Viktória Hornok

Growing and stability of gold nanoparticles and their functionalization by cysteine

Gold nanoparticles in aqueous dispersion were prepared using the trisodium citrate reduction method to control the size of particles by changing the concentration of HAuCl4. The average particle size measured by DLS is higher than that obtained by TEM at a zeta potential of -40 mV. When trisodium citrate concentration is kept constant, the particle size increases with gold concentration. The kinetics of growth was studied and apparent kinetic rate constants were determined at various gold/citrate ratios. Gold nanoparticles were attached to silanized glass surfaces; Au rods were grown (ca. 200 nm) by adding more precursors and the rods’ growth rate was monitored by UV-Vis spectroscopy as well as by AFM. Surface functionalization of gold surface was influenced by cysteine. The surface modification by cysteine at pH=6.0 results in aggregation and the red shift of absorption maximum is nearly 200 nm. When glutathione molecules are bound onto the cysteinelinked Au rods on the glass surface, the spectral shift reaches only an amount of 5–10 nm, because the surface attachment hinders the tendency to aggregate.

Effect of pH on stability and plasmonic properties of cysteine-functionalized silver nanoparticle dispersion.

Citrate-stabilized spherical silver nanoparticles (Ag NPs) with d=8.25±1.25 nm diameter were prepared and functionalized with L-cysteine (Cys) in aqueous dispersion. The nanosilver-cysteine interactions have been investigated by Raman and (1)H NMR spectroscopy. The effect of pH on stability of biofunctionalized Ag NPs was investigated. The cysteine-capped nanosilver dispersions remain stable at higher pH (pH>7), while the degree of aggregation increased as the pH decreased. Below pH ~7, the characteristic surface plasmon band of bare silver nanoparticles was back-shifted from λ(measured)(bareAgNP)=391 nm to λ(measured)(1)=387-391 nm, while the presence of a new band at λ(measured)(2)=550-600 nm was also observed depending on pH. Finite element method (FEM) was applied to numerically compute the absorption spectra of aqueous dispersions containing bare and cysteine-functionalized Ag NPs at different pH. Both the dynamic light scattering (DLS) measurements, Zeta potential values and the transmission electron microscopic (TEM) images confirmed our supposition. Namely, electrostatic interaction arose between the deprotonated carboxylate (COO(-)) and protonated amino groups (NH(3)(+)) of the amino acid resulting in cross-linking network of the Ag NPs between pH ~3 and 7. If the pH is measurable lower than ~3, parallel with the protonation of citrate and L-cysteine molecules the connection of the particles via l-cysteine is partly decomposed resulting in decrease of second plasmon band intensity.

Structural, optical, and adsorption properties of ZnO2/poly(acrylic acid) hybrid thin porous films prepared by ionic strength controlled layer-by-layer method

ZnO(2)/poly(acrylic acid) sandwich structures were prepared by layer-by-layer (LbL) self-assembly. The structure and optical behavior of the hybrid films were controlled by changing the surface charge and conformation of the poly(acrylic acid). The buildup of the films was followed by UV-vis absorption and reflection spectroscopy, atomic force microscopy (AFM), X-ray diffraction (XRD), and quartz crystal microbalance (QCM) measurements. It was found that the ionic strength of the polymer solution had a great influence on the film thickness which, in turn, affected the optical properties. The water vapor adsorption isotherms of the films determined by QCM showed an adsorption hysteresis characteristic of porous thin layer structures. The adsorption of water molecules inside the films changed the effective refractive index resulting in a change of the reflection properties. This phenomenon is shown to be exploited for the application of the films as optical sensors. The polarizability of water molecules in the adsorption layer was also determined. It was found that polarization of water molecules in the adsorption layer is much lower than in the liquid water when the surface coverage (Theta) is low.

Optical and structural properties of protein/gold hybrid bio-nanofilms prepared by layer-by-layer method

Lysozyme/gold thin layers were prepared by layer-by-layer (LbL) self-assembly method. The build-up of the films was followed by UV-vis-absorbance spectra, quartz crystal microbalance (QCM) and surface plasmon resonance (SPR) techniques. The structural property of films was examined by X-ray diffraction (XRD) measurements, while their morphology was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). It was found that gold nanoparticles (NPs) had cubic crystalline structure, the primary particles form aggregates in the thin layer due to the presence of lysozyme molecules. The UV-vis measurements prove change in particle size while the colour of the film changes from wine-red to blue. The layer thickness of films was determined using the above methods and the loose, porous structure of the films explains the difference in the results. The vapour adsorption property of hybrid layers was also studied by QCM using different saturated vapours and ammonia gas. The lysozyme/Au films were most sensitive for ammonia gas among the tested gases/vapours due to the strongest interaction between the functional groups of the protein.

Comprehensive study on the structure of the BSA from extended-to aged form in wide (2–12) pH range

In this work we studied the structure of the bovine serum albumin (BSA) and the protein-ligand interactions since researchers prefer to use them as carriers in drug delivery systems. Systematic study (between pH 2-12, in double distilled water and physiological salt solution) was carried out to determine the changes in the secondary and the tertiary structures of the BSA, the apparent molecular weight (Mw), the size (dLS) and the electrokinetic potential (ζ). At pH 7, the BSA has higher stability in the absence (ζ=-69mV, dLS=2.2nm, A2=1.4×10(-3)mlmol/g(2)) than in the presence of salt solution (ζ=-2.4mV, dLS=5.3nm, A2=-3.2×10(-4)mlmol/g(2)). The Mw strongly depends on the pH and the ionic strength (at pH 3 in the absence of salt, the Mw is 54.6kDa while in the presence of salt is 114kDa) which determines the geometry of the protein. The protein-ligand interactions were characterized by fluorescence (FL) and isothermal microcalorimetry (ITC) methods; these independent techniques provided similar thermodynamic parameters such as the binding constant (K) and the Gibbs free energy (ΔG).

Hydrothermal synthesis and humidity sensing property of ZnO nanostructures and ZnOIn(OH)3 nanocomposites

Prism- and raspberry-like ZnO nanoparticles and ZnO-In(OH)(3) nanocomposites were prepared by template free hydrothermal method. XRD investigations and microscopic studies showed that pill-like In(OH)(3) particles with body-centered cubic crystal structure formed on the surface of ZnO nanoparticles resulting in increased specific surface area. TEM-EDX mapping images demonstrated that not only nanocomposite formation took place in the course of the synthesis, but zinc ions were also built into the crystal lattice of the In(OH)(3). However, only undoped In(OH)(3) was found on the surface of the pill-like particle aggregates by XPS analyses. The raspberry- and prism-like ZnO particles exhibit strong visible emission with a maximum at 585 and 595 nm, respectively, whose intensity significantly increase due to nanocomposite formation. Photoelectric investigations revealed that photocurrent intensity decreased with increasing indium ion concentration during UV light excitation, which was explained by increase in visible fluorescence emission. QCM measurements showed that morphology of ZnO and concentration of In(OH)(3) had an influence on the water vapor sensing properties.

Sensitive detection of aflatoxin B1 molecules on gold SPR chip surface using functionalized gold nanoparticles

Due to the warm and favourably humid climate of Southern Hungary, the maize is one of the most important crops. The protection against crop damage caused by fusarium and Aspergillus species is essential. Detection of aflatoxin B1 (AFB1) molecules in cereal crops by selective sensors is important, while they can cause serious diseases in humans and animals if they enter the food chain. Our main objective was to develop selective AFB1 sensor with increased sensitivity applying βCD-functionalized gold nanoparticles (AuβCD NPs) in surface plasmon resonance (SPR) measuring apparatus. The nanoparticles ca. 10 nm in diameter were prepared in the presence of thiol-modified cyclodextrin. The adsorption isotherms of AFB1 on bare, thiol-modified cyclodextrin and AuβCD NPs covered Au film surface were calculated using SPR platform. The AFB1 concentration can be quantitatively determined in the 0.001–23.68 ng/mL range. The AuβCD NPs were found to be highly sensitive and exhibited a remarkably low limit of detection (LOD; 1 pg/mL) without using other analytical reagents.