Indirect heavy metals determination in water samples based on alkaline phosphatase inhibition using a novel modified surface by laser-induced fluorescence detection

Abstract

During the last few years, the need for fast, sensitive, specific, automated and in situ detection methodologies has led to the development of easy-to-use and inexpensive devices. Paper has attracted great interest as a potential support material for sensors in analytical chemistry and clinical chemistry due to its versatility, abundance and low cost. Its fibrous and porous structure generates a high surface-to-volume ratio that increases immobilization capability compared to traditional materials for the fabrication of sensor devices. Considering these advantages, the development of Paper-based Analytical Devices (PADs) has had a great growth. This work is based on the modification of paper through the impregnation of porous materials that provide greater advantages. In this sense, Metal-Organic-Frameworks (MOFs) are one of the new materials with major advantages that have been synthesized in the last decades. Among them, we can mention their defined crystalline structure, simple synthesis and high capacity to increase the surface area trough functionalization with reactive groups. The problem of certain metal ions, such as copper, mercury and lead, is growing, as their detection corroborates environmental contamination. Various sources of water samples are of great interest in this study, as multiple life forms depend on the toxicity found in water. Consequently, one of the main objectives in water quality monitoring is the determination of metal concentration. For this purpose, a paper biosensor modified with UiO-66-NH2 was developed. This support was characterized by several techniques to determine the crystalline structure, morphology and different immobilizations on it (SEM, EDS, XRD, FTIR). Metals were indirectly determined by measuring the enzymatic response of the enzyme alkaline phosphatase (ALP), because the activity of this enzyme decreases in the presence of the mentioned metal ions (Cu2+, Hg2+ and Pb2+). Serial decreasing concentration solutions of the metals under study were evaluated, starting from a solution of 2 mg mL-1. The evaluation of the enzymatic response was performed by measuring the fluorescence generated by the activity of alkaline phosphatase (FAL) against its substrate 4-methylumbelliferyl phosphate (MUP), which is converted into methylumbelliferone. Detection was performed by laser-induced fluorescence using a solid-state laser (λ excitation 430 nm, λ emission 458 nm). The optimum concentration of FAL was 0.005 mg mL-1 in DEA buffer 0.1 M pH 8. A decrease in its fluorescent response was found to be proportional to the increase in the metal ion solutions concentrations.