(463a) Magnetic Plasmonic Core-Shell Nanoparticles for Fast Condensation and Detection of Bacteria Using SERS

Noble metal nanoparticles (NPs) such as gold and silver have been widely used to enhance the Raman scattering of molecules adsorbed on their surfaces to the order of 10⁶ -10¹⁴ folds, known as surface-enhanced Raman scattering (SERS). These colloid NPs have also been used as SERS-active substrates for the detection and identification of microorganisms such as bacteria and spores. In the real applications, however, samples containing microorganisms are always in low concentration. Therefore, the ability to condense microorganisms is highly desired. In addition, the uniformly and densely covering microorganisms with colloid NPs could greatly increase the detection sensitivity and reproducibility. We developed a novel method that is capable of condensation and detection bacteria simultaneously by using the Fe₃O₄-Au magnetic plasmonic core-shell NPs. The superparamagnetic Fe₃O₄ NP clusters with the average size of 70 nm were synthesized and then a gold shell was attached by reducing chloroauric acid. The size and morphology of the final multi-functional NPs were examined by SEM and TEM, showing the NPs are about 200 nm with spiky gold shells. After plasma cleaning of the NPs, they were mixed with 10⁴-10⁶ cfu/mL bacteria. A 10 μL drop of the solution was added on a clean silicon substrate. By putting a corner of the magnet underneath the silicon substrate, the NPs were concentrated to a ~500 μm spot and thereby condensing the bacteria. SEM images showed that the number of bacteria in the condensed spot area is about 120 folds higher than those outside the NP spot. Moreover, all bacteria were surrounded by uniformly and densely packed NPs. SERS spectra were taken from the spot area by using a 785 nm laser focused to a 2 μm x 15 μm rectangle spot. Multiple spectra were taken for each sample and highly reproducible spectra of each bacterium were obtained. To further use this method to identify bacterial species, Escherichia coli K12, Pseudomonas aeruginosa, and Acinetobacter calcoaceticus at the concentration of 10⁵ cfu/mL were tested. The results showed that they could be clearly discriminated by the Raman spectra due to the high sensitivity of this method. The bacteria can be further differentiated by applying the principle component analysis (PCA). The success of the preliminary tests shows that this novel platform holds great potential to be applied in environment monitoring, food safety, and homeland security.