Microfluidic paper-based device for colorimetric determination of glucose based on a metal-organic framework acting as peroxidase mimetic

Droplet Impedance Feedback-Enabled Microsampling Microfluidic Device for Precise Chemical Information Monitoring

Microelectrodes have transformed our understanding of spatiotemporal responses to electrical stimulation. However, biological signals are often molecular, complicating the capture of intricate chemical signals. The microfluidic chip developed in this paper accurately measures droplet volume by using impedance analysis. The utilization of droplet volume as a feedback signal for precise microsampling pressure control ensures that microsampling remains unaffected by droplet volume influence. Once the microsampling is complete, chemiluminescence detection enables high temporal resolution and continuous and sensitive monitoring of chemical information within the droplets. Experimental verification shows that the chip can avoid volume influence through impedance feedback, achieving consistent and stable microampling at the nanoliter level (0-3 nL). In just 0.3 s, it can perform sensitive chemiluminescence detection of H2O2 and glucose within droplets. The linear detection ranges for these analytes are 10-50,000 and 20-600 μM, respectively, with the limit of detection being 0.648 and 0.334 μM. The significance of this chip lies in its ability to reveal changes in both electrical and chemical signals during transient biological processes. Its potential applications are numerous, encompassing a wide range of emerging areas such as single-cell analysis, cell communication, and cellular immunity.

Bifunctional oxidase-peroxidase mimicking Fe-Ce MOF on paper-based analytical devices to intensify luminol chemiluminescence: Application for measuring different sugars with a smartphone readout

This study presents a potent paper-based analytical device (PAD) for quantifying various sugars using an innovative bi-nanozyme made from a 2-dimensional Fe/Ce metal-organic framework (FeCe-BTC). The MOF showed excellent bifunctional peroxidase-oxidase activities, efficiently catalyzing luminol's chemiluminescence (CL) reaction. As a peroxidase-like nanozyme, FeCe-BTC could facilitate the dissociation of hydrogen peroxide (H2O2) into hydroxyl radicals, which then oxidize luminol. Additionally, it was also discovered that when reacting with H2O2, the MOF turns into a mixed-valence MOF, and acts as an oxidase nanozyme. This activity is caused by the generated Ce4+ ions in the structure of MOF that can directly oxidize luminol. The MOF was directly synthesized on the PAD and cascaded with specific natural enzymes to establish simple, rapid, and selective CL sensors for the measurement of different sugars. A cell phone was also used to record light intensities, which were then correlated to the analyte concentration. The designed PAD showed a wide linear range of 0.1-10 mM for glucose, fructose, and sucrose, with detection limits of 0.03, 0.04, and 0.04 mM, respectively. It showed satisfactory results in food and biological samples with recovery values ranging from 95.8 to 102.4 %, which makes it a promising candidate for point-of-care (POC) testing for food control and medicinal purposes.

Integrating of analytical techniques with enzyme-mimicking nanomaterials for the fabrication of microfluidic systems for biomedical analysis

Bioanalysis faces challenges in achieving fast, reliable, and point-of-care (POC) determination methods for timely diagnosis and prognosis of diseases. POC devices often display lower sensitivity compared to laboratory-based methods, limiting their ability to quantify low concentrations of target analytes. To enhance sensitivity, the synthesis of new materials and improvement of the efficiency of the analytical strategies are necessary. Enzyme-mimicking materials have revolutionized the field of the fabrication of new high-throughput sensing devices. The integration of microfluidic chips with analytical techniques offers several benefits, such as easy miniaturization, need for low biological sample volume, etc., while also enhancing the sensitivity of the probe. The use enzyme-like nanomaterials in microfluidic systems can offer portable strategies for real-time and reliable detection of biological agents. Colorimetry and electrochemical methods are commonly utilized in the fabrication of nanozyme-based microfluidic systems. The review summarizes recent developments in enzyme-mimicking materials-integrated microfluidic analytical methods in biomedical analysis and discusses the current challenges, advantages, and potential future directions.

Bimetallic FeOx-TiO2@Carbon hybrid structure materials with notable peroxidase enzyme mimics applied to one-step colorimetric detection of glucose

FeOx-TiO2@Carbon hybrid structure materials (FeOx-TiO2@CHs) with high peroxidase (POD)-like activity have been prepared by one-pot hydrothermal method. Based on the excellent POD activity of FeOx-TiO2@CHs, one pot colorimetric detection for glucose was constructed by using TMB as substrate with the synergistic reaction of glucose oxidase; the linear range and the limit of detection (LOD) are 25 ~ 1000 and 1.77 µM, respectively. Using this method, the glucose in serum real samples was detected with satisfactory results, and the results are consistent with that of the glucometer method in the hospital. The recovery in diabetic and artificial urine samples was 95.71 ~ 104.67% and 99.01 ~ 103.16%, respectively. The mechanism of the catalytic colorimetric reaction was also investigated by multiple measurements, and the results indicated that superoxide anions (O2•-) between FeOx-TiO2@CHs and substrate play a main role, but a small quantity of hydroxyl radical •OH and singlet oxygen 1O2 is also generated simultaneously. The one-pot reaction method is simple and fast; the detection process only requires a simple mixing, which is suitable for application in special environment.

Recent Progress and Prospect of Metal-Organic Framework-Based Nanozymes in Biomedical Application

A nanozyme is a nanoscale material having enzyme-like properties. It exhibits several superior properties, including low preparation cost, robust catalytic activity, and long-term storage at ambient temperatures. Moreover, high stability enables repetitive use in multiple catalytic reactions. Hence, it is considered a potential replacement for natural enzymes. Enormous research interest in nanozymes in the past two decades has made it imperative to look for better enzyme-mimicking materials for biomedical applications. Given this, research on metal-organic frameworks (MOFs) as a potential nanozyme material has gained momentum. MOFs are advanced hybrid materials made of inorganic metal ions and organic ligands. Their distinct composition, adaptable pore size, structural diversity, and ease in the tunability of physicochemical properties enable MOFs to mimic enzyme-like activities and act as promising nanozyme candidates. This review aims to discuss recent advances in the development of MOF-based nanozymes (MOF-NZs) and highlight their applications in the field of biomedicine. Firstly, different enzyme-mimetic activities exhibited by MOFs are discussed, and insights are given into various strategies to achieve them. Modification and functionalization strategies are deliberated to obtain MOF-NZs with enhanced catalytic activity. Subsequently, applications of MOF-NZs in the biosensing and therapeutics domain are discussed. Finally, the review is concluded by giving insights into the challenges encountered with MOF-NZs and possible directions to overcome them in the future. With this review, we aim to encourage consolidated efforts across enzyme engineering, nanotechnology, materials science, and biomedicine disciplines to inspire exciting innovations in this emerging yet promising field.

Low-tech vs. high-tech approaches in μPADs as a result of contrasting needs and capabilities of developed and developing countries focusing on diagnostics and point-of-care testing

Paper-based analysis has captivated scientists' attention in the field of analytical chemistry and related areas for the last two decades. Arguably no other area of modern chemical analysis is so broad and diverse in its approaches spanning from simple 'low-tech' low-cost paper-based analytical devices (PADs) requiring no or simple instrumentation, to sophisticated PADs and microfluidic paper-based analytical devices (μPADs) featuring elements of modern material science and nanomaterials affording high selectivity and sensitivity. Correspondingly diverse is the applicability, covering resource-limited scenarios on the one hand and most advanced approaches on the other. Herein we offer a view reflecting this diversity in the approaches and types of devices. The core idea of this article rests in dividing μPADs according to their type into two groups: A) instrumentation-free μPADs for resource-limited scenarios or developing countries and B) instrumentation-based μPADs as futuristic POC devices for e-diagnostics mainly aimed at developed countries. Each of those two groups is presented and discussed with the view of the main requirements in the given area, the most common targets, sample types and suitable detection approaches either implementing high-tech elements or low-tech low-cost approaches. Finally, a socioeconomic perspective is offered in discussing the fabrication and operational costs of μPADs, and, future perspectives are offered.

Engineering hetero-structural iron nanozyme decorated liposome with a self-cascade catalysis performance

Metal-based enzyme mimics are considered as acceptable agents in fabricating heterogeneous biocomposites through valency integrations because of their biomedical or biological properties. As the basic substitute, it delights us to utilize Fe3O4 nanoparticles (NPs) as metallic enzymes and overcome the limitation of peroxide-like enzymatic activity in physiological conditions. In this work, we present the fabrication of a soy phosphatidylcholine/Fe3O4@Ag/GOx (SFAG) biocomposite as a cascade enzyme, which exhibits a peroxidase-like property in kinetic processes, as shown from an analysis of the glucose detection processes. We also explored the mechanism of an ultrasound & microfluidic approach for the synthesis of SFAG. The resultant SFAG implies a characteristic absorption peak (652 nm), size (55 μm), and surface charge (-32.93 ± 2.58 mV). This is utilized to confirm the peroxidase-like activity by catalyzing 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2 under physiological conditions. But also, SFAG conveys a positive effect on the peroxidase-like activity at pH = 5.8, 7.4, and 8.0. The Michaelis-Menten parameters (Km) and the Vmax values of H2O2 are 1.914 mM and 1.429 × 10-7 M s-1, which further confirms the catalytic performances of the SFAG structure. The established platform was also used successfully for the determination of glucose in PBS and diluted synthetic blood with excellent sensitivity and stability. The relative selection and sensitivity show that the SFAG structure has a great possibility as a cascade metallic enzyme in chemokinetic works.

A Nitro Functionalized MOF with Multi-Enzyme Mimetic Activities for the Colorimetric Sensing of Glucose at Neutral pH

Benefiting from the advantages like large surface area, flexible constitution, and diverse structure, metal-organic frameworks (MOFs) have been one of the most ideal candidates for nanozymes. In this study, a nitro-functionalized MOF, namely NO₂-MIL-53(Cu), was synthesized. Multi-enzyme mimetic activities were discovered on this MOF, including peroxidase-like, oxidase-like, and laccase-like activity. Compared to the non-functional counterpart (MIL-53(Cu)), NO₂-MIL-53(Cu) displayed superior enzyme mimetic activities, indicating a positive role of the nitro group in the MOF. Subsequently, the effects of reaction conditions on enzyme mimetic activities were investigated. Remarkably, NO₂-MIL-53(Cu) exhibited excellent peroxidase-like activity even at neutral pH. Based on this finding, a simple colorimetric sensing platform was developed for the detection of H₂O₂ and glucose, respectively. The detection liner range for H₂O₂ is 1-800 μM with a detection limit of 0.69 μM. The detection liner range for glucose is linear range 0.5-300 μM with a detection limit of 2.6 μM. Therefore, this work not only provides an applicable colorimetric platform for glucose detection in a physiological environment, but also offers guidance for the rational design of efficient nanozymes with multi-enzyme mimetic activities.

Combining microfluidic paper-based platform and metal-organic frameworks in a single device for phenolic content assessment in fruits

A microfluidic paper-based device (µPAD) has been combined with metal-organic frameworks (MOFs) for total phenolic compounds (TPC) quantification in fruit samples for the first time. The performance of the µPAD, based upon the vertical flow approach, was enhanced in order to determine the TPC content with high accuracy in fruit samples. The method was based on the traditional Folin-Ciocalteu Index using gallic acid or oenotannin as reference phenolic compounds. This novel design and construction of the device are in agreement with the principles of Green Chemistry avoiding wax technology (lower toxicity). The analytical parameters that affect the colorimetric method (using digital imaging of the colored zone) performance were optimized including design, sample volume, and MOF amount. Then, the analytical features of the developed method were investigated such as dynamic range (1.6-30 mg L^-1), limit of detection (0.5 mg L^-1), and precision (RSD < 9%). Besides, the in-field analysis is achievable with a color stability up to 6 h after the loading process of the sample and storage stability for at least 15 days without performance losses (under vacuum at - 20 °C). Furthermore, the MOF ZIF-8@paper was characterized to study its composition and the successful combination. The feasibility of the proposed method was demonstrated by determining the TPC in 5 fruit samples using oenotannin as reference solute. The accuracy was validated by comparison of the data with the results obtained with the recommended protocol proposed by the International Organisation of Vine and Wine (OIV).

Paper-based optical sensors paired with smartphones for biomedical analysis

The traditional analytical methods used for biomedical analysis are expensive and not easy to handle and require sophisticated instruments, thus their application is limited in resource-limited settings. Due to their portability, low cost, and ability to be applied to different analytical techniques, paper-based analytical devices are becoming valuable tools for biomedical analysis. The integration of smartphones into analytical devices has provided the ability to build portable, cost-effective, straightforward analytical devices for biomedical analysis and mobile health. The key aim of this review is to emphasize the recent applications of PADs combined with a smartphone for the optical analysis of biomedical species. We started this review by highlighting the type of papers and their modifications with different materials to prepare the PADs. After that, this review presents various detection methods including colorimetry, fluorescence, and luminescence where the smartphone is used for read-out. In the end, we provided the recent applications of the analysis of different biomedical compounds such as cancer and cardiovascular biomarkers, metal ions, glucose, viruses, etc. We believe that the present review will attract a wide scientific community in the areas of analytical chemistry, sensors, and clinical testing.

Biomedical Applications of Microfluidic Devices: A Review

Both passive and active microfluidic chips are used in many biomedical and chemical applications to support fluid mixing, particle manipulations, and signal detection. Passive microfluidic devices are geometry-dependent, and their uses are rather limited. Active microfluidic devices include sensors or detectors that transduce chemical, biological, and physical changes into electrical or optical signals. Also, they are transduction devices that detect biological and chemical changes in biomedical applications, and they are highly versatile microfluidic tools for disease diagnosis and organ modeling. This review provides a comprehensive overview of the significant advances that have been made in the development of microfluidics devices. We will discuss the function of microfluidic devices as micromixers or as sorters of cells and substances (e.g., microfiltration, flow or displacement, and trapping). Microfluidic devices are fabricated using a range of techniques, including molding, etching, three-dimensional printing, and nanofabrication. Their broad utility lies in the detection of diagnostic biomarkers and organ-on-chip approaches that permit disease modeling in cancer, as well as uses in neurological, cardiovascular, hepatic, and pulmonary diseases. Biosensor applications allow for point-of-care testing, using assays based on enzymes, nanozymes, antibodies, or nucleic acids (DNA or RNA). An anticipated development in the field includes the optimization of techniques for the fabrication of microfluidic devices using biocompatible materials. These developments will increase biomedical versatility, reduce diagnostic costs, and accelerate diagnosis time of microfluidics technology.

New Insight into Assembled Fe3O4@PEI@Ag Structure as Acceptable Agent with Enzymatic and Photothermal Properties

Metal-based enzyme mimics are considered to be acceptable agents in terms of their biomedical and biological properties; among them, iron oxides (Fe₃O₄) are treated as basement in fabricating heterogeneous composites through variable valency integrations. In this work, we have established a facile approach for constructing Fe₃O₄@Ag composite through assembling Fe₃O₄ and Ag together via polyethyleneimine ethylenediamine (PEI) linkages. The obtained Fe₃O₄@PEI@Ag structure conveys several hundred nanometers (~150 nm). The absorption peak at 652 nm is utilized for confirming the peroxidase-like activity of Fe₃O₄@PEI@Ag structure by catalyzing 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂. The Michaelis–Menten parameters (K_m) of 1.192 mM and 0.302 mM show the higher catalytic activity and strong affinity toward H₂O₂ and TMB, respectively. The maximum velocity (V_max) value of 1.299 × 10⁻⁷ M∙s⁻¹ and 1.163 × 10⁻⁷ M∙s⁻¹ confirm the efficiency of Fe₃O₄@PEI@Ag structure. The biocompatibility illustrates almost 100% cell viability. Being treated as one simple colorimetric sensor, it shows relative selectivity and sensitivity toward the detection of glucose based on glucose oxidase. By using indocyanine green (ICG) molecule as an additional factor, a remarkable temperature elevation is observed in Fe₃O₄@PEI@Ag@ICG with increments of 21.6 °C, and the absorption peak is nearby 870 nm. This implies that the multifunctional Fe₃O₄@PEI@Ag structure could be an alternative substrate for formatting acceptable agents in biomedicine and biotechnology with enzymatic and photothermal properties.

Nanozyme-based colorimetric biosensor with a systemic quantification algorithm for noninvasive glucose monitoring

Diabetes mellitus accompanies an abnormally high glucose level in the bloodstream. Early diagnosis and proper glycemic management of blood glucose are essential to prevent further progression and complications. Biosensor-based colorimetric detection has progressed and shown potential in portable and inexpensive daily assessment of glucose levels because of its simplicity, low-cost, and convenient operation without sophisticated instrumentation. Colorimetric glucose biosensors commonly use natural enzymes that recognize glucose and chromophores that detect enzymatic reaction products. However, many natural enzymes have inherent defects, limiting their extensive application. Recently, nanozyme-based colorimetric detection has drawn attention due to its merits including high sensitivity, stability under strict reaction conditions, flexible structural design with low-cost materials, and adjustable catalytic activities. This review discusses various nanozyme materials, colorimetric analytic methods and mechanisms, recent machine learning based analytic methods, quantification systems, applications and future directions for monitoring and managing diabetes.

A handheld 3D-printed microchip for simple integration of the H2O2-producing enzymatic reactions with subsequent chemiluminescence detection: Application for sugars

Herein, a novel lab-on-a-chip (LoC) device fabricated by 3D printing based on H₂O₂-producing enzymatic reactions with sensitive chemiluminescence (CL) detection was developed to measure different sugars, including glucose, fructose, sucrose, and maltose, in honey, juice, and rice flour samples. The pumpless microchip included two main parts, separated by new cone-shape blocking valves; part A for sample introduction and subsequent enzymatic reaction, besides the CL reagent (luminol) container, and part B for detection. The specific enzyme(s) were embedded into the pores of the zinc zeolite-imidazole framework (ZIF-8) to improve their storage stability. By opening the valves, H₂O₂ produced by enzymatic reaction and luminol could flow through the designed channels into the detection zone on part B, where a 2D cobalt-imidazole framework was embedded to improve the luminol-H₂O₂ CL emission. The obtained signal was proportional to the considered sugar concentration, with the detection limits range of 20-268 µM.

Sensing Capacity in Dysprosium Metal-Organic Frameworks Based on 5-Aminoisophthalic Acid Ligand

Two novel metal-organic frameworks (MOFs), based on dysprosium as the metal and the 5-aminoisophthalic acid (5aip) ligand, have been solvothermally synthesized, with the aim of studying and modulating their luminescence properties according to the variation of solvent in the structure. These materials display intense photo-luminescence properties in the solid state at room temperature. Interestingly, one fascinating sensory capacity of compound 2 regards obtaining a variation of the signal, depending on the solvent to which it is exposed. These results pave the way for a new generation of sensitive chemical sensors.

Reversal of a Fluorescent Fluoride Chemosensor from Turn-Off to Turn-On Based on Aggregation Induced Emission Properties

Here we present a new approach for the development of fluoride chemosensors taking advantage of aggregation induced emission (AIE) properties. Although AIE-based chemosensors have been described, they rely primarily on the analyte causing aggregation and hence fluorescence. We propose a new concept in the use of AIE for the development of fluorescent sensors. Our hypothesis is based on the fact that a turn-off chemosensor in solution can be transformed into turn-on in the solid state if the properties of ACQ and AIE are properly combined between the fluorescent molecules involved. To demonstrate this hypothesis, we have selected a fluorescent chemosensor for the fluoride anion with a conjugated structure of bis(styryl)pyrimidine that, while showing turn-off behavior in solution, becomes turn-on when it is brought to the solid state. We have also combined it with the advantages of a detection system based on the microfluidic paper-based analytical devices (μPAD). The system is fully characterized spectroscopically both in solution and in the solid state, and quantum mechanical calculations were performed to explain how the sensor works. The prepared device presents a high sensitivity, with no interference and with an LoD and LoQ that allow determination of fluoride concentrations in water 2 orders of magnitude below the maximum allowed by WHO.

A template-free assembly of Cu,N-codoped hollow carbon nanospheres as low-cost and highly efficient peroxidase nanozymes

Utilizing Cu2+-coordinated poly(m-phenylenediamine) as the precursor, Cu,N-codoped hollow carbon nanospheres as low-cost and highly efficient peroxidase nanozymes were assembled through a template-free strategy for the first time.

Metal-organic frameworks for chemical sensing devices

Metal-organic frameworks (MOFs) are exceptionally large surface area materials with organized porous cages that have been investigated for nearly three decades. Due to the flexibility in their design and predisposition toward functionalization, they have shown promise in many areas of application, including chemical sensing. Consequently, they are identified as advanced materials with potential for deployment in analytical devices for chemical and biochemical sensing applications, where high sensitivity is desirable, for example, in environmental monitoring and to advance personal diagnostics. To keep abreast of new research, which signposts the future directions in the development of MOF-based chemical sensors, this review examines studies since 2015 that focus on the applications of MOF films and devices in chemical sensing. Various examples that use MOF films in solid-state sensing applications were drawn from recent studies based on electronic, electrochemical, electromechanical and optical sensing methods. These examples underscore the readiness of MOFs to be integrated in optical and electronic analytical devices. Also, preliminary demonstrations of future sensors are indicated in the performances of MOF-based wearables and smartphone sensors. This review will inspire collaborative efforts between scientists and engineers working within the field of MOFs, leading to greater innovations and accelerating the development of MOF-based analytical devices for chemical and biochemical sensing applications.

Lab-on-Paper Devices for Diagnosis of Human Diseases Using Urine Samples-A Review

In recent years, microfluidic lab-on-paper devices have emerged as a rapid and low-cost alternative to traditional laboratory tests. Additionally, they were widely considered as a promising solution for point-of-care testing (POCT) at home or regions that lack medical infrastructure and resources. This review describes important advances in microfluidic lab-on-paper diagnostics for human health monitoring and disease diagnosis over the past five years. The review commenced by explaining the choice of paper, fabrication methods, and detection techniques to realize microfluidic lab-on-paper devices. Then, the sample pretreatment procedure used to improve the detection performance of lab-on-paper devices was introduced. Furthermore, an in-depth review of lab-on-paper devices for disease measurement based on an analysis of urine samples was presented. The review concludes with the potential challenges that the future development of commercial microfluidic lab-on-paper platforms for human disease detection would face.

Smartphone-based colorimetric detection systems for glucose monitoring in the diagnosis and management of diabetes

Diabetes is a group of metabolic conditions resulting in high blood sugar levels over prolonged periods that affects hundreds of millions of patients worldwide. Measuring glucose concentration enables patient-specific insulin therapy, and is essential to reduce the severity of the disease, potential complications, and related mortalities. Recent advances and developments in smartphone-based colorimetric glucose detection systems are discussed in this review. The importance of glucose monitoring, data collection, transfer, and analysis, via non-invasive/invasive methods is highlighted. The review also presents various approaches using 3D-printed materials, screen-printed electrodes, polymer templates, designs allowing multiple glucose analysis, bioanalytes and/or nanostructures for glucose detection. The positive effects of advances in improving the performance of smartphone-based platforms are introduced along with future directions and trends in the application of emerging technologies in smartphone-based diagnostics.

NiCo2S4 microflowers as peroxidase mimic: A multi-functional platform for colorimetric detection of glucose and evaluation of antioxidant behavior

The regular design of atomic composition of materials endows a diverse range of enzyme mimics types and increases a broader application prospect. In this study, we designed a nickel-cobalt mixed metal sulfide and demonstrated that the as-prepared NiCo₂S₄ microflowers possessed intrinsic peroxidase-like activity. Nickel-cobalt sulfide (NiCo₂S₄) possessed high electron transfer capacity, which lead to good peroxidase-like activity. Compared with the reported enzyme-like materials, NiCo₂S₄ exhibited a smaller Km and a stronger affinity with substrate. A colorimetric assay was developed for the direct detection of hydrogen peroxide and indirect detection of glucose over a wide linear range (H₂O₂ was 20-200 μM, and glucose was 20-1000 μM) with a low detection limit (H₂O₂ was 5.19 μM, and glucose was 8.24 μM). Furthermore, a NiCo₂S₄ based platform was established to study the antioxidant behavior of three antioxidants. The antioxidant capacities of the antioxidants were found to rank in the order: tannic acid (TA) > ascorbic acid (AA)> gallic acid (GA). Moreover, the three antioxidants showed different inhibition mechanisms. This study indicated a new and important application field for NiCo₂S₄ and provides a basis for the rational design of enzyme-like mimics in future research.

Enzyme embedded microfluidic paper-based analytic device (μPAD): a comprehensive review

Low-cost paper-based analytical devices are the latest generation of portable lab-on-chip designs that offers an innovative platform for the on/off-site analysis (biosensing) of target analytes, especially in rural and remote areas. Recently, microfluidic paper-based analytical devices (μPADs) have attained significant recognition owing to their exciting fundamental features such as: ease of fabrication, rapid operation, and precise interpretations. The incorporation of enzymes with paper-based analytical devices significantly improves analytical performance while exhibiting excellent chemical and storage stability. In addition to that, these devices are highly compact, portable, easy-to-use, and do not require any additional sophisticated equipment for the detection and quantification of target analytes. This review provides a holistic insight into design, fabrication, and enzyme immobilization strategies for the development of enzyme-μPADs, which enables them to be widely implemented for in-field analysis. It also highlights the recent application of enzyme-μPADs in the area of: biomedical, food safety, and environmental monitoring while exploring the mechanisms of detection involved. Further, in order to improve the accuracy of analysis, researchers have designed a smartphone-based scanning tool for multi-variant point-of-care devices, which is summarized in the latter part of the review. Finally, the future perspectives and outlook of major challenges associated with enzyme-μPADs are discussed with their possible solutions. The development of enzyme integrated μPADs will open a new avenue as an exceptional analytical tool to explore various applications.HIGHLIGHTSEnzyme embedded paper-based analytical devices are a revolution in the field of biosensing.The design, fabrication, and enzyme immobilization on μPADs have been comprehensively discussed.The application of enzyme-μPADs food safety, environmental monitoring, and clinical diagnostic have been reviewed.Smartphones can be used as an on-site, user-friendly, and compact next-gen scanning tool for biosensing.

Strategies for the detection of target analytes using microfluidic paper-based analytical devices

Microfluidic paper-based analytical devices (μPADs) have developed rapidly in recent years, because of their advantages, such as small sample volume, rapid detection rates, low cost, and portability. Due to these characteristics, they can be used for in vitro diagnostics in the laboratory, or in the field, for a variety of applications, including food evaluation, disease screening, environmental monitoring, and drug testing. This review will present various detection methods employed by μPADs and their respective applications for the detection of target analytes. These include colorimetry, electrochemistry, chemiluminescence (CL), electrochemiluminescence (ECL), and fluorescence-based methodologies. At the same time, the choice of labeling material and the design of microfluidic channels are also important for detection results. The construction of novel nanocomponents and different smart structures of paper-based devices have improved the performance of μPADs and we will also highlight some of these in this manuscript. Additionally, some key challenges and future prospects for the use of μPADs are briefly discussed.

Paper microfluidic device using carbon dots to detect glucose and lactate in saliva samples

Bioanalyses are commonly performed with blood or serum samples. However, these analyses often require invasive and painful blood collection using a needle or finger pricking. Saliva is an alternative and very attractive biological medium for performing clinical analyses, since it contains many types of clinically relevant biomarkers and compounds. Its collection is straightforward and can be achieved in a non-invasive and stress-free way. However, the analytes are frequently present at low concentrations, while the viscosity of whole saliva hinders its analysis using paper devices, especially those with multiple layers (3D-μPADs). This work explores the use of a simple, fast, and low-cost saliva sample pretreatment using a cotton-paper-syringe filtration system, allowing the analysis of saliva samples using multilayer paper devices. The proposed methodology employs the oxidation of glucose and lactate, catalyzed by specific oxidase enzymes, producing hydrogen peroxide. The detection is based on the fluorescence quenching of carbon dots in the presence of hydrogen peroxidase. The concentrations of the analytes showed good linear correlations with the fluorescence quenching, with LODs of 2.60 × 10^-6 and 8.14 × 10^-7 mol L^-1 for glucose and lactate, respectively. The proposed method presented satisfactory intra-day and inter-day repeatabilities, with %RSD values in the range 3.82-6.61%. The enzymatic systems proved to be specific for the analytes and the matrix had no significant influence on the glucose and lactate determinations. The proposed methodology was successfully applied to saliva and serum samples and was validated using certified material.

A colorimetric immunoassay for determination of Escherichia coli O157:H7 based on oxidase-like activity of cobalt-based zeolitic imidazolate framework

Cobalt-based zeolitic imidazolate framework nanosheets (ZIF-67) with oxidase-like catalytic activities as an immunoprobe were employed to enhance the sensitivity of an immunoassay. ZIF-67 was synthesized via the solvothermal method using 2-methylimidazole and cobalt dichloride as substrates. A colorimetric immunoassay for Escherichia coli (E. coli) O157:H7 was designed. Preparation of the immunoprobe involved self-polymerized dopamine being applied for the surface modification of ZIF-67 nanosheets in order to bind to the antibody, which was used to identify E. coli O157:H7. ZIF-67 catalyze the oxidation of 3,3',5,5'-tetramethylbiphenyl (TMB) and produced a color change from colorless to blue. Upon reaction termination, the absorbance was measured at 450 nm. By combining ZIF-67@PDA catalyzed chromogenic reaction with antibody recognition and magnetic separation, the limit of determination is 12 CFU mL^-1 and the linear range is 30 to 3.0 × 10⁸ CFU mL^-1. The proposed colorimetric immunoassay was successfully utilized to detect E. coli O157:H7 of spiked food samples. Graphical abstract.

Nanozyme-based electrochemical biosensors for disease biomarker detection

In recent years, a new group of nanomaterials named nanozymes that exhibit enzyme-mimicking catalytic activity has emerged as a promising alternative to natural enzymes. Nanozymes can address some of the intrinsic limitations of natural enzymes such as high cost, low stability, difficulty in storage, and specific working conditions (i.e., narrow substrate, temperature and pH ranges). Thus, synthesis and applications of hybrid and stimuli-responsive advanced nanozymes could revolutionize the current practice in life sciences and biosensor applications. On the other hand, electrochemical biosensors have long been used as an efficient way for quantitative detection of analytes (biomarkers) of interest. As such, the use of nanozymes in electrochemical biosensors is particularly important to achieve low cost and stable biosensors for prognostics, diagnostics, and therapeutic monitoring of diseases. Herein, we summarize the recent advances in the synthesis and classification of common nanozymes and their application in electrochemical biosensor development. After briefly overviewing the applications of nanozymes in non-electrochemical-based biomolecular sensing systems, we thoroughly discuss the state-of-the-art advances in nanozyme-based electrochemical biosensors, including genosensors, immunosensors, cytosensors and aptasensors. The applications of nanozymes in microfluidic-based assays are also discussed separately. We also highlight the challenges of nanozyme-based electrochemical biosensors and provide some possible strategies to address these limitations. Finally, future perspectives on the development of nanozyme-based electrochemical biosensors for disease biomarker detection are presented. We envisage that standardization of nanozymes and their fabrication process may bring a paradigm shift in biomolecular sensing by fabricating highly specific, multi-enzyme mimicking nanozymes for highly sensitive, selective, and low-biofouling electrochemical biosensors.

Nanozyme-Modified Metal-Organic Frameworks with Multienzymes Activity as Biomimetic Catalysts and Electrocatalytic Interfaces

Many metal-organic frameworks have been designed and synthesized for biosensors because of high surface area and porosity, suitable size, and good biocompatibility. Despite recent advances, however, most of them are only used as a nanocarrier. In this work, a new artificial nanozyme was constructed on a metalloporphyrinic metal-organic framework (PMOF(Fe)), which was formed by Fe porphyrin and Zr⁴⁺ ions. Then, ultrasmall Pt nanoparticles (Pt NPs) were loaded on the surface of PMOF(Fe) to form Pt@PMOF(Fe). Because of the high surface area and exposed Fe activity center, PMOF(Fe) works as a nanocarrier to hinder the Pt NP aggregation and exhibits high peroxidase-mimicking activity. Hence, Pt NPs decorated on the surface of PMOF(Fe) possessed high stability and exhibited high activity. Due to the synergistic effect between PMOF(Fe) and Pt NPs, Pt@PMOF(Fe) exhibits superior catalase- and peroxidase-like activities. Moreover, Pt@PMOF(Fe) possesses high electrocatalytic activity toward the reduction of H₂O₂ and the oxygen reduction reaction (ORR). This strategy may serve as a strong foundation to design MOF-based artificial nanozymes and develop an ideal platform for MOFs and nanozymes toward artificial enzymatic catalytic systems, fuel cells and new analytical applications.

Modified Ti3C2 nanosheets as peroxidase mimetics for use in colorimetric detection and immunoassays

Since being discovered in 2011, a large class of two-dimensional materials, labeled MXenes, has received increased research enthusiasm both theoretically and experimentally due to the unique physical, optical and electrical properties. Here, we prepared few-layered Ti3C2 nanosheets by a facile two-step liquid exfoliation method and, for the first time, demonstrated their intrinsic peroxidase-like activity in a Ti3C2-TMB-H2O2 system. The as-produced Ti3C2 nanosheets, especially after histidine modification, were characterized with excellent water dispersibility, large specific surface area, and high stability, which contribute to their much higher affinity to both substrates when compared to HRP. We have also established the catalytic mechanism whereby Ti3C2 nanosheets, where Ti switched spontaneously from an oxidized to reduced state, promoted the electron transfer from TMB to H2O2. Given the color reaction of Ti3C2 nanosheets, we have fabricated a colorimetric paper-based sensor integrated with a smartphone to detect glucose and an immunoassay to detect IR-β, enabling Ti3C2 nanosheets to be a powerful tool in the biodetection field.

Simultaneous quantification of multiple biomarkers on a self-calibrating microfluidic paper-based analytic device

In this study, we developed a point-of-care assay platform with simultaneous detection and self-calibration capabilities for multiple targets based on a microfluidic paper-based analytical device (μPAD). This system is easily manufactured using a wax printing method on chromatographic paper. The design pattern consists of a zone of detection and a calibrant zone for controlled loading using wax barriers with different thicknesses. We showed the utility and applicability of this approach by a proof-of-concept study for two clinically important markers: glucose and lactate. With the naked eye, the results could be fully distinguished and recorded to evaluate the analytical performance with a flatbed scanner. The detection limits of glucose and lactate were 0.3125 mM and 0.2975 mM, respectively, and simultaneous detection was possible from a small sample (0.4 μL) with high sensitivity. Furthermore, this device has a self-calibration function, which minimizes the influence of environmental conditions (i.e., ambient light intensity, temperature, humidity, and pressure). Therefore, the developed multiplex paper-based device is promising for clinical multianalyte point-of-care testing since it is easy to manufacture, cost-effective, user-friendly, and highly sensitive.

Acceleration of Persulfate Activation by MIL-101(Fe) with Vacuum Thermal Activation: Effect of FeII/FeIII Mixed-Valence Center

In this work, the activation effect of vacuum thermal treatment on MIL-101(Fe) (MIL: Materials of Institute Lavoisier) was investigated for the first time. It demonstrated that vacuum thermal activation could accelerate the activation of persulfate (PS) by MIL-101(Fe), and the enhancement of the catalytic capacity of MIL-101(Fe) was mainly attributed to the change in the FeII/FeIII mixed-valence center. The results of the SEM and XRD showed that vacuum thermal activation had a negligible effect on the crystal structure and particle morphology of MIL-101(Fe). Meanwhile, the higher temperature of vacuum thermal activation caused a higher relative content ratio of FeII/FeIII. A widely used azo dye, X-3B, was chosen as the probe molecule to investigate the catalytic performance of all samples. The results showed that the activated samples could remove X-3B more effectively, and the sample activated at 150 °C without regeneration could effectively activate PS to remove X-3B for at least 5 runs and approximately 900 min. This work highlights the often-overlooked activation effect of vacuum thermal treatment and provides a simple way to improve the catalytic capacity and reusability of MIL-101(Fe) which is beneficial for the application of MIL-101(Fe)/PS systems in azo dye wastewater treatment.

New Single-Layered Paper-Based Microfluidic Devices for the Analysis of Nitrite and Glucose Built via Deposition of Adhesive Tape

A simple, low-cost technique has been developed for the rapid fabrication of single-layered paper-based microfluidic devices (μPADs). This technique, for the first time, made use of the deposition of patterned adhesive tape into the filter paper to construct hydrophobic barriers, with the help of toluene. Unlike other reported multi-layered μPADs that merely made use of adhesive tape as a separate layer for sealing or fluid flow controlling, the patterned adhesive tape was simultaneously dissolved and penetrated into the filter paper, which resulted in the successful transfer of the pattern from the tape to the filter paper. To demonstrate the effectiveness of this approach, nitrite and glucose were individually measured; detection limits as low as 0.015 ± 0.004 mM and 0.022 ± 0.006 mM were reported for nitrite and glucose, respectively. Multiplexed analysis of both analytes was also carried out with respective detection limits of 0.048 ± 0.005 mM and 0.025 ± 0.006 mM for nitrite and glucose. The application of the method was demonstrated by measuring nitrite and glucose in spiked artificial urine samples and satisfied recovery results were obtained.

Enhanced peroxidase-like activity of platinum nanoparticles decorated on nickel- and nitrogen-doped graphene nanotubes: colorimetric detection of glucose

A nanostructured catalyst is introduced that demonstrates peroxidase mimicking activity. It consists of nickel- and nitrogen-doped graphene nanotubes loaded with platinum nanoparticles. Pt-decorated Ni-doped nitrogen-rich graphitic nanotube (Pt/Ni@NGT) was synthesized using a two-step procedure in which the precursors were first refluxed to form a supramolecular assembly followed by a pyrolysis and leaching step to form nanotubes. Afterwards, Pt was decorated on the outer surface of nanotube by an ultrasound assisted method. Pt/Ni@NGT was characterized by XPS, TEM, SEM, and HAADF-STEM. The as-prepared Pt/Ni@NGT nanostructure was used for the detection of glucose via catalyzing the oxidation of a substrate, 3,3',5,5'-tetramethylbenzidine (TMB), to form a blue product (ox-TMB), thereby enabling colorimetric assay for enzymatically generated H₂O₂. The nanostructure exhibited excellent biocompatibility and led to highly efficient immobilization and retention of GOx. The method has a linear response in the 43 pM to 220 μM glucose concentration range, a detection limit as low as 1 pM and a limit of quantification of 3.4pM, along with good reproducibility(< 3%). A paper based visual microfluidic assay was also worked out that has an analytical range that extends from 0.1-50 mM. It is simple and rapid enough to be useful as a glucose home test.. The method was successfully applied to the determination of glucose in tear and saliva samples. Graphical abstract Graphene nanotubes doped with nitrogen and nickel (Ni@NGT) have been synthesized as the support to construct the unique Pt/Ni@NGT for providing artificial peroxidase activity for the GOx-based detection of glucose, which was further used for the construction of a glucose paper assay.

A bifunctional metal organic framework of type Fe(III)-BTC for cascade (enzymatic and enzyme-mimicking) colorimetric determination of glucose

A metal organic framework (MOF) of type Fe(III)-BTC (where BTC is 1,3,5-benzenetricarboxylic acid) was utilized to construct an integrated system for cascade colorimetric determination of glucose. The MOF performs a dual function in acting (a) as a peroxidase (POx) mimic, and (b) as a solid support for immobilization of glucose oxidase (GOx). The MOF was prepared by a one-pot method. Glucose is consumed while H₂O₂ is produced during the enzymatic oxidation by GOx. In the presence of H₂O₂, the POx mimic catalytically oxidizes 3,3',5,5'-tetramethylbenzidine (TMB) to form a blue-green product. The absorbance of oxidized TMB (measured at 652 nm) increases linearly in the 5.0-100 μM glucose concentration range, and the detection limit is 2.4 μM. The GOx@Fe-BTC MOF was successfully applied to the determination of glucose in serum. Graphical abstract Schematic presentation of a bifunctional metal organic framework of type Fe-BTC for cascade (enzymatic and enzyme-mimicking) colorimetric determination of glucose. The Fe-BTC performs a dual function in acting as both a peroxidase mimic and support for immobilizing glucose oxidase. Using the integrated enzyme, a colorimetric method was successfully applied to one-step detection of glucose in human serum.