Aqueous amination of track-etched polycarbonate membranes for tuneable nanochannel surface charge density

Track-etched polycarbonate (PC) membranes with nanochannels are versatile materials for electrochemical, energy-harvesting, and separation applications. Precise control over their surface charge is critical, as it governs ion selectivity, electroosmotic flow, and overall ionic transport behaviour in confined nanochannels. However, environmentally friendly and scalable strategies to precisely tune their surface charge remain limited. Amination is a practical approach for PC membrane functionalisation, as it introduces protonatable amine groups that enhance the positive surface charge and enable further chemical modifications via mild, aqueous reactions. Here, we report a simple aqueous amination method that enables systematic control of surface charge density in PC membranes between 0.0015-0.0034 C cm^-2. Commercial PC membranes with nominal pore sizes of 0.015, 0.05, and 0.1 µm were functionalised with a series of amines, hexamethylenediamine (HMDA), triethylenetetramine (TETA), polyethyleneimine (PEI), and glycine (Gly), through urethane-bond formation with surface carbonyl groups under mild aqueous conditions. Elemental and spectroscopic analyses confirmed efficient functionalisation and tuneable nitrogen content (9.7-22.6 at%), related to variable surface charge density, achieved by varying reaction parameters such as concentration, time, temperature, and amine type. The highest surface charge density of 0.0034 C cm^-2 was achieved using 5% w/v TETA on PC membranes with 0.1 µm diameter. This scalable, low-energy pathway for PC membrane functionalisation is even compatible with ultrasmall pores, down to ∼15 nm. The charge densities achieved through this green aqueous functionalisation are the highest among other surface charge-tuning methods, such as plasma, ultraviolet, or polymer-grafting methods. Aqueous amination-based functionalisation is suitable for fabricating charge-tuneable, ion-selective membranes for nanofluidic energy conversion, electrochemical sensing, and other surface-charge-governed applications.