Serine-Modified Au@Cu2O Core–Shell Nanoparticles for Catalysis-Mediated Colorimetric Detection of Small Molecules

3,3′,5,5′-Tetramethylbenzidine (TMB)-based catalysis-mediated colorimetric analysis for the detection of small molecules has poor specificity. In this study, well-prepared Au@Cu2O core–shell nanocatalysts (NCs) were fabricated that have excellent peroxidase-like activity and can efficiently catalyze the oxidation of TMB (colorless) to form oxidized TMB (oxTMB, blue). Moreover, serine was successfully coordinated on the Cu2O shell as a probe that can conjugate with ketone bodies, forming serine-modified Au@Cu2O nanoparticles (NPs) with exposed 1° amine and hydroxyl groups. These exposed groups enable specific intermolecular pairings with β-hydroxybutyric acid (βHBA) and volatile acetone. The click-chemical conjugations of serine−βHBA (amide bonding) and serine–acetone (Schiff base) were verified through X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and nuclear magnetic resonance analyses. Analyte capping on the surface of NCs can efficiently reduce their catalytic activity, thereby achieving ketone-body-concentration-dependent TMB colorimetry that has high sensitivity and a wide linear dynamic range of >4 logs for the ketone body system. Furthermore, the excellent specificity of the serine-modified Au@Cu2O NCs was demonstrated, proving the concept of alternative click-chemistry-promoted colorimetric analysis and opening an avenue to the development of advanced biosensors for small-molecule detection.