Modulation of wettability, gradient and adhesion on self-assembled monolayer by counterion exchange and pH

Journal of Colloid and Interface Science 512 (2018) 511–521

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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Modulation of wettability, gradient and adhesion on self-assembled monolayer by counterion exchange and pH Jaroslav Mosnácˇek a,b, Anton Popelka a, Josef Osicka a, Jaroslav Filip c, Marketa Ilcikova a,b, Jozef Kollar a,b, Ammar B. Yousaf a, Tomas Bertok d, Jan Tkac d, Peter Kasak a,⇑ a

Center for Advanced Materials, Qatar University, P.O. Box 2713, Doha, Qatar Polymer Institute, Slovak Academy of Sciences, Dubravska cesta 9, 845 41 Bratislava, Slovak Republic Department of Environment Protection Engineering, Tomas Bata University, Vavreckova 275, 760 01 Zlin, Czech Republic d Department of Glycobiotechnology, Institute of Chemistry, Slovak Academy of Sciences, Dubravská cesta 9, 845 38 Bratislava, Slovak Republic b c

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 13 September 2017 Revised 18 October 2017 Accepted 23 October 2017 Available online 25 October 2017 Keywords: Superwettability Self-assembled monolayer Wettability gradient Counterion exchange Adsorption

a b s t r a c t In this study, two quaternary ammonium salts derived from L-lipoic acid were applied for self-assembled monolayers formation on rough structured gold surface. The derivatives differ in functionality since one possesses simple quaternary ammonium group whereas the other one is carboxybetaine ester containing quaternary ammonium group with pH hydrolysable ester group as a pendant. The response of surface wettability to ion exchange between Cl
and perfluorooctanoate, kinetics and gradient wettability were examined by water contact angle measurement and confirmed by X-ray photoelectron spectroscopy. Furthermore, adhesion forces related to applied counterion on the entire surface and after hydrolysis were investigated by atomic force microscopy measurement at nanometer scales. A dramatic change in wettability upon counterion exchange from superhydrophilic for Cl
to very or superhydrophobic for perfluorooctanoate in a repeatable manner was observed for both derivatives. Kinetics of counterion exchanges revealed faster hydration of simple quaternary derivate. The wettability gradient could be designed from superhydrophobic to superhydrophilic either in a reversible manner by simple immersion of the modified surface in a counterion solution modulated by ionic strength or in an irreversible manner for carboxybetaine ester derivate by time-controlled hydrolysis to charge balanced carboxybetaine. Ó 2017 Elsevier Inc. All rights reserved.

1. Introduction

⇑ Corresponding author. E-mail address: [email protected] (P. Kasak). https://doi.org/10.1016/j.jcis.2017.10.086 0021-9797/Ó 2017 Elsevier Inc. All rights reserved.

In the last decades, there has been an increase in importance of special wettability on surface systems, especially for repeatable, tunable and gradient, due to increased application of these systems

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in bioengineering and biomedical or industrial technology [1,2]. The smartness of these surface systems allowed for their employment in self-cleaning systems, oil-water separation systems [3,4], microfluidic components [5] and for controlled interaction with biological materials [6,7]. The development of such smart surfaces includes utilization of systems responsive to external trigger [8], such as light, temperature, electrical potential, pH or ions [9–13]. Another methodology for designing such surfaces is counterion exchange, where wettability can be tuned dynamically by immersing a surface into solutions containing different counterions. As a platform for dramatic wettability switches, charged polymer brushes [14–18], organic films [19], layer by layer architectures (LBL) [20–23], modified carbon nanotubes [24–26] and selfassembled monolayer (SAM) [27] were utilized. These surfaces can be modified employing materials consisting of amine or nitrogen heterocycles with a quaternized moiety, such as polypyrrole [18], quaternary ammonium polymers [13–16], ionic liquidbased polymers [17] or quaternary ammonium thiolated molecules [27]. SAM can provide a highly structured film on a surface in nanoscale with high density of entire molecules and allows for control of wettability, dewetting, corrosion, chemical gradient, friction, adhesion, biocompatibility and protect functionality of such surfaces [28–32]. The counterion exchange in self-assembled monolayers (SAM) was carried out on imidazole-based ionic liquid-like derivatives on SAM with switches on silicone [33], Au [34,35] or glass surfaces [36] and by derivate with long aliphatic molecule possessing quaternary ammonium head group with thiol anchoring group on rough gold surface [27]. Additionally to the chemical character of a surface, its morphology is an even more essential factor for modulation and improvement wettability especially to achieve a superwettability behavior [37,38]. The modifications of surfaces in order to form a superhydrophobic surface (contact angle (CA) > 150°) require combination of the roughness and topology in the range of micro and nanoscale to ensure Cassie state rather than Wenzel state and thus to pledge low contact area between the water drop and the surface state [39]. Recently we have studied combination of rough microstructured gold surface with SAM from derivative possessing quaternary ammonium group on long aliphatic tail with thiol functionality for anchoring to surface [27]. We have shown a reversible exchange in superwettability between superhydrophilic state with contact angle <5° to superhydrophobic one with contact angle 151° by means of counterion exchange from Cl
to perfluorooctanoate (PFO
). Rewritable and ionic strength-controlled gradient from superhydrophilic to superhydrophobic state was achieved by simple immersion of the substrate in a counterion solution. Moreover, different adsorption properties of charged gold particles were observed based on the applied counterion. Here we explored switchability, kinetics and chemical surface gradient of tunable wettability on rough micro-structured gold with SAM consisting of two lipoic acid derivatives with quaternary ammonium groups. They differ in functionality since the first one consists of simple quaternary ammonium group and the second one is carboxybetaine ester with quaternary ammonium group with hydrolysable ester group. The later derivative, exhibiting pH-induced switch, allows achieving not only rewritable manner of gradient, as in case of passive immersion, but also permanent chemical gradient formation by controlled hydrolysis of the surface. The difference in adhesion force of modified surfaces with regard to the used counterion type or chemical transformation was elucidated by AFM on a nanometer scale. Moreover, superoleophobicity of the surfaces was also proven.

2. Experimental part 2.1. Materials (R)-Lipoic acid, 3-(dimethylamino)-1-propylamine, ethyl chloroacetate, iodoethane, N-hydroxysuccinimide (NHS), N,N0 -dicy clohexylcarbodiimide (DCC), HAuCl43H2O, L-cysteine, dichloromethane, acetonitrile, chloroform, tetrahydrofuran (THF), perfluorooctanoic acid, phosphate buffer, NaOH, KOH, NaCl, HCl, HF, H2O2, Na2SO4 were purchased from Aldrich and used as received. Perfluorooctanoate sodium salt (PFO
) was prepared by mixing equimolar amounts of NaOH and perfluorooctanoic acid solution. Ultrapure deionized water (DI) was obtained from a Millipore system (Direct Q3, France).

2.2. Synthesis 2.2.1. Synthesis of tertiary amine (R)-N-(2-(dimethylamino)propyl)-5(1,2-dithiolan-3-yl)pentanamide and (R)-3-((2-(5-(1,2-dithiolan-3yl)pentanamido)propyl)dimethylammonio) ethyl acetate (DAEAc) Synthesis of tertiary amine (R)-N-(2-(dimethylamino)propyl)5-(1,2-dithiolan-3-yl)pentanamide and (R)-3-((2-(5-(1,2-dithio lan-3-yl)pentanamido)propyl)dimethylammonio) ethyl acetate (DAEAc) was performed according to literature [40] and [41], respectively.

2.2.2. Synthesis of (R)-3-((2-(5-(1,2-dithiolan-3-yl)pentanamido) propyl)dimethyl-ethylammonio)iodide (DAE) To a stirred solution of tertiary amine (R)-N-(2-(dimethyla mino)propyl)-5-(1,2-dithiolan-3-yl)pentanamide (0.29 g, 1 mmol) in dried THF (2.0 mL), iodoethane (0.6 g, 4 mmol) was added dropwise under an argon atmosphere and the reaction mixture was stirred overnight at room temperature. The solvent was evaporated and the residue was triturated with 20 mL of diethyl ether to form a white suspension. The mixture was centrifuged and the supernatant was discarded. The residue was dried under reduced pressure, to form a product resembling slightly yellowish wax (0.35 g, 75%). 1 H NMR (400 MHz, D2O) d, 3.65 (m, 1H, H-3), 3.30 (m, 2H, NHACH2A), 3.25–3.15 (m, 4H, 2 N+-CH2 and m, 2H, H-5), 3.05 (m, 2H, N+ACH2), 3.00 (s, 6H, N+Me2), 2.45 (m, 1H, H-4), 2.20 (m, 2H, H-20 ), 2.00–1.90 (m, 2H, CH2ACH2AN and m, 1H, H-4), 1.65– 1.50 (m, 4H, H-30 and H-50 ), 1.40–1.25 (m, 2H, H-40 and m, 3H, CH3) ppm. 13 C NMR (100 MHz, D2O) d 177.1 (NACO), 61.0 (NACH2), 59.8 (N+ACH2), 56.6 (CH, C-3), 50.0 (NACH3), 40.3 (CH2, C-4), 38.2 (CH2, C-5), 36.0 (CH2), 35.5 (CH2), 30.2 (CH2), 27.9 (CH2), 25.1 (CH2), 22.2 (CH2), 7.4 (CH3) ppm. IR (ATR): cm
1 – 3440, 3276, 2925, 2859, 2056, 1641, 1533, 1447, 1242, 1017.

2.3. Preparation of gold structured surfaces The p-type silicon wafer was cleaned by washing in acetone, ethanol and finally in a piranha solution (a mixture of concentrated H2SO4 and concentrated H2O2 in 3 + 1 ratio, handle with special care). The wafers were then immersed into a mixture of aqueous solutions of 25 mM HAuCl43H2O and 2.5 M HF (handle with special care since it is very toxic, corrosive and destructive for tissues), heated to 50–60°C for 60 min, washed extensively with DI water and dried in vacuum.

J. Mosnácˇek et al. / Journal of Colloid and Interface Science 512 (2018) 511–521

2.4. SAM formation 4 pcs of 1  1 cm gold structured wafers were immersed into 10 mL of 4 mM aqueous solution of lipoic acid derivative (DAE or DAEAc) for 3.5 h under dark and argon atmosphere at RT. After modification, the wafers were thoroughly washed with DI water, dried and used for further experiments. 2.5. Counterion exchange experiments Wafers modified with lipoic acid derivatives were immersed into 40 mL of 0.5 mM NaCl for 5 min and thoroughly washed in DI water and dried. Subsequently, their contact angle was measured and the wafers were immersed for 10 s into 40 mL of 0.05 M perfluorooctanoate (PFO
) solution, washed with DI water and dried. This was followed by another contact angle measurement. The treatment was repeated with different incubation times. Subsequently, the wafer was immersed into 0.01 M NaCl and the progress of the exchange of PFO
ions for Cl
ions was monitored by contact angle measurements until the contact angle of 0° was reached. The same cycle of exchange of Cl
ions for PFO
ions and PFO
to Cl
was also performed using 0.1, 0.5, 1 and 2 M of NaCl solution. In all cases, the concentration of 0.05 M PFO
ions was used. 2.6. Gradient wettability by capillary forces A sample with dimensions 10  1 cm of gold wafer with microstructured surface modified with SAM of DAE was immersed into 0.05 M PFO
solution for 2 min to obtain a superhydrophobic surface. After washing in DI water and drying, 1 cm of the wafer was immersed into 1 or 2 M NaCl solution, perpendicular to solution surface, and was incubated for 10 min. The wettability gradient was measured starting at the solution front in the direction of evolution. 2.7. Gradient wettability by hydrolysis A buffer solution with pH 9.0 was prepared by dissolving 1.74 g of potassium dihydrogen phosphate in 80 mL of DI water, pH was adjusted with 1 M KOH and the mixture was diluted to 100 mL with DI water. Wafer modified with DAEAc was immersed into 40 mL of 0.05 mM PFO
solution for 2 min to exchange the Cl
ions for PFO
ions. The exchange was confirmed by contact angle measurement. The hydrolysis was performed by immersion of the wafer into the phosphate buffer solution with pH 9.0 or NaOH solution at pH 12. The progress of hydrolysis with time was followed by immersion of the wafer into the buffer for different time periods with thorough washing in DI water and the surface was subsequently hydrophobized in 0.05 M PFO
solution for 2 min. 2.8. Adhesion test The information about the interaction between the tip and the surface in a nano-scale was obtained using atomic force microscopy (AFM). The AFM device MFP-3D Asylum research (USA) equipped with a silicon probe (Al reflex coated Veeco model – OLTESPA, Olympus Model AC160TS) was used in this study. The interaction between the tip and the surface was measured in contact mode allowing for study of adhesive forces. The force volume test was carried out to obtain the force map in the specific area within 1  1 µm with the 56  56 contact areas between the tip and the sample surface. The surface adhesion related to the surface wettability was then displayed on the map image and characterized by related histograms. The mean of the adhesion force was

513

calculated based on these measurements using the Gaussian approach.

2.9. Surface characterization methods Scanning electron microscopy (SEM) was used to determine the surface morphology by (SEM, FEI Quanta 200). A contact angle (CA) was used to characterize the wetting properties of the samples and CA measurement was done by goniometer OCA 35 (DataPhysics, Germany) applying a 3 µL DW droplet and evaluated using the software SCA202. All measurements were done in triplicate to ensure assay reproducibility. The XPS signals on square shaped gold chips of 10  10  0.3 mm modified with SAM were recorded using AXIS ULTRA DLD (Kratos Analytical Ltd, UK) equipped with a Al Ka X-ray source. The spectra were acquired in the constant analyzer energy mode with pass energy of 160 eV, 10 kV and 10 mA emission current for the survey. The individual scans were performed with pass energy of 10 eV, 15 kV and 15 mA emission current. The Vision Manager 2 software was used for digital acquisition and data processing. The spectral calibration was determined using automated calibration routine and internal C1s standard. The surface compositions (in atomic %) were determined by considering the integrated peak areas of detected atoms and the respective sensitivity factors. Surface density of derivatives was performed by electrochemical measurements by using potentiostat VSP (Biologic, France) connected to an electrochemical cell. Reductive desorption of the derivative-modified Au in Ar-bubbled NaOH (0.1 M) was performed in order to determine the amount of lipoic acid derivatives anchored to the gold surface. Three electrode connections were employed, with modified gold surface electrode and platinum wire as working and auxiliary electrodes, respectively, while Ag/AgCl/ sat. KCl electrode was used as a reference electrode. The derivatives surface density was calculated from the charge value measured during the reduction obtained from integration of the cathodic peak between
1275 and
870 mV (vs. Ag/AgCl/saturated KCl reference electrode) of the first CV scan.

3. Results and discussion One of the crucial requirements for obtaining superhydrophobic and/or superhydrophilic surfaces is the nano/micro-structured roughness of the surface [42]. Therefore, in our investigations, we used silicon wafers deposited with gold and etched using galvanic reaction as was described previously [23,43] with electrochemically active area of 6.7 ± 0.2 cm2 per 1 cm2 of geometric area as an indicator of roughness [27]. The 3D microstructure of the surface after galvanic reaction can be also seen from SEM micrograph in Fig. S1 Supporting Information. It should be pointed out that several other methods such as AFM or profilometry were attempted for surface roughness evaluation however these techniques are not accurate due to 3D structured rough surface as it is in this case. The rough Au surface was utilized for formation of selfassembled monolayers (SAM) using (R)-lipoic acid derivatives containing ammonium quaternary salts, where dithiolane moiety ensures stable bounding to the gold surface. These derivatives differ in head functional group. The first one, (R)-3-((2-(5-(1,2-dit hiolan-3-yl)pentanamido)propyl)dimethyl-ethylammonio)iodide (DAE), contains only quaternary ammonium group and the second one, (R)-3-((2-(5-(1,2-dithiolan-3-yl)pentanamido)propyl)dimethy lammonio)ethyl acetate (DAEAc), contains also a pH hydrolysable ester group.

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Both derivatives with an anchoring dithiolane group DAE and DAEAc were used as a single (R)-enantiomer for SAM formation and synthetized from a tertiary amine derivative by quaternization with iodoethane and ethyl chloroacetate, respectively, as can be seen in Fig. 1. The final new product was characterized by 1H and 13 C NMR and FTIR spectrometry (Figs. S2–S4, Supporting Information). Both DAE and DAEAc were subsequently used for formation of SAM and investigations of various processes as depicted in Fig. 2. SAM was formed by immersion of structured wafer into 4 mM derivatives solution. SEM micrographs of the SAM formed on micro/nanostructured Au on the silicon wafers surfaces are shown in Fig. S5, Supporting information, and revealed an intact structured gold surface. To define the quantity of lipoic acid derivatives attached to the gold-coated silicon wafer, electrochemical reductive desorption was performed in deaerated NaOH (100 mM) using cyclic voltammetry in reduction potential range. The derivatives surface density was calculated from an electric charge passed during the derivatives reduction obtained from integration of a cathodic CV peak at about
1.1 V vs. Ag/AgCl/sat KCl reference electrode. The surface density of DAE was found to be approximately 1.5 ± 0. 5 molecules per nm2 while only 1.2 ± 0.5 molecules of DAEAc per nm2 were revealed, obviously due to bulkier ethyloxycarbonylmethyl moiety compared to the ethyl one. Interestingly, measurements on a plain electrode surface for DAEAc revealed a surface density of 0.86 ± 0.04 molecules per nm2 [41]. This difference indicates that repulsion forces between quaternary ammonium head groups and/or organization of individual molecules play significant role in affecting density of chemisorbed thiols. In the case of rough surface, the geometric curvature on edges allows for differences in organization with higher density of molecules similar to an edgecurved gold nanoparticle [44] as is proposed in Fig. S6, Supporting

Information. Additionally, the obtained values are higher than the reported value of 0.7 molecules per nm2 for spiropyran derivative based on lipoic acid [45], which is most probably the result of smaller head group of DAE and DAEAc derivatives compared to a large spiropyran moiety. On the other hand, lipoic acid derivative containing sulfobetaine moiety was reported to form a SAM with density of 1.97 molecules per nm2 on a plane polycrystalline gold surface with an increase to 2.66 molecules per nm2 when the sulfobetaine derivate was deposited together with linear 11mercaptoundecanoic acid shielding an electrostatic repulsion between the sulfobetaine molecules [40], showing an influence of derivative electric charge on SAM density and counterion presence. It can be concluded that both DAE and DAEAc derivatives formed dense SAM monolayers on structured gold surface. The modified microstructured surfaces with formed SAM were further investigated for exchange between perfluorooctanoate (PFO
) and chloride (Cl
) counterions. First, the surfaces were immersed into 0.1 M NaCl solution in order to exchange the original counterions present in DAE and DAEAc for chloride ones. Wettability changes during the Cl
/PFO
exchange were followed by contact angle measurements. As seen in Fig. 3 for both derivatives, the modified structured surfaces showed a dramatic change in contact angles. Originally, the modified surfaces with Cl
counterions were superhydrophilic, with contact angle <5°. After immersing the samples into a 0.1 M PFO
solution for 1 min and subsequent drying, the character of the surfaces changed from superhydrophilic to superhydrophobic in case of DAE with CA of 149.6° ± 1.0 or almost superhydrophobic for DAEAc with CA of 147.3° ± 2.2. As also shown in Fig. 3, the process is reversible, i.e. the superhydrophilic character of the surface was renewed after the immersion of the samples into the 0.1 M Cl
solution for 10 min. The reversibility showed a slight decrease in the case of DAE derivative

Fig. 1. Synthesis of DAE and DAEAc.

Fig. 2. Structure of DAE and DAEAc and schematic presentation of (A) SAM formation on the rough microstructured surface; (B) Switchable wettability by ion exchange between Cl
and perfluorooctanoate ions (PFO
); and (C) Chemical modification of DAEAc to carboxybetaine derivative to enable formation of a chemical irreversible gradient.

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(a) 160

Rough surface

(b)

Plain surface

Rough surface

160

Plain surface

140

60

-

-

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-

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l

O PF

6

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C 5

l

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l

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O

C 3

l

PF

2

C 2

C 1

1

PF

l

O

-

6

PF O

-

Cl 6

-

O PF

5

Cl

5

4

PF O

-

Cl 4

O

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PF

-

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O

2

PF

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O

Cl

PF

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Contact angle (°)

515

Cycle

Fig. 3. Reversible behavior on structured (square) and plain (circle) gold surface modified with (a) DAE, and (b) DAEAc over sequential couterion exchange cycle with Cl
and PFO
counterions. The insets show photographs of 3 µL water droplets present on the surface during measurements.

with decrease of the CA down to 142° ± 4 after 6 cycles, as is demonstrated in Fig. 3a. The surface modified with DAEAc showed higher stability, with CA of 147° ± 5 after 6 switching cycles (Fig. 3b). It should be pointed out that the structured surface is essential for such dramatic changes in wettability, as has been proven by an experiment on a plain surface when in the case of Cl
counterion the CA values were 72.0° ± 2 and 62.3° ± 1.9 and for PFO
counterion the CA values were 76.4° ± 0.8 and 69° ± 2 for DAE and DAEAc, respectively. Reversible alteration of superwettability by counterion exchange has been performed also on different polymeric systems, but only LBL architecture [20] has similar feature with regard to simplicity and time needed to prepare and switch such surfaces. The changes in chemical composition based on the counterion exchange at the structured surface with SAM formed from DAE and DAEAc were also monitored by XPS analysis. Both SAMs formed from DAE and DAEAc derivatives were studied after exchange by PFO
and subsequent exchange by Cl
solution. In a wide survey, the XPS spectra of the prepared samples for DAE with Cl
and PFO
counterions showed anticipated chemical elements, see Fig. S7a and S7b, respectively. Similarly, DAEAc samples with Cl
and PFO
counterions are shown in Fig. S8a and S8b, respectively. Fig. 4 shows high resolution spectra for C1s, and F1s. These elements were deeply investigated due to substantial changes after counterion exchanges. The changes in C1s spectra for DAE and DAEAc are depicted in Fig. 4a and c, respectively. In spectra for SAM with DAE and DAEAc with PFO
individual signals at 285, 286, 287, 289, 292 and 293 eV associated with CAC, CAN, CAS, C@O, CF2 and CF3 signals, respectively, were observed. The signals corresponding to CF2 and CF3 disappeared after the exchange for Cl
counterion. Moreover, a decrease in signal attributed to C@O was observed in spectra for both derivatives. Completed exchange is also visible in high resolution F 1s spectra, where, in the case of PFO
counterion, strong F signal at 689 eV corresponding to a CAF bond is present, while after the exchange to Cl
, the signal disappears for both derivatives. It should be pointed out that after the exchange to Cl
counterion, a very low to negligible signal for Cl ion at about 197 eV was observed in both derivates, even in high resolution spectra. It is known that counterion such as Cl
is barely detectable or usually undetectable by XPS techniques [46]. During the exchange process, no changes were observed in high resolution spectra of N1s (Fig. S9a and S9b Supporting Information for DAE and DAEAc, respectively) S2p (Fig. S10a and S10b of the Supporting Information for DAE and DAEAc, respectively). In N1s spectra peaks at 399 and 402 eV corresponding to amide and quaternary

ammonium functionalities, respectively, were observed. In S2p spectra, typical signals at about 162 and 164 eV were observed. These observations clearly showed stability of the formed SAMs and the complete exchange of counterions during the exchange process. The kinetics of the counterion exchange for both derivatives were investigated as well, since these specify the ability of wettability modulation. First, the modified wafers were immersed in various concentrations of NaCl and subsequently, after washing and the contact angle measurement, the wafers were immersed in 0.05 M PFO
for various time periods and their contact angles were again measured. In Fig. 5a, the exchange from Cl
to PFO
for surfaces modified with DAE is shown. As can be seen, the dramatic change from superhydrophilic to superhydrophobic behavior was observed already after 10 s of immersion independently from the ionic strenght of the NaCl solution used in the previous immersion step. This shows that there is a strong electronic affinity between the quaternary ammonium and PFO
due to the hydrophobic characters of both counterions moieties leading to decreased hydration energy [27,47]. As expected, the counterion exchange from PFO
to Cl
was slower and this was proven by progressive increase in wettability (see Fig. 5b). Moreover, the kinetics of the exchange depended on the NaCl concentration used in the exchange experiment. The exchange was slowest at the lowest investigated concentration (0.01 M NaCl) and the exchange rate increased with the concentration up to 1 M NaCl. A slight decrease in exchange rate was observed when 2 M NaCl solution was used. It is worth mentioning that the exchange was much faster than it was observed previously for surfaces modified with linear quaternary ammonium thiol derivate. A plausible reason is the denser binding of thiols derivatives on the gold surface (3.4 molecules per nm2) [27] than that of lipoic acid derivative (1.5 molecules per nm2), determined by electrochemical reductive desorption measurements as describe above. In the case of the thiol derivative, the higher density of hydrophobic PFO
counterions significantly decreased the diffusion of ions from water into the superhydrophobic surface and some induction period was observed until the SAM became wet and the ion exchange became possible. Thus, the SAM based on ammonium derivative of lipoic acid described here provides a system enabling a much faster switching between superhydrophilic and superhydrophobic nature of a surface. Moreover, with DAE derivative, the effect of ionic strength did not become more pronounced with the increase of exchange time as was observed for linear quaternary derivate and DAEAc, and as is discussed below.

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a)

b)

3

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C-C DBA Cl DBA PFO

3

32

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C-N

CF

C-S C=O

CF2

CF3

DAE Cl DAE PFO

30

35 28

a. u.

a. u.

30

26 24

25 22

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20

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c)

d)

3

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686

684

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680

x10

CF

CF2

676

3

20

C-C C-N C-S C=O

678

Binding energy (eV)

x10

30

688

DAEAc Cl DBEAc PFO

18

CF3

16

a. u.

a. u.

25

14

20

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10

15 280

285

290

295

300

694

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688

686

684

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Fig. 4. XPS spectra for DAE in C1s (a) and F1s (b) and for DAEAc in C1s (c) and F1s regions (d) on SAM on structured surface in the presence of different counterions Cl
(black lines) and PFO
(red lines). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

a)

b)

160

180

0.01M NaCl 0.1M NaCl 0.5M NaCl 1M NaCl 2M NaCl

160

Contact angle (°)

Contact angle (°)

140

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0.01M NaCl 0.1M NaCl 0.5M NaCl 1M NaCl 2M NaCl

5

120 100 80 60 40 20

0 0 0

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120

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0

50

100

150

200

250

300

Time (s)

Fig. 5. Contact angle measurement of the microstructured surface modified with DAE, as a function of time of counterion exchange using different concentrations of NaCl. (a) Counterion exchange from Cl
to PFO
by immersion in 0.05 M PFO
solution after pre-treatment of the wafer with 0.01, 0.1, 0.5, 1 and 2 M NaCl solution; (b) Exchange from PFO
to Cl
by immersion in 0.01, 0.1, 0.5, 1 and 2 M NaCl solution.

The kinetics of counterion exchange from Cl
to PFO
for DAEAc derivative was similar as for DAE (Fig. 6). However, the reverse exchange, from PFO
to Cl
, was slower for DAEAc than for DAE. The rate of the counterion exchange increased with increasing con-

centration of NaCl, from 0.01 M up to 0.5 M. With subsequent increase of NaCl concentration up to 2 M, the exchange rate decreased. The increased salt concentration can increase hydration of the ions and thus decrease the amount of water molecules able

J. Mosnácˇek et al. / Journal of Colloid and Interface Science 512 (2018) 511–521

b)

160

160

140

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120

Contact angle (°)

Contact angle (°)

a)

100 80 60

0.01M NaCl 0.1M NaCl 0.5M NaCl 1M NaCl 2M NaCl

40 20

517

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100 80 60 40 20

0

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300

0

5

10

15

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25

30

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50

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Fig. 6. Contact angle measurement of the microstructured surface modified with DAEAc, as a function of time of counterion exchange using different concentration of NaCl. (a) Counterion exchange from Cl
to PFO
by immersion in 0.05 M PFO
solution after pre-treatment of the wafer with 0.01, 0.1, 0.5, 1 and 2 M NaCl solution; (b) Exchange from PFO
to Cl
by immersion in 0.01, 0.1, 0.5, 1 and 2 M NaCl solution.

Fig. 7. Photograph of the 3 lL water droplets deposited along the wettability gradient formed on the structured gold substrate modified with DAE in various distances from the part of the substrate immersed in 2 M NaCl solution for 10 min. The sequence combines nine photographs due to the limitations of the goniometer view not allowing for collecting the entire gradient length. The numbers below the photographs are contact angle values.

tion range of 0–60 mm from the solution surface. The shortening of the distance is ascribed to the lower hydration capacity in the case of higher concentration of NaCl, as has already been discussed above. This result has shown the differences in capillary forces from the wettability gradient as described previously for quaternary ammonium derivative with a linear thiol anchoring moiety [27], where in the case of 1 M and 2 M NaCl the gradient was observed at the position ranges 0–60 mm and 0–40 mm, respectively. This difference is in accordance with slower PFO
to Cl
counterion exchange due to higher density of molecules bonded on the gold surface in the case of the linear thiol derivative. It is worth emphasizing here that the wettability gradient could be simple exchanged to the superhydrophilic or superhydrophobic

160 140 120

Contact angle (°)

to wet the surface, which prolongs the counterion exchange as was also observed for linear thiol derivate [27]. The slower PFO
to Cl
exchange observed in the case of DAEAc as compared to DAE can be caused by stronger interaction between PFO
and ammonium cation in the case of DAEAc due to electron withdrawing effect of the ester group [48] and higher hydrophobic van der Wall interactions of ethyl ester moiety with perfluorinated chain from counterions [49]. Due to these interactions, a more integrated monolayer can be formed in the case of DAEAc, thus partially hindering hydration and ion exchange. It is also worth comparing the time of ion exchange among different derivatives and modification types published previously. The time needed for counterion exchange between PFO
to Cl
in the LBL architecture was lower, i.e. in tens of seconds [50], because of a less coherent and uniform layer [51] compared to SAM, and thus contact of exchanging counterions to positive charged groups was easier. As a result from current data, lipoic acid derivatives can modulate the time of exchange by moiety on quaternary ammonium group quickly, in tens of seconds, in the presence of ethyl pendant, to a slow process taking several tens of minutes with exchange of ethyloxycarbonylmethyl pendant due to possible interaction with PFO
ion and aggregation based on an ionic strength. In case of linear derivative, wettability is modulated by ionic strength to tens of minutes, due to a denser monolayer on the surface. [27] Taking advantage of the fast PFO
to Cl
counterion exchange in the case of DAE, it was used in the study aimed to form a wettability gradient on the structured gold surface as shown in Fig. 7. One end of the sample was submerged into a solution with various NaCl concentrations and the gold surface was soaked with the NaCl solution by means of capillary forces. The dependence of contact angle on the distance from the surface of 1 M and 2 M solution of NaCl is shown in Fig. 8. In good agreement with the kinetics of PFO
to Cl
exchange, in the case of 1 M NaCl solution the exchange was faster and the wettability gradient was observed at the position range of 30–80 mm from the solution surface. Contrary to that, for 2 M NaCl the gradient was observed at the posi-

100 80 60 40 20

2M NaCl 1M NaCl

0 0

10

20

30

40

50

60

70

80

90

Position (mm) Fig. 8. Change of the contact angle on the structured gold substrate, modified with DAEAc, with the wettability gradient along the position at various distances from the part of the wafer immersed in 1 M or 2 M NaCl solution. The lines serve only as a guide to the eye.

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518

character of the surface by its immersion into a solution of Cl
or PFO
, respectively, and subsequently used for new gradient formation. As was proven and shown above, wettability gradient could be formed by capillary force of solution to SAM at surface. This process is rewritable and repeatable. Furthermore, chemically modulated gradient can be formed in case of DAEAc since it contains hydrolysable ester moiety. Irreversible gradient with chemical modification can be applied for development of diverse types of smart materials and can potentially be used for research as a surface-based transportation platform or for investigation of biologically-related processes such as different migration (haptotaxis), differentiation or polarization of cells [32,52]. To examine pH-driven fabrication of surface gradient, a SAM consisting of

DAEAc derivative was immersed in NaOH solution for various time periods, while solutions with two different pH were tested, as is indicated in Fig. 9. In this case, the ability of DAEAc derivative to be hydrolyzed was used. As can be seen from Fig. 9b, the hydrolysis performed in the NaOH solution at pH = 12, the hydrolysis-driven wettability modulation was very fast. On the other hand, using the NaOH solution buffered at pH = 9 provided progressive hydrolysis and it was possible to control the wettability by varying the time of hydrolysis. At the same time, it is important to note that the course of hydrolysis was not perfectly homogenously distributed along the wafer length, as can be seen from relatively high standard deviation values. The hydrolysis was completed after 50 min, when the surface retained its superhydrophilic character even after immersion in PFO
solution. It should be pointed out that the SAM from DAE was also tested under the same condition and after 50 min treatment at pH 9 and subsequent immersion into PFO
solution, the surface restored the same contact angle of 145° and a negligible deviation from the contact angle to the value of 142° was observed after treatment at pH 12. The progress of hydrolysis of the SAM formed by DAEAc was examined by XPS techniques in C1s region. The sample was similarly immersed to PFO
solution and change in intensity of signal at 292 and 293 eV associated with CF2 and CF3 bond, respectively, during the course of hydrolysis was observed. After 25 min, the signals for CAF bonds dramatically lowered and after 50 min they became undetectable. The complete hydrolysis was also confirmed by FTIR spectra, where the ester peak at 1750 cm
1 disappeared, as can be seen in Fig. 10. Stability of the amide group was proven by the presence of absorbance peak at 1630 cm
1. The new peak at 1540 cm
1 in spectrum after hydrolysis corresponded to carboxylate functionality. As investigated previously, pH induced dramatic changes in gradient also on surfaces created by LBL [53], mixing different components on SAM [54] or degradation of polymeric acid from block polymer [55]. However, our studied approach combines faster, irreversible and more modulated process when compared to the ones mentioned. Further, it was examined contact angle with CCl4 as heavy oil type liquid (Fig. 11) on pre-wetted rough surfaces with DAE and DAEAc bearing Cl
as a counterion and DAEAc after hydrolysis in zwiterionic state covered with water (Fig. 11a) and artificial seawater (Fig. 11b) and all surfaces possess superoleophobic character. The underwater contact angles with CCl4 were 165.0° ± 1.9, 158.1° ± 2.2 and 170.6° ± 2.2 and under seawater exhibited 161. 5° ± 1.6, 160.4° ± 2.0 and 162.0° ± 1.8 for DAE, DAEAc bearing Cl
and DAEAc after hydrolysis, respectively. The lowest CA was for both cases for DAEAc probably due to shielding of cationic group

a)

b)

180 pH = 9 pH = 12

160

Contact angle (°)

140 120 100 80 60 40 20 0 0

500

1000

1500

2000

2500

3000

Time (s) Fig. 9. (a) Photographs of 3 lL water droplets deposited on the structured gold substrate modified with DAEAc after hydrolysis in buffered NaOH solution (pH = 9) for various times followed by dipping for 5 min into 0.5 M PFO
solution. The sequence combines eight photographs due to the limitations of the goniometer view not allowing for collecting the entire gradient length; and (b) Contact angle measured on the structured gold substrate modified with DAEAc after hydrolysis in NaOH solution buffered at pH = 9 or 12 for various time periods, followed by dipping for 5 min into 0.5 M PFO
solution.

b) 0 min

C=O ester

C=O amide

x103

25

CF2

C=O

CF3

20

50 min

a. u.

Transmitance (a. u.)

a)

0 min 15

25 min 10

1800

1700

1600

Wavelenght (cm -1)

1500

50 min 280

285

290

295

300

Binding energy (eV)

Fig. 10. (a) FTIR spectra from the surface of rough gold wafers modified with DAEAc before (black line) and after hydrolysis (blue line); (b) XPS spectra for DAEAc in C1s region during the course of hydrolysis with NaOH solution with pH 9 and subsequent immersion to PFO
solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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519

Fig. 11. Photographs of 3 µl CCl4 droplet taken on the surface during the under water (a) and under seawater (b) measurements for DAE (left), DAEAc bearing Cl
(middle) and DAEAc after hydrolysis (right).

Fig. 12. Force-volume adhesion histograms: (a) DAEAc Cl
(black lines); (b) DAEAc PFO
(blue lines); and (c) DAEAc after hydrolysis (red lines). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

by ester pendant. Highest values were observed for DAEAc after hydrolysis in zwitterionic state with two ions in the same molecules, showing superior superoleophobic effect for such type of modification. Similarly as for silicone surface modified carboxybetaine zwitterions showed previously 120° for hexadecane in underwater [56] and LBL architecture CA 163.6° for 1,2-dichloromethane as a heavy oil type artificial seawater as a medium. In case of the LBL architecture, the superoleophobic behavior also occured in air after the PFO
exchange, which was not proven in our case. In addition to wettability, we assumed different adhesion ability can be anticipated based on ionic character as well as different counterions. This was therefore supported by force volume adhesion measurement by AFM. The prepared bare gold surfaces with SAM of DAEAc with different counterions, as well as after hydrolysis to carboxybetaine structure with zwitterionic character, were examined and the results can be seen in Fig. 12 as histograms and in Fig. S11 in Supporting Information as images. The force-volume adhesion was measured in the surface area 1 lm  1 lm for surface bearing Cl
counterion, after exchange to PFO
counterion and after hydrolysis to carboxybetaine derivate. The related adhesion histograms show the surface adhesion force distribution calculated from interactions between the AFM tip and the SAM of DAEAc bearing Cl
(Fig. 12a) and PFO
(Fig. 12b) as counterions and SAM after hydrolysis (Fig. 11c). The relatively uniform and homogenous adhesion forces throughout the surface area, as seen in Fig. S11, confirm robust and well-packed SAM on the gold surface in all cases. The SAM of DAEAc containing Cl
reveals the mean adhesive force of 51.6 ± 3.5 nN while after exchange to PFO
counterion, the force was only 34.3 ± 8.4 nN. Similar values and trends were observed in case of adhesion forces of ionic liquid imidazolium derivatives. [33] The sample after hydrolysis showed an

increased mean adhesive force of 117.2 ± 7.8 nN. It is known that perfluorated counterions are hydrophobic and increase adhesion resistance due to the lowering of capillary force between the tip and the surface. Moreover, the presence of carboxybetaine derivative on the surface increases the ionic character due to both charges being present on the surface. It can thus be concluded that these results correlate with the contact angle and adsorption investigation.

4. Conclusion Current investigation explored and exploited our previous finding on switchable superwettability, gradient and kinetics with linear quarternarly ammonium derivative anchoring by thiol group to gold rough surface [27]. In current study, two lipoic acid derivatives differing in type of ammonium salt, namely (R)-3-((2-(5-(1, 2-dithiolan-3-yl)pentanamido)propyl)dimethyl-ethylammonio)io dide (DAE), and (R)-3-((2-(5-(1,2-dithiolan-3-yl)pentanamido)pro pyl)dimethylammonio)ethyl acetate (DAEAc), were synthesized and used for formation of self-assembled monolayers on rough gold surface. A fast change from superhydrophilic to superhydrophobic behavior of the surface was achieved by a simple exchange of Cl
counterion with PFO
one for both derivatives. This process was reversible, with a faster change from superhydrophobic to superhydrophilic for the DAE derivative, probably due to stronger interactions of DAEAc with perfluorinated anion. The kinetics of counteranion exchange for the DAE and DAEAc derivative was significantly faster than previously published for linear aliphatic derivate [27] and slower than LBL architecture [50]. The reason for this is the dense SAM formation however with

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lower density of ammonium groups due to dithiolane character of anchoring moiety. The reversible and ionic strength modulated wettability gradient formation was subsequently successfully demonstrated using DAE and DAEAc. Additionally to physical formation of gradient, the hydrolysable ester group in the DAEAc structure allow to use this derivative for permanent pH modulation of the surface changes in the gradient. Dramatic modulation in gradient was showed also by LBL [53], mixing different components on SAM [54] or degradation of polymeric acid from block polymer [55], however, current study combines faster, irreversible and more modulated process. Moreover underwater and under seawater superoleophobicity of surface was proven. Thus, the approaches result in stable SAM on rough surface leading to a dramatic change in superwettability behavior, wettability gradient, adhesion and reversible or permanent modulation of superwettability. This simple, easy, fast and reproducible approach opens a novel pathway for utilization in migration (haptotaxis), differentiation or polarization of cells, biosensing and architecture for (nano) devices. Acknowledgements The authors gratefully acknowledge Mr Ahmed Suliman, Gas Processing Center Qatar University, for carrying out the XPS analysis. This publication was supported by Qatar University Grant QUUG-CAM-2017-1. This publication was made possible by NPRP grant # NPRP-6-381-1-078 and NPRP-9-219-2-105 from the Qatar National Research Fund (member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2017.10.086. References [1] M. Liu, S. Wang, L. Jiang, Nature-inspired superwettability systems, Nat. Rev. Mater. 2 (2017) 17036. [2] M. Cao, L. Jiang, Superwettability integration: concepts, design and applications, Surf. Innovat. 4 (2016) 186–191. [3] B. Wang, W. Liang, Z. Guo, W. Liu, Biomimetic super-lyophobic and superlyophilic materials applied for oil/water separation: a new strategy beyond nature, Chem. Soc. Rev. 44 (2015) 336–361. [4] P. Cao, J. Mangadlao, R. Advincula, Stimuli-responsive polymers and their potential applications in oil-gas industry, Polym. Rev. 55 (2015) 706–733. [5] M. Chaudhury, G. Whitesides, How to make water run uphill, Science 256 (1992) 1539–1541. [6] E. Cabane, X. Zhang, K. Langowska, C.G. Palivan, W. Meier, Stimuli-responsive polymers and their applications in nanomedicine, Biointerphases 7 (2012) 9. [7] H. Nandivada, A. Ross, J. Lahann, Stimuli-responsive monolayers for biotechnology, Prog. Polym. Sci. 35 (2010) 141–154. [8] H. Kuroki, I. Tokarev, S. Minko, Responsive surfaces for life science applications, Annu. Rev. Mater. Res. 42 (2012) 343–372. [9] T. Sun, G. Wang, L. Feng, B. Liu, Y. Ma, L. Jiang, D. Zhu, Reversible switching between superhydrophilicity and superhydrophobicity, Angew. Chem. Int. Ed. 43 (2004) 357–360. [10] J. Lahann, S. Mitragotri, T.N. Tran, H. Kaido, H.J. Sundaram, I.S. Choi, S. Hoffer, G. A. Somorjai, R. Langer, A reversibly switching surface, Science 299 (2003) 371– 374. [11] M. Ilcikova, J. Tkac, P. Kasak, Switchable materials containing polyzwitterion moieties, Polymers 7 (2015) 2344–2370. [12] S. Wang, Y. Song, L. Jiang, Photoresponsive surfaces with controllable wettability, J. Photochem. Photobiol. C 8 (2007) 18–29. [13] Y. Zhan, Y. Liu, J. Lv, Y. Zhao, Y. Yu, Photoresponsive surfaces with controllable wettability, Prog. Chem. 27 (2015) 157–167. [14] S. Moya, O. Azzaroni, T. Farhan, V. Osborne, W. Huck, Locking and unlocking of polyelectrolyte brushes: toward the fabrication of chemically controlled nanoactuators, Angew. Chem. Int. Ed. 44 (2005) 4578–4581. [15] Z. Hua, J. Yang, T. Wang, G. Liu, G. Zhang, Transparent surface with reversibly switchable wettability between superhydrophobicity and superhydrophilicity, Langmuir 29 (2013) 10307–10312. [16] O. Azzaroni, A. Brown, W. Huck, Tunable wettability by clicking counterions into polyelectrolyte brushes, Adv. Mater. 19 (2007) 151–154.

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