Letter pubs.acs.org/NanoLett
Ultrasensitive Flexible Graphene Based Field-Effect Transistor (FET)Type Bioelectronic Nose Seon Joo Park,†,§ Oh Seok Kwon,†,§ Sang Hun Lee,‡ Hyun Seok Song,‡ Tai Hyun Park,*,‡ and Jyongsik Jang*,† †
World Class University Program of Chemical Convergence for Energy & Environment, School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Korea ‡ School of Chemical and Biological Engineering, Seoul National University, 599 Gwanangro, Sillim-dong, Gwanak-gu, Seoul 151-742, Korea S Supporting Information *
ABSTRACT: Rapid and precise discrimination of various odorants is vital to fabricating enhanced sensing devices in the fields of disease diagnostics, food safety, and environmental monitoring. Here, we demonstrate an ultrasensitive and flexible field-effect transistor (FET) olfactory system, namely, a bioelectronic nose (B-nose), based on plasma-treated bilayer graphene conjugated with an olfactory receptor. The stable pand n-type behaviors from modified bilayer graphene (MBLG) took place after controlled oxygen and ammonia plasma treatments. It was integrated with human olfactory receptors 2AG1 (hOR2AG1: OR), leading to the formation of the liquid-ion gated FET-type platform. ORs bind to the particular odorant amyl butyrate (AB), and their interactions are specific and selective. The B-noses behave as flexible and transparent sensing devices and can recognize a target odorant with single-carbon-atom resolution. The B-noses are ultrasensitive and highly selective toward AB. The minimum detection limit (MDL) is as low as 0.04 fM (10−15; signal-to-noise: 4.2), and the equilibrium constants of OR-oxygen plasma-treated graphene (OR-OG) and ammonia plasma-treated graphene (-NG) are ca. 3.44 × 1014 and 1.47 × 1014 M−1, respectively. Additionally, the B-noses have long-term stability and excellent mechanical bending durability in flexible systems. KEYWORDS: Graphene, human olfactory receptor, flexible sensor, bioelectronic sensor, plasma treatment, odorants films have shown high carrier mobility at room temperature, leading to great promise for nanoscale applications such as electronic devices, energy storage, solar cells, display devices, and chemical/biological sensors.21−29 In particular, reduced graphene oxide (RGO)-based biological sensing systems have been developed that have advantages such as device structure simplicity and label-free detection.30−35 However, RGO-based sensors showed limited device performances in sensitivity because of the low purity of RGO. Modified graphene-based sensors have been developed as an alternative to RGO-based ones. Herein, we report the fabrication of an ultrasensitive and flexible B-nose based on bilayer graphene conjugated-2AG1 human olfactory receptors, which can bind with a particular odorant. The graphene was grown by chemical vapor deposition (CVD) and modified with oxygen and ammonia plasma treatments to control the bandgap. The transparent platforms based on the modified bilayer graphene (MBLG) integrated with the olfactory receptors had stable p-type (oxygen plasma-treated graphene; OG) and n-type (ammonia
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abrication of well-designed devices having sensitive odorant discrimination is an important and challenging task for various applications such as foods and beverages, environmental monitoring, and disease diagnostics because odorants can differ by only a single carbon atom in their structures.1−3 The olfactory system in humans or animals is capable of recognizing thousands of odorant compounds at very low concentrations (as low as parts per trillion: ppt).4,5 Recently, various sensing techniques with bulky or rigid substrates, such as surface plasmon resonance, quartz crystal microbalance, electrochemical impedance spectrometry, onedimensional (1D) field-effect transistor (FET)-type sensors, and light-addressable potentiometric sensors, have been introduced for developing artificial olfactory sensing devices, that is, bioelectronic noses (B-noses).3,6−14 Unfortunately, although each of these methods has individual strengths, significant limitations including low sensitivity, slow response times, and limited portability remain. Moreover, no flexible and transparent olfactory systems are currently suitable for flexible electrical devices. Graphene is a two-dimensional sheet of sp2-hybridized carbon.15 It has extraordinary thermal, mechanical, and electrical properties and has become an exciting experimental area.16 Graphene-based films on various flexible substrates can be highly flexible and transparent.17−20 Devices using graphene © XXXX American Chemical Society
Received: May 7, 2012 Revised: August 19, 2012
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Scheme 1. Synthetic Protocol of OR-Conjugated B-Nose Using Oxygen and Ammonia Plasma-Treated PBLG
plasma-treated graphene; NG) behaviors, which are suitable for FET-type devices. The liquid ion-gated FET-type B-noses showed excellent single-carbon-atom resolution to the target odorant (amyl butyrate: AB). Importantly, the FET B-nose with OG had the lowest detection limit, as low as ca. 0.04 fM (10−15; signal-to-noise: 4.2), which is ca. 2 orders of magnitude more sensitive than previously reported olfactory sensors. The response intensity of the B-nose also increased with increasing olfactory receptor loading. Additionally, the B-nose displayed long-term stability and had excellent mechanical bending durability in flexible systems. To the best of our knowledge, this is the first example of a high-performance flexible and transparent FET-type B-nose based on a modified bilayer graphene-conjugated olfactory system. We have previously manufactured B-noses using olfactory receptor-conjugated 1D conducting nanomaterials on rigid substrates.1,6,7,9 Continuous bilayer graphene (BLG) was grown by the CVD method using gaseous carbon sources under controlled temperatures and pressures (Supporting Information, Figures S1 and S2). Scheme 1 describes the experimental process for the fabrication of the B-nose substrate based on BLG modified by plasma treatments. First, pristine BLG (PBLG) was transferred onto the flexible substrate film, poly(ethylene terephthalate) (PET) using the dry-transfer method (Figure S3).36 To construct efficient electronic devices having logic operations, the PBLG was structurally modified to have p- and n-type behavior by doping foreign atoms through
Figure 1. (a) HR-TEM images of PBLG. (b) High-magnification AFM images of PBLG on a silicon wafer. The yellow line indicates a scanning trace of the PBLG, which is plotted in the inset. (c) UV− visible spectra of the PBLG film transferred on PET substrates (transmittance, Tr at 550 nm). The inset shows a transparent and flexible PBLG film deposited the gold electrodes.
O2 and NH3 plasma treatments. The human olfactory receptor 2AG1 (hOR2AG1: OR), which is a functionally wellcharacterized receptor and specific to amyl butyrate (AB; Figures S4 and S10), was then chemically attached to the modified BLG (MBLG) surface to selectively detect the odorant. One of the most efficient condensing agents, 1,5diaminonaphthalene (DAN), was stacked on the side plane of the MBLG by π−π interactions between the naphthalene group of DAN and its sp2-carbon plane to immobilize OR on the MBLGs. Glutaraldehyde (GA) was also added, and then the GA-conjugated DAN/MBLG were formed through a SchiffB
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Figure 2. Characterization of oxygen and ammonia plasma-treated PBLGs. (a) XPS spectra of the OG and NG. (b) XPS N 1s spectrum and (c) XPS C 1s spectrum of NG. (d) Raman spectra of OG and NG composed of two layers. The widths of the G (∼1587 cm−1) and 2D (∼2697 cm−1) peaks in plasma-treated PBLG are broader than those of bear PBLG. Importantly, the presence of D (∼1356 cm−1) or D′ (∼1615 cm−1) peaks indicates a breakdown of the k-selection rule, and their origin can be ascribed to a disorder-induced double resonance Raman scattering process.
grown graphene, which is obviously a bilayer. The number of layers of the graphene was also confirmed by highmagnification atomic force microscopy (AFM; Figure 1b); cross-sectional analysis showed a vertical thickness of ca. 1.3 nm (Figure 1b, inset). This is in good agreement with the HRTEM images that showed an average thickness of the PBLG as ca. 1.5 nm. Moreover, when the PBLG was transferred onto the flexible PET film, the optical transmittance decreased by ca. 3.7%, implying that the average thickness was approximately that of bilayer graphene (Figure 1c).42,43 These results indicate that the PBLG film used as the B-nose substrate was highly transparent. Plasma treatment can easily modify graphene intrinsically, which controls the density of foreign atoms in graphene.37,39 Xray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to characterize the plasma-treated PBLGs. Figure 2 shows the XPS and Raman results for oxygen (80 W, 10 s) and ammonia (80 W, 10 s) plasma-treated PBLGs (OG and NG) using a downstream inductively coupled plasma asher. The XPS survey scan spectrum displayed the principal C 1s, O 1s, and N 1s core levels, with no evidence of impurities (Figure 2a). The N 1s peak was clearly observed for the NG sample, whereas the OG sample displayed no detectable N peak.44 Table 1 shows the atomic percentages of nitrogen (N), carbon (C), and oxygen (O) in the PBLG, OG, and NG. In the case of both PBLG and OG, no N-containing species were detectable. However, after the ammonia plasma treatment, the N
Table 1. Atomic Percentages of C, O, and N of PBLG, OG, and NG C % (atom %) O % (atom %) N % (atom %)
PBLG
OG
NG
72.17 27.11
63.10 36.90
68.74 25.06 6.20
base reaction. Finally, through a similar chemical reaction, a covalent bond was formed between the amine group of the OR and the aldehyde group of the GA-DAN/MBLG. Graphene, a conductive material, must have the desired work function for efficient carrier injection, which is essential for electronics applications.15,16 In this study, an intrinsic modification of graphene was conducted using oxygen and ammonia plasma treatments.37−39 Generally, plasma treatment of graphene leads to structural defects. A thermal annealing process (above 100 °C for 6 h) is needed to recover the physical and electrical properties of the graphene.39 Plasma treatments were expected to also induce structural disorder in the upper layer of the graphene.38−41 Therefore, the bilayer, rather than a single layer of graphene, was used as the transducer part of the flexible and transparent B-nose. Observation of an edge by high-resolution transmission electron microscopy (HR-TEM) provides an accurate way to measure the number of layers and their thicknesses in the graphene film. Figure 1a shows an HR-TEM image of CVDC
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Figure 3. Typical FE-SEM images of the MBLG surface (a) before and (b) after immobilization of the OR on flexible substrate (the insets are the schematic diagrams of pristine and OR-conjugated MBLG). The arrow indicates an olfacty receptor (OR). Current−voltage (I−V) curves of (c) OG and (d) NG on the flexible substrate to various OR concentrations (Vds scan rate = 10 mV s−1).
These results indicate that the MBLG samples (OG and NG) were successfully fabricated. To utilize a MBLG sample as a signal transducer in a FETtype B-nose, we introduced an OR that can bind to a specific odorant with high selectivity, specificity, and affinity. In previous work, immobilization of biological molecules on the transducer through covalent bonding improved the stability of the sensing device for detecting the analyte in the liquid phase.7,47,48 For this work, the MBLGs were functionalized with DAN, and then the ORs were immobilized on the surface of the DAN/graphene by the Schiff-base reaction. Figure 3a and b displays typical field-emission scanning electron microscopy (FE-SEM) images before and after the introduction of the OR on the MBLG. The MBLG surface was considerably roughened by attaching the biological molecules. Moreover, to identify the electrical properties of the OR-MBLG platform, current− voltage (I−V) analyses were carried out. Linear I−V plots for OG (Figure 3c) and NG (Figure 3d) over a voltage range from −0.1 V to +0.1 V had ohmic behavior before and after the introduction of the OR on the MBLG surface. Moreover, controlled loading of OR showed stable I−V plots despite slightly increasing resistance induced by better OR-immobilization, which could improve the B-nose performance. These results indicated that OR-conjugated MBLGs were able to maintain reliable electrical contact after the immobilization process. Judging from these results, when the B-nose based on OR-conjugated MBLGs is assembled between the source and the drain electrodes, recognition of the odorants can be monitored by observing the current changes in the conductance.
percentage in NG increased to 6.2%, indicating successful nitrogen doping. Although the O 1s peak was observed for both NG (25.06%) and OG (36.90%) samples, it was caused by oxygen physisorbed on the NG surface. The N 1s peak in NG had three components centered at 398.9, 400.1, and 401.5 eV, corresponding to pyridinic-, pyrrolic-, and graphitic-N species, respectively (Figure 2b).44,45 Pyridinic- and pyrrolic-N species dominated the NG sample, consistent with the theoretical prediction that N atoms are more thermodynamically stable at the edges of a graphene lattice. As shown in Figure 2c, the C 1s core level peak for NG was located at 284.8 eV, corresponding to graphite-like sp2 C; small peaks at 285.9 and 287.1 eV were clearly visible in addition to the main sp2 C peak, indicating two different bond types, and corresponding to the N-sp2 C and Nsp3 C originating from the substitutional doping of N atoms.39−41 Raman spectroscopy is an effective tool to demonstrate the doping effect in graphene. The 2D/G height ratio is used because the 2D (ca. 2697 cm−1) and G (ca. 1587 cm−1) peaks have different doping dependences [the intensity of the D (∼1356 cm−1) peak increases rapidly, and the 2D decreases monotonically]. Figure 2d shows the Raman spectra of the two types (NG and OG) of graphene samples. The 2D/ G intensity ratios for OG (I2D/IG = 0.7590) and NG (I2D/IG = 0.3200) are lower than that for PBLG (I2D/IG = 1.1042). The most distinguishing feature of the NG Raman spectrum was the presence of the D and D′ bands. These bands indicate a disorder or a break in the planar sp2 symmetry of the graphene lattice.46 Such defects may be induced by nitrogen doping in the disordered structures and vacancies in the PBLG lattice. D
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Figure 4. (a) Schematic diagram of a liquid-ion gated FET B-nose using OR-conjugated MBLG (the capital “S”, “D”, and “Vg” indicate source/drain electrodes and gating voltage). Ids−Vds output characteristics of (b) B-nose OG at different Vg from 0.1 to −0.7 V in a step of −0.1 V (Vds: 0 to −1.0 V in a step of −50 mV) and (c) B-nose NG at different Vg from −0.1 to 0.5 V in a step of 0.1 V (Vds: 0 to 1 V in a step of +50 mV) in the phosphatebuffered solution.
addition of various odorant concentrations. OR specifically responds to AB, a common reagent for fruit flavor.1−5 Figure 5a displays the real-time responses to AB of B-nose OG, B-nose without OR (pristine-MBLG), and B-nose NG. The B-nose OG showed a concentration-dependent increase in ISD upon exposure to the target AB. As a control experiment, an identical test was performed with the pristine-MBLG: no significant current signals were observed. For the B-nose NG, the change in current was negatively recorded in real time on exposure to different AB concentrations. Although both B-nose OG and NG were sensitive to AB, the saturated concentration (400 pM) was much larger (over 7 orders of magnitude concentration increase) compared with the initial AB concentration (0.04 fM). This may have been caused by the ultrasmall olfactory receptor (hOR2AG1: OR) (diameter of under 4 nm) being expressed in the insoluble fraction of E. coli (Figure S7). A quantitative analysis was conducted to confirm the signal saturation, and then the amount of OR attached on the MBLG surface was calculated by a bicinchoninic acid (BCA) assay (see Supporting Information and Figure S8). The amount of OR attached on OG was 8.01 × 10−15 mol (4.82 × 109 OR molecules). The number of added AB molecules was ca. 5.35 × 109 (i.e., from 0.04 fM to 400 pM), which was larger than the amount of OR molecules attached on the MBLG surface. Thus, the experimental and theoretical concentrations for the signal saturation (Figure 5a) were in good agreement. Additionally, the B-nose OG showed better responses to AB than did the B-nose NG. The reason can be explained that the
To further examine the electrical characteristics of a B-nose, a liquid-ion gated FET geometry was constructed by surrounding it with a fixed phosphate-buffered solution (PBS: pH 7.4) that can efficiently provide a gate control.49−54 Figure 4a outlines the experimental setup utilized to evaluate the device performance of the B-nose, which was assessed under ambient conditions for more than 10 devices. Figure 4b demonstrates the output curves of the B-nose based on oxygen plasmatreated PBLG (B-nose OG) at room temperature. The drainto-source current (Ids) increased negatively with negatively increasing gate Vg, indicating p-type (hole-transporting) behavior, whereas for the PBLG with ammonia plasmatreatment (B-nose NG), the Ids could be positively enhanced with positively increasing Vg (Figure 4c), meaning that n-type (electron-transporting) behavior took place after nitrogen doping.53,55,56 Moreover, both B-nose OG and NG displayed ohmic-contact behavior at low voltage, indicating that the sensing mechanism in the B-nose system can be dominated by electrostatic gating effect by graphene channel, not contact resistance (Supporting Information, Figures S5 and S6).57 These electrical findings indicate that a specific odorant interacts with the OR attached on the MBLG surface, which in turn transduces a signal in the FET-type B-nose. Such odorant/OR interactions can affect the charge-carrier density on the surface of the MBLGs and thus may allow label-free recognition of target molecules using the FET configuration. To investigate the device characteristics of the liquid-ion gated B-nose graphene FET, the Ids was monitored upon the E
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Figure 5. (a) Real-time responses and a calibration curve of FET B-nose based on OR-conjugated OG and NG measured at Vds = 10 mV (Vg = 0 V) toward AB concentrations (0.04 fM to 4 nM). PMBLG without OR was a control sample. Transfer curves before and after adding AB (0.04 fM) to OR-conjugated (b) OG and (c) NG. The insets indicate expanded transfer curves. (d) Calibration curves of the FET B-nose based on OG and NG (S indicates ΔI/I0). The calibration curves were composed of the linear range (0.04 fM to 40 pM) and saturated range (over 400 pM), relatively.
nose NG. The negative gate voltage induced by OR-odorant binding events displayed the opposite effect with the NG device. The sensing mechanism was also confirmed by transfer curves.60−62 The OR-OG showed a positive shift (
