A MXene Hydrogel‐Based Versatile Microrobot for Controllable Water Pollution Management

Abstract The urgent demand for addressing dye contaminants in water necessitates the development of microrobots that exhibit remote navigation, rapid removal, and molecular identification capabilities. The progress of microrobot development is currently hindered by the scarcity of multifunctional materials. In this study, a plasmonic MXene hydrogel (PM‐Gel) is synthesized by combining bimetallic nanocubes and Ti3C2T x MXene through the rapid gelation of degradable alginate. The hydrogel can efficiently adsorb over 60% of dye contaminants within 2 min, ultimately achieving a removal rate of >90%. Meanwhile, the hydrogel exhibits excellent sensitivity in surface enhanced Raman scattering (SERS) detection, with a limit of detection (LOD) as low as 3.76 am. The properties of the plasmonic hydrogel can be further adjusted for various applications. As a proof‐of‐concept experiment, thermosensitive polymers and superparamagnetic particles are successfully integrated into this hydrogel to construct a versatile, light‐responsive microrobot for dye contaminants. With magnetic and optical actuation, the robot can remotely sample, identify, and remove pollutants in maze‐like channels. Moreover, light‐driven hydrophilic‐hydrophobic switch of the microrobots through photothermal effect can further enhance the adsorption capacity and reduced the dye residue by up to 58%. These findings indicate of a broad application potential in complex real‐world environments.

For pseudo-first-order kinetic model: For pseudo-second-order kinetic model: Here,  is the reaction time,   and   are the adsorption amounts at equilibrium and reaction time,  1 and  2 are the rate constants for the pseudo-first-order and pseudo-second-order model.
The fitting coefficient R 2 value of the pseudo-second-order kinetic model is higher than that of the pseudo-first-order kinetic model (Figure S12, Table S13).Therefore, the main adsorption mechanism should be the chemical adsorption.
In addition, calculation, the saturation adsorption rate (  ) of pure Alg hydrogel and S-16 PM-Gel are 1.727 and 4.484 mg/g for R6G, respectively.The enhanced adsorption capacity is evidently dependent on the incorporation of MXene material.Considering the mass ratio of 421:1 between the PM-Gel and MXene (see experimental section), the   of Ti3C2Tx MXene for R6G is calculated to be 1196.996mg/g, as estimated below: Similarly,  __ can be estimated as 548.934 mg/g.The enhancement factor (EF) of the proposed substrate was calculated using R6G.

S-17
The calculation of EF is demonstrated as follows.
According to the literatures (J.Phys.Chem. C. 2007, 111, 13794-13803), EF is calculated by: where,   and   are the number of R6G molecules in the illuminated spot for Raman and SERS detection, respectively.The intensity (  ,   ) is the SERS and Raman intensity at 1360 cm -1 peak normalized by acquisition time, respectively.
In the past experiments (ACS Nano 2021, 15, 12996-13006), we have obtained the data of   , which was calculated to be 5.6 × 10 9 .
for MXene-loaded substrate can be estimated as where,   is the surface area of laser spot (diameter ~ 3 μm),   is the volume of microrobot (diameter ~ 2 mm),  6 is the concentration of the solution (0.01 μM),   is the volume of the solution (2 mL), while  is the adsorption rate.Here,  was identified as 20%.So   is estimated to be 5.4 × 10 6 .
On the other hand, according to Raman and SERS spectra,   is 9.4,   is 50000.So, the analytical EF can be calculated as 5.5 × 10 6 .

Figure S6 .
Figure S6.The typical SEM images of the freeze-dried PM-Gel.

Figure S8- 11 .
Figure S8-11.The extinction data of standard dye solutions of different concentrations.

Figure S12 .
Figure S12.Kinetic modelling analysis for Alg and PM-Gel.

Figure S15 .
Figure S15.Extinction spectra of anionic solutions incubated with the microrobot.

Figure S16 .
Figure S16.The calculation of average enhancement factor.

Figure S18 .
Figure S18.Investigation of the SERS activity of the microrobot for anionic dyes.

Figure S19 .
Figure S19.Changes in SERS intensity acquired by the PM-Gel under different pH

Figure S20 .
Figure S20.Changes in SERS intensity acquired by the PM-Gel after 20 days of storage.

Figure S23 .
Figure S23.The typical SEM images of the freeze-dried spherical microrobot.

Figure S27 .
Figure S27.The changes in contact angles of the PM-Gel.

Figure S5 . 9 Figure S6 .
Figure S5.(a) The typical HAADF-TEM image and (b) EDX elemental mapping of AuAgAu nanocubes.(c-d) The typical elemental mapping results of Ag and Au, respectively.The scale bars are all 20 nm.

Figure S8 .
Figure S8.(a) Extinction spectra of R6G solutions of different concentrations.(b) The intensity of the extinction peak at 526 nm varies with concentration.

Figure S10 .
Figure S10.(a) Extinction spectra of AR solutions of different concentrations.(b) The intensity of the extinction peak at 522 nm varies with concentration.

Figure S11 .
Figure S11.(a) Extinction spectra of AB solutions of different concentrations.(b) The intensity of the extinction peak at 596 nm varies with concentration.

Figure S12 .
Figure S12.Kinetic modelling analysis for Alg and PM-Gel.(a) Pseudo-first-order model.(b) Pseudo-second-order model.The corresponding kinetic equations are as follows.

Figure S14 .
Figure S14.Extinction spectra of R6G solutions incubated with (a) pure alginate hydrogel and (b) MXene-doped microrobots for different time.Extinction spectra of MB solutions incubated with (c) pure alginate hydrogel and (d) MXene-doped microrobots for different time.

Figure S15 .
Figure S15.Extinction spectra of (a, c) AR or (b, d) AB solutions incubated with MXene-doped microrobots for different time.

Figure S16 .
Figure S16.The spectra of R6G detected by different substrates.

Figure S19 .
Figure S19.SERS intensity acquired by the PM-Gel soaked in R6G solutions (10 μM) with (a) various pH values and (b) different temperature.

Figure S20 .
Figure S20.Changes in sensing performance of the PM-Gel after a 20-day storage at 20℃ in air.The detection targets are all R6G solutions of 10 μM.

Figure S21 .
Figure S21.Reusability of the PM-Gel for R6G dye adsorption (10 μM).Before each assay, a 1-hour soak in ethanol was used to reset the hydrogel.

Figure S22 .
Figure S22.(a) The structure diagram of a spherical robot powered by plasmonic MXene hydrogel containing γ-Fe2O3 nanoparticles.(b) The schematic diagram of the cross-linked structure of the proposed hydrogel containing γ-Fe2O3 nanoparticles.

Figure S23 .
Figure S23.The typical SEM images of the freeze-dried spherical microrobot.

Figure S26 .
Figure S26.The hysteresis curves of γ-Fe2O3 nanoparticles.The inset photo is the PM-Gel-based microrobots dragged aside by a magnet.

Figure S27 .
Figure S27.The changes in contact angles of the PM-Gel.The power density of the light is 3 W/cm 2 .

Table S28 .
Overall features of our work in comparison with other typical materials.

Table S13 .
Overview of the Kinetic Parameters

Table S28 .
Overall features of our work in comparison with other typical materials.