In situ identification of intermediates of benzyl chloride reduction at a silver electrode by SERS coupled with DFT calculations

Aiming to deeply understand the electrocatalytic mechanism of silver on reduction of benzyl chloride, we carried out an in situ electrochemical surface-enhanced Raman spectroscopic study to characterize various surface species in different electrode potential regions. A further analysis with DFT calculation reveals that the benzyl radical and its anionic derivate bonded on a silver electrode are the key intermediates, implying that the pathway could drastically differ from the outer sphere concerted electron reduction at inert electrodes.

Controlled assembly of Janus nanoparticles

Janus nanoparticles were prepared by interfacial ligand exchange reactions of octanethiolate-protected gold (AuC8) nanoparticles with 3-mercapto-1,2-propanediol (MPD) at the air/water interface. AFM and TEM measurements showed that the resulting particles formed stable aggregates in water with dimensions up to a few hundred nanometers, in sharp contrast to the original AuC8 particles and bulk-exchange counterparts where the aggregates were markedly smaller. Consistent behaviors were observed in dynamic light scattering measurements. FTIR measurements of solid films of the nanoparticles suggested that the octanethiolate ligands were mostly of trans conformation, whereas the MPD ligands exhibited gauche defects as a consequence of the hydrogen-bonding interactions between the hydroxyl moieties of adjacent ligands. Raman spectroscopic measurements in an aqueous solution of pyridine showed that the pyridine ring breathing modes remained practically unchanged and the intensity profiles indicated minimal interactions between pyridine and the gold cores within the three nanoparticle ensembles. However, water bending vibrational features were found to be enhanced substantially with the addition of Janus nanoparticles, which was ascribed to the formation of clusters of water molecules that were trapped within the nanoparticle ensembles. No apparent enhancement was observed with the AuC8 or bulk-exchange particles.

Shell-isolated nanoparticle-enhanced Raman spectroscopy

Surface-enhanced Raman scattering (SERS) is a powerful spectroscopy technique that can provide non-destructive and ultra-sensitive characterization down to single molecular level, comparable to single-molecule fluorescence spectroscopy. However, generally substrates based on metals such as Ag, Au and Cu, either with roughened surfaces or in the form of nanoparticles, are required to realise a substantial SERS effect, and this has severely limited the breadth of practical applications of SERS. A number of approaches have extended the technique to non-traditional substrates, most notably tip-enhanced Raman spectroscopy (TERS) where the probed substance (molecule or material surface) can be on a generic substrate and where a nanoscale gold tip above the substrate acts as the Raman signal amplifier. The drawback is that the total Raman scattering signal from the tip area is rather weak, thus limiting TERS studies to molecules with large Raman cross-sections. Here, we report an approach, which we name shell-isolated nanoparticle-enhanced Raman spectroscopy, in which the Raman signal amplification is provided by gold nanoparticles with an ultrathin silica or alumina shell. A monolayer of such nanoparticles is spread as 'smart dust' over the surface that is to be probed. The ultrathin coating keeps the nanoparticles from agglomerating, separates them from direct contact with the probed material and allows the nanoparticles to conform to different contours of substrates. High-quality Raman spectra were obtained on various molecules adsorbed at Pt and Au single-crystal surfaces and from Si surfaces with hydrogen monolayers. These measurements and our studies on yeast cells and citrus fruits with pesticide residues illustrate that our method significantly expands the flexibility of SERS for useful applications in the materials and life sciences, as well as for the inspection of food safety, drugs, explosives and environment pollutants.

Probing surface and interface structure using optics

Optical techniques for probing surface and interface structure are introduced and recent developments in the field are discussed. These techniques offer significant advantages over conventional surface probes: all pressure ranges of gas-condensed matter interfaces are accessible and liquid-liquid, liquid-solid and solid-solid interfaces can be probed, due to the large penetration depth of the optical radiation. Sensitivity and discrimination from the bulk are the two challenges facing optical techniques in probing surface and interface structure. Where instrumental improvements have resulted in enhanced sensitivity, conventional optical techniques can be used to characterize heterogeneous adsorbed layers on a substrate, often with sub-monolayer resolution. Nanoscale lateral resolution is possible using scanning near-field optics. A separate class of techniques, which includes reflection anisotropy spectroscopy, and nonlinear optical probes such as second-harmonic and sum-frequency generation, uses the difference in symmetry between the bulk and the surface or interface to suppress the bulk contribution. A perspective is presented of likely future developments in this rapidly expanding field.

Dithiocarbamate-coated SERS substrates: sensitivity gain by partial surface passivation

The surface-enhanced Raman scattering (SERS) activity of nanoporous gold (NPG) can be boosted by controlled surface passivation. The SERS activities of unfunctionalized NPG were first optimized by etching substrates with NaI/I(2) (triiodide) and using 2-mercaptopyridine (2-MP) as the probing analyte. Gains in analyte sensitivity were then achieved by passivating the superficial regions of the NPG substrates with dimethyldithiocarbamate (Me(2)DTC) while leaving the more recessed "hot spots" available for SERS detection. Partial surface passivation with DTCs increased the substrate sensitivity to chemisorptive analytes such as 2-MP by an order of magnitude, whereas surface saturation lowered the sensitivity by an order of magnitude. The partially passivated NPG films can also be functionalized with supramolecular receptors for chemoselective SERS. Installation of a DTC-anchored terpyridine enabled the detection of divalent metal ions at trace levels, as determined by the complexation-induced shift of a characteristic Raman peak of the metal ion receptor.

Surface enhanced infrared absorption spectroscopy studies of DMAP adsorption on gold surfaces

Attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) measurements have been employed to study the adsorption of dimethylaminopyridine (DMAP) and its conjugate acid (DMAPH+) on gold surfaces as a function of applied potential and solution pH. Based on our transmission measurements, we have been able to demonstrate that the acid/base forms of this pyridine derivative can be readily differentiated due to their distinct IR signals. When the solution pH is equal to the pKa of DMAPH+, we demonstrate that the adsorbing species is DMAP, oriented with its heterocyclic ring perpendicular to the electrode surface. In acidic electrolytes, our SEIRAS data provide direct spectroscopic evidence of DMAP monolayer formation even though the pH is 5 units below the pKa of the conjugate acid. Our data support a potential induced deprotonation of the endocyclic nitrogen and resulting coordination of the nitrogen lone pair to the gold surface. Both of these results confirm our existing model of DMAP adsorption previously based solely on electrochemical measurements. However, the present SEIRAS study also indicates that, at low pH, DMAPH+ can electrostatically coordinate to very negatively charged surfaces. This mode of adsorption was previously unobserved, illustrating the ability of in situ spectroscopic techniques to reveal new information that is not apparent from traditional electrochemical techniques such as differential capacity and chronocoulometry.