Silica-Protected Hollow Silver and Gold Nanoparticles: New Material

Article pubs.acs.org/JPCC

Silica-Protected Hollow Silver and Gold Nanoparticles: New Material for Raman Analysis of Surfaces Heman Burhanalden Abdulrahman, Jan Krajczewski, Dorota Aleksandrowska, and Andrzej Kudelski*

Downloaded by TEXAS A&M INTL UNIV on August 28, 2015 | http://pubs.acs.org Publication Date (Web): August 19, 2015 | doi: 10.1021/acs.jpcc.5b03556

Faculty of Chemistry, Warsaw University, ul. Pasteura 1, 02-093 Warsaw, Poland ABSTRACT: The first example of Raman analysis of various surfaces using hollowsilver (h-Ag) and hollow-gold (h-Au) nanoparticles protected by the silica layer has been reported. Synthesized h-Ag@SiO2 and h-Au@SiO2 nanoparticles are efficient electromagnetic resonators, which locally enhance the electric field of the incident radiation. This effect leads to an increase of the efficiency of Raman scattering for molecules being in the close proximity to h-Ag@SiO2 and h-Au@SiO2 nanoresonators. In contrast to solid spherical Au nanoparticles typically used for construction of nanoresonators for shell-isolated nanoparticle-enhanced Raman scattering (SHINERS) measurements, for which a change of the diameter of the nanoparticle causes only a small change of the position of its plasmon band, the plasmon band for hollow spherical Au nanoparticles can be changed within broad range of the visible electromagnetic radiation. It suggests that silica-covered h-Au nanoparticles can be used in the broader wavelength range of the electromagnetic radiation than previously used “solid” nanoresonators. We also found that the samples of hollow Ag nanoparticles induce, on average, stronger enhancement of the Raman signal than the respective samples of solid nanoresonators synthesized from the same amount of metal. Furthermore, the efficiency of h-Ag@SiO2 and similar h-Au@SiO2 nanoresonators has been compared for the enhancing of Raman spectrum of 4-mercaptobenzoic acid (MBA) chemisorbed on the flat gold surface. For the excitation radiation of the wavelength of 632.8 nm, the increase of the Raman signal from MBA molecules that are in a close proximity to h-Ag@SiO2 nanoparticles is larger by ca. 2 orders of magnitude than when h-Au@SiO2 nanostructures are used.

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INTRODUCTION Analysis of surfaces of various materials is very important from the scientific and economic point of view. Surface analysis is especially difficult for so-called buried interfaces (for example, the surface of the solid sample submerged in the high pressure gas or the liquid). Important examples of such interfaces are interfaces of biological objects in situ. Raman scattering is one of the tools that are used for analysis of buried interfaces. A typical cross section for Raman scattering is very small (about 11 orders of magnitude smaller than the typical cross sections for UV−vis absorption).1 The efficiency of Raman scattering may be, however, increased significantly. In the 1950s, Shorygin reported that if the wavelength of the excitation radiation is tuned to the absorption band of the studied compound, the intensity of the measured Raman signal may increase even by 6−7 orders of magnitude.2 In the 1970s it was realized that the efficiency of Raman scattering can be also increased by 6−7 orders of magnitude using metallic nanoresonators.3,4 This nanoresonator induced effect was called SERS (acronym for surface-enhanced Raman scattering).3,4 When both resonance and nanoresonator-induced effects are combined, the efficiency of Raman scattering can be even about 15 orders of magnitude larger than in a “standard” Raman experiment. This makes possible observation of Raman spectra even of a single molecule,5−7 and hence, Raman spectroscopy is one of the most sensitive analytical tools, suitable even for analysis of “buried interfaces”. © 2015 American Chemical Society

For many years nanoresonator-enhanced Raman experiments have been carried out only on surfaces of so-called SERS-active metals (metals from which effective nanoresonators could be formed), sometimes covered with a thin layer of other metal.8−11 In 2000, Anderson12 and Zenobi et al.13 showed that it is possible to use external electromagnetic nanoresonator, which is brought to the proximity of the analyzed surface with the atomic force microscope or the scanning tunneling microscope. This approach is however difficult and expensive. In 2010, Tian et al. reported a significantly easier and cheaper method of Raman analysis of in principle any solid surface.14 In this method on the investigated surface the layer of Au nanoparticles protected by a thin layer of SiO2 or Al2O3 is deposited, and then the Raman spectrum of the analyzed surface is recorded. As mentioned above, metal nanoparticles act as electromagnetic resonators, significantly enhancing the electric field of the incident radiation and hence leading to a large increase of the intensity of Raman signal measured from the surface on which nanoparticles have been spread. A 2−4 nm thick layer of silica or alumina does not significantly damp electromagnetic enhancement, while separating metal nanoresonators from direct contact with the analyzed surface and keeps metal nanoparticles from agglomerating. This new Raman analytical method developed by Tian et al. is called Received: April 13, 2015 Revised: July 24, 2015 Published: August 12, 2015 20030

DOI: 10.1021/acs.jpcc.5b03556 J. Phys. Chem. C 2015, 119, 20030−20038

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The Journal of Physical Chemistry C

nanocrystals. Then, Ag2O nanoparticles were reduced by NaBH4. 0.18 mL of 10 mM freshly prepared NaBH4 solution was introduced at once, and the reaction mixture had turned orange. Analysis of transmission electron microscopic images of obtained h-Ag nanoparticles revealed that their average outer diameter is 31 ± 7 nm. Hollow gold nanospheres with various interior-cavity sizes were synthesized using cobalt nanoparticles as sacrificial templates. The size of the cavity was change by varying the molar ratio of used reagents.49 First, the Co nanoparticles were fabricated by reduction of CoCl2 with NaBH4 in the presence of sodium citrate. Briefly, 100 mL of water, 0.6 mL of 0.1 M sodium citrate solution, and 0.2 mL of 0.4 M CoCl2 solution were placed into a three-neck flask. Then, the reaction vessel was closed and carefully deareated by bubbling with argon for 20 min. To this, under rapid stirring, 0.1 mL of 1 M NaBH4 solution was added. The solution changed color from pale pink to brown/gray, which indicates formation of Co nanoparticles. The Co sol was allowed to react under constant argon flow for ca. 60 min. Then, the obtained cobalt sol has been added to the solutions of HAuCl4. 30 mL of the Co sol solution has been added to 10 mL of water, to which various amounts (between 0.025 and 0.17 mL) of 0.1 M HAuCl4 have been added (so, the ratio of numbers of moles of Au and Co was between 0.1:1 and 0.7:1). This solution was exposed to air, which caused complete oxidation of Co core leaving hollow Au nanoparticles. Samples with remaining Co cores are brown, whereas oxidized samples are blue, purple, or red depending on the molar ratio of CoCl2 and HAuCl4. Analysis of transmission electron microscopic images of synthesized gold nanostructures revealed that their average outer diameter is 27 ± 7 nm. We found that the diameter of formed gold structures is independent of the amount of added HAuCl4 when using the same cobalt sacrificial templates nanoparticles. “Solid” silver nanoparticles were prepared according to a modified method proposed by Lee and Meisel.50 100 mL of 5 × 10−4 M aqueous solution of AgNO3 was placed in a flask and heated to boiling under stirring. Next, 1 mL of a 1% solution of sodium citrate were quickly added to the boiling solution. The mixture was kept boiling for 90 min under stirring. Analysis of transmission electron microscopic images of obtained “solid” Ag nanoparticles revealed that their average diameter is 35 ± 8 nm. “Solid” gold nanoparticles were prepared using slightly modified standard Turkevich’s method of synthesis of gold sols,51 which is in principle the same procedure as presented above with the only difference that, instead of AgNO3, aqueous solution of HAuCl4 has been used. Deposition of Silica Layer on Metal Nanoparticles. For coating Au or Ag nanoparticles with SiO2 layer two silica deposition methods were used. In the first method SiO2 was produced by decomposition of Na2SiO3, whereas in the second method by decomposition of tetraethyl orthosilicate. Deposition of SiO2 from acidified solution of Na2SiO3 has been carried out very similarly to the procedure developed by Mulvaney et al.52 (a similar procedure has been used, for example, by Tian et al.14). Briefly speaking, to 120 mL of the metal sol 1.6 mL of a 1 mM aqueous solution of (3aminopropyl)trimethoxysilane was added under stirring. The obtained solution was stirred for 15 min. Then, 12.8 mL of 0.09 M aqueous solution of Na2SiO3 with pH adjusted to 10−11 by addition of HCl was added. Subsequently, the reaction solution was kept for 2−6 days in room temperature under stirring. The obtained silica-covered nanoparticles were cleaned by centrifug-

SHINERS (acronym for shell-isolated nanoparticle-enhanced Raman spectroscopy).14 Already in the first report concerning SHINERS measurements Tian et al. showed Raman spectra of various adsorbates on the surfaces of platinum and gold single crystals and even some Raman spectra of biological samples, as surfaces of orange fruits with methyl parathion residues or even walls of living cells.14 For the first SHINERS experiments only gold spherical nanoresonators have been used.14−22 In 2012, Tian et al.23 and Kudelski and Wojtysiak24 showed that using silver nanoresonators instead of gold one allows for large increase of the sensitivity of SHINERS measurements. The other interesting modification of the SHINERS measurements was using inhomogeneous metal nanoresonators (nanorods and nanocubes)25−27 instead of the spherical one. For some other examples of practical applications of SHINERS measurements the reader is referred to refs 28−42. In this paper we report on modification of SHINERS measurements by using hollow gold (h-Au) and hollow silver (h-Ag) nanoparticles as electromagnetic nanoresonators; in other words, we show the first example of using of h-Ag@SiO2 and h-Au@SiO2 nanoparticles for the SHINERS measurements. Hollow nanostructures exhibit surface plasmonic properties different (and, in some cases, superior to) their solid counterparts.43−47 For example, the position of surface plasmon resonance band of gold nanoshells can be changed by varying the shell diameter and its thickness in the significantly broader wavelength range than the plasmon band of solid spherical gold nanoparticles.43,45−47 The observed significant red-shift of the plasmon band for hollow gold nanoparticles facilitates various biomedical application of such nanostructures, since it allows to tune maximum absorption/scattering of nanoparticles to the transparent window of many biological tissues (800−1200 nm).45−47 Therefore, h-Ag@SiO2 and hAu@SiO2 nanoresonators may be especially useful in SHINERS experiments in which using excitation radiation with longer wavelength is preferred.

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EXPERIMENTAL SECTION Materials. Trisodium citrate dihydrate, isopropanol, 37% hydrochloric acid, 25% ammonia aqueous solution, CoCl2, AgNO3, and NaOH (all pure p.a.) were acquired from POCH S.A. O,O-Dimethyl-O-(4-nitrophenyl)phosphorothioate (common name: methyl parathion, >99.7%) and NaBH4 (≥99%) were purchased from Fluka. 4-Mercaptobenzoic acid (>99%), Lglutathione (>98%), tetraethyl orthosilicate (>99%), aqueous solution of Na2SiO3 (ca. 26.5% of SiO2, reagent grade), and (3aminopropyl)trimethoxysilane (>97%) were acquired from Sigma-Aldrich. HAuCl4 (30% solution in dilute HCl, 99.99% trace metals basis) and gold and platinum sheets (99.999%) were purchased from Mennica Państwowa. All of the reagents were used as received. Argon (≥99.999%) was purchased from Air Products. Water (with resistivity of 18.2 MΩ cm) was purified with Millipore Milli-Q system. Preparation of Metal Nanoparticles. Hollow silver nanoparicles were prepared according to a modified method proposed by Ben Moshe and Markovich.48 The preparation of the hollow Ag nanoparticles starts with the preparation of Ag2O nanoparticles: to 2.6 mL of ice cold water aqueous solution of AgNO3 (0.15 mL, 10 mM) an aqueous solution of glutathione (0.018 mL, 10 mM) was added. Subsequently, an aqueous solution of NaOH (0.5 mL, 0.1 M) was added while vigorously stirring. The pH of the solution increased to ca. 12; the solution had turned pale yellowish, signifying the formation of Ag2O 20031


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LIBRA 120 electron microscope working at an accelerating voltage of 120 kV. The electron microscope was equipped with the In-column OMEGA filter. The sample of obtained nanoparticles was deposited onto 400-mesh nickel grids coated by the Formvar layer. UV−vis spectra were recorded using a Shimadzu UV-2401PC spectrophotometer. Raman measurements were carried out using a Horiba Jobin-Yvon Labram HR800 spectrometer equipped with a Peltier-cooled CCD detector (1024 × 256 pixel), 600 grooves/mm holographic grating, and an Olympus BX40 microscope with a long distance 50× objective. A He−Ne laser provided the excitation radiation with the wavelength of 632.8 nm, and a diode pumped, frequency doubled Nd:YAG laser provided the excitation radiation with the wavelength of 532 nm.

ing, pouring off the supernatant, and again suspending of nanoparticles. Such a cleaning procedure was repeated four times. The average thickness of the SiO2 shells formed during the described above procedure is usually between 3 and 6 nm (an example of ca. 5 ± 1 nm thick layer of SiO2 on Ag nanoparticles formed during 6 days incubation is presented in Figure 1a).


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RESULTS AND DISCUSSION

Size and Optical Properties of Obtained Hollow Metal Nanoparticles. Figure 2A shows the UV−vis spectra and the visual appearance of Au colloids, and Figure 2B shows the respective TEM images of Au nanoparticles synthesized by the galvanic replacement reaction (see eq 1) between Co sol and HAuCl4.

Figure 1. TEM micrographs of silver clusters covered by a SiO2 layer: (a) silica layer formed in the acidified Na2SiO3 aqueous solution during 6 days; (b) silica layer formed in the isopropanol/water/ ammonia solution of tetraethyl orthosilicate during 1 h.

SiO2 deposition by decomposition of tetraethyl orthosilicate was carried out according to the procedure developed by Shanthil et al.53 Briefly speaking, metal sol in water was introduced to the 9 times larger volume of isopropanol under vigorous stirring. 1.9 mL of 25% ammonia aqueous solution and 40 μL of 50% tetraethyl orthosilicate solution were added to 100 mL of the above-mentioned mixture of metal sol and isopropanol. The obtained silica-covered nanoparticles were cleaned by centrifuging as in the previous synthesis. Using this method, one can obtain relatively quickly thick silica layers (Figure 1b shows ca. 30 ± 10 nm thick layer of SiO2 on Ag nanoparticles formed during only 1 h); however, this deposition process is significantly more difficult to control than SiO2 deposition by decomposition of Na2SiO3. Experimental Techniques. The transmission electron microscopy (TEM) analysis were carried out with a Zeiss

Figure 2. (A) UV−vis extinction spectra and visual appearance of gold colloids obtained by the galvanic replacement reaction between Co colloid and HAuCl4. For synthesis of all h-Au sols the same Co colloid (as a source of sacrificial templates Co nanoparticles) has been used. The ratio of numbers of moles of HAuCl4 and Co was equal to (a) 0.10:1, (b) 0.15:1, (c) 0.25:1, (d) 0.33:1, and (e) 0.50:1. (B) TEM images of gold nanoparticles obtained by the galvanic replacement reaction. The ratio of numbers of moles of HAuCl4 and Co was equal to (B1) 0.10:1, (B2) 0.25:1, and (B3) 0.50:1. For synthesis of all gold nanoparticles, the same Co colloid (as a source of sacrificial templates Co nanoparticles) has been used. 20032


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3Co + 2AuCl4 − → 3Co2 + + 8Cl− + 2Au

(1)

The galvanic replacement reaction has been carried out for various ratios of moles of HAuCl4 and Co (between 0.1:1 and 0.7:1). If the amount of added HAuCl4 was not sufficient to oxidize all metallic cobalt (the ratio of moles of HAuCl4 and Co below 0.67:1), in the next step of the process the remaining cobalt cores have been oxidized by the oxygen from air. We noticed on the basis of the TEM measurements that if the amount of added HAuCl4 was sufficient to react stoichiometricaly with Co nanoparticles, one can hardly see any nanoparticle with the interior cavity (three atoms of cobalt are replaced by two atoms of gold; however, the volume of 2 mol of gold is by ca. 3% larger that the volume of 3 mol of cobalt); see Figure 2B. Analysis of TEM images of gold nanoparticles obtained for different ratios of moles of HAuCl4 and Co revealed that if the ratio of moles of HAuCl4 and Co is above ca. 0.4:1, the majority of the formed nanostructures is without any visible hollow interior. If the ratio of moles of HAuCl4 and Co is below 0.33:1, nanoparticles with hollow interior clearly dominate in the obtained product (see Figure 2B). On the basis of TEM results, we calculated that if the galvanic replacement reaction has been carried out for the ratio of moles of HAuCl4 and Co equal to 0.1:1, the average volume of the hollow interior is equal to ca. 37 ± 4% of the total volume of the formed nanostructures. For the nanostructures obtained for the ratio of moles of HAuCl4 and Co equal to 0.25:1, the average volume of the hollow interior decreases to ca. 15 ± 2% of the total volume of the formed nanostructures. As can be seen from Figure 2A, the size of the hollow interior of gold nanostructures influences the coloration of their suspensions. The color of samples with large hollow interior is blue, while the samples with smaller hollow interior are purplish, red, and pink (pink color is observed also for the samples composed mainly of solid nanoparticles); see Figure 2A. This change of color of h-Au nanoparticles is due to change of their plasmonic properties. h-Ag nanoparticles have been synthesized by fast chemical reduction of silver oxide nanocrystals. Figure 3A shows TEM image of the obtained h-Ag nanostructures. This method of synthesis of h-Ag nanoparticles does not allow for the control of the size of the hollow interior. On the basis of TEM images of obtained nanostructures, we calculated that the average volume of the hollow interior is equal to ca. 32 ± 3% of the total volume of the formed nanostructures. The color of the aqueous suspension of the obtained h-Ag nanoparticles is orange, whereas the color of the aqueous suspension of the solid Ag nanoparticles having similar external diameter (ca. 25− 50 nm) is yellow or yellow-green (see Figure 3B). Optical Properties of the Obtained h-Ag@SiO2 and hAu@SiO2 Nanoparticles. As mentioned in the Experimental Section, the h-Ag and h-Au nanoparticles have been coated with silica layer using two different methods. In the first method SiO2 has been produced by decomposition of Na2SiO3 in the acidified solution and in the second method by decomposition of tetraethyl orthosilicate in the solution containing ammonia. On the basis of the TEM analysis of sols before and after deposition of SiO2 layer, we found that both methods of silica deposition do not cause significant (above 10%) change of the averaged diameter of the covered hollow metal nanostructures. On the other hand, Xue et al.54 showed that etching of silver nanoparticles is observed during deposition on them of SiO2 from the solution containing

Figure 3. (A) TEM image of synthesized silver hollow nanoparticles. (B) UV−vis absorption spectra and visual appearance of silver sols containing (a) solid and (b) hollow nanoparticles with the diameter of ca. 25−50 nm.

ammonia. It means that probably a part of metallic silver is dissolved during deposition of SiO2 by the decomposition of tetraethyl orthosilicate in the solution containing ammonia; however, due to quick formation of the protecting SiO2 layer, the silver dissolution process is quickly stopped and the changes in the structure of silver cores are not significant. The example TEM images of h-Ag@SiO2 nanoparticles are presented in Figure 4a,b, and the image of h-Au@SiO2 nanoparticle is given in Figure 4c. The thickness of the silica layer produced by decomposition of Na2SiO3 is relatively easy to control (see Figure 4, image a), whereas decomposition of tetraethyl orthosilicate allows for quick formation of relatively thick layers (see Figure 4, image b). Figures 5A and 5B show example UV−vis extinction spectra of h-Ag and h-Au nanoparticles, respectively, before and after deposition on them of SiO2 layer. Since the position of the plasmon peak depends on the size and shape of the nanoparticles, and therefore depends on the synthesis procedure (for example, Figure 2 shows how different the visual appearance of various gold colloids can be), the “reference” extinction spectra of the “unmodified” samples of metal nanoparticles were recorded for the sols that were later covered with the SiO2 layer. As can be seen in Figures 5A and 5B, after deposition of ca. 5 ± 2 nm thick SiO2 layer on h-Ag and h-Au nanoclusters, the position of the plasmon peak does not change significantly, although its width increases. The refractive index of SiO2 is higher than that of water; therefore, from a simple model of the plasmon resonance in metal nanoparticles one can expect that coating with the silica layer should induce the red-shift of the plasmon bands for metal nanoparticles.55,56 We observed red-shift of the plasmon band of metal nanoparticles only when the thickness of the SiO2 shell layer was relatively large; for example, Figure 5A shows red-shift by 20 nm of the plasmon band for silver hollow nanoparticles 20033


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Figure 5. (A) UV−vis extinction spectra of h-Ag nanoparticles before deposition of SiO2 (a1), covered with ca. 5 ± 2 nm thick layer of SiO2 (a2), and covered with ca. 30 ± 10 nm thick layer of SiO2 (a3). (B) UV−vis absorption spectra of “red” h-Au nanoparticles before deposition of SiO2 (b1) and covered with ca. 5 ± 2 nm thick layer of SiO2 (b2). Inset: UV−vis absorption spectra of “blue” h-Au nanoparticles before deposition of SiO2 (c1) and covered with ca. 30 ± 10 nm thick layer of SiO2 (c2).

from the respective TEM micrographs. For example, TEM image of two h-Ag nanoparticles fixed together with the silica layer is presented in Figure 4a. As can be seen in this micrograph, h-Ag nanoparticles do not directly touch each other, which suggests that the formation of such agglomerates appeared during deposition of silica. Comparison of the Efficiency of Solid and Hollow Nanoresonators. To compare efficiency of solid and hollow metal nanoresonators, we decided to record resonatorenhanced Raman spectra of monolayers formed on gold substrates from 4-mercaptobenzoic acid (MBA) and covered by hollow or solid Ag nanoparticles. To eliminate possible enhancement of the electromagnetic field from nanometersize defects of the gold substrates, Au substrates have been flame-annealed before deposition of the organic monolayer. MBA chemisorbs very strongly on the gold surface because the −SH group of MBA reacts chemically with Au forming stable Au−S bond (analogous chemisorption of thiols is also observed on other metals, such as Ag, Cu, or Pt).58−60 Therefore, only small part of the chemisorbed MBA molecules are desorbed from the metal surface during even 1 h soaking of the MBAmodified gold substrate in pure water.58 The efficiency of the enhancing of the Raman signal by hollow and solid nanoresonators has been compared before deposition of the SiO2 layer on their surfaces because even small changes of the thickness of the deposited oxide layer may cause significant changes of the efficiency of the obtained

Figure 4. TEM micrographs of hollow nanoparticles covered by a silica layer: (a) silver nanoparticles surrounded by 5 ± 2 nm SiO2 layer produced by the decomposition of Na2SiO3 during 4 days, (b) silver nanoparticles surrounded by ca. 30 ± 10 nm SiO2 layer produced by the decomposition of tetraethyl orthosilicate during 1 h, and (c) gold nanoparticles surrounded by ca. 6 ± 2 nm SiO2 layer produced by the decomposition of tetraethyl orthosilicate during 15 min.

after deposition on them of ca. 30 ± 10 nm thick layer of SiO2. Similar results have been obtained using h-Au nanoparticles (see Figure 5B) and have been also reported by other groups: a significant red-shift of the plasmon band for metal nanoparticles occurs after covering by SiO2 layers with the thickness of tens of nanometers,55,56 whereas for metal nanoparticles covered with very thin silica layer the position of the plasmon band does not practically change.24 The increase of the width of the plasmonic band after covering with SiO2 is probably due to aggregation/agglomeration of metal nanoparticles during deposition of SiO2 (as have been shown by many groups, formation of agglomerates of the silver or gold nanoparticles leads to appearance of a very broad plasmon band in the red part of their adsorption spectra).24,57 Agglomeration of metal nanoparticles during deposition of SiO2 may be also deduced 20034


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strong SERS signal. It means that the enhancement of the Raman spectra presented in Figure 6 not only is induced by the enhancement of the electromagnetic field caused by metal nanoparticles themselves but is also due to the electromagnetic coupling between surface plasmons in the underlying gold substrate and deposited nanoparticles. Therefore, we have repeated such measurements using MBA monolayers deposited on the weakly SERS-active metal substrate such as Pt (unfortunately, even in experiments using MBA chemisorbed on platinum substrate electromagnetic coupling could not be eliminated). In these measurements the recorded Raman signal was weaker by about 1 order of magnitude, which strongly supports large contribution of the electromagnetic coupling into the total SERS enhancement factor for these systems. Higher activity of hollow nanoresonators has been, however, observed even in experiments on Pt substrates; Figure 7 shows typical SERS spectra recorded for MBA adsorbed on Pt surface and covered with solid and hollow Au nanoparticles.


nanostructures in the enhancing of the Raman spectra from molecules being in the close proximity to such metal nanoresonators. As mentioned in the Experimental Section, suspensions of hollow and solid Ag nanoparticles containing the same amount of silver in the same volume have been prepared. Then, the same volumes of suspensions containing hollow or solid Ag nanoparticles have been dropped on the surface of the MBA-modified Au substrates. The deposition of nanoparticles has been repeated six times at various points of the substrate, and for every area on which the droplet of the metal sol has been deposited, the Raman spectra have been measured at 15 various points (it means that Raman measurements have been repeated 90 times for each system). The obtained average spectra are presented in Figure 6. As can

Figure 6. Averaged (from 90 measurements) Raman spectra of the MBA monolayer on gold covered with (a) solid Ag and (b) h-Ag. Solid and hollow nanoresonators were produced from the same amount of AgNO3. Excitation radiation λexc = 632.8 nm.

be seen in this figure, recorded Raman spectra are dominated by two bands at 1077 and 1587 cm−1, which are due to the ν12 and ν8a vibrations of the aromatic ring of MBA, respectively.58−60 As can be also seen in Figure 6, the averaged intensity of Raman spectra recorded using h-Ag nanoparticles is ca. 2.7 ± 0.2 times larger than the averaged intensity of the spectra obtained using solid Ag nanoparticles. It means that silver hollow nanoresonators are more efficient than the respective standard solid nanoresonators obtained from the same amount of metal. It is worth mentioning that deposition of silver nanoparticles on the monolayer of MBA molecules chemisorbed on the gold substrate causes increase of the intensity of measured Raman spectrum by a factor of ca. 104; therefore, contribution to the obtained spectrum from molecules which are not in the close proximity of the deposited silver nanoparticles may be neglected. As has been shown in previous works,61−63 when silver or gold nanoparticles are deposited on the flat gold surface, under illumination with a visible light there is an electromagnetic coupling between the localized surface plasmons in Ag or Au nanoparticles and the surface plasmons in the underlying gold substrate. This causes that molecules trapped in the gap between gold substrate and the metal nanoparticles give a very

Figure 7. (a) Raman spectrum of the MBA monolayer on platinum before deposition of gold nanoparticles. Typical Raman spectra of the MBA monolayer on platinum covered with (b) solid Au, and (c) h-Au nanoparticles. Solid and hollow nanoparticles were produced from the same amount of HAuCl4. Excitation radiation λexc = 532 nm.

SHINERS Measurements for Strongly Adsorbed Analytes. MBA chemisorbed on gold has been also used for SHINERS measurements with silica-covered h-Au and h-Au nanoresonators. Figure 8 shows Raman spectra of the MBA monolayer on gold before deposition of the metal nanoresonators and after deposition of h-Au@SiO2 or h-Ag@SiO2 nanoparticles. As can be seen in Figure 8 before deposition of metal nanoresonators Raman bands due to chemisorbed MBA molecules are hardly seen. When the MBA-modified gold substrate is covered with SiO2-protected Ag or Au hollow nanoresonators, the Raman spectrum of chemisorbed MBA molecules is well visible (we calculated that the increase of the intensity of Raman signal of the chemisorbed MBA molecules caused by the deposition of h-Au@SiO2 or h-Ag@SiO2 nanoparticles is equal to about 102 and 104, respectively). As mentioned above, under illumination of the system composed from gold or silver nanoresonators deposited on the gold 20035


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silica-covered nanostructures on the surface of contaminated orange fruit (0.5 μg of methyl parathion was deposited on the area of ca. 10 cm2 of the surface of orange fruit) and allowed to dry. Figure 9 shows Raman spectrum of solid methyl parathion

Figure 8. (a) Raman spectrum of the MBA monolayer on gold before deposition of nanoresonators. Raman spectra of the MBA monolayer on gold covered with (b) h-Au@SiO2 and (c) h-Ag@SiO2 nanoparticles. The thickness of the SiO2 layer on both h-Au and h-Ag nanoparticles was equal to ca. 4 ± 2 nm. Silver and gold nanoresonators were produced from the same number of moles of Ag and Au. Spectra are vertically shifted to enhance clarity of presentation. Excitation radiation λexc = 632.8 nm.

Figure 9. Raman spectra of (a) solid methyl parathion, (b) skin of the orange fruit, (c) skin of the orange fruit contaminated by methyl parathion, (d) skin of the orange fruit covered with h-Ag@SiO2 nanoparticles, and (e) skin of the orange fruit contaminated by methyl parathion and covered with h-Ag@SiO2 nanoparticles. The thickness of the SiO2 layer was equal to ca. 4 ± 2 nm. Spectra are vertically shifted to enhance clarity of presentation. Inset: enlarged part of (e). Excitation radiation λexc = 632.8 nm.

substrate there is an electromagnetic coupling between the localized surface plasmons in h-Au@SiO2 or h-Ag@SiO2 nanoparticles and the surface plasmons in the gold surface.61−63 Therefore, the above-reported increase of the efficiency of the Raman scattering not only is due to the enhancement of the electric field caused by the h-Au@SiO 2 or h-Ag@SiO 2 nanoparticles but also is due to the surface plasmons excited in the underlying gold substrate. We have also carried out analogous SHINERS measurements using MBA monolayers deposited on Pt. Similarly as in measurements with bare silver and gold nanoparticles the recorded Raman spectra of the MBA/Au systems covered with h-Au@SiO2 and h-Au@SiO2 nanoresonators were weaker by about 1 order of magnitude than Raman spectra of the MBA/Pt systems covered with hAu@SiO2 and h-Au@SiO2 nanoresonators. This strongly supports the assumption about the significance of the electromagnetic coupling effect in the h-Au@SiO2/MBA/Au and h-Ag@SiO2/MBA/Au systems. As can be seen in Figure 8, the intensity of Raman spectrum recorded using the h-Ag@SiO2 nanoparticles is about 2 orders of magnitude larger than the intensity of Raman spectrum recorded using the h-Au@SiO2 nanoparticles. This agrees well with the difference of the SERS activity of silver and gold nanoparticles that is equal to ca. 2 orders of magnitude for a red excitation radiation.64 SHINERS Detection of Methyl Parathion Using Hollow Nanoresonators. To verify applicability of h-Ag@SiO2 nanostructures for SHINERS measurements, we have also carried out detection of traces of methyl parathion (which is an efficient insecticide) on the surface of an orange fruit using hAg@SiO2 nanoresonators. Such an application of SHINERS technique has been already demonstrated in the first work concerning SHINERS measurements.14 We were unsuccessful in control deposition of nanoresonators as the solid phase (as “dust”); therefore, we decided to deposit the suspensions of

and spectra of clean and contaminated with methyl parathion skin of an orange fruit. Measurements of Raman spectra of skins of orange fruits have been carried out for fruits before and after deposition of h-Ag@SiO2 nanoparticles. The Raman spectrum of the surface of an orange fruit is dominated by two bands at 1156 and 1528 cm−1 (see Figure 9). These Raman bands are attributed to carotenoid molecules.14 There are no significant differences between “normal” Raman spectra obtained from the surface of the “noncontaminated” and contaminated with methyl parathion oranges. However, for the surface of oranges covered with h-Ag@SiO2 nanoresonators, in the spectra recorded from the surface of “contaminated” oranges one can clearly identify the Raman band due to the methyl parathion molecules at 1350 cm−1 (see also inset in Figure 9). Similar results have been previously obtained with solid Ag@SiO2 nanoparticles.24 As mentioned above, the actual SERS enhancement factor, which is correlated with the averaged efficiency of the nanoresonators, depends on the shape and size of the metal cores and the thickness of deposited SiO2 layer. Moreover, during deposition of the silica layer some silver nanoparicles may be lost (due to gluing to the mixing element or to the cells of the reactor), which may introduce additional error during the comparison of the efficiency of various silica-covered metal nanoresonators. Therefore, at the present stage, we are unable do precisely compare the efficiency of solid and hollow Ag@SiO 2 nanoresonators. However, taking into account comparison of the efficiency for SERS measurements of standard solid and hollow metal nanoparticles, one can expect that silica-covered hollow nanostructures should be better nanoresonators for practical SHINERS measurements than used so far solid nanostructures. Since we are able to carry out described above detection of methyl parathion using the part of the orange skin 20036


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The Journal of Physical Chemistry C with the area of 5 mm2, we can estimate the detection limit of methyl parathion in the SHINERS measurements as equal to ca. 3 ± 2 ng.

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CONCLUSIONS For the first time SHINERS measurements with silica-protected hollow-silver (h-Ag@SiO2) and hollow-gold (h-Au@SiO2) nanoparticles have been carried out. The results of our measurements suggest that hollow metal nanostructures may be better nanoresonators for practical SHINERS measurements than used so far solid nanoparticles. For used in this work excitation radiation (632.8 nm) h-Ag@SiO2 nanoresonators are about 2 orders of magnitude more efficient than h-Au@SiO2 nanoresonators. This agrees well with the difference of the SERS activity of gold and silver nanoparticles in this range of the optical spectrum.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Ph +48-228220211, Fax +048-228225996 (A.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Marianna Gniadek is acknowledged for her assistance in the TEM measurements. This work was financed from the funds of the National Science Centre (Poland) allocated on the basis of the decision number DEC-2013/11/B/ST5/02224. The electron microscope was purchased under CePT project, which was cofinanced by European Union from the European Regional Development Fund under the Operational Programme Innovative Economy 2007-2013.

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Silica-Protected Hollow Silver and Gold Nanoparticles: New Material

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