Article pubs.acs.org/JPCC
Silica-Covered Silver and Gold Nanoresonators for Raman Analysis of Surfaces of Various Materials Andrzej Kudelski* and Sebastian Wojtysiak Department of Chemistry, Warsaw University, Pasteur 1, PL-02-093 Warsaw, Poland ABSTRACT: Silver and gold colloids have been coated with ca. 3−6 nm thick layer of silica to form nanoparticles with core−shell structure. Synthesized Ag@SiO2 and Au@SiO2 nanoparticles are efficient electromagnetic resonators and can locally significantly enhance the electric field of the incident electromagnetic radiations, which leads to a very large increase of the Raman signal from species being in the close proximity to nanoresonators. In this contribution we report the first example of surface shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) analysis with Ag@SiO2 nanoresonators. Ag@SiO2 nanoresonators enhance locally the intensity of the exciting electromagnetic radiation in a significantly broader frequency range than the previously used Au@SiO2 nanoparticles; therefore, combination of the SHINERS technique with the resonance Raman effect is significantly easier with Ag@SiO2 clusters.
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INTRODUCTION Surface analysis of various materials (especially in the in situ conditions) is very important from the economic and scientific point of view. Such studies are especially difficult for so-called buried interfaces (e.g., the surface of the solid sample in the liquid or the high pressure gas), a situation that occurs in very important, from the practical point of view, interfaces of various biological samples in their “natural” environment. One of the tools that can be used for investigations of such interfaces is Raman spectroscopy. For many decades Raman spectroscopy has not been considered a useful analytical tool because of very low efficiency of “normal” Raman scattering. Typical total Raman scattering cross-section is ca. 10−29 cm2 per molecule, whereas typical cross sections for absorption in ultraviolet and infrared are ca. 10−18 and 10−21 cm2 per molecule, respectively.1 This limitation of Raman spectroscopy has been, however, overcome in two steps. First, Shorygin realized that if one tunes the frequency of the excitation radiation to the absorption band of an analyte, the intensity of measured Raman spectrum increases significantly (e.g., by a factor of 106).2 Then, it was realized that by utilizing special nanoresonators constructed from metal nanoclusters, the Raman scattering cross sections can be also increased by many orders of magnitude in so-called surface-enhanced Raman scattering (SERS).3,4 When both effects are combined, Raman cross section can be as large as 2 × 10−14 cm2 per molecule5 (i.e., about 15 orders of magnitude larger in comparison to the normal Raman scattering) in socalled surface-enhanced resonance Raman scattering (SERRS) effect. Therefore, SERRS spectroscopy makes possible observation of spectra even of a single molecule,5−7 and hence, comparably to single-molecule fluorescence spectroscopy, SERRS becomes one of the most sensitive analytical tools for analysis of mentioned above “buried interfaces”. © 2012 American Chemical Society
For many years highly sensitive SERS measurements have been carried out only on surfaces of materials from which effective nanoresonators could be formed (in principle nanostructured surfaces of silver, gold, and copper, sometimes covered with a few monolayers of other metal).8−11 The possibility of observation of strong SERS effect only on these few metals significantly limited the practical applications of SERS spectroscopy. This limitation was lifted in 2000 by Zenobi et al.12 and Anderson,13 who have shown that it is possible to record enhanced Raman signal on any substrate using external resonator from Au or Ag, which is brought to the close proximity of the studied surface with the STM or AFM device. This approach is, however, expensive, because it requires the use of a Raman microscope combined with a STM or AFM nanoscope. Recently Li et al. reported a cheaper but still sensitive method of chemical analysis of various surfaces.14 In this approach, the analyzed surface is covered with the layer of Au@SiO2 or Au@Al2O3 clusters, and then the Raman spectrum of the investigated sample is recorded. The ultrathin SiO2 or Al2O3 coating does not damp surface electromagnetic enhancement, however, separates nanoparticles from direct contact with the probed material and keeps them from agglomerating. Using this method (so-called shell-isolated nanoparticle-enhanced Raman spectroscopy − SHINERS), Li et al. recorded high-quality Raman spectra on various molecules adsorbed at Pt and Au single-crystal surfaces and from Si surfaces with hydrogen monolayers.14 Some biological samples, as walls of living cells, and surfaces of citrus fruits with pesticide residues were also characterized using this method.14 Even though the first SHINERS measurements have been reported Received: May 7, 2012 Revised: July 2, 2012 Published: July 4, 2012 16167
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in 2010, many other interesting contributions utilizing this technique and Au nanoparticles covered with SiO2 or Al2O3 layers have been already published.15−22 In this contribution we report on modification of SHINERS method by using silver nanoparticles coated with the layer of silica instead of the Au@SiO2 core−shell nanoclusters. Silver provides stronger plasmon resonance than gold, and SERS enhancement factors on silver are significantly higher than those on gold.23 Therefore, one may assume improving the analytical sensitivity when an Ag core instead of the Au one is used. Moreover, plasmon resonance on silver may be achieved for any visible electromagnetic radiation, whereas for gold nanoparticles efficient resonance may be only obtained with excitation radiation from the red part of the spectrum.23 This means that using Ag nanoparticles makes possible an easier combination of SHINERS with the “standard” resonance Raman scattering. Ag@SiO2 nanoparticles with various thicknesses of the silica layer (from 1 to 120 nm) have been already synthesized by other groups, and after covering by fluorescence molecules, have been used as fluorescent core−shell nanocomposites.24−29 In this paper we show the first example of using of Ag@SiO2 nanoparticles for the SHINERS measurements.
Figure 1. Size distribution histograms for (a) silver and (b) gold nanoparticles.
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Preparation of Ag@SiO2 and Au@SiO2 Nanoparticles. For coating gold or silver nanoparticles with amorphous silica shells, Mulvaney et al. have reported a procedure that involved the use of an amineterminated silane coupling agent as the primer to render metal surfaces vitreophilic.32,33 Although Hardikar and Matijevic later found that it is also possible to form uniform coatings of silica on silver nanoparticles without derivatizing their surfaces with any coupling agent,34 we decided to use for the deposition of silica layer on both silver and gold nanoparticles the same procedure (the original coating procedure of Mulvaney et al.,35 slightly modified by Li et al.15) with (3-aminopropyl)trimethoxysilane as a coupling agent. Briefly speaking, 1.6 mL of a 1 mM aqueous solution of (3aminopropyl)trimethoxysilane was added to 120 mL of the gold or silver nanoparticles solution and stirred at room temperature for 15 min. Subsequently, 12.8 mL of freshly prepared diluted sodium silicate solution (0.54% of SiO2) adjusted with HCl to pH 10−11 (for acidifying the solution obtained from 1 cm3 of Aldrich 26.5% Na2SiO3 solution 2.5 cm3 of 1 M HCl solution was used) was added with several more minutes of stirring. Then, the mixture was stirred for 3−4 days at room temperature. The Ag@SiO2 and Au@SiO2 nanoparticles were cleaned by centrifuging for 15 min, pouring off the supernatant, and adding water to suspend them again. This cleaning procedure was repeated four times (final Ag@ SiO2 and Au@SiO2 nanoparticles were obtained in concentrated solutions). We found that the average thickness of silica layer formed as a result of described above procedure is ca. 3−6 nm (Figure 2, which shows the TEM images of synthesized Ag@SiO2 and Au@SiO2 nanoparticles). As can be also noticed from Figure 2, some nanoparticles have been “glued” by silica during the process of the covering of the metal nanoparticles by the silica layer. On the basis of the analysis of TEM images we suppose that the less regular silica shells on the metal cores are obtained in experiments in which mixing of the reaction solution in the flask was less intensive (for example, when using the same flask, we carried out reaction in the larger volume). Electrochemical Pretreatment of Copper Substrates. Copper substrates were pretreated electrochemically in a conventional three-electrode cell with a large platinum sheet
EXPERIMENTAL SECTION Materials. AgNO3, CuCl2·2H2O, trisodium citrate, 37% hydrochloric acid and 65% nitric acid were purchased from POCH S.A. and were all puriss p.a. reagents. LiCl (puriss p.a.) was obtained from Chemapol. A 30 wt % HAuCl4 solution in dilute HCl (99.99% trace metals basis) was acquired from Mennica Panstwowa. (3-aminopropyl)trimethoxysilane (>97%) and an aqueous solution of sodium silicate (ca. 26.5% of SiO2, reagent grade) were purchased from Sigma-Aldrich. Rhodamine 6 G (>95%), sodium 2-mercaptoethanesulfonate (>98%), and methyl parathion (>99.7%, IUPAC name: O,O-dimethyl-O-(4nitrophenyl) phosphorothioate) were obtained from Fluka. All of the reagents were used without further purification. All glassware and Teflon-coated magnetic stirring bars were cleaned with aqua regia, followed by copious rinsing with purified water. Water was purified with Millipore Milli-Q water system (final resistivity: 18.2 MΩ cm). Preparation of Metal Nanoparticles. Silver sol was prepared according to a modified Lee and Meisel method.30 A 500 cm3 aliquot of a 5 × 10−4 M aqueous solution of AgNO3 was placed in a round-bottom flask and heated to boiling under stirring. Next, 5 cm3 of a 1% solution of sodium citrate was added quickly to the boiling solution, and the mixture was kept boiling gently for 90 min with continuous stirring. Then, the solution was cooled to room temperature and allowed to age overnight in the dark before further use. Gold sol was prepared using the same reduction procedure (which is in principle a slightly modified standard Turkevich’s method of synthesis of gold sols)31 with the only difference that instead of the silver salt, 5 × 10−4 M aqueous solution of HAuCl4 was used. Because the size of metal nanoparticles influences their properties as nanoresonators, and hence influences the intensity of the measured SERS signal (for example, SERS enhancement factors for silver sols obtained by citrate, borohydride or citrate/borohydride methods may differ even by 1 order of magnitude), we present in Figure 1 the size distribution histograms for silver and gold nanoparticles synthesized using above-described methods. 16168
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Figure 2. TEM micrographs of metal clusters covered by a silica layer: (a) two Ag clusters surrounded by a 3 nm SiO2 shell, (b) ca. 100 nm separate Au cluster surrounded by ca. 3 nm silica shell, and (c) two Au clusters covered by ca. 4−6 nm silica layer (in some places in the slit between metal clusters the thickness of the SiO2 layer is significantly larger, up to 12 nm).
as the counter-electrode and a 1 M KCl, AgCl|Ag electrode as the reference electrode (all potentials are quoted versus this electrode). The process was carried out by 3 successive positive−negative scans at a sweep rate of 20 mV s−1 from −0.55 to +0.08 to −0.55 V in a 0.2 M LiCl and 0.01 M CuCl2 solution. After electrochemical cycling, the copper electrode was removed at an open circuit and soaked for 30 min in a 2 M HCl to remove copper oxides. Then, the copper substrate was carefully washed in water. Experimental Techniques. Raman spectra were collected with a Horiba Jobin−Yvon Labram HR800 spectrometer equipped with an Olympus BX40 microscope with a 50× long distance objective, 600 grooves/mm holographic grating, and a Peltier-cooled (1024 × 256 pixel) CCD detector. A diode pumped, frequency doubled Nd:YAG laser (532 nm) and He− Ne laser (632.8 nm) provided the excitation radiation. The TEM studies were performed on electron microscope LIBRA 120 (Zeiss, Germany) equipped with the In-column OMEGA filter and CCD detector, working at an accelerating voltage of 120 kV. The sample of synthesized sol was dropped onto Formvar-coated 400-mesh nickel grids (Agar Scientific) and allowed to dry. UV−vis spectra were collected with a Shimadzu UV-2401PC spectrophotometer.
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RESULTS AND DISCUSSION Optical Properties of the Synthesized Ag@SiO2 and Au@SiO2 Nanoparticles. Figure 3 shows UV−visible absorption spectra of silver and gold nanoparticles before and after deposition of the SiO2 layer on them. For unmodified silver and gold sols used in this contribution, the positions of the characteristic plasmon peaks are at ∼409 and ∼578 nm, respectively (Figure 3). The position of the plasmon peak strongly depends on the actual shape and size of the nanoparticles and hence strongly depends on the synthesis procedure (for example, various groups reported the position of the absorption band for differently synthesized silver sols at 386,36 401,36 426,26 or 480 nm37). Therefore, the “reference” absorption spectra of the “unmodified” metal sols were measured for the sols that were actually covered with the silica layer. After the process of the deposition of thin silica layer on the Ag and Au nanoparticles, one can observe an increase in the absorption in the long wavelength part of the spectrum, and a small blue shift of the position of the plasmon peak (Figure 3). Increase in the absorption of the Ag and Au sols in the red part of the UV−vis spectrum is characteristic for increase in the number of conglomerated metal nanoparticles (for strongly agglomerated particles the absorption in the long wavelength
Figure 3. UV−visible absorption spectra of silver and gold nanoparticles before and after deposition on them of the SiO2 layer: (A) silver nanoparticles and (B) gold nanoparticles.
part of the spectrum strongly increases, whereas the plasmon peak at ca. 400 nm decreases).38 It is also worth noting that the appearance of the second (at longer wavelength) absorption band for the Ag@SiO2 clusters has been already observed by Ung et al., and has been ascribed to the multiple core Ag@SiO2 particles, in which “glued” together Ag nanoparticles are coupled optically.39 “Gluing” of some metal nanoparticles during deposition on them of the silica layer is observed in the respective TEM micrographs. Micrographs a and c in Figure 2 show TEM images of two silver and gold particles fixed 16169
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together with the SiO2 layer. Metal nanoparticles presented in Figure 2 do not directly touch each other (the thickness of the SiO2 layer between two metal cores is about 3 nm), which means that the formation of such “connected” nanoparticles undoubtedly appeared during deposition of the SiO2 layer. After deposition of the silica layer on Ag and Au nanoclusters, one can also observe a small blue shift of the plasmon peak. The refractive index of silica is slightly higher than that of water; therefore, one can rather expect that after coating with SiO2 the plasmon peaks for silver and gold nanoparticles should be red-shifted.26,40 The red shift of the plasmon band has been already observed by other groups when the thickness of deposited SiO2 layer was relatively large (tens of nanometers). For example, Aslan et al. showed that covering of Ag nanoparticles with the 35 nm thick SiO2 layer induces the red shift of the plasmon band by ca. 44 nm.26 Lu et al. found ca. 12 nm red shift for Au nanoclusters covered with the 70 nm thick SiO2 layer.40 On the other hand, Aslan et al. also noticed that the deposition of 2 nm SiO2 layer on silver nanoparticles causes small blue shift of the position of the plasmon band.26 In our opinion the blue shift observed in this contribution and by Aslan et al.26 is caused by the partial “gluing” of metal nanoparticles during deposition of the SiO2 layer. If one assumes that the probability of gluing of two silica-covered metal nanoparticles is proportional to their surface, the largest nanoparticles should glue preferentially, which means that the average size of isolated (not agglomerated) nanoparticles decreases. Because decreasing the average size of silver nanoparticles causes a blue shift of the plasmon absorption band,37 preferential gluing of the largest silica-covered nanoparticles may explain the observed spectral changes. Raman Detection of Methyl Parathion Using Ag@SiO2 and Au@SiO2 Nanoparticles. To verify the applicability of Ag@SiO2 nanoparticles for surface Raman analysis, we carried out detection of residues of methyl parathion (commercially used as an insecticide) on the surface of an orange fruit by means of Raman spectroscopy using Ag@SiO2 and Au@SiO2 nanoresonators. Such an application of Au@SiO2 nanoparticles has been already proposed by Li et al. in the first report concerning the SHINERS technique.14 To compare sensitivity of Raman analysis carried out with different nanoresonators, one has to control the amount of nanoresonators, which is deposited on the analyzed surface. We were unsuccessful in controlling the amount of Ag@SiO2 and Au@SiO2 nanoparticles deposited as the solid phase (as “dust”); therefore, we decided to deposit Ag@SiO2 and Au@SiO2 nanoparticles on the surface of an orange fruit as the aqueous suspensions of Ag@SiO2 and Au@SiO2 nanoparticles (the same volumes of the suspensions of Ag@SiO2 and Au@SiO2 have been obtained from the same number of moles of AgNO3 and HAuCl4; see the Experimental Section). Figure 4 shows Raman spectra of clean and contaminated with methyl parathion skins of an orange fruit recorded with 632.8 nm excitation radiation, and the Raman spectrum of solid methyl parathion. Raman measurements for clean and “contaminated” skins of orange fruits were carried out for samples before and after covering with the Ag@SiO2 or Au@SiO2 nanoparticles. All recorded spectra of skins of the orange fruit are dominated by two bands at 1156 and 1528 cm−1, attributed to carotenoid molecules contained in the orange skin (Figure 4).14 As can be also seen in Figure 4, the are no significant difference between the normal Raman spectrum of the “pure” orange skin and the spectrum of the skin contaminated with
Figure 4. Raman spectra of (a) the skin of the orange fruit, (b) the skin of the orange fruit contaminated by methyl parathion, (c) solid methyl parathion, (d) the skin of the orange fruit covered with Ag@ SiO2, (e) the skin of the orange fruit contaminated by methyl parathion and covered with Ag@SiO2, (f) the skin of the orange fruit covered with Au@SiO2, and (g) the skin of the orange fruit contaminated by methyl parathion and covered with Au@SiO2. Excitation radiation λexc = 632.8 nm. tac denotes the accumulation time of the spectrum. Spectra are vertically shifted to enhance the clarity of presentation.
methyl parathion. However, when the surface of the fruit is covered with Ag@SiO2 or Au@SiO2 nanoparticles, in the Raman spectrum recorded from the “contaminated” surface of the fruit one can clearly identify the band at 1350 cm−1 due to the pesticide molecules. As can be seen in Figure 4, the relative intensity of the methyl parathion band at 1350 cm−1 in the spectrum measured with Ag@SiO2 nanoparticles is not lower than in the spectrum measured with Au@SiO2 clusters. It means that Ag@SiO2 clusters may be successfully used instead of the Au@SiO2 nanoparticles for surface Raman characterization. Because the actual SERS enhancement factor depends on many parameters (size and shape of the metal core, the thickness of the SiO2 layer), which are all slightly different for Ag@SiO2 and Au@SiO2 nanoparticles (Figure 2), more quantitative comparison of the Ag@SiO2 and Au@SiO2 nanoresonators is not possible at the present stage. Silver nanoparticles support plasmon resonance for any visible excitation radiation, whereas gold nanoparticles support plasmon resonance only for red and infrared excitation radiation.8 Therefore, SHINERS measurements with Ag@ SiO2 nanoparticles may be carried out using significantly broader range of the excitation radiation. One can demonstrate it, for example, by carrying out Raman detection of methyl parathion on the “contaminated” orange skin with the green excitation radiation of 532 nm. We found that, when Ag@SiO2 nanoparticles are deposited on the “contaminated” orange fruit, in the spectra recorded using 532 nm excitation radiation the band at 1350 cm−1 due to molecules of methyl parathion can be identified. However, because Au clusters do not support plasmon resonance at this frequency, we were not able to 16170
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In measurements with Au@SiO2 nanoparticles, the green excitation radiation does not support surface plasmon resonance, and hence, there is no SERS enhancement for the spectra measured with 532 nm excitation radiation. However, when the SERS effect for the gold nanoresonators is “switched on” in measurements with the red excitation radiation, the resonance Raman effect for R6G molecules is “switched off” (the inset in Figure 5 shows absorption spectrum of the aqueous solution of R6G). Therefore, very sensitive detection of R6G is not possible when Au@SiO2 nanoresonators are used. On the other hand, using Ag@SiO2 nanoparticles allows us to have both the plasmon resonance for metal nanoresonators and the resonance Raman effect for analyte working simultaneously, and hence very sensitive detection of R6G is possible in this case. Diffusion of the Analyte via the Silica Layer. As mentioned in the Introduction, various Ag@SiO2 nanoparticles have been already synthesized and some of their properties have been investigated.25,39 For example, previous works showed that various molecules may diffuse via the silica shell in spite of relatively large thickness of used SiO2 layers (even above 0.1 μm), and then interact directly with the metal core (in some cases a chemical reaction between molecules that passed through the silica layer and the metal core has been observed).39 The diffusion coefficients for cyanide ions and R6G via the SiO2 layer were estimated as equal to about 10−16 and 3 × 10−18 m2 s−1, respectively.39,25 On the other hand, previous contributions show that silica shells deposited on gold cores using a very similar deposition method are pinhole-free (at least in SHINERS measurements).14,15 It suggests a strong sensitivity of the essential properties of formed silica layers on a small variation of the deposition procedure. Therefore, a simple method for testing the integrity of silica shells should be developed. We propose using thiols for such “test” measurements because it is possible to easily discriminate between thiol molecules in the solution (or adsorbed on the SiO2 layer) and those directly adsorbed on the metal surface. As a model thiol we chose substituted aliphatic thiol easily soluble in water: sodium 2-mercaptoethanesulfonate (MES). The Raman spectra of thiols in the spectral region between 2000 and 2700 cm−1 are dominated by a very strong band at ca. 2560−2580 cm−1 due to the S−H stretching vibration.44,45 However, when thiols attach to such metals as gold or silver, they react chemically with them forming very stable metal− sulfur bond,41−45 and the band due to the S−H stretching vibration is, of course, not observed in the Raman spectra of chemisorbed thiols.44,45 Figure 6 shows Raman spectra measured from a thin layer of Ag@SiO 2 or Au@SiO 2 nanoparticles on the glass substrate immersed in a 10 mM MES aqueous solution, and the Raman spectrum of a 1 M MES aqueous solution. As can be seen in Figure 6, Raman spectra of MES adsorbed on Ag@SiO2 and Au@SiO2 nanoclusters are significantly different than the Raman spectrum of the MES aqueous solution. The most prominent difference between the spectra enhanced by the layer of Ag@SiO2 or Au@SiO2 nanoparticles and the Raman spectrum of MES aqueous solution is the lack of any observable band due to the S−H stretching vibration in the range between 2500 and 2600 cm−1, in the nanoparticles-enhanced spectra, which is clearly perceptible in the Raman spectrum of MES aqueous solution at 2583 cm−1. The absence of the S−H stretching band indicate (i) a chemisorption of MES via its thiol group at the surface of silver and gold cores or (ii) a formation of disulfides on the
identify such band in the spectra measured from the surface of the “contaminated” orange fruit covered with Au@SiO2 nanoparticles. Combination of the Surface Plasmon Resonance with the Resonance Raman Scattering. Because Ag clusters support surface plasmon resonance in significantly wider wavenumber range than Au nanoparticles, it should be easier to combine the plasmon resonance of the metal core with the resonance Raman scattering of an analyte when Ag@SiO2 nanoresonators are used instead of the Au@SiO2 nanoparticles. We decided to combine the resonance Raman effect for the analyte with the plasmon resonance for metal nanoresonators to detect rhodamine 6G (R6G) deposited on the surface of the platinum sheet. Before Raman measurements the Pt sheet was immersed for 10 s in a 10−8 M R6G aqueous solution. Figure 5
Figure 5. Raman spectra of platinum surface covered with R6G molecules. (a)−(c) Spectra measured with 632.8 nm excitation radiation. (d)−(f) spectra measured with 532 nm excitation radiation. (b, e) Spectra measured for Pt/R6G samples covered with Au@SiO2 nanoparticles. (c, f) Spectra measured for Pt/R6G samples covered with Ag@SiO2 nanoparticles. Raman spectra are vertically shifted to enhance the clarity of presentation. Inset shows absorption spectrum of the aqueous solution of R6G with the marked positions of the radiation of 532 and 632.8 nm.
shows Raman spectra of the platinum surfaces covered with R6G molecules recorded with green (532 nm) and red (632.8 nm) excitation radiation. The measurements have been carried out before and after deposition of Ag@SiO2 and Au@SiO2 nanoparticles on the analyzed surfaces. As can be seen in Figure 5, the intensity of the spectrum measured for the Pt/R6G sample covered with Ag@SiO2 nanoparticles is high, and many bands of R6G are well visible. When the spectrum of the Pt/ R6G sample covered with Ag@SiO2 nanoparticles is recorded using excitation radiation of 632.8 nm, the Raman peaks of rhodamine 6G are very weak (Figure 5) despite using in both measurements the laser beams with similar powers at the sample. Also in experiments carried out with gold nanoresonators and in measurements for Pt/R6G samples which are not covered with any metal nanoresonators, the Raman peaks of rhodamine 6G can be hardly seen (Figure 5). 16171
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silver surface49). We were able to observe a good quality Raman spectrum of R6G adsorbed on silver even after several cycles of washing of the silver substrate in water. Roughened silver surface is highly SERS-active for any visible excitation radiation; so, it would be very difficult to demonstrate the SHINERS effect on this substrate. Therefore, we decided to verify whether an analogous effect is observed for R6G on copper (it means whether part of R6G molecules deposited on the copper is also strongly trapped on the surface). Copper belongs also to the IB group; however, is not active in SERS measurements when a green excitation radiation is used (strong SERS effect on copper is only observed for red or infrared excitation radiation; therefore, when green excitation radiation is used, coupling between the surface plasmons in the copper and in the silver particles may be excluded).1,11 Spectrum a in Figure 7 shows normal Raman spectrum measured on the roughened copper substrate soaked for 1 min
Figure 6. (a) Raman spectrum of MES adsorbed on Ag@SiO2 nanoparticles, (b) Raman spectrum of MES adsorbed on Au@SiO2 nanoparticles, and (c) Raman spectrum of 1 M MES aqueous solution. Spectra are vertically shifted to enhance the clarity of presentation.
surface of nanoresonators. Aliphatic disulfides give strong Raman band in the region around 510 cm−1.46 However, as can be seen in Figure 6, in the recorded SERS spectra there is no band that can be ascribed to the S−S stretching vibration. It means that one can exclude that the disappearance of the ν(S− H) band is due to the formation of disulfides. Other spectral differences between normal Raman spectrum of MES aqueous solution and the nanoparticles-enhanced spectra also strongly support the hypothesis of the direct chemisorption of MES molecules on the metal cores. For example, the band due to the ν(C−S) stretching vibration (where S is from the thiol group) of the trans conformer of MES molecules (this band is marked by an asterisk in Figure 6) is observed in the spectra measured on clusters with the silver core at 705 cm−1, in the spectra measured on clusters with the gold core at 732 cm−1, whereas in the spectra of MES aqueous solution this band is at 750 cm−1 (for bands assignments see refs 44 and 45). This significant shift toward lower wavenumbers of the ν(C−S) stretching band in the nanoparticles-enhanced spectra is characteristic for the direct chemisorption of thiol molecules on the metal surface and is related to a withdrawal of electron density from the C−S bond because of bonding of the thiol sulfur to the metal surface.43,46−48 SHINERS Measurements for Strongly Adsorbed Analytes. In the previous contribution we investigated roughened silver surfaces covered with R6G molecules.49 We found that part of R6G molecules (probably located in the slits between metal nanoparticles) remain on the silver surface even after many cycles of washing in water (in other words, part of R6G molecules are adsorbed very strongly on nanostructured
Figure 7. (a) Raman spectrum of the electrochemically roughened copper substrate covered with the submonolayer of R6G molecules. (b) Raman spectrum of the electrochemically roughened copper substrate covered with the submonolayer of R6G molecules on which some Ag@SiO2 nanoparticles have been deposited. (c) Raman spectrum of R6G adsorbed on the Ag nanoparticles. Excitation radiation λexc = 532 nm. Spectra are vertically shifted to enhance the clarity of presentation.
in a 10−7 M R6G aqueous solution, and then carefully rinsed in water also for 1 min. As can be seen in Figure 7, in the Raman spectrum of such a surface one can only identify broad bands at ca. 540 and 620 cm−1 due to the copper oxides.50−52 However, after deposition of Ag@SiO2 nanoresonators on the investigated Cu/R6G surface, one can observe also bands characteristic for R6G molecules in the measured Raman spectra (spectrum b in Figure 7). The Raman spectrum recorded from the Cu/R6G surface covered by Ag@SiO2 nanoparticles is, however, in many details different from the spectrum of R6G directly adsorbed on the silver nanoparticles (the spectrum of R6G adsorbed on the Ag nanoparticles is presented as spectrum c in Figure 7). For example, the ratio of intensities 16172
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The Journal of Physical Chemistry C of the R6G bands at 610 and 1652 cm−1 (these bands are due to the C−C−C ring in-plane bending, and aromatic C−C stretching vibrations, respectively)53,54 is equal to 6.2 in the spectrum of R6G adsorbed on silver, and 1.4 in the spectrum measured for the Cu/R6G/Ag@SiO2 system. The band observed in the spectrum of R6G adsorbed on silver at 1503 cm−1 (this band is due to the aromatic C−C stretching vibration)53,54 shifts to 1513 cm−1 when R6G is adsorbed on copper. The observed differences suggest that for the Cu/R6G/ Ag@SiO2 system R6G molecules remain adsorbed on the copper surface and do not interact directly with the silver nanoparticles that provide strong local enhancement of the electromagnetic fieldand hence the increase of the intensity of the measured Raman spectrum of R6G. It means that when adsorbates are strongly bonded to the analyzed surface, SHINERS measurements can be realized even using metal clusters covered with silica layers containing some “pinholes” (see the previous paragraph).
ACKNOWLEDGMENTS
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REFERENCES
We thank dr. Marianna Gniadek for assistance in TEM measurements. This project was financed from the funds of the National Science Centre (Poland) allocated on the basis of the decision number DEC-2011/01/B/ST5/03870. S.W. is thankful for the support from the Foundation for Polish Science MPD Programme cofinanced by the EU European Regional Development Fund. The TEM studies have been carried out using research equipment (electron microscope LIBRA 120) purchased under the CePT project cofinanced by European Union from the European Regional Development Fund under the Operational Programme Innovative Economy 2007−2013.
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CONCLUSIONS In this contribution we report the first SHINERS measurement with the silica-covered silver nanoresonators. Even though TEM images show that formed SiO2 layers are compact, the molecules of analyte may diffuse through some silica layers deposited on the surface of gold and silver nanoparticles, and then interact directly with the metal core. However, other contributions14,15 show that silica shells deposited on gold cores using a very similar deposition method are pinhole-free. It suggests a strong sensitivity of the essential properties of formed silica layers on a small variation of the deposition procedure. We also show that, when adsorbates are strongly bonded to the analyzed surface, “actual” SHINERS measurements can be even realized using metal clusters covered by silica layers containing some “pinholes”. The advantage of Ag@SiO2 nanoresonators instead of Au@ SiO2 nanoresonators originally used for SHINERS measurements is that silver provides stronger plasmon resonance than gold, and that plasmon resonance, and hence strong SERS effect, may be achieved on silver for any visible electromagnetic radiation, whereas for gold substrates efficient resonance (and hence strong SERS effect) may be only obtained with the red excitation radiation. It means that using Ag@SiO2 nanoresonators allows for easier combination of the SHINERS technique with standard resonance Raman scattering. Hence, we suppose that using Ag@SiO2 nanoresonators will broaden significantly the field of applications of isolated SHINERS analysis. Raman spectrometers have recently significantly profited from technical development and now it is possible to construct low-cost, battery-powered, portable Raman spectrometers, which have many of the spectral capabilities of laboratorybased systems. Taking that into account, we anticipate that SHINERS technique may be widely used for many on-site practical analysis, like the inspection of food safety and environment pollutants.
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The authors declare no competing financial interest. 16173
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