Enhancement of photoacoustic and upconversion emission in Mg2+/Zn2+ codoped Gd2O3: Er3+/Yb3+ phosphor using 980 nm excitation

In this work, Er³⁺/Yb³⁺ doped Gd₂O₃ phosphor samples were synthesized through a combustion method and then characterized through X-ray diffraction and FE-SEM techniques. The sample was studied for photoacoustic spectroscopy (PAS) and upconversion (UC) emission spectroscopy techniques using a frequency modulated 980 nm excitation source. A correlation between PA signal (produced due to non-radiative transitions) and UC emission intensity (produced due to radiative transition) is made in order to optimize the sample for both these properties. The phosphor was codoped with Mg²⁺ and Zn²⁺ ions to see enhancement in upconversion emission intensity and these ions have enhanced the upconversion emission. Studies show that the present phosphor is appropriate for producing strong upconversion emission intensity along with a strong photoacoustic signal which is beneficial for upconversion-imaging as well as photothermal therapy. The present sample has also shown its potential for detection of fingerprints.

An overview of the intracellular localization of high-Z nanoradiosensitizers

Radiation therapy (RT) is a method commonly used for cancer treatment worldwide. Commonly, RT utilizes two routes for combating cancers: 1) high-energy radiation to generate toxic reactive oxygen species (ROS) (through the dissociation of water molecules) for damaging the deoxyribonucleic acid (DNA) inside the nucleus 2) direct degradation of the DNA. However, cancer cells have mechanisms to survive under intense RT, which can considerably decrease its therapeutic efficacy. Excessive radiation energy damages healthy tissues, and hence, low doses are applied for cancer treatment. Additionally, different radiosensitizers were used to sensitize cancer cells towards RT through individual mechanisms. Following this route, nanoparticle-based radiosensitizers (herein called nanoradiosensitizers) have recently gained attention owing to their ability to produce massive electrons which leads to the production of a huge amount of ROS. The success of the nanoradiosensitizer effect is closely correlated to its interaction with cells and its localization within the cells. In other words, tumor treatment is affected from the chain of events which is started from cell-nanoparticle interaction followed by the nanoparticles direction and homing inside the cell. Therefore, passive or active targeting of the nanoradiosensitizers in the subcellular level and the cell-nano interaction would determine the efficacy of the radiation therapy. The importance of the nanoradiosensitizer's targeting is increased while the organelles beyond nucleus are recently recognized as the mediators of the cancer cell death or resistance under RT. In this review, the principals of cell-nanomaterial interactions and which dominate nanoradiosensitizer efficiency in cancer therapy, are thoroughly discussed.

Microwave-Assisted Preparation of Luminescent Inorganic Materials: A Fast Route to Light Conversion and Storage Phosphors

Luminescent inorganic materials are used in several technological applications such as light-emitting displays, white LEDs for illumination, bioimaging, and photodynamic therapy. Usually, inorganic phosphors (e.g., complex oxides, silicates) need high temperatures and, in some cases, specific atmospheres to be formed or to obtain a homogeneous composition. Low ionic diffusion and high melting points of the precursors lead to long processing times in these solid-state syntheses with a cost in energy consumption when conventional heating methods are applied. Microwave-assisted synthesis relies on selective, volumetric heating attributed to the electromagnetic radiation interaction with the matter. The microwave heating allows for rapid heating rates and small temperature gradients yielding homogeneous, well-formed materials swiftly. Luminescent inorganic materials can benefit significantly from the microwave-assisted synthesis for high homogeneity, diverse morphology, and rapid screening of different compositions. The rapid screening allows for fast material investigation, whereas the benefits of enhanced homogeneity include improvement in the optical properties such as quantum yields and storage capacity.

Influence of calcination temperature on the fluorescence and magnetic properties of Gd2O3:Tb3+, K+ nanoparticles

Gd₂O₃:Tb³⁺ nanoparticles were synthesized by using diethylene glycol as a solvent and doped with 3 mol% K⁺ ions. Gd₂O₃:Tb³⁺, K⁺ nanoparticles were calcinated at 600°C, 700°C, 800°C, and 900°C and subjected to the analysis of x-ray diffractometer, transmission electron microscope, Fourier transform infrared spectrometer, fluorescence spectroscopy, and magnetization. The experimental results showed that as the calcination temperature increased from 600°C to 800°C, the morphology and particle size of the Gd₂O₃:Tb³⁺, K⁺ nanoparticles did not change significantly; whereas when the calcination temperature rose from 800°C to 900°C, the structure of Gd₂O₃ particles changed from cubic to monoclinic. As the temperature increased (below 800°C), the crystallinity of the cubic particles increased and the surface defects of the particles decreased, resulting in an increase in fluorescence intensity. For the monoclinic particles, the fluorescence intensity was significantly decreased and the magnetization was increased. The measured magnetic results confirmed the good paramagnetism of the synthesized nanoparticles.

Synthesis and characterization of Gd2O3: Er3+, Yb3+doped with Mg2+, Li+ions-effect on the photoluminescence and biological applications

Gd₂O₃:1% Er³⁺, 18% Yb³⁺,x% Mg²⁺(x = 0; 2.5; 4; 5; 6; 8;10; 20; 25; 50) and Gd₂O₃:1% Er³⁺, 18% Yb³⁺, 2,5% Mg²⁺,y% Li⁺(y = 0.5-2.5) nanoparticles were synthesized by homogenous precipitation method and calcined at 900 °C for 3 h in air atmosphere. Powder x-ray diffraction, scanning electron microscopy, cathodoluminescence, transmission electron microscopy, energy dispersive x-ray spectroscopy and photoluminescence techniques were employed to characterize the obtained nanoparticles. We observed a 8-fold increase in red luminescence for samples suspended in DMSO solution for 2.5% of Mg²⁺doping. The x-ray analysis shows that for the concentration of 2.5% Mg, the size of the crystallites in the NPs is the largest, which is mainly responsible for the increase in the intensity of the upconversion luminescence. But the addition of Li⁺ions did not improve the luminescence of the upconversion due to decreasing of crystallites size of the NPs. Synthesized nanomaterials with very effective upconverting luminescence, can act as luminescent markers inin vivoimaging. The cytotoxicity of the nanoparticles was evaluated on the 4T1 cell line for the first time.

Structural, optical and magnetic properties of Y3-0.02-xEr0.02Yb x Al5O12 (0 < x < 0.20) nanocrystals: effect of Yb content

The paramagnetic Y_3-0.02-x Er_0.02Yb _x Al₅O₁₂ (x = 0.02, 0.06, 0.10, 0.12, 0.18, 0.20) nanocrystals (NCs) were synthesized by the microwave-induced solution combustion method. The XRD, TEM and SEM techniques were applied to determine the NCs' structures and sizes. The XRD patterns confirmed that the NCs have for the most part a regular structure of the Y₃Al₅O₁₂ (YAG) phase. The changes of the distance between donor Yb³⁺ (sensitizer) and acceptor Er³⁺ (activator) were realized by changing the donor's concentration with a constant amount of acceptor. Under 980 nm excitation, at room temperature, the NCs exhibited strong red emission near 660 and 675 nm, and green upconversion emission at 550 nm, corresponding to the intra 4f transitions of Er³⁺ (⁴F_9/2, ²H_11/2, ⁴S_3/2) → Er³⁺ (⁴I_15/2). The strongest emission was observed in a sample containing 18% Yb³⁺ ions. The red and green emission intensities are respectively about 5 and 12 times higher as compared to NCs doped with 2% of Yb³⁺. In order to prove that the main factor responsible for the increase of the upconversion luminescence efficiency is reduction of the distance between Yb³⁺ and Er³⁺, we examined, for the first time the influence of hydrostatic pressure on luminescence and luminescence decay time of the radiative transitions inside donor ion. The decrease of both luminescence intensity and luminescence decay times, with increasing hydrostatic pressure was observed. After applying hydrostatic pressure to samples with e.g. 2% and 6% Yb³⁺, the distance between the donor and acceptor decreases. However, for higher concentrations of the donor, this distance is smaller, and this leads to the effective energy transfer to Er³⁺ ions. With increasing pressure, the maximum intensity of near infrared emission is observed at 1029, 1038 and 1047 nm, what corresponds to ²F_5/2 → ²F_7/2 transition of Yb³⁺.

Concentration and pump power-mediated color tunability, optical heating and temperature sensing via TCLs of red emission in an Er3+/Yb3+/Li+ co-doped ZnGa2O4 phosphor

The Er³⁺/Yb³⁺/Li⁺ co-doped ZnGa₂O₄ phosphor gives intense red upconversion photoluminescence, color tunability with Er³⁺ ion concentration and incident pump power, R/G ratio, induced optical heating and temperature sensing characteristics.

Enhanced upconversion emission of Er3+/Yb3+ and Er3+/Yb3+/Zn2+ doped calcium aluminate for use in optical thermometry and laser induced optical heating

There are two key factors to design an efficient green upconversion (UC) emission based optical sensor for temperature. The primary need is to develop a thermally stable and economical material, for a stable sensor, and the second essence is to get an efficient green UC emission, for high sensitivity of the sensor. The proof of this concept is demonstrated on a model system CaAl₂O₄: Er³⁺, co-doped with Yb³⁺ and Zn²⁺. UC emission of Er³⁺ ion is enhanced, primarily, through co-operative energy transfer from Yb³⁺ to Er³⁺ ions. Secondly, we prove that, incorporation of Zn²⁺ ions alters local crystal field environment around Er³⁺ ions which causes an enhancement in green UC emission. The variation in intensity ratio of ²H_11/2 → ⁴I_15/2 (green) and ⁴S_3/2 → ⁴I_15/2 (green) transitions with temperature is studied to report the sensing property. We show that, sensitivity becomes better with an increase in UC efficiency and the best sensitivity is attained for CaAl_(0.793)2Er_0.007Yb_0.05Zn_0.15O₄ sample, ∼0.0154 K^-1 at 308 K. The obtained result is compared with other works and implies its better suitability. Further, the laser induced optical heating is also observed. The laser induced optical heating has been observed experimentally at 400 K above 1 W laser power. This has been further verified by theoretical justification of heating at various pump powers.

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] (∣²F_7/2, 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.

Improved photon upconversion photoluminescence and intrinsic optical bistability from a rare earth co-doped lanthanum oxide phosphor via Bi3+ doping

An Er³⁺, Yb³⁺, Bi³⁺ co-doped La₂O₃ phosphor shows upconverted emission and intrinsic optical bistability properties.

Effect of the concentration of the dopants (Er3+, Yb3+ and Zn2+) and temperature on the upconversion emission behavior of Er3+/Yb3+ co-doped SrAl2O4 phosphor

Er³⁺/Yb³⁺ co-doped SrAl₂O₄ (SRA: Er³⁺, Yb³⁺) phosphor has been synthesized by high temperature solid state reaction technique. The pure phase formation has been confirmed by X-ray diffraction (XRD) measurements. The surface morphology is studied by scanning electron microscopy (SEM) technique. The FTIR measurements give the information of vibrational bands arising due to sample. The intense UC emission from SRA: Er³⁺, Yb³⁺ phosphor has been monitored on excitation with 980nm diode laser. The SRA: Er³⁺, Yb³⁺ samples prepared at 1473K show a dominant green emission. On the other hand it shows dominant red emission when the sample is heated to 1623K. Variation of concentration of Er³⁺ and Yb³⁺ ions in SRA: Er³⁺, Yb³⁺ phosphor suggests two possible mechanisms involved in UC emission process viz. cross relaxation (CR) process and energy back transfer (EBT) process, respectively. The cross relaxation mechanism seems to play a major role. The UC emission efficiency is enhanced several times on co-doping of Zn²⁺ ion replacing Al³⁺ or Sr²⁺ in SRA: Er³⁺, Yb³⁺ phosphor sample. The color of the UC emission can be tuned from green to red region by varying the concentration of zinc.

Upconversion fluorescence imaging of HeLa cells using ROS generating SiO2-coated lanthanide-doped NaYF4 nanoconstructs

Multicolor upconversion of SiO₂-coated nanoparticles using for cells imaging and reactive oxygen species generation.

Solution Combustion Synthesis of Nanoscale Materials

Solution combustion is an exciting phenomenon, which involves propagation of self-sustained exothermic reactions along an aqueous or sol-gel media. This process allows for the synthesis of a variety of nanoscale materials, including oxides, metals, alloys, and sulfides. This Review focuses on the analysis of new approaches and results in the field of solution combustion synthesis (SCS) obtained during recent years. Thermodynamics and kinetics of reactive solutions used in different chemical routes are considered, and the role of process parameters is discussed, emphasizing the chemical mechanisms that are responsible for rapid self-sustained combustion reactions. The basic principles for controlling the composition, structure, and nanostructure of SCS products, and routes to regulate the size and morphology of the nanoscale materials are also reviewed. Recently developed systems that lead to the formation of novel materials and unique structures (e.g., thin films and two-dimensional crystals) with unusual properties are outlined. To demonstrate the versatility of the approach, several application categories of SCS produced materials, such as for energy conversion and storage, optical devices, catalysts, and various important nanoceramics (e.g., bio-, electro-, magnetic), are discussed.

Polymeric nanocapsules with up-converting nanocrystals cargo make ideal fluorescent bioprobes

An innovative approach for up-converting nanoparticles adaptation for bio-related and theranostic applications is presented. We have successfully encapsulated multiple, ~8 nm in size NaYF₄ nanoparticles inside the polymeric nanocarriers with average size of ~150 nm. The initial coating of nanoparticles surfaces was preserved due to the hydrophobic environment inside the nanocapsules, and thus no single nanoparticle surface functionalization was necessary. The selection of biodegradable and sugar-based polyelectrolyte shells ensured biocompatibility of the nanostructures, while the choice of Tm³⁺ and Yb³⁺ NaYF₄ nanoparticles co-doping allowed for near-infrared to near-infrared bioimaging of healthy and cancerous cell lines. The protective role of organic shell resulted in not only preserved high up-converted emission intensity and long luminescence lifetimes, without quenching from water environment, but also ensured low cytotoxicity and high cellular uptake of the engineered nanocapsules. The multifunctionality of the proposed nanocarriers is a consequence of both the organic exterior part that is accessible for conjugation with biologically important molecules, and the hydrophobic interior, which in future application may be used as a container for co-encapsulation of inorganic nanoparticles and anticancer drug cargo.