Synthesis of cadmium sulphide superlattices using self-assembled bacterial S-layers

Abstract

Methods for organizing materials at the nanometre scale have advanced tremendously in recent years¹,². One important objective is the synthesis of patterned arrays of inorganic nanocrystals^3,4,5,6, whose optical, electronic and magnetic properties might find technological uses, for example as memory elements. Techniques such as colloidal crystallization^7,8,9, monolayer deposition^10,11,12, multilayer casting¹³, molecular crosslinking¹⁴,¹⁵, the use of complementary interactions¹⁶,¹⁷ and the synthesis of nanoparticles in patterned etch pits¹⁸ have been used to organize nanocrystals into superlattices. Here we describe the use of bacterial S-layers — self-assembled, two-dimensionally ordered films of proteins that feature in many bacterial cell walls — as templates for the in situ nucleation of ordered two-dimensional arrays of cadmium sulphide nanocrystals about 5 nm in size. Nucleation of the inorganic phase is confined to the pores between subunits in the S-layers. Two-tier stacks of nanoparticles can be formed in the presence of double-layered protein crystals. The structural diversity of S-layers¹⁹,²⁰, their ease of self-assembly on a wide range of substrates and the potential for surface chemical modification suggest that this approach could be exploited to offer a wide range of ordered nanoparticle arrays.