The wide use of antibiotics in aquaculture has led to the emergence of resistant microbial species. It should be avoided/minimized by controlling the amount of drug employed in fish farming. For this purpose, the present work proposes test-strip papers aiming at the detection/semi-quantitative determination of organic drugs by visual comparison of color changes, in a similar analytical procedure to that of pH monitoring by universal pH paper. This is done by establishing suitable chemical changes upon cellulose, attributing the paper the ability to react with the organic drug and to produce a color change. Quantitative data is also enabled by taking a picture and applying a suitable mathematical treatment to the color coordinates given by the HSL system used by windows.As proof of concept, this approach was applied to oxytetracycline (OXY), one of the antibiotics frequently used in aquaculture. A bottom-up modification of paper was established, starting by the reaction of the glucose moieties on the paper with 3-triethoxysilylpropylamine (APTES). The so-formed amine layer allowed binding to a metal ion by coordination chemistry, while the metal ion reacted after with the drug to produce a colored compound. The most suitable metals to carry out such modification were selected by bulk studies, and the several stages of the paper modification were optimized to produce an intense color change against the concentration of the drug. The paper strips were applied to the analysis of spiked environmental water, allowing a quantitative determination for OXY concentra-tions as low as 30 ng/mL. In general, this work provided a simple, method to screen and discriminate tetracycline drugs, in aquaculture, being a promising tool for local, quick and cheap monitoring of drugs.

This work describes a novel sensing system using eggshells as substrate for the first time, targeting the detection and semiquantitative determination of antibiotics in waters from aquaculture, enabling simple, inexpensive, and in situ drug monitoring. Eggshell was ground and the resulting powder was modified by adsorption of suitable reagents, and it takes a typical colour after contact with the antibiotic. The colour intensity is correlated with the concentration of the antibiotic. This novel approach was applied to oxytetracycline, one of the antibiotics commonly used in aquaculture. The chemical changes on the eggshell powder were evaluated and optimised to produce an intense colour change as a function of the concentration of the antibiotic. The colour changes were evaluated by visual comparison with images taken with a digital camera, applying an appropriate mathematical treatment to the colour coordinates of the HSL system used by Windows. The selectivity of the response was tested against other antibiotic drugs. The materials were also used in the analysis of a spiked environmental water sample. Overall, this work presents a rapid, inexpensive, simple and equipment-free method for screening and discrimination of tetracycline drugs in aquaculture. The method is a green approach by reusing eggshells and decreasing the level of contamination correlated to analytical methods, thus being a promising tool for local, rapid, and cost-effective antibiotic monitoring.