Air filtration proposed for nanofibers require their adequately thin fiber diameters and porous packing structure. The ability to construct stable and large cavity structures by extremely thin fibers would have broad technological implications for areas ranging from individual protection and industrial security to environmental governance; however, it has proved extremely challenging. Here we report a strategy to design and create sandwich structured polyamide-6/polyacrylonitrile/polyamide-6 (PA-6/PAN/PA-6) composite membrane for effective air filtration via sequential electrospinning. Our approach allows the PAN bead-on-string fibers and two-dimensional (2D) PA-6 nanonets to assemble into stable filtration medium with tunable porous structure and mechanical properties on a large scale, by facilely regulating the solution concentration, applied voltage, combined with weight ratio of PA-6/PAN. With integrated features of ultrathin (similar to 20 nm) nanonets and bead-on-string fibers supported cavity, the resulting PA-6/PAN/PA-6 composite membrane exhibits robust mechanical properties, high filtration efficiency of 99.9998%, and low pressure drop of 117.5 Pa for filtrating ultrafine airborne particles in multiple capture manners; and it successfully gets rid of the potential safety hazards caused by unexpected electret failure. The successful synthesis of PA-6/PAN/PA-6 medium would not only make it a promising candidate for air filtration, but also provide new insights into the design and development of composite membranes for various applications. (C) 2015 Elsevier B.V. All rights reserved.