K. Ciszak

Upconverting/magnetic: Gd2O3:(Er3+,Yb3+,Zn2+) nanoparticles for biological applications: effect of Zn2+ doping

Upconverting Gd2O3 nanoparticles (NPs) doped 1% Er3+ and 18% Yb3+ permits one to perform optical imaging. Because of the presence of Gd3+ they are useful in MRI. The main challenge is to enhance the NPs upconversion efficiency. As a result of co-doping the NPs with Zn2+ ions, achieved using microwave-induced solution combustion synthesis, we obtained optimal upconversion quantum yields (UQYs). The breakdown of the local crystal field symmetry around the rare earth ions, maximal in the presence of 5% of zinc, may be responsible for the highest observed UQY. The upconversion of IR light results in emission of visible red light mainly at 660 nm and at 550 nm. Optimized red photoluminescence of the samples observed in an organic environment was examined as a function of the laser power density to explain the mechanism of the upconversion emission. Paramagnetic properties of the NPs were determined by superconducting quantum interference device measurements. The non-functionalized nanoparticles incubated with HeLa cells were endocytosed and imaged by confocal laser scanning microscopy. We investigated their localization inside HeLa cells for various incubation times and NPs concentrations. PrestoBlue toxicity assay was performed to test the NPs bio-efficacy.

PsbS is required for systemic acquired acclimation and post-excess-light-stress optimization of chlorophyll fluorescence decay times in Arabidopsis

Systemic acquired acclimation (SAA) is an important light acclimatory mechanism that depends on the global adjustments of non-photochemical quenching and chloroplast retrograde signaling. As the exact regulation of these processes is not known, we measured time-resolved fluorescence of chlorophyll a in Arabidopsis thaliana leaves exposed to excess light, in leaves undergoing SAA, and in leaves after excess light episode. We compare the behavior induced in wild-type plants with null mutant of non-photochemical quenching (npq4–1). The wild type rosettes exhibit a small reduction of fluorescence decay times in leaves directly exposed to excess light and in leaves undergoing SAA in ambient low light. However in npq4–1 exposition to excess light results in much faster fluorescence decay, which is insensitive to excitation power. At the same time npq4–1 leaves undergoing SAA displayed intermediate fluorescence decay. The npq4–1 plants also lost the ability to optimize florescence decay, and thus chlorophyll a dynamics up to 2 h after excess light episode. The fluorescence decay dynamics in both WT and npq4–1 can be described by a set of 3 maximum decay times. Based on the results, we concluded that functional PsbS is required for optimization of absorbed photon fate and optimal light acclimatory responses such as SAA or after excess light stress.

Contribution of PsbS Function and Stomatal Conductance to Foliar Temperature in Higher Plants.

Natural capacity has evolved in higher plants to absorb and harness excessive light energy. In basic models, the majority of absorbed photon energy is radiated back as fluorescence and heat. For years the proton sensor protein PsbS was considered to play a critical role in non-photochemical quenching (NPQ) of light absorbed by PSII antennae and in its dissipation as heat. However, the significance of PsbS in regulating heat emission from a whole leaf has never been verified before by direct measurement of foliar temperature under changing light intensity. To test its validity, we here investigated the foliar temperature changes on increasing and decreasing light intensity conditions (foliar temperature dynamics) using a high resolution thermal camera and a powerful adjustable light-emitting diode (LED) light source. First, we showed that light-dependent foliar temperature dynamics is correlated with Chl content in leaves of various plant species. Secondly, we compared the foliar temperature dynamics in Arabidopsis thaliana wild type, the PsbS null mutant npq4-1 and a PsbS-overexpressing transgenic line under different transpiration conditions with or without a photosynthesis inhibitor. We found no direct correlations between the NPQ level and the foliar temperature dynamics. Rather, differences in foliar temperature dynamics are primarily affected by stomatal aperture, and rapid foliar temperature increase during irradiation depends on the water status of the leaf. We conclude that PsbS is not directly involved in regulation of foliar temperature dynamics during excessive light energy episodes.

Extending light-harvesting of poly(3-hexylthiophene) through efficient energy transfer from infra-red absorbing nanocrystals: Single nanoparticle study

We report on single nanocrystal fluorescence microscopy of blends composed of colloidal up-converting NaYF4 nanocrystals doped with rare-earth ions embedded in poly(3-hexylthiophene) (P3HT) polymer. By probing both steady-state and time-resolved fluorescence properties of individual nanocrystals excited with infra-red 980 nm laser, we demonstrate that upon up-conversion to the visible spectral range, the energy is efficiently transferred from the nanocrystals to P3HT. From the analysis of fluorescence lifetimes, the energy transfer efficiency for 550 nm emission of the nanocrystals was estimated to be 60%. This observation renders the up-converting nanocrystals as potential structures for improving light-harvesting efficiency of polymers in the near-infrared spectral region.

Enhanced up-conversion in nanocrystals coupled to silver nanowires

We studied physical properties of hybrid nanostructures consisting of α-NaYF4:Tm3+/Yb3+ nanocrystals coupled to single silver nanowires. By using confocal fluorescence microscope we observed much higher intensity of the upconversion emission form nanocrystals placed in the vicinity of nanowires as compared to isolated ones. At the same time Fluorescence Life-time Imaging Microscopy demonstrates shortening of the fluorescence decays times for metal-coupled nanocrystals. We conclude that plasmon-enhanced emission rates are mainly responsible for enhanced up-conversion efficiency.

Single-step synthesis of Er3+ and Yb3+ ions doped molybdate/Gd2O3 core–shell nanoparticles for biomedical imaging

Nanostructures as color-tunable luminescent markers have become major, promising tools for bioimaging and biosensing. In this paper separated molybdate/Gd2O3 doped rare earth ions (erbium, Er3+ and ytterbium, Yb3+) core-shell nanoparticles (NPs), were fabricated by a one-step homogeneous precipitation process. Emission properties were studied by cathodo- and photoluminescence. Scanning electron and transmission electron microscopes were used to visualize and determine the size and shape of the NPs. Spherical NPs were obtained. Their core-shell structures were confirmed by x-ray diffraction and energy-dispersive x-ray spectroscopy measurements. We postulated that the molybdate rich core is formed due to high segregation coefficient of the Mo ion during the precipitation. The calcination process resulted in crystallization of δ/ξ (core/shell) NP doped Er and Yb ions, where δ-gadolinium molybdates and ξ-molybdates or gadolinium oxide. We confirmed two different upconversion mechanisms. In the presence of molybdenum ions, in the core of the NPs, Yb3+-[Formula: see text] (∣2F7/2, 3T2〉) dimers were formed. As a result of a two 980 nm photon absorption by the dimer, we observed enhanced green luminescence in the upconversion process. However, for the shell formed by the Gd2O3:Er, Yb NPs (without the Mo ions), the typical energy transfer upconversion takes place, which results in red luminescence. We demonstrated that the NPs were transported into cytosol of the HeLa and astrocytes cells by endocytosis. The core-shell NPs are sensitive sensors for the environment prevailing inside (shorter luminescence decay) and outside (longer luminescence decay) of the tested cells. The toxicity of the NPs was examined using MTT assay.

Luminescence enhancement and energy propagation in plasmonic networks

We describe the optical properties of a simple plasmonic network, which consists of silver nanowires coupled with nanocrystals doped with rare-earth-ions. The nanocrystals exhibit anti-Stokes emission called up-conversion. First of all, we demonstrate efficient coupling between a single silver nanowire and nearby nanocrystals via plasmonic excitations. These plasmonic interactions result in an enhancement of the up-converted emission intensity of nanocrystals located in the close vicinity of the nanowires, mostly due to increased radiative emission rates. We prove that luminescence can be either emitted directly by the nanocrystals or transferred to the nanowire. Imaging of angular-resolved emission patterns in the Fourier plane reveals plasmon-mediated luminescence, where the up-converted radiation is emitted via the nanowire antenna as leakage radiation. The luminescence signal can be distributed by plasmonic network for quite long distances reaching tens of micrometers.

Plasmonic networks based on metallic nanowires

In this paper we demonstrate direct excitation of propagating plasmons in silver nanowires. The plasmons are excited either directly by an infrared excitation or indirectly via up-conversion process in rare-earth doped nanocrystals placed in the vicinity of the silver nanowires. In order to map propagating plasmons in silver nanowires, we designed an experimental setup which enables precise, diffraction-limited focusing of the laser, while at the same time both wide-field and confocal imaging of fluorescence.

Plasmonic influence on the up-conversion luminescence in NaYF4:Er3+/Yb3+ nanocrystals

We study the fluorescence properties of up-converting α-NaYF4:Er3+/Yb3+ nanocrystals (NCs) in the presence of spherical Au nanoparticles. The effects of plasmon excitations in the Au nanoparticles were studied using scanning confocal fluorescence microscope. The results show that the 4S3/2→ 4I15/2 transition in erbium ions, which appears at 540 nm, resonant with the plasmon band of the Au nanoparticles, is efficiently quenched. In contrast, in the case of 4F9/2→ 4I15/2 transition, which appears at 650 nm, we find the increase of emission intensity. Complementary measurement of fluorescence lifetimes of individual NaYF4:Er3+/Yb3+ nanocrystals reveals that plasmonic excitations in Au nanoparticles influence the dynamics of both emission band in erbium ions.