Sandwich-type immunosensors and immunoassays exploiting nanostructure labels: A review

Diagnostics on acute myocardial infarction: Cardiac troponin biomarkers

Acute myocardial infarction or myocardial infarction (MI) is a major health problem, due to diminished flow of blood to the heart, leads to higher rates of mortality and morbidity. Data from World Health Organization (WHO) accounted 30% of global death annually and expected more than 23 million die annually by 2030. This fatal effects trigger the need of appropriate biomarkers for early diagnosis, thus countermeasure can be taken. At the moment, the most specific markers for cardiac injury are cardiac troponin I (cTnI) and cardiac troponin T (cTnT) which have been considered as 'gold standard'. Due to higher specificity, determination of the level of cardiac troponins became a predominant indicator for MI. Several ways of diagnostics have been formulated, which include enzyme-linked immunosorbent assay, chemiluminescent, fluoro-immunoassays, electrical detections, surface plasmon resonance, and colorimetric protein assay. This review represents and elucidates the strategies, methods and detection levels involved in these diagnostics on cardiac superior biomarkers. The advancement, sensitivity, and limitations of each method are also discussed. In addition, it concludes with a discussion on the point-of care (POC) assay for a fast, accurate and ability of handling small sample measurement of cardiac biomarker.

Macrocyclic host-dye reporter for sensitive sandwich-type fluorescent aptamer sensor

We describe herein a novel approach for the fluorescent detection of small molecules using a sandwich-type aptamer strategy based on a signaling macrocyclic host-dye system. One split adenosine aptamer fragment was 5'-conjugated to a β-cylodextrin (CD) molecule while the other nucleic acid fragment was labeled at the 3'-end by a dansyl molecule prone to be included into the macrocycle. The presence of the small target analyte governed the assembly of the two fragments, bringing the dye molecule and its specific receptor in close proximity and promoting the inclusion interaction. Upon the inclusion complex formation, the microenvironment of dansyl was modified in such a way that the fluorescent intensity increased. Concomitantly, this supplementary interaction at the aptamer extremities induced stabilizing effects on the ternary complex. We next proposed a bivalent signaling design where the two extremities of one split aptamer fragment were conjugated to the β-CD molecule while those of the other fragment were tagged by the dansyl dye. The dual reporting dye inclusion promoted an improvement of both the signal-to-background change and the assay sensitivity. Owing to the vast diversity of responsive host-macrocycle systems available, this aptasensor strategy has potential to be extended to the multiplexed analysis and to other kinds of transducers (such as electrochemical).

A sensitive electrochemical immunosensor for the detection of squamous cell carcinoma antigen by using PtAu nanoparticles loaded on TiO2colloidal spheres as labels

A sensitive sandwich-type electrochemical immunosensor for detection of squamous cell carcinoma antigen (SCCA) was developed by using PtAu nanoparticles loaded on TiO₂colloidal spheres (PtAu/TiO₂) as secondary-antibody (Ab₂) labels.

An electrochemical immunoassay based on trepang-like gold electrodes and nanogold functionalized flower-like hierarchical carbon materials with improved sensitivity

A novel electrochemical immunosensor based on a 3D-TG/G-PDDA modified electrode was designed for sensitive detection of CEA using AuNP functionalized FCM as a nanocarrier.

An ultrasensitive electrochemical immunosensor for the detection of CD146 based on TiO2 colloidal sphere laden Au/Pd nanoparticles

An ultrasensitive electrochemical immunosensor was developed for detecting CD146. rGO-TEPA enhanced the loading capacity of Ab₁ and facilitated the electron transfer. Au and Pd nanoparticles on the TiO₂ colloidal sphere facilitated the decomposition of H₂O₂. The immunosensor exhibited an extremely low detection limit of 1.6 pg mL⁻¹ for CD146.

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.

Signal-enhanced electrochemical immunosensor for CD36 based on cascade catalysis of a GOx labeled Prussian blue functionalized Ceria nanohybrid

A novel electrochemical immunosensor was constructed for human scavenger receptor B type CD36 with glucose oxidase labeled Prussian blue nanoparticles functionalized ceria nanoparticles as the signal enhancer.

Current Trends in Nanomaterial-Based Amperometric Biosensors

The last decade has witnessed an intensive research effort in the field of electrochemical sensors, with a particular focus on the design of amperometric biosensors for diverse analytical applications. In this context, nanomaterial integration in the construction of amperometric biosensors may constitute one of the most exciting approaches. The attractive properties of nanomaterials have paved the way for the design of a wide variety of biosensors based on various electrochemical detection methods to enhance the analytical characteristics. However, most of these nanostructured materials are not explored in the design of amperometric biosensors. This review aims to provide insight into the diverse properties of nanomaterials that can be possibly explored in the construction of amperometric biosensors.

Diagnostic tests for hepatitis C: Recent trends in electrochemical immunosensor and genosensor analysis

Hepatitis C is a liver disease that is transmitted through contact with the blood of an infected person. An estimated 150 million individuals worldwide have been chronically infected with the hepatitis C virus (HCV). Hepatitis C shows significant genetic variation in the global population, due to the high rate of viral RNA mutation. There are six variants of the virus (HCV genotypes 1, 2, 3, 4, 5, and 6), with 15 recorded subtypes that vary in prevalence across different regions of the world. A variety of devices are used to diagnose hepatitis C, including HCV antibody test, HCV viral load test, HCV genotype test and liver biopsy. Rapid, inexpensive, sensitive, and robust analytical devices are therefore essential for effective diagnosis and monitoring of disease treatment. This review provides an overview of current electrochemical immunosensor and genosensor technologies employed in HCV detection. There are a limited number of publications showing electrochemical biosensors being used for the detection of HCV. Due to their simplicity, specificity, and reliability, electrochemical biosensor devices have potential clinical applications in several viral infections.

A promising road with challenges: where are gold nanoparticles in translational research?

Nanoenabled technology holds great potential for health issues and biological research. Among the numerous inorganic nanoparticles that are available today, gold nanoparticles are fully developed as therapeutic and diagnostic agents both in vitro and in vivo due to their physicochemical properties. Owing to this, substantial work has been conducted in terms of developing biosensors for noninvasive and targeted tumor diagnosis and treatment. Some studies have even expanded into clinical trials. This article focuses on the fundamentals and synthesis of gold nanoparticles, as well as the latest, most promising applications in cancer research, such as molecular diagnostics, immunosensors, surface-enhanced Raman spectroscopy and bioimaging. Challenges to their further translational development are also discussed.

Electrochemical Immunoassay of Escherichia coli O157:H7 Using Ag@SiO2 Nanoparticles as Labels

Silica coated silver (Ag@SiO2) nanoparticles were prepared and characterized by transmission electron microscope (TEM) and UV-vis absorption, and the nanoparticles were used as labels in sandwich-type immunosensor of Escherichia coli O157:H7 (E. coli O157:H7). The labels involved in immunoreaction were dissolved by mixed acid of hydrofluoric acid and nitric acid, and the released Ag(+) ions were electrochemical stripping analyzed (via differential pulse voltammetry, DPV) at poly(acrylic acid)/poly(diallyldimethylammonium chloride)/carbon nanotubes (PAA/PDCNT) modified glass carbon electrode (GCE), which obviously enhanced the signal of Ag(+) stripping. Then, the number of E. coli O157:H7 could be indirectly reflected by the signal intensity of labeled Ag(+). And the results showed that the DPV signals were proportional to the logarithm of the E. coli O157:H7 concentration in the range from 20 cfu/mL to 8.0 × 10(3) cfu/mL with the detection limit of 13 cfu/mL.

Recent developments in antibody-based assays for the detection of bacterial toxins

Considering the urgent demand for rapid and accurate determination of bacterial toxins and the recent promising developments in nanotechnology and microfluidics, this review summarizes new achievements of the past five years. Firstly, bacterial toxins will be categorized according to their antibody binding properties into low and high molecular weight compounds. Secondly, the types of antibodies and new techniques for producing antibodies are discussed, including poly- and mono-clonal antibodies, single-chain variable fragments (scFv), as well as heavy-chain and recombinant antibodies. Thirdly, the use of different nanomaterials, such as gold nanoparticles (AuNPs), magnetic nanoparticles (MNPs), quantum dots (QDs) and carbon nanomaterials (graphene and carbon nanotube), for labeling antibodies and toxins or for readout techniques will be summarized. Fourthly, microscale analysis or minimized devices, for example microfluidics or lab-on-a-chip (LOC), which have attracted increasing attention in combination with immunoassays for the robust detection or point-of-care testing (POCT), will be reviewed. Finally, some new materials and analytical strategies, which might be promising for analyzing toxins in the near future, will be shortly introduced.

Amperometric carbohydrate antigen 19-9 immunosensor based on three dimensional ordered macroporous magnetic Au film coupling direct electrochemistry of horseradish peroxidase

A sandwich-type electrochemical immunosensor for the detection of carbohydrate antigen 19-9 (CA 19-9) antigen based on the immobilization of primary antibody (Ab1) on three dimensional ordered macroporous magnetic (3DOMM) electrode, and the direct electrochemistry of horseradish peroxidase (HRP) that was used as both the label of secondary antibody (Ab2) and the blocking reagent. The 3DOMM electrode was fabricated by introducing core-shell Au-SiO2@Fe3O4 nanospheres onto the surface of three dimensional ordered macroporous (3DOM) Au electrode via the application of an external magnet. Au nanoparticles functionalized SBA-15 (Au@SBA-15) was conjugated to the HRP labeled secondary antibody (HRP-Ab2) through the Au-SH or Au-NH3(+) interaction, and HRP was also used as the block reagent. The formation of antigen-antibody complex made the combination of Au@SBA-15 and 3DOMM exhibit remarkable synergistic effects for accelerating direct electron transfer (DET) between HRP and the electrode. Under the optimal conditions, the DET current signal increased proportionally to CA 19-9 concentration in the range of 0.05 to 15.65 U mL(-1) with a detection limit of 0.01 U mL(-1). Moreover, the immunosensor showed high selectivity, good stability, satisfactory reproducibility and regeneration. Importantly, the developed method was used to assay clinical serum specimens, achieving a good relation with those obtained from the commercialized electrochemiluminescent method.

Mesoporous nanogold–MnO2–poly(o-phenylenediamine) hollow microspheres as nanotags and peroxidase mimics for sensing biomolecules

Mesoporous MnO₂ nanotags as peroxidase mimics for sensing biomolecules.

Current trends in the development of the electrochemiluminescent immunosensors

This review presents a general picture of the current trends and developments (2008-2013) related to electrochemiluminescence-based immunosensors. It briefly covers the milestones of qualitative changes in the field of electrochemiluminescent immunosensors; the peculiarities of the electrochemiluminescent immunoassay formats; the basic mechanisms of ECL detection, main features of early and ongoing approaches in electrochemiluminescent immunoassay commercial instruments, and the recent developments in fabrication of solid-state electrochemiluminescent immunosensors. Moreover, systematized data on biomarkers, immunoassay formats, and novel types of electrochemiluminescent label and immobilization support, such as semiconductor nanocrystals, porous noble metals, graphene, TiO2 nanotube arrays, metal-organic composites, multiwall carbon nanotubes, liposomes, photolummonescent carbone nanocrystals are presented as a table. Considerable efforts have also been devoted towards the following two key points: multiplexing analysis (multi-label, and the multianalyte strategies) and integration in microfluidic lab-on-paper devices with capabilities for point-to-care diagnostics. An immuno-like electrochemiluminescent sensor (based on synthetic receptors-molecularly imprinted polymers), as a new alternative to traditional electrochemiluminescent immunoassay is highlighted. Future perspectives and possible challenges in this rapidly developing area are also discussed.

Additional molecular biological amplification strategy for enhanced sensitivity of monitoring low-abundance protein with dual nanotags

A new signal-on immunoassay protocol for sensitive electronic detection of alpha-fetoprotein (AFP) was developed by coupling with metal sulfide nanolabels and a silver nanocluster (AgNC)-based rolling circle amplification (RCA) strategy. Initially, a sandwiched immunocomplex was formed on a primary antibody-coated microplate using a PbS nanoparticle-labeled polyclonal anti-AFP antibody (PbS-pAb2) as the detection antibody, and then the carried PbS-pAb2 was dissolved by acid to release a large number of lead ions, which could induce the cleavage of lead-specific DNAzyme immobilized on the electrode. The residual single-stranded DNA on the electrode could be used as the primer to produce numerous repeated oligonucleotide sequences via the RCA reaction for the hybridization with many AgNC-labeled detection probes, resulting in the amplification of the electronic signal due to the unique properties of silver nanoclusters. Under optimal conditions, the developed immunoassay exhibited high sensitivity for the detection of AFP with a dynamic range of 0.001-200 ng mL(-1) and a detection limit (LOD) of 0.8 pg mL(-1). Intra-assay and interassay coefficients of variation were below 8.0% and 10%, respectively. Importantly, the methodology was evaluated by analyzing 12 clinical serum specimens, and no significant differences were encountered in comparison with the conventional enzyme-linked immunosorbent assay (ELISA) method.