Lina Mikoliunaite

Luminescence and luminescence quenching of highly efficient Y2Mo4O15:Eu3+ phosphors and ceramics

A good LED phosphor must possess strong enough absorption, high quantum yields, colour purity, and quenching temperatures. Our synthesized Y2Mo4O15:Eu3+ phosphors possess all of these properties. Excitation of these materials with near-UV or blue radiation yields bright red emission and the colour coordinates are relatively stable upon temperature increase. Furthermore, samples doped with 50% Eu3+ showed quantum yields up to 85%, what is suitable for commercial application. Temperature dependent emission spectra revealed that heavily Eu3+ doped phosphors possess stable emission up to 400 K and lose half of the efficiency only at 515 K. In addition, ceramic disks of Y2Mo4O15:75%Eu3+ phosphor with thickness of 0.71 and 0.98 mm were prepared and it turned out that they efficiently convert radiation of 375 and 400 nm LEDs to the red light, whereas combination with 455 nm LED yields purple colour.

The influence of localized plasmons on the optical properties of Au/ZnO nanostructures

Optical and structural experiments have been carried out on Si/ZnO thin films modified with ultra-thin gold layers of different thicknesses. ZnO was produced via Atomic Layer Deposition (ALD) and Au via Physical Vapor Deposition (sputtering). The structural properties of nanostructures were studied by XRD and AFM. Optical characterization was performed by absorbance, photoluminescence (PL) and spectroscopic ellipsometry (SE). A transition from cluster-to-thin films with the increase of Au thickness has been revealed from an analysis of optical and structural parameters. The analysis of optical features of the system has shown that slight changes of the localized plasmon absorption peaks in spectra make a significant contribution to complex refractive index of gold film and, as a result, leads to a strong enhancement of UV PL peak in the ZnO layer. The mechanism of the tailoring of ZnO optical features changes by varying the Au layer thickness was discussed. Our studies have shown that through the changes of structural properties of thin gold layer between the Si substrate and the ZnO film, we can tune the optical dispersion of each layer and hence the control of ZnO PL spectra enhancement and quenching in UV-Vis wavelengths region is possible. In order to apply the hybrid structure under consideration in various optical applications, such as LED, the dispersion of the complex refractive index of the components should be optimized taking into account a particular target.

The substrate matters in the Raman spectroscopy analysis of cells

Raman spectroscopy is a powerful analytical method that allows deposited and/or immobilized cells to be evaluated without complex sample preparation or labeling. However, a main limitation of Raman spectroscopy in cell analysis is the extremely weak Raman intensity that results in low signal to noise ratios. Therefore, it is important to seize any opportunity that increases the intensity of the Raman signal and to understand whether and how the signal enhancement changes with respect to the substrate used. Our experimental results show clear differences in the spectroscopic response from cells on different surfaces. This result is partly due to the difference in spatial distribution of electric field at the substrate/cell interface as shown by numerical simulations. We found that the substrate also changes the spatial location of maximum field enhancement around the cells. Moreover, beyond conventional flat surfaces, we introduce an efficient nanostructured silver substrate that largely enhances the Raman signal intensity from a single yeast cell. This work contributes to the field of vibrational spectroscopy analysis by providing a fresh look at the significance of the substrate for Raman investigations in cell research.

Quartz Crystal Microbalance-Based Evaluation of the Electrochemical Formation of an Aggregated Polypyrrole Particle-Based Layer

Electrochemical quartz crystal microbalance (EQCM) was used for the evaluation of conducting polymer polypyrrole (Ppy), which was formed by a sequence of potential pulses on a Au-plated EQCM disc. The Ppy layer was obtained from freshly prepared polymerization solution consisting of pyrrole that was dissolved in phosphate buffer. The main aim of the study was to determine some aspects of the Ppy layer formation process. The polymerization process was estimated by EQCM and chronoamperometry. The Cottrell equation was used for the integration of total charge that was passing through the electrochemical cell during the formation of the Ppy-based layer. It was found that the charge of the electrical double layer, which was estimated while applying an Anson plot, is negative. From this observation, it could be assumed that the pyrrole oxidation process could be well described by principles of heterogeneous kinetics. The negative value of the electrical double layer was the result of a charge-transfer restriction. This restriction of charge transfer could occur due to partial blocking of the electrode surface by an aggregated Ppy particle-based layer. Quartz crystal motional resistance (R) was taken into account during this research. Ppy layer formation is represented schematically on the basis of the obtained experimental results and analytical data.

Synthesis of polypyrrole within the cell wall of yeast by redox-cycling of [Fe(CN)6](3-)/[Fe(CN)6](4-).

Yeast cells are often used as a model system in various experiments. Moreover, due to their high metabolic activity, yeast cells have a potential to be applied as elements in the design of biofuel cells and biosensors. However a wider application of yeast cells in electrochemical systems is limited due to high electric resistance of their cell wall. In order to reduce this problem we have polymerized conducting polymer polypyrrole (Ppy) directly in the cell wall and/or within periplasmic membrane. In this research the formation of Ppy was induced by [Fe(CN)6](3-)ions, which were generated from K4[Fe(CN)6], which was initially added to polymerization solution. The redox process was catalyzed by oxido-reductases, which are present in the plasma membrane of yeast cells. The formation of Ppy was confirmed by spectrophotometry and atomic force microscopy. It was confirmed that the conducting polymer polypyrrole was formed within periplasmic space and/or within the cell wall of yeast cells, which were incubated in solution containing pyrrole, glucose and [Fe(CN)6](4-). After 24h drying at room temperature we have observed that Ppy-modified yeast cell walls retained their initial spherical form. In contrast to Ppy-modified cells, the walls of unmodified yeast have wrinkled after 24h drying. The viability of yeast cells in the presence of different pyrrole concentrations has been evaluated.

Rotating disk electrode-based investigation of electroluminescence of tris(2,2′-bipiridin)dichlorruthenium(II)hexahydrate, luminol, and N -(4-aminobuthyl)- N -ethyl-isoluminol

AbstractElectrochemically induced luminescence (ECL) is an attractive analytical technique, which could be used for many applications. Introduction of reactants and/or removal of formed products both are very important issues in the most ECL-based systems. The introduction/removal of chemicals could be achieved by flow-through cells. Flow-through cells are not efficient in all designs of ECL systems. Therefore, rotating disk electrode (RDE) could be a valuable alternative, which could increase the efficiency of ECL-based devices. In this work, the RDE was used for the evaluation of electroluminescence of tris(2,2′-bipiridin)dichlorruthenium(II)hexahydrate ([Ru(bpy)3]2+), 5-Amino-2,3-dihydro-1,4-phthalazinedione (luminol), and N-(4-aminobuthyl)-N-ethyl-isoluminol (ABEI). Detection limits, optimal pH and potential values, and emission spectra were determined for each compound.Graphical abstract

Photochemical synthesis of CeO2 nanoscale particles using sodium azide as a photoactive material: effects of the annealing temperature and polyvinylpyrrolidone addition

A novel and simple method for CeO2 nanoscale particle synthesis in aqueous solutions via a photochemical route is reported in this paper. To this end, CeCl3·7H2O or Ce(NO3)3·6H2O was used as a Ce precursor, while NaN3 was chosen as the photoactive compound. Synthesis was carried out without any surfactants or by using polyvinylpyrrolidone (PVP). The synthesized samples were subsequently thermally treated at different temperatures between 100 and 900 °C. XRD patterns and Raman spectra indicated that CeO2 samples possess the fluorite structure. TEM analysis revealed that synthesis without surfactants leads to formation of highly agglomerated particles, while adding PVP to the primary solution resulted in decreased agglomeration and reduced particle size. The particle size was calculated from XRD and Raman line broadening and confirmed by TEM analysis. The average crystallite size for the unheated samples prepared without surfactant was hardly radiation exposure dependent and varied from 6.5 to 8.9 nm. Even smaller particles (3.3–7.0 nm) were formed by using PVP. It turned out that an increase of the calcination temperature causes significant crystallite growth. A strong interaction between CeO2 nanoparticles and PVP was revealed by TG analysis. The UV/VIS absorption spectra showed a strong absorption below 400 nm (3.10 eV) with a well-defined absorption peak at around 295–320 nm. The estimated band gap (Eg) of the obtained nanoscale particles was in the range of 2.90–3.57 eV, i.e. the values are higher than that of a bulk CeO2 powder (Eg = 3.19 eV), except for the sample calcined at 900 °C.

Towards the application of Al2O3/ZnO nanolaminates in immunosensors: total internal reflection spectroscopic ellipsometry based evaluation of BSA immobilization

In this research Al2O3/ZnO nanolaminates were evaluated for possible application in the design of optical immunosensors. Total internal reflection ellipsometry (TIRE) was utilized to study the optical response during the formation of a bovine serum albumin (BSA) based monolayer on the surface of Al2O3/ZnO nanolaminates, which were pre-modified with a N-(3-aminopropyl)triethoxysilane (APTES) layer. The influence of the thicknesses of Al2O3 and ZnO layers on the performance of Al2O3/ZnO nanolaminate based structures has been assessed. This research has shown the noticeable contribution of multiple reflections from the interfaces between Al2O3 and ZnO for the enhancement of the optical response in a total internal reflection configuration. Al2O3/ZnO nanolaminates of 200 nm total thickness based on four 50 nm thick alternating Al2O3 and ZnO layers have shown better sensitivity than the nanolaminate based on two 100 nm oxide-layers. Real-time monitoring of the ellipsometric parameters Ψ(λ) and Δ(λ) has shown that BSA was successfully covalently attached to the nanolaminate/APTES surface. The surface concentration of immobilized BSA was evaluated from the real-time data of the ellipsometric parameters Ψ(λ) and Δ(λ). The expected advantages and disadvantages of Al2O3/ZnO nanolaminates during expected application in optical immunosensors are discussed.

Yeast-assisted synthesis of polypyrrole: Quantification and influence on the mechanical properties of the cell wall

In this study, the metabolism of yeast cells (Saccharomyces cerevisiae) was utilized for the synthesis of the conducting polymer - polypyrrole (Ppy).Yeast cells were modified in situ by synthesized Ppy. The Ppy was formed in the cell wall by redox-cycling of [Fe(CN)6]3-/4-, performed by the yeast cells. Fluorescence microscopy, enzymatic digestions, atomic force microscopy and isotope ratio mass spectroscopy were applied to determine both the polymerization reaction itself and the polymer location in yeast cells. Ppy formation resulted in enhanced resistance to lytic enzymes, significant increase of elasticity and alteration of other mechanical cell wall properties evaluated by atomic force microscopy (AFM). The suggested method of polymer synthesis allows the introduction of polypyrrole structures within the cell wall, which is build up from polymers consisting of carbohydrates. This cell wall modification strategy could increase the usefulness of yeast as an alternative energy source in biofuel cells, and in cell based biosensors.

3D opto-structuring of ceramics at nanoscale

Ceramics as advanced materials play an important role in science and technology as they are mechanically robust, can withstand immense heat, are chemically inert. Consequently, there is a direct end-user driven need to find ways for efficiently acquiring free-form 3D ceramic structures. Recently, stereo-lithographic 3D printing of hybrid organic-inorganic photo-polymer and subsequent heating was demonstrated to be capable of providing true 3D ceramic and glass structures. Up to now, this was limited to (sub-)millimeter scale and naturally the next step is to acquire functional glass-/ceramic-like 3D structures in micro-/nano-dimensions. In this paper, we explore a possibility to apply ultrafast 3D laser nanolithography followed by heating to acquire ceramic 3D structures down to micro-/nano-dimension. Laser fabrication is employed for the production of initial 3D structures with varying (ranging within hundreds of nm) feature sizes out of hybrid organic-inorganic material SZ2080. Then, a post-fabrication heating at different temperatures up to 1500 °C in an air atmosphere facilitates metal-organic framework decomposition, which results in the glass-ceramic hybrid material. Additionally, annealing procedure densifies the obtained objects providing an extra route for size control. As we show, this can be applied to bulk and free-form objects. We uncover that the geometric downscaling can reach up to 40%, while the aspect ratio of single features, as well as filling ratio of the whole object, remains the same regardless of volume/surface-area ratio. The structures proved to be qualitatively resistant to dry etching, hinting at significantly increased resiliency. Finally, Raman spectrum and X-ray diffraction (XRD) analysis were performed in order to uncover undergoing chemical processes during heat-treatment in order to determine the composition of material obtained. Revealed physical and chemical properties prove the proposed approach paving a route towards 3D opto-structuring of ceramics at the nanoscale for diverse photonic, microfluidic and biomedical applications.