Towards carbonate detection with a distance-based paper analytical sensor Zakia Tebetyo 1 , Samantha Richardson 1 , Leigh Madden 2 , Mark Lorch 1 , Nicole Pamme 1,3 1 University of Hull, UK, 2 Centre for Biomedicine, University of Hull, UK 3 Department of Materials and Environmental Chemistry, Stockholm University, Sweden Monitoring the concentration of carbonate levels enables prediction of the levels of acidification in aquatic eco system [1] . Measurement of carbonate levels is typically carried out using acid-base titrations that are laboratory- based, time consuming and require skilled personnel [2] . Here we have transferred the readout chemistry onto a distance-based paper analytical device (dPAD) that could be used on site by lower skilled persons. Hydrophobic wax barriers were printed onto filter paper to create hydrophilic regions, i.e., a reaction channel onto which titration reagents were deposited and a circular base for sample addition. The wicking of the sodium carbonate solution (60 µL) from the circular base to the reaction channel led to a colour change from yellow to blue. The optimised detection reagent concentrations were citric acid/ citrate buffer (0.5 M, pH 2.5), bromocresol green (BCG) indicator (0.10 %w/v) and polydiallyldimethylammonium chloride (1.0% v/v) dissolved in 20 % ethanol. Reaction channel dimensions of 1 mm x 30 mm as well as 2 mm x 30 mm were investigated with the wider channel chosen for further experiments as it gave a more distinct yellow to blue colour change. This therefore enabled a higher visual acuity since measurements were obtained manually. Two filter papers, Whatman 4 with pore size 23 µm and Whatman 1 with pore size 11 µm were then compared. The Whatman 4 was chosen for further experiments as it gave a faster wicking rate of 7 min compared to 42 min on the Whatman 1. A calibration curve was constructed at carbonate concentrations between 0 – 1000 mg L -1 . A linear response was observed between 0 – 100 mg L -1 . Repeatability tests were carried out at three different times in one day with 120 mg L -1 of CO 3 2- (n = 12) giving a maximum relative standard deviation of 1.84 %. Interference tests were subsequently conducted with 500 mg L -1 CO 3 2- containing either 250 mg L -1 SO 4 2- , 250 mg L -1 Cl - or 50 mg L -1 of NO 3 - . Distance signals were measured relative to 500 mg L -1 CO 3 2- and were found to be within % RSD of 2.7% of the 500 mg L -1 CO 3 2- distance thus showing minimum interference. Tests showed that the devices can be used for at least 21 days when kept in dark conditions. Future work will involve benchmarking with real environmental samples and in-the-field use by minimally trained personnel. References 1. W. J. Cai et al. , "Natural and Anthropogenic Drivers of Acidification in Large Estuaries," Ann Rev Mar Sci, 2021 , 13, 23-55. 2. L. He, C. Liu, and J. H. Xin, "A novel turn-on colorimetric and fluorescent sensor for Fe 3+ and Al 3+ with solvent-dependent
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