The Extended Receptive Field of Blends of Metalloporphyrins and pH Indicators

Molecular cooperation is a fascinating methodology to design functional materials. In chemical sensors, The sensitive unit can be formed by two interacting molecules whose interaction is altered by the adsorption of an analyte. This principle is conveniently exploited in the Indicator Displacement Assay (IDA) where the sensing unit is a colorimetric indicator bound to a colorless receptor. The interaction of the receptor with an analyte may than displace the dye giving rise to a detectable color change [1]. This technique has been demonstrated for instance for the colorimetric detection of sugars [2].

This working principle can be extended preparing an assembly constituted of two chemoresponsive dyes, namely molecules that can change their spectrum as a consequence of the interaction with volatile compounds. AS an example of this apporahc we discuss the case of blends of a porphyrin and an acid-base indicator. Metalloporphyrins allow for the recognition of analytes with Lewis acid/base capabilities, while pH indicators respond to changes in the proton acidity or basicity of the surrounding environment. Molecules that possess none of these properties are not expected to be detected by these chemoresponsive dyes; but they can be optically detected by proper blends of dyes.

Solid layers of blends can be spotted onto a transparent substrate to measure the light transmitted through the spote.  The layer of spots can be coinveniently matched with a transduction technique known as Computer Screen Photoassisted Technique (CSPT). CSPT arrangement consists of a LCD screen that illuminates the sensing film and a computer camera that records pictures of it during the exposure to vapors of target analytes. Such arrangement was already demonstrated as a suitable system for VOCs classification or for food freshness evaluation [3].

Different experiments aimed at evaluating the sensing capabilities of dye blends compared to those of single components were set up. Results indicate that for most part of the tested volatile compounds the variations of the absorbance spectrum of porphyrin-indicator blends exceed those of the single components.

Noteworthy, for some of the compounds a response is found also when no color change is elicited in the membrane based on pure indicators.

This is the most interesting characteristic of porphyrin-pH indicator blends as it overcomes the indicator failure in detecting certain chemical species through an optical transduction setup. The blends with different dye ratios show peculiar responses to the volatile compounds. In general the composition with a weight ratio 1:1 guarantees the best performance in terms of intensity response towards the largest number of compounds. Since the molar mass of the two components is similar, this is in agreement with the observation that the binding is established between single molecules.

The sensitivity of porphyrins-pH indicator blends was tested with alcohols, amines, ketones, alcohols, ethers, aldehydes. Principal component analysis of the array data shows the effective recognition of different compounds.

The blends with different dye ratios show peculiar responses to the volatile compounds. In general the composition with a weight ratio 1:1 guarantees the best performance in terms of intensity response towards the largest number of compounds. Since the molar mass of the two components is similar, this is in agreement with the observation that the binding is established between single molecules.

The sensitivity of porphyrins-pH indicator blends was tested with alcohols, amines, ketones, alcohols, ethers, aldehydes. Principal component analysis of the array data shows the effective recognition of different compounds.

References

[1]        E. V. Aslyn, Supramolecular Analytical Chemistry, JOC Perspective 72, 687-699 (2007)

[2]        T. James, P. Linnane, S. Shinkai, Fluorescent saccharide receptors: a sweet solution to the design, assembly and evaluation of boronic acid derived PET sensors,  Chem Comm 281-288 (1996)

[3]        D. Filippini, A. Alimelli, C. Di Natale, R. Paolesse, A. D’Amico, I Lundström. Chemical Sensing with Familiar Devices, Angew. Chemie. Int. Ed. 45, 3800-3803 (2006)