Electrochemiluminescent Quenching of Quantum Dots for Ultrasensitive Immunoassay through Oxygen Reduction Catalyzed by Nitrogen-Doped Graphene-Supported Hemin

Realizing the label-free sensitive detection of carcinoembryogenic antigen (CEA) in blood serum via a MNC-decorated flexible immunosensor

A label-free electrochemical immunosensor utilising nitrogen-rich mesoporous carbon (MNC) as the substrate material was developed for the sensitive quantification of carcinoembryonic antigen (CEA). The synergic interactions between MNC and AbCEA also eliminated the need for coupling agents such as EDC/NHS. The novel immunosensor demonstrated a wide detection range from 500 fM (9.04 pg mL-1) to 50 nM (1 μg mL-1) and a low detection limit (LOD) of 500 fM. Moreover, the immunosensor showed sensitivities of 12.27 mA nM-1 cm-2 and 0.066 mA nM-1 cm-2 for detecting CEA in the linear ranges 10 pM to 1 nM and 2 nM to 50 nM, respectively, while maintaining long-term storage stability of 6 weeks. Analysis of real serum sample analysis yielded highly accurate results with recovery rates ranging from 99.3% to 103.7%. Furthermore, the developed paper-based screen-printed electrode exhibited a similar detection range, suggesting its potential for use in point-of-care detection devices in future applications.

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.

Electrochemiluminescence nanoemitters for immunoassay of protein biomarkers

The family of electrochemiluminescent luminophores has witnessed quick development since the electrochemiluminescence (ECL) phenomenon of silicon nanoparticles was first reported in 2002. Moreover, these developed ECL nanoemitters have extensively been applied in sensitive detection of protein biomarker by combining with immunological recognition. This review firstly summarized the origin and development of various ECL nanoemitters including inorganic and organic nanomaterials, with an emphasis on metal-organic frameworks (MOFs)-based ECL nanoemitters. Several effective strategies to amplify the ECL response of nanoemitters and improve the sensitivity of immunosensing were discussed. The application of ECL nanoemitters in immunoassay of protein biomarkers for diagnosis of cancers and other diseases, especially lung cancer and heart diseases, was comprehensively presented. The recent development of ECL imaging with the nanoemitters as ECL tags for detection of multiplex protein biomarkers on single cell membrane also attracted attention. Finally, the future opportunities and challenges in the ECL biosensing field were highlighted.

Nanomaterial Gas Sensors for Biosensing Applications: A Review

BACKGROUND

Nanomaterial is one of the most used materials for various gas sensing applications to detect toxic gases, human breath, and other specific gas sensing. One of the most important applications of nanomaterial based gas sensors is biosensing applications. In this review article, the gas sensors for biosensing are discussed on the basis of crystalline structure and different categories of nanomaterial.

METHODS

In this paper, firstly, rigorous efforts have been made to find out research questions by going through a structured and systematic survey of available peer reviewed high quality articles in this field. The papers related to nanomaterial based biosensors are then reviewed qualitatively to provide substantive findings from the recent developments in this field.

RESULTS

In this mini-review article, firstly, classifications of nanomaterial gas sensors have been presented on the basis of the crystalline structure of nanomaterial and different types of nanomaterial available for biosensing applications. Further, the gas sensors based on nanomaterial for biosensing applications are collected and reviewed in terms of their performance parameters such as sensing material used, target gas component, detection ranges (ppm-ppb), response time, operating temperature and method of detection, etc. The different nanomaterials possess slightly different sensing and morphological properties due to their structure; therefore, it can be said that a nanomaterial must be selected carefully for a particular application. The 1D nanomaterials show the best selectivity and sensitivity for gases available in low concentration ranges due to their miniaturised structure compared to 2D and 3D nanomaterials. However, these 2D and 3D nanomaterials also so good sensing properties compared to bulk semiconductor materials. The polymer and nanocomposites which are also discussed in this patent article have opened the door for future research and have great potential for new generation gas sensors for detecting biomolecules.

CONCLUSION

These nanomaterials extend great properties towards sensing the application of different gases for a lower concentration of particular gas particles. Nano polymer and nanocomposites have great potential to be used as gas sensors for the detection of biomolecules.

DNA Nanotweezers for Biosensing Applications: Recent Advances and Future Prospects

DNA nanotweezers (DTs) are reversible DNA nanodevices that can optionally switch between opened and closed states. Due to their excellent flexibility and high programmability, they have been recognized as a promising platform for constructing a diversity of biosensors and logic gates, as well as a versatile tool for molecular biology studies. In this review, we provide an overview of biosensing applications using DTs. First, the design and working principle of DTs are introduced. Next, the signal producing principles of DTs are summarized. Furthermore, biosensing applications of DTs for varying targets and purposes, both in buffers and complex biological environments, are highlighted. Finally, we provide potential opportunities and challenges for the further development of DTs.

Highly active iron-nitrogen-boron-carbon bifunctional electrocatalytic platform for hydrogen peroxide sensing and oxygen reduction

An iron-nitrogen-boron-carbon (Fe-N-B-C) bifunctional electrocatalyst was prepared by means of a facile one-step hydrothermal reduction of graphene oxide using dimethylamine borane as doping agent. In addition, hemins were efficiently anchored during doping/reducing process on this modified graphene. The as-prepared Fe-N-B-C electro-catalyst showed enhanced response as regards its potential for reduction of H₂O₂ and O₂. In view of its catalytic activity, this Fe-N-B-C material was tested for the determination of H₂O₂ with a chronoamperometry method, obtaining a detection limit as low as 0.055 μM, which is better than that of some Hemin-N-C materials. Regarding O₂ reduction reaction, a study performed using a rotating disk electrode indicated that this material exhibits a positive onset potential (0.90V vs. RHE), high selectivity (4e^- process), high limiting-current density (4.75 mA cm^-2) and strong resistance against the crossover-effect from methanol in alkaline medium, making it to be the promising candidate as alternative for commercial Pt/C catalysts. These results could have commercial and environmental relevance and would deserve further complementary investigation.

Sandwich electrochemical carcinoembryonic antigen aptasensor based on signal amplification of polydopamine functionalized graphene conjugate Pd-Pt nanodendrites

Carcinoembryonic antigen (CEA) is considered as a disease biomarker, which is related to various cancers and tumors in the human bodies. Sensitive detection of CEA is significant for clinical diagnosis and treatment. Herein, we proposed an electrochemical aptasensor for CEA detection based on the amplification driven by polydopamine functional graphene and Pd-Pt nanodendrites (PDA@Gr/Pd-PtNDs), conjugated hemin/G-quadruplex (hemin/G4), which possess mimicking peroxidases activity. Firstly, PDA@Gr was modified on the electrode surface for fixing CEA aptamer 1 (Apt1). Then, PDA@Gr/Pd-PtNDs with large surface area served as matrix for immobilization of hemin/G4 to obtain the secondary aptamer. In virtue of the sandwich-type specific reaction between CEA and the corresponding aptamers, the second aptamer was captured on the sensing interface, which can catalyze the oxidation of signal probe hydroquinone (HQ) with H₂O₂ and amplify current signal. Furthermore, the electrochemical signals of HQ were proportional with CEA concentrations. Under the optimal conditions, a dynamic response range from 50 pg/mL to 1.0 μg/mL and a detection limit of 6.3 pg/mL for CEA were obtained. Moreover, the proposed strategy represented satisfactory sensitivity and stability, and showed a good precision in real samples application.

Paper-Based Immunosensors with Bio-Chemiluminescence Detection

Since the introduction of paper-based analytical devices as potential diagnostic platforms a few decades ago, huge efforts have been made in this field to develop systems suitable for meeting the requirements for the point-of-care (POC) approach. Considerable progress has been achieved in the adaptation of existing analysis methods to a paper-based format, especially considering the chemiluminescent (CL)-immunoassays-based techniques. The implementation of biospecific assays with CL detection and paper-based technology represents an ideal solution for the development of portable analytical devices for on-site applications, since the peculiarities of these features create a unique combination for fitting the POC purposes. Despite this, the scientific production is not paralleled by the diffusion of such devices into everyday life. This review aims to highlight the open issues that are responsible for this discrepancy and to find the aspects that require a focused and targeted research to make these methods really applicable in routine analysis.

Nanostructures for Biosensing, with a Brief Overview on Cancer Detection, IoT, and the Role of Machine Learning in Smart Biosensors

Biosensors are essential tools which have been traditionally used to monitor environmental pollution and detect the presence of toxic elements and biohazardous bacteria or virus in organic matter and biomolecules for clinical diagnostics. In the last couple of decades, the scientific community has witnessed their widespread application in the fields of military, health care, industrial process control, environmental monitoring, food-quality control, and microbiology. Biosensor technology has greatly evolved from in vitro studies based on the biosensing ability of organic beings to the highly sophisticated world of nanofabrication-enabled miniaturized biosensors. The incorporation of nanotechnology in the vast field of biosensing has led to the development of novel sensors and sensing mechanisms, as well as an increase in the sensitivity and performance of the existing biosensors. Additionally, the nanoscale dimension further assists the development of sensors for rapid and simple detection in vivo as well as the ability to probe single biomolecules and obtain critical information for their detection and analysis. However, the major drawbacks of this include, but are not limited to, potential toxicities associated with the unavoidable release of nanoparticles into the environment, miniaturization-induced unreliability, lack of automation, and difficulty of integrating the nanostructured-based biosensors, as well as unreliable transduction signals from these devices. Although the field of biosensors is vast, we intend to explore various nanotechnology-enabled biosensors as part of this review article and provide a brief description of their fundamental working principles and potential applications. The article aims to provide the reader a holistic overview of different nanostructures which have been used for biosensing purposes along with some specific applications in the field of cancer detection and the Internet of things (IoT), as well as a brief overview of machine-learning-based biosensing.

Enhancing the peroxidase-mimicking activity of hemin by covalent immobilization in polymer nanogels

A polymeric nanozyme that can closely mimic peroxidase is presented. The coordination between pendant hemins and primary amines together with the synergistic interactions between substrates and nanogels contribute to the enhanced catalytic activity.

A visual electrochemiluminescence biosensor based on CuInZnS quantum dots for superoxide dismutase detection

Superoxide dismutase (SOD), also known as liver protein, is a substance widely distributed in various biological cells. It has the function of catalyzing the disproportionation reaction of superoxide free radicals. SOD can form an antioxidant chain together with peroxidase, catalase, and other substances in the body of organisms, and thus, is one of the indispensable important substances in the body of organisms. In this work, we provided a simple and fast visual electrochemiluminescence (ECL) sensor for SOD detection. CuInZnS quantum dots (QDs) worked as the ECL luminophore with hydrogen peroxide as co-reactant. In the sensing process, SOD and CuInZnS QDs on a glassy carbon electrode (GCE) competed with each other for hydrogen peroxide to produce superoxide during electrochemical luminescence, thus quenching the ECL signal of CuInZnS QDs. The proposed sensor can quantify SOD with a limit of detection (LOD) of 0.03 μg/mL. In addition, the change in the CuInZnS QDs ECL signal was easily observed with a smartphone camera. The results indicated that this sensor could effectively work in the detection of SOD in human blood. Graphical abstract.

DNA-Programmed plasmonic ELISA for the ultrasensitive detection of protein biomarkers

We report a novel DNA-programmed plasmonic enzyme-linked immunosorbent assay (ELISA) for the ultrasensitive detection of protein biomarkers with the naked eye.

Arginine-Induced Self-Assembly of Protoporphyrin to Obtain Effective Photocatalysts in Aqueous Media Under Visible Light

The fabrication of controlled supramolecular nanostructures via self-assembly of protoporphyrin IX (PPIX) was studied with enantiomerically pure l-arginine and d-arginine, and we have shown that stoichiometry controlled the morphology formed. The nanostructure morphology was mainly influenced by the delicate balance of π-π stacking interactions between PPIX cores, as well as H-bonding between the deprotonated acidic head group of PPIX with the guanidine head group of arginine. PPIX self-assembled with l-/d-arginine to create rose-like nanoflower structures for four equivalents of arginine that were 5–10 μm in length and 1–4 μm diameter. We employed UV-vis, fluorescence spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), dynamic light scattering (DLS) and Fourier transform infrared spectroscopy (FT-IR) techniques to characterize the resulting self-assembled nanostructures. Furthermore, we investigated the catalytic activity of PPIX and arginine co-assembled materials. The fabricated PPIX–arginine nanostructure showed high enhancement of photocatalytic activity through degradation of rhodamine B (RhB) with a decrease in dye concentration of around 78–80% under simulated visible radiation.

An indirect ELISA-inspired dual-channel fluorescent immunoassay based on MPA-capped CdTe/ZnS QDs

To meet the need for high-throughput immunoassays, many multiplex fluorescent immunoassays have been proposed. Most of them need different kinds of fluorescent label indicators during the test. In this work, a novel indirect ELISA-inspired dual-channel fluorescent immunoassay based on 3-mercaptopropionic acid capped CdTe/ZnS quantum dots (QDs) was constructed. The ELISA wells were coated with two kinds of antigen-QD complex. When the primary antibodies were present in a sample, they mediated the binding of a secondary antibody-DNA-gold nanoparticle complex to the antigen-QD complex. Then the gold nanoparticles quenched the fluorescence of the QDs and a decrease in fluorescence intensity was observed. Thus, the amount of primary antibody could be estimated from the decrease of fluorescence intensity. Owing to the wide absorption range and the relatively narrow emission band of the QDs, the dual-channel fluorescent immunoassay system could work at the same excitation wavelength and the emission wavelengths of each channel had no interference. As a result, two different kinds of primary antibody could be detected at the same time in one ELISA well, which simplified the operation and greatly improved the efficiency. Besides, only one type of secondary antibody needs to be added to the prepared microtiter plates, which further simplified the operation during the detection procedure. This dual-channel fluorescent immunoassay system will provide new insights into high-throughput immunodetection. Graphical abstract.

AuCu xO-Embedded Mesoporous CeO2 Nanocomposites as a Signal Probe for Electrochemical Sensitive Detection of Amyloid-Beta Protein

A sandwich-type electrochemical immunosensor for detecting amyloid-beta protein was fabricated based on Au NP-functionalized reduced graphene oxide (Au@rGO) as an effective sensing platform and AuCu _xO-embedded mesoporous CeO₂ (AuCu _xO@m-CeO₂) nanocomposites as the catalytic matrix. The AuCu _xO@m-CeO₂ composites were obtained by adjusting the amount of m-CeO₂ in the reaction to expose enormous active sites. Also, AuCu _xO@m-CeO₂ was applied as a matrix to immobilize antibodies by forming bridged bonds between m-CeO₂ and carboxyl functional groups of antibodies without additional agents. Furthermore, AuCu _xO with prominent catalytic activities dramatically improved the performance of the fabricated immunosensor. Also, the morphology, structure, and electronic state of the surface were characterized by SEM, XRD, TEM, and XPS. In addition, the immunosensor demonstrated a wide linear range of 100 fg mL^-1 to 10 ng mL^-1. This study may provide a way for sensitively detecting various biomarkers.

A dual amplification electrochemical immunosensor based on HRP-Au@Ag NPs for carcinoembryonic antigen detection

A sensitive sandwich-type electrochemical immunosensor based on dual amplification strategy was constructed. The dual amplification strategy has been used secondary antibody(Ab₂)-horseradish peroxidase(HRP)-Au@Ag nanoparticles (Au@Ag NPs) for carcinoembryonic antigen(CEA) detection. Ab₂-HRP-Au@Ag NPs as dual amplification markers triggered the disproportionation of H₂O₂, which could facilitate the catalytic oxidation of hydroquinone to quinone(BQ). In addition, due to their large surface area and excellent conductivity, nitrogen-doped graphene were used as a platform to firmly assemble primary antibody (Ab₁). Above mentioned generated amout of BQ are corresponding to trace CEA, resulting in the highly electrochemical reduction signal. Under the optimal conditions, the linear range of CEA concentration was 0.0001-100 ng mL^-1, and the limit of detection (LOD) could be as low as 0.05 pg mL^-1. Importantly, the immunosensor also showed acceptable stability, reproducibility and selectivity.

Hapten-Grafted Programmed Probe as a Corecognition Element for a Competitive Immunosensor to Detect Acetamiprid Residue in Agricultural Products

We have developed an effective competitive electrochemical immunosensor assay based on hapten-grafted programmed probe (HGPP) as a corecognition element for highly sensitive and selective detection of acetamiprid. Starting with the synthesis of hapten, HGPP was prepared using carboxyl group in the hapten and amino group in the 5' end of the programmed probe through covalent conjugation. Acetamiprid present in samples competes with HGPP to bind with capture antibody on the electrodes by specific recognition interaction. Methylene blue probe (MBP) was used as the electrochemical redox probe to capture the hybridized HGPP on the electrodes. The competitive reaction changes in accordance with the quantity of the target acetamiprid in the sample, as the amounts of the hybridized HGPP and the immobilized antibody are constant, i.e., the more acetamiprid samples are added, the less MBP is combined on the electrodes. In the optimal conditions, thus, biosensor output showed a linear relationship from 5 to 10⁵ ng L^-1 for the acetamiprid assay with a detecting limit of 3.2 ng L^-1. The biosensor was successful in quantifying the amount of acetamiprid in spiked strawberry and cabbage extracts. This competitive immunosensor assay represents a rapid and sensitive technology for acetamiprid assay or other small molecule targets in food.

Quenching Electrochemiluminescence Immunosensor Based on Resonance Energy Transfer between Ruthenium (II) Complex Incorporated in the UiO-67 Metal-Organic Framework and Gold Nanoparticles for Insulin Detection

This work describes a sandwich-type electrochemiluminescence (ECL) strategy for insulin detection by using Ru(bpy)₃²⁺ as the luminophore which was encapsulated in the UiO-67 metal-organic framework (UiO-67/Ru(bpy)₃²⁺). Because UiO-67 possesses the characteristics of large specific surface area and porosity, more Ru(bpy)₃²⁺ could be loaded onto its surface and holes, thus greatly improving the ECL efficiency. Furthermore, the ECL resonance energy transfer (ECL-RET) could occur between UiO-67/Ru(bpy)₃²⁺ (ECL donor) and Au@SiO₂ nanoparticles (ECL acceptor), resulting in a conspicuously decreased ECL response. The ECL spectrum of UiO-67/Ru(bpy)₃²⁺ which exhibited strong ECL intensity has suitable spectral overlap with the absorption spectrum of Au@SiO₂, which further proved the occurrence of the ECL-RET action. The ECL intensity decreased with the increase of the concentration of insulin. In addition, the sandwich-type ECL immunosensor was applied to insulin detection, and the ECL decrease efficiency was found to be logarithmically related to the concentration of the insulin antigen in the range of 0.0025 to 50 ng mL^-1 with the limit of detection of 0.001 ng mL^-1. Meanwhile, this work provides an important reference for the application of metal-organic frameworks in the ECL and ECL-RET study and also exhibits potential capability in the detection of other hormones.

Development of quantum dot-based biosensors: principles and applications

We review the recent advances in quantum dot-based biosensors and focus on quantum dot-based fluorescent, bioluminescent, chemiluminescent, and photoelectrochemical biosensors.

Recent advances and developments on integrating nanotechnology with chemiluminescence assays

Chemiluminescence (CL) techniques are extensively utilized for detection of analytes due to their high sensitivity, rapidity and selectivity. With the advent of nanotechnology and incorporation of the nanoparticles in the CL system has revolutionized the assays due to their unique optical and mechanical properties. Several CL-based reactions have been developed where these nanoparticle based CL sensors have evolved as excellent prospects for sensing in various analytical applications. This review article addresses the nanoparticles based CL detection system that are recently developed, the mechanisms has been summarized and the role of luminophors have been discussed. This article critically analyzes the optimal conditions for the CL detection along with quantitative assessment of the analytes. We have included the use of semiconductor nanoparticles, metal nanoparticles, graphene based nanostructures, mesoporous nanospheres, layered double hydroxides, clays for CL detection. The scope and application of these nanoscale material based CL system in various branches of science and technology including chemistry, biomedical applications, pharmaceutics, food, environmental and toxicological applications has been critically summarized.

Energy-Related Small Molecule Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based Systems

Globally increasing energy demands and environmental concerns related to the use of fossil fuels have stimulated extensive research to identify new energy systems and economies that are sustainable, clean, low cost, and environmentally benign. Hydrogen generation from solar-driven water splitting is a promising strategy to store solar energy in chemical bonds. The subsequent combustion of hydrogen in fuel cells produces electric energy, and the only exhaust is water. These two reactions compose an ideal process to provide clean and sustainable energy. In such a process, a hydrogen evolution reaction (HER), an oxygen evolution reaction (OER) during water splitting, and an oxygen reduction reaction (ORR) as a fuel cell cathodic reaction are key steps that affect the efficiency of the overall energy conversion. Catalysts play key roles in this process by improving the kinetics of these reactions. Porphyrin-based and corrole-based systems are versatile and can efficiently catalyze the ORR, OER, and HER. Because of the significance of energy-related small molecule activation, this review covers recent progress in hydrogen evolution, oxygen evolution, and oxygen reduction reactions catalyzed by porphyrins and corroles.

Smart Nanocomposites of Cu-Hemin Metal-Organic Frameworks for Electrochemical Glucose Biosensing

Herein, a smart porous material, Cu-hemin metal-organic-frameworks (Cu-hemin MOFs), was synthesized via assembling of Cu²⁺ with hemin to load glucose oxidase (GOD) for electrochemical glucose biosensing for the first time. The formation of the Cu-hemin MOFs was verified by scanning electron microscopy, X-ray powder diffraction, Fourier transform infrared spectroscopy, N₂ adsorption/desorption isotherms, UV-vis absorption spectroscopy, fluorescence spectroscopy, thermal analysis and electrochemical techniques. The results indicated that the Cu-hemin MOFs showed a ball-flower-like hollow cage structure with a large specific surface area and a large number of mesopores. A large number of GOD molecules could be successfully loaded in the pores of Cu-hemin MOFs to keep their bioactivity just like in a solution. The GOD/Cu-hemin MOFs exhibited both good performance toward oxygen reduction reaction via Cu-hemin MOFs and catalytic oxidation of glucose via GOD, superior to other GOD/MOFs and GOD/nanomaterials. Accordingly, the performance of GOD/Cu-hemin MOFs-based electrochemical glucose sensor was enhanced greatly, showing a wide linear range from 9.10 μM to 36.0 mM and a low detection limit of 2.73 μM. Moreover, the sensor showed satisfactory results in detection of glucose in human serum. This work provides a practical design of new electrochemical sensing platform based on MOFs and biomolecules.

Efficient double-quenching of electrochemiluminescence from CdS:Eu QDs by hemin-graphene-Au nanorods ternary composite for ultrasensitive immunoassay

A novel ternary composite of hemin-graphene-Au nanorods (H-RGO-Au NRs) with high electrocatalytic activity was synthesized by a simple method. And this ternary composite was firstly used in construction of electrochemiluminescence (ECL) immunosensor due to its double-quenching effect of quantum dots (QDs). Based on the high electrocatalytic activity of ternary complexes for the reduction of H₂O₂ which acted as the coreactant of QDs-based ECL, as a result, the ECL intensity of QDs decreased. Besides, due to the ECL resonance energy transfer (ECL-RET) strategy between the large amount of Au nanorods (Au NRs) on the ternary composite surface and the CdS:Eu QDs, the ECL intensity of QDs was further quenched. Based on the double-quenching effect, a novel ultrasensitive ECL immunoassay method for detection of carcinoembryonic antigen (CEA) which is used as a model biomarker analyte was proposed. The designed immunoassay method showed a linear range from 0.01 pg mL⁻¹ to 1.0 ng mL⁻¹ with a detection limit of 0.01 pg mL⁻¹. The method showing low detection limit, good stability and acceptable fabrication reproducibility, provided a new approach for ECL immunoassay sensing and significant prospect for practical application.

Low potential detection of indole-3-acetic acid based on the peroxidase-like activity of hemin/reduced graphene oxide nanocomposite

An amperometric sensor was firstly established for the detection of indole-3-acetic acid (IAA) at low potential based on the hemin/reduced graphene oxide (hemin/rGO) composite. The hemin/rGO nanocomposite was prepared by a simple and facile hydrothermal method without using any reducing agent. It exhibited peroxidase-like activity for the catalytic oxidation of IAA in the presence of oxygen. The consumption of oxygen has a linear relationship with the concentration of IAA in the range from 0.1 to 43μM and from 43 to 183μM. The detection limit was down to 0.074μM. This sensor was unaffected by many interfering substances and stable over time. Such work broadened the application of hemin/rGO and provided a new method for IAA detection.

Cytotoxicity investigation of luminescent nanohybrids based on chitosan and carboxymethyl chitosan conjugated with Bi2S3 quantum dots for biomedical applications

Bioengineered hybrids are emerging as a new class of nanomaterials consisting of a biopolymer and inorganic semiconductors used in biomedical and environmental applications. The aim of the present work was to determine the cytocompatibility of novel water-soluble Bi2S3 quantum dots (QDs) functionalized with chitosan and O-carboxymethyl chitosan (CMC) as capping ligands using an eco-friendly aqueous process at room temperature. These hybrid nanocomposites were tested for cytocompatibility using a 3-(4,5-dimethylthiazol-2yl) 2,5-diphenyl tetrazolium bromide (MTT) cell proliferation assay with cultured human osteosarcoma cells (SAOS), human embryonic kidney cells (HEK293T cells) and a LIVE/DEAD® viability-cytotoxicity assay. The results of the in vitro assays demonstrated that the CMC and chitosan-based nanohybrids were not cytotoxic and exhibited suitable cell viability responses. However, despite the "safe by design" approach used in this research, we have proved that the impact of the size, surface charge and biofunctionalization of the nanohybrids on cytotoxicity was cell type-dependent due to complex mechanisms. Thus, these novel bionanocomposites offer promising prospects for potential biomedical and pharmaceutical applications as fluorescent nanoprobes.

Fluorescence ELISA for sensitive detection of ochratoxin A based on glucose oxidase-mediated fluorescence quenching of CdTe QDs

The present study described a novel fluorescence enzyme-linked immunosorbent assay (ELISA) used to detect ochratoxin A (OTA) by using the glucose oxidase (GOx)-mediated fluorescence quenching of mercaptopropionic acid-capped CdTe quantum dots (MPA-QDs), in which GOx was used as an alternative to horseradish peroxidase (HRP) for the oxidization of glucose into hydrogen peroxide (H2O2) and gluconic acid. The MPA-QDs were used as a fluorescent signal output, whose fluorescence variation was extremely sensitive to the presence of H2O2 or hydrogen ions in the solution. Under the optimized conditions, the proposed fluorescence ELISA demonstrated a good linear detection of OTA in corn extract from 2.4 pg mL(-1) to 625 pg mL(-1) with a limit of detection of 2.2 pg mL(-1), which was approximately 15-fold lower than that of conventional HRP-based ELISA. Our developed fluorescence immunoassay was also similar to HRP-based ELISA in terms of selectivity, accuracy, and reproducibility. In summary, this study was the first to use the GOx-mediated fluorescence quenching of QDs in immunoassay to detect OTA, offering a new possibility for the analysis of other mycotoxins and biomolecules.

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

Increased electrocatalyzed performance through hairpin oligonucleotide aptamer-functionalized gold nanorods labels and graphene-streptavidin nanomatrix: Highly selective and sensitive electrochemical biosensor of carcinoembryonic antigen

We report a triplex signal amplification strategy for sensitive biosensing of cancer biomarker by taking advantage of hairpin-shaped oligonucleotide-functionalized gold nanorods (HO-GNRs), graphene and the avidin-biotin reation. The strategy expands electrochemical detection of carcinoembryonic antigen (CEA) by using an aptamer as biosensor's recognition element and HO-GNRs as signal enhancer. To construct this biosensor, the GNR was used as a carrier of horseradish peroxidase (HRP) and HO aptamer with a biotin at the 3'-end and a thiol at the 5'-end, which amplified the electrochemical response because of a large molar ratio of HRP to HO. In the presence of target CEA, the binding reactions of CEA with the loop portions of the HOs caused HOs' loop-stem structure opened and exposed the biotins, and then HRP-GNRs-HO conjugates were captured on graphene and streptavidin modified electrodes via the reaction between the exposed biotins and preimmobilized streptavidins. The accumulation of HRP effectively catalyzed the hydrogen peroxide-mediated oxidation of o-phenylenediamine to generate an electrochemical reduction current for CEA detection. Under optimal conditions, the electrochemical biosensor exhibited a wide dynamic range of 5pgmL(-1) and 50ngmL(-1) toward CEA standards with a low detection limit of 1.5pgmL(-1) (signal-to-noise ratio of 3). The proposed biosensor accurately detected CEA concentration in 8 human serum samples from patients with lung diseases, showing excellent correlations with standard chemiluminescence immunoassay. Furthermore, these results of target DNA detection made it abundantly clear that the proposed strategy can also be extended for detection of other relative biomarkers using different functional DNA structures, which shows great prospects in single-nucleotide polymorphisms analysis, biomedical sensing and application for accurate clinical diseases diagnostic.