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Light-induced actuating nanotransducers
Edited by Vinothan N. Manoharan, Harvard University, Cambridge, MA, and accepted by the Editorial Board March 30, 2016 (received for review December 9, 2015)

Significance
Scientists have dreamt of nanomachines that can navigate in water, sense their environment, communicate, and respond. Various power sources and propulsion systems have been proposed but they lack speed, strength, and control. We introduce here a previously undefined paradigm for nanoactuation which is incredibly simple, but solves many problems. It is optically powered (although other modes are also possible), and potentially offers unusually large force/mass. This looks to be widely generalizable, because the actuating nanotransducers can be selectively bound to designated active sites. The concept can underpin a plethora of future designs and already we produce a dramatic optical response over large areas at high speed.
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
Actuators 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/nm2) 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 (
To overcome this we design a colloidal actuating transducer system with high-energy storage (1,000
Results and Discussion
Colloidal Actuators.
ANTs are assembled by functionalizing 60-nm-diameter citrate-stabilized Au NPs with pNIPAM via ligand exchange above
Reversible assembly of ANTs. (A) Formation of pNIPAM-coated Au nanoparticles by mixing in solution, and heating above
No such huge spectral shifts (
Scanning electron microscopy (SEM) images taken at different stages confirm this assembly process (Materials and Methods; Fig. 1 D–F). Initially the Au NPs remain well dispersed (Fig. 1D) but above
Reversible Clustering.
Zeta potential and dynamic light-scattering (DLS) measurements (Fig. 2A and B) confirm our model of light-induced reversible tuning (Fig. 1A). Initially, a sparse coating of amino-terminated pNIPAM displaces some of the charged citrate originally attached to each Au NP (○). When the solution is heated above
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
Actuation Forces.
Actuation works when heating and cooling the solution around
Surveying macroscale to nanoscale actuators (32) shows forces scale with mass
Optical Actuation.
Light-triggered actuation allows tuning of the nanoassembly by varying pNIPAM concentration, laser irradiation time, and power (Fig. 3). The initial pNIPAM concentration controls the surface charge of the Au NPs (SI Appendix, Fig. S10), and is crucial in determining the cluster saturation size. For pNIPAM concentrations below 20 μM, the plasmon resonance peak can redshift to 745 nm, but this redshift decreases at higher concentration (Fig. 3 A and B). With excess pNIPAM the coating thickness increases, spacing the Au NP cores further apart within the cluster and decreasing the maximum redshift. In all cases, the ANTs recover to their initial state around 535 nm (blue, Fig. 3B and SI Appendix, Fig. S11).
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.
Irradiation times influence the temperature of the ANTs (SI Appendix, Fig. S1), changing the kinetics of pNIPAM assembly onto Au NPs (Fig. 3 C and D). As irradiation times increase, the clusters grow, limited by their charge balance and diffusion (SI Appendix, Fig. S15). Similar effects are seen with increasing laser powers providing they exceed the
Actuator Performance.
This colloidal actuator enables remote light control of nanodevices through reversible expansion between AuNPs. Fabrication of the actuator nanoparticles on a large scale, and their operational mechanism, are both simple. They are compatible with aqueous environments and work at room temperature, with
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.
Upon cooling, the agarose is found to be forced up around the cluster edges by the swelling ANTs, which require forces
Van der Waals forces are crucial in providing sufficient attractive force in the collapsed pNIPAM state to bind NPs, while being not too strong to prevent them being thrust apart when switching the pNIPAM to the inflated state. The high optical cross-section of plasmonic Au NP cores enhances the local energy absorbed from the incident light, reducing the total power needed to switch the pNIPAM surrounding each NP. Whereas Au cores are thus ideal, van der Waals forces between other metallic cores also work. Critical for reversibility here is the charging limit on cluster size, without which clusters grow large and insoluble. Such nanoactuators are expected to prove of great utility in on-demand remotely controlled, fully reversible dynamic assembly, for nanomachines such as DNA origami (SI Appendix, Fig. S16), for overcoming the problematic surface tension in microdroplets (Fig. 4F and SI Appendix, Fig. S9) and microelectronic mechanical devices, for optically controlled microfluidic pumps and valves, as well as for wallpaper-scale optics such as nonfading large-area photochromics for buildings (color changes in Fig. 1A). Although we demonstrate here reversible expansion and contraction, adapting this for nanomachinery requires reconfiguring the isotropic-into directional-forces, for instance by nanoconfinement, attachment to scaffolds, or nonisotropic pNIPAM coating.
Materials and Methods
Methods and any associated references are available in the SI Appendix. The raw data of the figures in this paper can be found at https://www.repository.cam.ac.uk/handle/1810/254762.
Acknowledgments
We thank Edward Booker for help with temperature measurements; Rohit Chikkaraddy for simulations; and Elisa Hemmig, Vivek Thacker, and Ulrich Keyser for providing DNA origami samples. This research is supported by UK Engineering and Physical Sciences Research Council Grants EP/G060649/1 and EP/L027151/1, and ERC Grants LINASS 320503 and EMATTER 280078. V.K.V. acknowledges support from The Royal Society through the University Research Fellowships.
Footnotes
- ↵1To whom correspondence may be addressed. Email: dt413{at}cam.ac.uk or jjb12{at}cam.ac.uk.
Author contributions: T.D., V.K.V., and J.J.B. designed research; T.D., V.K.V., and A.R.S. performed research; A.R.S., C.J.F., S.K.S., and O.A.S. contributed new reagents/analytic tools; T.D., V.K.V., D.F., and J.J.B. analyzed data; and T.D., V.K.V., O.A.S., D.F., and J.J.B. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. V.N.M. is a guest editor invited by the Editorial Board.
Data deposition: The raw data of the figures in this paper can be found at https://www.repository.cam.ac.uk/handle/1810/254762.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1524209113/-/DCSupplemental.
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