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An optical biosensing system based on a standard CMOS technology and absorption photometry is proposed. Within the compact prototype, a CMOS sensor consisting a P+/Nwell finger photodiode and a transimpedance amplifier was implemented in a standard 0.35-mu m CMOS technology. The ABTS/H2O2/HRP method was adopted to be coupled with various analytes of interest, such as glucose and histamine, which can serve as the basis for medical diagnosis. The experimental results demonstrate that the minimum concentrations successfully detected for H2O2 , glucose, and histamine are 1 muM, 1 muM, and 10 muM, respectively. The detection limits are at least one order of magnitude better than those of reported CMOS biosensors, and are even comparable to those of a commercial spectrophotometer.