Gold nanoparticles can act as excellent catalysts for biological reactions due to their large surface-to-volume. Uncoordinated gold atoms present on the surface allow the transfer of charge to nearby reactive species.
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Catalysis by gold nanoparticles
Gold nanoparticles can mimic the action of some catalysis enzymes, including glucose oxidase during the oxidation of glucose to glucolactone. Certain blood sugar meters use gold nanoparticles.
The catalysis of glucose oxidation by gold nanoparticles has also been used as an indicator of DNA hybridization. The unhybridized ssDNA present in solution binds strongly to the gold nanoparticle surface, pacifying it.
Hydrogen peroxide produced during the oxidation of glucose reduces AuCl4 in solution to its metallic form. This contributes to the growth of gold nanoparticles not coated with ssDNA. Therefore, the growth of gold nanoparticles in solution indicates the absence of ssDNA, and successful hybridization.
The size of the nanoparticle can be inferred by the wavelength of the surface plasmon resonance of the nanoparticle.
Interaction of nanoparticles with proteins
Gold nanoparticles interact with proteins, forming a ‘protein corona’ of semi-permanent and loosely bound proteins at the surface of the nanoparticle. This results in structural changes and denaturation of the protein.
Specially engineered proteins have been synthesized that change conformation in the presence of gold nanoparticle catalysts. One such example is trp-RNA binding attenuation protein (TRAP), a toroidal protein found in Bacillus bacteria that has been mutated to bear an exposed cysteine amino acid. Cysteine contains a thiol group that forms strong covalent bond with gold.
The structural transformation of the protein from 8 nm toroid to 20 nm hollow spheres is catalysed by 1.4 nm gold nanoparticles following the formation of this covalent bond.
Catalysis by ligands
Gold nanoparticles may be coated using a variety of ligand molecules that may themselves be catalysts, bound to the nanoparticle surface by electrostatic forces, covalent bonds, or conjugated using a linking molecule. This allows a high density of catalyst molecules to be present at the nanoparticle surface.
Catalysts conjugated to gold nanoparticles
Catalysts that depend on a particular structural arrangement, such as access to the binding pocket of enzymes, are usually conjugated to gold nanoparticles using a linker chain molecule, such as polyethylene glycol, as direct binding with the gold surface can denature the protein or induce conformational changes.
The binding pocket must also be accessible to the substrate, and thus non-wild type proteins modified to allow easy “click chemistry” conjugation via a site that will not interfere with the binding pocket are frequently employed.