Highly Enantioselective Graphene-Based Chemical Sensors Prepared

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Highly Enantioselective Graphene-Based Chemical Sensors Prepared by Chiral Noncovalent Functionalization Xiaobo Shang, Cheol Hee Park, Gwan Yeong Jung, Sang Kyu Kwak, and Joon Hak Oh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13517 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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ACS Applied Materials & Interfaces

Highly

Enantioselective

Graphene-Based

Chemical

Sensors

Prepared by Chiral Noncovalent Functionalization

Xiaobo Shang,†,‡,§ Cheol Hee Park,†,‡,§ Gwan Yeong Jung,‖ Sang Kyu Kwak,*,‖ and Joon Hak Oh*,‡

†

Department of Chemical Engineering, Pohang University of Science and Technology

(POSTECH), 77 Cheongam-ro, Pohang 37673, Korea ‡

School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National

University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea ‖

Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National

Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Korea

Keywords: graphene transistors, chiral sensors, chemical sensors, noncovalent interactions, enantioselectivity.

1 Environment ACS Paragon Plus

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract As a basic characteristic of the natural environment and living matter, chirality has been used in various scientific and technological fields. Chiral discrimination is of particular interest owing to its importance in catalysis, organic synthesis, biomedicine, and pharmaceutics. However, it is still very challenging to effectively and selectively sense and separate different enantiomers. Here, enantio-differentiating chemosensor systems have been developed through spontaneous chiral functionalization of the surface of graphene field-effect transistors (GFETs). GFET sensors functionalized using noncovalent interactions between graphene and a newly synthesized chiralfunctionalized pyrene material, Boc-L-Phe-Pyrene, exhibit highly enantioselective detection of natural acryclic monoterpenoid enantiomers, i.e., (R)-(+)- and (S)-(–)-β-citronellol. Based on a computational study, the origin of enantio-differentiation is assigned to the discriminable charge transfer from (R)-(+)- or (S)-(–)-β-citronellol into graphene with significant difference in binding strength depending on surface morphology. The chemosensor system developed herein has great potential to be applied in miniaturized and rapid enantioselective sensing with high sensitivity and selectivity.


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1. Introduction Chirality plays a very important role in various fields of scientific and technological research, and has great potential to be applied in key areas such as chemical synthesis, pharmaceutics, catalysis, fundamental physics, and biomedicine.1 As most biological compounds and many pharmaceuticals are chiral, sensing and separating chiral compounds is of great importance.2 In many cases, one enantiomer of chiral drugs such as 3,4-dihydroxyphenylalanine has the desired effect (e.g., healing properties) while the other has an undesirable effect (e.g., toxicity).3 Therefore, there is a need for selective discrimination and separation of enantiomers. While chiral discrimination has great potential to be applied in many areas, it remains challenging because of the difficulty of discriminating between enantiomers that have nearly identical properties and the same chemical composition, different only in steric conformation.4 However, when chiral chemicals interact with another chiral entity (typically a molecular receptor) to produce enantiospecific interactions, their biological and physiological properties can differ dramatically. By making use of this principle, separation and detection of optical isomers can be carried out using offline analytical techniques such as chromatography. Other chiral discrimination methods include microwaves,5-6 photoelectron circular dichroism,7 chiral light,8-10 cavity-enhanced polarimeters,11 and magnetic substrates.12 Compared to other methods, amperometric chemosensors based on field-effect transistors (FETs) are attractive owing to advantages such as their rapid sensing ability, high sensitivity, ease of handling, small size, and need for only a small volume of samples.13-15 To date, FET-based sensors have been widely used in artificial skin technology, environmental monitoring, light sensing, drug delivery, and food safety detection.15 However, among the various applications of FET-based sensing, chiral sensing is still one of the most challenging applications of FET-based



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sensors. Torsi et al. successively reported a bilayer-structured organic field-effect transistor (OFET)-based sensing technology for enantioselective detection of terpene flavor molecules of biological origin at an unprecedentedly low concentration and a sensitive and quantitative capacitance-modulated transistor based on functionalization of gate electrodes using odorant binding proteins for the differential detection of chiral odorant molecules.4,

16

Recently,

molecularly imprinted polymer-based extended-gate OFET chemosensors have also been reported to enantioselectively sense chiral phenylalanine.3 Further in-depth research on topics such as the synthesis of novel chiral materials, as well as chiral sensing methods and mechanisms, will expedite the development of the chiral sensing platforms. In particular, graphene-based field-effect transistors (GFETs) are promising for sensing applications owing to the intrinsic chemical inertness of graphene and the high sensitivity of FET platform.17-19 However, sensitizer molecules and structures need to be introduced for high selectivity and broad sensing applications. Noncovalent functionalization of graphene by πinteractions is an attractive synthetic method because it offers the possibility of attaching functional groups to graphene while preserving the graphene lattice and electrical performance.2021

π–π interactions, which can be affected by several factors related to aromatic molecules, such as

electron-donating or withdrawing ability, planarity, and the substituents, are one of the most intriguing noncovalent interactions. For example, pyrene-substituted materials have larger planar aromatic structures that strongly anchor them to the hydrophobic surface of graphene sheets via π–π interactions without disrupting graphene’s conjugation.21-25 In the present study, we synthesized a pyrene-functionalized material (Boc-L-Phe-Pyrene) and utilized it as UV-Vis and fluorescent probes for enantioselective sensing of the biomolecule βcitronellol, which is prevalent in essential oils of various aromatic plant species and has various


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pharmacological and anti-anxiety effects, such as antibacterial, antifungal, antispasmodic, hypotensive, vasorelaxant, anticonvulsant, antinociceptive, and anti-inflammatory activities.26-29 More interestingly, Boc-L-Phe-Pyrene was used as the chiral functional layer on the graphene surface of GFET-based sensors using π–π noncovalent interactions for the enantioselective sensing of β-citronellol. Based on computational studies, including density functional theory (DFT) and molecular dynamics (MD) simulations, we suggest that an enantioselective sensing mechanism: Boc-L-Phe-Pyrene with oblique morphology exhibits different binding strengths with enantiomeric β-citronellol, leading to differences in the charge transfer amount into graphene. For future applications, we also demonstrate the fabrication of a flexible enantioselective sensor device. The highly selective and sensitive detection of β-citronellol enantiomers with low concentration using the Boc-L-Phe-Pyrene/graphene bilayer-structured system will enable new strategies for FET-based chiral sensing.

2. Experimental Section Materials and Instrumentation. All starting materials were purchased from Aldrich and used without further purification. All solvents were ACS grade unless otherwise noted. The absorption spectra were measured using a Cary 5000 UV-Vis-NIR spectrophotometer for solutions. Photoluminescence (PL) spectra were recorded on an FP-6500 spectrofluorometer (JASCO). An Agilent 5500 scanning probe microscope running with a Nanoscope V controller was used to obtain AFM images. AFM images were recorded in a high-resolution tapping mode under ambient conditions. Raman spectroscopic analysis was conducted at a wavelength of 532 nm (WITec, Micro Raman). The electrical performances of fabricated GFETs and GFET-based



enantioselective sensors were measured using a Keithley 4200-SCS semiconductor parametric analyzer. Synthesis and Characterization of Boc-L-Phe-Pyrene. 1-(3-Dimethylaminopropyl)-3ethylcarbodiimide hydrochloride (EDC·HCl) (6.2 mmol), 4-dimethylaminopyridine (DMAP) (2.8 mmol), trietheylamine (4.1 mmol), and 1-pyrenemethanol (2.1 mmol) were successively added to a solution of N-(tert-Butoxycarbonyl)-L-phenylalanine (1.89 mmol) in 25 mL dichloromethane (DCM) at room temperature, and the mixture was stirred overnight. Then the reaction mixture was concentrated in vacuo. The residue was diluted with DCM and sequentially extracted with saturated NaHCO3 aqueous solution, water, and brine. The final product was purified by column chromatography

(yield:

57%).

(S)-pyren-1-ylmethyl

2-((tert-butoxycarbonyl)amino)-3-

phenylpropanoate: 1H NMR (400 MHz, CDCl3): δ 8.00–8.26 (m, 9 H), 6.86–7.12 (m, 5 H), 5.80– 5.95 (m, 2 H), 4.92–5.03 (m, 1 H), 4.61–4.71 (m, 1 H), 2.95–3.10 (m, 2 H), 1.40 (s, 9 H) ppm. 13C NMR (100 MHz, CDCl3): 171.9, 131.2, 130.7, 129.7, 129.2, 128.4, 128.4, 128.0, 127.3, 126.2, 125.6, 125.6, 124.9, 124.6, 124.6, 122.8, 65.5, 54.5, 38.3, 28.3 ppm (Figure S1). Elemental analysis (%) calcd. for C24H23NO3: C 77.19, H 6.21, N 3.75, found (%): C 77.15, H 6.20, N 3.68. HRMS (MALDI-TOF, m/z): M+ calcd. for 479.2097, found: 479.2095. Sensor Device Fabrication on SiO2/n++Si wafer. Graphene film was grown on the copper foil by the conventional chemical vapor deposition (CVD) method using H2 and CH4 gas. After the growth of graphene, poly(methyl methacrylate) (PMMA) was coated on the graphene surface. Copper foil under the graphene film was etched using ammonium persulfate ((NH4)2S2O8) aqueous solution (0.1 M). Then the PMMA/graphene was wet-transferred on the SiO2 (300 nm)/n++Si wafer and annealed at 130°C for 30 min. PMMA-supporting film was removed using acetone. For deposition of the Boc-L-Phe-Pyrene layer, the devices were immersed in a N,N-


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dimethylformamide (DMF) solution of Boc-L-Phe-Pyrene for 12 h. Next, the devices were rinsed in an excess amount of DMF and annealed to remove residual solvent. Source and drain electrodes were deposited in a thickness of 40 nm by thermal evaporation of Au using a patterned shadow mask. Fabrication of Flexible Sensor Device. A flexible sensor was fabricated on an indium tin oxide (ITO)-deposited polyethylene naphthalate (PEN) substrate, in which ITO and PEN serve as gate electrode and substrate, respectively. As a flexible dielectric layer, SU-8 10 diluted with γbutyrolactone (γ-butyrolactone:SU-8 10 = 7:3 by volume ratio) was spin-coated at 7000 rpm for 120 s and films were annealed at 95°C for 1 min.30 SU-8 10 film was exposed to UV light to induce cross-linking, and annealed at 110°C for 1 h. The preparation processes for the graphene film, Boc-L-Phe-Pyrene layer, and electrodes were the same as described above. MD Simulation. The induced dipole moments of Boc-L-Phe-Pyrene on the graphene for each chiral enantiomer of β-citronellol were investigated using MD simulations. First, three types of morphologies were chosen as model systems by varying the angle between the aromatic plane and functional group in the Boc-L-Phe-Pyrene molecules (i.e., ‘vertical’, ‘oblique,’ and ‘horizontal’ shapes). After full optimization by DFT calculation, a 3×3×1 supercell structure was created, and the vacuum was fully introduced in the z-direction (i.e., 44.5 × 51.3 × 100.0 Å3). Subsequently, the equilibrium binding configurations for each (R)-(+)- or (S)-(–)-β-citronellol at a fixed pressure condition of 1 atmosphere were identified by grand canonical Monte Carlo (GCMC) simulation31 (i.e., ~70 chiral citronellol molecules adsorbed on each Boc-L-Phe-Pyrene/graphene system). Using these configurations as the initial structure, NVT (isothermal) MD simulations were performed for 10 ns at room temperature with a time-step of 1 fs. All MD simulations were carried out using COMPASSII forcefield.32-33 Atom-based cutoff (a cutoff of 15.5 Å) and the Ewald



scheme34-35 were applied to estimate the van der Waals and electrostatic interactions, respectively. Temperature was controlled using a Nosé-Hoover-Langevin thermostat.36 DFT Calculation. DFT calculations were used to investigate the binding energy and charge transfer of Boc-L-Phe-Pyrene/graphene in the presence of (R)-(+)- or (S)-(–)-β-citronellol. First, three morphological systems consisting of Boc-L-Phe-Pyrene and graphene were constructed by applying a vacuum in the z-direction (i.e., 14.8×17.1×35.0 Å3). With these models, the initial binding configurations of a single (R)-(+)- or (S)-(–)-β-citronellol were identified by GCMC simulations. Subsequently, after full optimization of the systems, the binding energies (BEs) of (R)-(+)- or (S)-(–)-β-citronellol on the Boc-L-Phe-Pyrene/graphene systems were calculated as follows: BE= Epyrene⁄G+cit. − Epyrene⁄G − Ecit. where Epyene/G+cit., Epyene/G, and Ecit. represent the total energies of Boc-L-Phe-Pyrene/graphene systems adsorbed by (R)-(+)- or (S)-(–)-β-citronellol, bare Boc-L-Phe-Pyrene/graphene systems, and an isolated single (R)-(+)- or (S)-(–)-β-citronellol molecule, respectively. All DFT calculations were performed using the DMol3 program37-38 with the generalized gradient approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) functional.39 The spin-polarized calculations were performed using double numerical basis set with polarization functions (DNP) and real-space cutoff of 4.4 Å. All electron relativistic effects were included for the treatment of core electrons in all model systems. The convergence criteria for energy, force, and displacement were set as 1.0×10–5 Ha, 0.002 Ha Å-1, and 0.005 Å, respectively. The atomic charges were obtained from Mulliken population analyses.40 GCMC Simulation. GCMC simulations of the (R)- and (S)-enantiomers of β-citronellol were performed using the Sorption program.41 The equilibrium binding configurations of (R)-(+)- or


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(S)-(–)-β-citronellols were investigated at a fixed pressure of 1 atmosphere and at room temperature. The systems were equilibrated for 1.0×106 MC steps and analyzed for the next 1.0×106 MC steps. The non-bond interactions, including electrostatic and van der Waals forces, were estimated by group-based summation and an atom-based cutoff (a radius of 15.5 Å) scheme.

3. Results and Discussion Pyrene-based materials are widely used in photochemical research because of their unique properties.42 Our pyrene-based chiral material, Boc-L-Phe-Pyrene, was synthesized from esterification between 1-pyrenemethanol and N-(tert-Butoxycarbonyl)-L-phenylalanine. The UVVis and PL spectra of pyrene-functionalized material in tetrahydrofuran (THF) solutions (5 × 10– 7

M) are illustrated in Figure 1. For the Boc-L-Phe-Pyrene, the absorption spectra in THF exhibited

three main peaks at 344 nm, 327 nm, and 315 nm, corresponding to the 0-0, 0-1, and 0-2 transitions of well-resolved vibronic structures of the chiral molecule, respectively. Fluorescence studies of Boc-L-Phe-Pyrene in THF (5 ×10–7 M) showed the mirror image of the UV-Vis spectra with three main peaks at the wavelengths of 415 nm, 396 nm, and 376 nm. UV-Vis spectroscopy is widely used in molecular sensing and has various advantages for determining enantiomeric excess (ee), such as rapid measurement and simple instrumentation. When the absorbance lies in the visible range, it is possible to quickly detect the ee value by the naked eye.43 Fluorescence is another optical technique that has been used to determine ee. The fluorescence instrument is highly sensitive, rapid, relatively inexpensive, and can be used for realtime analyses. Therefore, the development of enantioselective UV-Vis and fluorescence sensors for the recognition of chiral organic molecules has attracted great interest. 43-45 To assess the enantioselective response of Boc-L-Phe-Pyrene towards β-citronellol, we measured the absorption



and emission spectra of Boc-L-Phe-Pyrene (Figure 1c and 1d) with each enantiomer of βcitronellol. In the presence of either (R)-(+)- or (S)-(–)-β-citronellol at a concentration of 5 × 10–4 M, the maximum wavelength of fluorescence and absorbance kept constant, while the intensities of both fluorescence and absorbance of the Boc-L-Phe-Pyrene (5 × 10–7 M) were quenched. Interestingly, the intensities were always reduced more for (R)-(+)-β-citronellol than (S)-(–)-βcitronellol in both UV-Vis and PL spectra. We surmise the enantiomeric sensing mechanism of Boc-L-Phe-Pyrene can be related to the different association constants of Boc-L-Phe-Pyrene with the enantiomeric analyte of β-citronellol.46-48 For the use of Boc-L-Phe-Pyrene in enantioselective sensing, we fabricated a GFET sensor device using functionalized graphene with a Boc-L-Phe-Pyrene layer via noncovalent π–π interactions.25 A schematic illustration of the chiral-functionalized GFET sensor fabrication process is shown in Figure 2a (see details in the Experimental section). The film characteristics of pristine graphene and Boc-L-Phe-Pyrene/graphene were analyzed using atomic force microscope (AFM) and Raman spectroscopy. The AFM height image of pristine graphene shows a clean and flat surface with root-mean-square roughness (Rq) of 0.253 nm (Figure 2b), while Boc-L-Phe-Pyrene/graphene film exhibits a clearly different surface morphology with higher Rq of 0.447 nm, which explains the successful attachment of Boc-L-Phe-Pyrene onto the surface (Figure 2c). The thickness of Boc-L-Phe-Pyrene layers deposited on graphene is about 2 nm, when estimated from the AFM height profile (Figure S2). Raman spectra of the pristine graphene film showed a 2D band (2675 cm–1), G band (1582 cm– 1

), and high intensity ratio of 2D/G (I2D/IG) over 4, which indicates typical characteristics of

monolayer graphene (Figure 2d).49 In addition, the high quality of graphene is confirmed by the low intensity of the D band (1343 cm–1), which is an indicator of graphene defects. However, Boc-


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L-Phe-Pyrene/graphene films show apparent differences from pristine graphene film in Raman spectra. For example, the Raman spectra of the Boc-L-Phe-Pyrene/graphene film exhibits an increase in the intensity of the G band and position shifts of the 2D and G bands from 2675 cm–1 and 1582 cm–1 to 2680 cm–1 and 1588 cm–1, respectively, which might be related to the stacking of Boc-L-Phe-Pyrene on the top of the graphene film and the p-type doping effect of graphene originated from pyrene derivative functional layer.50-51 Electrical properties of pristine graphene- and Boc-L-Phe-Pyrene/graphene-based transistor devices are shown in Figure S3 (Supporting Information). The source-drain current (ID) as a function of gate voltage (VG) for the pristine graphene transistor shows typical properties of pchannel-operated graphene transistor devices, with a Dirac point of 10 V. However, the Boc-LPhe-Pyrene/graphene transistor device clearly shows a current-voltage characteristic of p-type doped devices, with a Dirac point shifted to a positive VG region above 50 V. This reflects the ptype doping effect of Boc-L-Phe-Pyrene, which is consistent with the upshift of 2D band and G band positions in Raman spectra of Boc-L-Phe-Pyrene/graphene film (Figure 2d).51-52 The hole carrier mobility (μh) of the devices was calculated based on current-voltage characteristics using the following equation: 𝜇h =

𝑔m 𝐿 𝑉D 𝑊𝐶g

where gm is the transconductance, L is the channel length, VD is the source-drain voltage, W is the channel width, and Cg is the capacitance of the dielectric layer.25 The average hole mobility of the pristine graphene transistor was 1510 ± 86 cm2 V–1 s–1, while that of the Boc-L-PhePyrene/graphene transistor was 1006 ±74 cm2 V–1 s–1. Although the Boc-L-Phe-Pyrene/graphene device showed p-type doped characteristics, the carrier mobility was relatively lower than that of the pristine device, which can be attributed to the impeded charge carrier transport owing to the



nonconductive property of the Boc-L-Phe-Pyrene layer positioned between the Au electrodes and graphene layer. To demonstrate the enantioselective chemical sensing properties of the Boc-L-PhePyrene/graphene system, we used the vapor of (R)-(+)-β-citronellol and (S)-(–)-β-citronellol enantiomers as the sensing analytes in the fabricated devices. For vapor phase sensing, the devices were operated in p-channel mode (VD = –0.1 V, VG = –20 V) and the analyte liquids were vaporized using customized sequential vaporization tools as reported by Torsi et al.4 For the analyte vapor concentration estimation, the theoretical saturation concentration of β-citronellol vapor was simply calculated using the Clausius-Clapeyron equation. Analyte vapor concentration was controlled by mixing analyte vapor with nitrogen gas using a customized gas flow control system. The schematic diagram of the customized vapor phase sensing system is shown in Figure S4 (Supporting Information). Figure 3a shows the change in drain current in the pristine graphene-based sensor devices as a function of analyte vapor concentration. The change in channel current (I0–ID) of the pristine graphene sensor showed a decrease in p-channel current when both (R)-(+)- and (S)-(–)-βcitronellol vapors were exposed to the sensor devices, and the increase in current reduction was consistent with the enhancement of analyte vapor concentration. As expected, the pristine graphene sensor shows almost the same level of current reduction for (R)-(+)- and (S)-(–)-βcitronellol vapor, which means the pristine graphene itself is not able to enantioselectively sense enantiomeric citronellol. However, the Boc-L-Phe-Pyrene/graphene-based sensor device exhibits obvious differences under exposure to β-citronellol enantiomer vapors compared to pristine sensors (Figure 3b). The Boc-L-Phe-Pyrene/graphene-based sensor device always reduced the channel current more for (S)-(–)-β-citronellol vapor than the other enantiomer (R)-(+)-β-citronellol.


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We tested vapor concentration ranges from 100 ppm to 2 ppm; however, we could not test vapor concentrations lower than 2 ppm owing to the limited vapor dilution properties of our customized vapor sensing system. The sensitivity for enantiomeric analytes, which was defined as the ratio of the channel current change with regard to the analyte concentration change, was measured from the slope of calibration curves in Figure 3b. Measured sensitivities were –6.7 ± 0.57 nA per ppm for (R)-(+)-β-citronellol and –9.1 ± 0.87 nA per ppm for (S)-(–)-β-citronellol, respectively. These sensitivities obtained from our sensor system showed remarkable enhancement when compared with the previously reported sensitivity of β-citronellol enantiomers obtained from OTFT-based sensor system (–0.04 ±0.01 nA per ppm for (R)-(+)-β-citronellol and –0.08 ±0.01 nA per ppm for (S)-(–)-β-citronellol, respectively).4 The theoretical limit of detection for both enantiomers, estimated from the calibration curves, was 1.45 ppm for (R)-(+)-β-citronellol and 1.06 ppm for (S)-(–)-β-citronellol, respectively.53 The response time, defined as the time required to reach 36.8% (= e–1) of the maximum current change54 for (R)-(+)- and (S)-(–)-β-citronellol sensing, was 2.25 s and 1.53 s, respectively. Interestingly, the level of current reduction for the Boc-L-Phe-Pyrene/graphene sensor for analyte vapor exposure was lower than that of the pristine graphene sensor. The lower level of current reduction with the Boc-L-Phe-Pyrene/graphene sensor can be attributed to the existence of the Boc-L-Phe-Pyrene interlayer, which limits the direct approach of the analyte molecule to the graphene layer and interactions between analyte vapor and graphene channels. In addition, we investigated multiple sensing responses at 100 ppm of (R)-(+)- and (S)-(–)-β-citronellol vapor using the Boc-L-Phe-Pyrene/graphene-based sensor, and found stable and repeatable multiple sensing signals with obvious differences in intensity for (R)-(+)- and (S)-(–)-β-citronellol vapors (Figure 3c and 3d). A comparison of sensing signals between the Boc-L-Phe-Pyrene/graphene-



based sensor and pristine graphene-based sensor is presented in Table 1. These results clearly demonstrate the enantioselective chemical sensing properties of the sensor device, and also show that the adsorption of a chiral functionalized Boc-L-Phe-Pyrene layer on the graphene film can provide enantioselective sensing properties to graphene-based sensor devices. The results plotted in Figure 3 clearly show that the interaction between Boc-L-Phe-Pyrene and β-citronellol vapor on the graphene-based sensor reduces the amount of channel current. We speculated that the difference in the level of channel current reduction occurred because the electron donating effects of chiral analyte differed for the (R)- and (S)-enantiomers of β-citronellol when the analytes were adsorbed on the Boc-L-Phe-Pyrene layer (Figure 4a). Thus, to verify a signature of enantioselective sensing, we investigated the induced dipole moment on the Boc-LPhe-Pyrene layer by performing MD simulations. We considered highly plausible morphological systems (i.e., ‘vertical’, ‘oblique,’ and ‘horizontal’ shapes) by varying the angle between the aromatic plane and functional group of Boc-L-Phe-Pyrene molecules (see Figure S5 and “MD simulation” in the Experimental Section). Interestingly, the induced dipole moments exhibited different trends depending on the surface morphology (Figure 4b and S6). In the vertical morphology, the dipole moment differences induced by (R)-(+)- or (S)-(–)-β-citronellol were small, whereas in other morphologies, the induced polarity of Boc-L-Phe-Pyrene was predominant to one side (i.e., (R) > (S) for horizontal morphology and (R) < (S) for oblique morphology). Thus based on our results, we suggest that the oblique morphology may have been dominantly present on the surface which is consistent to experimental results in Figure 3. To investigate this further, we compared the binding strengths of (R)-(+)- or (S)-(–)-β-citronellol adsorbed on the Boc-L-PhePyrene/graphene system using DFT calculations (see Figure S7 and “DFT calculation” in the Experimental Section). The oblique morphology exhibited a significant difference in binding


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energy according to the chirality of β-citronellol (i.e., –37.4 kcal mol–1 for (R)-(+)-β-citronellol and –45.3 kcal mol–1 for (S)-(–)-β-citronellol), resulting in the substantial differences in charge transfer to graphene. Notably, Mulliken charge analyses40 showed that electron donation to graphene, which was induced by the interaction between Boc-L-Phe-Pyrene and β-citronellol, was observed for all model systems regardless of surface morphology. Based on these results, the differences in binding energy due to the different chirality of β-citronellol induced differences in the level of charge transfer and subsequently the dipole moment, which was the cause of the different levels of channel current reduction in the device. To enable further applications of the enantioselective chemical sensor, we fabricated a flexible version using a plastic substrate and a polymer dielectric layer (Figure S8) that could easily be bent, as shown in Figure 5a. To demonstrate the enantioselective chemical sensing capability of the flexible sensor, we exposed it to (R)-(+)- and (S)-(–)-β-citronellol vapor and measured current changes. Similar to the results obtained from wafer-based sensors, we found clear differences in the level of drain current reduction depending on the chirality of the β-citronellol enantiomer vapor. The average change in drain current for (R)-(+)-β-citronellol vapor sensing it was –1.33 μA, while for (S)-(–)-β-citronellol vapor sensing was –1.62 μA (Figure 5b and 5c). This shows that flexible sensors can easily be fabricated with reproducible enantioselective sensing responses that are consistent with those obtained using wafer-based sensor devices.

4. Conclusion In summary, we synthesized the chiral-functionalized pyrene molecule Boc-L-Phe-Pyrene and utilized it for enantioselective discrimination of β-citronellol enantiomers. The Boc-L-Phe-Pyrene molecule itself showed chiral discrimination properties with different UV-Vis and PL responses



in solution-phase sensing of (R)-(+)- and (S)-(–)-β-citronellol. Device-based chiral discrimination of given β-citronellol enantiomers was accomplished by GFETs via noncovalent interactions between Boc-L-Phe-Pyrene and graphene. Both pristine graphene films and graphene/Boc-L-PhePyrene films were characterized by AFM and Raman spectroscopy to establish the successful deposition of Boc-L-Phe-Pyrene on the graphene surface. The resulting chemosensors could successfully identify β-citronellol enantiomers with high sensitivity. Based on computational studies, the binding interactions between (R)-(+)- or (S)-(–)-β-citronellol and the Boc-L-PhePyrene/graphene system significantly differ according to the chirality of β-citronellol, leading to substantial differences in the charge transfer from enantiomeric citronellol to graphene and in the induced dipole moments. Furthermore, we successfully fabricated a flexible GFET-based enantioselective sensor. Given the fact that FET-based chiral sensing is still rare and challenging, this study paves the way for the development of highly sensitive and enantioselective chemosensors.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website. NMR spectra; GFET performances; experimental process; simulation results; (PDF)

Author Information Corresponding Author *E-mail: [email protected] *E-mail: [email protected]


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ORCID Joon Hak Oh: 0000-0003-0481-6069 Sang Kyu Kwak: 0000-0002-0332-1534 Author Contributions §

X. S. and C. H. P. contributed equally to this work. The manuscript was written through

contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1602–03. S.K.K. acknowledges financial support from the NRF of Korea (NRF-2014R1A5A1009799) and computational resources from UNIST-HPC.



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Page 25 of 31

(b)

(a)

Boc-L-Phe-Pyrene

(R)-(+)-β-citronellol

(d)

1.0 0.8

PL Intensity (a.u.)

(c)

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Boc-L-Phe-Pyrene Boc-L-Phe-Pyrene with ((S)-(–)-β-citronellol with S)-()- -citronellol with ((R)-(+)-β-citronellol with R)-()- -citronellol

0.6 0.4 0.2 300

310

320

330

340

350

(S)-(–)-β-citronellol

1.0 0.8

Boc-L-Phe-Pyrene with ((S)-(–)-β-citronellol S)-()- -citronellol with ((R)-(+)-β-citronellol R)-()- -citronellol

0.6 0.4 0.2 370 380 390 400 410 420 430

Wavelength (nm)

Wavelength (nm)

Figure 1. Molecular structure of (a) Boc-L-Phe-Pyrene and (b) β-citronellol enantiomers. (c) UVVis spectra and (d) photoluminescence spectra of Boc-L-Phe-Pyrene solution with addition of chiral analytes. The concentration of Boc-L-Phe-Pyrene solution and chiral analyte was fixed at 5 × 10–7 M and 5 × 10–4 M, respectively.



(a) Graphene transfer

Boc-L-Phe-Pyrene deposition

S/D electrode evaporation

(b)

5 nm

(c)

5 nm

(d) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

200 nm

1588 cm –1 2680 cm–1 Graphene with Boc-L-Phe-Pyrene 2675 cm–1

1582 cm–1 Pristine graphene

200 nm 1200 1500 1800 2100 2400 2700 3000

Raman shift (cm-1)

Figure 2. (a) Schematic depiction of the device structure and fabrication process of a Boc-L-PhePyrene/graphene FET-type sensor. AFM height images of (b) pristine graphene and (c) Boc-LPhe-Pyrene deposited graphene on SiO2/Si substrate. (d) Raman spectra of pristine graphene (black) and Boc-L-Phe-Pyrene/graphene (red).


(c)

-1.8

(R)-(+)-β-citronellol (R)-()- -citronellol (S)-()- -citronellol (S)-(–)-β-citronellol

-1.6

IDD (μA) (A) II00-I

-8 -7 -6 -5 -4

0.4 0.0

-1.4 -1.2 -1.0 -0.8

-0.8 -1.2

-0.6

-1.6

-2

-0.4

-2.0

20

40

60

80

100

Concentration (ppm)

0

20

40

60

80 100

Concentration (ppm)

(d)

-0.4

-3 0

(R)-(+)-β-citronellol (R)-( )- -citronellol (S)-(–)-β-citronellol (S)-( )- -citronellol

0.4

(R)-(+)-β-citronellol (R)-( )- -citronellol (S)-(–)-β-citronellol (S)-( )- -citronellol

0.0

0 D

(b)

(R)-(+)-β-citronellol (R)-()- -citronellol (S)-()- -citronellol (S)-(–)-β-citronellol

-9

0 D

-10

A) II0-IID ((μA)

(a)

(A) II00-IIDD (μA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A) II0-IID ((μA)

Page 27 of 31

-0.4 -0.8 -1.2 -1.6

0

50

100

150

200

Time (s)

250

-2.0 25

30

35

40

45

50

55

Time (s)

Figure 3. Concentration dependent chiral β-citronellol vapor sensing signals from (a) pristine graphene sensors and (b) Boc-L-Phe-Pyrene/graphene sensors fabricated on SiO2/Si wafer. I0 is the baseline current. Solid lines are the interpolating calibration curves. Ten sensor devices were tested for statistical analysis. (c) Continuous enantioselective sensing responses of a Boc-L-PhePyrene/graphene sensor device for chiral β-citronellol vapor with a concentration of 100 ppm (VD = –0.1 V; VG = –20 V) and (d) magnified plot of the dotted area in (c).



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with (R)-(+)-β-citronellol with (S)-(–)-β-citronellol

Figure 4. (a) Schematic illustrations of Boc-L-Phe-Pyrene/graphene with adsorbed (R)-(+)- or (S)(–)-β-citronellol molecule. Note that Boc-L-Phe-Pyrene is the oblique form. The yellow colored arrows represent the direction of charge transfer (CT) from β-citronellol to graphene. The (R)-(+)or (S)-(–)-β-citronellol is colored in sky blue and pink, except that the oxygen and hydrogen atoms of –OH group are colored in red and white, respectively. The carbon, hydrogen, oxygen and nitrogen atoms of Boc-L-Phe-Pyrene are colored in black, red, cyan and blue, respectively. The graphene below the Boc-L-Phe-Pyrene is colored in gray. (b) Comparison of average dipole moment of Boc-L-Phe-Pyrene for three morphological systems (i.e., vertical, oblique, and horizontal forms (Figure S5)) obtained from MD simulations for last 5 ns. The error bars represent the standard deviation of each system.


Page 29 of 31

(b)

(a)

(R)-(+)-β-citronellol (R)-( )--citronellol (S)-(–)-β-citronellol (S)-( )--citronellol

0.4

(c)

5 mm

(A) I0I0-I-IDD (μA)

-0.4 -0.8 -1.2

(R)-(+)-β-citronellol (R)-( )--citronellol (S)-(–)-β-citronellol (S)-( )--citronellol

0.4 0.0

0.0

A) -ID ((μA) II00-I

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.4 -0.8 -1.2 -1.6

-1.6 0

40

80 120 160 200 240

Time (s)

30

35

40

45

50

55

Time (s)

Figure 5. (a) Photograph of a flexible enantioselective sensor device fabricated on an ITO/PEN substrate with a SU-8 dielectric layer. (b) Continuous enantioselective sensing responses for enantiomeric β-citronellol vapor with a concentration of 100 ppm (VD = 0.1 V; VG = 20 V) and (c) magnified plot of the dotted area in (b).



Page 30 of 31

Table 1. Average drain current changes for Boc-L-Phe-Pyrene/graphene-based sensors and pristine graphene sensors upon exposure to various concentrations of β-citronellol enantiomer vapor. I0-ID (μA) Active Layer

a

Analyte

2 ppm

10 ppm

20 ppm

40 ppm

60 ppm

80 ppm

100 ppm

(R)-(+)-βcitronellol

–0.53 (±0.03)b

–0.64 (±0.02)

–0.70 (±0.02)

–0.79 (±0.03)

–0.89 (±0.03)

–1.01 (±0.07)

–1.27 (±0.09)

(S)-(–)-βcitronellol

–0.61 (±0.03)

–0.73 (±0.04)

–0.82 (±0.02)

–0.91 (±0.03)

–1.04 (±0.03)

–1.28 (±0.10)

–1.60 (±0.10)

(R)-(+)-βcitronellol

–3.16 (±0.13)

–4.05 (±0.21)

–4.89 (±0.12)

–5.52 (±0.24)

–6.40 (±0.25)

–8.12 (±0.23)

–9.49 (±0.25)

(S)-(–)-β–3.19 –4.04 –4.95 –5.57 citronellol (±0.19) (±0.21) (±0.12) (±0.13) a Ten sensor devices for each active layer were tested for statistical analysis. b The standard deviation.

–6.43 (±0.13)

–8.11 (±0.30)

–9.51 (±0.23)

Chiral Functionalized Graphene

Pristine Graphene


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Graphical Abstract for TOC


Highly Enantioselective Graphene-Based Chemical Sensors Prepared

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