Two-dimensional β-MnOOH nanosheets with high oxidase-mimetic activity for smartphone-based colorimetric sensing

Manganese (Mn) is a versatile transition element with diverse oxidation states and significant biological importance. Mn-based nanozymes have emerged as promising catalysts in various applications. However, the direct use of manganese oxides as oxidase mimics remains limited and requires further improvement. In this study, we focus on hydroxylated manganese (MnOOH), specifically the layered form β-MnOOH which exhibits unique electronic and structural characteristics. The two-dimensional β-MnOOH nanosheets were synthesized through a hydrothermal approach and showed remarkable oxidase-like activity. These nanosheets effectively converted the oxidase substrate, 3,3',5,5'-tetramethylbenzidine (TMB), into its oxidized form by initiating the conversion of dissolved oxygen into ·O2-, 1O2 and ·OH. However, in the presence of L-cysteine (L-Cys), the catalytic activity of β-MnOOH was significantly inhibited, enabling highly sensitive detection of L-Cys. This sensing strategy was successfully applied for smartphone-based L-Cys assay, offering potential utility in the diagnosis of Cys-related diseases. The exploration of layered β-MnOOH nanosheets as highly active oxidase mimics opens up new possibilities for catalytic and biomedical applications.

Unveiling the role of heterophase nanostructure in MnO2-based colorimetric sensors for ascorbic acid detection

Nanozymes based on manganese oxide (MnO2) are demonstrated to be promising probes in colorimetric sensing applications. In this study, the r-MnO2/β-MnO2 heterophase nanostructure was simply prepared by a calcination process with controllable temperature. The characterization of the nanostructured material was confirmed by SEM, UV-vis spectroscopy, Raman, TGA-DSC, and XRD analysis. The r-MnO2/β-MnO2 exhibits a remarkably good catalytic activity in the oxidation process of 3,3',5,5'-tetramethylbenzidine (TMB) compared with the r-MnO2 or Mn2O3 nanostructure owing to its heterophase junctions. The enhanced performance of the colorimetric sensor for ascorbic acid (AA) detection was investigated using the r-MnO2/β-MnO2 heterophase nanostructure as probe. The r-MnO2/β-MnO2 material enhanced the monitoring of AA in the wide linear range from 1 µM to 50 μM with a limit of detection of 0.84 µM. This work presents a promising and straightforward approach for the construction of MnO2-based colorimetric sensor and their practical application in plant growth monitoring.

A CRISPR/Cas12 trans-cleavage reporter enabling label-free colorimetric detection of SARS-CoV-2 and its variants

We present a label-free colorimetric CRISPR/Cas-based method enabling affordable molecular diagnostics for SARS-CoV-2. This technique utilizes 3,3'-diethylthiadicarbocyanine iodide (DISC2(5)) which exhibits a distinct color transition from purple to blue when it forms dimers by inserting into the duplex of the thymidine adenine (TA) repeat sequence. Loop-mediated isothermal amplification (LAMP) or recombinase polymerase amplification (RPA) was used to amplify target samples, which were subsequently subjected to the CRISPR/Cas12a system. The target amplicons would activate Cas12a to degrade nearby TA repeat sequences, preserving DISC2(5) in its free form to display purple as opposed to blue in the absence of the target. Based on this design approach, SARS-CoV-2 RNA was colorimetrically detected very sensitively down to 2 copies/μL, and delta and omicron variants of SARS-CoV-2 were also successfully identified. The practical diagnostic utility of this method was further validated by reliably identifying 179 clinical samples including 20 variant samples with 100% clinical sensitivity and specificity. This technique has the potential to become a promising CRISPR-based colorimetric platform for molecular diagnostics of a wide range of target pathogens.

Enhanced oxidase-mimic constructed by luminescent carbon dots loaded on MIL-53(Fe)-NO2 for dual-mode detection of gallic acid and biothiols in food and humans

Monitoring of gallic acid (GA) in food and biothiols in humans is crucial for body health. Nanozyme-mediated colorimetric strategy for evaluating them has been widely applied nowadays, however, the inferior efficient and susceptible single-signal recognition limit its further application. Herein, a sensitive biosensor was first constructed for bimodal detection of GA and biothiols based on CDs@MIL-53(Fe)-NO2, prepared through a facile and time-saving microwave treatment. Benefiting from the excellent fluorescent and electron transfer properties of CDs, CDs@MIL-53(Fe)-NO2 exhibited significant enhanced blue fluorescence and oxidase-like activity, which could oxide colorless 3,3',5,5'-tetramethylbenzidine without H2O2, and the blue product could quench the fluorescence of composite. The dual-mode assay based on such bifunctional nanozyme showed an extremely sensitivity towards GA/l-cysteine/homocysteine with the detection limit of 62/65/124 nM and 17/16/27 nM in colorimetric/fluorescent modes, respectively. The practicability in real samples and portability based on a smartphone of the analysis has been investigated with reliable results.

A smartphone-assisted ratiometric colorimetric and fluorescent probe for triple-mode determination of nitrite based on MnO2 nanoparticles and carbon quantum dots

A triple-mode colorimetric and fluorescent sensing scheme based on manganese dioxide nanoparticles (MnO₂NPs) and carbon quantum dots (CQDs) were developed to determine nitrite. MnO₂NPs can oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into a blue oxidation product (TMBox), which is further oxidized into a yellow diimine derivative by nitrite. The ratio of absorbance at 652 nm to 452 nm was monitored as signal response for UV-vis detection mode. A "turn-off" CQDs fluorescence probe was also constructed for fluorescent detection mode. Smartphone tool kit was used to capture the color of sample for smartphone-based measurement. Various analytical performance under different detection modes were obtained and compared. The proposed methods were applied to food samples with satisfactory recoveries (83.3-106 %). The results were validated with AOAC standard spectrophotometric method. The current triple-mode detection were accurate, convenient, low-cost and fast for analyzing nitrite in foods and water samples on-site.

A simple, cost-effective colorimetric assay for aluminum ions via complexation with the flavonoid rutin

Aluminum has been linked to deleterious health effects with high concentration, chronic exposure, creating a need for innovative detection techniques. Colorimetric assays are an ideal approach since they are simple, cost-effective, and field adaptable. Yet, commercially available colorimetric assays for aluminum are limited since it forms few colored chelation complexes. Flavonoids, a class of polyphenolic compounds, are one of the few examples that create colored aluminum complexes. Aluminum ions (Al3+) are the main constituent in colorimetric assays for flavonoid detection in food or plant samples. Our assay design was based on colorimetric flavonoid assays, where the assay reported herein was optimized. Specifically, the flavonoid rutin concentration and sample-to-rutin volume ratio (295:5 µL) were optimized to detect Al3+ at low µM concentrations in samples. The assay performed comparably, and in some instances better, than those requiring advanced instrumentation and previously reported colorimetric assays, with a linear range (1–8 µM), sensitivity (7.6 nM), limit of detection (79.8 nM), and limit of quantification (266 nM) for Al3+. The colorimetric assay was accurate (99 ≤ 108 ± 4 ≤ 6% Al3+ recovery), precise (low intra- and inter-assay coefficient of variation (CV) of 3.1 ≤ 5.9% and 4.4%, respectively), and selective for Al3+ ions compared to solutions containing a variety of other mono-, di-, and tri-cations at much higher concentrations (10- to 100-fold higher). Lastly, the colorimetric assay was applicable to complex analysis. It was used to generate a chelation curve depicting the Al3+ chelation capacity of sodium alginate, a biologically derived polymer used as a bioink for 3D bioprinting.

Colorimetric detection of total antioxidants in green tea with oxidase-mimetic CoOOH nanorings

Green tea is a popular beverage and is widely consumed due to its taste and antioxidative polyphenols. Herein, a smartphone-based colorimetric reader using cobalt oxyhydroxide (CoOOH) nanorings has been successfully applied to detect antioxidants in green tea with high reliability and robustness. By exploiting the oxidase-mimicking activity, the as-synthesized CoOOH nanorings replaces natural enzymes to directly catalyze oxidate colorless 3,3 ´ ,5,5 ´ -tetramethylbenzidine (TMB), while antioxidants can disintegrate CoOOH, leading to an antioxidant concentration-dependent color change. Benefiting from the CoOOH nanorings-based colorimetric strategy, a smartphone-assistant nanosensor was devised for portable and visual detection of antioxidants in green tea. The proposed method can be extended to visual detection of a diverse range of diseases by responding to their specific antioxidant, and thus provide a pivotal disease toolbox that is compatible for development at the point-of-care.

Reversible regulation of enzyme-like activity of molybdenum disulfide quantum dots for colorimetric pharmaceutical analysis

Regulating the catalytic activity of nanozymes is significant for their applications in various fields. Here, we demonstrate a new strategy to achieve reversible regulation of the nanozyme's activity for sensing purpose. This strategy involves the use of zero-dimensional MoS₂ quantum dots (MQDs) as the building blocks of nanozymes which display very weak peroxidase (POD)-like activity. Interestingly, such POD-like activity of the MQDs largely enhances in the presence of Fe³⁺ while diminishes with the addition of captopril thereafter. Further investigations identify the mechanism of Fe³⁺-mediated aggregation-induced enhancement of the POD-like activity and the inhibitory effect of captopril on the enhancement, which is highly dependent on their concentrations. Based on this finding, a colorimetric method for the detection of captopril is developed. This sensing approach exhibits the merits of simplicity, rapidness, reliability, and low cost, which has been successfully applied in quality control of captopril in pharmaceutical products. Moreover, the present sensing platform allows smartphone read-out, which has promising applications in point-of-care testing devices for clinical diagnosis and drug analysis.

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A new post-synthesis strategy is developed to achieve reversible regulation of nanozyme's activity.

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Fe³⁺-mediated aggregation can enhance the peroxidase-like activity of MoS₂ quantum dots more than 10 times.

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Quality control of captopril in pharmaceutical products is realized by manipulating nanozyme's catalytical activity.

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This sensing approach allows smartphone read-out, which has promising applications in point-of-care testing.

A bifunctionalized NiCo2O4-Au composite: Intrinsic peroxidase and oxidase catalytic activities for killing bacteria and disinfecting wound

A NiCo₂O₄-Au composite was fabricated following the calcination-reduction method and testified to exhibit the intrinsic high peroxidase- and oxidase-like activities. The composite could activate a low level of H₂O₂ (100 μM) to kill E. coli and S. aureus. NiCo₂O₄-Au composite could be easily separated by an external magnet from the media and reused several times. NiCo₂O₄-Au composite could also effectively damage the existing biofilms and prevent the formation of new biofilms. The electron spin resonance tests showed that NiCo₂O₄-Au composite catalyzed H₂O₂ into reactive oxygen species (ROS), mainly including OH, O₂^-, and ¹O₂; while the oxidase-like activity of NiCo₂O₄-Au also stemmed from the ROS formation in the absence of H₂O₂. The radical trapping experiment confirmed that OH and ¹O₂ were the main radicals in the antibacterial process for NiCo₂O₄-Au in the presence of H₂O₂. A NiCo₂O₄-Au based Band-Aid was also designed, which exhibited high anti-infective and wound-healing properties. This study has demonstrated that NiCo₂O₄-Au composite can be a promising antibacterial agent in environmental and clinical applications.

Co nanoparticles decorated with N-doped carbon nanotubes as high-efficiency catalysts with intrinsic oxidase-like property for colorimetric sensing

Artificial nanozymes are designed for pursuing the functions of splendid catalytic efficiency and prominent selectivity of natural enzymes, meanwhile obtaining higher stability than that of natural enzymes. This emerging technology shows widespread application in the crossing field between nanotechnology and biomedicine. In this work, we employed a universal approach to fabricate a Co@N-CNTs hybrid nanocomposite as an oxidase mimic, in which fine Co nanoparticles were wrapped in N-doped carbon nanotubes, stacking on a hollow dodecahedron carbon skeleton. The synergistic effects of nanostructure engineering, N-doping and carbon coating, as well as the derived interfacial effect contribute to the glorious oxidase-like activity, stability and reusability. It can catalytically oxidize the colorless substrate 3,3′,5,5′-tetramethylbenzidine (TMB) to a blue oxidation product (ox-TMB). As a result, a colorimetric technique with excellent selectivity and sensitivity for detecting ascorbic acid (AA) with naked eyes was established, in view of specific inhibitory effects towards oxidation of TMB. Under optimal detection conditions, this method exhibits a good linearity ranging from 0.1 to 160 μM with a low limit of detection (LOD) of 0.076 μM. For practical applications, Co@N-CNTs hybrid catalyst as a mimic oxidase was used for the determination of AA in human serum, which yielded satisfactory results. This work may serve as a new research thought to guide the design of high-performance nanozymes and establish a sensing platform for the detection of AA.

In this work, we designed a Co@N-CNTs hybrid nanocomposite as an oxidase mimic for the colorimetric detection of ascorbic acid with the naked eye.

Molecule-gated surface chemistry of Pt nanoparticles for constructing activity-controllable nanozymes and a three-in-one sensor

Herein, a simple strategy for constructing activity-controllable nanozymes is proposed based on the glutathione (GSH)-gated surface chemistry of citrate-capped Pt nanoparticles (PtNPs). PtNPs have been shown to have oxidase-like activity that can effectively catalyze the oxidation of 3,3',5,5'-tertamethylbenzidine (TMB) by O2, resulting in a typical color reaction from colorless to blue. We found that GSH can inhibit the oxidase-like activity of PtNPs as a molecule-gated surface chemistry element, resulting in a dramatic decrease of the oxidation of TMB. The addition of copper ions (Cu2+) could oxidize GSH into glutathione disulfide (GSSG), resulting in the distinct suppression of GSH-modulated PtNP surface chemistry and oxidase-like activity inhibition, which further results in a significant acceleration of TMB oxidation and the obvious recovery of intense blue color. Furthermore, the color-based detection signal associated with the redox of TMB indicator here was found to show good fluorescence and a photothermal effect and exhibit sensitive and selective response toward the proposed molecule-gated surface chemistry and Cu2+ target. On the basis of this phenomenon, we successfully constructed a three-in-one sensor for Cu2+ with a triple signal readout, colorimetric, photothermal (temperature), and fluorescence, depending on the proposed in situ modulation method for PtNP catalysis. The applicability of the three-in-one sensor was also demonstrated by measuring Cu2+ in human serum with a standard addition method, and the results are of satisfactory accuracy as confirmed by ICP-MS measurements.

Adsorption enhanced the oxidase-mimicking catalytic activity of octahedral-shape Mn3O4 nanoparticles as a novel colorimetric chemosensor for ultrasensitive and selective detection of arsenic

Several researches have reported that Mn₃O₄ nanoparticles (NPs) could be used as adsorbent to remove arsenic from aqueous solution. However, we found that Mn₃O₄ NPs can not only adsorb arsenic, but also enhance the catalytic activity of Mn₃O₄ NPS, which enable us to establish a new method for the determination of arsenic. Herein, the adsorption of arsenic changes surface morphology of octahedral Mn₃O₄ NPs and further release Mn²⁺ to generate sufficient active sites, which enhances their oxidase-mimicking catalytic activity. Consequently, the solution changes to yellow and displays a characteristic absorption peak at 450 nm. This property enables us to construct a novel colorimetric chemosensor for arsenic detection. The limit of detection (LOD) of such colorimetric chemosensor for arsenic detection was determined as 1.32 μg⋅L^-1, which is lower than the threshold recommended by WHO. The chemosensor allows arsenic to be determined visually at the concentrations as low as 10 μg⋅L^-1, and displays excellent selectivity against other metal ions. Moreover, the chemosensor was successfully validated by analyzing several actual environmental and biological samples, indicating the excellent prospect of octahedral Mn₃O₄ NPs in the application of arsenic detection and removal.

Smartphone colorimetric assay of acid phosphatase based on a controlled iodine-mediated etching of gold nanorods

A simple but efficient colorimetric assay was developed for the detection and quantification of acid phosphatase (ACP) using a smartphone. This strategy is based on target-controlled iodine-mediated etching of gold nanorods (AuNRs). Due to effective hydrolysis of the substrate pyrophosphate (PPi) by ACP, chelated Cu²⁺ with PPi was released, which promoted the redox reaction with an iodide ion (I^-), leading to the formation of I₃^-. As the etching agent of AuNRs, I₃^- caused a blueshift of the localized surface plasmon resonance peak and, more importantly, an observable color change. The vivid colors were recorded with a smartphone camera and directly analyzed using an image-processing app. On the basis of the direct correlation between ACP concentration and the etching degree of AuNRs as well as color change, this smartphone nanocolorimetry technique showed a good linear response toward ACP over the range of 0-15.0 U/L, with a detection limit of 0.97 U/L. Using the standard addition method, the practical applicability of the proposed smartphone-based assay was successfully demonstrated by determining ACP in human serum samples, with results consistent with those obtained by UV-Vis spectrophotometry.

Peroxidase-like Activity of Metal-Organic Framework [Cu(PDA)(DMF)] and Its Application for Colorimetric Detection of Dopamine

A metal-organic framework (MOF) [Cu(PDA)(DMF)] was synthesized under mild mixed solvothermal conditions. It is constructed by 1,10-phenanthroline-2,9-dicarboxylic acid (H₂PDA) and Cu²⁺ ions. The complex exhibits high peroxidase-like activity and can catalytically oxidize the colorless substrate 3,3',5,5'-tetramethylbenzidine to a blue product in the presence of H₂O₂. However, the peroxidase-like activity of [Cu(PDA)(DMF)] can be potently inhibited in the presence of dopamine. Based on this phenomenon, the colorimetric detection of dopamine was demonstrated with good selectivity and high sensitivity. [Cu(PDA)(DMF)] showed good stability and robust catalytic activity, which has been employed in the detection of dopamine in human urine and pharmaceutical samples.

Recent Progress in the Development of Fluorescent Probes for Thiophenol

Thiophenol (PhSH) belongs to a class of highly reactive and toxic aromatic thiols with widespread applications in the chemical industry for preparing pesticides, polymers, and pharmaceuticals. In this review, we comprehensively summarize recent progress in the development of fluorescent probes for detecting and imaging PhSH. These probes are classified according to recognition moieties and are detailed on the basis of their structures and sensing performances. In addition, prospects for future research are also discussed.