Silver Nanoparticles with Many Sharp Apexes and Edges as Efficient

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Silver Nanoparticles with Many Sharp Apexes and Edges as Efficient Nanoresonators for Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy Karol Kołątaj, Jan Krajczewski, and Andrzej Kudelski* Faculty of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warsaw, Poland ABSTRACT: One of the tools used for investigations of various interfaces, especially in in situ conditions, is shellisolated nanoparticle-enhanced Raman spectroscopy SHINERS. To carry out SHINERS measurements, the surface under analysis is covered with electromagnetic nanoresonators, which locally significantly enhance the electric field of the incident electromagnetic radiation. This leads to a large increase in the Raman signal from molecules in close proximity to the deposited nanoresonators, thus making it possible to determine the composition of the surface phase on the basis of the Raman spectra obtained. We tested silver nanoparticles containing a large number of sharp apexes and edges (such as pentagonal bipyramids and triangular prisms) as surfaceenhanced Raman scattering (SERS) and SHINERS nanoresonators. We found that the SERS enhancement factors obtained in the experiments with these anisotropic nanoparticles were 1 order of magnitude larger than in analogous experiments with spherical Ag nanostructures having similar size. Silver pentagonal bipyramids are especially easy to covered with very thin silica layers, and so such silica-covered nanoparticles seem to be very promising nanoresonators for SHINERS measurements. We also present the direct evidence of the significantly higher chemical stability of silica-covered Ag nanoparticles in comparison with bare Ag nanoparticles in the presence of a model biological sample (a suspension of yeasts cells).

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INTRODUCTION Determining the composition of the surfaces of various materials is important, from both the economic and scientific points of view. Raman spectroscopy is one technique that can be used in investigations of various interfaces, especially in in situ conditions, such as the surfaces of solid samples in a liquid or high-pressure gas. Standard Raman spectroscopy is usually not a very useful analytical tool, because the efficiency of “normal” Raman scattering is very low. A typical total cross section for “standard” Raman scattering is ca. 10−29 cm2 per molecule, whereas cross sections in other standard spectroscopic methods, such as absorption in infrared and ultraviolet, are typically ca. 10−21 and 10−18 cm2 per molecule, respectively; this means they are 8−11 orders of magnitude larger.1 In some cases, however, the efficiency of Raman scattering may be considerably increased, and the Raman scattering cross sections can be as large as 2 × 10−14 cm2 per molecule,2 about 15 orders of magnitude larger than the scattering cross section in the “normal” Raman effect. In such cases, it is possible to observe the Raman spectra of even a single molecule,2−4 and Raman scattering, comparable to fluorescence single-molecule spectroscopy, becomes one of the most sensitive analytical tools. This enormous increase in the efficiency of Raman scattering has been achieved in two steps. First, Shorygin and Ivanova showed that, if the frequency of the excitation radiation is tuned to the absorption band of the molecules being analyzed, the © 2017 American Chemical Society

efficiency of the Raman scattering increases significantly, in some cases even by as many as 6 orders of magnitude.5 Later, it was realized that the efficiency of Raman scattering may be significantly increased for molecules deposited in the active area of electromagnetic nanoresonators, typically composed of nanoclusters of plasmonic metals. For historical reasons, this type of Raman spectroscopy is called surface-enhanced Raman scattering (SERS).6,7 When both of the above effects are combined, the efficiency of Raman scattering may be so large that, as mentioned above, it is possible to record reliable Raman spectra from even a single molecule.2−4 In 2010 Li et al. proposed an interesting new modification of SERS spectroscopy called shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS).8 In the first contribution describing SHINERS measurements, the surfaces analyzed were covered with a layer of gold plasmonic nanoparticles protected by an ultrathin transparent layer of SiO2 or Al2O3. The transparent coatings do not dampen the surface electromagnetic enhancement, though they do prevent the metal nanoparticles from coming into direct contact with the analyzed material and do prevent them from agglomerating. Using this technique, Li et al. obtained Raman spectra for various Received: March 21, 2017 Revised: May 17, 2017 Published: May 17, 2017 12383

DOI: 10.1021/acs.jpcc.7b02695 J. Phys. Chem. C 2017, 121, 12383−12391

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The Journal of Physical Chemistry C

also analyze the influence of silica layers on the stability of silver nanoparticles in contact with a biological sample.

molecules adsorbed at the surfaces of Pt and Au single crystals and from the hydrogen monolayers formed on the Si surface.8 Li et al. also analyzed certain biological objects, such as the walls of living cells,8 and also used SHINERS measurements to detect traces of a pesticide on the surface of citrus fruits.8 Since the development of SHINERS spectroscopy, there have been many attempts to improve its sensitivity and the chemical stability of the nanoresonators used. In 2012, Tian’s and Kudelski’s groups9,10 showed that, in certain cases, using nanoresonators formed from silver makes it possible to carry out SHINERS measurements that are significantly more sensitive than those carried out when gold nanoresonators are applied, as was the case initially. Later, nanoresonators with shapes other than spherical (nanorods and nanocubes) came to be used,11,12 as well as other transparent protecting layers more stable in alkaline media.13 Since the plasmon band for hollow nanoparticles can be changed more easily than that for analogous solid nanostructures, hollow Au and Ag nanoparticles have been also tested for SHINERS measurements.14 Quantitative measurements have shown that, in many cases, using hollow nanostructures makes it possible to increase the sensitivity of SHINERS measurements.14 For illuminated plasmonic nanoparticles, the highest electromagnetic field enhancement is observed at their sharp apexes and edges.15,16 Therefore, we decided to try to increase sensitivity of SERS and SHINERS measurements by using as nanoresonators plasmonic nanoparticles containing a large number of sharp apexes and edges. We used silver nanoparticles having the form of pentagonal bipyramids or triangular prisms (see Figure 1). Detailed analysis of the electromagnetic field

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EXPERIMENTAL SECTION Materials. Trisodium citrate dihydrate, ethanol, and silver nitrate were purchased from POCH SA. 16-Mercaptohexadecanoic acid, 4-mercaptobenzoic acid, dimethylamine, tetraethyl orthosilicate, and L-arginine were acquired from Sigma-Aldrich. NaBH 4 , sodium 2-mercaptoethanesulfonate, and poly(vinylpyrrolidone) (PVP) with an average molar mass of ca. 4 × 104 g mol−1 were acquired from Fluka. Platinum sheets were purchased from the Polish State Mint. Nitrogen was acquired from Air Products. The water used in all the experiments was purified in the Millipore Milli-Q manner. All materials were of high purity and were used as received without further purification or treatment. Preparation of Silver Nanoparticles. Silver nanoparticles in the form of pentagonal bipyramids were synthesized using the method developed by Pietrobon et al.20 In the first stage of this synthesis, “standard” spherical Ag nanoparticles were formed using the following procedure: 0.16 mL of a 0.1 M NaBH4 solution was added dropwise to a stirred solution obtained by mixing 15 mL of water with 0.1 mL of 0.5 M trisodium citrate, 0.1 mL of 0.025 M silver nitrate, 0.1 mL of 5 mM L-arginine, and 0.1 mL of 30 mM 40 kDa PVP solutions. After 10 min, about 9 mL of the just-formed Ag sol was transferred into a cylindrical glass reactor with a diameter of 2 cm and illuminated under constant stirring for 5 h by two lightemitting diode (LED) sets generating green radiation (λmax = 520 nm) with a relatively narrow spectral half-width (Δλ < 20 nm). The illuminance of the glass reactor was equal to ca. 95 kLux. The process was carried at various temperatures, typically 35 °C. The silver nanoprisms were synthesized using the method proposed by Xue et al.21 In the first stage, Ag sol was formed by the reduction of the solution of AgNO3 by the solution of NaBH4 in the presence of sodium citrate.21 The next day, this solution was adjusted to pH 11, transferred to a photochemical reactor (for details, see above), and irradiated for 5 h at 15 °C (as in the case of synthesizing decahedral nanoparticles). To synthesize the spherical silver nanoparticles, which were used as reference nanoresonators in the Raman measurements, 250 mL of a 1.06 mM AgNO3 solution was placed in a roundbottom flask. This solution was stirred, and then heated to boiling under reflux. Afterward, 10 mL of a 38.8 mM sodium citrate solution was added rapidly, and the mixture was kept boiling for 90 min. The average size of obtained spherical silver nanoparticles is 45 nm, with a standard deviation of 6.8 nm. Deposition of Silica Layer on Metal Nanoparticles. To create a thin silica layer on the resulting decahedral and prismatic nanoparticles, we used the deposition method described by Xue et al.21 In the first stage, an ethanolic solution of 16-mercaptohexadecanoic acid (MHA) was added to the freshly prepared sol containing silver nanoparticles. The final concentration of MHA added was ca. 20 μM. This solution was then centrifuged at 104 g for 30 min and redispersed in a 0.1 or 0.5 mM ethanolic solution of tetraethyl orthosilicate. Finally, dimethylamine (a 40% aqueous solution) was added while stirring until the dimethylamine concentration reached 0.6 M. After 3 h of the SiO2 deposition, the core−shell nanoparticles obtained were centrifuged for 30 min at 5 × 103 g and redispersed in distilled water.

Figure 1. (a) Pentagonal bipyramid and (b) triangular prism.

distribution around silver triangular nanoprisms (simulations have been based on the discrete dipole approximation) may be found, for example, in the work of Hao and Schatz,17 whereas distribution of the electromagnetic field around silver pentagonal bipyramids has been analyzed by Ye et al.18 (analogous pattern of the field distribution has been also obtained by Sánchez-Iglesias et al.19 for gold pentagonal bipyramids). Besides the large number of sharp apexes and edges of these forms, silver nanoparticles having such forms are relatively easy to synthesize. As mentioned above, the initial role of the layer of deposited silica or alumina in SHINERS nanoresonators is to prevent the metal cores from agglomerating and from coming into direct contact with the material being tested. In this contribution, we 12384


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The Journal of Physical Chemistry C Experimental Techniques. Transmission electron microscopy (TEM) measurements were carried out using a Zeiss LIBRA 120 electron microscope working at an accelerating voltage of 120 kV. This microscope was equipped with an Incolumn OMEGA filter. To carry out the TEM measurements, samples of the nanoparticles obtained were deposited onto 300mesh copper grids coated with a Formvar layer, and the deposited solution was allowed to dry. To determine the elemental composition of Ag@SiO2 nanostructures the samples of such nanoparticles have been deposited on the surface of the graphite substrate and studied with a Merlin field emission scanning electron microscope (Zeiss, Germany). The elemental composition was determined using an energy-dispersive X-ray microanalysis (EDS) probe (Bruker). UV−vis spectra were recorded using a Thermo Scientific Evolution 201 spectrophotometer. Raman spectra were taken using a Horiba Jobin-Yvon Labram HR800 spectrometer equipped with a Peltier-cooled CCD detector (1024 × 256 pixels), a 600 grooves/mm holographic grating, and an Olympus BX40 microscope with a long distance 50× objective. Raman spectra were recorded using two various wavelengths of excitation radiation: radiation with a wavelength of 633 nm generated by a He−Ne laser, and radiation with a wavelength of 532 nm generated by a diodepumped, frequency-doubled Nd:YAG laser.

As can be seen from Figure 2, in the reaction products one can identify many silver nanoparticles having the shape of pentagonal bipyramids (from 30 to 50 nm in size) or prisms (from 30 to 90 nm in size), irregular nanoparticles of a similar size, and some significantly smaller spherical or pseudospherical silver clusters. To quantitatively analyze the products obtained, we decided to assess only those nanoparticles larger than 30 nm and to evaluate the percentage of sharp-edged objects in this group. We found that the highest amount of decahedral silver nanoparticles (up to 85%) was obtained in the reaction products at ca. 35 °C, while the highest yield of prisms occurred at 15 and 25 °C (77% and 75%, respectively). The efficiency of the formation of decahedral and prismatic Ag nanoparticles was significantly lower at lower temperatures (for decahedral nanoparticles, 35% at 5 °C, 40% at 15 °C, and 74% at 25 °C; for prismatic nanoparticles, 63% at 5 °C). For temperatures above 45 °C (55 and 65 °C), the efficiency of the formation of decahedral nanoparticles was very low (very small Ag nanoparticles were mainly formed under these conditions). Moreover, using the procedure typical for synthesizing prismatic nanoparticles, when the process is carried out at temperatures above 30 °C, only very small amounts of nanoprisms are formed, and the product contains mainly round disks. To analyze the influence of illumination time on the efficiency of formation of silver pentagonal bipyramidal and prismatic nanoparticles, we analyzed only those processes carried out at 35 or 15 °C (when the efficiency of the formation of such nanoparticles is the highest). As mentioned above, with an illumination time of 5 h, the efficiency of the formation of sharp-edged silver nanoparticles was ca. 85% for decahedral nanoparticles and ca. to 77% for prismatic nanoparticles. When the illumination time was significantly shorter or longer, the efficiency of the formation of decahedral nanoparticles decreased, for example, to 57% for 3 h of illumination and to 68% for 7 h of illumination. Irradiation of 24 h led to the complete disappearance of decahedral nanoparticles in the reaction mixture. Similarly, in the case of prismatic nanoparticles, we found that the optimal irradiation time was 5 h, while the yield of anisotropic nanoparticles decreased to 45% after 3 h and to 33% after 7 h of irradiation. A longer irradiation time leads to a significant increase in the amount of round nanodisks and nanoprisms with “blunt” edges. Therefore, in our further experiments we used decahedral nanoparticles obtained after 5 h of irradiation carried out at 35 °C and nanoprisms obtained in a reaction carried out at 15 °C for the same length of time. Figure 3 shows size distribution histograms for the silver decahedral and prismatic nanoparticles obtained under these conditions. The average size of the silver pentagonal bipyramid nanoparticles is 43 nm, with a standard deviation of 4.6 nm, while for the nanoprisms the average size was calculated to be 48 nm, with a standard deviation of 15 nm. Deposition of Silica Layers. To cover the nanoparticles obtained with a thin silica layer, we used the method developed by Xue et al.21 Usually, when a silica layer is deposited on metal nanoparticles, an aqueous solution of ammonia is used to catalyze the hydrolysis of tetraethoxysilane to silica.22−25 This method is very efficient for creating a silica layer on gold nanoparticles, but it is useless for covering highly anisotropic silver nanoparticles because the ammonia solution induces significant etching of their edges (leading in many cases to a change in the shape of the anisotropic Ag nanoparticles) and usually to an agglomeration of Ag nanoparticles.21 In the case of the silver pentagonal bipyramidal and triangular prismatic

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RESULTS AND DISCUSSION Synthesis of Anisotropic Silver Nanoparticles. It is well-known that the process of the formation, growth, and transformation of silver nanoparticles is very sensitive to experimental conditions. Therefore, we decided to verify how the temperature at which the phototransformation is carried out and the duration of the illumination affect the composition of the mixture of silver nanoparticles formed. Figure 2 shows TEM micrographs of the silver nanoparticles obtained after 5 h of phototransformation carried out at 25 and 35 °C for decahedrons, and at 15 and 25 °C for prisms.

Figure 2. (a and b) TEM micrographs of decahedral silver nanoparticles obtained after 5 h of phototransformation carried out at (a) 25 and (b) 35 °C. (c and d) TEM micrographs of prismatic silver nanoparticles obtained after 5 h of phototransformation carried out at (c) 25 and (d) 35 °C. 12385


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Experimental Section) and produced from the same amount of silver. The rationale behind using platinum surfaces covered by monolayers of pMBA for such measurements is the high stability and reproducibility of such systems: the sulfhydryl group of pMBA reacts chemically with the platinum forming a metal−sulfur bond; therefore, the pMBA molecules are attached very strongly to the metal surface, and even after lengthy (for example, 1 h) immersion in water only a very small part of the pMBA is desorbed from the metal surface.26−28 Moreover, the excitation of surface plasmons in the platinum substrates illuminated with visible radiation is significantly less efficient than in other metals (e.g., Au, Ag, Cu), on which pMBA forms regular, stable monolayers. Therefore, the electromagnetic coupling between the localized surface plasmons in the Ag nanoparticles and the surface plasmons in the underlying substrate is significantly less efficient than when pMBA monolayers on gold are used (gold is a standard metal used for the formation of regular self-assembled thiols monolayers).29−31 As mentioned above, the pMBA monolayers on Pt were covered with silver nanoparticles that, when illuminated, locally enhance the intensity of the electromagnetic field of the incident radiation. To deposit films of Ag nanoparticles with the same surface concentration of Ag, the same volumes of suspensions containing the same mass of various Ag nanoparticles were dropped onto the surface being investigated (as mentioned above we used nanoparticles having roughly the same size, ca. 43−48 nm; for details see the Experimental Section), and the solution was then evaporated. On such Pt/ pMBA/Ag films, Raman (SERS) spectra were measured at 40 different, randomly chosen areas of the film. In experiments carried out using both 532 and 633 nm excitation radiation, the average intensity of the SERS signal recorded after the deposition of decahedral nanoparticles was larger by 1 order of magnitude (101) than when spherical nanoparticles were used. Typical SERS spectra obtained from such measurements are presented in Figure 5. To illustrate irreproducibility of the intensity of the individually measured SERS spectrum the logarithms of the intensity of the strongest pMBA band at 1587 cm−1 recorded in each from the series of 40 measurements is presented in Figure 6. We also observed similarly higher (also by a factor of 101) SERS activity of the prismatic nanoparticles in comparison with the spherical particles; however, due to above-described problems in covering the prismatic Ag nanoparticles with thin (10 nm) silica layers (see Figure 4d): during the slow deposition of SiO2 using a 0.2 mM tetraethoxysilane solution, the majority of silver nanoparticles agglomerated. As can be seen in Figure 4, in both cases the process of SiO2 deposition did not affect the shape of the metal nanoparticles. For some of the synthesized decahedral core− shell nanoparticles their elemental composition has been also determined from the respective EDS measurements. Obtained results of the elemental analysis (for example, the ratio of the relative atomic concentrations of Ag, Si, and O equals to 100:6.1:13.8) are in agreement with the assumed chemical composition (Ag@SiO2) of the synthesized nanostructures. Comparison of the Efficiency of Various Silver Nanoresonators. To compare the efficiency of various metal nanoresonators, we recorded SERS spectra of monolayers of 4-mercaptobenzoic acid (pMBA) formed on platinum surfaces and covered by spherical, decahedral, and prismatic nanoparticles having roughly the same size (for details see the 12386


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Figure 4. TEM micrographs of decahedral and prismatic Ag@SiO2 nanoparticles after deposition of silica from tetraethoxysilane solutions of different concentrations: (a and b) deposition from a 0.2 mM tetraethoxysilane solution, which resulted in a 3 nm thick silica layer; (c and d) deposition from a 0.5 mM tetraethoxysilane solution, which resulted in a 14 nm thick silica layer.

of the SiO2 shells. The measured ratio of the SHINERS activity of the silica-covered spherical and decahedral silver nanoparticles obtained in various experiments was less reproducible than in the case of the experiments with bare (not covered with silica) spherical and decahedral Ag nanoparticles. This larger irreproducibility is probably due to the appearance of the additional parameter influencing the intensity of the measured Raman spectrum; it means the thickness of the deposited SiO2 layer (even small changes in the thickness of the oxide layer significantly influence the efficiency of the nanoresonator). Decahedral-Ag@SiO2 nanoparticles were also tested in SHINERS measurements of model biological samples. Figure 9 shows Raman spectra of Saccharomyces boulardii (yeast) cells before and after deposition of decahedral-Ag@SiO2 nanoparticles. It can be seen that, in the Raman spectrum recorded with the decahedral-Ag@SiO2 nanoparticles, many Raman bands (e.g., at 1135, 1315, 1346, 1456, 1611, 1665 cm−1) can be clearly identified, whereas in the case of the Raman spectrum recorded without nanoresonators, it is very difficult to distinguish these bands from the noise, except for the weak band at 1135 cm−1. Probably all the Raman bands visible in the spectra presented in Figure 9 are due to the vibrations of mannoproteinsa group of proteins found in the cell walls of yeasts.8 Comparison of the Stability of Bare and SilicaCovered Silver Nanoparticles. Another interesting problem is that of comparing the stability of silver nanoparticles before and after an ultrathin silica layer is deposited on them. Where such nanoparticles are used for measurements of the SERS and

decahedral and prismatic nanoparticles in comparison to the spherical nanoparticles with similar size is even larger than the mentioned above ratio of the intensities of the SERS spectra. SHINERS Measurements. Next, the synthesized decahedral-Ag@SiO2 nanoparticles were tested as nanoresonators for model SHINERS experiments. Figure 7 shows a SHINERS spectrum of a pMBA monolayer on Pt covered with decahedral-Ag@SiO2 nanoresonators. In the standard Raman spectrum of the pMBA monolayer on Pt hardly any Raman band can be seen; however, after the deposition of decahedralAg@SiO2 nanoparticles, there are three clearly visible Raman bands characteristic for chemisorbed pMBA molecules (at 1077 and 1587 cm−1 assigned to the ν12 and ν8a vibrations of the aromatic ring, and at 1185 cm−1 due to the δ(C−H) vibrations).26 As one can expect, the intensity of the Raman spectrum of the pMBA monolayer on Pt recorded using decahedral-Ag@ SiO2 nanoresonators is lower than the intensity of the respective Raman spectrum recorded using decahedral-Ag nanoresonators before deposition on them of the silica layer; however, if the silica layer is very thin, the order of magnitude of intensities of both spectra is the same (see Figure 8), which means that decahedral-Ag@SiO2 nanoparticles are still very efficient nanoresonators. We have also observed that a SHINERS signal was significantly higher (about 1 order of magnitude) for the monolayer of pMBA chemisorbed on platinum covered with decahedral-Ag@SiO2 nanoparticles than for analogous systems covered with spherical-Ag@SiO2 nanoparticles with similar size of the Ag cores and similar thickness 12387


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Figure 7. Raman spectra of a pMBA monolayer on platinum (a) before deposition of electromagnetic nanoresonators and (b) after deposition of decahedral-Ag@SiO2 nanoparticles. Excitation radiation: 532 nm.

Figure 5. Raman spectra of a monolayer formed from 4mercaptobenzoic acid on a platinum surface before deposition of Ag nanoparticles (a), and covered with spherical (b) and decahedral (c) nanoparticles. Excitation radiation: (A) 532 nm; (B) 633 nm.

Figure 8. Raman spectra of a pMBA monolayer on platinum covered with decahedral-Ag nanoresonators before (a) and after (b) deposition on them of the 3 nm thick SiO2 layer. Excitation radiation: 532 nm.

time, and has practically disappeared after 12 h, whereas when using decahedral-Ag@SiO2 nanoparticles, the definite red color of the solution is still visible even after several days. The temporal evolution of the UV−vis spectra of the solutions containing yeasts and unmodified and silica-modified decahedral-Ag nanoparticles is shown in Figure 10, from which one can deduce that the deposition of a protecting SiO2 layer increases the stability of the Ag nanoparticles in the presence of the suspension of yeast cells by at least 1 order of magnitude. As far as we know, up to now the protecting role of the deposited SiO2 layers has been only observed in experiments in which made on purpose corrosive solution (solution of H2O2) has been used.32 In this contribution we show that, even when Ag nanoresonators are used in solutions that are not considered as highly corrosive (such as suspension of yeast cells), the protecting role of the SiO2 layer is very important. Decomposition of silver nanoparticles added to the suspension of the yeast cells may be also investigated by the analysis of the temporal decay of the SERS spectrum of so-

Figure 6. Logarithm of the intensity of the strongest pMBA band at 1587 cm−1 recorded in each from the series of 40 measurements for pMBA monolayer on Pt covered with spherical (●) and decahedral (□) silver nanoparticles. Solid and dashed lines show the average values calculated from 40 measurements for spherical and decahedral silver nanoparticles, respectively. Excitation radiation: 532 nm.

SHINERS spectra of yeast, the decomposition of silver nanoparticles (through oxidation) can be easily observed visually from a change in the intensity of the color of the solution containing yeast and unmodified and silica-modified Ag nanoparticles (for this study, we used decahedral silver nanoparticles). Where nanoparticles without a silica shell are added to a yeast suspension, the red color coming from the plasmon resonance in the Ag nanoparticles fades quickly over 12388


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Figure 9. Raman spectra of S. boulardii cells before deposition of nanoparticles (a) and after the addition of decahedral-Ag@SiO2 (b). Excitation radiation: 532 nm.

Figure 11. (A) Temporal evolution of the SERS spectrum of the mixture of solutions of yeasts, MES, and decahedral-Ag nanoparticles. Spectrum recorded (a) ca. 5 min, (b) 30 min, and (c) 2 h after mixing. (B) Temporal evolution of the SERS spectrum of the mixture of solutions of yeasts, MES, and decahedral-Ag@SiO2 nanoparticles. Spectrum recorded (a) ca. 5 min and (b) 12 h after mixing. Excitation radiation: 532 nm.

using of bare decahedral-Ag nanoparticles a significant decay of the SERS signal is observed already after 0.5 h, whereas for silica-protected nanoparticles there is not any significant change in the intensity of the SERS spectra even after 12 h. Diffusion of various compounds through the SiO2 layers deposited on silver nanoparticles has been already observed and analyzed in previous works.10,33,34 For example, Ung et al. showed that various molecules diffuse via the silica layer deposited on silver cores even when the thickness of the SiO2 layer is larger than 0.1 μm.33 The diffusion coefficients via the SiO2 layer for molecules of rhodamine 6G and cyanide ions were estimated as equal to 3 × 10−18 and 10−16 m2 s−1, respectively.33,34 We found that diffusion of various molecules is also possible via the SiO2 layers synthesized in this work. Figure 12 shows Raman spectra of a 1 M aqueous solution of MES and the spectra of a thin layer of decahedral-Ag@SiO2 nanoparticles deposited on the glass substrate and covered by a thin layer of a 10 mM MES aqueous solution. As can be seen in Figure 12, in the Raman spectrum of the MES aqueous solution a relatively strong Raman band due to the ν(S−H) stretching vibration at 2583 cm−1 is well-visible,35,36 whereas in the nanoparticles-enhanced spectra there is the lack of any observable band in this wavenumber range. The lack of any

Figure 10. (A) Temporal evolution of the UV−vis spectrum of mixture of solutions of yeasts and decahedral-Ag nanoparticles: (a) spectrum recorded immediately after mixing, (b) after 12 h, and (c) after 24 h. (B) Temporal evolution of the UV−vis spectrum of mixture of solutions of yeasts and decahedral-Ag@SiO2 nanoparticles: (a) spectrum recorded immediately after mixing and (b) after 24 h.

called Raman reporter introduced to the analyzed sample. Figure 11 shows temporal evolution of the SERS spectra of 2mercaptoethanesulfonate (MES), which was used as the Raman reporter in this experiment, introduced to the mixture of the silver sol (protected or not protected by the SiO2 layer) and the suspension of yeast cells. As one may expect, in the case of 12389


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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financed out of funds of the National Science Centre (Poland), allocated on the basis of Decision No. DEC2013/11/B/ST5/02224. The electron microscope was purchased under a CePT project cofinanced by the European Union from the European Regional Development Fund under the Innovative Economy Operational Programme 2007−2013.

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(1) Aroca, R. Surface-Enhanced Vibrational Spectroscopy; John Wiley & Sons Ltd.: Chichester, U.K., 2006. (2) Michaels, A. M.; Nirmal, M.; Brus, L. E. Surface Enhanced Raman Spectroscopy of Individual Rhodamine 6G Molecules on Large Ag Nanocrystals. J. Am. Chem. Soc. 1999, 121, 9932−9939. (3) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R.; Feld, M. S. Single Molecule Detection Using SurfaceEnhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78, 1667− 1670. (4) Nie, S. M.; Emery, S. R. Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275, 1102−1106. (5) Shorygin, P. P.; Ivanova, T. M. On the Simultaneous Observation of the Raman Spectrum and Fluorescence. Soviet Phys. Doklady 1958, 3, 764. (6) Jeanmaire, D. L.; van Duyne, R. P. Surface Raman Spectroelectrochemistry: Part I. Heterocyclic, Aromatic, and Aliphatic Amines Adsorbed on the Anodized Silver Electrode. J. Electroanal. Chem. Interfacial Electrochem. 1977, 84, 1−20. (7) Albrecht, M. G.; Creighton, J. A. Anomalously Intense Raman Spectra of Pyridine at a Silver Electrode. J. Am. Chem. Soc. 1977, 99, 5215−5217. (8) Li, J. F.; Huang, Y. F.; Ding, Y.; Yang, Z. L.; Li, S. B.; Zhou, X. S.; Fan, F. R.; Zhang, W.; Zhou, Z. Y.; Wu, D. Y.; et al. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature 2010, 464, 392− 395. (9) Uzayisenga, V.; Lin, X. D.; Li, L. M.; Anema, J. R.; Yang, Z. L.; Huang, Y. F.; Lin, H. X.; Li, S. B.; Li, J. F.; Tian, Z. Q. Synthesis, Characterization, and 3D-FDTD Simulation of Ag@SiO2 Nanoparticles for Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Langmuir 2012, 28, 9140−9146. (10) Kudelski, A.; Wojtysiak, S. Silica-Covered Silver and Gold Nanoresonators for Raman Analysis of Surfaces of Various Materials. J. Phys. Chem. C 2012, 116, 16167−16174. (11) Attard, G. A.; Bennett, J. A.; Mikheenko, I.; Jenkins, P.; Guan, S.; Macaskie, L. E.; Wood, J.; Wain, A. J. Semi-Hydrogenation of Alkynes at Single Crystal, Nanoparticle and Biogenic Nanoparticle Surfaces: the Role of Defects in Lindlar-Type Catalysts and the Origin of Their Selectivity. Faraday Discuss. 2013, 162, 57−75. (12) Lin, X. D.; Li, J. F.; Huang, Y. F.; Tian, X. D.; Uzayisenga, V.; Li, S. B.; Ren, B.; Tian, Z. Q. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy: Nanoparticle Synthesis, Characterization and Applications in Electrochemistry. J. Electroanal. Chem. 2013, 688, 5− 11. (13) Abdulrahman, H. B.; Kołątaj, K.; Lenczewski, P.; Krajczewski, J.; Kudelski, A. MnO2-Protected Silver Nanoparticles: New Electromagnetic Nanoresonators for Raman Analysis of Surfaces in Basis Environment. Appl. Surf. Sci. 2016, 388, 704−799. (14) Abdulrahman, H. B.; Krajczewski, J.; Aleksandrowska, D.; Kudelski, A. Silica-Protected Hollow Silver and Gold Nanoparticles: New Material for Raman Analysis of Surfaces. J. Phys. Chem. C 2015, 119, 20030−20038. (15) Kottmann, J. P.; Martin, O. J. F.; Smith, D. R.; Schultz, S. Dramatic Localized Electromagnetic Enhancement in Plasmon Resonant Nanowires. Chem. Phys. Lett. 2001, 341, 1−6.

Figure 12. (a) Raman spectrum of 1 M MES aqueous solution and (b) Raman spectrum of MES adsorbed on decahedral-Ag@SiO2 nanoparticles. Excitation radiation: 532 nm. Spectra have been vertically shifted to improve the clarity of the presentation.

observable ν(S−H) band in the nanoparticles-enhanced Raman spectra indicates a direct chemisorption of MES molecules on the silver core via thiol groups (in such case Ag−S bonds are formed). Direct bonding of MES molecules to the silver surface may be also deduced from a significant shift toward lower wavenumbers of the ν(C−S) stretching band (marked with an asterisk in Figure 12) from 750 cm−1 for MES molecules in aqueous solution to 713 cm−1 in the silver nanoparticlesenhanced Raman spectra. Such shift toward lower wavenumbers of the ν(C−S) stretching band is related to a withdrawal of electron density from the C−S bond during formation of the direct sulfur−metal bond.35,36

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CONCLUSIONS In this contribution we showed that, although they appear to be “significantly less anisotropic” than nanorods, decahedral-Ag@ SiO2 nanoparticles may also be used as very efficient anisotropic nanoresonators for SHINERS measurements: the SERS activity of such nanostructures is about 1 order of magnitude higher than the activity of standard spherical nanoparticles of a similar size (it means 45 nm). Probably, the higher SERS activity of decahedral-Ag nanoparticles is due to the many sharp apexes and edges on their surfaces. We also observed significantly higher stability of the silica-covered decahedral silver nanoparticles in the presence of the yeast cells than of those nanoparticles that were not protected even by a very thin silica layer. This means that even in an environment that is not consider as highly corrosive (such as suspension of yeast cells), it is recommended to increase stability of the silver plasmonic cores by deposition on them of the nanometers thick protecting layer (for example, from SiO2).

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +48-225526401. ORCID

Andrzej Kudelski: 0000-0003-1452-5951 12390


Article

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(35) Bryant, M. A.; Pemberton, J. E. Surface Raman Scattering of Self-Assembled Monolayers Formed from 1-Alkanethiols at Ag. J. Am. Chem. Soc. 1991, 113, 3629−3637. (36) Kudelski, A. Chemisorption of 2-Mercaptoethanol on Silver, Copper, and Gold: Direct Raman Evidence of Acid-Induced Changes in Adsorption/Desorption Equilibria. Langmuir 2003, 19, 3805−3813.

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Silver Nanoparticles with Many Sharp Apexes and Edges as Efficient

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