Wyniki wyszukiwania - BazTech

1

Calcium phosphate nanoparticles for gene transfer and silencing - a 2D and 3D cell culture model study

Bialas N., Sanchez L. R., Sokolova V., Epple M.

Engineering of Biomaterials

|

2017

|

Vol. 20, no. 143 spec. iss.

34

2

On the nature of silver ions in complex media

Loza K., Diendorf J., Sengstock C., Koller M., Epple M.

Engineering of Biomaterials

|

2014

|

Vol. 17, no. 128-129

80

EN

Antimicrobial biocides are commonly used to present the growth of bacteria on surfaces and within materials. They are typically added in small quantities to many applications to prevent bacterial growth on the treated object. Silver is increasingly used in many applications due to the aim to replace organic chemical agents by inorganic additives. Examples of applications are bacteriostatic water filters for household use or swimming pool algaecides and numerous devices, ranging from consumer commodities like mobile phones, refrigerators, and clothes to medical devices like catheters, implant surfaces, and plasters. To meet the diversity of application types, many different forms of silver compounds have been developed to serve this market. In particular, there is little information on the types of transformations that silver nanoparticles will undoubtedly undergo in real, complex environments during long-term aging, and the impact of these transformations on their distribution in the environment, bioavailability, and toxicity potential. The biocidal action results from the interaction of silver ions with bacteria. The most potent compounds for a high silver release are soluble silver salts like silver nitrate or silver acetate. These are fully water soluble with a high silver ion release rate. Therefore they are often used as control in cell experiments to elucidate the biological effect of silver nanoparticles. However, in the case of free silver nanoparticles the interactions can be more complex and catalytic reactions on the particle surface which depend on the size and shape of the nanoparticles can render the system very complex. If AgNO3 is used as control, it is tacitly assumed, that the free silver ion concentration is the same as that in the added AgNO3. This obviously cannot be true because of the presence of a whole set of proteins, biomolecules and inorganic ions like Cl- and H2PO4- in the biological medium. These will react with the silver ions in one or the other way. We report on experiments on the behaviour of silver ions in biologically relevant concentrations in different media, from physiological salt solution over phosphate-buffered saline solution to cell culture media. For dissolution and immersion experiments PVP-coated silver nanoparticles were synthesized by reduction with glucose in the presence of PVP. The final silver concentration in all dispersions was determined by atomic absorption spectroscopy. The dissolution of silver nanoparticles was followed in long-term experiments out of a dialysis tube which was permeable only for silver ions. In case of immersion experiments, the nanoparticles and all precipitates were isolated by ultracentrifugation, redispersed in pure water and again subjected to ultracentrifugation. The particles were analyzed by scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray powder diffraction. The dissolution requires the presence of dissolved oxygen. If no oxygen is present, only a very small fraction of silver is dissolved, possibly by traces of oxygen in the experimental setup. An oxidizing agent like H2O2 clearly enhances the dissolution. The presence of NaCl, either in pure form or as PBS, strongly slows down the dissolution, probably due to silver chloride formation. Cysteine has a clearly inhibiting effect with almost no dissolution of the silver nanoparticles whereas glucose has a decelerating effect but leads to a similar final dissolved fraction. This suggests that cysteine adsorbs onto the silver nanoparticle surface with its thiol group and prevents the oxidation. In contrast, glucose slows down the dissolution, but clearly did not prevent the oxidation on a longer time scale. We have extended the studies by mixing silver nanoparticle dispersions with different media of increasingly biological nature. The solutions/dispersions were stirred for equilibration and then subjected to ultracentrifugation. All precipitates and nanoparticles were isolated by this way and then analyzed. The results show that both initially present silver ions and released silver ions are mainly precipitated as AgCl if chloride is present. Only in the absence of chloride, glucose is able to reduce Ag+ to Ag0. The initially present silver nanoparticles were recovered in all cases. Silver phosphate was not observed in any case, probably due to the moderate pH (around 7) at which phosphate is mostly protonated to hydrogen phosphate and dihydrogen phosphate. We can conclude that released silver ions precipitate mostly as AgCl in biological media, and that most cell culture studies where silver ions are used as control are in fact studying the effect of colloidal silver chloride on the cells. To prove this assumption, human mesenchymal stem cells (hMSC) were cultured in the presence of silver chloride nanoparticles (diameter 120 nm), and the viability of the cells was analyzed by fluorescence microscopy. In general, we clearly observed that pure silver nanoparticles have lower toxicity to hMSC compared to silver chloride nanoparticles with a comparable total silver dose. Silver acetate in the biological medium had a comparable toxicity to hMSC compared to silver chloride nanoparticles with the same total silver dose.

3

Microstructure and properties of alloyed silver-gold nanoparticles

Prymak O., Ristig S., Meyer-Zaika W., Raabe D., Epple M.

Engineering of Biomaterials

|

2014

|

Vol. 17, no. 128-129

77

EN

Alloyed silver-gold nanoparticles recently raised an interest in biomedicine as potential antibacterial and surface-functionalized agents for imaging, drugdelivery, and tumor thermo-therapy [1,2]. The here synthesized alloyed AgAu nanoparticles with different compositions of silver and gold, as determined by atomic absorption spectroscopy (AAS), were prepared by reduction with citrate and tannic acid in aqueous media and subsequently functionalized by the addition of polyvinylpyrrolidone (PVP) [3]. UV spectroscopy confirmed that the particles consisted of alloyed Ag:Au and are not of a separate core-shell structure. The resulting nanoparticles were monodisperse and had a uniform size of ~6 nm, except pure Ag and Ag:Au-90:10, as shown by differential centrifugal sedimentation (DCS) and transmission electron microscopy (TEM). By means of X-ray powder diffraction (XRD) and use of Rietveld refinement [4], the precise lattice parameters, crystallite size and microstrain were determined. Based on the results by XRD, DCS and TEM it was shown, that the nanoparticles were not twinned, except pure Ag and Ag:Au-90:10. Additionally, a distinct deviation from Vegard’s linear rule of alloy mixtures for the lattice parameter was found for the nanoparticles. This effect was also found for AgAu bulk materials, but was much more pronounced in the nanostate. Further investigations of the crystal structure of the alloyed nanoparticles by means of synchrotron radiation might be helpful to gain more information about the interactions of silver and gold atoms.

4

Calcium phosphate nanoparticles for delivering synthetic drug molecules across the cell membrane

Rotan O., Sokolova V., Schrader T., Epple M.

Engineering of Biomaterials

|

2014

|

Vol. 17, no. 128-129

77--78

5

Correlation of composition and structure of shark teeth

Prymak O., Enax J., Fabritius H., Raabe D., Epple M.

Engineering of Biomaterials

|

2013

|

Vol. 16, no. 122-123 spec. iss.

68

EN

In contrast to mammalian teeth with the biomineral phase hydroxyapatite, the shark teeth contain harder mineral phase fluoroapatite with partial substitutions of phosphate by carbonate and of fluoride by hydroxide [1]. Their excellent mechanical properties are due to a special hierarchical structure of the constituting fluoroapatite crystals and organic matrix [2]. The two main structural elements of teeth, i.e. hard and mineral-rich enameloid on the outside and softer and less mineralized dentin on the inside, were structurally, chemically and mechanically characterized [3]. The teeth of two different shark species mako shark (Isurus oxyrinchus) and tiger shark (Galeocerdo cuvier) were investigated and their hierarchical structure by high-resolution scanning electron microscopy presented (Fig.1). X-ray diffraction showed that the inorganic matrix of both enameloid and dentin consisted of fluoroapatite, with a high crystalline phase in enameloid and nanocrystalline phase in dentin. FTIR-spectra of the shark teeth showed the characteristic bands of biological apatite. It was found by thermogravimetry that dentin had a higher content of water, organic matrix and carbonate than enameloid. To investigate the mechanical properties of the teeth in longitudinal and cross sections, nanoindentation and Vickers microhardness were carried out. Both methods gave comparable results: the enameloid of both shark teeth was approximately six times harder than the dentin with an isotropic hardness (longitudinal or cross section).