Simple and sensitive determination of papain by resonance light-scattering with CdSe quantum dots

In this contribution, a simple and sensitive method for papain detection was established based on the increment of the resonance light-scattering (RLS) intensity of mercaptoacetic acid-capped CdSe quantum dots (MAA-QDs) in aqueous solution. Meanwhile, the RLS characteristics and the optimal conditions were investigated in detail. Under the optimized conditions, the linear range of MAA-QDs RLS intensity versus the concentration of papain was 1.0 × 10−8 M to 6.0 × 10−7 M, with a correlation coefficient (R2) of 0.9977 and a limit of detection (3σ black) of 5.1 × 10−9 M. The relative standard deviation (R.S.D.) for 1.6 × 10−7 M papain was 1.0% (n = 5). There was almost no interference to coexisting foreign substances including common ions, proteins and 20 amino acids. The proposed method possessed the advantages of simplicity, rapidity and sensitivity. Three synthetic samples were analyzed by the methodology and the results were satisfying. The interaction between MAA-QDs and papain was also investigated systematically by using visual method, transmission electron microscopy and time-resolved fluorescence spectra, and the results indicated that the main force between MAA-QDs and papain was electrostatic interaction.