*2.2. Preparation of AuNPs, AuBSA-NPs and AuPEI-NPs*

AuNPs were synthesized similarly to [27]. In brief, the solution of Na3C6H5O7·3H2O (5 mL, 38.8 mM) (Fluka, Charlotte, NC, USA) was added under stirring to the boiled solution of HAuCl4·3H2O (45 mL, 1 mM) (Aurat, Moscow, Russia). The mixture was intensively stirred for 20 min and kept at room temperature for 24 h, and then filtered (pore size of 0.45 μm, MDI, Ambala Cantt, India). Extinction coefficient of resultant suspension was <sup>ε</sup><sup>260</sup> <sup>=</sup> 8.78 <sup>×</sup> <sup>10</sup><sup>8</sup> M–1 cm–1 (Shimadzu, Kyoto, Japan), which corresponds to a concentration of 3.6 <sup>×</sup> 10–9 M of Au [28]. The suspension was stored at 4 ◦C.

AuPEI-NPs were prepared by layer-by-layer approach. Initially, the reaction mixture (695 μL) containing AuNPs (3.6 nM) and 0.72 μM oligodeoxyribonucleotide (ON) was incubated for 30 min at 56 ◦C to prepare non-covalent AuON-NPs serving as an intermediate compound [29]. Oligodeoxyribonucleotide (5- -TTT TTT TTT TTT TTT TTT TTT TTT TT-3- ) was synthesized on an ASM-800 (Biosset, Novosibirsk, Russia) by the solid-phase phosphoroamidite protocol using phosphoramidites from ChemGenes (Wilmington, MA USA). The ON was purified by reversed phase high performance liquid chromatography (HPLC) on an Agilent 1200 Series (Santa Clara, CA, USA) using a Zorbax 5 <sup>μ</sup>m Eclipse-XDB-C18 80 Å column (150 <sup>×</sup> 4.6 mm2) by Agilent (Santa Clara, CA, USA).

The AuON-NPs were washed with 0.5 mL of 4 mM Na3C6H5O7 solution and precipitated by centrifugation for 30 min at 13,200 rpm. The precipitate was diluted with 0.57 mL of 4 mM Na3C6H5O7 solution, and pH of the suspension was adjusted to 10 with 12.5 μL of 1 M Na2HPO4 (AlfaChem Plus, Saint Petersburg, Russia). Solution of 100 μL of 0.8% 11-mercaptoundecanoic acid (MUA) (Sigma-Aldrich, St. Louis, MO, USA) in 10% ethanol (Kemerovo Pharmaceutical factory, Kemerovo, Russia) was added with shaking (1400 rpm) to AuON-NPs, and the mixture was incubated for 30 min at 25 ◦C to obtain AuON-MUA-NPs. The product was washed with 0.6 mL of 1 mM NaCl (Panreac, Barcelona, Spain), and precipitated by centrifugation for 10 min at 13,200 rpm. The precipitate was diluted with 0.25 mL of 1 mM NaCl. The final step was carried out in several tubes (Eppendorf, Hamburg, Germany). The AuON-MUA-NPs (50 μL) were added with shaking (1400 rpm) to 50 μL of 0.1% branched polyethylenimine (PEI) in each tube. The PEI (-NHCH2CH2-)x(-N(CH2CH2NH2)CH2CH2-)y, 10 kDa of molecular weight, was 99% of purity (Alfa Aesar, Ward Hill, MA, USA). The mixture was incubated for 30 min at 25 ◦C. Resulting product was washed with 0.5 mL of 1 mM NaCl, and then precipitated by centrifugation for 10 min at 13,200 rpm. The precipitate was diluted in 50 μL of 1 mM NaCl. The concentration of AuON-MUA-PEI-NPs (further designated as AuPEI-NPs) in the resulting suspension was 10 nM of AuNPs.

AuNPs coated with BSA were obtained by incubation of 250 μL 3 nM AuNPs with 50 μL of 10% BSA (Sigma, St. Louis, MO, USA) for 24 h on a Multi-rotator Multi Bio RS-24 at 10 rpm (Biosan, Riga, Latvia) [30]. The resulting AuBSA-NPs were washed with 1 mL of PBS (Sigma-Aldrich, St. Louis, MO, USA) and separated from the excess BSA by centrifugation for 30 min at 13,000 rpm on a Heraeus Biofuge pico (Thermo Fisher Scientific, Waltham, MA, USA). The AuBSA-NPs precipitate was suspended in PBS and the volume was brought to a concentration of AuNPs 10 nM. The stability of the AuBSA-NPs was confirmed by the absence of color changes when adding an equal volume of 3 M NaCl. The preparation was stored at 4 ◦C.
