In the three years of duration, the INITIO project aims to demonstrate the possible development of stereospecific chemical sensors, targeting enantiomeric pairs of analytes selected as test cases in gaseous and in liquid phases. The individual steps of sensor development also represent the intermediate objectives of the project, as listed in the following:
- Synthesis of chiral sensing materials.
TTU will target the preparation of selective chiral receptors, based on new chirogenic hosts, namely
hemicucurbituril and porphyrin derivatives (Figure 1) and further exploration of novel methodologies for optical signaling by combining the stereoselective and transducer subunits via supramolecular approach. This will create efficient and diverse platforms for the recognition of a wide range of chiral pollutants with different functionalities.
TCD unit, with its expertise in organic synthesis and porphyrin chemistry, will perform the design and synthesis of nonplanar porphyrins for sensor applications and the removal of pollutants. The concept relies on using the spatial accessibility of the inner nitrogen atoms – the primary structural motif of all free base porphyrins – to bind and recognize pollutant analytes. The goal is to exploit conformational design of free base porphyrins as a new tool to access highly efficient and selective nonplanar porphyrin sensors for the detection of pollutants by utilizing the porphyrin core itself as a recognition unit. The TCD and UNITOV units will design and synthetize achiral porphyrins and corroles to be applied as receptors onto chiral inorganic surfaces developed by CNRS, as described in point 3. Here, induction of chirality from the template allows for programming the desired asymmetry of the whole assembly.
- Characterization of the binding mechanisms of the developed receptors.
The formation of the receptor-analyte complex will be characterized for all receptors prepared in the first phase. This will involve photophysical and photochemical characterizations of the chemosensors themselves and their analyte adducts. Association constants will be determined and control experiments on the evaluation of interference of other substances will be performed. The TTU and CNRS units will carry out this study by applying circular dichroism and UV-vis spectroscopy, and, additionally, circularly polarized luminescence, recently acquired by the CNRS group as one of the first such apparatuses in Europe, to detect fluorescence active chiral systems. The JYU group will perform X-ray crystallographic characterizations of the adducts in the solid state to gain a better understanding of the possible mechanisms and sites of interactions. These studies will yield essential information on the receptor binding mechanisms, which is critical for the later transfer of the receptors to the solid state devices.
- Integration of the developed receptors into nanostructured materials.
The CNRS group in collaboration with other Research Units (RUs) will study the anchoring of chiral receptors developed in the first phase of project activities to hybrid/inorganic chiral nanostructures. The expertise of the CNRS group allows them to design and fabricate organic-inorganic hybrid chiral nanostructures as chiral template based on amphiphile self-assemblies. The dimension, chirality and the shape of the structures can be finely controlled (Figure 2). It was shown that these inorganic or hybrid nanoobjects can be used as the corresponding chirogenic platform to induce chirality toachiral functional molecules or nanoparticles which are organized /adsorbed/grafted on their surfaces (see Figure 2). In the context of the present project, such chiral templates can be used to induce chirality or stereoisomeric effect to the receptors grafted on their surfaces, which can result in the stereospecific recognition through chiral host-guest interactions for enantiomeric substances/pollutants. These nanostructures will lead to the production of new solid state systems that offer a significant boost in the sensing capacity of the hybrid material with respect to the properties of its separate components,as they can offer high volume/surface ratios and a stable anchoring of the receptors.
- Controlled deposition of chiral receptor systems onto transducer surfaces.
The consortium will utilize the experience of UNILEin the thin film depositions of sensing materials onto transducer surfaces. In this phase both the supramolecular aggregates and the hybrid inorganic-organic nanostructures developed in the previous steps will be deposited onto the surfaces of the chosen transducers – a key step in developing chemical sensors. Deposition of these materials can be achieved using various techniques. For example, whilst Langmuir-Blodgett or Langmuir-Shaefer methods could give more controlled morphologies, a simple spin-coating technique will be also employed, especially taking in consideration the mandatory requirements for the future realization of a commercial devices. The chiroptical properties of the obtained layers will be investigated after deposition onto transducer surfaces. The chiroptical properties of the obtained layers will be investigated after deposition onto transducer surfaces. UNILE will use Vibrational Circular Dichroism (VCD) investigations, studying the CD associated with infrared vibrational absorption bands. By this analysis the difference in absorption between left and right circularly polarized IR light of optically active compounds is detected and therefore fully matches the goals of the proposed research. VCD approach is very sensitive to the reciprocal orientation of the functional moieties within molecules thus providing details about the molecular organisation inside the transferred films and unravelling the supramolecular arrangement of the active layers. In this way this approach can give a fundamental contribution for the comprehension of relationships between the three-dimensional organisation of the thin layers and their activity and performances.
- Development of chemical sensors based on the prepared sensing materials.
The next step will be devoted to the development of chemical sensors targeting the detection of chiral analytes both in the gaseous and liquid phases. The transduction mechanism will be chosen to optimize the recognition properties of the sensing materials. For the gaseous phase, UNITOV will develop devices according to its expertise; one option is represented by quartz crystal microbalances (QCM), as these transducers are cheap, robust and simple to use. For the liquid phase, optical transduction will be the mechanism of choice. UNITOV will cooperate with Interspectrum and Eurochem Italia for the realization of an optical sensor platform for the project.
The sensing properties of the developed materials will be investigated by testing their response towards the enantiomeric pair chosen as model analytes. Figure 3 gives examples of these analytes, which are representatives of pollutant substance classes of interest known for their toxicological and environmental impact. Laboratory tests will first be carried out using measurement set-ups available at UNITOV, then the performances of the developed sensor arrays will be compared and validated with the protocols usually adopted by Eurochem Italia in the environmental analyses of pollutants. Other than the chiral discrimination, the comparison with commercial devices or other sensor systems available in the consortium, will give a clear idea of the suitability of the realized devices in real application conditions.