Theoretical simulation of CdTe nanocrystals in aqueous synthesis

At the B3LYP/LANL2DZ theoretical level, Cd₃Te₃, (Cd₃Te₃)₂, (Cd₃Te₃)₃, Cd₄Te₄, and Te–Cd–ligand clusters were optimized. Firstly, hexagon Cd₃Te₃ and tetrahedron Cd₄Te₄ structures (with T_D symmetry) may be the minimum units of CdTe nanocrystals. They have similar conformations with the experimental wurtzite and zinc blende structures, respectively. Secondly, the frequencies of calculated Raman peaks of four clusters appear in about 140 cm⁻¹, which is close to the experimental data. Following, analysis of Te–Cd–ligand molecules elucidates that all our ligands have similar effect to CdTe structure, because the main influence of ligands comes from thiol, which is also the result of experiment. Finally, considering the influence of solvent and ligand, we believe that our wavelengths of absorption peaks which are calculated using the time-dependent density functional theory are perfectly identical with those of CdTe nanocrystals, according to quantum size effect. Moreover, we have testified that all these absorption peaks are the transition from d to p orbitals.     

Studies of optical and structural properties of CdSe/polymer nanocomposites: evidence of charge transfer and photostability

In this work, the role of conducting [poly (p-phenylinevinylene) (PPV)] and nonconducting (polystyrene) polymers on the properties of their respective composites with CdSe quantum dots of varied sizes has been investigated. The emission and structural properties of polymer–CdSe composites are found to be dependent on the crystallite size and morphology of CdSe nanocrystallites. Smaller CdSe quantum dots (size, ∼5 nm) ensures efficient charge transfer process across polymer–CdSe interface as evident by almost complete quenching of photoluminescence (PL) emission as compared to larger CdSe quantum dots (size, ∼7 nm). Presence of residual trioctylphosphine (TOP)/ tri-n-octylphosphine-oxide (TOPO) species and agglomeration of particles act as a hindrance for quenching of emission and hence charge transfer for larger CdSe nanocrystallites. Emission studies indicated an increased conjugation length for PPV polymers in different solvents (toluene, pyridine) and in solid state. Nonconducting polymer polystyrene shows charge transfer across polymer–CdSe interface as well. However, polystyrene polymer has a shorter chain length, which ensures maximum coverage on the surface of CdSe nanocrystallites and provides better photostability to CdSe QDs within the polymer matrix as compared to that for PPV–CdSe nanocomposites.     

Role of surface modification of colloidal CdSe quantum dots on the properties of hybrid organic–inorganic nanocomposites

In this work, tri-octyl phosphine/tri-octyl phosphine oxide (TOPO)-capped cadmium selenide (CdSe) quantum dots (QDs) of varied sizes (5–9 nm), prepared by varying the input Cd:Se precursor ratio using chemical route, were dispersed in conducting polymer matrices viz. poly[2-methoxy, 5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly(3-hexylthiophene) (P3HT). By using a binary solvent mixture (pyridine–chloroform), homogeneous dispersion of CdSe nanocrystals in polymers (MEH-PPV, P3HT) could be realized. The properties of the resulting dispersions could be tailored by the composition and concentration of QDs in polymer. The emission and structural properties of polymer–CdSe nanocomposites are found to be dependent on the crystallite size and morphology of CdSe nanocrystallites. An effective quenching of photoluminescence emission in the polymer nanocomposite was observed for smaller CdSe quantum dots (size ∼6 nm) as compared to larger CdSe quantum dots (size ∼9 nm), thus ensuring efficient charge transfer process across the polymer–CdSe interface in the former case. The incomplete quenching, particularly for MEH-PPV:CdSe nanocomposites, could be as a result of insufficient coverage of polymers on the surface of CdSe nanocrystallites, mainly due to phase segregation for TOPO-stripped CdSe nanocrystallites. The superior morphology and optical properties of polymer nanocomposite (P3HT:CdSe QDs) could play a pivotal role for the realization of effective charge separation and transport in hybrid solar cells.     

Constraints in post-synthesis ligand exchange for hybrid organic (MEH-PPV)–inorganic (CdSe) nanocomposites

For an optimum charge/energy transfer performance of hybrid organic–inorganic colloidal nanocrystals for applications such as photonic devices and solar cells, the determining factors are the distance between the nanocrystal and polymer which greatly depends upon nanocrystal size/nanocrystal ligands. Short chain ligands are preferred to ensure a close contact between the donor and acceptor as a result of the tunnelling probability of the charges and the insulating nature of long alkyl chain molecules. Short distances increase the probability for tunnelling to occur as compared to long distances induced by long alkyl chains of bulky ligands which inhibit tunnelling altogether. The ligands on the as-synthesized nanocrystals can be exchanged for various other ligands to achieve desirable charge/energy transfer properties depending on the bond strength of the ligand on the nanocrystal compared to the replacement ligand. In this work, the constraints involved in post-synthesis ligand exchange process have been evaluated, and these factors have been tuned via wet chemistry to tailor the hybrid material properties via appropriate selection of the nanocrystal capping ligands. It has been found that both oleic acid and oleylamine (OLA)-capped cadmium selenide (CdSe) quantum dots (QDs) as compared with trioctylphosphine oxide (TOPO)-passivated CdSe QDs are of high quality, and they provide better steric stability against coagulation, homogeneity, and photostability to their respective polymer:CdSe nanocomposites. CdSe QDs particularly with OLA capping have relatively smaller surface energies, and thus, lesser quenching capabilities show dominance of photoinduced Forster energy transfer between donors (polymer) and acceptors (CdSe nanocrystals) as compared to charge transfer mechanism as observed in polymer:CdSe (TOPO) composites. It is conjectured that size quantization effects, stereochemical compatibility of ligands (TOPO, oleic acid, and oleyl amine), and polymer MEH-PPV stability greatly influence the photophysics and photochemistry of hybrid polymer–semiconductor nanocomposites.     

Temperature dependence and change over time of the spectral properties of CdSe/ZnS–organic dye nanocomposite quantum dots in solution and single nanostructures

A rearrangement of a surface capping layer consisting of trioctylphosphine oxide or long-chain amines was discovered in a spectral and kinetic study of CdSe/ZnS quantum dot semiconductors at 295–77 K. This rearrangement is more pronounced in nanocomposites with dye molecules such as porphyrins and perylene diimides, affecting the nonradiative exciton relaxation and exciton–phonon coupling. Spatially-resolved spectroscopy was used to find that the addition of one or several dye molecules in single nanocomposites leads to redistribution and/or generation of surface trap states, which is seen in a change in their spectrokinetic parameters over time.     

Features of the influence of stabilizing ligands on luminescence properties of cadmium selenide colloidal quantum dots

It has been shown that immediately after the synthesis, the luminescence efficiency of CdSe quantum dots stabilized by n-octadecylphosphonic acid together with one of the auxiliary ligands decreases in the order: oleylamine, hexadecylamine, trioctylphosphine oxide, 1-octadecene, and stearic acid, which is due to combination of the energy of their binding with surface atoms of the nanoparticles and the packing density of the ligands in the shell. In the course of post-synthetic ripening of the quantum dots in a solution, changes in their luminescence quantum yield occur, depending on the solvent polarity and due to rearrangement of the ligands in the shells. The effect of dark recovery of trap luminescence from UV-irradiated quantum dots stabilized by octadecylphosphonic acid and a long-chain amine has been found.     

Rational design,synthesis and biological evaluations of amino-noscapine: a high affinity tubulin-binding noscapinoid

Noscapine and its derivatives are important microtubule-interfering agents shown to have potent anti-tumor activity. The binding free energies (ΔG _bind) of noscapinoids computed using linear interaction energy (LIE) method with a surface generalized Born (SGB) continuum solvation model were in agreement with the experimental ΔG _bind with average root mean square error of 0.082 kcal/mol. This LIE–SGB model guided us in designing a novel derivative of noscapine, amino-noscapine [(S)-3-((R)-9-amino-4-methoxy-6-methyl-5,6,7,8-tetrahydro [1, 3] dioxolo[4,5-g]isoquinolin-5-yl)-6,7-dimethoxy isobenzo-furan-1(3H)-one] that has higher tubulin binding activity (predicted ΔG _bind = −6.438 kcal/mol and experimental ΔG _bind = −6.628 kcal/mol) than noscapine, but does not significantly change the total extent of the tubulin subunit/polymer ratio. The modes of interaction of amino-noscapine with the binding pocket of tubulin involved three hydrogen bonds and are distinct compared to noscapine which involved only one hydrogen bond. Also the patterns of non-bonded interactions are albeit different between both the lignads. The ‘blind docking’ approach (docking of ligand with different binding sites of a protein and their evaluations) as well as the reasonable accuracy of calculating ΔG _bind using LIE–SGB model constitutes the first evidence that this class of compounds binds to tubulin at a site overlapping with colchicine-binding site or close to it. Our results revealed that amino-noscapine has better anti-tumor activity than noscapine.     

In silico inspired design and synthesis of a novel tubulin-binding anti-cancer drug: folate conjugated noscapine (Targetin)

Our screen for tubulin-binding small molecules that do not depolymerize bulk cellular microtubules, but based upon structural features of well known microtubule-depolymerizing colchicine and podophyllotoxin, revealed tubulin binding anti-cancer property of noscapine (Ye et al. in Proc Natl Acad Sci USA 95:2280–2286, 1998). Guided by molecular modelling calculations and structure–activity relationships we conjugated at C9 of noscapine, a folate group—a ligand for cellular folate receptor alpha (FRα). FRα is over-expressed on some solid tumours such as ovarian epithelial cancers. Molecular docking experiments predicted that a folate conjugated noscapine (Targetin) accommodated well inside the binding cavity (docking score −11.295 kcal/mol) at the interface between α- and β-tubulin. The bulky folate moiety of Targetin is extended toward lumen of microtubules. The binding free energy (ΔG _bind) computed based on molecular mechanics energy minimization was −221.01 kcal/mol that revealed favourable interaction of Targetin with the receptor. Chemical synthesis, tubulin-binding experiments, and anti-cancer activity in vitro corroborate fully well with the molecular modelling experiments. Targetin binds tubulin with a dissociation constant (K _d value) of 149 ± 3.0 μM and decreases the transition frequencies between growth and shortening phases of microtubule assembly dynamics at concentrations that do not alter the total polymer mass. Cancer cells in general were more sensitive to Targetin compared with the founding compound noscapine (IC₅₀ in the range of 15–40 μM). Quite strikingly, ovarian cancer cells (SKOV3 and A2780), known to overexpress FRα, were much more sensitive to targetin (IC₅₀ in the range of 0.3–1.5 μM).     

The Effect of CdSe and InP Quantum Dots on the Interaction of ZnO with NO<Subscript>2</Subscript> under Visible Light Irradiation

Nanocomposites based on nanocrystalline ZnO and CdSe and InP nanocrystals (quantum dots) have been synthesized by chemical precipitation and high-temperature colloidal synthesis. The microstructure parameters of the oxide matrix and the size of the CdSe and InP nanocrystals have been determined. A correlation was established between the spectral dependence of the photoconductivity of nanocomposites and the optical absorption spectra of quantum dots. The influence of CdSe and InP quantum dots on the interaction of ZnO with NO₂ under visible light irradiation has been studied. It has been shown that the synthesized nanocomposites can be used to detect NO₂ under illumination with green light without additional thermal heating.     

Spectroscopic investigations on II–VI-semiconductor nanocrystals and their assemblies

This review points out that (magneto-)optical measurements may help to shine light on the recombination processes taking place in semiconductor nanocrystals. The surface capping with thiols creates a CdS shell around CdTe cores and forms a Cd site that is not fourfold-coordinated at the surface. It is pointed out how specific cappings such as thio-amines and thio-acids assist in coupling NCs and how we may distinguish between NC–NC interactions via electrostatic and covalent linking with the aid of the optical measurements. Furthermore, with static and time-resolved ODMR studies on IR-active core-shell HgTe/Hg _x Cd_1−x Te(S) particles it is demonstrated how the nature of the recombination emission being associated with a Cd–Hg mixed site is elucidated and by this yielding structural information on the NC core-shell interface. With these examples we show that and how nanomaterials of probable technological interest are studied beneficially with advanced spectroscopic techniques.     

A novel fluorescent assay for edaravone with aqueous functional CdSe quantum dots

Aqueous thiol-capped CdSe QDs with a narrow, symmetric emission were prepared under a low temperature. Based on the fluorescence enhancement of thiol-stabilized CdSe quantum dots (QDs) caused by edaravone, a simple, rapid and specific quantitative method was proposed to the edaravone determination. The concentration dependence of fluorescence intensity followed the binding of edaravone to surface of the thiol-capped CdSe QDs was effectively described by a modified Langmuir-type binding isotherm. Factors affecting the fluorescence detection for edaravone with thiol-stabilized CdSe QDs were studied, such as the effect of pH, reaction time, the concentration of CdSe QDs and so on. Under the optimal conditions, the calibration plot of C/(I − I₀) with concentration of edaravone was linear in the range of (1.45–17.42) μg/mL (0.008–0.1 μmol/L) with correlation coefficient of 0.998. The limit of detection (LOD) (3σ/κ) was 0.15 μg/mL (0.0009 μmol/mL). Possible interaction mechanism was discussed.     

A rapid and universal bacteria-counting approach using CdSe/ZnS/SiO2 composite nanoparticles as fluorescence probe

In this paper, a rapid, simple, and sensitive method was described for detection of the total bacterial count using SiO₂-coated CdSe/ZnS quantum dots (QDs) as a fluorescence marker that covalently coupled with bacteria using glutaraldehyde as the crosslinker. Highly luminescent CdSe/ZnS were prepared by applying cadmium oxide and zinc stearate as precursors instead of pyrophoric organometallic precursors. A reverse-microemulsion technique was used to synthesize CdSe/ZnS/SiO₂ composite nanoparticles with a SiO₂ surface coating. Our results showed that CdSe/ZnS/SiO₂ composite nanoparticles prepared with this method possessed highly luminescent, biologically functional, and monodispersive characteristics, and could successfully be covalently conjugated with the bacteria. As a demonstration, it was found that the method had higher sensitivity and could count bacteria in 3 × 10² CFU/mL, lower than the conventional plate counting and organic dye-based method. A linear relationship of the fluorescence peak intensity (Y) and the total bacterial count (X) was established in the range of 3 × 10²–10⁷ CFU/mL using the equation Y = 374.82X − 938.27 (R = 0.99574). The results of the determination for the total count of bacteria in seven real samples were identical with the conventional plate count method, and the standard deviation was satisfactory.     

Photocatalytic Hydrogen Evolution by Oleic Acid‐Capped CdS,CdSe, and CdS0.75Se0.25 Alloy Nanocrystals

Photocatalytic generation of hydrogen by using oleic acid‐capped CdS, CdSe, and CdS_0.75Se_0.25 alloy nanocrystals (quantum dots) has been investigated under visible‐light irradiation by employing Na₂S and Na₂SO₃ as hole scavengers. Highly photostable CdS_0.75Se_0.25 alloy nanocrystals gave the highest hydrogen evolution rate (1466 μmol h^?1 g^?1), which was about three times higher than that of CdS and seven times higher than that of CdSe.     

Organized quantum dot array through nanochemistry

Well defined tetrahedral cadmium sulfide nanocrystals were made in a rational way by organometallic chemical synthesis. Due to the fair degree of the ionic character in Cd—S bond, the sulfide S^2– can be replaced by the organothiolate RS^– without disrupting the CdS lattice structure. These ligands were delicately chosen to fabricate anisotropically capped nanocrystals. During solvent evaporation, these smart dots have the property to self-connect in a head-to-tail alignment leading to a new fibrous polymeric dot material. These quantum microcrystallites can be processed to make powder, free standing dots or optically transparent and anisotropic films. Optical spectroscopy, X-ray diffraction and electron microscopy have been used to characterize this organized quantum dot array.     

Stable α-heteroatom-free dialkylcarbenes: a DFT study

The stability of the singlet dicyclopropylcarbene (ΔE _S–T = 15.3 kcal/mol, 1) is increased not only by cyclization to 2,5-dicyclopropylcyclopentanylidene (ΔE _S–T = 20.3 kcal/mol, 2), but even more so by unsaturation to 2,5-dicyclopropylcyclopentenylidene (ΔE _S–T = 22.5 kcal/mol, 3). In a further attempt to pave the way toward synthesis of new stable dialkylcarbenes, we introduced different substituents on the α-cyclopropyls of 3, where the stability was increased over twice of 1 (ΔE _S–T = 37.8 kcal/mol) for 2,5-bis(2,3-dihydroxycyclopropyl)-3,4-dinitrocyclopentenylidene, 3_{\textOH-\textNO 2} {\mathbf{3}}_{{{\text{OH}}{-}{\text{NO}}_{ 2}}} .     

On Stabilization of Colloidal Quantum Dots of Cadmium Selenide in the Presence of Octadecylphosphonic Acid

Classical molecular dynamics has been used to study the stabilization of colloidal quantum dots of CdSe with n-octadecylphosphonic acid molecules in combination with different auxiliary ligands, such as trioctylphosphine, trioctylphosphine oxide, and hexadecylamine. The effects of different ligands that may be formed due to interactions of n-octadecylphosphonic acid with CdO and Se in an initial mixture have been considered. It has been shown that, among these ligands, the stabilizing effect increases with a rise in the charge of a ligand per n-octadecyl chain. The role of the auxiliary ligands, e.g. octadecene that devoid of functional groups, has been studied.     

Theoretical Study of Sticking Processes on Molecular Models of Silica Surfaces

The adsorption of small charged and neutral molecules on silica supports was modelled using perturbative post-Hartree–Fock quantum chemical methods (MP2 and MP4). The simplest spherosiloxane compound (H₄Si₄O₆) was used to mimic the surface while several molecules (namely CH₄, NH ₄ ⁺ , NH₃, OH ₃ ⁺ H ₃ ⁺ ) were considered as adsorbed species. Direct sticking of the molecules on one of the (Si–O)₃ ring leads to very different binding energies for cations (more than 11 kcal/mol) and neutral molecules (a few kcal/mol). These results indicate a dominant strong ion–multipole interaction for the first ones and a weak dispersion-type interaction for the latter. If the spherosiloxane cluster is screened by a mantle of accreted dust as it is the case in interstellar environment, the value of the binding energies, computed using the continuum dielectric theory, are predicted to be significantly reduced.     

Effect of P-Containing Ligands on the Structural and Optical Properties of (CdSe)<Subscript>n</Subscript> (n = 3, 6, 10) Clusters

The effect of phosphorus-containing ligands on the structure, energetics and properties of the (CdSe)_n clusters (n = 3, 6, and 10) with different number of PH₃ and PMe₃ ligands were studied by using density functional theory calculations. The P atom in the ligand interacts with Cd and forms a strong Cd–P coordination bond. The introduction of ligands does not change the cluster architecture, but leads to considerable changes in Cd–Se bondlength, charge distribution, binding energy, HOMO–LUMO gap and optical absorption. The ligand influence is enhanced with increasing ligand coverage. A blueshift in absorption band was predicted for the clusters with increasing ligands, resulting from the electron donating characteristics of the ligands that hamper electron transition from Se to Cd. As P-containing ligands are often used in the preparation of CdSe nanocrystals, our calculations reveal the influence of ligand-cluster interaction on the cluster geometrical and electronic properties, which would be helpful for the nanocrystal design and synthesis.     

Theoretical study of the neutral hydrolysis of methyl formate via a concerted and stepwise water-assisted mechanism using free-energy curves and molecular dynamics simulation

A procedure previously described by us is used for the theoretical study of chemical reactions in solution by means of molecular dynamics simulation, with solute–solvent interaction potentials LJ (12-6-1) derived from ab initio quantum calculations. We apply the procedure to the case of the neutral hydrolysis of methyl formate, HCOOCH₃ + 3H₂O → HCOOH + CH₃OH + 2H₂O in aqueous solution, via concerted and stepwise water-assisted mechanisms. We use the solvent as reaction coordinate, and the free-energy curves for the calculation of the activation energies. The theoretical calculation for the thermodynamics of this hydrolysis reaction in aqueous solution, assisted by three water molecules, is in agreement with the available experimental information. In particular our study gives values of ΔG ^≠ = 28.88 and 28.17 kcal/mol for the concerted and stepwise mechanisms, close to the experimental activation barrier of 28.8 kcal/mol, and a significant improvement over the values of 48.05 and 45.66 kcal/mol found in another similar study using the PCM model.