Highly photoluminescent multilayer QD-glass films prepared by LbL self-assembly

A novel and facile preparation method for layer-by-layer (LbL) self-assembled films incorporating quantum dots (QDs) and having intense photoluminescence (PL) from blue to red is presented. Functional sol-gel-derived glass layers prepared by the hydrolysis of 3-aminopropyltrimethoxysilane (APS) or 3-mercaptopropyltrimethoxysilane (MPS) have been used as a linkage between QD layers. Absorption, PL spectroscopy, transmission electron microscopy, and atomic force microscopy were employed for characterization, which revealed that the QDs in the prepared films had a nearly close-packed coverage and were not aggregated. The PL efficiencies of the QDs (CdTe or ZnSe, both are thioglycolic acid-stabilized) dispersed in the films were roughly half that of the initial colloidal solutions but reached 24% before a refractive index correction. The thickness of the red-emitting film with 10 CdTe QD layers was approximately 50 nm. The concentration of QDs in the film derived from the first absorption peak was approximately 0.01 M. Because the PL starts to show a red shift, the obtained concentration is practically the ultimate one in the glass matrix. The mercapto, amino, and carboxyl groups play important roles in LbL self-assembling processes.     

Polyelectrolyte multilayer films of different charge density copolymers with synergistic nonelectrostatic interactions prepared by the layer-by-layer technique

Random copolymers composed of diallyldimethylammonium chloride (DADMAC) and acrylamide with varying contents (8-100 mol %) of the cationic DADMAC component were alternated with polyanionic, fully charged poly(styrenesulfonate) to form multilayer thin films. UV-vis spectrophotometry, FTIR spectroscopy, and quartz-crystal microgravimetry (QCM) were employed to follow multilayer buildup. Atomic force microscopy was used to obtain structural information. Layer thicknesses have been determined with small-angle X-ray scattering and ellipsometry, in addition to values calculated from QCM. While in previous work, a critical charge density limit could be observed, below which no layer growth is possible; in this system, multilayer formation takes place with copolymers with charge densities as low as 8 mol %. Instead of a continuous increase of adsorbed amounts with decreasing charge density above the critical charge density, as found in previous work, similar layer thicknesses for films with 100 and 8 mol % charged polyelectrolytes and maximally adsorbed amounts for copolymers in an intermediate charge density region have been found. This adsorption behavior is explained in terms of synergistic nonelectrostatic interactions between the polyelectrolytes used.     

Near-infrared electrochromism of multilayer films of a cyclometalated diruthenium complex prepared by layer-by-layer deposition on metal oxide substrates

Science China Chemistry - A cyclometalated diruthenium complex 2 bridged by 1,2,4,5-tetra(pyrid-2-yl)benzene with six carboxylic acid groups at two ends was synthesized. Monolayer and multilayer...     

Selective layer-by-layer self-assembly on patterned porous films modulated by Cassie-Wenzel transition

We describe a robust and facile approach to the selective modification of patterned porous films via layer-by-layer (LBL) self-assembly. Positively charged honeycomb-patterned films were prepared from polystyrene-block-poly(N,N-dimethyl-aminoethyl methacrylate) (PS-b-PDMAEMA) and a PS/PDMAEMA blend by the breath figure method followed by surface quaternization. Alginate and chitosan were alternately deposited on the films via LBL self-assembly. The assembly on the PS-b-PDMAEMA film exhibits two stages with different growth rates, as elucidated by water contact angles, fluorescence microscopy, and quartz crystal microbalance results. The assembly can be controlled on the top surface or across all surfaces of the film by changing the number of deposition cycles. We confirm that there exists a Cassie-Wenzel transition with an increase in deposition cycles, which is responsible for the tunable assembly. For the PS/PDMAEMA film, the pores can be completely wetted and the polyelectrolytes selectively assemble inside the pores, instead of on the top surface. The controllable selective assembly forms unique hierarchical structures and opens a new route for surface modification of patterned porous films.     

Characterization of layer-by-layer self-assembled multilayer films of diblock copolymer micelles

The in situ layer-by-layer (LbL) self-assembly of low Tg diblock copolymer micelles onto a flat silica substrate is reported. The copolymers used here were a cationic poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (50qPDMA-PDEA; 50q refers to a mean degree of quaternization of 50 mol % for the PDMA block) and zwitterionic poly(methacrylic acid)-block-poly(2-(diethylamino)ethyl methacrylate) (PMAA-PDEA), which has anionic character at pH 9. Alternate deposition of micelles formed by these two copolymers onto a silica substrate at pH 9 was examined. The in situ LbL buildup of the copolymer micelle films was monitored using zeta potential measurements, optical reflectometry, and a quartz crystal microbalance with dissipation monitoring (QCM-D). For a six layer deposition, complete charge reversal was observed after the addition of each layer. The OR data indicated clearly an increase in adsorbed mass with each additional micelle layer and suggest that some interdiffusion of copolymer chains between layers and/or an increase in the film roughness, and hence in the effective surface area of the micellar multilayers, must take place as the film is built up. QCM-D data indicated that the self-assembled micellar multilayers on a flat silica substrate undergo structural changes over a prolonged period. This is attributed to longer-term interdiffusion of the copolymer chains between the outer two layers after the initial adsorption of each layer is complete. The QCM-D data further suggest that the outer adsorbed layers adopt a progressively more extended conformation, particularly for the higher numbered layers. The morphology of each successive layer was characterized using in situ soft-contact atomic force microscopy, and micelle-like surface aggregates are clearly observed within each layer of the complex film, suggesting the persistence of aggregate structures throughout the multilayer structure.     

Nanocomposite silica/polyamine films prepared by a reactive layer-by-layer deposition

Composite material nanofilms of controlled thickness constituted by ceramics and polymers find more and more applications to improve the properties and functionalities of material surfaces. In this paper we present a new way to deposit such composite coatings by alternated contacts of a surface with a polyamine solution (either poly(allylamine), poly(ethyleneimine), poly-l-lysine, or poly(diallydimethylammonium)chloride and silicic acid. The experiments are mainly realized by spraying of solutions onto the surface. The polyamines deposited in the first spraying step catalyze silica formation upon further spraying of a silicic acid-containing solution. The film thickness increases linearly with the number of deposition steps, the thickness increment being of the order of a few nanometers per silicic acid/polyamine layer. Infrared spectroscopy in the total attenuated reflection mode reveals spectra that are close to those of pure silica particles. The film morphology is further investigated by means of atomic force microscopy and environmental scanning electron microscopy. This reactive layer-by-layer deposition constitutes a new way to build, in an easy way, nanocomposite coatings with precise control of their thickness.     

Photoluminescence properties of multilayer oxide films intercalated with rare earth ions by the layer-by-layer technique

Multilayer oxide films consisting of a TiO-Eu3+-TiO-Tb3+-NbO-Tb3+-NbO-Eu3+ unit which was prepared by the layer-by-layer technique, showed photoluminescence with a high intensity containing both red and green lights.     

Adsorption of novel thermosensitive graft-copolymers: Core-shell particles prepared by polyelectrolyte multilayer self-assembly

The adsorption properties of thermosensitive graft-copolymers are investigated with the aim of developing self-assembled multilayers from these copolymers. The copolymers consist of a thermoreversible main chain of poly(N-isopropylacrylamid) and a weak polyelectrolyte, poly(2-vinylpyridine), as grafted side chains. Zeta-potential, single particle light scattering and adsorption isotherms monitor the adsorption of the thermoreversible copolymers to precoated colloidal particles. The results show a smaller surface coverage for a larger density of grafted chains. The surface coverage is discussed in terms of surface charge density in the adsorbed monolayer. Taking into account the monolayer adsorption properties, conditions are developed for the multilayer formation from these copolymers. A low pH provides a sufficient charge density of the grafted chains to achieve a surface charge reversal of the colloids upon adsorption. The charge reversal after each adsorbed layer is monitored by zeta-potential and the increase of the thickness is determined by light scattering. Stable and reproducible multilayers are obtained. The results imply that the conformation of the thermosensitive component in multilayers depends strongly on the grafting density, where the polymer with a higher grafting density adsorbs in a flat conformation while that with a lower grafting density adsorbs with more loops.     

Controlled growth of micropatterned, oriented calcite films on a self-assembled multilayer film

A micropatterned multilayer film, which was fabricated from layer-by-layer electrostatic self-assembly of nitrodiazoresin (NDR)/poly(acrylic acid) (PAA) followed by photolithography, was utilized as a structured template for the biomimetic mineralization of calcium carbonate. Micropatterned CaCO3 films consisting of regularly aligned calcite crystals oriented in the <104> direction were selectively deposited on the patterned NDR/PAA multilayer film.     

逐层交替自组装法制备葡萄糖氧化酶电极

采用了分子沉积法制备葡萄糖氧化酶电极.将带有正电荷的聚二甲基二丙烯铵盐(PDDA)和带有负电荷的葡萄糖氧化酶(GOD)交替沉积在修饰有3-巯基-1-丙基磺酸盐(MPS)的金电极表面.该电极以甲酸二茂铁衍生物(Fc-COOH)为电子媒介体,促进电子在葡萄糖氧化酶和金电极表面的传递.通过循环伏安法(CV)检测酶电极的活性,在0.332 V出现一对典型的可逆二茂铁氧化还原峰(见图1),其峰电流的大小和扫描速率的平方根成正比,即由扩散控制电流大小.该电极在溶液中对葡萄糖的线性响应范围上限为6.63 mmol/L(见图2),检测限为0.547mmol/L,响应时间为9.44s.在不同的pH值测试其对葡萄糖的响应,结果表明pH为7.0时为最佳反应条件.在同一浓度下重复测试电流响应,其标准偏差为0.152,其重复使用性能稳定.     

An electrochemiluminescence sensor for determination of durabolin based on CdTe QD films by layer-by-layer self-assembly

This work reported for the first time the use of flow injection electrochemiluminescence (FI-ECL) sensor for the determination of durabolin in an aqueous system based on CdTe quantum dot (QD) films. Aqueous CdTe colloidal solutions were prepared using thioglycolic acid as a capping agent. Zetasizer Nano ZS (Malvern, UK) was employed to characterize the size of CdTe QDs. The UV–vis and photoluminescence spectra of samples were systematically characterized. Indium tin oxide (ITO) slide glass was modified with CdTe QDs by layer-by-layer self-assembly. CdTe QD films were packed into a homemade cell and used as a recognizer of the FI-ECL sensor to determine durabolin. The intensive anodic ECL emission was obtained at a starting potential of +1.3 V (vs. Ag/AgCl) in a carbonate bicarbonate buffer solution with a pH of 9.93 at an ITO electrode. The ECL intensity was correlated linearly with the concentration of durabolin over the range of 1.0 × 10⁻⁸–1.0 × 10⁻⁵ g mL⁻¹, and the detection limit was 2.5 × 10⁻⁹ g mL⁻¹. The relative standard deviation for the determination of 1.0 × 10⁻⁶ g mL⁻¹ durabolin was 1.04% (n = 11). This simple and sensitive sensor revealed good reproducibility for ECL analysis. As a result, the new FI-ECL sensor had been successfully applied to the determination of durabolin in food samples. This strategy could be easily realized and opened new avenues for the applications of QDs in ECL biosensing.     

Polyelectrolyte layer-by-layer self-assembly enhanced by electric field and their multilayer membranes for separating isopropanol–water mixtures

An electric field enhanced method is developed for fabricating layer-by-layer (LbL) self-assembly polyelectrolyte multilayer membranes. Three kinds of electric field enhanced polyelectrolyte multilayer membranes (EPEMs), poly(diallyl dimethylammonium chloride)/poly(styrenesulfonate sodium salt) (PDDA/PSS), poly(diallyldimethylammonium chloride)/poly(acrylic acid sodium salt) (PDDA/PAA) and polyethylenimine/poly(acrylic acid sodium salt) (PEI/PAA), were self-assembled on a reverse osmosis membrane (ROM). The pervaporation performances of EPEMs for separating isopropanol–water mixtures (90/10, w/w) are all superior to those of corresponding normal self-assembled polyelectrolytes membranes (PEMs), and the selectivity increases with PDDA/PSS, PDDA/PAA and PEI/PAA in order. For (PEI/PAA)₄PEI EPEM, the separation factor is 1075 and permeation flux is 4.05 kg m⁻² h⁻¹ at 70 °C. This novel method speeds up the LbL process, which makes it promising for the practical application of the LbL multilayer membrane.     

Internal structure of nanoporous TiO2/polyion thin films prepared by layer-by-layer deposition

The internal structure of porous TiO2 films prepared by electrostatic layer-by-layer deposition was investigated. The films were prepared by alternate dipping of solid substrates into dispersions of TiO2 nanoparticles and polycations, polyanions, or pure buffer solution, respectively. The surface charge of the amphoteric TiO2 particles was controlled by the pH of the aqueous dispersions. The morphology of the film surface was investigated by means of scanning electron microscopy. It was found that the surface roughness strongly depends on the polymeric material used for the deposition process but is independent of the ionic strength of the solution or the molecular weight of the polyions. The samples with rough surfaces feature strong light scattering. The porosity and internal structure of the TiO2/polyelectrolyte films were investigated by adsorption/desorption of dye molecules. A crude estimate yields an internal surface that is up to 160 times the plane surface of the substrate for a film thickness of 1 microm. The composition of the films was investigated by X-ray photoelectron spectroscopy (XPS). Detection of the XPS signal after each deposition step of the first three dipping cycles shows a significant increase of the relative surface coverage of Ti after the TiO2 deposition step and of PSS after the PSS deposition step. For later dipping cycles, such an increase was also detectable but less prominent.     

Human serum albumin self-assembly on weak polyelectrolyte multilayer films structurally modified by pH changes

Adsorption of proteins onto film surfaces built up layer by layer from oppositely charged polyelectrolytes is a complex phenomenon, governed by electrostatic forces, hydrogen bonds, and hydrophobic interactions. The amounts of the interacting charges, however, both in polyelectrolytes and in proteins adsorbed on such films are a function of the pH of the solution. In addition, the number and the accessibility of free charges in proteins depend on the secondary structure of the protein. The subtle interplay of all these factors determines the adsorption of the proteins onto the polyelectrolyte film surfaces. We investigated the effect of these parameters for polyelectrolyte films built up from weak "protein-like" polyelectrolytes (i.e., polypeptides), poly(L-lysine) (PLL), and poly(glutamic acid) (PGA) and for the adsorption of human serum albumin (HSA) onto these films in the pH range 3.0-10.5. It was found that the buildup of the polyelectrolyte films is not a simple function of the pure charges of the individual polyelectrolytes, as estimated from their respective pKa values. The adsorption of HSA onto (PLL/PGA)n films depended strongly on the polyelectrolyte terminating the film. For PLL-terminated polyelectrolyte films, at low pH, repulsion, as expected, is limiting the adsorption of HSA (having net positive charge below pH 4.6) since PLL is also positively charged here. At high pH values, an unexpected HSA uptake was found on the PGA-ending films, even when both PGA and HSA were negatively charged. It is suggested that the higher surface rugosity and the decrease of the alpha-helix content at basic pH values (making accessible certain charged groups of the protein for interactions with the polyelectrolyte film) could explain this behavior.     

Tunable disintegration of layer-by-layer assembly multilayer films based on hydrolytical-polybetaine at wide-range time

Many efforts have been performed on the poly(vinylidene fluoride), PVDF, due to its piezoelectric, pyroelectric and ferroelectric potentials. In this regard, how to fabricate the PVDF with high content of β-phase, which is also the direct contribution to PVDF's prominent property, becomes a critical issue. In this study, starting with the α-phase dominated sample, the PVDF with extremely high content of β-crystalline phase was obtained by the incorporation of multiwalled carbon nanotubes (MWCNTs) modified by hyperbranched copolymers (HBCs). We proved that, via XRD, DSC as well as the structural characterizations from the polarized optical microscopy and transmission electron microscopy (TEM), the success of this strategy was ascribed to the enhanced dispersibility and stability of MWCNTs endowed by the HBCs, which significantly favors the formation of the β-crystalline phase of PVDF.     

Driving forces for layer-by-layer self-assembly of films of SiO2 nanoparticles and heme proteins

Heme protein hemoglobin (Hb) or myoglobin (Mb) and silica nanoparticles in a variety of charge states were assembled layer-by-layer into films on solid surfaces to investigate the driving forces for film assembly. Cyclic voltammetry (CV), quartz crystal microbalance (QCM), X-ray photoelectron spectroscopy (XPS), and UV-vis and reflectance absorption infrared (RAIR) spectroscopy were used to characterize the different [SiO2/protein]n films. Even when the proteins and silica were both negatively charged, stable layer-by-layer [SiO2/protein]n films were successfully fabricated, although amounts of protein were smaller than when nanoparticles and proteins had opposite charges. Results suggest the importance of localized Coulombic attractions between the negative nanoparticle surface and positively charged amino acid residues on the Mb or Hb surfaces in the assembly and for the stability of [SiO2/protein]n films.     

Macroscopic, free-standing ag-reduced, graphene oxide janus films prepared by evaporation-induced self-assembly

A facile, efficient, and unique self-assembly process for the preparation of the macroscopic, free-standing, Ag-reduced, graphene oxide (Ag-RGO) Janus films, which exhibit a unique asymmetry of their two surfaces with macroscopic dimensions, is presented. A novel strategy using an evaporation-induced, self-assembly (EISA) process is shown to be a powerful and flexible method for synthesizing well-defined Janus thin films.     

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

Seven different types of thio- and/or amine-modified cellulose resin materials were synthesized and their mercury (II) ion adsorption properties determined. All seven resins showed good mercury (II) adsorption capability in the more neutral pH regions. However, the o-benzenedithiol- and o-aminothiophenol-modified cellulosic resins were found to be very effective in removing mercury (II) ions from strongly acidic media. For example, 93.5-100% mercury (II) ion recoveries from very acid aqueous solutions (nitric acid concentration ranged from 0.1 to 2.0 mol/L) were obtained using the o-benzenedithiol-modified resin while recoveries ranged from ca. 50% to 60% for the o-aminothiophenol-modified resin. An adsorption capacity of 23 mg (as Hg atoms) per gram of resin was observed for the o-benzenedithiol-modified cellulose in the presence of 1.0 mol/L nitric acid. This same resin shows very good selectivity for mercury (II) as only ruthenium (II) also somewhat adsorbed onto it out of 14 other metal ions studied (Ag(+), Al(3+), As(3+), Co(2+), Cd(2+), Cr(3+), Cu(2+), Fe(3+), Mn(2+), Ni(2+), Pt(2+), Pb(2+), Ru(2+), and Zn(2+)).