Low-Temperature Base-Catalyzed Si-Capped ZnO Quantum Dots for pH-Switchable Dual Sensing of Amoxicillin and Cr³⁺

Amoxicillin (AMX), one of the most commonly used antibiotic drugs, can cause overwhelming hazards if abused, particularly when it contaminates the environment and affects individuals. As a result, devising an easy and accurate method for estimating AMX content is vital. In this work, we describe a turn off/on detection method for Cr3+ and AMX using a smart pH-switchable ZnO quantum dot (QD) nanoprobe. The ZnO QDs were synthesized using tetraethyl orthosilicate (TEOS) as a capping agent through a simple low-temperature base-hydrolysis reaction. The TEOS-capped ZnO QDs exhibited strong fluorescence with a peak at 520 nm under excitation at 330 nm. Numerous methods, such as energy-dispersive X-ray (EDX), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), UV–vis absorption, and UV–vis reflectance, have been used to demonstrate the successful synthesis of TEOS-capped ZnO QDs. The lowest excited state exhibits selective fluorescence quenching and enhancement upon the addition of Cr3+ and AMX, respectively, in the presence of other interferences at pH 7 and pH 1 respectively (both in liquid and solid-state PL). The estimated LOD of 15.94 nM (in case of Cr3+) and 77.86 nM (in case of AMX) was achieved. The recovery of various concentrations of AMX in real samples ranged from 95% to 101.1%, with a relative standard deviation of 0.05–1% (n = 3). This work harnesses the specific and excellent performance of TEOS-capped ZnO QDs, contributing to further progress in the analytical and materials research fields. There is ample scope for using this nanoprobe in the fabrication of both pH sensors (owing to its pH-switching ability) and capacitive sensors (owing to the semiconducting nature of ZnO).