Immobilization of Gold Nanoparticles on Aluminum Oxide Nanoporous Structure for Highly Sensitive Plasmonic Sensing

The localized surface plasmon resonance (LSPR) of nanoaggregates of gold or silver nanoparticles, particularly in dimerized nanoparticles, has received significant attention because of the coupling property of individual particles’LSPR.1–4)Moreover, theoretical studies of AuNDs show that their optical responses become singular when two single particles approach each other and weaken when they are separated.5–7)Basically, the LSPR bands of these anisotropic nanoparticles split into two oscillation modes; a transverse mode and a longitudinal mode.8–10)Interestingly, the strength and peak wavelength of the longitudinal LSPR band undergo larger spectral changes than those of spherical nanoparticles when the surrounding refractive index is altered. Therefore, it provides a highly sensitive plasmonic signal in a longer wavelength that is extremely neededfor molecular biosensing. The electromagnetic (EM) field enhancements from dimerized nanoparticles bring several advantages for in vitro and in vivo studies. 11-15)Recently, several investigations on the formation of AuNDs using multivalent thiol linkers,16)polymer encapsulated dimers of AuNPs,17)a solid bead support,18) two-phasemediated functionalization,19) functionalization on a two dimensional solid surface,20)and DNA-AuNP conjugates 21) have been reported. Since most of these methods are based on the modification of nanoparticle surfaces with synthetic organic or biological molecules, they are rather complex and require a considerable number of time-consuming steps.Likewise, the linker bridges of two nanoparticles in synthesized AuNDs prevent the functionalization of the gold surface to bind with analyte molecules in many plasmonic sensing applications. For this reason, there remain challenges in developing alternative immobilization methods that can capture ‘‘fresh’’ AuNDs on a novel gold nanostructure design for exploiting useful plasmonic properties.In this study, we immobilized AuNPs on aluminum oxide porous substrates made by the anodization of alumina. By several optimizing processes, we were able to fix 10% of immobilized AuNPs as nanoparticle dimers on the surface of a porous aluminum oxide (Al2O3) layer formed on an aluminum (Al) substrate using poly(allyl amine). We then conducted some investigations on the plasmonic properties of AuNP on photonic thin-film multilayers of Al2O3/Al.Analyses show that the thickness of the transparent Al2O3layer which could be controlled by adjusting electrochemical anodization time can be used to enhance the plasmonic properties of these AuNPs. Interestingly, the porous nano- structure partially trapped two nanoparticles, producing a small number of AuNDs on the substrate surface. Spectral bands attributed to the longitudinal plasmon of the AuNDs were found at a specific Al2O3 thickness and the LSPR band of dimers in the reflection spectra can be selectively enhanced by an optical interference effect from the thin-film multilayer structure. These experimental reflection spectra of our plasmonic substrates were validated by theoretical simulation.

The note is here