Correlation between the physical orientation of gold nanoparticles and their emitting dipoles by combining AFM with fluorescence microscope

Single molecule and particle rotational tracking by optical microscopy is of great significance to understand many important phenomena in biological and physical sciences. Defocused imaging is a highly efficient technique for orientation tracking since it can decode the direction of emitting dipoles within one frame. To realize the defocused imaging in orientation detection, however, the orientation of the nanoparticle itself and its emitting dipoles has to be correlated. Here, we propose a method to correlate the spatial orientation of the gold nanoparticles with their emitting dipoles by combing AFM with the defocused imaging technique which is realized in a normal wide-field fluorescence microscope. This correlating method does not require vacuum environment and sample pretreatment. Furthermore, the measuring process is not invasive to the sample. By using this method, the emission mechanism of gold nano-bipyramids (GNBs) is found to be similar to that of gold nanorods (GNRs), suggesting that GNBs can be applied as orientation sensors. This method can also be utilized to monitor the dynamics of photoluminescence from gold nanoparticles during its melting process under laser illumination. Our investigations will open a new way to explore the fluorescence mechanism of various nanoparticles at a single-particle level down to nanometer scale.