Nonadiabatic tapered optical fiber sensor for measurement of antimicrobial activity of silver nanoparticles against Escherichia coli

Silver nanoparticles (SNPs) exhibit antibacterial properties via bacterial inactivation and growth inhibition but the mechanism is not yet completely understood. In this study a label free and rapid detection method for study of antimicrobial activity of the SNP against Escherichia coli (E. coli K-12) is investigated using a nonadiabtic tapered fiber optic (NATOF) biosensor. The results show that SNPs interact with bacteria either by anchoring to or penetrating into the bacterial cell layer. These mechanism changes the refractive index (RI) of the tapered region, which in turn lead to the changes in the optical characteristics of the tapered fiber and output signals. With similar conditions for bacteria, the inhibition rate of the E. coli growth was measured by colony counting method as an experimental control and the results were compared with those obtained from the fiber sensor measurements. For SNP concentrations ranging from 0 to 50 μg ml−1 the inhibition rates of the E. coli growth were measured to be from 1.27 h−1 to −0.69 h−1 and from −3.00 × 10−3 h−1 to −1.98 × 10−2 h−1 for colony counting and optical fiber biosensor, respectively. The results demonstrate the potential of the proposed NATOF biosensor as a label free and rapid sensing platform for understanding the mechanism of antibacterial effects of SNPs.