In situ or colloidal prepared metal nanoparticles on E.coli and cyanobacteria for improved reproducible surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy with in situ or colloidal produced metallic nanoparticles is a powerful tool for fast and reliable selection of cyanobacterial strains, which produce desired product molecules, such as PHB or glycogen. [1, 2] To apply SERS properly the distribution of the analysed molecule on a metallic surface is decisive. It is possible to reduce pure salts of precious metals, such as gold and silver, in order to obtain colloidal metallic nanoparticles. [3] Some microorganisms like cyanobacteria or E. coli possess a negatively charged surface, which makes it possible to coat them with nanoparticles in situ and to investigate their status by Raman spectroscopy regarding the storage of specific molecules of interest. It is essential to distribute the metallic Raman signal enhancer regularly on the subject analysed. The aim is to improve the reproducibility and the quality of the gathered enhanced Raman scattering spectra, and to shorten the analysis time. Both, the Au-colloids (> 40 nm) and the in situ designed Ag-colloids (>100 nm) synthesized directly on the cell surface of the MOs, were evaluated. Using the Bruker Senterra instrument with 538 and 785 nm laser, the in situ Ag- colloids (Figure 1a) enabled an increased reproducibility of the data (98%) compared to classical 40 or 80-100 nm Agcolloids (58%), and the measurement time was reduced by a factor of 12. The Au-colloids (Figure 1b) mixed with cyanobacteria delivered reproducible measurements up to 82%. Here the reproducibility is defined as the percentage of measurements that delivered reasonable spectra in the wavenumber range of interest with signal to noise ratio of above 3. The acquired Raman data were pre-processed (detrend in prospectr, RStudio; 1280-920 1/cm) and assessed by principal component analysis (PCA). It was possible to distinguish between cyanobacterial strains producing different levels of desired molecule on a cellular level (Figure 2), revealing heterogeneities within sample groups as well. The tested SERS enhancers were applicable for specified microorganisms and according to the analyte of interest. For PHB producing cyanobacteria the in situ Ag-colloids performed the best. Goldcolloids, applied in solution, may be more useful in investigations of other cells.