The upconversion process is reported to occur via interparticle energy transfer (IPET) achieved with Ln3+‐doped BaLuF5 (Ln = Tm and Yb) nanocrystals in the size range of sub 5 nm. Both Yb3+ and Tm3+ ions are independently doped in BaLuF5 nanocrystals. Colloidal mixtures containing the above two nanocrystals show strong upconversion emission in the blue, red, and near‐IR regions via IPET.

entail a physical contact with the object or are limited to superficial and macroscale application. In contrast to traditional thermometers, temperature-sensitive photoluminescent nanoprobes, typically referred to as nanothermometers, allow for contactless, fast, and accurate temperature measurement at the sub-micrometer/nanoscale: [5,6] pertinent in the development of state-of-the-art electronics, in situ characterization of materials, unraveling of previously unexplored physical phenomena, and most of all biomedicine. [7-13] In biomedicine, there is a niche for photoluminescent nanothermometers operating in the nearinfrared (NIR) spectral region, within the biological imaging windows (BW-I: 750-950 nm, BW-II: 1000-1350 nm, and BW-III: 1500-1800 nm). [14,15] Biological tissues are partially translucent to NIR light, due to reduced optical scattering and absorption, thus leading to more accurate target visualization in the NIR. While nanothermometers working on the visible side of the optical spectrum are perfectly suited for in vitro microscopy, high-contrast deep-tissue imaging and temperature sensing ex/in vivo requires the development of all NIR nanothermometers (both excitation and emission in the NIR). [16-18] Among photoluminescent nanoprobes, rare-earth nanoparticles (RENPs) are the most studied and arguably the most promising nanothermometers. Over the last decade, NIR excited RENPs operating as nanothermometers in the UV-visible (via the process of upconversion), [19,20] BW-I, [21-26] BW-II, [27,28] and BW-III [17,29] regions have been developed and applied in biomedical research. In the case of the NIR, most BW-II/III nanothermometers rely on intensity-ratio-based nanothermometry between spectrally distant emission bands, such as those of Nd 3+ (1060 nm, 1340 nm)/Yb 3+ (1000 nm), Er 3+ (1550 nm)/Nd 3+ , Er 3+ /Yb 3+ , Tm 3+ (1230 nm)/Yb 3+ , and Nd 3+ /Ho 3+ (1180 nm). In light of the recent concerns about heterogeneous attenuation of these NIR bands by tissues, [16,30] such nanothermometers may lead to faulty temperature readout when applied in subcutaneous studies. One of the proposed workarounds to this issue is to define integration ranges for ratiometric nanothermometry that would be equally attenuated by tissues, thus maintaining their calibration despite the optical inhomogeneity of the tissue. To that effect, spectrally narrow single-band nanothermometry in the BW-II with Nd 3+-doped RENPs holds great promise. [31-35] Near-infrared (NIR) nanothermometers are sought after in biomedicine when it comes to measuring temperatures subcutaneously. Yet, temperature sensing within the third biological imaging window (BW-III), where the highest contrast images can be obtained, remains relatively unexplored. Here, LiErF 4 /LiYF 4 rare-earth nanoparticles (RENPs) are studied as NIR nanothermometers in the BW-III. Under 793 nm excitation, LiErF 4 /LiYF 4 RENPs emit around 1540 nm, corresponding to the 4 I 13/2 → 4 I 15/2 radiative transition of Er 3+. The fine Stark structure of this transition allows to de...

In this Research Article, we report for the first time the use of upconverting nanoparticles to detect melamine up to nanomolar concentration. Detection of melamine is important as it is one of the adulterant in protein rich food products due to its high nitrogen content. In this work, we have shown how the electron deficient 3,5-dinitrobenzoic acid (DNB)-coated Er/Yb-NaYF4 nanocrystals can specifically bind to electron rich melamine and alter the upconverting property of the nanocrystals. This selective binding led to the quenching of the upconversion emission from the nanocrystals. The high selectivity is verified by the addition of various analytes similar in structure with that of melamine. In addition, the selective quenching of the upconversion emission is reversible with the addition of dilute acid. This process has been repeated for more than five cycles with only a slight decrease in the sensing ability. The study was also extended to real milk samples, where the milk adulterated with melamine quenches the emission intensity of the DNB coated NaYF4:Er/Yb nanocrystals, whereas hardly any change is noted for the unadulterated milk sample. The high robustness and the sharp emission peaks make Er(3+)/Yb(3+)-doped NaYF4 nanocrystals a potential melamine sensing material over other organic fluorophores and nanocrystals possessing broad emissions.

Water-dispersible NIR-II dye (IR-1061) sensitized core/active shell upconversion nanoparticles (800 nm emission of Tm3+ ion).