A Novel Optical Method to Detect DNA Concentration Using Lock-In Amplifier

To detect DNA concentration, a novel fiber optic biosensor based on fluorescence quenching was designed and constructed by using lock-in amplifier for signal measurement. The sensor head was fixed on the sensing membrane prepared by the mixture of ethyl silicate, formamide, HCI and fluorescence indicator. The optimal film-forming conditions are that the contents of indicator [Ru(bipy)₂DAFND](CIO₄)₂ (Ru-DAFND, bipy is 2, 2-bipyridine, DAFND is 4, 5-diazafluorene-9-p nitrophenylhydrazone), HCI (0.01 mol/L) and formamide were 2.5 mg/mL, 11 % and 16 % respectively, the aging temperature and time of the sol-gel solution were 4 °C and 5 d, and the film forming temperature and time were 4 °C and 48 h, respectively. A linear relationship between the relative phase delay \\φ of the sensing membrane and calf thymus DNA (ctDNA) concentration was observed in range from 2.6×10^-8 to 5.4×10^-6 mol/L in Tris buffer (pH 7.1), and the detection limit was 8.4×10^-9 mol/L. The biosensor has a response time of 70 s and shows good stability and repeatability.