Measuring diffusion of molecules into individual polymer particles by confocal Raman microscopy.

Raman microscopy is a powerful method for providing spatially resolved, chemically selective information about the composition of materials. With confocal collection optics, the method is well suited to the analysis of small particles in contact with liquid solutions. In this work, the transport of an organic solvent component into small polystyrene particles is investigated. An inverted confocal Raman microscope is used to acquire spectra from individual 75-microm polystyrene particles in contact with acetonitrile/water mixtures. Monitoring the Raman scattering from the C[triple bond]N stretching mode of acetonitrile provides a measure of solvent uptake into the polymer material. The small collection volume defined by the confocal optics provides the micrometer spatial resolution needed to track solvent concentration at different locations within the particle with 30-s time resolution. The volume fraction of acetonitrile in water in the surrounding solution was varied in order to determine the concentration dependence of the diffusion kinetics. Modeling the transport of molecules into a particle was addressed by using finite element methods for the evaluation of the coupled space- and time-dependent differential equations that govern the molecular transport. The results indicate that the diffusion coefficient changes with the local solvent concentration in the polymer. At longer times, with the highest acetonitrile concentrations, an evolution of the solvent transport mechanism was observed, from a diffusive rate that depends on local concentration to a linear increase in concentration with time accompanied by measurable swelling of the particle volume.

Optical trapping of unilamellar phospholipid vesicles: investigation of the effect of optical forces on the lipid membrane shape by confocal-Raman microscopy.

Optical trapping of liposomes is a useful tool for manipulating these lipid vesicles for sampling, mechanical testing, spectroscopic observation, and chemical analysis. Through the use of confocal Raman microscopy, this study addresses the effects of optical forces on the structure of unilamellar, dipalmitoylphosphatidylcholine (DPPC) vesicles, both optically trapped in solution and adhered to a coverslip. The energy and forces involved in optical trapping of lipid vesicles were derived in terms of the dielectric contrast between the phospholipid membrane and the surrounding solution; reflection forces at the membrane/water interface were found to be negligible. At optical powers of 9 mW and greater, unilamellar liposomes trapped in bulk solution experience a gradient force sufficiently strong to bend the vesicle membrane, so that a second bilayer from the same vesicle is drawn into the optical trap, with an energy of approximately 6 x 10(-13) erg. For vesicles adhered to a coverslip, the confocal probe can be scanned through the attached vesicle. Optical forces are insufficient to detach the bilayer that is adhered to the glass; however, the upper DPPC bilayer can be manipulated by the optical trap and the shape of the vesicle distorted from a spherical geometry. The effect of calcium ion on the flexibility of membrane bilayers was also tested; with 5 mM calcium ion in solution, the lipid bilayer of a surface-attached liposome is sufficiently rigid so that it cannot be distorted at moderate laser powers.

Spatially resolved analysis of small particles by confocal Raman microscopy: depth profiling and optical trapping.

Raman microscopy is a powerful method to provide spatially resolved information about the chemical composition of materials. With confocal collection optics, the method is well suited to the analysis of small particles, either resting on a surface or optically trapped at a laser focus, where the confocal collection volume optimizes the signal from the particle. In this work, the sensitivity and spatial selectivity of detecting Raman scattering from single particles was determined as a function of particle size. An inverted confocal Raman microscope was used to acquire spectra of individual surface-bound and optically trapped polystyrene particles with sizes ranging between 200 nm and 10 microm. The particles are in contact with aqueous solution containing perchlorate ion that served as a solution-phase Raman-active probe to detect interferences from the surrounding medium. The collection volume is scanned through single particles that are attached to the surface of the coverslip, and the sensitivity and selectivity of detection are measured versus particle size. The results compare favorably with a theoretical analysis of the excitation profile and confocal collection efficiency integrated over the volumes of the spherical particles and the surrounding solution. This analysis was also applied to the detection of particles that are optically trapped and levitated above the surface of the coverslip. The results are consistent with the optical trapping of particles at or near the excitation beam focus, which optimizes excitation and selective collection of Raman scattering from the particle.