Surface plasmon enhanced auto-fluorescence and Raman spectroscopy for low-level detection of biological pathogens

Abstract

The current culture-based bacterial detection technique is time-consuming and requires an extended sample preparation methodology. We propose the potential of surface-enhanced Raman spectroscopy (SERS) and surface plasmon-enhanced auto-fluorescence spectroscopy (SPEAS) for the label-free identification and quantification of bacterial pathogens at low concentrations collecting its unique auto-fluorescence and Raman signatures utilising highly anisotropic three-dimensional nanostructures of silver nano dendrites (Ag-NDs). The SERS data facilitates qualitative bacterial identification using the spectral features coming from the bacterial cell wall compound, and the SPEAS data was utilised to gain unique auto-fluorescence spectra present on the bacterial cell wall with enhanced quantification. The enhancement of Raman and auto-fluorescence signals of Ag-NDs were first evaluated using rhodamine 6g (R6G) as a probe molecule that exhibits a significant enhancement of 106 and limit of detection (LOD) of 10-12 M for SERS and 15 fold intensity enhancement and LOD of 10-15 M for SPEAS measurements. Further, the SERS and SPEAS measurements of bacterial pathogens, such as Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), using the Ag-NDs were recorded, and the results exhibit high auto-fluorescence and Raman signal intensity for both the samples up to 100 cfu/ml for both modalities. The significant photon count and distinct emission range in SPEAS measurements of bacteria enables accurate quantification. Therefore, the comprehensive investigation of plasmonic enhancement of Ag-NDs for SPEAS and SERS techniques provides complementary information about molecules to enable accurate and quick identification and quantification of pathogens.

Keywords: Detection of E. coli; Enhancement factor; Localized surface plasmon resonance; Surface enhanced Raman spectroscopy (SERS); silver dendrites; surface plasmon enhanced auto-fluorescence spectroscopy (SPEAS).