DNA Detection by Flow Cytometry using PNA-Modified Metal-Organic Framework Particles.

A DNA-sensing platform is developed by exploiting the easy surface functionalization of metal-organic framework (MOF) particles and their highly parallelized fluorescence detection by flow cytometry. Two strategies were employed to functionalize the surface of MIL-88A, using either covalent or non-covalent interactions, resulting in alkyne-modified and biotin-modified MIL-88A, respectively. Covalent surface coupling of an azide-dye and the alkyne-MIL-88A was achieved by means of a click reaction. Non-covalent streptavidin-biotin interactions were employed to link biotin-PNA to biotin-MIL-88A particles mediated by streptavidin. Characterization by confocal imaging and flow cytometry demonstrated that DNA can be bound selectively to the MOF surface. Flow cytometry provided quantitative data of the interaction with DNA. Making use of the large numbers of particles that can be simultaneously processed by flow cytometry, this MOF platform was able to discriminate between fully complementary, single-base mismatched, and randomized DNA targets.

Cyclodextrin-based supramolecular nanoparticles for biomedical applications.

Supramolecular host-guest interactions are ideal for engineering supramolecular nanoparticles (SNPs), because their modular character offers the possibility of using the same basic SNPs made of very similar building blocks in a variety of applications. The most widely used host is cyclodextrin (CD), therefore, this review will focus on SNPs involving CD as the host entity. In the first part, particle formation and size control are described, and the forces that induce the assembly between the different components and, therefore, result in the formation of stable and controllable nanoparticles. In the second part, the use of CD-based SNPs for diagnostics and therapeutics is described. Here, the emphasis is on how the therapeutic agent/imaging component is included in the system and how it is released at the target site. CD-based SNPs provide great possibilities for the formulation of nanoparticles for biomedical applications because of their high flexibility, stability, modular character, and biocompatibility.

The effect of PEG length on the size and guest uptake of PEG-capped MIL-88A particles.

The surface functionalization of MOF particles with poly(ethylene glycol) (PEG) is important for their use in biomedical applications. Here, the effect of the molecular weight of a monovalent PEG-carboxylate capping ligand (MWPEG) was investigated in a newly developed one-step, stoichiometric procedure that aims at functionalizing MIL-88A particles and achieving size control at the same time. The bulk of the MIL-88A particles is composed of iron(iii) oxide metal clusters connected by fumaric acid as the organic ligand. The surface is functionalized with monovalent PEG-carboxylate capping ligands of different lengths. The size of the PEG-functionalized MIL-88A decreased with increasing MWPEG, and nanoMOFs were obtained for long (≥2 kDa) PEG chains. For lower MWPEG, higher concentrations of PEG were needed to obtain the maximum size effect, but the resulting sizes were still larger than for long PEGs. BET surface area, elemental analysis, zeta potential, and infrared spectroscopy measurements showed that the PEG chains were attached to the surface of the MOF particles and not in their interior. Moreover, it was demonstrated that longer chains occupy a larger surface area, and the PEG chains adopt the low-density brush conformation. Uptake and release experiments with sulforhodamine B dye (as a model drug) showed a higher and faster uptake and release for MIL-88A functionalized with PEG (20 kDa) than for native MIL-88A, which is attributed to a larger surface-to-volume ratio for the PEG-covered particles, and to the well-hydrated and accessible nature of the PEG layer in an aqueous medium. Complete release of the dye was achieved in phosphate buffered saline, the majority by counter ion exchange, and a smaller fraction in the salt form.

Metal-Organic Frameworks (MOFs) as Multivalent Materials: Size Control and Surface Functionalization by Monovalent Capping Ligands.

Control over particle size and composition are pivotal to tune the properties of metal organic frameworks (MOFs), for example, for biomedical applications. Particle-size control and functionalization of MIL-88A were achieved by using stoichiometric replacement of a small fraction of the divalent fumarate by monovalent capping ligands. A fluorine-capping ligand was used to quantify the surface coverage of capping ligand at the surface of MIL-88A. Size control at the nanoscale was achieved by using a monovalent carboxylic acid-functionalized poly(ethylene glycol) (PEG-COOH) ligand at different concentrations. Finally, a biotin-carboxylic acid capping ligand was used to functionalize MIL-88A to bind fluorescently labeled streptavidin as an example towards bioapplications.

Size-controlled and redox-responsive supramolecular nanoparticles.

Control over the assembly and disassembly of nanoparticles is pivotal for their use as drug delivery vehicles. Here, we aim to form supramolecular nanoparticles (SNPs) by combining advantages of the reversible assembly properties of SNPs using host-guest interactions and of a stimulus-responsive moiety. The SNPs are composed of a core of positively charged poly(ethylene imine) grafted with ß-cyclodextrin (CD) and a positively charged ferrocene (Fc)-terminated poly(amidoamine) dendrimer, with a monovalent stabilizer at the surface. Fc was chosen for its loss of CD-binding properties when oxidizing it to the ferrocenium cation. The ionic strength was shown to play an important role in controlling the aggregate growth. The attractive supramolecular and repulsive electrostatic interactions constitute a balance of forces in this system at low ionic strengths. At higher ionic strengths, the increased charge screening led to a loss of electrostatic repulsion and therefore to faster aggregate growth. A Job plot showed that a 1:1 stoichiometry of host and guest moieties gave the most efficient aggregate growth. Different stabilizers were used to find the optimal stopper to limit the growth. A weaker guest moiety was shown to be less efficient in stabilizing the SNPs. Also steric repulsion is important for achieving SNP stability. SNPs of controlled particle size and good stability (up to seven days) were prepared by fine-tuning the ratio of multivalent and monovalent interactions. Finally, reversibility of the SNPs was confirmed by oxidizing the Fc guest moieties in the core of the SNPs.

Formation of hybrid gold nanoparticle network aggregates by specific host-guest interactions in a turbulent flow reactor.

A multi-inlet vortex mixer (MIVM) was used to investigate the formation of hybrid gold nanoparticle network aggregates under highly turbulent flow conditions. To form aggregates, gold nanoparticles were functionalized with ß-cyclodextrin (CD) and mixed with adamantyl (Ad)-terminated poly(propyleneimine) dendrimers. Adamantyl-terminated poly(ethylene glycol) was added as a stabilizer to cap the supramolecular aggregates and to provide steric repulsion. Aggregates were characterized using dynamic light scattering, UV-vis spectroscopy, and transmission electron microscopy. It was demonstrated that the growth of the aggregates is driven by specific host-guest interactions between CD and Ad moieties. The size of the supramolecular gold aggregates (20-1000 nm) was controlled by varying the ratio of the Ad and CD concentrations. The turbulent flow was found to have a minor effect on the supramolecular aggregate size at relatively low Ad/CD ratios, whereas it had a substantial effect at high Ad/CD ratios, leading to larger agglomerates in solution compared to laminar flow and manual conditions.

Interplay of degradation, dissolution and stabilization of clarithromycin and its amorphous solid dispersions.

Clarithromycin (CLA) is an aminomacrolide antibiotic whose physical properties are fascinating and challenging. It has very poor solubility at neutral intestinal pH, but much better solubility under acidic conditions due to amine protonation. The improved solubility in an acid environment is confounded by the poor chemical stability of clarithromycin that is quite labile toward acid-catalyzed degradation. This creates a complex system under gastrointestinal (GI) conditions: dissolution in the stomach, degradation, potential for precipitation in the small intestine, and interplay with the formulation components. We report herein a study of amorphous solid dispersion (ASD) of CLA with carboxyl-containing cellulose derivatives, which have recently been shown to be excellent ASD matrices for maximizing oral bioavailability. This approach was intended to improve CLA solubility in neutral media while minimizing release in an acid environment, and thereby increase its uptake from the small intestine. Amorphous polymer/CLA nanoparticles were also prepared by high-shear mixing in a multi-inlet vortex mixer (MIVM). Different extents of release were observed at low pH from the various formulations. Thus the solubility increase from nanosizing was deleterious to the concentration of intact CLA obtained upon reaching small intestine conditions; the high extent of release at gastric pH led to complete degradation of CLA. Using pH-switch experiments, it was possible to separate the effects of loss of CLA from solution by crystallization vs. that from chemical degradation. It was found that the hydrophobic cellulose derivative cellulose acetate adipate propionate (CAAdP) was effective at protecting CLA from dissolution in the stomach, and preventing CLA decomposition at low pH; 54% of CLA in CAADP ASD was released intact, vs. 0% and 6% from HPMCAS and CMCAB ASDs, respectively. We conclude that protection against degradation is central to enhancing overall release of intact CLA from ASD formulations; the formulations studied herein have great promise for simultaneous CLA solubility enhancement and protection from loss to chemical degradation, thereby reducing dose requirements and potentially decreasing colonic exposure to CLA (reduced colonic exposure is expected to minimize killing of beneficial colonic bacteria by CLA).

Stability of polydimethylsiloxane-magnetite nanoparticle dispersions against flocculation: interparticle interactions of polydisperse materials.

The colloidal stability of dispersions comprised of magnetite nanoparticles coated with polydimethylsiloxane (PDMS) oligomers was investigated theoretically and experimentally. Particle-particle interaction potentials in a theta solvent and in a good solvent for the PDMS were predicted by calculating van der Waals, electrostatic, steric, and magnetic forces as functions of interparticle separation distances. A variety of nanoparticle sizes and size distributions were considered. Calculations of the interparticle potential in dilute suspensions indicated that flocculation was likely for the largest 1% of the population of particles. Finally, the rheology of these complexes over time in the absence of a solvent was measured to probe their stabilities against flocculation as neat fluids. An increase in viscosity was observed upon aging, suggesting that some agglomeration occurs with time. However, the effects of aging could be removed by exposing the sample to high shear, indicating that the magnetic fluids were not irreversibly flocculated.