Cascade Chromogenic System with Exponential Signal Amplification for Visual Colorimetric Detection of Acetone

FeMo2Ox(OH)y-based mineral hydrogels as a novel POD nanozyme for sensitive and selective detection of aromatic amines contaminants via a colorimetric sensor array

Developing convenient pathways to discriminate and identify multiple aromatic amines (AAs) remains fascinating and critical. Here, a novel three-channel colorimetric sensor array based on FeMo2Ox(OH)y-based mineral (FM) hydrogels is successfully constructed to monitor AAs in tap water. Benefiting from the substantial oxygen vacancies (VO), FM nanozymes exhibit extraordinary peroxidase (POD)-like activities with Km of 0.133 mM and Vmax of 2.518 × 10-2 mM·s-1 toward 3,3',5,5'-tetramethylbenzidine (TMB), which are much better than horseradish peroxidase and most of POD mimics. This reveals that doping Cu and Co into FM (FM-Cu and FM-Co) can change POD activity. Based on various POD activities, TMB and H2O2 are used to generate fingerprint colorimetry signals from the colorimetry sensor array. The analytes can accurately discriminate through linear discriminant analysis, with a detection limit as low as 2.12 × 10-2-0.14 μM. The sensor array can effectively identify and discriminate AA contaminants and their mixtures and has performed well in real sample tests.

Cupric oxide nanosheet as an oxidase mimic for fluorescent detection of acetone by a 3D-printed portable device

A cupric oxide (CuO) nanosheet-based chemical fluorescence sensor was developed to realize the detection of acetone in aqueous solutions. CuO is an oxidase mimic and can catalyze the oxidation of o-phenylenediamine (OPD) to form 2,3-diaminophenazine (oxOPD). Interestingly, acetone was found to possess the scavenging ability for superoxide anions generated in the CuO-catalyzed oxidation system, hence weakening the OPD oxidation and leading to a reduction in the fluorescence intensity of the catalyzing system at 574 nm under excitation at 425 nm. Based on this property of acetone, a fluorescent sensor was constructed to detect acetone. The sensor exhibits a linear range of 1.35 to 2 × 105 µmol L-1 and a detection limit of 1.08 µmol L-1. Additionally, a smartphone-free portable device was constructed to realize on-the-spot and rapid detection of acetone in cauliflower, mineral water, tap water, and lake water samples. The recoveries by the portable device are 93.2 to 108% for actual samples, with relative standard deviations of less than 4.3%, indicating a potential application prospect of the device in on-site detection.

Colorimetric Detection of Furfural with Enhanced Visible Absorption of Furfural-DNPH in Basic Conditions

Furfural is an intermediary toxic aldehyde compound produced during heat-induced food processing and storage. Furfural is also formed by the degradation of cellulosic insulation in oil-immersed electric potential transformers, whose level is an important indicator of aging for replacement. In this study, we report a new means to detect the trace level of furfural in a colorimetric manner. Furfural is reacted with dinitrophenylhydrazine (DNPH) in acid solutions. The colorless furfural-DNPH compound turns orange-colored as the solution changes to basic. The delocalization of the π-electron in the DNPH-aldehyde derivatives at the basic condition causes the shift of the absorption peak from 318 to 470 nm, which renders the solution orange-colored. The color and absorbance are saturated in 20 min of incubation. There is high linearity between the absorbance and the concentration of furfural in the range of 0-0.2 mM, which enables the quantitative detection of furfural. The limit of detection is estimated to be as low as 1.76 μM for the absorbance analysis and 10 μM for the naked eyes. The colorimetric assay protocol is applicable to the detection of various aromatic aldehydes, which show strong π-electron delocalization and is not applicable to aliphatic aldehydes due to lack of delocalization. This simple assay can be conducted in typical 96-well microplates using a microplate reader, which provides a low-cost and high-throughput screening. Therefore, we believe that our method is potentially applicable for the quantitative detection of aromatic aldehydes in various samples from foods, electronic devices, and so on.

A highly effective "naked eye" colorimetric and fluorimetric curcumin-based fluorescent sensor for specific and sensitive detection of H2O2in vivo and in vitro

Hydrogen peroxide (H₂O₂) is involved in many important tasks in normal cell metabolism and signaling. However, abnormal levels of H₂O₂ are associated with the occurrence of several diseases. Therefore, it is important to develop a new method for the detection of H₂O₂in vivo and in vitro. A turn-off sensor, 2,2-difluoro-4,6-bis(3-methoxy-4-((4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl)oxy)styryl)-2H-1,3,2-dioxaborine (DFCB), based on curcumin was developed for the detection of H₂O₂. The DFCB, an orange-emitting sensor, was constructed by employing 2,2-difluoro-4,6-bis(4-hydroxy-3-methoxystyryl)-2H-1,3,2-dioxaborine (DFC) as the main carrier, and 2-(4-bromomethylphenyl)-4,4,5,5-tetramethyl-1,3,2-doxaborolane as the recognition site. The recognition group on the DFCB sensor could be completely cleaved by H₂O₂ to generate the intermediate DFC, which would lead to a colorimetric change from bright orange to light blue accompanying by a significantly quenched fluorescence, which could be seen by the naked eye. This sensor exhibited a highly specific fluorescence response to H₂O₂, in preference to other relevant species, with an excellent anti-interference performance. The sensor DFCB also possessed some advantages including a wide pH response range (6-11), a broad linear range (0-300 μM), and a low detection limit (1.31 μM). The sensing mechanism of the DFCB sensor for H₂O₂ was verified by HRMS analysis, ¹H-NMR titration and DFT calculations. In addition, the use of the DFCB sensor was compatible with the fluorescence imaging of H₂O₂ in living cells and zebrafish.

Advances in immunoassay-based strategies for mycotoxin detection in food: From single-mode immunosensors to dual-mode immunosensors

Mycotoxin contamination in foods and other goods has become a broad issue owing to serious toxicity, tremendous threat to public safety, and terrible loss of resources. Herein, it is necessary to develop simple, sensitive, inexpensive, and rapid platforms for the detection of mycotoxins. Currently, the limitation of instrumental and chemical methods cannot be massively applied in practice. Immunoassays are considered one of the best candidates for toxin detection due to their simplicity, rapidness, and cost-effectiveness. Especially, the field of dual-mode immunosensors and corresponding assays is rapidly developing as an advanced and intersected technology. So, this review summarized the types and detection principles of single-mode immunosensors including optical and electrical immunosensors in recent years, then focused on developing dual-mode immunosensors including integrated immunosensors and combined immunosensors to detect mycotoxins, as well as the combination of dual-mode immunosensors with a portable device for point-of-care test. The remaining challenges were discussed with the aim of stimulating future development of dual-mode immunosensors to accelerate the transformation of scientific laboratory technologies into easy-to-operate and rapid detection platforms.

Shape-specific MOF-derived Cu@Fe-NC with morphology-driven catalytic activity: Mimicking peroxidase for the fluorescent- colorimetric immunosignage of ochratoxin

Ochratoxin A (OTA), which has strong hepatotoxicity and nephrotoxicity, can accumulate in the human body through the food chain; thus, the selective and effective detection of OTA is urgently required for food security. Nanozymes with hyperfine size and shape control have attracted attention because of their controllable structure and intrinsic activity. Herein, CuFe-bimetal coordinated N-doped carbon (Cu@Fe-NC) with morphology-driven peroxidase-mimicking activity was synthesized using Cu₂O with specific polygonal cubes and fully exposed {111} crystalline facets as the template to produce a CuFe-bimetallic metal organic framework (MOF) and further treating CuFe-MOF with high-temperature pyrolysis. N-doping can confer electronegativity to exhibit high affinity, while the large surface area of the porous carbon support can facilitate rapid adsorption-desorption equilibrium. Using the peroxidase-mimicking Cu@Fe_0.5-NC as a carrier, a versatile immunoassay for the detection of OTA was implemented based on the ratiometric fluorescence and the localized surface plasmon resonance peak shift, achieving a detection limit of 0.52 ng/L in the range of 0.001-10 μg/L. Therefore, the strategy of enhancing enzyme-mimicking activity using specific shapes and crystalline facets may open new avenues for food and environmental analysis.

A ratiometric solid AIE sensor for detection of acetone vapor

Acetone serves as a routine solvent and synthetic intermediate in chemical factories and laboratories. Monitoring the level of acetone vapor in working environment is of great necessity to employee health due to its strong volatility and toxicity, but there is still in lack of simple and easy-to-use portable sensors. In this study, we report a portable and intuitive indicator for real-time displaying acetone vapor concentration in air, based on the ratiometric fluorescence response of the designed organic molecule, PhB-SSB, to acetone. As an aggregation-induced emission (AIE) fluorophore, PhB-SSB underwent specific reaction with acetone through the salicylaldehyde Schiff base and phenylboronate groups to realize ratiometric fluorescence change from green to red after acetone vapor treatment. The reaction mechanism was proposed as acetone-induced breakage of the imine bond in PhB-SSB. We further fabricated PhB-SSB into a film fluorescent sensor for acetone vapor with good sensitivity and selectivity. Taking advantage of its intuitive fluorescent color contrast, acetone-specific response and small size, our sensor is practical in real-time alarming the acetone vapor hazard in the workplaces.

In situ synthesis of fluorescent polydopamine polymer dots based on Fenton reaction for a multi-sensing platform

The development of fluorescent nanosensors has attracted extensive research interest owing to their superior optoelectronic properties. However, current fluorescent nanoprobes generally involve complicated synthesis processes, background signal disturbance, and limited analyte detection. In this work, a facile and time-saving synthetic strategy for the preparation of green emitting polydopamine polymer dots (PDA-PDs) from dopamine via Fenton reaction at room temperature was proposed for the first time. The obtained PDA-PDs possessed excellent luminescence properties, with a long-wavelength emission of 522 nm, a large Stokes shift of 142 nm, and good photostability against ionic strength and UV irradiation. The formation mechanism of fluorescent PDA-PDs is as follows: in the presence of Fe2+ and H2O2, dopamine could rapidly undergo oxidation to its quinone derivatives and further polymerize to synthesize the fluorescent PDA-PDs with the acceleration of hydroxyl radicals produced from the Fenton reaction. Thus, a versatile turn-on fluorescence sensing method was developed for the detection of multi-analytes (including Fe2+, dopamine, H2O2, and glucose) based on monitoring the intrinsic fluorescence signal of the in situ formation of PDA-PDs. This sensing method could be efficiently applied for the detection of Fe2+, dopamine, and glucose in real human serum samples. Moreover, a three-input AND molecular logic gate based on this sensing platform was designed with the fluorescence signal of PDA-PDs as the gate. Finally, the proposed PDA-PDs could have immense broad prospects in nanomaterials and biosensors.

Sweetsop-like α-Fe2O3@CoNi catalyst with superior peroxidase-like activity for sensitive and selective detection of hydroquinone

Hydroquinone (HQ) is poorly degradable in the ecological environment and is highly toxic to human health even at a low concentration. The colorimetric method has the advantages of low cost and fast analysis, which provides the possibility for simple and rapid detection of HQ. In this work, a new colorimetric method has been developed for HQ detection based on a peroxidase-like catalyst, α-Fe2O3@CoNi. This sweetsop-like α-Fe2O3@CoNi catalyst enables H2O2 to produce hydroxyl (˙OH), leading to the oxidization of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxTMB. In the presence of HQ, the blue oxTMB is reduced to colorless, which allows for colorimetric detection of HQ in water samples. This method has been validated by detecting HQ in water samples with high selectivity, rapid response, broad detection range (0.50 to 30 μM), and low detection limit (0.16 μM).

Two-dimensional layered WS2 nanosheets as peroxidase mimetics in a colorimetric chemosensor for simple and rapid detection of acetone

Two-dimensional (2D) nanomaterials with catalytic activity have attracted considerable attention from researchers, but their application in the detection of hazardous substances needs to be further expanded. Herein, layered tungsten sulfide (WS₂) nanosheets with peroxidase-mimicking activity were used to construct a colorimetric chemosensor for rapid detection of acetone. WS₂ nanosheets can decompose H₂O₂ to generate hydroxyl radicals (·OH), which will further oxidize o-phenylenediamine (OPD) through hydrogen atom transfer (HAT) to form the yellow product 2,3-diaminophenazine. Acetone can block the HAT from OPD to ·OH, thus causing obvious inhibition of the peroxidase activity of WS₂ nanosheets, making the solution appear pale yellow or even colorless. The investigation of catalytic kinetics indicates that the catalytic reaction consists of the 'ping pong' mechanism, and the regulatory effect of acetone on WS₂ nanosheets is confirmed to be an irreversible inhibition. The chemosensor can easily distinguish a trace amount of acetone by the naked eye in less than 20 min, and has a limit of detection for acetone of as low as 3.08 mg  l^-1. The application in actual samples displays the accuracy and stability of the chemosensor, suggesting that such a method is promising for acetone detection.

Label-free colorimetric detection of glutathione by autocatalytic oxidation of o-phenylenediamine based on Au3+ regulation and its application

A label-free, rapid, and highly sensitive colorimetric assay for the detection of glutathione (GSH) was developed.

Photoacoustic-Based Miniature Device with Smartphone Readout for Point-of-Care Testing of Uric Acid

Real-time and rapid detection of various biomarkers in body fluids has important significance for early disease diagnosis, efficient monitoring of treatment, and evaluation of prognosis. However, traditional detection methods not only require bulky and costly instruments but also are not suitable for the analysis of heterogeneous samples (e.g., serum and urine), limiting their applications in point-of-care testing (POCT). Herein, an integrated photoacoustic (PA) device with a smartphone as the acoustic signal readout has been constructed, greatly reducing the volume and cost of the instrument, and providing a potential miniature platform for POCT of clinical samples. By exploiting the electron transfer product of 3,3',5,5'-tetramethylbenzidine (TMB) (i.e., TMB⁺⁺) as the PA probe and hemin-graphene oxide (H-GO) complex as the peroxidase, quantitative analysis of uric acid was successfully performed by using only 30 μL of a sample solution. Due to the favorable stability of artificial enzymes, reaction reagents could be effectively embedded in agar gel to make a portable "test strip". Therefore, operators just need to drop clinical samples on the "test strip" for PA analysis, which is user friendly without requiring complex sample preparation steps. In addition, since the acoustic change mainly comes from the PA effect, it has a lower background signal than UV-vis and fluorescence analysis, greatly improving the analytical performance. The simplicity, low cost, and broad adaptability make this miniature PA device attractive for on-site detection, particularly in resource-limited settings.