Photonic calcium and humidity array sensor prepared with reactive cholesteric liquid crystal mesogens

Ion and humidity sensor array films have been developed using an interpenetrating polymeric network (IPN) of photonic networked cholesteric liquid crystals (CLCs) intertwined with network-structured poly(acrylic acid). Selective curing of a photonic CLC network film infiltrated with an acrylic acid/tri(propylene glycol) diacrylate mixture produced an array film of dots with photonic IPN structures on a simple photonic CLC background. Water isolation by individual hydrophilic dots against a hydrophobic background allowed testing of small sample amounts. Subsequent acetone/KOH treatments of the IPN film (IPNacetone/KOH film) prevented hydrogen bonding between carboxylic groups, maximized swelling of the film with water, and increased the speed of photonic color change. A full range of reflected colors, i.e., blue (dry) to red (humid), allows this film to act as an optical humidity sensor. The replacement of K+ with divalent ions that bridge the carboxylic groups prevented film swelling. Given that the photonic band gap wavelength is inversely proportional to the CaCl2 concentration, the amount of Ca2+ can be determined from the reflected color by the naked eye. This cost-effective, convenient IPNacetone/KOH film has been successfully applied as a biosensor for detecting Ca2+ in human blood serum and saliva and as an anti-counterfeiting label that reveals a colorful image under humid conditions.