Synthesis of Multifunctional Mn‐Doped CdTe/ZnS Core/Shell Quantum Dots

The synthesis of a novel water‐soluble Mn‐doped CdTe/ZnS core‐shell quantum dots using a proposed ultrasonic assistant method and 3‐mercaptopropionic acid (MPA) as stabilizer is descried. To obtain a high luminescent intensity, post‐preparative treatments, including the pH value, reaction temperature, reflux time and atmosphere, have been investigated. For an excellent fluorescence of Mn‐doped CdTe/ZnS, the optimal conditions were pH 11, reflux temperature 100°C and reflux time 3 h under N₂ atmosphere. While for phosphorescent Mn‐doped CdTe/ZnS QDs, the synthesis at pH 11, reflux temperature 100°C and reflux time 3 h under air atmosphere gave the best strong phosphorescence. The characterizations of Mn‐doped CdTe/ZnS QDs were also identified using AFM, IR, powder XRD and thermogravimetric analysis. The data indicated that the photochemical stability and the photoluminescence of CdTe QDs are greatly enhanced by the outer inorganic ZnS shell, and the doping Mn²⁺ ions in the as‐prepared quantum dots contribute to strong luminescence. The strong luminescence of Mn‐doped CdTe/ZnS QDs reflected that Mn ions act as recombination centers for the excited electron‐hole pairs, attributing to the transition from the triplet state (⁴T₁) to the ground state (⁶A₁) of the Mn²⁺ ions. All the experiments demonstrated that the surface states played important roles in the optical properties of Mn‐doped CdTe/ZnS core‐shell quantum dots.     

Highly Luminescent N‐Doped Carbon Quantum Dots as an Effective Multifunctional Fluorescence Sensing Platform

The doping of carbon quantum dots with nitrogen provides a promising direction to improve fluorescence performance and broaden their applications in sensing systems. Herein we report a one‐pot solvothermal synthesis of N‐doped carbon quantum dots (NCQDs) and the synthesis of a series of NCQDs with different nitrogen contents. The as‐prepared NCQDs were compared with carbon quantum dots (CQDs); the introduction of nitrogen atoms largely increased the quantum yield of NCQDs and highest emission efficiency is up to 36.3 %. The fluorescence enhancement may originate from more polyaromatic structures induced by incorporated nitrogen atoms and protonation of nitrogen atoms on dots. It was found that NCQDs can act as a multifunctional fluorescence sensing platform because they can be used to detect pH values, Ag^I, and Fe^III in aqueous solution. The fluorescence intensity of NCQDs is inversely proportional to pH values across a broad range from 5.0 to 13.5, which indicates that NCQDs can be devised as an effective pH indicator. Selective detection of Ag^I and Fe^III was achieved based on their distinctive fluorescence influence because Ag^I can significantly enhance the fluorescence whereas Fe^III can greatly quench the fluorescence. The quantitative determination of Ag^I can be accomplished with NCQDs by using the linear relationship between fluorescence intensity of NCQDs and concentration of Ag^I. The sensitive detection of H₂O₂ was developed by taking advantage of the distinct quenching ability of Fe^III and Fe^II toward the fluorescence of NCQDs. Cellular toxicity test showed NCQDs still retain low toxicity to cells despite the introduction of a great deal of nitrogen atoms. Moreover, bioimaging experiments demonstrated that NCQDs have stronger resistance to photobleaching than CQDs and more excellent fluorescence labeling performance.     

Photoelectrochemical Immunosensor Array Based on Thioglycolic Acid Capped CdS Quantum Dots for Multiplexed Detection of Veterinary Drug Residues

A photoelectrochemical immunosensor based on multi‐electrode array was developed for simultaneous and sensitive determination of veterinary drug residues. In this system, poly(dimethyldiallylammonium chloride) (PDDA), Au nanoparticles (Au NPs) and thioglycolic acid (TGA)‐capped CdS quantum dots (QDs) were layer‐by‐layer assembled onto the home‐made Au electrode array. The assembling process of the (CdS/PDDA/Au NPs/PDDA)_n multilayer was characterized by electrochemical impedance spectroscopy. And then the antibodies for clenbuterol (CB), ractopamine (RAC) and chloramphenicol (CAP) were covalently immobilized onto the Au electrode array by 1‐ethyl‐3‐(3‐dimethylaminopropyl) carbodiimide (EDC) coupling reaction, respectively. The concentrations of CB, RAC and CAP were measured based on the photoelectrochemical effects of CdS QDs. Under the optimal conditions, the limits of detection (LOD) for CB, RAC and CAP were 25, 50 and 2.2 pg/mL (3Δ), respectively, with acceptable recovery over the range of 95.40%–105.5% in pig liver samples. All results indicate that the immunosensor array system has potential application for practical, effective and high throughput analysis of veterinary drugs residues.     

Paramagnetic Relaxation of Long‐Lived Coherences in Solution NMR

Long‐lived coherences (LLCs) are known to have lifetimes much longer than transverse magnetization or single quantum coherences (SQCs). The effect of paramagnetic ions on the relaxation of LLCs is not known. This is particularly important, as LLCs have potential applications in various fields like analytical NMR, in vivo NMR and MR imaging methods. We study here the behaviour of LLCs in the presence of paramagnetic relaxation agents. The stepwise increase in the concentration of the metal ion is followed by measuring various relaxation rates. The effect of paramagnetic ions is analysed in terms of the external random field’s contribution to the relaxation of two coupled protons in 2,3,6‐trichlorobenzaldehyde. The LLCs relax faster than ordinary SQCs in the presence of paramagnetic ions of varying character. This is explained on the basis of an increase in the contribution of the external random field to relaxation due to a paramagnetic relaxation mechanism. Comparison is also made with ordinary Zeeman relaxation rates like R₁, R₂, R_1ρ and also with rate of relaxation of long‐lived states R_LLS which are known to be less sensitive to paramagnetically induced relaxation. Also, the extent of correlation of random fields at two proton sites is studied and is found to be strongly correlated with each other. The obtained correlation constant is found to be independent of the nature of added paramagnetic impurities.     

Ascorbic Acid Induced Enhancement of Room Temperature Phosphorescence of Sodium Tripolyphosphate‐Capped Mn‐Doped ZnS Quantum Dots: Mechanism and Bioprobe Applications

Although quantum dot (QD)‐based room temperature phosphorescence (RTP) probes are promising for practical applications in complex matrixes such as environmental, food and biological samples, current QD‐based‐RTP probes are not only quite limited but also exclusively based on the RTP quenching mechanism. Here we report an ascorbic acid (AA) induced phosphorescence enhancement of sodium tripolyphosphate‐capped Mn‐doped ZnS QDs, and its application for turn‐on RTP detection. The chelating ability allows AA to extract the Mn and Zn from the surface of the QDs and to generate more holes which are subsequently trapped by Mn²⁺, while the reducing property permits AA to reduce Mn³⁺ to Mn²⁺ in the excited state, thereby enhancing the excitation and orange emission of the QDs. The enhanced RTP intensity of the QDs increases linearly with the concentration of AA in the range of 0.05–0.8 μM . Thus, a QD‐based RTP probe for AA is developed. The proposed QD‐based turn‐on RTP probe avoids tedious sample pretreatment, and offers good sensitivity and selectivity for AA in the presence of the main relevant metal ions and other molecules in biological fluids. The limit of detection (3s) of the developed method is 9 nM AA, and the relative standard deviation is 4.8 % for 11 replicate detections of 0.1 μM AA. The developed method is successfully applied to the analysis of real samples of human urine and plasma for AA with quantitative recoveries from 96 to 105 %.     

Application of Luminescent BSA‐Capped CdS Quantum Dots as a Fluorescence Probe for the Detection of Cu2+

CdS Quantum dots (QDs) are synthesized in aqueous solution with bovine serum albumin (BSA) as the stabilizing agents using a stepwise procedure at room temperature. They can be used as a new fluorescence probe in the determination of Cu²⁺, which is proved to be a simple, rapid and specific method. Approximate concentrations of other physiologically important cations, such as Zn²⁺, K⁺ etc, have no effect on the fluorescence. Under the optimum condition, the response is linear to the concentration of Cu²⁺ in the range of 0‐80.0 μol/L, and the detection limit is 50.0 nmol/L. This method is used to the determination of Cu²⁺ in human hair, the recovery is 98‐104%. And the possible detection mechanism is also discussed.     

Optically Transparent Electrodes Modified with Sulfide Ion‐Covered CdS Quantum Dots for Sensitive Photoelectrochemical Detection of Sulfhydryl‐Containing Compounds

A sensitive and selective photoelectrochemical sensor has been developed. Negatively charged, citrate‐capped CdS quantum dots (QDs) are assembled layer‐by‐layer onto indium tin oxide (ITO) electrodes precoated with positively charged poly(diallyldimethylammonium chloride). By exposing the modified photoelectrochemical electrodes to a Na₂S solution, QDs can be covered with excess S^2? ions. The weakly bound S^2? ions are easily replaceable with a sulphydryl‐containing analyte, and the photocurrent decrease is proportional to the analyte concentration. The detection limit is 0.4 nM for cysteine with a linear range of 1.0–100.0 nM. The level of cysteine in human serum samples has been quantified.     

Protein‐Directed Synthesis of Mn‐Doped ZnS Quantum Dots: A Dual‐Channel Biosensor for Two Proteins

Proteins typically have nanoscale dimensions and multiple binding sites with inorganic ions, which facilitates the templated synthesis of nanoparticles to yield nanoparticle–protein hybrids with tailored functionality, water solubility, and tunable frameworks with well‐defined structure. In this work, we report a protein‐templated synthesis of Mn‐doped ZnS quantum dots (QDs) by exploring bovine serum albumin (BSA) as the template. The obtained Mn‐doped ZnS QDs give phosphorescence emission centered at 590 nm, with a decay time of about 1.9 ms. A dual‐channel sensing system for two different proteins was developed through integration of the optical responses (phosphorescence emission and resonant light scattering (RLS)) of Mn‐doped ZnS QDs and recognition of them by surface BSA phosphorescent sensing of trypsin and RLS sensing of lysozyme. Trypsin can digest BSA and remove BSA from the surface of Mn‐doped ZnS QDs, thus quenching the phosphorescence of QDs, whereas lysozyme can assemble with BSA to lead to aggregation of QDs and enhanced RLS intensity. The detection limits for trypsin and lysozyme were 40 and 3 nM , respectively. The selectivity of the respective channel for trypsin and lysozyme was evaluated with a series of other proteins. Unlike other protein sensors based on nanobioconjugates, the proposed dual‐channel sensor employs only one type of QDs but can detect two different proteins. Further, we found the RLS of QDs can also be useful for studying the BSA–lysozyme binding stoichiometry, which has not been reported in the literature. These successful biosensor applications clearly demonstrate that BSA not only serves as a template for growth of Mn‐doped ZnS QDs, but also impacts the QDs for selective recognition of analyte proteins.     

Long‐Lived Spin States for Low‐Field Hyperpolarized Gas MRI

Parahydrogen induced polarization was employed to prepare a relatively long‐lived correlated nuclear spin state between methylene and methyl protons in propane gas. Conventionally, such states are converted into a strong NMR signal enhancement by transferring the reaction product to a high magnetic field in an adiabatic longitudinal transport after dissociation engenders net alignment (ALTADENA) experiment. However, the relaxation time T₁ of ～0.6 s of the resulting hyperpolarized propane is too short for potential biomedical applications. The presented alternative approach employs low‐field MRI to preserve the initial correlated state with a much longer decay time T_LLSS=(4.7±0.5) s. While the direct detection at low‐magnetic fields (e.g. 0.0475 T) is challenging, we demonstrate here that spin‐lock induced crossing (SLIC) at this low magnetic field transforms the long‐lived correlated state into an observable nuclear magnetization suitable for MRI with sub‐millimeter and sub‐second spatial and temporal resolution, respectively. Propane is a non‐toxic gas, and therefore, these results potentially enable low‐cost high‐resolution high‐speed MRI of gases for functional imaging of lungs and other applications.     

Localization and Dynamics of Long‐Lived Excitations in Colloidal Semiconductor Nanocrystals with Dual Quantum Confinement

Semiconductor nanocrystals consisting of a quantum dot (QD) core and a quantum well (QW) shell, where the QD and QW are separated by a tunneling barrier, offer a unique opportunity to engineer the photophysical properties of individual nanostructures. Using the thicknesses of the corresponding layers, the excitons of the first and second excited states can be separated spatially, localizing one state to the QD and the other to the QW. Thus the wave function overlap of the two states can be minimized, suppressing non‐radiative thermalization between the two wells, which in turn leads to radiative relaxation from both states. The molecular analogy to such dual emission would be the inhibition of internal conversion, a special case that violates Kasha′s rule. Using nanosecond time‐resolved spectroscopy of QDQW CdSe/ZnS onion‐like nanocrystals, an intermediate regime of exciton separation and suppressed thermalization is identified where the non‐radiative relaxation of the higher‐energy state is slowed, but not completely inhibited. In this intermediate thermalization regime, the temporal evolution of the delayed emission spectra resulting from trapped carriers mimic the dynamics of such states in nanocrystals that consist of only a QD core. In stark contrast, when a higher‐energy metastable state exists in the QW shell due to strongly suppressed interwell thermalization, the spectral dynamics of the long‐lived excitations in the QD and QW, which are spectrally distinct, are amplified and differ from each other as well as from those in the core‐only nanocrystals. This difference in spectral dynamics demonstrates the utility of exploiting well‐defined exciton localization to study the nature and spatial dependence of the intriguing photophysics of colloidal semiconductor nanocrystals, and illustrates the power of nanosecond gated luminescence spectroscopy in illuminating complex relaxation dynamics which are entirely masked in steady‐state or ultrafast spectroscopy.     

One‐Pot Green Synthesis of Nitrogen‐Doped Carbon Quantum Dots for Cell Nucleus Labeling and Copper(II) Detection

The doping of nitrogen into carbon quantum dots is vitally important for improved fluorescence performance. However, the synthesis of nitrogen‐doped carbon quantum dots (N‐CQDs) is usually conducted under strong acid and high temperature, which results in environmental pollution and energy consumption. Herein, the N‐CQDs were prepared by a mild one‐pot hydrothermal process. The hydrothermal reaction temperature was adjusted to control the particle size, nitrogen/carbon atomic ratio, and quantum yield. The products were water soluble with a narrow particle size distribution and good dispersion stability over a wide pH range. The N‐CQDs could penetrate into the HeLa cell nucleus without any further functionalization. Moreover, the fluorescence of N‐CQDs could be selectively quenched by Cu²⁺, which suggested applications for the detection of Cu²⁺ in human plasma.     

Bottom‐Up Fabrication of Single‐Layered Nitrogen‐Doped Graphene Quantum Dots through Intermolecular Carbonization Arrayed in a 2D Plane

A single‐layered intermolecular carbonization method was applied to synthesize single‐layered nitrogen‐doped graphene quantum dots (N‐GQDs) by using 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB) as the only precursor. In this method, the gas produced in the pyrolysis of TATB assists with speeding up of the reactions and expanding the layered distance, so that it facilitates the formation of single‐layered N‐GQDs (about 80 %). The symmetric intermolecular carbonizations of TATB arrayed in a plane and six nitrogen‐containing groups ensure small, uniform sizes (2–5 nm) of the resulting products, and provide high nitrogen‐doping concentrations (N/C atomic ratio ca. 10.6 %). In addition to release of the produced gas, TATB is almost completely converted into aggregated N‐GQDs; thus, relatively higher production rates are possible with this approach. Investigations show that the as‐produced N‐GQDs have superior fluorescent characteristics; high water solubility, biocompatibility, and low toxicity; and are ready for potential applications, such as biomedical imaging and optoelectronic devices.     

A Blinking Mesoporous TiO2−x Composed of Nanosized Anatase with Unusually Long‐Lived Trapped Charge Carriers

A mesoporous TiO_2?x material comprised of small, crystalline, vacancy‐rich anatase nanoparticles (NPs) shows unique optical, thermal, and electronic properties. It is synthesized using polymer‐derived mesoporous carbon (PDMC) as a template. The PDMC pores serve as physical barriers during the condensation and pyrolysis of a titania precursor, preventing the titania NPs from growing beyond 10 nm in size. Unlike most titania nanomaterials, during pyrolysis the NPs undergo no transition from the anatase to rutile phase and they become catalytically active reduced TiO_2?x. When exposed to a slow electron beam, the NPs exhibit a charge/discharge behavior, lighting up and fading away for an average period of 15 s for an extended period of time. The NPs also show a 50 nm red‐shift in their UV/Vis absorption and long‐lived charge carriers (electrons and holes) at room temperature in the dark, even long after UV irradiation. The NPs as photocatalysts show a good activity for CO₂ reduction.     

Aptamer‐Based Turn‐On Detection of Thrombin in Biological Fluids Based on Efficient Phosphorescence Energy Transfer from Mn‐Doped ZnS Quantum Dots to Carbon Nanodots

This paper presents the first example of a sensitive, selective, and stable phosphorescent sensor based on phosphorescence energy transfer (PET) for thrombin that functions through thrombin–aptamer recognition events. In this work, an efficient PET donor–acceptor pair using Mn‐doped ZnS quantum dots labeled with thrombin‐binding aptamers (TBA QDs) as donors, and carbon nanodots (CNDs) as acceptors has been constructed. Due to the π–π stacking interaction between aptamer and CNDs, the energy donor and acceptor are taken into close proximity, leading to the phosphorescence quenching of donors, TBA QDs. A maximum phosphorescence quenching efficiency as high as 95.9 % is acquired. With the introduction of thrombin to the “off state” of the TBA‐QDs‐CNDs system, the phosphorescence is “turned on” due to the formation of quadruplex‐thrombin complexes, which releases the energy acceptor CNDs from the energy donors. Based on the restored phosphorescence, an aptamer‐based turn‐on thrombin biosensor has been demonstrated by using the phosphorescence as a signal transduction method. The sensor displays a linear range of 0–40 nM for thrombin, with a detection limit as low as 0.013 nM in pure buffers. The proposed aptasensor has also been used to monitor thrombin in complex biological fluids, including serum and plasma, with satisfactory recovery ranging from 96.8 to 104.3 %. This is the first time that Mn‐doped ZnS quantum dots and CNDs have been employed as a donor–acceptor pair to construct PET‐based biosensors, which combines both the photophysical merits of phosphorescence QDs and the superquenching ability of CNDs and thus affords excellent analytical performance. We believe this proposed method could pave the way to a new design of biosensors using PET systems.     

CdSe Quantum Dots and N719‐Dye Decorated Hierarchical TiO2 Nanorods for the Construction of Efficient Co‐Sensitized Solar Cells

Three‐dimensional hierarchical TiO₂ nanorods (HTNs) decorated with the N719 dye and 3‐mercaptopropionic or oleic acid capped CdSe quantum dots (QDs) in photoanodes for the construction of TiO₂ nanorod‐based efficient co‐sensitized solar cells are reported. These HTN co‐sensitized solar cells showed a maximum power‐conversion efficiency of 3.93 %, and a higher open‐circuit voltage and fill factor for the photoanode with 3‐mercaptopropionic acid capped CdSe QDs due to the strong electronic interactions between CdSe QDs, N719 dye and HTNs, and the superior light‐harvesting features of the HTNs. An electrochemical impedance analysis indicated that the superior charge‐collection efficiency and electron diffusion length of the CdSe QD‐coated HTNs improved the photovoltaic performance of these HTN co‐sensitized solar cells.     

Electrochemistry in Carbon‐based Quantum Dots

Electrochemistry belongs to an important branch of chemistry that deals with the chemical changes produced by electricity and the production of electricity by chemical changes. Therefore, it can not only act a powerful tool for materials synthesis, but also offer an effective platform for sensing and catalysis. As extraordinary zero‐dimensional materials, carbon‐based quantum dots (CQDs) have been attracting tremendous attention due to their excellent properties such as good chemical stability, environmental friendliness, nontoxicity and abundant resources. Compared with the traditional methods for the preparation of CQDs, electrochemical (EC) methods offer advantages of simple instrumentation, mild reaction conditions, low cost and mass production. In return, CQDs could provide cost‐effective, environmentally friendly, biocompatible, stable and easily‐functionalizable probes, modifiers and catalysts for EC sensing. However, no specific review has been presented to systematically summarize both aspects until now. In this review, the EC preparation methods of CQDs are critically discussed focusing on CQDs. We further emphasize the applications of CQDs in EC sensors, electrocatalysis, biofuel cells and EC flexible devices. This review will further the experimental and theoretical understanding of the challenges and future prospective in this field, open new directions on exploring new advanced CQDs in EC to meet the high demands in diverse applications.     

Current Advances in Quantum‐Dots‐Based Photoelectrochemical Immunoassays

As a newly developed technique, photoelectrochemical (PEC) immunoassays have attracted great attention in recent years because of their low cost and desirable sensitivity. Because the detection signal originates from the photoelectric conversion of photoelectric materials, the appearance and application of quantum dots (QDs), which possess unique photophysical properties and regulated optoelectronic characteristics, has taken the development of PEC immunoassays to new heights. This review concisely introduces the general mechanism of QDs‐based photoelectric conversion for immunoassays and summarizes the current advances in QD applications in immunoassays. Given that signal strategies and photoactive materials are the key elements in PEC biosensor systems, we comprehensively highlight the state‐of‐the‐art signaling strategies and various applications of QDs in PEC immunoassays to introduce advances in QDs‐based PEC immunoassays. Finally, challenges and future developmental trends are briefly discussed