SERS-MS for quantitation on single cells
Surface-Enhanced Raman Scattering (SERS)-active nanotags are a new generation of optical tags that have become a central tool in bio-analytical research, particularly for marker detection on cancer samples. These SERS-nanotags offer many advantages, such as sharper spectroscopic signal (for multiplex analyses), resistance to photobleaching, biocompatibility, and high chemical and optical tunability compared to the traditionally used fluorescent dyes or quantum dots, while keeping similar detection sensitivity. Owing to the high spatial resolution offered by modern Raman microscopes (enabling subcellular resolution), coupled with the advantageous spectroscopic characteristics and nanometric size of SERS nanotags, these tags have been used to measure the level of expression of specific receptors (phenotyping) implied in cancer at the surface of single cells. The phenotypic information obtained from these measurements can be used to classify cells, to monitor treatment effects or to detect phenotypic heterogeneity among subpopulations of cells. Hence, accurately measuring the amount of marker of interest is of paramount importance to fine-tune cancer treatment. Despite almost two decades of development, extracting truly quantitative information about the abundance of cancer markers using SERS nanotags on single cells still remains highly challenging.
This project aims at investigating the quantitative capabilities of SERS-active nanotags for measuring overexpressed surface markers at the single cancer cell level by combining SERS with Mass Spectrometry (MS)-based techniques. Measurements with SERS nanotags are currently based on two main assumptions: (i) the SERS signal intensity is proportional to the amount of nanotags bound to cells; and (ii) the amount of nanotags bound to cells is proportional to the number of surface marker present on their surface. These assumptions will be challenged by combining SERS with complementary MS-based analytical techniques giving access to single cell information. SERS measurements will be followed by Laser Desorption/Ionisation MS (LDI-MS) analysis of the same cells, allowing full ablation of the nanotags to retrieve their absolute amount bound to cells. SERS measurements will also be correlated to single cell proteomics analysis, allowing to retrieve absolute amounts of a protein (surface marker) of interest. The innovative correlation between the SERS signal, the absolute number of nanotags and the absolute number of receptors, only accessible by complementary techniques as proposed in this project, is a milestone for the transition from qualitative to truly quantitative measurements using SERS nanotags on single cells.
