Nano-engineered electrochemical aptasensors for label-free detection of cryptosporidium, cadmium and arsenic in water

Abstract

Water is an essential resource for human survival, agriculture, and livestock. However, over the past thirty years, the World Health Organization has reported growing concerns about the impact of environmental pollution on water quality. Water quality is deteriorating due to contamination from microbes and heavy metals. Among the major microbial and potential heavy metals in water are Cryptosporidium (Crypto), cadmium (Cd2+) and arsenic. Crypto is an intestinal protozoan parasite that has become a significant cause of cryptosporidiosis, a gastrointestinal disease that can affect healthy adults and may be fatal for children and individuals with weakened immune systems. In contrast, Cd2+ and arsenic are amongst the most toxic and harmful metal ions found in the environment. These metals are highly mobile and can accumulate and spread throughout ecosystems. When ingested, they cause various health issues, including cardiovascular diseases, acute poisoning and cancer. These contaminants pose serious risks to aquatic species and the ecosystem at large. Early diagnostic methods for detecting Crypto, Cd2+, and arsenic were developed using microscopy, molecular, and spectroscopic techniques. However, these methods often yield false-negative results, are time-consuming, lack sensitivity and specificity, and have low detection limits. Crypto, arsenic, and Cd2+ pose a challenge to delivering safe drinking water due to their low concentrations in large volumes. Consequently, there is a need to develop portable, sensitive, and selective methods for detecting Crypto, arsenic, and Cd2+ at trace levels. This work develops a novel carbon quantum dot titanium dioxide (CQD-TiO2), Mil101(Fe)-CQD-TiO2 based-aptasensor platform capable of label-free simultaneous detection of Crypto and heavy metals at trace levels in phosphate buffer solutions and real water samples. The electrocatalysts used in this work were synthesized using precipitation and hydrothermal methods. Various characterization techniques, such as High-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), were employed to confirm the structural and morphological properties of the synthesized materials. The electrochemical properties of the modified electrodes were studied using cyclic voltammetry (CV), Electrochemical impedance spectroscopy (EIS), square-wave voltammetry (SWV), and Chronopotentiometry (CP), revealing enhanced reaction kinetics and improved stability. The use of various electrocatalysts as aptamer vehicles on the electrode surface has significantly improved the performance of aptasensors, resulting in greater selectivity, higher accuracy, and lower detection limits. As a result, a CQD-TiO2-based aptasensor platform was developed for detecting Crypto, achieving a detection limit of 0.0024 ng L-1, and a sensitivity of 0.27 mA μM-1. In another study, an electrochemical aptasensor platform based on Mil101(Fe)-CQD-TiO2 ternary composite achieved a detection limit of 0.001 ng L-1 for Crypto with a sensitivity of 0.529 mA μM-1. The aptasensor demonstrated excellent performance in detecting Cd2+ and arsenic, achieving low detection limits of 0.073 ng L-1 for Cd2+ and 0.092 ng L-1 for arsenic, with sensitivities of 0.127 mA μM-1 and 0.0065 mA μM-1. All developed aptasensor platforms demonstrated limits of detection within the limits reported in the literature. Thus, GCE-Mil101(Fe)-CQD-TiO2-Apt-BSA platform showed a low limit of detection, demonstrating high sensitivities and selectivity compared to conventional techniques. The aptasensor platforms showed acceptable recovery rates when tested with real water samples and demonstrated good stability, reproducibility, and selectivity. These aptasensors have significant potential for integration with microfluidic and on-chip technology, enabling the early detection of pathogens and trace metals.