Synthesis and characterization of ammonia-responsive polymer microgels

We report a type of polymer microgel that undergoes rapid, reversible, and highly sensitive volume phase transitions upon varying ammonia concentrations in milieu. Such an ammonia-responsive microgel is made by tethering of a phenoxazinium, N-(5-(3-azidopropylamino)-9H-benzo[a]-phenoxazin-9-ylidene)-N-methylmethanaminium chloride, to the network chains of poly(N-isopropylacrylamide-co-propargyl acrylate) via a copper(I)-catalyzed azide–alkene cycloaddition. Tethering of the ammonia-recognizable phenoxazinium onto the polymer network chains makes the microgels responsive to ammonia. While a fast (<0.1 s) and stable shrinkage of the microgels can be achieved upon addition of ammonia over a clinically relevant range (0.25–2.9 ppm), the microgels can convert the elevated concentrations of the solution/gas-phase ammonia into enhanced photoluminescence signals. This makes the microgels different from the phenoxazinium, or its analogs reported in previous studies, that exhibit ammonia-induced quenching of photoluminescence. With the microgels as probes, the detection limit was ca. 7.3 × 10−2 and 3.9 ppb for the solution and the gas-phase ammonia, respectively. These features enable “turn-on” photoluminescence detection of ammonia in breath.