Nanometer-sized liposomes decorated with macromolecules are increasingly used as drug delivery vehicles due to their long lifetimes and target cell specificity, but surface characterization methods often change their properties, which leads to incorrect results. Ligand binding is commonly applied for characterizing these surface modifications. Here, we use a nanofluidic-based label-free sensor for real-time sensing of ligands binding to liposomes. The liposomes are trapped in a nanochannel with a salt concentration gradient, and as the trapping position depends on the liposomes' zeta potential, it changes when charged ligands bind to the liposomes. Our sensing method does not require immobilization of the liposomes or labeling of the ligands with fluorophores, which may both affect the sensing. The zeta potential sensing is demonstrated by measuring hybridization of DNA targets with complementary DNA probes on liposome surfaces. DNA hybridization is monitored for both ensembles and individual liposomes, the latter allows for analysis of ensemble heterogeneity, and we demonstrate sensitivity to changes in surface charge down to 1.5%. DNA hybridization is used to demonstrate label-free sensing, but the method also has potential applications within exosome characterization, where biorecognition of, e.g., surface DNA, proteins, and antibodies is a promising candidate for early stage cancer diagnostics.
Keywords: diffusiophoresis; drug delivery; immobilization-free; label-free; mimetic membranes; nanofluidic sensor; zeta potential.