Simple and accurate quantification of quantum dots via single-particle counting

Quantification of quantum dots (QDs) is essential to the quality control of QD synthesis, development of QD-based LEDs and lasers, functionalizing of QDs with biomolecules, and engineering of QDs for biological applications. However, simple and accurate quantification of QD concentration in a variety of buffer solutions and in complex mixtures still remains a critical technological challenge. Here, we introduce a new methodology for quantification of QDs via single-particle counting, which is conceptually different from established UV-vis absorption and fluorescence spectrum techniques where large amounts of purified QDs are needed and specific absorption coefficient or quantum yield values are necessary for measurements. We demonstrate that single-particle counting allows us to nondiscriminately quantify different kinds of QDs by their distinct fluorescence burst counts in a variety of buffer solutions regardless of their composition, structure, and surface modifications, and without the necessity of absorption coefficient and quantum yield values. This single-particle counting can also unambiguously quantify individual QDs in a complex mixture, which is practically impossible for both UV-vis absorption and fluorescence spectrum measurements. Importantly, the application of this single-particle counting is not just limited to QDs but also can be extended to fluorescent microspheres, quantum dot-based microbeads, and fluorescent nano rods, some of which currently lack efficient quantification methods.     

Comprehensive Photophysical Behaviour of Ethynyl Fluorenes and Ethynyl Anthracenes Investigated by Fast and Ultrafast Time‐Resolved Spectroscopy

Detailed investigations by time‐resolved transient absorption and fluorescence spectroscopies with nano‐ and femtosecond time resolutions are carried out with the aim of characterising the lowest excited singlet and triplet states of three ethynyl fluorenes ( 1 – 3 ) and three ethynyl anthracenes ( 4 – 6 ) in solvents of different polarity. The solvent is found to modify the deactivation pathways of the lowest excited singlet state of compounds 1 – 4 , thus changing their fluorescence, intersystem crossing and internal conversion efficiencies. The fluorescence and triplet yields gradually decrease, while the internal conversion quantum yield increases upon increasing the solvent dielectric constant. These experimental results, coupled with the marked fluorosolvatochromic effect, point to the involvement of an emitting state with a charge‐transfer (CT) character, strongly stabilised by polar solvents. This is proved by ultrafast spectroscopic studies in which two transients, distinguished by characteristic spectral shapes assigned to locally excited (LE) and CT states, are detected, the CT state being the longer lived and fluorescent one in highly polar solvents. The intramolecular LE→CT process, operative in highly polar media, becomes particularly fast (up to ≈300 fs) in the case of the NO₂ derivative 1 . No push–pull character is found for 5 and 6 , which exhibit different photophysical behaviour; indeed, the solvent polarity does not modify significantly the dynamics of the lowest excited singlet states. Quantum mechanical calculations at the TDDFT level are also used to determine the state order and nature of the lowest excited singlet and triplet states and to rationalise the different photophysical behaviour of fluorine and anthracene derivatives, particularly concerning the intersystem crossing process.     

High‐Performance CsPb1−xSnxBr3 Perovskite Quantum Dots for Light‐Emitting Diodes

All inorganic CsPbBr₃ perovskite quantum dots (QDs) are potential emitters for electroluminescent displays. We have developed a facile hot‐injection method to partially replace the toxic Pb²⁺ with highly stable Sn⁴⁺. Meanwhile, the absolute photoluminescence quantum yield of CsPb_1−x Sn_x Br₃ increased from 45 % to 83 % with Sn^IV substitution. The transient absorption (TA) exciton dynamics in undoped CsPbBr₃ and CsPb_0.67Sn_0.33Br₃ QDs at various excitation fluences were determined by femtosecond transient absorption, time‐resolved photoluminescence, and single‐dot spectroscopy, providing clear evidence for the suppression of trion generation by Sn doping. These highly luminescent CsPb_0.67Sn_0.33Br₃ QDs emit at 517 nm. A device based on these QDs exhibited a luminance of 12 500 cd m⁻², a current efficiency of 11.63 cd A⁻¹, an external quantum efficiency of 4.13 %, a power efficiency of 6.76 lm w⁻¹, and a low turn‐on voltage of 3.6 V, which are the best values among reported tin‐based perovskite quantum‐dot LEDs.     

Ultrafast charge separation and recombination dynamics in lead sulfide quantum dot-methylene blue complexes probed by electron and hole intraband transitions

Lead salt quantum dots (QDs) have emerged as attractive materials for solar energy conversion because of their broad spectral response, long exciton lifetime, and efficient multiexciton generation. However, charge separation dynamics from these QDs remain poorly understood. In this study we investigate charge separation and recombination dynamics in PbS-methylene blue (MB(+)) complexes by femtosecond transient absorption spectroscopy. We show that while the 1S electrons and holes in excited PbS QDs lead to overlapping transient absorption features in the visible and near-IR regions, their intraband absorptions in the mid-IR can be monitored independently to directly follow the charge separation and recombination processes. The charge separation and recombination rates in PbS-MB(+) complexes were found to be (2.7 ± 0.2) × 10(12) and (1.1 ± 0.2) × 10(11) s(-1), respectively. The ultrafast charge separation rate suggests the possibility of hot electron injection and multiexciton dissociation from these strongly quantum confined QDs, consistent with recent reports of these phenomena at lead salt QD/TiO(2) interfaces.     

Ultrafast photoinduced intramolecular charge transfer in push-pull distyryl furan and benzofuran: solvent and molecular structure effect

The excited state deactivation pathways of push-pull distyryl furan and benzofuran derivatives in several organic solvents were investigated in detail by using time-resolved transient absorption and fluorescence spectroscopies, with nano- and femto-second time resolution. Solvent polarity was found to play a key role in determining the efficiencies of fluorescence, intersystem crossing and internal conversion. The triplet yield gradually decreased, while the internal conversion increased upon increasing the solvent dielectric constant. However the fluorescence showed a different solvent polarity effect in the low and high solvent polarity region, with a reversal of the trend of fluorescence properties (quantum yield and lifetime). This fact points to an emitting state of a different nature (smaller and larger dipole moments) in the two cases, as also suggested by the huge fluorosolvatochromism. In fact the ultrafast spectroscopic investigation evidenced the presence of two transients characterized by peculiar spectral shapes assigned to a locally excited (LE) and a charge transfer (CT) state. In the more polar solvents the CT state was the longer lived, fluorescent one and an intramolecular charge transfer process was found to be operative and to become faster (up to ～200-250 fs) in the higher polarity media. On the contrary, distyrylfuran, which exhibits the same molecular skeleton without the push-pull character showed a similar excited state dynamics in solvents of different polarities.     

PbS/CdS Core–Shell Quantum Dots Suppress Charge Transfer and Enhance Triplet Transfer

A sub‐monolayer CdS shell on PbS quantum dots (QDs) enhances triplet energy transfer (TET) by suppressing competitive charge transfer from QDs to molecules. The CdS shell increases the linear photon upconversion quantum yield (QY) from 3.5 % for PbS QDs to 5.0 % for PbS/CdS QDs when functionalized with a tetracene acceptor, 5‐CT . While transient absorption spectroscopy reveals that both PbS and PbS/CdS QDs show the formation of the 5‐CT triplet (with rates of 5.91±0.60 ns⁻¹ and 1.03±0.09 ns⁻¹ respectively), ultrafast hole transfer occurs only from PbS QDs to 5‐CT . Although the CdS shell decreases the TET rate, it enhances TET efficiency from 60.3±6.1 % to 71.8±6.2 % by suppressing hole transfer. Furthermore, the CdS shell prolongs the lifetime of the 5‐CT triplet and thus enhances TET from 5‐CT to the rubrene emitter, further bolstering the upconverison QY.     

光谱法研究核壳量子点CdTe/CdS与牛血清白蛋白的相互作用

应用荧光光谱、圆二色光谱和紫外吸收光谱等技术研究核壳量子点CdTe/CdS与牛血清白蛋白(BSA)相互作用的结果表明,CdTe/CdS对BSA的荧光猝灭机理为静态猝灭。根据不同温度下量子点对BSA的荧光猝灭作用计算了结合常数、热力学参数,证明了量子点与BSA相互作用力主要是范德华力或氢键作用力。探讨了量子点对BSA构象的影响。     

Investigation on Ligand Exchange Kinetics on CdSe/ZnS Quantum Dot Surface Utilizing Pyrene as Flourescent Probe

Direct ligand exchange kinetics between hydrophilic molecules and quantum dots(QDs) was investigated. Meanwhile, pyrene was exploited as probe to detect the efficiency of the ligand exchange reaction between octadecylamine-coated QDs(ODA-QDs) and different ligands[ligand 1: NH₂G3-OH, ligand 2: G4.5-PEG5-FA5, ligand 3: (COOH)₂G3-OH or ligand 4: G4.5-PEG1-FA1]. It was indicated that water-soluble QDs exhibit the same fluorescence and absorption spectra as ODA-QDs when they were dissolved in chloroform. Furthermore, the cellular experiments demonstrated that the folic acid(FA) targeting poly(amidoamine)(PAMAM) modified QD conjugates could be used as molecular targeting sensing systems for nanoparticle probes.     

Dual-Time Scale Photoinduced Electron Transfer from PbS Quantum Dots to a Molecular Acceptor

A combination of picosecond and microsecond transient absorption dynamics reveals the involvement of two mechanisms by which 1,4-benzoquinone (BQ) induces the decay of the excited state of PbS quantum dots (QDs): (i) electron transfer to BQ molecules adsorbed to the surfaces of PbS QDs and (ii) collisionally gated electron transfer to freely diffusing BQ. Together, these two mechanisms quantitatively describe the quenching of photoluminescence upon addition of BQ to PbS QDs in dichloromethane solution. This work represents the first quantitative study of a QD-ligand system that undergoes both adsorbed and collisionally gated photoinduced charge transfer within the same sample. The availability of a collisionally gated pathway improves the yield of electron transfer from PbS QDs to BQ by an average factor of 2.5 over that for static electron transfer alone.     

Study on excited states of hexamethoxybenzene-NO+ charge-transfer complex: incorporation of excited radical cation

A charge-transfer (CT) complex of NOBF₄ and hexamethoxybenzene (HMB), which gives out HMB^?+ as a “fluorescent radical cation probe,” upon one-electron oxidation, has been designed to explore the excited state dynamics of contact radical ion pairs by laser-induced fluorescence and femtosecond transient absorption spectroscopic techniques. The acetonitrile solution of the CT complex showed weak fluorescence with a similar spectrum to that observed for free excited HMB radical cation (HMB^?+*), suggesting the formation of HMB^?+* upon the one-photonic excitation of the CT complex. The laser-power dependence of the fluorescence intensity supported the one-photonic excitation event. We have also observed a short-lived transient species but no long-lived species by femtosecond laser flash photolysis of the CT complex. The lifetime (6.5 ps) was in good accordance with its fluorescence quantum yield (2.5 × 10^?5) and was able to assign the transient species to the fluorescent state, an excited radical ion pair [HMB ^?+*/NO^?]. All the events were completed within the inner sphere and the short lifetime of the transient species could be attributed to rapid back-electron transfer. It is concluded that the excited radical cation character in the excited state of the CT complex originates from the radical ion character in the CT complex in the ground state and that a relatively long lifetime of HMB^?+* facilitates its observation even in the contact ion pair.     

Investigation on Photophysical Properties of D–π–A–π–D‐Type Fluorenone‐Based Linear Conjugated Oligomers by Using Femtosecond Transient Absorption Spectroscopy

The effects of π‐spacer and electron donor groups on the photophysical behaviors of fluorenone‐based linear conjugated oligomers have been systemically investigated. Solvent‐dependent steady‐state measurements exhibit that the fluorene vinylene (FV) spacer and the electron‐donating ability of donor group are able to modulate the spectral features of oligomers and the fluorescence quantum yield could decrease with the increasing of the solvent polarity. Meanwhile, quantum chemical calculation simulates their absorption spectra, and analyzes their electron transition components simultaneously. The transient absorption measurements focus on the photoexcitation dynamics of these oligomers in the toluene solution, which show that an intramolecular charge transfer state exists in the relaxation process of excited states, and its generation process could accelerate with the introduction of FV spacer and the enhancement of donor strength.     

微波加热法水相合成梯度合金CdSeS量子点及其环境暴露下荧光稳定性研究

采用微波辅助加热法在水相中一步合成了梯度合金CdSeS量子点,荧光光谱和紫外吸收光谱表明,合成的CdSeS合金量子点具有较高的量子产率(30%左右)。X射线衍射分析和光电子能谱表征证明,合成的CdSeS量子点具有立方晶型,梯度合金结构,内部以CdSe为主,而外部主要以CdS为主;电子透射显微镜下观察到的量子点结晶度高,粒径小(仅2~3 nm),粒度分布均一。在此基础上探讨了所合成的合金量子点在氧化剂过氧化氢、不同的酸碱介质中及350 W氙灯的作用下其荧光光谱的稳定性。结果表明,该合金量子点的光稳定性与CdTe QDs相当,但具有更好的抗氧化性能和抗酸刻蚀性能,具有良好的应用前景。     

Excited State Relaxation and Stabilization of Hydrogen Terminated Silicon Quantum Dots

Silicon is the leading semiconductor material in microelectronic industry. Owing to the large surface to volume ratio, low-dimensional Si nanostructures, for instance, silicon quantum dots exhibit diverse electronic and optical properties. Passivating the surface of Si nanostructures by a suitable species is thereby required to stabilize and engineer the dot properties in different environment. Recent theoretical advances in the investigation of the excited state properties of silicon quantum dots (QDs) are reviewed in this article. The theoretical calculations reveal that the excited state relaxation is prevalent in hydrogenated silicon nanoparticles. Stokes shift due to structure relaxation in the excited state varies with the particle size. It is therefore desirable to minimize Stokes shift for the purpose of maximizing its quantum yield or efficiency in photoluminescence applications. Consequently, surface functionalization by a suitable species turns out to be the most effective avenue. Determination of proper passivating agent is of outmost importance to satisfy the practical necessity. All these intermingled factors are briefly addressed in this article.     

Magic sized ZnS quantum dots as a highly sensitive and selective fluorescence sensor probe for Ag+ ions

A green and simple chemical synthesis of magic sized water soluble blue-emitting ZnS quantum dots (QDs) has been accomplished by reacting anhydrous Zn acetate, sodium sulfide and thiolactic acid (TLA) at room temperature in aqueous solution. Refluxing of this mixture in open air yielded ZnS clusters of about 3.5 nm in diameter showing very strong and narrow photoluminescence properties with long stability. Refluxing did not cause any noticeable size increment of the clusters. As a result, the QDs obtained after different refluxing conditions showed similar absorption and photoluminescence (PL) features. Use of TLA as a capping agent effectively yielded such stable and magic sized QDs. The as-synthesized and 0.5 h refluxed ZnS QDs were used as a fluorescence sensor for Ag(+) ions. It has been observed that after addition of Ag(+) ions of concentration 0.5-1 μM the strong fluorescence of ZnS QDs was almost quenched. The quenched fluorescence can be recovered by adding ethylenediamine to form a complex with Ag(+) ions. The other metal ions (K(+), Ca(2+), Au(3+), Cu(2+), Fe(3+), Mn(2+), Mg(2+), Co(2+)) showed little or no effect on the fluorescence of ZnS QDs when tested individually or as a mixture. In the presence of all these ions, Ag(+) responded well and therefore ZnS QDs reported in this work can be used as a Ag(+) ion fluorescence sensor.     

量子点与蛋白质相互作用热力学

量子点(QDs)具有宽激发窄发射、量子产率高、发射波长可调和抗光漂白等优异的光学性质,因而在生物医学成像与示踪、生物传感等方面有着广泛的应用前景。量子点进入生命体系后,首先会遇到蛋白质,量子点与蛋白质之间发生相互作用后,蛋白质的结构和功能会因此发生变化,量子点的性能及应用也会发生改变。研究量子点与蛋白质的相互作用规律,可以为量子点的精细设计、高效应用以及生物安全性评价提供理论依据。本文在总结国内外相关文献和本课题组工作的基础上,介绍了量子点与蛋白质相互作用的热力学方法;重点从热力学角度揭示量子点与蛋白质相互作用的机制。     

Studying the interaction between CdTe quantum dots and Nile blue by absorption,fluorescence and resonance Rayleigh scattering spectra

Thioglycolic acid (TGA) capped CdTe quantum dots (QDs) with the diameter of 2–3 nm were synthesized. The interaction between CdTe QDs and Nile blue (NB) was investigated by ultraviolet–visible (UV–vis) absorption, resonance Rayleigh scattering (RRS) and fluorescence spectroscopy. UV–vis absorption spectrum of CdTe QDs and NB obviously changed, showing that CdTe QDs could associate with NB to form a new complex. At pH 6.8, NB effectively quenched the fluorescence of CdTe QDs. It was proved that the fluorescence quenching of CdTe QDs by NB was mainly result of the formation of CdTe QDs–NB complex, electrostatic attraction and hydrophobic forces played a major role in stabilizing the complex. The binding molar ratio of CdTe QDs and NB was 5:1 by a mole-ratio method. The interaction between CdTe QDs and NB lead to the remarkable enhancement of RRS and the enchantments were in proportional to the concentration of NB in a certain range. The mechanism of the interaction between CdTe QDs and NB, reasons for the enhancement of RRS intensity were also discussed. The obtained results suggested the more satisfactory mechanism for the interaction between CdTe QDs and NB.     

Microstructure-controlled aerosol-gel synthesis of ZnO quantum dots dispersed in SiO2 nanospheres

ZnO quantum dots dispersed in a silica matrix were synthesized from a TEOS:Zn(NO(3))(2) solution by a one-step aerosol-gel method. It was demonstrated that the molar concentration ratio of Zn to Si (Zn/Si) in the aqueous solution was an efficient parameter with which to control the size, the degree of agglomeration, and the microstructure of ZnO quantum dots (QDs) in the SiO(2) matrix. When Zn/Si ≤ 0.5, unaggregated quantum dots as small as 2 nm were distributed preferentially inside SiO(2) spheres. When Zn/Si ≥ 1.0, however, ZnO QDs of ～7 nm were agglomerated and reached the SiO(2) surface. When decreasing the ratio of the Zn/Si, a blue shift in the band gap of ZnO was observed from the UV/Visible absorption spectra, representing the quantum size effect. The photoluminescence emission spectra at room temperature denoted two wide peaks of deep-level defect-related emissions at 2.2-2.8 eV. When decreasing Zn/Si, the first peak at ～2.3 eV was blue-shifted in keeping with the decrease in the size of the QDs. Interestingly, the second visible peak at 2.8 eV disappeared in the surface-exposed ZnO QDs when Zn/Si ≥ 1.0.     

Hot‐Hole Extraction from Quantum Dot to Molecular Adsorbate

Ultrafast thermalized and hot‐hole‐transfer processes have been investigated in CdSe quantum dot (QD)/catechol composite systems in which hole transfer from photoexcited QDs to the catechols is thermodynamically favorable. A series of catechol derivatives were selected with different electron‐donating and ‐withdrawing groups, and the effect of these groups on hole transfer and charge recombination (CR) dynamics has been investigated. The hole‐transfer time was determined using the fluorescence upconversion technique and found to be 2–10 ps depending on the molecular structure of the catechol derivatives. The hot‐hole‐transfer process was followed after monitoring 2S luminescence of CdSe QDs. Interestingly, hot‐hole extraction was observed only in the CdSe/3‐methoxycatechol (3‐OCH₃) composite system owing to the higher electron‐donating property of the 3‐methoxy group. To confirm the extraction of the hot hole and to monitor the CR reaction in CdSe QD/catechol composite systems, ultrafast transient absorption studies have been carried out. Ultrafast transient‐absorption studies show that the bleach recovery kinetics of CdSe QD at the 2S excitonic position is much faster in the presence of 3‐OCH₃. This faster bleach recovery at the 2S position in CdSe/3‐OCH₃ suggests hot‐hole transfer from CdSe QD to 3‐OCH₃. CR dynamics in CdSe QD/catechol composite systems was followed by monitoring the excitonic bleach at the 1S position and was found to decrease with free energy of the CR reaction.     

Excited state properties of novel p- and n-type organic semiconductors with an anthracene unit

Photoinduced dynamics of novel p- and n-type organic semiconductors with an anthracene unit are theoretically investigated with quantum chemistry methods. The calculated vertical absorption and fluorescence frequencies of them are consistent with the experimental data. The changing tendencies of the dihedral angles between anthracene unit and trifluoromethylphenyl (or thiophene) in the photoinduced dynamics processes (vertical absorption and vertical fluorescence) are examined from the geometries of optimized ground and excited states. To study the influence of the individual units of the derivatives to the excited state properties of the derivatives, the energies and densities of frontier orbital HOMOs and LUMOs of the individual unit and the derivatives are studied in the processes of vertical absorption and fluorescence. The excited state properties of the two derivatives in the processes of vertical absorption and fluorescence are studied with 2D and 3D real space analysis methods, which are employed to study the electron–hole coherence and the excitation delocalization (with transition density matrix method), and charge and energy transfer (with transition and charge difference density method). Overall, the computed results remain in good agreement with the relevant experimental data, and the theoretical results promote deeper understanding to the optical and electronic properties of the semiconductor in the process of photoinduced dynamics.     

基于元素磷源的InP量子点的制备

以白磷作为磷源、醋酸铟为铟源、硬脂酸为表面包覆剂、十八烯为溶剂,采用胶体化学法合成了InP量子点。X射线衍射(XRD)和透射电子显微镜(TEM)分析测试显示InP量子点属于立方闪锌矿结构,并且是直径约为5 nm的球状纳米晶。紫外可见光谱和荧光光谱分析表明,InP量子点表现出明显的激子吸收和带边发射特征,荧光发射光谱在415~517 nm范围内连续可调,呈现明显的量子尺寸效应。