Noncovalent Monolayer Modification of Graphene Using Pyrene and

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Non-Covalent Monolayer Modification of Graphene Using Pyrene and Cyclodextrin Receptors for Chemical Sensing Xue V. Zhen, Emily G. Swanson, Justin T. Nelson, Yao Zhang, Qun Su, Steven J. Koester, and Philippe Buhlmann ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00420 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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ACS Applied Nano Materials

Non-Covalent Monolayer Modification of Graphene Using Pyrene and Cyclodextrin Receptors for Chemical Sensing Xue V. Zhen,1 Emily G. Swanson,1 Justin T. Nelson,2 Yao Zhang,1 Qun Su,3 Steven J. Koester,3 and Philippe Bühlmann1*

1

Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis, MN

55455, United States 2

Boston Scientific, 4100 Hamline Ave. N., St. Paul, MN 55112, United States

3

Department of Electrical and Computer Engineering, University of Minnesota, 200 Union St.

SE, Minneapolis, MN 55455, United States

ACS Paragon Plus Environment

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ACS Applied Nano Materials 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

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ABSTRACT: Surprisingly few details have been reported in the literature that help the experimentalist to determine conditions necessary for the preparation of self-assembled monolayers on graphene with a high surface coverage. With a view to graphene-based sensing arrays and devices, and in particular in view of graphene-based varactors for gas sensing, graphene was modified in this work by the π-π interaction-driven self assembly of ten pyrene and cyclodextrin derivatives from solution. The receptor compounds were pyrene, pyrene derivatives with hydroxyl, carboxyl, ester, ammonium, amino, diethylamino, and boronic acid groups, as well as the perbenzylated α-, β-, and γ-cyclodextrins. Adsorption of these compounds onto graphene was quantified by contact angle measurements and X-ray photoelectron spectroscopy (XPS). Data thus obtained were fitted with the Langmuir adsorption model to determine the equilibrium constants for surface adsorption and the concentrations of self-assembly solutions needed to form dense monolayers on graphene. The equilibrium constants of all pyrene derivatives fell into the range from 103.4 to 104.6 M-1. For the perbenzylated α-, β-, and γcyclodextrins, the equilibrium constants are 103.24, 102.97, and 102.95 M-1, respectively. Monolayers of 1-pyrenemethylammonium chloride on graphene were confirmed to be stable under heating up to 100 °C in a high vacuum (2 × 10-5 Torr). KEYWORDS: graphene, surface modification, self assembly, monolayer, Langmuir adsorption, adsorption isotherm, contact angle, X-ray photoelectron spectroscopy


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1. INTRODUCTION Graphene-based sensors are attractive for chemical sensing due to graphene’s high electrical conductivity and unique quantum properties.1-2 With different sensing mechanisms, graphene sensors include a wide range of electrochemical, electronic, and optical sensors, and have been shown suitable to detect inorganic, organic, and biomolecules.3-8 In particular, the sensitivity of graphene-based gas sensors towards a number of volatile organic compounds (VOCs) was shown to reach ppb and ppm levels, as the adsorption of VOCs onto graphene changes the resistance or quantum capacitance of such devices.9-14 Graphene sensors can also be utilized to detect analytes in solutions, such as electroactive species,15-17 large biomolecules,7,

18-19

and small organic

compounds.20-21 To make graphene-based sensors selective for particular compounds, it is essential to functionalize the graphene surface with receptors that can selectively interact with analytes of interest in the presence of other interference compounds. It is well documented that receptor molecules can be anchored onto graphene by either formation of covalent chemical bonds or self-assembly driven by non-covalent π-π stacking interactions between graphene and π-electronrich receptor molecules, such as pyrene derivatives and other compounds with aryl groups.22-28 The advantages of the noncovalent surface functionalization are that the mild surface chemistry does not affect the structure of graphene, that self-assembly is easy to control and generates a homogeneous functionalized graphene surface, and that surface adsorption is reversible.24, 29 A range of π electron-rich molecules have been self-assembled, not only onto graphene25, 30-31 but also onto other materials with aromatic carbon surfaces, such as carbon nanotubes32-34 and highly ordered pyrolytic graphite (HOPG).35-37 However, given the importance of surface functionalization for sensor applications, it is very


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surprising how little information can be found in the literature about the experimental conditions that are required to form dense self-assembled monolayers on graphene using non-covalent interactions.29, 38-40 For example, while over 130,000 articles have been published on the topic of graphene, we could find only 50 articles using the keywords graphene and monolayer coverage, and only three of these addressed self-assembly onto graphene from solution.41-43 No articles on the non-covalent self-assembly onto graphene from solution could be found using the combination of the keywords graphene, monolayer, and adsorption isotherm. Likewise, a very well cited review25 from 2012 on the functionalization of graphene with 459 references used the word monolayer in the context of noncovalent surface modification only for the discussion of two original publications.29, 44 Surface functionalization of graphene through self-assembly driven by non-covalent interactions has been used in a very large number of studies. Many publications explain that selfassembly was performed using solutions of the adsorbing species with a concentration in the millimolar range, and a wide variety of observations from solubilities and spectral properties to device performance make it clear that self-assembly was undoubtedly successful to some extent. However, only very few reports justify the concentration of the self-assembly solution, the selection of the self-assembly solvent, and the temperature and duration of the self-assembly.45 The majority of publications on this topic do not contain experimental evidence that can be used to determine whether the self-assembly resulted in partial coverage of graphene, formation of a monolayer of high coverage, or multilayer formation, as it has been observed, e.g., by scanning tunneling and atomic force microscopy,46-49 Raman spectroscopy,45 and electrochemical methods.43,

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The lack of such detailed information is of concern because submonolayer or

multilayer coverage will often reduce the selectivity and sensitivity of sensors based on graphene


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modified with receptor monolayers, especially in the case of resistance- or capacitance-based sensing systems. In this work, we used contact angle measurements and X-ray photoelectron spectroscopy (XPS) to quantitatively determine the conditions required for the formation of dense monolayers of ten pyrene and cyclodextrin derivatives on graphene. The self-assembly was quantified using Langmuir adsorption theory, with the view to provide, for all receptor compounds, equilibrium constants for surface adsorption and, therefrom, the concentrations of self-assembly solutions needed to form monolayers with a high surface coverage. In addition to the self-assembly of 1pyreneboronic acid (Pyr-B(OH)2), which we reported previously,51 we investigated pyrene derivatives with a range of functional groups, i.e., pyrene, 1-pyrenemethanol (Pyr-CH2OH), 1pyreneacetic acid (Pyr-CH2COOH), 1-pyreneacetic acid methyl ester (Pyr-CH2COOCH3), 1pyrenemethylammonium chloride (Pyr-CH2NH2·HCl), 1-pyrenemethylamine (Pyr-CH2NH2), and N,N-diethyl-1-pyrenemethylamine (Pyr-CH2N(CH2CH3)2). In addition, we characterized the self assembly of the perbenzylated α-, β- and γ-cyclodextrins (α-CDBn18, β-CDBn21, and γCDBn24, respectively), which have a rigid barrel shape with characteristic internal cavity size. Cyclodextrins have gained a great deal of attention in host–guest chemistry because their internal cavities permit the selective formation of inclusion complexes with analytes stabilized by van der Waals forces.52-55 The modification of graphene by β-cyclodextrins with one pyrene or 21 benzyl groups has been reported previously.10, 33 However, to the best of our knowledge, this is the first report of monolayers of α- and γ-cyclodextrins, and it is the first time that monolayers of the perbenzylated α-, β- and γ-cyclodextrins have been formed. Since the internal guest-binding cavities of these three relatively rigid cyclodextrins have distinctly different sizes,56 graphene sensors modified with monolayers of these cyclodextrins offer a venue to a range of chemical


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sensors and sensor arrays that provide size-selective analyte recognition. 2. EXPERIMENTAL SECTION 2.1 Materials For the self-assembly of the perbenzylated α-, β- and γ-cyclodextrins, monolayer graphene on Cu foil (grown by chemical vapor deposition) was purchased from Graphenea (Donostia, Spain). For all other self-assembly, monolayer graphene was grown on 25-µm-thick Cu foils (No. 46365, Alfa Aesar, Tewksbury, MA) by chemical vapor deposition using hydrogen and methane flow rates of 21 sccm and 0.105 sccm, respectively, at 1050 °C. Pyrene, 1-pyreneacetic acid, 1-pyrenemethylammonium chloride, 1-pyreneboronic acid, β-cyclodextrin, PBr3, and NaH (60% w/w dispersion in mineral oil) were obtained from Aldrich (St. Louis, MO). 1Pyrenemethanol and α- and γ-cyclodextrin were purchased from TCI (Cambridge, MA). The α-, β- and γ-cyclodextrins were dried in a vacuum overnight before use. Toluene, methanol, and dimethyl sulfoxide (DMSO) were dried over molecular sieves overnight before use. Deionized water (DI water, 0.18 MΩ m specific resistance) was obtained by purification with a Milli-Q PLUS reagent grade water system (Millipore, Billerica, MA). Borate buffer

(pH=9) was

prepared from 50 mL aqueous solution containing 0.1 M boric acid and 0.1 M KCl, 20.8 mL 0.1 M NaOH (aq), and 29.2 mL DI water. The syntheses of Pyr-CH2NH2, Pyr-CH2COOCH3, PyrCH2N(CH2CH3)2, α-CDBn18, β-CDBn21, and γ-CDBn24 are outlined in Scheme 1 and described in detail in the Supporting Information.57-60


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Scheme 1. Routes for syntheses of receptors.

2.2 Surface modification through self-assembly Graphene substrates were immersed overnight into solutions containing the pyrene or cyclodextrin derivative at various concentrations (0, 0.030, 0.10, 0.30, 1.0, 3.0, or 10 mM). The modified graphene substrates were then washed 3 times with small portions of the solvent to remove excess self-assembly solution. For the Pyr-CH2COOH, Pyr-CH2COOCH3, PyrCH2NH2·HCl, Pyr-CH2NH2, and Pyr-CH2N(CH2CH3)2 modifications, ethanol was used as the self-assembly solvent. For modifications with pyrene, Pyr-CH2OH, α-CDBn21, β-CDBn21, and γCDBn21, acetonitrile was used instead, because pyrene and Pyr-CH2OH could not be assembled well on graphene from ethanol, and the perbenzylated cyclodextrins were not soluble in ethanol.


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2.3 Surface Characterization Contact angle measurements. Contact angle measurements were performed with a contact angle goniometer (Erma, Tokyo, Japan). A drop of 4, 8, or 12 µL of an appropriate solvent was placed onto the graphene surface, and the average contact angle was obtained from 6 advancing contact angle readings at two spots.61 Contact angle images were collected with a microscopic contact angle meter (MCA-3, Kyowa Interface Science, Saitama, Japan). A drop of DI water (8 × 10-3 µL) was placed onto the graphene surface for imaging. X-ray photoelectron spectroscopy (XPS). XPS spectra of graphene modified with either one of the perbenzylated cyclodextrins or Pyr-B(OH)2 were collected on a VersaProbe III Scanning XPS Microprobe (PHI 5000, Physical Electronics, Chanhassen, MN). The elemental surface composition of bare graphene and modified graphene was obtained by averaging the elemental percentages as obtained from high resolution XPS spectra at three spots. 3. RESULTS AND DISCUSSION 3.1 Graphene modification with monolayers of pyrene derivatives Graphene samples modified with different pyrene derivatives were characterized by contact angle measurements, which are highly sensitive to the functional groups on sample surfaces. The contact angle of bare graphene, which was obtained in a control experiment after immersion of graphene into pure self-assembly solvents overnight, was found to be 91–92º with DI water as the wetting liquid.62 The contact angle changed after the graphene was immersed into selfassembly solutions containing the pyrene derivatives, confirming the success of the selfassembly (Figure S2). For instance, the contact angle of graphene decreased to 74.0º after


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exposure to 10 mM Pyr-CH2NH2·HCl solution (Table 1). The type of solution used to perform these measurements was chosen to maximize the difference between the contact angle of the solution on bare graphene and the contact angle on the modified graphene, thereby improving data accuracy for subsequent measurements of binding isotherms. Because of the different types of functional groups of interest in this study, a range of solutions were used to measure contact angles. For example, an aqueous 10 mM NaOH solution was used to characterize Pyr-CH2COOH modified graphene.31 The choice of the solution is straightforward, as deprotonation of the carboxyl groups results in a very polar surface that distinguishes itself strongly from the neutral graphene surface. DI water was the wetting liquid for graphene surfaces modified with Pyr-CH2COOCH3, Pyr-CH2NH2·HCl, and Pyr-CH2NH2. Since the latter will readily become protonated in water, the suitability of DI water to enhance the difference in wettability of the unmodified and modified graphene is evident here too. In contrast, when DI water was used as the wetting liquid, contact angles of graphene modified with Pyr-CH2OH and Pyr-CH2N(CH2CH3)2 were not very different from those of bare graphene. To enhance the effect of the solution on the wettability, borate buffer (pH 9) was used as the wetting liquid for graphene modified with Pyr-CH2OH, taking advantage of the covalent interactions borate may undergo with hydroxyl groups.63 To take advantage of hydrogen bonding interactions, 10% w/w CF3CH2OH (aq.), a strong hydrogen bonding donor, was chosen as the wetting liquid for graphene modified with Pyr-CH2N(CH2CH3)2, which is a strong hydrogen bonding acceptor. Importantly, the observed contact angles depended on the concentration of the receptor in the self-assembly solution, confirming the increasing surface concentration of receptor molecules on the modified graphene. As shown in Table 1, the contact angles of DI water on graphene


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modified with Pyr-CH2NH2·HCl, Pyr-CH2NH2, and Pyr-CH2COOCH3 were decreasing as the concentration of the receptors in the self-assembly solution increased. Moreover, the contact angle data illustrate that graphene modified with Pyr-CH2NH2·HCl, Pyr-CH2NH2, PyrCH2COOCH3, and Pyr-CH2COOH became more hydrophilic than bare graphene, which is consistent with the polarity of these receptor compounds (see Table 1 and Table S1). Interestingly, graphene modified with pyrene became more hydrophobic, which may be explained by the graphene samples having a small concentration of OH or COOH groups,64 making them slightly less hydrophobic than pyrene. This is consistent with the observation that use of the 10 mM NaOH solution to characterize the pyrene monolayers on graphene resulted in larger contact angle changes than use of DI water. According to Langmuir adsorption theory,65 monolayer modification of graphene can be controlled by varying the concentration of the adsorbate in the bulk of the self-assembly solution:

θ=

K *C 1+ K *C

(1)

where θ is the fractional surface coverage, C is the concentration of the adsorbate in the bulk of the self-assembly solution, and K is the equilibrium constant for adsorption of the adsorbate to graphene. Experimentally, the surface coverage can be expressed by the change in contact angle between bare graphene and modified graphene:

θ=

Φ ( i ) - Φ ( bare ) Φ ( sat.) - Φ ( bare )

(2)

where Φ(i) is the contact angle of the modified graphene as a function of the concentration in the self-assembly solution, Φ(bare) is the contact angle of bare graphene, and Φ(sat.) is the contact


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angle of graphene modified with a complete monolayer of receptor molecules (i.e., 100% surface coverage). Insertion of θ from eq 2 into eq 1 and solving for Φ(i) gives eq 3:

Φ ( i ) = Φ ( bare ) +

K ∗ C *[Φ ( sat.) - Φ ( bare )] (1 + K ∗ C )

(3)

This shows that the experimentally observed Φ(i) values can be fitted as a function of receptor concentration in the self-assembly solution, using the two fitting parameters K and Φ(sat.). Once these two parameters have been determined, relative surface coverages at different self-assembly concentrations can be predicted from eq 2, using for Φ(i) the right hand part of eq 3. Figure 1 shows a fit of the surface coverage of Pyr-CH2OH monolayers on graphene as a function of the Pyr-CH2OH concentration in the self-assembly solution (or, for the inset, its logarithm). The experimental data are in good agreement with Langmuir theory (see Figure S3–S6). As shown in Table 2, the concentrations needed for 90% monolayer coverage for all pyrene derivatives are estimated to be in the relatively narrow range of 0.30-3.6 mM, independently of whether self assembly was performed form acetonitrile or ethanol solution. The equilibrium constants varied between 103.4 and 104.6 M-1, values which are of the same order of magnitude as reported for other adsorption equilibria based on similar π-π stacking interactions.39, 66-68 For example, using an electrochemical method, the equilibrium constant for adsorption of 1pyrenebutyric acid N-hydroxysuccinimide ester from N,N-dimethylformamide solution to multiwalled carbon nanotubes was estimated to be 102.3 M-1.32 Importantly, the good fits based on the Langmuir adsorption theory confirm the formation of monolayers. Adsorption isotherms with different shapes would be observed if multilayers were formed, and the surface concentration of the pyrene derivative would not reach saturation.45


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Graphene substrates modified with Pyr-CH2NH2 were also characterized by XPS (Table S2). As the concentration of the self-assembly solution increased, the Cu mol % decreased, and the C and N mol % increased, confirming that the surface coverage of Pyr-CH2NH2 on graphene increased. The fit of the C mol % and Cu mol % data gave the equilibrium constant for adsorption of Pyr-CH2NH2 onto graphene as 104.29 M-1 (104.14 – 104.40 M-1). As shown in Table 2, by contact angle measurements, the equilibrium constant for adsorption of Pyr-CH2NH2 to graphene was determined as 103.81 M-1 (103.72 – 103.89 M-1). The presence of a small amount of Pyr-CH2NH2 in between the Cu substrate and the graphene may explain the small bias between the two values. As briefly mentioned previously in a report on glucose-sensitive graphene quantum capacitance varactors, Pyr-B(OH)2 too forms monolayers on graphene.51 In that case, a probing solvent that gave rise to sufficiently large contact angle changes could not be found, and XPS was used instead (see also Table S3 of the Supporting Information).

3.2 Graphene modification with perbenzylated α-, β- and γ-cyclodextrins Graphene samples modified with α-CDBn18, β-CDBn21, and γ-CDBn24 were characterized by XPS since the effect of monolayer formation only resulted in minimal changes in contact angle. For bare graphene grown on copper, the observed elemental surface composition was found to be 56.3 mol % carbon, 11.0 mol % oxygen, and 32.6 mol % copper. As shown in the high resolution O1s XPS spectrum (Figure 2b), four types of oxygen were observed, i.e., Cu–O (530.4 eV), C–O (531.4 eV), C=O (532.1 eV), and O–C=O (532.9 eV). The Cu–O peak was the largest, and integration of these peaks suggested that 61.2% oxygen came from the underlying copper substrate (Cu–O).69-71 The high resolution C1s XPS spectrum of bare graphene reveals five types of carbon atoms, which can be assigned to C=C (284.5 eV), C–OH (285.4 eV), C–O–


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C (286.4 eV), C=O (287.4 eV) and O–C=O (288.9 eV). This suggests the presence of a small amount of hydroxyl and carboxylate groups on the surface, likely due to graphene impurities (Figure 2a).64, 71 After the self-assembly of the perbenzylated cyclodextrins, the Cu percentage was reduced significantly, and the C and O percentages increased. This trend is consistent with the composition of the cyclodextrin monolayers, which is 84.4 mol % carbon and 15.6 mol % oxygen for all three cyclodextrins (Table S4–S6). The high resolution XPS spectra for C1s and O1s confirmed the surface modification by showing a much smaller Cu–O peak as compared to bare graphene and much larger peaks for the sp3 carbon and C–O–C carbon and oxygen atoms pertaining to the perbenzylated cyclodextrins (Figure 3). In analogy to the interpretation of the contact angle measurements as discussed above, the C, O, and Cu percentages as determined by XPS for the cyclodextrin monolayers were fitted as a function of the concentration of the cyclodextrins in the self-assembly solution. This provided, as fitted parameters, the C, O, and Cu mole percentages of the monolayer-modified graphene, along with the equilibrium constant for the cyclodextrin adsorption to the graphene surface. To maximize the accuracy of the fits for each cyclodextrin, the experimental C, O, and Cu mol % data were not fitted for each element independently. Instead, for each cyclodextrin, the three adsorption isotherms showing the experimentally determined C, O, or Cu mol % values as a function of the concentration of the self-assembly solution were fitted simultaneously with the analog of eq 3 and the Kronecker delta function (see Supporting Information for details), as similarly performed previously for fitting of multiple Job plots.72 This type of fitting provides more reliable fits of the equilibrium constant than individual fits of C, O, or Cu mol % data alone because it triples the number of data points used for the fit. As shown in Figure 4, plotting the data as the surface coverage versus the concentration of


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the cyclodextrin in the self-assembly solution (i.e., using again eq 2) gives three data points at each concentration, which correspond to the C, O and Cu mol %. As the plot shows, the data are in good agreement with the fit. The equilibrium constants were found to be 103.24, 102.97, and 102.95 M-1 for α-CDBn18, β-CDBn21, and γ-CDBn24, respectively, showing that overall the surface affinities of the three compounds are very similar. Somewhat unexpectedly, α-CDBn18 binds a little bit more strongly to graphene than β-CDBn21 and γ-CDBn24, even though it has fewer benzyl groups to interact with graphene. It is possible that the larger internal cavities of βCDBn21 and γ-CDBn24, and, in the case of γ-CDBn24, a less rigid macrocycle, allow the benzyl groups to interact more readily with the internal cyclodextrin cavity, making the benzyl groups less available for adsorption onto graphene than that is the case for the smaller and most rigid αCDBn18.

3.3 Stability and reversibility of the surface modification

Baking in a vacuum is a common process for the removal of water from graphene devices.73 To ensure the stability of the surface modification under heating or vacuum, graphene modified with Pyr-CH2NH2·HCl was baked overnight in a high vacuum (2 × 10-5 Torr) at 100 ºC. This was followed by contact angle measurements with DI water as the wetting liquid. Table 3 shows that the contact angles of graphene substrates modified with Pyr-CH2NH2·HCl did not change measurably as a result of the heat and vacuum exposure, suggesting that the Pyr-CH2NH2·HCl monolayer on graphene is not damaged under these conditions. To further confirm that the receptor layer was still on the graphene substrates, XPS measurements were performed. As shown by Table S7, the elemental surface composition of graphene substrates modified with PyrCH2NH2·HCl did not change after the heat and vacuum exposure.


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The reversibility of the non-covalent surface modification was also assessed by contact angle measurements. Instead of applying a potential to the modified surface24 or heating to a high temperature (600 ºC)29 to remove receptors by evaporation, as reported elsewhere, we immersed Pyr-CH2NH2 modified graphene into toluene for 1 h (replacing the toluene once after 0.5 h), and took contact angle measurements before and after the toluene immersion. The increase in contact angle from 85.5º ± 1.4º with the monolayer modification to 91.6º ± 0.5º after toluene immersion shows that, as expected, the toluene removed the receptor monolayer from graphene, exposing again the bare graphene surface. A control experiment was also performed immersing bare graphene into ethanol overnight and subsequently into toluene for 1 h. The contact angles were found to be 91.3º ± 1.5º and 92.5º ± 1.9º after the ethanol immersion and the toluene immersion, respectively, confirming that toluene did not damage the graphene-coated Cu substrates. XPS data also confirmed that the toluene exposure removed most of the Pyr-CH2NH2 but not all (Table S8). It is conceivable that small amounts of Pyr-CH2NH2 were trapped in between the graphene and the underlying Cu substrate and could not be removed readily by the toluene washing, as similarly suggested by the adsorption isotherm data (see Section 3.1). This may be an effect enhanced by ripples in the graphene monolayer74, which form as the copper contracts while graphene expands upon the cooling that follows the graphene synthesis at 1050 ºC, and could be an effect unique to this type of graphene/substrate combination used in this work.

4. CONCLUSIONS In summary, graphene was modified with monolayers of ten pyrene and cyclodextrin derivatives through self-assembly. The adsorption of the receptors onto graphene is reversible and consistent with Langmuir type adsorption. Fitting of contact angle and XPS data provided for each type of monolayer the equilibrium constant for adsorption onto graphene, which permits


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the calculation of the concentrations of these compounds in the self-assembly needed to achieve a specific monolayer coverage. The equilibrium constants fell in the range from 102.95 M-1 to 104.60 M-1, and the concentrations needed for 90% coverage were estimated to be between 0.30 and 10 mM. This shows that for the formation of such monolayers on graphene, self-assembly solutions of higher concentrations are needed than some investigators in the field appear to appreciate. In addition, stability and reversibility tests showed that self-assembled monolayers of Pyr-CH2NH2·HCl on graphene were not affected by heating (100 ºC) in a vacuum (2 × 10-5 Torr). Monolayers of Pyr-CH2NH2 could be removed from graphene by toluene immersion, but more detailed investigations may be needed in cases where quantitative removal from both the graphene and the underlying substrate is required. We believe that the quantitative recipes for the formation and characterization of receptor monolayers reported here should be useful to researchers working not only with graphene but also carbon nanotubes, HOPG, and other aromatic carbon surfaces, who have embraced the concept of non-covalent surface modification but rarely attempt optimization and confirmation of the self assembly. In view of chemical sensing, the receptors monolayers introduced here provide for a wide range of interactions between the receptor monolayers and analytes, including hydrogen bond acceptor, hydrogen bond donor, dipole–dipole, ligation, and steric repulsion interactions. Therefore, we expect these types of monolayers to be particularly useful for the fabrication of graphene-based sensing arrays.

ACKNOWLEDGMENT E.G.S. acknowledges an NSF REU summer fellowship (CHE-1359181). Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial


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support from NSF through the MRSEC program (DMR-1420013). This project was funded partially through Boston Scientific and the Medical Device Center, University of Minnesota. ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge: Synthesis of pyrene and cyclodextrin derivatives. Contact angle measurements. XPS analysis data and adsorption isotherms for monolayer-modified graphene. AUTHOR INFORMATION

Corresponding Author * Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Table 1. Contact angle of DI water on bare graphene and graphene modified with PyrCH2NH2·HCl, Pyr-CH2NH2, or Pyr-CH2COOCH3. Contact angle (º)

Concentration of self-assembly solution (mM)

Pyr-CH2NH2·HCl

Pyr-CH2NH2

Pyr-CH2COOCH3

0

92.0 ± 1.2

90.7 ± 3.9

90.7 ± 3.9

0.030

88.0 ± 2.2

89.3 ± 3.1

90.3 ± 2.2

0.10

81.0 ± 1.4

86.5 ± 1.2

85.7 ± 1.0

0.30

76.8 ± 2.2

81.8 ± 4.2

84.0 ± 2.3

1.0

76.6 ± 2.3

80.0 ± 4.3

81.7 ± 1.9

10

74.0 ± 1.2

79.2 ± 3.4

80.2 ± 2.6


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Table 2. Equilibrium constants and monolayer concentrations for adsorption of receptors to graphene. Log K Receptors

-1

(Log M )

Self-assembly solvent

Concentration of selfassembly solution needed for 90% surface coverage (mM)

Pyrene

3.87 (3.76-3.96)†

Acetonitrile

1.2

Pyr-CH2OH

3.76 (3.69-3.83)†

Acetonitrile

1.6

Pyr-CH2COOH

4.49 (4.28-4.63)†

Ethanol

0.30

Pyr-CH2COOCH3

3.79 (3.66-3.90)†

Ethanol

1.5

Pyr-CH2NH2·HCl

4.13 (4.03-4.21)†

Ethanol

0.70

Pyr-CH2NH2

3.81 (3.72-3.89)†

Ethanol

1.4

Pyr-CH2N(CH2CH3)2

4.60 (4.47-4.70)†

Ethanol

0.30

Pyr-B(OH)2

3.40 (2.97-3.61)‡

Acetonitrile

3.6a

α-CDBn18

3.24 (3.09-3.36)‡

Acetonitrile

5.2

β- CDBn21

2.97 (2.86-3.05)‡

Acetonitrile

9.6

γ-CDBn24

2.95 (2.84-3.04)‡

Acetonitrile

10

†

Determined by contact angle measurements. ‡ Determined by XPS; a From ref. 42.


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Table 3. Stability of graphene modified with Pyr-CH2NH2·HCl towards heat and vacuum treatments. Water contact angle (o) Modification conditions No heating/vacuum

After heating in a vacuum (100 ºC, 2 × 10-5 Torr)

Graphene immersed into ethanol overnight

92.0 ± 1.0

N/A

Graphene immersed into 0.10 mM Pyr-CH2NH2·HCl overnight

81.0 ± 1.4

81.4 ± 2.6

Graphene immersed into 1.0 mM Pyr-CH2NH2·HCl overnight

76.6 ± 2.3

77.2 ± 1.6


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Figure 1. Experimental relative monolayer coverage (red dots) along with a fit based on Langmuir adsorption theory (solid line) for adsorption of Pyr-CH2OH onto graphene, as determined from contact angle measurements. Inset: Plot of the relative surface coverage versus the logarithm of the concentration of the self-assembly solution.


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Figure 2. (a) High-resolution C1s XPS spectrum (black line) and fitted peaks (colored lines) of bare graphene. (b) High-resolution O1s XPS spectrum (black line) and fitted peaks (colored lines) of bare graphene.


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Figure 3. (a) High-resolution C1s XPS spectrum (black line) and fitted peaks (colored lines) of graphene modified with α-CDBn18 (3.0 mM). (b) High-resolution O1s XPS spectrum (black line) and fitted peaks (colored lines) of graphene modified with α-CDBn18 (3.0 mM).


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Figure 4. Experimental relative monolayer coverage (red dots) along with a fit based on the Langmuir adsorption theory (solid line) for adsorption of β-CDBn21 onto graphene, as determined from XPS data. Inset: Plot of the relative surface coverage versus the logarithm of the concentration of the self-assembly solution.


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TOC


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Noncovalent Monolayer Modification of Graphene Using Pyrene and

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