Structural properties: magic nanoclusters of gold.

nature nanotechnology | VOL 2 | MAY 2007 | www.nature.com/naturenanotechnology 273 MSNs and the green fl uorescent protein (GFP) marker encoded in the DNA was expressed in the cells and detected by microscopy. Delivery is effi cient because the minimum amount of DNA required to detect marker expression was 1,000-fold lower than that required when using conventional methods to deliver DNA into protoplasts. It seems that using MSNs as a means to deliver DNA in this way should gain popularity for protoplast-based gene expression studies. Although delivering material into protoplasts is important, it is not a particularly common approach in plant biotechnology because the cell walls must first be removed. A popular tool used to deliver materials into plants with intact cell walls is the ‘gene gun’. The carrier particles, usually coated with DNA, enter the cells through the walls by bombardment using high-pressure gases or, less commonly, explosive rounds. Despite the destructive nature of this method, recovery is efficient enough to allow the DNA to be expressed in the plants. Given the physical properties of the plant cell wall, which provides the strength required for plants to grow up to 130 m high8, particles that can penetrate this barrier must exceed some momentum threshold. Particles used for bombardment are typically made of gold (around 0.6 μm in diameter) because they readily adsorb DNA and are not toxic to cells. MSNs, being much smaller (and lighter) are not effective in delivering DNA to intact plant cells. However, Torney and colleagues found that capping the MSNs with gold nanoparticles increases their momentum after acceleration by the gene gun. Their experiments showed that the plasmid DNA delivered using gold-capped MSNs was successfully expressed in intact tobacco and maize tissues. The advantage of using MSNs with the gene gun is that both the DNA and small effector molecules can be delivered at the same time (Fig. 1b). Here, plasmid DNA carrying a regulated version of the GFP marker is adsorbed on the MSN surface and the small effector molecules that activate GFP expression (in this case β-oestradiol) are contained inside the gold-capped structure. After bombardment into the plant cell, the effector molecules are released from the gold-capped structure (Fig. 1c) by incubating the plant tissues on media containing dithiothreitol — a chemical that reduces the disulphide bonds that attach the gold caps to the MSNs. GFP expression is only observed under these conditions. This work stimulates a number of questions. What might be the effect of including combinations of effector molecules within the MSNs, and/or combinations of plasmid DNA on their surfaces? Can MSNs be designed to uncap under more selective conditions (for example, using laser light or in response to chemical changes in the plant cells)? Can MSNs be designed so they can be recapped? Answering these questions is by no means easy, but the promise shown by MSNs in general, and this work in particular, suggests many more breakthroughs will emerge in this area.