Visualizing the lower critical solution temperature phase transition of individual poly(nipam)-based hydrogel particles using near-infrared multispectral imaging microscopy.

This manuscript reports the use of near-infrared multispectral imaging (NIR-MSI) microscopy to provide the first direct observation and spectral measurement of individual poly(n-isopropylacrylamide-co-acrylic acid) (NIPAM-co-AAc) hydrogel particles. The high sensitivity and high spatial resolution (approximately 0.9 microm/pixel) of the NIR-MSI microscope, coupled with its ability to measure images and spectra directly and simultaneously, allows the unprecedented in situ monitoring of the size, morphology, and spectroscopic properties of individual hydrogel particles, which respond strongly to external stimuli (e.g., changes in temperature and/or pH). Importantly, this novel technique allows, for the first time, the direct measurement of the lower critical solution temperature (LCST) phase transition of individual hydrogel particles rather than that of a collection of hydrogel particles. Furthermore, NIR-MSI measurements reveal that the LCST value is unique for each individual hydrogel particle, depending strongly on particle size, with larger particles exhibiting higher LCST values.

Visualizing the size, shape, morphology, and localized surface plasmon resonance of individual gold nanoshells by near-infrared multispectral imaging microscopy.

We have successfully utilized the newly developed near-infrared multispectral imaging (NIR-MSI) microscope to observe and measure directly the localized surface plasmon absorption (LSPR) of individual gold nanoshells. The NIR-MSI is suited for this task because it can simultaneously record spectral and spatial information of a sample with high sensitivity (single pixel resolution) and high spatial resolution (approximately 0.9 microm/pixel). Importantly, the LSPR of individual nanoshells measured by the NIR-MSI microscope agrees well with the spectra calculated theoretically using Mie scattering for the nanoshells (i.e., nanoshells with silica cores approximately 800 nm in diameter and gold shell thicknesses of approximately 35 nm). Additionally, the NIR-MSI microscope enables measurement of LSPR at different positions within a single nanoshell. LSPR spectra were found to be distinct at various positions within a single nanoshell. Since LSPR spectra are known to depend on the shape and morphology of the nanoshells, these results seem to suggest that the individual nanoshells are not smooth and well-defined, but are rather rough and inhomogeneous. The LSPR spectra of single nanoshells in several different solvents were also examined using NIR-MSI and were found to depend on the dielectric constant of the medium. However, the relationship was discovered to be more complex than simply following the Drude equation. Specifically, when (lambda(max)/fwhm)(2) values of LSPR for single gold nanoshells were plotted as a function of 2n(2) (or 2epsilon) for nanoshells in six different solvents, a linear relationship was found for only three solvents: D(2)O, acetonitrile-d(3), and ethyl acetate. Acetone-d(6) showed a slight deviation, whereas formamide and pyridine-d(5) exhibited distinctly different correlations.

Porous silver-coated pNIPAM-co-AAc hydrogel nanocapsules.

This paper describes the preparation and characterization of a new type of core-shell nanoparticle in which the structure consists of a hydrogel core encapsulated within a porous silver shell. The thermo-responsive hydrogel cores were prepared by surfactant-free emulsion polymerization of a selected mixture of N-isopropylacrylamide (NIPAM) and acrylic acid (AAc). The hydrogel cores were then encased within either a porous or complete silver shell for which the localized surface plasmon resonance (LSPR) extends from visible to near-infrared (NIR) wavelengths (i.e., λmax varies from 550 to 1050 nm, depending on the porosity), allowing for reversible contraction and swelling of the hydrogel via photothermal heating of the surrounding silver shell. Given that NIR light can pass through tissue, and the silver shell is porous, this system can serve as a platform for the smart delivery of payloads stored within the hydrogel core. The morphology and composition of the composite nanoparticles were characterized by SEM, TEM, and FTIR, respectively. UV-vis spectroscopy was used to characterize the optical properties.

Ultrasmall hollow gold-silver nanoshells with extinctions strongly red-shifted to the near-infrared.

Hollow gold-silver nanoshells having systematically varying sizes between 40 and 100 nm were prepared. These particles consist of a hollow spherical silver shell surrounded by a thin gold layer. By varying the volume of the gold stock solution added to suspensions of small silver-core templates, we tailored the hollow gold-silver nanoshells to possess strong tunable optical extinctions that range from the visible to the near-IR spectral regions, with extinctions routinely centered at ∼950 nm. The size and morphology of these core/shell nanoparticles were characterized by dynamic light scattering (DLS), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). Separately, X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were used for measuring their elemental composition; UV-vis spectroscopy was used to evaluate their optical properties. Given their relatively small size compared to other nanoparticles that absorb strongly at near IR wavelengths, these easy-to-synthesize particles should find use in applications that require ultrasmall nanoparticles with extinctions comfortably beyond visible wavelengths (e.g., medicinal therapies, diagnostic imaging, nanofluidics, and display technologies).

Gold, palladium, and gold-palladium alloy nanoshells on silica nanoparticle cores.

The synthesis of gold, palladium, and gold-palladium alloy nanoshells (approximately 15-20 nm thickness) was accomplished by the reduction of gold and palladium ions onto dielectric silica core particles (approximately 100 nm in diameter) seeded with small gold nanoparticles (approximately 2-3 nm in diameter). The size, morphology, elemental composition, and optical properties of the nanoshells were characterized using field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and ultraviolet-visible spectroscopy. The results demonstrate the successful growth of gold, palladium, and gold-palladium alloy nanoshells, where the optical properties systematically vary with the relative content of gold and palladium. The alloy nanoshells are being prepared for use in applications that stand to benefit from photoenhanced catalysis.

Preparation, characterization, and optical properties of gold, silver, and gold-silver alloy nanoshells having silica cores.

This report describes the structural and optical properties of a series of spherical shell/core nanoparticles in which the shell is comprised of a thin layer of gold, silver, or gold-silver alloy, and the core is comprised of a monodispersed silica nanoparticle. The silica core particles were prepared using the Stöber method, functionalized with terminal amine groups, and then seeded with small gold nanoparticles (approximately 2 nm in diameter). The gold-seeded silica particles were coated with a layer of gold, silver, or gold-silver alloy via solution-phase reduction of an appropriate metal ion or mixture of metal ions. The size, morphology, and elemental composition of the composite nanoparticles were characterized by field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, thermal gravimetric analysis (TGA), dynamic light scattering (DLS), and transmission electron microscopy (TEM). The optical properties of the nanoparticles were analyzed by UV-vis spectroscopy, which showed strong absorptions ranging from 400 nm into the near-IR region, where the position of the plasmon band reflected not only the thickness of the metal shell, but also the nature of the metal comprising the shell. Importantly, the results demonstrate a new strategy for tuning the position of the plasmon resonance without having to vary the core diameter or the shell thickness.