3.3.3. Oil-Absorbance and Oil/Water-Separation Experiments with Hydrophobic Coated Cotton-Textile

Because of its remarkable water-repellent performance, **P4b**-coated textiles were then used for oil-absorbance tests and oil/water-separation experiments, simulating the removal of oily pollutants from aqueous phases. Two oily phases, tetradecane (TDC) and olive oil, colored with Disperse red 13, were used as non-volatile model pollutants upon the addition of 15 mL of distilled water. Afterwards, **P4b**-coated cotton fibers of known weight (dry) were soaked in the oil phase for 15 s, taken out, allowed to drain for 3 h, and weighted again. The coated fibers showed a 127% weight increase with TDC (25% for pristine cotton), and a 172% weight increase with olive oil (94% for pristine cotton). In accordance with this, SEM images of the fibers before and after absorbing TDC and olive oil reveal differences. A piece (1 × 2 cm) of cotton@**P4b** was also placed on top of a 20 mL vial, and used as a filter for phase separation of a Miglyol® 840 colored with a red-Disperse-13 and distilledwater mixture (1:1). Immediately after deposition with a syringe, oil quickly saturated the coated-cotton passing through, while water was retained at the top. Another piece of cotton@**P4b** (1 × 1 cm) was also successfully used to recover a microemulsion (20 µm average droplet-size) of Miglyol® 840 colored with Disperse red 13 in water stabilized with sodium dodecyl sulfate (SDS). For this, the textile was submerged in a 10× diluted aliquot of the stock emulsion, gently stirred by hand for 5 min, and subsequently taken out of the treated emulsion and left to dry in air. The coated cotton absorbed up to 97% of its own weight, and therefore acquired a remarkable red color, whereas the weight increase for the uncoated cotton was only 8% (Figure 2).

#### 3.3.4. Fluorescent Coating and Characterization of Glass Surface and Textile Pieces

We also tested our approach by replacing the alkyl chains with the fluorescein derivatives **4a**, **P4a** and **P6a,** using the experimental procedure previously described, but replacing the CH2Cl<sup>2</sup> with acetone. After rinsing the slides well, the fluorescent images of an inverted microscope using an Alexa Fluor 488 filter revealed that only the **P4a**-coated glass showed continuous intense fluorescence on the whole surface. The **4a**-coated glass slide showed low fluorescent intensity concentrated in some aggregates, while the styrenic derivative **P6a** was completely removed, as the final surfaces did not show fluorescence. Similar results were obtained by repeating the protocol with cotton and textiles, as shown in Figure 3.

#### 3.3.5. Dual-Modulated Hydrophobic and Fluorescent Coating of Textile Pieces

In addition to a multifunctional character, the fluorescent groups also provided higher polarity (WCAs of 31◦ and 34◦ for cotton and textiles, respectively), a fact that was then used to modulate the hydrophobic/hydrophilic balance of the textiles. For this, oligomers with different and defined **4a**/**4b** monomer ratios (80:20, 60:40, 40:60 and 20:80) were obtained in good yields (close to 65% in all the cases) following the same protocol previously described for **P4a** and **P4b** (see Supporting Information for experimental details and characterization). Afterwards, cotton fibers were coated with the resulting **C(4a-4b)** oligomers from different ratios upon immersion in ~7 mM solutions in CH2Cl<sup>2</sup> HPLC-grade for 4 h. Once removed from the solution, they were washed three times with fresh acetone/CH2Cl2, and dried in a gentle flux of argon. Interestingly, WCAs systematically increased from 0◦ to 120◦ , with the amount of the aliphatic derivative **4b** present in the **C(4a-4b)** oligomer (see Figure 3).

**Figure 2.** Top: Selective oil removal: (**1**) olive oil/water separation-phases; (**2**−**5**) soaking of a cotton@**P4b**. Middle: SEM images of pristine cotton weaves (**1**), cotton@**P4b** (**2**), and after absorption of TDC (**3**) and olive oil (**4**). Bottom: (**Left**) Sequence of an oil/water-phase separation by filtration with cotton@**P4b**. Water phase is retained on the top, whereas oil phase (in red) passes through our household filter into the vial. (**Right**) Oil-phase saturation onto coated cotton weaves (left), and water-repellent character (right). 3.3.4. Fluorescent Coating and Characterization of Glass Surface and Textile Pieces **Figure 2.** Top: Selective oil removal: (**1**) olive oil/water separation-phases; (**2**–**5**) soaking of a cotton@**P4b**. Middle: SEM images of pristine cotton weaves (**1**), cotton@**P4b** (**2**), and after absorption of TDC (**3**) and olive oil (**4**). Bottom: (**Left**) Sequence of an oil/water-phase separation by filtration with cotton@**P4b**. Water phase is retained on the top, whereas oil phase (in red) passes through our household filter into the vial. (**Right**) Oil-phase saturation onto coated cotton weaves (**left**), and water-repellent character (**right**). *Biomimetics* **2022**, *7*, x FOR PEER REVIEW 10 of 13

We also tested our approach by replacing the alkyl chains with the fluorescein deriv-

**Figure 3.** Top: Polymerization with the synthesized monomers **4a** and **4**b, to obtain final copolymers with two main properties: fluorescence and hydrophobicity (Scale bar: 200 m). Middle: images from an optical/fluorescence microscope of cotton fibers coated with copolymer 80:20 (**4b**:**4a**) (left), and homopolymer **p4a** (right). Bottom: WCA values of coated cotton weaves with some copolymers C8-9. **Figure 3.** Top: Polymerization with the synthesized monomers **4a** and **4**b, to obtain final copolymers with two main properties: fluorescence and hydrophobicity (Scale bar: 200 µm). Middle: images from an optical/fluorescence microscope of cotton fibers coated with copolymer 80:20 (**4b**:**4a**) (left), and homopolymer **p4a** (right). Bottom: WCA values of coated cotton weaves with some copolymers C8-9.

**C(4a-4b)** oligomer (see Figure 3).

**4. Conclusions**

3.3.5. Dual-Modulated Hydrophobic and Fluorescent Coating of Textile Pieces

In addition to a multifunctional character, the fluorescent groups also provided higher polarity (WCAs of 31°and 34°for cotton and textiles, respectively), a fact that was then used to modulate the hydrophobic/hydrophilic balance of the textiles. For this, oli-

were obtained in good yields (close to 65% in all the cases) following the same protocol previously described for **P4a** and **P4b** (see Supporting Information for experimental details and characterization). Afterwards, cotton fibers were coated with the resulting **C(4a-4b)** oligomers from different ratios upon immersion in ~7 mM solutions in CH2Cl<sup>2</sup> HPLCgrade for 4 h. Once removed from the solution, they were washed three times with fresh acetone/CH2Cl2, and dried in a gentle flux of argon. Interestingly, WCAs systematically increased from 0° to 120°, with the amount of the aliphatic derivative **4b** present in the

We designed a novel catechol-based modular synthetic approach to control the wettability of surfaces in a straightforward and systematic manner, using colorless coatings. For this, we used a unique basic scaffold, pentaerythritol tetrakis(3-mercaptopropionate)

## **4. Conclusions**

We designed a novel catechol-based modular synthetic approach to control the wettability of surfaces in a straightforward and systematic manner, using colorless coatings. For this, we used a unique basic scaffold, pentaerythritol tetrakis(3-mercaptopropionate) **1**, conjugated with both a catechol unit and a functional group of two thiol groups, while leaving the two additional free thiols available for polymerization, through the formation of disulfide bridges. As a proof of concept, we synthesized oligomers that confer a hydrophobic and/or fluorescent character to the surface of glass slides and cotton/textile weaves. Hydrophobic fabrics were, in fact, successfully tested on simulated oil-spill and emulsion samples. Moreover, the proper selection and combination of building block units combining both functionalities allowed us to systematically fine-tune at will the wettability of surfaces. All in all, the modular character of our synthetic approach and its rich and flexible chemistry open new opportunities for the development of colorless coatings with tailored properties. This is so thanks to the presence of the catechol moiety, which plays an important role in the adhesion processes, resulting in robust coatings even after vigorous washing processes.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/biomimetics8010003/s1.

**Author Contributions:** Conceptualization, J.M.-A., D.R.-M., F.B. and R.A.; methodology, J.M.-A., D.R.-M. and R.A.; formal analysis, C.C., J.M.-A. and M.M.-V.; investigation, C.C., A.L.-M. and M.M.-V.; resources, D.R.-M., F.B. and R.A.; data curation, C.C. and A.L.-M.; writing—original draft preparation, C.C., D.R.-M. and F.B.; writing—review and editing, all the team; supervision, D.R.-M., F.B. and R.A.; project administration, D.R.-M., F.B. and R.A.; funding acquisition, D.R.-M., F.B. and R.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors would like to acknowledge the funding support from grants: PID2019- 106403RB-I00 and PID2021-127983OB-C21 funded by MCIN/AEI/10.13039/501100011033, co-financed with the European Fund for Regional Development (FEDER). and by ERDF's A way of making Europe. The ICN2 is funded by the CERCA programme/Generalitat de Catalunya. The ICN2 is supported by the Severo Ochoa Centres of Excellence programme, and grant SEV-2017-0706 is funded by MCIN/AEI/10.13039/501100011033.

**Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We thank Salvio Suárez for AFM images.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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