A Selective and Accurate Ratiometric Electrochemical Biosensor for Monitoring of Cu2+ Ions in a Rat Brain

Universal Covalent Grafting Strategy of an Aptamer on a Carbon Fiber Microelectrode for Selective Determination of Dopamine In Vivo

The chemical information on brain science provided by electrochemical sensors is critical for understanding brain chemistry during physiological and pathological processes. A major challenge is the selectivity of electrochemical sensors in vivo. This work developed a universal covalent grafting strategy of an aptamer on a carbon fiber microelectrode (CFE) for selective determination of dopamine in vivo. The universal strategy was proposed by oxidizing poly(tannic acid) (pTA) to form an oxidized state (pTAox) and then coupling a nucleophilic sulfhydryl molecule of the dopamine-binding mercapto-aptamer with the o-quinone moiety of pTAox based on click chemistry for the interfacial functionalization of the CFE surface. It was found that the universal strategy proposed could efficiently graft the aptamer on a glassy carbon electrode, which was verified by using electroactive 6-(ferrocenyl) hexanethiol as a redox reporter. The amperometric method using a fabricated aptasensor for the determination of dopamine was developed. The linear range of the aptasensor for the determination of dopamine was 0.2-20 μM with a sensitivity of 0.09 nA/μM and a limit of detection of 88 nM (S/N = 3). The developed method has high selectivity originating from the specific recognition of the aptamer in concert with the cation-selective action of pTA and could be easily applicable to probe dopamine dynamics in the brain. Furthermore, complex vesicle fusion modes were first observed at the animal level. This work demonstrated that the covalently grafted immobilization strategy proposed is promising and could be extended to the in vivo analysis of other neurochemicals.

Challenges and strategies faced in the electrochemical biosensing analysis of neurochemicals in vivo: A review

Our brain is an intricate neuromodulatory network, and various neurochemicals, including neurotransmitters, neuromodulators, gases, ions, and energy metabolites, play important roles in regulating normal brain function. Abnormal release or imbalance of these substances will lead to various diseases such as Parkinson's and Alzheimer's diseases, therefore, in situ and real-time analysis of neurochemical interactions in pathophysiological conditions is beneficial to facilitate our understanding of brain function. Implantable electrochemical biosensors are capable of monitoring neurochemical signals in real time in extracellular fluid of specific brain regions because they can provide excellent temporal and spatial resolution. However, in vivo electrochemical biosensing analysis mainly faces the following challenges: First, foreign body reactions induced by microelectrode implantation, non-specific adsorption of proteins and redox products, and aggregation of glial cells, which will cause irreversible degradation of performance such as stability and sensitivity of the microsensor and eventually lead to signal loss; Second, various neurochemicals coexist in the complex brain environment, and electroactive substances with similar formal potentials interfere with each other. Therefore, it is a great challenge to design recognition molecules and tailor functional surfaces to develop in vivo electrochemical biosensors with high selectivity. Here, we take the above challenges as a starting point and detail the basic design principles for improving in vivo stability, selectivity and sensitivity of microsensors through some specific functionalized surface strategies as case studies. At the same time, we summarize surface modification strategies for in vivo electrochemical biosensing analysis of some important neurochemicals for researchers' reference. In addition, we also focus on the electrochemical detection of low basal concentrations of neurochemicals in vivo via amperometric waveform techniques, as well as the stability and biocompatibility of reference electrodes during long-term sensing, and provide an outlook on the future direction of in vivo electrochemical neurosensing.

Implantable Electrochemical Sensors for Brain Research

Implantable electrochemical sensors provide reliable tools for in vivo brain research. Recent advances in electrode surface design and high-precision fabrication of devices led to significant developments in selectivity, reversibility, quantitative detection, stability, and compatibility of other methods, which enabled electrochemical sensors to provide molecular-scale research tools for dissecting the mechanisms of the brain. In this Perspective, we summarize the contribution of these advances to brain research and provide an outlook on the development of the next generation of electrochemical sensors for the brain.

Recent Advances in Nanopore Technology for Copper Detection and Their Potential Applications

Recently, nanopore technology has emerged as a promising technique for the rapid, sensitive, and selective detection of various analytes. In particular, the use of nanopores for the detection of copper ions has attracted considerable attention due to their high sensitivity and selectivity. This review discusses the principles of nanopore technology and its advantages over conventional techniques for copper detection. It covers the different types of nanopores used for copper detection, including biological and synthetic nanopores, and the various mechanisms used to detect copper ions. Furthermore, this review provides an overview of the recent advancements in nanopore technology for copper detection, including the development of new nanopore materials, improvements in signal amplification, and the integration of nanopore technology with other analytical methods for enhanced detection sensitivity and accuracy. Finally, we summarize the extensive applications, current challenges, and future perspectives of using nanopore technology for copper detection, highlighting the need for further research in the field to optimize the performance and applicability of the technique.

In Vivo Electrochemical Biosensors: Recent Advances in Molecular Design, Electrode Materials, and Electrochemical Devices

Electrochemical biosensors provide powerful tools for dissecting the dynamically changing neurochemical signals in the living brain, which contribute to the insight into the physiological and pathological processes of the brain, due to their high spatial and temporal resolutions. Recent advances in the integration of in vivo electrochemical sensors with cross-disciplinary advances have reinvigorated the development of in vivo sensors with even better performance. In this Review, we summarize the recent advances in molecular design, electrode materials, and electrochemical devices for in vivo electrochemical sensors from molecular to macroscopic dimensions, highlighting the methods to obtain high performance for fulfilling the requirements for determination in the complex brain through flexible and smart design of molecules, materials, and devices. Also, we look forward to the development of next-generation in vivo electrochemical biosensors.

Development of a Plasmonic Sensor for a Chemotherapeutic Agent Cabazitaxel

Drug dosage is a crucial subject in both human and animal treatment. Administering less drug dosage may prevent treatment or make it less effective, and high drug dosage may cause a heightened risk of adverse effects, or in some cases, cost a patient's life. Also, even when the dosage is administered carefully, metabolic differences may cause different effects on different patients. Because of these considerations, monitoring drug dosage in the body is a critical and significant requirement in the health industry. Within the scope of this study, a reusable surface plasmon resonance (SPR) chip with fast response, high selectivity, and no pretreatment is produced for the chemotherapeutic agent cabazitaxel. A cabazitaxel-imprinted nanofilm was synthesized on the sensor chip surface and characterized by atomic force microscopy, ellipsometry, and contact angle measurements. Standard cabazitaxel solution and an artificial plasma sample were used for the kinetic analysis. Docetaxel, methylprednisolone, and dexamethasone were analyzed for their selectivity experiment. In addition, the repeatability and storage durability of the sensor were also evaluated. As a result of the adsorption studies, the limit of detection and limit of quantitation values were found to be 0.012 and 0.036 μg/mL, respectively. High-performance liquid chromatography analysis was used to validate the response of the cabazitaxel-imprinted sensor.

Structural and functional imaging of brains

Analyzing the complex structures and functions of brain is the key issue to understanding the physiological and pathological processes. Although neuronal morphology and local distribution of neurons/blood vessels in the brain have been known, the subcellular structures of cells remain challenging, especially in the live brain. In addition, the complicated brain functions involve numerous functional molecules, but the concentrations, distributions and interactions of these molecules in the brain are still poorly understood. In this review, frontier techniques available for multiscale structure imaging from organelles to the whole brain are first overviewed, including magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography (PET), serial-section electron microscopy (ssEM), light microscopy (LM) and synchrotron-based X-ray microscopy (XRM). Specially, XRM for three-dimensional (3D) imaging of large-scale brain tissue with high resolution and fast imaging speed is highlighted. Additionally, the development of elegant methods for acquisition of brain functions from electrical/chemical signals in the brain is outlined. In particular, the new electrophysiology technologies for neural recordings at the single-neuron level and in the brain are also summarized. We also focus on the construction of electrochemical probes based on dual-recognition strategy and surface/interface chemistry for determination of chemical species in the brain with high selectivity and long-term stability, as well as electrochemophysiological microarray for simultaneously recording of electrochemical and electrophysiological signals in the brain. Moreover, the recent development of brain MRI probes with high contrast-to-noise ratio (CNR) and sensitivity based on hyperpolarized techniques and multi-nuclear chemistry is introduced. Furthermore, multiple optical probes and instruments, especially the optophysiological Raman probes and fiber Raman photometry, for imaging and biosensing in live brain are emphasized. Finally, a brief perspective on existing challenges and further research development is provided.

Red-emitting carbon dots aggregates-based fluorescent probe for monitoring Cu2

R-CDAs have been synthesized in a one-pot solvothermal procedure starting from 3,4-diaminobenzoic acid in an acidic medium. Transmission electron microscopy (TEM) revealed that R-CDAs nanoparticles exhibited a much larger diameter of 7.2-28.8 nm than traditional monodisperse carbon dots. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) revealed the presence of polar functional groups (hydroxyl, amino, carboxyl) on the surface of R-CDAs. Upon excitation with visible light (550 nm), R-CDAs emit stable, red fluorescence with a maximum at 610 nm. Under the optimum conditions, Cu²⁺ ions quench the fluorescence of this probe, and the signal is linear in a concentration range of copper ions between 5 and 600 nM with the detection limit of only 0.4 nM. Recoveries from 98.0 to 105.0% and relative standard deviations (RSD) from 2.8 to 4.5% have been obtained for detection of Cu²⁺ in real water samples. Furthermore, the R-CDAs fluorescent probe showed negligible cytotoxicity toward HeLa cells and good bioimaging ability, suggesting its potential applicability as a diagnostic tool in biomedicine.

Real-Time Tracking of Electrical Signals and an Accurate Quantification of Chemical Signals with Long-Term Stability in the Live Brain

The development of in vivo analytical tools and methods for recording electrical signals and accurately quantifying chemical signals is a key issue for a comprehensive understanding of brain events. The electrophysiological microelectrode was invented to monitor electrical signals in free-moving brains. On the other hand, electrochemical assays with excellent spatiotemporal resolution provide an effect way to monitor chemical signals in vivo. Unfortunately, the in vivo electrochemical biosensors still have three limitations. First, many biological species such as reactive oxygen species (ROS) and neurotransmitters demonstrate large overpotentials at conventional electrodes. Thus, it is hard to convert the chemical/electrochemical signals of these molecules into electric signals. Second, the interfacial properties of the recognition molecules assembled onto the electrode surfaces have a great influence on the transmission of electric charge through the interface and the stability of the modified recognition molecules. Meanwhile, the surface of biosensors implanted in the brain is easily absorbed by many proteins present in the brain, resulting in the loss of signals. Finally, activities in the brain including neuron discharges and electrophysiological signals may be affected by electrochemical measurements due to the application of extra potentials and/or currents.This Account presents a deep view of the fundamental design principles and solutions in response to the above challenges for developing in vivo biosensors with high performance while meeting the growing requirements, including high selectivity, long-time stability, and simultaneously monitoring electrical and chemical signals. We aim to highlight the basic criteria based on a double-recognition strategy for the selective biosensing of ROS, H₂S, and H_nS through the rational design of specific recognition molecules followed by electrochemical oxidation or reduction. Recent developments in designing functionalized surfaces through a systematic investigation of self-assembly with Au-S bonds, Au-Se bonds, and Au≡C bonds for facilitating electrochemical properties as well as improving the stability are summarized. More importantly, this Account highlights the novel methodologies for simultaneously monitoring electrical and chemical signals ascribed to the dynamic changes in K⁺, Na⁺, and Ca²⁺ and pH values in vivo. Additionally, SERS-based photophysiological microarray probes have been developed for quantitatively tracking chemical changes in the live brain together with recording electrophysiological signals.The design principles and novel strategies presented in this Account can be extended to the real-time tracking of electrical signals and the accurate quantification of more chemical signals such as amino acids, neurotransmitters, and proteins to understand the brain events. The final part also outlines potential future directions in constructing high-density microarrays, eventually enabling the large-scale dynamic recording of the chemical expression of multineuronal signals across the whole brain. There is still room to develop a multifiber microarray which can be coupled with photometric methods to record chemical signals both inside and outside neurons in the live brains of freely moving animals to understand physiological processes and screen drugs.

A Simple Ratiometric Electrochemical Aptasensor Based on the Thionine–Graphene Nanocomposite for Ultrasensitive Detection of Aflatoxin B2 in Peanut and Peanut Oil

The accurate and reliable analysis of aflatoxin B2 (AFB2) is widely required in food and agricultural industries. In the present work, we report the first use of a ratiometric electrochemical aptasensor for AFB2 detection with high selectivity and reliability. The working principle relies on the conformation change of the aptamer induced by its specific recognition of AFB2 to vary the ratiometric signal. Based on this principle, the proposed aptasensor collects currents generated by thionine–graphene composites (ITHI) and ferrocene-labeled aptamers (IFc) to output the ratiometric signal of ITHI/IFc. In analysis, the value of ITHI remained stable while that of IFc increased with higher AFB2 concentration, thus offering a “signal-off” aptasensor by using ITHI/IFc as a yardstick. The fabricated aptasensor showed a linear range of 0.001–10 ng mL−1 with a detection limit of 0.19 pg mL−1 for AFB2 detection. Furthermore, its applicability was validated by using it to detect AFB2 in peanut and peanut oil samples with high rates of recovery. The developed ratiometric aptasensor shows the merits of simple fabrication and high accuracy, and it can be extended to detect other mycotoxins in agricultural products.

Free-electrodeposited anodic stripping voltammetry sensing of Cu(II) based on Ti3C2Tx MXene/carbon black

A proof-of-principle concept for free-electrodeposited anodic stripping voltammetry (ASV) sensing of Cu²⁺ is proposed by using Ti₃C₂T_x MXene/carbon black (Ti₃C₂T_x@CB) nanohybrids as electrode materials. Owing to the high adsorption and reduction capability of Ti₃C₂T_x towards Cu²⁺, Ti₃C₂T_x MXene enables Cu²⁺ to be immobilized and self-reduced directly to form Cu⁰ on the Ti₃C₂T_x@CB electrode surface. As a result an oxidation peak current appears from the re-oxidation of Cu⁰ via differential pulse voltammetry. Carbon black (CB) was introduced to prevent Ti₃C₂T_x Mxene aggregation and improve the related electron transfer as well as enhance their surface area. After optimizing various conditions, a considerable low limit of detection (4.6 nM) and a wide linear range (0.01-15.0 μM) for Cu²⁺ were achieved at the working potential from - 0.3 V to 0.0 V (vs SCE). Relative standard deviation (RSD) of eight individual Ti₃C₂T_x@CB electrodes is 3.72%, and the recoveries from tap water sample and lake water sample were in the ranges of 97.0-108% and 104-107%, respectively.

Nanozymes-Hitting the Biosensing "Target"

Nanozymes are a class of artificial enzymes that have dimensions in the nanometer range and can be composed of simple metal and metal oxide nanoparticles, metal nanoclusters, dots (both quantum and carbon), nanotubes, nanowires, or multiple metal-organic frameworks (MOFs). They exhibit excellent catalytic activities with low cost, high operational robustness, and a stable shelf-life. More importantly, they are amenable to modifications that can change their surface structures and increase the range of their applications. There are three main classes of nanozymes including the peroxidase-like, the oxidase-like, and the antioxidant nanozymes. Each of these classes catalyzes a specific group of reactions. With the development of nanoscience and nanotechnology, the variety of applications for nanozymes in diverse fields has expanded dramatically, with the most popular applications in biosensing. Nanozyme-based novel biosensors have been designed to detect ions, small molecules, nucleic acids, proteins, and cancer cells. The current review focuses on the catalytic mechanism of nanozymes, their application in biosensing, and the identification of future directions for the field.

Ratiometric Electrochemistry: Improving the Robustness, Reproducibility and Reliability of Biosensors

Electrochemical biosensors are an increasingly attractive option for the development of a novel analyte detection method, especially when integration within a point-of-use device is the overall objective. In this context, accuracy and sensitivity are not compromised when working with opaque samples as the electrical readout signal can be directly read by a device without the need for any signal transduction. However, electrochemical detection can be susceptible to substantial signal drift and increased signal error. This is most apparent when analysing complex mixtures and when using small, single-use, screen-printed electrodes. Over recent years, analytical scientists have taken inspiration from self-referencing ratiometric fluorescence methods to counteract these problems and have begun to develop ratiometric electrochemical protocols to improve sensor accuracy and reliability. This review will provide coverage of key developments in ratiometric electrochemical (bio)sensors, highlighting innovative assay design, and the experiments performed that challenge assay robustness and reliability.

Nonenzymatic Electrochemical Sensor with Ratiometric Signal Output for Selective Determination of Superoxide Anion in Rat Brain

There is still an urgent need to develop reliable analytical methods of O₂^•- in vivo for deeply elucidating the roles of O₂^•- playing in the brain. Herein, a nonenzymatic electrochemical sensor with ratiometric signal output was developed for an in vivo analysis of O₂^•- in the rat brain. Diphenylphosphonate-2-naphthol ester (ND) was designed and synthesized as a specific recognition molecule for the selective determination of O₂^•-. An anodic peak ascribed to the oxidation of 2-naphthol was generated via the nucleophilic substitution between ND and O₂^•- and was increased with the increasing concentration of O₂^•-. Meanwhile, the inner reference of methylene blue (MB) was co-assembled at the electrode surface to enhance the determination accuracy of O₂^•-. The anodic peak current ratio between 2-naphthol and MB exhibited a good linear relationship with the concentration of O₂^•- from 2 to 200 μM. Because of the stable molecule character of ND and its specific reaction with O₂^•-, the developed electrochemical sensor demonstrated excellent selectivity toward various potential interferences in the brain and good stability even after storage for 7 days. Accordingly, the present electrochemical sensor with high selectivity, high stability, and high accuracy was successfully exploited in monitoring the levels of O₂^•- in the rat brain and that of the diabetic model followed by cerebral ischemia.

Fluorescence switch of gold nanoclusters stabilized with bovine serum albumin for efficient and sensitive detection of cysteine and copper ion in mice with Alzheimer's disease

The near-infrared fluorescence of gold nanoclusters stabilized with bovine serum albumin (BSA -AuNCs) centered at 675 nm could be enhanced by cysteine and then effectively quenched by copper ion (Cu²⁺), therefore, cysteine and copper ion could be detected in sequence. At "on" state, fluorescence enhancement of BSA-AuNCs is generated due to the reaction between cysteine and BSA-AuNCs, via filling the surface defect of gold nanoclusters, while Cu²⁺ can further oxidize the reductive sulfydryl of cysteine and interact with amino acids presented in the BSA chain, inducing gold nanoclusters to aggregate, thus causing "off" state with fluorescence quenching. Fluorescence switch of BSA-AuNCs can be used for cysteine and Cu²⁺ detection in mice brain with Alzheimer's disease (AD) in vitro, with fast response, high chemical stability and sensitivity. Besides, it was able to image the endogenous Cu²⁺ in liver and heart of AD mice in situ. The results are promising, especially in the framework of early diagnosis of Alzheimer's disease.

Fabrication of the Ni-based composite wires for electrochemical detection of copper(Ⅱ) ions

Copper ions (Cu²⁺) pollution in the water environment poses a great threat to the health function of life-sustaining metabolic activities. However, the current detection methods need relatively expensive instruments, complex operation procedures and long time, so a facile and direct detection method is desired to be developed. In this work, the Ni-based composite wires with p-n junction (the Ni/NiO/ZnO/Chitosan wire) and Schottky junction (the Ni/NiO/Au/Chitosan wire) were fabricated, and the barrier driven electrochemical sensing mechanism was studied. The direct and facile detection of Cu²⁺ was achieved with a wide linear range (0-6000 nM) and a low LOD (0.81 nM). The excellent stability and recovery in real water samples made the Ni-based composite wires a promising candidate for the practical application. The interfacial barriers of semiconductor can be used as a special sensing factor to develop novel sensors.

Electrochemical Strategy for Analyzing the Co-evolution of Cu2+ and •OH Levels at the Early Stages of Transgenic AD Mice

As more researchers have acknowledged that the aggregation of amyloid β (Aβ) peptides might only be a pathological phenomenon that appears during the course of Alzheimer's disease (AD), it is therefore of great significance to have a preclinical or an early clinical diagnosis. Cu²⁺ dyshomeostasis and oxidative stress, such as hydroxyl radical (^•OH), are found to be associated with peptide aggregations. However, we still do not know how the levels of Cu²⁺ and ^•OH are altered in the brain before massive Aβ plaques appear. Herein, we demonstrated the design and application of a sensitive electrochemical sensor to monitor Cu²⁺ and ^•OH simultaneously in one system without obvious cross-talk. The electrode was fabricated using black phosphorus-loaded Au (BP-Au) nanoparticles, which were then sequentially linked with DNA1, DNA2-labeled Au (Au-DNA2) nanoparticles, and methylene blue (MB). Cu²⁺ was first recognized and captured onto the sensor by BP with high selectivity and then produced a reduction current at around -0.01 V. The ^•OH quantification was established on the cleavage of the hybrid structure between DNA1 and BP-Au upon the appearance of ^•OH in the phosphate-buffered saline (PBS), leading to the depletion of the voltammetric response of MB around -0.25 V. Good linear correlations were obtained over concentrations of 0.5-127.5 μM for Cu²⁺ and 0.5-96.0 μM for ^•OH. Most importantly, the developed sensor was successfully applied to track the variations of the two species in brain tissues from APP/PS1 transgenic AD mice at the early stages before massive Aβ plaques appeared.

Signal multi-amplified electrochemical biosensor for voltammetric determination of tau-441 protein in biological samples using carbon nanomaterials and gold nanoparticles to hint dementia

A signal multi-amplified electrochemical biosensor was fabricated for tau-441 protein, a dementia biomarker. It utilizes a carbon nanocomposite film modified gold electrode. The carbon nanocomposite film was composed of multi-walled carbon nanotubes (MWCNTs), reduced graphene oxide (rGO), and chitosan (CS). For the nanocomposite film, rGO improved the dispersibility of MWCNTs, and the effective surface area of MWCNTs was increased. On the other hand, MWCNTs also increased the interlayer spacing of rGO, resulting in a thinner rGO layer. MWCNTs-rGO had a better conductivity than that of MWCNTs and rGO due to the synergy effect. Biocompatible CS was employed for immobilization of the specific antibody. Tau-441 protein was modified with gold nanoparticles (AuNPs) for signal amplification again. The response of the electrochemical biosensor is linear in the range 0.5-80 fM (0.5, 1.5, 5, 10, 40, 80 fM) with a limit of detection (LOD) of 0.46 fM, using differential pulse voltammetry (DPV) in a potential range of - 100-500 mV. The biosensor was successfully applied to the analysis of serum samples of 14 normal people, 14 mild cognitive impairment (MCI) patients, and 14 dementia patients. Graphical abstract Schematic representation of signal multi-amplified electrochemical biosensor for determination of tau-441 protein in human serum.

Double-signal mode based on metal–organic framework coupled cascaded nucleic acid circuits for accurate and sensitive detection of serum circulating miRNAs

A double-signal mode based on metal–organic framework coupled cascaded nucleic acid circuits was developed for the accurate and sensitive detection of serum circulating miRNAs.

Design and Biosensing of a Ratiometric Electrochemiluminescence Resonance Energy Transfer Aptasensor between a g-C3N4 Nanosheet and Ru@MOF for Amyloid-β Protein

A dual-wavelength ratiometric electrochemiluminescence resonance energy transfer (ECL-RET) aptasensor based on the carbon nitride nanosheet (g-C₃N₄ NS) and metal-organic frameworks (Ru@MOFs) as energy donor-receptor pairs is first designed for the detection of the amyloid-β (Aβ) protein. The cathode ECL of g-C₃N₄ NS gradually decreased, whereas the anode ECL from Ru@MOF pyramidally enhanced along with the increasing concentration of Aβ in a 0.1 M phosphate-buffered saline solution containing 0.1 M S₂O₈^2-. Additionally, it is worth noting that 2-amino terephthalic acid from MOF not only can load abundant amounts of luminophor Ru(bpy)₃²⁺ but also promote the conversion of more amounts of S₂O₈^2- that served as a coreactant accelerator into SO₄^•-, further enhancing the ECL signal of Ru@MOF. Besides, the ECL intensity from the g-C₃N₄ NS had a tremendous spectrum overlap with the UV-vis spectrum of Ru@MOF, demonstrating the high-efficiency ECL-RET from g-C₃N₄ NS to Ru@MOF. According to the ratio of ECL_460nm/ECL_620nm, the constructed aptasensor for the detection of Aβ showed a wide linear range from 10^-5 to 500 ng/mL and a low detection limit of 3.9 fg/mL (S/N = 3) with a correction coefficient of 0.9965. The obtained results certified that the dual-wavelength ratiometric ECL sensor could provide a reliable direction and have the potential for application in biosensing and clinical diagnosis fields.

MXene with Great Adsorption Ability toward Organic Dye: An Excellent Material for Constructing a Ratiometric Electrochemical Sensing Platform

Ratiometric electrochemical sensors coupled with an intrinsic built-in correction have received much attention in biochemical analysis, which can effectively avoid potential impacts from both intrinsic and extrinsic factors. However, the complex modification procedure and the unstable reference signal limit development and application of ratiometric sensing. To address these issues, we proposed a novel ratiometric electrochemical platform based on MXene. Introduction of built-in correction was realized via simple one-step incubation of MXene in solution containing the reference molecule methylene blue (MB), and their firm electrostatic interaction ensures the strong adsorption capability of MXene toward MB. Remarkable enhancement in repeatibility and stability compared with nonratio sensor was proved by detecting the model analyte piroxicam. Furthermore, compatibility of the ratio sensor was demonstrated by integrating copper nanoparticles (CuNPs) into the platform. It turned out that sensing performance of the hybrid electrochemical sensor was significantly improved owing to synergistic effect of MXene and CuNPs, where the former affords a large specific surface area as well as quick electron transport, and the latter possess decent electrical catalytic ability. In all, the proposed ratiometric sensor based on MXene features easy preparation, superb reproducibility, robustness, and broad applicability, affording the platform highly competitive and reliable in the determination of a wide range of substances.

On-line regeneration of electrochemical biosensor for in vivo repetitive measurements of striatum Cu2+ under global cerebral ischemia/reperfusion events

The detection of Cu²⁺ ion, one of the metal ions substantial in cerebral physiology, is critical in studying brain activities and understanding brain functions. However, repetitive measurements of Cu²⁺ in the progress of physiological and pathological events is still challenging, because lack of the platform for repetitive on-line detection-regeneration cycle. Herein we report the design of a regenerated electrochemical biosensor combined with the in vivo microdialysis system. In this biosensor, hyperbranched polyethyleneimine (hPEI) acts as a regenerated recognition unit for Cu²⁺. Just by a simple rinse of ethylenediaminetetraacetic acid (EDTA) disodium salt, the Cu²⁺ and Cu⁺ ions on the biosensor interface were chelated with EDTA disodium salt, thus achieving the regeneration of the biosensor. In addition, 6-(ferrocenyl)hexanethiol (FcHT) serves as the inner reference moiety to elevate the sensing accuracy over regeneration cycles. As a result, this ratiometric electrochemical biosensor not only revealed high sensitivity and selectivity, but also exhibited excellent stability during multiple regeneration processing. This biosensor was capable of determining Cu²⁺ with a linear range between 0.05 and 12 μM and low detection limit (LOD) of 13 nM. Then, the platform has been successfully applied in repetitive Cu²⁺ analysis in rat brain under global cerebral ischemia/reperfusion events. The combination of results from 7 rats indicates global cerebral ischemia caused an obvious increase of the Cu²⁺ level, while reperfusion brought this level back to normal.

Real-time monitoring of peroxynitrite (ONOO−) in the rat brain by developing a ratiometric electrochemical biosensor

As a reactive oxygen species (ROS), peroxynitrite (ONOO⁻) generated by nitric oxide (NO) and superoxide anion (O₂˙⁻) plays important roles in physiological and pathological processes in the brain.

Combined determination of copper ions and β-amyloid peptide by a single ratiometric electrochemical biosensor

Copper ions (Cu²⁺) play a critical role in biological processes and are directly involved in β-amyloid peptide (Aβ) aggregation, which is responsible for the occurrence and development of Alzheimer's disease (AD). Therefore, combined determination of Cu²⁺ and Aβ in one analytical system is of great significance to understand the exact nature of the AD event. This work presents a novel ratiometric electrochemical biosensor for the dual determination of Cu²⁺ and Aβ_1-42. This unique sensor is based on a 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulphonate) (ABTS) and poly(diallyldimethylammonium chloride) (PDDA)-bi functionalized single-walled carbon nanotubes (ABTS-PDDA/CNTs) composite. The inclusion of ABTS not only enhanced the sensitivity, but it also acted as an inner reference molecule to improve detection accuracy. The specific recognition of Cu²⁺ was realized by neurokinin B (NKB) coatings on the ABTS-PDDA/CNTs surface to form a [Cu^II(NKB)₂] complex with Cu²⁺. The ABTS-PDDA/CNTs-NKB modified electrode also displayed an excellent electrochemical response toward the Aβ_1-42 monomer, when a certain amount of the Aβ_1-42 monomer was added to Cu²⁺-contained PBS buffer, which was due to the release of Cu²⁺ from the [Cu^II(NKB)₂] complex through Aβ binding to Cu²⁺. Meanwhile, our work showed that Cu²⁺ bound Aβ_1-42 was concentration-dependent. Consequently, the presented electrochemical approach was capable of quantifying two important biological species associated with AD by one single biosensor, with the detection limits of 0.04 μM for Cu²⁺ and 0.5 ng mL^-1 for Aβ_1-42, respectively. Finally, the ratiometric electrode was successfully applied for monitoring Cu²⁺ and Aβ_1-42 variations in plasma and hippocampus of normal and AD rats.

A ratiometric electrochemical strategy for sensitive determination of Furin activity based on dual signal amplification and antifouling nanosurfaces

Developing a sensitive and accurate method for Furin activity is still the bottleneck for understanding the role played by Furin in cell-surface systems and even in Alzheimer's disease. In this work, a ratiometric electrochemical biosensor was developed for sensitive and accurate determination of Furin activity in the cell based on dual signal amplification stemming from a peptide with multiple response sites and the antifouling gold nano-bellflowers (GBFs). A new peptide, HS-CMRVRR↓YKDFDFG (P3), was designed for the first time to be selectively cleaved by Furin at site↓. More importantly, this peptide P3 constitutes three amino acid residues with the -COOH group subsequently used to bind with the response molecule of ferrocene, and can remarkably improve the determination sensitivity by about 2.3 fold. Meanwhile, GBFs stabilized by PEG were taken as a second element to magnify the signal of the ferrocene group via a large ratio surface area and good conductivity, as well as an antibiofouling nanosurface to reduce the biofouling of the electrode surface in cells. This double amplification strategy can greatly enhance the sensitivity of Furin detection by 6.5-fold, which is favorable for detection of low amounts of Furin. In addition, 5'-MB-GGCGCGA(T)₁₃-SH-3' was co-assembled as an inner reference to provide a built-in element to correct the determination error resulting from a complicated analysis environment. Finally, this sensitive and accurate Furin biosensor was successfully applied to detect Furin activity in Furin overexpressed U251 and MDA-MB-468 cells. As far as we know, this is the first report to mention an electrochemical strategy to detect Furin activity in cells.

Designing Recognition Molecules and Tailoring Functional Surfaces for In Vivo Monitoring of Small Molecules in the Brain

The in vivo analysis of chemical signals in brain extracellular fluid (ECF) using implanted electrochemical biosensors is a vital way to study brain functions and brain activity mapping. This approach offers excellent spatial (10-200 μm) and temporal (approximately second) resolution and the major advantage of long-term stability. By implantation of a microelectrode in a specific brain region, changes in the concentration of a variety of ECF chemical species can be monitored through applying a suitable electrical signal and, typically, recording the resulting Faradaic current. However, the high performance requirements for in vivo biosensors greatly limit our understanding of the roles that biomolecules play in the brain. Since a large number of biological species, including reactive oxygen species (ROS), metal ions, amino acids, and proteins, coexist in the brain and interact with each other, developing in vivo biosensors with high selectivity is a great challenge. Meanwhile, it is difficult to quantitatively determine target molecules in the brain because of the variation in the distinct environments for monitoring biomolecules in vitro and in vivo. Thus, there are large errors in the quantification of concentrations in the brain using calibration curves obtained in artificial cerebrospinal fluid (aCSF). More importantly, to gain a full understanding of the physiological and pathological processes in the brain, the development of novel approaches for the simultaneous determination of multiple species in vivo is urgently needed. This Account provides insight into the basic design principles and criteria required to convert chemical/electrochemical reactions into electric signals, while satisfying the increasing requirements, including high selectivity, sensitivity, and accuracy, for the in vivo analysis of biomolecules in the brain. Recent developments in designing various functional surfaces, such as self-assembled monolayers, gold nanostructures, and nanostructured semiconductors for facilitating electron transfer from specific enzymes, including superoxide dismutase (SOD), and further application to an O₂^•- biosensor are summarized. This Account also aims to highlight the design principles for the selective biosensing of Cu²⁺ and pH in the brain through the rational design and synthesis of specific recognition molecules. Additionally, electrochemical ratiometric biosensors with current signal output have been constructed to correct the effect of distinct environments in a timely manner, thus greatly improving the accuracy of the determination of Cu²⁺ in the live brain. This method of using a built-in element has been extended to biosensors with the potential signal output for in vivo pH analysis. More importantly, the new concept of both current and potential signal outputs provides an avenue to simultaneously determine dual species in the brain. The extension of the design principles and developed strategy demonstrated in this Account to other biomolecules, which may be closely correlated to the biological processes of brain events, is promising. The final section of this Account outlines potential future directions in tailoring functional surfaces and designing recognition molecules based on recent advances in molecular science, nanoscience and nanotechnology, and biological chemistry for the design of advanced devices with multiple target species to map the molecular imaging of the brain. There are still opportunities to engineer surfaces that improve on this approach by constructing implantable, multifunctional nanodevices that promise to combine the benefits of multiple sensing and therapeutic modules.

A reusable ratiometric electrochemical biosensor on the basis of the binding of methylene blue to DNA with alternating AT base sequence for sensitive detection of adenosine

We develop a reusable ratiometric electrochemical biosensor on the basis of the binding of methylene blue (MB) to DNA with alternating AT base sequence for sensitive detection of adenosine. We design a strand 1 with MB-modified thymine (T) base in the proximal 3' termini as the capture probe for its immobilization on the gold electrode and a 3' termini ferrocene (Fc)-modified aptamer for the recognition of adenosine. The hybridization of strand 1 with the aptamer leads to the formation of a double-stranded DNA (dsDNA) and consequently the away of MB from the electrode surface and the close of Fc to the electrode surface, generating a small value of I_MB/I_Fc (I_MB and I_Fc are the peak currents of MB and Fc, respectively). In the presence of adenosine, its binding with the aptamer induces the release of Fc from the electrode surface and the close of MB to the electrode surface, generating a large value of I_MB/I_Fc. As a result, adenosine may be accurately quantified by the measurement of ratiometric signal (I_MB/I_Fc). This ratiometric electrochemical biosensor can be simply fabricated and exhibits high sensitivity with a limit of detection of as low as 90.8pM and a large dynamic range from 0.1nM to 100μM. Moreover, this biosensor demonstrates good performance with excellent selectivity, regeneration capability, high reliability and good reproducibility, and may become a universal platform for the detection of various biomolecules which can be recognized by aptamers, holding great potential for further applications in biomedical research and clinical diagnosis.

Highly sensitive electrochemical nuclear factor kappa B aptasensor based on target-induced dual-signal ratiometric and polymerase-assisted protein recycling amplification strategy

In this work, an amplified electrochemical ratiometric aptasensor for nuclear factor kappa B (NF-κB) assay based on target binding-triggered ratiometric signal readout and polymerase-assisted protein recycling amplification strategy is described. To demonstrate the effect of "signal-off" and "signal-on" change for the dual-signal electrochemical ratiometric readout, the thiol-hairpin DNA (SH-HD) hybridizes with methylene blue (MB)-modified protection DNA (MB-PD) to form capture probes, which is rationally introduced for the construction of the assay platform. On the interface, the probes can specifically bind to target NF-κB and expose a toehold region which subsequently hybridizes with the ferrocene (Fc)-modified DNA strand to take the Fc group to the electrode surface, accompanied by displacing MB-PD to release the MB group from the electrode surface, leading to the both "signal-on" of Fc (I_Fc) and "signal-off" of MB (I_MB). In order to improve the sensitivity of the electrochemical aptasensor, phi29-assisted target protein recycling amplification strategy was designed to achieve an amplified ratiometric signal. With the above advantages, the prepared aptasensor exhibits a wide linear range of 0.1pgmL^-1 to 15ngmL^-1 with a low detection limit of 0.03pgmL^-1. This strategy provides a simple and ingenious approach to construct ratiometric electrochemical aptasensor and shows promising potential applications in multiple disease marker detection by changing the recognition probe.

Colorimetric assay of copper ions based on the inhibition of peroxidase-like activity of MoS2 nanosheets

The peroxidase-like catalytic activity of MoS₂ nanomaterials has been utilized for colorimetric bioassays and medical diagnostics. However, the application of peroxidase-like catalytic activity of MoS₂ nanomaterials in environmental analysis was seldom explored. Herein, copper ions were found to inhibit the peroxidase-like catalytic activity of MoS₂ nanosheets, which can catalyze the oxidation of 3, 3', 5, 5'-tetramethylbenzidine by H₂O₂ to produce a colorimetric product. Based on this finding, a simple sensitive colorimetric method for the detection of copper ions was developed. In the presence of copper ions, the absorbance and color of the solution decreased with the increasing concentration of copper ions. The color of the solution can be used to semi-quantitative on-site assay of copper ions by naked eyes. A linear relationship between the absorbance and the concentration of copper ions was observed in the range of 0.4-4.0μmolL^-1 with a detection limit of 92nmolL^-1, which was much lower than the maximum contaminant level of copper in drinking water legislated by the Environmental Protection Agency of USA and the World Health Organization. The method was applied to detect copper ions in environmental water samples with satisfactory results.