A new ratiometric electrochemical immunoassay for reliable detection of nuclear matrix protein 22

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.

Dual-Mode Electrochemical Competitive Immunosensor Based on Cd2+/Au/Polydopamine/Ti3C2 Composite and Copper-Based Metal-Organic Framework for 17β-Estradiol Detection

Herein, a dual-mode electrochemical competitive immunosensor was constructed for the detection of 17β-estradiol (E2) based on differential pulse voltammetry (DPV) and chronoamperometry (i-t). During the immune recognition process, the E2 antibody (E2-Ab) was immobilized on the Cd²⁺/Au/polydopamine/Ti₃C₂ (Cd²⁺/Au/pDA/Ti₃C₂) composite-modified electrode; then, the E2-conjugated bovine serum albumin (E2-BSA) was labeled with a copper-based metal-organic framework (Cu-MOF) and competed with E2 in combining the E2-Ab. The Cu-MOF was not only an electroactive species but also possessed good electrocatalytic activity toward H₂O₂. Thus, E2 could be quantified according to the peak current change of the Cu-MOF in DPV curve or the variation of H₂O₂ reduction current. For DPV quantification, Cd²⁺ was introduced as an internal reference in this case, and a highly reproducible ratio readout was obtained. The as-prepared dual-mode E2 electrochemical immunosensor showed good linear relationship in the ranges of 1 pg mL^-1-10 ng mL^-1 (DPV) and 10 pg mL^-1-10 ng mL^-1 (i-t), and the detection limits were 0.47 and 5.4 pg mL^-1 (S/N = 3), respectively. Furthermore, the dual-mode electrochemical immunosensor exhibited good practicability in real sample analysis.

A homogeneous enzyme-free ratiometric immunoassay for the determination of C-peptide

In this study, a homogeneous enzyme-free ratiometric (HOMO- EF-RA) immunoassay was developed for the sensitive detection of C-peptide. In the immunoassay, there have been a miscible detection system by mixing with the fluorescent quantum dots conjugated antigen (QD-Ag conjugates) and the dylight dye conjugated antibody (DL-Ab conjugates). When connecting between Ag-QD conjugate and Ab-DL conjugate by specific recognition, the system emitted fluorescence resonant energy transfer (FRET). The target C-peptide can inhibit the connection and FRET formation between QD-Ag conjugates and DL-Ab conjugates, thus changing the dual fluorescence. By measuring the ratio dual fluorescence changes of the system, the content of C-peptide was evaluated without any enzyme used and multiple incubation and washing steps. This immunoassay realized the highly sensitive (as low as 0.12 ng mL^-1), selective and rapid (as less as 6 min) detection of C-peptide. Furthermore, the the simple and convenient immunoassay was applied successfully to the determination of C-peptide in real serum samples.

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).

Ratiometric Electrochemical Immunosensor Based on L-cysteine Grafted Ferrocene for Detection of Neuron Specific Enolase

In order to realize the ultra sensitive detection of Neuron specific enolase (NSE) in human serum, we chose electrochemical immunosensor as a simple analytical method. During the experiment, we found that the peak value signals of Cu-MOFs-Au and Fc-_L-Cys were significantly changed at -0.20 V and 0.20 V potentials by DPV. So a ratiometric electrochemical immunosensor for quantitative analysis of NSE was prepared for Cu-MOFs-Au as the electrode sensing surface and Fc-_L-Cys as the label of Ab₂. The data and performances of the immunosensor were tested and analyzed by DPV. Cu-MOFs not only provide the required signal for the immunosensor, but also have a large specific surface area, which can provide more sites for the placement of Au nanoparticles. _L-cysteine (_L-Cys) can prevent a large amount of Fc-COOH leakage, so that Fc⁺ can stably provide another required signal. With the beefing up of NSE concentration, redox peak of Cu-MOFs-Au decreased and that of Fc-_L-Cys raised. The ratio (ΔI=ΔI_Cu/ΔI_Fc) of two different signals was linear with the logarithm of NSE concentration in a certain value range. In brief, with the optimized experimental conditions, the immunosensor showed excellent performance in the concentration range of 1 pg/mL to 1 μg/mL, and the detection limit was 0.011 pg/mL. Compared with other immunosensors, it showed an unexpected high sensitivity. This method also provided a new idea for the ultra sensitive quantitative detection of other biomarkers.

A probe-free electrochemical immunosensor for methyl jasmonate based on ferrocene functionalized-carboxylated graphene-multi-walled carbon nanotube nanocomposites

Methyl jasmonate (MeJA) is an endogenous plant hormone, which plays an important role in agriculture production. A novel probe-free electrochemical immunosensor was fabricated for detecting of MeJA. Fc, carboxylated graphene (COOH-GR) and carboxylated multi-walled carbon nanotubes (COOH-MWNT) composite was formed and used to fabricate screen-printed electrode (SPE). Fc was used as the electronic medium. COOH-GR and COOH-MWNT were used to improve the conductivity and catalytic activity of the sensor and to immobilize the MeJA antibody. Thus, the immunosensor can be used to detect MeJA without external redox probe solution. The designed sensor can detect MeJA in a wide range of 100 fM-100 μM, and its detection limit is as low as 31.26 fM (S/N = 3). The as-prepared probe-free immunosensor is simple, low cost, and does not need redox probe solutions for measurements, which shows great promise for future application in precision agriculture.

Dual-signal electrochemiluminescence immunosensor for Neuron-specific enolase detection based on "dual-potential" emitter Ru(bpy)32+ functionalized zinc-based metal-organic frameworks

Neuron-specific enolase (NSE) is the preferred marker for monitoring small cell lung cancer and neuroblastoma. We devised a dual-signal ratiometric electrochemiluminescence (ECL) sensing strategy for sensitive detection of NSE. In this work, Ru (bpy)₃²⁺ functionalized zinc-based metal-organic framework (Ru-MOF-5) nanoflowers (NFs) with plentiful carboxyl groups provide an excellent biocompatible sensing platform for the construction of immunosensor. Importantly, Ru-MOF-5 NFs possess stable and efficient "dual-potential" ECL emission of cathode (-1.5 V) and anode (1.5 V) in the existence of co-reactant K₂S₂O₈. Simultaneously, the cathode ECL emitter ZnO-AgNPs are employed as the secondary antibody marker, whose participation amplify the cathode ECL signal as well attenuate the anode ECL emission of Ru-MOF-5 NFs. By monitoring the ECL dual-signal of -1.5 V and 1.5 V and calculating their ratios, a ratiometric strategy of quantified readout proportional is implemented for the proposed immunosensor to precise analyze NSE. Based on optimization conditions, the ECL immunosensor displays the wide linear range of 0.0001 ng/mL to 200 ng/mL and the minimum detection limit is 0.041 pg/mL. The "dual-potential" ratiometric ECL immunosensor effectively reduces system error or background signal by self-calibration from both emissions and improves detection reliability. The dual-signal ratiometric strategy with satisfactory reproducibility and stability provides further development possibilities for other biomolecular detection and analysis.

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.

Fe-MOFs as signal probes coupling with DNA tetrahedral nanostructures for construction of ratiometric electrochemical aptasensor

A ratiometric electrochemical aptasensor is proposed for the detection of thrombin. In the sensor, the iron metal-organic frameworks (Fe MOFs)-labeled aptamer as signal tags was used as signal probe (SP), and the electrolyte solution [Fe(CN)₆]^3-/4- was utilized as an inner reference probe (IR). In the presence of thrombin, the signal of Fe-MOFs can be detected. Meanwhile, the signal of [Fe(CN)₆]^3-/4_-IR almost remains stable. Accordingly, thrombin concentration can be monitored with the ratio response of I_Fe-MOFs-SP/I_[Fe(CN)6]^3-/4-_-IR. The proposed ratiometric biosensor owns a strong ability to eliminate the disturbance that arises from different DNA loading densities, environmental impact and instrumental efficiency. DNA nanotetrahedron (NTH) with three-dimensional (3D) scaffold can effectively eliminate nonspecific adsorption of DNA and protein. The accessibility of target molecules and loading amounts of signal substances could be increased because of the enhanced mechanical rigidity of well-designed 3D NTH. Thus, detection reproducibility and sensitivity can be further improved. Moreover, the biosensor only requires conjugation with one electroactive substance. The modification procedure can be greatly simplified. The biosensor owns high sensitivity with the detection limit of 59.6 fM. We expect that it will emerge as a generalized ratiometric sensor that may be useful for detecting target analytes.

Label-assisted chemical adsorption triggered conversion of electroactivity of sensing interface to achieve the Ag/AgCl process for ultrasensitive detection of CA 19-9

Efficient strategies in enhancing sensitivity are pivotal to ultrasensitive detection of tumor markers. In this work, based on the strategy of label-assisted chemical adsorption triggered conversion of electroactivity of sensing interface, a Ag/AgCl process was achieved to enhance sensitivity of the constructed sandwich-type amperometric immunosensor for ultrasensitive detection of carbohydrate antigen 19-9 (CA19-9). Briefly, polydopamine-Ag nanoparticles (PDA-Ag NPs), as signal precursor, combined with labeling antibody were served as labels and graphene oxide-melamine (GO-MA) substrate with chemical absorption capacity was applied as smart sensing interface. After successfully incubating labels, there was primitively no current response due to the poor conductivity between labels and electrode. However, in the presence of H₂O₂, Ag NPs from labels can be etched into Ag ions, which were adsorbed by GO-MA to form GO-MA-Ag as electroactive substrate. Then, the substrate exhibited a sharp and stable electrochemistry peak of solid-state Ag/AgCl process in the buffer containing KCl. The sensitivity toward detection of CA19-9 was notably enhanced based on the appearance of sharp peak. Under optimum conditions, the designed immunosensor demonstrated a wide working range from 0.0001 to 100 U mL^-1 and an ultralow detection limit 0.032 mU mL^-1. Thus, utilizing this strategy to construct immunosensor was highly promising in clinical diagnosis for ultrasensitive detection of tumor makers.