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Plasmonic Gratings


Figure 1 : (a) Schematic of a silver plasmonic GLAD grating on a fl uorescence-based application and an expanded schematic of the GLAD morphology with nanoprotrusions and nanogaps. The brighter yellow color of the image indicates higher fl uorescent emission intensity. (b) AFM 3D contour map of silver GLAD grating rapidly deposited at an oblique deposition angle (α = 60°), exhibiting controlled nanogaps and nanoprotrusions (hotspots). Reproduced from [ 19 ] with permission, copyright 2016 RSC.


Grating coatings. The microcontact lithography process for making PMSSQ gratings from profi les stored on DVDs is described in detail in our previous publications [ 20 – 25 , 28 ]. T is process can be used to fabricate PMSSQ gratings on both microscope slides and silicon wafers. Afterward, 2 nm thick chromium and 40 nm thick silver fi lms were deposited onto the PMSSQ grating using thermal evaporation and a custom-built variable angle deposition stage that rotates the sample surface such that the metal fl ux is incident at α = 60° above the surface. During our study of the GLAD technique, we found that the shape, distribution, and size of the nanoprotrusions and nanogaps can be further modifi ed by tuning the PMSSQ surface energy, incidence angle of the metal vapor, and the deposition rate. T e platform can also be coated with diff erent materials to improve its functionality for different applications. For example, the silver can be capped with silica (SiO 2 ) or gold to provide a binding site for biological studies. T e GLAD grating can also be coated with a PMSSQ matrix that incorporates a dye for fl uorescence studies. FDTD simulations. Based on fi nite diff erence time domain (FDTD) simulations of electric-field distribution ( Figure 2 ), the E-fi eld is highly concentrated into hotspots located on both nanoprotrusions and nanogaps. Because the E-fi eld is confi ned in both the axial and lateral directions, we can obtain better imaging resolution in both directions. It is important to note that the fl uorescence enhancement obtained by a GLAD grating is proportional to the product of both the excitation and emission E-fi eld enhancement [ 20 ]. T e wide coupling wavelength and angle ranges of the GLAD gratings are well suited to enhance the fl uorescence emission intensity. When this enhancement is combined with the extremely low background intensity due to the inherent absorptive nature of the plasmonic gratings, the resulting samples have an improved SNR. T is greater SNR enables imaging of single molecules over a wide range of fl uoro- phore concentration and improves the localization precision of super-resolution studies.


Single-molecule imaging . Typically, single-molecule imaging in the low fl uorophore concentration range requires amplified fluorescence emission intensity and/or increased camera sensitivity such that the signal of a single fl uorophore is distinguishable over background noise. On the other hand, this type of imaging in high fluorophore concentrations


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Figure 2 : Simulated E-fi eld (|Ez/Ez,0|) distribution for 40 nm thick silver GLAD gratings coated with 30 nm PMSSQ in water environment at Cy5 excitation (λ = 642 nm, θ = 0°) and emission (λ = 670 nm, θ = 12.2°). Reproduced from [ 19 ] with permission, copyright 2016 RSC.


requires that the excitation be confined to a small volume and imaged without bulk averaging. The spatial resolution is then determined by the volume of the confined light in a plasmon-related nanostructure, which can be quite small when a nanoprotrusion or nanogap is less than 100 nm in width [ 29 ]. We tested the single-molecule imaging capabilities of these GLAD gratings by imaging samples containing several Cy5 fluorophore concentrations including the following: a surface-immobilized DNA/RNA duplex labeled with Cyanine-3 (Cy3) and Cyanine-5 (Cy5) ( Figure 3 a and 3 b) and spin-coated PMSSQ fi lms (30 nm thick) laced with a low- and high-concentration range of Cy5 ( Figure 3 c and 3 d). T e fl uorescence images and videos were taken with either an Andor iXon+ EMCCD camera or an ORCAFlash 2.8 CMOS camera with a 1× or 2× magnifi cation collar. Because of the inherently


www.microscopy-today.com • 2017 January


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