Smart hydrogels for sensing and biosensing – Preparation, smart behaviours, and emerging applications – A comprehensive review

Hydrogels are three dimensional (3D) crosslinked hydrophilic polymer network structures with an excellent stimulus sensitivity. 3D networks of hydrogels can absorb high amount of water in their crosslinked structures. In the last few decades hydrogels are successfully gaining attention in tremendous applications by their nature, texture, smart behaviour and a number of developments are taking chance in the field of sensing and bio-sensing applications. Owing to their highly tunable swelling, self-healing, mechanical, porous-structure and conductive properties, the research has been advancing day-by-day in the field of hydrogels and their applications. Hydrogels are soft materials whch are prepared through physical or chemical crosslinking methods weak van der Waals forces, ionic bonds or covalent bonds. A successive progress and recent advancement in hydrogels from simple network to complex double network structure to development of smart hydrogels which is now a trending research area in this field. The versatility and ability of smart materials to responds to various external stimuli make them ideal for detecting and quantifying wide range of analytes. This review widely discusses hydrogel preparation techniques including self-assembly, ionic interaction, freeze-thawing, graft-copolymerization, chemical, electrochemical, radiation, template polymerization, photo crosslinking and by simple chemical interaction. This review also focusses on various types of hydrogel sensors such as, fluorescent, colorimetric, electrochemical, electrochemiluminescence, surface-enhanced Raman scattering along with their sensing mechanisms. In addition, the visco-elastic behaviour empowering the hydrogels to design 3D, 4D printable structures using additive manufacturing techniques for better sensing applications were discussed. Moreover, the review discusses the behaviour of multifunctional hydrogel composites incorporated with carbon-based nanomaterials, metal-oxides and novel 2D materials likeMXene for the development of flexible, and wearable sensors. The review also highlights the physico-chemical and bio-chemical stimuli sensitive mechanisms in response to external smart stimuli for today's cutting-edge applications in sensing and bio-sensing.