The photoacid makes the gel more acidic when the coloured light is shone on it, and when this happens in a melted gel, it causes the nanoparticles to form into chains.

This occurs thanks to a photoacid, a final “secret” ingredient in the gel. However, if you melt the gel while illuminating it with blue and red light, the nanoparticles stick together and form little chains. If you melt the gel and solidify it without illumination, they remain randomly distributed. The nanoparticles are initially spread at random throughout the gel. The gel can be trained because the gold nanoparticles in the mixture are sensitive to the acidity of their surroundings. The gel developed by teams from Aalto University and Tampere University mimics this process with the heating corresponding to the food and the coloured light corresponding to the bell. Pavlov trained the dog to behave like this by ringing a bell every time he fed the dog – the dog associated the sound of the bell with its food and would begin to drool when it heard the bell ring. In Ivan Pavlov’s famous classical conditioning experiment in experimental psychology dealing with simple forms of learning, a dog could be trained to salivate when it heard a bell. It has thus 'learned' to respond to a new stimulus. But if you melt the gel while illuminating it with red and blue light, then cool it back down into a gel, the gel will spontaneously melt the next time you shine red and blue light on it. If you melt the gel by heating, cool it back down so it solidifies again, and then shine red and blue light on it, nothing exciting happens either. When this gel has red and blue light shone on it, nothing happens. To explore whether classical conditioning could be achieved in artificial materials, the researchers made a solid gel based on agarose, a substance commonly extracted from seaweed, mixed with water and modified gold nanoparticles. Their work, recently published in Nature Communications, was inspired by the concept of classical conditioning in behavioural psychology, better known as Pavlov’s dog experiment. Researchers have successfully trained a material to respond to an originally neutral stimulus, a gel that can be taught to melt without needing heating.

Pavlov dog experiment