Shape-transformable liquid metal nanoparticles in aqueous solution

UV plasmonic properties of colloidal liquid-metal eutectic gallium-indium alloy nanoparticles

Nanoparticles made of non-noble metals such as gallium have recently attracted significant attention due to promising applications in UV plasmonics. To date, experiments have mostly focused on solid and liquid pure gallium particles immobilized on solid substrates. However, for many applications, colloidal liquid-metal nanoparticle solutions are vital. Here, we experimentally demonstrate strong UV plasmonic resonances of eutectic gallium-indium (EGaIn) liquid-metal alloy nanoparticles suspended in ethanol. We rationalise experimental results through a theoretical model based on Mie theory. Our results contribute to the understanding of UV plasmon resonances in colloidal liquid-metal EGaIn nanoparticle suspensions. They will also enable further research into emerging applications of UV plasmonics in biomedical imaging, sensing, stretchable electronics, photoacoustics, and electrochemistry.

Inorganic Nanoshell-Stabilized Liquid Metal for Targeted Photonanomedicine in NIR-II Biowindow

Gallium and gallium-based alloys, typical types of liquid metals with unique physiochemical properties, are emerging as a next generation of functional materials in versatile biomedical applications. However, the exploration of their biomedical performance is currently insufficient, and their intrinsic low oxidative resistance is a key factor blocking their further clinical translation. Herein, we report on the surface engineering of liquid metal-based nanoplatforms by an inorganic silica nanoshell based on a novel but facile sonochemical synthesis for highly efficient, targeted, and near-infrared (NIR)-triggered photothermal tumor hyperthermia in the NIR-II biowindow. The inorganic silica-shell engineering of liquid metal significantly enhances the photothermal performance of the liquid metal core as reflected by enhanced NIR absorption, improved photothermal stability by oxidation protection, and abundant surface chemistry for surface-targeted engineering to achieve enhanced tumor accumulation. Systematic in vitro cell-level evaluation and in vivo tumor xenograft assessment demonstrate that (Arg-Gly-Asp) RGD-targeted and silica-coated nanoscale liquid metal substantially induces phototriggered cancer-cell death and photothermal tumor eradication, accompanied by high in vivo biocompatibility and easy excretion out of the body. This work provides the first paradigm for surface-inorganic engineering of liquid metal-based nanoplatforms for achieving multiple desirable therapeutic performances, especially for combating cancer.

Light-Induced Shape Morphing of Liquid Metal Nanodroplets Enabled by Polydopamine Coating

Shape morphing nanosystems have recently attracted much attention and a number of applications are developed, spanning from autonomous robotics to drug delivery. However, the fabrication of such nanosystems remains at an early stage owing to limited choices of strategies and materials. This work reports a facile method to fabricate liquid metal (LM) nanodroplets by sonication of bulk LM in an aqueous dopamine hydrochloride solution and their application in light-induced shape morphing at the nanoscale. In this method, dopamine acts as a surfactant, which stabilizes the LM nanodroplets dispersion during the sonication, and results in downsizing of the nanodroplets. Furthermore, by adding 2-amino-2-(hydroxymethyl)-1,3-propanediol to the suspension, self-polymerization of dopamine molecules occurs, resulting in the formation of polydopamine (PDA)-coated LM nanodroplets. Owing to the high photothermal conversion of the PDA, PDA-coated LM nanodroplets are transformed from spherical shapes to ellipsoids by NIR laser irradiation. This study paves a simple and reliable pathway for the preparation of functional LM nanodroplets and their application as shape-morphing nanosystems.

Shape tunable gallium nanorods mediated tumor enhanced ablation through near-infrared photothermal therapy

To date, photothermal sensitizers include organic and inorganic nanomaterials for biomedical applications. However, the impediments of low biodegradability and potential toxicity hinder their further applications in clinics. Liquid metal nanospheres show superior photothermal effects under near-infrared laser irradiation, in addition, a transformation in shape can be triggered, which also promotes biodegradability that helps to avoid potential systemic toxicity. Here, we fabricated tunable liquid metal nanoparticles having sphere-shaped to rod-shaped characteristics, resulting in good biocompatibility, favorable photothermal conversion efficiency, and targeting capability to tumors. The synthesis strategy is easy to achieve through one-step sonication. We systematically evaluated the photothermal properties of these liquid metal nanoparticles as well as their destructive effects on tumors in a quantitative way both in vitro and in vivo under laser exposure. Results have shown for the first time in mice that gallium nanorods, regulated and controlled through the production of GaO(OH), displayed outstanding photothermal conversion efficiency and exhibited distinct temperature elevation compared to gallium nanospheres and gallium-indium alloy nanorods. These shape transformable and biocompatible gallium nanorods establish the basis for the future laser ablation of tumors to achieve enhanced therapeutic outcomes. This shape tunability of a smart nano-liquid metal directly contributes to enhanced photothermal therapy in mice and opens new opportunities for potential applications with tumor therapy and imaging.

Sonication-enabled rapid production of stable liquid metal nanoparticles grafted with poly(1-octadecene-alt-maleic anhydride) in aqueous solutions

Gallium-based liquid metals are attractive due to their unique combination of metallic and fluidic properties. Liquid metal nanoparticles (LM NPs), produced readily using sonication, find use in soft electronics, drug delivery, and other applications. However, LM NPs in aqueous solutions tend to oxidize and precipitate over time, which hinders their utility in systems that require long-term stability. Here, we introduce a facile route to rapidly produce an aqueous suspension of stable LM NPs within five minutes. We accomplish this by dissolving poly(1-octadecene-alt-maleic anhydride) (POMA) in toluene and mixing with deionized water in the presence of a liquid metal (LM). Sonicating the mixture results in the formation of toluene-POMA emulsions that embed the LM NPs; as the toluene evaporates, POMA coats the particles. Due to the POMA hydrophobic coating, the LM NPs remain stable in biological buffers for at least 60 days without noticeable oxidation, as confirmed by dynamic light scattering and transmission electron microscopy. Further stabilization is achieved by tuning the LM composition. This paper elucidates the stabilization mechanisms. The stable LM NPs possess the potential to advance the use of LM in biomedical applications.

Tunable Noble Metal Thin Films on Ga Alloys via Galvanic Replacement

Room-temperature liquid metals such as GaInSn or EGaIn present the most attractive properties for soft and highly stretchable electronics. Recently, several methods have been investigated to functionalize the surface of the liquid metal via coatings and encapsulation. However, most can hardly be extended to other samples than droplets. In this study, we focus on the tunability of the process of galvanic replacement of Ga alloys with gold to form thin-film encapsulation. We characterized in-depth the obtainable composition and structure of a noble metal shell formed on the liquid metal via scanning electron microscopy, energy-dispersive X-ray, and topographic laser microscopy and highlighted the change in mechanism of galvanic replacement in different pH ranges. We showed the tunability of the surface morphology selection of different pH ranges, the solutions concentrations, and the reaction time. The adjustment of the pH of KAuBr₄ solution to the preferential Ga₂O₃-free domain led to the successful formation of a sub-micrometer thin uniform coating with more than 60% of Au and reduced level of oxygen from 30% down to 10%. We finally demonstrated the effect of the coating composition on the electrical properties of the liquid metal using a simple and fast phase-drop measurement setup on the droplet and microchannels. A high correlation between the amount of noble metal deposited and the electrical properties of the droplets was demonstrated.

Liquid metals: fundamentals and applications in chemistry

Post-transition elements, together with zinc-group metals and their alloys belong to an emerging class of materials with fascinating characteristics originating from their simultaneous metallic and liquid natures. These metals and alloys are characterised by having low melting points (i.e. between room temperature and 300 °C), making their liquid state accessible to practical applications in various fields of physical chemistry and synthesis. These materials can offer extraordinary capabilities in the synthesis of new materials, catalysis and can also enable novel applications including microfluidics, flexible electronics and drug delivery. However, surprisingly liquid metals have been somewhat neglected by the wider research community. In this review, we provide a comprehensive overview of the fundamentals underlying liquid metal research, including liquid metal synthesis, surface functionalisation and liquid metal enabled chemistry. Furthermore, we discuss phenomena that warrant further investigations in relevant fields and outline how liquid metals can contribute to exciting future applications.

Cytotoxicity of Gallium-Indium Liquid Metal in an Aqueous Environment

Eutectic gallium-indium alloy (EGaIn) liquid metal is highly conductive, moldable, and extremely deformable and has attracted significant attention for many applications, ranging from stretchable electronics to drug delivery. Even though EGaIn liquid metal is generally known to have low toxicity, the toxicity of the metal, rather than a salt form of Ga or In, has not been systematically studied yet. In this paper, we investigate the time-dependent concentration of the ions released from EGaIn liquid metal in an aqueous environment and their cytotoxicity to human cells. It is observed that only the Ga ion is dominantly released from EGaIn when no external agitation is applied, whereas the concentration of the In ion drastically increases with sonication. The cytotoxicity study reveals that all human cells tested are viable in the growth media with naturally released EGaIn ions, but the cytotoxicity becomes significant with sonication-induced EGaIn releasates. On the basis of the comparative study with other representative toxic elements, that is, Hg and Cd, it could be concluded that EGaIn is reasonably safe to use in an aqueous environment; however, it should be cautiously handled when any mechanical agitation is applied.

Microfluidic Mass Production of Stabilized and Stealthy Liquid Metal Nanoparticles

Functional nanoparticles comprised of liquid metals, such as eutectic gallium indium (EGaIn) and Galinstan, present exciting opportunities in the fields of flexible electronics, sensors, catalysts, and drug delivery systems. Methods used currently for producing liquid metal nanoparticles have significant disadvantages as they rely on both bulky and expensive high-power sonication probe systems, and also generally require the use of small molecules bearing thiol groups to stabilize the nanoparticles. Herein, an innovative microfluidics-enabled platform is described as an inexpensive, easily accessible method for the on-chip mass production of EGaIn nanoparticles with tunable size distributions in an aqueous medium. A novel nanoparticle-stabilization approach is reported using brushed polyethylene glycol chains with trithiocarbonate end-groups negating the requirements for thiol additives while imparting a "stealth" surface layer. Furthermore, a surface modification of the nanoparticles is demonstrated using galvanic replacement and conjugation with antibodies. It is envisioned that the demonstrated microfluidic technique can be used as an economic and versatile platform for the rapid production of liquid metal-based nanoparticles for a range of biomedical applications.

Advances in liquid metals for biomedical applications

This tutorial review summarizes the common performances, featured properties and various state-of-the-art biomedical applications of liquid metals.

Manipulation of Biomolecule-Modified Liquid-Metal Blobs

Soft and deformable liquid metals (LMs) are building components in various systems related to uncertain and dynamic task environments. Herein we describe the development of a biomolecule-triggered external-manipulation method involving LM conjugates for the construction of future innovative soft robotics operating in physiological environments. Functional soft hybrids composed of a liquid-metal droplet, a thiolated ligand, and proteins were synthesized for the expression of diverse macroscopic commands, such as attachment to cells, binary fusion, and self-propelled movement through molecular recognition and enzymatic reactions. Our technology could be used to create new state-of-the-art soft robots for chemical and biomedical engineering applications.

Analysis of the Efficiency of Surfactant-Mediated Stabilization Reactions of EGaIn Nanodroplets

A methodology based on light scattering and spectrophotometry was developed to evaluate the effect of organic surfactants on the size and yield of eutectic gallium/indium (EGaIn) nanodroplets formed in organic solvents by ultrasonication. The process was subsequently applied to systematically evaluate the role of headgroup chemistry as well as polar/apolar interactions of aliphatic surfactant systems on the efficiency of nanodroplet formation. Ethanol was found to be the most effective solvent medium in promoting the formation and stabilization of EGaIn nanodroplets. For the case of thiol-based surfactants in ethanol, the yield of nanodroplet formation increased with the number of carbon atoms in the aliphatic part. In the case of the most effective surfactant system-octadecanethiol-the nanodroplet yield increased by about 370% as compared to pristine ethanol. The rather low overall efficiency of the reaction process along with the incompatibility of surfactant-stabilized EGaIn nanodroplets in nonpolar organic solvents suggests that the stabilization mechanism differs from the established self-assembled monolayer formation process that has been widely observed in nanoparticle formation.