Nanoparticle-assisted surface immobilization of phospholipid liposomes

Liposomes are artificially constructed spherical lipid vesicles with a small size, whose controllable diameter from tens to thousands of nanometers signifies that individual liposomes comprise com-partments with volumes from zeptoliters (10-21 L) to femtoliters (10-15 L) that can be used to encapsulate and store various cargoes, such as proteins, DNA, and drug molecules.1-3 When biomolecules are loaded inside, dynamical behavior such as protein folding-unfolding transitions, enzyme reaction kinetics, and pathways can be observed inside the liposome on the single-molecule level.4,5 Compared to studying individual biomolecules in buffer solution or those immobilized directly to a solid surface, trapping single biomolecules in surface-immobilized liposomes has advantages: (i) a biocompatible environment, (ii) prolonged observation time for each molecule, whose residence time within a laser focus volume would otherwise usually be brief due to its fast diffusion in bul