Self-assembled single-crystalline hydrogen-bonded frameworks: a potential metal-free molecule towards proton conductivity and electrochemical detection of organic and inorganic pollutants

Extensive efforts have been dedicated towards the electrochemical detection of inorganic (heavy metals) as well as organic pollutants (nitro compounds) since their presence at extensive levels notably affects human health and the environment. Although various nanomaterials have been explored as electrode materials for their detection, it is still challenging to develop metal-free or carbon-based electrodes that perform similarly or even surpass their metal-containing counterparts. In this contribution, a new class of hydrogen-bonded organic frameworks is reported via self-assembly of 1,2,4,5-benzenetetracarboxylic acid (BTC) as a hydrogen bond donor and 2,6-di(1H-imidazol-yl)pyridine (bimPy, hydrogen bond acceptor) to form reversible hydrogen bonds (BTC-bimPy@HOF). The obtained BTC-bimPy@HOF exhibits an excellent proton conductivity of 0.96 × 10⁻³ S cm⁻¹ at 55 °C@98% RH, making it a viable candidate for proton-exchange membrane fuel cells. Moreover, the BTC-bimPy@HOF individually detected both inorganic [cadmium (Cd²⁺) and lead (Pb²⁺)] and nitro compounds [para-nitroaniline (p-NA)] in an acetate buffer medium. The enhanced electrochemical signal response was due to the presence of an active binding site in BTC-bimPy@HOF, which enabled strong interaction between the hydroxyl groups and metal ions/nitro group (coordination bond and electrostatic interaction). The BTC-bimPy@HOF electrode displayed good linear detection ranges of 0.005 to 0.4 μM (Cd²⁺), 0.001 to 0.31 μM (Pb²⁺), and 0.01 to 5 μM (p-NA), respectively, coupled with low detection limits of 1.7 nM (Cd²⁺), 0.5 nM (Pb²⁺), and 23 nM (p-NA). Consequently, our work highlights a straightforward approach to design metal-free organic molecules via self-assembly, thus creating a notable impact in various electrocatalytic applications.