Surface lattice resonance in an asymmetric air environment of 2D Au near-spherical nanoparticle arrays: impact of nanoparticle size and its sensitivity

In numerous studies, it is imperative to establish a homogeneous refractive index (RI) surrounding environment to acquire a surface lattice resonance (SLR) characteristic peak for a plasmonic array. However, this requirement will limit the practical applicability of the SLR. This study successfully achieved the fabrication of cost-efficient, size-adjustable, and structure-transferable two-dimensional (2D) ordered gold (Au) near-spherical nanoparticle (NP) arrays with large NP size (>100 nm) through utilizing a colloidal crystal template and annealing process. In this method, the NP diameter can be adjusted by varying the thickness of the deposited Au film. Importantly, the fabricated 2D Au NP arrays with larger NP size supported an obvious and sharp SLR peak by exciting with normal incident light in an asymmetric air environment. These results indicated that if the arrays were well arranged and the NP size was large enough, index-matching was not necessary for the formation of far-field electromagnetic coupling of the plasmonic NPs. Furthermore, theoretical calculation demonstrated that the significant SLR signal in air environment was relevant to the NP size. Due to the obvious SLR peak in air, the 2D Au NP arrays were applied as a sensor to detect hemoglobin (Hb). It was found that the SLR peak was more sensitive than the localized surface plasmon resonance (LSPR) peak, and its offset was obviously observed when the Hb concentration was as low as 10 μM. Moreover, the Au NP arrays also displayed good uniformity and stability. Besides rigid nanophotonic sensor, the prepared 2D Au NP arrays on a quartz substrate can be transferred to a flexible substrate to form a flexible free-standing optical sensor, providing a real-time tunable SLR mode. With the decrease of water content, the SLR peak continuously blue-shifted from 1058 nm to 605 nm, displaying high sensitivity. This paper not only provides a cost-effective and controllable approach to fabricate next-generation rigid and flexible nanophotonic devices, but also paves the way for practical application of SLR in sensing.