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Posted: Sep 14, 2010
The supersensitive detection of explosives by nanowire nano-nose arrays
(Nanowerk Spotlight) The availability of raw materials for the preparation of explosives, together with the growing access to information on preparing these explosives, allows for almost anyone with sufficient will and internet access to prepare a bomb. The vast number of people passing through borders, public places, airports etc. poses a huge challenge for current day security screening technologies. The ultimate goal is of course to be able to rapidly and effectively screen every passing person, without the need to delay the traffic of people, and without human contact if possible.
Another important issue is that explosives, especially concealed ones, have a very low vapor pressure or 'signature' in the surrounding air, which makes their detection (without physically searching the person of course) extremely difficult. To achieve the goal of reliable online and real-time monitoring of large areas there is a need to develop sensors that are cheap, small, and simple, with ultra high sensitivities and with low power consumption to allow for continuous operation. These could be deployed in large numbers to cover the desired area and to prevent false positive/negative alerts.
Semiconducting nanowires are known to be extremely sensitive to chemical species adsorbed on their surfaces. For a nanowire device, the binding of a charged analyte to the surface of the nanowire leads to a conductance change, or a change in current flowing through these tinny wires. Their 1D (one dimensional) nanoscale morphology and their extremely high surface-to-volume ratio make this conductance change to be much greater for nanowire-based sensors versus planar FETs (field-effect transistors), increasing the sensitivity to a point that single molecule detection is possible. In the last decade, it has been demonstrated that these new nanostructures can be used for the detection of multiple biomolecular species of medical diagnostic relevance, such as DNA and proteins.
In our work "Super-sensitive sensing of explosives with silicon nanowire arrays", published recently in the September 10 issue of Angewandte Chemie International Edition, we used the ultrasensitive recognition properties of semiconducting silicon nanowires to demonstrate the most sensitive ever published sensing of explosives reported so far.
To achieve this goal, we chemically modified the nanowire devices with a monolayer of an amine-functionalized silane derivative, namely 3-aminopropyltriethoxy silane (APTES). TNT molecules can strongly bind to the surface of the nanowires through an acid–base pairing interaction between TNT and amino ligands on the sensor surface. The exceptional performance of the SiNW devices enables the detection of TNT with unprecedented sensitivities reaching sub-femtomolar (10-15 M) concentrations in the liquid phase or ∼10 fg/liter (10-15 gram) in the gas phase.
Schematic representation of TNT sensing. 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. These "NanoNoses" can be used to sense the presence of TNT at concentrations as low as 1x10-6 ppt, which is superior to that of sniffer dogs or any other known explosive detection method. (Reprinted with permission from Wiley-VCH Verlag)
In existing methods, pre-concentration of air or liquid samples is usually required for a measurable signal to be recorded by the sensor. These procedures are timely, and delay the operation of a sensor. Due to the extremely high sensitivities displayed by our "NanoNose" sensor, we do not have the requirement to perform such operations before detecting ultra low concentrations of TNT. Besides being extremely sensitive, this sensor can be rapidly 'regenerated' by a simple wash with water containing a minimal amount of organic solvent, so it can be used again and again for many cycles. Another important point worth mentioning is the fact that the entire volume of the sensing chamber in our device is ~10 µl. This means that very little sample is needed to perform analysis.
The target is to sense not only TNT but also other nitro aromatic containing explosives. This is where the "nose" concept comes into play. Our array has the potential to contain hundreds of silicon nanowire elements on a single chip (1 square millimeter area in total for this array).
Groups of nanowires are modified with different molecules, which have different chemical interactions with different explosive molecules resulting in a unique response pattern obtained for each of the explosives. In preliminary experiments we could indeed observe the different pattern response as expected. The large number of recognition elements provides not only the ability for pattern recognition, but also greatly reduces problems such as false positive (Phantom recognition) or false negative (missed recognition) alarms.
Pattern recognition is a known technique and has been used before, also for the sensing of different explosives, but signal processing is usually complex, since the signal is usually optical. With silicon nanowires, processing is very simple since we simply measure the currents passing through the wires, and the results are immediately transferred to a computer without any need for complex optical readers for signal transformation.
To achieve commercialization of sensing devices based on large arrays of silicon nanowires, precise assembly of the nanowire elements in predefined places on the device chip is required. Although several methods such as "fluidic alignment ", "electric field-directed assembly and "dry transfer" assembly have been developed, none has yet provided complete control over the density and interwire spacing on the chip.
Recently a new method developed in our lab was used to create large-scale ordered nanowire arrays with high yield by the so called "knocking-down" approach, based on the controlled in-place planarization of nanowire elements. Further development of this method or others is needed to successfully produce commercial devices based on silicon nanowire arrays. Another important issue is the selective modification of nanowires with different chemical functionalities for the detection of different materials within a single array.
Current work in our lab is focused in controlling the exact location of the nanowire elements and on their selective chemical modification to obtain an array that can differentiate between different explosives, and on using the ultrasensitive properties of silicon nanowires to develop new platforms for the detection of other interesting and relevant substances such as chemical warfare agents, e.g. toxins. These nanosensors, based on existing CMOS technology could be cheaply produced in large numbers and deployed in strategic locations such as airports, government buildings and public places to provide real-time and online monitoring of hazardous substances.