A photo timeline showing reaction monitoring using perovskite fluorescence. Photos: Syracuse University News Services.
A photo timeline showing reaction monitoring using perovskite fluorescence. Photos: Syracuse University News Services.

Chemists at Syracuse University and Brookhaven National Laboratory have come up with an innovative new way to visualize and monitor chemical reactions in real time. They have designed a nanomaterial that changes color when it interacts with ions and other small molecules during a chemical reaction.

Described in a paper in ACS Nano, their discovery allows researchers to monitor reactions qualitatively with the naked eye and quantitatively with simple instrumentation.

"In many cases, a chemical reaction between molecules occurs in a solution that is colorless and transparent or looks like a milky suspension," explains Mathew Maye, associate professor of chemistry at Syracuse University and the study's team leader. "The only way to know if a reaction has occurred or not is to perform extensive analysis after a multi-step purification."

In an attempt to find a simpler way to investigate why and how fast a reaction occurs (if at all), the group has designed a nanoparticle that reacts with by-products of the reaction. "When the reaction occurs, the nanoparticle fluoresces at a different color, allowing us to gauge kinetics by eye, instead of with a million-dollar spectrometer," Maye says.

Central to the group's work is an emerging class of nanomaterials called perovskites: a perovskite is a special class of crystal, typically made up of metal ions and oxygen. The group's perovskites are composed of metal ions and a halide (a bromine, chlorine or iodine ion).

At the nanoscale, perovskites are photo-luminescent, meaning they emit light when ‘excited’ by a laser or lamp. The fact that the colors they emit are determined, in part, by their ion concentrations makes perovskites unique among nanomaterials. It also makes them ripe for application: research groups in industry and academia see potential for perovskites in solar cells, light-emitting diodes, lasers and photo detectors.

"We knew about the potential of these materials in energy research," says Tennyson Doane, a post-doctoral researcher in Maye’s group and co-corresponding author of the paper with Maye. "We are interested in energy as well, and had this crazy idea of trying to use the ion concentration ratios of perovskites to detect ions in solution, and then perhaps monitor the chemical reaction, which is very difficult to do. We had no idea if it would work or not, so we just decided to go for it."

The group started by working with a very simple system that involved organic reactions of molecules called organohalides. When these molecules react, often forming carbon-carbon double bonds in what is known as an elimination reaction, they release a halide. Typically, the halide is an unimportant side-product of the reaction, until now.

"Our technology allows us to accurately detect the halide release," says Kevin Cruz, a chemistry major at Syracuse University and co-author of the article. "When the reaction starts, the perovskite fluoresces bright red. As the halide is released, or exchanged in the chemical reaction, our particle absorbs it, and the fluorescence color changes proportionally to the halide concentration – from red to orange to yellow to green. When the color is green, the reaction is over."

"Added to that is the fact that the perovskite concentration is very low, you just have to add a small amount to the reaction for observation," says Doane. "We have been able to calibrate the system very accurately, and from that can measure chemical kinetics in a new 'colorimetric' way."

Maye offers nothing but praise for Doane and Cruz, stating that what they have accomplished in a short amount of time and on a small budget is ‘amazing’. "No one, right now, is thinking about monitoring a chemical reaction this way," he states. "Our team is able to measure very precise chemical kinetics by monitoring the color change with nothing more than an ultraviolet lightbulb or a cheap fluorescence spectrometer."

The group's technology is patent-pending at Syracuse University. Maye says they are now testing the technique's applicability for a wide library of chemical reactions and its effectiveness at measuring low concentrations of ions and reactive molecules.

"Who knows, maybe in the future, every chemist will use a Syracuse-based perovskite for monitoring their reactions," he says.

This story is adapted from material from Syracuse University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.