Cells are very good at protecting their precious contents — and, as a result, it’s very difficult to penetrate their membrane walls to deliver drugs, nutrients, or biosensors without damaging or destroying the cell. One effective way of doing so, discovered in 2008, is to use nanoparticles of pure gold, coated with a thin layer of a special polymer. But nobody knew exactly why this combination worked so well, or how it made it through the cell wall. Now, researchers at MIT and the Ecole Polytechnique de Lausanne in Switzerland have figured out how the process works, and the limits on the sizes of particles that can be used. Their analysis was published online on August 5, 2013 in the journal Nano Letters, in a paper by graduate students Reid Van Lehn, Prabhani Atukorale, Yu-Sang Yang, and Randy Carney, and professors Alfredo Alexander-Katz, Darrell Irvine, and Francesco Stellacci. Until now, says Van Lehn, the paper’s lead author, “the mechanism was unknown. … In this work, we wanted to simplify the process and understand the forces” that allow gold nanoparticles to penetrate cell walls without permanently damaging the membranes or rupturing the cells. The researchers did so through a combination of lab experiments and computer simulations. The team demonstrated that the crucial first step in the process is for coated gold nanoparticles to fuse with the lipids — a category of natural fats, waxes, and vitamins — that form the cell wall. The scientists also demonstrated an upper limit on the size of such particles that can penetrate the cell wall — a limit that depends on the composition of the particle’s coating. The coating applied to the gold particles consists of a mix of hydrophobic and hydrophilic components that form a monolayer — a layer just one molecule thick — on the particle’s surface.
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