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An experimental approach for improving the sensitivity of the surface plasmon resonance (SPR) DNA hybridization sensor using gold nanoparticles (GNPs), modified by specific oligonucleotides, was elaborated. An influence of the ionic strength on the aggregation stability of unmodified GNPs and GNPs modified by the thiolated oligonucleotides was investigated by monitoring a value of light extinction at 520 nm that can be considered as a measure of a quantity of the non-aggregated GNPs. While the unmodified GNPs started to aggregate in 0.2 × saline-sodium citrate (SSC), GNPs modified by the negatively charged oligonucleotides were more stable at increasing ionic strength up to 0.5 × SSC. A bioselective element of the SPR DNA hybridization sensor was formed by immobilization on the gold sensor surface of the thiolated oligonucleotides P2, the sequence of which is a fragment of the rpoB gene of Mycobacterium tuberculosis. The injections into the measuring flow cell of the SPR spectrometer of various concentrations of GNPs modified by the complementary oligonucleotides T2-18m caused the pronounced concentration-dependent sequence-specific sensor responses. The magnitude of the sensor responses was much higher than in the case of the free standing complementary oligonucleotides. According to the obtained experimental data, the usage of GNPs modified by specific oligonucleotides can amplify the sensor response of the SPR DNA hybridization sensor in ~1200 times.

In this study, we applied two stringency control strategies for surface plasmon resonance (SPR) detection of DNA hybridization and discrimination of completely and partially complementary 24-mer sequences. These sequences are specific to the human normal bcr and the hybrid bcr-abl genes, protein products of which are responsible for some leukemia. SPR sensors based on resonance phenomena in nanoscale gold films are well suited for label-free, real-time investigations of the macromolecule interactions. Thermodynamic parameters obtained using the web server DINAMelt allowed supposing the possibility for realization (a) stringency control based on the ionic strength of the hybridization buffer and (b) stringency control based on the temperature elevation. The first one resulted in that the discrimination index of completely complementary and partially complementary oligonucleotides depending on the target concentration varied from 1.3 to 1.8 in 2 × SSC and from 2.0 to 2.9 in 0.5 × SSC. For implementation of the second stringency control strategy, SPR spectrometer measuring flow cell with built-in high-precision temperature control and regulation as well as corresponding software was created. It is shown that the duplexes formed by the immobilized probes mod-Ph and completely complementary oligonucleotides P1 remained without significant changes until ~50 °C, while the duplexes formed with partially complementary oligonucleotide Bcrex14 almost entirely disrupted at 40 °C. Thus, the absolutely effective thermodiscrimination of this pair of oligonucleotides was achieved in this temperature range (40–50 °C).

To investigate the formation of an intermediate layer of the immunosensor bioselective element based on the recombinant protein A from Staphylococcus aureus with cysteine residue (SPA-Cys) and its interactions with human IgG using the SPR spectrometer «Plasmon». Methods. The activity of the immune components applied was tested by ELISA. The spectrometry of surface plasmon resonance was used for studying protein immobilization on a gold sensor surface and interactions between the immobilized SPA-Cys and human immunoglobulin. Results. A direct dependence of the sensor response on the concentration of SPA-Cys in the range of 0.2 to 2 μM at its immobilization was demonstrated. The efficiency of blocking nonspecifi c adsorption sites on the sensor surface with milk proteins and the direct dependence of the sensor response on IgG concentration and surface density of immobilized SPA-Cys were shown. Fitting the experimental data to a Langmuir plot yields a K d value for SPA-Cys/IgG binding 8.5 ± 0.7× 10 -8 M (K a = 1.2 ± 0.1× 10 7 M -1 ). The determined equilibrium binding constant indicates a quite strong interaction and its value is consistent with the literature data. Conclusions. A successful immobilization of SPA-Cys on a gold surface of the SPR spectrometer while preserving its high immunoglobulin-binding activity, selectivity and stability of the sensor response confi rms the effi ciency of SPA-Cys as an intermediate component for the creation of the immunosensor bioselective elements. K e y w o r d s: immunoglobulin, recombinant Staphylococcal protein A, surface plasmon resonance, protein immobilization, immunosensor, equilibrium binding constant.

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Aim.To investigate an influence of the oligonucleotide concentration on their immobilization on the surface of gold nanoparticles (AuNPs), and to study interactions between the AuNPs modified by various oligonucleotides and the oligonucleotides immobilized on the chip of the SPR-based DNA-sensor. Methods. Oligonucleotide immobilization on the surface of AuNPs was investigated by fluorescence spectrometry. The interactions of citrate-stabilized AuNPs modified by oligonucleotides with the oligonucleotides immobilized on the chip of the DNAsensor were studied by the surface plasmon resonance spectrometry. Results. The initial oligonucleotide concentration influences the level of their immobilization on the surface of citratestabilized AuNPs: up to 200 nM the dependence was close to linear, and then saturation was observed at ~ 26 molecules per particle or ~ 0.5×10 13 molecules cm -2 . In contrast, the efficiency of immobilization gradually decreased with an increase in the initial oligonucleotide concentration. Using the SPR-based DNA-sensor, the efficient hybridization between oligonucleotides immobilized on the sensor chip and complementary oligonucleotides of various length (short T2-11m and long T2-18m) immobilized on the surface of AuNPs was demonstrated. In case of AuNPs modified by short oligonucleotides, efficient thermal and chemical regenerations of the bioselective element of the DNA-sensor were achieved. Conclusions. Oligonucleotide immobilization on the surface of AuNPs directly depends on the initial oligonucleotide concentration, whereas the initial oligonucleotide concentration and

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