We have developed a method of semiconductor nanostructure fabrication relying on the size and shape of a polynucleotide to dictate the overall structure of an assembly of individual nanoparticles. This is exemplified by our use of the 3455-basepair circular plasmid DNA molecule pUCLeu4 which, when anchored to a suitably derivatized substrate, yields an array of semiconductor nanoparticles matching the shape of the biopolymer stabilizer. The viability of the methodology was confirmed using data from high resolution transmission electron microscopy, selected area electron diffraction, and linear optical absorption spectroscopy. This is a unique demonstration of the self-assembly of mesoscale semiconductor nanostructures using biological macromolecules as templates.

The biopolymer calf thymus deoxyribonucleic acid (DNA) is employed to stabilize cadmium sulfide ctystallites in the quantum confinement size regime (Q-CdS). In this work, the synthesis and characterization of these semiconductor 'quantum dots' is described. These O-CdS clusters are easily prepared in aqueous solution at room temperature and are extremely stable (for more than 17 months when stored at 5'C). High-resolution transmission electron microscopy shows that the crystallites have an average diameter of 5.6nm. with lattice images and diffraction patterns consistent with the zinc-blende structure of CdS. For approximately 15% of the particles, unique hollow-sphere-or hollow-hemisphereshaped CdS structures are observed. and their presence attributed to the influence of the DNA host. Spectroscopically. these clusters show an absorption edge blueshifted from that of the bulk, consistent with quantum confinement, and broad trap emission characteristic of an appreciable number of defect sites at t h e semiconductor ciuster interface. apparentiy induced in pari by the hosi polynucleotide. The effects of the Q-CdS clusters on the macroscopic propelties of the DNA are illustrated by the change in intrinsic viscosity upon addition of cadmium ions and subsequent CdS formation.

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The intrinsic interaction between cadmium hydroxide-layered Q-CdS clusters and polynucleotides has been examined via photoluminescence (PL) spectroscopy. For semiconductor clusters of 40-A average diameter and a narrow luminescence maximum near 480 nm, quenching of this emission can be accomplished by the addition of polynucleotides such as deoxyribonucleic acid (DNA) from E. coli or polyadenylic acid (poly [A]). The observed PL area changes can be fit to a Perrin model, with the calculated volume of the quenching sphere for poly [A] being more than an order of magnitude for that of DNA, 4.5 X 10' A3 versus 4.4 X lo3 A3. After exposure to polynucleotides, these CdS surfaces are shown to be reactive, as attempts to grow ZnS layers at the interface result in an overall enhancement of trap emission in the 500-700-nm region. IntroductionControl of particle size and surface structure continues to be of interest in the investigation of stable quantum confined ("Qsized") semiconductor clusters.' The surface of these particles is important to semiconductor cluster photophysics, since (1) the charge carriers can localize on surface defects and (2) a large percentage of the total atom composition exists on the surface of these clusters. Several different approaches to the deliberate surface modification of these Qstate materials have km reported. These include a layering of cadmium hydroxide on CdS2 and ZnS on CdSe3 to remove the defect sites responsible for lower energy trap emission. The binding of amines,' (dimethylamino). methylferrocene? and lanthanide &diketonate complexes6 to the surface of Q-CdS enhances its overall photoluminescence (PL), while aliphatic thiols' and ammonia8 reduce the number of surface defects when these mgenta are present during particle formation.One recent approach under investigation in our laboratories is the usc of polynuclsotide stabilizers as a uscfd and unique probe of the Q-size semiconductor cluster/stabilizer interface. Previously, we have reported the synthesis and characterization of To whom correspondence should be addressed. 50-A-diameter Q-CdS semiconductor clusters in the presence of calf thymus DNA, polyadenylic acid (poly[A]), a polycytidylic acid (poly[C]) and noted that the nature of the semiconductor luminescence is sensitive to the type of polynucleotide utilized? For the case of QCdS synthesized in the presence of calf thymus DNA, existing experimental results suggest a physical picture of a strong cluster-DNA interaction.1° This, in turn, brings about the question of the intrinsic affinity of polynucleotides for semiconductor surfaces. Thus, we have subsequently studied the interaction between hydroxide-layered Q-CdS clusters and the polynucleotides DNA and polyadenylic acid. The parent Q-CdS clusters in this case exhibit narrow band edge PL in the range 465-500 nm, We demonstrate that the addition of polynucleotides, E. coli DNA and poly[A], quenches overall emission in this region through a modification of the cadmium hydroxide-layered CIS surface. Subsequent attempts...

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