Viral assembly of oriented quantum dot nanowires

Abstract

The highly organized structure of M13 bacteriophage was used as an evolved biological template for the nucleation and orientation of semiconductor nanowires. To create this organized template, peptides were selected by using a pIII phage display library for their ability to nucleate ZnS or CdS nanocrystals. The successful peptides were expressed as pVIII fusion proteins into the crystalline capsid of the virus. The engineered viruses were exposed to semiconductor precursor solutions, and the resultant nanocrystals that were templated along the viruses to form nanowires were extensively characterized by using high-resolution analytical electron microscopy and photoluminescence. ZnS nanocrystals were well crystallized on the viral capsid in a hexagonal wurtzite or a cubic zinc blende structure, depending on the peptide expressed on the viral capsid. Electron diffraction patterns showed single-crystal type behavior from a polynanocrystalline area of the nanowire formed, suggesting that the nanocrystals on the virus were preferentially oriented with their [001] perpendicular to the viral surface. Peptides that specifically directed CdS nanocrystal growth were also engineered into the viral capsid to create wurtzite CdS virus-based nanowires. Lastly, heterostructured nucleation was achieved with a dual-peptide virus engineered to express two distinct peptides within the same viral capsid. This work represents a genetically controlled biological synthesis route to a semiconductor nanoscale heterostructure.

Fig. 1.

Images of A7–pVIII-engineered viruses directing ZnS nanocrystal synthesis at 0°C. (a) Illustration depicting A7 peptide expression on the pVIII protein upon phage amplification and assembly, then subsequent nucleation of ZnS nanocrystals. Call-outs depict insertion of the A7 nucleotide sequences, resulting in A7 fusion protein shown as green-shaded areas. Additionally, the call-out of the engineered virus shows detail of the WT pVIII protein (gray) and the A7-engineered pVIII protein (green) composing the viral coat. (b) ADF STEM images showing the morphology of the ZnS-virus nanowires. The ZnS-virus nanowires were 560 nm long and 20 nm wide. (c) EDS mapping of S (an identical EDS mapping of Zn can be found in Supporting Text). (d) Typical HAADF STEM images of interconnected and branched viral nanowires formed through the specific binding of preformed ZnS noncrystalline nanoparticles on the viruses expressing the A7 peptide. (Inset) ED pattern of the nanowires. (e) HAADF STEM image of the control experiment using the A7 peptide-engineered viruses in CdS nanocrystal growth conditions. No viral wires were visible, only randomly precipitated large CdS particles. (Inset) ED pattern of control experiment. (Scale bars: 800 nm.)

Fig. 2.

Higher-magnification images of A7–pVIII-engineered viruses directing ZnS nanocrystal synthesis at 0°C. (a) HAADF STEM image of an individual viral ZnS-virus nanowire. (b) HAADF STEM image of a straight region of a viral nanowire at higher magnification showing the close-packed ZnS nanocrystal morphology. In this image, some areas were brighter as a result of overlapped ZnS nanocrystals (Fig. 12, which is published as supporting information on the PNAS web site). (Inset) ED pattern, taken from the area shown in b, shows the hexagonal wurtzite ZnS structure. (c) HRTEM image of a less close-packed area on the virus shown in a. (Insets) FFT of the visible individual nanocrystals as denoted by arrows. (d) HRTEM image of a less close-packed area on the virus shown in a. (Insets) FFT of the visible individual nanocrystals as denoted by arrows.

Fig. 3.

Images and characterization of A7–pVIII-engineered viruses directing ZnS nanocrystal synthesis at —25°C. (a) TEM image of the layered structure taken at 80 kV accelerating voltage. (Inset) ED of the layered structures showing the wurtzite ZnS structure and the polycrystalline status of the sample. (b) HAADF STEM image of a layered structure. (c) HRTEM of the layered structure showing close-packed and less oriented ZnS nanocrystals. (Inset) FFT of the whole image, which is consistent with the ED pattern in a.(d) HAADF STEM image of the layered structure shown in b at a higher magnification. (Inset) Schematic illustration.

Fig. 4.

Images and characterization of the J140–pVIII-engineered viruses directing CdS nanocrystal synthesis at 0°C. (a–c) ADF STEM image (a) of viral CdS–virus nanowires and corresponding EDS mapping of elements S (b) and Cd (c). (a Inset) HAADF image of the framed portion in a.(d) HAADF STEM image of the control experiment, where J140-engineered viruses were exposed to ZnS growth conditions. No viral wires were seen, only large precipitates were visible. (Inset) ED of d. (e) A typical PL image of CdS virus nanowires on the silicon substrate. The parallel red lines indicate the entrance slit to the monochromator and thus the sample area detectable by a charge-coupled-device camera. Excitation source was at 350 nm. The color bar (Left) indicates the linear PL intensity scale. The width of the nanowires is larger than that resolved by STEM because the nanowires (20 nm wide) are narrower than the spatial resolution (200 nm) of PL imaging. (f) PL spectra (Upper) and polarization degree (Lower) of a single nanowire. The polarization degree (P) is calculated from P = (I _∥ — I_⊥)/(I _∥ + I_⊥), where I_∥ and I_⊥ are PL intensity in the polarization direction that is parallel (red line Upper) and perpendicular (black line Upper) to the length of an individual nanowire, respectively. (Insets) PL images detected along the direction parallel (Left) and perpendicular (Right) to the length of the nanowire. An integrated PL polarization anisotropy ratio of up to 1.5 was observed.

Fig. 5.

Images and characterization of ZnS–CdS hybrid nanowires prepared from viruses expressing a stochastic mixture of both the A7–pVIII and J140–pVIII fusion proteins by using CdS/ZnS nanocrystal synthesis at —25°C. (a) HAADF STEM image of a viral CdS and ZnS hybrid layered structure. (Inset)ED pattern of the layered structure showing the coexistence of wurtzite CdS and ZnS phases. (b) HAADF STEM image of the layered structure at higher magnification. (Inset) Cartoon illustrating the layered structure composed of viruses and nanocrystals. (c–f) HAADF STEM image (c) of the layer structure and its corresponding EDS mapping of elements S (d), Zn (e), and Cd (f).