Chun-Pong Chak

pH-Controllable Supramolecular Systems

This Focus Review surveys representative examples of pH-controllable supramolecular systems with interesting features and state-of-the-art applications such as 1) conformational changes within individual molecules; 2) folding/unfolding of polymers; 3) simultaneous binding of cations and anions; 4) logic function; 5) ON-OFF switchable colorimetric sensing; 6) translocation of macrocycle-in-rotaxane molecules; 7) large-scale movement within molecules; and 8) regulation of the substrate flow in nanocontainers. In particular, systems will be discussed that involve: pH-induced conformational changes of a resorcinarene cavitand and a bis(iron porphyrin) complex; pH control in assembly and disassembly of supramolecular systems stabilized with different major noncovalent interactions; pH-driven movements of interlocked molecules involving rotaxanes, molecular elevators, and molecular muscles; and, finally, multicomponent supramolecular systems immobilized on solid supports as pH-responsive nanovalves for the controlled release of specific substrates. Recent advances in the understanding of pH-controllable supramolecular systems have led to the construction of meaningful molecular machines for electronic and biological applications that are amenable to control by simple perturbation with acids and bases.

Durable Mesenchymal Stem Cell Labelling by Using Polyhedral Superparamagnetic Iron Oxide Nanoparticles

Small polyhedral superparamagnetic iron oxide (SPIO) nanoparticles (<10 nm) coated with a thin layer of silica were prepared (SPIO@SiO(2) and SPIO@SiO(2)-NH(2)). Surface modification of the small polyhedral silica-coated SPIO nanoparticles with amines led to substantially higher mesenchymal stem cell (MSC) labelling efficiency without the use of additional transfecting agents. Therefore, amine surface-modified nanoparticles (SPIO@ SiO(2)-NH(2)) appeared to be the preferred candidate for MSC labelling. In vitro studies demonstrated that controlled labelling of SPIO@SiO(2) and SPIO@SiO(2)-NH(2) did not cause MSC death or proliferation inhibition. MSCs labelled with SPIO@SiO(2)-NH(2) nanoparticles retained differentiation potential and showed osteogenic, adipogenic and chondrogenic differentiations. The noncytotoxic polyhedral SPIO@SiO(2)-NH(2) nanoparticle-labelled MSCs were successfully implanted in rabbit brain and erector spinae muscle, and demonstrated long-lasting, durable MRI labelling efficacy after 8-12 weeks.

Discrete Functional Gold Nanoparticles: Hydrogen Bond-Assisted Synthesis, Magnetic Purification, Supramolecular Dimer and Trimer Formation

Amine monofunctional gold nanoparticles (1-AuNPs) were synthesized by employing a solid-supported technique and pH-switchable pseudorotaxane formation. Purification was repeatedly facilitated using crown ether peripherally coated superparamagnetic iron oxide microspheres to yield the monofunctional gold nanoparticles in excellent yield. The product and its related intermediate superstructures were characterized by IR and X-ray photoelectron spectroscopies. Novel supramolecular dimers and trimers were prepared by titrating the 1-AuNPs with bisDB24C8 and trisDB24C8 at different ratios. UV/visible absorption spectroscopic analyses of the supramolecular dimer and trimer solutions, which were formed by mixing their separate components in different ratios, indicated the gradual appearance of two distinct plasmonic resonance bands at 620 and approximately 700 nm. Furthermore, TEM images of the dimers revealed a significant amount of dimer pairs on the surface, while the TEM images of the trimers demonstrated the presence of both dimers and trimers. The trimers appeared as triangular or near-linear shapes.

Biological and Magnetic Contrast Evaluation of Shape-Selective Mn–Fe Nanowires

One-dimensional composite Mn-Fe oxide nanostructures of different sizes (nanoneedles, nanorods, and nanowires) were prepared by a linker-induced organization of manganese-doped iron oxide nanoparticles. The nanostructures were obtained by the treatment of MnFe2O4 nanoparticles in the presence of cystamine. The average lengths of nanoneedle, nanorod, and nanowire are approximately 400, 800, and 1000 nm, respectively. High-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray (EDX) spectroscopy, and inductively coupled plasma-optical emission spectroscopy (ICP-OES) were employed to characterize the morphologies and the elemental contents of the nanostructures. As an example of their potential applications, these nanostructures were explored as the cell-labeling agents for magnetic resonance imaging (MRI). The magnetic contrast properties of the nanostructures were characterized by a 1.5 T (Tesla) whole body MR system. 10 mug/mL of the nanostructures caused substantial negative contrast. After in vitro incubation, the nanostructures could be effectively incorporated into the cells of a monocyte/macrophage cell line (RAW264.7). These cellspsila viability and proliferation potential were not affected when the labeling concentration was less than 50 mug/mL.

Efficacy and Durability in Direct Labeling of Mesenchymal Stem Cells Using Ultrasmall Superparamagnetic Iron Oxide Nanoparticles with Organosilica, Dextran, and PEG Coatings

We herein report a comparative study of mesenchymal stem cell (MSC) labeling using spherical superparamagnetic iron oxide (SPIO) nanoparticles containing different coatings, namely, organosilica, dextran, and poly(ethylene glycol) (PEG). These nanomaterials possess a similar SPIO core size of 6–7 nm. Together with their coatings, the overall sizes are 10–15 nm for all SPIO@SiO2, SPIO@dextran, and SPIO@PEG nanoparticles. These nanoparticles were investigated for their efficacies to be uptaken by rabbit bone marrow-derived MSCs without any transfecting agent. Experimentally, both SPIO@SiO2 and SPIO@PEG nanoparticles could be successfully uptaken by MSCs while the SPIO@dextran nanoparticles demonstrated limited labeling efficiency. The labeling durability of SPIO@SiO2 and SPIO@PEG nanoparticles in MSCs after three weeks of culture were compared by Prussian blue staining tests. SPIO@SiO2 nanoparticles demonstrated more blue staining than SPIO@PEG nanoparticles, rendering them better materials for MSCs labeling by direct uptake when durable intracellullar retention of SPIO is desired.

Nanoparticle–DNA–polymer composites for hepatocellular carcinoma cell labeling, sensing, and magnetic resonance imaging

This paper describes comparative studies and protocols in (1) self-assembling of ultrasmall superparamagnetic iron oxide nanoparticle (NP), circular plasmid DNA, and branched polyethylenimine (PEI) composites; (2) magnetofection; (3) gene delivery, (4) magnetic resonance imaging (MRI), and (5) cytotoxicity of the composites toward hepatocellular carcinoma HepG2 cells.

Increased Efficacies in Magnetofection and Gene Delivery to Hepatocellular Carcinoma Cells with Ternary Organic–Inorganic Hybrid Nanocomposites

In recent years, liver cancer (hepatocellular carcinoma) has been ranked as the leading cancer in Asia together with lung and stomach cancers. Towards the treatment of human hepatocellular carcinoma, a novel theranostic nanomaterial has yet to be developed for potential simultaneous gene transfer, drug delivery, and magnetic resonance imaging (MRI) enhancement. Superparamagnetic iron oxide nano/microparticles such as ferumoxides and ferucarbotran have been used clinically as contrast agents for MRI in liver. In light of treating human hepatocellular carcinoma, we report herein the use of ternary organic–inorganic hybrid nanocomposites PEI/DNA/NP that contain deferoxamine-coated superparamagnetic iron oxide nanoparticles (NP), circular plasmid DNA (pEGFP-C1 and pRL-CMV), and branched polyethylenimine (PEI, 25 kDa, PDI =2.5), which are potentially suitable for the proposed applications together with transfection studies in HepG2 cells. Only a small amount of branched PEI is employed for forming the stable ternary complex to reduce its cytotoxicity. Plasmid pEGFP-C1 (~4.7 kb) encodes a red-shifted variant of wild-type green fluorescence protein (GFP) for expression in mammalian cells, while pRL-CMV (~4.0 kb) encodes Renilla luciferase (RLU) for expression in various cell types. These two DNAs are commonly used as markers for studying transfection efficacies. The fluorescence/luminescence intensity is directly proportional to the amount of GFP/ RLU, which can be easily detected owing to the strong and constitutive expression of the reporters. By using two signaldetection methods, any reduction of the enhancement of PEI/DNA/NP uptake by cells can be estimated. Deferoxamine is used clinically to treat iron poisoning, as it binds ferric ions in the blood stream and enhances its elimination in urine. Ultrasmall NPs of 8–10 nm in diameter (Figure 1) were attached to deferoxamine ligand at their peripheries in three synthetic steps; their hydroxy functional groups can interact with DNA and PEI, mainly through amide hydrogen bonds and electrostatic interactions. Stable ternary organic– inorganic hybrid nanocomposites of approximately 250 nm in diameter with different component ratios have been prepared in phosphate-buffered saline (PBS) and characterized by light scattering, zeta-potential analyses, mobility tests, etc. Typical Prussian blue staining images of HepG2 cells internalized with six ternary complexes that possess different ratios of components are shown in Figure 2. According to the visual assessment of these images, the cellular uptake efficacy increases as the amount of NPs was increased from 0.1 to 1.0 mg per well. By contrast, the PEI amount of 0.2 ng gives the best cellular uptake efficacy among other amounts [a] Prof. Dr. K. C.-F. Leung, Dr. X.-M. Zhu, C.-H. Wong Department of Chemistry The Hong Kong Baptist University Kowloon Tong, Kowloon, Hong Kong SAR (P. R. China) Fax: (+852) 3411-7348 E-mail : cfleung@hkbu.edu.hk [b] Dr. C.-P. Chak, S.-F. Lee Department of Chemistry The Chinese University of Hong Kong Shatin, NT, Hong Kong SAR (P. R. China) [c] J. M. Y. Lai, K. W. Y. Sham, Prof. Dr. C. H. K. Cheng School of Biomedical Sciences and Center of Novel Functional Molecules The Chinese University of Hong Kong Shatin, NT, Hong Kong SAR (P. R. China) [d] Dr. X.-M. Zhu, Prof. Dr. Y.-X. J. Wang Department of Imaging and Interventional Radiology Prince of Wales Hospital, The Chinese University of Hong Kong Shatin, NT, Hong Kong SAR (P. R. China) [e] Prof. Dr. K. C.-F. Leung Institute of Creativity The Hong Kong Baptist University Kowloon Tong, Kowloon, Hong Kong SAR (P. R. China) [f] Prof. Dr. K. C.-F. Leung Institute of Molecular Functional Materials Areas of Excellence, University Grants Committee Hong Kong SAR (P. R. China) Figure 1. Scheme of the synthetic route to obtain PEI/DNA/NP ternary organic–inorganic hybrid nanocomposites from deferoxamine-coated superparamagnetic iron oxide nanoparticles. Reagents and conditions: a) succinic anhydride, DMF, rt, 24 h; b) N,N’-dicyclohexylcarbodiimide, N-hydroxysuccinimide, THF, rt, 24 h; and c) deferoxamine, DMF, rt, 24 h.

DNA hybridization of pathogenicity island of vancomycin -resistant Enterococcus faecalis with discretely functionalized gold nanoparticles in organic solvent mixtures

Short single-strand (ss) DNA sensors that involve mono-functionalized gold nanoparticles are reported for the first time. By way of an example, the hybridization and detection of a 10 base-pair oligonucleotide (TGACTCTTAA), which relates to the pathogenicity island of vancomycin-resistant Enterococcus faecalis, has been demonstrated in the organic solvent mixture (DMF/CHCl3/H2O/NaCl) at sub-nanomole concentrations. The formation of the resulting double-strand (ds) DNA–AuNP complexes which involve either tail-to-tail or head-to-head alignment, are sensitive to salt (NaCl) and solvent (CHCl3) concentrations. By UV/visible absorption measurements, it has been found that the melting temperature of the dsDNA–AuNP complexes increases with increasing salt concentrations as well as the chloroform ratio, for both tail-to-tail and head-to-head alignments. Transmission electron microscopic images of the dsDNA–AuNP complexes with mono-functionalized AuNPs reveal significant amounts of dimeric AuNP species while that of randomly functionalized AuNPs reveal an agglomerated AuNP nanostructures.

Mesenchymal stem cell intracellular labeling using silica-coated superparamagnetic iron oxide nanoparticles with amine functional peripheries

Two types of superparamagnetic iron oxide nanoparticles designed for mesenchymal stem cell labeling and magnetic resonance imaging in vivo tracking were constructed in our laboratories. Both types of nanoparticles have an iron oxide core and a thin layer of silica coating, one of them has additional surface modification with amine. The overall particle size was around 8 nanometers. The in vitro study results demonstrated that, without the application of a transfection agent, both nanoparticles could be incorporated into bone marrow derived mesenchymal stem cells and located in lysosomes. Surface modification with amine significantly increased mesenchymal stem cell labeling efficiency, leading to an increased magnetic resonance imaging detection sensitivity of 2-5 folds.

Simultaneous purification and surface plasmon resonance characterization of mechanoresponsive, discretely functionalized gold nanoparticles

Discretely functionalized gold nanoparticles (AuNPs) with an amine surface functional group on each AuNP have been synthesized as a crude mixture with a significant amount of nonfunctional AuNPs. By employing the mechanoresponsive nature and reversible self-assembling properties of the mono-functionalized AuNP on a crown ether-coated gold surface, mono-functionalized AuNPs have been successfully isolated from a crude mixture. A real-time surface plasmon resonance (SPR) with organic solvent-compatible microfluidic device/instrumentation has been built together with a three-step denoising procedure for the detection of SPR absorption dip shifts with high sensitivity. With this instrument, pH-driven self-assembly and disassembly processes of the mono-functionalized gold nanoparticles (AuNPs), non-functional/mono-functionalized (crude) AuNPs, and randomly functionalized AuNPs on a crown ether-coated gold surface have been monitored simultaneously. From the real-time SPR traces, in general, the self-assembled AuNPs are stable on the surface with a solvent flow for a long period of time. These AuNPs possess different self-assembling and disassembling properties. In particular, SPR response traces obtained from mono-functionalized AuNPs reveal complete pH-driven self-assembly and disassembly processes on the gold surface. This device has enabled real-time monitoring of mono-functionalized AuNPs characteristics with the exclusion of non-functional AuNPs in a crude mixture. The SPR trace of self-assembled, randomly functionalized AuNPs indicates that only partial AuNPs were disassembled from the surface and eluted away after base treatment. Residue randomly functionalized AuNPs that were intact with the gold surface might possess higher number of functional group on each NP, rendering multivalent binding with enhanced stability towards the base. This observation has also been confirmed by atomic force microscopy (AFM). SPR monitoring of discretely functionalized AuNPs enables a real-time characterization other than the conventional transmission electron microscopy (TEM) and UV/visible absorption spectroscopic methods. The reported mono-functionalized AuNPs may find applications in single molecular detection and analysis of molecular binding events with a quantification capability with higher sensitivity.