Surface Capping Agents and Their Roles in Shape‐Controlled Synthesis of Colloidal Metal Nanocrystals

Surfactant Removal for Colloidal Nanocrystal Catalysts Mediated by N-Heterocyclic Carbenes

We report the facile removal of surfactants from colloidally synthesized nanocrystals via ligand exchange with N-heterocyclic carbenes (NHCs). Subsequent protonation of the NHC ligands in acid efficiently cleans the nanocrystals' surface while preserving their uniform morphology and structure for catalysis. The broad efficacy of this strategy is validated using monodisperse Pt, Pd, and Au nanocrystals, each prepared with strongly bound phosphine stabilizers. The surface-activated nanocrystals exhibit significantly improved catalytic activities, superior to those obtained with other surface cleaning methods, as demonstrated in two centrally important electrochemical reactions (glycerol oxidation and CO₂ reduction). This work highlights a new surface activation strategy for catalysis and other applications that enables the efficient use of well-defined nanocrystal libraries prepared by colloidal chemistry.

Using Reduction Kinetics to Control and Predict the Outcome of a Colloidal Synthesis of Noble-Metal Nanocrystals

Improving the performance of noble-metal nanocrystals in various applications critically depends on our ability to manipulate their synthesis in a rational, robust, and controllable fashion. Different from a conventional trial-and-error approach, the reduction kinetics of a colloidal synthesis has recently been demonstrated as a reliable knob for controlling the synthesis of noble-metal nanocrystals in a deterministic and predictable manner. Here we present a brief Viewpoint on the recent progress in leveraging reduction kinetics for controlling and predicting the outcome of a synthesis of noble-metal nanocrystals. With a focus on Pd nanocrystals, we first offer a discussion on the correlation between the initial reduction rate and the internal structure of the resultant seeds. The kinetic approaches for controlling both nucleation and growth in a one-pot setting are then introduced with an emphasis on manipulation of the reduction pathways taken by the precursor. We then illustrate how to extend the strategy into a bimetallic system for the preparation of nanocrystals with different shapes and elemental distributions. Finally, the influence of speciation of the precursor on reduction kinetics is highlighted, followed by our perspectives on the challenges and future endeavors in achieving a controllable and predictable synthesis of noble-metal nanocrystals.

Structural complexity induced by {110} blocking of cysteine in electrochemical copper deposition on silver nanocubes

Morphology evolution into intricate structures at the nanoscale is hard to understand, but we can get critical information from the combination of ex situ and in situ spectroelectrochemical techniques. In this study, we investigated the structural complexity generated during electrochemical Cu deposition on individual Ag nanocubes, which was driven by surface regulating cysteine molecules. During the deposition process, selective nucleation occurred on the Ag nanocubes by underpotential deposition, and then sequential structural evolution to a windmill morphology was observed. By adjusting the cysteine coverage, diverse structures were yielded, including face-overgrown, four-leaf clover, and octapod-like structures. Structural analysis along the crystallographic directions demonstrated that cysteine molecules exclusively blocked the growth along 110 and relatively promoted the growth along 100 and 111, respectively. Interestingly, all morphologies maintained a highly symmetric nature from the pristine cube, despite being diverse and sophisticated. These findings would be essential to design complex morphologies and achieve desirable optical and catalytic properties.

One-pot synthesis of mesoporous palladium/C nanodendrites as high-performance oxygen reduction eletrocatalysts through a facile dual surface protecting agent-assisted strategy

Palladium (Pd) is regarded as a potential non-platinum electrocatalyst to drive oxygen reduction in fuel cells. The development of Pd-based electrocatalysts with high performances through structural engineering is still highly desirable. Herein, a facile one-pot synthesis strategy with the assistance of dual surface protecting agents was developed to fabricate carbon-supported Pd (Pd/C) nanodendrites with high mesoporosity. The mesoporous spherical Pd/C nanodendrites are built with connected nanoparticles with a small size of several nanometers and coated by simultaneously formed carbon layers. The used dual protecting agents, glycine and oleylamine, exhibit synergistic effects to engineer Pd growth to form the unique mesoporous dendritic structure. Benefiting from the mesoporous feature, small size, defect-rich surface and carbon coating, the obtained mesoporous Pd/C nanodendrites exhibit great electrocatalytic performance toward the oxygen reduction reaction (ORR).

Completely green synthesis of rose-shaped Au nanostructures and their catalytic applications

A novel protocol for the one-pot, template/seed-free, and completely green synthesis of rose-shaped Au nanostructures with unique three-dimensional hierarchical structures was developed.

Ultrasmall Pd and PtPd nanoparticles for highly efficient catalysis directed by predesigned Morchella-inspired encapsulation

Although bio-inspired designs for ultrasmall metal nanoparticles (NPs) are likely to play an important role in exploring future heterogeneous catalysis materials, synthesizing these structures while retaining surface activity and avoiding aggregation is challenging. Inspired by the Morchella with the spatially and well-organized porous structures, we proposed a biological strategy to yield NPs with ultrasmall and highly dispersed while maintaining high catalytic activity through surfactin self-assembly. Here, multifunctional Morchella-like biological pores (MBP) nanomaterials (~28 nm) with reduction and encapsulation has been synthesized by surfactin self-assembly, then, ultrasmall PtPd (~2.90 nm) and Pd NPs (~2.87 nm) with coordinated sizes and well-dispersed have been successfully reduced and encapsulated inside the MBP. Notably, the synthesis possesses distinct advantages such as mild reaction conditions, strong controllability, good biological compatibility, low-toxicity and environmental friendliness. The as-prepared MBP-encapsulated ultrasmall PtPd and Pd NPs (M@MBP NPs) exhibited excellent catalytic activity and toxicity resistance for the ethanol oxidation reaction (EOR) in KOH, due to the synergistic effect of MBP and ultrasmall metal NPs. The current density of PtPd@MBP and Pd@MBP NPs were 3.35 and 2.72 A mg^-1, respectively. Such MBP synthesized and encapsulated nanoparticles open a new frontier for the design and preparation of NPs for various applications, such as catalysis, bioremediation and drug delivery.

Nanocrystals of platinum-group metals as peroxidase mimics for in vitro diagnostics

Peroxidase mimics of nanoscale materials as alternatives to natural peroxidases have found widespread uses in biomedicine. Among various types of peroxidase mimics, platinum-group metal (PGM) nanocrystals have drawn considerable attention in recent years due to their superior properties. Particularly, PGM nanocrystals display high catalytic efficiencies, allow for facile surface modifications, and possess excellent stabilities. This feature article summarizes our recent work on development of PGM nanocrystals as peroxidase mimics and exploration of their applications in in vitro diagnostics. We begin with a brief introduction to controlled synthesis of PGM nanocrystals in solution phase. We then elaborate on a variety of physicochemical parameters that can be carefully tuned to optimize the peroxidase-like properties of PGM nanocrystals. Then, we highlight the applications of PGM nanocrystals in different in vitro diagnostic platforms. We conclude this article with personal perspectives on future research directions in this emerging field, where challenges and opportunities are remarked.

Synthesis of Citrate-Coated Penta-twinned Palladium Nanorods and Ultrathin Nanowires with a Tunable Aspect Ratio

Green and scalable methodologies for the preparation of metal nanoparticles with fine control of shape and size are of high interest in many areas including catalysis, nanomedicine, and nanodiagnostics. In this contribution, we describe a new synthetic method for the production of palladium (Pd) penta-twinned nanowires and nanorods utilizing sodium citrate, formic acid, ascorbic acid, and potassium bromide (KBr) in water, without the use of surfactants or polymers. The synthesis is green, fast, and without the need of complex setups. Interestingly, a microwave-assisted scale-up process has been developed. The combination of a synthetic protocol for seeds and the seed-mediated growth process allows us to synthesize nanorods and nanowires by modulating the concentration of KBr. The synthesized nanomaterials have been physicochemically characterized. High-resolution transmission electron microscopy shows that the nanorods and nanowires have a penta-twinned structure enclosed by {100} lateral facets. Moreover, the absence of sticky molecules or toxic byproducts guarantees the biocompatibility of the nanomaterials, while leaving the surface clean to perform enzymatic activities.

Rationally designed dual-plasmonic gold nanorod@cuprous selenide hybrid heterostructures by regioselective overgrowth for in vivo photothermal tumor ablation in the second near-infrared biowindow

NIR-II plasmonic materials offer multiple functionalities for in vivo biomedical applications, such as photothermal tumor ablation, surface-enhanced Raman scattering biosensing, photoacoustic imaging, and drug carriers. However, integration of noble metals and plasmonic semiconductors is greatly challenging because of the large lattice-mismatch. This study reports the regioselective overgrowth of Cu_2-xSe on gold nanorods (GNRs) for preparation of dual-plasmonic GNR@Cu_2-xSe hybrid heterostructures with tunable NIR-II plasmon resonance absorption for in vivo photothermal tumor ablation.

Methods: The regioselective deposition of amorphous Se and its subsequent conversion into Cu_2-xSe on the GNRs are performed by altering capping agents to produce the GNR@Cu_2-xSe heterostructures of various morphologies. Their photothermal performances for NIR-II photothermal tumor ablation are evaluated both in vitro and in vivo.

Results: We find that the lateral one- and two-side deposition, conformal core-shell coating and island growth of Cu_2-xSe on the GNRs can be achieved using different capping agents. The Cu_2-xSe domain size in these hybrids can be effectively adjusted by the SeO₂ concentration, thereby tuning the NIR-II plasmon bands. A photothermal conversion efficiency up to 58-85% and superior photostability of these dual-plasmonic hybrids can be achieved under the NIR-II laser. Results also show that the photothermal conversion efficiency is dependent on the proportion of optical absorption converted into heat; however, the temperature rise is tightly related to the concentration of their constituents. The excellent NIR-II photothermal effect is further verified in the following in vitro and in vivo experiments.

Conclusions: This study achieves one-side or two-side deposition, conformal core-shell coating, and island deposition of Cu_2-xSe on GNRs for GNR@Cu_2-xSe heterostructures with NIR-II plasmonic absorption, and further demonstrates their excellent NIR-II photothermal tumor ablation in vivo. This study provides a promising strategy for the rational design of NIR-II dual-plasmonic heterostructures and highlights their therapeutic in vivo potential.

Magnetic Fe3O4-Supported Gold Nanoflowers with Lattice-Selected Surfaces: Preparation and Catalytic Performance

Nanoflowers (NFs)-shape-controlled noble metal nanocrystals-have garnered significant attention because of their novel catalytic properties and applicability. In this paper, we report the preparation and catalytic performance of a magnetic Fe₃O₄-supported AuNF catalyst with a clean surface. The magnetically supported AuNFs were obtained by using magnetic Fe₃O₄ as the support. However, when nonmagnetic γ-Al₂O₃ was utilized as the support, the AuNFs did not exhibit a magnetic response. These supported AuNFs were utilized to catalyze the oxidation of 1-phenylethyl alcohol to acetophenone using air (1 atm) as the oxidant. The rate of formation of acetophenone using supported AuNFs was 8-fold higher than that of acetophenone using supported spherical Au nanoparticles of comparable size. In addition, the Fe₃O₄-supported AuNFs exhibited a higher rate of formation of acetophenone than the Al₂O₃-supported AuNFs. The Fe₃O₄-supported AuNFs were recovered using a magnet, and the recovered catalyst was reused under identical catalytic reaction conditions. The rate of formation of acetophenone using recovered Fe₃O₄-supported AuNFs remained unchanged, demonstrating no loss of catalytic activity.

Monovalent sulfur oxoanions enable millimeter-long single-crystalline h-WO3 nanowire synthesis

Here, we discuss a misunderstanding regarding chemical capping, which has intrinsically hindered the extension of the length of hexagonal (h)-WO₃ nanowires in previous studies. Although divalent sulfate ions (SO₄^2-) have been strongly believed to be efficient capping ions for directing anisotropic h-WO₃ nanowire growth, we have found that the presence of SO₄^2- is highly detrimental to the anisotropic crystal growth of the h-WO₃ nanowires, and a monovalent sulfur oxoanion (HSO₄^-) rather than SO₄^2- only substantially promotes the anisotropic h-WO₃ nanowire growth. Ab initio electronic structure simulations revealed that the monovalent sulfur oxoanions were preferentially able to cap the sidewall plane (100) of the h-WO₃ nanowires due to the lower hydration energy when compared with SO₄^2-. Based on this capping strategy, using the monovalent sulfur oxoanion (CH₃SO₃^-), which cannot generate divalent sulfur oxoanions, we have successfully fabricated ultra-long h-WO₃ nanowires up to the millimeter range (1.2 mm) for a wider range of precursor concentrations. We have demonstrated the feasibility of these millimeter-long h-WO₃ nanowires for the electrical sensing of molecules (lung cancer biomarker: nonanal) on flexible substrates, which can be operated at room temperature with mechanical flexibility with bending cycles up to 10⁴ times due to the enhanced textile effect.