Mechanistic insights into the photocatalytic properties of metal nanocluster/graphene ensembles. Examining the role of visible light in the reduction of 4-nitrophenol.

Metal nanoclusters (MNCs) based on silver and gold, abbreviated as AgNCs and AuNCs, respectively, were synthesized and combined with functionalized graphene, abbreviated as f-G, forming novel MNC/f-G ensembles. The preparation of MNCs/f-G was achieved by employing attractive electrostatic interactions developed between negatively charged MNCs, attributed to the presence of carboxylates due to α-lipoic acid employed as a stabilizer, and positively charged f-G, attributed to the presence of ammonium units as addends. The realization of MNC/f-G ensembles was established via titration assays as evidenced by electronic absorption and photoluminescence spectroscopy as well as scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDX) spectroscopy analyses. Photoinduced charge-transfer phenomena were inferred within MNCs/f-G, attributed to the suppression of MNC photoluminescence by the presence of f-G. Next, the MNC/f-G ensembles were successfully employed as proficient catalysts for the model reduction of 4-nitrophenol to the corresponding 4-aminophenol as proof for the photoinduced hydrogen production. Particularly, the reduction kinetics decelerated by half when bare MNCs were employed vs. the MNC/f-G ensembles, highlighting the beneficial role of MNCs/f-G in catalysing the process. Furthermore, AuNCs/f-G displayed exceptionally higher catalytic activity both in the dark and under visible light illumination conditions, which is ascribed to three synergistic mechanisms, namely, (a) hydride transfer from Au-H, (b) hydride transfer from photogenerated Au-H species, and (c) hydrogen produced by the photoreduction of water. Finally, recycling and re-employing MNCs/f-G in successive catalytic cycles without loss of activity toward the reduction of 4-nitrophenol was achieved, thereby highlighting their wider applicability.

Toxicity determinants of multi-walled carbon nanotubes: The relationship between functionalization and agglomeration.

The elucidation of toxicity determinants of multi-walled carbon nanotubes (MWCNT) is still incomplete. Functionalization with carboxyl groups is, however, commonly used to mitigate MWCNT toxicity, although the rationale for the mitigating effect has not been fully clarified yet. In this work, two optimized chemical vapor deposition methods were employed to obtain MWCNT of comparable length but different diameter, which were subsequently functionalized. For MWCNT of diameter larger than 40 nm, no detrimental effects on cell viability of macrophages were observed, while mild cytotoxicity was recorded for diameters between 15 and 40 nm, with a mitigating effect of functionalization. To investigate the factors responsible for the mitigation, we used the thinnest MWCNT preparation on different cell models, evaluating several endpoints, such as viability, production of nitric oxide (NO), expression of pro-inflammatory markers, the Trans-Epithelial Electrical Resistance (TEER), and clonogenic activity. Substantial mitigation of the changes caused by pristine MWCNT was observed not only with carboxyl- but also with amino-functionalized MWCNT, suggesting that negative or positive surface charge was not the main factor responsible for the effect. Instead, either functionalized preparation exhibited a stronger tendency to agglomerate that was strictly dependent on the presence of proteins. Moreover, we found that either carboxyl- or amino-functionalized MWCNT adsorbed a larger amount of serum proteins than pristine counterparts, with a distinctive pattern for each type of MWCNT. We propose, therefore, that the formation of larger agglomerates, dependent upon different protein coronae, contributes to mitigate the biological effects of functionalized MWCNT in protein-rich biological media.