Self-assembled Multilayer Films Research Articles

Preparation and Electrochemical and Electrocatalytic Properties of Nanocomposite Multilayer Film Based on a Keggin-Type Phosphomolybdate

An electrostatically self-assembled multilayer film has successfully been prepared on quartz or indium-tin-oxide substrates by alternating adsorption of a Keggin-type phosphomolybdate H3PMo12O40 (PMo12) and 1,10-diaminodecane (1,10-DAD). The formation of (PMo12/1,10-DAD), film was investigated by ultraviolet (UV) spectroscopy. The UV spectra showed that the deposition of the films was uniform and reproducible. The cyclic voltammetry on the PMo12-based multilayer film indicated that the PMo12 anions in the film undergo rapid four 2e(-)/2H+ redox processes, and electrocatalytically active towards the reduction of bromate.

Modulated insulin release from glucose-sensitive multilayer films

Summary Self-assembled multilayer films were fabricated to modulate insulin release in response to glucose concentration changes. These films showed remarkable glucose-sensitive characteristics and were nontoxic in vitro.

DNA Biosensors Based on Layer-by-Layer Self-Assembled Multilayer Films of Carbon Nanotubes and Gold Nanoparticles

The paper has been withdrawn do to double publication. The original was published by SPIE Proceedings of SPIE, vol 7493, 74935A in 2009 (DOI:10.1117/12.840316)

A sensitive and regenerable biosensor for organophosphate pesticide based on self‐assembled multilayer film with CdTe as fluorescence probe

A fluorescence biosensor for organophosphorus pesticides was developed. A pH indicator, CdTe quantum dots, were used as an optical transducer of the inhibition of enzyme by analyte. Through the intervening agency of chitosan, the recognition elements (acetylcholinesterase and CdTe) were immobilized onto the surface of quartz by electrostatic attraction to form a self-assembled multilayer film. In the absence of pesticide, acetylcholine was biocatalytically hydrolysed to yield acetic acid and choline. The released acid resulted in pH decrease, which was sensed by the immobilized pH indicator (CdTe). In the presence of pesticide, the action of acetylcholine was reduced; the fluorescence intensity of the film changed and was related to the concentration of pesticide. This multilayer film could be used as the biosensor for monocrotophos, with a detection limit of 3.20 × 10(-8) mol/L; the sensitivity was 100 times higher than that of CdTe in aqueous solution. The sensor was easily regenerated, and had good stability and selectivity for organophosphorus pesticides.

Nano-TiO2 based multilayer film deposition on cotton fabrics for UV-protection

Nano-TiO2 based multilayer nanocomposite films were fabricated on cationically modified woven cotton fabrics by layer-by-layer molecular self-assembly technique. Cationization process was used to obtain cationic surface charge on cotton fabrics. Attenuated total reflectance Fourier transform infrared spectroscopy analyses were used to verify the presence of cationic surface charge and multilayer films deposited on the fabrics. Scanning electron microscope micrographs of poly(sodium 4-styrene sulfonate)/TiO2, nano polyurethane/TiO2, and TiO2/poly(diallyldimethylammonium chloride) multilayer films deposited on cotton fabrics were taken. With nano-TiO2 based multilayer film deposition, the protection of cotton fabrics against UV radiation is enhanced. The UV protection durability of the self-assembled multilayer films deposited on the cotton fabrics was analyzed after 10 and 20 washing cycles at 40 °C for 30 min. Air permeability and whiteness value analysis were performed on the untreated and multilayer film deposited cotton fabrics. The effect of layer-by-layer deposition process on tensile strength properties of the warp and weft yarns was determined.

Layer-by-layer self-assembly polytungstogermanate multilayer films and their photocatalytic properties under sunlight irradiation

The dilacunary decatungstogermanate [γ-GeW 10O 36] 8− (GeW 10) and the monolacunary undecatungstogermanate [β 2-GeW 11O 39] 8− (GeW 11) were employed as polyanions to layer-by-layer self-assembly with cationic thionine (TH) dye. The (GeW 10/TH) n and (GeW 11/TH) n composite films were characterized by UV–vis spectra, atomic force microscopy images (AFMs) and cyclic voltammograms (CV). Their photocatalytic activities were evaluated by photocatalytic degradation of methyl orange aqueous solution under sunlight irradiation. The results indicated that the (GeW 10/TH) n and (GeW 11/TH) n films exhibited the sunlight photocatalytic activities for methyl orange (MO) solution. The dye molecule TH extended the light absorption spectrum of polyoxometalates toward the visible light region. The photocatalytic mechanism of composite films was also discussed.

Controllable disintegration of temperature-responsive self-assembled multilayer film based on polybetaine

A layer-by-layer (LbL) self-assembly technique was used to construct temperature-responsive multilayer films of polybetaine poly-3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate (PDMAPS) and poly(acrylic acid) (PAA). The assembly and disintegration behaviors of PDMAPS/PAA multilayer films were investigated by quartz crystal microbalance (QCM) and atomic force microscopy (AFM). The results indicated that the assembly behaviors of PDMAPS/PAA multilayer films could be well regulated by the pH and temperature of dipping solutions. The disintegration of PDMAPS/PAA multilayer films could be well controlled by varying the disintegration temperature ( T d) and the pH of buffer solution. The critical disintegration temperature ( T cd) decreased with increasing pH of buffer solution used. The ionization of carboxylic acid groups (COOH) of PAA and the breakage of intra-chain associations of PDMAPS in multilayer films are two main reasons for the film disintegration. Based on the controllable disintegration of the PDMAPS/PAA multilayer, free-standing multilayer films were successfully prepared by LbL self-assembly of poly(4-vinylpyridiniomethanecarboxylate) (PVPMC) and PAA using PDMAPS/PAA as a sacrificial sublayer.

Fabrication of robust multilayer films by triggering the coupling reaction between phenol and primary amine groups with visible light irradiation

We prepared robust cross-linked (x-linked) multilayer films under visible light irradiation with the catalysis of a Ru(ii) complex. The x-linking is achieved by the coupling reaction between phenol group and primary amine group within the self-assembled multilayer films that were prepared beforehand. Three kinds of polymers, i.e., poly(4-vinylphenol), poly(allylamine) and poly(ethyleneimine), were selected as the model system to illustrate the concept of this strategy. Upon visible light irradiation, the chemical stability of the x-linked films towards solution etching was greatly enhanced. In previous studies, horseradish peroxidase (HRP) is often utilized to catalyze the C-C, C-O and C-N coupling structures, which is useful to prepare polymers, capsules and bulk hydrogels. We also tried to prepare the x-linked films by the catalysis of HRP. The comparison of the two methods suggests that the Ru(ii) complex method is more ideal for fabricating x-linked films. In addition, the photo-triggered chemical reaction within the films was confirmed by the solid-state (13)C NMR, XPS and FT-IR measurements. Without UV light irradiation or thermal treatment, this strategy brings many advantages. It is anticipated that this approach can be easily extended to the applications of the biological related fields in the future.

Preparation of Self-Assembled Zinc Oxide Nanoparticles Multilayer Films Under Ultrasonic Irradiation

Zinc oxide nanoparticles were synthesized and self-assembled on the reactive surface of a glass slide functionalized with (3-mercaptopropyl)-trimethoxysilane under ultrasonic irradiation. The structure, morphology, and optical property of the zinc oxide nanoparticles were investigated by TEM, XRD, and UV-vis spectroscopy. The functionalized glass slide was soaked in an aqueous solution which dispersed zinc oxide nanoparticles under ultrasonic irradiation. Zinc oxide multilayer films grew up to several layers (up to 5 layers) depending on the immersion time. The self-assembled zinc oxide nanoparticles multilayer films were characterized using UV-vis spectroscopy and SEM. Ultrasonic irradiation was an efficient method to make multilayer films on the functionalized glass slide with zinc oxide nanoparticles.

Electrostatic layer-by-layer self-assembly multilayer films based on graphene and manganese dioxide sheets as novel electrode materials for supercapacitors

A new class of multilayer films was constructed by electrostatic layer-by-layer self-assembly, using poly(sodium 4-styrenesulfonate) mediated graphene sheets (PSS-GS), manganese dioxide (MnO2) sheets, and poly(diallyldimethylammonium) (PDDA) as building blocks. UV-vis spectroscopy, field-emission scanning electron microscopy and X-ray photoelectron spectroscopy were used to characterize the microstructures and morphologies of the multilayer films. Capacitive properties of the synthesized multilayer film electrodes were studied using cyclic voltammetry and galvanostatic charge/discharge in 0.1 M Na2SO4 electrolyte. The specific capacitance of the ITO/(PDDA/PSS-GS/PDDA/MnO2)10electrode reached 263 F g−1 at a discharge current density of 0.283 A g−1; moreover, this film electrode also shows a good cyclic stability and high Coulombic efficiency. Anticipatedly, the synthesized multilayer films will find promising applications as a novel electrode material in supercapacitors and other devices in virtue of their outstanding characteristics of controllable capacitance, good cycle stability, low cost and environmentally benign nature.

Controlled insulin release from glucose-sensitive self-assembled multilayer films based on 21-arm star polymer

A glucose-sensitive multilayer film was fabricated by the layer-by-layer (LbL) assembly method with positively charged 21-arm poly[2-(dimethylamino)ethyl methacrylate] (star PDMAEMA) and negatively charged insulin and glucose oxidase (GOD) in the form of {(Star PDMAEMA/Insulin) 4 + (Star PDMAEMA/GOD) 4 + Star PDMAEMA}. The multilayer film shows an on-off regulation of insulin release in response to stepwise glucose challenge in vitro. It is found that the unique structure of star PDMAEMA and interdiffusion of charged insulin are the main factors to control the on-off status of the film. Reversible surface morphology transitions of the multilayer film were also observed, revealing a phase separation and large-scale reorganization process. Furthermore, the multilayer film could continuously release enough insulin in vivo after being subcutaneously implanted in streptocozotin-induced diabetic rats and reduce the blood glucose level for at least two weeks. It is indicated that such system may have substantial potential as a glucose-sensitive carrier for insulin due to its distinct mechanism.

Mechanism of Formation and Construction of Self-Assembled Myoglobin/Hyaluronic Acid Multilayer Films: An Electrochemical QCM, Impedance, and AFM Study

Self-assembled multilayer films of hyaluronic acid (HA) and the protein myoglobin (Mb) were built up layer by layer on Au covered quartz crystal microbalance (AuQCM) electrode substrates. Film formation and growth were monitored by an electrochemical quartz crystal microbalance (EQCM), and the step-by-step construction was investigated through quantification of the mass variation corresponding to adsorption of each monolayer together with cyclic voltammetry. A decrease of friction at the liquid/electrode interface was observed, indicating that the electrode surface becomes less rough after deposition of several monolayers. The properties of the {HA/Mb}(n) films were evaluated by electrochemical impedance spectroscopy (EIS). Modeling of the impedance spectra shows smoothing of the modified electrode surface with reorganization of the film structure after few monolayers, and the contribution of each layer to the electron transfer process was analyzed. The smoothing of the surfaces and the structural differences between successive bilayers was confirmed by morphological observations by using atomic force microscopy.

Fabrication and Optoelectronic Properties of Novel Films Based on Functionalized Multiwalled Carbon Nanotubes and (Phthalocyaninato)Ruthenium(II) via Coordination Bonded Layer-by-Layer Self-Assembly

4-(2-(4-pyridinyl)Ethynyl)benzenic diazonium salt (PBD) was used to modify multiwalled carbon nanotubes (MWCNTs) by the self-assembly technique. After the decomposition of the diazonium group in PBD under UV irradiation, the PBD monolayer film covalently anchored on multiwalled carbon nanotubes is very stable. The obtained pyridine-modified MWCNTs (Py(Ar)-MWCNTs) have good solubility in common organic solvents. Furthermore, the layer-by-layer (LBL) self-assembled fully conjugated films of Py(Ar)-MWCNTs and (phthalocyaninato)ruthenium(II) (RuPc) were fabricated on the PBD-modified substrates, and characterized using UV-vis absorption spectroscopy, scanning electron microscopy (SEM), and electrochemistry. The UV-vis analysis results indicate that the LBL RuPc/Py(Ar)-MWCNTs self-assembled multilayer films with axial ligands between the ruthenium atom and pyridine group were successfully fabricated, and the progressive assembly runs regularly with almost equal amounts of deposition in each cycle. A top view SEM image shows a random and homogeneous distribution of Py(Ar)-MWCNTs over the PBD-modified silicon substrate, which indicates well independence between all Py(Ar)-MWCNTs. Moreover, the opto-electronic conversion was also studied by assembling RuPc/Py(Ar)-MWCNTs multilayer films on PBD-modified ITO substrate. Under illumination, the LBL self-assembled films on ITO showed an effective photoinduced charge transfer because of their conjugated structure and the ITO current density changed with the number of bilayer. As the number of bilayers was increased, the photocurrent increases and reaches its maximum value (∼300 nA/cm(2)) at nine bilayers. These results allow us to design novel materials for applications in optoelectronic devices by using LBL self-assembly techniques.

The fabrication of nanocomposite thin films withTiO2 nanoparticles by the layer-by-layer deposition method for multifunctional cottonfabrics

A multilayer nanocomposite film composed of anataseTiO2 nanoparticles was fabricated on cationically modified woven cotton fabrics by thelayer-by-layer molecular self-assembly technique. For cationic surface charge, cotton fabricswere pre-treated with 2,3-epoxypropyltrimethylammonium chloride (EP3MAC) by apad-batch method. Attenuated total reflectance Fourier transform infrared spectroscopy(FTIR), x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM)were used to verify the presence of deposited nanolayers. Photocatalytic activities of thenanocomposite films were evaluated through the degradation of red wine pollutant. Nano-TiO2 deposition enhanced the protection of cotton fabrics against UV radiation incomparison with the untreated cotton fabrics. Air permeability and whitenessvalue analysis was performed on the fabrics before and after the treatment withTiO2 nanoparticles by the layer-by-layer deposition method. Tensile strength tests ofthe warp and weft yarns were performed to evaluate the effect of solution pHvalue changes during the alternate dipping procedures. For the first time thedurability of the effect of the self-assembled multilayer films on the cottonfabric functional properties was analyzed after 10 and 20 washing cycles at40 °C for 30 min.

Electrochromic properties of self-assembled nanoparticle multilayer films

Hexagonal tungsten bronze (HTB) nanocrystal and TiO 2 nanoparticles were assembled into thin films by layer-by-layer self-assembly method. HTB nanocrystals were synthesized by hydrothermal route at 155 °C. UV–Vis spectra showed that the HTB/TiO 2 films exhibit a linear increase in film thickness with assembly exposure steps. The electrochromic property of the film was carefully investigated. Cyclic voltammetry indicated that the redox peak was around −0.5 V. The electrochromic contrast, coloration efficiency, switching speed, stability and optical memory were carefully investigated. The films vary from white to blue and finally dark brown. The electrochromic contrast is 63.9% at 633 nm. The coloration efficiency of the films is relatively high. The response time is less than 3 s.

Electrochemical behaviour of self-assembly multilayer films based on iron-substituted α-Keggin polyoxotungstates

Ultrathin multilayer films containing metal-substituted polyoxometalates, [PW 11Fe III(H 2O)O 39] 4 − (PW 11Fe) or [SiW 11Fe III(H 2O)O 39] 5 − (SiW 11Fe), and poly(ethylenimine) (PEI) were prepared by the electrostatic layer-by-layer self-assembly method on a glassy carbon electrode. The multilayer films were characterized by cyclic voltammetry and scanning electron microscopy and UV–Vis absorption spectroscopy on a quartz slide was used to monitor film growth. Cyclic voltammetry indicates that the electrochemical properties of the polyoxometalates are completely maintained in the multilayer films, and the influence of scan rate on the voltammetric features showed that the first tungsten reduction process for immobilized PW 11Fe and SiW 11Fe is a surface-confined process. Studies with [Fe(CN 6)] 3 − /4 − as electrochemical probe showed that their permeability depends on the thickness of the multilayer films, if the outermost layer is negatively charged. Additionally, the (PEI/SiW 11Fe) n multilayer films showed electrocatalytic properties towards nitrite reduction.

Novel Azobenzene-Functionalized Polyelectrolytes of Different Substituted Head Groups 2: Control of Surface Wetting in Self-Assembled Multilayer Films

A novel set of light-responsive polyelectrolytes has been developed and studied, to control and tune surface wettability by introducing various types of substituted R head-groups of azo polyelectrolytes in self-assembled multilayer (SAMU) films. As part of a larger project to develop polymer surfaces where one can exert precise control over properties important to proteins and cells in contact, photo-reversibly, we describe here how one can tune quite reliably the contact angle of a biocompatible SAMU, containing a photo-reversible azo chromophore for eventual directed cell growth. The azo polyelectrolytes described here have different substituted R head-group pairs of shorter-ionized hydrophilic COOH and SO3H, shorter non-ionized hydrophobic H and OC2H5, and larger non-ionized hydrophobic octyl C8H17 and C8F17, and were employed as polyanions to fabricate the SAMU onto silicon substrates by using the counter-charge polycation PDAC. The prepared SAMU films were primarily characterized by measurement of their contact angles with water. The surface wetting properties of the thin films were found to be dependent on the type of substituted R-groups of the azo polyelectrolytes through their degree of ionization, size, hydrophobicity/hydrophilicity, solubility, conformation, and inter-polymeric association and intra-polymeric aggregation. All these factors appeared to be inter-related, and influenced variations in hydrophobic/hydrophilic character to different extents of aggregates/non-aggregates in solution because of solvation effects of the azo polyanions, and were thus manifested when adsorbed as thin films via the SAMU deposition process. For example, one interesting observation is significantly higher contact angles of ⩾79° for SAMU films of larger octyl R groups of PAPEA-C8F17 and PAPEA-C8H17 than for others with contact angles of ⩽64° observed for non-polar R-groups of OC2H5 and H. Furthermore, lower contact angle values of ⩽59° for SAMU films with polar R-groups of COOH and SO3H relative to that of non-polar R-groups are in accordance with their expected order of the hydrophilicity or hydrophobicity. It is possible that the large octyl groups are more effective in shielding the ionic functional groups on the substrate surface, and contributed less to the water drop-molecule interactions with ionic groups of the PDAC and/or AA groups. In addition, higher hydrophobicity of the SAMU films may be due to the incorporation of bulky and hydrophobic groups in these polyelectrolytes, which can produce aggregates on the surfaces of the SAMU films. Through understanding and controlling the complex aggregation behavior of the different substituted R-groups of these azo polyelectrolytes, and hence their adsorption on substrates, it appears possible to finely tune the surface energy of these biocompatible films over a wide range, enhance the photo-switching capabilities of the SAMU films, and tailor other surface properties for the development and application of new devices in diverse areas of microfluidics, specialty coatings, sensors, and biomedical sciences.

Novel Azobenzene-Functionalized Polyelectrolytes of Different Substituted Head Groups 3: Control of Properties of Self-Assembled Multilayer Thin Films

Novel azobenzene polyelectrolytes have been used to fabricate biocompatible self-assembled multilayer (SAMU) thin films of variable absorbance, thickness, organization, and morphology. The prepared SAMU films are useful for directed cell growth, and this application relies directly on control of contact and surface energy, and requires the ability to tune the surface characteristics which are critical to their development. The azo polyelectrolytes employed here were similar in their degree of polymerization and repeat unit composition of acrylic acid monomer and azo monomers, and only differ from each other due to the presence of different substituted head R-groups present on the p-position of the aromatic ring of the azo chromophores. Possession of characteristics of both the self-assembly due to acrylic acid groups, and photoswitchability of the azo monomer enable the azobenzene functionalized polyelectrolytes to exhibit novel photo-reversible applications. The azo polyelectrolytes with the substituted R-group pairs of shorter-ionized hydrophilic COOH and SO3H, shorter-non-ionized hydrophobic H and OC2H5, and larger-nonionized hydrophobic octyl C8H17 and C8F17 were used as polyanions and counter charge PDAC used as polycation to fabricate the layer-by-layer SAMU films onto glass and silicon substrates. The fabricated SAMU films were also characterized by various techniques. The UV absorption maxima, λmax p of the SAMU films move to lower wavelength relative to solution to exhibit a blue shift for the hydrophobic R-groups, while this behaviour was not observed for the hydrophilic R-groups. Similarly, the thickness, organization, morphology and other properties of the thin films were found to be dependent on the type of substituted R-groups of the azo polyelectrolytes due to the inter-related factors of ionization, hydrophobicity/hydrophilicity, solubility, and aggregation of azo PEL in the dipping solutions used for fabrication of the SAMU films. Understanding and controlling the adsorption characteristics of azo multilayer thin–film of switchable functionalities are vital to explore their potential for the development and application of new devices in diverse areas of biosensor, drug delivery systems, on-chip microscale chemical process and microfluidics systems.

Stability and electrochemical properties of full-conjugated self-assembled films based on polyaniline and its acid-substituted derivates

The stability of full-conjugated self-assembled (SA) multilayer films based on partially doped polyaniline (PANI) as a polycation and poly(o-aminobenzoic acid) (PCAN), poly(aniline-2-sulfonic acid) (PSAN) as polyanions is investigated in alkali aqueous solutions. The self-assembled PANI-PCAN films keep their stability within 24 h in 1 mol/L NaOH solution, -the PANI-PSAN films, however, maintain the stability for 20 min in the same condition because the solubility of PSAN in alkali solutions is much higher than that of PCAN. The electrochemical properties of the SA films are determined, and the film-CdS composites formed in situ are also reported.

층상자기조립법을 이용한 나노구조체의 제조와 응용

층과 층 사이의 정전기적인력, 수소결합 또는 공유결합을 이용하여 층당 두께를 수 옹스트롱에서부터 수십 나노미터까지 제조할 수 있으며 박막의 표면 형태를 흡착시키고자 하는 물질 및 박막 후처리 공정을 통해 제어할 수 있으며 더 나아가, 삽입하는 물질의 특성에 따라 박막의 기능성을 집적화 및 다양화시킬 수 있다. 본 연구에서는 이러한 층상자기조립방법의 특성을 이용하여 반사방지막, 초소수성 필름 및 전기화학센서로의 응용가능성을 제시하였다. 반사방지막의 경우, 구형의 블록공중합체를 유리기판 위에 다층박막으로 적층시킴으로써 박막 굴절률을 1.25까지 감소시켰고 이를 통해 약 99.5%의 빛 투과도를 달성할 수 있었다. 더 나아가 바이오물질인 엔자임을 다층박막에 삽입시킬 경우, 활성 산소를 분해시키는 전기화학센서로의 제조가 가능함을 보인다. 본 연구는 본인이 이미 발표한 논문(J. Am. Chem. Soc. 128, 9935 (2006); Adv. Mater. 19, 4364 (2007); Electro. Mater. Lett. 3, 163 (2007))들을 정리하여 층상자기조립법에 관해 소개하는 논문이다. We introduce a novel and versatile approach for preparing self-assembled nanoporous multilayered films with antireflective properties. Protonated polystyrene-block-poly (4-vinylpyrine) (PS-b-P4VP) and anionic polystyrene-block-poly (acrylic acid) (PS-b-PAA) block copolymer micelles (BCM) were used as building blocks for the layer-by-layer assembly of BCM multilayer films. BCM film growth is governed by electrostatic and hydrogen-bonding interactions between the oppositely BCMs. Both film porosity and film thickness are dependent upon the charge density of the micelles, with the porosity of the film controlled by the solution pH and the molecular weight (Mw) of the constituents. PS7K-b-P4VP28K/PS2K-b-PAA8K films prepared at pH 4 (for PS7K-b-P4VP28K) and pH 6 (for PS2K-b-PAA8K) are highly nanoporous and antireflective. In contrast, PS7K-b-P4VP28K/PS2K-b-PAA8K films assembled at pH 4/4 show a relatively dense surface morphology due to the decreased charge density of PS2K-b-PAA8K. Films formed from BCMs with increased PS block and decreased hydrophilic block (P4VP or PAA) size (e.g., PS36K-b-P4VP12K/PS16K-b-PAA4K at pH 4/4) were also nanoporous. Furthermore, we demonstrate that the nanostructured electrochemical sensors based on patterning methods show the electrochemical activities. Anionic poly(styrene sulfonate) (PSS) layers were selectively and uniformly deposited onto the catalase (CAT)-coated surface using the micro-contact printing method. The pH-induced charge reversal of catalase can provide the selective deposition of consecutive PE multilayers onto patterned PSS layers by causing the electrostatic repulsion between next PE layer and catalase. Based on this patterning method, the hybrid patterned multilayers composed of platinum nanoparticles (PtNP) and catalase were prepared and then their electrochemical properties were investigated from sensing <TEX>$H_2O_2$</TEX> and NO gas. This study was based on the papers reported by our group. (J. Am. Chem. Soc. 128, 9935 (2006); Adv. Mater. 19, 4364 (2007); Electro. Mater. Lett. 3, 163 (2007)).