doi:10.1038/nindia.2011.154 Published online 29 October 2011
Researchers have designed a new type of microcapsule-embedded fluorescent molecule that can monitor the concentration of hydrogen ions and ammonia present in environmental and biological samples.
Ammonia is a toxic chemical that is released into the environment by farming and chemical industries. Ammonia sensors are therefore needed to monitor ammonia levels in the food industry. In addition, pH monitoring is essential because it is a key aspect in most industrial and biological processes.
Unfortunately, however, the dye molecules used in fluorescent sensors are destroyed when exposed to light. To design a sensor that can escape light-induced destruction, the researchers embedded light-emitting hydroxypyrene trisulphonate (HPTS) molecules in microcapsules made from poly-L-lysine (PLL), trisodium citrate and silica nanoparticles.
The researchers tested the efficacy of the HPTS-containing microcapsules for monitoring concentrations of hydrogen ions and ammonia by irradiating phosphate buffer solutions containing the microcapsules with white light. The colour of the microcapsules changed gradually from nearly colourless to fluorescent green as the increasing ammonia concentration caused the pH to rise from 5 to 7. The microcapsules were able to sense ammonia concentrations as small as 20 parts per million.
The microcapsules formed an ordered structure in which the shell wall acquired a composite structure consisting of positively charged PLL chains attached to negatively charged silica nanoparticles. The HPTS molecules were bound to the PLL and remained at the inner part of the shell wall. Negatively charged HPTS molecules were not found in the outer shell, which is made mostly of negatively charged silica. This microcapsule approach therefore protects HPTS — the probe molecule — from light-induced destruction.
The researchers say that their microcapsule has the potential to measure hydrogen ion concentration in cells under physiological conditions and also to estimate ammonia concentration in environmental samples.
The authors of this work are from: Department of Chemistry, Faculty of Arts and Sciences, American University of Beirut, Beirut, Lebanon and Nanomaterials Laboratory, I & PC Division, Indian Institute of Chemical Technology, Hyderabad, India.
- Amali, A. J. et al. Nanoparticle assembled microcapsules for application as pH and ammonia sensor. Anal. Chim. Acta. doi: 10.1016/j.aca.2011.10.001 (2011)