Electroless Deposition of Palladium Nanoparticles on Graphdiyne Boosts Electrochemiluminescence.

Modulating the electronic structure of metal nanoparticles via metal-support interaction has attracted intense interest in the field of catalytic science. However, the roles of supporting substrates in regulating the catalytic properties of electrochemiluminescence (ECL) remain elusive. Here, we find that the use of graphdiyne (GDY) as the substrate for electroless deposition of Pd nanoparticles (Pd/GDY) produces the most pronounced anodic signal enhancement in luminol-dissolved oxygen (O2) ECL system as co-reactant accelerator over other carbon-based Pd composite nanomaterials. Pd/GDY exhibits electrocatalytic activity for the reduction of O2 through a four-electron pathway at approximately -0.059 V (vs Ag/AgCl) in neutral solution forming reactive oxygen species (ROS) as intermediates. The study shows that the interaction of Pd and GDY increases the amount and stability of ROS on the Pd/GDY electrode surface and promotes the reaction of ROS and luminol anion radical to generate excited luminol, which significantly boosts the luminol anodic ECL emission. Based on quenching of luminol ECL through the consumption of ROS by antioxidants, we develop a platform for the detection of intracellular antioxidants. This study provides an avenue for the development of efficient luminol ECL systems in neutral media and expands the biological application of ECL systems.

General In Situ Engineering of Carbon-Based Materials on Carbon Fiber for In Vivo Neurochemical Sensing.

Developing real-time, dynamic, and in situ analytical methods with high spatial and temporal resolutions is crucial for exploring biochemical processes in the brain. Although in vivo electrochemical methods based on carbon fiber (CF) microelectrodes are effective in monitoring neurochemical dynamics during physiological and pathological processes, complex post modification hinders large-scale productions and widespread neuroscience applications. Herein, we develop a general strategy for the in situ engineering of carbon-based materials to mass-produce functional CFs by introducing polydopamine to anchor zeolitic imidazolate frameworks as precursors, followed by one-step pyrolysis. This strategy demonstrates exceptional universality and design flexibility, overcoming complex post-modification procedures and avoiding the delamination of the modification layer. This simplifies the fabrication and integration of functional CF-based microelectrodes. Moreover, we design highly stable and selective H+, O2, and ascorbate microsensors and monitor the influence of CO2 exposure on the O2 content of the cerebral tissue during physiological and ischemia-reperfusion pathological processes.

Biodegradable Microelectrodes for Monitoring the Dynamics of Extracellular Ca2+ in Rat Brain.

In vivo electrochemical analysis is of great significance in understanding the dynamics of various physiological and pathological activities. However, the conventional microelectrodes for electrochemical analysis are rigid and permanent, which comes with increased risks for long-term implantation and secondary surgery. Here, we develop one biodegradable microelectrode for monitoring the dynamics of extracellular Ca2+ in rat brain. The biodegradable microelectrode is prepared by sputtering gold nanoparticles (AuNPs) on a wet-spun flexible poly(l-lactic acid) (PLLA) fiber for conduction and transduction and coating a Ca2+ ion-selective membrane (ISM) with a PLLA matrix on the PLLA/AuNPs fiber, forming PLLA/AuNPs/Ca2+ISME (ISME = ion-selective microelectrode). The prepared microelectrode shows excellent analytical properties including a near-Nernst linear response toward Ca2+ over the concentration range from 10 µM to 50 mM, good selectivity, and long-term stability for weeks as well as biocompatibility and biodegradability. The PLLA/AuNPs/Ca2+ISME can monitor the dynamics of extracellular Ca2+ following spreading depression induced by high potassium even if in the fourth day. This study provides a new design strategy for the biodegradable ISME and promotes the development of biodegradable microelectrodes for long-term monitoring of chemical signals in brain.

Enhancing Enzyme-like Activities of Prussian Blue Analog Nanocages by Molybdenum Doping: Toward Cytoprotecting and Online Optical Hydrogen Sulfide Monitoring.

Artificial nanozymes have been designed to solve the problems of high cost and poor stability involving natural enzymes in analytical applications. Nevertheless, the catalytic efficiency of the nanozyme still needs to be improved so that it can meet the stability and sensitivity requirements of continuous biological detection. We presented an effective tailoring strategy to enhance the enzyme-like activities of Prussian-blue-analog-based nanozymes. Molybdenum-polysulfide-deposited nickel-iron bimetal Prussian-blue-analog-based hollow nanocages (Nanocages) with peroxidase-, catalase-, and laccase-mimicking activities were synthesized. The doping of molybdenum successfully tailored the size, morphology, composition, and complex structure of the Nanocage, and the peroxidase- and laccase-mimicking activities of the Nanocage nanozyme were enhanced by over 37 and 27 times, respectively, compared with pristine Prussian blue analogs. Moreover, in environments of harsh pH, high temperature, and high salt concentration, Nanocages exhibited much higher stability than natural enzymes. The peroxidase- and catalase-mimicking activities were applied to eliminate reactive oxygen species in cells, whereas the laccase-like activity of Nanocages was integrated with an online sensing platform for in vivo and continuous optical hydrogen sulfide monitoring in the brains of living rats. Our findings may provide possibilities for advancing the design strategy of highly active nanozymes as well as nanozyme-based in vivo detection methods and will offer unique opportunities for their involvement in bioanalytical chemistry.

V2O5 Nanobelts Mimick Tandem Enzymes To Achieve Nonenzymatic Online Monitoring of Glucose in Living Rat Brain.

The continuous detection of glucose is significant for revealing its role in neuron protection and for diagnosis of various diseases. In this study, for the first time, a nonenzymatic online optical detection platform (OODP) for glucose measurement in rat brain utilizing the tandem enzyme activity of V2O5 nanobelts is developed. V2O5 nanobelts were synthesized via a facile solvothermal strategy, and for the first time it is found that the V2O5 nanobelts possess dual enzyme-like activity, i.e., glucose oxidase (GOx)-like and peroxidase-like activity, and can act as a "tandem nanozyme". To investigate the mechanisms of the GOx-like property, we built an adsorption model, and the RPBE density functional calculations indicate that the glucose molecule can be adsorbed on the V2O5 plane. Based on the ability of V2O5 nanobelts to mimick tandem enzymes, a nonenzymatic online optical detection platform (OODP) for the continuous monitoring of glucose in rat brain was designed, which exhibits excellent stability, high selectivity, and a wide linear detection range from 0.2 to 5 mM and records cerebral glucose alterations in the calm/ischemia model. This facile but reliable nonenzymatic online optical glucose measurement compares favorably with natural enzyme-based online electrochemical glucose analytical systems, and its ready adoption by physiologists and pathologists will facilitate the understanding of brain function and the pathogenesis of diabetes.

Bioinspired Cu-based metal-organic framework mimicking SOD for superoxide anion sensing and scavenging.

Superoxide anion (O2•-) is typically produced in living cells and organisms, while excess O2•- may cause unexpected damage, so monitoring and scavenging the O2•- is of considerable significance to exploring physiological and pathological process. In this study, a Cu-based metal-organic framework (Cu-MOF) which comprise sequential Cu metal ion and conductive organic 2,5-dicarboxylic acid-3,4-ethylene dioxythiophene is synthesized to mimic superoxide dismutase (SOD), in which Cu is the essence of active site. On one hand, the Cu-MOF possesses excellent electrocatalytic activity to detect O2•- at -0.05 V, biased at which potential the electrode showed good linearity toward O2•- with detection limit of 0.283 µM and interference immunity for AA, DA, UA, 5-HT and H2O2. The Cu-MOF modified microelectrode was applied for measuring the O2•- released from living cells real time and monitoring O2•- generation in rat brain. On the other hand, this Cu-MOF has the catalytic activity to mimic the superoxide dismutase for scavenging O2•- in HeLa cells effectively. This work provides a methodology to design metal ion based enzyme mimetic for analyzing and scavenging O2•- in cells and in vivo.

Ascorbate oxidase-like nanozyme with high specificity for inhibition of cancer cell proliferation and online electrochemical DOPAC monitoring.

Despite the extensive investigation of the nanozymes exhibit their favorable performance compared to natural enzymes, nevertheless, the highly specific nanozyme still needs to be developed so that it can meet the requirements of exploring the mechanism as well as administration of related diseases and selective monitoring in biological system. In this study, self-assembled glutathione-Cu/Cu2O nanoparticles (GSH-Cu/Cu2O NPs) that exhibits specific ascorbic acid (AA) oxidase-like catalytic activity were constructed for AA-activated and H2O2-reinforced cancer cell proliferation inhibition and selective neurochemical monitoring. Cu/Cu2O NPs demonstrates effective AA oxidase-like activity and no common characteristics of other redox mimic enzymes often present in nanozyme. In particular, we found that the AA oxidase-like activity of GSH-Cu/Cu2O nanozyme was significantly improved by about 40% by improving the activation ability toward oxygen. The synthesized nanozyme can induce the generation of active oxygen by accelerating the oxidation of AA, which effectively suppresses the proliferation of cancer cells. We constructed an online electrochemical system (OECS) though loading nanozyme with enhanced ascorbate oxidase activity into a microreactor and setting it in the upstream of the detector. This GSH-Cu/Cu2O NPs-integrated microreactor can completely eliminate AA interference of the physical level toward 3,4-dihydroxy phenylacetic acid (DOPAC) electrochemical measurement, and the nanozyme-based OECS is able to continuously capture DOPAC alteration in rat brain acidosis model. Our findings may inspire rational design of nanozymes with high specificity as well as nanozyme-based selectivity solution for in vivo detection and show promising opportunities for their involvement in neurochemistry investigation.

Three-dimensional flexible and stretchable gold foam scaffold for real-time electrochemical sensing in cells and in vivo.

Compared with typical two-dimensional electrodes, the three-dimensional (3D) cell culture platform can simulate the real cell survival environment for cell growth to accurately reproduce cell functions. Moreover, considering that living cells are exposed to various of mechanical force in the microenvironment, the construction of 3D electrodes with excellent flexible, stretchable, and biocompatibility is of great significance to real-time monitor mechanically evoked biomolecule release from cells. Herein, we demonstrated a straightforward and effective three-step approach to fabricate three-dimensional flexible and stretchable gold foam scaffold (3D Au foam scaffold) for construction of 3D cell culture integrated electrochemical sensing platform. The excellent biological and electrical properties of Au nanostructures and porous networks of the 3D scaffold endow the platform with desirable biocompatibility and sensitive electrochemical sensing performance. As a proof of concept, the 3D Au foam scaffold functionalized with cobalt based nanocubes (Co NCs/Au foam scaffold) was validated to provide 3D culture for human umbilical vein endothelial cells (HUVECs), and synchronously real-time monitor superoxide anion (O2•-) released by HUVECs under mechanical stretching. Furthermore, 3-mercaptopropionic acid (3-MPA) modified 3D Au foam (3-MPA/Au foam scaffold) was successfully used for real-time monitoring of catecholamines in rat brain. The results demonstrate the great potential of this 3D Au foam scaffold for real-time electrochemical monitoring biomolecules in vitro and in vivo, providing convenience for future research on mechanotransduction relevant processes.

Cobalt Single-Atom Nanozyme Co-Administration with Ascorbic Acid Enables Redox Imbalance for Tumor Catalytic Ablation.

The elevated antioxidant defense system in cancer cells can lead to resistance to treatments involving ROS. Breaking the redox balance of the cell system through a "open up the source and regulate the flow" strategy can inhibit the growth of cancer cells and thus design a cancer treatment strategy. Here, cobalt single atom-supported N-doped carbon nanozymes (Co SA-N/C) were synthesized via a simple sacrificial template method, which can mimic the properties of ascorbate oxidase and glutathione oxidase effectively. The synthesized Co SA-N/C can induce the generation of active oxygen by accelerating the oxidation of ascorbic acid (AA) and destroy the endogenous active oxygen scavenging system by consuming the main antioxidant, glutathione (GSH). In-depth in vitro and in vivo investigations indicate that compared with solo therapy, Co SA-N/C together with AA can significantly enhance the anti-tumor efficiency by simultaneously elevating oxidative stress and consuming the overexpressed glutathione (GSH) through the redox reaction catalyzed by Co SA-N/C. This work provides a promising route for developing nanozyme-guided and ascorbate-based antitumor agents.

Structure Defect Tuning of Metal-Organic Frameworks as a Nanozyme Regulatory Strategy for Selective Online Electrochemical Analysis of Uric Acid.

Nanozymes have been designed to address the limitations of high cost and poor stability involving natural enzymes in analytical applications. However, the catalytic efficiency of the nanozyme still needs to be improved so that it can meet the selectivity and stability requirements of accurate biomolecule analysis. Here, we presented structure defects of metal-organic frameworks (MOFs) as a tuning strategy to regulate the catalytic efficiency of artificial nanozymes and investigated the roles of defects on the catalytic activity of oxidase-like MOFs. Structural defects were introduced into a novel Co-containing zeolitic imidazolate framework with gradually loosened morphology (ZIF-L-Co) by doping cysteine (Cys). It was found that with the increase in defect degree, the properties of materials such as ascorbate oxidase-like, glutathione oxidase-like, and laccase-like were obviously enhanced by over 5, 2, and 3 times, respectively. In-depth structural investigations indicate that the doping of sulfur inducing structural defects which may destroy the equilibrium state between cobalt and nitrogen in 2-methylimidazole and distort the crystal lattice, thereby enhancing the adsorption of oxygen and thus promoting the oxidase-like activity. The ZIF-L-Co-10 mg with enhanced ascorbate oxidase- and laccase-like activity was loaded into a microreactor and integrated into an online electrochemical system (OECS) in the upstream of the detector. This nanozyme-based microreactor can completely remove ascorbic acid, dopamine, and 3,4-dihydroxyphenylacetic acid which are the main interference toward uric acid (UA) electrochemical measurement, and the ZIF-L-Co-10 mg Cys-based OECS system is capable of continuously capturing UA change in rat brain following ischemia-reperfusion injury. Structure defect tuning of ZIF-L-Co not only provides a new regulatory strategy for artificial nanozyme activity but also provides a critical chemical platform for the investigation of UA-related brain function and brain diseases.

Role of rare-earth elements in enhancing bioelectrocatalysis for biosensing with NAD+-dependent glutamate dehydrogenase.

Dehydrogenases (DHs) are widely explored bioelectrocatalysts in the development of enzymatic bioelectronics like biosensors and biofuel cells. However, the relatively low intrinsic reaction rates of DHs which mostly depend on diffusional coenzymes (e.g., NAD+) have limited their bioelectrocatalytic performance in applications such as biosensors with a high sensitivity. In this study, we find that rare-earth elements (REEs) can enhance the activity of NAD+-dependent glutamate dehydrogenase (GDH) toward highly sensitive electrochemical biosensing of glutamate in vivo. Electrochemical studies show that the sensitivity of the GDH-based glutamate biosensor is remarkably enhanced in the presence of REE cations (i.e., Yb3+, La3+ or Eu3+) in solution, of which Yb3+ yields the highest sensitivity increase (ca. 95%). With the potential effect of REE cations on NAD+ electrochemistry being ruled out, homogeneous kinetic assays by steady-state and stopped-flow spectroscopy reveal a two-fold enhancement in the intrinsic reaction rate of GDH by introducing Yb3+, mainly through accelerating the rate-determining NADH releasing step during the catalytic cycle. In-depth structural investigations using small angle X-ray scattering and infrared spectroscopy indicate that Yb3+ induces the backbone compaction of GDH and subtle ß-sheet transitions in the active site, which may reduce the energetic barrier to NADH dissociation from the binding pocket as further suggested by molecular dynamics simulation. This study not only unmasks the mechanism of REE-promoted GDH kinetics but also paves a new way to highly sensitive biosensing of glutamate in vivo.

ATP-responsive laccase@ZIF-90 as a signal amplification platform to achieve indirect highly sensitive online detection of ATP in rat brain.

A novel electrochemical online system for indirect, highly sensitive and selective online monitoring of ATP in the cerebral microdialysate is presented based on the particular reaction of ATP with zeolitic imidazole framework-90 (ZIF-90) encapsulated laccase microcrystals (laccase@ZIF-90) and the natural catalytic activity of laccase.

ZIF-67 as a Template Generating and Tuning "Raisin Pudding"-Type Nanozymes with Multiple Enzyme-like Activities: Toward Online Electrochemical Detection of 3,4-Dihydroxyphenylacetic Acid in Living Brains.

Due to its unique structure and high porosity, metal-organic frameworks (MOFs) can act not only as nanozyme materials but also as carriers to encapsulate natural enzymes and thus have received extensive attention in recent years. However, a few research studies have been conducted to investigate MOF as a template to generate and tune nanozymes in the structure and performance. In this work, the "raisin pudding"-type ZIF-67/Cu0.76Co2.24O4 nanospheres (ZIF-67/Cu0.76Co2.24O4 NSs) were obtained by rationally regulating the weight ratio of ZIF-67 and Cu(NO3)2 in the synthesis process. Here, ZIF-67 not only acts as a template but also provides a cobalt source for the synthesis of cobalt copper oxide on the surface of ZIF-67/Cu0.76Co2.24O4 NSs with multiple enzyme-like activities. The ZIF-67/Cu0.76Co2.24O4 NSs can mimic four kinds of enzymes with peroxidase-like, glutathione peroxidase-like, superoxide dismutase-like, and laccase-like activities. Based on its laccase-like activity, an online electrochemical system for continuous monitoring of 3,4-dihydroxyphenylacetic acid with good linearity in the range of 0.5-20 µM and a detection limit of 0.15 µM was established. Furthermore, the alteration of DOPAC in the brain microdialysate before and after ischemia of the rats' brain was also successfully recorded. This work not only raises a new idea for the synthesis of nanozyme materials with multiple enzyme activities but also provides a new solution for the detection of neurotransmitters in living brains.