Single-Atom Enzyme-Functionalized Solution-Gated Graphene Transistor for Real-Time Detection of Mercury Ion

Exploring Single-Atom Nanozymes Toward Environmental Pollutants: Monitoring and Control

As environmental pollutants pose a serious threat to socioeconomic and environmental health, the development of simple, efficient, accurate and cost-effective methods for pollution monitoring and control remains a major challenge, but it is an unavoidable issue. In the past decade, the artificial nanozymes have been widely used for environmental pollutant monitoring and control, because of their low cost, high stability, easy mass production, etc. However, the conventional nanozyme technology faces significant challenges in terms of difficulty in regulating the exposed crystal surface, complex composition, low catalytic activity, etc. In contrast, the emerging single-atom nanozymes (SANs) have attracted much attention in the field of environmental monitoring and control, due to their multiple advantages of atomically dispersed active sites, high atom utilization efficiency, tunable coordination environment, etc. To date, the insufficient efforts have been made to comprehensively characterize the applications of SANs in the monitoring and control of environmental pollutants. Building on the recent advances in the field, this review systematically summarizes the main synthesis methods of SANs and highlights their advances in the monitoring and control of environmental pollutants. Finally, we critically evaluate the limitations and challenges of SANs, and provide the insights into their future prospects for the monitoring and control of environmental pollutants.

Chiral plasmonic Au@Pt nanoparticles for detection of H2O2 and Hg2+ and enantiomeric differentiation

Chiral Au@Pt nanoparticles (NPs) with optically plasmonic and catalytic active surfaces were sustainably prepared to serve as label-free surface enhanced Raman scattering (SERS) platform to distinguish D- and L-enantiomers of alanine and tartaric acid. Surface morphologies were characterized by high-angle annular dark-field imaging-scanning transmission electron microscopy (HADDF-STEM) and selected area energy diffraction (SAED) patterns. The amounts of Pt on chiral Au NPs were estimated by the inductively coupled plasma-optical emission spectrometer (ICP-OES) and X-ray diffraction (XRD). The versatile applications including the environmental hazardous species monitoring and the fundamental enantiomeric differentiation are demonstrated for the novelty of prepared materials. Au@Pt NPs were used as a unique platform of H2O2 and Hg2+ concentrations using UV-Vis absorption and surface enhanced Raman scattering (SERS). The chiral Au@Pt NP platform may provide an efficient approach to achieve chiral nanocatalysis for numerous chemical reactions.

Materials Containing Single-, Di-, Tri-, and Multi-Metal Atoms Bonded to C, N, S, P, B, and O Species as Advanced Catalysts for Energy, Sensor, and Biomedical Applications

Modifying the coordination or local environments of single-, di-, tri-, and multi-metal atom (SMA/DMA/TMA/MMA)-based materials is one of the best strategies for increasing the catalytic activities, selectivity, and long-term durability of these materials. Advanced sheet materials supported by metal atom-based materials have become a critical topic in the fields of renewable energy conversion systems, storage devices, sensors, and biomedicine owing to the maximum atom utilization efficiency, precisely located metal centers, specific electron configurations, unique reactivity, and precise chemical tunability. Several sheet materials offer excellent support for metal atom-based materials and are attractive for applications in energy, sensors, and medical research, such as in oxygen reduction, oxygen production, hydrogen generation, fuel production, selective chemical detection, and enzymatic reactions. The strong metal-metal and metal-carbon with metal-heteroatom (i.e., N, S, P, B, and O) bonds stabilize and optimize the electronic structures of the metal atoms due to strong interfacial interactions, yielding excellent catalytic activities. These materials provide excellent models for understanding the fundamental problems with multistep chemical reactions. This review summarizes the substrate structure-activity relationship of metal atom-based materials with different active sites based on experimental and theoretical data. Additionally, the new synthesis procedures, physicochemical characterizations, and energy and biomedical applications are discussed. Finally, the remaining challenges in developing efficient SMA/DMA/TMA/MMA-based materials are presented.

Efficient 2D MOFs nanozyme combining with magnetic SERS substrate for ultrasensitive detection of Hg2

Biomimetic inorganic nanoenzyme is a kind of nanomaterial with long-term stability, easy preparation and low cost, which could instead of natural biological enzyme. Metal-organic framework (MOFs) as effectively nanoenzyme was attracted more attention for the adjustability and large specific surface area. This design is based on the catalase-like catalytic activity of 2D metal-organic frameworks (MOFs) and the high sensitivity of surface enhanced Raman spectroscopy (SERS) biosensors to construct a novel SERS biosensor capable of efficiently detecting mercury (Hg2+). In this study, 2D MOFs nanozyme was instead of 3D structure with more effecient catalytic site, which can catalyze o-Phenylenediamine (OPD) to OPDox with the assistance of H2O2. Besides, a magnetic composite nanomaterial Fe3O4@Ag@OPD was prepared as a signal carrier. In the presence of Hg2+, T-Hg2+-T base pairs were used to connect the two materials to realize Raman signal change. Based on this principle, the SERS sensor can realize the sensitive detection of Hg2+, the detection range is 1.0 × 10-12 ∼ 1.0 × 10-2 mol‧L-1, and the detection limit is 1.36 × 10-13 mol‧L-1. This method greatly improves the reliability of SERS sensor for detecting the target, and provides a new idea for detecting metal ions in the environment.

Fe Single-Atom Nanozyme-Modified Wearable Hydrogel Patch for Precise Analysis of Uric Acid at Rest

Resting sweat analysis could provide unique insight into the metabolic levels of physiological and pathological states. However, the low secretion rate of resting sweat and the low concentration of metabolic molecules pose challenges for the development of noninvasive wearable sensors. Here, we demonstrated a wearable patch for the precise analysis of uric acid at rest. Fe single-atom nanozymes (FeSAs) with excellent electrocatalytic activity were used to develop a sensor for selective catalysis of uric acid (UA, 1-425 μM), and the catalytic mechanism of UA was later explored by density functional theory. In addition, polyaniline was integrated into the wearable patch for pH detection; thus, accurate analysis of sweat UA molecules can be achieved by pH calibration. Then, we explored the possibility of collecting resting sweat with different ratios of agarose hydrogels to reduce the sweat accumulation time. Finally, the possibility of a wearable patch for accurate UA detection in volunteer sweat samples was experimentally verified. We believe that our work provides novel insights and ideas for the analysis of resting sweat using wearable devices, further driving advancements in the field of personalized medicine.

An Electrochemical Sensor Based on a Porous Biochar/Cuprous Oxide (BC/Cu2O) Composite for the Determination of Hg(II)

Mercuric ion (Hg²⁺) in aqueous media is extremely toxic to the environment and organisms. Therefore, the ultra-trace electrochemical determination of Hg²⁺ in the environment is of critical importance. In this work, a new electrochemical Hg²⁺ sensing platform based on porous activated carbon (BC/Cu₂O) modified with cuprous oxide was developed using a simple impregnation pyrolysis method. Differential pulse anodic stripping voltammetry (DPASV) was used to investigate the sensing capability of the BC/Cu₂O electrode towards Hg²⁺. Due to the excellent conductivity and large specific surface area of BC, and the excellent catalytic activity of Cu₂O nanoparticles, the prepared BC/Cu₂O electrode exhibited excellent electrochemical activity. The high sensitivity of the proposed system resulted in a low detection limit of 0.3 ng·L^-1 and a wide linear response in the ranges from 1.0 ng·L^-1 to 1.0 mg·L^-1. In addition, this sensor was found to have good accuracy, acceptable precision, and reproducibility. All of these results show that the BC/Cu₂O composite is a promising material for Hg²⁺ electrochemical detection.

Engineering Single-Atom Nanozymes for Catalytic Biomedical Applications

Nanomaterials with enzyme-mimicking properties, coined as nanozymes, are a promising alternative to natural enzymes owing to their remarkable advantages, such as high stability, easy preparation, and favorable catalytic performance. Recently, with the rapid development of nanotechnology and characterization techniques, single atom nanozymes (SAzymes) with atomically dispersed active sites, well-defined electronic and geometric structures, tunable coordination environment, and maximum metal atom utilization are developed and exploited. With superior catalytic performance and selectivity, SAzymes have made impressive progress in biomedical applications and are expected to bridge the gap between artificial nanozymes and natural enzymes. Herein, the recent advances in SAzyme preparation methods, catalytic mechanisms, and biomedical applications are systematically summarized. Their biomedical applications in cancer therapy, oxidative stress cytoprotection, antibacterial therapy, and biosensing are discussed in depth. Furthermore, to appreciate these advances, the main challenges, and prospects for the future development of SAzymes are also outlined and highlighted in this review.

Utilization of nitrogen, sulfur co-doped porous carbon micron spheres as bifunctional electrocatalysts for electrochemical detection of cadmium, lead and mercury ions and oxygen evolution reaction

The development of high-performance bifunctional electrocatalysts for oxygen evolution reaction and heavy metal ion (HMI) detection is significant and challenging. Here, a novel nitrogen, sulfur co-doped porous carbon sphere bifunctional catalyst was designed and fabricated by hydrothermal followed by carbonization using starch as carbon source and thiourea as nitrogen, sulfur source for HMI detection and oxygen evolution reactions. Under the synergistic effect of pore structure, active sites and nitrogen, sulfur functional groups, C-S_0.75-HT-C₈₀₀ demonstrated excellent HMI detection performance and oxygen evolution reaction activity. Under optimized conditions, the detection limits (LODs) of C-S_0.75-HT-C₈₀₀ sensor were 3.90, 3.86 and 4.91 nM for Cd²⁺, Pb²⁺ and Hg²⁺ when detected individually; and the sensitivities were 13.12, 19.50 and 21.19 μA/μM. The sensor also obtained high recoveries of Cd²⁺, Hg²⁺ and Pb²⁺ in river water samples. During the oxygen evolution reaction, a Tafel slope of 70.1 mV/dec and a low overpotential of 277 mV were obtained for C-S_0.75-HT-C₈₀₀ electrocatalyst with a current density of 10 mA/cm² in basic electrolyte. This research offers a neoteric and simple strategy in the design as well as fabrication of bifunctional carbon-based electrocatalysts.

Combining quasi-ZIF-67 hybrid nanozyme and G-quadruplex/hemin DNAzyme for highly sensitive electrochemical sensing

Zeolitic imidazolate frameworks (ZIFs), a famous subfamily of metal-organic frameworks (MOFs), are considered promising electrocatalysts. Herein, ZIF-67 was selected as an electrocatalyst for designing electrochemical sensors due to having the best electrocatalytic activity in ZIFs. To overcome the insufficient electrocatalytic activity of ZIFs, ZIF-67 derivatives (QZIF-67-X, where X represents calcination time) were obtained by calcining at 250 °C for a certain time. The porous structure of the precursor in QZIF-67-X is maintained, exposing more active centers. QZIF-67-X could accelerate electron transfer and lead to improve the electrocatalytic performance. Moreover, QZIF-67-2 was chosen as an Au nanoparticle-supported nanocarrier to further bind G-quadruplex/hemin DNAzymes with strong catalytic activity due to the best supporting activity of QZIF-67-2 among QZIF-67-X. The synergistic catalysis of QZIF-67-2 and G-quadruplex/hemin DNAzymes effectively amplified the reduction current signal of H₂O₂. The linear range of the prepared electrochemical sensor was 2 μM-65 mM, and the detection limit was 1.2 μM. Moreover, the real-time detection of H₂O₂ from HepG2 cells was achieved by the sensor, providing a novel technique for efficient anticancer drug evaluation. These results suggested that QZIF-67 can be utilized as an efficient electrocatalyst for improving the sensitivity of sensors.

Emerging single-atom iron catalysts for advanced catalytic systems

Due to the elusive structure-function relationship, traditional nanocatalysts always yield limited catalytic activity and selectivity, making them practically difficult to replace natural enzymes in wide industrial and biomedical applications. Accordingly, single-atom catalysts (SACs), defined as catalysts containing atomically dispersed active sites on a support material, strikingly show the highest atomic utilization and drastically boosted catalytic performances to functionally mimic or even outperform natural enzymes. The molecular characteristics of SACs (e.g., unique metal-support interactions and precisely located metal sites), especially single-atom iron catalysts (Fe-SACs) that have a similar catalytic structure to the catalytically active center of metalloprotease, enable the accurate identification of active centers in catalytic reactions, which afford ample opportunity for unraveling the structure-function relationship of Fe-SACs. In this review, we present an overview of the recent advances of support materials for anchoring an atomic dispersion of Fe. Subsequently, we highlight the structural designability of support materials as two sides of the same coin. Moreover, the applications described herein illustrate the utility of Fe-SACs in a broad scope of industrially and biologically important reactions. Finally, we present an outlook of the major challenges and opportunities remaining for the successful combination of single Fe atoms and catalysts.

General Strategies to Construct Highly Efficient Sensing Interfaces for Metal Ions Detection from the Perspective of Catalysis

Constructing high-effective electrode sensing interfaces has been considered an effective method for electrochemical detection toward heavy metal ions (HMIs). However, most research has been devoted to enhancing the stripping currents of HMIs by simply improving the adsorptive capacity and conductivity of the electrode modified materials, while lacking theoretical guidelines in fabricating catalytic sensing interfaces. Besides, the understanding of detection mechanisms is quite unscientific from the perspective of catalysis. This perspective summarizes five general strategies in designing highly efficient sensing interfaces in the recent five years, including modulating crystal phases, orientations and planes, defect engineering, ionic valence state cycle engineering, adsorption in situ catalysis strategy, and construction of atomic level catalytic active sites. What's more, the catalytic mechanisms for improving the signals of HMIs, such as boosting the electron transfer rates and conversion rates, lowering the energy barriers, etc., are introduced and emphasized. This study has a great significance in directionally controlling functionalized electrochemical sensors to achieve excellent sensitivity and selectivity in detecting environmental pollutants from the view of catalysis, and it also brings enlightenments and guidance to develop new electroanalytical methods.

Two-Dimensional Field-Effect Transistor Sensors: The Road toward Commercialization

The evolutionary success in information technology has been sustained by the rapid growth of sensor technology. Recently, advances in sensor technology have promoted the ambitious requirement to build intelligent systems that can be controlled by external stimuli along with independent operation, adaptivity, and low energy expenditure. Among various sensing techniques, field-effect transistors (FETs) with channels made of two-dimensional (2D) materials attract increasing attention for advantages such as label-free detection, fast response, easy operation, and capability of integration. With atomic thickness, 2D materials restrict the carrier flow within the material surface and expose it directly to the external environment, leading to efficient signal acquisition and conversion. This review summarizes the latest advances of 2D-materials-based FET (2D FET) sensors in a comprehensive manner that contains the material, operating principles, fabrication technologies, proof-of-concept applications, and prototypes. First, a brief description of the background and fundamentals is provided. The subsequent contents summarize physical, chemical, and biological 2D FET sensors and their applications. Then, we highlight the challenges of their commercialization and discuss corresponding solution techniques. The following section presents a systematic survey of recent progress in developing commercial prototypes. Lastly, we summarize the long-standing efforts and prospective future development of 2D FET-based sensing systems toward commercialization.

The Gate-Modified Solution-Gated Graphene Transistors for the Highly Sensitive Detection of Lead Ions

Lead ions are heavy metal ions that are extremely harmful to the human body and ecological environment. They can cause irreversible damage to the human nervous system and blood system at low concentrations. It is very important to develop a simple, rapid, and sensitive detection method of Pb²⁺. Solution-gated graphene transistors (SGGTs) have been widely studied in recent years due to their ultra-high sensitivity in chemical sensing. Herein, we have demonstrated a sensitive sensor of Pb²⁺ based on the SGGTs through the glutathione gate modification. When Pb²⁺ are added into the electrolyte solution, the electrical double layer capacitance near the gate electrode changes because Pb²⁺ can be strongly chelated, leading to the channel current change. The detection of Pb²⁺ can be realized. The detection limit of sensors for Pb²⁺ can reach 1 × 10^-18 M, and the response time is about 1 s. The channel current change and the logarithm of Pb²⁺ concentration exhibit a good linear relationship in the concentration range of 1 × 10^-18 and 1 × 10^-6 M. Because the glutathione molecule can well recognize Pb²⁺, the devices also demonstrate good selectivity to Pb²⁺. Compared with the convention detection, our method shows easy operation, high sensitivity, and high selectivity. Therefore, it has great potential in the analysis of trace samples for health and environment monitoring.

Perspective for Single Atom Nanozymes Based Sensors: Advanced Materials, Sensing Mechanism, Selectivity Regulation, and Applications

Nanozymes are a kind of nanomaterial mimicking enzyme catalytic activity, which has aroused extensive interest in the fields of biosensors, biomedicine, and climate and ecosystems management. However, due to the complexity of structures and composition of nanozymes, atomic scale active centers have been extensively investigated, which helps with in-depth understanding of the nature of the biocatalysis. Single atom nanozymes (SANs) cannot only significantly enhance the activity of nanozymes but also effectively improve the selectivity of nanozymes owing to the characteristics of simple and adjustable coordination environment and have been becoming the brightest star in the nanozyme spectrum. The SANs based sensors have also been widely investigated due to their definite structural features, which can be helpful to study the catalytic mechanism and provide ways to improve catalytic activity. This perspective presents a comprehensive understanding on the advances and challenges on SANs based sensors. The catalytic mechanisms of SANs and then the sensing application from the perspectives of sensing technology and sensor construction are thoroughly analyzed. Finally, the major challenges, potential future research directions, and prospects for further research on SANs based sensors are also proposed.

Engineering DNA/Fe-N-C single-atom nanozymes interface for colorimetric biosensing of cancer cells

Single atom nanozymes (SAzymes) represent the state-of-the-art technology in nanomaterial-based catalysis, which have attracted attentions in catalysis, cancer treatment, disinfection and biosensing fields. However, numerous SAzymes suffered from low aqueous dispersion and without recognition capacity, which impeded their applications in bioanalysis. Herein, we engineered DNA onto SAzymes to obtain the DNA/SAzymes conjugates, which significantly improved the aqueous dispersion and recognition ability of SAzymes. We synthesized iron SAzymes (Fe-N-C SAzymes) as the catalytic nanomaterials, and investigated the interactions between Fe-N-C SAzymes and DNA. We compared A₁₅, T₁₅ and C₁₅ adsorption of Fe-N-C SAzymes in HEPES containing 2 mM MgCl₂. We found that 50 μg mL^-1 Fe-N-C SAzymes produced nearly 100% A₁₅ adsorption, 90% T₁₅ adsorption and only 69% C₁₅ adsorption, indicating that adenine and thymine had higher adsorption affinity on Fe-N-C SAzymes. More importantly, DNA modification did not affect the peroxidase-like activity of Fe-N-C SAzymes and the bioactivity of the adsorbed DNA. Taking the advantage of the diblock DNA with one DNA sequence (adenine) binding to Fe-N-C SAzymes and the other DNA sequence (i.e., aptamer) binding to cancer cells, we designed Apt/Fe-N-C SAzymes for colorimetric detection of cancer cells, which offered new insights for the use of SAzymes in biomedicine.

Ultrasensitive colorimetric strategy for Hg2+ detection based on T-Hg2+-T configuration and target recycling amplification

A novelty aptasensor for ultrasensitive detection of Hg²⁺ is developed, exploiting the combination of plasmonic properties of gold nanoparticles (AuNPs) and exonuclease III (Exo III)-assisted target recycling for signal amplification. In the presence of Hg²⁺, a DNA duplex can be formed due to the strong coordination of Hg²⁺ and T bases of single-stranded DNA (ssDNA) probe. Exo III digests the DNA duplex from the 3' to 5' direction, resulting in the releasing of Hg²⁺. Then, the released Hg²⁺ binds with another ssDNA probe through T-Hg²⁺-T coordination. After Exo III-assisted Hg²⁺ cycles, numerous ssDNA probes are exhausted, which promotes poly(diallyldimethylammonium chloride) (PDDA)-induced AuNP aggregation, leading to an obvious color change and aggregation-induced plasmon red shift of AuNPs (from 520 to 610 nm). Therefore, this biosensor is ultrasensitive, which is applicable to the detection of trace level of Hg²⁺ with a linear range from 5 pM to 0.6 nM and an ultralow detection limit of 0.2 pM. Furthermore, it enables visual detection of Hg²⁺ as low as 50 pM by the naked eye. More importantly, the assay can be applied to the reliable determination of spiked Hg²⁺ in sea water samples with good recovery.

Highly Sensitive Detection for Mercury Ions Using Graphene Oxide (GO) Sensors

The mercury ion (Hg²⁺) is one of the heavy metal ions, and its presence in trace amounts can cause physiological damage to an organism. Traditional methods of Hg²⁺ detection have been useful but have also had numerous limitations and challenges, and as a result, it is important to design new and sophisticated methods that can aid in the detection of Hg²⁺. In this paper, two fluorescent dyes, carboxyfluorescein (FAM) and SYBR Green I, were used to label and intercalate DNA probes immobilized on the surface of graphene oxide (GO) for sensors to detect Hg²⁺. FAM and SYBR Green I dye share close excitation and emission wavelength spectra, which can promote and amplify the detection of signals, and also increase the limit of detection (LOD). The results showed that the limit of detection in this method was 0.53 nM. Moreover, when the sensors with double amino groups on the surface of GO were carried out to detect Hg²⁺, a limit of detection was improved to 0.43 nM. The sensors were then applied in the real sample. The results show that this method has a promising potential in Hg²⁺ detection.

Single-atom catalysts boost signal amplification for biosensing

Development of highly sensitive biosensors has received ever-increasing attention over the years. Due to the unique physicochemical properties, the functional nanomaterial-enabled signal amplification strategy has made some great breakthroughs in biosensing. However, the sensitivity and selectivity still need further improvement. Single-atom catalysts (SACs) containing atomically dispersed metal active sites demonstrate distinctive advantages in catalytic activity and selectivity for various catalytic reactions. As a consequence, the SAC-enabled signal amplification strategy holds great promise in biosensors, demonstrating satisfactory sensitivity and selectivity with the assistance of tunable metal-support interactions, coordination environments and geometric/electronic structures of active sites. In this tutorial review, we briefly discuss the structural advantages of SACs. Then, the catalytic mechanism at the atomic scale and signal amplification effects of SACs in the colorimetric, electrochemical, chemiluminescence, electrochemiluminescence, and photoelectrochemical biosensing applications are highlighted in detail. Finally, opportunities and challenges to be faced in the future development of the SAC-enabled signal amplification strategy for biosensing are discussed and outlooked.