Linker-enhanced binding of metalloporphyrins to cadmium selenide and implications for oxygen detection

Adsorption of trivalent metalloporphyrins from nitrogen-saturated chlorofor m solution onto etched n-CdSe crystals causes a profound reversible quenchi ng of the semiconductor's photoluminescence (PL). The PL responses due to t he presence of (MDMPPCl)-D-III and MmTPPCl (DMPPCl = protoporphyrin IX dime thyl eater chloride; TPP = tetraphenylporphyrin; M = Fe, Mn) exhibit a conc entration dependence that can be fit to the Langmuir adsorption isotherm mo del to yield binding constants of 10(4)-10(5) M-1. The CdSe surface may be modified by adsorption from solution of specifically designed linker ligand s (1-4). These ligands are able to bind to the semiconductor surface throug h one end and to ligate a heme analogue axially on the other end. Surfaces derivatized by each of the linkers showed concentration-dependent metallopo rphyrin-induced PL changes, corresponding to roughly order-of-magnitude inc reases in binding constants to 10(5) to 10(6) M-1. Films of linker-metallop orphyrin complexes were coated onto the semiconductor substrates and charac terized by X-ray photoelectron (XPS) spectroscopy. The linker-metalloporphy rin films can be used as transducers for dioxygen detection. Relative to a nitrogen ambient, the PL of CdSe samples coated with 1-3 is reversibly quen ched by exposure to oxygen (binding constants of similar to 1-10 atm(-1); d etection limit of similar to 0.1 atm), while bare CdSe surfaces show no res ponse to dioxygen. These coated CdSe samples were further characterized by contact potential difference (CPD) and time-resolved photoluminescence (TRP L), which suggest that oxygen-induced PL changes are due to variations in t he electric field present in the semiconductor substrate.