Direct Electrochemical Sensing of Phosphate in Aqueous Solutions Based on Phase Transition of Calcium Phosphate

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.

A unique fluorescence metal-organic framework for ultrasensitive fluorescent and colorimetric bimodal detection of phosphate

Monitoring the concentration of phosphate is crucial for environmental protection and human health due to its severe ecological and health risks associated with elevated concentrations. Herein, a fluorescent-colorimetric bimodal nanoprobe based on the unique fluorescent metal-organic frameworks (Zr-PDI) has been developed for high-efficiency quantification of phosphate. The metal-oxygen coordination in Zr-PDI effectively diminished its fluorescence. However, the introduction of phosphate could weaken the metal-oxygen coordination interaction, leading to fluorescence recovery and absorption spectra changes of Zr-PDI. Taking advantage of these characteristics, Zr-PDI was exploited as a fluorescent-colorimetric bimodal detection tool for phosphate, offering excellent selectivity, a wide detection range, and high accuracy. Notably, the detection limit of fluorescence detection mode was as low as 0.023 μM, enabling ultrasensitive detection of phosphate. Furthermore, the Zr-PDI-based nanoprobe has achieved sensitive and reliable quantification of phosphate in Yong River and diabetic mouse serum samples. This proposed strategy provides a powerful, convenient, and practical tool for detecting phosphate in environmental and biological samples.

Integration of Eu-based metal-organic frameworks and carbon dots for multicolor visual intelligent detection of phosphate

Phosphate (Pi) has an important influence on the water environment and physiological processes. Therefore, developing fluorescent probe for quantitative detection of Pi is crucial for water environment monitoring and human health assessment. This work designed a dual-emission ratio nano-fluorescent probe GCDs/Eu-BDC based on europium-based metal-organic frameworks (Eu-MOFs) and blue carbon dots (GCDs) for multicolor fluorescence detection of Pi. The GCDs/Eu-BDC realized multicolor fluorescence detection of Pi based on the red-to-blue fluorescence change. The probe has high selectivity and a detection limit of 70 nM in the range of 0-45 μM. GCDs/Eu-BDC can be used to detect Pi in environmental water samples and serum samples, proving the feasibility of quantitative analysis of Pi in real samples. In addition, a portable paper-based sensor was prepared in this work. Combined with the chromaticity analysis App in smartphones, the intelligent real-time detection of Pi can be realized, which has certain practical application potential.

Robust (hydrogen) phosphate sensing based on reversible redox of cobalt(II) hydroxide

Response mechanism of the electrode is elucidated in terms of (hydrogen) phosphate accelerating oxidation of CoII to CoIII for the first time. Cyclic voltammetric techniques in conjunction with XRD, XPS and Raman characterizations have demonstrated unambiguously the response of cobalt (II) hydroxide electrode involves a phosphate and hydrogen ion dependent charge transfer process. Phosphate ions induce Co(OH)2 transformed into CoOOH within interlayer adsorption and restored the initial state after reduction. Meanwhile, the in common structural between Co(OH)2 and CoOOH prevents extensive structural convertibility upon cycling, result in the advantage of reversibility in phase transformation. Demonstrated sustainable technique offered the determination of phosphate with robust reproducibility (1000 cycles), long storage stability (6 months) and selectivity (potential interference: Cl-, NO3-, SO42- and HCO3-), achieving a detection limit of 5 × 10-8 M over a wide linear range up to 1.28 mM. Presented work provided insights into the unique selectivity towards phosphate in cobalt based sensors, which may inspire the rational design of Co(OH)2-based electrodes with superior electrochemical performance or extended applications.

Improved electrochemical detection of levofloxacin in diverse aquatic samples using 3D flower-like Co@CaPO4 nanospheres

The misuse of antibiotics has become a concerning environmental issue, posing a significant threat to public health. Levofloxacin (LFX), a fluoroquinolone antibiotic, is particularly worrisome due to its detrimental impact on human health and the ecosystem. Therefore, the selective and accurate identification of LFX is of utmost importance. In this study, we have developed an electrochemical sensor based on cobalt-doped calcium phosphate (Co@CaHPO) for the sensitive and selective detection of LFX in various water samples. Under optimized conditions, the Co@CaHPO-modified glassy carbon electrode (GCE) exhibited exceptional electrochemical activity, low charge transfer resistance, and a fast electron transfer rate, outperforming the unmodified GCE. The proposed Co@CaHPO-modified GCE demonstrated remarkable electrochemical characteristics, including a wide linear range (0.3-460 μM) and a lower detection limit (0.151 μM) with high sensitivity (0.676 μAμM-1 cm2). This detection approach may enable the direct detection of LFX in the pharmaceutical environment. Furthermore, the resulting sensor exhibited good selectivity, excellent cyclic and storage stability, reproducibility, and repeatability. The practical application of this LFX sensor can be extended to various water samples, yielding reliable and satisfactory results.

Upconversion luminescence nanosensor for detection of Fe3+ and phosphate ion based on the inner-filter effect

In this work, an upconversion luminescence (UCL) nanosensor for fast detection of ferric ion (Fe3+) and phosphate ion (Pi) is developed based on the inner-filter effect (IFE) between NaYF4:Yb/Er upconversion nanoparticles (UCNPs) and Fe3+-hypocrellin B (HB) complex. Fe3+-HB complex has strong absorption band (450-650 nm), which overlaps with the green emission peak of UCNPs at 545 nm. By adding Fe3+ and Pi, the UCNPs-HB system produces the red-shift change of absorption spectrum, which leads to the "on-off-on" process of IFE. So, with the specific recognition ability of HB for Fe3+ and the competitive complexation of Pi for Fe3+, the proposed nanosensor utilizes the UCL change to achieve the detection of the targets. For the detections of Fe3+, the linear range is 10-600 μM with a limit of detection (LOD) of 2.62 μM, and for Pi, the linear range is 5-100 μM with a LOD of 1.25 μM. The results for selectivity, precision, and recovery test are also satisfactory. Furthermore, the real sample detection shows that the proposed nanaosensor has a great potential in environmental and biological systems. An upconversion luminescence (UCL) nanosensor based on the inner-filter effect (IFE) between upconversion nanoparticles (UCNPs) and Fe3+-hypocrellin B (HB) complex for the detection of Fe3+ and phosphate ion has been proposed, which is promising to be a convenient and sensitive assay for monitoring Fe3+ and phosphate ion in different environments and biological systems.

Enhanced Ultrasensitive Photoelectrochemical Probe for Phosphate Detection in Water Based on a Zirconium-Porphyrin Framework

Excessive phosphate poses a serious ecological and human health risk, and thereby, monitoring its trace concentration is of great significance to environmental protection and human health. In this work, a zirconium-porphyrin framework (PCN-222) with excellent stability and unique luminescence properties was designed to modify the surface of the indium tin oxide electrode, which was first used as a photoelectrochemical (PEC) probe for phosphate detection. The PCN-222-modified PEC probe demonstrated an excellent selectivity and stability and indicated a linear response to phosphate in the range of 0-10⁶ nM with a limit of detection (LOD) as low as 1.964 nM. To the best of our knowledge, this is the phosphate probe with the lowest LOD, and this is also the first signal-on PEC probe toward phosphate based on PCN-222. More importantly, the PEC probe can be validated for the good applicability of trace phosphate detection in real water samples, indicating a good application prospect. Finally, a series of electrochemical and spectroscopic studies have proved that phosphate can bind to the indium tin oxide (ITO)/PCN-222 electrode, which shortens the distance of the space charge region while reducing the bandwidth and thus facilitates the transfer of photogenerated electrons across the energy band barrier to reduce O₂ in the electrolyte, producing an enhanced cathodic photocurrent signal. The proposed strategy of the highly sensitive PEC probe provides a promising platform for more effective label-free phosphate monitoring in the environment and organisms.

Synthesis, structural and luminescent properties of Mn-doped calcium pyrophosphate (Ca2P2O7) polymorphs

In the present work, three different Mn²⁺-doped calcium pyrophosphate (CPP, Ca₂P₂O₇) polymorphs were synthesized by wet co-precipitation method followed by annealing at different temperatures. The crystal structure and purity were studied by powder X-ray diffraction (XRD), Fourier-transform infrared (FTIR), solid-state nuclear magnetic resonance (SS-NMR), and electron paramagnetic resonance (EPR) spectroscopies. Scanning electron microscopy (SEM) was used to investigate the morphological features of the synthesized products. Optical properties were investigated using photoluminescence measurements. Excitation spectra, emission spectra, and photoluminescence decay curves of the samples were studied. All Mn-doped polymorphs exhibited a broadband emission ranging from approximately 500 to 730 nm. The emission maximum was host-dependent and centered at around 580, 570, and 595 nm for γ-, β-, and α-CPP, respectively.

Electrochemical Detection of Phosphate Ion in Body Fluids with a Magnesium Phosphate Modified Electrode

An electrochemical sensor for phosphate detection in body fluids was developed based on the hydration transition of magnesium hydrogen phosphate (newberyite, MgHPO₄·3H₂O). The sensor was fabricated through incubation of a multi-walled carbon nanotube/Nafion (MWCNT/Nafion) modified glassy carbon electrode (GCE) in magnesium phosphate solution, where MgHPO₄·3H₂O was self-assembled on the electrode surface (denoted as MgP/MWCNT/Nafion). An electrooxidation peak at 1.0 V vs. Ag/AgCl was observed when the as-prepared electrode was subjected to a differential pulse voltammetry (DPV) scan in the presence of phosphate in acetate solution. When the DPV scan was performed in 0.4 - 1.3 V vs. Ag/AgCl, a linear relationship was observed between the peak height and the phosphate concentration in the range from 0.01 to 25 μM in the presence of 0.1 mM Mg²⁺ in the acetate solution with a limit of detection of 32 nM. And the sensor was successfully applied for phosphate detection in human urine and saliva samples with recoveries of 94.7 - 104.4 and 96 - 103.3%, respectively.

Alcohol-Triggered Capillarity through Porous Pyrolyzed Paper-Based Electrodes Enables Ultrasensitive Electrochemical Detection of Phosphate

The sensing field has shed light on an urgent necessity for field-deployable, user-friendly, sensitive, and scalable platforms that are able to translate solutions into the real world. Here, we attempt to meet these requests by addressing a simple, low-cost, and fast electrochemical approach to provide sensitive assays that consist of dropping a small volume (0.5 μL) of off-the-shelf alcohols on pyrolyzed paper-based electrodes before adding the sample (150 μL). This method was applied in the detection of phosphate after the formation of the phosphomolybdate complex (250-860 nm in size). Prior drops of isopropanol allow for the fast penetration of the sample through pores of this hydrophobic paper, delivering hindrance-free redox reactions across increasing active areas and ultimately improving the detection performance. The sensitivity (-1.9 10^-6 mA cm^-2 ppb^-1) and limit of detection (1.1 ppb) were improved, respectively, by factors of 33 and 99 over the data achieved without the addition of isopropanol, listing among the lowest values when compared with those results reported in the literature for phosphate (expressed in terms of the concentration of phosphorus). The approach enabled the quantification of this analyte in real samples with accuracies ranging from 87 to 103%. Furthermore, preliminary measurements demonstrated the successful performance of the electrodes with prior addition of other widely used alcohols, that is, methanol and ethanol. These results may extend the applicability of the method. In special, the scalability and eco-friendly character of the electrode fabrication combined with the sensitivity and simplicity of the analyses make the developed platform a promising alternative that may help to pave the way for a new generation of disposable sensors toward the daily monitoring of phosphate in water samples, thus contributing to prevent ecological side effects.

Ratiometric fluorescence for sensitive detection of phosphate species based on mixed lanthanide metal organic framework

Phosphate (PO₄^3-) plays a major role in aquatic ecosystems and biosystems. Developing a highly sensitive and selective ratiometric fluorescence probe for detection of PO₄^3- is of great significance to the ecological environment and human health. In this work, a novel dual lanthanide metal organic framework was synthesized via hydrothermal reaction based on Tb³⁺ and Ce³⁺ as the center metal ions and terephthalic acid as the organic ligand (designated as Tb-Ce-MOFs). The fluorescence of Tb-Ce-MOFs shows emission at 375 nm. In the presence of PO₄^3-, with increased concentration of PO₄^3-, the fluorescence intensity of Tb-Ce-MOFs at 500 nm and 550 nm increased, while the intensity at 375 nm was reduced. Hence, ratiometric fluorescence detecting of PO₄^3- can be achieved by measuring the ratio of fluorescence at 550 nm (FL₅₅₀) to 375 nm (FL₃₇₅) in the fluorescent spectra of the Tb-Ce-MOFs. In this sensing approach, the Tb-Ce-MOFs probe exhibits highly sensitive and selective for detection of PO₄^3-. The limit of detection is calculated to be 28 nM and the detection range is 0.1 to 10 μM. In addition, the Tb-Ce-MOFs were used in the detection of PO₄^3- in real samples. We design and synthesize a mixed lanthanide metal organic framework fluorescence probe (Tb-Ce-MOFs) for ratiometric fluorescence for the detection of PO₄^3- based on Tb³⁺ and Ce³⁺ as the center metal ions and terephthalic acid as the organic ligand.

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.