Modulating the Reactivity of Liquid Ga Nanoparticle Inks by Modifying Their Surface Chemistry

Tailoring Heterostructure Growth on Liquid Metal Nanodroplets through Interface Engineering

Liquid metal (LM) nanodroplets possess intriguing surface properties, thus offering promising potential in chemical synthesis, catalysis, and biomedicine. However, the reaction kinetics and product growth at the surface of LM nanodroplets are significantly influenced by the interface involved, which has not been thoroughly explored and understood. Here, we propose an interface engineering strategy, taking a spontaneous galvanic reaction between Ga0 and AuCl4- ions as a representative example, to successfully modulate the growth of heterostructures on the surface of Ga-based LM nanodroplets by establishing a dielectric interface with a controllable thickness between LM and reactive surroundings. Combining high-resolution electron energy-loss spectroscopy (EELS) analysis and theoretical simulation, it was found that the induced charge distribution at the interface dominates the spatiotemporal distribution of the reaction sites. Employing tungsten oxide (WOx) with varying thicknesses as the demonstrated dielectric interface of LM, Ga@WOx@Au with distinct core-shell-satellite or dimer-like heterostructures has been achieved and exhibited different photoresponsive capabilities for photodetection. Understanding the kinetics of product growth and the regulatory strategy of the dielectric interface provides an experimental approach to controlling the structure and properties of products in LM nanodroplet-involved chemical processes.

Liquid Metal Encased in Biomimic Polydopamine Armor to Reinforce Photothermal Conversion and Photothermal Stability

Liquid metal (LM) faces numerous obstacles like spontaneous coalescence, prone oxidizability, and deterioration in photothermal conversion, impeding the potential application as photothermal agent. To tackle these issues, several studies have focused on surface engineering strategy. Developing a feasible and efficient surface engineering strategy is crucial to prevent the aggregation and coalescence of LM, while also ensuring exceptional photothermal conversion and biosecurity. In order to achieve these goals in this work, the biomimetic polydopamine (PDA) armor was chosen to encase a typical LM (eutectic gallium-indium-tin alloy) via self-polymerization. Characterization results showed that the PDA encased LM nanoparticle exhibited enhanced photothermal stability, photothermal conversion, and biosecurity, which could be derived from the following factors: (1) The PDA protective shell acted as an "armor", isolating LM from dissolved oxygen and water, inhibiting heating-accelerated oxidation and shape morphing. (2) The exceptional near-infrared absorption of PDA was conducive to the photothermal conversion. (3) The biomimetic characteristic of polydopamine (PDA) was advantageous for improving the biosecurity. Hence, this work presented a new surface engineering strategy to reinforce LM for photothermal conversion application.

Functional micro-/nanostructured gallium-based liquid metal for biochemical sensing and imaging applications

In recent years, liquid metals (LMs) have garnered increasing attention for their expanded applicability, and wide application potential in various research fields. Among them, gallium (Ga)-based LMs exhibit remarkable analytical performance in electrical and optical sensors, thanks to their excellent conductivity, large surface area, biocompatibility, small bandgap, and high elasticity. This review comprehensively summarizes the latest advancements in functional micro-/nanostructured Ga-based LMs for biochemical sensing and imaging applications. Firstly, the electrical, optical, and biocompatible features of Ga-based LM micro-/nanoparticles are briefly discussed, along with the manufacturing and functionalization processes. Subsequently, we demonstrate the utilization of Ga-based LMs in biochemical sensing techniques, encompassing electrochemistry, electrochemiluminescence, optical sensing techniques, and various biomedical imaging. Lastly, we present an insightful perspective on promising research directions and remaining challenges in LM-based biochemical sensing and imaging applications.

Voltage-Driven Chemical Reactions Enable the Synthesis of Tunable Liquid Ga-Metal Nanoparticles

Nanosized particles of liquid metals are emerging materials that hold promise for applications spanning from microelectronics to catalysis. Yet, knowledge of their chemical reactivity is largely unknown. Here, we study the reactivity of liquid Ga and Cu nanoparticles under the application of a cathodic voltage. We discover that the applied voltage and the spatial proximity of these two particle precursors dictate the reaction outcome. In particular, we find that a gradual voltage ramp is crucial to reduce the native oxide skin of gallium and enable reactive wetting between the Ga and Cu nanoparticles; instead, a voltage step causes dewetting between the two. We determine that the use of liquid Ga/Cu nanodimer precursors, which consist of an oxide-covered Ga domain interfaced with a metallic Cu domain, provides a more uniform mixing and results in more homogeneous reaction products compared to a physical mixture of Ga and Cu NPs. Having learned this, we obtain CuGa2 alloys or solid@liquid CuGa2@Ga core@shell nanoparticles by tuning the stoichiometry of Ga and Cu in the nanodimer precursors. These products reveal an interesting complementarity of thermal and voltage-driven syntheses to expand the compositional range of bimetallic NPs. Finally, we extend the voltage-driven synthesis to the combination of Ga with other elements (Ag, Sn, Co, and W). By rationalizing the impact of the native skin reduction rate, the wetting properties, and the chemical reactivity between Ga and other metals on the results of such voltage-driven chemical manipulation, we define the criteria to predict the outcome of this reaction and set the ground for future studies targeting various applications for multielement nanomaterials based on liquid Ga.

Ga-Pt supported catalytically active liquid metal solutions (SCALMS) prepared by ultrasonication - influence of synthesis conditions on n-heptane dehydrogenation performance

Supported catalytically active liquid metal solution (SCALMS) materials represent a recently developed class of heterogeneous catalysts, where the catalytic reaction takes place at the highly dynamic interface of supported liquid alloys. Ga nuggets were dispersed into nano-droplets in propan-2-ol using ultrasonication followed by the addition of Pt in a galvanic displacement reaction - either directly into the Ga/propan-2-ol dispersion (in situ) or consecutively onto the supported Ga droplets (ex situ). The in situ galvanic displacement reaction between Ga and Pt was studied in three different reaction media, namely propan-2-ol, water, and 20 vol% water containing propan-2-ol. TEM investigations reveal that the Ga-Pt reaction in propan-2-ol resulted in the formation of Pt aggregates on top of Ga nano-droplets. In the water/propan-2-ol mixture, the desired incorporation of Pt into the Ga matrix was achieved. The ex situ prepared Ga-Pt SCALMS were tested in n-heptane dehydrogenation. Ga-Pt SCALMS synthesized in pure alcoholic solution showed equal dehydrogenation and cracking activity. Ga-Pt SCALMS prepared in pure water, in contrast, showed mainly cracking activity due to oxidation of Ga droplets. The Ga-Pt SCALMS material prepared in water/propan-2-ol resulted in high activity, n-heptene selectivity of 63%, and only low cracking tendency. This can be attributed to the supported liquid Ga-Pt alloy where Pt atoms are present in the liquid Ga matrix at the highly dynamic catalytic interface.

Shaping a Soft Future: Patterning Liquid Metals

This review highlights the unique techniques for patterning liquid metals containing gallium (e.g., eutectic gallium indium, EGaIn). These techniques are enabled by two unique attributes of these liquids relative to solid metals: 1) The fluidity of the metal allows it to be injected, sprayed, and generally dispensed. 2) The solid native oxide shell allows the metal to adhere to surfaces and be shaped in ways that would normally be prohibited due to surface tension. The ability to shape liquid metals into non-spherical structures such as wires, antennas, and electrodes can enable fluidic metallic conductors for stretchable electronics, soft robotics, e-skins, and wearables. The key properties of these metals with a focus on methods to pattern liquid metals into soft or stretchable devices are summari.

Plate-Like Colloidal Metal Nanoparticles

The pseudo-two-dimensional (2D) morphology of plate-like metal nanoparticles makes them one of the most anisotropic, mechanistically understood, and tunable structures available. Although well-known for their superior plasmonic properties, recent progress in the 2D growth of various other materials has led to an increasingly diverse family of plate-like metal nanoparticles, giving rise to numerous appealing properties and applications. In this review, we summarize recent progress on the solution-phase growth of colloidal plate-like metal nanoparticles, including plasmonic and other metals, with an emphasis on mechanistic insights for different synthetic strategies, the crystallographic habits of different metals, and the use of nanoplates as scaffolds for the synthesis of other derivative structures. We additionally highlight representative self-assembly techniques and provide a brief overview on the attractive properties and unique versatility benefiting from the 2D morphology. Finally, we share our opinions on the existing challenges and future perspectives for plate-like metal nanomaterials.

Alloying as a Strategy to Boost the Stability of Copper Nanocatalysts during the Electrochemical CO2 Reduction Reaction

Copper nanocatalysts are among the most promising candidates to drive the electrochemical CO₂ reduction reaction (CO₂RR). However, the stability of such catalysts during operation is sub-optimal, and improving this aspect of catalyst behavior remains a challenge. Here, we synthesize well-defined and tunable CuGa nanoparticles (NPs) and demonstrate that alloying Cu with Ga considerably improves the stability of the nanocatalysts. In particular, we discover that CuGa NPs containing 17 at. % Ga preserve most of their CO₂RR activity for at least 20 h while Cu NPs of the same size reconstruct and lose their CO₂RR activity within 2 h. Various characterization techniques, including X-ray photoelectron spectroscopy and operando X-ray absorption spectroscopy, suggest that the addition of Ga suppresses Cu oxidation at open-circuit potential (ocp) and induces significant electronic interactions between Ga and Cu. Thus, we explain the observed stabilization of the Cu by Ga as a result of the higher oxophilicity and lower electronegativity of Ga, which reduce the propensity of Cu to oxidize at ocp and enhance the bond strength in the alloyed nanocatalysts. In addition to addressing one of the major challenges in CO₂RR, this study proposes a strategy to generate NPs that are stable under a reducing reaction environment.

Surface Modification of Liquid Metal with p-Aniline Derivatives toward Bioapplications: Biosensing as an Example

It is a long-lasting research topic to avoid the formation of oxidation layers on gallium-based liquid metals. This study has developed a simple general method for modification of the eutectic gallium-indium (EGaIn) surface with p-aniline derivatives to introduce a monolayer of organic molecules with versatile functional groups. The binding affinity of carboxylic acid groups, amine groups, or thiol groups with EGaIn is in the order SH > NH₂ > COOH. For the first time, it is evidenced that both NH₂ and SH groups can coexist on the EGaIn nanoparticle surface with the binding affinities of 30 and 70%, respectively. The formation of these organic molecules on the EGaIn surface antioxidizes and thus stabilizes the EGaIn nanoparticles, while increasing the conductivity of EGaIn significantly. The resulting EGaIn nanoparticles have very good distribution in both ethanol and aqueous solutions and rich surface chemistry, making them suitable for the following attachment of biomolecules such as aptamers, antibodies, or enzymes for biomedical applications. As an example, the EGaIn surface is successfully modified with p-aminobenzoic acid followed by the attachment of an insulin aptamer, which can be used for the electrochemical detection of insulin with the lowest detectable concentration limit of 1 pM. This study reveals the modification of EGaIn nanoparticles with p-aniline derivatives with versatile functional groups to antioxidize EGaIn in a biological environment, opening a door for gallium-based liquid metals toward biomedical applications.

Emerging Liquid Metal Biomaterials: From Design to Application

Liquid metals (LMs) as emerging biomaterials possess unique advantages including their favorable biosafety, high fluidity, and excellent electrical and thermal conductivities, thus providing a unique platform for a wide range of biomedical applications ranging from drug delivery, tumor therapy, and bioimaging to biosensors. The structural design and functionalization of LMs endow them with enhanced functions such as enhanced targeting ability and stimuli responsiveness, enabling them to achieve better and even multifunctional synergistic therapeutic effects. Herein, the advantages of LMs in biomedicine are presented. The design of LM-based biomaterials with different scales ranging from micro-/nanoscale to macroscale and various components is explored in-depth to promote the understanding of structure-property relationships, guiding their performance optimization and applications. Furthermore, the related advanced progress in the development of LM-based biomaterials in biomedicine is summarized. Current challenges and prospects of LMs in the biomedical field are also discussed.

The Native Oxide Skin of Liquid Metal Ga Nanoparticles Prevents Their Rapid Coalescence during Electrocatalysis

Liquid metals (LMs) have been used in electrochemistry since the 19th century, but it is only recently that they have emerged as electrocatalysts with unique properties, such as inherent resistance to coke poisoning, which derives from the dynamic nature of their surface. The use of LM nanoparticles (NPs) as electrocatalysts is highly desirable to enhance any surface-related phenomena. However, LM NPs are expected to rapidly coalesce, similarly to liquid drops, which makes their implementation in electrocatalysis hard to envision. Herein, we demonstrate that liquid Ga NPs (18 nm, 26 nm, 39 nm) drive the electrochemical CO₂ reduction reaction (CO₂RR) while remaining well-separated from each other. CO is generated with a maximum faradaic efficiency of around 30% at -0.7 V_RHE, which is similar to that of bulk Ga. The combination of electrochemical, microscopic, and spectroscopic techniques, including operando X-ray absorption, indicates that the native oxide skin of the Ga NPs is still present during CO₂RR and provides a barrier to coalescence during operation. This discovery provides an avenue for future development of Ga-based LM NPs as a new class of electrocatalysts.