Development of highly thermoresponsive fluorescent sensors consisting of plasmonic silver nanoprisms and poly(N-isopropylacrylamide)-fluorophore composites.

We developed a new hybrid consisting of Ag nanoprisms, poly(N-isopropylacrylamide) (PNIPAm), and fluorophores via layer-by-layer assembly. The fluorescence intensity below the lower critical solution temperature (LCST) of PNIPAm was 6.4 times stronger than that above the LCST, meaning that the hybrids can function as nanosized highly thermoresponsive fluorescent sensors.

Particle size dependence of the surface-enhanced Raman scattering properties of densely arranged two-dimensional assemblies of Au(core)-Ag(shell) nanospheres.

We investigated the dependence of the surface-enhanced Raman scattering (SERS) activity of densely arranged two-dimensional assemblies of spherical Au(core)-Ag(shell) nanoparticles (Au/AgNSs) on the nanoparticle diameter. The size-controlled Au/AgNSs were synthesized using the Au nanosphere seed-mediated growth method without any bulky stabilizers. The diameters of the Au/AgNSs were 38, 53, and 90 nm and the ratio of the total diameter to the Au core diameter was adjusted to ca. 2.0. Extinction spectra of the colloidal solutions of these nanoparticles exhibited the prominent peak of the localized surface plasmon resonance (LSPR) of Ag and therefore the Au/AgNSs exhibited LSPR properties almost the same as Ag nanospheres. It was confirmed from SEM observation that the organic solvent-mediated liquid-liquid interface assembly technique easily generated densely arranged two-dimensional assemblies of the nanospheres. The extinction spectra of all the assemblies exhibited a prominent broad peak ranging from 500 nm to the near-infrared region, which is assigned to the longitudinal LSPR mode of the coupling nanospheres. The extinction intensity increased with increasing nanosphere diameter. The SERS activities of these assemblies were investigated using p-aminothiophenol as a probe molecule. The result revealed that the enhancement factor (EF) of the Raman signal dramatically increased upon increasing the particle diameter. The maximum EF obtained with a laser excitation wavelength of 785 nm was 1.90 × 10(6) for a nanosphere diameter of 90 nm. This renders the two-dimensional assemblies of the plasmonic Au/AgNSs promising for the development of highly sensitive SERS sensor platforms due to their strong electromagnetic effect.

Fabrication of dense two-dimensional assemblies over vast areas comprising gold(core)-silver(shell) nanoparticles and their surface-enhanced Raman scattering properties.

Fabrication of dense two-dimensional assemblies consisting of gold(core)-silver(shell) nanoparticles and the resulting peculiar surface-enhanced Raman scattering (SERS) activity are reported. The assemblies were prepared via assembly at air-toluene interfaces by drop-casting toluene solutions containing the nanoparticles protected with octadecylamine molecules onto glass plates. This simple process, which does not require special apparatus or significant fabrication time, leads to uniform assemblies over vast areas (~34 cm(2)). In the SERS measurements, the high spatial reproducibility of the SERS signals from p-aminothiophenol adsorbed on the assemblies over vast areas demonstrates that this method is useful for the quantitative investigation of SERS mechanisms. Under 532 nm laser excitation, the difference in the enhancement factors of the SERS signals at the a1 mode between assemblies consisting of gold, silver, and core-shell nanoparticles can be explained by the degree of overlap of the excitation wavelength with their plasmon coupling modes. In contrast, under 785 nm excitation, even though the plasmon band of the core-shell nanoparticle assemblies does not significantly overlap with the excitation wavelength as compared with that of gold nanoparticle assemblies, the enhancement factor from the core-shell nanoparticle assemblies was stronger than those from the gold nanoparticle assemblies. Therefore, we have demonstrated that the gold(core)-silver(shell) nanoparticle assemblies are excellent SERS active materials, which have strong electromagnetic mechanism (EM) as well as chemical mechanism (CM) effects due to the silver shells.

Densely arranged two-dimensional silver nanoparticle assemblies with optical uniformity over vast areas as excellent surface-enhanced Raman scattering substrates.

Dense two-dimensional assemblies of silver nanoparticles were fabricated over vast areas (~19 cm(2)) by utilizing a simple liquid-liquid interface assembly technique. The Raman signal for p-aminothiophenol immobilized on the assemblies was drastically enhanced by plasmon coupling-induced hot spots, and the enhanced signal showed high spatial reproducibility.