Suppressing infiltration and coffee-ring effects of colorimetric reagents on paper for trace-level detection of Ni (II)

Abstract

Abstract PVA matrix was induced to suppress infiltration and coffee-ring effects during the colorimetric analysis on a paper substrate and was applied to improve the trace-level detection of nickel ions (Ni2+) in environmental samples. To improve the color response, cross-linked PVA matrix was used to anchor the indicator reagent of Ni2+ on the surface of the porous paper substrate as well as reduce evaporation flow with increasing hydrogen bonding in the sample droplet. These phenomena mitigated the loss of color signal by suppression of the infiltration and coffee-ring effects. Under an optimized condition — in terms of crosslinker's concentration and addition order, type of commercial filter paper, and loading volume of the PVA/indicator mixture — the sensor obtained a limit of detection (LOD) as low as 0.92 ppm and a notable linear behavior of R2>0.97 at a linear range of 0.5-50 ppm which is sufficient for the detection of legal maximum residue limit (MRL) of Ni2+ in wastewaters in China. The PVA-assisted sensor showed good selectivity to most metal ions and could normally work at different pH levels ranging from 3 to 9. The sensor also possessed highly sensitive and stable performance at a normalized level in repetitive measurements and a shelf-life of more than 3 months. These results suggest that our work can provide a facile approach to boost the sensitivity and reliability of paper-based sensor to monitor Ni2+ concentration at trace level in aqueous samples with good selectivity and prolonged shelf-life.