Abstract
Lanthanide (Ln3+)-doped upconversion nanoparticles (UCNPs) have attracted tremendous interest owing to their potential bioapplications. However, the intrinsic photophysics responsible for upconversion (UC) especially the cooperative sensitization UC (CSU) in colloidal Ln3+-doped UCNPs has remained untouched so far. Herein, we report a unique strategy for the synthesis of high-quality LiYbF4:Ln3+ core-only and core/shell UCNPs with tunable particle sizes and shell thicknesses. Energy transfer UC from Er3+, Ho3+ and Tm3+ and CSU from Tb3+ were comprehensively surveyed under 980 nm excitation. Through surface passivation, we achieved efficient non-cooperative sensitization UC with absolute UC quantum yields (QYs) of 3.36%, 0.69% and 0.81% for Er3+, Ho3+ and Tm3+, respectively. Particularly, we for the first time quantitatively determined the CSU efficiency for Tb3+ with an absolute QY of 0.0085% under excitation at a power density of 70 W cm-2. By means of temperature-dependent steady-state and transient UC spectroscopy, we unraveled the dominant mechanisms of phonon-assisted cooperative energy transfer (T > 100 K) and sequential dimer ground-state absorption/excited-state absorption (T < 100 K) for the CSU process in LiYbF4:Tb3+ UCNPs.
Highlights
Highquality LiYbF4:Ln3+ (Ln = Er, Tm, Ho, or Tb) core-only and core/shell upconversion nanoparticles (UCNPs) were synthesized via a modified high-temperature co-precipitation method in the presence of Oleic acid (OA) and TOA as the surfactant and the solvent, respectively
The transmission electron microscopy (TEM) images show that the LiYbF4:30%Tb core-only UCNPs are roughly spherical with a mean size of 12.2 ± 0.6 nm (Fig. 1c and S3†), while the LiYbF4:2%Er UCNPs synthesized under identical conditions are rhombohedral with a mean size of (16.9 ± 0.8) × (21.7 ± 1.3) nm (Fig. 1d and S4†)
The NIRtriggered cooperative sensitization UC (CSU) luminescence from Tb3+ that is free of surface quenching might exhibit great potential for applications in a variety of UC luminescence (UCL) biosensing such as homogeneous UC-FRET bioassays, despite its current limitation with extremely low efficiency
Summary
Ions without metastable levels as the energy storage reservoir, such as Tb3+ and Eu3+, cooperative sensitization UC (CSU) takes place.[18,19,20,21,22] In a typical CSU process, the excitation energy of two adjacent Yb3+ is simultaneously transferred to Tb3+ or Eu3+.19 CSU luminescence from activators such as Tb3+ or Eu3+ is less influenced by the deleterious surface quenching effect than ETU luminescence from Er3+ or Tm3+, due to the large energy gap between the emitting levels and their low-lying levels of Tb3+ or Eu3+, thereby demonstrating unique advantages and promises for applications in UC-FRET biodetection and bioimaging.[23,24,25,26,27] Thanks to the pioneering work reported by H. Nanoscale low-concentration Yb3+-doped counterparts.[34,35,36] due to the stringent synthetic conditions and the surface quenching effect, currently it remains difficult for the synthesis of high-quality LiYbF4:Ln3+ UCNPs with controlled particle size and efficient UCL. We report a unique strategy for the synthesis of monodisperse LiYbF4:Ln3+ core-only and core/shell UCNPs with controlled particle sizes and shell thicknesses through a modified high-temperature co-precipitation method. By means of low-temperature (10 K) high-resolution steady-state and transient UC spectroscopy, we unravel the excited-state dynamics involved in the CSU process in LiYbF4:Tb3+ UCNPs. different temperatures (e.g., 300 °C and 310 °C) and for different times (e.g., from 20 min to 120 min). After cooling down to 80 °C, 0.5 mmol of LiYbF4:Ln3+ core-only UCNPs in 10 mL of cyclohexane was added and maintained at 80 °C for 30 min to remove cyclohexane. To control the shell thickness, different amounts of the shell precursors were weighed for the synthesis
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