Surface Photoluminescence of Oxidized Nanodiamonds: Influence of Environment pH

Nanoparticles' environment, in particular pH, can strongly affect their photoluminescence (PL). This can be especially true for the surface PL of nanodiamonds (NDs) known to be dependent on the composition of surface groups. In this paper, the effect of environmental pH in the range from 2 to 12.5 on the surface photoluminescence of oxidized NDs of various syntheses and surface treatments was investigated. For the first time, the varying changes of the NDs' PL with the pH were shown, having, however, an essential common feature: they were observed in the same pH regions from 2 to 5 and from 9 to 12.5, while in the pH region from 5 to 9, the PL intensity of all samples was almost constant. Obtained dependences of NDs' zeta-potentials on pH and quantum mechanical modeling showed that all changes of NDs' PL are due to the (de)protonation of surface groups: carboxyl at pH from 2 to 5 and hydroxyl on sp(2)-hybridized carbon at pH from 9 to 12.5. The varying response of NDs' PL to such deprotonation shows the possibility of tuning the surface photoluminescence of NDs for use in industry and biomedicine beyond what is possible by only controlling the nanodiamonds' surface groups.

Automated summary

This study investigates the effect of environmental pH on the surface photoluminescence of oxidized NDs of various syntheses and surface treatments, showing that the PL of NDs varies with pH in specific ranges. The changes in NDs' PL are attributed to the (de)protonation of surface groups, carboxyl at pH 2-5 and hydroxyl at pH 9-12.5, indicating the potential for tuning NDs' surface photoluminescence beyond surface group control.