Photo‐Chemical Stimulation of Neurons with Organic Semiconductors

Abstract Recent advances in light‐responsive materials enabled the development of devices that can wirelessly activate tissue with light. Here it is shown that solution‐processed organic heterojunctions can stimulate the activity of primary neurons at low intensities of light via photochemical reactions. The p‐type semiconducting polymer PDCBT and the n‐type semiconducting small molecule ITIC (a non‐fullerene acceptor) are coated on glass supports, forming a p–n junction with high photosensitivity. Patch clamp measurements show that low‐intensity white light is converted into a cue that triggers action potentials in primary cortical neurons. The study shows that neat organic semiconducting p–n bilayers can exchange photogenerated charges with oxygen and other chemical compounds in cell culture conditions. Through several controlled experimental conditions, photo‐capacitive, photo‐thermal, and direct hydrogen peroxide effects on neural function are excluded, with photochemical delivery being the possible mechanism. The profound advantages of low‐intensity photo‐chemical intervention with neuron electrophysiology pave the way for developing wireless light‐based therapy based on emerging organic semiconductors.


Figure
Figure S2: a) Schematic of the bilayer organic solar cell devices used -ITO/NiO/PDBCT/ITIC/Al.b) Current density versus voltage characteristics of the organic solar cells measured with different overall PDCBT-ITC thickness.The maximum short circuit current (2.2.mA/cm 2 ) as well as the maximum open-circuit voltage (0.9 V) is achieved with a total PDCBT-ITIC thickness of 50 nm.

Figure
Figure S3: a) A schematic of the setup used to study the photo-response of the diodes in PBS 1X.b) The optical absorption spectra of the diodes and the wavelengths of LED's used for photo-esxitationand c) the photovoltage response of the system when photoexcited at different wavelengths.In all cases the applied light pulse was 100 seconds long with an intensity of 100 mW/cm 2 .

Figure S5 :
Figure S5: Ultraviolet photoelectron spectroscopy measurements of the polymer semiconductors thin films used in this study, i.e.PDCBT (p-type), ITIC (n-type).

Figure
Figure S6: a) Optical absorption of the PDCBT (red) and ITIC (blue) thin films as cast (solid lines) and after sterilization with 70% ethanol for 1 hour (dashed lines) b) Water contact angle of the PDCBT and ITIC films before and after coating their surface with poly-d-lysine (P-d-L).

Figure S7 :
Figure S7: Madin-Darby Canine Kidney cells (MDCK II) cells cultured on the surface of control glass substrates (top raw), on PDCBT surface (middle raw) and on ITIC surface (bottom raw).The use of a thin adhesion layer (rat tail collagen) improves cell adhesion on the polymer surfaces and the formation of an epithelial layer.Scale bars = 100 μm.

Figure S8 :
Figure S8: Live/dead assay of Human Embryonic Kidney cells (HEK 293) cells cultured on the surface of control glass substrates (top raw), and on PDCBT/ITIC surface (bottom raw).Scale bars = 100 μm.

Figure S9 :
Figure S9: Photovoltage measurements of PDCBT/ITIC right after fabrication (grey line), after ethanol sterilization (dark grey line) and after surface modification with P-d-L.All samples were immersed in PBS 1X and illuminated with white light at 40 mW/cm 2 .

Figure S10 :
Figure S10: Cumulative distribution of frequency of action potentials in neurons cultured on top of glass coverslips (control, gray line, inset) and PDCBT-ITIC-P-d-L coated glass coverslips (blue line) after light exposure.

Figure S11 :
Figure S11: Voltage-time course traces, recorded from an individual neuron cultured on top of PDCBT-ITIC-P-d-L coated glass cover slips several minutes after light exposure.

Figure S12 :
Figure S12: Membrane potential recordings of a mouse cortical neuron cultured on top of glass -PDCBT-ITIC-P-d-L coverslips and exposed to white light for several second.Action potentials are induced with significantly smaller initial depolarization of membrane potential.

Figure S13 :
Figure S13: Representative voltage-time course traces, recorded from the same individual neuron cultured on glass -P-d-L substrates before, and after exposure in H2O2.

Figure S14 :
Figure S14: Representative voltage-time course traces, recorded from the same individual neuron cultured on half-coated glass -PDCBT-ITIC-P-d-L substrate before and after light irradiation.

Figure S15 :
Figure S15: H-NMR spectra of ITIC as fabricated (top), after being exposed for 2 minutes in 40 mW/cm 2 white light irradiation in cell culture media (middle), and after being exposed in 500 um H2O2 added in cell culture media (bottom).

Figure S16 :
Figure S16: H-NMR spectra of PDCBT as fabricated (top), after being exposed for 2 minutes in 40 mW/cm 2 white light irradiation in cell culture media (middle), and after being exposed in 500 um H2O2 added in cell culture media (bottom).

Figure S17 :
Figure S17: Representative voltage-time course traces, recorded from the same individual neuron cultured on fully coated PDCBT-ITIC-P-d-L glass cover slips in normal cell media containing 10 mM of HEPES (left), in cell media containing low concentration of HEPES (1 mM -middle) and again in normal media containing 10 mM of HEPES after being exposed in several consecutive light pulses for several seconds.

Figure
Figure S18: a) Representative membrane potential traces of a mouse cortical neuron cultured on glass cover slip half-coated with PDCBT-ITIC-P-d-L, in cell culture media with low HEPES concentration (1 mM) and in normal cell media containing 10 mM of HEPES.Action potential generation is induced only after light explosure in cell media containing 10 mM of HEPES.