Supramolecular Switching of Ion-Transport in Nanochannels - ACS

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Supramolecular Switching of Ion-Transport in Nanochannels Pavan Kumar V.V.S. Bosukonda, Sonu Kizhakeppura, K. Venkata Rao, Srinivasan Sampath, Subi J. George, and Muthusamy Eswaramoorthy ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07098 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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ACS Applied Materials & Interfaces

Supramolecular Switching of Ion-Transport in Nanochannels B.V.V.S. Pavan Kumar, † K.P. Sonu, † K. Venkata Rao, ‡ S. Sampath, ¥ Subi J. George, ‡,* M. Eswaramoorthy†,* † Nanomaterials and Catalysis lab, Chemistry and Physics of Materials Unit, School of Advanced Materials (SAMat), Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O, Bangalore 560064 (India) ‡ Supramolecular Chemistry Laboratory, New Chemistry Unit, School of Advanced Materials (SAMat), JNCASR, Jakkur P.O, Bangalore 560064 (India) ¥ Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012 (India) Email: [email protected], [email protected]

KEYWORDS: ion transport, nanochannels, non-covalent functionalization, supramolecular gating, charge transfer interaction, multiple gating states.

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ABSTRACT: Non-covalent approaches to achieve smart ion-transport regulation in artificial nanochannels have garnered significant interest in the recent years owing to their advantages over conventional covalent routes. Here in, we demonstrate a simple and generic approach to control the surface charge in mesoporous silica nanochannels by employing π-electron-rich charged motifs (pyranine-based donors) to interact with the surface of mesoporous silica modified with π-electron-deficient motifs (viologen-based acceptors) through a range of noncovalent forces viz. charge-transfer, electrostatic and hydrophobic interactions. The extent of each of these interactions was independently controlled by molecular design and pH, while employing them in a synergistic or antagonistic fashion to modulate the binding affinity of the charged motifs. This enabled the precise control of the surface charge of the nanochannels to achieve multiple ion-transport states.

Protein nanochannels in biological systems1 are capable of modulating the flux, nature and even selectively transport specific ions. Inspired by such efficient means to regulate ion transport, many abiotic mimics have been developed in recent years to modulate the flux, nature and direction of ion transport. Apart from geometry of the nanochannels which is an important factor governing the direction of ion transport, the control of surface charge was pivotal in controlling the flux and nature of ion transport in these abiotic analogues.2-4 Typically, the modulation of surface charge was achieved through covalently linking moieties capable of switching their charge in response to a single stimulus such as pH,5-15 light,16-18 electrical19-22 or temperature,23-25 or a combination of stimuli.26-27 More recently, the moieties appended to the nanochannel walls have been designed using the knowledge of supramolecular chemistry or biochemistry to bind to specific molecular species such as biomolecules,28-29 ions,30-32 polymers33 or complementary organic molecules,34-36 through


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electrostatic forces, ligation or other non-covalent interactions, to reversibly modulate the surface charge in the nanochannels.37-38 However, as most of these strategies rely on just one or two forces defining the interaction of the appended moieties to their charge-modifying counterparts in solution, they have a limited scope of attaining multiple ion transport states in terms of flux or nature of ion transport. One strategy of having multiple transport states was illustrated by Fang R. et al. where they used a layer-by-layer strategy to reversibly construct supramolecular networks inside the nanochannels.39 Here, we illustrate an alternative strategy using three different tuneable, non-covalent forces to modulate the binding of chargemodifying aromatic moieties donors to their π-complementary motifs on the nanochannel surface. This enabled the realization of different surface charge densities to control not only the nature of ion transport but also the flux of ions passing through the nanochannel. Effective modulation of surface charge inside nanochannels using non-covalent interactions relies heavily on the binding energy between the anchoring motifs attached to the nanochannel and the incoming complimentary charged motifs. Stronger the binding energy, higher the surface charge densities. We had previously used strong charge-transfer (CT) interactions40-42 between the multiply charged donors and acceptor (viologen) moieties anchored onto the pore walls of mesoporous silica films to reversibly modulate the surface charge through temporal control of binding and pH-responsive functionality of the donors used. In the current work, we employed additional “secondary” forces which are either repulsive or attractive in addition to the “primary” charge-transfer interactions between viologen and pyranine derivatives, to effectively modulate the binding energy to enable access to different surface charge densities which ultimately manifests as control of ion flux and its nature. Modulation of the binding energy was achieved by use of two “secondary” forces viz. electrostatic and hydrophobic forces, in a synergistic or antagonistic fashion to vary the



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overall binding energy between the non-covalently binding motifs. The pH-responsive charging of the silanols on the pore wall was used to constitute a switchable electrostatic repulsive force between the pore wall and the negatively charged pyranine derivatives (Scheme 1) and by employing pyranine derivatives having different lengths of alkyl chains (hexyl or dodecyl; Figure S1) we harness the hydrophobic interactions between alkyl chains to act as an attractive force between the pyranine derivatives bound to the viologen. The strength of the hydrophobic interactions could be programmed through the length of the alkyl chain appended to pyranine. By using the combination of these three forces, viz. chargetransfer interactions (primary), electrostatic repulsion between the silanols and the donors (secondary), and hydrophobic interactions (secondary) between the alkyl chains of the donors/pyranine derivatives, we tune the energy landscape of binding between the viologen mesoporous scaffold and the charge modifying donors to achieve different levels of occupancy or surface charge density to exert not only a qualitative control over the ion transport but also a quantitative one. The modulation of binding energy was further verified by carrying out competitive binding experiments. It is also worth noting that the choice of hydrophobic forces as a stabilising force was found to make the non-covalent assemblies inside the pores well suited for operation in aqueous media and stabilise them for extended periods without the necessity of addition of donor to the external solution to maintain the equilibrium towards the bound state. Most of the studies of ion transport through the mesoporous silica films in this work have been carried out without the addition of donor to the external solution in contrast to our previous study. Mesoporous silica films supported on ITO substrates43 were synthesized by dip coating a sol of tetraethylorthosilicate in the presence of non-ionic block copolymer template, Pluronic F127.44 The films were calcined to remove the surfactant templates and the FESEM images of the calcined films show very uniform crack-free films of thickness 180 nm (Figure S2).


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TEM images show arrays of pores of size range 8-10 nm (Figure S3). The pores were then covalently functionalised with viologen containing silane (Figure S4) as against the iodopropyl silane route used in our earlier work36 so that the silanols on the walls are not passivated by the silane functionalisation. The incorporation of viologen was confirmed through IR spectroscopy and digestive analysis of the viologen functionalised mesoporous silica films (MF-V) (Figure S5). The TEM images of the MF-V show preservation of the mesoporous structure on functionalization (Figure S3). The retention of the pH responsive silanols on the pore walls of MF-V was investigated by monitoring the diffusion of positively and negatively charged redox probes at pH 6 and pH 3 using cyclic voltammetry. All measurements were carried out within a potential window of 0.40 V to 0.45 V, where neither the donor nor the acceptor would show any electrochemical response thus enabling us to attribute the measurements to reflect the transport properties of the films 36 Figure 1a and 1b show the cyclic voltammograms recorded on MF-V film in the presence of 1 mM [Fe(CN)6]3- (blue trace) and 1 mM [Ru(NH3)6]3+ at pH 3 and pH 6 respectively with 0.1 M KCl as the supporting electrolyte. At both pH 3 and pH 6, MF-V showed an anion selective transport indicating a predominantly positively-charged pore surface. The decrease in the electrochemical response for [Fe(CN)6]3- from 230 µA/cm2 (anodic current density) at pH 3 to 140 µA/cm2 (anodic current density) at pH 6, indicates decrease in the overall positive charge on the pore walls of MF-V as the surface charge density regulates the ion currents through the nanochannels.4 It suggests that the silanol groups present on the pore walls are actively participating in defining the surface charge of the nanochannels in response to the pH as viologen doesn’t respond to changes in pH. The retention of anion selective transport at pH 6, where more silanols are dissociated than at pH 3, indicates that the negative charge from the silanolate groups at pH 6 is less than the positive charge from the viologen moieties as shown in the schematic (Figure 1c,d). Thus, by



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carrying out a single step post-modification with a viologen containing silane we retained a significant number of pH responsive silanol groups which could be used to control the affinity of charged donors to the viologen moieties anchored to the pore wall. To employ hydrophobic interactions as secondary forces to modulate the binding energy of donors to the viologen scaffold, a pyranine donor appended with a dodecyl alkyl group (PC12) through an ether linkage was used. The MF-V films have the viologen moieties to provide the primary binding force, C-T interactions (with negatively charged pyranine donor), along with the pH-responsive silanol groups to provide secondary repulsive force. So, to illustrate the use of two “secondary” forces viz. electrostatic and hydrophobic forces to modulate the binding energy between donor and acceptor, we carried out the non-covalent functionalisation of the MF-V films with PC12 at pH 3 (MF-V-PC12@pH3) and at pH 6 (MF-V-PC12@pH6). At pH 6, a large number of silanol groups are ionized as compared to the condition at pH 3 leading to destabilization of PC12 interaction with viologen. The cyclic voltammograms recorded at pH 3 on MF-V-PC12@pH3 and at pH 6 on MF-V-PC12@pH6, both show a strong electrochemical response for positively charged [Ru(NH3)6]3+ and almost no discernible response for negatively charged [Fe(CN)6]3- indicating predominant negative charge on the pore wall (Figure 2a,c). It is interesting to note that even though they have been non-covalently functionalised at different pH, the resultant surface charge is quite similar. In the case of MF-V-PC12@pH3, where the silanolate contribution to the negative surface charge is minimal, but still the observation of predominantly negative surface charge implies that most of the viologen moieties are bound to negatively charged PC12 (Figure 2e). This would also imply that since both MF-V-PC12@pH3 and MF-V-PC12@pH6 exhibit similar surface charge, the latter would have more free viologens than the former as silanolate groups have a greater contribution towards the negative charge at pH 6. To discern the contribution of silanolate groups towards the overall negative charge on the pore wall, the transport


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properties of MF-V-PC12@pH6 were studied at pH 3 by bringing down the pH to 3. The cyclic voltammograms recorded at pH 3, show that the transport is ambipolar or nonselective, indicating near neutral surface charge (Figure 2d). This implies that at pH 3 where the silanolate contribution is minimal, the binding of PC12 molecules is only neutralising/balancing the positive charge of the viologen moieties. This would suggest a two-third occupancy of viologen moieties by PC12 in MF-V-PC12@pH6 since for every three viologen molecules (bearing two positive charges) two PC12 molecules (bearing three negative charges) required for charge neutralization. The excess negative charge at pH 6 is contributed by the pH-responsive charging of the silanolate moieties leading to a predominantly negatively charged surface at pH 6 (Figure 2g,h). Soaking MF-V with negatively charged PC12 at pH 3 allows more number of PC12 molecules to bind to the viologen moieties compared to the one soaked at pH 6, as PC12 face less repulsion from the low density of silanolate moieties at pH 3. Hydrophobic interactions are short range interactions and hence we expect that MF-V-PC12@pH3 with a larger number of PC12 molecules bound to viologen would be stabilised to a greater extent. To investigate the effects of hydrophobic forces on the non-covalent assembly, transport studies were carried out at pH 6 on MF-V-PC12@pH3. It was interesting to note that an enhanced positive ion transport was observed at pH 6 indicating that the additional silanol mediated negative charging of the pore wall did not cause significant unbinding of the PC12 (Figure 2b,f). This implies that the binding energy of the viologen scaffold for the PC12 is strong enough to overcome the additional repulsive interactions triggered between the silanolate groups on the pore wall and the negatively charged PC12 by the change of pH from 3 to 6. The diffuse reflectance infra-red fourier transform spectra (DRIFTS) of the MF-VPC12@pH3 and MF-V-PC12@pH6 both show peaks for the assymetric and symmetric stretches at lower wavenumbers indicating semi-crystalline packing of alkyl chains (Figure



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S6). Apart from the intensity of the peaks which could be attributed to larger amount of PC12 bound to MF-V-PC12@pH3, there doesn’t seem to be much difference in the extent of the red shift observed due to packing of the alkyl chains. Despite the larger amount of the PC12 bound to MF-V at pH3 (MF-V-PC12@pH3) then at pH6 (MF-V-PC12@pH6), the peak current density for positively charged redox probe ([Ru(NH3)6]3+) of the former (Figure 2b) at pH 6 is roughly twice that of the later (Figure 2c). This implies that the ion-enrichment effects mediated by surface charge are more significant in this case than steric effects. To further understand the contribution of the hydrophobic interactions in the binding to the viologen scaffold, pyranine appended with a hexyl chain (PC6) and pyranine itself were used to non-covalently modify the MF-V films. It is known that the strength of the hydrophobic effect increases steeply with the length of the alkyl chain. The use of a hexyl chain appended pyranine (PC6) or pyranine itself (P) to modify MF-V could only neutralise the pore walls to allow ambipolar transport (Figure 3a,b). In both cases, the soaking with the donor was done at pH 3 to allow easy access to the viologens and the transport studies were carried out at pH 6. The observation of ambipolar transport in the case of MF-V-PC6 and MF-V-P implies that the binding energy of the viologen scaffold is not strong enough to be able to support charge reversal on the pore surface but only charge neutralisation (Figure S7). Furthermore, the DRIFTS spectrum of MF-V-PC6 didn’t show any distinct signature of alkyl chain packing unlike MF-V-PC12 (Figure 3d and S8a). This clearly indicates that hydrophobic interactions between donors in the case of PC12 are key for charge reversal by aiding the binding of the donors to the viologen scaffold. This additional stabilization could be suggested to have a greater contribution towards the binding energy in the case of MF-V-PC12@pH3, due to the increased possibility of having attractive hydrophobic interactions between the donor molecules when the donor occupancy is high. It could also be speculated that the ‘hydrophobic cohesion’ strengthens the charge-transfer interactions by providing a


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hydrophobic environment known to further stabilise C-T interactions. It should be noted that in the current work, we have carried out all the transport studies without adding free donors to the external solution. To provide a comparison to our previous work, we carried out the transport studies on MF-V-P in the presence of free pyranine (0.5 mM) in solution. The cyclic voltammograms showed a strong cationic transport and a weak anionic transport indicating that the charge reversal is not complete and that a significant amount of viologens are still unoccupied by donors (Figure 3c). This is in contrast to the observation of complete charge reversal in our previous work with pyranine and viologen scaffold synthesised using the iodopropyl silane route which passivates the surface silanols. So, the repulsive interactions of the donors with the silanolate moieties are responsible for the weaker binding in the case of pyranine and PC6. Furthermore, the Langmuir binding constant obtained from the binding titrations of pyranine (at pH 6) with the bulk mesoporous silica (SBA-15) modified with viologen silane was found to be 8 times smaller than the viologen silica prepared through the iodopropyl route, thus confirming that the silanolate moieties are responsible for the modulation/lowering of binding strength (Figure S9). So, we can see that by using the three forces, viz. C-T interactions, repulsive forces between pH-responsive silanol moieties and donor molecules, and donor dependent cohesive-hydrophobic forces, we can control the overall binding energy to modulate the donor occupancy of the viologens to qualitatively and quantitatively control the ion transport to achieve multiple ion transport states (Figure 4). The kinetics of binding of PC12 to MF-V was studied by following the surface charge of the pores by recording cyclic voltammograms at regular intervals. It was found that within 20 minutes of soaking with PC12, the cation transport reached saturation and even so, in 10 minutes the anion transport was suppressed (Figure S10). The faster kinetics of binding observed here in contrast to our previous work36 may be attributed to the fact that the pores in



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MF-V are not crowded with unreacted iodopropyl groups which could make for easier diffusion of the donors through the film. Furthermore, the non-covalent assemblies of MF-VPC12@pH3 were found to be quite stable even on soaking in water for extended periods of time. The transport properties of the film remained cation selective even on soaking in water for around 10 h. There was a 30 % decrease in the electrochemical current for positively charged [Ru(NH3)6]3+ but no significant change in the anion current density was observed (Figure 3e and S11). The decrease in cation flux could be attributed to the unbinding of PC12 from MF-V causing a reduction of the net negative charge density on the pore walls. Soaking the films for just another 10 minutes in PC12 could immediately refresh the non-covalent assemblies inside the pores (Figure 3e and S11). After establishing that the primary C-T interactions between viologen and the donors can be modulated by the silanolate moieties and cohesive hydrophobic interaction between appropriately chosen donors, further studies were carried out to validate the modulation of binding strength through reconfiguration of the non-covalent assemblies within the pores. The dynamic nature of non-covalent assemblies stems from their reversible nature which allows the stronger interactions to prevail over the weaker interactions lending these assemblies the dynamic reconfigurability. As noted before the non-covalent assemblies of PC6 and pyranine with MF-V were found to be weak and gave a near-neutral surface charge on the pores. So, when these films, MF-V-PC6 and MF-V-P, were subsequently soaked in PC12 solution at pH 3 for 2 h, the transport properties switched to strongly cation selective as in the case of MF-V-PC12 indicating the competitive displacement of weakly binding PC6 or pyranine from viologen by PC12 (Figure 3f and S12). This further confirms that the binding strength of MF-V for PC12 is greater than that for PC6 and pyranine. It is also worth noting that the non-covalent nature of these assemblies is responsible for their dynamic nature and


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ability to reorganise and reconfigure in the presence of a stronger stimulus to attain the most stable state under the given conditions. To examine the importance of charge-transfer interactions, an electrostatic blank (MF-B) was prepared by functionalising the pores walls with a quarternary ammonium silane (Figure S4). The transport properties of MF-B were similar to that of MF-V showing anion selective transport (Figure 3g). MF-B soaked in PC12 for 3 h, showed ambipolar transport indicating neutralization of charges on the pore wall unlike MF-V-PC12 which shows cation selective transport (Figure 3h). On subsequent soaking in water for 30 minutes, the transport switched back to anion selective (Figure 3h) but the anion selectivity is still lower than MF-B indicating that PC12 is still bound to MF-B causing a larger electrochemical response for positively charged [Ru(NH3)6]3+ than in MF-B. The DRIFTS spectra of PC12 soaked MF-B showed signs of alkyl chain packing (Figure S8b). It is worth noting that the soaking of MFB in PC12 switched the transport from anion selective to ambipolar even though it was not stable. The disparity can be understood by considering the fact that the binding of PC12 has a hydrophobic component which is independent of C-T and hence MF-B soaked in PC12 shows some amount of binding. On the other hand, when MF-B was soaked in PC6, no significant change was observed in the nature of ion transport (Figure S13). This further confirms that the binding of PC12 to MF-B was due to better hydrophobic cohesion between the donor molecules. So, we see that C-T interactions even though they contribute only a part of the binding energy are nevertheless very important in achieving charge reversal and in maintaining the stability of these assemblies. In conclusion, we have demonstrated a simple generic strategy to achieve multiple ion transport states by using a combination of three forces, viz. charge-transfer, electrostatic and hydrophobic interactions, to modulate the binding energy of the non-covalently binding motifs giving access to multiple surface charge densities to control ion transport through



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nanochannels (Figure 4). The supramolecular assembly of charge modifying donors could thus be controlled to exert a qualitative as well a quantitative control over the ion transport. The dynamic nature of these assemblies has also been illustrated, where, in the presence of a stronger binding donor the assembly re-organized to incorporate the stronger binding component and the permselectivity of the films could be switched accordingly. This study introduces the use of a “force dynamics” approach to achieving multiple ion transport states. The number of states can be increased just by changing one of the elements of the design, for example by simply attaching an amphoteric moiety to the pore wall would allow for attractive/repulsive forces between the pore wall and the negatively charged donors. In principle, the whole library of non-covalent motifs and interactions can be employed in this way to generate multiple ion transport states in response to different stimuli. The non-covalent design principles illustrated in this study would enable the construction of smart and re-configurable nanofluidic devices with precise control over ion transport. Such devices are employed for various applications such as bio-sensing,45-48 ion pumps,49-51 energy harvesting52-53 etc. that demand fine control over flux and direction of ion transport. We believe that the use of such a ‘molecular print-board’ strategy for control of surface charge would provide new opportunities for the fabrication of sophisticated iontronics devices by virtue of its modular design.


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Scheme 1. The use of three non-covalent forces, viz., charge-transfer interactions between viologen and donors (pyranine-based), electrostatic repulsion between the donors and pHresponsive silanols, and hydrophobic interactions between the alkyl chains of donors, to modulate the binding energy of the mesoporous viologen scaffold for the donors which determines the extent of charge reversal achieved.



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Figure 1. Cyclic voltammograms were recorded in the presence of 1 mM [Ru(NH3)6]3+ (red trace) and 1 mM [Fe(CN)6]3- (blue trace) on MF-V films at (a) pH 3 and (c) pH 6. Scan Rate: 200 mV/s. Supporting electrolyte: 0.1 M KCl. Schematics qualitatively depicting the charge on the pore walls as indicated by the nature of transport through the MF-V at (b) pH 3 and (d) pH 6. Please note that it is possible that there are silanols which are ionised at pH 3 also but they have not been indicated since they would be ionised at pH 6 as well.


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Figure 2. (a-d) Cyclic voltammograms recorded in the presence of 1 mM [Ru(NH3)6]3+ (red trace) and 1 mM [Fe(CN)6]3- (blue trace) on MF-V-PC12@pH3 films at pH 3 (a) and pH 6 (b), and on MF-V-PC12@pH6 films at pH 6 (c) and pH 3 (d). Scan Rate: 200 mV/s. Supporting electrolyte: 0.1 M KCl. The corresponding schematics (e-h) qualitatively depicting the charge on the pore walls at different pH values as indicated by the asymmetry of transport seen in the cyclic voltammograms (a-d) through the donor (PC12) modified MFV films.



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Figure 3. (a,b,c,f,g,h) Cyclic voltammograms recorded at pH 6 in the presence of 1 mM [Ru(NH3)6]3+ (red trace) and 1 mM [Fe(CN)6]3- (blue trace) on MF-V-PC6 before (a) and after (f) soaking in PC12, on MF-V-P in the absence (b) and presence (c) of 0.5 mM pyranine, and on MF-B before (g) and after (h) soaking in PC12. (e) Cyclic voltammograms recorded in the presence of 1 mM [Ru(NH3)6]3+at pH 6 on MF-V after soaking in PC12 for 1h, subsequently in water for 10 h and in PC12 again for 10 min. Scan Rate: 200 mV/s. Supporting electrolyte: 0.1 M KCl. (d) Diffuse reflectance infra-red fourier transform spectra of MF-V-PC12@pH3 and MF-V-PC6 films.


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Figure 4. The variation of peak current densities of [Fe(CN)6]3- and [Ru(NH3)6]3+ in MF-V, MF-V-PC6 and MF-V-PC12 at different pH showing the control over the cation or anion selective nature of ion transport as well the flux through the nanochannels.

ASSOCIATED CONTENT Supporting Information. Experimental procedure for the preparation of mesoporous silica thin film supported on ITO, Instrumentation details, voltammetry measurements, TEM and FE-SEM images of mesoporous silica thin film, chemical structures donor and acceptor molecules used in this study and other supporting figures (PDF). The following files are available free of charge.



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AUTHOR INFORMATION

Corresponding Author *M. Eswaramoorthy. Nanomaterials and Catalysis lab, Chemistry and Physics of Materials Unit, School of Advanced Materials (SAMat), Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O, Bangalore 560064 (India) [email protected] * Subi J. George. Supramolecular Chemistry Laboratory, New Chemistry Unit, School of Advanced Materials (SAMat), Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O, Bangalore 560064 (India) [email protected]

Present Addresses B.V.V.S. Pavan Kumar Centre for Organized Matter Chemistry, School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS (United Kingdom).

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We thank Prof. C. N. R. Rao, F. R. S. for his support and encouragement, JNCASR and Department of Science and Technology, Government of India for financial support. We thank M. Kumar and A. Jain for helpful discussions. We thank Sheikh Saqr Career Award Fellowships. B.V.V.S.P.K. thanks CSIR and Sheikh Saqr Junior Fellowships.


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