Temperature-Responsive Hydrogel-Coated Gold Nanoshells.

Gold nanoshells (~160 nm in diameter) were encapsulated within a shell of temperature-responsive poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAM-co-AA)) using a surface-bound rationally-designed free radical initiator in water for the development of a photothermally-induced drug-delivery system. The morphologies of the resultant hydrogel-coated nanoshells were analyzed by scanning electron microscopy (SEM), while the temperature-responsive behavior of the nanoparticles was characterized by dynamic light scattering (DLS). The diameter of the P(NIPAM-co-AA) encapsulated nanoshells decreased as the solution temperature was increased, indicating a collapse of the hydrogel layer with increasing temperatures. In addition, the optical properties of the composite nanoshells were studied by UV-visible spectroscopy. The surface plasmon resonance (SPR) peak of the hydrogel-coated nanoshells appeared at ~800 nm, which lies within the tissue-transparent range that is important for biomedical applications. Furthermore, the periphery of the particles was conjugated with the model protein avidin to modify the hydrogel-coated nanoshells with a fluorescent-tagged biotin, biotin-4-fluorescein (biotin-4-FITC), for colorimetric imaging/monitoring.

Thermal stability of mono-, bis-, and tris-chelating alkanethiol films assembled on gold nanoparticles and evaporated "flat" gold.

The thermal stability of SAMs generated from the adsorption of n-octadecanethiol (n-C18), 2-hexadecylpropane-1,3-dithiol (C18C2), 2-hexadecyl-2-methylpropane-1,3-dithiol (C18C3), and 1,1,1-tris(mercaptomethyl)heptadecane (t-C18) on colloidal gold and evaporated "flat" gold was investigated. The optical extinction of the monolayer-protected nanoparticles (MPCs) was monitored as a function of thermal stress by using ultraviolet-visible (UV-vis) spectroscopy, which revealed that the evolution of the surface plasmon resonance varied with the nature of the adsorbate. Specifically, MPCs functionalized with monodentate n-C18 showed the fastest red shift of the surface plasmon resonance while those functionalized with tridentate t-C18 showed the slowest red shift, with those derived from the bidentates C18C2 and C18C3 falling in between, suggesting a correlation between film stability and the degree of chelation. In separate studies, X-ray photoelectron spectroscopy (XPS) was used to evaluate the desorption of the monolayers on both colloidal gold and flat gold as a function of thermal stress. In these studies, SAMs generated from monodentate n-C18 showed the fastest desorption while SAMs generated from tridentate t-C18 showed the slowest desorption, with those derived from the bidentates C18C2 and C18C3 falling in between, again suggesting a correlation between film stability and the degree of chelation. As a whole, the following trend in thermal stability was observed: t-C18 > C18C2 approximately C18C3 > n-C18.

SAMs on gold derived from the direct adsorption of alkanethioacetates are inferior to those derived from the direct adsorption of alkanethiols.

Self-assembled monolayers (SAMs) on gold derived from the direct adsorption of thioacetic acid S-decyl ester (C10SAc) and thioacetic acid S-octadecyl ester (C18SAc) were compared to the corresponding SAMs derived from the analogous adsorption n-decanethiol (C10SH) and n-octadecanethiol (C18SH). All SAMs were characterized using ellipsometry, contact angle goniometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). The comparison revealed that the SAMs generated from the thioacetates are not as densely packed and well ordered as the SAMs generated from the thiols. Furthermore, studies of the kinetics of adsorption found that the thioacetates adsorb more slowly than the corresponding thiols.

Preparation, characterization, and chemical stability of gold nanoparticles coated with mono-, bis-, and tris-chelating alkanethiols.

A systematically varying series of monolayer-protected clusters (MPCs) was prepared by exposing small gold nanoparticles ( approximately 2 nm in diameter) to the following four adsorbates: n-octadecanethiol ( n - C18), 2-hexadecylpropane-1,3-dithiol ( C18C2), 2-hexadecyl-2-methylpropane-1,3-dithiol ( C18C3), and 1,1,1-tris(mercaptomethyl)heptadecane ( t - C18). The resultant MPCs were characterized by solubility studies, UV-vis spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FT-IR). All of the MPCs were soluble in common organic solvents; moreover, analysis by TEM showed that the core dimensions were unaffected by exposure to any of the adsorbates. Separate studies by XPS revealed that the sulfur atoms in all MPCs were predominantly bound to the surface of gold (i.e., approximately 85% or better). Analysis by FT-IR showed that MPCs functionalized with n - C18 possessed alkyl chains having the greatest conformational order in both the solid-state and dispersed in solution; in contrast, those generated from the other three adsorbates were more liquid-like with reduced conformational order (or crystallinity). The rate of nanoparticle decomposition induced by cyanide ions was monitored by UV-vis spectroscopy. While MPCs functionalized with n - C18 showed the fastest rate of decomposition, those functionalized with C18C3 were the most resistant to decomposition. Overall, the following trend in chemical stability was observed, C18C3 >> C18C2 > t - C18 >> n - C18.

Rationally designed ligands that inhibit the aggregation of large gold nanoparticles in solution.

Hexadecanethiol (n-C16), 2,2-dimethylhexadecane-1-thiol (DMC16), and the multidentate thiol-based ligands 2-tetradecylpropane-1,3-dithiol (C16C2), 2-methyl-2-tetradecylpropane-1,3-dithiol (C16C3), and 1,1,1-tris(mercaptomethyl)pentadecane (t-C16) were evaluated for their ability to stabilize large gold nanoparticles (>15 nm) in organic solution. Citrate-stabilized gold nanoparticles (20-50 nm) treated with the ligands were extracted from aqueous solution and dispersed into toluene. The degree of aggregation of the gold nanoparticles was monitored visually and further confirmed by UV-vis spectroscopy and dynamic light scattering (DLS). The bidentate ligands (C16C2 and C16C3) and particularly the tridentate ligand (t-C16) showed enhanced abilities to inhibit the aggregation of large gold nanoparticles in organic solution. For gold nanoparticles modified with these multidentate ligands, bound thiolate (S2p3/2 binding energy of 162 eV) was the predominant sulfur species (>85%) as evaluated by X-ray photoelectron spectroscopy (XPS). Although an entropy-based resistance to ordering of the loosely packed surfactant layers was initially considered to be a plausible mechanism for the enhanced stabilization afforded by the multidentate ligands, when taken as a whole, the data presented here support a model in which the enhanced stabilization arises largely (if not solely) from the multidentate chelate effect.