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Highly Reliable Passive RFID-Based Inductor–Capacitor Sensory System Strengthened by Solvatochromism for Fast and Wide-Range Lactate Detection

A passive radio frequency-identification-based inductor-capacitor (LC) lactate sensory system with a specific dye-containing interdigitated capacitor (IDC) in which the sensing signal is amplified by the solvatochromic effect is proposed. When a lactate solution contacts the IDC of the LC lactate sensor, the capacitance of the IDC changes, changing the resonance frequency of the sensor. This changes the oscillation frequency of the Colpitts oscillator in the readout circuit. By analyzing the frequency changes, the concentration of the lactate solution can be measured quickly and accurately over a wide range. To our knowledge, the proposed device is the first passive, battery-free LC lactate sensor that uses solvatochromic dye-containing IDC sensing elements to detect lactate solution concentrations. Four solvatochromic dyes were tested and incorporated into a polymer as the lactate-sensitive membranes of the IDCs. The proposed LC sensor tag offers excellent sensitivity and linearity over a wide lactate concentration range of about $10 \mu \text{M}$ to 1 M. The response and recovery times of our LC sensory system were significantly shorter than those of previously reported lactate sensors. Our results are useful for the development of reliable wearable devices capable of real-time lactate detection at a low cost.