Abstract:This study reported the frequency characteristics of series photodetector frequency circuit system for detection of fluorescence dye concentration. In the condition of the same fluorescence intensity, the series photodetector frequency circuit system with higher responsivity of photodetector had higher frequency shift. The 100 MHz series photodetector frequency circuit system was applied to determine the fluorescence dye concentration of HEX by frequency shift. The correlation curve showed that the frequency shift was linearly related to the logarithm of fluorescence dye concentration from 100 pmol 3 μl−1 to 10 amol 3 μl−1. The proposed method can be applied simply and the detection limit of fluorescence dye concentration was lower than the conventional fluorescence technique by 2–3 orders.