Form In Aqueous Media Research Articles

High-Quantum-Yield Ultrasmall Ln2O3 (Ln = Eu, Tb, or Dy) Nanoparticle Colloids in Aqueous Media Obtained via Photosensitization.

Fluorescent nanoparticles used in biomedical applications should be stable in their colloidal form in aqueous media and possess a high quantum yield (QY). We report ultrasmall Ln2O3 (Ln = Eu, Tb, or Dy) nanoparticle colloids with high QYs in aqueous media. The nanoparticles are grafted with hydrophilic and biocompatible poly(acrylic acid) (PAA) to ensure colloidal stability and biocompatibility and with organic photosensitizer 2,6-pyridinedicarboxylic acid (PDA) for achieving a high QY. The PAA/PDA-Ln2O3 nanoparticle colloids were nearly monodispersed and ultrasmall (particle diameter: ∼2 nm). They exhibited excellent colloidal stability with no precipitation after synthesis (>1.5 years) in aqueous media, very low cellular toxicity, and very high absolute QYs of 87.6, 73.6, and 2.8% for Ln = Eu, Tb, and Dy, respectively. These QYs are the highest reported so far for lanthanides in aqueous media. Therefore, the results suggest their high potential as sensitive optical or imaging probes in biomedical applications.

Water-Controlled Keto-Enol Tautomerization of a Prebiotic Nucleobase.

Barbituric acid is believed to be a proto-RNA nucleobase that was used for biological information transfer on prebiotic earth before DNA and RNA in their present forms evolved. Nucleobases have various tautomeric forms and the relative stability of these forms is critical to their biological function. It has been shown that barbituric acid has a tri-keto form in the gas phase and an enol form in the solid state. However, its dominant tautomeric form in aqueous medium that is most relevant for biology has been investigated only to a limited extent and the findings are inconclusive. We have used multiple approaches, namely, molecular dynamics, quantum chemistry, NMR, and IR spectroscopy to determine the most stable tautomer of barbituric acid in solution. We find a delicate balance in the stability of the two tautomers, tri-keto and enol, which is tipped toward the enol as the extent of solvation by water increases.

Self-Assembly of Octanuclear PtII/PdII Coordination Barrels and Uncommon Structural Isomerization of a Photochromic Guest in Molecular Space.

Two tetragonal molecular barrels TB1 and TB2 were successfully synthesized by coordination-driven self-assembly of a tetrapyridyl donor (L) of the thiazolo[5,4-d]thiazole backbone with cis-blocked 90° Pd(II) and Pt(II) acceptors, respectively. The single-crystal structure analysis of TB1 revealed the formation of a two-face opened tetragonal Pd8 molecular barrel architecture. In contrast, the isostructural Pt(II) barrel (TB2) is water-soluble. The large confined hydrophobic molecular cavity including wide open windows and good water solubility of the barrel TB2 made it a potential molecular container for the encapsulation of guests with different sizes and properties. This has been exploited to encapsulate and stabilize the open form of a photochromic molecule (G2) in water, while the same photochromic molecule exists exclusively in a cyclic zwitterionic form in aqueous medium in the absence of the barrel TB2. This cyclic form is very stable in water and does not go back to its parent open form under common external stimuli. Surprisingly, reverse switching of the cyclic form to a colored hydrophobic open form was also possible instantly in water upon addition of the solid barrel TB2 into an aqueous solution of G2. Such a fast reverse isomerization of an irreversible process in aqueous medium by utilizing host–guest interaction of the barrel TB2 and the guest G2 is interesting. The barrel TB2 was also capable of encapsulating the water-insoluble radical initiator G1 in aqueous medium.

AIEgen-based nanoprobe for the ATP sensing and imaging in cancer cells and embryonic stem cells

A turn-on fluorescent nanoprobe (named AAP-1), based on an aggregation-induced emission luminogen (AIEgen), is disclosed for the detection of adenosine triphosphate (ATP), which is an essential element in the biological system. Organic fluorophore (named TPE-TA) consists of tetraphenylethylene (TPE, sensing and signaling moiety) and mono-triamine (TA, sensing moiety), and it forms an aggregated form in aqueous media as a nanoprobe AAP-1. The nanoprobe AAP-1 has multiple electrostatic interactions as well as hydrophobic interactions with ATP, and it displays superior selectivity toward ATP, reliable sensitivity, with a detection limit around 0.275 ppb, and fast responsive (signal within 10 s). Such a fluorescent probe to monitor ATP has been actively pursued throughout fundamental and translational research areas. In vitro assay and a successful cellular ATP imaging application was demonstrated in cancer cells and embryonic stem cells. We expect that our work warrants further ATP-related studies throughout a variety of fields.

Small-angle scattering model analysis of cage-like uranyl peroxide nanoparticles

Uranyl peroxide nanoparticles (NPs) with spherical cage structure form in aqueous medium by the assembly of uranyl hexagonal bipyramids. Morphological evolution of uranyl peroxide NPs prepared with different LiOH/UO2 molar ratios is studied by small-angle X-ray scattering. Guinier analysis indicated that the overall size of the uranyl peroxide NPs increased with storage time. Their structure is further analyzed by model fitting to a spherical shell particle. Analysis of the particle size shows that U24 particles containing 24 uranium atoms are formed in dispersion with Li/U = 7.7, and U28 particles are generated in dispersion with Li/U = 3.1. With increasing storage time, the interactions between uranyl peroxide NPs changes from repulsion to attraction for samples with Li/U ratio 5.5 and 7.7, leading to their agglomeration and precipitation. Certain amount of large aggregates remained in the dispersions in ambient condition for over one year, as confirmed by dynamic light scattering. This study gives insight into the structural evolution and interactions of uranyl peroxide NPs.

Unusual Behavior of Donor-Acceptor Stenhouse Adducts in Confined Space of a Water-Soluble PdII8 Molecular Vessel.

Donor-acceptor Stenhouse adducts (DASA) are new-generation photochromic compounds discovered recently. DASA exist normally in open form (blue/violet) and readily convert to cyclic (light yellow/colorless) zwitterionic form reversibly in the presence of green light in toluene/dioxane. In aqueous medium, the open form is not stable and converts to the cyclic zwitterionic form irreversibly. We report here a new self-assembled Pd8 molecular vessel (MV) that can stabilize and store the open form of DASA even in aqueous medium. Reaction of the 90° acceptor cis-(tmeda)Pd(NO3)2 (M) [tmeda = N, N, N', N'-tetramethylethane-1,2-diamine] with a symmetric tetraimidazole donor (L, 3,3',5,5'-tetra(1 H-imidazol-1-yl)-1,1'-biphenyl) in a 2:1 molar ratio yielded a water-soluble [8+4] self-assembled M8L4 molecular barrel (MV). This barrel (MV) is found to be a potential molecular vessel to store and stabilize the open forms of DASA in aqueous medium over the more stable zwitterionic cyclic form, while in the absence of the barrel the same DASA exist in cyclic zwitterionic form in aqueous medium. The hydrophobic interaction between the cavity and the open form of DASA molecules benefits reaching an out-of-equilibrium or reverse equilibrium state in aqueous medium. The presence of excess MV could even drive the conversion of the stable cyclic form to the open form in aqueous medium. The host-guest complex is stable upon irradiating with green light. To the best of our knowledge, this is the first successful attempt to stabilize the open form of DASA molecules in aqueous medium and the first report on the fate of DASA in a confined space discrete molecular architecture. Furthermore, the molecular vessel has been utilized for catalytic Michael addition reactions of a series of nitrostyrene derivatives with 1,3-indandione in aqueous medium.

Fabrication of functional biomass carbon aerogels derived from sisal fibers for application in selenium extraction

Selenium (Se) is a rare element, usually in ionic form in aqueous media. The separation of selenium from aqueous solution has received considerable attention due to the large-batch application of selenide. In this work, the Mg–Al layered double hydroxides (LDHs)-functionalized biomass carbon aerogels (CAs) were fabricated by combining freeze-drying, carbonization with in-situ growth method. And the process for extracting selenium based on adsorption of CAs and “memory effect” of LDHs nanosheets has been developed. Firstly, CAs was prepared by alkalization, bleaching, freeze-drying and carbonization using the sisal fiber as precursor. Then, Mg–Al LDHs-functionalized CAs were fabricated by in situ growth of LDHs platelets on the CAs surfaces. The physical and chemical properties of the LDHs-functionalized CAs were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET). The SEM results showed that sisal cellulose fibers used in this paper have a smooth surface with a diameter about 8μm and a length of several millimeters. Furthermore, the flowerlike nanosheets are uniform in both size and shape, with each LDHs nanoplatelet grafted to the CAs. The nitrogen sorption analysis confirmed that LDHs-functionalized CAs is mesoporous structure with the mesopores centered at about 3.5–5nm and BET surface area of 165.26m2/g. It is indicated that the maximum adsorption capacity of selenium on LDHs-functionalized CAs reached 73.6mg[Se]/g. The adsorption time profile of selenium onto LDHs-functionalized CAs ndicates that the adsorption equilibrium was achieved within 4h. After recycling 9 times, the regenerated LDHs-functionalized CAs still displayed high selenium absorption properties. Therefore, the high absorption capacity and excellent recycling capabilities of LDHs-functionalized CAs composites give the material an ideal choice for the selenium recovery in practical applications.

Bioinspired co-crystals of Imatinib providing enhanced kinetic solubility

Realizing the full potential of co-crystals enhanced kinetic solubility demands a comprehensive understanding of the mechanisms of dissolution, phase conversion, nucleation and crystal growth, and of the complex interplay between the active pharmaceutical ingredient (API), the coformer and co-existing forms in aqueous media. One blueprint provided by nature to keep poorly water-soluble bases in solution is the complexation with phenolic acids. Consequently, we followed a bioinspired strategy for the engineering of co-crystals of a poorly water-soluble molecule – Imatinib – with a phenolic acid, syringic acid (SYA). The dynamics of dissolution and solution-mediated phase transformations were monitored by Nuclear Magnetic Resonance (NMR) spectroscopy, providing mechanistic insights into the 60 fold-increased long lasting concentrations achieved by the syringate co-crystals as compared to Imatinib base and Imatinib mesylate. This lasting effect was linked to SYA’s ability to delay the formation and nucleation of Imatinib hydrate – the thermodynamically stable form in aqueous media – through a metastable association of SYA with Imatinib in solution. Results from permeability studies evidenced that SYA did not impact Imatinib’s permeability across membranes while suggesting improved bioavailability through higher kinetic solubility at the biological barriers. These results reflect that some degree of hydrophobicity of the coformer might be key to extend the kinetic solubility of co-crystals with hydrophobic APIs. Understanding how kinetic supersaturation can be shaped by the selection of an interactive coformer may help achieving the needed performance of new forms of poorly water-soluble, slowly dissolving APIs.

PREPARATION AND EVALUATION OF CO-CRYSTALS OF CARBAMAZEPINE WITH GLUCOMANNAN

Objective: The objective of the present work was to inhibit transformation of carbamazepine anhydrous to its dihydrate form in aqueous medium by adopting the co-crystal approach.Methods: Co-crystallization of carbamazepine and glucomannan as co-former was carried out by solution mediated phase transformation. The solution of carbamazepine and glucomannan in ethanol (95%) was agitated for 2 h and the co-crystals obtained were recovered after 24 h.Results: Co-crystal formation due to hydrogen bonding between carbamazepine and glucomannan as a co-former was confirmed by FTIR study. Inhibition of transformation of co-crystal of carbamazepine to carbamazepine dihydrate in aqueous medium was confirmed by SEM.Conclusion: Inhibition of transformation of carbamazepine co-crystal to its dihydrate form resulted in its improved dissolution. Dissolution efficiency of carbamazepine in its co-crystal was increased up to 79.26% within 30 min.

Modulation of the activity and selectivity of the immobilized lipases by surfactants and solvents

Most of lipases are in equilibrium between a majority inactive closed form and a minority active open form in aqueous media. Perhaps, a certain stabilization of these open forms of lipases could be achieved in the presence of cosolvents or surfactants in the reaction medium. Three commercial lipases were studied (from Thermomyces lanuginosa (TLL), Candida Antarctica fraction B (CALB) and Lecitase (LEC)). Different derivatives were tested: TLL and LEC were adsorbed on an anionic exchanger and their activity strongly depends on the equilibrium between their open and closed form and CALB was adsorbed on a hydrophobic support when the open form was already stabilized by the support. Derivatives ionically adsorbed were hyperactivated by surfactans as well as by cosolvents: the activity of LEC increased 12 times in the presence of 15–20% of ethanol. CALB adsorbed on hydrophobic supports was hardly hyperactivated and even it was inhibited. The modification of the rate of covalent modification of the catalytic Ser seems to confirm that the observed hyperactivations were due to a stabilization of the open form of the adsorbed lipases (TLL and LEC). The hydrolysis of sardine oil was also studied in the presence or absence of surfactants and cosolvents. An interesting improvement in the ability of derivatives to discriminate the release of eicosipentaenic acid (EPA) and docosahexaenicacid (DHA) was found.

An efficient and reusable Li/NiO heterogeneous catalyst for ethanolysis of waste cottonseed oil

In order to use ethanol in place of methanol for biodiesel production a series of Li+ impregnated NiO (Li/NiO) was prepared in nano particle form in aqueous medium without adding any organic solvent or templates. The structure of the catalyst was established by powder XRD, surface morphology, and particle size by field emission scanning and high‐resolution transmission electron microscopy studies. Basic strengths of the catalysts were measured by Hammett indicators and found to be maximum in case of catalyst prepared with 5 wt% Li+ in NiO followed by calcination at 600°C (5‐Li/NiO‐600). Catalyst characterization supported the oxidation of Ni2+ into Ni3+ upon lithium impregnation, which impart the enhanced ethanolysis activity to the Li/NiO catalyst. Under optimized reaction condition of 5 wt% catalyst, ethanol‐to‐oil molar ratio of 12:1 and at 65°C reaction temperature, >98% fatty acid ethyl ester yield was obtained in 3 h. The catalyst was found to be effective for the ethanolysis of vegetable oils (VO) having up to 8.3 wt% free fatty acids. During reusability experiments, Li/NiO catalyst was able to catalyze seven reaction cycles without major loss in activity. The ethanolysis of VO was found to follow pseudo first order kinetics and activation energy for the same reaction was found to be 74.2 kJ/mol. Thus, the present work has demonstrated the development of a “greener” and completely renewable biofuel, fatty acid ethyl esters, using waste cottonseed oil as feedstock.Practical applications: Due to high free fatty acid and water contents, homogenous catalysts could not be employed for the transesterification of waste cooking oils. The present study demonstrates the preparation of Li/NiO and its application as reusable solid catalyst for the ethanolysis of waste cottonseed oil to produce fatty acid ethyl esters.The prepared catalyst 5‐Li/NiO‐600 was able to show the ethanolysis of waste cottonseed oil during seven successive catalytic runs. Thus, catalyst was found to be effective for the preparation of renewable diesel fuel substitute, fatty acid ethyl esters.

A Critical Comparison of Methods for the Analysis of Indigo in Dyeing Liquors and Effluents.

Indigo is one of the most important dyes in the textile industry. The control of the indigo concentration in dyeing liquors and effluents is an important tool to ensure the reproducibility of the dyed fabrics and also to establish the efficiency of the wastewater treatment. In this work, three analytical methods were studied and validated with the aim to select a reliable, fast and automated method for the indigo dye determination. The first method is based on the extraction of the dye, with chloroform, in its oxidized form. The organic solution is measured by Ultraviolet (UV)-visible spectrophotometry at 604 nm. The second method determines the concentration of indigo in its leuco form in aqueous medium by UV-visible spectrophotometry at 407 nm. Finally, in the last method, the concentration of indigo is determined by redox titration with potassium hexacyanoferrate (K3(Fe(CN)6)). The results indicated that the three methods that we studied met the established acceptance criteria regarding accuracy and precision. However, the third method was considered the most adequate for application on an industrial scale due to its wider work range, which provides a significant advantage over the others.

Polymer-grafted lignin surfactants prepared via reversible addition-fragmentation chain-transfer polymerization.

Kraft lignin grafted with hydrophilic polymers has been prepared using reversible addition-fragmentation chain-transfer (RAFT) polymerization and investigated for use as a surfactant. In this preliminary study, polyacrylamide and poly(acrylic acid) were grafted from a lignin RAFT macroinitiator at average initiator site densities estimated to be 2 per particle and 17 per particle. The target degrees of polymerization were 50 and 100, but analysis of cleaved polyacrylamide was consistent with a higher average molecular weight, suggesting not all sites were able to participate in the polymerization. All materials were readily soluble in water, and dynamic light scattering data indicate polymer-grafted lignin coexisted in isolated and aggregated forms in aqueous media. The characteristic size was 15-20 nm at low concentrations, and aggregation appeared to be a stronger function of degree of polymerization than graft density. These species were surface active, reducing the surface tension to as low as 60 dyn/cm at 1 mg/mL, and a greater decrease was observed than for polymer-grafted silica nanoparticles, suggesting that the lignin core was also surface active. While these lignin surfactants were soluble in water, they were not soluble in hexanes. Thus, it was unexpected that water-in-oil emulsions formed in all surfactant compositions and solvent ratios tested, with average droplet sizes of 10-20 μm. However, although polymer-grafted lignin has structural features similar to nanoparticles used in Pickering emulsions, its interfacial behavior was qualitatively different. While at air-water interfaces, the hydrophilic grafts promote effective reductions in surface tension, we hypothesize that the low grafting density in these lignin surfactants favors partitioning into the hexanes side of the oil-water interface because collapsed conformations of the polymer grafts improve interfacial coverage and reduce water-hexanes interactions. We propose that polymer-grafted lignin surfactants can be considered as random patchy nanoparticles with mixed hydrophilic and hydrophobic domains that result in unexpected interfacial behaviors. Further studies are necessary to clarify the molecular basis of these phenomena, but grafting of hydrophilic polymers from kraft lignin via radical polymerization could expand the use of this important biopolymer in a broad range of surfactant applications.

Reversible Rearrangements of Cu(II) Cage Complexes: Solvent and Anion Influences

The macrobicyclic mixed donor cage ligand AMME-N3S3sar (1-methyl-8-amino-3,13,16-trithia-6,10,19-triazabicyclo[6.6.6]eicosane) is capable of binding to Cu(II) as either a hexadentate (N3S3) or tetradentate (N2S2) ligand. The "Cu-in" (hexadentate)/"Cu-out" (tetradendate) equilibrium for the {Cu(AMME-N3S3sar)}(2+) units is strongly influenced by both solvent (DMSO, MeCN, and water) and halide ions (Br(-) and Cl(-)). We have established a crucial role of the solvent in these processes through the formation of intermediate solvato complexes, which are substituted by incoming halide ions triggering a final isomerization reaction. Surprisingly, for reactions carried out in the usually strongly coordinating solvent water, the completely encapsulated N3S3-bound "Cu-in" form is dominant. Furthermore, the small amounts of the "Cu-out" form present in equilibrated DMSO or MeCN solutions revert entirely to the "Cu-in" form in aqueous media, thus preventing reaction with halide anions which otherwise lead to partial or even complete decomposition of the complex. From the kinetic, electrochemical, and EPR results, the existence of an outer-sphere H-bonded network of water molecules interacting with the complex inhibits egress of the Cu(II) ion from the cage ligand. This is extremely relevant in view of outer sphere interactions present in strongly hydrogen bonding solvents and their effects on Cu(II) complexation.

Indirect electrochemical oxidation of aliphatic alcohols to carboxylic acids by active oxygen forms in aqueous media

Indirect electrochemical oxidation of aliphatic alcohols (butanol, hexanol, nonanol, decanol) to the corresponding carboxylic acids by active oxygen forms (AOFs) generated in situ in electrochemical cells from O2, H2O2, H2O is carried out in aqueous electrolyte using anodes of lead dioxide, a nickel oxide electrode, and boron-doped diamond electrode (BDDE). It is found that selectivity of the process of indirect electrosynthesis of carboxylic acids depends on the chemical nature of the anode material and structure of the initial alcohol and is determined by the conditions of AOF generation. Coupled electrosynthesis with simultaneous in situ generation of AOFs on the cathode and anode occurs more effectively with formation of the corresponding carboxylic acids.

Nitrofurantoin–p‐aminobenzoic acid cocrystal: Hydration stability and dissolution rate studies

Nitrofurantoin (NF) drug is known to transform to a hydrate form in aqueous medium. The hydration stability and dissolution rate of a few cocrystals of NF were compared with that of its stable β polymorph and hydrate form II. Hydrogen bonding and molecular packing in the novel cocrystal structures were analyzed. Pharmaceutical cocrystals of NF with p-aminobenzoic acid (PABA) and urea showed superior physicochemical properties compared with the known L-arginine salt hydrate. All the solid-state adducts were characterized by single-crystal X-ray diffraction, X-ray powder diffraction, differential scanning calorimetry, and thermogravimetric analysis. NF-PABA cocrystal was found to be superior among the compounds studied in terms of minimal transformation to NF hydrate and comparable dissolution rate to the reference drug.

Water-Induced Switching of β-Structure to Polyproline II Conformation in the 4S-Aminoproline Polypeptide via H-Bond Rearrangement

4S-Aminoproline polypeptide 2 forms unusual β-structure in trifluoroethanol that switches to the polyproline II (PPII) form in aqueous medium, while 4R-aminoproline peptide 1 retains PPII form in both solvents. This first instance of a polyproline derivative showing a β-structure is attributed to competitive pH-dependent (4-NH(3)(+)/NH(2)) stereoelectronic effect (4R vs 4S) and the overriding importance of stereospecific intra/intermolecular H-bonding in (2,4)-cis-4S-aminoproline in contrast to (2,4)-trans-4R-aminoproline oligomers.

Nanomicelle with long-term circulation and enhanced stability of camptothecin based on mPEGylated α, β-poly ( l-aspartic acid)-camptothecin conjugate

To enhance the stability and long-term circulation of camptothecin (CPT), mPEGylated α, β-poly ( l-aspartic acid)-CPT conjugates were synthesized, and used to fabricate nanomicelle. Firstly, α, β-poly ( l-aspartic acid) derivative (PAA-der) containing alkyne groups was synthesized via the ring-opening of PSI with propargyl amine. And then, azide-functionalized CPT derivatives (CPT-N 3) and azide-terminated poly (ethylene glycol) methyl ether (mPEG-N 3) were conjugated with PAA-der by click cycloaddition to give mPEG- graft-PAA-CPT conjugates. The formation of mPEG- graft-PAA-CPT nanomicelles was confirmed by fluorescence spectrophotoscopy and particle size measurements. It was found that all the nanomicelles showed spherical shapes with size about 178 nm. MPEG- graft-PAA-CPT nanomicelles showed good storage stability, even incubation at 37 °C for 60 days, and improved the stability of CPT lactone form in aqueous media. A steady release rate of CPT was kept for 72 h, suggested the great potential of mPEG- graft-PAA-CPT nanomicelles as polymer prodrug of CPT.

Rapid conversion of API hydrates to anhydrous forms in aqueous media

Three anhydrous polymorphs, a monohydrate and a dihydrate of an active pharmaceutical ingredient, N-{[(5S)-3-(4-{6-[(1R,5S)-6-cyano-3-oxabicyclo[3.1.0]hex-6-yl]pyridin-3-yl}phenyl)-2-oxo-1,3-oxazolidin-5-yl]methyl}acetamide (Compound 1), have been crystallized and characterized. Slurry experiments and thermal data have been used to determine their relative thermodynamic stability. The hydrates of Compound 1 were found to be less stable than the most stable anhydrous Form I and converted into Form I in water within 15 min. The rate of conversion in a dry state was found to depend on the relative humidity (RH) and was highest at the two RH extremes examined, 5% and 97.5% RH. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 98:4111–4118, 2009

Preparation, Spectroscopic Properties, and Stability of Water‐Soluble Subphthalocyanines

A series of subphthalocyanines containing an axial pyridyl, amino, or carboxy group have been prepared. They undergo N-methylation or deprotonation readily to give a new series of water-soluble subphthalocyanines. All the compounds have been characterized with various spectroscopic methods and elemental analysis. The molecular structure of the amino analogue SPc(OCH(2)CH(2)NMe(2)) (6) has also been determined by X-ray diffraction analysis. All these compounds are essentially non-aggregated in DMF as shown by the strong Q-band absorption. The low aggregation tendency of these compounds also results in strong fluorescence emission and high efficiency in generating singlet oxygen. In the presence of Cremophor EL, these compounds also remain in the monomeric form in aqueous media and can sensitize the formation of singlet oxygen. The carboxy and carboxylate derivatives exhibit a relatively higher photostability than the amino and cationic pyridinium and ammonium counterparts, making them potentially useful as photosensitizers in aqueous media.