A portable optical sensor combining smartphone with phycocyanin-based fluorescent test paper for rapid, visual and on-site detection of CO.

With the development of global industry, carbon dioxide emissions surged. The conversion of carbon dioxide from the air results in some CO, which can exacerbate environmental disasters like ocean acidification. Therefore, the content of CO in seawater is an important indicator of the degree of ocean acidification. In this study, natural fluorescent protein phycocyanin (PC) was used as a fluorescent probe, and a fluorescence detection method was established for quantitative monitoring of CO with quick response time (within 50 s), high sensitivity, and selectivity. The fluorescence quenching phenomenon between PC and CO was mainly attributed to static quenching. The limit of detection (LOD) was 0.42 μM and the method was successfully applied to monitor CO in tap water and seawater, acquiring satisfactory recovery between 99.28 % and 106.40 %. More importantly, paper-based test strips were easily fabricated using PC, enabling the rapid, visual, and on-site detection of CO with the aid of a smartphone. The visual detection integrated with the smartphone was converted to data information (RGB value) through a Color Picker APP and successfully used for quantitative identification of CO. By capturing fluorescent images and analyzing the corresponding RGB value via a smartphone, the linear calibration ranged from 0.5 μM to 500.0 μM with LOD of 0.11 μM was obtained. Satisfactory recoveries were acquired in tap water (98.00 %-107.50 %) and seawater (97.30 %-101.74 %), respectively. Therefore, integrating the PC fluorescent paper with a smartphone realizes the rapid, visual, and on-site detection of CO in the water environment, which is expected to broaden application prospects of monitoring ocean acidification degree.