Photoresponsive Systems Based on Self-Assembly
Raghavan, Srinivasa R
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Photoresponsive systems based on self-assembled nanostructures have received considerable attention recently. There is a wide range of applications for these fluids such as in drug delivery, coatings, sensors, or microfluidic valves and dampers. Current photoresponsive systems have typically required the use of specialized molecules with various chemical modifications. However, the requirement of complicated chemical synthesis prevents these systems from being used widely for practical applications. In this study, we focus on creating photoresponsive systems using only commercially available photoresponsive molecules and structure-forming components. In our first study, we describe simple reversible photorheological (PR) fluids, i.e., fluids whose rheological properties can be tuned by light. Our PR fluids are created by combining an azobenzene derivative, 4-azobenzene carboxylic acid (ACA), into micelles of the cationic surfactant erucyl bis(2-hydroxyethyl)methyl ammonium chloride (EHAC). We show that certain aqueous mixtures of EHAC and ACA, which are low-viscosity solutions at the outset, undergo nearly a million-fold increase in viscosity when irradiated with UV light. The same solutions revert to their initial viscosity when subsequently exposed to visible light. The above changes in viscosity are repeatable, and the sample can be reversibly cycled back and forth between low and high viscosity states. In our second study, we report a class of photoresponsive vesicles composed of inexpensive and commercially available cationic and anionic surfactants. The mixture of these amphiphiles forms vesicles due to ionic interactions between the cationic and anionic headgroups. When irradiated by UV light, the cationic surfactant loses its charge and, in turn, the vesicles are converted into micelles due to the loss of ionic interactions. In addition, a mixture of these photoresponsive vesicles and a hydrophobically modified biopolymer gives a photoresponsive vesicle-gel that can undergo a gel-to-sol transition with UV light. In our final study, we demonstrate practical applications of PR fluids as a light-activated fluidic valve in microchannels. Current PR fluid formulations are not suitable for this application. Here, we report a PR fluid composed of alginate, a photoacid generator (PAG) as a photo-trigger, and a chelated complex of divalent strontium (Sr2+) (Sr-EGTA). Upon exposure to UV, free Sr2+ ions are released, which self-assemble with the alginate chains to give a nanostructured gel with a high gel modulus (~ 2000 Pa). We flow our PR fluid in a microchannel and expose a specific point to UV light. At this point, alginate is converted into a gel, which blocks the flow. When the UV light is removed, the gel is gradually dissolved by the flow and the channel reopens. The above concept relies entirely on physical (non-covalent) bonds, i.e., on self-assembly, rather than on covalent crosslinking of dissolved monomers or polymers.