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 temperatureresponsive 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 T c = 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 T c 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.ctuators are needed to turn energy sources into physical movement. These can be for microrobotics, sensing, storage devices, smart windows and walls, or more general functional and active materials. Such artificial muscles have gained rapidly increasing interest (1, 2) leading to micropropellers (3, 4), gas jets from catalytic surfaces (5), and DNA machines (6). However, the actuation methods, delivery of energy, and forces obtained (typically 10 fN/nm 2 ) are limited so far (7): Magnetic fields are inconvenient to apply locally for actuation, as is >200°C heating to actuate polymer fibers; the nanocatalysis of chemical fuels lacks controllability, whereas DNA machines rely on "fuel" DNA strands to competitively bind and operate on very slow (second) timescales. Piezoelectric-type materials used in high-end instrumentation (such as atomic force microscopy or nanopositioning stages) provide short travel but with inorganic materials that are dense, delicate, expensive, hard to fabricate, and demand high voltages (150-300 V), as is also true for electrostrictive rubbers and relaxor ferroelectrics (8, 9). Many biological systems such as Escherichia coli (10), cilia (11), or nematocysts (12) provide sophisticated models for nanomachines (13). Although molecular motors and artificial muscles from hydrogels (14, 15), colloids (16), or liquid crystalline elastomers (17, 18) successfully mimic such behaviors, they are very slow (on the order of seconds) and the forces generated are very small (∼ pN). This is because either the energy density stored in the system is low or the energy release is inefficient.To overcome this we design a colloidal actuating transducer system with high-energy storage (1,000 k B T/cycle) and fast (>MHz) release mechanism....

Polymer-assisted self-assembly of gold nanoparticle monolayers and their dynamical switching

“…This self-assembly process is accelerated through several cycles of heating and cooling, bringing PNIPAM to thermodynamic equilibration with the aqueous phase. 30 The resulting coated particles along with water are mixed with hexadecane through vigorous shaking and settle at the H 2 O/hexadecane interface (Fig. 1b and c) within 10 min.…”

“…Separate measurements have shown the capability for switching such PNIPAM AuNP constructs on microsecond timescales. 30,39 …”

“Hot” in Plasmonics: Temperature‐Related Concepts and Applications of Metal Nanostructures

“…The actuating nanotransducers (ANTs), consist of thermo‐sensitive polymers—poly( N ‐isopropylacrylamide), abbreviated as pNIPAM—attached to plasmonic (gold) nanoparticles and dispersed in water. The plasmonic nanoparticles efficiently and quickly convert light into heat, triggering a strong response from the polymer.…”

Resolving Optical and Catalytic Activities in Thermoresponsive Nanoparticles by Permanent Ligation with Temperature‐Sensitive Polymers

“…These studies lead us to conclude that the contradictory phenomena reported in the literature described above may be ascribed to the quality of thermoresponsive polymer passivation on the surface of the NP.When PNIPAM ligands are not strongly attached to the Au NP surface, [7,21,22] the partial dissociation of PNIPAM into the solution occurs and thus becomes free linear PNIPAM chains.T hey function as the physical crosslinkers to facilitate the aggregation of Au NPs when temperature increases above LCST,a nd correspondingly ar ed-shift of plasmonic absorption with decreased intensity and an on/off catalytic behavior are observed. In contrast, with as trong tethering of PNIPAM ligands on the Au NP surface [5,19,20] and the temperature rising over the LCST,only the collapse (i.e., shrinkage) of surface PNIPAM ligands occurs and thus no aggregation of Au NPs,h ence as maller red-shift of plasmonic absorption with increased intensity and an on-monotonic catalytic action are yielded.…”