Complex Colloid Crystal Structures by Vertical Transfer Deposition

U. Jonas

Fabrication of nanostructured materials using submicrometer colloidal particles has attracted great interest of scientists, not only for the possibility to lead to advanced materials such as photonic crystals which may be used to confine, switch and amplify light in optical devices as the result of the periodicity. They also show great potential in the field of sensors, when modifyed with appropriate functional groups, like specific biological ligands for biosensors. Binary colloidal crystals consisting of large (L) and small (S) spheres possess more complex crystal structures and exhibit richer phase behaviours, however unlike mono-colloidal crystals (composed of only one type of particles), binary colloidal crystals have not been investigated extensively because they are much more difficult to grow and characterize. In such binary colloidal crystals proteins might be immobilized as specific sensing groups for the optical detection of biological targets. By vertical lifting deposition, binary colloidal crystals with layer numbers ranging from one to 87 over an area of several square centimeters were directly co-crystallized in a reasonable time (within 15 hours). The lattice geometry was varied by changing the size ratio (γS/L) and relative concentrations between small and large particles. The morphology of the small particle clusters in the voids at the crystal surface could further be changed from singlets to triplets by simply varying the relative concentration of the large particles and small particles. This versatile method could also be used for the fabrication of inverse opals, when the size ratio was small enough so that the small particles can homogeneously fill the interstitial space. When using silica nanoparticles with an average diameter of 10 nm the inverse opals were obtained by calcination of the binary mixture to remove the polystyrene template. As demonstrated by UV-vis spectra, the shift of the stop band for the neat polystyrene opal, the binary mixture, and the silica inverse opal is due to the change of the effective refractive index (which depends on the volume filling fraction). Current experiments explore the potential to obtain systems with a higher structural hierarchy by modifying the inner surface of the inverse silica opals with silane layers and by electroless metal deposition.
In a second strategy, ordered 3D arrays of polyaniline (PANI) inverse opals were fabricated via electrochemical methods by using colloidal crystals of polystyrene beads as sacrificials templates. Compared with films obtained by wet chemical synthesis, the inverse opaline samples obtained by electrochemistry showed a superior structural quality. To explore potential biosensing applications, PANI composite inverse opals were fabricated for the first time by modifying the structure with different dopants, such as poly(acrylic acid) (PAA), poly(styrenesulfonate) (PSS), etc. It was found that these additives had a major effect on the structural stability of the obtained opaline films, with suitable dopants (like PSS) leading to PANI composite inverse opals with very high quality. These films remained electroactive in buffer solutions of neutral pH, and owing to their huge surface area might be ideal candidates for biosensing applications, e.g., as electrocatalyst or bioreactors. First effort to use such mesoporous structures as electrocatalysts for the oxidation of reduced ß-nicotinamide adenine dinucleotide (NADH) showed a substantially higher electrocatalytic efficiency of the inverse opline film compared to an unpatterned film.

Figure:
top/left: colloid crystal film of polystyrene paricles (diameter of dL = 839 nm) obtained by vertical lifting deposition;
top/middle: top view of binary colloidal crystal prepared from two types of polystyrene spheres (dL = 839 nm, dS = 187 nm);
top/right: inverse PANI opal from electropolymerization after solvent extraction of the PS template;
bottom: UV-vis spectra with corresponding stop bands for the pure PS template (middle), the PS-silica cocrystal (right) and the inverse silica opal (left) after pyrolysis of the PS template particles.

Charles-André Fustin, Gunnar Glasser, Hans W. Spiess, Ulrich Jonas; "Parameters Influencing the Templated Growth of Colloidal Crystals on Chemically Patterned Surfaces", Langmuir 2004, 20 (21), 9114-9123; Web Release Date: 11-Sep-2004; (Research Article) DOI: 10.1021/la0489413
W. Cheng, J. J. Wang, U. Jonas, W. Steffen, G. Fytas, R. S. Penciu, E. N. Economou; "The spectrum of vibration modes in soft opals", J. Chem. Phys. 2005, 123 (12), 121104-1-4
S. Tian, J. Wang, U. Jonas and W. Knoll; "Inverse Opals of Polyaniline and Its Copolymers Prepared by Electrochemical Techniques", Chem. Mater., 2005, 17 (23), 5726-5730