SERS nanotags for folate receptor α detection at the single cell level: discrimination of overexpressing cells and potential for live cell applications.

Folate receptor α (FRα) is a high affinity folate membrane receptor that is overexpressed in a wide variety of cancers. Detecting the overexpression of this receptor is important for cancer cells identification and to potentially guide the choice of treatment since several FRα-targeted drugs are currently in clinical trials. In this work, we built SERS nanotags based on core@shell Au@Ag nanoparticles labelled with resonant Raman-reporter and functionalised with a thiolated PEG linker bearing folic acid at the chain end. Using SERS mapping on single cells, we showed that the nanotags (FR-nanotags) could specifically target FRα on overexpressing HeLa cells and could measure the gradual blocking of FRα by free folic acid introduced in the media along the nanotags. With a control nanotag, we showed that the SERS response was 10-fold higher on HeLa cells when folic acid is present on the PEG linker compared to PEG chains without folic acid. Non-specific binding of the FR-nanotags was demonstrated to be low and mainly caused by the folic acid molecule at the PEG chain end. When comparing cancer cells with different expression levels of FRα, we obtained 4-fold higher SERS response on overexpressing HeLa cells compared to non-overexpressing A549 cells, allowing the discrimination of both cell lines with a high contrast. Owing to the biocompatibility of the developed nanotags, we demonstrated that measurements of FRα on live HeLa cells were also possible and gave similar results to measurements on fixed cells, indicating the versatility of the developed nanotags for detecting FRα under various experimental conditions.