Journal of Materials Chemistry C This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online PAPER View Journal | View Issue Carbon-paste nanocomposites as unconventional gate electrodes for electrolyte-gated organic Open Access Article. Published on 11 November 2019. Downloaded on 6/15/2020 3:36:45 PM. Cite this: J. Mater. Chem. C, 2019, 7, 14993 field-effect transistors: electrical modulation and bio-sensing† Jose Muñoz, a Francesca Leonardi, a Tayfun Özmen,a Marta Riba-Moliner, a Arantzazu González-Campo, a Mireia Baeza b and Marta Mas-Torrent *ac Nanocomposite carbon-paste electrodes (NC-CPEs) have been investigated for the first time in electrolyte- gated organic field-effect transistors (EGOFETs) as a replacement of conventional metal gate electrodes, using carbon nanotubes (CNTs) as a model carbon filler. Interestingly, the electrical properties of the resulting devices have been modulated by changing the loading percentage of CNTs within the insulating polymeric matrix. The potential of using such non-conventional gate electrodes for sensing purposes has also been evaluated by investigating, as a proof of concept, the formation of a supramolecular complex between a functionalized CNT-based NC-CPE containing b-cyclodextrin (b-CD) as a bio-recognition element and Received 6th September 2019, tryptophan (TRP). This approach, in synergism with the amplification function of an EGOFET, affords a shift in Accepted 7th November 2019 the threshold voltage (VTH) of the transistor, giving promising analytical results with detection limits at 12 9 DOI: 10.1039/c9tc04929k picomolar levels (1.0  0.1 pM) as well as a linear response from 10 to 10 M. Accordingly, NC-CPEs have been demonstrated to be a potential alternative to metal gate electrodes for the development of a new rsc.li/materials-c generation of highly sensitive carbon-based EGOFET bio-sensors. Electrolyte-gated organic field-effect transistors (EGOFETs) are Interfaces are pivotal for ruling the EGOFET operation and one of the most promising technologies in the field of bio- are exploited for the development of sensors. From an electro- electronics.1–3 EGOFETs are three-terminal electronic devices analytical point of view, the key point to achieve a highly consisting of two electrodes (source, S, and drain, D) connected sensitive EGOFET bio-recognition platform relies on the possi- throughout a thin organic semiconducting (OSC) layer. The OSC bility of properly functionalizing either the gate electrode or the is covered by an electrolyte solution that acts as a dielectric, OSC, the two electronic active interfaces of the transistor.7 where a third electrode (gate, G) is immersed. The operation Engineering of the gate electrode/electrolyte interface by means principle is based on tuning the electrical current flowing along of binding a proper bio-receptor has been demonstrated to be a the OSC by the application of a gate voltage that leads to the viable route in bio-sensing.8–10 The formation of a host–guest formation of two electrical double layers at the OSC/electrolyte supramolecular interaction between the target analyte and the and electrolyte/gate interfaces.4 The main advantage of this anchored bio-receptor changes the potential of the gate electrode technology is the possibility to work in water or buffered media (VGS), affecting directly the current flowing between the S–D at low voltages (o1 V), permitting its direct application in the electrodes (IDS) due to a threshold voltage (VTH) shift. field of bio-sensing since the biological agents remain active.5,6 Gate electrodes with high specific surface area are desirable for a high current modulation in EGOFETs. In this regard, previously the use of active carbon-based electrodes was demonstrated to be an a Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), exciting approach to achieve low voltage channel modulation in Campus de la Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain. E-mail: mmas@icmab.es EGOFETs.11 However, carbon-based electrodes have been mainly b GENOCOV Research Group, Department of Chemistry, Faculty of Science, exploited in transistors with inorganic semiconductors and non- Edifici C-Nord, Universitat Autònoma de Barcelona, Carrer dels Tillers, aqueous electrolytes12,13 or implemented in the so-called organic 08193 Bellaterra (Cerdanyola del Vallès), Barcelona, Spain electrochemical transistors (OECTs),14,15 which exploit a conducting c Networking Research Center on Bioengineering, Biomaterials and Nanomedicine polymer as the active channel material. Additionally, the chemical (CIBER-BBN), Campus de la Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain functionalization of the gate electrode of EGOFETs for bio-sensing † Electronic supplementary information (ESI) available: Experimental procedures approaches has been limited to the use of self-assembled bio-active and further characterization data. See DOI: 10.1039/c9tc04929k molecules grafted on conventional metal gate electrode surfaces.7 This journal is © The Royal Society of Chemistry 2019 J. Mater. Chem. C, 2019, 7, 14993--14998 | 14993 View Article Online Paper Journal of Materials Chemistry C The use of different forms of carbon nanostructures response (10 12 to 10 9 M) with a picomolar (1.0  0.1 pM) (i.e., carbon nanotubes (CNTs), graphene, graphite and related detection limit. structures) for the development of nanocomposite carbon paste Hand-made NC-CPEs were fabricated by dispersing different electrodes (NC-CPEs) has played a leading role in analytical bio- composition ratios of CNTs within the insulating epoxy resin by This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. sensing electrochemical devices.16,17 Specifically, CNTs arise manual homogenization as explained in the Experimental as ideal conducting phase since their dispersion within an section in the ESI.† 21 The carbon-paste was then placed into inert polymeric matrix is a very attractive way to combine a cylindrical polyvinyl chloride (PVC) tube containing a copper the mechanical, chemical, structural, electrical and tunability disk to mount the electrode (see Experimental details in ESI†). properties of individual nanotubes with the processing advan- As shown in Fig. 1A, the percolation curve was constructed by tages of polymers.18,19 NC-CPEs benefit from several intrinsic varying the CNT loading from 3% to 20% (w/w). While nano- Open Access Article. Published on 11 November 2019. Downloaded on 6/15/2020 3:36:45 PM. features, including (i) robustness, (ii) renewable exposed surface composites containing less than 3% of CNTs presented insulating by polishing, (iii) ability to customize their electrical properties properties with resistivity values BN, those comprising more by modifying the filler/polymer composition ratio and (iv) com- than 20% of CNTs were not robust enough. Four different zones patibility with a large variety of functionalization approaches;20 were distinguished from the percolation curve: (a) high resistance however, their exploitation in electronic devices is nowadays an zone composites (HRCs), (b) percolation zone composites (PCs), unexplored field. (c) near-percolation zone composites (NPCs) and (d) low resis- Considering the mentioned advantages of NC-CPEs and tivity zone composites (LRCs), where conductivity reaches its motivated by the possibility to widen the current limited library maximum. Thus, representative nanocomposite compositions of gate electrodes, a novel EGOFET employing a carbon-based for each zone (3% for HRC, 5% for PC, 10% for NPC and 20% nanocomposite gate terminal is proposed, using CNTs as a for LRC) were chosen for performing further electrochemical model conducting filler. The NC-CPE was fabricated following and electronic studies. an easy protocol by dispersing CNTs within an insulating epoxy The electrochemical performances of the developed NC- resin.21 As a first demonstration of sensing applicability, a CPEs regarding their composition ratios were studied by cyclic functionalized CNT-based NC-CPE with gold nanoparticles voltammetry (CV) and electrochemical impedance spectroscopy (Au-NPs) carrying a thiolated b-cyclodextrin (b-CD) has been (EIS), using [Fe(CN)6]3 /4 as the redox marker, as summarized developed taking into account the well-known supramolecular in Table 1. The CVs from Fig. 1B reveal information about the complex formation between tryptophan (TRP) and b-CD (see accessibility of the redox marker to be oxidized or reduced at Scheme 1).22,23 Novelly, this approach achieves a renewable the electrode surface by analyzing the peak currents (Ip), where sensing gate surface by a simple polishing step owing to the the reversibility of the redox couple is defined in terms of the fact that the bio-recognition unit is included directly in the redox peak separation (DEp).25 Thus, a more reversible electro- carbon-paste before the electrode mounting.24 This approach chemical behavior (lower DEp) also accompanied by higher Ip led to an EGOFET bio-sensor exhibiting an excellent linear values was achieved with increasing the CNT loading.26 This fact Scheme 1 Schematic procedure of the EGOFET sensing approach. (A) Activated-CNTs containing carboxylic groups were firstly functionalized with Au-NPs as nanotemplates (i) in order to attach the bio-recognition element, a thiolated b-CD (ii). Subsequently, the hybrid nanomaterial was Fig. 1 (A) Percolation curve from the different NC-CPE composition dispersed throughout the polymeric matrix for the NC-CPE fabrication (iii), ratios (3% to 20% w/w in CNTs) showing four different regions: (a) HRC which after a 10 minute incubation time with different TRP concentrations, (3%), (b) PC (5%), (c) NPC (10%) and (d) LRC (20%). (B) CV and (C) Nyquist was incorporated as a gate electrode into the electronic transistor (iv). plots (inset: equivalent Randles circuit) for the selected NC-CPEs representing (B) Real photographs of the EGOFET setup showing the images of the three the four different percolation regions. Electrochemical experiments were individual components and the assembly of these components to build the recorded in 0.1 M KCl aqueous solution containing 0.01 M [Fe(CN)6]3 /4 . EGOFET device. The scan rate in the CV measurements was 10 mV s 1. 14994 | J. Mater. Chem. C, 2019, 7, 14993--14998 This journal is © The Royal Society of Chemistry 2019 View Article Online Journal of Materials Chemistry C Paper Table 1 Electrochemical parameters for the four different NC-CPE com- double-layer capacitance (Cdl), which is directly related to the position ratios obtained by CV (Ip, A and D Ep) and EIS (RO, RCT and Cdl) charging or background current. As expected from the percolation CNTs Percolation curve, a RO decrease with increasing CNT loading was observed. (%) zone Ip (mA) Aa (cm2) DEp (V) RO (O) RCT (kO) Cdl (mF) This fact can be explained since at lower carbon loads, the ohmic This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 3 HRC 198 0.15 0.43 866 4.5 0.44 resistance is dominated by the nanocomposite resistance, whereas 5 PC 330 0.26 0.27 405 1.9 1.33 at higher carbon loads, this parameter is more dominated by the 10 NPC 439 0.37 0.17 162 0.71 4.56 solution resistance.26 Concerning Cdl, an increase of the electrode 20 LRC 542 0.46 0.16 85 0.26 9.64 capacitance with increasing the %CNT load was observed since this a A: electroactive area, which was calculated in terms of peak current parameter is determined nearly exclusively by the exposed carbon (Ip, A) and scan rate (n, V s 1), according to the modified Randles–Sevčik on the electrode surface. Open Access Article. Published on 11 November 2019. Downloaded on 6/15/2020 3:36:45 PM. equation.21 Subsequently, EGOFETs based on a thin film of the organic semiconductor dibenzo-tetrathiafulvalene (DB-TTF) blended is mainly due to the enhancement of the electroactive area (A) with with polystyrene (4% w/w, ratio 1 : 2) and deposited by bar-assisted increasing the amount of conducting carbon filler (i.e., CNTs). meniscus shearing (BAMS) on a flexible Kapton substrate with Thus, the electron transfer rate of the NC-CPEs can be tailored by pre-patterned source–drain electrodes were fabricated as pre- simply modifying the carbon/polymer composition ratio. viously reported (see ESI† for further details).28–30 As the gate Fig. 1C depicts the Nyquist plots registered for the four terminal, the four types of CNT-based NC-CPEs were employed. selected NC-CPEs, which show the typical semicircle feature Scheme 1 shows the EGOFET configuration. For electronic characteristic of a Randles cell (see Fig. 1C, inset). The end of performance comparison, Pt and glassy carbon (GC) conven- the semicircle (low frequency domain) corresponds to the tional electrodes were utilized. charge transfer resistance (RCT), a parameter that is inversely Transfer and output characteristics were acquired using Milli-Q proportional to the electron transfer rate.27 In agreement with water as the electrolyte medium (Fig. 2A and Fig. S1, ESI†). All the the voltammetric experiments, the RCT value decreases with EGOFETs display a clear p-type modulation and the overall perfor- increasing CNT loading because of the increase of the electro- mances of the NC-CPE-based EGOFETs were comparable with the active sites on the electrode surface. Additional electrochemical ones achieved with the conventional Pt and GC gates (Fig. 2B and parameters related to the EIS performance were also evaluated Table 2). It can be noted that the transfer characteristics recorded in (Table 1), such as (i) the solution resistance (RO), which is the saturation regime (VDS = 0.4 V) display a marked VTH shift. dependent on the ionic concentration and directly related to Such a VTH trend seems to be directly related with the CNT loading the dry resistance taken from the percolation curve, and (ii) the of the gate electrode and, hence, with its intrinsic Cdl value derived Fig. 2 (A) I/V transfer characteristics of NC-CPEs loaded from 3% to 20% in CNTs (w/w) (inset: correlation A vs. VTH of the NC-CPEs). (B) Electrical comparison of EGOFETs employing: (a) raw CNTs and (d) b-CD/Au-NP@CNTs using the optimum NC-CPE composition ratio (20% w/w in carbon filler), and two standard electrodes, (b) GC and (c) Pt. (C) Electrical sensing response of the EGOFET at increasing [TRP] at the bio-recognition gate system. Reference curves present the pre-conditioning step in the electrolyte media (Milli-Q water and the PBS buffer) employed during the experiment (inset: negative EGOFET response at the bare NC-CPE). (D) Calibration curve of VTH vs. log [TRP] at the bio-recognition system, which is represented with its corresponding error bars (n = 9). Experiments recorded in the saturation regime (VDS = 0.4 V). This journal is © The Royal Society of Chemistry 2019 J. Mater. Chem. C, 2019, 7, 14993--14998 | 14995 View Article Online Paper Journal of Materials Chemistry C from the electroactive area (A) (Fig. 2A, inset). The regression the gate electrode was immersed in an Eppendorfs tube filled with equation obtained is: VTH (V) = 1.65–1.36 A (cm2) with r 2 = 0.996. 1 mL of each solution for 10 min in order to promote the formation Leakage current (IGS) represents an important feature of EGOFETs of the supramolecular complex between b-CD and TRP and, and a qualitative indicator of its field-effect operation mode. In the subsequently, incorporated in the EGOFET. In order to exclude This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. present case, the IGS response remains constant regardless of the any possible interference coming from the buffer solution, the type of gate electrode (Fig. S2, ESI†). Moreover, the low IGS current first experiment before the sensing tests consisted in immersing and the almost absent hysteresis suggest that no electrochemical the functionalized NC-CPE in only PBS for 10 min. A series of doping is occurring. Importantly, the electrical device properties and, 20 transfer scans every 30 s were performed with this gate in in particular, the operation voltage window, could be successfully Milli-Q water showing a perfect overlap between the initial and modulated with the %CNT loading of the gate NC-CPEs, shifting the final curves (Fig. S4, ESI†) and, hence, demonstrating the Open Access Article. Published on 11 November 2019. Downloaded on 6/15/2020 3:36:45 PM. VTH up to 0.4 V (see Table 2). stability of the EGOFET device. Such I–V characteristics were Having determined the excellent electronic behavior of the also used as reference for the successive measurements. CNT-based NC-CPEs as novel gate electrodes and taking into As it was expected from the impedimetric experiments (see Fig. account their great potential for applications in bio-sensors, S5, ESI†), a progressive shift of the transfer characteristics was NC-CPEs were exploited for the first time as novel bio-sensing observed with increasing the [TRP], as shown in Fig. 2C. This gates in an EGOFET device.31 In the present case, NC-CPEs behavior could be ascribed to the influence of the dipole moment containing a 20% load of the conducting filler were selected as of TRP on the device electrical characteristics, since it is in a the optimum ones due to their lower operational voltage (see zwitterionic form (pKa1 = 2.38, and pKa2 = 9.39) at the working Fig. 2A) as well as higher A and Cdl values (see Table 1), pH.35 The dipolar moment of the molecules on the gate electrode is ensuring an excellent analytical response in the EGOFET known to modify the effective potential applied to the channel.30 ln device. Accordingly, a renewable bio-recognition sensing gate the present case, the supramolecular interactions formed between was developed for L-tryptophan (TRP) determination as a proof TRP and b-CD lead to an effective gate potential decrease. At a fixed of concept, which is a relevant amino acid for humans since it VGS, a clear decrease of the current in the transfer characteristics is is a neurotransmitter and hormone precursor.32 For this aim, observed with increasing the analyte concentration, being reduced Au-NPs were incorporated on the CNT walls and afterwards by one order of magnitude at nM concentrations. It should be functionalized with a thiolated b-CD (see Experimental section highlighted that the control experiment carried out with the bare from ESI† and Fig. S3 for the microscopic and X-ray spectro- NC-CPE (20% w/w in raw CNTs) did not present any response scopy characterizations), which is known to form a host–guest towards [TRP] due to the absence of the bio-recognition element supramolecular complex with TRP.33 Au-NPs were used as (Fig. 2C, inset). The sensing properties of these devices were nanotemplates in order to ensure an appropriate orientation evaluated by extracting VTH from the transfer characteristics in the of the b-CD empty cavity. Afterwards, NC-CPEs were fabricated linear regime (VDS = 0.1 V). The VTH vs. [TRP] calibration curve was by dispersing 20% (w/w) of the functionalized CNT-filler (b-CD/ constructed per triplicate using three different electrodes (n = 9) in Au-NP@CNTs) throughout the insulating epoxy resin and then order to evaluate both repeatability of the sensing system after the applied as a bio-sensing gate for TRP determination. Importantly, polishing step and reproducibility during the fabrication process. this approach allows the development of bio-sensing gate electro- The calibration curve from Fig. 2D illustrates excellent sensitivity, des that can be easily reset by a simple polishing step since the reproducibility and regression line (r 2 = 0.9996), achieving promising active bio-recognition unit (b-CD) is incorporated into the precursor analytical results by means of detection limits at picomolar levels conducting paste. This aspect represents a unique property of (1.0  0.1 pM) as well as a wide range of linear response from 10 12 NC-CPEs in comparison with common metal or GC electrodes.34 to 10 9 M. Interestingly, while common carbon-based electrochemi- As evidenced in Fig. 2B (curves a vs. d), adding the bio- cal sensing devices determine TRP in the nM range, the present recognition unit on the paste electrode shifts the transfer approach based on a carbon-based EGOFET platform is capable of characteristics towards more negative values. Sensing performances achieving the detection of pM concentrations, demonstrating the were carried out by preparing different [TRP] values from 10 12 to high sensitivity of the transistor.36–38 It is also important to point out 10 9 M in a phosphate buffer saline (PBS) solution at pH 7.4. Then, the renewability of the bio-recognition system by a simple polishing step without losing sensibility. As it is depicted in Fig. S6 (ESI†), the initial EGOFET performance was recovered after the sensing analy- Table 2 Electrical characteristics of the different gate electrodes tical experiments, verifying that the NC-CPE gate electrode can be employed in this study used again for sensing. All these results elucidate the wide perspec- Electrode type Mobility, m (cm2 V 1 s 1) VTH (V) tives that NC-CPEs might offer in the field of renewable gates for GC 2.8  10 2 ( 0.4  10 2 ) 1.06 ( 0.11) EGOFET bio-sensing. 2 2 Pt 2.9  10 ( 2.7  10 ) 1.07 ( 0.34) 2 2 NC-CPE (3%) 0.6  10 ( 0.4  10 ) 1.36 ( 0.14) 2 2 NC-CPE (5%) 0.7  10 ( 0.4  10 ) 1.24 ( 0.12) NC-CPE (10%) NC-CPE (20%) 0.8 1.0   10 10 2 2 ( ( 0.4 0.7   10 10 2 2 ) ) 1.04 0.95 ( ( 0.15) 0.14) Conclusions 2 2 b-CD/Au-NP@CNTs (20%) 3.0  10 ( 0.2  10 ) 0.50 ( 0.33) Overall, NC-CPEs have been exploited for the first time in Note: % is calculated by means of CNTs (w/w). EGOFET technology as novel gate electrodes showing that they 14996 | J. Mater. Chem. C, 2019, 7, 14993--14998 This journal is © The Royal Society of Chemistry 2019 View Article Online Journal of Materials Chemistry C Paper can be a good alternative to conventional electrodes. Using 6 C. Diacci, M. Berto, M. D. Lauro, E. Bianchini, M. Pinti, CNTs as a model carbon filler, it has been demonstrated that D. T. Simon, F. Biscarini and C. A. Bortolotti, Biointerphases, the electrical properties of a transistor can be tailored by 2017, 12, 05F401. changing the filler/polymer composition ratio. It was found 7 D. Wang, V. Noël and B. Piro, Electronics, 2016, 5, 9. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. that the NC-CPE electroactive area is directly related to the 8 E. Macchia, K. Manoli, B. Holzer, C. Di Franco, device threshold voltage, and thus, allows for the tuning of the M. Ghittorelli, F. Torricelli, D. Alberga, G. F. Mangiatordi, device operation voltage window. G. Palazzo and G. Scamarcio, Nat. 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