Statement 1 is Correct: Carbon nanotubes (CNTs) can be functionalized with various bioactive molecules, such as drugs and antigens. This allows them to act as efficient delivery systems that can cross biological membranes to reach specific target sites in the body.

Statement 2 is Incorrect: While carbon nanotubes have been explored as scaffolds in tissue engineering, their use as artificial blood capillaries remains largely experimental and has not been established as a validated application. Blood capillaries require precise biological compatibility, flexibility, and permeability that CNTs alone cannot replicate. Hence, the claim that they can be made into functional artificial blood capillaries for injured parts of the human body is not sufficiently supported, and UPSC's official answer key treats this statement as incorrect.

Statement 3 is Correct: Due to their high surface-to-volume ratio and unique electrical properties, carbon nanotubes are ideal components for biochemical sensors used to detect biological molecules like glucose, proteins, or DNA with high sensitivity.

Statement 4 is Correct: Research has demonstrated that carbon nanotubes can be broken down by oxidative enzymes such as myeloperoxidase (MPO) found in human neutrophils, establishing their biodegradability under certain conditions.

⚠️ Caveat: Statement 4 remains a subject of ongoing scientific debate. While certain studies confirm enzymatic biodegradation of CNTs, this is not universally accepted, as biodegradability is highly dependent on the type, length, surface functionalization, and structural properties of the nanotube. In many real-world conditions, CNTs are considered persistent and non-biodegradable. The UPSC answer key treats Statement 4 as correct, but candidates should be aware of this nuance.