Methylene Blue Dimers Research Articles

Tangerine peel modified with sodium hydroxide for the removal of methylene blue: A calorimetric approach

Biosorbents have been studied as low-cost and efficient materials for removing dyes from wastewater. However, thermodynamic investigations into the interactions between these materials and dyes remain scarce, particularly concerning the effects of chemical modifications to the biosorbents. This article describes the biosorption of methylene blue (MB) by unmodified tangerine peel (BN) and tangerine peel modified with sodium hydroxide (BM). The materials were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and by assessing the point of zero charge as well as the number of acidic and basic functional groups. The biosorption studies combined kinetic and equilibrium analyses with isothermal titration calorimetry (ITC) measurements to characterize the sorption process in detail. BM exhibited a sorption capacity (328 mg g−1) approximately three times higher than that of BN. The adsorption process was enthalpically driven and at least 3 kJ mol−1 more exothermic for the untreated material, indicating that treatment with NaOH was unfavorable for the interaction between the dye and biosorption sites but favored the process entropically. The enthalpy of sorption depended on the amount adsorbed, indicating a high heterogeneity of biosorption sites within the materials. The results suggest that the lower availability of biosorption sites in BN led to the sorption of MB dimers, whereas in BM, the sorption of monomers was the preferred mechanism at less hydrophilic sites.

Pore Nanoarchitectonics of Carbon Nitrides for the Excited‐State Deactivation of Confined Methylene Blue

AbstractThe confinement of organic chromophores within mesoporous material architectures can exert a considerable alteration on their physico‐chemical properties. This study presents a detailed spectroscopic investigation of methylene blue (MB) entrapped in mesoporous polymeric carbon nitrides (mPCNs) with different pore architecturesusing transient absorption spectroscopy (TAS). The spatial confinement of MB molecules results in a prominent change in absorption spectra, characterized by both redshifts and the appearance of additional shoulder peaks, arising from the formation of MB dimers (MB2) concomitant with a distortion of the MB structure. Upon photoexcitation, entrapped MB molecules exhibit a notable altered excited‐state absorption feature, along with a drastic excited‐state quenching within 2 ns compared to molecues in bulk solutions. In contrast to the ultrafast quenching of sole MB2 with a lifetime of ~3.6 ps in highly concentrated solutions, the concentration‐dependent quenching behavior of MB aggregates in confined environments suggests the effect is caused by excimers formed in close proximity. The findings of this work highlight the impact of constrained environments and intermolecular interactions on the relaxation pathways of the excited states in photoactive molecules.

In-situ monitoring of thiazine molecular aggregation in various solvents via a free-standing acoustic levitator

In this work, we explored the in-situ reaction modeling of the molecular self-aggregation of methylene blue (MB), which is a cationic thiazine dye, in different solvents via a container-less acoustic levitator by floating of a single droplet. Our in-situ spectroscopic study revealed that the dimer essentially has a sandwich structural geometry with a deviation from parallel stacking and horizontal arrangements in the molecular planes. The real time conversion of the monomer in MB into a dimer and their dynamics in water and ethanol media were monitored using a free-standing acoustic levitator droplet system. The absorption spectra revealed changes in the two resolved peaks (monomer and dimer) and orderliness when water and ethanol were used as the media. Interestingly, the enhancement in the dimerization of MB could be attributed to droplet evaporation, which is difficult to observe in typical reactor containers. Moreover, acidic protonation resulted in a change in the aggregation orientation direction of the MB molecules, forming an unusual J-aggregation. Theoretical DFT calculations revealed that MB underwent typical H-aggregation and J-aggregation in the different solvent environments, and their orientations well matched the spectroscopic data.

Studies of monomer–dimer equilibria of methylene blue as a probe to investigate the solute–solvent and hydrophobic interactions in aqueous solutions of polyethylene glycols at 298 K

Electronic absorption spectrums of methylene blue (MB) in the concentration range wherein monomer–dimer equilibria exist have been systematically investigated in aqueous solutions of polyethylene glycols (PEG's) having molar mass 400, 1000 and 4000 g∙mol–1, at 298 K. The data on the absorption spectrums (550–700 nm) of MB are resolved to obtain monomer and dimer spectrums as well the geometrical details of the species (such as oscillatory strength, dipole moment etc.) are calculated. The molar absorption extinction coefficients for monomer and dimer cations of MB, the dimer dissociation constant values are calculated as a function of PEG concentrations. It has been noted that, absorption maximum occurs at 664 ± 1 and 605 ± 1 nm, respectively, for monomer and dimers which do not get affected much by presence of polymeric molecules and the data show perfect obedience of Beer–Lambert's law. The changes in dimer geometry from sandwich type to end–on–end type have been studied by applying exciton theory of Kasha et al. The dimer dissociation constant values at finite as well for infinitely dilute concentration of PEG's exhibit orderly amendment with respect to molar mass of PEG's and alteration in chain length of molecules. We report the free energy changes of transfer of dimeric cations of different concentrations from water to aqueous solutions of PEG's of fixed concentration, that is, to probe the interactions of water and PEG molecules with a hope of getting information about salting–in/salting–out effects, as well of binding of dye molecules with PEG's. We have suggested complexation of end hydroxylic groups of PEG's with dimer cations. The results are further interpreted as salting–in of PEG complex with MB–dimers only in the limit of infinite dilution (solute–solvent interactions) while salting–out of these complexed species is observed as a function of MB–dimer concentration.

Surface-functionalization of hydrogen titanate nanowires for efficiently selective adsorption of methylene blue

Polydopamine coated hydrogen titanate nanowires (PDA-HTN) are prepared via dopamine self-polymerization. The obtained PDA-HTN is a cheap adsorbent for selective adsorption of methylene blue (MB). The adsorption characteristics of PDA-HTN are studied from three aspects: isotherm, kinetics and thermodynamics. Date show that PDA-HTN adsorbs MB spontaneously, and the adsorption process is agreement with the fitting results of Langmuir and pseudo-second-order model. PDA-HTN is used to selectively adsorb MB from dye mixtures, the removal rate of MB can reach as high as 99.8% and adsorption capacity can reach 241.3 mg g−1. PDA-HTN can be used to adsorb MB in 0.5 mol L−1 NaCl solution, and the increase of ionic concentration in NaCl solution will improve the adsorption performance of PDA-HTN for MB, because salt ions will induce the dimerization of MB. PDA-HTN exhibits excellent cyclic adsorption property, the removal rate of MB remains above 97.7% after 12 cycles of adsorption-desorption. The adsorption is explained by cooperative effects of π–π stacking, electrostatic interaction, 1,4–Michael addition reaction and hydrogen bonding. Compared to the pristine HTN, the adsorption capacity of PDA-HTN for MB is greatly increased. In addition, PDA-HTN can also be used to remove other cationic dyes, and its adsorption capacity for basic yellow 1 (BY1) and crystal violet (CV) is 586.7 mg g−1 and 865.1 mg g−1, respectively.

A New Insight into the Mechanisms Underlying the Discoloration, Sorption, and Photodegradation of Methylene Blue Solutions with and without BNOx Nanocatalysts.

Methylene blue (MB) is widely used as a test material in photodynamic therapy and photocatalysis. These applications require an accurate determination of the MB concentration as well as the factors affecting the temporal evolution of the MB concentration. Optical absorbance is the most common method used to estimate MB concentration. This paper presents a detailed study of the dependence of the optical absorbance of aqueous methylene blue (MB) solutions in a concentration range of 0.5 to 10 mg·L-1. The nonlinear behavior of optical absorbance as a function of MB concentration is described for the first time. A sharp change in optical absorption is observed in the range of MB concentrations from 3.33 to 4.00 mg·L-1. Based on the analysis of the absorption spectra, it is concluded that this is due to the formation of MB dimers and trimers in the specific concentration range. For the first time, a strong, thermally induced discoloration effect of the MB solution under the influence of visible and sunlight was revealed: the simultaneous illumination and heating of MB solutions from 20 to 80 °C leads to a twofold decrease in the MB concentration in the solution. Exposure to sunlight for 120 min at a temperature of 80 °C led to the discoloration of the MB solution by more than 80%. The thermally induced discoloration of MB solutions should be considered in photocatalytic experiments when tested solutions are not thermally stabilized and heated due to irradiation. We discuss whether MB is a suitable test material for photocatalytic experiments and consider this using the example of a new photocatalytic material-boron oxynitride (BNOx) nanoparticles-with 4.2 and 6.5 at.% of oxygen. It is shown that discoloration is a complex process and includes the following mechanisms: thermally induced MB photodegradation, MB absorption on BNOx NPs, self-sensitizing MB photooxidation, and photocatalytic MB degradation. Careful consideration of all these processes makes it possible to determine the photocatalytic contribution to the discoloration process when using MB as a test material. The photocatalytic activity of BNOx NPs containing 4.2 and 6.5 at.% of oxygen, estimated at ~440 μmol·g-1·h-1. The obtained results are discussed based on the results of DFT calculations considering the effect of MB sorption on its self-sensitizing photooxidation activity. A DFT analysis of the MB sorption capacity with BNOx NPs shows that surface oxygen defects prevent the sorption of MB molecules due to their planar orientation over the BNOx surface. To enhance the sorption capacity, surface oxygen defects should be eliminated.

Spectrophotometric study of the sodium dodecyl sulfate in the presence of methylene blue in the methanol–water mixed solvent system

The methylene blue (MB) dye is very sensitive to changes in the polarity of its surroundings. The interaction of MB with sodium dodecyl sulfate (SDS) was studied spectrophotometrically in 0.1, 0.2, 0.3 and 0.4 vol fractions of methanol in an aqueous medium. The dielectric constant value decreases with the addition of methanol in the water and hence there is the lower stabilization of the lowest unoccupied molecular orbital which results in the hypsochromic shift in the monomeric form of methylene blue. In a 0.1 vol fraction of methanol, the peaks are seen at 665 and 612 nm, which are monomeric and dimeric MB peaks, respectively. The dimerization of MB is reduced with the increasing volume fraction of methanol in the solvent and ceases at/above 0.3 vol fraction of methanol. In the 0.4 vol fraction of methanol, the turbidity appears with the addition of SDS and there were no significant changes in the absorbance. In the presence of methanol, MB prefers to stay with the sulfate groups of SDS at the peripheral regions of the premicelle and micelle. At very low surfactant concentrations, far below their critical micelle concentration (CMC), the formation of MB-MB dimers and/or MB-SDS ion pairs or clusters occurs, and a corresponding shift in absorption and decrease in absorbance is observed. On increasing the concentration of SDS, micellization occurs, which disrupts the MB-MB dimers and MB-SDS ion pairs are incorporated with the micelles. On further addition of SDS, lone MB monomers become associated with these micelles and the absorbance increases slightly higher than those of the original dye in the solvent. This is the case of dye solubilization where monomers are completely incorporated or bounded by micelles. The CMC of SDS decreases in the presence of dye suggesting the strong interaction between the oppositely charged dye-surfactant ion pairs. The value of binding constant and partition coefficient decreased with the increasing volume fraction of methanol. The negative value of ΔGb indicates that the interaction between SDS and MB is spontaneous but the negative values of ΔGb decreased with the increasing volume fraction of methanol.

Monitoring of methylene blue monomers and dimers to control the bacterialogical water quality including application to photocatalysis.

In this study, we propose the development of a rapid and reliable method to control and to monitor microbial water quality. The methylene blue (MB) decolorization assay was based on the analysis of spectral profiles of dye in interaction with a different bacterial concentration. The determination of dye decolorization rate (DDR) shows a correlation between the MB reduction rate and the bacterial density. Moreover, the kinetic of the monomer and dimer equilibrium of MB in water mainly, the monitoring of bounded MB species in relationship with a knowed concentration of target bacteria, was allowed to establish a relationship between MB decolorization rate and bacterial density. Furthermore, this method was applied to evaluate the water quality after photocatalysis. Based on this method, the photocatalytic effects on bacterial density was highlighted by the decrease in DDR after photocatalytic treatment with fractioned times (0 to 5 h); this increase was followed by a decrease of bounded MB species and, an increase in free MB forms miming the reduction of bacterial density due to the biocide effects of photocatalysis process. However, the analysis of spectra profiles shows a weak but a continuous decrease in bounded MB dimer and monomer forms in the treated water samples exempt of culturable bacteria. Moreover, the MB spectra profiles were tended toward a negative control spectrum without superposition. Thus, the possibility of the presence of viable but non-culturable bacteria was expected; therefore, to optimize this tertiary water treatment process, an extending on proceeding time was recommended to avoid the bacterial resuscitation after photocatalysis.

Synergistic effect of a spinel ferrite on the adsorption capacity of nano bio-silica for the removal of methylene blue

ABSTRACT The synergistic effect of CoFe2O4 on the capacity of bio-silica extracted from rice husk for the removal of methylene blue (MB) was investigated. The novel composite of cobalt ferrite/nano bio-silica was prepared by dispersing cobalt and iron salt in ratio 1:2 in a solution containing bio-silica, calcined at 700°C and characterized. The adsorption capacity of the composite (253.6 mg g−1) was higher than that of bio-silica (52.6 mg g−1), and the process was exothermic and spontaneous. Langmuir and Freundlich models were applicable to explain the adsorption isotherm, while pseudo-second-order and Elovich are best applicable for the kinetics mechanism. The amount of MB that was removed, increased with an increase in ionic strength due to dimerization of MB. Regeneration and reusability of the adsorbents showed that they are economically viable. Energy-filtered transmission electron microscopy (EFTEM) and Fourier transformed infrared (FTIR) analysis of MB-loaded adsorbent confirmed the adsorption of MB.

Modulation of release mechanisms of methylene blue (MB) monomers and dimers from silica-MB@shellac synthesized by antisolvent crystallization

Release behaviors of drugs from drug deliveries are crucial for the enhancement of therapy efficiency, reduction of toxicity and patient compliance. Herein, antisolvent crystallization is employed to coat methlyene blue (MB)-loaded silica by shellac precipitation (silica-MB@shellac), which is simultaneously induced by outward diffusion of H+ ions from particular silica-MB. The encapsulation of shellac shell on silica-MB modulates the aggregation state of MB, which endows silica-MB@shellac a decreased MB's thermal stability, enhanced photoluminescence intensity, improved stability against in vitro reduction by ascorbic acid and retained photodynamic therapy activity. From the absorbance of MB supernatant obtained during incubation, the concentrations of MB monomers and dimers are determined via a non-linear regression analysis to investigate the influence of shellac coating on MB's release mechanisms from silica-MB@shellac. According to the simulated models, small diffusion constants of MB are caused by limited diffusion through shellac shells with high compaction degrees. These are observed for samples synthesized under high supersaturation degree during antisolvent crystallization. High degree of supersaturation is achieved through increasing shellac concentration, additive amount and dropping rate of antisolvent, as well as decreasing pH values of aqueous buffers as antisolvent. Furthermore, a combined mechanism of Fickian diffusion and Case-IΙ relaxation is proposed to describe the release behaviors of MB monomer and dimers from silica-MB@shellac. Therefore, this work may shed light on the encapsulation method of polymer on drug-loaded powders and the control of aggregation states of photosensitizers to promote the photoluminescence intensity, photodynamic therapy efficiency and controlled release behaviors.

Improved photodynamic efficiency for methylene blue from silica-methylene blue@tannic acid-Fe(III) ions complexes in aqueous solutions

To avoid multidrug resistance and tumour recurrence, photodynamic therapy (PDT) was emerging as an alternative therapy and its efficiency was related to photosensitizer (PS) efficiency, oxygen concentration and light characteristic. Methylene blue (MB) molecules as PSs were loaded in silica (silica-MB) and followed by encapsulation by coordination complexes of tannic acid (TA) and Fe(III) ions. In comparison with those of silica-MB, decreased condensation of Si-O-Si, shifted infrared absorbance frequencies of chemical bands, delayed thermal degradation and modulated release behavior of MB were observed for silica-MB@TA with a core–shell structure. Although MB dimers were dominantly released from silica-MB, release of MB monomers from silica-MB@TA was significantly promoted, which was described by the Higuchi model. The promotion of release of MB monomers from silica-MB@TA indicated the well control of aggregate states of MB by the encapsulation of TA and Fe(III) ions complexes. Through monitoring the oxidation of uric acid, generation efficiency of singlet oxygen (1O2) by MB released from silica-MB@TA was fairly higher than that from silica-MB. A facile method to encapsulate silica-MB with complexes of TA and Fe(III) ions was herein demonstrated to raise the generation efficiency of singlet oxygen.

Graphene oxide-methylene blue nanocomposite in photodynamic therapy of human breast cancer

The interaction of methylene blue (MB) as a photosensitizer with graphene oxide nano-sheets (GO) was examined in aqueous solution using UV-vis spectrophotometric techniques. MB–GO composites were prepared by mixing the solutions of GO nano-sheets and methylene blue due to interacting of the cationic methylene blue photosensitizer via electrostatic and π–π stacking or hydrophobic cooperative interactions. The cell killing potential of nanocomposite was examined on the MDA-MB-231 breast cancer cells in the absence and presence of red LED irradiation. The results demonstrated that the MB-GO nanocomposite has good performance in photodynamic therapy (PDT) during red LED irradiation. The cytotoxicity of nanocomposite caused reducing cell viability up to 20%. These effects would be due to the nano size structure of composite that could lead to effective cellular penetration. Also the significant difference has seen in lower concentrations of MB and MB-GO nanocomposite. The results show more than 40% increases in cell killing potential in lower concentrations of nanocomposite by using 2.5 μg/mL of each compound. The ratio of GO/MB can affect the interaction and higher ratios of graphene oxide (GO/MB > 1) can induce dimerization of MB. In lower concentrations and ratios of (GO/MB < 1) the free MB concentration increases and the electron shuttling effect of GO in photo activity decreases – which could affect the photocatalytic yield in PDT. The cell viability measurements confirm these effects on cancer cell killing potential of nanocomposite. According to microscopic and PDT assay results, the nanocomposite distribution and diffusion in cells enhanced the photochemical reaction yield in photodynamic therapy of MDA-MB-231 breast cancer cell line.

Raman imaging studies on the adsorption of methylene blue species onto silver modified linen fibers

We demonstrate in this research that surface‐enhanced resonance Raman scattering combined with Raman imaging can be effectively used for analysis of distinct forms of organic dyes in antimicrobial Ag‐loaded textile fibers. The potential of this approach, as a non‐destructive characterization method of fabrics, was evaluated with Raman studies performed on the molecular forms of methylene blue (MB), used here as the organic dye model. On the basis of the surface‐enhanced Raman scattering spectra of MB monomers and dimers, the Raman imaging of Ag‐loaded linen fibers previously treated with MB solution was performed and then used for identification of the adsorbate species in distinct regions of the substrates. A semi‐quantitative analysis is then performed by considering the area of the Raman bands ascribed to the MB molecular forms and image analysis applied to Raman images. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Study of electrolytic effect on the interaction between anionic surfactant and methylene blue using spectrophotometric and conductivity methods

The interaction studies of methylene blue (MB) as a cationic dye with an anionic surfactant were conducted. Conductometric and spectroscopic methods were used for investigations. The studies were performed at a wide range of SDS concentrations from pre to post micellar concentrations (1st and 2nd CMC of SDS). The large temperature range (275–313K) was selected and thermodynamic parameters were evaluated. The dye recovery experiment was performed to observe the strength of interactions within the surfactants by using cetyltrimethylammonium bromide (CTAB) as a cationic surfactant. The role of salt effect using strong electrolytes was examined during the interaction. Interaction of surfactant with dye at 2nd CMC and the dimerization of MB in the presence of SDS were also witnessed.

Methylene Blue Exciton States Steer Nonradiative Relaxation: Ultrafast Spectroscopy of Methylene Blue Dimer.

The photochemistry and aggregation properties of methylene blue (MB) lead to its popular use in photodynamic therapy. The facile formation of strongly coupled "face-to-face" H-aggregates in concentrated aqueous solution, however, significantly changes its spectroscopic properties and photophysics. The photoinitiated dynamics of the simplest MB aggregate, MB2, was investigated over femtosecond to nanosecond time scales revealing sequential internal conversion events that fully relax the excited population. MB monomer dynamics were analyzed in tandem for a direct comparison. First, ultrafast internal conversion from the electric-dipole allowed upper exciton state to the lower forbidden exciton state was evaluated by use of broadband transient absorption (BBTA) and two-dimensional electronic spectroscopy (2DES) with a time resolution of ∼ 10 fs. Lineshape analysis of MB and MB2 2DES bands at 298 and 77 K show effectively no difference in the diagonal/antidiagonal line width ratio for the dimer, in marked contrast to the distinct reduction of the homogeneous line width for MB. This result is interpreted as ultrafast population relaxation imposing a limitation to the homogeneous line width, instead of pure dephasing as in the case of the monomer. Narrowband transient absorption was performed with the aid of target analysis, to model the dynamics at longer times. The MB dynamics were described by a sequential model featuring vibrational relaxation (1-10 ps) followed by intersystem crossing and internal conversion (τ ∼ 370 ps) leaving behind MB triplet species. Alternatively, the dimer dynamics were entirely quenched within ∼ 10 ps, yielding a ground state recovery time of 3-4 ps. Such fast and complete relaxation to the ground state demonstrates the effect of concentration quenching when monomers are brought into close proximity. The formation of exciton states introduces an initial energy funnel that eventually leads to population relaxation to the ground state, preventing even the dissociation of dimers despite having internal energies well above its binding energy.

Methylene Blue Adsorption on the Basal Surfaces of Kaolinite: Structure and Thermodynamics from Quantum and Classical Molecular Simulation

AbstractOrganic dyes such as methylene blue (MB) are often used in the characterization of clays and related minerals, but details of the adsorption mechanisms of such dyes are only partially understood from spectroscopic data, which indicate the presence of monomers, dimers, and higher aggregates for varying mineral surfaces. A combination of quantum (density functional theory) and classical molecular simulation methods was used to provide molecular detail of such adsorption processes, specifically the adsorption of MB onto kaolinite basal surfaces. Slab models with vacuum-terminated surfaces were used to obtain detailed structural properties and binding energies at both levels of theory, while classical molecular dynamics simulations of aqueous pores were used to characterize MB adsorption at infinite dilution and at higher concentration in which MB dimers and one-dimensional chains formed. Results for the neutral MB molecules are compared with those for the corresponding cation. Simulations of the aqueous pore indicate preferred adsorption on the hydrophobic siloxane surface, while charge-balancing chloride ions adsorb at the aluminol surface. At infinite dilution and in the gas-phase models, MB adsorbs with its primary molecular plane parallel to the siloxane surface to enhance hydrophobic interactions. Sandwiched dimers and chains are oriented perpendicular to the surface to facilitate the strong hydrophobic intermolecular interactions. Compared with quantum results, the hybrid force field predicts a weaker MB adsorption energy but a stronger dimerization energy. The structure and energetics of adsorbed MB at infinite dilution are consistent with the gas-phase binding results, which indicate that monomer adsorption is driven by strong interfacial forces rather than by the hydration properties of the dye. These results inform spectroscopic studies of MB adsorption on mineral surfaces while also revealing critical areas for development of improved hybrid force fields.

Release behavior of methylene blue dimers from silica-methylene blue@octacalcium phosphate powders in phosphate-buffered saline and lysosome-like buffer

Methylene blue-loaded silica (silica-MB) was coated by octacalcium phosphate (silica-MB@OCP) through a facile sol–gel method. Both MB monomers and dimers were released from silica-MB, while release of MB dimers from silica-MB@OCP was predominant in phosphate-buffered saline and lysosome-like buffer as an indicative of the influence of the molecular weight of polyethylene glycol (PEG) on the interaction density of shell network consisted of PEG, citric acid, ethylenediaminetetraacetic acid and OCP. The ratio evolution of absorbance of dimers over that of monomers was caused by the radical distribution of MB molecules in silica-MB and silica-MB@OCP. The investigation of MB dimers/carrier system would shed light on the investigation of MB dimers in photodynamic therapy for the specific treatment to outgrowing solid tumors with the disproportion between oxygen supply and consumption.

Urea enhances the photodynamic efficiency of methylene blue

Methylene blue (MB) is a well-known photosensitizer used mostly for antimicrobial photodynamic therapy (APDT). MB tends to aggregate, interfering negatively with its singlet oxygen generation, because MB aggregates lean towards electron transfer reactions, instead of energy transfer with oxygen. In order to avoid MB aggregation we tested the effect of urea, which destabilizes solute–solute interactions. The antimicrobial efficiency of MB (30μM) either in water or in 2M aqueous urea solution was tested against a fungus (Candida albicans). Samples were kept in the dark and irradiation was performed with a light emitting diode (λ=645nm). Without urea, 9min of irradiation was needed to achieve complete microbial eradication. In urea solution, complete eradication was obtained with 6min illumination (light energy of 14.4J). The higher efficiency of MB/urea solution was correlated with a smaller concentration of dimers, even in the presence of the microorganisms. Monomer to dimer concentration ratios were extracted from the absorption spectra of MB solutions measured as a function of MB concentration at different temperatures and at different concentrations of sodium chloride and urea. Dimerization equilibrium decreased by 3 and 6 times in 1 and 2M urea, respectively, and increased by a factor of 6 in 1M sodium chloride. The destabilization of aggregates by urea seems to be applied to other photosensitizers, since urea also destabilized aggregation of Meso-tetra(4-n-methyl-pyridyl)porphyrin, which is a positively charged porphyrin. We showed that urea destabilizes MB aggregates mainly by causing a decrease in the enthalpic gain of dimerization, which was exactly the opposite of the effect of sodium chloride. In order to understand this phenomenon at the molecular level, we computed the free energy for the dimer association process (ΔGdimer) in aqueous solution as well as its enthalpic component in aqueous and in aqueous/urea solutions by molecular dynamics simulations. In 2M-urea solution the atomistic picture revealed a preferential solvation of MB by urea compared with MB dimers while changes in ΔHdimer values demonstrated a clear shift favoring MB monomers. Therefore, MB monomers are more stable in urea solutions, which have significantly better photophysics and higher antimicrobial activity. This information can be of use for dental and medical professionals that are using MB based APDT protocols.

Effect of the TiO2 Crystallite Size, TiO2 Polymorph and Test Conditions on the Photo-Oxidation Rate of Aqueous Methylene Blue

This study systematically re-examines the titania-catalysed photo-oxidation of methylene blue (MB) in aqueous solution at 20 °C, placing particular emphasis on the effects of TiO2 crystallite size, TiO2 polymorph (anatase, brookite, rutile and combinations thereof) and experimental test conditions on the rate of MB photo-oxidation. For all TiO2 samples tested, the highest rate of MB photo-oxidation was observed at pH 6, slightly above the isoelectric point of TiO2 (~5.8 for P25 TiO2). Increasing the ionic strength at pH 6 induced MB dimer formation in solution, and lowered the rate of MB photo-oxidation by TiO2. For all TiO2 polymorphs, the surface area normalised rate increased with crystallite size reflecting the corresponding reduction in surface and bulk defects (electron–hole pair recombination sites). The optimum crystallite sizes were ~20–25 nm for anatase and ~50 nm for brookite. The photocatalytic activity of the different TiO2 powders followed the general order P25 > anatase > brookite ≫ rutile, with the high activity of P25 TiO2 providing strong evidence that anatase–rutile heterojunctions act as “hotspots” for MB photo-oxidation. Mixed phase anatase–rutile or brookite–rutile powders, each containing ~5 wt% rutile, demonstrated superior area normalized photocatalytic activities for MB photo-oxidation compared to pure phase anatase or brookite powders of comparable crystallite size. Finally, deposition of Pd, Pt or Au nanoparticles decreased the activity of P25 TiO2 for MB photo-oxidation. This paper clarifies long-standing confusion in the scientific literature about the photo-oxidation of aqueous MB over TiO2 and M/TiO2 (M = Pd, Pt and Au) photocatalysts.

Enhanced photodynamic therapy and effective elimination of cancer stem cells using surfactant-polymer nanoparticles.

Photodynamic therapy is a potentially curative treatment for various types of cancer. It involves energy transfer from an excited photosensitizer to surrounding oxygen molecules to produce cytotoxic singlet oxygen species, a process termed as type II reaction. The efficiency of photodynamic therapy is greatly reduced because of the reduced levels of oxygen, often found in tumor microenvironments that also house cancer stem cells, a subpopulation of tumor cells that are characterized by enhanced tumorigenicity and resistance to conventional therapies. We show here that encapsulation of a photosensitizer, methylene blue, in alginate-Aerosol OT nanoparticles leads to an increased production of reactive oxygen species (ROS) under both normoxic and hypoxic conditions. ROS generation was found to depend on the interaction of the cationic photosensitizer with the anionic alginate polymer. Dye-polymer interaction was characterized by formation of methylene blue dimers, potentially enabling electron transfer and a type I photochemical reaction that is less sensitive to environmental oxygen concentration. We also find that nanoparticle encapsulated methylene blue has the capacity to eliminate cancer stem cells under hypoxic conditions, an important goal of current cancer therapy.