The study of the interaction between biologically active molecules and plasmonic nanoparticles has attracted an increasing interest due to their wide application in biosensors and biomedical diagnostics. In addition to their practical applications in medicine and pharmacology they are also used in fundamental research carried out in studying processes occurred in biological systems. Glutathione is a complexing agent that plays a key role in the functioning of a living cell. This molecule is a tripeptide one having the structure as L-glutamine-L-cysteine-glycine (γ-Glu-Cys-Gly; GSH) and performs intra-and intercellular transport of metal ions in biological systems. Thiol (“reduced”) glutathione and disulphide (“oxidized”) glutathione often act as actuators of molecular nano-switches that activate, regulate or inhibit the activity of enzymes as a response to the presence of different levels of reactive species, such as ROS or electrophilic organic metabolites.
Gold nanoparticles are often used as transducers of nanosized biosensors, since they have the optical and chemical properties necessary for sensory applications and make it possible to form a sensitive layer with the desired functionality on its surface using various molecular recognition species, in particular, glutathione molecule. Due to the presence of thiol groups in this molecule, direct irreversible immobilization of glutathione on the gold surface occurs at the room temperature without any additional activation procedures. Such coatings are in demand in medical preparations. For example, the coating of gold nanoparticles (AuNPs) with glutathione led to a new class of inorganic nanoscale carriers excreted by the kidneys with high resistance to serum protein adsorption, which can deliver drugs to or directly effectively affect the tumour under the influence of external factors (electromagnetic fields).
Silver nanoparticles also have attracted an increasing interest to sensor applications due to different spectral range, typically the narrower plasmon resonance band compared to gold nanoparticles, different surface chemistry and agglomeration behaviour. For example, conjugates of glutathione with silver NPs are characterized by the presence of circular dichroism, which indicates the instability of the attachment of glutathione molecules to the silver surface. Features of the behaviour of glutathione molecules on the surface of silver nanoparticles determined the purpose of this work, which was focused on studying the sequential evolution of glutathione on the surface of silver nanoparticles.
Despite the apparent simplicity of the procedure for modifying silver nanoparticles with glutathione, spectrophotometry and TEM data indicate that the interaction of glutathione molecules with silver nanoparticles is a rather complex chain of chemical transformations. At the initial stage, as in the case of gold nanoparticles, clustering of nanoparticles is observed. However, then, the formed clusters are destroyed with the formation of an insoluble precipitate. In this case, the disappearance of the plasmon resonance band and the appearance of additional absorption are observed that indicates the transformation of silver nanoparticles into silver sulphide. This compound is characterized by extremely low solubility in aqueous media and is not any more a source of silver ions, which are toxic to living cells. So, the use of silver nanoparticles in an environment where glutathione is present reduces the toxic effect of silver nanoparticles themselves. It should be noted that the formation of metal sulphides as detoxification forms of some thiol-binding metals has been recognized in several organisms, from microorganisms and plants to mammals.