Fig. 4: nBSC as a self-powered pH sensor.

a Microscope image of pH sensor with all integrated components (ring oscillator (RO) with nBSC sensing element connected to nBSC energy storage via drain-to-drain voltage (VDD), ground (GND)) before roll-up and (b), after roll-up, inset shows the operational circuit. Scale bars, 200 µm (a, b). c Volumetric capacitance and relative frequency change of nBSC as a function of electrolyte pH. d Frequency spectral response of the nBSC based pH sensor at various electrolyte pH. e Output voltage swing of a five-stage oscillator working at 3 V DC input provided by the on board nBSC as a function of electrolyte pH. (in Fig. 4c, error bars represent variation in data over three measured devices. All measurements are performed under ambient conditions at 25 °C in artificial plasma flow of 2 ml/min).