The intrinsic band gap photoluminescence of semiconducting single-walled carbon nanotubes (SWNTs) makes them promising biological imaging probes in the second near-infrared (NIR-II, 1.0-1.4 μm) window. Thus far, SWNTs used for biological applications have been a complex mixture of metallic and semiconducting species with random chiralities, preventing simultaneous resonant excitation of all semiconducting nanotubes and emission at a single well-defined wavelength. Here, we developed a simple gel filtration method to enrich semiconducting (12,1) and (11,3) SWNTs with identical resonance absorption at ~808 nm and emission near ~1200 nm. The chirality sorted SWNTs showed ~5-fold higher photoluminescence intensity under resonant excitation of 808 nm than unsorted SWNTs on a per-mass basis. Real-time in vivo video imaging of whole mouse body and tumor vessels was achieved using a ~6-fold lower injected dose of (12,1) and (11,3) SWNTs (~3 μg per mouse or ~0.16 mg/kg of body weight vs 1.0 mg/kg for unsorted SWNTs) than a previous heterogeneous mixture, demonstrating the first resonantly excited and chirality separated SWNTs for biological imaging.

In recent years, a new generation of quantum confined colloidal semiconductor structures has emerged, with more complex shapes than simple quantum dots 1, 2 . These include nanorods 3 and tetrapods 4 . Beyond shape, it is also now possible to spatially vary the electron and hole potentials within these nanoparticles by varying the composition. Examples of these new structures include seeded dots, rods, and tetrapods, which contain a CdSe core embedded within a CdS shell [5][6][7][8] . These structures may have many uses beyond those envisioned for simple quantum dots, which are frequently employed in luminescent applications 9 . This paper is concerned with changes in the optoelectronic properties of tetrapods when the arms are bent. We demonstrate that seeded tetrapods can serve as an optical strain gauge, capable of measuring forces on the order of nanonewtons. We anticipate that a nanocrystal strain gauge with optical readout will be useful for applications ranging from sensitive optomechanical devices to biological force investigations.A tetrapod nanocrystal consists of a central core with four arms branching out at the tetrahedral angle 4 . We have previously shown a few ways in which tetrapod arms can be bent. For instance, when a CdTe tetrapod (4 nm wide and 100 nm long arms) is deposited on a substrate through solvent evaporation, the fluid exerts a capillary force which pulls the tetrapod towards the substrate, in some cases permanently deforming the arms 10 . Salmeron and coworkers used an atomic force microscope to press on the outward-projecting arms of surface-immobilized CdTe tetrapods, and have shown that for forces below 100 nN, the tetrapod flexes elastically 11 . Motivated by these observations, Wang and coworkers calculated the electronic level structure of a CdSe tetrapod with different degrees of arm bending, induced by nanonewton forces, and predicted a red-shift of the energy gap with increasing strain 12 .Seeded tetrapods consisting of a CdSe core with CdS arms are highly luminescent 6 and are very symmetric objects. When placed under an anisotropic stress, we expect a reduction in symmetry, which will influence the electronic level structure. To fully quantify such effects, we have examined a series of samples under diverse conditions of stress and strain. Specifically, we have examined the luminescence from seeded dots, rods, and tetrapods placed in a diamond anvil cell (DAC) as a function of applied pressure in a highly hydrostatic medium, which transmits pressure nearly isotropically, as well as a non-hydrostatic medium, which transmits pressure anisotropically. We compare the tetrapods to rods to separate out stress-induced strain effects in a single rod from effects specifically arising in a tetrapod consisting of connected *apalivisatos@lbl.gov. Supporting Information Available. Experimental methods for nanocrystal characterization, diamond anvil cell studies, and spectral analysis; emission peak maxima for CdSe/CdS rods of different lengths in toluene in a diamond anvil cell (Fig....