pubs.acs.org/Langmuir © 2009 American Chemical Society
A General Approach Combining Diazonium Salts and Click Chemistries for Gold Surface Functionalization by Nanoparticle Assemblies Helene Gehan, Laure Fillaud, Nordin Felidj, Jean Aubard, Philippe Lang, Mohamed M. Chehimi, and Claire Mangeney* ITODYS, Universit e Paris Diderot and CNRS (UMR 7086), 15 rue Jean de Baı¨f, 75013 Paris, France Received September 5, 2009. Revised Manuscript Received November 19, 2009 This paper describes a general stepwise strategy combining diazonium salt and click chemistries for an efficient gold surface functionalization by gold nanoparticles. The procedure first involves the strong covalent bonding to gold electrodes of OH-terminated aryl layers derived from the electroreduction of the parent diazonium salts. The following step consists in transforming the OH end-groups to azides in order to obtain “clickable”-active gold surfaces, which could further be used as versatile platforms for the subsequent grafting of acetylene-bearing molecules. The practical interest of the gold surfaces functionalized by this stepwise strategy was evidenced through the self-assembly of surfaceenhanced Raman scattering (SERS)-active gold nanoparticles. SERS activity was shown to be amplified by the presence of a very strong local electric field confinement between the particles and the gold surface.
1. Introduction The ability to tailor material surface properties with desired physicochemical functions is an important field of research with a broad spectrum of applications. These applications range from the modification of wetting properties, over the alteration of optical properties, to the fabrication of molecular electronic devices. In each of these fields, it is of specific importance to be able to control the homogeneity, the thickness, and the functionality of the layers with high accuracy. Controlled chemisorption of alkanethiols to form selfassembled monolayers (SAMs) has been one of the most popular methods for gold surface functionalization.1 However, these films can prove to be unstable, because they result from relatively weak bonding to the solid surface.2 The degree of thermal and electrochemical instability of such SAMs can also be problematic.1-4 An increasingly popular alternative to SAMs for surface modification is the use of electroreduction of aryldiazonium salts, as it provides higher stability with respect to long-term storage in air, potential cycling under acidic conditions, and a wider potential window for subsequent electrochemistry.5-10 *To whom correspondence should be addressed. E-mail: mangeney@ univ-paris-diderot.fr. Phone: 33-01-57276878. Fax: 33-01-57277263. (1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (2) Barcelo, D.; Gorton, L. Comprehensive Analytical Chemistry: Biosensors and Modern Biospecific Techniques; Elsevier: Amsterdam, 2005; Vol. XLIV, Chapter 1. (3) Liu, G.; Bocking, T.; Gooding, J. J. J. Electroanal. Chem. 2007, 600, 335. (4) Liu, G.; Liu, J.; Bocking, T.; Eggers, P. K.; Gooding, J. J. Chem. Phys. 2005, 319, 136. (5) Dubois, L.; Viel, P.; Bureau, C.; Palacin, S. J. Am. Chem. Soc. 2004, 126, 12194. (6) Liu, Y.-C.; McCreery, R. L. Anal. Chem. 1997, 69, 2091. (7) Itoh, T.; McCreery, R. L. J. Am. Chem. Soc. 2002, 124, 10894. (8) Saby, C.; Ortiz, B.; Champagne, G. Y.; Belanger, D. Langmuir 1997, 13, 6805. (9) Boukerma, K.; Chehimi, M. M.; Pinson, J.; Blomfield, C. Langmuir 2003, 19, 6333. (10) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1997, 119, 201. (11) Adenier, A.; Cabet-Deliry, E.; Chausse, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C. Chem. Mater. 2005, 17, 491. (12) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1992, 114, 5883.
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The reduction can occur in aqueous and organic media, and can be either spontaneous8,11 or electroinduced.12 The mechanism for the electroinduced deposition involves the formation of an aryl radical at the vicinity of the surface following the reduction of the diazonium salt, leading to the departure of a N2 molecule. The following step leads to the formation of a covalent bond 9 between the aryl group and an atom of the substrate, allowing a strong attachment of the deposited layers. This mechanism can yield coatings of variable thicknesses (from the monolayer to thicker layers) depending on the charge allowed to reduce the diazonium and/or the reaction time. This technique has also the advantage of controlling the functionalization of the surface by an appropriate selection of functional group in the para position of the grafted molecule. However, despite a large choice of substituents available by this method, further selective derivatization is often required to enable the surfaces to be subsequently used as platforms to which further molecular or nanoscale components can be attached with controlled surface density. Indeed, the straightforward surface reaction cannot meet the demand of well-defined surface characterization because the efficiency and reliability of the conventional coupling techniques on solid surface is not easy to control (for example, low yield and poor selectivity). Recently, click chemistry, proposed by Sharpless and coworkers,13 has generated enormous interest13-16 for surface modification, as it uses only the most practical and reliable chemical reactions to connect a diversity of structures. Especially, the Huisgen reaction, a newly developed copper(I)-catalyzed 1,2,3-triazole formation reaction between azides and alkynes, is a prototype of click chemistry for its properties of high reaction yield, simple reaction, and purification conditions. It is a potential strategy for surface modification because of its fast, mild reaction conditions, high chemoselectivity, and tolerance toward functional groups.13,17 (13) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004. (14) Service, R. F. Science 2008, 320, 868. (15) Lipinski, C.; Hopkins, A. Nature 2004, 432, 855. (16) Ciampi, S.; Bocking, T.; Kilian, K. A.; James, M.; Harper, J. B.; Gooding, J. J. Langmuir 2007, 23, 9320. (17) Rostovtsev, V. V.; Green, J. G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2596.
Published on Web 12/29/2009
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Figure 1. General scheme summarizing the stepwise strategy for surface modification of gold electrodes by (i) electrochemical grafting of hydroxyethyl-aryl groups, (ii) esterification with 2-bromopropionyl bromide, (iii) azidation, (iv) click chemistry with acetylene-bearing molecules, and (v) immobilization of gold NPs.
Although both of these techniques, diazonium salt reduction and click chemistry, continue to stimulate increased effort of the scientific community, combining them for surface functionalization appears to be a new challenge in order to provide both a strong bonding with the surface and a high surface reaction efficiency. This combined strategy was first explored very recently by Evrard et al.18 who reported the electrochemical reduction of phenylazide or phenylacetylene diazonium salts onto the surface of carbon electrodes. In the presence of copper(I) catalyst, these azide- or alkynemodified surfaces were shown to react efficiently and rapidly with compounds bearing an acetylene or azide function, thus forming a covalent 1,2,3-triazole linkage by means of click chemistry. This was illustrated with the surface coupling of ferrocenes functionalized with an ethynyl or azido group and the biomolecule biotin terminated by an acetylene group. The purpose of the present work is to further demonstrate the utility of the combined approach diazonium/click chemistry on another type of substrate, i.e., gold electrodes, in order to create
new gold-active platforms for surface immobilization of nanoparticles (NPs). The presence of the gold substrate is important, as we can expect a strong optical coupling and enhanced electric fields between the NPs and the gold surface, making such systems very attractive in surface enhanced spectroscopies. We explored a stepwise strategy (summarized in Figure 1) based on the covalent functionalization of Au surfaces by functional aryl groups derived from the electroreduction of diazonium salts and subsequent derivatization to obtain azide-terminated layers. The originality of our strategy compared to the work of Evrard et al. lies in the hierarchy of the functionalization steps. Indeed, these authors used diazonium salts directly bearing the azide moieties, while we introduce, in the present work, the azide moieties on the bromo-terminated ester groups tethered on the gold surface. Although the method of Evrard et al. is more straigthforward, our multistep strategy offers the interest that it could be transposed to various bromine-terminated layers, such as, for example, polymer brushes synthesized by surface-initiated atom transfer radical polymerization (SI-ATRP).19
(18) Evrard, D.; Lambert, F.; Policar, C.; Balland, V.; Limoges, B. Chem.;Eur. J. 2008, 14, 9286.
(19) Tsarevsky, N. V.; Sumerlin, B. S.; Matyjaszewski, K. Macromolecules 2005, 38, 3558.
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More importantly, although the first steps of our strategy could be useful for in situ ATRP (-O-C(O)-CH(CH3)Br moiety is a known ATRP initiator), one can take advantage of the facile local eletrochemical reduction of C-Br bonds to pattern surfaces with polymer brushes.20,21 In our case, the proposed strategies of surface functionalization by aryl diazonium salts could be readily applied to pattern surfaces with gold NPs and other molecular or macromolecular functional species. The clickable-active gold surfaces could then serve as a versatile platform for quantitative and stable attachment of a variety of acetylene-bearing functional molecules (CtC-R-NH2 and CtC-R-COOH), via a selective, reliable, and robust click reaction. The product of the reaction is the stable heterocyclic linker, 1,4-disubstituted 1,2,3-triazole, which is essentially inert to molecular oxygen, various solvents, and common reaction conditions. Finally, the practical interest of these active gold electrodes was further demonstrated trough the self-assembly of Au NPs. High surface-enhanced Raman scattering (SERS) properties were observed in these Au NP assembly coated gold electrodes, arising from a strong electromagnetic coupling between the optical properties of the NPs and the substrate. The untreated, pretreated and NPs decorated gold surfaces were characterized by X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection-adsorption spectroscopy (PM-IRRAS), and scanning electron microscopy (SEM).
2. Experimental Section 2.1. Materials. Chemicals. Reagent-grade solvents were purchased from VWR-Prolabo or Alfa Aesar. Chemicals were used without further purification: (2) 2-bromopropionyl bromide (BPB) (97%, Aldrich), triethylamine (TEA) (99%, Merck), (3) sodium azide (NaN3) (99%, Prolabo), (4a) propargylamine (99%, Acros Organics), (4b) 4-pentynoic acid (98% Alfa Aesar), Cu(II) sulfate pentahydrate (99%, Prolabo), and L-ascorbic acid sodium salt (99%, Alfa Aesar). The synthesis of 4-hydroxyethylbenzene diazonium tetrafluoroborate salt (denoted as OH(CH2)2BD) is reported in the Supporting Information. Wafer. Gold-coated silicon wafers (Æ111æ oriented, 1000 A˚ coating, titanium adhesion layer, 4 in. 500 μm) were purchased from Aldrich. 2.2. Prefunctionalization of the Gold-Coated Silicon Wafers. Gold-coated silicon wafers were cleaned using a UV-ozone light for 10 min in order to destroy organic contaminations on the surface. All reaction steps were carried out at room temperature and followed by rinsing with the reaction solvent and ethanol. The samples were then dried under a flush of argon. (i). Electrochemical Treatment. Electrochemical grafting of OH(CH2)2BD was achieved on cleaned gold-coated silicon wafer by chronoamperometry for 30 s in acetonitrile (ACN) at a potential of 300 mV negative relative to the peak reduction potential measured on carbon. The -OH-terminated gold surfaces are abbreviated as Au-OH. (ii). Esterification. The hydroxyl groups tethered on gold electrodes were treated with 2-bromopropionyl bromide (0.1 M, dichloromethane) in the presence of TEA (0.12 M) for 5 min to produce bromo-terminated ester groups. The -Br-terminated gold surfaces are abbreviated as Au-Br. (iii). Nucleophilic Substitution Reaction. Substitution of the bromo-terminated esters was carried out by reacting in NaN3 (20) Slim, C.; Tran, Y.; Chehimi, M. M.; Garraud, N.; Roger, J.-P.; Combellas, C.; Kanoufi, F. Chem. Mater. 2008, 20, 6677. (21) Hauquier, F.; Matrab, T.; Kanoufi, F.; Combellas, C. Electrochim. Acta 2009, 54, 5127.
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solution (0.1 M, N,N-dimethylformamide (DMF)) for 6 h in order to obtain the click active platform, abbreviated as Au-N3.
2.3. Functionalization of the Click Active Platform. (iv). Azide-Alkyne Cycloaddition. The azide-terminated substrate was immersed in a solution containing the appropriated alkyne (10 mM, propargylamine (4a) or 4-pentynoic acid (4b), ethanol/water 1:1), copper(II) sulfate pentahydrate (0.1 mM), and L-ascorbic acid sodium salt (2.5 mM). The solutions were degassed by argon bubbling while being stirred for 5 h.
(v). Immobilization of Gold NPs onto the Amino-Terminated Substrates (a). Gold NP suspensions were prepared
following the method described by Frens22 (see Supporting Information). The substrate was immersed in the colloidal solution (pH 4.5) for 2 h and then thoroughly rinsed with water. 2.4. Instrumentation. X-ray photoelectron spectra were recorded using a Thermo VG Scientific ESCALAB 250 system fitted with a microfocused, monochromatic Al KR X-ray source (hν = 1486.6 eV; spot size = 650 μm; power = 15 kV 200 W). The pass energy was set at 150 and 40 eV for the survey and the narrow regions, respectively. Spectral calibration was determined by setting the main C1s component at 285 eV. The surface composition was determined using the integrated peak areas and the corresponding Scofield sensitivity factors corrected for the analyzer transmission function. PM-IRRAS spectra were recorded on a Nicolet 860 FTIR (Thermo-Electron) at a resolution of 8 cm-1 by coadding 1500 scans with a optical mirror velocity of 0.474 cm-1. The spectrometer was equipped with a commercial accessory PEM module (Thermo Electron) including a Zn Se photoelastic modulator. PM-IRRAS was performed at an incident angle of 80. After reflection on the sample, the beam was focused with a ZnSe lens (f = 5 cm) onto an MCT-A detector cooled at 77 K. The PEM oscillates at 50 kHz and changes polarization from parallel to perpendicular at 100 kHz. The polarization-modulated signal was separated from the low frequency signal (ωl between 380 and 3800 Hz) with a 48 kHz high pass filter and then demodulated (GWC Technologies). The two interferograms are high-pass and low-pass filtered by the spectrometer and simultaneously sampled in the dual channel electronics of the spectrometer. SEM images of Au NPs were recorded on a Zeiss Supra 40 microscope at an accelerating voltage of 15 kV. Raman spectra were obtained with a LABRAM Jobin-Yvon microspectrometer using a He-Ne excitation laser (632.8 nm, 1 mW power). In our experiments, the microscope was equipped with an X 100 objective (NA # 0.90), which leads to a focused laser spot area onto the substrate of ca.1 μm2 and ensures a very efficient collection of the Raman light. All spectra were taken with 1 s integration time in the 250-2500 cm-1 spectral range.
3. Results and Discussion Preparation of Azide-Active Gold Surfaces. The electrochemical grafting of (hydroxyethyl)phenyl groups derived from OH(CH2)2BD diazonium salt reduction on gold plates was achieved by chronoamperometry in ACN þ 0.1 M nBu4BF4. A 300 mV negative potential relative to the peak reduction potential measured for the diazonium salt on carbon (Epc = -0.33 V/SCE (saturated calomel electrode)) was maintained for 30 s. A very steep decrease of the current is observed with time, which is characteristic of the formation of an organic layer, which blocks the electron transfer from the electrode. The gold electrodes were then thoroughly rinsed under sonication during 3 min in deaerated acetone, before being immersed in a 2-bromopropionyl bromide solution during 5 min, in order to achieve the esterification of the hydroxyl groups. The azidation step was performed by (22) Frens, G. Nature 1973, 241, 20.
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A low-intensity nitrogen signal (N1s at ca. 400 eV) appears after the electrochemical reduction of the diazonium salt, possibly as a result of the formation of azo groups (-NdN-), as already suggested previously.23 After reaction with sodium azide, the relative nitrogen signal intensity significantly increases, with the N/C atomic ratio going from 0.014 to 0.06. This observation, which parrallels the quasi total disappearance of the bromine signal, confirms the nucleophilic substitution of bromine atoms by azide groups. The gold signal attenuation, observed all along the different chemical steps, is due to the covering of the electrode by the organic overlayer and can be used to provide an estimation of the organic layer thickness. Indeed, the relative attenuation of the Au 4f7/2 signal can be expressed as I=I0 ¼ expð-d=λ sin θÞ
Figure 2. XPS survey spectra of (a) bare gold substrate, (b) (hydroxyethyl)phenyl-coated gold, (c) (EBrP)phenyl-coated gold, and (d) azide-active gold surfaces. Table 1. Apparent Surface Chemical Composition (Atomic Percents) Determined by XPS for the Gold Electrodes before (Bare Au) and after Electroreduction of OH(CH2)2BD (Au-OH), Esterification by EBrP (Au-Br), and Azidation (Au-N3) materials
Au
C
O
Br
N
Bare Au Au-OH Au-Br Au-N3
76 21 13 14
19 62 69 69
5 16 14 13
0 0 3 traces
0 1 1 4
reacting the modified gold plates in a NaN3/DMF solution for 6 h, followed by rinsing with DMF and water. XPS. Figure 2 displays the XPS survey spectra of gold electrodes before and after modifications leading to Au-N3. All survey spectra display the characteristic gold peaks, with main ones at 84 and 88 eV assigned to Au4f7/2 and Au4f5/2, respectively. Carbon and oxygen are also detected at 285 eV (C1s) and 533 eV (O1s), first as traces due to the presence of contamination species on the bare gold electrode and then as major elemental components associated with the electrografted organic layers. Table 1 gathers the atomic surface composition obtained by the integration of the core level peaks as well as the surface concentration of the grafted groups, estimated as depicted below. Esterification of the OH-terminated aryl groups with 2-bromopropionyl bromide was confirmed by the appearance of peaks characteristic of bromide atoms in a C-Br environment (Br3d and Br3p3/2 at 71 and 182 eV, respectively). It is interesting to compare the experimental Br/C and Br/O atomic ratios (0.04 and 0.2, respectively) to the ones that would be expected in the case of a 100% reaction yield (0.1 and 0.5, respectively). These results indicate that the esterification reaction has indeed occurred but with a reaction yield close to around 50%. Nevertheless, it is difficult to give accurate calculations on the bromine signal, as the C-Br bond suffers photoinduced cleavage under the spectrometer X-ray spot leading to Br- species. Such a photoreduction during the XPS analysis results in broad peaks that are, on top of that, noisy as a result of the very low extent of bromine after azidation of the surface (see inset in Figure 2). The Br atomic percents are therefore only indicative, particularly in the case of low surface concentration of this halogen atom. 3978 DOI: 10.1021/la9033436
where d is the layer thickness, λ is the mean free path of the substrate-specific photoelectron in the organic layer, θ is the analysis takeoff angle relative to the surface, and I/I0 is the ratio of the Au4f7/2 peak intensities (modified surface/bare surface). Details of this calculation are reported in the Supporting Information. For Au-OH, the aryl adlayer thickness is found to be ∼3.5 nm, indicating that multilayers of (hydroxyethyl)phenyl groups are attached to the surface. Although the structure of the layer is far from fully established, it should look somewhat like a substituted polyphenylene, in agreement with the structure proposed by McDermott.24 This average thickness is reasonable because the gold core level peaks are all detected. Particularly, the sampling depth of Au4f is ∼12 nm (3 times the mean free path), much higher than 3.5 nm the thickness of the top layer. For this reason, the survey spectrum does not show a very intense background at low kinetic energy side (high apparent binding energy side of the spectra) due to inelastically scattered Au4f photoelecrons. As the organic modifiers employed in steps ii and iii (in Figure 1) are subnanometer sized small molecules, clearly the thickness of the top organic coating should have a thickness that reasonably matches that of the first aryl layer electrochemically grafted. By considering a density of 1.02 g/cm3 for the grafted aryl adlayer (approximately the density of 2-phenylethanol given by the supplier) and the thickness of 3.5 nm estimated by XPS, one can estimate the electrochemical grafting-induced surface coverage ΓΟH to be 3.0 10-9 mol 3 cm-2. The approximation that consists of assimilating the density of aryl grafted groups to that of nongrafted groups or other types of grafted-organic molecules is commonly used in the literature.26 It is noteworthy that the surface coverage value deduced from this approximation is around 2-3 times higher than the surface concentration of a close-packed monolayer ΓCPML of phenyl (or 4-substituted phenyl) groups estimated from molecular models: ΓCPML = 1.35 10-9 mol 3 cm-2.25 Click Chemistry. The azide-modified gold surfaces were further used as click-active platforms for the subsequent grafting of acetylene molecules bearing functional end groups such as amines (propargylamine, CtC-R-NH2) or carboxylic acids (23) Saby, C.; Ortiz, B.; Champagne, G. Y.; Belanger, D. Langmuir 1997, 13, 6805. (24) (a) Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 6534. (b) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947. (25) Pinson, J.; Podvorica, F. Chem. Soc. Rev. 2005, 34, 429. (26) (a) de Villeneuve, C. H.; Pinson, J.; Bernard, M. C.; Allongue, P. J. Phys. Chem. B 1997, 101, 2415. (b) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. B 1989, 93, 1670. (c) Matrab, T.; Save, M.; Charleux, B.; Pinson, J.; Cabet-deliry, E.; Adenier, A.; Chehimi, M. M.; Delamar, M. Surf. Sci. 2007, 601, 2357.
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Figure 3. XPS survey spectra of azide-modified gold surfaces (a) before click chemistry, (b) after reaction with CtC-R-COOH, and (c) after reaction with CtC-R-NH2.
Figure 4. PM-IRRAS spectra of azide-active gold electrodes (a) before click reaction, (b) after reaction with CtC-R-NH2, and (c) after reaction with CtC-R-COOH.
Table 2. Apparent Surface Chemical Composition Determined by XPS for the Azide-Active Gold Electrodes after Reaction with CtC-R-NH2 (Au-NH2) and CtC-R-COOH (Au-COOH) materials
Au
C
O
N
Au-NH2 Au-COOH
13 12
61 61
15 22
11 5
(4-pentynoic acid, CtC-R-COOH). The click chemistry reactions, expected to lead to the formation of a covalent 1,2,3-triazole linkage (see Figure 1), were performed in the presence of copper(I) catalyst. Figure 3 displays the XPS survey spectra of the azide-active gold surface before and after reaction with the alkyne-derived molecules. After click reaction, one observes the signature of the functional end groups carried by the alkyne molecules, with an increase in nitrogen content for the product of reaction with CtC-R-NH2 and an increase in oxygen content in the case of CtC-R-COOH. Atomic surface compositions of the product samples are summarized in Table 2. The high proportions of nitrogen and oxygen observed in Au-NH2 and Au-COOH, respectively, suggest a high reaction yield for the click chemistry step but could also arise from loosely adsorbed alkyne-derived species. It is noteworthy that a Cu2p3/2 emission at ∼933 eV (∼0.1% of the total carbon) is also observed for NH2-coated gold surfaces and is associated with traces of residual copper catalyst despite copious rinsing. In the case of COOH-coated gold electrodes, a Na1s peak appears, indicating that the COOH groups are in their deprotonated form. Figure 4 displays the PM-IRRAS spectra of the azide-active gold surface before and after reaction with the alkyne-derived molecules. All the spectra display the strong intensity stretching vibrational band of the CdO ester function (at 1740 cm-1), formed during the esterification reaction (with 2-bromopropionyl bromide). For Au-N3 (Figure 4a), one also observes the prominent asymmetric stretch of the organic azide group (at ca. 2110 cm-1). The 1,3-cycloaddition of the alkyne molecules can be monitored conveniently and accurately by the disappearance of the later band and the concomitant appearance of the characteristic vibrations associated to the functional end groups: 1605 cm-1 for the N-H bending vibration and 1600 and 1410 cm-1 for the CO Langmuir 2010, 26(6), 3975–3980
Figure 5. SEM image of self-assembled Au NPs on NH2-coated gold surfaces.
asymmetric and symmetric stretch of carboxylate CO2- groups, respectively. Assuming that the azide disappearance is correlated only to product formation, this provides evidence that the reaction is nearly quantitative and that these modified gold electrodes are reactive surfaces onto which acetylene-bearing molecules can be “clicked.” Immobilization of Au NPs and SERS Activity. Figure 5 illustrates a SEM image of the self-assembled Au NP monolayers on amine-coated gold surfaces. It shows a uniform coverage of well-dispersed NPs on the surface. The number of NPs (N) per area (A) of the sample was counted by zooming a part of the SEM image, and the surface coverage (j) was calculated to be ∼25%, from the following equation: j ¼ 100%Nπd 2 =4A where d is the average diameter of the NPs. The “SERS activity” of the Au NP-coated gold substrates (Au NP-Au) was evaluated using adsorbed methylene blue (MB) as molecular probes. The MB molecules were deposited on the DOI: 10.1021/la9033436
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Figure 6. SERS of MB on (a) Au NP-coated gold substrates (632.8 nm, tacc = 1 s, PL = 10 mW) and (b) Au NP-coated glass slides (632.8 nm, tacc=1 s, PL=1 mW).
substrates by dipping into a 10-4 M MB aqueous solution. The substrates were then washed and dried with N2 gas. Figure 6 displays the SERS spectrum of MB adsorbed on Au NP-coated gold substrates. It is compared to that obtained, in the same experimental conditions, on gold NPs immobilized on silanized glass slides (Au NP-SiO2). This later substrate is known to yield high-intensity SERS spectra.27 For a reasonable comparison of the SERS intensities, both substrates were covered with similar amount of gold NPs, the surface coverage of NPs being estimated to be ∼25%. It is observed that both SERS spectra (Figure 6a,b) display the same Raman features, associated with MB molecules, but a weak band at ca. 2130 cm-1 for Au-Au NPs (Figure 6a). This last band could be ascribed to remaining traces of alkyne reactants (CtC stretching mode) adsorbed onto Au NP-Au substrates, the Raman spectrum of which would be greatly enhanced. The spectrum of MB appears highly enhanced when deposited on Au NP-Au substrates, as compared to Au NP-SiO2. In order to estimate the relative enhancement factors (rEFs) of both substrates, we considered the ratio of the integrated intensities of two characteristic bands of MB. For this calculation, we chose the 1618 cm-1 and 443 cm-1 lines, which are the most intense MB Raman bands. The intensity ratios Ia/Ib (where Ia represents the integrated intensity on Au NP-Au, and Ib represents the integrated intensity on Au NP-SiO2 substrate) were found to be ∼500 and 1000 for the 1618 cm-1 and 443 cm-1 lines, respectively. Such high rEFs underline the influence of the gold substrate. The close proximity between Au NPs and the Au film, linked by the (27) Felidj, N.; Truong, S.; Aubard, J.; Levi, G.; Krenn, J. R.; Hohenau, A.; Leitner, A.; Aussenegg, F. R. J. Chem. Phys. 2004, 120, 7141. (28) Felidj, N; Aubard, J; Levi, G; Krenn, J. R.; Schider, G; Leitner, A; Aussenegg, F. R. Phys. Rev. B 2002, 66, 245407.
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functionalized aryl layer (particle-substrate distance is ca. 3-4 nm), favors strong near-field interactions, so-called hot-spots. This leads to an important intensity increase of the SERS spectrum of MB, in agreement with previous studies reported on lithographically prepared NP arrays 28. The observed variation in EF with Raman wavenumbers is due to the wavenumber dependence of the local electric field intensity, as reported recently by Le Ru et al.29 From this study and others found in recent literature, it appears that Au NP-coated gold substrates represent favorable platforms for obtaining hot spots formed in the gaps between the gold particles and the planar gold surface.28,30 Recently, Marzan’s group was able to reach zeptomol detection of 1,5-naphtalenedithiol by SERS through gold NPs with a separation of 1 nm from the gold film (the linker was made of 1-octanethiol SAMs). This separation corresponds to the gap size in our system, which ensures very high SERS activity.30
4. Conclusion A new stepwise strategy has been developed for the surface modification of gold electrodes by gold NPs. This strategy, based on a combination of diazonium salt electrografting and click chemistry, was fully characterized using XPS and PM-IRRAS. It was shown to provide a large surface coverage of gold NPs, strongly linked to the gold surface through triazole-functionalized aryl grafted groups. More interestingly, the thickness of this organic linker layer, around 3-4 nm, is expected to allow a coupling between the optical properties of the gold surface and the NPs, as suggested by the strong SERS activity of these substrates. Beyond these results, this work firmly highlights the interest of using aryl diazonium salts together with click chemistry in order to functionalize gold surfaces with nanostructures and to elaborate new materials with high added-value optical properties. Acknowledgment. The authors gratefully acknowledge the financial support from Ministere de la Defense, Direction Generale de l’Armement DGA (Project DGA-REI 07 34024 and Ph.D. Support 2007-2010). Supporting Information Available: Detailed description of the diazonium salt and gold NPs synthesis. XPS estimation of organic layer thickness. This material is available free of charge via the Internet at http://pubs.acs.org. (29) Le Ru, E. C.; Etchegoin, P. G.; Grand, J.; Felidj, N.; Aubard, J.; Levi, G.; Hohenau, A.; Krenn, J. R. Curr. Appl. Phys. 2008, 8, 467. (30) Rodrihuez-Lorenzo, L.; Alavrez-Puebla, R.; Pastoriza-Santos, I.; Mazzucco, S.; Stephan, O.; Kociak, M.; Liz-Marzan, L.; Garcia de Abajo, J. J. Am. Chem. Soc. 2009, 131, 4616. (31) Practical Surface Analysis: Auger and X-Ray Photoelectron Spectroscopy, 2nd ed.; Briggs, D., Seah, M. P., Eds.; John Wiley: Chichester, U.K., 1990; Vol. 1, p 209.
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