Abstract
Hierarchical organization of gold nanorods was previously obtained on a substrate, allowing precise control over the morphology of the assemblies and macroscale spatial arrangement. Herein, a thorough description of these gold nanorod assemblies and their orientation within supercrystals is presented together with a sol-gel technique to protect the supercrystals with mesoporous silica films. The internal organization of the nanorods in the supercrystals was characterized by combining focused ion beam ablation and scanning electron microscopy. A mesoporous silica layer is grown both over the supercrystals and between the individual lamellae of gold nanorods inside the structure. This not only prevented the detachment of the supercrystal from the substrate in water, but also allowed small molecule analytes to infiltrate the structure. These nanocomposite substrates show superior Raman enhancement in comparison with gold supercrystals without silica owing to improved accessibility of the plasmonic hot spots to analytes. The patterned supercrystal arrays with enhanced optical and mechanical properties obtained in this work show potential for the practical implementation of nanostructured devices in spatially resolved ultradetection of biomarkers and other analytes.
Highlights
The organization of gold nanorods (GNRs) into supercrystals is dictated by external parameters that influence the drying time, the chemistry of the colloid, as well as the size and shape of the mold used to dry the particles.[10,16,32]
In this work, micropatterned GNR supercrystals were coated and embedded with mesoporous silica to form a composite material with a remarkable surface enhanced Raman scattering (SERS) enhancement
Gold nanorod organization in the supercrystals was investigated, and longrange smectic ordering was observed in micron sized supercrystals, with long-range order increasing with cavity size and initial concentration of nanorods in solution
Summary
The design of materials with hierarchical organization ranging from the nanoscale to the macroscale is of interest for the development of novel sensing platforms and optical devices.[1,2,3,4,5] The organization of particles into self-assembled structures results in a material with increased overall size, dimensionality, and complexity, and the emergence of collective properties different from those of the individual components.[2,6] Noble metal nanoparticles organized into supercrystalline structures (i.e. supercrystals) can lead to significant improvements in targeted plasmonic-sensing applications, as the amplification of optical signals characteristic of single metal nanoparticles is highly enhanced when collective plasmon coupling occurs.[7,8,9]An ideal fabrication method requires simple, cost-effective, and parallelizable processes.[2]. The organization of gold nanorods (GNRs) into supercrystals is dictated by external parameters that influence the drying time (e.g. temperature or humidity), the chemistry of the colloid ( particle concentration and surface ligands), as well as the size and shape of the mold used to dry the particles.[10,16,32] All GNRs used in this work had dimensions of 57 ± 5 nm in length and 17 ± 2 nm in width (ESI†) and were dried inside PDMS cavities having a fixed height of 4.8 μm and lateral dimensions of either 2, 6, 12 or 20 μm.
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