Ultrafast Studies of Different Oxidation and Protonation States of Rhodamine 6G and Implications for Photocatalysis

The long-lived radical R6G•, derived from the cationic dye rhodamine 6G (R6G+) by reduction, is of growing interest in photoredox catalysis. This manuscript discusses three methods of its preparation in dimethylsulfoxide, highlighting spectral differences due to solvatochromism, co-solutes, and basicity of the solution. Upon excitation, R6G•* can release an electron to a substrate molecule or as a solvated electron, leading back to R6G+. However, a second reduction of R6G• is irreversible, decreasing the overall concentration of R6G• and R6G+ with time. R6G+ can also be deprotonated to R6G1 under basic conditions, and even double deprotonation to R6G2– is possible, though this may undergo irreversible reaction over time. Excitation of R6G1 leads to the formation of a photoproduct stable for seconds which then reforms R6G1. If R6G• is exposed to basic conditions in the presence of oxygen, it is oxidized to R6G+, which is then quickly deprotonated to yield R6G1 again. Hence, in basic solution R6G1 is the predominant species, so that other light-induced reaction pathways than with R6G+ are accessible. It remains to be determined whether the photoproduct of R6G1 could be beneficial for a photocatalytic application under strongly basic conditions.

Degradation of a cationic dye (Rhodamine 6G) using hydrodynamic cavitation coupled with other oxidative agents: Reaction mechanism and pathway

The cationic dyes rhodamine 6G (R6G) and oxazine 4 (Ox4) were intercalated into oriented lithium hectorite (LiHT, a synthetic fluor-mica) films by ion-exchange, and their orientation was studied by x-ray and polarized spectroscopy. Orientation of dyes was determined by basal spacing obtained by x-ray diffraction data, showing that angles of the long axis were 60° for R6G and 47° for Ox4 against the layer. Polarized ultraviolet-visible spectroscopy showed that the high-order H-aggregate of R6G and Ox4 were oriented at 64° and 52° against layers, respectively; other states of dyes were oriented at much lower angles. The interlayer distance was mostly determined by dimensions of the high-order H-aggregate.

Adsorption characteristics of cross-linked lysozyme crystals of different morphologies (tetragonal, orthorhombic, triclinic and monoclinic) were examined using four anionic dyes (fluorescein, eosin, erythrosin, and rose bengal), one zwitterionic dye (rhodamine B), and one cationic dye (rhodamine 6G). The adsorption isotherms were of the Langmuir type for all examined systems with the exception of rhodamine B adsorption by monoclinic crystals. The weakest adsorption was observed for the cationic dye, rhodamine B, whereas dianionic dyes, eosin, rose bengal, and erythrosin were strongly adsorbed on the protein surface. The adsorption capacities of the crystals for the dyes were found to depend on both charge and hydrophobicity of the dye, reflecting the heterogeneous character of the lysozyme pore surface. The adsorption affinity of the crystals for the dyes was a function of the dyes' hydrophobicity. Furthermore, the crystal morphology was identified as an additional factor determining capacity and affinity for dye adsorption. Differences between crystals prepared in the presence of the same precipitant were lower than between morphologies prepared with different precipitants.

Graphene oxide (GO)/polyacrylamide (PAM) composite hydrogels as efficient cationic dye adsorbents

Dodecahydro-closo-dodecaborate anion (B12H122–) organic polymers (BOPs) were facilely obtained from aqueous solution via a simple and effective method. Single crystal X-ray diffraction results insinuated that the BOPs were formed by intermolecular forces between 4,4′-bipyridine and B12H122–. The presence of B12H122– gave the BOPs a certain degree of electronegativity, which possessed excellent stability and high adsorption capacity toward the cationic dyes methylene blue (MB), rhodamine B (RB), and rhodamine 6G (RB6G). The adsorption capacity of BOPs for MB, RB, and RB 6G dyes reached 3250 mg·g–1, 1388 mg·g–1, and 2033 mg·g–1, respectively. Through adsorption kinetics and adsorption isotherm results, BOPs exhibited rapid kinetic adsorption and ultrahigh adsorption capacity. Moreover, the BOPs materials could separate MB/MO, MB/RB, MB/RB6G, RB/MO, and RB6G/MO mixed components efficiently by adjusting the pH of the dye solution. It was worth mentioning that BOPs adsorbents could be easily and rapidly regene...

A systematic study of the permeation of small molecules through Pdop microcapsules is reported. The zwitterionic Pdop microcapsules are prepared by oxidative polymerization of dopamine on polystyrene microspheres followed by core removal with THF. Rhodamine 6G, methyl orange and alizarin red are chosen as differently charged probing dyes. The loading amount is affected by pH and dye concentration. Highly selective and unidirectional uptake and release of charged molecules through Pdop microcapsules can be achieved by controlling pH value: at low pH, the Pdop particles incorporate cationic dye (rhodamine 6G); at high pH, they incorporate anionic dyes (methyl orange and alizarin red). In each case, the uptake is highly selective.

Surface-Enhanced Raman and Surface-Enhanced fluorescence of charged dyes based on alginate silver nanoparticles and its calcium alginate hydrogel beads

Basic dyes have been found to be the most soluble dyes used in textile industries that, with their tinctorial values being high and even in small quantities, produce obvious coloration. Adsorption has often been used as a method to remove dissolved contaminated organic compounds because of simplicity of design, ease of operation, and insensitivity to toxic substances. When a cation-exchange material is used as a sorbent, it is believed that the interaction of the functional groups present in the dye with the matrix material (sorbent) being used could be anywhere from covalent to Coulombic, hydrogen bonding, or weak van der Waals forces. In the present study, titanium phosphate (TiP), an inorganic ion-exchange material of the class of tetravalent metal acid salt has been synthesized by sol−gel method, characterized, and used as a sorbent. The sorption behavior of cationic dyes Crystal Violet (CV), Rhodamine 6G (R6G), Methylene Blue (MB), and Pink FG (PFG) toward TiP has been studied, based on thermodynamic parameters evaluated and adsorption isotherms (Langmuir and Fruendlich). Breakthrough capacity and elution behavior of dyes have also been studied. Sorption affinity of dyes toward TiP is found to be MB > CV> R6G > PFG.

Effective removal of Rose Bengal using Ni-Co-Zn layered triple hydroxide: Studies on batch adsorption, mechanism, selectivity, co-ions, and reusability

On the role of humic acids’ carboxyl groups in the binding of charged organic compounds

Using video-intensified fluorescence microscopy and a pseudocolor display of fluorescence intensity, we analyzed the distribution of microinjected molecules within the nurse-cell/oocyte syncytium of Drosophila ovarian follicles. We varied the composition and the osmolarity of the culture solution as well as the electrical charge and the molecular mass of the microinjected fluorescent probe. As culture solutions, we used four simple salines (IMADS) and a complex tissue-culture medium (R-14) that matched the osmolarity of adult hemolymph. Small amounts of two anionic dyes (Lucifer Yellow CH and Lucifer Yellow dextran) as well as of two cationic dyes (rhodamine 6G and tetramethylrhodamine dextran-lysine) were iontophoretically microinjected either into a nurse cell or into the oocyte of stage-10 follicles. In the tissue-culture medium, within a few seconds following microinjection, all tested dyes passed through the intercellular bridges in both the anterior direction (to the nurse cells) and the posterior direction (to the oocyte), independent of their electrical charge or molecular mass. In all simple salines, irrespective of their osmolarity, Lucifer Yellow CH was found to preferentially migrate in the posterior direction and to accumulate in the oocyte due to progressive binding to yolk spheres. Thus, with this sensitive method, no correlation was detectable between the external osmolarity, the electrical charge and the preferential direction of migration of a microinjected probe. Our results indicate that the electrical gradient described by other authors does not exert significant influence on the migration of charged molecules through intercellular bridges in situ.

Chitosan hydrogels embedding hyper-crosslinked polymer particles as reusable broad-spectrum adsorbents for dye removal

In the presence of a cationic fluorophore (rhodamine 6G) whose absorption has a significant spectral overlap with the emission of a room temperature ionic liquid (RTIL), the emission of the latter gets quenched, and the quenching has been shown to be dynamic in nature. It has been shown that resonance energy transfer (RET) indeed happens between the RTIL (donor) and rhodamine 6G (cationic acceptor), and RET is the reason for the quenching of the RTIL emission. The spectral and temporal aspects of the RET (between neat RTILs as the donors and rhodamine 6G as the acceptor) were closely studied by steady-state and picosecond time-resolved fluorescence spectroscopy. The influence of the alkyl chain length of the cation, size of the anion, excitation wavelength and concentration of the acceptor on the RET dynamics were also investigated. The energy transfer time (obtained from the rise time of the acceptor) was noted to vary from 2.5 ns to 4.1 ns. By employing the Förster formulation, the donor-acceptor distance was obtained, and its magnitude was found to vary between 31.8 and 37.1 Å. The magnitude of the donor-acceptor distance was shown to be independent of the alkyl chain length of the cation but dependent on the size of the anion of the RTIL. Moreover, the donor-acceptor distance was observed to be independent of the excitation wavelength or concentration of the acceptor. It was shown that the Förster formulation can possibly account for the mechanism and hence can explain the experimental observables in the RET phenomenon. Following the detailed experiments and rigorous analysis, a model has been put forward, which can successfully explain the nanoscopic environment that a cationic fluorophore experiences in an RTIL. Moreover, the nanoscopic environment experienced by the cationic probe has been noted to be different from that experienced by a neutral fluorophore.

Synergism of Aliquat336-D2EHPA as carrier on the selectivity of organic compound dyes extraction via emulsion liquid membrane process