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Gold nanoparticles have attracted great interests in the fields of biological and medical applications in past few years. Comparing with quantum dots and other materials, gold nanoparticles have been investigated and utilized in several biotechnology applications, such as sensory probes, drug delivery, and therapy techniques. In addition, gold nanoparticles can be excited by light at Near-IR absorbing, which makes them to be the next generation contrast agents for diagnostic and phototherapeutic applications, such as two-photon luminescence imaging, light-scattering imaging, surface-enhanced Raman scattering, and photothermal therapy. Moreover, by changing the particle size, surface chemistry, or aggregation state, the optical and electronic properties of gold nanoparticles can be adjustable and applicable for different uses, which present a promising potential in biological and clinical research.
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1. Surface Plasmon Resonance (SPR)
When gold nanoparticles are exposed to a specific wavelength of light, the oscillating electromagnetic field of the light induces a collective coherent oscillation of the free electrons, which causes a charge separation with respect to the ionic lattice, forming a dipole oscillation along the direction of the electric field of the light. The amplitude of the oscillation reaches the maximum at a specific frequency, called surface plasmon resonance (SPR).
When the size of gold nanoparticles change from 10 nm to 50 nm, the maximum extinction of the SPR Band shifts from 517 nm to 532 nm in the visible region, which indicates that the fluorescence intensity and the absorption band of gold nanoparticles are concentration and particle size dependent. In addition, SPR enhances the radiative properties such as absorption and scattering, offering multiple applications for biological and medical applications. For example, gold nanoparticles/titanium dioxide system has been proved to be efficient photo catalyst for some reactions.
2. Electrochemical Properties
The electrical property of gold particles has been intensively studied in past few years. Electron transport is not confined to the discrete energy levels of several atoms but appears as a continuum energy level. Therefore, surface charging and electron transport processes in gold nanoparticles may be understood with relatively simple classical physical expressions, as for resistance/capacitor electronic circuit diagrams. The electrical property of gold nanoparticles only depends on their size and surrounding medium, which has been used for many applications, such as electrical biosensors and electronic chips.
Gold nanoparticles can strongly absorb light as the result of the SPR. The absorbed light can efficiently be converted to heat by the fast electron–phonon and phonon–phonon processes, which makes gold nanoparticle a useful tool for photothermal therapy of cancers or other diseases. For example, when excited by light at wavelengths from 700 to 800 nm, near-IR absorbing gold nanoparticles can produce heat and eradicate tumors. Based on the enhanced permeability and retention effect and the explosion to the appropriate laser light, gold nanoparticles can be precisely accumulated in tumor cells and targeted treat tumor, which contribute much to the “see and treat” approach.
2. Drug and Gene Delivery
Gold nanoparticles conjugated with therapeutic agents improve the pharmacokinetics of the “free” drug and provides controlled or sustained release properties, which makes them an attractive tools for drug delivery and gene delivery. The large surface area-to-volume ratio of gold nanoparticles enables their surface to be coated with hundreds of molecules, including therapeutics, targeting agents, and anti-fouling polymers. Especially, DNA combined assembly gold nanoparticles have been successful used as efficient gene transfection tools.
Gold nanoparticles are widely used probes for immunogold staining in transmission electron microscopy (TEM) due to their high electron density. Additionally, the significant SPR-based light scattering capability of gold nanoparticles makes them probes for dark-field microscopy and Raman spectroscopy. Various studies has proved gold nanoparticles to be effective probes for cancer imaging based on their two-photon luminescence imaging, light-scattering imaging, surface-enhanced Raman scattering applications.
Gold nanoparticles have also been used as colorimetric probes. Typically, gold nanoparticle biosensing is based on the interaction of cross-linker with a receptor molecule on nanoparticles or the interaction between nanoparticles containing receptors when an ligand added in. Especially, gold nanoparticles protected by bovine serum albumin have been introduced as the ratiometric fluorescent probe for in vivo detection. This strategy could also be applied for the detection of proteins, pollutants, and other label-free molecules.
Gold nanoparticles are used as contrast agents in the diagnosis of heart diseases, cancers, and infectious agents. For example, X-ray computer tomography (CT) is a common diagnostic imaging tool for gold nanoparticles in vivo detection, which is used to visualize tissue density differences that provide image contrast by X-ray attenuation between soft tissues and electron-dense bone. Gold nanoparticles also exhibit good signal intensity and stability when acting as the promising materials for NIR imaging.
Gold nanoparticles are the most commonly used nanoparticles for lateral flow assays. Due to the optical properties of gold nanoparticles, detection with the naked eye can be achieved with excellent sensitivity. The assay can also be adapted to run both in non-competitive and competitive mode.
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Gold nanoparticles are used as photocatalysts in a number of chemical reactions. Because of the unique surface plasmon resonance property, the surface of gold nanoparticles can be used for selective oxidation or reduce a reaction in certain cases. Normally, gold nanoparticles are raised as photocatalyst with the combination of titanium dioxide, which can be useful in the chemical industry.
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