These authors contributed equally to the work. AbstractWith a low optical background, high loading capacity, and good biocompatibility, hydrogels are ideal materials for immobilization of biopolymers to develop optical biosensors. We recently immobilized mercury and lead binding DNAs within a monolithic gel and demonstrated ultra-sensitive visual detection of these heavy metals. The high sensitivity was attributed to the enrichment of the analytes into the gels.The signaling kinetics was slow, however, taking about one hour to obtain a stable optical signal due to a long diffusion distance. In this work we aim to understand the analyte enrichment process and improve the signaling kinetics by preparing hydrogel microparticles. DNA-functionalized gel beads were synthesized using an emulsion polymerization technique and most of the beads were between 10 and 50 μm. Acrydite-modified DNA was incorporated by co-polymerization. Visual detection of 10 nM Hg 2+ was still achieved and a stable signal was obtained in just two minutes. The gel beads could be spotted to form a microarray and dried for storage. A new visual sensor for adenosine was designed and immobilized within the gel beads. The adenosine aptamer binds its target about 1000-fold less tightly compared to the mercury binding DNA, allowing a comparison to be made on analyte enrichment by aptamer-functionalized hydrogels.

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Supported lipid bilayers have found a diverse range of applications in understanding membrane biophysics, biosensor development, drug screening, and drug delivery. While silica has been the most frequently used supporting material, hydrogels might act as a superior alternative not only because of their soft nature allowing better interfacing with transmembrane proteins but also their porous interior for molecular containment. Unlike silica, where neutral and even same charged liposomes can readily fuse, electrostatic attraction is crucial for liposome fusion onto hydrogels. In addition to fusion, we systematically characterized other interactions including liposome adsorption onto and diffusion into hydrogels. The fused membrane forms a continuous bilayer for the most part, which is demonstrated by the observation that the diffusion of DNA is blocked but small dyes could still enter the gel. The kinetics of liposome-gel interaction is characterized using calcein loaded liposomes, where liposome rupture is observed only when the oppositely charged gel is added. With this work, a more complete picture about the interaction between liposomes and hydrogels is obtained.

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