Smart microsubmarine shells operated by controlled buoyancy force through metal-ion adsorption/desorption and their biosensor applications

Small and uniform-sized poly(polyethylene glycol (PEG)–co-acrylic acid (AA)) shells engulfing a mineral oil droplet (oil@poly(PEG-co-AA) shells, ∼180 μm) are prepared using the in situ polymerization of the O(mineral oil)–W(PEGDA/AA, PEGDA = polyethylene glycol diacrylate)–O(mineral oil) double emulsion droplets produced in a microfluidic chip. The lightweight oil core and heavy-weight poly(PEG-co-AA) shells exhibit positive buoyancy and negative gravitational forces in water, respectively. Densities of oil@poly(PEG-co-AA) shells were finely tuned in water to be sensitive to a small weight change by controlling microfluidic parameters. The poly(PEG-co-AA) shells are pH-responsive in terms of charged state and shell size. The pH-controlled metal adsorption/desorption at poly(PEG-co-AA) shells breaks the density balance, leading to the sinking or floating of oil@poly(PEG-co-AA) shells in the water. Herein, this levitation and size change of oil@poly(PEG-co-AA) shells are applied to glucose and urea biosensors by immobilizing their enzymes. The divalent (or higher) metal adsorption is distinguishable from monovalent metal adsorption by the diminished size of the oil@poly(PEG-co-AA) shells and their different levitation behavior in water. Thus, these oil@poly(PEG-co-AA) shells facilitate smart motile sensor applications with an unexplored concept, which can be further expanded to other applications such as smart valves in microfluidics and smart windows.