Correction to: Nature Communications 9:641; https://doi.org/10.1038/s41467-017-02715-6; Published online 13 February 2018

The original version of this Article contained an error in Fig. 4c, in which the right-most chemical structure included an ‘N+’ rather than an ‘N’. The correct version of Fig. 4c is:

which replaces the previous incorrect version:

This has been corrected in both the PDF and HTML versions of the Article.