The Application of Metal-Organic Frameworks in Water Treatment and Their Large-Scale Preparation: A Review

Over the last few decades, there has been a growing discourse surrounding environmental and health issues stemming from drinking water and the discharge of effluents into the environment. The rapid advancement of various sewage treatment methodologies has prompted a thorough exploration of promising materials to capitalize on their benefits. Metal-organic frameworks (MOFs), as porous materials, have garnered considerable attention from researchers in recent years. These materials boast exceptional properties: unparalleled porosity, expansive specific surface areas, unique electronic characteristics including semi-conductivity, and a versatile affinity for organic molecules. These attributes have fueled a spike in research activity. This paper reviews the current MOF-based wastewater removal technologies, including separation, catalysis, and related pollutant monitoring methods, and briefly introduces the basic mechanism of some methods. The scale production problems faced by MOF in water treatment applications are evaluated, and two pioneering methods for MOF mass production are highlighted. In closing, we propose targeted recommendations and future perspectives to navigate the challenges of MOF implementation in water purification, enhancing the efficiency of material synthesis for environmental stewardship.

Alkyne-tagged SERS nanoprobe for understanding Cu+ and Cu2+ conversion in cuproptosis processes

Simultaneously quantifying mitochondrial Cu+ and Cu2+ levels is crucial for evaluating the molecular mechanisms of copper accumulation-involved pathological processes. Here, a series of molecules containing various diacetylene derivatives as Raman reporters are designed and synthesized, and the alkyne-tagged SERS probe is created for determination Cu+ and Cu2+ with high selectivity and sensitivity. The developed SERS probe generates well-separated distinguishable Raman fingerprint peaks with built-in corrections in the cellular silent region, resulting in accurate quantification of Cu+ and Cu2+. The present probe demonstrates high tempo-spatial resolution for real-time imaging and simultaneously quantifying mitochondrial Cu+ and Cu2+ with long-term stability benefiting from the probe assembly with designed Au-C≡C groups. Using this powerful tool, it is found that mitochondrial Cu+ and Cu2+ increase during ischemia are associated with breakdown of proteins containing copper as well as conversion of Cu+ and Cu2+. Meanwhile, we observe that parts of Cu+ and Cu2+ are transported out of neurons by ATPase. More importantly, cuproptosis in neurons is found including the oxidative stress process caused by the conversion of Cu+ to Cu2+, which dominates at the early stage (<9 h), and subsequent proteotoxic stress. Both oxidative and proteotoxic stresses contribute to neuronal death.

Advancements in Brain Research: The In Vivo/In Vitro Electrochemical Detection of Neurochemicals

Neurochemicals, crucial for nervous system function, influence vital bodily processes and their fluctuations are linked to neurodegenerative diseases and mental health conditions. Monitoring these compounds is pivotal, yet the intricate nature of the central nervous system poses challenges. Researchers have devised methods, notably electrochemical sensing with micro-nanoscale electrodes, offering high-resolution monitoring despite low concentrations and rapid changes. Implantable sensors enable precise detection in brain tissues with minimal damage, while microdialysis-coupled platforms allow in vivo sampling and subsequent in vitro analysis, addressing the selectivity issues seen in other methods. While lacking temporal resolution, techniques like HPLC and CE complement electrochemical sensing's selectivity, particularly for structurally similar neurochemicals. This review covers essential neurochemicals and explores miniaturized electrochemical sensors for brain analysis, emphasizing microdialysis integration. It discusses the pros and cons of these techniques, forecasting electrochemical sensing's future in neuroscience research. Overall, this comprehensive review outlines the evolution, strengths, and potential applications of electrochemical sensing in the study of neurochemicals, offering insights into future advancements in the field.

Electrochemical Nanopipette Sensor for In Vitro/In Vivo Detection of Cu2+ Ions

In vitro/in vivo detection of copper ions is a challenging task but one which is important in the development of new approaches to the diagnosis and treatment of cancer and hereditary diseases such as Alzheimer's, Wilson's, etc. In this paper, we present a nanopipette sensor capable of measuring Cu2+ ions with a linear range from 0.1 to 10 μM in vitro and in vivo. Using the gold-modified nanopipette sensor with a copper chelating ligand, we evaluated the accumulation ability of the liposomal form of an anticancer Cu-containing complex at three levels of biological organization. First, we detected Cu2+ ions in a single cell model of human breast adenocarcinoma MCF-7 and in murine melanoma B16 cells. The insertion of the nanoelectrode did not result in leakage of the cell membrane. We then evaluated the distribution of the Cu-complex in MCF-7 tumor spheroids and found that the diffusion-limited accumulation was a function of the depth, typical for 3D culture. Finally, we demonstrated the use of the sensor for Cu2+ ion detection in the brain of an APP/PS1 transgenic mouse model of Alzheimer's disease and tumor-bearing mice in response to injection (2 mg kg-1) of the liposomal form of the anticancer Cu-containing complex. Enhanced stability and selectivity, as well as distinct copper oxidation peaks, confirmed that the developed sensor is a promising tool for testing various types of biological systems. In summary, this research has demonstrated a minimally invasive electrochemical technique with high temporal resolution that can be used for the study of metabolism of copper or copper-based drugs in vitro and in vivo.

Ratiometric electrochemical detection of kojic acid based on glassy carbon modified MXene nanocomposite

The significance of developing a selective and sensitive sensor for quality control purposes is underscored by the prevalent use of kojic acid (KA) in cosmetics, pharmaceuticals, and food items. KA's utility stems from its ability to inhibit tyrosinase activity. However, the instability of KA and its potential adverse effects have created a pressing need for accurate and sensitive sensors capable of analyzing real samples. This research introduces an electrochemical ratiometric sensor designed to accurately detect KA in actual cosmetic and food samples. The ratiometric sensor offers distinct advantages such as enhanced selectivity, reproducibility, and sensitivity. It achieves this by leveraging the ratio between two output signals, thereby producing reliable and undistorted results. The sensor is constructed by modifying a Glassy Carbon Electrode (GCE) with a nanocomposite consisting of Ti3C2 MXene, Prussian blue, and gold nanoparticles. The incorporation of MXene and gold nanoparticles heightens sensitivity and reduces impedance. Meanwhile, the Prussian blue signal diminishes proportionally with increasing KA concentration, forming the basis for the ratiometric sensing mechanism. The outcomes of the study reveal a broad linear range (1-600 μM), a low detection limit (1 μM), and strong selectivity for KA. These findings suggest the sensor's potential efficacy in quality control across cosmetics, pharmaceuticals, and food products.

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.

CuS quantum dots activated DNAzyme for ratiometric electrochemical detection of telomerase activity

Telomerase activity detection has attracted much attention concerning its importance for early cancer diagnosis. Here, we established a ratiometric electrochemical biosensor for telomerase detection based on CuS quantum dots (CuS QDs) dependent DNAzyme-regulated dual signals. The telomerase substrate probe was used as the linker to combine the DNA fabricated magnetic beads and CuS QDs. In this way, telomerase extended the substrate probe with repeated sequence to from hairpin structure, releasing CuS QDs as an input to DNAzyme modified electrode. DNAzyme was cleaved with high current of ferrocene (Fc) and low current of methylene blue (MB). On the basis of the obtained ratiometric signals, telomerase activity detection was achieved in the range of 1.0 × 10^-12-1.0 × 10^-6 IU/L, with the limit of detection down to 2.75 × 10^-14 IU/L. Moreover, telomerase activity from HeLa extracts was also tested to verify the clinical application.

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.

Electrochemical Biosensors in the Diagnosis of Acute and Chronic Leukemias

Until now, morphological assessment with an optical or electronic microscope, fluorescence in situ hybridization, DNA sequencing, flow cytometry, polymerase chain reactions, and immunohistochemistry have been employed for leukemia identification. Nevertheless, despite their numerous different vantages, it is difficult to recognize leukemic cells correctly. Recently, the electrochemical evaluation with a nano-sensing interface seems an attractive alternative. Electrochemical biosensors measure the modification in the electrical characteristics of the nano-sensing interface, which is modified by the contact between a biological recognition element and the analyte objective. The implementation of nanosensors is founded not on single nanomaterials but rather on compilating these components efficiently. Biosensors able to identify the molecules of deoxyribonucleic acid are defined as DNA biosensors. Our review aimed to evaluate the literature on the possible use of electrochemical biosensors for identifying hematological neoplasms such as acute promyelocytic leukemia, acute lymphoblastic leukemia, and chronic myeloid leukemia. In particular, we focus our attention on using DNA electrochemical biosensors to evaluate leukemias.

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.

Echem methods and electrode types of the current in vivo electrochemical sensing

For a long time, people have been eager to realize continuous real-time online monitoring of biological compounds. Fortunately, in vivo electrochemical biosensor technology has greatly promoted the development of biological compound detection. This article summarizes the existing in vivo electrochemical detection technologies into two categories: microdialysis (MD) and microelectrode (ME). Then we summarized and discussed the electrode surface time, pollution resistance, linearity and the number of instances of simultaneous detection and analysis, the composition and characteristics of the sensor, and finally, we also predicted and prospected the development of electrochemical technology and sensors in vivo.

A ratiometric fluorescence-scattering sensor for rapid, sensitive and selective detection of doxycycline in animal foodstuffs

The residue problem of tetracycline antibiotics, especially doxycycline (DC), in animal foodstuffs has attracted much attention. This paper reported ZIF-8 and bovine serum albumin (BSA) as a ratiometric fluorescence-scattering sensor for DC. The mechanism relied on the disassembly of ZIF-8 caused by DC, bringing weakened second-order scattering, and the double fluorescence amplification of DC under ZIF-8 with BSA, inducing enhanced fluorescence. The response of the sensor was completed within 1 min, and the detection limit for DC (3.4 nM) was 1-2 orders of magnitude lower than the reported ones. The distinguishment of DC from other tetracycline antibiotics was also achieved by the sensor. The sensor was applied to detecting DC in animal foodstuffs with satisfactory recoveries (80.0-104.0%). Hence, this work develops a rapid, sensitive and selective ratiometric sensor to monitor the DC residue in animal foodstuffs, also opens the window to construct ratiometric DC sensors with the fluorescence-scattering strategy.

Raman Fiber Photometry for Understanding Mitochondrial Superoxide Burst and Extracellular Calcium Ion Influx upon Acute Hypoxia in the Brain of Freely Moving Animals

Developing a novel tool capable of real-time monitoring and simultaneous quantitation of multiple molecules in mitochondria across the whole brain of freely moving animals is the key bottleneck for understanding the physiological and pathological roles that mitochondria play in the brain events. Here we built a Raman fiber photometry, and created a highly selective non-metallic Raman probe based on the triple-recognition strategies of chemical reaction, charge transfer, and characteristic fingerprint peaks, for tracking and simultaneous quantitation of mitochondrial O₂ ^.- , Ca²⁺ and pH at the same location in six brain regions of free-moving animal upon hypoxia. It was found that mitochondrial O₂ ^.- , Ca²⁺ and pH changed from superficial to deep brain regions upon hypoxia. It was discovered that hypoxia-induced mitochondrial O₂ ^.- burst was regulated by ASIC1a, leading to mitochondrial Ca²⁺ overload and acidification. Furthermore, we found the overload of mitochondrial Ca²⁺ was mostly attributed to the influx of extracellular Ca²⁺ .

Gold nanostructures: synthesis, properties, and neurological applications

Recent advances in technology are expected to increase our current understanding of neuroscience. Nanotechnology and nanomaterials can alter and control neural functionality in both in vitro and in vivo experimental setups. The intersection between neuroscience and nanoscience may generate long-term neural interfaces adapted at the molecular level. Owing to their intrinsic physicochemical characteristics, gold nanostructures (GNSs) have received much attention in neuroscience, especially for combined diagnostic and therapeutic (theragnostic) purposes. GNSs have been successfully employed to stimulate and monitor neurophysiological signals. Hence, GNSs could provide a promising solution for the regeneration and recovery of neural tissue, novel neuroprotective strategies, and integrated implantable materials. This review covers the broad range of neurological applications of GNS-based materials to improve clinical diagnosis and therapy. Sub-topics include neurotoxicity, targeted delivery of therapeutics to the central nervous system (CNS), neurochemical sensing, neuromodulation, neuroimaging, neurotherapy, tissue engineering, and neural regeneration. It focuses on core concepts of GNSs in neurology, to circumvent the limitations and significant obstacles of innovative approaches in neurobiology and neurochemistry, including theragnostics. We will discuss recent advances in the use of GNSs to overcome current bottlenecks and tackle technical and conceptual challenges.

A label-free and modification-free ratiometric electrochemical strategy for enhanced natural enzyme detection using a bare electrode and nanozymes system

Ratiometric electrochemical assays have been demonstrated to be more sensitive and selective in various sensing events, mainly due to their affordable built-in correction and good self-reference capability. But it is known that complicated modification and labeling operations usually are necessary for the construction of ratiometric electrochemical assays, therefore is a hot and important issue needing consideration carefully. We herein report a new yet simple bare electrode-based ratiometric electrochemical bioassay to achieve sensitive and selective analysis of alkaline phosphatase (ALP), using a liquid phase system that contains CoOOH nanozymes and commercially available indicator substrate. This proposed bioassay works based on the ratiometric change of dual electrochemical signals, arising from an exclusive target ALP-triggered hydrolysis of electrochemical substrate p-nitrophenyl phosphate (PNPP). In this design, the two hydrolyzed products of electrochemically active p-nitrophenol (PNP) and electrochemically inactive phosphate anion (PO₄^3-) are responsible together for the ratiometric electrochemical analysis of ALP. PNP exhibits a straightforward current response toward ALP content; however, PO₄^3- cannot show a direct electrochemical signal thus is rationally designed to offer an alternative response by linking it with the specific CoOOH nanozyme-catalyzed reaction of 3,3',5,5'-tetramethylbenzidine (TMB) and H₂O₂, in which the nanozyme-catalyzed product oxTMB shows a direct reduction current at the GCE, and significantly decreases with increasing PO₄^3- species due to the good inhibition of PO₄^3- toward CoOOH nanozyme activity. As a result, a ratiometric electrochemical strategy for ALP analysis with a low limit of detection of 0.366 U/L (S/N = 3) was successfully achieved by integrating the above direct and indirect dual electrochemical responses. This developed bioassay can allow the quantitative diagnosis of ALP activity especially with a label-free and modification-free merit, therefore paving the way for simple, convenient, and portable electroanalytical tools in biosensing design and application.

Highly Stable Electrochemical Probe with Bidentate Thiols for Ratiometric Monitoring of Endogenous Polysulfide in Living Mouse Brains

The lack of reliable approaches for real-time measurement and quantification of polysulfides (H₂S_n) in vivo greatly limits the exploration of their potential roles in brain functions. Herein, an electrochemical probe, 4-(5-(1,2-dithiolan-3-yl)pentanamido)-1,2-phenylene bis(2-fluoro-5-nitrobenzoate) (FP2), was rationally designed and created for determination of H₂S_n. The bis-electrophilic groups of FP2 could specifically recognize two -SH groups in H₂S_n and trigger the generation of an electroactive pyrocatechol moiety, resulting in a well-defined faradic current signal at ∼0.24 V (vs Ag/AgCl). Meanwhile, bidentate thiols were designed as anchoring sites to greatly improve the assembled stability of FP2 at the Au surface, which efficiently defended the interference of glutathione (GSH) with a current decrease of less than 5.2% even after long-term measurements in 5 mM GSH for 3 h. In addition, a stable inner reference molecule with dithiols, α-lipoic acid ferrocenylamide (FcBT), was synthesized to construct a ratiometric electrochemical strategy for in vivo determination of H₂S_n through one-step coassembling with FP2 via double S-Au bonds. The present ratiometric strategy demonstrated high selectivity for real-time tracking of H₂S_n in a linear range of 0.25-20 μM. Eventually, the developed microelectrode with high selectivity, accuracy, and stability was employed for in vivo assaying of H₂S_n in mouse brains with ischemia.

Real-time Tracking and Sensing of Cu+ and Cu2+ with a Single SERS Probe in the Live Brain: Toward Understanding Why Copper Ions Were Increased upon Ischemia

The imbalance of Cu⁺ and Cu²⁺ in the brain is closely related to neurodegenerative diseases. However, it still lacks of effective analytical methods for simultaneously determining the concentrations of Cu⁺ and Cu²⁺ . Herein, we created a novel SERS probe (CuSP) to real-time track and accurately quantify extracellular concentrations of Cu⁺ and Cu²⁺ in the live brain. The present CuSP probe demonstrated specific ability for recognition of Cu⁺ and Cu²⁺ in a dual-recognition mode. Then, a microarray consisting of 8 CuSP probes with high tempo-spatial resolution and good accuracy was constructed for tracking and simultaneously biosensing of Cu⁺ and Cu²⁺ in the cerebral cortex of living brain. Using our powerful tool, it was found that that the concentrations of Cu²⁺ and Cu⁺ were increased by ≈4.26 and ≈1.80 times upon ischemia, respectively. Three routes were first discovered for understanding the mechanisms of the increased concentrations of Cu⁺ and Cu²⁺ during ischemia.

Sensitive Dual-Mode Biosensors for CYFRA21-1 Assay Based on the Dual-Signaling Electrochemical Ratiometric Strategy and "On-Off-On" PEC Method

Herein, an electrochemical (EC)-photoelectrochemical (PEC) dual-mode biosensor was constructed for cytokeratin 19 fragment 21-1 (CYFRA21-1) assay based on the dual-signaling electrochemical ratiometric strategy and "on-off-on" PEC method. The indium tin oxide (ITO) electrode was modified by 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)@C₆₀ and gold nanoparticles (Au NPs), and the double-stranded DNA composed of thiol/methylene blue (MB)-labeled single-stranded DNA (ssDNA) (S0-MB) and antibody/ferrocene (Fc)-labeled ssDNA (Ab1-S1-Fc) was immobilized on the Au NPs/PTCDA@C₆₀/ITO electrode via the Au-S bond between Au NPs and thiol of S0-MB. With the help of another antibody-labeled ssDNA (Ab2-S2), the presence of CYFRA21-1 triggered a typical antigen-antibody sandwich immune reaction (Ab1, CYFRA21-1, and Ab2) and proximity hybridization between Ab1-S1-Fc and Ab2-S2. This caused the release of Ab1-S1-Fc from the modified electrode and the change of S0-MB to a hairpin structure, resulting in a decrease (an increase) of the oxidation peak current of Fc (MB) and an increase of the photocurrent due to the enhancing (inhibiting) effect of MB (Fc) on the photoelectric performance of the Au NPs/PTCDA@C₆₀/ITO electrode. Thus, CYFRA21-1 was detected by the developed EC-PEC dual-mode sensing platform sensitively, and the linear response ranges of 0.001-40 ng/mL with a detection limit of 0.3 pg/mL for the EC technique and 0.0001-4 ng/mL with a detection limit of 0.03 pg/mL for the PEC method were obtained. Furthermore, by changing the specific antibodies of disease-related biomarkers, the developed dual-mode biosensing platform could be readily extended to detect other antigens, implying its great potential applications in biological analysis and early disease diagnosis.

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.

Interaction between the functionalized probes: The depressed efficiency of dual-amplification strategy on ratiometric electrochemical aptasensor for aflatoxin B1

Signal amplification is one of the most effective ways to develop the high-performance electrochemical sensors. However, it can be more complicated for ratiometric detections. Herein, a ratiometric electrochemical aptasensor for aflatoxin B1 (AFB1) was proposed by taking advantage of a dual-amplification strategy by coupling of DNA walker (DW) with hybridization chain reaction (HCR). The special binding of AFB1 with ferrocene (Fc)-labelled aptamer triggers DW on hairpin DNA (hDNA) tracks to produce abundant double-stranded DNA (dsDNA). HCR-based strand amplification occurs on these dsDNA to absorb more methylene blue (MB). Then current ratio of MB (I_MB) and Fc (I_Fc) is designed as a yardstick to detect AFB1. Our experiments reveal that the interaction between Fc and MB (i.e., steric hindrance, electron mediator) varies. In addition to steric hindrance, the presence of MB also acts as electron mediator, thereby facilitating the electron transfer between Fc and electrode. Such combined effect consequently depresses the efficiency of dual-amplification strategy to improve the detection. The developed ratiometric electrochemical aptasensor allows the accurate detection of AFB1 in the 0.003-3 pg mL^-1 range. Our work has shed light on the amplification strategy for ratiometric sensing, and provided a new route in integrating different amplification strategies.

Stimuli-Responsive Plasmonic Assemblies and Their Biomedical Applications

Among the diverse development of stimuli-responsive assemblies, plasmonic nanoparticle (NP) assemblies functionalized with responsive molecules are of a major interest. In this review, we outline a comprehensive and up-to-date overview of recently reported studies on in vitro and in vivo assembly/disassembly and biomedical applications of plasmonic NPs, wherein stimuli such as enzymes, light, pH, redox potential, temperature, metal ions, magnetic or electric field, and/or multi-stimuli were involved. Stimuli-responsive assemblies have been applied in various biomedical fields including biosensors, surfaced-enhanced Raman scattering (SERS), photoacoustic (PA) imaging, multimodal imaging, photo-activated therapy, enhanced X-ray therapy, drug release, stimuli-responsive aggregation-induced cancer therapy, and so on. The perspectives on the use of stimuli-responsive plasmonic assemblies are discussed by addressing future scientific challenges involving assembly/disassembly strategies and applications.

A Robust Au-C≡C Functionalized Surface: Toward Real-Time Mapping and Accurate Quantification of Fe2+ in the Brains of Live AD Mouse Models

Described here is that Au-C≡C bonds showed the highest stability under biological conditions, with abundant thiols, and the best electrochemical performance compared to Au-S and Au-Se bonds. The new finding was also confirmed by theorical calculations. Based on this finding, a specific molecule for recognition of Fe²⁺ was designed and synthesized, and used to create a selective and accurate electrochemical sensor for the quantification of Fe²⁺ . The present ratiometric strategy demonstrates high spatial resolution for real-time tracking of Fe²⁺ in a dynamic range of 0.2-120 μM. Finally, a microelectrode array with good biocompatibility was applied in imaging and biosensing of Fe²⁺ in the different regions of live mouse brains. Using this tool, it was discovered that the uptake of extracellular Fe²⁺ into the cortex and striatum was largely mediated by cyclic adenosine monophosphate (cAMP) through the CREB-related pathway in the brain of a mouse with Alzheimer's disease.

Bifunctional MOFs-Based Ratiometric Electrochemical Sensor for Multiplex Heavy Metal Ions

In this study, a ratiometric electrochemical sensor based on metal-organic frameworks (MOFs) was developed for sensing of multiplex metal ions. The bifunctional MOFs were prepared in a way to integrate two signal tags and a detection probe. In the presence of target metal ions, the target metal ions can replace the framework metal-ion center in the original MOFs through an ion-exchange reaction, leading to ratiometric electrochemical signals under different applied potentials. One consisted of the Cu²⁺ signal generated from electroactive MOFs selected as internal reference signals. The other consisted of the signal induced by other target metal ions. Using the I_metal ions/I_Cu²⁺ signal as the output, the prepared ratiometric probe was able to eliminate disturbance caused by the sensing environment. Moreover, the large surface area and abundant active sites in MOFs produced a multiplex ratiometric electrochemical sensor with improved characteristics in terms of reproducibility, stability, and sensitivity. The sensor was also simple without sophisticated instrumentation, amplification processes, or an acid dissolution/preconcentration procedure, hence promising for practical applications.

Phenyl doped graphitic carbon nitride nanosheets for sensing of copper ions in living cells

Copper (Cu) is a vital metal element for humans and animals. Monitoring and evaluating the concentration level of Cu2+ in a biological body is an effective way to prevent a variety of diseases. In this work, phenyl doped graphitic carbon nitride (PDCN) nanosheets with strong green fluorescence exhibited a sensitive and selective detection for Cu2+ with a linear range from 0.1-2.0 μmol L-1. Furthermore, fluorescent imaging was applied to semiquantitatively detect Cu2+ in HeLa cells using PDCN nanosheets as the probe, which can avoid the interference of background autofluorescence. This work provided a low-cost and biologically friendly fluorescent probe to monitor the concentration level of Cu2+ in living cells.

In Vivo Electrochemical Sensors for Neurochemicals: Recent Update

In vivo electrochemical sensing based on implantable microelectrodes is a strong driving force of analytical neurochemistry in brain. The complex and dynamic neurochemical network sets stringent standards of in vivo electrochemical sensors including high spatiotemporal resolution, selectivity, sensitivity, and minimized disturbance on brain function. Although advanced materials and novel technologies have promoted the development of in vivo electrochemical sensors drastically, gaps with the goals still exist. This Review mainly focuses on recent attempts on the key issues of in vivo electrochemical sensors including selectivity, tissue response and sensing reliability, and compatibility with electrophysiological techniques. In vivo electrochemical methods with bare carbon fiber electrodes, of which the selectivity is achieved either with electrochemical techniques such as fast-scan cyclic voltammetry and differential pulse voltammetry or based on the physiological nature will not be reviewed. Following the elaboration of each issue involved in in vivo electrochemical sensors, possible solutions supported by the latest methodological progress will be discussed, aiming to provide inspiring and practical instructions for future research.

High frequency, real-time neurochemical and neuropharmacological measurements in situ in the living body

The beautiful and complex brain machinery is perfectly synchronized, and our bodies have evolved to protect it against a myriad of potential threats. Shielded physically by the skull and chemically by the blood brain barrier, the brain processes internal and external information so that we can efficiently relate to the world that surrounds us while simultaneously and unconsciously controlling our vital functions. When coupled with the brittle nature of its internal chemical and electric signals, the brain's "armor" render accessing it a challenging and delicate endeavor that has historically limited our understanding of its structural and neurochemical intricacies. In this review, we briefly summarize the advancements made over the past 10 years to decode the brain's neurochemistry and neuropharmacology in situ, at the site of interest in the brain, with special focus on what we consider game-changing emerging technologies (eg, genetically encoded indicators and electrochemical aptamer-based sensors) and the challenges these must overcome before chronic, in situ chemosensing measurements become routine.

Recent advances in the construction of functionalized covalent organic frameworks and their applications to sensing

Covalent organic frameworks (COFs), as an emerging class of porous crystalline polymers, are built by the combination of the light elements through the strong covalent bonds. In the past decade, COFs have been reported to show plenty of unique properties (such as ordered channels, large specific surface area, highly tunable porosity, optional building blocks, predictable and stable structure, and abundant functional groups), and have been widely applied in multiple fields. Recently, to further improve the potential performances of COFs and extend their applicability, a number of COFs with various functionalities have been successfully developed through the functionalization modification. In this review, we summarized the advanced design and construction of functionalized COFs, including COFs with post-synthetic modification, COFs-based composites (e.g. COFs-metal nanoparticles composites, COFs-metal oxide nanoparticles composites, COFs-MOFs composites, and COFs-enzyme composites), and molecularly imprinted COFs. Impressively, the applications of functionalized COFs to sensing also have been comprehensively summarized, including colorimetric sensing, fluorescent sensing, electrochemical sensing, and other sensing (such as quartz crystal microbalance (QCM) sensing, photoelectrochemical sensing, and humidity sensing). In the end, future opportunities and challenges in this promising field are tentatively proposed.

Embedded Au Nanoparticles-Based Ratiometric Electrochemical Sensing Strategy for Sensitive and Reliable Detection of Copper Ions

The ratiometric method allows the measurement of ratio changes between two signals, which can reduce the detection signal fluctuations caused by distinct background conditions and greatly improve the reproducibility and reliability of detection. However, in contrast with the emerging dual excitation or dual emission dyes applied in ratiometric luminescence measurement, only a few internal reference probes have been exploited for ratiometric electrochemical detection. In this paper, a gold nanoparticles@carbonized resin nanospheres composite with thermally reduced graphene oxide as scaffold (AuNPs@CRS-TrGNO) has been fabricated, and the AuNPs embedded in the CRS were first used as an internal reference probe for ratiometric electrochemical detection. The detachment and aggregation of AuNPs is suppressed by embedding in the CRS, so its redox signal is very stable, which provides feasibility for ratiometric detection. Moreover, the embedment of AuNPs, carbonization of resin spheres, and hybridization with TrGNO all have played positive roles in improving the charge transfer rate, which leads to excellent electrochemical performance of the composite. Based on these characteristics of the AuNPs@CRS-TrGNO, a new ratiometric electrochemical detection platform was constructed, and copper ions (Cu²⁺) in simulated seawater were successfully detected. This ratiometric method has the advantages of simple design and convenient operation, and obviously it improves the reproducibility and reliability of the electrochemical sensor.

Rational Design of Specific Recognition Molecules for Simultaneously Monitoring of Endogenous Polysulfide and Hydrogen Sulfide in the Mouse Brain

A biosensor was created for the simultaneous monitoring of endogenous H₂ S_n and H₂ S in mouse brains and exploring their roles in activation of the TRPA1 channel under two types of brain disease models: ischemia and Alzheimer's disease (AD). Based on DFT calculations and electrochemical measurements, two probes, 3,4-bis((2-fluoro-5-nitrobenzoyl)oxy)-benzoic acid (M_PS-1 ) and N-(4-(2,5-dinitrophenoxy) phenyl)-5-(1, 2-dithiolan-3-yl)pentanamide (M_HS-1 ), were synthesized for specific recognition of H₂ S_n and H₂ S. Through co-assembly of the two probes at the mesoporous gold film with good anti-biofouling ability and electrocatalytic activity, this microsensor showed high selectivity for H₂ S_n and H₂ S against potential biological interferences. The biosensor can simultaneously determine the concentration of H₂ S_n from 0.2 to 50 μm, as well as that of H₂ S from 0.2 to 40 μm. The expression of TRPA1 protein positively correlated with levels of H₂ S_n under both ischemia and AD.

A ratiometric fluorescent DNA nanoprobe for cerebral adenosine triphosphate assay

A silver nanocluster-based ratiometric fluorescent nanosensor was developed for the determination of ATP in the cerebrospinal fluid of a mouse brain. Using this useful tool with good stability and high selectivity as well as a wide linear detection range, it was found that the ATP concentration in a mouse brain with Alzheimer's disease was 2300-fold higher than that in a normal one.

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.

Electrochemical sensor based on a three dimensional nanostructured MoS2 nanosphere-PANI/reduced graphene oxide composite for simultaneous detection of ascorbic acid, dopamine, and uric acid

A three dimensional (3D) nanostructured composite based on the self-assembly of MoS₂ nanospheres and polyaniline (PANI) loaded on reduced graphene oxide (denoted by 3D MoS₂-PANI/rGO) was prepared via a feasible one-pot hydrothermal process. The 3D MoS₂-PANI/rGO nanocomposite not only exhibits good functionality and bioaffinity but also displays high electrochemical catalytic activity. As such, the developed 3D MoS₂-PANI/rGO nanocomposite can be employed as the sensing platform for simultaneously detecting small biomolecules, i.e., ascorbic acid (AA), dopamine (DA), and uric acid (UA). The peak currents obtained from the differential pulse voltammetry (DPV) measurements depended linearly on the concentrations in the wide range from 50 μM to 8.0 mM, 5.0 to 500 μM, and 1.0 to 500 μM, giving low detection limits of 22.20, 0.70, and 0.36 μM for AA, DA, and UA, respectively. Furthermore, the 3D MoS₂-PANI/rGO-based electrochemical sensor also exhibited high selectivity, good reproducibility and stability toward small molecule detection. The present sensing strategy based on 3D MoS₂-PANI/rGO suggests a good reliability in the trace determination of electroactive biomolecules.

A three dimensional (3D) nanostructured composite based on the self-assembly of MoS₂ nanospheres and polyaniline (PANI) loaded on reduced graphene oxide (denoted by 3D MoS₂-PANI/rGO) was prepared via a feasible one-pot hydrothermal process.