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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - Jellyclock (Light Actuated Self-Pulsing Mircogels)

Teaser

Summary

In the ERC project Jellyclock we prepare micrometer sized hydrogels that respond under non-equilibrium conditions to IR-light effected temperature changes by fast contraction. So far we achieved reciprocal changes by which the small objects were able to perform work either by active swimming like it is known from bacteria or by pumping a fluid. In order to achieve the main objective of the project, i.e. to enable such gels to undergo self-oscillation at continuous IR-irradiation we focused on two tasks during the first period of the project: (i) Designing a feed-back mechanism where the IR-effected raise in temperature ceases heating, so that a cooling phase is introduced automatically. (ii) Designing the structure of the hydrogel in such a way, that structural changes follow a hysteresis path corresponding to a bistability that prevents the system to reach an equilibrium state. Besides the ability to perform mechanical work in an energy dissipative reciprocal action, the implementation of an efficient feed-back mechanism (i) and the bistability (ii) are the fundamental requirements for the self-oscillating gel, Jellyclock. For the feed back mechanism we successfully prepared two systems: a) Aligned gold nanorods on the surface of a thin microgel disc that bends upon heating whereby the distance between the ends of the nanorods is increased and the plasmon absorption is blue shifted so that IR-light heating is stopped. b) Gold nanorods that were specifically modified either on their longitudinal sides or at the tips to adhere upon raising temperature temperature side-to-side or end-to-end. For the first time we have been able to demonstrate reversibility of such processes. For the bistability we have been able to prepare small gel discs that undergo a snap-type transformation from a concave to convex bending along the direction of the aligned gold nanorods. In a second approach we have developed a process for an interpenetrating gel structure where a continuous thermoresponsive gel is interpenetrated by a network of tethered gold nanorods that cluster side to side or end to end. The heterogeneous structure of these networks has to optimized further for the reciprocal and hysteretic formation of the clusters. For the detailed characterization of such structures and their response to temperature changes we have implemented the STED super high resolution optical microscopy and new NMR-studies. Furthermore we have set up an analytical model to describe the far from equilibrium changes of the gel structures based on the coupling of the swelling depending elasticity of the gels, the rates of temperature changes, and the rates of volume changes. The model has been shown to be helpful to define geometries for which the establishment of self-oscillation is most likely.

Work performed

Synthetic strategies have been developed to decorate gold nanorods with a thermosensitive polymer brush that reversibly assemble themselves in response to near-infrared light and forms orientationally defined cluster without precipitation. A major challenge has been to elaborate a reliable protocol for selective grafting of thermosensitive polymer only at the ends of the nanorods while another polymer is grafted on the side and vice versa. Near-infrared light causes nanorods to assemble and disassemble on demand, allowing the dynamics organization of the nanoparticle and providing a path for preparing a dispersion that displays a self-deactivate heat upon irradiation with NIR-light. Indeed, we have shown that end-to-end or side-by-side assembly/disassembly of nanorods induces clear difference in plasmonic resonance profiles causing reversible shifts of the maximum absorption bands. Irradiation with NIR-light induces fast heating of the dispersion and subsequent AuNRs association. This in turn inhibits photothermal conversion and thus stops the heating of the medium. (Manuscripts ready to be submitted: a) SjÃ¶ren Schweizerhof, Alexander Nedliko, Ahmed Mourran, Khosrow Rahimi, Dimitry N. Chigrin, Helmut Keul, Gero von Plessen, Martin MÃ¶ller, How to control the temperature-driven, reversible end-to end association of gold nanorods in aqueous dispersion, b) SjÃ¶ren Schweizerhof, Ahmed Mourran, Alexander Nedilko, Helmut Keul, Gero von Plessen, and Martin MÃ¶ller, 1,2Poly(N-isopropylacrylamide) modified gold nanorods: Crucial parameters for reversibly controlling particle association by light or heat.)
Currently we focus on controlling the degree of association and the inter-particle distance as it control the shift in the absorption maximum for optimum contrast between the absorbing and the non absorbing state of the dispersion. In parallel, a synthetic pathway is being worked out to combine the AuNRs aggregates with thermosensitive microgel.

In a parallel bottom-up approach we have successfully decorated small gel discs (100 micron in diamter, 5 micron in height) with lines of aligned gold nanorods. It could be demonstrated that the heat induced shape and volume variations stopped the heating effect and that thus the anticpated feed back mechanism was achieved. End-to-end prealigned gold nanorods were transferred from a silicon template to a PNIPAm (micro) gel and covalently linked in parrallel lines. The light adsorption and thus the heating effect by the nanoparticles is now controlled by the state and the dynamic of the three-dimensional swollen hydrogel network. At low temperatures, the hydrogel is swollen and the nanoparticles behave as isolated with a high NIR absorption state. Whereas at high temperature, the collapse of the hydrogel reduces the inter-particles distance that shift the absorption plasmon bands, resulting in a non-absorbing state. These microgels are produced using a process based on the PRINT-process (Particle Replication In Non-wetting Templates), which defines the shape and dimensions of the produced microgel based on a polyfluoropolyether-based microstructured mold. We are able to measure a strong temperature dependent spectral shift (200 nm) by the AuNR containing microgels. Additionally we measured the effect of temperature on the photothermal heating efficiency, which shows lower photothermal heating when the gel is collapsed compared to when it is swollen, generating the aforementioned feedback mechanism.

The system will be further developed to exhibit snap buckling in order to demonstrate hysteresis and to allow self-osscialltion. (Mansucript for peer reviewed publication is in preparation)

In a a third attempt we develop an analytical model for the dynamic interplay of the eleasticity variations, the heat volume/concentration dependence of the heating cooling and the volume change of defined microgel obejects under nonequilibrium effects. here the challenge is that all three effects ar couple

Final results

We have been able to design microswimmers that swim by body shape deformation seeking their direction autonomously. We will furthermore a demonstrate chemotaxis for the swimming direction. Progress so far provides the basis to achieve te self-oscillation effect that is the most important objective of the project. As a result we expect that we can develop a fully autonomous microswimmer whose activity is is based on the harvesting of light energy and that operates independendly from outside forces and time controlling pulses.