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).

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.

Host-guest complexes between cucurbit[n]urils and acetanilides having aminopropyl units.

2-(Propylamino)acetamide of aniline (1a), and bis-2-(propylamino)acetamide of ortho- (1b) and para-(1c) phenylenediamine form host-guest complexes with CB[6], CB[7] and CB[8] as evidenced by the variations in the (1)H NMR spectroscopy chemical shifts and observation in MALDI-TOF-MS and ESI-MS of ions at the corresponding mass. Binding constants for the 1:1 complexes were estimated from fluorescence titrations and were in the range 10(5)-10(6)M(-1). Models based on molecular mechanics for these supramolecular complexes are provided. In spite of the different geometries arising from the ortho- or para-substitution, phenylenediamides form complexes of similar strength in which the hydrophobic alkyl chains are accommodated inside the host cavity. Formation of these host-guest complexes in the solid state was also achieved by modifying an aminopropyl silica with chloroacetanilides and preparing three silica having analogues of compounds 1a-c anchored to the solid particles. Titrations showed, however, that these solids can adsorb a large percentage of CBs by unselective interactions that are not related to the formation of inclusion complexes.

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.

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.

Cucurbituril complexes cross the cell membrane.

Cucurbiturils (CBs) of the appropriate size (CB[7] and CB[8]) form strong guest-host complexes in phosphate buffer solution (PBS) with acridine orange (AO) and pyronine Y (PYY) with 1 : 1 and 2 : 1 stoichiometries for CB[7] and CB[8] complexes, respectively. Binding constants in the range 0.87-1.60 x 10(6) M(-1) and 5.2-6.3 x 10(13) M(-2) were determined by titration with fluorescence spectroscopy for 1 : 1 and 2 : 1 complexes, respectively. These binding constants in PBS and the eight-fold excess of CBs minimize the presence of free dye in solution and also stabilize the host-guest complex in the culture medium. Images showing that the CB complexes can cross the cell membrane of 3T3 cells have been acquired using fluorescence microscopy. Given the current importance of supramolecular CB complexes and the search for new drug delivery systems, the present findings open avenues for the use of CBs as nanocapsules to transport drugs into the cells.

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.

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.

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.

Specific binding effects for cucurbit[8]uril in 2,4,6-triphenylpyrylium-cucurbit[8]uril host-guest complexes: observation of room-temperature phosphorescence and their application in electroluminescence.

2,4,6-Triphenylpyrylium (TP(+)) forms host-guest complexes with cucurbiturils (CBs) in acidic aqueous solutions. (1)H NMR spectroscopic data indicates that complexation takes place by encapsulation of the phenyl ring at the four position within CB. Formation of the complex with CB[6] and CB[7] leads to minor shifts in the fluorescence wavelength maximum (lambda(fl)) or quantum yield (Phi(fl)). In sharp contrast, for complexes with CB[8], the emission results in the simultaneous observation of fluorescence (lambda(fl)=480 nm, Phi(fl)=0.05) and room-temperature phosphorescence (lambda(ph)=590 nm, Phi(ph)=0.15). The occurrence of room-temperature phosphorescence can be used to detect the presence of CB[8] visually in solution. Molecular modeling and MM2 molecular mechanics calculations suggest that this effect arises from locking the conformational mobility of the 2- and 6-phenyl rings as a result of CB[8] encapsulation. The remarkably high room-temperature phosphorescence quantum yield of the TP(+)@CB[8] complex has been advantageously applied to develop an electroluminescent cell that contains this host-guest complex. In contrast, analogous cells prepared with TP(+) or TP(+)@CB[7] fail to exhibit electroluminescence.

Gold nanoparticles in organic capsules: a supramolecular assembly of gold nanoparticles and cucurbituril.

Gold nanoparticles (