Determination of nanograms of proteins based on decreased resonance light scattering of zwitterionic gemini surfactant

A new high-sensitivity detection of protein assay at the nanogram level is proposed based on the decreased resonance light scattering (RLS) signals of zwitterionic gemini surfactant (phosphodiesters quaternary ammonium salt [PQAS]). It was found that PQAS self-assembled into nanometer-scale PQAS aggregates, which induced intense RLS signal in Britton–Robinson (BR) buffer solution (pH 10.5). Under the optimum conditions, the RLS intensity quenching extent of PQAS aggregation was in proportion to the concentration of proteins in the range of 0.0012–1.08 μg/ml for bovine serum albumin, 0.0015–0.95 μg/ml for human serum albumin, and 0.0025–1.3 μg/ml for γ-globulin. The detection limits were 0.8, 1.2, and 2.0 ng/ml, respectively. The proposed method was successfully applied to determine total protein in human serum samples, and the results were identical to those obtained by the Bradford assay. The mechanism of interaction between PQAS and protein was studied using RLS, fluorescence, and time-resolved fluorescence, which indicated that the new complex formed between them, disaggregating self-aggregation of PQAS, resulted in the dominated quenching of RLS signal of the assay system.