This work describes a model of the piezoresistive behavior in nanocomposite sensors. These sensors are also called flexible sensors because the polymer matrix allows for large deformations without failure. The sensors have conductive nanoparticles dispersed through an insulative polymer matrix. The insulative polymer gaps between nanoparticles are assumed to be possible locations for electron tunneling. When the distance between two nanoparticles is small enough, electrons can tunnel from one nanoparticle to the next and ultimately through the entire sensor. The evolution of this gap distance with strain is important to understand the overall conductivity of the strain sensor. The gap evolution was modeled in two ways: (1) applying Poisson's contraction to the sensor as a homogenous material, referred to as Simple Poisson's Contraction (SPC) and (2) modeling the nanoparticle-polymer system with Finite Element Analysis (FEA). These two gap evolution models were tested in a random resistor network model where each polymer gap was treated as a single resistor in the network. The overall resistance was calculated by solving the resistor network system. The SPC approach, although much simpler, was sufficient for cases where various orientations of nanoparticles were used in the same sensor. The SPC model differed significantly from the FEA, however, in cases where nanoparticles had specific alignment, e.g. all nanoparticles parallel to the tensile axis. It was also found that the distribution used to determine initial gap sizes for the polymer gaps as well as the mean of that distribution significantly impacted the overall resistivity of the sensor.Another key part of this work was to determine if the piezoresistivity in the sensors follows a percolation type behavior under strain. The conductance versus strain curve showed the characteristic s-curve behavior of a percolative system. The conductance-strain curve was also compared to the effective medium and generalized effective medium equations and the latter (which includes percolation theory) fit the random resistor network much more closely. Percolation theory is, therefore, an accurate way to describe this polymer-nanoparticle piezoresistive system.Finally, the FEA and SPC models were compared against experimental data to verify their accuracy. There are also two design problems addressed: one to find the sensor with the largest gauge factor and another to determine how to remove the characteristic initial spike in resistivity seen in nanocomposite sensors.

Volatile organic compounds (VOCs) in air and heavy metal ions, anions, microorganism in water are environmental contaminants that require detection at certain low concentrations to avoid detrimental effects. Analytical instruments can accurately determine the concentration and composition of the contaminants at trace levels. However, these methods require skilled personnel to operate. Hence sensors should have fast response, low maintenance and easy to handle. In the present work, environmental monitoring sensor for volatile organic compounds, anion and E. coli was developed. The sensor is resistive sensor architecture with organic nanocomposite as sensing layer. The conjugated organic molecule with receptor moieties that can interact and exhibit affinity to each of the analyte was designed and synthesized. A new conducting polymer for sensing toluene, aldehyde is designed and fabricated. The sensor shows highest sensitivity and selectivity for targeted analyte. The sensor response is explained by molecular dynamics simulation. The solubility parameter of the new polymers is calculated by molecular dynamics and is used for elucidation of rationale of the mechanism for selectivity. The interaction energy of the sensing layer calculated by simulation is higher for targeted analyte than that for other analytes. The adsorption of vapors on the sensing layer results in volumetric change of the sensing layer. The effect can be experimentally determined by monitoring the thickness of the film and the change in the parameters such as mass change, capacitance, resistance change, refractive index change that occurs due to absorption of vapors in the polymers. Here, laser Doppler vibrometry, a non contact method is used to measure the displacement occurring due to interaction of a new polymer film with analyte vapors. A sensor for real time monitoring of nitrate ion concentration in water is fabricated. A new conjugated polymer is designed for selection of nitrate is used as a sensing molecule. The sensor is tested for various concentrations of nitrate ions and possible interferents effect. An organic nanocomposite based resistive sensor is designed and fabricated to detect E. coli in water. The organic molecule with receptor groups was selected such that receptor group would interact and exhibit affinity to the functional groups present on outer membrane of the cell wall of the E. coli. The resistance change is caused due to interaction and is because of E. coli acting as p-dopant to sensor molecule. The electrostatic interaction between positively charged amine groups on sensor molecule and negatively charged E. coli is believed to be the interaction mechanism. This work demonstrates that the conjugated molecules with suitable moieties can bind with analyte like VOCs, heavy metal ions, anions, microorganism, that can be used as sensing element in resistive sensor architecture.