Role of non-linear optics and multiple photon absorbance in enhancing sensitivity of enzyme-based chemical agent detectors

Real-time chemical sensors have been developed based on the binding of the analyte to monolayers of either porphyrin alone or porphyrins incorporated into the active site of enzymes. Binding of an analyte to porphyrin alone causes a redistribution of electrons in the porphyrin, altering the energy levels of the electrons which manifests as a change in the absorbance spectrum of the porphyrin. Porphyrins incorporated into the active site of enzymes such as cholinesterases are displaced when a competitive inhibitor such as nerve agents binds to the active site; this results in the porphyrin experiencing a different microenvironment than in the protein, resulting in a change in absorbance spectrum. Based on the Beer-Lambert relationship of concentration and absorbance, the limit of detection (LOD) for porphyrin-based sensors should be approximately 2 nM although LODs several orders of magnitude lower have been published. This increased sensitivity is explained as the result of multiple photon absorbance by the porphyrin and limiting self-quenching energy transfer reactions in the evanescent monolayer.