Novel synthesis of polycationic gold nanoparticles and their potential for microbial optical sensing

The aim of this research was to describe a facile protocol to obtain biocompatible gold nanoparticles (AuNPs) suitable for microbial optical sensing. For this purpose, polycationic poly-L-lysine (PLL) was employed as both reducing and stabilizing agent in order to obtain an optically active microbial nanotag based on the electrostatic interaction with negatively charged cell envelopes. A one-pot procedure was developed to produce homogeneous, positively charged AuNPs. The as-synthesized particles, named PLL@AuNPs, exhibited maximal surface plasmon resonance (SPR) at 532 nm, a FCC crystalline nature, and sizes ranging from 20 to 25 nm, according to spectroscopy, X-ray diffractometry (XRD), transmission electron microscopy (TEM), and dynamic light scattering (DLS) analyses. The reduction of gold ions by PLL was featured by Fourier-transform infrared (FTIR) absorption bands of various functional groups. Zeta potential analysis confirmed the high cationic feature with a value of + 57 mV. The applicability of the particles to tag bacterial cell surfaces was exemplified by their adherence to Escherichia coli, a bacterial species commonly used to monitor fecal pollution in water sources. Finally, the potential of this tagging approach for microbial sensing through surface-enhanced Raman scattering (SERS) was explored.