Fluorescent Gold Nanoparticle-Based Bioconjugate for the Detection of Salmonella

An easily prepared fluorescent bioconjugate colloidal solution based on gold nanoparticles is reported for the detection of Salmonella. This bioconjugate was prepared by conjugation of staphylococcal protein A on the surface of gold nanoparticles, followed by biofunctionalization over this layer with fluorescein isothiocyanate-labeled antibodies. The surface plasmon resonance band observed in the ultraviolet–visible spectrum of the gold nanoparticles showed a shift to low energy after covering the surface with protein A and a broadening of the band after biofunctionalization with the antibody. Surface-enhanced infrared spectroscopy showed the absorption bands of the conjugate and bioconjugate due to their proximity to the nanoparticles. In particular, vibrations at 1649, 1588, and 1400 cm⁻¹ were from C˭O, N–H, and C–NH₂ groups from the staphylococcal protein A covering the gold nanoparticles. The final bioconjugate colloidal solution was also used as probe to detecting Salmonella by confocal microscopy of solutions containing 1E3 and 1E5 CFU/ml. The total time required to detect these bacteria was less than 1 h. Fluorescent emission of the bacillus suggests that this methodology may have potential to develop a universal method for detecting different bacteria.