The field of medical diagnostics and therapeutics is being revolutionized by nanotechnology, from targeted drug delivery to cancer immunotherapy. Inorganic nanoparticles are widely used, albeit problems with agglutination, cytotoxicity, free radical generation, and instability in some biological environments limits their utility. Conjugation of biomolecules such as peptides to the surface of nanoparticles can mitigate such problems, as well as confer specialized theranostic (therapeutic and/or diagnostic) properties, useful across biomedical applications such as vaccines, drug delivery, and in vivo imaging. Coating with amino acids, rather than peptides, offers further a highly cost-effective approach (due to their ease of purification and availability), but is currently an underutilized way to decrease toxicity and enhance stability. Amino acid molecules are small (<200 Da) and have both positive and negative charge groups (zwitterionic) facilitating charge-specific molecule binding. Additionally, amino acids exert by themselves some useful biological functions, with antibacterial and viability enhancing properties (for eukaryotic cells). Overall particle size, nanoparticle core, and the specific amino acid used to functionalize their surface influence their biodistribution, and their effects on host immunity. In this review, we provide for the first time an overview of this emerging field, and identify gaps in knowledge for future research.