Explaining the influence of dopant concentration and excitation power density on the luminescence and brightness of β-NaYF4:Yb^3+,Er^3+ nanoparticles:Measurements and simulations

We assessed the influence of Yb^3+ and Er34 dopant concentration on the relative spectral distribution, quantum yield (Фuc), and decay kinetics of the upconversion luminescence (UCL) and particle brightness (BUC) for similarly sized (33 nm) oleate-capped p-NaYF4:Yb3t,Er^3+ upconversion (UC) nanoparticles (UCNPs) in toluene at broadly varied excitation power densities (F). This included an Yb^3+ series where the Yb^3+ concentration was varied between 11%—21% for a constant Er^3+ concentration of 3%, and an Er^3+ series, where the Er^3+ concentration was varied between 1%-4% for a constant Yb^3+ concentration of 14%. The results were fitted with a coupled rate equation model utilizing the UCL data and decay kinetics of the green and red Er34 emission and the Yb34 luminescence at 980 nm. An increasing Yb34 concentration favors a pronounced triphotonic population of 4F9/2 at high P by an enhanced back energy transfer (BET) from the 4G11/2 level. Simultaneously, the Yb^3+-controlled UCNPs absorption cross section overcompensates for the reduction in Фuc with increasing Yb^3+ concentration at high P, resulting in an increase in Sue. Additionally, our results show that an increase in Yb34 and a decrease in Er^3+ concentration enhance the color tuning range by P. These findings will pave the road to a deeper understanding of the energy transfer processes and their contribution to efficient UCL, as well as still debated trends in green-to-red intensity ratios of UCNPs at different P.

Upconversion FRET quantitation:the role of donor photoexcitation mode and compositional architecture on the decay and intensity based responses

Lanthanide-doped colloidal nanoparticles capable of photon upconversion(UC)offer long luminescence lifetimes,narrowband absorption and emission spectra,and efficient anti-Stokes emission.These features are highly advantageous for F?rster Resonance Energy Transfer(FRET)based detection.Upconverting nanoparticles(UCNPs)as donors may solve the existing problems of molecular FRET systems,such as photobleaching and limitations in quantitative analysis,but these new labels also bring new challenges.Here we have studied the impact of the core-shell compositional architecture of upconverting nanoparticle donors and the mode of photoexcitation on the performance of UC-FRET from UCNPs to Rose Bengal(RB)molecular acceptor.We have quantitatively compared luminescence rise and decay kinetics of Er3+emission using core-only NaYF4:20%Yb,2%Er and core-shell NaYF4:20%Yb@NaYF4:20%Yb,5%Er donor UCNPs under three photoexcitation schemes:(1)direct short-pulse photoexcitation of Er3+at 520 nm;indirect photoexcitation of Er3+through Yb3+sensitizer with(2)980 nm short(5-7 ns)or(3)980 nm long(4 ms)laser pulses.The donor luminescence kinetics and steady-state emission spectra differed between the UCNP architectures and excitation schemes.Aiming for highly sensitive kinetic upconversion FRET-based biomolecular assays,the experimental results underline the complexity of the excitation and energy-migration mechanisms affecting the Er3+donor responses and suggest ways to optimize the photoexcitation scheme and the architecture of the UCNPs used as luminescent donors.

Lanthanide-based bulky counterions against aggregation-caused quenching of dyes in fluorescent polymeric nanoparticles

Dye-loaded polymeric nanoparticles(NPs)are promising bioimaging agents because of their available surface chemistry,high brightness,and tunable optical properties.However,high dye loadings can cause the aggregation-caused quenching(ACQ)of the encapsulated fluorophores.Previously,we proposed to mitigate the ACQ inside polymeric NPs by insulating cationic dyes with bulky hydrophobic counterions.In order to implement new functionalities into dye-loaded NPs,here,we extend the concept of bulky counterions to anionic lanthanide-based complexes.We show that by employing Gd-based counterions with octadecyl rhodamine B loaded NPs at 30 wt% versus polymer,the fluorescence quantum yield can be increased to 10-fold(to 0.34).Moreover,Gd-anion provides NPs with enhanced contrast in electron microscopy.A combination of a luminescent Eu-based counterion with a far-red to near-infrared cyanine 5 dye(DiD)yields Forster resonance energy transfer NPs,where the UV-excited Eu-based counterion transfers energy to DiD,generating delayed fluorescence and large stokes shift of -340 nm.Cellular studies reveal low cytotoxicity of NPs and their capacity to internalize without detectable dye leakage,in contrast to leaky NPs with small counterions.Our findings show that the aggregation behavior of cationic dyes in the polymeric NPs can be controlled by bulky lanthanide anions,which will help in developing bright luminescent multifunctional nanomaterials.