R. Aroca

Single molecule analysis by surfaced-enhanced Raman scattering

Our main objective in this tutorial review is to provide insight into some of the questions surrounding single molecule detection (SMD) using surface-enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scattering (SERRS). Discovered thirty years ago, SERS is now a powerful analytical tool, strongly tied to plasmonics, a field that encompasses and profits from the optical enhancement found in nanostructures that support localized plasmon excitations. The spectrum of the single molecule carries the quantum fingerprints of the system modulated by the molecule-nanostructure interactions and the electronic resonances that may result under laser excitation. This information is embedded in vibrational band parameters. The dynamics and the molecular environment will affect the bandwidth of the observed Raman bands. In addition, the localized surface plasmon resonances (LSPR) empower the nanostructure with a number of optical properties that will also leave their mark on the observed inelastic scattering process. Therefore, controlling size, shape and the formation of the aggregation state (or fractality) of certain metallic nanostructures becomes a main task for experimental SERS/SERRS. This molecule-nanostructure coupling may, inevitably, lead to spectral fluctuations, increase photobleaching or photochemistry. An attempt is made here to guide the interpretation of this wealth of information when approaching the single molecule regime.

Surface-enhanced Raman scattering for ultrasensitive chemical analysis of 1 and 2-naphthalenethiols

The results of the search for the optimal experimental conditions for ultrasentitive chemical analysis of 1-naphthalenethiol (1-NAT) and 2-naphthalenethiol (2-NAT) using surface-enhanced Raman scattering (SERS) are discussed. The report begins with a review of the vibrational spectra, including infrared and Raman spectra of the target molecules, and the interpretation of the observed frequencies aided by local density functional theory (DFT) calculations at the B3LYP/6-311G(d,p) level of theory. Several metal nanostructures were tested for SERS activity, including island films and colloids of silver, gold and copper. Correspondingly, the most effective laser line for excitation in the visible and near infrared region was sought. The achieved detection limit for 1-naphthalenethiol, and for 2-naphthalenethiol, on silver nanostructures is in the zeptomole regime.

Surface-Enhanced Fluorescence on SiO2-Coated Silver Island Films

Fluorescence is one of the molecular spectroscopic properties that is enhanced by placing the molecule on the rough surface of a coinage metal. The surface-enhanced fluorescence (SEF) can be directly observed in steady-state fluorescence experiments. The observations are the results of a delicate balance between the enhanced emission and the quenching due to energy transfer to nonradiative surface plasmons. In the present report, SiO2-coated silver films were fabricated at varying dielectric thickness. The surface of the films was analyzed with the use of atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM confirms the surface roughness and XPS analysis indicates that the SiO2 coverage was successful. SEF and SERS (surface-enhanced Raman scattering) were observed on active 6, 10, and 14 nm silver films coated with SiO2. Similar results were obtained with a 6 nm silver film coated with 6 nm SiO. The SEF work was carried out on fluorescent molecules with different quantum yield, and the typical enhancement factor obtained for the fluorescent signal was approximately 10. Both the SiO2 and SiO overlayers provide stable surfaces with well-defined hydrophilic properties. Such stable constructions have applicability towards the advancement of SERS and SEF as routine analytical techniques in bio- and chemical sensors.

Synthesis of Silver Nanoparticles with Controllable Surface Charge and Their Application to Surface-Enhanced Raman Scattering

The objective of the present work was to explore new methods of synthesis of silver nanocolloids using amino acids as reducing agents. The goal of the study was to fabricate nanostructures with controllable surface charge (zeta potential) that may allow optimizing the adsorption of target molecules for ultrasensitive analysis using surface-enhanced Raman scattering (SERS). The average SERS properties of these colloids are tested with two organic analytes and compared with those obtained by using the most commonly used citrate Ag sols.

Surface-enhanced Raman scattering of p-nitrothiophenol: Molecular vibrations of its silver salt and the surface complex formed on silver islands and colloids

The interpretation of the surface-enhanced Raman scattering (SERS) spectra of p-nitrothiophenol (p-NTP) is reported. SERS spectra were obtained by vacuum evaporation and casting of p-NTP onto silver island films, and also from colloidal silver solutions. The vibrational spectra of the silver salt (p-NTP-silver) were obtained for a direct comparison with the SERS of p-NTP chemically adsorbed onto Ag. Chemisorption of p-NTP through S-Ag bonding is indirectly proven by the disappearance of the S-H stretching mode. The Raman scattering spectrum of the silver salt is in good agreement with the SERS spectra of the silver surface complex. The spectral interpretation was aided using density functional theory calculations of the molecular spectra of p-NTP and that of the Ag-p-NTP.

SERS detection of environmental pollutants in humic acid–gold nanoparticle composite materials

Humic acid (HA) solutions provide an unexpected medium for direct fabrication of gold nanoparticles (HA-AuNP) and a clear window for surface-enhanced Raman scattering (SERS) with many potential applications in the ultrasensitive chemical analysis of environmental pollutants. It is demonstrated that the HA-AuNP fabrication can be easily achieved in a wide range of pH (2 to 12). The background SERS spectra of HA is relatively weak in absolute intensity, allowing the detection of the enhanced Raman signal from trace amount of contaminants. An in-situ approach is illustrated where the HA-AuNP fabrication is carried out with a HA solution containing the target pollutant. The technique may allow for the direct detection of organic pollutants present in the humic fraction of soil.

Controlling the size and shape of gold nanoparticles in fulvic acid colloidal solutions and their optical characterization using SERS

The size and shape of nanoparticles can be controlled by varying the pH and concentration of the fulvic acid (FA) colloidal solution. FA thin solid films with embedded Au nanoparticles have been fabricated, and their optical field enhancing properties tested in surface-enhanced Raman scattering (SERS). High-resolution transmission electron microscopy data indicated that the size of nanoparticles ranges from 4 to 15 nm for films obtained at pH 11 and 8. At pH 5, however, hexagonal and triangular particles of ∼50 nm and truncated triangles of ∼200 nm are produced with a high concentration of fulvic acid (250 mg L−1). The number and size of truncated triangles can be diminished by decreasing the concentration of FA in the colloidal solution. This unprecedented control of size and shape was preserved in thin solid FA films embedding the gold nanoparticles. The plasmon-assisted enhancing optical properties are demonstrated by SERS of 2-naphthalenethiol, an analyte with a well defined SERS spectrum.

Vibrational Spectra and Ion-Pair Properties of Lithium Hexafluorophosphate in Ethylene Carbonate Based Mixed-Solvent Systems for Lithium Batteries

The vibrational spectra of LiPF6 1M solutions formed in aprotic mixed solventsobtained by mixing ethylene carbonate with dimethyl carbonate and diethylcarbonate are discussed. The Raman and infrared spectra of lithium hexafluorophosphate(LiPF6) and the quantum chemical computations of the vibrationalwavenumbers and intensities are reported. Due to the nature of the solutions,attenuated total internal reflectance spectroscopy was used to obtain the infraredspectra. The infrared active vibrational fundamentals of PF6− provided evidencefor the anion—solvent interaction as well as ion-pair formation. Similarly, theinfrared active modes of the solvent showed significant changes due to thecation—solvent interaction. The computations of the most energetically favorablegeometry in the formation of the Li+ PF6 ion pair are also presented. Conductivitymeasurements carried out for the 1M solutions scanning a wide temperatureinterval (−30 to + 60 °C) confirm the viability of these electrolytes forpractical applications.

Extended Hückel molecular orbital model for lanthanide bisphthalocyanine complexes

Abstract The electronic structure of lanthanide bisphthalocyanines (LnPc 2 ) is examined using the frontier molecular orbitals of the phthalocyanine (Pc) ligand fragment in the framework of the extended Huckel Hamiltonian. Correlation diagrams are constructed for a staggered conformation. It was found that the major component in the splitting of the energy levels was the ring-ring interaction. The largest splitting was calculated for the a 1u HOMOs of the two rings. Molecular geometry-electronic structure relationships are constructed using perturbation theory and overlap integrals.

Vibrational spectra of lutetium and ytterbium bis-phthalocyanine in thin solid films and SER(R)S on silver island films

Abstract Thin solid films of lutetium and ytterbium diphthalocyanine (Pc 2 ) complexes have been studied using vibrational and electronic absorptions, spontaneous and resonant Raman (RR) and surface enhanced Raman scattering (SERS) on silver island films. Surface enhanced resonance Raman spectra (SERRS) are identical to the RRS spectra with an enhancement factor of about 500. Vibrational data were consistent with a C 4 v molecular symmetry.