ and the repetition frequency always less than 1kHz, the ablation is carried out for mutual interaction of many phenomena, i.e. evaporation, thermionic transmission and ionization heat assisted by photons. The ablated material is mainly consisting of ions and atoms that are surrounded by the liquid in a thin thickness of evaporated liquid solvent. Such zone surrounding the laser beam above the target in which move the detached nanoparticles is called "plume". The production of nanoparticles is carried out in the plume during the expansion and, actually, ends when the pressure of the plume is equal to the pressure of the environment. The ablated material is mainly made up of ions and atoms. The atoms are produced by phases of the expansion. A certain amount of clusters is also produced during the evaporation .
[0050] The ratio between said species depends upon the energy of the pulse and upon the illumination region, i.e. by the fluence of the laser energy that hits the target at each pulse, since the duration of each laser pulse is always comprised between 15-40 picoseconds. At the beginning of the expansion and before reaching a diffusive conditions, i.e. when the pressure of the plume is equal to the liquid, the density of the species is large enough to permit an aggregation forming the nanoparticles. The ions and the atoms are attracted by each other by Van-der- Waals forces, like dipole-dipole links, including permanent dipoles and rotors, and ione-dipole links (permanent or rotors) . The latter type of force is considered forming the first seeds about which the nanoparticles grow, attracting other seeds with forces of Van-der-Waals type. These phenomena proceed until the average distance between the species is short enough to keep such forces significant. It is noted that the lack of Shockwaves and subsequent cavitation has the effect of not disturbing the plume allowing the colloid to form and stabilize in an optimal way.
[0051] The stabilization of the nanoparticles is attained during the expansion of the plume after each laser pulse so that the final characteristics of the colloid, i.e. the size and their statistical distribution and concentration, is obtained from the sum of nanoparticles formed in a single pulse multiplied by the number of pulses. After several hundreds of microseconds, no further significant formation of nanoparticles is observed. This is another evidence that the nanoparticles are stabilized during the expansion of the plume.
[0052] The diameter of the illuminated region 13 is preferably adjustable by optical unit 11, and an increase of the nanoparticle size can be obtained responsive to an increase of the diameter of the illumination region 13.
[0053] With reference to Fig. 1A, a possible exemplary embodiment of an apparatus for making nanoparticles with laser ablation, provides a picosecond pulsed laser beam 1 focused by optical unit 11 that can comprise a lens 3 with a prism 4, directed towards target 7. Alternatively to the prism, preferably an anti-reflective prism, a mirror or a path of mirrors can be used.
[0054] The diameter of the illumination region is preferably comprised between 0.5 mm and 5 mm, preferably between 0.5 mm and 2 mm. The focus of the laser beam is located under target 7, which is immersed in liquid volume 5, for example defined in a cuvette 6. The height of the liquid of volume 5 above target 7 is preferably set between 1 and 2 cm.
[0055] The adjustment of the illuminated region on target 7 can be set by a micrometric screw 2.
[0056] The volume of liquid can be in motion, for example between an inlet 8 and an outlet 9. The flow between the ducts 8 and 9 is selected in such a way that in the workspace is has a fixed flow rate less than 10 ml/min, allowing a full liquid substitution in the cuvette within several minutes, and then the substantial absence of turbulence .
[0057] Cuvette 6 can be located on a platform 10 connected to a step motor (not shown) , controlled by computer, which can move platform 10 and, accordingly, the target directions x, y.
[0058] The step motor has been chosen in order to cause platform 10 to move not continuously but in a stepped way. Each step can be triggered after a certain number of laser pulses 1, which depends upon the repetition frequency of the laser source 15 and upon the material of target 7. In particular, the motor of platform 10 can cause a transmission screw (not shown) to transmit a stepped motion in the direction x or y of about 1-2 mm, so that the target is struck by the laser beam 1 in a not ablated region, and to stop again.
[ 0059 ] Obviously, exemplary alternative embodiments are possible, such as a relative movement of the optical unit with respect to target 7, orientable mirrors, etc.3
[ 0060 ] Fig. 2 shows a detail of the above described cuvette 6 of Fig. 1 with an apparatus 20 for measuring the absorbance for each pulse. The light 24 emitted by the UV- Vis lamp 23 is conveyed via a fibre and a collimator (not shown) through the liquid 5 and is collected by another collimator (not shown) connected to the fibre 25 with a spectrometer 26 broadband that covers a spectral range of 200-1100 nm. The collected spectrum is treated by a personal computer 27 that operates also the laser 15 similar to that of Fig. 1.
[ 0061 ] Before starting the ablation, the spectrum of the lamp 23, through the liquid 5 still without nanoparticles in colloidal suspension, is acquired as reference 25 to calculate then the absorbance. When laser 1 (Fig. 1, Fig. 1A) starts the ablation, a control program in the PC 27 shows on a screen the evolution, for each pulse, of the absorbance, through an optical path of 1 cm, both for the whole spectrum and for each selected wavelength.
[ 0062 ] According to the invention, the UV-Vis absorbance, computed with a so-called Mie fit algorithm, a control of quality of the generated nanoparticles is attained. For values of the concentration having an absorbance larger than 1-2 in cuvette 6, the absorbance can be determined in an auxiliary system in fluid communication with the cuvette (see Fig. 3) . For many metals, a value of 10 of the absorbance corresponds to the maximum concentration which ensures stability of the colloid. Furthermore, the growth of the absorbance makes it possible to control the amount of ablated material.
[0063] For example, the oxidation of the surface of target 7, during the production of nanoparticles of Ag, can slow down the ablation of material so, causing a grow of absorbance in a way less than linear. In this case, the motor moves the target exposing in a zone not yet ablated and it stops.
[0064] In Fig. 3 shown a possible exemplary embodiment of a system with liquid circulation is diagrammatically shown. A container 31 contains at first the liquid solvent 5. A peristaltic pump 32 fills cuvette 6 until the height of the column of liquid 5 reaches typically 1-2 cm. An inlet valve 33 is open whereas an exit valve 35 is closed. The laser 1 starts the ablation to form the colloid 14 (Fig. 1 or 1A) and said system measures in situ the growth of the absorbance. When the absorbance in the cuvette (optical path 1 cm) reaches a pre-selected value, the valve 5 opens and the peristaltic pump 36 generates a non- turbulent a flow, so that in the cuvette a flow rate less than 10 ml/min is attained. An auxiliary peristaltic pump 27 injects a fraction of liquid in an auxiliary cuvette 28 where the absorbance is determined with the same apparatus of Fig. 2, or with a similar apparatus, after controlled dilution . [0065] Fig. 4 shows an example of the absorbance responsive to the number of pulses during the ablation of Au (upper box) . The lower box shows the growth of the peak value of plasmonic resonance that has a clearly linear behaviour.
[0066] Fig. 5 shows the operation of the system of Fig. 3 during the production of nanoparticles of Ag. The sawtooth shape of the diagram in the lower box defines the operation of the flow conveyance system as described in Fig. 3.
[0067] Figs. 6-6A-6B, 7-7A-7B, 8-8A-8B, show the characterisation of Au nanoparticles generated in double distilled water with waves at 1064 nm of a Nd:YAG laser with 25 ps duration and repetition frequency 10 Hz, diameter of the illuminated region 1.4 mm. The average diameter decreases when the pulse energy increases as expected on the basis of the previous description of the production method. As the delivered energy grows, the fraction of ions in the ablated material grows as well. The number of nanoparticle seeds increases but, at the same time, the amount of neutral atoms available that is responsible to the growth of a nanoparticle about the seed decreases. Similarly, the statistical distribution of the diameters narrows down. Even if the statistical distribution of the diameters can be defined by a log norm function, it is preferable to use a Gaussian asymmetric function, with standard deviations different for diameters that are smaller or larger than the average value, for showing better the dispersivity . On the other hand, the absence of nanoparticles of big diameter makes the two functions practically coincident to each other. [0068] The reproducibility of the samples is attained by avoiding, according to the invention, any phenomenon which can affect the expansion of the plume by the choice of the production parameters, i.e. diameter of the illuminated region and energy for pulse. The duration of each pulse is always in the range of 15-40 ps . A continuous flow and/or a continuous movement of the target, as disclosed in US2012/ 0282134A1 , would cause turbulence of the liquid that would change the shape and the statistical distribution of the colloid, reducing the stability with time .
[0069] The diameter of the illuminated region between 0.5 mm and 5 mm, preferably between 0.5 mm and 2 mm, permits to limit the fluctuations of the amount of ablated material, which would be caused by the roughness of the target. More precisely, the diameter of the illuminated region has to be sufficiently large to mediate such roughness in such a way that the amount of ablated material is about the same in different zones of the target. Furthermore, the illuminated region is selected sufficiently large to manufacture a sufficient amount of material for each pulse.
[0070] With the method above described, the production of nanoparticles resembles the growth of crystals. In Fig. 9- 9A-9B images obtained from TEM at a high resolution
(HRTEM) are shown, where the crystalline structure is perfectly visible.
[0071] The nanoparticles of gold (Au) generated in double distilled water, at 1064 nm wavelength, are essentially spherical and their average diameter decreases by increasing the energy for each pulse. Typical average diameters are 3-8 nm. Larger diameters are obtained by changing the illuminated region on the target and/or the energy for each pulse.
[ 0072 ] The above described Au nanoparticles with average diameter 3-8 nm in double distilled water are adapted to biomedical applications because they are not cyto- and/or genotoxic. The cyto- and geno- toxicity of the above described nanoparticles has been evaluated by the Department of Translational Research on New Technologies in Medicine and Surgery, Division of Medical Genetics Medic, of the University of Pisa.
[ 0073 ] The Au nanoparticles in acetone, generated with the second harmonic of said laser to 532 nm, have spherical shape with an average diameter of 2.5 nm and are monodispersed (Fig. 10-lOA-lOB) . Said nanoparticles can be transferred in double distilled water maintaining the same characteristics and without aggregation.
[ 0074 ] The above described Au nanoparticles, generated in acetone, show a limited geno- and cyto-toxicity due to the residual of amorphous coal on the surface, which is caused by the photo degradation of the acetone by laser irradiation. Such particles can be transferred in water maintaining the same characteristics. In this case, the toxicity is reduced but not eliminated.
[ 0075 ] The nanoparticles of silver (Ag) are generated in double distilled water, with the 1064 nm wavelength, mainly with spherical shape with an average diameter 1-5 nm with pulse energy between 8-30 mJ. Owing to the strong tendency to the oxidation, larger than Au, the plasmonic resonance is shifted towards the red of about 10-20 nm with respect to the calculated value of 385 nm expected for metal Ag.
[ 0076 ] Said shift towards the red shows a variety of types of aggregation. When the shift exceeds 10-20 nm, the plasmonic band is widened and the ablated material starts aggregating. The absorbance in situ allows the control of many regimens and then of the final quality of the colloid .
[ 0077 ] The nanoparticles of Ag, with average diameter 1-5 nm, have a strong bactericide action. Then, the colloids of Ag produced in water with the above described characteristics can be effectively used as disinfectants. Said product not present chemical by-products nor it requires additives for stabilizing the nanoparticles nor it for shipping. Said method is therefore "green" in all the steps from the production to the shipping.
[ 0078 ] The stability of the above described nanoparticles of silver extends for more than one year without addition of stabilizers [Fig. 11] .
[ 0079 ] The surface of the silver nanoparticles can be functionalized, without loss of stability, by adding Lithium chloride or Sodium chloride for increasing bactericide and anti-mould characteristics of the colloid. Up to a 10-3 Molar concentration of the salt, the main mechanism relates to the change of hydroxylic ions with chloride ions. This increases the negative charge, and then the stability, of said nanoparticles and increases the active surface. A similar mechanism of activation surface occurs in the formation of bimetal nanoparticles for galvanic exchange.
[ 0080 ] The method according to the invention applied to a metal titanium target (Ti) can produce directly Ti02 nanoparticles , with waves at 1064 nm. The oxidation of the ablated material is very quick and only a few nanoparticles of metal Ti can be observed. Ti02 (Titania) is considered an effective catalyst, in particular in the Anatase crystalline form.
[ 0081 ] Nanoparticles of Ti02 can be generated, with said method, in a prevailing crystalline form, either Rutile or Anatase, simply changing the pulse energy. The average diameter is 5 nm. Also particles larger than 10 nm and with hollow structure can be produced. The above described characteristics can be modulated by changing the diameter of the illuminated region between 0.5-5 mm, in particular 0.5-2 mm.
[ 0082 ] Owing to their their size and the possibility of production in a prevailing crystalline form of either Rutile or Anatase, the above described nanoparticles of Ti02 are particularly indicated for photocatalysis and production of pigments.
[ 0083 ] The palladium nanoparticles (Pd) can be generated with said method, at the wavelength of 1064 nm, in bi- distilled water where they are strongly oxidized and polydispersed with diameters of about 5-15 nm. The oxidation reduces the active surface and prevent their use in catalysis and hydrogen storage.
[ 0084 ] According to an aspect of the invention, the Pd nanoparticles generated in 2-propanol, with the second harmonic at 532 nm of said picosecond laser, have an average diameter of 2 nm and low dispersivity . By keeping the illumination region fixed, and changing the pulse energy and the number of pulses, the metal Pd nanoparticles become largely dominating with respect to the of oxide PdO nanoparticles . Then, such nanoparticles are ideal for catalysis and hydrogen storage for their degree of purity and the big surface-volume ratio. The metal Pd nanoparticles are steady for at least 4 months.
[0085] The concentration of Pd in the colloid can be controlled and calculated, from the value of the UV-Vis absorbance, after calibration by ICP.
[0086] The ablation with the aforementioned method of an iron target (Fe) in double distilled water, at the wavelength of 1064 nm of said picosecond laser, produces nanoparticles of magnetite. Said nanoparticles have strong magnetic characteristics so that they can be gathered with a magnetic field. When the magnetic field is withdrawn, the nanoparticles of magnetite can be separated from each other by stirring the sample [Fig. 12] . The successions of Fig. 12 shows the freshly produced colloid (Fig. 12A) , after application of the magnetic field (Fig. 12B) . After one night, the nanoparticles of magnetite have been gathered by a magnetic field (Fig. 12C-D) . Finally, the colloid is restored after stirring (Fig. 12E) .
[0087] The comparison of the absorbance of the sample freshly produced and after stirring, subsequently to the application of a magnetic field, shows the reversibility of the process (Fig. 13) . The average diameter of the nanoparticles of magnetite is 5-10 nm.
[0088] After many cycles, the above described nanoparticles of magnetite aggregate with each other and precipitate owing to the magnetization induced by the magnetic field.
[0089] The concentration of nanoparticles of magnetite can be controlled and calculated, from the value of the UV-Vis absorbance, after calibration by ICP.
[0090] Owing to the "green" nature of the method and to the many uses of the colloids in various fields, the industrial applicability is apparent. In particular, the applications of said colloidal solutions range from the pharmaceutical industry, where the NPs can be used as carrier for detection of drugs (AuNP) or for making AgNP colloidal solutions as disinfectant, to the food industry, for example using the NPs in the packaging field for protecting the freshness of products in the packages, and to the agricultural industry, for example using the AgNPs for increasing the duration the cut flowers or as antiparasitic aid.
[0091] The foregoing description of specific exemplary embodiments will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt in various applications the specific exemplary embodiments without further research and without parting from the invention, and, accordingly, it is meant that such adaptations and modifications will have to be considered as equivalent to the specific embodiments. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention, it is to be understood that the phraseology or terminology that is employed herein is for the purpose of description and not of limitation.

Claims

1. A method for making colloids consisting of metal nanoparticles , of metal or metal oxide, comprising the steps of:
- prearranging a source (15) of a Nd:YAG laser beam (1), with pulse duration comprised between 15 and 40 picoseconds, pulse energy comprised between 2 and 100 mJ, at the essential wavelength of 1064 nm or at the second harmonic of 532 nm or at the third harmonic of 355 nm, and repetition frequency up to a maximum of 1 kHz in said range of energy for pulse;
- prearranging a metal target (7) made of said metal or metal oxide;
- prearranging a workspace (6) where said target (7) is put immersed in a liquid solvent (5) ;
- prearranging an optical unit (11) configured for deflecting and concentrating on said target (7) said laser beam (1) emitted by said laser source (10) ;
- irradiating said target (7) with said laser beam (1) making a colloid (14) of said metal nanoparticles in said liquid solvent (5) ;
characterized in that
said optical unit (11) irradiates said target (7) with focus (12) of said laser beam (1) located beyond the target (7), so that said target (7) is irradiated in an illumination region (13) of predetermined diameter not less than 0.5 mm, in order to produce said colloid (14) in said region with predetermined dimension, statistical distribution and concentration.
Method according to claim 1, wherein said illumination region (13) of predetermined diameter is comprised between 0.5 mm and 5 mm, preferably between 0.5 mm and 2 mm, in particular the adjustment of the diameter of said illumination region carried out through said optical unit (11) .
Method according to claim 1, wherein said liquid solvent (5) is selected from the group consisting of: water, acetone, 2-propanol, PAMAM, PEG, oil for microscopy, and said target (7) is made of a metal or metal oxide, selected from the group consisting of: Au, Ag, Ti, Cu, Fe and Pd, pure or in the form of alloys of the above described metals.
Method according to claim 1, wherein said liquid solvent (5) is double distilled water, in particular said double distilled water is preliminarily treated keeping it at a predetermined conditioning temperature and then sterilized, in particular said conditioning temperature is about 100°C and said sterilization temperature is about 120°C.
Method according to claim 1, wherein said liquid solvent (5) in said workspace (6) has a fixed flow in a predetermined direction and the flow rate of the liquid solvent (5) in motion in said workspace (6) is less than 10 ml/min.
Method according to claim 1, wherein said workspace is defined by a cuvette (6) where said target (7) is arranged, in particular said target (7) in said cuvette (6) is covered by a column of liquid solvent (5) of height comprised between 1-2 cm.
7. Method according to claim 1, wherein said target (7) has a translation movement having a speed with respect to the laser beam (1) less than 0.1 cm/sec, in particular said target (7) is movable with a stepped translation movement in two directions x, y (10) .
8. Method according to claim 1, wherein said optical unit
(11) is selected from the group consisting of:
- a lens (3) and a prism (4) with anti-reflective coating,
- a lens (3) and a deflection mirror (4),
in particular, said optical unit (11) is located for deflecting the laser beam (1) for perpendicularly hitting on said target (7) .
9. Method according to claim 1, wherein said laser beam
(1) is converted from said essential harmonic to said second harmonic, with an energy efficiency of about 50%, said second harmonic being used for remodulating the shape of the nanoparticles and limiting the size, in particular to size values less than 4 nm and statistical distribution very narrow, in particular the second harmonic being used for Au in acetone or for Pd in 2-propanol.
10. Method according to claim 1, wherein a spectroscopic system (20) is provided for determining an absorbance spectrum in situ of said colloid (14) in said workspace (6) downstream of said illumination region (13), said laser beam (1) and said optical unit (11) being adjusted responsive to said absorbance spectrum so that said absorbance spectrum in situ corresponds to a predetermined range of metal nanoparticles, in particular with an average diameter comprised between 1 and 10 nm, spherical shape, crystalline structure and substantial absence of aggregations, in particular said absorbance spectrum in situ corresponding to a range of nanoparticles with an average diameter comprised between 1 and 5 nm for biomedical applications .
11. Method according to claim 10, wherein said spectroscopic system operates with UV-Vis spectroscopy, and said absorbance spectrum is sampled for each pulse, in the UV-Vis range from 200 to 1100 nanometres, in particular said measurement in situ of the UV-Vis absorbance of the colloid for controlling the stability provides an analysis of the spectral position of the plasmonic resonance and an analysis of the width of the plasmonic resonance.
12. Method according to claim 11, wherein said absorbance is determined by means of optical fibres, which end with a collimator, which collimates a light beam coming from a UV-Vis lamp (23) and travelling across said colloid (6) in said volume, until said light beam has been collected by another collimator connected, through an optical fibre (25) , to a low resolution wideband spectrometer (26) which operates between 200 and 1100 nm; in particular said collected light beam (25) at said spectrometer (26) is converted by program means (27) to obtain said absorbance, in particular the concentration of the colloid being measured by an Mie equation fit starting from the absorbance.
13. Method according to claim 11, wherein said absorbance is less than 10, in particular said measurement in situ of said absorbance being carried out for values of absorbance between 1 and 2, whereas for values of absorbance between 3 and 10 a step is provided of collecting said colloid, diluting it in an auxiliary container, which is crossed by a collimated light beam coming from a UV-Vis lamp.
14. Method according to claim 10, wherein said target (7) is displaced of one step after a predetermined number of laser pulses according to a predetermined function according to values of recorded absorbance, in particular said function being configured for adjusting the movement of the cuvette (6) in at least one of said directions x, y responsive to said absorbance so that, if a, incipient decrease of the absorbance, or a stop in the growth of the absorbance are observed, the target is displaced for exposing to the laser beam not ablated parts, until the absorbance starts growing again.
15. Method according to claim 1, wherein for increasing the production keeping a maximum quality of the colloid, said frequency range of pulses is increased between 50-100 Hz under a same energy for pulse.
16. Method according to claim 1, wherein said liquid solvent (5) is acetone and said laser emits the second harmonic at 532 nm with pulse energy about 15 mJ and diameter of the illuminated region 0.5-5 mm, in particular 0.5-2 mm, such that nanoparticles are obtained with an average diameter of 2.5 nm with dispersivity within 10% of the average diameter.
17. Method according to claim 1, wherein said nanoparticles are obtained from a target (7) of silver and to said liquid solvent (5) is added a solute selected from the group consisting of: Sodium chloride, Lithium chloride, in particular said solute is added to said liquid solvent (5) up to a 10~3 Molar concentration .
18. Method according to claim 1, wherein said nanoparticles are obtained from a target (7) of titanium and said laser has a pulse energy selected from the group consisting of:
— 2 and 10 mJ to obtain Rutile nanoparticles;
— 10 and 50 mJ to obtain Anatase nanoparticles;
— 50 and 100 mJ to obtain Rutile nanoparticles.
19. Method according to claim 1, wherein said nanoparticles are obtained from a iron target (7) to obtain a colloidal suspension of nanoparticles of Fe that include iron oxides in the form of magnetite with strong magnetic characteristics and a reversible behaviour in an external magnetic field.
20. Method according to claim 1, wherein said nanoparticles are obtained from a target (7) of Palladium and said liquid solvent (5) is 2-propanol and said laser emits the second harmonic at 532 nm with pulse energy between 5-40 mJ and diameter of the illuminated region 0.5-5 mm, in particular 0.5-2 mm, such that nanoparticles are obtained with an average diameter of 2 nm with dispersivity within 10% of the average diameter.
21. Method according to claim 1, wherein said nanoparticles are obtained from a target (7) of Palladium and said liquid solvent (5) is 2-propanol and said laser emits the second harmonic at 532 nm with pulse energy between 50-100 mJ and diameter of the illuminated region 0.5-5 mm, in particular 0.5-2 mm, such that nanoparticles are obtained of metal Pd dominating with respect to nanoparticles of palladium oxide PdO, for applications in catalysis and hydrogen storage .
22. An apparatus for making colloids consisting of metal nanoparticles, of metal or metal oxide configured for carrying out a method according to any of the previous claims .
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CN106077674A (en) * 2016-06-16 2016-11-09 中国科学院合肥物质科学研究院 A kind of silver nano-grain colloid solution with efficient sterilizing performance
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CN106492715B (en) * 2016-12-19 2023-02-10 广东工业大学 Method and device for preparing particles
CN112756938A (en) * 2021-01-20 2021-05-07 深圳市鑫镭创科自动化科技有限公司 Double-beam laser screen-disassembling method and device based on mobile phone software control
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CN114751649A (en) * 2022-04-25 2022-07-15 哈尔滨工业大学 Method for preparing material surface nano-particles by using laser
CN114751649B (en) * 2022-04-25 2023-08-25 哈尔滨工业大学 Method for preparing material surface nano particles by utilizing laser
CN115283684A (en) * 2022-08-05 2022-11-04 大连理工大学 Device for continuously preparing nano particles by using liquid-phase laser ablation method
CN115368894A (en) * 2022-08-18 2022-11-22 北京科技大学 Method for preparing sulfur oxide fluorescent powder by laser

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