Progress in the synthesis, characterisation, property enhancement techniques and application of gold nanoparticles: A review

Gold nanoparticles have been the solution to various hurdles today's scientific fraternity faces. Interestingly, reducing its size (1-100 nm) shows potential improvement in its chemical, physical and optical properties, suggesting its remarkable application in biopharmaceuticals, biosensors, photothermal therapy and chemotherapy, optical imaging and theranostics. This review aims to summarise the nuances associated with the synthesis (physical and chemical) and application of this remarkable material for advanced technological development. Emphasis has been given to the existing green methods like synthesis from microorganisms (Bacillus subtilis, Pseudomonas aeruginosa, Escherichia coli etc.) and plant extracts (grapes, cumin seeds, soybeans, garlic, mangosteen etc.). This review also summarises the main characterisation methods (qualitative and quantitative), their limitations, parameters and materials involved (chemicals, raw materials and process conditions) that play a significant role in preparing gold nanoparticles. Further, the various properties like optical, structural, electronic and chemical have been discussed, along with the detailed detection process mechanism to substantiate the capabilities of these engineered nanoparticles.