Tuning of silica nanoparticle–lysozyme protein complexes in the presence of the SDS surfactant

Abstract

The structures of the complexes of anionic silica nanoparticle (size ∼ 16 nm)–lysozyme (cationic) protein, tuned by the addition of the anionic surfactant sodium dodecyl sulfate (SDS), have been investigated by dynamic light scattering (DLS) and small-angle neutron scattering (SANS). The unique advantage of contrast variation SANS has been used to probe the role of individual components in binary and ternary systems. The cationic lysozyme protein (at pH ∼ 7) adsorbs on the anionic silica nanoparticles and forms mass fractal aggregates due to the strong attractive interaction, whereas similarly charged SDS does not interact physically with silica nanoparticles. The presence of SDS, however, remarkably affects the nanoparticle–protein interactions via binding with the oppositely charged segments of lysozyme. In general, the SDS–lysozyme complexes possess a variety of structures (e.g., insoluble complexes of Ly(DS)₈, crystalline structure, or micelle-like structure) depending on the surfactant-to-protein molar ratio (S/P). In the ternary system (HS40–lysozyme–SDS), lysozyme preferentially binds with SDS, instead of directly to nanoparticles. At low S/Ps (0 ≤ S/P ≤ 10), the SDS concentration is not enough to fully neutralize the charge of lysozyme, leading to the formation of cationic SDS–lysozyme complex-mediated nanoparticle aggregation. The morphology of the nanoparticle–(lysozyme–SDS) complexes is also found to be mass fractal kind where the fractal dimension increases with increasing SDS concentration. At S/P > 10, there is sufficient SDS to fully neutralize the lysozyme in the absence of competing charges from the particle but it is at S/P = 50 before all lysozyme desorbs from the particle and binds completely to the overwhelming amount of SDS, creating an oppositely charged lysozyme–SDS complex, which is repelled from the particle.