more selected reviews

Polymers with upper critical solution temperature
J. Seuring, S. Agarwal, Macromol. Rapid Commun. 2012, 33, 1898 – 1920 

This review focuses on polymers with upper critical solution temperature (UCST) in water or electrolyte solution and provides a detailed survey of the yet few existing examples. A guide for synthetic chemists for the design of novel UCST polymers is presented and possible handles to tune the phase transition temperature, sharpness of transition, hysteresis, and effectiveness of phase separation are discussed. This review tries to answer the question why polymers with UCST remained largely underrepresented in academic as well as applied research and what requirements have to be fulfilled to make these polymers suitable for the development of smart materials with a positive thermoresponse.

Controlled antibody/(bio-)conjugation of inorganic nanoparticles for targeted delivery.
J.-M. Montenegro, V. Grazu, A. Surkhanova, S. Agarwal, J. M. Fuente, I. Nabiev, A. Greiner. W. J. Parak, Adv. Drug Deliv. Rev. 2012, 65, 677-688.

Arguably targeting is one of the biggest problems for controlled drug delivery. In the case that drugs can be directed with high efficiency to the target tissue, side effects of medication are drastically reduced. Colloidal inorganic nanoparticles (NPs) have been proposed and described in the last 10 years as new platforms for in vivo delivery. However, though NPs can introduce plentiful functional properties (such as controlled destruction of tissue by local heating or local generation of free radicals), targeting remains an issue of intense research efforts. While passive targeting of NPs has been reported (the so-called enhanced permeation and retention, EPR effect), still improved active targeting would be highly desirable. One classical approach for active targeting is mediated by molecular recognition via capture molecules, i.e. antibodies (Abs) specific for the target. In order to apply this strategy for NPs, they need to be conjugated with Abs against specific biomarkers. Though many approaches have been reported in this direction, the controlled bioconjugation of NPs is still a challenge. In this article the strategies of controlled bioconjugation of NPs will be reviewed giving particular emphasis to the following questions: 1) how can the number of capture molecules per NP be precisely adjusted, and 2) how can the Abs be attached to NP surfaces in an oriented way. Solution of both questions is a cornerstone in controlled targeting of the inorganic NPs bioconjugates.

Chemistry on Electrospun Polymeric Nanofibers: Merely Routine Chemistry or a Real Challenge?
S. Agarwal, J. H. Wendorff, A. Greiner, Macromol. Rapid Commun. 2010, 31, 1317-1331.

Nanofiber-based non-wovens can be prepared by electrospinning. The chemical modification of such nanofibers or chemistry using nanofibers opens a multitude of application areas and challenges. A wealth of chemistry has been elaborated in recent years on and with electrospun nanofibers. Known methods as well as new methods have been applied to modify the electrospun nanofibers and thereby generate new materials and new functionalities. This Review summarizes and sorts the chemistry that has been reported in conjunction with electrospun nanofibers. The major focus is on catalysis and nanofibers, enzymes and nanofibers, surface modification for biomedical and specialty applications, coatings of fibers, crosslinking, and bulk modifications. A critical focus is on the question: what could make chemistry on or with nanofibers different from bulk chemistry?

Progress in the Field of Electrospinning for Tissue Engineering Applications.
S. Agarwal, J. H. Wendorff, A. Greiner, Adv. Mater. 2009, 21, 3343-3351.

Electrospinning is an extremely promising method for the preparation of tissue engineering (TE) scaffolds. This technique provides nonwovens resembling in their fibrillar structures those of the extracellular matrix (ECM), and offering large surface areas, ease of functionalization for various purposes, and controllable mechanical properties. The recent developments toward large-scale productions combined with the simplicity of the process render this technique very attractive. Progress concerning the use of electrospinning for TE applications has advanced impressively. Different groups have tackled the problem of electrospinning for TE applications from different angles. Nowadays, electrospinning of the majority of biodegradable and biocompatible polymers, either synthetic or natural, for TE applications is straightforward. Different issues, such as cell penetration, incorporation of growth and differentiating factors, toxicity of solvents used, productivity, functional gradient, etc. are main points of current considerations. The progress in the use of electrospinning for TE applications is highlighted in this article with focus on major problems encountered and on various solutions available until now.

Electrospinning of Manmade and Biopolymer Nanofibers-Progress in Techniques, Materials, and Applications.
S. Agarwal, A. Greiner, J. H. Wendorff, Adv. Funct. Mater. 2009, 19, 2863-2879.

Electrospinning of nanofibers has developed quickly from a laboratory curiosity to a highly versatile method for the preparation of a wide variety of nanofibers, which are of interest from a fundamental as well as a technical point of view. A wide variety of materials has been processed into individual nanofibers or nanofiber mats with very different morphologies. The diverse properties of these nanofibers, based on different physical, chemical, or biological behavior, mean they are of interest for different applications ranging from filtration, antibacterial coatings, drug release formulations, tissue engineering, living membranes, sensors, and so on. A particular advantage of electrospinning is that numerous non-fiber forming materials can be immobilized by electrospinning in nanofiber nonwovens, even very sensitive biological objects such as virus, bacteria, and cells. The progress made during the last few years in the field of electrospinning is fascinating and is highlighted in this Feature Article, with particular emphasis on results obtained in the authors' research units. Specific areas of importance for the future of electrospinning, and which may open up novel applications, are also highlighted.

Electrospinning: A Fascinating Method for the Preparation of Ultrathin Fibers.
A. Greiner, J. H. Wendorff, Angew. Chem. Int. Ed. 2007, 46, 5670-5703.

Electrospinning is a highly versatile method to process solutions or melts, mainly of polymers, into continuous fibers with diameters ranging from a few micrometers to a few nanometers. This technique is applicable to virtually every soluble or fusible polymer. The polymers can be chemically modified and can also be tailored with additives ranging from simple carbon-black particles to complex species such as enzymes, viruses, and bacteria. Electrospinning appears to be straightforward, but is a rather intricate process that depends on a multitude of molecular, process, and technical parameters. The method provides access to entirely new materials, which may have complex chemical structures. Electrospinning is not only a focus of intense academic investigation; the technique is already being applied in many technological areas.

Unusual Complex Chemistry of Rare-Earth Elements: Large Ionic Radii – Small Coordination Numbers.
K. Dehnicke, A. Greiner, Angew. Chem. Int. Ed. 2003, 42, 1340-1354.

Because of their large ionic radii and relatively low oxidation states rare-earth elements generally form complexes which have high coordination numbers and weak metal–ligand bonds. They are often not suitable for homogeneous catalysis on account of their instability of configuration in solution. Complexes of the corresponding metal atoms with low coordination numbers may be an improvement. This type of complex can be obtained in the classical way by the introduction of bulky ligands, and recently, they were also prepared in reactions with ligand groups which offer remarkable metal–ligand bond features. This concept is demonstrated for complexes with bulky bis(trimethylsilyl)amido ligands {N(SiMe3)2}− and “slim” phosphoraneiminato ligands (NPR3−). Their suitability as catalysts for the ring-opening polymerization of lactones is reported as well.

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