Biopolymer Networks for the Solid‐State Production of Porous Magnetic Beads and Wires

Aqueous solutions of sodium carboxymethyl cellulose are used for the morphosynthesis of spherical and wire‐shaped biopolymer networks, in which Fe³⁺ cations serve as a crosslinking and hardening agent. Their morphology remains intact upon drying, resulting in monolithic beads (1 mm) and wires (ca. 80 μm), which are exploited as reaction vessels to pre‐encapsulate poly(ethylene glycol) 400 (PEG 400) and cobalt cations. A solid‐state reaction in an inert atmosphere at 600 °C affords porous carbonaceous xerogels, macroscopically shaped as beads or wires and decorated with nanocrystalline magnetic iron oxide, metallic iron, or iron–cobalt alloy particles, thus imparting magnetic properties to the products. As such the reduction of Fe³⁺ species to α‐Fe nanoparticles can be achieved without H₂ treatment, since poly(ethylene glycol) serves as a reducing agent and the encapsulated Co²⁺ aids in the subsequent growth of the metallic iron particles. Particularly interesting are the magnetic properties of the carbon–α‐Fe composite, in which the size of the magnetic particles, estimated near the boundaries of the single magnetic domain, gives rise to increased coercivity compared with that of bulk iron.     

Temperature‐Sensitive Nanocapsules for Controlled Drug Release Caused by Magnetically Triggered Structural Disruption

Self‐assembled nanocapsules containing a hydrophilic core and a crosslinked yet thermosensitive shell are successfully prepared using poly(ethylene‐oxide)‐poly(propylene‐oxide)‐poly(ethylene‐oxide) block copolymers, 4‐nitrophenyl chloroformate, gelatin, and 1‐ethyl‐3‐(3‐dimethylaminopropyl) carbodiimide. The core is further rendered magnetic by incorporating iron oxide nanoparticles via internal precipitation to enable externally controlled actuation under magnetic induction. The spherical nanocapsules exhibit a hydrophilic‐to‐hydrophobic transition at a characteristic but tunable temperature reaching 40 °C, triggering a size contraction and shrinkage of the core. The core content experiences very little leakage at 25 °C, has a half life about 5 h at 45 °C, but bursts out within a few minutes under magnetic heating due to iron oxide coarsening and core/shell disruption. Such burst‐like response may be utilized for controlled drug release as illustrated here using a model drug Vitamin B12.     

Large‐Scale Synthesis,Annealing, Purification,and Magnetic Properties of Crystalline Helical Carbon Nanotubes with Symmetrical Structures

Crystalline helical carbon nanotubes (HCNTs) are synthesized as the main products in the pyrolysis of acetylene at 450 °C over Fe nanoparticles generated by means of a combined sol–gel/reduction method. Transmission electron microscopy (TEM) images reveal that there are two HCNTs attached to each Fe₃C nanoparticle, and that the two HCNTs are mirror images of each other. Annealing in Ar at 750 °C and purification by immersion in hot (90 °C) HCl solution do not significantly change the structure of the HCNTs, despite the partial removal of Fe nanoparticles by the latter treatment. The magnetic properties of the as‐prepared, annealed, and purified HCNTs have been systematically examined. The annealed sample shows relatively high magnetization due to the ferromagnetic α‐Fe nanoparticles encapsulated in the HCNT nodes. In the case of HCl treatment, relatively pure HCNTs are obtained by the removal of ferromagnetic nanoparticles from the double‐HCNT nodes. The effects of the amount of catalyst used in the synthesis process on the morphology and yield of the carbon products have also been investigated.     

Heparinized Magnetic Nanoparticles: In‐Vitro Assessment for Biomedical Applications

Superparamagnetic magnetite nanoparticles are of great interest owing to their numerous existing and potential biomedical applications. In this study, superparamagnetic magnetite nanoparticles with average diameters of 6–8 nm have been prepared and surface‐functionalized with poly(N‐isopropylacrylamide) (poly(NIPAAM)) via a surface‐initiated atom‐transfer radical polymerization, followed by immobilization of heparin. The success of the various surface‐functionalization steps has been ascertained using Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy. The rate of internalization of the as‐synthesized and surface‐functionalized magnetite nanoparticles by mouse macrophage cells has been investigated. The nanoparticle internalization into the macrophages has been visualized using optical microscopy and quantified by inductively coupled plasma spectroscopy. The effectiveness of the heparinized nanoparticles in preventing thrombosis has been determined using the plasma recalcification time. The results indicate that the above‐mentioned surface modifications of the magnetite nanoparticles are effective in delaying phagocytosis and preventing blood clotting in vitro. Such properties can be expected to enable their use in biomedical applications.     

Vapor Sorption and Electrical Response of Au‐Nanoparticle– Dendrimer Composites

Films comprising Au nanoparticles and polyphenylene dendrimers (first and second generation) are deposited onto transducer substrates via layer‐by‐layer self‐assembly and characterized by atomic force microscopy and X‐ray photoelectron spectroscopy. Their sorption behavior is studied by measuring the uptake of solvents from the vapor phase with quartz crystal microbalances (QCMs). The resistance of the films is simultaneously monitored. Both sensor types, QCMs and chemiresistors, give qualitatively very similar response isotherms that are consistent with a combination of Henry‐ and Langmuir‐type sorption processes. The sorption‐induced increase in relative differential resistance scales linearly with the amount of analyte accumulated in the films. This result is in general agreement with an activated tunneling process for charge transport, if little swelling and only small changes in the permittivity of the film occur during analyte sorption (a first‐order approximation). The relative sensitivity of the films to different solvents decreases in the order toluene ≈ tetrachloroethylene > 1‐propanol ? water. Films containing the larger second‐generation dendrimers show higher sensitivity than films containing first‐generation dendrimers.     

Kinetically Controlled Synthesis of Hexagonally Close‐Packed Cobalt Nanorods with High Magnetic Coercivity

High‐quality monodisperse metallic cobalt nanorods are obtained by the reduction of carboxylate salts of Co^II in 1,2‐butanediol using a rapid, simple, and solid‐template‐free procedure. In this polyol process, particle shape can be controlled via the growth rate, which depends on three parameters: i) the nature of the cobalt carboxylate, ii) the temperature ramp, and iii) the basicity of the medium. Cobalt in the hexagonally close‐packed phase favored the growth of anisotropic particles. Magnetic measurements of the cobalt nanorods indicate they are ferromagnetic at room temperature. They have a very high coercivity of 9.0 kOe at 140 K, much higher than that observed for wires prepared with solid templates. This can be attributed to their small mean diameter and high crystallinity.     

A Smart Supramolecular Hydrogel Exhibiting pH‐Modulated Viscoelastic Properties

The macroscopic viscoelastic properties of a physical hydrogel are reversibly modulated by tuning the microscopic hydrogen‐bonding interactions with pH. The hydrogel forms at a rather low concentration of the multi‐pyridyl‐based gelator, N, N′, N″‐tris(3‐pyridyl)trimesic amide. The yield stress of the hydrogel is greatly enhanced from 10 to 769 Pa by changing the pH from 7.0 to 5.0. At pH 7.0, the amide molecules are assembled into an ordered structure as a result of the hydrogen bonds between the amide N–H bond and the nitrogen on the pyridyl group (N–H…Py). Fourier transform (FT) IR spectroscopy indicates that hydrogen bonds of N–H…Py are partially broken because the pyridyl groups are partly protonated at pH 5.0. This condition leads to a highly branched and homogeneous fibrillar network, which is confirmed by X‐ray diffraction (XRD) measurements and field‐emission scanning electron microscopy (FESEM) images. Highly branched fibrillar networks create more compartments and greatly increase the interfacial tension that is required to hold the solvent in the gel, thereby increasing the yield stress to 769 Pa. By further increasing the acidity of the hydrogel to pH < 3.0, the gel becomes a sol. Both the change in the viscoelastic properties and the sol–gel transition are reversible and controllable in the material.     

Cd1–xMnxS Diluted Magnetic Semiconductors as Nanostructured Guest Species in Mesoporous Thin‐Film Silica Host Media

Recently, we demonstrated the possibility of synthesizing ordered nanowires of diluted magnetic II/VI semiconductors inside the channels of mesoporous silica host structures. Here, we expand this procedure from mesoporous powders to mesoporous thin films. Diluted magnetic semiconductors Cd_1–xMn_xS were synthesized within the pores of mesoporous thin‐film silica host structures by a wet‐impregnation technique using an aqueous solution of the respective metal acetates, followed by drying steps and a conversion to sulfide by thermal H₂S treatment. The presence of Cd_1–xMn_xS nanoparticles inside the pores was proved by powder X‐ray diffraction, infrared and Raman spectroscopy, and transmission electron microscopy. Photoluminescence excitation measurements clearly demonstrate the quantum size effect of the incorporated nanostructured guest species. The quality of the nanoparticles incorporated into the mesoporous films is comparable to that of those inside the mesoporous powders.     

Magnetic‐Field‐Induced Phase‐Selective Synthesis of Ferrosulfide Microrods by a Hydrothermal Process: Microstructure Control and Magnetic Properties

Microrods of the ferrosulfide minerals greigite (Fe₃S₄) and marcasite (FeS₂) are selectively synthesized by an in situ magnetic‐field‐assisted hydrothermal route. Each complex microrod is composed of fine building blocks with different shapes. The unique magnetic properties of the microrods and electrical performance of a single microrod are studied. The results demonstrate that the magnetic properties of the ferrosulfide minerals are strongly related to their corresponding microstructures. The value of the low‐temperature transition increases as the greigite component in the product decreases. The combination of small‐molecule sulfur precursors and an applied magnetic field makes possible the selective synthesis of ferrosulfide minerals with different phases and distinct microstructures, underlining the fact that the magnetic field can be a useful tool as well as an independent parameter for the phase‐selective synthesis and self‐assembly of inorganic building blocks in solution chemistry.     

Porous Polymersomes with Encapsulated Gd‐Labeled Dendrimers as Highly Efficient MRI Contrast Agents

The use of nanovesicles with encapsulated Gd as magnetic resonance (MR) contrast agents has largely been ignored due to the detrimental effects of the slow water exchange rate through the vesicle bilayer on the relaxivity of encapsulated Gd. Here, the facile synthesis of a composite MR contrast platform is described; it consists of dendrimer conjugates encapsulated in porous polymersomes. These nanoparticles exhibit improved permeability to water flux and a large capacity to store chelated Gd within the aqueous lumen, resulting in enhanced longitudinal relaxivity. The porous polymersomes, ～130 nm in diameter, are produced through the aqueous assembly of the polymers, polyethylene oxide‐b‐polybutadiene (PBdEO), and polyethylene oxide‐b‐polycaprolactone (PEOCL). Subsequent hydrolysis of the caprolactone (CL) block resulted in a highly permeable outer membrane. To prevent the leakage of small Gd‐chelate through the pores, Gd was conjugated to polyamidoamine (PAMAM) dendrimers via diethylenetriaminepentaacetic acid dianhydride (DTPA dianhydride) prior to encapsulation. As a result of the slower rotational correlation time of Gd‐labeled dendrimers, the porous outer membrane of the nanovesicle, and the high Gd payload, these functional nanoparticles are found to exhibit a relaxivity (R1) of 292 109 mM ^?1 s^?1 per particle. The polymersomes are also found to exhibit unique pharmacokinetics with a circulation half‐life of >3.5 h and predominantly renal clearance.     

Formation of Highly Luminescent Supramolecular Architectures Possessing Columnar Order from Octupolar Oxadiazole Derivatives: Hierarchical Self‐Assembly from Nanospheres to Fibrous Gels

The synthesis and study of the liquid crystalline, photophysical, and aggregation behavior of novel octupolar oxadiazole derivatives are reported. These molecules formed columnar mesophases at elevated temperatures which transformed into a glassy state at ambient temperatures wherein the columnar order was retained. Their spontaneous concentration dependent hierarchical self‐assembly from spheres to fibrous gels has been investigated using TEM, SEM, and XRD. Retention of the hexagonal columnar (Col_h) order was also observed in the fibrous aggregates. Concentration dependent luminescence spectral studies indicated that the change in morphology from spheres to fibrous aggregates was associated with a shift in chromophore packing from predominantly H‐type to J‐type aggregates. Time resolved anisotropic investigations revealed that the columnar stacking of molecules in the aggregated state provided a pathway for excitation energy migration to the lower energy J‐traps.     

Cover Picture: Vapor Sorption and Electrical Response of Au‐Nanoparticle– Dendrimer Composites (Adv. Funct. Mater. 6/2007)

The cover shows chemiresistors and mass‐sensitive vapor sensors coated with Au‐nanoparticle/dendrimer composites. The Au nanoparticles provide the film with electrical conductivity and the dendrimers control the chemical selectivity, as reported by Nadjedja Krasteva and co‐workers on p. 881. Analyte sorption follows a combined Henry–Langmuir model, and measurements reveal that sorption‐induced increase in film resistance scales linearly with the concentration of analyte sorbed in the film. The background shows a silicon wafer with lithographically defined microelectrode structures for chemiresistor fabrication. Films comprising Au nanoparticles and polyphenylene dendrimers (first and second generation) are deposited onto transducer substrates via layer‐by‐layer self‐assembly and characterized by atomic force microscopy and X‐ray photoelectron spectroscopy. Their sorption behavior is studied by measuring the uptake of solvents from the vapor phase with quartz crystal microbalances (QCMs). The resistance of the films is simultaneously monitored. Both sensor types, QCMs and chemiresistors, give qualitatively very similar response isotherms that are consistent with a combination of Henry‐ and Langmuir‐type sorption processes. The sorption‐induced increase in relative differential resistance scales linearly with the amount of analyte accumulated in the films. This result is in general agreement with an activated tunneling process for charge transport, if little swelling and only small changes in the permittivity of the film occur during analyte sorption (a first‐order approximation). The relative sensitivity of the films to different solvents decreases in the order toluene ≈ tetrachloroethylene > 1‐propanol ? water. Films containing the larger second‐generation dendrimers show higher sensitivity than films containing first‐generation dendrimers.     

Stimuli‐Responsive Colloidal Systems from Mixed Brush‐Coated Nanoparticles

We report on the application of mixed polymer brushes to the reversible in situ switching colloidal systems (suspensions of responsive 200 nm in diameter particles in individual solvents and immiscible liquids). We used mixed copolymer brushes to fabricate responsive nanoparticles and employed the particles to prepare responsive colloidal systems, which demonstrated drastic transformation/switching of material properties upon external stimuli. The interaction between the particles themselves and the particles and their environment can be precisely tuned by a change of solvent and pH. We show that this behavior can be used for a reversible formation of particle aggregates, stabilization and switching between w/o and o/w emulsions, and regulation of the particle transport between immiscible liquids across the interface. We demonstrate an example of the application of the responsive colloids for the fabrication of ultrahydrophobic coatings with textured surfaces from aqueous dispersion, and no surfactant application, using the switching properties of the responsive particles.     

Robust Self‐Healing Host–Guest Gels from Magnetocaloric Radical Polymerization

Given the increasing environmental and energy issues, materials with the ability to repair themselves following damage are highly desirable because this self‐healing property can prolong the lifespan of materials and reduce replacement costs. Host–guest assemblies are a powerful approach to create supramolecular materials with versatile functions. Here, a new mode of radical polymerization is demonstrated which is achieved via magnetocaloric effect to fabricate novel host–guest supramolecular gels within 5 min. The resulting gels can repair themselves spontaneously when damaged, without the assistance of any external stimuli, and possess great mechanical strength. Moreover, the Fe₃O₄‐doped supramolecular gels show accelerated self‐healing (from 24 h to 3 h) under an applied magnetic field, which is attributed to the synergy between host–guest healing and a magnetocaloric effect. This strategy might open a promising avenue for accelerating the use of host–guest assemblies to rapidly build robust materials.     

Multifunctional Core/Shell Nanoparticles Self‐Assembled from pH‐Induced Thermosensitive Polymers for Targeted Intracellular Anticancer Drug Delivery

Core/shell nanoparticles that display a pH‐sensitive thermal response, self‐assembled from the amphiphilic tercopolymer, poly(N‐isopropylacrylamide‐co‐N,N‐dimethylacrylamide‐co‐10‐undecenoic acid) (P(NIPAAm‐co‐DMAAm‐co‐UA)), have recently been reported. In this study, folic acid is conjugated to the hydrophilic segment of the polymer through the free amine group (for targeting cancer cells that overexpress folate receptors) and cholesterol is grafted to the hydrophobic segment of the polymer. This polymer also self‐assembles into core/shell nanoparticles that exhibit pH‐induced temperature sensitivity, but they possess a more stable hydrophobic core than the original polymer P(NIPAAm‐co‐DMAAm‐co‐UA) and a shell containing folate molecules. An anticancer drug, doxorubicin (DOX), is encapsulated into the nanoparticles. DOX release is also pH‐dependent. DOX molecules delivered by P(NIPAAm‐co‐DMAAm‐co‐UA) and folate‐conjugated P(NIPAAm‐co‐DMAAm‐co‐UA)‐g‐cholesterol nanoparticles enter the nucleus more rapidly than those transported by P(NIPAAm‐co‐DMAAm)‐b‐poly(lactide‐co‐glycolide) nanoparticles, which are not pH sensitive. More importantly, these nanoparticles can recognize folate‐receptor‐expressing cancer cells. Compared to the nanoparticles without folate, the DOX‐loaded nanoparticles with folate yield a greater cellular uptake because of the folate‐receptor‐mediated endocytosis process, and, thus, higher cytotoxicity results. These multifunctional polymer core/shell nanoparticles may make a promising carrier to target drugs to cancer cells and release the drug molecules to the cytoplasm inside the cells.     

Uniform Rattle‐type Hollow Magnetic Mesoporous Spheres as Drug Delivery Carriers and their Sustained‐Release Property

A novel kind of rattle‐type hollow magnetic mesoporous sphere (HMMS) with Fe₃O₄ particles encapsulated in the cores of mesoporous silica microspheres has been successfully fabricated by sol–gel reactions on hematite particles followed by cavity generation with hydrothermal treatment and H₂ reduction. Such a structure has the merits of both enhanced drug‐loading capacity and a significant magnetization strength. The prepared HMMSs realize a relatively high storage capacity up to 302 mg g^?1 when ibuprofen is used as a model drug, and the IBU–HMMS system has a sustained‐release property, which follows a Fick's law.     

Large‐Scale Synthesis of Uniform and Crystalline Magnetite Nanoparticles Using Reverse Micelles as Nanoreactors under Reflux Conditions

We have synthesized uniform and highly crystalline magnetite nanoparticles from the reaction of iron salts in microemulsion nanoreactors. The particle size can be controlled from 2 nm to 10 nm by varying the relative concentrations of the iron salts, surfactant, and solvent. Transmission electron microscope images of the nanoparticles reveal that they are very uniform in size distribution. Structural characterization using X‐ray diffraction and X‐ray magnetic circular dichroism shows that the nanoparticles are magnetite. The magnetic characterization of the nanoparticles showed that they are superparamagnetic at room temperature. Using a similar synthetic procedure, we have been able to synthesize nanoparticles of several mixed metal ferrites including cobalt ferrite, manganese ferrite, nickel ferrite, and zinc ferrite.     

Surfactant‐Free,Self‐Assembled PVA‐Iron Oxide/Silica Core–Shell Nanocarriers for Highly Sensitive,Magnetically Controlled Drug Release and Ultrahigh Cancer Cell Uptake Efficiency

Surfactant‐free, self‐assembled iron oxide/silica core–shell (SAIO@SiO₂) nanocarriers were synthesized as bifunctional magnetic vectors that can be triggered for the controlled release of therapeutic agents by an external magnetic field. In addition, drug release profiles can be well‐regulated through an ultrathin layer of silica shell. The hydrophobic drug molecules were encapsulated within the iron oxide‐PVA core and then further covered with a thin‐layer silica shell to regulate the release pattern. Remote control of drug release from the SAIO@SiO₂ nanocarriers was achieved successfully using an external magnetic field where the core phase being structurally disintegrated to a certain extent while subjected to magnetic stimulus, resulting in a burst release of the encapsulated drug. However, a relatively slow and linear release restored immediately, directly after removal of the stimulus. The nanostructural evolution of the nanocarriers upon the stimulus was examined and the mechanism for controlled drug release is proposed for such a core–shell nanocarrier. Surprisingly, the surfactant‐free SAIO@SiO₂ nanocarriers demonstrated a relatively high uptake efficiency from the HeLa cell line. Together with a well‐regulated controlled release design, the nanocarriers may provide great advantages as an effective cell‐based drug delivery nanosystem for biomedical applications.