# Systematic Evaluation of Different Coating Chemistries Used in Thin-Film Microextraction
## 2 Results and Discussion
### Comparison of Extraction Efficiency of Various Coatings at pH 7.4  
Figure 2 presents normalized extraction efficiencies obtained for each analyte with all the studied coatings (the normalization method of extraction efficiencies was seen in Table S2). Overall, SPE-based coatings showed better extraction performance toward most of the selected probes in comparison to carbon-based coatings. No extraction or negligible recovery of Ala-Ala (<0.1%), alanine (<0.1%) and methionine (<0.5%) at physiological pH in all cases evidenced the poor extraction capabilities of the studied coatings toward highly polar small compounds. Among all the extraction phases, the lowest recoveries for most polar probes were observed for carbon-based coatings (Figure 2a). These carbon-based sorbents are characterized for their highly hydrophobic surfaces, and therefore improved extraction efficiencies along with increasing log P values are expected. Additionally, the delocalized \(\pi\)-electron system of such materials allows for \(\pi\)-\(\pi\) interactions, leading to high affinity toward compounds with aromatic rings [16]. Considering these features, effective extraction of compounds such as diazepam, propranolol, testosterone, phenanthrene, and arachidonic acid was expected. However, all coatings prepared with carbon-based sorbents showed normalized extraction recoveries below 7%, and particularly graphene and MWCNTs displayed recoveries under  
Figure 1: Absolute recovery (**a**) and carryover values (**b**) obtained for each model analyte when using a polystyrene-divinylbenzene (PS-DVB) coating and four different desorption solvents. Solvent 1: methanol/acetonitrile/water (25/25/50, _v/v/v_), solvent 2: methanol/acetonitrile/water (40/40/25, _v/v_), solvent 3: methanol/acetonitrile (80/20, _v/v_), solvent 4: acetonitrile/water (80/20, _v/v_). Formic acid (0.1%, _v/v_) was added to all the desorption solvents. Extractions were performed in triplicate at pH 7.4 for 2 h, and desorption time was 1 h.  
3% (Figure 2a). These substantially low values might be possibly attributed to agglomeration or aggregation of graphene sheets or MWCNTs during the coating preparation, which deteriorates their sorptive characteristics and effective elution of extracted analytes, as reported elsewhere [16; 19]. As a practical solution to this issue, covalent binding of graphene and graphene oxide to silica particles was proposed [20]. Although, in this study, graphene-based materials were immobilized in an open-bed geometry using PAN as glue, it would be valuable to further determine the possible effect of aggregation by comparing the performance of the already evaluated graphene and graphene oxide coatings with others prepared using silica-bound graphene. As shown in Figure 2a (dot bar), the graphene oxide coating showed the best performance among all tested carbon-based extraction phases. As a matter of fact, graphene oxide and MWCNTs-COOH own in their structure a high density of oxygen-containing functional groups. These groups add hydrophilicity to such materials, hence favoring the extraction of more polar compounds due to electrostatic and hydrogen-bond interactions [17]. Normalized recoveries observed for riboflavin, for instance, evidence the affinity of graphene oxide and MWCNTs-COOH coatings toward multiple hydroxyl moieties via hydrogen-bond interactions. It is also worth emphasizing that besides improving the affinity of the coating for more polar functionalities, the oxygenated groups on the graphene oxide surface also facilitate their desorption process [16]. Polar moieties on the structure of such adsorbents reduce hydrophobic interactions, which can still be considered as the main contributors to the extraction process. In the case of SPE-based extraction phases, PS-DVB and HLB coatings yielded balanced coverage for hydrophilic and hydrophobic compounds, and similar normalized extraction recoveries for all the tested probes.