Balancing the Effects of Extinction and Enhancement for Optimal

Dec 8, 2016 - The field of ultrafast surface-enhanced Raman spectroscopy (SERS) is rapidly expanding; however, few applications for these new techniqu...
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Balancing the Effects of Extinction and Enhancement for Optimal Signal in Surface-Enhanced Femtosecond Stimulated Raman Spectroscopy Natalie L. Gruenke, Michael O. McAnally, George C. Schatz, and Richard P. Van Duyne* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: The field of ultrafast surface-enhanced Raman spectroscopy (SERS) is rapidly expanding; however, few applications for these new techniques have been demonstrated. One obstacle for the widespread application of ultrafast SERS is the addition of highly enhancing and scattering plasmonic substrates to already complex nonlinear spectroscopies. The competition between extinction and enhancement in ultrafast SERS techniques complicates the optimization of a number of experimental parameters. Here we study the concentration and path length dependences of signal quality in surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS). We find that in contrast to previous studies of spontaneous SERS which use signal magnitudes to define optimal experimental parameters, signal-to-noise ratios (SNRs) are the best measure of ideal experimental parameters in SE-FSRS. We report ideal concentrations and path lengths to use in transmissive geometry SE-FSRS experiments with colloidal nanoparticle substrates. Our results indicate that despite competing effects from SERS and FSRS mechanisms, similarly performed SE-FSRS and SERS experiments yield maximum SNRs using the same concentration and path length due to the overwhelming effects of extinction. By understanding how to optimize SE-FSRS experimental parameters, ultrafast SERS, and SE-FSRS in particular, can be more readily applied to future plasmonically enhanced spectroscopic studies.

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INTRODUCTION The use of surface-enhanced Raman spectroscopy (SERS) has expanded exponentially for a range of applications in the past 35 years1−3 including significant progress using plasmonic colloidal nanoparticle suspensions for sensing applications.4−7 This progress has become more relevant with the growth of the new field of ultrafast SERS, which combines the plasmonic sensitivity of SERS techniques with the potential to follow molecular dynamics on femtosecond timescales.8 Surfaceenhanced femtosecond stimulated Raman spectroscopy (SEFSRS) in particular has been used to study molecule−plasmon coupling and molecular vibrational dephasing lifetimes near metal surfaces using colloidal SERS substrates.9 By combining the femtosecond time resolution of FSRS with the high sensitivity of SERS, SE-FSRS can be used to study ultrafast molecular plasmonics and to improve our understanding of the enhancement mechanisms in plasmonic photovoltaic and photocatalytic systems.10−13 However, a number of questions remain as to how to best implement SE-FSRS for highest enhancement and sensitivity when using colloidal suspensions of nanoparticles as plasmonic substrates. The questions surrounding optimization and applications of SE-FSRS are reminiscent of earlier studies discussing extinction and enhancement in colloidal SERS,14−17 which explore the relationships between sample concentration and incident wavelength with observed signal and enhancement. For optimal SERS signal in colloidal solutions, ideal experimental conditions © XXXX American Chemical Society

must balance extinction and enhancement. As depicted in Figure 1, extinction is caused by scattering and absorption of both the incident and scattered light by the colloidal plasmonic substrates. As concentration increases, extinction increases, leading to less signal being generated as well as collected. However, increased concentration provides more plasmonic hot spots and analyte molecules in the focal volume of the probe beam, leading to an increase in both generated and enhanced signal. The effects of extinction and enhancement compete such that increasing concentrations initially increase SERS signal but then reach an ideal concentration, after which further concentration increases lead to reduction of SERS signal.15 The wavelength dependence of these two effects has also been investigated, as the relative energies of the substrate localized surface plasmon resonance (LSPR) and the incident and scattered light will affect not only the observed enhancement but also the observed extinction. Discussions of the extinction behavior of SERS experiments cannot be simply applied to the SE-FSRS case due to complications introduced by the stimulated nature of SEFSRS scattering. Complications arise in part because two beams with different wavelengths, the pump and probe, must overlap to generate the signal. Extinction caused by the plasmonic Received: October 24, 2016 Revised: December 5, 2016 Published: December 8, 2016 A

DOI: 10.1021/acs.jpcc.6b10727 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. SE-FSRS sample schematic. (a) The localized surface plasmon resonance (LSPR) of the substrates used for these experiments (black). The aggregate LSPR (850−1200 nm) is off-resonance with the Raman pump (orange, 795 nm) and preresonant with the probe (red, 850−1000 nm). (b) Illustration of the aggregates used for SE-FSRS. The substrates used for SE-FSRS experiments are an aqueous suspension of aggregated, silicaencapsulated Au particles with an adsorbed layer of a molecular analyte (trans-1,2-bis(4-pyridyl)ethylene). (c) Schematic of the sample geometry in these experiments and the extinction experienced by the Raman pump and stimulating probe beams. The SE-FSRS experiments here are performed in a transmission geometry, as illustrated, with collinear pump and probe beams.

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METHODS SE-FSRS and SERS Measurements. The experimental setup used to collect the SE-FSRS spectra has been described in detail elsewhere.9,18 The ground state SE-FSRS measurements are performed using a regenerative amplifier (Coherent RegA) with a 100 kHz repetition rate and an output power of 1 W at 795 nm. The amplifier output is split into two pulses: a 795 nm picosecond Raman pump pulse produced by filtering the fundamental with two narrow bandpass filters and a broadband (850−1000 nm) femtosecond stimulating probe pulse produced via continuum generation in sapphire and filtered for a selected NIR spectrum (see Figure 1a for pump and probe spectra). These pulses are temporally and spatially overlapped in a collinear geometry and focused into the sample using a 100 mm focal length lens. The effects of cross-phase modulation are minimized by modulating the pump pulse time delay with broadband, low-frequency vibrations from a speaker attached to the retroreflector in the Raman pump path. The collected signal is dispersed and detected with a spectrometer (Princeton

particles affects the two beams differently. Recent theoretical work has highlighted the importance of considering the differences specifically between spontaneous and stimulated Raman scattering when finding optimal experimental parameters in surface-enhanced stimulated Raman spectroscopy (SESRS).17 However, these theoretical findings necessarily use assumptions that are not amenable to SE-FSRS experiments, and thus their results have not yet been experimentally validated. Here we present a detailed investigation of a variety of experimental parameters affected by both extinction and enhancement to produce optimal signal in SE-FSRS, comparing spontaneous and stimulated surface-enhanced Raman spectroscopies. We also compare the use of Raman gain and signal-tonoise ratio (SNR) as measures of signal quality in SE-FSRS. This work is relevant not only for the future application of ultrafast SERS techniques but also for optimizing the sensitivity of any surface-enhanced stimulated Raman technique. B

DOI: 10.1021/acs.jpcc.6b10727 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Instruments Acton SP253) and CCD (PIXIS 100F). Raman gain spectra are obtained by chopping the pump pulse at 500 Hz and dividing “pump-on” spectra by “pump-off” spectra to reveal Raman gain features on top of an intense probe spectral envelope. Powers used are approximately 400−500 pJ/pulse Raman pump and 50−100 pJ/pulse probe with acquisition times of 3−6 min. Spontaneous continuous wave (CW) SERS measurements are made using a 785 nm laser (R-Type, single longitudinal mode (SLM), Innovative Photonic Solutions), focused into the sample and detected using the same optics and equipment as the SE-FSRS spectra. Substrate System. The colloidal nanoparticle samples used for these measurements have been previously characterized and were purchased from Cabot Security Materials.19−21 They consist of an aqueous solution of aggregated Au particles 90 nm in diameter. trans-1,2-Bis(4-pyridyl)ethylene (BPE) is adsorbed to the gold surface, and the aggregates are encapsulated in a silica shell, as illustrated in Figure 1b. Over half of the sample is estimated to be monomers. However, a significant portion consists of aggregates with highly enhancing SERS hot spots, and these aggregates cause the signals reported here. For a typical SE-FSRS measurement 0.2 mM Au suspensions of aggregated nanoparticles were stirred in a 2 mm quartz cuvette to avoid damage from long exposures. In concentration dependence measurements the concentration of gold was varied from 0.001 to 1 mM. For the path length measurements the samples were not stirred, and cuvettes of path lengths 1, 2, 5, and 10 mm were used. A schematic of the experimental system is shown in Figure 1c.

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RESULTS AND DISCUSSION Concentration Dependence of SERS and SE-FSRS Measurements. We recorded SE-FSRS spectra of colloidal nanoparticle solutions with concentrations ranging from 1 μM to 1 mM Au to find the concentration that yielded the highest signals for each technique. Figure 2a shows SE-FSRS spectra for a range of sample concentrations, increasing from 0.001 to 1 mM. Here we see asymmetric line shapes that have been the subject of significant previous interest9,18 and which can be understood in terms of coherent coupling of the driven vibrational mode with the dispersive plasmon enhanced field.22 In the present context, we note (see Supporting Information for further details) that the line shapes in the figure are welldescribed in terms of Fano profiles, as recently justified by a theoretical analysis.22 Using the same samples, we also measured spontaneous SERS spectra for a comparison of their concentration dependences. Figure 2b shows a comparison of signal intensity as a function of sample concentration for the 1200 cm−1 mode of BPE using both SERS and SE-FSRS. SERS measurements have a lower ideal concentration than SE-FSRS. The SERS signal reaches a maximum at a Au concentration of 0.3 mM as determined by the area under each peak. For SE-FSRS, a concentration of 0.7 mM yields maximum signal, defined by the amplitude of a fitted Fano line shape to the SE-FSRS gain spectrum (detailed information on the fitting of Fano line shapes can be found in the Supporting Information). These results deviate from expected concentration dependence in FSRS without plasmonic particles, in which signal increases with increasing concentrations. In FSRS, the natural logarithm of Raman gain increases linearly as a function of increasing concentration, Raman pump power, and path length.23 Therefore, finding ideal experimental parameters is a

Figure 2. (a) SE-FSRS spectra at a range of sample concentrations. Spectra are vertically offset for clarity, with Au concentration increasing vertically along the right axis from 0.001 to 1 mM Au. (b) Concentration dependences of signal intensity in SERS (blue, circles, left axis) and SE-FSRS (red, squares, right axis) as a function of Au concentration. For optimal gain amplitudes, SE-FSRS has a higher ideal concentration. (c) Comparison of signal-to-noise ratios (SNRs) and gain as measures of ideal concentration for SE-FSRS. SNRs (purple, diamonds, left axis) suggest a lower ideal concentration for SE-FSRS than gain (red, squares, right axis). C

DOI: 10.1021/acs.jpcc.6b10727 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The contrast between gain and SNR as measures of SE-FSRS signals arise from characteristics of both spontaneous SERS and normal FSRS. As discussed above, extinction effects will cause signal loss at high concentrations in spontaneous SERS experiments, due to loss of both incident light and scattered signal. Thus, it is unsurprising to see qualitatively similar results in SE-FSRS when measuring signal. However, signal and noise are affected differently by sample concentration as it is the gain that is measured in SE-FSRS. Raman gain is calculating by dividing the collected probe spectrum with the Raman pump by the probe spectrum without the Raman pump (and hence without any Raman signal), which introduces noise not present in spontaneous SERS experiments. In FSRS, a decrease in probe power will not affect the measured gain but instead leads to increased noise.23 In SE-FSRS, increasing the sample concentration will not only extinguish the incident pump and the probe beams (leading to less generated signal, analogous to spontaneous SERS) but also attenuate the stimulating probe beam after signal generation (leading to a noisier spectrum with the same Raman gain). When using SNR as a measure of signal, the increasing noise and signal at high concentrations balance out, leaving SE-FSRS with similar target concentrations as SERS for optimal signals. Path Length Dependence in SE-FSRS. Similar to concentration dependence, there should be an ideal path length for surface-enhanced stimulated Raman of colloidal solutions. This has not been discussed in previous studies of extinction and enhancement in colloidal SERS and SE-SRS17 but is an equally important component of experimental design as concentration. Without plasmonic substrates, FSRS signals would have the same dependence on both concentration and path length.23 The colloidal plasmonic substrates used in SERS and SE-FSRS could cause deviations from this expectation because the Raman pump and stimulating probe beams will scatter differently from the substrate. Stimulated Raman processes are complicated by the assumption that a change in sample path length will lead to a change in the distance over which the pump and probe beams are interacting since both beams are needed to generate the stimulated signal. This is the case for perfectly collinear pump and probe beams in FSRS but may not be the case in SE-FSRS as the two beams have different wavelengths and consequently scatter differently due to the plasmonic substrate. We varied the path length of our sample cuvette from 1 to 10 mm using a 0.2 mM Au concentration of colloidal nanoparticle substrates. We observed a similar trend as with the concentration dependence, where signal first increases with increasing path length and then decreases at the longest path length. To compare the path length and concentration dependences, Figure 3 shows FSRS gain as a function of path length and concentration for an SE-FSRS experiment with the x-axis normalized by the product of the concentration and path length. This is equivalent to plotting the data as a function of sample absorbance. Because the concentration and path length curves reach a maximum at the same point, our results illustrate that for collinear beams path length and concentration have the same effects on SE-FSRS signals. For noncollinear beams, however, the path length would have a far smaller effect on signal because the beam interaction volume would not depend on path length when the path length is longer than the overlapping focal spots of the pump and probe beams. Comparing readily available cuvette path lengths, we observed the highest SNRs with a 2 mm path length. Thus, the ideal

relatively simple function of the solubility of the analyte and laser power. These straightforward trends for FSRS are complicated, however, by the addition of plasmonic substrates in SE-FSRS experiments. For both SERS and SE-FSRS as sample concentration increases, signal increases, reaches a maximum, and then quickly decreases. This trend arises from two competing factors. As more gold aggregates are added to the suspension, the increased number of molecules and hot spots allows for more opportunities for signal generation as well as more opportunities for plasmonic enhancement of both the incident beams and the signal. However, increasing the concentration of nanoparticles increases the probability that the signal and incident light will be scattered or absorbed rather than transmitted to the detector. This extinction is a wavelength-dependent effect illustrated in the substrate extinction spectrum in Figure 1. At very high concentrations extinction overwhelms the effect of enhancement and leads to the decrease in signal observed at the highest concentrations. Consequently, there is an ideal concentration which optimizes signal in these experiments. Our results indicate a different ideal concentration for spontaneous SERS as compared to SE-FSRS. This result agrees well with recent calculations for measurements using colloidal plasmonic substrates, where Chng et al. modeled extinction and enhancement in SERS as well as surface-enhanced stimulated Raman scattering (SE-SRS) and predicted a higher ideal concentration for SE-SRS as compared to SERS.17 Stimulated Raman spectroscopy yields higher signals than spontaneous Raman spectroscopy due to the stimulated nature of the scattering process, and thus it is expected that the addition of hot spots to enhance the already higher coherent Raman signals will more easily outweigh the extinction due to increased concentrations in stimulated SERS. Consequently, SE-FSRS provides the highest absolute signal at higher concentrations than a comparable spontaneous SERS process because enhancement plays a larger role than extinction in determining signal magnitudes in SE-FSRS in the systems examined herein. Optimizing Raman gain is not the only consideration for obtaining the best signal in SE-FSRS experiments, however, because the goal of ultrafast SERS measurements is often to study a very small number of molecules, leading to small overall signal amplitudes compared to noise. Consequently, signal-tonoise ratios (SNRs) are a more useful observable for determining ideal experimental conditions, as they help determine the best experimental conditions to see even the smallest signals. In the SE-FSRS experiments shown in Figure 2a the most concentrated sample has nearly 4 times higher absolute Raman gain than the 0.1 mM sample. However, the most concentrated sample is also significantly noisier than the 0.1 mM sample. Thus, despite higher Raman gain at the highest concentration, it is not obvious which concentration would be ideal for SE-FSRS when looking at the spectra. A SNR would better determine ideal concentrations. To illustrate this effect, we have plotted both the SE-FSRS SNR and gain amplitudes as a function of concentration in Figure 2c. Here we define SNR as the fitted SE-FSRS gain amplitude divided by the standard deviation of the measured signal around that signal (details of SNR calculations can be found in the Supporting Information). If using SNRs instead of gain amplitude as a measure of signal quality, a lower concentration, closer to that used for spontaneous SERS, is seen to be ideal for SE-FSRS measurements. D

DOI: 10.1021/acs.jpcc.6b10727 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10727. Fits for the SE-FSRS data; signal-to-noise ratio calculations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(R.P.V.D.) E-mail [email protected]. ORCID

Natalie L. Gruenke: 0000-0001-6071-6666 Michael O. McAnally: 0000-0002-8681-2952 Richard P. Van Duyne: 0000-0001-8861-2228 Present Address

N.L.G.: Department of Chemistry, University of California, Berkeley, CA 94720, and Molecular Biophysics & Integrated Biology Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was made possible through the NSF Center for Chemistry at the Space-Time Limit (CaSTL), through Grant CHE-1414466. N.L.G. and M.O.M. acknowledge support from the National Science Foundation Graduate Fellowship Research Program under Grant DGE-0824162, while N.L.G., M.O.M., and R.P.V.D. acknowledge funding from NSF CHE1506683.

Figure 3. (a) Dependence of SE-FSRS gain at 1607 cm−1 as a function of path length. (b) Comparison of concentration and path length as variables to determine ideal SE-FSRS sample concentrations. SNRs for SE-FSRS as path length is varied and concentration is kept constant (green, diamonds, left axis) and SNRs for SE-FSRS as concentration is varied and path length is kept constant (purple, squares, right axis) are both plotted as a function of the product of Au concentration and sample path length (i.e., sample absorbance).

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experimental parameters for this system are approximately 2 mm path length with a concentration of 0.2 mM Au.

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CONCLUSIONS We have explored a number of experimental parameters to better understand the balance of extinction with enhancement for signal optimization in SE-FSRS. For a suspension of aggregated nanoparticles, maximum gain amplitude for SEFSRS occurs at a higher sample concentration than for spontaneous SERS. This is due to higher signals in stimulated over spontaneous SERS, which helps overcome extinction at higher sample concentrations. However, our results indicate that SNR is a better measure of signal quality for stimulated SERS experiments because the SNR accounts for increases in noise caused by SE-FSRS probe attenuation. SE-FSRS SNRs peak at the same sample concentrations that give the highest SERS signals. We found ideal path lengths and nanoparticle concentrations for SE-FSRS colloidal experiments and saw that the effects of increasing sample concentration and path length are the same. This implies that a collinear geometry allows for pump−probe interactions for the entirety of path lengths up to 10 mm, despite differences in extinction due to the plasmonic substrate at pump and probe wavelengths. Overall the work presented in this paper explores ideal experimental parameters for SE-FSRS and compares the effects of enhancement and extinction in SERS and SE-FSRS to inform future applications of this new ultrafast SERS technique. E

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DOI: 10.1021/acs.jpcc.6b10727 J. Phys. Chem. C XXXX, XXX, XXX−XXX