Vertically Aligned Carbon Nanotube-Sheathed Carbon Fibers as Pristine Microelectrodes for Selective Monitoring of Ascorbate in Vivo

Highly Sensitive and Biocompatible Microsensor for Selective Dynamic Monitoring of Dopamine in Rat Brain

Highly selective and sensitive in vivo neurotransmitter dynamic monitoring of the central nervous system has long been a challenging endeavor. Here, an implantable and biocompatible microsensor with excellent performances was reported by electrodepositing poly(3,4-ethylenedioxythiophene)-electrochemically reduced graphene oxide (PEDOT-ERGO) nanocomposites and poly(tannic acid) (pTA) sequentially on the carbon fiber electrode (CFE) surface, and its feasibility in in vivo electrochemical sensing applications were demonstrated. Due to the synergistic electrocatalytic effect of PEDOT-ERGO nanocomposites with the negative-charged pTA on dopamine (DA) redox reaction, the microsensor exhibits high detection sensitivities of 1.1 and 0.37 nA μM-1 in the detection ranges of 0.02-0.5 and 0.5-20 μM with a low limit of detection of 9.2 nM. Also, the microsensor shows excellent selectivity, good sensing stability, repeatability, and reproducibility. In addition, the highly hydrophilic and negative-charged pTA inhibits the nonspecific adsorption of hydrophobic proteins, which endows the microsensor with good antifouling ability. Moreover, DA dynamics in rat brain were successfully monitored in real time, and the selective sensing ability of the microsensor in vivo was also demonstrated. The present study provides a new method for selective dynamics monitoring of DA in the brain, which would help to better understand the pathological and physiological functions of DA.

Glass nano/micron pipette-based ion current rectification sensing technology for single cell/in vivo analysis

Glass nano/micron pipettes, owing to their easy preparation, unique confined space at the tip, and modifiable inner surface of the tip, can capture the ion current signal caused by a single entity, making them widely used in the construction of highly sensitive and highly selective electrochemical sensors for single entity analysis. Compared with other solid-state nanopores, their conical nano-tip causes less damage to cells when inserted into them, thereby becoming a powerful tool for the in situ analysis of important substances in cells. However, glass nanopipettes have some shortcomings, such as poor mechanical properties, difficulty in precise preparation (aperture less than 50 nm), and easy blockage during complex real sample detection, limiting their practicability. Therefore, in recent years, researchers have conducted a series of studies on glass micropipettes. Ionic current rectification technology is a novel electrochemical analysis technique. Compared with traditional electrochemical analysis methods, it does not generate redox products during the detection process; therefore, it can not only be used for the determination of non-electrochemically active substances, but also causes less damage to the cell/living body in situ analysis, becoming a powerful analysis technology for the in situ analysis of cells/in vivo in recent years. In this review, we summarize the preparation and functionalization of glass nano/micron pipettes and introduce the sensing mechanisms of two electrochemical sensing platforms constructed using glass nano/micron pipette-based ion current rectification sensing technology as well as their applications in single cell/in vivo analysis, existing problems, and future prospects.

General In Situ Engineering of Carbon-Based Materials on Carbon Fiber for In Vivo Neurochemical Sensing

Developing real-time, dynamic, and in situ analytical methods with high spatial and temporal resolutions is crucial for exploring biochemical processes in the brain. Although in vivo electrochemical methods based on carbon fiber (CF) microelectrodes are effective in monitoring neurochemical dynamics during physiological and pathological processes, complex post modification hinders large-scale productions and widespread neuroscience applications. Herein, we develop a general strategy for the in situ engineering of carbon-based materials to mass-produce functional CFs by introducing polydopamine to anchor zeolitic imidazolate frameworks as precursors, followed by one-step pyrolysis. This strategy demonstrates exceptional universality and design flexibility, overcoming complex post-modification procedures and avoiding the delamination of the modification layer. This simplifies the fabrication and integration of functional CF-based microelectrodes. Moreover, we design highly stable and selective H+, O2, and ascorbate microsensors and monitor the influence of CO2 exposure on the O2 content of the cerebral tissue during physiological and ischemia-reperfusion pathological processes.

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.

Electrochemical treatment in KOH improves carbon nanomaterial performance to multiple neurochemicals

Carbon-fiber microelectrodes (CFMEs) are primarily used to detect neurotransmitters in vivo with fast-scan cyclic voltammetry (FSCV) but other carbon nanomaterial electrodes are being developed. CFME sensitivity to dopamine is improved by applying a constant 1.5 V vs. Ag/AgCl for 3 minutes while dipped in 1 M KOH, which etches the surface and adds oxygen functional groups. However, KOH etching of other carbon nanomaterials and applications to other neurochemicals have not been investigated. Here, we explored KOH etching of CFMEs and carbon nanotube yarn microelectrodes (CNTYMEs) to characterize sensitivity to dopamine, epinephrine, norepinephrine, serotonin, and 3,4-dihydroxyphenylacetic acid (DOPAC). With CNTYMEs, the potential was applied in KOH for 1 minute because the electrode surface cracked with the longer time. KOH treatment increased electrode sensitivity to each cationic neurotransmitter roughly 2-fold for CFMEs, and 2- to 4-fold for CNTYMEs. KOH treatment decreased the background current of the CFMEs by etching the surface carbon; however, KOH-treatment increased the CNTYME background current because the potential separates individual nanotubes. For DOPAC, the current increase was smaller at CNTYMEs because it is anionic and was repelled by the negative holding potential and did not access the crevices. XPS and Raman spectroscopy showed that KOH treatment changed the CNTYME surface chemistry by increasing defect sites and adding oxide functional groups. KOH-treated CNTYMEs had less fouling to serotonin than normal CNTYMEs. Therefore, KOH treatment activates both CFMEs and CNTYMEs and could be used in biological measurements to increase the sensitivity and decrease fouling for neurochemical measurements.

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.

A novel electrochemical aptasensor based on polyaniline and gold nanoparticles for ultrasensitive and selective detection of ascorbic acid

Ascorbic acid (AA) is involved in many physiological activities of the body and plays an important role in maintaining and promoting human health. It is also present in many natural and artificial foods. Therefore, the development of highly sensitive and accurate AA sensors is highly desirable for human health monitoring, as well as other commercial application fields. Herein, an ultrasensitive and selective electrochemical sensor based on an aptamer was developed for the determination of AA for the first time. The aptasensor was fabricated by modifying a composite made of polyaniline (PANI) and gold nanoparticles (AuNPs) on a glassy carbon electrode. The morphologies and electrochemical properties of the resulting electrodes were characterized by various analytical methods. The results indicated relatively good electrical conduction properties of PANI for accelerated electron transfer. The modification with AuNPs provided signal amplification, suitable for applications as novel platforms for the sensitive sensing of AA. Under optimized conditions, the proposed aptasensor displayed a wide linear response toward the detection of AA from 1.0 to 1.0 × 105 ng L-1 coupled with a low detection limit of 0.10 ng L-1. The sensor also exhibited excellent selectivity and high stability, with at least 2000-fold higher sensitivity than similar previously reported methods. Importantly, the aptasensor exhibited promising properties for the determination of AA in real fruits, vegetables, and infant milk powder, thereby showing potential for food analysis.

A Novel Poly(3-hexylthiophene) Engineered Interface for Electrochemical Monitoring of Ascorbic Acid During the Occurrence of Glutamate-Induced Brain Cytotoxic Edemas

Although neuroelectrochemical sensing technology offers unique benefits for neuroscience research, its application is limited by substantial interference in complex brain environments while ensuring biosafety requirements. In this study, we introduced poly(3-hexylthiophene) (P3HT) and nitrogen-doped multiwalled carbon nanotubes (N-MWCNTs) to construct a composite membrane-modified carbon fiber microelectrode (CFME/P3HT-N-MWCNTs) for ascorbic acid (AA) detection. The microelectrode presented good linearity, selectivity, stability, antifouling, and biocompatibility and exhibited great performance for application in neuroelectrochemical sensing. Subsequently, we applied CFME/P3HT-N-MWCNTs to monitor AA release from in vitro nerve cells, ex vivo brain slices, and in vivo living rat brains and determined that glutamate can induce cell edema and AA release. We also found that glutamate activated the N-methyl-d-aspartic acid receptor, which enhanced Na⁺ and Cl^- inflow to induce osmotic stress, resulting in cytotoxic edema and ultimately AA release. This study is the first to observe the process of glutamate-induced brain cytotoxic edema with AA release and to reveal the mechanism. Our work can benefit the application of P3HT in in vivo implant microelectrode construction to monitor neurochemicals, understand the molecular basis of nervous system diseases, and discover certain biomarkers of brain diseases.

Aptamer functionalized cell membrane for brain and nerve cell sensing with high sensitivity and stability

Accurate dopamine (DA) monitoring with high stability is essential for investigating the chemical basis of brain function and pathology. Electrochemical-based tissue-implantable carbon fiber electrodes (CFEs) show great potential in sensing the dynamics of neurochemicals at a sub-second timescale. However, their anti-fouling property, selectivity, and stability pose challenges. Here, we presented a novel strategy to enhance electrode biocompatibility and stability by modifying CFE with a chitosan (CS) film, brain cell membrane (M), and aptamer cholesterol amphiphiles (DNA-cho). We found that CFE was uniformly covered by a cicada-like membrane after being modified. Electrochemical characterizations indicated that DNA-cho-M-CS-CFE exhibited a wide linear range of DA concentration and showed high sensitivity, specificity, and stability. The electrode also presented excellent fouling resistance and biocompatibility. Moreover, the biosensor was used to detect DA in K⁺-induced brain slices and PC12 cells with a satisfactory stability and sensitivity and to prove that LPS treatment leads to the delayed and decreased release of DA. DNA-cho-M-CS-CFE showed excellent electrochemical performance and unique advantages for long-term in vivo sensing of living cells, thus providing a new feasible scheme for studying neurochemical kinetics and brain diseases.

Recent Development of Neural Microelectrodes with Dual-Mode Detection

Neurons communicate through complex chemical and electrophysiological signal patterns to develop a tight information network. A physiological or pathological event cannot be explained by signal communication mode. Therefore, dual-mode electrodes can simultaneously monitor the chemical and electrophysiological signals in the brain. They have been invented as an essential tool for brain science research and brain-computer interface (BCI) to obtain more important information and capture the characteristics of the neural network. Electrochemical sensors are the most popular methods for monitoring neurochemical levels in vivo. They are combined with neural microelectrodes to record neural electrical activity. They simultaneously detect the neurochemical and electrical activity of neurons in vivo using high spatial and temporal resolutions. This paper systematically reviews the latest development of neural microelectrodes depending on electrode materials for simultaneous in vivo electrochemical sensing and electrophysiological signal recording. This includes carbon-based microelectrodes, silicon-based microelectrode arrays (MEAs), and ceramic-based MEAs, focusing on the latest progress since 2018. In addition, the structure and interface design of various types of neural microelectrodes have been comprehensively described and compared. This could be the key to simultaneously detecting electrochemical and electrophysiological signals.

A Review on Electrochemical Microsensors for Ascorbic Acid Detection: Clinical, Pharmaceutical, and Food Safety Applications

Nowadays, micro-sized sensors have become a hot topic in electroanalysis. Because of their excellent analytical features, microelectrodes are well-accepted tools for clinical, pharmaceutical, food safety, and environmental applications. In this brief review, we highlight the state-of-art electrochemical non-enzymatic microsensors for quantitative detection of ascorbic acid (also known as vitamin C). Ascorbic acid is a naturally occurring water-soluble organic compound with antioxidant properties and its quantitative determination in biological fluids, foods, cosmetics, etc., using electrochemical microsensors is of wide interest. Various electrochemical techniques have been applied to detect ascorbic acid with extremely high sensitivity, selectivity, reproducibility, and reliability, and apply to in vivo measurements. This review paper aims to give readers a clear view of advances in areas of electrode modification, successful strategies for signal amplification, and miniaturization techniques used in the electroanalytical devices for ascorbic acid. In conclusion, current challenges related to the microelectrodes design, and future perspectives are outlined.

Self-Terminated Electroless Deposition of Surfactant-Free and Monodispersed Pt Nanoparticles on Carbon Fiber Microelectrodes for Sensitive Detection of H2O2 Released from Living Cells

We report a self-terminated electroless deposition method to prepare surfactant-free and monodispersed Pt nanoparticle (NP)-modified carbon fiber microelectrodes (Pt NP/CFEs) for electrochemical detection of hydrogen peroxide (H₂O₂) released from living cells. The surfactant-free and monodispersed Pt NPs with a uniform size of 65 nm are spontaneously deposited on a CFE surface by immersing an exposed carbon fiber (CF) of CFE in the PtCl₄^2- solution, in which an exposed CF can be used as the reducing agent and stabilizer. A self-terminated electroless deposition method is demonstrated, in which the density and size of Pt NPs on a CFE surface do not increase when the reaction time increases from 20 to 60 min. The self-terminated electroless deposition process not only can effectively avoid any manual electrode modification and thus largely minimize person-to-person and electrode-to-electrode deviations but also can avoid the use of any extra reductant or surfactant in the fabrication process. Therefore, Pt NPs/CFEs, with good reproducibility and sensitivity, not only exhibit high electrocatalytic activity toward the oxidation of H₂O₂ but also maintain the spatial resolution of CFEs. Moreover, Pt NPs/CFEs can detect H₂O₂ with a wide linear range of 0.5-80 μM and a low detection limit of 0.17 μM and then can be successfully applied in the monitoring of H₂O₂ released from RAW 264.7 cells. The self-terminated electroless deposition method can also be extended to selectively prepare other metal NP-modified CFEs, such as Au NPs/CFEs or Ag NPs/CFEs, by choosing the metal ions with higher reduction potential as precursors. This work provides a simple, straightforward, and general method for the preparation of small, surfactant-free, and monodispersed metal NP-modified CFEs with high sensitivity, reproducibility, and spatial resolution.

Nanostructured Geometries Strongly Affect Fouling of Carbon Electrodes

Electrode fouling is a major factor that compromises the performance of biosensors in in vivo usage. It can be roughly classified into (i) electrochemical fouling, caused by the analyte and its reaction products, and (ii) biofouling, caused by proteins and other species in the measurement environment. Here, we examined the effect of electrochemical fouling [in phosphate buffer saline (PBS)], biofouling [in cell-culture media (F12-K) with and without proteins], and their combination on the redox reactions occurring on carbon-based electrodes possessing distinct morphologies and surface chemistry. The effect of biofouling on the electrochemistry of an outer sphere redox probe, [Ru(NH₃)₆]³⁺, was negligible. On the other hand, fouling had a marked effect on the electrochemistry of an inner sphere redox probe, dopamine (DA). We observed that the surface geometry played a major role in the extent of fouling. The effect of biofouling on DA electrochemistry was the worst on planar pyrolytic carbon, whereas the multiwalled carbon nanotube/tetrahedral amorphous carbon (MWCNT/ta-C), possessing spaghetti-like morphology, and carbon nanofiber (CNF/ta-C) electrodes were much less seriously affected. The blockage of the adsorption sites for DA by proteins and other components of biological media and electrochemical fouling components (byproducts of DA oxidation) caused rapid surface poisoning. PBS washing for 10 consecutive cycles at 50 mV/s did not improve the electrode performance, except for CNF/ta-C, which performed better after PBS washing. Overall, this study emphasizes the combined effect of biological and electrochemical fouling to be critical for the evaluation of the functionality of a sensor. Thus, electrodes possessing composite nanostructures showed less surface fouling in comparison to those possessing planar geometry.

A proposed implantable voltammetric carbon fiber-based microsensor for corticosteroid monitoring by cochlear implants

A novel carbon fiber microsensor (CFMS) with the capability of being inserted in the cochlear implant structure is introduced for in situ measurement of corticosteroid concentration. The microsensor structure is composed of a carbon microfiber, an Ag wire, and a Pt wire acting respectively as a working electrode, a reference electrode, and a counter electrode. In addition, a silicone septum is used for isolation purposes in place of the epoxy resin. The septum-insulated microsensor is capable of monitoring the concentration of the corticosteroids in the perilymph fluid without a need for sampling from the inner ear fluid and the consequent ex vivo analysis. The electrochemical determination of the corticosteroids was investigated on the carbon fiber electrode surface by differential pulse voltammetry. During the reduction of dexamethasone (DEX), a cathodic peak with a peak potential of -1.3 V appeared at the CFMS. Using the CFMS under optimized conditions, a calibration plot of the dexamethasone (DEX) in the artificial perilymph solution exhibited two linear ranges from 10 nM to 2 μM and 2 to 40 μM (sensitivity equal to 16.55 μA μM-1 cm-2; LOD = 4 nM) conforming with the DEX concentration range inside the inner ear after the insertion of a drug-eluting cochlear implant electrode (CIE). Furthermore, the interferences occurring in the hearing functions of the CIE after the presence and function of the CFMS were simulated numerically using the finite element method. According to our results, decreasing the size of the microsensor introduces lower interferences with the auditory function of the cochlear implant electrode.

Electrochemical treatment in KOH renews and activates carbon fiber microelectrode surfaces

Carbon fiber microelectrodes (CFMEs) are the standard electrodes for fast-scan cyclic voltammetry (FSCV) detection of neurotransmitters. CFMEs are generally used untreated but the surface can be activated with different treatments to improve electrochemical performance. In this work, we explored electrochemical treatments to clean and activate the CFME surface. We used different solution conditions for electrochemical treatment and found that electrochemical pretreatment in KOH outperforms treatment in KCl, H₂O₂, or HCl by accelerating the surface renewal process. The etching rate of carbon with electrochemical treatment in KOH is 37 nm/min, which is 10 times faster than that in the other solutions. Electrochemical treatment in KOH for several minutes regenerates a new carbon surface, which introduces more oxygen functional groups beneficial for adsorption and electron transfer. The KOH-treated CFMEs improved the limit of detection (LOD) to 9 ± 2 nM from 14 ± 4 nM for untreated CFMEs, and they successfully detected stimulated dopamine release in rat brain slices, demonstrating that they are stable and sensitive enough to use in biological systems. Electrochemical treatment in KOH completely restores the electrode sensitivity after biofouling. The proposed electrochemical treatment is simple and fast and can be applied prior to using CFMEs or after use to restore the surface. Thus, the method has potential to be a standard step to clean the carbon surface, or restore the sensitivity of electrodes from biofouling.

A colorimetric and fluorescence turn-on probe for the detection of ascorbic acid in living cells and beverages

A colorimetric and fluorescence turn-on dual-signal assay was developed for the determination of ascorbic acid (AA). Because the ultraviolet absorption of the oxidized 3,3',5,5'-tetramethylbenzidine (oxTMB) overlapped with the fluorescence emission of glutathione stabilized Au nanoclusters (AuNCs), the fluorescence of AuNCs can be quenched by oxTMB. When AA was added, the blue oxTMB was reduced to colorless TMB, and the fluorescence of AuNCs was restored simultaneously. The decrease in absorbance and increase in fluorescence signal depended on the concentration of AA. In the determination range of 0.5 to 200 μM, the detection limits (LOD) for AA were as low as 0.15 µM and 0.22 µM for fluorometric and colorimetric, respectively. The established probe was used successfully for AA detection in living cells and beverages.

Dual-Response Ratiometric Electrochemical Microsensor for Effective Simultaneous Monitoring of Hypochlorous Acid and Ascorbic Acid in Human Body Fluids

Redox homeostasis between hypochlorous acid (HClO/ClO^-) and ascorbic acid (AA) significantly impacts many physiological and pathological processes. Herein, we report a new electrochemical sensor for the simultaneous determination of HClO and AA in body fluids. We first coated a carbon fiber microelectrode (CFME) with a three-dimensional nanocomposite consisting of graphene oxide (GO) and carbon nanotubes (CNTs) to fabricate the CFME/GO-CNT electrode. After the electrochemical reduction of GO (ERGO), we integrated a latent 1-(3,7-bis(dimethylamino)-10H-phenothiazin-10-yl)-2-methylpropan-1-one (MBS) electrochemical molecular recognition probe to monitor HClO and employed anthraquinone (AQ) as an internal reference. The compact CFME/ERGO-CNT/AQ + MBS sensor enabled the accurate and simultaneous measurement of HClO and AA with excellent selectivity and sensitivity. Measurements were highly reproducible, and the sensor was stable and exceptionally biocompatible. We successfully detected changes in the redox cycles of HClO and AA in human body fluids. This sensor is a significant advance for the investigation of reactions involved in cellular redox regulation. More importantly, we have devised a strategy for the design and construction of ratiometric electrochemical biosensors for the simultaneous determination of various bioactive species.

Electrochemically Probing Dynamics of Ascorbate during Cytotoxic Edema in Living Rat Brain

Cytotoxic edema is the initial and most important step in the sequence that almost inevitably leads to brain damage. Exploring the neurochemical disturbances in this process is of great significance in providing a measurable biological parameter for signaling specific pathological conditions. Here, we present an electrochemical system that pinpoints a critical neurochemical involved in cytotoxic edema. Specially, we report a molecularly tailored brain-implantable ascorbate sensor (CFE_AA2.0) featuring excellent selectivity and spatiotemporal resolution that assists the first observation of release of ascorbate induced by cytotoxic edema in vivo. Importantly, we reveal that this release is associated with an increase in the amount of cytotoxic edema-inducing agent and that blockage of cytotoxic edema abolishes ascorbate release, further supporting that ascorbate efflux is cytotoxic edema-dependent. Our study holds the promise for understanding the molecular basis of cytotoxic edema that can lead to the discovery of biomarkers or potential therapeutic strategies of brain diseases.

Optimization of graphene oxide-modified carbon-fiber microelectrode for dopamine detection

Graphene oxide (GO) is a carbon-based material that is easily obtained from graphite or graphite oxide. GO has been used broadly for electrochemistry applications and our hypothesis is that GO coating a carbon-fiber microelectrode (CFME) will increase the sensitivity for dopamine by providing more adsorption sites due to the enhancement of oxygen functional groups. Here, we compared drop casting, dip coating, and electrodeposition methods to directly coat commercial GO on CFME surfaces. Dip coating did not result in much GO coating and drop casting resulted in large agglomerations that produced noisy signals and slow rise times. Electrodeposition method with cyclic voltammetry increase the current for dopamine and this method was the most reproducible and had the least noise compared to the other two coating methods. The optimized method used a triangular waveform scanned from -1.2 V to 1.5 V at 100 mV/s for 5 cycles in 0.2 mg/mL GO in water. With fast-scan cyclic voltammetry (FSCV), the optimized GO/CFME enhanced the dopamine oxidation peak two-fold. The sensitivity of the modified electrode is 41±2 nA/μM with a linear range from 25 nM to 1 μM, and a limit of detection of 11 nM. The optimized electrodes were used to detect electrically-stimulated dopamine in brain slices to demonstrate their performance in tissue. Thus, GO can be used to enhance the sensitivity of electrodes for dopamine and improve biological measurements.

Fluorescence Turn-On Detection of Ascorbic Acid Using a Self-Assembled Lanthanide Polymer Nanoparticle

Ascorbic acid (AA), or vitamin C, is an important reactive biological molecule in vivo, and an abnormal level of AA is associated with many diseases. Therefore, the rapid, sensitive, and selective detection of AA levels is of significance in cases of medical assay and diagnosis. Compared with other nanoparticles, lanthanide coordination polymer nanoparticles (Ln-CPs) have been demonstrated as the excellent biomolecule sensing platforms due to their unique optical properties and intrinsic porosities. In this work, the cerium coordination polymer nanoparticles ATP-Ce-Tris were synthesized in a simple and quick way. The synthesized ATP-Ce-Tris nanoparticle shows the characteristic peak of Ce³⁺ located at 365 nm, which is corresponding to the 4f→5d transition of Ce³⁺. In the presence of Fe³⁺, the fluorescence of ATP-Ce-Tris quenched, and the following added ascorbic acid (AA) makes it restoring effectively. Based on this, we constructed a fluorescence probe with excellent sensitivity for AA sensing in a wide linear relationship from 0.05 to 500 μM. The detection limit was as low as 18 nM (signal-to-noise ratio of three), which is one or two orders of magnitude lower than those of reported sensors. The proposed sensing systems also exhibits excellent sensitivity for AA detection in human serum sample, exploiting a valuable platform for AA analysis in clinic diagnostic and drug screening.

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.

A cobalt corrole/carbon nanotube enables simultaneous electrochemical monitoring of oxygen and ascorbic acid in the rat brain

It is of interest to in vivo monitor the co-dynamics of different substances. However, the tracking of multiple species is still challenging. In this work, we demonstrate an in vivo electrochemical method by using multi-potential step amperometry to in vivo detect ascorbic acid (AA) and oxygen (O2) simultaneously. In order to achieve good selectivity and high sensitivity for both AA and O2, we design a cobalt corrole [Co(tpfc)(py)2] (tpfc = 5,10,15-tris(penta-fluorophenyl) corrole, py = pyridine, denoted as Co-TPFC) and carbon nanotube nanocomposite to modify a carbon fiber microelectrode (Co-TPFC/MWNT/CFE). This Co-TPFC/MWNT/CFE exhibits excellent electrocatalytic properties towards the reduction of O2 preceding a 4e process and facilitates the oxidation of AA at low potential in the physiological environment. Based on this, we realize simultaneous detection of AA and O2 using two-potential steps (one cathodic (-0.2 V) and the other anodic (+0.05 V)) with 1 second step time. Both in vitro and in vivo experiments proved the feasibility of this method. This demonstrated strategy is useful for us to understand various physiological and pathological processes associated with O2 and AA co-dynamics, and also provides an idea for detecting multiple substances simultaneously.

Carbon nanospikes have better electrochemical properties than carbon nanotubes due to greater surface roughness and defect sites

Carbon nanomaterials are used to improve electrodes for neurotransmitter detection, but what properties are important for maximizing those effects? In this work, we compare a newer form of graphene, carbon nanospikes (CNSs), with carbon nanotubes (CNTs) grown on wires and carbon fibers (CFs). CNS electrodes have a short, dense, defect-filled surface that produces remarkable electrochemical properties, much better than CNTs or CFs. The CNS surface roughness is 5.5 times greater than glassy carbon, while CNTs enhance roughness only 1.8-fold. D/G ratios are higher for CNS electrodes than CNT electrodes, an indication of more defect sites. For cyclic voltammetry of dopamine and ferricyanide, CNSs have both higher currents and smaller ΔEp values than CNTs and CFs. CNS electrodes also have a very low resistance to charge transfer. With fast-scan cyclic voltammetry (FSCV), CNS electrodes have enhanced current density for dopamine and cationic neurotransmitters due to increased adsorption to edge plane sites. This study establishes that not all carbon nanomaterials are equally advantageous for dopamine electrochemistry, but that short, dense nanomaterials that add defect sites provide improved current and electron transfer. CNSs are simple to mass fabricate on a variety of substrates and thus could be a favorable material for neurotransmitter sensing.

Nanoskiving fabrication of size-controlled Au nanowire electrodes for electroanalysis

Nanoskiving, benefiting from its simple operation and high reproducibility, is a promising method to fabricate nanometer-size electrodes. In this work, we report the fabrication of Au nanowire electrodes with different shapes and well-controlled sizes through nanoskiving. Au nanowire block electrodes, membrane electrodes and tip electrodes are prepared with good reproducibility. Steady-state cyclic voltammograms (CVs) demonstrate that all these electrodes behave well as nanoband ultramicroelectrodes. A fast heterogeneous electron transfer rate constant can be extracted reliably from steady-state CVs at various size Au nanowire block electrodes by the Koutecký-Levich (K-L) method. The Au nanowire membrane electrodes demonstrate good sensitivity toward the oxidation of catecholamine and could monitor catecholamine released from rat adrenal chromaffin cells stimulated by high K⁺.

Defect Sites Modulate Fouling Resistance on Carbon-Nanotube Fiber Electrodes

Carbon nanotube (CNT) fiber electrodes have become increasingly popular electrode materials for neurotransmitter detection with fast-scan cyclic voltammetry (FSCV). The unique properties of CNT fiber electrodes like increased electron transfer, sensitivity, waveform application frequency independence, and resistance to fouling make them ideal biological sensors for FSCV. In particular, their resistance to fouling has been observed for several years, but the specific physical properties which aid in fouling resistance have been debated. Here, we investigate the extent to which the presence of defect sites on the surface attenuate both chemical and biological fouling with FSCV. We compared traditional carbon-fiber microelectrodes (CFMEs) to pristine CNTs and functionalized CNTs. CFMEs and functionalized CNTs are highly disordered with a great deal of defect sites on the surface. The pristine CNTs have fewer defects compared to the purposefully functionalized CNTs and CFMEs. All electrode surfaces were characterized by a combination of scanning electron microscopy (SEM), Raman spectroscopy, and energy dispersive spectroscopy (EDS). Chemical fouling was studied using serotonin, a popular neurotransmitter notoriously known for electrode fouling. To assess biological fouling, electrodes were implanted in brain tissue for 2 h. Defect sites on the carbon were shown to resist biofouling compared to pristine CNTs but were detrimental for serotonin detection. Overall, we provide insight into the extent to which the electrode surface dictates fouling resistance with FSCV. This work provides evidence that careful considerations of the surface of the CNT material are needed when designing sensors for fouling resistance.

Carbon Nanomaterials for Energy and Biorelated Catalysis: Recent Advances and Looking Forward

Along with the wide investigation activities in developing carbon-based, metal-free catalysts to replace precious metal (e.g., Pt) catalysts for various green energy devices, carbon nanomaterials have also shown great potential for biorelated applications. This article provides a focused, critical review on the recent advances in these emerging research areas. The structure-property relationship and mechanistic understanding of recently developed carbon-based, metal-free catalysts for chemical/biocatalytic reactions will be discussed along with the challenges and perspectives in this exciting field, providing a look forward for the rational design and fabrication of new carbon-based, metal-free catalysts with high activities, remarkable selectivity, and outstanding durability for various energy-related/biocatalytic processes.

Highly Uniform, Flexible Microelectrodes Based on the Clean Single-Walled Carbon Nanotube Thin Film with High Electrochemical Activity

Electrochemical sensors based on carbon nanotubes (CNTs) have great potential for use in wearable or implantable biomedical sensor applications because of their excellent mechanical flexibility and biocompatibility. However, the main challenge associated with CNT-based sensors is their uniform and reproducible fabrication on the flexible plastic film. Here, we introduce and demonstrate a highly reliable technique to fabricate flexible CNT microelectrodes on a plastic film. The technique involves a process whereby the CNT film is formed by the dry transfer process based on the floating-catalyst chemical vapor deposition. An oxide protection layer, which is used to cover the CNT thin film during the fabrication process, minimizes contamination of the surface. The fabricated flexible CNT microelectrodes show almost ideal electrochemical characteristics for microelectrodes with the average value of the quartile potentials, Δ E = | E_3/4 - E_1/4|, being 60.4 ± 2.9 mV for the 28 electrodes, while the ideal value of Δ E = 56.4 mV. The CNT microelectrodes also showed enhanced resistance to surface fouling during dopamine oxidation in comparison to carbon fiber and gold microelectrodes; the degradation of the oxidation current after 10 consecutive cycles were 1.8, 8.3, and 13.9% for CNT, carbon fiber, and gold microelectrodes, respectively. The high-sensitivity detection of dopamine is also demonstrated with differential-pulse voltammetry, with a resulting limit of detection of ∼50 nM. The reliability, uniformity, and sensitivity of the present CNT microelectrodes provide a platform for flexible electrochemical sensors.

Review: New insights into optimizing chemical and 3D surface structures of carbon electrodes for neurotransmitter detection

The carbon-fiber microelectrode has been used for decades as a neurotransmitter sensor. Recently, new strategies have been developed for making carbon electrodes, including using carbon nanomaterials or pyrolyzing photoresist etched by nanolithography or 3D printing. This review summarizes how chemical and 3D surface structures of new carbon electrodes are optimized for neurotransmitter detection. There are effects of the chemical structure that are advantageous and nanomaterials are used ranging from carbon nanotube (CNT) to graphene to nanodiamond. Functionalization of these materials promotes surface oxide groups that adsorb dopamine and dopants introduce defect sites good for electron transfer. Polymer coatings such as poly(3,4-ethylenedioxythiophene) (PEDOT) or Nafion also enhance the selectivity, particularly for dopamine over ascorbic acid. Changing the 3D surface structure of an electrode increases current by adding more surface area. If the surface structure has roughness or pores on the micron scale, the electrode also acts as a thin layer cell, momentarily trapping the analyte for redox cycling. Vertically-aligned CNTs as well as lithographically-made or 3D printed pillar arrays act as thin layer cells, producing more reversible cyclic voltammograms. A better understanding of how chemical and surface structure affects electrochemistry enables rational design of electrodes. New carbon electrodes are being tested in vivo and strategies to reduce biofouling are being developed. Future studies should test the robustness for long term implantation, explore electrochemical properties of neurotransmitters beyond dopamine, and combine optimized chemical and physical structures for real-time monitoring of neurotransmitters.

Analyte-triggered cyclic autocatalytic oxidation amplification combined with an upconversion nanoparticle probe for fluorometric detection of copper(II)

The authors describe an upconversion nanoparticle-based (UCNP-based) fluorometric method for ultrasensitive and selective detection of Cu²⁺. The UCNPs show a strong emission band at 550 nm under near-infrared excitation at 980 nm. The principle of the strategy is that gold nanoparticles (AuNP) can quench the fluorescence of UCNP. In contrast, the addition of L-cysteine (Cys) can induce the aggregation of AuNP, resulting in a fluorescence recovery of the UCNPs. On addition of Cu²⁺, it oxidizes Cys to cystine and is reduced to Cu⁺. The Cu⁺ thusformed can be oxidized cyclically to Cu²⁺ by dissolved O₂, which catalyzes and recycles the whole reaction. Thus, the aggregation of AuNP is inhibited and the fluorescence recovered by Cys is quenched. Under the optimal condition, the quenching efficiency shows a good linear response to the concentrations of Cu²⁺ in the 0.4-40 nM range. The limit of detection is 0.16 nM, which is 5 orders of magnitude lower than the U.S. Environmental Protection Agency limit for Cu²⁺ in drinking water (20 μM). The method has been further applied to monitor Cu²⁺ levels in real samples. The results of detection are well consistent with those obtained by atomic absorption spectroscopy. Graphical abstract Gold nanoparticles (AuNP) as a high efficient fluorescence quenching reagent of upconversion nanoparticles (UCNP) were used in a fluorometric method for detection of Cu²⁺ based on a cyclic catalytic oxidation amplification strategy.

An interdigitated electrode with dense carbon nanotube forests on conductive supports for electrochemical biosensors

A highly sensitive interdigitated electrode (IDE) with vertically aligned dense carbon nanotube forests directly grown on conductive supports was demonstrated by combining UV lithography and a low temperature chemical vapor deposition process (470 °C). The cyclic voltammetry (CV) measurements of K4[Fe(CN)6] showed that the redox current of the IDE with CNT forests (CNTF-IDE) reached the steady state much more quickly compared to that of conventional gold IDE (Au-IDE). The performance of the CNTF-IDE largely depended on the geometry of the electrodes (e.g. width and gap). With the optimum three-dimensional electrode structure, the anodic current was amplified by a factor of ∼18 and ∼67 in the CV and the chronoamperometry measurements, respectively. The collection efficiency, defined as the ratio of the cathodic current to the anodic current at steady state, was improved up to 97.3%. The selective detection of dopamine (DA) under the coexistence of l-ascorbic acid with high concentration (100 μM) was achieved with a linear range of 100 nM-100 μM, a sensitivity of 14.3 mA mol-1 L, and a limit of detection (LOD, S/N = 3) of 42 nM. Compared to the conventional carbon electrodes, the CNTF-IDE showed superior anti-fouling property, which is of significant importance for practical applications, with a negligible shift of the half-wave potential (ΔE1/2 < 1.4 mV) for repeated CV measurements of DA at high concentration (100 μM).

Carbon Nanohorn-Modified Carbon Fiber Microelectrodes for Dopamine Detection

Carbon nanohorns (CNHs), closed cone-shaped cages of sp 2-hybridized carbons, are a promising nanomaterial to improve carbon-fiber microelectrode (CFME) dues to their high specific surface area and edge planes, but few studies have tested their electrochemical properties. Here, we tested the dopamine detection at electrodeposited CNHs on CFME (CNH/CFME). The optimized concentration of CNHs in the deposition solution is 0.5 mg/mL, and the optimized electrodeposition waveform is 10 cycles of triangular waveform scanned from -1.0 V and +1.0 V at 50 mV/s. Using fast-scan cyclic voltammetry, the optimized CNH/CFME enhances dopamine peak current to 2.3 ± 0.2 times that of the CFME. To further increase the current, CNH/CFMEs were oxidized in NaOH (ox-CNH/CFME), which creates more defects and surface oxide groups to adsorb dopamine. The oxidative etching further increases the peak current to 3.5 ± 0.2 times of the CFME, and ox-CNH/CFME had a limit of detection of 6 ± 2 nM. The dopamine anodic current at ox-CNH/CFME was stable for 8 h of continuous scanning. The ox-CNH/CFME enhanced the anodic peak current for other cationic neurotransmitters including epinephrine, norepinephrine, and serotonin, but less enhancement was found for ascorbic acid, showing higher selectivity for cationic molecules. CNHs also decreased tissue biofouling at CFME. Thus, electrodeposited CNHs are a promising new method for increasing the surface area and current of CFMEs for dopamine detection.

Static and Dynamic Measurement of Dopamine Adsorption in Carbon Fiber Microelectrodes Using Electrochemical Impedance Spectroscopy

In this study, electrochemical impedance spectroscopy was used for the first time to study the adsorption of dopamine in carbon fiber microelectrodes. In order to show a proof-of-concept, static and dynamic measurements were taken at potentials ranging from -0.4 to 0.8 V versus Ag|AgCl to demonstrate the versatility of this technique to study dopamine without the need of its oxidation. We used electrochemical impedance spectroscopy and single frequency electrochemical impedance to measure different concentrations of dopamine as low as 1 nM. Moreover, the capacitance of the microelectrodes surface was found to decrease due to dopamine adsorption, which is dependent on its concentration. The effect of dissolved oxygen and electrochemical oxidation of the surface in the detection of dopamine was also studied. Nonoxidized and oxidized carbon fiber microelectrodes were prepared and characterized by optical microscopy, scanning electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. Optimum working parameters of the electrodes, such as frequency and voltage, were obtained for better measurement. Electrochemical impedance of dopamine was determined at different concentration, voltages, and frequencies. Finally, dynamic experiments were conducted using a flow cell and single frequency impedance in order to study continuous and real-time measurements of dopamine.

O2 Plasma Etching and Antistatic Gun Surface Modifications for CNT Yarn Microelectrode Improve Sensitivity and Antifouling Properties

Carbon nanotube (CNT) based microelectrodes exhibit rapid and selective detection of neurotransmitters. While different fabrication strategies and geometries of CNT microelectrodes have been characterized, relatively little research has investigated ways to selectively enhance their electrochemical properties. In this work, we introduce two simple, reproducible, low-cost, and efficient surface modification methods for carbon nanotube yarn microelectrodes (CNTYMEs): O2 plasma etching and antistatic gun treatment. O2 plasma etching was performed by a microwave plasma system with oxygen gas flow and the optimized time for treatment was 1 min. The antistatic gun treatment flows ions by the electrode surface; two triggers of the antistatic gun was the optimized number on the CNTYME surface. Current for dopamine at CNTYMEs increased 3-fold after O2 plasma etching and 4-fold after antistatic gun treatment. When the two treatments were combined, the current increased 12-fold, showing the two effects are due to independent mechanisms that tune the surface properties. O2 plasma etching increased the sensitivity due to increased surface oxygen content but did not affect surface roughness while the antistatic gun treatment increased surface roughness but not oxygen content. The effect of tissue fouling on CNT yarns was studied for the first time, and the relatively hydrophilic surface after O2 plasma etching provided better resistance to fouling than unmodified or antistatic gun treated CNTYMEs. Overall, O2 plasma etching and antistatic gun treatment improve the sensitivity of CNTYMEs by different mechanisms, providing the possibility to tune the CNTYME surface and enhance sensitivity.

The Electrochemical Behavior of Carbon Fiber Microelectrodes Modified with Carbon Nanotubes Using a Two-Step Electroless Plating/Chemical Vapor Deposition Process

Carbon fiber microelectrode (CFME) has been extensively applied in the biosensor and chemical sensor domains. In order to improve the electrochemical activity and sensitivity of the CFME, a new CFME modified with carbon nanotubes (CNTs), denoted as CNTs/CFME, was fabricated and investigated. First, carbon fiber (CF) monofilaments grafted with CNTs (simplified as CNTs/CFs) were fabricated in two key steps: (i) nickel electroless plating, followed by (ii) chemical vapor deposition (CVD). Second, a single CNTs/CF monofilament was selected and encapsulated into a CNTs/CFME with a simple packaging method. The morphologies of as-prepared CNTs/CFs were characterized by scanning electron microscopy. The electrochemical properties of CNTs/CFMEs were measured in potassium ferrocyanide solution (K₄Fe(CN)₆), by using a cyclic voltammetry (CV) and a chronoamperometry method. Compared with a bare CFME, a CNTs/CFME showed better CV curves with a higher distinguishable redox peak and response current; the higher the CNT content was, the better the CV curves were. Because the as-grown CNTs significantly enhanced the effective electrode area of CNTs/CFME, the contact area between the electrode and reactant was enlarged, further increasing the electrocatalytic active site density. Furthermore, the modified microelectrode displayed almost the same electrochemical behavior after 104 days, exhibiting remarkable stability and outstanding reproducibility.