Chemical Contribution to SERS Enhancement: An Experimental Study

Publication Date (Web): June 13, 2016. Copyright © 2016 American Chemical Society. *E-mail: [email protected]; phone +49 151 52485604. This article is p...
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Chemical Contribution to SERS Enhancement: An Experimental Study on a Series of Polymethine Dyes on Silver Nanoaggregates Katrin Kneipp* Hildegard-Jadamowitz-Straße 26, 10243 Berlin, Germany ABSTRACT: We investigate the chemical contribution to SERS on a series of polymethine dyes attached to silver nanoparticles in aqueous solution exploiting a total average enhancement level of 107−108. On top of uniform plasmonic and molecular resonance enhancement, we obtain changes in SERS signal levels and particularly changes in relative scattering intensities for selected Raman lines in the SERS spectra of very similar polymethine dyes. This variation in chemical SERS enhancement within the dye series correlates with the generation of dye radicals of the same dyes on photographic silver halides reported in the literature, where the dye radicals were determined by ESR spectroscopy. The correlation between these two experimentally independent observables suggests transfer and/or exchange/sharing of electrons between the metal and the molecule as basic process for chemical SERS enhancement, where the efficiency of the process depends on the molecular structure. Despite that the main contribution in SERS is caused by plasmonic field enhancement, the chemical contribution to SERS can be a sensitive tool to extract information on interest for processes that rely on charge transfer between surfaces/nanostructures and molecules such as catalysis or spectral sensitization.

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INTRODUCTION Surface enhanced Raman scattering (SERS)1,2 increases Raman signals of molecules attached to metal nanostructures, in particular of silver and gold. The main contribution to enormous total enhancement comes from enhanced local fields due to resonances between optical fields and the collective oscillations of the conduction electrons in the metal nanostructures, so-called surface plasmons.3 The plasmonsupported enhancement accounts for the strong dependence of SERS enhancement factors on the morphology of the metal nanostructures and hence on their plasmon spectrum, including also mode interaction and damping.4,5 For example, an increase in SERS enhancement by several orders of magnitude occurs for aggregates of silver and gold nanoparticles compared to isolated nanoparticles of those metals.6,7 In general, plasmonsupported field enhancement describes SERS and related effects very well. However, since the early days of SERS, experimental observations have been made that cannot be explained in an electromagnetic field enhancement model, and that suggest additional chemical or electronic effects.8−11 For example, in SERS in experiments on electrodes, changes in SERS enhancement factors for different vibrational modes depending on electrode potential point to chemical effects.12 Also, single molecule SERS obtained under molecular nonresonant conditions was explained to arise by superposition of a very strong electromagnetic enhancement and a strong chemical enhancement.13 Chemical enhancement is also discussed as a contribution to SERS on semiconductors14 and as reason for the observation of enhanced Raman signals from molecules on graphene.15 © XXXX American Chemical Society

Chemical enhancement effects are related to specific interactions between the molecule and the surface, where transfer and exchange or sharing of charges between molecule and substrate are considered as most likely basic process. Studies of chemical SERS at the single molecule level reveal that only a small fraction of molecules experiences chemical enhancement,16 which supports the hypothesis of so-called “active sites”, i.e., atomic scale local structures, where molecules chemically/electronically interact with the metal.9,11,17 There are different models for understanding and describing chemical contributions to SERS. One possible mechanism, also called “static charge transfer”, considers changes in the polarizability in a molecule-metal complex compared to the isolated molecule, giving rise to larger intrinsic Raman cross sections of the complex. Theoretical approaches using electronic structure methods18 have been applied to model the polarizability in metal−molecule systems for molecular nonresonant Raman scattering, including also hyper Raman scattering.19,20 Calculated chemical enhancement factors have been compared with experimentally separated chemical contributions to SERS.21 Another electronic mechanism called “transient charge transfer” suggests Raman scattering of adsorbed molecules via temporary electron or hole transfer between metal and molecule.11 A third possible explanation for a chemical SERS enhancement is based on new intermediate Special Issue: Richard P. Van Duyne Festschrift Received: April 13, 2016 Revised: June 5, 2016

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The Journal of Physical Chemistry C electronic states due to charge transfer transitions between the molecule and the metal. These states enable new resonance Raman processes, which would not exist in the molecule alone. A single, unified expression for the SERS intensity using the vibronic charge transfer coupling model describes all contributions to SERS together, including the effect related to molecule-metal charge-transfer resonance, as well as plasmon resonance and molecular resonance.22−24 In order to examine and to quantify the various contributions to SERS, the “degree of charge transfer” has been introduced, which describes to what extent charge transfer contributes to the overall SERS signal of a specific Raman line.23 Inferring the relative contribution of charge transfer enhancement requires a set of SERS data collected, for example, at different excitation wavelengths and/or from SERS experiments performed on electrodes at different potentials.23 The concept has been applied for p-aminothiophenol, a molecule for which considerable data has been obtained23 and for a quantitative description of the influence of intermolecular H-bonding on the contribution of charge transfer enhancement to SERS of pmercaptobenzoic acid on silver nanoparticles.25 Overall, studies on chemical SERS enhancement show that the effect is mainly determined by energetic resonance conditions in the molecule-metal complex, and by the symmetry of the vibrational modes. In this experimental study, we separate and quantify variations in the chemical SERS effect in a series of polymethine dyes on silver nanoparticles. Polymethines have many applications in science and technology, such as fluorescence labels, laser dyes, and spectral sensitizers in solar cells. In particular, they also play a role as spectral sensitizers of photographic silver halide materials, an application which has initialized also this study.26 Here, we observe SERS signals of polymethine dyes on top of a uniform level of plasmonic field enhancement and molecular resonance Raman enhancement. In order discuss the chemical enhancement in the context of underlying processes, we compare the SERS signals of a series of polymethine dyes with another, independently experimentally inferred property of the same molecules, namely their capability to form radicals, as reported in electron spin resonance (ESR) data in the literature.26 We compare variations in relative signals in the SERS spectra of a set of polymethines dyes with the yield of radical generation of the same dyes on photographic silver halides measured by ESR.26

Figure 1. Structures of the set of polymethines used in this study.

plasmonic and chemical SERS enhancement factors.29 The total average enhancement level was inferred from a comparison between SERS signals and nonenhanced methanol lines in the same spectrum. Figure 2 shows three typical extinction spectra of sample solutions with a maximum at 460 nm. The addition of the dyes

Figure 2. Extinction spectra of aqueous solutions of silver nanoaggregates and polymethine dyes used in SERS experiments. Solid, dashed, and dotted lines display samples with dyes 7, 2, and 1, respectively.

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EXPERIMENTAL SECTION Colloidal silver solutions used in our experiments were prepared by a standard citrate reduction procedure.27 The outcome of this process are silver nanoaggregates formed by 2− 8 silver nanoparticles in sizes between 20 and 50 nm 7. These structures can provide extremely high SERS enhancement factors and are one of the most popular enhancing plasmonic nanostructures for single molecule SERS experiments.13,28 The polymethine dyes used in this study were gifts from the research and development branch of former ORWO Photochemical Kombinat Wolfen. Figure 1 summarizes the structures of the molecules. SERS sample solutions were prepared by adding methanolic or aqueous solutions of dyes in 1:15 ratios to the solutions of silver nanoaggregates resulting, in a final dye concentration of 5 × 10−7 M. The concentration of silver nanoaggregates was on the order of 10−10 M. This means that per nanoaggregate there are on the order of 1000 dye molecules, experiencing average

did not result in broadening of the extinction band measured from the pure solution of nanoaggregates. Raman spectra were collected in 90°-scattering geometry from ∼10 nL probed volume within a 1 mL sample solution using 50 mW 514.5 nm excitation. We determine intensity ratios between different Raman lines by comparing areas under the Raman lines in average SERS spectra for each dye. Areas ([height × half width]/2) of the SERS peaks were determined in average spectra from at least three independent measurements performed with different experimental systems.

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RESULTS AND DISCUSSION Polymethines represent a class of dyes which contain a chain of methine groups as basic constitutive element. The methine chain also forms the chromophoric system, and the number of methine groups mainly determines the color of the dye. In Figure 1, all dyes with n = 1 have absorption maxima between B

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The Journal of Physical Chemistry C 540 and 560 nm and extinction coefficients on the order of 105 mol−1 L cm−1. Dye 9 with n = 0 has its absorption maximum at 422 nm. Polymethine dyes can give rise to strong SERS spectra.30−33 In most cases, the fluorescence inherent to resonant Raman spectra of dyes is quenched and does not superimpose their (resonant) surface enhanced Raman spectra measured on silver nanostructures. Raman spectra of polymethine dyes have been published and spectral features have been discussed and tentatively assigned previously,34,35 in particular also SERS spectra of the dyes shown in Figure 1.31,36,37 Due to their obvious structural relationship, the SERS spectra of these dyes show strong similarities but do also still allow to distinguish between them (see, for example, Figure 9 in ref 37. Their characteristic SERS spectra make polymethine dyes attractive for the use in SERS labels.38 Shifts in Raman frequencies between normal Raman spectra of polymethine dyes in solution and SERS spectra on silver nanoparticles are within ±8 cm−1 and on the same extent as it can be observed in their normal Raman scattering or CARS spectra in solution by changing solvents.30,31 In general, SERS spectra measured from the dyes on silver nanostructures at 514.5 nm excitation benefit from plasmonic SERS enhancement, resonance enhancement, particularly for the dyes with n = 1, and possible chemical enhancement effect(s). Figure 3a allows to infer a rough total enhancement factor by comparing SERS signals of dye 1 and normal Raman scattering of methanol. Taking into account the concentration of methanol of 3 M and a dye concentration of 5 × 10−7 M, total SER(R)S enhancement is between 107 −108, depending on the specific Raman band. Despite a different resonant Raman contribution for dye 9 at 514 nm excitation, the SERS signals of this molecule occur at about the same level, only a factor of 2 to 3 below those of the corresponding resonant trimethine dyes. Overall, the total SERS enhancement factors for different Raman lines of the dyes in Figure 1 vary within about 1 order of magnitude. Here we want to focus on relative SERS intensities for selected Raman lines in the SERS spectra and exploit the ratio of two SERS lines that appear in the spectra of all dyes with only small shifts of 2−8 cm−1 from dye to dye: a line around 1338 cm−1 (labeled with the letter A in Figure 3), which can be assigned to the polymethine chain, and a line at 1233 cm−1 (labeled with the letter B in Figure 3), which can be assigned to the ethyl group.35,31 In the comparison of structurally similar molecules, we first focus on SERS of dyes 1−8, i.e., we compare the spectra of the trimethines with sulfur as the heteroatom. For this dye series, equal plasmon absorbance of their sample solutions for all dyes (see, for example, 3 samples in Figure 2) strongly support that the dyes experience the same average plasmonic field enhancement. Furthermore, the absorption maxima of the dyes are shifted very little relative to each other, suggesting very similar molecular resonance Raman contributions to the total enhancement factors. In this way, we can study variations in chemical SERS enhancement factors on a constant level of electromagnetic and molecular resonance enhancement. Note that for both lines, the plasmonic field enhancement delivers the main contribution to the total SERS enhancement. This is supported by the observation that SERS experiments of the same polymethine dyes performed at isolated silver nanoparticles instead of nanoaggregates did not result in detectable Raman signals, even at higher dye concentrations. The intensity ratio of the two lines A and B varies within about 1 order of magnitude, with dye 2 at the

Figure 3. Average SERS spectra of selected polymethine dyes on silver nanoaggregates in solution using 514.5 nm excitation (for dye structures, see Figure 1). The letters A and B indicate the SERS bands used for comparing scattering signals, the asterisk labels a methanol band.

lower and dye 7 at the upper end of the scale. The obtained hierarchy in signal ratios of lines A and B for dyes 1−8 does not correlate with the small differences in the wavelengths of the absorption maxima of these dyes. Therefore, the variations in the ratios for dyes 1−8 obviously reflect different chemical contributions to the total SERS enhancement, due to the small structural variations between dyes. Our experiment quantifies differences in chemical SERS for two modes in dependence on small structural variations of SERS molecules. In order to explore the process behind chemical enhancement, we compare the ratios of SERS intensities with another property of the SERS molecules that can be inferred from an independent ESR experiment performed on the same series of polymethines on photographic silver halides during irradiation:26 The ESR signal appears due to radicals formed by the reduction of the excited polymethine dyes. The proposed process includes excitation of the positively charged dye and then, an electron transfer from C

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sulfur by oxygen. This change is on the same order of magnitude, as it occurs also for the other structural variations within the dye series 1−8. (No ESR data are available for dye 10). Also dye 9, which was measured under preresonant conditions fits into the correlation displayed in Figure 4. This suggests that resonant excitation of the electronic transition of the dye is not essential for the chemical enhancement obtained for polymethine dyes. It might be interesting to look at the hierarchy of chemical SERS enhancement within the dye series in the light of computations performed for a static charge transfer enhancement.20 In that work, it has been inferred that the chemical enhancement related to orbital interactions between the metal and molecule scales as (ωX/ωe)4, where ωX is the HOMO− LUMO excitation energy of the free molecule and ωe is lowest charge-transfer excitation energy of the metal−molecule complex. For the considered dye series, while ωX is almost the same, ωe can vary due to relatively different HOMO levels of the dyes.26 The polymethine dyes used in this study were of interest as spectral sensitizers for silver halide photographic materials.26 Connections and possible relations between SERS and silver halide photography have been discussed since a long time ago.42,33,43,44,39,40,45,41 Specifically, SERS studies on silver halides with sensitizing dyes as “SERS-active” molecules have been reported.33,43,44,39,46 In general, SERS can provide two interesting observables for silver halide photography:39,40 Print out silver can act as enhancing plasmonic nanostructure. The increase of the SERS signal of sensitizing dyes allows one to monitor growth and morphological changes of print out silver in photographic materials.46 The chemical SERS enhancement might provide insight into the process of spectral sensitization and can allow one to probe the capabilities of dyes as potential spectral sensitizers. For example, the relative SERS signals reported above also correlate with the sensitization effect of the dyes for photographic silver halides.39

an outer donor to the excited dye resulting in a dye radical following the scheme: Dye+ (excitation)→ Dye +* (electron transfer)→ Dye●.26 The intensity-time curve of the ESR signal enabled the determination of the quantum yield of the radical generation.26 In the ESR experiments, the samples consist of photographic silver halides with print-out silver and polymethine dyes as spectral sensitizers. This is very similar to the SERS-active system described here, particularly also regarding the presence of silver nanoparticles.39−41 Figure 4 displays the quantum yield

Figure 4. Normalized quantum yield for the generation of radicals in silver halides in the case of irradiation with 564 nm measured by ESR versus normalized SERS signal ratios of the two lines denoted with A and B in Figure 3 on silver nanoparticles for the set of polymethine dyes No 1−9 (see Figure 1). The dotted line shows a linear fit to the data, with a correlation coefficient of 0.85.

(normalized to the minimum observed number) for the generation of radicals inferred from ESR signals vs SERS intensity ratios for lines A and B (normalized to the minimum observed ratio) of the whole series of polymethine dyes. In the ESR study, the quantum yield quantifies the capability of the dye molecules to form radicals due to transfer of an electron from an outer donor. In our SERS study, the ratio between two Raman lines quantifies the chemical SERS. Note that the band labeled with A is related to the polymethine chain, i.e., to a region in the molecule that is directly involved in electron transfer because of the positive charge localized in the chain. In contrast, line B can be assigned to the ethyl group that is much less involved. The correlation between the two experimentally independent observables, generation of dye radicals measured by ESR, and chemical SERS enhancement suggests a structurally dependent predisposition within the series of the polymethine dyes for both, formation of radicals as well as chemical SERS enhancement. This strongly hints to transfer/exchange of charges as basic process for the chemical SERS enhancement. The obtained correlation does not reveal the detailed mechanism of the charge-transfer related contribution to chemical SERS and can support different models, such as static19,20 and transient9,11 charge transfer, and also the vibronic charge transfer coupling model.24 For a check of a potential dependence of the contribution to chemical SERS on other structural variations, Figure 3c shows SERS spectra of dyes 4 and 10, which only differ with respect to the heteroatom, with oxygen replacing sulfur in dye 10. The ratio IA/I B decreases by a factor of 2.6 upon substitution of

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CONCLUSION On top of uniform plasmonic and molecular resonance enhancement, relative SERS intensities in spectra of polymethine dyes on silver nanoaggregates vary within 1 order of magnitude, depending on small structural variations of the dyes. The variation in chemical SERS enhancement within the dye series correlates with the generation of dye radicals on photographic silver halides measured by ESR spectroscopy.26 This correlation suggests chemical enhancement mechanism(s) related to the exchange/transfer of charges between the metal and the molecule. Despite that the main contribution in SERS is caused by plasmonic field enhancement, the chemical contribution to SERS can be a sensitive probe to explore processes that rely on transfer or exchange of charges between nanostructures/ surfaces and molecules. This suggests chemical SERS as a tool to extract information on interest for technologically relevant processes such as catalysis and spectral sensitization in various systems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone +49 151 52485604. Notes

The authors declare no competing financial interest. D

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ACKNOWLEDGMENTS

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REFERENCES

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I am grateful to my former colleagues from Friedrich-Schiller University Jena, the Academy of Sciences Berlin, and from the research and development branch of the former ORWO Photochemical Kombinat Wolfen, in particular to Jörg Siegel, for interesting discussions about spectral sensitization and silver halide photography as well as for providing the polymethine dyes. I thank Harald Kneipp for support with data analysis and useful discussions.

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