Self-assembled, excitonically coupled aggregates of 5,10,15,20-tetrakis­(4-sulfonatophenyl)­porphyrin (TSPP) in aqueous solutions of HCl and of the alkali chloride salts LiCl, NaCl, KCl, and CsCl were studied by absorption and resonance light scattering (RLS) spectroscopy. Aggregates deposited from these solutions were imaged by transmission electron microscopy and atomic force microscopy. In NaCl and KCl, the J band in the absorption spectrum was broadened, while in CsCl it was narrowed, as compared to aggregates in HCl. In LiCl and NaCl, the relative excitonic coherence as determined from RLS was increased as compared to aggregates in HCl, while in KCl and CsCl it was reduced. The discrepancy between measures of coherence based on exchange narrowing in the absorption spectrum and those on RLS intensity points to morphology-dependent composite J band structures. Delocalization of excitons between nanotubular subunits of bundled aggregates appears to increase excitonic coherence. Loosening of intermonomer forces, observed as bending in the images, and possibly the inclusion of alkali and/or chloride ions within composite structures, appear to limit excitonic coherence by increasing disorder. The dependence of morphologies on counterion species can be explained, in part, by the cations’ differing kosmotropic versus chaotropic ion-pairing properties and effects on water structure. The results may inform methods to tune the spatial and spectral properties of excitons in these systems.

Polymer particles are promising particulate materials for renowned biomedical applications such as targeted drug delivery, tissue engineering and biosensing. Surface properties of the polymer particles are of key importance for biomedical applications because they directly interact with biological systems. Particularly, wrinkled as well as porous surfaces possess an enhanced ability for cell attachment without any additional chemical modification. Therefore, a key objective is to fabricate the particles with desired degree of wrinkles and porosity.Many methods such as solvent evaporation, plasma treatment, emulsion instability, and electro-spraying are being employed for the generation of porous, wrinkled and/or textured surfaces. Advantageously, an application of microfluidics can support the induction of surface instabilities on droplets in a case of droplet-based systems. Furthermore, microfluidics allows tuning of size and shape of the generated droplets as well as particles with desired surface textures. In this mini-review article, surface characteristics (especially surface wrinkles and porosity) of the hydrophobic and hydrophilic polymer particles are presented for the potential applications toward biological as well as biomedical field. In addition, the impact of microfluidics is highlighted in order to produce the polymer particles of functional surface properties.

Self-assembled, excitonically coupled aggregates of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) prepared in aqueous LiCl, NaCl, KCl, CsCl, and HCl were studied with solution phase and single-particle resonance Raman (RR) spectroscopy. Solution phase excitation profiles are sharply peaked at 488.0 nm excitation for samples in HCl, LiCl, and CsCl, which show narrow J-bands in the corresponding absorption spectra, but more gently peaked for those induced by NaCl or KCl, which show broader absorption J-bands. The former three samples also exhibit larger low to high frequency mode intensity ratios with excitation near the J-band peak and smaller depolarization ratios compared to the latter two samples. Polarized spectra of individual aggregates correlate with the solution phase results, exhibiting an increase in intensities involving either incident or scattered light polarized transversely to the aggregate long axes in conditions shown previously to induce bundling of nanotubes or greater disorder. These results, along with previous absorption, resonance light scattering (RLS), and imaging data, suggest that excitonic coupling across nanotubular components in bundled aggregates leads to spectral broadening. This is attributed to increased spectral density of allowed excitonic transitions, particularly those polarized transversely to the aggregate length. Disorder leads to deviation of excitonic transition polarizations from pure axial and transverse directions, resulting in greater transverse relative to axial polarization but smaller excitonic coherence as measured by RLS intensity. These results suggest that environment-induced morphological variations can affect the energies, polarizations, and spatial structure of excitons in dye aggregates.

Self-assembled, excitonically coupled aggregates of 5,10,15,20tetrakis(4-sulfonatophenyl)porphyrin (TSPP) in acidified alcohols were studied by absorption, resonance light scattering (RLS), and resonance Raman (RR) spectroscopy along with scanning probe microscopy. In ethanol, the major absorption bands of aggregates in 0.5 mM HCl (sample E1) were narrower, with smaller total RLS intensity, compared to those of aggregates in 50 mM HCl (sample E2). RR scattering cross sections were smaller, and depolarization ratio dispersion greater, for E2 than E1. Two distinct types of aggregates at differing acidity in 1-propanol, analogous to the ethanolic samples, but only one type in methanol, were also identified. Atomic force microscopy (AFM) images for E1 showed small, single layered structures (∼5−20 nm diameter, ∼1.5−2 nm thickness). Variable morphologies for E2 included double-layered, elongated structures consistent with collapsed nanotubes (∼25 nm width, ∼3.5−4 nm thickness, ∼100−200 nm length) and single molecule thick sheets (∼50−200 nm in characteristic diameter). The former were also observed in ultrahigh vacuum scanning tunneling microscopy (UHV-STM) images. Aggregate morphology appears to depend on protonation and deprotonation kinetics of porphyrin monomers. Spectroscopic observations suggest that a larger subset of J-band excitonic transitions are significantly active in E2 than in E1. These variations in excitonic properties are attributed to the differing aggregate sizes, shapes, and possibly molecular packing. Formation of long nanotubes appears to require a solvent able to donate two hydrogen bonds, such as water. An open sheet structure is favored otherwise, as in ethanol.