Approaches to Quantifying and Visualizing Polyelectrolyte Multilayer Film Formation on Particles

Evaporation-Induced Polyelectrolyte Complexation: The Role of Base Volatility and Cosolvents

Film formation is a vital step for coating applications where a homogeneous, defect-free solid phase should be obtained, starting from a liquid casting formulation. Recently, an alternative waterborne-coating approach was proposed, based on the formation of a polyelectrolyte complex film. In this approach, an evaporating base induces a pH change during drying that initiates the complexation of oppositely charged polyelectrolytes, followed by further densification. In previous studies, ammonia was used as the evaporative base, leading to relatively fast evaporation and resulting in films showing significant brittleness, which tended to crack at low relative humidity or larger thicknesses. We hypothesize that slower complexation and/or evaporation can reduce the problematic stress build-up in the prepared polyelectrolyte complex coatings. For this reason, we studied the changes in the film formation process when there are different bases and cosolvents. We found that reducing the evaporation rate by changing ammonia to the slower evaporating dimethylamine or by adding DMSO as a cosolvent, led to less internal stress build-up during film formation, which could be beneficial for film application. Indeed, films prepared with ammonia showed cracking after 1 h, while films prepared with dimethylamine only showed cracking after one month. The fast evaporation of ammonia was also found to cause a temporary turbid phase, indicating phase separation, while for the slower evaporating bases, this did not occur. All prepared films remained sensitive to humidity, which poses the next challenge for these promising coatings.

Construction of a Self-Assembled Polyelectrolyte/Graphene Oxide Multilayer Film and Its Interaction with Metal Ions

In this study, a composite multilayer film onto gold was constructed from two charged building blocks, i.e., negatively charged graphene oxide (GO) and a branched polycation (polyethylenimine, PEI) via layer-by-layer (LbL) self-assembly technology, and this process was monitored in situ with quartz crystal microbalance (QCM) under different experimental conditions. This included the differences in frequency (Δf) as well as the changes in dissipation to yield information on the absorbed mass and viscoelastic properties of the formed PEI/GO multilayer films. The experimental conditions were optimized to obtain a high amount of the adsorbed mass of the self-assembled multilayer film. The surface morphology of the PEI/GO multilayer film onto gold was studied with atomic force microscopy (AFM). It was found that the positively charged PEI chains were combined with the oppositely charged GO to form an assembled film on the QCM sensor surface, in a wrapped and curled fashion. Raman and UV-vis spectra also showed that the intensities of the GO-characteristic signals are almost linearly related to the layer number. To explore the films for their use in divalent ion detection, the frequency response of the PEI/GO multilayer-modified QCM sensor to the exposure of aqueous solutions solution of Cu²⁺, Ca²⁺, Zn²⁺, and Sn²⁺ was further studied using QCM. Based on the Sauerbrey equation and the weight of different ions, the number of metal ions adsorbed per unit area on the surface of QCM sensors was calculated. For metal ion concentrations of 40 ppm, the adsorption capacities per unit area of Cu²⁺, Zn²⁺, Sn²⁺, and Ca²⁺ were found to be 1.7, 3.2, 0.7, and 4.9 nmol/cm², respectively. Thus, in terms of the number of adsorbed ions per unit area, the QCM sensor modified by PEI/GO multilayer film shows the largest adsorption capacity of Ca²⁺. This can be rationalized by the relative hydration energies.

Visualization of materials using the confocal laser scanning microscopy technique

The development of materials science always benefits from advanced characterizations. Currently, imaging techniques are of great technological importance in both fundamental and applied research on materials. In comparison to conventional visualization methods, confocal laser scanning microscopy (CLSM) is non-invasive, with macroscale and high-contrast scanning, a simple and fast sample preparation procedure as well as easy operation. In addition, CLSM allows rapid acquisition of longitudinal and cross-sectional images at any position in a material. Therefore, the CLSM-based visualization technique could provide direct and model-independent insight into material characterizations. This review summarizes the recent applications of CLSM in materials science. The current CLSM approaches for the visualization of surface structures, internal structures, spatial structures and reaction processes are discussed in detail. Finally, we provide our thoughts and predictions on the future development of CLSM in materials science. The purpose of this review is to guide researchers to build a suitable CLSM approach for material characterizations, and to open viable opportunities and inspirations for the development of new strategies aiming at the preparation of advanced materials. We hope that this review will be useful for a wide range of research communities of materials science, chemistry, and engineering.

Combatting implant-associated biofilms through localized drug synthesis

Bacterial contamination of implantable biomaterials is a significant socioeconomic and healthcare burden. Indeed, bacterial colonization of implants after surgery has a high rate of incidence whereas concurrent prophylaxis using systemic antibiotics has limited clinical success. In this work, we develop enzyme-prodrug therapy (EPT) to prevent and to treat bacteria at interfaces. Towards the overall goal, novel prodrugs for fluoroquinolone antibiotics were developed on a privileged glucuronide scaffold. Whereas carbamoyl prodrugs were not stable and not suitable for EPT, glucuronides containing self-immolative linker between glucuronic acid masking group and the antibiotic were stable in solution and readily underwent bioconversion in the presence of β-glucuronidase. Surface coatings for model biomaterials were engineered using sequential polymer deposition technique. Resulting coatings afforded fast prodrug conversion and mediated antibacterial measures against planktonic species as evidenced by pronounced zone of bacterial growth inhibition around the biomaterial surface. These biomaterials coupled with the glucuronide prodrugs also effectively combatted bacteria within established biofilms and also successfully prevented bacterial colonization of the surface. To our knowledge, this is the first report of EPT engineered to the surface of biomaterials to mediate antibacterial measures.

Comparison of drug release from liquid crystalline monoolein dispersions and solid lipid nanoparticles using a flow cytometric technique

Colloidal lipid particles such as solid lipid nanoparticles and liquid crystalline nanoparticles have great opportunities as drug carriers especially for lipophilic drugs intended for intravenous administration. In order to evaluate drug release from these nanoparticles and determine their behavior after administration, emulsion droplets were used as a lipophilic compartment to which the transfer of a model drug was measured. The detection of the model drug transferred from monoolein cubic particles and trimyristin solid lipid nanoparticles into emulsion droplets was performed using a flow cytometric technique. A higher rate and amount of porphyrin transfer from the solid lipid nanoparticles compared to the monoolein cubic particles was observed. This difference might be attributed to the formation of a highly ordered particle which leads to the expulsion of drug to the surface of the crystalline particle. Furthermore, the sponge-like structure of the monoolein cubic particles decreases the rate and amount of drug transferred. In conclusion, the flow cytometric technique is a suitable technique to study drug transfer from these carriers to large lipophilic acceptors. Monoolein cubic particles with their unique structure can be used successfully as a drug carrier with slow drug release compared with trimyristin nanoparticles.

Biocatalytic polymer thin films: optimization of the multilayered architecture towards in situ synthesis of anti-proliferative drugs

We report on the assembly of multi-layered polyelectrolyte thin films containing an immobilized enzyme to perform conversion of externally administered prodrugs and achieve delivery of the resulting therapeutics to adhering cells. Towards this goal, multi-layered coatings were assembled using poly(sodium styrene sulfonate) and poly(allylamine hydrochloride). Activity of the incorporated enzyme was quantified as a function of the assembly conditions, position of the enzyme within the multi-layered architecture, concentration of the enzyme in the adsorption solution, and concentration of the administered prodrug. Biocatalytic coatings exhibited sustained levels of enzymatic activity over at least one week of incubation in physiological buffers without signs of loss of activity of the enzyme. Developed enzyme-containing polymer films afforded zero-order release of the in situ synthesized cargo with kinetics of synthesis (nM per hour) covering at least 3 orders of magnitude. Internalization of the synthesized product by adhering cells was visualized using a fluorogenic enzyme substrate. Therapeutic utility of biocatalytic coatings was demonstrated using a myoblast cell line and a prodrug for the anti-proliferative agent, 5-fluorouridine. Taken together, this work presents a novel approach to delivery of small molecule drugs using multi-layered polymer thin films with utility in surface-mediated drug delivery, assembly of therapeutic implantable devices, and tissue engineering.

Perylene derivative-bridged Au–graphene nanohybrid for label-free HpDNA biosensor

A novel PDI-bridged Au–graphene nanohybrid was synthesized and used to fabricate a label-free hpDNA biosensor.

Probing the dynamic nature of DNA multilayer films using Förster resonance energy transfer

DNA films are of interest for use in a number of areas, including sensing, diagnostics, and as drug/gene delivery carriers. The specific base pairing of DNA materials can be used to manipulate their architecture and degradability. The programmable nature of these materials leads to complex and unexpected structures that can be formed from solution assembly. Herein, we investigate the structure of DNA multilayer films using Förster resonance energy transfer (FRET). The DNA films are assembled on silica particles by depositing alternating layers of homopolymeric diblocks (polyA(15)G(15) and polyT(15)C(15)) with fluorophore (polyA(15)G(15)-TAMRA) and quencher (polyT(15)C(15)-BHQ2) layers incorporated at predesigned locations throughout the films. Our results show that DNA films are dynamic structures that undergo rearrangement. This occurs when the multilayer films are perturbed during new layer formation through hybridization but can also take place spontaneously when left over time. These films are anticipated to be useful in drug delivery applications and sensing applications.

Tuning smart microgel swelling and responsive behavior through strong and weak polyelectrolyte pair assembly

The layer-by-layer (LbL) assembly of polyelectrolyte pairs on temperature and pH-sensitive cross-linked poly(N-isopropylacrylamide)-co-(methacrylic acid), poly(NIPAAm-co-MAA), microgels enabled a fine-tuning of the gel swelling and responsive behavior according to the mobility of the assembled polyelectrolyte (PE) pair and the composition of the outermost layer. Microbeads with well-defined morphology were initially prepared by synthesis in supercritical carbon dioxide. Upon LbL assembly of polyelectrolytes, interactions between the multilayers and the soft porous microgel led to differences in swelling and thermoresponsive behavior. For the weak PE pairs, namely poly(L-lysine)/poly(L-glutamic acid) and poly(allylamine hydrochloride)/poly(acrylic acid), polycation-terminated microgels were less swollen and more thermoresponsive than native microgel, whereas polyanion-terminated microgels were more swollen and not significantly responsive to temperature, in a quasi-reversible process with consecutive PE assembly. For the strong PE pair, poly(diallyldimethylammonium chloride)/poly(sodium styrene sulfonate), the differences among polycation and polyanion-terminated microgels are not sustained after the first PE bilayer due to extensive ionic cross-linking between the polyelectrolytes. The tendencies across the explored systems became less noteworthy in solutions with larger ionic strength due to overall charge shielding of the polyelectrolytes and microgel. ATR FT-IR studies correlated the swelling and responsive behavior after LbL assembly on the microgels with the extent of H-bonding and alternating charge distribution within the gel. Thus, the proposed LbL strategy may be a simple and flexible way to engineer smart microgels in terms of size, surface chemistry, overall charge and permeability.

Template-free synthesis and encapsulation technique for layer-by-layer polymer nanocarrier fabrication

The encapsulation of therapeutic molecules within multiple layers of biocompatible and biodegradable polymeric excipients allows exquisite design of their release profile, to the extent the drug can be selectively delivered to a specific target location in vivo. Here, we develop a novel technique for the assembly of multilayer polyelectrolyte nanocarriers based on surface acoustic wave atomization as a rapid and efficient alternative to conventional layer-by-layer assembly, which requires the use of a sacrificial colloidal template over which consecutive polyelectrolyte layers are deposited. Polymer nanocarriers are synthesized by atomizing a polymer solution and suspending them within a complementary polymer solution of opposite charge subsequent to their solidification in-flight as the solvent evaporates; reatomizing this suspension produces nanocarriers with a layer of the second polymer deposited over the initial polymer core. Successive atomization-suspension layering steps can then be repeated to produce as many additional layers as desired. Specifically, we synthesize nanocarriers comprising two and three, and up to eight, alternating layers of chitosan (or polyethyleneimine) and carboxymethyl cellulose within which plasmid DNA is encapsulated and show in vitro DNA release profiles over several days. Evidence that the plasmid's viability is preserved and hence the potential of the technique for gene delivery is illustrated through efficient in vitro transfection of the encapsulated plasmid in human mesenchymal progenitor and COS-7 cells.

Introduction to nanocoatings produced by layer-by-layer (LbL) self-assembly

Studies on the adsorption of oppositely charged colloidal particles ultimately resulted in multilayered polyelectrolyte self-assembly. The inception of layer-by-layer constructed particles facilitated the production of multifunctional, stimuli-responsive carrier systems. An array of synthetic and natural polyelectrolytes, metal oxides and clay nanoparticles is available for the construction of multilayered nanocoats on a multitude of substrates or removable cores. Numerous substrates can be encapsulated utilizing this technique including dyes, enzymes, drugs and cells. Furthermore, the outer surface of the particles presents and ideal platform that can be functionalized with targeting molecules or catalysts. Some processing parameters determining the properties of these successive self-assembly constructs are the surface charge density, coating material concentration, rinsing and drying steps, temperature and ionic strength of the medium. Additionally, the simplicity of the layer-by-layer assembly technique and the availability of established characterization methods, render these constructs extremely versatile in applications of sensing, encapsulation and target- and trigger-responsive drug delivery.

Exploiting fluorescent polymers to probe the self-assembly of virus-like particles

The inside surfaces of the protein shells of many viruses are positively charged, thereby enhancing the self-assembly of capsid proteins around their (oppositely charged) RNA genome. These proteins have been shown to organize similarly around a variety of nonbiological, negatively charged, polymers, for example, poly(styrene sulfonate) (PSS), forming virus-like particles (VLPs). We have demonstrated recently that the VLPs formed from cowpea chlorotic mottle virus (CCMV) capsid protein increase in size (from T=2 to T=3 structures) upon increase in PSS molecular weight (from 400 kDa to 3.4 MDa), and that the total charge on the PSS exceeds that of the capsid protein by as much as a factor of 9. Here, we extend studies of this kind to PSS molecules that are sufficiently small that two or more can be packaged into VLPs. The use of 38 kDa PSS polymers that have been fluorescently labeled with Rhodamine B allows us to determine the number of PSS molecules per capsid. Electron micrographs of the VLPs show a bimodal distribution of particle diameters, with one peak centered around 19 nm, typical of a T=1 triangulation number, and the other around 21 nm, consistent with a pseudo T=2 structure; increasing the molar ratio of protein to PSS in the reaction mix shifts the VLP distribution from T=1 to T=2 structures. By combining fluorescence and gel electrophoresis measurements, it is determined that, on average, there are two polymers in each T=1 capsid and three in each T=2, with the PSS charge less than that of the capsid protein by as much as a factor of 2. VLPs of this kind provide a versatile model system for determining the principles underlying self-assembly of controlled numbers of cargo molecules in nanocontainers of increasing size.

Modular assembly of layer-by-layer capsules with tailored degradation profiles

Herein we report the preparation of layer-by-layer (LbL) assembled, biodegradable, covalently stabilized capsules with tunable degradation properties. Poly(L-glutamic acid) modified with alkyne moieties (PGA(Alk)) was alternately assembled with poly(N-vinyl pyrrolidone) (PVPON) on silica particles via hydrogen-bonding. The films were cross-linked with a bis-azide linker, followed by removal of the sacrificial template and PVPON at physiological pH through hydrogen bond disruption, yielding one-component PGA(Alk) capsules. To control the kinetics and location of capsule degradation, a number of approaches were investigated. First, a degradable bis-azide cross-linker was incorporated into the inherently enzymatically degradable capsules. Second, we assembled low-fouling capsules composed of nondegradable poly(N-vinyl pyrrolidone-ran-propargyl acrylate) (PVPON(Alk)) via hydrogen bonding with poly(methacrylic acid) (PMA) and combined this with the aforementioned system (PGA(Alk)/PVPON) to produce stratified hybrid capsules. The degradation profiles of these stratified capsules can be closely controlled by the number as well as the position of nondegradable barrier layers in the systems. The facile tailoring of the degradation kinetics makes this stratified LbL approach promising for the design of tailored drug-delivery vehicles.

The preparation of layered double hydroxide wrapped carbon nanotubes and their application as a flame retardant for polypropylene

Carbon nanotubes (CNTs) wrapped with layered double hydroxide (LDH-w-CNTs) were facilely obtained through in situ introduction of CNTs into the hydrothermal reaction system of LDH, with the goal of combining their unique physical and chemical characteristics to meet new advanced applications. Morphological observations indicated that LDH lamellae enwrapped the surface of CNTs and the wrapping degree was dependent on the functionalization of CNTs. Zeta-potential measurements showed that the interaction between the positive charge of LDH and the negative charge of CNTs was the main driving force of the wrapping process. Both hybrids led to a reduction in the peak heat release rate (PHRR) of polypropylene, indicating that they could confer better flame retardancy on polypropylene with respect to LDH and CNTs.

Subcompartmentalized polymer hydrogel capsules with selectively degradable carriers and subunits

Subcompartmentalized hydrogel capsules (SHCs) with selectively degradable carriers and subunits are designed for potential applications in drug delivery and microencapsulated biocatalysis. Thiolated poly(methacrylic acid) and poly(N-vinyl pyrrolidone) are used to assemble 3-microm-diameter carrier capsules and 300-nm-diameter subunits, independently stabilized by a diverse range of covalent linkages. This paper presents examples of SHCs with tens of subcompartments and their successful drug loading, as well as selective degradation of the SHC carrier and/or subunits in response to multiple chemical stimuli.

Flow cytometry as a new approach to investigate drug transfer between lipid particles

Lipid nanoparticles and liposomal carrier systems are of growing interest for intravenous drug delivery due to their biocompatibility and targetability. It is, however, difficult to investigate their release behavior for lipophilic drugs under physiological conditions. This study describes a novel flow cytometric method studying drug transfer from such carrier systems to particles simulating physiological receptor sites. For this purpose, liquid and solid trimyristin nanoparticles or soybean phospholipid liposomes were loaded with the lipophilic fluorescent substances Nile red, temoporfin, and DiI. The transfer of these model drugs to large emulsion droplets was examined by flow cytometry. Transfer of DiI to differently sized acceptor emulsions was also monitored by separating donor and acceptor particles using ultracentrifugation. Flow cytometry revealed a completion of transfer within a few minutes for Nile red and temoporfin at considerable amounts of transferred dye. In contrast, the highly lipophilic DiI transferred over a period of weeks only for a small percentage of the dye. Ultracentrifugation results confirmed this for DiI and indicated a dependence of transfer characteristics on the acceptor surface area. Nile red transfer into a bulk oil phase as alternative acceptor system was also very slow. Flow cytometry seems to be well suited to study the intrinsic transfer of fluorescent lipophilic substances, as no kinetic hindrances like dialysis bags nor separation steps are required. Additional detailed experiments will, however, be necessary to elucidate the prevalent transfer mechanisms completely.

Biodegradable click capsules with engineered drug-loaded multilayers

We report the modular assembly of a polymer-drug conjugate into covalently stabilized, responsive, biodegradable, and drug-loaded capsules with control over drug dose and position in the multilayer film. The cancer therapeutic, doxorubicin hydrochloride (DOX), was conjugated to alkyne-functionalized poly(l-glutamic acid) (PGA(Alk)) via amide bond formation. PGA(Alk) and PGA(Alk+DOX) were assembled via hydrogen bonding with poly(N-vinyl pyrrolidone) (PVPON) on planar and colloidal silica templates. The films were subsequently covalently stabilized using diazide cross-linkers, and PVPON was released from the multilayers by altering the solution pH to disrupt hydrogen bonding. After removal of the sacrificial template, single-component PGA(Alk) capsules were obtained and analyzed by optical microscopy, transmission electron microscopy, and atomic force microscopy. The PGA(Alk) capsules were stable in the pH range between 2 and 11 and exhibited reversible swelling/shrinking behavior. PGA(Alk+DOX) was assembled to form drug-loaded polymer capsules with control over drug dose and position in the multilayer system (e.g., DOX in every layer or exclusively in layer 3). The drug-loaded capsules could be degraded enzymatically, resulting in the sustained release of active DOX over approximately 2 h. Cellular uptake studies demonstrate that the viability of cells incubated with DOX-loaded PGA(Alk) capsules significantly decreased. The general applicability of this modular approach, in terms of incorporation of polymer-drug conjugates in other click multilayer systems, was also demonstrated. Biodegradable click capsules with drug-loaded multilayers are promising candidates as carrier systems for biomedical applications.

Influence of salt concentration on the assembly of DNA multilayer films

DNA multilayer films are promising candidates for a plethora of applications, including sensing, diagnostics, and drug/gene delivery. Fabricated solely from DNA, the use of salt in forming DNA multilayers is crucial in promoting and maintaining hybridization of complementary base pairs by minimizing the repulsive forces between the oligonucleotides and preventing disassembly of the layers once formed. Herein, we examine the role of salt on the assembly of DNA films assembled from oligonucleotides composed of two homopolymeric diblocks (polyA(n)G(n) and polyT(n)C(n)) in salt concentrations ranging from 0.1 to 2 M. Using quartz crystal microgravimetry (QCM) and flow cytometry, we show that films assembled at high salt concentrations (2 M salt) exhibit a different morphology and are denser than those assembled from lower (1 M salt) salt solutions. Formation of the T x A*T triplex in solution and within the DNA film was also studied using circular dichroism (CD) and QCM, respectively. DNA films assembled using oligonucleotides of various lengths (20- to 60-mer) at high salt concentration (2 M salt) showed no significant influence on the film growth. This work shows that salt plays an important role in the assembly and final morphology of DNA multilayer films, hence enabling films with different properties to be tailored.

Low-fouling, biofunctionalized, and biodegradable click capsules

We report the synthesis of covalently stabilized hollow capsules from biodegradable materials using a combination of click chemistry and layer-by-layer (LbL) assembly. The biodegradable polymers poly(L-lysine) (PLL) and poly(L-glutamic acid) (PGA) were modified with alkyne and azide moieties. Linear film buildup was observed for both materials on planar surfaces and colloidal silica templates. A variation of the assembly conditions, such as an increase in the salt concentration and variations in pH, was shown to increase the individual layer thickness by almost 200%. The biodegradable click capsules were analyzed with optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Capsules were uniform in size and had a regular, spherical shape. They were found to be stable between pH 2 and 11 and showed reversible, pH-responsive shrinking/swelling behavior. We also show that covalently stabilized PLL films can be postfunctionalized by depositing a monolayer of heterobifunctional poly(ethylene glycol) (PEG), which provides low-fouling properties and simultaneously enhances specific protein binding. The responsive, biodegradable click films reported herein are promising for a range of applications in the biomedical field.

Prevention of H-Aggregates Formation in Cy5 Labeled Macromolecules

H-aggregates of the cyanine dye Cy5 are formed during covalent linkage to the cationic macromolecule Poly(allylamine) (PAH). The nonfluorescent H-aggregates strongly restrict the usage of the dye for analytical purposes and prevent a quantitative determination of the labeled macromolecules. The behavior of the H-aggregates has been studied by investigation of the absorption and fluorescence spectra of the dye polymer in dependence on solvent, label degree and additional sulfonate groups. H-aggregate formation is caused by an inhomogeneous distribution of the Cy5 molecules on the polymer chain. The H-aggregates can be destroyed by conformational changes of the PAH induced by interactions with polyanions or in organic solvents. It has been found that the polymer labeling process in high content of organic solvents can prevent the formation of H-aggregates. The results offer a better understanding and improvement of the use of the Cy5 dye for labeling purposes in fluorescence detection of macromolecules.

Assembly of erodible, DNA-containing thin films on the surfaces of polymer microparticles: toward a layer-by-layer approach to the delivery of DNA to antigen-presenting cells

We report a layer-by-layer approach to the assembly of ultrathin and erodible DNA-containing films on the surfaces of polymer microparticles. DNA-containing multilayered films were fabricated layer-by-layer on the surfaces of polystyrene microspheres (approximately 6 microm) by iterative and alternating cycles of particle suspension, centrifugation and resuspension in solutions of plasmid DNA and a hydrolytically degradable polyamine. Film growth occurred in a stepwise manner, as demonstrated by characterization of the zeta potentials and fluorescence intensities of film-coated particles during film assembly. Characterization of film-coated particles by confocal fluorescence microscopy and scanning electron microscopy revealed the multilayered particle coatings to be smooth, uniform and free of large-scale physical defects. Film-coated microparticles sustained the release of transcriptionally active DNA into solution for approximately three days when incubated in physiologically relevant media. Previous studies have demonstrated that the adsorption of DNA onto the surfaces of cationic microparticles can be used to target the delivery of DNA to antigen-presenting cells. As a first step toward the application of this layer-by-layer approach to the development of methods for the delivery of DNA to antigen-presenting cells, we demonstrated that film-coated microparticles could be used to transport DNA into macrophage cells in vitro using a model mouse macrophage cell line. Our results suggest the basis of a general approach that could, with further development, prove useful for the delivery of DNA-encoded antigens to macrophages, or other antigen-presenting cells, and provide new materials-based methods for the formulation and delivery of DNA vaccines.

Manipulating the salt and thermal stability of DNA multilayer films via oligonucleotide length

DNA films are promising materials for diverse applications, including sensing, diagnostics, and drug/gene delivery. However, the ability to tune the stability of DNA films remains a crucial aspect for such applications. Herein, we examine the role of oligonucleotide length on the formation, and salt and thermal stability, of DNA multilayer films using oligonucleotides of homopolymeric diblocks (polyAG and polyTC), with each block (A, G, T, or C) ranging from 5 to 30 bases (10-, 20-, 30-, 40-, and 60-mer). Using a combination of quartz crystal microgravimetry, dual polarization interferometry, and flow cytometry, we demonstrate that at least 10 bases per hybridizing block in the DNA diblocks (that is, 20-mer) are required for successful hybridization and, hence, DNA multilayer film formation. Films assembled using longer oligonucleotide blocks were more stable in low salt conditions, with the DNA multilayer films assembled from the 60-mer oligonucleotides remaining intact in solutions of about 25 mM NaCl. A systematic increase in film melting temperature ( T m) was observed for the DNA multilayer films (assembled on colloids) with increasing oligonucleotide length, ranging from 38.5 degrees C for the 20-mer films to 53 degrees C for the 60-mer films. Further, an alternating trend in T m of the DNA multilayer films was observed with layer number (AG or TC); DNA multilayer films terminated with an AG layer exhibited a higher T m (44-49 degrees C) than films with an outermost TC layer (ca. 38 degrees C), suggesting a rearrangement of the film structure upon hybridization of the outermost layer. This work shows that the stability of DNA multilayer films can be tuned by varying the length of the oligonucleotide building blocks, thus providing a versatile means to tailor the salt and thermal stability of DNA films, which is necessary for the application of such films.

Characterization of the growth of polyelectrolyte multilayers formed at interfaces between aqueous phases and thermotropic liquid crystals

Polyelectrolyte multilayers (PEMs) formed at interfaces between aqueous solutions and thermotropic (water-immiscible) liquid crystals (LCs) offer the basis of a new method to tailor the nanometer-scale structure and chemical functionality of these interfaces. Toward this end, we report a study that compares the growth of PEMs formed at mobile and deformable interfaces defined by LCs relative to growth observed at model (rigid) solid surfaces. Experiments aimed at determining if polyelectrolytes such as poly(sodium-4-styrenesulfonate) (PSS) can partition from the aqueous phase into the bulk of the LC yielded no evidence of such partitioning. Whereas measurements of the growth of PEMs formed from poly(allylamine hydrochloride) (PAH) and PSS at the aqueous-LC interface revealed growth characteristics similar to those measured at both hydrophobic and hydrophilic interfaces of solids, the growth of PEMs from PAH and poly(acrylic acid) (PAA) at the aqueous-LC interface was found to differ substantially from the solids investigated: (i) the linear growth of PEMs of PAH/PAA that was measured at the aqueous-LC interface under conditions that did not lead to the growth of PEMs at the interface of octadecyltrichlorosilane (OTS)-treated glass (a hydrophobic solid surface), and (ii) in comparison to the growth of PEMs of PAH/PAA at the surface of glass (a hydrophilic charged surface), a higher rate of growth was observed at the aqueous-LC interface. The finding that the growth rate of PEMs of PAH/PAA at aqueous-LC interfaces is greater than on solid surfaces is supported by additional measurements of growth as a function of pH. Finally, the pH-triggered reorganization of PAH/PAA PEMs supported at the aqueous-LC interface led to changes in the order and optical properties of the LC. These data are discussed in light of the nature of aqueous-LC interfaces, including the mobility and deformability of the interface and recent measurements of the zeta-potentials of aqueous-LC interfaces.

A general approach for DNA encapsulation in degradable polymer microcapsules

We report a general and facile method for the encapsulation of DNA in nanoengineered, degradable polymer microcapsules. Single-stranded (ss), linear double-stranded (ds), and plasmid DNA were encapsulated into disulfide-cross-linked poly(methacrylic acid) (PMA) capsules. The encapsulation procedure involves four steps: adsorption of DNA onto amine-functionalized silica (SiO(2)(+)) particles; sequential deposition of thiolated PMA (PMA (SH)) and poly(vinylpyrrolidone) to form multilayers; cross-linking of the thiol groups of the PMA (SH) in the multilayers into disulfide linkages; and removal of the sacrificial SiO(2)(+) particles. Multilayer growth was dependent on the surface coverage of DNA on the SiO(2)(+) particles, with stable capsules formed from particles with up to 50% DNA surface coverage. The encapsulation strategy applies to nucleic acids with varied size and conformation and allows DNA to be concentrated over 100-fold from dilute solutions into monodisperse, uniformly loaded polymer capsules. The capsule loading can be controlled by the DNA:SiO(2)(+)particle ratio, and for 1 microm diameter capsules, loadings of approximately 1000 chains of 800 bp dsDNA and more than 10,000 chains of 20-mer ssDNA can be achieved. The encapsulated DNA was released and successfully used in polymerase chain reactions as both templates (linear dsDNA and plasmid DNA) and primer sequences (ssDNA), confirming the functionality and structural integrity of the encapsulated DNA. These DNA-loaded polymer microcapsules hold promise as delivery vehicles for gene therapy and diagnostic applications.

Direct evidence of layer-by-layer assembly of polyelectrolyte multilayers on soft and porous temperature-sensitive PNiPAM microgel using fluorescence correlation spectroscopy

We describe the layer-by-layer assembly of polyelectrolyte multilayers on soft and porous temperature-sensitive poly(N-isopropylacrylamide) (PNiPAM) microgel. Microgels are not hard and rigid but rather are soft and porous particles, and polyelectrolytes not only interdigitate with each other during multilayer formation but also with the microgel. Because of this difference, there could be concerns about the feasibility of the layer-by-layer technique on these systems. The argument is that the layer being deposited is stripping the underlying layer instead of anchoring to the latter, and common methods of characterizing film growth on particles such as zeta-potentials will still show "successful" charge reversal. To address this issue, we used two differently labeled polyelectrolytes during the deposition. Because of the small size of the microgel (400 nm) studied, we cannot distinguish between polyelectrolytes adsorbed on or in the microgel. However, with fluorescence correlation spectroscopy, we can clearly distinguish between free labeled polyelectrolytes and those that are bound to the microgel. Dual-color correlation confirms the presence of both polyelectrolytes bound to the same particle while fluorescence imaging (on a dry sample) provides the visual proof.