Research Focus

Polyaniline pH Sensing Films<br />Aileen C. Vandenberg*a<br />a Civil and Environmental Engineering, University of California, Davis, CA, USA <br />Abstract <br />Cost of corrosion of civil infrastructure is ever increasing. Maintaining correct pH levels in concrete is crucial to monitoring the health of concrete. Developing better pH sensors relies on exploring new sensing materials. Conducting polymers, such as polyaniline, have been gaining momentum as new pH sensing materials. This paper reviews the chemistry of polyaniline, the synthesis of polyaniline, different transducing pH sensing mechanisms used with polyaniline, and the next chapter in developing polyaniline/carbon nanotube pH sensors. <br />Keywords: Conducting polymers, polyaniline, pH sensor, corrosion monitoring, carbon nanotubes<br />INTRODUCTION <br />The financial impact corrosion has on civil and aerospace infrastructure in the United States is continuing to increase each year. The estimated direct cost of corrosion is totaled around $276 billion USD. ADDIN EN.CITE <EndNote><Cite><Author>Koch</Author><Year>2001</Year><RecNum>286</RecNum><DisplayText><style face=quot;
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>27</ref-type><contributors><authors><author>Gerhardus H. Koch</author><author>Michiel P.H. Brongers </author><author>Neil G. Thompson</author><author>Y. Paul Virmani</author><author>Joe H. Payer</author></authors></contributors><titles><title>Corrosion Cost and Preventive Strategies in the United States</title></titles><keywords><keyword>Cost of Corrosion, Preventive Strategies, Sector Studies,</keyword><keyword>Corrosion Cost, Direct Cost, Indirect Cost, Economic</keyword><keyword>Analysis, Control, Management, Technology, Design,</keyword><keyword>Practice, Corrosion</keyword></keywords><dates><year>2001</year><pub-dates><date>September 30, 2001</date></pub-dates></dates><publisher>CC Technologies Laboratories, Inc, NACE International, Federal Highway Administration</publisher><isbn>FHWA-RD-01-15</isbn><urls></urls></record></Cite></EndNote>1 For highway bridges this amounts to a cost between $6.42 billion and $10.15 billion USD. ADDIN EN.CITE <EndNote><Cite><Author>Yunovich</Author><Year>2005</Year><RecNum>288</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Yunovich, Mark</author><author>Thompson, Neil G.</author><author>Virmani, Y. Paul</author></authors></contributors><auth-address>CC Technologies Laboratories, Inc., 5777 Frantz Road, Dublin, OH 43017, United States</auth-address><titles><title>Corrosion protection system for construction and rehabilitation of reinforced concrete bridges</title><secondary-title>International Journal of Materials and Product Technology</secondary-title></titles><periodical><full-title>International Journal of Materials and Product Technology</full-title></periodical><pages>269-285</pages><volume>23</volume><number>Compendex</number><keywords><keyword>Concrete bridges</keyword><keyword>Bars (metal)</keyword><keyword>Cathodic protection</keyword><keyword>Corrosion protection</keyword><keyword>Costs</keyword><keyword>Highway bridges</keyword><keyword>Metallizing</keyword><keyword>Preventive maintenance</keyword></keywords><dates><year>2005</year></dates><publisher>Inderscience Enterprises Ltd.</publisher><isbn>02681900</isbn><urls><related-urls><url>http://dx.doi.org/10.1504/IJMPT.2005.007731</url></related-urls></urls></record></Cite></EndNote>2 Since corrosion is a natural thermodynamic process, preventative measures against corrosion are controlling and monitoring the rate of corrosion. To fight this increasing cost, better corrosion protection and monitoring techniques are needed that are economically feasible, environmentally friendly, and easy to fabricate. Promising new materials, such as conducting polymers, are providing solutions to these problems. ADDIN EN.CITE <EndNote><Cite><Author>Talaie</Author><Year>1997</Year><RecNum>232</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Talaie, Afshad</author></authors></contributors><titles><title>Conducting polymer based pH detector: A new outlook to pH sensing technology</title><secondary-title>Polymer</secondary-title></titles><periodical><full-title>Polymer</full-title></periodical><pages>1145-1150</pages><volume>38</volume><number>5</number><keywords><keyword>conducting polymers</keyword><keyword>resistance</keyword><keyword>pH sensor</keyword></keywords><dates><year>1997</year></dates><isbn>0032-3861</isbn><urls><related-urls><url>http://www.sciencedirect.com/science/article/B6TXW-3SPD3MF-M/2/4790c740cc04a273be4d6810c4c61b79</url></related-urls></urls><electronic-resource-num>Doi: 10.1016/s0032-3861(96)00612-x</electronic-resource-num></record></Cite></EndNote>3<br />Carbonation of Concrete<br />Concrete made with Portland cement maintains a high alkalinity due to the carbon hydroxide in the cement. ADDIN EN.CITE <EndNote><Cite><Author>Bohni</Author><Year>2005</Year><RecNum>289</RecNum><DisplayText><style face=quot;
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>12</ref-type><contributors><authors><author>Bohni, Hans</author></authors></contributors><titles><title>Corrosion in reinforced concrete structures</title></titles><dates><year>2005</year></dates><pub-location>Cambridge, England</pub-location><publisher>Woodhead</publisher><isbn>9781439823439 143982343X 0849325838 9780849325830</isbn><urls></urls><language>English</language></record></Cite></EndNote>4 When carbon dioxide from the atmosphere reacts with the cement carbonation occurs. Carbonation is when calcium hydroxide reacts with carbon dioxide to form calcium carbonate, <br />CaOH2+CO2->CaCO3+H2O. (1)<br />The water that is produced in this reaction in turn also reacts with the carbon dioxide to form hydrogen ions and carbonate ions, <br />H2O+ CO2->2H++CO32-,(2)<br />resulting in the pore water’s pH level to dip below 12.5; the pH level healthy concrete is typically at. If the pH level continues to decline past a pH of 9, the passivating pH level for steel, then corrosion of the reinforcement bars will occur. ADDIN EN.CITE <EndNote><Cite><Author>Bohni</Author><Year>2005</Year><RecNum>289</RecNum><DisplayText><style face=quot;
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>12</ref-type><contributors><authors><author>Bohni, Hans</author></authors></contributors><titles><title>Corrosion in reinforced concrete structures</title></titles><dates><year>2005</year></dates><pub-location>Cambridge, England</pub-location><publisher>Woodhead</publisher><isbn>9781439823439 143982343X 0849325838 9780849325830</isbn><urls></urls><language>English</language></record></Cite></EndNote>4 Thus, one way to monitoring corrosion of reinforcement steel in concrete is to monitor the pH levels with a pH sensor.<br />The pH Scale<br />The pH of a solution indicates the hydrogen ion concentration (H+) in a solution through the logarithmic relation ADDIN EN.CITE <EndNote><Cite><Author>Kostiner</Author><Year>2003</Year><RecNum>290</RecNum><DisplayText><style face=quot;
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>6</ref-type><contributors><authors><author>Kostiner, Edward</author><author>Jespersen, Neil D.</author></authors></contributors><titles><title>Chemistry</title></titles><dates><year>2003</year></dates><pub-location>Hauppauge, N.Y.</pub-location><publisher>Barron's</publisher><isbn>0764120069 9780764120060</isbn><urls></urls><remote-database-name>/z-wcorg/</remote-database-name><remote-database-provider>http://worldcat.org</remote-database-provider><language>English</language></record></Cite></EndNote>5<br />pH= -log⁡(H+). (3)<br />For example, if the hydrogen ion concentration is of the order of 10-3 moles per liter, then the pH of the solution is 3, indicating an acidic solution. If the concentration is of the order of 10-9 moles per liter, then the pH is 9, resulting in a more alkaline solution. The range of the pH scale is from 0 to 14, with pH of 7 being neutral.<br />The Glass Electrode pH sensor<br />The most common pH sensor is the glass electrode. A typical pH glass electrode is a thin walled glass bulb which contains a silver wire immersed in a chloride ion concentrated buffer solution. The potential difference between the buffer solution and the solution the glass bulb is immersed in directly correlates to the pH level of that solution (Fig. 1).<br />Figure 1 Schematic of a pH electrode. ADDIN EN.CITE <EndNote><Cite><Author>Monk</Author><Year>2004</Year><RecNum>291</RecNum><DisplayText><style face=quot;
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>6</ref-type><contributors><authors><author>Monk, Paul M. S.</author></authors></contributors><titles><title>Physical chemistry : understanding our chemical world</title></titles><dates><year>2004</year></dates><pub-location>Chichester [u.a.]</pub-location><publisher>Wiley</publisher><isbn>0471491802 9780471491804 0471491810 9780471491811</isbn><urls></urls><remote-database-name>/z-wcorg/</remote-database-name><remote-database-provider>http://worldcat.org</remote-database-provider><language>English</language></record></Cite></EndNote>6<br />While the glass electrode works well in the laboratory, it has many disadvantages. One disadvantage is that it does not measure correct pH levels at high or low hydrogen ion concentrations. ADDIN EN.CITE <EndNote><Cite><Author>Zhang</Author><Year>2008</Year><RecNum>292</RecNum><DisplayText><style face=quot;
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>6</ref-type><contributors><authors><author>Zhang, Xueji</author><author>Ju, Huangxian</author><author>Wang, Joseph</author></authors></contributors><titles><title>Electrochemical sensors, biosensors, and their biomedical applications</title></titles><dates><year>2008</year></dates><pub-location>Amsterdam; Boston</pub-location><publisher>Academic Press</publisher><isbn>9780123737380 0123737389</isbn><urls></urls><remote-database-name>/z-wcorg/</remote-database-name><remote-database-provider>http://worldcat.org</remote-database-provider><language>English</language></record></Cite></EndNote>7 Another disadvantage of this sensor is that it is fragile and hard to miniaturize, making it hard to embed into concrete.PEVuZE5vdGU+PENpdGU+PEF1dGhvcj5Tb25nPC9BdXRob3I+PFllYXI+MjAwNzwvWWVhcj48UmVj
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ADDIN EN.CITE.DATA 8 In addition, the buffer solution will react with the cement in the concrete, increasing the carbonation rate. Thus, other pH sensors have been developed to address these problems. <br />Conducting Polymers<br />Conducting polymers are long chained polymers that contain π-electrons delocalized along the polymer’s backbone that are the key to the polymer’s conductivity. ADDIN EN.CITE <EndNote><Cite><Author>Zarras</Author><Year>2003</Year><RecNum>109</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Zarras, P.</author><author>Anderson, N.</author><author>Webber, C.</author><author>Irvin, D. J.</author><author>Irvin, J. A.</author><author>Guenthner, A.</author><author>Stenger-Smith, J. D.</author></authors></contributors><titles><title>Progress in using conductive polymers as corrosion-inhibiting coatings</title><secondary-title>Radiation Physics and Chemistry</secondary-title></titles><periodical><full-title>Radiation Physics and Chemistry</full-title></periodical><pages>387-394</pages><volume>68</volume><number>3-4</number><keywords><keyword>Conductive polymer</keyword><keyword>Synthesis</keyword><keyword>Corrosion protection</keyword></keywords><dates><year>2003</year></dates><isbn>0969-806X</isbn><work-type>doi: DOI: 10.1016/S0969-806X(03)00189-0</work-type><urls><related-urls><url>http://www.sciencedirect.com/science/article/B6TVT-48NX3N4-1/2/f325317e3398e195c38117d198af29eb</url></related-urls></urls><access-date>2003/11//</access-date></record></Cite></EndNote>9 By adding electrons (doping) or protons (protonating) conducting polymers are transformed into their conductive state. The most widely studied conducting polymer is polyaniline. ADDIN EN.CITE <EndNote><Cite><Author>Inzelt</Author><Year>2011</Year><RecNum>153</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Inzelt, György</author></authors></contributors><titles><title>Rise and rise of conducting polymers</title><secondary-title>Journal of Solid State Electrochemistry</secondary-title></titles><periodical><full-title>Journal of Solid State Electrochemistry</full-title></periodical><pages>1-8</pages><keywords><keyword>Physics and Astronomy</keyword></keywords><dates><year>2011</year></dates><publisher>Springer Berlin / Heidelberg</publisher><isbn>1432-8488</isbn><urls><related-urls><url>http://dx.doi.org/10.1007/s10008-011-1338-3</url></related-urls></urls><electronic-resource-num>10.1007/s10008-011-1338-3</electronic-resource-num></record></Cite></EndNote>10<br />Polyaniline<br />Aniline is the monomer unit of polyaniline (Fig 2). It is an organic aromatic compound consisting of a benzene ring with a nitrogen unit attached. <br />Figure 2 Aniline, the monomer unit of polyaniline. ADDIN EN.CITE <EndNote><Cite><Author>Sorrell</Author><Year>2006</Year><RecNum>274</RecNum><DisplayText><style face=quot;
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>6</ref-type><contributors><authors><author>Thomas N. Sorrell</author></authors></contributors><titles><title>Organic Chemistry, Second Edition</title></titles><edition>2</edition><dates><year>2006</year></dates><pub-location>Sausalitio, California</pub-location><publisher>University Science Books</publisher><isbn>1-891389-38-6</isbn><urls></urls></record></Cite></EndNote>11<br />The base form of polyaniline (PANI, PA, PAn, PANi) is often represented as,<br />Figure 3 Generalize form of polyaniline. ADDIN EN.CITE <EndNote><Cite><Author>Macdiarmid</Author><Year>1989</Year><RecNum>302</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Macdiarmid, A. G.</author><author>Epstein, A. J.</author></authors></contributors><auth-address>OHIO STATE UNIV, DEPT PHYS, COLUMBUS, OH 43210 USA. OHIO STATE UNIV, DEPT CHEM, COLUMBUS, OH 43210 USA.MACDIARMID, AG, UNIV PENN, DEPT CHEM, PHILADELPHIA, PA 19104 USA.</auth-address><titles><title>Polyanlines-A Novel Class of Conducting Polymers</title><secondary-title>Faraday Discussions</secondary-title><alt-title>Faraday Discuss.</alt-title></titles><periodical><full-title>Faraday Discussions</full-title><abbr-1>Faraday Discuss.</abbr-1></periodical><alt-periodical><full-title>Faraday Discussions</full-title><abbr-1>Faraday Discuss.</abbr-1></alt-periodical><volume>88</volume><dates><year>1989</year></dates><isbn>1364-5498</isbn><accession-num>ISI:A1989DB79700021</accession-num><work-type>Article</work-type><urls><related-urls><url><Go to ISI>://A1989DB79700021</url></related-urls></urls><language>English</language></record></Cite></EndNote>12<br />where y determines the state the polymer is in and x represents the number of polymer units (~1000). ADDIN EN.CITE <EndNote><Cite><Author>Macdiarmid</Author><Year>1989</Year><RecNum>302</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Macdiarmid, A. G.</author><author>Epstein, A. J.</author></authors></contributors><auth-address>OHIO STATE UNIV, DEPT PHYS, COLUMBUS, OH 43210 USA. OHIO STATE UNIV, DEPT CHEM, COLUMBUS, OH 43210 USA.MACDIARMID, AG, UNIV PENN, DEPT CHEM, PHILADELPHIA, PA 19104 USA.</auth-address><titles><title>Polyanlines-A Novel Class of Conducting Polymers</title><secondary-title>Faraday Discussions</secondary-title><alt-title>Faraday Discuss.</alt-title></titles><periodical><full-title>Faraday Discussions</full-title><abbr-1>Faraday Discuss.</abbr-1></periodical><alt-periodical><full-title>Faraday Discussions</full-title><abbr-1>Faraday Discuss.</abbr-1></alt-periodical><volume>88</volume><dates><year>1989</year></dates><isbn>1364-5498</isbn><accession-num>ISI:A1989DB79700021</accession-num><work-type>Article</work-type><urls><related-urls><url><Go to ISI>://A1989DB79700021</url></related-urls></urls><language>English</language></record></Cite></EndNote>12 Polyaniline exists in three states: the fully reduced (y =1) luecoemeraldine state, the fully oxidized (y=0) pernigraniline state, and the partially reduced (y= ½) emeraldine state. (Fig 4-6).<br />Figure 4 Fully reduced polyaniline (leucoemeraldine). ADDIN EN.CITE <EndNote><Cite><Author>Macdiarmid</Author><Year>1989</Year><RecNum>302</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Macdiarmid, A. G.</author><author>Epstein, A. J.</author></authors></contributors><auth-address>OHIO STATE UNIV, DEPT PHYS, COLUMBUS, OH 43210 USA. OHIO STATE UNIV, DEPT CHEM, COLUMBUS, OH 43210 USA.MACDIARMID, AG, UNIV PENN, DEPT CHEM, PHILADELPHIA, PA 19104 USA.</auth-address><titles><title>Polyanlines-A Novel Class of Conducting Polymers</title><secondary-title>Faraday Discussions</secondary-title><alt-title>Faraday Discuss.</alt-title></titles><periodical><full-title>Faraday Discussions</full-title><abbr-1>Faraday Discuss.</abbr-1></periodical><alt-periodical><full-title>Faraday Discussions</full-title><abbr-1>Faraday Discuss.</abbr-1></alt-periodical><volume>88</volume><dates><year>1989</year></dates><isbn>1364-5498</isbn><accession-num>ISI:A1989DB79700021</accession-num><work-type>Article</work-type><urls><related-urls><url><Go to ISI>://A1989DB79700021</url></related-urls></urls><language>English</language></record></Cite></EndNote>12<br />Figure 5 Fully oxidized polyaniline (pernigraniline). ADDIN EN.CITE <EndNote><Cite><Author>Macdiarmid</Author><Year>1989</Year><RecNum>302</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Macdiarmid, A. G.</author><author>Epstein, A. J.</author></authors></contributors><auth-address>OHIO STATE UNIV, DEPT PHYS, COLUMBUS, OH 43210 USA. OHIO STATE UNIV, DEPT CHEM, COLUMBUS, OH 43210 USA.MACDIARMID, AG, UNIV PENN, DEPT CHEM, PHILADELPHIA, PA 19104 USA.</auth-address><titles><title>Polyanlines-A Novel Class of Conducting Polymers</title><secondary-title>Faraday Discussions</secondary-title><alt-title>Faraday Discuss.</alt-title></titles><periodical><full-title>Faraday Discussions</full-title><abbr-1>Faraday Discuss.</abbr-1></periodical><alt-periodical><full-title>Faraday Discussions</full-title><abbr-1>Faraday Discuss.</abbr-1></alt-periodical><volume>88</volume><dates><year>1989</year></dates><isbn>1364-5498</isbn><accession-num>ISI:A1989DB79700021</accession-num><work-type>Article</work-type><urls><related-urls><url><Go to ISI>://A1989DB79700021</url></related-urls></urls><language>English</language></record></Cite></EndNote>12<br />Figure 6 Partially reduced polyaniline (emeraldine). ADDIN EN.CITE <EndNote><Cite><Author>Macdiarmid</Author><Year>1989</Year><RecNum>302</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Macdiarmid, A. G.</author><author>Epstein, A. J.</author></authors></contributors><auth-address>OHIO STATE UNIV, DEPT PHYS, COLUMBUS, OH 43210 USA. OHIO STATE UNIV, DEPT CHEM, COLUMBUS, OH 43210 USA.MACDIARMID, AG, UNIV PENN, DEPT CHEM, PHILADELPHIA, PA 19104 USA.</auth-address><titles><title>Polyanlines-A Novel Class of Conducting Polymers</title><secondary-title>Faraday Discussions</secondary-title><alt-title>Faraday Discuss.</alt-title></titles><periodical><full-title>Faraday Discussions</full-title><abbr-1>Faraday Discuss.</abbr-1></periodical><alt-periodical><full-title>Faraday Discussions</full-title><abbr-1>Faraday Discuss.</abbr-1></alt-periodical><volume>88</volume><dates><year>1989</year></dates><isbn>1364-5498</isbn><accession-num>ISI:A1989DB79700021</accession-num><work-type>Article</work-type><urls><related-urls><url><Go to ISI>://A1989DB79700021</url></related-urls></urls><language>English</language></record></Cite></EndNote>12<br />Each state of polyaniline can also exist as a salt, however, usually only the emeraldine salt form is considered since it is in this state that polyaniline exhibits its highest conductivity. Hence, polyaniline is often mentioned as containing four main states, where the fourth state is emeraldine salt.PEVuZE5vdGU+PENpdGU+PEF1dGhvcj5aYXJyYXM8L0F1dGhvcj48WWVhcj4yMDAzPC9ZZWFyPjxS
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ADDIN EN.CITE.DATA 9 Polyaniline switches between states by oxidation or protonation (Fig 7).<br />Figure 7 Switching between states of polyaniline. ADDIN EN.CITE <EndNote><Cite><Author>Ge</Author><Year>2007</Year><RecNum>278</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Ge, Chenhao</author><author>Armstrong, Neal R.</author><author>Saavedra, S. Scott</author></authors></contributors><titles><title>pH-Sensing Properties of Poly(aniline) Ultrathin Films Self-Assembled on Indium−Tin Oxide</title><secondary-title>Analytical Chemistry</secondary-title></titles><periodical><full-title>Analytical Chemistry</full-title></periodical><pages>1401-1410</pages><volume>79</volume><number>4</number><dates><year>2007</year></dates><publisher>American Chemical Society</publisher><isbn>0003-2700</isbn><urls><related-urls><url>http://dx.doi.org/10.1021/ac061740e</url></related-urls></urls><electronic-resource-num>10.1021/ac061740e</electronic-resource-num></record></Cite></EndNote>13<br />1.3 Synthesis of Polyaniline<br />The emeraldine salt form of polyaniline can be synthesized either chemically or electrochemically. (For other methods the reader is referred to ADDIN EN.CITE <EndNote><Cite><Author>Bhadra</Author><Year>2009</Year><RecNum>306</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Bhadra, Sambhu</author><author>Khastgir, Dipak</author><author>Singha, Nikhil K.</author><author>Lee, Joong Hee</author></authors></contributors><titles><title>Progress in preparation, processing and applications of polyaniline</title><secondary-title>Progress in Polymer Science</secondary-title></titles><periodical><full-title>Progress in Polymer Science</full-title></periodical><pages>783-810</pages><volume>34</volume><number>8</number><keywords><keyword>Polyaniline</keyword><keyword>Synthesis</keyword><keyword>Processing</keyword><keyword>Application</keyword></keywords><dates><year>2009</year></dates><isbn>0079-6700</isbn><urls><related-urls><url>http://www.sciencedirect.com/science/article/pii/S0079670009000355</url></related-urls></urls><electronic-resource-num>10.1016/j.progpolymsci.2009.04.003</electronic-resource-num></record></Cite></EndNote>14.) Emeraldine salt can be achieved chemically by polymerizing aniline in an acidic medium. The polyaniline characteristics such as solubility, conductivity, and stability depend strongly on the acid chosen. ADDIN EN.CITE <EndNote><Cite><Author>Stejskal</Author><Year>2002</Year><RecNum>303</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>J. Stejskal</author><author>R. G. Gilbert</author></authors></contributors><titles><title>Polyanline: Preparation of a conducting polymer (IUPAC Technical Report)</title><secondary-title>Pure Appl. Chem.</secondary-title></titles><periodical><full-title>Pure Appl. Chem.</full-title></periodical><pages>857-867</pages><volume>74</volume><number>5</number><dates><year>2002</year></dates><urls></urls><electronic-resource-num>10.1351/pac200274050857</electronic-resource-num></record></Cite></EndNote>15 The most common oxidant/protonic acid combination is ammonium peroxydisulfate ,(NH4)2S2O8, with hydrochloric acid, HCl (Fig 8). ADDIN EN.CITE <EndNote><Cite><Author>Cao</Author><Year>1989</Year><RecNum>304</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Cao, Yong</author><author>Andreatta, Alejandro</author><author>Heeger, Alan J.</author><author>Smith, Paul</author></authors></contributors><titles><title>Influence of chemical polymerization conditions on the properties of polyaniline</title><secondary-title>Polymer</secondary-title></titles><periodical><full-title>Polymer</full-title></periodical><pages>2305-2311</pages><volume>30</volume><number>12</number><keywords><keyword>polyaniline</keyword><keyword>synthesis</keyword><keyword>conductivity</keyword><keyword>electrically conducting polymers</keyword><keyword>viscosity</keyword></keywords><dates><year>1989</year></dates><isbn>0032-3861</isbn><urls><related-urls><url>http://www.sciencedirect.com/science/article/pii/0032386189902668</url></related-urls></urls><electronic-resource-num>10.1016/0032-3861(89)90266-8</electronic-resource-num></record></Cite></EndNote>16<br />Figure 8 Oxidation of aniline hydrochloride with ammonium peroxydisulfate yields polyaniline (ES) hydrochloride. ADDIN EN.CITE <EndNote><Cite><Author>Stejskal</Author><Year>2002</Year><RecNum>303</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>J. Stejskal</author><author>R. G. Gilbert</author></authors></contributors><titles><title>Polyanline: Preparation of a conducting polymer (IUPAC Technical Report)</title><secondary-title>Pure Appl. Chem.</secondary-title></titles><periodical><full-title>Pure Appl. Chem.</full-title></periodical><pages>857-867</pages><volume>74</volume><number>5</number><dates><year>2002</year></dates><urls></urls><electronic-resource-num>10.1351/pac200274050857</electronic-resource-num></record></Cite></EndNote>15<br />Polyaniline can also be synthesized electrochemically by oxidizing aniline in an aqueous acidic medium on metal or conducting glass electrodes. However, it is harder to control the degree of the electroactive state of the polymer. ADDIN EN.CITE <EndNote><Cite><Author>Macdiarmid</Author><Year>1989</Year><RecNum>302</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Macdiarmid, A. G.</author><author>Epstein, A. J.</author></authors></contributors><auth-address>OHIO STATE UNIV, DEPT PHYS, COLUMBUS, OH 43210 USA. OHIO STATE UNIV, DEPT CHEM, COLUMBUS, OH 43210 USA.MACDIARMID, AG, UNIV PENN, DEPT CHEM, PHILADELPHIA, PA 19104 USA.</auth-address><titles><title>Polyanlines-A Novel Class of Conducting Polymers</title><secondary-title>Faraday Discussions</secondary-title><alt-title>Faraday Discuss.</alt-title></titles><periodical><full-title>Faraday Discussions</full-title><abbr-1>Faraday Discuss.</abbr-1></periodical><alt-periodical><full-title>Faraday Discussions</full-title><abbr-1>Faraday Discuss.</abbr-1></alt-periodical><volume>88</volume><dates><year>1989</year></dates><isbn>1364-5498</isbn><accession-num>ISI:A1989DB79700021</accession-num><work-type>Article</work-type><urls><related-urls><url><Go to ISI>://A1989DB79700021</url></related-urls></urls><language>English</language></record></Cite></EndNote>12 Both these processes produce the emeraldine salt form of polyaniline. To produce polyaniline emeraldine base, the salt form can be treated with a base such as ammonium hydroxide (NH4OH).<br />1.4 Solubility of Polyaniline<br />Polyaniline is very difficult to process since it tends to agglomerate due to the de-localized π-electronic structure allowing for large interchain π-π attraction. ADDIN EN.CITE <EndNote><Cite><Author>Skotheim</Author><Year>1998</Year><RecNum>300</RecNum><DisplayText><style face=quot;
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>6</ref-type><contributors><authors><author>Skotheim, T.A.</author><author>Elsenbaumer, R.L.</author><author>Reynolds, J.R.</author></authors></contributors><titles><title>Handbook of conducting polymers</title></titles><dates><year>1998</year></dates><publisher>M. Dekker</publisher><isbn>9780824700508</isbn><urls><related-urls><url>http://books.google.com/books?id=6GRovXHas_MC</url></related-urls></urls></record></Cite></EndNote>17 Angelopoulos et al. found that polyaniline can be dissolved in the organic solvent N-methylpyrrolidione (NMP) when it is in its emeraldine base state. Since then other solvents such as N, N’-dimethylurea (DMPU), formic acid, dimethlyformamide (DMF), and dimethyl sulphoxide (DMSO) have been used to dissolve polyaniline emeraldine base. ADDIN EN.CITE <EndNote><Cite><Author>Angelopoulos</Author><Year>1987</Year><RecNum>312</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Angelopoulos, Marie</author><author>Ray, Anjan</author><author>Macdiarmid, Alan G.</author><author>Epstein, Arthur J.</author></authors></contributors><titles><title>Polyaniline: Processability from aqueous solutions and effect of water vapor on conductivity</title><secondary-title>Synthetic Metals</secondary-title></titles><periodical><full-title>Synthetic Metals</full-title></periodical><pages>21-30</pages><volume>21</volume><number>1-3</number><dates><year>1987</year></dates><isbn>0379-6779</isbn><urls><related-urls><url>http://www.sciencedirect.com/science/article/pii/0379677987900622</url></related-urls></urls><electronic-resource-num>10.1016/0379-6779(87)90062-2</electronic-resource-num><access-date>1987/10//</access-date></record></Cite></EndNote>18<br />Polyaniline based pH Sensors<br />Since almost all chemical processes have an associated pH level to them, a variety of transducing methods can be applied to pH sensors. Polyaniline is unique among conducting polymers in that the number of electrons on its backbone does not change in its conductive state. This unique trait allows polyaniline to be used in a variety of different pH sensors.<br />Gravimetric Sensing<br />Quartz crystal microbalance (QCM) is a method that measures mass changes on the nanoscale range by relating the change in its resonance frequency to the change in mass. Since polyaniline switches from its emeraldine base state to emeraldine salt by protonation, pH levels can be directly related to the mass changes of polyaniline. <br />Zhou et al. ADDIN EN.CITE <EndNote><Cite><Author>Zhou</Author><Year>1996</Year><RecNum>313</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Zhou, Xingyao</author><author>Cha, Hongying</author><author>Yang, Cheng</author><author>Zhang, Wuming</author></authors></contributors><titles><title>Determination of pH using a polyaniline-coated piezoelectric crystal</title><secondary-title>Analytica Chimica Acta</secondary-title></titles><periodical><full-title>Analytica Chimica Acta</full-title></periodical><pages>105-109</pages><volume>329</volume><number>1-2</number><keywords><keyword>Polyaniline</keyword><keyword>Quartz crystals</keyword><keyword>Voltammetry</keyword><keyword>Sensors</keyword></keywords><dates><year>1996</year></dates><isbn>0003-2670</isbn><urls><related-urls><url>http://www.sciencedirect.com/science/article/pii/0003267096000980</url></related-urls></urls><electronic-resource-num>10.1016/0003-2670(96)00098-0</electronic-resource-num></record></Cite></EndNote>19 conducted one of the first experiments that validated this concept. In their experiment, polyaniline was electrochemically deposited onto a gold coated quartz crystal electrode by cyclic voltammetry. This polyaniline coated gold electrode acted as the working electrode in a standard three electrode electrochemical cell set-up. The cell contained a potassium chloride saturated calomel electrode (SCE) as the reference electrode and a platinum wire as the counter electrode. Sulfuric acid (H2SO4) was the electrolyte solution. The electrode potential was swept between -0.2 and +0.9 V at a sweeping rate of 100mV/s. The number of sweeps determined the thickness of the PANI film. The authors found that 10-15 sweeps was sufficient. The PANI coated electrode was tested in different pH solutions. The authors found that as the change in resonance frequency was linearly proportional to the change in pH for the pH range of 2-11. (Fig 9)<br />Figure 9 Frequency change as a function of solution pH. ADDIN EN.CITE <EndNote><Cite><Author>Zhou</Author><Year>1996</Year><RecNum>313</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Zhou, Xingyao</author><author>Cha, Hongying</author><author>Yang, Cheng</author><author>Zhang, Wuming</author></authors></contributors><titles><title>Determination of pH using a polyaniline-coated piezoelectric crystal</title><secondary-title>Analytica Chimica Acta</secondary-title></titles><periodical><full-title>Analytica Chimica Acta</full-title></periodical><pages>105-109</pages><volume>329</volume><number>1-2</number><keywords><keyword>Polyaniline</keyword><keyword>Quartz crystals</keyword><keyword>Voltammetry</keyword><keyword>Sensors</keyword></keywords><dates><year>1996</year></dates><isbn>0003-2670</isbn><urls><related-urls><url>http://www.sciencedirect.com/science/article/pii/0003267096000980</url></related-urls></urls><electronic-resource-num>10.1016/0003-2670(96)00098-0</electronic-resource-num></record></Cite></EndNote>19<br />In addition, the authors compared this QC pH electrode to a standard glass pH electrode by measuring the pH of tap water and rain water. They found that the results were very close. (Fig 10)<br />Figure 10 Comparison of pH levels in tap and rain water.PEVuZE5vdGU+PENpdGU+PEF1dGhvcj5aaG91PC9BdXRob3I+PFllYXI+MTk5NjwvWWVhcj48UmVj
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ADDIN EN.CITE.DATA 19<br /> HYPERLINK quot;
_ENREF_19quot;
quot;
Zhou, 1996 #313quot;
<br />Optical Sensing<br />Optical pH sensing is based on a material’s optical properties changing as pH levels change. Polyaniline is a very colorful polymer in that it appears to be blue in the emeraldine base state, green in the emeraldine salt state, violet in the pernigraniline state, and opaque/yellow in the leucoemeraldine state. ADDIN EN.CITE <EndNote><Cite><Author>Talaie</Author><Year>2000</Year><RecNum>310</RecNum><DisplayText><style face=quot;
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ENquot;
db-id=quot;
>17</ref-type><contributors><authors><author>Talaie, A.</author><author>Lee, J. Y.</author><author>Lee, Y. K.</author><author>Jang, J.</author><author>Romagnoli, J. A.</author><author>Taguchi, T.</author><author>Maeder, E.</author></authors></contributors><titles><title>Dynamic sensing using intelligent composite: an investigation to development of new pH sensors and electrochromic devices</title><secondary-title>Thin Solid Films</secondary-title></titles><periodical><full-title>Thin Solid Films</full-title></periodical><pages>163-166</pages><volume>363</volume><number>1-2</number><keywords><keyword>Electrochromic devices</keyword><keyword>Intelligent composite</keyword><keyword>Dynamic modelling</keyword><keyword>Polyaniline</keyword><keyword>Polypyrrole</keyword></keywords><dates><year>2000</year></dates><isbn>0040-6090</isbn><urls><related-urls><url>http://www.sciencedirect.com/science/article/pii/S0040609099009876</url></related-urls></urls><electronic-resource-num>10.1016/s0040-6090(99)00987-6</electronic-resource-num></record></Cite></EndNote>20 Thus, it has been found that pH levels can be monitored through PANI’s optical properties. Pringsheim et al. ADDIN EN.CITE <EndNote><Cite><Author>Pringsheim</Author><Year>1997</Year><RecNum>321</RecNum><DisplayText><style face=quot;
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ENquot;
db-id=quot;
>17</ref-type><contributors><authors><author>Pringsheim, Erika</author><author>Terpetschnig, Ewald</author><author>Wolfbeis, Otto S.</author></authors></contributors><titles><title>Optical sensing of pH using thin films of substituted polyanilines</title><secondary-title>Analytica Chimica Acta</secondary-title></titles><periodical><full-title>Analytica Chimica Acta</full-title></periodical><pages>247-252</pages><volume>357</volume><number>3</number><keywords><keyword>pH sensor</keyword><keyword>Polyanilines</keyword><keyword>Optical sensor</keyword><keyword>NIR sensor</keyword></keywords><dates><year>1997</year></dates><isbn>0003-2670</isbn><urls><related-urls><url>http://www.sciencedirect.com/science/article/pii/S0003267097005631</url></related-urls></urls><electronic-resource-num>10.1016/s0003-2670(97)00563-1</electronic-resource-num></record></Cite></EndNote>21 developed an optical pH sensor using polyaniline. In their experiment polyaniline was chemically synthesized using ammonium peroxodisulfate, (NH4)2S2O8, as the oxidant and hydrochloric acid, HCl, as the protonic acid. The chemical polymerization was carried out in polystyrene cuvettes. The deposited film was then scratched out and allowed to dry. Absorption spectroscopy was carried out over a range of 400 – 1100 nm at a pH 7. The authors found that the spectrum changed with pH changes. (Fig. 11a)<br /> Prangsheim et al. found that while the PANI kept stable in water for months, it immediately started to break down in air. Thus, indicating this type of pH sensor would not be good for long term sensing. In order to address this instability in air, Jin et al. ADDIN EN.CITE <EndNote><Cite><Author>Jin</Author><Year>2000</Year><RecNum>315</RecNum><DisplayText><style face=quot;
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ENquot;
db-id=quot;
>17</ref-type><contributors><authors><author>Jin, Zhe</author><author>Su, Yongxuan</author><author>Duan, Yixiang</author></authors></contributors><titles><title>An improved optical pH sensor based on polyaniline</title><secondary-title>Sensors and Actuators B: Chemical</secondary-title></titles><periodical><full-title>Sensors and Actuators B: Chemical</full-title></periodical><pages>118-122</pages><volume>71</volume><number>1-2</number><keywords><keyword>Optical fiber</keyword><keyword>Sensor</keyword><keyword>pH</keyword><keyword>Polyaniline</keyword></keywords><dates><year>2000</year></dates><isbn>0925-4005</isbn><urls><related-urls><url>http://www.sciencedirect.com/science/article/pii/S0925400500005979</url></related-urls></urls><electronic-resource-num>10.1016/s0925-4005(00)00597-9</electronic-resource-num></record></Cite></EndNote>22 increased the polymerization time from 30 minutes to 12 hours. This shifted the maximum absorption wavelength (Fig. 10b) and resulted in a PANI pH sensing film that was stable in air.<br />3017520142875(b)020000(b)-41970120638(a)020000(a) <br />Figure 11 Absorbance change of polyaniline film versus pH; polymerization time for (a) 30 min ADDIN EN.CITE <EndNote><Cite><Author>Pringsheim</Author><Year>1997</Year><RecNum>321</RecNum><DisplayText><style face=quot;
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ENquot;
db-id=quot;
>17</ref-type><contributors><authors><author>Pringsheim, Erika</author><author>Terpetschnig, Ewald</author><author>Wolfbeis, Otto S.</author></authors></contributors><titles><title>Optical sensing of pH using thin films of substituted polyanilines</title><secondary-title>Analytica Chimica Acta</secondary-title></titles><periodical><full-title>Analytica Chimica Acta</full-title></periodical><pages>247-252</pages><volume>357</volume><number>3</number><keywords><keyword>pH sensor</keyword><keyword>Polyanilines</keyword><keyword>Optical sensor</keyword><keyword>NIR sensor</keyword></keywords><dates><year>1997</year></dates><isbn>0003-2670</isbn><urls><related-urls><url>http://www.sciencedirect.com/science/article/pii/S0003267097005631</url></related-urls></urls><electronic-resource-num>10.1016/s0003-2670(97)00563-1</electronic-resource-num></record></Cite></EndNote>21 and (b) 12 hours. ADDIN EN.CITE <EndNote><Cite><Author>Jin</Author><Year>2000</Year><RecNum>315</RecNum><DisplayText><style face=quot;
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ENquot;
db-id=quot;
>17</ref-type><contributors><authors><author>Jin, Zhe</author><author>Su, Yongxuan</author><author>Duan, Yixiang</author></authors></contributors><titles><title>An improved optical pH sensor based on polyaniline</title><secondary-title>Sensors and Actuators B: Chemical</secondary-title></titles><periodical><full-title>Sensors and Actuators B: Chemical</full-title></periodical><pages>118-122</pages><volume>71</volume><number>1-2</number><keywords><keyword>Optical fiber</keyword><keyword>Sensor</keyword><keyword>pH</keyword><keyword>Polyaniline</keyword></keywords><dates><year>2000</year></dates><isbn>0925-4005</isbn><urls><related-urls><url>http://www.sciencedirect.com/science/article/pii/S0925400500005979</url></related-urls></urls><electronic-resource-num>10.1016/s0925-4005(00)00597-9</electronic-resource-num></record></Cite></EndNote>22<br />Conductimetric Sensing<br />Another way to monitoring the pH of a solution is by measuring the resistance or conductivity across a pH sensitive film. Gill et al. ADDIN EN.CITE <EndNote><Cite><Author>Gill</Author><Year>2007</Year><RecNum>272</RecNum><DisplayText><style face=quot;
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ENquot;
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>17</ref-type><contributors><authors><author>Gill, Edric</author><author>Arshak, Arousian</author><author>Arshak, Khalil</author><author>Korostynska, Olga</author></authors></contributors><titles><title>pH Sensitivity of Novel PANI/PVB/PS3 Composite Films</title><secondary-title>Sensors</secondary-title></titles><periodical><full-title>Sensors</full-title></periodical><pages>3329-3346</pages><volume>7</volume><number>12</number><dates><year>2007</year></dates><isbn>1424-8220</isbn><accession-num>doi:10.3390/s7123329</accession-num><urls><related-urls><url>http://www.mdpi.com/1424-8220/7/12/3329/</url></related-urls></urls></record></Cite></EndNote>23 developed a pH conductimetric sensor using a pH sensitive composite film compromised of polyaniline, polyvinyl butyral (PVB), and the solvent PS3. In this experiment, the authors fabricated two sensors; one where the PANI-PVB-PS3 solution was drop-casted onto an interdigital electrode (IDE) structure and the second where the solution was screen-printed onto the IDE structure. The authors performed DC and AC conductivity characteristic tests, as well as, analyzing the films with x-ray photon spectroscopy (XPS) and scanning electron microscopic (SEM) images. The authors found that the conductivity of the film decreased as pH increased (Fig. 12), validating that the polyaniline is in its protonated ES form in acidic conditions and transitions to its de-protonated EB form as the solution becomes more basic. In addition, it was found that the drop-casted film did not perform as well as the thick film screen printed film. The authors confirmed with SEM images that this was because higher agglomeration of the polyaniline in the film occurred in the drop-cast film, while the mask for the screen-printed film tended to filter out larger particles.<br />Figure 12 Conductivity versus pH for 120μm thick film at various times of exposure to solution. Other film thicknesses showed similar trends. ADDIN EN.CITE <EndNote><Cite><Author>Gill</Author><Year>2007</Year><RecNum>272</RecNum><DisplayText><style face=quot;
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ENquot;
db-id=quot;
>17</ref-type><contributors><authors><author>Gill, Edric</author><author>Arshak, Arousian</author><author>Arshak, Khalil</author><author>Korostynska, Olga</author></authors></contributors><titles><title>pH Sensitivity of Novel PANI/PVB/PS3 Composite Films</title><secondary-title>Sensors</secondary-title></titles><periodical><full-title>Sensors</full-title></periodical><pages>3329-3346</pages><volume>7</volume><number>12</number><dates><year>2007</year></dates><isbn>1424-8220</isbn><accession-num>doi:10.3390/s7123329</accession-num><urls><related-urls><url>http://www.mdpi.com/1424-8220/7/12/3329/</url></related-urls></urls></record></Cite></EndNote>23<br />Potentiometric Sensing<br />The sensing mechanism that the glass electrode depends on is potentiometric sensing, the measurement of the potential difference between a working electrode and a reference electrode. It is one of the most common types of pH sensors used. Sensitivity is important for potentiometric sensors. Sensitivity is change in potential per pH level. Glass electrodes are widely used because they have a high sensitivity range of 55-58mV/pH ADDIN EN.CITE <EndNote><Cite><Author>Karyakin</Author><Year>1996</Year><RecNum>314</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Karyakin, Arkady A.</author><author>Bobrova, Oksana A.</author><author>Lukachova, Lylia V.</author><author>Karyakina, Elena E.</author></authors></contributors><titles><title>Potentiometric biosensors based on polyaniline semiconductor films</title><secondary-title>Sensors and Actuators B: Chemical</secondary-title></titles><periodical><full-title>Sensors and Actuators B: Chemical</full-title></periodical><pages>34-38</pages><volume>33</volume><number>1-3</number><keywords><keyword>Biosensor</keyword><keyword>Potentiometric</keyword><keyword>Polyaniline</keyword><keyword>Glucose oxidase</keyword><keyword>Pesticides</keyword><keyword>Glucose</keyword><keyword>Organophosphorus</keyword></keywords><dates><year>1996</year></dates><isbn>0925-4005</isbn><urls><related-urls><url>http://www.sciencedirect.com/science/article/pii/0925400596019296</url></related-urls></urls><electronic-resource-num>10.1016/0925-4005(96)01929-6</electronic-resource-num></record></Cite></EndNote>24 that works for increasing and decreasing pH levels. An important factor in sensitivity is the sensing material. Various materials such as metal oxides and ion sensitive field effect transistors (ISFETs) are frequently used. However, they do have limitations. Because of its ease of fabrication, flexibility, and unique doping mechanisms, polyaniline is becoming a more popular potentiometric pH sensing material, especially for biosensors. ADDIN EN.CITE <EndNote><Cite><Author>Zhang</Author><Year>2008</Year><RecNum>292</RecNum><DisplayText><style face=quot;
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>6</ref-type><contributors><authors><author>Zhang, Xueji</author><author>Ju, Huangxian</author><author>Wang, Joseph</author></authors></contributors><titles><title>Electrochemical sensors, biosensors, and their biomedical applications</title></titles><dates><year>2008</year></dates><pub-location>Amsterdam; Boston</pub-location><publisher>Academic Press</publisher><isbn>9780123737380 0123737389</isbn><urls></urls><remote-database-name>/z-wcorg/</remote-database-name><remote-database-provider>http://worldcat.org</remote-database-provider><language>English</language></record></Cite></EndNote>7 <br />Polyaniline potentiometric pH sensors are often synthesized by electrochemically depositing aniline onto a working electrode through cyclic voltammetry with a sweep range between 20mV/s to 50mV/s in a three electrode electrochemical cell. The potential difference of the working electrode and the reference electrode is measured in relation to the change in pH of the buffer solution. For example, Karyakin et al. ADDIN EN.CITE <EndNote><Cite><Author>Karyakin</Author><Year>1996</Year><RecNum>314</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Karyakin, Arkady A.</author><author>Bobrova, Oksana A.</author><author>Lukachova, Lylia V.</author><author>Karyakina, Elena E.</author></authors></contributors><titles><title>Potentiometric biosensors based on polyaniline semiconductor films</title><secondary-title>Sensors and Actuators B: Chemical</secondary-title></titles><periodical><full-title>Sensors and Actuators B: Chemical</full-title></periodical><pages>34-38</pages><volume>33</volume><number>1-3</number><keywords><keyword>Biosensor</keyword><keyword>Potentiometric</keyword><keyword>Polyaniline</keyword><keyword>Glucose oxidase</keyword><keyword>Pesticides</keyword><keyword>Glucose</keyword><keyword>Organophosphorus</keyword></keywords><dates><year>1996</year></dates><isbn>0925-4005</isbn><urls><related-urls><url>http://www.sciencedirect.com/science/article/pii/0925400596019296</url></related-urls></urls><electronic-resource-num>10.1016/0925-4005(96)01929-6</electronic-resource-num></record></Cite></EndNote>24 electrochemically polymerized polyaniline onto a glass carbon working electrode in a three electrode electrochemical cell that had a silver/silver chloride reference electrode and a platinum auxiliary (counter) electrode. Potassium chloride (KCl) was used as the protonic acid and sulfuric acid (H2SO4) was the oxidant. The applied potential was between -0.3 to +0.8 V and the sweep rate was 50mV/s. A self-doped polyaniline pH electrode was also prepared. The results for this experiment showed high sensitivity (70-75 mV/pH) in a pH range of 4 to 9 (Fig. 13).<br />Figure 13 Steady-state potential versus pH of self-doped Pan. ADDIN EN.CITE <EndNote><Cite><Author>Karyakin</Author><Year>1996</Year><RecNum>314</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Karyakin, Arkady A.</author><author>Bobrova, Oksana A.</author><author>Lukachova, Lylia V.</author><author>Karyakina, Elena E.</author></authors></contributors><titles><title>Potentiometric biosensors based on polyaniline semiconductor films</title><secondary-title>Sensors and Actuators B: Chemical</secondary-title></titles><periodical><full-title>Sensors and Actuators B: Chemical</full-title></periodical><pages>34-38</pages><volume>33</volume><number>1-3</number><keywords><keyword>Biosensor</keyword><keyword>Potentiometric</keyword><keyword>Polyaniline</keyword><keyword>Glucose oxidase</keyword><keyword>Pesticides</keyword><keyword>Glucose</keyword><keyword>Organophosphorus</keyword></keywords><dates><year>1996</year></dates><isbn>0925-4005</isbn><urls><related-urls><url>http://www.sciencedirect.com/science/article/pii/0925400596019296</url></related-urls></urls><electronic-resource-num>10.1016/0925-4005(96)01929-6</electronic-resource-num></record></Cite></EndNote>24<br />Improving Polyaniline Based pH Sensors with Carbon nanotubes<br />While potentiometric pH sensors have high sensitivity, they do depend on a reference electrode and the pH range for linear behavior is typically only between pH levels of 3-9. Since concrete needs to typically stay above a pH of 11, better pH sensors are still needed. Carbon nanotubes, since coming onto the nanotechnology scene ADDIN EN.CITE <EndNote><Cite><Author>Iijima</Author><Year>1991</Year><RecNum>325</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Iijima, S.</author></authors></contributors><auth-address>IIJIMA, S, NEC CORP LTD,FUNDAMENTAL RES LABS,34 MIYUKIGAOKA,TSUKUBA,IBARAKI 305,JAPAN.</auth-address><titles><title>Helical Microtubles of Graphite Carbon</title><secondary-title>Nature</secondary-title><alt-title>Nature</alt-title></titles><periodical><full-title>Nature</full-title><abbr-1>Nature</abbr-1></periodical><alt-periodical><full-title>Nature</full-title><abbr-1>Nature</abbr-1></alt-periodical><pages>56-58</pages><volume>354</volume><number>6348</number><keywords><keyword>c-60</keyword></keywords><dates><year>1991</year><pub-dates><date>Nov</date></pub-dates></dates><isbn>0028-0836</isbn><accession-num>ISI:A1991GN82900055</accession-num><work-type>Article</work-type><urls><related-urls><url><Go to ISI>://A1991GN82900055</url></related-urls></urls><language>English</language></record></Cite></EndNote>25, have been used in a multitude of applications due to their remarkable mechanical, electrical, and thermal properties. Combining carbon nanotubes with polyaniline has been shown to improve polyanilne’s mechanical strength, conductivity, and thermal stability. ADDIN EN.CITE <EndNote><Cite><Author>Gajendran</Author><Year>2008</Year><RecNum>320</RecNum><DisplayText><style face=quot;
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>17</ref-type><contributors><authors><author>Gajendran, P.</author><author>Saraswathi, R.</author></authors></contributors><auth-address>[Gajendran, Pandi; Saraswathi, Ramiah] Madurai Kamaraj Univ, Dept Mat Sci, Madurai 625021, Tamil Nadu, India.Saraswathi, R, Madurai Kamaraj Univ, Dept Mat Sci, Madurai 625021, Tamil Nadu, India.</auth-address><titles><title>Polyaniline-carbon nanotube composites</title><secondary-title>Pure and Applied Chemistry</secondary-title><alt-title>Pure Appl. Chem.</alt-title></titles><alt-periodical><full-title>Pure Appl. Chem.</full-title></alt-periodical><pages>2377-2395</pages><volume>80</volume><number>11</number><keywords><keyword>polyaniline</keyword><keyword>conducting polymers</keyword><keyword>carbon nanotubes</keyword><keyword>composites</keyword><keyword>electroactive polymers&

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