The mu opioid receptor (MOR) system plays a crucial role in pain relief but is also associated with addiction and respiratory depression. In search for better and safer analgesics, but also better treatments to counter overdoses, scientists are constantly seeking ways to better understand and manipulate this receptor. A recent breakthrough involves using covalent ligands – molecules that can irreversibly bind with the receptor - to create altered (e.g. longer-lasting) effects. These ligands essentially stick to the receptor, prolonging (blockage of) its activity and potentially reducing the need for frequent dosing. By exploiting furan-oxidation triggered crosslinking chemistry, this project aims at the rational design of a series of covalent ligands for MOR, as well as related opioid receptors. More specifically, a furan-containing amino acid will be incorporated into known peptide or protein (nanobody) ligands for MOR, and a furan group will be incorporated into ‘nitazenes’, a highly potent class of MOR agonists, aiming at generating small molecule agonists capable of covalently binding MOR. These novel tools will be assessed for their crosslinking potential via mass spectrometry and will be extensively characterized via cell-based assays. Last, these tools will be used to set up an innovative opioid flow cytometry-based screening strategy in biological matrices. Together, this will increase our knowledge of MOR, which is crucial for the development of better opioids.