Optical sensing plays an important role in theranostics due to its capability to detect hint biochemical entities or molecular targets as well as to precisely monitor specific fundamental psychological processes. Rare-earth (RE) doped upconversion nanoparticles (UCNPs) are promising for these endeavors due to their unique frequency converting capability; they emit efficient and sharp visible or ultraviolet (UV) luminescence via use of ladder-like energy levels of RE ions when excited at near infrared (NIR) light that are silent to tissues. These features allow not only a high penetration depth in biological tissues but also a high detection sensitivity. Indeed, the energy transfer between UCNPs and biomolecular or chemical indicators provide opportunities for high-sensitive bio- and chemical-sensing. A temperature-sensitive change of the intensity ratio between two close UC bands promises them for use in temperature mapping of a single living cell. In this work, we review recent investigations on using UCNPs for the detection of biomolecules (avidin, ATP, etc.), ions (cyanide, mecury, etc.), small gas molecules (oxygen, carbon dioxide, ammonia, etc.), as well as for in vitro temperature sensing. We also briefly summarize chemical methods in synthesizing UCNPs of high efficiency that are important for the detection limit.

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Photovoltaic cells are able to convert sunlight into electricity, providing enough of the most abundant and cleanest energy to cover our energy needs. However, the efficiency of current photovoltaics is significantly impeded by the transmission loss of sub-band-gap photons. Photon upconversion is a promising route to circumvent this problem by converting these transmitted sub-band-gap photons into above-band-gap light, where solar cells typically have high quantum efficiency. Here, we summarize recent progress on varying types of efficient upconversion materials as well as their outstanding uses in a series of solar cells, including silicon solar cells (crystalline and amorphous), gallium arsenide (GaAs) solar cells, dye-sensitized solar cells, and other types of solar cells. The challenge and prospect of upconversion materials for photovoltaic applications are also discussed.

The inability to utilize near infrared (NIR) light has posed a stringent limitation for the efficiencies of most single-junction photovoltaic cells such as dye-sensitized solar cells (DSSCs). Here, we describe a strategy to alleviate the NIR light harvesting problem by upconverting non-responsive NIR light in a broad spectral range (over 190 nm, 670-860 nm) to narrow solar-cell-responsive visible emissions through incorporated dye-sensitized upconversion nanoparticles (DSUCNPs). Unlike typically reported UCNPs with narrow and low NIR absorption, the organic dyes (IR783) anchored on the DSUCNP surface were able to harvest NIR photons broadly and efficiently, and then transfer the harvested energy to the inorganic UCNPs (typically reported), entailing an efficient visible upconversion. We show that the incorporation of DSUCNPs into the TiO photoanode of a DSSC is able to elevate its efficiency from 7.573% to 8.568%, enhancing the power conversion efficiency by about 13.1%. We quantified that among the relative efficiency increase, 7.1% arose from the contribution of broad-band upconversion in DSUCNPs (about ∼3.4 times higher than the highest previously reported value of ∼2.1%), and 6.0% mainly from the scattering effect of DSUCNPs. Our strategy has immediate implications for the use of DSUCNPs to improve the performance of other types of photovoltaic devices.

Various energy trapping centers are employed to simultaneously suppress concentration quenching and tune luminescence output through efficiently confining the excitation energy in Er3+-sensitized upconversion nanoparticles.

Noncontact optical thermometers based on the luminescence intensity ratio of two thermally coupled energy levels, exhibiting high sensitivity, excellent accuracy, fast response, and low environment dependence, have attracted great interests in scientific research, life activities, and industrial manufacturing processes. However, the use of optical thermometers in extreme atmospheres (below 150 K) is usually limited by the required large temperature activation because of the relatively big energy difference (200 cm–1 ≤ ΔE ≤ 2000 cm–1). Here, we propose a strategy to alleviate the ultralow temperature-sensing problem by exploiting and utilizing the near-infrared (NIR) thermally coupled Stark sublevels of Tm3+ (3H4|0 → 3H6/3H4|1 → 3H6, ΔE ≈ 300 cm–1) that is much sensitive to minimal temperature variation, especially at ultralow temperatures because of the tiny energy difference. The integration of ultralow temperature-sensitive Tm3+ ions and room-temperature-sensitive Er3+ ions in an ultrasmall α-NaYbF4:Tm3+@CaF2@NaYF4:Yb3+/Er3+@CaF2 core/multishell nanoparticle (∼15 nm) as a dual-mode upconversion luminescent nanoprobe enables the broad-range temperature detection from 10 to 295 K. This structure induces ∼14 times NIR emission and ∼sixfold green upconversion luminescence output in comparison with the α-NaYbF4:Tm3+ core and α-NaYbF4:Tm3+@CaF2@NaYF4:Yb3+/Er3+ core/shell/shell nanoparticles. The maximum absolute and relative sensitivities of this dual-mode temperature sensor reach 0.67% and 3.06% K–1, respectively, showing the advantage of the concurrent utilization of the Tm3+ NIR 801/820 nm band ratio and the typical Er3+ visible 521/538 nm band ratio for a wide-range temperature-sensing purpose. This work provides a promising strategy to develop accurate and effective, contactless broad-range/ultralow temperature sensors.

Here lanthanide-doped tetragonal nanocrystals of BaYF 5 with strong near infrared-to-visible multicolor upconversion emissions have been controllably synthesized by a hydrothermal method. Opposite to well-established NaYF 4 nanocrystals, we found that the sizes of the BaYF 5 nanocrystals increase in proportion to that of the molar ratios of EDTA to lanthanide nitrates and decrease as the pH value of the precursor solution increases in presence of EDTA. These findings reveal that the nucleation process of BaYF 5 nanocrystals is governed by the pH-sensitive chelating ability of EDTA with both Ba 2+ and Ln 3+ . Additionally, it was found that single wavelength laser excitation of 980 nm can induce strong red, green, and blue upconversion emissions in BaYF 5 nanocrystals doped with 18% Yb 3+ /2%Er 3+ , 18%Yb 3+ /2%Ho 3+ , and 18%Yb 3+ /2%Tm 3+ , respectively. Furthermore, after a facile surface poly(acrylic acid) ligand exchange reaction, such multicolor nanoparticles can be transferred into water and multiple other solvents for applications in biology and materials science. . † Electronic supplementary information (ESI) available: The actual results of the effect of reaction conditions (Table S1); the XRD patterns of BaYF 5 with the different RE 3+ /EDTA, RE 3+ /F À ratio and pH; TEM and FESEM images of NaYF 4 nanorods; the relative distribution of species of EDTA vs. pH; TEM of BaYF 5 after ligand exchange; FT-IR of BaYF 5 before and after ligand exchange upconversion emission as a function of the pumping intensity for the BaYF 5 : Yb 3+ /Er 3+ , Yb 3+ /Ho 3+ , Yb 3+ /Tm 3+ system under 980 nm excitation and corresponding to upconversion mechanisms. See

We present a simple method to gradually tune the size and to induce the shape change of CeO(2) nanoparticles via increasing the content of Yb(3+) doping.