Ab Initio Studies on Amplified Fluorescence Polymers for the Detection of Explosives

Development of sensors for explosives to reduce the threat posed by Improvised Explosive Devices (IEDs) has become an international priority for obvious security reasons. Discovering new sensing materials with improved sensitivity and selectivity is critical for expediting the deployment of new sensing devices in the field. The Cagin research group has used methods from quantum chemistry for investigating the amplified quenching phenomena for a class of polymers and for studying properties of their excited states. The large scale calculations required have enabled researchers to study the behavior of TNT and analogous explosives, as well as the identification of false positives. Most of the computation involved in this work was carried out using the resources of the Texas A&M Supercomputing Facility.

Electron density maps of amplifying fluorescent polymer (AFP) at the monomer level and TNT. Red regions are high and blue regions are low density in electrons. This electrostatic type binding interaction between electron-rich polymer and the electron deficient nitroaromatics plays one of the main roles in the sensing mechanism. Gaussian was used for the the quantum calculations and Gaussview for the visualization on the Hydra supercomputer.

The basic transduction mechanism of high energetic material sensing using fluorescent conjugated polymers is based on the quenching mechanism. The transfer of excited electrons from the conjugated polymer or namely AFP to the lowest unoccupied molecule orbital (LUMO) of the target molecule (TNT) leads to quenching of fluorescence intensities of the polymer.

The repeating unit of the amplified fluorescence polymer (AFP) is on the left. The collective properties of analyte-receptor interactions in conjugated polymers ensure enhanced sensitivity over the isolated monoreceptor type (I) sensors. In the wired-receptor type conjugated polymer sensor (II), an even one bound receptor creates a perturbation in the electronic structure of the whole polymer due to the additive property resulting in much more increased sensitivities. The extended transportation pathway of the excited electrons (excitons) enhances the probability of catching an analyte, which makes these polymers sense TNT traces at 10 ppb level (Nomadics Inc). Additionally, the polymer has a porous structure, which provides rapid diffusion of analyte molecules.

Forming of the AFP monomer-TNT complex results in a low-lying charge transfer (CT) state which is a non-radiative sink that leads to the quenching of the fluorescence of the AFP monomer which normally gives strong oscillator strength (0.178) around a wavelength of 363 nm. Frontier orbital analysis (upper right) shows a charge transfer from HOMO of the monomer to the LUMO of the TNT indicating the CT state. Calculations are performed using the TD-DFT method (B3LYP with 6-31+G* basis set) using Gaussian on the Hydra supercomputer.