Development of colorimetric PEG-based hydrogel sensors for urea detection†

Urea has environmental, agricultural and clinical importance being present in many bodily fluids including blood, urine, tears and sweat. Monitoring urea levels is crucial, serving as an early warning for many health issues such as dehydration, kidney and liver malfunctions. Herein, semi-interpenetrating network (semi-IPN) poly(ethylene glycol) (PEG) based hydrogels (PEG700-DA/PEGx, x = Mn) were developed as sensors for the colorimetric detection of urea. Urea was detected using the urease/phenol red assay where a colorimetric change to fuchsia occurred due to pH shifts. Hydrogels were synthesised through photo-induced free radical polymerisation where the phenomenon of polymerisation-induced phase separation (PIPS) occurred. Both pristine and sensor gels were characterised. Stability and kinetic experiments on free urease were performed giving further insights into the sensors response to urea. Finally, the detection of urea by the naked-eye from model sweat mixtures was successful for concentrations as low as 0.3 mM, while preliminary results from a proposed smartphone-based RGB quantification demonstrated an LOD of 0.8 mM and an LOQ of 2.7 mM in the green channel.