Supramolecular polymers1-5 refer to molecular self-assembly into larger, more complex structures driven by reversible non-covalent interactions. This field has attracted continuous attention as these supramolecular structures could endow materials with unique properties, such as self-healing6, 7 and responsivity8-10. Most synthetic supramolecular polymers are formed under equilibrium or as metastable (kinetically trapped) states, whereas living organisms use biofuel-driven dissipative supramolecular structures far from equilibrium to perform complex biological functions. For example, guanosine-5’-triphosphate (GTP) is used as fuel to power transient microtubes for controlling cellular machinery. Inspired by nature’s out-of-equilibrium self-assembled supramolecular systems, researchers developed chemical reaction cycles to drive assembly and feedback-controlled disassembly of small molecules or nanoparticles11, 12. Various high-energy chemicals have been used as fuels in chemical reaction cycles for dissipative self-assembly, such as EDC,13-15 DNA,16, 17 amino acid,18, 19 ATP,20-23 light,24-29 and other chemicals.30-34 The development of fuel-driven dissipative self-assembly opened broad perspectives for smart materials with life-like features, such as oscillations,35-39 replications,40-42 transient nanostructures,18 dissipation,43-45 and adaption.46 However, these chemical systems typically consume all their ‘fuel’ within a single cycle, and repeated operation fuel additions lead to the accumulation of waste. To date, only the well-known Belousov-Zhabotinsky and a few related oscillators can show oscillations in a closed system – but there are no design rules that allow a rational design of fuel-driven closed systems.
Here, we overcome above mentioned difficulties by creating a dissipative supramolecular polymerization from a gas-liquid two-phase redox reaction system. In our proposed closed system, the chemical fuel, oxygen, is stored in the gas phase while inactive monomers and hydrazine are stored in the aqueous solution. Diffusion of aerial oxygen into the liquid phase is accelerated by shaking, which instantly leads to oxidation of viologens which subsequently co-assemble with pyranine into supramolecular nanotubes. Without mechanical agitation, the diffusion of aerial oxygen into the liquid phase slows down and reduction of viologens by hydrazine present in the liquid phase becomes the dominating reaction, leading to disassembly of supramolecular polymers into inactivated monomers. Of particular importance is the waste, nitrogen gas, which is released from solution in each cycle, ensuring that no waste accumulates after each reaction cycle. The principle presented here may provide a general and promising route to dissipative supramolecular self-assembly in closed systems, advancing the development of transient functional materials with life-like behaviors.
Shake-induced out-of-equilibrium redox reaction of viologens in open and closed chemical systems
We first investigated the dissipative performance of viologen28, 47, which is the central building block for our proposed dissipative supramolecular polymerization, in both open and closed chemical systems (Fig. 1a). The dissipative reaction cycles are driven by shaking-induced redox reactions in an air-aqueous two phase, in which O2 is the oxidant and N2H4•H2O is the reductant. The first experiment was done with 3 mM of alkyl substituted viologen (C12-MV•+) at pH = 12 in the presence of N2H4•H2O (10% v/v). Time dependent UV-vis spectra were recorded as shown in Fig. 1b. Upon shaking a sudden disappearance of absorbance intensity at 603 nm (corresponding to C12-MV•+) is clearly seen, and then the absorption band gradually increased, signifying the decay phase. A decayed color change (kreduction= 6.18×10− 4 L·mol− 1·s− 1, koxidation = 1.50×104 L·mol− 1·s− 1) was observed during the shake-induced redox reaction of C12-MV2+, as shown in Fig. 1d and Video S1. To demonstrate the repeatability of this transient system, refueling by subsequent shake was carried out and multiple cycles were obtained, in which the number of cycles could be further increased after reloading of reductant (Fig. 1e). The lifetime of the shake-induced transient redox reaction of C12-MV•+ is highly dependent on the number of reductants. As the equivalent of N2H4•H2O increased from 5–20% (v/v), it was observed that the lifetime of the transient C12-MV2+ drastically decreased from 300 minutes to 35 minutes (Fig. 1c, and Figure S6), which is because the high concentration of the reducing agent resulted in a faster reduction of C12-MV2+. The conversion associated with the color change mechanism is further investigated by time-dependent proton NMR. The addition of N2H4•H2O into the solution of C12-MV2+ (pH = 12) induced the gradual disappearance of peaks at 8.5 and 9.1 ppm corresponding to the protons in C12-MV2+ over time, indicating the formation of radicals (C12-MV•+) (Figure S7). The corresponding signals (8.5 and 9.1 ppm) re-appeared upon shaking the sample in the NMR tube. Additionally, EPR experiments on the basic solution of C12-MV•+ in the presence of excess amount of N2H4•H2O also supported the transient formation of C12-MV2+: signal (378 mT) corresponding to C12-MV•+ disappeared by shaking and gradually increased back (Figure S8).
Next, we took advantage of the low solubility of air in water to develop a shake-triggered dissipative reaction in a closed configuration. UV-vis measurements were performed to investigate the reaction process. As displayed in Fig. 1f, the UV-vis spectra exhibited shake induced reversible cycles (disappear and gradually raise of absorption band at 603 nm) which is similar to the open system described above. However, compared to the open system, the volume ratio of gas-to-liquid (Vg/ Vl) has a tremendous effect on the repeatability of these cycles since the air is fixed and limited in a closed system. When Vg/ Vl = 1:2, the redox reaction cycles could only be repeated 2 times and additional shake did not trigger the redox reaction of C12-MV2+, which attributed to the complete consumption of oxygen (fuel) in the closed system (Fig. 1f and 1g). As the volume ratio of gas-to-liquid increased from 1:2 to 2:1, it was observed that the repeated dissipative process of C12-MV2+ increased from 2 times to 6 times. It is worth noting that the redox cycles could continue by simply introducing fresh air, in which O2 was reloaded as the fuel in the gas phase (Figure S9).
Shake-induced transient supramolecular polymerization
We constructed the mechanosensitive supramolecular self-assembly system based on the study of thermodynamically stable supramolecular polymerization of alkane substituted viologen (C12-MV2+) and pyranine (PN), (Supporting Information Figure S1-S5), in which the self-assembly was driven by charge transfer interaction (CT interaction) and amphiphilic interaction. The mechanism of transient polymerization was shown in Fig. 2a, which was similar to the monomer C12-MV•+. During the process of transient supramolecular polymerization, shake is the key trigger for accelerating the diffusion of O2 from air into the solution. Upon shaking, similar to the individual monomer, the C12-MV•+ turned to C12-MV2+ by oxidation and followed by co-assembly with pyranine into supramolecular polymers. After shake is stopped, C12-MV2+ gradually turned back C12-MV•+ by reduction in the presence of excess amount of N2H4•H2O, which led to the decayed disassembly of C12-MV2+/ PN supramolecular polymers. Here too, nitrogen gas evolved as the waste but would not accumulate in the reaction solution because of its low solubility in water, which endowed a unique property, waste traceless, with our proposed system. The kinetics of transient polymerization and the degree of aggregation (α) were characterized by viscosity measurement and TEM. As shown in Fig. 2b, the degree of aggregation (α) after shaking in the solution containing C12-MV2+ (3 mM) and PN (3 mM) in the presence of N2H4•H2O (10% v/v) decreased (disassemble) gradually over 7 h, but another shake triggered another cycle of supramolecular polymerization. The reversible nature of supramolecular polymerization-depolymerization was displayed by performing at least six cycles in a closed system (Fig. 2c). The number of cycles of shake-induced supramolecular polymerization could be further increased by the re-addition of N2H4•H2O and fresh air. To monitor the morphological changes of transient polymerization in real-time, the system was characterized using TEM at different time points (Fig. 2d-2g). TEM images showed shake-triggered molar equivalent C12-MV2+ and PN (1 mM) self-assembled into well-defined nanofibers with 80 nm in diameter and several hundreds of microns in length. Subsequently, the fibers gradually shrunk and transformed into short fibers and vesicles when the solution was kept stagnant. The fibers re-appeared after mechanical stimulation (Figure S10). The evolution in structure of supramolecular polymers over time seen from TEM is in line with the above viscosity measurements. These results provide clear evidence of dynamic supramolecular polymerization.
Transient supramolecular helix
The above design of charged supramolecular polymer also offers additional electrostatic interactions, which could accommodate host-guest interactions for the formation of transient chiral supramolecular structures via introduction of charged chiral molecule into the reaction solution. To verify this hypothesis, chiral L-(-)-Phenyllactic acid and D-(+)-Phenyllactic acid were introduced as an additive in two separate dissipative processes, respectively (Fig. 3a). Circular dichroism (CD) was used to measure the time-dependent variation in the chirality of supramolecular polymers. Interestingly, a strong CD signal (λ max = 450 nm) was observed when the aqueous solution with C12-MV2+, PN, L-(-)-Phenyllactic acid and N2H4•H2O upon shaking (Fig. 3b), indicating the formation of single handed supramolecular helix by the addition of L-(-)-Phenyllactic acid into supramolecular polymerization system. The CD signals gradually disappeared as disassembly of C12-MV2+/ PN based supramolecular polymer by reduction of N2H4•H2O. Similarly, a shake-induced transition supramolecular helix of opposite handed helix is formed when D-(+)-Phenyllactic acid is introduced into supramolecular polymerization system (Fig. 3e). Additionally, supramolecular helix was also characterized by TEM (Fig. 3c and 3f). Twisted micrometers long fibers instead of nanotubes were formed when solutions (C12-MV2+/ PN, N2H4•H2O) with additional chiral L-(-)-Phenyllactic or D-(+)-Phenyllactic acid were shaken. Gradual disassembly of the fibers was observed when the reaction solution stayed still over time (Fig. 3d, 3g, S11 and S12), which was consistent with CD measurement.
Transient fluorescence and ultrasound guided patterns
EPR measurement on equimolar C12-MV2+/ PN (1 mM) samples in the presence of N2H4•H2O (20% v/v) was employed to characterize the dynamic behavior of radical formation (C12-MV•+) during the disassembly process which eventually influences the supramolecular polymerization. The EPR spectrum (Figure S14) of the solution measured immediately after the shaking show the quenching of radical signal at 379 mT and the signal is gradually increased over time while the solution stands still during the measurement.
In general, the radicals of viologens known to quench on fluorescent dyes, rather, in our case the solution of C12-MV•+/ PN showed stronger emission than C12-MV2+/ PN (Table S1 fluorescent quantum yield φf : 1.43% vs 0.29%). This is because C12-MV•+ itself tended to form nanoaggregates and is likely to precipitate from the solution, which leaves little opportunity for energy transfer between C12-MV•+ and PN. Moreover, the fluorescence intensity of the solution was rapidly quenched upon shaking the solution for few seconds, which was the result of aggregation caused quenching (ACQ) of PN during the supramolecular polymerization process48 (Fig. 4b, 4d, S15, S16 and Video S2). Fluorescence of the solution gradually turned on when stayed it still over time, which indicated the system had unique shake-induced quenching properties (Fig. 4a). Strikingly, in contrast to the C12-MV2+/ PN solution, the MV2+/ PN exhibited a behavior of shaking-induced fluorescence enhancement (Fig. 4a and Video S3). In the system of MV•+/ PN/ N2H4•H2O, due to the MV•+ is highly soluble and would not aggregate, the interaction between MV•+ and PN is strong, hence the increased energy transfer contributing to greater fluorescence quenching. And the fluorescence intensity of solution of MV•+/ PN/ N2H4•H2O raised rapidly after shaking the sample for 2 seconds (Figs. 4c, 4e and S17), which is because the reduced energy transfer between MV•+and PN when the radical MV•+ were turned to MV2+ by oxidation of O2 (Figure S18).
Next, we sought to produce a naked-eye visible transient fluorescent pattern inspired by the above shake-induced fluorescence cycles of quenching and enhancement. First, a sonication linked template was utilized to generate patterns on the solution of (MV2+ or MV•+)/ PN/ N2H4•H2O, unfortunately, which could not produce patterns because the patterns were destroyed by template when it was taken away from the aqueous solution. Alternatively, inspired by Kim et al use audio sound to produce transient patterns49, we used the ultrasound (an untouched shake) to drive the mechano-sensitive redox reaction for pattern formation (Figure S22 – S31 and Video S4 – S6).
Ultrasound is mechanical waves that propagate in an elastic medium, dissipative solution in our case, which accelerates the diffusion of oxygen from air to aqueous solution at specific regions. We observed that ultrasound wave in the reaction medium affects the diffusion rate of the gas in a certain direction of the solution, and subsequently induces a directional oxidation process which results in unique visible patterns (Fig. 4f and S25). Depending on the viscosity of the medium, different patterns under visible light can be obtained. Take the system (MV•+/ PN/ N2H4•H2O) as a model, the solution exhibited islands when the solution was subjected to sonication for 2.5 minutes and stayed for 1 minute. The patterns of islands gradually changed to concentric circles of diffusion patterns when the solution was loaded with more PEG (10kDa, 0–5%, weight contents) (Figure S22, S26 and Video S4). The changes in the pattern are due to the variation in solution viscosity with increased as loading more PEG, which reduces the diffusion rate of oxygen and makes its oxidation flow more continuous. A continuous flow pattern can be obtained under sonication using C12-MV2+ alone without the addition of PEG due to the surface activity of C12-MV2+. Moreover, the size of the central pattern is directly related to the concentration of C12-MV2+ which affects the surface tension of the solution (Figure S23, S24 and Video S6). However, during the attempted patterning of C12-MV2+/ PN, the aggregated C12-MV•+ was difficult to be dispersed again by ultrasound and stable patterns could not be obtained (Figure S28, S29). Best patterns also can be seen under both visible and UV (365 nm) light since the solution (MV•+/ PN / N2H4•H2O) had shaken-induced emission enhancement properties. Especially, a clearly reproduced concentric “heart” shape pattern was generated when the solution was on sonication for around 2 min and stayed a min. When the solution was on sonication, ultrasound induced liquid flow was accompanied by directional diffusion of oxygen to oxidize MV•+/ PN for discoloration, which led to formation of patterns (Fig. 4f, Video S5). The patterns spread throughout the surface of the solution when the sonication for longer time (ten minutes) leading to bright green fluorescent from the entire solution.
The reversibility of this “dark-fluorescent patterns-fully bright” cycle depends on whether the system is opened or sealed. The fully bright green fluorescent-on state would turn back to dark state once the solution was sealed and stayed still for around 30 min (Fig. 4g). However, in the case of the system exposed to the air, it turned to bright island-type patterns when the solution was kept still for about 30 min, and then gradually turned to bright green throughout of surface instead of dark state when keeping the solution for a longer time. Interestingly, the shape of mold affects the directional pattern. The mold changed from round to triangle or square shape mold for ultrasound also induced transient patterning but not equivalent to the circular shape. It turned out that only islands or short lines patterns formed (Figure S30, S31) when the solution containing C12-MV•+/ PN / N2H4•H2O in triangle or square shape mold, which was the result of mechanic waves from ultrasound took place different reflections on different shapes of molds.