Sensitive detection of E. coli using bioconjugated fluorescent silica nanoparticles

Abstract

Fluorescent dye-doped silica nanoparticles have been a major focus of research and development as potential fluorescent probes in biological systems. This is due to their strong fluorescence intensity, excellent photostability, biocompatibility, and ease of surface modification for bioconjugation. In this study, spherical silica nanoparticles doped with fluorescein isothiocyanate (FITC) were synthesized using reverse microemulsion method. The average diameter of the nanoparticles was 78 nm with a narrow size distribution (78 ± 49.50 nm). FITC was incorporated into the silica sphere through 3-aminopropyltriethoxysilane, which minimized the leakage of the dye. Only about 30% of the dye leaked out from these nanoparticles, after they were sonicated in aqueous medium for 60 min and the fluorescent intensity remained comparatively constant over time, showing an increased photostability (∼ 1.5 times higher) compared to the free dye in solution. The fluorescence emission spectrum of nanoparticles showed no apparent shift (maximum emission at 518 nm) when compared with the free dye in solution. The nanoparticle surface was chemically modified with amine and carboxyl functionalities to allow for Escherichia coli antibody conjugation. E. coli bacteria is widely used as a biological indicator to identify fecal contamination in food and water samples. Nanoparticle-based biosensors for the detection of E. coli have a vital importance in water quality, public health, and food safety. The E. coli in water was successfully observed under a fluorescence microscope using the antibody conjugated fluorescent silica nanoparticles synthesized, proving the feasibility of using them in a biosensor for qualitative determination of E. coli. The significance of this study lies in the relatively simple synthetic procedures as compared to those that have been in other studies reported in the literature. The ability to detect live cells under the fluorescence microscope further proves the suitability of these functionalized nanoparticles in live cell imaging.