Luminescence lifetime refers to the average time the molecule spends in the excited state returning to the ground state
[22][26]. The lifetime of a phosphor, τ, refers to the time at which the intensity decays to 1/e of its maximum
[23][27]. The decay of one emitting state in an upconversion energy transfer process is determined by its intrinsic decay and the intermediate states
[24][28]. Susceptive to their intrinsic properties and local environment, the lifetime of lanthanide-doped nanoparticles could be tuned by adjusting core–shell structure, the content of sensitizer and emitter, internal energy transfer channel, FRET system and temperature. Nanocrystals with a controllable lifetime have been widely used in biosensing to eliminate background autofluorescence and light scattering interference. However, the reported methods allow only a limited range of lifetime adjustments.
2.2. Changing Concentration of Sensitizer and Emitter
Excessive emitters would increase the probability of cross-relaxation, leading to concentration quenching. For a Yb
3+-Er
3+-Ho
3+ tri-doped nanoparticle, higher content of Ho
3+ could shorten the interionic distances of Yb
3+-Er
3+/Ho
3+, Er
3+-Ho
3+ and Ho
3+-Ho
3+, which could enhance the non-radiative process and energy transfer (ET) sensitization. A decrease in the lifetime of Ho
3+ (643 nm, 750 nm, 895 nm), Er
3+(525 nm, 545 nm, 655 nm, 810 nm, 845 nm) and Yb
3+ (1020 nm) was observed when increasing the concentration of Ho
3+ [40][44]. Similarly, as the mole content of Tm
3+ rose from 0.2% to 8% in NaYF
4: 20%Yb,x%Tm (40 nm), the lifetime of the blue emission was reduced from 662.4 µs to 25.6 µs (
Figure 3a). Meanwhile, as the mole percentage of Yb
3+ rose from 10% to 30% in NaYF
4: x%Yb,1%Tm, the lifetime of the same blue emission decreased from 206.7 µs to 120.2 µs
[18][20]. The higher concentration of part of emitters would likely cause self-aggregation and cross relaxation, resulting in a shortened lifetime. When the Er
3+ concentration was varying from 1% to 70% in the β-NaYbF
4@NaY
0.8−xEr
xGd
0.2F
4@ NaY
0.8Gd
0.2F
4, the lifetime of Er
3+ at the level of
4S
3/2 (540 nm) decreased sequentially. The shorter lifetime was attributed to the noneffective passivation on the surface when dopant with a higher concentration of emitters. Due to the greater energy from Yb
3+ (
2F
5/2) being transferred to Er
3+ (
4S
3/2,
4S
9/2), the lifetime of Yb
3+ was shortened. With the lack of cross-relaxation pathways for the
4S
9/2 level of Er
3+, the lifetime rarely has significant changes with the Er
3+ concentration variation. Interestingly, the lifetime of Er
3+ at 654 nm in the β-NaYbF
4@NaY
0.8−xEr
xGd
0.2F
4 structure had a long lifetime due to more incredible energy transferring to the surface
[41][45].
Simultaneous excitation of two Yb
3+ ions can produce Yb
3+ dimers with higher excitation energy, which could upconvert photons to Tb
3+. To study the composition-dependent emission lifetimes and the effect on the energy transfer efficiency, Yan et al. employed the Tb
3+-Yb
3+-Nd
3+ co-doped NaGdF
4: 80%Yb,10%Tb@NaGdF
4: 50%Nd,10%Yb nanoparticles with varied doping concentrations as the study model. The lifetime of Tb
3+ reached 1.76 ms when the content of Yb
3+ rose from 20% to 80% because more Yb
3+ facilitated the formation of Yb
3+ dimer. The lifetime at 542 nm is slightly prolonged by 0.06 ms when the proportion of Tb
3+ increased from 5% to 10% due to promoted energy transfer from Yb
3+ dimer to Tb
3+. In addition, when the content of Nd
3+ increased from 10% to 50%, near-infrared absorption intensity improved, the lifetime of Tb
3+ at 542 nm and Yb
3+ at 1000 nm both increased, indicating more energy was transferred to Tb
3+ and Yb
3+ [42][46].
A constant lifetime can be obtained when the doping content of the sensitizer is changed. To investigate the relationship between lifetime decay behavior and luminescence emission intensity, core–shell structure of NaYF
4@NaYF
4: x%Yb,1%Tm@ NaYF
4: y%Yb@NaYF
4 nanoparticles was developed by Zhang et al. Changing the mole content of Yb
3+ in the first shell from 20% up to 80%, the emissive intensity changed while the luminescence lifetime at 475 nm kept constant, suggesting a constant lifetime with different emissive intensity could be obtained. When the concentration of Yb
3+ in the first and second shell layers was changed, the varied lifetime (1256 ms to 310 ms) was obtained with a constant emission intensity (
Figure 3b)
[43][47]. For the mentioned nanoparticles, the declined Yb
3+ concentration increased the mean value of the distance between Yb
3+ and Tm
3+ ions, leading to a longer lifetime due to the weakened back energy transfer process. The Yb
3+ concentration in the energy transfer upconversion layers was decreased from 99%, 70%, 50%, 40%, 20% to 10%, resulting in the lifetime at 808 nm being increased from 1282, 1315, 1481, 1618, 1721 to 2157 μs, respectively
[37][41]. Besides, the Tm
3+ could be served as sensitizers and transfer energy to Yb
3+, Ho
3+ and Er
3+ when excited at 808 nm and 1208 nm, respectively. The lifetime of Yb
3+ (980 nm), Ho
3+ (1180 nm) and Er
3+ (1525 nm) decreased with the increase of the Tm
3+ molar ratio
[44][48].
Figure 3. (
a) The time-resolved confocal images of NaYF
4: Yb,Tm nanoparticles and the lifetime tuning scheme by changing Tm
3+ doping concentrations. Reproduced with permission from
[18][20]. Copyright 2014, Nature Publishing Group. (
b) Diagrammatic illustration of consistent lifetime with tunable intensity by doping various sensitive gradients in the shell. Reproduced with permission from
[43][47]. Copyright 2021, WILEY-VCH.
2.3. Adjusting the Energy Transfer Channel
The decay process of an excited state is inversely proportional to the energy transfer rate and the radiative and non-radiative transition rates. Tri-doped nanoparticles of NaYF
4@NaYF
4: Er
3+/Yb
3+/Mn
2+@NaYF
4 were synthesized by Mao et al., exhibiting the new energy transfer process between Mn
2+ and Er
3+. With a non-radiative energy transfer channel created between the level of Mn
2+ (
4T
1) and Er
3+ (
2H
11/2 and
4S
3/2), the overall transition rate increased owing to the resonance energy transfer, leading to a reduced lifetime of Er
3+ at 550 nm. The presence of Mn
2+ ions promoted the relaxation of the
4S
3/2 energy level, and the red emission of Er
3+ increased with the shortening of the decay time. Therefore, the increased population density of Mn
2+ caused the decreased radiative transition rate of Er
3+ (
4S
3/2 and
2H
11/2) turning down to ground state, resulting in an enhancement of red emission due to energy transfer from Mn
2+ (
4T
1) to Er
3+ (
4F
9/2) (
Figure 4a). Meanwhile, the increased lifetime of Er
3+ at 650 nm also verified the role of Mn
2+ in energy transfer trace according to the decay curves of various content of Mn
2+(0%, 10%, 20%, 30%) (
Figure 4b)
[30][34].
Introducing transition metal ions with a long lifetime into conventional UCNPs is particularly attractive. The lifetime of Mn
2+ ions could be modulated by crystal-site engineering. Liu et al. tuned the luminescence properties of Mn
2+ in core–multishell nanoparticles by doping Ca
2+ or Mg
2+, changing the output color from green to yellow and prolonging its lifetime
[45][49]. The spin-forbidden transition of Mn
2+ occurs between
4T
1→
6A
1, allowing a longer fluorescence lifetime than lanthanide emitters. Because of the larger energy mismatch between Yb
3+ and Mn
2+, the Yb
3+-Mn
2+ dimer is difficult to form. However, Zhang et al. reported the successful preparation of Yb
3+-Mn
2+ dimers, obtaining a substantial long lifetime of Eu
3+ (91 ms), while the normal UC lifetime of Eu
3+ is only about 7 ms. In addition, there is a dynamic population balance between the energy state |
2F
7/2,
4T
1(4G)⟩(Yb
3+–Mn
2+ dimers) and
5D
0 (Eu
3+), causing the sustained energy transferring from Yb
3+–Mn
2+ dimers to Eu
3+ [46][50].
The long-lived Mn
2+ integrated with the short-lived lanthanide particle platform could establish a new energy transfer pathway, and then affect the whole decay process. In a NaGdF
4: 30%Mn@NaGdF
4: 49%Yb,1%Tm@NaYF
4 nanoparticle, Gd sublattice-mediated energy migration facilitates Mn
2+ upconversion luminescence, leading to a decrease in the lifetime of Gd
3+ at 311 nm (
6P
7/2→
8S
7/2) from 6.5 to 4 ms (
Figure 4c)
[47][51]. As a result, the lifetime of lanthanide ions may be affected when the external ions introduced and interfered with the energy transfer channels.
Figure 4. (
a) The diagram for energy transfer mechanism among Yb
3+, Er
3+ and Mn
2+ under 980 nm excitation. (
b) Luminescence decay curves of Er
3+ at 550 nm and 650 nm in NaYF
4: Yb
3+/Er
3+ nanoparticles with different Mn
2+ concentrations (0, 10, 20 and 30 mol%). Reproduced with permission from
[30][34]. Copyright 2016, Royal Society of Chemistry. (
c) Structural design of core–multishell nanoparticle and the energy transfer pathway among the Yb
3+, Tm
3+, Gd
3+ and Mn
2+ ions for the short- and long-lived upconversion luminescence under 980 nm excitation. Reproduced with permission from
[47][51]. Copyright 2017, Nature Publishing Group.
Doping various amounts of Gd
3+ into the NaYF
4 host nanocrystals could regulate the upconversion photoluminescence lifetimes. Xie et al. prolonged the lifetime of Er
3+ (
4S
3/2→
4I
15/2,
4F
9/2→
4I
15/2) by utilizing the Gd
3+, substituting for Y
3+ and Yb
3+ in the crystal lattice of NaYF
4 host, which attributed to the energy transfer rate decrease caused by the average sensitizer-activator separation increasing
[48][52].
2.4. Fluorescence Resonance Energy Transfer
An efficient energy transfer pathway could be established in fluorescence dye-loaded rare-earth nanocrystals, which enables luminescence lifetime tuning
[49][53]. In contrast to conventional molecular donor–acceptor pairs, the energy transfer efficiency is related to the distances between lanthanide-doped nanoparticles, and thus significantly depends on the nanoparticle diameter. Hirsch et al. synthesized NaYF
4: 20%Yb,2%Er with the precisely controlled size spanning from 10 to 43 nm, and coated with sulforhodamine B and rose bengal by ligand exchange. The nanoparticles with a mean diameter ranging from 20 to 25 nm possessed an optimum efficiency of 50–60%. The lifetime of Er
3+ at 600 nm decreased primarily due to the competition of non-radiative surface deactivation at the smaller surface-to-volume ratios (
Figure 5a)
[50][54]. Li et al. loaded the IR-820 on NaYF
4: Tm to construct a FRET system. Luminescence decay from
3H
6→
3H
4 transition was used as a detection signal. When the energy accepter (IR-820) was attached to the donor (Tm
3+:
3H
4) under 785 nm excitation, the lifetime of Tm
3+ at 800 nm decreased because of luminescence resonance energy transfer (
Figure 5b)
[51][55]. Su et al. loaded the IR-806 on the NaGdF
4: 49%Yb,1%Tm@NaYF
4: 20%Yb@NaGdF
4: 50%Nd,10%Yb@NaGdF
4 nanoparticles to improve the ultraviolet luminescence intensity. With the back energy transferred from the nanoparticles to dye molecules, the decreased lifetimes of Gd
3+ and Tm
3+ ions were observed at 253, 276, 290, 310, 360 and 475 nm
[52][56]. An organic fluorescent dye as an antenna could be used to broaden and increase absorption for UCNPs, allowing the energy to flow to dye molecules. Meanwhile, the hybrid system between dye molecules and UCNPs creates a new energy diffusion pathway, increasing the radiative transition process. Li et al. added the Cy3-SO
3 into a NaYF
4: 20%Yb,2%Er@CaF
2 solution to construct an energy dissipation channel, which could transfer energy to the dye by the radiative transition. As a result, the luminescence lifetime of Er
3+ at 488 nm decreased with the concentration of Cy3-SO
3 increase (0.67, 2, 4, and 5.33 μM). The reduced lifetime value verified the non-radiative energy transfer process between Er
3+ and Cy3-SO
3 [53][57].
The triplet excitons could be trapped by inter- or intra-molecular interactions and prolong organic phosphorescence. For example, Yb
3+ luminescence could be generated by organic Yb
3+ complexes and hybrid organic-conjugated Yb
3+-doped nanoparticles. Ye et al. prepared a composite thin film, in which the Yb
3+ ions are incorporated with tetrakis-(pentafluorophenyl)imidodiphosphinate to form the Yb(F-TPIP)
3 chelate, while zinc salt of 2-(tetrafluoro-2-hydroxyphenyl)tetrafluorobenzothiazole (Zn(F-BTZ)
2) served as the organic chromophore. The Zn(F-BTZ)
2 possessed the emission ranging from 450 nm to 900 nm under 405 nm excitation, and gave rise of the Yb
3+ emission centered at 975 nm from the transition of Yb
3+:
2F
5/2→
2F
7/2. Note that the intrinsic lifetime of Yb
3+ at 1 μm is about ~1 ms. The lifetime of sensitized organic Yb
3+ compounds could prolong up to ~0.3 s. The prolonged emission lifetime was demonstrated by dynamic equilibrium due to the energy transfer process from long-lived organic triplet excitons
[54][58].
The surface ligand coordination could reconstruct the crystal-field splitting and orbital hybridization, and narrow the gap of the 4d orbitals between inner and surface lanthanide sensitizers. For example, after the bidentate picolinic acid (2PA) molecules coordinated to NaGdF
4: Yb nanoparticles, a longer lifetime (289 μs) at 980 nm was observed. The results confirmed that 2PA molecules could activate the dark surface layers and facilitate energy migration in the Yb
3+ sublattices. The Yb
3+-2PA coupling facilitated energy migration by 4f-orbital energy resonance within the ytterbium sublattice, which can reduce surface defects to hinder energy diffusion. Density functional theory (DFT) verified that 2PA coating could narrow the gap between the superficial and inner Yb
3+ by lowering the empty 4f levels. Rigid ligands also stabilized the excited state of superficial Yb
3+, prevented the superficial lanthanide ions from the solvent and fluoride vacancy-induced quenching, and thus significantly suppressing multiphonon non-radiative decay (
Figure 5c)
[55][59].
Figure 5. (
a) Schematic diagram of the lifetime changing due to the existence of FRET process from UCNPs to dye molecules. Reproduced with permission from
[50][54]. Copyright 2017, American Chemical Society. (
b) Schematic representation of the structure of the NaYF
4: Tm@PC-IR-820 nanocomposites, the absorbance and emission in the same transition (
3H
6–
3H
4) of NaYF
4: Tm nanoparticles and the variation of lifetime affected by the amount of IR-820 dye molecules. Reproduced with permission from
[51][55]. Copyright 2019, WILEY-VCH. (
c) The optimized structure shows ytterbium atoms located in the interior (Yb
in) and exterior (Yb
surf) position, the simulated 4f energy levels of ytterbium atoms, the spatial distribution of the charge densities for coupling states and the lifetime decay curves of Yb
3+ at 980 nm with and without 2PA capping on the NaGdF
4: 5%Yb nanoparticles (10 nm) excited at 965 nm. Reproduced with permission from
[55][59]. Copyright 2021, Nature Publishing Group.
2.5. Changing Temperature
The synthesized process would affect the crystallinity, phase state and volume of nanoparticles and thus influence the decay behaviors. Vatsa et al. studied the decay process of GdVO
4: Dy
3+ nanoparticles after heat treatment. When heated from 500 °C to 900 °C, the lifetime of Dy
3+ (
4F
9/2 level) extended from 114 μs to 260 μs due to the reduction of the non-radiative process by surface inhomogeneities. This increase in lifetime can also be ascribed to the decrease in the surface defects with the particle size increases in the heat treatment process (
Figure 6a)
[56][60]. For YVO
4: Ln
3+ (Ln
3+ = Dy
3+, Eu
3+) nanoparticles, the increase in covalent bond interaction caused by heat treatment led to a red shift in V–O charge transfer (CT). Similarly, the lifetimes of Dy
3+ at
4F
9/2 and Eu
3+ at
5D
0 increase with temperature from 500 °C to 900 °C due to the reduction of the non-radiative process on the surface of the particles
[57][61].
As is well known, the decay time constant is inversely proportional to the radiative and non-radiative transition rates in the cross-relaxation process. The luminescence lifetime decreases with the increase of ambient temperature in most cases. The decay time with specific emissions produced by radiative transition rarely varies with temperature, while the non-radiative decay rate changes significantly with temperature
[58][62]. For example, the lifetime of Yb
3+ at 1000 nm reduced from 470 ± 11 μs to 390 ± 12 μs in NaYF
4: Nd
3+, Yb
3+ nanoparticles as the temperature rose from 25 °C to 45 °C. While the thermal coefficient α
τ calculated by the TGI system was almost unchanged (−0.0092~−0.010 °C
−1) (
Figure 6b,e)
[59][63]. Moreover, cross-relaxation between Tm
3+ (
1G
4) usually occurs when raising the emitter concentration or temperature. Yu et al. compared the sensitivity of β-PbF
2: Tm
3+/Yb
3+ with different Tm
3+ doping concentrations. They found that the relative sensitivity maximum values of
1G
4 state lifetime in 0.0005Tm, 0.01Tm and 0.05Tm nanoparticles are 0.16%K
−1, 0.26%K
−1 and 0.46%K
−1 at 488K, respectively, indicating the potential ability as an indicator of upconversion luminescence lifetime-based thermometer
[60][64].
In addition, the host matrix has a significant effect on thermal sensitivity. Díaz et al. found that oxide materials are more sensitive than fluoride ones by comparing the decay curves of NaYF
4: Er,Yb and NaY
2F
5O: Er,Yb nanoparticles at room temperature and 60 °C.
[61][65]. The temperature dependency of Yb
3+ emission lifetime in NaYF
4@NaYF
4: Yb
3+,Nd
3+@CaF
2 nanoparticles was determined by the energy transfer and back energy transfer rate, the energy migration process (among Yb
3+), as well as radiative and non-radiative transition. Both the concentration of Nd
3+ and Yb
3+ affected the temperature sensitivity by changing the distance of Yb
3+-Yb
3+ and Yb
3+-Nd
3+, which in turn affected the back energy transfer processes from Yb
3+ to Nd
3+ and energy migration between Yb
3+ ions. NaYF
4@NaYF
4: 20%Yb
3+,60%Nd
3+@CaF
2 as the nanoprobe possessed optimum thermal sensitivity through varying doping concentrations of Yb
3+ and Nd
3+, in which the lifetime of Yb
3+ at 980 nm descended from 898 μs to 450 μs when the temperature increased from 10 °C to 64 °C. (
Figure 6c,f)
[62][66].
Interestingly, Li et al. demonstrate the lifetime compensation with temperature in NaErF
4@NaGdF
4 core–shell nanoparticles. The temperature-independent lifetime is attributed to the balance between lattice expansion (prolonging the lifetime) and thermal quenching (shortening the lifetime). A considerable energy migration process occurs in the high-doping concentration of Er
3+, and the efficiency is proportional inversely to the average donor-acceptor distance with sixth order of magnitude. As a consequence, elevated temperature induces the lattice to expand, leading to a longer transfer distance, and ultimately prolonging the lifetime of Er
3+. However, the prolonged lifetime caused by lattice expansion compensated for the difference value of the shorter lifetime aroused by thermal quenching, resulting in the temperature-independent lifetime (
Figure 6d)
[63][67].
Figure 6. (
a) Intensity (left) and the corresponding decay lifetime of Dy
3+ at
4F
9/2 states (right) with the dependent variable of Dy
3+ concentration when excited at 310 nm. Reproduced with permission from
[56][60]. Copyright 2009, AIP Publishing. (
b) Luminescence decay curves of NaYF
4: Nd
3+,Yb
3+ at different experimental temperatures and (
e) the corresponding calibration curve of temperature vs. luminescence lifetime. Reproduced with permission from
[59][63]. Copyright 2019, Nature Publishing Group. (
c) The decay curves of the measured NaYF
4@NaYF
4: 20%Yb
3+,60%Nd
3+@CaF
2 nanoprobe at various temperatures and (
f) corresponding nonlinear fitted curves between measured lifetime and temperature ranging from 10 °C to 70 °C. Reproduced with permission from
[62][66]. Copyright 2020, WILEY-VCH. (
d) Negative correlation curves of the lifetime of Er
3+ at
4S
3/2 versus ambient temperature for NaErF
4@NaGdF
4 and NaErF
4: 18%Yb,2%Er@NaGdF
4 nanoparticles. Reproduced with permission from
[63][67]. Copyright 2020, MDPI.