Comparisons of platinum, gold, palladium and glassy carbon as electrode materials in the design of biosensors for glutamate

Tuning N-heterocyclic carbene wingtips to form electrochemically stable adlayers on metals

Self-assembled monolayers (SAMs) are employed in electrochemical biosensors to passivate and functionalize electrode surfaces. These monolayers prevent the occurrence of undesired electrochemical reactions and act as scaffolds for coupling bioaffinity reagents. Thiols are the most common adlayer used for this application; however, the thiol-gold bond is susceptible to competitive displacement by naturally occurring solvated thiols in biological fluids, as well as to desorption under continuous voltage interrogation. To overcome these issues, N-heterocyclic carbene (NHC) monolayers have been proposed as an alternative for electrochemical biosensor applications due to the strong carbon-gold bond. To maximize the effectiveness of NHCs for SAMs, a thorough understanding of both the steric effects of wingtip substituents and NHC precursor type to the passivation of electrode surfaces is required. In this study, five different NHC wingtips as well as two kinds of NHC precursors were evaluated. The best performing NHC adlayers can be cycled continuously for four days (over 30 000 voltammetric cycles) without appreciably desorbing from the electrode surface. Benchmark thiol monolayers, in contrast, rapidly desorb after only twelve hours. Investigations also show NHC adlayer formation on other biosensor-relevant electrodes such as platinum and palladium.

MnFe@N-CNTs Based Lactate Biomicrochips for Nonintrusive and Onsite Periodontitis Diagnosis

In clinical settings, saliva has been established as a straightforward, noninvasive medium for diagnosing periodontitis. However, the precise diagnosis is often hampered by the absence of a specialized analyzer capable of detecting low concentrations of biomarkers typically found in saliva. In this study, we present a noninvasive, on-site screen-printed biomicrochip specifically engineered for the precise and sensitive quantification of lactate concentrations in saliva, a critical biomarker in the diagnosis of periodontitis. The microchip is constructed using a nanostructured ink formulation that includes MnFe@N-doped carbon nanotubes (MnFe@N-CNTs). These MnFe@N-CNTs exhibit a high degree of graphitization and low electrical resistance, significantly augmenting the electrocatalytic efficiency of the enzymatic reaction of lactate. This results in doubled sensitivity and a detection limit that surpasses those of the current advanced salivary assay methods. Remarkably, within just 30 s, the biomicrochip can quantitatively and precisely measure lactate concentrations in the saliva of 10 patients, which provides valuable insights into the severity of their periodontitis. This biosensor holds excellent potential for large-scale production and could broaden the scope of biomarker recognition, paving the way for the analysis of a wider range of oral diseases.

Effect of dendrimer-based interlayers for enzyme immobilization on a model electrochemical sensing system for glutamate

In this paper, we discuss dendrimer usage in enzyme-based electrochemical biosensors, particularly with respect to biomolecule loading on the sensing surface. A novel approach to design bioactive layers with immobilized enzymes for electrochemical biosensors using the surface plasmon resonance (SPR) method in combination with electrochemical impedance spectroscopy was presented. The gold surface was modified with linear linkers (various mercaptoalkanoic acids and aminoalkanethiols) and poly(amidoamine) dendrimers from the first- to fifth-generation. The best functionalization procedure was established by detailed SPR studies and transferred onto gold electrodes to electrochemically examine the model enzymatic reaction catalysed by glutamate oxidase. In the case of the chronoamperometric method, the determined sensitivity was 3.36 ± 0.08 μA∙mM^-1, and the low limit of detection (LOD) was 1.52 μM. Comparing the sensitivity and LOD obtained for CV measurements, the values of these parameters were 2.5 times higher and 4 times lower, respectively, for the fourth-generation dendrimer-based biosensor and the biosensor with a linear linker. The positive impact of the dendrimer interlayer on the long-term enzyme activity was also confirmed. The research results indicate the possibility of a significant increase in the sensor response using the dendrimer itself without enriching it with electrochemical components.

Fabrication of Highly Sensitive Pt-black Electrochemical Sensors for GABA Detection

GABA (Gamma-aminobutyric acid) is the main inhibitory neurotransmitter in the central nervous system of mammals. It is known to be related with various neurological disorders. GABA plays a crucial role in normal neuronal activity, information processing and plasticity, and neuronal network synchronization. To date, microdialysis has been widely used to monitor the level of GABA but the temporal and spatial resolution is limited. Besides, electrochemical sensors for neurotransmitter measurement, having high temporal and spatial resolution, overcome this problem. Here, using a cost-effective method of electrodeposition of platinum black (Pt-black), a highly sensitive, GABA specific, amperometric electrochemical sensor is fabricated. Nanostructured Pt-black increases the active surface area of the electrode contributing to higher sensitivity. Along with that, a self-referencing site and an exclusion layer are integrated to increase the selectivity and the signal-to-noise ratio (SNR) of the biosensor. This provides a prototype for a highly sensitive GABA sensor that could later be used to study various neurological disorders related to GABA concentrations.Clinical Relevance- This electrochemical sensor allows real-time monitoring of major inhibitory neurotransmitter (GABA) with high sensitivity which can be used for studying various neurological disorders.

Role of rare-earth elements in enhancing bioelectrocatalysis for biosensing with NAD+-dependent glutamate dehydrogenase

Dehydrogenases (DHs) are widely explored bioelectrocatalysts in the development of enzymatic bioelectronics like biosensors and biofuel cells. However, the relatively low intrinsic reaction rates of DHs which mostly depend on diffusional coenzymes (e.g., NAD+) have limited their bioelectrocatalytic performance in applications such as biosensors with a high sensitivity. In this study, we find that rare-earth elements (REEs) can enhance the activity of NAD+-dependent glutamate dehydrogenase (GDH) toward highly sensitive electrochemical biosensing of glutamate in vivo. Electrochemical studies show that the sensitivity of the GDH-based glutamate biosensor is remarkably enhanced in the presence of REE cations (i.e., Yb3+, La3+ or Eu3+) in solution, of which Yb3+ yields the highest sensitivity increase (ca. 95%). With the potential effect of REE cations on NAD+ electrochemistry being ruled out, homogeneous kinetic assays by steady-state and stopped-flow spectroscopy reveal a two-fold enhancement in the intrinsic reaction rate of GDH by introducing Yb3+, mainly through accelerating the rate-determining NADH releasing step during the catalytic cycle. In-depth structural investigations using small angle X-ray scattering and infrared spectroscopy indicate that Yb3+ induces the backbone compaction of GDH and subtle β-sheet transitions in the active site, which may reduce the energetic barrier to NADH dissociation from the binding pocket as further suggested by molecular dynamics simulation. This study not only unmasks the mechanism of REE-promoted GDH kinetics but also paves a new way to highly sensitive biosensing of glutamate in vivo.

A glucose biosensor based on glucose oxidase fused to a carbohydrate binding module family 2 tag that specifically binds to the cellulose-modified electrode

The method of immobilization of glucose oxidase (GOD) on electrodes is especially important for the fabrication and performance of glucose biosensors. In this study, a carbohydrate binding module family 2 (CBM2) was successfully fused to the C terminal of GOD with a natural linker (NL) in endo-β-xylanase by genetic recombination, and a fusion GOD (GOD-NL-CBM2) was obtained. The CBM2 was used as an affinity adsorption tag for immobilization of the GOD-NL-CBM2 on a cellulose modified electrode. The specific activity of GOD-NL-CBM2 was comparable to that of the wild type GOD. In addition, the CBM2 tag of fusion GOD almost maintained its highest binding capacity under optimal catalytic conditions (pH 5.0, 50 °C). The morphology and composition analysis of the cellulose film reacted with and without GOD or GOD-NL-CBM2 confirmed the immobilization of GOD-NL-CBM2. The electrochemical properties of the GOD-NL-CBM2/cellulose film bioelectrode, with a characteristic peak of H₂O₂ at +0.6 V in the presence of glucose, revealed the capability of the immobilized GOD-NL-CBM2 to efficiently catalyze glucose and produce H₂O₂. Additionally, the current signal response of the biosensor to glucose was linear in the concentration range from 1.25 to 40 mM (r² ≥ 0.99). The sensitivity and detection limit of the GOD-NL-CBM2/cellulose film bioelectrode were 466.7 μA mol^-1 L cm^-2 and 0.475 mM (S/N = 3), respectively. Moreover, the glucose biosensor exhibited a rapid current change (< 5 s), high reproducibility (Relative standard deviation, RSD < 5%), substrate selectivity and stability, and retained about 80 % of the original current response after 2 months. The affinity adsorption-based immobilization strategy for GOD provides a promising approach to develop a high performance glucose biosensor.

Phasic oxygen dynamics confounds fast choline-sensitive biosensor signals in the brain of behaving rodents

Cholinergic fast time-scale modulation of cortical physiology is critical for cognition, but direct local measurement of neuromodulators in vivo is challenging. Choline oxidase (ChOx)-based electrochemical biosensors have been used to capture fast cholinergic signals in behaving animals. However, these transients might be biased by local field potential and O₂-evoked enzymatic responses. Using a novel Tetrode-based Amperometric ChOx (TACO) sensor, we performed highly sensitive and selective simultaneous measurement of ChOx activity (COA) and O₂. In vitro and in vivo experiments, supported by mathematical modeling, revealed that non-steady-state enzyme responses to O₂ give rise to phasic COA dynamics. This mechanism accounts for most of COA transients in the hippocampus, including those following locomotion bouts and sharp-wave/ripples. Our results suggest that it is unfeasible to probe phasic cholinergic signals under most behavioral paradigms with current ChOx biosensors. This confound is generalizable to any oxidase-based biosensor, entailing rigorous controls and new biosensor designs.

In Vivo Glutamate Sensing inside the Mouse Brain with Perovskite Nickelate-Nafion Heterostructures

Glutamate, one of the main neurotransmitters in the brain, plays a critical role in communication between neurons, neuronal development, and various neurological disorders. Extracellular measurement of neurotransmitters such as glutamate in the brain is important for understanding these processes and developing a new generation of brain-machine interfaces. Here, we demonstrate the use of a perovskite nickelate-Nafion heterostructure as a promising glutamate sensor with a low detection limit of 16 nM and a response time of 1.2 s via amperometric sensing. We have designed and successfully tested novel perovskite nickelate-Nafion electrodes for recording of glutamate release ex vivo in electrically stimulated brain slices and in vivo from the primary visual cortex (V1) of awake mice exposed to visual stimuli. These results demonstrate the potential of perovskite nickelates as sensing media for brain-machine interfaces.

A platinum-nickel bimetallic nanocluster ensemble-on-polyaniline nanofilm for enhanced electrocatalytic oxidation of dopamine

We report a new approach to design flexible functional material platforms based on electropolymerized polyaniline (PANI) polymer nanofilms modified with bimetallic nanoclusters (NCs) for efficient electro-oxidation of small organic molecules. Composition defined ligand free Pt_0.75Ni_0.25 NCs were synthesized in the gas phase using the Cluster Beam Deposition (CBD) technology and characterized using RToF, HAADF-STEM, XAFS and XPS. NCs were then directly deposited on PANI coated templates to construct electrodes. Dopamine (DP) molecules were used as a representative organic analyte and the influence of the NC-PANI hybrid atomistic structure on the electrochemical and electrocatalytic performance was investigated. The as prepared, nearly monodispersed, Pt_0.75Ni_0.25 NCs of ca. 2 nm diameter featuring a PtOx surface combined with a shallow platelet-like Ni-O(OH) phase formed a densely packed active surface on PANI at ultralow metal coverages. Electrochemical measurements (EIS and CV) show a 2.5 times decrease in charge transfer resistance and a remarkable 6-fold increase at lower potential in the mass activity for Pt_0.75Ni_0.25 NCs in comparison with their pure Pt counterparts. The enhanced electrochemical performance of the Pt_0.75Ni_0.25 NC hybrid's interface is ascribed to the formation of mixed Pt metal and Ni-O(OH) phases at the surface of the alloyed PtNi cores of the bimetallic NCs under electrochemical conditions combined with an efficient charge conduction pathway between NCs.

A Comprehensive Review: Development of Electrochemical Biosensors for Detection of Cyanotoxins in Freshwater

Cyanobacteria harmful algal blooms are increasing in frequency and cyanotoxins have become an environmental and public concern in the U.S. and worldwide. In this Review, the majority of reported studies and developments of electrochemical affinity biosensors for cyanotoxins are critically reviewed and discussed. Essential background information about cyanobacterial toxins and electrochemical biosensors is combined with the rapidly moving development of electrochemical biosensors for these toxins. Current issues and future challenges for the development of useful electrochemical biosensors for cyanotoxin detection that meet the demands for applications in field freshwater samples are discussed. The major aspects of the entire review article in a prescribed sequence include (i) the state-of-the-art knowledge of the toxicity of cyanotoxins, (ii) important harmful algal bloom events, (iii) advisories, guidelines, and regulations, (iv) conventional analytical methods for determination of cyanotoxins, (v) electrochemical transduction, (vi) recognition receptors, (vii) reported electrochemical biosensors for cyanotoxins, (viii) summary of analytical performance, and (ix) recent advances and future trends. Discussion includes electrochemical techniques and devices, biomolecules with high affinity, numerous array designs, various detection approaches, and research strategies in tailoring the properties of the transducer-biomolecule interface. Scientific and engineering aspects are presented in depth. This review aims to serve as a valuable source to scientists and engineers entering the interdisciplinary field of electrochemical biosensors for detection of cyanotoxins in freshwaters.

Portable biosensor for monitoring cortisol in low-volume perspired human sweat

A non-faradaic label-free cortisol biosensor was demonstrated using MoS₂ nanosheets integrated into a nanoporous flexible electrode system. Low volume (1–5 μL) sensing was achieved through use of a novel sensor stack design comprised of vertically aligned metal electrodes confining semi-conductive MoS₂ nanosheets. The MoS₂ nanosheets were surface functionalized with cortisol antibodies towards developing an affinity biosensor specific to the physiological relevant range of cortisol (8.16 to 141.7 ng/mL) in perspired human sweat. Sensing was achieved by measuring impedance changes associated with cortisol binding along the MoS₂ nanosheet interface using electrochemical impedance spectroscopy. The sensor demonstrated a dynamic range from 1–500 ng/mL with a limit of detection of 1 ng/mL. A specificity study was conducted using a metabolite expressed in human sweat, Ethyl Glucuronide. Continuous dosing studies were performed during which the sensor was able to discriminate between four cortisol concentration ranges (0.5, 5, 50, 500 ng/mL) for a 3+ hour duration. Translatability of the sensor was shown with a portable form factor device, demonstrating a comparable dynamic range and limit of detection for the sensor. The device demonstrated a R² correlation value of 0.998 when comparing measurements to the reported impedance values of the benchtop instrumentation.

Microfabricated, amperometric, enzyme-based biosensors for in vivo applications

Miniaturized electrochemical in vivo biosensors allow the measurement of fast extracellular dynamics of neurotransmitter and energy metabolism directly in the tissue. Enzyme-based amperometric biosensing is characterized by high specificity and precision as well as high spatial and temporal resolution. Aside from glucose monitoring, many systems have been introduced mainly for application in the central nervous system in animal models. We compare the microsensor principle with other methods applied in biomedical research to show advantages and drawbacks. Electrochemical sensor systems are easily miniaturized and fabricated by microtechnology processes. We review different microfabrication approaches for in vivo sensor platforms, ranging from simple modified wires and fibres to fully microfabricated systems on silicon, ceramic or polymer substrates. The various immobilization methods for the enzyme such as chemical cross-linking and entrapment in polymer membranes are discussed. The resulting sensor performance is compared in detail. We also examine different concepts to reject interfering substances by additional membranes, aspects of instrumentation and biocompatibility. Practical considerations are elaborated, and conclusions for future developments are presented. Graphical Abstract ᅟ.

Enzyme Biosensors for Biomedical Applications: Strategies for Safeguarding Analytical Performances in Biological Fluids

Enzyme-based chemical biosensors are based on biological recognition. In order to operate, the enzymes must be available to catalyze a specific biochemical reaction and be stable under the normal operating conditions of the biosensor. Design of biosensors is based on knowledge about the target analyte, as well as the complexity of the matrix in which the analyte has to be quantified. This article reviews the problems resulting from the interaction of enzyme-based amperometric biosensors with complex biological matrices containing the target analyte(s). One of the most challenging disadvantages of amperometric enzyme-based biosensor detection is signal reduction from fouling agents and interference from chemicals present in the sample matrix. This article, therefore, investigates the principles of functioning of enzymatic biosensors, their analytical performance over time and the strategies used to optimize their performance. Moreover, the composition of biological fluids as a function of their interaction with biosensing will be presented.

A Micro-Platinum Wire Biosensor for Fast and Selective Detection of Alanine Aminotransferase

In this study, a miniaturized biosensor based on permselective polymer layers (overoxidized polypyrrole (Ppy) and Nafion^®) modified and enzyme (glutamate oxidase (GlutOx)) immobilized micro-platinum wire electrode for the detection of alanine aminotransferase (ALT) was fabricated. The proposed ALT biosensor was measured electrochemically by constant potential amperometry at +0.7 V vs. Ag/AgCl. The ALT biosensor provides fast response time (~5 s) and superior selectivity towards ALT against both negatively and positively charged species (e.g., ascorbic acid (AA) and dopamine (DA), respectively). The detection range of the ALT biosensor is found to be 10–900 U/L which covers the range of normal ALT levels presented in the serum and the detection limit and sensitivity are found to be 8.48 U/L and 0.059 nA/(U/L·mm²) (N = 10), respectively. We also found that one-day storage of the ALT biosensor at −20 °C right after the sensor being fabricated can enhance the sensor sensitivity (1.74 times higher than that of the sensor stored at 4 °C). The ALT biosensor is stable after eight weeks of storage at −20 °C. The sensor was tested in spiked ALT samples (ALT activities: 20, 200, 400, and 900 U/L) and reasonable recoveries (70%~107%) were obtained.

Glutamate biosensors based on diamond and graphene platforms

l-Glutamate is one of the most important neurotransmitters in the mammalian central nervous system, playing a vital role in many physiological processes and implicated in several neurological disorders, for which monitoring of dynamic levels of extracellular glutamate in the living brain tissues may contribute to medical understanding and treatments. Electrochemical sensing of glutamate has been developed recently mainly using platinum, carbon fibre and carbon nanotube electrodes. In the present work, we explore the fabrication and properties of electrochemical glutamate sensors fabricated on doped chemical vapour deposition diamond electrodes and graphene nanoplatelet structures. The sensors incorporate platinum nanoparticles to catalyse the electrooxidation of hydrogen peroxide, glutamate oxidase to oxidise glutamate, and a layer of poly-phenylenediamine to impart selectivity. The performance of the devices was compared to a similar sensor fabricated on glassy carbon. Both the diamond and the graphene sensor showed very competitive performance compared to the majority of existing electrochemical sensors. The graphene based sensor showed the best performance of the three investigated in terms of sensitivity, linear dynamic range and long term stability, whereas it was found that the diamond device showed the best limit of detection.

Phase transformation fabrication of a Cu2S nanoplate as an efficient catalyst for water oxidation with glycine

The synthesis of semiconducting nanoplates (NPs) with defined crystal phase is of particular interest, especially their intriguing properties related to the size, shape, and crystal phase. Herein, a liquid-state transformation process from hexagonal-phase CuS NPs is employed to fabricate the cubic-phase Cu2S NPs. The CuS NPs were converted into Cu2S NPs but maintained the morphology. The Cu2S NPs exhibit better oxygen evolution reaction (OER) activity than CuS NPs. Furthermore, the OER activity of Cu2S NPs can be improved by the addition of a glycine (Gly) solution. The Cu2S NPs with Gly in a phosphate buffer solution exhibit excellent OER activity and durability, which approaches that of the best known commercial Ir/C (20%) nanocatalyst. In this work, a good strategy for fabricating a noble-metal-free OER catalyst has been proposed, which could provide insight into developing new water oxidation catalysts with high activity.

Multidimensional mapping method using an arrayed sensing system for cross-reactivity screening

When measuring chemical information in biological fluids, challenges of cross-reactivity arise, especially in sensing applications where no biological recognition elements exist. An understanding of the cross-reactions involved in these complex matrices is necessary to guide the design of appropriate sensing systems. This work presents a methodology for investigating cross-reactions in complex fluids. First, a systematic screening of matrix components is demonstrated in buffer-based solutions. Second, to account for the effect of the simultaneous presence of these species in complex samples, the responses of buffer-based simulated mixtures of these species were characterized using an arrayed sensing system. We demonstrate that the sensor array, consisting of electrochemical sensors with varying input parameters, generated differential responses that provide synergistic information of sample. By mapping the sensing array response onto multidimensional heat maps, characteristic signatures were compared across sensors in the array and across different matrices. Lastly, the arrayed sensing system was applied to complex biological samples to discern and match characteristic signatures between the simulated mixtures and the complex sample responses. As an example, this methodology was applied to screen interfering species relevant to the application of schizophrenia management. Specifically, blood serum measurement of antipsychotic clozapine and antioxidant species can provide useful information regarding therapeutic efficacy and psychiatric symptoms. This work proposes an investigational tool that can guide multi-analyte sensor design, chemometric modeling and biomarker discovery.

Laser-scribed graphene presents an opportunity to print a new generation of disposable electrochemical sensors

Graphene application within electrochemical sensing has been widely reported, but mainly as a composite, which adds summative effects to an underlying electrode. In this work we report the use of laser-scribed graphene as a distinct electrode patterned on a non-conducting flexible substrate. The laser-scribed graphene electrode compared favourably to established carbon macroelectrodes when evaluating both inner sphere and outer sphere redox probes, providing promise of extensive utility as an electrochemical sensor. The laser-scribed graphene electrode demonstrated the fastest heterogeneous electron transfer rate of all the electrodes evaluated with a k(0) of 0.02373 cm s(-1) for potassium ferricyanide, which exceeds commercially available edge plane pyrolytic graphite at 0.00260 cm s(-1), basal plane pyrolytic graphite at 0.00033 cm s(-1) and the very slow and effectively irreversible electrochemistry observed using single layer graphene. Finally and most significantly, a proof of principle system was fabricated using the laser-scribed graphene as working electrode, counter electrode and underlying base for the Ag/AgCl reference electrode, all in situ on the same planar flexible substrate, removing the requirement of macroscale external electrodes. The planar three electrode format operated with the same optimal electrode characteristics. Furthermore, the fabrication is inexpensive, scalable and compatible with a disposable biosensor format, considerably widening the potential applications in electrochemical bio-sensing for laser-scribed graphene.

Implantable (Bio)sensors as new tools for wireless monitoring of brain neurochemistry in real time

Implantable electrochemical microsensors are characterized by high sensitivity, while amperometric biosensors are very selective in virtue of the biological detecting element. Each sensor, specific for every neurochemical species, is a miniaturized high-technology device resulting from the combination of several factors: electrode material, shielding polymers, applied electrochemical technique, and in the case of biosensors, biological sensing material, stabilizers, and entrapping chemical nets. In this paper, we summarize the available technology for the in vivo electrochemical monitoring of neurotransmitters (dopamine, norepinephrine, serotonin, acetylcholine, and glutamate), bioenergetic substrates (glucose, lactate, and oxygen), neuromodulators (ascorbic acid and nitric oxide), and exogenous molecules such as ethanol. We also describe the most represented biotelemetric technologies in order to wirelessly transmit the signals of the above-listed neurochemicals. Implantable (Bio)sensors, integrated into miniaturized telemetry systems, represent a new generation of analytical tools that could be used for studying the brain’s physiology and pathophysiology and the effects of different drugs (or toxic chemicals such as ethanol) on neurochemical systems.

Glutamate oxidase biosensor based on mixed ceria and titania nanoparticles for the detection of glutamate in hypoxic environments

We report on the design and development of a glutamate oxidase (GmOx) microelectrode for measuring l-glutamic acid (GluA) in oxygen-depleted conditions, which is based on the oxygen storage and release capacity of cerium oxides. To fabricate the biosensor, a nanocomposite of oxygen-rich ceria and titania nanoparticles dispersed within a semi-permeable chitosan membrane was co-immobilized with the enzyme GmOx on the surface of a Pt microelectrode. The oxygen delivery capacity of the ceria nanoparticles embedded in a biocompatible chitosan matrix facilitated enzyme stabilization and operation in oxygen free conditions. GluA was measured by amperometry at a working potential of 0.6 V vs Ag/AgCl. Detection limits of 0.594 µM and 0.493 µM and a sensitivity of 793 pA/µM (RSD 3.49%, n=5) and 395 pA/µM (RSD 2.48%, n=5) were recorded in oxygenated and deoxygenated conditions, with response times of 2s and 5s, respectively. The biosensor had good operational stability and selectivity against common interfering substances. Operation of the biosensor was tested in cerebrospinal fluid. Preliminary in vivo recording in Sprague-Dawley rats to monitor GluA in the cortex during cerebral ischemia and reperfusion demonstrate a potential application of the biosensor in hypoxic conditions. This method provides a solution to ensure functionality of oxidoreductase enzymes in oxygen-free environments.

Dumbbell-like PtPd-Fe₃O₄ nanoparticles for enhanced electrochemical detection of H₂O₂

Dumbbell-like Pt(x)Pd(100-x)-Fe(3)O(4) nanoparticles (NPs) were synthesized and studied for electrocatalytic reduction and sensing of H(2)O(2). In 0.1 M phosphate buffered saline (PBS) solution, the 4-10 nm Pt(x)Pd(100-x)-Fe(3)O(4) NPs showed the Pt/Pd composition-dependent catalysis with Pt(48)Pd(52)-Fe(3)O(4) NPs having the best activity. The Pt(48)Pd(52)-Fe(3)O(4) NPs were tested for H(2)O(2) detection, and their H(2)O(2) detection limit reached 5 nM, which was suitable for monitoring H(2)O(2) generated from Raw 264.7 cells. These dumbbell-like PtPd-Fe(3)O(4) NPs are the most sensitive probe ever reported and can be used to achieve real-time quantitative detection of H(2)O(2) in biological environment for biological and biomedical applications.

Implantable Microprobe with Arrayed Microsensors for Combined Amperometric Monitoring of the Neurotransmitters, Glutamate and Dopamine

An implantable, micromachined microprobe with a microsensor array for combined monitoring of the neurotransmitters, glutamate (Glut) and dopamine (DA), by constant potential amperometry has been created and characterized. Microprobe studies in vitro revealed Glut and DA microsensor sensitivities of 126±5 nA·μM(-1)·cm(-2) and 3250±50 nA·μM(-1)·cm(-2), respectively, with corresponding detection limits of 2.1±0.2 μM and 62±8 nM, both at comparable ~1 sec response times. No diffusional interaction of H(2)O(2) among arrayed microelectrodes was observed. Also, no responses from the electroactive interferents, ascorbic acid (AA), uric acid (UA), DOPA (a DA catabolite) or DOPAC (a DA precursor), over their respective physiological concentration ranges, were detected. The dual sensing microbe attributes of size, detection limit, sensitivity, response time and selectivity make it attractive for combined sensing of Glut and DA in vivo.

Fully integrated biochip platforms for advanced healthcare

Recent advances in microelectronics and biosensors are enabling developments of innovative biochips for advanced healthcare by providing fully integrated platforms for continuous monitoring of a large set of human disease biomarkers. Continuous monitoring of several human metabolites can be addressed by using fully integrated and minimally invasive devices located in the sub-cutis, typically in the peritoneal region. This extends the techniques of continuous monitoring of glucose currently being pursued with diabetic patients. However, several issues have to be considered in order to succeed in developing fully integrated and minimally invasive implantable devices. These innovative devices require a high-degree of integration, minimal invasive surgery, long-term biocompatibility, security and privacy in data transmission, high reliability, high reproducibility, high specificity, low detection limit and high sensitivity. Recent advances in the field have already proposed possible solutions for several of these issues. The aim of the present paper is to present a broad spectrum of recent results and to propose future directions of development in order to obtain fully implantable systems for the continuous monitoring of the human metabolism in advanced healthcare applications.

Evaluation of permselective membranes for optimization of intracerebral amperometric glutamate biosensors

Monitoring of extracellular brain glutamate concentrations by intracerebral biosensors is a promising approach to further investigate the role of this important neurotransmitter. However, amperometric biosensors are typically hampered by Faradaic interference caused by the presence of other electroactive species in the brain, such as ascorbic acid, dopamine, and uric acid. Various permselective membranes are often used on biosensors to prevent this. In this study we evaluated the most commonly used membranes, i.e. nafion, polyphenylenediamine, polypyrrole, polyaniline, and polynaphthol using a novel silica-based platinum electrode. First we selected the membranes with the highest sensitivity for hydrogen peroxide in vitro and an optimal selectivity against electrochemical interferents. Then we evaluated the performances of these membranes in a short lasting (3-4h) in vivo experiment. We found that best in vitro performance was accomplished with biosensors that were protected by a poly(m-phenylenediamine) membrane deposited onto the platinum electrode by cyclic voltammetry. However, post-implantation evaluation of these membranes showed poor selectivity against dopamine. Combination with a previously applied nafion layer did not protect the sensors against acute biofouling; indeed it was even counter effective. Finally, we investigated the ability of our biosensors to monitor the effect of glutamate transport blocker DL-TBOA on modulating glutamate concentrations in the prefrontal cortex of anaesthetized rats. The optimized biosensors recorded a rapid 35-fold increase in extracellular glutamate, and are considered suitable for further exploration in vivo.

Ceramic-based microelectrode arrays: recording surface characteristics and topographical analysis

Amperometric measurements using microelectrode arrays (MEAs) provide spatially and temporally resolved measures of neuromolecules in the central nervous system of rats, mice and non-human primates. Multi-site MEAs can be mass fabricated on ceramic (Al₂O₃) substrate using photolithographic methods, imparting a high level of precision and reproducibility in a rigid but durable recording device. Although the functional capabilities of MEAs have been previously documented for both anesthetized and freely moving paradigms, the performance enabling intrinsic physical properties of the MEA device have not heretofore been presented. In these studies, spectral analysis confirmed that the MEA recording sites were primarily composed of elemental platinum (Pt°). In keeping with the precision of the photolithographic process, scanning electron microscopy revealed that the Pt recording sites have unique microwell geometries post-fabrication. Atomic force microscopy demonstrated that the recording surfaces have nanoscale irregularities in the form of elevations and depressions, which contribute to increased current per unit area that exceeds previously reported microelectrode designs. The ceramic substrate on the back face of the MEA was characterized by low nanoscale texture and the ceramic sides consisted of an extended network of ridges and cavities. Thus, individual recording sites have a unique Pt° composition and surface profile that has not been previously observed for Pt-based microelectrodes. These features likely impact the physical chemistry of the device, which may influence adhesion of biological molecules and tissue as well as electrochemical recording performance post-implantation. This study is a necessary step towards understanding and extending the performance abilities of MEAs in vivo.

Effects of applied potential on the mass of non-conducting poly(ortho-phenylenediamine) electro-deposited on EQCM electrodes: comparison with biosensor selectivity parameters

An electrochemical quartz-crystal microbalance (EQCM) was used to determine the mass of poly-(o-phenylenediamine) (PoPD) layers electro-deposited at different applied potentials in neutral buffered monomer solution, conditions that produce the insulating form of the polymer used as a permselective membrane in biosensor applications. There was a systematic increase in the total, steady state PoPD mass deposited for fixed applied potentials from 0.05 to 0.6 V vs. SCE, followed by a plateau up to 0.8 V. Comparison of PoPD mass and permselectivity parameters indicates that the ability of the passivating form of PoPD to block interference species in biosensor applications is not related in a simple way to the mass of material deposited on the surface. Instead, effects of the applied electropolymerisation potential in driving the electro-oxidation of oPD dimers and oligomers formed during the electro-deposition process are likely to have a more direct impact on the selectivity characteristics of the PoPD layer. The results highlight the usefulness of apparent permeabilities, especially of ascorbic acid, in revealing differences between PoPD layers electro-deposited under different conditions.

Biosensors as analytical tools in food fermentation industry

The food industries need rapid and affordable methods to assure the quality ofproducts and process control. Biosensors, combining a biological recognition element and a sensitive transducer, are versatile analytical tools that offer advantages as classical analytical methods due to their inherent specificity, selectivity and simplicity. This paper reviews the recent trends in the development and applications of biosensors used in food fermentation industry, focusing on amperometric enzymatic and microbial sensors.

Review of recent advances in analytical techniques for the determination of neurotransmitters

Methods and advances for monitoring neurotransmitters in vivo or for tissue analysis of neurotransmitters over the last five years are reviewed. The review is organized primarily by neurotransmitter type. Transmitter and related compounds may be monitored by either in vivo sampling coupled to analytical methods or implanted sensors. Sampling is primarily performed using microdialysis, but low-flow push-pull perfusion may offer advantages of spatial resolution while minimizing the tissue disruption associated with higher flow rates. Analytical techniques coupled to these sampling methods include liquid chromatography, capillary electrophoresis, enzyme assays, sensors, and mass spectrometry. Methods for the detection of amino acid, monoamine, neuropeptide, acetylcholine, nucleoside, and soluble gas neurotransmitters have been developed and improved upon. Advances in the speed and sensitivity of these methods have enabled improvements in temporal resolution and increased the number of compounds detectable. Similar advances have enabled improved detection at tissue samples, with a substantial emphasis on single cell and other small samples. Sensors provide excellent temporal and spatial resolution for in vivo monitoring. Advances in application to catecholamines, indoleamines, and amino acids have been prominent. Improvements in stability, sensitivity, and selectivity of the sensors have been of paramount interest.