Using a Macroporous Silver Shell to Coat Sulfonic Acid Group

Article pubs.acs.org/Langmuir

Using a Macroporous Silver Shell to Coat Sulfonic Acid GroupFunctionalized Silica Spheres and Their Applications in Catalysis and Surface-Enhanced Raman Scattering Guohong Ren, Wenqin Wang,* Mengying Shang, Hanzhi Zou, and Shengwei Cheng Department of Polymer Science and Engineering, Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, PR China

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 15, 2015 | http://pubs.acs.org Publication Date (Web): September 15, 2015 | doi: 10.1021/acs.langmuir.5b02218

S Supporting Information *

ABSTRACT: In this paper, novel organic sulfonic acid groupfunctionalized silica spheres (SiO2−SO3H) were chosen as a template for fabricating core−shell SiO2−SO3H@Ag composite spheres by the seed-mediated growth method. The SiO2− SO3H spheres could be obtained easily by oxidation of the thiol group-terminated silica spheres (SiO2−SH) with H2O2. Due to the presence of sulfonic acid groups, the [Ag(NH3)2]+ ions were captured on the surface of the silica spheres, followed by in-site reduction to silver nanoseeds for further growth of the silver shell. By this strategy, the complete silver shell could be obtained, and the surface morphologies and structures of the silver shell could be controlled by adjusting the number of sulfonic acid groups on the silica spheres. A large number of sulfonic acid groups on the SiO2−SO3H spheres favored the formation of the macroporous silver shell, which was unique and exhibited good catalytic performance and a high surface-enhanced Raman scattering (SERS) enhancement ability.

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[Ag(NH3)2]+ ions could be adsorbed on the surface of the bare silica spheres via electrostatic attraction. Recently, some research groups have fabricated SiO2/Ag composite spheres based on the nature of bare silica spheres. Fox example, Kim et al. soaked silica spheres in an ethanolic solution of AgNO3 and butylamine to fabricate Ag-deposited silica spheres.9 Deng et al. used silica spheres to adsorb [Ag(NH3)2]+ ions, which were reduced to silver NPs in the presence of polyvinylpyrrolidone (PVP) at 70 °C.2 Wang et al. utilized a similar mechanism to adsorb [Ag(NH3)2]+ and PVP on the silica spheres and prepared SiO2/Ag composite spheres by a one-pot hydrothermal reaction.12 Although the high silver coverage could be achieved by the above methods, there still exist limitations to control the surface morphology and structure of the silver shell. There is also little discussion in the literature regarding the introduction of negatively charged functional groups on the silica spheres to guide the silver coating. According to our previous studies,14,15 negatively charged functional groups on the core materials not only enhanced the interaction between the metal coating and core but also effectively tuned the shape and structure of metal shell. However, by the layer-by-layer (LbL) self-assembly technique involving the electrostatic association between alternately deposited oppositely charged species, the desirable negatively charged shell could be

INTRODUCTION Using a noble metal (mainly, gold and silver) shell to coat dielectric silica spheres to form core−shell composite materials is of great interest in nanoscience due to their potential applications in antibacterials, catalysis, and surface-enhanced Raman scattering (SERS).1−12 The choice of the silica sphere as a core material is due to its low cost, easy preparation, uniform spherical shape, high monodispersity, and large particle size range. However, it is quite difficult to form a noble metal shell directly on bare silica spheres due to mutual incompatibility between them. To obtain a complete silver or gold shell, surface modification of the silica spheres has been the focus of research in this field. Utilizing a silane coupling agent [R1−Si(OR2)3] such as aminopropyl, where R1 is a functional group and R2 is CH3 or CH2CH3, to modify the silica spheres via well-known silane chemistry is one of the most popular methods.13 To date, amine-terminated silica (SiO2−NH2) spheres have been widely investigated because the amine groups could act as attachment points for small colloidal gold or silver, which then served as nucleation sites for the growth of a gold or silver nanoshell.3,7,8 However, the amine group is limited to the binding anionic precursors (e.g., AuCl4−) or negatively charged gold or silver nanoparticles (NPs) and therefore cannot be used to bind cationic precursors (e.g., [Ag(NH3)2]+ or Ag+). Actually, the silica spheres obtained by the Stöber method from the sol−gel process of tetraethyl orthosilicate (TEOS) under base catalysis exhibit a certain electronegativity due to the negatively charged silanol groups.9,10,12 Therefore, Ag+ or © XXXX American Chemical Society

Received: January 26, 2015 Revised: September 6, 2015

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DOI: 10.1021/acs.langmuir.5b02218 Langmuir XXXX, XXX, XXX−XXX

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solution (0.12 M). The as-prepared silver nanoseed-decorated SiO2− SO3H spheres were dispersed into 14 mL of the growth solution for 1 h under stirring at 0 °C. The products were collected by centrifugation (2000 rpm, 10 min) and washed several times with deionized water. Catalysis of the Reduction of MB Dye. The catalytic performance of SiO2−SO3H@Ag composite spheres was explored by monitoring the change in the absorbance intensity at the maximum absorbance wavelength (λmax) of the MB dye with a UV−visible spectrometer. In a typical procedure, 0.01 g of the SiO2−SO3H@Ag composite spheres was dispersed in 10 mL of water. Then, 1 mL of the above solution was added to 10 mL of the MB dye solution (6 × 10−4 M) and then injected rapidly with 1 mL of NaBH4 solution (0.1M) under stirring. The color of the mixture vanished gradually, indicating the reduction of the MB dye. Preparation of Samples for Surface-Enhanced Raman Scattering. For the SERS measurement, the as-obtained SiO2− SO3H@Ag composite spheres (about 10 mg) were first dispersed in 10 mL of solution as stock solution, and then different concentrations of 50 μL of 4-ATP solution were dropped into 2 mL of the above stock solution for 6 h. After the samples were separated twice by centrifugation/redispersion cycles, they were dropped onto a glass slide substrate and dried with high-purity flowing N2 before Raman examination. Characterization. The morphology of the products was observed by field-emission scanning electron microscopy (FE-SEM, SU70) at 5.0 kV. The composition of the samples was analyzed using X-ray photoelectron spectroscopy (XPS, Axis Ultra dld). The phase and the crystallographic structure of the products were characterized by X-ray diffraction (XRD) on a Rigaku D/max-RA X-ray diffraction meter. UV−visible absorption spectra were recorded using a UV−visible spectrophotometer (UV-2600, Shimadzu). SERS spectra were recorded with a confocal Raman spectrometer equipped with an HR800 microscope and employing a 632.8 nm laser beam and a charge-coupled detector (CCD) with 4 cm−1 resolution. The spectra were obtained by focusing a 1 μm laser spot on the sample using a 20× objective lens and were accumulated three times for 30 s each.

obtained.16,17 However, this approach is too time-consuming for practical application. Attaching carboxyl groups (−COOH) on silica spheres has also been reported, but the complex reaction process needs to be carried out.18,19 Thus, the facile introduction of negatively charged function groups onto silica spheres can still be crucial to both the fundamental understanding and fabrication of the silver shell with different shapes and structures for various applications. Herein, we select sulfonic acid group-terminated silica spheres (denoted as SiO2−SO3H) as the template, and various morphologies of silver shells can be obtained via the seedmediated growth method. The SiO2−SO3H spheres were obtained by oxidating the thiol group-terminated silica spheres (denoted as SiO2−SH) using hydrogen peroxide. [Ag(NH3)2]+ ions were absorbed on the SiO2−SO3H spheres and then followed via in situ reduction by NaBH4 to tiny silver NPs. These tiny silver NPs served as seeds for the subsequent silver shell growth in the growth solution containing [Ag(NH3)2]+ and glucose. The advantage of our strategy is that the SiO2−SH spheres can be obtained by a one-step process, and the oxidation process that is followed is also very simple because H2O2 gradually decomposes to H2O and O2, which do not contaminate the resulting products and environment. The shapes and structures of the silver shell on the silica spheres could be easily tuned by altering the oxidation time of SiO2− SH spheres, that is, the number of sulfonic acid groups on silica spheres. The applications of the as-prepared SiO2−SO3H@Ag composite spheres as a catalyst and SERS substrate were investigated, and the results reveal that the products have good catalytic capability for methylene blue dye and a high SERS signal enhancement for 4-aminothiophenol.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION The process of coating the SiO2−SO3H spheres with a silver shell is illustrated in Scheme 1: (1) 3-MPTMS was used as a

Chemicals. 3-Mercaptopropyl trimethoxysilane (MPTMS) and 4aminothiophenol (4-ATP) were purchased from Sigma-Aldrich. AgNO3 (≥99.8%), glucose, ammonium hydroxide (NH4OH, 25%), NaBH4, methylene blue (MB), and hydrogen peroxide (H2O2, 30%) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All chemicals were used as received without further purification. Synthesis of MPTMS Silica (SiO2−SH) Spheres. The synthesis procedure was based on a modified version of the previous method :20 1 mL of MPTMS was added to 100 mL of water under vigorous stirring at 40 °C until a transparent solution was obtained. Subsequently, 1 mL of NH4OH was added to the solution under stirring. After 6 h, the solution was centrifuged (2000 rpm, 10 min) and the precipitate was rinsed several times using ethanol. Finally, the product was vacuum dried at 50 °C for 12 h to a constant weight. Synthesis of SiO2−SO3H Spheres. SiO2−SH spheres (0.02 g) were dispersed in 10 mL of deionized water. Subsequently, 20 mL of 30% H2O2 was added to the above solution under stirring for different times at ambient temperature. The oxidation of SiO2−SH spheres for 2, 6, and 10 h resulted in SiO2−SO3H spheres denoted as S1, S2, and S3, respectively. The products were separated by centrifugation (2000 rpm, 10 min), rinsed with deionized water, and then vacuum dried at 40 °C for 12 h. Fabrication of Silver Nanoseed-Decorated SiO2−SO3H Spheres. SiO2−SO3H spheres (0.01 g) were dispersed in 10 mL of [Ag(NH3)2]+ solution (0.01 M), and the mixed solution was stirred for 1 h. Then, 5 mL of NaBH4 solution (0.01 M) was injected rapidly into the above solution under stirring. After 1 h, the products were centrifuged (2000 rpm, 10 min) and washed with deionized water for the next experiment. Fabrication of SiO2−SO3H@Ag Composite Spheres via Silver Mirror Deposition. The growth solution was prepared by mixing 30 mL of glucose aqueous solution (0.56 M) and 20 mL of [Ag(NH3)2]+

Scheme 1. Schematic Illustration of the Formation of SiO2− SO3H@Ag Composite Spheres

silane precursor to prepare SiO2−SH spheres. (2) SiO2−SH spheres were oxidized using H2O2 to form SiO2−SO3H spheres. (3) [Ag(NH3)2]+ ions were complexed with sulfonic acid groups of the SiO2−SO3H spheres, followed by in situ reduction with NaBH4 to silver “nanoseeds” on SiO2−SO3H spheres. (4) Silver nanoseeds on SiO2−SO3H spheres acted as B


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Langmuir nuclei, and [Ag(NH3)2]+ ions in the growth solution were reduced at the surface of the seeds via heterogeneous nucleation during the particle-growth reaction. SEM images of the obtained highly monodisperse SiO2−SH spheres with a diameter of 680 nm by the MPTMS silane precursor are shown in Figure S1 (Supporting Information). The oxidation of thiol groups to form sulfonic acid groups with H2O2 is conducted according to the following reaction:21

ence makes XPS a useful tool for evaluating the degree of oxidation of thiol groups to sulfonic acid groups from Figure 1b. When SiO2−SH spheres are oxidized with H2O2 at ambient temperature for 2 h (S1 sample), the percentage compositions of the sulfonic acid groups and thiol groups on the silica spheres are 4.8 and 95.2%, respectively, implying that a very small number of thiol groups are oxidized. Prolonging the oxidation time to 6 h (S2 sample) results in an increase in the number of sulfonic acid groups (−SO3H, 52.1%; −SH, 47.9%). After oxidation for 10 h, the percentage of sulfonic acid groups reaches 66.6%, which indicates that most thiol groups on silica spheres have been converted to sulfonic acid groups. An excess degree of oxidation may cause SiO2−SH particles to lose their spherical contour because all thiol groups are converted to sulfonic acid groups, and the relevant SEM images are shown in Figure S2 (Supporting Information). The SEM images of the silver shell on the SiO2−SO3H spheres with different numbers of sulfonic acid groups via the seed-mediated growth method are shown in Figure 2. It is found that complete silver shells are obtained by this strategy. Moreover, the silver shells exhibit different shapes and structures depending on the number of sulfonic acid groups on silica spheres. Figure 2a shows that the formed silver shell consists of large silver NPs, and Figure 2b shows that the silver shell consists of small silver NPs. When S3 samples were used as templates, a macroporous silver shell could be formed (Figure 2c). Figure S3 (Supporting Information) exhibits small cavities with a size ranging from 10 to 100 nm on the exterior surface of each sphere. To confirm the core−shell structure of as-prepared SiO2−SO3H@Ag composite spheres, a partial silver shell was peeled off of the SiO2−SO3H sphere surface by extensive sonication (or the SiO2−SO3H sphere core was selectively removed). The relevant SEM images shown in Figures S4 and S5 (Supporting Information) demonstrate that the silver coating exists on the surface of the SiO2−SO3H spheres. It is worth noting that if the SiO2−SH spheres are used as templates in the experiment, then there are a lot of free silver NPs in the final product (Figure S6, Supporting Information). This result indicates that homogeneous nucleation occurred rather than preferential growth around the SiO2−SH spheres when [Ag(NH3)2]+ ions were reduced in growth solution. Therefore, on the basis of SEM observation and the above XPS analysis, we think that the morphology evolution is mainly a result of the increasing number of sulfonic acid groups on the silica spheres. Chen et al. have demonstrated that more silver nanoseed nuclei located on the surface of silica spheres can result in the production of smaller and denser Ag NPs.11 In our experiment, S1@Ag spheres and S2@Ag spheres agree well with this theory because S2 spheres with more sulfonic acid groups could absorb more [Ag(NH3)2]+ ions, which results in the generation of more silver nanoseeds. However, this theory does not apply to S3@Ag spheres. According to a previous report,25 SiO2−SH spheres are composed of a cross-linked long-chain polymer. After the SiO2−SH spheres are oxidized by H2O2, a cross-linked long-chain polymer layer containing sulfonic acid groups (polymer−SO3H) on the surface of SiO2− SH spheres is created (Figure S7, Supporting Information). The outer layer is completely hydrophilic and can effectively adsorb [Ag(NH3)2]+ ions. Because S3 spheres contain enough of the thick polymer−SO3H layer, some [Ag(NH3)2]+ ions might permeate the layer inside. A thicker polymer−SO3H layer and more silver nanoseeds distributed on the surface and inside the layer might be the cause of the fabrication of the

−SH + 3H 2O2 → −SO3H + 3H 2O


The process is safe and reliable since the reaction condition is very mild (ambient temperature). Outside-in oxidation of SiO2−SH spheres could create different numbers of sulfonic acid groups with different oxidation times. The oxidative conditions were examined using X-ray photoelectron spectra (XPS), which could reveal the degree of oxidation of thiol to sulfonic acid. Figure 1a shows the survey spectra of the samples.

Figure 1. (a) XPS spectra of samples. (b) Curve fitting of S 2p XPS binding-energy peaks for the samples.

The binding energy of C 1s, O 1s, and Si 2p is observed at 284.27, 532.1, and 102.7 eV, respectively. Figure 1b shows the curve fitting of S 2p binding energies for two types of sulfur species: one at higher BE (168.5 eV), corresponding to sulfonic acid groups in anchored Si(CH2)3SO3H (Si represents Si bonded to three framework atoms), and another at lower BE (163.7 eV) associated with sulfur remaining in the original state (i.e., in unoxidized Si(CH2)3SH22−24). This energy differC


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Figure 2. High- and low-SEM images of the obtained SiO2−SO3H@Ag composite spheres by a seed-mediated growth method. (A, a) S1@Ag spheres, (B, b) S2@Ag spheres, and (C, c) S3@Ag spheres.

solution can preferentially deposit on these seeds for the growth of the silver shell. The typical XRD pattern of SiO2−SO3H@Ag is illustrated in Figure S9 (Supporting Information), which exhibits peaks at 2θ angles of 37.9, 44.1, 64.3, 77.2, and 81.4° corresponding to the reflections of the (111), (200), (220), (211), and (222) crystalline planes of the fcc structure of silver.2 Compared to the XRD pattern of the SiO2−SO3H sphere, the result indicates that the crystalline silver NPs have been formed on the surface of SiO2−SO3H spheres. Catalytic Property of SiO2−SO3H@Ag Composite Spheres. It has been experimentally demonstrated that Ag NPs could act as a catalyst for the reduction of various dyes and exhibit high catalytic activity.2,26 Here, we investigated the catalytic property of the SiO2−SO3H@Ag composite spheres by studying the evolution of the absorbance intensity at λmax. Their evolution of the UV−visible spectra of the MB dye during the reduction reaction in the presence of the SiO2−

macroporous silver shell. S1 and S2 contain thinner polymer− SO3H layers, and Ag nanoseeds are located only on the surface of SiO2−SO3H spheres, resulting in a common silver shell rather than a macroporous silver shell. A control experiment was also carried out to investigate if the complete silver shell could be formed directly on SiO2−SO3H spheres not decorated with silver nanoseeds by the silver-mirror reaction. Under the same reaction conditions, the SEM images of the obtained SiO2−SO3H/Ag composite spheres are shown in Figure S8 (Supporting Information). It is found that silver coverage is very low on SiO2−SO3H spheres, and no continuous silver shell is formed. In addition, many silver particles dispersed in the solution are detected, which implies that some silver nucleation occurs in the solution phase. The above results confirm that the silver nanoseeds tethered to the SiO2−SO3H spheres play a crucial role in fabricating a complete silver shell and that the reduced silver in the growth D


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Figure 3. UV−visible spectra of an MB dye reduced by NaBH4 as functions of reaction time and catalyst.

molecule because it has a distinct Raman feature,30 and −SH of the 4-ATP molecule can easily bond to form a strong −S− metal bond when it is adsorbed on the metal surface. Figure 4(A) shows the SERS spectra of 4-ATP molecules obtained from SiO2−SO3H@Ag composite spheres. For comparison, the normal Raman spectra of solid 4-ATP and pure SiO2@Ag (S3@Ag) spheres are also given in Figure 4(A). SERS signals are observed in all cases, but noticeable differences between SERS and the normal spectrum are frequency shifts for some bands and changes in the relative intensity.32 The υcs band shifts from 1087 to 1073 cm−1, and a visible frequency shift from 1592 to 1577 cm−1 occurs, which indicates that the −SH bond in 4-ATP directly contacts the SiO2−SO3H@Ag spheres by forming a strong Ag−S bond.33,34 From Figure 4(A), the S1@Ag spheres give only a very weak enhancement. As for S2@Ag spheres, the SERS spectrum becomes more intense because S2@Ag spheres have rougher surfaces and more gaps generated by small silver NPs. The most intense spectrum is observed using S3@Ag spheres as substrates. The results may be attributed to two reasons: (1) Among three types of SiO2− SO3H@Ag spheres, the sample S3@Ag spheres have the roughest surface, which provides more hot spots favoring SERS signal enhancement. (2) More importantly, the macroporous structure of the sample S3@Ag spheres increases the accessible surface area for the detected molecules, which can also enhance the SERS signal.7 To measure the SERS results quantitatively, we calculated the surface enhancement factor (G) of 4-ATP on the S3@Ag composite spheres. The value of G was calculated using the following expression.35

SO3H@Ag composite spheres is illustrated in Figure 3. It is found that the absorbance at λmax of the MB dye decreases with the reaction time. For S1@Ag and S2@Ag composite spheres, the absorbance in intensity at λmax of MB decreases but does not completely disappear within 15 min. After S3@Ag composite spheres as catalysts were added to the mixture, the absorbance at λmax quickly declined and completely vanished within 15 min. The results may be attributed to the macroporous structures of S3@Ag composite spheres. It is widely accepted that the catalytic performance is highly dependent on the surface-to-volume ratio and the morphology of noble metal nanostructures.27,28 To three samples, although the SiO2−SO3H spheres were all covered completely with silver NPs, S2@Ag and S3@Ag spheres exhibit more rough surfaces than do S1@Ag spheres. In S3@Ag spheres, the interior of the macroporous silver shell contains a lot of void spaces and connected channels, resulting in an increase in the accessible surface area for MB and NaBH4 molecules and promoting the rate of reaction. When the dyes were reduced with NaBH4 solutions in the absence of SiO2−SO3H@Ag composite spheres, the color of the dye remained unchanged for 12 h. Therefore, the experimental results confirm that the obtained SiO2−SO3H@Ag composite spheres exhibit good catalytic performance. SERS Property of the SiO2−SO3H@Ag Composite Spheres. According to previous work, the SERS signals can be significantly enhanced in “hot spot” regions, which are present in the “gap” between noble metal NPs and sharp angles or the tip of noble metal nanostructures.29−31 In our work, asprepared SiO2−SO3H@Ag spheres exhibit a rough surface, which could provide more “hot spots” for SERS signal enhancement. We evaluated the performance of the as-prepared SiO2−SO3H@Ag composite spheres as SERS substrates using 4-ATP as a probe molecule. 4-ATP is chosen as the probe

G= E

ISERS Nbulk Ibulk Nads DOI: 10.1021/acs.langmuir.5b02218 Langmuir XXXX, XXX, XXX−XXX

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fairly good quality, and its main bands have high signal-to-noise ratios. This detection limit is identical to the result reported by other research groups.36,37 The evidence indicates that the S3@ Ag composite spheres as SERS-active substrates have a high sensitivity to 4-ATP molecules and have potential applications in single-molecule detection.


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CONCLUSIONS We presented a novel SiO2−SO3H sphere template for the uniform coating of a silver shell via the seed-mediated growth method. The shapes and structures of the silver shell depended on the number of sulfonic acid groups on the silica spheres. The applications of the SiO2−SO3H@Ag core−shell composite spheres as catalysts and SERS substrates were investigated, and the results demonstrated they had excellent catalytic ability with respect to MB and high SERS activity with respect to 4-ATP. The novel SiO2−SO3H spheres may provide a versatile platform for the deposition of other metals or functional materials due to the presence of sulfonic acid groups.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02218. SEM images, XRD patterns, and Raman spectra of the relevant samples (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest.

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Figure 4. (A) SERS spectra obtained from the samples: the intensities were magnified 3- and 8-fold for normal solid 4-ATP and pure S3@Ag composite spheres, respectively. (B) SERS spectra of lower concentrations of 4-ATP on S3@Ag composite spheres.

ACKNOWLEDGMENTS This work was sponsored by the K. C. Wong Magna Fund in Ningbo University, the Scientific Research Fund of Education of Zhejiang Province (Y201327028), the Fund for Science and Technology Innovative Team in Zhejiang Province (2011R50001-03), the Ningbo Natural Science Foundation (2014A610142), and the Postgraduate Innovation Fund of Ningbo University (G15043).

where ISERS and Ibulk denote the intensities of the same vibrational mode for adsorbed molecules and solid 4-ATP, respectively. Nads is the density of adsorbed molecules in the laser spot, and Nbulk represents the density of molecules in the bulk solid. If we assume that the S3@Ag composite spheres are closely packed on the glass to form a compact film and 4-ATP is dispersed on the film uniformly, then the density of 4-ATP is assumed to be 10−6 M × 50 μL × NA/cm2 (i.e., 3.01 × 1013/ cm2 according to previous reports33). Because the surface area of the laser spot (1 μm diameter) is about 7.85 × 10−9 cm−2, the number of adsorbed molecules within the laser spot is 2.36 × 105. The spot diameter of the laser beam was 1 μm, and its penetration depth was also 2 μm. Therefore, Nbulk is calculated to be 8.9 × 109 considering the density of 4-ATP to be 1.17 g/ cm3.34 The ratio of Nbulk to Nads was about 3.77 × 104. The intensity of the vibrational mode (vcc) at 1592 cm−1 is used to calculate the values, and the ratio of ISERS to Ibulk is about 3.1, Therefore, G is calculated to be 1.17 × 105. The G value is higher than that of the assembled silver NPs (three layers) and is equal to that of the assembled silver NPs (six layers).34 To investigate the limit of detection of 4-ATP on S3@Ag composite spheres, we prepared a solution ranging from 1 × 10−7 to 1 × 10−9 M. The relevant Raman spectra are shown in Figure 4(B); the spectrum of the 1 × 10−9 M sample also has

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Using a Macroporous Silver Shell to Coat Sulfonic Acid Group

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