Chemical Synthetic Strategy for Single-Layer Transition-Metal Chalcogenides

A solution-phase synthetic protocol to form two-dimensional (2D) single-layer transition-metal chalcogenides (TMCs) has long been sought; however, such efforts have been plagued with the spontaneous formation of multilayer sheets. In this study, we discovered a solution-phase synthetic protocol, called “diluted chalcogen continuous influx (DCCI)”, where controlling the chalco­gen source influx (e.g., H₂S) during its reaction with the transition-metal halide precursor is the critical parameter for the formation of single-layer sheets as examined for the cases of group IV TMCs. The continuous influx of dilute H₂S throughout the entire growth period is necessary for large sheet formation through the exclusive <i>a-</i> and <i>b-</i>axial growth processes. By contrast, the burst influx of highly concentrated H₂S in the early stages of the growth process forms multilayer TMC nanodiscs. Our DCCI protocol is a new synthetic concept for single-layer TMCs and, in principle, can be operative for wide range of TMC nanosheets

Well-Defined Colloidal 2‑D Layered Transition-Metal Chalcogenide Nanocrystals via Generalized Synthetic Protocols

While interesting and unprecedented material characteristics of two dimensionality (2-D) layered nanomaterials are emerging, their reliable synthetic methodologies are not well developed. In this study we demonstrate general applicability of synthetic protocols to a wide range of colloidal 2-D layered transition-metal chalcogenide (TMC) nanocrystals. As distinctly different from other nanocrystals, we discovered that 2-D layered TMC nanocrystals are unstable in the presence of reactive radicals from elemental chalcogen during the crystal formation. We first introduce the synthesis of titanium sulfide and selenide where well-defined single crystallinity and lateral size controllability are verified, and then such synthetic protocols are extended to all of group IV and V transition-metal sulfide (TiS₂, ZrS₂, HfS₂, VS₂, NbS₂, and TaS₂) and selenide (TiSe₂, ZrSe₃, HfSe₃, VSe₂, NbSe₂, and TaSe₂) nanocrystals. The use of appropriate chalcogen source is found to be critical for the successful synthesis of 2-D layered TMC nanocrystals. CS₂ is an efficient chalcogen precursor for metal sulfide nanocrystals, whereas elemental Se is appropriate for metal selenide nanocrystals. We briefly discuss the effects of reactive radical characteristics of elemental S and Se on the formation of 2-D layered TMC nanocrystals

Magnetic Tandem Apoptosis for Overcoming Multidrug-Resistant Cancer

Multidrug resistance (MDR) is a leading cause of failure in current chemotherapy treatment and constitutes a formidable challenge in therapeutics. Here, we demonstrate that a nanoscale magnetic tandem apoptosis trigger (m-TAT), which consists of a magnetic nanoparticle and chemodrug (e.g., doxorubicin), can completely remove MDR cancer cells in both in vitro and in vivo systems. m-TAT simultaneously activates extrinsic and intrinsic apoptosis signals in a synergistic fashion and downregulates the drug efflux pump (e.g., P-glycoprotein) which is one of the main causes of MDR. The tandem apoptosis strategy uses low level of chemodrug (in the nanomolar (nM) range) to eliminate MDR cancer cells. We further demonstrate that apoptosis of MDR cancer cells can be achieved in a spatially selective manner with single-cell level precision. Our study indicates that nanoscale tandem activation of convergent signaling pathways is a new platform concept to overcome MDR with high efficacy and specificity

Detection of α‑Thrombin with Platelet Glycoprotein Ibα (GP1bα) for the Development of a Coagulation Marker

The detection of prothrombotic markers is crucial for understanding thromboembolism and assessing the effectiveness of anticoagulant drugs. α-Thrombin is a marker that plays a critical role in the coagulation cascade process. However, the detection of this enzymatic molecule was hindered by the absence of an efficient modality in the clinical environment. Previously, we reported that one α-thrombin interacts with two α-chains of glycoprotein Ib (GPIbα), i.e., multivalent protein binding (MPB), using bioresponsive hydrogel nanoparticles (nanogels) and optical microscopy. In this study, we demonstrated that GPIbα-mediated platforms led to the highly sensitive and quantitative detection of α-thrombin in various diagnostic systems. Initially, a bioresponsive nanogel-based surface plasmon resonance (nSPR) assay was developed that responds to the MPB of α-thrombin to GPIbα. The use of GPIbα for the detection of α-thrombin was further validated using the enzyme-linked immunosorbent assay, which is a gold-standard protein detection technique. Additionally, GPIbα-functionalized latex beads were developed to perform latex agglutination (LA) assays, which are widely used with hospital diagnostic instruments. Notably, the nSPR and LA assays exhibited a nearly 1000-fold improvement in sensitivity for α-thrombin detection compared to our previous optical microscopy method. The superiority of our GPIbα-mediated platforms lies in their stability for α-thrombin detection through protein–protein interactions. By contrast, assays relying on α-thrombin enzymatic activity using substrates face the challenge of a rapid decrease in postsample collection. These results suggested that the MPB of α-thrombin to GPIbα is an ideal mode for clinical α-thrombin detection, particularly in outpatient settings

Colloidal Single-Layer Quantum Dots with Lateral Confinement Effects on 2D Exciton

Controlled lateral quantum confinement in single-layer transition-metal chalcogenides (TMCs) can potentially combine the unique properties of two-dimensional (2D) exciton with the size-tunability of exciton energy, creating the single-layer quantum dots (SQDs) of 2D TMC materials. However, exploring such opportunities has been challenging due to the limited ability to produce well-defined SQDs with sufficiently high quality and size control, in conjunction with the commonly observed inconsistency in the optical properties. Here, we report an effective method to synthesize high-quality and size-controlled SQDs of WSe₂ via multilayer quantum dots (MQDs) precursors, which enables grasping a clear picture of the role of lateral confinement on the optical properties of the 2D exciton. From the single-particle optical spectra and polarization anisotropy of WSe₂ SQDs of varying sizes in addition to their ensemble data, we reveal how the properties of 2D exciton in single-layer TMCs evolve with increasing lateral quantum confinement

Photoinduced Separation of Strongly Interacting 2‑D Layered TiS₂ Nanodiscs in Solution

Colloidal 2-D layered transition metal dichalcogenide (TMDC) nanodiscs synthesized with uniform diameter and thickness can readily form the vertically stacked assemblies of particles in solution due to strong interparticle cohesive energy. The interparticle electronic coupling that modifies their optical and electronic properties poses a significant challenge in exploring their unique properties influenced by the anisotropic quantum confinement in different directions taking advantage of the controlled diameter and thickness. Here, we show that the assemblies of 2-D layered TiS₂ nanodiscs are efficiently separated into individual nanodiscs via photoexcitation of the charge carriers by pulsed laser light, enabling the characterization of the properties of noninteracting TiS₂ nanodiscs. Photoinduced separation of the nanodiscs is considered to occur via transient weakening of the interparticle cohesive force by the dense photoexcited charge carriers, which facilitates the solvation of each nanodisc by the solvent molecules

Colloidal Synthesis of Single-Layer MSe₂ (M = Mo, W) Nanosheets via Anisotropic Solution-Phase Growth Approach

The generation of single-layer 2-dimensional (2D) nanosheets has been challenging, especially in solution-phase, since it requires highly anisotropic growth processes that exclusively promote planar directionality during nanocrystal formation. In this study, we discovered that such selective growth pathways can be achieved by modulating the binding affinities of coordinating capping ligands to the edge facets of 2D layered transition-metal chalcogenides (TMCs). Upon changing the functional groups of the capping ligands from carboxylic acid to alcohol and amine with accordingly modulated binding affinities to the edges, the number of layers of nanosheets is controlled. Single-layer MSe₂ (M = Mo, W) TMC nanosheets are obtained with the use of oleic acid, while multilayer nanosheets are formed with relatively strong binding ligands such as oleyl alcohol and oleylamine. With the choice of appropriate capping ligands in the 2D anisotropic growth regime, our solution-based synthetic method can serve a new guideline for obtaining single-layer TMC nanosheets

Unveiling Chemical Reactivity and Structural Transformation of Two‑Dimensional Layered Nanocrystals

Two-dimensional (2D) layered nanostructures are emerging fast due to their exceptional materials properties. While the importance of physical approaches (e.g., guest intercalation and exfoliation) of 2D layered nanomaterials has been recognized, an understanding of basic chemical reactions of these materials, especially in nanoscale regime, is obscure. Here, we show how chemical stimuli can influence the fate of reaction pathways of 2D layered nanocrystals. Depending on the chemical characteristics (Lewis acid (¹O₂) or base (H₂O)) of external stimuli, TiS₂ nanocrystal is respectively transformed to either a TiO₂ nanodisc through a “compositional metathesis” or a TiO₂ toroid through multistage “edge-selective structural transformation” processes. These chemical reactions can serve as the new design concept for functional 2D layered nanostructures. For example, TiS_2(disc)-TiO_2(shell) nanocrystal constitutes a high performance type II heterojunction which not only a wide range solar energy coverage (∼80%) with near-infrared absorption edge, but also possesses enhanced electron transfer property