Carbon nanotubes (CNT) represent one of the most unique materials in the field of nanotechnology. CNT are the allotrope of carbon having sp2 hybridization. CNT are considered to be rolled-up graphene with a nanostructure that can have a length to diameter ratio greater than 1,000,000. CNT can be single-, double-, and multi-walled. CNT have unique mechanical, electrical, and optical properties, all of which have been extensively studied. The novel properties of CNT are their light weight, small size with a high aspect ratio, good tensile strength, and good conducting characteristics, which make them useful for various applications. The present review is focused on the structure, properties, toxicity, synthesis methods, growth mechanism and their applications. Techniques that have been developed to synthesize CNT in sizeable quantities, including arc discharge, laser ablation, chemical vapor deposition, etc., have been explained. The toxic effect of CNT is also presented in a summarized form. Recent CNT applications showing a very promising glimpse into the future of CNT in nanotechnology such as optics, electronics, sensing, mechanical, electrical, storage, and other fields of materials science are presented in the review.

Carbon nanotubes: synthesis, properties and engineering applications
    Abstract
    1 Introduction
    2 Type and structure
    3 Properties
        3.1 Physical properties
            3.1.1 Electrical property
            3.1.2 Thermal property
            3.1.3 Mechanical property
            3.1.4 Vibrational property
        3.2 Chemical properties
            3.2.1 Solubility
            3.2.2 Functionalization
    4 Toxicity
    5 Advantages of CNT
    6 Disadvantages of CNT
    7 Synthesis of CNT
        7.1 Arc discharge technique
        7.2 Laser ablation technique
        7.3 Chemical vapor deposition (CVD)
        7.4 Plasma-enhanced CVD (PECVD)
    8 Growth mechanism of CNT
    9 Applications of CNT
        9.1 Field emission devices
            9.1.1 CNT array emitters
            9.1.2 Lighting
            9.1.3 X-ray generators
            9.1.4 Microwave devices
        9.2 Optical devices
            9.2.1 Photonic crystals
            9.2.2 Optical antennae
            9.2.3 Solar cells
        9.3 Nanoelectrode-based sensors
            9.3.1 Ion sensors
            9.3.2 Gas sensors
                9.3.2.1 Hydrogen gas sensor 
            9.3.3 Biosensors
        9.4 Mechanical devices
            9.4.1 CNT ropes
            9.4.2 TEM grid
        9.5 Electrical devices
        9.6 Storage devices
            9.6.1 Fuel cells
            9.6.2 Supercapacitors
            9.6.3 Lithium-ion batteries
            9.6.4 Hydrogen storage
        9.7 Electromechanical devices
            9.7.1 Artificial implants
            9.7.2 Actuators
        9.8 Separation techniques
        9.9 Heat transfer applications
    10 Current challenge and need for future research
    11 Conclusions
    Acknowledgements 
    References

• Nikita Gupta(USBAS, Guru Gobind Singh Indraprastha University)
• Shipra Mital Gupta(USBAS, Guru Gobind Singh Indraprastha University)
• S. K. Sharma(USCT, Guru Gobind Singh Indraprastha University)