Methods of preparation of metal-doped and hybrid tungsten oxide nanoparticles for anticancer, antibacterial, and biosensing applications

Tungsten oxide (WO_x) is one of the most important and commonly used n-type wide band-gap oxygen-deficient metal oxide semiconductors with several important physicochemical properties, which enabled them to be used in numerous biomedical applications. Recently, researchers have demonstrated that: (I) suitable modifications in their dimension, crystal structure and morphology by doping metals into the WO_x lattice framework play a critical role in improving their applications by tuning the physicochemical properties; (ii) metal doped WO_x nanoparticles (NPs) can generate reactive oxygen species (ROS) upon absorption of NIR light even in the absence of photosensitizer and molecular oxygen; (iii) doping of noble metals into WO_x films increase their electrical conductivity for improved electrochemical sensing; and (iv) the binary and ternary conjugates of WO_x or tungsten-bronze exhibit increased photoelectric conversion ability and ROS production rate as well as improved photothermal conversion efficiency. However, no comprehensive review article has been reported yet on metal-doped WO_xNPs and hybrid WO_x nanocomposites. Therefore, herein we aimed to discuss firstly the several methods of metal-doped and hybrid WO_xNPs synthesis such as precipitation, hydrothermal, microwave and solvothermal methods, etc. with appropriate conditions that control their size, shape, crystal structure and defects upon which the physicochemical properties depend. After that, we have discussed about some of their important physicochemical properties. Finally, we thoroughly reviewed the biomedical applications of WO_xNPs in the field of biosensing, imaging, antibacterial and anticancer therapy with probable mechanisms. Finally, we have discussed the lacunae and the anticipated applications of WO_xNPs. This review will provide a platform for understanding the different synthesis routes and the mechanistic details of the biomedical applications of metal-doped/hybrid WO_xNPs to design novel nanocomposites with improved physico-chemical properties for their future applications.