Abstract Lanthanide-doped upconversion nanoparticles (UCNPs) have been gaining increasing research interests in chemistry, materials, optics, and interdisciplines due to their excellent photostability, adequate anti-Stokes emission efficiency, multicolor sharp-band emissions and tunable long emission lifetimes. Constructive applications on miniaturized optical devices, biomedicine, solar energy conversion, etc. have been demonstrated. Nonetheless, previous studies are mainly based on the properties from nanoparticle ensembles, which ignore the interparticle discrepancy in size, composition, surface status, optical properties, etc. Recently, with the development of optical detection with high sensitivity and spatial resolution, upconversion emission investigations step into a single particle level. This reveals intrinsic upconversion properties of UCNPs instead of leveling of particles. Here, we provide a critical review on the recent advances of upconversion in single nanoparticles and propose the perspectives of this emerging field. Specifically, we start by the introduction of detection methods of upconversion emissions in single nanoparticles. Subsequently, we summarized the very recent progress on photostability, steady-state and transient-state, polarized upconversion emissions, interparticle energy transfers as well as interdisciplinary studies on optical tracking, optical tweezer, lasing, and super-resolution nanoscopy of single UCNPs. Meanwhile, upconversion on a single microparticle is also included for the typical photophysical studies, such as optical waveguide and multiplexing. We further discussed future directions in terms of photophysical properties and microscopic/nanoscopic applications of upconversion in single particles. This review is expected to inspire investigations on disclosing intriguing upconversion properties of lanthanides and implanting UCNPs in advanced microscopic/nanoscopic applications.

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单个粒子的上转换发射研究

摘要 镧系元素掺杂的上转换纳米粒子 (UCNPs) 由于其优异的光稳定性、足够的反斯托克斯发射效率、多色锐带发射和可调谐的长发射寿命，在化学、材料、光学和跨学科领域的研究越来越受到关注。在微型光学器件、生物医学、太阳能转换等方面的建设性应用已经得到证明。尽管如此，以前的研究主要基于纳米粒子集合的性质，忽略了粒子间在尺寸、组成、表面状态、光学性质等方面的差异。 近年来，随着具有高灵敏度和空间分辨率的光学检测的发展，上转换发射研究步入单个粒子级别。这揭示了 UCNP 的固有上转换特性，而不是粒子的整平。在这里，我们对单个纳米粒子上转换的最新进展进行了批判性评论，并提出了这一新兴领域的观点。具体来说，我们首先介绍了单个纳米粒子中上转换发射的检测方法。随后，我们总结了光稳定性、稳态和瞬态、极化上转换发射、粒子间能量转移以及光学跟踪、光镊、激光和单个 UCNP 的超分辨率纳米显微镜的跨学科研究的最新进展。同时，对于典型的光物理研究，例如光波导和多路复用，还包括对单个微粒的上转换。我们进一步讨论了光物理特性和单粒子上转换的微观/纳米应用的未来方向。这篇综述有望激发关于揭示镧系元素有趣的上转换特性和在先进的显微/纳米应用中植入 UCNP 的研究。