Effect of Graphene Characteristics on Morphology and Performance of Composite Noble Metal-Reduced Graphene Oxide SERS Substrate.

Graphene/noble metal substrates for surface enhanced RAMAN scattering (SERS) possess synergistically improved performance, due to the strong chemical enhancement mechanism accounted to graphene and the electromagnetic mechanism raised from the metal nanoparticles. However, only the effect of noble metal nanoparticles characteristics on the SERS performance was studied so far. In attempts to bring a light to the effect of quality of graphene, in this work, two different graphene oxides were selected, slightly oxidized GOS (20%) with low aspect ratio (1000) and highly oxidized (50%) GOG with high aspect ratio (14,000). GO and precursors for noble metal nanoparticles (NP) simultaneous were reduced, resulting in rGO decorated with AgNPs and AuNPs. The graphene characteristics affected the size, shape, and packing of nanoparticles. The oxygen functionalities actuated as nucleation sites for AgNPs, thus GOG was decorated with higher number and smaller size AgNPs than GOS. Oppositely, AuNPs preferred bare graphene surface, thus GOS was covered with smaller size, densely packed nanoparticles, resulting in the best SERS performance. Fluorescein in concentration of 10-7 M was detected with enhancement factor of 82 × 104. This work demonstrates that selection of graphene is additional tool toward powerful SERS substrates.

Dry sonication process for preparation of hybrid structures based on graphene and carbon nanotubes usable for chemical sensors.

The combination of graphene (G) and multi-walled carbon nanotubes (MWCNTs) creates three-dimensional hybrid structures particularly suitable as next-generation electrical interface materials. Nevertheless, efficient mixing of the nanopowders is challenging, unless previous disaggregation and eventual surface modification of both is reached. To avoid use of solvents and multistep purification process for synthesis of stable G/MWCNTs hybrids, herein, a novel dry method based on an air sonication process was used. Taking advantage from the vigorous turbulent currents generated by powerful ultrasonication in air that induces strong thermal convection or radiation to and from the particles, it simultaneously ensures disentanglement of the large MWCNT bundles and G exfoliation and their only mild surface modifications. By changing the ratio between MWCNTs and G, a range of hybrids was obtained, different in surface morphology and chemistry. These hybrids have shown great potential as sensing material for designing mass-based sensors for toxic gases and chemiresistor for vapors detection.

Covalent-Bonded Reduced Graphene Oxide-Fluorescein Complex as a Substrate for Extrinsic SERS Measurements.

When graphene is used as SERS substrates, it contributes to the chemical mechanism (CM) of enhancement of Raman signal, owing to which the detection limit is very low (lower than mM content of probe molecules). The CM of enhancement depends largely on the interactions between the substrate and the probe molecules. Therefore, in this work, we have investigated the possibility of increasing the SERS activity of graphene by improving the interaction between the probe molecule and the graphene substrate by establishing exclusively strong covalent bonding between them. Fluorescein (Fl) was selected as a probe molecule because it is one of the most commonly used fluorophore in bioscience. As a graphene substrate, reduced graphene oxide (rGO) platelets were used. In addition, silver nanoparticles (AgNPs) were added onto the hybrids to further increase the enhancement by electromagnetic mechanism. Highly enhanced Raman signal of Fl onto neat rGO was achieved for micromolar concentration of the probe molecules. This was attributed to the covalent bonding between them, which introduced hole doping to rGO, decreasing the Fermi level of rGO and bringing it more closely to the LUMO of Fl. This induces aligning of their energy levels, resulting in higher contribution of the nonresonance effect to the charge transfer mechanism of enhancement, which, in this case, occurred intramolecularly. When AgNPs were added onto the rGO substrate, the expected enhancement performance was not observed. On the one hand, this was attributed to small size (∼20 nm) of AgNPs and lack of aggregates and, on the other, due to the unusually high contribution of CM determined.