A field-effect transistor (FET) based on ultrathin Ti3C2–MXene micropatterns is developed and utilized as a highly sensitive biosensor. The device is produced with the microcontact printing technique, making use of its unique advantages for easy fabrication. Using the MXene–FET device, label-free probing of small molecules in typical biological environments and fast detection of action potentials in primary neurons is demonstrated. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.