## 1 Introduction
As siloxane bonds form, molecules aggregate into a sol, which transitions into a gel as particles interconnect, trapping solvent molecules within the network [17; 18]. Despite its apparent simplicity, the sol-gel process is influenced by factors such as water-to-precursor ratio, solvent type, catalyst concentration, pH, temperature, aging, and drying conditions. Controlling these parameters allows for precise structural tuning of the final network [19; 20]. Sol-gel materials offer advantages, including compatibility with organic and inorganic reagents, chemical and thermal stability, controlled particle size distribution, and application as thin films, enhancing the stability of immobilized molecules while preventing leaching [18; 21]. Materials produced via the sol-gel method are crucial for encapsulating various molecules, including organic, inorganic, and biomolecules [22]. Optical pH sensors typically work by immobilizing an indicator within a solid matrix, which is applied as a thin film. Methods for immobilizing the indicator in the sol-gel matrix include impregnation, covalent bonding, and doping [23; 24]. Covalent bonding is particularly advantageous, as it prevents leaching, ensuring the long-term stability of sensor films. However, it may reduce indicator responsiveness, leading to weaker signal changes [25]. Doping, where the indicator is incorporated during the sol-gel reaction, offers rapid response times and enhanced stability, while impregnation, though simpler, can lead to leaching, making it less suitable for continuous-use sensors [26; 27]. Static mixers are critical components in millireactors, designed to enhance mixing efficiency by facilitating fluid interaction without requiring mechanical agitation. These passive mixing elements improve mass and heat transfer within the reactor, leading to better process control and reaction outcomes [28]. Various static mixer designs exist, each optimized for specific flow conditions. Common types include helical mixers, which generate rotational flow to improve radial mixing; grid mixers, which create multiple flow divisions to enhance dispersion; and Kenics-type mixers, which employ alternating left- and right-handed helical elements to promote efficient blending. The selection of a suitable mixer depends on factors such as fluid viscosity, flow regime, and desired mixing intensity [29]. The performance of static mixers is typically assessed using key parameters such as pressure drop, residence time distribution (RTD), mixing index, and coefficient of variation (CoV). Pressure drop indicates the energy required for mixing, and RTD characterizes fluid residence time and distribution, while the mixing index and CoV quantify the degree of homogeneity achieved within the reactor. These parameters provide valuable insights into mixer efficiency and process optimization [30]. Traditional glass-electrode pH sensors are fragile and unsuitable for dynamic microreactor environments, where mechanical stress and small sample volumes pose significant challenges. Additionally, they provide only single-point measurements, limiting their ability to capture spatial pH variations within microfluidic channels. While miniaturized probes exist, their diameters exceed the sub-millimeter scale required for true microreactor integration. Fluorescence-based pH sensors offer high sensitivity but require expensive, complex instrumentation and suffer from issues such as dye leaching and photostability, making them impractical for continuous, long-term monitoring in microreactors [31]. Microfluidic devices, particularly millireactors, are essential for advanced chemical processing due to their enhanced mass and heat transfer capabilities. Static mixers improve mixing efficiency in these systems, but their performance evaluation often relies on computational simulations or indirect measurements. This study presents an innovative approach by integrating pH-sensitive films into 3D printed millireactors to directly assess mixing efficiency.