Carbon Nanoparticle-Induced Changes to Lipid Monolayer Structure at Water-Air Interfaces

Light on the interactions between nanoparticles and lipid membranes by interface-sensitive vibrational spectroscopy

Nanoparticles are produced in natural phenomena or synthesized artificially for technological applications. Their frequent contact with humans has been judged potentially harmful for health, and numerous studies are ongoing to understand the mechanisms of the toxicity of nanoparticles. At the macroscopic level, the toxicity can be established in vitro or in vivo by measuring the survival of cells. At the sub-microscopic level, scientists want to unveil the molecular mechanisms of the first interactions of nanoparticles with cells via the cell membrane, before the toxicity cascades within the whole cell. Unveiling a molecular understanding of the nanoparticle-membrane interface is a tricky challenge, because of the chemical complexity of this system and its nanosized dimensions buried within bulk macroscopic environments. In this review, we highlight how, in the last 10 years, second-order nonlinear optical (NLO) spectroscopy, and specifically vibrational sum frequency generation (SFG), has provided a new understanding of the structural, physicochemical, and dynamic properties of these biological interfaces, with molecular sensitivity. We will show how the intrinsic interfacial sensitivity of second-order NLO and the chemical information of vibrational SFG spectroscopy have revealed new knowledge of the molecular mechanisms that drive nanoparticles to interact with cell membranes, from both sides, the nanoparticles and the membrane properties.

Surface Activity and Aggregation Behavior of Polyhydroxylated Fullerenes in Aqueous Solutions

Polyhydroxylated fullerene (PHF) surface activity and aggregation behavior at the air-water interface were examined using surface tension and resonance-enhanced second harmonic generation (SHG). Surface tension data showed that PHFs are surface active with a limiting surface excess corresponding to 130 Ų/molecule in aqueous (Millipore water) solutions. Increasing the solution-phase ionic strength (through the addition of NaCl) reduces the PHF surface excess. Conductivity measurements show that PHFs carry a single charge, presumably negative. Surface-specific SHG experiments show a small but measurable fixed wavelength, nonlinear response from solutions having surface excess coverages as low as ∼400 Ų/molecule. The SHG response of PHF solutions in the low-concentration limit shows unexpected behavior, implying that at bulk concentrations below 0.06 mg/mL, PHF monomers adsorb to the surface and interfere destructively with the intrinsic nonlinear susceptibility of the aqueous/vapor interface, leading to a ∼75% reduction in the SH signal. Above a PHF concentration of 0.0.06 mg/mL, the SH signal begins to rise in the Millipore and 50 mM NaCl solutions but remains very low in the 500 mM NaCl solutions. From this behavior, we infer that an increased nonlinear optical response is due to adsorbed aggregates.