Differential SERS Activity of Gold and Silver Nanostructures Enabled

Jan 7, 2012 - citrate-reduced Ag and Au NPs are similar for rhodamine 6G (R6G) ... higher than BPE,s pKa2 of 5.65, where the analyte is largely unprot...
1 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Langmuir

Differential SERS Activity of Gold and Silver Nanostructures Enabled by Adsorbed Poly(vinylpyrrolidone) Polina Pinkhasova,† Liu Yang,‡ Yong Zhang,‡ Svetlana Sukhishvili,*,‡ and Henry Du*,† †

Department of Chemical Engineering and Materials Science and ‡Department of Chemistry, Chemical Biology, and Biomedical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States S Supporting Information *

ABSTRACT: We report that poly(vinylpyrrolidone) (PVP), a common stabilizer of colloidal dispersions of noble metal nanostructures, has a dramatic effect on their surface-enhanced Raman scattering (SERS) activity and enables highly selective SERS detection of analytes of various type and charge. Nanostructures studied include PVP-stabilized Au−Ag nanoshells synthesized by galvanic exchange reaction of citrate-reduced Ag nanoparticles (NPs), as well as solid citrate-reduced Ag and Au NPs, both before and after stabilization with PVP. All nanostructures were characterized in terms of their size, surface plasmon resonance wavelength, surface charge, and chemical composition. While the SERS activities of the parent citrate-reduced Ag and Au NPs are similar for rhodamine 6G (R6G) and 1,2-bis(4-pyridyl)ethylene (BPE) at various pH values, PVP-stabilized nanostructures demonstrate large differences in SERS enhancement factors (EFs) between these analytes depending on their chemical nature and protonation state. At pH values higher than BPE’s pKa2 of 5.65, where the analyte is largely unprotonated, the PVP-coated Au−Ag nanoshells showed a high SERS EF of >108. In contrast, SERS EFs were 103- to 105-fold lower for the protonated form of BPE at lower pH values, or for the usually highly SERS-active cationic R6G. The differential SERS activity of PVP-stabilized nanostructures is a result of discriminatory binding of analytes within-adsorbed PVP monolayer and a subsequent increase of analyte concentration at the nanostructure surface. Our experimental and theoretical quantum chemical calculations show that BPE binding with PVPstabilized Au−Ag nanoshells is stronger when the analyte is in its unprotonated form as compared to its cationic, protonated form at a lower pH.

■

INTRODUCTION Polymers along with low molecular weight surfactants have proven indispensible for size- and shape-controlled synthesis of noble metal colloidal nanostructures for a variety of applications including chemical and biological sensing using surfaceenhanced Raman scattering (SERS).1−3 Poly(vinylpyrrolidone) (PVP), a neutral polymer, is among the most commonly used to obtain stable intricate nanostructures such as Au−Ag nanocubes,4 Au−Ag nanocages,5 and dendritic Au nanoparticles (NPs).6 One recent example of PVP-assisted nanostructure growth is the synthesis of hollow Au nanospheres via galvanic replacement of Co nanoparticles with Au salt.7 Adsorbed PVP sustains colloidal stability during synthesis via steric (entropic) interparticle repulsion between PVP loops. Surface chemistry of the noble nanostructures plays a critical role in SERS as it has its origin stemming primarily from the socalled electromagnetic and chemical enhancements.8−10 The effect of surface chemistry on SERS has been mostly probed from the standpoint of charge manipulation to maximize electrostatic attraction between the analyte and the metal nanostructures. For example, several studies have indicated the importance of electrostatic interactions in SERS measurements.11,12 We reported earlier that adsorption of either cationic or anionic © 2012 American Chemical Society

species on Ag NPs can be promoted by controlling the particle surface charge for high SERS sensitivity.13 Exploring SERS sensing using colloidal solutions, Aroca and co-workers14 showed that the SERS intensity for several analytes is strongly related to the ζ potential and the solution pH. The strongest SERS signal was obtained at ζ potential values where electrostatic attraction between the particles and the analyte was maximized. With SERS substrates containing immobilized Ag NPs of controlled surface chemistry and charge, we also illustrated previously that ultrasensitive detection of anions is achievable exclusively in the case of positively charged Ag NPs.15 Unlike electrostatic interactions, the role of other types of interactions between analyte molecules and SERS-active nanostructures has not yet been explored. One evident example is the analyte interactions with commonly used stabilizing agent PVP. Here, we explore the role of PVP, adsorbed at the noble metal surface, and their SERS activity. Using pH-sensitive trans-1, 2-bis(4-pyridyl)ethylene (BPE) and rhodamine 6G (R6G) as analytes, and PVP-stabilized Ag/Au nanostructures, as well as Received: December 5, 2011 Revised: January 6, 2012 Published: January 7, 2012 2529

dx.doi.org/10.1021/la2047992 | Langmuir 2012, 28, 2529−2535

Langmuir

Article

Immobilization of NPs and Nanoshells on Planar Substrates. Cylindrical glass liquid cells custom-made with glass or quartz substrate as the cell bottom were used for immobilization of Ag or Au NPs and Au−Ag nanoshells on the bottom surface. Briefly, the cell was filled with aqueous solution of PAH (2 mg mL−1) at pH 9 for 20 min to allow its adsorption on the cell bottom. The cell was subsequently rinsed thoroughly with Milli-Q water at pH 4.5 to remove any free or loosely bound PAH. Next, NPs were attached to the PAHmodified surface using a solution of 1016 particles mL−1 at pH 5.5 for 4 h. All NPs were at the same concentration and pH. Nanostructures were immobilized through electrostatic interaction of the positively charged PAH surface with the negatively charged NPs/nanoshells as well as via the binding affinity of Ag and Au NPs to the amino groups in PAH. We note that there is excellent reproducibility in terms of the distribution and the coverage density of immobilized particles of the same structuretype under the same conditions as ascertained by SEM. This is critically important for the study of the effect of PVP treatment while keeping all other factors, especially nanoparticle aggregation, consistent. The cells with immobilized Ag and Au NPs or Au−Ag nanoshells at the glass and quartz bottom surface were rinsed five times with Milli-Q water and were used right away for SERS measurements. SERS and FTIR Measurements. SERS measurements were conducted at either 632.8 or 785 nm excitation wavelength using a custom-built Raman spectrometer.19 These measurements aimed to study the substrate-to-substrate reproducibility and surface adsorbates of the as-synthesized nanostructure surface as well as to quantitatively evaluate analytical SERS EF of the nanostructures for different analytes. Analytical EFs were calculated using EF = (ISERS/CSERS)/(INR/CNR), where ISERS and INR are the integrated intensities of a characteristic band from SERS and from normal Raman, and CSERS and CNR are concentrations of analytes used in SERS and normal Raman experiments, respectively. Both intensities of normal Raman and SERS were collected under the same conditions (same power, wavelength, and acquisition time), and the volume of the analyte solution was kept constant. Intensities of the BPE peak (1610 cm−1) and R6G peak (1510 cm−1) were used to calculate the EFs. The intensities of these peaks were normalized to the appropriate concentration of the analyte present in the liquid cell. To ensure statistical significance, SERS measurements and calculated EFs for each analyte were done on three identically prepared substrates of the same nanostructure-type, and five measurements were done on each of those substrates; variation between identically prepared substrates and spot-to-spot variation was less than 15%. The SERS data were processed using Origin 8.0 software. FTIR measurements were carried out using a PARAGON 1000 PC FTIR spectrometer (Perkin-Elmer) to further delineate the different surface characteristics of the nanostructures. FTIR sample preparation entailed washing and centrifuging at 10 000 rpm for 30 min of Ag and Au NPs and Au−Ag nanoshells. This procedure was repeated two times. Finally the colloidal solutions were freeze-dried, mixed with KBr, and compressed into a pellet for FTIR analysis. Binding of BPE to Nanoshells. BPE solutions with concentrations ranging from 2.0 × 10−5 to 2.0 × 10−6 M were prepared with 20 mL of nanoshells (1014 particles mL−1). For each concentration two identical solutions were prepared, one at pH 3.0 and the other at pH 6.3. Solutions were left overnight to allow complete binding. Next, each solution was centrifuged at 10 000 rpm for 10 min in order to separate BPE bound to

PVP-free solid Ag and Au NPs as controls, we show that adsorbed PVP layer enables selective binding of analytes, based on their chemical nature and ionization state. For example, for the case of PVP-stabilized hollow Ag−Au nanoshells we show that optimized binding between a neutral form of BPE and adsorbed PVP layer results in a high analytical SERS enhancement factor (EF) of ≥108, while the same nanostructure exhibited a surprisingly low EF of 105 for the commonly used Raman probe R6G. Our experimental results demonstrate and theoretical results confirm that PVP absorbed at the surface of the Au/Ag nanostructure plays an important role in discriminative binding and differential SERS sensing of analytes.

■

EXPERIMENTAL SECTION Materials. The following reagents were purchased from the indicated suppliers and used without further purification: silver nitrate (ultrapure grade, Across), sodium chloride (99.999%, Sigma), sodium citrate (Fisher), hydrogen tetrachloroaurate(III) (30 wt % solution in dilute hydrochloric acid, 99.999%), poly(vinylpyrrolidone) (PVP; weight-average molecular weight of 40 000 g mol−1, Sigma), poly(amine hydrochloride) (PAH; weight-average molecular weight of 15 000 g mol−1, Aldrich), rhodamine 6G (R6G; Aldrich), 1,2-bis(4-pyridyl)ethylene (BPE; Sigma). All glassware was cleaned in nochromix solution and sulfuric acid for 24 h, followed by thorough washing with Milli-Q water. Synthesis and Characterization of Ag and Au NPs. Ag and Au NPs were synthesized by a modified Lee and Meisel method16 with aid of UV irradiation, details of which can be found elsewhere.17 Briefly, for Ag NP synthesis, 1% aqueous sodium citrate (0.8 mL) was added to AgNO3 solution (1 mM, 40 mL). The mixture was placed under a UV lamp (UV Flood Curing System, Cure Zone 2 (CON-TROL-CURE, Chicago, IL)) for 4 h with gentle stirring. For Au NP preparation, 1% aqueous sodium citrate (4 mL) was added to HAuCl4 solution (5 mM, 40 mL). The mixture was placed under UV lamp for 30 min under stirring. The size and distribution, surface charge, and plasmon resonance of the resultant Ag and Au NPs were determined by transmission electron microscopy (TEM, Philips CM20), scanning electron microscopy (SEM, Auriga Ziess), dynamic light scattering (Zetasizer Nano Series, Malvern Instruments), and UV−visible absorption spectroscopy (Synergy HT multidetection microplate reader, BioTek Instruments). For PVP-coated Ag and Au NPs, 9 mL of synthesized NPs was mixed with 1 mL of 1 mg/mL PVP solution. The solution was left overnight under constant stirring. Next, it was centrifuged twice under 10 000 rpm for 10 min and washed with Milli-Q water in order to remove free PVP from solution. Synthesis and Characterization of Au−Ag Nanoshells. Au−Ag hollow nanoshells were obtained via galvanic replacement reaction,18 using Ag NPs as a template. A 10 mL mixture of Ag colloids (0.3 mM) and PVP (1 mg mL−1) was heated to boil. Immediately upon boiling a specific amount of aqueous HAuCl4 (0.5 mM) was added dropwise under stirring, with a colloidal Ag to HAuCl4 molar ratio ranging from 2.85 to 20 in the solution mixture. The color changed instantly. The solution was boiled for another minute to ensure completion of reaction as evidenced by color stabilization. Once cooled to room temperature, the resultant nanostructures were centrifuged and washed one time with Milli-Q water to remove silver chloride byproduct. The Au−Ag nanoshells were similarly characterized as the Ag and Au NPs. 2530

dx.doi.org/10.1021/la2047992 | Langmuir 2012, 28, 2529−2535

Langmuir

Article

obtained at colloidal Ag to HAuCl4 molar ratio of 2.85, yielding nanoshells with 53:47 Au:Ag atomic composition according to energy-dispersive X-ray (EDX) measurements (Figure S1 in Support information). Figure 1 shows UV−visible absorption spectra of the Ag and Au colloids as well as the chosen Au−Ag nanoshells. The plasmon resonances of the Ag and Au NPs are located at 420 and 525 nm, respectively, in agreement with literature results.16,28 A shift in the plasmon resonance took place from 420 nm for the Ag NP template to 720 nm for the subsequently grown Au−Ag nanoshells. The TEM image in Figure 1 reveals the hollow core of the nanoshells. The redshifted plasmonic band of the nanoshells as compared to solid NPs results from the changes in the structure and the composition, rather than the size of the nanostructures. Table 1 and Figure 1 show that Au−Ag nanoshells had approximately the same overall diameter as compared to solid Ag NPs. We note that while only a single nanoshell type is shown in Table 1 and Figure 1, a continued red shift in the plasmon resonance is observed as the HAuCl4 to Ag NPs molar ratio is increased from 2.85 to 20 (Figure S1 in Support information), corresponding to gradual enrichment of Au in the nanoshells, consistent with literature findings.18,29 One important observation is that in spite of a low value of ζ potential of Ag−Au nanoshells (−15 mV, Table 1), nanoshells remained highly dispersed as individual particles both in solution and after surface immobilization. Such exceptional colloidal stability of the nanoshells stems from steric hindrance of these nanostructures with adsorbed neutral PVP chainsa feature very useful and routinely exploited in synthesis of a variety of nanostructures. We then aimed to obtain further proof of the presence of PVP at the surface of nanoshells, as well as to shed light on the competitive strength of binding to Ag/Au surfaces of residual products of citrate oxidation used in synthesis of Ag and Au NPs and PVP-stabilized Ag particles used in galvanic reaction. To that end, we also added, postsynthesis, PVP to citrate-reduced Ag and Au NPs and explored the effect of such addition on the particle size, change, and colloidal stability. Table 1 and Figure 2 both show that

nanoshells. The collected supernatant was monitored with UV absorption, using BPE absorption peak at 330 nm. In addition, identical concentrations of BPE in water without any nanoshells were prepared and monitored with UV absorption. All of the UV data were processed using Origin 8.0 software. Theoretical Calculations. Quantum chemical simulations were carried out to assess the effect of the protonation state on binding between the monomer of PVP and BPE and provide theoretical insights into the role of the absorbed PVP in SERS sensitivity under different pH conditions. The solvent effect in the aqueous solutions was included in both the geometry optimization and energetic property calculations, using a PCM method.20−24 Normal vibrational mode analysis was performed to ensure each structure is indeed an energy minimum stationary state in the respective potential energy surface. The reaction enthalpies and Gibbs free energies were calculated at room temperature as with the experiments. All of the calculations were done using the commonly adapted hybrid density functional theory and Hartree−Fock method B3LYP25 and the triple-ζ basis set 6-311G plus an additional d function for each heavy atom and an additional p function for each hydrogen. The Gaussian 09 program26 was used to carry out these calculations.

■

RESULTS AND DISCUSSION We initially synthesized three types of nanostructures: (a) citrate-reduced solid Ag NPs (used as a template for synthesis of Au−Ag nanoshells); (b) citrate-reduced solid Au NPs; (c) Au−Ag nanoshells. The nanoshells were made by galvanic reaction of Ag NPs with HAuCl4, as described elsewhere.27 While synthesis of solid Ag and Au NPs did not involve the use of stabilizing PVP, the use of PVP was imperative to ensure colloidal stability of Au−Ag nanoshells during galvanic reaction. Table 1 and Figure 1 summarize size, charge, and plasmonic Table 1. Size and Charge Characterization of Ag/Au Nanostructures nanostructure type

diameter, nm (from TEM)

ζ-potential, mV (pH 5.5)

Ag NPs Au NPs Au−Ag nanoshells

45 ± 5 85 ± 8 45 ± 5

−35 −40 −15

Figure 2. ζ potential and hydrodynamic diameter (DH) as a function of pH of citrate-reduced or PVP-adsorbed Ag NPs (A) and PVPstabilized Au−Ag nanoshells (B). Figure 1. UV−visible absorption spectra of colloidal solutions of Ag NPs (a), Au NPs (b), and Au−Ag nanoshells (c) with 53:47 Au:Ag atomic composition. The insets are their corresponding TEM images.

PVP-free, citrate-reduced Ag and Au NPs (data for Au NPs are shown in Figure S2 in the Supporting Information) exhibit relatively high (−35 to −40 mV) ζ-potential values as measured in deionized water. The negative surface charge for both Ag and Au NPs results from the adsorbed citrate remnants.30 Figure 2A

characteristics of these initially synthesized nanostructures. Included here are results of only one type of Au−Ag nanoshells 2531

dx.doi.org/10.1021/la2047992 | Langmuir 2012, 28, 2529−2535

Langmuir

Article

carboxylate group (νs(COO−) and νas(COO−), respectively) present at the Ag NP surface. Similar bands are also seen in the FTIR spectrum of a reference sodium citrate (Figure S3a in the Supporting Information). However, in the case of Ag NPs, we suggest that caboxylate groups belong to a product of oxidation of sodium citrate−acetoacetic acidrather than to initial sodium citrate. Indeed, it has been postulated and experimentally demonstrated by Munro et al.30 that under boiling conditions citrate undergoes thermal and photooxidation32 resulting in acetonedicarboxylic acid, which further decomposes at elevated temperatures to form acetoacetic acid. The FTIR spectrum of Au NPs (Figure 3b) is notably different from that of Ag NPs in that two additional bands have emerged at 1236 and 1722 cm−1, assigned to the bending vibration of carboxylate ion δ(COO−) and stretching vibration of carbonyl group ν(CO), respectively, which may correspond to products of further oxidation and decomposition of sodium citrate. Interestingly, the addition of PVP in Ag and Au colloids followed by subsequent boiling has resulted in displacement of citrate remnants and related products adsorbed at the surfaces of both Ag and Au NPs by PVP (Figure 3c,d, respectively). Note that displacement of citrate remnants was complete in the case of Au NPs, while a small amount of products of citrate oxidation remained at the surface of Ag NPs, indicating slightly higher PVP affinity to Au rather than to Ag NPs. Incorporation of PVP within the adsorbed layer is evident from the appearance of two new bands at 1290 and 1655 cm−1 (Figure 3c,d), corresponding to the vibration of C−N and carbonyl group in the pyrrolidone ring.33 The corresponding spectrum of bulk PVP containing the same vibrational bands is shown in Figure S3b. Importantly, vibrational features of Au−Ag nanoshells (Figure 3e) resemble those of the PVP-treated Ag and Au NPs (Figure 3c,d). These results indicate that PVP is the dominant adsorbate on the resultant Au−Ag nanoshells following galvanic replacement reactions with PVP as a stabilizer. To evaluate the SERS activity of PVP-stabilized Ag−Au nanoshells as well as Ag and Au NPs with or without adsorbed PVP, these nanostructures were immobilized on glass and quartz substrates using procedures described in the Experimental Section. Insets in Figure 4 are representative SEM images of the resultant substrates. While the solid NPs typically lead to mixed discrete and clustered particles on the substrate (Figure 4A,B), Ag−Au nanoshells yield discrete nanostructure coverage (Figure 4C). It should be realized that SERS enabled by colloidal nanoparticles has a strong dependence particle aggregation, with EF variation by orders of magnitude possible that stems from the well-known hot-spot effect. Because the focus of our study is on the effect of PVP treatment, we purposefully carried out SERS measurements using the same substrates but before and after the treatment process, effectively eliminating the aggregation effect as a variable for the observed SERS response. As a result the change in the SERS enhancement could only be attributed to the change in the surface chemistry of the nanostructures through adsorption of PVP. SERS measurements involving Ag and Au NPs were carried out at 633 nm excitation wavelength in accordance with their UV− visible absorption characteristics. Similarly, measurements using Au−Ag nanoshells were carried out at 785 nm. Shown in Figure 4 are the Raman background spectra of the Ag and Au NPs as well as the Au−Ag nanoshells, all in the absence of any analyte. Upper spectra on Figure 4A,B represent the background of citrate-reduced Ag and Au nanoparticles, and the lower spectra

and Figure S2 additionally show that citrate-reduced Ag and Au colloids became unstable and aggregated readily below pH 4.0 due to diminishing negative surface charge (consistent with pKa values for ionization of the carboxylic group of citric acid (pKa1 ∼ 3.1, pKa2 ∼ 4.8, and pKa3 ∼ 6.4)). At the same time, PVPtreated NPs (see Experimental Section for details) prevented such aggregation at low pH values (Figure 2), in spite of a decrease in ζ potential below −20 mV. This suggests adsorption of PVP at the NPs. Enhanced steric repulsions between NPs by adsorbed PVP opened a wide pH window for galvanic reactions of Ag NPs while ensuring colloidal stability throughout the replacement reaction. Importantly, absolute values of ζ potential of Ag−Au nanoshells were similarly low to those of PVP-stabilized Ag and Au NPs (∼−15 mV at pH 4−10, and lower still at pH < 4), and Ag−Au nanoshells remained individaully dispersed in the entire range of solution pH from 2 to 10 (Figure 2B), also in agreement with the steric stabilization argument. Significantly, the measurements of the hydrodynamic diameter of NPs and nanoshells (at pH > 4−5) showed relatively low polydispersity (with number-averaged DH of 58 ± 5, 97 ± 5, and 78 ± 1 nm at pH 5.5 for Ag and Au NPs and Ag−Au nanoshells, respectively), indicating the almost exclusive presence of individually dispersed particles. Therefore, an increase in DH (ΔDH) of citrate-reduced Ag and Au NPs after addition of PVP of 5−15 nm is statistically significant and reflects formation of adsorbed PVP layer around NPs. The ΔDH = 5−15 nm (for the range of pH 5.5−9.0) is determined by the extension of loops and tails of adsorbed PVP. This result agrees well with the previously reported thickness in the case of PVP adsorbed on Pd NPs.31 To explore differences in the chemical composition of species adsorbed at the surfaces of Ag and Au NPs before and after PVP addition and to compare those changes with the chemical species adsorbed at the surface of Au−Ag nanoshells, we performed FTIR analysis of dried dispersions of these nanostructures. Prior to FTIR analysis, samples were centrifuged and rinsed with Milli-Q water, as described in Experimental Section, in order to remove free chemical species in solution. Illustrated in Figure 3 are the corresponding FTIR spectra (intensity not

Figure 3. FTIR spectra of citrate-reduced Ag NPs (a), citrate-reduced Au NPs (b), PVP-coated Ag NPs (c), PVP-coated Au NPs (d), and Ag−Au nanoshells stabilized by PVP (e). All dispersions were centrifuged and re-dispersed in water at 10 000 rpms for 30 min (two times), prior to measurements. Measurements were carried out using freeze-dried samples.

in scale). The FTIR spectrum of Ag NPs (Figure 3a) consists mainly of two bands at 1385 and 1590 cm−1, corresponding to the symmetric and asymmetric stretching modes of the 2532

dx.doi.org/10.1021/la2047992 | Langmuir 2012, 28, 2529−2535

Langmuir

Article

Figure 4. SERS spectra of citrate-reduced and PVP-adsorbed Ag NPs (A), Au NPs (B), and PVP-stabilized Au−Ag nanoshells (C), all immobilized on glass substrates with their SEM micrographs in the insets. Spectra A and B were acquired at an excitation wavelength of 633 nm (4 mW, 20 s). Spectrum C was obtained using 785 nm (10 mW, 20 s).

show the same substrate but after treatment with PVP. Careful examination of SEM images (Figure S4 in Supporting Information) before and after treatment with PVP shows that such treatment does not alter the coverage density nor the distribution of immobilized nanoparticles, allowing for comparative study of the effect of PVP and PVP alone from the SERS response. Citrate-reduced Ag NPs exhibit the richest Raman background, with the bands at 955, 1385, and 1497 cm−1 corresponding to ν(C−COO−), symmetric ν(COO−), and asymmetric ν(COO−) of citrate remnants, respectively (Figure 4A, upper spectrum). Citrate-reduced Au NPs show considerably different Raman background from their Ag counterpart, with the distinct δ(COO−) band at 1218 cm−1 (Figure 4B, upper spectrum). Consistent with the FTIR data in Figure 4, treatment with PVP resulted in a significant decrease in SERS intensities for Ag NPs (Figure 4A, lower spectrum) and complete disappearance of features from citrate remnant for Au NPs (Figure 4B, lower spectrum). These results are another illustration of stronger affinity of PVP to Au NPs as compared to Ag NPs. Importantly, Raman features were virtually absent in the background spectrum of Au−Ag nanoshells (Figure 4C), reflecting the use of PVP as well as the generation of chlorine anions during galvanic reaction. Cl− ions were shown to be very effective in displacing adsorbates on citrate-reduced Ag NPs (Figure S5 in the Supporting Information). While PVP was present at the surfaces of all nanostructures (see Figures 2 and 3c), its features did not show up in SERS spectra. One possible reason is its intrinsically low Raman scattering cross-section (data not shown).34,35 The synergistic effect of PVP and chlorine anion thus led to the effective cleansing of unwanted spectral features from the Raman baseline of the Au−Ag nanoshells. Intriguing SERS sensitivity to two commonly studied model analytes, R6G and BPE was demonstrated with the five substrate types investigated. Representative SERS intensities for BPE (upper and middle spectra) and R6G (lower spectra) are shown in Figure 5A−C for citrate-reduced Ag NPs, PVPcovered Ag NPs, and Au−Ag nanoshells, while SERS data for citrate-reduced Au NPs and PVP-covered Au NPs are shown in Figure S6 in the Supporting Information. The corresponding analytical EFs are summarized in Figure 5D. The EFs for BPE have been determined at two different values of pH 6.3 and 3.0 at which BPE (pKa1 ∼ 4.3 and pKa2 ∼ 5.65)36 is neutral or positively charged, respectively. With citrate-reduced Ag and Au NPs, EFs for both types of analytes in solutions at pH > 6.0 were similar. Strikingly, PVP adsorption led to differential SERS sensitivity of all types of nanostructures (Figure 5D). With Ag and Au NPs, the EF of R6G was ∼3 orders of magnitude lower with PVP-coated NPs as compared to the initial citrate-reduced

Figure 5. SERS spectra of 20 ppb BPE at pH 6.3 (upper spectra), 20 ppb BPE at pH 3.0 (middle spectra), and 50 ppb R6G at pH 6.8 (lower spectra) obtained using Ag NPs (A), Ag NPs covered with PVP (B), and Au−Ag nanoshells (C). The characteristic bands for BPE at ∼1610 and ∼1643 cm−1 correspond to ν(CN) and δ(C−N), ν(C−C), δ(C−H), respectively. The characteristic bands for R6G at ∼1365, ∼1510, and ∼1650 cm−1 correspond to ν(C−C) vibration in aromatic ring. Spectra A and B were acquired at 633 nm (4 mW, 20 s). Spectra C were acquired at 785 nm (10 mW, 20 s). (D) Summary of calculated SERS EFs for R6G and BPE using substrates with immobilized Ag and Au NPs with or without PVP and PVP-stabilized Au−Ag nanoshell.

NPs. Obviously, adsorbed PVP segments blocked adsorption sites at the NP surface, reducing R6G adsorption. Binding of positively charged R6G with adsorbed PVP is unlikely according to ample literature evidence on the absence of binding affinity between cationic dyes and PVP.37,38 Similarly low EFs are observed with BPE at pH 3, where BPE becomes positively charged (pKa1 value of 4.3). At the same time, for all nanostructures with surface-adsorbed PVP, the EFs for BPE at pH 6.3 were 103- to 105-fold higher than those at pH 3, Figure 5A−C (upper and middle spectra) demonstrating this trend. This is in spite of the expectation that at both pH values adsorbed PVP chains should equally efficiently block the surface adsorption sites and impede analyte adsorption. The cause of a dramatic difference in EFs for BPE at various pH values should therefore stem from pH-dependent binding affinity of BPE to adsorbed PVP chains. We conducted additional SERS measurements with Ag nanoparticles and Ag− Au nanoshells at 532 nm excitation wavelength. The SERS results for Ag nanoparticles with respect to the effect of PVP at 2533

dx.doi.org/10.1021/la2047992 | Langmuir 2012, 28, 2529−2535

Langmuir

Article

this wavelength are consistent with our findings at 633 and 785 nm reported in the paper (Figure S7 and Figure S8 in the Supporting Information). However, because 532 nm is far from the surface plasmon resonance wavelength of 720 nm of the nanoshells, no SERS enhancements were observed with the nanoshells at 532 nm, as expected. To confirm this hypothesis, we performed direct experimental evaluation of binding of PVP with PVP-stabilized Ag− Au nanoshells at various pH values. Figure 6 shows UV−vis

Figure 7. Optimized molecular structures of (a) A-H2O-B-H2O and (b) A-H2O-HBH-H2O. Color schemes: C, cyanl N, bluel O, red; H, gray; hydrogen bond, dashed green line.

Figure 6. UV absorption of BPE in supernatant as a function of concentration in water in the absence of nanoshells (squares), as well as after adsorption on nanoshells at pH 3.0 (circles) and pH 6.3 (triangles). Solutions with nanoshells were centrifuged at 10 000 rpm for 10 min, and unbound BPE after separation was monitored with UV absorption at 330 nm.

■

CONCLUSIONS In conclusion, our study has shown that PVP plays a central role in differential SERS activity of noble metal nanostructures. By contrast, in the absence of PVP the SERS sensitivity of Ag and Au NPs to several analytes was not affected by the pH change. Surface-adsorbed PVP performs a dual function: it provides steric hindrance to promote colloidal stability and results in selective analyte binding which enables differential SERS activity. Since PVP is commonly used in the synthesis of a variety of Ag and Au nanostructures with novel architectures, our findings are thus broadly applicable to exploiting and especially tailoring the SERS activity of these diverse nanostructures.

absorption data of BPE in supernatant as a function of BPE concentration at pH 6.3 and pH 3.0. Details of these experiments can be found in the Experimental Section. In brief, the procedure included mixing BPE and Ag−Au nanoshells in such a way that, for at least several BPE/nanoshell compositions, the amount of BPE was not sufficient to saturate PVPcoated nanoshells with BPE, to incubate these mixtures to allow BPE binding, and to separate nanoshells from supernatant solution by centrifugation. Unbound BPE remaining in the supernatant was then monitored by UV−vis spectrometry using the characteristic BPE band at 330 nm. Figure 6 indicates that BPE at pH 6.3 has higher affinity for PVP-adsorbed nanoshells than BPE at pH 3, consistent with the observed SERS sensitivity variations. This experimental result is also supported by the quantum chemical calculations using BPE and the PVP monomer, Figure 7 and Table S1 in Supporting Information. To investigate the protonation state effect on the binding, the following two reactions were evaluated:

A‐H2O + B‐2H2O → A‐H2O − B‐H2O + H2O

(1)

A‐H2O + HBH‐2H2O → A‐H2O‐HBH‐H2O + H2O

(2)

■

ASSOCIATED CONTENT

S Supporting Information *

Text describing synthesis and characterization of nanoshells, additional experimental FTIR and SERS data, and theoretical calculations, figures showing EDX sprectra and corresponding TEM images of Ag−Au nanoshells, ζ potential and DH as a function of pH of citrate-reduced or PVP-adsorbed Au NPs, FTIR spectra of citrate and PVP, SEM image of citrate-reduced Ag nanoparticles, and SERS spectra of the effects of NaCl, of BPE and R6G, and of citrate-reduced and PVP-adsorbed Ag nanoparticles, and table listing computed reaction properties. This material is available free of charge via the Internet at http://pubs.acs.org.

■ ■ ■

where A is a model for the monomer of PVP and B represents BPE. The optimized structures of A-H2O-B-H2O and A-H2OHBH-H2O are shown in Figure 7. The simulation indicated that when hydrated water molecules are taken into account, the overall Gibbs free energy change of vinylpyrrolidone binding with the high-pH form of BPE is more favorable than that for the low-pH form of BPE (Table S1). Our calculations also showed that the preference in free energy of binding for the unprotonated form of BPE as compared to its protonated form is driven by a greater entropy gain when hydrated unprotonated BPE interacts with the hydrated low molecular weight analogue of monomer PVP units.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.D.); [email protected] (S.S.).

ACKNOWLEDGMENTS We thank Dr. Tsengming Chou for his help with TEM work. REFERENCES

(1) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (2) Alexandridis, P. Chem. Eng. Technol. 2011, 34, 15.

2534

dx.doi.org/10.1021/la2047992 | Langmuir 2012, 28, 2529−2535

Langmuir

Article

(3) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (4) Siekkinen, A. R.; McLellan, J. M.; Chen, J.; Xia, Y. Chem. Phys. Lett. 2006, 432, 491. (5) Ma, Y.; Li, W.; Cho, E. C.; Li, Z.; Yu, T.; Zeng, J.; Xie, Z.; Xia, Y. ACS Nano 2010, 4, 6725. (6) Tang, X.-L.; Jiang, P.; Ge, G.-L.; Tsuji, M.; Xie, S.-S.; Guo, Y.-J. Langmuir 2008, 24, 1763. (7) Preciado-Flores, S.; Wang, D.; Wheeler, D. A.; Newhouse, R.; Hensel, J. K.; Schwartzberg, A.; Wang, L.; Zhu, J.; Barboza-Flores, M.; Zhang, J. Z. J. Mater. Chem. 2011, 21, 2344. (8) Qian, X. M.; Nie, S. M. Chem. Soc. Rev. 2008, 37, 912. (9) Moskovits, M. J. Raman Spectrosc. 2005, 36, 485. (10) Banholzer, M. J.; Millstone, J. E.; Qin, L.; Mirkin, C. A. Chem. Soc. Rev. 2008, 37, 885. (11) Faulds, K.; Littleford, R. E.; Graham, D.; Dent, G.; Smith, W. E. Anal. Chem. 2003, 76, 592. (12) Lévi, G.; Pantigny, J.; Marsault, J. P.; Aubard, J. J. Raman Spectrosc. 1993, 24, 745. (13) Tan, S., Pristinski, D., Sukhishvili, S., Du, H., Islam, M. S., Achyut, K. D., Eds. SPIE: Bellingham, WA, USA, 2005; Vol. 6008, p 600808. (14) Alvarez-Puebla, R. A.; Arceo, E.; Goulet, P. J. G.; Garrido, J. J.; Aroca, R. F. J. Phys. Chem. B 2005, 109, 3787. (15) Tan, S.; Erol, M.; Sukhishvili, S.; Du, H. Langmuir 2008, 24, 4765. (16) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (17) Erol, M.; Han, Y.; Stanley, S. K.; Stafford, C. M.; Du, H.; Sukhishvili, S. J. Am. Chem. Soc. 2009, 131, 7480. (18) Au, L.; Lu, X.; Xia, Y. Adv. Mater. 2008, 20, 2517. (19) Han, Y.; Tan, S.; Oo, M. K. K.; Pristinski, D.; Sukhishvili, S.; Du, H. Adv. Mater. 2010, 22, 2647. (20) Cossi, M.; Barone, V.; Cammi, R.; Tomasi, J. Chem. Phys. Lett. 1996, 255, 327. (21) Cossi, M.; Barone, V.; Mennucci, B.; Tomasi, J. Chem. Phys. Lett. 1998, 286, 253. (22) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. New developments in the polarizable continuum model for quantum mechanical and classical calculations on molecules in solution; AIP: College Park, MD, USA, 2002; Vol. 117. (23) Cances, E.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032. (24) Mennucci, B.; Tomasi, J. Continuum solvation models: A new approach to the problem of solute's charge distribution and cavity boundaries; AIP: College Park, MD, USA, 1997; Vol. 106. (25) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange; AIP: College Park, MD, USA, 1993; Vol. 98. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski.,V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision B.01 ed.; Gaussian: Wallingford, CT, USA, 2010. (27) Lu, X.; Chen, J.; Skrabalak, S. E.; Xia, Y. Proc. Inst. Mech. Eng., Part N: J. Nanoeng. Nanosyst. 2007, 221, 1. (28) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 1999, 12, 306. (29) Xianmao Lu, J. C.; Skrabalak, S E; Xia, Younan Proc. Inst. Mech. Eng., Part N: J. Nanoeng. Nanosyst. 2007, 221, 1.

(30) Munro, C. H.; Smith, W. E.; Garner, M.; Clarkson, J.; White, P. C. Langmuir 1995, 11, 3712. (31) Hirai, H.; Yakura, N. Polym. Adv. Technol. 2001, 12, 724. (32) Redmond, P. L.; Wu, X.; Brus, L. J. Phys. Chem. C 2007, 111, 8942. (33) Borodko, Y.; Habas, S. E.; Koebel, M.; Yang, P.; Frei, H.; Somorjai, G. A. J. Phys. Chem. B 2006, 110, 23052. (34) Kim, K.; Lee, H. B.; Park, H. K.; Shin, K. S. J. Colloid Interface Sci. 2008, 318, 195. (35) Mahmoud, M. A.; Tabor, C. E.; El-Sayed, M. A. J. Phys. Chem. C 2009, 113, 5493. (36) Zou, W.-s.; Shen, Q.-j.; Wang, Y.; Jin, W.-j. Chem. Res. Chin. Univ. 2008, 24, 712. (37) Frank, H. P.; Barkin, S.; Eirich, F. R. J. Phys. Chem. 1957, 61, 1375. (38) Pinto, J. R.; Novak, S. W.; Nicholas, M. J. Phys. Chem. B 1999, 103, 8026.

2535

dx.doi.org/10.1021/la2047992 | Langmuir 2012, 28, 2529−2535