Phantom nanoparticles as probes of biomolecular interactions

A new light-scattering-based method to detect molecular interactions at the surface of low-refractive-index nanoparticles was recently proposed. Water-dispersed nanoparticles functionalized with receptors typical of immature bacteria cell walls were used to study the activity of the antibiotic vancomycin. This method subtly depends on the specific properties of the nanoparticles. Here we discuss, by comparative experiments and through theoretical evaluation, the effects of size, refractive index, electric charge, and dilution on the reliability and accuracy of the method. Quite surprisingly, perfect index matching and minimal size (i.e., maximum surface), which is almost attained in one of the colloids here employed, do not represent the ideal conditions. Rather, we show that a nanoparticle radius of 100 nm and a refractive index slightly below that of water yields the best signal/background amplitude. We also show that repulsive interactions can lead to artifacts in the adsorption isotherm, thus indicating that electrostatic stabilization should be kept at a minimum. The close agreement between the interaction strengths, as measured with two different nanoparticle systems, testifies to the reliability of the method.