Analytical applications of nanomaterials in electrogenerated chemiluminescence

Functionalized nanomaterials-based electrochemiluminescent biosensors and their application in cancer biomarkers detection

To detect a range of trace biomarkers associated with human diseases, researchers have been focusing on developing biosensors that possess high sensitivity and specificity. Electrochemiluminescence (ECL) biosensors have emerged as a prominent research tool in recent years, owing to their potential superiority in low background signal, high sensitivity, straightforward instrumentation, and ease of operation. Functional nanomaterials (FNMs) exhibit distinct advantages in optimizing electrical conductivity, increasing reaction rate, and expanding specific surface area due to their small size effect, quantum size effect, and surface and interface effects, which can significantly improve the stability, reproducibility, and sensitivity of the biosensors. Thereby, various nanomaterials (NMs) with excellent properties have been developed to construct efficient ECL biosensors. This review provides a detailed summary and discussion of FNMs-based ECL biosensors and their applications in cancer biomarkers detection.

Efficient Electrochemiluminescence Sensing in Microfluidic Biosensors: A Review

Microfluidic devices are capable of handling 10-9 L to 10-18 L of fluids by incorporating tiny channels with dimensions of ten to hundreds of micrometers, and they can be fabricated using a wide range of materials including glass, silicon, polymers, paper, and cloth for tailored sensing applications. Microfluidic biosensors integrated with detection methods such as electrochemiluminescence (ECL) can be used for the diagnosis and prognosis of diseases. Coupled with ECL, these tandem devices are capable of sensing biomarkers at nanomolar to picomolar concentrations, reproducibly. Measurement at this low level of concentration makes microfluidic electrochemiluminescence (MF-ECL) devices ideal for biomarker detection in the context of early warning systems for diseases such as myocardial infarction, cancer, and others. However, the technology relies on the nature and inherent characteristics of an efficient luminophore. The luminophore typically undergoes a redox process to generate excited species which emit energy in the form of light upon relaxation to lower energy states. Therefore, in biosensor design the efficiency of the luminophore is critical. This review is focused on the integration of microfluidic devices with biosensors and using electrochemiluminescence as a detection method. We highlight the dual role of carbon quantum dots as a luminophore and co-reactant in electrochemiluminescence analysis, drawing on their unique properties that include large specific surface area, easy functionalization, and unique luminescent properties.

Electrochemiluminescence (ECL) biosensor based on tris(2,2'-bipyridyl)ruthenium(II) with glucose and lactate dehydrogenases encapsulated within alginate hydrogels

Two dehydrogenase enzymes (glucose, GDH, and lactate, LDH, dehydrogenases) encapsulated within alginate hydrogels were deposited on glassy carbon electrodes. The as-prepared enzyme modified alginate hydrogels were utilized as electrochemiluminescence (ECL)-based biosensors for the indirect detection of glucose and lactic acid upon reaction between NADH and tris(2,2'-bipyridyl) ruthenium (II) [Ru(bpy)₃]²⁺. The ECL response was obtained from the redox reaction between the substrate, the cofactor NAD⁺ and the encapsulated enzyme. The production of NADH resulting from the enzymatic reaction led to the ECL emission upon reaction with [Ru(bpy)₃]²⁺. The biosensors showed good stability and repeatability, with linear range between 0.56 and 4.2 µM and limit of detection of 0.84 µM for glucose, and linear range between 5 and 30 µM with a limit of detection of 2.52 µM for lactic acid. These ECL-based biosensors showed good sensitivity when tested in the presence of common interfering species. These biosensors were utilized in artificial sweat and were characterized by good reproducibility and repeatability. The results herein presented suggest that the dehydrogenases encapsulated within alginate hydrogels have potential for the development of biocompatible sensors for detection of glucose and lactic acid in physiological fluids.

Advances in electrochemiluminescence luminophores based on small organic molecules for biosensing

Electrochemiluminescence (ECL) has several advantages, such as a near-zero background signal, high sensitivity, wide dynamic range, simplicity, and is widely used for sensing, imaging, and single cell analysis. ECL luminophores are the key factors in the performance of various applications. Among various luminophores, small organic luminophores exhibit many intriguing features including good biocompatibility, facile modification, well-defined molecular structure, and sustainable raw materials, making small organic luminophores attractive for the use in the ECL field. Although many great achievements have been made in the synthesis of new small organic luminophores, solving various challenges, and expanding new applications, there are almost no comprehensive reviews on small organic ECL luminophores. In this review, we briefly introduce the advantages and emission mechanisms of small organic ECL luminophores, summarize the main types, molecular characteristics, and ECL properties of most existing small organic ECL luminophores, and present the important applications and design principles in sensors, imaging, single cell analysis, sterilization, and other fields. Finally, the challenges and outlook of organic ECL luminophores to be popularized in biosensing applications are also discussed.

Low-Triggering-Potential Electrochemiluminescence from a Luminol Analogue Functionalized Semiconducting Polymer Dots for Imaging Detection of Blood Glucose

In recent years, semiconducting polymer dots (Pdots) as environmentally friendly and high-brightness electrochemiluminescence (ECL) nanoemitters have attracted intense attention in ECL biosensing and imaging. However, most of the available Pdots have a high ECL excitation potential in the aqueous phase (>1.0 V vs Ag/AgCl), which causes poor selectivity in actual sample detection. Therefore, it is particularly important to construct a simple and universal strategy to lower the trigger potential of Pdots. This work has realized the ECL emission of Pdots at low-trigger-potential based on the electrochemiluminescence resonance energy transfer (ERET) strategy. By covalently coupling the Pdots with a luminol analogue, N-(4-aminobutyl)-N-ethylisoluminol (ABEI), the ABEI-Pdots showed an anodic ECL emission with a low onset potential of +0.34 V and a peak potential at +0.45 V (vs Ag/AgCl), which was the lowest trigger potential reported so far. We further explored this low-triggering-potential ECL for imaging detection of glucose in buffer and serum. By imaging the ABEI-Pdots-modified screen-printed electrodes (SPCE) at +0.45 V for 16 s, the ECL imaging method could quantify the glucose concentration in buffer from 10 to 200 μM with detection limits of 3.3 μM, while exhibiting excellent selectivity. When applied to real serum, the results of our method were highly consistent with a commercial blood glucose meter, with the relative errors ranging from 3.2 to 13%. This work provided a universal strategy for constructing low potential Pdots and demonstrated its application potential in complex biological sample analysis.

Electrochemiluminescence sensors and forensic investigations: a viable technique for drug detection?

Novel psychoactive substances (NPS) are today considered one of the major ticking public health time bombs in regard to drug abuse. The inability to identify these substances with current screening methods, sees their distribution remain uninterrupted and contributes to the high death rates amongst users. To tackle this problem, it is vital that new robust screening methods are developed, addressing the limitation of those currently in place, namely colour subjectivity and lack of compatibility with the complex matrices these substances may be found within. To this avail, electrochemical methods have been assessed. These low cost and extremely portable sensors have been successfully applied for the direct detection of a broad range of compounds of interest in a range of matrices including, herbal material, commercial drinks and biological fluids (serum, saliva, sweat and urine). With their high versatility, gifted through a significant degree of flexibility in regard to electrode material a range of sensors have to date been reported. In this review the various electrochemical sensors developed to date for NPS detection will be compared and contrasted, with a special focus upon those utilising electrochemiluminescence (ECL) technology.

Electrochemiluminescence with semiconductor (nano)materials

Electrochemiluminescence (ECL) is the light production triggered by reactions at the electrode surface. Its intrinsic features based on a dual electrochemical/photophysical nature have made it an attractive and powerful method across diverse fields in applied and fundamental research. Herein, we review the combination of ECL with semiconductor (SC) materials presenting various typical dimensions and structures, which has opened new uses of ECL and offered exciting opportunities for (bio)sensing and imaging. In particular, we highlight this particularly rich domain at the interface between photoelectrochemistry, SC material chemistry and analytical chemistry. After an introduction to the ECL and SC fundamentals, we gather the recent advances with representative examples of new strategies to generate ECL in original configurations. Indeed, bulk SC can be used as electrode materials with unusual ECL properties or light-addressable systems. At the nanoscale, the SC nanocrystals or quantum dots (QDs) constitute excellent bright ECL nano-emitters with tuneable emission wavelengths and remarkable stability. Finally, the challenges and future prospects are discussed for the design of new detection strategies in (bio)analytical chemistry, light-addressable systems, imaging or infrared devices.

Sensitive electrochemiluminescence biosensing of polynucleotide kinase using the versatility of two-dimensional Ti3C2TX MXene nanomaterials

Electrochemiluminescence (ECL) is a powerful readout method for the development of (bio)sensors, whose performances depend on the electrode materials and the architecture of its surface. Herein, we demonstrate that the precise control of the sensing interface using the versatility of two-dimensional (2D) transition metal carbides (Ti₃C₂T_X MXene) leads to the enhancement of the ECL signal. This electrode material, which exhibits remarkable structural and electrochemical properties was decorated by the in situ formation of gold nanoparticles (AuNPs) owing to the Ti reducibility. Then, a large amount of the luminophore, Ru(bpy)₃²⁺, was immobilized on Ti₃C₂T_X MXene thanks to its unique negative charge and large specific surface area to obtain Ru-Ti₃C₂T_X-AuNPs. The presented approach exploits the high catalytic activity and excellent conductivity of this 2D nanomaterial as illustrated by the enhanced ECL emission performance of the Ru-Ti₃C₂T_X-AuNPs nanoprobes. Finally, DNA phosphorylated with polynucleotide kinase (PNK) was recognized efficiently by the chelation between Ti and phosphate group. A highly sensitive and selective ECL biosensor was developed for the detection of PNK and the screening of its inhibitors. A lower detection limit of 0.0002 U mL^-1 and wide linear relationship ranged from 0.002 to 10 U mL^-1 were obtained. Furthermore, the practicality of our method was tested in MCF-7 cell lysate, which opens enticing perspectives for future applications of Ti₃C₂T_X materials in the ECL bioanalysis field.

Electrochemiluminescence of Ru(bpy)32+/thioacetamide and its application for the sensitive determination of hepatotoxic thioacetamide

Thioacetamide (TAA) is a well-known hepatotoxic substance, so it is important to determine its presence and content in food and environmental samples. Herein, we report a highly sensitive determination method for TAA based on the electrochemiluminescence (ECL) of tris(2,2'-bipyridyl)ruthenium(ii) (Ru(bpy)32+) for the first time by using TAA as a new coreactant for Ru(bpy)32+ ECL via an anodic route. The developed Ru(bpy)32+-TAA ECL system allows the determination of TAA with a good dynamic linear range and low limit of detection (LOD) of 0.1 μM to 1000 μM and 0.035 μM (3σ/m), respectively. In addition, the established ECL system can be applied to detect TAA in fruit juice and waste water samples with outstanding recoveries.

DNA-Based Nanoswitches: Insights into Electrochemiluminescence Signal Enhancement

Electrochemiluminescence (ECL) is a powerful transduction technique that has rapidly gained importance as a powerful analytical technique. Since ECL is a surface-confined process, a comprehensive understanding of the generation of ECL signal at a nanometric distance from the electrode could lead to several highly promising applications. In this work, we explored the mechanism underlying ECL signal generation on the nanoscale using luminophore-reporter-modified DNA-based nanoswitches (i.e., molecular beacon) with different stem stabilities. ECL is generated according to the "oxidative-reduction" strategy using tri-n-propylamine (TPrA) as a coreactant and Ru(bpy)₃²⁺ as a luminophore. Our findings suggest that by tuning the stem stability of DNA nanoswitches we can activate different ECL mechanisms (direct and remote) and, under specific conditions, a "digital-like" association curve, i.e., with an extremely steep transition after the addition of increasing concentrations of DNA target, a large signal variation, and low preliminary analytical performance (LOD 22 nM for 1GC DNA-nanoswtich and 16 nM for 5GC DNA-nanoswitch). In particular, we were able to achieve higher signal gain (i.e., 10 times) with respect to the standard "signal-off" electrochemical readout. We demonstrated the copresence of two different ECL generation mechanisms on the nanoscale that open the way for the design of customized DNA devices for highly efficient dual-signal-output ratiometric-like ECL systems.

Recent developments in electrochemiluminescence nanosensors for cancer diagnosis applications

In recent years, electrochemiluminescence (ECL) nanosensing systems have undergone rapid development and made significant progress in ultrasensitive analysis and cell imaging. Because of the unique advantages of high selectivity, ultra-sensitivity, and good reproducibility, ECL nanosensors can open new paths for cancer diagnosis. With the development of ECL nanosensors, high-throughput analysis, visual detection and spatially resolved ECL imaging of single cells are being realized. The innovations of ECL nanosensors consist of electrochemical excitation, coreactant catalysis, light radiation and luminescence signal amplification, which involve several fields such as nanotechnology, catalysis, optics, and electrochemistry. The developments of ECL instruments also relate to imaging technology. Herein, we review the construction modes, sensing strategies and cancer diagnosis applications of ECL nanosenors. Firstly, the nano-components of the ECL sensing system are discussed. The construction and signal amplification methods of the nanosensing system are emphasized. Secondly, the high-efficiency cancer identification strategies are presented, including protein tumor marker detection, nucleic acid assay, cancer cell identification and exosome detection. The recent advances in representative examples of ECL nanosenors in cancer diagnosis are highlighted, including high-throughput ECL analysis, in situ assay, visual ECL detection, single-cell imaging diagnosis, and so on. Finally, the challenges are featured based on the recent development of the ECL nanosensing system in the clinical diagnosis. The ECL nanosensors provide effective and reliable analytical methods and open new paths for cancer diagnosis. It is noteworthy that the prospects of the ECL nanosensing system in clinical diagnosis are instructive to the developments of other nanosensor research.

Photophysics, Electrochemistry and Efficient Electrochemiluminescence of Trigonal Truxene-Core Dyes

We synthesized and characterized a series of dyes built from a spirofluorene or truxene core. The quadrupolar spirofluorene system is the initial building unit for the design and preparation of more complex star-shaped dyes consisting of a truxene core bearing three di- or triphenylamine moieties with or without a thiophene connector. Their photophysical, electrochemical, and electrochemiluminescence (ECL) properties were first investigated in solution. Structure/activity relationships were derived and rationalized by comparing the quadrupolar system and trigonal truxene-core derivatives using computational studies. The photophysical and redox characteristics are drastically tuned by the introduction of a thiophene bridge and electron-donor substituents at their terminal branches. These comparative studies show the essential role of the stability of both radical cations and anions to obtain efficient ECL dyes. The stabilization of the radicals is directly related to the charge delocalization due to the π-conjugation by the thiophene bridge. The brightest ECL is achieved by annihilation and coreactant (benzoyl peroxide) pathways with the blue-emitting truxene dye, which is 2- and 4.5-times greater than that of the quadrupolar compound and reference [Ru(bpy)₃ ]²⁺ emitter, respectively. Such an extensive study on these extended π-conjugated molecules presenting different core structures may guide the design and synthesis of new ECL dyes with a strong efficiency.

Conductive Metal-Organic Framework Thin Film Hybrids by Electropolymerization of Monosubstituted Acetylenes

1-Hexyne monomers were potentiostatically electropolymerized upon confinement in 1D channels of a surface-mounted metal-organic framework Cu(BDC) (SURMOF-2). A layer-by-layer deposition method allowed for SURMOF depostition on substrates with prepatterned electrodes, making it possible to characterize electrical conductivity in situ, i.e., without having to delaminate the conductive polymer thin film. Successful polymerization was evidenced by mass spectroscopy, and the electrical measurements demonstrated an increase of the electrical conductivity of the MOF material by 8 orders of magnitude. Extensive DFT calculations revealed that the final conductivity is limited by electron hopping between the conductive oligomers.

An electrochemiluminescence cytosensor for sensitive detection of HeLa cells based on a signal amplification strategy of Au-NaYF4:Yb,Er nanocomposites

A novel electrochemiluminescence (ECL) cytosensor was proposed for the quantitative detection of HeLa cells (human cervical cancer cells) with the help of a signal amplification strategy. Firstly, the Au-NaYF4:Yb,Er nanocomposites were prepared by a simple in situ hydrothermal method and characterized by transmission electron microscopy (TEM) images, X-ray diffraction (XRD) patterns, UV-vis spectra and Fourier transform infrared (FTIR) spectra. Compared with the bare NaYF4:Yb,Er nanocomposites, the ECL intensity of Au-NaYF4:Yb,Er nanocomposites was greatly enhanced by about 4.2-fold which can be attributed to the good conductivity of gold nanoparticles (Au NPs). The nanocomposites showed high and stable ECL emission, fast response and superior conductivity, all of which were advantageous to the ECL detection. Furthermore, HeLa cells were immobilized on the modified electrode via the interaction between folic acid and a folate receptor present on the cell surface. The ECL cytosensor showed satisfactory sensitive response to HeLa cells in a linear range of 4.25 × 102-4.25 × 105 cells per mL with a low detection limit of 326 cells per mL. The proposed cytosensor had good sensitivity and stability, which can offer a great potential platform for bioassay analysis.

A specific electrochemiluminescence sensor for selective and ultra-sensitive mercury(ii) detection based on dithiothreitol functionalized copper nanocluster/carbon nitride nanocomposites

A novel electrochemiluminescence sensor based on the combination of copper nanoclusters and carbon nitride nanosheets was fabricated for detecting Hg²⁺.

A molecularly imprinted sensor with enzymatic enhancement of electrochemiluminescence of quantum dots for ultratrace clopyralid determination

A new molecularly imprinted polymer electrochemiluminescence (ECL) sensor was developed for the detection of clopyralid (CPD) based on enzyme-biocatalyzed amplification. CdTe quantum dots were immobilized on the surface of an electrode by reaction with p-aminothiophenol preadsorbed on the electrode. Then a molecularly imprinted film was formed by electrochemical polymerization of o-phenylenediamine in the presence of CPD on the CdTe-modified gold electrode. During the analytical cycle, horseradish peroxidase (HRP)-labeled CPD was replaced by CPD in the sample. The amount of HRP on the molecularly imprinted polymer electrode decreased, and then less H₂O₂ was catalytically decomposed. Subsequently, the ECL intensity of the CdTe-H₂O₂ system was enhanced. There was a good linear relationship between ECL intensity and the concentration of CPD in the range from 2.0 × 10^-11 to 2.5 × 10^-10 mol/L and in the range from 2.5 × 10^-10 to 3.5 × 10^-8 mol/L. The detection limit was 4.1 × 10^-12 mol/L. The sensor was applied to determine CPD in vegetable samples. Graphical abstract A molecularly imprinted electrochemiluminescence sensor was fabricated for ultratrace clopyralid determination. The sensitivity was significantly improved with the enhancement of the ECL intensity of quantum dot via the enzymatic reaction of HRP.

A single-electrode electrochemical system for multiplex electrochemiluminescence analysis based on a resistance induced potential difference

A single-electrode electrochemical system uses only one electrode for multiplex experiments, and is a highly cheap platform for high throughput analysis.

Developing low-cost and simple electrochemical systems is becoming increasingly important but still challenged for multiplex experiments. Here we report a single-electrode electrochemical system (SEES) using only one electrode not only for a single experiment but also for multiplex experiments based on a resistance induced potential difference. SEESs for a single experiment and multiplex experiments are fabricated by attaching a self-adhesive label with a hole and multiple holes onto an ITO electrode, respectively. This enables multiplex electrochemiluminescence analysis with high sensitivity at a very low safe voltage using a smartphone as a detector. For the multiplex analysis, the SEES using a single electrode is much simpler, cheaper and more user-friendly than conventional electrochemical systems and bipolar electrochemical systems using electrode arrays. Moreover, SEESs are free from the electrochemiluminescent background problem from driving electrodes in bipolar electrochemical systems. Since numerous electrodes and cover materials can be used to fabricate SEESs readily and electrochemistry is being extensively used, SEESs are very promising for broad applications, such as drug screening and high throughput analysis.

Trends and Advances in Electrochemiluminescence Nanobiosensors

The rapid and increasing use of the nanomaterials (NMs), nanostructured materials (NSMs), metal nanoclusters (MNCs) or nanocomposites (NCs) in the development of electrochemiluminescence (ECL) nanobiosensors is a significant area of study for its massive potential in the practical application of nanobiosensor fabrication. Recently, NMs or NSMs (such as AuNPs, AgNPs, Fe₃O₄, CdS QDs, OMCs, graphene, CNTs and fullerenes) or MNCs (such as Au, Ag, and Pt) or NCs of both metallic and non-metallic origin are being employed for various purposes in the construction of biosensors. In this review, we have selected recently published articles (from 2014-2017) on the current development and prospects of label-free or direct ECL nanobiosensors that incorporate NCs, NMs, NSMs or MNCs.

Building a Three-Dimensional Nano-Bio Interface for Aptasensing: An Analytical Methodology Based on Steric Hindrance Initiated Signal Amplification Effect

The development of novel detection methodologies in electrochemiluminescence (ECL) aptasensor fields with simplicity and ultrasensitivity is essential for constructing biosensing architectures. Herein, a facile, specific, and sensitive methodology was developed unprecedentedly for quantitative detection of microcystin-LR (MC-LR) based on three-dimensional boron and nitrogen codoped graphene hydrogels (BN-GHs) assisted steric hindrance amplifying effect between the aptamer and target analytes. The recognition reaction was monitored by quartz crystal microbalance (QCM) to validate the possible steric hindrance effect. First, the BN-GHs were synthesized via self-assembled hydrothermal method and then applied as the Ru(bpy)₃²⁺ immobilization platform for further loading the biomolecule aptamers due to their nanoporous structure and large specific surface area. Interestingly, we discovered for the first time that, without the aid of conventional double-stranded DNA configuration, such three-dimensional nanomaterials can directly amplify the steric hindrance effect between the aptamer and target analytes to a detectable level, and this facile methodology could be for an exquisite assay. With the MC-LR as a model, this novel ECL biosensor showed a high sensitivity and a wide linear range. This strategy supplies a simple and versatile platform for specific and sensitive determination of a wide range of aptamer-related targets, implying that three-dimensional nanomaterials would play a crucial role in engineering and developing novel detection methodologies for ECL aptasensing fields.

Optimising electrogenerated chemiluminescence of quantum dots via co-reactant selection

We demonstrate that for quantum dot (QD) based electrochemiluminescence (ECL), the commonly used co-reactant does not perform as effectively as potassium persulfate. By exploiting this small change in co-reactant, ECL intensity can be enhanced dramatically in a cathodic-based ECL system. However, TPA remains the preferential co-reactant-based system for anodic ECL. This phenomenon can be rationalised through the relative energy-level profiles of the QD to the co-reactant in conjunction with the applied potential range. This work highlights the importance of understanding the co-reactant pathway for optimising the application of ECL to bioanalytical analysis, in particular for near-infrared (NIR) QDs which can be utilised for analysis in blood.

Graphical Abstract

Electrochemiluminescent detection of methamphetamine and amphetamine

Direct detection of amphetamine type stimulants (ATSs) including methylamphetamine (MA) in street samples and biological matrices without the need for pretreatment or extraction is a great challenge for forensic drug analysis. Electrochemical techniques, such as electrochemiluminescence (ECL), are promising tools for this area of analysis. This contribution focuses on the electrochemical and photochemical properties of [Ru(bpy)3](2+) Nafion composite films and their subsequent use for the detection of ATS in particular MA. Under optimised conditions, the response linearly increased with the concentration over the concentration range 50pM≤[MA]≤1mM while an equivalent dynamic range was obtained for amphetamine with a correlation coefficient of 0.9903 and 0.9948 respectively. The ECL signal was monitored at ∼620nm, representing the λmax for the [Ru(bpy)3](2+) Nafion composite films. This wavelength is shifted by approximately 15nm compared to the photoexcited λmax for the same system. The modified films were formed by direct interaction with the electrode surface without the need for surface modification or chain linkers. This is a major advantage for the fabrication of any sensor as it reduces the synthesis times resulting in more economically and cheaper production costs. This technique is simple, rapid, selective and sensitive, and shows potential for the high-throughput quantitation of ATS as well as possibilities for adaptation with other techniques such as FIA or LC systems.

A sensitive electrogenerated chemiluminescence biosensor for galactosyltransferase activity analysis based on a graphitic carbon nitride nanosheet interface and polystyrene microsphere-enhanced responses

An electrogenerated chemiluminescence biosensor for galactosyltransferases activity analysis was developed based on g-C₃N₄ nanosheet interface and polystyrene microsphere enhanced responses.

Fabrication and multifunctional properties of ultrasmall water-soluble tungsten oxide quantum dots

Novel ultrasmall water-soluble tungsten oxide quantum dots with multifunctional properties have been successfully developed by a facile and green method.