Structural and Size Effects on the Spectroscopic and Redox Properties of CdSe Nanocrystals in Solution: The Role of Defect States

Two series of CdSe quantum dots (QDs) with different diameters are prepared, according to frequently used protocols of the same synthetic procedure. For each sample the photophysical properties and the potentials for the first reduction and oxidation processes in organic solution are determined. The band gap obtained from electrochemical experiments is compared with that determined from the absorption and luminescence spectra. While the optical band gap decreases upon increasing the nanocrystal diameter, as expected on the basis of quantum confinement, the redox potentials and the electrochemical band gap are not monotonously related to the QD size. For both series, the smallest and largest QDs are both easier to oxidize and reduce than mid‐sized QDs. In fact, the latter samples exhibit very broad voltammetric profiles, which suggests that the heterogeneous electron‐transfer processes from/to the electrode are kinetically hindered. Conversely, the electrochemical band gap for the smallest and largest particles of each series is somewhat smaller than the optical band gap. These results indicate that, while the optical band gap depends on the actual electron–hole recombination within the nanocrystal, and therefore follows the size dependence expected from the particle‐in‐a‐box model, the electrochemical processes of these QDs are strongly affected by other factors, such as the presence of surface defects. The investigations suggest that the influence of these defects on the potential values is more important for the smallest and largest QDs of each series, as confirmed by the respective luminescence bands and quantum yields. An interpretation for the size‐dependent evolution of the surface defects in these nanocrystals is proposed based on the mechanism of their formation and growth.     

Development of a CD‐MEKC method for investigating the metabolism of tamoxifen by flavin‐containing monooxygenases and the inhibitory effects of methimazole,nicotine and DMXAA

A selective and low‐cost CD‐MEKC method under acidic conditions was developed for investigating the N‐oxygenation of tamoxifen (TAM) by flavin‐containing monooxygenases (FMOs). The inhibitory effects of methimazole (MMI), nicotine and 5,6‐dimethylxanthenone‐4‐acetic acid (DMXAA) on the given FMO reaction were also evaluated; 100 mM phosphate buffer (pH 8.6) was used for performing the enzymatic reaction and the separation of TAM and its metabolite tamoxifen N‐oxide (TNO) was obtained with a BGE consisting of 100 mM phosphoric acid solution adjusted to pH 2.5 with triethanolamine containing 50 mM sodium taurodeoxycholate, 20 mM carboxymethyl β‐CD and 20% ACN. The proposed method was applied for the kinetics study of FMO1 using TAM as a substrate probe. A Michaelis–Menten constant (K_m) of 164.1 μM was estimated from the corrected peak area of the product, TNO. The calculated value of the maximum reaction velocity (V_max) was 3.61 μmol/min/μmol FMO1; 50% inhibitory concentration and inhibition constant (K_i) of MMI, the most common alternate substrate FMO inhibitor, were evaluated and the inhibitory effects of two other important FMO substrates, nicotine and DMXAA, a novel anti‐tumour agent, were investigated.     

Intermolecular Magnetic Spin–Spin Interactions in Solution State at 1.53 mT

Overhauser dynamic nuclear polarization (DNP) technique can provide a dramatic increase in the signal obtained from nuclear magnetic resonance experiments owing to the magnetic spin–spin interactions between ¹H nuclei of the solvent and electrons delocalized on the asphaltene in crude petroleum or asphalt. Studies on ¹H Overhauser DNP enhancements at 1.53 mT are reported for benzene solvent medium with three different radical sources: Iran crude petroleum, MC30 liquid asphalt, and MC800 liquid asphalt for a range of radical concentrations. The results show that protons of benzene are good detectors for dipolar coupling.     

Synthesis, IR spectral studies and quantum-chemical calculations on 1,2-dihydronaphto[1,2-e]oxazine-3-thiones and 3,4-dihydrobenzo[e][1,3]oxazine-2-thione

2-Hydroxy-1-naphthaldehyde (1) was reacted with substituted anilines to afford 1-(substituted phenyliminomethyl)naphthalen)-2-ols (2). The reduction of these imines by NaBH₄ gave 1-((substituted phenylaminomethyl)naphthalen)-2-ols (3) which were cyclized with thiophosgene to give corresponding 2-substituted phenyl-1,2-dihydronaphto[1,2-e]oxazine-3-thiones (4). 3-p-Tolyl-3,4-dihydrobenzo[e][1,3]oxazine-2-thione (8) was also obtained by the same way. The structures of these new compounds were determined by ¹H NMR, IR spectroscopic data and elemental analyses. AM1, PM3 and ab initio (at Hartree–Fock level with 3-21G basis set) methods were used to study the molecular geometry of the compounds. A complete infrared spectral analysis of the oxazines has been performed in this paper. Observed frequencies of the molecules were compared with calculated normal mode analysis which was carried out on the basis of RHF/3-21G method. Assignments of vibrational bands (in the range of 1760–400 cm⁻¹) have been performed by taking into account the results of the ab initio vibrational analysis. The mechanism of the cyclization reaction between (3a) and thiophosgene was studied by the semi-empirical AM1 and ab initio (RHF) calculations.     

Determination and correlation of liquid–liquid equilibria for the (water + carboxylic acid + dimethyl maleate) ternary systems at T = 298.2 K

(Liquid–liquid) equilibrium (LLE) data for the ternary systems of {water + carboxylic acid (formic, acetic, propionic or butyric acid) + dimethyl maleate} were measured at T = 298.2 K and atmospheric pressure. Selectivity values for solvent separation efficiency were derived from the tie-line data. A comparison of the extracting capabilities of the solvent was made with respect to distribution coefficients, separation factors, and solvent-free selectivity bases. The reliability of the data was ascertained from Othmer–Tobias plots. The experimental data were correlated using the UNIQUAC and NRTL (α = 0.2) equations, and the binary interaction parameters were reported. The phase diagrams for the ternary mixtures including both the experimental and calculated tie-lines were presented.     

Poly(p‐phenylene) with Poly(ethylene glycol) Chains and Amino Groups as a Functional Platform for Controlled Drug Release and Radiotherapy

Conventional cancer treatments such as chemotherapy, radiotherapy, or combination of these two result in side effects, which lower the quality of life of the patients. To overcome problems with these methods, altering the drug properties by conjugating them to carrier polymers has emerged. Such polymeric carriers also hold the potential to make tumor cells more sensitive to radiation therapy. Herein, poly(p‐phenylene) (PPP) polymer with poly(ethylene glycol) (PEG) chains and primary amino groups (PPP‐NH₂‐g‐PEG) is synthesized and conjugated with anticancer drug Doxorubicin (DOX). pH dependent drug release experiments are performed at pH 5.3 and pH 7.4, respectively. Cell viability studies on human cervix adenocarcinoma cells show that lower doses of DOX inhibit cell proliferation when conjugated with nontoxic doses of PPP‐NH₂‐g‐PEG polymer. Additionally, PPP‐NH₂‐g‐PEG/Cys/DOX bioconjugate significantly increases radiosensitive properties of DOX. It is possible to use lower doses of DOX when conjugated to PPP‐NH₂‐g‐PEG in combination with radiotherapy.

     

A Bifunctional Photosensitizer for Enhanced Fractional Photodynamic Therapy: Singlet Oxygen Generation in the Presence and Absence of Light

The photosensitized generation of singlet oxygen within tumor tissues during photodynamic therapy (PDT) is self‐limiting, as the already low oxygen concentrations within tumors is further diminished during the process. In certain applications, to minimize photoinduced hypoxia the light is introduced intermittently (fractional PDT) to allow time for the replenishment of cellular oxygen. This condition extends the time required for effective therapy. Herein, we demonstrated that a photosensitizer with an additional 2‐pyridone module for trapping singlet oxygen would be useful in fractional PDT. Thus, in the light cycle, the endoperoxide of 2‐pyridone is generated along with singlet oxygen. In the dark cycle, the endoperoxide undergoes thermal cycloreversion to produce singlet oxygen, regenerating the 2‐pyridone module. As a result, the photodynamic process can continue in the dark as well as in the light cycles. Cell‐culture studies validated this working principle in vitro.     

pH-sensitive ligand for luminescent quantum dots

We developed a strategy to switch the luminescence of semiconductor quantum dots with chemical stimulations. It is based on the photoinduced transfer of either energy from CdSe-ZnS core-shell quantum dots to [1,3]oxazine ligands or electrons from the organic to the inorganic components. The organic ligands incorporate a dithiolane anchoring group, an electron-rich indole, and a 4-nitrophenylazophenoxy chromophore in their molecular skeleton. Their adsorption on the surface of the quantum dots results in partial luminescence quenching. Electron transfer from the indole fragment to the nanoparticles is mainly responsible for the decrease in luminescence intensity. Upon addition of base, the [1,3]oxazine ring of the ligands opens to generate a 4-nitrophenylazophenolate chromophore, which absorbs in the range of wavelengths where the quantum dots emit. This transformation activates an energy-transfer pathway from the excited nanoparticles to the ligands. In addition, the oxidation potential of the ligand shifts in the negative direction, improving the efficiency of electron transfer. The overall result is a decrease in the luminescence quantum yield of 83%. Addition of acid also opens the [1,3]oxazine ring of the ligands. However, the resulting 4-nitrophenylazophenol does not absorb in the visible region and cannot accept energy from the excited nanoparticles. Furthermore, the oxidation potential shifts in the positive direction, lowering the electron-transfer efficiency. In fact, the luminescence quantum yield increases by 33% as a result of this transformation. These changes are fully reversible and can be exploited to probe the pH of aqueous solutions from 3 to 11. Indeed, our sensitive quantum dots adjust their luminescence in response to variations in pH within this particular range of values. Thus, our general design strategy can eventually lead to the development of pH-sensitive luminescent probes for biomedical applications based on the unique photophysical properties of semiconductor quantum dots.