Abstract
We showed that the anisotropic disk shape of nanoplasmonic upconverting nanoparticles (NP-UCNPs) creates changes in fluorescence intensity during rotational motion. We determined the orientation by a three-fold change in fluorescence intensity. We further found that the luminescence intensity was strongly dependent on the particle orientation and on polarization of the excitation light. The luminescence intensity showed a three-fold difference between flat and on-edge orientations. The intensity also varied sinusoidally with the polarization of the incident light, with an Imax/Imin ratio of up to 2.02. Both the orientation dependence and Imax/Imin are dependent on the presence of a gold shell on the UCNP. Because the fluorescence depends on the NP’s orientation, the rotational motion of biomolecules coupled to the NP can be detected. Finally, we tracked the real-time rotational motion of a single NP-UCNP in solution between slide and coverslip with diffusivity up to 10−2 μm2s−1.
Highlights
UCNPs are not as efficient as traditional Stokes dyes; several studies have tried to enhance the Anti-Stokes emission by coupling gold and silver NPs and shells to rare earth ion doped upconverting cores (NP-UCNP)[10,11,12,13,14] in order to increase overall emission intensity and modify emission color[8, 10, 11, 13,14,15,16]
Earlier we found that nanoplasmonic particles are considerably brighter, up to five fold increase, compared with unmodified UCNPs8, 21
Anisotropically shaped NP-UCNPs oriented with flat configurations showed stronger emission intensity than those with edge orientations
Summary
UCNPs are not as efficient as traditional Stokes dyes; several studies have tried to enhance the Anti-Stokes emission by coupling gold and silver NPs and shells to rare earth ion doped upconverting cores (NP-UCNP)[10,11,12,13,14] in order to increase overall emission intensity and modify emission color[8, 10, 11, 13,14,15,16]. Two different types of plasmonic structures are used for enhanced upconversion, and their plasmon resonances overlap spectrally with either the excitation or the emission wavelength of the UCNP. Depending on the orientation of the nanodisk, the emission is polarized and directional[23], lending further complexity and richness to the photophysics of UCNPs. According to Chen et al, the fluorescence intensity of an on-edge oriented UCNP nanodisk shows a sinusoidal dependence on excitation polarization[24, 25]. According to Chen et al, the fluorescence intensity of an on-edge oriented UCNP nanodisk shows a sinusoidal dependence on excitation polarization[24, 25] They attributed this effect to the variation of the dipole oscillator strength spatially within the nanodisk due to the anisotropic nature of the surrounding crystal lattice. Our current experimental observations correlate well with our earlier calculations on similar nanoplasmonic upconverting NPs with different orientations to the incident excitation and excitation polarization[8]
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