Natural oxidase-mimicking copper-organic frameworks for targeted identification of ascorbate in sensitive sweat sensing

Ultrasensitive detection of chloramphenicol using a performance-enhanced Fe/Zn-MIL-88B driven dual-mode biosensing platform

Chloramphenicol (CAP) used in aquaculture poses a great threat to human health. In this study, a novel biosensing platform was proposed by Fe/Zn-MIL-88B with performance-enhanced for fluorescent/colorimetric dual-mode detection of CAP. Zinc was incorporated into the metal-organic framework to modulate its internal electronic structure, resulting in the formation of Fe/Zn-MIL-88B. This modification notably boosted its peroxidase-like activity, enhancing the colorimetric sensing capabilities. Simultaneously, Fe/Zn-MIL-88B exhibited excellent fluorescence quenching properties, when combined with upconversion nanomaterials and aptamers to enable a fluorescent sensing system. The composite materials were integrated into a three-dimensional folded microfluidic analyzer created a biosensing platform for ultrasensitive sensing of CAP. The results demonstrated that the dynamic response of the biosensing platform for CAP ranged from 0.5 to 1000 ng/mL, with the detection limits of 0.064 ng/mL (fluorescence) and 0.095 ng/mL (colorimetry), respectively. The work offers valuable insights into the further development of advanced biosensing platforms.

Electrochemistry sensing of ascorbic acid based on conductive metal-organic framework (Cu3(benzenehexathiol)) nanosheets modified electrode

BACKGROUND

Bulk-type conductive metal-organic frameworks (c-MOFs) had been applied to modify the electrode and used in electrochemical sensing because of the high conductive properties. But which still suffer from low mass permeability, restricted active site exposure, and poor accessibility due to the coordination saturation at metal sites in the bulk-type c-MOF. Recent studies have demonstrated that transforming bulk-type MOFs into MOFs nanosheets (NSs) can maximize the exposure of active sites and mass transfer. However, c-MOF NSs have rarely been applied in electrochemical sensing.

RESULTS

This study presents NSs type c-MOF Cu3(benzenehexathiol) (CuBHT), synthesized using a simple sacrificial template method. CuBHT NSs were modified onto a glassy carbon electrode (GCE) to prepare CuBHT NSs/GCE, which was then applied to sense the model target ascorbic acid (AA), the system exhibits high sensitivity of 1.521 mA mM-1 cm-2 and a wide linear range of 1-789 μM, low detection limit of 0.46 μM. The sensitivity is 1.90 times higher than that of bulk-type CuBHT nanoparticles (NPs) modified GCE, which can be attributed to the CuBHT NSs having more exposed Cu sites on their surfaces. CuBHT NSs/GCE was then used to monitor AA levels in human sweat during daily activities or exercise, and the results indicated high reliability compared to the vitamin C ASA kit method.

SIGNIFICANCE

The design of c-MOF CuBHT NSs/GCE lead to better performance in terms of sensitivity and low detection limit in AA sensing compared to bulk-type nanoparticles. The AA sensing mechanism based on CuBHT was investigated, and the sensing system was demonstrated by detecting AA in sweat. This work advances both the fundamental understanding and practical applications of c-MOF NSs in AA sensing.

Smartphone-assisted fluorescent film based on the Flu grafted on Eu-MOF for real-time monitoring of fresh-cut fruit freshness

Indicator migration within intelligent packaging systems can compromise the safety of the food matrix and the accuracy of coloration, particularly in high humidity packaging. Herein, Eu-BDC-NH2-Fluorescein (MOF-Flu) nanofillers were synthesized by the amide coupling, followed by an analysis of their structural, morphological characteristics, and optical response. The MOF-Flu and microcrystalline cellulose (MCC) nanomaterials were embedded into a sodium carboxymethyl cellulose (CMC-Na) substrate to fabricate enhanced packaging films. Several beneficial properties, including superior hydrophobicity and water resistance, improved mechanical properties, and enhanced thermal stability, were observed for CMC-Na/MOF-Flu compared to CMC-Na/MCC. Additionally, the MOF-Flu composite film exhibited improved UV-visible barrier properties, exceptional resistance to pigment migration, and good time-temperature stability. Finally, a significant linear correlation (R2 = 0.9938, LOD: 1.79-2.90 N/cm2, RSD: 2.16%, recovery: 103.93%) was established using a smartphone application to display the relationship between the SRGB-values of MOFs-based films and hardness of fresh-cut Narcissus mangoes at 4 °C. The digital sensing platform utilizing smartphones has pioneered a powerful approach for the on-site rapid quantitative assessment of fresh-cut fruit freshness, significantly enhancing the precision and convenience of intelligent packaging. Furthermore, the developed indicator material MOF-Flu can accurately and non-destructively monitor changes of fruit freshness, alleviating concerns regarding dye contamination during migration.

Nanocellulose Hybrid Membranes for Green Flexible Electronics: Interface Design and Functional Assemblies

Flexible electronics have garnered significant attention in recent years. The emergence of membrane electronics addresses several limitations of rigid counterparts, such as high Young's modulus, poor biocompatibility, and poor responsiveness. Nevertheless, the development of traditional polymer and semiconductor membranes faces serious limitations. Nanocellulose (NC), known for its multifunctionality, biocompatibility, biodegradability, high mechanical strength, structural flexibility, and reinforcing capabilities, presents an excellent possibility to develop flexible electronics depending on the self-assembly behavior. Meanwhile, the combination of NC and functional fillers enables the fabrication of high-performance membranes with amplification capabilities, making them suitable for application in conductive materials for sensing and energy storage applications. The creation includes preparation strategies and potential applications. Moreover, the interface reaction mechanism and micro/nano scale morphology structure of carbon-based materials, polymers, and metal oxides combined with NC hybrid membranes are summarized from a molecular perspective. We discuss the design strategies and performance trends for improving mechanical properties, thermal conductivity, heat resistance, optical performance, and electrical conductivity of NC hybrid membranes. The recent advancements in nanocellulose for flexible sensors, thermal management, supercapacitors, and solar cells are evaluated along with perspectives on the current challenges and future directions in the development of NC membrane-based multifunctional flexible electronics. It will help improve the development of green flexible electronics, thereby advancing future investigations of this field.

Amino Acid-Regulated Biomimic Fe-MOF Nanozyme with Enhanced Activity and Specificity for Colorimetric Sensing of Uranyl Ions in Seawater

Nanozymes are attracting widespread attention as effective alternatives to overcome the limitations of natural enzymes. However, their catalytic performance is unsatisfactory due to the low catalytic activity and specificity. In this work, an efficient metal-organic framework (MOF) nanozyme mimicking the active centers of natural enzymes has been developed and its catalysis mechanism has been thoroughly investigated. The partial histidine- and arginine-doped Fe-MOF (HA Fe-MOF) is demonstrated to activate structure reconstruction with abundant oxygen vacancy generation, which promotes the binding capacity of HA Fe-MOF. The Fe sites in HA Fe-MOF act as catalytic sites for decomposition of H2O2. Intriguingly, histidine and arginine in the HA Fe-MOF can form hydrogen bonds with H2O2 as observed in natural enzymes, constituting a unique microenvironment that increases the local concentration of H2O2. Benefiting from the establishment of such enzyme-mimicking active centers, HA Fe-MOF exhibits high peroxidase-like specificity and activity. In addition, HA Fe-MOF holds great potential for detecting uranyl ions with a limit of detection as low as 0.012 μM, surpassing most reported nanozymes. This work achieves the rational design of highly specific peroxidase-like nanozymes by mimicking the structure-selectivity relationship of natural peroxidases, which provides new insights into the design of nanozymes with advanced configurations.

Self-Powered Glucose Biosensor Based on Non-Enzymatic Biofuel Cells by Au Nanocluster/Pd Nanocube Heterostructure and Fe3C@C-Fe Single-Atom Catalyst

Self-powered biosensors (SPBs) based on biofuel cells (BFCs) use electrical output as a sensing signal without the need of external power supplies, providing a feasible approach to constructing miniaturized implantable or portable devices. In this work, a novel nanozyme of gold nanoclusters/palladium nanocubes (AuNCs/PdNCs) heterostructure is successfully fabricated to develop an innovatively self-powered and non-enzymatic glucose sensing system. The AuNCs/PdNCs with glucose oxidase (GOD)-like activity exhibits superior electrocatalytic and non-enzymatic sensing performance toward glucose. The non-enzymatic BFCs-based SPBs system, established on the AuNCs/PdNCs (anodic catalyst) and single atomic Fe sites coupled with carbon-encapsulated Fe3C crystals (Fe3C@C-Fe SACs as a cathodic catalyst) platform, exhibits an exceptional sensitivity to glucose with 0.151 µW cm-2 mm-1 (3.4 times higher than the PdNCs), outstanding selectivity and robust stability. The outstanding performance of the BFCs-based SPBs system can be attributed to the synergistic cooperation between the PdNCs and AuNCs.

Substrate Specificity of Adenine-Cu-PO4 Nanozyme: Ascorbic Acid Oxidation and Selective Cytotoxicity

Though nanozymes are becoming promising alternatives to natural enzymes due to their superior properties, constructing nanozyme with high specificity is still a great challenge. Herein, with Cu2+ as an active site and adenine as a ligand, Adenine-Cu-PO4 is synthesized in phosphate-buffered saline. As an oxidase mimic, Adenine-Cu-PO4 could selectively catalyze oxidation of ascorbic acid (AA) to dehydroascorbic acid, but not universal substrates (3,3',5,5'-tetramethylbenzidine (TMB), 2,2'-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) and 2,4-dichlorophenol (2,4-DP)), small biomolecules (dopamine, glutathione, glucose, galactose), other vitamins (vitamin A acid, vitamin B1, vitamin K1) and even dithiothreitol (a common interference of AA). Such the specific AA catalytic oxidation is revealed that Adenine-Cu-PO4 selectively binds with AA through hydrogen bonds, accompanied with catalyzing AA oxidation, and concurrently O2 transferring to H2O2 via O2⋅-, further to H2O via ⋅OH. Based on the produced reactive oxygen species, with AA as a pro-oxidant, Adenine-Cu-PO4 nanozyme efficiently triggers severe intratumor oxidative stress to induce tumor cell death. This work opens a new avenue to design intrinsic nanozymes with high specificity, and also presents a promising application in the field of AA oxidation induced cancer therapy.

Nanozyme-based wearable biosensors for application in healthcare

Recent years have witnessed tremendous advances in wearable sensors, which play an essential role in personalized healthcare for their ability for real-time sensing and detection of human health information. Nanozymes, capable of mimicking the functions of natural enzymes and addressing their limitations, possess unique advantages such as structural stability, low cost, and ease of mass production, making them particularly beneficial for constructing recognition units in wearable biosensors. In this review, we aim to delineate the latest advancements in nanozymes for the development of wearable biosensors, focusing on key developments in nanozyme immobilization strategies, detection technologies, and biomedical applications. The review also highlights the current challenges and future perspectives. Ultimately, it aims to provide insights for future research endeavors in this rapidly evolving area.

Machine Learning Assisted-Intelligent Lactic Acid Monitoring in Sweat Supported by a Perspiration-Driven Self-Powered Sensor

Lactic acid has aroused increasing attention due to its close association with serious diseases. A real-time, dynamic, and intelligent detection method is vital for sensitive detection of lactic acid. Here, a machine learning (ML)-assisted perspiration-driven self-powered sensor (PDS sensor) is fabricated using Ni-ZIF-8@lactate oxidase and pyruvate oxidase (Ni-ZIF-8@LOx&POx)/laser-induced graphene (LIG), bilirubin oxidase (BOD)/LIG, and a microchannel for highly sensitive and real-time monitoring of lactic acid in sweat. Driven by the oxidation reaction of lactic acid, PDS sensors exhibit excellent sensitivity, a wide detection range, good reproducibility, and excellent selectivity for lactic acid detection in sweat. When subjects with different body mass index (BMI) undergo aerobic or anaerobic exercise or maintain a sedentary state, PDS sensors can monitor lactic acid in sweat wirelessly and in real-time. Moreover, a ML algorithm was employed to assist PDS sensors to detect lactic acid in the subjects' sweat with a high prediction accuracy of 96.0%.

Facet engineering of Cu2O for efficient electrochemical glucose sensing

BACKGROUND

Accurate monitoring glucose level is significant for human health management, especially in the prevention, diagnosis, and management of diabetes. Electrochemical quantification of glucose is a convenient and rapid detection method, and the crucial aspect in achieving great sensing performance lies in the selection and design of the electrode material. Among them, Cu2O, with highly catalysis ability, is commonly used as electrocatalyst in non-enzymatic glucose sensing. This feature has attracted great attentions for improving sensing performances by increasing exposed surface area and modifying with other active materials, but the intrinsic electrochemical activity of Cu2O is often diminished.

RESULT

In this work, we synthesized Cu2O cubes with exposed (100) faces, Cu2O octahedron with exposed (111) faces and Cu2O rhombic dodecahedron with exposed (110) faces of similar sizes to investigate the influence of expose facets on its glucose sensing performance. By employing the facets-regulation strategy, the anti-interference performance was largely improved by the Cu2O octahedron when compared with the Cu2O cube and the Cu2O rhombic dodecahedron, while the sensitivity and stability were also improved. Eventually, the Cu2O octahedron with exposed (111) faces based on glucose sensor displayed great practicability in human serum and the relative deviation was less than 3 % when compared with biochemical analyzer. Experimental, calculation and simulation result elucidate that Cu2O octahedron with exposed (111) faces, possessing stronger intrinsic electrochemical activity, comparatively favors the glucose adsorption, electron transfer and current density enhancement, rather than more active sites.

SIGNIFICANCE AND NOVELTY

This work focuses on the exploration of the intrinsic electrochemical activity of Cu2O and confirms that a facets-regulation strategy is an effective approach for boosting performance of non-enzymatic electrochemical sensors, especially for their sensitivity and selectivity. Moreover, it will lay the foundation for advancing the key field of precision medicine.

Synergistic color-changing and conductive photonic cellulose nanocrystal patches for sweat sensing with biodegradability and biocompatibility

Given the ongoing requirements for versatility, sustainability, and biocompatibility in wearable applications, cellulose nanocrystal (CNC) photonic materials emerge as excellent candidates for multi-responsive wearable devices due to their tunable structural color, strong electron-donating capacity, and renewable nature. Nonetheless, most CNC-derived materials struggle to incorporate color-changing and electrical sensing into one system since the self-assembly of CNCs is incompatible with conventional conductive mediums. Here we report the design of a conductive photonic patch through constructing a CNC/polyvinyl alcohol hydrogel modulated by phytic acid (PA). The introduction of PA significantly enhances the hydrogen bonding interaction, resulting in the composite film with impressive flexibility (1.4 MJ m-3) and progressive color changes from blue, green, yellow, to ultimately red upon sweat wetting. Interestingly, this system simultaneously demonstrates selective and sensitive electrical sensing functions, as well as satisfactory biocompatibility, biodegradability, and breathability. Importantly, a proof-of-concept demonstration of a skin-adhesive patch is presented, where the optical and electrical dual-signal sweat sensing allows for intuitive visual and multimode electric localization of sweat accumulation during physical exercises. This innovative interactive strategy for monitoring human metabolites could offer a fresh perspective into the design of wearable health-sensing devices, while greatly expanding the applications of CNC-based photonic materials in medicine-related fields.

LDHzyme-assisted high-performance on-site tracking of levodopa pharmacokinetics for Parkinson's disease management

Parkinson's disease (PD) is a progressive neurodegenerative disorder marked by the loss of dopaminergic neurons and the consequent decline in motor and cognitive functions. The primary therapeutic agent levodopa necessitates precise dosing due to its narrow therapeutic window and complex pharmacokinetics. This study presents the development of a novel CuCoFe-LDHzyme-based sweat sensor for real-time monitoring of levodopa concentration in PD patients. Employing differential pulse voltammetry (DPV) technique, the sensor demonstrates high sensitivity and selectivity, achieving a detection limit of 28.1 nM. The sensor's design allows for non-invasive, continuous monitoring, significantly enhancing patient convenience compared to traditional blood sampling methods. Through pH correction, precise quantification of levodopa in sweat is accomplished, and a strong correlation (Pearson coefficient = 0.833) with blood levodopa levels is established. The pharmacokinetic profile of levodopa is reconstructed in real-time, offering a promising tool for optimizing PD treatment regimens. This study highlights the potential of CuCoFe-LDHzyme sensors to advance personalized treatment strategies, aiming to improve the quality of life for PD patients by providing clinicians with real-time data for medication adjustments.

A skin-mountable flexible biosensor based on Cu-MOF/PEDOT composites for sweat ascorbic acid monitoring

Continuous monitoring of sweat nutrients offers valuable insights into metabolic cycling and health levels. However, existing methods often lack adaptability and real-time capabilities. Here, we propose a skin-mountable flexible biosensor integrated with metal-organic framework (MOF)-derived composites for real-time monitoring of sweat ascorbic acid (AA) levels. The biosensor features a miniaturized, highly integrated system capable of an imperceptible, stretchable skin patch with dimensions of 16.9 × 9.9 × 0.1 mm3, ensuring conformal integration with curvilinear skin contours. The introduction of a copper-based MOF anchored with poly(3,4-ethylenedioxythiophene) (Cu-MOF/PEDOT) significantly enhances sensing performance toward AA, achieving a detection limit of 0.76 μM and a sensitivity of 725.7 μA/(mM·cm2). Moreover, a miniaturized flexible circuit enables wireless communication, resulting in a lightweight, wearable platform weighing only 1.3 g. Structural and electrochemical analyses confirm the favorable sensitivity, reversibility, and stability of the biosensor, while in-vivo validation in human subjects further reveals the capability to track sweat AA variations during nutrient intake and sustained exercise, showcasing its potential in metabolic cycle assessment and health management. The biosensor presents a promising avenue for scalable health monitoring using adaptable and user-friendly technologies.

Peroxynitrite-Responsive Near-Infrared Fluorescent Imaging Guided Synergistic Chemo-Photodynamic Therapy via Biomimetic Metal-Organic Frameworks

Peroxynitrite (ONOO-) plays a crucial role in maintaining cellular redox homeostasis and regulating diffusive processes, cellular transport, and signal transduction. Extensive studies have revealed that increased ONOO- levels during tumor progression are associated with heightened levels of oxidative stress. However, current methods lack noninvasive visualization, immediate reporting, and highly sensitive fluorescence sensing. In light of this, we have designed a biomimetic fluorescent nanoplatform, named Z-C-T@CM, for peroxynitrite-responsive near-infrared fluorescent imaging guided cancer treatment. The nanoplatform comprises tetrakis(4-carboxyphenyl) porphyrin (TCPP) and curcumin (CCM) encapsulated within a zeolitic imidazolate framework-8 (ZIF-8), which is coated with a mouse breast cancer cell membrane for enhanced biocompatibility and targeting, while evading immune clearance. In vitro experimental results demonstrate that the as-prepared nanoplatform exhibits enhanced near-infrared fluorescence emission upon exposure to ONOO-, indicating a significant potential for noninvasive in vivo imaging of ONOO- during tumor progression. Additionally, Z-C-T@CM readily degrades in the tumor microenvironment, releasing TCPP and CCM, enabling a synergistic chemo-photodynamic therapy with near-infrared illumination. Further investigations indicate that Z-C-T@CM efficiently stimulates a tumor immune response and facilitates therapeutic efficiency. Collectively, this work introduces a novel noninvasive strategy for ONOO- detection, shedding new light on the integration of cancer diagnosis and efficient treatment.

Recent Advances in Nanozyme Sensors Based on Metal-Organic Frameworks and Covalent-Organic Frameworks

Nanozymes are nanomaterials that exhibit enzyme-like catalytic activity, which have drawn increasing attention on account of their unique superiorities including very high robustness, low cost, and ease of modification. Metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) have emerged as promising candidates for nanozymes due to their abundant catalytic activity centers, inherent porosity, and tunable chemical functionalities. In this review, we first compare the enzyme-mimicking activity centers and catalytic mechanisms between MOF and COF nanozymes, and then summarize the recent research on designing and modifying MOF and COF nanozymes with inherent catalytic activity. Moreover, typical examples of sensing applications based on these nanozymes are presented, as well as the translation of enzyme catalytic activity into a visible signal response. At last, a discussion of current challenges is presented, followed by some future prospects to provide guidance for designing nanozyme sensors based on MOFs and COFs for practical applications.

Construction of Stable 2D Cationic Breathing Ni-MOF for Cr(VI) Trapping and Electrochemical Sensing

Pollution of surface water by heavy metal hexavalent chromium ions poses a serious threat to human health; herein, a two-dimensional (2D) cationic breathing Ni-MOF with free nitrate ions between the layers was designed and synthesized according to the characteristics of hexavalent chromium ions, {[Ni(L)2](NO3)2·5H2O}n (L = 1,3,5-tris[4-(imidazol-1-yl)phenyl]benzene). The flexible layer spacing of the 2D breathing Ni-MOF allows the exchange of NO3- by CrO42- without destroying the original structure. Electrostatic and hydrogen bonding interactions between CrO42- and Ni-MOF facilitate its exchange with NO3-. Moreover, CrO42- exhibits a higher binding energy with Ni-MOF compared to NO3-, and the hydrophobic channels of Ni-MOF favor CrO42- trapping due to its lower hydration energy. Consequently, Ni-MOF demonstrates both effective sorption and electrochemical sensing of Cr(VI), achieving a sensitivity of 2.091 μA μM-1 and a detection limit of 0.07 μM.

Freeze-Cast MIL-53(Al) Porous Materials with High Thermal Insulation and Flame Retardant Properties

The development of materials with superior thermal insulation and flame retardancy is critical for industrial applications and daily life. Metal-organic framework (MOF)@poly(vinyl alcohol) (PVA) (MOF@PVA) aerogel composites have demonstrated remarkable thermal insulation and flame retardancy properties. In this work, MIL-53(Al) was directly mixed with PVA and formed by freeze-drying, and the influence of the pore structure on the thermal insulation and flame retardancy properties of the materials was investigated. The incorporated MIL-53(Al) nanoparticles introduced abundant micro- and mesopores, enhancing the complexity of the pore structure and improving the thermal insulation and flame retardancy properties of the aerogels. The directionally freeze-cast aerogel achieved a thermal conductivity of 0.040 W·mK-1, and maintained excellent thermal insulation ability even at 220 °C. Furthermore, the aerogel exhibited nonflammable and self-extinguishing characteristics. This environmentally friendly manufacturing method provides new ideas for the design of MOF-based composites, thereby expanding their application areas.

Rise of Metal-Organic Frameworks: From Synthesis to E-Skin and Artificial Intelligence

Metal-organic frameworks (MOFs) have attained broad research attention in the areas of sensors, resistive memories, and optoelectronic synapses on the merits of their intriguing physical and chemical properties. In this review, recent progress on the synthesis of MOFs and their electronic applications is introduced and discussed. Initially, the crystal structures and properties of MOFs encompassing optical, electrical, and chemical properties are discussed in brief. Subsequently, advanced synthesis methods for MOFs are introduced, categorized into hydrothermal approach, microwave synthesis, mechanochemical synthesis, and electrochemical deposition. After that, the various roles of MOFs in widespread applications, including sensing, information storage, optoelectronic synapses, machine learning, and artificial intelligence, are discussed, highlighting their versatility and the innovative solutions they provide to long-standing challenges. Finally, an outlook on remaining challenges and a future perspective for MOFs are proposed.

Sensing patches for biomarker identification in skin-derived biofluids

In conventional clinical disease diagnosis and screening based on biomarker detection, most analysis samples are collected from serum, blood. However, these invasive collection methods require specific instruments, professionals, and may lead to infection risks. Additionally, the diagnosis process suffers from untimely results. The identification of skin-related biomarkers plays an unprecedented role in early disease diagnosis. More importantly, these skin-mediated approaches for collecting biomarker-containing biofluid samples are noninvasive or minimally invasive, which is more preferable for point-of-care testing (POCT). Therefore, skin-based biomarker detection patches have been promoted, owing to their unique advantages, such as simple fabrication, desirable transdermal properties and no requirements for professional medical staff. Currently, the skin biomarkers extracted from sweat, interstitial fluid (ISF) and wound exudate, are achieved with wearable sweat patches, transdermal MN patches, and wound patches, respectively. In this review, we detail these three types of skin patches in biofluids collection and diseases-related biomarkers identification. Patch classification and the corresponding manufacturing as well as detection strategies are also summarized. The remaining challenges in clinical applications and current issues in accurate detection are discussed for further advancement of this technology (Scheme 1).

Visual Sensor Array for Multiple Aromatic Amines via Specific Ascorbic Acid Oxidase Mimic Triggered Schiff-Base Chemistry

Redox nanozymes have exhibited various applications in recognizing environmental pollutants but not aromatic amines (a type of typical pollutant). Herein, with Cu2+ as a node and tryptophan (Trp) as a linker, Cu-Trp as a specific ascorbic acid oxidase mimic was synthesized, which could catalyze ascorbic acid (AA) oxidation to dehydroascorbic acid (DHAA). Alternatively, with other natural amino acids as linkers to synthesize Cu-based nanozymes, such catalytic performances are also observed. The as-produced DHAA could react with o-phenylenediamine (OPD) and its derivatives (2,3-naphthalene diamine (NDA), 4-nitro-o-phenylenediamine (4-NO2-OPD), 4-fluoro-o-phenylenediamine (4-F-OPD), 4-chloro-o-phenylenediamine(4-Cl-OPD), and 4-bromo-o-phenylenediamine(4-Br-OPD)) to form a Schiff base and emit fluorescence. Based on the results, with Cu-Trp + AA and Cu-Arg (with arginine (Arg) as a linker) + AA as two sensing channels and extracted red, green, and blue (RGB) values from emitted fluorescence as read-out signals, a visual sensor array was constructed to efficiently distinguish OPD, NDA, 4-NO2-OPD, 4-F-OPD, 4-Cl-OPD, and 4-Br-OPD as low as 10 μM. Such detecting performance was further confirmed through discriminating binary, ternary, quinary, and senary mixtures with various concentration ratios, recognizing 18 unknown samples, and even quantitatively analyzing single aromatic amine. Finally, the discriminating ability was further validated in environmental waters, providing an efficient assay for large-scale scanning levels of multiple aromatic amines.

An origami microfluidic paper device based on core-shell Cu@Cu2S@N-doped carbon hollow nanocubes

BACKGROUD

The global prevalence of diabetes mellitus, a serious chronic disease with fatal consequences for millions annually, is of utmost concern. The development of efficient and simple devices for monitoring glucose levels is of utmost significance in managing diabetes. The advancement of nanotechnology has resulted in the indispensable utilization of advanced nanomaterials in high-performance glucose sensors. Modulating the morphology and intricate composition of transition metals represents a viable approach to exploit their structure/function correlation, thereby achieving optimal electrocatalytic performance of the synthesized catalysts.

RESULTS

Herein, a sensitive and rapid Cu-encapsulated Cu2S@nitrogen-doped carbon (Cu@Cu2S@N-C) hollow nanocubes-functionalized microfluidic paper-based analytical device (μ-PAD) was fabricated. Through a delicate sacrificial template/interface technique and thermal decomposition, inter-connected hollow networks were formed to boost the active sites, and the carbon shell was coated to protect Cu from being oxidation. For application, the constructed μ-PAD is used for glucose sensing utilizing an origami automated sample pretreatment system enabled by a simple application of strong alkaline solution on wax paper. Under optimal circumstances, the Cu@Cu2S@N-C electrochemical biosensor exhibits broad detection range of 2-7500 μM (R2 = 0.996) with low detection limit of 0.16 μM (S/N = 3) and high sensitivity of 1996 μA mM-1 cm-2. Additionally, the constructed μ-PAD also exhibited excellent selectivity, stability, and reproducibility.

SIGNIFICANCE

By rationally designing the double-shell hollow nanostructure and introducing Cu-encapsulated inner layer, the synthesized Cu@Cu2S@N-C hollow nanocubes show large specific surface area, short diffusion channels, and high stability. The proposed origami μ-PAD has been successfully applied to serum samples without any additional sample preparation steps for glucose determination, offering a new perspective for early nonenzymatic glucose diagnosis.

Molecule Engineering Metal-Organic Framework-Based Organic Photoelectrochemical Transistor Sensor for Ultrasensitive Bilirubin Detection

The functionalization of metal-organic frameworks (MOFs) with organic small molecules by in situ postsynthetic modification has garnered considerable attention. However, the precise engineering of recognition sites using this method remains rarely explored in optically controlled bioelectronics. Herein, employing the Schiff base reaction to embed the small molecule (THBA) into a Zr-MOF, we fabricated a hydroxyl-rich MOF on the surface of titanium dioxide nanorod arrays (U6H@TiO2 NRs) to develop light-sensitive gate electrodes with tailored recognition capabilities. The U6H@TiO2 NR gate electrodes were integrated into organic photoelectrochemical transistor (OPECT) sensing systems to tailor a sensitive device for bilirubin (I-Bil) detection. In the presence of I-Bil, coordination effects, hydrogen bonding, and π-π interactions facilitated strong binding between U6H@TiO2 NRs and the target I-Bil. The electron-donating property of I-Bil influenced the gate voltage, enabling precise control of the channel status and modulation of the channel current. The OPECT device exhibited exceptional analytical performance toward I-Bil with wide linearity ranging from 1 × 10-16 to 1 × 10-9 M and a low limit detection of 0.022 fM. Leveraging the versatility of small molecules for boosting the functionalization of materials, this work demonstrates the great potential of the small molecule family for OPECT bioanalysis and holds promise for the advancement of OPECT sensors.

Approach of a small protein to the biomimetic bis-(μ-oxo) dicopper active-site installed in MOF-808 pores with restricted access perturbs substrate selectivity of oxidase nanozyme

Advances in nanozymes have taken shape over the past few years in several domains. However, persisting challenging limitations of selectivity, specificity, and efficiency necessitate careful attention to aid in the development of next-generation artificial enzymes. Despite nanozymes having significant therapeutic and biotechnological prospects, the multienzyme mimetic activities can compromise their intended applications. Furthermore, the lack of substrate selectivity can hamper crucial biological pathways. While working on addressing the challenges of nanozymes, in this work, we aim to highlight the interplay between the substrates and bis-(μ-oxo) dicopper active site-installed MOF-808 for selectively mimicking oxidase. This oxidase mimetic with a small pore-aperture (1.4 nm), similar to the opening of enzyme binding pockets, projects a tight control over the dynamics and the reactivity of substrates, making it distinct from the general oxidase nanozymes. Interestingly, the design and the well-regulated activity of this nanozyme effectively thwart DNA from approaching the active site, thereby preventing its oxidative damage. Crucially, we also show that despite these merits, the oxidase selectivity is compromised by small proteins such as cytochrome c (Cyt c), having dimensions larger than the pore aperture of MOF-808. This reaction lucidly produces water molecules as a result of four electron transfer to an oxygen molecule. Such unintended side reactivities warrant special attention as they can perturb redox processes and several cellular energy pathways. Through this study, we provide a close look at designing next-generation artificial enzymes that can address the complex challenges for their utility in advanced applications.

User-Friendly and Responsive Electrochemical Detection Approach for Triclosan by Nano-Metal-Organic Framework

Antimicrobial resistance poses a significant challenge to public health, and is worsened by the widespread misuse of antimicrobial agents such as triclosan (TCS) in personal care and household products. Leveraging the electrochemical reactivity of TCS's phenolic hydroxyl group, this study investigates the electrochemical behavior of TCS on a Cu-based nano-metal-organic framework (Cu-BTC) surface. The synthesis of Cu-BTC via a room temperature solvent method, with triethylamine as a regulator, ensures uniform nanoparticle formation. The electrochemical properties of Cu-BTC and the signal enhancement mechanism are comprehensively examined. Utilizing the signal amplification effect of Cu-BTC, an electrochemical sensor for TCS detection is developed and optimized using response surface methodology. The resulting method offers a simple, rapid, and highly sensitive detection of TCS, with a linear range of 25-10,000 nM and a detection limit of 25 nM. This research highlights the potential of Cu-BTC as a promising material for electrochemical sensing applications, contributing to advancements in environmental monitoring and public health protection.

Metal-ligand dual-site single-atom nanozyme mimicking urate oxidase with high substrates specificity

In nature, coenzyme-independent oxidases have evolved in selective catalysis using isolated substrate-binding pockets. Single-atom nanozymes (SAzymes), an emerging type of non-protein artificial enzymes, are promising to simulate enzyme active centers, but owing to the lack of recognition sites, realizing substrate specificity is a formidable task. Here we report a metal-ligand dual-site SAzyme (Ni-DAB) that exhibited selectivity in uric acid (UA) oxidation. Ni-DAB mimics the dual-site catalytic mechanism of urate oxidase, in which the Ni metal center and the C atom in the ligand serve as the specific UA and O2 binding sites, respectively, characterized by synchrotron soft X-ray absorption spectroscopy, in situ near ambient pressure X-ray photoelectron spectroscopy, and isotope labeling. The theoretical calculations reveal the high catalytic specificity is derived from not only the delicate interaction between UA and the Ni center but also the complementary oxygen reduction at the beta C site in the ligand. As a potential application, a Ni-DAB-based biofuel cell using human urine is constructed. This work unlocks an approach of enzyme-like isolated dual sites in boosting the selectivity of non-protein artificial enzymes.

Exploring Enzyme-Mimicking Metal-Organic Frameworks for CO2 Conversion through Vibrational Spectra-Based Machine Learning

In pursuing the benefits of natural enzyme catalysts while overcoming their limitations, we find metal-organic frameworks (MOFs), renowned for their highly tunable functionalities, stand out in biomimetic applications. We used unsupervised machine learning on density functional theory-computed vibrational infrared and Raman spectral features to screen 300 Zn-MOFs for CO2 conversion, similar to carbonic anhydrase (CA). Our findings confirmed that MOFs with spectroscopic attributes closely resembling those of CA hold the potential for replicating CA's electronic and catalytic properties. Unlike previous studies that relied on heuristic or trial-and-error methods and focused on geometric configurations, our research uses vibrational spectral features to explore structure-property relationships, making them more accessible through spectroscopy. Moreover, we highlight vibrational spectral features as efficient carriers for highly dimensional chemical information, enabling the simultaneous optimization of multiple performance parameters. These findings pave the way for pioneering designs of enzyme-mimetic MOFs and concurrently expand the application scope of spectroscopic tools in biomimetic catalysis.

Modulation of copper sites in porphyrin metal-organic frameworks for electrochemical ascorbic acid sensing

Two metal-organic frameworks (MOFs) with different Cu-centered coordination structures were synthesized. By introducing 4,4-bipyridine as a linker in the Cu-MOFs, we have discovered that Cu-O, instead of Cu-N, is the active site with higher electrocatalytical activity towards ascorbic acid, which is essential to understand and develop Cu-based ascorbic acid sensors.

Recent Advances in Conductive Polymers-Based Electrochemical Sensors for Biomedical and Environmental Applications

Electrochemical sensors play a pivotal role in various fields, such as biomedicine and environmental detection, due to their exceptional sensitivity, selectivity, stability, rapid response time, user-friendly operation, and ease of miniaturization and integration. In addition to the research conducted in the application field, significant focus is placed on the selection and optimization of electrode interface materials for electrochemical sensors. The detection performance of these sensors can be significantly enhanced by modifying the interface of either inorganic metal electrodes or printed electrodes. Among numerous available modification materials, conductive polymers (CPs) possess not only excellent conductivity exhibited by inorganic conductors but also unique three-dimensional structural characteristics inherent to polymers. This distinctive combination allows CPs to increase active sites during the detection process while providing channels for rapid ion transmission and facilitating efficient electron transfer during reaction processes. This review article primarily highlights recent research progress concerning CPs as an ideal choice for modifying electrochemical sensors owing to their remarkable features that make them well-suited for biomedical and environmental applications.

The combination of DNA nanostructures and materials for highly sensitive electrochemical detection

Due to the wide range of electrochemical devices available, DNA nanostructures and material-based technologies have been greatly broadened. They have been actively used to create a variety of beautiful nanostructures owing to their unmatched programmability. Currently, a variety of electrochemical devices have been used for rapid sensing of biomolecules and other diagnostic applications. Here, we provide a brief overview of recent advances in DNA-based biomolecular assays. Biosensing platform such as electrochemical biosensor, nanopore biosensor, and field-effect transistor biosensors (FET), which are equipped with aptamer, DNA walker, DNAzyme, DNA origami, and nanomaterials, has been developed for amplification detection. Under the optimal conditions, the proposed biosensor has good amplification detection performance. Further, we discussed the challenges of detection strategies in clinical applications and offered the prospect of this field.

A Bioinspired Copper-Pair Catalyst in Metal-Organic Frameworks for Molecular Dioxygen Activation and Aerobic Oxidative C-N Coupling

Open Cu sites were loaded to the UiO-67 metal-organic framework (MOF) skeleton by introduction of flexible Cu-binding pyridylmethylamine (pyma) side chains to the biphenyldicarboxylate linkers. Distance between Cu centers in the MOF pores was tuned by controlling the density of metal-binding side chains. "Interacted" Cu-pair or "isolated" monomeric Cu sites were achieved with high and low (pyma)Cu side chain loading, respectively. Spectroscopic and theoretical studies indicate that "interacted" Cu pairs can effectively bind and activate molecular dioxygen to form Cu2O2 clusters, which showed high catalytic activity for aerobic oxidative C-N coupling. On the contrary, MOF catalyst bearing isolated monomeric Cu sites only showed modest catalytic activity. Enhancement in catalytic performance for the Cu-pair catalyst is attributed to the remote synergistic effect of the paired Cu site, which binds molecular dioxygen and cleaves the O═O bond in a collaborative manner. This work demonstrates that noncovalently interacted metal-pair sites can effectively activate inert small molecules and promote heterogeneous catalytic processes.

Atomic Engineering of Single-Atom Nanozymes for Biomedical Applications

Single-atom nanozymes (SAzymes) showcase not only uniformly dispersed active sites but also meticulously engineered coordination structures. These intricate architectures bestow upon them an exceptional catalytic prowess, thereby captivating numerous minds and heralding a new era of possibilities in the biomedical landscape. Tuning the microstructure of SAzymes on the atomic scale is a key factor in designing targeted SAzymes with desirable functions. This review first discusses and summarizes three strategies for designing SAzymes and their impact on reactivity in biocatalysis. The effects of choices of carrier, different synthesis methods, coordination modulation of first/second shell, and the type and number of metal active centers on the enzyme-like catalytic activity are unraveled. Next, a first attempt is made to summarize the biological applications of SAzymes in tumor therapy, biosensing, antimicrobial, anti-inflammatory, and other biological applications from different mechanisms. Finally, how SAzymes are designed and regulated for further realization of diverse biological applications is reviewed and prospected. It is envisaged that the comprehensive review presented within this exegesis will furnish novel perspectives and profound revelations regarding the biomedical applications of SAzymes.

Fabrication of NiCo Bimetallic MOF Films on 3D Foam with Assistance of Atomic Layer Deposition for Non-Invasive Lactic Acid Sensing

Lactic acid (LA) is an important downstream product of glycolysis in living cells and is abundant in our body fluids, which are strongly associated with diseases. The development of enzyme-free LA sensors with high sensitivity and low consumption remains a challenge. 2D metal-organic frameworks (MOFs) are considered to be promising electrochemical sensing materials and have attracted much attention in recent years. Compared to monometallic MOFs, the construction of bimetallic MOFs (BMOFs) can obtain a larger specific surface area, thereby increasing the exposed active site. 3D petal-like NixCoy MOF films on nickel foams (NixCoy BMOF@Ni foams) are successfully prepared by combining atomic layer deposition-assisted technology and hydrothermal strategy. The established NixCoy BMOF@Ni foams demonstrate noticeable LA sensing activity, and the study is carried out on behalf of the Ni1Co5 BMOF@Ni foam, which has a sensitivity of up to 9030 μA mM-1 cm-2 with a linear range of 0.01-2.2 mM and the detection limit is as low as 0.16 μM. Additionally, the composite has excellent stability and repeatability for the detection of LA under a natural air environment with high accuracy and reliability. Density functional theory calculation is applied to study the reaction process between composites and LA, and the result suggests that the active site in the NiCo BMOF film favors the adsorption of LA relative to the active site of monometallic MOF film, resulting in improved performance. The developed composite has a great potential for the application of noninvasive LA biosensors.

Copper Nanodots-Based Hybrid Hydrogels with Multiple Enzyme Activities for Acute and Infected Wound Repair

Effectively controlling bacterial infection, reducing the inflammation and promoting vascular regeneration are all essential strategies for wound repair. Nanozyme technology has potential applications in the treatment of infections because its non-antibiotic dependent, topical and noninvasive nature. In wound management, copper-based nanozymes have emerged as viable alternatives to antibiotics. In this study, an ultrasmall cupric enzyme with high enzymatic activity is synthesized and added to a nontoxic, self-healing, injectable cationic guar gum (CG) hydrogel network. The nanozyme exhibits remarkable antioxidant properties under neutral conditions, effectively scavenging reactive nitrogen and oxygen species (RNOS). Under acidic conditions, Cu NDs have peroxide (POD) enzyme-like activity, which allows them to eliminate hydrogen peroxides and produce free radicals locally. Antibacterial experiments show that they can kill bacteria and remove biofilms. It reveals that low concentrations of Cu ND/CG decrease the expression of the inflammatory factors in cells and tissues, effectively controlling inflammatory responses. Cu ND/CG hydrogels also inhibit HIF-1α and promote VEGF expression in the wound with the ability to promote vascular regeneration. In vivo safety assessments reveal a favorable biosafety profile. Cu ND/CG hydrogels offer a promising solution for treating acute and infected wounds, highlighting the potential of innovative nanomaterials in wound healing.

Research progress of metal-organic framework nanozymes in bacterial sensing, detection, and treatment

The high efficiency and specificity of enzymes make them play an important role in life activities, but the high cost, low stability and high sensitivity of natural enzymes severely restrict their application. In recent years, nanozymes have become convincing alternatives to natural enzymes, finding utility across diverse domains, including biosensing, antibacterial interventions, cancer treatment, and environmental preservation. Nanozymes are characterized by their remarkable attributes, encompassing high stability, cost-effectiveness and robust catalytic activity. Within the contemporary scientific landscape, metal-organic frameworks (MOFs) have garnered considerable attention, primarily due to their versatile applications, spanning catalysis. Notably, MOFs serve as scaffolds for the development of nanozymes, particularly in the context of bacterial detection and treatment. This paper presents a comprehensive review of recent literature pertaining to MOFs and their pivotal role in bacterial detection and treatment. We explored the limitations and prospects for the development of MOF-based nanozymes as a platform for bacterial detection and therapy, and anticipate their great potential and broader clinical applications in addressing medical challenges.

Artificial enzyme innovations in electrochemical devices: advancing wearable and portable sensing technologies

With the rapid evolution of sensing technologies, the integration of nanoscale catalysts, particularly those mimicking enzymatic functions, into electrochemical devices has surfaced as a pivotal advancement. These catalysts, dubbed artificial enzymes, embody a blend of heightened sensitivity, selectivity, and durability, laying the groundwork for innovative applications in real-time health monitoring and environmental detection. This minireview penetrates into the fundamental principles of electrochemical sensing, elucidating the unique attributes that establish artificial enzymes as foundational elements in this field. We spotlight a range of innovations where these catalysts have been proficiently incorporated into wearable and portable platforms. Navigating the pathway of amalgamating these nanoscale wonders into consumer-appealing devices presents a multitude of challenges; nevertheless, the progress made thus far signals a promising trajectory. As the intersection of materials science, biochemistry, and electronics progressively intensifies, a flourishing future seems imminent for artificial enzyme-infused electrochemical devices, with the potential to redefine the landscapes of wearable health diagnostics and portable sensing solutions.

Enhancing Local CO2 Adsorption by L-histidine Incorporation for Selective Formate Production Over the Wide Potential Window

Electrochemical carbon dioxide reduction reaction (CO2 RR) to produce valuable chemicals is a promising pathway to alleviate the energy crisis and global warming issues. However, simultaneously achieving high Faradaic efficiency (FE) and current densities of CO2 RR in a wide potential range remains as a huge challenge for practical implements. Herein, we demonstrate that incorporating bismuth-based (BH) catalysts with L-histidine, a common amino acid molecule of proteins, is an effective strategy to overcome the inherent trade-off between the activity and selectivity. Benefiting from the significantly enhanced CO2 adsorption capability and promoted electron-rich nature by L-histidine integrity, the BH catalyst exhibits excellent FEformate in the unprecedented wide potential windows (>90 % within -0.1--1.8 V and >95 % within -0.2--1.6 V versus reversible hydrogen electrode, RHE). Excellent CO2 RR performance can still be achieved under the low-concentration CO2 feeding (e.g., 20 vol.%). Besides, an extremely low onset potential of -0.05 VRHE (close to the theoretical thermodynamic potential of -0.02 VRHE ) was detected by in situ ultraviolet-visible (UV-Vis) measurements, together with stable operation over 50 h with preserved FEformate of ≈95 % and high partial current density of 326.2 mA cm-2 at -1.0 VRHE .

Two-Site Enhanced Porphyrinic Metal-Organic Framework Nanozymes and Nano-/Bioenzyme Confined Catalysis for Colorimetric/Chemiluminescent Dual-Mode Visual Biosensing

The rational design of efficient nanozymes and the immobilization of enzymes are of great significance for the construction of high-performance biosensors based on nano-/bioenzyme catalytic systems. Herein, a novel V-TCPP(Fe) metal-organic framework nanozyme with a two-dimensional nanosheet morphology is rationally designed by using V2CTx MXene as a metal source and iron tetrakis(4-carboxyphenyl)porphine (FeTCPP) ligand as an organic linker. It exhibits enhanced peroxidase- and catalase-like activities and luminol-H2O2 chemiluminescent (CL) behavior. Based on the experimental and theoretical results, these excellent enzyme-like activities are derived from the two-site synergistic effect between V nodes and FeTCPP ligands in V-TCPP(Fe). Furthermore, a confined catalytic system is developed by zeolitic imidazole framework (ZIF) coencapsulation of the V-TCPP(Fe) nanozyme and bioenzyme. Using the acetylcholinesterase (AChE) as a model, our constructed V-TCPP(Fe)/AChE@ZIF confined catalytic system was successfully used for the colorimetric/CL dual-mode visual biosensing of organophosphorus pesticides. This work is expected to provide new insights into the design of efficient nanozymes and confined catalytic systems, encouraging applications in catalysis and biosensing.

Valence-Engineered Oxidase-Mimicking Nanozyme with Specificity for Aromatic Amine Oxidation and Identification

Oxidase-mimicking nanozymes with specificity for catalyzing oxidation of aromatic amines are of great significance for recognition of aromatic amines but rarely reported. Herein, Cu-A nanozyme (synthesized with Cu²⁺ as a node and adenine as a linker) could specifically catalyze oxidation of o-phenylenediamine (OPD) in Britton-Robinson buffer solution. Such a specific catalytic performance was also corroborated with other aromatic amines, such as p-phenylenediamine (PPD), 1,5-naphthalene diamine (1,5-NDA), 1,8-naphthalene diamine (1,8-NDA), and 2-aminoanthracene (2-AA). Moreover, the presence of salts (1 mM NaNO₂, NaHCO₃, NH₄Cl, KCl, NaCl, NaBr, and NaI) greatly mediated the catalytic activity with the order of NaNO₂ < blank ≈ NaHCO₃ < NH₄Cl ≈ KCl ≈ NaCl < NaBr < NaI, which was due to anions sequentially increasing interfacial Cu⁺ content via anionic redox reaction, while the effect of cations was negligible. With the increased Cu⁺ content, K_m decreased and V_max increased, indicating valence-engineered catalytic activity. Based on high specificity and satisfactory activity, a colorimetric sensor array with NaCl, NaBr, and NaI as sensing channels was constructed to identify five representative aromatic amines (OPD, PPD, 1,5-NDA, 1,8-NDA, and 2-AA) as low as 50 μM, quantitatively analyze single aromatic amine (with OPD and PPD as model analysts), and even identify 20 unknown samples with an accuracy of 100%. In addition, the performance was further validated through accurately recognizing various concentration ratios of binary, ternary, quaternary, and quinary mixtures. Finally, the practical applications were demonstrated by successfully discriminating five aromatic amines in tap, river, sewage, and sea water, providing a simple and feasible assay for large-scale scanning aromatic amine levels in environmental water samples.

UiO-66 framework with an encapsulated spin probe: synthesis and exceptional sensitivity to mechanical pressure

Probes sensitive to mechanical stress are in demand for the analysis of pressure distribution in materials, and the design of pressure sensors based on metal-organic frameworks (MOFs) is highly promising due to their structural tunability. We report a new pressure-sensing material, which is based on the UiO-66 framework with trace amounts of a spin probe (0.03 wt%) encapsulated in cavities. To obtain this material, we developed an approach for encapsulation of stable nitroxide radical TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl) into the micropores of UiO-66 during its solvothermal synthesis. Pressure read-out using electron paramagnetic resonance (EPR) spectroscopy allows monitoring the degradation of the defected MOF structure upon pressurization, where full collapse of pores occurs at as low a pressure as 0.13 GPa. The developed methodology can be used in and ex situ and provides sensitive tools for non-destructive mapping of pressure effects in various materials.