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Organic Semiconductors Interfaced with Biological Environments

Periodic Reporting for period 2 - OSIRIS (Organic Semiconductors Interfaced with Biological Environments)

Reporting period: 2019-02-01 to 2020-07-31

Transducing information to and from biological environments is essential for bioresearch, neuroscience and healthcare. There has been recent focus on using organic semiconductors to interface the living world, since their structural similarity to bio-macromolecules strongly favours their biological integration. Versatile applications have been demonstrated – sensing, neural stimulation, transduction of brain activity, and photo-stimulation of cells.

However, progress in the organic biosensing and bioelectronics field is limited by poor understanding of the underlying fundamental working principles. Given the complexity of the disordered, hybrid solid-liquid systems of interest, gaining mechanistic knowledge presents a considerable scientific challenge. The objective of OSIRIS is to overcome this challenge with a high-end spectroscopic approach, at present essentially missing from the field.

Via spectroscopy, we target relevant processes with time-resolution, structurally characterize the solid-liquid interface using non-linear optical effects, exploit shifts in the absorption spectra related to interfacial fields, determine nanoscale charge mobility using terahertz spectroscopy and simultaneously measure ionic transport.

Specific objectives are:
1) Investigating how the properties of organic semiconductors are affected upon exposure to aqueous biological environments.
2) Investigating the relevant interfaces between organic semiconductors and aqueous biological environments.
3) Investigating how information is transduced across the interface (ion penetration, optical signals, thermal effects, charge transfer, electric fields).
Our main results and experimental advances are summarized below:

• We have investigated complexes of single-stranded DNA with conjugated polyelectrolytes in solution, showing how the assembly of the two affects the photophysical behaviour of the polymer backbone, and thus establishing structure-property relations for biosensing applications.

• We have investigated the excited-state properties and terahertz (THz) conductivity of doped organic thin films (including self-doped conjugated polyelectrolytes), which are important in organic bioelectronics and in particular for organic electrochemical transistors (OECTs).

• We have developed the capacity to prepare and characterize OECTs for bioelectronic applications.

• We have developed steady-state and time-resolved techniques for in-situ measurements of processes occurring in those OECTs (visible, Raman and THz).

• We have developed a highly sensitive THz detection scheme to carry out the above OECT in-situ measurements in spite of the strong attenuation of the THz radiation by water present in OECTs.

• We have set up a non-linear sum-frequency generation (SFG) experiment to study molecular vibrations selectively at the interface of organic conjugated polymer films with water.
Beyond state-of-the-art results:

• While it has been known for some time that conjugated polyelectrolytes (CPEs) can assemble with biological macromolecules, leading to a strong optical response due to structural changes in the polymer backbone, we managed to gain unprecedented atomistic-level understanding of how single DNA strands impact the conformation of thiophene-based CPEs, allowing to predict which combinations of DNA sequences with specific CPEs yield the largest photophysical changes.

• The electronic structure and origin of the optical transitions in doped organic semiconductors have recently been put into question by theoretical work. Our excited-state study on a self-doped CPE has allowed first direct experimental evidence for this new interpretation, which radically changes the understanding of the optoelectronics in those doped systems.

• We have put together a unique palette of experiments to gain comprehensive insights to the functioning of organic electrochemical transistors (OECTs). In particular, the development and full characterization of a highly sensitive terahertz (THz) detection scheme represent a significant technological advance, that has allowed us to carry out in-situ THz measurements on OECTs, which has not yet been achieved by any other group.

Expected future results:

• We are combining optical and (temperature-dependent) THz studies on a large variety of doped thin films, expecting significant findings on the involved doping mechanisms and the impact of short-range conductivity.

• We expect a full picture of the mechanisms ruling OECT operation, by measuring simultaneously the overall device performance (I-V characterization), structural aspects of the bulk (in-situ Raman) and at the interface (SFG), the ion-induced doping processes (time-resolved optical spectra following gate switching), and the electronic conductivity (THz measurements and transient currents).

• We are using similar measurements on more ion-impermeable thin films, to distinguish between field effects and electrochemical doping in the corresponding transistors.

• We will add photoexcitation to the above techniques, in order to investigate interfacial processes that can lead to cell stimulation in artificial vision applications.
Illustration: Spectroscopic investigation of organic bioelectronic devices