Supersensitive Detection of Explosives by Silicon Nanowire Arrays

Dog on a chip: Explosives can be detected with unprecedented sensitivity by using arrays of silicon nanowire field-effect transistors modified with an electron-rich aminosilane monolayer, which form complexes with the analytes (see picture). These “nanosniffers” can be used to sense the presence of TNT at concentrations as low as 1×10−6 ppt, which is superior to that of sniffer dogs or any other known explosive detection method. There has been a great increase in the development of trace and ultra-trace explosive detection in the last decade, mainly because of the globalization of terrorist acts, and the reclamation of contaminated land previously used for military purposes. In this regard, detection methods for traces of explosives continue to be hampered by the low volatility of the analytes and thus, the analytical problem remains challenging.1 One of the most commonly used high explosives in the last 100 years is 2,4,6-trinitrotoluene (TNT), which not only poses a security threat, but is also of great environmental concern because of soil and water contamination. Thus, TNT is a suitable target analyte for chemical sensing devices. Analytical procedures in use today for the trace detection of explosives typically involve the collection of vapor samples and their analysis by using a sensitive method. Although several sensitive and selective strategies have been reported for the detection of TNT and other explosives,2 these methods are usually time consuming, require bulky equipment, tedious sample preparation, and an expert operator. Furthermore, and of critical importance for practical use, these systems cannot be miniaturized and lack the ability to perform an automated high-throughput analysis. A successful chemical sensor for TNT, and any other explosives, must: 1) be extremely sensitive given that the vapor pressure of TNT at 25 °C is 5.8×10−6 Torr (5 μM), thus suggesting the formation of surface dipoles of opposite sign caused by the interaction of this molecule with the surface APTES layer. At concentrations lower than 5 nM, interfering materials (2–6) do not give rise to appreciable signals, thus making our sensor highly suitable for the ultra-trace detection of TNT. These data provide strong support for the proposed charge-transfer interaction of TNT with the APTES-modified SiNW device. Top: response of an APTES-functionalized silicon nanowire device towards (red) 5 μM solutions and (blue) 5 nM solutions of: TNT (1), 2,6-DNT (2), 2,4-dinitrophenol (2,4-DNP, 3), aniline (4), RDX (5), and p-nitrophenol (p-NP, 6). Bottom: molecular analytes used in this study. A key factor when considering a real-time field sensor is its ability to be quickly regenerated after operation, namely, its reversibility. When the reference washing solution containing no TNT is introduced into the system, after the interaction of TNT with the sensor, the device responded again very rapidly, and the conductance returns to its baseline value. Importantly, we have performed approximately 100 repeated TNT injection/wash cycles with the same nanowire device for over more than a week, and found remarkable sensing stability and reproducibility (see Figure 2 S b in the Supporting Information). In order to improve the detection limit of a sensor, it is not only necessary to have a high gain but also to reduce the noise level in order to provide a high signal-to-noise ratio, thus also preventing the high number of false positive and false negative incidents, which are intrinsic to current sensing technologies. One strategy to achieve this goal is by employing a large number of identical sensors that simultaneously sense the same analyte molecule in order to enhance the signal-to-noise level. Our sensor chip is designed to contain close to 200 devices that can potentially perform the simultaneous detection of TNT. To demonstrate this detection, we performed the simultaneous detection of TNT with three nanowire devices (Figure 4). Clearly, all devices behave similarly, and show the expected decrease in conductance upon exposure to TNT and subsequent increase when the TNT is washed away; this behavior also true for most of the working devices in a single chip (see Figure 2 Sa in the Supporting Information). Relative conductance change recorded simultaneously from three APTES functionalized p-type SiNW FETs. The decrease and increase in conductance correspond to times where a 0.1 % DMSO/H2O solution with added TNT (5 μM) and a reference solution, respectively, were delivered into the fluidic channel. In addition, we tested our sensing arrays for their ability to sense TNT directly from air samples, as TNT is able to form Meisenheimer complexes and acid–base pair complexes with amino groups, even in the gas phase.9 The gas-phase detection of TNT-containing vapors was conducted with the same detection set-up, but using either a nitrogen gas or dry air stream as the TNT vapor carrier. Clearly, the presence of TNT vapors is easily and rapidly detected by the nanowire sensor array (Figure 5; note that the vapor pressure of TNT at 20 °C is approximately 10 ng L−1 or 1 ppb). The TNT delivery line is open to the detection system for only a few seconds before it is closed again, thus exposing the sensing array to very short pulses of TNT-containing vapor of controlled concentration (see the Supporting Information for details). This result shows that the nano-sensors are extremely sensitive to the presence of TNT in air samples and that long sampling and most importantly, that preconcentration steps are not required in our sensing platform. The sensing of TNT could be performed repeatedly (>50 cycles) at low concentrations of TNT (between ppb and ppt concentrations), with unprecedented sensitivities down to at least 10−2 ppt in air (Figure 5). If required, the sensor surface can be readily reactivated by a short washing step in water/0.1 % DMSO solution. Additionally, no influence of humidity and odor materials (compounds 7–9) was detected at our experimental conditions (see Figure 3 S and Experimental Section in the Supporting Information). The possibility of sensing TNT vapors directly and rapidly from samples that were collected from the air but do not require preconcentration, and the effective complete regeneration of sensing elements, are of fundamental importance in the future deployment of our sensor in the practical detection of explosives. Relative percent conductance change versus time for an APTES functionalized p-type SiNW FET sensor after 5 s short pulses of ca. 1 ppt TNT vapors in carrier air samples (arrows denote the time when the TNT vapor pulses were applied). In conclusion, the present study has demonstrated a rapid, label-free, real-time, supersensitive, and selective detection for TNT with the use of large arrays of chemically modified SiNW-FETs with a detection limit that reaches the attomolar concentration range. Moreover, our results show that TNT could be distinguished from other related compounds, with or without nitro groups, and exhibit a clear concentration-dependent conductance response for TNT. This approach represents the first generation of selective and supersensitive electronic sensing arrays intended for the detection of TNT and other explosive-chemical analytes (see Table 1S in the Supporting Information for a comparison with existing TNT detection approaches). The sensor arrays allow for the detection of TNT vapors directly from samples in air, as no solution is required for sensing. Current experiments are focused on the modification of nano-sensor subgroups in a single array with a broad number of amine derivatives, each with different electron-donating capabilities, as this approach will allow the differentiation between different nitro containing explosives10 (see Figure 4 S in the Supporting Information for preliminary results). We thus hope, in the near future, to create a universal platform for the label-free simultaneous detection of a larger spectrum of explosive chemical agents, each selectively identified by the specific electrical signal pattern measured by the nano-sensor array. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. 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