# Advances in Wearable Smart Chemical Sensors for Health Monitoring
## 2 Wearable Chemical Sensors Based on Different Working Principles
### Wearable Chemical Sensors Based on Thermal Energy
With an increase in the Hg concentration, the proportion of HgTe on Te NWs rises, thereby leading to a stronger electrical output. Consequently, the variation in electrical output can be exploited for the detection of the Hg\({}^{2+}\) concentration. Throughthis mechanism, the thermoelectric nano-sensor can accomplish highly sensitive detection of mercury ions. The thermoelectric nano-sensor utilizes Te NWs as a highly specific substance in the reaction with mercury ions, enabling it to achieve a detection limit of 1.7 nM and a wide linear range (from 10 nm to 1 \(\upmu\)m) for detecting mercury ions with a high sensitivity. The thermoelectric nano-sensor exhibited a strong selectivity towards mercury ions, with its voltage increasing ninefold in their presence compared to other ions. The analysis of these studies clearly shows that thermoelectric-based chemical sensors show clear potential for future wearable and flexible miniaturized sensing systems, as proposed by [PERSON] et al. [72] and demonstrated in Figure 5i. A flexible self-powered sensing component was created through the combination of an organic transistor chemical sensor with a flexible power-organic thermoelectric (OTE) generator. The OTE array constructed on a paper substrate has a maximum open-circuit voltage of 0.52 V and a maximum power output of 0.32 \(\upmu\)W. According to reports, the device can power gas sensors utilizing organic field-effect transistors at a very low voltage and accurately detect the existence of ammonia. The traditional lithium battery, currently the primary energy source for wearable electronic gadgets, faces significant challenges like environmental harm and the need for frequent recharging. To address these issues, [PERSON] et al. [73] described a versatile hybrid photo-thermal electric generator (PTEG) that utilizes a basic design comprising a thermoelectric generator (TEG) and a photo-thermal conversion layer to capture both thermal and radiant energy using a unified operating mechanism, as shown in Figure 5j. Proven screen-printing techniques are used to fabricate large-scale n-type (Bi\({}_{2}\)Te\({}_{2.7}\)Se\({}_{0.3}\)) and p-type (Sb\({}_{2}\)Te\({}_{3}\)) thermoelectric inks on polyimide substrates, creating TEGs with serpentine-shaped thermocouple chains. These TEGs are then coated with a photo-thermal conversion layer to form PTEGs. It has been reported that PTFGs can be used to harvest both biofermal and infrared radiant energies to power electronic devices, providing new solutions to the current problem of powering wearable electronic devices. Advancements in wearable chemical sensors utilizing thermal energy offer new potential for enhancing our daily lives. By utilizing thermal energy from the environment or the human body, electronic devices enable us to continuously monitor ourselves and our surroundings in real-time, regardless of location. The further development and application of this technology will bring more convenience and safety to our lives.