*3.4. Atomic Force Microscopy*

The morphology, dispersity, and relative size of the DOC micelles were investigated by AFM. As shown in Figure 3a, the D-17 sample contains spherical structures with a diameter of 30.5 ± 5.2 nm. The average height of 1.4 ± 0.6 nm for these particles corresponds to those obtained for nucleic acid samples [16,22,23,43]. The average diameter value is in line with our DLS results and correlates with the results of some previous AFM studies of the LOC-formed micellar structures [16,22].

**Figure 3.** Characterization of D-17 (**a**,**b**) and D-17PG (**c**,**d**) micelles in TAM buffer by atomic force (AFM) (**a**) and transmission electron (TEM) (**b**–**d**), contrasting using uranyl acetate) microscopies. Scale bars are indicated. Each sample contained 1.5 μM (AFM) or 5 μM (TEM) of the corresponding oligomer.

TEM visualization of the D-17 conjugate revealed discrete spherical particles and aggregates (Figure S17) with a size of about 30 nm (Figure 3b). The aggregates represent clusters of individual particles (<10) (Figure S17f). D-17PG particles varied in size from 25 to 50 nm (Figure 3c) and contained more aggregates than the D-17 sample (Figure 3d, Figure S18). This result can also be partly associated with the influence of two uncharged phosphoryl guanidine groups in the sugar-phosphate backbone of D-17PG conjugate, probably facilitating particle aggregation.

In concordance with DLS results, AFM and TEM data indicated that the self-association of DOCs in aqueous solutions leads to the formation of micellar particles with a high tendency to aggregate.
