Advances in Microfluidic Paper-Based Analytical Devices (µPADs): Design, Fabrication, and Applications.

Microfluidic Paper-based Analytical Devices (µPADs) have emerged as a new class of microfluidic systems, offering numerous advantages over traditional microfluidic chips. These advantages include simplicity, cost-effectiveness, stability, storability, disposability, and portability. As a result, various designs for different types of assays are developed and investigated. In recent years, µPADs are combined with conventional detection methods to enable rapid on-site detection, providing results comparable to expensive and sophisticated large-scale testing methods that require more time and skilled personnel. The application of µPAD techniques is extensive in environmental quality control/analysis, clinical diagnosis, and food safety testing, paving the way for on-site real-time diagnosis as a promising future development. This review focuses on the recent research advancements in the design, fabrication, material selection, and detection methods of µPADs. It provides a comprehensive understanding of their principles of operation, applications, and future development prospects.

Spatially Decoupled H2O2 Activation Pathways and Multi-Enzyme Activities in Rod-Shaped CeO2 with Implications for Facet Distribution.

CeO2, particularly in the shape of rod, has recently gained considerable attention for its ability to mimic peroxidase (POD) and haloperoxidase (HPO). However, this multi-enzyme activities unavoidably compete for H2O2 affecting its performance in relevant applications. The lack of consensus on facet distribution in rod-shaped CeO2 further complicates the establishment of structure-activity correlations, presenting challenges for progress in the field. In this study, the HPO-like activity of rod-shaped CeO2 is successfully enhanced while maintaining its POD-like activity through a facile post-calcination method. By studying the spatial distribution of these two activities and their exclusive H2O2 activation pathways on CeO2 surfaces, this study finds that the increased HPO-like activity originated from the newly exposed (111) surface at the tip of the shortened rods after calcination, while the unchanged POD-like activity is attributed to the retained (110) surface in their lateral area. These findings not only address facet distribution discrepancies commonly reported in the literature for rod-shaped CeO2 but also offer a simple approach to enhance its antibacterial performance. This work is expected to provide atomic insights into catalytic correlations and guide the design of nanozymes with improved activity and reaction specificity.

Reduction of Li+ within a borate anion.

Group 1 elements exhibit the lowest electronegativity values in the Periodic Table. The chemical reduction of Group 1 metal cations M+ to M(0) is extremely challenging. Common tetraaryl borates demonstrate limited redox properties and are prone to decomposition upon oxidation. In this study, by employing simple yet versatile bipyridines as ligands, we synthesized a series of redox-active borate anions characterized by NMR and X-ray single-crystal diffraction. Notably, the borate anion can realize the reduction of Li+, generating elemental lithium metal and boron radical, thereby demonstrating its potent reducing ability. Furthermore, it can serve as a powerful two-electron-reducing reagent and be readily applied in various reductive homo-coupling reactions and Birch reduction of acridine. Additionally, this borate anion demonstrates its catalytic ability in the selective two-electron reduction of CO2 into CO.

Applying thallium isotopic compositions as novel and sensitive proxy for Tl(I)/Tl(III) transformation and source apportionment.

Thallium is a rare metal known for its highly toxic nature. Recent research has indicated that the precise determination of Tl isotopic compositions using Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP MS) provides new opportunities for understanding Tl geochemical behavior. While isotopic fractionation of Tl derived from anthropogenic activities (e.g., mining, smelting) have been reported, there is limited information regarding Tl influenced by both natural weathering processes and anthropogenic origins. Herein, we investigated, for the first time, the Tl isotopic compositions in soils across a representative Tl-rich depth profile from the Lanmuchang (LMC) quicksilver mine (southwest China) in the low-temperature metallogenesis zone. The results showed significant variations in Tl isotope signatures (ε205Tl) among different soil layers, ranging from -0.23 to 3.79, with heavier isotope-205Tl enrichment observed in the bottom layers of the profile (ε205Tl = 2.18-3.79). This enrichment of 205Tl was not solely correlated with the degree of soil weathering but was also partially associated with oxidation of Tl(I) by Fe (hydr)oxide minerals. Quantitative calculation using ε205Tl vs. 1/Tl data further indicated that the Tl enrichment across the soil depth profile was predominantly derived from anthropogenic origins. All these findings highlight that the robustness and reliability of Tl isotopes as a proxy for identifying both anthropogenic and geogenic sources, as well as tracing chemical alterations and redox-controlled mineralogical processes of Tl in soils. The nascent application of Tl isotopes herein not only offers valuable insights into the behavior of Tl in surface environments, but also establishes a framework for source apportionment in soils under similar circumstances.

Thermally Induced Persistent Covalent-Organic Frameworks Radicals.

Persistent covalent-organic framework (COF) radicals hold important applications in magnetics and spintronics; however, their facile synthesis remains a daunting challenge. Here, three p-phenylenediacetonitrile-based COFs (named CityU-4, CityU-5, and CityU-6) were synthesized. Upon heat treatment (250 °C for CityU-4 and CityU-5 or 220 °C for CityU-6), these frameworks were brought into their persistent radical forms (no obvious changes after at least one year), together with several observable factors, including color changes, red-shifted absorption, the appearance of electron spin resonance (ESR) signals, and detectable magnetic susceptibility. The theoretical simulation suggests that after heat treatment, lower total energy and nonzero spin density are two main factors to guarantee persistent COFs radicals and polarized spin distributions. This work provides an efficient method for the preparation of persistent COF radicals with promising potentials.

Regulating the H2O2 Activation Pathway on a Well-Defined CeO2 Nanozyme Allows the Entire Steering of Its Specificity between Associated Enzymatic Reactions.

Nanozymes are promising alternatives to natural enzymes, but their use remains limited owing to poor specificity. For example, CeO2 activates H2O2 and displays peroxidase (POD)-like, catalase (CAT)-like, and haloperoxidase (HPO)-like activities. Since they unavoidably compete for H2O2, affecting its utilization in the target application, the precise manipulation of reaction specificity is thus imperative. Herein, we showed that one can simply achieve this by manipulating the H2O2 activation pathway on pristine CeO2 in well-defined shapes. This is because the coordination and electronic structures of Ce sites vary with CeO2 surfaces, wherein the (100) and (111) surfaces display nearly 100% specificity toward POD-/CAT-like and HPO-like activities, respectively. The antibacterial results suggest that the latter surface can well-utilize H2O2 to kill bacteria (cf., the former), which is promising for anti-biofouling applications. This work provides atomic insights into the synthesis of nanozymes with improved activity, reaction specificity, and H2O2 utilization.

Removing Perfluoro Pollutants PFOA and PFOS by Two-Pronged Design of a Ni8-Pyrazolate Porous Framework.

Inspired by the practical need to remove persistent perfluoro pollutants from the environment, we leverage cutting-edge crystal engineering approaches. For this, we set our eyes on a recent porous coordination framework system based on the Ni8-oxo cluster and pyrazolate linkers as it is known for its stability to bases and other harsh environmental conditions. Our designer linker molecule here features (1) pyrazole donors masked by t-butyloxycarbonyl and (2) ethynyl side units protected by triisopropylsilyl groups. The former solvothermally demasks to assemble the Ni8-pyrazolate framework, in which the triisopropylsilyl groups can be post-synthetically cleaved by guest fluoride ions to unveil the terminal alkyne group (-CCH). The ethynyl groups of the framework solid offer versatile reactions for functionalization, as with perfluorophenyl azide (via a click reaction) to afford the two prongs of the 1,2,3-triazole base unit and the perfluoro unit. Together, these two functions make for an effective adsorbent for the topical acid pollutants of perfluorooctanoic acid and perfluorooctanesulfonic acid, with a high apparent rate constant (kobs) of 0.99 g mg-1 h-1 and large maximum uptake capacity (qmax) of 268.5 mg g-1 for perfluorooctanoic acid and kobs of 0.77 g mg-1 h-1 and qmax of 142.1 mg g-1 for perfluorooctanesulfonic acid.

Salt-Induced High-Density Vacancy-Rich 2D MoS2 for Efficient Hydrogen Evolution.

Emerging non-noble metal 2D catalysts, such as molybdenum disulfide (MoS2 ), hold great promise in hydrogen evolution reactions. The sulfur vacancy is recognized as a key defect type that can activate the inert basal plane to improve the catalytic performance. Unfortunately, the method of introducing sulfur vacancies is limited and requires costly post-treatment processes. Here, a novel salt-assisted chemical vapor deposition (CVD) method is demonstrated for synthesizing ultrahigh-density vacancy-rich 2H-MoS2 , with a controllable sulfur vacancy density of up to 3.35 × 1014  cm-2 . This approach involves a pre-sprayed potassium chloridepromoter on the growth substrate. The generation of such defects is closely related to ion adsorption in the growth process, the unstable MoS2 -K-H2 O triggers the formation of sulfur vacancies during the subsequent transfer process, and it is more controllable and nondestructive when compared to traditional post-treatment methods. The vacancy-rich monolayer MoS2 exhibits exceptional catalytic activity based on the microcell measurements, with an overpotential of ≈158.8 mV (100 mA cm-2 ) and a Tafel slope of 54.3 mV dec-1 in 0.5 m H2 SO4 electrolyte. These results indicate a promising opportunity for modulating sulfur vacancy defects in MoS2 using salt-assisted CVD growth. This approach represents a significant leap toward achieving better control over the catalytic performances of 2D materials.

Next-generation diagnostic test for dengue virus detection using an ultrafast plasmonic colorimetric RT-PCR strategy.

The current global COVID-19 pandemic once again highlighted the urgent need for a simple, cost-effective, and sensitive diagnostic platform that can be rapidly developed for distribution and easy access in resource-limited areas. Here, we present a simple and low-cost plasmonic photothermal (PPT)-reverse transcription-colorimetric polymerase chain reaction (RTcPCR) for molecular diagnosis of dengue virus (DENV) infection. The assay can be completed within 54 min with an estimated detection limit of 1.6 copies/µL of viral nucleic acid. The analytical sensitivity and specificity of PPT-RTcPCR were comparable to that of the reference RT-qPCR assay. Moreover, the clinical performance of PPT-RTcPCR was evaluated and validated using 158 plasma samples collected from patients suspected of dengue infection. The results showed a diagnostic agreement of 97.5% compared to the reference RT-qPCR and demonstrated a clinical sensitivity and specificity of 97.0% and 100%, respectively. The simplicity and reliability of our PPT-RTcPCR strategy suggest it can provide a foundation for developing a field-deployable diagnostic assay for dengue and other infectious diseases.

An ultrasound-activatable platinum prodrug for sono-sensitized chemotherapy.

Despite the great success achieved by photoactivated chemotherapy, eradicating deep tumors using external sources with high tissue penetration depth remains a challenge. Here, we present cyaninplatin, a paradigm of Pt(IV) anticancer prodrug that can be activated by ultrasound in a precise and spatiotemporally controllable manner. Upon sono-activation, mitochondria-accumulated cyaninplatin exhibits strengthened mitochondrial DNA damage and cell killing efficiency, and the prodrug overcomes drug resistance as a consequence of combined effects from released Pt(II) chemotherapeutics, the depletion of intracellular reductants, and the burst of reactive oxygen species, which gives rise to a therapeutic approach, namely sono-sensitized chemotherapy (SSCT). Guided by high-resolution ultrasound, optical, and photoacoustic imaging modalities, cyaninplatin realizes the overall theranostics of tumors in vivo with superior efficacy and biosafety. This work highlights the practical utility of ultrasound to precisely activate Pt(IV) anticancer prodrugs for the eradication of deep tumor lesions and broadens the biomedical uses of Pt coordination complexes.

A Dye-Assisted Paper-Based Assay to Rapidly Differentiate the Stress of Chlorophenols and Heavy Metals on Enterococcus faecalis and Escherichia coli.

Biological toxicity testing plays an essential role in identifying the possible negative effects induced by substances such as organic pollutants or heavy metals. As an alternative to conventional methods of toxicity detection, paper-based analytical device (PAD) offers advantages in terms of convenience, quick results, environmental friendliness, and cost-effectiveness. However, detecting the toxicity of both organic pollutants and heavy metals is challenging for a PAD. Here, we show the evaluation of biotoxicity testing for chlorophenols (pentachlorophenol, 2,4-dichlorophenol, and 4-chlorophenol) and heavy metals (Cu2+, Zn2+, and Pb2+) by a resazurin-integrated PAD. The results were achieved by observing the colourimetric response of bacteria (Enterococcus faecalis and Escherichia coli) to resazurin reduction on the PAD. The toxicity responses of E. faecalis-PAD and E. coli-PAD to chlorophenols and heavy metals can be read within 10 min and 40 min, respectively. Compared to the traditional growth inhibition experiments for toxicity measuring which takes at least 3 h, the resazurin-integrated PAD can recognize toxicity differences between studied chlorophenols and between studied heavy metals within 40 min.

Single-layer HNb3O8 with strong and nearby Lewis and Brønsted acid sites boosts amide bond hydrolysis for urease mimicking.

Urea pollution is a growing environmental concern, and its removal via catalytic hydrolysis is challenging due to the resonance-stabilized amide bonds. In nature, this reaction is catalyzed by ureases in many soil bacteria. However, the remedy of this problem with natural enzymes is not feasible as they are easily denatured and require high costs for both preparation and storage. Given this, the development of nanomaterials bearing enzyme-like activity (nanozymes) with advantages such as low production cost, simple storage, and pH/thermal stability has attracted much attention over the past decade. As inspired by the mechanism of urease-catalyzed urea hydrolysis, the co-presence of Lewis acid (LA) and Brønsted acid (BA) sites is imperative to proceed with this reaction. Herein, layered HNb3O8 samples with intrinsic BA sites were adopted for investigation. The layer reduction of this material to few-/single layers can expose Nb sites with various LA strengths depending on the degree of NbO6 distortion. Among the catalysts examined, single-layer HNb3O8 bearing strong LA and BA sites displays the best hydrolytic activity towards acetamide and urea. This sample with high thermal stability was found to outperform urease at temperatures higher than 50 °C. The acidity-activity correlation established in this study is believed to guide the future design of industrial catalysts to remediate urea pollution.

Plasmonic Cross-Linking Colorimetric PCR for Simple and Sensitive Nucleic Acid Detection.

Simple, low-cost, and accurate nucleic acid assay platforms hold great promise for point-of-care (POC) pathogen detection, disease surveillance, and control. Plasmonic photothermal polymerase chain reaction (PPT-PCR) is a powerful and efficient nucleic acid amplification technique, but it lacks a simple and convenient analysis method for POC applications. Herein, we propose a novel plasmonic cross-linking colorimetric PCR (PPT-ccPCR) assay by integrating plasmonic magnetic nanoparticle (PMN)-based PPT-PCR with gold nanoparticle (AuNP)-based cross-linking colorimetry. AuNPs form assembled structures with the PMNs in the presence of amplicons and collect in a magnetic field, resulting in color changes to the supernatant. Target DNA with concentrations as low as 5 copies/µL can be visually detected within 40 min. The achieved limit of detection was 1.8 copies/µL based on the absorption signals. This simple and sensitive strategy needs no expensive instrumentation and demonstrates high potential for POC detection while enabling further applications in clinical diagnostics.

Cu-Co Dual-Atom Catalysts Supported on Hierarchical USY Zeolites for an Efficient Cross-Dehydrogenative C(sp2)-N Coupling Reaction.

A cross-coupling reaction via the dehydrogenative route over heterogeneous solid atomic catalysts offers practical solutions toward an economical and sustainable elaboration of simple organic substrates. The current utilization of this technology is, however, hampered by limited molecular definition of many solid catalysts. Here, we report the development of Cu-M dual-atom catalysts (where M = Co, Ni, Cu, and Zn) supported on a hierarchical USY zeolite to mediate efficient dehydrogenative cross-coupling of unprotected phenols with amine partners. Over 80% isolated yields have been attained over Cu-Co-USY, which shows much superior reactivity when compared with our Cu1 and other Cu-M analogues. This amination reaction has hence involved simple and non-forceful reaction condition requirements. The superior reactivity can be attributed to (1) the specifically designed bimetallic Cu-Co active sites within the micropore for "co-adsorption-co-activation" of the reaction substrates and (2) the facile intracrystalline (meso/micropore) diffusion of the heterocyclic organic substrates. This study offers critical insights into the engineering of next-generation solid atomic catalysts with complex reaction steps.

Polynitrosoarene Radical as an Efficient Cathode Material for Lithium-Ion Batteries.

Organic radical batteries (ORBs) with radical-branched polymers as cathode materials represent a valuable alternative to meet the continuously increasing demand on energy storage. However, the low theoretical capacities of current radical-contained compounds strongly hamper their practical applications. To address this issue, a chemically robust polynitrosoarene (tris(4-nitrosophenyl)amine) with a pronounced radical property is rationally designed as an efficient cathode for ORBs. Its unique multi-nitroso structure displays remarkably reversible charge/discharge capability and a superior capacity up to 300 mA h g-1 (93% theoretical capacity) after 100 cycles at 100 mA g-1 within a broad potential window of 1.3-4.3 V (vs Li+/Li). Moreover, the ultra-long cycle life is also achieved at 1000 mA g-1 with 85% preservation of the capacity after 1000 cycles, making it the best-reported organic radical cathode material for lithium-ion batteries.

An Atomic Insight into the Confusion on the Activity of Fe3O4 Nanoparticles as Peroxidase Mimetics and Their Comparison with Horseradish Peroxidase.

Although Fe3O4 nanoparticles were early reported to outperform horseradish peroxidase (HRP), recent studies suggested that this material bears a very poor activity instead. Here, we resolve this disagreement by reviewing the definition of descriptors used and provide an atomic view into the origin of Fe3O4 nanoparticles as peroxidase mimetics. The redox between H2O2 and Fe(II) sites on the Fe3O4 surface was identified as the key step to producing OH radicals for the oxidation of colorimetric substrates. This mechanism involving free radicals is distinct from that of HRP oxidizing substrates with a radical retained on its Fe-porphyrin ring. Surprisingly, the distribution and chemical state of Fe species were found to be very different on single- and polycrystalline Fe3O4 nanoparticles with the latter bearing not only a higher Fe(II)/Fe(III) ratio but also a more reactive Fe(II) species at surface grain boundaries. This accounts for the unexpected gap in the catalytic constant (kcat) observed for this material in the literature.

Facet-Dependent Activity of CeO2 Nanozymes Regulate the Fate of Human Neural Progenitor Cell via Redox Homeostasis.

Neural progenitor cells (NPCs) therapy, a promising therapeutic strategy for neurodegenerative diseases, has a huge challenge to ensure high survival rate and neuronal differentiation rate. Cerium oxide (CeO2) nanoparticles exhibit multienzyme mimetic activities and have shown the capability of regulating reactive oxygen species (ROS), which is a pivotal mediator for intracellular redox homeostasis in NPCs, regulating biological processes including differentiation, proliferation, and apoptosis. In the present study, the role of facet-dependent CeO2-mediated redox homeostasis in regulating self-renewal and differentiation of NPCs is reported for the first time. The cube-, rod-, and octahedron-shaped CeO2 nanozymes with different facets are prepared. Among the mentioned nanozymes, the cube enclosed by the (100) facet exhibits the highest CAT-like activity, causing it to provide superior protection to NPCs from oxidative stress induced by H2O2; meanwhile, the octahedron enclosed by the (111) facet with the lowest CAT-like activity induces the most ROS production in ReNcell CX cells, which promotes neuronal differentiation by activated AKT/GSK-3ß/ß-catenin pathways. A further mechanistic study indicated that the electron density of the surface Ce atoms changed continuously with different crystal facets, which led to their different CAT-like activity and modulation of redox homeostasis in NPCs. Altogether, the different surface chemistry and atomic architecture of active sites on CeO2 exert modulation of redox homeostasis and the fate of NPCs.

Cluster Nanozymes with Optimized Reactivity and Utilization of Active Sites for Effective Peroxidase (and Oxidase) Mimicking.

Single-atom catalysts have attracted attention in the past decade since they maximize the utilization of active sites and facilitate the understanding of product distribution in some catalytic reactions. Recently, this idea has been extended to single-atom nanozymes (SAzymes) for the mimicking of natural enzymes such as horseradish peroxidase (HRP) often used in bioanalytical applications. Herein, it is demonstrated that those SAzymes without constructing the reaction pocket of HRP still undergo the OH radical-mediated pathway like most of the reported nanozymes. Their positively charged single-atom centers resulting from support electronegative oxygen/nitrogen hinder the reductive conversion of H2 O2 to OH radicals and hence display low activity per site. In contrast, it is found that this step can be facilitated over their metallic counterparts on cluster nanozymes with much higher site activity and atom efficiency (cf. SAzymes with 100% atom utilization). Besides the mimicking of HRP in glucose detection, cluster nanozymes are also demonstrated as a better oxidase mimetic for glutathione detection.

Disclosing the Origin of Transition Metal Oxides as Peroxidase (and Catalase) Mimetics.

Since Fe3O4 was reported to mimic horseradish peroxidase (HRP) with comparable activity (2007), countless peroxidase nanozymes have been developed for a wide range of applications from biological detection assays to disease diagnosis and biomedicine development. However, researchers have recently argued that Fe3O4 has no peroxidase activity because surface Fe(III) cannot oxidize tetramethylbenzidine (TMB) in the absence of H2O2 (cf. HRP). This motivated us to investigate the origin of transition metal oxides as peroxidase mimetics. The redox between their surface Mn+ (oxidation) and H2O2 (reduction) was found to be the key step generating OH radicals, which oxidize not only TMB for color change but other H2O2 to produce HO2 radicals for Mn+ regeneration. This mechanism involving free OH and HO2 radicals is distinct from that of HRP with a radical retained on the Fe-porphyrin ring. Most importantly, it also explains the origin of their catalase-like activity (i.e., the decomposition of H2O2 into H2O and O2). Because the production of OH radicals is the rate-limiting step, the poor activity of Fe3O4 can be attributed to the slow redox of Fe(II) with H2O2, which is two orders of magnitude slower than the most active Cu(I) among common transition metal oxides. We further tested glutathione (GSH) detection on the basis of its peroxidase-like activity to highlight the importance of understanding the mechanism when selecting materials with high performance.