  The interaction between metallic and biological nanoparticles (NPs) is widely
used in various biotechnology and biomedical applications. However, detailed
characterization of this type of interaction is challenging due to a lack of
high-throughput techniques that can quantify both size and composition of
suspended NP complexes. Here, we introduce a technique called ``twilight
nanoparticle tracking analysis'' (tNTA) and demonstrate that it provides a
quantitative relationship between the measured optical signal and the
composition of suspended dielectric-metal NP complexes. We assess the
performance of tNTA by analyzing the selective binding of tannic acid-modified
gold nanoparticles (TaAuNPs) to herpes simplex viruses (HSV). Our results show
that TaAuNPs bind specifically to HSV without causing substantial changes in
the size or refractive index of the virus, suggesting that the binding does not
cause virus disruption. Instead, the anti-viral properties of TaAuNPs appear to
stem from direct particle binding to the virus.
