Light-induced actuating nanotransducers

Abstract

Nanoactuators and nanomachines have long been sought after, but key bottlenecks remain. Forces at submicrometer scales are weak and slow, control is hard to achieve, and power cannot be reliably supplied. Despite the increasing complexity of nanodevices such as DNA origami and molecular machines, rapid mechanical operations are not yet possible. Here, we bind temperature-responsive polymers to charged Au nanoparticles, storing elastic energy that can be rapidly released under light control for repeatable isotropic nanoactuation. Optically heating above a critical temperature [Formula: see text] = 32 °C using plasmonic absorption of an incident laser causes the coatings to expel water and collapse within a microsecond to the nanoscale, millions of times faster than the base polymer. This triggers a controllable number of nanoparticles to tightly bind in clusters. Surprisingly, by cooling below [Formula: see text] their strong van der Waals attraction is overcome as the polymer expands, exerting nanoscale forces of several nN. This large force depends on van der Waals attractions between Au cores being very large in the collapsed polymer state, setting up a tightly compressed polymer spring which can be triggered into the inflated state. Our insights lead toward rational design of diverse colloidal nanomachines.

Keywords: colloidal; nanoactuator; nanomachine; pNIPAM; plasmonics.

Fig. 1.

Reversible assembly of ANTs. (A) Formation of pNIPAM-coated Au nanoparticles by mixing in solution, and heating above $T_c$ = 32 °C to attach pNIPAM onto Au. In deflated state, NPs aggregate tightly together (blue sol). Cooling then explosively splits clusters into individual ANTs (red sol). Further heating and cooling results in reversible fission and aggregation. (B) Extinction spectra of Au NPs initially (black) and in 40-μM pNIPAM (orange), under laser heating (red) and cooled (purple). (Inset) Peak wavelength changes over successive cycles of laser heating and cooling. (C) Extinction spectral kinetics of Au NP–pNIPAM mixture through one cycle of laser irradiation. (D–F) SEM images of ANTs before (D), during (E), and after (F), irradiating with 10 W cm⁻² for 5 min. (D, Inset) magnifies assembled ANT cluster.

Fig. 2.

Mechanism of reversible assembly. (A) Change of hydrodynamic size from DLS and (B) zeta potential, of Au–pNIPAM assembly (initial state marked ○) for four cycles of heating and cooling measured at 25 °C and 40 °C. (C) Potential energy when bringing extra ANT nanoparticle closer to a single cluster, in both hot (red) and cold (blue) states near $T_c$. In the cold state swelled ANTs bounce from each other. In the hot state, the potential energy depends on the number of NPs in the cluster as each contribute more repulsive charge (Right).

Fig. 3.

ANT tunability. (A–F) Extinction spectra of Au NP–pNIPAM system at (A and B) different concentrations of pNIPAM, (C and D) different irradiation times at 5 W, and (E and F) different irradiation powers at 10 min. B, D, and F show corresponding extracted longitudinal coupled plasmon mode wavelengths from A, C, and E.

Fig. 4.

Dynamics of nanomachines. (A) SEM of agarose-encapsulated ANT cluster on Si, with (B) schematic. (C) Scattering spectra of the agarose-encapsulated ANT cluster on Si when cycling the temperature between 28 °C and 35 °C, with (D) scattering dynamics (integrated from 700 to 900 nm) when modulated by 0.5-mW 635-nm laser (Top), and (E) dark-field images. (F) Absorbance profile across a single microdroplet (Inset, images) containing pNIPAM and 60-nm AuNPs, when thermally cycled to drive the ANTs onto and off the oil/water interface.