Fluorescent nanodiamonds (FNDs) have attracted recent interest for biological applications owing to their biocompatibility and photostability (absence of photoblinking and bleaching). For optical thermometry, nitrogen-vacancy (NV) color centers and silicon-vacancy color centers in diamonds have demonstrated potential, where the NV has the highest sensitivity. However, NV is often excited with green light, which can cause heating and photodamage to tissues, as well as autofluorescence that decreases sensitivity. To overcome these limitations, we report temperature sensing using NV centers excited by deep red light (660 nm), plus another color center that can be excited with NIR light; the nickel (Ni) complex. The NV center measures temperature using diamond lattice expansion while the Ni complex measures temperature using phonon sideband strength.

Luminescence thermometry is a promising approach using upconversion nanoparticles (UCNPs) with a nanoscale regime in biological tissues. UCNPs are superior to conventional fluorescent markers, benefiting from their autofluorescence suppression and deep imaging in tissues. However, they are still limited by poor water solubility and weak upconversion luminescence intensity, especially at a small particle size. Recently, ${\mathrm{YVO}}_4\text{:}{\mathrm{Er}}^{+3},{\mathrm{Yb}}^{+3}$ nanoparticles have shown high efficiency upconversion (UC) luminescence in water at single-particle level and high contrast imaging in biological models. Typically, a 980-nm laser triggers the UC process in the UCNPs, which overlaps with maximum absorption of water molecules that are dominant in biological samples, resulting in biological tissues overheating and possible damaging. Interestingly, neodymium $({\mathrm{Nd}}^{+3})$ possesses a large absorption cross section at the water low absorption band (808 nm), which can overcome overheating issues. In this Letter, we introduce ${\mathrm{Nd}}^{+3}$ as a new near-infrared absorber and UC sensitizer into ${\mathrm{YVO}}_4\text{:}{\mathrm{Er}}^{+3},{\mathrm{Yb}}^{+3}$ nanoparticles in a core/shell structure to ensure successive energy transfer between the new UC sensitizer $({\mathrm{Nd}}^{+3})$ to the upconverting activator $({\mathrm{Er}}^{+3})$. Finally, we synthesized water-tolerant ${\mathrm{YVO}}_4\text{:}{\mathrm{Er}}^{+3},{\mathrm{Yb}}^{+3}@{\mathrm{Nd}}^{+3}$ core/shell nanoparticles (average size 20 nm) with strong UC luminescence at a biocompatible excitation wavelength for optical temperature sensing where overheating in water is minimized.

Upconversion nanoparticles (UCNPs) are of interest because they allow suppression of tissue autofluorescence and are therefore visible deep inside biological tissue. Compared to upconversion dyes, UCNPs have a lower pump intensity threshold, better photostability, and less toxicity. Recently, YVO₄: Er⁺³, Yb⁺³ nanoparticles were shown to exhibit strong up-conversion luminescence with a relatively low 10 kW cm⁻² excitation intensity even in water, which makes them excellent bio-imaging candidates. Herein, we investigate their use as internal probes in insects by injecting YVO₄ : Er⁺³, Yb⁺³ nanoparticles into fire ants as a biological model, and obtain 2D optical images with 980 nm illumination. High-contrast images with high signal-to-noise ratio are observed by detecting the up-conversion fluorescence as the excitation laser is scanned.

The exceptional optical properties of lanthanide-doped upconversion nanoparticles (UCNPs) make them among the best fluorescent markers for many promising bioapplications. To fully utilize the unique advantages of the UCNPs for bioapplications, recent significant efforts have been put into improving the brightness of small UCNPs crystals by optimizing dopant concentrations and utilizing the addition of inert shells to avoid surface quenching effects. In this work, we engineered bright and small size upconversion nanoparticles in a core–shell–shell (CSS) structure. The emission of the synthesized CSS UCNPs is enhanced in the biological transparency window under biocompatible excitation wavelength by optimizing dopant ion concentrations. We also investigated the biosafety of the synthesized CSS UCNP particles in living cell models to ensure bright and non-toxic fluorescent probes for promising bioapplications.

Upconversion nanoparticles (UCNPs) and carbon quantum dots (CQDs) have recently received a lot of attention as promising materials to improve the stability and efficiency of perovskite solar cells (PSCs). This is because they can passivate the surfaces of perovskite-sensitive materials and act as a spectrum converter for sunlight. In this study, we mixed and added both promising nanomaterials to PSC layers at the ideal mixing ratios. When compared to the pristine PSCs, the fabricated PSCs showed improved power conversion efficiency (PCE), from 16.57% to 20.44%, a higher photocurrent, and a superior fill factor (FF), which increased from 70% to 75%. Furthermore, the incorporation of CQDs into the manufactured PSCs shielded the perovskite layer from water contact, producing a device that was more stable than the original.