www.afm-journal.de FULL PAPER © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.MaterialsViews.com wileyonlinelibrary.com Ying Wang, Anthony La, Yu Ding, Yixin Liu, and Yu Lei* 1. Introduction One pressing concern in anti-terrorism and homeland security is explosive detection. Most high explosives are nitro-substituted organic compounds. Typically, nitroaromatics, such as 2,4,6-trin- itrotoluene (TNT) and 2,4-dinitrotoluene (2,4-DNT), are the pri- mary military explosives and also the principle components in the unexploded landmines worldwide. [1] Nitramines and nitrate esters (e.g., 3,5-trinitroperhydro-1,3,5-triazine (RDX) and pen- taerythritol tetranitrate (PETN)), are main components of highly energetic plastic explosives, such as C-4 (91% RDX) and Semtex (40–76% PETN). As nitro explosives are extremely sensitive to shock, friction and impact, and therefore, detection methods that permit contact-free analysis are desirable. Moreover, the demands of detecting hidden explosives in transportation hubs and buried explosives in warzones also have led to an intense interest in the low cost and ultrasensitive explosive detection techniques. Compared to the detection in solution and solid phases, the detection of nitro explosives in vapor phase is more chal- lenging since most of them have substan- tially low volatility (Table S1, Supporting Information). Although current nitro- explosive vapor detection heavily relies on ion mobility spectrometry (IMS) [2] and gas chromatography coupled with mass spec- trometry (GC-MS), [3] their sophisticated protocols, poor portability and high-cost have restricted their broad applications. Thus, it is in great demand to develop innovative sensing systems that are cheap, easy to use, highly sensitive and selective for a broad spectrum of nitro-explosives. The stability of energetic materials is often assessed by their trigger linkage, which is generally the C-NO 2 (or N-NO 2 and O-NO 2 ) bond in nitro explosives, and consequently, a high nitro substitution has become their most important character and renders nitro explo- sives electrophilic, which could quench fluorophores through photoinduced electron transfer. [4] Fluorescent conjugated poly- mers have been considered as the leading structures in explo- sive detection due to their efficient exciton migration along the polymer chains, which could result in fluorescence quenching over long range by a single quencher-binding, or called “molec- ular wire” signal amplification. [5–8] It has been reported that by spin-casting pentiptycene derived poly(phenylenethynylene) (PPE) into an ultrathin film, the trace detection of particulate TNT at 10–100 femtograms level can be realized, and its com- mercialized product, FidoXT, has been applied in war zones of Afghanistan and Iraq. [9] Nevertheless, fluorescence quenching based methods have been mainly limited to nitroaromatic (e.g., TNT, 2,4-DNT) vapors, and extension of these methods to vapor detection of nitramine and nitrate ester explosives is still remaining a chal- lenge, largely owing to their ultra-low volatility (e.g., the satura- tion vapor concentrations for HMX, RDX, and PETN are 0.1, 5 and 7 ppt, respectively), [10] unfavorable reduction potential, [11] and the lacking of conjugated electrons to engage in π -stacking. Recently, Andrew and Swager have reported a fluorescence turn-on explosive sensor for nitramines and nitrate esters through their photolytic cleavage products. [12] This system is one of a few examples to detect nanogram particulate RDX and Novel Signal-Amplifying Fluorescent Nanofibers for Naked- Eye-Based Ultrasensitive Detection of Buried Explosives and Explosive Vapors A novel electrospun fluorescent nanofiberous membrane with a function like “molecular wires” was developed via electrospinning for the detection of ultra-trace nitro explosive vapors and buried explosives by naked eye under UV excitation. The high binding affinity between the electron-deficient nitro explosives and the sensing film results in a rapid, dramatic quenching in its fluorescence emission. A wide spectrum of nitro explosives, in particular, TNT, Tetryl, RDX, PETN and HMX could be “visually” detected at their sub- equilibrium vapors (less than 10 ppb, 74 ppt, 5 ppt, 7 ppt and 0.1 ppt, respec- tively) released from 1 ng explosives residues. Such outstanding sensing performance could be attributed to the proposed “sandwich-like” conforma- tion between pyrene and phenyl pendants of PS which may allow efficient long-range energy migration similar to “molecular wire”, thus achieving amplified fluorescence quenching. Its application for the detection of buried explosives in soil by naked eye was also demonstrated, indicating its potential application for landmine mapping. To the best of our knowledge, this is the first report about the detection of buried explosives without the use of any advanced analytical instrumentation. DOI: 10.1002/adfm.201200047 Y. Wang, A. La, Y. Ding, Y. Liu, Prof. Y. Lei Department of Chemical Materials and Biomolecular Engineering University of Connecticut Storrs, CT, 06269-3222, USA, 191 Auditorium Road, U3222, Storrs, CT 06269, USA E-mail: ylei@engr.uconn.edu Adv. Funct. Mater. 2012, DOI: 10.1002/adfm.201200047