In-Plane Plasmonic Antenna Arrays with Surface Nanogaps for Giant Fluorescence Enhancement

Abstract

Optical nanoantennas have a great potential for enhancing light-matter interactions at the nanometer scale, yet fabrication accuracy and lack of scalability currently limit ultimate antenna performance and applications. In most designs, the region of maximum field localization and enhancement (i.e., hotspot) is not readily accessible to the sample because it is buried into the nanostructure. Moreover, current large-scale fabrication techniques lack reproducible geometrical control below 20 nm. Here, we describe a new nanofabrication technique that applies planarization, etch back, and template stripping to expose the excitation hotspot at the surface, providing a major improvement over conventional electron beam lithography methods. We present large flat surface arrays of in-plane nanoantennas, featuring gaps as small as 10 nm with sharp edges, excellent reproducibility and full surface accessibility of the hotspot confined region. The novel fabrication approach drastically improves the optical performance of plasmonic nanoantennas to yield giant fluorescence enhancement factors up to 10 4 −10 5 times, together with nanoscale detection volumes in the 20 zL range. The method is fully scalable and adaptable to a wide range of antenna designs. We foresee broad applications by the use of these in-plane antenna geometries ranging from large-scale ultrasensitive sensor chips to microfluidics and live cell membrane investigations. KEYWORDS: Optical nanoantennas, template stripping, electron beam lithography, fluorescence enhancement, plasmonics O ptical nanoantennas take advantage of the plasmonic response of noble metals to strongly confine light energy into nanoscale dimensions and breach the classical diffraction limit. 1−3 This confinement leads to a drastic enhancement of the interactions between a single quantum emitter and the light field, 4−7 enabling large fluorescence gains above a thousand fold, 8−13 ultrafast picosecond emission, 14−16 and photobleaching reduction. 17,18 As such, optical antennas hold great interest for ultrasensitive biosensing, especially for the detection of single molecules at biologically relevant micromolar concentrations. 19−21 Biosensing applications of nanoantennas require the largescale availability of narrow accessible gaps. Not only should nanogaps with sub-20 nm dimensions be reproducibly fabricated but also the gap region (plasmonic hotspot) must remain accessible to probe the target molecules. Despite impressive recent progress using electron beam, 22 focused ion beam, 23 or stencil lithographies 24−26 or alternatively with bottom-up self-assembly techniques, 6,7,9,13,16,27−30 the challenges of reliable narrow gap fabrication and hotspot accessibility remain major hurdles limiting the impact and performance of optical nanoantennas. For instance, when aiming for the fabrication of aperture antennas, electron beam lithography (EBL) using a positive-tone resist requires metal dry etching, which produces high line-edge roughness that are not suited for the definition ...

Transient Nanoscopic Phase Separation in Biological Lipid Membranes Resolved by Planar Plasmonic Antennas

“…The antenna design consisted of gold dimers of 80 nm in diameter separated by nanogaps of different sizes (from 10 nm to 45 nm) and surrounded by nano-apertures to further constrain the excitation area and reduce background contribution from fluorescent molecules diffusing outside the antenna hotspots (Supporting information Figure S2a). 32 For the experiments reported here we took advantage of the in-plane antenna geometry to have access to the maximum spatial confinement at the gap regions. In addition, considering that the height difference between the gold dimers and the filling polymer at the hotspot measurement site is below 1nm (Supporting information Figure S2b), we regard these substrates as of excellent planarity and thus suitable for membrane studies.…”

“…Recently, we overcame this drawback by fabricating in-plane dimer antenna arrays where the gap region is located at the sample surface. 32 This design provides direct accessibility to the antenna hotspot region and drastically improves the optical performance to yield fluorescence enhancement factors of up to 10 4 −10 5 together with nanoscale detection volumes in the zeptoliter range. 32 Here, we take advantage of the strong optical confinement occurring on plasmonic antenna arrays together with their remarkable planarity to inquire on the nanoscale dynamics of multicomponent lipid bilayers.…”

Optical Antenna-Based Fluorescence Correlation Spectroscopy to Probe the Nanoscale Dynamics of Biological Membranes

“…We recently overcame this issue by combining electron beam lithography with planarization, etch back and template stripping. 118 The planarization strategy fills the aperture volume with a transparent polymer, yielding a flat top surface (of a planarity better than 3 nm, see Fig. 2 c), compatible with membrane studies on living cells.…”

Deep Ultraviolet Plasmonic Enhancement of Single Protein Autofluorescence in Zero-Mode Waveguides

“…19 Plasmonic optical nanoantennas provide powerful means to overcome the limits of diffraction-limited microscopes, as they enable concentrating light at the nanoscale and enhancing the fluorescence emission of single molecules. [20][21][22][23][24] Additionally, plasmonic nanostructures offer the opportunity to detect single molecules at high micromolar concentrations compatible with biologically-relevant conditions. [25][26][27] All these features are highly appealing to improve the tryptophan autofluorescence detection of single proteins and extend plasmonics into the UV range.…”