We propose to measure and detect the energetic materials - explosives and compounds used for their fabrication in sewage and in suspected unauthorized dumpsites (information of the uniformed services). The continuous monitoring points: sewage treatment plants, airports, railway stations, the main sanitary facilities are chosen for detection system installation.
To achieve low limit of detection (LOD) the authors propose the use of new, multimodal detection systems based of mutually independent and complementary applied methods: optical spectroscopic techniques and current-voltage measurements in DC and AC mode. The combination of optical detection with simultaneous electrochemical measurement will allow increase in sensitivity demanded in the analysis of samples with low content of analytes.
Hybrid sensing systems prepared in this way will allow detection of analyte using two independent groups of techniques:
optical spectroscopy - the measurement of enhanced Raman and absorption spectroscopy,
electrochemical - voltamperometry techniques and electrochemical impedance spectroscopy.
It is expected that the developed detection system will achieve sensitivity in the range of 10-6 - 10-15 M of specific compounds.
The proposed hybrid sensing system consists of dual detection:
Semi-remote optical detection based on light interaction with vaporized sewage or dumps by use of ultrasonic actuator or infrared radiation.
Direct electrochemical detection based on novel FET heterostructures based on innovative thin-film carbon materials (graphene and diamond).
Graphene has demonstrated great promise and capabilities as a sensor, including at the single bacterium level using graphene derived from graphite oxide .However, the graphite oxide-derived graphene has intrinsically much lower mobility than that derived from large area chemical vapor deposition or exfoliated graphene. Therefore, our planned sensors have the potential to reach higher sensitivity into low concentrations of IED compounds to facilitate the realization of sensing system.
Graphene field effect transistor with a chemically functionalized diamond layer on top.
The diamond films will be grown without doping to make insulating dielectric layers. After these diamond films are grown, we will transfer them to graphene. Electrochemical measurement of designed FET structure will be performed using: potentiostat-galvanostat. Complementary electrochemical and spectroscopy measurements will be performed using new unique research station contain purchased universal analytical equipment and laser excitation source.
Electrochemical studies include measurements using (I) voltammetry techniques, (II) impedance spectroscopy and (III) electrochemical polarisation studies of hybrid diamond-organic system in determined potential range.
(I) voltamperometry techniques (i.e. cyclic and stripping voltamperometry)
Reactivity studies of the FET electrodes formed by nanodiamond-graphene structure to varying degrees will be performed in the basic electrolyte (0.5 M Na2SO4) and in the presence standard redox system Fe (CN)63-/4-. The Fe (CN)63-/4- system is relatively sensitive to the surface properties of carbon type electrodes and the overall mechanism for heterogeneous electron transfer is complicated by pronounced cation/electrolyte effects.
(II) electrochemical impedance spectroscopy
The electrochemical impedance spectroscopy measurements will be conducted in typical basic electrolyte (0.5 M Na2SO4) with and without presence of redox active species (Fe (CN)63-/4-). For collected spectra there will be proposed equivalent electric circuit (EqEC) giving the same electric response as investigated electrode. According to our preliminary studies concerning impedance spectroscopy of FET electrodes prepared by GUT, EqEC will be based on the electrolyte resistance connected in series with parallel arrangement of frequency independent capacitance (C) or constant phase element (CPE) and resistance. The fitting procedure will be conducted according to Levenberg-Marquardt algorithm.
There are many optical methods of explosives detection. For example, explosives sensing can be obtained using surface enhanced Raman scattering (SERS), laser-induced fluorescence (LIF), laser induced break-down spectroscopy (LIBS) or by use of terahertz radiation. Application of absorption spectroscopy to explosive detection is used as well. For example, there was reported application photoacoustic spectroscopy (PAS) system to TNT spectrum investigation. The detection limit corresponds to saturated TNT vapor pressure at 5°C was reached, i.e., about 0.1 ppb. These instruments have some drawbacks i.e. insensitivity to some key compounds, masking by interferes, practical deployment concerns, detection limit, etc. The nature of the threat from explosives is also increasing by developments in improvised explosive devices (IEDs) which use explosives manufactured from common domestic chemicals rather than those produced commercially. One example is TATP (triacetone triperoxide) which was used in the London public transport bombings. Development of sensitive optical methods to detect these substances with a minimum of sampling effort therefore is a challenge.
Our explosive detection method is based either on the reaction of the sensors to the nitrogen oxides directly emitted by the explosives or on the reaction to the nitrogen oxides produced during thermal decomposition of explosive vapors. For TNT, PETN, RDX and HMX a detection limit better than 1 ng has been achieved. Some successful research with nitroglycerine (NG), nitrocellulose and TNT has been already performed.
Method novelty relies on application of one of the most sensitive optoelectronic technique called cavity enhanced absorption spectroscopy (CEAS) associated with concentrator unit (CU) consisting of explosives vapors preconcentrator and explosive decomposition unit.
CEAS based on the light absorption phenomenon. Absorption spectroscopy is a simple and in situ technique for obtaining information about different species. However, the sensitivity of traditional spectroscopic methods is limited to the range of 10-4 – 10-5 cm-1. To improve the sensitivity, a longer absorption path length should be used. Cavity enhanced systems require a high-finesse stable optical cavity. In this technique, the laser pulses are injected into the optical cavity (resonator) consisting of two spherical and high-reflective mirrors. Radiation is multiply reflected inside the resonator. After each reflection, a small part of laser radiation leaves the optical cavity due to the residual transmission of mirrors, and registered by a photo-receiver. CEAS sensors attain the detection limit of about 10-9 cm-1.
Optoelectronic sensors employing CEAS designed to measure trace concentration of nitrogen dioxide (NO2), nitric oxide (NO), and nitrous oxide (N2O) are able to trace explosives detection. They provide possibility of non-invasive, continuous and fast monitoring of the threats. Because they use the phenomenon of optical radiation absorption to detect and measure the concentrations of the molecules, they provide achieving low detection limits and high selectivity. For this purpose, it is necessary to apply radiation, the wavelength of which is matched to the spectral range characterized by strong absorption of the tested molecules. Nevertheless, this task is not so simply. It is very important to minimize the impact of the absorption spectra of other molecules.
Additionally, to enhance the sensitivity of the optoelectronic sensors, a special concentrator unit can be applied. Therefore, operating procedure includes measurements of the NOx concentration, which are products of the thermal decomposition processes. In the unit, explosives vapours are collected from a large incoming flow (preconcentration process) using a special sorptive material (adsorbent). Next, the adsorbent is heated up a temperature affecting the thermal decomposition of the explosives. Thanks to this, additional compounds including NO, NO2 and N2O can be observed. Finally, the products of this process is analysed using the NOx sensors.
The capabilities of laser absorption spectroscopy system will be recognized using laser-based system consisting of concentrator unit and NO2 sensor. Project of the proposed system is presented in below.
Project of CEAS-CU system
In case of explosives containing NO2 groups, which are characterized by lower vapor pressure, it will be necessary to increase temperature of the testing object (liquid or solid). Such explosive can be for example TATB – triaminotrinitrobenzene (Table 1), which is more powerful than TNT, but it has low sensitivity to shock, vibration, fire and impact. Next, concentrator will capture and accumulates the explosives from air and will convert (decompose) them to the nitrogen oxides. The gas concentration will be measured by the NO2 sensor that is characterized by high sensitivity (ppb level), selectivity and real time measurements (< 3 s).
The surface of the sensor structure must be protected to achieve required properties. The most important is to provide a sufficient electrically insulating coating of the sensor body that also fulfils other criteria, e.g. mechanical and temperature stability, resistance against dirt and chemicals etc. Another request on the sensor surface is the presence of an active chemical grafts, based on amine (-NHx) and/or carboxyl (-COOH) groups, that allows bounding of organic systems. The low temperature plasma will be utilized for deposition of protective coatings followed by its surface activation.
Physical Vapour Deposition (PVD) is the most common plasma-assisted deposition technique. PVD processes typically employ planar magnetrons and hollow cathode sputtering. The basic principle is a sputtering of the cathode made of the material to be deposited. Ejected cathode atoms subsequently condense on a substrate forming a thin film. In last decade the new trend, called High Power Pulsed Sputtering (HiPPS), is widely and intensively explored. HiPPS systems are driven in dc-pulse modulated regime operated at low repetition frequency to achieve high power density in the pulse. High instantaneous power results in production of dense plasma with large fraction of ionized sputtered material. Large quantity of ionized sputtered atoms leads to the growth of smooth and dense films and allows controlling the crystallography, phase composition, microstructure and mechanical properties. It was already reported that HiPPS is used to be use for deposition of hard and protective coatings, e.g. BN, TiN etc. Our laboratories perform extensive research of magnetron as well as hollow cathode HiPPS deposition techniques, which can be demonstrated by more than two dozens of papers published in the last years.
Sputtering techniques allow depositing not only metal-based films but also plasma polymers. Usually rf sputtering (sputtering power driven at radio frequency) of polymeric materials in an inert gas or in its own fragmented polymer vapours is used. Recently, amino-rich coatings were prepared by sputtering of nylon in reactive Ar/N2 and N2/H2 atmosphere. However, the reactive groups can be achieved also by so-called surface processing during which the material is neither added nor removed in any significant amount. However, the composition and structure of the surface or of the near-surface layers are controllably modified. Plasma surface processing allows adding specific chemical functional groups that have strong bonds with the bulk material. It was demonstrated in numerous papers that the surface activation can introduce e.g. amine (-NHx), carboxyl (-COOH), carbonyl (-C=O), fluorine containing (-CFx), hydroxyl (-OH) etc. groups with respect of discharge used. Hence, activated surfaces with proper functional groups enable to immobilize molecules or increase the adhesion on the surface due to chemical grafts.