Observing Single, Heterogeneous, One-Electron Transfer Reactions

Understanding electrochemical events on the single-molecule level is crucial for fields such as catalysis and biological systems. A variety of techniques exist to study the electrochemistry of single molecules, but few provide correlated chemical information. Herein, we study the electrochemistry of rhodamine 6G in nonaqueous conditions and demonstrate the first statistic electrochemical single-molecule SERS (EC-SMSERS) proof of single-electron transfer events. We find that the distribution of reduction events is broader than that in a bulk electrochemical experiment. The distribution of the reduction potentials can be explained by molecular reorientation and variations of the local surface site or chemical potential of the Ag nanoparticle. Our results contribute toward understanding electrochemical behavior of single molecules on the nanoscale monitored by SERS and the ultimate goal of controlling single-electron transfer processes

Tip-Enhanced Raman Spectroscopy (TERS) for <i>in Situ</i> Identification of Indigo and Iron Gall Ink on Paper

Confirmatory, nondestructive, and noninvasive identification of colorants <i>in situ</i> is of critical importance for the understanding of historical context and for the long-term preservation of cultural heritage objects. Although there are several established techniques for analyzing cultural heritage materials, there are very few analytical methods that can be used for molecular characterization when very little sample is available, and a minimally invasive approach is required. Tip-enhanced Raman spectroscopy (TERS) is a powerful analytical technique whose key features include high mass sensitivity, high spatial resolution, and precise positioning of the tip. In the current proof-of-concept study we utilized TERS to identify indigo dye and iron gall ink <i>in situ</i> on Kinwashi paper. In addition, TERS was used to identify iron gall ink on a historical document with handwritten text dated to the 19th century. We demonstrate that TERS can identify both of these colorants directly on paper. Moreover, vibrational modes from individual components of a complex chemical mixture, iron gall ink, can be identified. To the best of our knowledge, this is the first demonstration of <i>in situ</i> TERS for colorants of artistic relevance directly on historical materials. Overall, this work demonstrates the great potential of TERS as an additional spectroscopic tool for minimally invasive compositional characterization of artworks <i>in situ</i> and opens exciting new possibilities for cultural heritage research

Single Molecule Electrochemistry: Impact of Surface Site Heterogeneity

Probing the electrochemistry of single molecules is a direct pathway toward a microscopic understanding of a variety of electron transfer processes related to energy science, such as electrocatalysis and solar fuel cells. In this context, Zaleski et al. recently studied the single electron transfer reaction of the dye molecule rhodamine-6G (R6G) by electrochemical single molecule surface-enhanced Raman spectroscopy (EC-SMSERS) (J. Phys. Chem. C 2015, 119, 28226−28234). In that work, the reductions of the dye molecule R6G were not only observed in the same potential range as in the ensemble surface cyclic voltammogram but also seen under some less negative potentials. Aiming to understand and explain this experiment theoretically, we relate the binding energy of R6G⁺ adsorbed on a silver nanoparticle (AgNP) to its reduction potential and further use periodic density functional theory to calculate this adsorption energy at different local surface sites. Well-defined crystal facets and defective surfaces, are considered. We find that the calculated adsorption energy distribution of the strongest binding states at each surface site closely matches the potential range of the experimentally observed Faradaic events. Moreover, the underpotential events are explained by the metastable adsorption states with less binding strength compared with those corresponding to Faradaic events. Our study reveals the importance of the heterogeneity of surface structures on the AgNP and offers a new perspective on understanding single molecule electrochemical behavior

Identification and Quantification of Intravenous Therapy Drugs Using Normal Raman Spectroscopy and Electrochemical Surface-Enhanced Raman Spectroscopy

Errors in intravenous (IV) drug therapies can cause human harm and even death. There are limited label-free methods that can sensitively monitor the identity and quantity of the drug being administered. Normal Raman spectroscopy (NRS) provides a modestly sensitive, label-free, and completely noninvasive means of IV drug sensing. In the case that the analyte cannot be detected within its clinical range with Raman, a label-free surface-enhanced Raman spectroscopy (SERS) approach can be implemented to detect the analyte of interest. In this work, we demonstrate two individual cases where we use NRS and electrochemical SERS (EC-SERS) to detect IV therapy analytes within their clinically relevant ranges. We implement NRS to detect gentamicin, a commonly IV-administered antibiotic and EC-SERS to detect dobutamine, a drug commonly administered after heart surgery. In particular, dobutamine detection with EC-SERS was found to have a limit of detection 4 orders of magnitude below its clinical range, highlighting the excellent sensitivity of SERS. We also demonstrate the use of hand-held Raman instrumentation for NRS and EC-SERS, showing that Raman is a highly sensitive technique that is readily applicable in a clinical setting

Toward Monitoring Electrochemical Reactions with Dual-Wavelength SERS: Characterization of Rhodamine 6G (R6G) Neutral Radical Species and Covalent Tethering of R6G to Silver Nanoparticles

The combination of electrochemistry (EC) and single molecule surface-enhanced Raman spectroscopy (SMSERS) has recently proven to be a sensitive method to investigate electron transfer (ET) reactions at the single molecule level. SMSERS can both detect single redox-active molecules and potentially monitor both the oxidized (O) and reduced (R) forms of a one-electron ET reaction in a single experiment. Herein, we report progress toward complete monitoring of single ET reactions with EC-SMSERS. We first obtained the solution phase resonance Raman (RR) spectrum of the Rhodamine 6G (R6G) neutral radical (R) with thin-layer resonance Raman spectroelectrochemistry (EC-RRS). The experimental spectrum was then correlated with the spectrum calculated by density functional theory (DFT). We then describe our approach to address the problem of adsorbate (R) loss caused either by desorption or reaction of the neutral radical with trace water or oxygen during the EC-SMSERS experiment. We have investigated a covalent cross-linking reaction which tethers R6G to SERS-active substrates (Ag nanoparticles). Covalently tethered R6G is subsequently characterized by surface cyclic voltammetry (CV) and SERS. Lastly, an optimized cross-linking reaction is devised which enabled the first direct detection of the one-electron reduced form of R6G with SERS. Our findings demonstrate that SERS can simultaneously monitor both O and R of a one-electron ET reaction