Colloidal crystal hydrogels with tunable optical properties are useful materials to construct thermoresponsive tunable
photonic crystals<sup>1 </sup>and optical switch that responds in nanoseconds<sup>2</sup>. We report here instead, the fabrication of optically
tunable colloidal crystal hydrogel material that can also behave as biosensors, responding to the presence of antigens or
antibodies. Optically tunable hydrogel material was fabricated from submicron sized, thermoresponsive poly-N-isopropyl
acrylamide (pNIPAM) particles and pNIPAM-co-acrylic acid (pNIPAM-AA) copolymer particles. The color of this
translucent hydrogel material can be tuned from blue to green to red by changing the concentration of pNIPAM particles.
PNIPAM-AA with covalently linked protein A (pNIPAM-AA-protein A) formed light blue colored hydrogel material
that respond to the presence of anti-Protein A antibodies to form an opaque, crosslinked networked hydrogel particles
aggregate which could be detected visually. A similar response was observed when pNIPAM-AA particles, with attached
anti-protein A antibodies (pNIPAM-AA-Anti Protein A), were treated with protein A. The lowest limit of detection,
based on visual observation of aggregate, of anti-protein A antibodies was 0.5μg or a concentration of 1μg/μl, whereas
for protein A the lowest limit of detection was 0.25μg or a concentration of 0.5μg/μl. We expect this optically tunable
hydrogel biosensor material to find applications as photonic crystals that respond to the presence of microbial antigens or
human antibodies, to control laser transmission and the optically switching of devices.