Real-time monitoring of extracellular adenosine using enzyme-linked microelectrode arrays

Characterisation of a microelectrochemical biosensor for real-time detection of brain extracellular d-serine

A modified development protocol and concomitant characterisation of a first generation biosensor for the detection of brain extracellular d-serine is reported. Functional parameters important for neurochemical monitoring, including sensor sensitivity, O2 interference, selectivity, shelf-life and biocompatibility were examined. Construction and development involved the enzyme d-amino acid oxidase (DAAO), utilising a dip-coating immobilisation method employing a new extended drying approach. The resultant Pt-based polymer enzyme composite sensor achieved high sensitivity to d-serine (0.76 ± 0.04 nA mm-2. μM-1) and a low μM limit of detection (0.33 ± 0.02 μM). The in-vitro response time was within the solution stirring time, suggesting potential sub-second in-vivo response characteristics. Oxygen interference studies demonstrated a 1 % reduction in current at 50 μM O2 when compared to atmospheric O2 levels (200 μM), indicating that the sensor can be used for reliable neurochemical monitoring of d-serine, free from changes in current associated with physiological O2 fluctuations. Potential interference signals generated by the principal electroactive analytes present in the brain were minimised by using a permselective layer of poly(o-phenylenediamine), and although several d-amino acids are possible substrates for DAAO, their physiologically relevant signals were small relative to that for d-serine. Additionally, changing both temperature and pH over possible in vivo ranges (34-40 °C and 7.2-7.6 respectively) resulted in no significant effect on performance. Finally, the biosensor was implanted in the striatum of freely moving rats and used to monitor physiological changes in d-serine over a two-week period.

Spreading depolarizations pose critical energy challenges in acute brain injury

Spreading depolarization (SD) is an electrochemical wave of neuronal depolarization mediated by extracellular K+ and glutamate, interacting with voltage-gated and ligand-gated ion channels. SD is increasingly recognized as a major cause of injury progression in stroke and brain trauma, where the mechanisms of SD-induced neuronal injury are intimately linked to energetic status and metabolic impairment. Here, I review the established working model of SD initiation and propagation. Then, I summarize the historical and recent evidence for the metabolic impact of SD, transitioning from a descriptive to a mechanistic working model of metabolic signaling and its potential to promote neuronal survival and resilience. I quantify the energetic cost of restoring ionic gradients eroded during SD, and the extent to which ion pumping impacts high-energy phosphate pools and the energy charge of affected tissue. I link energy deficits to adaptive increases in the utilization of glucose and O2, and the resulting accumulation of lactic acid and CO2 downstream of catabolic metabolic activity. Finally, I discuss the neuromodulatory and vasoactive paracrine signaling mediated by adenosine and acidosis, highlighting these metabolites' potential to protect vulnerable tissue in the context of high-frequency SD clusters.

A novel "off-on" ratiometric fluorescent aptasensor for adenosine detection based on FRET between quantum dots and graphene oxide

A novel "off-on" ratiometric fluorescent aptasensor was established for adenosine detection based on fluorescence resonance energy transfer (FRET) between CdS QDs, DNA QDs as donor and graphene oxide (GO) as acceptor. Amino-riched DNA QDs covalently bonded to the carboxyl group on the edge of the GO, and with the absorption of the TGA-modified CdS QDs with aptamer (CdS QDs-apt) onto the GO surface via the π-π stacking interaction. The fluorescence of both CdS QDs and DNA QDs were efficiently quenched due to FRET (turn off). When adenosine was present, the specific binding of the aptamer to the target preferentially that released the CdS QDs-apt from GO. The process would inhibit the FRET which contribute to the fluorescence of CdS QDs-apt recovery again (turn on), while the fluorescence intensity of DNA QDs only slightly altered and acted as the reference signal. Thus, a novel "off-on" ratiometric fluorescent aptasensor for adenosine detection was constructed accordingly. There was a good linearity relationship between the ratio of the FL intensity (F595 nm/F464 nm) and the concentration of adenosine in the range of 20.00-180.0 nmol/L with a detection limit of 1.3 nmol/L (S/N = 3, n = 9). Importantly, the feasibility of the developed aptasensor for selective detection of adenosine in serum and urine samples with satisfactory results. The recoveries were observed to be 97.04-100.2 %.

Aptamer-Magnetic Nanoparticle Complexes for Powerful Biosensing: A Comprehensive Review

The selective and sensitive diagnosis of diseases is a significant matter in the early stages of the cure of illnesses. To elaborate, although several types of probes have been broadly applied in clinics, magnetic nanomaterials-aptamers, as new-generation probes, are becoming more and more attractive. The presence of magnetic nanomaterials brings about quantification, purification, and quantitative analysis of biomedical, especially in complex samples. Elaborately, the superparamagnetic properties and numerous functionalized groups of magnetic nanomaterials are considered two main matters for providing separation ability and immobilization substrate, respectively. In addition, the selectivity and stability of aptamer can present a high potential recognition element. Importantly, the integration of aptamer and magnetic nanomaterials benefits can boost the performance of biosensors for biomedical analysis by introducing efficient and compact probes that need low patient samples and fast diagnosis, user-friendly application, and high repeatability in the quantification of biomolecules. The primary aim of this review is to suggest a summary of the effect of the employed other types of nanomaterials in the fabrication of novel aptasensors-based magnetic nanomaterials and to carefully explore various applications of these probes in the quantification of bioagents. Furthermore, the application of these versatile and high-potential probes in terms of the detection of cancer cells and biomarkers, proteins, drugs, bacteria, and nucleoside were discussed. Besides, research gaps and restrictions in the field of biomedical analysis by magnetic nanomaterials-aptamers will be discussed.

Flexible needle-type Microbiosensor for real-time monitoring traditional acupuncture-mediated adenosine release In vivo

Rapid adenosine (ADO) signaling, on the time frame of seconds, regulates physiological and pathological processes, including the therapeutic efficacy of acupuncture. Nevertheless, standard monitoring strategies are limited by poor temporal resolution. Herein, an implantable needle-type microsensor capable of monitoring ADO release in vivo in response to acupuncture in real time has been developed. Electrocatalytic Prussian Blue nanoparticles, an immobilized multienzyme system, and a permselective poly-o-phenylenediamine-based membrane were used for the sequential modification of the sensing region of the electrode. The resultant sensor can perform amperometric measurements of ADO levels in response to a very low level of applied potential (-0.05 V vs Ag/AgCl). This microsensor also functioned across a broad linear range (0-50 μM) and exhibited good sensitivity (1.1 nA/μM) with a rapid response time of under 5 s. Importantly, the sensor also exhibited good reproducibility and high selectivity. For in vivo animal studies, the microsensor was employed for the continuous assessment of instantaneous ADO release at the ST36 (Zusanli) acupoint when this acupoint was subjected to twirling-rotating acupuncture manipulation. Benefiting from superior sensor in vivo performance and stability, the positive correlation between the variability in acupuncture-induced ADO release and the stimulus intensity levels that affect the clinical benefit can be demonstrated for the first time. Overall, these results highlight a powerful approach to analyzing the in vivo physiological effects of acupuncture, expanding application realm of micro-nano sensor technology on a fast time scale.

The adenosine hypothesis of schizophrenia into its third decade: From neurochemical imbalance to early life etiological risks

The adenosine hypothesis of schizophrenia was conceptualized about two decades ago in an attempt to integrate two prominent theories of neurochemical imbalance that attribute the pathogenesis of schizophrenia to hyperfunction of the mesocorticolimbic dopamine neurotransmission and hypofunction of cortical glutamate neurotransmission. Given its unique position as an endogenous modulator of both dopamine and glutamate signaling in the brain, adenosine was postulated as a potential new drug target to achieve multiple antipsychotic actions. This new strategy may offer hope for improving treatment, especially in alleviating negative symptoms and cognitive deficits of schizophrenia that do not respond to current medications. To date, however, the adenosine hypothesis has yet led to any significant therapeutic breakthroughs. Here, we address two possible reasons for the impasse. First, neither the presence of adenosine functional deficiency in people with schizophrenia nor its causal relationship to symptom production has been satisfactorily examined. Second, the lack of novel adenosine-based drugs also impedes progress. This review updates the latest preclinical and clinical data pertinent to the construct validity of the adenosine hypothesis and explores novel molecular processes whereby dysregulation of adenosine signaling could be linked to the etiology of schizophrenia. It is intended to stimulate and revitalize research into the adenosine hypothesis towards the development of a new and improved generation of antipsychotic drugs that has eluded us for decades.

ATP-responsive laccase@ZIF-90 as a signal amplification platform to achieve indirect highly sensitive online detection of ATP in rat brain

A novel electrochemical online system for indirect, highly sensitive and selective online monitoring of ATP in the cerebral microdialysate is presented based on the particular reaction of ATP with zeolitic imidazole framework-90 (ZIF-90) encapsulated laccase microcrystals (laccase@ZIF-90) and the natural catalytic activity of laccase.

Micro/Nano Electrode Array Sensors: Advances in Fabrication and Emerging Applications in Bioanalysis

Due to the rapid development of micro/nano manufacturing techniques and the greater understanding in electrochemical principles and methods, micro/nano electrode array sensing has received much attention in recent years, especially in bioanalysis. This review aims to explore recent progress in innovative techniques for the construction of micro/nano electrode array sensor and the unique applications of various types of micro/nano electrode array sensors in biochemical analysis. Moreover, the new area of smart sensing benefited from miniaturization of portable micro/nano electrode array sensors as well as wearable intelligent devices are further discussed.

Galvanic Redox Potentiometry Based Microelectrode Array for Synchronous Ascorbate and Single-Unit Recordings in Rat Brain

Neuronal communication relies on cooperation between the chemical and electrical patterns of neurons. Thus, techniques for illustrating the linkage of the neurochemical events and action potentials with high temporal and spatial resolution is imperative to gain a comprehensive understanding of the intricacies of brain function. Herein, we integrate galvanic redox potentiometry (GRP) and electrophysiological recording onto a 16-site Au microelectrode array (MEA), one of which is for indicating the ascorbate concentration while the others for single-unit activity assessment. The electrochemical probing site was modified with single-walled carbon nanotubes to promote electron-transfer kinetics of ascorbate at low overpotential so as to enlarge the driving force for the spontaneous ascorbate/O₂ cell reaction. The resulting GRP-based MEA outputs open-circuit potential that is in a linear relationship with the logarithmic ascorbate concentration and exhibits high selectivity against a set of coexisting electroactive species. Furthermore, no reciprocal interference between the two recording systems is observed during concurrent GRP sensing of ascorbate and single-unit recording in a rat brain. In vivo feasibility of the GRP-based MEA is demonstrated by synchronous real-time measurement of ascorbate release and electrical activity from multiple neuronal populations during spreading depression. Our GRP-based MEA sensor creates new opportunities to realize high-throughput screening or mapping of neurochemical patterns in a larger dimension and correlate them to neuron functions across a spatial scale.

Imaging Guidance for Therapeutic Delivery: The Dawn of Neuroenergetics

Modern neurocritical care relies on ancillary diagnostic testing in the form of multimodal monitoring to address acute changes in the neurological homeostasis. Much of our armamentarium rests upon physiological and biochemical surrogates of organ or regional level metabolic activity, of which a great deal is invested at the metabolic-hemodynamic-hydrodynamic interface to rectify the traditional intermediaries of glucose consumption. Despite best efforts to detect cellular neuroenergetics, current modalities cannot appreciate the intricate coupling between astrocytes and neurons. Invasive monitoring is not without surgical complication, and noninvasive strategies do not provide an adequate spatial or temporal resolution. Without knowledge of the brain's versatile behavior in specific metabolic states (glycolytic vs oxidative), clinical practice would lag behind laboratory empiricism. Noninvasive metabolic imaging represents a new hope in delineating cellular, nigh molecular level energy exchange to guide targeted management in a diverse array of neuropathology.

Approaches to Monitor Circuit Disruption after Traumatic Brain Injury: Frontiers in Preclinical Research

Mild traumatic brain injury (TBI) often results in pathophysiological damage that can manifest as both acute and chronic neurological deficits. In an attempt to repair and reconnect disrupted circuits to compensate for loss of afferent and efferent connections, maladaptive circuitry is created and contributes to neurological deficits, including post-concussive symptoms. The TBI-induced pathology physically and metabolically changes the structure and function of neurons associated with behaviorally relevant circuit function. Complex neurological processing is governed, in part, by circuitry mediated by primary and modulatory neurotransmitter systems, where signaling is disrupted acutely and chronically after injury, and therefore serves as a primary target for treatment. Monitoring of neurotransmitter signaling in experimental models with technology empowered with improved temporal and spatial resolution is capable of recording in vivo extracellular neurotransmitter signaling in behaviorally relevant circuits. Here, we review preclinical evidence in TBI literature that implicates the role of neurotransmitter changes mediating circuit function that contributes to neurological deficits in the post-acute and chronic phases and methods developed for in vivo neurochemical monitoring. Coupling TBI models demonstrating chronic behavioral deficits with in vivo technologies capable of real-time monitoring of neurotransmitters provides an innovative approach to directly quantify and characterize neurotransmitter signaling as a universal consequence of TBI and the direct influence of pharmacological approaches on both behavior and signaling.

Challenges of simultaneous measurements of brain extracellular GABA and glutamate in vivo using enzyme-coated microelectrode arrays

BACKGROUND

Although GABA is the major inhibitory neurotransmitter in the CNS, quantifying in vivo GABA levels has been challenging. The ability to co-monitor both GABA and the major excitatory neurotransmitter, glutamate, would be a powerful tool in both research and clinical settings.

NEW METHOD

Ceramic-based microelectrode arrays (MEAs) were used to quantify gamma-aminobutyric acid (GABA) by employing a dual-enzyme reaction scheme including GABase and glutamate oxidase (GluOx). Glutamate was simultaneously quantified on adjacent recording sites coated with GluOx alone. Endogenous glutamate was subtracted from the combined GABA and glutamate signal to yield a pure GABA concentration.

RESULTS

Electrode sensitivity to GABA in conventional, stirred in vitro calibrations at pH 7.4 did not match the in vivo sensitivity due to diffusional losses. Non-stirred calibrations in agarose or stirred calibrations at pH 8.6 were used to match the in vivo GABA sensitivity. In vivo data collected in the rat brain demonstrated feasibility of the GABA/glutamate MEA including uptake of locally applied GABA, KCl-evoked GABA release and modulation of endogenous GABA with vigabatrin.

COMPARISON WITH EXISTING METHODS

Implantable enzyme-coated microelectrode arrays have better temporal and spatial resolution than existing off-line methods. However, interpretation of results can be complicated due to the multiple recording site and dual enzyme approach.

CONCLUSIONS

The initial in vitro and in vivo studies supported that the new MEA configuration may be a viable platform for combined GABA and glutamate measures in the CNS extending the previous reports to in vivo GABA detection. The challenges of this approach are emphasized.

Electrochemical aptamer-based microsensor for real-time monitoring of adenosine in vivo

In this work, an implantable and minimally invasive micro-aptasensor for adenosine monitoring in vivo, based on flexible integrated electrodes, was developed. Firstly the sensor was made by the modification of a needle-type electrode with reduced graphene oxide and gold nanoclusters (rGO-AuNCs) using two-step electrodeposition. Secondly Sulfhydryl-terminated capture probe (ssDNA1) was immobilized on rGO-AuNCs modified electrode surface by self-assembly, and then it was hybridized with adenosine aptamer (ssDNA2). Lastly methylene blue (MB) as an electrochemical indicator was adsorbed on the aptamer through specific interaction of MB with guanine base. The peak current of MB decreased linearly with increasing adenosine concentration due to the formation of aptamer-adenosine complex and displacement of the aptamer from the modified electrode surface. The sensor showed a low detection limit of 0.1 nM with signal-to-noise ratio equal to 3 as well as a wide linear response range (0.1 nM-1 mM) in vitro. Also, a high selectivity was demonstrated for adenosine in relation to uridine, guanosine, and cytidine. Experiments in vivo demonstrated fast responses for a range of adenosine concentrations. This work demonstrates a promising path for implantable devices for the determination of biomolecules in vivo, thus allowing for health tests, detection of infectious diseases, and other medical conditions.

Continuous monitoring of adenosine and its metabolites using microdialysis coupled to microchip electrophoresis with amperometric detection

Rapid monitoring of concentration changes of neurotransmitters and energy metabolites is important for understanding the biochemistry of neurological disease as well as for developing therapeutic options. This paper describes the development of a separation-based sensor using microchip electrophoresis (ME) with electrochemical (EC) detection coupled to microdialysis (MD) sampling for continuous on-line monitoring of adenosine and its downstream metabolites. The device was fabricated completely in PDMS. End-channel electrochemical detection was accomplished using a carbon fiber working electrode embedded in the PDMS. The separation conditions for adenosine, inosine, hypoxanthine, and guanosine were investigated using a ME-EC chip with a 5-cm long separation channel. The best resolution was achieved using a background electrolyte consisting of 35 mM sodium borate at pH 10, 15% dimethyl sulfoxide (DMSO), and 2 mM sodium dodecyl sulphate (SDS), and a field strength of 222 V/cm. Under these conditions, all four purines were separated in less than 85 s. Using a working electrode detection potential of 1.4 vs Ag/AgCl, the limits of detection were 25, 33, 10, and 25 μM for adenosine, inosine, hypoxanthine, and guanosine, respectively. The ME-EC chip was then coupled to microdialysis sampling using a novel all-PDMS microdialysis-microchip interface that was reversibly sealed. This made alignment of the working electrode with the end of the separation channel much easier and more reproducible than could be obtained with previous MD-ME-EC systems. The integrated device was then used to monitor the enzymatic conversion of adenosine to inosine in vitro.

Bio-electrochemical microelectrode arrays for glutamate and electrophysiology detection in hippocampus of temporal lobe epileptic rats

Temporal Lobe Epilepsy (TLE) is a chronic neurological disorder, characterized by sudden, repeated and transient central nervous system dysfunction. For better understanding of TLE, bio-nanomodified microelectrode arrays (MEA) are designed, for the achievement of high-quality simultaneous detection of glutamate signals (Glu) and multi-channel electrophysiological signals including action potentials (spikes) and local field potentials (LFPs). The MEA was fabricated by Micro-Electro-Mechanical System fabrication technology and all recording sites were modified with platinum black nano-particles, the average impedance decreased by nearly 90 times. Additionally, glutamate oxidase was also modified for the detection of Glu. The average sensitivity of the electrode in Glu solution was 1.999 ± 0.032 × 10^-2pA/μM·μm²(n = 3) and linearity was R = 0.9986, with a good selectivity of 97.82% for glutamate and effective blocking of other interferents. In the in-vivo experiments, the MEA was subjected in hippocampus to electrophysiology and Glu concentration detection. During seizures, the fire rate of spikes increases, and the interspike interval is concentrated within 30 ms. The amplitude of LFPs increases by 3 times and the power increases. The Glu level (4.22 μM, n = 4) was obviously higher than normal rats (2.24 μM, n = 4). The MEA probe provides an advanced tool for the detection of dual-mode signals in the research of neurological diseases.

N2O micro-profiles in biofilm from a one-stage autotrophic nitrogen removal system by microelectrode

Emission of nitrous oxide (N₂O), a greenhouse gas, is of growing concern in biological wastewater treatment. N₂O emission from biofilm in a one-stage completely autotrophic nitrogen removal system was investigated using microelectrodes in this study. It is indicated that the pathways of nitrogen transformation in biofilm mainly included partial nitrification and anaerobic ammonium oxidation (anammox), also included nitrification and heterotrophic denitrification (HD). Ammonium-oxidizing bacteria (AOB) denitrification and HD were the main pathways resulting in N₂O production in the biofilm, and hydroxylamine (NH₂OH) oxidation was a subordinate pathway. In addition, the amount of N₂O emission in test in which both NH₄⁺ and NO₂^- were added (NH₄⁺-N: NO₂^--N = 1:1) was about 2 times greater than that in test with NH₄⁺ addition only. This result expressed that NO₂^- is an important factor affecting N₂O production in the biofilm. In conclusion, the present study provides a theoretical support for reducing N₂O production in one-stage completely autotrophic nitrogen removal system.

Evidence that adenosine contributes to Leao's spreading depression in vivo

Leao's spreading depression of cortical activity is a propagating silencing of neuronal activity resulting from spreading depolarization (SD). We evaluated the contributions of action potential (AP) failure and adenosine A₁ receptor (A₁R) activation to the depression of evoked and spontaneous electrocorticographic (ECoG) activity after SD in vivo, in anesthetized mice. We compared depression with SD-induced effects on AP-dependent transmission, and synaptic potentials in the transcallosal and thalamocortical pathways. After SD, APs recovered rapidly, within 1-2 min, as demonstrated by evoked activity in distant projection targets. Evoked corticocortical postsynaptic potentials recovered next, within ∼5 min. Spontaneous ECoG and evoked thalamocortical postsynaptic potentials recovered together, after ∼10-15 min. The duration of ECoG depression was shortened 20% by systemic (10 mg/kg) or focal (30 µM) administration of A₁R competitive antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). ECoG depression was also shortened by focal application of exogenous adenosine deaminase (ADA; 100 U/mL), and conversely, was prolonged 50% by the non-competitive ADA inhibitor deoxycoformycin (DCF; 100 µM). We concluded that while initial depolarization block is brief, adenosine A₁R activation, in part, contributes to the persistent secondary phase of Leao's cortical spreading depression.

The adenosine generating enzymes CD39/CD73 control microglial processes ramification in the mouse brain

Microglial cells invade the brain as amoeboid precursors and acquire a highly ramified morphology in the postnatal brain. Microglia express all essential purinergic elements such as receptors, nucleoside transporters and ecto-enzymes, including CD39 (NTPDase1) and CD73 (5'-nucleotidase), which sequentially degrade extracellular ATP to adenosine. Here, we show that constitutive deletion of CD39 and CD73 or both caused an inhibition of the microglia ramified phenotype in the brain with a reduction in the length of processes, branching frequency and number of intersections with Sholl spheres. In vitro, unlike wild-type microglia, cd39^-/- and cd73^-/- microglial cells were less complex and did not respond to ATP with the transformation into a more ramified phenotype. In acute brain slices, wild-type microglia retracted approximately 50% of their processes within 15 min after slicing of the brain, and this phenomenon was augmented in cd39^-/- mice; moreover, the elongation of microglial processes towards the source of ATP or towards a laser lesion was observed only in wild-type but not in cd39^-/- microglia. An elevation of extracellular adenosine 1) by the inhibition of adenosine transport with dipyridamole, 2) by application of exogenous adenosine or 3) by degradation of endogenous ATP/ADP with apyrase enhanced spontaneous and ATP-induced ramification of cd39^-/- microglia in acute brain slices and facilitated the transformation of cd39^-/- and cd73^-/- microglia into a ramified process-bearing phenotype in vitro. These data indicate that under normal physiological conditions, CD39 and CD73 nucleotidases together with equilibrative nucleoside transporter 1 (ENT1) control the fate of extracellular adenosine and thereby the ramification of microglial processes.

3-D Printed Adjustable Microelectrode Arrays for Electrochemical Sensing and Biosensing

Printed Electronics has emerged as an important fabrication technique that overcomes several shortcomings of conventional lithography and provides custom rapid prototyping for various sensor applications. In this work, silver microelectrode arrays (MEA) with three different electrode spacing were fabricated using 3-D printing by the aerosol jet technology. The microelectrodes were printed at a length scale of about 15 μm, with the space between the electrodes accurately controlled to about 2 times (30 μm, MEA30), 6.6 times (100 μm, MEA100) and 12 times (180 μm, MEA180) the trace width, respectively. Hydrogen peroxide and glucose were chosen as model analytes to demonstrate the performance of the MEA for sensor applications. The electrodes are shown to reduce hydrogen peroxide with a reduction current proportional to the concentration of hydrogen peroxide for certain concentration ranges. Further, the sensitivity of the current for the three electrode configurations was shown to decrease with an increase in the microelectrode spacing (sensitivity of MEA30: MEA100: MEA180 was in the ratio of 3.7: 2.8: 1), demonstrating optimal MEA geometry for such applications. The noise of the different electrode configurations is also characterized and shows a dramatic reduction from MEA30 to MEA100 and MEA180 electrodes. Further, it is shown that the response current is proportional to MEA100 and MEA180 electrode areas, but not for the area of MEA30 electrode (the current density of MEA30 : MEA100 : MEA180 is 0.25 : 1 : 1), indicating that the MEA30 electrodes suffer from diffusion overlap from neighboring electrodes. The work thus establishes the lower limit of microelectrode spacing for our geometry. The lowest detection limit of the MEAs was calculated (with S/N = 3) to be 0.45 μM. Glucose oxidase was immobilized on MEA100 microelectrodes to demonstrate a glucose biosensor application. The sensitivity of glucose biosensor was 1.73 μAmM^-1 and the calculated value of detection limit (S/N = 3) was 1.7 μM. The electrochemical response characteristics of the MEAs were in agreement with the predictions of existing models. The current work opens up the possibility of additive manufacturing as a fabrication technique for low cost custom-shaped MEA structures that can be used as electrochemical platforms for a wide range of sensor applications.

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.