Optical manipulation of plasmonic nanoparticles provides opportunities for fundamental and technical innovation in nanophotonics. Optical heating arising from the photon-to-phonon conversion is considered as an intrinsic loss in metal nanoparticles, which limits their applications. We show here that this drawback can be turned into an advantage, by developing an extremely low-power optical tweezing technique, termed opto-thermoelectric nanotweezers (OTENT). Through optically heating a thermoplasmonic substrate, alight-directed thermoelectric field can be generated due to spatial separation of dissolved ions within the heating laser spot, which allows us to manipulate metal nanoparticles of a wide range of materials, sizes and shapes with single-particle resolution. In combination with dark-field optical imaging, nanoparticles can be selectively trapped and their spectroscopic response can be resolved in-situ. With its simple optics, versatile low-power operation, applicability to diverse nanoparticles, and tuneable working wavelength, OTENT will become a powerful tool in colloid science and nanotechnology.

Precise nanometre-scale imaging of soft structures at room temperature poses a major challenge to any type of microscopy because fast thermal fluctuations lead to significant motion blur if the position of the structure is measured with insufficient bandwidth. Moreover, precise localization is also affected by optical heterogeneities, which lead to deformations in the imaged local geometry, the severity depending on the sample and its thickness. Here we introduce quantitative thermal noise imaging, a three-dimensional scanning probe technique, as a method for imaging soft, optically heterogeneous and porous matter with submicroscopic spatial resolution in aqueous solution. By imaging both individual microtubules and collagen fibrils in a network, we demonstrate that structures can be localized with a precision of ∼10 nm and that their local dynamics can be quantified with 50 kHz bandwidth and subnanometre amplitudes. Furthermore, we show how image distortions caused by optically dense structures can be corrected for.

13Collagen is the most abundant protein in humans and the primary component of the 14 extracellular matrix, a meshwork of biopolymer networks, which provides structure and 15 integrity to tissues. Its mechanical properties profoundly influence the fate of cells. The 16 cell-matrix interaction, however, is not well understood due to a lack of experimental 17 techniques to study the mechanical interplay between cells and their local environment. 18Here we introduce Activity Microscopy, a new way to visualize local network mechanics 19 with single filament resolution. Using collagen I networks in vitro, we localize fibril 20 positions in two-dimensional slices through the network with nanometer precision and 21 quantify the fibrils' transverse thermal fluctuations with megahertz bandwidth. Using a 22 fibril's thermal fluctuations as an indicator for its tension, we find a heterogeneous stress 23 distribution, where "cold" fibrils with small thermal fluctuations surround regions of highly 24 fluctuating "hot" fibrils. We seed HeLa cells into collagen networks and quantify the 25 anisotropy in the propagation of their forces. 26

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