Development of a novel fluorescent aptasensor based on the interaction between hexagonal β-Co(OH)2 nanoplates and nitrogen-doped carbon dots for ultrasensitive detection of patulin

There is an urgent need to develop an economical and convenient method for the ultrasensitive detection of patulin (PAT), a mycotoxin that can potentially harm human health when it is found in fruits and their derivatives. In this study, we have developed a novel fluorescent aptasensor that utilizes nitrogen-doped carbon dots (N-CDs) as the fluorescent donor and hexagonal β-Co(OH)2 nanoplates as the fluorescent acceptor. N-CDs were synthesized through the hydrothermal method, resulting in spherical particles with a diameter of 7.6 nm. These nanoparticles exhibited excellent water solubility and displayed a vibrant blue emission at 448 nm when excited at 360 nm. Cobalt hydroxide nanoplates with a beta crystal structure [β-Co(OH)2] were synthesized using a simple co-precipitation method, exhibiting hexagonal plate-like shapes with uniform lateral sizes of 4-5 μm. The fluorescence of N-CDs can be efficiently quenched by hexagonal β-Co(OH)2 nanoplates through Förster resonance energy transfer mechanism. The maximum quenching-recovery capability can be achieved when the concentrations of N-CDs-Apt and β-Co(OH)2 nanoplates are 150 nmol/L and 100 μg/mL, respectively. The pH of the TE buffer should be 8.0, and the incubation time should be 10 min at 25 °C. The developed fluorescent aptasensor displayed an excellent selectivity for PAT determination with a detection limit of 0.57 pg/mL in the linear range of 1.25 pg/mL-100 ng/mL. The rapid PAT determination in fruit juice samples was realized with good recoveries (96.9-105.8%). The developed fluorescent aptasensor based on the interaction between N-CDs and hexagonal β-Co(OH)2 nanoplates can be a promising method for the rapid and ultrasensitive detection of PAT in agricultural products.

Electrochemical Determination of Hazardous Herbicide Diuron Using MWCNTs-CS@NGQDs Composite-Modified Glassy Carbon Electrodes

Diuron (DU) abuse in weed removal and shipping pollution prevention always leads to pesticide residues and poses a risk to human health. In the current research, an innovative electrochemical sensor for DU detection was created using a glassy carbon electrode (GCE) that had been modified with chitosan-encapsulated multi-walled carbon nanotubes (MWCNTs-CS) combined with nitrogen-doped graphene quantum dots (NGQDs). The NGQDs were prepared by high-temperature pyrolysis, and the MWCNTs-CS@NGQDs composite was further prepared by ultrasonic assembly. TEM, UV-Vis, and zeta potential tests were performed to investigate the morphology and properties of MWCNTs-CS@NGQDs. CV and EIS measurements revealed that the assembly of MWCNTs and CS improved the electron transfer ability and effective active area of MWCNTs. Moreover, the introduction of NGQDs further enhanced the detection sensitivity of the designed sensor. The MWCNTs-CS@NGQDs/GCE electrochemical sensor exhibited a wide linear range (0.08~12 μg mL-1), a low limit of detection (0.04 μg mL-1), and high sensitivity (31.62 μA (μg mL-1)-1 cm-2) for DU detection. Furthermore, the sensor demonstrated good anti-interference performance, reproducibility, and stability. This approach has been effectively employed to determine DU in actual samples, with recovery ranges of 99.4~104% in river water and 90.0~94.6% in soil. The developed electrochemical sensor is a useful tool to detect DU, which is expected to provide a convenient and easy analytical technique for the determination of various bioactive species.

Dual-quenching effects of methylene blue on the luminophore and co-reactant: Application for electrochemiluminescent-electrochemical ratiometric zearalenone detection

Methylene blue (MB) is a common multifunctional indicator, which can be applied as a quencher for electrochemiluminescence (ECL) analysis as well as a classical redox probe. Although it is relatively prevalent for MB to study the mechanism with Ru-based luminophores in ECL systems, there are few studies on the effects between MB and co-reactants. In this work, we proposed the first investigation of MB on the luminophore and co-reactant of the self-enhanced ECL composites (nitrogen-doped graphene quantum dots on Ru(bpy)₃²⁺-doped silica nanoparticles, NGQDs-Ru@SiO₂), respectively. The relatively narrow ECL spectrum of luminophore (Ru@SiO₂) and the suitable ultraviolet-visible absorption spectrum of MB led to the ECL resonance energy transfer between them, meanwhile the appropriate energy levels among them facilitated the electron transfer, resulting in a decreased ECL signal (quench mode I). Additionally, the co-reactant (NGQDs) was prone to π-π conjugation with MB due to its abundant π-electrons, which reduced the concentration of NGQDs' intermediates and triggered a weakened ECL signal (quench mode II). Therefore, the dual-quenching effects are ingeniously integrated and designed in one ECL-electrochemical (ECL-EC) ratiometric aptasensor for zearalenone detection, for demonstrating its efficacy in enhancing the sensitivity, which is 4.8-fold higher than Ru@SiO₂ alone. This innovative ratiometric aptasensor achieved a relatively wide linear range from 1.0 × 10^-15 to 5.0 × 10^-8 g mL^-1, and obtained a low detection limit of 8.5 × 10^-16 g mL^-1. Our proposed dual-quenching interactions between MB and NGQDs-Ru@SiO₂ will open a new prospective for ECL-EC ratiometric aptasensor, which further broaden the application in sensitive and precise analysis of mycotoxins.

The role of doping strategy in nanoparticle-based electrochemiluminescence biosensing

Doping plays a crucial role in electrochemiluminescence (ECL) due to the followings: (1) Modulation of electronic structure, alteration of the surface state of nanoparticles (NPs), providing effective protection from the surrounding environment, thereby leading to ECL emitters with exceptional properties including tunable spectra, high luminescence efficiency, low excitation potential, and good stability. (2) Employment of doped NPs as promising coreactant alternatives due to the presence of functional groups such as amines induced by NP doping. (3) Serving as novel co-reaction accelerators (CRAs) for ECL through doping induced high catalytic properties. (4) Behaving as excellent carriers to load ECL emitters, recognition elements, and catalysts due to doping-induced larger surface area, higher conductivity and better biocompatibility of NPs. As a consequence, doped NPs have aroused broad interest and found wide applications in various ECL sensing platforms. In this review, the current promising improvements, concepts, and excellent applications of doped NPs for ECL biosensing are addressed. We aim to bring to light the physicochemical characteristics of various doped NPs that endow them with appealing ECL performance, leading to diverse applications in biosensing.

Au-doped MOFs catalyzed electrochemiluminescence platform coupled with target-induced self-enrichment for detection of synthetic cannabinoid RCS-4

A ternary composite material with Au, Co-based organic frameworks (ZIF-67) and perylene derivatives (PTCD-cys) has been synthesized for identification of synthetic cannabinoids. Through contact with Au-S, Au-ZIF-67 increased electrochemiluminescence (ECL) sensitivity and stability and efficiently catalyzed the ECL of PTCD-cys. Compared with the ECL response of PTCD-cys monomer, the ECL signal value of the composite material was significantly increased, and the onset potential of Au-ZIF-67/PTCD-cys favorably shifted more than that of PTCD-cys/GCE. When the target cannabinoid molecule RCS-4 appeared, Au-ZIF-67 captured and immobilized it on the sensor surface by adsorption to achieve target-induced self-enrichment of RCS-4. Under optimal conditions, the ECL sensor was found to be linearly related to the logarithm of the RCS-4 concentration ranging from 3.1 × 10^-15 to 3.1 × 10^-9 mol/L with a detection limit (LOD) of 6.0 × 10^-16 mol/L (S/N = 3). The approach had the advantages of being simple to use, having a high sensitivity, a wide detection range, and good stability, making it a novel platform for RSC-4 detection in public health safety monitoring.

Shared-cathode closed bipolar electrochemiluminescence cloth-based chip for multiplex detection

Electrochemiluminescence (ECL) chips have been widely used in the field of medical diagnosis. However, most of these chips currently in use are costly and require high amounts of sample. In this work, we present, for the first time, a shared-cathode closed bipolar electrochemiluminescence (SC-CBP-ECL) cloth-based chip, which can be used for multiplex detection. The SC-CBP-ECL chips ($0.03-0.05 for each chip) are manufactured using carbon ink- and wax-based screen-printing techniques, without the need for expensive and complex fabrication equipment. Under optimised conditions, the SC-CBP-ECL chips were successfully used for coinstantaneous detection of glucose in double ECL systems (i.e., Ru(bpy)₃²⁺ and luminol), with corresponding linear ranges of 0.05-1 mM and 0.05-10 mM, and detection limits of 0.0382 mM and 0.0422 mM. To our knowledge, this is the first report on the application of fibre material-based closed bipolar electrodes (C-BPE) combined with double ECL systems. Furthermore, the SC-CBP-ECL chips exhibit an acceptable specificity and good reproducibility and stability and can be used for glucose detection in human serum samples with a good agreement compared with the clinical method. Finally, the SC-CBP-ECL chips could be successfully used for simultaneous detection of seven glucose samples and also show potential for simultaneous detection of three different targets (hydrogen peroxide [H₂O₂], glucose, and uric acid [UA]). Therefore, we believe that the chip described in this study has broad potential application in the field of cost-effective multiplex detection.

A sensitive electrochemiluminescence biosensor for assay of cancer biomarker (MMP-2) based on NGQDs-Ru@SiO2 luminophore

A sensitive biosensor that can be used for the determination of matrix metalloproteinase 2 (MMP-2) was proposed. The biosensor was developed by using an excellent self-enhanced nanocomposites as an illuminant and a peptide as a recognition element. For the electrostatic attraction between Ru(bpy)₃²⁺ and nitrogen-doped graphene quantum dots (NGQDs), the self-enhanced electrochemiluminescence (ECL) nanocomposites of NGQDs-Ru(bpy)₃²⁺-doped silica nanoparticles (NGQDs-Ru@SiO₂) were synthesized through a simple sol-gel process. Then, a specific peptide (labeled sulfhydryl) was combined with the self-enhanced ECL nanocomposites (carboxyl in NGQDs) via acylation reaction to obtain the peptide-NGQDs-Ru@SiO₂ nanoprobe, which was fabricated onto the gold electrode surface via Au-S bond. The peptide of the ECL nanoprobe was exposed to cleavage in the presence of MMP-2, which caused the signal substance to move farther away from the electrode, leading to a decrease of the ECL signal. The proposed NGQDs-Ru@SiO₂-labeled peptide ECL biosensor displayed a lower detection limit of 6.5 pg mL^-1 than those of reported ECL methods. The proposed biosensor provided an outlook for future applications in other disease-associated biomarkers.

An intermolecular hydrogen-bond-induced quench-type Ru(dcbpy)32+/TPA electrochemiluminescence system by nitrogen-doped carbon quantum dots

Here, we show that nitrogen-doped carbon quantum dots (NCQDs) strongly inhibits the anodic electrochemiluminescence (ECL) signal of a tris(4,4'-dicarboxylic acid-2,2'-bipyridyl) ruthenium(II) (Ru(dcbpy)₃²⁺)/tripropylamine (TPA) aqueous system. To determine the ECL-quenching mechanism, we used photoluminescence spectroscopy, UV-Visible absorption spectroscopy and dynamic simulation technology. Quenching of the ECL signal of Ru(dcbpy)₃²⁺/TPA by NCQDs was predominantly attributed to the interaction between Ru(dcbpy)₃²⁺ and NCQDs rather than that between TPA and NCQDs. Specifically, when Ru(dcbpy)₃²⁺ and NCQDs were in aqueous solution together, the carboxyl (-COOH) groups of Ru(dcbpy)₃²⁺ were in contact with oxygen- and nitrogen-containing groups on the surface of NCQDs and formed intermolecular hydrogen bonds. This process involved energy transfer from the excited-state Ru(dcbpy)₃²⁺ to the intermolecular hydrogen bonds, thus resulting in a decrease in the Ru(dcbpy)₃²⁺ ECL signal. On this basis, a quenching-type ECL sensor for the quantification of NCQDs was fabricated. The sensor had a wide linear range and an estimated detection limit of 0.0012 mg mL^-1, as well as excellent stability and selectivity. Satisfactory recoveries of 97.0-99.5% were obtained using the ECL sensor to quantify NCQDs in tap water. NCQDs could potentially be used as a quenching probe of Ru(dcbpy)₃²⁺ to construct various biosensors with widespread applications in the sensing field.

Induced self-enhanced electrochemiluminescence aptamer sensor for 17β-estradiol detection based on nitrogen-doped carbon quantum dots as Ru(dcbpy)32+ coreactant: What role of intermolecular hydrogen bonds play in the system?

Herein, an induced self-enhanced electrochemiluminescence (ECL) sensor with superior ECL performances was simply fabricated by just dropping the ECL reagent (tris(4,4'-dicarboxylicacid-2,2'-bipyridyl) ruthenium (II) dichloride, Ru(dcbpy)₃Cl₂) and coreactant (nitrogen-doped carbon quantum dots, NCQDs) pair onto the surface of glassy carbon electrode. In this strategy, based on the carboxyl (-COOH) groups in Ru(dcbpy)₃²⁺ and oxygen, nitrogen-containing groups on NCQDs surface, an intermolecular hydrogen bonds-induced self-enhanced ECL composite was generated in the solid contact layer for the first time. Since Ru(dcbpy)₃²⁺ and NCQDs were co-existing in the same composite, the electron-transfer distance between them was shortened and the energy loss was decreased, thereby higher ECL efficiency was acquired. This working process greatly avoided the introduction of signal amplifier and simplified the experimental operation. On this basis, 17β-estradiol (E2) was selected as a target model to fabricate a self-enhanced ECL aptamer sensor for the investigation of its analytical performances. Resultantly, excellent detection properties of E2, including wider linear range of 1.0 × 10^-14 - 1.0 × 10^-6 mol L^-1 and lower detection limit of 1.0 × 10^-15 mol L^-1 with superior selectivity, were successfully achieved. Finally, E2 spiked into milk powder was quantified to assess the practicability of this sensor. Prospectively, this strategy could be extensively applied for other analytes determination by adjusting the corresponding target aptamers.

A simple and convenient electrochemiluminescence immunoassay by using gold nanoparticles as both label and co-reactant

A simple and convenient electrochemiluminescence immunoassay (ECLIA) has been developed by using gold nanoparticles (AuNPs) as both label and co-reactant. In this novel ECLIA, the detection antibodies labeled with AuNPs as co-reactant reacts directly with ruthenium complex to produce electrochemiluminescence (ECL), which avoids the weakness of luminophore as a label. The feasibility of the new method was investigated through the sandwich ECL immunosensor for the determination of human immunoglobulin (HIgG), and excitation of luminescence was respectively driven by linear sweep voltammetry and step potential. Under the optimal conditions, ECL intensity of the immunosensor increased with HIgG concentration in a wide range from 0.01 to 10.0 ng/mL and displayed linear response to logarithm of HIgG concentration. The results showed that the detection limit was 5 pg/mL, and the relative error was no more than 4.6% for the repeated measurements. The relative standard deviation was less than 2.9% for the determination of the standard sample, which can meet the requirement of ECLIA.

An ultrasensitive electrochemiluminescence aptasensor for the detection of diethylstilbestrol based on the enhancing mechanism of the metal-organic framework NH2-MIL-125(Ti) in a 3,4,9,10-perylenetetracarboxylic acid/K2S2O8 system

In this work, a sensitive and selective electrochemiluminescent aptasensor was proposed based on the enhancing mechanism of the metal-organic framework NH₂-MIL-125(Ti) in a 3,4,9,10-perylenetetracarboxylic acid/K₂S₂O₈ system for a diethylstilbestrol assay. Herein, 3,4,9,10-perylenetetracarboxylic acid was selected as the major luminophore, and the metal-organic framework NH₂-MIL-125(Ti) displayed a large specific surface area to immobilize abundant PTCA molecules to facilitate electrochemiluminescence efficiency. Besides, the metal-organic framework NH₂-MIL-125(Ti) was used as a novel catalyst in the 3,4,9,10-perylenetetracarboxylic acid/K₂S₂O₈ system, which could react with the co-reactant K₂S₂O₈ to produce more SO₄˙^-. In addition, we introduced the amino-aptamer of diethylstilbestrol; due to the specific binding affinity between the aptamer and diethylstilbestrol, a selective electrochemiluminescent aptasensor for diethylstilbestrol was thus developed here. Under the optimal conditions, a wide detection range from 1.0 fM to 1.0 μM with a low detection limit of 0.28 fM (S/N = 3) was obtained. More importantly, the residual diethylstilbestrol in water was detected by the developed aptasensor; this confirmed that this method has good performance and potential applications in real samples.

Highly Luminescent and Self-Enhanced Electrochemiluminescence of Tris(bipyridine) Ruthenium(II) Nanohybrid and Its Sensing Application for Label-Free Detection of MicroRNA

Inspired by the coreactive activity of carbon nanodots (CDs) and branched polyethylenimine (BPEI) toward electrochemiluminescence (ECL) of Ru(bpy)₃²⁺, a highly luminescent and self-enhanced ECL nanohybrid (Ru-BCDs) was synthesized through covalently linking BPEI-coated carbon dots (BCDs) with Tris (4,4'-dicarboxylic acid-2,2'-bipyridyl) ruthenium(II) dichloride (Ru(dcbpy)₃²⁺). The composition and morphological characterization demonstrated that the spherical Ru-BCDs particles with 12.1 ± 1.4 nm diameter were obtained. The enhanced ECL property of Ru-BCDs was proved to originate from the dual coreactive contribution of BPEI and CDs as coreactants as well as the intramolecular electron transfer process, which could shorten the electron transfer path and minimize energy loss. A carbon nitride nanosheet (CNN) was utilized to stabilize the Ru-BCDs-modified glassy carbon electrode, which greatly improved the stability of solid-state ECL. By utilizing the affinity discrepancy of the CNN to single-stranded and double-stranded nucleic acids, a label-free and signal-on ECL biosensor was constructed for the determination of microRNA-133a (miR-133a), a potential biomarker of acute myocardial infarction. The designed biosensor exhibited good performance of miR-133a detection with a detection limit of 60 fM and could be used for the detection of real human serum with satisfactory results. The self-enhanced ECL nanohybrid with distinguished ECL efficiency holds a promising prospect in biosensing and bioimaging applications.

Monitoring zearalenone in corn flour utilizing novel self-enhanced electrochemiluminescence aptasensor based on NGQDs-NH2-Ru@SiO2 luminophore

Accurate and early diagnosis of mycotoxin is particularly significant to the food and agricultural product safety. In the present work, a sensitive and effective monitoring method for zearalenone (ZEN) was exploited based on a novel self-enhanced electrochemiluminescence (ECL) aptasensor. The self-enhanced lumonophore was compounded by electrostatically combining amine-functionalized Ru(bpy)₃²⁺-doped silica nanoparticles (NH₂-Ru@SiO₂ NPs) and nitrogen doped graphene quantum dots (NGQDs) together. Since the emitter and co-reactant simultaneously existed in the same nanoparticle, shortened electron-transfer distance and decreased energy loss was obtained. Therefore, self-enhanced ECL aptasensor based on the novel complex expressed the widest linear range of 10 fg mL^-1-10 ng mL^-1 and the lowest detection limit of 1 fg mL^-1 for ZEN detection. More importantly, ZEN produced during the mildew process of corn flour was monitored by the developed aptasensor, which exhibited superior determination and potential application in real samples.