Sensors containing chromatic materials change color in response to external conditions such as temperature and pH and are useful because they can be read by the naked eye. Conjugated polymers, such as polydiacetylene (PDA), are being widely investigated for use in these devices because under stimuli their π-electron-delocalized backbone shortens, inducing a color change. However, PDA sensors normally only respond to a limited range of stimuli and the color change is typically slow and non-reversible.

Now, Huisheng Peng and Xuemei Sun at Fudan University in China1 have made a carbon nanotube/PDA nanocomposite fiber that responds rapidly and reversibly to electric current. “This should be the first example of current-induced chromatism of PDA,” says Peng.

No current-induced color changes had been previously observed for PDA alone, possibly because of its extremely low conductivity, Peng explains. He found that composite fibers containing carbon nanotubes, which have high electrical conductivity, were able to carry sizeable currents.

Fig. 1: Schematic diagram of electrochromatic PDA/carbon nanotube composite fibers.

To make the fibers, the researchers coated pure carbon nanotubes with diacetylenic precursors and then polymerized the precursors under ultraviolet light. They demonstrated that the fibers changed rapidly and reversibly from blue to red in response to electric current greater than 10 mA (Fig. 1). The color change from blue to red took just two seconds when a current was applied, and in another two seconds the fibers reverted back to the original color once the current was stopped. Critically, they also found that this reversible color change can continue for up to 14 cycles provided a relatively low current is used.

It isn’t just blue to red, as Peng explains. “A broad spectrum of colors is available by designing PDA with different side chains.” These fibers still respond to other stimuli that are well known for inducing color change in bare PDA, such as mechanical abrasion, temperature and chemical and organic vapor.

Peng foresees a large number of potential applications for his team’s colorful sensing fibers, including use as safety indicators for electronic devices and in optoelectronic devices. Due to their strength, light weight and flexibility, he also suggests that they could be woven into cloth, opening up even more potential uses.