M. Mushfiq, M.M. Alam
Keywords: conducting polymer nanowires, electrochemical polymerization, nanowire growth and morphology, microelectrode devices, organic and ingornic acids, electrolytes, chemical and biological sensors
Summary:Conductive polymer (CP)-based nanostructured materials in thin films and nanowires have been utilized in numerous chemical sensor and biosensor applications.(1) Their beneficial attributes include: lightweight, large surface area, adjustable transport properties, chemical specificity, low cost, ease of processing, tunable conductivities, material flexibilities, and readily scalable production.(1,2) These nanomaterials have thus become prime candidates for replacing conventional bulk materials in micro- and nanoelectronic devices and chemical sensors.(2,3) CPs show the following features: (1) conjugated backbone allows for the flow of electrons, (2) able to maintain polymer-associated physical characteristics, (3) usable in a wide range of applications including sensors and biosensors, (4) high environmental and thermal stability, straightforward synthesis, and low-cost materials and/or monomers, and (5) conductivity can increase by over 10 orders of magnitude upon exposure to analytes via the change in ionic species, charge carrier, and transport mechanism.(1,2,3) However, nanowire dimension and morphology of CP nanowires play a critical role on their sensing performance. Although CP nanowires have been grown by different methods using different electrolytes, their uniform and reproducible growth is still a challenge.(1,2,3) To understand and overcome this challenge, we conducted a systematic studies on in-situ growth of CP nanowires on microelectrode devices using a patent-pending site-specific electrochemical process in different acid electrolyte solutions.(1c) Briefly, the electrochemical process uses a three-necked cell filled with monomer solution in an aqueous acidic electrolyte solution where we used a wire-bonded microelectrode device submerged in the electrolyte as a working electrode, a platinum coil as a counter electrode, and a silver/silver chloride as a reference electrode. Detailed process will be discussed in the conference. We investigated five acidic electrolyte solutions: (1) formic acid (HCOOH), (2) acetic acid (CH3COOH), (3) perchloric acid (HClO4), (4) hydrochloric acid (HCl) and (5) nitric acid (HNO3)—for the growth of CP nanowires at the electrode junction devices. The organic acids, (1) and (2) did not show any growth, at any electroactive monomer and electrolyte concentrations, on the microelectrode devices confirmed by current-voltage measurements and scanning electron microscopy analysis. Therefore, we concluded that organic acids in general did not facilitate any electroactive oxidation and electro-polymerization processes for the growth of CP nanowires. All of the inorganic acids we investigated grew CP nanowires with diameters ~50–150 nm and length ≥2 µm, particularly in the monomer concentration range of 0.2–0.6 M and electrolyte concentration range of 0.2–0.6 M in various degrees. These data along with further results will be presented at the upcoming conference. We thank the Missile Defense Agency-Small Business Innovation Research (SBIR) Contract (#HQ0147-14-C-7012). References: (1) (a) Alam MM, Wang J, Gao Y, Lee SP, Tseng H-R. Journal of Physical Chemistry B. 2005, 109:12777–12784. (b) Alam MM, Sampathkumaran U, Modular Chemiresistive Sensor. 2015, U.S. Pat. Application 14/658,034 (approved). (2) Skotheim TA, Reynolds JR, “Handbook of Conducting Polymers, Conjugated Polymers: Processing and Applications. 3rd ed.,” CRC Press: Boca Raton, 2007. (3) Virji S, Huang JX, Kaner RB, Weiller BH, “Polyaniline nanofiber gas sensors: examination of response mechanisms,” Nano Letters, 2004, 4:491–496.