# Bioreactors: Applications and Innovations for a Sustainable and Healthy Future--A Critical Review
## 4 Advances in Optical Sensors in Bioreactors  
Optical chemosensors, also known as optodes, are devices that operate through the interaction between an analyte and a chemical indicator incorporated into a matrix immobilized at the tip of the sensor. The operation of these sensors is based on illuminating the indicator with a light-emitting diode (LED) via an optical fiber. Changes in the optical properties of the indicator, such as photoluminescence intensity, absorption, or reflection, are detected by a photodiode. These changes are directly correlated with the concentration of the analyte of interest, enabling precise and reliable measurements [44; 56]. The versatility of optodes allows for their in situ use in stainless steel bioreactors, utilizing standard ports that facilitate integration with existing systems. In small-scale contexts, such as deep-well plates and shake flasks, optical sensors can be applied via adhesive patches. This application is particularly valuable in low-volume systems where in situ sensors may not be feasible or could interfere with the system's hydrodynamics. These sensors are widely used to monitor critical parameters such as dissolved oxygen, carbon dioxide, and pH, with few innovations in their operating mechanisms. The ability to be sterilized by gamma radiation before use ensures that they meet the stringent sterility standards required in bioprocesses [37; 43]. Significant innovations include reducing the size of spectrometry sensors to a miniaturized form, capable of performing microscopic scans and optical density measurements. Additionally, the development of optical biosensors involves immobilizing enzymes, substrates, or bacteria in a stationary phase, which can be a resin or some nanostructure. Figure 3 shows the main technologies being studied in the field of optical sensors [38; 43; 56]. Figure 3: Key technologies being studied in the field of optical sensors. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Models** & **Description** & **Applications** & **References** \\ \hline Support Vector Machine (SVM) & The Support Vector Machine (SVM) is a classification algorithm that identifies a hyperplane separating data sets into different classes, aiming to find the maximum margin of distance between the classes. & Optimization of Membrane Permeability of a Membrane Rotating Biological Contactor for Wastewater Treatment, including optimization of microbial lipid fermentation from cellulosic ethanol wastewater by & [48; 54; 55] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._The ability to integrate with remote monitoring systems and industrial automation is another strong point of optical chemosensors. With the growing adoption of Industry 4.0 technologies, the ability to collect and analyze data in real time is crucial. Optodes can be easily connected to sensor networks and control systems, allowing continuous monitoring and optimization of biotechnological processes. New projects are studying the transformation of smartphones into optical sensors, turning an everyday device into a precise measurement instrument [42; 43; 56; 57]. Table 4 below lists some new optical sensors being developed and their applications. Conventional optical microscopy is constrained by the diffraction limit of light, which prevents the resolution of objects smaller than approximately 200 nanometers. Super-resolution microscopy overcomes this limitation by employing strategies that precisely pinpoint the location of individual fluorophores, enabling imaging at resolutions far beyond the diffraction barrier. This technique has revolutionized bioimaging, allowing for the visualization of cellular structures with extraordinary detail. In super-resolution microscopy, specific cellular structures are labeled with fluorescent molecules, and these fluorophores are activated one at a time, enabling the precise localization of each molecule and resulting in a much higher-resolution image.