Test strip for determination of nitrite in water

A disposable test strip is proposed for the determination of nitrite in waters. The strip is an inert rectangular strip of polyester with a 6 mm o.d. circular, transparent and colorless film attached to its surface. This film contains the chemicals required for reaction and fixation of the dye formed, sulfanilamide, N-(1-naphthyl)ethylenediamine on Nafion. When the test strip is placed in an acidified (pH 2.0) sample solution containing nitrite a red-violet color develops; the absorbance of this is measured at 536 nm. The linear range of the method depends on the time of equilibration of the test strip with the sample solution. When the equilibration time was 45 min, the linear range was 8.9-500 microg L(-1) whereas for an equilibration time of 60 min it was 4.7-200 microg L(-1). The detection limit was 1.4 microg L(-1) for an equilibration time of 60 min. The precision of the method, expressed as RSD, was 8.8 % at 100.0 microg L(-1). The method was applied, and validated chemometrically, for the determination of nitrite in different types of water (spring, mineral, tap, well, and sea).     

A test strip for chloride analysis in environmental water

A disposable and reversible test strip for chloride is proposed. It is based on a polyester strip containing a circular sensing zone, 6 mm in diameter and 9.5 microm in thickness, with all the reagents necessary to produce a selective response to chloride. This sensing zone comprises a plasticized poly(vinyl chloride) (PVC) membrane that incorporates trioctyltin chloride as ionophore and 4',5'-dibromofluorescein octadecyl ester as chromoionophore. The prepared test strip works on a co-extraction system of chloride and hydrogen ions in the sensing zone. This disposable test strip can determine chloride simply by introducing the strip into a water sample containing a pH 2.0 buffer and measuring the absorbance at 534 nm as the analytical signal because the colour changes from red to orange. Experimental variables that influence the sensor response have been studied, especially those related to selectivity and response time. The sensor responds linearly in activities in the range 0.15-24.7 mM. The detection limit is 0.15 mM, the intermembrane reproducibility at a mid-level of the range is 6.4% relative standard deviation (RSD) of log aCl- and the intramembrane reproducibility is 4.5%. The procedure was applied to the determination of chloride in different types of water (tap, well, stream and sea), validating results against a reference procedure. This proposed test system for chloride determination in waters is inexpensive, selective and sensitive and uses only conventional instrumentation.     

Calcium selective test strip for water and milk

We have developed a selective and reversible test strip based on an ion-exchange mechanism to determine calcium. The optical test strip contains a polymeric film of plasticised PVC that contains all of the reagents necessary to produce a response to calcium, among them the new ionophore, 4,13-bis[(N-adamantylcarbamoyl)propionyl]-1,7,10,16-tetraoxa-4,13-diazacyclooctadecane. The measurement of the absorbance at 655 nm in a standard photometer makes it possible to determine calcium activities. The composition of the membrane and reaction conditions have been adjusted to obtain adequate selectivity. The test strip responded linearly to calcium between 0.050 and 135 mM in activities. The reproducibility intermembrane at a medium level of the range was 6.2%, as RSD, of log a(Mg(2+)), and 3.4% as RSD intramembrane. The procedure was applied to the determination of calcium ion in different types of waters (tap, well, spring and mineral) and milks (whole, skimmed, skimmed with calcium added, special types) validating the results against a reference procedure.     

Determination of hypochlorite in water using a chemiluminescent test strip

We have developed a selective and reusable chemiluminescent test strip to determine hypochlorite. The hypochlorite-sensitive test strip contains a 10 mm × 9 mm piece of anionic cellulose paper in fluoresceinate cycle, glued to a 10 mm × 4 cm × 0.5 mm polyester strip. The measurement of the chemiluminescence in a luminometer when 1 ml of sample is injected into a conventional cell containing the strip makes it possible to determine hypochlorite. The composition of the membrane and reaction conditions have been adjusted to obtain adequate sensitivity and selectivity. The test strip responded linearly to hypochlorite over two linear ranges, the first 2.0-10.3 mg L⁻¹ and the second 10.3-51.4 mg L⁻¹, with a detection limit of 0.4 mg L⁻¹. The reproducibility using the same disposable test strip at a medium level of the range was 6.6%, as relative standard deviation (R.S.D.), and 12.3% using different test strips. The procedure was applied to the determination of hypochlorite in different types of waters.     

Enzyme-linked immunosorbent assay and immunochromatographic strip for rapid detection of atrazine in water samples

We have developed a heterologous direct competitive enzyme-linked immunosorbent assay (ELISA) and a colloidal gold-based immunochromatographic (ICG) strip for the determination of the herbicide atrazine in water samples. The ELISA had a half-maximum inhibition concentration (IC₅₀) of 0.12 ng mL^?1 and a limit of detection (LOD, calculated as the IC₁₅ value) of 0.01 ng mL^?1. The average of recoveries for all spiked water samples was 96.5%. There was a good correlation between the data determined by this ELISA and those obtained by high performance liquid chromatography (HPLC) (r ² ?=?0.996). The visual LOD of the ICG strip assay was 2 ng mL^?1. The assay process only took 10 min, and no sample pretreatment was required. Its high specificity, sensitivity and fast detection made the strip well suited for on-site screening of atrazine in water samples. Both the ELISA and the ICG strip assay are useful for rapid analysis of a large number of water samples at low cost.

Characterisation of a transparent optical test strip for quantification of water hardness

An optical and reversible test strip that uses an ion-exchange mechanism which responds equally to calcium and magnesium and makes it possible to determine water hardness is described. The transparent test strip, made of a polyester sheet, has a circular polymeric film of plasticised poly(vinyl chloride) (PVC) that contains all of the reagents necessary to produce an equal response to calcium and magnesium, namely, a cation-selective neutral ionophore, such as 4,13-[bis(N-adamantylcarbamoyl)acetyl]-1,7,10,16-tetraoxa-4,13-diazacyclooctadecane, a chromoionophore, such as lipophilised Nile Blue, and potassium tetrakis (4-chlorophenyl)borate as a lipophilic salt, which it is evaluated by absorbance measurement at 655 nm in a standard photometer.All experimental variables that influence test strip response, especially in terms of selectivity and response time, have been studied. The sensor responded linearly to hardness up to 14,800 mg l⁻¹, in activities, expressed as CaCO₃. The detection limit is 1.9 mg l⁻¹ as CaCO_3, the reproducibility intermembrane at a medium level of the range was 7.0%, as R.S.D., of and 2.6% as intramembrane. The procedure was applied to the determination of hardness in different types of waters (tap, well, mineral and spring) validating results against a reference procedure. This proposed method is quick, inexpensive, selective and sensitive and uses only conventional instrumentation.     

Development of non-enzymatic strip for simple and selective determination of urea in water and biological samples

A simple and selective method for the determination of urea based on the paptode technique is described. The sensor was constructed by immobilizing an ionophore on a TLC strip. The procedure is based on the nucleophilic displacement of urea with tetrachloro-p-benzoquinone (chloranil) as an ionophore, and the formed violet-color product was detected using a flatbed scanner. The color of each spot was analyzed to red (R), green (G) and blue (B) values from 0 to 255 using a program written in visual basic (VB) programming language. The calibration graph obtained with the proposed sensor was linear over the range of 0.05?C10.00?mg?L^?1 with a detection limit of 0.01?mg?L^?1 for urea. Parameters such as pH and concentration of chloranil were optimized. The proposed sensor was successfully applied for the determination of urea in bovine serum, urine and tap water samples.     

Paper test strip for silver ions detection in drinking water samples based on combined fluorometric and colorimetric methods

In this study, a portable silver ion (Ag⁺) sensor was fabricated based on a dual signal output system using black phosphorus quantum dots (BPQDs) as probes. It is the first work for Ag⁺ detection using paper test strip based on BPQDs. The color change of BPQDs paper sensor for the determination of Ag⁺ was easily identified by naked eye. BPQDs were synthesized from bulk black phosphorus (BP) by mechanical exfoliation combined with a solvothermal method. BPQDs exhibited blue fluorescence with a quantum yield of 8.82 %. The fluorescence of BPQDs can be quenched by Ag⁺, and the absorbance of BPQDs is increased with increasing Ag⁺ concentration. The mechanism of the interaction between BPQDs and Ag⁺ involving fluorescence quenching and bonding was investigated by experimental and computational methods. The detection limit of Ag⁺ was 1.56 μg/mL and 0.19 μg/mL using fluorometry and colorimetry methods, respectively. A portable visual sensor based on paper test strip was constructed for Ag⁺ detection using the colorimetric approach. The strategy was employed to determine Ag⁺ successfully in drinking water samples. Therefore, the proposed portable Ag⁺ sensor can be potentially utilized for the lab-free analysis of drinking water and even dietary samples.     

Magnesium-selective test strip

A selective test strip based on an ion-exchange mechanism to determine magnesium is described. This optical and reversible test strip, made on a transparent polyester sheet, has a circular polymeric film of plasticised PVC that contains all of the reagents necessary to produce a response to magnesium, namely, a cation-selective neutral ionophore, 2-amino-N,N′-diheptyl-N,N′-dimethyl-butanediamide, a chromoionophore, N,N-diethyl-5-(octadecanoylimino)-5H-benzo[a]phenoxazine-9-amine, and potassium tetrakis (4-chlorophenyl)borate (TCPB) as a lipophilic salt, which is evaluated by absorbance measurement at 655 nm in a standard photometer. The experimental variables that influence test strip response, especially in terms of selectivity and response time, have been studied. The test strip responded linearly to magnesium between 0.009 and 160 mM in activities. The reproducibility intermembrane at a medium level of the range was 8.0%, as R.S.D., of log aMg2+, and 5.9% as R.S.D. intramembrane. The procedure was applied to the determination of magnesium in different types of waters (tap, well and mineral) validating the results against a reference procedure. This proposed method is quick, inexpensive, selective and sensitive and uses only conventional instrumentation.     

A test strip for diamines in tuna.

This study describes the production of a solid-phase assay (test strip/dipstick test) for putrescine and cadaverine in tuna based on the coupling of an amine oxidase to a peroxidase/dye system. The assay was linear to 75 microM in phosphate buffer, and the minimum detectable concentration was 0.5 microM (< 0.1 ppm), corresponding to 0.01 mg% in spiked extracts. Intra- and interassay precisions were < 20%. Test strips were stable at 4 degrees C for at least 12 months. Lysine, ornithine, and histidine did not react with the assay, and histamine reacted only minimally. Sixteen fish samples were tested by test strip and the standard AOAC protocol, and results were in good agreement.     

Self-assembly of a molecular figure-of-eight strip

A hexanuclear copper(ii) complex with a figure-of-eight strip topology is formed by metal-directed self-assembly of tritopic ligand L, bis-bidentate glycine hydroxamic acid and Cu(ii) ions in a 2:2:6 ratio.     

Screening nitrate in forages with a test strip: collaborative study

A collaborative study was conducted for screening nitrate in forages with a commercially available test strip. The method involves extracting a finely ground sample with deionized water. The test strip is dipped in the sample extract. The color of the reaction zone on the test strip changes from white to pink or purple depending on the nitrate concentration in sample extract. The nitrate present in the extract is determined by comparing the color of the test strip to the color scale on the test strip container. Six blind quintuplicates of forage samples were analyzed by 20 collaborators. Nitrate concentrations in forage samples tested ranged from < 1000 ppm nitrate to > 10,000 ppm nitrate on dry matter basis. Each collaborator was asked to assign each sample to one of the 4 following nitrate concentration ranges: (1) < 1000 ppm, (2) 1000 to 5000 ppm, (3) > 5000 ppm to 10,000 ppm, and (4) > 10,000 ppm. Nineteen of 20 collaborators reported results. Results from 2 laboratories were rejected as outliers by inspection and chi 2 test. Sensitivity rates (p+) ranged from 0.965 to 0.998, with standard errors of 0.006 to 0.16. Specificity rates (p-) ranged from 0.991 to 0.997 for the 4 ranges, with standard errors of 0.003 to 0.006. False-positive rates (pf+) ranged from 0.006 to 0.046, with standard errors of 0.006 to 0.025. False-negative rates (pf-) ranged from 0.003 to 0.007, with standard errors of 0.003 to 0.006. Screening nitrate in forages with a test strip has been adopted first action by AOAC INTERNATIONAL.     

Multiplexed paper test strip for quantitative bacterial detection

Rapid, sensitive, on-site detection of bacteria without a need for sophisticated equipment or skilled personnel is extremely important in clinical settings and rapid response scenarios, as well as in resource-limited settings. Here, we report a novel approach for selective and ultra-sensitive multiplexed detection of Escherichia coli (non-pathogenic or pathogenic) using a lab-on-paper test strip (bioactive paper) based on intracellular enzyme (β-galactosidase (B-GAL) or β-glucuronidase (GUS)) activity. The test strip is composed of a paper support (0.5 × 8 cm), onto which either 5-bromo-4-chloro-3-indolyl-β-D: -glucuronide sodium salt (XG), chlorophenol red β-galactopyranoside (CPRG) or both and FeCl(3) were entrapped using sol-gel-derived silica inks in different zones via an ink-jet printing technique. The sample was lysed and assayed via lateral flow through the FeCl(3) zone to the substrate area to initiate rapid enzyme hydrolysis of the substrate, causing a change from colorless-to-blue (XG hydrolyzed by GUS, indication of nonpathogenic E. coli) and/or yellow to red-magenta (CPRG hydrolyzed by B-GAL, indication of total coliforms). Using immunomagnetic nanoparticles for selective preconcentration, the limit of detection was ~5 colony-forming units (cfu) per milliliter for E. coli O157:H7 and ~20 cfu/mL for E. coli BL21, within 30 min without cell culturing. Thus, these paper test strips could be suitable for detection of viable total coliforms and pathogens in bathing water samples. Moreover, inclusion of a culturing step allows detection of less than 1 cfu in 100 mL within 8 h, making the paper tests strips relevant for detection of multiple pathogens and total coliform bacteria in beverage and food samples.     

Fluorescent strip sensor for rapid determination of toxins

Here, we report a simple fluorescent strip sensor based on aptamer-quantum dots technology that can meet toxin monitoring demands using ochratoxin A (OTA) as a model toxin. The limit of the detection (LOD) for the fluorescent strip was 1.9 ng mL(-1), while the time needed for the detection is only 10 min; this conforms to the standards of World Health Organization (WHO) or better. Overall functional parameters are also better than the analogous characteristics of gold nanoparticle strips. High selectivity was maintained as well, making them suitable for the samples with complex solution composition.     

A visual strip sensor for determination of iron

A visual strip has been developed for sensing iron in different aqueous samples like natural water and fruit juices. The sensor has been synthesized by UV-radiation induced graft polymerization of acrylamide monomer in microporous poly(propylene) base. For physical immobilization of iron selective reagent, the in situ polymerization of acrylamide has been carried out in the presence of 1,10-phenanthroline. The loaded strip on interaction with Fe(II) in aqueous solution turned into orange red color and the intensity of the color was found to be directly proportional to the amount of Fe(II) in the aqueous sample. The minimal sensor response with naked eye was found for 50 ng mL⁻¹ of Fe in 15 min of interaction. However, as low as 20 ng mL⁻¹ Fe could be quantified using a spectrophotometer. The detection limit calculated using the 3s/S criteria, where ‘s’ is the standard deviation of the absorbance of blank reagent loaded strip and ‘S’ is the slope of the linear calibration plot, was 1.0 ng mL⁻¹. The strip was applied to measure Fe in a variety of samples such as ground water and fruit juices.     

Electric fields in an electrolyte solution near a strip of fixed potential

Electrostatic fields produced by flat electrodes are often used to manipulate particles in solution. To study the field produced by such an electrode, we consider the problem of an infinite strip of width 2a with imposed constant potential immersed in an electrolyte solution. Sufficiently close to the edge of the strip, the solution is determined by classical electrostatics and results in a field singularity. We examine two limiting cases, (a) when strip width a<1k, the Debye screening length, and (b) when strip width is much greater than the Debye screening length, a>1k. We present exact results for the two cases in the limit of small potentials where the Poisson-Boltzmann equation can be linearized. By drawing on an analogy with antiplane shear deformations of solids, and by employing the path-independent J integral of solid mechanics, we present a new method for determining the strength of the edge singularity. The strength of the singularity defines an exact near-field solution. In the far field the solution goes to that of a line of charge. The accuracy of the solution is demonstrated by comparison with the numerical solutions of the Poisson-Boltzmann equation using the finite element method.     

Disposable amperometric glucose sensor electrode with enzyme-immobilized nitrocellulose strip

Electrochemical properties of screen-printed carbon paste electrodes (CPEs) with a glucose oxidase-immobilized and hexamineruthenium (III) chloride ([Ru(NH₃)₆]³⁺) containing nitrocellulose (NC) strip were examined. The NC strip (2×8 mm) placed on the CPEs printed on polyester (PE) film is tightly sealed using another PE film on the top with open edges on both sides. Samples containing macromolecules and particles (e.g. proteins and blood cells) are applied at one edge of the NC strip and reach the detection area, chromatographically separating small molecules (e.g. glucose, ascorbate, acetaminophen, and uric acid) of analytical interests. Since sample volumes and the amount of catalytic reagents (mediator and glucose oxidase) are precisely predefined by the dimension and pore size (8 μm) of the NC strip, the sensor-to-sensor reproducibility and accuracy of analysis are greatly improved. The use of [Ru(NH₃)₆]³⁺ mediator, which exhibits characteristic substantially lowers the applied potential (0.0 V vs Ag/AgCl) for glucose determination and eliminates the interference from other oxidizable species, providing improved analytical results.