Visual Colorimetry Research Articles

Headspace-single drop microextraction based visual colorimetry for non-chromatographic speciation analysis of nitrite and nitrate in environmental water sample

In this work, a visual colorimetric method based on headspace-single drop microextraction (HS-SDME) was developed for the non-chromatographic speciation analysis of nitrite and nitrate. The method adequately exploited the individual chemical vapor conditions of nitrite and nitrate that were converted to NO by specific reductants. Gaseous NO underwent a diazotization reaction with chromogenic reagent through the HS-SDME process to change from colorless to purplish-red. Instead of using large, time-consuming and relatively complicated chromatographic equipment for separation, the species of nitrite and nitrate could be analyzed by simply changing the reagents. Meanwhile, the extraction method effectively mitigated the matrix interference of solution and improved the sensitivity of method, with intuitive discrimination of color by the naked eye. The limits of detection were 0.07 μg/mL for nitrite and 0.2 μg/mL for nitrate, respectively. This method has been applied to analysis of real environmental water samples, which also effectively extended the application scope of HS-SDME device.

Visual colorimetric label for real-time monitoring of SO2 concentration change in grape and mango during storage

Sulfur dioxide (SO2) is widely utilized as a preservative in food transportation and storage, but excessive consumption poses health risks. This study presents a novel and efficient method for the real-time detection of SO2 using a sensor named TK, synthesized from triphenylamine and 2-cyanomethyl-1-methyl-quinolinium. The core mechanism involves the Michael addition reaction of the CC bond in TK with SO2, which disrupts the intramolecular charge transfer process, resulting in a significant color change and a blue shift in fluorescence emission. Methodologically, the sensor's response was quantified by the change in fluorescence intensity ratio (I425/I647) within a SO2 concentration range of 0–180 μM. The sensor exhibited high sensitivity and selectivity. For practical application, TK was incorporated into hydrophilic polyvinyl alcohol to create a smart label capable of visual colorimetry and fluorescence analysis. SO2 concentration changes were monitored by using this label, demonstrated by the color transition from burgundy red to colorless, yielding a maximum color difference (ΔE) of 73.6. The smart label was successfully used to monitor the quality of various grapes and mangoes during long-term storage, providing a reliable, equipment-independent method suitable for household use. The study offers a new tool for enhancing food safety and mitigating health risks associated with SO2 exposure.

Highly sensitive hydrolytic nanozyme-based sensors for colorimetric detection ofaluminum ions.

Hydrolytic nanozyme-based visual colorimetry has emerged as a promising strategy for the detection of aluminum ions. However, most studies focus on simulating the structure of natural enzymes while neglecting to regulate the rate of hydrolysis-related steps, leading to low enzyme-like activity for hydrolytic nanozymes. Herein, we constructed a ruthenium dioxide (RuO2) in situ embedded cerium oxide (CeO2) nanozyme (RuO2/CeO2) with aLewis acid-base pair (Ce-O-Ru-OH), which can simulate the catalytic behavior of phosphatase (PPase) and can be quantitatively quenched by Al3+ to achieve accurate and sensitive Al3+ colorimetric sensing detection. The incorporation of Ru into CeO2 nanorods accelerates the dissociation of H2O, followed by subsequent combination of hydroxide species to Lewis acidic Ce-O sites. This synergistic effect facilitates substrate activation and significantly enhances the hydrolysis activity of the nanozyme. The results show that the RuO2/CeO2 nanozyme exhibits a limit of detection as low as 0.5ng/mL. We also demonstrate their efficacy in detecting Al3+ in various practical food samples. This study offers novel insights into the advancement of highly sensitive hydrolytic nanozyme engineering for sensing applications.

Highly Sensitive Whole-Cell Mercury Biosensors for Environmental Monitoring.

Whole-cell biosensors could serve as eco-friendly and cost-effective alternatives for detecting potentially toxic bioavailable heavy metals in aquatic environments. However, they often fail to meet practical requirements due to an insufficient limit of detection (LOD) and high background noise. In this study, we designed a synthetic genetic circuit specifically tailored for detecting ionic mercury, which we applied to environmental samples collected from artisanal gold mining sites in Peru. We developed two distinct versions of the biosensor, each utilizing a different reporter protein: a fluorescent biosensor (Mer-RFP) and a colorimetric biosensor (Mer-Blue). Mer-RFP enabled real-time monitoring of the culture's response to mercury samples using a plate reader, whereas Mer-Blue was analysed for colour accumulation at the endpoint using a specially designed, low-cost camera setup for harvested cell pellets. Both biosensors exhibited negligible baseline expression of their respective reporter proteins and responded specifically to HgBr2 in pure water. Mer-RFP demonstrated a linear detection range from 1 nM to 1 μM, whereas Mer-Blue showed a linear range from 2 nM to 125 nM. Our biosensors successfully detected a high concentration of ionic mercury in the reaction bucket where artisanal miners produce a mercury-gold amalgam. However, they did not detect ionic mercury in the water from active mining ponds, indicating a concentration lower than 3.2 nM Hg2+-a result consistent with chemical analysis quantitation. Furthermore, we discuss the potential of Mer-Blue as a practical and affordable monitoring tool, highlighting its stability, reliance on simple visual colorimetry, and the possibility of sensitivity expansion to organic mercury.

Practical Research on Soil Nutrient Detection Technology Based on Visual Colorimetry

Practical Research on Soil Nutrient Detection Technology Based on Visual Colorimetry

Selective detection of ionic liquid fluorescence probes for visual colorimetry of different metal ions

At present, the fast distinction of different metal ions in pure water media is not only a great challenge, but also drives the protection of water quality in environmental water bodies. In this paper, a novel ionic liquid fluorescent probe Glycolic Acid-L-Arginine (GA-L-Arg) was rationally created and designed through an in-depth study of ionic liquids. It is also used as an innovative multi-ion fluorescent probe for colorimetric detection and separate identification of Fe3+ and Co2+ in aqueous solutions of various metal ions. GA-L-Arg has excellent water solubility due to the strong hydrophilicity of Glycolic Acid and L-Arginine. The probe showed high sensitivity, extremely significant selectivity, and great pH stability for Fe3+ and Co2+ in pure water. The GA-L-Arg structure and the mechanism of Fe3+ and Co2+ detection were analyzed by infrared spectroscopic characterization and quantum chemical calculations. More importantly, the distinct colorimetric partitioning of Fe3+ and Co2+ was performed by the unique extraction of Fe3+ in the presence of the fluorescent probe and buffer solution.

Headspace single drop microextraction based visual colorimetry for highly sensitive, selective and matrix interference-resistant determination of sulfur dioxide in food samples.

Excessive intake of SO2, a widely-used food additive, is able to cause respiratory, cardiovascular and neurological disease. For effectively monitoring SO2 level, we have developed a headspace single drop microextraction based visual colorimetry for highly sensitive and selective sensing of SO2 with TMB (3,3',5,5'-tetramethylbenzidine) as a classic chromogenic reagent. A combination of single drop and headspace microextraction integrated merits of high extraction efficiency, low consumption of reagents and excellent matrix interference-resistant ability. The colorimetric principle was based on oxidation of TMB, and SO2 could compete with TMB to preferentially react with ·OH, resulting in the fading of color blue that could be easily read out by naked eye. LOD was calculated to be 0.53μM and 5μM by UV-vis and naked eye, respectively. The method was successfully utilized to analysis of food samples, and the experimental device was miniaturized and easy to construct, thus showing a promising potential in field analysis.

Graphene oxide-enhanced colorimetric detection of Mec A gene based on toehold-mediated strand displacement

Mec A, as a representative gene mediating resistance to β‐lactam antibiotics in methicillin-resistant Staphylococcus aureus (MRSA), allows a new genetic analysis for the detection of MRSA. Here, a sensitive, prompt, and visual colorimetry is reported to detect the Mec A gene based on toehold‐mediated strand displacement (TMSD) and the enrichment effect of graphene oxide (GO). The Mec A triggers to generate the profuse amount of signal units of single-stranded DNA (SG) composed of a long single‐stranded base tail and a base head: the tail can be adsorbed and enriched on the surface of GO; the head can form a G quadruplex structure to exert catalytic function towards 2,2′‐azino‐bis (3‐ethylbenzthiazoline‐6‐sulphonic acid). Therefore, through the enrichment effect of GO, the signal units SG reflects different degrees of signal amplification on different substrates (such as aqueous solution or filter membrane). This strategy demonstrates a broad linear working range from 100 pM to 1.5 nM (solution) and 1 pM to 1 nM (filter membrane), with a low detection limit of 39.53 pM (solution) and 333 fM (filter membrane). Analytical performance in real samples suggests that this developed colorimetry is endowed with immense potential for clinical detection applications.

Chiral Recognition of D/L-Ribose by Visual and SERS Assessments.

Ribose is the central molecular unit in ribose nucleic acid (RNA). Ribose is a key molecule in the study of many persistent scientific mysteries, such as the origin of life and the chiral homogeneity of biological molecules. Therefore, the chiral recognition of ribose is of great significance. The traditional method of chiral recognition of ribose is HPLC, which is time-consuming, expensive, and can only be operated in the laboratory. There is no report on optical analytical techniques that can quickly detect the chirality of ribose. In this study, a simple and convenient approach for the chiral recognition of ribose has been developed. β-cyclodextrin(β-CD)-coated Ag NPs aggregate after adding D-ribose, so that D-/L-ribose can be identified using visual colorimetry and/or surface-enhanced Raman spectroscopy (SERS). The color change visible to the naked eye can readily distinguish the chirality of ribose, while the SERS method can provide the more sensitive analysis of enantiomeric ribose. The advantages of this method are that it is fast, convenient, low cost, and can be operated outside the laboratory. DFT calculations show that D-ribose and cyclodextrin have the same chirality, forming multiple strong hydrogen bonds between them; thus, D/L-ribose will induce different optical effects.

Syntheses and fluorescence properties of lanthanide isostructural complexes derived from aspartic acid.

To protect ecosystem balance and human health, the development of fluorescent sensing materials with high sensitivity and portability has attracted wide attention in several decades. Herein, six lanthanide isostructural complexes {[Ln(μ6-Hcaa)(H2O)]Cl}n (H3caa = N-(4-carboxylbenzyl)-L-aspartic acid, Ln3+ = Ce3+ (1), Pr3+ (2), Nd3+ (3), Sm3+ (4), Eu3+ (5), and Tb3+ (6)) with optical properties based on aspartic acid derivative (H3caa) were synthesized by the solvothermal method and characterized in detail. It is worth noting that complex 6 can not only specifically recognize Cr(VI) with very low detection limits (LODs) of 3.66 nM (Cr2O72-) & 5.35 nM (CrO42-) but also selectively recognize TCs with LODs of 0.24 μM (CTC = chlortetracycline) and 0.25 μM (TC = tetracycline) based on the method of fluorescence detection. In addition, the identification of Cr(VI) and TCs by visual colorimetry may be realized through the combination of a smartphone and portable test strips. This study suggests that complex 6 is a good optical material for detecting heavy metals and antibiotic contaminants in aqueous systems and broadens the development of amino acid derivatives.

A microplasma converter-based spectrophotometry and visual colorimetry for nonchromatographic speciation analysis of H2S/SO2 or S2-/[formula omitted] in environmental water samples

Pollution of sulfur compounds could result in serious environmental issues, while the excessive intake tended to damage the respiratory and nervous system of living beings. In this article, a microplasma converter with small size, easy operation and low gas and power consumption was constructed to fulfill the high-efficient oxidation process of H2S to SO2, which was applied to both spectrophotometry and visual colorimetry to achieve a simple and quick nonchromatographic speciation analysis of H2S/SO2 or S2-/SO32-. Especially, analytes were introduced by acidified to vapor, greatly alleviating the matrices interferences, and visual colorimetry could attain direct naked eye detection without expensive instruments and tedious operation procedures. The lowest limit of detection was 3.47 μmol/L for both aqueous S2- and SO32- ions, and the integration of two methods covered a wide detection range of 0.05–7 mmol/L. These developed methods were applied to analysis of real environmental water samples successfully.

PVC dechlorination residues as new peroxidase-mimicking nanozyme and chemiluminescence sensing probe with high activity for glucose and ascorbic acid detection

Polyvinyl chloride (PVC) dechlorination residues urgently need to be used to improve PVC recycling efficiency. Herein, PVC dechlorination residues were used for visual colorimetry and chemiluminescence (CL) sensing for the first time. It was found that the dechlorination residues of PVC and iron chips treated by subcritical water showed excellent peroxidase-mimicking activity and could catalyze enzyme substrate 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation to produce blue product. Based on this, residues colorimetric analysis was developed to detect hydrogen peroxide in the range of 3.2 × 10−8–9.5 × 10−5 M and glucose in the range of 8.9 × 10−6–9.2 × 10−4 M. The detection limits were estimated to be 4.5 × 10−9 M for hydrogen peroxide and 6.8 × 10−7 M for glucose, respectively. The kinetic investigation of enzymatic reaction confirmed that the simulated enzyme activity of the residues was much stronger than that of the previously synthesized nanocomposite (The Km and Vmax of the residue system with TMB as the substrate were 0.298 mM and 207.3 × 10−8 M S−1, respectively). In addition, the residues could alone trigger luminol CL reaction to produce strong CL signal with no need for oxidant, and the residues CL analysis was developed for detection of ascorbic acid (AA) in the range of 1.5 × 10−12 M to 7 × 10−8 M with the detection limit of 0.32 pM. The results affirmed that the residues were fantastic colorimetric and CL sensing material, which not only provided vital reference for residues applications in other sensing assays, but also was important for construction of the sensing platforms using wastes.

Ultra-Low Detection of Infectious Disease Nucleic Acid Biomarkers By a Visual Colorimetric Sensor

The gold standard method for detecting nucleic acid markers is the polymerase chain reaction. However, this method demands high technical skills, labor-intensive, time-consuming, and expensive. Many resource-limited countries around the globe do not have access to such laboratory-based PCR tests. To address the global test needs of virus and other pathogen infections, as well as address the early biomarker detection of diseases. We have developed an affordable nanoparticle colorimetric sensor approach. In our system, the virus RNA marker is captured by the complementary oligonucleotide attached to low fouling gold-coated iron oxide nanoparticles. Further specificity is achieved through a second complementary hybridization probe conjugated with streptavidin-horseradish peroxidase labels. Visual colorimetry is performed using a chromogenic substrate allowing the visual conversion of the colorless solution to a blue color confirming a positive test. The intensity of the color is in direct correlation to the concentration of the virus RNA marker. We have successfully demonstrated attomolar detection in buffer and picomolar detection in real undiluted biofluids such as in neat human serum. Our ongoing efforts focus on saliva-based non-invasive detection with ultra-low capabilities, optimization of the assay, repeatability, and expanding this work for a multiplex detection format

Sensitive optical thermometer via efficient energy transfer in dual-emitting thermochromic YNbO4:Bi3+, Eu3+ phosphor

The remote and non-invasive optical thermometer exploiting fluorescence intensity ratio technology is of great importance in various intelligent optoelectronic sensors. Dual-emitting YNbO4:Bi3+, Eu3+ phosphor is synthesized and investigated for the development of temperature sensing materials. The phosphor excited at 318 nm shows a blue broad band emission and several red sharp peaks. The value of Bi3+→Eu3+ energy transfer can reach 98.2% when Eu3+ doping content is 0.14. Benefiting from diverse thermal response behaviors due to Bi3+ and Eu3+ ions completely different electronic configurations, a dual-emitting optical thermometer is designed. The maximum values of absolute and relative sensitivities are as high as 2.51% K−1 at 473 K and 1.43% K−1 at 423 K, respectively. The thermochromic phosphor possesses a great chromaticity shift (ΔE = 317×10−3) with temperature. Based on superior color discriminability, visual colorimetry temperature measurement relied on thermal characteristic is proposed. All the results demonstrate that YNbO4:0.01Bi3+, 0.005Eu3+ phosphor as a sensitive and reliable optical thermometer has great potentials in temperature measurement.

Lanthanide-based complexes as efficient physiological temperature sensors

A new molecular thermometric sensor based on the terbium(III) complex [C2mim][Tb(fod)4] (C2mim – 1-methyl-3-ethylimidazolium, fod− - tetrakis-6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate), doped with 0.015% of its europium(III) analogue (1, [C2mim][Tb(fod)4]0.99985:[C2mim][Eu(fod)4]0.00015), was prepared and its thermochromic behaviour evaluated from ambient temperature up to 75 °C, including in the physiological range (35–45 °C). It was found that the intensity ratio of the 5D4→7F5 (TbIII) and 5D0→7F2 (EuIII) transitions is correlated with temperature having three different linear regimes. Visual colorimetry allowed the evaluation of the temperature in different ranges from green at ambient temperature, to yellow and finally red at higher temperatures. The TbIII complex emission intensity is extremely sensitive to small temperature variations, particularly between 25 and 35 °C, were it reaches only 40% of the initial intensity.Confinement of the dopped TbIII tetrakis-complex in the organic polymeric matrix poly(methylmethacrylate) (PMMA) induced higher thermal stability in 1, together with a strong temperature dependence of the most intense emissive transition of the TbIII complexes. The photoluminescence quantum yield of polymer-lanthanide hybrid materials increased significantly compared with that of 1. Under 366 nm irradiation, the hybrid material presents a green colour at 25 °C that evolves to yellow at 30 °C and to a white tone at 35 °C.

The development of thermal immunosensing for the detection of food-borne pathogens E. coli O157:H7 based on the novel substoichiometric photothermal conversion materials MoO3-x NPs

A novel substoichiometric photothermal conversion materials MoO3-x nanoparticles (4-MBA MoO3-x NPs) was first prepared by 4-mercaptobenzoic acid modified based on the oxygen vacancy defects of molybdenum oxide and used to develop a thermal immunosensor for the detection of pathogens E. coli O157:H7.4-mercaptobenzoic acid (4-MBA) being selected as the reducing agent functionalized surface modifier to prepare 4-MBA MoO3-x NPs, which was an excellent photothermal conversion materials and had 42.9 % photothermal conversion efficiency under 808 nm laser. Photothermal sensor probes prepared by 4-MBA-MoO3-x NPs being coupled with antibodies was used to develop the immunoassay for the detection of E. coli O157:H7 by means of immunomagnetic separation technology. The method could be used for semi-quantitative and quantitative determine of the E. coli O157:H7 by visual colorimetry and infrared thermal imaging. For the quantitative determination, the E. coli O157:H7 was quantified by recording the temperature response of the target bacteria combined with the nanomaterial under 808 nm laser with infrared camera. Under the optimal conditions, the low limit of detection (LOD) of the method was 14 CFU/mL with a linearity range of 1.4 × 101-1.4 × 106 CFU/mL, and the recoveries was 82.45 %−105.08 % in real samples analysis.

Orange red-emitting carbon dots for enhanced colorimetric detection of Fe3.

Carbon dots (CDs) have demonstrated considerable potential for use in sensing and bioimaging applications based on their unique intrinsic properties. However, CDs have typically been used as fluorescence sensors, as opposed to indicators, based on their ultraviolet absorption and discoloration performance. In this study, orange red-emitting CDs with a high quantum yield (QY) of 53% were constructed from 2,3-diaminopyridine via solvothermal synthesis. Different from the fluorescence quenching behavior reported previously, the constructed CDs demonstrated a unique character in their ultraviolet absorption and fluorescence emission in the presence of Fe3+. The color of the CD solution varied from mauve to orange following the addition of Fe3+ at concentrations exceeding 1 μM, which enabled these CDs to be used in the determination of the presence of Fe3+ in lake water. In addition, due to their negligible cytotoxicity, good solubility, and adequate dispersity, an outstanding cellular probe with near-infrared fluorescence was established. Overall, this study presents a unique CD-based sensor, details its fluorescence mechanism, visual colorimetry, and ultraviolet absorption variations, and confirms its applicability in near-infrared cellular imaging and environmental analyses.

5,10,15,20-tetrakis (4-carboxylphenyl) porphyrin functionalized NiCo2S4 yolk-shell nanospheres: Excellent peroxidase-like activity, catalytic mechanism and fast cascade colorimetric biosensor for cholesterol

5,10,15,20-tetrakis (4-carboxylphenyl) porphyrin functionalized NiCo2S4 yolk-shell nanospheres (Por-NiCo2S4) were fabricated by a facile and efficient solvothermal strategy and demonstrated to possess excellent peroxidase-like activity. Compared with pure NiCo2S4 nanospheres, Por-NiCo2S4 exhibits the superior peroxidase activity, which was evaluated by the fast oxidation of 3,3′,5,5′-tetramethylbenzidine (a chromogenic substrate, TMB) by H2O2 to form the blue product (oxTMB) only in 10 s. The systematic studies including radical scavenger, fluorescence probe, and electrochemical experiments confirm that superoxide free radicals (O2−) play a key role in catalytic reactions, in addition to the synergistic effect of holes (h+). Based on the developed peroxidase activity of Por-NiCo2S4, and combined with cholesterol oxidase (Chox), a fast cascade reaction sensing platform has been constructed to quantitatively determination cholesterol in a wide linear range of 0.1−9.0 mM with the detection limit of 19.36 μM. The Por-NiCo2S4 based colorimetric biosensing platform has been used for determining the levels of total cholesterol in serum samples, indicating Por-NiCo2S4 is expected to be used as the promising materials for fast visual colorimetry in clinical medical detection.

Field analysis free chlorine in water samples by a smartphone-based colorimetric device with improved sensitivity and accuracy

It is highly desirable to regularly monitor the concentration of free chlorine in water samples because residual free chlorine can greatly affect the health of human and the growth of aquatic baby animals. Visual colorimetry is a cost-effective method to monitor the concentration of residue chlorine in water. To reduce the errors from estimating the hue by naked eyes and to enhance the sensitivity, we propose a smartphone-based portable colorimetric device with long-path and self-calibration ability. The key idea relies on a flow colorimetric tube with the illumination in empty tube or at a low absorption wavelength as the internal references to correct the baseline drift due to light source intensity decay. The self-focusing ability in the round-bottom colorimetric tube is helpful to simplify the design of the portable device. The absorbance measured by proposed device obeyes the Lambert-Beer law in the range of 0–1. By using N,N‑diethyl‑p‑phenylenediamine as the colorimetric regent, the proposed method demonstrates a promising result for the determination of free chlorine with the linear range of 0.8–14 μM and the limit of detection of 0.23 μM. This simple colorimetric device offers the advantages of cost effectiveness, ease of implementation, portability, sensitivity and reliability for field analysis.

Novel methods for the quantification of toxic, residual phase transfer catalyst in fluorine-18 labeled radiotracers

IntroductionFluorine-18 labeled radiopharmaceuticals undergo quality control testing for residual phase-transfer-catalyst content. The almost universally used quality-control test is a silica plate spot-test comparison of the radiopharmaceutical beside a 50-ppm standard. Once developed by staining, the radiopharmaceutical spot must be of equal or less intensity to pass the test. There is currently a need for a quantitative, inexpensive, and less subjective quality control method that allows the automatic incorporation of the acquired measurement directly into electronic batch reports. ResultsIn the developed method, a resazurin test solution is mixed with an aliquot of the radiopharmaceutical analyte along with dichloromethane (DCM). The mixture is vortexed. The potassium resazurin-phase transfer catalyst complex solubilizes into the DCM imparting a blue color. The organic layer is then removed for analysis.Three measurement methods were utilized: visual colorimetry against pre-prepared standards, spectrophotometric measurement of transmittance, and electrical conductance. A simple prototype spectrophotometer and an electrical test cell were constructed to acquire data.Sodium Resazurin dye was found to be a suitable test chromophore for residual phase transfer catalyst analysis of aqueous solutions. Quantitative spectrophotometric measurements are possible in the 0–100-ppm range (18-crown-6) and 0–150-ppm range (Kryptofix® or tetrabutylammonium). Electrical resistance measurements of the phase transfer-catalyst resazurin complex in DCM are also a viable method, allowing quantitative phase transfer catalyst measurements in the 0–100-ppm range. ConclusionThe methodologies developed are more quantitative alternatives to the current spot-test method. The spectrophotometric method was determined to be the most accurate method.