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

Adsorption of trivalent metalloporphyrins from nitrogen-saturated chloroform solution onto etched n-CdSe crystals causes a profound reversible quenching of the semiconductor's photoluminescence (PL). The PL responses due to the presence of M^IIIDMPPCl and M^IIITPPCl (DMPPCl = protoporphyrin IX dimethyl ester chloride; TPP = tetraphenylporphyrin; M = Fe, Mn) exhibit a concentration dependence that can be fit to the Langmuir adsorption isotherm model to yield binding constants of 10^4-10⁵M^-1. The CdSe surface may be modified by adsorption from solution of specifically designed linker ligands (1-4). These ligands are able to bind to the semiconductor surface through one end and to ligate a heme analogue axially on the other end. Surfaces derivatized by each of the linkers showed concentration-dependent metalloporphyrin-induced PL changes, corresponding to roughly order-of-magnitude increases in binding constants to 10⁵ to 10⁶ M^-1. Films of linker-metalloporphyrin complexes were coated onto the semiconductor substrates and characterized by X-ray photoelectron (XPS) spectroscopy. The linker-metalloporphyrin 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 quenched by exposure to oxygen (binding constants of approx. 1 - 10 atm^-1; detection limit of approx. 0.1 atm), while bare CdSe surfaces show no response to dioxygen. These coated CdSe samples were further characterized by contact potential difference (CPD) and time-resolved photoluminescence (TRPL), which suggest that oxygen-induced PL changes are due to variations in the electric field present in the semiconductor substrate.