Selective detection of explosive compounds is critical for national defense and homeland security. Nitroaromatic compounds pose a particular threat; 2,4,6-trinitrotoluene (TNT), for example, is an inexpensive and readily available component of fifteen of the most widely used blends ^[1] . Existing methods to detect explosives include biosensors ^[2] , electrochemical sensors  ^[3] and fiber optic ^[4] sensors. Devices utilizing chromatography ^[5] and Raman spectroscopy ^[6] are used for the same purpose. However, sensors using the aforementioned techniques require complicated sensing and readout components; moreover, they are comparatively large in size and consume significant amounts of power during operation. In this work we describe the fabrication and demonstration of a chemical sensor capable of detecting nitroaromatic explosives in air. The aim of this work is the development of a simple sensor that has the unique features of micro-scale dimensions, simple and inexpensive fabrication, and low power consumption. It consists of a nano-patterned conductive metal line placed on top of a patterned responsive polymer, poly(4-vinylpyridine) (P4VP), as shown in Figure 1. Due to polymer-solvent interactions, P4VP swells when it encounters the target analyte, producing a large stress. Detection takes place by monitoring the change in device resistance as the metal nano line deforms or fractures when P4VP swells and transfers mechanical stress.

Fabricated devices were tested for their response to nitroaromatic exposure using a previously described system((W. E Tenhaeff, L. D. McIntosh, and K. K. Gleason, “Synthesis of poly(4-vinylpyridine) thin films by initiated chemical vapor deposition (iCVD) for selective nanotrench-based sensing of nitroaromatics,” Advanced Functional Materials, vol. 20, no. 7, pp. 1144-1151, 2010.)) ^[7] . Test devices were located on a cooled stage within a flow cell; swelling responses of P4VP films were measured via in situ interferometry. Figure 2 illustrates the change in device resistance for a 200-nm-thick, 5-μm-wide P4VP line intersected by a 100-nm-thick, 300-nm-wide Au line sensor upon exposure to 500 ppm of nitrobenzene. The concentration was increased to 650 ppm at t=15mins. The change in resistance corresponds well to the calculated change in exposure concentration. A permanent increase (8.5%) in resistance is clearly observed as the result of permanent deformation and micro-cracks; this change is large enough to be easily detected.

1. S. J. Toal, and W. C. Trogler, “Polymer sensors for nitroaromatic explosives detection,” Journal of Materials Chemistry, vol. 16, no. 28, pp. 2871-2883, Apr. 2006.
2. R. M. Wadkins, J. P. Golden, L. M. Pritsiolas, F., and S. Ligler, “Detection of multiple toxic agents using a planar array immunosensor,” Biosensors and Bioelectronics, vol. 13, no. 3, p. 407, 1998.
3. K. Masunaga, K. Hayama, T. Onodera, K. Hayashi, N. Miura, K. Matsumoto, and K. Toko, “Detection of aromatic nitro compounds with electrode polarization controlling sensor,” Sens. Actuators B, vol. 108, nos. 1-2, p. 427, 2005.
4. R. A. Ogert, L. C. Shriver-Lake, and F. S. Ligler, “Toxin detection using a fiber-optic-based biosensor,” in Proc. SPIE, 1993, vol. 1885, Mar. 1993, p. 11-17.
5. A. Hilmi, J. H. T. Luong, and A. L. Nguyen, “Determination of explosives in soil and ground water by liquid chromatography–amperometric detection,” Journal of Chromatogr. A, vol. 844, nos. 1-2, p. 97, 1999.
6. I. R. Lewis, N. W. Daniel Jr., N. C. Chaffin, P. R. Griffiths, and M. W. Tungol, “Raman spectroscopic studies of explosive materials: towards a fieldable explosives detector,” Spectrochimica Acta Part A, vol. 5, no. 12, p. 1985, 1995.
7. W. J. Arora, W. E. Tenhaeff, K. K. Gleason, and G. Barbastathis., “Integration of reactive polymeric nanofilms into a low-power electromechanical switch for selective chemical sensing,” Journal of Microelectromechical Systems, vol. 18, no. 1, pp. 97-102, 2009.

Collaborators

• V. Bulovic, MIT
• H.A. Clark, Northeastern Univ.
• T.A. Hatton, MIT
• M.C. Demirel, Penn State Univ.

Graduate Students

• M. Alf, Res. Asst., ChemE
• M. C. Barr, Res. Asst., ChemE
• D. Borelli, Res. Asst., ChemE
• R. Howden, Res. Asst., ChemE
• C. Petuczok, Res. Asst., ChemE
• D. Smith, Res. Asst., PPST
• J. Xu, Res. Asst., ChemE
• R. Yang, Res. Asst., ChemE

Support Staff

• G. Wilcox, Admin. Asst. II

Publications

Bhattacharyya, D.; Gleason, K.K.; Single-step oxidative chemical vapor deposition of –COOH functional conducting copolymer and immobilization of biomolecule for sensor application, Chemistry of Materials 2011, 23 (10) 2600-2605.

Sreenivasan, R.; Basset, E.K.; Hoganson, D.M.; Vacanti, J.P.; Gleason, K.K.; Ultra-thin, gas permeable free-standing and composite membranes for microfluidic lung assist devices, Biomaterials 2011, 32 (16) 3883-3889.

Ozaydin-Ince, G.; Tsinman, T.; Gleason, K.K.; Langer, R.; Khademhosseini, A.; Demirel, M.C.; Responsive microgrooves for the formation of harvestable tissue constructs, Langmuir 2011, 27 (9) 5671-5679

Ozaydin-Ince, G.; Dubach, J.M.; Gleason, K.K.; Clark, H.A.; Microworm optode sensors limit particle diffusion to enable in vivo measurements, PNAS 2011, 108 (7) 2656-2661.

Im, S.G.; Gleason, K.K.; Solvent-free modification of surfaces with polymers: The case for initiated and oxidative chemical vapor deposition (CVD), AIChE Journal 2011, 57 (2) 276-285.

Ozaydin-Ince, G.; Gleason, K.K.; Demirel, M.C.; A stimuli-responsive coaxial nanofilm for burst release, Soft Matter 2011, 7 (2) 638-643.

Asatekin, A.; Gleason, K.K.; Polymeric nanopore membranes for hydrophobicity-based separations by conformal initiated chemical vapor deposition, Nano Letters 2011, 11 (2) 677-686.

Alf, M.E.; Godfrin, P.D.; Hatton, T.A.; Gleason, K.K.; Sharp hydrophilicity switching and conformality on nanostructured surfaces prepared via initiated chemical vapor deposition (iCVD) of a novel thermally responsive copolymer, Macromolecular Rapid Communications 2010, 31 (24) 2166-2172.

Kramer, N.J.; Sachteleben, E.; Ozaydin-Ince, G.; van de Sanden, R.; Gleason, K.K.; Shape memory polymer thin films deposited by initiated chemical vapor deposition, Macromolecules 2010, 43, 8344-8347.

Coclite, A.M.; Ozaydin-Ince, G.; Palumbo, F.; Gleason, K.K.; Single-chamber deposition of multi-layer barriers by plasma enhanced and initiated chemical vapor deposition of organosilicones, Plasma Processes and Polymers 2010, 7 (7) 561-570.

Figure 1: Scheme of the microworm fabrication process. The bare AAO template (a) is first conformally coated with the iCVD hydrogel layer (b). The optode solution is filled in the pores of the template (c) and the excess optode and the hydrogel layer are etched away (d). A final hydrogel layer is deposited on both sides of the template to cap the optode (e). As the final step, the membrane is dipped in HCl solution to etch the AAO template and release the microworms (f).

The development of biosensors is considered a focal subject for clinical applications. The main efforts are being addressed towards in-vivo continuous monitoring of different analytes. Implantation of nanoparticles, consisting of an optode embedded in a polymer, for minimally invasive physiological monitoring has been already tested ^[1] . Nevertheless, the small size of the probes also results in them diffusing rapidly away from the desired location. In this work, we present a novel method to fabricate cylindrical-shape sensors (microworms). Microworms combine a long axis (tens of microns), which provides a higher hydrodynamic radius to prevent diffusion, with a nanostructured shell, which facilitates the diffusion of the analyte inside the cylinder to interact with a specific optode. The fabrication of the microworm sensors, using an anodic aluminum oxide (AAO) membrane as template, is depicted in Figure 1. The biocompatible hydrogel thin layer (50 nm) is deposited on the inner walls of the membrane by initiated chemical vapor deposition (iCVD). iCVD is a solvent-free polymerization method, which yields conformal coatings ^[2] with a full retention of functionality ^[3] . Therefore, the optode is fully encapsulated in the hydrogel layer to study its performance as sensor. These microworms can be implanted under the skin to monitor multiple types of ions and molecules by tracking the changes in the fluorescence signal. Recently, a sensor for in-vivo monitoring of sodium has been successfully created ^[4] . In addition, the latest investigations are being focused on the fabrication of microworm sensors for glucose detection.

1. H. A. Clark, M. Hoyer, M. A. Philbert, R. Kopelman, “Optical nanosensors for chemical analysis inside single living cells. 1. Fabrication, characterization, and methods for intracellular delivery of PEBBLE sensors,” Analytical Chemistry, vol. 71, pp. 4831-4836, Nov. 1999.
2. S. H. Baxamusa and K. K. Gleason, “Initiated chemical vapor deposition of polymer films on nonplanar substrates,” Thin Solid Films, vol. 517, pp. 3536-3538, Apr. 2009.
3. W. E. Tenhaeff and K. K. Gleason, “Initiated and oxidative chemical vapor deposition of polymeric thin films: iCVD and oCVD,” Advanced Functional Materials, vol. 18, pp. 979-992, Apr. 2008.
4. G. Ozaydin-Ince, J. M. Dubach, K. K. Gleason, H. A. Clark, “Microworm optode sensors limit particle diffusion to enable in vivo measurements,” Proc. National Academy of Sciences of the United States of America, vol. 108, pp. 2656-2661, Feb. 15 2011.

Protective coatings that prevent the ingress of water into electronic devices fabricated on flexible plastic substrates are essential to extend the device’s lifetime. Widely investigated barrier protective coatings are multilayer stacks where dense, inorganic layers are alternated with soft, organic ones ^[1] . The possibility of a single-chamber system may greatly simplify the production and allows the quicker and cheaper roll-to-roll deposition.

A new technique consisting of coupling initiated CVD (iCVD) and PECVD was investigated for the multilayer deposition, maintaining the same precursor and the same reactor configuration ^[2] . Multilayer coatings of alternating silica-like and organosilicon layers were deposited using hexavinyldisiloxane (HVDSO) as a precursor.

Chemical and morphological characterization of the organic layer showed that the iCVD of HVDSO resulted in a very crosslinked film with high carbon content (79% from XPS analysis), low roughness (0.7 nm), and planarizing properties. PECVD of the same monomer, highly diluted in oxygen, gave an inorganic coating with a low content of OH terminal groups. When the inorganic layer was deposited over the organic layer, a graded interphase (40 nm) was detected, due to plasma ion bombardment.

Figure 1 shows the results on the moisture barrier permeation tests for the single inorganic PECVD layers and for the multistack structures. A barrier improvement factor of 100 over the single inorganic layer was obtained with a hexalayer. The ‘‘orders-of-magnitude’’ improvement in the barrier performance made by adding organic/inorganic dyads is a signature of an effective defect decoupling. The organic layers, despite lacking the intrinsic barrier properties, produce a synergistic improvement in the performance when coupled with inorganic layers. Interesting is the comparison between the WVTR of the 300-nm-thick single PECVD layer and of the hexalayer containing three 100-nm-thick PECVD layers (whose cross-sectional micrograph is shown in Figure 2). These two structures have the same total inorganic layer thickness, but the multilayer has ten times better performance.

1. P. E. Burrows, G. L. Graff, M. E. Gross, P. M. Martin, M. K. Shi, M. Hall, E. Mast, C. Bonham, W. Bennett, and M. B. Sullivan, Ultra barrier flexible substrates for flat panel displays, Displays, vol. 22, p. 65, 2001.
2. A. M. Coclite, G. Ozaydin-Ince, F. Palumbo, A. Milella, and K. K. Gleason, Single-Chamber Deposition of Multilayer Barriers by Plasma Enhanced and Initiated Chemical Vapor Deposition of Organosilicones, Plasma Proc. Polym., vol. 7, p. 561, 2010.

There is emerging interest in the ability to produce low-cost and lightweight solar cells and other electronics on flexible, stretchable, and foldable substrates.  Rigid glass or silicon substrates in the current designs represent a large fraction of the overall module cost ^[1] and also restrict how and where modules can be deployed. Thus, a shift to the design of modules specifically on low-cost substrates could open untapped locations for solar deployment, including formats that are ubiquitous in our society (e.g., textiles, window curtains, printed paper documents, and wall paper). For example, photovoltaic (PV) devices fabricated directly on common fiber-based paper substrates are also foldable and rollable for storage and portability, easily shaped for three-dimensional applications, and able to be stapled to the roof structures or glued onto walls. To this end, there is significant interest in integrating various electronics to low-cost paper substrates, including transistors, storage devices, and displays ^{[2] [3]} .  In our work, we examine the use of oxidative chemical vapor deposition (oCVD) ^{[4] [5]} in conjunction with organic photovoltaics to fabricate PV cells ^[6] directly on fiber-based and paper substrates that are both flexible and foldable ^[7] .  For example, in Figure 1 we show a paper PV cell that was folded into a paper airplane (see corresponding video ^[8] ).   Furthermore, we have designed paper-based, monolithically series-integrated arrays that are capable of powering common electronics such as small-format LCD displays under ambient light as shown in Figure 2, with shelf lifetimes greater than several months.  These demonstrations allow us to rethink how and where lightweight and potentially low-cost photovoltaics can be deployed.

1. K. Zweibel, “The terawatt challenge for thin film photovoltaics,” Thin Film Solar Cells, Hoboken, NJ, John Wiley and Sons Ltd., 2006, pp. 427-459.
2. U. Zschieschang, T. Yamamoto, K. Takimiya, H. Kuwabara, M. Ikeda, T. Sekitani, T. Someya, and H. Klauk, “Organic electronics on banknotes,” Advanced Materials, vol. 23, pp. 654-658, 2011.
3. D. Tobjörk and R. Österbacka, “Paper electronics,” Advanced Materials, vol. 23, pp. 1935-1961, 2011.
4. M. E. Alf, A. Asatekin, M. C. Barr, S. H. Baxamusa, H. Chelawat, G. Ozaydin-Ince, C. D. Petruczok, R. Sreenivasan, W. E. Tenhaeff, N. J. Trujillo, S. Vaddiraju, J. J. Xu, and K. K. Gleason, “Chemical vapor deposition of conformal, functional, and responsive polymer films,” Advanced Materials, vol. 22, pp. 1993-2027, 2010.
5. S. H. Baxamusa, S. G. Im, and K. K. Gleason, “Initiated and oxidative chemical vapor deposition: a scalable method for conformal and functional polymer films on real substrates,” Physical Chemistry Chemical Physics, vol. 11, pp. 5227-5240, 2009.
6. C. W. Tang, “2-Layer Organic Photovoltaic Cell,” Applied Physics Letters, vol. 48, pp. 183-185, 1986.
7. M. C. Barr, J. A. Rowehl, R. R. Lunt, J. J. Xu, A. Wang, C. M. Boyce, S. G. Im, V. Bulovic, and K. K. Gleason, “Paper-thin, organic photovoltaic circuits fabricated directly on ubiquitous, everyday substrates,” Advanced Materials, accepted for publication.
8. Available: http://web.mit.edu/newsoffice/component/mitmultimedia/?type=video&videoid=233