Measuring Diffusion of Molecules into Individual Polymer Particles by Confocal Raman Microscopy

Abstract

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.

Estimating Diffusion Coefficients of Probe Molecules into Polyelectrolyte Brushes by Electrochemical Impedance Spectroscopy

“…Within this framework, we have shown that EIS provides a powerful alternative tool for exploring dynamical aspects and elucidating the magnitude of the molecular transport process occurring at macromolecular interfaces and polymer thin films. 37,38 ACKNOWLEDGMENT C.A.G. gratefully acknowledges the Comisió n de Investigaciones Científicas y Té cnicas Buenos Aires (CICBA) for his position as a member of the Carrera del Investigador Científico.…”

Lacritin proteoforms prevent tear film collapse and maintain epithelial homeostasis

“…For Raman confocal microscopy, 10 -20 µm meibum films were created by application of 10 µl of 10 mg/ml meibum in chloroform to a coverslip on a 1 mm x 7 mm flow channel (82) followed by drying under nitrogen and then briefly under vacuum with thickness between coverslip-meibum and meibum-buffer interfaces estimated by incident laser beam reflection. The Raman microscopic probe (83,84) has a depth resolution of ±1.2 µm (FWHM), a diameter of ~500 nm, and a probe volume of ~500 fL which is well-matched to the meibum film deposited on the coverslip interface. For analyses, the probe was brought to 1.5 µm below the meibum-buffer interface with spectra collected at a laser power of 100 mW with an integration time of two minutes.…”

Confocal Raman Microscopy: Performance, Pitfalls, and Best Practice

“…Numerous workers have confirmed the benefits of using immersion objectives to depth profile complicated materials ranging from polymers and paper products to human skin. [29][30][31][32][33][34][35][36][37][38][39][40] As well as reducing the depth-scale compression, the oil has an additional beneficial effect of wetting out particle/air boundaries in porous, highly scattering particulate coatings, improving transparency and permitting confocal profiling of samples that would be opaque to a dry objective. 28 Nonetheless, a significant mismatch between the index of the coupling oil and the sample will result in a slight axial shift and distortion in the shape of individual layers, manifested by an attenuation that increases with depth.…”