Quenching of the Electrochemiluminescence of Tris(2,2′-bipyridine)ruthenium(II)/Tri-n-propylamine by Pristine Carbon Nanotube and Its Application to Quantitative Detection of DNA

A review of nanomaterials for biosensing applications

A biosensor is a device that reacts with the analyte to be analyzed, detects its concentration, and generates readable information, which plays an important role in medical diagnosis, detection of physiological indicators, and disease prevention. Nanomaterials have received increasing attention in the fabrication and improvement of biosensors due to their unique physicochemical and optical properties. In this paper, the properties of nanomaterials such as the size effect, optical and electrical properties, and their advantages in the field of biosensing are briefly summarized, and the application of nanomaterials can effectively improve the sensitivity and reduce the detection limit of biosensors. The advantages of commonly used nanomaterials such as gold nanoparticles (AuNPs), carbon nanotubes (CNTs), quantum dots (QDs), graphene, and magnetic nanobeads for biosensor applications are also reviewed. Besides, the two main types of biosensors using nanomaterials involved in their construction and their working principles are described, and the toxicity and biocompatibility of nanomaterials and the future direction of nanomaterial biosensors are discussed.

Highly Sensitive, Portable Detection System for Multiplex Chemiluminescence Analysis

Chemiluminescence (CL) has emerged as a critical tool for the sensing and quantification of various bioanalytes in virtually all clinical fields. However, the rapid nature of many CL reactions raises challenges for typical low-cost optical sensors such as cameras to achieve accurate and sensitive detection. Meanwhile, classic sensors such as photomultiplier tubes are highly sensitive but lack spatial multiplexing capabilities and are generally not suited for point-of-care applications outside a standard laboratory setting. To address this issue, in this paper, a miniaturized and versatile silicon-photomultiplier-based fiber-integrated CL device (SFCD) was designed for sensitive multiplex CL detection. The SFCD comprises a silicon photomultiplier array coupled to an array of high numerical aperture plastic optical fibers to achieve 16-plex detection. The optical fibers ensure efficient light collection while allowing the fixed detector to be mated with diverse sample geometries (e.g., circular or grid), simply by adjusting the fiber configuration. In a head-to-head comparison with a lens-based camera system featuring a cooled detector, the SFCD achieved a 14-fold improved limit of detection in both direct and enzyme-mediated CL reactions. The SFCD also features improved compactness and lower cost, as well as faster temporal resolution compared with camera-based systems while preserving spatial multiplexing and good environmental robustness. Thus, the SFCD has excellent potential for point-of-care biosensing applications.

In situ growth of TiO2/Ti3C2 MXene Schottky heterojunction as a highly sensitive photoelectrochemical biosensor for DNA detection

In this work, a heterojunction composed of a TiO₂ nanosheet and layered Ti₃C₂ was synthesized by directly growing TiO₂ in Ti₃C₂ MXene. Compared with pure TiO₂, TiO₂/Ti₃C₂ composites had increased surface area, and a light absorption range that extended from ultraviolet to visible light, which greatly extended the life of photogenerated carriers. A photoelectrochemical biosensor for DNA detection was constructed based on the TiO₂/Ti₃C₂ heterogeneous structure, which was comprehensively studied based on photocurrent responses. In the absence of the target, the CdSe QDs were close to the surface of the electrode, resulting in enhanced sensitization and increased photocurrent. In the presence of the target, the photocurrent decreases due to the formation of rigid double strands with the probe DNA, which caused the CdSe QDs to be far away from the electrode surface. The sensor had stability and sensitivity for DNA detection in the range of 10 nM-10 fM, and the lower detection limit was 6 fM. Its outstanding characteristics also provided ideas for detecting various other target DNA for early diagnosis of various diseases.

Mixed-Ligand-Regulated Self-Enhanced Luminous Eu-MOF as an ECL Signal Probe for an Oriented Antibody-Decorated Biosensing Platform

The self-luminescence behavior of lanthanide MOFs (Ln-MOFs) due to the unique antenna effect is considered to be a promising electrochemiluminescence (ECL) emission for biosensors. It is more challenging for Ln-MOFs on account of the difficulty to stimulate Ln ions with the desired energy-transfer efficiency to produce stronger ECL emissions at a low potential. Here, guided by a second ligand-assisted energy-transfer strategy, we present an efficient self-enhanced luminescence mixed-ligand Eu-MOF as an ECL signal probe for an oriented antibody-decorated biosensing platform with a low detection limit and a broad detection range. Diamino terephthalic acid (NH₂-H₂BDC) and 1,10-phenanthroline (Phen) were selected as the first and second ligands, respectively, to form highly conjugated structures, as well as suppress the nonradiative energy transfer. Impressively, Phen precisely adjusts the energy gap between the triplet ligand and the excited state of Eu³⁺, realizing the self-enhancement of ECL efficiency of the Eu-MOF. The mixed ligand adjusted the molar ratio to obtain the stable and strong ECL signal at a lowered triggering potential (0.83 V). In addition, FeCo@CNT features densely active FeCo sites along with a rich hierarchy conductive carbon nanotube (CNT) network, which is efficiently a co-reaction accelerator to produce more TPA^•+ radicals to accelerate the reduction process of the Eu-MOF for achieving the ECL emission amplification. FeCo@CNT with heptapeptide HWRGWVC (HWR) constructed a matrix biosensing interface that allowed the fragment antigen-binding (Fab) regions to target specific antigens and enhance the incubation efficiency. The present study has gone some way toward designing a self-enhanced luminous Eu-MOF, thus giving new fresh impetus to develop high-performance ECL emitters for biological analysis.

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.

Regulation of Ru(bpy)32+ Electrochemiluminescence Based on Distance-Dependent Electron Transfer of Ferrocene for Dual-Signal Readout Detection of Aflatoxin B1 with High Sensitivity

Ferrocene (Fc) is a common quencher of Ru(bpy)₃²⁺ luminescence. However, interactions between Fc and Ru(bpy)₃²⁺ can be extremely complicated. In this work, we reported the first use of Fc to regulate the electrochemiluminescence (ECL) of Ru(bpy)₃²⁺ by tuning the length of the DNA sequence between Fc and the luminophore of nitrogen-doped graphene quantum dots-Ru(bpy)₃²⁺-doped silica nanoparticles (SiO₂@Ru-NGQDs). The ECL of SiO₂@Ru-NGQDs was depressed when the distance between Ru(bpy)₃²⁺ and Fc was less than 8 nm; a stronger ECL was observed when the distance was more than 12 nm. The switching of the ECL of Ru(bpy)₃²⁺ by Fc was attributed to the electron transfer mechanism, in which Fc participated in the redox of Ru(bpy)₃²⁺ for "signal-off" ECL; this favored electron transfer at the electrode fabricated with an Fc-labeled aptamer (Fc-apt) and SiO₂@Ru-NGQDs for "signal-on" ECL depending on the length of the DNA sequence. Here, a dual-signal readout aptasensor for aflatoxin B1 (AFB1) detection was developed via the enhanced ECL of SiO₂@Ru-NGQDs by Fc-apt. The redox currents of Fc and the ECL of Ru(bpy)₃²⁺ were simultaneously collected as yardsticks, and both decreased with higher concentrations of AFB1. The aptasensor allowed linear ranges of 3 × 10^-5 to 1 × 10² ng mL^-1 for ECL mode and 1 × 10^-3 to 3 × 10³ ng mL^-1 for electrochemical mode. Our work provides insight into the interactions between Fc and Ru(bpy)₃²⁺. The dual-signal readout strategy is a potential platform for the versatile design of aptasensors.

Phosphate Group-Derivated Bipyridine-Ruthenium Complex and Titanium Dioxide Nanoparticles for Electrochemical Sensing of Protein Kinase Activity

Monitoring of protein kinase activity is of significance for fundamentals of biochemistry, biomedical diagnose, and drug screening. To reduce the usage of a relatively complicated bio-labeled signal probe, the phosphate group-derivated bipyridine-ruthenium (Pbpy-Ru) complex and titanium dioxide nanoparticles (TiO₂ NPs) were employed as signal probes to develop an electrochemical sensor for evaluating the protein kinase A (PKA) activity. Through the specific interaction between the phosphate groups and TiO₂ NPs, the preparation of a Pbpy-Ru-TiO₂ NP signal probe and its linkage with the phosphorylated PKA substrate peptides could be performed in a simple and effective way. The tethering of Pbpy-Ru onto the TiO₂ NP surface does not degrade the electrochemical property of the complex. The Pbpy-Ru-TiO₂ NP probe exhibits well-defined redox signals at about 1.0 V versus Ag/AgCl reference and notably has about fivefold current response than that of the TiO₂ NPs with physically adsorbed tris-(bipyridine)-Ru. The PKA activity evaluation was realized by measuring the electrochemical response of the Pbpy-Ru-TiO₂ NPs at the phosphorylated peptide-assembled electrode. Operating at optimal conditions, the cathodic signals at the potential of 1.03 V exhibit a good linearity with the PKA concentrations of 0.5-40 U mL^-1. The electrochemical sensor shows good selectivity, low detection limit (0.2 U mL^-1, signal/noise = 3), qualified reproducibility, and satisfactory applicability for PKA determination in the cell lysate. The Pbpy-Ru-TiO₂ NPs/electrode system would be an excellent electrochemical platform for protein phosphorylation monitoring and sensing.

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.

Mass spectrometric detection of fleeting neutral intermediates generated in electrochemical reactions

Towards the goal of on-line monitoring of transient neutral intermediates during electrochemical reactions, an electrochemistry-neutral reionization-mass spectrometry (EC-NR-MS) technique was developed in this work. The EC-NR setup consisted of a customized EC flow cell, a sonic spray ionization source, a heating tube, an ion deflector and an electrospray ionization source, which were respectively used for the precise control of the electrochemical reaction, solution nebulization, droplet desolvation, ion deflection and neutral intermediate ionization. Based on the EC-NR-MS approach, some long-sought neutral radicals including TPrA˙, DBAE˙ and TEOA˙, which belong to important reductive intermediates in electrochemiluminescence (ECL) reactions, were successfully identified which helps to clarify the previously unproven ECL reaction mechanism. These findings were also supported by spin-trapping experiments and the tandem MS technique. Accordingly, the EC-NR-MS method provides a direct solution for studying complicated electrochemical reactions, especially for detecting short-lived neutral radicals as well as ionic intermediates.

An electrochemistry-neutral reionization-mass spectrometry (EC-NR-MS) technique was developed for on-line studying the long-sought neutral radicals generated in electrochemical reactions.

DNA-targeted formation and catalytic reactions of DNAzymes for label-free ratiometric electrochemiluminescence biosensing

A label-free ratiometric electrochemiluminescence (ECL) sensing strategy for the sensitive detection of target DNA (T-DNA) was proposed on the basis of G-quadruplex/hemin-regulated ECL emissions of CdS quantum dots (QDs) and luminol with their common coreactant of H₂O₂. The ECL biosensor was constructed through stepwise assemblies of CdS QDs and hairpin DNA (H-DNA) on a glassy carbon electrode, and subsequent introduction of T-DNA resulted in the development of G-quadruplex/hemin DNAzymes via the specific recognition of T-DNA and H-DNA in the presence of hemin and K⁺ ions. The formed DNAzymes not only prompted the catalytic oxidation of hydroquinone followed by deposition of insoluble oxidation oligomers on the electrode surface to attenuate the cathodic ECL emission of CdS QDs but also triggered the catalytic oxidation of luminol to enhance the anodic ECL emission. The label-free ratiometric ECL biosensor for the detection of T-DNA showed a wide response range from 1 to 10,000 fM (10^-15 M) with a low detection limit of 0.2 fM and exhibited excellent selectivity against mismatched base sequences. This work provides a reliable and sensitive sensing platform for the detection of targets in analytical community by means of rational design of DNA sequences.

Electrochemiluminescence resonance energy transfer between luminol and black phosphorus nanosheets for the detection of trypsin via the “off–on–off” switch mode

In this work, the electrochemiluminescence behavior of a luminol–H₂O₂ system was studied on a black phosphorus nanosheet modified electrode.

A highly sensitive signal-on biosensor for microRNA 142-3p based on the quenching of Ru(bpy)32+–TPA electrochemiluminescence by carbon dots and duplex specific nuclease-assisted target recycling amplification

CDs have been used to quench the ECL of Ru(bpy)₃²⁺–TPA for the first time and a sensitive biosensor was developed.

Coupling aptazyme and catalytic hairpin assembly for cascaded dual signal amplified electrochemiluminescence biosensing

Developing reliable and sensitive detection methods for adenosine triphosphate (ATP) is vital for both clinical diagnosis and food safety. In this work, by coupling aptazyme- and catalytic hairpin assembly (CHA)-based signal amplification and electrochemiluminescence (ECL), an ultrasensitive biosensor for sensing ATP was fabricated using Ru(bpy)₃²⁺-doped silica nanoparticles (RuSiO₂) as ECL probes and a ferrocene-functionalized hairpin DNA (hairpin-Fc) as quencher. The aptazyme-triggered cleavage of the DNA substrate and the CHA reaction both led to the circular release of trigger DNA, resulting in a significant dual signal amplification, with unprecedented enhancement up to 940-fold. Moreover, the bioconjugation of the DNA substrate with Au@Fe₃O₄ facilitated the separation and purification of the released trigger DNA, and effectively reduced the background signal. As a result, the as-prepared ECL biosensor exhibited a much lower detection limit of 0.054 pM for ATP, compared to those in previous reports, and showed high reliability for ATP detection in both spiked serum samples and Staphylococcus aureus. This work offers a new perspective for designing nucleic acid-based signal amplification for detecting ATP in bacterial analysis and clinical diagnosis.

Lanthanide terbium complex: synthesis, electrochemiluminescence (ECL) performance, and sensing application

In this study, a new lanthanide terbium complex, Tb(pzda)3(NO3)3·nH2O, was synthesized by a hydrothermal method and characterized by Fourier transform infrared spectroscopy (FT-IR) and energy-dispersive X-ray spectroscopy (EDS). It was found that the as-synthesized Tb-complex exhibited good electrochemiluminescence (ECL) behavior in the presence of triethanolamine (TEOA) in a HAc-NaAc buffer solution on a glassy carbon electrode. The possible reaction mechanism has been discussed based on the fluorescence spectra and ECL spectra. For sensing applications, it was found that protocatechuic acid (PCA) had an obvious quenching effect on the ECL signal of the Tb-complex, and this resulted in a decreased ECL signal associated with the concentration of PCA. Therefore, a highly sensitive method for the detection of PCA was established with a linear range of 1.283 × 10-10 M to 3.845 × 10-4 M and a detection limit of 0.085 nM at an S/N ratio of 3. This novel ECL assay strategy with an outstanding ECL efficiency offers great potential for pharmaceutical analyses.

Quantum material accompanied nonenzymatic cascade amplification for ultrasensitive photoelectrochemical DNA sensing

A novel cascade photoelectrochemical (PEC) signal amplification sensing strategy was designed and applied in target DNA detection by introducing quantum dots (QD) as the accompanying tag.

An efficient signal-on aptamer-based biosensor for adenosine triphosphate detection using graphene oxide both as an electrochemical and electrochemiluminescence signal indicator

An efficient aptasensor was developed in which graphene oxide (GO) was employed as an indicator for both electrochemical impedance spectroscopy and electrochemiluminescence (ECL) signal generation. The aptasensor was fabricated by self-assembling the ECL probe of a thiolated adenosine triphosphate binding aptamer (ABA) tagged with a Ru complex (Ru(bpy)3(2+) derivatives) onto the surface of gold nanoparticle (AuNP) modified glassy carbon electrode (GCE). ABA immobilized onto AuNP modified GCE could strongly adsorb GO due to the strong π-π interaction between ABA and graphene oxide; ECL quenching of the Ru complex then takes place because of energy transfer and electron transfer, and a large increase of the electron transfer resistance (Ret) of the electrode. While in the presence of target adenosine triphosphate (ATP), the ABA prefers to form ABA-ATP bioaffinity complexes, which have weak affinity to graphene oxide and keep the graphene oxide away from the electrode surface, thus allowing the ECL signal enhancement, and in conjunction with the decrease of the Ret. Because of the high ECL quenching efficiency, unique structure, and electronic properties of graphene oxide, the Ret and ECL intensity versus the logarithm of ATP concentration was linear in the wide range from 10 pM to 10 nM with an ultra-low detection limit of 6.7 pM to 4.8 pM, respectively. The proposed aptasensor exhibited excellent reproducibility, stability, and outstanding selectivity, and ATP could be effectively distinguished from its analogues. More significantly, this efficient ECL aptasensor strategy based on GO acting both as an electrochemical and ECL signal indicator is general and can be easily extended to other biological binding events.

Electrochemiluminescent molecular logic gates based on MCNTs for the multiplexed analysis of mercury(ii) and silver(i) ions

In this paper, logic gates with electrochemiluminescence (ECL) signal as outputs were constructed based on the use of the thymine (T)-rich (S₁) or cytosine (C)-rich (S₂) oligonucleotides for the selective analysis of mercury ions (Hg²⁺) or silver ions (Ag⁺).

Label-free signal-on ATP aptasensor based on the remarkable quenching of tris(2,2'-bipyridine)ruthenium(II) electrochemiluminescence by single-walled carbon nanohorn

The quenching of electrochemiluminescence by single-walled carbon nanohorn has been demonstrated for the first time. Moreover, a sensitive, label-free, and signal-on electrochemiluminescence ATP aptasensor was developed using single-walled carbon nanohorn as both quencher and scaffold.

Recent advances in electrochemiluminescence

The great success of electrochemiluminescence (ECL) for in vitro diagnosis (IVD) and its promising potential in light-emitting devices greatly promote recent ECL studies. More than 45% of ECL articles were published after 2010, and the first international meeting on ECL was held in Italy in 2014. This critical review discusses recent vibrant developments in ECL, and highlights novel ECL phenomena, such as wireless ECL devices, bipolar electrode-based ECL, light-emitting electrochemical swimmers, upconversion ECL, ECL resonance energy transfer, thermoresponsive ECL, ECL using shape-controlled nanocrystals, and ECL as an ion-selective electrode photonic reporter, a paper-based microchip, and a self-powered microfluidic ECL platform. We also comment on the latest progress in bioassays, light-emitting devices and, the computational approach for the ECL mechanism study. Finally, perspectives and key challenges in the near future are addressed (198 references).

Novel electrochemi-/photo-luminescence of Ag3Cu5 heterometallic alkynyl clusters

Two windmill-like Ag₃Cu₅ alkynyl clusters were synthesized and characterized. They display novel PL and ECL properties, which could be modified by changing the substituent on the alkynyl ligands. According to the study of EC, ECL behaviours and ECL emission spectra of the Ag₃Cu₅ clusters, a possible ECL mechanism was proposed.

Label-free and highly sensitive electrochemiluminescence biosensing using quantum dots/carbon nanotubes in ionic liquid

A universal and label-free electrochemiluminescence biosensing platform with high sensitivity was developed based on quantum dots/carbon nanotubes in ionic liquid.

In situ generation of electron acceptor for photoelectrochemical biosensing via hemin-mediated catalytic reaction

A novel photoelectrochemical sensing strategy is designed for DNA detection on the basis of in situ generation of an electron acceptor via the catalytic reaction of hemin toward H2O2. The photoelectrochemical platform was established by sequential assembly of near-infrared CdTe quantum dots, capture DNA, and a hemin-labeled DNA probe to form a triple-helix molecular beacon (THMB) structure on an indium tin oxide electrode. According to the highly catalytic capacity of hemin toward H2O2, a photoelectrochemical mechanism was then proposed, in which the electron acceptor of O2 was in situ-generated on the electrode surface, leading to the enhancement of the photocurrent response. The utilization of CdTe QDs can extend the absorption edge to the near-infrared band, resulting in an increase in the light-to-electricity efficiency. After introducing target DNA, the THMB structure is disassembled and releases hemin and, thus, quenches the photocurrent. Under optimized conditions, this biosensor shows high sensitivity with a linear range from 1 to 1000 pM and detection limit of 0.8 pM. Moreover, it exhibits good performance of excellent selectivity, high stability, and acceptable fabrication reproducibility. This present strategy opens an alternative avenue for photoelectrochemical signal transduction and expands the applications of hemin-based materials in photoelectrochemical biosensing and clinical diagnosis.