Optical sensors based on neutral carriers

In a chemical sensor/biosensor there has to be some reversible analyte recognition process of relevant selectivity and a means for translating this process into an adequate signal (transduction). Electrically neutral molecules for the recognition of ions (neutral ion carriers, neutral ionophores) are widely used in ion-selective electrodes (ISEs). In addition to their performance in sensors with potentiometric transduction, they offer exciting possibilities for sensors with optical transduction as well. Even if the analyte recognition by the ionophore does not furnish an adequate optical signal, there are schemes to use them in sensors with optical transduction. Such sensors respond to ratios or products of the activities of the ions participating in the reaction of the sensor membrane with the sample. So far, analytically relevant chemical sensors for Cl^-, NO₃^-, Ca²⁺, Na⁺, K⁺, Pb²⁺ (detection limit < 10^-9 mol/l), Zn²⁺ and for the recognition of chiral organic cations have been described. The sensor scheme may be extended to electrically neutral analytes. Utilizing electrically neutral analyte carriers, we have been able to design optodes for glucose, ethanol water ammonia (detection limit ∼ 1 ppbv), H₂S and other species. A creatinine carrier has been developed and its implementation into an optode is in progress. Speculations about attractive possibilities will be made.