Telling Solvents Apart Easily
Identifying organic solvents in an environmental, laboratory, manufacturing, or other sample usually revolves around a relatively sophisticated analytical technique. Mass spectrometry or nuclear magnetic resonance spectroscopy (NMR) are possible approaches. Other spectroscopic methods coupled with chromatographic techniques might be used. However, these usually involve the use of costly equipment, lab time, and expertise and do not always offer a quick or definitive solution to solvent identification. Moreover, in fieldwork away from the analytical laboratory they become untenably cumbersome.
“Sensors that work without recourse to instrumentation and use visual indicators are very important in the development of cheap and reliable methods to measure solvents and anions,” says Tony James, University of Bath, UK. “In particular for environmental field work where the rapid analysis of a potentially dangerous solvent contamination of an important water source could avoid (or speed up) the need to turn off domestic water supplies.”
Jonathan L. Sessler, Eric V. Anslyn, both University of Texas at Austin, TX, USA, and colleagues point out that push-pull dyes, fluorescent chromophores, functional polymers, and solid metal complexes have been used as chemosensors in systems discussed quite colloquially as “artificial noses”. Such approaches do not need complex and unwieldy instrumentation and are portable. The detection techniques exist, but they are usually focused on the gas phase and not solutions.
For work in solution, molecules that can act as hosts for individual solvent molecule guests have been sought in the realm of supramolecular chemistry. Tie a chromophore to such an entity and if the supramolecular complex is sufficiently selective in its hosting of said guest, it might be possible to produce a glowing flag to indicate the presence of a given solvent in a mixture. A different host for each solvent tagged with a different colored chromophore flag might provide a way to identify the presence of different solvents in the mixture through visual inspection or a relatively simple optical sensing approach relayed via fiber optics to a suitable camera-like sensor and a computer.
One group of molecules that have been used widely in supramolecular chemistry are porphyrins, the central unit of the light-gathering photosynthetic system in the chlorophyll of green plants. This heritage hints at their intense absorption and emission features, making them their own chromophore units for chemosensing for detection in the ultraviolet, visible and near-infrared spectral regions. An individual porphyrin is planar, conjugated, and rigid, which facilitates interactions with small molecules via its π electrons. It would seem to be the perfect host in a supramolecular assembly for detecting multiple solvents in solution.
Assemblies with a Unique Responsive Behavior to Solvents and Anions
Porphyrins are a good starting point. However, Sessler, Anslyn, and colleagues were hoping to find a supramolecular building block that could have more distinct optical features for distinguishing solvents with similar structures and for detecting anions. Earlier supramolecular assemblies have commonly been used to recognize cations (sodium, potassium, calcium, magnesium etc.). The researchers were also looking for greater flexibility in their hosts and an ability to work under conditions of simple mixing.
Based on work funded by the National Science Foundation (NSF), the National Institutes of Health (NIH), and the Welch Foundation, the team now reports on a series of expanded porphyrin-anion supramolecular assemblies based on the cyclo[m]pyridine[n]pyrroles (where m + n = 6). They report that “addition of polar organic solvents or common anions to the ensembles leads to either a visible color change, a change in the fluorescence emission features, or differences in solubility.” The researchers point out that the specific response is easy to see with the naked eye and depends on the structure of the assembly and the particular analyte chosen. In their proof of principle experiments, they were able to distinguish between the commonly used small molecule organic solvents diethyl ether, tetrahydrofuran (THF), ethyl acetate, acetone, methanol, acetonitrile, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). The assemblies could be used to identify complex solvent systems. They were also able to distinguish between various anions: fluoride, chloride, bromide, nitrate, and sulfate.
“The present systems may have a role to play as chemosensors that allow certain salts and various solvents to be identified easily by optical or visual means,” the team concludes.
Towards Simple Solvent Testing
“The work demonstrates the power of the confluence of supramolecular polymers with statistical analysis,” says Bruce C. Gibb, Tulane University, New Orleans, LA, USA. “Solvatochromism is a tried and trusted analytical technique, but by utilizing a supramolecular dye based on expanded porphyrins and dicarboxylic acids, they have created a system possessing a much richer response-space than is possible with simple molecular dyes … they hit a sweet spot.” He adds that, “Much work remains to be done. However, this development beautifully demonstrates that in the not too distant future test strips capable of identifying which organic solvents are present within a complex mixture may be as common as pH paper is today.”
A. P. de Silva, Queen’s University Belfast, UK, is equally enthused by the work. “Identifying a solvent, whether in the vapor or in the liquid phase, by a quick optical experiment is a tough ask,” he told ChemViews Magazine. “The current work goes beyond previous efforts using dye sensor arrays,” he says, and adds, “Chemometric analyses can further improve such discrimination when the solvents are present in mixtures.”
- Expanded Porphyrin-Anion Supramolecular Assemblies: Environmentally Responsive Sensors for Organic Solvents and Anions,
Zhan Zhang, Dong Sub Kim, Chung-Yon Lin, Huacheng Zhang, Aaron D. Lammer, Vincent M. Lynch, Ilya Popov, Ognjen Š. Miljanić, Eric V. Anslyn, Jonathan L. Sessler,
J. Am. Chem. Soc. 2015.