Solar radiation-promoted selective photocatalytic degradation of Congo red dye by a novel amorphous Cr-based metal-organic framework serving as sensor for 2,4,6-trinitrophenol explosive detection

Synthesis of novel benzene-1,2,4-tricarboxylic acid-based chromium metal-organic framework (designated as Cr-BTC MOF) by solvothermal method using water:ethanol:dimethylformamide (1:1:2) as solvent media has been undertaken with an aim to exploit its role as photocatalyst in degradation of some anionic dyes along with sensing potential of some explosives. The MOF has been characterized by Fourier transform infra-red, scanning electron microscopy, Brunauer-Emmett Teller and powder X-Ray diffraction techniques and has shown high thermal stability, upto 373 °C. The prepared MOF was utilized as photocatalyst in selective degradation of Congo red (CR) dye. The effects of pH, source of radiation, initiator and concentration of catalyst were monitored and the results have shown that catalyst exhibits maximum efficiency of 93.3% in the presence of sunlight in neutral medium. The stability and reusability of the catalyst, after four cycles of reusability, renders it to be a highly efficient photocatalyst in the treatment of wastewater under the effect of sunlight. Photoluminescence-detection of explosives viz. 2,4,6-trinitrophenol and nitromethane, has been carried out, wherein Stern-Volmer equation was used to assess the quenching efficiency evaluated. The results have shown exceptional efficiency and selectivity of Cr-BTC MOF towards detection of 2,4,6-trinitrophenol (94%). The reusability has shown the synthesized MOF to display excellent recyclability upto 5 cycles. Minimum inhibitory concentration (MIC) method was investigated to establish their antibacterial efficacy against some Gram-positive and Gram-negative strains. The MOF has showed good efficacy towards Bacillus cereus and Staphylococcus aureus, displaying a MIC value of 7.81 µg/mL, and Pseudomonas aeruginosa (15.625 µg/mL) similar to the standard antibacterial drug, chloramphenicol, thereby establishing their biological efficacy.

Colorimetric/fluorescent dual-mode assay for antioxidant capacity of gallnuts based on CuCo nanozyme and AIE luminogen

In this work, we present a dual-mode assay system consisting of a nanozyme and a luminogen with the aggregation-induced emission (AIE) feature. In the assay system, the chosen nanozyme named CuCo-0 catalyzes the substrate to produce colorimetric signals, while the aggregates of H4ETTC (4,4',4″,4‴-(ethene-1,1,2,2-tetrayl) tetrakis ([1,1'-biphenyl]-4-carboxylic acid), a typical AIE luminogen, generate fluorescent signals. The peroxidase-like activity of the CuCo-0 nanozyme can be remarkably suppressed with sequential additions of antioxidants, leading to a dual-signal response characterized by enhanced fluorescence emission and reduced UV-vis absorbance. On this basis, a dual-mode assay capable of producing both colorimetric and fluorescent signals for the assessment of antioxidant capacity using gallic acid as a representative antioxidant was exploited. Good linearity can be obtained in the 0-60 μM range for both colorimetric analysis and fluorescent analysis, with detection limits of 1.3 μM and 0.35 μM, respectively. Furthermore, this dual-mode assay was successfully applied to real gallnut samples, yielding satisfactory results.

Upconversion luminescence nanosensor for detection of Fe3+ and phosphate ion based on the inner-filter effect

In this work, an upconversion luminescence (UCL) nanosensor for fast detection of ferric ion (Fe3+) and phosphate ion (Pi) is developed based on the inner-filter effect (IFE) between NaYF4:Yb/Er upconversion nanoparticles (UCNPs) and Fe3+-hypocrellin B (HB) complex. Fe3+-HB complex has strong absorption band (450-650 nm), which overlaps with the green emission peak of UCNPs at 545 nm. By adding Fe3+ and Pi, the UCNPs-HB system produces the red-shift change of absorption spectrum, which leads to the "on-off-on" process of IFE. So, with the specific recognition ability of HB for Fe3+ and the competitive complexation of Pi for Fe3+, the proposed nanosensor utilizes the UCL change to achieve the detection of the targets. For the detections of Fe3+, the linear range is 10-600 μM with a limit of detection (LOD) of 2.62 μM, and for Pi, the linear range is 5-100 μM with a LOD of 1.25 μM. The results for selectivity, precision, and recovery test are also satisfactory. Furthermore, the real sample detection shows that the proposed nanaosensor has a great potential in environmental and biological systems. An upconversion luminescence (UCL) nanosensor based on the inner-filter effect (IFE) between upconversion nanoparticles (UCNPs) and Fe3+-hypocrellin B (HB) complex for the detection of Fe3+ and phosphate ion has been proposed, which is promising to be a convenient and sensitive assay for monitoring Fe3+ and phosphate ion in different environments and biological systems.

Peroxidase-like activity of biosynthesized silver nanoparticles for colorimetric detection of cysteine

Cysteine is one of the important amino acids that is involved in various physiological processes, food industries, pharmaceuticals, and personal care. It also serves as a biomarker for some diseases. The large use of cysteine necessitates rapid, cheap, and accurate determination of cysteine in a range of samples. Although many techniques have been employed for the detection of cysteine, they suffer from limitations that make them unsuitable for routine analysis. Here we report on a cheap colorimetric method using biosynthesized silver nanoparticles (AgNPs) as nanozymes. The AgNPs were characterized by UV/visible spectrophotometry, scanning electron microscopy (SEM), and surface-enhanced Raman spectroscopy (SERS). The AgNPs exhibit peroxidase-like activity using o-phenylenediamine (OPD) as a chromogenic reagent. The low K_m values observed for OPD and H₂O₂ (0.9133 and 61.56 mM respectively) show strong affinity of the substrates to AgNPs. The peroxidase-like activity of AgNPs, however, was inhibited on the addition of cysteine. The results show that the absorption intensity of the oxidized OPD decreased linearly with the concentration of cysteine in the range of 0.5-20 μM. The limit of detection (LOD) in this linear range was found to be as low as 90.4 nM. The recovery from urine sample (spiked with cysteine) analyses demonstrated the feasibility of the method in real sample application. From our findings, we anticipate that our method can be applied for the analysis of cysteine in various samples.

Color-deconvolution-based feature image extraction and application in water quality analysis

We propose a feature color extraction method that improves the accuracy of water quality analysis using a digital image and eliminates the effect of interfering ions and chromogenic agents on the color after a color reaction. The proposed method is based on color deconvolution (CD) combined with machine learning for substance measurement in water. After an ordinary camera acquires the solution image after color reaction, the CD algorithm is applied to extract the feature image, calculate the first-order, second-order, and third-order color moments corresponding to RGB channels, and construct a gradient boosting regression tree prediction model based on color moment features to detect substances in water. In predicting ammonia, nitrite, and orthophosphate concentrations, the mean square error values were 0.01029, 0.00063, and 0.1361, and the mean absolute error values were 0.08103, 0.02231, and 0.32886, respectively. There was no significant difference in the results of the comparative spectrophotometric method on the actual water samples. The spiked recoveries of the samples ranged from 94% to 120%, confirming that the method can effectively measure the content of substances in water.

Colorimetric assay of phosphate using a multicopper laccase-like nanozyme

A new nanozyme (Cu-NADH) is reported composed of Cu-coordinated nicotinamide adenine dinucleotide (NADH) exhibiting laccase-like activity. The Cu-NADH nanozyme had higher heat tolerance and catalytic efficiency than natural laccase, and its catalytic activity can be enhanced by high concentration of Cl ions and it is intensely inhibited by phosphate. Therefore, a colorimetric method based on Cu-NADH and indigo carmine was successfully developed to detect phosphate in water. This method showed an excellent selectivity for phosphate, and it had a linear relationship in the phosphate concentration range 2-50 μM with a detection limit of 0.37 μM. We believe that this example of coordination between metal ions and biomolecules to mimic natural enzymes can inspire more effective and alternative strategies in nanozyme design and expand their use in sensing and determination.

Cobalt-Based Metal-Organic Framework Nanoparticles with Peroxidase-like Catalytic Activity for Sensitive Colorimetric Detection of Phosphate

Appropriate addition of phosphate salt in food can improve the food quality and taste. However, extensive intake of phosphate salt may lead to some human diseases such as hyperphosphatemia and renal insufficiency. Thus, it is essential to establish a cost-effective, convenient, sensitive, and selective method for monitoring phosphate ion (Pi) to ensure food quality control. In this work, a Co-based metal-organic frameworks (Co-MOF) nanomaterial with dual functions (peroxidase-like activity and specific recognition) was designed for acting as a catalytic chromogenic platform for sensitive detection of Pi. The Co2+ nodes not only provide high enzyme-like activity to catalyze the 3,3′,5,5′--tetramethylbenzidine (TMB) substrate to blue oxTMB (652 nm) but also act as selective sites for Pi recognition. The use of cationic organic ligands (2-methylimidazole) and cationic metal ions (Co2+) endows the Co-MOF with a strong positive surface charge, which is beneficial to the capture of negative-charged Pi and the dramatically suppressed TMB oxidation. When Pi exists, it specifically adsorbs onto the Co-MOF through the Co-O-P bond and the strong electrostatic interaction, leading to the change of surface charge on Co-MOF. The peroxidase-like catalytic activity of Co-MOF is thus restrained, causing a different catalytic effect on TMB oxidation from that without Pi. Based on this principle, a colorimetric assay was established for rapid and sensitive detection of Pi. A good linear relationship was obtained between Pi concentration and the absorbance at 652 nm, with a linear range of 0.009–0.144 mg/L and a detection limit of 5.4 μg/L. The proposed assay was applied to the determination of Pi in actual food samples with recoveries of 92.2–108% and relative standard deviations (RSDs) of 2.7–7.3%, illustrating the promising practicality for actual samples analysis.

Multicolor hydrogen sulfide sensor for meat freshness assessment based on Cu2+-modified boron nitride nanosheets-supported subnanometer gold nanoparticles

Hydrogen sulfide (H₂S) has emerged as an important indicator in the spoilage process of meat. In this study, a mimetic enzyme based on Cu²⁺-modified boron nitride nanosheets-supported gold nanoparticles (AuNPs/Cu²⁺-BNNS) was synthesized, which can be used to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). The H₂S gas can inhibit the activity of AuNPs/Cu²⁺-BNNS toward catalytic oxidation of TMB. Meanwhile, the usage of headspace method could avoid most interferences in the rotten sample. Various concentrations of TMB⁺ could change the aspect ratio of the gold nanoroads (AuNRs), which results in vivid color changing and UV-vis spectra shifting. The sensor had a good linear relationship with H₂S concentration ranging from 10.0 to 90.0 μmol/L, and the detection limit is 7.8 μmol/L. The AuNPs/Cu²⁺-BNNS sensors were successfully applied to detect H₂S produced by meat spoilage with satisfying results.

Enzymatic Behavior Regulation-Based Colorimetric and Electrochemiluminescence Sensing of Phosphate Using the Cobalt Oxyhydroxide Nanosheet

In this work, a convenient and flexible assay for colorimetric and electrochemiluminescence (ECL) sensing of phosphate was proposed based on the enzymatic behavior regulation of the cobalt oxyhydroxide (CoOOH) nanosheet. CoOOH as a novel nanoenzyme exhibited a peroxidase-like activity, which could catalyze different substrates such as 2, 2'-azinobis-3-ethylbenzthiazoline-6-sulfonate (ABTS) and 4-chloro-1-naphthol (4-CN) with hydrogen peroxide (H₂O₂) as the electron acceptor. Phosphate could specifically regulate the enzymatic behavior of the CoOOH nanosheet via the deactivating effect. A high level of phosphate enabled a weak color change of ABTS, which offered a "turn-off" model of the colorimetric assay with a limit of detection of 0.673 μM. Based on the similar enzymatic behavior, this strategy could then be applied in the ECL assay utilizing l-arginine-6-aza-2-thiothymine-protected gold nanoclusters (Arg-ATT-AuNCs) as ECL signal indicators. Specifically, 4-CN was catalyzed to generate the precipitate and lead to the quenching on ECL emission. Different from colorimetric behavior, phosphate with a high concentration could induce strong ECL performance, which enabled the "turn-on" model of the ECL assay with a more sensitive determination down to 0.434 nM. This flexible enzymatic behavior regulation could then allow the phosphate measurement in environmental samples including tap water and river water with satisfactory accuracy, which holds the potential in the field of environmental protection.

A fast phosphate colorimetric sensor based on MoS2/UiO-66 (Fe/Zr) nanocomposites as oxidase-/peroxidase-like nanoenzymes

According to the excellent peroxidase activity of MoS2/UiO-66(Fe/Zr) as well as specific recognition towards phosphate (Pi), a fast cheap colorimetric sensing platform has been established to determine phosphate.