pH indicators are more than just tools for helping students visualize the differences between acids and bases. They can also be used in sensors to monitor bacterial growth in packaged foods. Bacteria growth causes food spoilage, endangering the health of consumers. Real-time monitoring of bacterial growth is needed for food storage and to ensure freshness.
Researchers at the University of Regensburg in Germany have developed a new method for making pH nanosensors that can image the pH value during the growth of E. coli bacteria. The sensors consist of nanometer sized particles made up of two fluorescent dyes. One dye acts as a green-emitting pH probe and the second acts as a red-emitting reference dye (pH insensitive).
On average bacteria are 2 to 10 microns in diameter, so nanosized sensors are needed—not conventional imaging sensor layers or microsized sensor particles. The nanoparticles in the nanosensors are about 12 nanometers in diameter and are the smallest size reported to date according to the researchers.
The nanoparticles have a hydrophilic outer polymer (polyethylene glycol) coating. The coating keeps the particles stable in water-based solutions but more importantly reduce cytotoxicity and prevent penetration of cell membranes. This means the nanosensors won’t interfere with the growth or metabolism of bacteria. The researchers report the non-toxic feature of the nanosensors is unique compared to existing pH-sensitive nanosensors.
The nanoparticles were then embedded in an agarose gel, a biocompatible sensing material for fluorescence imaging. This allows bacterial growth to be monitored and imaged in real-time. There’s a green emission of light (luminescence) by the pH probe and its brightness intensity increases with a rising pH. The red luminescence is a reference signal since the dye is pH insensitive. So the pH probe changes from red to green when is pH increased from 5.5-9.0.
Bacteria produce protons during growth that changes the local pH value. For example, the growth of an E. coli colony lowers the pH value. The growth of E. coli bacteria decreases the brightness of the green luminescence.
While the new nanosensors improve existing sensor technologies, there are more features that need to be incorporated before the nanosensors are truly useful. The researchers believe their nanosensors will be adapted in the future to detect oxygen or carbon dioxide, as well as distinguish different species of bacteria. Furthermore, they envision the nanosensors to be integrated into the barcodes of packaged foods to monitor food freshness.
Wang, X., Meier, R., & Wolfbeis, O. (2012). Fluorescent pH-Sensitive Nanoparticles in an Agarose Matrix for Imaging of Bacterial Growth and Metabolism Angewandte Chemie DOI: 10.1002/ange.201205715