The effects of three sulfonate gemini surfactants with different hydrophobic chain lengths (8, 10, and 12 carbon atoms) on the optical properties of a fluorene-based conjugated cationic polymer poly{[9,9-bis(6′-N,N,N-trimethylammonium)hexyl]-fluorene-phenylene} bromide (PFP) dissolved in DMSO–water solutions (4% v/v) or water were investigated, respectively. When surfactants with PFP dissolved in DMSO–water solutions (4% v/v) are incubated, a decrease in photoluminescence (PL) intensity and a red shift of emission maxima are obtained at low surfactant concentrations. Intriguingly, two different Stern–Volmer constants (KSV1 and KSV2) are obtained and analyzed in detail for the first time. Further increase in the surfactant concentration enhanced PL intensity, and distinct blue shifts of both absorption and emission maxima are observed. Importantly, the turning point between the emission quenching and enhancement is closely related to the hydrophobic chain length: the longer the chain length, the earlier the turning point appears. Simulation studies provide strong evidence to explain these phenomena. Surface tension measurements show more insight on the interactions between PFP and surfactant. On the contrary, no emission quenching is obtained at low surfactant concentrations for PFP dissolved in water.

•The interactions of BSA with alkyl sulfate gemini surfactants are firstly studied.•The spacer length has an important influence on the binding of gemini to BSA.•Gemini surfactant with a short spacer could enhance BSA fluorescence.•Gemini surfactants play two opposite roles in the unfolding of the BSA structure.Protein–surfactant interactions carry greater importance in a number of process and industries. However, most researches mainly focused on the interactions of protein with traditional surfactants such as sodium dodecyl sulfate, and the researches on gemini surfactants, a new generation of surfactants, with proteins are relatively limited. The present study aimed to explore the interactions of bovine serum albumin (BSA) with four alkyl sulfate gemini surfactants (designated as C12CnC12, where n is the spacer carbon number of 3, 4, 6 and 10) using fluorescence spectroscopy, circular dichroism (CD) and dynamic light scattering (DLS). Results showed that the spacer length had an important influence on the binding of gemini surfactants to BSA. The binding of C12C3C12 and C12C4C12 to BSA at low surfactant concentrations showed an enhancement of fluorescence, which was contrary to the effect of gemini surfactant with a longer spacer (n = 6 and 10) to BSA. CD experiments showed that C12CnC12 could play two opposite roles when they interacted with BSA. At low C12CnC12/BSA molar ratios, C12CnC12 could stabilize the secondary structure of BSA, while C12CnC12 could mainly act as denaturants at higher molar ratios. The micropolarities of C12CnC12–BSA complexes were lower than the corresponding free gemini micelles. The hydrodynamic radii of the C12CnC12–BSA complexes were also measured and discussed. Further studies using molecular modeling are recommended to obtain detailed information about the binding sites of BSA, which are mainly influenced by alkyl sulfate gemini surfactants with different spacer length.