Fabrication strategies, sensing modes and analytical applications of ratiometric electrochemical biosensors

Organic fluorophores-based molecular probes with dual-fluorescence ratiometric responses to in-vitro/in-vivo pH for biosensing, bioimaging and biotherapeutics applications

In recent years, organic fluorophores-based molecular probes with dual-fluorescence ratiometric responses to in-vitro/in-vivo pH (DFR-MPs-pH) have been attracting much interest in fundamental application research fields. More and more scientific publications have reported the exploration of various DFR-MPs-pH systems that have unique dual-fluorescence ratiometry as the signal output, in-built and signal self-calibration functions to improve precise detection of targets. DFR-MPs-pH systems possess high-performance applications in biosensing, bioimaging and biomedicine fields. This review has comprehensively summarized recent advances of DFR-MPs-pH for the first time. First of all, the compositions and types of DFR-MPs-pH are introduced by summarizing different organic fluorophores-based molecule systems. Then, construction strategies are analyzed based on specific components, structures, properties and functions of DFR-MPs-pH. Afterward, biosensing and bioimaging applications are discussed in detail, primarily referring to pH sensing and imaging detection at the levels of living cells and small animals. Finally, biomedicine applications are fully summarized, majorly involving bio-toxicity evaluation, bio-distribution, biomedical diagnosis and therapeutics. Meanwhile, the current status, challenges and perspectives are rationally commented after detailed discussions of representative and state-of-the-art studies. Overall, this present review is comprehensive, in-time and in-depth, and can facilitate the following further exploration of new and versatile DFR-MPs-pH systems toward rational design, facile preparation, superior properties, adjustable functions and highly efficient applications in promising fields.

An electrochemical ratiometric immunosensor for the detection of NMP22 based on ZIF-8@MWCNTs@Chit@Fc@AuNPs and AuPt-MB

Nuclear matrix protein 22 (NMP22) is one of the most important tumor markers of bladder cancer and is significantly elevated in the urine of bladder cancer patients. Therefore, in this work, a highly sensitive ratiometric electrochemical immunosensor was constructed to detect NMP22 based on ZIF-8@MWCNTs@Chit@Fc@AuNPs composites. ZIF-8 had a large surface area and good adsorption ability. Multi-Walled Carbon Nanotubes (MWCNTs) can optimize the electrical conductivity of ZIF-8, so that the electrode surface of ferrocene (Fc) obtains a stable and strong electrochemical signal. In addition, AuPt-MB provided another strong detection signal methylene blue (MB) while immobilizing the secondary antibody (Ab2) through Au-N and Pt-N bonds. A ratiometric electrochemical sensor was formed based on ZIF-8@MWCNTs@Chit@Fc@AuNPs and AuPt-MB, which showed a great linear connection between IMB/IFc and the logarithmic concentration of NMP22 with a detection limit of 3.33 fg mL-1 (S/N = 3) under optimized specifications in the concentration interval of 0.01 pg mL-1 to 1000 ng mL-1. In addition, the ratiometric immunosensor showed good selectivity and stability.

Boronate crosslinking-based ratiometric electrochemical assay of glycated albumin

As a product of nonenzymatic glycation, glycated albumin (GA) is a promising serum marker for the short-term glycemic monitoring in patients with diabetes. On the basis of the boronate crosslinking (BCL)-enabled direct labeling of ferrocene (Fc) tags to the nonenzymatically glycated (NEG) sites, we report herein a novel aptamer-based ratiometric electrochemical (apt-REC) platform for the point-of-care (POC) assay of GA. This apt-REC platform is based on the recognition of GA proteins by the methylene blue (MB)-modified aptamer receptors and the labeling of the Fc tags to the NEG sites via the BCL. Using MB as the reference tag and Fc as the quantification tag, the ratio of the oxidation currents (i.e., IFc/IMB) can serve as the yardstick for the ratiometric assay of GA. Due to the presence of tens of the NEG sites, each GA protein can be labeled with tens of quantification tags, permitting the amplified assay in a simple, time-saving, and low-cost manner. The ratiometric signal exhibited a good linear response over the range from 0.1 to 100 μg/mL, with a detection limit of 45.5 ng/mL. In addition to the superior reproducibility and robustness, this apt-REC platform is highly selective (capable of discriminating GA against human serum albumin (HSA)) and applicable to GA assay in serum samples. Due to its low cost, high reproducibility and robustness, simple operation, and high sensitivity and selectivity, this apt-REC platform holds great promise in the POC assay of GA for diabetes management.

Nuclear matrix protein 22 in bladder cancer

Bladder cancer (BC) is ranked as the ninth most common malignancy worldwide, with approximately 570,000 new cases reported annually and over 200,000 deaths. Cystoscopy remains the gold standard for the diagnosis of BC, however, its invasiveness, cost, and discomfort have driven the demand for the development of non-invasive, cost-effective alternatives. Nuclear matrix protein 22 (NMP22) is a promising non-invasive diagnostic tool, having received FDA approval. Traditional methods for detecting NMP22 require a laboratory environment equipped with specialized equipment and trained personnel, thus, the development of NMP22 detection devices holds substantial potential for application. In this review, we evaluate the NMP22 sensors developed over the past decade, including electrochemical, colorimetric, and fluorescence biosensors. These sensors have enhanced detection sensitivity and overcome the limitations of existing diagnostic methods. However, many emerging devices exhibit deficiencies that limit their potential clinical use, therefore, we propose how sensor design can be optimized to enhance the likelihood of clinical translation and discuss the future applications of NMP22 as a legacy biomarker, providing insights for the design of new sensors.

Ratiometric electrochemical immunoassay based on 2D Co/Fe MOF decorated with toluidine blue and Fc-labeled Schiff base for accurate assay of alpha-fetoprotein in clinical serum

The high level of alpha-fetoprotein (AFP) expression is closely related to hepatocellular carcinoma (HCC). Herein, a dual signal ratiometric electrochemical immunosensor based on chitosan-ferrocenecarboxaldehyde-spindle gold (Chit-Fc-SAu) and Co/Fe metal-organic framework-toluidine blue/polydopamine (Co/Fe MOF-TB/PDA) was proposed for quantitative analysis of AFP. Specifically, Chit-Fc-SAu worked as a substrate to trap more primary antibodies (Ab1) generating the first electrochemical signal from Fc. Thanks to the large specific surface area, the synergistic and electronic effects of Co/Fe MOF nanosheets, and the rich functional groups of PDA, Co/Fe MOF-TB/PDA could load more secondary antibodies (Ab2) and signal molecules (TB) providing another amplified electrochemical signal. In the presence of AFP, Ab1-AFP-Ab2 formed a sandwich structure, and as the AFP concentration increased, the peak current ratio of TB to Fc (ITB/IFc) also increased. The dual signal ratiometric strategy can avoid environmental signal interference and achieve signal self-calibration, thereby improving the accuracy and reproducibility of detection. After a series of exploration, this self-calibrated ratiometric immunosensor exhibited a wide linear range (0.001-200 ng mL-1), a low detection limit (0.34 pg mL-1), and good repeatability. When applied to the assay of clinical serum samples, the detection results of ratiometric sensor were consistent with that of commercial electrochemiluminescence (ECL) immunoassay, significantly superior to that of non-ratiometric sensor. The self-calibrated strategy based on ratiometric sensor helps to improve the accuracy of AFP in clinical diagnosis.

A novel differential ratiometric molecularly imprinted electrochemical sensor for determination of sulfadiazine in food samples

Herein, we report a novel differential ratiometric molecularly imprinted polymer (MIP) electrochemical sensor for sulfadiazine (SDZ). An MIP membrane with double templates, SDZ and propyl gallate (PG), was fabricated on glassy carbon electrode (GCE) modified by CuInS2/ZnS nanocomposites. After adding PG in the samples as the reference, the current differences between MIP@CuInS2/ZnS/GCE and non-imprinted polymer@CuInS2/ZnS/GCE at the potentials of 0.18 V (ΔIPG) and 0.92 V (ΔISDZ) were measured. The ratio of ΔISDZ/ΔIPG was used for SDZ determination in the differential and ratiometric dual-mode. The influence of the variations in electrode modification and sample enrichment conditions on the determination of SDZ can be suppressed by 2.8 ∼ 13.2-fold, enhancing the reproducibility and stability of the MIP sensor. The interference level was reduced by one order of magnitude compared with the normal MIP mode. The proposed sensors were used to determine SDZ in food samples, with the detection limit of 2.1 nM.

A novel and sensitive dual signaling ratiometric electrochemical aptasensor based on nanoporous gold for determination of Ochratoxin A

Ochratoxin A (OTA) is a toxic pollutant in foods, and its actual detection is crucial. A novel and sensitive dual signaling ratiometric electrochemical aptasensor based on nanoporous gold (NPG) was proposed to determine OTA. NPG, with high specific surface area and conductivity, improved the sensitivity by immobilizing more aptamers. Meanwhile, the dual signaling ratiometric strategy improved the detection reproducibility through self-referencing and built-in correction. NPG and ratiometric strategy multi-amplified the dual signal changes. The sensitivity of OTA was evaluated by the ratio of methylene to ferrocene current values. Under the optimal conditions, the NPG-based aptasensor demonstrated excellent sensitivity with a wide linear range of 1 pg/mL to 2 ng/mL and the limit of detection (LOD) of 0.4 pg/mL for OTA. This developed aptasensor also effectively detected OTA in spiked Cordyceps sinensis and grape juice samples, with recovery values falling in the 98.49-108.0% range.

Machine learning-powered electrochemical aptasensor for simultaneous monitoring of di(2-ethylhexyl) phthalate and bisphenol A in variable pH environments

Plastic waste is a pernicious environmental pollutant that threatens ecosystems and human health by releasing contaminants including di(2-ethylhexyl) phthalate (DEHP) and bisphenol A (BPA). Therefore, a machine-learning (ML)-powered electrochemical aptasensor was developed in this study for simultaneously detecting DEHP and BPA in river waters, particularly to minimize the electrochemical signal errors caused by varying pH levels. The aptasensor leverages a straightforward and effective surface modification strategy featuring gold nanoflowers to achieve low detection limits for DEHP and BPA (0.58 and 0.59 pg/mL, respectively), excellent specificity, and stability. The least-squares boosting (LSBoost) algorithm was introduced to reliably monitor the targets regardless of pH; it employs a layer that adjusts the number of multi-indexes and the parallel learning structure of an ensemble model to accurately predict concentrations by preventing overfitting and enhancing the learning effect. The ML-powered aptasensor successfully detected targets in 12 river sites with diverse pH values, exhibiting higher accuracy and reliability. To our knowledge, the platform proposed in this study is the first attempt to utilize ML for the simultaneous assessment of DEHP and BPA. This breakthrough allows for comprehensive investigations into the effects of contamination originating from diverse plastics by eliminating external interferent-caused influences.

Electric field-enabled aptasensing interfacial engineering to simultaneously enhance specific preconcentration and electrochemical detection of mercury and lead ions

Aptamers with strong affinity to heavy metal ions (HMIs) allow fabrication of electrochemical sensors with high selectivity and sensitivity, while controllable regulation of aptamer-HMI recognition at the sensing interface, which is vital for better analytical performance, remains challenging. Here, an electric field-based strategy for engineering an aptasensing interface was proposed to realize the specific preconcentration and accurate detection of mercury (Hg2+) and lead (Pb2+) ions with a ratiometric electrochemical sensor. The working principle is to apply an electric field to drive HMIs to approach the aptamer and retain the orientation of the DNA structure. Anthraquinone-2-carboxylic acid (AQ)-labeled complementary DNA was designed to simultaneously bind a ferrocene (Fc)-labeled aptamer for Hg2+ and a methylene blue (MB)-labeled aptamer for Pb2+, and the sensing interface was fabricated with this presynthesized DNA structure. For preconcentration, an electric field of 3.0 V pushed HMIs to approach the aptamer and retained the orientation of DNA to favor the following recognition; for detection, the oriented DNA in 2.5 V electric field offered a stable current of AQ as a reference. In this way, currents of AQ, Fc and MB were used to produce ratiometric signals of IAQ/IFc and IAQ/IMB for Hg2+ and Pb2+, respectively. Such a strategy allowed the simultaneous detection of Hg2+ and Pb2+ within 30 min with detection limits of 0.69 pM and 0.093 pM, respectively. The aptasensor was applied for soil, water, and crayfish analysis in paddy fields. The electric field-enabled strategy offers a new way to fabricate high-performance electrochemical aptasensor for HMIs detection.

Highly Coupled Seven-Core Fiber for Ratiometric Anti-Phase Sensing

A ratiometric fiber optic temperature sensor based on a highly coupled seven-core fiber (SCF) is proposed and experimentally demonstrated. A theoretical analysis of the SCF's sinusoidal spectral response in transmission configuration is presented. The proposed sensor comprises two SCF devices exhibiting anti-phase transmission spectra. Simple fabrication of the devices is shown by just splicing a segment of a 2 cm long SCF between two single-mode fibers (SMFs). The sensor proved to be robust against light source fluctuations, as a standard deviation of 0.2% was registered in the ratiometric measurements when the light source varied by 12%. Its low-cost detection system (two photodetectors) and the range of temperature detection (25 °C to 400 °C) make it a very attractive and promising device for real industrial applications.

Recent advances of ratiometric sensors in food matrices: mycotoxins detection

The public health problem caused by mycotoxins contamination has received a great deal of attention worldwide. Mycotoxins produced by filamentous fungi widely distributed in foodstuffs can cause adverse impacts on humans and livestock, posing serious health threats. Particularly worth mentioning is that mycotoxins can accumulate in organisms and be enriched through the food chain. Improving early trace detection and control from the source is a more desirable approach than the contaminated food disposal process to ensure food safety. Conventional sensors are susceptible to interference from various components in intricate food matrices when detecting trace mycotoxins. The application of ratiometric sensors avoids signal fluctuations, and reduce background influences, which casts new light on developing sensors with superior performance. This work is the first to provide an overview of the recent progress of ratiometric sensors in the detection of mycotoxins in intricate food matrices, and highlight the output types of ratiometric signal with respect to accurate quantitative analysis. The prospects of this field are also included in this paper and are intended to have key ramifications on the development of sensing detection conducive to food safety.

Recent Aptamer-Based Biosensors for Cd2+ Detection

Cd²⁺, a major environmental pollutant, is heavily toxic to human health. Many traditional techniques are high-cost and complicated; thus, developing a simple, sensitive, convenient, and cheap monitoring approach is necessary. The aptamer can be obtained from a novel method called SELEX, which is widely used as a DNA biosensor for its easy acquisition and high affinity of the target, especially for heavy metal ions detection, such as Cd²⁺. In recent years, highly stable Cd²⁺ aptamer oligonucleotides (CAOs) were observed, and electrochemical, fluorescent, and colorimetric biosensors based on aptamers have been designed to monitor Cd²⁺. In addition, the monitoring sensitivity of aptamer-based biosensors is improved with signal amplification mechanisms such as hybridization chain reactions and enzyme-free methods. This paper reviews approaches to building biosensors for inspecting Cd²⁺ by electrochemical, fluorescent, and colorimetric methods. Finally, many practical applications of sensors and their implications for humans and the environment are discussed.

A ratiometric electrochemical sensor for detecting lead in fish based on the synergy of semi-complementary aptamer pairs and Ag nanowires@zeolitic imidazolate framework-8

This work describes the synergistic application of semi-complementary aptamer pairs and signals on-off ratio strategies on glassy carbon electrodes (GCE) for detecting lead ions (Pb²⁺) in fish. Gold nanoparticles (AuPNs) as the electrode substrate can provide added binding sites for the aptamers and improve the conductivity of the electrodes. Pb²⁺ aptamers containing ferrocene (Fc) molecules act as molecular recognizers in the sensing system. In the presence of target ions, Fc signals are affected by conformational changes of the aptamer. The "Ag nanowires@zeolitic imidazolate framework-8 with methylene blue (AgNWs@ZIF-8/MB)" can be semi-complementary to the Pb²⁺ aptamer after binding to single-stranded DNA (S₁). However, S₁/AgNWs@ZIF-8/MB self-assembled with Pb²⁺ aptamer (Apt) by hybridization incubation was quickly replaced by Pb²⁺ competitively, resulting in the loss of methylene blue (MB) signaling molecules. Hence, the internal reference signal (MB) and conformation change signal (Fc) comprise the ratio sensing system well. Morphology, spectroscopy, and electrochemistry methods have validated the modification and sensing behaviors. The used Apt has made considerable progress in analytical performance. In interference studies and stability checks, the ratio measurement signal I_Fc/I_MB is a more reliable signal than the single signal readout. Following a log-linear relationship, this sensor provides a wide linear range. Furthermore, the proposed sensor can be used to determine Pb²⁺ in fish samples, and the results agree with those obtained using ICP-MS and recovery tests.

Recent Advances in Recognition Receptors for Electrochemical Biosensing of Mycotoxins-A Review

Mycotoxins are naturally occurring toxic secondary metabolites produced by fungi in cereals and foodstuffs during the stages of cultivation and storage. Electrochemical biosensing has emerged as a rapid, efficient, and economical approach for the detection and quantification of mycotoxins in different sample media. An electrochemical biosensor consists of two main units, a recognition receptor and a signal transducer. Natural or artificial antibodies, aptamers, molecularly imprinted polymers (MIP), peptides, and DNAzymes have been extensively employed as selective recognition receptors for the electrochemical biosensing of mycotoxins. This article affords a detailed discussion of the recent advances and future prospects of various types of recognition receptors exploited in the electrochemical biosensing of mycotoxins.

Recent advances in electrochemical proximity ligation assay

Proximity ligation assay (PLA) is a vigorously developed homogeneous immunoassay assisted by DNA combining dual recognition of target protein by pairs of proximity probes, in which the detection of protein is tactfully converted to the detection of DNA. The booming developments in PLA have enabled a variety of ultrasensitive assays for the detection of protein and this concept of PLA is also extended to the detection of nucleic acids and some small molecule. The association between PLA and electrochemical method, defined as electrochemical proximity ligation assay (ECPLA), has gained much interests in disease diagnosis, food safety and environmental assays with the advantages, such as broad range of targets, simplicity, low cost and rapid response. In this review, we took a different perspective to present the history of PLA, the classical ECPLA biosensing methodology as well as the developments of ECPLA based on several key parameters, such as sensitivity, selectivity, reusability and generalization. In addition, the developments of PLA with electrochemiluminescence as readout are also presented. Finally, perspective and some unresolved challenges in ECPLA that can potentially be addressed have also been discussed.

Ratiometric Electrochemical Sensor for Butralin Determination Using a Quinazoline-Engineered Prussian Blue Analogue

A ratiometric electrochemical sensor based on a carbon paste electrode modified with quinazoline-engineered ZnFe Prussian blue analogue (PBA-qnz) was developed for the determination of herbicide butralin. The PBA-qnz was synthesized by mixing an excess aqueous solution of zinc chloride with an aqueous solution of precursor sodium pentacyanido(quinazoline)ferrate. The PBA-qnz was characterized by spectroscopic and electrochemical techniques. The stable signal of PBA-qnz at +0.15 V vs. Ag/AgCl, referring to the reduction of iron ions, was used as an internal reference for the ratiometric sensor, which minimized deviations among multiple assays and improved the precision of the method. Furthermore, the PBA-qnz-based sensor provided higher current responses for butralin compared to the bare carbon paste electrode. The calibration plot for butralin was obtained by square wave voltammetry in the range of 0.5 to 30.0 µmol L^-1, with a limit of detection of 0.17 µmol L^-1. The ratiometric sensor showed excellent precision and accuracy and was applied to determine butralin in lettuce and potato samples.

Ratiometric upconversion luminescence nanoprobes from construction to sensing, imaging, and phototherapeutics

In terms of the combined advantages of upconversion luminescence (UCL) properties and dual-signal ratiometric outputs toward specific targets, the ratiometric UCL nanoprobes exhibit significant applications. This review summarizes and discusses the recent advances in ratiometric UCL nanoprobes, mainly including the construction of nanoprobe systems for sensing, imaging, and phototherapeutics. First, the construction strategies are introduced, involving different types of nanoprobe systems, construction methods, and ratiometric dual-signal modes. Then, the sensing applications are summarized, involving types of targets, sensing mechanisms, sensing targets, and naked-eye visual detection of UCL colors. Afterward, the phototherapeutic applications are discussed, including bio-toxicity, bio-distribution, biosensing, and bioimaging at the level of living cells and small animals, and biomedicine therapy. Particularly, each section is commented on by discussing the state-of-the-art relevant studies on ratiometric UCL nanoprobe systems. Moreover, the current status, challenges, and perspectives in the forthcoming studies are discussed. This review facilitates the exploration of functionally luminescent nanoprobes for excellent sensing, imaging, biomedicine, and multiple applications in significant fields.

A novel ratiometric electrochemical aptasensor for highly sensitive detection of carcinoembryonic antigen

A novel ratiometric electrochemical aptasensor was proposed for carcinoembryonic antigen (CEA) detection based on exonuclease III (Exo III)-assisted recycling and rolling circle amplification (RCA) strategies. A thiolated ferrocene-labeled hairpin probe 2 (Fc-HP2) was fixed on the gold nanoparticles (AuNPs)-modified gold electrode (AuE) surface through Au-S bonds. The presence of CEA led to the release of trigger, which hybridized with the 3'-protruding of hairpin probe 1 (HP1) and triggered the Exo III cleavage reaction, accompanied by the releasing of trigger and generation of new DNA fragment which was used for the successive hybridization with Fc-HP2. After the Exo III cleavage process, the remaining Fc-HP2 fragments hybridized as primers with the RCA template to initiate the RCA process, and long single-stranded polynucleotides were produced for methylene blue (MB) binding. Such changes resulted in the signal of Fc (I_Fc) decreased and that of MB (I_MB) increased, achieving a linear relationship between I_MB/I_Fc and logarithm of CEA concentrations ranging from 1.0 pg mL^-1 to 100.0 ng mL^-1 with a detection limit of 0.59 pg mL^-1. Additionally, the developed aptasensor had been successfully applied to detect CEA in human serum samples. Therefore, the proposed strategy might provide a new platform for clinical detections of CEA.

Programmable dual-electric-field immunosensor using MXene-Au-based competitive signal probe for natural parathion-methyl detection

Immunosensor is a promising tool for natural parathion-methyl (PTM) detection, and its analytical advantages can be magnified by introducing flexibly-fabricating technique. Herein, we present a dual-electric-field PTM immunosensor on highly-compatible screen-printed electrode (SPE). MXene-Au, the product of in-situ gold nanoparticle growth on MXene, provides considerable binding sites for PTM antigen (ATG) and methylene blue (MB). During sensing, the MXene-Au-MB-ATG probe competitively binds antibody against PTM, composing a ratiometric immune-system. With DC-biased sine excitations from complementary waveforms, on-chip electric field couple improves immunoreactions among PTM, probe, and antibody. Electric field distribution is programmed by trimming bypass resistors to pursue optimal performance. Probe synthesis is solidly proven with morphological examinations, and competition mechanism between the probe and target PTM is clarified in electrochemical analyses. Remarkably, this method brings less consumption of immune time than electric-field-free or solo-electric-field setup (50 s vs. 900 or 70 s), and simultaneously provides more powerful ratiometric signal than the rivals. Log-linear relationship, between PTM level and sensor readout, is established in 0.02-38 ng/mL, and limit of detection is found as 0.01 ng/mL. This method is applied in laboratorial and natural PTM analyses, and the readouts are consistent with high performance liquid chromatography and recovery test.

Dual-Color Fluorescent Hydrogel Microspheres Combined with Smartphones for Visual Detection of Lactate

Since it is difficult for human eyes to distinguish between two identical colors with only <15% variation in brightness, mono-color fluorescent hydrogel microspheres have some limitations in the detection of lactate. Herein, we prepared novel dual-color fluorescent hydrogel microspheres, which can achieve hue transformation. Microspheres were prepared by introducing a fluorescent nanoparticle as the reference signal while CdTe QDs were used as the response signal. We used smartphones with image processing software to collect and analyze data. In this way, the signal of lactate was converted to RGB (red, green, and blue) values, which can be quantitatively read. Within 10 to 1500 μM, the R/G values of the microspheres had a linear relationship with the logarithm of the lactate concentration. Moreover, color cards for lactate detection were prepared, from which the color change and concentration of lactate could be easily read by the naked eye. It is worth mentioning that this method was successfully applied to screen patients with hyperlactatemia.

Picomolar glutathione detection based on the dual-signal self-calibration electrochemical sensor of ferrocene-functionalized copper metal-organic framework via solid-state electrochemistry of cuprous chloride

Chemical biosensing techniques are essential for food analysis and disease diagnosis. Nanomaterials with redox activity show great potential in electrochemical analysis, acting as signal labels or signal amplification unit, which can reflect the targets concentration in foods and biological samples. Here, an ultra-sensitive dual-signal intrinsic self-calibration electrochemical platform for GSH was firstly fabricated based on the novel electroactive nanomaterial of ferrocene-functionalized copper metal-organic framework (Fc-Cu-MOF). Due to the solid-state electrochemical property of cuprous chloride (CuCl), a sharp characteristic peak with an increased signal appears with the coexistence of chloride ions in solution. The stronger specific affinity between Cu⁺ and GSH than that of Cu⁺ and Cl^- triggers a "crowding effect" that causes the current signal of CuCl decrease greatly. Meanwhile, the peak current of ferrocene keeps unchanged as an internal reference. Based on the ratio of the peak current variation (ΔI_Cu/ΔI_Fc) as the signal output, Fc-Cu-MOF modified electrode showed wider linear range in 0.1 nM -1 μM for GSH with the detection limit as low as 0.025 nM. And the sensor was successfully applied in the determination of GSH with excellent recoveries in various real samples such as food and serum samples, providing good prospect in application of bioanalysis and food screening.

Boronate-Affinity Cross-Linking-Based Ratiometric Electrochemical Detection of Glycoconjugates

Despite the widespread application of the boronate-affinity cross-linking (BAC) in the separation, enrichment, and sensing of glycoconjugates, it remains a huge challenge to integrate the BAC into the selective electrochemical detection of glycoconjugates due to the poor selectivity of the BAC. Herein, we demonstrate a BAC-based ratiometric electrochemical method for the simple, low-cost, and highly sensitive and selective detection of glycoconjugates. Briefly, the methylene blue (MB)-tagged nucleic acid aptamer is exploited as the recognition element to selectively capture target glycoconjugate, to which a large number of ferrocene (Fc) tags are subsequently labeled via the BAC between the phenylboronic acid (PBA) group and the cis-diol site of the oligosaccharide chains on the captured targets. Using the MB tag as the internal reference and the Fc tag as the reporter of the target capture, the dual-signal output enables the ratiometric detection. Due to the presence of a high density of the cis-diol sites on a glycoconjugate, sufficiently high sensitivity can be obtained even without using any amplification strategies. Using glycoprotein mucin 1 (MUC1) as the model target, the signal ratio (I_Fc/I_MB) exhibits good linearity over the range from 0.05 to 50 U/mL, with a detection limit of 0.021 U/mL. In addition to the high sensitivity and selectivity, the results of the analysis of MUC1 in serum samples are acceptable. By virtue of its simplicity, cost-effectiveness, and high robustness and reproducibility, this BAC-based ratiometric electrochemical method holds great promise in the highly sensitive and selective detection of glycoconjugates.

Ratiometric Luminescence Aptasensor Based on Dual-Emissive Persistent Luminescent Nanoparticles for Autofluorescence- and Exogenous Interference-Free Determination of Trace Aflatoxin B1 in Food Samples

Sensitive and accurate determination of aflatoxin B1 (AFB1) is of great significance to food safety and human health as it is recognized as the most toxic mycotoxin and carcinogenic. Herein, we report a ratiometric luminescence aptasensor based on dual-emissive persistent luminescent nanoparticles (PLNP) for the accurate determination of trace AFB1 in complex food samples without autofluorescence and exogenous interference. Dual-emissive PLNP ZnGa₂O₄:Cr_0.0001 was prepared first and acted as the donor for energy transfer as well as the signal unit with phosphorescence at 714 and 508 nm (the detection and the reference signal, respectively). AFB1 aptamer was then bonded on the surface of PLNP to offer specific recognition ability. Aptamer complementary DNA modified with Cy5.5 was employed as the acceptor for energy transfer and the quenching group to eventually develop a turn-on ratiometric luminescence aptasensor. The developed ratiometric luminescence aptasensor combined the merits of long-lasting luminescence, in situ excitation and autofluorescence-free of PLNP, exogenous interference-free and self-calibration reading of ratiometric sensor, as well as the high selectivity of aptamer, holding great promise for accurate determination of trace AFB1 in complex matrix. The developed ratiometric aptasensor exhibited excellent linearity (0.05-70 ng mL^-1), low limit of detection (0.016 ng mL^-1), and good precision (2.3% relative standard deviation for 11 replicate determination of 1 ng mL^-1 AFB1). The proposed ratiometric aptasensor was successfully applied for the determination of AFB1 in corn, wheat, peanut, millet, oats, and wheat kernels with recoveries of 95.1-106.5%.

Specifically triggered dissociation based ratiometric electrochemical sensor for H2O2 measurement in food samples

A novel ratiometric strategy based electrochemical sensor was developed to quantitative assay of H₂O₂ in different food samples. 4-aminophenylboronic acid pinacol ester (ABAPE) dissociation was specifically triggered by H₂O₂ to generate electro-active 4-aminophenol (4-AP), which not only can be oxidized to indirectly indicate the concentration of H₂O₂, but also endowed the sensor with high selectivity. Meanwhile, a reference probe of poly(thionine) (TH) was modified with ketjen black (KB) and gold nanoparticles (AuNPs) on electrode surface. KB and AuNPs displayed high electrocatalytic activity to 4-AP. A current ratio between 4-AP and TH (i/i_TH) showed a good linear relationship with the concentration of H₂O₂ in a range of 3.0 × 10^-7 - 1.0 × 10^-4 mol/L (0.010 ppm - 3.40 ppm) with a limit of detection of 2.6 × 10^-7 mol/L (0.009 ppm) (S/N = 3). Moreover, the ratiometric strategy based sensor possessed good accuracy, reliability, and stability, and successfully determined H₂O₂ in food samples with satisfactory results.

Label-free ratiometric homogeneous electrochemical aptasensor based on hybridization chain reaction for facile and rapid detection of aflatoxin B1 in cereal crops

Aflatoxin B1 (AFB1) contamination has raised global concerns in agricultural and food industry; thus, sensitive, accurate and rapid AFB1 sensors are essential in many circumstances. Herein, we developed a label-free and immobilization-free ratiometric homogeneous electrochemical aptasensor based on hybridization chain reaction (HCR) for facile and rapid determination of AFB1. Methylene blue (MB) and ferrocene (Fc) were used as label-free probes to produce a response signal (I_MB) and a reference signal (I_Fc) in solution phase, respectively. The ratio of I_MB/I_Fc was used as a yardstick to quantify AFB1. HCR was exploited to enlarge the intensity of I_MB as well as ratiometric signal. By combining label-free homogeneous assay and ratiometric strategy, the resulting aptasensor offered sensitive, rapid, and reliable determinations of AFB1 with a detection limit of 38.8 pg mL^-1. The aptasensor was then used to determine AFB1 in cereal samples with comparable reliability as HPLC-MS.

Molecularly imprinted ratiometric electrochemical sensor based on carbon nanotubes/cuprous oxide nanoparticles/titanium carbide MXene composite for diethylstilbestrol detection

Conventional molecularly imprinted polymers (MIP)-based electrochemical sensors are generally susceptible to the changes of personal operation, electrode surface, and solution conditions. Herein, a ratiometric strategy was employed through introducing Cu₂O nanoparticles (NPs) as inner reference probe to realize the reliable detection of diethylstilbestrol (DES). MIP film was prepared by electropolymerization of 1H-pyrrole-3-carboxylicacid in the presence of DES on carbon nanotubes/cuprous oxide/titanium carbide (CNT/Cu₂O NPs/Ti₃C₂T_x) modified electrodes. The Ti₃C₂T_x with accordion-like structure not only possessed good electrical conductivity, but also facilitated the immobilization of Cu₂O NPs, which contributed to stabilizing the signal. CNT was introduced to further improve the sensitivity of the sensor. Under optimum conditions, the MIP/CNT/Cu₂O NPs/Ti₃C₂T_x electrochemical sensors showed a broad linear response range of 0.01 to 70 μM, and a low detection limit of 6 nM (S/N = 3). Moreover, the sensor was applied to detect DES in real samples including lake water, milk, and pork, and the recoveries for spiked standard were 88-112%. Thus, this work provides a new way for reliable DES detection.

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-labeling ratiometric electrochemical strategy initiated with ISDPR for accurate screening MecA gene

An outstanding dual-labeling ratiometric electrochemical biosensor based on isothermal strand displacement polymerization reaction (ISDPR) for highly sensitive and selective detection of mecA gene has been proposed. Concretely, in the presence of mecA gene, the addition of methylene blue (MB)-labeled primer and polymerase induced recycling amplification to change the structure of the ferrocene (Fc)-labeled hairpin probe, thereby releasing abundant target gene to realize the signal amplification and dual-signal output. Through this process, the electrochemical responses of Fc (I_Fc) and MB (I_MB) were both substantially reduced and increased proportionally, ensuring that the value of I_MB/I_Fc can accurately reflect the true detection level of mecA gene. Benefiting from the "signal-on/off" strategy, the fabricated biosensor exhibited outstanding sequence specificity to discriminate mismatched mecA gene, which verified to be 2.72 times that of single-label detection for perfect match/single base mismatch (PM/MM) discrimination ratio. This strategy effectively integrated the advantages of signal amplification and ratiometric modes, making the biosensor exhibit a broad working range with 10 fM - 3000 pM and a limit of detection (LOD) with 3.33 fM (S/N = 3). Moreover, the proposed biosensor has good feasibility for mecA gene determination in water samples due to acceptable recoveries (95-115%) and repeatability relative standard deviations (RSD) value of 4%. This will provide a powerful sensing platform for improving accuracy and decreasing background signal of sensor for ARGs screening in environmental monitoring.

Rational engineering the DNA tetrahedrons of dual wavelength ratiometric electrochemiluminescence biosensor for high efficient detection of SARS-CoV-2 RdRp gene by using entropy-driven and bipedal DNA walker amplification strategy

Fast and effective detection of epidemics is the key to preventing the spread of diseases. In this work, we constructed a dual-wavelength ratiometric electrochemiluminescence (ECL) biosensor based on entropy-driven and bipedal DNA walker cycle amplification strategies for detection of the RNA-dependent RNA polymerase (RdRp) gene of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The entropy-driven cyclic amplification reaction was started by the SARS-CoV-2 RdRp gene to generate a bandage. The bandage could combine with two other single-stranded S1 and S2 to form a bipedal DNA walker to create the following cycle reaction. After the bipedal DNA walker completed the walking process, the hairpin structures at the top of the DNA tetrahedrons (TDNAs) were removed. Subsequently, the PEI-Ru@Ti₃C₂@AuNPs-S7 probes were used to combine with the excised hairpin part of TDNAs on the surface of Au-g-C₃N₄, and the signal change was realized employing electrochemiluminescence resonance energy transfer (ECL-RET). By combining entropy-driven and DNA walker cycle amplification strategy, the ratiometric ECL biosensor exhibited a limit of detection (LOD) as low as 7.8 aM for the SARS-CoV-2 RdRp gene. As a result, detecting the SARS-CoV-2 RdRp gene in human serum still possessed high recovery so that the dual-wavelength ratiometer biosensor could be used in early clinical diagnosis.

Tailoring Oxygen-Containing Groups on Graphene for Ratiometric Electrochemical Measurements of Ascorbic Acid in Living Subacute Parkinson's Disease Mouse Brains

Ascorbic acid (AA), a major antioxidant in the central nervous system (CNS), is involved in withstanding oxidative stress that plays a significant role in the pathogenesis of Parkinson's disease (PD). Exploring the AA disturbance in the process of PD is of great value in understanding the molecular mechanism of PD. Herein, by virtue of a carbon fiber electrode (CFE) as a matric electrode, a three-step electrochemical process for tailoring oxygen-containing groups on graphene was well designed: potentiostatic deposition was carried out to fabricate graphene oxide on CFE, electrochemical reduction that assisted in removing the epoxy groups accelerated the electron transfer kinetics of AA oxidation, and electrochemical oxidation that increased the content of the carbonyl group (C═O) generated an inner-reference signal. The mechanism was solidified by ab initio calculations by comparing AA absorption on defected models of graphene functionalized with different oxygen groups including carboxyl, hydroxyl, epoxy, and carbonyl. It was found that epoxy groups would hinder the physical absorption of AA onto graphene, while other functional groups would be beneficial to it. Biocompatible polyethylenedioxythiophene (PEDOT) was further rationally assembled to improve the antifouling property of graphene. As a result, a new platform for ratiometric electrochemical measurements of AA with high sensitivity, excellent selectivity, and reproducibility was established. In vivo determination of AA levels in different regions of living mouse brains by the proposed method demonstrated that AA decreased remarkably in the hippocampus and cortex of a subacute PD mouse than those of a normal mouse.

Ratiometric electrochemical sensor for bisphenol A detection using a glassy carbon electrode modified with a poly(toluidine blue)/gold nanoparticle composite

A ratiometric electrochemical sensor for bisphenol A (BPA) detection is developed using a glassy carbon electrode modified with a poly(toluidine blue)/gold nanoparticle composite (PTB/AuNP/GCE). The ratiometric signal, namely, the oxidation peak current ratio of BPA to PTB, increases linearly with BPA concentration in the 0.2-5.0 μM range, with a detection limit of 0.15 μM. The electrochemical mechanism of BPA is studied at the PTB/AuNP/GCE, and the results show that BPA undergoes an electrooxidation process of two electrons and two protons at the PTB/AuNP/GCE. The proposed sensor has high sensitivity, high stability and good selectivity. The application of BPA in water samples is successfully verified using the proposed ratiometric electrochemical sensor.

Integration of Multiple Redox Centers into Porous Coordination Networks for Ratiometric Sensing of Dissolved Oxygen

The application of porphyrin metal-organic frameworks (MOFs) as a ratiometric electrochemical sensing platform is still unexplored. In this paper, we report a ratiometric electrochemical sensor by the integration of multiple redox centers into porphyrin MOFs for the detection of dissolved oxygen (DO). Specifically, the ferrocene (Fc) group was integrated into the nanosized PCN-222(Fe) (PCN = porous coordination networks) via acid-base reaction to synthesize the Fc@PCN-222(Fe) composite with two redox centers of the Fc group and Fe-porphyrin. The Fc group that is insensitive to DO serves as an internal reference, and the Fe-porphyrin in PCN-222(Fe) is a DO indicator. The ratios of the cathodic currents for the two redox centers exhibit a linear relationship with DO concentrations from 2.8 to 28.9 mg mL^-1 and a limit of detection of 0.3 mg mL^-1. In addition, the ratiometric electrochemical sensor has high selectivity and stability for DO sensing results from the Fc@PCN-222(Fe) composite. Because there are numerous redox centers, such as methylene blue and thionine, which can be integrated into MOFs, many MOF-based ratiometric electrochemical sensors can be simply developed for high-performance biosensing.

Substrate Materials for Biomolecular Immobilization within Electrochemical Biosensors

Electrochemical biosensors have potential applications for agriculture, food safety, environmental monitoring, sports medicine, biomedicine, and other fields. One of the primary challenges in this field is the immobilization of biomolecular probes atop a solid substrate material with adequate stability, storage lifetime, and reproducibility. This review summarizes the current state of the art for covalent bonding of biomolecules onto solid substrate materials. Early research focused on the use of Au electrodes, with immobilization of biomolecules through ω-functionalized Au-thiol self-assembled monolayers (SAMs), but stability is usually inadequate due to the weak Au-S bond strength. Other noble substrates such as C, Pt, and Si have also been studied. While their nobility has the advantage of ensuring biocompatibility, it also has the disadvantage of making them relatively unreactive towards covalent bond formation. With the exception of Sn-doped In2O3 (indium tin oxide, ITO), most metal oxides are not electrically conductive enough for use within electrochemical biosensors. Recent research has focused on transition metal dichalcogenides (TMDs) such as MoS2 and on electrically conductive polymers such as polyaniline, polypyrrole, and polythiophene. In addition, the deposition of functionalized thin films from aryldiazonium cations has attracted significant attention as a substrate-independent method for biofunctionalization.

A label-free ratiometric immunoassay using bioinspired nanochannels and a smart modified electrode

Labeling with redox reporter is often required in developing electrochemical bioassay for most proteins or nucleic acid biomarkers. Herein, a label-free ratiometric immunosensing platform is firstly developed by integrating the antibody-conjugated nanochannels with a smart modified electrode. The electrode modifier is the composite of C₆₀, tetraoctylammonium bromide (TOA⁺) and Prussian blue (PB). Cyclic voltammograms of the ultimate C₆₀-TOA⁺/PB modified electrode exhibited two pairs of peaks at 0.15 V and -0.13 V, ascribing to the redox of PB and C₆₀, respectively. With the addition of K₃[Fe(CN)₆] in the electrolyte solution, the peaks of PB decreased due to the adsorption of [Fe(CN)₆]^3- while the peaks of C₆₀ increased because of the formation of the ternary complex (TC) C₆₀-TOA⁺-[Fe(CN)₆]^3-. As a result, the peak current ratio I_PB/I_TC decreased gradually with the increment of the concentration of [Fe(CN)₆]^3-. For the nanochannels-based immunosensing platform, the steric hindrance of the bioconjugated nanochannels varied with the loading amount of the target CA125, and thus [Fe(CN)₆]^3- passing through the channels was quantitatively affected. And the higher CA125 level was, the less [Fe(CN)₆]^3- concentration was. And thus, the ratio I_PB/I_TC monitored at the C₆₀-TOA⁺/PB modified electrode increased with the increase of the concentration of CA125. The ratiometric immunoassay featured a linear calibration range from 1.0 U mL^-1 to 100 U mL^-1 with a low detection limit of 0.86 U mL^-1. In addition, the ratiometric immunosensing platform demonstrated good specificity and stability as well as acceptable accuracy in overcoming the effect of electrode passivation which was an inherent problem of electroanalysis.