Synthesis of Ti₂CO₂ MXene and Its Application in Photoelectrochemical Biosensors with Ultrahigh Sensitivity and Long-Term Stability

Theoretical chemists have predicted that Ti₂CO₂ is a promising semiconducting MXene with high stability and surface activity, making it suitable as the bonding layer in biosensors. However, its synthesis remains challenging due to difficulties in accurately controlling the oxidation of the initial Ti₂C phase. Here, we successfully synthesized Ti₂CO₂ MXene via pulsed ozone treatment (POT). The resulting Ti₂CO₂ MXene is an n-type semiconductor with a bandgap of 0.49 eV, exhibiting a high probe adsorption capacity of ∼3.03 × 10¹⁴ molecules/cm². The successful synthesis of Ti₂CO₂ MXene is attributed to a strategy of controlling the activation energy window during the "Ti₂C → Ti₂CO₂ → TiO₂" multistep reaction. The strong oxidizing ability of ozone reduces the activation energy barrier for the "Ti₂C → Ti₂CO₂" reaction, while precise control of short-time POT pulses inhibits the formation of TiO₂. By employing the Ti₂CO₂ MXene as a bonding layer in a photoelectrochemical sensor for serotonin detection, the sensor exhibits an ultrahigh sensitivity of 0.433 aM (oligomolecular level) and considerable long-term stability for over 240 h. Given its high adsorption capacity and robust long-term stability, Ti₂CO₂ MXene becomes a positive candidate for high-precision biosensing applications.