Molecularly imprinted polymer ZnO nanostructures derived from Pseuderanthemum palatiferum leaf extract for paper-based colorimetric sensing of chlortetracycline.

Chlortetracycline (CTC) is a widely used tetracycline antibiotic in veterinary medicine, livestock production, and aquaculture, yet its excessive use and environmental persistence have raised increasing concerns regarding food safety, ecological contamination, and antibiotic resistance. In this study, we developed a portable and selective colorimetric sensing platform based on molecularly imprinted ZnO nanozymes integrated into a paper-based analytical device (PAD) for rapid on-site detection of CTC. ZnO nanoparticles (ZnO NPs) were synthesized via a green route using Pseuderanthemum palatiferum (P. palatiferum) leaf extract and subsequently surface-imprinted using 3-aminopropyltriethoxysilane (APTES) and tetraethyl orthosilicate (TEOS) in the presence of CTC as the template molecule. Structural characterization by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) confirmed the successful formation of the imprinted polymer layer while preserving the hexagonal wurtzite crystalline structure of ZnO. The resulting molecularly imprinted polymer-modified ZnO (MIP@ZnO) retained intrinsic peroxidase-like activity and efficiently catalyzed the H2O2-mediated oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), generating a blue oxidized product with a characteristic absorbance at 652 nm. Upon selective rebinding of CTC to the imprinted cavities, the catalytic activity was suppressed through a binding-induced inhibition mechanism, resulting in a concentration-dependent decrease in absorbance. Under optimized conditions, the sensor exhibited a linear response over the range of 0-15 µM with a limit of detection (LOD) of 0.35 µM. Integration into a capillary-driven PAD enabled rapid visual analysis within minutes with minimal reagent consumption and no need for complex instrumentation. The platform also demonstrated good selectivity against structurally related antibiotics and common interfering species. Practical applicability was verified using tap water and milk samples, affording satisfactory recoveries and acceptable precision without extensive sample pretreatment. These results demonstrate that the proposed MIP@ZnO-based PAD provides a low-cost, field-deployable, and effective strategy for selective monitoring of CTC residues in complex real samples.