Matching Capacitors to Self-Powered Biosensors for Signal Amplification: Toward Ultrasensitive Electrochemical Detection for MicroRNA-21-Triggered Catalytic Hairpin Assembly

A capacitor coupled with enzymatic biofuel cells (EBFCs) can deliver high-power instant output to obtain an amplified detection signal. Herein, matching a capacitor to a self-powered electrochemical biosensor for ultrasensitive determination of microRNA-21 is investigated. The detection system mainly combines a capacitor, EBFCs, and biological amplification technologies. Concretely, EBFCs are integrated into a capacitor-joined circuit, and the capacitor is automatically shorted by a switching regulator to provide an instantaneous current that is rapidly detected with a digital multimeter. A sensitivity of 38.72 μA/pM is achieved with the contribution of the matching capacitor, which is 6.96 times that without a capacitor. Moreover, the redox reaction on the anode leads to high-voltage outputs at low substrate concentrations and generates electrons when the target miRNA triggers a catalytic hairpin assembly cycle, while the release of the [Fe(CN)6]3– electron acceptor on the cathode is catalyzed to obtain a higher detection signal. As a result, the limit of detection of the developed biosensor is 0.18 fM (S/N = 3) with a linear range of 0.5–10 000 fM, which indicates that the innovative capacitor-matched self-powered biosensor holds great promise for ultrasensitive electrochemical biosensing.