Improving the biological interfacing capability of diketopyrrolopyrrole polymers via p-type doping

Polydiketopyrrolopyrrole terthiophene (DPP3T) is an organic semiconducting polymer that has been widely investigated as the active layer within organic electronic devices, such as photovoltaics and bioelectronic sensors. To facilitate interfacing between biological systems and organic semiconductors it is crucial to tune the material properties to support not only cell adhesion, but also proliferation and growth. Herein, we highlight the potential of molecular doping to judiciously modulate the surface properties of DPP3T and investigate the effects on Schwann cell behaviour on the surface. By using p-type dopants FeCl3 and Magic Blue, we successfully alter the topography of DPP3T thin films, which in turn alters cell behaviour of a Schwann cell line on the surfaces of the films over the course of 48 hours. Cell numbers are significantly increased within both DPP3T doped films, as well as cells possessing larger, more spread out morphology indicated by cell size and shape analysis. Furthermore, the viability of the Schwann cells seeded on the surfaces of the films was not significantly lowered. The use of dopants for influencing cell behaviour on semiconducting polymers holds great promise for improving the cell-device interface, potentially allowing better integration of cells and devices at the initial time of introduction to a biological environment.


S2. Experimental procedures
Scheme 1: Synthetic route to monomer 3, used in Stille coupling reaction to form DPP3T.
The organics were extracted with CHCl3 and washed with H2O (2 x 75 mL), brine (75 mL) and dried (MgSO4). After filtration, the organic layer was concentrated under vacuum and purified When cooled, the vial was uncapped the reaction mixture extracted with CHCl3 (20 mL).
Sodium thiodicarbamate was added and the solution heated to 60 °C under N2 (2 h). Deionised water (60 mL) was added and the solution stirred at 60 °C overnight. After cooling to room temperature, the polymer was extracted with CHCl3 (100 mL) and washed with deionised water (3x 100 mL). The organic solvent was concentrated under vacuum and the polymer precipitated into an excess of stirring MeOH. The precipitate was filtered into a glass fibre thimble and the polymer purified via Soxhlet extraction with MeOH, acetone and hexane (24 h, each) before extracting with CHCl3. The polymer was precipitated into stirring MeOH and collected by vacuum filtration to yield DPP3T as a dark blue solid (0.136 g, 66%). Mn = 79 kDa, Figure S1: Proton NMR spectra of compound 1 recorded in DMSO-d6 (500 MHz).

Suppl. 1 UV-vis spectra of DPP3T doped with increasing concentrations of A) iron tosylate hexahydrate, B) anhydrous FeCl3, C) Magic Blue.
Suppl. 1 reports the UV-vis-NIR spectra of DPP3T doped with the 3 p-type dopants employed within this study. Firstly, Iron (III) tosylate hexahydrate was used to attempt to dope DPP3T thin films, however no alteration of the absorbance features was seen, indicating that no electron transfer occurred from the film to the p-type dopants. Secondly, FeCl3, a commonly FeCl3 successfully doped the DPP3T film in air, however the concentration was higher than that usually seen within the literature. FeCl3 is cytotoxic, and therefore once we had seen successful doping, we did not chose to increase the dose of FeCl3 any higher. Lastly, Magic Blue, a less common p-type dopant was utilized to dope DPP3T. Magic Blue required a 100 fold lower concentration to completely saturate the primary absorbance band of neutral DPP3T, and it is this concentration that was taken forward to the cell study.  We then studied the composition of elements on the surface of the neutral and doped films using XPS (Suppl. 2 & 3). In the survey scan, the neutral DPP3T films show peaks associated to oxygen, nitrogen, carbon and sulphur arising from the polymer. FeCl3 doped films showed the same peaks as the neutral films however, peaks for Fe 2p and Cl 2p are also present indicating FeCl3 is on the surface. Magic Blue doped films also tell a similar story, but the new peaks arise from Sb 3d3/2 around 540 eV and Cl 2p. The Sb 3d5/2 overlaps with O 1s.
Interestingly we observed no bromine present in the survey scan of the Magic Blue doped films indicating that the tris(4-bromophenyl)ammoniumyl counter cation is no longer present possibly due to the washing step 3 . We also found that the S 2p and N 1s peaks for the MB and FeCl3 doped DPP3T films shift to higher binding energies by around 2 eV compared with the undoped film, which has been associated to oxidation of the polymer in the literature 4 .

Suppl. 4 AFM images of the DPP3T (A) pristine film surface, (B) doped with FeCl3 and (C) doped
with Magic Blue.    ion 'trapping' of the dopant through counter ion switching would cause any difference in the doping kinetics and longevity of the doped films. We first uncovered that it is still possible to dope DPP3T within this ionic liquid using the same dopants according to the UV-vis-NIR data, and interesting that doping efficiency was altered within the FeCl3 sample in comparison to using neat acetonitrile as the solvent. However, we uncovered that this was not the case, and the thin films de-doped after 48 hours incubation.