In this study, DNA polyfluorophores were used as optical chemosensors for the detection of complex mixtures of hydrocarbons in contaminated soil. A library of 4096 olygodeoxyfluorosides (ODFs) sequences conjugated on PEG-PS micro-beads (130 µm) was analyzed by fluorescence hyperspectral microscopy (Figure 1).
The hyperspectral data were collected with IMA-Fluorescence™ on an epifluorescence inverted microscope, with a 4X objective. For the excitation, a 340-380 nm filter cube was used, with a long-pass filter at 420 nm. The spectral acquisition was recorded between 400-700 nm, with a step of 1 nm, on an EMCCD camera, at an exposure time of 0.1 s, and an average of 15 images.
Each image covers an area of 2 x 2 mm2 and contains more than 250,000 spectra. Figure 2 shows a series of monochromatic images of the micro-beads at 460 nm, 510 nm, 570 nm and 650 nm that correspond to the blue, green, yellow and red channel, respectively. The various ODFs sequences can be identified by studying the variations in their emission spectra (Figure 3): shape, intensity quenching or enhancement, and wavelength shifts.
A fast analytical method is paramount for the analysis of soils contaminated by petroleum-based products. In fact, during the cleanup of a contaminated area, it is necessary to determine rapidly the nature of the components of the spill and understand its origin. It is also very important to control the limits of the spill during the cleaning in order to fully recover the area. Hyperspectral imaging is an essential technique for rapid quantitative fluorescence imaging.
ODFs-conjugated PEG-PS micro-beads were kindly provided by Prof. Eric Kool, at Stanford University.
Further reading on DNA-based chemosensors for the analysis of soil contamination:
W. Jiang, S. Wang, L. H. Yuen, H. Kwon, T. Ono and E. T. Kool Chem. Sci. 4, 3184 (2013)