Polarised Raman Imaging of Living Cells for Chemical Contrast Manipulation

摘要

Raman spectral imaging has become a more and more popular technique in biological studies because it can extract chemical information from living cells in a label-free manner. One of the most challenging issues in the label-free Raman imaging of biological samples is to increase the molecular specificity in the spectra for better chemical contrast. Usually, the Raman spectrum from a cell is dominated by a few strong Raman bands such as the amide I band around 1650 cm~(-1), CH_2 bend around 1445 cm~(-1) or the amide Ⅲ band around 1300 cm~(-1) and it is not easy to get chemical contrast from other Raman bands that overlap with them. In this study, we aim to manipulate the chemical contrast in a living cell by exploiting the polarisation effects in Raman spectroscopy. By simply putting an analyser before the spectrometer, we can take the Raman image at the parallel and perpendicular polarisation against the incident light at the sample. The Raman spectra at the two orthogonal polarisations represent the Raman signals with different molecular orientation and symmetry of vibrations. Our experimental results demonstrate that at certain Raman shifts the two orthogonal polarisations indeed present different chemical contrasts. This indicates that polarized Raman imaging can help us visualise the different chemical contrasts that overlap at the same Raman shift and therefore increase the amount of chemical information we can get from cells.