Smart Metal Organic Frameworks for Sensing Volatile Organic Compounds

Periodic Reporting for period 1 - SmartMOFs (Smart Metal Organic Frameworks for Sensing Volatile Organic Compounds)

Période du rapport: 2017-06-01 au 2019-05-31

Metal-organic frameworks (MOFs) constitute one of the most exciting developments in recent nanoporous material science. Synthesised in a self-assembly process from metal centers and organic linkers, a nearly infinite number of materials can be created by combining different units. The porous nature of MOFs allows to trap guests in the voids of their structure and, playing on the choice of the building blocks, to finely tune the host-guest interaction. MOFs are therefore considered promising materials for many applications such as gas separation, drug delivery or sensing.

Sensitive and selective detection systems for harmful volatile organic compounds (VOCs) are highly needed for many applications, such as industrial process management, chemical threat detection, medical diagnostics, food quality control, occupational safety, and environmental monitoring.

SmartMOFs aimed at producing new MOFs with interesting optical and electrical properties and enhanced selectivity and sensitivity towards harmful volatile organic compounds (VOCs). The project included the full characterization (structural, optical, electrical) of the new MOFs using specialised techniques, and the study of the possibility to integrate these MOFs into sensory devices. More specifically, the main targets of the project were:
- Proper design and selection of metal centers and ligands to create MOFs with the desired properties in terms of absorption/luminescence, electrical conductivity and gas adsorption;
- Preparation of MOFs with combined transduction mechanisms and validation of their selective and sensible recognition activity towards the most harmful VOCs;
- Integration of the final stimuli-responsive MOFs (SR-MOFs) into devices by employing novel thin-film growth techniques.

The innovation of SmartMOFs is based on a combined sensing approach, where both optical and electrical changes in the MOF material enable the recognition event. The primary step to meet the required combination of specific absorption/emission and conductivity properties is the selection of the MOF structural units. The starting building units are selected to offer extensive connectivity, flexibility, tuneable porosity and functionalization of the inner surface of the network, with groups interacting with the analytes by means of coordination bonds, p-p interactions or hydrogen bonds.

Project Results:
WP1- Design and synthesis of multifunctional SR-MOFs.
Mostly, we employed solvothermal synthesis to get single crystals. Cu(I) was mainly used as metal, since it is able to interact with thiolates ligands known as 2,2'-dithiobis(5-nitropyridine) (SNP) and 6,6'-dithiodinicotinic acid (H2dtdn), to obtain MOFs with suitable optical and/or electrical properties.
Working with copper(I) iodide and 6,6'-dithiodinicotinic acid (H2dtdn) and its reduced form 6-mercaptonicotinic acid (6mna), three new coordination polymers, named CP1-3, were synthesized.
On the other hand, the interaction of 2,2'-dithiobis(5-nitropyridine) (SNP) with copper(I) iodide gave rise to three new coordination complexes, named complexes 4-6.

WP2- Structural and optical/electrical characterization.
The structures of the six newly synthetized complexes were fully characterized by different techniques: i) single crystal and powder X-ray diffraction, ii) infrared spectroscopy and iii) thermal stability studies.
The optical properties of all complexes, such as absorption, emission, quantum yield and excited-state lifetime, were evaluated by means of steady-state and time-resolved spectroscopic and photophysical techniques.
The electrical properties of the complexes were measured on single crystals using a two-contact set-up.

From the structural point of view, CP1 and CP3 are 2D-CPs while CP2 is 1D-CP, with an interesting crystal arrangement. Complexes 4 and 5 are 2D-CPs while 6 is a particular hexanuclear cluster.
The three CPs 1-3 showed a semiconductive behavior and CP1 and CP2 displayed an intense emission, in the 600-700 nm range, at low temperature (77 K). Complex 6 was found to be strongly emissive in the near-infrared region and complex 4 has shown an interesting photo-induced electrical behaviour. Theoretical calculations were performed in order to describe the electronic transitions at the origin of the observed optical outcomes.

WP3- Test of the sensing properties towards selected VOCs.
Test studies on the response of the prepared complexes towards selected analytes have been performed.
Among the prepared materials, complex 6 showed good sensitivity towards NaHS, in terms of photoluminescence quenching and colour change, from red to black. This interesting response predicts its application for the detection of H2S, a known poisonous gas.
On the other hand, the room-temperature non-emissive CP1 complex has shown a switch-on emission behaviour when immersed in acetic acid. Due to the low crystallinity of the obtained new phase, it was difficult to clarify the possible structural changes induced by the guest.

While most of the achieved deliverables contributed to expand considerably the knowledge on synthesis, characterization and sensing application of MOFs, some limitations were encountered in some specific domains, such as:
- Obtaining porous MOFs/CPs;
- Deposition of the obtained CPs on surfaces (WP4) due to the lack of solubility.

Exploitation and dissemination of the results.
The outcomes of the project have been presented at nine national and international events. One paper has been recently submitted and forthcoming papers are in preparation. Dissemination to the large public of the importance of MOFs and of their applications in sensing and cultural heritage preservation has been achieved through the participation of the ER and the Supervisor in the European Researchers’ Nights events in 2017 and 2018 in Bologna with a stand, presentations and activities.

The work carried out within the SmartMOFs project represents a progress in the field of design, synthesis and characterization of MOFs and coordination polymers with interesting optical and electrical properties. We could show that the preparation conditions strongly affect the final structure of the material, which can contain new ligands, different from the starting ones, due to cleavage reactions occurring during the solvothermal process. From the same starting units it was thus possible to obtain complexes with completely different structures and properties.
The promising preliminary results obtained with the new materials in terms of sensing of harmful compounds allow to envisage important applications in the preparation of sensing devices, once the materials are incorporated in suitable matrices, attracting industrial interest. The attention of the society in health risks due to exposure to harmful or toxic VOCs, in fact, has drastically increased in the last two decades and the European Parliament has enacted strict regulations to limit VOC emissions from industrial sources and commercial products.

Scheme of the SmartMOFs methodology

Periodic Reporting for period 1 - SmartMOFs (Smart Metal Organic Frameworks for Sensing Volatile Organic Compounds)

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