Opto- Electrochemical Detection of Waterborne Bacteria Using Bi-Functional Magnetic Nanoparticle Conjugates and Nile Red

Detection of microbial contamination in water is imperative to ensure water quality. We have developed an opto- electrochemical sensor for the detection of E. coli using bi-functional magnetic nanoparticle (MNP) conjugates and Nile Red (NR). The bi-functional MNP conjugates were prepared by terminal-specific conjugation of anti-E. coli IgG antibody and the electroactive marker ferrocene. The bi-functional MNP conjugate possesses both E. coli-specific binding and electroactive properties, which were studied in detail. The conjugation efficiency of ferrocene and IgG antibodies with amine-functionalized MNPs was investigated. Square-wave voltammetry enabled the detection of E. coli concentrations ranging from 10 1 –10 7 cells/mL in a dose-dependent manner, as ferrocene-specific current signals were inversely dependent on E. coli concentrations, completely suppressed at concentrations higher than 10 7 cells/mL. MNP bioconjugate captured E.coli was also detected by fluorescence methods using NR staining which can bind with both membrane and cytoplasmic lipids. This MNP bioconjugate captured NR stained E.coli emits high intensity fluorescence with maximum wavelength at 635 nm when excitation with optimum wavelength at 559 nm and it appeared in deep red to golden –yellow color in fluorescence microscopic analysis. Fluorescence intensity has a direct relationship with concentrations of E.coli cells. The developed opto-electrochemical method is highly sensitive (10 cells/mL) and, coupled to magnetic separation, provides specific signals within 1h. Overall, the bi-functional conjugates serve as ideal candidates for opto-electrochemical detection of waterborne bacteria. This approach can be applied for the detection of other bacteria and viruses. Figure 1