US scientists have developed nanoparticle sensors that change colour when heated above a specific temperature and don’t change back again when cool. The sensor could tell customers what has happened to their food before they buy it.
Nanoparticles are used in many sensors including temperature sensors; in the past these have been mainly based on the melting or freezing of DNA ligands attached to the particles which causes alterations in the aggregation of the particles and hence in the apparent colour. Such changes are reversible which means that no-one can tell the history of the sensor.
Bartosz Grzybowski and his team at Northwestern University, USA, have developed a new kind of sensor that can remember if it was previously heated above a certain critical temperature. They took gold nanoparticles and covered them with a mixture of two different ligand attachments, one of which contained an azobenzene group.
The nanoparticles are distributed in a solid support of frozen solvent. UV light is shone on the system, which causes the azobenzene part of the first ligand to change shape to a less stable cis form. The nanoparticles are trapped in the solid and the ligand cannot revert back to its more stable trans form without more UV light. Once the temperature is raised above the melting point of the frozen solid solvent, the nanoparticles with their cis ligands can move about. The nanoparticles collect together and the second ligand falls off and enables the particles to cross-link to each other. The cross-linking causes the nanoparticles to precipitate and removes them from solution, which makes the process irreversible.
Depending on the amount of each ligand used, and which solvent is chosen, the critical temperature can be tuned between -55 and +16.5 oC. The team have also made sensors that tell them not only if the temperature was raised above the critical one, but for how long it was held there. They do this by studying the speed at which the sensor sample melts when it defrosts.
The authors accept that their sensors are not long-lived enough for long-term storage but believe they could be used to show if the temperature has ever been too high over a shorter time period, e.g., when transporting high-value food or scientific samples.
An expert in gold nanoparticles, Vincent Rotello from University of Massachusetts, agrees. He says “This paper describes a nifty use of nanoparticle aggregation to create a nanoscale recording thermometer that not only records what temperature is reached but also how long the system has been held at that temperature. This sort of recording technology is very timely, providing a very pragmatic way of validating safe storage of both traditional biologics (e.g. plasma) but also next-generation protein and antibody-based therapies." This development should save customers money and give them greater confidence in the quality of the goods they buy.