Impedimetric immunoglobulin G immunosensor based on chemically modified graphenes

Ultrasensitive Electrochemical Impedance Chiral Discrimination and Sensing of Tryptophan Isomers Based on Core-Shell-Structured Au-Ag Nanoparticles

Au-Ag nanoparticles (Au-Ag NPs) with a core-shell structure are prepared and used for ultrasensitive electrochemical impedance (EI) discrimination of the isomers of tryptophan (Trp). As revealed by circular dichroism, rotary polarization caused by the Au-Ag NPs is consistent with D-Trp but opposite to L-Trp, and thus, the Au-Ag NPs can selectively combine with D-Trp through preferential interactions. Compared with Au-Ag NPs, the composites of D-Trp and Au-Ag NPs (Au-Ag NPs/D-Trp) display significantly increased charge transfer resistance (R_ct); differently, the R_ct of Au-Ag NPs/L-Trp remains almost unchanged because the Au-Ag NPs exhibit poor affinity toward L-Trp. Therefore, ultrasensitive EI enantiodiscrimination of the isomers of Trp is realized even at an extremely low concentration of the Trp isomers (0.1 nM). In addition, it is successfully applied in the ultrasensitive determination of D-Trp at a low concentration level (0.1 nM∼10 μM).

Flexible and wearable electrochemical biosensors based on two-dimensional materials: Recent developments

The research interest in wearable sensors has tremendously increased in recent years. Amid the different biosensors, electrochemical biosensors are unparalleled and ideal for the design and manufacture of such flexible and wearable sensors because of their various benefits, including convenient operation, quick response, portability, and inherent miniaturization. A number of studies on flexible and wearable electrochemical biosensors have been reported in recent years for invasive/non-invasive and real-time monitoring of biologically relevant molecules such as glucose, lactate, dopamine, cortisol, and antigens. To attain this, novel two-dimensional nanomaterials and their hybrids, various substrates, and detection methods have been explored to fabricate flexible conductive platforms that can be used to develop flexible electrochemical biosensors. In particular, there are many advantages associated with the advent of two-dimensional materials, such as light weight, high stretchability, high performance, and excellent biocompatibility, which offer new opportunities to improve the performance of wearable electrochemical sensors. Therefore, it is urgently required to study wearable/flexible electrochemical biosensors based on two-dimensional nanomaterials for health care monitoring and clinical analysis. In this review, we described recently reported flexible electrochemical biosensors based on two-dimensional nanomaterials. We classified them into specific groups, including enzymatic/non-enzymatic biosensors and affinity biosensors (immunosensors), recent developments in flexible electrochemical immunosensors based on polymer and plastic substrates to monitor biologically relevant molecules. This review will discuss perspectives on flexible electrochemical biosensors based on two-dimensional materials for the clinical analysis and wearable biosensing devices, as well as the limitations and prospects of the these electrochemical flexible/wearable biosensors.Graphical abstract.

Recent Advances in the Fabrication and Application of Graphene Microfluidic Sensors

This review reports the progress of the recent development of graphene-based microfluidic sensors. The introduction of microfluidics technology provides an important possibility for the advance of graphene biosensor devices for a broad series of applications including clinical diagnosis, biological detection, health, and environment monitoring. Compared with traditional (optical, electrochemical, and biological) sensing systems, the combination of graphene and microfluidics produces many advantages, such as achieving miniaturization, decreasing the response time and consumption of chemicals, improving the reproducibility and sensitivity of devices. This article reviews the latest research progress of graphene microfluidic sensors in the fields of electrochemistry, optics, and biology. Here, the latest development trends of graphene-based microfluidic sensors as a new generation of detection tools in material preparation, device assembly, and chip materials are summarized. Special emphasis is placed on the working principles and applications of graphene-based microfluidic biosensors, especially in the detection of nucleic acid molecules, protein molecules, and bacterial cells. This article also discusses the challenges and prospects of graphene microfluidic biosensors.

A simple gold nanoplasmonic SERS method for trace Hg2+ based on aptamer-regulating graphene oxide catalysis

The as-prepared graphene oxide (GO) exhibited a strong catalytic effect on reduction of HAuCl₄ by trisodium citrate to form gold nanoplasmons (AuNPs) with a strong surface-enhanced Raman scattering (SERS) effect at 1615 cm^-1 in the presence of molecular probe Victoria blue 4R (VB4r). SERS intensity increased with nanocatalyst GO concentration due to the formation of more AuNP substrates. The aptamer (Apt) of Hg²⁺ can bind to GO to form Apt-GO complexes, which can strongly inhibit nanocatalysis. When target Hg²⁺ is present, the formed stable Hg²⁺ -Apt complexes are separated from the GO surface, which leads to GO catalysis recovery. The enhanced SERS signal was linear to Hg²⁺ concentration in the range 0.25-10 nmol/L, with a detection limit of 0.08 nmol/L Hg²⁺ . Thus, a new gold nanoplasmon molecular spectral analysis platform was established for detecting Hg²⁺ , based on Apt regulation of GO nanocatalysis.

Current Technologies of Electrochemical Immunosensors: Perspective on Signal Amplification

An electrochemical immunosensor employs antibodies as capture and detection means to produce electrical charges for the quantitative analysis of target molecules. This sensor type can be utilized as a miniaturized device for the detection of point-of-care testing (POCT). Achieving high-performance analysis regarding sensitivity has been one of the key issues with developing this type of biosensor system. Many modern nanotechnology efforts allowed for the development of innovative electrochemical biosensors with high sensitivity by employing various nanomaterials that facilitate the electron transfer and carrying capacity of signal tracers in combination with surface modification and bioconjugation techniques. In this review, we introduce novel nanomaterials (e.g., carbon nanotube, graphene, indium tin oxide, nanowire and metallic nanoparticles) in order to construct a high-performance electrode. Also, we describe how to increase the number of signal tracers by employing nanomaterials as carriers and making the polymeric enzyme complex associated with redox cycling for signal amplification. The pros and cons of each method are considered throughout this review. We expect that these reviewed strategies for signal enhancement will be applied to the next versions of lateral-flow paper chromatography and microfluidic immunosensor, which are considered the most practical POCT biosensor platforms.

Immunocontrolling Graphene Oxide Catalytic Nanogold Reaction and Its Application to SERS Quantitative Analysis

The gold nanoreaction between HAuCl4 and H2O2 is very slow at 50 °C, and the nanoenzyme of graphene oxide (GO) greatly catalyzes the nanoreaction to form gold nanoparticles (AuNPs) with high SERS activity in the presence of Vitoria blue 4R (VB4r) molecular probes, strong resonance Rayleigh scattering (RRS), and surface plasmon resonance (SPR) absorption effect. With the increase of GO, the SERS, RRS, and SPR absorptions were enhanced linearly due to the formation of more AuNPs. The rabit antibody of human chorionic gonadotropin (RHCG) strongly adsorbed on the GO surface to inhibit its catalysis. Upon addition of human chorionic gonadotropin (HCG), the RHCG is separated from the GO surface due to the formation of HCG-RHCG specific immunocomplexes, which led to the recovery of GO catalysis. Using the new strategy of immunocontrolling GO catalysis, three types of resonance methods including SERS, RRS, and surface plasmon resonance (SPR) absorption have been developed for detection of HCG.

Recent Advances in Electrochemical Immunosensors

Immunosensors have experienced a very significant growth in recent years, driven by the need for fast, sensitive, portable and easy-to-use devices to detect biomarkers for clinical diagnosis or to monitor organic pollutants in natural or industrial environments. Advances in the field of signal amplification using enzymatic reactions, nanomaterials such as carbon nanotubes, graphene and graphene derivatives, metallic nanoparticles (gold, silver, various oxides or metal complexes), or magnetic beads show how it is possible to improve collection, binding or transduction performances and reach the requirements for realistic clinical diagnostic or environmental control. This review presents these most recent advances; it focuses first on classical electrode substrates, then moves to carbon-based nanostructured ones including carbon nanotubes, graphene and other carbon materials, metal or metal-oxide nanoparticles, magnetic nanoparticles, dendrimers and, to finish, explore the use of ionic liquids. Analytical performances are systematically covered and compared, depending on the detection principle, but also from a chronological perspective, from 2012 to 2016 and early 2017.

Graphene as a signal amplifier for preparation of ultrasensitive electrochemical biosensors

Early diagnostics of diseases performed with minimal money and time consumption has become achievable due to recent advances in development of biosensors. These devices use biorecognition elements for selective interaction with an analyte and signal readout is obtained via different types of transducers. Operational characteristics of biosensors have been reported to improve substantially, when a diverse range of nanomaterials was employed. This review presents construction of electrochemical biosensors based on graphene, atomically thin 2D carbon crystals, which is currently intensively studied nanomaterial. The most attractive directions of graphene applications in biosensor preparation are discussed here including novel detection and amplification schemes exploiting graphene's unique electrochemical, physical and chemical properties. The future of graphene-based biosensors is most likely bright, but there is still a lot of work to do to fulfill high expectations.

A simple and sensitive impedimetric aptasensor for the detection of tumor markers based on gold nanoparticles signal amplification

A simple and sensitive electrochemical impedimetric aptasensor based on gold nanoparticles (AuNPs) signal amplification was developed for the ultrasensitive detection of tumor markers (mucin 1 protein, MUC1 as a model). The designed cDNA, which is partly complementary with the aptamer of MUC1 was immobilized on the gold electrode. The detection of MUC1 could be carried out by virtue of switching structures of aptamers from DNA/DNA duplex to DNA/target complex. The change of the interfacial feature of the electrode was characterized by electrochemical impedance analysis (EIS) with the redox probe [Fe(CN)6](3-/4-). The quantitative detection of MUC1 protein was obtained from the changes of electron-transfer resistance (ΔRet). Moreover, as the signal enhancer, the aptamer-modified AuNPs (Apt@AuNPs) conjugates was introduced on the electrode by the hybridization of cDNA with aptamer. As expected, the detection sensitivity for MUC1 was greatly improved, which may be due to the specific binding of MUC1 onto the surface of the Apt@AuNPs modified electrode. This proposed simple aptasensor has a low detection limit of 0.1 nM, and also exhibits several advantages of high sensitivity and good selectivity. This present work may provide a general model for the detection of tumor marker based on impedimetric aptasensor.

Electrochemical biosensors on platforms of graphene

In recent years, graphene, the two-dimensional closely packed honeycomb carbon lattice, has been attracting much attention in the field of electrochemistry due to its intrinsic properties and merits. Efforts to create novel graphene based electrochemical biosensors have led to the establishment of effective strategies for diverse bioassays, from simple molecules to complex biotargets. In this Feature Article, we provide an overview of electrochemical biosensing with graphene related materials, and discuss the role of graphene in different sensing protocols.

CVD graphene based immunosensor

Herein, we aim to draw attention to employing chemical vapour deposition (CVD) method grown graphene as a potential platform for immunosensing of IgG.

Signal amplification for thrombin impedimetric aptasensor: sandwich protocol and use of gold-streptavidin nanoparticles

In this work, we report a highly specific amplification strategy demonstrated for the ultrasensitive biosensing of thrombin with the use of gold-streptavidin nanoparticles (strep-AuNPs) and silver reduction enhancement. The biotinylated aptamer of thrombin was immobilized onto an avidin-graphite epoxy composite (AvGEC) electrode surface by affinity interaction between biotin and avidin; electrochemical impedance measurements were performed in a solution containing the redox marker ferrocyanide/ferricyanide. The change in interfacial charge transfer resistance (Rct) experimented by the redox marker, was recorded to confirm aptamer complex formation with target protein, thrombin (Thr), in a label-free first stage. A biotinylated second thrombin aptamer, with complementary recognition properties was then used in a sandwich approach. The addition of strep-AuNPs and silver enhancement treatment led to a further increment of Rct thus obtaining significant signal amplification. The AptThrBio1-Thr-AptThrBio2 sandwich formation was inspected by confocal microcopy after incubation with streptavidin quantum dots. In order to visualize the presence of gold nanoparticles, the same silver enhancement treatment was applied to electrodes already modified with the nanoparticle-sandwich conjugate, allowing direct observation by scanning electron microscopy (SEM). Results showed high sensitivity and selectivity for thrombin detection, with an improvement from ca. 4.7 pM in a simple assay to 0.3 pM in the amplified reported scheme.

Silver nanowire-based electrochemical immunoassay for sensing immunoglobulin G with signal amplification using strawberry-like ZnO nanostructures as labels

The quick development of nanoscience and nanotechnology has paved the way for ultrasensitive biosensing and analysis. In this work, an ultrasensitive electrochemical immunosensor was developed for the detection of human immunoglobulin G (IgG) by combining with a newly designed trace tag on a disposable immunosensor array. The array was prepared by immobilizing captured antibodies on ultralong Ag nanowires, whilst the trace tag was prepared by loading horseradish peroxidase (HRP)-labeled goat anti-human IgG (HRP-anti-IgG) on thionine (TH)-doped mesoporous ZnO nanostrawberries (MP-ZnO). With a sandwich-type immunoassay format, mainly due to crystalline framework and high surface area of the mesoporous (MP) materials, as well as the superconductivity of silver nanowires, the electrochemical signal was significantly amplified. The linear range of the developed immunosensor is 0.01-200 ng mL(-1) and the detection limit is 4 pg mL(-1) IgG, which make the hierarchically nanostructured composites very promising candidates for the next-generation sandwich-type electrochemical immunoassays.