In recent years single-molecule localization super-resolution microscopy (SMLM) has become an indispensable tool for many fields of research. Here, for any image of a single molecule one determines its center position with much higher accuracy than the size of that image itself. A challenge of SMLM is to achieve super-resolution also along the third dimension. Recently, Metal-Induced Energy Transfer or MIET [1,2] was introduced. It exploits the energy transfer from an excited fluorophore to plasmons in a thin metal film. Similar to Förster Resonance Energy Transfer (FRET), this coupling shows a strong distance dependence, but over a range up to 150 nm and enables axial localization of fluorophores with 5-6 nm resolution at a photon budget of 1000 photons. [3,4] Here, we show that using a graphene layer the localization accuracy of MIET reaches Ångström accuracy. At such accuracy, minute details such as nanometer-scale roughness of the sample surfaces becomes important.
For proof of principle, we determined absolute distances of single molecules from a surface for samples with an a priori well-known sample geometry. We spin-coated fluorescent dye molecules (Atto655) on top of three different substrates with spacer thickness values of 10, 15, and 20 nm, defining the distance of the molecules from the graphene layer. Next, we determined the thickness of supported lipid bilayers (SLBs) by localizing fluorescent dyes attached to lipid head groups in the bottom and top leaflet of the SLB.
We have demonstrated that by using graphene as the energy acceptor in MIET, the axial localization accuracy and resolution reaches sub-nanometer levels at photon budgets which are typical in conventional SMLM experiments. An interesting feature of graphene-MIET is that it provides an axial localization accuracy which now surpasses significantly that of lateral localization provided by most SMLM approaches.
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