Upconversion Nanocrystals with High Lanthanide Content: Luminescence Loss by Energy Migration versus Luminescence Enhancement by Increased NIR Absorption

Abstract

Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted a lot of interest due to their benefits in biological applications: They are not suffering from intermittence and provide nearly background-free luminescence. The progress in synthesis nowadays enables access to complex core-shell particles of controlled size and composition. Nevertheless, the frequently used doping ratio dates back to where mostly core-only particles of relatively large size have been studied. Especially at low power excitation as needed in biology, a decrease in particle size leads to a drastic decrease in the upconversion efficiency. An enhancement strategy based on an increased absorption rate of near-infrared light provided by an increase of the sensitizer content, together with the simultaneous blocking of the energy migration pathways to the particle surface, is presented. NaYbF 4 (20%Er) particles of 8.5 nm diameter equipped with an about 2 nm thick NaYF 4 shell show significantly enhanced upconversion luminescence in the red (660 nm) compared to the most commonly used particles with only 20% Yb 3+ and 2% Er 3+ . The impact of size, composition, and core-shell architecture on photophysical properties are studied. The findings demonstrate that an increase in doping rates enables the design of small, bright UCNPs useful for biological applications.

Lanthanide-Doped Luminescent Nanoparticles with Dual-Mode Color Modulation Enable Cross-Correlation Anticounterfeiting

“…For nanoparticles upon excitation, the ground-state Er 3+ was generally pumped to the high-lying 4 F 7/2 level via sequential ground-state absorption (GSA) and excited-state absorption (ESA) processes and energy transfer process from the neighboring high-lying Yb 3+ in the case of Yb 3+ -contained nanoparticles . The subsequent multiphonon relaxation (MPR) processes that such high-lying Er 3+ species underwent typically led to the population of green ( 2 H 11/2 and 4 S 3/2 ) and red ( 4 F 9/2 ) emitting states with the former enabling emission bands centered at 525 and 540 nm ( 2 H 11/2 / 4 S 3/2 → 4 I 15/2 ), respectively, while the latter imparting red emission feature with wavelength around 650 nm ( 4 F 9/2 → 4 I 15/2 ). − …”

An overview of boosting lanthanide upconversion luminescence through chemical methods and physical strategies

“…In general, loss of emission intensity for UCNPs occurs when raising the dopant concentration beyond a threshold value, which is known as concentration quenching effect. 27,28 To minimize the detrimental concentration quenching, UCNPs are typically doped with a relatively low concentration of Ln 3+ ions, leading to insufficient harvesting of excitation light and consequently weak emission. Encouragingly, recent advances have demonstrated the effective alleviation of energy loss at high dopant concentration through several complementary strategies, thereby making heavy Ln 3+ doping a feasible way to enhance the UCL of UCNPs.…”

Modulating the Rise and Decay Dynamics of Upconversion Luminescence through Controlling Excitations

“…After decades of fast development, lanthanide‐doped upconversion luminescence (UCL) has been extensively studied in a wide range of areas from previous bioimaging, contactless temperature probing, and anticounterfeiting to nowadays compact lasing, optogenetics, single nanoparticle (NP) tracking, and X‐ray detection [1–12] . Obviously, a deep understanding of the working principles behind those individual applications is the key to successful modulation of UCL emission with expected parameters in terms of lifetime, relative intensity, and wavelength, toward different purposes [13–20] …”

“…[1][2][3][4][5][6][7][8][9][10][11][12] Obviously, a deep understanding of the working principles behind those individual applications is the key to successful modulation of UCL emission with expected parameters in terms of lifetime, relative intensity, and wavelength, toward different purposes. [13][14][15][16][17][18][19][20] Unfortunately, UCL process was empirically assumed to have the same time-resolved output profile as that of organic dye/quantum dot, [21][22][23][24] which works as one luminescing unit and is composed of a sharp rise and a steep decay edge originated from population and depopulation process, [25] respectively. In detail, electrons are pumped to excited state under proper excitation, and (after a quick relaxation) return immediately to the ground state accompanied with luminescence emission (Figure 1a).…”