Polymeric Membrane Ion-Selective Electrode for Butyrylcholinesterase Based on Controlled Release of Substrate

A highly sensitive dual-mode detection platform based on the novel copper/molybdenum bimetallic nanoclusters and Co-Fe layered doubled hydroxide nanozyme for butyrylcholinesterase activity sensing

Herein, a novel copper/molybdenum bimetallic nanoclusters (Cu/Mo NCs) with intense blue emission were synthesized by using polyvinylpyrrolidone (PVP) as template and ascorbic acid as reducing agent. Owing to the synergistic effect between Cu and Mo, the fluorescence intensity of Cu/Mo NCs was significantly improved about 6-time than monometallic copper nanoclusters. A novel and sensitive ratiometric fluorescence and colorimetric dual-mode sensing platform for monitoring butyrylcholinesterase (BChE) was strategically constructed by the integration of Cu/Mo NCs with excellent optical properties and Co-Fe layered doubled hydroxide (CoFe-LDH) with superior peroxidase-like activity for the first time. In the presence of H2O2, nonfluorescent and colorless o-phenylenediamine (OPD) was oxidized to fluorescent and yellow 2,3-diaminophenazine (DAP) with maximum fluorescence emission peak at 564 nm and ultraviolet absorption peak at 418 nm by CoFe-LDH with peroxidase-like activity. Simultaneously, the generation of DAP could effectively quench Cu/Mo NCs fluorescence at 444 nm through the inner-filter effect (IFE). The hydrolysis of S-butyrylthiocholine iodide (BTCh) can be catalyzed by butyrylcholinesterase (BChE) to generate thiocholine (TCh) that could hinder the oxidation of OPD, leading to the fluorescence and ultraviolet absorption of DAP decreased, meanwhile, the fluorescence of Cu/Mo NCs recovered. The ratiometric fluorescence signal F564/F444 and colorimetric system both performed a satisfactory response to the concentration of BChE in the range 0.5 to 90 U L-1 and 1 to 100 U L-1 with the LOD of 0.18 U L-1 and 0.36 U L-1, respectively. The dual-mode sensing for BChE exhibited outstanding application potential in biosensing.

Ratiometric fluorescence and colorimetric strategies for assessing activity of butyrylcholinesterase in human serum using g-C3N4 nanosheets, silver ion and o-phenylenediamine

Ratiometric fluorescence and colorimetric strategies for detecting activity of butyrylcholinesterase (BChE) in human serum were developed by using g-C3N4 nanosheets, silver ion (Ag+) and o-phenylenediamine (OPD) as chromogenic agents. The oxidation-reduction reaction of OPD and Ag+ generates 2,3-diaminophenazine (oxOPD). Under exciation at 370 nm, g-C3N4 nanosheets and oxOPD emit fluorescence at 440 nm (F440) and 560 nm (F560), respectively. Additionally, oxOPD exhibits quenching ability towards g-C3N4 nanosheets via photoinduced electron transfer (PET) process. Thiocholine (TCh), as a product of BChE-catalyzed hydrolysis reaction of butylthiocholine iodide (BTCh), can coordinate with Ag+ intensively, and consequently diminish the amount of free Ag+ in the testing system. Less amount of free Ag+ leads to less production of oxOPD, resulting in less fluorescence quenching towards g-C3N4 nanosheets as well as less fluorescence emission of oxOPD. Therefore, by using g-C3N4 nanosheets and oxOPD as fluorescence indicators, the intensity ratio of their fluorescence (F440/F560) was calculated and employed to evaluate the activity of BChE. Similarly, the color variation of oxOPD indicated by the absorbance at 420 nm (ΔA420) was monitored for the same purpose. These strategies were validated to be sensitive and selective for detecting BChE activity in human serum, with limits of detection (LODs) of 0.1 U L-1 for ratiometric fluorescence mode and 0.7 U L-1 for colorimetric mode.

Quantification of underivatized amino acids in solid beverages using high-performance liquid chromatography and a potentiometric detector

The simultaneous quantification of amino acids (AAs) in solid beverages without prior derivatization was explored by high-performance liquid chromatography (HPLC) coupled to a potentiometric detector. Included were threonine, leucine, methionine, phenylalanine, and histidine. The potentiometric detector was made consisting of a copper(II)-selective electrode based on a polyvinyl chloride (PVC) membrane, and the potential changes in the detector were determined according to the coordination interactions between cupric copper ions released from the inner filling solution of the electrode and AAs. Conditions were optimized for effective separation and sensitive detection. Fundamental characteristics such as linearity, limits of detection, limits of quantitation, accuracy, precision, and robustness were validated experimentally. The calibration curves showed a linear relationship between peak heights and the injection concentrations of the AAs. The detection limits down to the sub-micromolar range were achieved under isocratic conditions, outperforming ultraviolet detection. The copper(II)-selective electrode had a minimum lifetime of one month. Some real samples were examined to further demonstrate the feasibility of the proposed approach. The measurement results obtained by the present method were in good agreement with those obtained by the HPLC-mass spectrometry (MS), indicating that the combined HPLC-potentiometric method is a potential option for quantifying AAs.

Determination of butyrylcholinesterase activity based on thiamine luminescence modulated by MnO2 nanosheets

In this paper, a novel strategy for biosensing butyrylcholinesterase (BChE) activity is developed based on manganese dioxide (MnO₂) nanosheets to modulate the photoluminescence of thiamine (TH). The oxidase-like activity of MnO₂ nanosheets enables them to catalyze the oxidation of non-fluorescent substrate TH to generate strong fluorescent thiochrome (TC). When the target BChE is introduced to form thiocholine in the presence of S-butyrylthiocholine iodide (BTCh), MnO₂ nanosheets are reduced by thiocholine to Mn²⁺, resulting in the loss of their oxidase-like activity and the reduction of TC fluorescence. Based on this, a BChE activity fluorescence biosensor is constructed utilizing the luminescence behavior variation of TH and the oxidase-like activity of MnO₂ nanosheets. The fluorescence biosensor shows a sensitive response to BChE, and the detection limit reaches 0.036 U L^-1. In addition, the feasibility of the biosensor in real samples analysis is studied with satisfactory results.

Redox-Controlled Fluorescent Nanoswitch Based on Reversible Disulfide and Its Application in Butyrylcholinesterase Activity Assay

Butyrylcholinesterase (BChE) mainly contributing to plasma cholinesterase activity is an important indicator for routinely diagnosing liver function and organophosphorus poisoning in clinical diagnosis, but its current assays are scarce and frequently suffer from some significant interference and instability. Herein, we report a redox-controlled fluorescence nanoswtich based on reversible disulfide bonds, and further develop a fluorometric assay of BChE via thiol-triggered disaggregation-induced emission. Thiol-functionalized carbon quantum dots (thiol-CQDs) with intense fluorescence is found to be responsive to hydrogen peroxide, and their redox reaction transforms thiol-CQDs to nonfluorescent thiol-CQD assembly. The thiols inverse this process by a thiol-exchange reaction to turn on the fluorescence. The fluorescence can be reversibly switched by the formation and breaking of disulfide bonds caused by external redox stimuli. The specific thiol-triggered disaggregation-induced emission enables us to assay BChE activity in a fluorescence turn-on and real-time way using butyrylthiocholine iodide as the substrate. As-established BChE assay achieves sufficient sensitivity for practical determination in human serum, and is capable of avoiding the interference from micromolar glutathione and discriminatively quantifying BChE from its sister enzyme acetylcholinesterase. The first design of reversible redox-controlled nanosiwtch based on disulfide expands the application of disulfide chemistry in sensing and clinical diagnostics, and this novel BChE assay enriches the detection methods for cholinesterase activity.

Pulsed galvanostatic control of a polymeric membrane ion-selective electrode for potentiometric immunoassays

Pulsed galvanostatic control of ion fluxes across polymeric membrane ion-selective electrodes (ISEs) is an emerging field for potentiometric sensing. Herein we report a novel potentiometric enzyme immunoassay based on current-controlled release of an enzyme substrate, which eliminates the addition of marker ions in the sample solution. In this method, the carboxylated poly(vinyl chloride) matrix at the outer layer of the ISE membrane is employed to attach a primary antibody. A sandwich immunoassay with an alkaline phosphatase labeled antibody (ALP-Ab) as the reporter is used for the determination of human IgG (as a model protein). The large difference between the lipophilicity of the substrate ion and that of the product ion allows p-nitrophenyl phosphate to be used as the enzyme substrate for potentiometric immunosensors. After the immunoreactions, the captured ALP-Ab catalyzes the hydrolysis of the substrate ions released at the sample-membrane interface by using the pulsed galvanostatic technique. This process can be potentiometrically determined by measuring the open circuit potential of the ISE. Under optimal conditions, the potential response of the proposed immunosensor is proportional to the concentration of human IgG in the range of 50-1000 ng/mL with a detection limit of 30 ng/mL (3σ). Owing to simplicity and independence of sample volume and sample turbidity, the proposed potentiometric immunoassay offers a viable alternative to those based on optical absorbance.

Potentiometric aptasensing based on target-induced conformational switch of a DNA probe using a polymeric membrane silver ion-selective electrode

In this article, we introduce a general, sensitive, facile, and label-free potentiometric assay based on metal-mediated DNA base pairs. A nucleic acid with one adenosine-5'-triphosphate (ATP) binding sequence (aptamer) in the middle and two cytosine(C)-rich sequences at the lateral portions was employed as a model. A rigid hairpin structure can be formed in the presence of Ag(+) ions, in which the C residues of the spatially separated nucleotides are linked by the ions. The strong interaction between Ag(+) ions and cytosines forms a stable C-Ag(+)-C structure, which could reduce the concentration of silver ions released from the polymeric membrane silver ion-selective electrode (ISE) at the sample-membrane interface and decrease the potential response. In the presence of its target, the aptamer (the loop sequence of the probe) binds specifically to the target via reaction incubation. Such target-binding induced aptamer conformational change prevents the formation of C-Ag(+)-C structure, leaving more silver ions at the sample-membrane interface, which can be detected by the silver ISE. ATP can be quantified in the range of 0.5-3.0 μM with a detection limit of 0.37 μM. The relative standard deviation for 5 μM ATP is 5.5%. For the proposed method, the combination of using ion fluxes of silver ions as modulating reagents and as signal reporters greatly simplifies the detection procedures. In addition, by changing the binding sequence in the middle of the probe, the present detection method will be able to explore new applications of ISE for the detection of a large variety of targets.

Potentiometric flow injection system for determination of reductants using a polymeric membrane permanganate ion-selective electrode based on current-controlled reagent delivery

A polymeric membrane permanganate-selective electrode has been developed as a current-controlled reagent release system for potentiometric detection of reductants in flow injection analysis. By applying an external current, diffusion of permanganate ions across the polymeric membrane can be controlled precisely. The permanganate ions released at the sample-membrane interface from the inner filling solution of the electrode are consumed by reaction with a reductant in the sample solution thus changing the measured membrane potential, by which the reductant can be sensed potentiometrically. Ascorbate, dopamine and norepinephrine have been employed as the model reductants. Under the optimized conditions, the potential peak heights are proportional to the reductant concentrations in the ranges of 1.0×10(-5) to 2.5×10(-7)M for ascorbate, of 1.0×10(-5) to 5.0×10(-7)M for dopamine, and of 1.0×10(-5) to 5.0×10(-7)M for norepinephrine, respectively with the corresponding detection limits of 7.8×10(-8), 1.0×10(-7) and 1.0×10(-7)M. The proposed system has been successfully applied to the determination of reductants in pharmaceutical preparations and vegetables, and the results agree well with those of iodimetric analysis.