Remote Metal Ion Sensors

Ryan Supino and Joseph J. Talghader

This project involves the development of an autonomous remote metal ion sensor capable of measuring metal ion concentrations, specifically copper ions, in a remote environment. The end-use for the sensor system is to detect metal ion contamination in water or chemicals, especially in environments that are difficult to sample directly due to their location or small size. Sensor particles are dispersed in the liquid where they determine ion concentration either at a specific location or as a time-average. At the end of the sensing operation, the liquid is sampled and any collected sensors are self-assembled and dried for read-out.

The design of the device as incorporates three individual solar cell mesas with a resonant cantilever mass sensor. An electrode is present at the tip of the cantilever and a counter-electrode surrounds the cantilever. When in a solution where metal ions are present, the solar cells provide current to the sensor, driving an electrodeposition process plating the tip of the cantilever with metal. The total mass deposited on the cantilever beam, which correlates to the concentration of metal ions in the solution, can be determined by measuring the shift in resonant frequency of the cantilever using a common optical lever technique.

The total volume of the sensor system including power supply is below 0.046 mm³ with no dimension exceeding 360 microns in length. Current levels greater than 65 nA are achievable under daylight conditions. This current level is sufficient to power the diffusion-limited electrodeposition process used in the associated sensor. The power-scavenging system consists of an array of solar cells fabricated using a SOI process that allows integration with a micromachined mass-resonance sensor.

Figure 1 - Optical photograph of released microsensor. Solar cell mesas surround cantilever and counter electrode of metal ion sensor. Total sensor block is 360 microns in width.

Figure 2 - Drawing of complete sensor device. (a) Solar cell mesa. (b) Cantilever with electrode at tip connected to solar cell array. (c) Counter electrode connected to solar cell array.

Figure 3 - I-V curve of solar cell array at 3 distinct light intensity levels.