This paper presents a dual-mode adjustable high- gain transimpedance amplifier (TIA). The TIA is designed with a high sensitivity to current bio-signals for a fine-dust detection system Integrated Circuit (IC) based on the operation of a photodiode module with high gain amplification. Gain adjustment is implemented by coarse gain steps using selective loads with four different gain values and fine gain steps by 42 dBΩ during 16 fine steps. To improve the settling time, a capacitive compensation is applied for the last stage. An off-state circuitry is proposed to avoid any off-current leakage. This TIA is designed in a 0.18 μm standard CMOS technology, output reached high gains from 149 dBΩ up to 216 dBΩ with 67 dBΩ gain variation range, input-referred noise less than 600 fA/√Hz in low frequencies and less than 27 fA/√ Hz at 20 kHz, a minimum detectable current signal of 4 pA, and has a power consumption of 2.71 mW.