Recent Advances Based on Nanomaterials as Electrochemiluminescence Probes for the Fabrication of Sensors

Carboxyl groups on carbon nanodots as co-reactant sites for anodic electrochemiluminescence of tris(2,2-bipyridine)ruthenium(II)

Carbon nanodots (C-dots) with good biocompatibility have been extensively utilized as co-reactants for electrochemiluminescence (ECL) of the tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)32+) system. However, the ECL intensity of this system is still relatively low and the mechanism of C-dots as co-reactants remains unclear, which greatly limits its further application in bio-analysis. In this work, we revealed that the carboxyl groups on C-dots are co-reactant sites for Ru(bpy)32+ ECL by systematically investigating the contribution of carboxyl, hydroxyl and carbonyl groups on the surface of C-dots to the ECL intensity. Further treatment with hydrogen peroxide to increase the carboxyl-group content on C-dots resulted in a 10-fold increase in ECL intensity over the original Ru(bpy)32+/C-dots system. This work provides new insights for the rational design of ECL systems with C-dots as co-reactants and offers new chances for further applications of C-dots in the field of ECL.

Luminophore-Surface-Engineering-Enabled Low-Triggering-Potential and Coreactant-Free Electrochemiluminescence for Protein Determination

Coreactant-free electrochemiluminescence (ECL) is promising for removing the exogenous effects of coreactant and simplify the operation procedures and setups of commercialized ECL bioassays. Herein, an electrosterically involved strategy for achieving a low-triggering-potential (+0.21 V vs Ag/AgCl) and coreactant-free ECL from dual-stabilizer-capped CdTe nanocrystals (NCs) is proposed with mercaptopropionic acid (MPA) and hexametaphosphate (HMP) as the capping agents of luminophores. Upon employing the CdTe NCs as the ECL tag for the immunoassay, all the tags in the bioconjugates of the CdTe NCs and the secondary antibody (Ab₂|CdTe) as well as in the final achieved sandwich-type immunocomplexes can exhibit efficient coreactant-free ECL with an electrosterically involved triggering potential nature. The bioconjugates of Ab₂|CdTe with Ab₂ no more than 30 kDa, such as the thyroid stimulating hormone (30 kDa) and the recombinant pro-gastrin releasing peptide (ProGRP, 14 kDa), merely exhibit coreactant-free ECL around +0.24 V, while bioconjugates of Ab₂|CdTe with an Ab₂ beyond 30 kDa only give off coreactant-free ECL around +0.82 V. Due to the further enhanced electrosteric effect in sandwich-type immunocomplexes, only the ECL immunosensor with ProGRP as the target can give off coreactant-free ECL around +0.24 V. The electrosterically involved and coreactant-free ECL of CdTe NCs is consequently utilized to sensitively and selectively determine the molecular protein ProGRP, which demonstrates a wide linearity range from 0.1 to 2000 pg/mL and a low limit of detection at 0.05 pg/mL (S/N = 3). This low-triggering-potential and coreactant-free combined ECL platform indicates that engineering the surface of CdTe NCs with a protein can improve the performance of ECL tags in a protein-weight-involved electrosterical way.

Coreactant-free electrochemiluminescence of polyfluorene nanoparticle coupling double quencher for β-amyloid1-42 detection

β-amyloid_1-42 (Aβ_1-42) is a humoral biomarker for early diagnosis of Alzheimer's disease (AD), and exists at a low level in human body. Its sensitive detection is very valuable. The electrochemiluminescence (ECL) assay of Aβ_1-42 has attracted special attention owing to high sensitivity and simple operation. However, currently reported ECL assays for Aβ_1-42 usually required the introduction of exogenous coreactants to improve the detection sensitivity. Introducing exogenous coreactants will lead to non-negligible repeatability and stability problems. This work exploited poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1',3}-thiadazole)] nanoparticles (PFBT NPs) as coreactant-free ECL emitters for detecting Aβ_1-42. The PFBT NPs, first antibody (Ab₁) and antigen Aβ_1-42 were successively assembled on glassy carbon electrode (GCE). Silica nanoparticles served as a carrier to grow polydopamine (PDA) in situ, and further assembled Au nanoparticles (Au NPs) and second antibody (Ab₂), producing the secondary antibody complex (SiO₂@PDA-Au NPs-Ab₂). With its assembly on the biosensor, the ECL signal decreased since both PDA and Au NPs could quench ECL emission from PFBT NPs. The limit of detection (LOD) of 0.55 fg/mL and limit of quantification (LOQ) of 37.45 fg/mL for Aβ_1-42 were obtained. PFBT NPs coupling dual-quencher PDA-Au NPs created an excellent ECL system for bioassays, and constructed a sensitive analytical method for Aβ_1-42.

NaBiF4 upconversion nanoparticle-based electrochemiluminescent biosensor for E. coli O157 : H7 detection

Foodborne or water-borne pathogens pose great threats to human beings and animals. There is an urgent need to detect pathogens with cheap, rapid and sensitive point-of-care diagnostic assays. Herein, we report the electrochemiluminescent (ECL) behaviors of NaBiF₄ : Yb³⁺/Er³⁺ upconversion nanoparticles (UCNPs) which were synthesized via a fast and environment-friendly method at room temperature for the first time. The UCNPs together with K₂S₂O₈ exhibit high ECL intensity and stable cathodic signals. Further, the Au nanoparticles (Au NPs) and Anti-E. coli O157 : H7 antibody were assembled on the surface of UCNPs successively to construct a novel ECL immunosensor for the detection of deadly E. coli O157 : H7. The as-prepared ECL immunosensor reveals high sensitivity to E. coli O157 : H7 in a linear range of 200–100 000 CFU mL⁻¹, and the minimum detection limit could reach up to 138 CFU mL⁻¹. The designed UCNP-based biosensor demonstrates high specificity, good stability and remarkable repeatability, and the strategy will provide a sensitive and selective method for rapid detection of E. coli O157 : H7 in food safety and preclinical diagnosis.

The ECL behaviors of NaBiF₄ : Yb³⁺/Er³⁺ UCNPs synthesized via a fast and environment-friendly method are reported for the first time. UCNPs-based ECL biosensor shows a wide detection range with low detection limit of 138 CFU mL⁻¹ for E. coli O157 : H7.

Quantum dots for electrochemiluminescence bioanalysis - A review

Electrochemiluminescence (ECL) bioanalysis has become increasingly important in various fields from bioanalysis to clinical diagnosis due to its outstanding merits, including low background signal, high sensitivity, and simple instrumentation. Quantum dots (QDs) are a significant theme in ECL bioanalysis since their excellent optical, electrochemical properties, and ease of functionalization endow them with versatile roles and new mechanisms of signal transduction in ECL. Herein, this review details recent advances of QDs-based ECL bioanalysis by using QDs as ECL emitters, coreactants, or ECL resonance energy transfer donors/acceptors, mainly focused on their optical and electrochemical properties and ECL reaction mechanism. In the end, we will discuss the current limitations and future developments in QDs ECL bioanalysis to address the requirement about selectivity, sensitivity, toxicity, and emerging applications.

A Flexible Electrochemiluminescence Sensor Equipped With Vertically Ordered Mesoporous Silica Nanochannel Film for Sensitive Detection of Clindamycin

Fast, convenient, and highly sensitive detection of antibiotic is essential to avoid its overuse and the possible harm. Owing to enrichment effect and antifouling ability of ultrasmall nanochannels, the vertically ordered mesoporous silica nanochannel film (VMSF) has great potential in the development of the facile electrochemiluminescence (ECL) sensor for direct and sensitive analysis of antibiotics in complex samples. In this study, we demonstrated a flexible ECL sensor based on a cost-effective electrode covered with a VMSF for sensitive detection of clindamycin. Polyethylene terephthalate coated with indium tin oxide (PET-ITO) is applied as a flexible electrode to grow VMSF using the electrochemically assisted self-assembly (EASA) method. The negatively charged VMSF nanochannels exhibit significant enrichment toward the commonly used cationic ECL luminophores, tris(2,2-bipyridyl) dichlororuthenium (II) (Ru (bpy)₃²⁺). Using the enhanced ECL of Ru (bpy)₃²⁺ by clindamycin, the developed VMSF/PET-ITO sensor can sensitively detect clindamycin. The responses were linear in the concentration range of 10 nM–25 μM and in the concentration range of 25–70 μM. Owing to the nanoscale thickness of the VMSF and the high coupling stability with the electrode substrate, the developed flexible VMSF/PET-ITO sensor exhibits high signal stability during the continuous bending process. Considering high antifouling characteristic of the VMSF, direct analysis of clindamycin in a real biological sample, human serum, is realized.

A solid-state electrochemiluminescence aptasensor for β-lactoglobulin using Ru-AuNP/GNP/Naf nanocomposite-modified printed sensor

An electrochemiluminescence (ECL) aptasensor for the detection of the milk protein allergen β-lactoglobulin (β-LG) using nanocomposite as luminophore was fabricated. The Ru-AuNPs/GNP/Naf complex was formed by combining the Rubpy₃²⁺-AuNPs complex (Ru-AuNPs), prepared by modifying the negatively charged surface of gold nanoparticles (AuNPs) with positively charged Rubpy₃²⁺ through electrostatic interactions and the graphene nanoplatelets-Nafion (GNP/Naf) at a ratio of 2:1. The nanocomposite was coated on the surface of the screen-printed electrode (SPCE) through the film-forming properties of Nafion. A layer of chitosan (CS) was coated onto this modified electrode, and later amine-terminated β-LG aptamers were covalently attached to the CS/Ru-AuNP/GNP/Naf via glutaraldehyde (GLUT) cross-linking. When β-LG was incubated with the aptasensor, a subsequent decrease in ECL intensity was recorded. Under the optimal conditions, the ECL intensity of the aptasensor changed linearly with the logarithmic concentration of β-LG, in the range 0.1 to 1000 pg/ml, and the detection limit was 0.02 pg/mL (3σ/m). The constructed aptasensor displayed simple and fast determination of β-LG with excellent reproducibility, stability, and high specificity. Additionally, the proposed ECL aptasensor displayed high recoveries (92.5-112%) and low coefficients of variation (1.6-7.8%), when β-LG fortified samples were analyzed. Integrating Ru-AuNPs/GNP/Naf nanocomposite in the ECL aptasensor paves the way towards a cost-effective and sensitive detection of the milk allergen β-LG.

Electrochemiluminescent determination of CYFRA21-1 serum levels using Ti-Fe-O nanotubes immunoassay

Prominent electrochemiluminescence (ECL) in Ti-Fe-O nanotube arrays (Ti-Fe-O NTs) with K₂S₂O₈ as the cathode coreactant is reported for the first time. Compared with pure titanium dioxide nanotubes (TiO₂ NTs), this heterojunction could effectively reduce the band gap, facilitate electronic transitions, and move the ECL potential to a positive direction. The ECL performance motivated the development of an ultrasensitive ECL immunosensor for detecting cytokeratin fragment 21-1 (CYFRA21-1). Magnetic beads loaded with conductive carbon black (CCB/MNTs) were used to efficiently quench the ECL signal of a Ti-Fe-O NTs electrode and were combined with an ECL immunoassay to realize sensitive detection of CYFRA21-1. Over a CYFRA21-1 concentration range of 1.0 pg·mL^-1 ~ 100 ng·mL^-1, the change in the ECL signal was highly linear with the logarithm of the CYFRA21-1 concentration, and the limit of detection (LOD) was 0.114 pg·mL^-1. This ECL immunosensor was used to successfully determine the CYFRA21-1 content in serum. The recovery of CYFRA21-1 in actual serum was 88.6 - 104.4%, and the RSD was 1.4 - 3.0%. The coreaction solution used in this work was PBS (0.1 M, pH = 7.4) containing 0.05 M K₂S₂O₈, the scanning range was -1.0 - 0 V, the photomultiplier tube (PMT) was set to 750 V, and the scanning rate was 100 mV·s^-1.

An ultra-high-sensitivity electrochemiluminescence aptasensor for Pb2+ detection based on the synergistic signal-amplification strategy of quencher abscission and G-quadruplex generation

Signal amplification provides an effective way to improve detection performance. Herein, an ultrasensitive electrochemiluminescence (ECL) aptasensor for Pb²⁺ detection was developed based on a dual signal-amplification strategy of the abscission of a quencher and the generation of a G-quadruplex by one-step and simultaneous way. Nitrogen-doped carbon quantum dots linked with complementary DNA (cDNA-NCQDs) at the sensing interface was applied as the quencher of a tris(4,4'-dicarboxylic acid-2,2'-bipyridyl)ruthenium(II) (Ru(dcbpy)₃²⁺)/tripropylamine system to minimize the ECL signal due to the intermolecular hydrogen bond-induced energy-transfer process. Upon the addition of Pb²⁺, its specific binding with the aptamer triggered the abscission of cDNA-NCQDs, accompanied by the formation of G-quadruplex on the surface of the electrode, both of which amplified the intensity of the light emission. The ECL amplification efficiency induced by the above two mechanisms (78.6%) was valuably greater than that of their sum value (69.3%). This synergistic effect resulted in high detection sensitivity of the ECL aptasensor, which allowed to thereby obtain Pb²⁺ measurements in the range of 1 fM - 10 nM with an ultra-low detection limit of 0.19 fM. The Pb²⁺-mediated synergistic signal-amplification ECL strategy can provide a new approach for integrating various amplification strategies.

Facile synthesis of nitrogen-doped graphene quantum dots as nanocarbon emitters for sensitive detection of catechol

Development of nanomaterial-based electrochemiluminescence (ECL) emitters is highly desirable for the fabrication and wide applications of ECL sensors. Herein, nitrogen-doped graphene quantum dots (NGQDs) were easily synthesized as nanocarbon emitters with anodic ECL for sensitive ECL determination of catechol (CC). Facile synthesis of NGQDs was easily achieved using molecular fusion of a carbon precursor in a one-step hydrothermal process. The synthesis has advantages of simple and convenient operation and high yield. The as-prepared NGQDs have uniform size, good crystallinity, single-layered graphene structure, and excitation-independent fluorescence. In the presence of hydrogen peroxide (H₂O₂), NGQDs exhibit high anodic ECL owing to the presence of functional hydrazide groups. As CC could significantly reduce the ECL intensity of NGQDs, sensitive determination of CC was realized with a linear range from 100 nM to 10 μM and 10 μM to 60 μM with a low limit of detection (LOD, 42 nM). The determination of CC in environmental water was also achieved with high reliability.

Nitrogen-doped graphene quantum dots are easily synthesized and can be used as nanocarbon emitters for sensitive electrochemiluminescence detection of catechol.

Hydrogen Bond Organic Frameworks as a Novel Electrochemiluminescence Luminophore: Simple Synthesis and Ultrasensitive Biosensing

Nowadays, continuous efforts have been devoted to searching highly efficient electrochemiluminescence (ECL) emitters for applications in clinical diagnosis and food safety. In this work, triazinyl-based hydrogen bond organic frameworks (Tr-HOFs) were synthesized by N···H hydrogen bond self-assembly aggregation, where 6,6'-(1,4-phenylene)bis(1,3,5-triazine-2,4-diamine) (phenyDAT) was prepared via the cyclization reaction and behaved as a novel ligand. Impressively, the resulting Tr-HOFs showed strong ECL responses with highly enhanced ECL efficiency (21.3%) relative to the Ru(bpy)₃²⁺ standard, while phenyDAT hardly showed any ECL emission in aqueous phase. The Tr-HOFs innovatively worked as a new ECL luminophore to construct a label-free biosensor for assay of kanamycin (Kana). Specifically, the ECL response greatly weakened upon assembly of captured DNA with ferrocene (cDNA-Fc) onto the Tr-HOFs-modified electrode, while the ECL signals were adversely recovered by releasing linked DNA (L-DNA) from double-stranded DNA (dsDNA, hybridization of aptamer DNA (aptDNA) with L-DNA) due to the specific recognition of Kana with the aptDNA combined by the linkage of L-DNA and cDNA-Fc on the electrode. The as-built sensor showed a broadened linear range (1 nM-10 μM) and a limit of detection (LOD) down to 0.28 nM, which also displayed satisfactory results in the analysis of Kana in the milk and diluted human serum samples. This work offers a novel pathway to design an ECL emitter with organic molecules, holding great promise in biomedical analysis and food detection.

Low-Triggering-Potential Electrochemiluminescence from Surface-Confined CuInS2@ZnS Nanocrystals and their Biosensing Applications

Electrochemiluminescence (ECL) of low triggering potential is strongly anticipated for ECL assays with less inherent electrochemical interference and improved long-term stability of the working electrode. Herein, effects of the thiol capping agents and the states of luminophores, i.e., the thiol-capped CuInS₂@ZnS nanocrystals (CuInS₂@ZnS-Thiol), on the ECL triggering potential of CuInS₂@ZnS-Thiol/N₂H₄·H₂O were explored on the Au working electrode. The thiol capping agent of glutathione (GSH) not only enabled CuInS₂@ZnS-Thiol/N₂H₄·H₂O with the stronger oxidative-reduction ECL than other thiol capping agents but also demonstrated the largest shift for the ECL triggering potential of CuInS₂@ZnS-Thiol/N₂H₄·H₂O upon changing the luminophores from the monodispersed state to the surface-confined state. CuInS₂@ZnS-GSH/N₂H₄·H₂O exhibited an efficient oxidative-reduction ECL around 0.78 V (vs Ag/AgCl) with CuInS₂@ZnS-GSH of the monodispersed state. Upon employing CuInS₂@ZnS-GSH as the ECL tag and immobilizing them onto the Au working electrode, the oxidative-reduction ECL of CuInS₂@ZnS-GSH/N₂H₄·H₂O was lowered to 0.32 V (vs Ag/AgCl), which was about 0.88 V lower than that of traditional Ru(bpy)₃²⁺/TPrA (typically ∼1.2 V, vs Ag/AgCl). The ECL of the CuInS₂@ZnS-GSH/N₂H₄·H₂O system with the luminophore of both monodispersed and surface-confined states was spectrally identical to each other, indicating that this surface-confining strategy exhibited negligible effect on the excited state for the ECL of CuInS₂@ZnS-GSH. A surface-confined ECL sensor around 0.32 V was fabricated with CuInS₂@ZnS-GSH as a luminophore, which could sensitively and selectively determine the K-RAS gene from 1 to 500 pM with a limit of detection at 0.5 pmol L^-1 (S/N = 3).

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.

Liposome-assisted enzyme catalysis: toward signal amplification for sensitive split-type electrochemiluminescence immunoassay

Developing an efficient signal amplification strategy is very important to improve the sensitivity of bioanalysis. In this paper, a liposome-assisted enzyme catalysis signal amplification strategy was developed for electrochemiluminescence (ECL) immunoassay of prostate specific antigen (PSA) in a split-type mode. The sandwich immunoreaction occurred in a 96-well plate, and glucose oxidase (GOx) encapsulated and antibody-modified liposomes were used as labels. The ECL detection was carried out using a rGO-Au NP modified glassy carbon electrode (GCE). The large amount of generated H2O2, i.e. the coreactant of the luminol system, and the excellent catalytic behavior of rGO-Au NPs greatly boosted the ECL signal, resulting in the signal amplification. The developed ECL immunosensor for detecting PSA achieved a wider linear range from 1.0 × 10-13 to 1.0 × 10-8 g mL-1 and a detection limit of 1.7 × 10-14 g mL-1. The application of the proposed strategy was demonstrated by analyzing PSA in human serum samples with recoveries from 89.0% to 113.0%, and relative standard deviations (RSDs) were less than 6.6%. This work provides a new horizon to expand the application of liposomes for ECL bioanalysis.

One-Step Preparation of Nitrogen-Doped Graphene Quantum Dots With Anodic Electrochemiluminescence for Sensitive Detection of Hydrogen Peroxide and Glucose

Simple and efficient synthesis of graphene quantum dots (GQDs) with anodic electrochemiluminescence (ECL) remains a great challenge. Herein, we present an anodic ECL-sensing platform based on nitrogen-doped GQDs (N-GQDs), which enables sensitive detection of hydrogen peroxide (H₂O₂) and glucose. N-GQDs are easily prepared using one-step molecular fusion between carbon precursor and a dopant in an alkaline hydrothermal process. The synthesis is simple, green, and has high production yield. The as-prepared N-GQDs exhibit a single graphene-layered structure, uniform size, and good crystalline. In the presence of H₂O₂, N-GQDs possess high anodic ECL activity owing to the functional hydrazide groups. With N-GQDs being ECL probes, sensitive detection of H₂O₂ in the range of 0.3–100.0 μM with a limit of detection or LOD of 63 nM is achieved. As the oxidation of glucose catalyzed by glucose oxidase (GOx) produces H₂O₂, sensitive detection of glucose is also realized in the range of 0.7–90.0 μM (LOD of 96 nM).

A Review on van der Waals Boron Nitride Quantum Dots

Boron nitride quantum dots (BNQDs) have gained increasing attention for their versatile fluorescent, optoelectronic, chemical, and biochemical properties. During the past few years, significant progress has been demonstrated, started from theoretical modeling to actual application. Many interesting properties and applications have been reported, such as excitation-dependent emission (and, in some cases, non-excitation dependent), chemical functionalization, bioimaging, phototherapy, photocatalysis, chemical, and biological sensing. An overview of this early-stage research development of BNQDs is presented in this article. We have prepared un-bias assessments on various synthesis methods, property analysis, and applications of BNQDs here, and provided our perspective on the development of these emerging nanomaterials for years to come.

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.

Recent Progress of Novel Electrochemiluminescence Nanoprobes and Their Analytical Applications

High-performance nanomaterials have been seen as a new generation of electrochemiluminescence (ECL) probes or emitters for their finely tunable structure and concomitant remarkable properties, guaranteeing the prospective applications in the analysis and diagnosis devices with superior performances. The structure-activity relationships of ECL nanoprobes in nanoscale are presenting milestone in understanding of the ECL microscopic behaviors and mechanisms, and guide the exploitation of novel ECL probes. In this mini-review, we summarized the recent development of novel ECL probes based on the nanomaterials. The mechanism and relationships between their structure as well as the active sites and functionality were revealed. In addition, the design and regulation of the ECL nanoprobes were emphasized for the biosensing and imaging application. Finally, the potential prospect of the ECL nanoprobes, design, and their applications were discussed.

A ratiometric electrochemiluminescence strategy based on two-dimensional nanomaterial-nucleic acid interactions for biosensing and logic gates operation

Two-dimensional (2D) nanomaterial-nucleic acid interactions have been widely used in the construction of fluorescent sensors, but they are rarely used in the construction of electrochemiluminescent (ECL) sensors and have never been used in the design of ratiometric ECL sensors. Therefore, a ratiometric ECL sensing platform was developed in this study based on the ECL resonance energy transfer (ECL-RET) of graphitic carbon nitride nanosheets (GCNNs)/Ru(bpy)₃²⁺ donor/acceptor pair. The 2D GCNNs showed much weaker affinity to the long dsDNA duplexes formed by hybridization chain reaction (HCR) than Ru(bpy)₃²⁺-lableled fuel hairpin DNAs (H1 and H2) for HCR. Therefore, the target-initiated HCR resulted in the luminescence enhancement of the GCNNs at 460 nm and the luminescence attenuation of the Ru(bpy)₃²⁺ at 610 nm. By measuring the I_{460 nm}/I_{610 nm} ratios, quantitative analysis of microRNA-133a was realized with a limit of detection of 0.41 pM. In addition, this ECL-RET sensing platform can be easily extended to detect metal ions or aptamer substrates by simply redesigning helper DNAs without changing the sequences of fuel hairpin DNAs. Moreover, due to the programmability of HCR, a series of sensitive logic gates ("OR", "INHIBIT", "AND", "NAND" and "INHIBIT-OR") based on the ECL-RET ratiometry can be constructed and responded to as low as 100 pM of Hg²⁺ or Ag⁺.

Enhanced Near-Infrared Electrochemiluminescence from Trinary Ag-In-S to Multinary Ag-Ga-In-S Nanocrystals via Doping-in-Growth and Its Immunosensing Applications

Screening toxic-element-free and biocompatible electrochemiluminophores was crucial for electrochemiluminescence (ECL) evolution. Herein, l-glutathione (GSH)-capped Ag-Ga-In-S (AGIS) nanocrystals (NCs) were prepared by doping Ag-In-S (AIS) NCs in a doping-in-growth way and utilized as a model for both ECL modulating and developing multinary NC-based electrochemiluminophores with enhanced ECL performance than trinary NCs. AGIS NCs not only primarily preserved the morphology, size, phase structure, and water monodisperse characteristics of AIS NCs with broadened band gap but also demonstrated obviously enhanced oxidative-reduction ECL than AIS NCs. Importantly, ECL of AGIS NCs was located at the near-infrared region with a maximum emission wavelength of 744 nm and could be utilized for an ECL immunoassay with human prostate-specific antigen (PSA) as a model, which exhibited a linearity range from 0.05 pg/mL to 1.0 ng/mL and a low limit of detection of 0.01 pg/mL (S/N = 3). This work provided a promising alternative to the traditional binary NCs for developing toxic-element-free and biocompatible electrochemiluminophores with efficient near-infrared ECL.

Sensitive and Programmable "Signal-Off" Electrochemiluminescence Sensing Platform Based on Cascade Amplification and Multiple Quenching Mechanisms

A versatile and sensitive quantum dot (QD)-based "signal-off" electrochemiluminescence (ECL) sensing system was constructed using target-initiated dual Mg²⁺-dependent DNAzyme (MNAzyme) recycling and catalytic hairpin assembly (CHA) amplification strategies. After the cascade amplification, numerous ferrocene-labeled Y-shaped DNA complexes generated on the QD-modified electrode surface. In the presence of hemin, moreover, the terminal sequence of the formed complex could assemble into hemin/G-quadruplex. Therefore, the highly efficient ECL quenching was achieved due to the multiple quenching mechanisms, including electron/energy transfer between ferrocene and QDs, the steric hindrance effects, and the horseradish peroxidase-mimicking activity of hemin/G-quadruplex. Furthermore, owing to the flexibility in regulating the recognition sequences of MNAzyme, the assaying targets can be programmed. Based on the cascade amplification and multiple ECL quenching mechanisms, the developed programmable "signal-off" ECL sensing platform demonstrates excellent sensitivity and the detection limits of 35.00 aM, 3.71 fM, and 0.28 pM (S/N = 3) for target DNA, aptamer substrate (ATP as a model), and ion (Ag⁺ as a model), respectively.

Progress in electrochemiluminescence of nanoclusters: how to improve the quantum yield of nanoclusters

Classification of nanoclusters and methods to improve their quantum yield and applications.

Recent Advances in Electrochemiluminescence and Chemiluminescence of Metal Nanoclusters

Metal nanoclusters (NCs), including Au, Ag, Cu, Pt, Ni and alloy NCs, have become more and more popular sensor probes with good solubility, biocompatibility, size-dependent luminescence and catalysis. The development of electrochemiluminescent (ECL) and chemiluminescent (CL) analytical methods based on various metal NCs have become research hotspots. To improve ECL and CL performances, many strategies are proposed, from metal core to ligand, from intermolecular electron transfer to intramolecular electron transfer. Combined with a variety of amplification technology, i.e., nanostructure-based enhancement and biological signal amplification, highly sensitive ECL and CL analytical methods are developed. We have summarized the research progresses since 2016. Also, we discuss the current challenges and perspectives on the development of this area.

Sensors and microarrays in protein biomarker monitoring: an electrochemical perspective spots

The development of clinically applicable portable sensors and multiplex protein biomarker assays is one of the most important goals of laboratory medicine today. Sensing strategies based on electrochemical devices are discussed in this overview, with special emphasis on detection principles derived from voltammetry, electrogenerated chemiluminescence, bipolar electrochemistry and impedance-based measurements. Up-to-date examples of electrochemical methods in biomedical research and development are highlighted here, including critical evaluation and future directions of the analysis, development and validation of new protein biomarkers.

Electrochemiluminescence Biosensors Using Screen-Printed Electrodes

Electrogenerated chemiluminescence (also called electrochemiluminescence (ECL)) has become a great focus of attention in different fields of analysis, mainly as a consequence of the potential remarkably high sensitivity and wide dynamic range. In the particular case of sensing applications, ECL biosensor unites the benefits of the high selectivity of biological recognition elements and the high sensitivity of ECL analysis methods. Hence, it is a powerful analytical device for sensitive detection of different analytes of interest in medical prognosis and diagnosis, food control and environment. These wide range of applications are increased by the introduction of screen-printed electrodes (SPEs). Disposable SPE-based biosensors cover the need to perform in-situ measurements with portable devices quickly and accurately. In this review, we sum up the latest biosensing applications and current progress on ECL bioanalysis combined with disposable SPEs in the field of bio affinity ECL sensors including immunosensors, DNA analysis and catalytic ECL sensors. Furthermore, the integration of nanomaterials with particular physical and chemical properties in the ECL biosensing systems has improved tremendously their sensitivity and overall performance, being one of the most appropriates research fields for the development of highly sensitive ECL biosensor devices.

Cathodic electrogenerated chemiluminescence of tris(2,2'-bipyridine)ruthenium(ii) and peroxydisulfate at pure Ti3C2Tx MXene electrodes

We demonstrate the first use of pure films of two-dimensional (2D) transition metal carbides and nitrides (Ti₃C₂T_x MXene) as an electrode material for electrogenerated chemiluminescence (ECL). The Ti₃C₂T_x MXene electrodes exhibited excellent electrochemical stability in the cathodic scan range and produced bright reductive-oxidation ECL using peroxydisulfate as a co-reactant with the tris(2,2'-bipyridine)ruthenium(ii) ([Ru(bpy)₃]²⁺) luminophore.

Electrochemically Lighting Up Luminophores at Similar Low Triggering Potentials with Mechanistic Insights

Electrochemiluminescence (ECL) with high electrode compatibility and less electrochemical interference has conventionally been envisioned by lowering the oxidative potential of luminophores and/or screening luminophores with a low oxidative potential. Herein, an alternative was developed by employing the environmental-friendly carbohydrazide as a coreactant, which enabled serial luminophores with oxidative-reduction ECL at one similar low triggering potential around 0.55 V versus Ag/AgCl, including Ru(bpy)₃²⁺ as well as CdTe, CdSe, CuInS₂/ZnS, and Au nanocrystals. Because the eight-electron releasing process of carbohydrazide was electrochemically triggered at ∼0.25 V versus Ag/AgCl, the radicals generated via electrochemical oxidation of carbohydrazide could reduce the luminophores at a much lower potential than those of traditional coreactants. All the luminophore/carbohydrazide systems exhibited one ECL process around 0.55 V, which was about 0.65 V lower than that of a traditional Ru(bpy)₃²⁺/tri-n-propylamine system (typically around +1.2 V), and even lower than the oxidative potential of some luminophores. The ECL of the luminophore/carbohydrazide system was spectrally close to that of the corresponding luminophore/tri-n-propylamine system; the maximum emission wavelength of the low triggering potential ECL could shift from 540 to 783 nm via the selection of luminophores in this case. The coreactant screening strategy would be a favorable addition to the expected luminophore screening strategy for achieving enhanced ECL performance. This work created an avenue toward a deeper understanding of the ECL mechanism.

Highly Selective Electrochemiluminescence Sensor Based on Molecularly Imprinted-quantum Dots for the Sensitive Detection of Cyfluthrin

A highly selective and sensitive molecularly imprinted electrochemiluminescence (MIECL) sensor was developed based on the multiwall carbon nanotube (MWCNT)-enhanced molecularly imprinted quantum dots (MIP-QDs) for the rapid determination of cyfluthrin (CYF). The MIP-QDs fabricated by surface grafting technique exhibited excellent selective recognition to CYF, resulting in a specific decrease of ECL signal at the MWCNT/MIP-QD modified electrode. Under optimal conditions, the MIECL signal was proportional to the logarithm of the CYF concentration in the range of 0.2 µg/L to 1.0 × 10³ µg/L with a determination coefficient of 0.9983. The detection limit of CYF was 0.05 µg/L, and good recoveries ranging from 86.0% to 98.6% were obtained in practical samples. The proposed MIECL sensor provides a novel, rapid, high sensitivity detection strategy for successfully analyzing CYF in fish and seawater samples.

Ultrasound-Enhanced Chemiluminescence for Bioimaging

Tissue imaging has emerged as an important aspect of theragnosis. It is essential not only to evaluate the degree of the disease and thus provide appropriate treatments, but also to monitor the delivery of administered drugs and the subsequent recovery of target tissues. Several techniques including magnetic resonance imaging (MRI), computational tomography (CT), acoustic tomography (AT), biofluorescence (BF) and chemiluminescence (CL), have been developed to reconstruct three-dimensional images of tissues. While imaging has been achieved with adequate spatial resolution for shallow depths, challenges still remain for imaging deep tissues. Energy loss is usually observed when using a magnetic field or traditional ultrasound (US), which leads to a need for more powerful energy input. This may subsequently result in tissue damage. CT requires exposure to radiation and a high dose of contrast agent to be administered for imaging. The BF technique, meanwhile, is affected by strong scattering of light and autofluorescence of tissues. The CL is a more selective and sensitive method as stable luminophores are produced from physiochemical reactions, e.g. with reactive oxygen species. Development of near infrared-emitting luminophores also bring potential for application of CL in deep tissues and whole animal studies. However, traditional CL imaging requires an enhancer to increase the intensity of low-level light emissions, while reducing the scattering of emitted light through turbid tissue environment. There has been interest in the use of focused ultrasound (FUS), which can allow acoustic waves to propagate within tissues and modulate chemiluminescence signals. While light scattering is decreased, the spatial resolution is increased with the assistance of US. In this review, chemiluminescence detection in deep tissues with assistance of FUS will be highlighted to discuss its potential in deep tissue imaging.

Mechanistic investigations into synergistically enhanced radiative-charge-transfer in Au–Ag bimetallic nanoclusters

Synergetic effects in Au–Ag bimetallic nanoclusters more favorably enhanced electrochemical redox induced electrochemiluminescence over photoexcitation induced photoluminescence.

Recent advances in co-reaction accelerators for sensitive electrochemiluminescence analysis

In electrochemiluminescence sensing platforms, co-reaction accelerators are specific materials used to catalyze the dissociation of co-reactants into active radicals, which can significantly boost the ECL emission of luminophores.

Enhanced biosensing strategies using electrogenerated chemiluminescence: recent progress and future prospects

An overview of enhancement strategies for highly sensitive ECL-based sensing of bioanalytes enabling early detection of cancer.

Unveiling the interfacial electrochemiluminescence behavior of lead halide perovskite nanocrystals

In this study, a three-phase heterostructure interface including glassy carbon (conducting medium), CsPbBr₃ perovskite nanocrystals (PNCs, emitter) and acetonitrile (electrolyte) is constructed for fully investigating the interfacial electrochemiluminescence (ECL) behavior of CsPbBr₃ PNCs. We find that these interfaces serve as bridges for efficient electron-hole transfer during the ECL process. As a proof of concept, the increase of the heterostructure interface area will accordingly enhance the ECL intensity of CsPbBr₃ PNCs. About seven-fold enhancement of the ECL intensity could be achieved when the interface area has triple-fold increase, which provides a new perspective to construct more efficient ECL systems via interface engineering.

An electrochemiluminescence immunosensor for the N-terminal brain natriuretic peptide based on the high quenching ability of polydopamine

A sandwich-type electrochemiluminescence (ECL) immunosensor for the N-terminal brain natriuretic peptide (NT-proBNP) is described. The assay is based on the quenching of the ECL of graphite-like carbon nitride (g-C₃N₄) by polydopamine (PDA). Two-dimensional g-C₃N₄ is grown in-situ on titanium dioxide nanoflowers (TiO₂ NFs). The macroporous structure of the NFs enhances the interfacial stability of g-C₃N₄, and also promotes the ECL reaction of g-C₃N₄ with the co-reactant. The introduction of gold nanoparticles into the matrix further enhances the ECL and facilitates the immobilization of capture antibodies. The nanoquencher used to label the secondary antibody is synthesized by catalytic polymerization of dopamine in the presence of bimetallic NiPd nanoparticles. The nanoquencher preserves the high reactivity of polydopamine and quenches the ECL of the g-C₃N₄/TiO₂ system. Compared to other methods, the detection limit for NT-proBNP is decreased to 50 fg∙mL^-1. Graphical abstract Schematic presentation of the electrochemiluminescence (ECL) process of the immunosensor: titanium dioxide nanoflowers@graphite-like carbon nitride-gold nanoparticles (TiO₂ NFs@g-C₃N₄-Au) as luminophor, and polydopamine (PDA) as nanoquencher.

Review of Carbon and Graphene Quantum Dots for Sensing

Carbon and graphene quantum dots (CQDs and GQDs), known as zero-dimensional (0D) nanomaterials, have been attracting increasing attention in sensing and bioimaging. Their unique electronic, fluorescent, photoluminescent, chemiluminescent, and electrochemiluminescent properties are what gives them potential in sensing. In this Review, we summarize the basic knowledge on CQDs and GQDs before focusing on their application to sensing thus far followed by a discussion of future directions for research into CQDs- and GQD-based nanomaterials in sensing. With regard to the latter, the authors suggest that with the potential of these nanomaterials in sensing more research is needed on understanding their optical properties and why the synthetic methods influence their properties so much, into methods of surface functionalization that provide greater selectivity in sensing and into new sensing concepts that utilize the virtues of these nanomaterials to give us new or better sensors that could not be achieved in other ways.