Table 2 Summary and comparison of properties of NPs mentioned in this review
Classifications of NPs | NP name | Size (nm) | Materials | Advantages | Disadvantages |
|---|---|---|---|---|---|
Organic NPs | Polymer-based cationic NPs | 100–200 | Polymers, such as polyethylenimine (PEI), polyamidoamine (PAMAM) dendrimers, chitosan, polyethylene glycol (PEG) | PAMAM enhances the loading capacity, protects the cargo from degradation and lessens systemic toxicity. PEG enhances structural stability, electrostatic binding, and hydrophobicity, and can be tuned to specifically meet the cargo delivery requirements. PEI exploits a large positive surface charge in gene transfection | Some evidence of cytotoxicity and side effects due to residual material aggregation in tissues. Elimination routes and in vivo metabolism have not been elucidated |
Inorganic NPs | Nanodiamonds (NDs) | 2–8 | Carbon with truncated octahedral architecture | Low cost, fluorescent capability, low cytotoxicity, and provides long-term stability without causing cell death and oxidative stress | There exist differences and complications in characterizing NDs in dry state and in living organisms. Few in vivo studies done |
Inorganic NPs | Gold NPs | 1–100 | Gold | Excellent chemical stability, good biocompatibility, tunable size, and large specific surface area | More information about uptake, biocompatibility and low cytotoxicity is required for clinical translation |
Inorganic NPs | Graphene oxide NPs | 165 | A single-layer graphene oxide sheet (graphene is a layer of carbon arranged in a 2D crystal structure) | High surface area, mechanical and chemical stability, and biocompatibility. Able to directly penetrate the cell membrane or enter the cell through endocytosis | Some evidence of dose-dependent cytotoxicity and cell apoptosis |
Inorganic NPs | Artificial virus NPs | 130–135 | Virus-like core (composed of plasmid DNA, condensing agent, and functional peptides) and a hydrophilic shell | Improved infection capacity; naturally occurring nanomaterials and hence biocompatible and biodegradable | Further modifications are necessary to improve the delivery efficiency in vivo |
Inorganic NPs | Supramolecular NPs (SMNPs) | 110–127 | PAMAM, PEI, and TAT, assembled through specific non-covalent interactions and molecular recognition properties | Tunable particle size, optimizable surface charge, and enhanced delivery efficiency | More information about uptake, biocompatibility and low cytotoxicity is required for clinical translation |
Inorganic NPs | Lipid-based cationic NPs | 100–200 | A mixture of cationic lipids (such as DOTAP, MVL5) and neutral lipids (such as DOPE, cholesterol) | Low toxicity, biodegradable, able to transport both hydrophobic and hydrophilic molecules | Crystallize after prolonged storage conditions, and poor diffusibility in the negatively charged vitreous due to excessive positive charges |
Other NPs | DNA Nanoclews | 56 | Long-chain ssDNAs with palindromic sequences | Intrinsically biocompatible and degradable | More information about immune- related issues is required for clinical translation |
Other NPs | Nanoscale zeolitic imidazole frameworks (ZIFs) | 100–200 | Made of divalent metal cations and imidazolate bridging ligands | 3D network with a porous structure that facilitates endosomal escape | No known report on this delivery system for the retina yet |