Quantitative determination of glycine in aqueous solution using glutamate dehydrogenase-immobilized glyoxal agarose beads

Nanomolar level electrochemical detection of glycine on a miniaturized modified screen-printed carbon-based electrode: a comparison of performance with glassy carbon electrode system

In this work, we demonstrate the electrochemical (EC) sensing of glycine (GLY) on a gold-copper nanocluster on nitrogen-doped graphene quantum dot-modified (indigenously fabricated) screen-printed electrode (AuCuNC@N-GQD/SPE). SPE was fabricated by step-by-step printing of reference, working, and counter electrodes to develop an all-printed SPE. A comparison strategy between SPE and the glassy carbon electrode (GCE) towards the EC sensing of GLY was carried out. The sensing performance was enhanced while replacing GCE with SPE. The limit of detection (LOD) for GLY obtained by EC sensing with AuCuNC@N-GQD/GCE was 10 nM and that with AuCuNC@N-GQD/SPE was 10 times lower, 1 nM, and is the lowest LOD value reported hitherto. Compared with AuCuNC@N-GQD/GCE, the current response of AuCuNC@N-GQD/SPE exhibited a ∼2.6-times enhancement with a sensitivity of 0.206 μA μM-1 cm-2. Thus, the successful shift from GCE to SPE not only miniaturizes the sensor device but also enhances the electrochemical detection performance.

Ca@Cu-CD nanoprobe for dual detection of glycine and ex vivo glycine imaging

Hydrothermally prepared copper-doped carbon dots (Cu-CDs) were modified with Ca2+, which serve as an excellent platform for the recognition of glycine. The feeble emission of Ca@Cu-CD increases substantially in the presence of glycine due to aggregation-induced emission. At the same time, there was a 5-fold increase in the current response of the Ca@Cu-CD modified electrode as compared to the control. The exceptional combination of fluorescence and conducting properties, along with Ca-glycine interaction, establishes our probe as a dual sensor for the detection of glycine in real serum samples. The limit of detection for this nonenzymatic fluorescence and electrochemical sensing are 17.2 and 4.1 nM, respectively. Furthermore, an extensive evaluation of the toxicity and bioimaging properties in fruit fly Drosophila melanogaster shows that the Ca@Cu-CD probe is not cytotoxic and can be applied for ex vivo imaging of glycine.

Modification of substrate specificity of L-arginine oxidase for detection of L-citrulline

Enzymatic detection of citrulline, a potential biomarker for various diseases, is beneficial. However, determining citrulline levels requires expensive instrumental analyses and complicated colorimetric assays. Although L-amino acid oxidase/dehydrogenase is widely used to detect L-amino acids, an L-citrulline-specific oxidase/dehydrogenase has not been reported. Therefore, in this study, we aimed to develop an L-citrulline-specific enzyme by introducing a mutation into L-arginine oxidase (ArgOX) derived from Pseudomonas sp. TPU 7192 to provide a simple enzymatic L-citrulline detection system. The ratio of the oxidase activity against L-arginine to that against L-citrulline (Cit/Arg) was 1.2%, indicating that ArgOX could recognize L-citrulline as a substrate. In the dehydrogenase assay, the specific dehydrogenase activity towards L-arginine was considerably lower than the specific oxidase activity. However, the specific dehydrogenase activity towards L-citrulline was only slightly lower than the oxidase activity, resulting in improved substrate specificity with a Cit/Arg ratio of 49.5%. To enhance the substrate specificity of ArgOX, we performed site-directed mutagenesis using structure-based engineering. The 3D model structure indicated that E486 interacted with the L-arginine side chain. By introducing the E486 mutation, the specific dehydrogenase activity of ArgOX/E486Q for L-citrulline was 3.25 ± 0.50 U/mg, which was 3.8-fold higher than that of ArgOX. The Cit/Arg ratio of ArgOX/E486Q was 150%, which was higher than that of ArgOX. Using ArgOX/E486Q, linear relationships were observed within the range of 10-500 μM L-citrulline, demonstrating its suitability for detecting citrulline in human blood. Consequently, ArgOX/E486Q can be adapted as an enzymatic sensor in the dehydrogenase system.

Photolysis of N-chlorourea and its effect on urea removal in a combined pre-chlorination and UV254 process

Urea is one of the most important nitrogenous organic pollutants in water, and its removal attracts attention because of a growing concern related to water eutrophication. Urea has previously been considered to be largely unaffected by the UV-chlorine process. However, N-chlorourea, an intermediate of urea chlorination, has been shown to absorb ultraviolet radiation, and as such its photolysis is possible. Experiments were conducted to quantify the kinetics of N-chlorourea degradation under UV₂₅₄ irradiation. The results showed that about 92% of N-chlorourea was degraded under UV₂₅₄ irradiation. Ammonia and nitrate were detected as the primary nitrogen containing products of the photolysis of N-chlorourea. Solution pH ranging from 3.0 to 7.5 influenced the distribution of these products but not on the degradation rate. Based on these data, a possible pathway of photodegradation of N-chlorourea under UV₂₅₄ is proposed. The degradation of urea was also achieved by the photolysis of N-chlorourea during the combined pre-chlorination and UV₂₅₄ process. Insights gained in this study may be useful for exploring the potential of combined pre-chlorination and UV₂₅₄ process on urea removal in water treatment.

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.

The effects of acetaldehyde, glyoxal and acetic acid on the heterogeneous reaction of nitrogen dioxide on gamma-alumina

Heterogeneous reactions of nitrogen oxides on the surface of aluminium oxide result in the formation of adsorbed nitrite and nitrate. However, little is known about the effects of other species on these heterogeneous reactions and their products. In this study, diffuse reflectance infrared spectroscopy (DRIFTS) was used to analyze the process of the heterogeneous reaction of NO2 on the surface of aluminium oxide particles in the presence of pre-adsorbed organic species (acetaldehyde, glyoxal and acetic acid) at 298 K and reveal the influence of these organic species on the formation of adsorbed nitrite and nitrate. It was found that the pre-adsorption of organic species (acetaldehyde, glyoxal and acetic acid) on γ-Al2O3 could suppress the formation of nitrate to different extents. Under the same experimental conditions, the suppression of the formation of nitrate by the pre-adsorption of acetic acid is much stronger than that by pre-adsorption of acetaldehyde and glyoxal, indicating that the influence of acetic acid on the heterogeneous reaction of NO2 is different from that of acetaldehyde and glyoxal. Surface nitrite is formed and identified to be an intermediate product. For the heterogeneous reaction of NO2 on the surface of γ-Al2O3 with and without the pre-adsorption of acetaldehyde and glyoxal, it is firstly formed and then gradually disappears as the reaction proceeds, but for the reaction with the pre-adsorption of acetic acid, it is the final main product besides nitrate. This indicates that the pre-adsorption of acetic acid would promote the formation of nitrite, while the others would not change the trend of the formation of nitrite. The possible influence mechanisms of the pre-adsorption of acetaldehyde, glyoxal and acetic acid on the heterogeneous conversion of NO2 on γ-Al2O3 are proposed and atmospheric implications based on these results are discussed.