Cu‐doped quantum dots: a new class of near‐infrared emitting fluorophores for bioanalysis and bioimaging

Transition metal ion‐doped quantum dots (QDs) exhibit unique optical and photophysical properties that offer significant advantages over undoped QDs, such as larger Stokes shift to avoid self‐absorption/energy transfer, longer excited‐state lifetimes, wider spectral window, and improved chemical and thermal stability. Among the doped QDs emitters, Cu is widely introduced into the doped QDs as novel, efficient, stable, and tunable optical materials that span a wide spectrum from blue to near‐infrared (NIR) light. Their unique physical and chemical characteristics enable the use of Cu‐doped QDs as NIR labels for bioanalysis and bioimaging. In this review, we discuss doping mechanisms and optical properties of Cu‐doped QDs that are capable of NIR emission. Applications of Cu‐doped QDs in in vitro biosensing and in in vivo bioimaging are highlighted. Moreover, a prospect of the future of Cu‐doped QDs for bioanalysis and bioimaging are also summarized.     

Dark‐red‐emitting CdTe/Cd1‐xZnxS core/shell quantum dots: preparation and properties

CdTe nanocrystals (NCs) were fabricated through an organic synthesis. The growth and properties of CdTe NCs depended strongly on the preparation conditions. In a reaction system of octadecene and tetradecylphosphonic acid (TDPA), the growth was slow. CdTe NCs with cubic‐like morphology were created in trioctylamine (TOA) using octadecylphosphonic acid (ODPA)‐CdO or TDPA‐CdO as precursors. The TOA and ODPA system gives rise to NCs with high photoluminescence (PL) efficiencies (12%). A Cd_xZn_1‐xS shell coating on the CdTe core, gave rise to tunable dark red PL (630–670 nm). The morphology and PL properties of the CdTe cores were drastically affected by shell coating and this determined the properties of CdTe/Cd_xZn_1‐xS NCs. Small CdTe NCs were easily coated with Cd_xZn_1‐xS shells. The resulting core/shell NCs revealed a spherical morphology. However, shell growth became slow when large CdTe cores were used. This is ascribed to the cores with a cubic‐like morphology. CdS interlayer plays an important role for the formation of the CdTe/Cd_xZn_1‐xS NCs because the experimental result indicated it is difficult to coat CdTe NCs with a ZnS shell. The core/shell NCs benefited from a Cd_xZn_1‐xS composite shell because CdTe/CdS NCs created via a similar procedure revealed a low PL efficiency. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis of carbon‐based quantum dots from starch extracts: Optical investigations

Carbon‐based quantum dots (C‐QDs) were synthesized through microwave‐assisted carbonization of an aqueous starch suspension mediated by sulphuric and phosphoric acids. The as‐prepared C‐QDs showed blue, green and yellow luminescence without the addition of any surface‐passivating agent. The C‐QDs were further analyzed by UV?vis spectroscopy to measure the optical response of the organic compound. The energy gaps revealed narrow sizing of C‐QDs in the semiconductor range. The optical refractive index and dielectric constant were investigated. The C‐QDs size distribution was characterized. The results suggested an easy route to the large scale production of C‐QDs materials.     

Synthesis and luminescence of novel near‐infrared emitting BaZrSi3O9:Cr3+ phosphors

The BaZrSi₃O₉:Cr³⁺ phosphors were prepared by a high temperature solid state method. Their structures were confirmed with XRD and their luminescence properties were investigated. Under excitation at 455 nm, BaZrSi₃O₉:Cr³⁺ phosphors exhibited a broad near infrared emission band peaked at 800 nm, which was assigned to the ⁴T₂→⁴A₂ transition of Cr³⁺. The near infrared emission intensity reached a maximum at Cr³⁺ concentration of 0.7%. There was a concentration quenching phenomenon of Cr³⁺ in BaZrSi₃O₉ matrix and the corresponding concentration quenching mechanism was investigated. With efficient near infrared emission in the range of 700–1000 nm, BaZrSi₃O₉:Cr³⁺ phosphors may find applications in solar energy conversion.     

Size‐dependent temperature sensitivity of photoluminescence peak position of CdTe quantum dots

The size dependence of the temperature coefficient (sensitivity) of the photoluminescence (PL) peak position of CdTe quantum dots stabilized by thioglycolic acid in aqueous solution has been investigated. Temperature sensitivity increases as the average radius of CdTe quantum dots decraeases. This must be taken into account in the design of solar light concentrators and light‐emmiting diode‐monitors as well as other technologies in which a fine tuning of the light emission is important. Copyright © 2013 John Wiley & Sons, Ltd.     

Lead sulfide near-infrared quantum dot bioconjugates for targeted molecular imaging

In this paper, we report the use of lead sulfide quantum dot (PbS QD) bioconjugates as near infrared (NIR) contrast agents for targeted molecular imaging with expanded emission wavelengths beyond 1000 nm. The red-shifted emission band, coupled with the small particle size, which will facilitate clearance, both afford PbS QDs unique properties for noninvasive, high resolution in vivo NIR imaging applications. We have performed imaging experiments at the molecular level using surface-modified PbS NIR QDs, together with our lab-built NIR imaging system. This novel instrumentation and fluorescent contrast agent have enabled us to study the relatively unexplored NIR biomedical imaging spectral region of 900-1200 nm. Preliminary experimental results indicate that PbS-QD/antibody bioconjugates are promising candidates for targeted NIR molecular imaging and future in vivo NIR tissue imaging applications.     

Tracking bio‐molecules in live cells using quantum dots

Single particle tracking (SPT) techniques were developed to explore bio‐molecules dynamics in live cells at single molecule sensitivity and nanometer spatial resolution. Recent developments in quantum dots (Qdots) surface coating and bio‐conjugation schemes have made them most suitable probes for live cell applications. Here we review recent advancements in using quantum dots as SPT probes for live cell experiments. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Near‐Infrared,Heavy Metal‐Free Colloidal “Giant” Core/Shell Quantum Dots

“Giant” core/shell quantum dots (g‐QDs) are a promising class of materials for future optoelectronic technologies due to their superior chemical‐ and photostability compared to bare QDs and core/thin shell QDs. However, inadequate light absorption in the visible and near‐infrared (NIR) region and frequent use of toxic heavy metals (e.g., Cd and Pb) are still major challenges for most g‐QDs (e.g., CdSe/CdS) synthesized to date. The synthesis of NIR, heavy metal‐free, Zn‐treated spherical CuInSe₂/CuInS₂ g‐QDs is reported using the sequential cation exchange method. These g‐QDs exhibit tunable NIR optical absorption and photoluminescence (PL) properties. Transient fluorescence spectroscopy shows prolonged lifetime with increasing shell thickness, indicating the formation of quasi type‐II band alignment, which is further confirmed by simulations. As a proof‐of‐concept, as‐synthesized g‐QDs are used to sensitize TiO₂ as a photoanode in a photoelectrochemical (PEC) cell, demonstrating an efficient and stable PEC system. These results pave the way toward synthesizing NIR heavy metal‐free g‐QDs, which are very promising components of future optoelectronic technologies.     

High quantum yield and well‐dispersed quantum dots luminescent composite through sodium carboxymethyl starch

It is a challenging task to prepare well‐dispersed and highly luminescent quantum dots (QDs) powder and a new strategy is reported in this article. Sodium carboxymethyl starch (CMS‐Na) was employed in this work to prepare the QDs–starch composite. Ultraviolet (UV) light shows that the blank starches had no fluorescence, while the QDs‐starches were highly luminescent. Scanning electron microscopy (SEM) observation shows that the QDs–starch composite has the typical particle morphology with the diameter around 200 nm. Energy dispersive X‐ray spectroscopy (EDX) results show that there are intensive tellurium (Te) and cadmium (Cd) element signals. Combined fluorescent lifetime and steady‐state spectrometer show that the QDs–starch quantum yields (QYs) increase when the QDs loading increases from 1 × 10^?6 mol/g to 2 × 10^?6 mol/g, but when the loadings further increase, the QYs decrease slightly. For the red colour (λ_em = 660 nm) QDs, the QYs can reach to as high as 28.2%, and for the other colour QDs they can also have the QYs above 22%. Time‐resolved photobleaching experiments show that the fluorescent QDs–starch composite has a half‐decay time of 40.23 s. These results indicate that the CMS‐Na is a promising QDs dispersant to obtain high QY QD composites.     

Interaction of digestive enzymes with tunable light emitting quantum dots: a thorough Spectroscopic investigation

In this article, we have examined the direct spectroscopic and microscopic evidence of efficient quantum dots‐ α‐chymotrypsin (ChT) interaction. The intrinsic fluorescence of digestive enzyme is reduced in the presence of quantum dots through ground‐state complex formation. Based on the fluorescence data, quenching rate constant, binding constant, and number of binding sites are calculated under optimized experimental conditions. Interestingly, fluorescence quenching method clearly illustrated the size dependent interaction of MPA‐CdTe quantum dots. Conformational change of ChT was traced using synchronous fluorescence measurements, circular dichroism and FTIR spectroscopic methods. Furthermore, the AFM results revealed that the individual enzyme molecule dimensions were changed after interacting with quantum dot. Consequently, this result could be helpful for constructing safe and effective utilisation of QDs in biological applications. Copyright © 2015 John Wiley & Sons, Ltd.     

Preparation and application of solvent‐modulated self‐doped N–S multicolour fluorescence carbon quantum dots

In this paper, two types of carbon quantum dot (CQDs) were prepared using biocompatible l ‐methionine as the carbon source and urea as the nitrogen source and a one‐step hydrothermal treatment. By changing the reaction solvents (deionized (DI) water and dimethylformamide (DMF)), the maximum emission of the resulting CQDs shifted from blue to red light. Specifically, the emission wavelength of the CQDs moved from 433 nm to 625 nm following embedding of a new functional group (–CONH–) on the surface of the CQDs. Photoluminescence quantum yields of the CQDs with blue and red emission reached 64% and 61%, respectively. The R‐CQDs were used to detect metal ions and a linear relationship was demonstrated between ln(F/F₀) and Fe³⁺ concentration in the range 0–0.5 mmol/L with a detection limit of 0.067 μM. Therefore these R‐CQDs have great potential as fluorescent probes for Fe³⁺ detection. We expect that the excellent water‐soluble, biocompatible and optical properties of the CQDs developed in this work mean that they will be widely used to detect biological cells.     

Synthesis of CdSe quantum dots with luminescence in the violet region of the solar spectrum

We have designed a simple, one‐step synthesis of CdSe quantum dots with photoluminescence frequencies ranging from the red through to the violet region of the solar spectrum. The photoluminescence peaks have FWHM of 30 nm indicating absorption over a narrow range of wavelengths. The effect of solvent type and solvent boiling point on the physical and photoluminescence properties of the quantum dots has been studied. High boiling point, non‐polar solvents shift the photoluminescence peak to longer wavelengths and low boiling point, polar solvents shift the photoluminescence peak to shorter wavelengths. Copyright © 2009 John Wiley & Sons, Ltd.     

Graphite oxide‐dispersed CdTe quantum dots nanocomposite for flexible display luminescent membranes

A quantum dot (QD) dispersant material was prepared using graphite oxide (GO). Luminescent films were prepared using polyvinyl alcohol as the polymer matrix. First, water‐soluble CdTe QDs were prepared by wet chemistry and GO was synthesized using a modified Hummers method. X‐Ray diffraction tests showed that the GO reflection peak [001] was 11.9°, which indicates that the d‐spacing is 0.7431 nm; atomic force microscopy showed a GO thickness of 200 nm. Fourier transform infrared spectra showed vibrations at 1624 cm^?1 for the carbonyl groups, and 3260 cm^?1 for the GO samples; the ‐C–O vibration was at 1320 cm^?1 and ‐COOH, ?OH vibrations were at 950 cm^?1. Fluorescent tests showed that pH had an impact on the QD colloidal stability. GO was neutralized before use as the host media for the GO/QDs nanocomposite. The results proved that the resultant nanocomposite is promising for use in brightness enhancement films in flexible displays.     

Using quantum dots to visualize clathrin associations

Our current understanding of clathrin-mediated endocytosis proposes that the process is initiated at a specialized anatomical structure called a coated pit. Electron microscopy has been required for elucidation of the morphology of coated pits and the vesicles produced therein, and the presence of a bristle coat has been taken as suggestive of clathrin surrounding these vesicles. More recently, immunocytochemical methods have confirmed that endocytic vesicles are surrounded by clathrin and its adaptor proteins, but there is a need to identify precisely and to follow the fate of the cellular organelles seen by fluorescence microscopy. We used quantum immune-electron microscopy to localize clathrin in a human adrenal cortical cell line (SW-13). Clathrin was shown to associate with a variety of vesicle types including the classic clathrin-coated vesicles and pits used in receptor internalization, pentilaminar annular gap junction vesicles, and multivesicular bodies. The images obtained with quantum dot technology allow accurate and specific localization of clathrin and the clathrin adaptor protein, AP-2, with cellular organelles and suggest that some of the structures classified as typical coated vesicles by immunocytochemical light microscopic techniques actually may be membrane bound pits.     

The synthesis and modification of highly fluorescent carbon quantum dots for reversible detection of water‐soluble phosphonate‐1‐hydroxyethane‐1,1‐diphosphonic acid by fluorescence spectroscopy

Photoluminescent (PL) carbon quantum dots (CQDs) were prepared successfully using a facile and green procedure. They exhibited striking blue fluorescence and excellent optical properties, with a quantum yield as high as 61.44%. Due to the fluorescence quenching effect and the stronger complexing ability of the phosphoric acid group of 1‐hydroxyethane‐1,1‐diphosphonic acid (HEDP) to Fe³⁺ , CQDs doped with Fe³⁺ were adequately constructed as an efficient and sensitive fluorescent probe for HEDP‐specific sensing. The proposed fluorescent probe had a sensitive and rapid response in the range 5–70 μ M. Furthermore, quantitative molecular surface (QMS) analysis based on the Multiwfn program was applied to explore the complexation mode of HEDP and metal ions. The distribution of electrostatic potential (ESP), average local ionization energy (ALIE), the minimum value points and the position of the lone pair electrons on the surface of molecular van der Waals were further determined. More strikingly, this experiment achieved the quantitative detection of water‐soluble phosphonate‐HEDP, for the first time using fluorescence spectrometry. It has been proved to be an effective and intuitive sensing method for the detection of HEDP in real samples.     

荧光量子点探针及其标记技术

量子点作为一种新型荧光标记物,与有机染料和荧光蛋白质相比,它们具有可调谐且宽的吸收光谱,激发可产生多重荧光颜色、强荧光信号、抗光漂白能力强等独特的光学特性,使其广泛应用在生物和医学领域。该文就量子点探针的表面修饰和功能化及其标记技术的研究进展进行了阐述。     

H2O2‐ and pH‐sensitive CdTe quantum dots as fluorescence probes for the detection of glucose

A novel fluorescence assay system for glucose was developed with thioglycollic acid (TGA)‐capped CdTe quantum dots (QDs) as probes. The luminescence quantum yield of the TGA‐capped CdTe QDs was highly sensitive to H₂O₂ and pH. In the presence of glucose oxidase, glucose is oxidized to yield, gluconic acid and H₂O₂. H₂O₂ and H⁺ (dissociated from gluconic acid) intensively quenched the fluorescence of QDs. The experimental results showed that the quenched fluorescence was proportional to the glucose concentration within the range of 0.01–5.0 mm under optimized experimental conditions. Compared with most of the existing methods, this newly developed system possesses many advantages, including simplicity, low cost, high flexibility, and good sensitivity. Furthermore, no complicated chemical modification of QDs and enzyme immobilization was needed in this system. Copyright © 2012 John Wiley & Sons, Ltd.     

Water‐soluble mercaptoundecanoic acid (MUA)‐coated CdTe quantum dots: one‐step microwave synthesis,characterization and cancer cell imaging

ABSTRACT: In this study, a one‐step approach for aqueous synthesis of highly luminescent semiconductors, CdTe quantum dots (QDs), using long‐chain thiols‐mercaptoundecanoic acid (MUA) as surface ligand, was developed in a microwave irradiation system. The synthetic conditions were systematically investigated. The as‐prepared MUA‐coated QDs were characterized by various spectroscopy techniques, transmission electron microscopy (TEM) and X‐ray powder diffraction (XRD). The experimental results document that MUA‐coated CdTe QDs have small diameter, good stability, high luminescence and long lifetime. Particularly, it was confirmed, using fluorescence correlation spectroscopy (FCS) that, compared with other ligand, MUA formed a thicker ligand layer on the QD surfaces, which will help their stability and conjugation with biomolecules. Furthermore, MUA‐coated QDs were successfully used for HeLa cell imaging. Copyright © 2011 John Wiley & Sons, Ltd.     

A sensitive fluorescent nanosensor for chloramphenicol based on molecularly imprinted polymer‐capped CdTe quantum dots

A novel fluorescent nanosensor using molecularly imprinted silica nanospheres embedded CdTe quantum dots (CdTe@SiO₂@MIP) was developed for detection and quantification of chloramphenicol (CAP). The imprinted sensor was prepared by synthesis of molecularly imprinting polymer (MIP) on the hydrophilic CdTe quantum dots via reverse microemulsion method using small amounts of solvents. The resulting CdTe@SiO₂@MIP nanoparticles were characterized by fluorescence, UV–vis absorption and FT‐IR spectroscopy and transmission electron microscopy. They preserved 48% of fluorescence quantum yield of the parent quantum dots. CAP remarkably quenched the fluorescence of prepared CdTe@SiO₂@MIP, probably via electron transfer mechanism. Under the optimal conditions, the relative fluorescence intensity of CdTe@SiO₂@MIP decreased with increasing CAP by a Stern–Volmer type equation in the concentration range of 40–500 µg L^–1. The corresponding detection limit was 5.0 µg L^–1. The intra‐day and inter‐day values for the precision of the proposed method were all <4%. The developed sensor had a good selectivity and was applied to determine CAP in spiked human and bovine serum and milk samples with satisfactory results. Copyright © 2015 John Wiley & Sons, Ltd.