**1. Introduction**

For over 6000 years and prior to the introduction of penicillin in the early 1940s, silver has been the main antimicrobial used by mankind [1]. Few people today are aware that, by 1940 prior to the introduction of penicillin, in the USA alone more than 50 silver-based antimicrobial products had been marketed in different formulations (solutions, ointments, colloids, or foils) for topical, oral, and intramuscular injections [2]. In brief, between 1900 and 1940, tens of thousands of patients were treated with colloidal silver, with several million doses of silver administered intravenously [1]. Since the early 2000s, antibiotic resistance of microbes has been of increasing concern. For both antiviral and antimicrobial

applications, silver ion (Ag<sup>+</sup> )- and silver nanoparticle (AgNP)-based formulations have displayed an advantage in this respect, attacking bacteria and viruses in multiple ways and thereby limiting the chances of both viruses and bacteria to develop resistance to such formulations [2, 3].

This chapter describes the virucidal efficacy of a silver ion formulation, evaluated by using the standardized ASTM International (ASTM) E1052 methodology [4], for inactivating severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and feline calicivirus (FeCV). The chapter also includes a discussion of the proposed mechanism of inactivation, with reference to some previously published articles. Some characteristics of Ag+ - and AGNP-based virucides also are discussed in this research report/minireview.

#### **2. Rediscovery of the antimicrobial potential of silver ion (Ag+ ) and AgNP**

The discovery that the antibacterial activity of AgNP is chiefly due to Ag<sup>+</sup> nonetheless led Xiu et al. [5] to recommend the use of AgNP in antimicrobial formulations because AgNP are less prone than Ag+ to binding and sequestering by naturally occurring ligands. For this reason, it was thought that AgNP might better deliver Ag+ to the bacterial cytoplasm via the acidic cell membrane.

The rediscovery of silver as a powerful and broad-spectrum antimicrobial since the early 2000s has several lessons to teach us. The demonstration of the efficacy of silver, this time in the form of AgNP, against drug-resistant bacteria such as Pseudomonas aeruginosa, ampicillin-resistant Escherichia coli*,* erythromycin-resistant Streptococcus pyogenes, and methicillin-resistant Staphylococcus aureus (MRSA) is encouraging, in view of the continuous increase in multidrug-resistant human pathogenic microbes [6]. Research advances suggesting new medical uses of silver, including nanocrystalline silver, have been rapid and numerous products have been marketed, especially for healing wounds. The rediscovery of the medical uses of silver provides a noticeable example of the interface of chemistry and medicine in enhancing the real (and nonlinear) progress of scientific research.

The use of AgNP for various biological and biomedical applications, such as antibacterial, antifungal, antiviral, anti-inflammatory, anticancer, and antiangiogenic has now been described [7]. Under these circumstances, there have not been many efficacy studies for Ag<sup>+</sup> compared to AgNP. In our development work, we have conducted a series of virucidal tests of Ag<sup>+</sup> formulations against SARS-CoV-2 and FeCV. These are enveloped and non-enveloped viruses, respectively, which are human pathogens or surrogate viruses for human pathogens that continue to impact health. Additional virucidal agents with broad-spectrum efficacy might be useful for infection prevention and control (IPAC) during the present or future viral epidemics/pandemics.
