A colorimetric sensor array includes a CMOS image sensor having a surface including pixels and a multiplicity of colorimetric sensing elements. Each sensing element has a sensing material disposed directly on one or more of the pixels. The colorimetric sensing elements are distributed randomly on the surface of the CMOS image sensor. Fabricating the colorimetric sensor array includes spraying a sensing fluid in the form of droplets directly on a surface of a CMOS image sensor and removing the solvent from the droplets to yield a multiplicity of sensing elements on the surface of the CMOS image sensor. Each droplet covers one or more pixels of the CMOS image sensor with the sensing fluid. The sensing fluid includes a solvent and a sensing material. The droplets are distributed randomly on the surface of the CMOS image sensor.