Customizing oil-wettability in air—without affecting extreme water repellency

Avijit Das a and Uttam Manna *ab
aDepartment of Chemistry, Indian Institute of Technology-Guwahati, Kamrup, Assam 781039, India. E-mail: umanna@iitg.ac.in
bCentre for Nanotechnology, Indian Institute of Technology-Guwahati, Kamrup, Assam 781039, India

Received 14th August 2020 , Accepted 22nd October 2020

First published on 22nd October 2020


Lotus leaf inspired superhydrophobic interfaces strongly repel the aqueous phase—but inherently display super-oil-affinity in air. However, superamphiphobic interfaces repel both the aqueous phase and the oil/oily phase strongly, due to their contact angles of above 150°. The fundamental criteria for optimizing such distinct super liquid wettabilities are different. Thus, in the past, distinct synthetic approaches were adopted to achieve these two different types of liquid-repellent interfaces for different prospective and relevant applications. Here, in this communication, a rapid and scalable spray deposition process is introduced for tailoring different oil-wettabilities in air, without perturbing the superhydrophobicity. An appropriate dilution of a reaction mixture of strategically selected two small molecules that readily reacted through the 1,4 conjugate addition reaction provided a facile basis for customizing oil wettability—starting from superoleophilicity to superoleophobicity, keeping intact the super water repellence. The synthesized superhydrophobic and superamphiphobic interfaces remained efficient for sustaining exposures of various practically relevant physical manipulations and abrasions and chemically complex aqueous phases. Furthermore, both the superhydrophobic and superamphiphobic interfaces were successfully extended for comparing the oil/water separation, anti-fouling and self-cleaning performances. Such a simple and common synthetic approach for preparing extremely water repellent interfaces that have differences in oil-wettability in air would be useful for practically relevant outdoor applications.


Introduction

Lotus leaf inspired interfaces that simultaneously display extreme repellence towards the aqueous phase and a super affinity for the oil/oily phase are widely recognized as superhydrophobic interfaces,1–10 whereas interfaces that strongly repel both water and oil phases with contact angles of above 150° are formally defined as superamphiphobic interfaces.11–20 While a hierarchically featured interface with a low surface energy atop provides contrasting and extreme wettability for water and oil/oily phases,1 superamphiphobic interfaces demand more specific structural features apart from regular co-optimization of hierarchical topography and low surface energy.11 Thus, the design of a superamphiphobic interface is more challenging, and generally, distinct strategies have been adopted to synthesize superhydrophobic/superoleophilic and superamphiphobic interfaces.1–20 In the past, generally different polymers and metal oxides have been used to achieve the essential topography, and further co-optimization of the essential low surface energy conferred the desired super-liquid repellent interfaces.1–12 The separate association of superoleophilicity or superoleophobicity with superhydrophobicity is important for diverse and relevant applications.1–20 For example, a superhydrophobic interface that is embedded with a super-oil-affinity allows selective filtration-based oil/water separation,21,22 whereas the self-cleaning and antifouling performances of superamphiphobic interfaces remain superior over those of lotus leaf inspired superhydrophobic interfaces.11–20 Nevertheless, a common synthetic approach for the selective and controlled integration of the desired oil-wettability with an extremely water-repellent interface is unprecedented.

In this communication, two strategically selected small molecules—(3-aminopropyl)trimethoxysilane (APTMS) and 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-heneicosafluorododecyl acrylate (HFDDA) (Scheme 1A) that readily react in toluene through a 1,4 conjugate addition reaction (Scheme 1B) under ambient conditions provide a simple and rapid basis for the controlled tailoring of the desired oil wettability without altering the embedded superhydrophobicity, as shown in Scheme 1C. A simple dilution of the reaction mixture of the selected two small molecules, prior to spray deposition, allows a single step co-optimization of the essential topography and chemistry to achieve three distinct liquid-repelling coatings, where oil wettability varies from superoleophilicity to superoleophobicity—without perturbing the superhydrophobicity, as shown in Scheme 1C. The optically transparent and extremely water repellent coatings that were embedded with tailored oil-wettability in air, continued to display an unaltered extreme liquid repellence even under various practically relevant and severely challenging settings, including the exposure of physical manipulations, abrasions, chemically complex liquids, high temperature, UV-radiation, etc. Furthermore, such interfaces were extended to compare oil/water separation, self-cleaning and antifouling performances.


image file: d0nr05964a-s1.tif
Scheme 1 (A and B) Schematic illustration for the rapid and facile tailoring of three different types of extremely water repellent interfaces having differences in oil-wettability in air through the rational use of a 1,4-conjugate addition reaction between two strategically selected small molecules (3-Aminopropyl)trimethoxysilane (APTMS) and 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-Heneicosafluorododecyl acrylate (HFDDA). (C) Depending on the dilution of the reaction mixture of the selected small molecules, a simple and common spray deposition process provided superhydrophobic/superoleophobic, superhydrophobic/oleophobic and superhydrophobic/superoleophilic interfaces.

Results and discussion

In the past, the 1,4-conjugate addition reaction between amine and acrylate was an elegant avenue for synthesizing functional polymeric coatings.23–27 Recently, our lab extended this chemical approach for the controlled optimization of both (1) water wettability in-air and (2) oil-wettability under water through a strategic selection of the post covalent modification of a chemically reactive multi-layered coating of polymeric nanocomplexes.26,27 In the earlier reported approach, bio-inspired (lotus leaf and fish-scale) liquid (water and oil) wettabilities were acheived by adopting two distinct chemical optimization processes.26,27 In contrast to the earlier report, here two selected small molecules—APTMS and HFDDA—were mixed with a molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 to allow the spontaneous 1,4-conjugate addition reaction between amine and acrylate groups at ambient condition. In this current design, HFDDA is expected to provide an appropriate low surface energy and APTMS that recognized for facile polymerization28,29 (as evident from the infra-red (IR) spectral signature for the Si–O–Si bridge30 at 1028 cm−1 (Fig. S1)) is likely to allow a uniform and robust coating. Thereafter, standard and widely accepted Fourier-transform infrared attenuated total reflectance (FTIR-ATR) spectra were recorded at a regular time interval to monitor the progress of the chemical reaction between selected small molecules (HFDDA and APTMS) through 1,4-conjugate addition reaction. The characteristic IR signatures for (a) the C–H deformation of the β carbon of a vinyl group and (b) the stretching of the carbonyl moiety at 1410 and 1730 cm−1, respectively, confirmed the existence of acrylate groups in the reaction mixture of APTMS and HFDDA at t = 0 min, similar to the solution of HFDDA (Fig. 1A). During the course of the 1,4 conjugate addition reaction between the acrylate and amine groups, only the vinyl moiety of the acrylate group was compromised and the carbonyl moiety remained unperturbed at the end of the reaction. As a result, with progression of the reaction between the selected reactants (APTMS and HFDDA), the IR peak intensity at 1410 cm−1 gradually reduced, and a significant depletion of the IR peak intensity was noted at t = 20 min, as shown in Fig. 1A. Parallelly, the IR signature at 1601 cm−1 for N–H bending for primary amine groups of APTMS completely disappeared in the reaction mixture at t = 20 min. Further, 1H NMR studies (Fig. S2) revalidated the successful and complete reaction between the two reactants through a 1,4 conjugate addition reaction. The characteristic signal for the vinyl moiety (δ 5.5–7.0) was not observed after the reaction between the selected small molecules (Fig. S2). Thereafter, the reaction mixture was spray deposited on a selected fibrous substrate that readily soaks up both oil and water in air. After air-drying of the deposited reaction mixture, the coated fibrous substrate displayed super repellence to liquids having both high (water: 72 mN m−1) and low (Dodecane (a liner hydrocarbon): 25 mN m−1) surface tensions. The droplets of water and dodecane (DD) beaded with CAs of 160° and 155° (Fig. 1B–D), respectively, and the beaded droplets rolled away on tilting the interface below 10° (Fig. S3A–F). Further, the facile and rapid deposition of the superamphiphobic coating is demonstrated in Movie 1. The submerged and shiny interface (Fig. S4A and B) of the synthesized superamphiphobic coating in both the water and the DD revealed the appropriate entrapment of an external third phase—air, which provided a heterogeneous and extreme water and oil wettability.1,11 Thereafter, the reaction mixture was diluted 2 times of its initial concentration, keeping the composition (molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1) of the selected reactants unaltered, and this specific dilution is denoted as ‘Dilution-I’ in the rest of the text. The spray deposition of this diluted reaction solution yielded a superhydrophobic, but oleophobic interface (Fig. 1E), where the water droplet beaded with a CA of ∼158° and the droplet of DD wet the same interface with a CA of ∼114°, as shown in Fig. 1F and G. On increasing the dilution of the reaction mixture 4 times (which is referred as Dilution-II), the same spray deposition process provided a lotus-leaf inspired superhydrophobic interface, where water was strongly repelled with a CA of ∼157°—but the droplet of DD immediately spread with a CA of 0°, as shown in Fig. 1H–J. Thus, the same reaction mixture of small molecules with identical composition provided three different types of water repellent coatings, depending on the appropriate dilution of the same reaction mixture, prior to spray deposition. Further, the liquid wettabilities of the small molecules derived all three types of extremely water repellent coatings having differences in their oil-wettability (i.e. superoleophilicity and oleophobicity and superoleophobicity) were examined using other various liquids including alcohol (EtOH), toluene, (1,2-dichloroethane (DCE)), diiodomethane (CH2I2), PEG and glycerol, as accounted in Fig. S5. However, this interface fails to display extreme repellence of liquids that have a surface tension below 23 mN m−1. Furthermore, the small molecules derived all three types of coatings embedded with a distinct combination of both oil- and water-wettability, remained highly optically transparent (99%, Fig. S6), very similar to bare glass, as shown in Fig. 1K. The past studies validated the difference in the requirement of topography towards super water/oil wettabilities. While the lotus leaf inspired superhydrophobic interfaces require a hierarchical topography with an arbitrary arrangement of nano/micro-domains,31 the springtails-inspired superoleophobic interfaces mostly demand a more specific and re-entrant texture.11,16 In our current-study, the small molecules derived all three different coatings that displayed completely distinct oil-wettabilities in air have been examined under a scanning electron microscope (SEM). The topography of the coatings on the feature-less fibrous substrates (Fig. 1L) was observed to be significantly different (Fig. 1M–O) depending on the dilution of the reaction mixture, where the composition of the selected reactants remained unperturbed. The reaction mixture—without any dilution—provided an appropriate topography that conferred both superhydrophobicity and superoleophobicity in air (Fig. 1M), whereas the same reaction mixture obtained after ‘Dilution-II’, yielded an interface with a random arrangement of granular domains and displayed superhydrophobicity and superoleophilicity in air, as shown in Fig. 1O. Thus, this simple study revalidated the crucial role of topography for achieving the superoleophobicity. Again, the energy-dispersive X-ray spectroscopic (EDX) analysis (Fig. S7) of the coated substrate confirmed the presence of key elements (F and Si) that belong to the used reactants (HFDDA and APTMS) in the reaction mixture, irrespective of the dilution of the deposition solution. However, a change in the density of HFDDA is expected with the dilutions of the reaction mixture. The percentage of fluorine in the coated fibrous substrates decreased with dilution of the reaction mixture, as confirmed by elemental analysis (Fig. S7). Thus, both the change in the topography and the fluorine chemistry (HFDDA) played crucial roles in tailoring the oil wettability. Furthermore, the small molecule derived coating on the rigid and flat substrates (glass, Al-foil and silicon wafer) provided another anti-fouling property—and the coated substrate became highly slippery to beaded droplets of water and dodecane, as shown in Fig. S8. However, the same beaded liquid droplets of water and dodecane were either immediately spread or firmly adhered to the uncoated selected substrates (Fig. S9).
image file: d0nr05964a-f1.tif
Fig. 1 (A) The Fourier-transform infrared attenuated total reflectance (FTIR-ATR) spectra accounting the 1,4 conjugate addition reaction in the reaction mixture (1[thin space (1/6-em)]:[thin space (1/6-em)]1) of HFDDA (yellow) and APTMS (violet) in toluene, where FTIR-ART spectra were recorded at regular time intervals, including 0 min (blue), 10 min (red) and 20 min (black). In the FTIR-ATR spectra, the IR peaks at 1730 cm−1, 1601 cm−1 and 1410 cm−1 characterize the signatures for C[double bond, length as m-dash]O stretching, primary N–H bending and C–H deformation of the β-carbon of the vinyl group, respectively. (B–J) Digital images (B, E and H) and contact angle images (C, D, F, G, I and J) of beaded water (C, F and I) and beaded dodecane (D, G and J) droplets on the three distinct water repellent interfaces: (1) superhydrophobic/superoleophobic (B–D), (2) superhydrophobic/oleophobic (E–G) and (3) superhydrophobic/superoleophilic (H–J), where the reaction solutions of the selected small molecules were spray deposited on the selected fibrous substrate without dilution (B–D) and with dilution-I (E–G) & dilution-II (H–J). (K) Optical transmittance spectra of uncoated (black) and coated (red, blue and grey) glass slides, where the coating was achieved by depositing the reaction mixture without dilution (grey) and with dilution-I (blue) & dilution-II (red). (L–O) FESEM images of bare fabric (L) and small molecule derived coated fabric (M–O), where the coatings were achieved on the selected fibrous substrate by spray deposition of the reaction solution without dilution (M) and with dilution-I (N) & dilution-II (O).

The durabilities of both lotus-leaf-inspired superhydrophobic and superamphiphobic interfaces were investigated in detail, for comparing their performances at practically relevant different and difficult circumstances, including exposures to various physical manipulations, physical abrasions, UV-light, high temperature, extremes of pH, sea water, etc. Firstly, both the superhydrophobic and superamphiphobic interfaces were separately exposed to creasing, twisting, tissue paper wiping and finger wiping multiple times, prior to examining the liquid wettability with droplets of water and dodecane. The treated interfaces repelled both the beaded droplets of water and of dodecane with a contact angle above 155°, as shown in Fig. 2A–G. Similar to the superamphiphobic interface, the synthesized superhydrophobic coating also displayed an uninterrupted super water repellence. Thereafter, more challenging and different physical abrasive exposures were incurred on both the superhydrophobic and superamphiphobic coatings. Some arbitary physical scratches were introduced on the freshly prepared liquid-repellent interfaces, using a sharp knife, as shown in Fig. 2H. The physically damaged interfaces displayed extreme oil and water repellence with CAs above 150° (Fig. 2I and Fig. S10A, B), and the beaded droplets of water and organic solvents readily rolled off on physically damaged superamphiphobic interfaces, as demonstrated in Movie 2. Next, a freshly exposed adhesive tape was separately applied on both the superhydrophobic and superamphiphobic interfaces with an external load of 25 kPa that facilitated a uniform contact between the liquid-repellent interface and the adhesive surface. The embedded and extreme liquid-wettability remained unperturbed, even after the adhesive tape peeling test (Fig. 2J, K, Fig. S10C, D and Movie 3). Furthermore, an abrasive sandpaper was rubbed on the small molecule derived superamphiphobic interfaces with a distance of 250 cm; however, the droplets of water and dodecane beaded on the sandpaper treated superamphiphobic interface with CAs above 150° (Fig. 2L, M and Fig. S10E, F), and, further, the beaded droplets immediately rolled off on a tilted interface, as shown in Movie 4. The topography of the coated fibrous substrate was examined with SEM after incurring different physical manipulations (creasing, twisting, tissue paper wiping and finger wiping) and physical abrasive exposures—including adhesive tape peeling, sand-paper abrasions and knife tests. As shown in Fig. S11. The topography of the coating that was exposed to different physical manipulations and the adhesive tape peeling test remained almost unaltered. As expected, some changes in the topography of the coating were noted after the treatment with severe abrasive exposures—i.e. sandpaper abrasion and knife tests (Fig. S11C and D). A very similar durability was observed for the synthesized superhydrophobic interface as well (Fig. S12A–I). Next, small molecule derived and physical-abrasion tolerant superhydrophobic and superamphiphobic interfaces were exposed to UV light (254 nm and 365 nm) and a relatively high (80 °C) temperature for 10 days. However, both the superhydrophobic and superamphiphobic interfaces remained capable of sustaining such treatments without perturbing their embedded super oil/water wettability, as shown in Fig. 2N and Fig. S13. Furthermore, both the water-repelling interfaces that embedded with either superoleophilicity or superoleophobicity were exposed to various and complex aqueous phases, including DI water, highly acidic (pH 1) & alkaline (pH 12) media, artificial sea-water, river water (Brahmaputra, Guwahati, and Assam) and a surfactant contaminated aqueous phase for 10 days. Even after such prolonged and harsh treatments, both the small molecule derived superhydrophobic (Fig. S13) and superamphiphobic (Fig. 2O) coatings continued to perform with unaltered super-liquid (oil/water) repellence, where the contact angle of the beaded droplets of water and dodecane remained above 150°. The small molecule derived superamphiphobic coating successfully sustained the standard washing test for 25 times, as shown in Fig. S14. Thus, the strategic use of a 1,4-conjugate addition reaction between selected small molecules followed by a simple spray-deposition process yielded highly tolerant water-repellent interfaces that were embedded with tailored oil-wettability in air.


image file: d0nr05964a-f2.tif
Fig. 2 (A–D) Digital images depicting the various physical manipulations, including creasing (A), twisting (B), tissue paper-wiping (C) and finger-wiping (D) on the synthesized superamphiphobic coating. (E–G) Digital image (E) and contact angle images (F and G) of the beaded water (F) and dodecane (G) droplets on the superamphiphobic fabric after incurring creasing, twisting, finger wiping, and tissue paper wiping tests. (H, J and L) Digital images of the experimental setup of knife scratching test (H), adhesive tape test (J), and sandpaper abrasion test (L) on the superamphiphobic coating. (I, K and M) Digital images of beaded water and dodecane droplets on a superamphiphobic interface after performing the knife scratching test (I), adhesive tape test (K), and sandpaper abrasion test (M). (N and O) Plots accounting the impact of practically relevant various harsh exposures on the superamphiphobic fabric (no dilution) including UV irradiation (at λmax = 254 and 365 nm) and high (80 °C) temperature, DI water, highly acidic (pH 1) & alkaline (pH 12) media, artificial-sea water, river water (Brahmaputra, Guwahati and Assam), and surfactant contaminated aqueous phase (SDS, CTAB, Triton-X; 2 mM), for 10 days.

A high tolerance on the embedded extreme liquid repellence in the synthesized superhydrophobic and superamphiphobic coatings is appropriate for achieving an uninterrupted performance at practically relevant diverse outdoor scenarios. The abilities of both the synthesized superhydrophobic and superamphiphobic interfaces were extended to compare their abilities for some important and relevant applications—including oil/water separation, self-cleaning and anti-fouling of different liquids. The superamphiphobic interface that strongly repelled various oil/oily and aqueous phases is highly useful for preventing fouling caused by spillages of various commercially available and relevant aqueous and oily phases, including juice, coffee, milk, kerosene, petrol and vegetable oil, as demonstrated in Fig. 3A–L and Movie 5. In comparison to superamphiphobic coating, the superhydrophobic interface remained only effective to prevent aqueous-fouling (Fig. 3M–R), but the same interface failed to prevent fouling caused by oil/oily spillages, as shown in Fig. 3S–X—likely due to the existence of an embedded super-oil-affinity in the lotus-leaf inspired superhydrophobic coatings (Fig. 1H–J). Next, the self-cleaning ability of both interfaces was compared under streams of both water and oil. The synthesized superamphiphobic coating displayed an ability for self-cleaning of the deposited dust and dirt under the exposures of both water and oil phases, and at the end, a dry and clean interface was recovered, as shown in Fig. 4A–F and Movie 6. In comparison, the superhydrophobic interface lost the self-cleaning ability under the exposure of the oil-stream (Fig. 4J–L), though it remained efficient to self-clean the deposited dust and dirt under aqueous exposure, as shown in Fig. 4G–I and Movie 6. In the past, lotus-leaf inspired strongly water repellent interface embedded with superoleophilicity have been explored for gravity-driven oil/water separation.21,22 The small molecule derived superhydrophobic coating remained efficient for the selective filtration of the oil phase from an oil/water mixture, as demonstrated in Fig. 4P–R. A three-phase oil/water mixture that consists of heavy model oil (DCE, pink-coloured bottom layer), aqueous layer (blue coloured-middle layer) and petrol (yellow-coloured top layer) poured in the lab-made prototype, where the superhydrophobic fibrous interface was used as a selective oil-filtrating membrane. As expected, the extreme water repellence restricted the passage of the aqueous phase but the embedded super oil-affinity allowed the oil/oily phase to selectively pass through the synthesized superhydrophobic membrane, as shown in Fig. 4P–R and Movie 7. In contrast, the synthesized superamphiphobic interface that strongly repelled both oil and aqueous phases prevented the passage of both the phases. Eventually, the superamphiphobic interface became inappropriate for environmentally friendly and gravity-driven oil/water separation, as shown in Fig. 4M–O. Thus, both the superhydrophobic and the superamphiphobic interfaces associated with contrasting oil wettability have a superior performance over one another, depending on their strategic and relevant uses. Further, the superamphiphobic coating can be achieved on various fibrous substrates following the simple and scalable spray deposition of the reaction mixture (Fig. S15).


image file: d0nr05964a-f3.tif
Fig. 3 (A–L) Digital images depicting the successful antifouling performance of superamphiphobic coating, where the stream of various aqueous phases including cold (4 °C) juice (A and B), hot (60 °C) coffee (C and D), cow's milk (E and F) and commonly used oil/oily phases including kerosene (G and H), petrol (I and J), and veg oil (K and L), were poured on the synthesized coating. (M–X) Digital images depicting both the success (M–R) and failure (S–X) in antifouling performance of superhydrophobic coating (prepared following dilution-II) with various aqueous phases including a stream of cold (4 °C) juice (M and N), hot (60 °C) coffee (O and P), cow's milk (Q and R) and with various oil/oily phases including kerosene (S and T), petrol (U and V), and veg oil (W and X).

image file: d0nr05964a-f4.tif
Fig. 4 (A–F) Illustrating the self-cleaning of deposited dust and dirt on the superamphiphobic fabric under the exposures of water (A–C) and veg oil (D–F). (G–L) Digital images illustrating the success in the self-cleaning performance of the superhydrophobic fabric with water (G–I) and the failure in the self-cleaning performance of the superhydrophobic fabric with vegetable oil (denoted as veg oil; J–L). (M–O) Digital images depicting the failure in the selective separation of oil from a three-phase (light oil/water/heavy oil) oil–water mixture using a superamphiphobic coating, where superamphiphobicity prevented the passage of both oil and aqueous phases. (P–R) Digital images illustrating the successful separation of three phases (light oil/water/heavy oil) oil–water mixtures using a superhydrophobic coating, where extreme water repellency prevented the passage of the aqueous phase—but the embedded super-oil-affinity allowed the selective passage of the oil phase under a gravitation force.

Conclusion

In conclusion, here, a facile and common synthetic approach has been introduced for achieving different water repellent coatings that are embedded with a distinct oil-wettability in air. A 1,4-conjugate addition reaction between rationally selected small molecules at ambient conditions, followed by spray deposition, allowed us to co-optimize essential topography and chemistry for achieving optically transparent, durable and different oil-wettability (in air) starting from superoleophilicity to superoleophobicity, keeping the embedded superhydrophobicity unperturbed. The synthesized superhydrophobic and superamphiphobic interfaces remained capable of sustaining a diverse range of practically relevant challenging exposures. Further, such interfaces were extended for comparing their performances towards various important and relevant applications. Such an elegant synthetic strategy could be useful in developing various functional interfaces.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The generous financial support from the Science and Engineering Research Board (CVD/2020/000018), Government of India is acknowledged. We thank CIF and the Department of Chemistry, Indian Institute of Technology-Guwahati, for their generous assistance in executing various experiments and for the infrastructure. We thank Dr Akshai Kumar Alape Seetharam for providing us with dodecane. Mr Avijit Das is grateful for a CSIR SRF fellowship (Award No. 09/731(0148)/2015-EMR-1).

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Footnote

Electronic supplementary information (ESI) available: The attached ESI with detailed experimental procedures, illustrating the polymerization of APTMS, rolling of beaded droplets of water and dodecane, entrapped air in the synthesized coatings, optical transparency of the coatings, wettability after incurring various and standard durability tests, and coating on various fibrous substrates. See DOI: 10.1039/d0nr05964a

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