DNA
methyltransferases may function as important biomarkers of
cancers and genetic diseases. Herein, we develop a dye-sensitized
and gold plasmon-enhanced cathodic photoelectrochemical (PEC) biosensor
on the basis of p-type covalent organic polymers (COPs) for the signal-on
measurement of M.SssI methyltransferase (M.SssI MTase). The cathodic
PEC biosensor is constructed by the in situ growth
of p-type COP films onto a glass coated with indium tin oxide and
the subsequent assembly of biotin- and HS-labeled double-stranded
DNA (dsDNA) probes onto the COP film via biotin–streptavidin
interaction. The dsDNA probe contains the recognition sequence of
M.SssI MTase. The COP thin films possess a porous ultrathin nanosheet
structure with abundant active sites, facilitating the generation
of a high photocurrent compared with the hydrothermally synthesized
ones. The presence of DNA methyltransferases can prevent the digestion
of restriction endonuclease HpaII, consequently inducing the introduction
of gold nanoparticles (AuNPs) to the dsDNA probes via the S–Au bond and the intercalation of rhodamine B (RhB)
into the DNA grooves to produce a high photocurrent due to the dye-photosensitized
enhancement and AuNP-mediated surface plasmon resonance. However,
in the absence of M.SssI MTase, HpaII digests the dsDNA probes, and
neither AuNPs nor RhB can be introduced onto the electrode surface,
leading to a low photocurrent. This cathodic PEC biosensor possesses
high sensitivity and good selectivity, and it can screen the inhibitors
and detect M.SssI MTase in serum as well.