**Acknowledgements**

This research study was supported by a grant of Ministery of Research and Innovation, CNCS - UEFISCDI, project number PN-III-P4-ID-PCE-2016-0818, within PNCDI III. The authors gratefully acknowledge financial support of the GEX project no. 75/25.09.2017, CH 38-17-05. All data presented were recorded using the Renishaw confocal spectrometer with 473 nm laser excitation wavelength and 1.25 laser power. For this study graphene oxides with different types of functionalities were used as received from NanoInnova Technologies. Graphite, SWCNTs and MWCNTs were purchased from Aldrich.
