Quantum Dot-Catalyzed Photoreductive Removal of Sulfonyl-Based Protecting Groups

This Communication describes the use of CuInS₂/ZnS quantum dots (QDs) as photocatalysts for the reductive deprotection of aryl sulfonyl-protected phenols. For a series of aryl sulfonates with electron-withdrawing substituents, the rate of deprotection for the corresponding phenyl aryl sulfonates increases with decreasing electrochemical potential for the two electron transfers within the catalytic cycle. The rate of deprotection for a substrate that contains a carboxylic acid, a known QD-binding group, is accelerated by more than a factor of ten from that expected from the electrochemical potential for the transformation, a result that suggests that formation of metastable electron donor–acceptor complexes provides a significant kinetic advantage. This deprotection method does not perturb the common NHBoc or toluenesulfonyl protecting groups and, as demonstrated with an estrone substrate, does not perturb proximate ketones, which are generally vulnerable to many chemical reduction methods used for this class of reactions.     

CuO Quantum‐Dot‐Sensitized Mesoporous ZnO for Visible‐Light Photocatalysis

How to extend ultraviolet photocatalysts to the visible‐light region is a key challenge for solar‐driven photocatalysis. Herein, we show that ultraviolet ZnO photocatalysts can present high visible‐light photocatalytic activity when combined with CuO quantum dots (QDs; <3 nm). Theoretical analysis demonstrates that the quantum size effect plays a key role in the photoactivity of the CuO/ZnO composite. For CuO QDs smaller than 3 nm, the separated charges could transfer from CuO QDs to the conduction bands of ZnO due to quantum splitting of the CuO energy level and phonon compensation for the difference in the conduction band minimum of CuO and ZnO; however, this process would not occur with the disappearance of the quantum size effect. Further structural analysis demonstrates that interfacial charge separation and transfer between ZnO and CuO dominate the photocatalytic processes instead of a single CuO or ZnO surface. Compared with ZnO? noble metal structures (e.g., ZnO? Ag or ZnO? Au), these ZnO? CuO QD composites present wider absorption bands, higher visible photocatalytic efficiencies, and lower costs.     

Clean Donor Oxidation Enhances the H2 Evolution Activity of a Carbon Quantum Dot–Molecular Catalyst Photosystem

Carbon quantum dots (CQDs) are new‐generation light absorbers for photocatalytic H₂ evolution in aqueous solution, but the performance of CQD‐molecular catalyst systems is currently limited by the decomposition of the molecular component. Clean oxidation of the electron donor by donor recycling prevents the formation of destructive radical species and non‐innocent oxidation products. This approach allowed a CQD‐molecular nickel bis(diphosphine) photocatalyst system to reach a benchmark lifetime of more than 5 days and a record turnover number of 1094±61 mol_H2 (mol_Ni)^?1 for a defined synthetic molecular nickel catalyst in purely aqueous solution under AM1.5G solar irradiation.     

Tuning Optical Properties and Photocatalytic Activities of Carbon‐based “Quantum Dots” Through their Surface Groups

We report recent progress in tuning optical properties and photocatalytic activities of carbon‐based quantum dots (carbon‐based QDs) through their surface groups. It is increasingly clear that the properties of carbon‐based QDs are more dependent on their surface groups than on their size. The present challenge remains as to how to control the type, number, and conformation of the heterogeneous groups on the surface of carbon‐based QDs when considering their target applications. By reviewing the related achievements, this personal account aims to help us understand the roles different surface groups play in tuning the properties of carbon‐based QDs. A number of significant accomplishments have demonstrated that surface groups possess strong power in engineering electronic structure and controlling photogenerated charge behaviors of carbon‐based QDs. However, effective strategies for modifying carbon‐based QDs with diverse heterogeneous groups are still needed.     

Quantum‐Dot‐Sensitized Solar Cells

Quantum‐dot‐sensitized solar cells (QDSCs) are a promising low‐cost alternative to existing photovoltaic technologies such as crystalline silicon and thin inorganic films. The absorption spectrum of quantum dots (QDs) can be tailored by controlling their size, and QDs can be produced by low‐cost methods. Nanostructures such as mesoporous films, nanorods, nanowires, nanotubes and nanosheets with high microscopic surface area, redox electrolytes and solid‐state hole conductors are borrowed from standard dye‐sensitized solar cells (DSCs) to fabricate electron conductor/QD monolayer/hole conductor junctions with high optical absorbance. Herein we focus on recent developments in the field of mono‐ and polydisperse QDSCs. Stability issues are adressed, coating methods are presented, performance is reviewed and special emphasis is given to the importance of energy‐level alignment to increase the light to electric power conversion efficiency.     

Positively Charged Compact Quantum Dot–DNA Complexes for Detection of Nucleic Acids

Selective DNA detection: The fluorescence, from stable cationic QDs, is quenched by 90% on complexation with modified DNA molecules. The QD–DNA probe is capable of detecting pathogenic DNA fragments at concentrations as low as 200 nM in solution and shows selective fluorescence recovery in the presence of target DNA (see spectrum c in figure) vs noncomplementary DNA (spectrum d).

     

Halide–Amine Co‐Passivated Indium Phosphide Colloidal Quantum Dots in Tetrahedral Shape

Wet chemical synthesis of covalent III‐V colloidal quantum dots (CQDs) has been challenging because of uncontrolled surfaces and a poor understanding of surface–ligand interactions. We report a simple acid‐free approach to synthesize highly crystalline indium phosphide CQDs in the unique tetrahedral shape by using tris(dimethylamino) phosphine and indium trichloride as the phosphorus and indium precursors, dissolved in oleylamine. Our chemical analyses indicate that both the oleylamine and chloride ligands participate in the stabilization of tetrahedral‐shaped InP CQDs covered with cation‐rich (111) facets. Based on density functional theory calculations, we propose that fractional dangling electrons of the In‐rich (111) surface could be completely passivated by three halide and one primary amine ligands per the (2×2) surface unit, satisfying the 8‐electron rule. This halide–amine co‐passivation strategy will benefit the synthesis of stable III‐V CQDs with controlled surfaces.     

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.     

Mechanistic Insights into the Interface‐Directed Transformation of Thiols into Disulfides and Molecular Hydrogen by Visible‐Light Irradiation of Quantum Dots

Quantum dots (QDs) offer new and versatile ways to harvest light energy. However, there are few examples involving the utilization of QDs in organic synthesis. Visible‐light irradiation of CdSe QDs was found to result in virtually quantitative coupling of a variety of thiols to give disulfides and H₂ without the need for sacrificial reagents or external oxidants. The addition of small amounts of nickel(II) salts dramatically improved the efficiency and conversion through facilitating the formation of hydrogen atoms, thereby leading to faster regeneration of the ground‐state QDs. Mechanistic studies reveal that the coupling reaction occurs on the QD surfaces rather than in solution and offer a blueprint for how these QDs may be used in other photocatalytic applications. Because no sacrificial agent or oxidant is necessary and the catalyst is reusable, this method may be useful for the formation of disulfide bonds in proteins as well as in other systems sensitive to the presence of oxidants.     

Large‐Scale Synthesis of High‐Quality Metal Sulfide Semiconductor Quantum Dots with Tunable Surface‐Plasmon Resonance Frequencies

High‐quality CdS and Cu₇S₄ quantum dots (QDs) were synthesized with N,N‐dibutylthiourea (DBTU) as an organic sulfur source. In this method, nucleation and growth reactions were controlled simply by the heating rate of the reaction. The mild oxidation conditions gave monodisperse CdS QDs exhibiting pure band‐edge emission with relatively high photoluminescence quantum yield. During the synthesis of Cu₇S₄ QDs, the addition of dodecanethiol to the reaction system controlled the reaction rate to give monodisperse spherical or disk‐shaped QDs. A hundred‐gram scale of copper precursor could be used to generate the high‐quality Cu₇S₄ QDs, indicating that an industrial‐scale reaction is achievable with our method. As observed in anisotropic noble‐metal nanocrystals, larger disk‐shaped Cu₇S₄ QDs showed lower localized‐surface‐plasmon resonance energy in the near‐infrared region. The disk‐shaped Cu₇S₄ QDs could be used effectively as templates to form cation‐exchanged monodisperse disk‐shaped CdS QDs.     

Highly Photoluminescent Amino‐Functionalized Graphene Quantum Dots Used for Sensing Copper Ions

Herein, we report a new kind of highly fluorescent probe for Cu²⁺ sensing generated by hydrothermal treatment of graphene quantum dots (GQDs). After hydrothermal treatment in ammonia, the greenish‐yellow fluorescent GQDs (gGQDs) with a low quantum yield (QY, 2.5 %) are converted to amino‐functionalized GQDs (afGQDs) with a high QY (16.4 %). Due to the fact that Cu²⁺ ions have a higher binding affinity and faster chelating kinetics with N and O on the surface of afGQDs than other transition‐metal ions, the selectivity of afGQDs for Cu²⁺ is much higher than that of gGQDs. Furthermore, afGQDs are biocompatible and eco‐friendly, and the afGQDs with a positive charge can be easily taken up by cells, which makes it possible to sense Cu²⁺ in living cells. The strategy presented here is simple in design, economical, and offers a “mix‐and‐detect” protocol without dye‐modified oligonucleotides or complex chemical modification.     

Electrophoretic Deposition of a Reduced Graphene–Au Nanoparticle Composite Film as Counter Electrode for CdS Quantum Dot‐Sensitized Solar Cells

A reduced graphene (RG)‐Au nanoparticle composite film is successfully fabricated by electrophoretic deposition and used as counter electrode for quantum dot‐sensitized solar cells. The RG‐Au composite is prepared by one‐step microwave‐assisted reduction of chloroaurate in alkaline solution with graphite oxide dispersion. Under one sun illumination (AM 1.5 G, 100 mW cm^?2), the cell with a RG‐Au counter electrode shows an energy conversion efficiency of 1.36 %, which is higher than those of cells employing conventional Pt or Au counter electrodes, due to the superior combination of highly catalytic Au nanoparticles and the conductive graphene network structure.     

Room‐Temperature Synthesis of Cu2−xE (E=S,Se) Nanotubes with Hierarchical Architecture as High‐Performance Counter Electrodes of Quantum‐Dot‐Sensitized Solar Cells

Copper chalcogenide nanostructures (e.g. one‐ dimensional nanotubes) have been the focus of interest because of their unique properties and great potential in various applications. Their current fabrications mainly rely on high‐temperature or complicated processes. Here, with the assistance of theoretical prediction, we prepared Cu_2?xE (E=S, Se) micro‐/nanotubes (NTs) with a hierarchical architecture by using copper nanowires (Cu NWs), stable sulfur and selenium powder as precursors at room temperature. The influence of reaction parameters (e.g. precursor ratio, ligands, ligand ratio, and reaction time) on the formation of nanotubes was comprehensively investigated. The resultant Cu_2?xE (E=S, Se) NTs were used as counter electrodes (CE) of quantum‐dot‐sensitized solar cells (QDSSCs) to achieve a conversion efficiency (η) of 5.02 and 6.25 %, respectively, much higher than that of QDSSCs made with Au CE (η=2.94 %).     

CdSeTe@CdS@ZnS Quantum‐Dot‐Sensitized Macroporous Tio2 Film: A Multisignal‐Amplified Photoelectrochemical Platform

A macroporous TiO₂ film (M‐TiO₂), which was prepared by burning off the polystyrene microsphere (PS) template from a PS/TiO₂ composite film, can provide a large active surface, improve electron‐transport performance, and increase the photocurrent. Furthermore, core–shell–shell CdSeTe@CdS@ZnS quantum dots (QDs) were introduced to sensitize the M‐TiO₂ film, which can efficiently broaden the absorption spectra range, separate and transfer charge carriers, reduce recombination loss, and improve photovoltaic response, with a sensitization shell of CdS and a passivation shell of ZnS. A multisignal‐amplified photoelectrochemical platform was fabricated by further modifying this film with a combination of biotin–DEVD–peptide (Biotin–Gly–Asp–Gly–Asp–Glu–Val–Asp–Gly–Cys) (which is specifically cleaved by caspase‐3) and streptavidin‐labeled alkaline phosphatase (SA‐ALP). Under the enzymatic catalysis of ALP with the substrate 2‐phospho‐L ‐ascorbic acid trisodium salt (AAP), ascorbic acid (AA) was generated as a better electron donor, leading to increased photocurrent output. The activity of caspase‐3, which depends on the amount of residual peptide on the electrode, was inversely proportional to the amount of AA. By monitoring the variation of photocurrent caused by AA, caspase‐3 activity and the therapeutic effect of nilotinib (a special medicine of chronic myeloid leukemia, CML) were indirectly detected and evaluated. The photoelectrochemical platform can be used as a potential evaluation system for monitoring caspase‐3 activity and drug effects.     

An Efficient Templating Approach for the Synthesis of Redispersible Size‐Controllable Carbon Quantum Dots from Graphitic Polymeric Micelles

Access to high‐quality, easily dispersible carbon quantum dots (CQDs) is essential in order to fully exploit their desirable properties. Copolymers based on N‐acryloyl‐D ‐glucosamine and acrylic acid prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization are self‐assembled into micelle‐like nanoreactors. After a facile graphitization process (170 °C, atmospheric pressure), each micellar template is transformed into a CQD through a 1:1 copy process. These high‐quality CQDs (quantum yield=22 %) with tunable sizes (2–5 nm) are decorated by carboxylic acid moieties and can be spontaneously redispersed in water and polar organic solvents. This preparation method renders the mass production of multifunctional CQDs possible. To demonstrate the versatility of this approach, CQDs hybridized TiO₂ nanoparticles with enhanced photocatalytic activity under visible‐light have been prepared.