Synthesis of core-shell nanoparticles and applications of said nanoparticles for surface enhanced Raman scattering

Abstract

A method of synthesizing of gold-silver core-shell nanoparticles, from a colloidal aqueous solution of gold seeds with surfactant, the gold-silver core-shell nanoparticles being produced from anisotropic gold seeds, said method comprising adding to the colloidal aqueous solution a precursor of silver and a reducing agent, to produce the deposition of silver on the gold seeds in a step called principal, characterized in that the method has an incubation step of the colloidal aqueous solution containing the gold seeds with surfactant in the DMSO, prior to the principal step.

Claims

1. A method of synthesizing gold-silver core-shell nanoparticles from a colloidal aqueous solution of gold seeds with surfactant, the gold-silver core-shell nanoparticles being produced from anisotropic gold seeds, characterized in that the method comprises successively: an incubation step of the colloidal aqueous solution containing the gold seeds with an initial surfactant, in a mixture of water solvents and DMSO, for a first given period of time, in order to modify the organization of the initial surfactant and the assembly of the gold seeds; a step adding an additional surfactant to the previous resultant mixture; a heating step for the resultant mixture, for a second given period of time; a step adding to the resultant mixture a precursor of silver and a reducing agent, to produce the deposition of silver onto the gold seeds in a step called principal during a third period of time; an extraction step of the nanoparticles.

2. The method as claimed in claim 1, characterized in that the ratio between the volume of DMSO and the total volume of water is less than 2 and greater than 0.1, the total volume of water being the volume contributed by the colloidal aqueous solution of gold seeds with the initial surfactant and by the water present in the mixture of solvents water+DMSO for incubation.

3. The method as claimed in claim 1, characterized in that the ratio between the volume of DMSO and the total volume of water is less than or equal to 0.33, the total volume of water being the volume contributed by the colloidal aqueous solution of gold seeds with the initial surfactant and by the water present in the mixture of solvents water+DMSO for incubation.

4. The method as claimed in either claim 1, characterized in that the ratio between the volume of DMSO and the total volume of water is 1, the total volume of water being the volume contributed by the colloidal aqueous solution of gold seeds with the initial surfactant and by the water present in the mixture of solvents water+DMSO for incubation.

5. The method as claimed in claim 1, characterized in that the ratio between the volume of DMSO and the total volume of water is 1.5, the total volume of water being the volume contributed by the colloidal aqueous solution of gold seeds with the initial surfactant and by the water present in the mixture of solvents water+DMSO for incubation.

6. The method as claimed in claim 1, characterized in that the ratio between the volume of DMSO and the total volume of water is less than 10 and greater than 2, the total volume of water being the volume contributed by the colloidal aqueous solution of gold seeds with the initial surfactant and by the water present in the mixture of solvents water+DMSO for incubation.

7. The method as claimed in claim 6, characterized in that the ratio between the volume of DMSO and the total volume of water is Greater than or equal to 4, the total volume of water being the volume contributed by the colloidal aqueous solution of gold seeds with the initial surfactant and by the water present in the mixture of solvents water+DMSO for incubation.

8. The method of synthesizing core-shell nanoparticles according to claim 1, characterized in that the step adding the additional surfactant takes place after a first optimal incubation time ranging from a few minutes to one hour and determined by spectroscopy, making it possible to control the assembly of the gold seeds and to obtain either one gold seed per silver shell, or two gold seeds per silver shell.

9. The method as claimed in claim 1, characterized in that the extraction step is achieved by centrifuging.

10. The method as claimed in claim 1, characterized in that the additional surfactant is chosen from the following list: cetyltrimethylammonium chloride (CTAC), cetyltrimethylammonium bromide (CTAB) or benzyldimethylhexadecylammonium chloride (BDAC).

11. The method as claimed in claim 1, characterized in that, after the extraction step, the method has a new step adding a precursor of silver and a reducing agent to the nanoparticles, in order to produce an overgrowth of silver.

12. The method as claimed in claim 1, characterized in that the reducing agent is a solution of ascorbic acid (AA) and surfactant, and in that the precursor of silver is silver nitrate.

13. Gold-silver core-shell nanoparticles produced from gold seeds of elongated shape, obtained by the method defined according to claim 1, and having traces of DMSO visible by spectroscopy.

14. The gold-silver core-shell nanoparticles as claimed in claim 13, characterized in that each gold-silver core-shell nanoparticle has at least two gold seeds encapsulated in the same silver shell enclosing the core.

15. The gold-silver core-shell nanoparticles as claimed in claim 14, characterized in that for each nanoparticle, the gold seeds are disposed head-to-head in the same silver shell.

16. The gold-silver core-shell nanoparticles as claimed in claim 14, characterized in that for each nanoparticle, the gold seeds are disposed face-to-face in the same silver shell.

17. The gold-silver core-shell nanoparticles as claimed in claim 14, characterized in that the gold seeds are nanorods.

18. The gold-silver core-shell nanoparticles as claimed in claim 17, characterized in that the nanorods have an average aspect ratio between 2 and 5.

19. A solid substrate for Surface Enhanced Raman Scattering (SERS) comprising gold-silver core-shell nanoparticles resulting from claim 13, the nanoparticles being organized in one or more 2D or 3D networks on surface areas of more than 10 m.sup.2, advantageously more than 40 m.sup.2, for each 2D or 3D network.

20. A solid substrate for Surface Enhanced Raman Scattering (SERS) comprising gold-silver core-shell nanoparticles resulting from claim 13, the nanoparticles being organized in one or more 1D chains, the 1D change having characteristic dimensions ranging from 2 to 3 m.

21. The application of the substrate as claimed in claim 19 to the detection by SERS of analytes such as organic pollutants.

22. The application as claimed in claim 21, the analyte being atrazine, the cold-silver core-shell nanoparticles being used with beta-cyclodextrin (CAS 7585-39-9) or alpha-cyclodextrin (CAS 10016-20-3).

23. The application as claimed in claim 21, the analyte being chosen from the following list: thiram (CAS 137-26-8), phosmet (CAS 732-11-6), malathion (CAS 121-75-5), (4,4)-BPE (CAS 13362-78-2), 4-mercaptobenzoque acid (CAS 1074-36-8).

24. The method as claimed in claim 1, where the initial surfactant is the same as the additional surfactant.

25. The method as claimed in claim 1, where the initial surfactant is different from the additional surfactant.

Description

LIST OF FIGURES

(1) Other objects and advantages of the invention will be seen from the following description of an embodiment, provided with reference to the appended drawings in which:

(2) FIG. 1 is a view by scanning electronic microscopy of gold nanoparticles in the form of nanorods of different aspect ratios (ratio of the largest dimension to the smallest dimension);

(3) FIGS. 2-A and 2-B present a view by scanning electronic microscopy of gold/silver core-shell nanoparticles, according to one implementation of the method according to the invention;

(4) FIG. 3 is a view by scanning electronic microscopy of gold/silver core-shell nanoparticles resulting from the overgrowth of silver on gold/silver core-shell nanoparticles, according to another implementation of the method according to the invention;

(5) FIG. 4 is an absorption spectrum of the gold nanoparticles solutions for different additions of DMSO;

(6) FIG. 5-A is a graph representing the development of the absorption spectrum of the colloidal solutions over time after addition of silver nitrate and ascorbic acid; FIG. 5-B is a graph representing the development of the wavelength of the maximum of the longitudinal resonance as a function of time, following the addition of silver nitrate and ascorbic acid in solutions containing gold nanoparticles, using different co-solvents (MeCN, DMSO, EtOH), or without the use of co-solvent;

(7) FIGS. 6 and 7 are views by scanning electronic microscopy of arrangements of nanoparticles obtained according to a synthesis method according to the invention;

(8) FIG. 8 is a schematic representation of the synthesis method according to the proportions of DMSO in the reactional medium and the resulting core-shell particles;

(9) FIG. 9 is a schematic representation of the effect of the DMSO on the properties of gold nanoparticles, with DMSO=(Volume of DMSO)/(Total volume) with Total volume=Volume of water+Volume of DMSO.

DETAILED DESCRIPTION OF EMBODIMENTS

(10) In a first step, a synthesis of gold nanorods is carried out according to a conventional method.

(11) As has been mentioned, the synthesis of gold nanorods, in colloidal solution, is known per se, and will therefore not be described again here.

(12) The gold nanorods are of the general shape represented in FIG. 1. By way of illustration, three families of nanorods used in the syntheses are presented from left to right in FIG. 1 for aspect ratios of 4.0 (5514 nm), 2.0 (7637 nm) and 2.4 (9640 nm). The sizes between parentheses expressed in nanometers represent, in order, the average length along the large axis of the rod and along the small axis.

(13) The gold nanorods are incubated in a dipolar aprotic solvent.

(14) In one implementation, said solvent is dimethyl sulfoxide DMSO.

(15) By way of example, the gold nanorods (120 l to 1.82 nM) are incubated in 100 l of dimethyl sulfoxide DMSO for 30 minutes.

(16) Then, the gold nanorods are transferred to a solution of CTAC (cetyltrimethylammonium chloride) and heated.

(17) For example, the nanorods are transferred to a solution of CTAC (26 mg in 3.8 mL of water) and heated at 60 C. for 20 minutes.

(18) The deposition of silver is initiated, by addition to the previous preparation, of a solution of silver nitrate AgNO.sub.3, and a solution containing ascorbic acid AA and CTAC.

(19) For example, the deposition of silver is initiated by addition of a solution of silver nitrate AgNO.sub.3 (2 mM, 1.2 mL), and a solution containing ascorbic acid (AA, 45 mM, 1.2 mL) and CTAC (40 mM).

(20) A complete preparation protocol P is as follows, in one embodiment.

(21) In a tube containing 300 l of gold nanorods (C=0.15 nM), 100 L of DMSO is introduced, and the solution is left to rest for 5 minutes, then added to an aqueous solution of CTAC (26 mg of CTAC in 4.6 mL of Milli-Q water previously heated to 60 C. The resulting solution is agitated in a bain marie at 60 C. for 20 minutes. The deposition step is initiated by the addition of 1.2 mL of a solution of AgNO.sub.3 at 2 mM and 1.2 mL of an AA solution (44 mM)/CTAC (40 mM) protected from light. The addition is done drop by drop under moderate agitation. The deposition of silver is stopped after 100 minutes, centrifuging at 6000 RPM for eight minutes.

(22) Another preparation protocol makes it possible to work at a larger scale, as a first step towards the industrialization of the synthesis method. This method, in an implementation enabling up to 1000 times more gold-silver core-shell particles to be produced, is as follows:

(23) In 16 tubes each containing 150 L gold nanorods (C=17 nM), 1.15 mL of DMSO is introduced, and the solution is allowed to rest for 10 minutes. The contents of the 16 tubes is added to an aqueous solution of CATC (320 mg of CATC in 22 mL water) previously heated to 60 C. The resulting solution is held at 60 C. under agitation for 20 minutes. The deposition step is initiated by the addition of 10 mL of a solution of AgNO.sub.3 at 4 mM and 10 mL of an AA solution (100 mM)/CTAC (80 mM) protected from light. The addition is done drop by drop under moderate agitation. The deposition of silver is stopped after a period of between 90 minutes and 20 hours (depending on the desired shape of particle), centrifuging at 6000 RPM for five minutes.

(24) The shape of the final object and the thickness of deposited silver are modulated by the proportion of DMSO in the mixture, the quantity of silver precursor and the reaction time prior to centrifuging the mixture.

(25) The shape of the gold/silver core-shell nanoparticles is that of an ogive, as shown in FIG. 2.

(26) The deposition of silver is done essentially along the small axis and the points of the gold rods outcrop at the surface of the core-shells. These anisotropic depositions in both directions result in a lowering of the aspect ratio, which is 1.4 for the core-shell nanoparticles, compared to a ratio of 3.9 for the initial gold nanorods.

(27) The deposition of silver onto the gold is followed by UV-vis light extinction spectroscopy with a progressive shift of the longitudinal plasmon resonance towards the blue.

(28) The optical properties of the core-shell nanoparticles obtained, the position of the extinction band corresponding to the longitudinal plasmon resonance of the nanoparticles can be modulated between 510 and 800 nm, depending on the thickness of the deposition of silver and the aspect ratio of the initial gold seed.

(29) Taking advantage of the localized surface plasmon resonance of the synthesized gold/silver nanoparticles and the better enhancement expected for the silver (compared to the gold alone), surface enhanced Raman scattering characterizations have been carried out in solution in presence of molecular probes.

(30) The spectra show an enhancement of the Raman signal between 4 and 100 times, compared to what was recorded for gold/silver core-shell nanoparticles obtained in water with no co-solvent.

(31) By this synthesis, the gold/silver core-shell nanoparticles obtained by the method according to the invention contain traces of DMSO that are visible by spectroscopy.

(32) Once isolated and deposited on a solid support (silicon slide for example), the core-shells thus obtained self-organize spontaneously into 3D network, 2D structure or 1D chain, of lengths (or surface areas) greater than 1 micrometer, depending on the concentration of solution deposited and the shape of the core-shell nanoparticles. FIGS. 6 and 7 show some examples of arrangement obtained.

(33) As illustrated in FIGS. 6 and 7, the networks of gold/silver core-shell nanoparticles (NP) organized in 1D, 2D or 3D have a long-range order going from a few microns to several tens of microns. For the 2D and 3D arrangements, they can have surface areas of at least 10 square micrometers, advantageously 40 square micrometers, and even greater, and which are repeated on the substrate. These large surface areas obtained can be explained in particular by the fact of the homogeneity of shape of the nanoparticles obtained by the method.

(34) As illustrated on the 3D structure in FIG. 6, the NPs have colors that are white, somewhat grayed or more grayed depending on their distance to the microscope.

(35) These networks are obtained in particular with gold/silver core-shell nanoparticles having a single seed of gold encased in a shell of silver.

(36) Depending in particular on the concentrations used of surfactants, gold-silver core-shell nanoparticles, thickness of the silver layer, size of the gold seeds, certain arrangements are obtained. For example, gold seeds encased by a small deposition of silver (less than 5 nm) give 1D chains, while gold/silver core-shell particles with a thicker layer of silver (10 nm and more) are organized rather in 2D or 3D networks. Moreover, these ordered ranges will be remote on the silicon substrate if the deposition is done from diluted solutions of gold-silver core-shell nanoparticles in water.

(37) Influence of the Quantity of Co-Solvent

(38) Tests were conducted, by maintaining the type and quantity of initial gold seed, the quantity of the reducing agent and of the silver precursor, the reaction time prior to centrifuging; varying the DMSO/H.sub.2O ratio

(39) Depending on the proportion of DMSO added to the medium, three limiting behaviors have been identified in solution, by absorption spectroscopy, as illustrated in FIG. 4.

(40) For DMSO/H.sub.2O ratios lower than 0.33 (here, ratio by volume=Volume DMSO/Total volume of water), the stability of the gold nanorods is maintained in solution and they remain at a distance from one another.

(41) For a DMSO/H.sub.2O ratio of 1 in the initial solution, a point-to-point assembly in solution of the gold nanorods is evidenced (FIG. 9).

(42) This assembly is detected by the appearance in a short time of a band, shifted towards the red relative to the longitudinal Plasmon resonance band of the initial nanorods. Point-to-point assembly (or head-to-head HH), here denotes the observation of the convergence of the end parts of the nanorods, contiguous nanorods being aligned between them or forming an angle between them.

(43) Finally, for a larger DMSO-H.sub.2O ratio (1.5) in the starting solution, kinetic monitoring by absorption spectroscopy over time shows an increasing shift towards the blue of the longitudinal resonance and an increasing shift towards the red of the transverse resonance. These spectral changes are characteristics of a face-to-face (FF) assembly of the nanoparticles (FIG. 9). Face-to-face assembly here designates the observation of the convergence of the nanorods by their lateral faces, the thin directions of contiguous nanorods being substantially parallel.

(44) FIG. 9 illustrates the different states of the gold nanorods, as a function of the percentage of DMSO (proportion of DMSO in the water+DMSO mixture).

(45) At a value of 50% DMSO, the gold nanorods are assembled in solution point-to-point.

(46) At a value of 60% DMSO, the gold nanorods appear assembled in solution paired face-to-face.

(47) At a value of 25% (or less than 25%) of DMSO, the gold nanorods appear dispersed in solution. For proportions of DMSO between these characteristic values, the gold nanorods are in the form of two types of populations, for example assembled face-to-face and point-to-point, for proportions of DMSO between 50 and 60%.

(48) For percentages of DMSO in the mixture between 80 and 95%, the gold nanorods again appear dispersed in solution and the nanoparticles produced subsequently have a single seed of gold per shell of silver.

(49) Thus, using a water-DMSO mixture as described in the invention makes it possible: to selectively orient the growth of the silver on the gold surface, and/or to assemble the gold seeds in face-to-face (FF) or head to head (HH) solution, or leave them isolated from each other before making the layer of silver grow around them.

(50) After incubation in the water-DMSO mixture, the particles preserve their layer of surfactants without which the particles would precipitate irreversibly, making the mixture unusable. The addition of DMSO plays two parts. On the one hand, the DMSO makes it possible to modify the organization of the surfactant (by playing on the zeta potential of the colloidal suspension) at the surface of the gold seeds, and on the other hand, it is adsorbed on some facets of the gold seeds as revealed by the SERS studies. The surfactants (CTAB, CTAC) are always present on the surface of the seeds, but adopt a different organization in the presence of the DMSO.

(51) The origin of the observed phenomena is not completely clear, the following hypotheses being advanced.

(52) The DMSO is a dipolar aprotic solvent of highly dissociative nature and capable of solubilizing polarizable, polar and ionic compounds.

(53) This type of solvent has mesomeric effect charges and is coordinated on one particular face (111) of the gold mono crystal (CFC) of the gold, by ionic interaction between its negatively charged oxygen and one or two positively charged ad-atoms of gold. Consequently, said layer may contaminate the surface of the gold, thus inhibiting the growth of the silver. The points of the nanorods, essentially composed of facets (111), can be coordinated by the DMSO, thus significantly disturbing the diffusion towards the surface and the deposition of silver on said facets The coordination of the DMSO on the gold would occur via the two possible coordination modes for this ligand (S-donor or O-donor).

(54) Comparison of the Effects of Different Co-Solvents

(55) Tests have been carried out to compare the effects of the DMSO to that of other co-solvents: acetonitrile (MeCN, CAS 75-05-8), aprotic polar solvent; ethanol (EtOH, CAS 64-17-5), aprotic polar solvent; tetrahydrofuran (THF, CAS 109-99-9), aprotic polar solvent.

(56) The protocol P previously presented was applied by replacing the DMSO with ethanol, or by replacing the volume of DMSO with 75 l of MeCN and 25 l of Milli-Q water, or by replacing the volume of DMSO with the Milli-Q water.

(57) Different depositions of silver were carried out on gold seeds preincubated in a water/co-solvent mixture (hereinafter identified as NRsEtOH, NRsMeCN and NRsDMSO respectively for a mixture with EtOH, MeCN and DMSO).

(58) These depositions were compared to the same silver deposition on gold nanorods incubated in water only.

(59) The kinetic monitoring of the reactional media was performed by UV-vis absorption spectroscopy, as shown in FIG. 5-A.

(60) The variations of the wavelength of the maximum of the longitudinal resonance band (LLSP) as a function of time show a shift towards the blue of this band, correlated with a deposition of silver along the small axis of the rod.

(61) After 45 min, the largest shift of the LLSP is recorded for gold nanorods incubated in the DMSO (LLSP) DMSO=227 nm, (LLSP) H2O, MeCN=197 nm, (LLSP) EtOH=183 nm); as represented in FIG. 5-B.

(62) The speed of deposition of silver on these NRsDMSO is faster than that of deposition of silver on NRsMeCN, NRsEtOH and NRsH.sub.2O (used as reference).

(63) Disturbance of the surfactant layer by adding organic solvent such as ethanol or acetonitrile does not directly affect the shape of the final core-shells. The modifications are restricted to slight differences in the level of kinetics of silver deposition onto the gold and the wavelength of the longitudinal resonance of the objects, attributed to the difference in solubility of the surfactant present on the surface of the gold in the water, ethanol and acetonitrile.

(64) In contrast, for the DMSO, kinetic monitoring shows a displacement towards the blue of the longitudinal and transverse resonances, and resonances that are narrower and more intense. These spectral modifications may be due to a deposition of silver that is greater, or to morphological differences of the objects obtained.

(65) Tests of Silver Overgrowth on Gold/Silver Core-Shells to Give Particular Shapes to the Nanoparticles

(66) Tests of silver overgrowth on core-shells incubated in DMSO were performed. A shift towards the blue of the wavelength of the longitudinal resonance of initial objects is noted. The MEB images show that the deposition is done essentially along the small axis and in a single direction, and the final objects resemble pyramids the base of which is a gold nanorod, as illustrated in FIG. 3.

(67) Deposition of Silver on Dimers

(68) We have shown the role of DMSO in assembling gold nanorods in face-to-face FF dimers for a quite specific water/DMSO ratio, as well as its effect on the final shape of the core-shell particles. Thus, the addition of DMSO in the colloidal solution such that the DMSO/H.sub.2O ratio is equal to 1.5, results in this FF assembly of initial gold nanorods.

(69) It should be noted that the addition of surfactant during the incubation step makes it possible to stop the assembly of gold seeds either head-to-head (HH) or face-to-face (FF) (depending on the ratio of DMSO volume to water volume). Thus, depending on the moment selected for this addition, assemblies of two, three, four, etc. gold seeds can be produced for the core, then encased by the silver shell.

(70) This assembly is evidenced by increasing shift towards the blue of the longitudinal resonance and an increasing shift towards the red of the transverse resonance.

(71) The assembly of the Au NRs in FF configuration is probably due to the depletion (attractive) forces, which destabilize the colloidal solution. It is the solubilizing power of the DMSO that is involved in this phenomenon of partial desorption of the surfactant of the bilayer of surfactant around particles.

(72) The decrease in electrostatic repulsion between the rods, in addition to the Van der Waals attractive interactions between the hydrophobic chains thus exposed will be the driving force of the assembly first as dimer, then as oligomers over long periods of time.

(73) To freeze the system in the dimers state and avoid the formation of oligomers, the concentration of CTAC surfactant in the medium is sharply increased beyond the CMC.sub.CTAC.

(74) Indeed, the addition of CTAC in excess reforms the bilayer of CTAC with the hydrophobic chains exposed around dimers and avoids any aggregation by electrostatic repulsion.

(75) The encapsulation of FF dimers in a layer of silver was done by reduction of the AgNO.sub.3 precursor with ascorbic acid AA in the solution of dimers at 60 C.

(76) A protocol for the production of core-shell dimer nanoparticles is as follows, in one implementation: to 100 L of a solution of gold nanorods NRsAu (4 nM), a volume of 150 L of DMSO is added and the face-to-face assembly of the gold nanorods is stopped by adding 800 L of CTAC 8 mM. The solution is centrifuged and re-dispersed in 5 mL of CTAC 16 mM. To initiate the deposition, 1.2 mL of the aqueous solution of AgNO.sub.3 at 2 mM and 1.2 mL of the aqueous solution AA (44 mM)/CTAC (40 mM) is added, protected from light. The addition is done drop by drop under moderate agitation. The deposition is stopped after 60 minutes, centrifuging at 6000 RPM for 8 minutes.

(77) The deposition of silver is revealed during monitoring by absorption spectroscopy, by a shift of at least 100 nm towards the blue of the LLSP resonance after only three minutes of reaction.

(78) The core-shell particles obtained are in the form of octahedrons, with more pronounced points, which gives them more advantageous enhancement properties, as shown in FIG. 3.

(79) Applications

(80) Comparative studies by SERS of particles obtained by this new synthesis method show an increase in SERS gain, relative to cuboid particles obtained conventionally in water. The SERS substrates of the invention have greater homogeneity in terms of response relative to those of the state of the art, due to the greater homogeneity of arrangement of the gold-silver core-shell nanoparticles deposited.

(81) The gold-silver core-shell nanoparticles obtained (NRsAu@Ag) having more pointed ends will induce a stronger confinement of the field to the points.

(82) Application to Detection of Atrazine (ATR) To 183 l of basic Milli-Q (pH equal to 6 adjusted by addition of NaOH 0.1 M), a volume X of ATR (1, 0.1, 0.01, 0.001 mM in the acetonitrile) and Y of -CD or -cyclodextrine (6, 0.66, 0.066, 0.0066 mM in water) are added. The solutions prepared in advance are added to 100 l of NRsAu@Ag. The values of X and Y (between 4 and 16 l) as well as the concentrations of the -CD and ATR are chosen in such a way that the final concentrations of ATR and -CD are equal to 0.05, 1, 3, 6, 12.5, 25, 100, 200 M. The solutions are incubated for 1 hour prior to characterizing them by SERS. Alpha-cyclodextrin can also be used to detect atrazine.

(83) Advantages of the Invention

(84) The invention has numerous advantages.

(85) The invention allows the modulation of the shapes and self-assembly of gold-silver core-shell nanoparticles, by an original synthesis method in a water/organic co-solvent medium.

(86) This synthesis method makes it possible to include one or more gold seeds of elongated shape (nanorod of dimension ranging from 15 to 150 nm), in a monocrystalline silver matrix.

(87) It is possible to incorporate more than one nanorod gold seed. Indeed, the relative proportions of water/DMSO induce, for some compositions, a face-to-face assembly of the gold nanorods during the incubation step.

(88) This process is remarkably simpler than those commonly used in the literature, to obtain more exotic shapes than the conventional cuboids.

(89) During the overgrowth of the silver onto the gold, the shape of the core-shell particles is controlled by the deposition kinetics of the silver atoms onto the surface of gold.

(90) The invention has numerous advantages for producing SERS substrates.

(91) The bottom-up approach of preparation allows the modulation of the size, shape and therefore the position of the plasmon resonance of the substrate, making it possible to consider a plurality of laser excitation wavelengths (532, 632.8 and 785 nm).

(92) Furthermore, the silver shell of the nanoparticles is an advantage in terms of enhancement factor, since the silver is known to be more effective than gold (interband responsible for losses by attenuation of the plasmon situated in the visible spectrum for gold and in the UV for the silver).

(93) Moreover, the self-assembly on solid substrate of the nanoparticles obtained by the synthesis method opens it to SERS sensors applications.

(94) A bottom-up production approach of these SERS substrates from colloids constitutes an alternative to the top-down approaches by physical lithography, which although expensive currently remain favored for commercial substrates (better reproducibility). One of the disadvantages of colloids, relative to lithographic substrates, is the more random and non-reproducible nature of the placement of the hot points. The self-assembly mechanisms of the nanoparticles allow a greater constancy of the properties of the substrates.

(95) The spontaneous surface arrangements of the core-shell nanoparticles allow new SERS substrates to be prepared, for sensor applications, for example for the following applications: agri-food: for example to detect melamine in milk; scientific police/security: drugs, poison, explosives; environmental analysis of contaminants in water: PCB, atrazine, pesticide, hormones; biosensors (early detection of markers).

(96) The intended applications fall within the field of chemical sensors using surface enhanced Raman scattering as a tool to quantify and identify the analyte of interest.