A facile colorimetric sensor for ultrasensitive and selective detection of Lead(II) in environmental and biological samples based on intrinsic peroxidase-mimic activity of WS2 nanosheets

Transforming crystal structures of cobalt molybdate to generate electron-rich sites for electrochemical detection of Pb(II)

BACKGROUND

Most of the investigations on distinct crystal structures of catalysts are individually focused on the difference of surface functional groups or adsorption properties, but rarely explore the changes of active sites to affect the electrocatalytic performance. Catalysts with diverse crystal structures had been applied to modified electrodes in different electrocatalytic reactions. However, there is currently a lack of an essential understanding for the role of real active sites in catalysts with crystalline structures in electroanalysis, which is crucial for designing highly sensitive sensing interfaces.

RESULTS

Herein, cobalt molybdate with divergent crystal structures (α-CoMoO4 and β-CoMoO4) were synthesized by adjusting the calcination temperature, indicating that α-CoMoO4 (800 °C) (60.00 μA μM-1) had the highest catalytic ability than β-CoMoO4 (700 °C) (38.68 μA μM-1) and α-CoMoO4 (900 °C) (29.55 μA μM-1) for the catalysis of Pb(II). It was proved that the proportion of Co(II) and Mo(IV) as electron-rich sites in α-CoMoO4 (800 °C) were higher than β-CoMoO4 (700 °C) and α-CoMoO4 (900 °C), possessing more electrons to participate in the valence cycles of Co(II)/Co(III) and Mo(IV)/Mo(VI) to boost the catalytic reduction of Pb(II). Specifically, Co(II) transferred a part of electrons to Mo(VI), promoting the formation of Mo(IV). Co(II) and Mo(IV), as the electron-rich sites, providing electrons to Pb(II), further accelerating the conversion of Pb(II) into Pb(0).

SIGNIFICANCE

In the process of detecting Pb(II), the CoMoO4 structures under different temperatures have distinct content of electron-rich sites Co(II) and Mo(IV). α-CoMoO4 (800 °C), with the highest content are benefited to detect Pb(II). This work is conducive to understanding the effect of the changes of active sites resulting from crystal transformation on the electrocatalytic performance, and provides a way to construct sensitive electrochemical interfaces of distinct active sites.

Recent advances and trends in innovative biosensor-based devices for heavy metal ion detection in food

Low-cost, reliable, and efficient biosensors are crucial in detecting residual heavy metal ions (HMIs) in food products. At present, based on distance-induced localized surface plasmon resonance of noble metal nanoparticles, enzyme-mimetic reaction of nanozymes, and chelation reaction of metal chelators, the constructed optical sensors have attracted wide attention in HMIs detection. Besides, based on the enrichment and signal amplification strategy of nanomaterials on HMIs and the construction of electrochemical aptamer sensing platforms, the developed electrochemical biosensors have overcome the plague of low sensitivity, poor selectivity, and the inability of multiplexed detection in the optical strategy. Moreover, along with an in-depth discussion of these different types of biosensors, a detailed overview of the design and application of innovative devices based on these sensing principles was provided, including microfluidic systems, hydrogel-based platforms, and test strip technologies. Finally, the challenges that hinder commercial application have also been mentioned. Overall, this review aims to establish a theoretical foundation for developing accurate and reliable sensing technologies and devices for HMIs, thereby promoting the widespread application of biosensors in the detection of HMIs in food.

Mechanistic investigation and dual-mode colorimetric-chemiluminescent detection of glyphosate based on the specific inhibition of Fe3O4@Cu nanozyme peroxidase-like activity

In this study, a dual-mode colorimetric/CL nanosensor was developed for glyphosate detection based on the specific inhibition of Fe3O4@Cu peroxidase-like activity. Synthesized Fe3O4@Cu exhibited high levels of peroxidase-like activity that triggered the oxidation of luminol/3,3',5,5'-tetramethyl benzidine dihydrochloride (TMB) to excited-state 3-aminophthalic acid/blue oxTMB, thereby delivering a CL signal/visible colorimetric signal, however, the presence of glyphosate inhibited this activity, resulting in a decrease in signal strength. In-depth investigation revealed that this inhibitory mechanism occurs via two pathways: one in which glyphosate chelates with Fe(III)/Cu(II) and occupy the catalytical active sites of Fe3O4@Cu, thereby decreasing the generation of OH, and another in which glyphosate competes with TMB to consume generated OH, thus reducing the oxidation of TMB. This mechanism formed the basis of our novel dual-mode colorimetric/CL glyphosate nanosensor, which achieved limits of detection (LODs) of 0.086 µg/mL and 0.019 µg/mL in tests, thus demonstrating its significant potential for on-site glyphosate monitoring.

Sensing lead ions in water: a comprehensive review on strategies and sensor materials

It is well-known fact that elevated lead ions (Pb2+), the third most toxic among heavy metal ions in aqueous systems, pose a threat to human health and aquatic ecosystems when they exceed permissible limits. Pb2+ is commonly found in industrial waste and fertilizers, contaminating water sources and subsequently entering the human body, causing various adverse health conditions. Unlike being expelled, Pb2+ accumulates within the body, posing potential health risks. The harmful impact of presence of Pb2+ in water have prompted researchers to diligently work toward maintaining water quality. Recognizing the importance of Pb2+, this review article makes a sincere and effective effort to address the issues associated with Pb2+. This overview article gives insights into various sensing approaches to detect Pb2+ in water using different sensing materials, including 2-dimensional materials, thiols, quantum dots, and polymers. Herein, different sensing approaches such as electrochemical, optical, field effect transistor-based, micro-electromechanical system-based, and chemi resistive are thoroughly explained. Field effect transistor-based and chemiresistive work on similar principles and are compared on the basis of their fabrication processes and sensing capabilities. In conclusion, future directions for chemiresistive sensors in Pb2+ detection are proposed, emphasizing their simplicity, portability, straightforward functionality, and ease of fabrication. Notably, it sheds light on various thiol and ligand compounds and coupling strategies utilized in Pb2+ detection. This comprehensive study is expected to benefit individuals engaged in Pb2+ detection.

Tunable Structured 2D Nanobiocatalysts: Synthesis, Catalytic Properties and New Horizons in Biomedical Applications

2D materials have offered essential contributions to boosting biocatalytic efficiency in diverse biomedical applications due to the intrinsic enzyme-mimetic activity and massive specific surface area for loading metal catalytic centers. Since the difficulty of high-quality synthesis, the varied structure, and the tough choice of efficient surface loading sites with catalytic properties, the artificial building of 2D nanobiocatalysts still faces great challenges. Here, in this review, a timely and comprehensive summarization of the latest progress and future trends in the design and biotherapeutic applications of 2D nanobiocatalysts is provided, which is essential for their development. First, an overview of the synthesis-structure-fundamentals and structure-property relationships of 2D nanobiocatalysts, both metal-free and metal-based is provided. After that, the effective design of the active sites of nanobiocatalysts is discussed. Then, the progress of their applied research in recent years, including biomedical analysis, biomedical therapeutics, pharmacokinetics, and toxicology is systematically highlighted. Finally, future research directions of 2D nanobiocatalysts are prospected. Overall, this review to provide cutting-edge and multidisciplinary guidance for accelerating future developments and biomedical applications of 2D nanobiocatalysts is expected.

Facile colorimetric sensor using oxidase-like activity of octahedral Ag2O particles for highly selective detection of Pb(II) in water

Pb(II) is a prevalent heavy metal ion classified as a 2B carcinogen. Excessive intake of Pb(II) in the human body can damage the central nervous system, kidneys, liver, and immune system, leading to permanent brain damage, anemia, and cancer. Colorimetry can be applied to rapidly determine Pb(II) residues, but there are still many challenges in the accuracy and sensitivity of detection. Based on the inhibitory impact of Pb(II) on the oxidase-like activity of octahedral silver oxide (Ag2O), a colorimetric sensor with smartphone-assisted analysis for the Pb(II) detection was first developed. Herein, it has been found that Pb(II) can adsorb onto the surface of octahedral Ag2O, hindering the production of O2- in the reaction system. This ultimately results in the suppression of oxidase-like activity, leading to a lighter purple appearance of the colorimetric reaction solution. The sensor exhibits a high degree of sensitivity and a limit of detection (LOD) for Pb(II) was calculated as 2.2 μg L-1. Hence, the developed colorimetric sensor with high sensitivity, excellent specificity, and high tolerance to sodium ions is hopeful to have practical applications in Pb(II) detection in environmental water samples. Moreover, the sensor will provide a novel strategy for heavy metal ion detection and other substances.

Recent Advances in Design and Application of Nanomaterials-Based Colorimetric Biosensors for Agri-food Safety Analysis

A colorimetric sensor detects an analyte by utilizing the optical properties of the sensor unit, such as absorption or reflection, to generate a structural color that serves as the output signal to detect an analyte. Detecting the refractive index of an analyte by recording the color change of the sensor structure on its surface has several advantages, including simple operation, low cost, suitability for onsite analysis, and real-time detection. Colorimetric sensors have drawn much attention owing to their rapidity, simplicity, high sensitivity and selectivity. This Review discusses the use of colorimetric sensors in the food industry, including their applications for detecting food contaminants. The Review also provides insight into the scope of future research in this area.

Reactive Template-Engaged Synthesis of NiSx/MoS2 Nanosheets Decorated on Hollow and Porous Carbon Microtubes with Optimal Electronic Modulation toward High-Performance Enzyme-like Performance

As a promising cost-effective nanozyme, MoS₂ nanosheets (NSs) have been considered as a good candidate for the enzyme-like catalysis. However, their catalytic activity is still restricted by the insufficient active sites and poor conductivity, and thus, the comprehensive performances are still unsatisfactory. To address these issues, herein, we design and fabricate an intelligent tubular nanostructure of hierarchical hollow nanotubes, which are assembled by NiS_x/MoS₂ NSs encapsulated into N-doped carbon microtubes (NiS_x/MoS₂@NCMTs). The N-doped carbon microtubes (NCMTs) serve as a conductive skeleton, integrating with NiS_x/MoS₂ NSs and ensuring their well-distribution, thereby maximally exposing more active sites. Additionally, the tube-like structure is favorable for increasing the mass transfusion to ensure their excellent catalytic performance. Profiting from their component and structural advantages, the obtained NiS_x/MoS₂@NCMTs exhibit a surprisingly enhanced enzyme-like activity. Based on these, a facile colorimetric sensing platform to detect H₂O₂ and GSH has been developed. This proposed approach can be expected to synthesize a series of tubular heterostructured MoS₂-based composites, which will be widely applied in catalysis, energy storage, disease diagnosis, etc.

A novel TMD-based peroxidase-mimicking nanozyme: From naked eye detection of leukocytosis-related diseases to sensing different bioanalytes

Being emerged as alternatives to natural enzymes, nanozymes have recently drawn much attention in sensing. Herein, the first multicomponent transition metal dicalchogenide (TMD)-based nanozyme (MCFS/rGO) was synthesized by a facile hydrothermal method and characterized. This peroxidase-mimic nanozyme follows the typical Michaelis-Menten kinetics, showing a higher affinity for H₂O₂ substrate (K_m = 9 μM) compared to that of natural peroxidase (K_m = 3700 μM). The remarkable potential of the MCFS/rGO nanozyme to detect H₂O₂ provided us with a great opportunity to design some simple and fast colorimetric sensing systems. Coupling the efficient peroxidase-mimicking activity of the nanozyme with the H₂O₂ production capacity of white blood cells (WBCs) leads to the development of a novel, simple, rapid, and efficient colorimetric method to distinguish leukocytosis-related patients from healthy people by the naked eye. This pioneering diagnostic technique can also be utilized to quantitatively measure the WBC count. Moreover, we coupled the mentioned nanozyme-based system with the activity of glucose oxidase enzyme available in different types of honey samples, an innovative mechanism proved to be an effective quality indicator of the samples. Last but not least, the MCFS/rGO nanozyme is also able to determine the quantity of some biologically significant analytes, including glutathione (GSH), ascorbic acid (AA), and mercury ions (Hg²⁺), of which the limit of detection (LOD) was 9.3 nM, 22.5 nM, and 0.32 μM, respectively. Our results, however, demonstrated the superior performance of the MCFS/rGO nanozyme to determine the first two mentioned bioanalytes compared with other TMDs. Overall, this novel nanozyme-based sensor system can be considered a suitable candidate for developing multipurpose biosensors for medical and biochemical applications.

Downregulation of Peroxidase Activity of Platinum Cube Enables Minute-Time Scale Colorimetric Signaling of Hypoxanthine for Fish Freshness Monitoring

Due to its unique biological composition, aquatic products, especially fish, are extremely perishable compared to other muscle products. Herein, we proposed an artificial nanozyme-based colorimetric detection of hypoxanthine (Hx), the indicator of fish freshness, in a minute-time scale without the assistance of a natural enzyme (hypoxanthine oxidase). The principle is based on the interaction between Hx and polyvinylpyrrolidone-modified platinum cubic nanomaterials (PVP-PtNC), in which the catalytic active sites of PVP-PtNC's surface were blocked by Hx. This causes the downregulation of PVP-PtNC's catalytic ability and weakened its ability to catalyze the oxidization of 3,3',5,5'-Tetramethylbenzidine (TMB) by H₂O₂. Accordingly, the decrease in the UV-vis absorption and the weakening of the colorimetric reaction color is proportional to the Hx concentration. On this basis, a target-triggered colorimetric method for detecting Hx is developed for fish freshness monitoring with a fast detection speed, low cost, high accuracy, and simplified operation. Experiments reveal that the correlation response of Hx is from 0.5 μM to 10 mM with a limit of detection of 0.16 μM. In particular, the Hx detected from real fish indicates that the method possesses a promising potential for practical application. All of these features are expected to promote the development of online detection tools for food safety monitoring.

Nanozyme-based pollutant sensing and environmental treatment: Trends, challenges, and perspectives

Nanozymes are defined as nanomaterials exhibiting enzyme-like properties, and they possess both catalytic functions and nanomaterial's unique physicochemical characteristics. Due to the excellent stability and improved catalytic activity in comparison to natural enzymes, nanozymes have established a wide base for applications in environmental pollutants monitoring and remediation. Nanozymes have been applied in the detection of heavy metal ions, molecules, and organic compounds, both quantitatively and qualitatively. Additionally, within the natural environment, nanozymes can be employed for the degradation of organic and persistent pollutants such as antibiotics, phenols, and textile dyes. Further, the potential sphere of applications for nanozymes traverses from indoor air purification to anti-biofouling agents, and even they show promise in combatting pathogenic bacteria. However, nanozymes may have inherent toxicity, which can restrict their widespread utility. Thus, it is important to evaluate and monitor the interaction and transformation of nanozymes towards biosphere damage when employed within the natural environment in a cradle-to-grave manner, to assure their utmost safety. In this context, various studies have concluded that the green synthesis of nanozymes can efficiently overcome the toxicity limitations in real life applications, and nanozymes can be well utilized in the sensing and degradation of several toxic pollutants including metal ions, pesticides, and chemical warfare agents. In this seminal review, we have explored the great potential of nanozymes, whilst addressing a range of concerns, which have often been overlooked and currently restrict widespread applications and commercialization of nanozymes.

Determination Methods of the Risk Factors in Food Based on Nanozymes: A Review

Food safety issues caused by foodborne pathogens, chemical pollutants, and heavy metals have aroused widespread concern because they are closely related to human health. Nanozyme-based biosensors have excellent characteristics such as high sensitivity, selectivity, and cost-effectiveness and have been used to detect the risk factors in foods. In this work, the common detection methods for pathogenic microorganisms, toxins, heavy metals, pesticide residues, veterinary drugs, and illegal additives are firstly reviewed. Then, the principles and applications of immunosensors based on various nanozymes are reviewed and explained. Applying nanozymes to the detection of pathogenic bacteria holds great potential for real-time evaluation and detection protocols for food risk factors.

Metal–Organic Frameworks for Electrocatalytic Sensing of Hydrogen Peroxide

The electrochemical detection of hydrogen peroxide (H₂O₂) has become more and more important in industrial production, daily life, biological process, green energy chemistry, and other fields (especially for the detection of low concentration of H₂O_2). Metal organic frameworks (MOFs) are promising candidates to replace the established H₂O₂ sensors based on precious metals or enzymes. This review summarizes recent advances in MOF-based H₂O₂ electrochemical sensors, including conductive MOFs, MOFs with chemical modifications, MOFs-composites, and MOF derivatives. Finally, the challenges and prospects for the optimization and design of H₂O₂ electrochemical sensors with ultra-low detection limit and long-life are presented.

A liquid colorimetric chemosensor for ultrasensitive detection of glyphosate residues in vegetables using a metal oxide with intrinsic peroxidase catalytic activity

The control of pesticide residues in food is of increasing importance nowadays due to the over-use and misapplication of herbicides in agricultural production. However, the current colorimetric method for rapid detection of glyphosate still faces many challenges like the low sensitivity and stability. Herein, a simple and ultrasensitive liquid colorimetric chemosensor for glyphosate detection was successfully constructed. Glyphosate pesticide can interact with metallic oxidelike porous Co3O4 nanodisc, and inhibit its inherent peroxidase-mimicking activity, making the colour of the solution change from blue to light blue or even colourless. The colour variation of the colorimetric chemosensor enables us to easily distinguish in less than 20 min even by the naked eye whether glyphosate exceeds the allowable level. The limit of detection (LOD) of the chemosensor for glyphosate was calculated as low as 2.37 μg·L-1, and the chemosensor displays excellent selectivity against other competitive pesticides and metal ions. Further studies have also validated the applicability of the colorimetric chemosensor in actual samples like tomato, cucumber and cabbage, indicating that the proposed strategy may have promising application prospects for detecting glyphosate residues in agricultural products.

2D material-based peroxidase-mimicking nanozymes: catalytic mechanisms and bioapplications

The boom in nanotechnology brings new insights into the development of artificial enzymes (nanozymes) with ease of modification, lower manufacturing cost, and higher catalytic stability than natural enzymes. Among various nanomaterials, two-dimensional (2D) nanomaterials exhibit promising enzyme-like properties for a plethora of bioapplications owing to their unique physicochemical characteristics of tuneable composition, ultrathin thickness, and huge specific surface area. Herein, we review the recent advances in several 2D material-based nanozymes, such as carbonaceous nanosheets, metal-organic frameworks (MOFs), transition metal dichalcogenides (TMDs), layered double hydroxides (LDHs), and transition metal oxides (TMOs), clarify the mechanisms of peroxidase (POD)-mimicking catalytic behaviors, and overview the potential bioapplications of 2D nanozymes.

Signal amplification of SiO2 nanoparticle loaded horseradish peroxidase for colorimetric detection of lead ions in water

In this work, we developed an aptamer-based optical assay for the analysis of Pb²⁺, a hazardous heavy metal that may be present in the food chain and harmful to human health. An aptamer targeted against Pb²⁺ was immobilized onto the microplate as the capture probe. SiO₂ nanoparticles (NPs) were synthesized and used as carriers of the signaling horseradish peroxidase (HRP) to achieve amplification of the optical signal. Complementary DNA (cDNA) of the aptamer was also linked to the above mentioned SiO₂ nanoparticle (NPs) as the signal probe. The aptamers were found to be able to capture Pb²⁺, and the unbound aptamers were subsequently hybridized with cDNA-HRP-SiO₂ conjugates. As a result, the addition of TMB-H₂O₂ promoted the formation of blue products in the catalytic system. The assay adopting SiO₂ NPs as an enhancer resulted in higher sensitivity with an LOD of 2.5 nM compared to normal procedures. The feasibility of the aptamer-based colorimetric assay was verified by successful detection of Pb²⁺ in water samples with recoveries in the range of 97.4-103.52%.

Transition Metal Dichalcogenides (TMDC)-Based Nanozymes for Biosensing and Therapeutic Applications

Nanozymes, a type of nanomaterial with enzyme-like properties, are a promising alternative to natural enzymes. In particular, transition metal dichalcogenides (TMDCs, with the general formula MX2, where M represents a transition metal and X is a chalcogen element)-based nanozymes have demonstrated exceptional potential in the healthcare and diagnostic sectors. TMDCs have different enzymatic properties due to their unique nano-architecture, high surface area, and semiconducting properties with tunable band gaps. Furthermore, the compatibility of TMDCs with various chemical or physical modification strategies provide a simple and scalable way to engineer and control their enzymatic activity. Here, we discuss recent advances made with TMDC-based nanozymes for biosensing and therapeutic applications. We also discuss their synthesis strategies, various enzymatic properties, current challenges, and the outlook for future developments in this field.

Tannic Acid-Capped Gold Nanoparticles as a Novel Nanozyme for Colorimetric Determination of Pb2+ Ions

In this study, tannic acid-modified gold nanoparticles were found to have superior nanozyme activity and catalyze the oxidation reaction of 3,3′,5,5′-tetramethylbenzidine in the presence of hydrogen peroxide. Enhancing the catalytic activity of the nanozyme by Pb2+ ions caused by selectively binding metal ions by the tannic acid-capped surface of gold nanoparticles makes them an ideal colorimetric probe for Pb2+. The parameters of the reaction, including pH, incubation time, and concentration of components, were optimized to reach maximal sensitivity of Pb2+ detection. The absorption change is directly proportional to the Pb2+ concentration and allows the determination of Pb2+ ions within 10 min. The colorimetric sensor is characterized by a wide linear range from 25 to 500 ng×mL−1 with a low limit of detection of 11.3 ng×mL−1. The highly sensitive and selective Pb2+ detection in tap, drinking, and spring water revealed the feasibility and applicability of the developed colorimetric sensor.

Porous Co3O4 nanodisks as robust peroxidase mimetics in an ultrasensitive colorimetric sensor for the rapid detection of multiple heavy metal residues in environmental water samples

The current method for rapid and ultrasensitive detection of multiple heavy metals in environmental water still face challenge. Herein, the porous Co₃O₄ nanodisks with robust peroxidase-mimicking activity were prepared, and its catalytic activity can be significantly inhibited by the heavy metals like Cd(II), Hg(II), Pb(II) and As, which makes us to establish an ultrasensitive and rapid colorimetric sensor for the detection of multiple heavy metals. Further investigation reveals the anticompetitive inhibition effect of heavy metals on peroxidase-mimicking activity. The colorimetric sensor displays excellent sensitivity and selectivity, and the limits of detection (LOD) for Cd(II), Hg(II), Pb(II) and As are 0.085 μg·L^-1, 0.19 μg·L^-1, 0.2 μg·L^-1 and 0.156 μg·L^-1, respectively. Notably, the absorbance variation will be greater than 0.5 as the concentration of heavy metals exceeds 5 μg·L^-1, which can be clearly discriminated by the naked eyes. Moreover, the average recovery range of heavy metals in actual water samples is from 86.9% to 98.3%. The above results indicate that the proposed sensor exhibits excellent practical applicability for the rapid and ultrasensitive detection of multiple harmful heavy metals in several environmental water samples, which has potential bright application in protecting the environment and human health.

Therapeutic Applications of Nanozymes in Chronic Inflammatory Diseases

Since the discovery of horseradish peroxidase-like activity of magnetite nanoparticles in 2007, many researchers have investigated different types of nanoparticles that show enzyme-like activities, namely, nanozymes. Nanozymes possess high efficiency, stability, and low production costs compared to natural enzymes. Thus, nanozymes have already been widely studied in various domains including medical science, food industry, chemical engineering, and agriculture. This review presents the utilization of nanozymes in medicine and focuses particularly on their therapeutic applications in chronic inflammatory diseases because of their antioxidant-like activity. Furthermore, the treatment of chronic inflammatory diseases with nanozymes of different materials was introduced emphatically.

G-quadruplex DNAzyme as peroxidase mimetic in a colorimetric biosensor for ultrasensitive and selective detection of trace tetracyclines in foods

The colorimetric method can determine the initial results even by the naked eyes, but its main challenge for antibiotics detection in food at present is the relatively low sensitivity. Herein, an ultrasensitive colorimetric biosensor based on G-quadruplex DNAzyme was firstly proposed for the rapid detection of trace tetracycline antibiotics like tetracycline, oxytetracycline, chlortetracycline and doxycycline. DNAzyme composed of hemin and G-quadruplex has peroxidase-like activity, and tetracyclines can combine with hemin to form a stable complex and reduce catalytic activity, making the color of solution changes from yellow to green. The limits of detection (LOD) of the proposed colorimetric biosensor for tetracyclines is determined as low as 3.1 nM, which is lower than most of the other colorimetric methods for antibiotics detection. Moreover, the average recovery range of tetracyclines in actual samples is from 89% to 99%, indicating that such strategy may has bright application prospects for tetracyclines detection in foods.

Development of a microplate-format direct optode sensor for ultra-high-throughput environmental and wastewater monitoring of Pb2

Although many Pb²⁺-selective optodes have been developed so far, methods using optical sensor membranes have not become widespread in environmental analytical practice. In order to create a bulk optode sensor, which can overcome all of the main drawbacks in the application of conventional optode membranes, - i.e., pH-dependence, long response time and the leakage of the ionic components - unusually thick PVC membrane was developed, embedded in microtiter plates and operated on a novel concept. This is the first reported work, which applies a plate-format optode as well as a direct optode-type sensing membrane for determination of Pb²⁺. We reported here also the first example for the application of an ionic component-free bulk optode membrane to avoid the membrane leakage, improve the regenerability and extend the lifetime of the membrane. The reported sensor has a LOD above 4.0 × 10^-7 M (∼83 μg L^-1), thus it is unsuitable for the effective monitoring of drinking waters, but considered to be a promising method for monitoring contamination episodes. On the other hand, the widest pH-independent working range of 4.3 < pH < 7.0 among bulk optodes reported in the literature was realized and an unprecedentedly fast response time of <10 s was achieved. The effectiveness of the applied method was investigated by measuring Pb²⁺-spiked multicomponent aqueous solutions as simulated environmental or wastewater samples containing near equimolar amounts of Ag⁺, Ca²⁺, Co²⁺, Cu²⁺, K⁺, Mg²⁺, Na⁺ and Zn²⁺ as acetate salts. In the presence of these potential competing ions with a concentration not greater than the typical ionic strength of surface freshwaters (∼10^-3 M) the reported sensor proved to be appropriate for the selective detection of Pb²⁺ without any preparation of the samples (e.g., preconcentration, buffering, addition of excipients, etc.) with a required sample volume of only 100 μL. An outstanding analytical performance could be achieved within an average time of less, than 5 s/sample. The reported fluorescent probe is considered to be a promising method for replacing atomic absorption spectroscopy- (AAS), anodic stripping voltammetry- (ASV) or inductively coupled plasma- (ICP) based techniques as well as conventional ion-selective bulk membranes in high-throughput preliminary environmental monitoring of Pb²⁺, as it provides a cheap and unprecedentedly fast qualitative analysis of contaminated surface and wastewaters.

A novel liquid colorimetric probe for highly selective and sensitive detection of lead (II)

A novel liquid colorimetric sensor based on deep eutectic solvent (DES) was developed for the preconcentration and detection of Pb²⁺ in fruit juice, milk and cereal samples. The colorimetric probe was simply fabricated by adding dithizone (DZ) into DES, prepared from choline chloride and phenol. Pb²⁺ was formed complex with DZ in the probe, providing hydrophobic complex of [Pb-DZ] which was simultaneously extracted into DES and the color was changed from light orange to carmine red. The enriched [Pb-DZ] in DES was detected using spectrophotometer at 520 nm and naked-eyes. In addition, a smartphone in combination with an Image J program was used as an alternative detection device. Under optimal conditions, the enrichment factor was 92 with LOD of 2.1 µg L^-1 and the linear range was 0.007-0.075 mg L^-1. The proposed liquid colorimetric sensor was successfully applied for Pb²⁺ detection in various food samples and the results were in good agreement with those obtained by FAAS. The advantages of this method are simple, rapid, environmental friendly and low cost.

Colorimetric Detection of Kanamycin Residue in Foods Based on the Aptamer-Enhanced Peroxidase-Mimicking Activity of Layered WS2 Nanosheets

Although the colorimetric methods can easily meet the demands of point-of-care and ease-of-use for antibiotic detection, they still face many challenges in the accuracy and stability of assay. Herein, a facile and stable colorimetric aptasensor is first developed for kanamycin residue detection based on the aptamer-enhanced peroxidase-mimicking activity of layered WS₂ nanosheets. The investigation confirmed that aptamer sequences can improve the affinity of nanosheets to the chromogenic substrate 3,3'',5,5''-tetramethylbenzidine, resulting in a significant increase of the peroxidase-mimicking activity. Under the optimal conditions, the limit of detection of the proposed colorimetric aptasensor for kanamycin was determined to be as low as 0.6 μM, and such an aptasensor displays excellent selectivity against other competitive antibiotics. Moreover, further studies have verified the applicability of the established colorimetric aptasensor in several actual samples, indicating that the aptasensor may have bright application prospects for kanamycin detection in livestock husbandry and agriculture samples.

Nanozymes for Environmental Pollutant Monitoring and Remediation

Nanozymes are advanced nanomaterials which mimic natural enzymes by exhibiting enzyme-like properties. As nanozymes offer better structural stability over their respective natural enzymes, they are ideal candidates for real-time and/or remote environmental pollutant monitoring and remediation. In this review, we classify nanozymes into four types depending on their enzyme-mimicking behaviour (active metal centre mimic, functional mimic, nanocomposite or 3D structural mimic) and offer mechanistic insights into the nature of their catalytic activity. Following this, we discuss the current environmental translation of nanozymes into a powerful sensing or remediation tool through inventive nano-architectural design of nanozymes and their transduction methodologies. Here, we focus on recent developments in nanozymes for the detection of heavy metal ions, pesticides and other organic pollutants, emphasising optical methods and a few electrochemical techniques. Strategies to remediate persistent organic pollutants such as pesticides, phenols, antibiotics and textile dyes are included. We conclude with a discussion on the practical deployment of these nanozymes in terms of their effectiveness, reusability, real-time in-field application, commercial production and regulatory considerations.

A review on metal nanozyme-based sensing of heavy metal ions: Challenges and future perspectives

Large scale mining, manufacturing industries, exploitation of underground water, depletion of groundwater level, and uncontrolled discharge of industrial wastes have caused severe heavy metal ion pollution to the environment throughout the world. Therefore, the rapid detection of such toxic metal ions is inevitable. However, conventional methods require sophisticated instruments and skilled manpower and are difficult to operate in on-field conditions. Recently, metal nanozyme-based assays have been found to have the potential as an alternative to conventional methods due to their portability, simplicity, and high sensitivity to detect metal ion concentration to as low as parts per trillion (ppt). Metal nanozyme-based systems for heavy metal ions enable rapid and cheap screening on the spot with a very simple instrument such as a UV-vis absorption spectrophotometer and therefore, are convenient for use in field operations, especially in remote parts of the world. The sensing mechanism of a nanozyme-based sensor is highly dependent on its surface properties and specific interactions with particular metal ion species. Such method often encounters selectivity issues, unlike natural enzyme-based assays. Therefore, in this review, we mainly focus our discussion on different types of target recognition and inhibition/enhancement mechanisms, and their responses toward the catalytic activity in the sensing of target metal ions, design strategies, challenges, and future perspectives.

Two-Dimensional Nanomaterials With Enzyme-Like Properties for Biomedical Applications

Recently, remarkable progress has been made in nanozyme research due to the rapid development of nanomaterials. Two-dimensional nanomaterials such as metal nanosheets, graphene-based materials, transition metal oxides/dichalcogenides, etc., provide enhanced physical and chemical functionality owing to their ultrathin structures, high surface-to-volume ratios, and surface charges. They have also been found to have high catalytic activities in terms of natural enzymes such as peroxidase, oxidase, catalase, and superoxide dismutase. This review provides an overview of the recent progress of nanozymes based on two-dimensional nanomaterials, with an emphasis on their synthetic strategies, hybridization, catalytic properties, and biomedical applications. Finally, the future challenges and prospects for this research are discussed.