Novel hybrid nanocomposites based on graphene derivatives and colloidal nanoparticles for sensing applications

The research activity of this PhD course reports on the synthesis and chemical physical characterization of a novel hybrid nanocomposite formed of Histidine (His) functionalized Reduced Graphene Oxide (RGO) sheets, decorated with Ag nanowires (NWs) (His-RGO/Ag NWs), and its applications in sensors devices. Graphene is among the carbonaceous nanostructures, one of the most interesting for its high specific surface area, high conductivity, high electrocatalytic activity, SERS activity, high Young’s modulus and mechanical strength, high thermal and electrical conductivity, all amiable properties that have been exploited in electrochemical and SERS sensors, devices for energy conversion and storage, Field-Effect Transistors (FETs), touch panels, and membranes. On the other hand, Ag NWs show interesting plasmonic properties, high electric and thermal conductivity, optical transparency and good mechanical flexibility, properties that have been exploited in Surface Enhanced Raman Scattering (SERS), electrochemical, pressure and temperature sensors, transparent heaters, electrodes for solar cells and touch screen panels. Our interest in developing hybrid nanocomposites formed of graphene and Ag NWs, relays on the possibility to merge their outstanding properties, resulting in materials with novel properties or with enhanced the functionalities of the pristine components. In particular, nanocomposites based on inorganic nanoparticles and graphene derivatives found wide applications in the aerospace industry for the manufacturing of composites having high Young’s modulus to integrate in vehicles, advanced thermoregulatory textiles for manufacturing sophisticated clothes for individual thermal comfort in extreme thermal conditions, components for energy storage as battery and supercapacitors, electromagnetic shielding materials, sensors for monitoring atmosphere conditions, temperature and pressure and individual physiological parameters. In the frame of this research project, the hybrid nanocomposite has been synthesized starting from the exfoliation of RGO sheets with His in water. The amino acid intercalates among the RGO multilayers, anchors onto the RGO basal plane by π-π interactions and grafts it by -COOH and -NH2 groups allowing its exfoliation. The use of His prevented the use of the toxic organic solvents that are typically used in the exfoliation of RGO, because its binding to the RGO scaffold allows RGO dispersion in aqueous solutions. Then, the Ag NWs were synthesized in situ onto the RGO basal plane by the polyol approach, using AgNO3 as precursor, ethylene glycol (EG) as solvent and reducing agent, and polyvinylpyrrolidone (PVP) as capping and steric stabilizing agent. The synthesized NWs anchor RGO basal plane binding, by coordination, the -COOH groups of His, and RGO behaves as support and protective coating layer, avoiding NWs aggregation and oxidation thanks to its gas and moisture barrier properties. The synthesis of the His-RGO/Ag NWs has been optimized by using PVP of 360 kDa, and ca. 3.4 ± 0.9 µm long and 0.06 ± 0.01 µm thick Ag NWs, with aspect ratio of 57, were achieved. The synthesized nanocomposite His-RGO/Ag NWs have been tested for the electrochemical detection of the pesticide carbofuran and for the SERS detection of probe molecules. Such a study has been conceived to test and validate, as a proof-of-concept demonstration, the potentialities of the novel engineered nanocomposite material, opening perspectives to its leveraging in sensors having applications closer to those related to aerospace industry. Electrochemical sensors When integrated on screen-printed carbon electrodes (SPCEs) and further modified by the electropolymerization of the polymer PEDOT:PSS, the achieved SPCE/His-RGO/Ag NW/PEDOT:PSS electrodes have shown an increased conductivity, a higher heterogeneous charge transfer constant and an higher electrocatalytic activity, favoring oxidation of carbofuran at the electrode surface. Thus, the fabricated SPCEs/His-RGO/Ag NW/PEDOT:PSS electrodes have shown a high sensitivity in the detection of carbofuran, with a limit of detection of 17.3 nM, that is lower than the U.S. EPA recommended concentration in drinking water with a relatively good %RSD of selectivity, repeatability, reproducibility and storage stability. This improved sensitivity is due to the electrocatalytic properties of Ag NWs and the high conductivity of both the nanocomposite and the electropolymerized PEDOT:PSS film, opening the venue to the application of the fabricated electrode in the detection of other molecules of environment interest. SERS sensors When deposited by drop-casting onto hydrophobic paper substrates, the nanocomposite has been tested in the SERS detection of model molecules having a different chemical structure, namely 1-naphthalenethiol (1-Nat), rhodamine 6G (R6G), and benzo[a]pyrene), against paper substrates modified by neat Ag NWs based samples. The study reveals that 1-Nat, possessing a high affinity for silver, exhibits strong SERS signals on both His-RGO/Ag NWs and the neat Ag NWs substrates. Meanwhile, R6G generates even more intense SERS peaks compared to 1-Nat on both substrates due to its high affinity with silver and its interaction with RGO. Both molecules 1-Nat and R6G reach a limit of detection (LOD) of 10⁻⁷ M on both His-RGO/Ag NWs and Ag NWs substrates. In contrast, benzo[a]pyrene produced no detectable SERS signal. This absence of response was attributed to the absence of functional groups in benzo[a]pyrene having chemical affinity for silver and to the reduced number of aromatic rings in the molecule's structure that undergoes weak aromatic π- π stacking interactions. The developed hybrid nanocomposite materials designed for the detection of pollutant molecules hold significant potential in the aerospace sector, which increasingly relies on advanced sensing technologies to enhance operational safety, environmental compliance, and crew health in complex environments. Monitoring air quality in aircraft cabins and space vehicles is crucial due to the potential accumulation of toxic chemicals, which may arise from system-related emissions, prolonged closed-loop life support, and even contaminants like pesticides in agricultural aviation settings. Detecting and managing these pollutants in real-time requires sensor technologies that are both highly sensitive and robust under challenging aerospace conditions.