Measurements of Particle-Surface Interactions in Both Equilibrium and Nonequilibrium Systems

Parametric Study of Colloidal Particle Confinement near a Surface in the Presence of DLVO and Structural Interactions Using Brownian Dynamic Simulations

Total internal reflection microscopy (TIRM) has become a crucial technique for understanding the surface interactions and dynamics of Brownian colloidal particles near a surface. However, for select colloidal systems, experimental limitations associated with TIRM can occlude exploration of nano- and submicrometer colloids dispersed in complex or structured fluids. It should be possible to use Brownian dynamic simulations to quantify, explore, or circumvent these limitations to extend the TIRM technique further. A Brownian dynamics algorithm based on the Langevin equation was utilized to identify favorable colloidal systems for conducting TIRM experiments in electrolyte and nonadsorbing polyelectrolyte solutions. In electrolyte solution, the motion of polystyrene and silica particles of nanometer- and micrometer-sized radii was simulated near a glass slide in the presence of retarded van der Waals and electric double-layer forces to develop potential energy profiles. In the case of nonadsorbing polyelectrolyte solutions, a structural force was also implemented into the simulation, and the influence of structural interactions on particle confinement was explored as a function of particle size, particle density, and polyelectrolyte concentration. In electrolyte solutions, our results were able to identify the minimum particle size required for TIRM experiments as well as insight into particle selection based on material density. For structural or oscillatory forces, our results show that prior to conducting TIRM experiments, Brownian dynamics simulation can be used to select the appropriate particle size, material, and polyelectrolyte concentration range where the colloidal particle can sample multiple structural energy wells without confinement. These results provide insight into the colloidal system suitable to experimentally study near-surface particle diffusion dynamics for a range of separations in the presence of structural interactions.

Total internal reflection microscopy: a powerful tool for exploring interactions and dynamics near interfaces

The occurrence of many micro/macrophenomena is closely related to interactions and dynamics near interfaces. Hence, developing powerful tools for characterizing near-interface interactions and dynamics has attached great importance among researchers. In this review, we introduce a noninvasive and ultrasensitive technique called total internal reflection microscopy (TIRM). The principles of TIRM are introduced first, demonstrating the characteristics of this technique. Then, typical measurements with TIRM and the recent development of the technique are reviewed in detail. At the end of the review, we highlight the great progress of TIRM during the past several decades and show its potential to be more influential in measuring interactions and dynamics near interfaces in various research fields.

Measurements of interactions between fluorescent molecules and polyethylene glycol self-assembled monolayers

Blocking the non-specific binding of fluorescent biomolecules to substrates is one of the most important approaches to minimize the background noise in single-molecule fluorescence detection. Polyethylene glycol (PEG) and its derivatives are the most frequently used self-assembled monolayers (SAMs) for surface passivation because they are particularly effective to reduce the adsorption of a majority of biomolecules. Most studies related to PEG SAMs focus only on the interactions between biomolecules and substrates, while few reports exist in which the interactions between fluorophores and organosilane SAMs are directly examined. The objective of this study is to try to clarify the interactions between fluorescein isothiocyanate (FITC) and PEG SAMs at different ionic strengths. Total internal reflection microscopy (TIRM) was utilized for quantitative analysis of the interactions. At low ionic strength, long-range attractions between FITC-modified polystyrene-silica particles and PEG SAM grafting substrates were observed, even though both of them had an ensemble-averaged negative charge. The origin of this attraction could be correlated to their nonuniformly charged surfaces. At high ionic strength, van der Waals attraction at short distances was measured as the electrostatic interactions were completely screened. Due to the polarizability of the FITC molecule, the van der Waals attractions increased with the thickness of the PEG SAMs. This phenomenon is explained by the hydration shell of the PEG SAMs.