Fast and stable optical pH sensor materials for oceanographic applications

The study reports preparation and detailed comparison of four new pH sensor materials for seawater measurements. The composition of the sensors is optimized in several iterations to ensure optimal dynamic range, fast response time at low temperatures, low cross-sensitivities to temperature (dpKa/dT similar to -0.013 pH units/K) and negligible cross sensitivity to ionic strength above salinity 15 PSU. The first generation (material pH-1) utilizes a pH indicator which is physically entrapped into a polyurethane hydrogel. This material shows satisfactory performance only at comparably high temperatures with response times being extremely long at low temperatures (t(95) > 2 h at 5 degrees C). The three other materials utilize cross-linked hydrophilic polymers based on poly(acryloylmorpholine) with indicator dye covalently coupled to the polymer. They feature fast response times at low temperatures (t(90) < 1 min at 5 degrees C). Moreover, the last two generations (pH-3 and pH-4) showed no drift over 54 days at 10 degrees C and only a drift of 0.003 pH units/day at 25 degrees C. Although the stability and the sensing properties of these materials are rather similar, the synthetic effort varies considerably. The material of the fourth generation pH-4 explores a novel approach of covalent coupling via B-O linkage and is characterized by low synthetic effort and the pK(a) value optimal for seawater measurements (8.05 at 20 degrees C). Importantly, all new sensor materials are spectrally compatible to a recently presented seawater optode system for combined pH, pO(2) and pCO(2) measurements which facilitates their application in marine environment.