### Nanosensors: An Overview  
Nanosensors have emerged as a promising technology for structural health monitoring in bridge maintenance due to their unique properties and capabilities. These miniature devices, typically on the nanometer scale, utilize nanoscale materials and innovative sensing principles to detect and measure various physical and chemical parameters relevant to bridge structures.  
The field of nanosensors encompasses a wide range of sensor types, each with its own advantages and applications [18]. Carbon nanotube-based sensors are one such example, known for their exceptional mechanical strength, high electrical conductivity, and sensitivity to strain and deformation [19]. These sensors can be embedded within bridge components to monitor structural changes and provide valuable data on the structural health and integrity of the bridge [20].  
Graphene-based sensors have also garnered significant attention in recent years [21]. Graphene, a single layer of carbon atoms arranged in a two-dimensional honeycomb lattice, exhibits remarkable electrical, mechanical, and sensing properties. Graphene-based nanosensors offer high sensitivity, rapid response times, and excellent electrical conductivity, making them suitable for detecting a wide range of physical and chemical parameters, including strain, temperature, humidity, and gas concentrations [22]. There are many other types such as nanowire-based sensors [23], quantum dot-based sensors [22], etc., where each type of nanosensor offers unique properties and capabilities for monitoring structural health and detecting potential issues in bridges.  
The fabrication techniques employed to create nanosensors vary depending on the specific sensor type and desired properties [24]. Top-down approaches, such as lithography-based fabrication and chemical etching methods, involve patterning and shaping bulk materials into nanoscale structures [25]. Bottom-up approaches, including chemical vapor deposition (CVD) and molecular self-assembly techniques, involve the growth of nanoscale materials from atomic or molecular building blocks [26]. Hybrid fabrication techniques combine both top-down and bottom-up approaches to achieve precise control over sensor properties [27].  
Figure 1: Importance of structural health monitoring in bridge maintenance.  
Integrating nanosensors into bridge maintenance systems poses its own set of challenges. Issues such as sensor placement, data communication, power supply, and signal processing need to be addressed for successful implementation [28]. Additionally, calibration, reliability, and the long-term performance of nanosensors in harsh environmental conditions are areas of ongoing research [29]. Nanosensors offer a promising avenue for structural health monitoring in bridge maintenance [30]. Their unique properties and sensitivity make them ideal for detecting and monitoring various parameters critical to bridge integrity. As research and development of nanosensors continue to advance, their integration into bridge maintenance practices holds significant potential for enhancing safety, extending bridge lifespan, and optimizing maintenance strategies.  
From the literature above, it appears that the topic of nanosensors for structural health monitoring in the context of bridge maintenance is still relatively nascent and has not been extensively covered in the existing literature. While there may be some papers addressing this topic, the number of publications specifically focusing on nanosensors in bridge maintenance is limited. Therefore, this review paper fills a significant gap in the literature by providing a comprehensive analysis and detailed insights into this emerging field.