Abstract
Multilayers of well-ordered and close-packed 2D nanostructures of gold nanoparticles (NPs) were fabricated using a layer-by-layer technique. Colloidal spherical Au NPs of 5 and 15 nm diameters were synthesized and, respectively, self-assembled in multilayers. The robustness of these systems was insured by a combination of electrostatic and covalent bonds between nanoparticles and linker molecules. The compacity of the superlattice was characterized by AFM observation and ellipsometry measurements. Evidence of stronger cohesion of multilayers of smaller NPs size was brought by submitting the systems to sonication test. The multilayers have also proved analytical potential when used to detect low concentration methylene blue molecules adsorbed on the Au nanoparticles, by surface-enhanced Raman spectroscopy (SERS). The detection sensitivity of these two sized Au NPs architectures was directly compared to an evaporated “bulk” Au thin film of equivalent thickness. Results have displayed a strong increase of the electromagnetic field enhancement with a decrease of the NPs size, whereas the bulk thin Au film was shown to be inefficient as a SERS substrate. These results bring a nice evidence of size effects on the global performance (SERS, cohesion) and hopefully on the stability of NPs based nanostructures.
Highlights
In recent years, considerable effort has been devoted to the design and fabrication of structured materials with functional properties
When used to enhance the optical susceptibility of molecules that are embedded in a nanoparticulate surface which is irradiated by an electromagnetic field, this optical property can be used as a powerful spectroscopic tool, known as surface-enhanced Raman spectroscopy (SERS)
Gold colloidal nanoparticles were first characterized by transmission electron microscopy (TEM) and ultraviolet light (UV)-Vis spectrophotometry in order to determine the morphology and size of the gold nanoparticles
Summary
Considerable effort has been devoted to the design and fabrication of structured materials with functional properties. The long-range and large-scale arrangement of nanoparticles (NPs) is an important issue in the development of nanostructured materials with new electronic, magnetic, and photonic properties [1]. Different top-down techniques like photolithography [2] and microcontact printing [3] have been developed to organize nanoparticles on substrates. In this field, noble metal nanoparticles, and especially gold nanoparticles, have attracted a great deal of interest. Metal particles of size smaller than the wavelength of light exhibit specific optical properties due to the resonant excitation of coherent electron oscillations, a phenomenon known as localized surface plasmons. When used to enhance the optical susceptibility of molecules that are embedded in a nanoparticulate surface which is irradiated by an electromagnetic field (light), this optical property (surface plasmons) can be used as a powerful spectroscopic tool, known as surface-enhanced Raman spectroscopy (SERS)
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