Pumping between phases with a pulsed-fuel molecular ratchet

The sorption of species from a solution into and onto solids underpins the sequestering of waste and pollutants, precious metal recovery, heterogeneous catalysis, analysis and separation science, and other technologies 1 , 2 . The transfer between phases tends to proceed spontaneously in the direction of equilibrium. For example, alkyl ammonium groups mounted on silica nanoparticles are used to chemisorb cucurbituril macrocycles from solution through host–guest binding 3 , 4 . Molecular ratchet mechanisms 5 – 7 , in which kinetic gating 8 – 12 inhibits or accelerates particular steps, makes it possible to progressively drive dynamic systems 13 – 16 away from equilibrium 17 – 21 . Here we report on molecular pumps 22 immobilized on polymer beads 23 – 25 that use an energy ratchet mechanism 5 , 9 , 19 – 21 , 26 – 30 to directionally transport substrates from solution onto the beads. On the addition of trichloroacetic acid (CCl 3 CO 2 H) 19 , 31 – 33 fuel 19 , 34 – 37 , micrometre-diameter polystyrene beads functionalized 38 with solvent-accessible molecular pumps sequester from the solution crown ethers appended with fluorescent tags. After fuel consumption, the rings are mechanically trapped in a higher-energy, out-of-equilibrium state on the beads and cannot be removed by dilution or exhaustive washing. This differs from dissipative assembled materials 11 , 13 – 16 , which require a continuous supply of energy to persist, and from conventional host–guest complexes. The addition of a second fuel pulse causes the uptake of more macrocycles, which drives the system further away from equilibrium. The second macrocycle can be labelled with a different fluorescent tag, which confers sequence information 39 on the absorbed structure. The polymer-bound substrates can be released back to the bulk either one compartment at a time or all at once. Non-equilibrium 40 sorption by immobilized artificial molecular machines 41 – 45 enables the transduction of energy from chemical fuels for the use, storage and release of energy and information. Artificial molecular machines bound on polymer beads enable non-equilibrium sorption of species from solution onto a solid surface.