Graphene Nanomaterials for Chemical Sensors

Medical diagnostics, environmental protection, and detection of explosives in security controls at airports are examples of fields with a demand for detection of chemicals in very low concentrations. It has been suggested that nano-sensors will be well suited for the task, as they will be able to detect extremely small quantities, in principle down to a single molecule. The project presents new active sensing nanotechnology materials for the purpose.

With crucial properties such as high surface area, high conductivity, and low production cost with easy up-scaling, graphene-like materials are promising for many applications. By functionalization with molecular receptors already known from other types of chemical sensing, graphene-like materials can be endowed with increasing selectivity to form better and cheaper sensing composite materials.

In the project, reduced graphene oxide (RGO) was covalently functionalized with supramolecular moieties to create active sensing materials. Two different strategies were applied.

The first approach consisted of covalently attaching crown-ethers – a type of chemically resistant supramolecular moieties – to graphene oxide (GO). The functionalized GO was then reduced chemically. This resulted in monolayer RGO nano-sheets functionalized with crown-ether to an extent of up to 30 % of the theoretically available surface area. These materials were shown to selectively bind alkali metal ions. Potentiometric sensing based on the materials was achieved with a detection limit of 10-5 M.

The described first approach is, however, only applicable to supramolecular moieties that are not subjected to chemical reduction. In order to widen the possibilities of functionalization to include less chemically stable moieties an alternative strategy was developed.

In the second approach, Azido-RGO was prepared as a general platform for post reduction modification. GO was here functionalized with a short linker terminated in an alcohol. The intermediate material was then reduced effectively with NaBH4, followed by chemical transformation of the alcohol into azide, thus providing a chemical handle for click chemistry in the form of CuAAC.

This platform material was then functionalized with ferrocene as a redox probe to accurately determine surface coverage in which one azido functionalized group was attached per 16 carbon atoms in the RGO sheet.

This Azido-RGO platform was then used in successful functionalization with a large supramolecular receptor molecule, TTF-calix[4]pyrrole. The resulting composite material can sense Cl- and potentially TNB. The coverage achieved was one molecule per 50-60 carbon atoms in the RGO sheet. In view of the size of this molecular moiety, the coverage is actually very high. The material was used for Cl- sensing showing sensitivity at very low concentration with linear response in the concentration range 10-8 to 10-5 M.


Illustration:

Crown-ether functionalized graphene nanosheets and sensor for selective sensing of potassium ions.