Electrostatic Self-assembled Films Research Articles

Effect of CTAB on the aggregation properties of Congo Red organized in layer-by-layer electrostatic self-assembled film: a spectroscopic and Atomic force microscopy study

Effect of CTAB on the aggregation properties of Congo Red organized in layer-by-layer electrostatic self-assembled film: a spectroscopic and Atomic force microscopy study

Optical Conduction Resonance in Self-Assembled Metal Nanoparticle Array-Dielectric Thin Films

Optical conduction resonance- (OCR-) enhanced third-order optical nonlinearity of two dimensional (2D) periodic gold nanoparticle array-dielectric thin films has been investigated. The third-order optical susceptibility of periodic gold nanoparticle array embedded in silica thin film shows ~104 enhancement comparing to gold nanoparticle colloids. The 2D gold nanoparticle arrays were synthesized by using the electrostatic self-assembly (ESA) technique. During the fabrication process, the positively or negatively functionalized gold nanoparticles are automatically self-aligned to establish a 2D array with a very small interparticle spacing due to the polymer shell on the metal particles. Then, a monolayer of silica can be coated on the top of the 2D metal nanoparticle array. This type of 2D gold nanoparticle array-dielectric thin films has high volume fraction of gold nanoparticles. According to the extended Maxwell-Garnett theory, this kind of films can exhibit OCR. The OCR frequency can be tuned from visible to mid-infrared by controlling the gold nanoparticle volume fraction. During OCR, the real part of the composite dielectric constant is zero to make the induced electromagnetic waves in gold nanoparticles to couple effectively within the film. The open-aperture z-scan technique is used to measure the nonlinear optical properties of the ESA films.

Organic-inorganic hybrid layer-by-layer electrostatic self-assembled film of cationic dye Methylene Blue and a clay mineral: Spectroscopic and Atomic Force microscopic investigations

The metachromic cationic dye Methylene Blue (MB) belongs to the thiazine class of organic dye. This dye cation can interact electrostatically with inorganic clay mineral Montmorillonite (MMT) to form organic-inorganic hybrid layer-by-layer (LbL) self-assembled film onto a poly(allylamine hydrochloride) coated quartz substrate. The degree of dye aggregations in LbL films was found to depend on concentrations of both MMT clay and MB in LbL self-assembled films as evidenced by UV–vis absorption spectroscopic technique. The adsorption of dye molecules to the LbL films was found to a two-step exponential process: first step was very fast and the later process had a much delay period. Also the assembling behavior and organizations of MB molecules in the host clay matrix in LbL films significantly depend on pH of dye solution from which LbL film was fabricated. For MMT/MB mixed solution the π-π* transition of absorption bands of MB shifted to lower wavelength due to the formation of large number of H-aggregates in their mixed aqueous solution. Fourier Transform infrared (in ATR mode) spectroscopic method was employed to explore the electrostatic interaction between clay MMT and MB in LbL films. Atomic Force microscopic image of hybrid LbL films revealed the surface morphology and roughness profile of the organic-inorganic hybrid molecular assemblies onto the solid substrate.

Measurement of Nonlinear Properties for PEI Tetra Sulfonated Indium Phthalocyanine Compounds Self-Assembly Film Using Improved Tophat Z-Scan Technique

The nonlinear refraction and absorption properties of a PEI (phthalocyanine indium) tetrasulphonated chlorinated phthalocyanine electrostatic self-assembled film were studied using a simple and high sensitivity technique. We adopted picosecond laser pulses as source light and modified top-hat Zscan technique with a disk and a small aperture to explore the nonlinear refraction of the film. Experimental results show that the measuring sensitivity is increased by two orders of magnitude. Through theoretical fitting, we obtain the nonlinear refraction index and nonlinear absorption coefficients of the film are 2.1×10-14 m2/W and 2.4×10-7 m/W, respectively.

Photophysical behavior of layer-by-layer electrostatic self-assembled film of azo dye Chromotrope-2R and a polycation

This communication reports the fabrication of layer-by-layer electrostatic self-assembled films of an azo dye Chromotrope-2R (CH2R) and a Polycation poly(allylamine hydrochloride) (PAH) onto solid substrate. UV–vis absorption and steady state fluorescence emission spectroscopy successfully confirm the incorporation of dye molecules onto the PAH coated quartz substrate. The adsorption behavior of CH2R onto PAH backbone in LbL films highly depends upon the variation of the microenvironment namely pH of the dye solution from which the film was fabricated. PAH layer onto quartz substrate was able to swell sufficiently in the dye solution at very high pH. The Density functional theory was also utilized here to explain the origin of various spectral transitions from the ground electronic states for both in neutral and anionic form of CH2R. In LbL films the more closure association of dye molecules causes their aggregations which are reflected in their absorption and steady state fluorescence emission spectra when compared to those of pure dye solution. Atomic force microscopic images of LbL films assembled from CH2R aqueous solution at different pH clearly reveal the change in the surface morphology of the films and different degree of association of dye molecules in LbL films deposited at various pH of CH2R.

Layer-by-layer films and colloidal dispersions of graphene oxide nanosheets for efficient control of the fluorescence and aggregation properties of the cationic dye acridine orange

Chemically derived graphene oxide (GO) nanosheets have received great deal of interest for technological application such as optoelectronic and biosensors. Aqueous dispersions of GO become an efficient template to induce the association of cationic dye namely Acridine Orange (AO). Interactions of AO with colloidal GO was governed by both electrostatic and π–π stacking cooperative interactions. The type of dye aggregations was found to depend on the concentration of GO in the mixed ensemble. Spectroscopic calculations revealed the formation of both H and J-type dimers, but H-type aggregations were predominant. Preparation of layer-by-layer (LbL) electrostatic self-assembled films of AO and GO onto poly (allylamine hydrochloride) (PAH) coated quartz substrate is also reported in this article. UV–Vis absorption, steady state and time resolve fluorescence and Raman spectroscopic techniques have been employed to explore the detail photophysical properties of pure AO, AO/GO mixed solution and AO/GO LbL films. Scanning electron microscopy was also used for visual evidence of the synthesized nanodimensional GO sheets. The fluorescence quenching of AO in the presence of GO in aqueous solution was due to the interfacial photoinduced electron transfer (PET) from photoexcited AO to GO i.e. GO acts as an efficient quenching agent for the fluorescence emission of AO. The quenching is found to be static in nature. Raman spectroscopic results also confirmed the interaction of AO with GO and the electron transfer. The formation of AO/GO complex via very fast excited state electron transfer mechanism may be proposed as to prepare GO-based fluorescence sensor for biomolecular detection without direct labeling the biomolecules by fluorescent probe.

The fabrication and enhanced nonlinear optical properties of electrostatic self-assembled film containing water-soluble chiral polymers

An ultra-thin film containing a water-soluble chiral PPV derivative and oligo-thiophene derivative was fabricated through the electrostatic self-assembly technique. The PPV and thiophene derivatives are poly{(2,5-bis(3-bromotrimethylammoniopropoxy)-phenylene-1,4-divinylene)-alt-1,4-(2,5-bis((3-hydroxy-2-(S)-methyl)propoxy)phenylenevinylene) (BHP-PPV) and 4′,3″-dipentyl-5,2′:5′,2″:5″,2″′-quaterthiophene-2,5″′-dicarboxylic acid (QTDA), respectively. The circular dichroism (CD) spectrum of BHP-PPV cast film on quartz substrate proved the chirality of BHP-PPV. The UV–vis spectra showed a continuous deposition process of BHP-PPV and QTDA. The film structure was characterized by small angle X-ray diffraction (XRD) measurement and atomic force microscopy (AFM) images. The nonlinear optical (NLO) properties of BHP-PPV/QTDA ultra-thin film with different number of bilayers were investigated by the Z-scan technique with 8ns laser pulse at 532nm. The Z-scan experimental data were analyzed with the double-sided film Z-scan theory. The BHP-PPV/QTDA film exhibits enhanced reverse saturable absorption (RSA) and self-defocusing effects, which may be attributed to the conjugated strength, chirality and well-ordered film structure. The chirality may lead to the RSA of BHP-PPV/QTDA film contrary to the SA of the other electrostatic self-assembled films without chiral units. The self-defocusing effect should be due to the thermal effect.

Preparation and Characterization of Ta2O5Films Surface Modified by Electrostatic Self-assembly Ultrathin Films

Ultrathin films of poly-diallyldimethylammonium chloride (PDDA)/polystyrene sulfonate (PSS) and PDDA/poly(3,4-ethylene dioxythiophene)-polystyrene sulfonate (PEDOT-PSS) were constructed on Ta2O5 films through layer-by-layer electrostatic self-assembly (ESA) method for a surface modification purpose. The self-assembly ability of PDDA/PSS and PDDA/PEDOT-PSS ultrathin films on Ta2O5 film was investigated. The results showed that the ESA ultrathin films can be constructed on Ta2O5 film surface with a layer-by-layer deposition method, and the Ta2O5 film covered with ESA film exhibited smoother surface roughness compared with Ta2O5 film without surface modification. Moreover, the electronic performance of Ta/Ta2O5/ESA films/polymer film capacitor structure was investigated. The results revealed that the capaci- tor composed of ultrathin ESA film exhibited better anti-voltage endurance ability and less leakage current, which resulted from the isolation of leakage current channel in Ta2O5 films because of surface coverage of ultrathin ESA film on Ta2O5. It also can be found that the thicker ultrathin ESA film for surface modification resulted in better anti-voltage endurance ability and higher equivalent series resistance (ESR) of Ta2O5 ca- pacitor. The further investigation confirmed that lower ESR of capacitor composed of PDDA/PEDOT-PSS ESA films because of high conductive ability of PEDOT-PSS. Keywords electrostatic self-assembly; Ta2O5; anti-voltage endurance ability; equivalent series resistor;

Self-Assembled Films of a Biferrocenyl-Containing Hemicyanine Derivative with SiW12O4−40: Preparation, UV-Visible Spectroscopy and Electrochemical Properties

Electrostatic self-assembled films were successfully prepared by alternating deposition of SiW12O40(4-) and a biferrocenyl-containing hemicyanine derivative on quartz or indium-tin-oxide coated glass substrates. The films were characterized by UV-visible absorption spectroscopy and cyclic voltammetry.

Electrochemical and Photoelectrochemical Investigation of New Electrostatic Self-Assembled Films Based on Prussian Blue and a Binuclear Ru(II) Complex

New hybrid multilayer films containing Prussian blue (PB) and a binuclear Ru(II) complex were successfully fabricated using the electrostatic layer-by-layer self-assembly technique. The process was carefully monitored by ultraviolet-visible (UV-vis) spectroscopy. A linear increase in the UV-vis absorbance with the number of deposited layers indicated that the film deposition was uniform and reproducible. The electrochemical and photoelectrochemical properties of these hybrid multilayer films were investigated in an aqueous Na2SO4 solution. PB and the binuclear Ru(II) complex assembled in the film were redox active with surface confined characteristics. The surface concentration and molecular area of RuL in the film were calculated according to a UV-vis absorption spectrum and cyclic voltammetry, respectively. The photocurrent generated by switching on the light irradiation of the film increased linearly as the deposited bilayers increased. The photocurrent action spectrum was in agreement with metal-to-ligand charge-transfer (MLCT) band of RuL in the absorption spectrum, which indicated that the photocurrent was generated based on MLCT excitation of the RuL in the (PB/RuL) film.

Preparation, Characterization, and Photoelectric Properties of an Electrostatically Self-Assembled Film Based on Colloidal Tungsten Trioxide and a Dinuclear Ru(II) Complex

An electrostatic self-assembled film based on a tungstic acid colloidal solution and a dinuclear Ru(II) complex [(bpy)2Ru(bpbh)Ru(bpy)2](CIO4)4 [where bpy = 2,2'-bipyrizine and bpbh = 1,6-bis-(2-(2-pyridyl)benzimidazoyl)hexane] has been prepared and characterized by means of ultraviolet-visible absorption and emission spectroscopy and cyclic voltammetry. The photoelectrochemical properties of the films have also been studied. A large anodic photocurrent of 0.8 microA/cm2 was observed when a monolayer film of ITO/WO3/the Ru complex was irradiated with polychromatic light (730 nm > lambda > 325 nm) at a positive applied potential of 0.6 V versus a saturated calomel electrode, four-fold that observed under the same experimental condition for the ITO electrode covered with only a monolayer of WO3 without the Ru(II) complex, indicating that the the anodic photocurrent mainly arose from the electron injection from the photoexcited Ru(II) complex to the conduction band of WO3.

THE THIRD- AND FIFTH-ORDER NONLINEAR REFRACTION IN THE SINGLE- AND DOUBLE-SIDED TMAF/PSS ELECTROSTATIC SELF-ASSEMBLY FILMS

The nonlinear refraction in the single- and double-sided 40-bilayer TMAF/PSS electrostatic self-assembly films were measured by closed-aperture Z-scan technique with picosecond laser pulses (with duration of 40 ps at 532 nm). The materials contain a positively charged material trifluoroacetic acid (TFA) salt of amino-substitute methanofullerene derivative monoadduct fullerene (TMAF) and a negatively charged material ploy (sodium 4-styrenesulfon ate) (PSS). The experimental results show that under the conditions of the peak intensity I0 = 7.0 GW/cm2, both the single- and double-sided films demonstrate third- and fifth-order non-linearities, and the fifth-order non-linearity has greater contribution to the nonlinear refraction than the third-order nonlinearity. The theoretical fittings also prove that the nonlinear refraction coefficients both γ and η are almost the same for the same materials no matter what the measurements are from the single- or the double-sided films.

Preparation and Second-Harmonic Generation Properties of a Self-Assembled Multilayer Film Based on Nanoporous Isopolyoxomolybdate and Bipolar Hemicyanine

An electrostatic self-assembled multilayer film has successfully been prepared by alternating adsorption of a wheel-shaped nanoporous isopolyoxomolybdate and a bipolar hemicyanine derivative, as monitored by absorption spectroscopy. The square roots of second-harmonic intensities of the film exhibit quadratic dependence on film thickness up to 10 layers. The average titled angle of the nonlinear optical chromophores to the substrate normal is derived to be 34 degrees by polarized second-harmonic generation measurements.

Investigating into a Ca2+-sensitive electrostatic self-assembly films to fabricate reproducible immunobeads for flow immunoassay

Abstract A novel strategy has been developed for fabricating reproducible bead-based flow immunoassay system by combining the merits of Ca 2+ -induced cross-linkage of sodium alginate with negative carboxyl groups of alginate. The sodium alginate was applied to modify the polyallyamine beads and render beads negative charge to immobilize antibodies via electrostatic/hydrogen bonding interaction between alginate and antibodies to realize the fabrication of immunobeads in buffer solution of 0.01 mol/l acetic-acetate (pH 7.5). The Ca 2+ -induced cross-linkage of alginate on the immunobeads can easily release antibodies from beads and regenerate a fresh alginate-modified beads for next fabrication of new immunobeads. The electrochemical flow immunoassay system based on the so-made immunobeads has been fabricated and the feasibility of the proposed strategy and qualities of so-fabricated flow immunoassay system was evaluated by applying to a competitive transferrin–transferrin antiserum immunoassay system.

Preparation, UV−Visible Spectroscopic and Electrocatalytic Properties of Electrostatic Self-Assembled Film Made of [Tb(PMo<sub>9</sub>V<sub>2</sub>O<sub>39</sub>)<sub>2</sub>]<sup>15−</sup> and a Bipolar Hemicyanine Cation

A novel electrostatic self-assembled multilayer film was successfully prepared on quartz and indium-tin-oxide (ITO) substrates by alternating adsorption of [Tb(PMo 9 V 2 O 39 ) 2 ] 15- and a bipolar hemicyanine cation of l,6-bis((E)-4-(2-(4'-dimethylaminophenyl)pyridinum))hexane. The films were investigated by UV-visible spectroscopy and cyclic voltammetry. The UV-visible spectra showed that the film was uniformly deposited up to at least 12 layers and the charge transfer between two film-forming components might occur in the film. The cyclic voltammetry on the [Tb(PMo9V2039)2] 15- -based multilayer film indicated that the film was electrocatalytically active towards the reduction of bromate.

Dielectric constant and breakdown field studies of electrostatic self-assembledmaterials

Electrical properties of electrostatic self-assembled (ESA) films were investigated using aconductive polymer and metal bar. These nanostructured polymeric films were fabricatedon gold-coated glass slides using the ESA method. The thicknesses of the films were inthe range 120–630 nm, and the films were obtained by depositing numbers ofbilayers of negatively charged Poly s-119 (PS-119) or heparin and positively chargedPoly(diallyldimethylammonium chloride) (PDDA). Measurement electrodes were fabricatedon the ESA films using conductive silver grease and a brass bar. Capacitancemeasurements were conducted to determine the dielectric constant of the ESA films overvarious temperature and frequency ranges at 1 mV and 15% relative humidity, whileelectric field breakdown tests were performed at 1000 Hz, 15% relative humidity andvarying temperatures and voltages. The test results showed that dielectric constantvalues were between 1.8 and 2.4 and breakdown field values were approximately9 kV mm−1. Based on the test results, it is concluded that this is a technique that might prove usefulin estimating the capacitance, dielectric constant and breakdown field values ofnanostructured ESA films.

Preparation of noble metal nanoparticles @ titania precursor composite electrostatic self-assembled film

Noble metal nanoparticles as Au, Pd, and Pt were successfully incorporated throughout titania precursor composite layer-by-layer electrostatic self-assembled (LbL-ESA) film under basic condition because the LbL-ESA film is rich in ‘free’ imine groups, which can behave as ion-adsorption sites to the anionic precursors. The precursors were bound to the imine groups, resulting in formation of nanoparticles in the film through reduction. In case of Au composites, repetition of ion exchange–reduction treatment led to increase of the concentration of Au loaded and growth of these crystal sizes. The subsequent formation of nano particles at LbL-ESA film was convenient process to fabricate nanocomposites, and has a wide range of applications in the fields of catalysis, optoelectronics, and sensing materials.

Nonresonant Saturation Absorption in ElectrostaticSelf-Assembly Films Containing Methanofullerene

Ultrathin films containing methaofullerene are prepared byelectrostatic layer-by-layer self-assembly of a positively chargedtrifluoroacetic acid salt of monoamino-substituted methanofullerene(TMAF) derivative and a negatively charged poly (sodium4-styrenesulfonate) (PSS). Absorptive optical nonlinearities of 30bilayer TMAF/PSS films and TMAF in water solution are measured usingthe Z-scan technique at 532 nm. Nonresonant saturation absorption isobserved in the film sample, whereas reverse saturation absorption wasfor TMAF in water. The saturation-absorption behaviour is interpretedby a special formula. The saturation intensity is extracted to be15.2±0.8 MW/cm2. The mechanism of saturation absorption in theTMAF/PSS film is discussed.

Enhanced absorption and electro-optic Pockels effect of electrostatically self-assembled CdSe quantum dots

The spectrum and electro-optic properties of CdSe quantum dots are studied. Spectrum wavelength shifts that are due to the quantum size effect and to the electro-optic Stark effect are investigated. It is found that CdSe quantum dot-polymer composites formed by an electrostatic self-assembly (ESA) technique exhibit high internal electric fields. Using the second-order perturbation theory of the 1s-1s energy shift (Stark effect), we estimate the internal field of the ESA film to be as high as 2.6 x 10(8) V/m. This value results in a much higher absorption coefficient and electro-optic coefficients for ESA films than for their bulk crystal counterparts or for spin-coated film samples. The relationships among unusual spectra, film structure, and high electro-optic response are analyzed. These results are useful both for understanding the physical mechanisms of semiconductor quantum dots and for developing high-performance photonic devices.

Dielectric Properties of Electrostatic Self-Assembled (ESA) Films

AbstractCapacitance measurements were conducted to determine the dielectric constant of electrostatic self assembly (ESA) films over various temperature and frequency ranges at 1 mV and 15% relative humidity. Measurement electrodes were fabricated on the ESA films using silver grease and a brass bar. In these tests, polymeric nanofilms were fabricated on gold-coated glass slides using the ESA process. Thicknesses of the films were between 100 nm and 600 nm obtained by depositing different number bilayers of negatively charged Poly s-119 (PS-119) or heparin and positively charged poly(diallyldimethylammonium chloride) (PDDA). The test results showed that dielectric constant values were around 2. Based on the test results, we concluded that this is a technique that might prove useful to estimate the capacitance and dielectric constant values of nanostructured ESA films, which can be largely used in the near future.