I think the main thing is to add more depth and realistic details rather than just using simple vector shapes. For liposomes, you can show the lipid bilayer with some variation in thickness, highlights, and small structural details to make it look more three-dimensional. It also helps to use a few different visual layers for the nanoparticles, cargo, and surrounding environment instead of trying to draw everything with the same style. If you don't have much experience with scientific illustration, you could also look at examples on https://scientific-illustrations.com/ for inspiration on how to present this kind of structure more professionally. For a grant proposal, I'd focus on making the graphic scientifically accurate but still easy to understand at a glance. Sometimes a clean semi-realistic 3D style works much better than trying to make a complex 2D vector illustration.