45523-AC7
Colloids Painted Black and White: Rotational Diffusion of MOON Particles
This research led to major advances in a subject on which this laboratory had not worked previously; this success has spawned an entirely new line of research in this laboratory. This initial success, spawned by PRF support, led to independent funding that will allow this work to continue in the future.
Careers of the personnel involved in this project benefited materially as follows. First, the postdoc developed new areas of research that currently augur well for developing independent lines of research as he seeks an independent research career. Second, an NSF Fellowship holder who needed no salary support was supported in the sense of materials and supplies. Third, some laboratory infrastructure developed in the course of this project produced unanticipated, valuable spinoff for other imaging projects.
On the research side, one goal was to develop practical approaches to understand an undeveloped subject: so-called Janus colloids, which are spherical in geometry but whose hemispheres differ chemically. Coating one hemisphere of fluorescent microspheres with a thin layer of high reflective metal results in modulation of the excitation and emission of fluorescence in the course of a particle’s ensuing motion. This enables one to produce and study colloidal probes whose size, limited only by the fact that the coating should not so thick as to compete in size with the particle, can be as large as one likes, and as small as submicron. During the course of our research, we learned that phase contrast microscopy also works, thus removing the need for fluorescent-labeled particles. In colloid science, the historical assumption that the relevant interactions are non-directional is the premise for analyzing a vast number of ubiquitous technological and environmental problems. However, surface chemistry is commonly spotty, patchy, and heterogeneous. Rather than dismiss this as imperfection, we showed that qualitatively new behavior follows when spherical symmetry is broken by anisotropic chemical composition. In the course of this research, this concept was extended in the direction of computer algorithms that allowed us to discriminate rotation and translation. We developed algorithms and an experimental method to discriminate optically anisotropic colloidal spheres under situations where diffraction owing to their close proximity causes overlapping images, and this data analysis was applied to modulated optical nanoprobes (MOONs) that are coated with metal on one hemisphere.
This led us to introduce the notion of “Janus balance” defined as the dimensionless ratio of work to transfer an amphiphilic colloidal particle from the oil-water interface into the oil phase, normalized by the work needed to move it into the water phase. Its value can be calculated simply from the interfacial contact angle and the geometry of Janus particles, without need to know interfacial energy. The Janus balance concept may enable predictions of how a Janus particle behaves with respect to efficiency and function as a solid surfactant, as the Janus balance of solid surfactants is the analogue of the classical HLB (hydrophile-lipophile balance) of small surfactant molecules.
Another significant advance was to develop a simple, generalizable, inexpensive method to synthesize “Janus” colloidal particles in large quantity. At the liquid-liquid interface of emulsified molten wax and water, untreated particles adsorb and are frozen in place when the wax solidifies. The exposed surfaces of the immobilized particles are modified chemically. Finally, wax is dissolved and the inner surfaces are modified chemically. We showed that gram-sized quantities or more of Janus particles can be synthesized by taking this approach. Later during this project, the approach was extended to a solvent-free synthesis.
