Nanotoxicology: nanoparticles reconstruct lipids.

news & views a n emerging 'rule of thumb' suggests that nanoparticles less than 100 nm in diameter can enter cells, those with diameters below 40 nm can enter the cell nucleus and those that are smaller than 35 nm can pass through the blood–brain barrier and enter the brain. Understanding the way nanoparticles interact with living matter will open up fundamentally new opportunities in medicine and diagnostics. This knowledge equally imposes on us the necessity for consideration, without excessive and unscientific alarm, of key safety issues in implementing nanoscience. It is early days in this field and much is still unknown. 1 at the University of Illinois in Urbana report that nanoparticles can actively modulate the phase structure of lipid membranes so that the stiffness differs from spot-to-spot. This variation in stiffness is functionally important for material and sensor applications, but the findings could also have broader implications for understanding nanoparticle–cell interactions and their safety issues. Granick and co-workers mixed positively or negatively charged polystyrene nanoparticles (~20 nm in diameter) with different suspensions of liposomes — spherical lipid bilayer membranes that contain aqueous compartments — and measured the state of the membrane phases using fluorescence and calorimetry. A charge-dependent reconstruction of the membrane surface was observed at the local spots where the nanoparticles bound; negatively charged nanoparticles bound to a fluid area of the membrane induced gelation, whereas positively charged nanoparticles turned gelled areas into a fluid state. Experiments with liposomes made from different types of lipids showed that the local phase-change did not depend on the choice of lipids, the size of the liposomes or the size of the nanoparticles. Rather, it seemed to depend on the density and placement of charges on the surface of the nanoparticles; nanoparticles with a higher density of surface charge resulted in a greater degree of membrane gelation, whereas DNA, which is a flexible rod-like molecule, did not induce this effect. The Illinois team suggests that the rigid placement of charges on the surface enables the nanoparticles to induce structural reorganization of the lipids and change their local state. Although these studies relate to a very simple model system of a single component, the clarity of the conclusions prompt broader questions. It is now appreciated that, below a certain size, a broad range of nanoparticle materials enter a variety of cells by different processes — the details of which remain to be …