Enzyme-Linked Immunosorbent Assay (ELISA)

Enzyme-linked immunosorbent assay (ELISA) is one of the most specific and straightforward assays for detecting biomolecules in research and clinics. With advances in analytical methods, ELISA assay has been constantly optimized to improve its sensitivity, and different types of ELISA are now available to detect various biomarkers. This chapter provides an overall summary of the basic principle of ELISA, discusses different components of ELISA assay, and clearly outline protocols for different types of ELISA assays, including direct, indirect, sandwich, competitive, and nanoparticle-based ELISA.

Multiplexed chemiluminescence determination of three acute myocardial infarction biomarkers based on microfluidic paper-based immunodevice dual amplified by multifunctionalized gold nanoparticles

Acute myocardial infarction (AMI) causes significant mortality and morbidity. The determination of multiple AMI biomarkers is very important for the timely diagnosis of AMI. In this work, simultaneous determination of three AMI biomarkers were achieved by virtue of a three-dimensional (3D) microfluidic paper-analytical device (μPAD) with temporally resolved chemiluminescence (CL) emissions for the first time. A dual-signal amplification strategy was introduced including by employing primary antibody functionalized gold nanoparticles (Ab₁-GNPs) immobilized on the detection zone as amplified capture probes, and Co(II) catalyst, secondary antibody, luminol multifunctionalized gold nanoparticles (Co(II)-Ab₂-luminol-GNPs) with excellent CL activity as amplified signal probes. CL immunoreactions were performed at three detection zone of the fabricated 3D μPAD by assembling Ab₁-GNPs, antigen, and Co(II)-Ab₂-luminol-GNPs to form sandwich-type immunocomplexes. Auto separated CL signals with temporal resolution were obtained by time delayed transport of H₂O₂ to different detection zones for multiplexed analysis. The CL signal obtained by using Co(II)-Ab₂-luminol-GNPs as signal probe (10576 a.u.) were about 20-fold higher than that by using conventional horseradish peroxidase labeled antibody modified luminol-GNPs as signal probe (531 a.u.). Finally, three AMI biomarkers including heart-type fatty acid-binding protein (H-FABP), cardiac troponin I (cTnI) and copeptin were quantitatively analyzed in one CL detection run by reading the CL intensity of the obtained three CL emission peaks. The detection range were ultra-wide ranged from 0.1 pg/mL to 1 μg/mL, 0.5 pg/mL to 1 μg/mL and 1 pg/mL to 1 mg/mL with the detection limits down to 0.06 pg/mL, 0.3 pg/mL and 0.4 pg/mL for H-FABP, cTnI and copeptin detection, respectively. The developed μPAD based immunoassay performing multiplexed analysis ability, high sensitivity, ultra-wide dynamic range, favorable selectivity, accessible accuracy and reproducibility, have great application potential for the early diagnosis of AMI.

Assembly of Multifunctionalized Gold Nanoparticles with Chemiluminescent, Catalytic, and Immune Activity for Label-Free Immunoassays

In this study, we report a universal label-free immunoassay to detect antigen based on multifunctionalized gold nanoparticles (MF-GNPs), which were obtained by successive assembly of N-aminobutyl- N-ethylisoluminol functionalized gold nanoparticles (ABEI-GNPs) with antibody, bovine serum albumin (BSA) and Co²⁺. MF-GNPs exhibited excellent chemiluminescent (CL), catalytic and immune activity. It was demonstrated that the CL signal of MF-GNPs decreased in the presence of antigens via antigen-antibody specific binding using human immunoglobulin G (hIgG) and corresponding antibody goat anti-human IgG (anti-hIgG) as a model system, due to that immunoreaction led to the aggregation of GNPs. According to the decreased CL intensity, hIgG could be determined in the range of 1.0 fM to 1.0 nM with a low detection limit of 0.13 fM. Furthermore, this CL strategy was also confirmed to be a general one by replacing hIgG with heart-type fatty acid-binding protein (H-FABP), which is a biomarker of early acute myocardial infarction (AMI). The CL strategy could be employed to detect H-FABP ranging from 10.0 fM to 10.0 nM, and the detection limit is 7.8 fM. The CL strategy also showed good selectivity. It might be extended to detect other antigens if their corresponding antibodies are available.

Design a New Strategy Based on Nanoparticle-Enhanced Chemiluminescence Sensor Array for Biothiols Discrimination

Array-based sensor is an interesting approach that suggests an alternative to expensive analytical methods. In this work, we introduce a novel, simple, and sensitive nanoparticle-based chemiluminescence (CL) sensor array for discrimination of biothiols (e.g., cysteine, glutathione and glutathione disulfide). The proposed CL sensor array is based on the CL efficiencies of four types of enhanced nanoparticle-based CL systems. The intensity of CL was altered to varying degrees upon interaction with biothiols, producing unique CL response patterns. These distinct CL response patterns were collected as “fingerprints” and were then identified through chemometric methods, including linear discriminant analysis (LDA) and hierarchical cluster analysis (HCA). The developed array was able to successfully differentiate between cysteine, glutathione and glutathione disulfide in a wide concentration range. Moreover, it was applied to distinguish among the above analytes in human plasma.

The chemiluminescence determination of 2-chloroethyl ethyl sulfide using luminol-AgNO3-silver nanoparticles system

A highly sensitive chemiluminescence (CL) method for the determination of 2-chloroethyl ethyl sulfide (2-CEES) was presented. It was found that 2-chloroethyl ethyl sulfide (2-CEES) could inhibit the CL of the luminol-AgNO3 system in the presence of silver nanoparticles in alkaline solution, which made it applicable for determination of 2-CEES. The presented method is simple, convenient, rapid and sensitive. Under the optimized conditions, the calibration curve was linear in the range of 0.0001-1ngmL(-1), with the correlation coefficient of 0.992; while the limit of detection (LOD), based on signal-to-noise ratio (S/N) of 3, was 6×10(-6)ngmL(-1). Also, the relative standard deviation (RSD, n=5) for determination of 2-CEES (0.50ngmL(-1)) was 3.1%. The method was successfully applied for the determination of 2-CEES in environmental aqueous samples.

Highly sensitive homogenous chemiluminescence immunoassay using gold nanoparticles as label

Homogeneous immunoassay is becoming more and more attractive for modern medical diagnosis because it is superior to heterogeneous immunoassay in sample and reagent consumption, analysis time, portability and disposability. Herein, a universal platform for homogeneous immunoassay, using human immunoglobulin G (IgG) as a model analyte, has been developed. This assay relies upon the catalytic activity of gold nanoparticles (AuNPs) on luminol-AgNO3 chemiluminescence (CL) reaction. The immunoreaction of antigen and antibody can induce the aggregation of antibody-functionalized AuNPs, and after aggregation the catalytic activity of AuNPs on luminol-AgNO3 CL reaction is greatly enhanced. Without any separation steps, a CL signal is generated upon addition of a trigger solution, and the CL intensity is directly correlated to the quantity of IgG. The detection limit of IgG was estimated to be as low as 3pg/mL, and the sensitivity was better than that of the reported AuNPs-based CL immunoassay for IgG.

An ultrasensitive chemiluminescence biosensor for carcinoembryonic antigen based on autocatalytic enlargement of immunogold nanoprobes

A sensitive flow injection chemiluminescence assay for carcinoembryonic antigen (CEA) detection based on signal amplification with gold nanoparticles (NPs) is reported in the present work. The sandwich system of CEA/anti-CEA/goat-anti-mouse IgG functionalized Au nanoparticles was used as the sensing platform. In order to improve detection sensitivity, a further gold enlargement step was developed based on the autocatalytic Au deposition of gold nanoprobes via the reduction of AuCl₄⁻ to Au⁰ on their surface in the presence of NH₂OH·HCl. AuCl₄⁻, which is a soluble product of gold nanoprobes, served as an analyte in the CL reaction for the indirect measurement of CEA. Under optimized conditions, the CL intensity of the system was linearly related to the logarithm of CEA concentration in the range of 100 pg·mL⁻¹ to 1,000 ng·mL⁻¹, with a detection limit of 20 pg·mL⁻¹.

Silver nanoparticle based label-free colorimetric immunosensor for rapid detection of neurogenin 1

Neurogenin 1 (ngn1), with the functions of controlling the differentiation of neurons, determining specific neuronal subtype, and inhibiting glial differentiation, is quantitatively detected for the first time. By using specifically modified silver nanoparticles (AgNP) as the signaling element, a label-free, rapid and sensitive colorimetric immunoassay for the synthetic peptide fragment of ngn1 (amino acid sequence: AQDDEQERRRRRGRTR) is reported. The detection procedure is based on an anti-aggregation mechanism, by which ngn1 inhibits the aggregation of the probe in the presence of salt (NaClO(4)). The anti-ngn1 antibody conjugated AgNP (denoted as AgNP-Ab) is negatively charged, and mono-binding of the like-charged ngn1 to the probe will increase the surface charge density, hence enhancing the interparticular electrostatic repulsion. Along with the increase of ngn1 concentration, the color of the solution varies from red to yellow, thereby developing a feasible approach for the detection of ngn1. Using a UV/vis spectrophotometer, this assay exhibits a linear response range of two orders of magnitude, from 50 to 800 ng mL(-1), and a detection limit of 30 ng mL(-1). On the basis of these qualities, the antibody-conjugated AgNP may become a useful tool for point-of-care diagnosis of ngn1 and such a method offers a new insight on the detection of the analogous antigen fragment as well.

Sensitive competitive flow injection chemiluminescence immunoassay for IgG using gold nanoparticle as label

A sensitive competitive flow injection chemiluminescence (CL-FIA) immunoassay for immunoglobulin G (IgG) was developed using gold nanoparticle as CL label. In the configuration, anti-IgG antibody was immobilized on a glass capillary column surface by 3-(aminopropyl)-triethoxysilane and glutaraldehyde to form immunoaffinity column. Analyte IgG and gold nanoparticle labeled IgG were passed through the immunoaffinity column mounted in a flow system and competed for the surface-confined anti-IgG antibody. CL emission was generated from the reaction between luminol and hydrogen peroxide in the presence of Au (III), generated from chemically oxidative dissolution of gold nanoparticle by an injection of 0.10 mol L(-1) HCl-0.10 mol L(-1) NaCl solution containing 0.10 mmol L(-1) Br(2). The concentration of analyte IgG was inversely related to the amount of bound gold nanoparticle labeled IgG and the CL intensity was linear with the concentration of analyte IgG from 1.0 ng mL(-1) to 40 ng mL(-1) with a detection limit of 5.2×10(-10) gm L(-1). The whole assay time including the injections and washing steps was only 30 min for one sample, which was competitive with CL immunoassays based on a gold nanoparticle label and magnetic separation. This work demonstrates that the CL immunoassay incorporation of nanoparticle label and flow injection is promising for clinical assay with sensitivity and high-speed.

Aptameric system for the highly selective and ultrasensitive detection of protein in human serum based on non-stripping gold nanoparticles

A novel approach is proposed in this study for the development of an aptameric assay system for protein based on non-stripping gold nanoparticles (NPs)-triggered chemiluminescence (CL) upon target binding. The strategy chiefly depends on the formation of a sandwich-type immunocomplex among the capture antibody immobilized on the polystyrene microwells, target protein and aptamer-functionalized gold NPs. Introduction of target protein into the assay system leads to the attachment of gold NPs onto the surface of the microwells and thus the assembled gold NPs could trigger the reaction between luminol and AgNO(3) with a CL emission. Further signal amplification was achieved by a simple gold metal catalytic deposition onto the gold NPs. Such an amplified CL transduction allowed for the detection of model target IgE down to the 50 fM, which is better than most existing aptameric methods for IgE detection. This new protocol also provided a good capability in discriminating IgE from nontarget proteins such as IgG, IgA, IgM and interferon. The practical application of the proposed gold NPs-based immunoassay was successfully carried out for the determination of IgE in 35 human serum samples. Overall, the proposed assay system exhibits excellent analytical characteristics (e.g., a detection limit on the attomolar scale and a linear dynamic range of 4 orders of magnitude), and it is also straightforward to adapt this strategy to detect a spectrum of other proteins by using different aptamers. This new CL strategy might create a novel technology for developing simple biosensors in the sensitive and selective detection of target protein in a variety of clinical, environmental and biodefense applications.

Silver nanoparticle-initiated chemiluminescence reaction of luminol-AgNO3 and its analytical application

Ag(+) has been regarded as an inert chemiluminescent oxidant. In this work, it was found that in the presence of silver nanoparticles (AgNPs), AgNO(3) could react with luminol to produce strong chemiluminescence (CL). The AgNPs with smaller size could initiate stronger CL emission. To investigate the CL mechanism of the AgNPs-luminol-AgNO(3) system, the UV-visible spectra and the CL spectrum of the CL system were obtained. The CL reaction mechanism involving catalysis was proposed. Compared with the reported nanoparticles-luminol-H(2)O(2) CL system, the AgNPs-luminol-AgNO(3) CL system has the advantages of low background and good stability. Moreover, the new CL system was used in immunoassay for IgG.

Nanoparticles in molecular diagnostics

The aim of this chapter is to provide an overview of the available and emerging molecular diagnostic methods that take advantage of the unique nanoscale properties of nanoparticles (NPs) to increase the sensitivity, detection capabilities, ease of operation, and portability of the biodetection assemblies. The focus will be on noble metal NPs, especially gold NPs, fluorescent NPs, especially quantum dots, and magnetic NPs, the three main players in the development of probes for biological sensing. The chapter is divided into four sections: a first section covering the unique physicochemical properties of NPs of relevance for their utilization in molecular diagnostics; the second section dedicated to applications of NPs in molecular diagnostics by nucleic acid detection; and the third section with major applications of NPs in the area of immunoassays. Finally, a concluding section highlights the most promising advances in the area and presents future perspectives.

Hydroxylamine-amplified gold nanoparticles for the homogeneous detection of sequence-specific DNA

Herein, we report the development of a simple, sensitive, inexpensive, and homogeneous detection method for the analysis of DNA hybridization based on the optical properties of hydroxylamine-amplified gold nanoparticles (Au NPs) in solution phase. The assay relies on a sandwich-type DNA hybridization in which DNA targets are first hybridized with capture DNA probes immobilized on the surface of magnetic beads and then sandwiched with Au NPs modified with biotinylated reporter DNA. Au NPs, after being anchored on the magnetic beads, are then dispersed in solution by the dehybridization and enlarged by using a mixture of HAuCl(4) and NH(2)OH. The Au NP growth signal which is used for the quantitative analysis of sequence-specific DNA can be easily monitored by the naked eye directly or an UV-vis spectrophotometer. Surface plasmonic signature of the enlarged Au NPs and the kinetics of the Au NP growth in the homogenous phase containing of HAuCl(4) and NH(2)OH have also been studied. As a result, such a homogeneous assay allows the detection of 30-base DNA targets down to the 100 amol level, which offers great promise for facilitating sensitive detection of other biorecognition events.

Chemiluminescence platforms in immunoassay and DNA analyses

Chemiluminescent (CL) detection techniques for DNA assays and immunoassays have become very popular in recent years. This review discusses recent advances in those CL assays that have occurred over the last few years. In the monoplex assay section, different classes of CL labels including nanoparticle, DNAzyme, acridinium ester, enzyme and luminol-based CL assays are reviewed concerning the detection of DNAs and proteins. In the multiplex assay section, both spatial resolution and substrate zone-resolved techniques are discussed.