Because of their well-known optical properties, upconversion nanoparticles (UCNPs) are regarded as some of the most promising nanomaterials for bioimaging, biosensors, and solar cells. The nonblinking nature of their upconversion emissions has been a particularly beneficial advantage for live-cell imaging. However, the origin of this unique property has never been seriously investigated. We report, for the first time, the observation of stochastic photon emission (SPEM) in core/shell UCNPs (NaYF₄:Yb³⁺,Er³⁺/NaYF₄) on the microsecond and nanosecond time scales, even under continuous irradiation at 980 nm. This SPEM was attributed to slow “upconversion cycles”. We consider that the conventionally reported, nonblinking nature of UCNP emissions can be attributed to the averaging of SPEMs from multiple Er³⁺ ions and the low temporal resolution of previous observation. The off-time distribution, which possesses kinetics information for the upconversion pathways, was well fitted to a single exponential indicating involvement of a single rate-determining step. The distinct behaviors of the green and red emissions confirm their different photophysical pathways.

中文翻译：

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非闪烁上转换纳米粒子的随机光子发射。

由于其众所周知的光学特性，上转换纳米粒子（UCNP）被认为是用于生物成像，生物传感器和太阳能电池的一些最有前途的纳米材料。它们的上转换发射的非闪烁性质对于活细胞成像是特别有利的优势。但是，从未对这种独特属性的起源进行过认真的调查。我们首次报告了核/壳UCNPs（NaYF ₄：Yb ³⁺，Er ³⁺ / NaYF ₄），即使在980 nm的连续照射下也是如此。该SPEM归因于缓慢的“上转换周期”。我们认为，UCNP排放的常规报道，非闪烁性质可以归因于来自多个Er ³⁺离子的SPEM的平均值以及先前观察到的低时间分辨率。具有上转换途径动力学信息的非时间分布非常适合单个指数，表明涉及单个速率确定步骤。绿色和红色发射的不同行为证实了它们不同的光物理路径。