Alkyne-DNA-Functionalized Alloyed Au/Ag Nanospheres for Ratiometric Surface-Enhanced Raman Scattering Imaging Assay of Endonuclease Activity in Live Cells

A novel ratiometric surface-enhanced Raman scattering (SERS) nanosensor has been developed to probe the activity of endonuclease under in vitro and in living cells conditions. The optimized alloyed Au/Ag nanoparticles (NPs) were synthesized as the SERS substrate, which combined the superior properties of both pure Au and pure Ag nanoparticles: they exhibit excellent plasmonic property with high chemical stability and low cytotoxicity. They were then employed for quantitative detection of endonuclease through functionalization with single-stranded DNA (ssDNA) carrying 3-[4-(phenylethynyl)benzylthio]propanoic acid (PEB) as endonuclease-responsive SERS signaling molecule and 4-thiophenylacetylene (TPA) as the internal standard SERS signaling molecule. In the presence of endonuclease, the ssDNA was cleaved, releasing PEB molecules from the particle surface and decreasing the SERS signal at 2215 cm^-1 from PEB. Since the SERS signal at 1983 cm^-1 from alkynyl TPA remained the same, quantitative detection of endonuclease was achieved, based on the ratiometric peak intensity of I₁₉₈₃/I₂₂₁₅, with a detection limit as low as 0.056 unit/mL. A highly biocompatible and antijamming ratiometric SERS sensor was established by combining the alloyed Au/AgNPs with two unique alkynes molecules with Raman signals in the cellular silent region. The ratiometric sensor was successfully employed to detect intracellular endonuclease activity as well as endonuclease in living cells for the first time.