2.1.3. Functionalization of SiNPs with Carboxy (COOH) Groups

The modification of SiNPs with carboxy functional groups was performed by following a slightly modified protocol by Barczak et al. [25]. First, 0.1 g of SiNPs, 0.024 g of Meldrum's acid (MA), and 33.3 mL of toluene were added to a round-bottom flask attached to a reflux condenser and heated to 110 ◦C in an oil bath with continuous stirring for 3 h. Then, the yellowish reaction mixture was centrifuged at 6000× *g* and washed once with 1% chloroform (diluted in Millipore water) and twice with absolute ethanol to assure that the unreacted species derived from the Meldrum's acid decomposition were removed. The washed mixture was then placed into a clean round-bottom flask with 33.3 mL of water and boiled for another 3 h. Finally, the contents of the flask were poured into a crystallization cup and dried overnight at 70 ◦C. The SiNP\_M-functionalized particles obtained were then stored in an airtight container.
