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Abstract
The detection of molecular and nanoparticle labels with nanometre spatial resolution is of great interest for biomolecular and material sciences. Nanosensors capable of monitoring bending and rotations of biomolecules or characterizing soft materials assembled using DNA as scaffolds are highly desirable. A powerful idea incorporated in optical spectroscopic rulers is to transduce changes in spatial arrangement into spectral differences. With few exceptions, current spectroscopic rulers such as fluorescent resonant energy transfer and the recently demonstrated plasmonic ruler provide merely one-dimensional information about the distance between labelling entities. Here, we propose and demonstrate a three-dimensional spectroscopic nanosensor, called a 'plasmonic protractor', based on a plasmonic nanostructure formed between a plasmonic sphere and a nanolabel attached to it. A polarization-resolved scattering technique enables the reconstruction of the nanolabel's location and orientation with deep subdiffraction spatial resolution. This plasmonic far-field, in situ spatial arrangement sensor greatly expands the capability of existing spectroscopic rulers.