polymers Review Chemo-Electrical Gas Sensors Based on Conducting Polymer Hybrids Seon Joo Park 1, Chul Soon Park 1,2 and Hyeonseok Yoon 2,3,* 1 Hazards Monitoring Bionano Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB), 125 Gwahak-ro, Yuseong-gu, 34141 Daejeon, Korea; [email protected] (S.J.P.); [email protected] (C.S.P.) 2 Department of Polymer Engineering, Graduate School, Chonnam National University, 77 Yongbong-ro, Buk-gu, 61186 Gwangju, Korea 3 School of Polymer Science and Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, 61186 Gwangju, Korea * Correspondence: [email protected]; Tel.: +82-62-530-1778 Academic Editor: Po-Chih Yang Received: 10 March 2017; Accepted: 24 April 2017; Published: 26 April 2017 Abstract: Conducting polymer (CP) hybrids, which combine CPs with heterogeneous species, have shown strong potential as electrical transducers in chemosensors. The charge transport properties of CPs are based on chemical redox reactions and provide various chemo-electrical signal transduction mechanisms. Combining CPs with other functional materials has provided opportunities to tailor their major morphological and physicochemical properties, often resulting in enhanced sensing performance. The hybrids can provide an enlarged effective surface area for enhanced interaction and chemical specificity to target analytes via a new signal transduction mechanism. Here, we review a selection of important CPs, including polyaniline, polypyrrole, polythiophene and their derivatives, to fabricate versatile organic and inorganic hybrid materials and their chemo-electrical sensing performance. We focus on what benefits can be achieved through material hybridization in the sensing application. Moreover, state-of-the-art trends in technologies of CP hybrid sensors are discussed, as are limitations and challenges. Keywords: conducting polymers; composites; sensors; polypyrrole; polyaniline; polythiophene 1. Introduction Many candidate materials have been explored for chemosensing materials, and, accordingly, many different transduction mechanisms have been developed [1]. Conducting polymers (CPs) have been studied extensively since their discovery in 1977 [2]. In 2000, Heeger, MacDiarmid, and Shirakawa jointly received the Nobel Prize in Chemistry, which reignited considerable interest in CP research. At the beginning of 2000, so-called nanoscience and nanotechnology were also hot topics in research worldwide. Many unique and surprising properties of materials were found at the nanoscale. Naturally, various CP nanomaterials have been developed and have novel and fascinating properties [3]. CPs are a new class of electronic materials and are completely different from most existing inorganic electronic materials. Simply, CPs have the advantages of tunable electrical/electronic properties, easy synthesis, structural diversity, flexibility, and environmental stability (e.g., non-corrosive) [4–6]. These advantages allow CPs to be explored as alternatives to the inorganic electronic materials that are used in diverse application fields. Of course, CPs have intrinsic limitations, such as a relatively low conductivity and a lack of consistency in properties. Therefore, there has been considerable effort to couple CPs with other heterogeneous species, which has resulted in many remarkable research findings [7–10]. Regarding sensor applications, the hybridization of CPs with other components has been conducted Polymers 2017, 9, 155; doi:10.3390/polym9050155 www.mdpi.com/journal/polymers Polymers 2017, 9, x FOR PEER REVIEW 2 of 24 withPolymers other2017 components, 9, 155 has been conducted to improve the sensitivity and selectivity [3,4].2 of 24For example, CPs can achieve sensitive responses to various external stimuli, including chemical species, at torelatively improve low the sensitivitytemperatures, and which selectivity corresponds [3,4]. For to example, one of the CPs greatest can achieve advantages sensitive of responses using CPs to as thevarious sensing external material stimuli, [11,12]. including However, chemical unfortunately, species, at relatively the sensitive low temperatures, signal transduction which corresponds of CPs is availableto one of for the all greatest external advantages stimuli, ofnot using just CPsa target as the species. sensing materialThus, it [is11 ,not12]. easy However, to achieve unfortunately, selective, specificthe sensitive responses signal to transductiona target species of CPsusing is availableonly pure for CPs. all To external overcome stimuli, this not limitation, just a target the species.chemical structureThus, it of is CPs not easycan be to achievemodified selective, or functional specific heterogeneous responses to amaterials target species can be using coupled only with pure CPs. CPs. A varietyTo overcome of heterogeneous this limitation, thematerials, chemical structureincluding of CPsquantum can be modifieddots, two-dimensional or functional heterogeneous inorganic nanomaterials,materials can and be couplednanocarbons with (e.g., CPs. fullerene, A variety carbon of heterogeneous nanotubes, graphene), materials, includinghave been quantum combined withdots, CPs two-dimensional to achieve synergistic inorganic effects nanomaterials, [7-10]. and nanocarbons (e.g., fullerene, carbon nanotubes, graphene),Figure 1 have shows been the combined latest research with CPs trend to achieve in CPs. synergistic CPs have effectsattracted [7– 10increasing]. interest over the last decade,Figure and1 shows the proportion the latest research of hybrids trend or incompos CPs. CPsites havein CP attracted research increasing has continuously interest over increased. the Thelast improved decade, and properties the proportion of the CP of hybrids hybrids or and composites composites in CP have research made has it possible continuously to fabricate increased. new, high-performanceThe improved properties sensors. ofMany the CP efforts hybrids have and been composites devoted haveto the made development it possible of to various fabricate kinds new, of sensorshigh-performance with different sensors. transduction Many efforts modes have using been the devoted hybrids to and the composites development [13,14]. of various In this kinds review, of wesensors cover the with electrical different transduction transduction of modes CP hybrids using the to hybridsdetect gas-ph and compositesase chemical [13 ,species,14]. In this and review, discuss thewe transduction cover the electrical mechanisms transduction according of CP to hybridsthe type to of detect hybrid. gas-phase Several chemicalcomprehensive species, review and discuss articles the transduction mechanisms according to the type of hybrid. Several comprehensive review articles have been published regarding CP-based sensors [12-14]. Therefore, we focus on what benefits can have been published regarding CP-based sensors [12–14]. Therefore, we focus on what benefits can be obtained in using CP hybrids in chemo-electrical sensor applications. In addition, examples are be obtained in using CP hybrids in chemo-electrical sensor applications. In addition, examples are introduced to present the recent research trends, most of which have been conducted since 2012. introduced to present the recent research trends, most of which have been conducted since 2012. FigureFigure 1. 1. TheThe proportion proportion of of articles articles published onon hybrids/compositeshybrids/composites inin thethe body body of of CP-related CP-related Commented [M3]: Please replace with a clearer figure. researchresearch over over the the last last decade. decade. Data Data fr fromom the ISI Web ofof KnowledgeKnowledge database. database. Answer) replaced. 2. 2.CP CP Chemosensors Chemosensors CPs,CPs, including including polypyrrole polypyrrole (PPy), (PPy), polyaniline polyaniline (PANI), polythiophenepolythiophene (PTh),(PTh), poly(3,4-poly(3,4- ethylenedioxythiopheneethylenedioxythiophene (PEDOT), (PEDOT), and and their their derivatives, asas shownshown inin FigureFigure2, 2, have have received received increasingincreasing attentionattention because because of theirof easytheir fabrication easy fabrication and inherent and physical inherent properties, physical biocompatibility, properties, biocompatibility,and optical properties. and optical Their mainproperties. structures Their consist main of structures single and doubleconsist bonds,of single which and are double conjugated bonds, whichπ–electron are conjugated systems. Although π–electron these systems. conjugated Although polymer these chains conjugated have conductivity, polymer theirchains values have conductivity,are very low. their To values increase are their very conductivity, low. To increa a dopingse their process conductivity, by redox a doping reaction process or protonation by redox is needed to remove the electrons on the backbone of the CPs. Positive charges in a CP backbone reaction or protonation is needed to remove the electrons on the backbone of the CPs. Positive charges are accomplished as a charge carrier, leading to conducting properties (conductivity, ca. 100 to in a CP backbone are accomplished as a charge carrier, leading to conducting properties (conductivity, 105 S·cm–1)[4,13]. In comparison, copper has a conductivity of approximately 106 S·cm–1 at ca. 100 to 105 S·cm–1) [4,13]. In comparison, copper has a conductivity of approximately 106 S·cm–1 at room temperature. The unique electrical properties of doped CPs enable energy and electrical Polymers 2017,