Reducing the size of upconversion nanoparticles (UCNPs) down to a few nm yields luminescent materials containing a very small number of emitters. Considering the bottom limit of one activator per particle ultrasmall UCNPs offer an unprecedented platform to study the contributions of the different energy transfers at play in upconversion luminescence. Maintaining detectable emission despite the limited number of emitting ions and the high surface‐to‐volume ratio requires suitable particle architectures. Na(Gd‐Yb)F₄:Tm³⁺ emissive sub‐3 nm diameter β‐phase UCNPs are prepared using a gadolinium‐rich composition in situ mixing of the precursors and a microwave high‐temperature cycling sequence allowing precise control of the particle size and dispersity. These cores are coated with a NaGdF₄ inert shell to minimize the deleterious influence of surface quenching (SQ). Time‐resolved luminescence measurements combining standard NIR excitation of the Yb³⁺ sensitizer and direct UV excitation of the Tm³⁺ activator are performed to quantify cross relaxation and surface quenching processes. The fine tuning of the number of activators per particle via an optimized synthesis pathway along with the use of an appropriate excitation scheme enabled to provide an accurate analysis of the different mechanisms at play in these model nanoparticles and to characterize the structure of the core‐shell architecture.

Polyethylene model particle is synthesized by a top-down approach with effective incorporation of upconverting nanoparticles. The particles with irregular shapes, similar to those found in the environment, are trackable under 980 nm irradiation.

A novel, one-step method for the synthesis of functional, organic–inorganic hybrid nanoparticles is reported. The quench ionic Flash NanoPrecipitation (qiFNP) method enables the straightforward synthesis of nanoparticles by decoupling the formation of the inorganic core and surface functionalization. As a proof-of-concept, the qiFNP method was successfully applied for the tunable and highly controlled synthesis of various LnPO₄-based nanomaterials for bioimaging applications.

Upconverting nanoparticles are a rising class of non-linear luminescent probes burgeoning since the beginning of the 2000’s, especially for their attractiveness in theranostics. However, the precise quantification of the light delivered remains a hot problem in order to estimate their impact on the biological medium. Sophisticated photophysical measurements under near infrared excitation have been developed only by few teams. Here, we present the first attempt towards a simple and cheap photochemical approach consisting of an actinometric characterization of the green emission of NaYF₄:Yb,Er nanoparticles. Using the recently calibrated actinometer 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)-3,3,4,4,5,5-hexafluoro-1-cyclopentene operating in the green region of the visible spectra, we propose a simple photochemical experiment to get an accurate estimation of the efficiency of these green-emitting “nanolamps”. The agreement of the collected data with the previous published results validates this approach.