Covalent molecular assembly of multilayer dendrimer ultrathin films in supercritical medium.

Ultrathin films containing dendrimers are fabricated on amine- and anhydride-derivatized silicon dioxide surface through alternate layer-by-layer (LbL) assembly of pyromellitic dianhydride (PMDA) and poly(amidoamine) (PAMAM) dendrimer in supercritical carbon dioxide (SCCO2) with interlayer linkage established by covalent bonds. X-ray photoelectron and UV-visible absorption spectroscopies, atomic force microscopy (AFM), and ellipsometry were employed to study the interfacial chemistry, growth, morphology, and thickness of the assembled film. XPS analysis suggests that the PMDA/PAMAM interlayer covalent bond is established to completion, and functional surfaces for immobilization of the next layer are available after deposition of each layer. UV-visible absorption and ellipsometry revealed layer-by-layer growth of the film. The functional property film as a porous matrix was manifested in the reduction of the refractive index upon introduction of the dendrimer.

Polyion multilayers with precise surface charge control for antifouling.

We report on a molecular fabrication approach to precisely control surface ζ potentials of polymeric thin layers constructed by electrostatic layer-by-layer (LbL) assembly methods. The protocol established allows us to achieve surface isoelectric points (IEP) in the pH range of 6-10. Poly(acrylic acid) (PAA, a weak polyanion) and poly(diallyldimethylammonium chloride) (PDADMAC, a strong polycation) were chosen to build up the bulk films. The weak polycation polyethylenimine (PEI) was applied as a top layer. A unique feature of this approach is that the chemical composition of the top layer is not affected by the manipulation of the ζ potential of the films. Surface charge tuning is achieved by controlling the degree of ionization of the weak polyelectrolytes at various pH values and subsequent manipulation of the amount of polyelectrolyte deposited in the penultimate and last layers, respectively. Following assembly and characterization, the films were used as candidates for antifouling surfaces. The fouling behavior of barnacle cyprids and bacteria on the LbL films with similar hydrophilicity and roughness but different surface charge densities were studied. We found that more cyprids of Amphibalanus amphitrite settled on the negatively charged LbL film compared to the neutral or positively charged LbL film. In bacterial adhesion tests employing Pseudomonas, Escherichia coli, and Staphylococcus aureus, more bacteria were observed on the positively charged LbL film compared with the neutral and negatively charged LbL films, possibly as a result of the negative potential of the bacterial cell wall. The procedures proposed allow one to adjust surface isoelectric points of LbL architectures to achieve optimal antifouling performance of a given material taking into account specific pH values of the environment and the character of the fouler.

Deposition of zwitterionic polymer brushes in a dense gas medium.

Poly(sulfobetaine methacrylate) (PSBMA) films known for their resistance to nonspecific protein adsorption, cell/bacterial adhesion and biofilm formation were produced by surface initiated polymerization on a silicon surface via a batch reaction system in CO2 expanded liquid (CO2-EL) medium. Atom transfer radical polymerization (ATRP) was carried out using 2,2'-bipyridyl as ligand and CuBr as a catalyst in water/methanol mixture with trichloro[4-(chloromethyl)phenyl]silane (CMPS) used as the initiating species. The films were grown in the CO2-EL environment at a range of conditions and thickness up to 10nm. In contrast to films produced by conventional solvent systems at atmospheric pressure, the polymer films grown by the CO2-EL process showed uniform thickness and pin-hole free topography. Most importantly, the CO2-EL processed PSBMA films showed no trace of copper (used as the catalyst), thus obviating the need for post-deposition processing and avoiding adverse effects of the metal leaching during service. Finally, PSBMA films from both the conventional and CO2-EL processes were exposed to Human mesenchymal stem cells (hMSCs) and the results showed that, while in both the cases the cell proliferation rate was inhibited by the charged polymeric brush surface, the CO2-EL-processed brush exhibited inhibition to a larger extent due to the reduced occurrence of pinholes. The process can be easily exploited effectively when carrying out surface initiated polymerization on non-flat topographies, such as in trenches and nanostructured features with high aspect ratios.

Electrochemically exfoliated graphene as solution-processable, highly conductive electrodes for organic electronics.

Solution-processable thin layer graphene is an intriguing nanomaterial with tremendous potential for electronic applications. In this work, we demonstrate that electrochemical exfoliation of graphite furnishes graphene sheets of high quality. The electrochemically exfoliated graphene (EG) contains a high yield (>80%) of one- to three-layer graphene flakes with high C/O ratio of 12.3 and low sheet resistance (4.8 kΩ/□ for a single EG sheet). Due to the solution processability of EG, a vacuum filtration method in association with dry transfer is introduced to produce large-area and highly conductive graphene films on various substrates. Moreover, we demonstrate that the patterned EG can serve as high-performance source/drain electrodes for organic field-effect transistors.

Covalent molecular assembly of oligoimide ultrathin films in supercritical and liquid solvent media.

An ultrathin film of oligoimide has been fabricated on amine-modified substrates of silicon and quartz through alternate layer-by-layer (LBL) assembly of pyromellitic dianhydride (PMDA) and diaminodiphenyl ether (DDE), with interlayer links established by covalent bonds. The assembly was formed in supercritical carbon dioxide (SCCO2) and in solution (dimethyl acetamide, DMAc), and the imidization reaction was performed by thermal and chemical methods, in benzene and in the supercritical medium. X-ray photoelectron and UV-visible absorption spectroscopies, atomic force microscopy (AFM), and ellipsometry were employed to study the interfacial chemistry, growth, morphology, and thickness of the assembled film. XPS analysis confirmed the sequential deposition of PMDA and DDE through formation of amic acids. At each deposition step, surface functionalities for the assembly of the next layer were generated. The interfacial chemical reaction was almost complete in the SCF (supercritical fluid) medium, as compared to the conversions observed in conventional assembly. Both the PMDA and DDE molecules were assembled in an organized manner, resulting in uniform surface morphology. Uniform film growth was revealed from the increase of UV absorption intensity and film thickness. The overall growth and quality of the films in SCF medium were greater than that for films formed in DMAc. The results of this novel study show that an environmentally friendly solvent can be used to obtain mechanically robust and thermally stable ultrathin films with little loss of material during the imidization step. In contrast to conventional deposition of the molecular layers that utilizes liquid solvents, use of SCCO(2) avoids solvent effects and posttreatment for solvent removal, while ensuring facile transport during contact.