Shape-transformable liquid metal nanoparticles in aqueous solution

Engineering Polymers via Understanding the Effect of Anchoring Groups for Highly Stable Liquid Metal Nanoparticles

Liquid metal nanoparticles (LMNPs) have recently attracted much attention as soft functional materials for various biorelated applications. Despite the fact that several reports demonstrate highly stable LMNPs in aqueous solutions or organic solvents, it is still challenging to stabilize LMNPs in biological media with complex ionic environments. LMNPs grafted with functional polymers (polymers/LMNPs) have been fabricated for maintaining their colloidal and chemical stability; however, to the best of our knowledge, no related work has been conducted to systematically investigate the effect of anchoring groups on the stability of LMNPs. Herein, various anchoring groups, including phosphonic acids, trithiolcarbonates, thiols, and carboxylic acids, are incorporated into brush polymers via reversible addition-fragmentation chain transfer (RAFT) polymerization to graft LMNPs. Both the colloidal and chemical stability of such polymer/LMNP systems are then investigated in various biological media. Moreover, the influence of multidentate ligands is also investigated by incorporating different numbers of carboxylic or phosphonic acid into the brush polymers. We discover that increasing the number of anchoring groups enhances the colloidal stability of LMNPs, while polymers bearing phosphonic acids provide the optimum chemical stability for LMNPs due to surface passivation. Thus, polymers bearing multidentate phosphonic acids are desirable to decorate LMNPs to meet complex environments for biological studies.

Self-Standing, Photothermal-Actuating, and Motion-Monitoring Janus Films One-Pot Synthesized by Green Carboxymethyl Glucomannan/Liquid Metal Nanoinks

Downsizing bulk liquid metals (LM) at the nanometer scale with biocompatibility and multifunction is a key process for electronic or medical applications. Here, we report a stable and green LM aqueous colloidal ink by wrapping eutectic gallium-indium alloys (EGaIn) with carboxymethyl glucomannan (CGM) derived from radiata pine chip, which is capable of being prepared into a free-standing, photothermal-actuating, and motion-monitoring Janus film. With the assistance of CGM, the bulk EGaIn was ultrasonicated into stable nanodroplets (∼500 nm) with a typical "core-shell" structure, in which the colloidal inks can be stored for more than 1 week under room temperature. The stable CGM/EGaIn inks can be patterned on different substrates to form coating layers or self-assembled into free-standing Janus films with high mechanical strength and modulus (∼94 MPa and ∼3.8 GPa) by density deposition. Such a Janus film with anisotropic thermal conductivity made it a potential photothermal actuator. In addition, the biocompatible film demonstrated both high conductivity and large resistance variation in response to strain change (gauge factor >500), allowing for human motion monitoring. This work provides a new prospect for the development of biocompatible and high-performance nano-LM materials.

Antibacterial Longevity of a Novel Gallium Liquid Metal/Hydroxyapatite Composite Coating Fabricated by Plasma Spray

Hydroxyapatite (HAp)-coated metallic implants are known for their excellent bioactivity and osteoconductivity. However, infections associated with the microstructure of the HAp coatings may lead to implant failures as well as increased morbidity and mortality. This work addresses the concerns about infections by developing novel composite coatings of HAp and gallium liquid metal (GaLM) using atmospheric plasma spray (APS) as the coating technique. Five weight percent Ga was mixed into a commercially supplied HAp powder using an orbital shaker; then, the HAp-Ga particle feedstock was coated onto Ti₆Al₄V substrates using the APS technique. The X-ray diffraction results indicated that Ga did not form any Ga-related phases in either the HAp-Ga powder or the respective coating. The GaLM filled the pores of the HAp coating presented both on the top surface and within the coating, especially at voids and cracks, to prevent failures of the coating at these locations. The wettability of the surface was changed from hydrophobic for the HAp coating to hydrophilic for the HAp-Ga composite coating. Finally, the HAp-Ga coating presented excellent antibacterial efficacies against both initial attachments and established biofilms generated from methicillin-resistant Staphylococcus aureus and Pseudomonas aeruginosa after 18 h and 7 days of incubation in comparison to the control HAp coating. This study shows that GaLM improves the antibacterial properties of HAp-based coatings without sacrificing the beneficial properties of conventional HAp coatings. Thus, the HAp-Ga APS coating is a viable candidate for antibacterial coatings.

Responsive Liquid Metal Droplets: From Bulk to Nano

Droplets exist widely in nature and play an extremely important role in a broad variety of industrial processes. Typical droplets, including water and oil droplets, have received extensive attention and research, however their single properties still cannot meet diverse needs. Fortunately, liquid metal droplets emerging in recent years possess outstanding properties, including large surface tension, excellent electrical and thermal conductivity, convenient chemical processing, easy transition between liquid and solid phase state, and large-scale deformability, etc. More interestingly, liquid metal droplets with unique features can respond to external factors, including the electronic field, magnetic field, acoustic field, chemical field, temperature, and light, exhibiting extraordinary intelligent response characteristics. Their development over the past decade has brought substantial breakthroughs and progress. To better promote the advancement of this field, the present article is devoted to systematically summarizing and analyzing the recent fundamental progress of responsive liquid metal droplets, not only involving droplet characteristics and preparation methods, but also focusing on their diverse response behaviors and mechanisms. On this basis, the challenges and prospects related to the following development of liquid metal droplets are also proposed. In the future, responsive liquid metal droplets with a rapid development trend are expected to play a key role in soft robots, biomedicine, smart matter, and a variety of other fields.

Liquid Metal Based Nano-Composites for Printable Stretchable Electronics

Liquid metal (LM) has attracted prominent attention for stretchable and elastic electronics applications due to its exceptional fluidity and conductivity at room temperature. Despite progress in this field, a great disparity remains between material fabrication and practical applications on account of the high surface tension and unavoidable oxidation of LM. Here, the composition and nanolization of liquid metal can be envisioned as effective solutions to the processibility-performance dilemma caused by high surface tension. This review aims to summarize the strategies for the fabrication, processing, and application of LM-based nano-composites. The intrinsic mechanism and superiority of the composition method will further extend the capabilities of printable ink. Recent applications of LM-based nano-composites in printing are also provided to guide the large-scale production of stretchable electronics.

Synthesis of Liquid Gallium@Reduced Graphene Oxide Core-Shell Nanoparticles with Enhanced Photoacoustic and Photothermal Performance

This report presents nanoparticles composed of a liquid gallium core with a reduced graphene oxide (RGO) shell (Ga@RGO) of tunable thickness. The particles are produced by a simple, one-pot nanoprobe sonication method. The high near-infrared absorption of RGO results in a photothermal energy conversion of light to heat of 42.4%. This efficient photothermal conversion, combined with the large intrinsic thermal expansion coefficient of liquid gallium, allows the particles to be used for photoacoustic imaging, that is, conversion of light into vibrations that are useful for imaging. The Ga@RGO results in fivefold and twofold enhancement in photoacoustic signals compared with bare gallium nanoparticles and gold nanorods (a commonly used photoacoustic contrast agent), respectively. A theoretical model further reveals the intrinsic factors that affect the photothermal and photoacoustic performance of Ga@RGO. These core-shell Ga@RGO nanoparticles not only can serve as photoacoustic imaging contrast agents but also pave a new way to rationally design liquid metal-based nanomaterials with specific multi-functionality for biomedical applications.

Controlled Transformation of Liquid Metal Microspheres in Aqueous Solution Triggered by Growth of GaOOH

Liquid metals (LMs) are playing an increasingly important role in the fields of flexible devices, electronics, and thermal management due to their low melting point and excellent thermal and electrical conductivity, and the transformation of LMs in deionized water has recently received much attention. In this paper, we investigate the transformation process of EGaIn microspheres in deionized water and propose a two-step process of microspherical transformation, whereby the microspheres are first deformed into a spindle shape and then into lamellar nanorods. It is also shown that the growth of GaOOH crystals drives the transformation. Based on this result, EGaIn microspheres with controllable transformation could be prepared, such as spindle or lamellar rod shapes, extending the application area of LMs.

Liquid metals: an ideal platform for the synthesis of two-dimensional materials

The surfaces of liquid metals can serve as a platform to synthesise two-dimensional materials. By exploiting the self-limiting Cabrera-Mott oxidation reaction that takes place at the surface of liquid metals exposed to ambient air, an ultrathin oxide layer can be synthesised and isolated. Several synthesis approaches based on this phenomenon have been developed in recent years, resulting in a diverse family of functional 2D materials that covers a significant fraction of the periodic table. These straightforward and inherently scalable techniques may enable the fabrication of novel devices and thus harbour significant application potential. This review provides a brief introduction to liquid metals and their alloys, followed by detailed guidance on each developed synthesis technique, post-growth processing methods, integration processes, as well as potential applications of the developed materials.

Applications of liquid metals in nanotechnology

Post-transition liquid metals (LMs) offer new opportunities for accessing exciting dynamics for nanomaterials. As entities with free electrons and ions as well as fluidity, LM-based nanomaterials are fundamentally different from their solid counterparts. The low melting points of most post-transition metals (less than 330 °C) allow for the formation of nanodroplets from bulk metal melts under mild mechanical and chemical conditions. At the nanoscale, these liquid state nanodroplets simultaneously offer high electrical and thermal conductivities, tunable reactivities and useful physicochemical properties. They also offer specific alloying and dealloying conditions for the formation of multi-elemental liquid based nanoalloys or the synthesis of engineered solid nanomaterials. To date, while only a few nanosized LM materials have been investigated, extraordinary properties have been observed for such systems. Multi-elemental nanoalloys have shown controllable homogeneous or heterogeneous core and surface compositions with interfacial ordering at the nanoscale. The interactions and synergies of nanosized LMs with polymeric, inorganic and bio-materials have also resulted in new compounds. This review highlights recent progress and future directions for the synthesis and applications of post-transition LMs and their alloys. The review presents the unique properties of these LM nanodroplets for developing functional materials for electronics, sensors, catalysts, energy systems, and nanomedicine and biomedical applications, as well as other functional systems engineered at the nanoscale.

Effect of Surrounding Solvents on Interfacial Behavior of Gallium-Based Liquid Metal Droplets

Gallium-based liquid metal (GaLM) alloys have been extensively used in applications ranging from electronics to drug delivery systems. To broaden the understanding and applications of GaLMs, this paper discusses the interfacial behavior of eutectic gallium-indium liquid metal (EGaIn) droplets in various solvents. No significant difference in contact angles of EGaIn is observed regardless of the solvent types. However, the presence or absence of a conical tip on EGaIn droplets after dispensing could indirectly support that the interfacial energy of EGaIn is relatively low in non-polar solvents. Furthermore, in the impact experiments, the EGaIn droplet bounces off in the polar solvents of water and dimethyl sulfoxide (DMSO), whereas it spreads and adheres to the substrate in the non-polar solvents of hexane and benzene. Based on the dimensionless We number, it can be stated that the different impact behavior depending on the solvent types is closely related to the interfacial energy of EGaIn in each solvent. Finally, the contact angles and shapes of EGaIn droplets in aqueous buffer solutions with different pH values (4, 7, and 10) are compared. In the pH 10 buffer solution, the EGaIn droplet forms a spherical shape without the conical tip, representing the high surface energy. This is associated with the dissolution of the “interfacial energy-reducing” surface layer on EGaIn, which is supported by the enhanced concentration of gallium ion released from EGaIn in the buffer solution.

Nanotechnology for Targeted Detection and Removal of Bacteria: Opportunities and Challenges

The emergence of nanotechnology has created unprecedented hopes for addressing several unmet industrial and clinical issues, including the growing threat so-termed "antibiotic resistance" in medicine. Over the last decade, nanotechnologies have demonstrated promising applications in the identification, discrimination, and removal of a wide range of pathogens. Here, recent insights into the field of bacterial nanotechnology are examined that can substantially improve the fundamental understanding of nanoparticle and bacteria interactions. A wide range of developed nanotechnology-based approaches for bacterial detection and removal together with biofilm eradication are summarized. The challenging effects of nanotechnologies on beneficial bacteria in the human body and environment and the mechanisms of bacterial resistance to nanotherapeutics are also reviewed.

Electrostatic Stabilization of Nano Liquid Metals in Doped Nonpolar Liquids

Liquid metals and alloys are attracting renewed attention owing to their potential for application in various advanced technologies. Eutectic gallium-indium (EGaIn) has been focused on in particular because of its integrated advantages of high conductivity, low melting point, and low toxicity. In this study, the colloidal behavior of nano-dispersed EGaIn in nonpolar oils is investigated. Although the nonpolar oil continuous phase is commonly considered to be free of electric charges, electrostatic repulsion appears to be crucial in the colloidal stabilization of the nano-dispersed EGaIn phases, the modulation of which is possible by doping the oil phases with different types of oil-soluble surfactants. The qualitative correlation between the observed colloidal stabilities and the "zero field" particle mobilities inferred from the field-dependent electrophoretic mobilities indicates that the electric charging of EGaIn particles in surfactant-doped nonpolar oils is a static phenomenon that is maintained in equilibrium, rather than a solely field-induced process. A systematic investigation of the charging properties of these unique biphasic particles, consisting of the liquid Ga-In bulk and the solid Ga₂ O₃ surface that formed spontaneously, reveals the complicated system-dependent nature of the charging mechanisms mediated by ionic and nonionic surfactants in nonpolar media.

Polydopamine Shell as a Ga3+ Reservoir for Triggering Gallium-Indium Phase Separation in Eutectic Gallium-Indium Nanoalloys

Low melting point eutectic systems, such as the eutectic gallium-indium (EGaIn) alloy, offer great potential in the domain of nanometallurgy; however, many of their interfacial behaviors remain to be explored. Here, a compositional change of EGaIn nanoalloys triggered by polydopamine (PDA) coating is demonstrated. Incorporating PDA on the surface of EGaIn nanoalloys renders core-shell nanostructures that accompany Ga-In phase separation within the nanoalloys. The PDA shell keeps depleting the Ga³⁺ from the EGaIn nanoalloys when the synthesis proceeds, leading to a Ga³⁺-coordinated PDA coating and a smaller nanoalloy. During this process, the eutectic nanoalloys turn into non-eutectic systems that ultimately result in the solidification of In when Ga is fully depleted. The reaction of Ga³⁺-coordinated PDA-coated nanoalloys with nitrogen dioxide gas is presented as an example for demonstrating the functionality of such hybrid composites. The concept of phase-separating systems, with polymeric reservoirs, may lead to tailored materials and can be explored on a variety of post-transition metals.

Liquid metal assisted sonocatalytic degradation of organic azo dyes to solid carbon particles

Room temperature liquid metals are an emerging class of materials for a variety of heterogeneous catalytic reactions. In this work we explore the use of Ga based liquid metals as a sonochemical catalyst for the degradation of organic azo dyes such as methyl orange, congo red and eriochrome black T. Rapid degradation to non toxic solid carbon particles was achieved over a large dye concentration range to produce differently sized particles via both bath and probe sonication which could be repeated multiple times with the same catalyst.

Single Particle Color Switching by Laser-Induced Deformation of Liquid Metal-derived Microcapsules

Active controlling of optical properties of metallic particles holds great promise for nonlinear nanophotonics and compact optoelectronic devices. Except for the electronic and chemical tuning of their properties, active control through fast and reversible shape modulation remains a significant challenge. Here, we report on the concept for changing the color and brightness of single particles by reversible/irreversible tuning of their shapes. As a family of plasmonic materials with low melting points and high flexibility, we synthesized liquid metal microparticles with different interior (dense/hollow) and morphology from Ga and its alloys (GaNi, GaCu). Utilizing near-infrared femtosecond laser pulses, we achieve two regimes for reversible/irreversible optical tuning due to consequent weak/strong perturbation of the microcapsules (MC) shapes. The chemical composition and MCs morphology significantly affect the tuning of color and brightness, as well as the rigidity of the MCs to extreme laser conditions.

Photothermal photodynamic therapy and enhanced radiotherapy of targeting copolymer-coated liquid metal nanoparticles on liver cancer

The maximized therapeutic efficacy in tumor treatment can be achieved with combination therapy. Herein, a metronidazole (MN) and RGD peptides were linked with the copolymer chains of polyacrylic acid (PAA) and polyethylene glycol (PEG) by condensation and Michael addition reactions, respectively, named as RGD-PEG-PAA-MN. Subsequently, liquid-metal (LM) nanoparticles broken by ultrasonication were coated with modified copolymer, forming RGD-PEG-PAA-MN@LM nanoparticles. These nanoparticles with the degradation under an acidic condition could target to tumor cells, and LM of these composited nanoparticles could not only efficiently convert the photoenergy of near infrared (NIR) into thermal energy, but also produce more reactive oxygen species under NIR or X ray irradiation. Furthermore, MN in the composited nanoparticles could enhance their radiation sensitivity of tumor tissues with hypoxia condition. The synergic effect of these nanoparticles on cancer limitation after the sequential radiations of NIR and X ray was significantly higher than the single radiation. In the experiments of tumor bearing mice, the volume of the tumor in RGD-PEG-PAA-MN@LM group at 14th day after two radiations of NIR and X-ray were significantly smaller than LM group, and the tumor of RGD-PEG-PAA-MN@LM group at 14th day after two radiations almost disappeared, suggesting better synergistic effect of RGD-PEG-PAA-MN@LM nanoparticles on photothermal conversion, photodynamics under two irradiations and their enhanced sensitization of X-ray radiation. Our results indicated that the prepared nanoparticles would be applied in the combinational therapy of liver tumor by the photothermal, photodynamic and sensitized radiation.

Flexible and Stretchable Liquid Metal Electrodes Working at Sub-Zero Temperature and Their Applications

We investigated characteristics of highly flexible and stretchable electrodes consisting of Galinstan (i.e., a gallium-based liquid metal alloy) under various conditions including sub-zero temperature (i.e., <0 °C) and demonstrated solar-blind photodetection via the spontaneous oxidation of Galinstan. For this work, a simple and rapid method was introduced to fabricate the Galinstan electrodes with precise patterns and to exfoliate their surface oxide layers. Thin conductive films possessing flexibility and stretchability can be easily prepared on flexible substrates with large areas through compression of a dried suspension of Galinstan microdroplets. Furthermore, a laser marking machine was employed to facilitate patterning of the Galinstan films at a high resolution of 20 μm. The patterned Galinstan films were used as flexible and stretchable electrodes. The electrical conductivity of these electrodes was measured to be ~1.3 × 106 S m-1, which were still electrically conductive even if the stretching ratio increased up to 130% below 0 °C. In addition, the surface oxide (i.e., Ga2O3) layers possessing photo-responsive properties were spontaneously formed on the Galinstan surfaces under ambient conditions, which could be solely exfoliated using elastomeric stamps. By combining Galinstan and its surface oxide layers, solar-blind photodetectors were successfully fabricated on flexible substrates, exhibiting a distinct increase of up to 14.7% in output current under deep ultraviolet irradiation (254 nm wavelength) with an extremely low light intensity of 0.1 mW cm-2, whereas no significant change was observed under visible light irradiation.

Transformable Spinose Nanodrums with Self-Supplied H2 O2 for Photothermal and Cascade Catalytic Therapy of Tumor

Advances in enzymes involve an efficient biocatalytic process, which has demonstrated great potential in biomedical applications. However, designing a functional carrier for enzymes equipped with satisfactory degradability and loading efficiency, remains a challenge. Here, based on transformable liquid metal (LM), a spinose nanodrum is designed as protein carrier to deliver enzyme for tumor treatment. With the assistance of spines and a special drum-like shape, it is found that the spiny LM can carry much more enzymes than spherical LM under the same condition. Benefiting from the satisfactory enzyme loading efficiency of spiny LM, a plasma amine oxidase immobilized spinose LM nanosystem enveloped with epigallocatechin gallate (EGCG)-Fe³⁺ (LMPE) is fabricated for photothermal and cascade catalytic tumor therapy. Activated by the acidic condition in the tumor microenvironment, the LMPE can oxidize spermine (Spm) and spermidine (Spd) to generate hydrogen peroxide (H₂ O₂ ) for Fenton catalytic reaction to produce the lethal hydroxyl radical (•OH) for tumor cell killing. Combined with remarkable photothermal performance of LM, LMPE exhibits significant inhibition of tumor in vivo.

An exploration into two-dimensional metal oxides, and other 2D materials, synthesised via liquid metal printing and transfer techniques

Two-dimensional (2D) metal oxides can be difficult to synthesise, and scaling up production using traditional methods is challenging. However, a new liquid metal-based technique, that utilises both "top-down" and "bottom-up" processes, has recently been introduced. These liquids oxidise to form an oxide surface "skin" which may be exfoliated as a 2D flake and subsequently used in various electronic devices and chemical reactions.

Production of Uniformly Sized Gallium-Based Liquid Alloy Nanodroplets via Ultrasonic Method and Their Li-Ion Storage

Gallium-based liquid alloys are attractive due to their unique properties, and they can potentially be applied in the field of flexible electronics as coolant materials for nuclear and liquid batteries, due to the high thermal conductivity and excellent fluid properties of liquid metals. However, it is still challenging to fabricate gallium-based liquid alloy nanodroplets with uniform and small size. Here, we performed a systematical study on the influence of various factors affecting the size of nanodroplets. Liquid metal nanodroplets with an average size of 74 nm and narrow size distribution were successfully fabricated. Li-ion half-cells were assembled with eutectic GaIn (eGaIn) nanodroplets as anode active materials, which showed higher specific capacity than the bulk eGaIn alloy under the same testing conditions.

Mini/Micro/Nano Scale Liquid Metal Motors

Swimming motors navigating in complex fluidic environments have received tremendous attention over the last decade. In particular, liquid metal (LM) as a new emerging material has shown considerable potential in furthering the development of swimming motors, due to their unique features such as fluidity, softness, reconfigurability, stimuli responsiveness, and good biocompatibility. LM motors can not only achieve directional motion but also deformation due to their liquid nature, thus providing new and unique capabilities to the field of swimming motors. This review aims to provide an overview of the recent advances of LM motors and compare the difference in LM macro and micromotors from fabrication, propulsion, and application. Here, LM motors below 1 cm, named mini/micro/nano scale liquid metal motors (MLMTs) will be discussed. This work will present physicochemical characteristics of LMs and summarize the state-of-the-art progress in MLMTs. Finally, future outlooks including both opportunities and challenges of mini/micro/nano scale liquid metal motors are also provided.

Galvanic replacement reaction for in situ fabrication of litchi-shaped heterogeneous liquid metal-Au nano-composite for radio-photothermal cancer therapy

With tremendous research advances in biomedical application, liquid metals (LM) also offer fantastic chemistry for synthesis of novel nano-composites. Herein, as a pioneering trial, litchi-shaped heterogeneous eutectic gallium indium-Au nanoparticles (EGaIn-Au NPs), served as effective radiosensitizer and photothermal agent for radio-photothermal cancer therapy, have been successfully prepared using in situ interfacial galvanic replacement reaction. The enhanced photothermal conversion efficiency and boosted radio-sensitization effect could be achieved with the reduction of Au nanodots onto the eutectic gallium indium (EGaIn) NPs surface. Most importantly, the growth of tumor could be effectively inhibited under the combined radio-photothermal therapy mediated by EGaIn-Au NPs. Inspired by this approach, in situ interfacial galvanic replacement reaction may open a novel strategy to fabricate LM-based nano-composite with advanced multi-functionalities.

Surface modification of liquid metal as an effective approach for deformable electronics and energy devices

The fields of flexible or stretchable electronics and energy devices, reconfigurable and compliant soft robotics, wearable e-textiles or health-care devices have paid significant attention to the need of deformable conductive electrodes due to its critical role in device performances. Gallium-based liquid metals, such as the eutectic gallium-indium (EGaIn) being an electrically conductive liquid phase at room temperature, have attracted immense interests as a promising candidate for deformable conductor. However, in the case of bulk liquid metal, there are several limitations such as the need of encapsulation, dispersion in a polymer matrix, or accurate patterning. For these reasons, the preparation of liquid metal particles in harnessing the properties in a non-bulk form and surface modification is crucial for the success of incorporating liquid metal into functional devices. Herein, we discuss the current progress in chemical surface modification and interfacial manipulations of liquid metal particles. The physical and chemical properties of the surface modification-assisted liquid metal polymer composite are also reviewed. Lastly, the applications of the surface-modified liquid metal particles such as flexible electrode, soft robotics, energy storage or harvester, thermal conductor, dielectric sensor, and bioelectronics are discussed, and the corresponding perspectives of deformable electronics and energy devices are provided. In particular, we focus on the functionalization method or requirement of liquid metal particles in each application. The challenging issues and outlook on the applications of surface-modified liquid metal particles are also discussed.

Aerosol Spray Deposition of Liquid Metal and Elastomer Coatings for Rapid Processing of Stretchable Electronics

We report a spray deposition technique for patterning liquid metal alloys to form stretchable conductors, which can then be encapsulated in silicone elastomers via the same spraying procedure. While spraying has been used previously to deposit many materials, including liquid metals, this work focuses on quantifying the spraying process and combining it with silicones. Spraying generates liquid metal microparticles (~5 μm diameter) that pass through openings in a stencil to produce traces with high resolution (~300 µm resolution using stencils from a craft cutter) on a substrate. The spraying produces sufficient kinetic energy (~14 m/s) to distort the particles on impact, which allows them to merge together. This merging process depends on both particle size and velocity. Particles of similar size do not merge when cast as a film. Likewise, smaller particles (<1 µm) moving at the same speed do not rupture on impact either, though calculations suggest that such particles could rupture at higher velocities. The liquid metal features can be encased by spraying uncured silicone elastomer from a volatile solvent to form a conformal coating that does not disrupt the liquid metal features during spraying. Alternating layers of liquid metal and elastomer may be patterned sequentially to build multilayer devices, such as soft and stretchable sensors.

A universal signature in the melting of metallic nanoparticles

Predicting when phase changes occur in nanoparticles is fundamental for designing the next generation of devices suitable for catalysis, biomedicine, optics, chemical sensing and electronic circuits. The estimate of the temperature at which metallic nanoparticles become liquid is, however, a challenge and a standard definition is still missing. We discover a universal feature in the distribution of the atomic-pair distances that distinguishes the melting transition of monometallic nanoparticles. We analyse the solid-liquid change of several late-transition metals nanoparticles, i.e. Ni, Cu, Pd, Ag, Au and Pt, through classical molecular dynamics. We consider various initial shapes from 146 to 976 atoms, corresponding to the 1.5-4.1 nm size range, placing the nanoparticles in either a vacuum or embedded in a homogeneous environment, simulated by an implicit force-field. Regardless of the material, its initial shape, size and environment, the second peak in the pair-distance distribution function, expected at the bulk lattice distance, disappears when the nanoparticle melts. As the pair-distance distribution is a measurable quantity, the proposed criterion holds for both numerical and experimental investigations. For a more straightforward calculus of the melting temperature, we demonstrate that the cross-entropy between a reference solid pair-distance distribution function and the one of nanoparticles at increasing temperatures present a quasi-first order transition at the phase-change temperature.

Synthesis and application of core-shell liquid metal particles: a perspective of surface engineering

Liquid metal micro/nano particles (LMPs) from gallium and its alloys have attracted tremendous attention in the last decade due to the unique combination of their metallic and fluidic properties at relatively low temperatures. Unfortunately, there is limited success so far in realizing the highly controllable fabrication and functionalization of this emerging material, posing great obstacles to further promoting its fundamental and applied studies. This review aims to explore solutions for the on-demand design and manipulation of LMPs through physicochemically engineering their surface microenvironment, including compositions, structures, and properties, which are featured by the encapsulation of LMPs inside a variety of synthetic shell architectures. These heterophase, core-shell liquid metal composites display adjustable size and structure-property relationships, rendering improved performances in several attractive scenarios including but not limited to soft electronics, nano/biomedicine, catalysis, and energy storage/conversion. Challenges and opportunities regarding this burgeoning field are also disclosed at the end of this review.

Biomedical Applications of Liquid Metal Nanoparticles: A Critical Review

This review is focused on the basic properties, production, functionalization, cytotoxicity, and biomedical applications of liquid metal nanoparticles (LMNPs), with a focus on particles of the size ranging from tens to hundreds of nanometers. Applications, including cancer therapy, medical imaging, and pathogen treatment are discussed. LMNPs share similar properties to other metals, such as photothermal conversion ability and a propensity to form surface oxides. Compared to many other metals, especially mercury, the cytotoxicity of gallium is low and is considered by many reports to be safe when applied in vivo. Recent advances in exploring different grafting molecules are reported herein, as surface functionalization is essential to enhance photothermal therapeutic effects of LMNPs or to facilitate drug delivery. This review also outlines properties of LMNPs that can be exploited in making medical imaging contrast agents, ion channel regulators, and anti-pathogenic agents. Finally, a foresight is offered, exemplifying underexplored knowledge and highlighting the research challenges faced by LMNP science and technology in expanding into applications potentially yielding clinical advances.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Gallium-Based Liquid Metal Particles for Therapeutics

Gallium (Ga) and Ga-based liquid metal (LM) alloys offer low toxicity, excellent electrical and thermal conductivities, and fluidity at or near room temperature. Ga-based LM particles (LMPs) synthesized from these LMs exhibit both fluidic and metallic properties and are suitable for versatile functionalization in therapeutics. Functionalized Ga-based LMPs can be actuated using physical or chemical stimuli for drug delivery, cancer treatment, bioimaging, and biosensing. However, many of the fundamentals of their unique characteristics for therapeutics remain underexplored. We present the most recent advances in Ga-based LMPs in therapeutics based on the underlying mechanisms of their design and implementation. We also highlight some future biotechnological opportunities for Ga-based LMPs based on their extraordinary advantages.

Broad-spectrum treatment of bacterial biofilms using magneto-responsive liquid metal particles

The formation and proliferation of bacterial biofilms on surfaces, particularly those on biomedical devices, is a significant issue that results in substantial economic losses, presenting severe health risks to patients. Furthermore, heterogeneous biofilms consisting of different bacterial species can induce the increase in pathogenicity, and the resistance to antimicrobial agents due to the synergistic interactions between the different species. Heterogeneous bacterial biofilms are notoriously difficult to treat due to the presence of extracellular polymeric substances (EPS) and, in conjunction with the rapid rise of multi-drug resistant pathogens, this means that new solutions for anti-biofilm treatment are required. In this study, we investigate the application of magneto-responsive gallium-based liquid metal (GLM-Fe) nanomaterials against a broad range of Gram-positive and Gram-negative bacterial mono-species and multi-species biofilms. The GLM-Fe particles exhibit a magneto-responsive characteristic, causing spherical particles to undergo a shape transformation to high-aspect-ratio nanoparticles with sharp asperities in the presence of a rotating magnetic field. These shape-transformed particles are capable of physically removing bacterial biofilms and rupturing individual cells. Following treatment, both mono-species and multi-species biofilms demonstrated significant reductions in their biomass and overall cell viability, demonstrating the broad-spectrum application of this antibacterial technology. Furthermore, the loss of integrity of the bacterial cell wall and membranes was visualized using a range of microscopy techniques, and the leakage of intracellular components (such as nucleic acids and protein) was observed. Insights gained from this study will impact the design of future liquid metal-based biofilm treatments, particularly those that rely on magneto-responsive properties.

Oxidation of Gallium-based Liquid Metal Alloys by Water

Gallium alloys with other low melting point metals, such as indium or tin, to form room-temperature liquid eutectic systems. The gallium in the alloys rapidly forms a thin surface oxide when exposed to ambient oxygen. This surface oxide has been previously exploited for self-stabilization of liquid metal nanoparticles, retention of metastable shapes, and imparting stimuli-responsive behavior to the alloy surface. In this work, we study the effect of water as an oxidant and its role in defining the alloy surface chemistry. We identify several pathways that can lead to the formation of gallium oxide hydroxide (GaOOH) crystallites, which may be undesirable in many applications. Furthermore, we find that some crystallite formation pathways can be reinforced by typical top-down particle synthesis techniques like sonication. This improved understanding of interfacial interactions provides critical insight for process design and implementation of advanced devices that utilize the unique coupling of flexibility and conductivity offered by these gallium-based liquid metal alloys.

Liquid Metal Initiator of Ring-Opening Polymerization: Self-Capsulation into Thermal/Photomoldable Powder for Multifunctional Composites

Liquid metal nanodroplets not only share similar metallic properties and nanoscale effect with solid metal nanoparticles, but also possess the additional uniqueness in nonvolatile fluidity and ambient sintering ability into continuous conductors. In most cases, liquid metal nanodroplets are encapsulated into ultrathin and fragile shells of oxides and amphiphile monolayers, and may be hindered from incorporating homogeneously into various composites through conventional processing methods. In this study, ring-opening polymerization is found to be initiated by sonicating the liquid metal EGaIn in fluidic lactones. By this in situ polymerization, EGaIn nanodroplets are encapsulated into polylactone shells with tunable thickness, which can further be dried into a solid powder. Besides high chemical stability and dispersibility in organic solvents, the powder of the EGaIn capsules combines the exceptional properties of the EGaIn droplets (e.g., photothermal effect) and the polylactone shells (e.g., biocompatibility, biodegradability, and compatibility with different polymer matrixes), being capable of being introduced into thermoplastic composites through liquid casting and thermal- or photomolding for the notch-insensitive tearing property, sintering-induced electric conductivity, and photothermal effect. Thus, the EGaIn initiator of ring-opening polymerization may start a pathway to produce stable andthermal/photomoldable powders of EGaIn capsules and their multifunctionalcomposites, applicable in biomedicines, soft electronics, and smart robots.

Low-Temperature Triggered Shape Transformation of Liquid Metal Microdroplets

Shape transformable materials that can respond to external environments have attracted widespread interest over the fields of soft robotics, flexible electronics, and tissue engineering. Among stimuli-responsive materials, liquid metals exhibit rather unique characteristics of versatile morphological changes upon diverse stimuli, including chemicals, electrical field, and mechanical force, etc. Herein, a superfast (few milliseconds), large-scaled (13.8% deformation increase), and fierce (cracks formation) transformation of liquid metal microdroplets (LMMs) with strong impulse expanded force due to liquid-solid phase transition in a dual fluid system composed of LMMs and aqueous solution is reported. When subject to low-temperature stimulus, LMM would transform from ellipsoidal shape to amorphous shape induced by thermal stress, driving the shape morphing. Furthermore, the phase changes of LMMs as well as the formation of surrounding ice crystals are proven to be responsible for this phenomenal behavior. The densification of ice crystals is demonstrated to play a significant role in the transformable behavior. In particular, these nonconductive LMMs in aqueous solutions are discovered to turn into conducive materials with an impedance change of about 10⁵ times. The present discovery is of fundamental and practical significance, and would open new venues in fields such as fluid mechanics, thermal science, flexible electronics, biomedicine, and so forth.

A Simple and Cost-Effective Method for Producing Stable Surfactant-Coated EGaIn Liquid Metal Nanodroplets

Liquid metals show unparalleled advantages in printable circuits, flexible wear, drug carriers, and electromagnetic shielding. However, the efficient and large-scale preparation of liquid metal nanodroplets (LM NDs) remains a significant challenge. Here, we propose a simple and efficient method for the large-scale preparation of stable eutectic gallium indium nanodroplets (EGaIn NDs). We compared different preparation methods and found that droplets with smaller particle sizes could quickly be produced using a shaking technique. The size of EGaIn NDs produced using this technique can reach 200 nm in 30 min and 100 nm in 240 min. Benefiting from the simple method, various surfactants can directly modify the surface of the EGaIn NDs to stabilize the prepared droplets. In addition, we discovered that shaking in an ice bath produced spherical nanodroplets, and after shaking for 30 min in a non-ice bath, rod-shaped gallium oxide hydroxide (GaOOH) appeared. Furthermore, the EGaIn NDs we produced have excellent stability-after storage at room temperature for 30 days, the particle size and morphology change little. The excellent stability of the produced EGaIn NDs provides a wider application of liquid metals in the fields of drug delivery, electromagnetic shielding, conductive inks, printed circuits, etc.

Laser-Generated Supranano Liquid Metal as Efficient Electron Mediator in Hybrid Perovskite Solar Cells

Creating colloids of liquid metal with tailored dimensions has been of technical significance in nano-electronics while a challenge remains for generating supranano (<10 nm) liquid metal to unravel the mystery of their unconventional functionalities. Present study pioneers the technology of pulsed laser irradiation in liquid from a solid target to liquid, and yields liquid ternary nano-alloys that are laborious to obtain via wet-chemistry synthesis. Herein, the significant role of the supranano liquid metal on mediating the electrons at the grain boundaries of perovskite films, which are of significance to influence the carriers recombination and hysteresis in perovskite solar cells, is revealed. Such embedding of supranano liquid metal in perovskite films leads to a cesium-based ternary perovskite solar cell with stabilized power output of 21.32% at maximum power point tracing. This study can pave a new way of synthesizing multinary supranano alloys for advanced optoelectronic applications.

Water-processable liquid metal nanoparticles by single-step polymer encapsulation

Stabilizing metastable colloids such as gallium-based liquid metal nanoparticles (LM NPs) and using them in complex machining processes is a great challenge due to their simple and sensitive surface chemistry against ligand modification and solution processing. Water, a green solvent, is unfortunately not favored by LM NPs in the entire workflow from synthesis to functionalization and application. This dilemma is relieved herein by presenting a sonochemical polymer deposition technique that can produce a water-dispersible LM@polymer hybrid nanomaterial in one step. Water-soluble polymers such as poly(vinyl pyrrolidone) (PVP) can be directly deposited onto eutectic gallium indium (EGaIn) as a coating shell (up to 20 nm) and prevent their rapid and continuous oxidation in aqueous solutions. EGaIn@PVP NPs are stable after preservation for 30 days in water and 60 days in ethanol, and the polymer coating shows solvent-responsive swelling/shrinking behaviors. The LM@polymer NPs can be stably composited with other materials through complex processing and perform desired functions after composition. For demonstration, EGaIn@PVP NPs are employed as active electrode materials for lithium-ion batteries by compositing with water-soluble binders, and exhibit excellent cycling performances with no obvious decay (capacity retention >95%) in more than 700 cycles at 4 A g-1. This strategy enriches the toolbox for surface engineering of LMs especially under harsh conditions, which can benefit the applications of LM NPs in various scenarios.

Attributes, Fabrication, and Applications of Gallium-Based Liquid Metal Particles

This work discusses the attributes, fabrication methods, and applications of gallium-based liquid metal particles. Gallium-based liquid metals combine metallic and fluidic properties at room temperature. Unlike mercury, which is toxic and has a finite vapor pressure, gallium possesses low toxicity and effectively zero vapor pressure at room temperature, which makes it amenable to many applications. A variety of fabrication methods produce liquid metal particles with variable sizes, ranging from nm to mm (which is the upper limit set by the capillary length). The liquid nature of gallium enables fabrication methods-such as microfluidics and sonication-that are not possible with solid materials. Gallium-based liquid metal particles possess several notable attributes, including a metal-metal oxide (liquid-solid) core-shell structure as well as the ability to self-heal, merge, and change shape. They also have unusual phase behavior that depends on the size of the particles. The particles have no known commercial applications, but they show promise for drug delivery, soft electronics, microfluidics, catalysis, batteries, energy harvesting, and composites. Existing challenges and future opportunities are discussed herein.

Surfaces and Interfaces of Liquid Metal Core-Shell Nanoparticles under the Microscope

Eutectic gallium indium (EGaIn), a Ga-based liquid metal alloy holds great promise for designing next generation core-shell nanoparticles (CSNs). A shearing assisted ligand-stabilization method has shown promise as a synthetic method for these CSNs; however, determining the role of the ligand on stabilization demands an understanding of the surface chemistry of the ligand-nanoparticle interface. EGaIn CSNs have been created functionalized with aliphatic carboxylates of different chain length allowing a fundamental investigation on ligand stabilization of EGaIn CSNs. Raman and diffuse reflectance Fourier transform spectroscopies (DRIFTS) confirm reaction of the ligand with the oxide shell of the EGaIn nanoparticles. Changing the length of the alkyl chain in the aliphatic carboxylates (C2-C18) may influence the size and structural stability of EGaIn CSNs, which is easily monitored using atomic force microscopy (AFM). No matter how large the carboxylate ligand, there is no obvious effect on the size of the EGaIn CSNs, except the particle size got more uniform when coated with longer chain carboxylates. The AFM force distance (F-D) measurements are used to measure the stiffness of the carboxylate coated EGaIn CSN. In corroboration with DRIFTS analysis, the stiffness studies show that the alkyl chains undergo conformational changes upon compression.

Ga-Based Liquid Metal Micro/Nanoparticles: Recent Advances and Applications

Liquid metals are emerging as fluidic inorganic materials in various research fields. Micro- and nanoparticles of Ga and its alloys have received particular attention in the last decade due to their non toxicity and accessibility in ambient conditions as well as their interesting chemical, physical, mechanical, and electrical properties. Unique features such as a fluidic nature and self-passivating oxide skin make Ga-based liquid metal particles (LMPs) distinguishable from conventional inorganic particles in the context of synthesis and applications. Here, recent advances in the bottom-up and top-down synthetic methods of Ga-based LMPs, their physicochemical properties, and their applications are summarized. Finally, the current status of the LMPs is highlighted and perspectives on future directions are also provided.

Antibacterial Liquid Metals: Biofilm Treatment via Magnetic Activation

Antibiotic resistance has made the treatment of biofilm-related infections challenging. As such, the quest for next-generation antimicrobial technologies must focus on targeted therapies to which pathogenic bacteria cannot develop resistance. Stimuli-responsive therapies represent an alternative technological focus due to their capability of delivering targeted treatment. This study provides a proof-of-concept investigation into the use of magneto-responsive gallium-based liquid metal (LM) droplets as antibacterial materials, which can physically damage, disintegrate, and kill pathogens within a mature biofilm. Once exposed to a low-intensity rotating magnetic field, the LM droplets become physically actuated and transform their shape, developing sharp edges. When placed in contact with a bacterial biofilm, the movement of the particles resulting from the magnetic field, coupled with the presence of nanosharp edges, physically ruptures the bacterial cells and the dense biofilm matrix is broken down. The antibacterial efficacy of the magnetically activated LM particles was assessed against both Gram-positive and Gram-negative bacterial biofilms. After 90 min over 99% of both bacterial species became nonviable, and the destruction of the biofilms was observed. These results will impact the design of next-generation, LM-based biofilm treatments.

Galvanic replacement of liquid metal Galinstan with copper for the formation of photocatalytically active nanomaterials

Galvanic replacement of liquid metal Galinstan under mechanical agitation with copper creates a multi-elemental system that is photocatalytically active for the degradation of organic dyes where reuseability is achieved via immobilisation on a solid support.

Synthesis of sub-micrometer biphasic Au-AuGa2/liquid metal frameworks

A novel biphasic sub-micrometer Au-AuGa₂/liquid metal framework, consisting of solid nanoparticles encapsulating liquid metal (LM) droplets, is introduced. By utilizing oxide-free galvanic replacement of a Ga-alloy LM with alkaline KAuBr₄, the controllable process of gold-based encapsulation of individual sub-micrometer LM droplets capped with polyvinylpyrrolidone, lysozyme or sodium alginate is demonstrated. The morphology, structure and composition of the encapsulated droplets are characterized in-depth via scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. For the first time, the existence of two kinetic regimes leading to the controllable synthesis of either intermetallic AuGa₂ or Au nanoparticles to encapsulate LM droplets is highlighted via the study of the evolution of the pH of the reaction medium. The as-formed biphasic sub-micrometer Au-LM frameworks demonstrate moderate stability in an aqueous environment and formidable structural and chemical stability in ambient air at room temperature. The encapsulation process can be adapted to the micro-patterning of LM droplet surfaces to produce conductive Au-encapsulated LM droplets then sintered to recover LM lines, while the sub-micrometer Au-LM droplets can be reintegrated onto larger LM surfaces and volumes to create multi-metallic and multi-scale biphasic structures with controllable electrical properties. The adaptability of the encapsulation process to the macro-, micro- and nanoscale of liquid metal features with a wide range of surfactants could in principle allow its use in various systems and applications.

Interfacial Rheology of Gallium-Based Liquid Metals

Gallium and its alloys react with oxygen to form a native oxide that encapsulates the liquid metal with a solid "skin". The viscoelasticity of this skin is leveraged in applications such as soft electronics, 3D printing, and components for microfluidic devices. In these applications, rheological characterization of the oxide skin is paramount for understanding and controlling liquid metals. Here, we provide a direct comparison of the viscoelastic properties for gallium-based liquid metals and illustrate the effect of different subphases and addition of a dopant on the elastic nature of the oxide skin. The du Noüy ring method is used to investigate the interfacial rheology of oxide skins formed by gallium-based liquid metal alloys. The results show that the oxide layer on gallium, eutectic gallium-indium, and Galinstan are viscoelastic with a yield stress. Furthermore, the storage modulus of the oxide layer is affected by exposure to water or when small amounts of aluminum dopant are added to the liquid metals. The former scenario decreases the interfacial storage modulus of the gallium by 35-85% while the latter increases the interfacial storage modulus by 25-45%. The presence of water also changes the chemical composition of the oxide skin. Scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy suggest that a microstructural evolution of the interface occurs when aluminum preferentially migrates from the bulk to the surface. These studies provide guidance on selecting liquid metals as well as simple methods to optimize their rheological behavior for future applications.