Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2023 / Paper 1 / Section B / 10 / i

The surface carboxylic acid is less reactive toward direct nucleophilic acyl substitution: hydroxide is a poor leaving group, and an amine initially tends to form an ammonium carboxylate salt. An alcohol also does not normally give rapid, quantitative ester formation merely by mixing with the acid.

Conversion to an acyl chloride activates the carbonyl and provides chloride as a substantially better leaving group. Amine or alcohol attack can then form a tetrahedral intermediate followed by elimination and proton transfer, giving the grafted amide or ester. Other coupling or esterification methods are possible; acid-chloride activation supplies the efficient route used in this scheme. Surface functionalization with the corresponding alkylamine and diol was demonstrated in the underlying carbon-material study.^[ref]

CNS-DDA contains an amide linkage, $\mathrm{surface}-\mathrm{C}(\mathrm{O})-\mathrm{NH}-{\mathrm{C}}_{12}{\mathrm{H}}_{25}$. The amine nitrogen becomes bonded to the surface carbonyl carbon.

CNS-EG contains an ester linkage, $\mathrm{surface}-\mathrm{C}(\mathrm{O})-\mathrm{O}-\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$. One alcohol group forms the ester while the other remains an outer hydroxyl group. The latter is not an ether linkage at the surface carbonyl.

Centrifuge the dispersion, separate the particle-rich sediment from the supernatant, wash the particles with a compatible solvent and repeat separation as necessary to remove soluble reagents and byproducts. Use conditions appropriate to the actual particle size and colloidal stability; the aim is particle separation rather than chemical decomposition.

A suitable membrane filtration process is another possibility if its pore size retains the particles. Simple solvent evaporation alone would leave soluble impurities together with the product, so it is not a selective isolation method.