Quantitative comparison of luminescence probes for biomedical applications

Optical imaging holds great promise for the early-stage detection of diseases. It plays an important role in the process of protecting the patient's health. Most of the organic dyes suffer due to photobleaching, light scattering, short light penetration depth, and autofluorescence of specimen, thus, need to be replaced with alternative nanoprobes emitting light in the optical biological window (700-1350 nm). The group of candidates which can challenged described problems are colloidal quantum dots (e.g. CdSe and PbS) and upconverting nanocrystals (e.g. NaGdF₄:Er, Yb). This paper presents comprehensive and systematic studies of the aforementioned probes, using specially designed tissue phantom, and custom-built wide-field fluorescence microscope. We investigated how the absorption and scattering of light at the water, hemoglobin, and intralipid may affect the intensity of luminescence probes and the quality of optical images. We propose a protocol, that could be easily implemented for investigating other nanoprobes that allow for comparison of their optical performance.

Luminescence of Porous Nanostructured Strontium Titanate Films Doped with Eu3+ Ions

Porous nanostructured strontium titanate films doped with europium SrTiO3:Eu[Formula: see text] fabricated on silicon and porous anodic alumina by the sol–gel route are presented. The films with the thicknesses of 430[Formula: see text]nm and 510[Formula: see text]nm after depositions of three and five layers, respectively, have the perovskite structure confirmed by X-ray diffraction. The porous films demonstrate Eu[Formula: see text] photoluminescence peaks at 593 ([Formula: see text]), 615 ([Formula: see text]) and 705 ([Formula: see text])[Formula: see text]nm. The porosity and PL intensity of the fabricated films noticeably reduce with the number of deposited layers.

Percolation limited emission intensity from upconverting NaYF4:Yb3+,Er3+ nanocrystals - a single nanocrystal optical study

Upconverting nanocrystals (UCNC) have recently been subjected to intensive investigation due to their interesting optical properties and high potential for practical applications. Despite the level of attention paid to these materials, very low quantum yield is still an important issue. In order to break through this limitation, understanding of the emission intensity limitation is crucial. In this paper, we investigate the influence of percolation phenomena on the limitation of the emission intensity from NaYF₄:Yb³⁺,Er³⁺ nanocrystals. We propose a numerical model and support this experimentally at the single nanocrystal level, explaining the influence of Yb³⁺ concentration on the optical properties of UCNC. Moreover, based on the experimental and numerical results, we explain the existence of the optimal Yb³⁺ concentration in the core architecture often reported in the literature. All the measurements have been performed using a custom-built wide-field fluorescence microscope to analyze the emission from hundreds of single nanocrystals and thus make analysis independent of UCNC concentration.

Multiband Monte Carlo modeling of upconversion emission in sub 10 nm β-NaGdF4:Yb3+, Er3+ nanocrystals-Effect of Yb3+ content

In this work, we report the results of theoretical modeling supported and confirmed by experimentally measured emission, emission decay curves, and power dependent emission spectra for sub 10 nm β-NaGdF₄:Er³⁺,Yb³⁺ nanocrystals with different Yb³⁺ content (0.5%-15%). For the theoretical analysis, we develop a stochastic Monte Carlo model which is based on two components: (i) formation of clusters composed of Er³⁺ ion and Yb³⁺ neighbors, which gives insight into the role of local parameters and (ii) a simplified kinetic model of excitation and relaxation phenomena in pairs of Er³⁺and Yb³⁺ ions. The quantitative agreement between experimental data and modeling was obtained for the relative emission ratio of upconversion luminescence in green, red, and blue spectral ranges. Theoretical predictions of impact of excitation pulse duration and pumping light power on upconversion luminescence are presented.

Enhanced photoluminescence stability of CdS nanocrystals through a zinc acetate reagent

An enhancement in the photoluminescence temporal stability was observed upon zinc acetate addition to CdS nanocrystals in one pot synthesis.

Structural and emission properties of Tb3+-doped nitrogen-rich silicon oxynitride films

Terbium doped silicon oxynitride host matrix is suitable for various applications such as light emitters compatible with CMOS technology or frequency converter systems for photovoltaic cells. In this study, amorphous Tb³⁺ ion doped nitrogen-rich silicon oxynitride (NRSON) thin films were fabricated using a reactive magnetron co-sputtering method, with various N₂ flows and annealing conditions, in order to study their structural and emission properties. Rutherford backscattering (RBS) measurements and refractive index values confirmed the silicon oxynitride nature of the films. An electron microscopy analysis conducted for different annealing temperatures (T _A) was also performed up to 1200 °C. Transmission electron microscopy (TEM) images revealed two different sublayers. The top layer showed porosities coming from a degassing of oxygen during deposition and annealing, while in the region close to the substrate, a multilayer-like structure of SiO₂ and Si₃N₄ phases appeared, involving a spinodal decomposition. Upon a 1200 °C annealing treatment, a significant density of Tb clusters was detected, indicating a higher thermal threshold of rare earth (RE) clusterization in comparison to the silicon oxide matrix. With an opposite variation of the N₂ flow during the deposition, the nitrogen excess parameter (N_ex) estimated by RBS measurements was introduced to investigate the Fourier transform infrared (FTIR) spectrum behavior and emission properties. Different vibration modes of the Si-N and Si-O bonds have been carefully identified from the FTIR spectra characterizing such host matrices, especially the 'out-of-phase' stretching vibration mode of the Si-O bond. The highest Tb³⁺ photoluminescence (PL) intensity was obtained by optimizing the N incorporation and the annealing conditions. In addition, according to these conditions, the integrated PL intensity variation confirmed that the silicon nitride-based host matrix had a higher thermal threshold of rare earth clusterization than its silicon oxide counterpart. Analysis of time-resolved PL intensity versus T _A showed the impact of Tb clustering on decay times, in agreement with the TEM observations. Finally, PL and PL excitation (PLE) experiments and comparison of the related spectra between undoped and Tb-doped samples were carried out to investigate the impact of the band tails on the excitation mechanism of Tb³⁺ ions.

Unified Model of Nanosecond Charge Recombination in Closed Reaction Centers from Rhodobacter sphaeroides: Role of Protein Polarization Dynamics

Ongoing questions surround the influence of protein dynamics on rapid processes such as biological electron transfer. Such questions are particularly addressable in light-activated systems. In Rhodobacter sphaeroides reaction centers, charge recombination or back electron transfer from the reduced bacteriopheophytin, HA(-), to the oxidized dimeric bacteriochlorophyll, P(+), may be monitored by both transient absorption spectroscopy and transient fluorescence spectroscopy. Signals measured with both these techniques decay in a similar three-exponential fashion with lifetimes of ∼0.6-0.7, ∼2-4, and ∼10-20 ns, revealing the complex character of this electron transfer reaction. In this study a single kinetic model was developed to connect lifetime and amplitude data from both techniques. The model took into account the possibility that electron transfer from HA(-) to P(+) may occur with transient formation of the state P(+)BA(-). As a result it was possible to model the impact of nanosecond protein relaxation on the free energy levels of both P(+)HA(-) and P(+)BA(-) states relative to that of the singlet excited state of P, P*. Surprisingly, whereas the free energy gap between P* and P(+)HA(-) increased with time in response to protein reorganization, the free energy gap between P* and P(+)BA(-) decreased. This finding may be accounted for by a gradual polarization of the protein environment which stabilizes the state P(+)HA(-) and destabilizes the state P(+)BA(-), favoring productive charge separation over unproductive charge recombination.

Rice oil as a green source of capping ligands for GdF3 nanocrystals

The thermal decomposition of triglycerides allows control of the amount of ligands in the synthesis of GdF₃ nanocrystals.

β-NaGdF4:Eu3+ nanocrystal markers for melanoma tumor imaging

Europium doped nanocrystals can be optimized to be successfully used as visualization markers for i.e. melanoma tumor.

Ion-ion interactions in β-NaGdF4:Yb(3+),Er(3+) nanocrystals--the effect of ion concentration and their clustering

In this work we report co-thermolysis as a suitable method for nanomaterial synthesis which allows the creation of hexagonal upconverting nanocrystals, NaGdF4:Yb(3+),Er(3+), in a wide range of sizes (20-120 nm). Only a very high Yb(3+) concentration (above 70%) results in pure cubic-phase nanocrystals with irregular shape. Additionally, we showed that the impact of Yb(3+), Er(3+) and Gd(3+) ions on the size and optical properties of nanocrystals is significant. We found that the main changes in optical properties do not depend on the nanocrystal size mostly, but are determined by the ion-ion interactions which include both Er(3+)-Er(3+) and Er(3+)-Yb(3+) cross relaxation.

Green light emission from terbium doped silicon rich silicon oxide films obtained by plasma enhanced chemical vapor deposition

The effect of silicon concentration and annealing temperature on terbium luminescence was investigated for thin silicon rich silicon oxide films. The structures were deposited by means of plasma enhanced chemical vapor deposition. The structural properties of these films were investigated by Rutherford backscattering spectrometry, transmission electron microscopy and Raman scattering. The optical properties were investigated by means of photoluminescence and photoluminescence decay spectroscopy. It was found that both the silicon concentration in the film and the annealing temperature have a strong impact on the terbium emission intensity. In this paper, we present a detailed discussion of these issues and determine the optimal silicon concentration and annealing temperature.

Passivation of lanthanide surface sites in sub-10 nm NaYF(4):Eu(3+) nanocrystals

We examined in detail the optical properties of NaYF(4):Eu(3+) nanocrystals of ~9 nm in diameter. For such small nanocrystals roughly 17 % of Y(3+) ions occupy surface sites and can be efficiently substituted by optically active Eu(3+) ions. In order to determine the influence of surface Eu(3+) on the optical properties of the whole nanocrystal, small β-NaYF(4):Eu(3+) nanocrystals with homogenous size distribution were prepared using trioctylphosphine oxide as a coordinating solvent. In order to passivate the surface sites, a thin β-NaYF(4) shell was further deposited on nanocrystals core and the optical properties were investigated. For this purpose absorption, photoluminescence, photoluminescence excitation, and photoluminescence decays were recorded and analyzed. The optical characteristics of surface Eu(3+) significantly diminish for surface passivated nanocrystals. We calculated the increase of quantum yield to the value of 64 % when NaYF(4):Eu(3+) core was capped by undoped shell. The optical spectroscopy techniques were shown to be sufficient in determination of surface passivation of nanocrystals with high surface to volume ratio.

Correlation between stress and carrier nonradiative recombination for silicon nanocrystals in an oxide matrix

Silicon nanocrystals embedded in an oxide matrix formed in a multilayer architecture were deposited by the magnetron sputtering method. By means of Raman spectroscopy we have found that compressive stress is exerted on the silicon nanocrystal core. The stress varies as a function of silicon concentration (O/Si ratio) in the silicon-rich oxide (SRO) layers, which can be attributed to the changing nanocrystal environment. By conducting the time-resolved spectroscopy experiment, we demonstrate that, depending on the nanocrystal surroundings, a different amount of nonradiative recombination sites participates in the excited carrier relaxation process, leading to changes of the relative quantum yield of photoluminescence.

On the nature of the stretched exponential photoluminescence decay for silicon nanocrystals

The influence of hydrogen rate on optical properties of silicon nanocrystals deposited by sputtering method was studied by means of time-resolved photoluminescence spectroscopy as well as transmission and reflection measurements. It was found that photoluminescence decay is strongly non-single exponential and can be described by the stretched exponential function. It was also shown that effective decay rate probability density function may be recovered by means of Stehfest algorithm. Moreover, it was proposed that the observed broadening of obtained decay rate distributions reflects the disorder in the samples.

Quantitative evaluation of boron-induced disorder in multilayers containing silicon nanocrystals in an oxide matrix designed for photovoltaic applications

The effect of doping by boron on optical properties of multilayers containing Si-NCs were studied by means of photoluminescence (PL), time-resolved PL, photoluminescence excitation (PLE), transmission and reflection measurements. It was found that PL decay is strongly non-single exponential and can be described by means of Laplace transform of log-normal decay rates distribution. It was also proposed that changes observed in the distribution central moments reflect the disorder induced by boron-doping.

Temperature dependent emission quenching for silicon nanoclusters

Silicon reach-silicon-oxide (SRSO) film containing silicon nanoclusters was obtained by the reactive magnetron sputtering. Photoluminescence (PL) spectra were measured as a function of temperature at different excitation wavelengths and additionally at different excitation power densities. Obtained PL spectra characterize by two emission bands centered at 1.6 and 2.4 eV. For these bands, temperature behaviour of PL intensities strongly differs but clearly correlate each other. Moreover, it has been observed that obtained PL intensities versus temperature exhibit a strong dependence on the excitation power density in the low temperature range.