Polyelectrolyte microshells as carriers for fluorescent sensors: loading and sensing properties of a ruthenium-based oxygen indicator

The rise of bio-inspired polymer compartments responding to pathology-related signals

Self-organized nano- and microscale polymer compartments such as polymersomes, giant unilamellar vesicles (GUVs), polyion complex vesicles (PICsomes) and layer-by-layer (LbL) capsules have increasing potential in many sensing applications. Besides modifying the physicochemical properties of the corresponding polymer building blocks, the versatility of these compartments can be markedly expanded by biomolecules that endow the nanomaterials with specific molecular and cellular functions. In this review, we focus on polymer-based compartments that preserve their structure, and highlight the key role they play in the field of medical diagnostics: first, the self-assembling abilities that result in preferred architectures are presented for a broad range of polymers. In the following, we describe different strategies for sensing disease-related signals (pH-change, reductive conditions, and presence of ions or biomolecules) by polymer compartments that exhibit stimuli-responsiveness. In particular, we distinguish between the stimulus-sensitivity contributed by the polymer itself or by additional compounds embedded in the compartments in different sensing systems. We then address necessary properties of sensing polymeric compartments, such as the enhancement of their stability and biocompatibility, or the targeting ability, that open up new perspectives for diagnostic applications.

Polymer- and Hybrid-Based Biomaterials for Interstitial, Connective, Vascular, Nerve, Visceral and Musculoskeletal Tissue Engineering

In this review, materials based on polymers and hybrids possessing both organic and inorganic contents for repairing or facilitating cell growth in tissue engineering are discussed. Pure polymer based biomaterials are predominantly used to target soft tissues. Stipulated by possibilities of tuning the composition and concentration of their inorganic content, hybrid materials allow to mimic properties of various types of harder tissues. That leads to the concept of "one-matches-all" referring to materials possessing the same polymeric base, but different inorganic content to enable tissue growth and repair, proliferation of cells, and the formation of the ECM (extra cellular matrix). Furthermore, adding drug delivery carriers to coatings and scaffolds designed with such materials brings additional functionality by encapsulating active molecules, antibacterial agents, and growth factors. We discuss here materials and methods of their assembly from a general perspective together with their applications in various tissue engineering sub-areas: interstitial, connective, vascular, nervous, visceral and musculoskeletal tissues. The overall aims of this review are two-fold: (a) to describe the needs and opportunities in the field of bio-medicine, which should be useful for material scientists, and (b) to present capabilities and resources available in the area of materials, which should be of interest for biologists and medical doctors.

Classification of analytics, sensorics, and bioanalytics with polyelectrolyte multilayer capsules

Polyelectrolyte multilayer (PEM) capsules, constructed by LbL (layer-by-layer)-adsorbing polymers on sacrificial templates, have become important carriers due to multifunctionality of materials adsorbed on their surface or encapsulated into their interior. They have been also been used broadly used as analytical tools. Chronologically and traditionally, chemical analytics has been developed first, which has long been synonymous with all analytics. But it is not the only development. To the best of our knowledge, a summary of all advances including their classification is not available to date. Here, we classify analytics, sensorics, and biosensorics functionalities implemented with polyelectrolyte multilayer capsules and coated particles according to the respective stimuli and application areas. In this classification, three distinct categories are identified: (I) chemical analytics (pH; K⁺, Na⁺, and Pb²⁺ ion; oxygen; and hydrogen peroxide sensors and chemical sensing with surface-enhanced Raman scattering (SERS)); (II) physical sensorics (temperature, mechanical properties and forces, and osmotic pressure); and (III) biosensorics and bioanalytics (fluorescence, glucose, urea, and protease biosensing and theranostics). In addition to this classification, we discuss also principles of detection using the above-mentioned stimuli. These application areas are expected to grow further, but the classification provided here should help (a) to realize the wealth of already available analytical and bioanalytical tools developed with capsules using inputs of chemical, physical, and biological stimuli and (b) to position future developments in their respective fields according to employed stimuli and application areas. Graphical abstract.

Luminescence-Based Optical Sensors Fabricated by Means of the Layer-by-Layer Nano-Assembly Technique

Luminescence-based sensing applications range from agriculture to biology, including medicine and environmental care, which indicates the importance of this technique as a detection tool. Luminescent optical sensors are required to be highly stable, sensitive, and selective, three crucial features that can be achieved by fabricating them by means of the layer-by-layer nano-assembly technique. This method permits us to tailor the sensors' properties at the nanometer scale, avoiding luminophore aggregation and, hence, self-quenching, promoting the diffusion of the target analytes, and building a barrier against the undesired molecules. These characteristics give rise to the fabrication of custom-made sensors for each particular application.

Gold Nanocluster Containing Polymeric Microcapsules for Intracellular Ratiometric Fluorescence Biosensing

A new approach to sensing and imaging hydrogen peroxide (H₂O₂) was developed using microcapsule-based dual-emission ratiometric luminescent biosensors. Bovine serum albumin-capped gold nanoclusters (BSA-AuNCs) sensitive to H₂O₂ were coencapsulated with insensitive FluoSpheres (FSs) within polymeric capsules fabricated via the layer-by-layer method. Under single-wavelength excitation, the microcapsule-based biosensors exhibited emission bands at ∼516 and ∼682 nm resulting from the FSs and BSA-AuNCs, respectively. The polyelectrolyte multilayers lining the microcapsules were effective in protecting BSA-AuNCs from the degradation catalyzed by proteases (chymotrypsin, trypsin, papain, and proteinase K) and subsequent luminescent quenching, overcoming a key limitation of prior BSA-AuNC-based sensing systems. The luminescent response of the sensors was also found to be independent of local changes in pH (5-9). Quenching of the AuNCs in the presence of H₂O₂ enabled the spectroscopic quantification and imaging of changes in H₂O₂ concentration from 0 to 1 mM. The microcapsule sensors were easily phagocytized by murine macrophage cells (RAW 264.7), were effective as intracellular H₂O₂ imaging probes, and were successfully used to detect local release of H₂O₂ in response to an external chemical stimulus.

Microbial Activation of Bacillus subtilis-Immobilized Microgel Particles for Enhanced Oil Recovery

Microbially enhanced oil recovery involves the use of microorganisms to extract oil remaining in reservoirs. Here, we report fabrication of microgel particles with immobilized Bacillus subtilis for application to microbially enhanced oil recovery. Using B. subtilis isolated from oil-contaminated soils in Myanmar, we evaluated the ability of this microbe to reduce the interfacial tension at the oil-water interface via production of biosurfactant molecules, eventually yielding excellent emulsification across a broad range of the medium pH and ionic strength. To safely deliver B. subtilis into a permeable porous medium, in this study, these bacteria were physically immobilized in a hydrogel mesh of microgel particles. In a core flooding experiment, in which the microgel particles were injected into a column packed with silica beads, we found that these particles significantly increased oil recovery in a concentration-dependent manner. This result shows that a mesh of microgel particles encapsulating biosurfactant-producing microorganisms holds promise for recovery of oil from porous media.

Cellular uptake of poly(allylamine hydrochloride) microcapsules with different deformability and its influence on cell functions

It is important to understand the safety issue and cell interaction pattern of polyelectrolyte microcapsules with different deformability before their use in biomedical applications. In this study, SiO2, poly(sodium-p-styrenesulfonate) (PSS) doped CaCO3 and porous CaCO3 spheres, all about 4μm in diameter, were used as templates to prepare microcapsules with different inner structure and subsequent deformability. As a result, three kinds of covalently assembled poly(allylaminehydrochloride)/glutaraldehyde (PAH/GA) microcapsules with similar size but different deformability under external osmotic pressure were prepared. The impact of different microcapsules on cell viability and functions are studied using smooth muscle cells (SMCs), endothelial cells (ECs) and HepG2 cells. The results demonstrated that viabilities of SMCs, ECs and HepG2 cells were not significantly influenced by either of the three kinds of microcapsules. However, the adhesion ability of SMCs and ECs as well as the mobility of SMCs, ECs and HepG2 cells were significantly impaired after treatment with microcapsules in a deformability dependent manner, especially the microcapsules with lower deformability caused higher impairment on cell functions. The cellular uptake kinetics, uptake pathways, intracellular distribution of microcapsules are further investigated in SMCs to reveal the potential mechanism. The SMCs showed faster uptake rate and exocytosis rate of microcapsules with lower deformability (Cap@CaCO3/PSS and Cap@CaCO3), leading to higher intracellular accumulation of microcapsules with lower deformability and possibly larger retardation of cell functions. The results pointed out that the deformability of microcapsules is an important factor governing the biological performance of microcapsules, which requires careful adjustment for further biomedical applications.

Fabrication of nanocapsule carriers from multilayer-coated vaterite calcium carbonate nanoparticles

Nanosized luminescent sensors were prepared as reagents for optical sensing and imaging of oxygen using ratiometric emission properties of a two-dye system. Polymeric capsules were fabricated utilizing poly(vinylsulfonic acid) (PVSA)-stabilized vaterite CaCO3 nanoparticles (CCNPs) as sacrificial templates. The buffer and polymeric surfactant requirements of the layer-by-layer (LbL) process were evaluated toward deposition of multilayer coatings and, ultimately, formation of hollow capsules using these interesting materials. CCNPs were found to be more stable in alkaline NaHCO3 buffer after repeated cycles of washing under sonication and resuspension. An intermediate PVSA concentration was required to maximize the loading of oxygen-sensitive porphyrin and oxygen-insensitive fluorescent nanoparticles in the CCNPs while maintaining minimal nanoparticle size. The CCNPs were then coated with polyelectrolyte multilayers and subsequent removal of the CaCO3 core yielded nanocapsules containing dye and fluorescent nanoparticles. The resulting nanocapsules with encapsulated luminophores functioned effectively as oxygen sensors with a quenching response of 89.28 ± 2.59%, and O2 (S = 1/2) = 20.91 μM of dissolved oxygen.

Biological applications of LbL multilayer capsules: from drug delivery to sensing

Polyelectrolyte multilayer (PEM) capsules engineered with active elements for targeting, labeling, sensing and delivery hold great promise for the controlled delivery of drugs and the development of new sensing platforms. PEM capsules composed of biodegradable polyelectrolytes are fabricated for intracellular delivery of encapsulated cargo (for example peptides, enzymes, DNA, and drugs) through gradual biodegradation of the shell components. PEM capsules with shells responsive to environmental or physical stimuli are exploited to control drug release. In the presence of appropriate triggers (e.g., pH variation or light irradiation) the pores of the multilayer shell are unlocked, leading to the controlled release of encapsulated cargos. By loading sensing elements in the capsules interior, PEM capsules sensitive to biological analytes, such as ions and metabolites, are assembled and used to detect analyte concentration changes in the surrounding environment. This Review aims to evaluate the current state of PEM capsules for drug delivery and sensing applications.

25th anniversary article: double emulsion templated solid microcapsules: mechanics and controlled release

How droplet microfluidics can be used to fabricate solid-shelled microcapsules having precisely controlled release behavior is described. Glass capillary devices enable the production of monodisperse double emulsion drops, which can then be used as templates for microcapsule formation. The exquisite control afforded by microfluidics can be used to tune the compositions and geometrical characteristics of the microcapsules with exceptional precision. The use of this approach to fabricate microcapsules that only release their contents when exposed to a specific stimulus--such as a change in temperature, exposure to light, a change in the chemical environment, or an external stress--only after a prescribed time delay, and at a prescribed rate is reviewed.

Synthesis and characterization of ratiometric ion-sensitive polyelectrolyte capsules

Micrometer-sized polyelectrolyte capsules are synthesized, which have ion-sensitive fluorophores embedded in their cavities. As the membranes of the capsules are permeable to ions, the fluorescence of the capsules changed with the ion concentration. In particular, capsules sensitive to protons, sodium, potassium, and chloride ions are fabricated and their fluorescence response analyzed. In order to allow for ratiometric measurements, additional fluorophores whose emission do not depend on the ion concentration and which emit a different wavelength are co-embedded in the capsule cavities.

Polypeptide multilayer film co-delivers oppositely-charged drug molecules in sustained manners

The current state-of-the-art for drug-carrying biomedical devices is mostly limited to those that release a single drug. Yet there are many situations in which more than one therapeutic agent is needed. Also, most polyelectrolyte multilayer films intended for drug delivery are loaded with active molecules only during multilayer film preparation. In this paper, we present the integration of capsules as vehicles within polypeptide multilayer films for sustained release of multiple oppositely charged drug molecules using layer-by-layer nanoassembly technology. Calcium carbonate (CaCO(3)) particles were impregnated with polyelectrolytes, shelled with polyelectrolyte multilayers, and then assembled onto polypeptide multilayer films using glutaraldehyde. Capsule-integrated polypeptide multilayer films were obtained after decomposition of CaCO(3) templates. Two oppositely charged drugs were loaded into capsules within polypeptide multilayer films postpreparation based on electrostatic interactions between the drugs and the polyelectrolytes impregnated within capsules. We determined that the developed innovative capsule-integrated polypeptide multilayer films could be used to load multiple drugs of very different properties (e.g., opposite charges) any time postpreparation (e.g., minutes before surgical implantation inside an operating room), and such capsule-integrated films allowed simultaneous delivery of two oppositely charged drug molecules and a sustained (up to two weeks or longer) and sequential release was achieved.

Retrieval of a metabolite from cells with polyelectrolyte microcapsules

To monitor cellular processes in individual cells, it is important to measure the concentrations of intracellular metabolites and to retrieve them for analysis. The use of functionalized polyelectrolyte microcapsules as intracellular sensors for in vivo reporting is persented. Capsules loaded with streptavidin-rhodamine, which was introduced into fibroblasts by electroporation, autonomously escaped from an endocytic compartment and efficiently recruited biotin-fluorescein from the cytosol. This work demonstrates the utility of polyelectrolyte microcapsules for intracellular capture of metabolites and eventually for drug delivery on an organismic level.

LbL multilayer capsules: recent progress and future outlook for their use in life sciences

In this review we provide an overview of the recent progress in designing composite polymer capsules based on the Layer-by-Layer (LbL) technology demonstrated so far in material science, focusing on their potential applications in medicine, drug delivery and catalysis. The benefits and limits of current systems are discussed and the perspectives on emerging strategies for designing novel classes of therapeutic vehicles are highlighted.

Nanomanufacturing by layer-by-layer assembly - from nanoscale coating to device applications

Layer-by-layer (LbL) assembly is emerging as a key nanomanufacturing technique that is finding a broad range of applications. It is a versatile, simple, and easy to use technique, allowing the realization of novel nanometer-scale multi-layered materials and structures that can be made to have highly desirable properties, including chemical, mechanical, electrical, magnetic, thermal, and optical. The present paper discusses the LbL assembly technique and its applications, including for nanoelectromechanical systems (NEMS) and micro-electromechanical systems (MEMS) (in combination with microfabrication), for biocompatible coating, for nanoengineered capsules, for pulp microfibre nanocoating to obtain better paper, and for polymer-based electronic devices.

Microcapsule biosensors using competitive binding resonance energy transfer assays based on apoenzymes

This paper reports the first demonstration of a fluorescence resonance energy transfer based glucose sensor, wherein a competitive binding (CB) assay is encapsulated into polyelectrolyte microcapsules. The work supports the concept that microcapsules are superior to hydrogel systems or other matrixes for competitive-binding-based system, as they provide free movement of the sensing elements within the capsule interior while constant total sensing assay concentration is maintained. The transduction approach employed in these preliminary experiments is also a novel CB system based on a model apoenzyme, apo-glucose oxidase (AG), which is highly specific to beta-d-glucose, as the model target-binding protein. The glucose sensitivity of the fluorescein isothiocyanate (FITC)-dextran and tetramethylrhodamine isothiocyanate-AG encapsulated in microcapsules showed 5 times greater specificity for beta-D-glucose over other sugars, with sensitivity (change in intensity ratio) in the range of 2-6%/mM. It was observed that the sensitivity and range of the response can be tailored by controlling the assay concentration using different FITC-dextran molecular weight and total capsule concentration. The findings support the concepts of using microcapsules to encapsulate CB assays for reversible and stable sensors and the use of apoenzymes as specific molecular recognition elements in CB assays. Further, characterization results for microcapsule glucose sensors demonstrate their suitability for monitoring physiological glucose levels.

Using electrostatic self-assembled gradient nanosensor phantoms for calibration of an optical intrinsic signal imaging system

Wide-field optical intrinsic signal (OIS) imaging has been used in functional cortex mapping for its excellent spatial resolution. To compensate for its low temporal resolution, extrinsic dye signals have been introduced. Fluorescence spectroscopy in the form of nanoengineered sensors has also been used to detect biochemical signals of molecular interactions. In this paper, oxygen-sensitive dye Ru(dpp)/sub 3//sup 2+/ was immobilized into nano-sized spheres by electrostatic layer-by-layer (LbL) self-assembly, and then deposited on glass slides as intensity gradients. By comparing gradient ratios and ratios of function dye Ru(dpp)/sub 3//sup 2+/ and reference dye between epi-fluorescence microscope and an OIS imaging system, the feasibility and efficiency of nano-sized oxygen sensors in OIS imaging were studied.

Layer-by-Layer Enzyme/Polyelectrolyte Films as a Functional Protective Barrier in Oxidizing Media

The influence of a catalase (Cat) layer located at different depths in the layer-by-layer hemoglobin/polystyrene sulfonate films with an (Hb/PSS)(20)(-)(x)/(Cat/PSS)/(Hb/PSS)(x) (x = 0-20) architecture on kinetics of hemoglobin degradation under treatment with hydrogen peroxide solutions of different concentrations and features of H(2)O(2) decay in surrounding solutions has been studied. While assembled on the top of the multilayers, the catalase layer shows the highest activity in hydrogen peroxide decomposition. Hemoglobin in such films retains its nativity for a longer period of time. The effect of catalase layers is compared with that of protamine, horseradish peroxidase, and inactivated catalase. Positioning an active layer with catalytic properties as an outer layer is the best protection strategy for layer-by-layer assembled films in aggressive media.

Equilibrium Distribution of Permeants in Polyelectrolyte Microcapsules Filled with Negatively Charged Polyelectrolyte: The Influence of Ionic Strength and Solvent Polarity

The effects of ionic strength and solvent polarity on the equilibrium distribution of fluorescein (FL) and FITC-dextran between the interior of polyelectrolyte multilayer microcapsules filled with negatively charged strong polyelectrolyte and the bulk solution were systematically investigated. A negatively charged strong polyelectrolyte, poly(styrene sulfonate) (PSS), used for CaCO3 core fabrication, was entrapped inside the capsules. Due to the semipermeability of the capsule wall, a Donnan equilibrium between the inner solution within the capsules and the bulk solution was created. The equilibrium distribution of the negatively charged permeants was investigated by means of confocal laser scanning microscopy as a function of ionic strength and solvent polarity. The equilibrium distribution of the negatively charged permeants could be tuned by increasing the bulk ionic strength to decrease the Donnan potential. Decreasing the solvent polarity also could enhance the permeation of FL, which induces a sudden increase of permeation when the ethanol volume fraction was higher than 0.7. This is mainly attributed to the precipitation of PSS. A theoretical model combining the Donnan equilibrium and Manning counterion condensation was employed to discuss the results.

Redox properties of the ferricyanide ion on electrodes coated with layer-by-layer thin films composed of polysaccharide and poly(allylamine)

Polyelectrolyte multilayer thin films were prepared by an alternate deposition of poly(allylamine hydrochloride) (PAH) and anionic polysaccharides {carboxymethylcellulose (CMC) and alginic acid (AGA)} on the surface of a gold (Au) disk electrode, and the binding of ferricyanide [Fe(CN)(6)](3)(-) and hexaammine ruthenium ions [Ru(NH(3))(6)](3+) to the films was evaluated. Poly(acrylic acid) (PAA) was also employed as a reference polyanion bearing carboxylate side chains. A quartz-crystal microbalance study showed that PAH-CMC and PAH-AGA multilayer films grow exponentially as the number of depositions increases. The thicknesses of five bilayers of (PAH-CMC)(5) and (PAH-AGA)(5) films were estimated to be 150 +/- 20 and 90 +/- 15 nm, respectively, in the dry state. The PAH/polysaccharide multilayer film-coated Au electrodes exhibited a redox response to the [Fe(CN)(6)](3)(-) ion dissolved in solution, irrespective of the sign of the surface charge of the film, suggesting the high permeability of the films to the [Fe(CN)(6)](3)(-) ion. In contrast, the PAH-PAA film-coated Au electrodes exhibited a redox response only when the outermost surface of the film was covered with a positively charged PAH layer. However, the permeation of the [Ru(NH(3))(6)](3+) cation was severely suppressed for all of the multilayer films. It was possible to confine the [Fe(CN)(6)](3)(-) ion in the films by immersing the film-coated electrodes in a 1 mM [Fe(CN)(6)](3)(-) solution for 15 min. Thus, the [Fe(CN)(6)](3)(-)-confined electrodes exhibited a cyclic voltammetric response in the [Fe(CN)(6)](3)(-) ion-free buffer solution. The loading of the [Fe(CN)(6)](3)(-) ion in the films was higher when the surface charge of the film was positive and increased with increasing film thickness. It was also found that the [Fe(CN)(6)](3)(-) ion confined in the films serves as an electrocatalyst that oxidizes ascorbic acid in solution.

Stable encapsulation of active enzyme by application of multilayer nanofilm coatings to alginate microspheres

In an effort to improve the stability for long-term biosensor use, layer-by-layer self-assembly was explored as a potential technique to provide a diffusion barrier to encapsulated glucose oxidase inside alginate microspheres (<5 microm), fabricated using an emulsification technique. The total loss of encapsulated enzyme was reduced to less than 25 and 15% with the application of single PAH/PSS and crosslinked PAH/PAA coatings, respectively, in comparison to at least a 45% loss observed with uncoated and PDDA/PSS-coated microspheres. Furthermore, it was found that enzyme within PAH/PSS- and crosslinked PAH/PAA-coated spheres retained more than 84 and 60% of initial activity, respectively, after three months, whereas uncoated and PDDA/PSS-coated microspheres retained less than 20%.

Nanoscale internally referenced oxygen sensors produced from self-assembled nanofilms on fluorescent nanoparticles

A novel sensor fabrication concept using indicators trapped in nanoengineered ultrathin films deposited on fluorescent nanoparticles is demonstrated for oxygen sensing. The nanoscale systems are based on the quenching of tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) immobilized within polyelectrolyte multilayers deposited on the surface of nanoparticle templates. The ionically bound nanofilms create a porous scaffold into which controlled precipitation of the chromophore is achieved using a combination of electrostatic attraction and dye insolubility in water. The fluorescent nanoparticles act as physical scaffolds and also provide a complementary spectral signature for use as an internal intensity reference. Oxygen sensors created on 100-nm yellow-green fluorescent particles exhibit a linear Stern-Volmer behavior with a quenching constant of 1.06 mM and sensitivity of 60%, which demonstrates that the dye is still partially accessible to oxygen following immobilization. To demonstrate the feasibility of intracellular metabolic monitoring with such nanoprobes, the oxygen sensors were chemically delivered into human dermal fibroblasts with no apparent loss in cell viability. The results prove that the approach to sensor production is facile and leads to sensitive systems that can be further optimized for improved response, and these findings support the further development of similar self-referenced probes toward quantitative intracellular analysis.

Stimuli-sensitive thin films prepared by a layer-by-layer deposition of 2-iminobiotin-labeled poly(ethyleneimine) and avidin

Layered thin films composed of avidin and 2-iminobiotin-labeled poly(ethyleneimine) (ib-PEI) were prepared by a layer-by-layer deposition of avidin and ib-PEI on a solid surface, and the disintegration induced by changing environmental pH and adding biotin in the solution was studied. The avidin/ib-PEI layered film could be deposited only from the solutions of pH 10-12. The film did not form in pH 9 or more acidic media because of a low affinity of protonated 2-iminobiotin residues in ib-PEI to avidin. The avidin/ib-PEI layered films were stable in pH 8-12 solutions, while in pH 5-7 media the film decomposed spontaneously as a result of the protonation to 2-iminobiotin residues in ib-PEI. The avidin/ib-PEI films were disintegrated also upon addition of biotin and analogues in the solution owing to the preferential binding of biotin or analogues to the binding site of avidin. The decomposition rate was arbitrarily controlled by changing the type of stimulant (biotin or analogues) and its concentration. The avidin/ib-PEI films were disintegrated rapidly by addition of 10(-)(5) M of biotin or desthiobiotin, while the rate was slower upon adding the same concentration of lipoic acid or 2-(4'-hydroxyphenylazo)benzoic acid. On the other hand, the film was fully decomposed within 1 min in the 10(-)(3) M lipoic acid or 2-(4'-hydroxyphenylazo)benzoic acid solution. Thus, the decomposition rate is highly dependent on the concentration of the stimulants. It was observed that the stimuli-induced decomposition of the films is slow at pH 12, in contrast to a rapid decomposition in pH 8 medium due to a low affinity of the protonated 2-iminobiotin to avidin. The present system may be useful for constructing stimuli-sensitive devices that can release drug or other functional molecules.

Spontaneous Loading of Positively Charged Macromolecules into Alginate-Templated Polyelectrolyte Multilayer Microcapsules

A simple and high-efficiency approach to loading macromolecules into microscale carriers is presented. Calcium-cross-linked alginate hydrogel microspheres were fabricated by an emulsification technique and then used as negatively charged templates to form polyelectrolyte multilayer coatings. A calcium ion chelator, EDTA, was used to free the Ca(2+)-cross-linked alginate hydrogel within {poly(allylamine hydrochloride)/poly (styrene sulfonate)}(4) ({PAH/PSS}(4)) coating, allowing partial release of alginate. The retention of alginate in {PAH/PSS}(4) microcapsule was confirmed by FTIR spectroscopy and confocal microscopy. Real-time confocal microscopy was used to investigate the loading process of positively charged macromolecules (dextran-amino, and peroxidase) into alginate-templated microcapsules, which showed the loading occurred in <2 min for dextran-amino and <10 min for peroxidase, respectively. A high loading efficiency of 25 mug peroxidase in approximately 1.0 x 10(7) microcapsules (2.5 pg POx/capsule) was achieved with a low concentration of peroxidase loading solution (10 mug/mL). This spontaneous loading technique for encapsulating positively charged molecules in alginate-templated polyelectrolyte microcapsules shows strong potential for biosensor and drug delivery applications.

Interactions between self-assembled polyelectrolyte shells and tumor cells

Layer-by-layer self-assembled polyelectrolyte shells are a new class of micro/nanocapsules with unique physicochemical properties for potential applications in drug/gene delivery. The objective of this study was to investigate the interactions of polyelectrolyte shells ( approximately 1 mum in diameter) with MCF-7 breast cancer cells and identify key parameters that affect such interactions. Tailoring of surface properties of polyelectrolyte shells was achieved by choosing different outermost layer materials, including cationic polymers, anionic polymers, and lipid bilayers. Different surface compositions led to a wide range of electrostatic potentials from -46 to +47 mV in phophate-buffered saline buffer. Confocal microscopy studies showed that the polyelectrolyte shells were internalized into the cell cytoplasm, but not into the nuclei. Correlation of cell uptake with shell surface compositions was complicated by the adsorption of serum proteins on the surface of polyelectrolyte shells, particularly polycation-coated shells. To prevent protein adsorption, poly(ethylene glycol) (PEG) grafted poly(ethyleneimine) (PEI) copolymers (1:1, 1:5, 1:10 graft ratios) were synthesized and introduced on the shell surface. Shells coated with PEI-PEG copolymers effectively reduced protein adsorption whereas PEI-PEG copolymers with lower graft ratios achieved higher cell uptake efficiency after 24 h of incubation with MCF-7 cells.

Macromolecule encapsulation in diazoresin-based hollow polyelectrolyte microcapsules

A stable enzyme encapsulation technique based on the conversion of weak interactions between diazo resin/poly(styrene sulfonate) to covalent bonds was explored. Photosensitive diazoresin-based polyelectrolyte microcapsules were prepared via layer-by-layer electrostatic self-assembly of poly(styrene sulfonate) and diazoresin on MnCO(3) templates. UV-vis and zeta-potential measurements confirmed the alternate deposition of {PSS/DAR} multilayers on the micrometer-sized dissolvable templates. The DAR-based microcapsules were demonstrated to be permeable to enzymes prior to UV irradiation, while the permeability of the multilayer wall was changed substantially after photo-cross-linking. Encapsulated molecules were stably entrapped after UV irradiation, as shown by confocal microscopy and atomic force microscopy images. Activity assays revealed that encapsulated glucose oxidase possessed 52.8% of the catalytic activity exhibited by the same amount of free enzyme, proving the preservation of native conformation and accessibility of substrate. This encapsulation technique is promising for many biomedical and biotechnological applications, particularly enzyme biosensors, which require stable immobilization of functional components while allowing sufficient transport rates for substrate molecules.

Disintegration of Layer-by-Layer Assemblies Composed of 2-Iminobiotin-Labeled Poly(ethyleneimine) and Avidin

A layer-by-layer thin film composed of avidin and 2-iminobiotin-labeled poly(ethyleneimine) (ib-PEI) was prepared and their sensitivity to the environmental pH and biotin was studied. The avidin/ib-PEI multilayer assemblies were stable at pH 8-12, whereas the assemblies were decomposed at pH 5-6 due to the low affinity of the protonated iminobiotin residue to avidin. The avidin/ib-PEI assemblies can be disintegrated upon addition of biotin and analogues in the solution as a result of the preferential binding of biotin or analogues to the binding site of avidin. The decomposition rate was arbitrarily controlled by changing the type of stimulant (biotin or analogues) and its concentration. The avidin/ib-PEI assemblies were disintegrated rapidly by the addition of biotin or desthiobiotin, whereas the rate of decomposition was rather slow upon addition of lipoic acid or 2-(4'-hydroxyphenylazo)benzoic acid. The present system may be useful for constructing the stimuli-sensitive devices that can release drug or other functional molecules.

Structural and optical investigations of SiO2–CdS core–shell particles

Cadmium sulfide nanoparticles (approximately 5 nm), chemically capped using thioglycerol molecules, have been anchored onto silica particles (approximately 80 nm) functionalized with 3-aminopropyltrimethoxysilane. Transmission electron microscopy clearly showed that at a low concentration of cadmium sulfide, nanoparticles were discretely and more or less uniformly attached onto the silica particles. At a high concentration of cadmium sulfide nanoparticles, an approximately 6-nm-thick compact shell of cadmium sulfide was formed on the silica particles. In both cases the nanocrystalline nature of cadmium sulfide particles was preserved, as is evident from X-ray diffraction and optical absorption spectra.

Glucose-sensitive nanoassemblies comprising affinity-binding complexes trapped in fuzzy microshells

A new design for glucose monitoring with "smart" materials based on self assembly, competitive binding, and resonance energy transfer (RET) is presented. The basic transduction principle is changing RET efficiency from fluorescein isothiocyanate (FITC) to tetramethylrhodamine isothiocyanate (TRITC), as FITC-dextran is displaced from TRITC-Concanavalin A (Con A) with the addition of glucose. Nanoscale fabrication by self-assembly of Con A/dextran into multilayer films, followed by polymer multilayers. The advantages of this approach include physical localization and separation of sensing molecules from the environment via entrapment of the biosensorelements in a semi-permeable polymeric shell, and only functional molecules are included in the sensors. To realize these nanostructures, dissolvable resin microparticles were coated with FITC-dextran+TRITC-Con A multilayers, followed by polyelectrolyte multilayers, and the core particles were then dissolved to yield hollow capsules. The nanoassembly process was studied using microbalance mass measurements, fluorescence spectroscopy, confocal fluorescence microscopy, and zeta-potential measurements. The key findings are that the specific binding between Con A and dextran can be used to deposit ultrathin multilayer films, and these exhibit changing RET in response to glucose. Fluorescence spectra of a microcapsules exhibited a linear, glucose-specific, 27% increase in the relative fluorescence of FITC over the 0-1800 mg/dL range. These findings demonstrate the feasibility of using self-assembled microcapsules as optical glucose sensors, and serve as a basis for work toward better understanding the properties of these novel materials.

Potential for glucose monitoring with nanoengineered fluorescent biosensors

Possibilities for engineering of fluorescent micro/nanoscale devices for glucose sensing are presented. A description of the potential for clinical use is given in terms of overall concept, current knowledge, advantages of the fabrication approach proposed, and challenges that must be addressed prior to clinical use. Deployment of micro/nanoparticles in the dermis may allow transdermal monitoring of glucose changes in interstitial fluid. Using electrostatic self-assembly, an example of nanotechnology for fabrication, two types of sensors are being studied: (1). solid nanoparticles coated with fluorescent enzyme-containing thin films and (2). hollow micro/nanocapsules containing fluorescent indicators and enzymes or glucose-binding proteins. Nanoengineering of the coated colloids and microcapsules allows precision control over optical, mechanical, and catalytic properties to achieve sensitive response using a combination of polymers, fluorescent indicators, and glucose-specific proteins. Challenges to in vivo use include understanding of material toxicity and failure modes, and determining methods to overcome fouling, protein inactivation, and material degradation.