Sub-ppm evanescent waveguide sensor for heavy metal detection in water

We present an assessment of the performance of a lab-on-chip optoelectronic device consisting of a polymeric waveguide and a hydrogenated amorphous silicon (a-Si:H) photodetector, both integrated on a compact BK7 glass substrate with an overall surface area of a few square centimeters. The system detects the concentration of an analyte in a sample by measuring the light power drop as a result of the optical interaction between the solution droplet and the guided radiation. The power variation is related to changes in refractive index of the sample and is measured by an a-Si:H photosensor. The efficiency of such device has been assessed for a solution of copper (Cu) and mercury (Hg) performing numerical simulations in the red spectrum (632.8 nm). The sample-chip interaction is inspected for different parameters and the outcomes are combined with the a-Si:H detector's experimentally-obtained performance. The reported sensitivity of the sensor reaches up to 25 pA/ppm for Cu and 17 pA/ppm for Hg. Additionally, limits of detection of 6 ppb for Cu and 9 ppb for Hg were achieved. Performances shown by this extremely compact, inexpensive and easy-to-use chip seem very promising for applications related to health and environment monitoring, such as in-situ detection of heavy metals in water.