Disk-shaped amperometric enzymatic biosensor for in vivo detection of D-serine

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.

Transient potentiometry based d-serine sensor using engineered d-amino acid oxidase showing quasi-direct electron transfer property

d-Serine biosensing has been extensively reported based on enzyme sensors using flavin adenine dinucleotide (FAD) -dependent d-amino acid oxidase (DAAOx), based on the monitoring of hydrogen peroxide generated by the enzymatic reaction, which is affected by dissolved oxygen concentration in the measurement environment in in vivo use. Here we report a novel sensing principle for d-serine, transient potentiometry based d-serine sensor using engineered DAAOx showing quasi-direct electron transfer (DET) property. DAAOx Gly52Val mutant, revealed to possess dye-mediated dehydrogenase activity using artificial synthetic electron acceptors, while its oxidase activity was negligible. The enzyme was immobilized on electrode and was modified with amine-reactive phenazine ethosulfate, resulted an enzyme electrode showing quasi-DET type response. Although OCP based monitoring took more than several minutes to obtain steady state OCP value, the time dependent OCP change monitoring, transient potentiometry, provided rapid and sensitive sensor signals. While dOCP/dt based monitoring was suitable for sensing with longer than 5 s time resolution with d-serine concentration range between 0.5 mM and 5 mM, dOCP/d t based monitoring is suitable for d-serine monitoring with much shorter time resolution (less than 1 s) with high sensitivity with wider dynamic range (20 μM-30 mM). The maximum dOCP/d t was -39.2 ± 2.0 mV/s^1/2, the K_m(app) was 1.9 mM, and the lower limit of detection was 20 μM. In addition, d-serine monitoring was also possible in the artificial cerebrospinal fluid. The transient potentiometry based sensing reported in this study will be further utilized to realize miniaturized, continuous, real-time, in vivo sensor for d-serine monitoring.

High-Throughput Strategy for Glycine Oxidase Biosensor Development Reveals Glycine Release from Cultured Cells

Glycine is an important biomarker in clinical analysis due to its involvement in multiple physiological processes. As such, the need for low-cost analytical tools for glycine detection is growing. As a neurotransmitter, glycine is involved in inhibitory and excitatory neurochemical transmission in the central nervous system. In this work, we present a 10 μM Pt-based electrochemical enzymatic biosensor based on the flavoenzyme glycine oxidase (GO) for localized real-time measurements of glycine. Among GO variants at position 244, the H244K variant with increased glycine turnover was selected to develop a functional biosensor. This biosensor relies on amperometric readouts and does not require additional redox mediators. The biosensor was characterized and applied for glycine detection from cells, mainly HEK 293 cells and primary rat astrocytes. We have identified an enzyme, GO H244K, with increased glycine turnover using mutagenesis but which can be developed into a functional biosensor. Noteworthy, a glycine release of 395.7 ± 123 μM from primary astrocytes was measured, which is ∼fivefold higher than glycine release from HEK 293 cells (75.4 ± 3.91 μM) using the GO H244K biosensor.

Enantiospecific Detection of D-Amino Acid through Synergistic Upconversion Energy Transfer

D-amino acids (DAAs) are indispensable in regulating diverse metabolic pathways. Selective and sensitive detection of DAAs is crucial for understanding the complexity of metabolic processes and managing associated diseases. However, current DAA detection strategies mainly rely on bulky instrumentation or electrochemical probes, limiting their cellular and animal applications. Here we report an enzyme-coupled nanoprobe that can detect enantiospecific DAAs through synergistic energy transfer. This nanoprobe offers near-infrared upconversion capability, a wide dynamic detection range, and a detection limit of 2.2 μM, providing a versatile platform for in vivo noninvasive detection of DAAs with high enantioselectivity. These results potentially allow real-time monitoring of biomolecular handedness in living animals, as well as developing antipsychotic treatment strategies.

Enhancing Electrochemical Biosensor Selectivity with Engineered d-Amino Acid Oxidase Enzymes for d-Serine and d-Alanine Quantification

d-Amino acid oxidase (DAAO) enzymes bind a range of d-amino acids with variable affinity. As such, the design of selective DAAO-based enzymatic biosensors remains a challenge for real-world biosensor application. Herein, a methodology for developing biosensors with varying substrate selectivity is presented. First, we address DAAO-based biosensor selectivity toward d-serine by introducing point mutations into DAAO using rational design. Next, the wild-type yeast DAAO (RgDAAO WT) and variants human DAAO W209R and yeast M213G are characterized for their selectivity and activity toward d-serine and d-alanine, the preferred DAAO substrates. The DAAO enzymes have been immobilized for final biosensor design, where they demonstrate selectivity comparable to free DAAO. The cross-linking procedure impacts on DAAO structure and function and the use of a regeneration strategy allows the biosensor response to be improved.

Quantitative measurements of free and immobilized RgDAAO Michaelis-Menten constant using an electrochemical assay reveal the impact of covalent cross-linking on substrate specificity

Challenges facing enzyme-based electrochemical sensors include substrate specificity, batch to batch reproducibility, and lack of quantitative metrics related to the effect of enzyme immobilization. We present a quick, simple, and general approach for measuring the effect of immobilization and cross-linking on enzyme activity and substrate specificity. The method can be generalized for electrochemical biosensors using an enzyme that releases hydrogen peroxide during its catalytic cycle. Using as proof of concept RgDAAO-based electrochemical biosensors, we found that the Michaelis-Menten constant (K_m) decreases post immobilization, hinting at alterations in the enzyme kinetic properties and thus substrate specificity. We confirm the decrease in K_m electrochemically by characterizing the substrate specificity of the immobilized RgDAAO using chronoamperometry. Our results demonstrate that enzyme immobilization affects enzyme substrate specificity and this must be carefully evaluated during biosensor development.

Synthesis of Carbon Dots by Varying Doped Elements and Application in Serine Detection

Carbon dots (CDs) with different doping elements were successfully synthesized via a simple hydrothermal strategy. 3-amino-4-chlorophenylboronic acid, 3-aminobenzeneboronic acid, aniline, and benzene were used as precursors, respectively. The B/N co-doping CDs (BNCDs) derived from 3-aminobenzeneboronic acid show brightest fluorescence among the CDs products with quantum yield at 0.15. The fluorescence of BNCDs exhibits good photostability and excitation-independent emission behavior. The bright blue emission of BNCDs can be quenched by serine, which is a kind of neutral aliphatic amino acid containing hyroxyl groups with polarity. It is possibly due to the molecular collision between excited state of BNCDs and the ground state of serine. BNCDs can be served as fluorophore probe for the assay of serine based on the efficient quenching effect. The approach for the determination of serine shows a high sensitivity with a detection limit at 0.14 nM, which is lower than those of previous works. Furthermore, the present BNCDs system can be employed to monitor serine in real food and biological samples. The strategy may be a potential way for the application in food safety and biomedicine fields.

Biosensors for D-Amino Acids: Detection Methods and Applications

D-enantiomers of amino acids (D-AAs) are only present in low amounts in nature, frequently at trace levels, and for this reason, their biological function was undervalued for a long time. In the past 25 years, the improvements in analytical methods, such as gas chromatography, HPLC, and capillary electrophoresis, allowed to detect D-AAs in foodstuffs and biological samples and to attribute them specific biological functions in mammals. These methods are time-consuming, expensive, and not suitable for online application; however, life science investigations and industrial applications require rapid and selective determination of D-AAs, as only biosensors can offer. In the present review, we provide a status update concerning biosensors for detecting and quantifying D-AAs and their applications for safety and quality of foods, human health, and neurological research. The review reports the main challenges in the field, such as selectivity, in order to distinguish the different D-AAs present in a solution, the simultaneous assay of both L- and D-AAs, the production of implantable devices, and surface-scanning biosensors. These innovative tools will push future research aimed at investigating the neurological role of D-AAs, a vibrant field that is growing at an accelerating pace.

Recent advances in the biosensing of neurotransmitters: material and method overviews towards the biomedical analysis of psychiatric disorders

Neurotransmitters are the most important messengers of the nervous system, and any changes in their balances and activities can cause serious neurological, psychiatric and cognitive disorders such as schizophrenia, Alzheimer's disease and Parkinson's disease.

Analysing single live cells by scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) offers single live cell activities along its topography toward cellular physiology and pathology.

Electrochemical imaging of cells and tissues

This minireview summarizes the recent achievements of electrochemical imaging platforms to map cellular functions in biological specimens using electrochemical scanning nano/micro-probe microscopy and 2D chips containing microelectrode arrays.

The technological and experimental progress in electrochemical imaging of biological specimens is discussed with a view on potential applications for skin cancer diagnostics, reproductive medicine and microbial testing. The electrochemical analysis of single cell activity inside cell cultures, 3D cellular aggregates and microtissues is based on the selective detection of electroactive species involved in biological functions. Electrochemical imaging strategies, based on nano/micrometric probes scanning over the sample and sensor array chips, respectively, can be made sensitive and selective without being affected by optical interference as many other microscopy techniques. The recent developments in microfabrication, electronics and cell culturing/tissue engineering have evolved in affordable and fast-sampling electrochemical imaging platforms. We believe that the topics discussed herein demonstrate the applicability of electrochemical imaging devices in many areas related to cellular functions.

Amperometric Self-Referencing Ceramic Based Microelectrode Arrays for D-Serine Detection

D-serine is the major D-amino acid in the mammalian central nervous system. As the dominant co-agonist of the endogenous synaptic NMDA receptor, D-serine plays a role in synaptic plasticity, learning, and memory. Alterations in D-serine are linked to neuropsychiatric disorders including schizophrenia. Thus, it is of increasing interest to monitor the concentration of D-serine in vivo as a relevant player in dynamic neuron-glia network activity. Here we present a procedure for amperometric detection of D-serine with self-referencing ceramic-based microelectrode arrays (MEAs) coated with D-amino acid oxidase from the yeast Rhodotorulagracilis (RgDAAO). We demonstrate in vitro D-serine recordings with a mean sensitivity of 8.61 ± 0.83 pA/µM to D-serine, a limit of detection (LOD) of 0.17 ± 0.01 µM, and a selectivity ratio of 80:1 or greater for D-serine over ascorbic acid (mean ± SEM; n = 12) that can be used for freely moving studies.

Chiral Measurement of Aspartate and Glutamate in Single Neurons by Large-Volume Sample Stacking Capillary Electrophoresis

d-Amino acids (d-AAs) are endogenous molecules found throughout the metazoan, the functions of which remain poorly understood. Measurements of low abundance and heterogeneously distributed d-AAs in complex biological samples, such as cells and multicellular structures of the central nervous system (CNS), require the implementation of sensitive and selective analytical approaches. In order to measure the d- and l-forms of aspartate and glutamate, we developed and applied a stacking chiral capillary electrophoresis (CE) with laser-induced fluorescence detection method. The achieved online analyte preconcentration led to a 480-fold enhancement of detection sensitivity relative to capillary zone electrophoresis, without impacting separation resolution or analysis time. Additionally, the effects of inorganic ions on sample preconcentration and CE separation were evaluated. The approach enabled the relative quantification of d-aspartate and d-glutamate in individual neurons mechanically isolated from the CNS of the sea slug Aplysia californica, a well characterized neurobiological model. Levels of these structurally similar d-AAs were significantly different in subpopulations of cells collected from the investigated neuronal clusters.

The Gliotransmitter d-Serine Promotes Synapse Maturation and Axonal Stabilization In Vivo

The NMDAR is thought to play a key role in the refinement of connectivity in developing neural circuits. Pharmacological blockade or genetic loss-of-function manipulations that prevent NMDAR function during development result in the disorganization of topographic axonal projections. However, because NMDARs contribute to overall glutamatergic neurotransmission, such loss-of-function experiments fail to adequately distinguish between the roles played by NMDARs and neural activity in general. The gliotransmitter d-serine is a coagonist of the NMDAR that is required for NMDAR channel opening, but which cannot mediate neurotransmission on its own. Here we demonstrate that acute administration of d-serine has no immediate effect on glutamate release or AMPA-mediated neurotransmission. We show that endogenous d-serine is normally present below saturating levels in the developing visual system of the Xenopus tadpole. Using an amperometric enzymatic biosensor, we demonstrate that glutamatergic activation elevates ambient endogenous d-serine levels in the optic tectum. Chronically elevating levels of d-serine promoted synaptic maturation and resulted in the hyperstabilization of developing axon branches in the tadpole visual system. Conversely, treatment with an enzyme that degrades endogenous d-serine resulted in impaired synaptic maturation. Despite the reduction in axon arbor complexity seen in d-serine-treated animals, tectal neuron visual receptive fields were expanded, suggesting a failure to prune divergent retinal inputs. Together, these findings positively implicate NMDAR-mediated neurotransmission in developmental synapse maturation and the stabilization of axonal inputs and reveal a potential role for d-serine as an endogenous modulator of circuit refinement.SIGNIFICANCE STATEMENT Activation of NMDARs is critical for the activity-dependent development and maintenance of highly organized topographic maps. d-Serine, a coagonist of the NMDAR, plays a significant role in modulating NMDAR-mediated synaptic transmission and plasticity in many brain areas. However, it remains unknown whether d-serine participates in the establishment of precise neuronal connections during development. Using an in vivo model, we show that glutamate receptor activation can evoke endogenous d-serine release, which promotes glutamatergic synapse maturation and stabilizes axonal structural and functional inputs. These results reveal a pivotal modulatory role for d-serine in neurodevelopment.

The Rise and Fall of the d-Serine-Mediated Gliotransmission Hypothesis

d-Serine modulates N-methyl d-aspartate receptors (NMDARs) and regulates synaptic plasticity, neurodevelopment, and learning and memory. However, the primary site of d-serine synthesis and release remains controversial, with some arguing that it is a gliotransmitter and others defining it as a neuronal cotransmitter. Results from several laboratories using different strategies now show that the biosynthetic enzyme of d-serine, serine racemase (SR), is expressed almost entirely by neurons, with few astrocytes appearing to contain d-serine. Cell-selective suppression of SR expression demonstrates that neuronal, rather than astrocytic d-serine, modulates synaptic plasticity. Here, we propose an alternative conceptualization whereby astrocytes affect d-serine levels by synthesizing l-serine that shuttles to neurons to fuel the neuronal synthesis of d-serine.

Stabilization of Prussian blue with polyaniline and carbon nanotubes in neutral media for in vivo determination of glucose in rat brains

This study demonstrates a new electrochemical microbiosensor for selectivein vivomonitoring of glucose in rat brains.

Achieving reproducible performance of electrochemical, folding aptamer-based sensors on microelectrodes: challenges and prospects

Combining specific recognition capabilities with the excellent spatiotemporal resolution of small electrodes represents a promising methodology in bioanalytical and chemical sensing. In this paper, we report the development of reproducible electrochemical, aptamer-based (E-AB) sensors on a gold microelectrode platform. Specifically, we develop microscale sensors (25 μm diameter) for two representative small molecule targets–adenosine triphosphate and tobramycin. Furthermore, we report on the challenges encountered at this size scale including small-magnitude signals and interference from the irreversible reduction of dissolved oxygen and present methods to circumvent these challenges. Through the electrochemical deposition of dendritic gold nanostructures, we demonstrate microscale sensors with improved performance by increasing signal-to-noise and consequently sensitivity. Finally, we report on the use of the nonspecific adsorption of serum proteins as an additional layer of surface passivation for stable sensor performance. The sensor development here represents general guidelines for fabricating electrochemical, folding aptamer-based sensors on small-scale electrodes.

Calixarene modified montmorillonite: a novel design for biosensing applications

Here we report the synthesis, characterization and application of calixarene (Calix) modified montmorillonite (Mt) as a platform for bio-applications such as biomolecule immobilization and biosensing technologies.