Protein determination using methylene blue in a synchronous fluorescence technique

A new method for detecting protein by synchronous fluorescence enhancement was developed, based on the combination of near infrared (NIR) fluorescence and the dedimerization phenomenon of methylene blue (MB). Under analytical conditions, there are linear relationships between the enhancing extent of synchronous fluorescence of MB–sodium dodecyl benzene sulfonate (SDBS)–protein at 667 nm and the concentration of protein in the range of 8.0 × 10−8–4.0 × 10−5 g mL−1 for bovine serum albumin (BSA), 1.0 × 10−7–3.5 × 10−5 g mL−1 for egg albumin (EA). The detection limits (S/N = 3) of BSA and EA are 8.9 ng mL−1 and 10.0 ng mL−1, respectively. The fluorescence enhancement mechanism is discussed in detail. Results from multiple techniques indicate that the fluorescence enhancement of the system originates from the hydrophobic microenvironment provided by BSA and SDBS, and the formation of an MB–SDBS–BSA complex, as well as the deaggregation of some MB dimer.