Intrinsic oxidase-mimicking activity of nitrite upon visible light illumination and its colorimetric detection in saliva

Small molecules with enzyme-like properties have recently attracted considerable attention. Herein, we discovered that nitrite possesses intrinsic oxidase-mimicking activity upon visible light, catalyzing the oxidation of the typical chromogenic substrate in the absence of H2O2. Notably, nitrite exhibited a markedly high value of Kcat, approximately 4, 7, and 4000-fold greater than that of Acr+-Mes, Eosin Y, and Diacetyl, respectively. Comprehensive investigation elucidated that O2•⁻ and •OH are the primary reactive oxygen species (ROS) responsible for the oxidation of 3,3',5,5'-tetramethylbenzidine dihydrochloride hydrate (TMB). Leveraging the linear correlation between the absorbance of oxidized TMB (oxTMB) at 652 nm and nitrite concentration, a simple colorimetric approach for nitrite detection was successfully established in the range of 1-75 μM with a detection limit of 0.14 μM. Moreover, the proposed strategy could be applied to determine the nitrite concentration in saliva, exhibiting a great prospect for clinical diagnosis. This work contributes novel insights into the exploration of small-molecule enzyme mimics.

Iron Gall Ink Revisited: Visible Light-Induced, Eosin-Mediated Acceleration of Fe2+ Oxidation in Pattern Nanoarchitectonics of Fe3+-Tannic Acid Films

Inspired by iron gall ink (IGI), the Fe2+ ion is utilized for the continuous formation of Fe3+-tannic acid (TA) films through its in situ oxidation to the Fe3+ ion. Although several IGI-based strategies have been developed to meet different application requirements, there remains a need for the precise kinetic control for Fe2+ oxidation, along with one-step spatial controllability. This work demonstrates a visible light-induced method for oxidizing Fe2+ to Fe3+ ions, achieving the kinetic control over Fe3+-TA film formation. Photoexcitation of eosin Y leads to significantly accelerated film formation in a continuous manner, as shown by an 11-fold increase in film thickness compared with the air oxidation-based IGI method. Mechanistic studies reveal that the process is O2-dependent, and the kinetics of the film-forming process could be easily tuned by changing the light intensity or eosin Y concentration. Furthermore, the spatiotemporal controllability of the photoreaction allows for the pattern generation of Fe3+-TA complexes, proteins, and cells.

Gold Nanoparticles-Based Colorimetric Immunoassay of Carcinoembryonic Antigen with Metal-Organic Framework to Load Quinones for Catalytic Oxidation of Cysteine

This work reported gold nanoparticles (AuNPs)-based colorimetric immunoassay with the Cu-based metal-organic framework (MOF) to load pyrroloquinoline quinone (PQQ) for the catalytic oxidation of cysteine. In this method, both Cu2+ and PQQ in the MOF could promote the oxidation of inducer cysteine by redox cycling, thus limiting the cysteine-induced aggregation of AuNPs and achieving dual signal amplification. Specifically, the recombinant carcinoembryonic antigen (CEA) targets were anchored on the MOF through the metal coordination interactions between the hexahistidine (His6) tag in CEA and the unsaturated Cu2+ sites in MOF. The CEA/PQQ-loaded MOF could be captured by the antibody-coated ELISA plate to catalyze the oxidation of cysteine. However, once the target CEA in the samples bound to the antibody immobilized on the plate surface, the attachment of CEA/PQQ-loaded MOF would be limited. Cysteine remaining in the solution would trigger the aggregation of AuNPs and cause a color change from red to blue. The target concentration was positively related to the aggregation and color change of AuNPs. The signal-on competitive plasmonic immunoassay exhibited a low detection limit with a linear range of 0.01-1 ng/mL. Note that most of the proteins in commercial ELISA kits are recombinant with a His6 tag in the N- or C-terminal, so the work could provide a sensitive plasmonic platform for the detection of biomarkers.

Colorimetric aptasensor based on temporally controllable light-stimulated oxidase-mimicking fluorescein for the sensitive detection of exosomes in mild conditions

Analysis of exosomes provides important information for rapid and non-invasive screening of tumors. However, sensitive and convenient detection of exosomes remains technically challenging to date. Herein, a colorimetric aptasensor based on the light-stimulated oxidase-mimicking activity of FITC was constructed for detecting ovarian cancer (OC) exosomes. The aptasensor contained an EpCAM aptamer to capture OC exosomes. Cholesterol and fluorescein (FITC) were used to modify either end of the DNA (DNA anchor). The DNA anchor could combine with exosomes through a hydrophobic reaction between cholesterol and the lipid membrane. FITC oxidized 3,3',5,5'-tetramethylbenzidine (TMB) under a 365 nm LED light source in a temporally controllable manner under mild conditions, causing the solution to change from colorless to blue, and the corresponding UV-vis absorbance increased. Based on this principle, the exosomes were qualitatively analyzed by observing the color change with the naked eye. In parallel, the exosome concentration was also detected using UV-vis spectrophotometry. The linear range was from 2 × 105 to 100 × 105 particles per mL with a limit of detection of 1.77 × 105 particles per mL. The developed aptasensor also exhibited favorable selectivity and could discriminate the exosomes from OC cells and normal cells. Besides, the receiver operating characteristic (ROC) curve demonstrates that it is possible to distinguish between patients with OC and healthy donors (HDs) using exosomes as the biomarker. Our technology may expand the applications of DNA-based detection method-enabled OC diagnostic tools.

Biotemplated fabrication of N/O co-doped porous carbon confined spinel NiFe2O4 heterostructured mimetics for triple-mode sensing of antioxidants and ameliorating packaging properties

In this work, shrimp shell-derived magnetic NiFe2O4/N, O co-doped porous carbon nanozyme with superior oxidase (OXD)-like activity was prepared and used for colorimetric/photothermal/smartphone dual-signal triple-mode detection of antioxidants in fruits and beverages. The magnetic NiFe2O4/N, O co-doped porous carbon (MNPC) material was triumphantly fabricated using a combined in-situ surface chelation and pyrolysis method. The resultant MNPC composite exhibits a superior OXD-like activity, which can effectively oxidize 3,3',5,5'-tetramethylbenzidine (TMB) for yielding colorimetric/temperature dual-signal (CTDS) in absence of H2O2. This CTDS output sensor was successfully used for the determination of ascorbic acid and tannic acid. The proposed CTDS sensor with good specificity and high sensitivity can satisfy different on-site analysis requirements. Interestingly, the MNPC as a sustainable filler was further used for improving packaging properties of polyvinyl alcohol film. In short, this work offers a large-scale and cheap method to fabricate magnetic carbon-based nanozyme for monitoring antioxidants and ameliorating packaging properties.

On-Chip Electrochemical Sensing with an Enhanced Detecting Signal Due to Concentration Polarization-Based Analyte Preconcentration

Here, we integrated two key technologies within a microfluidic system, an electrokinetic preconcentration of analytes by ion Concentration Polarization (CP) and local electrochemical sensors to detect the analytes, which can synergistically act to significantly enhance the detection signal. This synergistic combination, offering both decoupled and coupled operation modes for continuous monitoring, was validated by the intensified fluorescent intensities of CP-preconcentrated analytes and the associated enhanced electrochemical response using differential pulse voltammetry and chronoamperometry. The system performance was evaluated by varying the location of the active electrochemical sensor, target analyte concentrations, and electrolyte concentration using fluorescein molecules as the model analyte and Homovanillic acid (HVA) as the target bioanalyte within both phosphate-buffered saline (PBS) and artificial sweat solution. The combination of on-chip electrochemical sensing with CP-based preconcentration renders this generic approach adaptable to various analytes. This advanced system shows remarkable promise for enhancing biosensing detection in practical applications while bridging the gap between fundamental research and practical implementation.

Molybdenum disulfide nanosheet induced reactive oxygen species for high-efficiency luminol chemiluminescence

BACKGROUND

Luminol chemiluminescence (CL) sensing system remains an excellent candidate for application in bioanalysis due to its good water solubility. However, traditional luminol CL system usually requires the addition of oxidizing agents and sensitizers to obtain high efficiency for the improvement of detection sensitivity. Although numerous studies on the nanomaterial-enhanced luminol CL systems have been carried out, there is still great potential to develop inexpensive, readily available and easily handled catalysts to construct simple and effective CL platform for biomolecular sensing.

RESULTS

Few-layered MoS2 nanosheets (NS) prepared by sonication-assisted exfoliation of commercially available bulk MoS2 were found to significantly enhance the CL of luminol‒dissolved oxygen in the absence of additional oxidants. The mechanism study demonstrated that exfoliated MoS2 NS could catalyze the decomposition of dissolved oxygen by virtue of its exposed active sites on the surface, generating increased reactive oxygen intermediates, which then oxidize luminol and produce intense CL emission. The proposed high-efficiency luminol CL system was then employed for the extremely sensitive identification of dopamine based on the quenching of CL by dopamine. The limit of detection (LOD) for dopamine can be as low as 2.07 nM. Besides, it also works well in the actual urine sample with good recoveries (99.6-100.6 %), confirming the practicability of the method.

SIGNIFICANCE

As a new type of CL catalyst, MoS2 NS developed in this work are easy to obtain, simple to prepare and can be produced in large quantities, which lays a foundation for extending applicability of MoS2 NS in the CL field, and provides a new idea for developing simple and highly sensitive CL sensing system.

Molecular Engineering to Boost the Photo-Oxidase Activity of Molecular Rotors in Colorimetric Sensing of Temperatures

Some organic dyes and photosensitizers with strong visible absorption can behave as photo-responsive oxidase mimics. However, the relationship between the photo-oxidase activity and molecular structure remains unclear to date. In this work, a new type of photosensitizer with the characteristics of molecular rotors, namely DPPy, served as the molecular scaffold for further investigation. To adjust the photocatalytic oxidation ability, DAPy and CBPy were designed and synthesized based on the enhancement and diminishment of the intramolecular charge transfer (ICT) process, respectively. Kinetic studies revealed that DAPy and CBPy both exhibited highly efficient photo-activated oxidase-like activity with 3,3',5,5'-tetramethylbenzidine (TMB) as the substrate, which were in good accordance with their molecular engineering to promote either type I or type II reactive oxygen species (ROS) generation. Impressively a colorimetric method based on the visible light induced oxidase-like activity of molecular rotors was developed to determine the environmental temperature for the first time. Both DAPy and CBPy showed distinct sensitivities toward temperature as compared with several molecular rotors based on the typical fluorimetric detection. This work provides a new strategy for the application of molecular rotors to overcome the non-emissive challenge in temperature sensing.

Photoredox-catalyzed 1,2-oxo-alkylation of vinyl arenes with 1,3-diketones: an approach to 1,4-dicarbonyls via C-C activation

A mild and inexpensive approach to synthesising a series of 1,4-diketones in moderate to excellent yields via 1,2-oxo alkylation has been developed using fluorescein as a photocatalyst and air as an oxidant. The key features include (i) varied substrate scope (39 examples); (ii) good functional group tolerance; (iii) unsymmetrical 1,4-dicarbonyls; (iv) late-stage functionalization of thymol and ibuprofen derivatives; and (v) the synthetic expansion to 5- and 6-membered N-, O- and S-containing heterocycles.

Light-gated specific oxidase-like activity of a self-assembled Pt(II) nanozyme for environmental remediation

Artificial enzyme equivalents, also known as nanozymes, are a practical tool for environmental remediation when compared to their natural counterparts due to their high operational stability, efficiency, and cost-effectiveness. Specific oxidase mimicking nanozymes are well suited to degrade toxic chemicals from industrial waste such as phenols and azo dyes. Therefore, photocatalytic nanozymes using visible/sunlight would provide a viable strategy for sustainable environmental remediation. Herein, we introduce an aggregation-induced emissive Pt(II) complex, which self-assembles in water providing NanoPtA nanotapes. These structures exhibit a specific oxidase-like nanozyme activity driven by light. The NanoPtA structure assists in the photogeneration of singlet oxygen in water via a triplet excited 3MMLCT state, leading to a specific oxidase-like activity instead of a peroxidase-like activity. The self-assembled nanozyme showed great stability under harsh environmental conditions and exhibited photo-induced specific oxidase-mimetic activity, which was considerably more efficient than the natural enzyme or other specific nanozymes. We demonstrated efficient NanoPtA-induced photocatalytic degradation of various phenolic compounds and azo dyes within 5-10 minutes of light irradiation. Notably, the system operates under sunlight and exhibits reusability over twenty cycles of catalytic reactions. Another fascinating aspect of NanoPtA is the unaltered catalytic performance for more than 75 days, providing a robust enzyme-equivalent for practical sustainable environmental remediation.

Facile Strategy to Output Fluorescein from Nucleic Acid Interactions

Biomolecular operations, which involve the conversion of molecular signals or interactions into specific functional outputs, are fundamental to the field of biology and serve as the important foundation for the design of diagnostic and therapeutic systems. To maximize their functionalities and broaden their applicability, it is crucial to develop novel outputs and facile chemical transformation methods. With this aim, in this study, we present a straightforward method for converting nucleic acid signals into fluorescein outputs that exhibit a wide range of functionalities. This operation is designed through a DNA-templated reaction based on riboflavin-photocatalyzed oxidation of dihydrofluorescein, which is readily prepared by simple NaBH4 reduction of the fluorescein with no complicated chemical caging steps. The templated photooxidation exhibits high efficiency (kapp = 2.7 × 10-3/s), generating a clear fluorescein output signal distinguishable from a low background, originating from the high stability of the synthesized dihydrofluorescein. This facile and efficient operation allows the nucleic acid-initiated activation of various fluorescein functions, such as fluorescence and artificial oxidase activity, which are applied in the design of novel bioanalytical systems, including fluorescent and colorimetric DNA sensors. The operation presented herein would expand the scope of biomolecular circuit systems for diagnostic and therapeutic applications.

Nanoclusterzyme for Dual Colorimetric Sensings: A Case Study on [Au14 (Dppp)5 I4 ]2

The enzymatic activity of atomically precise metal nanoclusters has recently been recognized; however, the number of nanoclusterzymes is very small. Besides, the applications of nanoclusterzyme wait to be explored. Herein, a novel nanoclusterzyme is synthesized and its structure is majorly resolved by single-crystal X-ray diffraction and mass spectrometry, which reveal that the nanocluster consists of an Au₁₃ icosahedron capped by an exterior shell including four I, three Dppp (1,3-bis(diphenylphosphino) propane) ligands, and a rarely reported Dppp-Au-Dppp handle staple, which contributes a lot to the enzyme activity of [Au₁₄ (Dppp)₅ I₄ ]²⁺ nanocluster. The as-obtained nanocluster can catalyze oxygen to O₂ ^•- under visible light irradiation with a specific activity up to 0.182 U·mg^-1 and lead to the blue color of 3,3',5,5'-tetramethylbenzidine (TMB) in both solution and solid states. With the addition of acetylcholinesterase (AChE), the blue color of (Au₁₄  + TMB) solution system disappears due to the nanoclusterzyme activity inhibition, but the further addition of organophosphorus pesticides (OPs) into the above mixture can restore the nanoclusterzyme and recover the blue color. Based on the color turn-off and on, the various nanoclusterzyme-containing systems are used to colorimetrically sense AChE and OPs with the detection limits reaching 0.04 mU·mL^-1 and 0.02 ng·mL^-1 , respectively.

In situ formed and switchable enzymatic activity of BiOBr under light stimulation for homogeneous and label-free bioassay

A new concept to construct photoresponsive nanozyme through the in situ deposition of electron transporting material (ETM) on BiOBr nanoplates was proposed. That was, the spontaneous coordination of ferricyanide ions (i.e., [Fe(CN)₆]^3-) onto the surface of BiOBr formed electron transporting material (ETM), which efficiently prevented electron-hole recombination and led to efficient enzyme mimicking activity under light stimuli. Moreover, the formation of the photoresponsive nanozyme was regulated by pyrophosphate ions (PPi) due to the competitive coordination of PPi with [Fe(CN)₆]^3- onto the surface of BiOBr. This phenomenon allowed the construction of an engineerable photoresponsive nanozyme that was coupled with the rolling circle amplification (RCA) reaction to elucidate a novel bioassay for chloramphenicol (CAP, taken as a model analyte). The developed bioassay manifested the merits of label-free, immobilization-free and with efficiently amplified signal. Quantitative analysis of CAP in a wide linear range from 0.05 to 100 nM with the detection limit of 0.015 nM was realized, which endowed the methodology with sufficiently high sensitivity. It is expected to be a powerful signal probe in bioanalytical field by virtue of its switchable and fascinating visible-light-induced enzyme mimicking activity.

Ru(bpy)32+ as a photoinduced oxidase mimic for colorimetric detection of biothiols

We have found that tris (2,2'-bipyridyl) ruthenium (II) (Ru(bpy)₃²⁺) possesses a high photo-induced oxidase-like activity and is capable of catalyzing the color reaction of 3,3',5,5'-tetramethylbenzidine (TMB) with dissolved oxygen. Ru(bpy)₃²⁺ has a catalytic constant (K_cat) that is twice as high as that of fluorescein, 170 and 275-fold higher than that of 9-mesityl-10-methyl acridine and Eosin Y, respectively. Electron spin resonance spectroscopy (ESR) and radical scavenging experiments have verified the major active radicals involved in the color reaction are •OH. A colorimetric biothiol assay has been successfully developed for the oxidase-like activity of Ru(bpy)₃²⁺ can be suppressed by sulfhydryl compounds. A linear dependence between the decrease in absorbance and the logarithm of thiol concentrations can be found ranging from 5.0 to 50 μM, with a detection limit of 1.0 μM. This work reveals a new oxidase mimic with high catalytic activity and will facilitate the utilization of this oxidase mimic in biochemical analysis.

An aptamer-based sandwich assay for detection of alpha-defensin human neutrophil protein 1 on a microfluidic platform

The diagnosis of periprosthetic joint infection (PJI) remains a labor-intensive and challenging issue, with life-threatening complications associated with misdiagnoses. Superior diagnostic approaches are therefore urgently needed, and synovial biomarkers are gaining substantial attention in this capacity. A new aptamer-based sandwich assay was developed where the aptamer probes specific to one such biomarker, alpha-defensin human neutrophil protein 1 (HNP 1), was integrated herein into a new microfluidic platform. The magnetic beads coated with the primary aptamer probe were able to bind the target protein with high affinity and high specificity in synovial fluid and a fluorescent-labelled secondary aptamer were further used to quantify HNP 1 in a sandwich approach. Up to four clinical samples with low volume (∼50 μL each) in a much faster assay including detection within <60 min with 100% accuracy (with totally 13 clinical samples without the need of sample pretreatment) through the use of the aptamer-based sandwich assay were automatically detected on a single chip. The wide dynamic range of this compact device, 0.5-100 mg/L, highlights its utility for future PJI diagnostics in the clinic.

Morphology and Enzyme-Mimicking Activity of Copper Nanoassemblies Regulated by Peptide: Mechanism, Ultrasensitive Assaying of Trypsin, and Screening of Trypsin Inhibitors

To regulate nanostructure synthesis is of crucial importance for developing various applications, including catalysis, bioanalysis, and optical devices. Herein, the morphology and peroxidase (POD)-mimicking activity of peptide-templated copper nanoassemblies (Cu NAs) are regulable with peptide types. The Cu NAs templated with peptide containing single cysteine are uniform nanoclusters with strong POD-like activity. However, the Cu NAs templated with peptide containing two cysteines are fusiform-like with very weak POD-like activity. Unexpectedly, the POD-like activity of Cu NAs templated with peptide containing two cysteines with lysine between the cysteines is significantly enhanced when trypsin is incubated, which is unchanged for the Cu NAs templated with peptide containing two cysteines without lysine between the cysteines. The remarkably enhanced POD-mimicking activity originates from trypsin specifically shearing the peptide bond on the lysine, thereby allowing the aggregated Cu NAs to unravel into individual nanoclusters. Therefore, a robust colorimetric sensing platform was constructed for sensitive and selective detection of trypsin, which showed a linear concentration range of 3-1000 nM and a detection limit of 0.82 nM (S/N = 3). More interestingly, featured by trypsin inhibitor restraining trypsin activity, it enabled us to screen trypsin inhibitors as well. Subsequently, the developed assay was applied to detect trypsin in serum samples with good accuracy and reproducibility. Thus, this strategy shows great potential application in the clinic for diagnosis of trypsin-indicating diseases as well as the screening of trypsin inhibitor-based anti-cancer drugs.

Highly Selective Colorimetric Detection of Cu2+ Using EDTA-Complexed Chlorophyll-Copper/ZnO Nanorods with Cavities Specific to Cu2+ as a Light-Activated Nanozyme

In this study, chlorophyll-copper (ChlCu)-modified ZnO nanorods (ChlCu/ZnO) were prepared, and then sodium ethylenediamine tetraacetate (EDTA) was used to remove part of Cu²⁺ in ChlCu, leaving cavities with specific adsorption activity for Cu²⁺ in E-ChlCu/ZnO. Appropriate EDTA treatment improved the photoactivity of ChlCu/ZnO and the adsorption selectivity to Cu²⁺. However, excessive EDTA treatment might lead to the collapse of the ChlCu structure, resulting in a decrease in photoactivity. The E-ChlCu/ZnO sample with 8 h of ChlCu treatment and 2 h of EDTA treatment showed optimal photoactivity. The as-prepared E-ChlCu/ZnO exhibited activity as a light-activated nanozyme, which could oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to blue under illumination, but when Cu²⁺ was present in the solution, this colorimetric reaction was inhibited; therefore, E-ChlCu/ZnO could be used for colorimetric detection of Cu²⁺. Because of the existence of specific cavities, E-ChlCu/ZnO showed excellent detection selectivity, a wide linear detection range (0-1 and 1-15 μM), and a low detection limit (0.024 μM) in the colorimetric detection of Cu²⁺.

Construction of Donor-Acceptor Heteroporous Covalent Organic Frameworks as Photoregulated Oxidase-like Nanozymes for Sensing Signal Amplification

Nanomaterials with enzyme-like characteristics (called nanozymes) show their extreme potentials as alternatives to natural enzymes. Covalent organic frameworks (COFs) as metal-free nanozymes have attracted huge attention for catalytic applications due to their flexible molecular design and synthetic strategies and conjugated, porous, and chemically stable architectures. Designing high-performance two-dimensional (2D) porous COF materials embedded with functional building units for modulating nanozymes' catalytic activity is of immense importance in contemporary research. The proper combination of donor-acceptor (D-A) fragments within a porous COF skeleton is an effective strategy to decrease the band gap and provide a strong charge-transfer pathway for highly effective charge separation. Herein, two donor-acceptor heteroporous COFs using an electron-deficient 4,4'-(thiazolo[5,4-d]thiazole-2,5-diyl)dibenzaldehyde (Tz) unit or 4,4'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzaldehyde (Td) unit and electron-rich tetrakis(4-aminophenyl)ethane (ETTA) linkers were presented. The resulting crystalline and heteroporous COFs showed outstanding oxidase-like activity under light irradiation, which can catalyze the oxidation of typical substrates and corresponding evolution in color and absorption. The light-activatable ETTA-Tz COF with prominent oxidase-like activity can serve as a colorimetric probe for quantitative detection of sulfide ions with a linear range of 1-50 μM and a detection limit of 0.27 μM within 3 min. The colorimetric approach could also be used for sulfide ion detection in human serum samples. The research demonstrated the future potential of D-A motifs within fully conjugated COFs to obtain excellent mimic enzyme activity.

Acetylcholinesterase Activity Monitoring and Natural Anti-neurological Disease Drug Screening via Rational Design of Deep Eutectic Solvents and CeO2-Co(OH)2 Nanosheets

The activity monitoring of acetylcholinesterase (AChE) and the screening of its inhibitors are critical for the diagnosis and therapy of neurological diseases. Herein, CeO₂-Co(OH)₂ nanosheets were synthesized for the first time in a newly designed deep eutectic solvent (DES) composed of l-proline and Ce(NO₃)₃·6H₂O, and a colorimetric assay was developed for quantitative detection of AChE and anti-neurological disease drug screening. Impressively, CeO₂-Co(OH)₂ composites prepared in DESs have more prominent oxidase-like activity than Co(OH)₂, CeO₂, and CeO₂-Co(OH)₂ produced in aqueous solution. The mechanism study shows that the oxygen vacancies of CeO₂-Co(OH)₂ play a vital role in oxidase-like catalysis. Based on their excellent oxidase-like activity, the CeO₂-Co(OH)₂ nanosheets have been successfully applied for highly sensitive and selective detection of AChE with a linear range of 0.2-20 mU/mL. This strategy can also be used for inhibitor screening. The sensor displays an excellent linear response in the range of 0.001-2 μg/mL toward an irreversible inhibitor (paraoxon-ethyl). Moreover, five alkaloids, namely, berberine hydrochloride, caffeine, camptothecin, matrine, and evodiamine, were screened by using neostigmine bromide as a control; berberine hydrochloride exhibited a good inhibitory effect on AChE with an IC₅₀ of 0.94 μM, while the other four had no obvious inhibitory effect. The mechanism of the different effects of alkaloids on inhibiting acetylcholinesterase activity was explored via molecular docking and kinetic simulation.

A fluorescence nanoplatform for the determination of hydrogen peroxide and adenosine triphosphate via tuning of the peroxidase-like activity of CuO nanoparticle decorated UiO-66

A novel nanocomposite of CuO nanoparticle-modified Zr-MOF (CuO/UiO-66) was synthesized and developed as a fluorescence nanoplatform for H₂O₂ and adenosine triphosphate (ATP) via the "turn-on-off" mode in the presence of terephthalic acid (TA). The structure of CuO/UiO-66 was thoroughly characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and other techniques. The CuO/UiO-66 with enhanced peroxidase-like (POD) activity obtained due to the Zr⁴⁺ in UiO-66 is beneficial to the aggregation of CuO NPs on its surface. As a result, the strengthened fluorescence at 425 nm with the excitation of 300 nm was found due to the highly fluorescent species of TAOH. This is produced by the oxidation of TA by ·OH that came from the catalysis of H₂O₂ via the peroxidase mimic of CuO/UiO-66. Hence the modification of CuO NPs on porous UiO-66 can provide a friendly and sensitive physiological condition for H₂O₂ detection. However, upon addition of ATP, the fluorescence intensity of TAOH at 425 nm effectively declined owing to the formation of complexation of Zr⁴⁺-ATP and the interaction of CuO to ATP which hampers the catalytic reaction of CuO/UiO-66 to H₂O₂. The specific interaction induced "inhibition of the peroxide-like activity" endows the sensitive and selective recognition of ATP. The detection limits were 16.87 ± 0.2 nM and 0.82 ± 0.1 nM, and linear analytical ranges were 0.02-100 μM and 0.002-30 μM for H₂O₂ and ATP, respectively. The novel strategy was successfully applied to H₂O₂ and ATP determination in serum samples with recoveries of 97.2-103.8% for H₂O₂ and 97.6-101.7% for ATP, enriching the avenue to design functional MOFs and providing new avenue of multicomponent bioanalysis.

Recent advances in enzyme-enhanced immunosensors

Among the products for rapid detection in different fields, enzyme-based immunosensors have received considerable attention. Recently, great efforts have been devoted to enhancing the output signals of enzymes through different strategies that can significantly improve the sensitivity of enzyme-based immunosensors for the need of practical applications. In this manuscript, the significance of enzyme-based signal transduction patterns in immunoassay and the central role of enzymes in achieving precise control of reaction systems are systematically described. In view of the rapid development of this field, we classify these strategies based on the combination of immune recognition and enzyme amplification into three categories, namely enzyme-based enhancement strategies, combination of the catalytic amplification of enzymes with other signal amplification methods, and substrate-based enhancement strategies. The current focus and future direction of enzyme-based immunoassays are also discussed. This article is not exhaustive, but focuses on the latest advances in different signal generation methods based on enzyme-initiated catalytic reactions and their applications in the detection field, which could provide an accessible introduction of enzyme-based immunosensors for the community with a view to further improving its application efficiency.

Nanoscale Covalent Organic Frameworks with Donor-Acceptor Structures as Highly Efficient Light-Responsive Oxidase-like Mimics for Colorimetric Detection of Glutathione

Although organic artificial enzymes have been reported as biomimetic oxidation catalysts and are widely used for colorimetric biosensors, developing organic artificial enzymes with high enzymatic activity is still a challenge. Two-dimensional (2D) covalent organic frameworks (COFs) have shown superior potential in biocatalysts because of their periodic π-π arrays, tunable pore size and structure, large surface area, and thermal stability. The interconnection of electron acceptor and donor building blocks in the 2D conjugated COF skeleton can lead to narrower band gaps and efficient charge separation and transportation and thus is helpful to improve catalytic activity. Herein, a donor-acceptor 2D COF was synthesized using tetrakis(4-aminophenyl)pyrene (Py) as an electron donor and thieno[3,2-b]thiophene-2,5-dicarbaldehyde (TT) as an electron acceptor. Under visible light irradiation, the donor-acceptor 2D COF exhibited superior enzymatic catalytic activity, which could catalyze the oxidation of chromogenic substrates such as 3,3',5,5'-tetramethylbenzidine (TMB) by the formation of superoxide radicals and holes. Based on the above property, the photoactivated donor-acceptor 2D COF with enzyme-like catalytic properties was designed as a robust colorimetric probe for cheap, highly sensitive, and rapid colorimetric detection of glutathione (GSH); the corresponding linear range of GSH was 0.4-60 μM, and the limit of detection was 0.225 μM. This study not only presents the construction of COF-based light-activated nanozymes for environmentally friendly colorimetric detection of GSH but also provides a smart strategy for improving nanozyme activity.

Light-responsive Au nanoclusters with oxidase-like activity for fluorescent detection of total antioxidant capacity

A fluorescent assay for total antioxidant capacity (TAC) detection based on the light-responsive oxidase-like activity of bovine serum albumin-stabilized gold nanoclusters (BSA-AuNCs) has been developed. Thiamine (TH) as the peroxidase substrate usually works at alkaline conditions and thus limits its practical applications. Here, by utilization the light-responsive oxidase-like activity of BSA-AuNCs, TH is oxidized to fluorescent thiochrome under neutral condition in two minutes due to the single oxygen generated by BSA-AuNCs upon light irradiation. After the introduction of antioxidants into the BSA-AuNCs-TH system, the formation of thiochrome is inhibited resulting in the fluorescence decrease. On the basis of the above facts, BSA-AuNCs-TH-based assay has been fabricated and applied successfully to detect antioxidants and the TAC of vitamin C tablets as well as some commercial fruit juice with satisfied results. This work may provide novel insights into developing light-responsive nanozymes-based fluorescent assays.

Lysosome targeting metal-organic framework probe LysFP@ZIF-8 for highly sensitive quantification of carboxylesterase 1 and organophosphates in living cells

Herein, a lysosomal targeting LysFP@ZIF-8 metal-organic framework (MOF) was fabricated using fluorescent protein chromophore-based probe (LysFP) for selectively detection of carboxylesterase 1 (CES1) in living cells. Unlike the regular small molecule fluorescent probes, LysFP@ZIF-8 showed wide range pH tolerabiligy, high selectivity and sensitivity to CES1 in bio-samples, and was successfully applied to achieve the visual monitoring of CES1 activity in living cells. Low detection limit and high fluorescence quantum yield was calculated as 79 ng/mL and 0.76 for LysFP@ZIF-8, respectively. Furthermore, LysFP@ZIF-8 can also serve as a fluorescence indicator of organophosphates pesticide exposure in the way of hydrolyzing the carboxylic acid ester group in LysFP. This type of probe can inspire the development of fluorescent tools for further explore many pathological processes.

Ultrathin PdCu alloy nanosheet-assembled 3D nanoflowers with high peroxidase-like activity toward colorimetric glucose detection

Enzyme-mimetic properties of nanomaterials can be efficiently tuned by controlling their size, composition, and structure. Here, ultrathin PdCu alloy nanosheet-assembled three-dimensional (3D) nanoflowers (Pd₁Cu_x NAFs) with tunable surface composition are obtained via a generalized strategy. In presence of H₂O₂, the as-synthesized Pd₁Cu_x NAFs can catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to the oxidized form of TMB (oxTMB) with a characteristic absorption peak at 652 nm. Interestingly, Pd₁Cu_x NAFs show obviously composition-dependent peroxidase-like catalytic activities because of the synergistic interaction of nanoalloy. Additionally, different from 2D Pd nanosheets, the distinctive 3D superstructures are featured with rich approachable sites and proper layer spacing, which are in favor of fast mass transport and electron transfers during the catalytic process. Among the studied Pd₁Cu_x NAFs, the Pd₁Cu_1.7 NAFs show the highest enzyme-like activities and can be successfully applied for the colorimetric detection of glucose with a low detection limit of 2.93 ± 0.53 μM. This work provides an efficient avenue to fabricate PdCu NAF nanozymes in biosensing toward glucose detection. Two-dimensional (2D) PdCu ultrathin nanosheet-assembled 3D nanoflowers (Pd₁Cu_x NAFs) with tunable surface composition exhibit substantially enhanced intrinsic peroxidase-like catalytic activities. The Pd₁Cu_1.7 NAFs are successfully used as peroxidase mimic catalyst for the colorimetric detection of glucose with low detection limit of 2.93 μM.

A colorimetric sensor for acid phosphatase activity detection based on acridone derivative as visible-light-stimulated oxidase mimic

Currently, organic artificial enzymes as biocatalysts have been extensively used to construct various colorimetric sensors. However, exploiting a potential organic artificial enzyme with high catalytic efficiency still remains a challenge. To address this issue, herein, we synthesize an acridone derivative 10-benzyl-2-amino-acridone (BAA). The synthesized BAA exhibits an intrinsic visible-light-stimulated oxidase-like activity, which is capable of oxidizing various chromogenic substrates without destructive hydrogen peroxide (H₂O₂) under visible light stimulation, resulting in colored products. The reaction system can be regulated by switching light on and off, which is milder and more reliable means than others H₂O₂-dependent. The photocatalytic mechanism of BAA is investigated in detail. However, l-ascorbic acid (AA), an antioxidant generating from the acid phosphatase (ACP)-mediated hydrolysis of 2-phospho-l-ascorbic acid (AAP), is able to inhibit the catalytic activity of BAA. Based on the above properties, a facile, photo-switchable and low-cost colorimetric sensing strategy is developed for ACP detection. The linear range is 0.05-2.5 U/L (r = 0.9994), and the limit of detection (LOD) is 0.0415 U/L. Moreover, the proposed sensing system can be applied for monitoring ACP activity in practical samples, demonstrating promising applications in clinical analysis and biosensor platform.

Paper-based Chemiluminescence Device with Co-Fe Nanocubes for Sensitive Detection of Caffeic Acid

In this work, a new chemiluminescence (CL) system of Co-Fe prussian blue analogs nanocubes (Co-Fe PBA NCs) that can catalyze luminol to produce strong CL in the absence of H₂O₂ was established. Co-Fe PBA NCs have the property of oxidase-like activity, and it can catalyze the generation of active oxygen radicals in a dissolved oxygen system. Since caffeic acid (CA) can remove reactive oxygen species in the system, a sensitive detection method for CA on a paper-based chip was developed. Under the optimal conditions, this method showed a good linear response to CA in the range of 10 - 800 ng mL^-1 with a limit of 3 ng mL^-1. The proposed method had been used for the determination of CA in tea samples. The results may open a new avenue for the catalytic property on luminol CL system without extra oxidants.

Construction of a red emission fluorescent protein chromophore-based probe for detection of carboxylesterase 1 and carbamate pesticide in culture cells

Designing fluorescent probe for detecting carboxylesterase 1 is remains challenging. Herein, a red emission human carboxylesterase 1 (CES1) probe (CAE-FP) was synthesized based on fluorescent protein chromophore. Probe CAE-FP can specific detect human CES1 with high selectively. The fluorescence quantum yield was calucated as 0.19. The carboxylic acid ester in CAE-FP could be easily hydrolyzed by CES1 under physiological conditions, and this process could induce the obvious fluorescence signal in red emission region. The detection limit of CES1 was calculated as 84.5 ng/mL. Due to the biological detoxification mechanism of carboxylesterase and the obvious inhibitory effect of pesticides on its activity, CAE-FP was applied to detect carbamate pesticide and have achieved good application results. Since fluorescent protein chromophore has excellent biocompatibility, probe CAE-FP with good cell membrane permeable and was successfully applied to monitor the real activities of CES1 in living cells. In summary, this is one of the few reported fluorescent probes that can specific detect the real-time activity of CES1 in biological samples. Besides, we first applied the fluorescent protein chromophore to construct the specific target enzyme probe. This work would contribute to further investigate CES1-associated physiological and pathological processe.

Light-accelerating oxidase-mimicking activity of black phosphorus quantum dots for colorimetric detection of acetylcholinesterase activity and inhibitor screening

A feasible and sensitive colorimetric platform was established for the assay of acetylcholinesterase (AChE) activity and evaluation of its inhibitor screening, based upon the light-accelerating oxidase-mimicking activity of black phosphorus quantum dots (BP QDs). The BP QDs were synthesized through a thermal exfoliation method and characterized using various techniques. The BP QDs exhibit oxidase-mimicking catalytic activity on dissolved oxygen-mediating oxidation of 3,3',5,5'-tetramethylbenzidine, a typical substrate of oxidase. This results in a transformation of 3,3',5,5'-tetramethylbenzidine into its blue oxidized product, which has a visible absorption peak at 652 nm. The exposure of 365 nm light irradiation significantly accelerates the oxidase-mimicking activity of the BP QDs and speeds up the reaction efficiency. AChE can specifically catalyze the decomposition of its substrate acetylthiocholine chloride to thiocholine. Thiocholine has reducing capacity and can thus reduce the oxidase-mimicking activity of the BP QDs. As a result, the oxidation of 3,3',5,5'-tetramethylbenzidine is hindered and the blue solution becomes paler. This gives a linear response for AChE ranging from 0.5 to 10.0 mU mL-1 and a detection limit of 0.17 mU mL-1. The assay was successfully applied to evaluate inhibitor screening with neostigmine as the model.

Facile Preparation of Homogeneous Copper Nanoclusters Exhibiting Excellent Tetraenzyme Mimetic Activities for Colorimetric Glutathione Sensing and Fluorimetric Ascorbic Acid Sensing

Nanozymes are artificial enzymes, which can substitute traditional biological enzymes for multifield applications. However, to date, it remains challenging to search novel mimic enzymes or multienzyme mimics. Herein, a facile and green method for preparing monodisperse, homogeneous copper nanoclusters (Cu NCs) with smaller size was developed, which used cysteamine as a template and hydrazine hydrate as a reductant to reduce Cu²⁺. The as-prepared Cu NCs exhibited excellent tetraenzyme-like activities, including peroxidase (POD)-, catalase (CAT)-, superoxide dismutase (SOD)-, and ascorbic acid oxidase (AAO)-mimic activities. The mechanisms, kinetics, and catalytic performances of Cu NCs were systematically studied. Moreover, based on the POD-like activity of Cu NCs, sensitive and simple colorimetric sensing glutathione (GSH) was explored, with the low limit of detection of 0.89 μM GSH (S/N = 3). Additionally, a novel fluorimetric ascorbic acid (AA) sensor was developed with the linear range of 0.5-30 μM and limit of detection (LOD) of 0.144 μM, on the basis of the principle that AA is oxidized to dehydroascorbic acid (DHAA) specifically catalyzed by the AAO-like activity of Cu NCs, while DHAA can further react with o-phenylenediamine (OPDA) to generate a highly fluorescent quinoxaline (DFQ) derivative. The as-proposed colorimetric GSH sensor and the fluorimetric AA sensor were capable of detecting GSH and AA, respectively, in real samples accurately and reproducibly. Thus, the Cu NCs-based multienzyme mimic is a promising candidate for biocatalysis and biosensing.

Copper-based metal-organic xerogels on paper for chemiluminescence detection of dopamine

In this work, copper(ii)-containing metal-organic xerogels (Cu-MOXs), which were composed of copper as the central ion and 2,2'-bipyridine-6,6'-dicarboxylic acid as the ligand, were quickly synthesized by a mild facile strategy. The Cu-MOXs exhibited superior catalytic performance for the luminol-H₂O₂ chemiluminescence (CL) system. The possible mechanism was studied via CL spectra, UV-Vis absorption and electron paramagnetic resonance (ESR). Since dopamine (DA) can inhibit the reaction of this system, a sensitive paper-based CL device for the detection of DA was established. Under the optimal experimental conditions, the linear range of this method was 40-200 nM with a detection limit of 10 nM. The proposed method was used for the determination of DA in urine samples.

Reducing background absorbance via a double-lock strategy for detection of alkaline phosphatase and α-fetoprotein

Lowering the background signal for more sensitive analysis of determinands is as important as amplifying the target signal. The photoinduced oxidase of fluorescein has been reported, which can catalyze the oxidization of common substrates in a few minutes. As a metaphor for locks and keys, we designed double locks confining the activity of fluorescein to reduce the background absorbance during colorimetric detection. The first lock inhibits the main activity of fluorescein by phosphating. The second lock almost completely deactivates fluorescein by forming coordination nanoparticles (CNPs) via the self-assembly of cerium chloride and fluorescein diphosphate (FDP). The Ce-FDP CNPs are characterized by scanning electron microscope (SEM), dynamic light scattering (DLS), Fourier transform infrared spectrometer (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy dispersive spectrum (EDS), which show electrostatic formation and amorphous character in the morphology. Alkaline phosphatase (ALP), the key to release fluorescein, can destroy Ce-FDP CNPs along with decomposing FDP by degrading phosphate groups. Therefore, a novel colorimetric strategy for sensitive detection of ALP is established. The detection of α-fetoprotein (AFP) is further succeeded by labeling AFP antibody with ALP. By dramatically reducing the background absorbance, the detection limits of ALP and AFP are as low as 0.014 mU/mL and 0.023 ng/mL, respectively. This convenient, brief, sensitive assay provides a promising prospect for clinical diagnosis. Graphical abstract.

Bifunctional antibody and copper-based metal-organic framework nanocomposites for colorimetric α-fetoprotein sensing

Cu²⁺ are found to greatly reduce the photoinduced oxidase activity of fluorescein and then inhibit the chromogenic reaction catalyzed by fluorescein. A simple colorimetric assay for Cu²⁺ is established. Based on this, bifunctional nanocomposites of α-fetoprotein (AFP) antibody (Ab) and copper-based metal-organic framework (Ab2@Cu-MOF) are synthesized by the simple self-assembly of AFP Ab2, Cu²⁺, and 4,4'-dipyridyl: the binding site of AFP Ab2 exposed on the surface of the nanocomposites can specifically recognize AFP antigen; Cu²⁺ in nanocomposites can inhibit the visible light-induced activity of fluorescein. The structure of Ab2@Cu-MOF disintegrate and Cu²⁺ is released in an acetate buffer solution. The higher the amount of AFP antigens, the more significant the inhibitory effect. Thus, the Ab2@Cu-MOF immunoassay for AFP determination is established using 3,3',5,5'-tetramethylbenzidine as chromogenic substrate with a detection limit of 35 pg.mL^-1. This simple, cheap, and sensitive method sheds substantial light on practical clinical diagnosis. Meanwhile, the mechanism of inhibition is revealed to facilitate the targeted selection of enzyme regulators. Graphical abstract Diagrammatic illustration of Cu²⁺ detection (part a) and Ab2@Cu-MOF immunoassay for sensing α-fetoprotein based on the synthesized Ab2@Cu-MOF nanocomposites (parts a and b).

Fe(III) bipyridyl or phenanthroline complexes with oxidase-like activity for sensitive colorimetric detection of glutathione

In this study, three types of Fe(III) bipyridyl or phenanthroline (Fe(III)-L₃ ) complex could directly catalyze 3,3',5,5'-tetramethylbenzidine (TMB) to induce blue chromogenic changes without H₂ O₂ . Fe(III)-L₃ complex could induce a colour change in TMB directly after a short incubation time. Due to the high oxidase-like activity of the Fe(III)-L₃ complexes, superoxide anion radicals (O₂ ^•- ) were formed in solution. Intermediates radical involving oxo-iron species were then produced that oxidized TMB to its oxidation products (oxTMB), which had an absorbance maximum at 652 nm. Glutathione (GSH) could inhibit the oxidation reaction of the Fe(III)-L₃ complex-TMB system, a rapidly colorimetric method was established for the specific detection of GSH that had a detection limit of 0.1 μM. Furthermore, Fe(III)-L₃ complexes could catalyze TMB to oxTMB directly without H₂ O₂ . This fast and simple colorimetric method may open a new avenue for application in the point-of-care diagnosis field using the TMB chromogenic system.

A signal amplification strategy for prostate specific antigen detection via releasing oxidase-mimics from coordination nanoparticles by alkaline phosphatase

A novel signal amplification method for prostate specific antigen (PSA) is developed by freeing fluorescein with photoinduced oxidase-like activity from coordination nanoparticles (CNPs) in the presence of alkaline phosphatase (ALP). CNPs loaded with fluorescein (F@CNPs) are obtained in aqueous solution by self-assembly using Tb³⁺ as metal ion, guanosine monophosphate (5'-GMP) as ligand, and fluorescein as signal molecule. The F@CNPs display outstanding properties of simple synthesis, low cost, good water solubility, negligible leakage and satisfactory load capacity. Fluorescein is quantitatively encapsulated in CNPs with a binding ratio of 92.72%. Meanwhile, ALP can specifically hydrolyze the phosphate group of 5'-GMP ligand, triggering the destruction of F@CNPs and leakage of fluorescein. Fluorescein, a photoinduced oxidase mimic, can catalyze the oxidation of non-fluorescent Amplex UltraRed (AUR) into fluorescent resorufin under LED lamp. This strategy exhibits good sensitivity for ALP detection. In addition, a new immunoassay for PSA is validated by labelling ALP on PSA antibody. The low detection limit of 0.04 ng mL^-1 in detecting PSA is appropriate for PSA detection in real samples. Therefore, the work not only establishes a new strategy for ALP and PSA determination, but also provides a new conception for putting photoinduced oxidase-like fluorescein in practical application.

Light-activated semiconducting polymer dots as mimic oxidases with remarkable catalytic efficiency: characteristics, mechanisms, and applications

Herein we demonstrate the characteristics, mechanisms, and applications of light-activated semiconducting polymer dots as mimic oxidases with remarkable catalytic efficiency.

CeO2 -Encapsulated Hollow Ag-Au Nanocage Hybrid Nanostructures as High-Performance Catalysts for Cascade Reactions

Inspired by bio-enzymes, multistep cascade reactions are highly attractive in catalysis. Despite extensive research in recent years, it remains a challenge to promote the stability and activity of catalysts. Here, well-defined core-shell structured Ag-Au nanocage@CeO₂ (Ag-Au NC@CeO₂ ) are designed by a simple and facile self-assembly method. The results indicate that the Ag-Au NC@CeO₂ has glucose oxidase-like activity and intrinsic peroxidase-like activity at the same time. As expected, Ag-Au NC@CeO₂ hybrid nanomaterials exhibit cascade reactions activity. Moreover, the hybrid materials are promising to detect glucose without bio-enzymes. This research has potential applications in biomedicine and biomimetic catalysis.