Inductance-based sensing technique for wireless, remote-query measurement in liquid media

A novel inductance-based sensing technique is presented for remote query measurement in different liquid media including organic solvents and inorganic solutions. The inorganic solutions tested included salt solutions of different concentrations, and the organic solvents detected included 1,4-dioxane and tetrahydrofuran. To extend the application of the sensor, bacterial culture media were also detected, and the growth of Escherichia coli ( E. coli ) was controlled. The influential factors which may affect the inductance responses were studied in detail. It was found that quantitative relationships exist between the sensor’s inductance response and the physico-chemical parameters of the liquid media. The sensor’s inductance response ( L ) decreases with the increase of salt concentration ( C ) and its ionic valence ( e ) according to a semi-logarithmic equation Lg L = −a eC + b, where a and b are constants, which is in accordance with the theoretically deduced equation. The inductance variation rate (Δ K ) increases directly with the temperature ( T ): Δ K = a′ T + b′. As for organic solutions, the sensor’s inductance was found to increase with the increasing permittivity of the organic solution. The wireless sensor we designed is simple and easy to manipulate. It has the potential for remote determination of not only chemical substances but also microbiological species such as bacteria. Using the newly developed inductance-based sensor, the pathogenic E. coli was monitored with a limit of detection of 10 cells/mL and a linear semi-logarithmic range of 1.0 × 10 1 to 2.5 × 10 9 cells/mL.