3D-printed and fully portable fluorescent-based platform for sulfide determination in waters combining vapor generation extraction and digital images treatment.

The determination of sulfide anion in a variety of waters (e.g. wastewaters and natural waters) even at low concentration (i.e. in the μM range) is essential due to its high toxicity, corrosivity and unpleasant smelling proprieties. Despite several methodologies are dedicated to aqueous sulfide determination, most of them need sampling/transport steps - which is no adequate to sulfide due to its reactivity and instability - resulting in critical analytical bias. In this study, we present a fully modular and portable 3D-printed platform for in-situ aqueous sulfide determination. The analytical device is based on HS vapor generation from the sulfide sample solution by addition of HPO followed by collection in a miniaturized cuvette (μCuvette) containing few microliters of Fluorescein Mercury Acetate (FMA), a fluorescent dye. The chemical reaction results in fluorescence quenching of the dye at 530 nm when excited at 470 nm. A light-emitted diode (LED) emitting at 470 nm and powered with 9 V-battery based circuitry was employed to provide stable excitation light source at 20 mA. Digital images from the light emitted by FMA were captured by a smartphone and the Green channel intensity was used as analytical signal. Under optimized conditions, a linear relation (r > 0.99) from 0.1 to 5 μM of sulfide was obtained using 10 mL of standard/sample solution. The portable platform was applied to the in-situ monitoring of sulfide in tap water and river water with no loss of analyte, no need for external power supplies or powered pumps. and the analysis results were obtained in 20 min. The proposed device shows advantages in terms of high degree of portability, low-power consumption, easiness to use, minimal use of reagents yet enabling on-site determination of sulfide with high sensitivity. By using the vapor generation approach combined with the modular building blocks concept presented herein for the first time, we anticipate the development of a tailored "plug-and-play" platform enabling the multiplexed determination of volatile substances using absorbance, reflectance or fluorescence measurements with smartphones.