Penetration-then-growth enables label-free surface-enhanced Raman spectroscopic discrimination of fibrotic cells and kidney tissues

Surface-enhanced Raman spectroscopy (SERS) is a highly efficient optical technique that employs metallic nanostructures to amplify the inherently weak Raman signals. Despite its great potential in obtaining fingerprint spectral information in complex biological samples, examining subtle molecular change in fixed cells/tissues by SERS is difficult because large-sized metal nanoparticle (NPs) can generate strong SERS signals but lack the ability to penetrate into the samples, whereas small-sized NPs can easily infiltrate samples but do not exhibit SERS activity. Herein, we propose a penetration-then-growth strategy for SERS-based detection of molecular change in the fibrogenic process. In the proof of concept study, control and fibrotic cells / mouse kidney tissue sections were initially incubated with ∼15 nm Au NPs which were permeable, and then the deposited NPs were grown up in situ to act as SERS substrates for Raman signal amplification. After NP growth, the Raman signals were dramatically enhanced, enabling fast discrimination of molecular changes occurred during fibrosis, which were indiscernible using conventional Raman spectrum or SERS detected with ∼50 nm Au NPs. This method is simple, fast, and can be readily adapted for inspecting molecular changes in other biological events.