Colorimetric bioassay using the catalytic ester hydrolysis by esterase-like Cu2+

This paper reports an artificial enzyme-based assay using (i) rapid and selective electroless Cu deposition on DNA- and antibody-conjugated Au-nanoparticle (NP) labels and (ii) rapid catalytic hydrolysis of p-nitrophenyl picolinate (pNPP) by esterase-like Cu2+. Rapid and selective Cu deposition is achieved by the electroless Cu deposition in the presence of formaldehyde (reducing agent) and tartrate (complexing agent). Unwanted Cu deposition on DNA and sensing surface is significantly suppressed in pH 12 at 45 °C. Rapid catalytic hydrolysis of pNPP to a colored product, p-nitrophenol, is achieved in acetate buffer (pH 6.0), which also minimizes self-hydrolysis of pNPP. The optimum conditions are applied to the sensitive colorimetric detection of DNA in buffer and troponin T in human serum. The calculated detection limits are ca. 1 pM and 100 pg/mL for DNA and troponin T, respectively.