Diffusion and Conformation of Peptide-Functionalized Polyphenylene Dendrimers Studied by Fluorescence Correlation and 13C NMR Spectroscopy

Expanding the limits of synthetic macromolecular chemistry through Polyphenylene Dendrimers

Polyphenylene dendrimers (PPDs) are a unique class of macromolecules because their backbone is made from twisted benzene repeat units that result in a rigid, shape-persistent architecture as reported by Hammer et al. (Chem Soc Rev 44:4072-4090, 2015) and Hammer and Müllen (Chem Rev 116:2103-210, 2016) These dendrimers can be synthetically tailored at their core, scaffold, and surface to introduce a wide range of chemical functionalities that influence their applications. It is the balance between the macromolecular properties of polyphenylene dendrimers with grandiose synthetic ingenuity that presents a template for the next generation of synthetic dendrimers to achieve complex structures other chemistry fields cannot. This perspective will look at how advances in synthetic chemistry have led to an explosion in the properties of polyphenylene dendrimers from their initial stage, as PPDs that were used as precursors for nanographenes, to next-generation dendrimers for organic electronic devices, sensors for volatile organic compounds (VOCs), nanocarriers for small molecules, and even as complexes with therapeutic drugs and viruses, among others. Ideally, this perspective will illustrate how the evolution of synthetic chemistry has influenced the possible structures and properties of PPDs and how these chemical modifications have opened the door to unprecedented applications.

Accurate Diffusion Coefficients of Organosoluble Reference Dyes in Organic Media Measured by Dual-Focus Fluorescence Correlation Spectroscopy

Dual-focus fluorescence correlation spectroscopy (2fFCS) is a versatile method to determine accurate diffusion coefficients of fluorescent species in an absolute, reference-free manner. Whereas (either classical or dual-focus) FCS has been employed primarily in the life sciences and thus in aqueous environments, it is increasingly being used in materials chemistry, as well. These measurements are often performed in nonaqueous media such as organic solvents. However, the diffusion coefficients of reference dyes in organic solvents are not readily available. For this reason we determined the translational diffusion coefficients of several commercially available organosoluble fluorescent dyes by means of 2fFCS. The selected dyes and organic solvents span the visible spectrum and a broad range of refractive indices, respectively. The diffusion coefficients can be used as absolute reference values for the calibration of experimental FCS setups, allowing quantitative measurements to be performed. We show that reliable information about the hydrodynamic dimensions of the fluorescent species (including noncommercial compounds) within organic media can be extracted from the 2fFCS data.

How shape influences uptake: interactions of anisotropic polymer nanoparticles and human mesenchymal stem cells

Among several nanoparticle properties, shape is important for their interaction with cells and, therefore, relevant for uptake studies and applications. In order to further investigate such characteristics, fluorescently labeled spherical polymer nanoparticles are synthesized by free-radical polymerization via the miniemulsion process. The spherical nanoparticles are subsequently submitted to controlled mechanical deformation to yield quasi-ellipsoidal polymeric nanoparticles with different aspect ratios. The uptake behaviors of spherical and non-spherical particles with equal volume are investigated qualitatively and quantitatively by electron microscopy, confocal laser scanning microscopy, and flow cytometry measurements. Non-spherical particles show fewer uptake by cells than their spherical counterparts with a negative correlation between aspect ratio and uptake rate. This is attributed to the larger average curvature radius of adsorbed non-spherical particles experienced by the cells.

Tracer diffusion in silica inverse opals

We employed fluorescence correlation spectroscopy (FCS) to study the diffusion of small fluorescence tracers in liquid filled silica inverse opals. The inverse opals consisted of a nanoporous silica scaffold spanning a hexagonal crystal of spherical voids of 360 nm diameter connected by circular pores of 70 nm diameter. The diffusion of Alexa Fluor 488 in water and of perylene-3,4,9,10-tetracarboxylic diimide (PDI) in toluene was studied. Three diffusion modes could be distinguished: (1) Free diffusion limited by the geometric constraints given by the inverse opal, where, as compared to the free solution, this diffusion is slowed down by a factor of 3-4, (2) slow diffusion inside the nanoporous matrix of the silica scaffold, and (3) diffusion limited by adsorption. On the length scale of the focus of a confocal microscope of roughly 400 nm diffusion was non-Fickian in all cases.

Diffusion in polymer solutions studied by fluorescence correlation spectroscopy

We employed fluorescence correlation spectroscopy (FCS) to study the diffusion of molecular and macromolecular tracers in polystyrene solutions over a broad range of concentrations (c) and molecular weights (M(w,m)) of the matrix polymer. Molecular tracer diffusion scales only with the matrix concentration and superimposes on a single, nonpolymer specific, curve. On the contrary, the diffusion of macromolecular tracers in solutions of matrix polymers with M(w,m) sufficiently larger than the tracer molecular weight scales with c/c(p)*, where c(p)* is the tracer overlap concentration. We further demonstrate that FCS can address local and global dynamics simultaneously.

Diffusion in Model Networks as Studied by NMR and Fluorescence Correlation Spectroscopy

We have studied the diffusion of small solvent molecules (octane) and larger hydrophobic dye probes in octane-swollen poly(dimethyl siloxane) linear-chain solutions and end-linked model networks, using pulsed-gradient nuclear magnetic resonance (NMR) and fluorescence correlation spectroscopy (FCS), respectively, focusing on diffusion in the bulk polymer up to the equilibrium degree of swelling of the networks, that is, 4.8 at most. The combination of these results allows for new conclusions on the feasibility of different theories describing probe diffusion in concentrated polymer systems. While octane diffusion shows no cross-link dependence, the larger dyes are increasingly restricted by fixed chemical meshes. The simple Fujita free-volume theory proved most feasible to describe probe diffusion in linear long-chain solutions with realistic parameters, while better fits were obtained assuming a stretched exponential dependence on concentration. Importantly, we have analyzed the cross-link specific effect on probe diffusion independently of any specific model by comparing the best-fit interpolation of the solution data with the diffusion in the networks. The most reasonable description is obtained by assuming that the cross-link effect is additive in the effective friction coefficient of the probes. The concentration dependences as well as the data compared at the equilibrium degrees of swelling indicate that swelling heterogeneities and diffusant shape have a substantial influence on small-molecule diffusion in networks.