Deep Ultraviolet Standoff Photoacoustic Spectroscopy of Trace Explosives.

We demonstrate deep ultraviolet (UV) photoacoustic spectroscopy (PAS) of trace explosives using a sensitive microphone at meter standoff distances. We directly detect 10 µg/cm2 of pentaerythritol tetranitrate (PETN), 2,4,6-trinitrotoluene (TNT), and ammonium nitrate (AN) with 1 s accumulations from a 3 m standoff distance. Large PAS signals for standoff detection are achieved by exciting into the absorption bands of the explosives with a 213 nm laser. We also investigate the impact of the deep UV photochemistry of AN on the PAS signal strength and stability. We find that production of gaseous species during photolysis of AN enhances the PAS signal strength. This deep UV photochemistry can, however, limit the PAS signal lifetimes when detecting trace quantities.

Hydrophobic Collapse Initiates the Poly( N-isopropylacrylamide) Volume Phase Transition Reaction Coordinate.

The best-known examples of smart, responsive hydrogels derive from poly( N-isopropylacrylamide) (PNIPAM) cross-linked polymer networks. These hydrogels undergo volume phase transitions (VPTs) triggered by temperature, chemical, and/or environmental changes. PNIPAM hydrogels can undergo more than 50-fold volume changes within ∼1 µs intervals. Studies have tried to elucidate the molecular mechanism of these extraordinarily large responses. Nevertheless, the molecular reaction coordinates that drive the VPT remain unclear. Using visible nonresonance Raman temperature-jump spectroscopy, we determined the molecular ordering of this VPT. The PNIPAM hydrophobic isopropyl and methylene groups dehydrate with time constants of 109 ± 64 and 104 ± 44 ns, initiating the volume collapse of PNIPAM. The subsequent dehydration of the PNIPAM amide groups is significantly slower, as our group previously discovered (360 ± 85 ns). This determination of the ordering of the molecular reaction coordinate of the PNIPAM VPT enables the development of the next generation of super-responsive materials.

Observation of Single Molecule Plasmon-Driven Electron Transfer in Isotopically Edited 4,4'-Bipyridine Gold Nanosphere Oligomers.

We clarify mechanistic questions regarding plasmon-driven chemistry and nanoscale photocatalysis within optically confined near-field plasmonic systems. Using surface-enhanced Raman scattering (SERS), we directly monitor the photoinduced reaction dynamics of 4,4'-bipyridine molecules, localized in plasmonic hot spots within individual gold nanosphere oligomers. Our experiment generates surface electrons from the gold nanoparticle using an intense off-molecular resonance continuous wave pump field, and detects radical anion products via SERS. This is done by adopting a dual-wavelength spectroscopic approach. Empirical evidence of plasmon-driven electron transfer is provided for the first time by direct detection of the 4,4'-bipyridine radical anion species localized in the plasmonic hot spots of individual gold nanosphere oligomers, corroborated by open-shell density functional theory calculations. An isotopologue approach using both protonated and deuterated 4,4'-bipyridine molecules demonstrates the single molecule response of plasmon-driven electron transfer occurring in single nanosphere oligomer systems with a 3% yield, a phenomenon unobserved in ensemble measurements under analogous experimental conditions. This mechanism has broad applicability to using nanoscale chemical reactors for surface redox reactions on the subnanometer scale.

Single-Molecule Chemistry with Surface- and Tip-Enhanced Raman Spectroscopy.

Single-molecule (SM) surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) have emerged as analytical techniques for characterizing molecular systems in nanoscale environments. SERS and TERS use plasmonically enhanced Raman scattering to characterize the chemical information on single molecules. Additionally, TERS can image single molecules with subnanometer spatial resolution. In this review, we cover the development and history of SERS and TERS, including the concept of SERS hot spots and the plasmonic nanostructures necessary for SM detection, the past and current methodologies for verifying SMSERS, and investigations into understanding the signal heterogeneities observed with SMSERS. Moving on to TERS, we cover tip fabrication and the physical origins of the subnanometer spatial resolution. Then, we highlight recent advances of SMSERS and TERS in fields such as electrochemistry, catalysis, and SM electronics, which all benefit from the vibrational characterization of single molecules. SMSERS and TERS provide new insights on molecular behavior that would otherwise be obscured in an ensemble-averaged measurement.

Aluminum Film-Over-Nanosphere Substrates for Deep-UV Surface-Enhanced Resonance Raman Spectroscopy.

We report here the first fabrication of aluminum film-over nanosphere (AlFON) substrates for UV surface-enhanced resonance Raman scattering (UVSERRS) at the deepest UV wavelength used to date (λex = 229 nm). We characterize the AlFONs fabricated with two different support microsphere sizes using localized surface plasmon resonance spectroscopy, electron microscopy, SERRS of adenine, tris(bipyridine)ruthenium(II), and trans-1,2-bis(4-pyridyl)-ethylene, SERS of 6-mercapto-1-hexanol (as a nonresonant molecule), and dielectric function analysis. We find that AlFONs fabricated with the 210 nm microspheres generate an enhancement factor of approximately 104-5, which combined with resonance enhancement of the adsorbates provides enhancement factors greater than 106. These experimental results are supported by theoretical analysis of the dielectric function. Hence our results demonstrate the advantages of using AlFON substrates for deep UVSERRS enhancement and contribute to broadening the SERS application range with tunable and affordable substrates.

Molecular plasmonics for nanoscale spectroscopy.

Surface- and tip-enhanced Raman and LSPR spectroscopies have developed over the past 15 years as unique tools for uncovering the properties of single particles and single molecules that are unobservable in ensemble measurements. Measurements of individual events provide insight into the distribution of molecular properties that are averaged over in ensemble experiments. Raman and LSPR spectroscopy can provide detailed information on the identity of molecular species and changes in the local environment, respectively. In this review a detailed discussion is presented on single-molecule and single-particle Raman and LSPR spectroscopy focusing on the major developments in the fields and applications of the techniques.