Tuneable 2D self-assembly of plasmonic nanoparticles at liquid|liquid interfaces

Abstract

Understanding the structure and assembly of nanoparticles at liquid|liquid interfaces is paramount to their integration into devices for sensing, catalysis, electronics and optics. However, many difficulties arise when attempting to resolve the structure of such interfacial assemblies. In this article we use a combination of X-ray diffraction and optical reflectance to determine the structural arrangement and plasmon coupling between 12.8 nm diameter gold nanoparticles assembled at a water|1,2-dichloroethane interface. The liquid|liquid interface provides a molecularly flat and defect-correcting platform for nanoparticles to self-assemble. The amount of nanoparticles assembling at the interface can be controlled via the concentration of electrolyte within either the aqueous or organic phase. At higher electrolyte concentration more nanoparticles can settle at the liquid|liquid interface resulting in a decrease in nanoparticle spacing as observed from X-ray diffraction experiments. The plasmonic coupling between the nanoparticles as they come closer together is observed by a red-shift in the optical reflectance spectra. The optical reflectance and the X-ray diffraction data are combined to introduce a new 'plasmon ruler'. This allows extraction of structural information from simple optical spectroscopy techniques, with important implications for understanding the structure of self-assembled nanoparticle films at liquid interfaces.

Gold Nanofilms at Liquid–Liquid Interfaces: An Emerging Platform for Redox Electrocatalysis, Nanoplasmonic Sensors, and Electrovariable Optics

“…Comparison of SEM and TEM images obtained with transfer of the nanofilm on a solid substrate shows similar features, confirming that the structure of the film can stay more or less intact during the transfer. 7 Furthermore, Velleman et al 135 used a commercial reflectivity probe positioned at 90°to the liquid−liquid interface (see Figure 9D) to monitor the red-shift in SPR reflectance maxima as a result of enhanced plasmonic coupling between AuNPs due to decreasing interparticle spacing with increasing ionic strength in either the aqueous or organic phase (allowing calibration of a so-called "plasmonic ruler").…”

“…As detailed in section 3.4, decreasing the interparticle gap in a gold nanofilm causes a near-exponential increase in the sensitivity of the responses of SPR and SERS-based nanoplasmonic sensors, as well as an exponential increase in the conductivity of the gold nanofilm. Thus, the creation of calibrated plasmonic rulers using easily accessible optical spectroscopic techniques, as described by Velleman et al, 135 provides a precise guide to tune the interparticle gaps through addition of salt, applying an electric field, etc. in real time.…”

Monitoring plasmon coupling and SERS enhancement through in situ nanoparticle spacing modulation

“…The LSPR of GNPs assembled into the film shifted to 558 nm ( Figure 2b). From our previous study, 18 the theoretical model was adapted to 42 nm GNPs and predicts an LSPR position of 558 nm to correspond to GNPs spaced 30 nm apart. It should be noted that the gap distances calculated are averages over the NP array.…”

“…With these techniques, we have previously shown inter-NP surface to surface spacing at the LLI to be easily tuned between upwards of 30 nm down to 4.5 nm. 18 In this study we have again exploited the self-assembly of NPs at LLI's to obtain highly ordered, self-healing, low-defect NP arrays, exploring the distance-dependent properties of both spherical and anisotropic gold NPs (Figure 1a). At the LLI NPs are mobile on the XY plane, rearranging themselves to minimise energy.…”

Electrochemical plasmonic metamaterials: towards fast electro-tuneable reflecting nanoshutters

“…This term refers to plasmonic metasystems based on electrode/electrolyte interfaces, solid-liquid or liquid-liquid 1 , which involve adsorption-desorption of plasmonic nanoparticles from the solution to the interface, tuned by variation of electrode potential and controlled by electrolyte concentration and pH of the solution. It has been predicted theoretically and shown experimentally, that rearrangements of such nanoparticles -from dispersion in the bulk to formation of arrays at the interface, may dramatically change optical properties of the interface 10 , be it mirror-window switching 2 , tuneable colour mirrors 11 , or SERS sensors 12 . One can read more about the progress in this area in recent review articles 1 .…”