Application of Granular Materials in Immunoassay
As a powerful tool in bioanalysis, immunoassays rely on specific reactions between antigens and selective antibodies and use different labels such as radioisotopes, enzymes, and fluorophores for signal development. In recent years, nanomaterials are increasingly replacing these molecular tags due to their superior optical or electrochemical properties and substantially greater chemical stability. Several immunoassays and biosensors based on magnetic particles and nanoparticles have been developed to detect various targets (e.g., cells, proteins, pathogens, and small molecule toxins).
Magnetic particles (MPs) are widely used in immunoassays for the following reasons: 1) MPs help to increase sensitivity and shorten analysis time by manipulating particles with an external magnetic field, magnetic washing, and magnetic separation; 2) compared with traditional fluorescent markers Compared with enzyme markers, MPs show better performance in opaque or highly dispersed biological media. In general, there are two main uses of MPs in immunoassays: 1) MPs can be used as a solid phase for the formation of immune complexes; 2) MPs can be used as detection labels in assays (Figure 1).
Figure 1: Main methods of using MP in immunoassays: A) intrinsic MP as a marker or MP-coated enzyme marker; B) MP as a carrier
As for nanoparticles, they usually have a length scale of 1 ~ 100nm (but not limited to 100nm) and are used in many types of studies at the atomic, molecular or macromolecular scale. Nanoparticles have some unique physical, chemical, and biological properties that can be widely used in immunoassays. For example, nanoparticles generally have a large surface-to-volume ratio relative to their small size and can easily label a large number of different molecules. Furthermore, the physical properties of nanoparticles are chemically customizable. Over the past decade, nanoparticle-based immunoassays have been used to improve the sensitivity and specificity of medical detection and provide clinical Diagnostics provide new tools.
Magnetic particles and nanoparticles are increasingly used in immunoassays as novel labels that increase sensitivity and simplify detection. Details are presented and discussed below.
Magnetic Particles
Iron Oxide Magnetic Particles
Magnetic particles are widely used in magnetic resonance imaging, magnetic hyperthermia, drug and gene delivery, biological analysis, and other fields. Among numerous MPs, iron oxide MPs are the most widely used in bioanalysis due to their easy preparation and functionalization, and biocompatibility. MPs-based immunoassays combine MPs with signaling tags including enzymes, noble metal nanoparticles, and fluorescent nanoparticles. These immunoassays are typically fast and easy to use and are well suited for the detection of a wide variety of analytes including proteins, bacteria, viruses, hormones, and small molecule toxins.
Gold Magnetic Particles
Gold magnetic nanoparticles (GMPs) are composite particles with a typical core/shell structure, with iron oxide as the core and a layer of gold deposited on the surface of the core as the shell. GMPs have the advantages of convenient combination with biomolecules such as proteins or nucleic acids, and easy separation by using a magnetic field. At the same time, they also bring the characteristics of short reaction time and high reaction efficiency. GMPs can be used in microchips and have attracted considerable attention in the past few years due to their unique properties of tunable anisotropic interactions. It can be used as a carrier to transport in microchannels, solving the problem of manipulation of biomolecules. At the same time, low reagent consumption and short washing time can be achieved based on the miniaturization and integration of microchips.
Silica Magnetic Particles
The high surface-to-volume ratio and the availability of many biofunctionalization options make magnetic particles ideal for capturing analytes from biological samples. The value of magnetic silica particles in nucleic acid preparation and detection has been discovered. The capture process relies on the physical adsorption of nucleic acids to particles, followed by a fluid exchange step to achieve isolation and purification. Specific capture requires the functionalization of particles with specific capture molecules, such as antibodies, with a high affinity to the analyte to be detected.
Superparamagnetic Polystyrene (SPP) Particles
Lee et al. demonstrated a fast and simple method for magnetic particle immunoassay in a capillary mixing system. They used antibody-coated micron-sized superparamagnetic polystyrene (SPP) particles for detection in a sandwich (non-competitive) format rabbit IgG assay. They found that competing magnetic and viscous drag forces help enhance the interaction between the analyte and the antibody captured on the particle. Furthermore, it was shown that the formation of chains of SPP particles improves immunoassay kinetics under conditions corresponding to the critical Mersenne number.
Nanoparticles
Nanogold
Au NPs are able to provide a microenvironment similar to biomolecules in natural systems, thus maintaining the activity of biomolecules after immobilization. Therefore, Au NPs are usually modified by various biomolecules (such as enzymes, antibodies, and DNA) to construct specific nanoprobes for the detection of various analytes.
Carbon Nanomaterials
Carbon nanomaterials generally refer to carbon nanotubes (CNTs), graphene and its derivatives graphene oxide (GO), and reduced graphene oxide (rGO). They feature extraordinary mechanical strength, good biocompatibility, large surface area, and high electrical/thermal conductivity. Therefore, carbon nanotubes and graphene nanomaterials are widely used to fabricate electrochemical biosensors and improve the analytical performance of biosensors. Carbon nanomaterials have also been used in fluorescent and chemiluminescent immunoassays.
Silicon Nanoparticles
Silica nanoparticles are widely used in the field of bioanalysis: cell targeting, biomarkers, DNA or RNA detection, and biosensor development. In addition, silica nanoparticles are capable of dispersing in aqueous solutions, and the silica surface facilitates a variety of surface reactions and allows the conjugation of biomolecules.
References
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