Single-molecule imaging with SPAD array detection

The resolution limit of about 250 nm in conventional optical microscopes is problematic in the study of structural biology, since proteins, macromolecules and nuclear acids are typically much smaller than 100 nm.

Single-molecule localization microscopy is able to circumvent this limit by sequentially and stochastically switching on/activating single fluorescent molecules and determining their position in the image plane. E.g. MINFLUX demonstrated a 3D resolution of 6 nm.

However, the point-detection system in MINFLUX does not allow directly recording the image of the fluorescent molecules in the image plane, which is required for the localization, thus a rather complex and slow beam-scanning approach is required.

Secondly, the technique does not leverage the fluorescence lifetime information, which can provide nanometer-scale information on the structure of interest, e.g. via Förster resonance energy transfer.

In this project, both limitations will be tackled:The main goal of the SM-SPAD project is to develop a 3D single-molecule fluorescence lifetime imaging technique for structural biology.

The goal will be reached by combining the concept of MINFLUX with three new ideas: (i) 3D motionless structured illumination and structured detection for improved spatial resolution in all three dimensions; (ii) fluorescence antibunching analysis to speed up the data acquisition by enabling simultaneous localization of several active molecules; (iii) SM level fluorescence lifetime analysis to extract the maximum amount of information from the sample.The proposed molecular-scale imaging technique, with very high spatial and temporal resolution, is perfectly suited for the study of complex biological samples.

SM-SPAD is, in particular, promising in the field of neuroscience research, in which the organization of large protein complexes are of high interest, as they may play a crucial role in the development of neurodegenerative diseases, such as Alzheimer’s and Parkinson’s .