In situ ratiometric SERS imaging of intracellular protease activity for subtype discrimination of human breast cancer

Accurate discrimination between different cells at the molecular level is of fundamental importance for disease diagnosis. Endogenous proteases are such molecular candidates for cancer cell subtype study. But in situ probing their activity in live cells remains challenging for surface-enhanced Raman scattering (SERS). Here, we present a sensitive ratio-type SERS nanoprobe for imaging of matrix metalloproteinase-2 (MMP-2) in different cancer cells subtypes. The nanoprobe contained three components: a plasmon-active gold nanoparticle as the SERS enhancing matrix, Raman dye rhodamine B (Rh B)-labelled substrate peptides as the specific MMP-2 recognizer, and 2-naphthalenethiol (2-NT) as the internal standard. MMP-2-responsive cleavage of peptides from the nanoprobe surface results in decrease or even disappearance of SERS emission of Rh B, which was ratioed over the emission of 2-NT for the quantification of MMP-2 activity. Both in-tube assay and in-cell imaging results show that the MMP-responsive nanoprobe can work and serve to differentiate the normal breast cells from the tumorous ones, to differentiate two breast cancer cell subtypes with a different degree of malignancy. We believe that this SERS nanoprobe could find a wide application in the fields of tumor biology and accurate disease diagnosis.