Chemical sensors based on molecular recognition events promise detection with high selectivity and sensitivity. Supramolecular guest-host chemistry provides an artificial means for mimicking the high specificity found in biological systems. For example, cyclophanes and metallacycles contain molecular-sized cavities that demonstrate selectivity in binding ionic and aromatic analytes. To transduce the binding event into a readable output, the electrical or photophysical properties of the complex must reflect the coordination state of the host molecule.
The objective of this project was to develop a fieldable, screening-level chemical assay for perchlorate ions and nitroaromatic explosives (2,4,6-trinitrotoluene [TNT] and 2,4-dinitrotoluene [DNT]). The optical response of lanthanide-modified cyclophanes was intended to transduce the presence of the analytes into a readable output.
Cyclophanes are cyclic compounds with molecular-sized cavities that can accommodate guest molecules. Novel macrocycles were engineered to incorporate optically active centers (i.e., lanthanide metals) around the edges of the ring structure. The photophysical properties of these metal centers are highly sensitive to their chemical and physical surrounding. Subtle changes in the electrostatic or chemical environment (e.g., molecule-analyte binding), are reflected in the fluorescent properties of the metal centers and provide a highly sensitive source for indicating a positive response.
A functionalized cyclophane receptor was synthesized to provide selective detection of nitroaromatics such as TNT. The organic cyclophane was functionalized with ligating capabilities along its periphery which allow for the binding of lanthanide chromophores. The sensitivity of the lanthanide-cyclophane complex to the aromatics was better than 10 ppm and was capable of distinguishing the nitroaromatics indicative of explosives from aromatic hydrocarbons with high sensitivity.
This project provides the basis for a screening-level chemical assay for perchlorate ions and a variety of aromatic explosives that is convenient, inexpensive, rapid, and environmentally benign. The versatility in the synthetic procedures will enable successive generations of cyclophanes to be immobilized on glass or beaded supports. These materials can be used to develop fiber optic “dipstick” probes for testing soil and water samples, or, as a “spray-on” method for wide-area screening from a standoff position. These technologies will support replacement of the timely and expensive ion-chromatographic methods used to detect perchlorate and gas chromatography-mass spectrometry (GC-MS) techniques used to determine the identity of organic explosives.