**3.3 XPS analysis of carbon dots**

A survey scan, ranging from 0 to 1200 eV is shown in **Figure 3(a)**. It is evident that two elements, namely C (286 eV) and O (543 eV) are present in the spectrum [25]. The detailed XPS spectra of C 1S and O 1S are shown in **Figure 3(b)** and **(c)** respectively.

The C 1S signal peak can be divided in to three peaks, located at 283.8, 284.83and 287.4 eV. These peaks correspond to C-C, C=C and C=O respectively [26]. Finally, in the XPS spectrum of O1S, the peak at 532.25 eV refers to oxygen singly bound to aliphatic carbon while the smaller or low intensity peak at 535.53 eV refers to COO<sup>−</sup> thus confirming the presence of carboxylate groups on the surface of carbon dots.
