As is well-known, the extraction capacity of adsorbents largely depends on the amount of effective action sites for analytes. To some extent, the thickness of PDA coating can be controlled by tuning the concentration of DA monomers and the polymerization time [31]. For obtaining higher extraction efficiency, different polymerization times ranging from 3–24 h were tested (Fig. 4a), further prolonging the reaction time was not investigated for it does not help to increase the thickness of the coating [40]. The experimental results (Fig. 4a) show that the extraction recovery for analytes reached a maximum when the polymerizaton time was 12 h; increasing the time over 12 h brought about slight decrease, probably due to lowered mass-transfer kinetics. Therefore, 12 h of polymerization was chosen for following studies.