Current Organic Chemistry - Volume 9, Issue 11, 2005

I have been invited to provide the present issue of Current Organic Chemistry by Professor Stanislaw Penczek, the Executive Guest Editor of COC. The papers focus on chemical reactions leading to nano- and microparticles with tailored properties allowing for using these objects as useful tools for various applications, in particular in the area of medicine. Each particle consists of at least two zones - an interior and periphery. Selective control of chemical structure of these zones leads to particles with required internal integrity and properly adjusted interactions with molecules in particle' environment. Authors of the first paper (I. Gitsov, C. Lin) report on synthesis of dendrimers - the most precisely defined synthetic unimolecular nanoparticles. Since their discovery dendrimers attracted attention of many researchers and many reviews were published in this area. Authors are specialist in post-synthesis modification of dendrimers, in particular at their periphery, and in the review they discuss this subject in details. The following paper (Q. Zhang, M. Wang, and K.L. Wooley) discusses preparation and properties of nanoconfined crystalline organic polymers. Crosslinking of core-crystallized polymer micelles in solution is presented as a method for preparation of nanocrystalline objects. Authors of the next paper (J. Forcada and R. Hidalgo-Álvarez) present a comprehensive review on synthesis of polymer colloids with functional groups in the interfacial layer and on relation between chemical structure of synthesized particles and their colloidal stability. Importance of tailored chemical structure of interfacial layer of colloidal particles for their suitability for immunodiagnostics has been highlighted. Authors of the following review (M.P. Byron, K. Fromell and K.D. Caldwell) report on controlled modification of surfaces, in particular surfaces of colloidal particles, by attachment of surfactants with different types of reactive groups. The discussed approach allows preparation of surfaces with pre-determined levels of functionalization. Finally, Authors of the last review on hybrid nanoparticles (R. Veyret, Th. Delair C. Pichot and A. Elaissari) report on preparation of this new class of polymeric organic-inorganic materials with core containing magnetic nanocrystals and tight shell formed by layer-by-layer adsorption of purposely synthesized polyelectrolytes and on using the core-shell magnetic colloids for binding, extraction, concentration, and enzymatic amplification nucleic acids with desired oligonucleotide sequences.

Dendritic macromolecules or “dendrimers” are artificial nanoparticles with precisely defined covalent macromolecular architecture and broad application potential. Their discovery, construction strategies, postsynthetic peripheral modifications, surface-related properties and selected potential applications are the subject of this review. The standard dendrimer is characterized by three distinct zones, the periphery, the interior, and the core. All formation approaches ultimately yield structures with these spatial arrangements. The specific interactions of the dendritic molecules with their local nanoenvironment is determined by factors that include the nature of starting building blocks, the synthetic pathway, and more importantly - the postformation modifications on the periphery of the dendritic globules or to a minor extent - on their interior. The review examines the different chemistries used to tailor dendrimers and their properties for specific applications.

Nanocrystallinity is an important phenomenon in natural and synthetic systems. Whether an inorganic or organic material, the mechanism for confinement of crystal growth within nanoscopic dimensions usually employs an organic mediator, which adsorbs to and stabilizes the growing crystal. Following an introduction to nanocrystalline biomaterials and inorganic materials, this review focuses upon the preparation and study of nanoconfined crystalline organic polymers. Confinement is limited to nanoscopic domains, by the phase segregation of crystalline-amorphous block copolymers in the bulk or in solution. The effects of nanoconfinement upon the crystallization behaviors are discussed, including reports of small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) studies. Examples of nanoconfinement vs. break-out crystal growth are presented. Finally, irreversible trapping of nanocrystalline domains via covalent crosslinking of core-crystallized polymer micelles in solution is highlighted as a method for the preparation of permanent, discrete nanocrystalline objects. Confinement within the nanoscopic core domain of the shell crosslinked knedel-like (SCK) polymer micelle and covalent attachment to the crosslinked SCK shell impart interesting perturbations in local chain packing and disrupt the extent of crystallinity. In addition, SCKs with a toroidal morphology are prepared by micellization of diblock copolymers at high concentrations. A variety of physicochemical techniques are employed to characterize the thermal properties for SCKs of differing size and morphology. DSC is used to monitor the melting and crystallization transitions and to assess the percentage of the crystallinity of poly(η-caprolactone) (PCL) within the SCK cores, quenched at different temperatures from the melt. The core-shell morphology of the SCK imposes a fractionated crystallization onto the PCL and induces homogenous and heterogeneous nucleation. In addition, the low percentage of crystallinity found for PCL in SCKs, relative to that for PCL homopolymer and PCL-b-PAA copolymer is interpreted as the product of spatial confinement and covalent attachment to a crosslinked surface (the shell) provided by the containment of the PCL chains within the SCK core domain.

Functionalized polymer colloids were synthesized by emulsion polymerization having specific ionic groups on surface. In this review, a comprehensive study on the synthesis of functionalized polymer colloids is carried out. Monodisperse polymer colloids with acetal, aldehyde, chloromethyl, and amino functionalities were synthesized by a multi-step emulsion polymerization process. In the first step, the seeds were synthesized by batch emulsion polymerization of styrene; and in the following steps, onto the previously formed polystyrene latex particles, the functional monomers were co- and/or ter-polymerized. Some of the synthesized latexes were chosen as the polymeric support to carry out the covalent coupling with a protein and to test the utility of the latex-protein complexes formed in immunoassays. In addition, the colloidal stability of polymer colloids is theoretically and experimentally analyzed. This study shows that classical DLVO (Derjaguin, Landau, Verwey, Overbeek) theory can explain the stability of weakly charged polymer colloids. This is not sufficient in the case of highly charged polymer colloids using hydrophilic monomers. In such a system, the steric repulsion is not negligible and an electrosteric repulsion mechanism must be considered. Our interest has centered on studying this effect from a quantitative point of view. Although the method used contains five variables, it is possible to considerably reduce this number if some of them are calculated by alternative methods (ψd, δ) or taken from literature (A, x).

Because of their availability in discrete sizes and the relative ease with which they can be derivatized and characterized, polymeric nanoparticles have become important model surfaces in bioanalytical work. One finds that increased reaction rates and reduced steric hindrance are the results when proteins or other reactive macromolecules are attached to ever smaller particulate carriers in the sub-micron size range. The introduction of new chemical features to a surface can be followed and optimized through work with polymeric surfactants that adsorb in a stable manner to hydrophobic surfaces, such as those associated with the latex particles. By introducing reactive structures into the endgroups of such polymeric surfactants, and then mixing derivatized with underivatized product prior to adsorption, one can let the process yield surfaces with pre-determined levels of functionalization. By mixing surfactants with different types of reactive structures it is possible to engineer surfaces with distinctly optimal performance in specific situations. One such situation is the attachment of the particle to some other substrate, e.g. a flat surface for optical read-out. Such attachments have been performed and the bound particles have proven to resist shear-induced removal even at high rates of laminar flow. Through this strategy, particles attached in large arrays may be relied upon to facilitate the complex analyses mandated by the rapidly growing field of proteomics.

Narrowly size-distributed functionalized magnetic emulsions with diameters between 200 and 300 nm bearing reactive amine or carboxylic groups were prepared respectively via single adsorption or layer-by-layer adsorption process. The colloidal stability of the functionalized magnetic emulsion was related to the polymer adsorbed amount and to the adsorption methodology. The single adsorption of poly(ethyleneimine) onto negatively charged magnetic emulsion led to amino-containing magnetic nanodroplets for nucleic acid extraction, concentration and amplification. The enzymatic amplification of adsorbed nucleic acid molecules (i.e. RNA) was found to be related to both initial nucleic acid concentration and the used magnetic particles in the amplification step. The undesirable inhibition phenomena observed in the enzymatic amplification (i.e. RT-PCR) process was eliminated by the addition of appropriate negatively charged polyelectrolyte before the amplification step. The encapsulation of magnetic emulsion via layer-by-layer polyelectrolytes adsorption process was used to elaborate functionalized core-shell magnetic colloids. The characterized final magnetic dispersions were evaluated in specific nucleic acids capture and detection, and in proteins immobilization process.