In this review, we provide an overview of conjugated organic semiconductors and their applications in biological sensing with a primary focus on the role of the organic semiconductor.

Small-molecule organic semiconductors have displayed remarkable electronic properties with a multitude of π-conjugated structures developed and fine-tuned over recent years to afford highly efficient hole- and electron-transporting materials. Already making a significant impact on organic electronic applications including organic field-effect transistors and solar cells, this class of materials is also now naturally being considered for the emerging field of organic bioelectronics. In efforts aimed at identifying and developing (semi)­conducting materials for bioelectronic applications, particular attention has been placed on materials displaying mixed ionic and electronic conduction to interface efficiently with the inherently ionic biological world. Such mixed conductors are conveniently evaluated using an organic electrochemical transistor, which further presents itself as an ideal bioelectronic device for transducing biological signals into electrical signals. Here, we review recent literature relevant for the design of small-molecule mixed ionic and electronic conductors. We assess important classes of p- and n-type small-molecule semiconductors, consider structural modifications relevant for mixed conduction and for specific interactions with ionic species, and discuss the outlook of small-molecule semiconductors in the context of organic bioelectronics.

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Poly(3,4-ethylenedioxythiophene) (PEDOT) is a well-known semiconducting polymer with favorable properties which find it often applied as the active material in biological sensors and electrochromic devices. However, PEDOT has several drawbacks which can prohibit its effective or long-term use, including weak adhesion to substrates such as ITO-coated glass, poorly controlled surface morphology, and reduced electrochemical stability over time. While a diverse range of approaches have been explored to overcome these issues, most involve additives or substrate modification, while solutions based on direct covalent adaptation are relatively lacking. We present a novel polymer based on covalently modified EDOT (PEDOT-Crown), featuring polar motifs and a 15-crown-5 moiety. Compared to PEDOT, PEDOT-Crown demonstrates a wealth of advantageous properties including: superior adhesion to ITO under physical and electrochemical duress; a more uniform surface morphology; and electrochemical properties including a higher contrast ratio, red-shifted polaron and bipolaron absorption features, bleaching of the neutral absorption band across a narrower voltage range, and more Faradaic rather than capacitive behavior. Additionally, we note that in the presence of Na + , PEDOT-Crown appears to show modified behavior in long-term electrochemical experiments. These features make PEDOT-Crown a material with improved suitability for application in biological sensing and electrochromic devices, compared to PEDOT.

A cancer cell-targeting fluorescent sensor has been developed to image mobile Zn2+ by introducing a biotin group. It shows a highly selective response to Zn2+in vitro, no toxicity in cellulo and images 'mobile' Zn2+ specifically in cancer cells. We believe this probe has the potential to help improve our understanding of the role of Zn2+ in the processes of cancer initiation and development.