Chemical sensors based on temperature-responsive hydrogels - art. no. 61670T

Temperature responsive hydrogels show a strong ability to change their swelling degree in dependence on organic solvent or salt concentration in aqueous solutions. This behavior can be used for appropriate sensors if a suitable transducer transforms the volume change into an electrical output signal. In the present work., piezoresistive sensors were used where the hydrogel led to a deflection of a silicon membrane within the sensor chip. This principle allows for a strict separation of the fluid from the piezoresistors as well as from other electronic components at the front side of the sensor chip. Poly(N-isopropylacrylamide) (PNIPAAm) as well as photo cross-linkable poly(N-isopropylacrylamide-co-dimethyl-acrylamide-co-2-(dimethyl maleimido)-N-ethyl-acrylamide) (PNIPAAm-DMAAm-DMIAAm) terpolymer have been applied and investigated for organic solvent concentration sensors and salt concentration sensors. The sensor's output voltage was measured during the swelling of the hydrogel under influence of water solutions with different organic and inorganic solute concentrations at different temperatures. A complex ,,reentrant" swelling behavior of the hydrogel in mixed co-solvents as well as "salting in" and "salting out" effects of different salts were studied. It was found that the change in the gel volume phase transition temperature depends on the solution viscosity and the concentration of the additive affecting the stiffness of the polymer chain in the surrounding solution. The influence of an initial gel conditioning procedure, on the signal value and the sensitivity of the proposed chemical sensors was investigated and the measurement conditions necessary for high signal reproducibility and long-term stability were determined.