Biosensor for rapid determination of 3-hydroxybutyrate using bi-enzyme system

Nanozymes based on octahedral platinum nanocrystals with {111} surface facets: glucose oxidase mimicking activity in electrochemical sensors

The ability of shape-controlled octahedral Pt nanoparticles to act as nanozyme mimicking glucose oxidase enzyme is reported. Extended {111} particle surface facets coupled with a size comparable to natural enzymes and easy-to-remove citrate coating give high affinity for glucose, comparable to the enzyme as proven by the steady-state kinetics of glucose electrooxidation. The easy and thorough removal of the citrate coating, demonstrated by X-ray photoelectron spectroscopy analysis, allows a highly stable deposition of the nanozymes on the electrode. The glucose electrochemical detection (at -0.2 V vs SCE) shows a linear response between 0.36 and 17 mM with a limit of detection of 110 μM. A good reproducibility has been achieved, with an average relative standard deviation (RSD) value of 9.1% (n = 3). Similarly, a low intra-sensor variability has been observed, with a RSD of 6.6% (n = 3). Moreover, the sensor shows a long-term stability with reproducible performances for at least 2 months (RSD: 7.8%). Tests in saliva samples show the applicability of Pt nanozymes to commercial systems for non-invasive monitoring of hyperglycemia in saliva, with recoveries ranging from 92 to 98%.

Monitoring Metabolites Using an NAD(P)H-sensitive Polymer Dot and a Metabolite-Specific Enzyme

We introduce an NAD(P)H-sensitive polymer dot (Pdot) biosensor for point-of-care monitoring of metabolites. The Pdot is combined with a metabolite-specific NAD(P)H-dependent enzyme that catalyzes the oxidation of the metabolite, generating NAD(P)H. Upon UV illumination, the NAD(P)H quenches the fluorescence emission of Pdot at 627 nm via electron transfer, and also fluoresces at 458 nm, resulting in a shift from red to blue emission at higher NAD(P)H concentrations. Metabolite concentration is quantified ratiometrically-based on the ratio of blue-to-red channel emission intensities, with a digital camera-with high sensitivity and specificity. We demonstrate phenylalanine biosensing in human plasma for a phenylketonuria screening test, quantifying several other disease-related metabolites (lactate, glucose, glutamate, and β-hydroxybutyrate), and a paper-based assay with smartphore imaging for point-of-care use.

Clinic-on-a-Needle Array toward Future Minimally Invasive Wearable Artificial Pancreas Applications

In order to reduce medical facility overload due to the rise of the elderly population, modern lifestyle diseases, or pandemics, the medical industry is currently developing point-of-care and home medical device systems. Diabetes is an incurable and lifetime disease, accountable for a significant mortality and socio-economic public health burden. Thus, tight glucose control in diabetic patients, which can prevent the onset of its late complications, is of enormous importance. Despite recent advances, the current best achievable management of glucose control is still inadequate, due to several key limitations in the system components, mainly related to the reliability of sensing components, both temporally and chemically, and the integration of sensing and delivery components in a single wearable platform, which is yet to be achieved. Thus, advanced closed-loop artificial pancreas systems able to modulate insulin delivery according to the measured sensor glucose levels, independently of patient supervision, represent a key requirement of development efforts. Here, we demonstrate a minimally invasive, transdermal, multiplex, and versatile continuous metabolites monitoring system in the subcutaneous interstitial fluid space based on a chemically modified SiNW-FET nanosensor array on microneedle elements. Using this technology, ISF-borne metabolites require no extraction and are measured directly and continuously by the nanosensors. Due to their chemical sensing mechanism, the nanosensor response is only influenced by the specific metabolite of interest, and no response is observed in the presence of potential exogenous and endogenous interferents known to seriously affect the response of current electrochemical glucose detection approaches. The 2D architecture of this platform, using a single SOI substrate as a top-down multipurpose material, resulted in a standard fabricated chip with 3D functionality. After proving the ability of the system to act as a selective multimetabolites sensor, we have implemented our platform to reach our main goal for in vivo continuous glucose monitoring of healthy human subjects. Furthermore, minor adjustments to the fabrication technique allow the on-chip integration of microinjection needle elements, which can ideally be used as a drug delivery system. Preliminary experiments on a mice animal model successfully demonstrated the single-chip capability to both monitor glucose levels as well as deliver insulin. By that, we hope to provide in the future a cost-effective and reliable wearable personalized clinical tool for patients and a strong tool for research, which will be able to perform direct monitoring of clinical biomarkers in the ISF as well as synchronized transdermal drug delivery by this single-chip multifunctional platform.

Highly Sensitive Electrochemical Sensor for Diagnosis of Diabetic Ketoacidosis (DKA) by Measuring Ketone Bodies in Urine

In this report, we present an enzyme deposited Au electrode for an electrochemical measurement of acetylacetic acid (AcAc) in urine. The electrode has an immobilized layer of a mixture of D-β-hydroxybutyrate dehydrogenase (HBDH) and nicotinamide adenine dinucleotide (NADH) as sensing material to investigate its electroanalytical properties by means of cyclic voltammetry (CV). The modified electrodes are used for the detection of AcAc and present a linear current increase when the AcAc concentration increases. The electrode presents a limit of detection (LOD) of 6.25 mg/dL in the range of 6.25-100 mg/dL for investigation of clinical relevance. Finally, the electrode was evaluated using 20 patient samples. The measured results of urine ketone by the developed electrode were compared with the clinical results from a commercial kit, and the analysis showed good agreement. The proposed electrode was demonstrated to be a very promising platform as a miniaturized electrochemical analyzer for point-of-care monitoring of the critical biochemical parameters such as urine ketone.

An Electrochemical Enzyme Biosensor for 3-Hydroxybutyrate Detection Using Screen-Printed Electrodes Modified by Reduced Graphene Oxide and Thionine

A biosensor for 3-hydroxybutyrate (3-HB) involving immobilization of the enzyme 3-hydroxybutyrate dehydrogenase onto a screen-printed carbon electrode modified with reduced graphene oxide (GO) and thionine (THI) is reported here. After addition of 3-hydroxybutyrate or the sample in the presence of NAD⁺ cofactor, the generated NADH could be detected amperometrically at 0.0 V vs. Ag pseudo reference electrode. Under the optimized experimental conditions, a calibration plot for 3-HB was constructed showing a wide linear range between 0.010 and 0.400 mM 3-HB which covers the clinically relevant levels for diluted serum samples. In addition, a limit of detection of 1.0 µM, much lower than that reported using other biosensors, was achieved. The analytical usefulness of the developed biosensor was demonstrated via application to spiked serum samples.

Synthesis of robust electrochemical substrate and fabrication of immobilization free biosensors for rapid sensing of salicylate and β-hydroxybutyrate in whole blood

An electrochemical latent redox probe, SAF 5 was designed, synthesized and characterized. A rapid and sensitive solution-based assay was demonstrated for salicylate hydroxylase (SHL). In presence of NADH at aerobic conditions, SHL catalyzed the decarboxylative hydroxylation of SAF and released a redox reporter amino ferrocene (AF 6). The release of AF 6 was monitored at interference free potential region (-50 mV vs. Ag|AgCl) using differential pulse voltammetry as signal read-out. The current signal generated by this process is highly specific, and insensitive to other biological interfering compounds. Next, the SAF incorporated SHL assay was extended to fabricate immobilization-free biosensors for rapid sensing of salicylic acid (SA) and β-hydroxybutyrate (β-HB) in whole blood. The described method rapidly detects SA in a linear range of 35-560 μM with detection limit of 5.0 μM. For β-HB determination, the linear range was 10-600 μM and detection limit was 2.0 μM. Besides, the assay protocols are simple, fast, reliable, selective, sensitive and advantageous over existing methods. The whole blood assay did not required cumbersome steps such as, enzyme immobilization, pre-treatments and holds great practical potential in clinical diagnosis.

Structural insights into the catalytic reaction trigger and inhibition of D-3-hydroxybutyrate dehydrogenase

D-3-Hydroxybutyrate dehydrogenase catalyzes the reversible conversion of acetoacetate and D-3-hydroxybutyrate. These ketone bodies are both energy-storage forms of acetyl-CoA. In order to clarify the structural mechanisms of the catalytic reaction with the cognate substrate D-3-hydroxybutyrate and of the inhibition of the reaction by inhibitors, the enzyme from Alcaligenes faecalis has been analyzed by X-ray crystallography in liganded states with the substrate and with two types of inhibitor: malonate and methylmalonate. In each subunit of the tetrameric enzyme, the substrate is trapped on the nicotinamide plane of the bound NAD(+). An OMIT map definitively shows that the bound ligand is D-3-hydroxybutyrate and not acetoacetate. The two carboxylate O atoms form four hydrogen bonds to four conserved amino-acid residues. The methyl group is accommodated in the nearby hydrophobic pocket so that the formation of a hydrogen bond from the OH group of the substrate to the hydroxy group of Tyr155 at the active centre is facilitated. In this geometry, the H atom attached to the C(3) atom of the substrate in the sp(3) configuration is positioned at a distance of 3.1 Å from the nicotinamide C(4) atom in the direction normal to the plane. In addition, the donor-acceptor relationship of the hydrogen bonds suggests that the Tyr155 OH group is allowed to ionize by the two donations from the Ser142 OH group and the ribose OH group. A comparison of the protein structures with and without ligands indicates that the Gln196 residue of the small movable domain participates in the formation of additional hydrogen bonds. It is likely that this situation can facilitate H-atom movements as the trigger of the catalytic reaction. In the complexes with inhibitors, however, their principal carboxylate groups interact with the enzyme in a similar way, while the interactions of other groups are changed. The crucial determinant for inhibition is that the inhibitors have no active H atom at C(3). A second determinant is the Tyr155 OH group, which is perturbed by the inhibitors to donate its H atom for hydrogen-bond formation, losing its nucleophilicity.

A Paper-Based "Pop-up" Electrochemical Device for Analysis of Beta-Hydroxybutyrate

This paper describes the design and fabrication of a "pop-up" electrochemical paper-based analytical device (pop-up-EPAD) to measure beta-hydroxybutyrate (BHB)-a biomarker for diabetic ketoacidosis-using a commercial combination BHB/glucometer. Pop-up-EPADs are inspired by pop-up greeting cards and children's books. They are made from a single sheet of paper folded into a three-dimensional (3D) device that changes shape, and fluidic and electrical connectivity, by simply folding and unfolding the structure. The reconfigurable 3D structure makes it possible to change the fluidic path and to control timing; it also provides mechanical support for the folded and unfolded structures that enables good registration and repeatability on folding. A pop-up-EPAD designed to detect BHB shows performance comparable to commercially available plastic test strips over the clinically relevant range of BHB in blood when used with a commercial glucometer that integrates the ability to measure glucose and BHB (combination BHB/glucometer). With simple modifications of the electrode and the design of the fluidic path, the pop-up-EPAD also detects BHB in buffer using a simple glucometer-a device that is more available than the combination BHB/glucometer. Strategies that use a "3D pop-up"-that is, large-scale changes in 3D structure and fluidic paths-by folding/unfolding add functionality to EPADs (e.g., controlled timing, fluidic handling and path programming, control over complex sequences of steps, and alterations in electrical connectivity) and should enable the development of new classes of paper-based diagnostic devices.

Development of 3-hydroxybutyrate dehydrogenase enzyme biosensor based on carbon nanotube-modified screen-printed electrode

Precise detection of 3-hydroxybutyrate (HB) in biological samples is of great importance for management of diabetic patients. In this study, an HB biosensor based on single-walled carbon nanotubes (SWCNTs)-modified screen-printed electrode (SPE) was developed to determine the concentration of HB in serum. The specific detecting enzyme, HB dehydrogenase, was physically immobilised on SWCNTs deposited on the surface of SPEs. The electrochemical measurement of HB that involved cyclic voltammetry was based on the sAgnal produced by j3-nicotinamide adenine dinucleotide (NADH), one of the products of the enzymatic reaction. The application of SWCNT reduced the oxidation potential of NADH to about -0.05 V. Electrochemical measurements showed that the response of this biosensor had relevant good linearity in the range of 0.1-2 mM with a low detection limit of 0.009 mM. Investigation of biosensor response in the presence of interfering molecules verified its specificity. Furthermore, the study of long-term stability demonstrated the acceptable efficiency of this biosensor for about 100 days.

An electrochemical biosensor for 3-hydroxybutyrate detection based on screen-printed electrode modified by coenzyme functionalized carbon nanotubes

3-Hydroxybutyrate, one of the main blood ketone bodies, has been considered as a critical indicator for diagnosis of diabetic ketoacidosis. Biosensors designed for detection of 3-hydroxybutyrate with advantages of precision, easiness and speedy performance have attracted increasing attention. This study attempted to develop a 3-hydroxybutyrate dehydrogenase-based biosensor in which single-walled carbon nanotubes (SWCNT) was used in order to immobilize the cofactor, NAD(+), on the surface of screen-printed electrode. The formation of NAD(+)-SWCNT conjugates was assessed by electrochemistry and electron microscopy. Cyclic voltammetry was used to analyze the performance of this biosensor electrochemically. The considerable shelf life and reliability of the proposed biosensor to analyze real sample was confirmed by this method. The reduction in the over potential of electrochemical oxidation of NADH to -0.15 V can be mentioned as a prominent feature of this biosensor. This biosensor can detect 3-hydroxybutyrate in the linear range of 0.01-0.1 mM with the low detection limit of 0.009 mM. Simultaneous application of screen-printed electrode and SWCNT has made the biosensor distinguished which can open new prospects for detection of other clinically significant metabolites.

A novel, disposable, screen-printed amperometric biosensor for ketone 3-β-hydroxybutyrate fabricated using a 3-β-hydroxybutyrate dehydrogenase-mesoporous silica conjugate

A disposable amperometric biosensor for ketone 3-β-hydroxybutyrate (3HB) has been developed successfully. The sensor is based on a screen-printed carbon electrode containing Meldola's Blue (MB) and sensing components containing nicotinamide adenine dinucleotide (NAD(+)) and 3-β-hydroxybutyrate dehydrogenase (3HBDH) immobilized in mesoporous silica (FSM8.0) using an aqueous photo-cross-linkable polymer matrix of polyvinyl alcohol (O-391), and it requires only a small sample volume of 10 μL for the measurement. The behavior of a resulting biosensor, i.e., 3HBDH-FSM8.0/NAD(+)/MB-SPCE, was examined in terms of NAD(+) concentration for construction, pH, applied potential, operational range, selectivity, and storage stability. The sensor showed an optimum response at a pH of 7.6 and at an applied potential of -50 mV. The determination range and the response time for 3HB were from 30 μM to 8 mM and approximately 30 s, respectively. In addition, the sensor was quite stable and maintained >90% of its initial response after being stored for over 6 months. This result implies that our method provides a novel biosensor for ketone 3-β-hydroxybutyrate which is easy-to-use, cost-effective, and has good reproducibility, which are vital for commercial purposes.

Development of a sensitive long-wavelength fluorogenic probe for nitroreductase: a new fluorimetric indictor for analyte determination by dehydrogenase-coupled biosensors

Nitroreductase (NTR) is a flavin-containing enzyme that uses NADH as the electron source to reduce nitroaromatic compounds to the corresponding amines. Previous studies have shown that nitroreductase-targeted latent fluorophores exhibit low solubility in the aqueous media and fluoresce at lower wavelengths upon uncloaking, thus limiting their effective applications. Here, we have prepared a new switch-on long-wavelength latent fluorogenic substrate, NTRLF (4), for NTR. In the presence of NADH, NTR catalyzes the reduction of the nitroaromatic moiety in NTRLF (4), followed by the cascade reaction, 1,6-rearrangement-elimination reaction, cyclic urea formation, and concomitant ejects a long-wavelength fluorescence coumarin (8). However, this reaction was inhibited in the presence of nitroaromatic analogues. The fluorescence signal generated by the cascade reaction was specific and insensitive to various reductants. Accordingly, we propose that NTRLF and NTR in the presences of NADH constitute a useful switch-off high-throughput fluorescence sensor for screening nitroaromatic compounds. Furthermore, NTRLF in the NTR-coupled 3-hydroxybutyrate dehydrogenase and aldehyde dehydrogenase assay reactions was a sensitive fluorimetric indicator for the quantitatively measurement of 3-hydroxybutyrate and propionaldehyde, respectively within micromolar range. Our novel NTRLF and NTR-coupled dehydrogenase assay platform may thus be effectively applied for the quantitative estimation of a broad range of analytes.

Design and synthesis of a long-wavelength latent fluorogenic substrate for salicylate hydroxylase: a useful fluorimetric indicator for analyte determination by dehydrogenase-coupled biosensors

Salicylate hydroxylase (SHL) catalyzes the production of catechol (plus CO(2) and H(2)O) from salicylate, NADH, and O(2). Coimmobilization of SHL with a NAD(P)(+)-dependent dehydrogenase in front of a Clark-type oxygen electrode has been investigated in the development of a general type of dehydrogenase-based biosensors that can detect various biological analytes; however, currently, no fluorophores are available for these applications. We synthesized the first new long-wavelength latent fluorogenic substrate SHLF (3) for SHL. In the presence of NADH and under aerobic conditions, SHL catalyzes the decarboxylative hydroxylation of SHLF followed by a quinone-methide-type rearrangement reaction concomitant with the ejection of a fluorescence coumarin 2, which is spontaneous and irreversible at physiological temperatures in aqueous media. The fluorescence signal generated by this process is specific and, in the near red spectral region with an emission maximum at 595 nm, is suppressed by salicylic acid. The fluorescence response of SHLF is insensitive to various biological reactive oxygen species (ROS) and reductants. Furthermore, SHLF is a sensitive fluorimetric indicator for analyte determination in the SHL-coupled dehydrogenase assay in which NAD(+) is converted to NADH. This novel fluorescence assay detected 3-hydroxybutyrate and cholesterol in the nanomolar range and is more sensitive than the current SHL-dehydrogenase amperometric sensors, making it applicable to the construction of a fiber-optic fluorescence biosensor for clinical diagnostic uses.

Determination of Poly(3-hydroxybutyrate) Using a Combination of Enzyme-based Biosensor and Alkaline Hydrolysis

The combination of an enzyme-based biosensor and alkaline hydrolysis was developed for the measurement of poly(3-hydroxybutyrate) (PHB). The principle of the determination is based on that the alkaline condition converts PHB to produce its monomer, 3-hydroxybutyrate (3-HB), which generates a detectable current signal by an amperometric biosensor through coupled two-enzyme reactions on an electrode. This method takes less than 40 min, and results in a linear detection range of 0.5-110 mg L-1 PHB with a detection limit of 0.3 mg L-1 by the saturated production of 3-HB; it can also take less than 15 min and result in a linear detection range of 1.0-160 mg L-1 PHB with a detection limit of 0.5 mg L-1 by a part production of 3-HB. The method also shows simple operation and high reproducibility.