Modeling and experimental investigations of the sensitivity of piezoresistive chemical sensors based on polyelectrolytic hydrogels

Abstract

The application of stimuli-responsive or smart cross-linked gels in chemical sensors is based on their ability to a phase transition under the influence of external excitations (temperature, pH, concentration of additives in water). The external stimulus lowers the energy barrier between two possible gel states: a stable state (shrunk gel) and a metastable state (swollen gel), and thereby makes possible the gel transition into the swollen state. The amount of the solvent absorbed due to the external stimulus has been modeled and calculated taking into account the polymer parameters (concentrations of the hydrophilic, hydrophobic, ionisable and ionised groups as well as polymer cross-linking degree) and the solution parameters (analyte concentration, ionic strength, viscosity as well as temperature and temperature change rate). Combining a smart hydrogel and a micro fabricated pressure sensor chip allows to continuously monitor the analytedependent swelling of a hydrogel and hence the analyte concentration in ambient aqueous solutions. The sensitivity of hydrogels with regard to the concentration of such additives as H+-ions (pH sensor), transition-metal ions and salts in water was experimentally and numerically investigated at different temperatures. It has been demonstrated that the sensor's sensitivity depends on the polymer composition as well as on the polymer cross-linking degree. A higher sensitivity was observed for polyelectrolyte hydrogels with higher concentrations of ionisable groups. The long-term measurements have shown that the lifetime of piezoresistive chemical sensors can be prolonged up to several years provided that specific operation and storage conditions are fulfilled.