Highly sensitive gas-phase explosive detection by luminescent microporous polymer networks

Abstract

We propose microporous networks (MPNs) of a light emitting spiro-carbazole based polymer (PSpCz) as luminescent sensor for nitro-aromatic compounds. The MPNs used in this study can be easily synthesized on arbitrarily sized/shaped substrates by simple and low-cost electrochemical deposition. The resulting MPN afford an extremely high specific surface area of 1300 m(2)/g, more than three orders of magnitude higher than that of the thin films of the respective monomer. We demonstrate, that the luminescence of PSpCz is selectively quenched by nitro-aromatic analytes, e.g. nitrobenzene, 2,4-DNT and TNT. In striking contrast to a control sample based on non-porous spiro-carbazole, which does not show any luminescence quenching upon exposure to TNT at levels of 3 ppm and below, the microporous PSpCz shows a clearly detectable response even at TNT concentrations as low as 5 ppb, clearly demonstrating the advantage of microporous films as luminescent sensors for traces of explosive analytes. This level states the vapor pressure of TNT at room temperature.

Figure 1

Principle of electrochemical polymerization into a microporous network PSpCz from a solution of the monomer SpCz (a). Photoluminescence (PL) spectra of the resulting microporous PSpCz in comparison to the PL of a thermally evaporated thin film of the monomer (SpCz).

Figure 2

Setup for the characterization of the photoluminescence of the sensors upon controlled exposure levels of various analytes (a). Molecular structures of the analytes considered in this study, along with their reported LUMO level positions (b). Principle of luminescence sensors: Energy level scheme of PSpCz and fate of an exciton without and with quenching analyte molecule. (c) Example of the PL spectra of PSpCz in the fresh state and after exposure to 100 ppm of NB (d). Relative change of the integrated PL intensity of the luminescent sensor before (I₀) and after (I) exposure to 100 ppm of the respective analyte (e). Note, the absolute value of the quenching efficiency, |I₀/I − 1|, is plotted.

Figure 3

PL spectra of PSpCz upon exposure to various concentrations of nitrobenzene ranging from no exposure (violet) to 1000 ppm (red) (a), and the resulting plot of the quenching efficiency vs. concentration of NB (b). For the determination of I and I₀ either the full spectrum or the spectral region between 455−80 nm (marked in (a)) has been chosen.

Figure 4

Comparison of the response of the integrated PL intensity of SpCz and PSpCz to different levels of TNT exposure (a). Quenching efficiency of PSpCz exposed to 5 ppb at short time scales. Exposure to TNT starts at t = 0 s (b).