Methanol Clipping Modification on Liquid Metal Surface Enhances Photothermal Performance and Biocompatibility

ABSTRACT

Photothermal conversion (PTC) enables materials to convert light energy, particularly in the near‐infrared region, into heat. Photothermal materials have been widely applied in energy harvesting, sensing, and cancer therapy, while photothermal therapy (PTT) has emerged as a noninvasive tumor treatment with high targeting capability and low toxicity. Liquid metals (LMs) are promising photothermal agents due to their low toxicity, degradability, and tunable photothermal properties. However, their high reflectivity and poor stability significantly hinder practical applications. Herein, the three‐phase system consisting of LMs, polytetrafluoroethylene (PTFE), and methanol (CH₃OH) is developed. Through contact electrification (CE) at the multiphase interface, mechanical energy is converted into chemical energy, thereby clipping CH₃OH molecules to synthesize LMs nanoparticles (LMNPs). LMNPs exhibit excellent photothermal performance and stability. Within 10 min, the photothermal conversion efficiency (PCE) is calculated to be 67.89% in the centrifugal liquid and 36.25% in deionized water, corresponding to heating rates of 4.74 and 5.96°C/min, respectively. Assays in vitro demonstrate that, at a concentration of 2000 µg/mL, the material reduces tumor cell viability to 54.57% while exhibiting no detectable cytotoxicity, indicating good biosafety. This work provides a novel surface engineering strategy for expanding the application of LMs in PTT.

Mechanical activation induced by stirring and ultrasonication triggers a contact electrification (CE) effect at the multiphase interfaces among liquid metals (LMs), polytetrafluoroethylene (PTFE), and methanol (CH₃OH). This process facilitates interfacial electron transfer, drives the conversion of CH₃OH molecules, and promotes the surface functionalization of LMs. Consequently, an LMs core–shell nanostructure is constructed, exhibiting enhanced photothermal performance and enabling efficient near‐infrared (NIR) light‐induced heating for cancer cell ablation.

1. Introduction

Photothermal conversion (PTC) refers to the process by which materials convert light energy, particularly in the near‐infrared (NIR) region, into heat with high efficiency [1]. Owing to this property, photothermal materials have been widely explored in applications such as solar energy harvesting and storage [2], environmental monitoring [3], and biosensing [4]. These materials have also demonstrated considerable potential for cancer therapy in the biomedical field [5]. Photothermal therapy (PTT) represents a noninvasive treatment modality that has garnered considerable research interest [6]. NIR light irradiates tumor tissues and triggers photothermal agents to convert light energy into localized hyperthermia, enabling precise cancer cell ablation while alleviating damage to surrounding healthy tissues. This strategy avoids the trauma associated with conventional surgery and systemic chemotherapy. Moreover, through targeted design of photothermal agents, such as enhancing the permeability and retention effect or introducing specific molecular ligands [7, 8], the therapeutic precision of PTT can be further improved. The efficacy of PTT relies on the synergistic interaction between light and materials [9], which reduces systemic side effects. In addition, many photothermal agents exhibit favorable biodegradability and metabolic clearance [10, 11]. During treatment, real‐time thermal imaging enables accurate monitoring of temperature distribution, allowing precise control of the irradiation region, minimizing collateral tissue damage, and improving patient tolerance. The technique is simple to administer, does not require complex surgical procedures, leaves minimal post‐treatment residues, and permits repeated treatments for recurrent tumors [12, 13, 14]. Laser power density and irradiation duration can be modulated to elevate tumor‐region temperatures to 42–60°C, eliciting cellular apoptosis or necrosis. Temperatures surpassing 46°C further expedite cell death and boost ablation efficacy [15, 16]. Furthermore, PTT can be combined with other therapeutic modalities, including photodynamic therapy [17], chemotherapy [18], immunotherapy [19], and radiotherapy [20]. It can also be integrated with photoacoustic and fluorescence imaging to achieve theranostic synergy [21], thereby significantly enhancing treatment outcomes. Benefiting from the deep tissue penetration of NIR‐II‐based photothermal agents, PTT shows great promise for treating deep‐seated tumors, overcoming the limitations of conventional minimally invasive therapies that are typically restricted to superficial lesions.

First‐generation photothermal materials are primarily based on noble metals such as gold, silver, and platinum, among which gold nanostructures (nanospheres, nanorods, and nanostars) have been extensively studied due to their high photothermal conversion efficiency (PCE) [22, 23]. However, their slow metabolic clearance and high cost limit clinical translation. Escalating demand for alternative materials enables liquid metals (LMs) to emerge as a promising class of candidates. LMs are materials that remain in a liquid state at or near room temperature while retaining metallic properties [24]. Elemental LMs include cesium, francium, rubidium, mercury, and gallium. However, cesium is highly reactive, francium is radioactive, rubidium is easily oxidized, and mercury is highly toxic, which severely limits their practical applications. In contrast, gallium‐based LMs, particularly alloys such as eGaIn and eGaInSn, have attracted increasing interest in biomedical research due to their low toxicity, excellent biocompatibility, high thermal conductivity, and fluidity [25, 26]. Their low melting point allows them to remain liquid at room temperature, enabling excellent deformability and processability. LMs possess high electrical and thermal conductivity, similar to those of typical metallic materials, rendering them promising candidates for applications in electronic transport and thermal management. Their ability to flow through microchannels and complex geometries provides unique advantages in advanced manufacturing [27, 28, 29, 30]. In addition, their high surface tension imparts self‐healing capability and facilitates the formation of stable metallic films [31, 32]. Upon exposure to air, LMs spontaneously form a thin oxide layer (Ga₂O₃), which stabilizes their structure and enhances biocompatibility. Moreover, the highly reactive surface of LMs enables interactions with various species, allowing functional modification and performance enhancement. Owing to these unique physicochemical properties, LMs hold great promise in diverse applications, including PTT, targeted drug delivery, self‐healing materials, biosensing, and biomedical imaging. In particular, in cancer treatment, LMs are anticipated to become an important alternative material, providing novel solutions for PTT and targeted drug delivery [33, 34, 35, 36]. However, the high surface reactivity of LMs also renders them susceptible to oxidation and environmental degradation when exposed to oxygen and moisture.

Methanol (CH₃OH), as a widely available and low‐cost one‐carbon alcohol, provides oxygen‐containing reactive species through its hydroxyl groups [37]. Leveraging the high oxygen sensitivity of LMs, this study proposes a novel nanoengineering strategy based on the construction of a multiphase interfacial system composed of LMs, polytetrafluoroethylene (PTFE), and CH₃OH. The interfacial contact electrification (CE) effect is exploited to achieve surface functionalization of LMs, thereby synergistically enhancing their photothermal performance and structural stability. Solid–solid, solid–liquid, and liquid–liquid friction among LMs, PTFE, and CH₃OH in this system induces CH₃OH molecular conversion accompanied by bond clipping, and the corresponding products from the reaction between LMs and CH₃OH in situ construct a composite coating layer on the surface of LMs. This coating effectively reduces the reflectivity of LMs while improving their stability, resulting in a significant enhancement in PCE. A top‐down approach is employed, wherein mechanical stirring and ultrasonication, coupled with the interfacial CE effect, enable the conversion of mechanical energy into chemical energy. The active sites on the LMs surface adsorb oxygen species, while CE‐induced generation of metal ions facilitates the cleavage and recombination of chemical bonds in CH₃OH molecules, ultimately yielding LMs nanoparticles (LMNPs). This construction strategy not only improves the photothermal performance of LMs but also provides new insights into interfacial reactions and structural evolution in photothermal materials. Moreover, it offers a promising material design framework and theoretical basis for the development of low‐toxicity, spatiotemporally controlled PTT for tumor treatment.

2. Experimental Section

2.1. Synthesis of LMs and LMNPs

PTFE, high‐purity gallium (Ga, 99.9%), and indium (In, 99.9%) were used as starting materials. Ga and In were mixed at a mass ratio of 75.5:24.5 and sealed under an argon (Ar) atmosphere. The mixture was heated to 60°C and stirred for at least 30 min to ensure complete alloying, forming LMs. To minimize the influence of surface oxidation, all subsequent characterizations and analyses were conducted using samples extracted from the bulk interior of the alloy. The reaction was carried out in a 250 mL glass reaction flask. Initially, 200 mL of high‐purity CH₃OH (99.99%) and PTFE were added, and the vessel was sealed. Subsequently, 12.5 g of LMs was slowly injected into the flask using a syringe to ensure complete immersion in the solution. Under the combined action of external mechanical forces and the CE effect, LMs reacted with CH₃OH and were dispersed in the solution. The resulting mixture was then centrifuged at 13,500 rpm (a schematic illustration and photographs before and after centrifugation are provided in Figures S1 and S2). The supernatant was collected and subsequently subjected to vacuum drying at 60°C, yielding the final product, hereafter designated as LMNPs.

2.2. Calculation of PCE for LMNPs

Based on previously reported methods [38], the PCE of LMNPs was determined under two conditions: a centrifuged solution (0.0625 g/mL) and a dispersion system prepared by dispersing 0.2 g of LMNPs NPs in 6.2 mL of deionized (DI) water. The samples were irradiated with an 808 nm laser at a power density of 1.5 W/cm² (10,490 mW) over a circular area (r = 1.5 cm) for 10 min. The PCE was calculated using the following equation:

$$\eta =\left[hS\left(T_{Max}-TSurr\right)-QDis\right]I\left(1-{10}^{-A808}\right)$$ (1)

$$hS=m_0{C}_o/\tau s$$ (2)

In this equation, h represents the heat transfer coefficient, S denotes the surface area of the container, and T_Surr and T_{M a x} correspond to the ambient and maximum temperatures of the system, respectively. Q_Dis refers to the heat dissipated by the solvent under light irradiation. The Q_Dis value is determined to be 106.68 mW for CH₃OH (as calculated from Figure 5a) and 10.2 mW for DI water [39]. I denotes the incident laser power, and A₈₀₈ represents the absorbance at 808 nm. where m₀ and C₀ are the mass (5 g) and heat capacity (the specific heat capacity equals 2.51 J g^{− 1} K^{− 1} for CH₃OH and 4.18 J g^{− 1} K^{− 1} for DI water) of the solvent, respectively, τs is the sample system time constant calculated by the equation.

FIGURE 5.

Systematically presents the photothermal performance characterization of the materials. (a) Temperature time profiles of various materials under 808 nm laser irradiation; (b) Corresponding infrared thermal images during irradiation; (c) Heating cooling curves of the centrifugal liquid at different power densities; (d) Five cycle stability test of the centrifugal liquid at 1.5 W/cm²; (e) Error band distribution of temperature fluctuations across five cycles for the supernatant system (mean ± SD, n = 5); (f) Heating cooling curves of LMNPs dispersed in DI water; (g) Five‐cycle stability profile of the LMNPs_(aq) system; (h) Error band distribution of temperature fluctuations over five cycles for the aqueous LMNPs system (mean ± SD, n = 5); i) PCE profile of the supernatant system: (j) PCE profile of the LMNPs_(aq) system; (k) Comparative bar‐chart illustration of five‐cycle photothermal PCE values for the two systems: (l) Heating cooling responses of LMNPs themselves at different power densities; (m) Their five cycle stability curve; (n) Comparative analysis of PCE.

2.3. Characterization Methods

In this study, the preparation of LMNPs was carried out using a magnetic stirrer (model HJ 6A) and an ultrasonic cell disruPCEr (model Xinyi 1000N). The carbon content of LMNPs was measured using a CS844 infrared carbon sulfur analyzer. Morphological and elemental analysis was conducted using a Zeiss Sigma 300 field emission scanning electron microscope (FESEM) equipped with an Oxford Xplore 30 energy dispersive x‐ray spectrometer (EDS). Microstructure and lattice information were obtained using a FEI Talos F200S transmission electron microscope (TEM) with an X‐Twin EDS detector. Surface chemical state analysis was performed with a Thermo Fisher Scientific K‐Alpha x‐ray photoelectron spectrometer (XPS), with charge correction based on the C1s (284.80 eV) reference. The crystal structure was analyzed using a Smart Lab SE x‐ray diffractometer (CuKα1 radiation, λ = 1.54056 A, 40 kV, 15 mA) with a scan range of 2θ = 10°–90°. Raman spectra were recorded with a Horiba LabRAM HR Evolution spectrometer, with a scan range of 50–4000 cm^{− 1}. The Ga concentration in CH₃OH was measured using inductively coupled plasma optical emission spectroscopy (ICP‐OES, Agilent 5100/5800) and inductively coupled plasma mass spectrometry (ICP‐MS, Agilent 7800). UV–visible NIR absorption spectra were acquired using a Shimadzu UV‐3600 spectrophotometer. Gas phase products were analyzed using a GC 9790II gas chromatograph. Fourier transform infrared (FTIR) spectra were obtained with an FTIR‐2000 spectrometer, with a scan range of 400–4000 cm^{− 1}. Photothermal performance was tested using a VCL 808 nm M2 30 W laser, and temperature distribution was recorded with a FLIR E1330 infrared thermal imager. In cell experiments, cell viability after photothermal treatment was measured using a Thermo Fisher 353 microplate reader, with a HW808AD2000 100F 808 nm laser as the NIR light source. Toxicity assays were conducted using a Thermo Fisher Multiskan microplate reader, and cell morphology was observed with an AE2000 inverted microscope. The short‐circuit current (I_SC), open‐circuit voltage (V_OC), and transferred charge (Q_SC) of LMs under different states were analyzed using a Keithley 6514 electrometer.

3. Results and Discussion

3.1. Preparation of LMNPs and Their Formation Mechanism

The CE effect is a ubiquitous interfacial physicochemical phenomenon that describes charge transfer and the formation of an electric double layer upon contact and separation between materials, including solid–solid, solid–liquid, and liquid–liquid systems [40]. This process occurs without the need for external electrical input [41]. In this work, the CE effect within a multiphase system composed of LMs, PTFE, and CH₃OH is exploited to design and fabricate LMNPs with enhanced photothermal performance. This strategy is expected to improve both the PCE of LMs and their tumor cell ablation capability. Initially, LMs were mixed with CH₃OH under magnetic stirring for 1 h, followed by ultrasonication for 30 min. Subsequently, alternating cycles of magnetic stirring and ultrasonication were conducted for a total duration of 24 h. Figure S3 delineates the relevant experimental outcomes, revealing that bulk LMs experience progressive disintegration into fine LMs fragments. Owing to the high oxygen sensitivity of LMs [42], CH₃OH molecules were adsorbed onto the surface of the as‐formed LMs. With the assistance of the CE effect at the interfaces among LMs, PTFE, and CH₃OH, the chemical conversion of CH₃OH molecules was induced. Conversion products derived from CH₃OH and LMs form a composite shell in situ on LMs, yielding LMNPs with robust light absorption and low reflectivity [43], as depicted in Figure 1a.

FIGURE 1.

Presents the synthesis and formation mechanism of the material. (a) Schematic illustration of the synthesis process of LMNPs; (b) Schematic diagram of the adsorption‐induced decomposition mechanism of CH₃OH molecules on the surface of LMs; (c) Schematic illustration of the entire process from the initial interfacial contact between LMs and CH₃OH molecules to the eventual formation of a complete coating layer.

Bulk LMs undergo progressive disintegration and homogeneous dispersion throughout CH₃OH upon external mechanical agitation. During this process, the CE effect at the interface between LMs and PTFE is accompanied by the continuous release of metal ions, such as Ga^{3 +} and In^{3 +}, into the surrounding medium. These highly reactive ions subsequently participate in interfacial coordination interactions with CH₃OH under the modulation of surface charges on PTFE. Mechanical forces not only promote the efficient dispersion of LMs but also continuously drive the dynamic formation and renewal of interfaces, thereby enhancing mass transfer and facilitating the adsorption of CH₃OH molecules onto the surface of LMs. Figure 1b displays the dissociation of adsorbed CH₃OH molecules at the interface driven by the synergistic effect of metal ions and surface charges. This process induces bond clipping and the selective cleavage of C─H, C─O, and O─H bonds, yielding active species including –H, O, and –CH_x. Figure 1c further elaborates that the entire route involves mechanical dispersion, interfacial adsorption, bond clipping, product formation, and surface deposition. Uniform composite shells form in situ on the surfaces of LMs, yielding compact and robust core–shell architectures(see Figure S4). This composite coating not only significantly enhances the chemical stability of the material but also improves its overall mechanical strength.

3.2. CE Effect in a Triphase System

Elucidating the charge transfer behavior at the LMs interface and further exploring its role in promoting CH₃OH conversion and enabling the transformation of mechanical energy into chemical energy, we systematically investigated the CE effect in three systems. Ga and In, LMs and PTFE, and a composite system comprising LMs, PTFE, and CH₃OH. The interfacial charge behavior was analyzed by measuring the contact current, contact voltage, and transferred charge of each system, in conjunction with schematic illustrations of electron transfer pathways. Figure 2a delineates preferential electron migration from In to Ga upon physical contact, a phenomenon stemming from disparate intrinsic work functions [44]. The successive contact–separation cycle produces prominent electrical outputs of current, voltage, and charge (Figure 2d–f), verifying efficient interfacial charge transport across the solid Ga/In boundary governed by Fermi‐level rearrangement [45]. Apart from accelerating such interfacial electron translocation, In exerts a dominant influence on depressing the melting point, retaining liquid configuration under ambient surroundings, and boosting inherent fluidity. Elemental In integration further elevates surface charge density and relieves lattice restraint, which expedites charge migration and storage and concurrently strengthens chemical durability of the alloy. Figure 2b depicts electron transfer from LMs to PTFE during contact, leaving the LMs surface positively charged and the PTFE surface negatively charged. PTFE simultaneously acts as a highly electronegative triboelectric layer that promotes interfacial charge separation and accumulation, thereby enhancing the localized electric field and facilitating CH₃OH activation during the mechanochemical process [46, 47]. Owing to the significant difference in electron affinity and surface electronic states between the two materials, substantial electron transfer occurs at the interface, resulting in a high triboelectric output. Figure 2g–i further illustrate the evolution of electrical signals during the contact–separation process at the LMs/PTFE interface. The contact current, voltage, and transferred charge all exhibit a marked enhancement, further corroborating the highly efficient interfacial charge transfer.

FIGURE 2.

Presents the CE effects of Ga/In, LMs/PTFE, and LMs/PTFE/CH₃OH systems, as well as the comprehensive mechanochemical pathway toward CH₃OH transformation and bond clipping. (a) Schematic illustration of electron transfer between solid Ga and In; (b) Schematic illustration of electron transfer between LMs and PTFE; (c) Schematic illustration of electron transfer among LMs, PTFE, and CH₃OH; (d–f) Contact current, contact voltage and transferred charge of solid Ga and In; (g–i) Contact current, contact voltage and transferred charge of LMs/PTFE; (j–l) Contact current, contact voltage and transferred charge of the LMs/PTFE/CH₃OH system; (m) Comprehensive mechanochemical pathway toward CH₃OH transformation and bond clipping.

Figure 2c shows that the adsorption of CH₃OH molecules onto the LMs surface modifies the interfacial charge distribution and electron transfer energy barrier after the introduction of CH₃OH. The interfacial layer formed by solvent molecules mediates charge dissipation and redistribution, thereby regulating the interfacial electric field strength and CE behavior. Figure 2j–l reveals that the contact current, contact voltage, and transferred charge of the LMs/PTFE system are significantly modulated in the presence of CH₃OH [48]. These results demonstrate that the introduction of CH₃OH effectively influences the interfacial charge transfer process and further regulates the triboelectric properties of the system. This study demonstrates efficient interfacial charge transfer in Ga/In, LMs/PTFE, and LMs/PTFE/CH₃OH systems. CH₃OH adsorbed on the LMs surface modulates the interfacial charge distribution and lowers the electron transfer barrier, thereby tailoring the triboelectric properties and enabling the cleavage of C─H, C─O, and O─H bonds within CH₃OH molecules (Figure 2m). These findings provide direct experimental evidence that interfacial charge transfer can boost CH₃OH conversion and realize the conversion of mechanical energy into chemical energy.

3.3. TEM and SEM Characterization of LMNPs

Within LMs, electrons exhibit a delocalized distribution, rendering them intrinsically non‐polar at the molecular level [49]. Nevertheless, when LMs come into contact with polar molecules or functional groups containing oxygen, nitrogen, or other heteroatoms, charge redistribution takes place under the driving force of interfacial interactions [50, 51]. This process further gives rise to the formation of stable charge‐asymmetric structures or dipole layers at the interface, thereby endowing LMs with distinct interfacial polarization behavior. In this work, leveraging this characteristic, CH₃OH molecules were adsorbed onto the LMs surface under mechanical force. The CE effect between LMs and PTFE then triggered the conversion of CH₃OH molecules and the formation of a coating layer (Figure 3k), enabling the dispersion of LMs in the CH₃OH solution. Figure 3d shows that the centrifuged solution exhibited a distinct brownish‐yellow color under visible light (the static evolution of the reaction solution and centrifuged supernatant is presented in Figures S5 and S6). ICP results of the centrifuged supernatant (Table S1) confirmed the significant dissolution of Ga in the system. This indicates that Ga in LMs was oxidized and released as Ga^{3 +} under the CE effect, thus participating in subsequent interfacial reactions (the schematic diagram, the correlation between C_o and C₁, and the relationship between C₁ and C_x are shown in Figures S7–S9).

FIGURE 3.

Presents the structural characterization results of LMNPs. (a) Photograph of Ga and In metallic particulates; (b) Photograph of the Ga–In alloy; (c) Photograph of LMNPs; (d) Color comparison of the centrifuged solution before and after light exposure; (e) Schematic illustration of the morphological transformation of LMs; (f–i) TEM images of the centrifuged solution at different magnifications; (j) EDS elemental mapping images of the corresponding region with a scale bar of 50.0 nm; (k) Schematic illustration of the adsorption interaction on the LMs surface; (l,m) SEM images of LMNPs; (n) EDS elemental mapping images of the relevant region with a scale bar of 5.0 µm.

Figure 3a–c,e demonstrates that the Ga–In metal particles, upon liquefaction, reacted with CH₃OH under externally applied mechanical forces and were subsequently transformed into uniformly dispersed nanoparticles. The CH₃OH‐induced encapsulation layer not only effectively modulated the mechanical properties of the particles, but also markedly suppressed particle oxidation and aggregation, thereby enhancing their colloidal stability and photothermal stability [52]. TEM images (Figure 3f–i) confirm the formation of the coating layer and verify the core–shell structure of the resulting material (see Figure S10 and Table S2). Distinct coating layers are clearly observed at different scales (200, 50, and 10 nm), with corresponding thicknesses of 16.55, 6.59, and 6.22 nm, respectively. In addition, SEM images (Figure 3l,m, and Figure S11) further corroborate that the as‐synthesized LMNPs exhibit a regular spherical morphology. Furthermore, EDS analysis (Figure 3j,n) reveals that LMs are predominantly distributed in the core region of the particles (see Figures S12 and S13). The coating shell is mainly composed of reaction products derived from CH₃OH and LMs, indicating that the surface modification process involves chemical interactions rather than simple physical adsorption (see Figures S14 and S15, and Table S3). Under normal circumstances, uncoated bare LMs are susceptible to oxidation and dealloying in air upon exposure to moisture and oxygen [53, 54], leading to irreversible agglomeration of the particles.

3.4. Composition Evolution and Stability Assessment During the Clipping Process

Systematic spectroscopic characterizations of the as‐prepared materials are performed to elucidate the formation mechanism of LMNPs. Figure 4a illustrates that, upon contact between CH₃OH and LMs, CH₃OH molecules adsorb onto the surface of LMs under mechanical stimulation. The CE effect induces charge accumulation on the surface, accompanied by the generation of Ga^{3 +} and In^{3 +} [55]. Subsequently, Ga^{3 +} further reacts with oxygen and hydrogen species derived from CH₃OH molecules, leading to the formation of GaO(OH). XPS analysis reveals that characteristic peaks corresponding to Ga 2p, In 3d, O 1s, and C 1s can be distinguished in the survey spectrum of LMNPs (the full XPS survey spectrum is shown in Figure S16). The Ga 2p spectrum (Figure 4b) was deconvoluted into four peaks located at 1115.30 eV (Ga 2p3/2, Ga), 1117.41 eV (Ga 2p3/2, Ga^{3 +}), 1141.61 eV (Ga 2p1/2, Ga), and 1144.34 eV (Ga 2p1/2, Ga^{3 +}). Similarly, the In 3d spectrum (Figure 4c) was fitted into four peaks at 442.30 eV (In 3d5/2, In), 444.47 eV (In 3d5/2, In^{3 +}), 450.53 eV (In 3d3/2, In), and 451.99 eV (In 3d3/2, In^{3 +}). Both the Ga 2p and In 3d spectra can be resolved into characteristic signals of metallic and ionic states. In the final product, the peak intensities of Ga^{3 +} and In^{3 +} are both significantly higher than those of their metallic counterparts, indicating that an obvious oxidation process has occurred in the system. In contrast, Ga exhibits a lower standard electrode potential and higher chemical reactivity, making it more likely to lose electrons and undergo preferential oxidation during interfacial reactions, thus dominating the formation of GaO(OH), Ga₂O₃, and other related species. Meanwhile, this oxidation process is further intensified by the synergistic effect of the CE effect and the interfacial electric field, promoting interfacial reactions and structural evolution in the system. The fitted O 1s spectrum (Figure 4d) displays a Ga–O peak at 530.49 eV and a Ga–OH peak at 531.63 eV, confirming the adsorption of oxygen species from CH₃OH onto the LMs surface and their subsequent chemical bonding. Furthermore, three peaks at 284.8 eV (C─C), 286.18 eV (C─O), and 287.53 eV (O─C═O) in the C 1s spectrum (Figure 4e) provide additional evidence for the presence of carbonaceous species on the material surface. We performed XRD analysis on the final product and the bottom precipitate after centrifugation (the sampling schematic is shown in Figure S17). Abundant GaO(OH) with high crystallinity was identified in the precipitate (the XRD pattern of the bottom precipitate is presented in Figure S18).

FIGURE 4.

Surface effects of LMs and characterization results of LMNPs. (a) Dynamic diagram of the surface effects of LMs; (b–e) High‐resolution XPS spectra of Ga 2p, In 3d, O 1s and C 1s, respectively; (f) XRD pattern of LMNPs; (g) Raman scattering spectrum of LMNPs; (h) Comparative XRD patterns of LMNPs stored at room temperature for 60 days at different time intervals; (i) Comparative FTIR spectra of LMNPs stored at room temperature for 60 days at different time intervals; (j) GC profile of hydrogen produced during the reaction.

Figure 4f illustrates the corresponding results, compared with pristine LMs (the comparative XRD patterns of LMNPs and bare LMs are provided in Figure S19). LMNPs show a broadened diffraction band in the 30–70° range, corresponding to GaO(OH) and carbon materials (see Figures S20 and S21). This result indicates the relatively low crystallinity of the product. Furthermore, the appearance of the D band and G band in the Raman spectrum (Figure 4g) provides conclusive evidence for the presence of carbonaceous species within the shell layer of the product. To further quantify the carbon content in the sample, carbon–sulfur analysis was performed on 0.0695 g of the sample (the schematic diagram of the carbon–sulfur analysis is shown in Figure S22). The results indicate that the mass fraction of carbon is determined to be 3.86 wt% (Table S4). This finding confirms the occurrence of chemical interactions between LMs and CH₃OH, as well as the successful formation of a coating layer on the surface of LMs. Such a coating layer can effectively inhibit the agglomeration and coalescence of LMs, thus helping to strengthen the mechanical stability of the core–shell structure. To evaluate the influence of surface modification on the chemical stability and antioxidation ability of the material, LMNPs were stored at room temperature for 60 days. XRD (Figure 4h) and FTIR (Figure 4i) characterizations were conducted at 0, 30, and 60 days, respectively. The results show that the material maintains its amorphous structure, and no obvious changes appear in the absorption peaks of the FTIR spectra, with no detectable signs of oxidation. This suggests that LMNPs possess excellent chemical stability and antioxidation performance. These results illustrate that the synthetic strategy assisted by magnetic stirring and ultrasound can remarkably improve the stability of LMs, and the CH₃OH‐derived coating layer provides effective chemical protection for the LMs core in the core–shell structure. To probe the reaction mechanism, GC was used to analyze the gaseous products generated during the reaction (Figure 4j). Detected hydrogen gas verifies that the combined effects of stirring and ultrasonication promote the reaction between LMs and CH₃OH molecules. Based on the identified products, we infer that hydrogen generation originates from the bond clipping of C─H and O─H bonds in CH₃OH, followed by the combination of free hydrogen atoms to form H₂. Comprehensive characterization results reveal that Distinct triboelectric current, voltage, and transferred charge signals solidly verify efficient interfacial charge generation and transport throughout CE. TEM, SEM, and spectral characterizations further evidence the growth of a GaO(OH)/carbonaceous hybrid shell on LMs surfaces, alongside carbonaceous and carboxylate moieties derived from CH₃OH conversion. Collective triboelectric and spectral data imply that charge transfer driven by CE can promote CH₃OH activation and successive bond scission. Sustained charge buildup and localized electric fields at the multiphase interface reduce the activation energy of interfacial reactions and accelerate the formation of hybrid shell components. The composite shell mainly consists of GaO(OH) and carbonaceous materials containing carboxylate species generated from methyl modification. The overall reaction involving the formation of GaO(OH), carbon materials, and hydrogen gas can be represented by Equations ((2), (3), (4)).

$$2Ga+2C{H}_{3}OH\to 2GaO\left(OH\right)+3H_2+2C$$ (3)

$$C{H}_{3}OH+O_2\to CHOOH+H_{2}O$$ (4)

$$6CHOOH+2Ga\to 2Ga{\left(CHOO\right)}_3+3H_2$$ (5)

3.5. Photothermal Performance Evaluation and PCE Calculation of LMNPs

Owing to their outstanding NIR absorption capability and favorable biocompatibility, LMs have emerged as promising photothermal conversion materials for PTT [56, 57]. Nevertheless, LMs tend to spontaneously form an oxide layer in ambient environments, which markedly suppresses their PCE and thus restricts their practical applications [58]. Enhancing the stability and photothermal performance of LMs, in this work, the surface of LMs was modified via the CE effect among LMs, PTFE, and CH₃OH. A stable core–shell structure was thereby constructed, and its photothermal properties were systematically evaluated. To evaluate the photothermal performance, photothermal experiments were conducted on CH₃OH, LMs, and the centrifuged supernatant for 10 min under NIR laser irradiation at a power density of 1.5 W/cm², with their temperature changes recorded (Figure 5a). The photothermal results indicate that the heat generation capacity of the system is significantly enhanced when CH₃OH is combined with LMs. Specifically, CH₃OH exhibited only a slight temperature increase. the temperature of LMs rose from 17.0°C to 57.7°C, whereas that of the centrifuged supernatant rapidly climbed from 18.9°C to 63.6°C. This finding suggests that the coating layer generated via the reaction between CH₃OH and LMs serves as an effective protective layer, enhancing the photothermal conversion performance through synergistic cooperation with the LMs core. Furthermore, the infrared thermal imaging results (Figure 5b) further corroborate that the centrifuged supernatant features a faster heating rate and a more prominent temperature distribution, which intuitively demonstrates its excellent PCE. We further explored the photothermal performance of individual constituents within the shell. We performed 10 min heating tests on GaO(OH), carboxylate species, and carbonaceous materials, and the relevant data are provided in Figure S23. The results demonstrate that carboxylate species show marginal heating capacity. GaO(OH) delivers a moderately higher temperature rise, while carbonaceous materials exhibit the strongest photothermal response. These observations verify that GaO(OH) and carbonaceous constituents act as the dominant functional components endowing LMNPs with superior photothermal properties, and carboxylate species play only a secondary role.

Based on the above experimental data, we further calculated the PCE of the two systems according to the conventional methods reported in the literature: one is the centrifuged supernatant, and the other is the system prepared by redispersing LMNPs in DI water. First, both systems were irradiated at various power densities (0.5, 1.0, 1.5, and 2.0 W/cm²), and their temperature rise and natural cooling processes were recorded (Figure 5c,f). Laser power increase led to a remarkable rise in the maximum temperature reached by the samples after 10 min of irradiation. The corresponding infrared thermal images (Figures S24 and S25) intuitively demonstrate the effect of laser irradiation on the temperature of the systems. To evaluate the photothermal stability, five on–off laser cycling tests were performed on the two systems under a power density of 1.5 W/cm² (Figure 5d,g). The heating‐cooling curves of each cycle overlapped significantly, indicating that both systems maintained stable performance under repeated thermal stress. The corresponding infrared thermal images (Figures S26 and S27) showed that the maximum temperatures remained nearly unchanged after five cycles, further confirming the excellent cycling stability and thermal reliability. Five‐cycle heating–cooling datasets from Figure 5d,g were processed to generate error‐band plots in Figure 5e,h for evaluating photothermal repeatability and dynamic behavior. LMNPs_(aq) and the supernatant underwent five sequential cyclic tests, with all temperature profiles expressed as mean ± SD (n = 5). Central curves reflect average temperature changes, and shaded regions mark the ±1 SD range from five independent measurements. The supernatant maintained minimal temperature drift during heating and cooling. Aqueous LMNPs, however, displayed notable fluctuations and wider error bands, which probably stem from external disturbances. Collectively, both systems delivered reliable photothermal responses during heating and cooling. Thereafter, the PCE of both systems during five cyclic tests is quantitatively evaluated. Corresponding PCE data maintain favorable consistency and stability in the two groups, as depicted in Figure 5k and Figures S28 and S29. Finally, the calculated PCE of the centrifuged supernatant system was 67.89% (Figure 5i; its UV–vis spectrum is provided in Figure S30), with a heating rate of 4.74°C/min within 10 min, while that of the system with LMNPs redispersed in DI water was 36.25% (Figure 5j; the UV–vis spectrum of LMNPs_(aq) is shown in Figure S31), corresponding to a heating rate of 5.96°C/min within 10 min. This finding demonstrates that the material exhibits favorable photothermal response capability in diverse solvent environments.

Specifically, the coexisting carboxylic acid‐derived carbonaceous component and GaO(OH) collectively tailor the surface topography of LMs to form a rough outer shell. Such rugged morphology creates plentiful interfacial light‐trapping sites, which suppress surface reflectance and intensify broadband NIR absorption. In addition, this hybrid coating facilitates efficient interfacial energy dissipation and insulates inner LMs against structural deterioration during cyclic operation. Benefiting from the above structural advantages, the resultant composites achieve superior PCE and improved cycling stability, as displayed in Figure S32. In comparison, LMNPs fabricated via CH₃OH modification exhibit distinct heating‐cooling responses under different laser power levels (Figure 5l), with a remarkable increase in the maximum temperature attained by the sample. Additionally, the material presents highly consistent performance curves without noticeable degradation during five on–off cycling tests (Figure 5m). The corresponding infrared thermal images (Figures S33 and S34) intuitively demonstrate the effect of laser power on the temperature elevation of LMNPs, and the LMNPs maintain excellent stability under repeated thermal stress. Currently documented photothermal materials inherently face an inherent dilemma: high photothermal efficiency usually demands harsh synthetic conditions, while mild and facile fabrication routes merely yield inferior photothermal performance [38, 59, 60, 61, 62] By contrast, this work ingeniously combines mild synthetic conditions, simple preparation procedures, and exceptional photothermal conversion capability, exhibiting overwhelming comprehensive superiority (Figure 5n and Table S5) [63]. In summary, surface modification with CH₃OH can significantly improve the PCE of LMs. The as‐prepared LMNPs retain structural integrity and stable performance over multiple thermal cycles, accompanied by favorable operational durability. These findings provide a critical basis for the practical application of LMs in PTT and confirm that the material exhibits excellent photothermal response capability in various solvent environments.

3.6. Biosafety Evaluation of LMNPs

Systematic investigation of the structure, composition, and photothermal properties of LMNPs demonstrates that CH₃OH‐functionalized LMs exhibit markedly enhanced PCE, superior antioxidative capability, and excellent long‐term stability. These advantages effectively overcome the intrinsic limitations of conventional LMs, including insufficient light‐harvesting capacity, high optical reflectance, and poor oxidative stability. Combined with the inherent biocompatibility of LMs, the as‐fabricated nanocomposites show considerable potential for PTT applications. To further evaluate the biological feasibility of LMNPs, comprehensive studies were conducted to assess their in vitro photothermal ablation efficacy and biocompatibility. Human breast cancer cells (MCF‐7) were employed to evaluate the photothermal therapeutic efficacy. Cells were seeded in 96‐well plates at a density of 1×10⁴ cells per well and cultured for 24 h, followed by incubation with varying concentrations of LMNPs for 6 h. Subsequently, the cells were exposed to 808 nm laser irradiation at a power density of 0.1 W cm^{− 2} for 5 min and then cultured for an additional 24 h. Cell viability was then quantitatively determined using the CCK‐8 assay, as shown in Figure 6a. As illustrated in Figure 6c,d, cell viability decreased progressively with increasing concentrations of both pristine LMs and LMNPs. At a concentration of 2000 µg/mL, photothermal treatment with pristine LMs resulted in a cell viability of 74.34%. By contrast, treatment with LMNPs led to a pronounced reduction in cell viability to 54.57%. It is anticipated that further increasing the laser power density would enhance the cell ablation effect. To evaluate the biosafety of the material, murine macrophages (RAW 264.7) were used for cytotoxicity assessment. Cells were seeded at a density of 1×10⁵ cells per well and cultured for 24 h. Subsequently, as presented in Figure 6f, LMNPs at concentrations of 100–400 µg/mL were added, followed by incubation for an additional 24 h prior to CCK‐8 analysis (the experimental procedure is shown in Figure 6b). The results indicate that the nanocomposites exhibit negligible cytotoxicity within the tested concentration range and even slightly enhance cellular metabolic activity (Figure 6e), suggesting favorable biocompatibility. Collectively, these results confirm that CH₃OH modification not only significantly enhances the photothermal performance and stability of LMs but also improves their biointerface compatibility, thereby providing strong experimental support for their application in tumor PTT. This work further assesses the translational potential of LMNPs via evaluation of their physiological compatibility. The outer hybrid shell (Figure 6g) physically separates the inner LMs core from biological surroundings and alleviates fluid‐triggered corrosion, preserving structural integrity, intrinsic physicochemical features, and photothermal robustness within physiological milieus. The core–shell architecture modulates the sustained release of Ga^{3 +} and In^{3 +} ions (Figure 6h), which circumvents toxic risks arising from ion overaccumulation and sustains dynamic interfacial stability. The material delivers potent tumor cell ablation alongside excellent biocompatibility over a broad range of therapeutic dosages. Such a favorable trade‐off between therapeutic performance and biosafety confirms its considerable prospects for biomedical implementation.

FIGURE 6.

Presents the in vitro cellular evaluation results of LMNPs. (a) Schematic illustration of the photothermal ablation procedure for MCF‐7 cells; (b) Schematic depiction of the cytotoxicity assay protocol for RAW 264.7 cells; (c) Quantitative analysis of the relative viability of MCF‐7 cells treated with various concentrations of LMs upon 808 nm laser irradiation; (d) Quantitative analysis of the relative viability of MCF‐7 cells treated with various concentrations of LMNPs upon 808 nm laser irradiation; (e) Quantitative analysis of the relative viability of RAW 264.7 cells incubated with different concentrations of LMNPs; (f) Representative optical microscopy images of RAW 264.7 cells after treatment with low, medium, and high concentrations of LMNPs; (g) Schematic illustration of the functional shell of LMNPs; (h) Schematic illustration of the advantages and potential of LMNPs.

4. Conclusion

In this work, a CE effect‐driven surface modification strategy for LMs mediated by CH₃OH is proposed, enabling the synergistic enhancement of photothermal performance and structural stability through the construction of a core–shell architecture. This strategy exploits the high sensitivity of LMs to oxygen‐containing species and the CE effect at the multiphase interface among LMs, PTFE, and CH₃OH. External mechanical forces induce interfacial charge separation and migration, thereby promoting the adsorption and activation of CH₃OH molecules on the surface of LMs. Subsequent interfacial reactions generate GaO(OH) together with carbonaceous species containing carboxylate groups derived from methyl transformation, which collectively assemble in situ into a uniform composite shell. Mechanistic studies reveal that mechanical force promotes the dispersion of LMs and triggers a pronounced CE effect, driving the accumulation and release of interfacial charges and accelerating the ionization of Ga^{3 +} and In^{3 +}. Resultant interfacial electric field and high‐energy state charges drive bond clipping and selective cleavage of C─H, C─O, and O─H bonds in CH₃OH molecules, enabling mechanically driven chemical conversion and the formation of a dense and stable protective layer. The obtained LMNPs exhibit excellent photothermal performance and long‐term stability, with PCE values of 67.89% in CH₃OH and 36.25% in DI water, along with enhanced oxidation resistance and cycling stability. Cellular experiments show that at a concentration of 2000 µg/mL, LMNPs reduce tumor cell viability to 54.57% under NIR irradiation at 0.1 W/cm² for 5 min, without obvious cytotoxicity, indicating good biosafety. Overall, this work demonstrates the coupling of mechanically driven interfacial charge regulation with chemical reactions via the CE effect, providing a core–shell design that integrates oxidation resistance and mechanical stability, and offering a new framework for LMs surface engineering with potential applications in PTT, catalysis, and related fields.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smll74478‐sup‐0001‐SuppMat.pdf.

Data Availability Statement

Research data are not shared.