Specific Hydration on p-Nitroaniline Crystal Studied by Atomic Force

Jan 21, 2013 - Specific Hydration on p-Nitroaniline Crystal Studied by Atomic Force Microscopy. Rina Nishioka, Takumi Hiasa*, Kenjiro Kimura, and Hiro...
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Specific Hydration on p‑Nitroaniline Crystal Studied by Atomic Force Microscopy Rina Nishioka, Takumi Hiasa,* Kenjiro Kimura, and Hiroshi Onishi Department of Chemistry, Graduate School of Science, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan S Supporting Information *

ABSTRACT: The molecular-scale structure of water was studied over the (101) surface of p-nitroaniline crystals using advanced atomic force microscopy. p-Nitroaniline contains two polar groups on opposite ends of the nonpolar benzene ring and presents a surface of controlled heterogeneity. The crosssectional distribution of force applied to the tip was precisely determined and was related to the local density of the structured water. Force modulations were present on the polar end-groups and absent on the benzene ring, suggesting water localization on the polar end-groups.

1. INTRODUCTION Interfacial water plays an important role in a broad range of chemistry, biology, and material sciences.1,2 Chemical functional groups of solid surfaces are believed to play a key role in structuring interfacial water, and are often categorized as hydrophilic or hydrophobic groups. Specific hydration of hydrophilic groups is thought to have important consequences for wetting behavior, the stability of the proteins, and biochemical functions. In this study, the molecular scale structure of water facing p-nitroaniline (H2NC6H4NO2) crystals was investigated. Crystalline p-nitroaniline has been widely investigated over the years;3−6 it has two polar groups, amino and nitro, present on opposite ends of a nonpolar benzene ring. We expect different hydration structures on the two end-groups versus the hydrophobic core. By observing the hydration structure over the surface of controlled heterogeneity, our knowledge about how interfacial water is structured on organic compounds can be improved. This is an important issue in dissolution and crystallization related to applications such as dyes, fertilizers, and pharmaceutical materials.7 It is not an easy task to experimentally observe the structure of liquid when interfaced with solid. Here, we employed frequency-modulation atomic force microscopy (FM-AFM). Recent developments8 enable us to trace the topography of a solid object as well as the local structure of the liquid facing the solid. The force applied to the tip is determined as a function of the tip−surface distance. Using advanced FM-AFM, the subnanometer resolution topography of water-soluble crystalline calcite9 and lysozyme10 was determined in an aqueous solution. In addition to topography, the interfacial water structure can be deduced from the tip−surface force distribution as a function of the vertical and lateral coordinates. This has been demonstrated for mica,11,12 sapphire,13 proteins,11 lipids,14 and hydrophilic monolayers.15 © 2013 American Chemical Society

2. EXPERIMENT In this study, crystals of p-nitroaniline were prepared by recrystallization. A saturated solution of p-nitroaniline (Tokyo Chemical Industry, >98%) was prepared in ethanol (Wako, >99.5%). Flat rhombic yellow crystals, which were typically 5 mm in width and 1 mm in thickness, were precipitated in 50 days. The prepared crystal was fixed on a fluorocarbon polymer plate for AFM scans in liquids.16 The plate was installed on the piezo stage of a microscope, covered by a purified water (Milli-Q) droplet and the cantilever assembly. All the AFM measurements were carried out at room temperature. A Shimadzu SPM 9600 microscope was modified to reduce the noise of cantilever deflection to as low as 20 fm Hz−1/2 by increasing the sensitivity with a higher laser power than is used in conventional AFMs.8 Silicon cantilevers (Nanosensors, NCH) with a nominal spring constant of 40 N m−1 were backside-coated with gold. Typically, the resonant frequency was 130 kHz and the quality factor (Q) of resonance was 10 in water. To stabilize a self-oscillation loop in the low-Q environment, bandpass filters (Q = 20) were inserted in the feedback loop. The absolute deflection of the cantilevers was calibrated using the theoretical amplitude of thermal Brownian motion of the cantilevers. The rate of thermal drift of the tip relative to the surface was checked, and the drift-induced distortion of topography was negligible, unless otherwise stated. The piezoelectric xyz-scanner was calibrated on mica in the lateral (xy) dimension, and on rutile TiO2(110) in the vertical (z) dimension. 3. RESULTS AND DISCUSSION 3.1. Topography. We start with a discussion of the topography of the prepared p-nitroaniline crystal facing water. Received: November 29, 2012 Revised: January 17, 2013 Published: January 21, 2013 2939

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Figure 1. Terraces and steps on a p-nitroaniline crystal in water. (a) Wide-area topography observed with a cantilever oscillation amplitude of 0.3 nm and a frequency-shift set point of +70 Hz. The acquisition time was 90 s frame−1. (b) A cross-sectional height profile averaged over the boxed area in image a.

one hydrogen of the amino group protrude from the plane. The alternately tilted molecules are symmetric with respect to the (010) plane perpendicular to the (101) surface. We can therefore expect a topography with two equivalent partial structures in one unit cell. This was what was found in the appearance of Figure 2b. With this interpretation, the spots of equal brightness represent the edges of a benzene ring protruding from the surface plane. The hydrophilic end-groups are located in the topographic pits. Consider the major appearance a with nonequivalent partial structures, i.e., bright and less bright protrusions. The apex of tips often plays an important role in creating AFM topography.17−19 We ascribe the different brightness to asymmetry in the scanning tip apex. Suppose that our tip apex was asymmetric with respect to the surface normal; the alternately tilted molecules would then present an uneven topography. In order to support the possible contribution of an asymmetric tip apex, a force simulation was conducted and is described in the Supporting Information. The limited probability for the symmetric appearance can be ascribed to the limited chance for a tip apex to be symmetric. Minor changes of the nonequivalent appearance were sometimes observed and ascribed to minor tip changes. An example of minor-changed appearance was added to the Supporting Information as Figure S2. 3.2. Hydration Structure. As the p-nitroaniline molecule lies with its molecular axis parallel to the (101) surface, the hydrophilic end-groups and the hydrophobic core of each molecule are exposed to water. The end-groups and the core are expected to interact differently with water. In order to determine the interfacial water structure on the (101) facet, cross-sectional Δf distributions were obtained using the following procedure. The tip was scanned vertically from the bulk liquid to the surface. The frequency shift was simultaneously recorded as a function of the vertical coordinate to obtain one Δf−distance curve at one lateral position. The vertical scans aborted when Δf reached +300 Hz to prevent severe contact with the surface. The tip then laterally shifted and another vertical scan was carried out. By repeating Δf−distance curve measurements along a lateral coordinate, a two-dimensional Δf distribution cross-sectional to the surface was constructed. Figure 3a presents Δf distributions cross-sectional to the (101) facet scanned by the asymmetric tip which provided the asymmetric topography shown in Figure 2a. The distribution was acquired along the [101]̅ direction. Positive (or negative) Δf is shown to be bright (or dark) in the image. Δf relates to the force applied to the tip on the basis of the Sader−Jarvis transformation.20 Here we can regard the Δf distribution as an approximation of the force distribution.

Topography was traced by regulation of the tip−surface distance to keep the shift of the resonant oscillation frequency (Δf) constant. Figure 1a shows a large-area topography of the p-nitroaniline crystal in water. By immersing in water, the surface of the crystal dissolved and steps in the topography quickly emerged. Typically 90 min after immersion, flat terraces separated by stable steps were observed, as shown in Figure 1a. The stable steps suggest that the surface was under equilibrium with the solution. The average height of the observed single steps was 0.7 nm, as shown in the cross-sectional profile of Figure 1b. The lattice spacing of the (101) plane of p-nitroaniline was determined to be 0.70 nm in an X-ray diffraction study.4 The observed step height reproduced this spacing. The (101) plane is known as the prominent cleavage plane of this compound and provides a well-developed facet for recrystallization.3 We thus assign the terraces as (101) planes. Narrow-area topography was repeatedly measured on the (101) terraces. Two appearance types were observed, as shown in Figure 2a,b. The thermal drift rate was 4 nm min−1 along the fast scan direction in imaging the topography of Figure 2b. This was not negligible, and a drift-corrected image is shown as Figure 2b. A rectangular unit cell, shown by the dotted line in each panel, was identified as being common to the two appearances. From the X-ray diffraction study,4 a rectangular unit cell of 0.61 nm in the [010] direction and 1.52 nm in the [101]̅ direction is predicted for the (101) plane. The size of the observed unit cell was 0.6 nm ×1.5 nm and consistent with the crystallographic ones. Figure 2a shows the major appearance of the topography, which presents a bright and a less bright spot in each unit cell. In limited cases, the brightness of the two spots was nearly equivalent, as shown in Figure 2b. The nonequivalent or equivalent appearance is clearly seen in the cross-sectional height profiles of panel c or d. The bright and less bright spots were on line i of appearance a, and the spots of equivalent brightness were on line iii of appearance b. The depth of the topographic pits was also sensitive to the appearance. Pits in appearance a presented two different depths on line ii. Those in appearance b were of equal depth. We ascribe the presence of major and minor appearances to a tip-induced change in the topography, as described below. Figure 2e illustrates the structure of the (101) plane based on a diffraction study4 without relaxation. The amino group of one molecule is hydrogen-bonded to the nitro group of a neighbor. The head-to-tail hydrogen-bonding should be the origin of the stability of the (101) plane. The 0.6 × 1.5 nm2 unit cell contains two molecules with their benzene rings alternately tilted by ±24° relative to the (101) plane. Due to the tilt, one side of the benzene ring, one oxygen of the nitro group, and 2940

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Figure 2. Subnanometer topography of p-nitroaniline crystal in water. Two appearances were observed as shown in a and b with cross-sectional height profiles in c and d. Cantilever oscillation amplitude: 0.2 nm. Frequency-shift set point: +200 Hz. The acquisition time was 8 s frame−1. The crystallographic structure of the (101) plane is illustrated in e.

bottom distance in the oscillatory modulation was 0.4 nm. This distance is comparable to the size of a water molecule. Such force oscillations can be related to the uneven density distribution of interfacial liquids.11−15 Hence, the Δf modulations over the grooves suggested that water localized over the hydrophilic end-groups of p-nitroaniline. On the other hand, the absence of Δf modulation over the protrusion inferred that water was not stabilized on the benzene ring. A possible picture of water localized on the end-groups is shown in Figure 3c. Δf distributions were obtained on cross-sectional planes parallel to the [010] direction, perpendicular to the planes observed in Figure 3. We observed two major appearances among consecutive data acquisitions; representative results are shown in panels a and b of Figure 4. Oscillatory modulations of Δf appeared as dark-to-bright changes in raw distributions. In distribution a, the modulated regions were located to the left of the topographic protrusions, whereas those in distribution b were to the right. The Δf modulations were neither solely on the protrusions nor solely on the trenches, but alternately appeared on the left of the protrusions in distribution a and on the right in b. We ascribe the alternate shifts as water structure sensitive to the tilted hydrophilic end-groups of the p-nitroaniline molecules.

The asymmetry of the tip helped us to determine the lateral coordinate of the cross-section. In the bottom of the distributions, major and minor grooves of the brightest region (indicated with arrows in the images) periodically appeared. The separation of the neighboring major grooves was 1.5 nm, which is consistent with the periodicity of the [101̅] direction. The trace qualitatively reproduced the cross-sectional height profile ii of Figure 2c. The major and minor grooves thus correspond to the deep and shallow pits in Figure 2c, where the hydrophilic end-groups are located. Figure 3b shows Δf−distance curves over a major groove, a minor groove, and a protrusion. Four curves collected in each boxed area were averaged and depicted. Δf was modulated by oscillation over the major or minor groove at a distance of about 0.3 nm from the threshold. On the other hand, no modulation was found over the protrusion. The Δf−distance curve over the major groove was converted to a force−distance curve, using Sadar−Jarvis transformation.20 The amplitude of force oscillations was less than 100 pN. The converted curve is provided in the Supporting Information as Figure S3. The oscillatory modulations appeared in the cross-sectional distribution as dark-to-bright changes along the vertical coordinate, some of which are marked with dotted circles. The bottom-to2941

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A possible relation of hydrogen-bonded water and tilted hydrophilic end-groups is illustrated in panel c. We have previously proposed that water hydrogen bonds in a bidentate fashion to the COO− or COOH end-groups of functionalized thiolate monolayers.15 Here, we assume similar water coordination to the hydrophilic end-groups of p-nitroaniline. When a cross section of the plane i was measured, the water molecule coordinating NO2 would be present at the left of the topographic protrusion, i.e., the oxygen atom is protruding from the (101) plane. When the other cross section was created on plane ii, the water molecule would be to the right. A similar asymmetry would be expected with water coordinating to NH2 end-groups. The alternate appearance of modulated water density can be interpreted thus, although this description is speculative at this stage of the study. Further effort including tip−surface force simulations in the presence of water is required to draw a full picture of the interface.

4. SUMMARY The topography of the (101) surface of p-nitroaniline was determined in water using state-of-the-art FM-AFM. The corrugations in the constant frequency-shift topography were interpreted as arising from the physical topography of the alternately tilted molecules at the surface. A possible contribution of tip apex symmetry to the observed topography was proposed. The crosssectional Δf distribution was observed to suggest interfacial water present on the hydrophilic end-groups rather than the hydrophobic benzene ring.

Figure 3. Cross-sectional Δf distribution of water over the p-nitroaniline (101) surface shown in Figure 2a. (a) The raw data observed on a crosssectional plane parallel to the [101]̅ direction. Topographic grooves are indicated by arrows. Some Δf modulations are marked with dotted circles. Cantilever oscillation amplitude: 0.2 nm. (b) Δf−distance curves averaged in the dotted area in distribution a. The curves are shifted in the vertical direction to aid visualization. (c) Illustration of a possible water structure. The solid line represents surface topography along the [101]̅ direction. Localized water is indicated by dotted circles.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the force simulation, a minor-changed topography of the surface, and a converted force−distance curve are provided

Figure 4. (a,b) Cross-sectional Δf distributions along the [010] direction. Some Δf modulations are marked with dotted circles. Cantilever oscillation amplitude: 0.2 nm. (c) Illustration of a possible aqueous interface of p-nitroaniline. The solid line represents the topography of the (101) facet along the [010] direction. Localized water is indicated by dotted circles. (d) Schematic illustration of a possible cross-section of the Δf distributions. 2942

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in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Phone/FAX: +81-78-8035674. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The microscope used in this study was developed by the Advanced Measurement and Analysis Project of the Japan Science Technology Agency in collaboration with Masahiro Ohta, Kazuyuki Watanabe, Ryohei Kokawa, Noriaki Oyabu, Kei Kobayashi, and Hirofumi Yamada. The software for tip−surface force simulation was developed by Advanced Algorithm and Systems Co., Japan, and loaned to the authors. This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas [477] “Molecular Science for Supra Functional Systems.” T.H. was supported by the Japan Society for the Promotion of Science Fellowship.

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REFERENCES

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