Study of Interfacial Charge Transfer from an Electron Rich Organic Molecule to CdTe Quantum Dot by using Stern-Volmer and Stochastic Kinetic Models.

Photoinduced electron transfer (PET) from N-methylaniline (NMA) to a photoexcited CdTe quantum dot (QD*) is studied in toluene. The PET mechanism at low to moderate quencher (NMA) concentrations (<0.08 M) remains mostly collisional with some contributions from QD-NMA complex formation. However, at high quencher concentrations (>0.10 M), QDs form larger numbers of static complexes with NMA molecules leading to a steep positive deviation in the steady-state Stern-Volmer curves. An isothermal titration calorimetry (ITC) study confirms the formation of QD-NMA complexes (K∼150 M-1 ) at high quencher concentrations. Fitting our experimental data using a stochastic kinetic model indicates that the number of NMA molecules attached per QD at highest NMA concentration (∼0.16 M) used in this study decreases from ∼0.76 to ∼0.47 with reducing the QD size from ∼5.2 nm to ∼3.2 nm. However, the PET rate increases with decreasing QD size, which is commensurate with the observation that the chemical driving force (ΔG) increases with decreasing the QD particle size. We have analyzed the PET kinetics mainly by using Stern-Volmer fittings. However, in some cases Tachiya's stochastic kinetic model is used for stoichiometric analysis, which seems to be useful only at high quencher concentrations. The measured PET rate coefficients in all the cases are found to be at least an order of magnitude lower when compared to the diffusion-controlled rate of the reaction medium.

Role of Emissive and Non-Emissive Complex Formations in Photoinduced Electron Transfer Reaction of CdTe Quantum Dots.

Bimolecular photoinduced electron transfer (PET) from excited state CdTe quantum dot (QD*) to an electron deficient molecule 2,4-dinitrotoluene (DNT) is studied in toluene. We observed two types of QD-DNT complex formations; (i) non-emissive complex, in which DNT is embedded deep inside the surface polymer layer of QD and (ii) emissive complex, in which DNT molecules are attached to QDs but approach to the QD core is shielded by polymer layer. Because of its non-emissive nature, the lifetime of QD is not affected by dark complex formation, though the steady-state emission is greatly quenched. However, emissive complex formation causes both, lifetime and steady-state emission quenching. In our fitting model, consideration of Poisson distribution of the attached quencher (DNT) molecules at QD surface enables a comprehensive fitting to our time resolved data. QD-DNT complex formation was confirmed by an isothermal titration calorimetry (ITC) study. Fitting to the time resolved data using a stochastic kinetic model shows moderate increase (0.05 ns-1 to 0.072 ns-1 ) of intrinsic quenching rate with increasing the QD particle size (from ≈3.2 nm to ≈5.2 nm). Our fitting also reveals that the number of DNT molecules attached to a single QD increases from ≈0.1-0.2 to ≈1.2-1.7, as the DNT concentration is increased from ≈1 mm to 17.5 mm. Complex formation at higher quencher concentration assures that the observed PET kinetics is a thermodynamically controlled process where solvent diffusion has no role on it.

Model-Free Estimation of Energy-Transfer Timescales in a Closely Emitting CdSe/ZnS Quantum Dot and Rhodamine†6G FRET Couple.

Analysis of Förster resonance energy transfer (FRET) in nanoparticles is often complicated by a number of factors, such as close emission band positions of the donor and acceptor, the presence of simultaneous photoinduced electron transfer, fluorescence blinking, and natural excited-state decay processes. To address these concerns, mostly from materials chemists and biologists, herein, a state-of-the-art FRET analysis method that utilizes the bright green emission of a CdSe/ZnS core/shell quantum dot (QD) is proposed. The uniqueness of this model-free FRET analysis demonstrates the potential of these QDs to be part of an efficient FRET assay in molecular ruler applications. Molecular distance calculations relying on the proposed FRET analysis complement nicely with the expected donor-to-acceptor separation distances; the QD, as a photoluminescent marker, is electrostatically attached to a compatible fluorescent dye, rhodamine 6G. The beneficial aspects of the model-free FRET analysis provide many possibilities, including the use of low cytotoxicity QD-based FRET assays as a next-generation molecular ruler for the accurate estimation of distances inside a biological system.

Spectroscopic and Calorimetric Studies of Molecular Recognitions in a Dendrimer-Surfactant Complex.

Molecular recognitions, causing supramolecular complex formation between a hyperbranched polymer molecule (polyamidoamine (PAMAM) dendrimer generation 3) with oppositely charged surfactant sodium dodecyl sulfate (SDS) in aqueous solution, were studied by using various spectroscopic techniques and calorimetric titration of heat change measurements. Spectroscopic measurements were performed using dynamic Stokes shift (DSS), rotational anisotropy decay, and translational diffusion of a fluorescent probe molecule coumarin 153 (C153) noncovalently attached to the dendrimer-surfactant complex. All these studies unanimously confirm that the critical aggregation concentration (CAC) of SDS falls to ∼0.8 mM (from its critical micelle concentration (CMC) ∼ 8 mM) in the presence of ∼0.2 mM dendrimer. Further studies of isothermal titration calorimetry (ITC) measurement show that the CAC of SDS in the presence of dendrimer remains invariant to the dendrimer concentration. Complexation reaction between SDS and dendrimer is highly exothermic in nature. A maximum heat release (ΔH∼ -6.6 kJ/mol of SDS binding) was observed at a SDS-to-dendrimer mole ratio of ∼3-5; where up to 3 to 5 SDS molecules were encapsulated by one dendrimer molecule to form dendrimer-SDS encapsulation complex. When negatively charged SDS was replaced with a positively charged surfactant dodecyl-trimethylammonium-bromide (DTAB), we found that the DTAB hardly interacted with positively charged dendrimer due to the charge-charge repulsions.