## 3 Results and Discussion  
The aim of our work was to design, build and validate a set-up for flow cell coupled DLS measurements that enables real-time monitoring of the size of nanoparticles during liquid phase synthesis. For flow cell coupled real-time measurements of dispersions, we used a DLS unit equipped with the 3D cross correlation geometry (the unit is based on the commercially available NanoLab 3D, LS Instruments AG, Fribourg, Switzerland). The technology applied in this instrument is based on two simultaneous light scattering experiments performed at the same scattering vector on the same sample volume in order to extract only the single scattering information common to both (Figure 1). It enables short measurement acquisition times (dark counts <250 s\({}^{-1}\)). To perform measurements in stirred fluids, a custom designed analysis loop was used to couple the DLS instrument to a stirred 2 L double-walled tank reactor. The dispersion is lead to the measurement module by means of an analysis loop (Figure 2). Figure 1: Schematic representation of the dynamic light scattering (DLS) instrument equipped with 3D cross correlation geometry. Reprinted with permission from LS Instruments AG. Figure 2: (**a**) Schematic representation of the analysis loop for coupling the DLS unit to the pilot reactor system and (**b**) detailed photograph of the setup. In the analysis loop, the reaction mixture is circulated continuously using a Watson pump with flexible tubing. The pump is placed after the cuvette to circumvent contamination of the sample with agglomerated particles due to scaling of the pump. After measurement, the DLS loop is rinsed by demineralized water to clean it before the next sample enters the measuring zone. To validate the set-up, we started monitoring the growth of silica nanoparticles in a Stober synthesis starting from the molecular precursor TEOS in a mixture of water, ethanol and ammonia. This is a prototypical liquid phase synthesis of nanoparticles. In this sol-gel synthesis, TEOS partially hydrolyses, and through subsequent polycondensation silica nanoparticles are formed as shown in the reaction Equations (1)-(3) [16]. \[\mathrm{Si(OC_{2}H_{5})_{4}}+x\mathrm{H_{2}O}\rightarrow\mathrm{Si(OC_{2}H_{5 })_{4-x}(OH)_{x}}+x\mathrm{C_{2}H_{5}OH}\;(0\leq x\leq 4) \tag{1}\]  
\[\equiv \mathrm{Si\text{-}O\text{-}H}+\mathrm{H\text{-}O\text{-}Si}\equiv \rightarrow\equiv\mathrm{Si\text{-}O\text{-}Si}\equiv+\mathrm{H_{2}O} \tag{2}\]  
\[\equiv \mathrm{Si\text{-}OC_{2}H_{5}}+\mathrm{H\text{-}O\text{-}Si}\equiv \rightarrow\equiv\mathrm{Si\text{-}O\text{-}Si}\equiv+\mathrm{C_{2}H_{5}OH} \tag{3}\]  
This process proceeds according to an aggregation-based kinetic model [17; 18]. The size of the silica nanoparticles resulting from this synthesis depends amongst others on the concentration of ammonia and TEOS. The average particle size increases with increasing concentration of ammonia. The initial concentration of TEOS is inversely proportional to the size of the resulting silica nanoparticles. We started validating the effect of flow rate on the particle size measured in the flow-cell coupled DLS set-up with 3D cross correlation geometry. Therefore, a 1 wt-% dispersion of \"large\" and \"small\" silica particles of a radius of 157 nm and 17.6 nm, respectively, were circulated in the analysis loop comprising the DLS at flow rates \(Q\) between 0 and 17 mL\(\cdot\)min\({}^{-1}\). The particle radii were determined using conventional DLS. The displayed particle size is measured with the 3D cross correlation DLS whilst the dispersion was pumped through the flow cell at a defined flow rate \(Q\) between 0 and 17 mL\(\cdot\)min\({}^{-1}\). For each flow rate, the presented value for the particle size represents an average of ten DLS measurements (Figure 3, error bars represent standard deviation). Figure 3: _Cont._  
The average particle radius measured using conventional DLS is 157 nm for the large particles and 17.6 nm for the small particles.