Among proteases, matrix metalloproteinases (MMPs) have been of significant interest because they are considered as one of the promising biomarkers in association with cancer metastasis, inflammation and other degenerative diseases. Many attempts based on the optical sensing have been made to analyze the activity of MMPs, but most of them require an expensive fluorescence readout and a labor-intensive process. To circumvent this issue, we demonstrated a simple colorimetric detection of protease activity by using carboxy gold nanoparticles (AuNPs) and histidine-containing peptides via metal-affinity coordination. Due to their higher surface-to-volume ratio, the nanometer size of AuNPs enables the surface ligands to function like a chelator, providing greater affinity with metal ions, even in the absence of chelators. With no additional modification by multidentate ligands, the carboxy AuNPs were easily aggregated and changed in color (from reddish-brown to violet) after adding peptide substrates with hexahistidine at both ends and metal ions, whereas the presence of proteases in solution prevented NP aggregation by cleaving the peptides, thereby retaining the original color of the AuNPs. When the extinction ratio (E(520)/E(700)) of the AuNP solution was measured as a function of matrix metalloproteinase concentration in a single reaction, there was good linearity from as low as 3 nM to 52 nM. This approach is anticipated to be useful in designing other diagnostic nanosensors.
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