Optical EngineeringAnalysis and improvement on frequency sensitivity of series photodetector frequency circuit system and its application for HEX fluorescence measurement
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This study reported the improvement of frequency sensitivity of series photodetector frequency circuit system for fluorescence detection. The 48 MHz series photodetector frequency circuit system matched with polydimethyl siloxane (PDMS) focusing lens was applied to the detection of emissive fluorescence intensity. MEMS technology processes were used to fabricate PDMS focusing lens, the mixing amount of PDMS and the curing temperature were adjusted, and self-improved mold-transfer technology was used to make a 3D planar lens of diameter of 3 to 10 mm. While the series photodetector frequency circuit system matched with PDMS focusing lens of diameter 10 mm was used for fluorescence detection, the relative fluorescence intensity was enhanced by approximately two times, which indicated that the frequency sensitivity of the 48 MHz sensor system was improved. Besides, the bias of the photodetector was also adjusted to improve the frequency sensitivity. According to the optimal conditions of PDMS focusing lens and bias of a photodetector, the detection limit of HEX fluorescence dye concentration 3.3 pmol/L can be measured. The results of the fluorescence experiment also demonstrated that the frequency shift of the 48 MHz sensor system was linearly related to the logarithm of fluorescence dye concentration from 333.3 nmol/L to 3.3 pmol/L.