# Single particle tracking

Principle of single particle tracking: The rectangles represent frames from an image acquisition at times ${\displaystyle t=0,1,2,...}$. The tracked particles are represented as red circles and in the last frame, the reconstructed trajectories are shown as blue lines

Single particle tracking (SPT) is the observation of the motion of individual particles within a medium. The coordinates (x, y, z) over a series of time steps is referred to as a trajectory. The trajectory can be analyzed to identify modes of motion or heterogeneities in the motion such as obstacles or regions of fast transport (e.g. due to active transport or flow), in the case of random motion trajectory analysis can provide a diffusion coefficient.

## Applications

1. Single Particle Tracking is used to quantify specific behaviour of a protein on the cell surface.

## Brownian diffusion

Main article: Brownian motion

## Membrane diffusion

Components of biological membranes diffuse in a viscous pseudo-two-dimensional environment. By labeling membrane component with either an optical label (such as a gold or polystyrene bead) or a fluorescent tag a trajectory can be observed. The trajectory of lipids and proteins within the membrane of live cells have been used to infer the mechanisms affecting their motion.[1] One of the methods to observe optical labels on cell membranes is known as nanovid microscopy. Another interesting technique associated with particle tracking is real-time 3D particle tracking, that was used in the context of observing small complexes as they are shuttled around in or near the cell membrane.[2]

### Methods

Particle tracking can be performed using fluorescent or optical labels. The use of optical labels, such as gold nanoparticles, in SPT experiments has been termed nanovid microscopy. In addition, there are several imaging modalities for 3D particle tracking including Multifocal plane microscopy,[3] and DH-PSF microscopy.[4]