# Advances in Wearable Smart Chemical Sensors for Health Monitoring
## 2 Wearable Chemical Sensors Based on Different Working Principles
### Wearable Chemical Sensors Based on Solar Cells  
Solar energy is an important renewable and clean energy source, which can be converted into electricity to meet the energy needs of human beings. Solar cells are one of the most important ways to collect solar energy. Thanks to its unique performance features, it is commonly used in photovoltaic power generation devices. It uses silicon-containing semiconductors to generate a photovoltaic effect, directly converting light energy into electricity. This provides a sustainable energy solution for wearable chemical sensors, enabling them to operate for long periods outdoors or in well-lit environments. The integration of solar cells and wearable chemical sensors promotes device miniaturization and multifunctionality. By integrating solar cells, chemical sensors and signal-processing units into a small wearable device, energy collection and chemical substance detection can be achieved simultaneously without increasing the device's size [66]. Based on this, solar cell-based photovoltaic systems are typically applied for monitoring human motion and health information, with a focus on flexible and wearable sensor technologies. Extensive research has been conducted on chemical sensors that can be worn and are powered by solar cells. Hybrid halide chalcogenides are promising light absorbers that have been intensively studied in many photovoltaic applications. They have recently been utilized for self-powered chemical sensors. [PERSON] et al. [67] demonstrated a prototype of self-sustaining inorganic halide-thalate for detecting chemical gasses under visible light at room temperature, as shown in Figure 4a. Comprising a permeable lattice of CsPbBr\({}_{3}\) (CPB), these gadgets generate a 0.87 V open-circuit voltage when exposed to visible light, enabling the detection of different levels of O\({}_{2}\) and medically significant volatile organic compounds like acetone and ethanol in parts per million, all while maintaining rapid response and recovery times (Figure 4b-d). Such a device has positive implications for the upcoming era of wearable technology. Similarly, [PERSON] et al. [43] synthesized two-dimensional black phosphorus nanosheets (NSs) through a simple yet highly efficient n--methylpyrrolidone (NMP) solvent exfoliation approach. Subsequently, these synthesized nanosheets were employed as an electrochemiluminescence (ECL) luminophore within the context of peroxydisulfate (S\({}_{2}\)O\({}_{8}\)\({}^{2-}\)). They developed a chalcogenide solar cell (PSC) device on paper to substitute the costly and large electrochemical workstations typically utilized, as shown in Figure 4e. Utilizing the PSC-based paper platform, the constant-potential ECL\(-\)RET system demonstrated an exceptional sensitivity in detecting miRNA-107 across a wide concentration range of 0.1 pM to 15 nM, as shown in Figure 4f. Crucially, this research not only established a model for creating a very responsive PSC-induced ECL detection system but also investigated the potential uses of BP nanomaterials in biological analysis. Additionally, a sensing platform that is self-powered and has dual-response capabilities was created using photoelectrochemical cells. g\(-\)C\({}_{3}\)N\({}_{4}-\)Au-B\(-\)TiO\({}_{2}\) nanorod probes were constructed as photoanodes and PB-modified FTOs served as cathodes. [PERSON] et al. [68] illustrated the development of g-C\({}_{3}\)N\({}_{4}-\)Au-B-TiO\({}_{2}\) nanorods, as shown in Figure 4g. The solar-to-chemicalenergy conversion efficiency in the device was enhanced by the gold nanoparticles placed between the B-TiO\({}_{2}\) nanorods and g-C\({}_{3}\)N\({}_{4}\), as illustrated in Figure 4h. [PERSON] et al. [57] developed a self-sustaining versatile detector by combining crystalline silicon IBC photovoltaic cells with a vertically stacked structure on a single chip. The IBC PV cells were arranged vertically in parallel with a humidity sensor and a photodetector (PD) as illustrated in Figure 4i.