We report on a fluorescent optoelectronic nose for the trace detection of nitroaromatic explosive vapors. The sensor arrays, fabricated by aerosol-jet printing, consist of six different polymers as transducers. We demonstrate the nose’s ability to discriminate between several nitroaromatics including nitrobenzene, 1,3-dinitrobenzene and 2,4-dinitrotoluene at three different concentrations using linear discriminant analysis (LDA). We assess the within-batch reproducibility of the printing process and we report that the sensor polymers show efficient fluorescence quenching capabilities with detection limits of a few parts-per-billion in air.
Our approach enables the realization of highly integrated optical sensor arrays in optoelectronic noses for the sensitive and selective detection of nitroaromatic explosive trace vapors using a potentially low-cost digital printing technique suitable for high-volume fabrication. An important challenge is temperature-dependence which is often neglected even though organic emitters are strongly affected by temperature. For some materials, even small changes of a few Kelvin can lead to large changes in the emission intensity making a temperature-control for sensing applications inevitable. Therefore, the temperature-dependence of these sensors is investigated via a heated transparent thin film on the back of such sensors allowing the active layer to be temperature controlled. All of these led to the development of a portable system.
In this work, we report on fluorescent sensor arrays fabricated by aerosol jet printing on glass substrates to detect explosives-related nitroaromatic species. The printed sensor arrays consist of six different fluorescent polymers responding to nitroaromatic vapors through a photo-induced electron transfer. This results in a quenched fluorescence proportional to the vapor concentration. Distinct fluorescence quenching patterns are detected for nitroaromatic species including nitrobenzene, 1,3-dinitrobenzene and 2,4-dinitrotoluene. The detected fingerprints are evaluated at low concentrations of only 1, 3 and 10 parts-per-billion in air. Linear discriminant analysis is used to train each sensor array enabling the discrimination of the target analyte vapors. To investigate the reproducibility of multiple sensor arrays on a single substrate, the measured fluorescence quenching patterns are used to benchmark the linear discriminant models. For this purpose, the target analytes and vapor concentrations are predicted for each sensor array. On average, we report low and reproducible misclassification rates of about 4 % indicating excellent discriminatory abilities at low concentrations close to the detection limits. We conclude that digital printing of fluorescent polymers offers the potential to realize low-cost sensor arrays for a reliable detection of trace explosives.
We report on lasing in conical microcavities, which are made out of the low-loss polymer poly (methyl methacrylate)
(PMMA) doped with the dye rhodamine 6G, and directly fabricated on silicon. Including a thermal reflow step during
fabrication enables a significantly reduced surface roughness, resulting in low scattering losses of the whispering gallery
modes (WGMs). The high cavity quality factors (above 2·106 in passive cavities) in combination with the large oscillator
strength gain material enable lasing threshold energies as low as 3 nJ, achieved by free-space excitation in the quasistationary
pumping regime. Lasing wavelengths are detected in the visible wavelength region around 600 nm. Finite
element simulations indicate that lasing occurs in fundamental TE/TM cavity modes, as these modes have - in
comparison to higher order cavity modes - the smallest mode volume and the largest overlap with the gain material. In
addition, we investigate the effect of dye concentration on lasing wavelength and threshold by comparing samples with
four different concentrations of rhodamine 6G. Observations are explained by modifying the standard dye laser model.