In Situ Electrochemical Tip-Enhanced Raman Spectroscopy with a

Jun 26, 2018 - Chemically modified tips in scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been used to improve the imaging...
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Cite This: J. Phys. Chem. Lett. 2018, 9, 3825−3828

In Situ Electrochemical Tip-Enhanced Raman Spectroscopy with a Chemically Modified Tip Guillaume Goubert,†,§,∥ Xu Chen,‡,∥ Song Jiang,† and Richard P. Van Duyne*,‡,† †

Department of Chemistry and ‡Applied Physics Graduate Program, Northwestern University, Evanston, Illinois 60208, United States

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S Supporting Information *

ABSTRACT: Chemically modified tips in scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been used to improve the imaging resolution or provide richer chemical information, mostly in ultrahigh vacuum (UHV) environments. Tip-enhanced Raman spectroscopy (TERS) is a nanoscale spectroscopic technique that already provides chemical information and can provide subnanometer spatial resolution. Chemical modification of TERS tips has mainly been focused on increasing their lifetimes for ambient and in situ experiments. Under UHV conditions, chemical functionalization has recently been carried out to increase the amount of chemical information provided by TERS. However, this strategy has not yet been extended to in situ electrochemical (EC)TERS studies. The independent control of the tip and sample potentials offered by ECSTM allows us to prove the in situ functionalization of a tip in EC-STM-TERS. Additionally, the Raman response of chemically modified TERS tips can be switched on and off at will, which makes EC-STM-TERS an ideal platform for the development of in situ chemical probes on the nanoscale.

C

recently been demonstrated in liquid under potential control by several groups.18−24 Furthermore, TERS offers exciting possibilities to study important surface processes in their native environment and to modulate the properties of adsorbates via potential control. STM-TERS is uniquely adapted to EC conditions because it allows for independent control of both the tip and sample potentials. Here we present an in situ ECSTM-TERS study of a chemically modified TERS tip. We show that the spectroscopic response of Nile Blue (NB) molecules covalently tethered to an Au TERS tip and to an Au(111) electrode can be separately addressed by controlling their EC potentials independently. Such capability makes ECSTM-TERS an ideal platform for the development of in situ chemical probes on the nanoscale. EC-STM-TERS was performed using a custom apparatus.24 Au tips were electrochemically etched before coated using Zaponlack varnish. A detailed description of the apparatus and the tip fabrication can be found in the Supporting Information. NB molecules were covalently attached to an Au(111) electrode using an EDC coupling procedure adapted from the recipe by Willets and coworkers,25,26 as described in more detail in the Supporting Information. We refer to the covalently tethered NB molecules as EDC-NB. Electrochemical surface-enhanced Raman spectroscopy (EC-SERS)25−28 and EC-TERS22−24 studies have shown that upon reduction the electronic resonance of NB at 633 nm is

hemically modified STM and AFM probes have been used to improve the imaging resolution or to harvest more detailed chemical information.1−4 In the AFM case, carbon monoxide (CO)-modified tips have been used to provide structural information on planar molecules with atomic resolution.1−3 In the STM case, a CO-terminated tip can be used to reveal internal and external bonding of molecules using inelastic tunneling spectroscopy (IETS) by monitoring the spatial variations in the C−O vibration as it interacts with adsorbates.4 IETS provides detailed spectroscopic information on the nanoscale but is limited to UHV and low-temperature conditions. TERS is a nanoscale spectroscopic technique that has demonstrated chemical imaging capability with subnanometer resolution.5−11 TERS tips have been chemically modified mostly to increase their lifetimes and protect them from environmental contaminations.12−15 Apkarian and coworkers have employed a CO-terminated TERS tip to chemically speciate an Ag atom and a neighboring CO molecule.11 At present, CO-terminated tips require UHV and cryogenic conditions to be stable.1−4,11 It is an open question as to the applicability of this approach in EC-TERS. Recently, thiophenol-functionalized Au-coated TERS tips have been used to evaluate the local temperature at the tip apex by monitoring the thermal desorption of thiophenol under laser excitation.16 Another recent study used chemically modified TERS tips to achieve local pH sensing with a resolution of 400 nm by monitoring the deprotonation of p-mercaptobenzoic acid (pMBA) and the dimerization of p-aminothiophenol (pATP).17 An attractive feature of TERS is its compatibility with a wide range of experimental conditions. In situ TERS has © XXXX American Chemical Society

Received: May 25, 2018 Accepted: June 26, 2018 Published: June 26, 2018 3825

DOI: 10.1021/acs.jpclett.8b01635 J. Phys. Chem. Lett. 2018, 9, 3825−3828

Letter

The Journal of Physical Chemistry Letters

Figure 1. TERS as a function of (A) tip potential with EDC-NB on the tip, (B) sample potential with EDC-NB on the tip, and (C) sample potential with EDC-NB only on the sample. The tunneling current is maintained at −1.0 nA. Only tip-engaged spectra are shown here. Tipretracted spectra can be found in Figure S3 in the Supporting Information. Each spectrum is accumulated over 120 s and background subtracted to remove the potential-dependent fluorescence. The areas of the 593 cm−1 band of the stacked spectra in panels A−C are normalized and presented as a function of tip potential in panel D and as a function of sample potential in panels E and F, respectively.

and b is an offset. R, F, and T are the gas constant, the Faraday constant, and the absolute temperature, respectively.

lost, which results in much weaker Raman scattering. Using potential-dependent TERS, we can follow the reduction of NB molecules at the tip-sample junction. Figure 1A,D presents the EC-TERS signal with an NBfunctionalized Au tip as a function of the tip potential. The sample potential is held constant at −0.35 V to ensure that all of the NB attached to the sample is reduced and therefore does not generate detectable TERS signal. In a control experiment, the dependence of EC-TERS on the sample potential is shown on Figure 1B,E. In this experiment, the potential of the NB-modified tip is fixed at +0.15 V to maintain EDC-NB in its oxidized form. The EC-TERS intensity is not substantially affected by the sample potential. Panels A−D in Figure 1 show that the molecule(s) responsible for the signal is(are) only sensitive to the tip potential, proving that the tip was functionalized by EDC-NB. Figure 1C,F shows the EC-TERS signal as a function of sample potential acquired with the same tip before chemical functionalization, while the tip potential is fixed at +0.15 V. In this case, the EC-TERS amplitude decreases with the sample potential because EDC-NB molecules are only bound to the sample, and, when reduced, they lose their resonance at 633 nm.22−28 Additionally, we observe that TERS intensity evolves with either the sample potential or the tip potential but not the bias (difference between the two potentials). In Figure 1A,D TERS intensity decreases as the amplitude of bias decreases. In Figure 1C,F TERS intensity decreases as the bias amplitude increases. In Figure 1B,E there is no monotonic change in TERS intensity with bias. From these observations we conclude that the change in bias does not play a significant role in our study. We use a simple Nernst equilibrium model (eq 1) to extract the formal potential associated with the potential-dependent TERS of NB shown in Figure 1A,D. S is the TERS signal amplitude, n is the number of electrons passed (n = 2 for NB), E is the applied potential at the electrode, E0 is the formal potential of the EDC-NB, α is the charge-transfer coefficient,

α

S= 1+e

−nF(E − E 0)/ RT

+b (1)

We define the TERS amplitude S as the signal integrated under the marker band of EDC-NB at 593 cm−1. The potential dependence of S for the NB-functionalized tip is shown on Figure 2, along with the best fit of S using eq 1. This allows us to define the formal potential associated with EDC-NB attached to the apex of the TERS tip, which is at E0TERS = 13 mV versus Ag/AgCl (3 M NaCl). This is very close to E0Sample = 34 mV, the formal potential of NB extracted from the

Figure 2. Cyclic voltammogram (CV) of the sample acquired before the EC-TERS experiments at 0.1 V/s in blue. Amplitude of the 593 cm−1 band of TERS from an NB-modified tip as a function of tip potential in red and the best fit of the TERS amplitude using the Nernst model presented as a black dashed line. The x axis represents the sample potential for the blue line and the tip potential for the red and black lines. The background of the CV has been subtracted to highlight the faradaic current emanating from surface-bound species. Examples of raw CVs are presented in Figure S2. 3826

DOI: 10.1021/acs.jpclett.8b01635 J. Phys. Chem. Lett. 2018, 9, 3825−3828

Letter

The Journal of Physical Chemistry Letters

a certain potential range without perturbing the sample being probed.

cyclic voltammograms of the sample acquired before the ECTERS experiments (Figure 2 and Figure S2). We hypothesize that the tip was chemically modified at its very apex by one or only a few EDC-NB molecules. The modification could have happened either by breaking the thiolAu bond at the sample surface or by the activated chemisorption of free EDC-NB that was present on the sample but was not chemisorbed via the thiol moiety. A recent TERS report has shown that irradiation of a tip-sample junction in TERS can cause the thermal desorption of some thiols.16 It is possible that in our setup the thermal desorption of EDC-coupled NB facilitated the chemical modification of the tip. Although the adsorption mechanism for the molecules on the tip is not known directly, it is likely that strong chemisorption via a thiol-Au bond occurs. This is supported by the long residence time of molecules on the tip, around 2 h. It is very unlikely that a small number of physisorbed molecules would stay for such an extended period of time in the hot spot. In a separate experiment, we observed that for physisorbed NB, reductive desorption occurs as soon as the potential reaches E0TERS. Two approaches are used to detect if a TER spectrum originates from sample-bound or tip-bound molecules: (1) compare spectra acquired when the tip is in tunneling contact with the sample with respect to when it is retracted far away and (2) test the same tip on a clean sample after the measurement. The latter method takes into account the effect of the gap-mode plasmon on the enhancement.29 In an ambient or UHV-TERS experiment, when a tip picks up molecule(s) from the substrate, it must be discarded or cleaned as it is no longer a neutral surface probe. Operating under EC conditions gives us control over the EC potentials experienced by molecules attached to the tip and the sample. We have shown that this new degree of freedom can be used to selectively reduce/oxidize molecules on the tip or the sample (Figure 1) and to measure their EC properties separately (Figure 2). Compared with the usual test for the presence of molecules on the tip, this permits the detection of tip-bound molecules in situ without leaving tunneling contact. This approach could provide a more complete analysis of the properties of adsorbates on the tip as well as a way to deconvolute TER signals originating from molecules both tipbound and sample-bound. If the signal-to-noise ratio of ECTERS permits high-frequency detection, then it would be possible to extract temporal information from repeated cycles of TERS voltammetry, as demonstrated in the kinetic ECSERS study of NB.28 For future experiments, we propose to employ the isotopologue strategy30,31 to statistically prove the single-molecule sensitivity of EC-TERS for both sample-bound and tip-bound molecules. The chemical modification of nanoscale probes such as TERS tips is still a relatively unexplored field of research that could draw inspiration from the rich literature on chemically modified SERS substrates.32,33 The results presented here show that in an EC environment STM-TERS offers unique opportunities for studies using chemically modified probes. It provides access to in situ studies of technologically relevant analytes at solid-liquid interfaces and the unique capability to separately control the sample and tip potentials. This opens a promising avenue for the development of in situ TERS because it permits us to separate signals coming from the tip and the sample. Chemically functionalized tips could also be used to create switchable chemical probes that would be active only in

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b01635.

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Experimental details and additional data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 847-491-3516. Fax: 847-491-7713. ORCID

Guillaume Goubert: 0000-0002-4325-0267 Xu Chen: 0000-0002-2603-4837 Richard P. Van Duyne: 0000-0001-8861-2228 Present Address §

G.G.: Laboratory of Organic Chemistry, D-CHAB, ETH Zurich, Vladimir-Prelog-Weg 3, 8093 Zürich, Switzerland. Author Contributions ∥

X.C. and G.G. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Air Force Office of Scientific Research MURI (FA9550-14-10003). We thank Dr. Allen J. Bard, Dr. Henry White, Dr. Martin Edwards, Dr. Michael Mattei, and Dr. Katherine A. Willets for helpful discussions.

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