Olga Kulakovich

DNA-assisted formation of quasi-nanowires from fluorescent CdSe/ZnS nanocrystals

Highly ordered quasi-nanowires from fluorescent semiconductor CdSe/ZnS spherical (quantum dots) or rod-like (quantum rods) nanoparticles were produced using DNA as a template. Positively charged nanoparticles were fixed along the negatively charged DNA backbone by electrostatic interaction. After incubation of the solution of DNA and nanoparticles at different stoichiometric ratios the complexes were applied to the hydrophobic surface and stretched using the molecular combing technique. Here, we demonstrate that fluorescent patterns with desirable morphology and properties can be formed by varying the nanoparticle charge and shape and their stoichiometry in the complex with DNA.

Nonresonant Surface-Enhanced Raman Scattering of ZnO Quantum Dots with Au and Ag Nanoparticles

Pronounced 10(4)-fold enhancement of Raman scattering has been obtained for ZnO nanocrystals on substrates coated with 50 nm Ag nanoparticles under nonresonant excitation with a commercial red-emitting laser. This makes feasible beyond 10(-18) mole detection of ZnO nanocrystals with a commercial setup using a 0.1 mW continuous wave laser and can be purposefully used in analytical applications where conjugated nanocrystals serve as Raman markers. For Au-coated surfaces the enhancement is much lower and the heating effects in the course of Raman experiments are pronounced.

Improved method for fluorophore deposition atop a polyelectrolyte spacer for quantitative study of distance-dependent plasmon-assisted luminescence

A layer-by-layer deposited polyelectrolyte spacer is used to attach CdSe nanocrystals (NCs), dye molecules and fluorescein-labelled bovine serum albumin (BSA-FITC) to flat glass and rough silver island surfaces in order to study the effect of spacer thickness on homogeneity and surface concentration of fluorophore coverage. Three different methods of fluorophore deposition atop the polyelectrolyte spacer are examined using steady-state spectroscopy, fluorescent microscopy and statistical analysis. The best homogeneous covering with nanocrystals at a controllable concentration was found for deposition from a solution of NCs which are electrostatically bound to polyelectrolyte macromolecules. This fluorophore deposition method allows one to avoid artefacts and to evaluate the fluorescence enhancement factor of BSA-FITC adsorbed on silver island films.

Raman scattering enhancement using crystallographic surface of a colloidal crystal

Deposition of coinage metals on a crystallographic surface of a colloidal crystal is proposed with the aim of fabricating metal surfaces with a regular relief on a scale of 200–300 nm to get strong surface-enhanced Raman scattering (SERS). The approach is implemented through thin gold-film deposition on a surface of a crystal consisting of silica globules. Mitoxantrone molecules, a DNA intercalator, were used to prove high SERS efficiency of the structures proposed. As compared to other SERS-active substrates, metal-dielectric colloidal crystal structures possess well-defined surface parameters (globule diameter and film thickness), high stability and reproducibility. These advantages are important for systematic analysis of SERS mechanisms in mesoscopic structures and its application in single-molecule detection.

Quasi-nanowires from fluorescent semiconductor nanocrystals on the surface of oriented DNA molecules

Ordered structures in the form of quasi-nanowires were obtained from CdSe/ZnS fluorescent semiconductor nanoparticles of spherical (quantum dots) or rodlike (quantum rods) form by their electrostatic deposition on DNA molecules with subsequent stretching of the molecules on a solid substrate. Positively charged nanoparticles were fixed along the negatively charged backbones of DNA molecules by electrostatic interactions in an aqueous solution of a mixture of DNA with quantum particles at different stoichiometric ratios. Strands of single DNA molecules with quantum particles fixed along them were immobilized and stretched on hydrophobic surfaces using the molecular combing technique. It is shown that, by varying the nanoparticle charge and the stoichiometry of complexes of DNA with particles, it is possible to create fluorescent structures with predetermined morphology and properties.

Photoluminescence of water-soluble CdSe/ZnS nanoparticles in complexes with cationic and anionic polyelectrolytes

The data on the influence of polyelectrolytes on the photon emission probability of water-soluble CdSe/ZnS nanoparticles are obtained. The decrease in the photoluminescence quantum yield of nanoparticles occurring upon their transfer to aqueous solutions from toluene (in the course of solubilization) depends on the ionic nature of an agent applied for the replacement of trioctylphosphine oxide residues on the surface of nanoparticles. It turns out that such a cationic modifying agent as cysteamine leads to an insignificant (∼10%) decrease in the photoluminescence quantum yield of nanoparticles. The use of such an anionic agent as mercaptoacetic acid causes a significant (∼80%) decrease in the quantum yield and the average decay time of photoluminescence. For nanoparticles modified by mercaptoacetic acid (anionic nanoparticles), this decrease is partially compensated if these particles interact with polyelectrolytes whose backbone is oppositely charged (cationic polyelectrolytes), such as polyallylamine and polydiallyldimethylammonium chloride. In this case, the photoluminescence quantum yield shows a reverse increase by 40%, remaining the same within a matter of months or longer. In contrast to this, cationic nanoparticles, only slightly quenched by cysteamine at the stage of solubilization, are appreciably degraded in complexes with anionic polyelectrolytes in solutions and upon immobilization of complexes on a substrate, so that their photoluminescence quantum yield irreversible decreases to zero within a few days. Possible mechanisms of the effects observed are discussed and their consideration in polyelectrolyte-based molecular lithography.

Photostability of complexes based on fluorescent CdSe/ZnS nanoparticles and polyelectrolytes

By means of fluorescence spectroscopy, it has been found that the oscillator strength of the electronic transition in CdSe/ZnS nanoparticles increases when the hydrophilic fluorescent CdSe/ZnS nanoparticles form complexes with cationic and anionic polyelectrolytes. However, an increase in the relative quantum yield and photostability is possible only in the complexes of the anionic shell-covered CdSe/ZnS nanoparticles with polycations. The mechanisms of the effect of ionic interactions on the probabilities and channels of electronic transitions in the nanoparticles are discussed.

Opal-based photonic crystals as substrates for ultrasensitive molecular spectroscopy

Deposition of coinage metals on a crystallographic surface of a colloidal crystal is proposed with the aim of fabricating metal surfaces with a regular relief on a scale of 200-300 nm to get strong surface enhanced Raman scattering (SERS). The approach is implemented through thin gold-film deposition on a surface of a crystal consisting of silica globules. Mitoxantrone, a DNA intercalator, malachite green and methylene blue molecules were used to prove high Raman and fluorescence enhancement efficiency of the structures proposed. Distance dependence measurements of the mitox secondary emission intensity show a long-range character of enhancement effects. As compared to other SERS-active substrates, metal-dielectric colloidal crystal structures possess well-defined surface parameters (globule diameter and film thickness), high stability and reproducibility. These advantages are important for systematic analysis of SERS mechanisms in mesoscopic structures and its application in single-molecule detection.

Plasmonic sensors for identification of inorganic microcrystals

A method of applying giant stimulated electronic Raman scattering (SERS) by plasmonic gold nanoparticles for identification of inorganic microcrystals in the structure of works of art is presented. The high signal-to-noise ratio in the SERS spectra, along with the low luminescent background, makes the method promising for implementation in practice of technical expertise of objects of cultural heritage.

Surface-Enhanced Fluorescence from Polypropylene Substrates

Surface-enhanced fluorescence of molecular probes is a highly developing field providing a number of new methods in chemical and biomedical analysis. Fluorescence enhancement effect depends on size and shape of the metallic nanoparticles, its optical absorption properties, distance between them and fluorophore (Gaponenko SV, Introduction to nanophotonics. Cambridge University Press, Cambridge, pp 456–467, 2010; Guzatov DV, Vaschenko SV, Stankevich VV, Lunevich AY, Glukhov YF, Gaponenko SV, J Phys Chem C. doi:10.1021/jp301598w). The properties of the substrate also influence on the optical response as the distribution of the nanoparticles depends on the surface properties of the substrate. The surface-enhanced fluorescence was investigated on glass (Guzatov DV, Vaschenko SV, Stankevich VV, Lunevich AY, Glukhov YF, Gaponenko SV, J Phys Chem C, doi: 10.1021/jp301598w), polystyrene 96-well microplates (Nooney RI, Stranik O, McDonagh C, MacCraith BD Langmuir, 10.1021/la801631w), quartz (Lakowicz JR, Malicka J, D’Auria S, Gryczynski I Anal Biochem. 10.1016/S0003-2697(03)00351-8) and modified polycarbonate films (Aslan K, Badugu R, Lakowicz JR, Geddes CD J Fluoresc, 10.1007/s10895-005-2515-5).