A general strategy to facilely design ratiometric electrochemical sensors in electrolyte solution by directly using a bare electrode for dual-signal sensing of analytes

A label-free ratiometric homogeneous electrochemical aptasensor based on dual catalytic hairpin self-assembly for rapid and sensitive detection of ochratoxin A in food

The food contamination with Ochratoxin A (OTA) has highlighted the need to create precise, sensitive, and convenient techniques. Herein, we proposed a label-free and immobilization-free ratiometric homogeneous electrochemical aptasensor based on dual catalytic hairpin self-assembly (CHA) for OTA detection. Methylene blue (MB) and ferrocene (Fc) in solution were utilized as label-free signaling molecules, generating a response signal (IMB) and a reference signal (IFc), respectively. The ratio of IMB/IFc was utilized as a measure to quantify OTA. Dual CHA was exploited to increase the ratiometric signal and enhance the amplification efficiency. This aptasensor achieved trace-level detection for OTA over a linear range of lower concentrations (1.0 × 10-3 ng/mL-1.0 × 103 ng/mL) with LOD of 92 fg/mL. The aptasensor was successfully applied to detect OTA in cereal and wine, with comparable results of HPLC-MS/MS. This strategy provided a viable platform for rapid, sensitive, and accurate detection of OTA in food.

Development of a highly sensitive aptamer-based electrochemical sensor for detecting saxitoxin based on K3Fe(CN)6 regulated silver nanoparticles

BACKGROUND

Saxitoxin (STX) is the most toxic marine toxin, which can pose several adverse effects on human health. High sensitivity, fast response, and low-cost detection of STX contamination are of significance to reducing the fishery and seafood industries' loss. Among the various types of biosensors, the electrochemical biosensors have been extensively studied in the detection of STX, but the electrode surface modification material is easy to fall off, resulting in unstable electrochemical signals and poor reproducibility. It is imperative to have a ratiometric electrochemical biosensor for STX.

RESULTS

In this study, we developed a novel aptamer-based electrochemical sensor (AECs) for the sensitive detection of STX based on a K3Fe(CN)6 regulated silver nanoparticles (Ag NPs) modified with aptamer. The AECs was constructed by immobilizing aptamer on Ag NPs surfaces. Under optimized conditions, the AECs showed a linear response towards STX in the range from 0.04 to 0.15 μM with the regression equation of Y = -8.0 + 233.7 X (R2 = 0.9956). The limit of detection (LOD) was calculated to be 1 nM (based on 3 N/S), which is significantly lower than the regulatory limits for STX in seafood. Moreover, the AECs showed excellent sensitivity, reproducibility and stability, as well as the detection in samples with acceptable recovery ranged from 71.2 % to 93.8 %, demonstrating its broad application prospects in detection of STX in seafood samples.

SIGNIFICANCE

This work proposed an AECs to achieve sensitive detection of STX. A reaction system of K3Fe(CN)6 etched Ag NPs was introduced and used as the signal source to avoid the instability of the electrochemical signal, which can produce a ratiometric electrochemical signal output mode, improving the stability and sensitivity of electrochemical detection of STX.

Electrochemistry as a Powerful Tool for Investigations of Antineoplastic Agents: A Comprehensive Review

Cancer is most frequently treated with antineoplastic agents (ANAs) that are hazardous to patients undergoing chemotherapy and the healthcare workers who handle ANAs in the course of their duties. All aspects related to hazardous oncological drugs illustrate that the monitoring of ANAs is essential to minimize the risks associated with these drugs. Among all analytical techniques used to test ANAs, electrochemistry holds an important position. This review, for the first time, comprehensively describes the progress done in electrochemistry of ANAs by means of a variety of bare or modified (bio)sensors over the last four decades (in the period of 1982-2021). Attention is paid not only to the development of electrochemical sensing protocols of ANAs in various biological, environmental, and pharmaceutical matrices but also to achievements of electrochemical techniques in the examination of the interactions of ANAs with deoxyribonucleic acid (DNA), carcinogenic cells, biomimetic membranes, peptides, and enzymes. Other aspects, including the enantiopurity studies, differentiation between single-stranded and double-stranded DNA without using any label or tag, studies on ANAs degradation, and their pharmacokinetics, by means of electrochemical techniques are also commented. Finally, concluding remarks that underline the existence of a significant niche for the basic electrochemical research that should be filled in the future are presented.

Signal on-off ratiometric electrochemical sensor coupled with a molecularly imprinted polymer for the detection of carbendazim

A stable ratiometric electrochemical sensor is introduced for the selective detection of carbendazim (CBD). Specifically, the proposed sensor employs a Co@Mo₂C bimetallic nanomaterial as the glassy carbon electrode substrate and a layer of molecularly imprinted polymer (MIP) was in situ fabricated on glassy carbon electrode by electropolymerization, with o-aminophenol as the functional monomer and CBD acting as template. A ratiometric MIP sensor was constructed by adding ferrocene (Fc) internal reference directly to the sample solution. The bimetallic nanomaterials provide a large loading platform for the MIP layer through synergistic effects, amplifying the signal. Excellent CBD binding selectivity is achieved by the templating effect of the three-dimensional (3D) MIP layer. The internal standard is added directly to the electrolyte solution to be tested, allowing the new type of ratiometric electrochemical sensor to avoid the cumbersome steps of other methods and reducing the difficulty and human error of the experimental procedure. Combining a ratiometric strategy with a 3D MIP structure realises the dual-signal detection of CBD. The optimised sensor showed an excellent linear relationship between 0.01 and 1 000 μM, with a correlation coefficient of 0.997 and a detection limit of 3.4 nM (S/N = 3).

Covalent organic framework modified carbon cloth for ratiometric electrochemical sensing of bisphenol A and S

A novel ratiometric electrochemical sensor was developed based on a carbon cloth electrodeposited with silver nanoparticles and drop-coated by covalent organic framework (COF-LZU1) for simultaneous determination of bisphenol A (BPA) and bisphenol S (BPS). Carbon cloth exhibited a significantly larger electrochemical active area than common glassy carbon electrodes (27.5 times). Silver nanoparticles not only provided a stable reference signal but also enhanced electroactivity for the oxidation of BPA and BPS. COF-LZU1 with good adsorption performance and large periodic π-arrays promoted the enrichment of BPA and BPS to further increase the current response. Compared with the traditional single-signal electrochemical sensor, the developed ratiometric sensor exhibited better reproducibility and a wider linear range for BPA and BPS from 0.5 to 100 μM with a limit of detection of 0.15 μM. Furthermore, the developed sensor showed excellent stability and superior anti-interference ability. The real sample analysis for BPA and BPS has been successfully carried out in mineral water, electrolyte drink, tea, juice, and beer with recoveries of 88.3-111.7%. The developed ratiometric sensor is expected to be a candidate for the preparation of other electrochemical sensors and the analysis of additional practical samples.

A label-free and modification-free ratiometric electrochemical strategy for enhanced natural enzyme detection using a bare electrode and nanozymes system

Ratiometric electrochemical assays have been demonstrated to be more sensitive and selective in various sensing events, mainly due to their affordable built-in correction and good self-reference capability. But it is known that complicated modification and labeling operations usually are necessary for the construction of ratiometric electrochemical assays, therefore is a hot and important issue needing consideration carefully. We herein report a new yet simple bare electrode-based ratiometric electrochemical bioassay to achieve sensitive and selective analysis of alkaline phosphatase (ALP), using a liquid phase system that contains CoOOH nanozymes and commercially available indicator substrate. This proposed bioassay works based on the ratiometric change of dual electrochemical signals, arising from an exclusive target ALP-triggered hydrolysis of electrochemical substrate p-nitrophenyl phosphate (PNPP). In this design, the two hydrolyzed products of electrochemically active p-nitrophenol (PNP) and electrochemically inactive phosphate anion (PO₄^3-) are responsible together for the ratiometric electrochemical analysis of ALP. PNP exhibits a straightforward current response toward ALP content; however, PO₄^3- cannot show a direct electrochemical signal thus is rationally designed to offer an alternative response by linking it with the specific CoOOH nanozyme-catalyzed reaction of 3,3',5,5'-tetramethylbenzidine (TMB) and H₂O₂, in which the nanozyme-catalyzed product oxTMB shows a direct reduction current at the GCE, and significantly decreases with increasing PO₄^3- species due to the good inhibition of PO₄^3- toward CoOOH nanozyme activity. As a result, a ratiometric electrochemical strategy for ALP analysis with a low limit of detection of 0.366 U/L (S/N = 3) was successfully achieved by integrating the above direct and indirect dual electrochemical responses. This developed bioassay can allow the quantitative diagnosis of ALP activity especially with a label-free and modification-free merit, therefore paving the way for simple, convenient, and portable electroanalytical tools in biosensing design and application.

Dual-signaling electrochemical ratiometric strategy for simultaneous quantification of anticancer drugs

A novel and unique ratiometric electrochemical sensing strategy for highly reliable and selective simultaneous quantification of Irinotecan (IRI) and 5-Fluorouracil (5-FU) has been developed based on Pd-Au/MWCNT-rGO nanocomposite. Introduction of Pd-Au/MWCNT-rGO significantly improved the speed of electron transport, specific surface area, and electrical catalytic ability of sensing system due to synergistic effect of Pd-Au bimetallic nanoparticles and MWCNT-rGO hybrid structure. The assay strategy was based on the use of ferrocene (Fc) as reference electroactive substance and IRI and 5-FU as analytes with three oxidation peaks at different potentials (Fc at +0.20 V, IRI at +0.58 V, and 5-FU at +1.17 V). The oxidation peak currents of the IRI and 5-FU were gradually enhanced while that of Fc remained almost constant with continuous adding of IRI and 5-FU. By using I_IRI/I_Fc and I_5-FU/I_Fc signals as output, the designed ratiometric system showed good performance with a wide linear range of 0.05-40 μM for IRI and 0.05-75 μM for 5-FU and low detection limit of 0.0061 μM and 0.0094 μM for IRI and 5-FU, respectively. This study proved that ratiometric strategy is able to eliminate disturbance caused by the sensing environment and possess high sensitivity, reproducibility, stability, and selectivity toward anticancer drugs detection, over potential interferents as well as opens a new procedure for reliable and selective simultaneous analysis of other analytes.

Ratiometric Electrochemistry: Improving the Robustness, Reproducibility and Reliability of Biosensors

Electrochemical biosensors are an increasingly attractive option for the development of a novel analyte detection method, especially when integration within a point-of-use device is the overall objective. In this context, accuracy and sensitivity are not compromised when working with opaque samples as the electrical readout signal can be directly read by a device without the need for any signal transduction. However, electrochemical detection can be susceptible to substantial signal drift and increased signal error. This is most apparent when analysing complex mixtures and when using small, single-use, screen-printed electrodes. Over recent years, analytical scientists have taken inspiration from self-referencing ratiometric fluorescence methods to counteract these problems and have begun to develop ratiometric electrochemical protocols to improve sensor accuracy and reliability. This review will provide coverage of key developments in ratiometric electrochemical (bio)sensors, highlighting innovative assay design, and the experiments performed that challenge assay robustness and reliability.

Enhanced performance of an electrochemical aptasensor for real-time detection of vascular endothelial growth factor (VEGF) by nanofabrication and ratiometric measurement

Achieving a biosensing interface without baseline drift caused by variables in matrix samples is essential for real-time detection of analytes. In this study, we developed a molecular beacon based electrochemical aptasensor to realize the ratiometric signal quantification of VEGF in serum by surface modification of nanocomposites of graphene oxide/methylene blue (GO/MB) and AuNPs followed by the attachment of ferrocene-labeled aptamer (aptamer-Fc) against VEGF. The presence of VEGF can trigger the configuration change of aptamer-Fc, resulting in the redox probe Fc being far away from the electrode surface to attenuate the electrochemical communication between electrode and Fc. Meanwhile, signal of MB also decreased due to the impediment of aptamer-Fc to electron transfer passage. The achieved GC-rGO/MB-AuNPs-aptamer-Fc sensing interface was successfully used for the sensitive detection of VEGF in real-time with a linear detection range 2-500 pg mL^-1 and detection limit of 0.1 pg mL^-1 based on ratiometric dual signal (Fc and MB) read-out. It was observed loading MB and AuNPs to the GO based sensing interface was favorable to enhance the analytical performance in terms of sensitivity and capability to effectively eliminate background interference. This electrochemical aptasensor provides a universal and reliable biosensing platform which is potential for real-time and sensitive tracking of various cytokines in vivo.

Ratiometric Strategy for Electrochemical Sensing of Carbaryl Residue in Water and Vegetable Samples

Accurate analysis of pesticide residue in real samples is essential for food safety and environmental protection. However, a traditional electrochemical sensor based on single-signal output is easily affected by background noise, environmental conditions, electrode diversity, and a complex matrix of samples, leading to extremely low accuracy. Hence, in this paper, a ratiometric strategy based on dual-signal output was adopted to build inner correction for sensing of widely-used carbaryl (CBL) for the first time. By comparison, Nile blue A (NB) was selected as reference probe, due to its well-defined peak, few effects on the target peak of CBL, and excellent stability. The effects of a derivatization method, technique mode, and pH were also investigated. Then the performance of the proposed ratiometric sensor was assessed in terms of three aspects including the elimination of system noise, electrode deviation and matrix effect. Compared with traditional single-signal sensor, the ratiometric sensor showed a much better linear correlation coefficient (r > 0.99), reproducibility (RSD < 10%), and limit of detection (LOD = 1.0 μM). The results indicated the introduction of proper reference probe could ensure the interdependence of target and reference signal on the same sensing environment, thus inner correction was fulfilled, which provided a promising tool for accurate analysis.

Nanotechnology and nanomaterial-based no-wash electrochemical biosensors: from design to application

Nanotechnology and nanomaterial based electrochemical biosensors (ECBs) have achieved great development in many fields, such as clinical diagnosis, food analysis, and environmental monitoring. Nowadays, the single-handed pursuit of sensitivity and accuracy cannot meet the demands of detection in many in situ and point-of-care (POC) circumstances. More and more attention has been focused on simplifying the operation procedure and reducing detection time, and thus no-wash assay has become one of the most effective ways for the continuous development of ECBs. However, there are many challenges to realize no-wash detection in the real analysis, such as redox interferences, multiple impurities, non-conducting protein macromolecules, etc. Furthermore, the complex detection circumstance in different application fields makes the realization of no-wash ECBs more complicated and difficult. Thanks to the updated nanotechnology and nanomaterials, in-depth analysis of the obstacles in the detection process and various methods for fabricating no-wash ECBs, most issues have been largely resolved. In this review, we have systematically analyzed the nanomaterial based design strategy of the state-of-the-art no-wash ECBs in the past few years. Following that, we summarized the challenges in the detection process of no-wash ECBs and their applications in different fields. Finally, based on the summary and analysis in this review, we also evaluated and discussed future prospects from the design to the application of ECBs.

A versatile ratiometric electrochemical sensing platform based on N-Mo2C for detection of m-nitrophenol

M-nitrophenol (m-NP) is a high priority environmental pollutant and poses a series of threats on human health. Accurate and rapid detection of m-NP in practical samples is very important as this is the key prerequisite for its effective monitoring. Eelectrochemical sensor, though long serving as highly sensitive and fast analytical tool, suffers from the bottleneck problems like low specificity, poor reproducibility, susceptibility to internal and external disturbances, etc. Herein, we developed a ratiometric electrochemical sensor (R-ECS) for m-NP detection, in which nitrogen-doped Mo₂C (N-Mo₂C) was deployed as the sensing agent and methylene blue (MB) as the internal reference. Full characterization of N-Mo₂C was carried out in the aspects of morphology, composition, chemical bonds and electrochemical behavior, and the sensing performance of the easy-to-operate R-ECS was evaluated. Complete separation of the oxidation peaks of m-NP and MB was achieved using the MB/N-Mo₂C composite modified electrode and their ratiometric signals were adopted for quantification of m-NP. The linear relation between the electrical signal and the concentration of m-NP is in the range of 1-1500 μM, with the detection limit of 0.256 μM (S/N = 3). The sensor was applied to measure m-NP in real samples from tap water and river. Experimental results demonstrate that it exhibits decent repeatability, reproducibility, stability and selectivity, which proves its great practical potential as an analytical detector.

Ratiometric Electrochemical Sensors Associated with Self-Cleaning Electrodes for Simultaneous Detection of Adrenaline, Serotonin, and Tryptophan

Electrochemical sensors have long suffered from issues such as nonspecific adsorption, poor anti-interference ability, and internal and external disturbances. To address these challenges, we developed a facile electrochemical method, which integrated a ratiometric strategy with self-cleaning electrodes. In the novel sensing system, the self-cleaning electrode was realized via forming a hydrophobic layer on carbonized ZIF-67@ZIF-8 (cZIF) by polydimethylsiloxane (PDMS) precursor vaporization. As for ratiometry, it is worth to mention that the measurements were conducted by adding an interior reference (methylene blue) directly into electrolyte solution, which is more facile and flexible to operate compared with conventional ones. Sensing performance of the self-cleaning electrode as well as the newly established ratiometric strategy was explored fully, and it turned out that PDMS@cZIF nanocomposites provided decent electrocatalytic ability, superhydrophobic property, and stability. Furthermore, the ratiometric strategy significantly elevated the robustness and reproducibility of electrochemical sensing. Simultaneous detection of Adr, 5-HT, and Trp was performed under the optimum experimental conditions with wide linear ranges and low detection limits. Finally, the original ratiometric electrochemical sensor was successfully applied for monitoring the three target molecules in biological samples.

Reduced graphene oxide/nile blue/gold nanoparticles complex-modified glassy carbon electrode used as a sensitive and label-free aptasensor for ratiometric electrochemical sensing of dopamine

In this work, glassy carbon electrode (GCE) surface was modified by drop-coating graphene oxide (GO) and nile blue (NB) to form GO/NB/GCE. By using a one-step coreduction treatment under cyclic voltammetry (CV) scanning, gold nanoparticles (AuNPs) were electrodeposited onto GO/NB/GCE surface, simultaneously generating reduced GO (rGO). AuNPs from the prepared rGO/NB/AuNPs/GCE was combined with 5'-SH-terminated aptamer of dopamine (DA) via Au-S coupling to fabricate aptamer-rGO/NB/AuNPs/GCE system. DA specifically combined with its aptamer modified on rGO/NB/AuNPs/GCE surface. CV, electrochemical impedance spectroscopy, square wave voltammetry responses of this system as the working electrode were measured. With the addition of DA, the peak current intensities located at -0.45 V (I_NB) and 0.15 V (I_DA) showed gradually decreased and increased changes, respectively. There was a good linear (R² = 0.9922) relationship between lg(I_DA/I_NB) and the logarithm of DA concentration (lgC_DA) in the C_DA range from 10 nM to 0.2 mM, showing a low detection limit of 1 nM. This system as a novel, sensitive and label-free aptasensor was used for ratiometric electrochemical sensing of DA. Experimental results verified that this aptasensor possessed high stability, selectivity and sensitivity towards DA detection, over potential interferents. This aptasensor efficiently determined DA in real biological samples, together with high detection recoveries of 97.0-104.0%.

A bimetallic nanoparticle/graphene oxide/thionine composite-modified glassy carbon electrode used as a facile ratiometric electrochemical sensor for sensitive uric acid determination

A novel and facile ratiometric electrochemical sensor was developed for sensitive determination of uric acid.

A ratiometric strategy -based electrochemical sensing interface for the sensitive and reliable detection of imidacloprid

A ratiometric electrochemical sensor was developed for selective and sensitive detection of imidacloprid. Modified poly(thionine) provided a built-in correction to endow the sensor with good accuracy and stability.