Microreactors are useful tools for optimizing and studying chemical reactions. [1][2][3][4][5][6][7][8][9][10][11] Compared to conventional reactors, microreactors improve chemical synthesis through advantages offered by their small length scales, such as reaction volumes, and enhanced heat and mass transfer. [1,2] These advantages can be very important for nanoparticle synthesis. Indeed, several kinds of inorganic particles have been synthesized in microreactors: quantum dots (as CdS [3] or CdSe/ZnS core/shell structures [4] ), metallic nanoparticles (Pd, Co, Ag, Au [5] ), and oxide nanoparticles (g-Fe 2 O 3 , [6,7] a-FeOOH, [8] SiO 2 , [9] TiO 2 , [10] and SiO 2 /TiO 2 core/shell nanoparticles [11] ). Nanoparticles with tailored structural, magnetic, fluorescence, and chemical properties have a wide range of applications in the biomedical field, including imaging, targeting, and drug delivery. [12] Superparamagnetic g-Fe 2 O 3 nanoparticles (MNPs) are a good example of such multifunctional particles and are used for magnetic separation, drug delivery, magnetic resonance imaging (MRI), and hyperthermia cancer treatment. [13][14][15][16] Most of these applications require proven chemical stability of the nanoparticles, a narrow particle size distribution, and good dispersion of the nanoparticles in the liquid medium to avoid any unspecific aggregation. Encapsulating the MNPs in silica shells provides a protective, biocompatible, inert, and hydrophilic surface with excellent anchoring points for derivatizing molecules. [17,18] Moreover, incorporation of chromophores in the silica shell provides magnetic and luminescent core/shell nanocomposites with applications as contrast agents for molecular imaging. [19,20] Chromophores can be organic fluorescent dyes, [21] or luminescent inorganic particles such as quantum dots. [19] Methods reported for formation of MNP/ silica nanocomposites in the bulk include the use of aerosol pyrolysis, [22,23] emulsions, [24] microemulsions, [25,26] and reactions performed under Stöber conditions. [27][28][29]