# Density filtered Fluorescence Correlation Spectroscopy for highly concentrated solutions

## TL;DR

This paper demonstrates that Fluorescence Correlation Spectroscopy (FCS) can be effectively used at high concentrations of fluorescent molecules by employing OD filters and statistical averaging, expanding its applicability beyond low-concentration systems.

## Contribution

The authors show that FCS can be applied to highly concentrated solutions up to 50μM by using neutral OD filters and averaging techniques, challenging the traditional low-concentration limitation.

## Key findings

- FCS remains effective at concentrations up to 50μM with OD filters.
- Signal-to-noise ratio can be improved through statistical averaging.
- High concentration FCS broadens the technique's applicability.

## Abstract

Fluorescence Correlation Spectroscopy (FCS) is widely used to detect and quantify diffusion processes at the molecular level. The molecules of which diffusion is studied are marked with fluorescent dyes. It is commonly maintained that this technique only applies to systems where the concentration of fluorescent molecules is low. Even if this is the optimal operational condition, we show that FCS can be used also at high concentrations (up to 50$\mu$M) of fluorescent molecules: the detector blinding due to highly fluorescent solutions of concentrated dyes can be avoided by using neutral optic density (OD) filters, and the initial condition of very bad signal to noise ratio (SNR) can be hampered by suitable statistical averaging, as usual in other contexts of signal analysis.