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.

Biosensors based on the localized surface plasmon resonance (LSPR) of individual metallic nanoparticles promise to deliver modular, low-cost sensing with high-detection thresholds. However, they continue to suffer from relatively low sensitivity and figures of merit (FOMs). Herein we introduce the idea of sensitivity enhancement of LSPR sensors through engineering of the material dispersion function. Employing dispersion and shape engineering of chiral nanoparticles leads to remarkable refractive index sensitivities (1,091 nm RIU−1 at λ=921 nm) and FOMs (>2,800 RIU−1). A key feature is that the polarization-dependent extinction of the nanoparticles is now characterized by rich spectral features, including bipolar peaks and nulls, suitable for tracking refractive index changes. This sensing modality offers strong optical contrast even in the presence of highly absorbing media, an important consideration for use in complex biological media with limited transmission. The technique is sensitive to surface-specific binding events which we demonstrate through biotin–avidin surface coupling.

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). KEYWORDS.Magneto-plasmonics, chiral plasmonics, chiroptical switch, nanorheologyThe resonant optical field enhancement in plasmonic nanostructures is of interest in research disciplines ranging from sensing to energy conversion, and is the basis for modern advances in metamaterials and the associated capabilities in shaping electromagnetic fields 1-3 .The scope of potential applications of nanoplasmonic structures is greatly extended by tailoring the nanostructure, for instance to shift the resonance frequency while keeping the overall size of the nanoparticle small 4-7 , or by incorporating diverse functionalities, such as magnetic 8 , chiral 9 , and electrical 10 . Recent fabrication advances have also allowed the programmatic growth of nanoparticles that lack mirror symmetry [11][12][13][14][15] . Such chiral nanoparticles mimic their molecular counterparts by exhibiting optical activity, but with One particularly challenging application is the measurement of rheological properties in complex fluids. Such systems contain a mixture of multiple phases; for instance, many biological fluids contain solids consisting of isolated microparticles or a network of macromolecules suspended in a fluid phase 20 . Because of the solid phase, macroscopic rheological measurements will generally show non-Newtonian viscoelastic behavior even if the liquid phase is a simple Newtonian fluid 21 . For instance, the viscosity of blood plasma is a crucial indicator for clinical diagnoses 22,23 , but cannot be determined in whole blood due to the presence of leukocytes (10-15 µm) and erythrocytes (6-8 µm) 24,25 . Furthermore, the solid phase in complex fluids, such as blood cells, contributes to absorption and scattering which complicate optical measurements.By combining multiple materials and shape control, we here introduce chiral magnetoplasmonic structures that can be actuated in solution. Since the chiroptical spectrum depends on the alignment of the chiral structure (Figure 1a), we can achieve chiroptical switching and exploit it for nanorheological measurements using picomolar probe concentrations. The scheme works by using...