<i>In Situ</i> Detection and Identification of Hair Dyes Using Surface-Enhanced Raman Spectroscopy (SERS)

Hair is one of the most common types of physical evidence found at a crime scene. Forensic examination may suggest a connection between a suspect and a crime scene or victim, or it may demonstrate an absence of such associations. Therefore, forensic analysis of hair evidence is invaluable to criminal investigations. Current hair forensic examinations are primarily based on a subjective microscopic comparison of hair found at the crime scene with a sample of suspect’s hair. Since this is often inconclusive, the development of alternative and more-accurate hair analysis techniques is critical. In this study, we utilized surface-enhanced Raman spectroscopy (SERS) to demonstrate that artificial dyes can be directly detected on hair. This spectroscopic technique is capable of a confirmatory identification of analytes with single molecule resolution, requires minimal sample, and has the advantage of fluorescence quenching. Our study reveals that SERS can (1) identify whether hair was artificially dyed or not, (2) determine if a permanent or semipermanent colorants were used, and (3) distinguish the commercial brands that are utilized to dye hair. Such analysis is rapid, minimally destructive, and can be performed directly at the crime scene. This study provides a novel perspective of forensic investigations of hair evidence

Probing Redox Reactions at the Nanoscale with Electrochemical Tip-Enhanced Raman Spectroscopy

A fundamental understanding of electrochemical processes at the nanoscale is crucial to solving problems in research areas as diverse as electrocatalysis, energy storage, biological electron transfer, and plasmon-driven chemistry. However, there is currently no technique capable of directly providing chemical information about molecules undergoing heterogeneous charge transfer at the nanoscale. Tip-enhanced Raman spectroscopy (TERS) uniquely offers subnanometer spatial resolution and single-molecule sensitivity, making it the ideal tool for studying nanoscale electrochemical processes with high chemical specificity. In this work, we demonstrate the first electrochemical TERS (EC-TERS) study of the nanoscale redox behavior of Nile Blue (NB), and compare these results with conventional cyclic voltammetry (CV). We successfully monitor the disappearance of the 591 cm^–1 band of NB upon reduction and its reversible reappearance upon oxidation during the CV. Interestingly, we observe a negative shift of more than 100 mV in the onset of the potential response of the TERS intensity of the 591 cm^–1 band, compared to the onset of faradaic current in the CV. We hypothesize that perturbation of the electrical double-layer by the TERS tip locally alters the effective potential experienced by NB molecules in the tip–sample junction. However, we demonstrate that the tip has no effect on the local charge transfer kinetics. Additionally, we observe step-like behavior in some TERS voltammograms corresponding to reduction and oxidation of single or few NB molecules. We also show that the coverage of NB is nonuniform across the ITO surface. We conclude with a discussion of methods to overcome the perturbation of the double-layer and general considerations for using TERS to study nanoscale electrochemical processes

Single Molecule Surface-Enhanced Raman Spectroscopy without Nanogaps

We provide conclusive proof of single molecule (SM) detection by surface-enhanced Raman spectroscopy (SERS) for discrete Ag triangular nanopyramids prepared via nanosphere lithography (NSL). While the observation of SMSERS has been well-demonstrated using various chemically synthesized nanoparticle substrates, they have a high degree of polydispersity in shape, size, and aggregation state resulting in an interest to develop more reproducible and uniform nanoparticles. Here triangular-based nanopyramids were characterized by scanning electron microscopy to confirm their geometry and interparticle spacing. Then the isotopologue approach with Rhodamine 6G was used to conclusively prove SM sensitivity for the individual nanoparticles. NSL’s straightforward, simple fabrication procedure produces large active arrays. Furthermore, the tunable localized surface plasmon resonance makes NSL prepared substrates effective for the detection of resonant molecules by SMSERS

Electrochemical STM Tip-Enhanced Raman Spectroscopy Study of Electron Transfer Reactions of Covalently Tethered Chromophores on Au(111)

The ability to study electron transfer reactions at the solid–liquid interface with nanometer resolution has the potential to critically improve our understanding of electrocatalytic processes. However, few techniques are capable of studying electrode surfaces <i>in situ</i> at the nanoscale. We study the redox reactions of Nile Blue (NB) covalently tethered to an Au(111) electrode using <i>in situ</i> tip-enhanced Raman spectroscopy (TERS) and show that TERS amplitude decreases reversibly as NB is reduced. The potential dependent TERS intensity allows us to associate an electrochemical wave with the loss of electronic resonance of NB and another with the peak of fluorescence of tethered NB, which we tentatively attribute to the disassembly of on-surface NB aggregates. The study of the electrochemical activity of immobile adsorbates at the solid–liquid interface with TERS is an essential step toward the realization of <i>in situ</i> spectroscopic mapping at the nanoscale

<i>In Situ</i> Electrochemical Tip-Enhanced Raman Spectroscopy with a Chemically Modified Tip

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 <i>in situ</i> 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 <i>in situ</i> electrochemical (EC)-TERS studies. The independent control of the tip and sample potentials offered by EC-STM allows us to prove the <i>in situ</i> 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 <i>in situ</i> chemical probes on the nanoscale

A Look at the Origin and Magnitude of the Chemical Contribution to the Enhancement Mechanism of Surface-Enhanced Raman Spectroscopy (SERS): Theory and Experiment

Normal and surface-enhanced Raman spectra for a set of substituted benzenethiols were measured experimentally and calculated from static polarizability derivatives determined with time-dependent density functional theory (TDDFT). Both silver and gold cluster–thiolate complexes were studied to investigate how the chemical enhancement varies with substituent. The experimental relative peak intensities and positions are well-matched by their theoretical counterparts. The static chemical enhancement of the ring stretching modes near 1600 cm^–1 is determined experimentally and computationally for each derivative, and it is found that the experimental enhancement varies by a factor of 10 as a result of chemical substitution, with stronger electron donating groups on the benzene unit leading to higher enhancements. The calculated trends with substitution match experiment well, suggesting that TDDFT is describing the chemical effect qualitatively, if not quantitatively, in the static (low-frequency) limit. A two-state model is developed, providing qualitative insight into the results in terms of the variation of ligand-to-metal charge-transfer excitation energy with substitution

Tip-Enhanced Raman Spectroscopy with Picosecond Pulses

Tip-enhanced Raman spectroscopy (TERS) can probe chemistry occurring at surfaces with both nanometer spectroscopic and submolecular spatial resolution. Combining ultrafast spectroscopy with TERS allows for picosecond and, in principle, femtosecond temporal resolution. Here we couple an optical parametric oscillator (OPO) with a scanning tunneling microscopy (STM)-TERS microscope to excite the tip plasmon with a picosecond excitation source. The plasmonic tip was not damaged with OPO excitation, and TER spectra were observed for two resonant adsorbates. The TERS signal under ultrafast pulsed excitation decays on the time scale of 10 s of seconds; whereas with continuous-wave excitation no decay occurs. An analysis of possible decay mechanisms and their temporal characteristics is given

A Look at the Origin and Magnitude of the Chemical Contribution to the Enhancement Mechanism of Surface-Enhanced Raman Spectroscopy (SERS): Theory and Experiment

Normal and surface-enhanced Raman spectra for a set of substituted benzenethiols were measured experimentally and calculated from static polarizability derivatives determined with time-dependent density functional theory (TDDFT). Both silver and gold cluster–thiolate complexes were studied to investigate how the chemical enhancement varies with substituent. The experimental relative peak intensities and positions are well-matched by their theoretical counterparts. The static chemical enhancement of the ring stretching modes near 1600 cm^–1 is determined experimentally and computationally for each derivative, and it is found that the experimental enhancement varies by a factor of 10 as a result of chemical substitution, with stronger electron donating groups on the benzene unit leading to higher enhancements. The calculated trends with substitution match experiment well, suggesting that TDDFT is describing the chemical effect qualitatively, if not quantitatively, in the static (low-frequency) limit. A two-state model is developed, providing qualitative insight into the results in terms of the variation of ligand-to-metal charge-transfer excitation energy with substitution

A 2D Semiquinone Radical-Containing Microporous Magnet with Solvent-Induced Switching from <i>T</i>_c = 26 to 80 K

The incorporation of tetraoxolene radical bridging ligands into a microporous magnetic solid is demonstrated. Metalation of the redox-active bridging ligand 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH₂) with Fe^II affords the solid (Me₂NH₂)₂­[Fe₂L₃]·​2H₂O·6DMF​. Analysis of X-ray diffraction, Raman spectra, and Mössbauer spectra confirm the presence of Fe^III centers with mixed-valence ligands of the form (L₃)^8– that result from a spontaneous electron transfer from Fe^II to L^2–. Upon removal of DMF and H₂O solvent molecules, the compound undergoes a slight structural distortion to give the desolvated phase (Me₂NH₂)₂­[Fe₂L₃], and a fit to N₂ adsorption data of this activated compound gives a BET surface area of 885(105) m²/g. Dc magnetic susceptibility measurements reveal a spontaneous magnetization below 80 and 26 K for the solvated and the activated solids, respectively, with magnetic hysteresis up to 60 and 20 K. These results highlight the ability of redox-active tetraoxolene ligands to support the formation of a microporous magnet and provide the first example of a structurally characterized extended solid that contains tetraoxolene radical ligands

Tip-Enhanced Raman Spectroscopy with Picosecond Pulses

Tip-enhanced Raman spectroscopy (TERS) can probe chemistry occurring at surfaces with both nanometer spectroscopic and submolecular spatial resolution. Combining ultrafast spectroscopy with TERS allows for picosecond and, in principle, femtosecond temporal resolution. Here we couple an optical parametric oscillator (OPO) with a scanning tunneling microscopy (STM)-TERS microscope to excite the tip plasmon with a picosecond excitation source. The plasmonic tip was not damaged with OPO excitation, and TER spectra were observed for two resonant adsorbates. The TERS signal under ultrafast pulsed excitation decays on the time scale of 10 s of seconds; whereas with continuous-wave excitation no decay occurs. An analysis of possible decay mechanisms and their temporal characteristics is given