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

A PVA matrix was used to suppress infiltration and coffee-ring effects during colorimetric analysis on a paper substrate and improve the trace-level detection of nickel ions (Ni2+) in environmental samples. To improve the color response, a 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 by increasing hydrogen bonding in the sample droplet. These phenomena mitigated the loss of color signal by suppressing infiltration and coffee-ring effects. Under optimized conditions, including crosslinker 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 R-2 > 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 work at different pH levels ranging from 3 to 9. The sensor also exhibited highly sensitive and stable performance in repetitive measurements and had a shelf-life of more than 3 months. These results suggest that our work provides a facile approach to improving the sensitivity and reliability of paper-based sensors for monitoring trace-level Ni2+ concentration in aqueous samples with good selectivity and prolonged shelf-life.