Magnetically Active Carbon Nanotubes at Work

Endohedral and exohedral assembly of magnetic nanoparticles (MNPs) and carbon nanotubes (CNTs) recently gave birth to a large body of new hybrid nanomaterials (MNPs-CNTs) featuring properties that are otherwise not in reach with only the graphitic or metallic cores themselves. These materials feature enhanced magnetically guided motions (rotation and translation), magnetic saturation and coercivity, large surface area, and thermal stability. By guiding the reader through the most significant examples in this Concept paper, we describe how researchers in the field engineered and exploited the synergistic combination of these two types of nanoparticles in a large variety of current and potential applications, such as magnetic fluid hyperthermia therapeutics and in magnetic resonance imaging to name a few.

Direct imaging of the magnetic polarity and reversal mechanism in individual Fe(3-x)O4 nanoparticles

This work reports on the experimental characterization of the magnetic domain configurations in cubic, isolated Fe3-xO4 nanoparticles with a lateral size of 25-30 nm. The magnetic polarity at remanence of single domain ferrimagnetic Fe3-xO4 nanoparticles deposited onto a carbon-silicon wafer is observed by magnetic force microscopy. The orientations of these domains provide a direct observation of the magneto-crystalline easy axes in each individual nanoparticle. Furthermore, the change in the domain orientation with an external magnetic field gives evidence of particle magnetization reversal mediated by a coherent rotation process that is also theoretically predicted by micromagnetic calculations.

C-H arylation of triphenylene, naphthalene and related arenes using Pd/C

A highly selective arylation of a number of polyaromatic hydrocarbons (PAHs) with aryliodonium salts and Pd/C as the only reagent is reported. The first C-H functionalization of triphenylene is explored, and proceeds at the most sterically hindered position. This non-chelate assisted C-H functionalization extends the reactivity profile of Pd/C and provides controlled access to π-extended PAHs, an important aspect of work towards the preparation of nanographenes. Mechanistic studies suggest in situ formation of catalytically active insoluble nanoparticles, and that the reaction likely proceeds via a Pd(0)/Pd(ii) type reaction manifold.

Structure evolution of nanoparticulate Fe2O3

The atomic structure and properties of nanoparticulate Fe2O3 are characterized starting from its smallest Fe2O3 building unit through (Fe2O3)n clusters to nanometer-sized Fe2O3 particles. This is achieved by combining global structure optimizations at the density functional theory level, molecular dynamics simulations by employing tailored, ab initio parameterized interatomic potential functions and experiments. With the exception of nearly tetrahedral, adamantane-like (Fe2O3)2 small (Fe2O3)n clusters assume compact, virtually amorphous structures with little or no symmetry. For n = 2-5 (Fe2O3)n clusters consist mainly of two- and three-membered Fe-O rings. Starting from n = 5 they increasingly assume tetrahedral shape with the adamantane-like (Fe2O3)2 unit as the main building block. However, the small energy differences between different isomers of the same cluster-size make precise structural assignment for larger (Fe2O3)n clusters difficult. The tetrahedral morphology persists for Fe2O3 nanoparticles with up to 3 nm in diameter. Simulated crystallization of larger nanoparticles with diameters of about 5 nm demonstrates pronounced melting point depression and leads to formation of ε-Fe2O3 single crystals with hexagonal morphology. This finding is in excellent agreement with the results obtained for Fe2O3 nanopowders generated by laser vaporization and provides the first direct indication that ε-Fe2O3 may be thermodynamically the most stable phase in this size regime.

Insights into mesoporous metal phosphonate hybrid materials for catalysis

Mesoporous metal phosphonates have received increasing attention as promising heterogeneous catalysts due to their abundant framework compositions and controllable porosity.

Magnetic nanoparticles-supported tungstic acid (MNP-TA): an efficient magnetic recyclable catalyst for the one-pot synthesis of spirooxindoles in water

This paper reports the preparation, characterization and catalytic application of a novel, magnetically separable catalyst consisting of tungstic acid supported on silica coated magnetic nanoparticles.

Quaternary ammonium functionalized Fe3O4@P(GMA–EGDMA) composite particles as highly efficient and dispersible catalysts for phase transfer reactions

We have revealed a high efficiency, high dispersibility, structural stability, easy recovery and reusable magnetic phase transfer catalyst.

Design of polystyrene latex particles covered with polyoxometalate clusters via multiple covalent bonding

Polyoxometalate clusters can be chemically grafted onto the surface of polymer latex via simple emulsion polymerization reaction. Such hierarchical nano-structures could serve as highly efficient quasi-homogeneous catalysts.

The effect of oleic acid on the synthesis of Fe3−xO4 nanoparticles over a wide size range

This work reports on the effect of the oleic acid concentration on the magnetic and structural properties of Fe_3−xO₄ nanoparticles synthesized by thermal decomposition of Fe(acac)₃ in benzyl-ether.

Immobilized Pd on magnetic nanoparticles bearing proline as a highly efficient and retrievable Suzuki–Miyaura catalyst in aqueous media

Immobilized Pd on magnetic nanoparticles bearing proline as a highly efficient and retrievable Suzuki–Miyaura catalyst in aqueous media.

Industrial applications of nanoparticles

This tutorial review analyses where nanoparticle research has left the laboratory and today contributes to valuable products.

Nano γ-Fe2O3-supported fluoroboric acid: a novel magnetically recyclable catalyst for the synthesis of 12-substituted-benzo[h][1,3]dioxolo[4,5-b]-acridine-10,11-diones as potent antitumor agents

Nano γ-Fe₂O₃-supported fluoroboric acid was synthesized as a novel magnetic catalyst, and was used for the efficient synthesis of benzoacridinediones.

TiO2-coated magnetite nanoparticle-supported sulfonic acid as a new, efficient, magnetically separable and reusable heterogeneous solid acid catalyst for multicomponent reactions

TiO₂-coated magnetite nanoparticles-supported sulfonic acid (nano-Fe₃O₄–TiO₂–SO₃H (n-FTSA)) is synthesized by the immobilization of –SO₃H groups on the surface of nano-Fe₃O₄–TiO₂.

Surfactant-free Pd@pSiO2 yolk–shell nanocatalysts for selective oxidation of primary alcohols to aldehydes

The surfactant-free Pd@pSiO₂ yolk–shell nanoparticles proved to be an efficient catalyst for the oxidation of substituted benzyl alcohols.

Catalytic applications of magnetic nanoparticles functionalized using iridium N-heterocyclic carbene complexes

The development of a synthetic modular methodology for the preparation of catalytic materials based on magnetic nanoparticles with iridium complexes and their application to transfer hydrogenation.

Efficient magnetic recycling of covalently attached enzymes on carbon-coated metallic nanomagnets

In the pursuit of robust and reusable biocatalysts for industrial synthetic chemistry, nanobiotechnology is currently taking a significant part. Recently, enzymes have been immobilized on different nanoscaffold supports. Carbon coated metallic nanoparticles were found to be a practically useful support for enzyme immobilization due to their large surface area, high magnetic saturation, and manipulatable surface chemistry. In this study carbon coated cobalt nanoparticles were chemically functionalized (diazonium chemistry), activated for bioconjugation (N,N-disuccinimidyl carbonate), and subsequently used in enzyme immobilization. Three enzymes, β-glucosidase, α-chymotrypsin, and lipase B were successfully covalently immobilized on the magnetic nonsupport. The enzyme-particle conjugates formed retained their activity and stability after immobilization and were efficiently recycled from milliliter to liter scales in short recycle times.

Soluble Polymers as Tools in Catalysis

Examples where soluble polymers have been used in homogeneous catalysis were first noted 50 years ago, but this role for soluble polymers remained relatively unexplored until the 1990s. Since then, the use of new polymers, new developments in polymer synthesis, new separation strategies, and the imaginative ways soluble polymers' structure and physical properties can be used to influence a covalent or ligated catalyst have led to increasing interest in soluble polymers as tools in catalysis.

Polymer- and dendrimer-coated magnetic nanoparticles as versatile supports for catalysts, scavengers, and reagents

The work-up of chemical reactions by standard techniques is often time consuming and energy demanding, especially when chemists have to guarantee low levels of metal contamination in the products. Therefore, scientists need new ideas to rapidly purify reaction mixtures that are both economically and environmentally benign. One intriguing approach is to tether functionalities that are required to perform organic reactions to magnetic nanoparticles, for example, catalysts, reagents, scavengers, or chelators. This strategy allows researchers to quickly separate active agents from reaction mixtures by exploiting the magnetic properties of the support. In this Account, we discuss the main attributes of magnetic supports and describe how we can make the different nanomagnets accessible by surface functionalization. Arguably the most prominent magnetic nanoparticles are superparamagnetic iron oxide nanoparticles (SPIONs) due to their biologically well-accepted constituents, their established size-selective synthesis methods, and their diminished agglomeration (no residual magnetic attraction in the absence of an external magnetic field). However, nanoparticles made of pure metal have a considerably higher magnetization level that is useful in applications where high loadings are needed. A few layers of carbon can efficiently shield such highly reactive metal nanoparticles and, equally important, enable facile covalent functionalization via diazonium chemistry or non-covalent functionalization through π-π interactions. We highlight carbon-coated cobalt (Co/C) and iron (Fe/C) nanoparticles in this Account and compare them to SPIONs stabilized with surfactants or silica shells. The graphene-like coating of these nanoparticles offers only low loadings with functional groups via direct surface modification, and the resulting nanomagnets are prone to agglomeration without effective steric stabilization. To overcome these restrictions and to tune the dispersibility of the magnetic supports in different solvents, we can introduce dendrimers and polymers on Co/C and Fe/C platforms by various synthetic strategies. While dendrimers have the advantage of being able to array all functional groups on the surface, polymers need fewer synthetic steps and higher molecular weight analogues are easily accessible. We present the application of these promising hybrid materials for the extraction of analytes or contaminates from complex aqueous solutions (e.g. waste water treatments or blood analytics), for metal-, organo-, and biocatalysis, and in organic synthesis. In addition, we describe advanced concepts like magnetic protecting groups, a multistep synthesis solely applying magnetic reagents and scavengers, and thermoresponsive self-separating magnetic catalysts. We also discuss the first examples of the use of magnetic scaffolds manipulated by external magnetic fields in flow reactors on the laboratory scale. These hold promise for future applications of magnetic hybrid materials in continuous flow or highly parallelized syntheses with rapid magnetic separation of the applied resins.

Pd/C as a catalyst for completely regioselective C-H functionalization of thiophenes under mild conditions

The completely C3-selective arylation of thiophenes and benzo[b]thiophenes was achieved by using Pd/C as a heterogeneous catalyst without ligands or additives under mild reaction conditions. The practicability of this transformation is demonstrated by notable functional group tolerance and the insensitivity of the reaction to H2 O and air. This method is also applicable to nitrogen- and oxygen-containing heterocycles, yielding the corresponding C2-arylated products. Three-phase tests along with Hg-poisoning and hot-filtration tests suggest that the catalytically active species is heterogeneous in nature.

Guanidine based self-assembled monolayers on Au nanoparticles as artificial phosphodiesterases

Gold nanoparticles passivated with a catalytic monolayer based on guanidine exhibit high cooperativity and efficiency in the cleavage of phosphodiesters.

Water-dispersible and magnetically separable gold nanoparticles supported on a magnetite/s-graphene nanocomposite and their catalytic application in the Ullmann coupling of aryl iodides in aqueous media

The water-dispersible and magnetic separable Au/Fe₃O₄/s-G nanocomposite was used as an effective and reusable heterogeneous catalyst for the Ullmann homocoupling.

Fe3O4@SiO2–imid–PMAnmagnetic porous nanospheres as recyclable catalysts for the one-pot synthesis of 14-aryl- or alkyl-14H-dibenzo[a,j]xanthenes and 1,8-dioxooctahydroxanthene derivatives under various conditions

Synthesis of 14-aryl- or alkyl-14H-dibenzo[a,j]xanthenes and 1,8-dioxooctahydroxanthene derivatives by Fe₃O₄@SiO₂–imid–PMA nanocatalysts.