Fabrication of Gold Nanosphere Oligomers for Surface-Enhanced

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Fabrication of Gold Nanosphere Oligomers for SurfaceEnhanced Femtosecond Stimulated Raman Spectroscopy Bogdan Negru, Michael O. McAnally, Hannah E Mayhew, Tyler Ueltschi, Lingxuan Peng, Emily Sprague-Klein, George C. Schatz, and Richard P. Van Duyne J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09664 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

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The Journal of Physical Chemistry

Fabrication of Gold Nanosphere Oligomers for Surface-Enhanced Femtosecond Stimulated Raman Spectroscopy Bogdan Negru†, Michael O. McAnally, Hannah E. Mayhew, Tyler Ueltschi, Lingxuan Peng, Emily Sprague-Klein, George C. Schatz, Richard P. Van Duyne* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States

ABSTRACT: Surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS) is a technique that has the potential to study the ultrafast dynamics of surface adsorbates with simultaneously high temporal and spectral resolutions. One of the greatest impediments to the further development of SE-FSRS is the lack of high quality plasmonic substrates that permit excitation and prove wavelength agility as well as flexible selection of the adsorbate molecule. Herein, we present a simple aggregation/stabilization procedure for the formation of gold nanosphere oligomers with tunable plasmonic properties and flexible choice of adsorbate that are ideal for SE-FSRS and other coherent Raman spectroscopies.

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INTRODUCTION Femtosecond stimulated Raman spectroscopy (FSRS) has been developed to study the ensemble-averaged structure of chemical systems on the time scale of molecular motion.1-3 This technique probes changes in molecular structure by acquiring Raman spectra with a temporal resolution in the range of 10 – 100 fs.1, 4-5 FSRS has been used to study the vibrational dynamics of many molecules,6-8 but has been limited to highly concentrated systems with large Raman scattering cross-sections.

This limitation was surpassed for ground-state FSRS by the

development of surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS).9 SE-FSRS has the capability of studying surface chemistry with high spectral and temporal resolution. SE-FSRS uses a combination of the electromagnetic and chemical mechanisms active in surface-enhanced Raman scattering10 (SERS) to amplify the FSRS signal by as much as four to six orders of magnitude.9 Ground state SE-FSRS was demonstrated on Au nanosphere oligomers (also called SERS nanotags) that were commercially available. These nanoparticles are Au cores (90 nm or 60 nm in diameter), surface functionalized with trans-1,2-bis(4-pyridyl)-ethylene (BPE) as the adsorbate, and capped with a thick (~60 nm) silica shell.9, 11 Numerous attempts were made in our laboratory to acquire SE-FSR spectra on Au film over nanosphere (AuFON) surface and nanotriangles fabricated by nanosphere lithography (NSL), all such attempts failed due to sample damage caused by the high peak powers of pulsed pump and probe beams. All SE-FSRS studies published thus far have been limited to Au nanosphere oligomers.9, 12-17 While these commercial substrates are very durable and highly enhancing, they are limited to only a few adsorbate molecules and have limited availability. A reliable method for substrate synthesis is, therefore, of utmost importance if SE-FSRS is to be used for the investigation of ultrafast dynamics of


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relevant plasmonic chemistry systems:18 such as plasmon-enhanced solar cells19-20 and plasmonenhanced photocatalysis.21-23 A wide variety of synthetic approaches for SERS substrates have been published in literature, including colloidal pastes,24 thin metal films, NSL nanotriangles,25-26 metal FONs,27 and polymer-fiber-nanoparticle assemblies.28 In order to implement time-resolved (TR)-SEFSRS two femtosecond and one picosecond pulse must be overlapped in space and time. Effectively, these pulses interrogate a volume of sample in which the three pulses are overlapped, necessitating substrates that occupy a volume that is greater than the diameter of the focal spot times an interaction length of a few millimeters.29-30 Depending on beam overlap, the FSRS signal can be observed as far as 1 cm or more from the focal spot created by a 10 cm focal length lens. The Au nanosphere oligomers are dispersed in solution with pathlengths of 1-10 mm, while two-dimensional substrates, such as FONs and NSL triangles, only minimally interact with the laser pulses, due to the near-zero interaction length. The short interaction length problem coupled with surface damage caused by the intrinsically high peak powers of laser pulses amplified at electromagnetic hot spots9, 17 accounts for the lack of success in using these substrates for SE-FSRS. For these reasons, the gold nanosphere oligomers have proven to be the best, and presently only, platform for SE-FSRS.9, 12-13, 15-17 Herein we present a simple and reliable protocol for the synthesis of gold nanosphere oligomers. This protocol can be used for the development of highly enhancing substrates that are exceptionally stable to ambient environments, femtosecond laser pulses, and contain a wide range of user chosen adsorbates. To demonstrate the utility of this protocol, the synthesis of gold nanosphere oligomers with the following adsorbates: tris-(2,2′-bipyridine)ruthenium(II) ([Ru(bpy)3]2+),

tris-(2,2′-bipyridine)iron(II)

([Fe(bpy)3]2+),

4,4′-azopyridine,

2,2′:6′,2″-


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terpyridine (Terpy), and β-carotene is provided. All of these molecules are of significant interest for time-resolved photodynamics and have been studied in solution by TR-FSRS.3 For example, previous studies explored the excited state dynamics of [Ru(bpy)3]2+ in solution and observed rapid formation of triplet metal to ligand charge transfer state (3MLCT) after 1MLCT 1A1 excitation.31-32 However, the impact of adsorbing [Ru(bpy)3]2+ to a plasmonic surface on the photophysics of this system is unknown. METHODS The synthesis of gold nanosphere oligomers followed a bottom-up approach that started with gold monomer solutions. Gold colloids of two different sizes, roughly 50 nm and 90 nm in diameter, were prepared by a standard citrate reduction method.33 Briefly, a 50 mL 0.1% wt/wt HAuCl4 in MilliQ water solution was brought to a boil. A single addition of either 0.40 mL or 0.22 mL 1% wt/wt Na3-citrate was made under vigorous stirring conditions. After 30 minutes of boiling, the solution was slowly cooled and stored in the dark. A previously described34 highresolution localized surface plasmon resonance instrument (HR-LSPR) was used to continuously monitor the extinction spectra of the gold monomer solution in-situ from 500 to 1100 nm. Aggregation of gold monomers was induced by the repeated addition of 10 µL aliquots of MilliQ water containing the adsorbate molecule of interest. In the case of BPE, the aggregating solution consisted of a 100-fold dilution of room temperature saturated BPE aqueous solution. The aggregation of gold monomers can be monitored in real time by the acquisition of extinction spectra, as shown in Figure 1. In addition, aqueous solutions of [Ru(bpy)3]2+, [Fe(bpy)3]2+, 4,4′azopyridine, and other analytes were successfully used to aggregate gold monomers. It is important to note that nanosphere oligomers were successfully created with a wide range of adsorbate molecule concentrations: from µM to mM regimes.


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Most importantly, monomer aggregation was stopped and newly formed nanosphere oligomers were stabilized by the addition of 0.2 mL of 10% wt/wt 55 kDa polyvinylpyrrolidone (PVP) solution in ethanol. PVP is a water soluble polymer commonly employed in nanoparticle synthesis.35 A thin (approximately 3 nm) polymer layer can be observed in the transmission electron microscopy (TEM) scan presented in Figure 2. All reagents were purchased from Sigma-Aldrich and used without further purification, with the exception of BPE, which was purified by sublimation.

Figure 1: Real time extinction spectra of 90 nm gold monomers aggregating due to the addition of dilute aqueous solution of trans-1,2-bis(4-pyridyl)-ethylene.

A detailed description of the experimental setup for SE-FSRS spectroscopy has been described previously.9,

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In short, the fundamental of a Coherent RegA 100 kHz ultrafast

amplifier was split into two pulses: pump and probe. The picosecond Raman pump pulse at 795 nm was created by passing the beam through two narrow bandpass filters, while the broadband probe pulse was created by continuum generation in an undoped YAG window. Both pulses are recombined in a macroscopic cuvette sample, with Raman gain determined by dividing the pump-on spectra by the pump-off spectra. Surface-enhanced Raman scattering spectra were


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collected on an inverted microscope.36 Briefly, the sample was mounted on a Nikon Eclipse TiU inverted microscope and illuminated with a 785 nm CW laser (Renishaw Inc.). A 20X objective was used to both focus the laser light onto the sample and collect the scattered signal. Stokes-shifted signal was focused into an ACTON 2300 spectrometer outfitted with a PIXIS 400BR CCD camera (Princeton Instrument).

Figure 2: Transmission electron microscopy (TEM) scan of nanosphere oligomer aggregated with 2,2′:6′,2″-terpyridine and stabilized with 55 kDa polyvinylpyrrolidone. The image was acquired with a Hitachi H-8100 at 200 kV.

RESULTS AND DISCUSSION The HR-LSPR setup allows for continuous monitoring of the extinction spectrum, and thus sample aggregation as shown in Figure 1. Initially, the monomer peak was observed near 560 nm. Aggregate formation can be tracked by monitoring the appearance of broad spectral features at longer wavelengths. Monomers aggregate to form dimers and trimers, then continue aggregating to form larger and larger nanosphere oligomers. For this reason, the LSPR peak corresponding to the oligomers shifts to longer and longer wavelengths.

As the average

nanosphere oligomer size increases, the size distribution also increases, leading to broader LSPR peaks.

Nanosphere oligomers are formed from monomers, so the monomer LSPR peak


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decreases in intensity but does not vanish as oligomers still retain a monomer LSPR band.11 Nanosphere oligomer solutions can be slowly aggregated over a period of days by the addition of a small number of adsorbate molecules. Alternatively, they can be aggregated in a few seconds if a large amount of adsorbate is added.

Step-wise addition of adsorbate solution with

continuous LSPR monitoring makes it easy to achieve the desired state of oligomerization. It is important to note that the oligomerization of gold monomers can be instantaneously stopped, at the macroscopic scale, by the addition of PVP, a weakly bound nitrogen-based capping agent.37 Figure 3a compares the LSPR spectra of BPE aggregated nanosphere oligomer samples. One sample was produced from smaller gold monomers roughly 50 nm in diameter (trace 3, red), and the rest from gold monomers approximately 90 nm in diameter. The latter samples (traces 1, 2, and 4) were arrested at different states of oligomerization resulting in substrates with varying plasmonic properties. In addition, Figure 3 displays the overlap between nanosphere oligomer LSPR and FSRS laser pulses. The spectral overlap between the LSPR of enhancing substrate and Raman pump/probe laser pulses influences both gain and dispersion of FSRS signal,16 as seen in Figure 3b. The dispersive peak shapes observed in SE-FSRS arise from coupling between adsorbed molecules and the plasmon resonances in the nanosphere oligomers.12, 16 The lineshapes observed in these samples appear very different, although they were collected sequentially with identical laser and spectrometer parameters. The LSPR characteristics of these highly enhancing substrates are crucial for SE-FSRS, and the preparation technique described herein can be used reliably to generate substrates that are optimized for the laser pulses available to the user.


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Figure 3: The effect of nanosphere oligomer LSPR on SE-FSRS lineshape and amplitude. a) Spectral overlap between Raman pump/probe and nanosphere oligomer LSPR. The colored traces labeled 1,2, and 4 represent 90 nm Au monomers arrested at different points in the aggregation process using PVP and trace 3 represents a sample made with 50 nm Au monomers. b) SE-FSRS signal from samples 1-4, showing varying Fano lineshapes and amplitudes dependent on the sample LSPR position relative to the Raman pump/probe pulse pair.

Close observation of the extinction spectrum of the nanosphere oligomer sample allows one to manipulate the rate of oligomerization by controlling adsorbate addition. Furthermore, since the LSPR has direct influence over the enhancing capabilities of the nanosphere oligomer,16 the extinction spectrum can be used to determine the optimal moment to stop aggregation by polymer addition. On the timescale of oligomerization, the gold nanospheres are stabilized instantaneously, and their plasmonic properties at the time of PVP addition are preserved indefinitely (at least to several months).

PVP stabilization of gold nanosphere


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oligomers provides unparalleled molecular control: making it possible to produce samples with desired plasmon resonances for stimulated Raman spectroscopies. The use of PVP for arresting oligomerization is a general and simple procedure. The addition of 200 µL solutions of 55 kDa PVP effectively stopped the aggregation of gold monomers driven by many adsorbates of interest, including BPE, Terpy, [Ru(bpy)3]2+, [Fe(bpy)3]2+, and 4,4′-azopyridine (Figure 4). Figure 4 displays ground-state SE-FSRS and SERS spectra of gold nanosphere oligomers aggregated with [Ru(bpy)3]2+ and 4,4′-azopyridine. These samples produced SER and SE-FSR spectra of excellent quality, opening the door for TRSE-FSRS experiment on chemical systems with ultrafast dynamics.


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Figure 4: SERS and SE-FSRS spectra of nanosphere oligomers aggregated with a) tris-(2,2′bipyridine)ruthenium(II), and b) 4,4′-azopyridine. SERS spectra were acquired for 10 seconds with 110 µW at 785 nm.

CONCLUSION We have demonstrated that Au nanosphere oligomers are the best substrate presently available for SE-FSRS experiments. The use of polyvinylpyrrolidone (PVP) to stop monomer oligomerization is a straight-forward approach for the fabrication of gold nanosphere oligomers containing user selectable adsorbate molecules. A variety of adsorbate molecules interest to us were used to prepare Au nanosphere oligomers. PVP (55 kDa) polymer was used to arrest and stabilize Au nanosphere oligomers. The stabilizing effect of the polymer is instantaneous on the


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timescale of oligomerization, facilitating the fabrication of SE-FSRS substrates with precisely tuned plasmonic properties. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Present Addresses †

B.N.: Department of Chemistry, Sonoma State University, 1801 East Cotati Avenue, Rohnert

Park, California 94928, United States Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Science Foundation Grant CHE-1414466 and the Center for Chemical Innovation (CCI) dedicated to Chemistry at the Space-Time limit (CaSTL). BN would also like to thank the School of Science and Technology at Sonoma State University for support. We thank Natalie L. Gruenke, Naihao Chiang, and Dmitry Kurouski for helpful discussions.

The authors also thank Northwestern University’s Atomic and Nanoscale

Characterization Experimental Center (NUANCE) for the use of their imaging facilities.


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Fabrication of Gold Nanosphere Oligomers for Surface-Enhanced

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