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Quantitative comparison of emission intensity between lanthanide doped upconverting (UC) nanoparticles is critical to evaluate the impact of host matrix and nanocrystal size, active- and passive- dopants, core-shell chemical architecture, surface biofunctionalization and others, which are typically a subjects to optimization. While relative luminescence intensity has been commonly used for that purpose, quantum yield (QY) is another accurate measure, because here, number of UC emission photons is normalized to the number of photons being absorbed. However, this type of measurements have been hindered by limited availibility of commercial instrumentation. Here we show a novel, simple and affordable UCQY setup design and versatilely characterize its performance using Yb3+ and Ho3+ co-doped colloidal NaYF4 nanoparticles dispersed in chloroform. The UCQY showed a liner dependence on excitation intensity, and reached up to 0.41% and 0.25% for the 2% Ho3+, 20% Yb3+ and 0.1% Ho3+, 20% Yb3+ materials, respectively. Interestingly, the UCQY values depend on the selection of reference material and concentration of NPs. In particular, the un-doped reference NPs of the same diameter as the sample NPs show higher UCQY values than the ones obtained for the pure solvent. Moreover, UCQY depend on the concentration of the NPs. These two effects were related to the scattering of photoexcitation beam in the sample and reference materials. Additionally, no saturation of power dependent UC was seen up to ~270 W/cm2 excitation intensities, which we relate to the photoexcitation methodology. In opposite to the Continuous Wave photoexcitation, we used Pulse Width Modulation of photoexcitation intensity, and confirm such behavior with a simple, time – resolved differential rate equations based model. These findings and reliable UCQY instruments are important to quantitatively compare and further optimize the performance of UCNPs.

The emulsification process leading to up-converting NaYF4:Er3+,Yb3+ NPs encapsulation, was performed using a modified water/oil/water double emulsion evaporation method, where poly(lactic-co-glycolic acid) was used as biocompatible polymer. Span 80 and Cremophor A25 were applied as non-ionic surfactants and dichloromethane as oily phase. The use of trioctylphosphine oxide ligands for the synthesis of up-converting NaYF4:Er3+,Yb3+ NPs allowed to obtain spherical particles with sizes below 10 nm, what further facilitated the efficient encapsulation process. Those newly designed nanosystems were subjected to analysis of their morphology, colloidal stability and optical properties by: dynamic light scattering, ζ-potential, atomic force microscopy, transmission electron microscopy and measuring the up-conversion emission spectra of free and loaded NaYF4:Er3+,Yb3+ NPs. The encapsulated NaYF4:Er3+,Yb3+ NPs showed increased colloidal stability for a long period of 60 days of storage in different conditions. Simultaneously, the encapsulation process did not significantly influenced their optical properties and strong visible emission could be observed upon nearinfrared excitation.

The ligand induced chirality in semiconducting nanoparticles has recently gained special attention, due to possible application of chiral nanoparticles as optically active probes in enantioselective bio-sensing. However, the exact mechanisms of chiral organic ligands interaction with quantum dots surfaces are still not fully described. We have performed ligand exchange procedure in order to obtain water soluble CdSe QDs functionalized with left- and right-handed enantiomers of penicillamine molecules and the influence of such post-synthetic surface treatment on the optical properties of CdSe QDs with respect to their size has been investigated. In particular, picosecond and nanosecond time scale kinetics revealed strong hole trapping at the thiol molecules near to the quantum dots surfaces. Additionally, possible applications of chiral drug discrimination are shown for L- and D-penicillamine functionalized CdSe quantum dots.

The concentration dependent up-conversion luminescence of 0.1; 0.2; 0.5; 1; 1.5; 2; 3; 4 and 5% Ho3+ and 20% Yb3+ co-doped β-NaYF4 colloidal nanocrystals have been studied. The obtained nanocrystals were homogenous in size and shape, as well as the crystallographic structure was pure. The absorption spectra, upconversion lifetimes, up-conversion emission and Quantum Yield power dependence have been measured to estimate the optimum doping concentration of Ho3+ ions. The experimental results were supported by the calculations based on Judd-Ofelt theory, in order to quantify the oscillator strengths in absorption and luminescence, excited-state radiative lifetimes, energy-transfer probabilities, and to estimate the quantum efficiencies of this interesting material. We have observed evident trend in rise- and luminescence decay profiles with rising Ho3+ activator concentration, and the 1–2% Ho3+ ions doping was found to be the most efficient nanophosphors.

We investigated new development in photodynamic therapy (PDT), aiming at enhanced tumor selectivity and biocompatibility, which included application of a third-generation photosensitizing agent, i.e. xanthene-origin Rose Bengal (RB) co-encapsulated with up-converting NaYF4 nanoparticles (NPs) co-doped with lanthanide ions: Er3+ (2%) and Yb3+ (20%). The hybrid fluorophores were applied as components of double core nanocarriers (NCs) obtained by double (multiple) emulsion solvent evaporation process. Next, to improve the biocompatibility and photodynamic activity, biodegradable polymer: poly(lactide-co-glycolide) - PLGA and non-ionic surfactants with different hydrophobicity: Span 80 and Cremophor A25, were used. After the engineering process, controlled by dynamic light scattering (DLS) measurements, ζ-potential evaluation, transmission electron and atomic force microscopy (TEM and AFM) imaging, as well as optical analysis provided by measurements of the up-conversion emission spectra and luminescence kinetics for encapsulated only NaYF4:Er3+,Yb3+ NPs and co-encapsulated RB + NaYF4:Er3+,Yb3+ molecules, spherical polyester NCs with average size

Hydrophobic CdSe x Te 1− x quantum dots with near infrared emission in the 700–750 nm range were synthesized by a wet chemistry technique. Their nonlinear optical properties were studied using Z-scan technique with a tunable femtosecond laser system. The peak value of the two-photon absorption cross section was found to be ∼2400 GM at 1400 nm. To demonstrate a possible way of utilizing the CdSe x Te 1− x quantum dots in aqueous environment we describe here a convenient method of preparation of Brij 58® micellar systems loaded with the quantum dots. The obtained nanoconstructs were characterized using optical spectroscopy, TEM and DLS. The micelles colloidal stability, and the influence of the encapsulation process on the spectroscopic properties of the quantum dots are discussed. In particular, we have observed a 60 nm blue-shift of the emission maxima upon loading quantum dots inside the micelles.

We demonstrate the use of thermal decomposition reaction to obtain Ga 2 O 3 nanoparticles with average size around 5 nm. The obtained nanoparticles presented a good colloidal stability and high optical transparency. We were also able to incorporate Eu 3+ and Tb 3+ ions inside the crystal host. The synthesized nanomaterials exhibited dual mode emission upon UV excitation, consisting of a broad band in the blue region and a characteristic series of sharp lines. The former resulted from donor–acceptor pairs recombination in Ga 2 O 3 host, while the latter from 4f–4f electronic transitions in lanthanide ions. For fuller optical characterization of the obtained nanoparticles, we have performed wide wavelength range Z-scan studies, and calculated the values of nonlinear absorption cross-sections. Gallium (III) oxide nanoparticles showed two-photon absorption in the range between 500 nm and 700 nm, with molecular weight scaled nonlinear optical parameters exceeding the values for other lanthanide-doped nanoparticles of similar size.

Synthesis, crystal structure, morphology and optical properties of three various NaYF 4 nanoparticle systems co-doped with Tm 3+ and Yb 3+ ions have been investigated. Characterization of ultrasmall (size ∼8 nm), small (∼25 nm) NaYF 4 :2%Tm 3+ , 20%Yb 3+ nanoparticles and core–shell structures (NaYF 4 :20%Yb 3+ –NaGdF 4 :2%Tm 3+ ) synthesized according to two different protocols is presented. We have found that the up-conversion luminescence is strongly influenced by the nanoparticle size, internal structure and spatial distribution of active Tm 3+ ions. Further spectroscopic studies revealed significant fluorescence rise and decay time lengthening of the 1 G 4 energy level of Tm 3+ ions in NaYF 4 :20%Yb 3+ –NaGdF 4 :2%Tm 3+ core–shell nanoparticles. The formation of the core–shell structures was proven indirectly through the average crystalline size increase between the bare core and core–shell nanoparticles, the observed changes in EDX composition as well as variation of Tm 3+ ions luminescence properties.

We have investigated the influence of gold nanorods (Au NRs) proximity on the up-conversion luminescence of lanthanide-doped NaYF 4 nanoparticles (NPs). The measurements were performed on two different systems. We used solvent evaporation method to assemble NaYF 4 :2%Tm 3+ , 20%Yb 3+ NPs/Au NRs thin films, and colloidal water blend suspensions of mixed NaYF 4 :2%Tm 3+ , 20%Yb 3+ or NaYF 4 :2%Er 3+ , 20%Yb 3+ NPs with Au NRs. The coupling of the NaYF 4 :2%Tm 3+ , 20%Yb 3+ and NaYF 4 :2%Er 3+ , 20%Yb 3+ NPs emission with the surface plasmon of Au NRs was found to lead to effective modulation of Tm 3+ and Er 3+ ions luminescence. As the result of NPs and Au NRs being in close contact the rate of excitation could be increased, but also quenching by energy transfer and nonradiative relaxation has been observed.

The possibility of tuning between red, green and broadband white color of up-conversion was demonstrated in thermally 450 °C treated Yb/Er co-doped nano NaYF 4 phosphor. The color variability was studied by means of power dependence of luminescence, which exhibited unusual behavior. Large hysteresis, as well as discrepancy from a power law indicated the important role the increased heating played during the up-conversion in nano-sized materials.

Transparent, chloroform dispersed α -NaYF 4 nanocrystals doped with neodymium ions were synthesized and characterized. XRD and TEM measurement confirmed cubic structure of α -NaYF 4 of the nanoparticles (NPs). The absorption and emission spectra as well as 4 F 3/2 level fluorescence decay curves were measured in order to estimate the influence of Nd 3+ concentration in the matrix on the optical properties of the NPs. With the increase of Nd 3+ doping level, the Judd–Ofelt Ω 4 parameter as well as the spectroscopic Nd 3+ parameter X Nd = Ω 4 / Ω 6 was growing. In the same time the Ω 2 and Ω 6 were decreasing. Theoretical luminescence lifetimes of the 4 F 3/2 level equal to ∼300 μs were also calculated and compared with the experimental values to quantify the concentration quenching. Based on this comparison, quantum efficiency was found to vary systematically between ∼100% and 4% for Nd 3+ content increasing from 2 up to 25%.