Contribution of the Loss of Nanocrystal Ligands to Interdot Coupling in Films of Small CdSe/1-Thioglycerol Nanocrystals

Ti3C2 MXene-anchored photoelectrochemical detection of exosomes by in situ fabrication of CdS nanoparticles with enzyme-assisted hybridization chain reaction

Exosomes that carry large amounts of tumor-specific molecular information have been identified as a potential non-invasive biomarker for early warning of cancer. In this work, we reported an enzyme-assisted photoelectrochemical (PEC) biosensor for quantification of exosomes based on the in situ synthesis of Ti₃C₂ MXene/CdS composites with magnetic separation technology and hybridization chain reaction (HCR). First, exosomes were specifically bound between aptamer-labeled magnetic beads (CD63-MBs) and a cholesterol-labeled DNA anchor. The properly designed anchor ends acted as a trigger to enrich the alkaline phosphatase (ALP) through HCR. It catalyzed more sodium thiophosphate to generate the sulfideion (S²⁻), which combined with Cd²⁺ for in situ fabrication of CdS on Ti₃C₂ MXene resulting in elevated photocurrent. The Ti₃C₂ MXene-anchored PEC method was realized for the quantitative detection of exosomes, which exhibited the dynamic working range from 7.3 × 10⁵ particles per mL to 3.285 × 10⁸ particles per mL with a limit of detection of 7.875 × 10⁴ particles per mL. The strategy showed acceptable stability, high sensitivity, rapid response and excellent selectivity. Furthermore, we believe that the PEC biosensor has huge potential as a routine bioassay method for the precise quantification of exosomes from breast cancer in the future.

An enzyme-assisted photoelectrochemical (PEC) biosensor was established for quantification of exosomes based on the in situ synthesis of Ti₃C₂ MXene/CdS composites with magnetic separation technology and hybridization chain reaction (HCR).

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.

Shell thickness modulation in ultrasmall CdSe/CdS(x)Se(1-x)/CdS core/shell quantum dots via 1-thioglycerol

In this study, we report on the synthesis of CdSe/CdS core-shell ultrasmall quantum dots (CS-USQDs) using an aqueous-based wet chemistry protocol. The proposed chemical route uses increasing concentration of 1-thioglycerol to grow the CdS shell on top of the as-precipitated CdSe core in a controllable way. We found that lower concentration of 1-thioglycerol (3 mmol) added into the reaction medium limits the growth of the CdSe core, and higher and increasing concentration (5-11 mmol) of 1-thioglycerol promotes the growth of CdS shell on top of the CdSe core in a very controllable way, with an increase from 0.50 to 1.25 nm in shell thickness. The growth of CS-USQDs of CdSe/CdS was confirmed by using different experimental techniques, such as optical absorption (OA) spectroscopy, fluorescence spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopy. Data collected from OA were used to obtain the average values of the CdSe core diameter, whereas Raman data were used to assess the average values of the CdSe core diameter and CdS shell thicknesses.

Temperature-dependent Hall and field-effect mobility in strongly coupled all-inorganic nanocrystal arrays

We report on the temperature-dependent Hall effect characteristics of nanocrystal (NC) arrays prepared from colloidal InAs NCs capped with metal chalcogenide complex (MCC) ligands (In2Se4(2-) and Cu7S4(-)). Our study demonstrates that Hall effect measurements are a powerful way of exploring the fundamental properties of NC solids. We found that solution-cast 5.3 nm InAs NC films capped with copper sulfide MCC ligands exhibited high Hall mobility values over 16 cm(2)/(V s). We also showed that the nature of MCC ligands can control doping in NC solids. The comparative study of the temperature-dependent Hall and field-effect mobility values provides valuable insights concerning the charge transport mechanism and points to the transition from a weak to a strong coupling regime in all-inorganic InAs NC solids.

Directly deposited quantum dot solids using a colloidally stable nanoparticle ink

We develop a photovoltaic colloidal quantum dot ink that allows for lossless, single-step coating of large areas in a manufacturing-compatible process. Our materials strategy involves a solution-phase ligand exchange to transport compatible linkers that yield 1-thioglycerol-capped PbS quantum dots in dimethyl sulfoxide with a photoluminescence quantum yield of 24%. A proof-of-principle solar cell made from the ink exhibits 2.1% power conversion efficiency.

Size dependence in tunneling spectra of PbSe quantum-dot arrays

Interdot Coulomb interactions and collective Coulomb blockade were theoretically argued to be a newly important topic, and experimentally identified in semiconductor quantum dots, formed in the gate confined two-dimensional electron gas system. Developments of cluster science and colloidal synthesis accelerated the studies of electron transport in colloidal nanocrystal or quantum-dot solids. To study the interdot coupling, various sizes of two-dimensional arrays of colloidal PbSe quantum dots are self-assembled on flat gold surfaces for scanning tunneling microscopy and scanning tunneling spectroscopy measurements at both room and liquid-nitrogen temperatures. The tip-to-array, array-to-substrate, and interdot capacitances are evaluated and the tunneling spectra of quantum-dot arrays are analyzed by the theory of collective Coulomb blockade. The current-voltage of PbSe quantum-dot arrays conforms properly to a scaling power law function. In this study, the dependence of tunneling spectra on the sizes (numbers of quantum dots) of arrays is reported and the capacitive coupling between quantum dots in the arrays is explored.

Strong electronic coupling in two-dimensional assemblies of colloidal PbSe quantum dots

Thin films of colloidal PbSe quantum dots can exhibit very high carrier mobilities when the surface ligands are removed or replaced by small molecules, such as hydrazine. Charge transport in such films is governed by the electronic exchange coupling energy (beta) between quantum dots. Here we show that two-dimensional quantum dot arrays assembled on a surface provide a powerful system for studying this electronic coupling. We combine optical spectroscopy with atomic force microscopy to examine the chemical, structural, and electronic changes that occur when a submonolayer of PbSe QDs is exposed to hydrazine. We find that this treatment leads to strong and tunable electronic coupling, with the beta value as large as 13 meV, which is 1 order of magnitude greater than that previously achieved in 3D QD solids with the same chemical treatment. We attribute this much enhanced electronic coupling to reduced geometric frustration in 2D films. The strongly coupled quantum dot assemblies serve as both charge and energy sinks. The existence of such coupling has serious implications for electronic devices, such as photovoltaic cells, that utilize quantum dots.

Nanocrystal plasma polymerization: from colloidal nanocrystals to inorganic architectures

Nanocrystal superstructures are increasingly becoming a subject of intense study. Such materials could constitute a new class of nanocomposites of designed structure, of homogeneous composition, and with unique properties. New phenomena are observed in these materials because of the interaction at such diminutive length scales. A common problem in the development of devices relying on colloidal nanocrystal assemblies is that the individual nanocrystal building blocks require organic molecules to control their size. These ligands are responsible for the colloidal stability of the individual nanocrystal building blocks and are thus necessary for their solution processibility. Because of the ligands' incompatibility with many solid state applications, it is important to develop post-processing techniques that mildly remove them from these nanocomposites, while maintaining the size-dependent properties of the building blocks. This Account highlights a new strategy, nanocrystal plasma polymerization (NPP), for processing colloidal nanocrystal assemblies. This technique exposes the nanocomposite to a mild air plasma and allows for the removal of the nanocrystals' capping ligands while preserving their size-dependent and material properties. As a result, the process yields a nearly all-inorganic flexible solid-state material with unprecedented characteristics. We describe early experiments, in which NPP was used to create arbitrarily complex 1D, 2D, and 3D inorganic free-standing architectures entirely composed of nanocrystals, as well as future directions and challenges. We expect this platform will be useful for the design of new materials and will be a valuable new addition to the nanoscientist's toolbox.