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1.
Nano Lett ; 2020 Mar 09.
Artigo em Inglês | MEDLINE | ID: mdl-32134676

RESUMO

Epitaxial growth of a protective semiconductor shell on a colloidal quantum dot (QD) core is the key strategy for achieving high fluorescence quantum efficiency and essential stability for optoelectronic applications and biotagging with emissive QDs. Herein we investigate the effect of shell growth rate on the structure and optical properties in blue-emitting ZnSe/ZnS QDs with narrow emission line width. Tuning the precursor reactivity modifies the growth mode of ZnS shells on ZnSe cores transforming from kinetic (fast) to thermodynamic (slow) growth regimes. In the thermodynamic growth regime, enhanced fluorescence quantum yields and reduced on-off blinking are achieved. This high performance is ascribed to the effective avoidance of traps at the interface between the core and the shell, which are detrimental to the emission properties. Our study points to a general strategy to obtain high-quality core/shell QDs with enhanced optical properties through controlled reactivity yielding shell growth in the thermodynamic limit.

2.
J Chem Phys ; 151(22): 224501, 2019 Dec 14.
Artigo em Inglês | MEDLINE | ID: mdl-31837660

RESUMO

Coupled colloidal quantum dot molecules composed of two fused CdSe/CdS core/shell sphere monomers were recently presented. Upon fusion, the potential energy landscape changes into two quantum dots separated by a pretuned potential barrier with energetics dictated by the conduction and valence band offsets of the core/shell semiconductors and the width controlled by the shell thickness and the fusion reaction conditions. In close proximity of the two nanocrystals, orbital hybridization occurs, forming bonding and antibonding states in analogy to the hydrogen molecule. In this study, we examine theoretically the electronic and optical signatures of such a quantum dot dimer compared to its monomer core/shell building-blocks. We examine the effects of different core sizes, barrier widths, different band offsets, and neck sizes at the interface of the fused facets on the system wave-functions and energetics. Due to the higher effective mass of the hole and the large valence band offset, the hole still essentially resides in either of the cores, breaking the symmetry of the potential for the electron as well. We found that the dimer signature is well expressed in a red shift of the band gap both in absorption and emission, in slower radiative lifetimes and in an absorption cross section which is significantly enhanced relative to the monomers at energies above the shell absorption onset, while remains essentially at the same level near the band-edge. This study provides essential guidance to predesign of coupled quantum dot molecules with specific attributes which can be utilized for various new opto-electronic applications.

3.
Nat Commun ; 10(1): 5401, 2019 12 16.
Artigo em Inglês | MEDLINE | ID: mdl-31844043

RESUMO

Coupling of atoms is the basis of chemistry, yielding the beauty and richness of molecules. We utilize semiconductor nanocrystals as artificial atoms to form nanocrystal molecules that are structurally and electronically coupled. CdSe/CdS core/shell nanocrystals are linked to form dimers which are then fused via constrained oriented attachment. The possible nanocrystal facets in which such fusion takes place are analyzed with atomic resolution revealing the distribution of possible crystal fusion scenarios. Coherent coupling and wave-function hybridization are manifested by a redshift of the band gap, in agreement with quantum mechanical simulations. Single nanoparticle spectroscopy unravels the attributes of coupled nanocrystal dimers related to the unique combination of quantum mechanical tunneling and energy transfer mechanisms. This sets the stage for nanocrystal chemistry to yield a diverse selection of coupled nanocrystal molecules constructed from controlled core/shell nanocrystal building blocks. These are of direct relevance for numerous applications in displays, sensing, biological tagging and emerging quantum technologies.

4.
ACS Nano ; 13(9): 10826-10834, 2019 Sep 24.
Artigo em Inglês | MEDLINE | ID: mdl-31487452

RESUMO

We report a solution for the challenge of having luminescence and metal conductivity from the same material. The fabrication of a hybrid metal-conductive luminescent film that manifests this dual property is described: the conductivity arising from a continuous gold thin film structure and luminescence originating from the embedded fluorescent emitters (nanoparticles of silica-coated CdSe/CdS quantum dots (QD/SiO2 NPs)). The embedding of the QD/SiO2 NPs is performed via a self-templating gold electroless process. The presence of the insulating silica layer on the QDs avoids quenching and enables luminescence, while still allowing plasmonic coupling of the QDs, as observed by luminescence lifetime analysis and by surface-enhanced Raman scattering. The potential applications of this special dual functionality are demonstrated by its used as a temperature probe: Passing current (heating the gold thin film) affects the emission intensity and induces a spectral red-shift of the QD/SiO2 NPs. All properties of this metal-conductive luminescent film required the special embedding architecture and are not observed with simple adsorption of QD/SiO2 NPs on a continuous Au film.

5.
Plant Methods ; 15: 48, 2019.
Artigo em Inglês | MEDLINE | ID: mdl-31139238

RESUMO

Background: Better understanding of the physiological and metabolic status of plants can only be obtained when metabolic fluxes are accurately assessed in a growing plant. Steady state 13C-MFA has been established as a routine method for analysis of fluxes in plant primary metabolism. However, the experimental system needs to be improved for continuous carbon enrichment from labelled sugars into metabolites for longer periods until complex secondary metabolism reaches steady state. Results: We developed an in vitro plant culture strategy by using peppermint as a model plant with minimizing unlabelled carbon fixation where growing shoot tip was strongly dependent on labelled glucose for their carbon necessity. We optimized the light condition and detected the satisfactory phenotypical growth under the lower light intensity. Total volatile terpenes were also highest at the same light. Analysis of label incorporation into pulegone monoterpene after continuous U-13C6 glucose feeding revealed nearly 100% 13C, even at 15 days after first leaf visibility (DALV). Label enrichment was gradually scrambled with increasing light intensity and leaf age. This study was validated by showing similar levels of label enrichment in proteinogenic amino acids. The efficiency of this method was also verified in oregano. Conclusions: Our shoot tip culture depicted a method in achieving long term, stable and a high percentage of label accumulation in secondary metabolites within a fully functional growing plant system. It recommends the potential application for the investigations of various facets of plant metabolism by steady state 13C-MFA. The system also provides a greater potential to study sink leaf metabolism. Overall, we propose a system to accurately describe complex metabolic phenotypes in a growing plant.

6.
J Phys Chem Lett ; 10(3): 335-345, 2019 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-30607959

RESUMO

The integrity of fluorescent carbon dot (FCD) emission deserves its highest appreciation when sample purification is performed with extreme care. Several controversial phenomena of FCD fluorescence including excitation-dependent emission, spectral migration with time, and thereby violation of the Kasha-Vavilov rule, which sparked intense debate during recent reports, disappeared when we rigorously purified the as-synthesized FCD sample. Purification was performed by first visual silica column chromatography (observing the emissions under UV illumination) and subsequently prolonged membrane dialysis. Most of the surprising phenomena of FCD fluorescence reported earlier apparently arose from ground-state spectral heterogeneity of FCD sample containing a large amount of fluorescent impurities (mostly polymeric or oligomeric in nature). Observation of our ensemble spectroscopic measurements, albeit nicely matched with recent reports based on single-particle experiments, differed largely from that of other ensemble measurements. Our results reconciled a number of long-standing controversies on FCD emission mostly by emphasizing the urgency of sample purification with more scientific rigor.

7.
Chem Asian J ; 13(21): 3296-3303, 2018 Nov 02.
Artigo em Inglês | MEDLINE | ID: mdl-30178522

RESUMO

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.

8.
Langmuir ; 34(3): 817-825, 2018 01 23.
Artigo em Inglês | MEDLINE | ID: mdl-28505448

RESUMO

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.

9.
J Phys Chem B ; 121(8): 1930-1940, 2017 03 02.
Artigo em Inglês | MEDLINE | ID: mdl-28165242

RESUMO

Tightly confined water within a small droplet behaves differently from bulk water. This notion is obtained on the basis of several reports showing unusual behaviors of water droplet residing at the core of a reverse micelle. In this study, we have shown a well-known hydrophobic dye, coumarin 153 (C153), which prefers to reside at the water-rich region inside the dendrimer molecule. Optical density (OD) measurement at the absorption peak of C153 shows that it is almost insoluble in bulk water but highly soluble in aqueous dendrimer solution. The OD of C153 increases several times in the latter case as compared to that in the former. We found the most interesting observation when we compared the data from fluorescence correlation spectroscopy (FCS) with the fluorescence anisotropy decay of C153 in aqueous dendrimer solution. The FCS measurement reveals a much slower translational diffusion time (τD) of C153 attached to a dendrimer molecule as compared to that of free C153 in bulk water in the absence of dendrimer. The slower τD in the former case is commensurate with the size of the dendrimer molecule. This is possible only when C153 is encapsulated by the dendrimer molecule. In contrast to the FCS study, the fluorescence anisotropy decay of C153 in water remains largely invariant after addition of the dendrimer. This can happen if a bulk-water-like environment at the C153 surroundings is preserved within the C153-dendrimer complex. This supports our institutive expectation that C153 resides within the water-rich peripheral cavities of the dendrimer molecule. A more expected binding of C153 to the hydrophobic core of dendrimer may not be possible here because of an inadequate size of the dendrimer core.

10.
Phys Chem Chem Phys ; 18(47): 32308-32318, 2016 Nov 30.
Artigo em Inglês | MEDLINE | ID: mdl-27849209

RESUMO

An insight study reveals the strong synergistic solvation behaviours from reporter dye molecules within the acetonitrile (ACN)-water (WT) binary mixture. Synergism of a binary mixture refers to some unique changes of the physical and thermodynamic properties of the solvent mixture, originating from the interactions among its cosolvents, which are absent within the pure cosolvents. Synergistic solvation of a binary mixture is likely to be fundamental for greater stabilization of an excited state solute dipole; at least to some extent greater as compared to one stabilized by any of its cosolvents alone. A dynamic Stokes shift due to the solvation of an excited dipole in the ACN-WT binary mixture is found to be highly relevant to the ground state physical properties of the solute molecule (polarity, hydrophilicity, acidity, etc.). Largely different solvation times in the ACN-WT mixture are observed from different dye molecules with widely varying polarities. However, earlier study shows that dye molecules, irrespective of their varying polarities, exhibit very similar solvation times within a pure solvent (J. Phys. Chem. B, 2014, 118, 7577-7785). On further study with fluorescence correlation spectroscopy (FCS) we observed that, unlike the translational diffusion coefficient (Dt) of a dye molecule within a pure solvent, which remains the same irrespective of the location of the dye molecule inside the solvent, a broad distribution among the Dt values of a dye molecule is obtained from different locations within the ACN-WT binary mixture. Lastly our 1H NMR study in the ACN-WT binary mixture shows the existence of strong hydrogen bond interactions among the cosolvents in the ACN-WT mixture.

11.
Phys Chem Chem Phys ; 18(36): 24830-24834, 2016 Sep 14.
Artigo em Inglês | MEDLINE | ID: mdl-27711522

RESUMO

The PAMAM dendrimer with a cage like structure acts as an excellent electron donor in the presence of an electron deficient molecule. Electron transfer (ET) causes significant quenching of dendrimer fluorescence. Trapping of quencher molecules within the dendrimer cage helps ET to take place through an expeditious route. Utilization of intrinsic fluorescence and sensing applications of dendrimers have been established here.

12.
Chemphyschem ; 16(16): 3518-26, 2015 Nov 16.
Artigo em Inglês | MEDLINE | ID: mdl-26403589

RESUMO

The solvation dynamics of three coumarin dyes with widely varying polarities were studied in acetonitrile-water (ACN-H2O) mixtures across the entire composition range. At low ACN concentrations [ACN mole fractions (X(ACN))≤0.1], the solvation dynamics are fast (<40 ps), indicating a nearly homogeneous environment. This fast region is followed by a sudden retardation of the average solvation time (230-1120 ps) at higher ACN concentrations (X(ACN)≈0.2), thus indicating the onset of nonideality within the mixture that continues until X(ACN)≈0.8. This nonideality regime (X(ACN)≈0.2-0.8) comprises of multiple dye-dependent anomalous regions. At very high ACN concentrations (X(ACN)≈0.8-1), the ACN-H2O mixtures regain homogeneity, with faster solvation times. The source of the inherent nonideality of the ACN-H2O mixtures is a subject of debate. However, a careful examination of the widths of time-resolved emission spectra shows that the origin of the slow dynamics may be due to the diffusion of polar solvent molecules into the first solvation shell of the excited coumarin dipole.


Assuntos
Acetonitrilos/química , Modelos Moleculares , Água/química , Cumarínicos/química , Dimetil Sulfóxido/química , Solventes/química , Espectrometria de Fluorescência , Espectrofotometria Infravermelho
13.
Phys Chem Chem Phys ; 16(40): 22352-63, 2014 Oct 28.
Artigo em Inglês | MEDLINE | ID: mdl-25223527

RESUMO

The solvation dynamics of coumarin dyes in dimethyl sulfoxide (DMSO)-glycerol (GLY) binary mixtures were studied across the GLY concentrations. Three coumarin dyes with widely different hydrophobicities were used for probing the entire polarity regions of this solvent mixture. Multiple anomalous concentration regions with significantly slow solvation times were detected from all three coumarin dyes. However, their precise positions were found to be probe molecule dependent. The solvation dynamics of the moderately hydrophobic dye coumarin 480 (C480) maintain a plateau region with a similar solvation time (∼550 ps) with the increase in GLY concentration until X(GLY) (the mole fraction of glycerol) reaches 0.5. This plateau region is followed by a sudden slowdown (to ∼975 ps) on the addition of more GLY to the DMSO-GLY mixture, and then this slow region persists from X(GLY)∼ 0.55 to 0.65 (peak at 0.6). On further addition of GLY (X(GLY) > 0.7), the solvation dynamics again become slower to ∼828 ps (at X(GLY)∼ 0.8) from ∼612 ps (at X(GLY)∼ 0.7). For very high GLY-content samples (X(GLY) > 0.85), the solvation times remain similar on further changes of the GLY concentrations. In contrast to C480, the most hydrophobic dye coumarin 153 (C153) shows a linear increase of solvation time in the DMSO-GLY mixture, from 102 ps (at X(GLY)∼ 0.1) to 946 ps (at X(GLY)∼ 0.9) with increase in GLY concentration, except for the concentration region, X(GLY)∼ 0.45-0.55 (peak at 0.5), where a substantial slowdown of the solvation time is observed. The highly hydrophilic probe coumarin 343 (C343) demonstrates multiple concentration regions (X(GLY)∼ 0.05-0.10, 0.25-0.35 and 0.55-0.65) where the solvation dynamics are significantly retarded. The presence of probe dependent anomalies in the DMSO-GLY mixture is a clear indication of there being different locations of probe molecules within this solvent mixture. We assume that the slowing-down of the solvation time could be a reflection of several aspects, including the inherit inhomogeneity, intriguing structural transformations in the DMSO-GLY mixture, percolation among DMSO molecules and network structure formation, where DMSO:GLY complexes contribute to the dynamical features.

14.
J Phys Chem B ; 118(27): 7577-7585, 2014 Jul 10.
Artigo em Inglês | MEDLINE | ID: mdl-24942350

RESUMO

Excited state dipole solvation of three coumarin dyes with different hydrophobicities was studied in DMSO-glycerol binary solvent. The solvation times obtained from the three dyes are remarkably different. The highly hydrophilic dye coumarin 343 (C343) exhibits the slowest solvation time (>12 ns) among all the dyes we used. This is in contrast to the most hydrophobic dye coumarin 153 (C153), where the solvated state is reached just within ∼104 ps. However, the moderately hydrophobic dye coumarin 480 (C480) demonstrates an intermediate (∼396 ps) solvation time. Unprecedented slowdown of solvation time of C343 is probably due to the slow diffusion of solvent molecules in the glycerol-rich first solvation shell followed by hydrogen bond rearrangements around the solute dipole. On the other hand, fast solvation of hydrophobic dye C153 is most likely caused by the fast reorganization dynamics of hydrophobic -CH3 groups of DMSO or the carbon backbone of the glycerol molecule around the solute dipole. Interestingly, a remarkable probe dependency in solvation dynamics was not observed in the case of DMSO-water binary solvent or in a neat solvent isopropanol. Probe dependent solvation in a DMSO-glycerol mixture is attributed to the microscopic phase segregation and different locations of coumarin dyes within this binary solvent.

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