An automated system controlled by a custom LabView (National Instruments) program is used to perform the sensing measurements. The sensors are tested simultaneously, in the same exposure chamber, using an Agilent 34980A multifunction switch. A Stanford Research System SR830 DSP lock-in amplifier controlled by an IEEE 488 bus is used to supply the AC voltage signal (0.2 V at 1 kHz) and to measure the corresponding current (<10μA in the studied devices). This setup allows for measuring normalized changes in conductance as small as 0.01%. Sensor resistance was continuously acquired during the experiments. Sensing experiments were continuously performed using subsequent exposure cycles (see SOI, section 2).