Porphyrins are a vast molecular family that play a key role in significant vital mechanisms such as the transport of oxygen in blood. In the past decade several authors demonstrated that the remarkable chemical richness of these molecules can be usefully exploited for chemical sensing and for odour recognition in artificial olfactory systems.
Most of the remarkable sensing properties of metalloporphyrin have been studied in non-structured molecular films. Nonetheless, porphyrins chemical versatility also offers the possibility to drive self-assembled molecular aggregations to a large variety of ordered geometrical structures. As an example it is known that the aggregation of oppositely charged porphyrins can lead to the formation of self-assembled molecular nanotubes. Among the three-dimensional structures, nanotubes are particularly appealing for applications as chemical sensors, because of the potential inclusion of different guests inside the cavity or the induced modification of the skeletal interaction after analytes binding.
In a recent paper, published in Nanotechnology, the authors, following the original idea of Shelnutt, demonstrated that nanotubular aggregates of two charged porphyrins exhibit a sensitivity largely exceeding that of the individual constituents. The reason of the increased sensitivity may be likely found in an additional sensing mechanism related to the modulation of the amplitude of the forces that keep the supramolecular ensemble together.
Porphyrin nanotubes are characterized by specific optical properties in the visible range than can change upon the interaction with a guest molecule. In their recent Nanotechnology paper, the authors detected these changes with a simple platform based on the combination of a computer screen, programmed to display millions of colour combining wavelengths in the optical range, and a digital camera, decomposing the light into three broad channels. The authors demonstrated in the past that this arrangement, known as Computer Screen Photoassisted Technology, gives the possibility to perform optical characterizations allowing a simultaneous evaluation of absorbance and fluorescence of samples. Video capture and display are nowadays embedded in a steadily growing number of equipments (from portable computers to cellular phones) that are densely distributed over the whole planet. This fact makes this technique a privileged solution for ubiquitous sensing capability.