The study of phospholipid phase transitions is important for understanding drug- and protein-membrane interactions as well as other phenomena such as trans-membrane diffusion and vesicle fusion. A temperature-controlled stage on a confocal Raman microscope has allowed phase transitions in optically trapped phospholipid vesicles to be monitored. Raman spectra were acquired and analyzed using self-modeling curve resolution, a multivariate statistical analysis technique. This method revealed the subtle spectral changes indicative of sub- and pretransitions and main transitions in vesicles composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). The Raman scattering results were compared to differential scanning calorimetry (DSC) experiments and found to be in good agreement. This method of observing lipid phase transition profiles requires little sample preparation and a minimal amount of lipid (

The oxidative adsorption of n-alkanethiolates (C n c H2 n c +1S-) at Ag(111) in aqueous and methanolic solutions containing 0.5 M NaOH has been investigated by cyclic voltammetry and in-situ surface-enhanced Raman spectroscopy (SERS). Reversible adsorption of C n c H2 n c +1S- under active potential control in solutions containing millimolar concentrations of C n c H2 n c +1S- provides a means to control the deposition of n-alkanethiolate monolayers, and allows for the direct voltammetric measurement of the free energy of monolayer formation. Oxidative adsorption of short chain alkanethiolates (n c ≤ 6) in aqueous 0.5 M NaOH is characterized by two voltammetric waves, demonstrating that monolayer formation involves at least two energetically distinct chemical steps. The first voltammetric wave corresponds to the reversible and rapid adsorption of C n c H2 n c +1S- at submonolayer coverages. The redox potential of this wave ( = −1.19 ± 0.02 V vs Ag/AgCl) is independent of n c , suggesting that the interactions between adsorbed molecules are minimal at low surface coverages and that the energetics of adsorption are determined primarily by the strength of the Ag(111)−S bond. A second voltammetric wave is observed at more positive electrode potentials, corresponding to further adsorption of C n c H2 n c +1S- to yield a complete monolayer (Γmax ∼ 7.7 × 10-10 mol/cm2). The redox potential for the second wave, , is a function of chain length, shifting to more negative potentials with increasing n c . The dependence of on n c reflects the influence of hydrophobic interactions and intermolecular forces between the hydrocarbon chains. For n c > 6, shifts to potentials negative of , and the two voltammetric waves merge into a single wave, suggesting that the more structurally ordered monolayer is energetically favored for longer chain lengths (i.e., n c > 6). In-situ SERS is used to establish the potential-dependent adsorption isotherm of n-hexanethiolate adsorbed on roughened Ag electrodes. The potential dependence of the SERS intensities of the trans and gauche ν(C−S) stretching modes provides a means to monitor the structural ordering of the alkanethiolate monolayer during electrochemical deposition. The electrochemical data are used to separate the total adsorption free energy (ΔG ads) into the individual contributions associated with the formation of the Ag(111)−S bond (−22.8 and −16.6 kcal/mol for the low- and high-density structures, respectively) and that associated with hydrophobic interactions and intermolecular forces between hydrocarbon chains (−1.02 ± 0.04 kcal/mol per n c ). Voltammetric data and ΔG ads values are also reported for the adsorption of C n c H2 n c +1S- (2 ≤ n c ≤ 16) onto Ag(111) from basic methanolic solutions.

Optical trapping of small structures is a powerful tool for the manipulation and investigation of colloidal and particulate materials. The tight focus excitation requirements of optical trapping are well suited to confocal Raman microscopy. In this work, an inverted confocal Raman microscope is developed for studies of chemical reactions on single, optically trapped particles and applied to reactions used in solid-phase peptide synthesis. Optical trapping and levitation allow a particle to be moved away from the coverslip and into solution, avoiding fluorescence interference from the coverslip. More importantly, diffusion of reagents into the particle is not inhibited by a surface, so that reaction conditions mimic those of particles dispersed in solution. Optical trapping and levitation also maintain optical alignment, since the particle is centered laterally along the optical axis and within the focal plane of the objective, where both optical forces and light collection are maximized. Hour-long observations of chemical reactions on individual, trapped silica particles are reported. Using two-dimensional least-squares analysis methods, the Raman spectra collected during the course of a reaction can be resolved into component contributions. The resolved spectra of the time-varying species can be observed, as they bind to or cleave from the particle surface.

Metal colloids immobilized on a glass support substrate are modified with a self-assembled alkylsilane (C18) layer to promote adsorption of polycyclic aromatic hydrocarbons from aqueous solutions. Detection of these compounds from low concentration solutions is accomplished by using surface-enhanced Raman scattering (SERS). SERS spectra of pyrene adsorbed to C18-modified immobilized silver colloids are dominated by Raman bands that are not consistent with pyrene and indicate that pyrene undergoes a chemical reaction at the surface. The origins of this surface product are investigated, and it is determined that silver and oxygen are required to form the product, whose Raman spectrum is consistent with oxidation to a quinone. When a C18-modified gold-colloid substrate is used, Raman scattering consistent with unreacted pyrene is observed. The adsorption and detection of pyrene adsorbed from low (2 ppb) concentration aqueous solutions onto C18-modified gold-colloid substrates is reported; naphthalene and phenanthrene are detected at approximately 5 ppb. Adsorption kinetics are rapid (<5 min), and the concentration-dependent SERS response is consistent with a Langmuir isotherm.

ATR-FT-IR spectroscopy was employed to the study the adsorption of ethyl acetate and 2-propanol to the surface of thin silica sol-gel films in contact with n-heptane solutions. In situ vibrational spectra of silica-adsorbed species provided information regarding the mechanisms of solute retention and elution in normal-phase chromatography. Previous normal-phase chromatographic studies of ethyl acetate adsorption revealed nonlinear isotherms which were explained by both bilayer and adsorbate delocalization models. Infrared spectra of ethyl acetate at the silica surface versus concentration showed that nonlinear adsorption can be attributed to site heterogeneity, where adsorption to free silanols and surface-adsorbed water can be distinguished. Least-squares modeling of the data produced resolved spectra for the two sites and adsorption equilibrium constants that differed by about an order of magnitude. Adsorption of 2-propanol was best modeled by a single Langmuir isotherm showing no significant difference in adsorption energy for the two sites; 2-propanol was shown to easily displace ethyl acetate from the silica surface. Ethyl acetate could also displace 2-propanol from the silica, and least-squares modeling again revealed two-adsorbed-component spectra for ethyl acetate that were indistinguishable from spectra obtained when ethyl acetate adsorbed directly onto the surface.