US20100224026A1 - A process for synthesising silver nanoparticles - Google Patents

A process for synthesising silver nanoparticles Download PDF

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US20100224026A1
US20100224026A1 US12/733,967 US73396708A US2010224026A1 US 20100224026 A1 US20100224026 A1 US 20100224026A1 US 73396708 A US73396708 A US 73396708A US 2010224026 A1 US2010224026 A1 US 2010224026A1
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silver
nanoparticles
seeds
solutions
nanoprisms
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Margaret Elizabeth Brennan Fournet
Patrick Fournet
Damian John Aherne
John Moffat Kelly
Deirdre Marie Ledwith
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College of the Holy and Undivided Trinity of Queen Elizabeth near Dublin
National University of Ireland Galway NUI
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Assigned to NATIONAL UNIVERSITY OF IRELAND, GALWAY reassignment NATIONAL UNIVERSITY OF IRELAND, GALWAY ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: BRENNAN FOURNET, MARGARET ELIZABETH, FOURNET, PATRICK
Assigned to PROVOST, FELLOWS AND SCHOLARS OF THE COLLEGE OF THE HOLY AND UNDIVIDED TRINITY OF QUEEN ELIZABETH, NEAR DUBLIN, THE reassignment PROVOST, FELLOWS AND SCHOLARS OF THE COLLEGE OF THE HOLY AND UNDIVIDED TRINITY OF QUEEN ELIZABETH, NEAR DUBLIN, THE ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: LEDWITH, DEIRDRE MARIE, AHERNE, DAMIAN JOHN, KELLY, JOHN MOFFAT
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F9/00—Making metallic powder or suspensions thereof
    • B22F9/16—Making metallic powder or suspensions thereof using chemical processes
    • B22F9/18—Making metallic powder or suspensions thereof using chemical processes with reduction of metal compounds
    • B22F9/24—Making metallic powder or suspensions thereof using chemical processes with reduction of metal compounds starting from liquid metal compounds, e.g. solutions
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
    • B22F1/05—Metallic powder characterised by the size or surface area of the particles
    • B22F1/054—Nanosized particles
    • B22F1/0551—Flake form nanoparticles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
    • B22F1/05—Metallic powder characterised by the size or surface area of the particles
    • B22F1/054—Nanosized particles
    • B22F1/0553—Complex form nanoparticles, e.g. prism, pyramid, octahedron
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy

Definitions

  • This invention relates to a process for the production of nanoparticles.
  • the invention relates to a process for the production of large quantities of nanoparticles.
  • Nanoparticles can be synthesised from a range of materials including dielectric inorganic, organic, polymer and metallic materials. Nanoparticles have been utilised in a number of different fields of technology ranging from paints to biomolecular diagnostics. Over the last few years there has been an increase in the number of uses of nanoparticles, such an increase has resulted in a need to producing nanoparticles in large quantities while maintaining batch reproducibility.
  • WO 04/086044 describes a two-step wet chemistry batch process for synthesising silver seeds and growing the synthesized silver seeds to produce a range of silver nanoparticles.
  • the quantity of silver nanoparticles produced by a wet chemistry batch reaction process are limited.
  • the invention provides a process for synthesising silver nanoparticles comprising the steps of:
  • Silver nanoparticles produced by the process may have an average diameter of between 5 nm and 100 nm and a UV-vis spectrum peak in the 420 nm to 900 nm region
  • both steps (a) and (b) may be performed using microfluidic flow chemistry.
  • the silver source may be a silver salt, for example silver nitrate.
  • the silver source may be a complexed silver compound or salt.
  • the silver source may be dissolved in a capping agent solution, for example a capping agent solution selected from the group consisting: Trisodium Citrate, Cetyl -trimethyl-ammonium-bromide.
  • a capping agent solution selected from the group consisting: Trisodium Citrate, Cetyl -trimethyl-ammonium-bromide.
  • the reducing agent may be selected from the group consisting: sodium borohydride, ascorbic acid.
  • the ratio of silver source: reducing agent may be about 1:8.
  • Step (a) may be performed using microfluidic flow chemistry with a flow rate of between 3 ml/min and 10 ml/min for the silver source.
  • Step (a) may be performed using microfluidic flow chemistry with a flow rate of about 1 ml/min for the reducing agent.
  • Step (a) may be performed at 0° C.
  • step (b) may further comprise the step of aging the silver seeds.
  • the aging step may comprise:
  • the silver source of the aging step may be the same as the silver source used in step (a).
  • the polymeric stabiliser may be water soluble.
  • the polymeric stabiliser may have a molecular weight between 10 kDa and 1300 kDa.
  • the polymeric stabiliser may be selected from one or more of the group consisting: poly(vinyl alcohol), poly(vinyl pyrollidone), poly(ethylene glycol), and poly(acrylic acid).
  • the polymeric stabiliser may be poly(vinyl alcohol).
  • the aging step may further comprise the step of reducing the silver source present in the silver source-polymeric stabiliser-silver seed mixture.
  • the silver source may be reduced by ascorbic acid.
  • Step (b) of the process may be carried out at a temperature of between 10° C. to 60° C.
  • step (b) is carried out at a temperature of 40° C.
  • the nanoparticles produced by the process may be stable in an aqueous solution.
  • the nanoparticles produced by the process may have a colour tunability throughout the visible and near infra red spectrum.
  • the nanoparticles produced by the process may be red in colour in a colloidal aqueous solution.
  • the nanoparticles produced by the process may comprise at least 30% non-spherical shaped nanoparticles.
  • the nanoparticles produced by the process may comprise at least 50% non-spherical shaped nanoparticles. For example, at least 70% non-spherical shaped nanoparticles.
  • the non-spherical shaped nanoparticles may be triangular and/or hexagonal and/or truncated triangular in shape.
  • the nanoparticles produced by the process may have a UV-vis spectral peak in the 345 nm region.
  • the nanoparticles produced by the process may have a UV-vis main spectral width FWHM of less than 300 nm.
  • a UV-vis main spectral width FWHM of less than 150 nm such as a UV-vis main spectral width FWHM of less than 120 nm or a UV-vis main spectral width FWHM of less than 100 nm.
  • the invention further provides a process for synthesising silver nanoparticles comprising the steps of:
  • the silver seeds may be grown in step (b) by mixing a silver source solution and a reducing agent solution.
  • the silver source solution may comprise silver seeds.
  • the reducing agent solution may comprise silver seeds.
  • the silver source solution may comprise a stabiliser.
  • the reducing agent solution may comprise a stabiliser.
  • the solutions may be pressurised to at least about 35 MPa such as in the range of between about 35 MPa to about 275 MPa.
  • the solutions may be pressurised at about 140 MPa.
  • the solutions may have a shear rate of at least about 1 ⁇ 10 6 s ⁇ 1 such as in the range of about 1 ⁇ 10 6 s ⁇ 1 to about 50 ⁇ 10 6 s ⁇ 1 .
  • the solutions may have a flow rate of at least about 10 ml/min such as at least about 100 ml/min or at least about 1 l/min, for example at least about 10 l/min.
  • the solutions may be introduced separately. Each solution may have a different flow rate or each solution may have the same flow rate. The solutions may be added at different concentrations.
  • the residence time of the solutions in a mixing chamber of a microfluidic system may be less than about 1 ms.
  • the residence time of the solutions in a mixing chamber of a microfluidic system may be between about 2 ⁇ s and about 1 ms.
  • Step (b) may be carried out at a temperature of between about 10° C. to about 60° C., for example about 40° C.
  • the silver source may be a silver salt such as silver nitrate.
  • the silver source may be present at a concentration between about 10 ⁇ 3 M to about 10 ⁇ 1 M.
  • the stabiliser may be a polymeric stabiliser.
  • the polymeric stabiliser may be water soluble.
  • the polymeric stabiliser may have a molecular weight of between about 10 kDa and about 1300 kDa.
  • the polymeric stabiliser may be selected from one or more of the group consisting: poly(vinyl alcohol), poly(vinyl pyrollidone), poly(ethylene glycol), poly(sodium styrenesulphonate) and poly(acrylic acid).
  • the polymeric stabiliser may be poly(vinyl alcohol).
  • the polymeric stabiliser may be present at a concentration of about 10 ⁇ 2 wt % to about 10 wt %.
  • the stabiliser may be trisodium citrate.
  • the trisodium citrate may be present at a concentration of between about 10 ⁇ 3 M to about 10 ⁇ 1 M.
  • the reducing agent may be ascorbic acid.
  • the reducing agent may be present at a concentration of about 10 ⁇ 3 M to about 10 ⁇ 1 M.
  • the silver seeds may be present at a concentration of between about 10 ⁇ 8 M to about 10 ⁇ 4 M of silver.
  • the silver source in step (a) may be a silver salt such as silver nitrate.
  • the silver source in step (a) may be dissolved in a capping agent solution.
  • the capping agent solution may comprise trisodium citrate and/or cetyl-trimethyl-ammonium-bromide.
  • the reducing agent in step (a) may be sodium borohydride and/or ascorbic acid
  • the ratio of silver source: reducing agent in step (a) may be 1:8.
  • Step (a) may be performed using microfluidic flow chemistry.
  • the solutions may be introduced separately. Each solution may have a different flow rate. Each solution may have the same flow rate.
  • the solutions may be added at different concentrations.
  • the flow rate of the silver source solution may be between about 3 ml/min and about 10 ml/min such as about 8 ml/min.
  • the flow rate of the reducing agent solution may be between about 0.5 ml/min and about 1.5 ml/min such as about 1 ml/min.
  • Step (a) may be performed using pressurised microfluidic flow chemistry.
  • the solutions may be pressurised to at least about 35 MPa.
  • the solutions may be pressurised in the range of about 35 MPa to about 275 MPa such as about 140 MPa.
  • the solutions may have a shear rate of at least about 1 ⁇ 10 6 s ⁇ 1 such as in the range of about 1 ⁇ 10 6 s ⁇ 1 to about 50 ⁇ 10 6 s ⁇ 1 .
  • the solutions may have a flow rate of at least about 10 ml/min.
  • the solutions may have a flow rate of at least about 100 ml/min.
  • the solutions may have a flow rate of at least about 1 l/min.
  • the solutions may have a flow rate of at least about 10 l/min.
  • the residence time of the solutions in a mixing chamber of a microfluidic system may be less than about 1 ms.
  • the residence time of the solutions in a mixing chamber of a microfluidic system may
  • Step (a) may be performed at about 0° C.
  • the invention also provides a process for producing silver nanoparticles comprising the steps of:
  • the polymer may be a polyanionic polymer.
  • the polymer may be a derivative of polysulphonate.
  • the polymer may be a derivative of polystyrene sulphonate.
  • the derivative may be an inorganic salt of polystyrene sulphonate.
  • the derivative may be a monovalent salt of polystyrene sulphonate.
  • the polymer may be poly (sodium styrenesulphonate) (PSSS).
  • PSSS may have a molecular weight between about 3 KDa to about 1,000 KDa such as about 1,000 KDa.
  • the concentration of polymer in the silver seed preparation may be about 2.5 mg/ml to about 250 mg/ml such as about 25 mg/ml.
  • the step of preparing silver seeds may be carried out at room temperature.
  • the step of growing the silver seeds may be carried out at room temperature.
  • the invention further provides a process for synthesising silver nanoparticles comprising the steps of:
  • the water soluble polymer may be a polyanionic polymer.
  • the polymer may be a derivative of polysulphonate.
  • the polymer may be a derivative of polystyrene sulphonate.
  • the derivative may be an inorganic salt of polystyrene sulphonate.
  • the derivative may be a monovalent salt of polystyrene sulphonate.
  • the polymer may be poly (sodium styrenesulphonate) (PSSS).
  • PSSS poly (sodium styrenesulphonate)
  • the PSSS may have a molecular weight of between about 3 KDa to about 1,000 KDa such as about 1,000 KDa.
  • the concentration of polymer may be between about 10 ⁇ 1 mg/l to about 250 g/l.
  • the concentration of polymer may be between about 10 ⁇ 1 mg/l to about 1 g/l.
  • the concentration of polymer may be between about 2.5 g/l to about 250 g/l.
  • Step (a) is performed in the presence of the water soluble polymer.
  • the silver seeds may be grown in step (b) by mixing a silver source solution and a reducing agent solution.
  • the silver source solution may comprise silver seeds.
  • the reducing agent solution may comprise silver seeds.
  • the silver source solution may comprise a stabiliser.
  • the reducing agent solution may comprise a stabiliser.
  • the silver source may be a silver salt such as silver nitrate.
  • the silver source may be present at a concentration between about 10 ⁇ 3 M to about 10 ⁇ 1 M.
  • the stabiliser may be a polymeric stabiliser.
  • the polymeric stabiliser may be water soluble.
  • the polymeric stabiliser may have a molecular weight between about 10 kDa and about 1300 kDa.
  • the polymeric stabiliser may be selected from one or more of the group consisting: poly(vinyl alcohol), poly(vinyl pyrollidone), poly(ethylene glycol), poly(sodium styrenesulphonate) and poly(acrylic acid).
  • the polymeric stabiliser may be poly(vinyl alcohol).
  • the polymeric stabiliser may be present at a concentration of between about 10 ⁇ 2 wt % to about 10 wt %.
  • the stabiliser may be trisodium citrate.
  • the trisodium citrate may be present at a concentration of between about 10 ⁇ 3 M to about 10 ⁇ 1 M.
  • the reducing agent may be ascorbic acid.
  • the reducing agent may be present at a concentration of between about 10 ⁇ 3 M to about 10 ⁇ 1 M.
  • the silver seeds may be present at a concentration of between about 10 ⁇ 8 M to about 10 ⁇ 4 M of silver.
  • Step (b) may be carried out at a temperature of between about 10° C. to about 60° C. such as about 40° C.
  • Step (b) may be preformed using microfluidics.
  • the solutions may be introduced separately. Each solution may have a different flow rate. Each solution may have the same flow rate.
  • the solutions may be added at different concentrations
  • Step (b) may be performed using pressurised microfluidic flow chemistry.
  • the solutions may be pressurised to at least about 35 MPa.
  • the solutions may be pressurised in the range of about 35 MPa to about 275 MPa such as at about 140 MPa.
  • the solutions may have a shear rate of at least about 1 ⁇ 10 6 s ⁇ 1 such as in the range of between about 1 ⁇ 10 6 s ⁇ 1 to about 50 ⁇ 10 6 s ⁇ 1 .
  • the solutions may have a flow rate of at least about 10 ml/min.
  • the solutions may have a flow rate of at least about 100 ml/min.
  • the solutions may have a flow rate of at least about 1 l/min.
  • the solutions may have a flow rate of at least about 10 l/min.
  • the residence time of the solutions in a mixing chamber of a microfluidic system may be less than about 1 ms.
  • the residence time of the solutions in a mixing chamber of a microfluidic system may be between about 2 ⁇ s and about 1 ms.
  • the silver source of step (a) may be a silver salt such as silver nitrate.
  • the silver source of step (a) may be dissolved in a capping agent solution.
  • the capping agent solution may comprise trisodium citrate and/or cetyl-trimethyl-ammonium-bromide.
  • the reducing agent of step (a) may comprise sodium borohydride and/or ascorbic acid.
  • the ratio of silver source: reducing agent in step (a) may be 1:8.
  • Step (a) may be carried out at a temperature of about 0° C.
  • Step (a) may be performed using microfluidic flow chemistry.
  • the solutions may be introduced separately. Each solution may have a different flow rate. Each solution may have the same flow rate.
  • the solutions may be added at different concentrations.
  • the flow rate of the silver source solution may be between about 3 ml/min and about 10 ml/min such as about 8 ml/min.
  • the flow rate of the reducing agent solution may be between about 0.5 ml/min and about 1.5 ml/min such as about 1 ml/min.
  • Step (a) may be performed using pressurised microfluidic chemistry.
  • the solutions may be pressurised to at least about 35 MPa.
  • the solutions may be pressurised in the range of about 35 MPa to about 275 MPa such as about 140 MPa.
  • the solutions may have a shear rate of at least about 1 ⁇ 10 6 s ⁇ 1 such as in the range of about 1 ⁇ 10 6 s ⁇ 1 to about 50 ⁇ 10 6 s ⁇ 1 .
  • the solutions may have a flow rate of at least about 10 ml/min.
  • the solutions may have a flow rate of at least about 100 ml/min.
  • the solutions may have a flow rate of at least about 1 l/min.
  • the solutions may have a flow rate of at least about 10 l/min.
  • the residence time of the solutions in a mixing chamber of a microfluidic system may be less than about 1 ms.
  • the residence time of the solutions in a mixing chamber of a microfluidic system may be between about 2 ⁇ s and about 1 ms.
  • the silver nanoparticles synthesised by the processes described herein may have an average diameter of between 5 nm and 100 nm and a UV-vis spectrum peak in the 420 nm to 900 nm region
  • the silver nanoparticles synthesised by the processes described herein may have an average diameter of between about 5 nm and about 100 nm and an optical absorption spectrum peak in the region of about 900 nm to about 1610 nm.
  • the nanoparticles may be stable in an aqueous solution.
  • the nanoparticles may have a colour tunability throughout the visible and near infra red spectrum.
  • the nanoparticles may be red in colour in a colloidal aqueous solution.
  • the nanoparticles may comprise at least 30% non-spherical shaped nanoparticles.
  • the nanoparticles may comprise at least 50% non-spherical shaped nanoparticles.
  • the nanoparticles may comprise at least 70% non-spherical shaped nanoparticles.
  • the nanoparticles may comprise at least 95% non-spherical shaped nanoparticles.
  • the non-spherical shaped nanoparticles may be triangular and/or hexagonal and/or truncated triangular in shape.
  • the non-spherical shaped nanoparticles may be triangular in shape (nanoprisms).
  • the non-spherical shaped nanoparticles may be plate like having an aspect ratio of between about 1:2 to about 1:10. for example triangle edge length of about 45 nm and height about 5 nm.
  • the nanoparticles may have an optical absorption spectrum peak in the region of about 340 nm ⁇ 10 nm.
  • the nanoparticles may have an optical absorption spectrum peak in the region of about 335 nm to about 338 nm.
  • the nanoparticles may have a UV-vis spectral peak in the 345 nm region.
  • the nanoparticles may have a main optical absorption spectrum peak width (FWHM) of less than about 300 nm.
  • FWHM main optical absorption spectrum peak width
  • the nanoparticles may have a main optical absorption spectrum peak width (FWHM) of less than about 150 nm.
  • FWHM main optical absorption spectrum peak width
  • the nanoparticles may have a main optical absorption spectrum peak width (FWHM) of less than about 120 nm.
  • FWHM main optical absorption spectrum peak width
  • the nanoparticles may have a main optical absorption spectrum peak width (FWHM) of less than about 100 nm.
  • FWHM main optical absorption spectrum peak width
  • microfluidic processes of the invention can produce large volumes of high definition silver nanoparticles.
  • the silver nanoparticles produced by a microfluidics process as described herein have improved physical properties, for example narrower size distribution, increased presence of shaped nanoparticles, higher uniformity of samples, better and higher batch to batch reproducibility, compared to nanoparticles produced by a conventional wet chemistry method including,
  • Improvements in the production of silver nanoparticles using the Microfluidics technology described herein compared to the conventional wet chemistry process include:
  • Properties of high definition silver nanoparticles include:
  • Microfluidics can be used to produce shaped silver nanoparticles with the advantage that the microfluidics synthesis process produced a half litre per batch (and is capable of producing several litres per hour at flow rates typically in the range 10 ml/minute to 500 ml/minute) while the wet chemistry method is limited to 100 ml production.
  • TEM images of microfluidics process produced silver nanoparticles confirmed a significant improvement in the size distribution of the nanoparticles compared to nanoparticles prepared using a conventional wet chemistry technique.
  • nanoparticles produced by the microfluidic processes described herein have a greater batch to batch reproducibility of physical characteristics compared to nanoparticles produced by a conventional wet chemistry process.
  • nanoparticles produced by the microfluidic process have long term stability in water or aqueous solution suspension, in particular the nanoparticles do not aggregate or sediment over time.
  • microfluidic processes both the chip and the processor routes, will enable a controlled scale-up of the production of high quality high definition silver nanoparticles in a range of shapes, sizes, colours, and/or surface chemistries.
  • the microfluidic processes described herein can be adapted for the scaled up production of a range of high quality nanoparticles.
  • FIG. 1 is an example of a microfluidic flow chemistry synthesis for silver seed synthesis (step (a)) of the process for the production of discrete high definition silver nanoparticles;
  • FIG. 2 is an example of a microfluidic flow chemistry synthesis for the step of growing silver seeds into silver nanoparticles (step (b)) of the process for the production of discrete high definition silver nanoparticles;
  • FIG. 3A is a schematic of a microfluidic system using a generic microfluidics chip for silver seed production (step (a));
  • FIG. 3B is a detailed schematic of the generic microfluidics chip used in the system of FIG. 3A ;
  • FIG. 4 is a schematic of an alternative microfluidic chip system for silver seed production (step (a));
  • FIG. 5 is a schematic of a microfluidic chip set up showing a sequence for reagent input in a process for producing nanoparticles
  • FIG. 6 is a schematic of a microfluidics chip set up showing reagent output sequencing specifically designed for discrete high definition silver nanoparticle synthesis
  • FIG. 7 is a schematic of a microfluidics chip showing the design requirements for stream input and mixing criteria
  • FIG. 8 (A) is a graph showing the UV-visible spectrum for nanoparticles produced;
  • (B) is a TEM micrograph of the nanoparticles produced;
  • (C) is a bar chart showing the size distribution of the nanoparticles produced;
  • (D) are bar charts showing the distribution of shaped hexagons and triangles/truncated triangles in the nanoparticles produced.
  • the discrete silver nanoparticles were produced from seeds synthesized using a generic microfluidic chip system with a flow rate ratio of 8:1 for solution 1 and 2 (step (a)) and conventional wet chemistry was used to grow the seeds into nanoparticles (step (b));
  • FIG. 9 is a graph showing the UV-visible spectrum for nanoparticles produced;
  • (B) is a TEM micrograph of the nanoparticles produced;
  • (C) is a bar chart showing the size distribution of the nanoparticles produced;
  • (D) are bar charts showing the distribution of shaped hexagons and triangles/truncated triangles in the nanoparticles produced.
  • the discrete silver nanoparticles were produced from seeds synthesized using a generic microfluidic chip system with a flow rate ratio of 8:1 for solution 1 and 2 (step (a)) and conventional wet chemistry was used to grow the seeds into nanoparticles (step (b)).
  • FIG. 10 is a line graph showing the UV-Visible spectrum of two sets of silver seeds produced with a flow rate ratio of 8:1 of solution 1 and 2 respectively (the two lines are superimposed);
  • FIG. 11 (A) is a line graph showing the main optical absorption spectrum peak width dependence (defined as full width at half maximum (FWHM)) and (B) is a line graph showing the dependence of the maximum wavelength for silver seeds produced using a microfluidic chip system with a variation in the flow rate of solution 1 (AgNO 3 and TSC) from 3 to 10 ml/min while keeping the flow rate of solution 2 constant at 1 ml/min;
  • FWHM full width at half maximum
  • FIG. 12 (A) is a graph showing the UV-visible spectrum for nanoparticles produced; and (B) is a TEM micrograph of the nanoparticles produced.
  • the discrete silver nanoparticles were produced from seeds synthesised by conventional wet chemistry (step (a)) and the seeds were grown into nanoparticles using a microfluidics processor (step (b));
  • FIG. 13 (A) is a graph showing the UV-Visible spectra for silver seeds synthesised in presence of PSSS using a microfluidics processor method (processed seeds); discrete silver nanoparticles prepared by a conventional wet chemistry growth in presence of PSSS (step (b)) of microfluidics synthesised silver seeds (step (a)) (beaker experiment); and discrete silver nanoparticles produced from microfluidics synthesised silver seeds (step (a)) and microfluidics grown nanoparticles (step (b)) (microfluidics reaction technology).
  • (B) is a graph showing the UV-Visible spectra of a silver seed solution synthesised using a conventional wet chemistry method (seeds); discrete silver nanoparticles prepared using a conventional wet chemistry method for both steps (a) and (b) (beaker experiment); and discrete silver nanoparticles prepared using a microfluidics process for both steps (a) and (b) (microfluidic reaction technology);
  • FIG. 14 is a graph showing the UV-Visible spectra for a range batches of discrete silver nanoparticles prepared under the same conditions by conventional wet chemistry method for both steps 1 and 2 (each line represents a different batch);
  • FIG. 15 (A) to (C) are TEM images for a range of discrete silver nanoparticles prepared under the same conditions by conventional wet chemistry method for both steps 1 and 2;
  • FIG. 16 is a graph showing the UV-Visible spectra for a range of batches of silver seeds (step (a)) synthesised by a conventional wet chemistry method (each line represents a different batch).
  • FIG. 17 A) is a TEM image of flat-lying silver nanoprisms from a typical sample.
  • B) is a TEM image of silver nanoprisms from another sample, made by the same procedure, that are stacked together and are oriented such that they are standing vertically on their edges.
  • C) is a UV-Vis spectrum of sample of nanoprisms shown in (A) showing the main SPR (in-plane dipole) at ⁇ 825 nm;
  • FIG. 18 A) is a TEM image from a sample of flat lying silver nanoprisms grown from seeds produced with PSSS present.
  • B) and C) are TEM images of silver nanoparticles grown from seeds produced without PSSS present, the sample comprises nanoprisms of a wide range of sizes and “spherical” particles of various sizes with 5-fold symmetry present. A “nanotape” is also visible in (C).
  • D) is a UV-Vis spectra of nanoprisms in (A) grown from seeds produced in presence of PSSS (labeled A) and nanoparticles in (B) and (C) grown from seeds produced in absence of PSSS (labeled B);
  • FIG. 19 A) is a UV-Vis spectra of 4 samples (1 to 4) of nanoprism.
  • B) is a plot of size data for silver nanoprisms from each of the 4 samples of (A).
  • C) is a plot of data (squares) for the position of the main SPR (in-plane dipole) against edge-length (L) divided by thickness (T) for the four samples of (A).
  • the dashed line is a linear fit to the data.
  • the edge-length and thickness were obtained by measuring the dimensions of vertically oriented nanoprisms.
  • the average thickness (T) of all samples is approximately 5.5 nm.
  • the error bars represent the uncertainty in L/T that arises for the standard deviation of measurements of the edge-length and thickness of the nanoprisms;
  • FIG. 20 A) is a photograph of a series of samples ( 1 to 10 ) illustrating the range of colours obtained.
  • the purple color in sample 10 is largely the result of extinction by the in-plane quadrupole.
  • B) is a normalized spectra of a series of as prepared samples obtained using different volumes of seed solution: 1) 650 ⁇ l, 2) 500 ⁇ l, 3) 400 ⁇ l, 4) 260 ⁇ l, 5) 200 ⁇ l, 6) 120 ⁇ l, 7) 90 ⁇ l, 8) 60 ⁇ A 9) 40 ⁇ l, 10) 20 ⁇ l.
  • C) is a spectra from (B) plotted against energy;
  • FIG. 21A is a graph showing the FWHM of the SPRs plotted against plasmon resonance energy for each of the nanoprism samples ( 1 to 10 ).
  • B) is a graph showing the FWHM of the SPRs plotted against nanoprism volume for each of the nanoprism samples ( 1 to 10 ).
  • a standard deviation of 19% (based on data in Table 3) for edge-length and 1 nm uncertainty for thickness was used to generate the error bars for nanoprism volume;
  • FIG. 22 is a TEM image of a flat-lying nanoprism. A close-up in the bottom left clearly shows the 2.5 ⁇ spacing between lattice fringes. The inset at bottom right is a Fourier transform of the whole image;
  • FIG. 23 is a schematic illustrating how intrinsic stacking faults along ⁇ 111>, i.e. faults in the successive stacking of the ABC layers ( ⁇ 111 ⁇ planes) of an fcc crystal, give rise to a hcp region.
  • the black dots represent atoms in the ⁇ 110 ⁇ plane while the grey dots represent atoms immediately below;
  • FIG. 24 A) is a TEM image of a stack of vertically oriented silver nanoprisms.
  • B) is a high resolution image of the nanoprism on the right hand side of (A) showing defect structure. This nanoprism is oriented such that the ⁇ 110 ⁇ plane is in the plane of the image, i.e. the electron beam is along ⁇ 110>.
  • C) shows the analysis of internal structure of nanoprism in (B).
  • a series of intrinsic stacking faults has resulted in a hexagonally close packed pattern emerging and gives rise to an arrangement of atoms that is aligned perpendicular to the surface with a spacing of 2.50 ⁇ . The correct spacing of 2.35 ⁇ has been obtained for ⁇ 111 ⁇ planes and also for the alternate ABAB . . . layers of the hcp region;
  • FIG. 25 is a graph showing X-ray diffraction data for silver nanoprisms, the peaks corresponding to fcc silver are labeled with * and the relevant miller indices. Two additional peaks (labeled with x) correspond to predicted positions from theoretical diffractograms, reported in the Supporting Information Section of reference 57 for a defect-induced hcp arrangement of silver atoms in silver nanoparticles;
  • FIG. 26 A is a schematic illustrating a ⁇ 110> oriented segment of a fcc crystal. The edges of a crystal cut in this manner have alternating pairs of ⁇ 100 ⁇ and ⁇ 111 ⁇ faces.
  • B) is a schematic of a nanoplate constructed from a single fcc crystal (no twin planes or defects). A singe crystal would not normally take up this structure but the schematic illustrates that a nanoplate cut from a fcc crystal could have edges consisting of alternating pairs of ⁇ 100 ⁇ and ⁇ 111 ⁇ faces.
  • C) is a schematic of a nanoplate with a defect-induced hcp layer sandwiched between two fcc layers of unequal thicknesses. The hcp layer determines the lateral growth. Within the two-dimensional growth plane, certain directions are preferred due to the asymmetric distribution of crystal faces. The block arrows indicated the proposed directions of preferred growth that lead to the familiar triangular shape of nanoprisms;
  • FIG. 27 shows TEM images of flat-lying and stacked silver nanoprisms for samples 1 to 4 . There is a clear trend of increasing edge-length of nanoprisms with the spectral position of the main SPR as shown in FIG. 19A . Scale bars are 20 nm;
  • FIG. 28 is a spectra of sample 10 from FIG. 20B (line A) and of sample 4 from FIG. 19A (line B).
  • the in-plane quadrupole SPRs are clearly visible with a shoulder at ⁇ 465 nm that is tentatively assigned as an in-plane octupole SPR.
  • the out-of-plane dipole and quadrupole SPRs are visible at ⁇ 400 nm and ⁇ 330 nm respectively;
  • FIG. 29 is a line graph showing the UV-Visible spectrum of silver seeds produced by microfluidics with a flow rate ratio of 1:1 of solution 1 (AgNO 3 ) and solution 2 (NaBH 4 , PSSS and TSC) respectively; and
  • FIG. 30 is a graph showing the UV-visible spectrum of size and colour tuned (spectral peak ranging from 656 nm to 500 nm) triangular silver nanoplates produced by varying the volume of generic microfluidic produced seeds from 100 ⁇ l to 600 ⁇ l in steps of 100 ⁇ l or 50 ⁇ l as listed in table 4.
  • the process for producing discrete high definition silver nanoparticles comprises two steps: step (a) synthesizing silver seeds and step (b) growing silver seeds into silver nanoparticles.
  • the invention provides a microfluidic process for producing discrete high definition silver nanoparticles.
  • Microfluidic technologies can be applied to at least the growth step (step (b)) or to both the silver seed production and growth steps (steps (a) and (b)).
  • the nanoparticles produced using the processes described herein are highly shaped, e.g. contain a high percentage of triangles and hexagons compared to spheres, and/or have a narrow size distribution in a desired size range such as about 25 nm or about 30 nm or about 40 nm or larger or smaller and/or have a UV-visible optical absorption spectrum with a main peak at wavelengths longer that about 400 nm.
  • the full width at half maximum (FHWM) of the main peak may be less than about 100 nm.
  • microfluidics chip and microfluidics processor methods for steps (a) and (b) we have devised a process that enables the scaled-up production of discrete high definition silver nanoparticles.
  • the silver nanoparticles produced by the microfluidic process described herein have a high batch to batch reproducibility and improved physical properties including a narrower size distribution, an increased presence of shaped nanoparticles and a higher uniformity between the silver nanoparticles.
  • the microfluidic methods also allow the size, shape, spectral profile and surface chemistries of the discrete high definition silver nanoparticles to be controlled.
  • the microfluidic synthesis processes described herein can be adapted for the scaled up production of a range of high quality nanoparticles both metallic and non metallic.
  • mixing of the reagents may be performed in small volumes in a microfluidic reactor at high or differential flow rates. For example at flow rates between about 1 ml/min to about 10 ml/min for low pressure systems and flow rates of at least 10 ml/min up to litres/min for high pressure systems.
  • the reagents used in step (a) and/or step (b) of the process may have differential flow rates.
  • the flow rate of individual reagents can be variably controlled within a microfluidic reaction system resulting in the reagent solutions being rapidly and thoroughly mixed.
  • the ratio of the reagents, and/or the ratio at which the reagents are mixed can impact the physical properties of the nanoparticles formed. For example, an excess of about eight times the reducing agent solution to the silver salt solution has been found to be optimum for the reaction chemistry for producing silver seeds in step (a) of the process.
  • step (b) of the process may require a microfluidic reactor which is capable of delivering reagent solutions under high pressures for example between about 35 MPa to about 275 MPa (about 5000 psi to about 40000 psi), such as about 140 MPa (about 20000 psi).
  • a microfluidic reactor which is capable of delivering reagent solutions under high pressures for example between about 35 MPa to about 275 MPa (about 5000 psi to about 40000 psi), such as about 140 MPa (about 20000 psi).
  • Mixing reagents under pressure in step (a) and/or step (b) of the process may assist with the rapid and thorough mixing of the reagents.
  • the use of high pressure flow and/or variable differential flow rates of reagents may allow for a uniform reaction to take place.
  • the pressure and flow rate of reagents and the dimensions of the microfluidic reactor may be such that a turbulent flow of reagents is generated at the point at which the reaction takes place. Turbulent flow of reagents may thereby promote thorough mixing of the reagents and maintain consistent control of the reaction chemistry in a continuous microfluidic flow process.
  • microfluidic reactor when designed and operated as described herein will maintain a continuous flow and through mixing of reagents under controlled conditions thereby allowing a true scaling up of the reaction chemistry without compromising the quality of the nanoparticles produced.
  • the thorough and rapid mixing of reagents according to the process described herein allows for certain desired characteristics of the silver nanoparticles to be controlled and reproducible produced.
  • Such controlled reproducibility is not possible in a conventional wet batch chemistry reaction in which reagents are mixed in higher volumes compared to the microfluidic process resulting in variations in nanoparticle characteristics both within a batch, and between batches.
  • Steps (a) and (b) can be combined in some embodiments of the invention to produce a single step microfluidic production method for nanoparticles.
  • the order of addition of the reagents, the type of reagents used, and/or the concentration of the reagents can all be varied and in some embodiments additional reagents can be introduced into the reaction.
  • additional reagents can be introduced into the reaction.
  • microfluidic methods described herein enable the reproducible production of high definition silver nanoparticles with predetermined, size, shape, narrow distribution of size and shape.
  • microfluidics processor method can be used to produce discrete high definition silver nanoparticles in large volume batches.
  • silver nanoparticles can be produced on an industrial scale while retaining control of the reaction chemistry conditions necessary to produce controlled size and shape range silver nanoparticles.
  • a high pressure for example in the range of about 35 MPa to about 275 MPa, such as about 140 MPa
  • high shear rate for example between about 1 ⁇ 10 6 s ⁇ 1 to about 50 ⁇ 10 6 s ⁇ 1 , typically about 10 7 s ⁇ 1
  • microfluidics processor method to produce silver nanoparticles in 500 ml batches.
  • the process is capable of producing several liters per hour at flow rates typically in the range of about 10 ml/minute to about 500 ml/minute, while a wet chemistry method is limited to 100 ml batch production.
  • FIG. 1 is a schematic illustrating the set up for microfluidic synthesis of silver seeds.
  • product 1 is a silver nitrate (AgNO 3 ) and Trisodium Citrate (TSC) solution and product 2 is a sodium borohydride (NaBH 4 ) solution.
  • a silver source in this case silver nitrate
  • TSC Trisodium Citrate
  • NaBH 4 sodium borohydride
  • product 1 is a sodium borohydride (NaBH 4 ) and Trisodium Citrate (TSC) solution and product 2 is a silver nitrate (AgNO 3 ) solution.
  • the microfluidics set-up for the production of the silver seeds is as shown in FIG. 3A , this consists of a microfluidic reactor chip system (micromixer glass or polymer chips) to which the component solutions, such as those described in FIG. 1 are added at a controlled rate using pumps.
  • the microfluidics chip may be of a generic type, i.e. an “off the shelf” chip. Details of a suitable generic chip are given in FIG. 3B and in Table 1 below.
  • a polymer such as poly(sodiumstyrene sulfonate) (PSSS) may be added to step (a).
  • PSSS poly(sodiumstyrene sulfonate)
  • PSSS could be included in one or more of the silver nitrate solution, trisodium citrate solution, and sodium borohydride solution at a concentration of about 10 ⁇ 4 M.
  • microfluidic chip Chip internal volume 250 ⁇ l Pressure rating 30 Bar (450 psi) Pressure drop across chip for 0.2 Bar water flowing at 100 ⁇ l/min Material
  • Number of glass layers 2
  • Channel fabrication Double isotropic etch and thermal bond Channel cross-section Rectangular, curved along shorter dimension In some preferred embodiments approximately 3:1 width to height ratio Hole fabrication Mechanical drill Channel shape Circular Channel depth/ ⁇ m 250 Mixing channel width/ ⁇ m 300 Mixing channel length/mm 532 Mixing channel pitch/ ⁇ m 500 Reaction channel width/ ⁇ m 400 Reaction channel length/mm 250 Reaction channel pitch/ ⁇ m 600
  • Set pump 1 and pump 2 flow rates for example at 1 ml/min and 8 ml/min respectively;
  • FIG. 5 A more generic setup for reagent input sequencing for general nanoparticle production is shown in FIG. 5 .
  • This setup can be applied to the production method for of a wide range of nanoparticles including high definition silver nanoparticles.
  • For general nanoparticle production the setup, conditions and reagents would need to be adjusted for each particular type of nanoparticle to be produced.
  • step (a) The results from experiments using a generic microfluidic chip system for the production of silver seeds (step (a)) are given below.
  • the second step, (the growth of these seeds to produce discrete high definition silver nanoparticles) was carried out using a conventional batch chemistry method.
  • silver seeds were synthesised using a generic microfluidic chip system according to the following method:
  • Pump 1 was run for 30 s and the by-product collected. With pump 1 still running, pump 2 was run for 30 s and the by-product collected. Prior to stopping the pumps, 5 ml of final seed product was then collected while pumps 1 and 2 were still running.
  • silver seeds were grown into nanoparticles (step (b)) using the conventional wet chemistry method below.
  • step (a) above was diluted with 5 ml PVA and added to the PVA-AgNO 3 solution. Approximately 30 s after the silver seed solution was added to the PVA-AgNO 3 solution 250 ⁇ l of 0.1M ascorbic acid was added to the mixture in one rapid shot.
  • FIG. 8 shows the UV-visible spectrum (A) and a TEM image (B) of discrete high definition silver nanoparticles produced using seeds produced by the generic microfluidic chip with a flow rate ratio of 8:1 for solution 1 (AgNO 3 and TSC) and 2 (NaBH 4 ).
  • the average nanoparticle size is about 21.6 ⁇ 7.5 nm, with about 75.4% of the nanoparticles being shaped, for example, triangular, truncated triangular and hexagonal.
  • the full width at half maximum (FWHM) of the main UV-visible spectral peak is about 105 nm.
  • the peak maximum wavelength is in the region of about 520 nm.
  • FIG. 9 shows the UV-visible spectrum (A) and a TEM image (B) of a different batch of discrete high definition silver nanoparticles produced using seeds produced by the generic microfluidic chip with a flow rate ratio of 8:1 for solution 1 (AgNO 3 and TSC) and 2 (NaBH 4 ).
  • the average nanoparticle size is about 24.0 ⁇ 8.9 nm, with about 44.9% of the nanoparticles being shaped, for example, triangular, truncated triangular and hexagonal.
  • the FWHM of the main UV-visible spectral peak is about 120 nm.
  • the peak maximum wavelength is in the region of about 519 nm.
  • the nanoparticles of FIGS. 8 and 9 demonstrate the ready reproducibility of using a microfluidic process for synthesising silver seeds (step (a)).
  • the nanoparticles grown from the solver seeds are discrete high definition silver nanoparticles with similar size, narrow size distribution and a high percentage of shaped nanoparticles, using a microfluidic method to produce the silver seed nanoparticles.
  • FIG. 10 Shows the UV-visible spectra of two different sets of microfluidic seeds produced using a generic microfluidic chip with a flow rate ratio of solution 1 and 2 of 8:1.
  • the UV-visible spectra for each set of microfluidic synthesisied seeds are superimposed demonstrating the reproducibility of the microfluidic method for synthesising silver seeds with controlled physical properties.
  • FIG. 11 shows the dependence of seed FWHM and maximum wavelength with variation of flow rate of solution 1 (AgNO 3 and TSC) from about 3 ml/min to about 10 ml/min while keeping flow rate of solution 2 (NaBH 4 ) constant at 1 ml/min.
  • flow rate of solution 1 AgNO 3 and TSC
  • NaBH 4 flow rate of solution 2
  • step (b) We used a microfluidics processor to carry out step (b), the growth of silver seeds to produce high quality discrete high definition silver nanoparticles.
  • the silver seeds were synthesised using a conventional wet chemistry method.
  • the average nanoparticle size was about 37 ⁇ 18 nm, with about 31% of the nanoparticles being shaped, for example, triangular, truncated triangular and hexagonal.
  • the FWHM of the main UV-visible spectral peak was about 98 nm.
  • step (b) Blocking and clogging difficulties were encountered in some experiments when a microfluidic chip systems was used for carrying out step (b), the growth of discrete high definition silver nanoparticles from silver seeds. It was found that blocking and clogging of the microfluidic system could be overcome if the reagents were under pressure for this step.
  • a limited number of suitable commercial microfluidics processors are available, and have heretofore been used for processes other than chemical reactions.
  • a microfluidics system supplied by a company now known as Microfluidics International Corporation located at 30 Ossipee Road, P.O. Box 9101 Newton, Mass. 02464-9101, U.S.A.
  • This microfluidics processor operates at very high pressures of the order of about 140 MPa (about 20,000 psi) and provides high shear rates in the range of about 1 ⁇ 10 6 s ⁇ 1 to about 50 ⁇ 10 6 s ⁇ 1 , thereby maximizing the energy-per unit fluid volume.
  • the microfluidics processor used allowed the reagent streams to be pressurized so that the reagent streams traveled at high velocities to meet in a reaction chamber where turbulent mixing took place.
  • the microfluidics processor also allowed for continuous flow of the reaction product (silver nanoparticles). Details of typical processor operating parameters are given in Table 2 below.
  • Suitable parameters of microfluidics processor Pressure range 35 MPa to 275 MPa (5,000 psi to 40,000 psi)
  • Flow rate range 10 ml/min to liters/min Typical fluid velocity 1.2-20 m/s up to 500 m/s
  • Typical resident time 0.5-1 ms down to 2 ⁇ s
  • Typical shear rate 1-50 ⁇ 10 6 s ⁇ 1
  • FIG. 2 shows a microfluidic system set up for growing nanoparticles from silver seeds.
  • product 3 is a silver nitrate (AgNO 3 ) polyvinyl alcohol (PVA) solution
  • product 4 is a silver nitrate (AgNO 3 ) polyvinyl alcohol (PVA) and silver seed solution
  • product 5 is a solution of the discrete high definition silver nanoparticles.
  • FIG. 12B A TEM image of the silver seeds produce (product 5 ) is shown in FIG. 12B .
  • microfluidics International Corporation microfluidics processor technology described in Example 4 above was also applied to the production of silver seeds (step (a)) and in a further stage these microfluidic processor produced seeds were grown to produce discrete silver nanoparticles (step (b)) also using a microfluidics processor.
  • a solution comprising 2.94 ⁇ 10 ⁇ 4 M AgNO 3 and 2.5 ⁇ 10 ⁇ 4 M TSC and 10 ⁇ 4 M PSSS in water was made and poured into the reservoir of a microfluidics processor.
  • a 0.01M solution of NaBH 4 was introduced into the microfluidics processor.
  • the NaBH 4 and AgNO 3 -TSC solutions were mixed at flow rates of 15 ml/min and 485 ml/min respectively with a continuously flowing stream of the AgNO 3 -TSC solution and the material was processed for one pass at 140 MPa (20,000 psi).
  • the PVA-AgNO 3 -PSSS-silver seed solution was placed in the reservoir of a microfluidics processor.
  • a 0.01M solution of ascorbic acid solution was introduced to a 475 ml/min continuously flowing stream of PVA-AgNO 3 -PSSS-silver seed solution at a rate of 25 ml/min.
  • the material was processed for 1 pass at 35 MPa (20,000 psi).
  • FIG. 13A The UV-visible spectrum of silver seeds produced using a microfluidics processor and the discrete high definition silver nanoparticles produced by the subsequent growth of these microfluidics processor produced silver seeds also using a microfluidics processor are shown in FIG. 13A . Also shown are discrete silver nanoparticles produced by the conventional wet chemistry growth of the microfluidic processor synthesized silver seeds. It is clear from the spectra shown in FIG. 13A that the microfluidic process of growing the microfluidic synthesized silver seeds (i.e.
  • step (a) and (b)) results in the production of discrete silver nanoparticles with a much higher presence of shaped silver nanoparticles compared to a conventional wet chemistry operation of the growth step as is signified by the much more distinct peak in the region of 345 nm in the case of the microfluidics processor produced discrete silver nanoparticles.
  • a silver seed solution was synthesised using a conventional wet chemistry method and discrete silver nanoparticles were prepared by either using a conventional wet chemistry method for growing the wet chemistry synthesised silver seeds or a microfluidic processor for growing microfluidic synthesized silver seeds (i.e. using a microfluidics process for both steps (a) and (b)). It is clear from the spectra shown in FIG. 13B that using a microfluidics process for both steps (a) and (b)) results in the production of discrete silver nanoparticles with a much higher presence of shaped silver nanoparticles compared to a conventional wet chemistry method.
  • a wet chemistry method was used to synthesize silver seeds (step (a)) and growing the silver seeds to form discrete silver nanoparticles (step (b)), as described in WO04/086044. Briefly, silver seeds were formed from vigorously stirring an aqueous mixture of silver nitrate, trisodium citrate and sodium borohydride. The typical ratio of silver nitrate: trisodium citrate was about 1:1 and the typical ratio of silver nitrate: sodium borohydride was about 1:8.
  • the wet chemistry method has a restricted production volume in the order of about 50 ml, with a maximum of up to about 100 ml of discrete silver nanoparticles being produced in any one batch.
  • Batch to batch reproducibility difficulties are experienced, as indicated by the diverse range of UV-Visible spectra of discrete silver nanoparticles using wet chemistry prepared under precisely the same conditions as shown in FIG. 14 .
  • These batches of discrete silver nanoparticles have spectra, whose maximum peak wave lengths range over 70 nm, have FWHM in excess of 150 nm and have spectra which vary between single peaked to twin peaked where both spherical and shaped associated peaks are of similar intensity to the case where the shaped associated peak is dominant.
  • FIG. 15 (A) to (C) show representative TEM images of discrete silver nanoparticles prepared using a wet chemistry method for both steps (a) and (b).
  • FIGS. 15 (A) and (B) illustrate the wide size distribution within and between batches and
  • FIG. 15 (C) illustrates the low presence of shaped nanoparticles.
  • FIG. 16 shows UV-visible spectra for three different batches of silver seeds produced under the same conditions using conventional wet chemistry.
  • the spectra profiles are very similar with a peak maximum wavelength in the region of 399 nm and a FWHM of the order of 65 nm.
  • These silver seeds are typical of those used in the wet chemistry step (b) resulting in discrete silver nanoparticles which have the range of variation and poor reproducibility shown in FIGS. 14 and 15 .
  • microfluidics methods for the production of silver seeds is important in achieving discrete silver nanoparticles with the required characteristics, controlled size, narrow size distribution, high presence of shaped nanoparticles and good batch to batch reproducibility.
  • microfluidics methods such as microfluidics processors, can be readily applied to produce litres per hour of the discrete silver nanoparticles with out sacrificing quality.
  • This hcp structure has a periodicity of 2.50 ⁇ , thus explaining the 2.50 ⁇ lattice fringes that are commonly observed in ⁇ 111> oriented flat-lying nanoprisms.
  • Nanoparticles of noble metals such as silver are of considerable interest in nanotechnology. This stems largely from the collective oscillation of the conduction electrons in resonance with certain frequencies of incident light, leading to an extinction known as a surface plasmon resonance (SPR). [1,2,3,4,5] The spectral position of the resonance is highly dependent on nanoparticle size and shape and also depends on the refractive index of the metal and the surrounding medium.
  • SPR surface plasmon resonance
  • One of the key, and most interesting, properties of highly-shaped metal nanoparticles is the fact that at the SPR of a metal nanoparticle, the electric field intensity near the surface of the nanoparticle is enhanced strongly relative to the applied field.
  • SEF Surface Enhanced Fluorescence
  • SERS Surface Enhanced Raman Spectroscopy
  • the degree of enhancement is dependent on a number of factors. One of these is shape. It has been shown by discrete dipole approximation (DDA) calculationst [6] that nanorods and nanoprisms show a much higher degree of enhancement of the local field than spheres.
  • DDA discrete dipole approximation
  • EELS electron energy-loss spectroscopy
  • the field enhancement factor if is directly proportional to the dephasing time T 2 , of the SPR (
  • the nanorods have a much lower volume than the corresponding nanospheres with the same plasmon resonance energy, and the radiative dephasing rate (radiation damping) is proportional to nanoparticle volume, [18,19,20,22] i.e. ⁇ rad ⁇ V. Since different nanoparticle shapes result in different nanoparticle volumes for a given plasmon resonance energy, it is clear that the degree of plasmon damping is highly influenced by nanoparticle shape and this is another route for nanoparticle shape to influence the degree of enhancement of the local field. The non-radiative contribution to plasmon damping increases with increasing plasmon resonance energy due to the frequency-dependent dielectric properties of silver. [19,20,23]
  • anisotropic growth that results from the preferential binding of organic species to certain crystal faces relies on the underlying twinning or defect structure of the seed particles since this is what determines the type and orientation of the crystal faces that are exposed to the growth medium. This is all the more apparent when we consider that in most syntheses a range of particle shapes are observed and yet the same shaped particle can be the major product of very different syntheses.
  • anisotropic structures such as nanoprisms present a particular challenge to the face-selective binding model in that gold and silver nanoprisms typically have large flat ⁇ 111 ⁇ faces, with two-dimensional growth from the edges.
  • the internal defect structure has been implicated as a direct factor influencing crystal growth. Specifically, defects such as twinning that arise during the early stages of particle formation give rise to preferred growth directions where the defects are exposed to the growth medium. In the case of nanoprisms, parallel stacking faults in the ⁇ 111> direction have been observed with these making contact with the growth medium at the edges, precisely where growth occurs.
  • the silver halide growth model has also been resurrected as a way of explaining particle growth in many synthesis methods. [54,55,56,57] In this model, twin planes form reentrant grooves, which are favorable sites for the attachment of adatoms.
  • a single twin plane is expected to direct growth in two dimensions but limit the final size of the nanoprism, while the presence of two parallel twin planes would allow the fast growing edges to regenerate one another, allowing shapes such as hexagonal nanoplates to form.
  • Rocha and Zanchet have studied the defects in silver nanoprisms in some detail and have shown that the internal structure can be very complex with many twins and stacking faults. [58] These defects are parallel to each other and the flat ⁇ 111 ⁇ face of the nanoprism, subdividing it into lamellae which are stacked in a ⁇ 111> direction, and are also present in the silver seeds. In that paper, it was demonstrated how the planar defects in the ⁇ 111> direction could give rise to local hexagonally close-packed (hcp) regions. These would in turn explain the 2.50 ⁇ lattice fringes that are observed in ⁇ 111> orientated nanoprisms, which have hitherto been attributed to formally forbidden 1 ⁇ 3 ⁇ 1422 ⁇ reflections. [52,53,54,59]
  • silver seeds are produced by combining aqueous trisodium citrate (5 ml, 2.5 mM), aqueous poly(sodium styrenesulphonate) (PSSS; 0.25 ml, 500 mg L ⁇ 1 ; Aldrich 1,000 kDa) and aqueous NaBH 4 (0.3 ml, 10 mM, freshly prepared) followed by addition of aqueous AgNO 3 (5 ml, 0.5 mM) at a rate of 2 ml min ⁇ 1 while stirring continuously.
  • PSSS poly(sodium styrenesulphonate)
  • NaBH 4 0.3 ml, 10 mM, freshly prepared
  • the nanoprisms are produced by combining 5 ml distilled water, aqueous ascorbic acid (75 ⁇ l, 10 mM) and various quantities of seed solution, followed by addition of aqueous AgNO 3 (3 ml, 0.5 mM) at a rate of 1 ml min ⁇ 1 . After synthesis, aqueous trisodium citrate (0.5 ml, 25 mM) is added to stabilize the particles and the sample is diluted with distilled water as desired. Distilled water is used throughout for all solutions. The synthesis is complete after the 3 minutes required for addition of the AgNO 3 during which time the colour of the solution changes as the SPR red-shifts in response to nanoprism growth.
  • Samples were prepared for XRD measurements by concentrating a nanoprism sample by centrifugation.
  • a viscous nanoprism mixture was prepared by adding the few drops of concentrated nanoprism solution to a few drops of aqueous 5% w/v poly(vinyl alcohol) (PVA). This was added to the glass slide for XRD analysis (Philips X′Pert Pro) and allowed to dry.
  • PVA poly(vinyl alcohol)
  • the method involves the silver seed-catalyzed reduction of Ag + by ascorbic acid, and surprisingly results in a minimal concentration of spherical nanoparticles being produced.
  • the spectral position of the SPR can be tuned by controlling the size of the nanoprisms, without any significant variation in thickness. This can be achieved through adjustment of the number of seeds in the growth mixture.
  • FIG. 17 A typical example of the nanoprisms produced with this method is shown in FIG. 17 .
  • PSSS poly(sodium styrenesulphonate)
  • FIG. 18 A key ingredient for production of high quality samples is poly(sodium styrenesulphonate) (PSSS), which is used as a stabilizer in the seed production step. If PSSS is left out or only added to the seed solution after seed production, then there is a diversity of nanoparticle shapes and sizes, this is shown clearly in FIG. 18 . This result is important as it shows that the PSSS is not simply playing a shape-directing role through preferential adsorption to certain crystal faces during the growth stage, but rather it must have a strong influence on the defect structure of the seeds and indeed a preference for seeds whose structure predisposes them for growth into nanoprisms. We believe that as PSSS is a charged polymer, it interacts relatively strongly with the silver surface thereby influencing the defect structure of the seeds.
  • the amount of citrate present in the synthesis of many of the samples is very low.
  • the amount of citrate present in the synthesis of many of the samples is very low.
  • a low citrate/Ag + ratio ( ⁇ 1) resulted in triangular and hexagonal structures with a broad range of sizes (30 to 300 nm) while a high citrate/Ag + ratio (>1) was required for nanoprisms to be the major product.
  • Citrate may therefore play an important role in anisotropic growth by influencing the defect structure of the seeds.
  • citrate is used in the absence of PSSS nanoparticles having a variety of different shapes are produced.
  • PSSS is that nanoparticles having a predominantly triangular shape are produced.
  • TEM analysis of statistically significant numbers of nanoprisms from four samples was carried out.
  • the positions of the main SPRs of these samples were well separated as can be seen in FIG. 19A .
  • TEM grids of samples were prepared such that many of the particles were arranged in a stacked formation with their flat faces parallel to the electron beam. To achieve this it was necessary to concentrate the nanoprisms by centrifugation so that it was possible to measure both their edge-length and thickness.
  • the edge-length measurement has a certain degree of uncertainty as it is possible that some nanoprisms are free to rotate about the ⁇ 111> axis perpendicular to the flat faces of the nanoprisms, although most are probably resting on an edge in the plane of the TEM grid.
  • the nanoprism measurement data are shown in FIG. 19B and in Table 3, it is clear from this data that there is a distribution of nanoparticle thicknesses within any sample but that the average thickness of nanoprisms from each sample is approximately the same for each sample.
  • the edge-length displays a clear trend; nanoprisms from each sample have higher average edge-lengths as the spectral position of the main SPR increases.
  • FIG. 27 Examples of TEM images of nanoprisms from samples 1 to 4 are shown in FIG. 27 which demonstrates that the triangular shape of the nanoprisms is established early on in the growth process and that growth proceeds through enlargement of these nanoprisms.
  • the position of the band should depend linearly on edge length and on the inverse of the thickness.
  • ⁇ max the edge-length
  • L the edge-length
  • T the nanoprism thickness
  • the lateral dimensions of the triangular nanoparticles can be controlled by adjusting the extent of growth. This is controlled by adjusting the number of seeds in the reaction, which in turn is determined by the volume of seed solution used in this growth stage. There is a linear relationship between the position of the in-plane dipole plasmon band and the dimensions of the nanoparticles.
  • the ultimate size of the nanoprisms can be tuned by controlling the ratio of silver ion: silver seed in the growth step.
  • the ratios may be used:
  • FIG. 27 Examples of four samples ( 1 to 4 ) with TEM analysis are shown in FIG. 27 .
  • the series of successively larger nanoprisms were synthesised according to the process described in this example with volumes of seed solution of 650 ⁇ l, 300 ⁇ l, 150 ⁇ l and 130 ⁇ l.
  • FIG. 21A the line widths (FWHM) of each of the SPRs from FIG. 20C are plotted against plasmon resonance energy. It can be seen that the width of the main SPR (in-plane dipole) increases as the energy of the resonance increases. This is consistent with measurements of the scattering spectra of individual silver nanoprisms by Munechika et al [20] who showed that the line width of the SPR of individual nanoprisms increased with plasmon resonance energy and that this was also correlated with nanoprism volume. Overall, this trend of increasing line width could be explained as due to both increased radiation damping and increased non-radiative decay.
  • FIG. 21B the SPR line widths are plotted against nanoprism volume and it can be seen that in our samples the line widths of the SPRs decrease as nanoprism volume increases (the SPR energy scales inversely with nanoprism edge-length).
  • the TEM studies of FIGS. 17 to 21 show that as the particles become larger, there is no decrease in polydispersity of the samples, i.e. no focusing of the growth conditions to produce a sample with a narrower size distribution. In fact, there is a steady increase in the edge-length distribution as edge-length increases, see Table 3, yet the line widths decrease. This means that the narrowing of the line widths with increasing nanoprism volume must be due to a narrowing of the line widths of the SPRs of the individual silver nanoprisms in the samples with increasing size.
  • the in-plane dipole SPR is sufficiently red-shifted and the samples are sufficiently monodisperse, then the in-plane quadrupole SPR should be visible. This is clearly the case for many of the spectra in FIG. 20A . Calculations have shown that as silver nanoprisms get even larger, higher order multipole resonances should become visible [3,60] Higher order multipole resonances in nanorods are well documented [61,62,63] and have also been observed in silver nanospheres. [64] A closer look at some of the spectra for the samples of our largest nanoprisms show a shoulder on the in-plane quadrupole resonance at 465 nm (see FIG. 28 ). While this could possibly be due to extinction by another species we provisionally assign this as an in-plane octupole resonance.
  • FIG. 17A A typical sample of flat-lying nanoprisms is shown in FIG. 17A .
  • the nanoprism is oriented such that a ⁇ 110 ⁇ plane is in the plane of the image.
  • two ⁇ 111 ⁇ planes and a ⁇ 100 ⁇ plane are aligned vertically with respect to the electron beam.
  • the defects can then be detected as discontinuities in either the ⁇ 100 ⁇ or ⁇ 111 ⁇ planes that propagate away from the flat face of the nanoprism. This is illustrated schematically in FIG. 23 .
  • the nanoprisms firstly need to be vertically orientated as in the stacked formation as shown in FIG. 17B and secondly need to have one edge parallel to the electron beam (see the left hand side of FIG. 23 ).
  • some nanoprisms do have the right orientation and a layered defect structure is visible in two of the stacked silver nanoprisms in FIG. 24A . Closer inspection of the nanoprism on the right reveals that it is indeed being observed along ⁇ 110> as the internal defect structure of the crystal is visible ( FIG. 24B ). An analysis of the defects is shown in FIG.
  • the reconstruction of the silver lattice is illustrated schematically in FIG. 23 .
  • a series of intrinsic stacking faults isf
  • ABABAB . . . stacking arrangement of the atomic planes in a region of the nanoprism.
  • the perpendicular arrangement of atoms with respect to the flat ⁇ 111 ⁇ face of the nanoprism is indicated and has a 2.50 ⁇ spacing.
  • XRD x-ray diffraction
  • the familiar triangular shape and constant thickness of nanoprisms results from highly selective lateral growth from the edges. Due to the lamellar defect structure of the nanoprisms, it is precisely at these edges where the defects are exposed to the growth solution. Thus the significant rearrangement of the crystal structure described here very likely plays a crucial role in giving rise to two-dimensional growth.
  • the hcp crystal faces (or defect-rich regions) at the edges must support a much faster rate for the addition of silver atoms during growth, compared to the ⁇ 111 ⁇ or ⁇ 100 ⁇ faces.
  • the hcp structure is not the natural crystal structure for silver, it must therefore be less stable than the fcc structure, making it likely that the edges where the hcp structure is exposed to the growth medium are less stable than the ⁇ 111 ⁇ or ⁇ 100 ⁇ faces. This higher degree of instability may be the basis of the faster two-dimensional growth at the edges.
  • the hcp and fcc crystal structures both have a hexagonal symmetry so it remains to be explained why triangles, and not hexagonal nanoplates, are the preferred outcome of two-dimensional growth.
  • fcc single crystal as shown in the schematic in FIG. 26A . It is not proposed that a single fcc crystal would take up such an anisotropic structure, but it is clear that it can be cut such that opposite sides could have alternating ⁇ 111 ⁇ / ⁇ 100 ⁇ pairs of faces.
  • the fcc crystal has six-fold symmetry around the ⁇ 111> axis so a hexagonal platelet could have the alternating faces as outlined in FIG. 26B , although the relative sizes of each face at an edge would not necessarily be as fixed as the diagram suggests.
  • FIG. 26C a hexagonal nanoplate that could be the result of initial two-dimensional growth from the seed, see FIG. 26C .
  • This possesses the hcp (or defect-rich) region sandwiched between two fcc regions, corresponding to what our TEM data suggest.
  • the schematic is drawn such that the regions on either side of the central hcp region are asymmetric.
  • the thickness of each fcc layer would then define the size of each of the respective crystal faces on each edge. This would mean that not all of the edges of the nanoplate are identical; three of them have a larger ⁇ 100 ⁇ face than the ⁇ 111 ⁇ face while the other three have a larger ⁇ 111 ⁇ face than the ⁇ 100 ⁇ face.
  • the nanoprism maintains its triangular shape, with both types of edges growing in a concerted fashion. In this manner smaller triangular nanoprisms grow continuously into larger triangular nanoprisms without any significant increase in thickness. In cases where there is no asymmetry in thickness between the fcc layers on either side of the hcp layer, hexagonal nanoplates are expected.
  • silver nanoprisms possess many defects in the ⁇ 111>direction perpendicular to the flat face of the nanoprisms and that these can combine to give rise to a hcp layer sandwiched between two fcc layers.
  • This hcp layer has a periodicity of 2.50 ⁇ that provides an explanation for the commonly observed 2.50 ⁇ lattice fringes in flat-lying nanoprisms.
  • this two-dimensional hcp layer is most likely the main explanation for the two-dimensional lateral growth, with the triangular shape of the nanoprisms being driven by the asymmetric distribution of crystal faces at the edges, which is in turn determined by the asymmetric thicknesses of the fcc layers on either side of the hcp layer.
  • the silver halide model is perhaps a good starting point for understanding anisotropic growth in as much as it identifies defects as crucial, however it apparently does not adequately explain the growth patterns of metal nanoprisms.
  • step (a) we used a generic microfluidic chip system for the production of silver seeds (step (a)) and step b) was carried out by systematically changing the volume of seeds added to the growth step.
  • the nanoparticles produced were size and colour tuned triangular nanoplates (nanoprisms).
  • solution 1 comprised 100 ml of 5 ⁇ M silver nitrate.
  • Solution 2 comprised a mixture of 3 mL of 10 mM sodium borohydride, 2.5 mL of 500 mgL ⁇ 1 poly(sodiumstyrene sulfonate) and 100 mL of 2.5 ⁇ 10 ⁇ 3 M trisodium citrate in water.
  • Solution 1 and solution 2 were connected to pump 1 and pump 2 respectively (see setup of FIGS. 3A and 4 ).
  • the flow rates of pump 1 and pump 2 were set for example at 1 ml/min and 1 ml/min respectively.
  • the pump lines were primed with the solution to be used in them and pump 1 and pump 2 were run in succession for ⁇ 2 min each such that an initial volume of ⁇ 2 mL of each solution was run through the chip and discarded.
  • Pump 1 and pump 2 were run together and the first 1 ml of the product solution was discarded. The subsequent 5 ml of seed product was collected and both the pumps were stopped.
  • FIG. 29 shows the UV-visible spectrum of the seeds produced.
  • the flow rates, the flow rate ratios, the reagents and their relative concentrations, volumes and mixing configuration, mixing order and conditions and reagents may be adjusted for each specific type of nanoparticle seeds to be produced.
  • Microfluidics was used for step (b) (growing the silver seeds synthesized in step (a) into nanoparticles) to produce colour tuned triangular nanoplates from the silver seed synthesised by microfluidics method with a flow rate ratio of 1:1 of solution 1 (AgNO 3 ) and solution 2 (NaBH 4 , PSSS and TSC) described above.
  • FIG. 30 shows the UV-visible spectrum of a range of triangular silver nanoplates which are size and colour tuned, produced as described above using the silver seed volumes given in Table 4.
  • Step (b) may be carried out using the high pressure microfluidics process which would enable the production of large volumes of size and shape controlled triangular silver nanoplates.
  • the flow rates, the flow rate ratios the reagents and their relative concentrations, volumes and mixing configuration, order and conditions may be adjusted for each specific type of nanoparticle seeds to be produced.
  • a polymer maybe added at any of solutions or at any of the preparation stages to further modify the surface chemistry, the stability or the durability of the silver nanoparticles for applications such as functionalisation or industrial processing.
  • a microfluidics processor technology as described above in examples 4 and 5 was used to create seven batches of discrete high definition silver nanoparticle solutions of varied formulations, as described in Table 5 below.

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