If the particles are moving faster, then it makes sense that the correlation curve should drop more quickly, because the light scattering signal is going to change more quickly as the particles diffuse at high speed. Now, remember that we said that DLS relies on the principle that larger particles move (or diffuse) more slowly than small particles? This comes into play in the analysis, because we can look at how quickly the correlation curve drops from 1 to 0. We use this correlation function to draw a curve depicting the correlation over time (also called a correllelogram), as demonstrated on the right hand side of Figure 2. This correlation function returns a value of 1 if the two signals are identical, and a value of 0 if they are completely different. We can compare subsequent measurements to the first measurement taken, and measure how similar they are using a correlation function. ![]() Okay, so we know that we are taking measurements in quick succession of the light that is scattered by the particles in solution. This idea is shown in Figure 2.įigure 2 - Source (The video on ’s website is an excellent 5 minute overview of DLS.) But after 50 nanoseconds (5 time steps later) the particles will have moved enough that the new measurement of the light intensity signal does not match the first measurement very much at all. The particles have not moved very much in 10 nanoseconds, so the light intensity signals at these two points in time are very similar to each other. Let’s imagine we are looking at the first snapshot, and the second one, taken after some very small time interval (let’s say 10 nanoseconds). We compare each snapshot of the light intensity to the first measurement we took, and we compare how similar they are. The particles in the fluid are scattering the light, generating a signal that varies over time. Figure 1 shows a schematic of the setup commonly found in DLS machines. Ī DLS machine collects data by shining a laser through a sample, and then recording the scattered light intensity many times over a very short time interval. By studying how the light is scattered, we can draw conclusions about the size of the particles in the fluid. ![]() ![]() DLS shines a light on the particles (both metaphorically and literally!) and measures how that light is scattered over time. At its core, DLS relies on the fact that big particles move more slowly than small particles. Brownian motion simply refers to the random motion of particles suspended in a fluid - the suspended particles are constantly moving (or diffusing) as they collide with molecules of the fluid. ![]() Let’s get started! The Basic Principlesĭynamic light scattering measures the Brownian motion of particles in solution, and relates that motion to a specific particle size. I actually first learned to use DLS two years ago, but I didn’t write down a good summary of what I had learned so I am making sure that I write down the basic operating principle this time! In this post, I’m going to give an outline of how the method works, then go into some of the mathematics used in the analysis. Dynamic light scattering (DLS) is used to measure very small particles, typically on the order of 0.6nm - 6um, by studying how they move in solution. There are many ways to characterize and measure DNA nanostructures, and today I wanted to write about one tool in particular, dynamic light scattering (or photon correlation spectroscopy).
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