Electrochemiluminescence of Semiconductor Quantum Dots and Its Biosensing Applications: A Comprehensive Review

Electrochemiluminescence (ECL) is the chemiluminescence triggered by electrochemical reactions. Due to the unique excitation mode and inherent low background, ECL has been a powerful analytical technique to be widely used in biosensing and imaging. As an emerging ECL luminophore, semiconductor quantum dots (QDs) have apparent advantages over traditional molecular luminophores in terms of luminescence efficiency and signal modulation ability. Therefore, the development of an efficient ECL system with QDs as luminophores is of great significance to improve the sensitivity and detection flux of ECL biosensors. In this review, we give a comprehensive summary of recent advances in ECL using semiconductor QDs as luminophores. The luminescence process and ECL mechanism of semiconductor QDs with various coreactants are discussed first. Specifically, the influence of surface defects on ECL performance of semiconductor QDs is emphasized and several typical ECL enhancement strategies are summarized. Then, the applications of semiconductor QDs in ECL biosensing are overviewed, including immunoassay, nucleic acid analysis and the detection of small molecules. Finally, the challenges and prospects of semiconductor QDs as ECL luminophores in biosensing are featured.

Tunable electrochemiluminescence properties of CsPbBr3perovskite nanocrystals using mixed-monovalent cations

Herein, we demonstrate a simple approach for tuning the elecrochemiluminescence (ECL) properties of CsPbBr₃perovskite nanocrystals by using mixed-monovalent cations.

Sensors and Biosensors for C-Reactive Protein, Temperature and pH, and Their Applications for Monitoring Wound Healing: A Review

Wound assessment is usually performed in hospitals or specialized labs. However, since patients spend most of their time at home, a remote real time wound monitoring would help providing a better care and improving the healing rate. This review describes the advances in sensors and biosensors for monitoring the concentration of C-reactive protein (CRP), temperature and pH in wounds. These three parameters can be used as qualitative biomarkers to assess the wound status and the effectiveness of therapy. CRP biosensors can be classified in: (a) field effect transistors, (b) optical immunosensors based on surface plasmon resonance, total internal reflection, fluorescence and chemiluminescence, (c) electrochemical sensors based on potentiometry, amperometry, and electrochemical impedance, and (d) piezoresistive sensors, such as quartz crystal microbalances and microcantilevers. The last section reports the most recent developments for wearable non-invasive temperature and pH sensors suitable for wound monitoring.

Signal enhancement and low oxidation potentials for miniaturized ECL biosensors via N-butyldiethanolamine

We present studies on ruthenium-based electrochemiluminescence (ECL) focusing on conditions supporting signal enhancement and low oxidation potentials. Low oxidation potentials (LOPs) are especially attractive for miniaturized ECL biosensors, as microfabricated electrodes tend to detach from their support when used with high currents and operated at high potentials. Furthermore, high potentials or current densities can lead to damage of typical biosensor surface coatings and biological probes. The possibility of generating LOP ECL signals at a potential below 900 mV was therefore studied for Ru(bpy)₃²⁺ with two typical coreactants, i.e. 2-(dibutylamino)ethanol (DBAE) and tripropylamine (TPA), as well as with the tertiary amine N-butyldiethanolamine (NBEA). Furthermore, the effect of buffer components and pH values on ECL signal generation was investigated. We could show a significant LOP ECL signal for NBEA. We found that Tris buffer, with its ability to form complexes with transition metal ions, has a positive influence on this ECL signal in terms of signal strength and LOP capabilities. Specifically, at basic pH values significant increases in ECL signals were observed at 900 mV and at 1.2 V. In fact, the ECL signal at 1.2 V was three times higher than the signal observed in phosphate buffer at a pH of 7, and it was thirty times higher than the ECL signal for TPA under these conditions. The LOP signal for NBEA in Tris buffer at pH 8.5 was similar to the signal obtained for TPA in phosphate buffer at pH 8.5 but three times higher than for TPA at pH 7.0. Interestingly, the coreactant DBAE was neither significantly influenced by the buffer system or pH nor did it present a valuable LOP ECL signal. Finally, it was found that high peak currents in cyclic voltammograms are not the indicators for high ECL signals, which should be obvious because the ECL mechanism requires more complex electron transfers. Overall, the standard TPA ECL at 1.2 V in phosphate buffer at pH 7.0 can successfully be replaced by NBEA ECL at 900 mV in Tris at pH 8.5 providing significantly higher signals accompanied by more gentle electrochemical conditions.

An ultrasensitive electrogenerated chemiluminescence-based immunoassay for specific detection of Zika virus

Zika virus (ZIKV) is a globally emerging mosquito-transmitted flavivirus that can cause severe fetal abnormalities, including microcephaly. As such, highly sensitive, specific, and cost-effective diagnostic methods are urgently needed. Here, we report a novel electrogenerated chemiluminescence (ECL)-based immunoassay for ultrasensitive and specific detection of ZIKV in human biological fluids. We loaded polystyrene beads (PSB) with a large number of ECL labels and conjugated them with anti-ZIKV monoclonal antibodies to generate anti-ZIKV-PSBs. These anti-ZIKV-PSBs efficiently captured ZIKV in solution forming ZIKV-anti-ZIKV-PSB complexes, which were subjected to measurement of ECL intensity after further magnetic beads separation. Our results show that the anti-ZIKV-PSBs can capture as little as 1 PFU of ZIKV in 100 μl of saline, human plasma, or human urine. This platform has the potential for development as a cost-effective, rapid and ultrasensitive assay for the detection of ZIKV and possibly other viruses in clinical diagnosis, epidemiologic and vector surveillance, and laboratory research.

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

Anodic Electrogenerated Chemiluminescence of Ru(bpy)3(2+) with CdSe Quantum Dots as Coreactant and Its Application in Quantitative Detection of DNA

In the present paper, we report that CdSe quantum dots (QDs) can act as the coreactant of Ru(bpy)₃²⁺ electrogenerated chemiluminescence (ECL) in neutral condition. Strong anodic ECL signal was observed at ~1.10 V at CdSe QDs modified glassy carbon electrode (CdSe/GCE), which might be mainly attributed to the apparent electrocatalytic effect of QDs on the oxidation of Ru(bpy)₃²⁺. Ru(bpy)₃²⁺ can be intercalated into the loop of hairpin DNA through the electrostatic interaction to fabricate a probe. When the probe was bound to the CdSe QDs modified on the GCE, the intense ECL signal was obtained. The more Ru(bpy)₃²⁺ can be intercalated when DNA loop has larger diameter and the stronger ECL signal can be observed. The loop of hairpin DNA can be opened in the presence of target DNA to release the immobilized Ru(bpy)₃²⁺, which can result in the decrease of ECL signal. The decreased ECL signal varied linearly with the concentration of target DNA, which showed the ECL biosensor can be used in the sensitive detection of DNA. The proposed ECL biosensor showed an excellent performance with high specificity, wide linear range and low detection limit.

Magnetic chemiluminescent immunoassay for human C-reactive protein on the centrifugal microfluidics platform

Schematic of the LabDisk-based hCRP MCIA. The antibody-coated dynabeads are sequentially transported through the immunoassay buffers by magnetic actuation. Finally the chemiluminescence signal is acquired from a detection cavity.

Strong anodic electrochemiluminescence from dissolved oxygen with 2-(dibutylamino) ethanol for glucose oxidase assay

A strong anodic electrochemiluminescence of dissolved oxygen with 2-(dibutylamino) ethanol is developed to detect glucose oxidase.

Mapping electrogenerated chemiluminescence reactivity in space: mechanistic insight into model systems used in immunoassays

Mapping the reactivity of a redox-sensitive luminescent microobject positioned in fluxes of reactive species allows analyzing complex mechanistic processes such as the electrogenerated chemiluminescence of model systems used in immunoassays.

Current analytical strategies for C-reactive protein quantification in blood

The measurement of serum C-reactive protein (CRP) levels has been given particular interest as a marker of inflammation associated with cardiovascular diseases. CRP belongs to the pentraxin family of proteins and the routine clinical analysis of CRP in blood samples is used as an important factor in primary prevention programmes together with causative and predisposing factors. This review focuses on the most representative methodologies and strategies for CRP detection and quantification that have been recently proposed, as well as reviewing those that are currently being developed for the specific, sensitive, inexpensive and high-throughput blood analysis of this protein.

Sensitive and rapid quantification of C-reactive protein using quantum dot-labeled microplate immunoassay

Background

High-sensitivity C-reactive protein (hs-CRP) assay is of great clinical importance in predicting risks associated with coronary heart disease. Existing hs-CRP assays either require complex operation or have low throughput and cannot be routinely implemented in rural settings due to limited laboratory resources.

Methods

We developed a novel hs-CRP assay capable of simultaneously quantifying over 90 clinical samples by using quantum dots-labeled immunoassay within a standard 96-well microplate. The specificity of the assay was enhanced by adopting two monoclonal antibodies (mAbs) that target distinct hs-CRP epitopes, serving as the coating antibody and the detection antibody, respectively. In the presence of hs-CRP antigen, the fluorescence intensity of the mAb-Ag-mAb sandwich complex captured on the microplate can be read out using a microplate reader.

Results

The proposed hs-CRP assay provides a wide analytical range of 0.001-100 mg/L with a detection limit of 0.06 (0.19) μg/L within 1.5 h. The accuracy of the proposed assay has been confirmed for low coefficient of variations (CVs), 2.27% (intra-assay) and 8.52% (inter-assay), together with recoveries of 96.7-104.2%. Bland-Altman plots of 104 clinical samples exhibited good consistency among the proposed assay, commercial high-sensitivity ELISA, and nephelometry, indicating the prospects of the newly developed hs-CRP assay as an alternative to existing hs-CRP assays.

Conclusion

The developed assay meets the needs of the rapid, sensitive and high-throughput determination of hs-CRP levels within a short time using minimal resources. In addition, the developed assay can also be used to detect and quantify other diagnostic biomarkers by immobilizing specific monoclonal antibodies.

Carbon optically transparent electrodes for electrogenerated chemiluminescence

This study investigates pyrolyzed photoresist film (PPF)-based carbon optically transparent electrodes (C-OTEs) for use in electrogenerated chemiluminescence (ECL) studies. Oxidative-reductive ECL is obtained with a well-characterized ECL system, C8S3 J-aggregates with 2-(dibutylamino)ethanol (DBAE) as coreactant. Simultaneous cyclic voltammograms (CVs) and ECL transients are obtained for three thicknesses of PPFs and compared to nontransparent glassy carbon (GC) and the conventional transparent electrode indium tin oxide (ITO) in both front face and transmission electrode cell geometries. Despite positive potential shifts in oxidation and ECL peaks, attributed to the internal resistance of the PPFs that result from their nanoscale thickness, the PPFs display similar ECL activity to GC, including the low oxidation potential (LOP) observed for amine coreactants on hydrophobic electrodes. Reductive-oxidative ECL was obtained using the well-studied ECL luminophore Ru(bpy)(3)(2+), where the C-OTEs outperformed ITO because of electrochemical instability of ITO at very negative potentials. The C-OTEs are promising electrodes for ECL applications because they yield higher ECL than ITO in both oxidative-reductive and reductive-oxidative ECL modes, are more stable in alkaline solutions, display a wide potential window of stability, and have tunable transparency for more efficient detection of ECL.