Selenium nanoparticles (Se NPs) have many biomedical applications as both therapeutic agents and delivery systems. The bio-functionality and toxicity of Se NPs are defined by their size, morphology, and surface chemical composition, which are associated with the capping layer on the surface of NPs. Capping agents can modify the characteristics of NPs and make them appropriate candidates for biomedical use. They play a critical role in preventing over-growth and aggregation of NPs, which is one of the major obstacles in NPs preparation. Capping agents stabilize the NPs interface, where is in contact with the surrounding medium and are responsible for the physicochemical and biological properties of NPs. A wide range of compounds with different sources can be used as capping agents for Se NPs synthesis to improve their pharmacokinetics and pharmacodynamics. Each capping agent has its unique characteristics leading to a particular NP formation. In this narrative review, different types of capping agents based on their sources and their effects on physicochemical and biological aspects of Se NPs are discussed.

Guanine-rich DNA and RNA sequences can fold into noncanonical nucleic acid structures called G-quadruplexes (G4s). Since the discovery that these structures may act as scaffolds for the binding of specific ligands, G4s aroused the attention of a growing number of scientists. The versatile roles of G4 structures in viral replication, transcription, and translation suggest direct applications in therapy or diagnostics. G4-interacting molecules (proteins or small molecules) may also affect the balance between latent and lytic phases, and increasing evidence reveals that G4s are implicated in generally suppressing viral processes, such as replication, transcription, translation, or reverse transcription. In this review, we focus on the discovery of G4s in viruses and the role of G4 ligands in the antiviral drug discovery process. After assessing the role of viral G4s, we argue that host G4s participate in immune modulation, viral tumorigenesis, cellular pathways involved in virus maturation, and DNA integration of viral genomes, which can be potentially employed for antiviral therapeutics. Furthermore, we scrutinize the impediments and shortcomings in the process of studying G4 ligands and drug discovery. Finally, some unanswered questions regarding viral G4s are highlighted for prospective future projects. Significance Statement G-quadruplexes (G4s) are noncanonical nucleic acid structures that have gained increasing recognition during the last few decades. First identified as relevant targets in oncology, their importance in virology is now increasingly clear. A number of G-quadruplex ligands are known: viral transcription and replication are the main targets of these ligands. Both viral and cellular G4s may be targeted; this review embraces the different aspects of G-quadruplexes in both host and viral contexts.