# 4D Single-particle tracking with asynchronous read-out single-photon avalanche diode array detector

## TL;DR

The paper introduces a new microscope system that tracks particles in real-time with high precision and measures their fluorescence lifetime.

## Contribution

The novel use of an asynchronous single-photon avalanche diode array detector enables 4D single-particle tracking with high spatiotemporal resolution.

## Key findings

- The system achieves 40 nm lateral and 60 nm axial localization precision with 100 photons.
- It enables sub-millisecond temporal sampling for real-time tracking and microsecond-scale offline tracking.
- The system was validated on fluorescent beads and used to study lysosome movement and fluorescence lifetime in living cells.

## Abstract

Single-particle tracking techniques enable investigation of the complex functions and interactions of individual particles in biological environments. Many such techniques exist, each demonstrating trade-offs between spatiotemporal resolution, spatial and temporal range, technical complexity, and information content. To mitigate these trade-offs, we enhanced a confocal laser scanning microscope with an asynchronous read-out single-photon avalanche diode array detector. This detector provides an image of the particle’s emission, precisely reflecting its position within the excitation volume. This localization is utilized in a real-time feedback system to drive the microscope scanning mechanism and ensure the particle remains centered inside the excitation volume. As each pixel is an independent single-photon detector, single-particle tracking is combined with fluorescence lifetime measurement. Our system achieves 40 nm lateral and 60 nm axial localization precision with 100 photons and sub-millisecond temporal sampling for real-time tracking. Offline tracking can refine this precision to the microsecond scale. We validated the system’s spatiotemporal resolution by tracking fluorescent beads with diffusion coefficients up to 10 μm2/s. Additionally, we investigated the movement of lysosomes in living SK-N-BE cells and measured the fluorescence lifetime of the marker expressed on a membrane protein. We expect that this implementation will open other correlative imaging and tracking studies.

Here, the authors upgrade a confocal laser scanning microscope with a single-photon array detector, achieving 40 nm lateral and 60 nm axial localisation precision with 100 photons and a sub-millisecond temporal sampling for real-time single-particle tracking with fluorescence lifetime measurement.