A Paper-Based Device for Ultrasensitive, Colorimetric Phosphate Detection in Seawater

Citizen-Based Water Quality Monitoring: Field Testing a User-Friendly Sensor for Phosphate Detection in Global Surface Waters

Widespread concern over surface water pollution has led to interest in developing easy-to-use accurate tools for citizen-based measurements that provide high spatial and temporal resolution while maintaining accuracy. Excessive anthropogenic phosphate significantly contributes to global eutrophication and necessitates regular on-site phosphate monitoring in surface waters. Traditional instrumentation for quantifying phosphate is labor-intensive, expensive, and performed in laboratories. Existing on-site testing methods relying on phosphomolybdenum blue (PMB) have limited sensitivity and stability under ambient conditions. To overcome these limitations, a novel low-cost, rapid, and user-friendly sensor for citizen-led phosphate monitoring in surface water is introduced and demonstrated with a global sampling campaign. The fast-flow microfluidic device provides user-friendly operation, achieving an environmentally relevant limit of detection (LoD) of 190 ppb, which is near the EPA-recommended maximum for phosphate. The dip-and-read operation reduces procedural steps while delivering accurate sample volume, making it well-suited for citizen-led science initiatives. This sensor exhibits high selectivity and prolonged stability for two months under ambient conditions. The sensor's performance was validated using the industry-standard UV-Vis method with 90% correlation. More than 1000 sensors were deployed in different continents, facilitating phosphate mapping in diverse water sources across multiple continents. The initiative covered much of the globe, including Thailand, Nepal, Brazil, Chile, the USA, and Germany. In some cases, phosphate levels exceeded legislative guidelines by 100-fold. Through the collaboration of citizen scientists, we analyzed regional topography and socioeconomic practices near water sources, identifying potential sources that could contribute to eutrophication in these areas.

Determination of phosphate in food based on molybdenum yellow derivatization coupled with resonance Rayleigh scattering method

This paper proposed a rapid, selective and sensitive molybdenum yellow derivatization coupled with Resonance Rayleigh scattering (MYD-RRS) method for detection of phosphate. Under the acidic condition, phosphate can be selectively transformed to Keggin type of phosphomolybdic acid (PMA, i.e., PMo12O403-) through molybdenum yellow derivatization reaction prior to RRS detection. The PMA can further react with cationic methyl violet (MV) to form larger PMA-MV ion association complexes, generating significant RRS signal. The concentration of phosphate was linearly related to the RRS signal in the range of 8-200 ng/mL, with the determining coefficient (R2) of 0.9973 and the detection limit of 4 ng/mL. The analytical procedure can be completed within 10 min and the RRS signal intensity can remain stable more than 4 h. The method showed good stability toward temperature and time, and good anti-interference capability. The method was applied to the determination of phosphate in real food samples with the recovery of 85-117% and RSD of 1-5.2%. With the advantages of rapidness, high sensitivity and good selectivity, the MYD-RRS method exhibits great potential to the determination of phosphate in food. It also provides an instructive strategy for detection of analytes with weak RRS signal.

Shining light on fluoride detection: a comprehensive study exploring the potential of coumarin precursors as selective turn-on fluorescent chemosensors

In this study, we report a fluoride chemosensor based on the use of a non-fluorescent pre-coumarin, compound 1. This compound undergoes selective fluoride-triggered formation of coumarin 2, with a concomitant turn-on fluorescence signal. Although compound 1 exists as a mixture of alkene isomers (2 : 1 in favor of the E isomer), only the minor Z-isomer undergoes cyclization. Nonetheless, comprehensive computational and experimental studies provide evidence that in situ isomerization of E-1 to Z-1, followed by fluoride-triggered phenolate evolution and intramolecular cyclization, facilitates the generation of coumarin 2 in high yield. Moreover, this system is an effective turn-on fluorescence sensor for fluoride anions, which displays outstanding selectivity (limited response to other commonly occurring analytes), sensitivity (lowest reported limits of detection for this sensor class), and practicality (works in solution and on paper to generate both fluorometric and colorimetric responses). Ongoing efforts are focused on expanding this paradigm to other pre-coumarin scaffolds, which also undergo analyte-specific coumarin formation accompanied by turn-on fluorescence.

Semi-quantitative microfluidic paper-based analytical device for ionic silica detection

Silicate ions (SiO₃^2-), or ionic silica, are known to cause silica scaling in industrial water applications when excess amounts are present; hence, concentrations must be monitored and kept at a constant low level. Ionic silica is conventionally measured by spectrophotometry in the form of its silicomolybdic complex based on the molybdenum blue reaction, but the operation process is complicated and not suitable for on-site detection. To solve these issues, microfluidic paper-based analytical devices (µPADs) have been gaining attention as portable, low-cost analytical devices suitable for on-site detection. The foldable origami type device described in this work enabled silica detection based on the molybdenum blue reaction, in the range of 50-1000 mg/L, with a practically detectable lowest concentration of 50 mg/L. The device showed selectivity for silicate ions and stability over 21 days when stored at 4 °C. The semi-quantitative analytical performance makes the proposed paper-based device attractive for on-site industrial monitoring.

Colorimetric analysis based on solid-phase extraction with sedimentable dispersed particulates: demonstration of concept and application for on-site environmental water analysis

A novel simple and functional colorimetric methodology for on-site environmental water analysis was proposed. This method combines coloration of the analyte and extraction of the colored species on dispersed particulates during their sedimentation in the same container. The whole analysis can be performed within 15 min by comprising the addition of 1 mL of sample solution into a 1.5-mL microtube containing the powders of coloring reagents and the sedimentable fine particulates as an adsorbent. The analyte is determined by comparing the sediment color with the standard color by visual inspection or the color information of the photo image. The potential of this methodology was demonstrated through developing colorimetry for Fe²⁺ with o-phenanthroline, NO₂^- by azo-dye formation, HCHO by the MBTH method, and PO₄^3- by the 4-aminoantipyrine method based on the enzyme reactions. The material, size, amount of the adsorbent particles, and other conditions were optimized for each analytes. The advantages of the methodology were as follows: high sensitivity, easy controllability of the sensitivity over the wide range by the amount, size, and material of the particulates, lower interference from the colored matrix components due to obtaining the color data from not the aqueous phase but the sedimented particulates, and acceleration of the color development rate by the particulates as seen in NO₂^- determination as consequence shorten the operation time. A simple device equipped with twin cells was proposed for on-site analysis which contains two successive different coloring operations. The developed methods were successfully applied to the environmental water samples with the good agreement of the results with those by the usual instrumental methods.

Boron carbon oxynitride quantum dots-based ratio fluorescent nanoprobe assisted with smartphone for visualization detection of phosphate

A ratio fluorescence nanoprobe was constructed by simple mixing BCNO QDs with 8-hydroxyquinoline-5-sulfonic acid (HQSA), which had an obvious fluorescence peak at 420 nm and a weak fluorescence peak at 500 nm, corresponding to the BCNO QDs and HQSA, respectively. This fluorescence probe takes stable fluorescence of BCNO QDs as an internal standard, based on HQSA chelating enhanced fluorescence and specificity of phosphate in the presence of Mg²⁺, which realizes a rapid and sensitive detection of phosphate with good linearity in the range 0.3-50 μM and 50-100 μM and a detection limit of 0.073 μM. The recovery is between 94.1 and 111% and the relative standard deviations (RSDs) below 10%. At the same time, we took color photos of the reaction solution under 310-nm UV lamp with smartphones for visual detection through RGB data image analysis, which make the detection easier and faster. The proposed method provides a new strategy for the intelligent online detection of other targets in complex environment samples.

Low-Cost Resistive Microfluidic Salinity Sensor for High-Precision Detection of Drinking Water Salt Levels

Rapid, inexpensive, and precise water salinity testing remains indispensable in water quality monitoring applications. Despite many sensors and commercialized devices to monitor seawater salinity, salt detection and quantification at very low levels of drinking water (below 120 ppm) have been overlooked. In this paper, we report on optimization of a low-cost microfluidic sensor to measure water salinity in the range of 1-120 ppm. The proposed design employs two copper microbridge wires suspended orthogonally in a PDMS microchannel to measure salinity based on the electrical resistance between the wires. The preliminary design of the sensor microchannel with a rectangular cross-section width (w) of 900 μm and height (h) of 500 μm could measure the water salinity in the range of 1-20 ppm in less than 1 min with detection sensitivity, limit of detection (LOD), and limit of quantification (LOQ) of 17.1 ohm/ohm·cm_, 0.31 ppm, and 0.37 ppm, respectively. Data from the preliminary design was used for developing and validating a numerical model which was subsequently used for parametric studies and optimization to improve the sensor's performance. The optimized design demonstrated an order of magnitude increase in sensitivity (385 ohm/ohm·cm), a 6-fold wider detection range (1-120 ppm), and a 15-fold enhancement in miniaturization of the microfluidic channel (w = 200 μm and h = 150 μm) with LOD and LOQ of 0.39 and 0.44 ppm, respectively. In the future, the sensor can be integrated into a hand-held device to remove present impediments for low-cost and ubiquitous salinity surveillance of drinking water.

Organic Anion Detection with Functionalized SERS Substrates via Coupled Electrokinetic Preconcentration, Analyte Capture, and Charge Transfer

Detecting ultralow concentrations of anionic analytes in solution by surface-enhanced Raman spectroscopy (SERS) remains challenging due to their low affinity for SERS substrates. Two strategies were examined to enable in situ, liquid phase detection using 5(6)-carboxyfluorescein (5(6)-FAM) as a model analyte: functionalization of a gold nanopillar substrate with cationic cysteamine self-assembled monolayer (CA-SAM) and electrokinetic preconcentration (EP-SERS) with potentials ranging from 0 to +500 mV. The CA-SAM did not enable detection without an applied field, likely due to insufficient accumulation of 5(6)-FAM on the substrate surface limited by passive diffusion. 5(6)-FAM could only be reliably detected with an applied electric field with the charged molecules driven by electroconvection to the substrate surface and the SERS intensity following the Langmuir adsorption model. The obtained limits of detection (LODs) with an applied field were 97.5 and 6.4 nM on bare and CA-SAM substrates, respectively. For the CA-SAM substrates, both the ligand and analyte displayed an ∼15-fold signal enhancement with an applied field, revealing an additional enhancement due to charge-transfer resonance taking place between the metal and 5(6)-FAM that improved the LOD by an order of magnitude.

Improvement of On-Site Sensor for Simultaneous Determination of Phosphate, Silicic Acid, Nitrate plus Nitrite in Seawater

Accurate, on-site determinations of macronutrients (phosphate (PO43-), nitrate (NO3-), and silicic acid (H4SiO4)) in seawater in real time are essential to obtain information on their distribution, flux, and role in marine biogeochemical cycles. The development of robust sensors for long-term on-site analysis of macronutrients in seawater is a great challenge. Here, we present improvements of a commercial automated sensor for nutrients (including PO43-, H4SiO4, and NO2- plus NO3-), suitable for a variety of aquatic environments. The sensor uses the phosphomolybdate blue method for PO43-, the silicomolybdate blue method for H4SiO4 and the Griess reagent method for NO2-, modified with vanadium chloride as reducing agent for the determination of NO3-. Here, we report the optimization of analytical conditions, including reaction time for PO43- analysis, complexation time for H4SiO4 analysis, and analyte to reagent ratio for NO3- analysis. The instrument showed wide linear ranges, from 0.2 to 100 μM PO43-, between 0.2 and 100 μM H4SiO4, from 0.5 to 100 μM NO3-, and between 0.4 and 100 μM NO2-, with detection limits of 0.18 μM, 0.15 μM, 0.45 μM, and 0.35 μM for PO43-, H4SiO4, NO3-, and NO2-, respectively. The analyzer showed good precision with a relative standard deviation of 8.9% for PO43-, 4.8% for H4SiO4, and 7.4% for NO2- plus NO3- during routine analysis of certified reference materials (KANSO, Japan). The analyzer performed well in the field during a 46-day deployment on a pontoon in the Kiel Fjord (located in the southwestern Baltic Sea), with a water supply from a depth of 1 m. The system successfully collected 443, 440, and 409 on-site data points for PO43-, Σ(NO3- + NO2-), and H4SiO4, respectively. Time series data agreed well with data obtained from the analysis of discretely collected samples using standard reference laboratory procedures and showed clear correlations with key hydrographic parameters throughout the deployment period.

A simple epoxy resin screen-printed paper-based analytical device for detection of phosphate in soil

This study develops a simple and low-cost 3D paper-based analytical device (3D PAD) for the detection of available phosphate in soil. Epoxy resin is presented as a new hydrophobic material for low-cost mass production of PADs using the screen-printing method. An optimized concentration of epoxy resin solution is screen printed onto Whatman filter paper no. 1 in an easy one-step process to create hydrophobic patterns on PADs. The epoxy resin is air dried at room temperature, without heating or UV curing. This method delivers high reproducibility, resolution, and stability, and the epoxy resin barrier is compatible with both organic solvents and aqueous solutions. The molybdenum blue method is used in this PAD to measure phosphate in a colorimetric assay. The developed 3D PAD attains a linear range of 0.5-40 mg L^-1, with a limit of detection (LOD) of 0.25 mg L^-1, and a limit of quantitation (LOQ) of 0.83 mg L^-1. The relative standard deviation of intra-day measurements is 1.52-2.46%, and the inter-day standard deviation is 1.89-2.74%, indicating satisfactory reproducibility. This 3D PAD was tested for its ability to detect phosphate in a variety of actual soil samples and the results were validated against spectrophotometric analysis using a paired t-test, which showed high accuracy. In short, the new analytical device described in this study is simple, fast, and inexpensive to make and use, providing a versatile phosphate detection tool for many soil types, even in situations when resources are limited.

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

Ion chromatograph with three-dimensional printed absorbance detector for indirect ultraviolet absorbance detection of phosphate in effluent and natural waters

An ion chromatography system employing a low-cost three-dimensional printed absorbance detector for indirect ultraviolet detection towards portable phosphate analysis of environmental and industrial waters has been developed. The optical detection cell was fabricated using stereolithography three-dimensional printing of nanocomposite material. Chromatographic analysis and detection of phosphate were carried out using a CS5A 4 × 250 mm analytical column with indirect ultraviolet detection using a 255 nm light-emitting diode. Isocratic elution using a 0.6 mM potassium phthalate eluent combined with 1.44 mM sodium bicarbonate was employed at a flow rate of 0.75 ml/min. A linear calibration range of 0.5 to 30 mg/L PO₄ ^3- applicable to environmental and wastewater analysis was achieved. For retention time and peak area repeatability, relative standard deviation values were 0.68% and 4.09%, respectively. Environmental and wastewater samples were analyzed with the optimized ion chromatography platform and the results were compared to values obtained by an accredited ion chromatograph. For the analysis of environmental samples, relative errors of <14 % were achieved. Recovery analysis was also carried out on both freshwater and wastewater samples and recovery results were within the acceptable range for water analysis using standard ion chromatography methods.

Amperometric Environmental Phosphate Sensors

We report a novel amperometric sensor for aqueous phosphate ions in freshwater systems based on the reductive square wave voltammetry of molybdate(VI) anions immobilized within a chitosan matrix deposited on a glassy carbon electrode. A sensitivity of 4.4 ± 0.1 μA/μM was realized together with a LOD of 0.15 μM. The sensor was insensitive to chloride and nitrate ions below a threshold concentration of 1.0 mM. Analytical measurements were successfully made in authentic samples of tap and pond water.

Recent advances of environmental pollutants detection via paper-based sensing strategy

Paper has become one of the most promising substrates for building low-cost and powerful sensing platforms due to its self-pumping ability and compatibility with multiple patterning methods. Paper-based sensors have been greatly developed in the field of environmental monitoring. In this review, we introduced the research and application of paper-based sensors in environmental monitoring, focusing on the deposition and patterning methods of building paper-based sensors, and summarized the applications of detecting environmental pollutants, including metal ions, anions, explosives, neurotoxins, volatile organic compounds, and small molecules. In addition, the development prospects and challenges of promoting paper-based sensors are also discussed. The current review will provide references for the construction of portable paper-based sensors, and has implications for the field of on-site real-time detection of the environment.

In situ SERS detection of dissolved nitrate on hydrated gold substrates

The accurate and fast measurement of nitrate in seawater is important for monitoring and controlling water quality to prevent ecologic and economic disasters. In this work we show that the in situ detection of nitrate in aqueous solution is feasible at nanomolar concentrations through surface enhanced Raman spectroscopy (SERS) using native nanostructured gold substrates without surface functionalization. Spectra were analyzed as collected or after standard normal variate (SNV) normalization, which was shown through Principal Component Analysis (PCA) to reduce spectral variations between sample sets and improve Langmuir adsorption model fits. An additional normalization approach based on the substrate silicon template showed that silicon provided an internal standard that accounted for the spectral variance without the need for SNV normalization. Nitrate adsorption was well-described by the Langmuir adsorption model, consistent with an adsorbed monolayer, and a limit of detection of 64 nM nitrate was obtained in ultrapure water, representing environmentally relevant concentrations. Free energy calculations based on the Langmuir adsorption constants, approximating equilibrium adsorption constants, and calculated self-energy arising from image charge, accounting for electrostatic interactions with a polarizable nanostructured substrate, suggest that nitrate adsorption was partially driven by an entropy gain presumably due to dehydration of the gold substrate and/or nitrate ion. This work is being extended to determine if similar statistical and normalization methods can be applied to nitrate detection in complex natural waters where non-target ions and molecules are expected to interfere.

[Applications of microfluidic paper-based chips in environmental analysis and detection]

近年来,微流控纸芯片由于低成本、便携化、检测快等优点,在需要快速检测的环境分析领域中展现出了巨大的应用前景。该综述从微流控纸芯片在环境分析中的应用角度,总结归纳了微流控纸芯片在环境分析中的最新研究进展,并展望了其在未来的发展趋势与挑战。论文内容引用150余篇源于科学引文索引(SCI)与中文核心期刊中的相关论文。该综述包括微流控纸芯片在环境检测中的优势与制造方法介绍;电化学法、荧光法、比色法、表面增强拉曼法、集成传感法等基于纸芯片的先进分析方法介绍;根据环境分析目标物种类,如重金属离子、营养盐、农药、微生物、抗生素以及其他污染物等,对纸芯片的最新应用现状进行了举例评述;基于微流控纸芯片的环境分析研究的未来发展趋势和前景展望。通过综述近期相关研究,表明微流控纸芯片从提出至今虽然只有十几年的发展历程,但其在环境分析研究中的发展却十分迅速。微流控纸芯片可以根据不同的环境条件和检测要求灵活选择制作与分析方法,实现最佳的检测效果。但是微流控纸芯片也面临一些挑战,如纸张机械强度不足、流体控制程度不佳等问题。这些问题指出了微流控纸芯片在环境检测领域的发展趋势,相信随着不断深入的研究,纸芯片将会在未来的环境分析中发挥更大作用。

All Silica Micro-Fluidic Flow Injection Sensor System for Colorimetric Chemical Sensing

This paper presents a miniature, all-silica, flow-injection sensor. The sensor consists of an optical fiber-coupled microcell for spectral absorption measurements and a microfluidic reagent injection system. The proposed sensor operates in back reflection mode and, with its compact dimensions, (no more than 200 µm in diameter) enables operation in small spaces and at very low flow rates of analyte and reagent, thus allowing for on-line or in-line colorimetric chemical sensing.

Enzymatic Behavior Regulation-Based Colorimetric and Electrochemiluminescence Sensing of Phosphate Using the Cobalt Oxyhydroxide Nanosheet

In this work, a convenient and flexible assay for colorimetric and electrochemiluminescence (ECL) sensing of phosphate was proposed based on the enzymatic behavior regulation of the cobalt oxyhydroxide (CoOOH) nanosheet. CoOOH as a novel nanoenzyme exhibited a peroxidase-like activity, which could catalyze different substrates such as 2, 2'-azinobis-3-ethylbenzthiazoline-6-sulfonate (ABTS) and 4-chloro-1-naphthol (4-CN) with hydrogen peroxide (H₂O₂) as the electron acceptor. Phosphate could specifically regulate the enzymatic behavior of the CoOOH nanosheet via the deactivating effect. A high level of phosphate enabled a weak color change of ABTS, which offered a "turn-off" model of the colorimetric assay with a limit of detection of 0.673 μM. Based on the similar enzymatic behavior, this strategy could then be applied in the ECL assay utilizing l-arginine-6-aza-2-thiothymine-protected gold nanoclusters (Arg-ATT-AuNCs) as ECL signal indicators. Specifically, 4-CN was catalyzed to generate the precipitate and lead to the quenching on ECL emission. Different from colorimetric behavior, phosphate with a high concentration could induce strong ECL performance, which enabled the "turn-on" model of the ECL assay with a more sensitive determination down to 0.434 nM. This flexible enzymatic behavior regulation could then allow the phosphate measurement in environmental samples including tap water and river water with satisfactory accuracy, which holds the potential in the field of environmental protection.

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

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.

Ultrasensitive electrochemical determination of phosphate in water by using hydrophilic TiO2 modified glassy carbon electrodes

In this paper, ultrasensitive electrochemical determination of phosphate in water is achieved by using hydrophilic TiO₂ modified glassy carbon electrodes for the first time. The differential pulse voltammetry (DPV) method is proposed to measure phosphate in water as pulse techniques offer higher sensitivity compared with the conventional cyclic voltammetry (CV) method. Hydrophilic TiO₂ films were obtained upon ultraviolet (UV) illumination after TiO₂ precursor emulsions were coated on the surfaces of glassy carbon electrodes, and used for phosphate determination. Contact angle measurements (around 23.4°) proved the good hydrophilicity of the TiO₂ modified surface upon UV illumination. A detection limit of 0.1 μg L^-1 is obtained, and a linear relationship (R² = 0.99) between the phosphate concentration (ranging from 0.1 μg L^-1 to 1 μg L^-1) and the peak current was observed.

A New Paper-Based Microfluidic Device for Improved Detection of Nitrate in Water

In this paper, we report a simple and inexpensive paper-based microfluidic device for detecting nitrate in water. This device incorporates two recent developments in paper-based technology suitable for nitrate detection and has an optimized microfluidic design. The first technical advancement employed is an innovative fibrous composite material made up of cotton fibers and zinc microparticles that can be incorporated in paper-based devices and results in better nitrate reduction. The second is a detection zone with an immobilized reagent that allows the passage of a larger sample volume. Different acids were tested—citric and phosphoric acids gave better results than hydrochloric acid since this acid evaporates completely without leaving any residue behind on paper. Different microfluidic designs that utilize various fluid control technologies were investigated and a design with a folding detection zone was chosen and optimized to improve the uniformity of the signal produced. The optimized design allowed the device to achieve a limit of detection and quantification of 0.53 ppm and 1.18 ppm, respectively, for nitrate in water. This accounted for more than a 40% improvement on what has been previously realized for the detection of nitrate in water using paper-based technology.

A Microfluidic Paper-Based Analytical Device for Type-II Pyrethroid Targets in an Environmental Water Sample

A detection method for type-II pyrethroids in an environmental water sample using a microfluidic paper-based analytical device (µPAD) is reported here. The detection approach is based on the formation of cyanide from the hydrolysis of type-II pyrethroids and the colorimetric detection of cyanide on a layer-based µPAD. Parafilm and inexpensive laminating pouches were used to create a hydrophobic barrier for the assay on the µPAD. This detection approach was selective to type-II pyrethroids in water for which an environmental water sample was tested. The calibration curves for cypermethrin, deltamethrin, cyhalothrin, and fenvalerate ranged from 2 to 40 µg/mL without sample preconcentration. The lower concentrations of type-II pyrethroids can be assessed by including a preconcentration step prior to the detection on a µPAD. This detection system provides an alternative platform for fast, semiquantitative testing for pesticide contamination in environmental surface water by allowing for portability, low reagent/sample consumption, and low-cost testing.