## 4 Innovation in Surface Disinfection Method
### Antimicrobial Spray Nanocating
#### 4.1.2 Possible Exploitation of Nanocoating to Protect against COVID-19  
In the last decimal, many laboratories and researchers have focused on the development of antiviral nanocoatings. As innovation, they tested nanopowder technology to create a more stable layer and ensure a large surface area compared to the volume ratio: a small number of particles induces an efficient antiviral surface [59; 60; 61]. To visualize the morphology of the nanopowder, observations of the powder with a resolution smaller than 1 nm and high magnification should be made [62]. The morphology of the nanosprayed powder is a spherical shape with a porous surface. The pore size is in the order of nanometers distributed on the spherical surface of the nanosprayed particle powder. The nanoparticles are produced with an irregular form and spherical shape. There is a slight difference  
Figure 1: Schematic representation of the drying nanospray principle.  
between the nanoparticle sizes [63]. To measure the size of the nanosprayed powder, there are different instruments such as dynamic and static light scattering ((DLS) and (SLS)), gravitational settling and centrifugation (GSC), laser-induced breakdown detection (LIBD), and laser particle analyzer (LPA) [58; 64; 65]. Using the old nanospray dryer classic instrument, the dried particle size is in the range of 0.3 \(\upmu\)m to 5 \(\upmu\)m [66; 67; 68; 69]. Indeed, using the new concept mechanical technology method can create powders in the nanometric size range with stable distributions and high formulation yields. This novel spray produces and collects submicron particles from a disinfect solution. The dried particle size is in the range of the 285 nm to 999 nm [70].  
Many different types of nanoparticles have been shown to be effective at inactivating viruses, including coronaviruses such as SARS-CoV-2. For example, nanoscale zinc oxide, cuprous oxide, silver, copper iodide, polymer, titanium oxide, gold on silica, and quaternary ammonium cations (quats) have all shown promise [71; 72; 73; 74; 75]. The choice of the sprayed nanoparticle protocol and methodology is depending on the application field. For example, last semester, researchers from Hong Kong University of Science and Technology developed a nanocapsule formed with heat-sensitive polymers that contain disinfectants [76]. This material is known by multilevel antibacterial polymer MAP-1. It is prepared from polymer encapsulated chlorine dioxide [77]. The polymer releases the biocides at the sites of contamination when warmed by human contact [78]. It was used in public places such as shopping malls, schools, and buses as a surface sterilization solution against COVID-19.  
Another innovative solution used a combination of nanoactive elements formed by positively charged silver nanoparticles sprayed on surfaces to create a low surface energy nanocoating [79]. The stages of the formation of these elements under ambient conditions were widely described in [80]. The mechanism of bacterial inactivation described in Figure 2 is based on the interaction of Ag nanoparticles with the inside and the outside of the cell membrane rich in sulfur containing amino acids to inhibit microorganism activities. Despite that this nanocoating was initially developed to protect against bacteria, fungi, and algae, it was tested to be effective against COVID-19 since February 2020 on many types of materials such as metals and plastics [81]. More than 99% of antiviral efficiency was noticed in 2 h of contact in different places (airports, schools, and markets). However, more tests of performance are in development to extend the use of this method for other materials.