The factors contributing to the survival
of enveloped viruses (e.g.,
influenza and SARS-CoV-2) on fomite surfaces are of societal interest.
The bacteriophage Phi6 is an enveloped viral surrogate commonly used
to study viability. To investigate how viability changes during the
evaporation of droplets on polypropylene, we conducted experiments
using a fixed initial Phi6 concentration while systematically varying
the culture concentration and composition (by amendment with 2% fetal
bovine serum (FBS), 0.08 wt % BSA, or 0.5 wt % SDS). The results were
consistent with the well-founded relative humidity (RH) effect on
virus viability; however, the measured viability change was greater
than that previously reported for droplets containing either inorganic
salts or proteins alone, and the protein effects diverged in 1×
Dulbecco’s modified Eagle’s medium (DMEM). We attribute
this discrepancy to changes in virus distribution during droplet evaporation
that arise due to the variable solute drying patterns (i.e., the “coffee-ring”
effect) that are a function of the droplet biochemical composition.
To test this hypothesis, we used surface-enhanced Raman spectroscopy
(SERS) imaging and three types of gold nanoparticles (pH nanoprobe,
positively charged (AuNPs(+)), and negatively charged (AuNPs(−)))
as physical surrogates for Phi6 and determined that lower DMEM concentrations,
as well as lower protein concentrations, suppressed the coffee-ring
effect. This result was observed irrespective of particle surface
charge. The trends in the coffee-ring effect correlate well with the
measured changes in virus infectivity. The correlation suggests that
conditions resulting in more concentrated coffee rings provide protective
effects against inactivation when viruses and proteins aggregate.
