Multicolor Changes Research Articles

A plasmonic ELISA for multi-colorimetric sensing of C-reactive protein by using shell dependent etching of Ag coated Au nanobipyramids

The etching of gold nanorods/nanobipyramid, or silver-coated nanorods/nanobipyramid inducing plasmon changes represents an efficient strategy to improve the performance of enzyme-linked immunosorbent assay (ELISA). However, the effect of shape on the sensitivity was negligible, especially the thickness of coated silver shell. Here, we propose a plasmonic ELISA for multi-colorimetric detection of CRP based on the etching of Ag-coated Au nanobipyramid (Au NBP@Ag). The effect of silver shell thickness on the sensitivity of plasmon peak shifting was investigated by experiments and DDA calculations. The relationship between the Ag shell thickness and the sensitivity of plasmon peak shifting was obtained. Our results reveal that the thickness of coated Ag shell acts as a key factor in the multi-color change of Au NBP@Ag etching. It is found that Au NBP@Ag with medium Ag shell thickness and rod-like shape has the higher sensitivity and is suitable for sensing. At the optimized most sensitive Ag shell, the detection limit of proposed plasmonic ELISA for CRP was determined to be 0.09 ng/mL with a spectrometer in the range from 0.09 ng/mL to 25 ng/mL. Importantly, the visual detection limit was 0.78 ng/mL, which allows the differential diagnosis with the naked eye. Compared with traditional ELISA with the monochromatic intensity variations, the multi-color ELISA proposed in this study has a large linear range and rich color variation for high-sensitivity and naked-eye semi-quantitative detection.

A novel Schiff's base conjugate with multicolor changes in multiple states and its multipurpose applications for selective detection of Cu2+ in aggregated state and on silica gel strip

Herein, a pair of Schiff's base conjugates were developed by embellishing benzil hydrazone with 4-dimethylaminobenzaldehyde and 4-(diethylamino)salicylaldehyde, respectively. Particularly, 4-(diethylamino)salicylaldehyde-functionalized Schiff's base featured multicolor changes in multiple states (solution, solid film and aggregation states). To be specific, it displayed the excellent photochromic property in CHCl3 solution under UV365nm irradiation, showed solvatochromic phenomena in solid film via CHCl3 or EtOH solvents/vapours, and exhibited bicolor emission (yellow to cyan) from aggregation-induced emission (AIE) characteristic to aggregation-induced quenching (ACQ) effect in THF/water system. Moreover, it served as a fluorescent probe for sensing Cu2+ in THF/water mixtures (v/v, 1:1) with the limit of detection (LOD) of 0.2 μM. Furthermore, the orange silica gel testing strip prepared by immersing silica gel plate in the photochromic CHCl3 solution, showed high-selectively fluorescent response toward Cu2+ (6 μM), which provided an effective strategy to detect metal ions.

Multicolor detection with vapochromism of platinum(II) 3,8-bis-(2-triethylsilylethynyl)-phenanthroline organometallic complexes with bis-arylethynyl derivatives

We synthesized platinum(II) organometallic complexes with 3,8-bis-(triethylsilyl)ethynyl-1,10-phenanthroline (3,8-Phen-TES) and six respective arylethynyl ligands with different substituents (Pt(3,8-Phen-TES)(-Ph-R)2: R = H (1TES), 4-Me (2TES), 4-CF3 (3TES), 4-t-Bu (4TES), 3,5-di-CF3 (5TES), 3,5-di-t-Bu (6TES)), and characterized their luminescence in solution and solid state by photoluminescence spectroscopy. The luminescence of the 1TES – 6TES complexes in solution was assigned to the phosphorescence from the mixed transition of 3MLCT/3LLCT (LLCT = ligand-to-ligand charge transfer), revealing the similarity to emission spectra of previously reported platinum(II) organometallic complexes Pt(L)(-aryl)2 (L = several diimines) in solution. However, the color of the six complexes in the solid state ranged from orange to dark red compared to the yellow or light orange of the respective complexes in solution. The emission spectra of 5TES and 6TES in the dried powder sample were obviously observed at a long wavelength area over a wide range (520–1000 nm) relative to the spectra of their corresponding complexes in solution. The luminescence of 5TES in the solid state was assigned to the phosphorescence from the transition of a metal−metal-to-ligand charge transfer (3MMLCT), while the emission of 6TES in the solid state was assigned to the emission from the 3MLCT/3LLCT mixed transition. Vapochromism and vapoluminescence experiments with 13 species of volatile organic compounds (VOCs) for 6TES clearly showed that the multicolor change in the case of several VOCs could be seen with the naked eye, and that 6TES is superior to previous complexes reported by our group for VOC detection. The results suggest that the vapochromism and vapoluminescence of 6TES are induced by sterically hindered ligands with (3,5-di-t-butyl-phenyl)ethynyl and 3,8-Phen-TES, and that the expansion of the space between molecules in the solid state was important for multicolor detection of VOCs with vapochromism.

Electrochemical Actuators with Multicolor Changes and Multidirectional Actuation.

Electrochemical (EC) actuators have garnered significant attention in recent years, yet there are still some critical challenges to limit their application range, such as responsive time, multifunctionality, and actuating direction. Herein, an EC actuator with a back-to-back structure is fabricated by stacking two membranes with bilayer V2 O5 nanowires/single-walled carbon nanotubes (V2 O5 NWs/SWCNTs) networks, and shows a synchronous high actuation amplitude (about ±9.7mm,±28.4°) and multiple color changes. In this back-to-back structure, the inactive SWCNTs layer is used as a conductive current collector, and the bilayer network is attached to a porous polymer membrane. The dual-responsive processes of V2 O5 nanowires (V2 O5 NWs) actuation films and actuators are also deeply investigated through in situ EC X-ray diffraction and Raman spectroscopy. The results show that the EC actuation of the V2 O5 NWs/SWCNTs film is highly related to the redox behavior of the pseudocapacitive V2 O5 NWs layer. At last, both V2 O5 NWs and W18 O49 nanowires (W18 O49 NWs)-based EC actuators are constructed to demonstrate the multicolor changes and multidirectional actuation induced by the opposite lattice changes of V2 O5 NWs and W18 O49 NWs during ionic de-/intercalation, guiding the design of multifunctional EC actuators in the future.

Multicolorimetric and fluorometric dual-modal immunosensor for histamine via enzyme-enabled metallization of gold nanorods and inner filter effect of carbon dots

Histamine (HA) is one of the key causative agents associated with allergies and food poisoning. The accurate and convenient detection of HA is highly required to avert HA toxicity and ensure food safety. Herein, a multicolorimetric and fluorometric dual-readout immunosensor for HA detection was developed based on an alkaline phosphatase (ALP) enabled multicolor change reaction and fluorescence quenching of carbon dots (CDs) resulting from the metallization of gold nanorods (Au NRs). In this immunosensor, ALP is employed as labeling enzyme which can efficiently catalyze the hydrolysis of ascorbic acid 2-phosphate into ascorbic acid (AA). Then, the resultant AA can reduce Ag+ into silver monomer, and a silver shell is deposited on the surface of Au NRs to form Au/Ag core-shell nanorods (Au NRs@Ag). This leads to the blue-shift of the longitudinal local surface plasmon resonance peak of Au NRs and a vivid multicolor variation. Meanwhile, the fluorescence of CDs can be gradually quenched via inner-filter effect by the resultant Au NRs@Ag. Multicolorimetric assay allows a semiquantitative detection with a visible detection limit of 0.25 mg/L, and fluorometric assay provides a sensitive and accurate quantitative detection with a detection limit of 0.08 mg/L. The immunosensor not only fulfills the reliable and accurate detection requirements but also fits different testing needs for both laboratory and on-site detection.

Colorimetric Biosensor Based on Magnetic Enzyme and Gold Nanorods for Visual Detection of Fish Freshness.

Histamine, an important safety index for aquatic products, can also be used as a freshness indicator for red-fleshed fish. In this work, magnetic graphene oxide (Fe3O4@GO, MGO) was applied to immobilize diamine oxidase (DAO) through a method of adsorption and covalent bonding. Under the optimized conditions, magnetic DAO prepared by adsorption immobilization had a higher enzyme activity than that of free enzyme, which was selected for the sensor construction. A colorimetric biosensor based on magnetic DAO induced etching of gold nanorods (AuNRs) was developed for the detection of histamine in fish. The developed biosensor showed an excellent response toward histamine with a low detection limit of 1.23 μM and had negligible interference from other diamines. With increasing the histamine concentration, the AuNRs after the reaction exhibited colors ranging from dark green to blue-green, blue, purple, red, and colorless. The etching induced multicolor change of AuNRs indicated the presence of different contents of histamine in mackerel during storage, and was consistent with the overall change in the content of the total volatile basic nitrogen (TVB-N). Thus, it was indicated that the proposed colorimetric biosensor with a naked-eye-detectable readout has a great potential to evaluate the freshness of red-fleshed fish high in histamine.

Alkaline phosphatase-triggered dual-signal immunoassay for colorimetric and electrochemical detection of zearalenone in cornmeal

Zearalenone (ZEN), as one of the most important foods and feeds pollutant mycotoxins, is threatening human and animal health. Here, we have developed a dual-signal immunoassay triggered by alkaline phosphatase (ALP) for detecting ZEN in cornmeal. The immobilized ZEN-bovine serum albumin and free ZEN in the sample were competitively bound to an anti-ZEN monoclonal antibody (McAb). Then, the ALP labeled goat anti-mouse IgG (ALP-IgG) combined with the McAb. ALP catalyzed ascorbic acid 2-phosphate to produce L-ascorbic acid (AA). AA converted potassium ferricyanide (K3[Fe(CN)6]) to potassium ferrocyanide (K4[Fe(CN)6]), which reacted with ferric ion (III) to promote the formation of Prussian blue nanoparticles (PB NPs). Consequently, the solution appeared multicolor changes. Meanwhile, PB NPs had a maximum absorption peak at 700 nm. It could be monitored by an UV–vis spectrometer. As an electron transfer medium, K3[Fe(CN)6] was consumed gradually along with PB NPs formation. Therefore, electrochemical detection was also used for detecting ZEN. Under the optimum conditions, the logarithm of ZEN concentration showed a good linear relationship with the absorbance from 0.2 to 0.8 ng/mL (R2 = 0.987) and the differential pulse voltammetry peak current from 0.125 to 0.5 ng/mL (R2 = 0.993). The limits of detection of colorimetric and electrochemical methods were 0.04 and 0.08 ng/mL, respectively. The recovery rates of ZEN from cornmeal samples were in the range of 80–120%, and relative standard deviations were lower than 10%. The results demonstrated that the dual-signal immunoassay has a high sensitivity for ZEN detection in corn samples.

Multiresponsive Tetra-Arylethene-Based Fluorescent Switch with Multicolored Changes: Single-Crystal Photochromism, Mechanochromism, and Acidichromism

In this article, we report a simple tetra-arylethene-based fluorescent switch TPS═C4 modified with a flexible alkyl chain by the Schiff base structure. The incorporation of C═N retains the excellent photochromic property of tetra-arylethene and endows TPS═C4 with new multiresponsiveness of mechanochromism and acidichromism and multicolor changes. TPS═C4 shows remarkable mechanochromism from a deep blue emission at 420 nm to bright blue with a new shoulder band at 450 nm, which arises from the force-induced phase transition from the crystal state to an amorphous form. Both the original crystalline powder and the ground amorphous sample exhibit interesting acidichromism, and their emission colors turn yellow (530 nm), due to the protonation of C═N. More interestingly, TPS═C4 displays fascinating photochromism in multiple states, especially in the single-crystal state. The flexible alkyl chain offers enough free space for molecular motion and facilitates single-crystal photochromism. Due to the multiresponsiveness and multicolor switch, TPS═C4 can be satisfactorily used for the multidimensional anticounterfeiting application. To the best of our knowledge, TPS═C4 is a rare multifunctional molecule with a simple structure but featuring multiresponsiveness and multicolor switch.

Electrochromic Metamaterials of Metal–Dielectric Stacks for Multicolor Displays with High Color Purity

Inorganic electrochromic (EC) materials with vibrant multicolor change that are compatible with large-scale processing have been at the forefront of EC technology and are crucial in a wide range of applications, such as displays and camouflage. However, limited strategies are available to realize such inorganic materials, and challenges such as low color purity are yet to be overcome. Here, we demonstrate multilayered metal-dielectric metamaterials (MMDMs) as a new family of inorganics-based EC materials to achieve dynamic alternation among multicolors with high contrast and high color purity, which are structurally realized by significantly enhancing the confinement of the incident light in specific optical frequencies. This multilayer structure renders high reflectivity (75%), high quality factor (7.4), and a full width at half-maximum of 60 nm before coloration and presents a color gamut at least 40% wider than that of previously reported metamaterials after coloration, indicating good color quality.

A pH-sensitive ESIPT molecule with aggregation-induced emission and tunable solid-state fluorescence multicolor for anti-counterfeiting and food freshness detection

Achieving stimulus-responsive multicolor solid-state photoluminescence based on a single fluorescence compound has potential applications in anti-counterfeiting labels and visual detection. However, few such organic chromophores have these features and been used in this fields. In this study, multiple pH-sensitive sites were incorporated into a single molecule of succinimide substituted 2-(2′-hydroxyphenyl)Benzothiazole derivative (BTSA) that was Aggregation induced emission (AIE) and Excited state intramolecular proton transfer (ESIPT) active, exhibiting multilevel and highly secured anti-counterfeiting abilities. Under ammonia atmosphere, BTSA exhibits reversible switching between the green and blue fluorescence. By rationally manipulating two stimuli (acid and base), BTSA labels incredibly shows four different emission colors. The multicolor fluorescence changes are irreversible and evidently depend on the order of acid-base added. Together with smartphone that can quickly and accurately identify the multiple colors, BTSA affords a new covert fluorescent anti-counterfeiting technique with high security through the combination of reversible and irreversible fluorescence multicolor changes, as well as order and stimuli-dependent luminescent patterns. In addition, BTSA labels are successfully applied for real-time and visual monitoring the food freshness of pork and shrimp. The molecular design rationale presented here provide a new design strategy for the purposes of multicolor anti-counterfeiting and food freshness detection.

Liquid/Liquid Interfacial Suzuki Polymerization Prepared Novel Triphenylamine-Based Conjugated Polymer Films with Excellent Electrochromic Properties

Preparing conjugated polymer films via interfacial Suzuki polymerization is a promising method for obtaining desirable electrochromic materials with desired structures. Here, a series of aryl boronic esters and triphenylamine-based aryl bromides were applied as precursors, and several polymer films were finally obtained via the liquid/liquid interfacial Suzuki polymerization reaction under mild conditions. FT-IR, UV, and Raman as well as electrochemistry, SEM, and EDS results all provide strong evidence for the formation of the desired polymer structures. Among them, the TPA-Wu (containing triphenylamine and alkyl-fluorene) film exhibits the best film-forming quality. Besides, these polymer films were applied in electrochromic applications. The results show that electrochromic properties can be affected by the quality of film formation. It is worth mentioning that the TPA-Wu film could achieve excellent electrochromic properties with reversible multicolor changes from transparent yellow to orange-red to blue-green under varying potentials. Compared to other triphenylamine-based electrochromic materials, the TPA-Wu film possessed the most desirable coloring efficiency, higher optical contrast, and shorter switching time. This work provides an existing general approach of liquid/liquid interfacial Suzuki polymerization for constructing conjugated polymer films toward electrochromic applications.

Multicolorimetric sensing of histamine in fishes based on enzymatic etching of gold nanorods

Histamine determination is of great significance for fish safety, quality and trade. In this work, we developed a novel multicolorimetric biosensor for the simple and rapid visual detection of histamine in fishes. With the help of iodide and molybdate, the hydrogen peroxide generated by the enzymatic reaction of histamine and diamine oxidase enzyme effectively etched the gold nanorods (AuNRs). This etching reaction caused a blueshift of the longitudinal peak of the nanorods, accompanied by an obvious multicolor change from blue-green to purple and red, which could be easily observed by the naked eye. After the optimization of reaction conditions, the linear range of the standard curve obtained from the longitudinal peak blueshift distance of the AuNRs was determined to be 20–100 μM. Attentively, the interference from other analytes is negligible, especially diamines such as putrescine and cadaverine. Furthermore, one can easily distinguish with the naked eye whether the histamine content measured in a fish sample exceeds the limit value of 50 mg/kg using the proposed method. The method is simple and fast, with high specificity and accuracy, and can be successfully applied to determine the histamine content in real fish samples, indicating the potential of its application in the on-site detection of various analytes.

Novel evaluation method to determine the local mixing time distribution in stirred tank reactors

Stirred tank reactors are frequently used for mixing as well as heat- and mass transfer processes in chemical and biochemical engineering due to their robust operation and extensive experiences in the past. However, for cell culture processes like mammalian cell expression systems, special requirements have to be met to ensure optimal cell growth and product quality. One of the most important requirements to ensure ideal transport processes is a proper mixing performance, characterized typically by the global mixing time tmix,global or the dimensionless global mixing time tmix,global·n. As an evaluation method for mixing time determination, the time is usually determined until a tracer signal (e.g. conductivity) has reached a constant value after a peak has been introduced (e.g. by adding a salt). A disadvantage of this method is, that the position of tracer feeding as well as the position of the probe significantly influences the detected mixing time. Further on, the global mixing time does not provide any information about the spatial and temporal ”history” of the mixing process to identify areas that are mixed poorly or areas that form stable compartments. To overcome this disadvantage, a novel image analysis will be presented in this study for the detailed characterization of mixing processes by taking into account the history of mixing. The method is based on the experimental determination of the local mixing time distribution by using a multi-color change caused by a pH-change in a bromothymol blue solution. A 3L transparent stirred tank reactor is used for the benchmark experiment. To demonstrate the suitability of the new characterization method for the validation of numerical simulations, a calculation with a commercial Lattice-Boltzmann approach (M-Star CFD) has been performed additionally and evaluated regarding mixing time distributions. The exemplary application of image analysis to a numerical mixing time simulation shows good agreement with the corresponding experiment. On the one hand, this shows that the method can also be interesting for numerical work, especially for experimental validation, and on the other hand, this allows much deeper insights into the mixing behavior compared to conventional mixing criteria. For example the new method enables the characterization of mixing on different scales as well as the identification of micro- and macroscopic flow structures. The strong influence of the acid to base ratio on mixing time experiments becomes clearly visible with the new method.

Colorimetric glucose sensing with multiple-color changes by using a MnO2 NSs-TMB nanosystem.

Glucose performs many essential functions associated with metabolic processes in the living system, and is closely related to many diseases such as diabetes and hypoglycemia. Most of the existing glucose concentration detection methods require complex instruments, which undoubtedly limit its widespread use. Here, we have designed a glucose colorimetric detection system composed of glucose, glucose oxidase (GOD), manganese dioxide nanosheets (MnO2 NSs) and 3,3',5,5'-tetramethylbenzidine (TMB) to achieve colorimetric detection with the naked eye. Compared with the single-color change of the colorimetric method in previous studies, multiple-color changes have been realized. MnO2 NSs, as a kind of nanomaterial imitating oxidase, can directly oxidize TMB to oxTMB. Because oxTMB showed a dark yellow color when strongly oxidized and light blue when weakly oxidized, this feature can achieve multiple-color changes rather than a single-color change, which is helpful for colorimetric observation with the naked eye. Finally, we successfully used MnO2 NSs for colorimetric detection of glucose and realized multiple-color changes, making it easier to achieve colorimetric observation with the naked eye. The linear detection range is 0-4000 μM and limit of detection is 5.0 μM. This is not only useful for glucose, but also has an important significance for other experiments considering colorimetric experiments with the naked eye.

Polyoxotungstate-based nanocomposite films with multi-color change and high volumetric capacitance toward electrochromic energy-storage applications

A facile fabrication strategy is proposed to construct a POMs-based nanocomposite film. It realizes multi-color transition during charging and discharging process, thereby links electrochromic behavior with energy storage performance.

High performance organic-inorganic hybrid material with multi-color change and high energy storage capacity for intelligent supercapacitor application

As a kind of multifunctional supercapacitor, electrochromic-supercapacitor device (ESD) can be used to visually estimate the energy storage level of the device through the color change. Currently, low energy storage capacity limits the practical application of ESD. Here, WO3 with oblate shape morphology formed by the aggregation of nanoclusters is successfully prepared by hydrothermal synthesis. Then WO3-poly (5-cyanoindole) (P5ICN/WO3) hybrid material is obtained by electrochemical polymerization P5ICN on the surface of WO3. Due to the synergistic effect of P5ICN and WO3, P5ICN/WO3 hybrid material shows enhanced charge transfer rate, multi-colors change (blue-green, yellow and green) and good supercapacitive properties (specific capacitance is 89.2 mF cm−2). An asymmetric ESD is successfully constructed based on P5ICN/WO3, which also shows good electrochromic and supercapacitive properties, such as high specific capacitance, energy density and power density. The color of ESD can change between yellow and dark green during charge-discharge process. In practical application, the intelligent monitoring of the energy storage state of device is achieved by the color variations or transmittance change of this ESD. And the charged ESD also can power a LED, indicating the feasibility of practical application.

Vapochromism and vapoluminescence of platinum(II) 3,8-bis-(3-hydroxy-3-methylbut-1-yn-1-yl)-phenanthroline organometallic complexes with bis-arylethynyl derivatives

A new series of platinum(II) organometallic complexes with 3,8-bis-(3-hydroxy-3-methylbut-1-yn-1-yl)-1,10-phenanthroline (3,8-Phen-C(CH3)2OH) and nine respective arylethynyl ligands with different substituents (Pt(3,8-Phen-C(CH3)2OH)(Ph-R2)2: R2 = H (1OH), 2-F (2OH), 3-F (3OH), 4-F (4OH), 4-Me (5OH), 4-CF3 (6OH), 4-t-Bu (7OH), 3,5-di-CF3 (8OH), 3,5-di-t-Bu (9OH)) were synthesized and their luminescences in solution and solid state were characterized by photoluminescence spectroscopy. The luminescence of each of the new complexes, 1OH – 9OH, in solution was assigned to the phosphorescence from the mixed transition of 3MLCT/3LLCT (LLCT = ligand-to-ligand charge transfer), revealing that two series had very similar emission spectra: Pt(3,8-PhenTMS)(-Ph-R)21TMS – 9TMS and Pt(3,8-PhenH)(Ph-R)21H – 9H in solution. In solid state, the emission spectra of three series of platinum organometallic complexes, 1OH – 9OH, 1H – 9H, and 1TMS – 9TMS, were observed over a wide range (500–1100 nm) because the phosphorescence of each of these complexes was assigned to the emission from the 3MLCT/3LLCT mixed transition and/or the transition of a metal−metal-to-ligand charge transfer, called 3MMLCT.Results of vapochromism and vapoluminescence experiments with 13 species of volatile organic compounds (VOCs) for three series of the 27 above-mentioned platinum complexes suggested that the vapochromism of the present complex systems was induced by sterically hindered substituents of the arylethynyl ligands as tert-butyl substituents, and that the expansion of the space among the molecules in solid state is important for selective and multicolor detection with vapochromism and vapoluminescence for VOCs. Especially, the vapoluminescence experiment with VOCs for 9TMS, which was composed of a combination of the bulkiest ligands, 3,8-bis-(trimethylsilyl)ethynyl-1,10-phenanthroline and 3,5-di-t-butyl-phenylehtynyl ligands, among the present 27 complexes showed multicolor changes from yellow to dark brown, in contrast with respective VOCs.

Visual detection of acid phosphatase based on hollow mesoporous manganese dioxide nanospheres

Acid phosphatase is widely used as a clinical indicator because of its close correlation with a variety of diseases. Herein, a label-free and colorimetric sensing method for detecting the activity of acid phosphatase was constructed based on hollow mesoporous manganese dioxide nanospheres. The nanospheres exhibit superior oxidase-like property and can oxidize colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to yellow TMB2+. Ascorbic acid from acid phosphatase-catalyzed hydrolysis of L-ascorbic acid-2-phosphate will inhibit the oxidization reaction, igniting vivid color variation. On the basis of this obvious multicolor change, the visual detection of acid phosphatase was achieved. Compared with the single-color change, the multicolor colorimetric method is more conducive for naked-eye discrimination. The absorbance difference at 450 nm exhibits a linear relationship with the concentration of acid phosphatase ranging from 1.0 to 25 U L−1, with a detection limit as low as 0.45 U L−1. Acid phosphatase in human serum samples was successfully determined. Moreover, the inhibition efficiency of NaF for acid phosphatase activity was investigated, proving the proposed colorimetric method will be a potential platform for screening acid phosphatase inhibitors and discovering new drugs.

3,3',5,5'-tetramethylbenzidine as multi-colorimetric indicator of chlorine in water in line with health guideline values.

Sanitizing solutions against bacterial and viral pathogens are of utmost importance in general and, in particular, in these times of pandemic due to Sars-Cov2. They frequently consist of chlorine-based solutions, or in the direct input of a certain amount of chlorine in water supply systems and swimming pools. Colorimetry is one of the techniques used to measure the crucial persistence of chlorine in water, including household chlorine test kits commonly based on colorimetric indicators. Here, we show a simple and cheap colorimetric method based on 3,3′,5,5′-tetramethylbenzidine (TMB), commonly used as chromogenic reagent for enzyme-linked immunosorbent assays. TMB is converted by chlorine to a colored molecule through a pH-dependent multi-step oxidation process where the chromaticity of TMB is directly proportional to chlorine content. This molecule offers several advantages over other commonly used reagents in terms of safety, sensitivity, and, peculiarly, hue modulation, giving rise to the detection of chlorine in water with a multi-color change of the indicator solution (transparent/blue/green/yellow). Moreover, through the appropriate setting of reaction conditions, such coloration is finely tunable to cover the range of chlorine concentration recommended by international health agencies for treatment of drinking water and swimming pools and to test homemade solutions prepared by dilution of household bleach during health emergency events such as during the current pandemic. Graphical abstractElectronic supplementary materialThe online version of this article (10.1007/s00216-020-02918-9) contains supplementary material, which is available to authorized users.

Multicolor colorimetric detection of ochratoxin A via structure-switching aptamer and enzyme-induced metallization of gold nanorods

Colorimetric aptasensors have been intensively studied for the ochratoxin A (OTA) detection, but they mostly exhibit just one-color change, resulting in poor visual resolution and limited use for semi-quantitative analysis. Thus, we designed a high-resolution colorimetric assay on the basis of aptamer structural switching and enzyme-induced metallization of gold nanorods (AuNRs). DNA-alkaline phosphatase (ALP)-immobilized magnetic beads were prepared. The aptamer bounded to OTA to form G-quadruplexes, releasing ALP-labelled complementary DNA (cDNA-ALP). After magnetic separation, cDNA-ALP catalyzed the decomposition of ascorbic acid 2-phosphate to ascorbic acid that reduced Ag+, forming an Ag shell on the surface of AuNRs. This caused a blue-shift of the longitudinal local surface plasmon resonance peak of the AuNRs and a naked eye visible multicolor change. Under optimal conditions, the assay exhibited a 9.0 nM detection limit for OTA, with high specificity. This method is promising for the on-site visual semi-quantitative detection of mycotoxins in foods.