Quantitative Comparison of Raman Activities, SERS Activities, and

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Quantitative Comparison of Raman Activities, SERS Activities, and SERS Enhancement Factors of Organothiols: Implication to Chemical Enhancement Siyam M. Ansar,†,⊥ Xiaoxia Li,‡,⊥ Shengli Zou,§ and Dongmao Zhang*,† †

Department of Chemistry, Mississippi State University, Mississippi State, Mississippi 39762, United States Department of Sciences and Mathematics, Mississippi University for Women, Columbus, Mississippi 39701, United States § Department of Chemistry, University of Central Florida, Orlando, Florida 32816, United States ‡

S Supporting Information *

ABSTRACT: Studying the correlation between the molecular structures of SERS-active analytes and their SERS enhancement factors is important to our fundamental understanding of SERS chemical enhancement. Using a common internal reference method, we quantitatively compared the Raman activities, SERS activities, and SERS enhancement factors for a series of organothiols that differ significantly in their structural characteristics and reported chemical enhancements. We find that while the tested molecules vary tremendously in their normal Raman and SERS activities (by more than 4 orders of magnitude), their SERS enhancement factors are very similar (the largest difference is less than 1 order of magnitude). This result strongly suggests that SERS chemical enhancement factors are not as diverse as initially believed. In addition to shedding critical insight on the SERS phenomena, the common internal reference method developed in this work provides a simple and reliable way for systematic investigation of the correlation between molecular structures and their normal Raman and SERS activities. SECTION: Nanoparticles and Nanostructures

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etermination of Raman activities,1,2 surface-enhanced Raman spectroscopic (SERS) activities, and SERS enhancement factors is an intrinsically challenging problem as it requires quantitative information regarding the number of probed molecules, the photon collection efficiency of the instruments, the laser power distribution on the samples, and so forth.3 Further complications arise when determining SERS activities and SERS enhancement factors because of the poor reproducibility of the SERS substrates.4 The SERS signal varies significantly from spot to spot even in the same SERS substrate. These technical difficulties have limited our ability to systematically investigate the correlation between the analyte structure and its normal Raman activity, SERS activity, and SERS enhancement factor. Filling this knowledge gap is important not only for deepening our fundamental understanding of Raman and SERS phenomena but also for developing Raman- and SERS-based analytical and bioanalytical techniques.5−7 While it has been widely accepted that both electromagnetic and chemical enhancement contribute to the signal enhancement in SERS,8−12 the magnitude and the exact origin of chemical enhancements remain elusive. Our poor understanding of chemical enhancement is manifested in the large disparities in chemical enhancement factors (10−107 fold) reported in the literature.13−15 © 2012 American Chemical Society

Unlike electromagnetic enhancement that is mainly a property of the SERS substrates (neglecting the effect of molecular orientation) and chemically nonselective, chemical enhancement should depend critically on the molecular structure of the analyte. As a result, studying the correlation between the chemical structures of the analytes and their SERS enhancement factors is the key to understanding the significance and mechanism of chemical enhancement.13,16,17 Instead of determining normal Raman activities, SERS activities, and SERS enhancement factors for individual analytes, reported herein is a common internal reference method for quantitative comparison of normal Raman activities, SERS activities, and SERS enhancement factors of different AB AB organothiols (Figure 1). RAB NR, RSERS, and REF in Figure 1 represent the relative normal Raman activity, SERS activity, and SERS enhancement factor between analytes A and B, respectively. I denotes the normal Raman or SERS intensity of analyte A or B. We chose organothiols as the probe molecules because they are the most commonly used model molecules for studying SERS phenomena including SERS chemical enhancements.18−20 By using mercaptobenzimidazole (MBI) as the common internal reference (analyte B in Figure Received: December 13, 2011 Accepted: February 6, 2012 Published: February 6, 2012 560

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the SERS activity comparison samples. This result is important as it allows us to use the concentration ratio of CA/CB in the SERS sample to calculate the SERS activity ratio of analytes A and B, as shown in Figure 1. The low organothiol coverage on the NPs in the SERS samples is also important to avoid premature NP aggregation, that is, NP aggregation before complete organothiol adsorption, as this may modify the uniformity of the analyte distribution on the NP surfaces. Conceivably, if one of the paired organothiols adsorbs faster onto the NPs and induces NP aggregation, the other analyte will be partially or entirely excluded from the NP junction areas (the SERS “hot spots”), which would make the SERS enhancement factor of the first adsorbed analyte falsely higher. In order to evaluate the correlation between the chemical structure and its SERS enhancement factor, we included a series of model molecules that differ significantly in their structural characteristics (aromaticity, thiol contents, etc.) and their reported chemical enhancement (Figure 2). A recent report

Figure 1. Scheme for quantitative comparison of normal Raman activities, SERS activities, and SERS enhancement factors of paired organothiols A and B. After acquiring its normal Raman spectrum, a diluted A/B mixture was prepared for the SERS measurement so that the total amount of organothiols (A + B) was below the monolayer binding capacity of the nanoparticles (NPs) to ensure complete analyte adsorption. The asterisks denote the compared molecules and their Raman/SERS features. The negative peaks in the normal Raman spectrum are due to solvent background subtraction.

1), we are able to quantitatively compare the normal Raman activities, SERS activities, and SERS enhancement factors of a large number of organothiols, which are all relative to the Raman activity, SERS activity, and SERS enhancement of MBI. Measurement errors in the relative Raman activity, SERS activity, and SERS enhancement factor caused by variation in instrument performance and in spot-to-spot SERS activity in the SERS substrates are canceled because the relative normal Raman or SERS activity between a specific analyte and MBI was determined ratiometrically on the basis of the analyte/MBI mixture normal Raman or SERS spectrum.21−23 Any variation in the laser excitation power, instrument performance, and SERS substrate will presumably change the analyte and MBI Raman or SERS intensity the same way. Organothiols are ideal model molecules for evaluating the significance of chemical enhancement. Because of their ability to covalently bond to gold and silver nanoparticles (AuNPs and AgNPs), all of the adsorbed organothiols are presumably in direct contact with the nanoparticle (NP) surfaces, thus essentially experiencing the same electromagnetic enhancement (neglecting the difference resulting from different molecular orientations). To ensure complete analyte adsorption onto the AuNPs or AgNPs in the SERS measurements, the total amount of the organothiol pair (A and B) mixed with AuNPs or AgNPs is less than 35% of the monolayer binding capacity of the NPs (assuming that the full monolayer packing density for all of the model molecules is 570 pmol/cm2, as we recently determined for MBI adsorption onto AuNPs24,25). Experimental confirmation of complete (>95%) organothiol adsorption was obtained from SERS measurements. There was no detectable SERS signal from organothiols in the supernatant after centrifuge removal of the AuNPs or AgNPs together with their surface-adsorbed organothiols. In contrast, high-quality SERS spectra were obtained when the organothiols were 20 times more dilute than the concentrations used for the comparative SERS activity study (Figure S1, Supporting Information). This indicates that at least 95% of the organothiols were adsorbed onto the AuNPs and AgNPs in

Figure 2. Molecular structure of the model organothiols.

claimed that because of its reported higher binding affinity to gold as a dithiol, the chemical enhancement factor of benzenedithiol (BDT) is 130 times higher than that of methylbenzenethiol (MBT), a monothiol with a similar structure to BDT.14 In addition to BDT, we also included TBA-MDA, another dithiol,26 to evaluate how the thiol content may affect the organothiol SERS enhancement factor. While it is generally believed that organothiols bind to AuNP and AgNP surfaces through the formation of a S−Au or S−Ag bond, the fate of the hydrogen atom in the thiol group remains controversial.27 Our recent study of MBI adsorption onto AuNPs shows that compared to MBI normal Raman spectra acquired at neutral, acidic, and basic pHs, the SERS spectrum of MBI most resembles the normal Raman spectrum of MBI acquired in a 0.1 M NaOH solution, supporting the hypothesis that organothiols adopt a thiolate form on the NP surface. To ensure that the molecular structures of the tested organothiols in their normal Raman samples are similar to those in their respective SERS samples, the normal Raman spectra were acquired with organothiol solutions prepared in either 0.1 M NaOH aqueous solutions (TBA-MDA, MBI, Cys, and TG) or in 0.1 M NaOH/ethanol (50/50) cosolvent (MBT and BDT due to their poor solubility in water). The alkalinity of normal Raman solutions is also important for ensuring that the analyte solubility is high enough for normal Raman spectral acquisition. The solutions used for the SERS activity studies were prepared with organothiols dissolved in 0.1 M NaOH solutions. 561

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Figure 3. Normal Raman, and SERS spectra of (left column) MBT/MBI, (middle column) BDT/MBI, and (right column) BDT/MBT, respectively. The spectra on the top and middle rows are SERS spectra acquired using AuNPs and AgNPs as the SERS substrates, respectively, while those on the bottom row are normal Raman spectra acquired using organothiol solutions prepared from either a 0.1 M NaOH aqueous solution (MBI) or 0.1 M NaOH/ethanol (50:50 in volume) cosolvent (MBT and BDT). The Raman features selected for calculation of the Raman activity, SERS activity, and SERS enhancement factor are marked with green and red asterisks. The negative peaks (marked with black asterisk) are due to solvent background subtraction in the normal Raman spectra. Concentrations of NPs, analytes A and B in the Raman, and SERS sample mixtures of A/B are shown in Table 2 in the Experimental Section.

Table 1. Relative Normal Raman Activity, SERS Activity, and SERS Enhancement Factors analyte (A/B) TG/MBI TBA-MDA/MBI Cys/MBI BDT/MBI MBT/MBI BDT/MBTf BDT/MBTg

a RAB NR

0.63 353.4 0.015 4.67 1.68 2.63 2.77

± ± ± ± ± ± ±

0.05 11.5 0.0013 0.13 0.03 0.05 0.09

b RAB SERS(AgNP)

0.33 134.7 0.012 1.92 1.35 1.36 1.42

± ± ± ± ± ± ±

c RAB SERS(AuNP)

0.05 26.8 0.002 0.14 0.15 0.21 0.19

0.92 309.27 NA 1.32 3.48 0.38 0.38

± 0.02 ± 14.8 ± ± ± ±

0.07 0.12 0.06 0.02

d RAB EF (AgNP)

0.52 0.38 0.81 0.41 0.81 0.52 0.50

± ± ± ± ± ± ±

0.09 0.08 0.15 0.03 0.09 0.08 0.07

e RAB EF (AuNP)

1.46 0.87 NA 0.28 2.07 0.146 0.136

± 0.22 ± 0.18 ± ± ± ±

0.02 0.24 0.03 0.02

a

The normal Raman activity ratio of analytes A and B. bThe SERS activity ratio of A and B on AgNPs. cThe SERS activity ratio of A and B on AuNPs. dThe SERS enhancement factor ratio of A and B on AgNPs. eThe SERS enhancement factor ratio of A and B on AuNPs. fObtained by direct experimental comparison of the Raman and SERS features of BDT and MBT. gCalculated on the basis of experimental results of BDT/MBI and MBT/MBI. The experimental conditions used for normal Raman and SERS acquisition are described in the Experimental Section.

to be proportional to analyte concentrations. The excellent linear correlation between the TG/MBI SERS intensity ratio and their concentration ratio indicates that the ratio of the amount of TG and MBI adsorbed onto the AuNPs is proportional to the concentration ratio of the TG and MBI added into the AuNPs. This result is not surprising because under our experimental conditions, all of the MBI and TG were presumably adsorbed onto the AuNPs because the total amount of MBI and TG added into the AuNPs is significantly less than the monolayer binding capacity of AuNPs. The data in Figure 4 is important as it demonstrates that the relative SERS enhancement factors can be reliably quantified using a combined ratiometric Raman and SERS method for analytes that can be quantitatively coadsorbed onto NP surfaces. It can be shown mathematically that the slope in Figure 4B corresponds to the normal Raman activity ratio between TG

Using the normal Raman and SERS spectra acquired with the mixtures of each of the organothiols paired with MBI (Figure 3 and Figure S2, Supporting Information), the relative normal Raman activities, SERS activities, and SERS enhancement factors were determined (Table 1). Validation of this common internal reference method comes from the excellent agreement between the relative normal Raman activities, SERS activities, and SERS enhancement factors calculated for DBT and MBT using MBI as the common reference and their counterpart value obtained by direct experimental comparison of BDT and MBT. (Table 1, rows 7 and 8). Further validation of this common internal reference method was demonstrated with the experimental results shown in Figure 4. The nearly perfect linear correlation between the normal Raman intensity ratio of TG/MBI and its concentration ratio is expected because the normal Raman intensity is known 562

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more or less the same degree of chemical enhancement despite their significant structural differences. Besides the possible chemical effects, differences in the electromagnetic enhancement of the vibrational modes selected for calculation of the SERS enhancement factors also contribute to the small analyte and SERS substrate dependence of the relative SERS enhancement factors (Table 1). According to the “surface selection rules”, the electromagnetic enhancement of a vibrational mode is the highest when the polarization tensor is parallel to the norm of the NP surface.28,29 Because the molecules tested in this work differ in their structural characteristics and the Raman vibrational modes selected for calculation of their relative SERS enhancement factors do not necessarily have the same orientation on the NP surface, the electromagnetic enhancement experienced by these vibrational modes can be different for different molecules. Conceivably, such an “electromagnetic” effect can be both analyte- and SERS-substrate-dependent. Unfortunately, quantitative determination of such an effect is not currently possible as it requires precise information about both the molecular orientation on the AuNP and AgNP surfaces and the electromagnetic field distribution on the aggregated AgNP and AuNPs. Conceivably, the relative SERS enhancement factors determined with this common internal reference method can have a strong dependence of the specific vibrational modes selected. Future work will focus on the comparison of the SERS activities of different vibrational modes using combined computational modeling and experimental investigation The TBA-MDA adduct has the highest normal Raman and SERS activities among all of the studied organothiols. This result is not surprising because TBA-MDA has the largest number of π-conjugated electrons. Another reason for the extraordinarily high Raman activity of TBA-MDA is the possibility of preresonance Raman and SERS enhancement under our experimental conditions. The peak TBA-MDA UV− vis absorption is 530 nm, which is only ∼100 nm blue-shifted from our 633 nm excitation laser. The low Raman and SERS activity of Cys is also expected. As an aliphatic amino acid, it has only one pair of π electrons. Indeed, because of its low SERS activity, obtaining SERS spectra of Cys on AuNPs has not been possible. In summary, we demonstrated a simple common internal reference method for quantitative comparison of the Raman activities, SERS activities, and SERS enhancement factors for different organothiols. Our experimental results suggest that the SERS enhancement factors of the organothiols are not strongly analyte-dependent. In addition, our method provides a robust and reliable analytical methodology that can be used for systematic investigation of the correlation between analyte structure and its normal Raman/SERS activities. The quantitative comparison of the Raman and SERS activities of different molecules should aid future selection and design of Raman and SERS tags for analytical and bioanalytical Raman and SERS applications.

Figure 4. (A) Raman spectra of TG and MBI mixtures with concentration ratios of (a) 0/10, (b) 3/7, (c) 5/5, (d) 7/3, (e) 8/2, (f) 9/1, and (g) 10/0. The overall TG and MBI concentration is 64 mM in 0.1 M NaOH. (B) Correlation between the TG and MBI Raman intensity ratio and their concentration ratio. (C) SERS spectra of TG and MBI mixtures with concentration ratios of (a) 0/10, (b) 2/8, (c) 3/7, (d) 5/5, (e) 7/3, (f) 8/2, and (g) 10/0. The overall TG and MBI concentration in the SERS sample is 2.5 μM. The concentration of AuNPs (50 nm in diameter) is 0.26 nM. (D) Correlation between the TG and MBI SERS intensity ratio and their concentration ratio in the SERS samples.

and MBI, and the slope in Figure 4D is the SERS activity ratio between TG and MBI on the AuNPs. The ratio between the slopes in Figure 4B and D, which is 1.53, is the relative SERS enhancement factor of TG and MBI on the AuNP, which is in agreement with the SERS enhancement factor ratio of TG and MBI shown in Table 1. Even though the tested organothiols differ significantly in their normal Raman activities and SERS activities (by more than 4 orders of magnitude), their SERS enhancement factors are similar (Table 1). The largest difference in the SERS enhancement factors among all of the analytes, which is between BDT and MBT on AuNPs, is less than 8 fold. More importantly, compared to monothiols, none of the dithiols (BDT and TBA-MDA) exhibit higher SERS enhancement factors, indicating that the number of thiol groups has no significant impact on the SERS enhancement factor. This result is in stark contrast to a recent report that the SERS enhancement factor of BDT is 130 times larger than that of MBT.14 One possible reason for this large discrepancy may be due to the difference in the SERS sample preparation in this work and that reported by Maitani et al.,14 where the SERS activities of MBT and DBT were measured individually with two separately prepared SERS substrates. Another critical difference is that Maitani et al.14 did not experimentally quantify either the normal Raman activities of MBT and BDT or the number of molecules adsorbed. Nevertheless, the remarkable similarity in the SERS enhancement factors among the tested organothiols indicates that under our experimental conditions, all of the tested organothiols have

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EXPERIMENTAL SECTION Preparation of the Normal Raman and SERS Samples. Table 2 shows the sample composition of the normal Raman and SERS solutions used for the comparative quantification of the relative normal Raman activity, SERS activity, and SERS enhancement factor between analytes A and B. The AuNPs and AgNPs (50 nm in diameter) were purchased from Nanocomposite LLC. The NPs were concentrated to 5 times for all of the SERS 563

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the submonolayer organothiol adsorption did not induce NP aggregation under our experimental conditions. UV−vis measurement was also used to confirm that the KCl induced NP aggregation. It showed that upon KCl addition, the LSPR feature of the submonolayer organothiol adsorbed NPs was red-shifted significantly, resulting from the LSPR coupling between NPs in the NP aggregates (Figure S3, Supporting Information).

Table 2. Concentrations of Organothiols A and B and the NPs in the Normal Raman and SERS Activity Comparison Samples normal Raman

SERS

analyte (A/B)

A (mM)

B (mM)

A (μM)

B (μM)

NP (nM)

TG/MBI TBA-MDA/MBI Cys/MBI BDT/MBI MBT/MBI BDT/MBT

32 0.9 500 45 105 26

32 32 10 32 32 60

1.25 0.05 37.5 1.25 1.25 1.25

1.25 1.25 0.75 1.25 1.25 1.25

0.26 0.26 2.60 0.26 0.26 0.26

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ASSOCIATED CONTENT

S Supporting Information *

SERS spectra of the supernatants of the SERS activity comparison samples and organothiol controls and Raman and SERS spectra of individual organothiols and their mixtures. This material is available free of charge via the Internet at http://pubs.acs.org.

samples except for the Cys-containing samples, in which the AgNPs were concentrated to 50 times. These NP concentrations were achieved by centrifugation precipitation of the asreceived NPs followed with sonication redispersion. Normal Raman and SERS Acquisition. Normal Raman spectra were acquired by transferring 10 μL of the normal Raman solutions of the A/B mixtures onto the Ramchip slides (Z&S Tech. LLC). The Ramchip slide is a normal Raman substrate that is free of fluorescence and Raman background. The SERS measurements of the A/B mixtures adsorbed onto AuNPs or AgNPs were performed as follows: After preparation of the SERS samples of A and B mixed with AuNPs or AgNPs, with the compositions shown in Table 2, the SERS solutions were left at room temperature overnight to allow complete ligand adsorption. Before SERS spectral acquisition, 5 μL of 1% KCl was added into 10 μL each of the overnight incubated SERS samples to induce AgNP or AuNP aggregation. After brief mixing (∼10 s) of the sample, 10 μL of this mixture was transferred to a Ramchip slide (Z&S Tech. LLC) for SERS acquisition. All of the spectra were acquired using a 633 nm HeNe laser. The laser power on the sample was 13 mW for the normal Raman spectra and 1.3 mW for the SERS measurements. The spectral integration time was varied from 10 to 200 s. Three independent measurements were made for each sample. Quantif ication of Organothiol Adsorption onto AuNPs and AgNPs in the SERS Samples. After mixing A/B organothiol pairs and AuNPs or AgNPs with the sample concentrations shown in Table 2, the solutions were left at room temperature for ∼5 h to allow analyte adsorption before centrifugation precipitation of the AuNPs or AgNPs. SERS spectra of the supernatants of the SERS activity comparison samples were acquired to confirm that the organothiols were completely adsorbed onto the AuNPs and AgNPs. A SERS spectrum was also acquired for each organothiol control in which the analyte concentration was 20 times smaller than that in the comparative SERS activity sample. While qualitative SERS spectra were obtained with the organothiol controls, there were no detectable SERS features in the supernatants of the SERS activity comparison samples (Figure S2, Supporting Information). This result indicates that more than 95% of each organothiol was adsorbed onto the NPs in the SERS activity comparison samples. UV−Vis Extinction Spectra of AuNPs and AgNPs. The UV−vis spectra were acquired with an Evolution 300 UV−vis spectrophotometer (Fisher Scientific) to study how submonolayer organothiol adsorption may affect the AgNPs and AuNPs stability (Figure S3, Supporting Information). The data showed that there was no significant change in the localized surface plasmonic resonance (LSPR) feature of the AgNP and AuNP before and after the organothiol addition. This indicates that

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ⊥

Authors equally contributed to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by NSF funds (EPS-0903787) provided to D.Z. S.Z. is thankful for support from the ACS Petroleum Research No. 48268-G6, NSF CBET 0827725, and ONR N00014-0-1-1118 fund. The authors thank Dr. Willard Collier for his editorial assistance and valuable discussion.

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