In the last 15 years, the exploitation of biological systems (i.e. plants, bacteria,
mycelial fungi, yeasts, and algae) to produce metal(loid) (Me)-based nanomaterials
has been evaluated as eco-friendly and a cost-effective alternative to the chemical
synthesis processes. Although the biological mechanisms of biogenic Me-nanomaterial
(Bio-Me-nanomaterials) production are not yet completely elucidated, a key advantage
of such bio-nanostructures over those chemically synthesized is related to their natural
thermodynamic stability, with several studies ascribed to the presence of an organic
layer surrounding these Bio-Me-nanostructures. Different macromolecules (e.g. proteins,
peptides, lipids, DNA, and polysaccharides) or secondary metabolites (e.g. flavonoids,
terpenoids, glycosides, organic acids, and alkaloids) naturally produced by organisms
have been indicated as main contributors to the stabilization of Bio-Me-nanostructures.
Nevertheless, the chemical-physical mechanisms behind the ability of these molecules
in providing stability to Bio-Me-nanomaterials are unknown. In this context, transposing
the stabilization theory of chemically synthesized Me-nanomaterials (Ch-Me-nanomaterials)
to biogenic materials can be used towards a better comprehension of macromolecules
and secondary metabolites role as stabilizing agents of Bio-Me-nanomaterials. According
to this theory, nanomaterials are generally featured by high thermodynamic instability
in suspension, due to their high surface area and surface energy. This feature leads
to the necessity to stabilize chemical nanostructures, even during or directly after
their synthesis, through the development of (i) electrostatic, (ii) steric, or (iii)
electrosteric interactions occurring between molecules and nanomaterials in suspension.
Based on these three mechanisms, this review is focused on parallels between the stabilization
of biogenic or chemical nanomaterials, suggesting which chemical-physical mechanisms
may be involved in the natural stability of Bio-Me-nanomaterials. As a result, macromolecules
such as DNA, polyphosphates and proteins may electrostatically interact with Bio-Me-nanomaterials
in suspension through their charged moieties, showing the same properties of counterions
in Ch-Me-nanostructure suspensions. Since several biomolecules (e.g. neutral lipids,
nonionic biosurfactants, polysaccharides, and secondary metabolites) produced by metal(loid)-grown
organisms can develop similar steric hindrance as compared to nonionic amphiphilic
surfactants and block co-polymers generally used to sterically stabilize Ch-Me-nanomaterials.
These biomolecules, most likely, are involved in the development of steric stabilization,
because of their bulky structures. Finally, charged lipids and polysaccharides, ionic
biosurfactants or proteins with amphiphilic properties can exert a dual effect (i.e.
electrostatic and steric repulsion interactions) in the contest of Bio-Me-nanomaterials,
leading to the high degree of stability observed.