## 3 Results
### Quantum Dot-Based Sensors
#### 3.4.2 Optical and Electrical Sensing Principles  
Quantum dot-based sensors operate based on optical or electrical sensing principles, taking advantage of the unique properties of quantum dots.  
In optical sensing, quantum dots exhibit size-dependent optical properties, including absorption and emission wavelengths [63]. By tuning the size and composition of the quantum dots, their absorption and emission spectra can be tailored to match specific analytes or environmental conditions. Changes in the surrounding environment, such as the presence of target molecules or variations in temperature, can induce shifts in the absorption or emission spectra of the quantum dots, enabling the detection and quantification of the analytes or parameters of interest [64].  
Electrical sensing in quantum dot-based sensors relies on the charge transport properties of the quantum dots [65]. By incorporating quantum dots into a conductive matrix or a field-effect transistor (FET) configuration, changes in charge carrier concentration or mobility can be detected. This can be achieved through mechanisms such as modulation of the quantum dots' charge transfer or capacitance in response to analyte interactions or changes in the surrounding environment [66].  
The optical or electrical signals generated by the quantum dot-based sensors are typically detected and analyzed using appropriate readout systems [67]. Optical sensing relies on spectroscopic techniques, such as absorption spectroscopy, fluorescence spectroscopy, or luminescence imaging. Electrical sensing involves measuring changes in current, voltage, or resistance using appropriate circuitry and amplification methods.  
The applications of quantum dot-based sensors in bridge maintenance are diverse. They can be utilized for the detection of specific analytes, such as gases or chemical species relevant to corrosion or pollutant monitoring. Quantum dots can also be incorporated into strain or deformation sensors, providing optical or electrical readouts for structural health monitoring. Furthermore, their tunable optical properties enable the development of multifunctional sensors capable of detecting multiple parameters simultaneously.  
Figure 4 combines two pie charts to provide a comprehensive overview of quantum dot-based sensors in the context of bridge maintenance. The left pie chart illustrates the principles of quantum dot-based sensors, highlighting their optical and electrical sensingcapabilities. The chart demonstrates that quantum dots can be utilized for both optical sensing, leveraging their size-dependent optical properties for detecting analytes or environmental conditions, and electrical sensing, relying on the charge transport properties of the quantum dots to detect changes in charge carrier concentration or mobility. The right pie chart focuses on the applications of quantum dot-based sensors in bridge maintenance. It showcases their versatility in bridge monitoring by highlighting three key application areas. These areas include gas/chemical species detection, enabling the identification and quantification of relevant gases or chemicals associated with corrosion or pollution; strain/deformation sensing, providing optical or electrical readouts for monitoring structural health and detecting potential issues in bridge components; and multifunctional sensors, capable of detecting multiple parameters simultaneously, further enhancing the overall monitoring capabilities of bridge maintenance systems.