A multi-sensor pixel is presented using ion-sensitive field-effect transistors (ISFETs) as ion sensors, photodiodes as optical sensors and MOSFETs as temperature sensors. The pixel is inspired from an Active Pixel Sensor (APS) topology previously reported as an ISFET front-end. Current mirrors and switches are used to multiplex sensors and encode the output in the time domain for in-pixel quantisation. A novel temperature compensation method, based on temperature coefficient cancellation, is implemented based on bandgap and translinear circuits. The non-idealities of ISFETs, such as drift and trapped charge, are compensated by source voltage modulation. The pixel is implemented using TSMC 0.18 um CMOS technology and occupies a 38 um x 36 um area with an ultra-low power consumption of 33.96 nW associated with weak inversion operation. The simulated results demonstrate a high pH sensitivity of 33.96 ns/pH and an ultra-stable temperature variation between 63 fA/oC and 2.3 pA/oC.