How do you stick a label on something as small as a cell? Cell-labeling techniques are explained, and the use of polymer-coated nanoparticles as environment-sensitive cell labels is demonstrated by scientists in Germany and Australia.
Many scientists are rising to the challenge of effective cell labeling by using nanoparticles as labels; these labels may be either fluorescent, magnetic, or radioactive. Nanoparticles are ideal for this purpose because they are smaller than a cell and can be taken up via the cell machinery and they can be made biocompatible. Nanoparticles can also have properties that make them easily observed from the outside, like fluorescence or magnetism. However water-soluble nanoparticles are not easy to make on a large scale, which can limit their use.
Paul Mulvaney (University of Melbourne, Australia), Wolfgang Parak (Philipps University of Marburg, Germany), and co-workers have reviewed attempts to date to make many sorts of nanoparticles soluble in water. They describe different approaches taken by various scientists, including ligand exchange and encapsulation.
Ligand exchange is useful because the smallest water-soluble nanoparticles can be made in this way, but it can reduce the quantum yield of fluorescent particles and the stability of all particles. This method can, however, be very useful to introduce new functional groups to the particle surface.
Encapsulation of nanoparticles with silica or polymers is another approach to increasing their water-solubility. Growing a silica shell around the particle adds both solubility and chemical functionality, but involves a complex procedure, whereas surrounding the nanoparticle with a layer of polymer that is attracted to both water and the particle can be relatively straightforward to achieve by self-assembly and the procedure does not change much for different particles, which makes it universal. Polymer-coated nanoparticles are very stable, but larger than those produced by ligand exchange.
The researchers don’t stop there; they expand upon work they’ve done using polymer-coated nanoparticles to make sense of cell behavior. In particular, if a pH-sensitive and fluorescent polymer is chosen to coat the nanoparticles, then information can be gleaned about uptake into cells of the particle and any changes in the environment once it is there. So scientists truly can put responsive labels into cells, which should enable us to understand cell behavior better in the future.