The surface charge of the gold nanoparticle becomes neutral, causing nanoparticles to aggregate. As a result, the solution color changes from red to blue.

Synthesis of gold nanoparticles developed a synthetic method for creating AuNPs in 1951 by treating hydrogen tetrachloroaurate (HAuCl4) with citric acid in boiling water, where the citrate acts as both reducing and stabilizing agent (Scheme 2B).

‘Green synthesis’ are required to avoid the production of unwanted or harmful by-products through the build-up of reliable, sustainable, and eco-friendly synthesis procedures. The use of ideal solvent systems and natural resources (such as organic systems) is essential to achieve this goal.

Gold nanoparticles are used in resonance scattering dark-field microscopy for the detection of microbial cells and their metabolites [37], the bio-imaging of tumor cells [38], and for the detection of receptors on their surface [39], and for the study of endocytosis [40].

Colloidal gold has been used by artists for centuries because of the nanoparticle’s interactions with visible light. These colors occur because of a phenomenon called localized surface plasmon resonance (LSPR), in which conduction electrons on the surface of the nanoparticle oscillate in resonance with incident light.

When exposed to heat or light, the particles stick together, changing the colour of the material. The geometry of the nanoparticles when they bind into clusters determines which colour they appear as: when the nanoparticles are spread apart they are red and when they cluster together they are dark blue.

Physical properties Gold nanoparticles absorb and scatter light resulting in colours ranging from vibrant reds (smaller particles) to blues to black and finally to clear and colorless (larger particles), depending on particle size, shape, local refractive index, and aggregation state.

Gold nanoparticles are made up of metallic gold atoms to create a crystal structure of 1-100 nm. Gold nanoparticles have been produced in the laboratory for several decades, and many approaches have since been developed.

According to the Mie Theory, different-sized gold nanoparticles exhibit unique light-scattering properties (Mie). By observing these trends by collecting their absorption at their Surface Plasmon Resonance (SPR) and at a wavelength of 450 nm, the average size of the gold nanoparticles can be estimated.

Gold nanoparticles absorb and scatter light with extraordinary efficiency. Their strong interaction with light occurs because the conduction electrons on the metal surface undergo a collective oscillation when they are excited by light at specific wavelengths.

Gold nanoparticles plays an essential role in spectroscpic applications and biological screening platform. Nontoxic and non-hazardous environmental friendly nanomaterials are focused towards a greener future. Novel premises of ‘Green Nanotechnology’ have tremendous impacts towards industrial scale revolution.

Green synthesis of nanomaterials are highly promising in the Nanotechnology as the benefits include reduced energy and minimal production cost compared to the traditional synthesis of nanomaterials using harmful chemicals like: sodium borohydride or hydrazine [ 9 ].

Nanoscience assures a promising future by its improvement in green chemistry to develop the ‘Greenary Nanoscience and Nanotechnology’.

Nanomaterials are usually synthesized by utilizing smart techniques that comprise the usage of hazardous solvents and toxic chemicals like pyrolysis, sol-gel technique, chemical vapor deposition, etching etc., and supercritical fluid [ 7 ].