High Colloidal Stable Carbon Dots Armored Liquid Metal Nano‐Droplets for Versatile 3D/4D Printing Through Digital Light Processing (DLP)

Liquid metal (LM) and liquid metal alloys (LMs) possess unique physicochemical features, which have become emerging and functionalized materials that are attractive applicants in various fields. Herein, uniform LM nanodroplets armored by carbon dots (LMD@CDs) were prepared and exhibited high colloidal stability in various solvents, as well as water. After optimization, LMD@CDs can be applied as functional additives for the 3D/4D printing of hydrogel and cross‐linked resin through digital light processing (DLP). The light absorption of LMD@CDs not only improved the printing accuracy, but also led to the cross‐linking density differential during the post‐curing process. Base on the cross‐linking density differential of soft hydrogel and photothermal performance of the LM, the 3D printed objects can exhibit stimulus responses to both water and laser irradiation. Additionally, the CDs shell and LM core of LMD@CDs provide the printed objects interesting photoluminescence and electric conductivity capabilities, respectively. We deduce this versatile 3D/4D printing system would provide a new platform for the preparation of multi‐functional and stimuli‐responsive advance materials.


Introduction
Eutectic gallium/indium (EGaIn) is a liquid metal alloys (LMs) which exhibits both metallicity and liquid flowability that benefits from its low viscosity near or below room temperature. [1]Owing to the fact that promise innovative platforms for diverse applications depend on the high metallic thermal/electrical conductivity and superior mechanical properties, [2,3] EGaIn have attracted tremendous attention of researchers over the past decade. [4,5]Recently, LM materials have also been adopted in additive manufacturing through various methods, such as stamp printing, [6,7] direct writing, [8][9][10] masked deposition, [11,12] and inkjet/spray printing. [13,14]17] However, the incorporation of LM with light curing 3D printing and four-dimensional (4D) composite material was rarely reported.The reason may due to the special raw material requirements of printing technology. [10,11]therwise, 4D printing that is attributed to the 3D printing with smart (stimuli-responsive) materials that can be significantly transformed over time shows promising capabilities and broad potential applications. [18,19]Based on, functional materials can be an extensive nanofillers to change shape and/or properties of matrix over time, by different external stimuli, including swelling, [20] temperature, [21,22] pH, [23,24] light, [25] and electricity, [26] etc.Take resin substrate as an example, LM should be well dispersed in the resin or monomer solution with low viscosity and kept stable during the printing period. [13,27,28]The most efficient method was producing surface-modified LM droplets.Besides small molecule surfactants, such as 1-dodecanethiol (C12) [29,30] and 3-mercapto-Nnonylpropionamide (1ATC9), [30] pioneer researchers also developed strategy of surface-initiated polymerization on LM droplets to obtain macromolecular shells, such as poly(methyl methacrylate), [31,32] poly (2-dimethylamino) ethyl methacrylate [33] and ε-caprolactone. [34]However, these methods were complicated and inefficient to get complete polymer shell on the surface of LM droplets. [35]Moreover, the polymer shell has negative effect on the thermal stability and electrical conductivity of LM droplets.[41] Through the interaction of functional groups (-NH 2 , -COOH, -OH) on the surface with Ga 2 O 3 , the surface of LM could be armored by GQDs or GO to obtain spherical micronlevel LM droplets.However, LM droplets with high colloidal stability have rarely been reported to the best of our knowledge.
Liquid metal (LM) and liquid metal alloys (LMs) possess unique physicochemical features, which have become emerging and functionalized materials that are attractive applicants in various fields.Herein, uniform LM nanodroplets armored by carbon dots (LMD@CDs) were prepared and exhibited high colloidal stability in various solvents, as well as water.After optimization, LMD@CDs can be applied as functional additives for the 3D/ 4D printing of hydrogel and cross-linked resin through digital light processing (DLP).The light absorption of LMD@CDs not only improved the printing accuracy, but also led to the cross-linking density differential during the post-curing process.Base on the cross-linking density differential of soft hydrogel and photothermal performance of the LM, the 3D printed objects can exhibit stimulus responses to both water and laser irradiation.Additionally, the CDs shell and LM core of LMD@CDs provide the printed objects interesting photoluminescence and electric conductivity capabilities, respectively.We deduce this versatile 3D/4D printing system would provide a new platform for the preparation of multi-functional and stimuli-responsive advance materials.
We herein report the preparation of uniform LM nanodroplets armored by carbon dots (LMD@CDs), which exhibited high colloidal stability in various solvents, as well as water.The nanodroplets were prepared through a simple Pickering miniemulsion process without other stabilizer, and the effect of different parameters on the stability and size of LMD@CDs were discussed, including the power and time of ultrasonication, concentration of CDs and solvents.After optimization, LMD@CDs can be applied as functional additives for the 3D/4D printing of hydrogel and cross-linked resin through digital light processing (DLP).The CDs shell and LM core of LMD@CDs provide the printed objects interesting photoluminescence and electric conductivity capabilities, respectively.Owing to the different cross-linking density and photothermal performance of the LMD@CDs, the printed soft hydrogel exhibited stimulus responses to both water and laser irradiation.This LMD@CDs assistant versatile 3D/4D printing system was deduced to provide a new platform for the preparation of multifunctional and stimuli-responsive advanced materials.

Preparation and Characterization of LMD@CDs
The synthetic process of CDs and LMD@CDs is schematically described in Figure 1a.CDs were prepared through a bottom-up hydrothermal approach, using citric acid monohydrate and urea as the precursors. [42,43]The obtained CDs exhibited a near-spherical morphology with an average diameters of 3.5 AE 0.2 nm (Figure S1a, Supporting Information).The aqueous solution of CDs showed distinct absorption peak at 365 nm and a strong fluorescence emission at 475 nm under the excitation of 365 nm (Figure S1b,c, Supporting Information).FTIR analysis (Figure S1d, Supporting Information) showed the hydroxyl group (-OH) vibration peak of CDs appearing at 3430 cm −1 , the appearance of N-H at 3216 cm −1 , and C=O bonds at 1653 cm −1 .Moreover, XPS survey spectra (Figure S2, Supporting Information) confirmed the strong signals originating from C-O/C=O, C-C/C=C, and C-N which reflected the existence of -OH, -NH 2 , and -COOH groups in the CDs.The types of N for the N-doped CDs were graphic and pyrrolic N.
Subsequently, LM (100 mg) was added to the N,N-Dimethylformamide (DMF) solution in the presence of as-prepared CDs (1.0 mg mL −1 ) without any other stabilizers.After 60 min of ultrasonic (100 W), more stable suspension was achieved when compared with the control experiment without CDs.The field-emission scanning electron microscopy (FE-SEM) image (Figure 1b,c, left images and Figure S3, Supporting Information) showed that the participation of CDs (Figure 1c) made the LM droplets to be more uniform and smaller and also proved by particle-size distribution (Figure S4a, Supporting Information).Dynamic light scattering (DLS) (Figure S4b, Supporting Information) results showed that the diameter of LM droplets decreased from 1.0 μm (control) to 90 nm (with CDs), and the polydispersity index (PDI) decreased from 0.65 (control) to 0.089 (with CDs).More details could be observed from TEM image (Figure 1c, right image, and Figure S5, Supporting Information), in which the average diameters of the spherical core-shell LMD@CDs particles was 79 AE 13 nm.In addition, CDs were observed to be armored on the surface of LM nanodroplets, and formed rough surfaces.The average thickness of CDs shell was measured to be 7.3 AE 0.8 nm by the TEM image (insert of Figure 1c, right).In situ elemental mapping analysis on the isolated composite nanodroplets revealed the detailed characterizations of the chemical elements of LMD@CDs was uniformly distributed in the core (Ga, In) of the composites, and the enriched N originated from CDs layer was deposited on the outer surfaces (Figure 1d).XPS analysis was used to investigate the detailed chemical composition of LMD@CDs, in which Ga 3d spectrum gave two peaks at 20.8 and 17.6 eV originating from Ga (III) and Ga (0), reflecting the existence of gallium oxide (formulated as Ga 2 O 3 ) in the composite nanoparticles (Figure 1e).The strong N 1s and N 2p signals at 154 and 103.6 eV are presented in Figure 1f, further indicating that the liquid metal nanodroplets have been coated by double layers of Ga 2 O 3 and CDs.After coating, CDs endow several functional groups to the surface of LMD@CDs, including hydroxyl (3430 cm −1 ), carbonyl (1653 cm −1 ), and amino (3216 cm −1 ) as shown in Figure 1g.
Since there was no other stabilizer, the formation of LMD@CDs was ascribed to the Pickering emulsion process [44][45][46][47] described in Figure 1a and Figure S6, Supporting Information.The formation of LMD@CDs suggested that the process was similar to the formation of Pickering emulsions that two immiscible phases were stabilized solely by solid particles (without surface modification).The bulk LM overcomes its own surface tension under the ultrasound to form small droplets, whose surface would be oxidized quickly by oxygen to form Ga 2 O 3 shell. [48]Since the dispersion process was carried out in the presence of CDs, the Ga 2 O 3 shell will form hydrogen bond with -COOH and -NH 2 groups on the CDs.Under the combined action of hydrogen bonding and surface energy, CDs were adsorbed on LM droplets to form LMD@CDs nanodroplets.On the one hand, the CDs shell hindered the LM droplets from immediate combination.On the other hand, the abundant functional groups on the CDs endow high colloidal stability to the LMD@CDs in solvents.

Effect of Different Parameters on the Formation of LMD@CDs
Here, we investigated the effect of sonication time, power, and CDs contents on the diameter and size distribution (PDI) of LMD@CDs nanodroplets.As shown in Figure 2a, with 1.0 mg mL −1 CDs and 100 mg LM in DMF, the diameter of the nanodroplets significantly decreased from 230 to 130 nm in 30 min and the PDI decreases from 0.4 to 0.2.More details could be found in the SEM and TEM images (insert Figure 2a and Figure S7a-c, Supporting Information), and a fraction of larger droplets (ca. 1 mm) was present after short periods of ultrasonic treatment, which disappeared during prolonged treatment (insert of Figure 2a down and Figure S7b,c, Supporting Information).With further extension of ultrasonic time, the decreasing trend of diameter slowed down and nanodroplets with an average size of 89 nm was achieved after 120 min of ultrasonic treatment.Meanwhile, the PDI of the nanodroplets decreased to be 0.1.The relationship of decrease of average diameters and the ultrasonication time fits the monoexponential function ð Þ=τ (see the Supporting Information).Then the ultrasonic power was verified from 50 to 100 W (Figure 2b).It seems that ultrasonic treatment with lower power took longer time to reach the platform, but the final particle size was not affected by the power after 120 min.The content of solid particles was another important factor that affected emulsion droplet size and stability.Average diameters of LMD@CDs prepared with different Energy Environ.Mater.2024, 7, e12609 concentrations of CDs (0, 0.1, 0.2, 0.5, 1, 2 mg mL −1 , corresponding mass ratio of LM:CDs = 100:1, 50:1, 20:1, 10:1, 5:1) are shown in Figure 2c.Notably, quite a few of CDs (0.1 mg mL −1 ) would decrease the diameter of LMD droplets form 1.0 μm to 230 nm, and significantly decreased the PDI from 0.68 to 0.35.The average droplet diameter decreased with the increase of CDs contents as expected, but the change was no longer noticeable when the concentrations of CDs increased to 0.5 mg mL −1 .
The versatility of this Pickering emulsion process was proved by using different solvents both moderate polar solvents (tetrahydrofuran, acetone, and 2-propanol) and strong polar solvents (DMF, methanol, ethanol, DMSO, water).Among these solvents, the suspensions of LMD@CDs exhibited superior chemical and colloidal stability after siting for long time (>15 d, Figure S8a, Supporting Information).
Likewise, the particle size distribution and PDI of LMD@CDs prepared in different solvents were also monitored by DLS (Figure S8b, Supporting Information).The final average diameters of LMD@CDs in THF, acetone, and DMF were less than 100 nm, while the average sizes in other solvents were between 150 and 400 nm.But the PDI for LMD@CDs in all these solvents were close to 0.1, indicating the uniformity of LMD@CDs nanodroplets achieved through Pickering emulsion.
It should be noted that LM nanodroplets can be formed in aqueous media in the presence of CDs, avoiding the chemical reaction between LM and water. [38,49]As shown in Figure 2d and Figure S7d-f, Supporting Information, with 0.5 mg mL −1 CDs and 50 mg LM in H 2 O, the diameter of the nanodroplets significantly decreased to 166 nm (PDI = 0.1) in 120 min.Another method of preparing LM emulsions in aqueous media was to collect nanodroplets formed in other solvents, Energy Environ.Mater.2024, 7, e12609 such as DMF, and re-dispersed them into water.DLS results showed that the LMD@CDs nanodroplets after re-dispersed still kept small particle size and monodispersity (Figure S9, Supporting Information).The slight increase of particle size and PDI may be due to the aggregation of some droplets during the centrifugation period.Further, we verified the effect of particle size and solvent on stability (Figure 2e,f), in which the LMD@CDs suspensions were more stable (>7 days) with smaller and more uniform particle size.It can be clearly seen from Figure 2f that the particle size of the original dispersion (0 day) is mainly distributed around 90 nm, and meanwhile, the particle size distribution does not change significantly after standing for 7 days, indicating that the asprepared LMD@CDs dispersion has a good stability.As mentioned above, this is mainly due to the complete coating of the liquid metal by the carbon dots, which prevents the liquid metal from directly contacting with water and air to cause chemical reactions.In addition, rheological property was an important parameter of a colloidal solution.Figure S10, Supporting Information, shows the viscosity changes with the shear rate, which indicated that the addition of LMD@CDs making the fluid showed a slight shear thinning.When the shear rate was greater than 10 S −1 , the fluid shear viscosity did not change significantly, so it could be considered that the viscosity did not change.In other words, at this high shear rate, the LMD@CDs nanoparticle suspension behaved as a Newtonian fluid.This also proved the high colloidal stability of the LMD@CDs suspension.It was a critical factor to select more beneficial conditions for the next step of 3D printing.Moreover, the optical stability of the LMD@CDs suspension was also verified by UV and fluorescence spectra (Figure S11, Supporting Information).

Photo-Curing and 3D Printing in the Presence of LMD@CDs
The application of LMD@CDs as filler in photocurable resins were formulated using acrylamide (AAm) as monomer, diphenyl(2,4,6trimethylbenzoyl)phosphine oxide (TPO) as photoinitiator and poly (ethyleneglycol) diacrylate (PEGDA, M n = 600 g/mol) as cross-linker.We first investigated the polymerization kinetics of photocuring under violet light irradiation (λ max = 405 nm) by measuring the vinyl bond conversions via attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy.As presented in Figure 3a, the monomer conversion rate decreased with the increase of LMD@CDs concentrations in the system, as well as the elongated induction period.Notably, all test groups displayed high vinyl bond conversions (>90%) after 60 s irradiation, except for the one with 10 mg mL −1 LMD@CDs.The reason was attributed to the light absorption of LMD@CDs which hindered the penetration of light in the system.Subsequently, the photoinitiator (TPO) concentration was varied from 0.05 to 0.2 mol% of vinyl in the presence of 5 mg mL −1 LMD@CDs (Figure 3b).The monomer conversion rate highly increased with 0.2 mol% TPO, and reached 90% within 20 s.
Inspired by the rapid monomer conversion rates, the LMD@CDs system was then applied for 3D printing with a commercially available DLP 3D printer (Figure 3d, λ max = 405 nm).The objects were printed at a 20 s layer cure time, with a sliced layer thickness of 100 μm, with the formulation of [AAM]:[PEGDA600] = 50:1 in 50% (v/v) deionized water, and 5 mg mL −1 of LMD@CDs and 0.2 mol% TPO.The initial printed object was "LMD@CDs" words with the thickness of (CDs = 1.0 mg mL −1 ) in DMF.c) Changes in average diameters and PDI with CDs concentration (0, 0.05, 0.1, 0.2, 0.5, 1.0, 2.0 mg mL −1 ) under the power 100 W at 60 min of ultrasonic time.d) Average diameters and PDI of the LMD@CDs in water at the mass ratio LM: CDs = 10:1 (CDs = 0.5 mg mL −1 ).The dashed lines correspond to monoexponential fitting curves.e) Digital photographs of LMD@CDs suspensions in DMF after 0 and 30 days (top) and H 2 O after 0 day and after 7 days (down) (prepared in DMF, and re-disperse them into water).f) DLS curves of LMD@CDs in H 2 O after 0 day and 7 days.
Energy Environ.Mater.2024, 7, e12609 1.0 mm.As expected, the object retained the characteristic gray of LMD@CDs and high resolutions for prints were achieved with sharp edges which were able to be inherited from the original model.Moreover, the "LMD@CDs" word under 365 nm UV light exhibited uniform and strong blue fluorescence emission, owing to the optical properties of CDs in the printed hydrogel.To demonstrate the capability of our resins to print large, a 54.2 × 54.2 × 27.5 mm Temple of Heaven (Figure 3f) were printed and a complex objects of hollowedout letters "EGaIn" (Figure 3g) also proved the precision of the printing.The object exhibited outstanding print resolution, demonstrated by the fine details around the Temple of Heaven that were able to be replicated from the original drawing (Figure 3f).Furthermore, the precision can be achieved to 0.5 mm (Figure 3g), due to the LMD@CDs as photoblocker in the resin, avoiding light scattering during the printing.To investigate the versatile of this LMD@CDs in 3D printing, we also printed some resin objects with the formulation of tert-Butyl acrylate (tBA) as monomer, ethylene dimethacrylate (EGDMA) as cross-linker, and [tBA]:[EGDMA] = 50:1 in 30% (v/v) DMF, with 5 mg mL −1 of LMD@CDs and 1.9 mol% TPO.Three square prisms with gradually decreased dimensions, while cylinder and star were printed to prove the stability of LMD@CDs in non-aqueous 3D printing system (Figure S12, Supporting Information).

4D Behavior of Printed Objects
As the mechanical properties of our 3D printed materials were dependent on the light exposure, we decided to exploit these differences to print a material with spatially resolved properties.For the convenience of observation, two objects were printed with petal-shaped and stripshape.After printing, the objects were placed under UV for post-curing.As a result, the UV-exposed side (UL) of the hydrogel had the higher chain density and cross-linking density than the other side (BL), as LMD@CDs block the transmission of UV.The petal was then placed in a water-filled petri dish with the UL dose facing up.As expected, the petal deformed in 3 min, and gradually flattened after exposing to air to evaporate water under room temperature.According to pioneer works, [18,50,51] this reversible deformation was attributed to the different swelling ratio of UL and BL during the swelling and dehydrating processes.In addition, since the PDMAEMA is known to be a thermalsensitive polymer and LMD@CDs have excellent photothermal effect (Figures S13, S14, Supporting Information), the body part of strip hydrogel was irradiated by a near-infrared (NIR) laser beam.As shown in Figure 4g, the strip got bent with the irradiation point as the bending point, and the bending angels were measured based on the images.Figure 4h shows that the bending started from 20 s and increased rapidly within the following 40 s.After 60 s, the increase of bending angel gradually became slower.Generally, the shape variation of the printed strip was depended on the photothermal effect of LMD@CDs, which induced the rising of local temperature and led to the contraction of thermal-sensitive hydrogels.Different cross-linking degrees led to different degrees of contraction and resulting in bending.Moreover, the bending degree and speed were also affected by crosslinking degree.
In Figure S15, Supporting Information, typical stress-strain curves of PAAm and PAAm-LMD@CDs hydrogels upon stretch are presented in a normal environment.The fracture strength and elongation increase significantly by the incorporation of LMD@CDs inclusions that maximum value reach to 0.470 MPa at 218% contrasted with pure PAAm hydrogel (0.236 MPa at 70%).The conductivity properties of the hydrogels were measured by digital prototyping (Figure S16, Supporting Information).The superiorly stretchable of hydrogel makes them applied in flexible electronics with special conditions, such as robotics and large bodily motion.Based on the stretchability and conductivity of LMD@CDs, composite hydrogel by 3D printing can be used as a flexible electronic device.A piece of composite hydrogel was integrated into a closed circuit with a light emitting diode (LED), the brightness of the LED continuously decreased along with the extension of the hydrogel, indicating that the composite hydrogels have potential application as force-sensors (Figure S17, Supporting Information).

Conclusion
In summary, uniform LM droplets with nano-level particle sizes were prepared through Pickering miniemulsion process in the presence of CDs.The achieved LM droplets exhibited distinct core@shell structure with complete CDs shell.Generally, the diameter and PDI of LMD@CDs nanodroplets decreased with the increase of sonication time, and higher power and CDs contents would induce the faster achievement to minimum particle size.Due to the well dispersity of CDs in various solvents, the Pickering emulsion process could be performed in different solvents including both moderate polar solvent and strong polar solvents, as well as water.After optimization, LMD@CDs can be applied as functional additives for the 3D/4D printing of hydrogel and cross-linked resin through DLP.The system can be used for printing complex models, such as Temple of Heaven and hollow structures, and all the printed objects show high resolution.Moreover, the printed objects exhibited uniform and strong blue fluorescence emission under 365 nm UV light, owing to the optical properties of CDs in the hydrogel.The light absorption of LMD@CDs not only improved the printing accuracy, but also led to the cross-linking density differential during the post-curing process.Base on the cross-linking density differential of soft hydrogel and photothermal performance of the LM, the 3D printed objects can exhibit stimulus responses to both water and NIR laser irradiation.Combined with the stretchability and conductivity of LMD@CDs, this versatile 3D/4D printing system would provide a new platform for the preparation of multifunctional and stimuli-responsive advanced materials.

Experimental Section
Detailed information related to the synthesis of active electrodes, physicochemical characterization, and electrochemical evaluation of bifunctional electrodes towards UOR and supercapacitor application is provided in Supporting Information.

Figure 2 .
Figure 2. a) Average diameters and PDI of the LMD@CDs in DMF at the mass ratio LM: CDs = 10:1 (CDs = 1.0 mg mL −1 ) under the power 100 W. b) Average diameters as a function of ultrasonication time for adjusting to 50, 80 and 100 W at 20 °C with the mass ratio LM: CDs = 10:1 (CDs = 1.0 mg mL −1 ) in DMF.c) Changes in average diameters and PDI with CDs concentration (0, 0.05, 0.1, 0.2, 0.5, 1.0, 2.0 mg mL −1 ) under the power 100 W at 60 min of ultrasonic time.d) Average diameters and PDI of the LMD@CDs in water at the mass ratio LM: CDs = 10:1 (CDs = 0.5 mg mL −1 ).The dashed lines correspond to monoexponential fitting curves.e) Digital photographs of LMD@CDs suspensions in DMF after 0 and 30 days (top) and H 2 O after 0 day and after 7 days (down) (prepared in DMF, and re-disperse them into water).f) DLS curves of LMD@CDs in H 2 O after 0 day and 7 days.

Figure 3 .
Figure 3. Polymerization kinetics and 3D printing for resin formulations used in this work.a) vinyl bond conversion versus time under violet light at various loadings of LMD@CDs for the resin with [AAm]:[PEGDA] = 50:1 and TPO ratio 0.2 mol%.b) Vinyl bond conversions under violet light (λ max = 405 nm) irradiation for various content of TPO for the resin formula [AAm]:[PEGDA] = 50:1, LMD@CDs = 5 mgÁmL −1 .c) Optical image of a printed "LMD@CDs" word under natural light (milled) and ultraviolet light (bottom).d) 3D printing by digital light projection (DLP) 3D printing.e) Panorama photos and the close-up of Temple of Heaven was 3D printed using the system as novel resins.f) Models (Top) and 3D printed object (down) of the hollow-carving "EGaIn."g) The close-up of Figure3f, which shows that printed objects have clear higher resolution details.

Figure 4 .
Figure 4. Swelling/desolvation and NIR-induced actuation of a material 3D printed with spatially resolved light doses.a) Designed geometrical properties (left) and top view of printed object (right) of petal.b) Diagram of dual-gradient by post-curing under UV.c) Petal with layer exposed to higher light dose on the top (UL, dark blue), after 3 min in water.d) Flipped swollen petal and after 90 s dehydration (layer exposed to lower light dose on top, BL, sky blue).e) Re-swelling in water after 10 min (UL layer on top).f) Schematic illustration of transformation of petal when swelling and desolvation.g) Continuous imaging of the LMD@CDs-striped composite hydrogel under 808 nm NIR laser stimulation.h) Experimental results of bending angles of the LMD@CDs composite hydrogel under NIR.i) Simulation bending angles and scheme of actuation by NIR.j) The process of petals deforming by NIR irradiated.