Uniform thin films of CdSe and CdSe(ZnS) core(shell) quantum dots by sol-gel assembly: enabling photoelectrochemical characterization and electronic applications

Quantum dots as advanced nanomaterials for food quality and safety applications: A comprehensive review and future perspectives

The importance of food quality and safety lies in ensuring the best product quality to meet consumer demands and public health. Advanced technologies play a crucial role in minimizing the risk of foodborne illnesses, contamination, drug residue, and other potential hazards in food. Significant materials and technological advancements have been made throughout the food supply chain. Among them, quantum dots (QDs), as a class of advanced nanomaterials with unique physicochemical properties, are progressively demonstrating their value in the field of food quality and safety. This review aims to explore cutting-edge research on the different applications of QDs in food quality and safety, including encapsulation of bioactive compounds, detection of food analytes, food preservation and packaging, and intelligent food freshness indicators. Moreover, the modification strategies and potential toxicities of diverse QDs are outlined, which can affect performance and hinder applications in the food industry. The findings suggested that QDs are mainly used in analyte detection and active/intelligent food packaging. Various food analytes can be detected using QD-based sensors, including heavy metal ions, pesticides, antibiotics, microorganisms, additives, and functional components. Moreover, QD incorporation aided in improving the antibacterial and antioxidant activities of film/coatings, resulting in extended shelf life for packaged food. Finally, the perspectives and critical challenges for the productivity, toxicity, and practical application of QDs are also summarized. By consolidating these essential aspects into this review, the way for developing high-performance QD-based nanomaterials is presented for researchers and food technologists to better capitalize upon this technology in food applications.

Characterization of the Interfacial Structures of Core/Shell CdSe/ZnS QDs

Core/shell quantum dots (QDs) have been extensively studied, yet their optical properties widely vary among studies. Such variation may arise from the variation in interfacial structures induced by the subtle difference in each synthetic procedure. Here, we studied the interfacial structures of CdSe/ZnS QDs using the time-of-flight medium energy ion-scattering spectroscopy (TOF-MEIS), which offers the radial elemental distributions as well as the overall elemental compositions of QDs. The TOF-MEIS spectra provided strong evidence for the existence of an alloyed layer at the interface between CdSe and ZnS in typical CdSe/ZnS QDs. On the basis of the emission and absorption spectra of QDs sampled during the synthesis, we conclude that such interfacial alloying is caused by the dissolution of CdSe seeds during the synthesis steps. Such a dissolution mechanism is further corroborated by the observation that the ligand environment of solvent (X or L type) leads to different shapes of interfaces.

Sensibilization of p-NiO with ZnSe/CdS and CdS/ZnSe quantum dots for photoelectrochemical water reduction

Core/shell quantum dots (QDs) paired with semiconductor photocathodes for water reduction have rarely been implemented so far. We demonstrate the integration of ZnSe/CdS and CdS/ZnSe QDs with porous p-type NiO photocathodes for water reduction. The QDs demonstrate appreciable enhancement in water-reduction efficiency, as compared with the bare NiO. Despite their different structure, both QDs generate comparable photocurrent enhancement, yielding a 3.8- and 3.2-fold improvement for the ZnSe/CdS@NiO and CdS/ZnSe@NiO system, respectively. Unraveling the carrier kinetics at the interface of these hybrid photocathodes is therefore critical for the development of efficient photoelectrochemical (PEC) proton reduction. In addition to examining the carrier dynamics by the Mott-Schottky technique and electrochemical impedance spectroscopy (EIS), we performed theoretical modelling for the distribution density of the carriers with respect to electron and hole wave functions. The electrons are found to be delocalized through the whole shell and can directly actuate the PEC-related process in the ZnSe/CdS QDs. The holes as the more localized carriers in the core have to tunnel through the shell before injecting into the hole transport layer (NiO). Our results emphasize the role of interfacial effects in core/shell QDs-based multi-heterojunction photocathodes.

Control over Structure and Properties in Nanocrystal Aerogels at the Nano-, Micro-, and Macroscale

The assembly of individual colloidal nanocrystals into macroscopic solvogels and aerogels introduced a new exciting type of material into the class of porous architectures. In these so-called nanocrystal gels, the structure and properties can be controlled and fine-tuned to the smallest details. Recently it was shown that by employing nanocrystal building blocks for such gel materials, the interesting nanoscopic properties can be conserved or even expanded to properties that are available neither in the nanocrystals nor in their respective bulk materials. In general, the production of these materials features the wet-chemical synthesis of stable nanocrystal colloids followed by their carefully controlled destabilization to facilitate arrangement of the nanocrystals into highly porous, interconnected networks. By isolation of the synthesis of the discrete building blocks from the assembly process, the electronic structure, optical properties, and structural morphology can be tailored by the myriad of procedures developed in colloidal nanocrystal chemistry. Furthermore, knowledge and control over the structure–property correlation in the resulting gel structures opens up numerous new ways for extended and advanced applications. Consequently, the amount of different materials converted to nanocrystal-based gel structures is rising steadily. Meanwhile the number of methods for assembly initiation is likewise increasing, offering control over the overall network structure and porosity as well as the individual nanocrystal building block connection. The resulting networks can be dried by different methods to obtain highly porous air-filled networks (aerogels) or applied in their wet form (solvogels). By now a number of different applications profiting from the unique advantages of nanocrystal-based gel materials have been realized and exploited in the areas of photocatalysis, electrocatalysis, and sensing.

In this Account, we aim to summarize the efforts undertaken in the structuring of nanocrystal-based network materials on different scales, fine-tuning of the individual building blocks on the nanoscale, the network connections on the microscale, and the macroscale structure and shape of the final construct. It is exemplarily demonstrated how cation exchange reactions (at the nanoscale), postgelation modifications on the nanocrystal networks (microscale), and the structuring of the gels via printing techniques (macroscale) endow the resulting nanocrystal gel networks with novel physicochemical, mechanical, and electrocatalytic properties. The methods applied in the more traditional sol–gel chemistry targeting micro- and macroscale structuring are also reviewed, showing their future potential promoting the field of nanocrystal-based aerogels and their applications.

Reversible Electrochemical Gelation of Metal Chalcogenide Quantum Dots

The ability to dictate the assembly of quantum dots (QDs) is critical for their integration into solid-state electronic and optoelectronic devices. However, assembly methods that enable efficient electronic communication between QDs, facilitate access to the reactive surface, and retain the native quantum confinement characteristics of the QD are lacking. Here we introduce a universal and facile electrochemical gelation method for assembling metal chalcogenide QDs (as demonstrated for CdS, ZnS, and CdSe) into macroscale 3-D connected pore-matter nanoarchitectures that remain quantum confined and in which each QD is accessible to the ambient. Because of the redox-active nature of the bonding between QD building blocks in the gel network, the electrogelation process is reversible. We further demonstrate the application of this electrogelation method for a one-step fabrication of CdS gel gas sensors, producing devices with exceptional performance for NO₂ gas sensing at room temperature, thereby enabling the development of low-cost, sensitive, and reliable devices for air quality monitoring.

Mechanistic insight into the non-hydrolytic sol-gel process of tellurite glass films to attain a high transmission

The development of amorphous films with a wide transmission window and high refractive index is of growing significance due to the strong demand of integrating functional nanoparticles for the next-generation hybrid optoelectronic films. High-index TeO₂-based glass films made via the sol–gel process are particularly suitable as their low temperature preparation process promises high compatibility with a large variety of nanoparticles and substrates that suffer from low thermal stability. However, due to the lack of in-depth understanding of the mechanisms of the formation of undesired metallic-Te (highly absorbing species) in the films, the preparation of high-transmission TeO₂-based sol–gel films has been severely hampered. Here, by gaining insight into the mechanistic chemistry of metallic-Te formation at different stages during the non-hydrolytic sol–gel process, we identify the chemical route to prevent the generation of metallic-Te in a TeO₂-based film. The as-prepared TeO₂-based film exhibits a high transmission that is close to the theoretical limit. This opens up a new avenue for advancing the performance of hybrid optoelectronic films via incorporating a large variety of unique nanoparticles.

This work develops a high-transparency amorphous film with a wide transmission window and high refractive index, which can potentially meet the strong demand of integrating functional nanoparticles for the next-generation hybrid optoelectronic films.

Exploiting kinetics for assembly of multicomponent nanoparticle networks with programmable control of heterogeneity

Kinetic control of metal chalcogenide nanoparticle oxidative assembly is realized by varying the redox potential of the chalcogenide, structure (wurtzite vs. zinc blende), and ligand chain length. This knowledge is exploited to form two-component (ZnS + CdSe) hybrid aerogels with minimal heterobonding (phase-segregated) or maximal heterobonding (intimately mixed).

Cu–Cd–Zn–S/ZnS core/shell quantum dot/polyvinyl alcohol flexible films for white light-emitting diodes

We present a facile route for the synthesis of water-soluble Cu–Cd–Zn–S/ZnS core/shell quantum dots (QDs) by simple pH regulation. The PL spectra of Cu–Cd–Zn–S/ZnS core/shell quantum dots can cover the whole visible light region in the case of only two ratios of Cu/Cd/Zn. The emission wavelength of Cu–Cd–Zn–S/ZnS QDs can be conveniently tuned from 474 to 515 and 548 to 629 nm by adjusting the pH value when the ratios of Cu/Cd/Zn are fixed at 1 : 5 : 80 and 1 : 5 : 10, respectively. It is worth noting that under the condition of a constant Cu/Cd/Zn ratio, the UV-vis absorption spectra do not change with the fluorescence spectra, indicating that the band gap of QDs remains unchanged during the change of pH value. The photoluminescence (PL) quantum yield of the as-prepared QDs with yellow emission is up to 76%. The QDs also show excellent chemical stability after the deposition of the ZnS shell. Luminescent and flexible films are fabricated by combining Cu–Cd–Zn–S QDs with polyvinyl alcohol (PVA). The QD/PVA flexible hybrid films are successfully applied on top of a conventional blue InGaN chip for remote-type warm-white LEDs. As-fabricated warm-white LEDs exhibit a higher color rendering index (CRI) of about 89.2 and a correlated color temperature (CCT) of 4308 K.

We present a facile route for the synthesis of water-soluble Cu–Cd–Zn–S/ZnS core/shell quantum dots (QDs) by simple pH regulation.

Effect of metal ion solubility on the oxidative assembly of metal sulfide quantum dots

The versatility of the oxidative assembly method for the creation of 2D and 3D quantum dot (QD) architectures represents both an opportunity and a challenge as a method enabling controlled placement of chemically distinct QDs in multicomponent systems. The opportunity lies in the ability to independently tune the kinetics of the different components so that they are similar (leading to well-mixed systems) or different (enabling gradient or phase-segregated composites) using a wide range of variables; the challenge lies in understanding those variables and how their interplay affects the overall kinetics. Here, we show that the identity of the cation in the sulfide matrix (M = Cd²⁺ vs Zn²⁺) plays a large role in the kinetics of assembly of mass spectrometry QDs, attributed to differences in solubility. Time resolved dynamic light scattering is used to monitor the hydrodynamic radius, R¯_h. ZnS shows an exponential growth associated with reaction-limited cluster aggregation (RLCA), whereas CdS demonstrates a significant induction period (10-75 min) followed by a growth step that cannot be distinguished between RLCA and diffusion limited cluster aggregation. These data correlate with relative solubilities of the nanoparticles, as probed by free-cation concentration. Data also confirm prior studies showing that cubic-closest-packed (ccp) lattices are kinetically slow relative to hexagonally closest-packed (hcp); using the slope of the ln R¯_h vs time plot for the rate constant, the values of 0.510 s^-1 and 3.92 s^-1 are obtained for ccp ZnS and hcp ZnS, respectively. Thus, both the structure and the solubility are effective levers for adjusting the relative reactivity of QDs toward oxidative assembly.

Nanoplatelet cryoaerogels with potential application in photoelectrochemical sensing

Semiconductor nanoparticle based porous 3D assemblies are interesting materials for various applications in the fields of photovoltaics, catalysis, or optical sensing. For use as photoelectrodes in photoelectrochemical sensors they need to be characterised by a high porosity, a good photostability, and a high charge carrier mobility. Our work reports on the preparation of cryoaerogel photoelectrodes based on CdSe nanoplatelets and their photoelectrochemical characterisation by means of linear sweep voltammetry (LSV) and intensity modulated photocurrent spectroscopy (IMPS). The obtained open-pored cryoaerogel films were observed to produce much higher photocurrents than comparable drop-cast films. By means of IMPS, the performance differences could be linked to the occurrence of charge carrier movement, which could solely be proven for the cryoaerogels. In a proof-of-principle experiment, the potential of the prepared photoelectrodes for application in photoelectrochemical sensing was moreover demonstrated.

Quantum Dot-Acrylic Acrylate Oligomer Hybrid Films for Stable White Light-Emitting Diodes

White light-emitting diodes (LEDs) have been achieved using photopolymerization. Red and green CdSe/ZnS core-shell quantum dots (QDs) are dispersed in photopolymerized aliphatic acrylic acrylate resins, cured by using 36 W UV light for 1.5 min producing QD-acrylate nanocomposites, and then a hybrid LED device is achieved using the QD-acrylate nanocomposite with two emission wavelengths excited by using a blue InGaN LED chip. The cured QD-acrylate nanocomposite films are characterized by using ultraviolet-visible, fluorescence, scanning electron microscopy, atomic force microscopy, and thermogravimetric analysis measurements. Photopolymerization is conveniently employed to adjust several parameters of the CIE-1931 coordinate (x, y), color temperature, and color rending index (CRI). Good white balance of the red-green hybrid device achieves a luminance of 7976 lm/m² at a 30 mA working current. The light emission efficiency, CRI, and color temperature of 14%, 47, and 11 204 K, respectively, are attained at this working current.

Optoelectronic Properties in Near-Infrared Colloidal Heterostructured Pyramidal "Giant" Core/Shell Quantum Dots

Colloidal heterostructured quantum dots (QDs) are promising candidates for next-generation optoelectronic devices. In particular, "giant" core/shell QDs (g-QDs) can be engineered to exhibit outstanding optical properties and high chemical/photostability for the fabrication of high-performance optoelectronic devices. Here, the synthesis of heterostructured CuInSe x S2-x (CISeS)/CdSeS/CdS g-QDs with pyramidal shape by using a facile two-step method is reported. The CdSeS/CdS shell is demonstrated to have a pure zinc blend phase other than typical wurtzite phase. The as-obtained heterostructured g-QDs exhibit near-infrared photoluminescence (PL) emission (≈830 nm) and very long PL lifetime (in the microsecond range). The pyramidal g-QDs exhibit a quasi-type II band structure with spatial separation of electron-hole wave function, suggesting an efficient exciton extraction and transport, which is consistent with theoretical calculations. These heterostructured g-QDs are used as light harvesters to fabricate a photoelectrochemical cell, exhibiting a saturated photocurrent density as high as ≈5.5 mA cm-2 and good stability under 1 sun illumination (AM 1.5 G, 100 mW cm-2). These results are an important step toward using heterostructured pyramidal g-QDs for prospective applications in solar technologies.

Emerging Hierarchical Aerogels: Self-Assembly of Metal and Semiconductor Nanocrystals

Aerogels assembled from colloidal metal or semiconductor nanocrystals (NCs) feature large surface area, ultralow density, and high porosity, thus rendering them attractive in various applications, such as catalysis, sensors, energy storage, and electronic devices. Morphological and structural modification of the aerogel backbones while maintaining the aerogel properties enables a second stage of the aerogel research, which is defined as hierarchical aerogels. Different from the conventional aerogels with nanowire-like backbones, those hierarchical aerogels are generally comprised of at least two levels of architectures, i.e., an interconnected porous structure on the macroscale and a specially designed configuration at local backbones at the nanoscale. This combination "locks in" the inherent properties of the NCs, so that the beneficial genes obtained by nanoengineering are retained in the resulting monolithic hierarchical aerogels. Herein, groundbreaking advances in the design, synthesis, and physicochemical properties of the hierarchical aerogels are reviewed and organized in three sections: i) pure metallic hierarchical aerogels, ii) semiconductor hierarchical aerogels, and iii) metal/semiconductor hybrid hierarchical aerogels. This report aims to define and demonstrate the concept, potential, and challenges of the hierarchical aerogels, thereby providing a perspective on the further development of these materials.

Surface Chemistry and Nano-/Microstructure Engineering on Photocatalytic In2S3 Nanocrystals

Colloidal nanocrystals (NCs) compete with molecular catalysts in the field of homogenous catalysis, offering easier recyclability and a number of potentially advantageous functionalities, such as tunable band gaps, plasmonic properties, or a magnetic moment. Using high-throughput printing technologies, colloidal NCs can also be supported onto substrates to produce cost-effective electronic, optoelectronic, electrocatalytic, and sensing devices. For both catalytic and technological application, NC surface chemistry and supracrystal organization are key parameters determining final performance. Here, we study the influence of the surface ligands and the NC organization on the catalytic properties of In₂S₃, both as a colloid and as a supported layer. As a colloid, NCs stabilized by inorganic ligands show the highest photocatalytic activities, which we associate with their large and more accessible surfaces. On the other hand, when NCs are supported on a substrate, their organization becomes an essential parameter determining performance. For instance, NC-based films produced through a gelation process provided five-fold higher photocurrent densities than those obtained from dense films produced by the direct printing of NCs.

Application of Aqueous-Based Covalent Crosslinking Strategies to the Formation of Metal Chalcogenide Gels and Aerogels

An aqueous-based metal ion crosslinking approach for assembly of metal chalcogenide nanoparticles (NPs) into robust gels is reported. Short chalcogenide ligands (S2−) undergo crosslinking with metal salts (Sn4+) to form a gel [NP/S2−/Sn4+]n (NP=PbTe, PbS, CdS, CdSe). The corresponding aerogel networks retain the crystallinity and quantum confinement effects of the native building blocks while achieving excellent porosity [Brunauer–Emmett–Teller (BET) surface areas of 160–238 m2/g]. Treatment of sulfide-capped PbTe nanoparticles with an excess of Sn4+ leads to ion exchange and formation of an amorphous “SnTe” gel.

CuGaS₂ and CuGaS₂-ZnS Porous Layers from Solution-Processed Nanocrystals

The manufacturing of semiconducting films using solution-based approaches is considered a low cost alternative to vacuum-based thin film deposition strategies. An additional advantage of solution processing methods is the possibility to control the layer nano/microstructure. Here, we detail the production of mesoporous CuGaS₂ (CGS) and ZnS layers from spin-coating and subsequent cross-linking through chalcogen-chalcogen bonds of properly functionalized nanocrystals (NCs). We further produce NC-based porous CGS/ZnS bilayers and NC-based CGS–ZnS composite layers using the same strategy. Photoelectrochemical measurements are used to demonstrate the efficacy of porous layers, and particularly the CGS/ZnS bilayers, for improved current densities and photoresponses relative to denser films deposited from as-produced NCs.

Dual-Hole Excitons Activated Photoelectrolysis in Neutral Solution

II-VI semiconductors exhibit unique behaviors that can generate dual-holes ("heavy and light"), but the application in photocatalysis is still missing. Herein, an empirical utilization of light/heavy holes in a hybrid metal cluster-2D semiconductor nanoplatelets is reported. This hybrid material can boost the hole-transfer at the surface and suppress the recombination. Different roles are enacted by light-holes and heavy-holes, in which the light-holes with higher energy and mobility can facilitate the slow kinetics of water oxidation and further reduce the onset voltage, while the massive heavy-holes can increase the resulting photocurrent by about five times, achieving a photocurrent of 2 mA cm^-2 at 1.23 V versus RHE under AM 1.5 G illumination in nonsacrificial neutral solution. These strategies can be the solutions for photoelectrolysis and be beneficial for sustainable development in solar conversion.

Bipyridine based metallogels: an unprecedented difference in photochemical and chemical reduction in the in situ nanoparticle formation

Metal co-ordination induced supramolecular gelation of low molecular weight organic ligands is a rapidly expanding area of research due to the potential in creating hierarchically self-assembled multi-stimuli responsive materials. In this context, structurally simple O-methylpyridine derivatives of 4,4'-dihydroxy-2,2'-bipyridine ligands are reported. Upon complexation with Ag(i) ions in aqueous dimethyl sulfoxide (DMSO) solutions the ligands spontaneously form metallosupramolecular gels at concentrations as low as 0.6 w/v%. The metal ions induce the self-assembly of three dimensional (3D) fibrillar networks followed by the spontaneous in situ reduction of the Ag-centers to silver nanoparticles (AgNPs) when exposed to daylight. Significant size and morphological differences of the AgNP's was observed between the standard chemical and photochemical reduction of the metallogels. The gelation ability, the nanoparticle formation and rheological properties were found to be depend on the ligand structure, while the strength of the gels is affected by the water content of the gels.

Role of Crystal Structure and Chalcogenide Redox Properties on the Oxidative Assembly of Cadmium Chalcogenide Nanocrystals

Oxidative assembly of metal chalcogenide nanocrystals (NCs) enables the formation of 2-D (dense) and 3-D porous structures without the presence of intervening ligands between particles that can moderate transport properties. This route has been demonstrated to be successful for a range of single-component structures including CdQ, PbQ, and ZnQ (Q = S, Se, Te). En route to the controllable assembly of multicomponent nanostructures, the roles of Q redox properties (2Q^2- → Q₂^2- + 2e) responsible for particle cross-linking and the native structure (cubic zinc blende vs hexagonal wurtzite) in the kinetics of assembly in single-component CdQ NCs are evaluated using time-resolved dynamic light scattering (TR-DLS). For wurtzite CdQ, the rates follow the ease of oxidation, with telluride as the fastest, followed by selenide and sulfide. However, when comparing CdS wurtzite (w) and zinc blende (zb), the cubic NCs exhibit surprisingly slow kinetics. NMR studies reveal the zb structure to have lower ligand coverage (by a factor of 4) relative to that of w, and the formation of free disulfide (the product of ligand oxidation) is slow. This is attributed to differences in the surface energies of w and zb facets, with w having polar (0001) facets of high energy compared to the neutral facets of the zb structure. The zb-CdS NCs prepared by low-temperature synthesis methods are likely to suffer from surface defects that may moderate reactivity. EPR studies suggest that zb-CdS has paramagnetic sulfur vacancies not present in w-CdS. These data suggest that structure plays an unexpectedly large role in the kinetics of CdQ NC oxidative assembly, providing a useful lever to moderate activities in multicomponent assemblies.

CdS Aerogels as Efficient Photocatalysts for Degradation of Organic Dyes under Visible Light Irradiation

Synthesis of efficient photocatalysts based on CdS nanomaterials for oxidative decomposition of organic effluents typically focuses on (a) enhancement of surface area of the catalysts and (b) promotion of the separation of photogenerated electron-hole pairs. CdS aerogel, which are synthesized by simple sol-gel assembly of discrete nanocrystals (NCs) into a porous network followed by supercritical drying, could provide higher surface area for photocatalytic reactions along with facile charge separation due to direct contact between NCs via covalent bonding. We evaluated the efficiency of CdS aerogel materials for degradation of organic dyes using methylene blue (MB) and methyl orange (MO) as test cases. CdS aerogel materials exhibited remarkable photocatalytic activity for dye degradation compared to typical, ligand-capped CdS NCs. The catalytic efficiency of CdS aerogels was further improved by decreasing the chain-length and extent of surface organics, leading to higher, and more hydrophilic, accessible surface area. The use of porous, chalcogenide-based solid state architectures for photocatalysis enables easy separation of catalyst while ensuring a high-interfacial surface area for analyte reactivity and visible light activation.

3D assembly of preformed colloidal nanoparticles into gels and aerogels: function-led design

Nanoparticle-based aerogels combine the properties of traditional aerogels with those of nanoparticles, and hold promise for various applications following a function-led design.

Electronic Processes within Quantum Dot-Molecule Complexes

The subject of this review is the colloidal quantum dot (QD) and specifically the interaction of the QD with proximate molecules. It covers various functions of these molecules, including (i) ligands for the QDs, coupled electronically or vibrationally to localized surface states or to the delocalized states of the QD core, (ii) energy or electron donors or acceptors for the QDs, and (iii) structural components of QD assemblies that dictate QD-QD or QD-molecule interactions. Research on interactions of ligands with colloidal QDs has revealed that ligands determine not only the excited state dynamics of the QD but also, in some cases, its ground state electronic structure. Specifically, the article discusses (i) measurement of the electronic structure of colloidal QDs and the influence of their surface chemistry, in particular, dipolar ligands and exciton-delocalizing ligands, on their electronic energies; (ii) the role of molecules in interfacial electron and energy transfer processes involving QDs, including electron-to-vibrational energy transfer and the use of the ligand shell of a QD as a semipermeable membrane that gates its redox activity; and (iii) a particular application of colloidal QDs, photoredox catalysis, which exploits the combination of the electronic structure of the QD core and the chemistry at its surface to use the energy of the QD excited state to drive chemical reactions.

Intracellular imaging of quantum dots, gold, and iron oxide nanoparticles with associated endocytic pathways

Metallic nanoparticles (NP) have been used for biomedical applications especially for imaging. Compared to nonmetallic NP, metallic NP provide high contrast images because of their optical light scattering, magnetic resonance, X-ray absorption, or other physicochemical properties. In this review, a series of in vitro imaging techniques for metallic NP will be introduced, meanwhile their strengths and weaknesses will be discussed. By utilizing these imaging methods, the cellular uptake of metallic NP can be easily visualized to better understand the endocytic mechanisms of NP intracellular delivery. Several types of metallic NP that are used for imaging or as contrast agents such as quantum dots, gold, iron oxide, and other metallic NP will be presented. Cellular uptake of metallic NP and associated endocytic mechanisms highly depends upon the NP size, charge, surface coating, shape, or other factors such as cell type, cell differentiation status, cell surface status, external forces, protein binding, temperature, and the biological milieu. Classical endocytic routes such as lipid raft-mediated pathways, clathrin or caveolae-mediated pathways, macropinocytosis, and phagocytosis have been investigated, yet there is still a demand to determine other endocytic pathways. Knowing the different methodologies used to determine the endocytic pathways will increase the understanding of NP toxicity, cancer cell targeting, and imaging, so that surface coatings can be created for efficient cell uptake of metallic NP with minimal cytotoxicity WIREs Nanomed Nanobiotechnol 2017, 9:e1419. doi: 10.1002/wnan.1419 For further resources related to this article, please visit the WIREs website.

Characterization of the Dynamics of Photoluminescence Degradation in Aqueous CdTe/CdS Core-Shell Quantum Dots

We investigate the effects of the excitation power on the photoluminescence spectra of aqueous CdTe/CdS core-shell quantum dots. We have focused our efforts on nanoparticles that are drop-cast on a silicon nitride substrate and dried out. Under such conditions, the emission intensity of these nanocrystals decreases exponentially and the emission center wavelength shifts with the time under laser excitation, displaying a behavior that depends on the excitation power. In the low-power regime a blueshift occurs, which we attribute to photo-oxidation of the quantum dot core. The blueshift can be suppressed by performing the measurements in a nitrogen atmosphere. Under high-power excitation the nanoparticles thermally expand and aggregate, and a transition to a redshift regime is then observed in the photoluminescence spectra. No spectral changes are observed for nanocrystals dispersed in the solvent. Our results show a procedure that can be used to determine the optimal conditions for the use of a given set of colloidal quantum dots as light emitters for photonic crystal optical cavities.

Micro- and nanostructures of photoelectrodes for solar-driven water splitting

Artificial photosynthesis of clean fuels has aroused great interest to meet the great demand for clean and renewable energy. Great advances have recently been made in various photoelectrodes with their efficiencies and stabilities significantly improved by the design and implementation of novel structures, which are determinative for the optical absorption, charge-transport path, surface area, and electronic conductivity. This Research News article discusses perspectives of the synthetic methods and micro- and nanostructures (planar structures, 1D structures, and mesoporous structures) of photoelectrodes, and their relationships with the photo-electrochemical performance. Structural features, such as particle size, crystallinity, morphology, and film thickness, as well as the trade-offs among them are also evaluated and discussed for each category of structure.

Single-step direct fabrication of luminescent Cu-doped Zn(x)Cd(1-x)S quantum dot thin films via a molecular precursor solution approach and their application in luminescent, transparent, and conductive thin films

Luminescent Cu-doped ZnxCd1-xS quantum dot thin films have been directly fabricated via a facile solution method in open air. Cu2O, ZnO, and Cd(OH)2 were used as starting materials, and 3-mercaptopropionic acid was used as the capping agent. The effects of Cu dopant concentration, sintering temperature, and sintering time on the photoluminescence properties of Cu-doped ZnxCd1-xS nanocrystal thin films have been systematically investigated. As-prepared quantum dot thin films exhibit tunable emission covering the whole visible light region and the absolute photoluminescence quantum yields can reach as high as 25.5%, which have high potential for applications in luminescent, transparent, and conductive thin films.

Transition metal ion induced hydrogelation by amino-terpyridine ligands

Hydrogelation behavior of a new class of terpyridine based metallogelators are explored. The gelation and the gel morphology was found to be critically dependent on divalent metal ions, anions and on subtle structural changes on the gelator molecule.