Pedro Montes-Navajas

Complexation and fluorescence of tricyclic basic dyes encapsulated in cucurbiturils.

Tricyclic basic dyes (proflavine, acridine orange, pyronine, pyronine Y, oxonine, thionine and methylene blue) often form one-to-one or two-to-one complexes with CB[7] and CB[8], respectively. In the case of pyronine Y, the complexes with CB[7] and CB[8] have a one-to-one and three-to-one stoichiometry, respectively. The binding constants for CB[7] complexes range from 3.07x10(6) to 1.70x10(7) m(-1). In the case of CB[8], the association constant varies between 3.24x10(13) and 2.50x10(16) m(-2). Overall, these binding constants are four orders of magnitude higher than those reported for the same dyes in beta and gamma-cyclodextrins. Formation of the host-guest complexes leads to an increase in the fluorescence quantum yields in the case of CB[7], while the dimeric or trimeric dye encapsulated in CB[8] are remarkably less fluorescent than the same dye in diluted solutions.

Surface area measurement of graphene oxide in aqueous solutions.

Graphene oxide (GO) forms persistent dispersions in aqueous solutions up to concentrations of 0.2 mg mL(-1). Addition of methylene blue (MB) to these aqueous dispersion of GO gives rise to the observation in optical spectroscopy of new absorption bands that are indicative of the formation of MB/GO conjugates. Four new absorption maxima have been characterized, and their intensity varies depending on the relative concentration of MB with respect to GO. Two of these bands appearing at 677 and 757 nm correspond to individual MB molecules adsorbed on neutral or acid sites of GO, respectively. Two other bands at 615 and 580 nm are attributable to adsorbed MB molecules showing interaction with other neighbor dye molecules at incomplete (615 nm) or complete (580 nm) surface coverage. Complete coverage of GO surface by MB causes the formation of a precipitate and the separation of the MB/GO conjugate. EDS mapping of carbon and sulfur atoms of MB/GO conjugate indicates the homogeneous distribution of MB molecules coating GO sheets. A simple and reliable protocol for surface area measurement and determination of the level of aggregation for GO dispersions in water has been proposed by determining the amount of MB that leads to the maximum intensity of the 580 nm band and precipitation of the MB/GO conjugate. Specific surface area as high as 736.6 m(2) g(-1) in the range of the theoretical value for GO has been experimentally measured for diluted GO solutions, but aggregation levels of 15% were estimated for GO concentration of 50 μg mL(-1).

Methylene Blue Encapsulation in Cucurbit[7]uril: Laser Flash Photolysis and Near-IR Luminescence Studies of the Interaction with Oxygen

The effect of methylene blue (MB) encapsulation in cucurbit[7]uril (CB[7]) on triplet excited-state behavior and singlet oxygen (1O2) generation has been studied by using laser flash photolysis (LFP) and time-resolved near-IR luminescence spectroscopy. The lifetime of the triplet excited state of MB is longer in the CB[7] cavity (140 micros for MB-CB[7] vs 79.5 micros for aqueous MB). Cucurbituril also protects the dye triplets from quenching by oxygen, reducing the quenching rate constant [kq(O2)] from 2.6x10(9) M(-1) s(-1) to 0.2x10(9) M(-1) s(-1). The quantum yield of 1O2 production in the air-equilibrated D2O solutions is similar for free MB and for MB-CB[7], and the singlet oxygen lifetime is approximately 70 micros, suggesting its decay occurs in the aqueous (D2O) phase. The generation of singlet oxygen is delayed by CB[7]; this is attributed to the time required for oxygen to access the CB[7] nanocavity and react with the MB triplet. Thus, the rate-limiting step for sensitization is the entry of oxygen into the CB[7] cavity. Encapsulation inside CB[7] increases the relative efficiency of photoinduced MB2+* dication-radical generation, for which a modest yield is observed.

A colorimetric sensor array for the detection of the date-rape drug γ-hydroxybutyric acid (GHB): a supramolecular approach.

γ-Hydroxybutyric acid (GHB), a colourless, odourless and tasteless chemical, has become one of the most dangerous illicit drugs of abuse today. At low doses, this drug is a central nervous system depressant that reduces anxiety and produces euphoria and relaxation, sedating the recipient. There is an urgent need for simple, easy-to-use sensors for GHB in solution. Here, we present a colorimetric sensor array based on supramolecular host-guest complexes of fluorescent dyes with organic capsules (cucurbiturils) for the detection of GHB.

Positron Annihilation Lifetimes in Cucurbiturils: Evidence of Internal Inclusion of Gold in CB[7]

Positron annihilation lifetime (PAL) measurements are carried out to determine the cavity dimensions of hydrated and dehydrated cucurbiturils (CB[n]; n=5,6,7,8). In the case of hydrated samples, the dimensions of the cavity are unrelated to the number of glycoluril units of the CB, whereas for dehydrated samples the cavity dimensions measured by PAL follow the expected order, although the experimental values obtained by PAL are significantly smaller than the dimensions of each CB determined by crystallography. For a host-guest complex in which colloidal gold nanoparticles are assumed to be included inside CB[7], a significant reduction of the cavity size is measured. This represents an experimental evidence of the inclusion of gold inside CB[7], which complements previous high-resolution transmission electron microscopy (TEM) images.

Unexpected solvent isotope effect on the triplet lifetime of methylene blue associated to cucurbit[7]uril

Methylene blue shows an isotope dependent triplet lifetime that is 50% longer in D(2)O compared with H(2)O as a result of electronic-to-vibrational relaxation. The effect is enhanced when the dye is bound to curcubit[7]uril due to a combination of restricted mobility and a unfavorable vibrational coupling.

Influence of CB[n] complexation on the quenching of 2,4,6-triphenylpyrylium excited states by Fe2+ ions.

This manuscript focuses on the influence that the addition of Fe(2+) as electron donor quencher exerts on the photophysics of 2,4,6-triphenylpyrylium (TP(+)) depending on the formation or not of supramolecular inclusion complexes with cucurbit[n]uril (n: 7 or 8). (1)H-NMR spectroscopy does not provide evidence supporting the formation of a ternary TP(+)@CB[n]-Fe(2+) complex. Emission quenching studies indicate that the prevalent deactivation mechanism for the quenching of TP(+) emission by Fe(2+) is by increasing the ionic strength of the solution, with no evidence for the occurrence of dynamic quenching. Laser flash photolysis indicates that while the triplet excited state of TP(+) is instantaneously quenched by Fe(2+) in the absence of CB[n], formation of the TP(+)@CB[n] inclusion complex protects TP(+) triplet excited state from quenching, an effect that is more pronounced in the case of CB[8] due to the deeper penetration and more complete encapsulation of the heterocyclic pyrylium core inside the larger CB[8] capsule.