Sensors are vital components in a wide variety of applications throughout our everyday lives. Two critical factors that contribute to their importance are sensitivity and selectivity. To achieve high-performance sensors, modification and functionalization of the sensing surface become imperative. The process involves altering the surface properties of the sensor and introducing a recognition element to optimize its response to the target analyte. However, when it comes to internal sensing surfaces, such as those found in microchannel-based sensors, challenges arise. Being narrow and confined makes the modification process particularly arduous. Accessing the internal surfaces for modification becomes difficult due to their small size, and achieving uniformity across the microchannel surface poses a considerable challenge. Overcoming these obstacles demands innovative fabrication techniques, specialized equipment, and precise control during the modification process. This paper will review the potential approaches for the modification and functionalization of the sensor surface, as well as offer a variety of strategies to effectively enhance the internal sensing surface.