Colorimetric assay of acetylcholinesterase inhibitor tacrine based on MoO2 nanoparticles as peroxidase mimetics

Fluorescent assay for acetylcholinesterase activity and inhibitor screening based on lanthanide organic/inorganic hybrid materials

It is of great significance for the clinical diagnosis of Alzheimer's disease (AD) to achieve the on-site activity evaluation of acetylcholinesterase (AChE), the hydrolase of acetylcholine (ACh). Herein, we have developed a biosensing method endowed with considerable superiority based on the organic-inorganic hybrid composite Eu(DPA)3@Lap with excellent stability and fluorescent properties for this purpose by loading Eu3+ ions and 2,6-dipicolinic acid (DPA) into LAPONITE® (Lap). Through the comprehensive consideration of the specific hydrolysis of acetylthiocholine (ATCh) into thiocholine (TCh) by AChE, the high binding affinity of TCh to copper ion (Cu2+), and the selective fluorescence quenching ability of Cu2+, a simple Eu(DPA)3@Lap-based assay was developed to realize the rapid and convenient evaluation of AChE activity. Owning to the facile signal on-off-on response mode with a clear PET-based sensing mechanism, our assay presents favorable selectivity and sensitivity (LOD of 0.5 mU mL-1). Furthermore, the fluorescent assay was successfully applied for assessing AChE activity in human serum samples and screening potential AChE inhibitors, showing potential for application in the early diagnosis and drug screening of AD, as a new development path of AD therapy.

TiO2-In-MIL-101(Cr) with Visible Light-Induced Peroxidase Activity for Colorimetric Detection of Blood Glucose

In this work, metal-organic framework MIL-101(Cr) with regular morphology, stable structure, and good dispersion was prepared by the hydrothermal method. MIL-101(Cr) has two different sizes of pores, but after TiO2 nanoparticles (NPs) were in situ prepared, the two pores disappear. The result demonstrates that TiO2 NPs were located in the pores of MIL-101(Cr). TiO2-decorated MIL-101(Cr) forms an inside type II heterojunction and the band gap energy is narrowed, which can promote electron-hole separation and enhance the light absorption. Therefore, the heterojunction shows a high visible light-induced peroxidase-like activity. Kinetic studies exhibit that the K m value of TiO2-in-MIL-101(Cr) to TMB is 0.17 mM, and the affinity of TiO2-in-MIL-101(Cr) is higher than that of natural horseradish peroxidase (HRP). Then, a "turn-on" colorimetric assay based on TiO2-in-MIL-101(Cr) was constructed for the detection of blood glucose. The detection range is 1-100 μM (R 2 = 0.9950) with a limit of detection (LOD) of 1.17 μM. Compared with the clinical method, the constructed colorimetric method has accurate and reliable results for the clinical detection. The anti-interference experiment confirms that the method has high selectivity to glucose.

Analyte-mediated formation and growth of nanoparticles for the development of chemical sensors and biosensors

The cornerstone of nanomaterial-based sensing systems is the synthesis of nanoparticles with appropriate surface functionalization that ensures their stability and determines their reactivity with organic or inorganic analytes. To accomplish these requirements, various compounds are used as additives or growth factors to regulate the properties of the synthesized nanoparticles and their reactivity with the target analytes. A different rationale is to use the target analytes as additives or growth agents to control the formation and properties of nanoparticles. The main difference is that the analyte recognition event occurs before or during the formation of nanoparticles and it is based on the reactivity of the analytes with the precursor materials of the nanoparticles (e.g., metal ions, reducing agents, and coatings). The transition from the ionic (or molecular) state of the precursor materials to ordered nanostructured assemblies is used for sensing and signal transduction for the qualitative detection and the quantitative determination of the target analytes, respectively. This review focuses on assays that are based on analyte-mediated regulation of nanoparticles' formation and differentiate them from standard nanoparticle-based assays which rely on pre-synthesized nanoparticles. Firstly, the principles of analyte-mediated nanomaterial sensors are described and then they are discussed with emphasis on the sensing strategies, the signal transduction mechanisms, and their applications. Finally, the main advantages, as well as the limitations of this approach, are discussed and compared with assays that rely on pre-synthesized nanoparticles in order to highlight the major advances accomplished with this type of nano-sensors and elucidate challenges and opportunities for further evolving new nano-sensing strategies.

Co, N co-doped porous carbon-based nanozyme as an oxidase mimic for fluorescence and colorimetric biosensing of butyrylcholinesterase activity

A Co, N co-doped porous carbon-based nanozyme (Co-N-C nanozyme) has been fabricated. Taking advantages of the excellent oxidase catalytic activity and significant stability of Co-N-C nanozyme, we propose a fluorescence and colorimetric system based on Co-N-C nanozyme and red-emitting carbon quantum dots (RCDs) for butyrylcholinesterase (BChE) sensing. As the chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB) was catalyzed and oxidized by Co-N-C nanozyme, the generated oxTMB had a new absorption peak at 652 nm, which resulted in the significant quenching of the fluorescence of the carbon quantum dots at 610 nm. Under the catalysis of BChE, thiocholine was generated from the hydrolysis of S-butyrylthiocholine iodide (BTCh), and the as-generated thiocholine effectively inhibited the oxidation of TMB catalyzed by Co-N-C nanozyme, leading to a decrease of the absorption of oxTMB at 652 nm and effective fluorescence recovery of RCDs. By measuring the absorbance of produced oxTMB at 652 nm and the fluorescence of RCDs at 610 nm, the fluorescence and colorimetric system both exhibited an outstanding linear response to the activity of BChE in the range 0.5 to 40 U L^-1, with a detection limit of 0.16 U L^-1 and 0.21 U L^-1, respectively. Furthermore, this established dual-channel biosensing strategy has been successfully applied to the determination of BChE in human serum samples. The present work has effectively expanded the development and application of nanozyme in biosensing.

Hierarchical porous MoS2 particles: excellent multi-enzyme-like activities, mechanism and its sensitive phenol sensing based on inhibition of sulfite oxidase mimics

It is important to exploit highly efficient methods for detecting pollutants selectively and sensitively. Artificial enzymes are promising to replace natural enzymes with diverse functions for sustainable developments and various applications. However, it remains the challenge to develop novel mimic enzymes or multi-enzyme mimics for pollutant detection. Herein we report hierarchical porous MoS₂ particles prepared by a simple hydrothermal method, which demonstrated excellent sulfite oxidase (SuO_x)-, nicotinamide adenine dinucleotide (NADH) oxidase- and superoxide dismutase-mimicking activities. In addition, the catalytic conditions for SuO_x-like and NADH oxidase-like activities of MoS₂ were optimized. The catalytic mechanism of the NADH oxidase mimics is that O₂ involves in the oxidation of NADH, to generate O₂^.- intermediate and finally turn to H₂O₂, while SuO_x mimics comes from that MoS₂ particles can effectively catalyze sulfite to reduce [Fe(CN)₆]^3-. Based on the excellent SuO_x-like activity of MoS₂ particles, while phenol can inhibit the oxidation of sulfite, a phenol colorimetric sensor was explored with the dynamic range of 2-1000 μM and the limit of detection of 0.72 μM, applicable to detect phenol in effluents. Therefore, MoS₂ particles with the SuO_x-like, NADH oxidase-like and SOD-like activities has broad application prospects in environmental monitoring and bio-analysis.

DiZyme: Open-Access Expandable Resource for Quantitative Prediction of Nanozyme Catalytic Activity

Enzymes suffer from high cost, complex purification, and low stability. Development of low-cost artificial enzymes of comparative or higher effectiveness is desired. Given its complexity, it is desired to presume their activities prior to experiments. While computational approaches demonstrate success in modeling nanozyme activities, they require assumptions about the system to be made. Machine learning (ML) is an alternative approach towards data-driven material property prediction achieving high performance even on multicomponent complex systems. Despite the growing demand for customized nanozymes, there is no open access nanozyme database. Here, a user-friendly expandable database of >300 existing inorganic nanozymes is developed by data collection from >100 articles. Data analysis is performed to reveal the features responsible for catalytic activities of nanozymes, and new descriptors are proposed for its ML-assisted prediction. A random forest regression (RFR) model for evaluation of nanozyme peroxidase activity is developed and optimized by correlation-based feature selection and hyperparameter tuning, achieving performance up to R²  = 0.796 for K_cat and R²  = 0.627 for K_m . Experiment-confirmed unknown nanozyme activity prediction is also demonstrated. Moreover, the DiZyme expandable, open-access resource containing the database, predictive algorithm, and visualization tool is developed to boost novel nanozyme discovery worldwide (https://dizyme.net).

Porous polymers from octa(amino-phenyl)silsesquioxane and metalloporphyrin as peroxidase-mimicking enzyme for malathion colorimetric sensor

Rapid and efficient pesticide detection methods are particularly important due to the growing problems of pesticide residues. Here, a new azo-based porous organic polymer, Azo(Fe)PPOP, was prepared from octa(amino-phenyl)silsesquioxane (OAPS) and iron(III) 5,10,15,20-tetrakis(4-nitrophenyl)porphyrin (FeTPP(NO₂)₄) via a simple coupling reaction without the participation of metal catalysts. The inorganic cage units of OAPS endowed Azo(Fe)PPOP a porous framework, high surface area, favorably thermal and chemical stability. In Azo(Fe)PPOP, iron(III) porphyrin units were individually isolated in a fixed location, which could effectively avoid dimerization or self-oxidation as happens as in the case of porphyrin monomers. Such a unique structure made Azo(Fe)PPOP exhibit an excellent peroxidase-like catalytic performance in the presence of H₂O₂ and 3,3',5,5'-tetramethylbenzidine (TMB). Because of these advantages, we established a selective, facile, and sensitive colorimetric platform for direct detection of malathion within a very short time (3 min) with a low detection limit (8.5 nM). In addition, the recognition mechanism between Azo(Fe)PPOP and malathion was verified using X-ray photoelectron spectroscopy spectra. The practicality of the constructed platform was further executed by the detection of the pesticide in soil and food samples.

Intrinsic peroxidase-like activity of 4-amino hippuric acid reduced/stabilized gold nanoparticles and its application in the selective determination of mercury and iron in ground water

Herein, we report a method for the synthesis of 4-aminohippuric acid (4-AHA) reduced/stabilized gold nanoparticles and their peroxidase mimicking properties for the colorimetric detection of Fe³⁺ and Hg²⁺. The synthesis of nanoparticles was evidenced by appearance of bright red color and an absorption peak at 518 nm. Transmission electron microscopic (TEM) characterization revealed the nanoparticles to be spherical with average size of about 5.9 ± 1.7 nm. X-ray diffraction (XRD) analysis established highly crystalline nature of the nanoparticles. The synthesized nanoparticles have shown very good peroxidase mimicking property; exhibiting the catalytic oxidation of the chromogen 3,3',5,5'-tetramethyl benzidine (TMB) to a blue color product, in the presence of hydrogen peroxide. The peroxidase mimicking activity of the nanoparticles was found to be selectivity enhanced in the presence of Fe³⁺ and Hg²⁺ while there was no change in the activity in the presence of other concomitant ions. The mechanism studies revealed that the synthesized gold nanoparticles assisted in electron transfer during the catalytic process however the stimulation of peroxidase-like activity in the presence of Fe³⁺ and Hg²⁺ is owed to both generation of hydroxyl radical and accelerated electron transfer. The assay was made selective for iron by the addition of cysteine in acetate buffer; whereas the selective detection of mercury was achieved by carrying out the assay in citrate buffer. The linear ranges for the determination of Fe³⁺ and Hg²⁺ in deionized water were found to be: 5-50 ppb and 5-200 ppb respectively. The limits of quantification (LOQ) for Fe³⁺ and Hg²⁺ were 4.0 and 2.5 ppb respectively. The assay was applied for the determination of Fe³⁺ and Hg²⁺ in drinking and ground water samples. The method holds potential for the on-field screening of these metal ions in real environmental samples.