pH sensor based on the crown heteropolyanion K28Li5H7P8W48O184·92H2O immobilized using a layer by layer assembly process

A new sensitive pH sensor based on immobilization of the crown heteropolyanion K28Li5H7P8W48O184·92H2O (P8W48) on a electrode surface through a layer by layer assembly process is described. The immobilization is based on the electrostatic adsorption of the complex in layers of charged polyelectrolyte poly(allylamine hydrochloride) (PAH). The deposited P8W48/LBL film was investigated by cyclic voltammetry, potentiometry and electrochemical impedance spectroscopy. Compared to the electrochemical behavior of dissolved P8W48, a slight shift in the redox peak towards negative potentials is observed, which have been attributed to a slight decrease in the acidity of the interior of the P8W48/LBL film compared to the testing buffer solution. The relationship between the peak currents of the deposited P8W48/LBL film and the number of layers is shown to be linear, which demonstrates that equal amounts of P8W48 are adsorbed in each deposition layer. The P8W48/LBL modified electrode showed high sensitivities toward pH. Therefore, such electrodes were tested as pH sensors using the titration method. The resulting pH sensor has a detection range of pH 1–13, a sensitivity of 69 ± 2 mV/pH, high repeatability (<3 mV), fast response time (<7 s), low sensitivity toward change in ionic strength and nature of the supporting electrolyte, low internal resistance and a working life time of at least 3 months. Moreover, the sensor is easy to manufacture and can be easily miniaturized for measurements in micro- and nano-systems.