Using a microfluidic paper-based analytical device and solid-phase extraction to determine phosphate concentration

Phosphate is an essential nutrient, but in high concentrations it contributes to water pollution. Traditional methods for phosphate measurement, such as absorption spectrophotometry and ion chromatography, require expensive equipment and skilled operators. This study introduces a microfluidic paper-based analytical device (μPAD) that is designed to accomplish field-based, low-concentration phosphate measurements. This μPAD utilizes colorimetric detection based on the molybdenum blue method. Herein, we describe how the conditions were optimized in terms of design and sensitivity by adjusting reagent concentrations, paper thickness, and the time frames for sample introduction, and reaction. The operation consists of simply dipping the μPAD into a sample, capturing images in a home-made photo studio box, and processing the images with ImageJ software to measure RGB intensity. An additional preconcentration step involves solid-phase extraction with an anion exchange resin that achieves a 10-fold enrichment, which enables detection that ranges from 0.05 to 1 mg L^-1 with a detection limit of 0.089 mg L^-1 and a quantification limit of 0.269 mg L^-1. The replicated measurements showed good reproducibility both intraday and interday (five different days) as 4.7 % and 3.0 % of relative standard deviations, respectively. After storage in a refrigerator for as long as 26 days, this μPAD delivered stable and accurate results for real-world samples of natural water, soil, and toothpaste. The results produced using this system correlate well with those produced via spectrophotometry. This μPAD-based method is a cost-effective, portable, rapid, and simple approach that allows relatively unskilled operators to monitor phosphate concentrations in field applications.