A Colorimetric Dip Strip Assay for Detection of Low Concentrations of Phosphate in Seawater

Abstract

Nutrient pollution remains one of the greatest threats to water quality and imposes numerous public health and ecological concerns. Phosphate, the most common form of phosphorus, is one of the key nutrients necessary for plant growth. However, phosphate concentration in water should be carefully monitored for environmental protection requirements. Hence, an easy-to-use, field-deployable, and reliable device is needed to measure phosphate concentrations in the field. In this study, an inexpensive dip strip is developed for the detection of low concentrations of phosphate in water and seawater. In this device, ascorbic acid/antimony reagent was dried on blotting paper, which served as the detection zone, and was followed by a wet chemistry protocol using the molybdenum method. Ammonium molybdate and sulfuric acid were separately stored in liquid form to significantly improve the lifetime of the device and enhance the reproducibility of its performance. The device was tested with deionized water and Sargasso Sea seawater. The limits of detection and quantification for the optimized device using a desktop scanner were 0.134 ppm and 0.472 ppm for phosphate in water and 0.438 ppm and 1.961 ppm in seawater, respectively. The use of the portable infrared lightbox previously developed at our lab improved the limits of detection and quantification by a factor of three and were 0.156 ppm and 0.769 ppm for the Sargasso Sea seawater. The device’s shelf life, storage conditions, and limit of detection are superior to what was previously reported for the paper-based phosphate detection devices.

Review—Solid State Sensors for Phosphate Detection in Environmental and Medical Diagnostics

“…9 These systems are large and expensive to purchase, need on-site reagents and waste storage systems, and only work within a narrow detection range. A few attempts have been made to use a solid-state colorimetric sensor using paper based devices [10][11][12]13 (Table I). A filter paper based device was fabricated with two regions to maximize the stability of the reagents used in the assay (molybdate/antimony tartrate and ascorbic acid).…”

The Use of Bi-Potentiostat as a Simple and Accurate Electrochemical Approach for the Determination of Orthophosphate in Seawater

“…The product complex is characterized by two spectral peaks at 720 nm and 880 nm, with a higher intensity at 880 nm, and the complex is stable for several hours. The Molybdenum Blue method is limited by the stability of ascorbic acid, whose lifetime under dark conditions is only two months [ 6 ].…”

Portable Colorimetric Hydrogel Test Kits and On-Mobile Digital Image Colorimetry for On-Site Determination of Nutrients in Water

“…Various reductants have been reported, such as titanium chloride, hydrazine, cadmium [ 13 ], and zinc powder, an environmentally friendly, cost-effective reductant [ 7 , 11 ]. For the molybdenum blue-based method for phosphate detection, ammonium molybdate reacts with orthophosphate under strong acidic conditions to form a Keggin ion and then is reduced by ascorbic acid to generate a molybdenum blue complex [ 14 , 15 ]. Potassium antimony tartrate is commonly used as a source of antimony to increase the reaction rate and eliminate the need for a heating process to form a stable molybdenum blue product [ 15 , 16 ].…”

“…For the molybdenum blue-based method for phosphate detection, ammonium molybdate reacts with orthophosphate under strong acidic conditions to form a Keggin ion and then is reduced by ascorbic acid to generate a molybdenum blue complex [ 14 , 15 ]. Potassium antimony tartrate is commonly used as a source of antimony to increase the reaction rate and eliminate the need for a heating process to form a stable molybdenum blue product [ 15 , 16 ]. Polymeric test kits involving the use of polymers to stabilize and/or support colorimetric reagents have also been reported for the detection of these nutrients [ 11 , 17 , 18 , 19 , 20 ].…”