What is the concept of fluctuation spectroscopy? I know this method is used to study protein-protein interactions.

Here is a link for a good summary of the principles, theory and applications of Fluctuation Correlation Spectroscopy (FCS).


Link: http://online.physics.uiuc.edu/courses/phys598OS/fall05/FinalPapers-05/Vinayak.pdf

One of the leaders in fluorescence correlation spectroscopy (FCS) is Prof Enrico Gratton. He used to be in the Physics Dept (LFD) at UIUC. Fortunately for those living in S. CA he and the Laboratory for Fluorescence Dynamics (LFD) has moved to UC Irvine.
Here is link to the LFD's website which will provide you with plenty of resources.

http://www.lfd.uci.edu/

Here is another summary I came across.

Fluorescence Correlation Spectroscopy Techniques

Enrico Gratton*, Claire Brown◊ and Michelle Digman*
*University of Illinois, Champaign-Urbana, Dept. of Physics, Laboratory for Fluorescence Dynamics. ◊ University of Virginia, Dept. of Cell Biology

Link: http://www.biophysics.org/discussions/2004/gratton.pdf

There is a new FCS Classroom at www.fcspxert.com that explains some of the theory of Fluorescence Correlation Spectroscopy and its experimental applications.

Here's a brief description of FCS:

What Does FCS Measure?
Fluorescence correlation spectroscopy (FCS) is a single-molecule detection technique that detects and correlates fluctuations in fluorescence intensity within a very small detection volume (on the order of femtoliters).

What Information Do FCS Data Provide?
Concentration
Because the fluorescence intensity inside the femtoliter detection volume is directly related to the number of fluorescent particles, FCS provides a sensitive measure of concentration down to picoMolar values.

Particle Size
By correlating the rate of fluctuations in the concentration within the detection volume, FCS also provides a measure of the rate of diffusion of particles in and out of the detection volume, which is directly related to the size of the particles.

What are the Applications of FCS?
By measuring changes in concentration and size, it's possible to monitor interactions and complexing between macromolecules such as proteins and polynucleic acids on the single-molecule scale. Basically, many of the things that you would use PCR or ELISA to detect, FCS can measure without the need for amplification or washing.