Raquel E. Galian

Nontemplate Synthesis of CH3NH3PbBr3 Perovskite Nanoparticles

To date, there is no example in the literature of free, nanometer-sized, organolead halide CH3NH3PbBr3 perovskites. We report here the preparation of 6 nm-sized nanoparticles of this type by a simple and fast method based on the use of an ammonium bromide with a medium-sized chain that keeps the nanoparticles dispersed in a wide range of organic solvents. These nanoparticles can be maintained stable in the solid state as well as in concentrated solutions for more than three months, without requiring a mesoporous material. This makes it possible to prepare homogeneous thin films of these nanoparticles by spin-coating on a quartz substrate. Both the colloidal solution and the thin film emit light within a narrow bandwidth of the visible spectrum and with a high quantum yield (ca. 20%); this could be advantageous in the design of optoelectronic devices.

Maximizing the emissive properties of CH3NH3PbBr3 perovskite nanoparticles

Highly luminescent and photostable CH3NH3PbBr3 nanoparticles have been prepared by fine-tuning the molar ratio between CH3NH3Br, PbBr2, a medium-size alkyl-chain ammonium salt, and 1-octadecene. The nanoparticles exhibit an excellent photoluminescence quantum yield (ca. 83%) and average recombination lifetime (ca. 600 ns) in toluene dispersion.

Non-linear effects in the quenching of fluorescent quantum dots by nitroxyl free radicals

Quenching of quantum dot luminescence by nitroxides is extremely non-linear and dependent on nanoparticle size, being most effective for the smaller particles; it is proposed to involve an electron exchange mechanism.

Blue-luminescent organic lead bromide perovskites: highly dispersible and photostable materials

The preparation of a blue-luminescent and photostable organic–inorganic hybrid perovskite with an X-ray powder diffraction spectrum consistent with a two-dimensional inorganic framework is reported. This perovskite can be produced with a high reaction yield and valuable optical properties, such as luminescence quantum yield over 20%, radiative rate constant of up to 80 × 106 s−1, and high photostability under UV light. This material remains stable as a solid, is toluene-dispersible, and can be reverted reversibly into its precursors by using dimethylformamide (DMF). Moreover, the DMF dispersion can be injected into toluene to produce a nanomaterial or be used to prepare films by spin-coating on a substrate; both, the nanomaterial and the film exhibit practically the same optical features as the initial perovskite.

Photoluminescence Enhancement of CdSe Quantum Dots: A Case of Organogel–Nanoparticle Symbiosis

Highly fluorescent organogels (QD-organogel), prepared by combining a pseudopeptidic macrocycle and different types of CdSe quantum dots (QDs), have been characterized using a battery of optical and microscopic techniques. The results indicate that the presence of the QDs not only does not disrupt the supramolecular organization of the internal fibrillar network of the organogel to a significant extent, but it also decreases the critical concentration of gelator needed to form stable and thermoreversible organogels. Regarding the photophysical properties of the QDs, different trends were observed depending on the presence of a ZnS inorganic shell around the CdSe core. Thus, while the core-shell QDs preserve their photophysical properties in the organogel medium, a high to moderate increase of the fluorescence intensity (up to 528%) and the average lifetime (up to 1.7), respectively, was observed for the core QDs embedded in the organogel. The results are relevant for the development of luminescent organogels based on quantum dots, which have potential applications as advanced hybrid materials in different fields.

Catalytic processes activated by light

Light-driven catalytic strategies are of great interest for the synthesis of useful products causing minimal environmental impact. Nature provides many examples of systems working in processes activated by light, and understanding them has been and, in the future, will be useful in the design of artificial catalysts.

Cyclodextrin enhanced fluorimetric method for the determination of tryptamine

The effect of native cyclodextrins (alpha-, beta- or gamma- with six, seven or eight glucose units, respectively), hydroxypropyl-beta-cyclodextrin, beta-cyclodextrin solubilized in urea, soluble starch and glucose in water solution on the fluorescence behaviour of tryptamine [3-(2-aminoethyl)indole] (T) was determined. In addition, the effect of methanol and propanol with and without beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin was ascertained. From the fluorescence changes with pH and with beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin, the values of the pKa of the ground and excited states and the association constants of T and TH [3-(2-ammoniumethyl)indole] with the two hosts were determined. The values are pKa = 9.5 +/- 0.2 and pKa* = 8.4 +/- 0.2; KAssocTH = (1.6 +/- 0.3) x 10(2) mol-1 dm3 and KAssocT = (2.8 +/- 0.3) x 10(2) mol-1 dm3 with beta-cyclodextrin, KAssocTH = (1.8 +/- 0.5) x 10(2) mol-1 dm3 and KAssocT = (4.9 +/- 0.9) x 10(2) mol-1 dm3 with hydroxypropyl-beta-cyclodextrin. The ratio of the fluorescence quantum yields for the bound and free substrate (phi b/phi f) were in the range 1.25-1.33. The detection limit for the better conditions where the host-guest interactions produce fluorescence enhancement was 0.454 +/- 0.002 ng ml-1 for the complex T-hydroxypropyl-beta-cyclodextrin in water. The method is simpler than others reported previously.

Highly fluorescent and photostable organic- and water-soluble CdSe/ZnS core-shell quantum dots capped with thiols

Highly fluorescent organic- and water-soluble CdSe/ZnS core-shell quantum dots (QDs) with thiol ligands chemisorbed on the QD surface were synthesized by the replacement of amine ligands by alkyl thiols under very mild conditions. The QDs exhibited an even greater photostability than the initial core-shell amine capped QDs.

Sensing Chiral Drugs by Using CdSe/ZnS Nanoparticles Capped with N‐Acetyl‐L‐Cysteine Methyl Ester

Chiral quantum dots (QDs), differing in their core or shell size and, consequently, in their optical properties, were synthesized by the treatment of commercially available amine-capped quantum dots with methyl ester N-acetyl-L-cysteine (CysP). Interestingly, their colloidal methanol solutions remain stable for several months. Their NMR and IR spectra were in accordance with CysP binding to the QD surface through two anchoring groups; its thiolate (strongly bound) and the carbonyl group of its ester (weaker bound) group, whereas their circular dichroism (CD) spectra showed a new broad redshifted band, suggesting that the attachment to the QD surface modified the conformational equilibrium towards conformer(s) with optical activity in this region. These QDs were sufficiently fluorescent to perform studies of the chiral recognition of drugs, in particular the aryl propionic acids (APAs) ketoprofen (KP), naproxen (NP), flurbiprofen (FP), and ibuprofen (IP). We used different drug concentration ranges, depending on the QD solubility. All the assayed drugs quenched the QD emission in a concentration-dependent mode. Quenching fluorescence assays with the chiral QDs (CS@CysP) showed their extraordinary capacity for the chiral recognition of KP, NP, and FP, and particularly in the case of KP and FP, a remarkable positive allosteric effect was detected for the R enantiomer. By using a drug/CS@CysP molar ratio of 5000:1 and 2500:1, the changes of intensity and the sign of the CD spectrum of the drug evidenced the dissociation of the drug carboxylic group in the presence of the QD.

Hydroxypropyl-β-cyclodextrin enhanced fluorimetric method for the determination of melatonin and 5-methoxytryptamine

The effects of native cyclodextrins (alpha, beta or gamma), hydroxypropyl-beta-cyclodextrin, beta-cyclodextrin solubilized in urea, soluble starch and glucose in water solution on the fluorescence behaviour of melatonin (N-acetyl-5-methoxytryptamine) (M) and 5-methoxytryptamine [5-methoxy-3-(2-aminoethyl)indole] (5M) were determined. In addition, the effect of methanol and propanol with and without beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin was assessed. From the fluorescence changes with pH, the values of the pKa for the ground (9.9 +/- 0.2) and the excited state (7.7 +/- 0.2) for 5M were determined. From the fluorescence changes with beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin, the association constants of M, 5MH [5-methoxy-3-(2-ammoniumethyl)indole] and 5M with the two hosts were determined. The values with beta-cyclodextrin were KAssoc5MH = (1.4 +/- 0.4) x 10(2) mol-1 dm3, KAssoc5M = (1.6 +/- 0.1) x 10(2) mol-1 dm3 and KAssocM = (1.1 +/- 0.2) x 10(2) mol-1 dm3, and with hydroxypropyl-beta-cyclodextrin KAssoc5MH = (1.1 +/- 0.3) x 10(2) mol-1 dm3, KAssoc5M = (2.5 +/- 0.1) x 10(2) mol-1 dm3 and KAssocM = (1.51 +/- 0.07) x 10(2) mol-1 dm3. The ratios of the fluorescence quantum yields for the bound and free substrate (phi b/phi f) were in the range 1.15-1.48. The detection limits under the optimum conditions were 0.381 +/- 0.001 ng cm-3 for the complex 5MH-hydroxypropyl-beta-cyclodextrin in water and 0.290 +/- 0.001 ng cm-3 for the complex M-hydroxypropyl-beta-cyclodextrin in water with 5% of methanol. The recovery of melatonin from pharmaceutical preparations was 98-103% with an RSD of 2%. The recovery from rat pineals was also good. The method is direct, simple and accurate.