Simple colorimetric ammonium assay employing well microplate with gas pervaporation and diffusion for natural indicator immobilized paper sensor via smartphone detection

Microfluidic paper-based analytical device for the speciation of inorganic nitrogen species

A microfluidic paper-based analytical device (μPAD) utilizing gas-diffusion separation and solid-phase reduction was developed for the first time for the determination of both ammonium and nitrate, which are the dominant inorganic nitrogen species in environmental waters. The device consists of 3 filter paper layers accommodating the sample, reagent and detection zones. The reagent zone is separated from the detection zone by a semipermeable hydrophobic membrane and acts as a solid-phase reactor where nitrate is reduced to ammonia by Devarda's alloy microparticles, integrated into a μPAD for the first time. The detection zone incorporates the acid-base indicators bromothymol blue (BTB) or nitrazine yellow (NY) and changes colour in two steps. Initially the colour change is caused by ammonia generated by the reaction of ammonium and sodium hydroxide in the sample zone. This colour change is followed by a subsequent colour change as a result of the ammonia produced by the reduction of nitrate by the Devarda's alloy microparticles. The corresponding reflectance value changes are used for the quantification of the two inorganic nitrogen species in the ranges 6.5-100.0 or 2.1-15.0 mg N L-1 for ammonium and 18.2-100.0 or 4.2-15.0 mg N L-1 for nitrate when BTB or NY are used, respectively. Under optimal conditions the limits of quantification of ammonium and nitrate in the case of BTB were determined as 6.5 and 18.2 mg N L-1, respectively, while the corresponding values in the case of NY were found to be 2.1 and 4.2 mg N L-1. The newly developed μPAD was stable for 62 days when stored in a freezer and 1 day at ambient temperature. It was validated with a certified reference material and successfully applied to the determination of ammonium and nitrate in spiked environmental water samples and soil extracts.

Structure Optimization and Data Processing Method of Electronic Nose Bionic Chamber for Detecting Ammonia Emissions from Livestock Excrement Fermentation

In areas where livestock are bred, there is a demand for accurate, real-time, and stable monitoring of ammonia concentration in the breeding environment. However, existing electronic nose systems have slow response times and limited detection accuracy. In this study, we introduce a novel solution: the bionic chamber construction of the electronic nose is optimized, and the sensor response data in the chamber are analyzed using an intelligent algorithm. We analyze the structure of the biomimetic chamber and the surface airflow of the sensor array to determine the sensing units of the system. The system employs an electronic nose to detect ammonia and ethanol gases in a circulating airflow within a closed box. The captured signals are processed, followed by the application of classification and regression models for data prediction. Our results suggest that the system, leveraging the biomimetic chamber, offers rapid gas detection response times. A high classification prediction accuracy, with a determination coefficient R2 value of 0.99 for single-output regression and over 0.98 for multi-output regression predictions, is achieved by incorporating a backpropagation (BP) neural network algorithm. These outcomes demonstrate the effectiveness of the electronic nose, based on an optimized bionic chamber combined with a BP neural network algorithm, in accurately detecting ammonia emitted during livestock excreta fermentation, satisfying the ammonia detection requirements of breeding farms.

Development of a colorimetric sensor based on the coupling of a microfluidic paper-based analytical device and headspace microextraction for determination of formaldehyde in textile, milk, and wastewater samples

A user-friendly, cost-effectively, portable, and environmentally friendly colorimetric sensor for the quantitative determination of formaldehyde was developed based on the combining of microfluidic paper-based analytical device (μPAD), headspace microextraction (HSME), and digital image colorimetry. Coupling HSME and μPAD led to enhancements in selectivity and sensitivity of the sensor through sample cleanup and analyte enrichment. To construct the μPAD-HSME device, two pieces of paper as the sample and detection zone were placed facing each other so that a small common and sealed space was created between them. The color change occurred when the analyte in the gaseous form crossed this gap and reached the detection zone. Colorimetric sensing in the detection zone was performed based on the Hantzsch reaction. The color change in the detection zone was recorded by a smartphone and digital images were processed using image analysis software based on the RGB model. The influence of some key variables on the sensitivity of the method including derivatization reagent composition, sample volume, extraction temperature, and extraction time was studied and optimized. The linear dynamic range of the method was obtained in two ranges of 0.10-0.75 and 0.75-5.0 mg L-1 with a limit of detection of 0.03 mg L-1. The recoveries were in the range 80-126% for the quantification of formaldehyde in textile, milk, and wastewater samples.

A polylactide based multifunctional hydrophobic film for tracking evaluation and maintaining beef freshness by an electrospinning technique

A nanofiber film was prepared by a facile electrospinning technique using polylactide (PLA), butterfly pea flower extract (BPA) and cinnamaldehyde (CIN). The as-prepared film shows the prominent antioxidative, antibacterial, colorimetric and hydrophobic properties so that the beef freshness can be monitored and maintained up to 6 days at 4 °C simultaneously. Besides, the nanofiber structure endows the film with a fast color responsiveness under acidic-alkaline atmospheres with different concentrations. Moreover, this film exhibits higher tensile strength (9.56 Mpa) than that of the pure PLA electrospinning film (4.40 Mpa). Especially the introduction of the BPA effectively boosts the antimicrobial ability of the CIN. The freshness, sub-freshness and spoilage levels of the beef can be easily testified by observing the color difference change of the film. So the polylactide based multifunctional film as an intelligent packaging has an excellent potential for the sub-freshness detection of meat.

Multi-well plate as headspaces for paper-based colorimetric detection of sulfur dioxide gas: An alternative method of sulfite titration for determination of formaldehyde

This work describes the analysis of formaldehyde using a 96-well microplate as multiple headspaces for the separation of sulfur dioxide gas generated from the sulfite remaining after its reaction with the formaldehyde in the sample. The quantitation of the gas is by colorimetric detection of an indicator paper placed over the microplate. The samples are aqueous extracts of various foods that are possibly adulterated with formaldehyde. A known excess amount of sulfite is added to the extract solution aliquoted in the well. The remaining sulfite is acidified with hydrochloric acid to generate sulfur dioxide gas which diffuses through the headspace above the solution to be absorbed at the moist strip of the indicator paper placed over the mouth of the wells. Anthocyanins extracted from the butterfly pea flower is used as the pH indicator giving a color change from the increase of hydrogen ions by hydrolysis of the absorbed sulfur dioxide gas. The exposed paper strip is scanned, and the digital images of the colored region analyzed using ImageJ software. The optimized method has a linear range of 200-1000 mg L^-1 formaldehyde with limit of detection ((2.57*SD of intercept)/(slope of calibration line)) of the aqueous extract of 40 mg L^-1 and coefficient of determination (r²) > 0.9979. Samples of fresh produce, such as seafood, meat, and vegetables, and various processed food were analyzed for their possible formaldehyde content. The results obtained from the headspace paper-based colorimetric detection are not statistically different from the values obtained from the titration method by paired t-tests.

Cost-effective formaldehyde assay platform for multi-sample analysis in one run based on pervaporation coupled with a biodegradable sensor and a simple heat control unit

A novel, cost-effective platform using a biodegradable sensor and a simple heat control unit was proposed for multi-sample formaldehyde (FA) assay in one run based on pervaporation. The biodegradable sensor was a composite starch gel attached to paper and immobilized with a mixture of color agents of modified 4-amino-3-hydrazino-5-mercapto-1,2,4-triazol (AHMT). The sensor was situated on the cap of a vial that served for pervaporation. Two types of heat control units were specially designed using the concepts of aluminum block and water bath heating. With these two designs, multi-sample assays together with standard calibration could be performed in the same run under the same conditions. An FA solution was placed in the vial of the pervaporation unit. After a heating period, FA vapor would change the color of the sensor to purple due to the reaction between AHMT and FA. As a result, the color intensity was proportional to the FA concentration. The change of the color (green or G intensity) was monitored using a smartphone camera and image processing software. Factors affecting the sensitivity of the assay, pervaporation time, pervaporation temperature, FA solution volume, and humidity, were studied. Under the chosen condition, the developed procedure, with a calibration of G intensity = 7.93[FA] + 198, R² = 0.98, was applied to analyze real samples of seafood and mushrooms available in local markets in Thailand. As there were 24 pervaporation units in the proposed platform, 5 working standards and 9 samples with duplicates could be included in a 1-run assay either in the laboratory or on-site. The developed assay offers green chemical analysis with a simple, cost-effective approach. This serves the UN-SDGs of #2, #3, #7, #10, and #12.

Development of a versatile smartphone-based environmental analyzer (vSEA) and its application in on-site nutrient detection

The citizen-science-based environmental survey can benefit from the smartphone technology used in chemical and biological sensing of a wide range of analytes. Quantification by smartphone-based colorimetric assays is being increasingly reported, however, most of the quantification uses empirical formula or complex exhaustive methods. In this study, a versatile and robust algorithm is proposed to overcome these limitations. A model is established to simulate and analyze the conversion process from the camera's spectral information into RGB (Red, Green, Blue) color information. Moreover, the feasibility of the algorithm for the quantification of different analytes is also explored. Based on this algorithm, a versatile smartphone-based environmental analyzer (vSEA) is built and its reliability, versatility, and analytical performance are comprehensively optimized. The good linearity (R² ≥ 0.9954) and precision (relative standard deviations < 5.3%) indicates that the vSEA is accurate enough to quantify the nutrients in most natural waters. Furthermore, the vSEA is used for the field measurement of five important nutrients, and the results show no significant difference compared to conventional methods. The vSEA offers a simpler and easier method for the on-site measurement of nutrients in natural water bodies, which can aid in the emergency monitoring of aqueous ecosystems and the performance of citizen-science-based research.

Towards citizen science. On-site detection of nitrite and ammonium using a smartphone and social media software

Citizen scientists-based water quality surveys are becoming popular because of their wide applications in environmental monitoring and public education. At present, many similar studies are reported on collecting samples for later laboratory analysis. For environmentally toxic analytes such as ammonium and nitrite, on-site detection is a promising choice. However, this approach is limited by the availability of suitable methods and instruments. Here, a simple on-site detection method for ammonium and nitrite is reported. The chemistry of this method is based on the classic Griess reaction and modified indophenol blue reaction. Digital image colorimetry is carried out using a smartphone with a custom-made WeChat mini-program or free built-in applications (APPs). Using a simple and low-cost analytical kit, the detection limit of 0.27 μmol/L and 0.84 μmol/L is achieved for nitrite and ammonium, respectively, which are comparable to those achieved with a benchtop spectrophotometer. Relative standard deviations (n = 7) for low and high concentrations of nitrite are 3.6% and 4.3% and for ammonium are 5.6% and 2.6%, respectively. Identical results with a relative error of less than 10% are obtained using different smartphones (n = 3), color extracting software (n = 6), and with multiple individual users (n = 5). These results show the robustness and applicability of the proposed method. The on-site application is carried out in an in-campus wastewater treatment plant and at a local river. A total of 40 samples are analyzed and the analytical results are compared with that obtained by a standard method and a spectrophotometer, followed by a paired t-test at a 95% confidence level. This proposed on-site analytical kit has the advantages of simplicity and portability and has the potential to be popular and useful for citizen science-based environmental monitoring.

A ninety-six well plate as headspaces with moist starch indicator paper as a cover for the determination of ascorbic acid by iodate oxidation and formation of volatile iodine

This work presents the use of a 96-well plate as headspaces for the determination of ascorbic acid in samples loaded in the 96-well plate. Ascorbic acid in the sample is oxidized to iodide by the addition of excess acidic iodate solution into the well. The iodide is further oxidized by the remaining iodate to molecular iodine. A single sheet of moist starch indicator paper is immediately placed over the 96-well plate after the addition of the iodate with the moisture forming a gas seal. The iodine gas in each well diffuses through the headspace to react with the starch paper producing circular areas of a colored starch-iodine complex. After 15 min the indicator paper is scanned, and the digital images of the complex are analyzed by using ImageJ software to obtain blue intensity values. The precision of the intensity values from 12 wells containing 20 μL of 2.84 mM standard ascorbic acid is <2% relative standard deviation. Optimal conditions for detection were investigated, including the starch concentration, the acidic iodate reagent, and the measurement time. The linear calibration range of ascorbic acid is 0.284-2.84 mM, based on the plot of concentration vs. -log(reflectance). The coefficient of determination (r²) is >0.998. Samples of fruit juice and dietary supplements were analyzed for their ascorbic acid contents. The results obtained from the headspace reflectance method are not statistically different from values obtained from the titration method using paired t-tests (α = 0.05).

Phosphate determination in environmental, biological and industrial samples using a smartphone as a capture device

A lab-made device was built to analyse phosphate in four different matrices using a smartphone as a capturing device.

New designs of paper based analytical devices (PADs) for completing replication analysis of a sample within a single run by employing smartphone

Paper-based analytical devices (PADs) with four new designs could be fabricated using commercially available home-based scan-and-cut printer. They serve for miniaturised platforms for chemical analysis. Replication analysis of a sample together with the calibration (using the analyte standards at different concentrations) can be completed in a single run, by utilising smartphone as the detector. Some new approaches for choosing detection zones were suggested. The four proposed PAD designs here were used as models in microliter scale operation to demonstrate the well-known chemistries of colorimetric determinations of iron, phosphate, and hardness using 1,10-phenanthroline and simple aqueous guava leaf extract; molybdate, and EBT-EDTA complexometric titration, respectively, through calibrations: where Blue (B) value = 88.2log [Fe³⁺] - 80.8, R² = 0.989; B value = 1.75 [Fe³⁺] + 0.198, R² = 0.999; Grey scale (I) value = 1.77 [Fe³⁺] - 1.22, R² = 0.997; Red (R) value = 16.1log [PO₄^3-] + 8.95, R² = 0.999; Hue (H) value = 43.3log [Ca²⁺] + 233, R² = 0.994, respectively. For the hardness, using one of the PAD designs, true titration was also possible. Applications of the proposed devices and procedures were demonstrated for real world samples with validation. Additionally, kinetic study of the molybdenum blue for phosphate was demonstrated using one of the PADs.

Assessing citric acid-derived luminescent probes for pH and ammonia sensing: A comprehensive experimental and theoretical study

The present work reports on the assessment of luminescent probes derived from citric acid (CA) and β-aminothiols (namely, l-cysteine (Cys) and cysteamine) for instrumental and smartphone-based fluorimetric sensing purposes. Remarkably, the evaluated luminescent probes derived from natural compounds showed pH-dependent dual excitation/dual emission features. Both fluorophores hold promise for the ratiometric fluorimetric sensing of pH, being especially convenient for the smartphone-based sensing of pH via ratiometric analysis by proper selection of B and G color channels. Time dependent density functional theory (TDDFT) calculations allowed to substantiate the pH dependent structure-property relationship and to unveil the critical role of the CA derived carboxyl group, these findings contributing to the fundamental knowledge on these systems for the rational design of new fluorophores and in establishing fluorescence sensing mechanisms of CA-derived systems. Besides, paper-based devices modified with CA-Cys were implemented in a three-phase separation approach for sensitive and selective ammonia sensing, yielding a remarkable enrichment factor of 389 and a limit of detection of 37 μM under optimal conditions. The proposed approach was successfully applied to the determination of ammonia nitrogen and extractable ammonium in water samples and marine sediments, respectively.

An "on-off" fluorescent probe based on cucurbit[7]uril for highly sensitive determination of ammonia nitrogen in aquaculture water

A novel "on-off" fluorescent probe was synthesized for highly sensitive and ultra-trace determination of ammonia nitrogen in aquaculture water. Ammonium can react with formaldehyde and sodium hydroxide to form a ring substance (urotropine), which shows no fluorescence signal. Palmatine hydrochloride (PAL) can enter the hydrophobic cavity of cucurbit[7]uril (CB[7]), eventually forming a 1 : 1 host guest complex called PAL@CB[7] under neutral or acidic conditions, which has strong green fluorescence with the maximum excitation (λ_ex) wavelength at 343 nm, and the maximum emission (λ_em) wavelength at 500 nm, while urotropine has a fluorescence quenching effect on the fluorescence enhancement system of PAL@CB[7]. Therefore, a fluorescent chemosensor based on PAL@CB[7] and the reaction of ammonia nitrogen with formaldehyde was developed. The results indicate that the linearity range and the limit of detection of the proposed method are 1-300 μg L^-1 with a good correlation coefficient (r² = 0.9966) and 1.8 × 10^-2 μg L^-1, respectively. Under the optimal conditions, the method was employed for the detection of ammonia nitrogen in real aquaculture water samples, revealing high selectivity and sensitivity. In the future, the combination of the "on-off" fluorescence method, a portable hardware system and intelligent algorithms will provide technology support for the design of on-line sensors for measuring ammonia nitrogen in aquaculture water.

Cost-effective and sensitive anthocyanin-based paper sensors for rapid ammonia detection in aqueous solutions

In this work, we developed a cost-effective and environmentally friendly anthocyanin-based paper sensor with high sensitivity and optical visibility for the rapid detection of ammonia in aqueous solutions. The detection principle is based on a color change upon ammonia exposure to an anthocyanin-containing paper, which can be recorded simply via a smartphone. The paper sensors were fabricated by extracting anthocyanin from different sources (i.e., red cabbage, blueberry, and blackberry) and immersing pre-cut paper in anthocyanin extracts. Anthocyanin was extracted from different sources into water and aqueous ethanolic solution (80%) using solid-liquid extraction (SLE) and sonication assisted extraction (SAE) methods. The sensor sensitivity and optical visibility were improved by selecting a suitable combination of anthocyanin source, extraction technique, and solvent and controlling the ammonia release from the samples via alkalinization using a suitable base. Sensors fabricated with anthocyanin extracted from red cabbage (Red-C) into water using the SLE method and samples alkalinized with NaOH showed higher sensor sensitivity and better optical visibility. The Red-C anthocyanin sensors also exhibited a visible color change from dark to light blue for ammonia samples with concentrations as low as 2 mg NH₃-N/L. Moreover, the spike recovery results of the sensors (101.9-109.4%) were in good agreement with those of the standard spectrophotometry method (105.4-112.2%), which suggest that these biosensors are a promising analytical tool as a replacement for time-consuming and environmentally unfriendly standard spectrophotometry methods for the on-site screening of ammonia.

An environmentally-benign flow-batch system for headspace single-drop microextraction and on-drop conductometric detecting ammonium

This work presents a lab-made automatic flow-batch system for headspace single-drop microextraction and on-drop conductometric sensing ammonium. Sample and NaOH solution are simultaneously pumped into a reaction chamber (RC), where ammonium is converted to ammonia by raising pH. The converted ammonia then diffuses into the headspace of the RC, and reacts with a 100 mM boric acid drop. The conductivity of the drop is measured by an on-drop conductivity probe, which is made by two stainless-steel contacting electrodes. The result shows that the increasing rate of conductivity has a linear relationship to the ammonium concentration in sample (R² = 0.9945). This method has a linear range up to 400 μM, a limit of detection 2.8 μM, a relative standard deviation of 3.0% (200 μM, n = 10) and carryover coefficient 0.028. Measurements of river waters, lake waters and wastewaters have been demonstrated. The recoveries have achieved from 99.0 to 114%. This method avoids using of harmful or odorous reagents and follows the concept of green chemistry.

Phytochemicals toward Green (Bio)sensing

Because of numerous inherent and unique characteristics of phytochemicals as bioactive compounds derived from plants, they have been widely used as one of the most interesting nature-based compounds in a myriad of fields. Moreover, a wide variety of phytochemicals offer a plethora of fascinating optical and electrochemical features that pave the way toward their development as optical and electrochemical (bio)sensors for clinical/health diagnostics, environmental monitoring, food quality control, and bioimaging. In the current review, we highlight how phytochemicals have been tailored and used for a wide variety of optical and electrochemical (bio)sensing and bioimaging applications, after classifying and introducing them according to their chemical structures. Finally, the current challenges and future directions/perspective on the optical and electrochemical (bio)sensing applications of phytochemicals are discussed with the goal of further expanding their potential applications in (bio)sensing technology. Regarding the advantageous features of phytochemicals as highly promising and potential biomaterials, we envisage that many of the existing chemical-based (bio)sensors will be replaced by phytochemical-based ones in the near future.

A simple paper-based analytical device using UV resin screen-printing for the determination of ammonium in soil

This study reports a convenient and low cost paper-based analytical device (PAD) for measuring ammonium in soil. This PAD has been developed using an inexpensive UV resin solution, as a new hydrophobic material, with a screen-printing method to create hydrophobic areas. The paper-based colorimetric device was developed using a modified Berthelot reaction in which salicylate and dichloroisocyanurate are used in order to produce a green compound of 2-2 dicarboxyindophenol in the presence of ammonium. The concentration of ammonium is proportional to the intensity of the resulting green color, as analysed by ImageJ software. A variety of tests was carried out to optimize and evaluate various aspects of the PAD's fabrication and utilization. A linear range was obtained in the range of 10-100 mg L-1 with a limit of detection and limit of quantitation of 0.5 mg L-1 and 1.7 mg L-1, respectively. The relative standard deviation of intra-day measurements was 3.0% and the inter-day precision was 3.2% with good reproducibility. When recovery of ammonium added to samples was evaluated it ranged from 95.5 to 107.5%. The PAD was applied to detect ammonium in a variety of actual soil samples, and the results were validated against spectrophotometric results using a paired t-test, showing good accuracy. A convenient color comparison chart was also created on paper to enable the user to interpret the color results without the need for a color scanner. This developed PAD is fast, easy, inexpensive, and provides an alternative to existing methods for the determination of ammonium in soil.

Rapid naked-eye colorimetric detection of gaseous alkaline analytes using rhodamine B hydrazone-coated silica strips

Development of rhodamine B hydrazone-coated silica strips for rapid detection of alkaline vapors by the naked-eye or using a smartphone camera.

Disposable paper strips for carboxylate discrimination

A fluorescent macromolecular chemosensor, built from readily available components, performs chemical fingerprinting of carboxylate anions on low-cost, disposable paper supports.