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Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Quantifying the Contribution of Chemical Enhancement to SERS: A Model Based on the Analysis of Light-Induced Degradation Processes Bo Liu,*,† Bonito Thielert,*,† Andreas Reutter,† Rainer Stosch,‡ and Peter Lemmens*,†,§ Institute for Condensed Matter Physics and §Laboratory for Emerging Nanometrology LENA, TU Braunschweig, 38106 Braunschweig, Germany ‡ Physikalisch-Technische Bundesanstalt (PTB), 38116 Braunschweig, Germany Downloaded via KEAN UNIV on July 26, 2019 at 01:43:00 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: Surface-enhanced Raman scattering (SERS) is of growing importance in different fields, from clinical analysis/chemistry to food industry. For a better insight into the complex light−matter interaction processes that are underlying the intensity enhancement, it is essential to experimentally distinguish and quantify the contributions based on the chemical mechanism (CM) with respect to the electromagnetic mechanism. Here, we present a model to estimate the relative CM response of Raman modes by analyzing light-induced degradation of target molecules on designed metal/semiconductor SERS substrates. The resulting intensity evolution is described by a biexponential function with two model parameters that allow a differentiation of the enhancement processes. Our work thereby provides a means for a better understanding of CM and will be advantageous for an application of intensity-based quantitative SERS techniques.

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INTRODUCTION Surface-enhanced Raman scattering (SERS) is a promising technique for applications in the fields of clinical analysis/ chemistry, biotechnology, pharmacy, food industry, and so forth.1−6 However, although 40 years have passed since the SERS effect was discovered, there still exist theoretical aspects that are not fully understood. Currently, this theory considers the signal enhancement by two separate mechanisms: the electromagnetic mechanism (EM) and the chemical mechanism (CM). The EM is based on the enhancement of the local electromagnetic field around the metallic nanostructure due to surface plasmons. In fact, after the first demonstration of SERS on roughened metallic surfaces, a large number of studies have been carried out using metal colloids or dispersed metal nanoparticles. The spatial extent of the enhanced local electromagnetic field is usually in the range of several tens of nanometers. If two or more metal nano-units on the surface are sufficiently close, the enhanced light fields associated with them may overlap and interact, resulting in a significant increase in the intensity of the electromagnetic field between the nano-units. Such spots between metal surfaces are named “hot spots”. If target molecules migrate to such hot spots, their Raman signal will consequently be enhanced and can reach even single molecule detection.7 The CM has been proposed as a complementary contribution to the plasmonic EM theory. Jeanmaire and Van Duyne observed rather early that SERS enhancement of pyridine is much stronger than the enhancement of water,8 and © XXXX American Chemical Society

such an observation cannot be explained by an EM effect only. Pioneering research has been carried out to understand CMbased enhancement.9−16 Mrozek and Otto named it a “first layer effect”.9,10 Doering and Nie examined the roles of CM at the single molecule level.17 In the following, SERS spectra were experimentally measured for a set of substituted molecules and compared with each other or with calculated spectra.14−16,18,19 The current qualitative understanding is that CM effects are based on specific interactions between the molecule and the surface that include charge transfer, exchange, or covalency. In detail, CM effects include metal−ligand complex formation, the transient transfer of hot electrons or holes out of the metal into the adsorbed molecules, or charge transfer resonances between metal and molecules.16,20−22 A quantification of the contributions from CM and EM is of fundamental and application-related interest. It can also be used to calibrate density functional theory calculations.23 Moreover, most of the SERS experiments are carried out for qualitative purposes, and quantitative SERS techniques are still under development due to challenges of intensity comparisons. Finally, the contribution of CM to each SERS system is different, always mixed and in coexistence with EM contributions. Therefore, it would be needful to have a direct intensity comparison between any two SERS systems with a precisely quantified CM contribution. Received: May 13, 2019 Revised: July 3, 2019 Published: July 12, 2019 A

DOI: 10.1021/acs.jpcc.9b04526 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C For this purpose, various studies have been carried out. Uetsuki et al. compared SERS spectra of 4-ATP adsorbed on bare Au films and on Au films with a PVP spacer layer. These layers prevent any chemical bonding between Au and 4-ATP molecules and a comparison to the bare Au film gives information about the CM enhancement.24 Saikin et al. characterized CM effects by approximating EM contribution through extinction intensities of the SERS substrate at the Raman mode frequencies and the |E|4 rule.25 However, a spacer layer modifies the electromagnetic field as well and simulations based on ideal models could also be different from reality. Conventional estimations of the CM contribution to Raman scattering work mainly as follows:26 1. Determine SERS and regular Raman spectra and read out the respective mode intensities. 2. Choose a mode as a standard which should not be affected by a CM and normalize all modes in the regular Raman spectrum to the standard mode. 3. Use the intensity difference between SERS and normalized regular Raman spectra to describe the absolute CM enhancement of each Raman mode. Strictly, this treatment works only when the following preconditions are accepted: first, the standard mode should not be affected by CM at all; second, all modes are enhanced uniformly by EM; and third, all molecules are bound to the metal surface and influenced by CM. In contrast, the experimental situation is less clear: it is challenging to achieve a uniform EM enhancement in a broad frequency range27,28 and the existence of a reference mode without CM. Molecules, which migrate into the hot-spot between two nanoparticles but bind to none of them, can also reveal a strong SERS effect by EM only. In such a case and following the treatment hereinabove, a “CM contribution” is resolved from a batch of molecules, instead of a variation of the Raman cross-section of a single target molecule. Therefore, the above-described conventional estimation of the CM contribution should be taken with caution. The purpose of our work is to identify and quantify the enhancement contribution related to the CM. This is based on a statistical analysis of experimental intensity data. The experiments are performed in a way to minimize external effects of intensity degradation. In this work, an advanced approach to identify and quantify the enhancement contribution related to the CM and a statistical analysis is introduced. We propose a basic degradation model for this advanced approach and demonstrate its validity using experimental data.

Figure 1. Mobile vs static SERS systems. Target molecules are shown as orange dots. Metallic nanoparticles are gray, numbered circles. Green circles correspond to the area of the excitation laser.

can easily or spontaneously enter or leave the laser spot, as shown in Figure 1a. In addition, the reorganization or deformation of hot-spots between nanoparticles may change the Raman intensity, too. Summing up these effects and averaging over all molecules during the measurement time, the Raman intensity may increase or decrease. In contrast, in a “static” system (e.g., dried metallic colloids), the molecules or nanostructures are not able to move in or out of the laser focus. Thereby, the local electric fields will not change assuming a stability of the excitation laser in intensity and position. As shown in Figure 1b, only a degradation (photobleaching or other photochemical reactions) could influence the number of emitting molecules, which are located in the laser focus. A decrease in Raman intensity would then be related to a decreasing number of molecules. In such a case, the Raman intensity decay is described by a first-order kinetics of degradation and characterized using a single degradation constant b. This leads to the following SERS intensity expression as a function of time t I(t ) = a ·e−bt = I0·α ·N0·e−bt

(1)

where a consists of all optical process parameters, as intensity of excitation on site I0, Raman cross-section of one molecule α, and number of molecules N0 at the initial time. The response function of the setup is considered as being linear and is not further considered here. In practice, a sequence of Raman spectra is measured. As Raman scattering integrates over a certain time, also eq 1 is integrated. This is included in the advanced approach by introducing a sequence index “order of measurement” n instead of t. As a precondition, each Raman spectrum in the sequence has to be measured with invariant setup parameters. Following the integration of eq 1, an exponential function with adapted parameters a′ and b′ is derived. More details of this approach can be found in the Supporting Information. In addition, we expect more than one independent light-induced degradation process, so that eq 1 has in general K-exponential decay functions

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COMPUTATIONAL DETAILS The advanced approach is based on the assumption that target molecules can be classified either as molecules that are affected by EM and CM (with valid adsorption on plasmonic surfaces) or only EM (without adsorption). When different levels of adsorption lead to different degrees of light-induced degradation, a differentiation of CM from EM−CM-mixtures and its quantification seem feasible. This is the basis of this approach. Categorizing SERS experiments either into “mobile” or “static” situations, as shown in Figure 1, we suggest here to use the latter. Mobility allows the target molecules to move around, while a “static” situation leads to a spatial confinement of the molecules. In “mobile” experiments (for instance, the use of metallic colloids as SERS substrate), target molecules

K

I(n) =

∑ ak′ e−b′n k

k

(2)

In the following, we assume the existence of two types of target molecules, which are affected by either CM + EM or EM B

DOI: 10.1021/acs.jpcc.9b04526 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

superlattice located in anodic aluminum oxide templates were prepared.29 All relevant procedures and preparation steps can be found in a previous work.30 A scanning electron microscopy (SEM) picture and representative magnitudes of the SERS substrates are given in the supporting information. Malachite green as a probe-molecule is dissolved in ethanol with 10−3 mol/L concentration, and 5 μL of the solution has been dropped on the top of the Au/AAO nanorod array substrate and dried in air for 12 h. Data of additional experiments on rhodamine 6G can be found in the Supporting Information. As shown in Figure 2, seven SERS spectra have been collected one by one. The following setup parameters were

only. Therefore, if there are different degradation constants of these two types, according to eq 2, the intensity of a Raman mode (mode i) as a function of the n-th measure is specified and rewritten as follows: Ii(n) = Ai e−Bn + Ci e−Dn = Iceα[i ,ce]N[0,ce] e−Bn + Ieα[i ,e]N[0,e] e−Dn

(3)

where Ai and Ci are the corrected initial intensities of mode i, as well as B and D are the corrected appropriate degradation constants. In the advanced approach, the two exponential functions are assigned to either molecules affected by “CM and EM” or “EM only”. This can be described by the Raman cross sections α[i,ce] and α[i,e], respectively. N[0,ce] and N[0,e] are the number of adsorbed (with CM and EM) and non-adsorbed molecules (with only EM) at the initial moment. Ice and Ie are the corrected intensities of the excitation at the appropriate molecules. The second term in eq 3 corresponds to the EM contribution. The intensity ratio between two modes of this term should be the same or similar as the intensity ratio observed in a regular Raman spectrum. This prediction corresponds to the conventional estimation of CM. Using this approximation facilitates one to assign both exponential terms to either “CM and EM” or “only EM” by its ratio deviation from the regular Raman spectrum. The advanced approach uses the regular Raman spectrum only as a reference. Furthermore, the determination of the numbers of molecules in eq 3 is difficult, and absolute cross sections are usually not determined. Therefore, a mode s is chosen to normalize the intensity and skip the pre-factors of the cross sections. The CM + EM-related normalized intensity ratio Ai′ = Ai/As of each mode is expressed as follows:

Figure 2. Sequence of SERS spectra of malachite green. The malachite green molecules are not subject to the laser excitation in between two consequent measures. A decreasing Raman scattering intensity is observed from measurement to measurement.

Ai′ = Ai /As

kept invariant: λ = 532 nm laser as an excitation source with a laser power of P = 33 μW, an accumulation number of 10, and an acquisition time of 10 s. This corresponds to an integration time of 100 s. In order to proof that the Raman intensity decay originates only from light-induced degradation processes, the time intervals between two consecutive measurements are chosen randomly from several seconds to tens of minutes. In these time intervals, the laser excitation is blocked and the molecules should stay unchanged. Additional degradation processes should lead to a random Raman intensity decay between consecutive measures. As we do not observe large scattering, the first order kinetics is dominant. Four Raman modes at 1368, 1400, 1592, and 1620 cm−1 are fitted by Lorentzian lines. The fitted intensities are presented in Figure 3. Our experiments show a monotonous decay of Raman intensity instead of a random behavior. This validates that the intensity decay occurs only during the measurements due to light-induced degradation. The existence of a CM contribution to the scattering intensity can be verified as follows: the mode selective intensity of the modes at 1368, 1400, and 1592 cm−1, shown in Figure 4, is normalized by the mode at 1620 cm−1. We notice that the modes show different tendencies. The intensity decays of the four modes in Figure 3 are fitted. A simple exponential decay following a·e−bn does not fit the observed dependence at all. In contrast, the derived biexponential function in eq 3 fit to every mode very well. The fitting results of the parameters A, B, C, and D are given in

= Iceα[i ,ce]N[0,ce]/Iceα[s ,ce]N[0,ce] = α[i ,ce]/α[s ,ce]

(4)

The only-EM-related normalized intensity ratio can be derived in a similar way. Finally, a chemical enhancement factor (CEF or - ) can be defined that indicates the CM-related Raman cross-section enhancement -i = α[i ,ce]/α[i ,e] for mode i due to the parallel contribution of EM and CM.11 The relative CEF - ′i = -i /-s normalized to mode s can be resolved as - ′i = =

α[i ,ce]/α[i ,e] -i = α[s ,ce]/α[s ,e] -s α[i ,ce]/α[s ,ce] α[i ,e]/α[s ,e]

= Ai′/Ci′

(5)

Using eq 5, a quantification of the CM contribution to the Raman cross-section of each mode is achieved.

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RESULTS AND DISCUSSION To verify this advanced approach, a statistical analysis of the Raman scattering data enhanced by Au/AAO nanorod plasmonic arrays has been performed. SERS substrates based on periodically arranged Au nanorods with a quasi-hexagonal C

DOI: 10.1021/acs.jpcc.9b04526 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 3. Reduction of Raman scattering intensity of the four modes as a function of the order of measurement. The data are fitted to a biexponential decay.

Figure 5. Comparison between degradation constants B and D from each mode, respectively. Error bars show the 95% confidence bounds.

Table 2. Corrected Fitting Results a and c in Arbitrary Units Corresponding to the Four Modes by Using the “Most Probable” Degradation Constants mode A C

1

2

3

4

1368 cm−1

1400 cm−1

1592 cm−1

1620 cm−1

110.4 ± 2 569.7 ± 15

148.4 ± 3.4 490.6 ± 25.7

163.3 ± 5.3 365.7 ± 40

300.3 ± 3.9 1249 ± 29

Figure 4. Intensity of each mode normalized to the mode at 1620 cm−1. The mode selective behavior evidences the existence of CM in this experiment.

Table 1. Both fittings are shown in the Supporting Information. The validity of the advanced approach is supported by comparing the parameters B and D from each Raman mode: although the degradation constants B (or D) are fitting results from data of individual modes, both of them should not be mode-dependent. This is due to the fact that they represent the degradation character of the molecules instead of the Raman modes. To highlight this character, both parameters are plotted as scatters with error bars (95% confidence interval) in Figure 5. A comparison shows that the constants B and D gain similar values for all modes, considering the 95% confidence interval. Their values are 0.125 and 1.5, respectively. Moreover, the four decay curves in Figure 3 were fitted again with the “most probable” degradation constants a·exp(−0.125· n) + c·exp(−1.5·n), and the corrected fitting results are shown in Table 2. A comparison among the normalized a and c values, as well as the normalized intensity ratios of a regular Raman experiment of a powder (see Figure 7a), is shown in Figure 6. The intensities are normalized to the 1368 cm−1 mode. This normalization is carried out to determine the corresponding decay term-contribution. In the following, we use the c· exp(−1.5·n) term as the “only EM” contribution, because the c′ values (green curve) show a similar nonmonotonous

Figure 6. Comparison of the normalized intensities of each mode from powder samples in a regular Raman experiment and the two terms of the biexponential decay function.

tendency as the powder sample (blue curve). This assignment is consistent with the conventional estimation of CM. The higher degradation constant of the “only EM” contribution compared to the “CM + EM” one can be confirmed visual as follows: a comparison between the relative intensities of the first two peaks reveals that the degradation sequence in Figure 2 move away from the powder spectrum in Figure 7a. The degradation constants are a by-product of the assignment, and their transfer to a theoretical basis warrants further research. According to eq 5, the Raman mode (4) at 1620 cm−1 is taken as a standard or reference mode, and the CEF (- ) on the Au/AAO substrate is calculated as shown in Table 3. For comparison, a conventional estimation of the CM contribution as previously described has been carried out. As a basis, we use the data shown in Figure 7 that contains both SERS and regular Raman data. In order to demonstrate the

Table 1. Fitting Results of Parameters A, B, C, and D Based on eq 3a mode A B C D

1

2

3

4

1368 cm−1

1400 cm−1

1592 cm−1

1620 cm−1

118 0.138 600 1.6

± ± ± ±

16 0.024 122 0.28

135 0.106 467 1.37

± ± ± ±

19 0.027 93 0.29

143 0.098 345 1.27

± ± ± ±

29 0.036 105 0.48

314 0.134 1295 1.57

± ± ± ±

30 0.018 229 0.24

a

B and D are unitless. A and C are in arbitrary units. D

DOI: 10.1021/acs.jpcc.9b04526 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The comparison in Table 5 shows that the first peak has a lower and the second as well as the third peak have a higher CEF with respect to the fourth peak. The parameters corresponding to the second peak are similar. This good agreement demonstrates that the advanced approach is valid and compatible with the conventional estimation of CM. While the 1368 and 1592 cm−1 modes show the same tendency in both cases, their parameters differ. This result can be reconciled with the extended methodology of the advanced approach: the Raman spectrum on powder has been used only as a reference. Therefore, both crystallization effects in the powder and orientation effects of the molecules on the SERS substrate do not significantly impact the data. On the other hand, the EM-contribution leads to a peakdependent enhancement in contrast to the conventional approximation. This effect is separated in the advanced approach. Finally, the results in the conventional estimation are weighted by molecules that are not affected by CM. Because of the exclusion of these perturbing factors, the advanced approach has the potential to increase the measurement accuracy and the understanding of the chemical contribution to SERS.

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CONCLUSIONS In this work, we describe a Raman scattering approach to quantify the chemical with respect to the electromagnetic fieldinduced enhancement in SERS. Based on a model of photodegradation, intensity variations are modeled and compared to experimental data. We highlight the need of a static SERS experiment to accurately determine the contribution from the CM. Furthermore, we assume that there exist two characteristic degradation constants for “CM and EM” and “only EM”. In addition, three experimental conditions are taken for granted: random time intervals between two SERS measurements that rule out the possibility of signal degradation from other sources, the observation of a mode-selective decay that evidences the existence of a chemical enhancement, and the common value of degradation constants derived from different Raman modes. SERS experiments using malachite green and rhodamine 6G have been carried out using Au nanorod SERS substrates to experimentally verify the advanced approach. The contribution from the CM has been quantified and compared to results from a conventional Raman experiment. This advanced approach is a tool to better differentiate and quantify the CM in SERS leading to an overall progress in intensity-based SERS techniques.

Figure 7. (a) Regular Raman spectrum of a malachite green fine powder and (b) SERS spectra of 5 μL of 10−4 mol/L malachite green/ ethanol solution on two different substrates.

Table 3. Normalized CEFs of Each Mode from Malachite Green mode

-′

1

2

3

4

1368 cm−1

1400 cm−1

1592 cm−1

1620 cm−1

0.81 ± 0.03

1.26 ± 0.08

1.86 ± 0.21

1

enhancement of the SERS-substrate, a droplet of malachite green solution on p-type silicon is measured in Figure 7b. The peak intensities of the four modes measured with regular Raman data are shown in Table 4. The corresponding Table 4. Intensities of Four Modes of Malachite Green Following a Conventional Estimation of CM mode

powder intensity

1 2 3 4

145.6 74.3 84.3 189.1

± ± ± ±

5.1 4.6 4.9 8.9

SERS intensity 24.5 30.7 34.2 61.9

± ± ± ±

1.2 1.3 1.5 1.7

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normalized CEFs, which are calculated based on the results from the conventional experiment, and the correctionpercentages compared to the advanced approach are shown in Table 5.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b04526.

Table 5. Comparison of the Advanced Approach and the Conventional Estimation of CM mode

-′

1

2

3

4

0.81 ± 0.03

1.26 ± 0.08

1.86 ± 0.21

1 1

- ′conv

0.51 ± 0.04

1.26 ± 0.12

1.24 ± 0.11

Δ/- ′conv

57% ± 11%

0.3% ± 11%

50% ± 20%

ASSOCIATED CONTENT

Detailed SEM picture of the Au/AAO SERS substrate, derivation of the time integration of the exponential decay function, details on malachite green, comparison of an exponential and a biexponential fit to the intensity degradation, and analysis of rhodamine 6G spectra including fitting results and relative chemical enhancement factors for two different R6G solutions (PDF) E

DOI: 10.1021/acs.jpcc.9b04526 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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copy (SERS): Theory and Experiment. J. Phys. Chem. Lett. 2013, 4, 2599−2604. (15) Zayak, A. T.; Hu, Y. S.; Choo, H.; Bokor, J.; Cabrini, S.; Schuck, P. J.; Neaton, J. B. Chemical Raman Enhancement of Organic Adsorbates on Metal Surfaces. Phys. Rev. Lett. 2011, 106, 083003. (16) Kneipp, K. Chemical Contribution to SERS Enhancement: An Experimental Study on a Series of Polymethine Dyes on Silver Nanoaggregates. J. Phys. Chem. C 2016, 120, 21076−21081. (17) Doering, W. E.; Nie, S. Single-Molecule and Single-Nanoparticle SERS: Examining the Roles of Surface Active Sites and Chemical Enhancement. J. Phys. Chem. B 2002, 106, 311−317. (18) Ansar, S. M.; Li, X.; Zou, S.; Zhang, D. Quantitative Comparison of Raman Activities, SERS Activities, and SERS Enhancement Factors of Organothiols: Implication to Chemical Enhancement. J. Phys. Chem. Lett. 2012, 3, 560−565. (19) Yu, X.; Cai, H.; Zhang, W.; Li, X.; Pan, N.; Luo, Y.; Wang, X.; Hou, J. G. Tuning Chemical Enhancement of SERS by Controlling the Chemical Reduction of Graphene Oxide Nanosheets. ACS Nano 2011, 5, 952−958. (20) Moskovits, M. Persistent Misconceptions Regarding SERS. Phys. Chem. Chem. Phys. 2013, 15, 5301−5311. (21) Otto, A. The ‘Chemical’ (Electronic) Contribution to SurfaceEnhanced Raman Scattering. J. Raman Spectrosc. 2005, 36, 497−509. (22) Boerigter, C.; Aslam, U.; Linic, S. Mechanism of Charge Transfer from Plasmonic Nanostructures to Chemically Attached Materials. ACS Nano 2016, 10, 6108−6115. (23) Zuloaga, J.; Prodan, E.; Nordlander, P. Quantum Plasmonics: Optical Properties and Tunability of Metallic Nanorods. ACS Nano 2010, 4, 5269−5276. (24) Uetsuki, K.; Verma, P.; Yano, T.-a.; Saito, Y.; Ichimura, T.; Kawata, S. Experimental Identification of Chemical Effects in Surface Enhanced Raman Scattering of 4-Aminothiophenol. J. Phys. Chem. C 2010, 114, 7515−7520. (25) Saikin, S. K.; Chu, Y.; Rappoport, D.; Crozier, K. B.; AspuruGuzik, A. Separation of Electromagnetic and Chemical Contributions to Surface-Enhanced Raman Spectra on Nanoengineered Plasmonic Substrates. J. Phys. Chem. Lett. 2010, 1, 2740−2746. (26) Liang, X.; Liang, B.; Pan, Z.; Lang, X.; Zhang, Y.; Wang, G.; Yin, P.; Guo, L. Tuning Plasmonic and Chemical Enhancement for SERS Detection on Graphene-Based Au Hybrids. Nanoscale 2015, 7, 20188−20196. (27) Lin, K.-Q.; Yi, J.; Zhong, J.-H.; Hu, S.; Liu, B.-J.; Liu, J.-Y.; Zong, C.; Lei, Z.-C.; Wang, X.; Aizpurua, J.; et al. Plasmonic Photoluminescence for Recovering Native Chemical Information from Surface-Enhanced Raman Scattering. Nat. Commun. 2017, 8, 14891. (28) Itoh, T.; Yoshida, K.; Biju, V.; Kikkawa, Y.; Ishikawa, M.; Ozaki, Y. Second Enhancement in Surface-Enhanced Resonance Raman Scattering Revealed by an Analysis of Anti-Stokes and Stokes Raman Spectra. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 085405. (29) Shukla, S.; Kim, K.-T.; Baev, A.; Yoon, Y. K.; Litchinitser, N. M.; Prasad, P. N. Fabrication and Characterization of Gold−Polymer Nanocomposite Plasmonic Nanoarrays in a Porous Alumina Template. ACS Nano 2010, 4, 2249−2255. (30) Liu, B.; Yan, H.; Stosch, R.; Wolfram, B.; Bröring, M.; Bakin, A.; Schilling, M.; Lemmens, P. Modelling Plexcitons of Periodic Gold Nanorod Arrays with Molecular Components. Nanotechnology 2017, 28, 195201.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.L.). *E-mail: [email protected] (B.T.). *E-mail: [email protected]. Phone: +49 531 3915131 (P.L.). ORCID

Bonito Thielert: 0000-0002-6364-4167 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of the Braunschweig International Graduate School of Metrology B-IGSM, DFG Research Training Group GrK1952 “Metrology for Complex Nanosystems” and DFG − EXS 2123 QuantumFrontiers, Light and Matter at the Quantum Frontier. We thank Stefan Wundrack for important discussion contributions.

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REFERENCES

(1) Sharma, B.; Frontiera, R. R.; Henry, A.-I.; Ringe, E.; Van Duyne, R. P. SERS: Materials, Applications, and the Future. Mater. Today 2012, 15, 16−25. (2) Li, J.-F.; Anema, J. R.; Wandlowski, T.; Tian, Z.-Q. Dielectric Shell Isolated and Graphene Shell Isolated Nanoparticle Enhanced Raman Spectroscopies and Their Applications. Chem. Soc. Rev. 2015, 44, 8399−8409. (3) Wang, X.-P.; Walkenfort, B.; König, M.; König, L.; KasimirBauer, S.; Schlücker, S. Fast and Reproducible iSERS Microscopy of Single HER2-Positive Breast Cancer Cells Using Gold Nanostars as SERS Nanotags. Faraday Discuss. 2017, 205, 377−386. (4) Stosch, R.; Yaghobian, F.; Weimann, T.; Brown, R. J. C.; Milton, M. J. T.; Güttler, B. Lithographical Gap-Size Engineered Nanoarrays for Surface-Enhanced Raman Probing of Biomarkers. Nanotechnology 2011, 22, 105303. (5) Zhang, Z.; Merk, V.; Hermanns, A.; Unger, W. E. S.; Kneipp, J. Role of Metal Cations in Plasmon-Catalyzed Oxidation: A Case Study of p-Aminothiophenol Dimerization. ACS Catal. 2017, 7, 7803−7809. (6) Zhang, Z.; Gernert, U.; Gerhardt, R. F.; Höhn, E.-M.; Belder, D.; Kneipp, J. Catalysis by Metal Nanoparticles in a Plug-In Optofluidic Platform: Redox Reactions of p-Nitrobenzenethiol and p-Aminothiophenol. ACS Catal. 2018, 8, 2443−2449. (7) Maier, S. A. Plasmonics: Fundamentals and Applications; Springer US: New York, 2007. (8) Jeanmaire, D.; Van Duyne, R. P. Surface Raman Spectroelectrochemistry. J. Electroanal. Chem. Interfacial Electrochem. 1977, 84, 1−20. (9) Mrozek, I.; Otto, A. SERS  A Long-Range Effect? Appl. Phys. A: Solids Surf. 1989, 49, 389−391. (10) Mrozek, I.; Otto, A. Quantitative Separation of the “Classical” Electromagnetic and the “Chemical” Contribution to Surface Enhanced Raman Scattering. J. Electron Spectrosc. Relat. Phenom. 1990, 54−55, 895−911. (11) Campion, A.; Ivanecky, J. E.; Child, C. M.; Foster, M. On the Mechanism of Chemical Enhancement in Surface-Enhanced Raman Scattering. J. Am. Chem. Soc. 1995, 117, 11807−11808. (12) Fromm, D. P.; Sundaramurthy, A.; Kinkhabwala, A.; Schuck, P. J.; Kino, G. S.; Moerner, W. E. Exploring the Chemical Enhancement for Surface-Enhanced Raman Scattering with Au Bowtie Nanoantennas. J. Chem. Phys. 2006, 124, 061101. (13) Zhao, L. L.; Jensen, L.; Schatz, G. C. Surface-Enhanced Raman Scattering of Pyrazine at the Junction between Two Ag20 Nanoclusters. Nano Lett. 2006, 6, 1229−1234. (14) Valley, N.; Greeneltch, N.; Van Duyne, R. P.; Schatz, G. C. A Look at the Origin and Magnitude of the Chemical Contribution to the Enhancement Mechanism of Surface-Enhanced Raman SpectrosF

DOI: 10.1021/acs.jpcc.9b04526 J. Phys. Chem. C XXXX, XXX, XXX−XXX