Insulator Polarization Mechanisms in Polyelectrolyte‐Gated Organic Field‐Effect Transistors

Electrolyte‐gated organic field‐effect transistors (OFETs) hold promise for robust printed electronics operating at low voltages. The polarization mechanism of thin solid electrolyte films, the gate insulator in such OFETs, is still unclear and appears to limit the transient current characteristics of the transistors. Here, the polarization response of a thin proton membrane, a poly(styrenesulfonic acid) film, is controlled by varying the relative humidity. The formation of the conducting transistor channel follows the polarization of the polyelectrolyte, such that the drain transient current characteristics versus the time are rationalized by three different polarization mechanisms: the dipolar relaxation at high frequencies, the ionic relaxation (migration) at intermediate frequencies, and the electric double‐layer formation at the polyelectrolyte interfaces at low frequencies. The electric double layers of polyelectrolyte capacitors are formed in ～1 µs at humid conditions and an effective capacitance per area of 10 µF cm^?2 is obtained at 1 MHz, thus suggesting that this class of OFETs might operate at up to 1 MHz at 1 V.     

Facile Photo‐Crosslinking of Azide‐Containing Hole‐Transporting Polymers for Highly Efficient,Solution‐Processed,Multilayer Organic Light Emitting Devices

A novel framework of azide containing photo‐crosslinkable, conducting copolymer, that is, poly(azido‐styrene)‐random‐poly(triphenylamine) (X‐PTPA), is reported as a hole‐transporting material for efficient solution‐processed, multi‐layer, organic light emitting diodes (OLEDs). A facile and energy‐efficient crosslinking process is demonstrated with UV irradiation (254 nm, 2 mW/cm²) at a short exposure time (5 min). By careful design of X‐PTPA, in which 5 mol% of the photo‐crosslinkable poly(azido‐styrene) is copolymerized with hole‐transporting poly(triphenylamine) (X‐PTPA‐5), the adverse effect of the crosslinking of azide moieties is prevented to maximize the performances of X‐PTPA‐5. Since the photo‐crosslinking chemistry of azide molecules does not involve any photo‐initiators, superior hole‐transporting ability is achieved, producing efficient devices. To evaluate the performances of X‐PTPA‐5 as a hole‐transporting/electron‐blocking layer, Ir(ppy)₃‐based, solution‐processable OLEDs are fabricated. The results show high EQE (11.8%), luminous efficiency (43.7 cd/A), and power efficiency (10.4 lm/W), which represent about twofold enhancement over the control device without X‐PTPA‐5 film. Furthermore, micro‐patterned OLEDs with the photo‐crosslinkable X‐PTPA‐5 can be fabricated through standard photolithography. The versatility of this approach is also demonstrated by introducing the same azide moiety into other hole‐transporting materials such as poly(carbazole) (X‐PBC).     

The Photo‐Stability of Polymer Solar Cells: Contact Photo‐Degradation and the Benefits of Interfacial Layers

The organic/electrode interfaces in organic solar cells are systematically studied for their light, heat, and electrical stability in an inert atmosphere. Various extraction layers are examined for their effect on device stability, including poly(3,4‐ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and MoO₃ for hole extraction layers, as well as LiF, Cs₂CO₃, and lithium acetylacetonate (Liacac) for electron extraction layers. The organic/metal interface is shown to be inherently photo‐unstable, resulting in significant losses in device efficiency with irradiation. X‐ray photoelectron spectroscopy measurements of the organic/aluminum interface suggest that the photo‐induced changes are chemical in nature. In general, interfacial layers are shown to substantially reduce photo‐degradation of the active layer/electrode interface. In spite of their photo‐stability, several interfacial layers present at the active layer/cathode interface suffer from thermal degradation effects due to temperature increases under exposure to light. Electrical aging effects are proven to be negligible in comparison to other major modes of degradation.     

Sub‐3 V ZnO Electrolyte‐Gated Transistors and Circuits with Screen‐Printed and Photo‐Crosslinked Ion Gel Gate Dielectrics: New Routes to Improved Performance

A facile, high‐resolution patterning process is introduced for fabrication of electrolyte‐gated transistors (EGTs) and circuits using a photo‐crosslinkable ion gel and stencil‐based screen printing. The photo‐crosslinkable gel is based on a triblock copolymer incorporating UV‐sensitive terminal azide functionality and a common ionic liquid. Using this material in conjunction with conventional photolithography and stenciling techniques, well‐defined 0.5–1 μm thick ion gel films are patterned on semiconductor channels as narrow as 10 μm. The resulting n‐type ZnO EGTs display high electron mobility (>2 cm² Vs^?1) and on/off current ratios (>10⁵). Further, EGT‐based inverters exhibit static gains >23 at supply voltages below 3 V, and five‐stage EGT ring oscillator circuits display dynamic propagation delays of 50 μs per stage. In general, the screen printing and photo‐crosslinking strategy provides a clean room‐compatible method to fabricate EGT circuits with improved sensitivity (gain) and computational power (gain × oscillating frequency). Detailed device analysis indicates that significantly shorter delay times, of order 1 μs, can be obtained by improving the ion gel conductance.     

Perfluorinated Ionomer‐Modified Hole‐Injection Layers: Ultrahigh‐Workfunction but Nonohmic Contacts

Recently it has been reported that Nafion oligomers, i.e., 2‐(2‐sulfonatotetrafluoroethoxy)‐2‐trifluoromethyltrifluoroethoxyfunctionalized oligotetrafluoroethylenes, also called perfluorinated ionomers (PFIs), can be blended into poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDT:PSSH) films to increase their workfunctions beyond 5.2 eV. These PFI‐modified films are useful for energy‐level alignment studies, and have been proposed as hole‐injection layers (HILs). It is shown here however that these HILs do not provide sufficiently fast hole transfer into adjacent polymer semiconductor layers with ionization potentials deeper than ≈5.2 eV. X‐ray and ultraviolet photoemission spectroscopies reveal that these HILs exhibit a molecularly‐thin PFI overlayer that sets up a surface dipole that provides the ultrahigh workfunction. This dipolar layer persists even when the subsequent organic semiconductor layer is deposited, as evidenced by measurements of the diode built‐in potentials. As a consequence, the PFI‐modified HILs produce a higher contact resistance, and a lower equilibrium density of holes at the semiconductor contact than might have been expected from simple thermodynamic considerations of the reduction in hole‐injection barrier. Thus the use of insulating dipolar surface layers at the charge‐injection contact to tune its workfunction to match the relevant transport level of the semiconductor is of limited utility to achieve ohmic contact in these devices.     

Photo‐ and Rubbing‐Alignable Brush Polyimides Bearing Three Different Chromophores

Three new photoreactive brush polyimides (PSPIs), each bearing a different type of chromophore (cinnamoyl (CA), 3‐(2‐furyl)acryloyl (FA), and methacryloyl (MA)) in their bristles (i.e., side groups), are successfully synthesized, and are found to produce good‐quality films with smooth surfaces through conventional spin‐casting and drying processes. These PSPI polymers are thermally stable up to 320 °C. This is the first quantitative investigation of the photoaligning and rubbing‐aligning processabilities of PSPI polymer films, and of the abilities of the resultant films to control the orientation and anchoring of liquid‐crystal (LC) molecules. The chromophores of both poly(1‐cinnamoyloxy‐2,4‐phenylene hexafluoroisopropylidenediphthalimide) (6F‐DAP‐CA) and poly(1‐3‐(2‐furyl)acryloyloxy‐2,4‐phenylene hexafluoroisopropylidenediphthalimide) (6F‐DAP‐FA) PSPIs are found to undergo photodimerization in thin films and, to a lesser extent, photoisomerization, resulting in insoluble, crosslinked films. The MA chromophores of 6F‐DAP‐MA PSPI are found to undergo photopolymerization in thin films, which might include photodimerization to a lesser extent, resulting in insoluble, crosslinked films. Thin films of the PSPI polymer chains are found to have excellent unidirectional orientation ability as a result of either photoexposure with linearly polarized UV light (LPUVL) or rubbing. Both the photoaligned and the rubbing‐aligned polymer chains in the PSPI films are demonstrated to effectively induce the alignment of nematic LCs along their orientation directors by anisotropic interactions between the preferentially oriented polymer chain segments and the LCs. The contribution to LC alignment of the microgrooves developed in the rubbed films is found to be very low. The anchoring energies of the LCs on the photoaligned film surfaces are comparable to those on the rubbing‐aligned film surfaces; the anchoring energies are found to be in the range 0.45–2.25 × 10^–5 J m^–2, and to depend on which film treatment process is used and which chromophore bristle is present. In summary, the new PSPIs reported in this paper are promising LC alignment‐layer candidates with rubbing‐free processing for the production of advanced LC‐display (LCD) devices, including LCD televisions with large display areas.     

Fabrication of Reversibly Crosslinkable, 3‐Dimensionally Conformal Polymeric Microstructures

Multifaceted porous materials were prepared through careful design of star polymer functionality and properties. Functionalized core crosslinked star (CCS) polymers with a low glass transition temperature (T_g) based on poly(methyl acrylate) were prepared having a multitude of hydroxyl groups at the chain ends. Modification of these chain ends with 9‐anthracene carbonyl chloride introduces the ability to reversibly photocrosslink these systems after the star polymers were self‐assembled by the breath figure technique to create porous, micro‐structured films. The properties of the low T_g CCS polymer allow for the formation of porous films on non‐planar substrates without cracking and photo‐crosslinking allows the creation of stabilized honeycomb films while also permitting a secondary level of patterning on the film, using photo‐lithographic techniques. These multifaceted porous polymer films represent a new generation of well‐defined, 3D microstructures.     

Copper‐Assisted Weak Polyelectrolyte Multilayer Formation on Microspheres and Subsequent Film Crosslinking

The formation of weak polyelectrolyte films on planar and spherical supports has recently evoked major interest, as such coatings allow novel material properties to be tunable by pH and salt adjustment of the polyelectrolyte deposition conditions. We report on the build up of multilayers of the weak polyelectrolytes poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) on submicrometer‐sized polystyrene (PS) and silica colloid spheres (～ 500 nm) with the aid of copper ion templating. The copper ions complex to the carboxylate groups of PAA, facilitating the formation of PAA/PAH multilayers on the particles. Regular growth of the layers on the colloid spheres with each polyelectrolyte deposition step was confirmed by microelectrophoresis, single‐particle light scattering (SPLS), and transmission electron microscopy (TEM), with an average bilayer thickness of ～ 3 nm. The polyelectrolyte multilayer‐coated particles formed stable colloidal dispersions, with ζ‐potentials ranging from 30 mV (PAH outer layer) and –50 mV (PAA outer layer). Complementary quartz‐crystal microbalance and UV‐vis spectrophotometry studies on PAA/PAH multilayers formed on planar supports were performed to examine the film formation and the role of copper ion binding to the layers. PAA/PAH multilayers formed on colloid particles were also chemically crosslinked by using the activator 1‐ethyl‐3‐(3‐dimethylaminopropyl) carbodiimide (EDC). The degree of film crosslinking could be readily controlled by varying the concentration of EDC employed. Following solvent decomposition of the template particles coated with crosslinked PAA/PAH multilayers, intact hollow polymer capsules were obtained. These capsules were found to be impenetrable to polystyrene.     

Mosaic,Single‐Crystal CaCO3 Thin Films Fabricated on Modified Polymer Templates

Mosaic, single‐crystal CaCO₃ thin films have been prepared on modified poly(ethylene terephthalate) (PET) templates. Surface modification of PET through the introduction of carboxylic acid groups (COOH‐PET), and the subsequent physical and chemical adsorption of poly(allylamine hydrochloride) (PAH) at pH 8 (PAH8‐PET) and pH 11 (PAH11‐PET), afford template surfaces that influenced the phase transition of an amorphous CaCO₃ (ACC) films during crystallization in air. Macroscopic ACC thin films are prepared on modified PET films in the presence of poly(acrylic acid). Polycrystalline, spherulitic vaterite (CaCO₃) films are observed to form on native PET and PAH11‐PET, while mosaic, single‐crystal calcitic (CaCO₃) films form on COOH‐PET and PAH8‐PET templates. These results confirm that single‐crystal CaCO₃ growth patterns are dependent on the surface characteristics of the PET template. We infer therefore, that the nucleation and growth of ceramic films on polymeric templates can be controlled by chemical modification of the polymeric template surface, and by the subsequent attachment of ionic polyelectrolytes.     

Green Emission in Ladder‐Type Quarterphenyl: Beyond the Fluorenone‐Defect

In polyfluorenes it is generally accepted that (photo)degradation leads to fluorenone type defects that accept the excitation energy and emit green‐to‐yellow light with rather low efficiency. Although initial spectroscopic studies suggest the same to hold true for ladder‐type poly(para‐phenylene)s (LPPPs), kinetic studies of the degradation process are not compatible with the established mechanism. In general, the observed green emission can be caused by the introduction of carbonyl groups; however, only if associated with an additional disruption of the backbone rigidity and hence planarity of the entire π‐system. This is clearly shown by comparison with synthesized model compounds, which are bearing the fluorenone motif yet possess very different optical properties as compared to the defects, which are actually formed. Degradation can be caused by solvent specific, yet substrate nonspecific aromatic formylation but mainly originates from reaction with in‐situ generated singlet oxygen, both in solution as well as in thin films. Time‐dependent photoluminescence measurements on thin films show that green emission is enhanced by energy transfer from intact molecules to defect centers.     

Pyridylamino‐β‐cyclodextrin as a Molecular Chaperone for Lipopolysaccharide Embedded in a Multilayered Polyelectrolyte Architecture

Layer‐by‐layer self‐assembled polyelectrolyte films containing a charged cyclodextrin and lipopolysaccharide (LPS) are developed for the first time as a potential model for local endotoxin antagonist delivery. We have examined the biological activity of a lipopolysaccharide from E. coli incorporated into multilayered architectures made of poly‐(L ‐lysine) and poly‐(L ‐glutamic acid). Used in such build‐ups, a polycationic cyclodextrin, heptakis(6‐deoxy‐6‐pyridylamino)‐β‐cyclodextrin showed molecular chaperone properties by enabling restoration of the LPS biological activity whenever lost upon interaction with poly‐(L ‐lysine).     

Fabrication of Robust Biomolecular Patterns by Reactive Microcontact Printing on N‐Hydroxysuccinimide Ester‐Containing Polymer Films

The fabrication of robust biomolecule microarrays by reactive microcontact printing (μCP) on spin‐coated thin films of poly(N‐hydroxysuccinimidyl methacrylate) (PNHSMA) on oxidized silicon and glass is described. The approach combines the advantages of activated polymer thin films as coupling layers, characterized by high reactivity and high molecular loading, with the versatility and flexibility of soft lithography. The transfer of amino end‐functionalized poly(ethylene glycol) (PEG) from oxidized poly(dimethylsiloxane) elastomer stamps to PNHSMA films is shown by Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, fluorescence microscopy, and ellipsometry measurements to result in covalent coupling and identical grafting densities, as found previously for coupling from solution. The PEG‐protected areas effectively inhibit the adsorption of fluoresceinamine, bovine serum albumin, as well as 25‐mer DNA, while the unreacted N‐hydroxysuccinimidyl methacrylate ester groups retain their reactivity towards primary amino groups. Biomolecule microarrays have been thus conveniently fabricated in a two‐step procedure. The hybridization of target DNA to immobilized probe DNA in micropatterns proves the concept of reactive μCP on activated polymer films for obtaining robust platforms for biomolecule immobilization and screening.     

A Novel Polymeric Ionomer as a Potential Biomaterial: Crystallization Behavior,Degradation, and In‐Vitro Cellular Interactions

To evaluate the potential of polyester‐based ionomers as biomaterials, we have characterized them in terms of crystallization behavior, degradation, and in‐vitro cellular interactions. The polymers used are poly(butylene succinate)‐based ionomers (PBSis) with 1 to 5 mol‐% dimethyl 5‐sodium sulfoisophthalate. Even a few incorporated ionic groups significantly decreases the folding surface energy, indicating that folding into crystalline lamellae is more difficult for chains restricted by ionic aggregates. Transmission electron microscopy (TEM) does not reveal any distinct aggregation of ionic clusters following hydrolytic degradation, which suggests that the physical crosslinkage due to ionic interactions is vulnerable to hydrolysis. The in‐vitro cellular interactions of polyester‐based ionomers is assessed by the culture of human dermal fibroblasts with PBSi extracts or in direct contact with the PBSi films. Cells on PBSi films and in their extracts exhibit appropriate specific growth rates and normal metabolic function regardless of the incorporated ionic content compared with poly[(D ,L ‐lactic acid)‐co‐(glycolic acid)] (75:25, PLGA), which is well known to be biocompatible. The cells growing on PBSi films spread to a sufficient extent, displaying relatively active filopodial growth, as compared to that of parent PBS. These results suggest that the conspicuous topology and hydrophilic nature of the ionomer surface affect cellular interactions, and that this ionomer therefore has potential applications as a biomaterial.     

Designing a New Generation of Proton‐Exchange Membranes Using Layer‐by‐Layer Deposition of Polyelectrolytes

All fuel cells utilizing the membrane‐electrode assembly have their ion‐conductive membrane sandwiched between bipolar plates. Unfortunately, applying conventional techniques to isolated polyelectrolyte membranes is challenging and difficult. A more practical alternative is to use the layer‐by‐layer assembly technique to fabricate a membrane‐electrode assembly that is technologically relatively simple, economic, and robust. The process presented here paves the way to fabricate ion‐conductive membranes tailored for optimum performance in terms of controlled thickness, structural morphology, and catalyst loading. Composite membranes are constructed through the layered assembly of ionically conductive multilayer thin films atop a porous polycarbonate membrane. Under ambient conditions, a fuel cell using a poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) composite membrane delivers a maximum power density of 16.5 mW cm^–2 at a relative humidity of 55 %, which is close to that of some commercial fuel cells operating under the same conditions. Further optimization of these systems may lead to new, ultrathin, flexible fuel cells for portable power and micropower applications.     

Primary Cell Adhesion on RGD‐Functionalized and Covalently Crosslinked Thin Polyelectrolyte Multilayer Films

Polyelectrolyte multilayers (PEMs) are now widely used for biomedical applications. In this work, we investigated the primary osteoblast adhesion properties of PEMs of poly(L ‐lysine) (PLL), poly(L ‐glutamic acid) (PGA), poly(alginic acid) (Palg), and poly(galacturonic acid) (Pgal). In order to compensate for the poor adhesion of the as‐synthesized films, two kinds of film modifications were achieved: a purely physical modification by film crosslinking, and a chemical modification by grafting a arginine–glycine–aspartic acid (RGD) peptide to PGA. Crosslinking was performed using a water‐soluble carbodiimide in combination with N‐hydroxysulfosuccinimide (sulfo‐NHS) to induce amide formation. This reaction was followed by Fourier‐transform IR spectroscopy. For film functionalization, a 15‐amino‐acid peptide was grafted to PGA and deposited as the top layer of the film. PLL/PGA, PLL/Palg, and PLL/Pgal films were crosslinked or functionalized. The films were tested for both short‐term adhesion properties and long‐term proliferation of primary osteoblasts. Whereas the effect of film crosslinking on short‐term adhesion was moderate, it was much more important for the RGD‐functionalized films. On the other hand, the long‐term proliferation was the same or even higher for the crosslinked films as compared with the functionalized films. This effect was particularly enhanced for the PLL/Palg and PLL/Pgal films. Finally, we functionalized PLL/PGA that had been crosslinked prior to PGA‐RGD deposition. These architectures exhibited even higher short‐term adhesion and proliferation. These results clearly show the important role of the physical properties of the films, besides their chemical properties, for the modulation of primary cell‐adhesion behavior.     

Effects of Photo‐oxidation on the Performance of Poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene]:[6,6]‐Phenyl C61‐Butyric Acid Methyl Ester Solar Cells

A study of the photo‐oxidation of films of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV) blended with [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM), and solar cells based thereon, is presented. Solar‐cell performance is degraded primarily through loss in short‐circuit current density, J_SC. The effect of the same photodegradation treatment on the optical‐absorption, charge‐recombination, and charge‐transport properties of the active layer is studied. It is concluded that the loss in J_SC is primarily due to a reduction in charge‐carrier mobility, owing to the creation of more deep traps in the polymer during photo‐oxidation. Recombination is slowed down by the degradation and cannot therefore explain the loss in photocurrent. Optical absorption is reduced by photo‐bleaching, but the size of this effect alone is insufficient to explain the loss in device photocurrent.     

Multivalent‐Ion‐Mediated Stabilization of Hydrogen‐Bonded Multilayers

Hydrogen‐bonding interactions are an important alternative to electrostatic interactions for assembling multilayer thin films of uncharged components. Herein, a new method is reported for rendering such films stable at pH values close to physiological conditions. Multilayer films based on hydrogen bonding are assembled by the alternate deposition of poly[(styrene sulfonic acid)‐co‐(maleic acid)] (PSSMA) and poly(N‐isopropylacrylamide) (PNiPAAm) at pH 2.5. The use of PSSMA results in multilayers that contain free styrene sulfonate groups, as these moieties do not interact with the PNiPAAm functional groups. Subsequent infiltration of a multivalent ion (Ce⁴⁺ or Fe³⁺) leads to an increase in the total film mass, with little impact on the film morphology, as determined by using atomic force microscopy. To examine the film stability, the resulting films have been exposed to elevated pH (7.1). While there is substantial swelling of the multilayers (25 % and 55 % for Ce⁴⁺‐ and Fe³⁺‐stabilized films, respectively), film loss is negligible. This provides a stark contrast with non‐stabilized films, which disassemble almost immediately upon exposure to pH 7.1. This method represents a simple and effective strategy for stabilizing hydrogen‐bonded structures non‐covalently. Further, the multivalent ions also render the films responsive to changes in the local redox environment, as demonstrated by film disassembly after exposure of Fe³⁺‐treated films to iodide solutions.     

Solid‐State Photovoltaic Thin Films using TiO2, Organic Dyes,and Layer‐by‐Layer Polyelectrolyte Nanocomposites

We report photovoltaic devices consisting of patterned TiO₂, porphyrin dyes, and layer‐by‐layer (LBL) polyelectrolyte multilayer/oligoethylene glycol dicarboxylic acid (OEGDA) composite films. A composite polyelectrolyte LBL/OEGDA film was fabricated by formation of an alternating multilayer of linear polyethyleneimine (LPEI) and polyacrylic acid (PAA), followed by immersion of the LBL film into an OEGDA aqueous solution. The ionic conductivity attained in this LBL LPEI/PAA and OEGDA composite film was approximately 10^–5 S cm^–1 at room temperature and humidity. Investigations of dye‐sensitized photovoltaic devices constructed with the LBL (LPEI/PAA)/OEGDA composite films, TiO₂, and four types of porphyrin dyes resulted in optimization of the dye molecule and its orientation at the interface with the ionically conductive composite. The photocurrent value of photovoltaic devices constructed with the composite LBL/OEGDA film from illumination of a xenon white light source exhibited a nearly 1.5 times enhancement over the device without OEGDA. This enhancement of the photocurrent was due to the high room‐temperature ionic conductivity of the multilayer composite film. Further marked improvements of the photovoltaic performance were achieved by patterning the TiO₂ electrode using polymer stamping as a template for TiO₂ deposition. The device with patterned TiO₂ electrodes exhibited almost 10 times larger conversion efficiencies than a similar device without patterning.     

Preparation and Organization of Nanoscale Polyelectrolyte‐Coated Gold Nanoparticles

The layer‐by‐layer (LbL) desposition of oppositely charged polyelectrolytes from adsorption solutions of different ionic strength onto ～7 nm diameter carboxylic acid‐derivatized gold nanoparticles has been studied. The polyelectrolyte‐modified nanoparticles were characterized by UV‐vis spectrophotometry, microelectrophoresis, analytical ultracentrifugation, and transmission electron microscopy. UV‐vis data showed that the peak plasmon absorption wavelength of the gold nanoparticles red‐shifted after each adsorption step, and microelectrophoresis experiments revealed a reversal in the surface charge of the nanoparticles following deposition of each layer. These data are consistent with the formation of polyelectrolyte layers on the nanoparticles. Analytical ultracentrifugation showed an increase in mean nanoparticle diameter on adsorption of the polyelectrolytes, confirming the formation of gold‐core/polyelectrolyte‐shell nanoparticles. Transmission electron microscopy studies showed no signs of aggregation of the polyelectrolyte‐coated nanoparticles. The adsorption of the polyelectrolyte‐coated gold nanoparticles onto oppositely charged planar supports has also been examined. UV‐vis spectrophotometry and atomic force microscopy showed increased amounts of nanoparticles were adsorbed with increasing ionic strength of the nanoparticle dispersions. This allows control of the nanoparticle surface loading by varying the salt content in the nanoparticle dispersions used for adsorption. The LbL strategy used in this work is expected to be applicable to other nanoparticles (e.g., semiconductors, phosphors), thus providing a facile means for their controlled surface modification through polyelectrolyte nanolayering. Such nanoparticles are envisaged to have applications in the biomedical and bioanalytical fields, and to be useful building blocks for the creation of advanced nanoparticle‐based films.     

Controlled Electrodissolution of Polyelectrolyte Multilayers: A Platform Technology Towards the Surface‐Initiated Delivery of Drugs

A novel method for the electrochemical dissolution of polyelectrolyte multilayers from the surface of an electrode for applications in controlled drug delivery is reported. Biodegradable and biocompatible multilayer films based on poly(L ‐lysine) and heparin have been selected as a model system, and have been built on an indium tin oxide semiconductor substrate. The build‐up and dissolution processes of the multilayers is followed by electrochemical optical waveguide light mode spectroscopy. The formation and stability of the polyelectrolyte multilayers have been found to depend on the applied potential and the ionic strength of the buffer. The application of potentials above a threshold of 1.8 V induces dissolution, which follows single‐exponential kinetics, of the polyelectrolyte multilayer film. The rate of this process can be varied by an on–off profile of the potential, leading to the controlled release of heparin into the bulk. Atomic force microscopy investigations show that the electrodissolution of the polyelectrolyte multilayers is a local phenomenon that leads to the formation of nanoporous films.