Heavy metal pollution is one of the more serious environmental problems; therefore, there is a constant demand for the development of new analytical tools for its monitoring. An optical chemical sensor represents a good alternative to classical instrumental methods. The mesoporous materials used in optical chemical sensors' fabrications have properties such as high porosity, exceptional adsorption capacity, tuneable 3D shape, geometry, and morphology, which enable improved limit of detection, response time, and selectivity properties of optical sensors. In this review, we firstly present the properties of mesoporous materials, provide a brief description of sensing mechanisms, and briefly discuss the importance of continuous monitoring. Recent advances in those mesoporous silica-based optical sensors used for heavy metal detection have been reported and their advantages and limitations also discussed. This review covers publications that have appeared since 2008.

In this work we demonstrate the efficient immobilization of histidine 6-tagged organophosphate hydrolase (His6-OPH), an organophosphate-degrading enzyme, on mesoporous titania thin films. This permits the use of the biocatalyst films as efficient tools in the detection/detoxification of paraoxon. His6-OPH was immobilized on mesoporous thin films with uniform (9 nm) and bimodal (13-38 nm) pore size distribution, through covalent attachment and physical adsorption. The biocatalyst films show good activity, and enhanced stability with respect to the free enzyme at extreme conditions of pH and temperature, especially around neutral pH and room temperature. In addition, the bioactive films can be easily separated from the reaction media and reused multiple times without significant loss of activity.

In this work, we report on the development of a bio-sensing film for the detection of organophosphorous compounds using sol-gel technology. A novel sol-gel immobilization method employing tetraethoxysilane/3-glycidoxypropyltrimethoxysilane/water hybrid material was developed and used to immobilize the hexahistidine-tagged organophosphorous hydrolase enzyme (His(6)-OPH). Bio-sensing layers with encapsulated His(6)-OPH of various structures (water/silane, precursor ratios) have been prepared. The optimal (P = 5:1, R = 188) bio-sensing layers retained 90% of the initial enzyme activity. Furthermore, the bio-sensing layer prepared by this method was able to maintain its activity at or above 80% of its initial activity for 2 weeks. The bio-hybrid film also showed excellent reusability and improved activity at neutral pH in comparison to the same enzyme in solution.

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