Spin Coating Photoactive Photosystem I–PEDOT:PSS Composite Films

Photosystem I (PSI), a naturally abundant multi-subunit protein complex known for its ability to harvest solar energy and transform it into chemical energy in photosynthesis, is mixed with an intrinsically conducting polymer (ICP) poly(3,4-ethylenedioxy­thiophene):polystyrene­sulfonate (PEDOT:PSS) to deposit well-mixed thin films via spin coating from aqueous solution. This process enables uniform, reproducible, and rapid film formation in which the composition and thickness of composite films can be readily tuned up to a few hundred nanometers. We assess the size distributions of the system in solution as well as the composition, thickness, conductivity, scalability, and photoactivity of the resulting biohybrid PSI–polymer films. The combination of the protein and ICP yields increased photocurrents and turnover numbers when compared to single-component films of the protein or ICP alone to reveal a synergistic combination of film components. Based on photocurrents and turnover numbers, the efficiency of integrating the protein with the polymer is highest at low PSI loadings where protein–polymer interactions are maximized

Controlled Levitation of Colloids through Direct Current Electric Fields

We report the controlled levitation of surface-modified colloids in direct current (dc) electric fields at distances as far as 75 μm from an electrode surface. Instead of experiencing electrophoretic deposition, colloids modified through metallic deposition or the covalent bonding of poly­(ethylene glycol) (PEG) undergo migration and focusing that results in levitation at these large distances. The levitation is a sensitive function of the surface chemistry and magnitude of the field, thus providing the means to achieve control over the levitation height. Experiments with particles of different surface charge show that levitation occurs only when the absolute zeta potential is below a threshold value. An electrodiffusiophoretic mechanism is proposed to explain the observed large-scale levitation

Rod Hydrodynamics and Length Distributions of Single-Wall Carbon Nanotubes Using Analytical Ultracentrifugation

Because of their repetitive chemical structure, extreme rigidity, and the separability of populations with varying aspect ratio, SWCNTs are excellent candidates for use as model rodlike colloids. In this contribution, the sedimentation velocities of length and density sorted single-wall carbon nanotubes (SWCNTs) are compared to predictions from rod hydrodynamic theories of increasing complexity over a range of aspect ratios from <50 to >400. Independently measuring all contributions to the sedimentation velocity besides the shape factor, excellent agreement is found between the experimental findings and theoretical predictions for numerically calculated hydrodynamic radius values and for multiterm analytical expansion approximations; values for the hydrodynamic radii in these cases are additionally found to be consistent with the apparent hydrated particle radius determined independently by buoyancy measurements. Lastly, we utilize this equivalency to calculate the apparent distribution of nanotube lengths in each population from their sedimentation coefficient distribution without adjustable parameters, achieving excellent agreement with distributions from atomic force microscopy. The method developed herein provides an alternative for the ensemble measurement of SWCNT length distributions and others rodlike particles

Interactive Forces between Sodium Dodecyl Sulfate-Suspended Single-Walled Carbon Nanotubes and Agarose Gels

Selective adsorption onto agarose gels has become a powerful method to separate single-walled carbon nanotubes (SWCNTs). A better understanding of the nature of the interactive forces and specific sites responsible for adsorption should lead to significant improvements in the selectivity and yield of these separations. A combination of nonequilibrium and equilibrium studies are conducted to explore the potential role that van der Waals, ionic, hydrophobic, π–π, and ion–dipole interactions have on the selective adsorption between agarose and SWCNTs suspended with sodium dodecyl sulfate (SDS). The results demonstrate that any modification to the agarose gel surface and, consequently, the permanent dipole moments of agarose drastically reduces the retention of SWCNTs. Because these permanent dipoles are critical to retention and the fact that SDS–SWCNTs function as macro-ions, it is proposed that ion–dipole forces are the primary interaction responsible for adsorption. The selectivity of adsorption may be attributed to variations in polarizability between nanotube types, which create differences in both the structure and mobility of surfactant. These differences affect the enthalpy and entropy of adsorption, and both play an integral part in the selectivity of adsorption. The overall adsorption process shows a complex behavior that is not well represented by the Langmuir model; therefore, calorimetric data should be used to extract thermodynamic information

Unique Toxicological Behavior from Single-Wall Carbon Nanotubes Separated via Selective Adsorption on Hydrogels

Over the past decade, extensive research has been completed on the potential threats of single-wall carbon nanotubes (SWCNTs) to living organisms upon release to aquatic systems. However, these studies have focused primarily on the link between adverse biological effects in exposed test organisms on the length, diameter, and metallic impurity content of SWCNTs. In contrast, few studies have focused on the bioeffects of the different SWCNTs in the as-produced mixture, which contain both metallic (m-SWCNT) and semiconducting (s-SWCNT) species. Using selective adsorption onto hydrogels, high purity m-SWCNT and s-SWCNT fractions were produced and their biological impacts determined in dose–response studies with <i>Pseudokirchneriella subcapitata</i> as test organism. The results show significant differences in the biological responses of <i>P. subcapitata</i> exposed to high purity m- and s-SWCNT fractions. Contrary to the biological response observed using SWCNTs separated by density gradient ultracentrifugation, it is found that the high-pressure CO conversion (HiPco) s-SWCNT fraction separated by selective adsorption causes increased biological impact. These findings suggest that s-SWCNTs are the primary factor driving the adverse biological responses observed from <i>P. subcapitata</i> cells exposed to our as-produced suspensions. Finally, the toxicity of the s-SWCNT fraction is mitigated by increasing the concentration of biocompatible surfactant in the suspensions, likely altering the nature of surfactant coverage along SWCNT sidewalls, thereby reducing potential physical interaction with algal cells. These findings highlight the need to couple sample processing and toxicity response studies

Analyzing Surfactant Structures on Length and Chirality Resolved (6,5) Single-Wall Carbon Nanotubes by Analytical Ultracentrifugation

The structure and density of the bound interfacial surfactant layer and associated hydration shell were investigated using analytical ultracentrifugation for length and chirality purified (6,5) single-wall carbon nanotubes (SWCNTs) in three different bile salt surfactant solutions. The differences in the chemical structures of the surfactants significantly affect the size and density of the bound surfactant layers. As probed by exchange of a common parent nanotube population into sodium deoxycholate, sodium cholate, or sodium taurodeoxycholate solutions, the anhydrous density of the nanotubes was least for the sodium taurodeoxycholate surfactant, and the absolute sedimentation velocities greatest for the sodium cholate and sodium taurodeoxycholate surfactants. These results suggest that the thickest interfacial layer is formed by the deoxycholate, and that the taurodeoxycholate packs more densely than either sodium cholate or deoxycholate. These structural differences correlate well to an observed 25% increase in fluorescence intensity relative to the cholate surfactant for deoxycholate and taurodeoxycholate dispersed SWCNTs displaying equivalent absorbance spectra. Separate sedimentation velocity experiments including the density modifying agent iodixanol were used to establish the buoyant density of the (6,5) SWCNT in each of the bile salt surfactants; from the difference in the buoyant and anhydrous densities, the largest hydrated diameter is observed for sodium deoxycholate. Understanding the effects of dispersant choice and the methodology for measurement of the interfacial density and hydrated diameter is critical for rationally advancing separation strategies and applications of nanotubes