Optical encoding together with color multiplexing benefits on-site detection, and enriching the components with narrow emissions from lanthanide could greatly increase the coding density. Here, we show a typical example to combine emission color and lifetime that are simultaneously integrated in a single lanthanide nanoparticle. With the multicompartment core/shell structure, the nanoparticles can activate different emitting pathways under varied excitation. This enables the nanoparticles to generate versatile excitation orthogonalized upconversion luminescence in both emission colors and lifetimes. As a typical example, green emission of Er3+ and blue emission of Tm3+ can be triggered with 808 and 980 nm lasers, respectively. Moreover, with incorporation of Tb3+, not only is emission from Tb3+ introduced but also the lifetime difference of 0.13 ms (Er3+) and 3.6 ms (Tb3+) is yielded for the green emission, respectively. Multiplexed fingerprint imaging and time-gated luminescence imaging were achieved in wavelength and lifetime dimensions. The spectral and lifetime encoding ability from lanthanide luminescence greatly broadens the scope of luminescent materials for optical multiplexing studies.
The development of rare-earth doped upconversion nanoparticles (RE-UCNPs) in various applications is fuelling the demand for nanoparticles with highly enhanced upconversion luminescence (UCL). Although the core/shell structure is proved to enhance the UCL effectively, there is still plenty of room to further improve the UCL by optimizing the doping ratio of the materials. In this article, a general strategy is demonstrated to achieve highly-enhanced visible UCL in core/shell nanostructured NaREF by increasing the doping ratio of Yb in the core region. The energy transfer from RE-UCNPs to surface quenching sites through Yb-Yb energy migration is demonstrated to be the main reason for restricting the doping ratio of Yb. Notable UCL enhancement (ca. 15 times) of core/shell structured α-NaYF:Yb,Er@CaF nanoparticles is observed by increasing the concentration of Yb to 98 mol%. The highly-enhanced visible UCL signal is used to guide the lymphatic vessel resection with the naked eye.
limited by either the few choices of fluorophores [8,9] or the difficulty of aqueous modification, [10] which is especially true when NIR absorption and emission are required. [11,12] Moreover, the above-mentioned fluorophores mostly interact with analytes in a quenching scheme which can be easily affected in complex biological media such as cytoplasm or body fluid, making it more challenging to produce reliable in vivo lifetime signals. [13] Alternatively, thanks to the well-shielded 4f-4f transition, the lifetime of rare-earthdoped nanocrystals is less likely to fluctuate in those conditions once the emitters are well protected, [14][15][16] and NIR-emissive Tm 3+ , Er 3+ , Yb 3+ , or Nd 3+ has been widely applied in in vivo imaging. [17][18][19][20][21] The longer lifetime of rare-earth-doped nanocrystals also benefits background-free bioimaging as the short-lived autofluorescence can be filtered by time gate. [22][23][24] However, the commonly used rareearth-doped nanoparticles usually have to be passivated by an inert shell to prevent the quenching of activators and emitters (Yb 3+ , Tm 3+ ), [25][26][27][28][29] while the fluorescence energy transfer is most sensitive of distance. [30] This causes a dilemma; when a thicker shell increases the fluorescence intensity, the energy transfer efficiency inevitably drops. Therefore, it is necessary to design a luminescent lifetime probe which can take both the strong luminescence signal and the appropriate energy transfer distance into account thus meeting the requirements of detection in vivo. Recently, we reported a family of rare-earth nanocrystals with the absorption and emission at the same energy level, which has inspired our design. With time-domain filtering technique, the NIR light transducers have strong luminescence and high quantum yield compared to upconversion materials, NIR dyes, or quantum dots. [31] Based on these findings, a highly luminescent (182 times compared to core upconversion particles and 33 times compared to core-shell upconversion particles under low power density, as shown in Figure 3) and lifetime-responsive LRET nanocomposite was built for effective in vivo sensing (Figure 1). NaYF 4 :Tm nanocrystal, which absorbs and emits photons in the same transition ( 3 H 6 -3 H 4 ), was employed as the donors (Figure 1b). It was further combined with a commercially available IR-820 dye as acceptors, which has a specific response to ClO − , forming an NIR ClO − -responsive LRET nanocomposite. The lifetime was affected by the number of window (660-950 and 1000-1500 nm). Herein, this work reports a lifetimeresponsive nanocomposite with both excitation and emission in the NIR I window (800 nm) and lifetime in the microsecond region. The incorporation of Tm 3+ -doped rare-earth nanocrystals and NIR dye builds an efficient energy transfer pathway that enables a tunable luminescence lifetime range. The NaYF 4 :Tm nanocrystal, which absorbs and emits photons at the same energy level, is found to be 33 times brighter than optimized core-shell upconversi...
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