Abstract
1. Introduction
In the era of artificial intelligence, high quality of life[
As one of the potential wearable electronic devices, photodetector and gas sensor play an indispensable role in health monitoring system, such as forecasting alarm and pre-diagnosis by tracking specific signals in the environment or from human body. Owing to their unique properties like electronic conduction and energy bandgap which are highly programmable via a low-temperature processing route, solution-processable metal chalcogenides emerge as ideal building blocks for flexible and stretchable photodetectors and gas sensors. The merits of metal chalcogenides are high sensitivity and mild preparation in the smart sensor application. Sensing response/sensitivity is the ratio of signal to noise. However, their demerit is low stability. Herein we reviewed the recent progress of flexible and stretchable sensors for wearable healthcare based on solution-processable metal chalcogenides.
2. Requirements of healthcare monitoring with photodetectors and gas sensors
2.1. Photodetectors for healthcare monitoring
Sensors are the main way and means for humans to obtain information in the natural and artificial fields. They are the basic devices of Internet of Things that promote the acceleration of digital and network to intelligence. As one of physical sensors, photodetectors can detect light simply through the change of materials conductivity upon radiation. In the field of health monitoring, photodetectors for ultraviolet radiation monitoring[
Figure 1.(Color online) Photodetectors for healthcare monitoring. (a) Schematic illustration of the wearable photodetector as a real-time UV monitor[
The ultraviolet rays by the sun are the main source of ultraviolet radiation in our daily life. Excessive ultraviolet radiation can cause skin wrinkles, aging and relaxation. In addition, ultraviolet radiation is also considered as the main cause of skin cancer[
Heart rate, a direct signal of heart health, can reflect person's blood circulation status. Heart rate detection, an irreplaceable medical physiological indicator in clinical medicine, occupies an extremely important position in body monitoring. Studies have shown that increased heart rate is one of the major risk factors for death of patients with cardiovascular disease[
Photodetectors used for the blood oxygen saturation monitoring have been demonstrated. Oxygen saturation is the ratio between the concentration of oxygenated (HbO2) hemoglobin and deoxygenated (Hb) hemoglobin. The saturation of blood oxygen is an important physiological parameter reflecting the function of breathing and circulation. The oxygen saturation value SvO2 in venous blood is of important clinical significance, related to the flow of blood to blood tissue and its metabolic rate[
2.2. Gas sensors for healthcare monitoring
The safety and quality of human life are closely related to the gas environment. Gas sensor is one of the most effective ways for real-time monitoring and disaster warning of flammable, explosive and toxic gases in real time[
With the development of the petrochemical industry, the types and applications of flammable, explosive, and toxic gases have increased. The leakage of these gases in the process of production, transportation and use could lead poisoning, fire and even explosion accidents, seriously endangering people's lives. In the event of a gas leak, appropriate measurement must be taken as soon as possible to reduce the accident loss to a lower level. This puts high demands on gas detection and monitoring equipment. The development of gas sensors makes them more and more widely used in industrial safety. The environmental problems also have great influence on our daily life. For example, as one of the toxic and harmful gases, NO2 is one of the main harmful gases released from automobile exhaust and fossil fuel combustion. This pollution not only causes serious respiratory problems in humans, such as pulmonary edema, but also threatens animals, plants and the environment[
In the fields of domestic gas leakage, coal mine safety, oil exploration, etc., gas sensors are of major need as safety alarms. Because of its high calorific value, natural gas, a widely used fuel, produces less smoke and does not cause much damage to the environment. However, it has serious leakage risks, which may cause an explosion. Inhalation of the human body may also cause suffocation[
Table 1 lists some of the gas markers used for respiratory testing, which can reflect certain physiological and pathological conditions of human body. Olopade et al.[
3. Solution-processed metal chalcogenides
In recent years, metal chalcogenides have been widely used in flexible wearable electronic devices. Common metal chalcogenides include PbS, PbSe, CdS, CdSe, HgTe, MoS2, Bi2S3 and so on. These materials could be synthesized by solution process. In the solution synthesis system, the solution and surfactant are reasonably selected based on different target metal chalcogenides, and the nucleation and growth process of the product can be controlled by adjusting parameters such as reaction temperature and reaction time[
3.1. Solution-processed synthesis
The widespread use of metal chalcogenides has benefited from the synthesis of nanostructured materials of different sizes and new forms. When the size of metal chalcogenides is reduced to the nanometer scale, new physical and chemical properties occur in well-known quantum size effects[
Quite a few journals have been published on the synthesis of metal chalcogenides with nanostructures. Liu et al.[
Figure 2.(Color online) Some solution-processed metal chalcogenides. (a) HRTEM of PbS CQDs[
In addition, 2D transition metal sulfides (TMDs) because of the special band structure, semiconductor or superconducting properties and excellent mechanical properties[
3.2. Gas-sensitive and photon-sensitive characteristics
Several metal chalcogenide compounds have been selected for analysis of their properties, including photoelectric and gas-sensitive properties. Liu et al.[
Figure 3.(Color online) (a) Absorption spectra of PbS CQD film and response curves of the sensor to NO2 of different concentrations[
Chen et al.[
Jin et al.[
Sargent et al.[
Franky et al.[
4. Device performance based on metal chalcogenides
4.1. Photodetectors performance
At present, metal chalcogenides have been reported as photodetector materials for ultraviolet and heart rate monitoring. Fang et al.[
Figure 4.(Color online) (a) Schematic diagram of the device configuration of p-CuZnS/n-TiO2 NTAs with Ag contacts[
Based on PbS QDs and multi-walled carbon nanotube hybrid film materials, wearable photodetectors designed by Gao et al.[
4.2. Gas sensors performance
Gas sensor has been widely used in flammable, explosive, toxic and harmful gas detection, environmental monitoring, security alarm and other fields as an indispensable part. Different materials can be used according to different gas detection objects. Duesberg et al.[
Figure 5.(Color online) (a) Sensor response plots show percentile resistance change versus time of the MoS2 film with a bias voltage of 0.5 V, upon consequent NH3 exposures from 2 to 30 ppm[
5. Conclusion
In this review, we highlight the need of wearable photodetectors and gas sensors for health monitoring in real time. Flexible and stretchable wearable electronic devices have new requirements on sensitive materials, including flexibility, stretchability, high mechanical properties and high electrical conductivity. We listed the reported materials used in wearable electronic devices and analyzed their properties. We further proposed the advantages of metal chalcogenides synthesized by solution method, and summarized the synthesis methods, morphology, photoelectric and gas-sensitive characteristics of some metal chalcogenides at present. Finally, some photodetectors and gas sensors based on metal chalcogenides which have been used for health monitoring (including ultraviolet radiation, heart rate, toxic and harmful gases, flammable and explosive gases, etc.) were listed and their performance was analyzed.
There is an urgent need on flexible, stretchable wearable electronic devices to monitor human activities and personal health care. However, due to the limitation of materials, the current wearable electronic devices can’t meet all the requirements of practical application. Therefore, the preparation of highly sensitive wearable electronic devices is still the main potential of future research. In addition, the integration of wearable electronic devices with other platforms, such as data storage, data transmission system can further play a huge role on monitoring human activities and personal health care. But integrating multiple functional components into wearable electronics is also a considerable challenge.
Acknowledgments
This work in this paper was supported by National Natural Science Foundation of China (61861136004) and the National Key R&D Program of China (2016YFB0402705). H. Liu acknowledges the Innovation Fund of WNLO, and Program for HUST Academic Frontier Youth Team (2018QYTD06).
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