Enhancing electrochemical sensing through the use of functionalized graphene composites as nanozymes

Graphene-based nanozymes possess inherent nanomaterial properties that offer not only a simple substitute for enzymes but also a versatile platform capable of bonding with complex biochemical environments. The current review discusses the replacement of enzymes in developing biosensors with nanozymes. Functionalization of graphene-based materials with various nanoparticles can enhance their nanozymatic properties. Graphene oxide functionalization has been shown to yield graphene-based nanozymes that closely mimic several natural enzymes. This review provides an overview of the classification, current state-of-the-art development, synthesis routes, and types of functionalized graphene-based nanozymes for the design of electrochemical sensors. Furthermore, it includes a summary of the application of functionalized graphene-based nanozymes for constructing electrochemical sensors for pollutants, drugs, and various water and food samples. Challenges related to nanozymes as electrocatalytic materials are discussed, along with potential solutions and approaches for addressing these shortcomings.

Ratiometric Detection of microRNA Using Hybridization Chain Reaction and Fluorogenic Silver Nanoclusters

miRNA (miR)-155 is a potential biomarker for breast cancers. We aimed at developing a nanosensor for miR-155 detection by integrating hybridization chain reaction (HCR) and silver nanoclusters (AgNCs). HCR serves as an enzyme-free and isothermal amplification method, whereas AgNCs provide a built-in fluorogenic detection probe that could simplify the downstream analysis. The two components were integrated by adding a nucleation sequence of AgNCs to the hairpin of HCR. The working principle was based on the influence of microenvironment towards the hosted AgNCs, whereby unfolding of hairpin upon HCR has manipulated the distance between the hosted AgNCs and cytosine-rich toehold region of hairpin. As such, the dominant emission of AgNCs changed from red to yellow in the absence and presence of miR-155, enabling a ratiometric measurement of miR with high sensitivity. The limit of detection (LOD) of our HCR-AgNCs nanosensor is 1.13 fM in buffered solution. We have also tested the assay in diluted serum samples, with comparable LOD of 1.58 fM obtained. This shows the great promise of our HCR-AgNCs nanosensor for clinical application.

Hydrogen-Bonded Biohybrid Framework-Derived Highly Specific Nanozymes for Biomarker Sensing

Nanozymes are of particular interest due to their enzyme-mimicking activity and high stability that are favorable in biomedical sensing and immunoassays. In this work, we report a highly specific N-doped nanozyme through pyrolysis of framework-confined bovine serum albumin (BSA). This strategy allows one to translate the low-cost and featureless BSA into a highly active enzyme mimic. The obtained carbon nanozyme (denoted as HBF-1-C800) displays 3- to 7-fold enhancement on peroxidase (POD) activity compared with the conventional carbon nanozymes and also shows ca. 5-fold activity enhancement compared to the reported N-doping graphene. Such excellent POD activity originates from high N-doping efficiency, protein-induced defective sites, and the intrinsic porous structure of HBF-1-C800, which provides abundantly accessible active sites and accelerates substrate diffusion simultaneously. Importantly, the HBF-1-C800 nanozyme has highly specific POD activity and also enables resistance to several harsh conditions that should denature natural enzymes. These features allow it with high accuracy, stability, and sensitivity for biosensing applications. Moreover, HBF-1-C800 has been designed as a promising platform for colorimetric biosensing of several biomarkers including H₂O₂, glutathione, and glucose, with wide linear ranges and low limits of detection that are satisfied with the disease diagnosis.

Nucleic acid-driven aggregation-induced emission of Au nanoclusters for visualizing telomerase activity in living cells and in vivo

Visual monitoring of telomerase activity in living cancer cells and in vivo is essential for clinical diagnosis and treatment. However, most detection methods were performed in vitro due to the difficulty of probes entering cells and the interferences from complex biological environments. Herein, we developed a novel probe based on Au nanoclusters (AuNCs) with a nucleic acid-driven aggregation-induced emission (AIE) property for the first time. The probe was applied for detection of telomerase with high sensitivity. Importantly, the probe could achieve telomerase imaging in living cells and in solid tumor tissue in vivo. The study provided a specific connection fashion of metal nanoclusters for AIE generation. It holds great potential for the development of AIE-active metal nanoclusters as a diagnostic tool for disease detection in vitro as well as in vivo.

Nanochemistry Modulates Intracellular Decomposition Routes of S-Nitrosothiol Modified Silica-Based Nanoparticles

Cellular delivery of nitric oxide (NO) using NO donor moieties such as S-nitrosothiol (SNO) is of great interest for various applications. However, understandings of the intracellular decomposition routes of SNO toward either NO or ammonia (NH₃ ) production are surprisingly scarce. Herein, the first report of SNO modified mesoporous organosilica nanoparticles with tetrasulfide bonds for enhanced intracellular NO delivery, ≈10 times higher than a commercial NO donor, is presented. The tetrasulfide chemistry modulates the SNO decomposition by shifting from NH₃ to NO production in glutathione rich cancer cells. This study provides a new strategy to control the NO level in biological systems.

Efficient β-Carboline Alkaloid-Based Probe for Highly Sensitive Imaging of Endogenous Glutathione in Wheat Germ Tissues

Discriminative detection of GSH is achieved by employing a highly sensitive and selective fluorescent probe (KL-DN) that bears β-carboline alkaloid as a potential fluorophore and an azide group as the recognition unit. A rapid fluorescence off-on change is caused by special redox reaction; KL-DN has the capability of monitoring endogenous GSH in wheat germ tissues, indicating that this probe holds great potential for biological applications in plant tissues.

Emerging Concepts of Nanobiotechnology in mRNA Delivery

Introducing mRNA into cells has attracted intense interest for diverse applications; however, success requires delivery solutions. Engineered nanomaterials have been applied as mRNA nanocarriers; their functions are designed mainly as delivery vehicles, but rarely in regulation of the protein translation. Recently, progress in nanobiotechnology has shifted the design principle of mRNA nanocarriers from simple delivery tools to translation modulators. Here, we review the emerging concepts of nanomaterials regulating mRNA translation and recent progress in mRNA delivery. Designer nanomaterials providing integrated functions for specific mRNA applications are also reviewed to provide insights for the design of next-generation nanomaterials to revolutionize mRNA technology.

Enzyme-free amplified detection of miRNA based on target-catalyzed hairpin assembly and DNA-stabilized fluorescent silver nanoclusters

A simple, cost-effective, sensitive, and selective strategy was developed for microRNA analysis using target-catalyzed hairpin assembly and fluorescent silver nanoclusters.

Ratiometric fluorescent nanoprobes for visual detection: Design principles and recent advances - A review

Signal generation techniques for visual detection of analytes have received a great deal of attention in various sensing fields. These approaches are considered to be advantageous when instrumentation cannot be employed, such as for on-site assays, point-of-care tests, and he althcare diagnostics in resource-constrained areas. Amongst various visual detection approaches explored for non-invasive quantitative measurements, ratiometric fluorescence sensing has received particular attention as a potential method to overcome the limitations of intensity-based probes. This technique relies on changes in the intensity of two or more emission bands (induced by an analyte), resulting in an effective internal referencing which improves the sensitivity of the detection. The self-calibration, together with the unique optophysical properties of nanoparticles (NPs) have made the ratiometric fluorescent nanoprobes more sensitive and reliable, which in turn, can result in more precise visual detection of the analytes. Over the past few years, a vast number of ratiometric sensing probes using nanostructured fluorophores have been designed and reported for a wide variety of sensing, imaging, and biomedical applications. In this work, a review on the NP-based ratiometric fluorescent sensors has been presented to meticulously elucidate their development, advances and challenges. With a special emphasis on visual detection, the most important steps in the design of fluorescent ratiometric nanoprobes have been given and based on different classes of analytes, recent applications of fluorescent ratiometric nanoprobes have been summarized. The challenges for the future use of the technique investigated in this review have been also discussed.

Nucleobase-mediated synthesis of nitrogen-doped carbon nanozymes as efficient peroxidase mimics

Carbon nanozymes are catalytic carbon nanomaterials with intrinsic enzyme-like activities. They are advantageous over their natural counterparts in terms of higher stability, lower preparation cost, and better robustness. However, the peroxidase-like activities of the most developed carbon nanozymes were moderate due to the imperfection of active centers and limited tuning strategies. Herein, we designed a novel class of efficient peroxidase-mimicking carbon nanozymes with nitrogen atom doping. The N-doped carbon nanozymes were facilely synthesized by direct pyrolysis of different nucleobases at controlled temperatures without other treatments. A high ratio of nitrogen atoms was doped into the carbon skeleton. For example, 8.77 wt% of N remained in the guanine-derived carbon nanozyme with a pyrolysis temperature of 900 °C. The dominant graphitic N species greatly boosted the peroxidase-like activities of nucleobase-derived carbon nanozymes. Moreover, nucleobases are cheap, abundant, and environmentally friendly. We have demonstrated that nitrogen-rich nucleobases are ideal starting materials for the large-scale and cost-effective synthesis of N-doped carbon nanozymes. The carefully designed N-doped carbon nanozymes with superior activities were further used to construct effective biosensors for bioactive molecules (i.e., H2O2 and glucose). Highly sensitive and selective detection of H2O2 and glucose was achieved using the N-doped carbon nanozymes as efficient peroxidase mimics. This study offers an economical and sustainable approach for the scalable preparation of N-doped carbon nanozymes and creates a new path for the rational design of efficient peroxidase-mimicking carbon nanozymes by heteroatom doping.

Specific and sensitive detection of Plasmodium falciparum lactate dehydrogenase by DNA-scaffolded silver nanoclusters combined with an aptamer

Innovative nanomaterials offer significant potential for diagnosis of severe diseases of the developing world such as malaria. Small sized silver nanoclusters have shown promise for diagnostics due to their intense fluorescence emission and photo-stabilities. Here, double-stranded DNA-scaffolded silver nanoclusters (AgNCs-dsDNA) were prepared to detect the established malaria biomarker, Plasmodium falciparum lactate dehydrogenase (PfLDH). Significant luminescence enhancement over a wide concentration range of PfLDH was demonstrated. In addition, a low limit of detection at 0.20 nM (7.4 pg μL^-1) was achieved for PfLDH in buffer solution, sensitive enough for practical use correlating with the clinical level of PfLDH in plasma from malaria-infected patients. Unique specificity was observed towards Plasmodium falciparum over Plasmodium vivax and human lactate dehydrogenase, as well as other non-specific proteins, by combining the use of AgNCs-dsDNA with a DNA aptamer against PfLDH. Moreover, the intrinsic mechanism was revealed in detail for the two-step luminescence response. The combination of DNA-scaffolded silver nanoclusters coupled to a selective single-stranded DNA aptamer allows for a highly specific and sensitive detection of PfLDH with significant promise for malaria diagnosis in future.

Formulation of DNA chimera templates: Effects on emission behavior of silver nanoclusters and sensing

Single strand DNA (ssDNA) chimeras consisting of a silver nanoclusters-nucleating sequence (NC) and an aptamer are widely employed to synthesize functional silver nanoclusters (AgNCs) for sensing purpose. Despite its simplicity, this chimeric-templated AgNCs often leads to undesirable turn-off effect, which may suffer from false positive signals caused by interference. In our effort to elucidate how the relative position of NC and aptamer affects the fluorescence behavior and sensing performance, we systematically formulated these NC and aptamer regions at different position in a DNA chimera. Using adenosine aptamer as a model, we tested the adenosine-induced optical response of each design. We also investigated the effect of linker region connecting NC and aptamer, as well as different NC sequence on the sensing performance. We concluded that locating NC sequence at 5'-end exhibited the best response, with immediate fluorescence enhancement observed over a wide linear range (1-2500 μM). Our experimental findings help to explain the emission behavior and sensing performance of chimeric conjugates of AgNCs, providing an important means to formulate a better aptasensor.

DNA metallization: principles, methods, structures, and applications

This review summarizes the research activities on DNA metallization since the concept was first proposed in 1998, covering the principles, methods, structures, and applications.

L-tyrosine methyl ester-stabilized carbon dots as fluorescent probes for the assays of biothiols

Over the past few decades, assays of biothiols had attracted much attention due to the essential role they played in human physiology, especially using the fluorescent analysis. In most cases, competitive mechanism was often employed, where the metal ions were often introduced as the quenchers and thiols competed with metal ions due to the high binding affinity and strong thiophilicity for 'signal-on' assays. To develop a metal ions-free approach for the assays of thiols, here, L-tyrosine methyl ester capped carbon dots (Tyr-CDs) were employed and prepared as the fluorescent probes. The as-prepared Tyr-CDs displayed narrow size distribution and distinct blue fluorescence with high quantum yield (12.9%) compared with the unmodified CDs. Moreover, Tyr-CDs exhibited higher quenching efficiency due to the efficient energy transfer between Tyr-CDs and the quinone products in the presence of tyrosinase. When the targeted biothiols was present, the catalytic reaction of the tyrosinase to the formation of quinone was inhibited and the fluorescence signal was recovered in a biothiols-concentration-dependent manner, which provided the basis for the analysis of biothiols. The practical application of the present system was demonstrated by testing the biothiols in human plasma samples and good recovery was obtained, indicating that the sensing platform we proposed hold great promise in the accurate detection of biothiols in complex biosystems.

A label-free ratiometric electrochemical DNA sensor for monitoring intracellular redox homeostasis

A label-free ratiometric electrochemical determination of GSH by DNA metallization-mediated HCR amplification is reported.

Fluorescent metal quantum clusters: an updated overview of the synthesis, properties, and biological applications

A brief history of metal quantum clusters, their synthesis methods, physical properties, and an updated overview of their applications is provided.

Fluorometric “Turn-On” glucose sensing through the in situ generation of silver nanoclusters

A fluorometric “turn-on” glucose detection is performed based on the Fenton reaction which can trigger the generation of Ag nanoclusters.

A novel universal colorimetric sensor for simultaneous dual target detection through DNA-directed self-assembly of graphene oxide and magnetic separation

A novel universal colorimetric sensor for simultaneous dual target detection through DNA-directed self-assembly of graphene oxide and magnetic separation was designed for the first time.