The novel sensing scheme, unlike previous ones that are based on the use of pH indicator probes, is making use of solvatochromic probe Nile Red (NR). Dissolved in a matrix of ethyl cellulose, it can report the polarity of its microenvironment that is modulated by an additive (a hydrophobic amidine) that is capable of reversibly binding carbon dioxide. The spectra of NR undergo a strong solvatochromic shift both in color (from brick-red to magenta) and in fluorescence (from orange to red) if the respective sensor layer is exposed to gaseous CO(2) (gCO(2)) or dissolved CO(2) (dCO(2)). Both visual and instrumental readouts are possible. The sensor layer responds to gCO(2) in the range from 0 to 100% and to dCO(2) in the range from 0 to 1 M solutions of bicarbonate (equivalent to a CO(2) partial pressure of up to 255 hPa). The detection limits are around 0.23% for gCO(2) and 1.53 hPa for dCO(2). The response time is in the order of 10 min in the forward direction and 3 min in the reverse direction for gCO(2) but up to 25 min in the case of dCO(2). The optical response also was quantified using a digital camera by extracting the spectral information using the blue and green color channels (in reflectometry) and the green and red channels (in fluorescence), respectively, and by generating pseudocolor pictures.
© 2011 American Chemical Society