Nanotechnology is a multidisciplinary field and has achieved breakthroughs in bioengineering, molecular biology, diagnostics, and therapeutics. A recent advance in nanotechnology is the development of a functional nanosystem by incorporation, adsorption, or covalent coupling of polymers, carbohydrates, endogenous substances/ligands, peptides, proteins, nucleic acids, and polysaccharides to the surface of nanoparticles. Functionalization confers a wide array of interesting properties such as stealth characteristics, a bioadhesive property, and that it prevents aggregation of nanoparticles, imparts biostability and solubility, reduces toxicity, and provides site-specific delivery. This makes the nanosystem an intelligent tool for diagnostics, prognostics, and controlled and sustained delivery of protein, peptide, pDNA, and other therapeutic agents to specific targets (tissue, cell, and intracellular). Various types of functional nanosystems, such as carbon nanotubes, quantum dots, polymeric micelles, dendrimers, metallic nanoparticles, and liposomes, are being extensively explored. However, high tissue accumulation of nonbiodegradable nanoparticles has caused toxicity problems and rendered them as not-so-popular therapeutic and diagnostic systems. The toxicity and safety of nonbiodegradable nanoparticles are subject to future research. Polymeric nanoparticles have offered attractive alternative modules due to biocompatibility, nonimmunogenicity, nontoxicity, biodegradability, simple preparation methods, high physical stability, possibility of sustained drug release, and higher probability for surface functionalization. Depending on properties that have been modified, polymeric nanoparticles can be grouped in to four classes, namely, stealth, polysaccharide decorated biomimetic, bioadhesive, and ligand-anchored functional polymeric nanoparticles (f-PNPs). This review explores the ligand-anchored f-PNP as a carrier for active delivery of bioactives, envisaged to date. This review also details the ligands available for conjugation, their method of coupling to nanoparticles, and applications of f-PNPs in anticancer drug delivery, oral delivery, gene delivery, vaccine delivery, and intracellular delivery; site-specific delivery to liver, macrophages, lymphatics, and brain; and miscellaneous applications. This review also addresses formidable challenges encountered, and proposes some future strategies for development of a promising site-specific active delivery system.
