Recent advances in pH sensors based on halochromic dyes

Accurate and reliable pH monitoring is crucial across diverse chemical, biological, and environmental processes. Halochromic dyes, which undergo reversible spectral changes in response to proton concentration, have emerged as powerful tools for constructing highly sensitive and versatile pH sensors. In this paper, the classification of halochromic dyes is presented, mainly including structural types, such as triarylmethane and azo dyes, along with representative fluorescent and natural dyes. The main synthesis methods of pH sensors based on halochromic dyes are also summarized, namely physical entrapment, absorption, covalent bonding, and microencapsulation, which largely determine the performance of the resulting pH sensors. The applications of these pH sensors in biomedical engineering, health monitoring, food safety, and environmental monitoring are then highlighted. Finally, the main challenges and future perspectives for the development of such sensors are discussed, providing insights into the design and application of more stable, reliable, and smart pH sensors.