Assessing inter lanthanide photophysical interactions in co-doped titanium dioxide nanoparticles for multiplex assays

Abstract

This study assesses the inter lanthanide photophysical interactions in trivalent lanthanide cations (Ln³⁺) co-doped titanium dioxide nanoparticles. As a case study, incorporation of neodymium (Nd³⁺) and samarium (Sm³⁺) to generate Ti(NdSm)O₂ nanoparticles has been considered. The presence of co-doping offers a promising avenue for multiplex assays. The co-doped nanoparticles have characteristic visible emission at 584, 612, 664 and 726 nm respectively from Sm³⁺ and near infrared (NIR) emission at 912 and 1094 nm respectively from Nd³⁺, thus presenting composite doped nanoparticles with six distinct emission wavelengths spanning both the orange-red and NIR spectral window, using a single excitation wavelength. The photophysical properties of the Ti(NdSm)O₂ nanoparticles have been compared with that observed in the singly doped Ti(Nd)O₂ and Ti(Sm)O₂ nanoparticles. Remarkable differences in the Ln³⁺ emission have been observed in the singly and doubly doped nanoparticles. Both the Nd³⁺ and Sm³⁺ emissions have been found to decrease in the Ti(NdSm)O₂ nanoparticles, compared to those observed in the singly doped Ti(Nd)O₂ and Ti(Sm)O₂ nanoparticles. However, the extent of decrease in emission was found to be unequal for Nd³⁺ and Sm³⁺, with a decrease being marginally more prominent in Nd³⁺. The results have been rationalized by considering the Ln³⁺ as charge traps in the nanoparticles and associated relaxation pathways that are dictated by the spin selection rule. This photophysical rationalization was further tested and verified by performing experiments with the Ti(NdEr)O₂ nanoparticles. The results presented provide important physical insight on the design criteria of co-doped semiconductor nanoparticles.