Fig. 1. (Color online) Photodetectors for healthcare monitoring. (a) Schematic illustration of the wearable photodetector as a real-time UV monitor^[14]. Copyright 2018, Adv Mater. (b) Photographs of a wearable real-time UV sensor in real life^[14]. Copyright 2018, Adv Mater. (c) Comparison between traditional heart-rate sensor probe (clipped at middle finger) and the wearable photodetector (worn at forefinger)^[15]. Copyright 2014, Appl Phys Lett. (d) Image of a device laminated on the skin of forehead, with an operating m-ILED (wavelength 650 nm) under room light illumination and in the dark^[16]. Copyright 2014, Nat Commun.

Fig. 2. (Color online) Some solution-processed metal chalcogenides. (a) HRTEM of PbS CQDs^[30]. Copyright 2016, Thin Solid Films. (b) Typical absorption spectra of PbSe Nanocrystal from different sizes and TEM image of PbSe^[43]. Copyright 2011, Adv Funct Mater. (c) TEM image of solution-synthesized CdSe NWs, with (inset) an AFM image of a network of such NWs formed on a SiO₂/Si substrate^[44]. Copyright 2007, Nano Lett. (d) SEM images of Bi₂S₃ Nanorods^[46]. Copyright 2017, J Photochem Photobiol A. (e) TEM image of HgTe CQDs with an absorption onset at 5 mm. Scale bar, 100 nm^[47]. Copyright 2011, Nature Photonics. (f) TEM image of 5 nm quasi-spherical ZnS nanocrystals^[48]. Copyright 2005, J Am Chem Soc. (g) SEM of Ag₂Se^[49]. Copyright 2001, J Am Chem Soc. (h) TEM of MoS₂ nanosheets^[59]. Copyright 2011, Science. (i) TEM of WS₂ nanosheets^[59]. Copyright 2011, Science.

Fig. 3. (Color online) (a) Absorption spectra of PbS CQD film and response curves of the sensor to NO₂ of different concentrations^{[30, 67]}. Copyright 2014, Adv Mater. (b) Performance of Bi₂S₃ nanowire sensor^[68]. Copyright 2008, J Phys Chem. (c) PbS QDs infrared photodetectors and photovoltaics^[70]. Copyright 2005, Nat Mater. (d) Dark current-voltage, responsivity and measured detectivity characteristics of PbSe nanocrystal photodetectors^[43]. Copyright 2011, Adv Funct Mater.

Fig. 4. (Color online) (a) Schematic diagram of the device configuration of p-CuZnS/n-TiO₂ NTAs with Ag contacts^[14]. Copyright 2018, Adv Mater. (b) The on–off switching tests of the fiber-shaped PD at 3 V under 350 nm^[14]. Copyright 2018, Adv Mater. (c) Heart-rate test of the PbS QD PDs under red LEDs^[15]. Copyright 2014, Appl Phys Lett. (d) and (e) The original heart-rate signal measured through the CAB PDs in the red and infrared spectral ranges^[15]. Copyright 2014, Appl Phys Lett.

Fig. 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^[71]. Copyright 2013, Adv Mater. (b) Gas response (%) versus time of n-CdS/p-polyaniline heterojunction at a fixed voltage of 2 V and at a concentration of 1040 ppm LPG^[72]. Copyright 2010, Sens Actuators B. (c) Gas sensor device attached on the finger joint with “paper” and “rock” state^[73]. Copyright 2018, ACS Sensors. (d) Real-time sensing curves and sense response toward different concentrations of NO₂ at room temperature^[73]. Copyright 2018, ACS Sensors. (e) Response curves toward 50 ppm of H₂S of the sensor at different temperatures^[74]. Copyright 2015, Sens Actuators B. (f) Response–recovery curves of the gas sensor towards 20, 50, 100, and 200 ppm ethanol, respectively^[75]. Copyright 2014, RSC Adv.

Table 1. Respiratory gas markers and pathology.

Table 2. Methods of synthesis of some metal chalcogenides.