A fluorescent biosensor for the determination of xanthine in tea and coffee via enzymatically generated uric acid

Highly sensitive detection for xanthine by combining single-band red up-conversion nanoparticles and cycle signal amplification strategy based on internal filtration effect

Up-conversion nanoparticles (UCNPs), especially single-band bright red UCNPs, have better penetration of biological tissues, absorb less lost energy, and have higher sensitivity and accuracy in the determination of actual biological samples in the field of biosensing. Here, a novel colorimetric and fluorescent dual-channel method based upon an internal filtration effect (IFE) quenching mechanism was proposed for the quantitative analysis of xanthine (XA) by using red UCNPs as fluorescence indicator and 3,3',5,5' -tetramethylbenzidine (TMB) as chromogenic substrate. The sensitivity of the detection system was also enhanced by a cycle signal amplification strategy based on the Fenton reaction. Under the best conditions, the detection limits of XA by fluorescent and colorimetric methods were 0.58 μM and 1.19 μM, respectively. The developed method was applied to the detection of XA in actual serum samples, and the recoveries of the spiked samples by fluorescent and colorimetric methods were in the range of 96.3-104.3 % and 94.3-105.4 %, respectively. In addition, the commercial ELISA method was used to verify the application of the proposed method and the test results of XA were close to those obtained by fluorescent and colorimetric methods, indicating that the accuracy of the developed nanosensing system was acceptable.

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.

Sensitive ratiometric fluorescence assay for detecting xanthine in serum based on the inner filter effect of enzyme-catalyzed oxidation products to silicon nanoparticles

A new type of fluorescent silicon nanoparticles (SiNPs) were prepared via a facile one-pot hydrothermal method by using N-[3-(trimethoxysilyl)propyl]-ethylenediamine (DAMO) and glucose as reagents, and were subsequently applied to construct a ratiometric fluorescence assay for sensitive and rapid determination of xanthine in human serum. Two catalytic oxidation reactions were employed to induce a fluorescence response of the testing system towards xanthine. Under the catalysis of xanthine oxidase (XOD), xanthine in serum samples was oxidized and produced hydrogen peroxide (H₂O₂). By utilizing o-phenylenediamine (OPD) as the substrate for horseradish peroxidase (HRP) in the presence of H₂O₂, fluorescent 2,3-diaminophenazine (DAP) was finally generated. A ratiometric fluorescence assay for xanthine was established by determining the ratio of the green-yellow fluorescence emission of DAP and the blue fluorescence emitted from SiNPs under the inner filter effect (IFE) of DAP. Instead of traditional multi-step procedures for adding reacting reagents to the testing solution, all the reaction reagents were mixed with serum samples in a single step for this assay to shorten the total reaction time. This assay demonstrates superiority over a solo DAP fluorescence-based assay as well as other reported methods, with excellent sensitivity and reduced testing time. The strategies proposed in this work for both synthesis and application of fluorescent SiNPs can be used in future fabrication of novel fluorescent probes, especially for sensing biological metabolites involved in H₂O₂-generation or consumption reactions.

A Simple and Cost Effective Turn off Fluorescence Sensor for Biliverdin and Bilirubin Based on L-Cysteine Modulated Copper Nanoclusters

The present article reports the efficiency of L-cysteine modulated copper nanoclusters (L-cys-CuNCs) as a fluorescent probe for the selective determination of naturally occurring bile pigments biliverdin (BVD) and bilirubin (BLR). These pigments were found to quench the fluorescence of L-cys-CuNCs through static processes. Under optimized conditions, the proposed strategy permitted the quantification of BVD and BLR in the range 4.00 × 10^-5 to 5.00 × 10^-7M and 1.00×10^-5 to 1.00×10^-6 M respectively with limits of detection 2.33 × 10^-7M and 2.29 × 10^-7 M. The practical utility of the developed sensor have been investigated in spiked blood and urine samples.

ATMP-induced three-dimensional conductive polymer hydrogel scaffold for a novel enhanced solid-state electrochemiluminescence biosensor

Reliable and sensitive detection of xanthine has important medical and biological significance. In this work, a novel three-dimensional (3D) conductive polymer hydrogel of polyaniline (PAni) was feasibly prepared using aniline (Ani), amino trimethylene phosphonic acid (ATMP) and ammonium persulfate ((NH₄)₂S₂O₈) as monomer, gelatinizing agent and oxidizing agent, respectively. Protonation of aniline can be achieved by ATMP, inducing good conductivity of the obtained hydrogel. ATMP remained the chelating abilities in the conductive hydrogel, enabling further immobilization with silver nanoparticles (AgNPs) functionalized by a luminol derivative, N-(aminobutyl)-N-(ethylisoluminol) (ABEI). ABEI-Ag@PAni-ATMP exhibited an enhanced performance of solid-state electrochemiluminescence (ECL). Integrated with xanthine oxidase (XOD), the proposed biosensor can be applied in the detection of xanthine via in-situ generated hydrogen peroxide (H₂O₂), and present a low detection limit of 9.6 nM, a wide linear range (from 0.01 to 200 μM) and excellent stability.

Co-localization of oxidase and catalase inside a porous support to improve the elimination of hydrogen peroxide: Oxidation of biogenic amines by amino oxidase from Pisum sativum

Diamine oxidase (DAO) from Pisum sativum is an enzyme that catalyzes the degradation of biogenic amines (BA) present in wine, producing harmless aldehydes and hydrogen peroxide (H2O2). H2O2 promotes a rapid inactivation of the immobilized enzyme. At first glance, co-immobilization of DAO and catalase (CAT) could improve the elimination of the released hydrogen peroxide. Two different co-immobilized derivatives were prepared: (a) both enzymes co-localized and homogeneously distributed across the whole structure of a porous support, and (b) both enzymes we de-localized inside the porous support: DAO immobilized on the outer part of the porous support and catalase immobilized in the inner part. Co-localized derivatives were seven-fold more effective than de-localized ones for the elimination of hydrogen peroxide inside the porous support. In addition to that, the degradation of putrescine by DAO was three-fold more rapid when using both co-localized enzymes. The optimal co-localized derivative (containing 1.25 mg of DAO plus 25 mg of CAT per g of support) promoted the instantaneous elimination of 91% H2O2 released inside the porous support during putrescine oxidation. This optimal derivative preserves 92% of activity after three reaction cycles and DAO immobilized without catalase only preserves 41% of activity. Co-localization seems to be the key strategy to immobilize two sequential enzymes. When enzymes are immobilized in close proximity to each other in a co-localized pattern, the generation of byproducts as H2O2 is strongly reduced.