<p>Ions are essential regulators of physiological processes and vital indicators of health in biological systems, yet conventional ion-selective electrodes and transistors face limitations in sensitivity, dynamic range, and integration. Here, we present a monolithically integrated dual-sensor platform that combines a carbon nanotube field-effect transistor (CNT FET) with a locally gated CNT amplifier for broad-range and ultrasensitive ion detection. The FET ion sensor enables wide-range detection through a concentration-dependent threshold voltage (<i>V</i>_th) shift, while the amplifier provides ultrahigh sensitivity via transition gain, with the local bottom gate (LBG) ensuring independent gate control and coordinated operation between the two integrated units. The FET sensor achieved a sensitivity of 50.5&#xa0;mV/pH across a wide pH range (3.6–9.8), whereas the amplifier sensor reached 3 663&#xa0;mV/dec for potassium ions within narrower ranges, demonstrating dramatically enhanced sensitivity. A handheld biosensor device was developed to support systematic operation of both sensors. Notably, the amplifier ion sensor exhibited a pH sensitivity of 1 923.1&#xa0;mV/pH at an operating voltage of −1&#xa0;V—approximately 40 times higher than that of the FET sensor. By using the FET sensor to dynamically adjust the amplification range of the amplifier, the platform accomplishes both wide-range and ultrasensitive detection. These monolithically integrated sensor platforms represent a versatile biosensing system that can be widely applied to the detection of other biochemical molecules.</p> Graphical Abstract <p></p>