Nanopores in geologic systems are often chemically bound with diverse functional groups. While the physical and chemical interactions between an aqueous solution and unconfined surfaces with functional groups are understood reasonably well, it is still challenging to describe how the pH and ions will behave in the nanoconfined space with different functional groups. Here, we designed a core-shell nanosensor with a gold nanorod (AuNR) as the core and functionalized mesoporous silica as the shell. AuNR can significantly enhance the Raman scattering of the probe molecules anchored on the surface: Para-mercaptobenzoic acid was used to measure pH, and ortho-mercaptoisonicotinic acid was utilized to determine concentrations of heavy metals. Our results show that the physical interaction originating from ion polarization dominates the hydrophobic functionalized nanopores (methyl- and phenyl-), causing suppressed concentrations of heavy metals and a lower pH in nanopores compared to those in bulk solution. In contrast, in hydrophilic functionalized nanopores (amine-, carboxyl-, and thiol-), pH and concentrations of heavy metals are respectively dependent on pK<sub>a</sub> values of the functional groups and chemical affinities of heavy metals towards these functional groups. Our new findings can help delineate the distribution of heavy metals in soil and help to design new remediation approaches that can utilize the unique characteristics of nanopore chemistry.

Hydrophilicity/hydrophobicity controls pH and concentrations of heavy metals in nanopores

ACS Spring 2022

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