Photoluminescent properties of cadmium selenide in contact with solutions and films of metalloporphyrins: Nitric oxide sensing and evidence for the aversion of an analyte to a buried semiconductor-film interface

The band-edge photoluminescence (PL) intensity of etched n-CdSe single crys tals is quenched reversibly by adsorption of the trivalent metalloporphyrin s, MTPPCl (TPP = tetraphenylporphyrin; M = Mn, Fe, Co) in nitrogen-saturate d methylene chloride solution. The PL responses are concentration dependent and can be fit to the Langmuir adsorption isotherm model to yield binding constants of similar to 10(3)-10(4) M-1. The MTPPCl compounds react irrever sibly with NO in solution to form nitrosyl adducts, and these compounds rev ersibly enhance the CdSe PL intensity when adsorbed onto the semiconductor surface, also with binding constants of similar to 10(3)-10(4) M-1. Films o f MTPPCl were prepared on CdSe substrates by solvent evaporation. These coa tings serve as transducers for NO detection: while the bare CdSe surface sh ows no response to NO gas relative to N-2, the coated surfaces reversibly e nhance the PL intensity (CoTPPCl) or quench it (MnTPPCl and FeTPPCl), with binding constants on the order of similar to 1 atm(-1). In contrast to the PL results, which are particularly sensitive to the semiconductor-film inte rface, electronic and IR spectral changes of the bulk film induced by NO bi nding were irreversible. The UV-vis and IR spectra could be spectroscopical ly mimicked by preformed nitrosyl adduct films that were prepared by solven t evaporation of MTPPCl (M = Co, Fe) and MTPP (M = Co) solutions that had b een exposed to NO. These films, however, lack transduction capability, as t he PL intensity is the same in NO and N-2 ambients. For the films prepared from FeTPPCl and CoTPPCl, the saturation of IR and UV-vis spectral changes occurs at NO pressures at least 10-fold lower than observed for PL changes. These results indicate that NO has a strong aversion to binding at the sem iconductor-film interface as opposed to the bulk film environment. Steric a nd electronic contributions to these observed effects are discussed.