Facile and efficient approach to speed up layer-by-layer assembly: dipping in agitated solutions

A facile and efficient approach has been developed to speed up the fabrication of LBL films through sequential dipping in vigorously agitated solutions. By this agitated-dipping (AD) LBL technique, the multilayer films of PAH and PSS were fabricated. The resulting films were explored by UV-vis spectroscopy, X-ray reflectivity, and AFM. Meanwhile, the comparison of the AD and conventional LBL films was made, which demonstrated that AD LBL can decrease dipping time by more than 15 times without reducing film quality remarkably. In addition, to verify the generality of AD LBL, we studied the AD LBL films of PDDA/PSS and PAH/PAA preliminarily as well. AD LBL promotes the efficiency of conventional LBL greatly while preserving its most advantages, such as simplicity, cheapness, precise control, universality in substrates, recycling use of sample solutions, and so on. It would be a promising alternative to build up LBL films rapidly.     

Mg-mediated self-assembly of an amphiphilic pyrene derivative with single-stranded DNA

An amphiphilic ethynyl-pyrene derivative modified with phosphonic acid groups (PyTOH) was synthesized and characterized. Single-stranded DNA (ssDNA), which carries negative charges, was found to induce the self-assembly of PyTOH through electrostatic interactions with Mg²⁺ as co-complexation ion in aqueous solution. UV-vis, fluorescence, and circular dichroism (CD) spectra revealed that left-handed helical architecture of PyTOH/Mg²⁺/DNA was formed. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that nanoscale structures could be fabricated by self-assembly.     

Surface modification of poly(ethylene terephthalate) via hydrolysis and layer-by-layer assembly of chitosan and chondroitin sulfate to construct cytocompatible layer for human endothelial cells

Surface modification of poly(ethylene terephthalate) (PET) film was performed by surface hydrolysis and layer-by-layer (LBL) assembly followed a mechanism of electrostatic adsorption of oppositely charged polymers, exemplified with chitosan and chondroitin sulfate (CS). Hydrolysis of PET in concentrated alkaline solution produced a carboxyl-enriched surface. The changes of weight loss and surface chemistry, morphology and wettability were monitored and verified by UV-vis spectroscopy, atomic force microscopy (AFM) and water contact angle. Assembly of positively charged chitosan and negatively charged CS was then conducted in a LBL manner to create multilayers on the hydrolyzed PET film. The process of layer growth and oscillation of surface wettability were monitored by UV-vis spectroscopy and water contact angle measurement, respectively. In vitro cell culture revealed that the adherence of endothelial cells was significantly enhanced on the biomacromolecules-modified PET film with preserved endothelial cell function, in particular on those assembled with larger number of chitosan/CS layers. However, with regard to cell proliferation and viability properties after cultured for 4 days, minor difference was determined between the modified and the unmodified PET films.     

Rapid microarray detection of DNA and proteins in microliter volumes with surface plasmon resonance imaging measurements

A four-chamber microfluidic biochip is fabricated for the rapid detection of multiple proteins and nucleic acids from microliter volume samples with the technique of surface plasmon resonance imaging (SPRI). The 18 mm × 18 mm biochip consists of four 3 μL microfluidic chambers attached to an SF10 glass substrate, each of which contains three individually addressable SPRI gold thin film microarray elements. The 12-element (4 × 3) SPRI microarray consists of gold thin film spots (1 mm(2) area; 45 nm thickness), each in individually addressable 0.5 μL volume microchannels. Microarrays of single-stranded DNA and RNA (ssDNA and ssRNA, respectively) are fabricated by either chemical and/or enzymatic attachment reactions in these microchannels; the SPRI microarrays are then used to detect femtomole amounts (nanomolar concentrations) of DNA and proteins (ssDNA binding protein and thrombin via aptamer-protein bioaffinity interactions). Microarrays of ssRNA microarray elements are also used for the ultrasensitive detection of zeptomole amounts (femtomolar concentrations) of DNA via the technique of RNase H-amplified SPRI. Enzymatic removal of ssRNA from the surface due to the hybridization adsorption of target ssDNA is detected as a reflectivity decrease in the SPR imaging measurements. The observed reflectivity loss is proportional to the log of the target ssDNA concentration with a detection limit of 10 fM or 30 zeptomoles (18?000 molecules). This enzymatic amplified ssDNA detection method is not limited by diffusion of ssDNA to the interface, and thus is extremely fast, requiring only 200 s in the microliter volume format.     

Layer-by-layer assembled multilayer of graphene/Prussian blue toward simultaneous electrochemical and SPR detection of H2O2

A new type of chemically converted graphene sheets, cationic polyelectrolyte-functionalized ionic liquid decorated graphene sheets (PFIL-GS) composite, was synthesized and characterized by Ultraviolet-visible (UV-vis) absorption, Fourier transform infrared, and Raman spectroscopy. It was found that the presence of PFIL enabled the formation of a very stable aqueous dispersion due to the electrostatic repulsion between PFIL modified graphene sheets. With respect to the excellent dispersibility of this material, we have fabricated a novel PFIL-GS/Prussian blue (PB) nanocomposite multilayer film via classic layer-by-layer (LBL) assembly. The assembly process was confirmed by UV-vis spectroscopy and surface plasmon resonance (SPR) spectroscopy, which showed linear responses to the numbers of the deposited PFIL-GS/PB bilayers. Moreover, the as-prepared composite films were used to detect hydrogen peroxide (H₂O₂) by electrochemical surface plasmon resonance (EC-SPR) spectroscopy. This real time EC-SPR technique can provide simultaneous monitoring of both optical SPR signal and electrochemical current responses upon injecting H₂O₂ into the reaction cell. The experimental results revealed that both the electrochemical and SPR signal exhibited splendid linear relationship to the concentration of the injected H₂O₂, and the detection limit could be up to 1 μM.     

A low-cost surface plasmon resonance instrument based on detection of resonance excitation wavelength

A laboratory-made surface plasmon resonance (SPR) instrument based on the detection of resonance excitation wavelength has been successfully fabricated. The performance and workability of the SPR instrument was demonstrated as a DNA biosensor. Biotinylated single-stranded oligonucleotides (ssDNA) were chemically immobilized on a gold-film surface of the SPR instrument as a DNA probe for the detection of its fully complementary, half-complementary and non-complementary ssDNA. The immobilization of the ssDNA probe was done by avidin-biotin linkage. The ssDNA used were 12-mer oligonucleotides. The sensing mechanism was based on the shift in resonance wavelength of an excitation light beam as the target ssDNA hybridized with the ssDNA on the gold-film surface. The linear dynamic ranges of the DNA biosensor for fully complementary and half-complementary ssDNA are 0.04-1.2 pM and 0.08-1.1 pM, respectively. The DNA biosensor showed higher sensitivity to fully complementary ssDNA than to half-complementary ssDNA. But no shift of resonance wavelength to the non-complementary ssDNA was observed.     

Evidence of DNA transfer through F-pilus channels during Escherichia coli conjugation

The mechanism of DNA transfer from Escherichia coli ( E. coli) Hfr donor strain AT2453 to recipient strain AB1157 during the conjugation process has been investigated by liquid atomic force microscopy (AFM). With the success of immobilizing both E. coli strains on gelatin-treated glass under aqueous solution, the F-pilus between an E. coli mating pair could be clearly imaged and dissected by an AFM probe. Another AFM probe functionalized with an anti-single-stranded DNA (ssDNA) antibody was then applied to detect transferring ssDNA. According to the AFM force spectrum, the transferring ssDNA could be detected only in the dissected area with a binding force of 109 +/- 5 pN measured. Our results provide direct evidence indicating that the DNA was transferred through the F-pilus channel between an E. coli mating pair during their conjugation.     

Fabrication and Characterization of DNA/QPVP‐Os Redox‐Active Multilayer Film

Calf thymus DNA was immobilized on functionalized glassy carbon, gold and quartz substrates, respectively, by the layer‐by‐layer (LBL) assembly method with a polycation QPVP‐Os, a quaternized poly(4‐vinylpyridine) partially complexed with osmium bis(2,2′‐bipyridine) as counterions. UV‐visible absorption and surface plasmon resonance spectroscopy (SPR) showed that the resulting film was uniform with the average thickness 3.4 nm for one bilayer. Cyclic voltammetry (CV) showed that the total surface coverage of the polycations increases as each QPVP‐Os/DNA bilayer added to the electrode surface, but the surface formal potential of Os‐centered redox reaction shifts negatively, which is mainly attributed to the intercalation of redox‐active complex to DNA chain. The electron transfer kinetics of electroactive QPVP‐Os in the multilayer film was investigated by electrochemical impedance experiment for the first time. The permeability of Fe(CN) in the solution into the multilayer film depends on the number of bilayers in the film. It is worth noting that when the multilayer film is up to 4 bilayers, the CV curves of the multilayer films display the typical characteristic of a microelectrode array. The nanoporous structure of the multilayer film was further confirmed by the surface morphology analysis using atomic force microscopy (AFM).     

Fluorescence enhancement for the complex PAMAM-BSA in the presence of photonic crystal heterostructures

The paper presents the optical characterization of the multilayer film composed of styrene (ST)-poly(ethylene glycol) methyl ether methacrylate (PEGMA 1100)-gold nanoparticles (Au)-poly(amidoamine) PAMAM (G4)-bovine serum albumin (BSA). The addition of the last layer composed of BSA resulted in an unusual optical behaviour, i.e. increase of the fluorescence emission intensity, respectively the intensity of the UV-vis reflection, compared with the ST-PEGMA 1100-Au-G4 film. The explanation could be attributed to the presence of photonic crystal heterostructures. The multilayer film has been characterized by optical microscopy, AFM, UV-vis, and fluorescence.     

Fabrication and characterization of gold nano particles for DNA biosensor applications

This research involves the preparation of a biosensor using silicon oxide for biomedical applications, and its effective use for the detection of target DNA hybridization. An electrochemical DNA biosensor was successfully fabricated by using(3-aminopropyl) tri-ethoxysilane(APTES) as a linker molecule combined with gold nanoparticles(GNPs) on a thermally oxidized SiO_2 thin film. The size of the GNPs was calculated by utilizing UV–vis data with an average calculated particle size within the range of 30±5 nm, and characterization by transmission electron microscopy(TEM) and atomic force microscopy(AFM). The GNP-modified SiO_2 thin films were electrically characterized through the measurement of capacitance, permittivity and conductivity using a low-cost dielectric analyzer. The capacitance, permittivity and conductivity profiles of the fabricated sensor clearly differentiated DNA immobilization and hybridization.     

Multilayered gold-nanoparticle/polyimide composite thin film through layer-by-layer assembly

A novel type of composite thin film consisting of gold nanoparticles (AuNPs) and polymide (PI) was fabricated through layer-by-layer (LBL) assembly. To fabricate such films, bare AuNPs and a poly (amic acid) bearing pendant amine groups, namely, amino poly (amic acid) or APAA, were synthesized and assembled in an LBL fashion. Without any organic encapsulation layer on their surface, AuNPs were bound directly to APAA chains at the amine sites; X-ray photoelectron spectroscopy study suggested that the binding was based on a combined effect of metal-ligand coordination and electrostatic interaction, with the former dominating over the latter. An approximately linear growth of the film started from the second layer of AuNP as revealed by the UV-vis spectroscopy, and the degree of particle aggregation was higher in the first AuNP layer than in the subsequent layers due to the differences in the density of binding sites. The resultant assembly was heated to imidize the APAA, thereby creating a robust composite structure.     

Photovoltaic cells based on sequentially adsorbed multilayers of conjugated poly(p-phenylene ethynylene)s and a water-soluble fullerene derivative

We describe the layer-by-layer (LBL) fabrication of multilayer films and photovoltaic cells using poly(phenylene ethynylene)-based anionic conjugated polyelectrolytes as electron donors and water-soluble cationic fullerene C60 derivatives as acceptors. LBL film deposition was found to be linearly related to the number of bilayers as monitored by UV-vis absorption. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) of the multilayer films revealed an aggregated but relatively uniform morphology devoid of any long-range phase separation. The maximum incident monochromatic photon to current conversion efficiency (IPCE) of the photovoltaic cells was 5.5%, the highest efficiency reported to date for cells fabricated by using the LBL fabrication technique, and since the thin film cells do not provide complete absorption of the incident light, the current generation per photon absorbed may be as much as 10%. The cells exhibited open circuit voltages of 200-250 mV with highest measured short circuit currents up to 0.5 mA/cm2 and fill factors around 30%. The power conversion efficiencies measured at AM 1.5 solar conditions (100 mW/cm2) varied between 0.01 and 0.04%, and similar to the IPCE results, the efficiency is a function of the thickness of the PV active layer.     

Multilayered films fabricated from plasmid DNA and a side-chain functionalized poly(beta-amino ester): surface-type erosion and sequential release of multiple plasmid constructs from surfaces

Hydrolytically degradable polyamines can be used to fabricate multilayered polyelectrolyte films that erode and release DNA in aqueous environments. Past studies have investigated films fabricated from poly(beta-amino ester) 1 and the influence of polymer backbone structure on film erosion and the release of anionic polyelectrolytes. This investigation sought to characterize the influence of polymer side-chain structure on the stability of multilayered films in physiologically relevant media. Here, we report on the fabrication and characterization of multilayered films approximately 150 nm thick assembled from plasmid DNA and side-chain functionalized polymer 2. We observed large differences in the behavior of films fabricated from polymer 2 as compared to films fabricated from polymer 1. Whereas films fabricated from polymer 1 erode and release DNA over approximately 2 days when incubated in phosphate-buffered saline, films fabricated from polymer 2 erode and release DNA over approximately 2 weeks. In addition, whereas films fabricated from polymer 1 undergo complex nanometer-scale physical transformations in aqueous media, characterization of the surfaces of films fabricated from polymer 2 by atomic force microscopy (AFM) demonstrates that the surfaces of these materials remain smooth and uniform during erosion. The apparent surface-type erosion of these materials permits the fabrication of ultrathin films with architectures that provide control over the timing and the order in which two different DNA constructs are released from surfaces. For example, the order in which two different DNA constructs are released from films and expressed by cells can be controlled to measurable extents by the relative order in which they are deposited during fabrication. These results suggest approaches to the localized and sequential release of multiple different DNA constructs to cells or tissues from the surfaces of tissue engineering scaffolds or implantable devices coated with multilayered films.     

Layer by layer assembly of DNA film via Zr(IV) ion and its intercalation property

In order to study the molecular recognition ability of DNA and different behavior of dyes incorporated into the base pairs, DNA molecule was assembled layer by layer via a Zr(IV) ion. The UV absorption spectra showed the uniform layer assembly of the DNA film. The fabricated DNA film was water-insoluble and maintained the native B-form structure. UV and CD measurements showed that the DNA film could intercalate ethidium bromide (EtBr).     

The preparation and characterization of poly(o-phenylenediamine)/gold nanoparticles interface for immunoassay by surface plasmon resonance and electrochemistry

The poly-o-phenylenediamine (PoPD) nonconducting film and gold nanoparticles (AuNPs) were combined to fabricate AuNPs/PoPD film, which is used as a novel biocompatible interface for the immobilization of antibody and develop a simple and sensitive label-free immunoassay for the detection of the related antigen (human immunoglobulin G (IgG)). Surface plasmon resonance (SPR) and electrochemical methods were used to provide the real-time information about the polymer film growth, assembling of various sizes of gold nanoparticles, anti-human IgG antibody (anti-hIgG) immobilization and the antigen–antibody interaction. The microstructures of the PoPD and AuNPs/PoPD films were characterized by atomic force microscopy (AFM). These results demonstrated that AuNPs were uniformly dispersed on the porous surface of PoPD film, which formed a nano-structure biocompatible AuNPs/PoPD interface. The use of gold nanoparticles and PoPD film could enhance the immunoassay sensitivity and anti-nonspecific property of the resulting immunoassay electrode. Additionally, the reproducibility and preliminary application of anti-hIgG/AuNPs/PoPD/Au electrode for SPR detection of hIgG was also evaluated.     

电化学石英晶体微天平实时表征和定量检测短序列DNA

利用电化学石英晶体微天平(EQCM)这一灵敏的质量和电化学传感器测定特定序列DNA。应用自组装膜技术在压电石英晶振表面自组装一带羧基的α－硫辛酸单层膜,通过盐酸1-乙基-3-(3-二甲基氨基丙基)碳二亚胺(EDC)及N-羟基琥珀酰亚胺(NHS)共价固化寡聚核苷酸为探针,用于测定与其碱基序列互补的DNA。实验中EQCM实时监测了α-硫辛酸的自组装过程、探针固化过程及其与cDNA杂交过程。定量得出了探针固化量及cDNA杂交量。在酸性、中性和碱性条件下,分别对固化和杂交过程进行表征,实验发现探针固化及DNA杂交都受pH影响,本文对此现象进行了解释。同时,利用染料Hoechst33258的电化学活性,使其与双链DNA嵌合,通过测量Hoechst33258的电化学信息进一步验证了DNA杂交关键步骤。     

The formation of fibers via complementary base pairing of DNA-conjugated bovine serum albumin

For the first time the conjugate of single stranded DNA and Bovine Serum Albumin, which could self-assemble into fibers via complementary base pairing of the DNA.     

Multilayer films of carbon nanotubes and redox polymer on screen-printed carbon electrodes for electrocatalysis of ascorbic acid

Multilayer films containing multiwall carbon nanotubes and redox polymer were successfully fabricated on a screen-printed carbon electrode using layer-by-layer (LBL) assembled method. UV-vis spectroscopy, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy and electrochemical method were used to characterize the assembled multilayer films. The multilayer films modified electrodes exhibited good electrocatalytic activity towards the oxidation of ascorbic acid (AA). Compared with the bare electrode, the oxidation peak potential negatively shifted about 350 mV (versus Ag/AgCl). Furthermore, the modified screen-printed carbon electrodes (SPCEs) could be used for the determination of ascorbic acid in real samples.     

A Novel pH/Light‐Triggered Surface for DNA Adsorption and Release

A simple strategy for the immobilization of Cy3‐labeled single strand DNA (Cy3‐ssDNA) on a Si(001) surface and its release under control of both light and pH stimuli is presented. In order to prepare a dual pH/light‐triggered surface, positively chargeable azobenzene molecules are self‐assembled on the Si(001) surface. The surface wettability of this substrate can be changed under influence of both light and pH conditions. The substrates can be positively charged under mildly acidic conditions. The pH‐sensitive behavior of the film allows binding of Cy3‐ssDNA on the functionalized Si(001) surface through e?ective electrostatic interactions with the negatively charged polynucleotide backbone. Moreover, irradiation of the film with UVA light induces trans–cis isomerization of the azobenzene units on the surface. As a result, the binding a?nity for DNA decreases due to the changing surface hydrophilicity. In order to understand and control the reversible photoswitchable mechanism of this surface, water contact angles are measured after UVA and visible light irradiation. The release of DNA from a dual pH/light‐sensitive sample is performed using fluorescence microscopy. The results show that irradiation of the film with UVA light induces trans–cis isomerization of the photoresponsive azobenzene units; this leads to significant changes in the surface hydrophilicity and reduces the binding affinity for DNA.     

Fluorescence staining and confocal laser scanning microscopy study of hydrogen-bonded poly(vinylpyrrolidone)/poly(acrylic acid) film

Poly(vinylpyrrolidone) (PVPON) and poly(acrylic acid) (PAA) were layer-by-layer (LBL) assembled to prepare the thin films based on hydrogen-bonding complexation. The hydrogen-bonded PVPON/PAA films were incubated in acidic, neutral and basic vapors separately. To study the morphologies after incubation, the films were stained by pH-sensitive fluorescent dyes using chemical and physical ways, and investigated with confocal laser scanning microscope (CLSM). The chemical way (labeling) was covalently linking fluoresceinamine (FAM) to some monomer units of PAA while the physical way was adsorbing rhodamine B (RB) molecules from dilute solution. Atomic force microscope (AFM) was combined with CLSM to find that after incubation in neutral or basic vapor the hydrogen-bonded PVPON/PAA films form porous structure and the pores are through the whole film.