Preparation and characterization of protein-nanotube conjugates

This chapter describes methods of immobilizing proteins on carbon nanotubes, using two different routes—physical adsorption and covalent attachment. We also provide an overview on how such conjugates can be characterized with the help of various techniques, such as Raman, Fourier transform infrared (FT-IR), circular dichroism (CD), and fluorescence spectroscopies, in addition to the standard enzyme kinetic analyses of activity and stability. Both the attachment routes—covalent and noncovalent—could be used to prepare protein conjugates that retained a significant fraction of their native structure and function; furthermore, the protein conjugates were operationally stable, reusable, and functional even under harsh denaturing conditions. These studies therefore corroborate the use of these immobilization methods to engineer functional carbon nanotube-protein hybrids that are highly active and stable

Adhesion induced mesoscale instability patterns in thin PDMS-metal bilayers

We show that the surface of a thin elastomer-metal (aluminum) hybrid bilayer becomes spontaneously patterned when brought in adhesive contact with a rigid surface. The self-organized surface patterns show three distinct morphological phases-columns, labyrinths, and holes-depending on the area of contact. The characteristic wavelength of these patterns is found to be 2.94±0.20 times the total film thickness, independent of the morphological phase and the surface properties of the contacting surface. Interestingly, the metal films 60-120 nm thick showed the same scaling, but the bilayers with thicker metal films were completely stable. This observation demonstrates for the first time a "hard" transition to the instability as the elastic stiffness of the film is varied. We also report a protocol for alignment of the instability patterns and for transferring the metal patterns to another surface

Contact instability of elastic bilayers: miniaturization of instability patterns

We show, both experimentally and theoretically, that the free surface of an elastic bilayer becomes spontaneously rough when brought in contact with another rigid surface. The lateral length scales of these self-organized structures were found to scale as: lamda =RF h, where h is the total bilayer thickness. The scale factor, RF could be modulated by the ratios of the individual film thicknesses and shear moduli. This is unlike the case of a single elastic film where the scale factor is independent of all material properties, RF ∼ 3. A linear stability analysis shows that the instability patterns in the bilayer can be tuned from the short waves (∼ 0.5 h) to long waves (∼ 8 h). Experiments show good agreement with the theoretical predictions regarding the existence of short wave deformations. Further, a rather catastrophic change in the wavelength from its minimum value, ∼h/2, to a limiting value of ∼ 3 h occurs by very small changes in the film thicknesses. This effect is explained by a switching of states in the bilayer energy curve which displays two minima at different wave numbers. Thus, the contact instability of elastic bilayers suggests novel strategies for the control of adhesion and engineering of feature sizes in a wide range. In particular, these findings have implications for further pattern miniaturization in the elastic contact lithography using pre-patterned stamps

Contact instability of elastic bilayers: Miniaturization of instability patterns

We show, both experimentally and theoretically, that the free surface of an elastic bilayer becomes spontaneously rough when brought in contact with another rigid surface. The lateral length scales of these self-organized structures were found to scale as: lambda = R(F)h, where h is the total bilayer thickness. The scale factor, R-F could be modulated by the ratios of the individual film thicknesses and shear moduli. This is unlike the case of a single elastic film where the scale factor is independent of all material properties, RF similar to 3. A linear stability analysis shows that the instability patterns in the bilayer can be tuned from the short waves (similar to 0.5 h) to long waves (similar to 8 h). Experiments show good agreement with the theoretical predictions regarding the existence of short wave deformations. Further, a rather catastrophic change in the wavelength from its minimum value, similar to h/2, to a limiting value of similar to 3 h occurs by very small changes in the film thicknesses. This effect is explained by a switching of states in the bilayer energy curve which displays two minima at different wave numbers. Thus, the contact instability of elastic bilayers suggests novel strategies for the control of adhesion and engineering of feature sizes in a wide range. In particular, these findings have implications for further pattern miniaturization in the elastic contact lithography using pre-patterned stamps

Contact instability of elastic bilayers: miniaturization of instability patterns

We show, both experimentally and theoretically, that the free surface of an elastic bilayer becomes spontaneously rough when brought in contact with another rigid surface. The lateral length scales of these self-organized structures were found to scale as: λ= RF h, where h is the total bilayer thickness. The scale factor, RF could be modulated by the ratios of the individual film thicknesses and shear moduli. This is unlike the case of a single elastic film where the scale factor is independent of all material properties, RF ˜ 3. A linear stability analysis shows that the instability patterns in the bilayer can be tuned from the short waves (˜ 0.5 h) to long waves (˜ 8 h). Experiments show good agreement with the theoretical predictions regarding the existence of short wave deformations. Further, a rather catastrophic change in the wavelength from its minimum value, ˜h/2, to a limiting value of ˜ 3 h occurs by very small changes in the film thicknesses. This effect is explained by a switching of states in the bilayer energy curve which displays two minima at different wave numbers. Thus, the contact instability of elastic bilayers suggests novel strategies for the control of adhesion and engineering of feature sizes in a wide range. In particular, these findings have implications for further pattern miniaturization in the elastic contact lithography using pre-patterned stamps

Self-organized meso-patterning of soft solids by controlled adhesion: elastic contact lithography

The surface of a soft elastic film becomes unstable and deforms when a rigid flat plate is brought into its contact proximity, without application of any external pressure. These isotropic undulations have a characteristic wavelength, lamda∼3H, where H is the film thickness. The wavelength is independent of the adhesive interactions and the mechanical properties of the film. We present here a mini-review of our recent work on techniques of aligning, modulating, and ordering the instability structures by the use of simple 1-D patterned stamps, by changing the stamp-surface separation, by slow shearing of a flat stamp and by confining the instability in soft narrow channels. The generality of the technique for different soft materials is illustrated by patterning cross-linked polydimethylsiloxane (PDMS), aluminum coated PDMS and hydrogels films. Use of a flexible stamp such as a metal foil provides enhanced conformal contact by adhesive forces, which aids large area patterning without critically maintaining a parallel configuration and uniform pressure between the stamp and the film. The technique has the potential to develop into a new soft lithography tool—“Elastic Contact Lithography” suitable for rapid, large area micron and sub-micron self-organized patterning of a variety of soft materials without any special equipments

Structure, Function, and Stability of Enzymes Covalently Attached to Single-Walled Carbon Nanotubes

We describe the structure, activity, and stability of enzymes covalently attached to single-walled carbon nanotubes (SWNTs). Conjugates of SWNTs with three functionally unrelated enzymeshorseradish peroxidase, subtilisin Carlsberg, and chicken egg white lysozymewere found to be soluble in aqueous solutions. Furthermore, characterization of the secondary and tertiary structure of the immobilized proteins by circular dichroism and fluorescence spectroscopies, respectively, and determination of enzyme kinetics revealed that the enzymes retained a high fraction of their native structure and activity upon attachment to SWNTs. The SWNT−enzyme conjugates were also more stable in guanidine hydrochloride (GdnHCl) and at elevated temperatures relative to their solution counterparts. Thus, these protein conjugates represent novel preparations that possess the attributes of both soluble enzymeshigh activity and low diffusional resistanceand immobilized enzymeshigh stabilitymaking them attractive choices for applications ranging from diagnostics and sensing to drug delivery