Ultrasound-Responsive Systems as Components for Smart Materials.

Smart materials can respond to stimuli and adapt their responses based on external cues from their environments. Such behavior requires a way to transport energy efficiently and then convert it for use in applications such as actuation, sensing, or signaling. Ultrasound can carry energy safely and with low losses through complex and opaque media. It can be localized to small regions of space and couple to systems over a wide range of time scales. However, the same characteristics that allow ultrasound to propagate efficiently through materials make it difficult to convert acoustic energy into other useful forms. Recent work across diverse fields has begun to address this challenge, demonstrating ultrasonic effects that provide control over physical and chemical systems with surprisingly high specificity. Here, we review recent progress in ultrasound-matter interactions, focusing on effects that can be incorporated as components in smart materials. These techniques build on fundamental phenomena such as cavitation, microstreaming, scattering, and acoustic radiation forces to enable capabilities such as actuation, sensing, payload delivery, and the initiation of chemical or biological processes. The diversity of emerging techniques holds great promise for a wide range of smart capabilities supported by ultrasound and poses interesting questions for further investigations.

Platinum-DNA Origami Hybrid Structures in Concentrated Hydrogen Peroxide.

The DNA origami technique allows fast and large-scale production of DNA nanostructures that stand out with an accurate addressability of their anchor points. This enables the precise organization of guest molecules on the surfaces and results in diverse functionalities. However, the compatibility of DNA origami structures with catalytically active matter, a promising pathway to realize autonomous DNA machines, has so far been tested only in the context of bio-enzymatic activity, but not in chemically harsh reaction conditions. The latter are often required for catalytic processes involving high-energy fuels. Here, we provide proof-of-concept data showing that DNA origami structures are stable in 5 % hydrogen peroxide solutions over the course of at least three days. We report a protocol to couple these to platinum nanoparticles and show catalytic activity of the hybrid structures. We suggest that the presented hybrid structures are suitable to realize catalytic nanomachines combined with precisely engineered DNA nanostructures.

Electron circular dichroism in hot electron emission from metallic nanohelix arrays.

We investigate the electron emission from 3D chiral silver alloy nanohelices initiated by femtosecond laser pulses with a central photon energy of hν = 1.65 eV, well below the work function of the material. We find hot but thermally distributed electron spectra and a strong anisotropy in the electron yield with left- and right-circularly polarized light excitations, which invert in sign between left- and right-handed helices. We analyze the kinetic energy distribution and discuss the role of effective temperatures. Measurements of the reflectance and simulations of the absorbance of the helices based on retarded field calculations are compared to the anisotropy in photoemission. We find a significant enhancement of the anisotropy in the electron emission in comparison to the optical absorption. Neither simple thermionic nor a multiphoton photoemission can explain the experimentally observed asymmetries. Single photon deep-UV photoemission from these helices together with a change of the work function suggests a contribution of the chirally induced spin selectivity effect to the observed asymmetries.

Response to Comment on "Following Molecular Mobility during Chemical Reactions: No Evidence for Active Propulsion" and "Molecular Diffusivity of Click Reaction Components: The Diffusion Enhancement Question".

In their Comment (DOI: 10.1021/jacs.2c02965) on two related publications by our groups (J. Am. Chem. Soc. 2021, 143, 20884-20890; DOI: 10.1021/jacs.1c09455) and another (J. Am. Chem. Soc. 2022, 144, 1380-1388; DOI: 10.1021/jacs.1c11754), Huang and Granick discuss the diffusion NMR measurements of molecules during a copper-catalyzed azide-alkyne cycloaddition (CuAAC) "click" reaction. Here we respond to these comments and maintain that no diffusion enhancement was observed for any species during the reaction. We show that the relaxation agent does not interfere with the CuAAC reaction kinetics nor the diffusion of the molecules involved. Similarly, the gradient pulse length and diffusion time do not affect the diffusion coefficients. Peak overlap was completely removed in our study with the use of hydrazine as the reducing agent. The steady-state assumption does not hold for these diffusion measurements that take several minutes, which is the reason monotonic gradient orders are not suitable. Finally, we discuss the other reactions where similar changes in diffusion have been claimed. Our conclusions are fully supported by the results represented in our original JACS Article and the corresponding Supporting Information.

Amplification of Acoustic Forces Using Microbubble Arrays Enables Manipulation of Centimeter-Scale Objects.

Manipulation of macroscale objects by sound is fundamentally limited by the wavelength and object size. Resonant subwavelength scatterers such as bubbles can decouple these requirements, but typically the forces are weak. Here we show that patterning bubbles into arrays leads to geometric amplification of the scattering forces, enabling the precise assembly and manipulation of cm-scale objects. We rotate a 1 cm object continuously or position it with 15 µm accuracy, using sound with a 50 cm wavelength. The results are described well by a theoretical model. Our results lay the foundation for using secondary Bjerknes forces in the controlled organization and manipulation of macroscale structures.

Holograms for acoustics.

Holographic techniques are fundamental to applications such as volumetric displays, high-density data storage and optical tweezers that require spatial control of intricate optical or acoustic fields within a three-dimensional volume. The basis of holography is spatial storage of the phase and/or amplitude profile of the desired wavefront in a manner that allows that wavefront to be reconstructed by interference when the hologram is illuminated with a suitable coherent source. Modern computer-generated holography skips the process of recording a hologram from a physical scene, and instead calculates the required phase profile before rendering it for reconstruction. In ultrasound applications, the phase profile is typically generated by discrete and independently driven ultrasound sources; however, these can only be used in small numbers, which limits the complexity or degrees of freedom that can be attained in the wavefront. Here we introduce monolithic acoustic holograms, which can reconstruct diffraction-limited acoustic pressure fields and thus arbitrary ultrasound beams. We use rapid fabrication to craft the holograms and achieve reconstruction degrees of freedom two orders of magnitude higher than commercial phased array sources. The technique is inexpensive, appropriate for both transmission and reflection elements, and scales well to higher information content, larger aperture size and higher power. The complex three-dimensional pressure and phase distributions produced by these acoustic holograms allow us to demonstrate new approaches to controlled ultrasonic manipulation of solids in water, and of liquids and solids in air. We expect that acoustic holograms will enable new capabilities in beam-steering and the contactless transfer of power, improve medical imaging, and drive new applications of ultrasound.

Diffusion mechanisms of DNA in agarose gels: NMR studies and Monte Carlo simulations.

We report on the diffusion mechanism of short, single-stranded DNA molecules with up to 100 nucleobases in agarose gels with concentrations of up to 2.0% with the aim to characterize the DNA-agarose interaction. The diffusion coefficients were measured directly, i.e., without any model assumptions, by pulsed field gradient nuclear magnetic resonance (PFG-NMR). We find that the diffusion coefficient decreases, as expected, with an increase in both DNA strand length and gel concentration. In addition, we performed Monte Carlo simulations of particle diffusion in a model network of polymer chains, considering our experimental conditions. Together, the Monte Carlo simulations and the PFG-NMR results show that the decrease in diffusion coefficients in the presence of the agarose gel is due to a temporary adhesion of the DNA molecules to the surface of gel fibers. The average adhesion time to a given gel fiber increases with the length of the DNA strands but is independent of the number of gel fibers. The corresponding magnitude of the binding enthalpies of DNA strands to gel fibers indicates that a mixture of van der Waals interactions and hydrogen bonding contributes to the decreased diffusion of DNA in agarose gels.

Following Molecular Mobility during Chemical Reactions: No Evidence for Active Propulsion.

The reported changes in self-diffusion of small molecules during reactions have been attributed to "boosted mobility". We demonstrate the critical role of changing concentrations of paramagnetic ions on nuclear magnetic resonance (NMR) signal intensities, which led to erroneous measurements of diffusion coefficients. We present simple methods to overcome this problem. The use of shuffled gradient amplitudes allows accurate diffusion NMR measurements, even with time-dependent relaxation rates caused by changing concentrations of paramagnetic ions. The addition of a paramagnetic relaxation agent allows accurate determination of both diffusion coefficients and reaction kinetics during a single experiment. We analyze a copper-catalyzed azide-alkyne cycloaddition "click" reaction, for which boosted mobility has been claimed. With our methods, we accurately measure the diffusive behavior of the solvent, starting materials, and product and find no global increase in diffusion coefficients during the reaction. We overcome NMR signal overlap using an alternative reducing agent to improve the accuracy of the diffusion measurements. The alkyne reactant diffuses slower as the reaction proceeds due to binding to the copper catalyst during the catalytic cycle. The formation of this intermediate was confirmed by complementary NMR techniques and density functional theory calculations. Our work calls into question recent claims that molecules actively propel or swim during reactions and establishes that time-resolved diffusion NMR measurements can provide valuable insight into reaction mechanisms.

Light- and magnetically actuated FePt microswimmers.

Externally controlled microswimmers offer prospects for transport in biological research and medical applications. This requires biocompatibility of the swimmers and the possibility to tailor their propulsion mechanisms to the respective low Reynolds number environment. Here, we incorporate low amounts of the biocompatible alloy of iron and platinum (FePt) in its [Formula: see text] phase in microstructures by a versatile one-step physical vapor deposition process. We show that the hard magnetic properties of [Formula: see text] FePt are beneficial for the propulsion of helical micropropellers with rotating magnetic fields. Finally, we find that the FePt coatings are catalytically active and also make for Janus microswimmers that can be light-actuated and magnetically guided.

Investigating photoresponsivity of graphene-silver hybrid nanomaterials in the ultraviolet.

There have been several reports of plasmonically enhanced graphene photodetectors in the visible and the near infrared regime but rarely in the ultraviolet. In a previous work, we have reported that a graphene-silver hybrid structure shows a high photoresponsivity of 13 A/W at 270 nm. Here, we consider the likely mechanisms that underlie this strong photoresponse. We investigate the role of the plasmonic layer and examine the response using silver and gold nanoparticles of similar dimensions and spatial arrangement. The effect on local doping, strain, and absorption properties of the hybrid is also probed by photocurrent measurements and Raman and UV-visible spectroscopy. We find that the local doping from the silver nanoparticles is stronger than that from gold and correlates with a measured photosensitivity that is larger in devices with a higher contact area between the plasmonic nanomaterials and the graphene layer.

Diffusion Measurements of Swimming Enzymes with Fluorescence Correlation Spectroscopy.

Self-propelled chemical motors are chemically powered micro- or nanosized swimmers. The energy required for these motors' active motion derives from catalytic chemical reactions and the transformation of a fuel dissolved in the solution. While self-propulsion is now well established for larger particles, it is still unclear if enzymes, nature's nanometer-sized catalysts, are potentially also self-powered nanomotors. Because of its small size, any increase in an enzyme's diffusion due to active self-propulsion must be observed on top of the enzyme's passive Brownian motion, which dominates at this scale. Fluorescence correlation spectroscopy (FCS) is a sensitive method to quantify the diffusion properties of single fluorescently labeled molecules in solution. FCS experiments have shown a general increase in the diffusion constant of a number of enzymes when the enzyme is catalytically active. Diffusion enhancements after addition of the enzyme's substrate (and sometimes its inhibitor) of up to 80% have been reported, which is at least 1 order of magnitude higher than what theory would predict. However, many factors contribute to the FCS signal and in particular the shape of the autocorrelation function, which underlies diffusion measurements by fluorescence correlation spectroscopy. These effects need to be considered to establish if and by how much the catalytic activity changes an enzyme's diffusion. We carefully review phenomena that can play a role in FCS experiments and the determination of enzyme diffusion, including the dissociation of enzyme oligomers upon interaction with the substrate, surface binding of the enzyme to glass during the experiment, conformational changes upon binding, and quenching of the fluorophore. We show that these effects can cause changes in the FCS signal that behave similar to an increase in diffusion. However, in the case of the enzymes F1-ATPase and alkaline phosphatase, we demonstrate that there is no measurable increase in enzyme diffusion. Rather, dissociation and conformational changes account for the changes in the FCS signal in the former and fluorophore quenching in the latter. Within the experimental accuracy of our FCS measurements, we do not observe any change in diffusion due to activity for the enzymes we have investigated. We suggest useful control experiments and additional tests for future FCS experiments that should help establish if the observed diffusion enhancement is real or if it is due to an experimental or data analysis artifact. We show that fluorescence lifetime and mean intensity measurements are essential in order to identify the nature of the observed changes in the autocorrelation function. While it is clear from theory that chemically active enzymes should also act as self-propelled nanomotors, our FCS measurements show that the associated increase in diffusion is much smaller than previously reported. Further experiments are needed to quantify the contribution of the enzymes' catalytic activity to their self-propulsion. We hope that our findings help to establish a useful protocol for future FCS studies in this field and help establish by how much the diffusion of an enzyme is enhanced through catalytic activity.

Absolute diffusion measurements of active enzyme solutions by NMR.

The diffusion of enzymes is of fundamental importance for many biochemical processes. Enhanced or directed enzyme diffusion can alter the accessibility of substrates and the organization of enzymes within cells. Several studies based on fluorescence correlation spectroscopy report enhanced diffusion of enzymes upon interaction with their substrate or inhibitor. In this context, major importance is given to the enzyme fructose-bisphosphate aldolase, for which enhanced diffusion has been reported even though the catalysed reaction is endothermic. Additionally, enhanced diffusion of tracer particles surrounding the active aldolase enzymes has been reported. These studies suggest that active enzymes can act as chemical motors that self-propel and give rise to enhanced diffusion. However, fluorescence studies of enzymes can, despite several advantages, suffer from artefacts. Here, we show that the absolute diffusion coefficients of active enzyme solutions can be determined with Pulsed Field Gradient Nuclear Magnetic Resonance (PFG-NMR). The advantage of PFG-NMR is that the motion of the molecule of interest is directly observed in its native state without the need for any labelling. Furthermore, PFG-NMR is model-free and thus yields absolute diffusion constants. Our PFG-NMR experiments of solutions containing active fructose-bisphosphate aldolase from rabbit muscle do not show any diffusion enhancement for the active enzymes, nor the surrounding molecules. Additionally, we do not observe any diffusion enhancement of aldolase in the presence of its inhibitor pyrophosphate.

Chemotaxis of Active Janus Nanoparticles.

While colloids and molecules in solution exhibit passive Brownian motion, particles that are partially covered with a catalyst, which promotes the transformation of a fuel dissolved in the solution, can actively move. These active Janus particles are known as "chemical nanomotors" or self-propelling "swimmers" and have been realized with a range of catalysts, sizes, and particle geometries. Because their active translation depends on the fuel concentration, one expects that active colloidal particles should also be able to swim toward a fuel source. Synthesizing and engineering nanoparticles with distinct chemotactic properties may enable important developments, such as particles that can autonomously swim along a pH gradient toward a tumor. Chemotaxis requires that the particles possess an active coupling of their orientation to a chemical gradient. In this Perspective we provide a simple, intuitive description of the underlying mechanisms for chemotaxis, as well as the means to analyze and classify active particles that can show positive or negative chemotaxis. The classification provides guidance for engineering a specific response and is a useful organizing framework for the quantitative analysis and modeling of chemotactic behaviors. Chemotaxis is emerging as an important focus area in the field of active colloids and promises a number of fascinating applications for nanoparticles and particle-based delivery.

Optical and Thermophoretic Control of Janus Nanopen Injection into Living Cells.

Devising strategies for the controlled injection of functional nanoparticles and reagents into living cells paves the way for novel applications in nanosurgery, sensing, and drug delivery. Here, we demonstrate the light-controlled guiding and injection of plasmonic Janus nanopens into living cells. The pens are made of a gold nanoparticle attached to a dielectric alumina shaft. Balancing optical and thermophoretic forces in an optical tweezer allows single Janus nanopens to be trapped and positioned on the surface of living cells. While the optical injection process involves strong heating of the plasmonic side, the temperature of the alumina stays significantly lower, thus allowing the functionalization with fluorescently labeled, single-stranded DNA and, hence, the spatially controlled injection of genetic material with an untethered nanocarrier.

Chiral Plasmonic Hydrogen Sensors.

In this article, a chiral plasmonic hydrogen-sensing platform using palladium-based nanohelices is demonstrated. Such 3D chiral nanostructures fabricated by nanoglancing angle deposition exhibit strong circular dichroism both experimentally and theoretically. The chiroptical properties of the palladium nanohelices are altered upon hydrogen uptake and sensitively depend on the hydrogen concentration. Such properties are well suited for remote and spark-free hydrogen sensing in the flammable range. Hysteresis is reduced, when an increasing amount of gold is utilized in the palladium-gold hybrid helices. As a result, the linearity of the circular dichroism in response to hydrogen is significantly improved. The chiral plasmonic sensor scheme is of potential interest for hydrogen-sensing applications, where good linearity and high sensitivity are required.

Structured light enables biomimetic swimming and versatile locomotion of photoresponsive soft microrobots.

Microorganisms move in challenging environments by periodic changes in body shape. In contrast, current artificial microrobots cannot actively deform, exhibiting at best passive bending under external fields. Here, by taking advantage of the wireless, scalable and spatiotemporally selective capabilities that light allows, we show that soft microrobots consisting of photoactive liquid-crystal elastomers can be driven by structured monochromatic light to perform sophisticated biomimetic motions. We realize continuum yet selectively addressable artificial microswimmers that generate travelling-wave motions to self-propel without external forces or torques, as well as microrobots capable of versatile locomotion behaviours on demand. Both theoretical predictions and experimental results confirm that multiple gaits, mimicking either symplectic or antiplectic metachrony of ciliate protozoa, can be achieved with single microswimmers. The principle of using structured light can be extended to other applications that require microscale actuation with sophisticated spatiotemporal coordination for advanced microrobotic technologies.

Magnetic Propulsion of Microswimmers with DNA-Based Flagellar Bundles.

We show that DNA-based self-assembly can serve as a general and flexible tool to construct artificial flagella of several micrometers in length and only tens of nanometers in diameter. By attaching the DNA flagella to biocompatible magnetic microparticles, we provide a proof of concept demonstration of hybrid structures that, when rotated in an external magnetic field, propel by means of a flagellar bundle, similar to self-propelling peritrichous bacteria. Our theoretical analysis predicts that flagellar bundles that possess a length-dependent bending stiffness should exhibit a superior swimming speed compared to swimmers with a single appendage. The DNA self-assembly method permits the realization of these improved flagellar bundles in good agreement with our quantitative model. DNA flagella with well-controlled shape could fundamentally increase the functionality of fully biocompatible nanorobots and extend the scope and complexity of active materials.

Active Nanorheology with Plasmonics.

Nanoplasmonic systems are valued for their strong optical response and their small size. Most plasmonic sensors and systems to date have been rigid and passive. However, rendering these structures dynamic opens new possibilities for applications. Here we demonstrate that dynamic plasmonic nanoparticles can be used as mechanical sensors to selectively probe the rheological properties of a fluid in situ at the nanoscale and in microscopic volumes. We fabricate chiral magneto-plasmonic nanocolloids that can be actuated by an external magnetic field, which in turn allows for the direct and fast modulation of their distinct optical response. The method is robust and allows nanorheological measurements with a mechanical sensitivity of ∼0.1 cP, even in strongly absorbing fluids with an optical density of up to OD ∼ 3 (∼0.1% light transmittance) and in the presence of scatterers (e.g., 50% v/v red blood cells).

A loop-gap resonator for chirality-sensitive nuclear magneto-electric resonance (NMER).

Direct detection of molecular chirality is practically impossible by methods of standard nuclear magnetic resonance (NMR) that is based on interactions involving magnetic-dipole and magnetic-field operators. However, theoretical studies provide a possible direct probe of chirality by exploiting an enantiomer selective additional coupling involving magnetic-dipole, magnetic-field, and electric field operators. This offers a way for direct experimental detection of chirality by nuclear magneto-electric resonance (NMER). This method uses both resonant magnetic and electric radiofrequency (RF) fields. The weakness of the chiral interaction though requires a large electric RF field and a small transverse RF magnetic field over the sample volume, which is a non-trivial constraint. In this study, we present a detailed study of the NMER concept and a possible experimental realization based on a loop-gap resonator. For this original device, the basic principle and numerical studies as well as fabrication and measurements of the frequency dependence of the scattering parameter are reported. By simulating the NMER spin dynamics for our device and taking the (19)F NMER signal of enantiomer-pure 1,1,1-trifluoropropan-2-ol, we predict a chirality induced NMER signal that accounts for 1%-5% of the standard achiral NMR signal.

Self-propelling nanomotors in the presence of strong Brownian forces.

Motility in living systems is due to an array of complex molecular nanomotors that are essential for the function and survival of cells. These protein nanomotors operate not only despite of but also because of stochastic forces. Artificial means of realizing motility rely on local concentration or temperature gradients that are established across a particle, resulting in slip velocities at the particle surface and thus motion of the particle relative to the fluid. However, it remains unclear if these artificial motors can function at the smallest of scales, where Brownian motion dominates and no actively propelled living organisms can be found. Recently, the first reports have appeared suggesting that the swimming mechanisms of artificial structures may also apply to enzymes that are catalytically active. Here we report a scheme to realize artificial Janus nanoparticles (JNPs) with an overall size that is comparable to that of some enzymes ∼30 nm. Our JNPs can catalyze the decomposition of hydrogen peroxide to water and oxygen and thus actively move by self-electrophoresis. Geometric anisotropy of the Pt-Au Janus nanoparticles permits the simultaneous observation of their translational and rotational motion by dynamic light scattering. While their dynamics is strongly influenced by Brownian rotation, the artificial Janus nanomotors show bursts of linear ballistic motion resulting in enhanced diffusion.