Grafting charged polymer (or polyelectrolytes or PEs) chains on a solid substrate in close proximity to one another enforces the PE chains to attain "brush"-like configuration, and more importantly, aided by the environmental-stimuli responsive capabilities of the brushes, impart tremendous functionalities to the substrate on which these brushes are grafted. Such functionalities have been leveraged to use brush-grafted planar surfaces and brush-grafted nanoparticles (NPs) for a myriad of applications such as designing surfaces with desired wettability and protein absorption capabilities, fabrication of brush-functionalized nanopores for sensing, current rectification, and diode action, devising novel strategies for drug delivery, water filtration, and oil recovery using brush-functionalized nanoparticles, etc.