Two-Dimensional Surface-Enhanced Raman Imaging of a Roughened Silver Electrode Surface with Adsorbed Pyridine and Comparison with AFM Images

Theoretical study on contribution of charge transfer effect to surface-enhanced Raman scattering spectra of pyridine adsorbed on Ag(n) (n = 2-8) clusters

We investigate surface-enhanced Raman scattering (SERS) spectra of pyridine-Ag(n) (n = 2-8) complexes by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. In simulated normal Raman scattering (NRS) spectra, profiles of pyridine-Ag(n) (n = 2-8) complexes are analogical with that of isolated pyridine. Nevertheless, calculated pre-SERS spectra are strongly dependent on electronic transition states of new complexes. Wavelengths at 335 nm, 394.8 nm, 316.9 nm and 342.6 nm, which are nearly resonant with pure charge transfer excitation states, are adopted as incident light when simulating pre-SERS spectra for pyridine-Ag(n) (n = 2-8) complexes, respectively. We obtain enhancement factors from 10(3) to 10(5) in pre-SERS spectra compared with corresponding NRS spectra. The obvious increase in Raman intensities mainly result from charge transfer resonance Raman enhancement. A charge difference densities (CDDs) methodology is adopted in describing chemical enhancement mechanism. This methodology aims at visualizing charge transfer from Ag(n) (n = 2-8) clusters to pyridine on resonant electronic transition, which is one of the most direct evidences for chemical enhancement mechanism.

Nanocrystal assembly for bottom-up plasmonic materials and surface-enhanced Raman spectroscopy (SERS) sensing

Plasmonic materials are emerging as key platforms for applications that rely on the manipulation of light at small length scales. Sub-wavelength metallic features support surface plasmons that can induce huge local electromagnetic fields at the metal surface, facilitating a host of extraordinary optical phenomena. Ag nanocrystals (NCs) and nanowires (NWs) are ideal building blocks for the bottom-up fabrication of plasmonic materials for photonics, spectroscopy, and chemical sensing. Faceted Ag nanostructures are synthesized using a colloidal approach to regulate nucleation and crystallographic growth direction. Next, new methods of nanoscale organization using Langmuir-Blodgett (LB) compression are presented where one- and two-dimensional assemblies can be constructed with impressive alignment over large areas. Using this method, plasmon coupling between Ag nanostructures can be controlled by varying spacing and density, achieving for the first time a completely tunable plasmon response in the visible wavelengths. Lastly, these assemblies are demonstrated as exceptional substrates for surface-enhanced Raman spectroscopy (SERS) by achieving high chemical sensitivity and specificity, exhibiting their utility as portable field sensors, and integrating them into multiplexed "lab-on-a-chip" devices.

Influences of composition on electroless deposition of silver nanoparticles on glass substrates for surface-enhanced Raman scattering measurements

A silver-mirror reaction was used to prepare active substrates for surface-enhanced Raman scattering (SERS). Glass plates were surface treated with a thin layer of silver nanoparticles (Ag-NPs). The factors influencing the performance of the SERS substrates were systematically studied. Factors included concentrations, species of complexing agents, and reducing agents. p-Nitrothiophenol (pNTP) was used to test the surfaces with Ag-NPs, with the observed signals used to compare the performance resulting from different reducing and complexing agents. The morphologies of the Ag-NPs formed by different reaction conditions were also examined by scanning electron microscope (SEM) and correlated with the SERS signals. Reducing agents included formaldehyde, sodium tartrate, and several carbohydrates. The results indicate that the use of glucose as a reducing agent produced the most suitable Ag-NPs for SERS measurements. Complexing agents of ammonia and ethyl amine offered the best performances. The optimal concentration of complexing agent was found to be approximately six times the concentration of silver ions. With a reaction time of 2 min, the optimized concentrations of glucose and silver nitrate were 0.5 M and 50 mM, respectively. In general, the enhancement factor was on the order of 10(5) to 10(6) for the substrates prepared in this work.

Raman Chemical Imaging for Ingredient-specific Particle Size Characterization of Aqueous Suspension Nasal Spray Formulations: A Progress Report

PURPOSE

This study was conducted to evaluate the feasibility of using Raman chemical imaging (i.e., Raman imaging microspectroscopy) to establish chemical identity, particle size and particle size distribution (PSD) for a representative corticosteroid in aqueous nasal spray suspension formulations.

MATERIALS AND METHODS

The Raman imaging PSD protocol was validated using polystyrene (PS) microsphere size standards (NIST-traceable). A Raman spectral library was developed for the active and inactive compounds in the formulation. Four nasal sprays formulated with beclomethasone dipropionate (BDP) ranging in size from 1.4 to 8.3 microm were imaged by both Raman and brightfield techniques. The Raman images were then processed to calculate the PSD for each formulation.

RESULTS

Within each region examined, active pharmaceutical ingredient (API) particles are unambiguously identified and the total number of those particles, particle size and PSD of API free of excipients and PSD of API particles adhered to other excipients are reported.

CONCLUSIONS

Good statistical agreement is obtained between the reported and measured sizes of the PS microspheres. BDP particles were clearly distinguishable from those of excipients. Raman chemical imaging (RCI) is able to differentiate between and identify the chemical makeup of multiple components in complex BDP sample and placebo mixtures. The Raman chemical imaging method (coupled Raman and optical imaging) shows promise as a method for characterizing particle size and shape of corticosteroid in aqueous nasal spray suspension formulations. However, rigorous validation of RCI for PSD analysis is incomplete and requires additional research effort. Some specific areas of concern are discussed.