LbL Assembly Research Articles

Urea Amperometric Biosensors Based on Nanostructured Polypyrrole

AbstractIn this work, we report the development of urea biosensors using macroporous polypyrrole as electrochemical transducer. Nanostructured polypyrrole was synthesized using monodispersed polystyrene spheres (460 nm) as template. Nanostructured urea biosensors were assembled after immobilization of urease (an enzyme that hydrolyses urea to ammonia) on macroporous polypyrrole, which was carried out using two different procedures: covalent attachment or layer‐by‐layer (lbl) deposition. Results show that lbl assembly of urease on macroporous polypyrrole gives rise to biosensors with improved detection kinetics (as demonstrated by higher Imax values) and tunable sensitivities. Comparison with bulk urea biosensors previously described in the literature demonstrates that the use of nanostructured polypyrrole as sensing platform enhances biosensor′s analytical performances, mainly due to the availability of more active sites for enzyme immobilization.

Spray-Layer-by-Layer Assembly Can More Rapidly Produce Optical-Quality Multistack Heterostructures

Automated spray-layer-by-layer (LbL) assembly was used to create highly reflective structurally colored thin films with high reflectance at near-UV light wavelengths. Reflectance peaks were tuned by fabricating alternating stacks of high (TiO(2) nanoparticles) and low (SiO(2) nanoparticles) refractive index materials using a non-quarter-wave design. Spray-assembled multilayer heterostructures fabricated with up to 840 individual polymer or nanoparticle deposition steps presented similar roughness and refractive index values compared to Bragg stacks obtained via immersion LbL assembly. Such complex multilayer heterostructures, however, could be fabricated in significantly shorter times; the time required to deposit a complete bilayer was only about 90 s, compared to 36 min for the immersion assembly of the same system. Optimization of the experimental parameters was performed to achieve uniform coatings and relatively smooth interfaces and surfaces. We observed that the spraying times of the nanoparticle and polymer solutions are the main parameters that determine the thickness, optical properties, and uniformity of the assembled films. Ellipsometry, atomic force microscopy (AFM), UV-vis spectroscopy, and transmission electron microscopy (TEM) were used to characterize the samples. The nanoparticle thin films were iridescent and presented relatively narrow peaks of high reflectance (∼90%) at visible and near-UV wavelengths of light.

Bioassisted synthesis of catalytic gold/silica nanotubes using layer-by-layer assembled polypeptide templates

We report the bioassisted synthesis of gold nanoparticle/silica (Au NP/silica) tubes using layer-by-layer (LBL) assembled poly( l-lysine)/poly( l-tyrosine) (PLL/PLT) multilayer films deposited on the polycarbonate (PC) membrane pores as both mediating agents and templates. The novelty of this approach is the in situ synthesis of Au NP/silica tubes using PLL/PLT multilayer films for sequential growth of Au NPs and silicas. The experimental data revealed that the buildup of the LBL multilayer films was mainly driven by the formation of hydrogen bond and the polypeptide macromolecular assemblies adopted mainly β-sheet conformation. The as-prepared Au NP/silica tubes possessed promising catalytic activity toward the reduction of p-nitrophenol. The synthesis conditions such as the concentration of gold precursor and polypeptide molecular weight were found to influence the gold weight ratio and particle size in the tubes and the catalytic properties of the Au NP/silica tubes. This approach provides a facile, robust, and green method to obtain nonaggregated metal nanoparticles immobilized in porous oxide network at ambient conditions. Using the synergy between biomimetic or bioassisted synthesis of nanostructured materials and LbL assembly technique, a variety of structures such as films, tubes, and capsules comprising of multiple compositions can be obtained.

Single polyelectrolyte multilayers deposited onto silica microparticles and silicon wafers

Single polyelectrolyte component multilayers, exemplified by poly(vinyl amine) (PVAm), have been prepared using a method of 3,3′,4,4′-benzophenonetetracarboxylic-dianhydride (BTCDA)-mediated electrostatics and hydrogen bonds LbL assembly. The PVAm with low (15,000 g/mol) and high (340,000 g/mol) molar mass was adsorbed at pH 9.5 onto silica microparticles with particle diameter of 0.015–0.040 mm. The BTCDA cross-linking of the PVAm adsorbed onto silica microparticles results in a surface covered with carboxylic groups. The silica/PVAm–BTCDA, negatively charged over a wide range of pH, can adsorb a new positively charged PVAm layer. A regular increase in the single (PVAm/BTCDA) n polyelectrolyte multilayer onto silica microparticles was observed by streaming potential measurements using a particle charge detector. X-ray photoelectron spectroscopy, the elemental and thermogravimetric analysis, were employed to monitor the (PVAm–BTCDA) n multilayer construction. The topography and thickness of the films deposited on silicon wafers, as a function of the number of PVAm layers, was investigated by atomic force microscopy. Both molar masses (15,000 and 340,000 g/mol) of PVAm were suitable for building (BTCDA)-mediated LbL assembly onto silica microparticles, even the behavior of PVAm on the multilayer formation strongly depends on the molar mass.

Controlled Fluorescence Resonance Energy Transfer from Multiple Fluorophores in Layer-by-Layer Assemblies

We demonstrate precise control of multiple FRET within LbL assemblies to produce simultaneous light emission with enhanced intensity from a single excitation with a blue wavelength. Three different fluorescent dyes were attached to neat polyelectrolytes and then incorporated into the LbL assembly. With designed deposition sequence, the layer of blue-absorbing dyes was placed in the adjacent layer of green- and red-emitting dyes, while the layers of emitting dyes were separated with each other beyond their Förster radius by inserting neat polyelectrolytes between them. The LbL assemblies not only allowed FRET from the absorbing dyes to the emitting dyes but also prohibited FRET between the emitting dyes to produce simultaneously enhanced green and red emission.

Self‐Assembly of Nanostructures on Surfaces Using Metal–Organic Coordination

AbstractLayer‐by‐layer (LbL) assembly of multilayers is an established method for the construction of layered nanostructures on surfaces, affording control of the thickness, composition, and organization in the vertical direction. Binding between layers is accomplished using various types of interactions, including electrostatic binding, hydrogen bonding, covalent bonding, metal–organic coordination, host–guest interactions, biospecific interactions, and others. Here we focus on LbL assembly using metal–organic coordination, and specifically on layered nanostructures based on bishydroxamate–M4+ binding. The coordination approach offers attractive features, such as a simple reaction, a defined geometry, and reversibility under certain conditions. The basic scheme includes self‐assembly of a ligand (anchor) monolayer on the surface, followed by alternate binding of metal ions and multi‐functional ligand layers, to form a coordination multilayer. This approach is demonstrated by the construction of a variety of coordinated nanostructures, including bilayers, multilayers, dendrimers, and nanoparticle assemblies, prepared on gold and oxide substrates.

Transparent, Flexible Conducting Hybrid Multilayer Thin Films of Multiwalled Carbon Nanotubes with Graphene Nanosheets

We developed a simple, versatile method of integrating hybrid thin films of reduced graphene oxide (RGO) nanosheets with multiwalled carbon nanotubes (MWNTs) via LbL assembly. This approach involves the electrostatic interactions of two oppositely charged suspensions of the RGO nanosheet with MWNTs. This method affords a hybrid multilayer of graphenes with excellent control over the optical and electrical properties. Moreover, the hybrid multilayer exhibits a significant increase of electronic conductivity after the thermal treatment, producing transparent and conducting thin films possessing a sheet resistance of 8 kOmega/sq with a transmittance of 81%. By taking advantage of the conducting network structure of MWNTs, which provides an additional flexibility and mechanical stability of RGO nanosheets, we demonstrate the potential application of hybrid graphene multilayer as a highly flexible and transparent electrode. Because of the highly versatile and tunable properties of LbL-assembled thin films, we anticipate that the general concept presented here offers a unique potential platform for integrating active carbon nanomaterials for advanced electronic, energy, and sensor applications.

Nonvolatile memory properties of Pt nanoparticle-embeddedTiO2 nanocomposite multilayers via electrostatic layer-by-layer assembly

It is demonstrated that notable resistive switching memory properties dependingon voltage polarity (i.e. bipolar switching properties) can be obtained from thelayer-by-layer (LbL) assembled multilayers based on transition metal oxidesand metal nanoparticles. Cationic poly(allylamine hydrochloride) and anionictitania precursor layers were deposited alternately onto Pt-coated Si substratesusing an electrostatic LbL assembly process. Anionic Pt nanoparticles (PtNP) with about 5.8 nm diameter size were also inserted within the multilayers usingthe same interactions mentioned above. These multilayers were converted toPtNP-embeddedTiO2 films by thermal annealing and the films were then coated with a topelectrode. When external bias was applied to the devices, bipolar switchingproperties were observed at low operating voltages showing the highON/OFF ratio(>104) and the stable device performance. These phenomena were caused by the presence ofPtNP inserted within TMO films.

Recent Developments in Supramolecular Approach for Nanocomposites

One of the most efficient ways to create new materials is the fabrication of nanocomposites and/or nanohybrids. Composites adopt some characteristics from the components that compose it; synergistic effects can also produce properties not present in any of the parts. A union of the worlds of organic, biological, and inorganic materials merges the versatility, fine structural precision, and mechanical and chemical stability of these respective areas. In this short review, we introduce recent developments of supramolecular approaches for nanocomposite fabrications mainly based on LbL assembly, LB films, and mesoporous structures, as well as incorporation of various supramolecular components into composites.

Layer-by-layer deposition of magnetic microgel films on plastic surfaces for the preparation of magnetic resonance visibility enhancing coatings

Magnetic resonance visibility enhancing coatings comprising layer-by-layer assembled multilayer films of magnetic microgels and poly(sodium 4-styrenesulfonate) (PSS) were successfully fabricated on hydrophobic plastic surfaces based on electrostatic interaction as the driving force. The magnetic microgels, which were chemically cross-linked poly(allylamine hydrochloride) (PAH) and dextran (PAH-D) encapsulated with superparamagnetic Fe3O4 nanoparticles (NPs) acted as the magnetic resonance imaging (MRI) contrast agents in the multilayer films. The magnetic film deposition is initiated with the deposition of the PAH-D microgels because of their strong affinity toward plastic surfaces and so complicated steps for plastic surface modification are avoided. The as-prepared multilayer films of magnetic microgels are stable and have satisfactory adhesion with the underlying plastic surface. The plastic substrates deposited with multilayer films of magnetic microgels are capable of being clearly visualized in vitro by MRI equipment. The easy fabrication of MR emitting coatings on hydrophobic plastic surfaces combined with the capability of the LbL assembly technique for film deposition on substrates of irregular shapes provides a facile way to deposit magnetic resonance visibility enhancing coatings on plastic intervention tools for their application in MRI-guided therapy.

Sustained delivery of doxorubicin by porous CaCO 3 and chitosan/alginate multilayers-coated CaCO 3 microparticles

Carboxymethyl cellulose (CMC)-doped CaCO 3 microparticles with an average diameter of 5 μm were prepared and coated by chitosan and alginate multilayers. The prepared CaCO 3 microparticles had a dominant phase of vaterite and a spherical morphology with nanopores on their surface. After LbL assembly of chitosan and alginate, the CaCO 3 microparticles were significantly smoothened. Treatment of the multilayers-coated particles yielded hollow microcapsules. These particles could spontaneously load positively charged doxorubicin (DOX) molecules, whose amount was 475 and 482 mg DOX/g CaCO 3 for the CaCO 3(CMC) microparticles and the (chitosan/alginate) 5 coated CaCO 3(CMC) microparticles, respectively. Brunauer–Emmett–Teller (BET) method was used to analyze the specific surface area and the pore size distribution of the CaCO 3(CMC) microparticles before and after DOX loading. After DOX loading, S BET and pore volume were reduced obviously, and the volume of smaller pores decreased significantly, whereas that of larger pores were increased. The increase of the volume of larger pores was explained by an electric charge screening effect. DOX release from the CaCO 3 microparticles in pH 5 was relatively slow within the first 15 h, and could be sustained to more than 150 h. The release amount at lower pH was larger at the same time. Coating of the CaCO 3(CMC) microparticles with the chitosan/alginate multilayers could obviously assuage the initial burst release and reduce the release rate.

Construction of Multifunctional Coatings via Layer-by-Layer Assembly of Sulfonated Hyperbranched Polyether and Chitosan

Layer-by-layer assembly has shown a great deal of promise in biomedical coatings, as well as local drug delivery systems. The poor loading capacity of hydrophobic drugs within the multilayers is a drawback in their potential applications. Herein, sulfonated hyperbranched polyether (HBPO-SO(3)) with a hydrophobic core was incorporated into LBL films to provide nanoreservoirs for hydrophobic guest molecules. HBPO-SO(3) was proven to form stable micelles in the sodium acetate and acetic acid buffer solution (HAc buffer) for LbL assembly. The QCM and ellipsometry experiments demonstrated that the LBL films can be fabricated via alternating deposition of HBPO-SO(3) micelles and chitosan. The fluorescence emission spectra verified that the hydrophobic pyrene can be incorporated both by pre-encapsulation in HBPO-SO(3) micelles and post-diffusion in preassembled multilayer films. Compared with the pre-encapsulation approach, the post-diffusion process was more efficient in incorporating hydrophobic guest molecules into the LbL films and carried out a much more controllable release of the guest molecules. A multifunctional coating with potential anticoagulation, antibacterial, and local release of hydrophobic drug Probucal, which has powerful antioxidant properties and can prevent restenosis after coronary angioplasty, was then developed via post-diffusion of the anti-restenosis agents into the multilayer films of HBPO-SO(3) and chitosan.

Design and in vitro Biodegradation of Novel Hepatocyte‐Targetable (Galactose Polycation/Hemoglobin) Multilayers and Microcapsules

AbstractThe formation of novel hepatocyte‐targetable and biodegradable polycation/protein multilayers and hollow microcapsules by LbL assembly is reported, in which galactose residues allow for targetability and Hb acts as a main degradable component. The in vitro biodegradability of multilayers via proteases was evaluated in detail on both planar substrates and CaCO3 particles followed by UV‐Vis, SEM, and AFM characterizations. The degradation approximately follows an exponential decay, and the relation between the kinetic constant k and the number n of deposited bilayers is well approximated by a power equation. Sustained release of the encapsulated model drug was attained by using enzymatic degradation of hollow capsules.magnified image

Polyelectrolyte Coated PLGA Nanoparticles: Templation and Release Behavior

Poly[(D,L-lactide)-co-glycolide] nanoparticles coated with polyethyleneimine on their surface were prepared by an emulsification-solvent evaporation method and subsequently surface modified by LBL assembly. The assembly of poly(acrylic acid) and polyethyleneimine on a planar substrate and on the PLGA nanoparticles was monitored by QCM-D, zeta-potential, flow cytometry and TEM. Carboxylic and amino groups in the multilayers were crosslinked by carbodiimide condensation, which was also later used to graft poly(ethylene glycol) (PEG). Rhodamine 6G, 5(6)-carboxyfluorescein and fluorescein were incorporated into the nanoparticles and their release profiles were recorded at 60 degrees C and at 37 degrees C for rhodamine 6G, for nanoparticles with a multilayer coating, and those that were crosslinked and grafted with PEG.

Electrically Conductive Thin Films Prepared from Layer‐by‐Layer Assembly of Graphite Platelets

AbstractLayer‐by‐layer (LBL) assembly of carbon nanoparticles for low electrical contact resistance thin film applications is demonstrated. The nanoparticles consist of irregularly shaped graphite platelets, with acrylamide/ββ‐methacryl‐oxyethyl‐trimethyl‐ammonium copolymer as the cationic binder. Nanoparticle zeta (ζζ) potential and thereby electrostatic interactions are varied by altering the pH of graphite suspension as well as that of the binder suspension. Film thickness as a function of zeta potential, immersion time, and the number of layers deposited is obtained using Monte Carlo simulation of the energy dispersive spectroscopy measurements. Multilayer film surface morphology is visualized via field‐emission scanning electron microscopy and atomic‐force microscopy. Thin film electrical properties are characterized using electrical contact resistance measurements. Graphite nanoparticles are found to self‐assemble onto gold substrates through two distinct yet overlapping mechanisms. The first mechanism is characterized by logarithmic carbon uptake with respect to the number of deposition cycles and slow clustering of nanoparticles on the gold surface. The second mechanism results from more rapid LBL nanoparticle assembly and is characterized by linear weight uptake with respect to the number of deposition cycles and a constant bilayer thickness of 15 to 21 nm. Thin‐film electrical contact resistance is found to be proportional to the thickness after equilibration of the bilayer structure. Measured values range from 1.6 mΩ cm−2 at 173 nm to 3.5 mΩ cm−2 at 276 nm. Coating volume resistivity is reduced when electrostatic interactions are enhanced during LBL assembly.

Layer-by-layer assembly of polyaniline nanofibers/poly(acrylic acid) multilayer film and electrochemical sensing

Polyaniline (PANI) nanofibers are synthesized by dilute polymerization and subsequently used for layer-by-layer assembly with poly(acrylic acid) (PAA). The chemical synthesized PANI nanofibers are characterized by SEM and TEM. In addition, the LBL assembly process is characterized by SEM, UV–vis spectrometry and electrochemical methods. PAA inside the multilayer film can dope nanostructured PANI effectively and shift its electroactivity to a neutral pH environment. Compared with PANI/PAA film co-deposited on the electrode by electropolymerization, the redox behavior of PANI/PAA multilayer via LBL assembly is more reversible, indicating the enhancement of electron transfer. The obtained nanostructured PANI/PAA multilayer films are very stable and show high electrocatalytic ability toward H 2O 2, which makes it an ideal substrate for H 2O 2 detection and offers great promise for biosensing.

Paclitaxel delivery by micro/nano encapsulation using layer-by-layer assembly

AbstractA novel formulation of paclitaxel (PTX) has been developed by providing multilayer assembly over drug loaded porous CaCO3 microparticles (CaCO3 MP) using combination of biocompatible and biodegradable polyelectrolytes (PE’s). PTX was encapsulated into the nanopores of preformed CaCO3 MP prepared by the co-precipitation method. Infrared (IR) and X-ray diffraction (XRD) provides evidences that PTX has been encapsulated into nanopores of CaCO3 MP and not crystallized on the surface. PTX loaded CaCO3 MP (CaCO3-PTX) was found to be highly stabilized against thermal decomposition as evinced by thermo gravimetric analysis (TGA) indicating decomposition at 600°C and 250°C for CaCO3-PTX and PTX respectively. The multilayer assembly over CaCO3-PTX was effectuated by alternate deposition of protamine sulfate (PRM) and sodium alginate (SA) using LBL technique followed by subsequent core removal [PTX- (PRM/SA)5]. The pay load efficiency of PTX in this system was found to be 78.98±2.14%. The developed system was further evaluated for surface morphology, size and size distribution, surface charge, core removal and layer-by-layer growth due to sequential adsorption of PE’s. The release data of PTX-(PRM/SA)5 was comparable with marketed formulation of PTX (PTX-M) and CaCO3-PTX when performed in simulated intestinal fluid (SIF pH=7.4). The release profile of PTX-(PRM/SA)5 indicates that PEs based multilayer matrix is capable to provide barrier to PTX release as it has been found to follow first order matrix diffusion kinetics with 64±4.8% release within 24 hrs. The t50% of PTX-M, CaCO3-PTX and PTX-(PRM/SA)5 was found to be 70, 90 and 480 minutes respectively. This alternative delivery system of PTX disguised in the form of LBL assembly could have immense application for the treatment of metastasized mammary glands vis-à-vis existing formulation of PTX which is by and large criticized for having certain toxic excipients to be given parentrally. Moreover, the proposed system provides ample of opportunity to modify the surface for targeted application of PTX.

Ordered Polyelectrolyte Multilayers: Unidirectional FRET Cascade in Nanocompartmentalized Polyelectrolyte Multilayers

Multifunctional polyelectrolyte (or layer-by-layer, LbL) multilayers consisting of a set of nanocompartments separated by impermeable ultrathin barriers, whereby the thickness of the compartments is tuned in the range 1-10 nm, are synthesized. Each compartment contains a different dye, introduced by co-adsorption during multilayer deposition. Different LbL barriers are tested for impermeability towards dye diffusion while simultaneously allowing energy transfer to occur between the compartmentalized dyes. Cross-linked LbL multilayers based on poly(acrylic acid) and poly(allyl amine) are shown to provide the desired impermeability for thicknesses as small as about 2.5 nm. A proof-of-concept system is then realized involving a cascade of two FRET processes, whereby the light energy is collected in a first nanocompartment containing pyranine, sent to a second nanocompartment loaded with fluorescein, before finally being transferred to a third, Nile blue-filled compartment located at the external surface of the film. This demonstrates the possibility to fabricate complex light-harvesting antenna systems by LbL assembly while controlling the architecture of the antenna down to a few nanometers.

Biomaterials and Biofunctionality in Layered Macromolecular Assemblies

Recent research in the field of LbL assembly is summarized and categorized as fabrication, sensing, drug release/delivery, and cell technology. Special emphasis is given to topics such as cell-membrane-mimic assembly, fabrication of free-standing biomolecular structures including protein microtubes, detection of DNA adducts and reactive metabolites, DNA hybridization analysis, sensing of toxic and bio-active chemicals, entrapment of proteins and DNA, biocomponent carriers with barcode encoding, release and delivery of DNA plasmids, multiagent delivery, smart defense capsules and oxidation-resistant films, vector introduction to cells, patterned cell culturing and microfluidic microreactors, stem cell differentiation, cellular uptake and degradation, and control of cellular apoptosis.

Thermal treatment of galactose-branched polyelectrolyte microcapsules to improve drug delivery with reserved targetability

A novel multilayered drug delivery system by LbL assembly of galactosylated polyelectrolyte, which is possible to have the potential in hepatic targeting by the presence of galactose residues at the microcapsule's surface, is designed. Thermal treatment was performed on the capsules and a dramatic thermal shrinkage up to 60% decrease of capsule diameter above 50 °C was observed. This thermal behavior was then used to manipulate drug loading capacity and release rate. Heating after drug loading could seal the capsule shell, enhancing the loading capacity and reducing the release rate significantly. Excellent affinity between galactose-binding lectin and heated galactose-containing microcapsules were observed, indicating a stable targeting potential even after high temperature elevating up to 90 °C.