Raman study of Ag nanoparticles coated with proteins synthesized by fungi using Surface-enhanced Raman spectroscopy and Tip-Enhanced Raman Spectroscopy

Abstract

Silver nanoparticles (AgNPs) have a large number of applications in technology and physical and biological sciences. These nanomaterials can be synthesized by chemical and biological methods. The biological synthesis that uses fungi represents an ecological approach to the production of nanomaterials that has the advantage of biocompatibility. This paper studies the silver nanoparticles (AgNPs) produced by the fungi Rhodotorula glutinis and Rhodotorula mucilaginosa found in the ordinary soil of the campus of the Federal University of Ceará (Brazil). Biosynthesized AgNPs have a protective layer of proteins that involves an Ag metal core. The objective of this work is to investigate the size and structure of the coverture protein layer, how it interacts with the Ag core and how sensitive the system is (core+protein) to visible light illumination, specifically your Raman response. For this, we use SEM, AFM, photoluminescence spectroscopy, SERS, dark field spectroscopy and TERS. AgNPs were isolated and SEM measurements showed the average size diameter between 58 nm for R. glutinis and 30 nm for R. mucilaginous. These values are in accordance with the AFM measurements, which also provided the average diameter of 85 nm for R. glutinis and 56 nm for R. mucilaginous, as well as additional information on the average size of the protein protection layers, whose values were 24 and 21 nm for R. mucilaginosa and R. glutinis nanoparticles, respectively. The layer structure of the proteins that coat the AgNPs seemed to be easily disturbed, and the SERS spectra were unstable but in TERS they were shown to be stable. It was possible to identify the Raman peaks that could be related to the helix, the sheet and the mixed protein structures, in addition to differences in formation structure. Finally, dark field microscopy showed that silver nuclei are very stable, but some are affected by laser energy due to heating or melting.