Simple and Cost-effective “Turn-on” Fluorescence Sensor for the Determination of Xanthine

Advances in xanthine biosensors and sensors: A review

Xanthine is derived from hypoxanthine by xanthine oxidase (XOD), a flavoprotein containing molybdenum and non-haem iron, sulfur and from guanine by guanine deaminase enzyme. Xanthine is oxidized into uric acid by XOD. Xanthine is used as an indicator of fish freshness, based on the reactions in which ATP is degraded into xanthine and its quantity increases with time of fish death. Fresh fish meat is required in food industry for making high quality items. The determination of xanthine in biological fluids is also used in diagnosing and curing many diseases like renal failure, gout, xanthinuria, hyperuricemia. Various methods are available for detection of xanthine but most of them are complicated, time consuming less sensitive & specific and require expensive instrumental setup and trained person to operate. Enzyme based biosensors and non enzymic sensors overcome these disadvantages, as these are simple, rapid, specific, sensitive and easy to operate. Present review describes xanthine biosensors, which work optimally between pH 3.5-9.0, temperature 25 °C-65 °C, xanthine concentration ranging from 0.001-50 × 104 µM. These biosensors have also been used to measure xanthine concentration in beverages, urine and serum samples. Various modified electrodes have been discussed for the detection of xanthine using both enzymatic and non-enzymatic approaches in the present review.

A comprehensive review of the spoilage of shrimp and advances in various indicators/sensors for shrimp spoilage monitoring

Shrimp is a popular internationally traded shellfish due to its unique taste, texture, and nutritional value. Shrimp is highly perishable because it has enough free amino acids, high moisture levels, non-nitrogenous compounds used for microbial growth, and melanosis. Shrimp spoilage after death is caused by various reasons, like autolysis (endogenous proteinases actions during shrimp storage), growth of spoilage microorganisms, ATP degradation, melanin formation, and lipid peroxidation. A microbial byproduct, total volatile basic nitrogen, is one of the major reasons for the generation of foul odors from shrimp spoilage. Shrimp freshness monitoring is crucial for market sellers and exporters. Traditional methods for estimating shrimp freshness are expensive and inaccessible to the general public. Sensors are rapid, sensitive, selective, and portable food toxins' detection tools, devoid of expensive instruments, skilled people, sample pretreatment, and a long detection time. This review addresses shrimp spoilage causes. The mechanisms of different stages of shrimp spoilage after death, like rigor mortis, dissolution of rigor mortis, autolysis, and microbial spoilage mechanisms, are discussed. This review highlights the last five years' advances in shrimp freshness detection sensors and indicators like colorimetric pH indicators, fluorescence sensors, electronic noses, and biosensors, their working principles, and their sensitivities. Commercially available indicators and sensors for shrimp spoilage monitoring are also discussed. A review highlighting the applications of the different sensors and indicators for monitoring shrimp freshness is unavailable to date. Challenges and future perspectives in this field are explained at the end.

Highly sensitive visual colorimetric sensor for xanthine oxidase detection by using MnO2-nanosheet-modified gold nanoparticles

In this study, a highly sensitive colorimetric assay has been constructed for the determination of xanthine oxidase (XOD) activity by the GNP@MnO₂ core-shell nanoparticles as probe. In the presence of XOD, xanthine can be oxidized to produce H₂O₂, which makes the MnO₂ shell fallen off. With the single particle detection (SPD) based dark field microscopy (DFM), the scattering color of GNP@MnO₂ NP probe shows obvious change before and after etching process. At the single particle level, noticeable color change of the single probe can be easily detected in the existence of trace XOD. This SPD-based colorimetric strategy displays broad linear dynamic range (0.02-4 mU/mL) and low detection limit of 7.82 μU/mL, which is more sensitive than the results from ensemble sample measurement. In addition, we tested the inhibitory effect of quercetin on the activity of XOD and obtained good inhibition effect. As a consequence, this SPD-based colorimetric strategy provides new perception for the ultrasensitive detection of molecules in complex system.

A copper nanoclusters probe for dual detection of microalbumin and creatinine

A fluorescent probe based on glutathione-capped copper nanoclusters (GSH-CuNCs) was developed for the detection of dual targets, human serum albumin (HSA) and creatinine, in human urine. The GSH-CuNCs were synthesized by a one-pot green method using ascorbic acid as a reducing agent. The detection of HSA was in a turn-on mode via electrostatic interaction in a basic condition while the detection of creatinine was in a turn-off mode via non-covalent bonding in an acidic condition. Under optimal conditions, the linear range and detection limit of HSA were 5.0 nM to 150 nM and 1.510 ± 0.041 nM, while those of creatinine were 30 μM to 1000 μM and 13.0 ± 1.0 μM. This easily fabricated nanocluster probe provided a fast response with high sensitivity, and good selectivity. Recoveries from urine samples were in the range of 81.44 ± 0.25 to 109.22 ± 0.57% for HSA and 80.57 ± 0.16 to 109.0 ± 0.10% for creatinine. The urinary analytical results from the fluorescent probe were in good agreement (P > 0.05) to those obtained from immunoturbidimetric and enzymatic methods, signifying the excellent performance of this sensing system.

Sensitive detection of glutathione through inhibiting quenching of copper nanoclusters fluorescence

A method for a sensitive fluorescence detection of glutathione was established. Glutathione-stabilized copper nanoclusters (CuNCs) were synthesized via a facile process. These CuNCs showed blue fluorescence with a peak around 450 nm. In the presence of p-benzoquinone (PBQ), the electron transfer from the copper nanoclusters to PBQ quenched the fluorescence of the CuNCs. Glutathione (GSH), as a reducing agent, formed a complex with PBQ. This formation inhibited the quenching from PBQ, and a restored fluorescence was obtained. This interaction provided a fluorescence enhancement for the measurement of GSH. Under the optimal condition, linear responses were obtained toward GSH in the ranges of 0.06-6.0 μM, with a limit of detection at 20 nM. This developed assay was easy in operation with high sensitivity and selectivity. The applicability was approved with successful glutathione measurements in real samples.

Self-assembly encapsulation of vanadium tetrasulfide into nitrogen doped biomass-derived porous carbon as a high performance electrochemical sensor for xanthine determination

A novel VS4@N-BPC platform was constructed, and demonstrated a high electrochemical response to xanthine due to the excellent synergistic effect.

Cobalt-doped cerium oxide nanocrystals shelled 1D SnO2 structures for highly sensitive and selective xanthine detection in biofluids

In this study, we prepared a three-dimensional self-supported electrocatalyst based on a thin layer of cerium oxide nanocrystals doped with cobalt heteroatoms (CeO₂-Co) and then uniformly shelled over one-dimensional tin oxide (SnO₂) nanorods supported by carbon cloth substrate. The material was used as a binder-free sensor that could nonenzymatically detect xanthine (XA) with an excellent sensitivity of 3.56 μA μM^-1, wide linear range of 25 nM to 55 µM, low detection limit of 58 nM, and good selectivity. A screen-printed electrode based on the material accurately detected XA in food samples as well. The achievements were resulted from synergistic effects coming from the unique core@shell formation and Co-doping strategy, which efficiently modified electronic structure of the material to expose more electroactive site numbers/types and fast charge transfer, thereby producing intrinsic catalytic properties for XA oxidation. These results suggested that the SnO₂@CeO₂-Co is potential for developing efficient sensor to detect XA with good sensitivity and accuracy in food-quality monitoring.