Recently, lanthanide-doped upconversion nanocrystals (UCNCs) have discovered immense potential in the applications of nonlinear optoelectronic and near-infrared bioimaging devices because of their exceptional photostability and tunable spectral characteristics.
To be more specific, their near-infrared excitation bands at 980 and 808 nm are within the first biological transparency window, indicating low photothermal damage and high penetration depth. However, the low quantum efficiency of such UCNCs (typically less than 1%) comprises of an intrinsic obstacle to practical use. A number of chemical and physical methods have been developed to overcome this limitation by enhancing the emission and absorption efficiencies, including host lattice manipulation, energy transfer management, and surface passivation.
Recently, this long-standing issue was handled by using the plasmonic enhancement effect in subwavelength metallic nanostructures. This effect enlarged the the absorption cross-section of lanthanide ions and accelerated their radiative decay rate. Additionaly, plasmonic nanostructures are capable of influencing the polarization state of the upconversion luminescence of close UCNCs, which cannot be attained with any of the other above-mentioned approaches and has till date remained comparatively unexplored.
In this work, the double plasmon resonances of a silica-coated gold nanorod were used to improve the upconversion luminescence intensity of CaF2:Yb3+,Er3+ nanocrystals and also simultaneously adjust the polarization state of the green and red emissions. Successful synthesization has been performed for hybrid plasmonic upconversion nanostructures comprising of sub-10 nm CaF2:Yb3+, Er3+ UCNCs fixed on silica-coated gold nanorods in a core-shell-satellite geometry. Adjusting the thickness of silica shell helped achieve a maximum luminescence enhancement factor of ~3-fold for the green emission band and ~7-fold for the red emission band.
The improved upconversion emissions allow such hybrid plasmonic upconversion nanostructures to exhibit improved multiphoton imaging contrast of HeLa cells in both green and red imaging channels, establishing their potential to be used as a promising nonlinear fluorescent probe for contrast bioimaging applications. The two emissions from single hybrid nanostructures were observed to be greatly polarized with unique polarization response, with the green emission polarization along the incidence polarization and the red emission polarization along the long axis of the gold nanorod. The physical origin responsible for the polarized upconversion emissions has been analyzed by incorporating full-wave electrodynamic simulations and Förster resonance energy transfer theory. It was discovered that the logical emission between the plasmonic dipoles of the GNR and the emission dipoles of UCNCs determines the emission polarization state in different situations and thus make room for accurately controlling the UC emission anisotropy for a broad range of biosensing and bioimaging applications as well as polarized illuminators in polarization-sensitive nanoscale photodetectors and spectrometers.
The related work features in Light: Science & Applications.