Intrinsic Time‐Tunable Emissions in Core–Shell Upconverting Nanoparticle Systems

Abstract

Color‐tunable luminescence has been extensively investigated in upconverting nanoparticles for diverse applications, each exploiting emissions in different spectral regions. Manipulation of the emission wavelength is accomplished by varying the composition of the luminescent material or the characteristics of the excitation source. Herein, we propose core–shell β‐NaGdF4: Tm3+, Yb3+/β‐NaGdF4: Tb3+ nanoparticles as intrinsic time‐tunable luminescent materials. The time dependency of the emission wavelength only depends on the different decay time of the two emitters, without additional variation of the dopant concentration or pumping source. The time‐tunable emission was recorded with a commercially available camera. The dynamics of the emissions is thoroughly investigated, and we established that the energy transfer from the 1D2 excited state of Tm3+ ions to the higher energy excited states of Tb3+ ions to be the principal mechanism to the population of the 5D4 level for the Tb3+ ions.

Luminescence dynamics and enhancement of the UV and visible emissions of Tm3+ in LiYF4:Yb3+,Tm3+ upconverting nanoparticles

“…3 These properties affect the overall optical characteristics of the materials, such as the absorption and emission bandwidth, the absorption crosssection, the probability of energy transfer and the population mechanisms of the different excited states. [4][5][6][7][8] Mahalingam et al rst described the synthesis of LiYF 4 :Yb 3+ ,Tm 3+ UCNPs and reported the upconverted emissions from the UV to the NIR upon 980 nm excitation. 9 Taking advantage of the high depth penetration, reduced photodamage and minimal autouorescence of the NIR excitation light, Tm 3+ -doped upconverting nanoparticles have found applications in drug delivery systems and in photodynamic therapy.…”

The Key Role of Intrinsic Lifetime Dynamics from Upconverting Nanosystems in Multiemission Particle Velocimetry

“…The intrinsic luminescence characteristics of UCNPs offers such a possibility, since the time interval between two intercorrelated emissions does not change with the velocity of the nanoparticle and can act as such a reference. [ 14 ] In this communication, we prove that the emissions from Tm 3+ and Tb 3+ in NaGdF 4 :49% Yb 3+ , 1% Tm 3+ /NaGdF 4 :20% Tb 3+ core–shell UCNPs respect the requirements for such a reference value. Consequently, velocity measurements were performed with these core–shell UCNPs in a toluene flow without the need for calibration or a sophisticated experimental setup.…”

“…The engineered core–shell UCNPs have a characteristic hexagonal prism shape (20 × 10 nm), forming colloidal suspensions in toluene. [ 14 ] The flow of toluene in the investigated velocity range between 1 and 1000 mm s ‐1 follows the so‐called laminar flow regime. The flow immediately around a nanoparticle with the same geometry of the proposed nanoprobes was preemptively simulated to verify that homogeneous flow [ 15,16 ] was not disrupted by the particles during the measurements and to evaluate the flow characteristics before performing the experiments (Appendix S1, Supporting Information).…”

“…The luminescent dynamics of NaGdF 4 :49% Yb 3+ , 1% Tm 3+ /NaGdF 4 :20% Tb 3+ core–shell nanoprobes have been thoroughly investigated in a previous article. [ 14 ] The green emission (540 nm) from the 5 D 4 state of Tb 3+ originates directly from the 1 D 2 emission of Tm 3+ , maximized in the blue region (450 nm). [ 14 ] The maxima of the Tm 3+ emission at 450 nm always occurs 252 ± 10 µs before the maxima of the Tb 3+ emission at 540 nm.…”

“…[ 14 ] The green emission (540 nm) from the 5 D 4 state of Tb 3+ originates directly from the 1 D 2 emission of Tm 3+ , maximized in the blue region (450 nm). [ 14 ] The maxima of the Tm 3+ emission at 450 nm always occurs 252 ± 10 µs before the maxima of the Tb 3+ emission at 540 nm. This time interval is independent of the pulse width for short pulses before reaching the emission steady‐state (Figures S4 and S5, Supporting Information), [ 14,21 ] , i.e., this time interval does not depend on the residence time of the particles within the excitation beam and is therefore independent of the velocity.…”