Monolithic fluid sensor system for sensing a fluid in the environment, comprising: a sensor arrangement having a thermal radiation emitter , an optical filter structure , a waveguide structure and a thermal radiation detector on a first main surface region (115-1) of a sensor substrate a cover substrate , wherein a recess is arranged in a first main surface region (120-1) of the cover substrate and a through-opening is arranged between the recess in the first main surface region (120-1) and a second main surface region (120-2) of the cover substrate , wherein the first main surface region (120-1) of the cover substrate is bonded to the first main surface region (115-1) of the sensor substrate (115; 105), and wherein the sensor arrangement is arranged below the recess of the cover substrate , wherein the recess forms a cavity and wherein the through-opening forms a fluidic connection from the cavity to the environment, a reference sensor arrangement having a reference thermal radiation emitter , a reference optical filter structure , a reference waveguide structure and a reference thermal radiation detector on a first main surface region (135-1) of a reference sensor substrate , and a reference cover substrate , wherein a reference recess is arranged in a first main surface region (140-1) of the reference cover substrate , wherein the first main surface region (140-1) of the reference cover substrate is bonded to the first main surface region (135-1) of the reference sensor substrate , and wherein the reference recess in the first main surface region (140-1) of the reference cover substrate forms a hermetically closed cavity for the reference sensor arrangement . a sensor arrangement having a thermal radiation emitter , an optical filter structure , a waveguide structure and a thermal radiation detector on a first main surface region (115-1) of a sensor substrate a cover substrate , wherein a recess is arranged in a first main surface region (120-1) of the cover substrate and a through-opening is arranged between the recess in the first main surface region (120-1) and a second main surface region (120-2) of the cover substrate , wherein the first main surface region (120-1) of the cover substrate is bonded to the first main surface region (115-1) of the sensor substrate (115; 105), and wherein the sensor arrangement is arranged below the recess of the cover substrate , wherein the recess forms a cavity and wherein the through-opening forms a fluidic connection from the cavity to the environment, a reference sensor arrangement having a reference thermal radiation emitter , a reference optical filter structure , a reference waveguide structure and a reference thermal radiation detector on a first main surface region (135-1) of a reference sensor substrate , and a reference cover substrate , wherein a reference recess is arranged in a first main surface region (140-1) of the reference cover substrate , wherein the first main surface region (140-1) of the reference cover substrate is bonded to the first main surface region (135-1) of the reference sensor substrate , and wherein the reference recess in the first main surface region (140-1) of the reference cover substrate forms a hermetically closed cavity for the reference sensor arrangement . <EOS>