Wolfgang Parak tells Journal of Materials Chemistry about his hot paper.

1. Can you briefly describe what you achieved in this article? 
We have embedded a pH-sensitive dye in a polymer capsule which acts as delivery vehicle for transporting the dye inside living cells. The colour of fluorescence depends on the pH. In this way capsules outside the cells in the alkaline medium emit in red, whereas capsules which have been ingested by cells emit in green due to the acidic pH in the endosomal / lysosomal compartments. The ingestion of the capsule can be followed online by monitoring its colour of fluorescence.

2. Could you explain the significance of your article to the non-specialist? 
For any kind of cellular diagnostics it would be helpful to have a noninvasive reporter inside cells which constantly measures the concentration of important molecules and send this information to a detector outside the cell. This is similar to the idea of having a miniature diagnosis submarine inside the body as it has been featured in some science fiction movies. This reporter would be like an artificial organelle. Ideally it would not disturb the function of the cells and one would have a noninvasive way to observe online what is going on inside a cell.

3. What has motivated you to conduct this work? 
In previous work (A. G. Skirtach, A. Muñoz Javier, O. Kreft, K. Köhler, A. Piera Alberola, H. Möhwald, W. J. Parak, G. B. Sukhorukov, "Laser-Induced Release of Encapsulated Materials inside Living Cells", Angewandte Chemie International Edition 45, 4612 - 4617 (2006)) we have reported the use of polymer capsules as a delivery system. As we are able to transfer capsules inside cells we thought about delivering something with an active function, such as the pH-sensitive dye reported in the present work.

4. Where do you see this work developing in the future? 
In the long run we envision a multiplexed reporter system which would allow for detecting the concentration of several important molecules inside cells online and noninvasively. This could be an important tool for diagnostics.

5. Are there any particular challenges facing future research in this area? 
So far all experiments were performed in vitro. It will be a long time involving a lot of modification of the system until we can think about using these capsules as in vivo diagnostics system.

Oliver Kreft has been at the MPI of Colloids and Interfaces (Potsdam, Germany) since 2004 in the group of Professor Sukhorukov. His current research activities concern the development of multifunctional microcapsules for biochemical and pharmaceutical applications: enzymatic reactions within microcontainers, biodegradable microcapsules, targeted delivery and release of encapsulated materials, and the development of capsule-based sensor systems.

Almudena Muñoz Javier has been in Professor Parak's group at the Center for Nanoscience at the Ludwig Maximilians Universität (München, Germany) since 2003. Her work is devoted to the investigation of particle uptake (colloidal quantum dots, colloidal gold, polymer microcapsules, magnetic bubbles) by living cells.

Gleb Sukhorukov joined Queen Mary University of London in 2005, where he took responsibility for the Chair in Biopolymers at the Department of Materials (Interdisciplinary Research Center for Biomedical Materials). His research field spans Physical Chemistry, Biophysics and Materials Science, comprising physics and (bio)-chemistry on submicron dimensions, the design of multifunctional colloidal particles and capsules and nano-engineered biomaterials. He has initiated research on layer-by-layer assembly of polyelectrolytes on colloidal particles, fabrication of micron-sized capsules and encapsulation of a variety of substances.

Wolfgang Parak has held the chair for Biophotonics at the Physics department of the Philipps Universität Marburg (Marburg, Germany) since 2007.  His group is developing new nanotechnology-based materials which are hybrids of functional inorganic nanoparticles and organic molecules.  In particular, the interaction of these materials with biological systems as macromolecules and living cells is investigated.