Anna Fucikova

Brightly Luminescent Organically Capped Silicon Nanocrystals Fabricated at Room Temperature and Atmospheric Pressure

Silicon nanocrystals are an extensively studied light-emitting material due to their inherent biocompatibility and compatibility with silicon-based technology. Although they might seem to fall behind their rival, namely, direct band gap based semiconductor nanocrystals, when it comes to the emission of light, room for improvement still lies in the exploitation of various surface passivations. In this paper, we report on an original way, taking place at room temperature and ambient pressure, to replace the silicon oxide shell of luminescent Si nanocrystals with capping involving organic residues. The modification of surface passivation is evidenced by both Fourier transform infrared spectroscopy and nuclear magnetic resonance measurements. In addition, single-nanocrystal spectroscopy reveals the occurrence of a systematic fine structure in the emission single spectra, which is connected with an intrinsic property of small nanocrystals since a very similar structure has recently been observed in specially passivated semiconductor CdZnSe nanoparticles. The organic capping also dramatically changes optical properties of Si nanocrystals (resulting ensemble photoluminescence quantum efficiency 20%, does not deteriorate, radiative lifetime 10 ns at 550 nm at room temperature). Optically clear colloidal dispersion of these nanocrystals thus exhibits properties fully comparable with direct band gap semiconductor nanoparticles.

On the origin of the fast photoluminescence band in small silicon nanoparticles

Colloidal suspensions of small silicon nanoparticles (diameter around 2nm) with fast and efficient ultraviolet-blue photoluminescence (PL) band are fabricated by enhanced electrochemical etching of Si wafers. The detailed study of photoluminescence excitation spectra in a wide range of excitation photon energies (270-420nm) reveals specific behavior of the Stokes shift of the fast PL band that agrees well with theoretical calculation of optical transitions in small silicon nanocrystals and is distinct from emission of silicon dioxide defects.

Microscopic Origin of the Fast Blue‐Green Luminescence of Chemically Synthesized Non‐oxidized Silicon Quantum Dots

The microscopic origin of the bright nanosecond blue-green photoluminescence (PL), frequently reported for synthesized organically terminated Si quantum dots (Si-QDs), has not been fully resolved, hampering potential applications of this interesting material. Here a comprehensive study of the PL from alkyl-terminated Si-QDs of 2-3 nm size, prepared by wet chemical synthesis is reported. Results obtained on the ensemble and those from the single nano-object level are compared, and they provide conclusive evidence that efficient and tunable emission arises due to radiative recombination of electron-hole pairs confined in the Si-QDs. This understanding paves the way towards applications of chemical synthesis for the development of Si-QDs with tunable sizes and bandgaps.

Photoluminescence quantum yield of PbS nanocrystals in colloidal suspensions

The absolute photoluminescence (PL) quantum yield (QY) of oleic acid-capped colloidal PbS quantum dots (QDs) in toluene is thoroughly investigated as function of QD size, concentration, excitation photon energy, and conditions of storage. We observed anomalous decrease of QY with decreasing concentration for highly diluted suspensions. The ligand desorption and QD-oxidation are demonstrated to be responsible for this phenomenon. Excess of oleic acid in suspensions makes the QY values concentration-independent over the entire reabsorption-free range. The PL emission is shown to be dominated by surface-related recombinations with some contribution from QD-core transitions. We demonstrate that QD colloidal suspension stability improves with increasing the concentration and size of PbS QDs.

Novel use of silicon nanocrystals and nanodiamonds in biology

The presented work is aimed at the development of nontoxic nanocrystalline silicon fluorescence labels, biodegradable in living body and long-term stable, and of fluorescent nanodiamonds mainly for in vitro use. These novel fluorescence labels could be very good substitutes for commercially used quantum dots (e.g. cadmium compound quantum dots) which can be toxic according to the latest results. In this work, manufacturing of porous nanocrystalline silicon (por-Si) is described, several basic optical properties of por-Si are presented and the influence of Si nanocrystals, nanodiamonds, and milled silicon on the growth of a cell culture of L929 mouse fibroblast and HeLa cells is compared. Bio-interaction of nanoparticles was studied by optical transmission microscopy, time-lapse microphotography of cell culture evolution, fluorescence microscopy, fluorescence microspectroscopy, and scanning electron microscopy. The size and shape of nanocrystals were determined using atomic force microscopy (AFM).

The impact of doped silicon quantum dots on human osteoblasts

Silicon (Si) nanostructures allow for the expansion of the application spectrum of this important semiconductor material with respect to the fields of optoelectronics and photonics. At the same time, the significant potential of Si quantum dots (SiQDs) has been revealed in terms of their potential application in the areas of biology and medicine due to their biocompatibility, low toxicity and natural biodegradability, unlike currently used semiconductor quantum dots. As far as this study is concerned, SiQDs co-doped with boron and phosphorus were used for the in vitro evaluation of their cytotoxicity in human osteoblasts. Two chemically identical types of SiQD differing in terms of their size and photoluminescence (PL) were studied. They both display long-lasting dispersion in methanol and even in aqueous media as well as PL which is not sensitive either to changes in the environment or surface modifications. Our experiments revealed significant differences between the two types of SiQD tested in regard to their behavior in a cell culture environment depending on increasing concentration (25–125 μg ml−1) and cultivation conditions (the presence or absence of proteins from the fetal bovine serum – a component of the cultivation medium). A detailed description of their optical parameters and the evaluation of zeta potential enhance the understanding of the complexities of the in vitro results obtained.

Silicon nanocrystals and nanodiamonds in live cells: photoluminescence characteristics, cytotoxicity and interaction with cell cytoskeleton

The number of newly developed nanomaterials is steadily increasing but only a few are suitable for applications in biology. These nanomaterials are often made from harmful compounds and there is no convenient technique to remove them from the body after application. This study is focused on silicon nanocrystals and nanodiamonds, which are two promising nanomaterials for bio-applications. Silicon nanocrystals (Si-NCs) are exceptional since they provide several desired properties: low cytotoxicity, suitability for chemical activation, efficient photoluminescence, and bio-degradability. All these parameters promote application of Si-NCs in living organisms and even human medicine. Nanodiamonds (NDs), on the other hand, are non-biodegradable which limits their use to mostly long term in vitro studies. However, when using these nanomaterials one needs to address the effect of accumulation and aggregation of such materials in cells and how it contributes to the overall cytotoxicity. Here, we present studies on the interaction of two promising nanomaterials Si-NCs and NDs with actin structure of mammalian cells, evaluation of their cytotoxicity by various methods, and observation of single nanoparticle luminescence spectra within living cells. According to our results Si-NCs are more promising for application in nanomedicine compared to NDs.

Single-dot spectroscopy of boron and phosphorus codoped silicon quantum dots

Boron (B) and phosphorous (P) codoped silicon quantum dots (Si QDs) are dispersible in polar solvents without organic ligands, and exhibit size controllable photoluminescence (PL) from 0.85 to 1.85 eV due to the electronic transitions between the donor and the acceptor states. We study the PL spectra of the codoped Si QDs at room temperature and at 77 K. We show that the broad PL band of codoped colloidal Si QDs (full width at half maximum is over 400 meV) is composed of narrower PL bands of individual QDs with different PL energies. We also show that the PL linewidth of individual codoped Si QDs is almost twice as large as those of undoped Si QDs. In contrast to the significant narrowing of the PL linewidth of undoped Si QDs at low temperatures, that of codoped Si QDs is almost independent of the temperature except for a few very small QDs. These results suggest that a large number of B and P are doped in a QD and there are a number of non-identical luminescence centers in each QD.

Modification of C Terminus Provides New Insights into the Mechanism of α-Synuclein Aggregation

Aggregation of neuronal protein α-synuclein leads to the formation of amyloid fibrils, which are associated with the development of Parkinsons disease. The mechanism of α-synuclein pathology is not fully understood and is a subject of active research in the field. To tackle this problem, the fusions of fluorescent proteins to α-synuclein C-terminus are often used in cellular and animal studies. The effects induced by such α-synuclein sequence extension on α-synuclein aggregation propensity are, however, not systematically examined despite the evidence that the negatively charged C-terminus plays a critical role in the regulation of α-synuclein aggregation. In this work, we investigated how the charge and length variations of the C-terminus affect the aggregation propensity of α-synuclein. To address these questions, we prepared mutants of α-synuclein carrying additional moieties of different charge and length at the protein C-terminus. We determined the rates of two different aggregation stages (primary nucleation and elongation) based on a thioflavin T kinetic assay. We observed that all mutants bearing neutrally charged moieties of different length fibrilized slower than wild-type α-synuclein. The primary nucleation and elongation rates strongly decreased with increase of the C-terminal extension length. Meanwhile, charge variation of the C-terminus significantly changed the rate of α-synuclein nucleation, but did not markedly affect the rate of fibril elongation. Our data demonstrate that both the charge and length of the C-terminus play an important role at the stage of initial fibril formation, but the stage of fibril elongation is affected mainly by the length of C-terminal extension. In addition, our results suggest that there are at least two steps of incorporation of α-synuclein monomers into the amyloid fibril: namely, the initial monomer binding to the fibril end (charge-dependent, relatively fast), and the subsequent conformational change of the protein (charge-independent, relatively slow, and thus the rate-limiting step).

Influence of non-thermal plasma on structural and electrical properties of globular and nanostructured conductive polymer polypyrrole in water suspension

Non-thermal plasma has proved its benefits in medicine, plasma assisted polymerization, food industry and many other fields. Even though, the ability of non-thermal plasma to modify surface properties of various materials is generally known, only limited attention has been given to exploitations of this treatment on conductive polymers. Here, we show study of non-thermal plasma treatment on properties of globular and nanostructured polypyrrole in the distilled water. We observe that plasma presence over the suspension level doesn’t change morphology of the polymer (shape), but significantly influences its elemental composition and physical properties. After 60 min of treatment, the relative concentration of chloride counter ions decreased approximately 3 and 4 times for nanostructured and globular form, respectively and concentration of oxygen increased approximately 3 times for both forms. Simultaneously, conductivity decrease (14 times for globular and 2 times for nanostructured one) and changes in zeta potential characteristics of both samples were observed. The modification evolution was dominated by multi-exponential function with time constants having values approximately 1 and 10 min for both samples. It is expected that these time constants are related to two modification processes connected to direct presence of the spark and to long-lived species generated by the plasma.