# Systematic Evaluation of Different Coating Chemistries Used in Thin-Film Microextraction
## 2 Results and Discussion
### pH Effect on the Extraction Performance of Different Coatings  
Table S3 illustrates results corresponding to the extraction performance of the studied coatings at different pH values (3.0, 7.4 and 10.0). As can be seen, different trends for each coating and for each analyte were found at such conditions. First of all, highly polar compounds bearing on their structure carboxylic and amine moieties, e.g., alanine and Ala-Ala, could be extracted by all the coatings under both acidic and basic conditions. For carbon-based coatings, under acidic condition (pH 3.0), protonated amino groups (-NH\({}_{3}\)+) could provide weak cation-\(\pi\)[23] types of interactions with conjugated carbon rings of adsorbents, which might contribute to the adsorption process. For basic conditions (pH 10.0), it is postulated that deprotonated carboxylate groups would strengthen the dipole-dipole interactions between amino acids and conjugated carbon-substrates in comparison to their nonionic counterparts at pH 7.4 [24], thus facilitating the extraction of amino acids onto these  
Figure 2: Comparison of extraction performances of the eight coatings for all analytes at pH 7.4, (**a**) carbon-based coatings, (**b**) solid phase extraction (SPE)-based coatings. Extractions were performed in triplicate for 2 h, and desorption was performed for 1 h using desorption solvent 2.  
adsorbents. Similar results were also observed for extraction of phenylalanine and tryptophan by using carbon-based coatings. For SPE-based coatings, the increase in extraction efficiencies towards polar amino acids under acidic and basic conditions indicated that there were weak ionic interactions between amino acids and the sorbents. It should be considered that the presence of polar moieties in the structures of all SPE sorbents, i.e., C-N and C=O for HLB, N-H and B-OH for PBA, residual Si-OH for C18 and functional polar groups for PS-DVB, might provide hydrophilic or weak ion exchange interactions with ionospheric amino acids in acidic or basic solutions, thus improving the extraction of amino acids onto SPE-based coatings. Another interesting observation was the dramatic increase in amounts of mandelic acid extracted by all coatings when pH was adjusted to 10. Mandelic acid contains a hydroxyl group that is very close to its carboxylic group, thus strong intra-molecular hydrogen bonds are formed between hydroxyl and carboxyl groups [25]. As the pH of the matrix increases, the intra-molecular hydrogen bond force would weaken and completely disappear under strong basic conditions [26], which might improve the hydrogen bonding and \(\pi\)-\(\pi\) electron donor-acceptor type of interactions between mandelic acid and adsorbents, and thus give rise to great improvements in extraction. Thirdly, the significant decrease in extraction efficiencies for riboflavin and aspartate by all coatings at pH 10.0 could be attributed to their instability and rapid degradation [27; 28] under strong basic conditions. Apart from the results discussed in the above section, several differences in extraction performance were observed for several compounds when using carbon-based coatings and SPE-based coatings under various pH conditions. These variations serve as an indication of the different interactions taking place between the studied analytes and the surface of each coating. In carbon-based coatings, graphene, MWCNTs, and MWCNTs-COOH exhibited a similar extraction trend for all model compounds at the studied pHs. This observation supports that these analytes were adsorbed onto the surface of these three coatings through similar binding forces, mainly hydrophobic and \(\pi\)-\(\pi\) types of interactions. Notably, arachidonic acid showed great enhancement of extraction on these coatings at pH 3.0. This indicates that protonated arachidonic acid preferred to be adsorbed onto the hydrophobic surfaces of carbon-based coatings. Improvements in the extraction of some drugs, i.e., atenol, pindolol, and propranolol, were also observed for carbon-based coatings at pH 10.0.