What does "acidic" mean in an article on hydrogen boride nanosheets?

From Rojas et al. [[1](https://doi.org/10.1038/s43246-021-00184-5)] (emphasis mine):



> 
> On the other hand, we found that proton exchange with HB occurs in water with the estimated $\mathrm{p}K\_\mathrm{a}$ of $3.5\pm 0.2,$ even after reactive sites for the hydrolysis reaction are terminated. Thus, for the practical application, one should keep in mind that HB sheets have a chemical stability against hydrolysis reaction, but become *acidic* in the presence of water
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Does “*acidic*” here refer to the solution? Or acidic sheets that are gaining protons?


### Reference


1. Rojas, K. I. M.; Cuong, N. T.; Nishino, H.; Ishibiki, R.; Ito, S.; Miyauchi, M.; Fujimoto, Y.; Tominaka, S.; Okada, S.; Hosono, H.; Arboleda, N. B.; Kondo, T.; Morikawa, Y.; Hamada, I. Chemical Stability of Hydrogen Boride Nanosheets in Water. *Commun. Mater.* **2021**, 2 (1), 1–8. DOI: [10.1038/s43246-021-00184-5](https://doi.org/10.1038/s43246-021-00184-5). (Open Access)


