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We provide evidence of a local synaptic nano-environment in the brain extracellular space (ECS) lying within 500 nm of postsynaptic densities. To reveal this brain compartment, we developed a correlative imaging approach dedicated to thick brain tissue based on single-particle tracking of individual fluorescent single wall carbon nanotubes (SWCNTs) in living samples and on speckle-based HiLo microscopy of synaptic labels. We show that the extracellular space around synapses bears specific properties in terms of morphology at the nanoscale and inner diffusivity. We finally show that the ECS juxta-synaptic region changes its diffusion parameters in response to neuronal activity, indicating that this nano-environment might play a role in the regulation of brain activity.

International audience; In localization microscopy, the position of isolated fluorescent emitters are estimated with a resolution better than the diffraction limit. In order to image thick samples, which are common in biological applications, there is considerable interest in extending the depth-of-field of such microscopes in order to make their accuracy as invariant as possible to defocus. For that purpose, we propose to optimize annular binary phase masks placed in the pupil of the microscope in order to generate a point spread function for which the localization accuracy is almost invariant along the optical axis. The optimization criterion is defined as the localization accuracy in the plane expressed in terms of the Cramér-Rao bound. We show that the optimal masks significantly increase the depth-of-field of single-molecule imaging techniques relatively to an usual microscope objective.

International audience; Single-molecule localization microscopy has become a prominent approach to study structural and dynamic arrangements of nanometric objects well beyond the diffraction limit. To maximize localization precision, high numerical aperture objectives must be used; however, this inherently strongly limits the depth-of-field (DoF) of the microscope images. In this work, we present a framework inspired by "optical co-design" to optimize and benchmark phase masks, which, when placed in the exit pupil of the microscope objective, can extend the DoF in the realistic context of single fluorescent molecule detection. Using the Cramér-Rao bound (CRB) on localization accuracy as a criterion, we optimize annular binary phase masks for various DoF ranges, compare them to Incoherently Partitioned Pupil masks and show that they significantly extend the DoF of single-molecule localization microscopes. In particular we propose different designs including a simple and easy-to-realize two-ring binary mask to extend the DoF. Moreover, we demonstrate that a simple maximum likelihood-based localization algorithm can reach the localization accuracy predicted by the CRB.The framework developed in this paper is based on an explicit and general information theoretic criterion, and can thus be used as an engineering tool to optimize and compare any type of DoF-enhancing phase mask in high resolution microscopy on a quantitative basis.

International audience; The brain extracellular space (ECS) is a complex network that constitutes a key microenvironment for cellular communication, homeostasis, and clearance of toxic metabolites 1. Signaling molecules, neuromodulators, and nutrients transit via the ECS, therefore mediating the communication between cells. Despite the relevance of this important part of the brain, its dynamics and structural organization at the nanoscale is still mostly unknown 2. We have recently demonstrated that single-walled carbon nanotubes (SWCNTs) can be used to image and probe live brain tissue, providing super-resolved maps of the brain ECS and quantitative information on the local diffusion environment 3,4. Here, we propose an important refinement of this approach by implementing a structured illumination technique (named HiLo microscopy 5) to image fluorescently labelled neuronal structures in parallel to SWCNT NIR imaging. This technique is based on speckle illumination and relies on the acquisition of one structured and one uniform illumination image to obtain images deep into tissues with good optical sectioning. Having access to spatially resolved SWCNT diffusivity around specific neuronal structures will provide more precise insights about the heterogeneity of the brain environment.

Luminescent single wall carbon nanotubes are now well established, as unique nanoreporters to probe the brain extracellular space[1]. On the imaging side, this comes from their rich near-infrared optical properties, which also eventually be improved by sp3-chemical functionalization[2]. In addition, their uncommon 1D morphology is an important asset for tissue penetration, yet understanding their diffusion behavior in the complex brain extracellular network is challenging and necessitates dedicated analysis tools. To this aim, we have developed two novel approaches (i) based on the local analysis of trajectory contours to locally measure the nanoscale dimensions of the brain network [3] and (ii) based on the transient evaluation of anomalous carbon nanotube diffusion to delineate the ECS molecular diffusion landscape [4]. The application of these analytical tools to extract relevant biological parameters in physiological and pathological brain models will be presented [5]. References [1] Godin et al Nat. Nanotechnol. 12 (2017) 238-243 ; Gao, et al, Nanomaterials 7, 11, (2017) 393 ; Danné et al ACS Photonics, 5, 2, (2018) 359-364 [2] Mandal et al. Scientific Reports, 10 (2020) 5286 [3] Paviolo et al Methods 174 (2020) 91-99 [4] Lee et al, in preparation [5] Soria et al Nat. Commun., 11 (2020) 3440

International audience; A new approach is presented to measure the length distribution of dispersed single-walled carbon nanotubes (SWCNT). In this method, the diffusive trajectories of individual SWCNTs in solution are reconstructed from high-frame rate video stacks. These trajectories allow the estimation of two key statistics for the SWCNTs: their translational diffusion coefficient, and the autocorrelation time of their fluorescence intensity. We show that the autocorrelation time is a measure of the rotational diffusion coefficient of the SWCNTs, and that the length of the SWCNTs can be estimated either from the rotational diffusion coefficients alone, or by combining translational and rotational diffusion coefficients. Moreover, this last estimate does not require knowledge of the solution viscosity or of the SWCNT hydrodynamic diameter.

International audience; The brain extracellular space (ECS) is a system of narrow compartments whose intricate nanometric structure has remained elusive until very recently. Understanding such a complex organisation represents a technological challenge that requires a technique able to resolve these nanoscopic spaces and simultaneously characterize their rheological properties. We recently used single-walled carbon nanotubes (SWCNTs) as near-infrared fluorescent probes to map with nanoscale precision the local organization and rheology of the ECS. Here we expand our method by tracking single nanotubes through super-resolution imaging in rat organotypic hippocampal slices and acute brain slices from adult mice, pioneering the exploration of the adult brain ECS at the nanoscale. We found a highly heterogeneous ECS, where local rheological properties can change drastically within few nanometres. Our results suggest differences in local ECS diffusion environments in organotypic slices when compared to adult mouse slices. Data obtained from super-resolved maps of the SWCNT trajectories indicate that ECS widths may vary between brain tissue models, with a looser, less crowded nano-environment in organotypic cultured slices.