Multimode scanning probe microscope

Samples for SEM were prepared as mentioned in section “Verification of the presence of active phages on a plastic surface”. First phages were mixed (800 rpm, concentration of T4 phages 4 × 10⁶ PFU/ml, 8 h for Eppendorf-type tubes, and 640 rpm, the concentration of T4 phages 2 × 10⁵ PFU/ml, 5 h for Falcon-type tubes). Afterward, a rectangular plastic piece was cut from Eppendorf-type tubes (E5, E6) or Falcon-type tubes (F1, F2). Atomic force microscopy imaging was performed with a MultiMode Scanning Probe Microscope (Bruker, USA). For SEM analysis, plastic pieces were covered with gold using sputter (EMITECH K550X) (thickness of the gold layer: 9–11 nm). Scanning electron microscopy images were taken with the use of NovaSEM (FEI USA).

NC:NA complexes were assembled under conditions used for EMSA with 1 ng/ $$\mathsf{\mu }$$ L of M13 ssDNA and in a binding solution containing 10 mM TrisAcetate pH 7.0, 50 mM sodium acetate, 2.5 to 5 mM magnesium diacetate, and 0.5 mM TCEP. A freshly cleaved muscovite mica surface was pre-treated for 2 min with a fresh dilution of spermidine (50 $$\mathsf{\mu }$$ M), extensively rinsed with water and dried under a nitrogen flow [96 (link)]. A 5 $$\mathsf{\mu }$$ L drop of the NP complexes was deposited on the surface and incubated for 3–5 min and dried with nitrogen gas. AFM images were carried out in air with a multimode scanning probe microscope (Bruker, USA) operating with a Nanoscope IIIa or V controller (Bruker, USA) and silicon AC160TS cantilevers (Olympus, Japan) using the tapping mode at their resonant frequency. The scan frequency was typically 1.0 Hz per line and the modulation amplitude was a few nanometers. A second-order polynomial function was used to remove the background with the AFM software (Bruker, USA).