Multicomponent supramolecular systems could be used to prepare exciting new functional materials, but it is often challenging to control the assembly across multiple length scales. Here we report a simple approach to forming patterned, spatially resolved multicomponent supramolecular hydrogels. A multicomponent gel is first formed from two low-molecular-weight gelators and consists of two types of fibre, each formed by only one gelator. One type of fibre in this ‘self-sorted network’ is then removed selectively by a light-triggered gel-to-sol transition. We show that the remaining network has the same mechanical properties as it would have done if it initially formed alone. The selective irradiation of sections of the gel through a mask leads to the formation of patterned multicomponent networks, in which either one or two networks can be present at a particular position with a high degree of spatial control.
Low-molecular-weight hydrogels are responsive materials in which the self-assembled small molecule gelators are held together by non-covalent forces1,2. These gels are typically responsive to changes in physical parameters, such as temperature or pH, in going back and forth between a gel and a liquid. Generally, single molecules self-assemble into one-dimensional fibres, which form the gel matrix. However, using two different molecules, each of which can individually form a gel, presents the opportunity to tune the properties of the gels further3,4,5,6. For example, specific functionality could be placed within the gel, or controlled fibrous architectures suitable for bulk heterojunctions7 and conductive materials could be formed. Such systems would be extremely useful if the internal spatial structure of the gel could be controlled such that both networks existed in some regions, but only one network existed elsewhere. This could allow gels to be formed with specific functional groups presented only within certain areas of the gel. To build such multicomponent networks requires that we are able to control both the primary fibres and their assembled structures in space. This is difficult enough for a single-component system, in which there are few reliable design rules, and it is becoming increasingly clear that the process of self-assembly is critical to the final gel properties8.
Conceptually, spatially resolved multicomponent networks could be generated if we could form two independent fibrous networks, and then selectively remove one of these networks only at specific points. Selective melting of one network in a two-component system has been demonstrated previously9. With a view to optimizing the spatial resolution, light-responsive gelators are a clear choice as one component of the system. A number of such gelators exist10,11,12,13,14,15,16,17,18,19,20,21; however, in all cases these have been investigated as single-component systems in which the light-triggered degelation results in a spatially resolved system with one region a gel and the other a liquid. One example exists in which the spectral properties of the gel can be altered using a light-sensitive gelator, with a gel being maintained throughout22. Light-responsive polymeric gel networks have also been reported23,24. To incorporate a light-sensitive gelator as one component of a two-gelator system, we need to be able to assemble and then disassemble such a network in the presence of a second gelator network.
Results and discussion
We recently showed that self-sorted networks can be formed by the slow acidification of a solution that contains two different pH-triggered gelators25,26,27. As the pH decreases slowly, the gelator with the highest pKa assembles first and buffers the system before the pH reaches the pKa of the second gelator. This method most often leads to the assembly of self-sorted fibres, in which each fibre contains exclusively one of the two gelators. We showed that this was the case using a combination of small-angle neutron scattering and fibre X-ray diffraction25. We hypothesized, therefore, that if one of the gelators was not only pH triggered, but also light triggered, this method could be used to form our required network (Fig. 1).
A stilbene-based gelator (1) was synthesized by the reaction of the acid chloride of 4,4′-stilbene dicarboxylic acid with phenylalanine ethyl ester, followed by deprotection with lithium hydroxide. The trans-isomer forms hydrogels when the pH of a solution at a concentration of 5 mg ml–1 of 1 is decreased from 10 to 4. To lower the pH controllably in a uniform manner throughout the sample, we used the slow hydrolysis of glucono-δ-lactone (GdL) to gluconic acid28, as we have described elsewhere29. A translucent hydrogel is formed (Fig. 2a, inset, referred to as gel-1 throughout), which has rheological properties that are typical of this type of low-molecular-weight hydrogel (Supplementary Fig. 1). The storage modulus (G′) and loss modulus (G″) are relatively independent of frequency, with G′ being approximately an order of magnitude higher than G″.
The apparent pKa of 1 is 5.8 and, in agreement with our previous work30, 1 forms gels when the pH is below this pKa. The corresponding cis-isomer of gelator 1 was formed by irradiation of a solution of 1 with a 365 nm light-emitting diode (LED) at pH 10 (see Supplementary Fig. 2). Attempts to carry out its gelation in a similar manner to that of trans-isomer 1 (by slowly decreasing the pH of the solutions that contained the cis-isomer) resulted in a precipitate rather than a gel, which shows that the cis-isomer is not an effective gelator. When the gel prepared using trans-isomer 1 was irradiated with the 365 nm LED, it converted into a liquid (Fig. 2b inset; see Fig. 4d for rheological data). When a mask was used, selective spatial conversion of the gel into a liquid could be achieved, similar to other light-triggered gelators (Supplementary Fig. 3). Xerogels of gel-1 were imaged using scanning electron microscopy (SEM), and the images showed that the gels are a result of the self-assembly of the gelator into a network of fibres (Fig. 2a). After irradiation, the fibres had transformed into ill-defined spherical aggregates (Fig. 2b). These aggregates are similar to those formed on lowering the pH of solutions that contained cis-1 by using GdL (see Supplementary Fig. 2).
We then combined 1 with a second gelator, 2, preparing a solution at pH 10 that contained both at a concentration of 5 mg ml–1 (a total gelator concentration, therefore, of 10 mg ml–1). Gelator 2 was chosen on the basis of a significantly different molecular structure and the apparent pKa of the terminal carboxylic acid (5.0)31; our previous data suggested that both of these factors should encourage self-sorting25,26. Control experiments showed that gels formed from 2 alone (gel-2) were not affected by irradiation with the 365 nm LED, either visually or rheologically (Supplementary Fig. 5).
Gels were formed from the mixed solution, again by the addition of GdL, and are referred to as gel-1,2 throughout. The use of GdL, and the therefore slow pH change, allows us to monitor the gelation process with time25,29. Characterization by 1H NMR spectroscopy can be used to monitor gelation; as the molecules self-assemble and form fibres, the structures become invisible to NMR spectroscopy. The hydrolysis of GdL is highly reproducible. Hence, the pH of a separate solution can be correlated with the NMR data. In a parallel experiment, the evolution of the rheological properties can be measured (Fig. 3a).
The evolution of 1H NMR spectra throughout the reaction demonstrates clearly that a sequential assembly process occurs, with 1 becoming NMR invisible before 2. The plot of the integral size for each gelator is shown in Fig. 3a. This sequential assembly is expected from the higher apparent pKa of 1 as compared with that of 2. As 1 self-assembles, the rheological data show that a gel is formed. After around eight minutes, there is an increase in both G′ and G″, with a second increase that occurs at around 20 minutes. Such a two-stage development in the moduli is common for this kind of gelator when using GdL to trigger the gelation29,32. As the pH drops to around 5, 2 also starts to become NMR invisible, as expected from the pKa of this gelator. Concurrently, there is a significant further increase in both G′ and G″. This correlates with gel-2 formed in a single-component system being significantly stronger than gel-1. It is noticeable that G′ at the point where only 1 has self-assembled is lower than might be expected from the data for 1 alone (Supplementary Fig. 1). We hypothesize that this is because the network forms in the presence of 2, which may be acting as a surfactant above its apparent pKa. Indeed, Ulijn and co-workers have shown that similar gels can be affected by the presence of surfactant-like gelators33. Nonetheless, it is clear that the self-assembly of 1 and 2 is a sequential process across multiple length scales. The final gel-1,2 formed from the self-sorting mixture is both translucent and homogeneous (Fig. 3b). Under a handheld ultraviolet lamp, the fluorescence from the gel is uniform in colour and intensity (Fig. 3c; the blue colour arises mainly from emission from the gels under irradiation), which indicates that there is no phase separation or segregation of the gelators over these longer length scales. Based on our previous work, we propose that the sequential assembly leads to a self-sorted two-component network25,26. To prove this is difficult; as we show below, the fibres from both networks appear very similar, which makes differentiation by microscopy impossible. However, if the networks are truly self-sorted then selective removal of one network should leave the other intact (Fig. 1). We show that this is what happens through mechanical data.
Unlike in the case of gel-1, when a self-sorted gel-1,2 is irradiated using the 365 nm LED, the gel retains its structural integrity, although the gel strength decreases (Fig. 4a). There was a slow decrease in the rheological data for the irradiated gel compared with the data from the as-prepared gel when the irradiation was carried out over time, with G′ decreasing from approximately 3.8 × 104 Pa to 1.9 × 104 Pa (Fig. 4b). After two hours of irradiation, the rheological data then stabilized, with G′ and G″ being significantly lower than for the initially as-prepared gel (Fig. 4b). The data were in close agreement with the data for gel-2 (Fig. 4b). As can be seen from the control experiments (Fig. 4c,d), the rheological properties of gel-2 are essentially unaffected by light irradiation, but those from gel-1 are strongly affected. The implication of this data is that under irradiation the fibres formed from 1 were removed selectively as 1 was isomerized and that the remaining network was essentially that which would have formed in the absence of 1. This shows a high degree of control over the fibre networks in a supramolecular gel. The temperature of the gels only rose by 2 °C after being irradiated for one hour, so drying out or an increased temperature of the gels is unlikely to be a factor in the changes in the rheology of the system. The photographs in Fig. 5a–c show the gels before and after irradiation.
To the best of our knowledge, this removal of one of the networks within a self-sorted low-molecular-weight gel is unprecedented. We can further achieve spatial patterning by combining the irradiation step with a mask. Example photographs are shown in Fig. 5d,e, for which a star-shaped mask (shown in Fig. 5f) was used to pattern the gel. In these only the star shape was exposed to ultraviolet light. As can be seen, the bulk gel structure is maintained, but (most clearly under ultraviolet light), it is clear that the network formed by 1 has been disrupted only where the mask did not cover the gel. SEM of the dried sample shows that the areas not irradiated by the LED are still composed of fibres (Fig. 5g) that are similar to those of the networks formed by 1 and 2. However, in the centre of the star where the self-sorted gel has been irradiated, a dense network was found, in which fibres covered in spheres can be seen (Fig. 5h). This is consistent with the spherical structures formed by cis-1 being deposited on the fibres formed by the self-assembly of 2.
To prove the spatial resolution further, we freeze-dried a gel in which one-half had been irradiated and the other not, again using a mask. The data (Supplementary Fig. 5) clearly show that where the gel was exposed to ultraviolet light, both cis-1 and trans-1 are present. The region of the gel not exposed to irradiation only contains trans-1.
We have shown how the concept of self-sorted low-molecular-weight gels can be combined with photoresponsive gelators to allow a high degree of control over the rheological properties of bulk gels. Spatially resolved gels can also be prepared; one network can be removed selectively, which is a step forward from the current state of the art. This demonstrates that the two networks formed initially must be truly independent, in close analogy with interpenetrating polymer hydrogels34. This is the first example, as far as we are aware, of this type of control over multiple-component low-molecular-weight gels. This methodology opens up the possibility of spatially controlling the rheological properties of a gel, and allows a significant advance over the simple gel/no-gel switch normally observed with photoswitchable gelators. We envisage that this methodology could be used to prepare complex structured gels.
The Supplementary Information gives details of the synthesis.
A pH-switch method was used to form the hydrogels. Single-component gels were prepared at a concentration of 5 mg ml–1. The gelator was dissolved at high pH in 2 ml of water. In the case of 1, two molar equivalents of a 0.1 M sodium hydroxide were used. For 2, one molar equivalent of 0.1 M sodium hydroxide was used. The solution was stirred until all the gelator had dissolved. This solution was next transferred to a vial that contained 10 mg of GdL, shaken gently and then 1 mL was transferred to a 20 ml plastic syringe with the nozzle removed (to act as a mould). The open top of the syringe was covered with Parafilm and the solution was left to gel overnight. The gel was removed from the syringe by gently pushing the plunger. For mixed-component gels, separate solutions of each gelator were prepared at 10 mg ml–1, and then 1 ml aliquots of each solution were combined to provide a 2 ml solution that contained each gelator at a concentration of 5 mg ml–1. This solution was then added to 20 mg of GdL and again allowed to gel as above.
Irradiation of the gels
Gels were placed onto a glass microscope slide, which was placed inside a plastic Petri dish together with a wet paper towel, which kept the air saturated with water and prevented the gel from drying out. The lid of the Petri dish had a hole cut out to allow the LED to be able to irradiate the sample. A 365 nm LED (LedEngin Inc., LZ1-10U600) with a light source powered by a TTi QL564P power supply operating at 1.0 W was used to irradiate the gel samples. When a mask was used, the shape was cut out of a sheet of opaque plastic and placed over the sample prior to irradiation.
Other experimental techniques are described in the Supplementary Information.
E.R.D. thanks the Engineering and Physical Sciences Research Council (EPSRC) for a Doctorial Training Accounts studentship. D.A. thanks the EPSRC for a Fellowship (EP/L021978/1).
About this article
Chinese Journal of Polymer Science (2017)