Photostability comparison of CdTe and CdSe/CdS/ZnS quantum dots in living cells under single and two-photon excitations

The photostability is an outstanding feature of quantum dots (QDs) used as fluorescence probes in biological staining and cell imaging. To find out the related factors in the QD photostability, the photobleaching of naked CdTe QDs and BSA coated CdSe/CdS/ZnS QDs in human hepatocellular carcinoma (QGY) cells and human nasopharynx carcinoma (KB) cells were studied under single photon excitation (SPE) and two-photon excitation (TPE). In these two cell lines the cellular QDs were irradiated by a 405 nm continuous wave laser for SPE or an 800 nm femto-second (fs) laser for TPE. The QD photobleaching with the irradiation time was found to fit a biexponential decay. The fast decay plays a dominant role in the bleaching course and thus can be used as the parameter to quantitatively evaluate the QD photostability. The TPE decreased the QD photobleaching as compared to SPE. The BSA coated core/shell QDs had improved the photostability up to 4-5 times than the naked QDs due to the shielding effect of the QD shell. Therefore, it is better to use core/shell structured QDs as the fluorescence probe combining with a TPE manner for those long-term monitoring studies.     

Thiol-Capped CdTe Quantum Dots with Two-Photon Excitation for Imaging High Autofluorescence Background Living Cells

To effectively image living cells with quantum dots (QDs), particularly for those cells containing high content of native fluorophores, the two-photon excitation (TPE) with a femto-second 800 nm laser was employed and compared with the single-photon excitations (SPE) of 405 nm and 488 nm in BY-2 Tobacco (BY-2-T) and human hepatocellular carcinoma (QGY) cells, respectively. The 405 nm SPE produced the bright photoluminescence (PL) signals of cellular QDs but also induced a strong autofluorescence(AF) from the native fluorophores like flavins in cells. The AF occupied about 30% and 13% of the total signals detected in QD imaging channel in the BY-2-T and QGY cells, respectively. With the excitation of 488 nm SPE, the PL signals were lower than those excited with the 405 nm SPE, although the AF signals were also reduced. The 800 nm TPE generated the best PL images of intracellular QDs with the highest signal ratio of PL to AF, because the two-photon absorption cross section of QDs is much higher than that of the native fluorophores. By means of the TPE, the reliable cellular imaging with QDs, even for the cells having the high AF background, can be achieved.     

Physical reasons of emission transformation in infrared CdSeTe/ZnS quantum dots at bioconjugation

The core/shell CdSeTe/ZnS quantum dots (QDs) with emission at 780–800 nm (1.55–1.60 eV) have been studied by means of photoluminescence (PL) and Raman scattering methods in the nonconjugated state and after conjugation to different antibodies (Ab): (i) mouse monoclonal [8C9] human papilloma virus Ab, anti-HPV 16-E7 Ab, (ii) mouse monoclonal [C1P5] human papilloma virus HPV16 E6+HPV18 E6 Ab, and (iii) pseudo rabies virus (PRV) Ab. The transformations of PL and Raman scattering spectra of QDs, stimulated by conjugated antibodies, have been revealed and discussed.The energy band diagram of core/shell CdSeTe/ZnS QDs has been designed that helps to analyze the PL spectra and their transformations at the bioconjugation. It is shown that the core in CdSeTe/ZnS QDs is complex and including the type II quantum well. The last fact permits to explain the nature of infrared (IR) optical transitions (1.55–1.60 eV) and the high energy PL band (1.88–1.94 eV) in the nonconjugated and bioconjugated QDs. A set of physical reasons has been analyzed with the aim to explain the transformation of PL spectra in bioconjugated QDs. Finally it is shown that two factors are responsible for the PL spectrum transformation at bioconjugation to charged antibodies: (i) the change of energy band profile in QDs and (ii) the shift of QD energy levels in the strong quantum confinement case. The effect of PL spectrum transformation is useful for the study of QD bioconjugation to specific antibodies and can be a powerful technique for early medical diagnostics.     

Re-charging the luminescence states in CdSe/ZnS quantum dots at the conjugation to Osteopontin antibodies

The paper presents the original study of photoluminescence (PL) and Raman scattering spectra of core–shell CdSe/ZnS quantum dots (QDs) covered by the amine-derivatized polyethylene glycol (PEG) with luminescence interface states. First commercially available CdSe/ZnS QDs with emission at 640 nm (1.94 eV) covered by PEG polymer have been studied in nonconjugated states. PL spectra of nonconjugated QDs are characterized by a superposition of PL bands related to exciton emission in a CdSe core and to the hot electron–hole recombination via high energy luminescence states. The study of high energy PL bands in QDs at different temperatures has shown that these PL bands are related to luminescence interface states at the CdSe/ZnS or ZnS/polymer interface. Then CdSe/ZnS QDs have been conjugated with biomolecules—the Osteopontin antibodies. It is revealed that the PL spectrum of bioconjugated QDs changed essentially with decreasing hot electron–hole recombination flow via luminescence interface states. It is shown that the QD bioconjugation process to Osteopontin antibodies is complex and includes the covalent and electrostatic interactions between them. The variation of PL spectra due to the bioconjugation is explained on the basis of electrostatic interaction between the QDs and biomolecule dipoles that stimulates re-charging QD interface states. The study of Raman scattering of bioconjugated CdSe/ZnS QDs has confirmed that the antibody molecules have the electric dipoles. It is shown that CdSe/ZnS QDs with luminescence interface states are promising for the study of bioconjugation effects with specific antibodies and can be a powerful technique in biology and medicine.     

Temperature-dependent photoluminescence of CuInS2 quantum dots

Temperature-dependent photoluminescence (PL) spectroscopy of CuInS₂ core and CuInS₂/ZnS core–shell quantum dots (QDs) was studied for understanding the influence of a ZnS shell on the PL mechanism. The PL quantum yield and lifetime of CuInS₂ core QDs were significantly enhanced after the QD surface was coated with the ZnS shell. The temperature dependences of the PL energy, linewidth, and intensity for the core and core–shell QDs were studied in the temperature range from 92 to 287 K. The temperature-dependent shifts of 98 meV and 35 meV for the PL energies of the QDs were much larger than those of the excitons in their bulk semiconductors. It was surprisingly found that the core and core–shell QDs exhibited a similar temperature dependence of the PL intensity. The PL in the CuInS₂/ZnS core–shell QDs was suggested to originate from recombination of many kinds of defect-related emission centers in the interior of the cores.     

Power density and temperature dependent multi-excited states in InAs/GaAs quantum dots

Self-assembled InAs/GaAs (001) quantum dots (QDs) were grown by molecular beam epitaxy using ultra low-growth rate. A typical dot diameter of around 28 ± 2 nm and a typical height of 5 ± 1 nm are observed based on atomic force microscopy image. The photoluminescence (PL) spectra, their power and temperature dependences have been studied for ground (GS) and three excited states (1–3ES) in InAs QDs. By changing the excitation power density, we can significantly influence the distribution of excitons within the QD ensemble. The PL peak energy positions of GS and ES emissions bands depend on an excitation light power. With increasing excitation power, the GS emission energy was red-shifted, while the 1–3ES emission energies were blue-shifted. It is found that the full width at half maximum of the PL spectra has unusual relationship with increasing temperature from 9 to 300 K. The temperature dependence of QD PL spectra shown the existence of two stages of PL thermal quenching and two distinct activation energies corresponding to the temperature ranges I (9–100 K) and II (100–300 K).     

Preparation of Highly Stable and Photoluminescent Cadmium-Free InP/GaP/ZnS Core/Shell Quantum Dots and Application to Quantitative Immunoassay

Indium phosphide (InP) quantum dots (QDs) are ideal substitutes for widely used cadmium-based QDs and have great application prospects in biological fields due to their environmentally benign properties and human safety. However, the synthesis of InP core/shell QDs with biocompatibility, high quantum yield (QY), uniform particle size, and high stability is still a challenging subject. Herein, high quality (QY up to 72%) thick shell InP/GaP/ZnS core/shell QDs (12.8 ± 1.4 nm) are synthesized using multiple injections of shell precursor and extension of shell growth time, with GaP serving as the intermediate layer and 1-octanethiol acting as the new S source. The thick shell InP/GaP/ZnS core/shell QDs still keep high QY and photostability after transfer into water. InP/GaP/ZnS core/shell QDs as fluorescence labels to establish QD-based fluorescence-linked immunosorbent assay (QD-FLISA) for quantitative detection of C-reactive protein (CRP), and a calibration curve is established between fluorescence intensity and CRP concentrations (range: 1–800 ng mL⁻¹, correlation coefficient: R² = 0.9992). The limit of detection is 2.9 ng mL⁻¹, which increases twofold compared to previously reported cadmium-free QD-based immunoassays. Thus, InP/GaP/ZnS core/shell QDs as a great promise fluorescence labeling material, provide a new route for cadmium-free sensitive and specific immunoassays in biomedical fields.     

The effect of bio-conjugation on aging of the photoluminescence in CdSeTe–ZnS core–shell quantum dots

The aging of the photoluminescence (PL) in bio-conjugated and non-conjugated CdSeTe–ZnS core–shell quantum dots (QDs) is studied by the micro-PL, micro-Raman and X-ray diffraction (XRD) in the samples of buffered QD solution dried on a crystalline Si wafer and stored in the atmospheric ambience for about 2 years. The aging of the PL consists in a “blue” spectral shift of the PL band, an increase in PL band half-width and the decrease in the PL intensity. These changes are more pronounced in the conjugated QD samples. The XRD analysis of the aged samples revealed that the QD core diameter is reduced by ∼1.5 nm in the conjugated QDs as compared to the non-conjugated ones. The possible mechanism of PL spectrum aging is the oxidation that decreases the QD core dimension. It is concluded that the bio-conjugation promotes QD oxidation and the mechanism of the effect is proposed.     

Biotoxicity of nanoparticles: effect of natural organic matter

Various natural organic matters (NOM) with different characteristics in aquatic environment may affect toxicity of leased nanoparticles, owing to interactions between NOM and nanoparticles. This study investigated the effect of NOM and physical characteristics of the effluent organic matter (EfOM) on the ecotoxicity of quantum dots (QD) using Daphnia magna. Organic matter samples were obtained from: Yeongsan River (YR-NOM), Dongbuk Lake (DL-NOM), Damyang wastewater treatment plant (EfOM), and Suwannee River NOM (SR-NOM). The QD was composed of a CdSe core, ZnS shell, and polyethylene glycol coating. The average size of the investigated QD was 4.8, 56.5, and 25.0 nm determined by transmission electron microscopy, dynamic light scattering, and asymmetric flow field-flow fractionation, respectively. The relative hydrophobicity of NOM was investigated using both specific UV absorbance at 254 nm and XAD-8/4 resins. The sorption of NOM on the QD was measured using a fluorescence quenching method. The highest hydrophobicity was exhibited by the SR-NOM, while the lowest was recorded for the DL-NOM. All tested NOMs significantly reduced the acute toxicity of D. magna when adsorbed to QD, and the order of effectiveness for each NOM was as follows: SR-NOM > EfOM > YS-NOM > DL-NOM. The sorption of NOM on the QD surface caused a decrease in the fluorescence intensity of QD at increasing NOM concentration. This suggests that the NOM coating influenced the physicochemical characteristics of QD in the internal organs of D. magna by inducing a reduced bioavailability. Results from this study revealed that NOM with relatively high hydrophobicity had a greater capability of inducing toxicity mitigation.     

Manganese Doped Zinc Sulfide Quantum Dots for Detection of Escherichia coli

A novel biocompatible chitosan passivated manganese doped zinc sulfide (Mn doped ZnS) nanophosphor has been synthesized through a simple aqueous precipitation reaction. Upon excitation with ultraviolet light, the quantum dots (QDs) emit an orange luminescence peaking at 590 nm, which is visible to the naked eye. These chitosan coated Mn doped ZnS QDs can have potential applications in bio-labeling, particularly in fluorescence-based imaging. One of the envisioned applications of these QDs is in improving the conventional, organic dye-reliant Fluorescence in situ Hybridization (FISH) technique, a widely used method for microbial detection. Here we demonstrate that the chitosan-capped Mn doped ZnS QDs are suitable for this purpose.     

Photoinduced relaxation processes in self-assembling complexes from CdSe/ZnS water-soluble nanocrystals and cationic porphyrins

Particular features and quenching mechanisms of exciton luminescence of water-soluble nanocomposites that are formed as a result of the interaction of surface charged semiconductor quantum dots (QDs) CdSe/ZnS (d _CdSe = 2.8 nm) and cationic porphyrins (H₂TMPyrP⁴⁺ and ZnTMPyrP⁴⁺) have been studied theoretically and experimentally. It has been found that, in CdSe/ZnS??Porphyrin conjugates, there occurs long-range inductive resonance electronic excitation energy transfer from surface modified (with thioglycolic or mercaptoundecanoic acid) QDs to porphyrins, which is accompanied by quenching of the exciton luminescence of QDs and an increase in the fluorescence intensity of porphyrin. It has been shown that, when mercaptoundecanoic acid is used as a QD shell, the QD luminescence quenching efficiency by porphyrins follows the F?rster-Galanin theory and depends on the overlap integral between the CdSe/ZnS luminescence band and the absorption spectra of free-base porphyrin H₂TMPyrP⁴⁺ and its metal complex ZnTMPyrP⁴⁺. It has been revealed that, as the QDs ? Zn-porphyrin intercenter distance decreases from 39.1 (mercaptoundecanoic acid) to 30.1), a considerable QD luminescence quenching is observed; however, the energy transfer efficiency substantially decreases, from 55% in the former case to 23% in the latter one. Based on the spectral-luminescent data and quantum-chemical calculations, it has been found that the chemical change of H₂TMPyrP⁴⁺ in the structure of the complex with CdSe/ZnS QDs passivated by thioglycolic or mercaptoundecanoic acid is caused by the formation of a metal complex ZnTMPyrP⁴⁺. Based on calculations of the redox-potentials, it has been concluded that the low luminescence quantum yield of CdSe/ZnS QDs passivated by residues of mercaptocarboxylic acids S^?(CH₂) _n COO^? and its dependence on the number of CH₂ groups are related to the possibility of photoinduced electron transfer from the HOMO of passivating molecules to QDs (QD* ? S^?(CH₂)nCOO^? hole transfer). It has been shown that the quenching of the exciton luminescence of QDs in heterogeneous structures CdSe/ZnS(thioglycolic acid)??ZnTMPyrP⁴⁺, which is complementary to the energy transfer, can be caused by the photoinduced electron transfer that involves the participation of the LUMO of the ZnTMPyrP⁴⁺ molecule (QD* ? ZnTMPyrP⁴⁺).     

Fabrication of rare-earth/quantum-dot nanocomposites for color-tunable sensing applications

We developed a new fluorescent nanocomposite by using a layer-by-layer approach to link NaYF₄:Ce,Tb rare-earth (RE) nanocrystals and CdSe/ZnSe semiconductor quantum dots (QDs) with opposite charges. Under ultraviolet light excitation, the nanocomposites exhibited both the green Tb emission centered at 550 nm, and the red QD emission at 650 nm. Sensing applications showed that the red QD emission was quenched by trace amount of Cu²⁺ (or Ag⁺) ions due to the ion displacement mechanism, while the green RE emission kept constant. Thus, the nanocomposites with the decreased QD/RE emission intensity ratio and changed fluorescence output color provided a visible “indicator” to detect metal ions quantificationally. In comparison with single emission materials, the dual emission nanocomposites can be a more reliable probe for various sensing applications.     

Determination of rifampicin based on fluorescence quenching of GSH capped CdTe/ZnS QDs

Aqueous glutathione (GSH)-capped CdTe/ZnS QDs with the diameter of 3–4 nm were synthesized. The fluorescence of CdTe/ZnS QDs at 577 nm was quenched in the presence of rifampicin (Rfp), with excitation wavelength at 350 nm. The mechanism of the interaction of CdTe/ZnS QDs with Rfp was investigated. Under the optimal conditions, the calibration plot of ln(F₀/F) was linear in the range 0.83–56 μg mL^?1 with concentration of Rfp, and the detection limit was 0.25 μg mL^?1. The proposed method was successfully applied to the determination of Rfp in its commercial capsules, and satisfactory results were obtained. The recovery of the method was in the range 98.6–103.2%.     

Facile synthesis of mercaptosuccinic acid-capped CdTe/CdS/ZnS core/double shell quantum dots with improved cell viability on different cancer cells and normal cells

Water-soluble, mercaptosuccinic acid (MSA)-capped CdTe/CdS/ZnS core/double shell quantum dots (QDs) were prepared by successive growth of CdS and ZnS shells on the as-synthesized CdTe/CdS_thin core/shell quantum dots. The formation of core/double shell structured QDs was investigated by ultraviolet-visible (UV–Vis) absorption and photoluminescence (PL) spectroscopy, PL decay studies, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The core/double shell QDs exhibited good photoluminescence quantum yield (PLQY) which is 70% higher than that of the parent core/shell QDs, and they are stable for months. The average particle size of the core/double shell QDs was ～3 nm as calculated from the transmission electron microscope (TEM) images. The cytotoxicity of the QDs was evaluated on a variety of cancer cells such as HeLa, MCF-7, A549, and normal Vero cells by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) cell viability assay. The results showed that core/double shell QDs were less toxic to the cells when compared to the parent core/shell QDs. MCF-7 cells showed proliferation on incubation with QDs, and this is attributed to the metalloestrogenic activity of cadmium ions released from QDs. The core/double shell CdTe/CdS/ZnS (CSS) QDs were conjugated with transferrin and successfully employed for the biolabeling and fluorescent imaging of HeLa cells. These core/double shell QDs are highly promising fluorescent probe for cancer cell labeling and imaging applications.     

All‐optical modulation based on silicon quantum dot doped SiOx:Si‐QD waveguide

All‐optical modulation based on silicon quantum dot doped SiO_x:Si‐QD waveguide is demonstrated. By shrinking the Si‐QD size from 4.3 nm to 1.7 nm in SiO_x matrix (SiO_x:Si‐QD) waveguide, the free‐carrier absorption (FCA) cross section of the Si‐QD is decreased to 8 × 10⁻¹⁸ cm² by enlarging the electron/hole effective masses, which shortens the PL and Auger lifetime to 83 ns and 16.5 ps, respectively. The FCA loss is conversely increased from 0.03 cm⁻¹ to 1.5 cm⁻¹ with the Si‐QD size enlarged from 1.7 nm to 4.3 nm due to the enhanced FCA cross section and the increased free‐carrier density in large Si‐QDs. Both the FCA and free‐carrier relaxation processes of Si‐QDs are shortened as the radiative recombination rate is enlarged by electron–hole momentum overlapping under strong quantum confinement effect. The all‐optical return‐to‐zero on‐off keying (RZ‐OOK) modulation is performed by using the SiO_x:Si‐QD waveguides, providing the transmission bit rate of the inversed RZ‐OOK data stream conversion from 0.2 to 2 Mbit/s by shrinking the Si‐QD size from 4.3 to 1.7 nm.     

CdS_xSe_(1-x)/ZnS(核/壳)量子点的光谱截面及其掺杂光纤的传光特性

测量了不同组份比例x的CdS_xSe_(1-x)/ZnS(核/壳)量子点的吸收谱和发射谱,确定了量子点的吸收系数、吸收截面和发射截面.量子点吸收截面随粒径的增大而增大、随x的增大而减小.采用紫外固化胶,制备了掺杂浓度为0.1~5mg/mL的CdS_(0.4)Se_(0.6)/ZnS量子点光纤,测量了不同掺杂浓度量子点光纤中473nm泵浦功率的吸收衰减速率.吸收衰减速率和吸收截面弱关联于掺杂浓度.测量了光致荧光光谱强度随光纤长度和量子点浓度的变化.量子点光纤的光致荧光峰值强度随掺杂浓度和光纤长度变化而变化,且存在一个与最大峰值强度对应的饱和掺杂浓度和光纤长度.本文的实验结果有助于进一步构建新型的CdS_xSe_(1-x)/ZnS量子点增益型光电子器件.     

Surface-initiated ring-opening metathesis polymerization (SI-ROMP) to attach a tethered organic corona onto CdSe/ZnS core/shell quantum dots

Core–shell CdSe/ZnS quantum dots (QDs) are useful as tunable photostable fluorophores for multiple applications in industry, biology, and medicine. However, to achieve the optimum optical properties, the surface of the QDs must be passivated to remove charged sites that might bind extraneous substances and allow aggregation. Here we describe a method of growing an organic polymer corona onto the QD surface using the bottom-up approach of surface-initiated ring-opening metathesis polymerization (SI-ROMP) with Grubbs catalyst. CdSe/ZnS QDs were first coated with mercaptopropionic acid by displacing the original tri-octylphosphine oxide layer, and then reacted with 7-octenyl dimethyl chlorosilane. The resulting octenyl double bonds allowed the attachment of ruthenium alkylidene groups as a catalyst. A subsequent metathesis reaction with strained bicyclic monomers (norbornene-dicarbonyl chloride (NDC), and a mixture of NDC and norbornenylethylisobutyl-polyhedral oligomeric silsesquioxane (norbornoPOSS)) allowed the construction of tethered organic homo-polymer or co-polymer layers onto the QD. Compounds were characterized by FT-IR, 1H-NMR, X-ray photoelectron spectroscopy, differential scanning calorimetry, and transmission electron microscopy. Atomic force microscopy showed that the coated QDs were separate and non-aggregated with a range of diameter of 48–53 nm.     

Energy transfer in associates of semiconductor quantum dots with tetrapyridinoporphyrazine molecules

The interaction of CdSe/ZnS quantum dots (QDs) with metal-free tetrapyridinoporphyrazine (TPPA) molecules in chloroform has been investigated. It was found that, at QD concentrations lower than ～3 × 10^?7 M, QD luminescence is quenched in the presence of TPPA and characteristic changes occur in the absorption and luminescence spectra of TPPA, which reflect the interaction of TPPA molecules with the QD surface. Along with the QD luminescence quenching, sensitized luminescence of TPPA molecules adsorbed on QDs was observed. The luminescence excitation spectra of adsorbed TPPA molecules unambiguously indicate the presence of energy transfer from QDs to TPPA. The efficiency of energy transfer from QDs to TPPA is estimated from the quantum yield of sensitized TPPA luminescence.     

Aqueous phase-synthesized small CdSe quantum dots: adsorption layer structure and strong band-edge and surface trap emission

We synthesized, in aqueous solution at room temperature, small water-soluble CdSe quantum dots (QDs) with strong photoluminescence (PL) and then correlated the PL with their adsorption layer structure. For synthesizing the QDs, their initial synthesis condition was controlled to form small Cd-containing species capable of passivating dangling bonds on the CdSe core surface. Each CdSe QD (d ~ 2.5 nm) consisted of a CdSe core (d ~ 2.1 nm), a cysteine (cys)-ligand shell, and an adsorption layer composed of Cd–cys complexes (mainly CdL(-H)⁻, cys ≡ H₂L), cys (as L²⁻), Cd(OH)₂, and CdO _x (x ≥ 1). Our CdSe QDs showed strong blue band-edge PL as well as strong green surface trap PL. Their PL quantum yield (QY) of ~18% was unexpectedly high, considering their extremely small core size and their absence of any wide-bandgap inorganic shell. We attributed the QY to their adsorption layer species. The small weakly charged Cd–cys complex and the small neutral cadmium oxides in the adsorption layer could relatively readily diffuse into the unprotected surface sites on the core. These wide-bandgap species coalesced selectively on the unprotected surface sites with minimal spatial disturbance to the preexisting surface Cd-ligand coordination, and passivated them effectively. These decreased nonradiative recombination of the excitons significantly and thus led to the unexpectedly high QYs.     

CdSe/ZnSe/ZnS多壳层结构量子点的制备与表征

展示了一种简捷的多壳层量子点合成路线。在含有过量Se源的CdSe体系中直接注入Zn源,"一步法"合成了CdSe/ZnSe量子点;进一步以CdSe/ZnSe为"核",表面外延生长ZnS壳层制备了核/壳/壳结构CdSe/ZnSe/ZnS量子点。相对于以往报道的多壳层结构量子点的制备方法,该方法通过减少壳层的生长步骤有效地简化了实验操作,缩短了实验周期,同时减少对原料的损耗。对量子点进行高温退火处理,能够大幅提高CdSe/ZnSe/ZnS量子点的发光量子产率。透射电镜、XRD以及光谱研究表明:所制备的量子点接近球形,核与壳层纳米晶均为闪锌矿结构,最终获得的CdSe/ZnSe/ZnS量子点的光致发光量子产率达到53％。为了实现量子点的表面生物功能化,通过巯基酸进行了表面配体交换修饰,使量子点表面具有水溶性的羧基功能团,并且能够维持较高的光致发光量子产率。