Low-Cost Automatic Sensor for in Situ Colorimetric Detection of Phosphate and Nitrite in Agricultural Water

Automation and Computerization of (Bio)sensing Systems

Sensing systems necessitate automation to reduce human effort, increase reproducibility, and enable remote sensing. In this perspective, we highlight different types of sensing systems with elements of automation, which are based on flow injection and sequential injection analysis, microfluidics, robotics, and other prototypes addressing specific real-world problems. Finally, we discuss the role of computer technology in sensing systems. Automated flow injection and sequential injection techniques offer precise and efficient sample handling and dependable outcomes. They enable continuous analysis of numerous samples, boosting throughput, and saving time and resources. They enhance safety by minimizing contact with hazardous chemicals. Microfluidic systems are enhanced by automation to enable precise control of parameters and increase of analysis speed. Robotic sampling and sample preparation platforms excel in precise execution of intricate, repetitive tasks such as sample handling, dilution, and transfer. These platforms enhance efficiency by multitasking, use minimal sample volumes, and they seamlessly integrate with analytical instruments. Other sensor prototypes utilize mechanical devices and computer technology to address real-world issues, offering efficient, accurate, and economical real-time solutions for analyte identification and quantification in remote areas. Computer technology is crucial in modern sensing systems, enabling data acquisition, signal processing, real-time analysis, and data storage. Machine learning and artificial intelligence enhance predictions from the sensor data, supporting the Internet of Things with efficient data management.

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

Portable colorimetric hydrogel test kits are newly developed for the on-site detection of nitrite, nitrate, and phosphate in water. Griess-doped hydrogel was prepared at the bottom of a 1.5 mL plastic tube for nitrite detection, a nitrate reduction film based on zinc powder was placed on the inner lid of a second 1.5 mL plastic tube for use in conjunction with the Griess-doped hydrogel for nitrate detection, and a molybdenum blue-based reagent was entrapped within a poly(vinyl alcohol) hydrogel matrix placed at the bottom of a third 1.5 mL plastic tube to detect phosphate. These test kits are usable with on-mobile digital image colorimetry (DIC) for the on-site determination of nutrients with good analytical performance. The detection limits were 0.02, 0.04, and 0.14 mg L−1 for nitrite, nitrate, and phosphate, respectively, with good accuracy (<4.8% relative error) and precision (<1.85% relative standard deviation). These test kits and on-mobile DIC were used for the on-site determination of nutrients in the Pak Bang and Bang Yai canals, the main canals in Phuket, Thailand. The concentrations of nitrite, nitrate, and phosphate were undetectable to 0.60 mg L−1, undetectable to 2.98 mg L−1, and undetectable to 0.52 mg L−1, respectively.

A Comprehensive Review on Water Quality Monitoring Devices: Materials Advances, Current Status, and Future Perspective

Water quality monitoring has become more critical in recent years to ensure the availability of clean and safe water from natural aquifers and to understand the evolution of water contaminants across time and space. The conventional water monitoring techniques comprise of sample collection, preservation, preparation, tailed by laboratory testing and analysis with cumbersome wet chemical routes and expensive instrumentation. Despite the high accuracy of these methods, the high testing costs, laborious procedures, and maintenance associated with them don't make them lucrative for end end-users and field testing. As the participation of ultimate stakeholders, that is, common man for water quality and quantity can play a pivotal role in ensuring the sustainability of our aquifers, thus it is essential to develop and deploy portable and user-friendly technical systems for monitoring water sources in real-time or on-site. The present review emphasizes here on possible approaches including optical (absorbance, fluorescence, colorimetric, X-ray fluorescence, chemiluminescence), electrochemical (ASV, CSV, CV, EIS, and chronoamperometry), electrical, biological, and surface-sensing (SPR and SERS), as candidates for developing such platforms. The existing developments, their success, and bottlenecks are discussed in terms of various attributes of water to escalate the essentiality of water quality devices development meeting ASSURED criterion for societal usage. These platforms are also analyzed in terms of their market potential, advancements required from material science aspects, and possible integration with IoT solutions in alignment with Industry 4.0 for environmental application.

Stainless steel electrochemical capacitive microneedle sensors for multiplexed simultaneous measurement of pH, nitrates, and phosphates

Concerns for agri-food safety and environmental management require development of simple to use and cost- and time-effective multiplex sensors for point-of-need (PON) chemical analytics by public end-user. Simultaneous detection of nitrates, phosphates, and pH is of importance in soil and water analysis, agriculture, and food quality assessment. This article demonstrates a suite of stainless steel microneedle electrochemical sensors for multiplexed measurement of pH, nitrate, and phosphate using faradaic capacitance derived from cyclic voltammetry as the mode of detection. The multi-target microneedle sensors were fabricated by layer-by-layer (LbL) assembly in a stainless steel hypodermic microneedle substrate. For nitrate sensing, the stainless steel was coated with carbon nanotube/cellulose nanocrystal (CNT)/CNC) decorated with silver nanoparticles (Ag). For pH measurement, the polyaniline (pANI) was coated onto the CNT/CNC@Ag film, while for phosphate detection, the CNT/CNC/Ag @pANI microneedle was further decorated with ammonium molybdenum tetrahydrate (AMT). The microelectrode platforms were characterized by FTIR, Raman, and microscopic techniques. The nitrate- and phosphate-based microneedle electrochemical sensors had excellent selectivity and sensitivity, with a determined limit of detection (LOD) of 0.008 mM and 0.007 mM, respectively. The pH microneedle sensor was responsive to pH in the linear range of 3-10. The three microneedle sensors yielded repeatable results, with a precision ranging from 4.0 to 7.5% RSD over the concentration ranges tested. The inexpensive (~ 1 $ CAD) microneedle sensors were successfully verified for use in quantification of nitrate, pH, and phosphate in brewed black coffee as a real sample. As such, the microneedle sensors are economical devices and show great promise as robust platforms for PON precision chemical analytics.

Advances in Technological Research for Online and In Situ Water Quality Monitoring—A Review

Monitoring water quality is an essential tool for the control of pollutants and pathogens that can cause damage to the environment and human health. However, water quality analysis is usually performed in laboratory environments, often with the use of high-cost equipment and qualified professionals. With the progress of nanotechnology and the advance in engineering materials, several studies have shown, in recent years, the development of technologies aimed at monitoring water quality, with the ability to reduce the costs of analysis and accelerate the achievement of results for management and decision-making. In this work, a review was carried out on several low-cost developed technologies and applied in situ for water quality monitoring. Thus, new alternative technologies for the main physical (color, temperature, and turbidity), chemical (chlorine, fluorine, phosphorus, metals, nitrogen, dissolved oxygen, pH, and oxidation–reduction potential), and biological (total coliforms, Escherichia coli, algae, and cyanobacteria) water quality parameters were described. It was observed that there has been an increase in the number of publications related to the topic in recent years, mainly since 2012, with 641 studies being published in 2021. The main new technologies developed are based on optical or electrochemical sensors, however, due to the recent development of these technologies, more robust analyses and evaluations in real conditions are essential to guarantee the precision and repeatability of the methods, especially when it is desirable to compare the values with government regulatory standards.

Construction of polythiophene-derivative films as a novel electrochemical sensor for highly sensitive detection of nitrite

Herein, a novel, convenient, and highly selective electrochemical sensor for determination of nitrite based on a polythiophene-derivative film-modified glassy carbon electrode (GCE) was established. In this work, 2,5-di-thiophen-3-yl-thiazolo[5,4-d]thiazole (DTT), a novel thiophene derivative, was synthesized and used to form an original and excellent polymer film (PolyDTTF) on GCE through one-step electropolymerization for the first time. The modified electrodes were characterized by electron microscopy (SEM), Fourier transform infra-red spectroscopy (FT-IR), UV-visible spectra, Raman spectroscopy, and electrochemical technologies, in which the electrochemical sensor based on PolyDTTF was successfully constructed and demonstrated a significant electrocatalytic effect on nitrite. The influence of pH value, electrodeposition scanning times, scanning speed, and potential on the electrochemical behavior of nitrite were investigated in detail. Furthermore, the nitrite sensor exhibits excellent responses proportional to nitrite concentrations (R² = 0.9972) over a concentration range of 5.5 × 10^-9 ~ 3.5 × 10^-5 M with a detection limit (LOD) of 2 nM, and has extremely good anti-interference ability for nitrite detection. This proposed sensor can be used to detect nitrite in actual samples, opening the possibility for applications in the food industry and environmental analysis.

Integrating electrochemical sensor based on MoO3/Co3O4 heterostructure for highly sensitive sensing of nitrite in sausages and water

Considering excess nitrites are detrimental to the human body and environment, designing a rapid, sensitive, and real-time quantitative determination for nitrite is of great significance for environmental preservation and public health. In this paper, Co₃O₄ nanoflowers coupled with ultrafine MoO₃ nanoparticles (MoO₃/Co₃O₄) are obtained via a hybrid electrochemical deposition strategy (HED). The as-designed MoO₃/Co₃O₄/CC integrating electrode exhibits superior electrocatalytic properties towards nitrite oxidation, owing to the synergistic effect between MoO₃ and Co₃O₄ caused by the heterostructure of MoO₃/Co₃O₄. The electrode achieved a low response time of 2 s, an excellent sensitivity of 1704.1 μA mM^-1 cm^-2, and a low limit of detection of 0.075 μM (S/N = 3). Furthermore, the electrode displays promise for nitrite detection in complex food such as water and sausages samples. Our study will provide a significant strategy for the application of bimetallic heterostructure to explore the design of sensing interfaces.

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.

Smartphone-based ratiometric fluorescent definable system for phosphate by merged metal-organic frameworks

Phosphate plays an important role in a wide range of chemical and biological processes, so the development of a new phosphate optical sensor with high sensitivity, specificity and visual recognition function has important practical significance. Herein, a ratiometric fluorescent (RF) probe and a smartphone-integrated colorimetric test paper sensing platform for assay phosphate was fabricated using hybrid fluorescent UiO-66-NH₂ and Eu³⁺@MOF-808 metal-organic frameworks. After continuous addition of phosphate, the blue fluorescence emission of UiO-66-NH₂ and the red emission of Eu³⁺@MOF-808 were regularly enhanced and quenched respectively, and the fluorescence response of the detection platform to phosphate exhibited a clear color change process (red → pink → blue). More importantly, the probe solution and test paper of the integrated smartphone are converted to digital values through RGB channels and successfully used to visualize semi-quantitative recognition of phosphate. In addition, an RF probe and a smartphone integrated fluorescent test paper were developed separately to devise logic gate devices for detecting phosphate. The multifunctional ratio sensing platform integrated by the smartphone furnishes a new strategy and broad prospects for the intelligent online identification of important targets in biological samples and environmental samples.

Headspace Solid-Phase Microextraction Following Chemical Vapor Generation for Ultrasensitive, Matrix Effect-Free Detection of Nitrite by Microplasma Optical Emission Spectrometry

A new chemical vapor generation method coupled with headspace solid-phase microextraction miniaturized point discharge optical emission spectrometry (HS-SPME-μPD-OES) for the sensitive and matrix effect-free detection of nitrite in complex samples is described. In an acidic medium, the volatile cyclohexene was generated from cyclamate in the presence of nitrite, which was volatilized to the headspace of the container, efficiently separated, and preconcentrated by HS-SPME. Consequently, the SPME fiber was transferred to a laboratory-constructed thermal desorption chamber wherein the cyclohexene was thermally desorbed and swept into μPD-OES for its sensitive quantification via monitoring the carbon atomic emission line at 193.0 nm. As a result, the quantification of nitrite was accomplished through the determination of cyclohexene. The application of HS-SPME as a sampling technique not only simplifies the experimental setup of μPD-OES but it also preconcentrates and separates cyclohexene from N₂ and sample matrices, thus eliminating the interference from water vapor and N₂ and significantly improving the analytical performance on the determination of nitrite. Under the optimum experimental conditions, a limit of detection of 0.1 μg L^-1 was obtained, which is much better than that obtained by conventional methods. The precision, expressed as relative standard deviation, was better than 3.0% at a concentration of 10 μg L^-1. The proposed method provides several advantages of portability, simplicity, high sensitivity, and low energy consumption and eliminates expensive instruments and matrix interference, thus retaining a promising potential for the rapid, sensitive, and field analysis of nitrite in various samples.

A Disposable Multiplexed Chip for the Simultaneous Quantification of Key Parameters in Water Quality Monitoring

On-site simultaneous quantification of multiple contaminants in a water body is challenging, especially for parameters requiring complicated chemical reactions to measure such as Chemical Oxygen Demand (COD), ammonia nitrogen, and phosphate. A novel disposable multiplexed microfluidic device has been developed herein that allows the quantitative detection of up to five parameters at once. Solid reagent rather than commonly used liquid reagent was used to ensure long shelf life, and a "flow to dissolve" mechanism was provided accordingly for the thorough dissolution and mixing of a solid reagent on chip. Samples from river water and industrial wastewater were tested using the microfluidic chip, showing less than 15% deviation from results acquired with the traditional standard method. The test time though was only 1/6 of that required by the traditional method. In addition, the feasibility of using a smartphone to collect the colorimetric signal was discussed, and a data analysis method was provided for quantification purposes. The combination of the multiplexed chip and smartphone imaging provides a convenient and practical way to obtain accurate information on the water quality within a short period of time without the use of any sophisticated instruments.

La-based-adsorbents for efficient biological phosphorus treatment of wastewater: Synergistically strengthen of chemical and biological removal

The present work demonstrated the invention of synergistically strengthen of chemical and biological removal of phosphorus (P) in biological wastewater treatment, which was achieved by exposure the bioreactors to different levels of La-based-adsorbents. We fabricated a high-performance La₂O₂CO₃ micro-adsorbent (H-La₂O₂CO₃) and added it into sequencing batch reactors. When activated sludge was exposed to 40 mg/L H-La₂O₂CO₃ for 40 d, effluent total phosphorus (TP) concentration significantly decreased to approximately 0.18 mg/L, with the steady removal efficiency of 96.4%, which is superior to the biological phosphorus removal (BPR). The effect of H-La₂O₂CO₃ dosages on P removal in biological wastewater treatment was also detailedly investigated. The H-La₂O₂CO₃ adsorbent could not only capture P by chemical bonding itself, but also increased protein (PN) contents of extracellular polymeric substances (EPS) and changed the functional group of EPS to chemically adsorb P. Additionally, the results of 16s rDNA molecular analysis revealed that the species richness and microbial diversity varied with the different dosages of adsorbent. Sequence analyses showed that the appropriate concentration of H-La₂O₂CO₃ addition increased the contents of several polyphosphate accumulating organisms (PAOs) at genus level in sludge.

Integrated Digital Microfluidic Platform for Colorimetric Sensing of Nitrite

In this paper, a palm-size digital microfluidic (DMF) platform integrated with colorimetric analysis was developed for quantifying the concentration of nitrite. To realize the on-chip repeatable colorimetric analysis, a novel printed circuit board (PCB)-based DMF chip was designed with an embedded aperture on the actuator electrode, forming a vertical light path for online measurement of the droplets. The capabilities of the DMF platform enable automatic manipulation of microliter-level droplets to implement Griess assay without the use of external systems such as syringe, pump, or valve, which provides the benefits including high flexibility, portability, miniature size, and low cost. Results indicated the characteristics of good linearity (R 2 = 0.9974), the ignorable crosstalk for reusability, and the limit of detection (LOD) of nitrite as low as 5 μg/L. Furthermore, the presented platform was successfully applied to determine nitrite levels in food products with reliable results and satisfactory recoveries. This integrated DMF platform can be a promising new tool for a wide range of applications involving step-by-step solution mixing and optical detection in environmental monitoring, food safety analysis, and point-of-care testing.

A multi-parameter in-situ water quality analyzer based on a portable document scanner and 3D printed self-sampling cells

This research introduced a new low-cost and multi-parameter analyzer for in-situ measurements of typical nutrients in water bodies. The analyzer consisted of color detection and chromogenic reaction modules. The self-sampling action of the 3D printed sampling/reaction cells was achieved with the cooperative application of rubber bands and dissolvable thread. The target analytes in the collected water sample reacted with the chromogenic reagents that were diffused from the pre-placed glass wool in the cell, producing color compounds. A portable document scanner was employed as a multi-parameter in-situ detector to record the image of the colored solutions in all five cells simultaneously. Based on the image, the corrected grayscale values were derived for target analyte quantitation. The relationships between grayscale values and concentrations of target analytes were established, and the temperature effects were studied. In addition, the practicability of the analyzer was demonstrated by in-situ experiments carried out in four different sites, including a creek, a river dock, a reservoir and a secondary settling tank in a wastewater treatment facility. The results indicated that the analyzer could be used for in-situ measuring of nutrients at μmol/L levels in the water. The nutrient concentrations obtained with the analyzer were comparable with those obtained with the standard methods. The presented analyzer provided new complementary ideas and methods for in-situ rapid measurement of nutrients and other target analytes in various water systems.

Highly sensitive and selective fluorescent detection of phosphate in water environment by a functionalized coordination polymer

Phosphate, due to its somatotrophic effect on organisms, can cause severe eutrophication with excessive content in water. Conventional methods for phosphate detection, which are based on complicated instruments, are time-consuming. Here we report a luminescent lanthanide functionalized coordination polymer (Eu@BUC-14) by doping Eu³⁺ cations to BUC-14 nanocrystals. This Eu³⁺ functionalized hybrid (Eu@BUC-14) presents excellent luminescence features of Eu³⁺ ion that originated from efficient energy transfer from the ligand. The detection results show that Eu@BUC-14 is a highly efficient luminescent probe for phosphate detection in aqueous solutions, exhibiting high sensitivity with a low detection limit of 0.88 μM and a short response time of within 5 min. More significantly, Eu@BUC-14 has a high specificity for PO₄^3- over fifteen other pollutant anions and eleven metal cations. The sensing mechanism is proposed via an in-depth analysis of the interaction between PO₄^3- and Eu³⁺. Simultaneously, it displays high adsorption ability toward PO₄^3- (57.9 mg P/g), making it an outstanding multifunctional material. And the adsorption process plays an important role in preconcentration of PO₄^3- which can lead to a quick fluorescent response with high quenching efficiency. The practicality of Eu@BUC-14 was also validated by sensing PO₄^3- in real environment water samples.

Nozzle-Jet-Printed Silver/Graphene Composite-Based Field-Effect Transistor Sensor for Phosphate Ion Detection

High concentration of dissolved phosphate ions is the main responsible factor for eutrophication of natural water bodies. Therefore, detection of phosphate ions is essential for evaluating water eutrophication. There is a need at large-scale production of real-time monitoring technology to detect phosphorus accurately. In this study, facile enzymeless phosphate ion detection is reported using a nozzle-jet-printed silver/reduced graphene oxide (Ag/rGO) composite-based field-effect transistor sensor on flexible and disposable polymer substrates. The sensor exhibits promising results in low concentration as well as real-time phosphate ion detection. The sensor shows excellent performance with a wide linear range of 0.005-6.00 mM, high sensitivity of 62.2 μA/cm²/mM, and low detection limit of 0.2 μM. This facile combined technology readily facilitates the phosphate ion detection with high performance, long-term stability, excellent reproducibility, and good selectivity in the presence of other interfering anions. The sensor fabrication method and phosphate detection technique yield low-cost, user-friendly sensing devices with less analyte consumption, which are easy to fabricate on polymer substrates on a large scale. Besides, the sensor has the capability to sense phosphate ions in real water samples, which makes it applicable in environmental monitoring.