CPs, such as poly (3,4-ethylenedioxythiophene) (PEDOT). However, excess PSS diminishes CP conductivity, long-term stability, specific capacity, and biocompatibility (see Supplementary section 1.1 for more details). Electropolymerisation produces CP thin films in a single step, but is limited to deposition onto conducting surfaces and less easily adapted to the synthesis of composite materials. Vapour phase techniques provide another single-step method, capable of producing CP thin films with record-high electrical conductivity, but require high vacuum and/or temperatures, can be complicated or expensive to implement, and incompatible with heat-sensitive substrates. A liquid|liquid (L|L) interface provides a reproducible and defect-free environment to prepare and process free-standing two-dimensional (2D) thin films of nanomaterials, such as CPs, in a single step. By polarising certain L|L interfaces, known as interfaces between two immiscible electrolyte solutions (ITIES), exquisite electrochemical control over the kinetics of interfacial electron transfer (IET) between oxidant and monomer redox couples in opposite phases is achieved. Progress to date in this field, interfacial electrosynthesis at an ITIES, is summarised in Supplementary section 1.2.