Formation of nanoscale size cadmium sulfide within a channel protein monolayer

Abstract

The size, structure and ease of manipulation of organized monolayer structures have been used to control reaction pathways for the formation of semiconductor particle clusters of cadmium sulfide and other semiconductors. The size and particle distributions are determined by the specific monolayer structure chosen. The present study reports a new technique for the formation of small clusters of cadmium and zinc sulfides. It combines monolayer methods with techniques based on inclusion formation in restricted volumes. Cadmium and zinc sulfides are formed within the cavity of a channel protein by a simple process in which a close-packed monolayer of the channel protein is formed on a neutral subphase, transported to a cadmium-chloride-containing subphase, transferred to a slide by Langmuir-Blodgett transfer and exposed to hydrogen sulfide. The channel protein is a membrane-bound entity with hydrophobic exterior walls and a hydrophilic inside channel (12 Å diameter). It is cylindrical and in compressed monolayers is oriented normal to the interface with the channel mouth in communication with the subphase. Small water-soluble ions are rapidly taken into the channel interior. The size of the cluster formed is limited by the small number of ions capable of assimilation into the channel and, because each group of ions is compartmentalized, there is no possibility of spontaneous aggregation to produce macroscopic particulate inside of the protein channel. The cadmium and zinc sulfides were produced in the “quantum dot” size range as evidenced by their markedly blue-shifted optical spectra. The results of this study indicate more general potential application of channel protein structures as nanoscale reactors; in the case of the protein used here the channel opening and closing can be modulated by pH changes, affording the possibility of sealing or encapsuliting reactants or products in small defined volumes. Other types of channel structure are closed by changes in potential, specific small ions or biogenic messenger molecules. Some of these have significantly larger channel volumes that the protein used here. These too may be likely future systems for specific small-scale reactions.