## 4 Conclusions  
The three MNPs coated with different crosslinked poly(ionic liquid)s were synthesized. They differed in functional ligands of bonded IL. All of them were imidazolium derivatives. Two of them have additional carboxyl groups, and the third one has a butyl sulfonate group. The polymers-coated MNP were characterized and used in further research on OGN extraction. Adsorption of unmodified OGN on the MNP was possible at a low pH (ion-exchange mode) or with the addition of an appropriate volume of organic solvent (ACN or ACE--hydrophilic interaction mode). Water and water alkalized with ammonia were tested for the desorption of OGNs adsorbed in HI mode. Unfortunately, obtained recovery values were low. Opposite results were obtained when the IE mode was applied. Regardless of MNP, desorption was possible with the use of alkaline water solutions of organic salt with the addition of methanol. However, the effects of salt concentration, pH, and organic solvent percentage were different for different MNP. The studies have shown that various interactions are \begin{table}
\begin{tabular}{c c c c} \hline \hline
**OGN** & **DNA-16** & **DNA-18** & **DNA-20** \\ \hline Concentration range & & 1.25–20.0 uM \\ Calibration curve equation & y = 1.3675x \(-\) 1.2946 & y = 2.0206x \(-\) 1.6838 & y = 1.8008x \(-\) 1.6348 \\ R\({}^{2}\) & 0.9996 & 0.9996 & 0.9995 \\ \hline LOD (uM) & 0.29 & 0.28 & 0.32 \\ LOQ (uM) & 0.95 & 0.92 & 1.08 \\ \hline RSD (\%) & 1.25 uM & 2.90 & 3.59 & 4.93 \\ intra-day & 7.5 uM & 2.12 & 2.40 & 3.59 \\  & 15.0 uM & 1.52 & 1.60 & 1.49 \\ \hline RSD (\%) & 1.25 uM & 7.45 & 6.67 & 7.26 \\ inter-day & 7.5 uM & 5.55 & 5.68 & 4.95 \\  & 15.0 uM & 3.75 & 3.54 & 3.18 \\ \hline Recovery (\%) & & 82.1 \(\pm\) 2.1 & 82.5 \(\pm\) 3.3 & 85.0 \(\pm\) 3.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Calibration curve parameters and validation parameters for determined OGNs. Abbreviations: OGN—oligonucleotide; R\({}^{2}\)—coefficient of determination; LOD—limit of detection; LOQ—limit of quantification; RSD—relative standard deviation. responsible for OGNs adsorption; thus, a diverse desorption mechanism occurs. Based on the performed investigations, the MDSPE procedure for OGN extraction with the use of MNP coated with zwitterionic poly(ionic liquid) was proposed. This procedure uses 10 mM NH4 OAc (pH = 9.5)/MeOH (50/50, \(v/v\)) as the elution solution. The proposed procedure was used in the extraction of different tested analytes. They differed in lengths and chemical modifications. The obtained recoveries were higher than 80% for all unmodified DNA, RNA, phosphorothioate, and 2\({}^{\prime}\)-O-methyl OGNs. The desorption effectiveness was lower for molecules with less polar modifications, such as 2\({}^{\prime}\)-O-(2-methoxyethyl) and locked nucleic acid. In general, the extraction of shorter OGN gives higher recovery. This information is useful considering the potential applications. Finally, the MDSPE procedure was tested in the serum sample preparation. The mixture of OGN and its synthetic metabolites was extracted from fortified serum with over 80% recoveries without any pretreatment or additional purification. Crosslinked poly(ionic liquid)s are promising adsorbents for extraction of OGNs. Coating magnetic nanoparticles with them greatly facilitates the extraction procedure when comparing with D-\(\upmu\)-SPE with pure adsorbents. The following are available online at [[https://www.mdpi.com/article/10](https://www.mdpi.com/article/10)]([https://www.mdpi.com/article/10](https://www.mdpi.com/article/10)) 3390/ma14123146/s1, schemes and detailed synthesis procedures, IR and 1\({}^{\prime}\)H NMR spectra, SEM images (Supplementary Materials Figures S1-S14). L.N.--conceptualization, methodology, formal analysis, investigation, writing--original draft, visualization, and funding acquisition. K.D.--methodology and investigation. Z.R.--methodology and resources. S.S.--conceptualization, methodology, resources, writing--review and editing, supervision, and funding acquisition.