Amperometric sensing of hydrogen peroxide vapor for security screening
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Rapid detection of the hydrogen peroxide precursor of peroxide explosives is required in numerous security screening applications. We describe a highly sensitive and selective amperometric detection of hydrogen peroxide vapor at an agarose-coated Prussian-blue (PB) modified thick-film carbon transducer. The sensor responds rapidly and reversibly to dynamic changes in the level of the peroxide vapor, with no apparent carry over and with a detection limit of 6 ppbv. The remarkable selectivity of the PB-based screen-printed electrode towards hydrogen peroxide leads to effective discrimination against common beverage samples. For example, blind tests have demonstrated the ability to selectively and non-invasively identify concealed hydrogen peroxide in drinking cups and bottles. The attractive performance of the new microfabricated PB-based amperometric peroxide vapor sensor indicates great potential for addressing a wide range of security screening and surveillance applications.
KeywordsHydrogen peroxide Prussian-blue Vapor sensor Screen-printed electrodes Explosives Security screening
Growing security concerns have generated urgent needs for innovative tools for field screening homemade explosives and their precursors . Particular recent attention has been given to peroxide explosives (particularly triacetone triperoxide (TATP)) due to their widespread use by terrorists and their straightforward synthesis from readily available hydrogen peroxide and acetone . In numerous security screening applications, it is crucial to detect rapidly the hydrogen peroxide precursor both in the liquid and vapor phases. Eliasson et al.  reported recently on the use of Raman Spectroscopy for detecting hydrogen peroxide concealed in bottles/containers. In another recent study, Bohrer et al.  described the use of phthalocyanine-based chemiresistors for the vapor-phase detection of hydrogen peroxide.
Here, we report on the use of a Prussian-blue (PB)-based thick-film amperometric sensor for rapid and selective detection of hydrogen peroxide vapor. Electrochemical devices have shown considerable promise for on-site detection of explosives . Such devices have been used previously for environmental monitoring of atmospheric hydrogen peroxide [6, 7] but not towards security screening of peroxide vapor. We demonstrated recently an “Add-Detect” electrochemical approach for detecting solid and liquid samples of the peroxide explosives TATP and hexamethylene triperoxide diamine based on the electrocatalytic detection of their acid-generated hydrogen-peroxide product at a PB-modified electrode . Prussian-blue acting as “artificial peroxidase” offers a highly selective, preferential low-potential electrocatalytic detection of hydrogen peroxide . It has been widely used for liquid-phase peroxide measurements but rarely for direct vapor analysis and not towards security screening applications.
In the following sections, we illustrate that an agarose-coated PB-modified screen-printed electrode (SPE) sensor offers a rapid, highly sensitive, and selective response to hydrogen peroxide vapor. Screen-printed (thick-film) electrochemical sensors are readily produced at low cost and are extremely attractive for decentralized testing . The potential of the new PB-screen-printed sensor for security screening applications is demonstrated by the sensor’s ability to distinguish non-invasively and rapidly concealed hydrogen peroxide from common beverages in drinking cups and other containers. Such high selectivity of the PB-based vapor electrochemical sensor holds great promise for minimizing false alarms common to airport screening or other threat-detection applications.
Chronoamperometric measurements were performed using a CHI 1030 Electrochemical Analyzer (CH Instruments, Austin, TX, USA) along with screen-printed electrode transducers. Gas dilutions were performed using a 1010 Precision Gas Diluter with an internal pump, Tedlar™ bags, Bev-A-Line tubing (6.5 mm o.d × 3.12 mm i.d.) and JACO polypropylene connectors (Custom Sensor Solutions, Oro Valley, AZ, USA). A semi-automatic screen printer (Model TF 100; MPM, Franklin, MA, USA) was used for printing the thick-film Prussian-blue modified carbon (working), carbon (counter), and Ag/AgCl (pseudo reference) electrodes. The carbon and silver inks [Ercon, E-3449 graphite and R-2412 Ag/AgCl, respectively] were printed through a patterned stencil on 10 × 10 cm ceramic plates containing 30 strips (3.3 × 1.0 cm each). All three electrodes were cured at 150°C for 1 h. The insulating ink (Ercon, E6165-116, Blue Insulayer) was subsequently printed on a portion of the plate, leaving 2 × 2 mm sections on both ends, defining the electrode area on one side and the silver electrical contacts (of the three electrodes) on the opposite end. The insulating layer was cured at 100°C for 1 h.
Stock solutions of hydrogen peroxide were prepared by diluting a 30% (w/w) H2O2 standard solution purchased from Mallinckrodt (Phillipsburg, USA). Deionized water from a Milli-Q system (Millipore, Bedford, MA, USA) was used to prepare all solutions. Potassium ferricyanide, iron (III) chloride, graphite powder, potassium chloride, and agarose I-A were all purchased from Sigma-Aldrich (St Louis, USA). Hydrochloric acid (12 M) was obtained from EMD (Darmstadt, Germany).
Preparation of the PB-modified ink
The PB-modified ink was prepared using the following procedure. One gram of graphite powder and 10 mL 0.1 M iron (III) chloride were mixed with stirring for 2 min. Next, 10 mL of 0.1 M potassium ferricyanide were added, and the resulting solution was stirred for 10 min. The resulting mixture was filtered, and the filtrand was dried at 100°C for 10 h. The resulting PB-coated graphite powder was then mixed with the commercial carbon ink at three different loadings (2.5%, 5.0%, and 10% w/w).
Hydrogel solid electrolyte preparation
One hundred milligrams of agarose I-A was dissolved under stirring in 5 mL 0.2 M KCl solution. The mixture was slowly heated until boiling and kept at that temperature for 5 min. Subsequently, the gel solution was cooled down to around 65°C and kept at this constant temperature with stirring (liquid phase). A 50-μL agarose droplet was cast on the end of the electrode, covering the three electrode areas. The droplet was allowed to cool to room temperature and rapidly solidified to form a 1-mm thin gel layer covering the three electrodes (over 6 × 10 mm).
Chronoamperometric measurements of H2O2 at PB-modified SPE
Transient measurements were performed in a cylindrical glass tube (internal diameter, 10 cm; total length, 60 cm; Fig. 5). Samples of hydrogen peroxide were placed inside the tube in a plastic weigh boat, and the SPE was inserted through a small slit in the glass tube approximately 50 cm above the sample. For transient measurements, the initial potential (0.2 V) was applied for 5 s, then the secondary potential (−0.05 V) was applied for 35 s, with the current sampling carried out after 30 s. There was a 20-s interval between runs.
On-line measurements of H2O2 in a flow system
To generate hydrogen peroxide vapor, 1 ml of a 0.0025% (w/w) H2O2 was added to a 4 L Tedlar™ bag, and the bag was subsequently filled with nitrogen. The sample was allowed to equilibrate for 6 h in the sealed environment and was then connected via a Bev-A-Line tubing to the gas diluter. A 40-L Tedlar™ bag, filled with the nitrogen background gas, was also connected to the gas diluter via Bev-A-Line tubing. The gas diluter was then connected to the flow cell with Bev-A-Line tubing. The electrochemical flow cell comprised a cylindrical glass tube (2.0 cm in length, 2.5 cm i.d.) affixed at the bottom to a ceramic plate that had been modified to allow insertion of the planar thick-film strip electrode. The other end of the flow cell was capped with a rubber stopper connected to the gas diluter and equipped for outlet flow. The stopper was placed approximately 0.75 cm above the hydrogel-covered PB-modified electrode, resulting in a total cell volume of approximately 3.7 mL. In addition to the 50 μL of hydrogel placed directly on the sensing area, an additional hydrogel (50 μL) aliquot was placed over the insulator area of the SPE to increase the humidity around the working sensing area. The sample vapor was diluted with nitrogen to alternating concentrations of 25 and 100 ppbv, and the flow rate of diluted H2O2 vapor to the flow cell was maintained at 4.1 scc/s. A PB-modified SPE was inserted into the flow cell for the amperometric measurement of the H2O2 vapor, carried out at an applied potential of −0.2 V.
Amperometric measurements of peroxide vapor at PB-modified SPE for screening applications
To generate hydrogen peroxide vapor, a 10-mL sample of a 30% w/w H2O2 solution was added to a 500-mL Styrofoam coffee cup (covered with a lid) and was allowed to equilibrate for 30 min. The PB-modified SPE was placed at different distances (0.5–2.5 cm) above the drinking opening in the lid, and quantitative current measurements were taken at an applied potential of −0.05 V. This potential was applied for 5 min to ensure a stable current baseline signal before placing the sensor above the lid opening.
Results and discussion
This article addresses the urgent needs to detect the hydrogen peroxide precursor of peroxide explosives. It demonstrates a highly selective and sensitive vapor detection of hydrogen peroxide at an agarose-coated PB-modified screen-printed carbon-electrode transducer. The new hydrogen-peroxide vapor sensor offers reliable non-invasive screening of concealed hydrogen peroxide in drinking cups and is not responding to constituents of a wide range of common beverages. Coupling such remarkable selectivity of the PB modifier, with high sensitivity and speed, as well as with portability and low cost of the SPE, makes this new electrochemical route extremely attractive for addressing a wide range of threat detection field scenarios.
We wish to thank Motorola Inc. and the US DIA for the financial support.
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