# Mussel Inspired Polydopamine as Silica Fibers Coating for Solid-Phase Microextraction
## 4 Discussion
First, the extraction aptitude of the fibers was tested on biphenyl and benzo(a)pyrene species, in relation with dopamine concentration used for producing the fibers, the thickness, and consistency of the active PDA layers (Figure 4), underlining that the extraction efficiencies increased along the dopamine concentration used for their preparation, and consequently the thickest PDA fiber was selected for all the experiments (dopamine 3 mg/mL). PDA fibers using the chosen dopamine concentration were tested using different optimization conditions, such as sample volume, temperature, ionic strength, and extraction time, before validating the analytical method. As previously mentioned, commenting on Figure 5, the volume obviously positively affected the extraction yields (Figure 5A), while the temperature, which influences both kinetics and thermodynamics of the extraction process, had a negative effect. In fact, a temperature increase (from 20 to 50 \({}^{\circ}\)C) produced a decrease in the extraction yields (Figure 5B), and 20 \({}^{\circ}\)C was consequently selected as the working thermal condition for the following experiments. It is likely that, in this case, even if higher temperatures reduce the time needed to reach equilibrium by enhancing the diffusion of the analyte towards the organic phase, they could have altered the distribution coefficients of the analytes between the aqueous phase and the fiber. The influence of salt addition has been also investigated, discovering a different behavior of the fibers for biphenyl and benzo(a)pyrene, respectively. The presence of salts led to a general decrease on the extraction efficiency of biphenyl, and a positive effect in the case of benzo(a)pyrene (Figure 5C). Extraction time profiles were visualized in Figure 5D as graphs obtained by plotting the area counts versus the extraction time, describing specific kinetics with huge slope variabilities vs time for biphenyl with respect to benzo(a)pyrene. SPME is a non-exhaustive process in which analytes are partitioned between the sample and the polymer coating. The equilibrium time refers to the time after which the amount of analyte extracted remains constant and corresponds to the amount extracted after an infinite time, within the limits of the experimental error. Apparently, equilibrium conditions for these molecules were reached after 60 min, and 30 min was chosen for further experiments. After the optimization of the analytical parameters, a complete investigation on the present PDA-based extractive systems in comparison with four commercial SPME fibers (polydimethylsiloxane (PDMS), polydimethylsiloxane/divinylbenzene (PDMS/DVB), polyacrylate (PA), and polyethylene glycol (PEG)) was performed. The PDA coating allowed better extraction efficiencies than commercial fibers for benzo(a)pyrene, while the most efficient extraction of biphenyl was shown by the PA and PDMS/DVB fibers, probably due to the higher thickness of their coatings. Finally, the PDA-based fiber was successfully used for the extraction of selected PAHs from soot samples exploiting the conditions optimized for biphenyl and benzo(a)pyrene. Even if great advantages in terms of extraction performances in comparison with commercial polymers were not obtained, the new fibers proposed here are clearly innovative in manufacture. Coatings of commercial silica fibers for SPME often involve pre-treating processes. For instance, the bulk fused silica should first undergo exposition of silanol groups (via acid oxidative treatments) onto the surface for further functionalization, or the creation of the primary sites using sol gel methods or organic activators [34], in addition to the use of some active organic solvents [35]. On the contrary, the polydopamine coating does not need particular pretreatments of the silica bulk fibers for material processing.