The formation of a hybrid metal-biopolymer material is described. The synthesis of this material consists of functionalizing the surface of gold nanoparticles through a series of steps that lead to epoxy-functionalized nanoparticles. These are subsequently reacted with hydroxyl moieties of the alpha-D-glucopyranosyl groups of dextran. Subsequently, the dextran chains are carboxylated through treatment with bromoacetic acid. The resultant material combines the unique optical properties of gold nanoparticles with the versatility that carboxylated dextran offers for further functionalization with biomolecules. The interaction of this material with three proteins was then investigated through changes in the plasmon resonance properties of the gold nanoparticles. Concanavalin A, a lectin that binds glucose and mannose by means of specific molecular recognition, interacts readily with this material and such interaction is easily detected using optical absorption spectroscopy. Through reaction of the carboxyl groups with (+)-biotinyl-3,6,9,-trioxaundecanediamine, a material bearing biotin groups was obtained. This could interact with streptavidin or antibiotin by means of specific molecular recognition. Further confirmation of biospecific interactions was obtained with control experiments in which the binding sites were blocked through preincubation of the proteins with the corresponding ligand in solution. Binding of these proteins was concentration-dependent over a wide concentration range. This material provides a simple and convenient colorimetric method for biospecific interaction analysis.
Thermal diffusivity measurements on two nanofluids and their base fluids were made using an optical technique called forced Rayleigh scattering. The nanofluids studied were a citrate-stabilized Au nanoparticle suspension in water and an Al2O3 nanoparticle suspension in a petroleum oil. Thermal diffusivity measurements on the nanofluids and base fluids were made at temperatures in the range of 25–75°C. From these data, it was possible to estimate the thermal conductivity enhancement in the nanofluids as a function of temperature. In contrast to previous reports on similar systems, our experiments are consistent with thermal conductivity enhancement predictions from effective medium theory. In particular, we find that the level of thermal conductivity enhancement is independent of temperature.
Gold nanoparticles linked to linear carboxylated dextran chains were attached to 3-aminopropyltriethoxysilane-functionalized glass surfaces. This method provides novel hybrid nanostructures on a surface with the unique optical properties of gold nanoparticles. The particles attached to the surface retain the capability to aggregate and disaggregate in response to their environment. This procedure presents an alternative method to the immobilization of gold nanoparticles onto planar substrates. Compared to gold nanoparticle monolayers, larger particle surface densities were obtained. Exposure to hydrophobic environments changes the conformation of the hydrophilic dextran chains, causing the gold nanoparticles to aggregate and inducing changes in the absorption spectrum such as red-shifting and broadening of the plasmon absorption peaks. These changes, characteristic of particle aggregation, are reversible. When the substrates are dried and then immersed in an aqueous environment, these changes can be visually observed in a reversible fashion and the sample changes color from the red color of colloidal gold to a bluish-purple color of aggregated nanoparticles. Surface-bound nanoparticles that retain their mobility when attached to a surface by means of a flexible polymer chain could expand the use of aggregation-based assays to solid substrates.
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