SERS detection of bacteria in water by in situ coating with Ag nanoparticles

The bio-sensing for the convenient detection of bacteria has been widely explored with the use of various sensing materials and techniques. It is still a challenge to achieve an ultrasensitive and selective, but simple, rapid, and inexpensive detection method for bacteria. We report on surface-enhanced Raman scattering (SERS) for the detection of living bacteria in drinking water by employing a synthesis of silver nanoparticles coating the cell wall of bacteria. We found that the Raman signals intensity of bacteria after AgNP synthesis mainly depends on the zeta potential of the cell wall. The enhancement of the Raman signal of bacteria using this strategy is about 30-fold higher than that in the case of a simply mixed colloid-bacterial suspension. The total assay time required is only 10 min and the total reactants' volume needed to analyze bacteria in a real environment is as low as 1 mL. Particularly, only one droplet of 3 μL sample is necessary for each SERS measurement. Furthermore, we can use this novel strategy to discriminate three strains of Escherichia coli and one strain of Staphylococcus epidermidis by hierarchy cluster analysis. Finally, we can detect bacteria down to 2.5 × 10(2) cells/mL on a hydrophobic glass slide by SERS mapping. Thus, our detection method offers prominent advantages, such as reduced assay time, simple handling, low reactant volumes, small amount of sample, and higher sensitivity and selectivity compared to previously reported label free methods. This novel strategy may be extended to open an avenue for developing various SERS-based biosensors.