Library Design of Ligands at the Surface of Colloidal Nanocrystals

Molecular-Scale Visualization of Steric Effects of Ligand Binding to Reconstructed Au(111) Surfaces

Direct imaging of single molecules at nanostructured interfaces is a grand challenge with potential to enable new, precise material architectures and technologies. Of particular interest are the structural morphology and spectroscopic signatures of the adsorbed molecule, where modern probes are only now being developed with the necessary spatial and energetic resolution to provide detailed information at the molecule-surface interface. Here, we directly characterize the adsorption of individual m-terphenyl isocyanide ligands on a reconstructed Au(111) surface through scanning tunneling microscopy and inelastic electron tunneling spectroscopy. The site-dependent steric pressure of the various surface features alters the vibrational fingerprints of the m-terphenyl isocyanides, which are characterized with single-molecule precision through joint experimental and theoretical approaches. This study provides molecular-level insights into the steric-pressure-enabled surface binding selectivity as well as its effect on the chemical properties of individual surface-binding ligands.

Understanding Flexdispersion: Structure-Function Relationship Studies of Organic Amphiphilic Ligands

Since inorganic nanoparticles have unique properties that differ from those of bulk materials, their material applications have attracted attention in various fields. In order to utilize inorganic nanoparticles for functional materials, they must be dispersed without agglomeration. Therefore, the surfaces of inorganic nanoparticles are typically modified with organic ligands to improve their dispersibility. Nevertheless, the relationship between the tail group structure in organic ligands and the dispersibility of inorganic nanoparticles in organic solvents remains poorly understood. We previously developed amphiphilic ligands that consist of ethylene glycol chains and alkyl chains to disperse inorganic nanoparticles in a variety of organic solvents. However, the structural requirements for amphiphilic ligands to "flexibly" disperse nanoparticles in less polar to polar solvents are still unclear. Here, we designed and synthesized several phosphonic acid ligands for structure-function relationship studies of flexdispersion. Dynamic light scattering analysis and visible light transmittance measurements revealed that the ratio of alkyl/ethylene glycol chains in organic ligands alone does not determine the dispersibility of the nanoparticles in organic solvents, but the arrangement of the individual chains also has an effect. From a practical application standpoint, it is preferable to design ligands with ethylene glycol chains on the outside relative to the particle surface.

Colloidal Quantum Dots for Explosive Detection: Trends and Perspectives

Sensitive, accurate, and reliable detection of explosives has become one of the major needs for international security and environmental protection. Colloidal quantum dots, because of their unique chemical, optical, and electrical properties, as well as easy synthesis route and functionalization, have demonstrated high potential to meet the requirements for the development of suitable sensors, boosting the research in the field of explosive detection. Here, we critically review the most relevant research works, highlighting three different mechanisms for explosive detection based on colloidal quantum dots, namely photoluminescence, electrochemical, and chemoresistive sensing. We provide a comprehensive overview and an extensive discussion and comparison in terms of the most relevant sensor parameters. We highlight advantages, limitations, and challenges of quantum dot-based explosive sensors and outline future research directions for the advancement of knowledge in this surging research field.

Alkyne-Functionalized Platinum Chalcogenide (S, Se) Nanoparticles

Metal chalcogenide nanoparticles play a vital role in a wide range of applications and are typically stabilized by organic derivatives containing thiol, amine, or carboxyl moieties, where the nonconjugated particle-ligand interfaces limit the electronic interactions between the inorganic cores and organic ligands. Herein, a wet-chemistry method is developed for the facile preparation of stable platinum chalcogenide (S, Se) nanoparticles capped with acetylene derivatives (e.g., 4-ethylphenylacetylene, EPA). The formation of Pt-C≡ conjugated bonds at the nanoparticle interfaces, which is confirmed by optical and X-ray spectroscopic measurements, leads to markedly enhanced electronic interactions between the d electrons of the nanoparticle cores and π electrons of the acetylene moiety, in stark contrast to the mercapto-capped counterparts with only nonconjugated Pt-S- interfacial bonds, as manifested in spectroscopic measurements and density functional theory calculations. This study underscores the significance of conjugated anchoring linkages in the stabilization and functionalization of metal chalcogenides, a unique strategy for diverse applications.

Recent Applications of Quantum Dots in Pharmaceutical Analysis

Nanotechnology has emerged as one of the most potential areas for pharmaceutical analysis. The need for nanomaterials in pharmaceutical analysis is comprehended in terms of economic challenges, health and safety concerns. Quantum dots (QDs)or colloidal semiconductor nanocrystals are new groups of fluorescent nanoparticles that bind nanotechnology to drug analysis. Because of their special physicochemical characteristics and small size, QDs are thought to be promising candidates for the electrical and luminescent probes development. They were originally developed as luminescent biological labels, but are now discovering new analytical chemistry applications, where their photo-luminescent properties are used in pharmaceutical, clinical analysis, food quality control and environmental monitoring. In this review, we discuss QDs regarding properties and advantages, advances in methods of synthesis and their recent applications in drug analysis in the recent last years.

Direct Optical Patterning of Quantum Dots: One Strategy, Different Chemical Processes

Patterning, stability, and dispersion of the semiconductor quantum dots (scQDs) are three issues strictly interconnected for successful device manufacturing. Recently, several authors adopted direct optical patterning (DOP) as a step forward in photolithography to position the scQDs in a selected area. However, the chemistry behind the stability, dispersion, and patterning has to be carefully integrated to obtain a functional commercial device. This review describes different chemical strategies suitable to stabilize the scQDs both at a single level and as an ensemble. Special attention is paid to those strategies compatible with direct optical patterning (DOP). With the same purpose, the scQDs' dispersion in a matrix was described in terms of the scQD surface ligands' interactions with the matrix itself. The chemical processes behind the DOP are illustrated and discussed for five different approaches, all together considering stability, dispersion, and the patterning itself of the scQDs.

Ligand dynamics on the surface of CdSe nanocrystals

Synthesis protocols of colloidal semiconductor nanocrystals (NCs) comprise the coordination of the semiconductive inorganic core by a layer of organic ligands, which play a crucial role in stabilizing the NCs in organic solvents. Understanding the distribution, binding and mobility of ligands on the different NC facets is key to prevent the formation of surface defects and to optimize the overall optoelectronic efficiency of these materials. In this paper, we employed classical molecular dynamics (MD) simulations to shed light on the plausible locations, binding modes and mobilities of carboxylate ligands on the different facets of CdSe nanocrystals. Our results suggest that these features are influenced by the temperature of the system and the coordination number of the surface (Cd and Se) atoms. High ligand mobilities and structural rearrangements are linked to a low coordination of the Cd atoms. Undercoordinated Se atoms, which are considered the culprit of hole trap states in the bandgap of the material, are instead found to spontaneously form on the nanosecond timescale, making them likely candidates for an efficient photoluminescence quenching mechanism.

Threonine functionalized colloidal cadmium sulfide (CdS) quantum dots: the role of solvent and counterion in ligand included chiroptical properties

The functionalization of semiconductor nanocrystals, quantum dots (QDs), with small organic molecules has been studied extensively to gain better knowledge on how to tune the electronic, optical and chiroptical properties of QDs. Chiral QDs have progressively emerged as key materials in a vast range of applications including biosensing and biorecognition, imaging, asymmetric catalysis, optoelectronic devices, and spintronics. To engage the full potential of the unique properties of chiral nanomaterials and be able to prepare them with tailorable chiroptical characteristics, it is essential to understand how chirality is rendered from chiral molecular ligands at the surface of nanocrystals to the electronic states of QDs. Using a series of polar protic and aprotic solvents together with ammonium (NH₄⁺), tetramethylammonium (TMA⁺), and tetrabutylammonium (TBA⁺) countercations in the preparation of threonine-functionalized cadmium sulfide (Thr-CdS) QDs by phase transfer ligand exchange approach, we demonstrated the significance of the role both the solvent and the countercations play in the transfer of chirality from chiral molecular ligand to achiral semiconductor QDs as apparent by the modulations of the signatures and anisotropy of the circular dichroism (CD) spectra. Moreover, we have utilized tetrabutylammonium countercation to successfully synthesize chiral QDs in nonpolar cyclohexane solvent for the first time. This study provides further insights into the origin of the ligand induced chirality of colloidal nanomaterials and facilitates the synthesis of tailormade chiral QDs.

Highly water-stable, luminescent, and monodisperse polymer-coated CsPbBr3 nanocrystals for imaging in living cells with better sensitivity

Recently, CsPbX₃ (X= Cl, Br, I) nanocrystals (NCs) have evolved as a potential contender for various optoelectronic applications due to some of their excellent photophysical properties. Their superior non-linear optical properties enable them to take part in bioimaging applications due to their longer penetration depth and less scattering effect in living cells. However, the poor stability of perovskite NCs in aqueous media still remains a great challenge for practical usage. Comparatively stable silica-coated NCs have a tendency to agglomerate among other NCs and transform into bigger particles. Such big particles clog the inside of narrow channels during the uptake and can't effectively reach the targeted cells. To tackle such issues, we introduce a fast and reproducible synthesis process of CsPbBr₃ NCs that are coated with different long-chained organic ligands/polymers and compared their photophysical properties. Among them, polyvinylpyrrolidone (PVP) encapsulated NCs are highly luminescent in the green spectral region and showed a maximum photoluminescence quantum yield (PLQY) of up to 84%. The incorporation of n-isopropyl acrylamide (NIPAM) along with PVP further improves the stability of the PVP-coated NCs against heat and moisture. These NCs exhibit higher water stability compared to silica-coated NCs and maintained their emission properties for about one week in DI water. The smaller particle size, uniform size distribution, higher structural stability, and better dispersivity of polymer-coated NCs in the aqueous media enable them to perform as fluorescent probes for live cell imaging in mammalian Chinese Hamster Ovary (CHO-K1) cells. There is no adverse affect in the cells' viability and morphology even after long incubation periods (∼72 hours). The dosage of Pb-ions contained in the polymer-coated NCs is calculated as below 5 μg mL^-1, which is suitable for live cell imaging. This work provides insight for expanding the use of these NCs significantly into bioimaging applications with higher sensitivity.

Dispersibility of TiO2 Nanoparticles in Less Polar Solvents: Role of Ligand Tail Structures

Nanoparticles (NPs) are inherently prone to aggregation and loss of their size-derived properties, thus it is essential to enhance their dispersibility for applications. In less polar solvents, organic ligands containing oleyl groups are known as good dispersants due to their inefficient shell packing and inhibition of chain-chain crystallization as well as interdigitation between adjacent NPs. However, reagents with oleyl structures, such as oleic acid and oleylamine, can contain trans double bonds and saturated impurities, which might affect the chemical and/or physical properties of the NPs. Nevertheless, the effect of slight differences in surface ligand structure, including isomers, on the dispersibility of NPs has been little studied. We have synthesized five phosphonic acid ligands to investigate the structure-dispersibility relationship in detail. Dynamic light scattering and visible light transmittance revealed that not only regio- but also the stereochemistries of the C=C double bond in the ligand molecule, as well as the choice of solvent, are key factors in enhancing dispersibility.

Electric, magnetic, and shear field-directed assembly of inorganic nanoparticles

Ordered assemblies of inorganic nanoparticles (NPs) have shown tremendous potential for wide applications due to their unique collective properties, which differ from those of individual NPs. Various assembly methods, such as external field-directed assembly, interfacial assembly, template assembly, biomolecular recognition-mediated assembly, confined assembly, and others, have been employed to generate ordered inorganic NP assemblies with hierarchical structures. Among them, the external field-directed assembly method is particularly fascinating, as it can remotely assemble NPs into well-ordered superstructures. Moreover, external fields (e.g., electric, magnetic, and shear fields) can introduce a local and/or global field intensity gradient, resulting in an additional force on NPs to drive their rotation and/or translation. Therefore, the external field-directed assembly of NPs becomes a robust method to fabricate well-defined functional materials with the desired optical, electronic, and magnetic properties, which have various applications in catalysis, sensing, disease diagnosis, energy conversion/storage, photonics, nano-floating-gate memory, and others. In this review, the effects of an electric field, magnetic field, and shear field on the organization of inorganic NPs are highlighted. The methods for controlling the well-ordered organization of inorganic NPs at different scales and their advantages are reviewed. Finally, future challenges and perspectives in this field are discussed.

Optical gas sensor based on the combination of a QD photoluminescent probe and a QD photodetector

We report on a sensor architecture for detection of hazardous gases. The proposed device is based on the integration of a solid-state quantum dot (QD) photoluminescent probe with a QD photodetector on the same substrate. The effectiveness of the approach is demonstrated by developing a compact optical sensor for trace detection of explosives in air. The proposed architecture is very simple and consists of a silicon substrate with both surfaces coated with QD films. The upper layer acts as photoluminescent probe, pumped by a blue LED. The change of photoluminescence intensity associated to the interaction between the QDs and the target analyte is measured by the QD photodetector fabricated on the opposite side of the substrate. The sensor is mounted into a small chamber provided with the LED and the front-end electronics. The device is characterized by using nitrobenzene as representative nitroaromatic compound. Extremely low concentrations (down to 0.1 ppm) can be detected by the proposed device, with a theoretical detection limit estimated to be as low as 2 ppb. Results are repeatable and no ageing effect is observed over a 70 d period. The proposed architecture may provide a promising solution for explosive detection in air as well as other sensing applications, thanks to its sensitivity, simple fabrication process, practical usability and cost effectiveness.

Strengthening Engineered Nanocrystal Three-Dimensional Superlattices via Ligand Conformation and Reactivity

Nanocrystal assembly into ordered structures provides mesostructural functional materials with a precise control that starts at the atomic scale. However, the lack of understanding on the self-assembly itself plus the poor structural integrity of the resulting supercrystalline materials still limits their application into engineered materials and devices. Surface functionalization of the nanobuilding blocks with organic ligands can be used not only as a means to control the interparticle interactions during self-assembly but also as a reactive platform to further strengthen the final material via ligand cross-linking. Here, we explore the influence of the ligands on superlattice formation and during cross-linking via thermal annealing. We elucidate the effect of the surface functionalization on the nanostructure during self-assembly and show how the ligand-promoted superlattice changes subsequently alter the cross-linking behavior. By gaining further insights on the chemical species derived from the thermally activated cross-linking and its effect in the overall mechanical response, we identify an oxidative radical polymerization as the main mechanism responsible for the ligand cross-linking. In the cascade of reactions occurring during the surface-ligands polymerization, the nanocrystal core material plays a catalytic role, being strongly affected by the anchoring group of the surface ligands. Ultimately, we demonstrate how the found mechanistic insights can be used to adjust the mechanical and nanostructural properties of the obtained nanocomposites. These results enable engineering supercrystalline nanocomposites with improved cohesion while preserving their characteristic nanostructure, which is required to achieve the collective properties for broad functional applications.

Automated Quantum Dots Purification via Solid Phase Extraction

The separation of colloidal nanocrystals from their original synthesis medium is an essential process step towards their application, however, the costs on a preparative scale are still a constraint. A new combination of approaches for the purification of hydrophobic Quantum Dots is presented, resulting in an efficient scalable process in regard to time and solvent consumption, using common laboratory equipment and low-cost materials. The procedure is based on a combination of solvent-induced adhesion and solid phase extraction. The platform allows the transition from manual handling towards automation, yielding an overall purification performance similar to one conventional batch precipitation/centrifugation step, which was investigated by thermogravimetry and gas chromatography. The distinct miscibility gaps between surfactants used as nanoparticle capping agents, original and extraction medium are clarified by their phase diagrams, which confirmed the outcome of the flow chemistry process. Furthermore, the solubility behavior of the Quantum Dots is put into context with the Hansen solubility parameters framework to reasonably decide upon appropriate solvent types.

Colloidal Bismuth Chalcohalide Nanocrystals

Here we present a colloidal approach to synthesize bismuth chalcohalide nanocrystals (BiEX NCs, in which E=S, Se and X=Cl, Br, I). Our method yields orthorhombic elongated BiEX NCs, with BiSCl crystallizing in a previously unknown polymorph. The BiEX NCs display a composition‐dependent band gap spanning the visible spectral range and absorption coefficients exceeding 10⁵ cm⁻¹. The BiEX NCs show chemical stability at standard laboratory conditions and form colloidal inks in different solvents. These features enable the solution processing of the NCs into robust solid films yielding stable photoelectrochemical current densities under solar‐simulated irradiation. Overall, our versatile synthetic protocol may prove valuable in accessing colloidal metal chalcohalide nanomaterials at large and contributes to establish metal chalcohalides as a promising complement to metal chalcogenides and halides for applied nanotechnology.

Entropy of Branching Out: Linear versus Branched Alkylthiols Ligands on CdSe Nanocrystals

Surface ligands of semiconductor nanocrystals (NCs) play key roles in determining their colloidal stability and physicochemical properties and are thus enablers also for the NCs flexible manipulation toward numerous applications. Attention is usually paid to the ligand binding group, while the impact of the ligand chain backbone structure is less discussed. Using isothermal titration calorimetry (ITC), we studied the effect of structural changes in the ligand chain on the thermodynamics of the exchange reaction for oleate coated CdSe NCs, comparing linear and branched alkylthiols. The investigated alkylthiol ligands differed in their backbone length, branching position, and branching group length. Compared to linear ligands, lower exothermicity and entropy loss were observed for an exchange with branched ligands, due to steric hindrance in ligand packing, thereby justifying their previous classification as "entropic ligands". Mean-field calculations for ligand binding demonstrate the contribution to the overall entropy originating from ligand conformational entropy, which is diminished upon binding mainly by packing of NC-bound ligands. Model calculations and the experimental ITC data both point to an interplay between the branching position and the backbone length in determining the entropic nature of the branched ligand. Our findings suggest that the most entropic ligand should be a short, branched ligand with short branching group located toward the middle of the ligand chain. The insights provided by this work also contribute to a future smarter NC surface design, which is an essential tool for their implementation in diverse applications.

Thermodynamics of nanocrystal–ligand binding through isothermal titration calorimetry

Manipulations of nanocrystal (NC) surfaces have propelled the applications of colloidal NCs across various fields such as bioimaging, catalysis, electronics, and sensing applications.

Surface Chemistry Impact on the Light Absorption by Colloidal Quantum Dots

At the size scale at which quantum confinement effects arise in inorganic semiconductors, the materials' surface-to-volume ratio is intrinsically high. This consideration sets surface chemistry as a powerful tool to exert further control on the electronic structure of the inorganic semiconductors. Among the materials that experience the quantum confinement regime, those prepared via colloidal synthetic procedures (the colloidal quantum dots - and wires and wells, too -) are prone to undergo surface reactions in the solution phase and thus represent an ideal framework to study the ensemble impact of surface chemistry on the materials' electronic structure. It is here discussed such an impact at the ground state by using the absorption spectrum of the colloidal quantum dots as a descriptor. The experiments show that the chemical species (the ligands) at the colloidal quantum dot surface induce changes to the optical band gap, the absorption coefficient at all wavelengths, and the ionization potential. These evidences point to a description of the colloidal quantum dot (the ligand/core adduct) as an indecomposable species, in which the orbitals localized on the ligands and the core mix in each other's electric field. This description goes beyond conventional models that conceive the ligands on the basis of pure electrostatic arguments (i. e., either as a dielectric shell or as electric dipoles) or as a mere potential energy barrier at the core boundaries.

Thermomechanically controlled fluorescence anisotropy in thin films of InP/ZnS quantum dots

Macroscopic scale sources of polarized light play a fundamental role in designing light-emitting devices. In this communication we report the formation of nano- and macro-scale ordered, layered assemblies of InP/ZnS quantum dots (QDs) exhibiting fluorescence anisotropy (FA), as well as thermo- and mechano-responsive properties. The long-range organization of small, quasi-isotropic nanoparticles was achieved by introducing liquid crystal molecules to the surface of QDs, without the need to use an organic matrix. Melting/crystallization of the ligand at 95 deg. C translated to a reversible reconfiguration of QDs thin film between 2D layered and body-centered cubic structures, characteristic for a temperature range below and above the melting point, respectively. The low-temperature, layered structure exhibited mechano-responsiveness which was key to introduce and control the sample alignment. Interestingly, transverse and parallel alignment modes of QDs layers were achieved, depending on the temperature of mechanical shearing. As prepared QD samples exhibited fluorescence anisotropy strongly correlated to the macroscopic orientation of the layers. Correlated small-angle X-ray diffraction (SAXRD) and fluorescence spectroscopy studies confirmed the mm-scale alignment of the thin films of QDs. Such films may be advantageous for developing efficient, densely packed, and uniform macro-scale FA sources.

Metal Cation Valency Dependence in Morphology Evolution of Cu2-x S Nanodisk Seeds and Their Pseudomorphic Cation Exchanges

A crucial parameter in the design of semiconductor nanoparticles (NPs) with controllable optical, magnetic, electronic, and catalytic properties is the morphology. Herein, we demonstrate the potential of additive metal cations with variable valency to direct the morphology evolution of copper-deficient Cu_2-x S nanoparticles in the process of seed-mediated growth. In particular, the djurleite Cu_1.94 S seed could evolve from disk into tetradecahedron in the presence of tin(IV) cations, whereas they merely formed sharp hexagonal nanodisks with tin(II) cations. In addition to djurleite Cu_1.94 S, the tin(IV) cations could be generalized to direct the growth of roxbyite Cu_1.8 S and covellite CuS nanodisk seeds into tetradecahedra. We further perform pseudomorphic cation exchanges of Cu_1.94 S tetradecahedra with Zn²⁺ and Cd²⁺ to produce polyhedral zinc sulfide (ZnS) and cadmium sulfide (CdS) NPs. Moreover, we achieve Cu_1.8 S/ZnS and Cu_1.94 S/CdS tetradecahedral heterostructures via partial cation exchange, which are otherwise inaccessible by traditional synthetic approaches.

Uncovering the Role of Countercations in Ligand Exchange of WSe2: Tuning the d-Band Center toward Improved Hydrogen Desorption

The role of countercations that do not bind to core nanocrystals (NCs) but rather ensure charge balance on ligand-exchanged NC surfaces has been rarely studied and even neglected. Such a scenario is unfortunate, as an understanding of surface chemistry has emerged as a key factor in overcoming colloidal NC limitations as catalysts. In this work, we report on the unprecedented role of countercations in ligand exchange for a colloidal transition metal dichalcogenide (TMD), WSe₂, to tune the d-band center toward the Fermi level for enhanced hydrogen desorption. Conventional long-chain organic ligands, oleylamine, of WSe₂ NCs are exchanged with short atomic S^2- ligands having countercations to preserve the charge balance (WSe₂/S^2-/M⁺, M = Li, Na, K). Upon exchange with S^2- ligands, the charge-balancing countercations are intercalated between WSe₂ layers, thereby serving a unique function as an electrochemical hydrogen evolution reaction (HER) catalyst. The HER activity of ligand-exchanged colloidal WSe₂ NCs shows a decrease in overpotential by down-shift of d-band center to induce more electron-filling in antibonding orbital and an increase in the electrochemical active surface area (ECSA). Exchanging surface functionalities with S^2- anionic ligands enhances HER kinetics, while the existence of intercalated countercations improves charge transfer with the electrolyte. The obtained results suggest that both anionic ligands and countercationic species in ligand exchange must be considered to enhance the overall catalytic activity of colloidal TMDs.