Substrate- and time-dependent photoluminescence of quantum dots inside the ultrathin polymer LbL film

The photoluminescence of CdSe/ZnS quantum dots (QDs) in different configurations at solid surfaces (glass, silicon, PDMS, and metals) is considered for three types of organization: QDs directly adsorbed on solid surfaces, separated from the solid surface by a nanoscale polymer film with different thickness, and encapsulated into a polymer film. The complete suppression of photoluminescence for QDs on conductive metal surfaces (copper, gold) indicated a strong quenching effect. The temporal variation of the photoluminescent intensity on other substrates (glass, silicon, and PDMS) can be tuned by placing the nanoscale (3-50 nm) LbL polymer film between QDs and the substrate. The photooxidation and photobleaching processes of QD nanoparticles in the vicinity of the solid surface can be tuned by proper selection of the substrate and the dielectric nanoscale polymer film placed between the substrate and QDs. Moreover, the encapsulation of QD nanoparticles into the polymer film resulted in a dramatic initial increase in the photoemission intensity due to the accelerated photooxidation process. The phenomenon of enhanced photoemission of QDs encapsulated into the ultrathin polymer film provides not only the opportunity for making flexible, ultrathin, QD-containing polymer films, transferable to any microfabricated substrate, but also improved light emitting properties.     

Antioxidant activity assay based on the inhibition of oxidation and photobleaching of l-cysteine-capped CdTe quantum dots

Quantum dots (QDs) have recently been the focus of attention of many investigators for development of diagnostic tools in many research areas. In this work, we established a new QD-based assay to evaluate the antioxidant/polyphenolic activity. This assay is based on measurement of the inhibitory effect of the antioxidant/polyphenolic compounds on the UV-induced bleaching of CdTe QDs with l-cysteine capping. QDs exhibited excellent photostability without any UV exposure, while they bleached rapidly under UV irradiation. Generation of reactive oxygen species (ROS) under UV irradiation is probably the main cause of the photobleaching of QDs. By comparing the photostability of QDs in buffer solution in the absence and presence of sodium azide, as a known (1)O(2) quencher, the involvement of (1)O(2) in photobleaching of QDs was confirmed. The photobleaching effect induced by ROS could be reduced in the presence of antioxidant/polyphenolic compounds. We tested several antioxidant/polyphenolic compounds as well as known antioxidants such as trolox and 4 different types of tea. The results obtained by the QD-based assay revealed a very good correlation with the data acquired by Folin-Ciocalteu assay. Furthermore, a deeper understanding of the mechanism and the solution for photobleaching of QDs under UV irradiation might be very meaningful in promoting their clinical applications.     

In vivo NIR imaging with CdTe/CdSe quantum dots entrapped in PLGA nanospheres

Luminescent near-infrared (NIR) CdTe/CdSe QDs were synthesized and encapsulated in poly(lactic-co-glycolic acid) (PLGA) nanospheres to prepare stable and biocompatible QDs-loaded nanospheres for in vivo imaging. QDs were encapsulated with PLGA nanospheres by a solid dispersion method and optimized to have high fluorescence intensity for in vivo imaging detection. The resultant QDs-loaded PLGA nanospheres were characterized by various analytical techniques such as UV-Vis measurement, dynamic light scattering (DLS), fluorescence spectroscopy, and transmission electron microscopy (TEM). Finally, we evaluated toxicity and body distribution of QDs loaded in PLGA nanospheres in vitro and in vivo, respectively. From the results, the QDs loaded in PLGA nanospheres were spherical and showed a diameter range of 135.0-162.3 nm in size. The QD nanospheres increased their stability against photooxidation and photobleaching, which have the high potential for applications in biomedical imaging. We have also attained non-invasive in vivo imaging with light photons, representing an intriguing avenue for obtaining biological information by the use of NIR light.     

Generation of singlet oxygen via the composites of water-soluble thiol-capped CdTe quantum dots-sulfonated aluminum phthalocyanines

Singlet oxygen (1O2), one of the reactive oxygen species, plays an important role in many biomedical applications. The various compounds including the phthalocyanines, quantum dots (QDs) and QD complex, which may have potential to produce 1O2, thus received more and more attentions in recent years. By means of the direct detection of near-infrared 1270 nm, we found that the water-soluble thiol-capped CdTe QDs can photoproduce 1O2 in deuterated water with a low quantum yield (QY) of 1%. When sulfonated aluminum phthalocyanines (AlSPc's) were connected to these QDs, forming water-soluble QD-Pc composites, the 1O2 QY of the composites increased to 15% under the excitation of 532 nm, while little 1O2 production can be found for AlSPc alone at the same excitation because of the poor absorption of AlSPc in this region. The results of indirect measurements of 1O2, obtained from the photodegradation of the 1O2 chemical trap anthracene-9,10-diyl-bis-methylmalonate (ADMA), confirmed 1O2 yields in both QD and QD-Pc composite solutions. The QD-Pc composites have the advantage of extending the excitation region to 400-600 nm with remarkably enhanced extinction coefficients as compared with that of AlSPc. Therefore QD-Pc composites can fully utilize visible region light excitation to effectively produce 1O2, which may facilitate the applications of QD-Pc composites in broad areas.     

Multidentate surface ligand exchange for the immobilization of CdSe/ZnS quantum dots and surface quantum dot-oligonucleotide conjugates

A method for synthesizing multidentate thiol ligands on fused silica surfaces (e.g., optical fibers) was developed for the immobilization of CdSe/ZnS quantum dots (QDs) capped with hydrophilic or hydrophobic ligands. This work was motivated by the poor stability of QDs immobilized via monodentate thiol ligands and the need for stable immobilization strategies in the development of sensor technologies based on QDs. Multi-dentate immobilization was able to withstand washing protocols, and surface ligand exchange occurred via self-assembly through the zinc-metal affinity interaction. Atomic force and scanning electron microscopy images suggested that the QDs were immobilized at high density, approximately 2-4 x 10 (13) cm (-2). It was possible to immobilize one, two, or three colors of QD. Upon immobilization, 1-2 nm bathochromic shifts in the PL spectra were observed. This was attributed to both ligand exchange and the change in local environment. The change in environment was accompanied by a decrease in PL lifetime. Self-assembly of immobilized QD-oligonucleotide and QD-avidin conjugates was also demonstrated. These conjugates were able to hybridize with complementary oligonucleotide and bind biotin, respectively. This versatile immobilization chemistry is an important step in the development of surface-based QD nanosensors. Such technology requires QDs to be immobilized such that they remain accessible to target molecules in solution.     

Time-dependent photoluminescence blue shift of the quantum dots in living cells: effect of oxidation by singlet oxygen

Time-dependent photoluminescence (PL) enhancement, blue shift, and photobleach were observed from the thiol-capped CdTe quantum dots (QDs) ingested in mouse myoblast cells and human primary liver cancer cells. It was revealed that the PL blue shift resulted from the photooxidation of the QD core by singlet oxygen molecules formed on the QD core surface.     

硫脲修饰法制备高发光性能CdTe量子点

通过巯基水解制备了具有优异荧光特性的碲化镉量子点. 详细研究前驱体镉离子与巯基丙酸(MPA)摩尔比、镉离子浓度等制备条件对大尺寸、高量子产率的亲水性碲化镉量子点光学性能的影响. 在不同的水热生长时间下, 可制备出荧光发射峰位于485-660 nm范围内的不同尺寸的碲化镉水溶性量子点, 荧光发射峰半高宽控制在40-75 nm之间, 量子点的最高量子产率(QY)达到了45%. 并利用硫脲缓慢水解和光解释放自由硫离子, 修饰碲化镉表面, 检测修饰后的量子点在12天内光学性能的变化情况. 通过考察硫脲用量对量子点修饰效果, 发现当n(CdTe)/n(thiourea)=1:4(量子点浓度以镉离子浓度计)时, 硫脲对发射峰为505 nm的碲化镉量子点修饰效果最为理想, 量子点荧光强度加强了5倍, 量子产率达到68.3%.     

C-O bond cleavage of benzophenone substituted by 4-CH2OR (R = C6H5 and CH3) with stepwise two-photon excitation

The C-O bond cleavage from benzophenone substituted with 4-CH2OR (p-BPCH2OR, 1-3), such as p-phenoxymethylbenzophenone (1, R= C6H5) and p-methoxymethylbenzophenone (2, R= CH3), occurred by a stepwise two-photon excitation during two-color, two-laser flash photolysis. On the other hand, no C-O bond cleavage occurred from p-hydroxymethylbenzophenone (3, R = H). The first 355-nm laser excitation of 1-3 generates p-BPCH2OR in the lowest triplet excited state (T1) which has an absorption at 532 nm. When p-BPCH2OR(T1) is excited with the second 532-nm laser to p-BPCH2OR in the higher triplet excited state (T(n)), the C-O bond cleavage occurred within the laser flash duration of 5 ns. The quantum yields of the C-O bond cleavage during the second 532-nm laser irradiation were found to be 0.015 +/- 0.007 and 0.007 +/- 0.003 for 1 and 2, respectively. Although these values are low, the diminishing 1(T1) or 2(T1) was found to convert, in almost 100% yield, to phenoxyl (C6H5O*) and p-benzoylbenzyl (BPCH2*) radicals or methoxyl (CH3O*) and BPCH2* radicals, respectively. The T(n) excitation energy, the energy barrier along the potential surface between the T(n) states and product radicals, and delocalization of the T(n) state molecular orbital including BP and CH2OR (R = C6H5, CH3, H) moieties are important factors for the occurrence of the C-O bond cleavage. It is found that the C-O bond cleavage and production of free radicals, such as BPCH2*, C6H5O*, and CH3O*, can be performed by a stepwise two-photon excitation. The present study is an example in which the chemical reactions can be selectively initiated from the T(n) state but not from the S1 and T1 states.     

Controlling the rate of electron transfer between a quantum dot and a tri-ruthenium molecular cluster by tuning the chemistry of the interface

Ultrafast transient absorption measurements reveal that the rate of photoinduced electron transfer (PET) from colloidal CdSe quantum dots (QDs) to oxo-centered triruthenium clusters (Ru(3)O) depends on the structure of the chemical headgroup by which the Ru(3)O clusters adsorb to the QDs. Complexes comprising QDs and Ru(3)O clusters adsorbed through a pyridine-4-carboxylic acid ligand (nic-Ru(3)O) have an intrinsic PET rate constant of (4.9 ± 0.9) × 10(9) s(-1) whereas complexes comprising QDs and Ru(3)O clusters adsorbed through a 4-mercaptopyridine ligand (thiol-Ru(3)O) have an intrinsic PET rate constant of (36 ± 7) × 10(9) s(-1). Cyclic voltammetry measurements of nic-Ru(3)O and thiol-Ru(3)O yield reduction potentials vs. Ag/AgCl of -0.93 V for both clusters, and density functional theory calculations of the nic-Ru(3)O and thiol-Ru(3)O clusters yield internal reorganization energies for the cluster radical anion of -0.17 eV and -0.19 eV, respectively. The small differences in driving force and reorganization energy between the two complexes rule out these parameters as possible explanations for the factor-of-seven difference in the rate constants for PET. The difference in the observed rates of PET for the two complexes is therefore attributable to a difference in donor-acceptor electronic coupling, which, according to electronic structure calculations, is modulated by the torsional angle between the Ru(3)O core of the cluster and the functionalized pyridine ligand that bridges the cluster to the QD surface.     

Zn_xCd_(1-x)S量子点组成与其光催化活性的关系（英文）

Aqueous colloidal dispersions containing Znx Cd1‐x S quantum dots (QDs) of different x compositions were prepared by precipitating zinc and cadmium acetates with sodium sulphide,in the presence of a cetyltrimethylammonium bromide stabilizer.Ultraviolet‐visible absorption spectroscopy was used to determine the transition energies of the QDs,which in turn were used to calculate their sizes,which depended on their composition.The QD size decreased with increasing Zn content.The photocatalytic activity of the Znx Cd1‐x S QDs was studied by the decomposition of methylene blue under ultraviolet irradiation,at a maximum intensity at 365 nm (3.4 e V).Three different photo‐catalytic activity regions were observed,which depended on the Zn content.The quantum levels of the QDs could be excited by incident irradiation,and influenced the resulting photocatalytic activity.Maximum photocatalytic activity was achieved at x = 0.6,where the QD transition energy was equal to the irradiation photon energy.The photocatalytic efficiency of the QDs depended on their surface area and arrangement of quantum levels,because of the quantum size effect.     

The Application of Amine‐Terminated Silicon Quantum Dots on the Imaging of Human Serum Proteins after Polyacrylamide Gel Electrophoresis (PAGE)

Novel amine‐terminated silicon (Si) quantum dots (QDs) were synthesized and applied for the detection of human serum proteins on gels directly after polyacrylamide gel electrophoresis (PAGE). The diameter of these stable amine‐terminated Si QDs was in the range of 0.5–2.0 nm. In this study, the fluorescent imaging conditions, such as the buffer solution, pH value, buffer concentration and quantity of Si QDs, were optimized and the possible mechanisms of Si QDs–protein interaction were analyzed. The mode of Si QDs and human serum albumin association was found to occur by hydrogen bond interactions; this was probably attributed to the interaction between the amino group of amine‐terminated Si QDs and the carboxyl group of proteins. Meanwhile, human serum proteins separated by native 1D and native 2D electrophoresis were detected by Si QD‐based fluorescent imaging. Some proteins, such as isoform 1 of α‐1‐antitrypsin, complement C3 (Fragment) and hemopexin, which were identified by mass spectrometry (MS), were easily detected by using Si QDs, but not with CBB‐R250 staining. The Si QDs‐based fluorescent imaging technique with high resolution is a sensitive and dependable method for direct detection of human serum proteins, and has enormous potential in clinical diagnosis.     

Photocatalysis with Quantum Dots and Visible Light: Selective and Efficient Oxidation of Alcohols to Carbonyl Compounds through a Radical Relay Process in Water

Selective oxidation of alcohols to aldehydes/ketones has been achieved with the help of 3‐mercaptopropionic acid (MPA)‐capped CdSe quantum dot (MPA‐CdSe QD) and visible light. Visible‐light‐prompted electron‐transfer reaction initiates the oxidation. The thiyl radical generated from the thiolate anion adsorbed on a CdSe QD plays a key role by abstracting the hydrogen atom from the C−H bond of the alcohol (R¹CH(OH)R²). The reaction shows high efficiency, good functional group tolerance, and high site‐selectivity in polyhydroxy compounds. The generality and selectivity reported here offer a new opportunity for further applications of QDs in organic transformations.     

Photoassisted synthesis of CdSe and core-shell CdSe/CdS quantum dots

This paper describes the synthesis of core-shell CdSe/CdS quantum dots (QDs) in aqueous solution by a simple photoassisted method. CdSe was prepared from cadmium nitrate and 1,1-dimethylselenourea precursors under illumination for up to 3 h using a pulsed Nd:YAG laser at 532 nm. The effects that the temperature and the laser irradiation process have on the synthesis of CdSe were monitored by a series of experiments using the precursors at a Cd:Se concentration ratio of 4. Upon increasing the temperature (80-140 degrees C), the size of the CdSe QDs increases and the time required for reaching a maximum photoluminescence (PL) is shortened. Although the as-prepared CdSe QDs possess greater quantum yields (up to 0.072%) compared to those obtained by microwave heating (0.016%), they still fluoresce only weakly. After passivation of CdSe (prepared at 80 degrees C) by CdS using thioacetamide as the S source (Se:S concentration ratio of 1) at 80 degrees C for 24 h, the quantum yield of the core-shell CdSe/CdS QDs at 603 nm is 2.4%. Under UV irradiation of CdSe/CdS for 24 h using a 100-W Hg-Xe lamp, the maximum quantum yield of the stable QDs is 60% at 589 nm. A small bandwidth (W1/2 < 35 nm) indicates the narrow size distribution of the as-prepared core-shell CdSe/CdS QDs. This simple photoassisted method also allows the preparation of differently sized (3.7-6.3-nm diameters) core-shell CdSe/CdS QDs that emit in a wide range (from green to red) when excited at 480 nm.     

Photoinduced fluorescence intensity oscillation in a reaction-diffusion cell containing a colloidal quantum dot dispersion

The nonlinear spontaneous oscillation of photoluminescence (PL) intensity in an ensemble of semiconductor quantum dots (QDs), which differs from the fluorescence intermittency of a single QD, is investigated. The PL intensity in a QD dispersion slowly oscillates with time under continuous illumination. The oscillatory behavior is found to vary with changing QD concentration, solvent viscosity, volume fraction of irradiated region, and irradiation intensity. On the basis of the Gray-Scott model [Chemical Oscillation and Instabilities: Non-linear Chemical Kinetics (Clarendon, Oxford, 1994); J. Phys. Chem. 89, 22 (1985); Chem. Eng. Sci. 42, 307 (1987)], and its comparison with the experimental results, it is revealed that the following processes are important for PL oscillation: (1) mass transfer of QDs between the illuminated and dark regions, (2) autocatalytic formation of vacant sites on QD surfaces via photodesorption of ligand molecules, and (3) passivation of vacant sites via photoadsorption of water molecules.     

Competition between photobleaching and fluorescence increase of photosensitizing porphyrins and tetrasulphonated chloroaluminiumphthalocyanine

The time-dependent fluorescence changes of photosensitizing porphyrins and tetrasulphonated chloroaluminiumphthalocyanine (A1C1SPc) were measured at different intracellular sites using video-enhanced microscopy and image processing. To obtain variations in intracellular fluorescence intensity, different radiant exposures of a Kr+ laser-pumped dye laser were delivered via a 600 microns plastic-clad silica fibre connected to the microscope. During irradiation, competition between photobleaching and fluorescence increase of the different dyes was observed. The porphyrins normally showed photobleaching, which was dependent on the sensitizer and its specific accumulation within the cell. Photobleaching was less pronounced for hydrophilic uroporphyrin than for more hydrophobic dyes. In contrast with an almost exponential decrease in porphyrin fluorescence with increasing light dose, the fluorescence intensity of A1C1SPc significantly increased at the beginning of irradiation, and could be correlated with intracellular deaggregation.     

Assembly kinetics of nanocrystals via peptide hybridization

The assembly kinetics of colloidal semiconductor quantum dots (QDs) on solid inorganic surfaces is of fundamental importance for implementation of their solid-state devices. Herein an inorganic binding peptide, silica binding QBP1, was utilized for the self-assembly of nanocrystal quantum dots on silica surface as a smart molecular linker. The QD binding kinetics was studied comparatively in three different cases: first, QD adsorption with no functionalization of substrate or QD surface; second, QD adsorption on QBP1-modified surface; and, finally, adsorption of QBP1-functionalized QD on silica surface. The surface modification of QDs with QBP1 enabled 79.3-fold enhancement in QD binding affinity, while modification of a silica surface with QBP1 led to only 3.3-fold enhancement. The fluorescence microscopy images also supported a coherent assembly with correspondingly increased binding affinity. Decoration of QDs with inorganic peptides was shown to increase the amount of surface-bound QDs dramatically compared to the conventional methods. These results offer new opportunities for the assembly of QDs on solid surfaces for future device applications.     

Ultraviolet light-induced hydrophilicity effect on TiO2(110)(1 x 1). Dominant role of the photooxidation of adsorbed hydrocarbons causing wetting by water droplets

The UV photoproduction of a hydrophilic TiO(2)(110)(1x1) surface has been investigated in a pressurized ultrahigh vacuum apparatus under controlled conditions of hydrocarbon concentration in oxygen gas at 1 atm pressure. Water droplet contact angles have been measured continuously as the droplet is exposed to UV irradiation, yielding the first observations of a sudden wetting process during irradiation. Using hexane as a model hydrocarbon, it is found that when low partial pressures of hexane are present, the sudden onset of surface wetting occurs during UV irradiation after an induction period under photooxidation conditions. The induction period to reach the critical condition for sudden wetting increases when the partial pressure (and equilibrium surface coverage) of hexane is increased. These results indicate that the removal of adsorbed hydrocarbons by photooxidation is the critical factor leading to the UV-induced hydrophilicity phenomenon on TiO(2). The phenomenon does not occur in the absence of O(2) gas. A concept concerned with kinetic screening of the TiO(2)-H(2)O interface from O(2) by water droplets is presented to explain the observation of sudden wetting in our experiments, compared to gradual wetting which is observed following UV irradiation in all other experiments reported in the literature. Complementary infrared spectroscopy measurements of the effect of UV irradiation in an O(2) atmosphere on adsorbed Ti-OH groups and on adsorbed H(2)O on the surface of a high-area TiO(2) powder show that no spectroscopic changes occur. This indicates that UV-induced changes in the -OH coverage or the nature of -OH bonding to TiO(2), as suggested by others, cannot be used to explain the photoinduced hydrophilicity effect.     

CsPbBr3 QD/AlOx Inorganic Nanocomposites with Exceptional Stability in Water,Light, and Heat

Herein, the assembly of CsPbBr₃ QD/AlO_x inorganic nanocomposites, by using atomic layer deposition (ALD) for the growth of the amorphous alumina matrix (AlO_x ), is described as a novel protection scheme for such QDs. The nucleation and growth of AlO_x on the QD surface was thoroughly investigated by miscellaneous techniques, which highlighted the importance of the interaction between the ALD precursors and the QD surface to uniformly coat the QDs while preserving the optoelectronic properties. These nanocomposites show exceptional stability towards exposure to air (for at least 45 days), irradiation under simulated solar spectrum conditions (for at least 8 h), and heat (up to 200 °C in air), and finally upon immersion in water. This method was extended to the assembly of CsPbBr_x I_3−x QD/AlO_x and CsPbI₃ QD/AlO_x nanocomposites, which were more stable than the pristine QD films.     

Generation of thiyl radicals by the photolysis of 5-iodo-4-thiouridine

The photochemistry of 2',3',5'-tri-O-acetyl-5-iodo-4-thiouridine (3) in deoxygenated 1:1 CH(3)CN-H(2)O pH 5.8 (phosphate buffer) solution has been studied by means of steady-state and nanosecond laser flash photolysis methods. Under steady-state irradiation (lambda > or = 334 nm), the stable photoproducts were iodide ion, 2',3',5'-tri-O-acetyl-4-thiouridine (4), and two disulfides. The disulfides were the symmetrical bis-(2',3',5'-tri-O-acetyl-5-iodo-4-thiouridine) (5) and unsymmetrical 6, which contains both 4-thiouridine and 5-iodo-4-thiouridine residues. The formation of the dehalogenated photoproduct suggests that C(5)-I bond cleavage is a primary photochemical step. Attempts to scavenge the resulting C(5)-centered radical by suitable addends, bis-(N-alpha-acetyl)cystine-bis-N-ethylamide or benzene, were unsuccessful. Analysis of the photoproducts formed under these conditions showed that the S-atom is the reactive center. The photoproduct 4, obtained by irradiation of 3 in CD(3)CN-H(2)O, followed by reversed-phase HPLC isolation using nonlabeled eluents, did not contain deuterium. An analogous experiment performed in CH(3)CN-D(2)O gave deuterated product 4-d with 88% of the deuterium incorporated at C(5). Transient absorption observed upon laser excitation (lambda= 308 nm) of 3 was assigned to the 4-uridinylthiyl radical on the basis of the similarity of this spectrum with that obtained upon laser photolysis of the disulfide: bis-(2',3',5'-tri-O-acetyl-4-thiouridine) 14. On the basis of the results of steady-state and laser photolysis studies, a mechanism of the photochemical reaction of 3 is proposed. The key mechanistic step is a transformation of the C(5)-centered radical formed initially by C(5)-I bond cleavage into a long-lived S-centered radical via a 1,3-hydrogen shift. Theoretical calculations confirmed that the long-lived S-centered radical is the most stable radical derived from the 4-thiouracil residue.     

CsPbBr3 QD/AlOx Inorganic Nanocomposites with Exceptional Stability in Water,Light, and Heat

Herein, the assembly of CsPbBr₃ QD/AlO_x inorganic nanocomposites, by using atomic layer deposition (ALD) for the growth of the amorphous alumina matrix (AlO_x ), is described as a novel protection scheme for such QDs. The nucleation and growth of AlO_x on the QD surface was thoroughly investigated by miscellaneous techniques, which highlighted the importance of the interaction between the ALD precursors and the QD surface to uniformly coat the QDs while preserving the optoelectronic properties. These nanocomposites show exceptional stability towards exposure to air (for at least 45 days), irradiation under simulated solar spectrum conditions (for at least 8 h), and heat (up to 200 °C in air), and finally upon immersion in water. This method was extended to the assembly of CsPbBr_x I_3−x QD/AlO_x and CsPbI₃ QD/AlO_x nanocomposites, which were more stable than the pristine QD films.