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Silver nanocrystals have been successfully fabricated by the bioreduction route using the ethanolic extract of Azadirachta indica (neem) leaves as the reducing and capping agent without solvent interference. The silver nanocrystals were grown in a single-step method, without the influence of external energy or surfactants, and at room temperature. The nanoparticles were prepared from different ratios of silver ions to reducing agent molecules and were characterized by UV–vis spectroscopy and transmission electron microscopy (TEM). The nanoparticles were roughly spherical and polydispersed with diameters of less than 40 nm, as determined with high-resolution transmission electron microscopy (HRTEM). Fast Fourier transform (FFT) analysis and X-ray diffraction (XRD) analysis elucidated the crystalline nature of the nanoparticles. The presence of participating functional groups was determined with Fourier transform infrared (FTIR) spectroscopy. The synthesized silver nanoparticles were analyzed as a potential surface-enhanced Raman spectroscopy (SERS) substrate by incorporating rhodamine B as the Raman reporter molecule. The bioreduction process was monitored through SERS fingerprint, which was evaluated by the change in vibrational energies of metal–ligand bonds. It was possible to detect the SERS spectral pattern of the probe molecules on the Ag nanoparticles without the use of any aggregating agent. Thus, the formation of probable intra- and interparticle hot spots was attributed to evaporation-induced aggregation. Furthermore, stirring and precursor salt concentration influenced the kinetics involved in the fabrication process. The thermal stability of the lyophilized nanoparticles prepared from 0.1 M AgNO3 was evaluated with thermogravimetric analysis (TGA) and had a residual mass of 60% at 600 °C. X-ray photoelectron spectroscopy (XPS) studies were used to validate the compositional and chemical-state information. The biomass-capped silver nanoparticles provided antimicrobial activity by inhibiting the growth of Pseudomonas nitroreducens, a biofilm-forming bacterium, and the fungus, Aspergillus unguis (NII 08123).

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The development of composite/nanocomposite coatings on various surfaces based on electroless process has much interest among researchers due to their enhanced mechanical, electrical, magnetic, and optical properties compared with their metallic or alloy counterparts. These coatings have shown many applications in automotive, aerospace and industrial fields with unique characteristics obtained by the incorporation of secondary particles as reinforcement at micro or nanoscale in the matrix. However, properties of the product depend upon not only the nature of secondary particle used but also the specific characteristics of the metal matrix. This paper critically reviews the recent developments in micro and nanostructured metallic composite coatings formed by electroless coating techniques and retrospect into the various mechanisms of the electroless coating process, its preparation with different metallic systems, properties, and applications in detail.

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