A novel immunoassay based on the dissociation of immunocomplex and fluorescence quenching by gold nanoparticles

Plasmonic Fluorescence Enhancement in Diagnostics for Clinical Tests at Point-of-Care: A Review of Recent Technologies

Fluorescence-based biosensors have widely been used in the life-sciences and biomedical applications due to their low limit of detection and a diverse selection of fluorophores that enable simultaneous measurements of multiple biomarkers. Recent research effort has been made to implement fluorescent biosensors into the exploding field of point-of-care testing (POCT), which uses cost-effective strategies for rapid and affordable diagnostic testing. However, fluorescence-based assays often suffer from their feeble signal at low analyte concentrations, which often requires sophisticated, costly, and bulky instrumentation to maintain high detection sensitivity. Metal- and metal oxide-based nanostructures offer a simple solution to increase the output signal from fluorescent biosensors due to the generation of high field enhancements close to a metal or metal oxide surface, which has been shown to improve the excitation rate, quantum yield, photostability, and radiation pattern of fluorophores. This article provides an overview of existing biosensors that employ various strategies for fluorescence enhancement via nanostructures and have demonstrated the potential for use as POCT. Biosensors using nanostructures such as planar substrates, freestanding nanoparticles, and metal-dielectric-metal nanocavities are discussed with an emphasis placed on technologies that have shown promise towards POCT applications without the need for centralized laboratories.

In situ H2O2 generation with gold nanoflowers as the coreactant accelerator for enzyme-free electrochemiluminescent immunosensing

In traditional electrochemiluminescence (ECL) analysis, gold nanomaterials are commonly used as a tool for signal amplification and linking antibodies due to their good electrical conductivity and biocompatibility. Here, we found that multitipped gold nanoparticles-gold nanoflowers (AuNFs) as coreactant accelerator have good catalytic activity for the reduction of dissolved oxygen (O₂) to hydrogen peroxide (H₂O₂) using tris (hydroxymethyl) aminomethane (Tris) as electron donor. Based on this, a new enzyme-free and label-free ECL immunosensor have been constructed for the detection of α-fetoprotein (AFP). In this system, due to the unique geometric and spatial effects of AuNFs, the dissolved O₂ as endogenous coreactant was catalyzed by AuNFs to produce H₂O₂ using Tris as an electron donor. The in situ generated H₂O₂ can more efficiently produce various electrogenerated reactive oxygen species (ROSs) as the important intermediates on the electrode surface. Then, oxidation of luminol reacts with ROSs significantly amplifies the luminol ECL signal. Under optimal experimental conditions, the proposed ECL immunosensor was able to detect the AFP concentration from 0.01 to 100 ng mL^-1, with a low detection limit of 3.4 pg mL^-1 (S/N = 3). In addition, the prepared ITO-based sensor is similar to a micro-test chip and convenient to use, thus making it suitable for clinical use as a disposable device in point-of-care tests (POCTs).

Gold nanocage-based lateral flow immunoassay for immunoglobulin G

The authors describe a gold nanocage-based lateral flow strip biosensor (LFSB) for low-cost and sensitive detection of IgG. This protein was used as a model analyte to demonstrate the proof-of-concept. The method combines the unique optical properties of gold nanocages (GNCs) with highly efficient chromatographic separation. A sandwich-type of immunoreactions occurs on the GNC-based LFSB which has the attractive features of avoiding multiple incubation, separation, and washing steps. The captured GNCs on the purple test zone and control zone of the biosensor are producing characteristic purple bands, and this enables IgG even to be visually detected. Quantitatation was accomplished by reading the intensities of the bands with a portable strip reader. The LFSB fabrication and assay parameters were optimized. The biosensor displays a linear response in the 0.5 to 50 ng·mL^-1 IgG concentration range, and it has a 15 min assay time. The detection limit is 0.1 ng·mL^-1 of IgG, which is 2.5 times lower than that when using a gold nanoparticle-based LFSB. In our perception, this assay has a wide potential for the detection of other proteins and species for which respective antibodies are available.

Novel Pyrimidine Tagged Silver Nanoparticle Based Fluorescent Immunoassay for the Detection of Pseudomonas aeruginosa

A simple pyrimidine-based fluorescent probe (R)-4-(anthracen-9-yl)-6- (naphthalen-1-yl)-1,6-dihydropyrimidine-2-amine (ANDPA) was synthesized through the greener one pot reaction and characterized by IR, NMR, and ESI-Mass. Glucose stabilized silver nanoparticles (Glu-AgNPs) were also synthesized and characterized using UV, IR, XRD, SEM, and TEM. When ANDPA was tagged with Glu-AgNPs, the fluorescent intensity of ANDPA decreased drastically. When the monoclonal antibody (Ab) [immunoglobulin G (IgG)] of Pseudomonas aeruginosa (PA) was attached with ANDPA/Glu-AgNPs, the original intensity of the probe was recovered with minimal enhancement at 446 nm. On further attachment of PA with ANDPA/Glu-AgNPs/PA, the fluorescence intensity of the probe was enhanced obviously at 446 nm with red shift. This phenomenon was further supported by SEM and TEM. The linear range of detection is from 8 to 10^-1 CFU/mL, and LOD is 1.5 CFU/mL. The immunosensor was successfully demonstrated to detect Pseudomonas aeruginosa in water, soil, and food products like milk, sugar cane, and orange juices.

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

Methods based on sandwich-type immunosensors and immunoassays have been developed for detection of multivalent antigens/analytes with more than one eptiope due to the use of two matched antibodies. High-affinity antibodies and appropriate labels are usually employed for the amplification of detectable signal. Recent research has looked to develop innovative and powerful novel nanoparticle labels, controlling and tailoring their properties in a very predictable manner to meet the requirements of specific applications. This articles reviews recent advances, exploiting nanoparticle labels, in the sandwich-type immunosensors and immunoassays. Routine approaches involve noble metal nanoparticles, carbon nanomaterials, semiconductor nanoparticles, metal oxide nanostructures, and hybrid nanostructures. The enormous signal enhancement associated with the use of nanoparticle labels and with the formation of nanoparticle-antibody-antigen assemblies provides the basis for sensitive detection of disease-related proteins or biomolecules. Techniques commonly rely on the use of biofunctionalized nanoparticles, inorganic-biological hybrid nanoparticles, and signal tag-doped nanoparticles. Rather than being exhaustive, this review focuses on selected examples to illustrate novel concepts and promising applications. Approaches described include the biofunctionalized nanoparticles, inorganic-biological hybrid nanoparticles, and signal tage-doped nanoparticles. Further, promising application in electrochemical, mass-sensitive, optical and multianalyte detection are discussed in detail.

Fluorescence in nanobiotechnology: sophisticated fluorophores for novel applications

Nanobiotechnology is one of the fastest growing and broadest-ranged interdisciplinary subfields of the nanosciences. Countless hybrid bio-inorganic composites are currently being pursued for various uses, including sensors for medical and diagnostic applications, light- and energy-harvesting devices, along with multifunctional architectures for electronics and advanced drug-delivery. Although many disparate biological and nanoscale materials will ultimately be utilized as the functional building blocks to create these devices, a common element found among a large proportion is that they exert or interact with light. Clearly continuing development will rely heavily on incorporating many different types of fluorophores into these composite materials. This review covers the growing utility of different classes of fluorophores in nanobiotechnology, from both a photophysical and a chemical perspective. For each major structural or functional class of fluorescent probe, several representative applications are provided, and the necessary technological background for acquiring the desired nano-bioanalytical information are presented.

Exploiting the light–metal interaction for biomolecular sensing and imaging

The ability of metal surfaces and nanostructures to localize and enhance optical fields is the primary reason for their application in biosensing and imaging. Local field enhancement boosts the signal-to-noise ratio in measurements and provides the possibility of imaging with resolutions significantly better than the diffraction limit. In fluorescence imaging, local field enhancement leads to improved brightness of molecular emission and to higher detection sensitivity and better discrimination. We review the principles of plasmonic fluorescence enhancement and discuss applications ranging from biosensing to bioimaging.

Gold nanoparticles in the clinical laboratory: principles of preparation and applications

In order to meet the challenges of effective healthcare, the clinical laboratory is constantly striving to improve testing sensitivity while reducing the required time and cost. Gold nanoparticles (AuNPs) are proposed as one of the most promising tools to meet such goals. They have unique optophysical properties which enable sensitive detection of biomarkers, and are easily amenable to modification for use in different assay formats including immunoassays and molecular assays. Additionally, their preparation is relatively simple and their detection methods are quite versatile. AuNPs are showing substantial promise for effective practical applications and commercial utilization is already underway. This article covers the principles of preparation of AuNPs and their use for development of different diagnostic platforms.

Gold nanoparticle-based fluorescence immunoassay for malaria antigen detection

The development of rapid detection assays for malaria diagnostics is an area of intensive research, as the traditional microscopic analysis of blood smears is cumbersome and requires skilled personnel. Here, we describe a simple and sensitive immunoassay that successfully detects malaria antigens in infected blood cultures. This homogeneous assay is based on the fluorescence quenching of cyanine 3B (Cy3B)-labeled recombinant Plasmodium falciparum heat shock protein 70 (PfHsp70) upon binding to gold nanoparticles (AuNPs) functionalized with an anti-Hsp70 monoclonal antibody. Upon competition with the free antigen, the Cy3B-labeled recombinant PfHsp70 is released to solution resulting in an increase of fluorescence intensity. Two types of AuNP-antibody conjugates were used as probes, one obtained by electrostatic adsorption of the antibody on AuNPs surface and the other by covalent bonding using protein cross-linking agents. In comparison with cross-linked antibodies, electrostatic adsorption of the antibodies to the AuNPs surfaces generated conjugates with increased activity and linearity of response, within a range of antigen concentration from 8.2 to 23.8 μg.mL(-1). The estimated LOD for the assay is 2.4 μg.mL(-1) and the LOQ is 7.3 μg.mL(-1). The fluorescence immunoassay was successfully applied to the detection of antigen in malaria-infected human blood cultures at a 3% parasitemia level, and is assumed to detect parasite densities as low as 1,000 parasites.μL(-1).

Acoustic wave immunosensing of a meningococcal antigen using gold nanoparticle-enhanced mass sensitivity

Bacterial meningitis is an infection of the thin membranes covering the brain and spinal cord by a number of microorganisms including Neisseria meningitidis, which can lead to permanent neurological damage in the event of late diagnosis. Given the quick onset and severity of the disease, there is a clear need for a rapid, sensitive and specific diagnostic technique. Here, we describe the development and evaluation of an acoustic wave sensor, the quartz crystal microbalance (QCM), as a rapid immunosensor employing antibodies against the cell surface outer membrane protein 85 (OMP85) of N. meningitidis as an immobilized selective layer. These antibodies were directionally orientated as receptors by thin film deposition of structured polyvinylidene fluoride and Protein A. The sensitivity of this QCM immunosensor was further increased by conjugation of the OMP85 antigen to 50 nm gold nanoparticles providing reproducible detection of the target down to 300 ng/mL. Subsequent treatment of the QCM surface with an acidic glycine solution regenerated the immunosensor allowing each crystal to be used several times.

Facile synthesis of biocompatible gold nanoparticles from Vites vinefera and its cellular internalization against HBL-100 cells

The remarkable health benefits of the chemical cocktails occluded within Vites vinefera (grapes) have been broadly used as dietary supplements and as natural pharmaceuticals in the treatment of various diseases including human cancer. Current discovery demonstrates the rapid formation of gold nanoparticles with the phytochemicals present in grapes, which serve a dual role as synergistic reducing agents to reduce gold salts into gold nanoparticles and also as stabilizers to provide a robust coating on the gold nanoparticles in a single step. Furthermore, the grape-generated gold nanoparticles (GAuNPs), have demonstrated remarkable in vitro stability on specific functionalization with peptides (GSH) and thiol-containing compounds (lipoic acid) followed by the induction of cell-specific response. In addition, the grape-generated gold nanoparticles (GAuNPs, GSH-GAuNPs, LA-GAuNPs) have demonstrated remarkable affinity towards human breast cancer cells (HBL-100) in the present study. These studies thus signified the cellular internalization of GAuNPs and its conjugates by transmission electron microscopy through endocytosis into cancer cells. Notably, at higher concentration of gold nanoparticles conjugate, there was an asymmetric accumulation of gold nanoparticles in the periphery of the cell nucleus of the HBL-100 cells which was confirmed by fluorescence microscopy. Other than gold salts, no "manmade" chemicals are used in this truly biogenic, green nanotechnological process which thereby paves the way for outstanding opening for their application in molecular imaging and cancer therapy.

Functionalized Gold Nanoparticles and Their Biomedical Applications

Metal nanoparticles are being extensively used in various biomedical applications due to their small size to volume ratio and extensive thermal stability. Gold nanoparticles (GNPs) are an obvious choice due to their amenability of synthesis and functionalization, less toxicity and ease of detection. The present review focuses on various methods of functionalization of GNPs and their applications in biomedical research. Functionalization facilitates targeted delivery of these nanoparticles to various cell types, bioimaging, gene delivery, drug delivery and other therapeutic and diagnostic applications. This review is an amalgamation of recent advances in the field of functionalization of gold nanoparticles and their potential applications in the field of medicine and biology.

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.

Effect of gold nanoparticles on the fluorescence excitation spectrum of α-fetoprotein: local environment dependent fluorescence quenching

The effect of colloid gold nanoparticles (AuNPs) on the fluorescence excitation spectrum of α-fetoprotein (AFP) has been investigated experimentally. The excitation spectral peaks of AFP with low concentration from 0.01 ng ml(-1) to 12 ng ml(-1) increase monotonically with increasing of AFP concentration. When some gold colloids were added to the AFP solution, the excitation peak at 285 nm decreases distinctly. By comparing the excitation peak intensity of AFP solution with gold colloids and without gold colloids at different AFP concentrations, the quenching effect from gold nanoparticle was more effective at lower AFP concentration. So the range of concentration from 0.01 ng ml(-1) to 0.09 ng ml(-1) will be the potential range of applications because of the higher sensitivity. The physical origin based on local field effect was investigated to illuminate this local environment dependent fluorescence quenching. The changing extent of quenching with different AFP concentrations can be attributed to the nonlinear decreasing of the local field factor of gold nanoparticles as a function of environmental dielectric constant.

Long-range fluorescence quenching by gold nanoparticles in a sandwich immunoassay for cardiac troponin T

We report the first homogeneous sandwich immunoassay with gold nanoparticles (AuNPs) as fluorescence quenchers. The sandwich assay is designed for the detection of the protein cardiac troponin T (cTnT) by its simultaneous interaction with two different antibodies, one attached to AuNPs and the other labeled with fluorescent dyes. We demonstrate the working principle of the assay and using time-resolved fluorescence spectroscopy, we determine the quenching efficiency of the gold nanoparticles. In spite of the relatively large separation distance between dye molecules and AuNPs, ranging from 3 to 22 nm, the AuNPs quench the fluorescence with efficiencies as high as 95%. A limit of detection of 0.02 nM (0.7 ng/mL) was obtained for cTnT, which is the lowest value reported for a homogeneous sandwich assay for cTnT. These results illustrate the use of metallic nanoparticles as fluorescence quenchers in immunoassays where the large biomolecules involved impose distances for which energy transfer between fluorophores would be inefficient.

Enhanced solid-phase immunoassay using gold nanoshells: effect of nanoparticle optical properties

Plasmon-resonant nanoparticle-labeled immunoassays provide a simple, low-cost and effective way of detecting target molecules in solutions. The optical mechanisms behind their efficiency, however, have not been addressed until now. We present the first theoretical description of nanoparticle-labeled dot immunoassay and its experimental verification with functionalized 15 nm colloidal gold nanospheres and silica/gold nanoshells (GNs). Three types of GNs, with silica core diameters of 100, 140 and 180 nm and a gold shell thickness of about 15 nm, were studied in our experiments. The fabricated markers were characterized by electron and atomic-force microscopy, UV-vis spectroscopy and dynamic light scattering. A normal rabbit serum (the target IgG molecules) and sheep antirabbit antibodies (the probing molecules) were used as a biospecific model. The minimal detection limit for IgG target molecules was about 15 ng in the case of a standard dot-assay protocol based on 15 nm colloidal gold particles conjugated with probing molecules. In contrast to this observation, a simple replacement of 15 nm gold labels by GN conjugates resulted in a drastic increase in detection sensitivity of up to 0.25 ng in the case of 180/15 nm GNs and of up to 0.5-1 ng for 100/15 and 140/15 GNs. By using the theory developed, we explained the dependences of the low detection limit, the maximal-color intensity and the probe-load saturation limit on the particle parameters.

Fluorescence Quenching/Enhancement Surface Assays: Signal Manipulation Using Silver-coated Gold Nanoparticles

Gold nanoparticles covalently attached to the indium tin oxide coated glass slide drastically quench fluorescence of a surface immunoassay (approximately 5-fold). Silver electrochemically deposited over the gold particles leads to fluorescence amplification: signal increases approximately 7-8 times if compared to the signal on gold particles not covered with silver. This phenomenon allows enhancing of the surface immunoassays utilizing both types of nanoparticles.