Crystals of borenium fluorescent dye, which emits light in the red and near-infrared part of the electromagnetic spectrum. Top images taken in ambient light, bottom in ultraviolet (UV). Scale bar 1,000 micrometres. Credit: MIT
A new kind of fluorescent dye which responds to changes in temperature by emitting different colours of light could be used as ‘molecular thermometers’ to monitor whether sensitive substances like drugs or vaccines have been exposed to extreme temperatures during transport.
The dye, which emits light in the red to near-infrared range, could be incorporated into thin films and used as organic light-emitting diodes (OLEDs) in flexible screens or encapsulated in polymers and injected into the body for use in biomedical imaging.
“One of the reasons why we focus on red to near-IR is because those types of dyes penetrate the body and tissue much better than light in the UV and visible range,” says Robert Gilliard, a professor of chemistry at Massachusetts Institute of Technology (MIT) in the US.
Red and near-infrared fluorescent dyes could produce clearer images of tumours and other structures deep within tissues.
Positively charged ions called ‘borenium cations’, which contain atom of boron attached to 3 other atoms or ligands (molecules or ions), emit light in this range.
However, most break down quickly when exposed to air or light. They’re also relatively dim, with quantum yields – the ratio of fluorescent photons emitted per photon of light absorbed – of only about 1%.
“Stability and brightness of those red dyes are the challenges that we tried to overcome in this study,” says Gilliard.
In a 2022 paper, Gilliard and his collaborators stabilised borenium cations with ligands called carbodicarbenes (CDCs), which made them stable enough to be handled in open air.
In a new study published in Nature Chemistry, Gilliard’s team experimented with changing the borenium ion’s ‘counter ion’ to further alter the dye’s properties.
The counter ion is the negatively charged ion which accompanies the borenium ion to make the overall substance electrically neutral.
In sodium chloride (NaCl) table salt, for example, the positively charged sodium ion is the counter ion to the negatively charged chloride ion, and vice versa.
They found that changing the counter ions shifted the dye’s emission and absorption properties toward the infrared end of the spectrum. It also generated a higher quantum yield.
“Not only are we in the correct region, but the efficiency of the molecules is also very suitable,” Gilliard says.
“We’re up to percentages in the 30s for the quantum yields in the red region, which is considered to be high for that region of the electromagnetic spectrum.”
The team showed the dye can be made into crystals, films, powders and as colloidal suspensions in liquid, which suggests it could be used in a range of applications. They are now working to extend the light emission further into the near-infrared by incorporating more boron atoms.