Reactivity and adsorption mechanisms in polymer-graphene nanocomposites: a theoretical investigation on polypyrrole...

Carbon quantum dots functionalized with conjugated polymers have emerged as promising platforms for chemical sensing due to their tunable electronic and optical properties. Here we present a theoretical investigation on how successive chemical functionalization and polymerization modulate the electronic structure and gas-sensing response of CQD-based nanocomposites. Electronic-structure calculations were conducted within the density functional theory framework. Local reactivity was assessed via condensed Fukui indices, and SO₂ adsorption was investigated at the most reactive sites. Born-Oppenheimer molecular dynamics (BOMD) simulations were conducted to assess cluster stability. The results reveal that carboxylation induces uniform stabilization of the frontier orbitals without significantly altering the fundamental electronic gap, whereas 2-(1H-pyrrol-1-yl) ethan-1-amine (PETN) anchoring shifts orbital energies upward through inductive effects. In contrast, polymerization into PPy networks produces pronounced HOMO-LUMO gap narrowing driven by HOMO destabilization and enhanced π-conjugation. Fukui analyses indicate that only polymerized systems exhibit electrophilic reactive sites on the polymer chains, highlighting the crucial role of PPy formation in sensing activity. SO₂ adsorption perturbs frontier orbital energies and introduces analyte-derived states near the LUMO, consistent with electron-trapping behavior and reduced electronic mobility. The complexes exhibit binding energies and recovery times consistent with reversible physisorption suitable for sensing applications, as further supported by BOMD simulations. TD-DFT analyses reveal analyte-induced charge-transfer excitations dominated by PPy-to-analyte electron transfer processes mediated by PETN pyrrole units, which are expected to modulate photoluminescence and exciton dynamics. Overall, the results demonstrate that controlled PPy polymerization on CQDs creates electronically responsive adsorption centers and enables efficient analyte-induced charge trapping, establishing CQD-PPy nanocomposites as promising candidates for sensitive optoelectronic sensors.