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Nanoscale pH-Profile at a Solution/Solid Interface by Chemically Modified Tip-Enhanced Raman Scattering (TERS) Prompong Pienpinijtham, Sanpon Vantasin, Yasutaka Kitahama, Sanong Ekgasit, and Yukihiro Ozaki J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03460 • Publication Date (Web): 21 Jun 2016 Downloaded from http://pubs.acs.org on June 25, 2016

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The Journal of Physical Chemistry

Nanoscale pH-Profile at a Solution/Solid Interface by Chemically Modified Tip-Enhanced Raman Scattering (TERS) Prompong Pienpinijtham,a,b,* Sanpon Vantasin,a Yasutaka Kitahama,a Sanong Ekgasitb and Yukihiro Ozakia,* a

Department of Chemistry, School of Science and Technology, Kwansei Gakuin University,

Sanda, Hyogo 669-1337, Japan. E-mail: [email protected] b

Department of Chemistry, Faculty of Science, Chulalongkorn University, Pathumwan, Bangkok

10330, Thailand. E-mail: [email protected]

1 Environment ACS Paragon Plus


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ABSTRACT. A nanoscale pH-profile on a 4×4 µm2 area of NH2-anchored glass slide in an aqueous solution is constructed using chemically modified tip-enhanced Raman scattering (TERS). Para-mercaptobenzoic acid (pMBA) and para-aminothiophenol (pATP) are bonded to the tip surface. A pH change can be detected from a peak at 1422 cm-1 due to the –COOstretching vibration from pMBA and that at 1442 cm-1 due to the N=N stretching vibration arising from the formation of 4,4′-dimercaptoazobenzene (DMAB) on the pATP-modified tip. The pMBA- and pATP-modified tip can be used to determine pH in the range of 7–9 and 1–2, respectively. The spatial resolution to differentiate pH of two areas can be considered as ~400 nm. The measured pH becomes the pH of the bulk solution when the tip is far by ~200 nm from the surface. This technique suggests a possibility for the pH sensing in wet biological samples. TERS tips could also be chemically modified with other molecules to determine other properties in a solution.


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INTRODUCTION Over the past 15 years, extensive developments in tip-enhanced Raman scattering (TERS) have greatly benefited studies on molecular structures and interactions in the world of nanotechnology.1-8 TERS is a technique that combines the advantages of scanning probe microscopy (SPM) and surface-enhanced Raman scattering (SERS).9-19 SPM, e.g. atomic force microscopy (AFM) and scanning tunneling microscopy (STM), provides topological information of a sample surface in the nanoscale region, while SERS yields molecular information, such as molecular structure, inter/intramolecular interactions, and chemical reactions on trace molecules attached to a rough metal surface.20-21 These two features can be obtained simultaneously at a given position by TERS. TERS applications have been demonstrated in many ways10,

12-15

including few- or single-molecule studies,14, 22-24 RNA sequencing,25-26 mechanistic studies of biological molecules,27-28 carbon nanotube and graphene characterization,19,

29-31

and TERS

imaging/mapping.15, 17-18, 32-34 Most of the TERS measurements were carried out to investigate dry samples. Only a few works involving TERS measurements in solution/liquid systems have been reported due to a number of limitations including problems with laser focusing, sample preparation, the feedback system of SPM, and adsorbed contaminants on the TERS tip.35-39 However, the use of TERS in solutions is still very important, especially in the studies of biological molecules or living samples in wet environments. Moreover, some properties, such as pH, appear only in aqueous systems. In this work, a TERS tip was chemically modified with pH-sensitive molecules i.e., paramercaptobenzoic acid (pMBA) and para-aminothiophenol (pATP), which are topics of much



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attention in several papers on pH/chemical probes using SERS techniques.40-45 For pMBA, a – COOH group transforms to a –COO- moiety when an acidic solution becomes alkaline. For pATP, in an alkaline solution, two pATP molecules react each other to form an azo (N=N) dimer, i.e., 4,4′-dimercaptoazobenzene (DMAB) while a pATP molecule is stable in acidic environments. EXPERIMENTAL METHOD Chemicals 3-aminopropyltrimethoxysilane (APTMS) was purchased from Sigma-Aldrich Co. Paramercaptobenzoic acid (pMBA), para-aminothiophenol (pATP), and other chemicals were purchased from Wako Pure Chemical Industries, Ltd. All chemicals were used as prior without any further purification. De-ionized (DI) water with a purity of 18 MΩ resistivity was used as a solvent. Sample preparation For measuring pH of bulk solutions, well-defined pH solutions were prepared (HCl-KCl buffers for pH 1–2, citrate buffers for pH 3–6, phosphate buffers for pH 7–8, and borate buffers for pH 9–11). To measure local pH, the surface of a glass slide was chemically modified to have an alkaline functional group. Briefly, the glass slide was cleaned by boiling in a piranha solution (conc. H2SO4:30% H2O2 = 3:1) at 80 °C for one hour in order to remove all organic contaminants on the surface. Then, it was rinsed with water for three times. A half of cleaned glass was immersed into 2% APTMS in ethanol for 24 hours. In this step, an amino group (–NH2) was


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anchored on the surface of glass slide via a Si–O–Si bond. Finally, the glass slide was rinsed with ethanol for three times to remove excess APTMS molecules. The modified glass slide was kept for further investigation. TERS Tip modification The TERS tips fabricated from a silver wire using an electrochemical etching technique were provided by UNISOKU Co., Ltd. The tip radius was approximately ∼50 nm (see an SEM image in Figure S1). The tips were chemically modified by dipping the tip apex into a 10 mM ethanol solution of pMBA or pATP for one hour. The pMBA or pATP molecules adsorbed self-assembly on a surface of the silver tip by anchoring with the Ag–S bond. Then, the tips were washed with ethanol for three times to remove excess molecules. After the chemical modification, the tips were employed as AFM tips to measure the topology of samples and as pH-probe tips to measure local pH at very small specific areas. Moreover, the tip modification also help to protect the tip surface from oxidation (from O2 in the atmosphere) and adsorbed contaminants. Each tip was employed for full-ranged pH measurement. TERS measurement TERS spectra were measured at a spectral resolution of 4 cm-1 by using an AFM/Raman instrument (Photon Design Nanostar NFRSM800). An 514.5 nm diode-pumped solid-state (DPSS) laser (Cobolt Fandago 25) was used for the excitation. The diameter of the laser spot on a sample surface was approximately 1 micrometer. The TERS setup was top-illumination and top-collection using the same objective lens (×90 magnification, 0.71 NA). This setup provides the advantage of no transparent sample requirement. Surface topology was acquired by noncontact mode AFM (UNISOKU Co., Ltd.) with a 45° angle between the tip and sample surface



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(see TERS measurement setup in Figure S1). The feedback mechanism is a frequency modulation mode. The estimated tip-sample separation in the noncontact mode was 2, the peak at 1442 cm-1 slightly increases with an increase in pH. This confirms that DMAB is more stable at higher pHs.

Figure 3. A plot of the ratio of two peak areas at 1422 and 1586 cm-1 measured from TERS spectra of pMBA-modified tip versus a distance from the surface of an NH2-anchored glass slide. To measure local pH on a surface, –NH2 groups were anchored on a glass slide surface using 3-aminopropyltrimethoxysilane (APTMS). The NH2-anchored glass slide and the pMBAmodified tip were immersed in water and TERS spectra were collected from the tip at different distances from the surface. The ratios of the peaks at 1422 and 1586 cm-1 versus the distances are shown in Figure 3. At a distance of < 1 nm, the average peak ratio is about 0.12, which indicates a pH of ~8 (comparing the value with Figure 1B). It is higher than pH of pure water (pH 6–7) because the –NH2 group drives the dissociation of water (H2O ↔ H+ + OH-) towards. Then, the –


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NH2 groups are protonated by H+ to form –NH3+ ions while more OH- ions exist in the solution, which raises up the pH. When the tip is moved away from the surface, the intensity ratio decays exponentially. From the plot, we can determine that the distance that can be reached by the OHis less than 200 nm. This is because the –NH2 group is a weak base, which does not fully ionize and instead exists in chemical equilibrium. Hence, OH- generated by the –NH2 group cannot move far away from the surface.52 However, the thickness of the electronic double layer (EDL) is approximately a few nanometers from the surface.53 These results go beyond the EDL thickness. In this case, the ions related to pH are H+ and OH-, which can be generated spontaneously via the auto-ionization of water. OH- is not alien ions that adsorb on a surface and diffuse into a bulk solution. OH- generated by the –NH2 group on the surface could induce the auto-ionization of further water molecules. Therefore, the OH- detected at a distance further than the EDL thickness would not be OH- generated by the –NH2 group but it should be OH- from the auto-ionization. It explains why the pH effect from the surface can go further than the thickness of the electronic double layer. This phenomenon is also found in other works.54 Moreover, in close tip-sample proximity, finite size effects in the confined liquid volume between the tip and the sample can change the local free energy landscape and thus locally modify the pH value. At distances further than 200 nm, the average intensity ratios are below 0.05 or the pH is approximately 6–7, which is the normal pH of pure water (bulk aqueous solution).



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Figure 4. (A) An AFM image of the glass slide where a half of surface was anchored by –NH2 group. (B) The height of the surface along selected line scan on x-axis (y = 0.5 µm) in Figure 4A. (C) A contour map constructed by plotting the ratio of two peak areas at 1422 and 1586 cm-1 measured from TERS spectra of pMBA-modified tip versus corresponding positions (+) in Figure 4A. To demonstrate how to construct nanoscale pH-profile, half the area of a glass slide surface was anchored with –NH2 groups. The topology at the interface between modified and non-modified areas was monitored using AFM technique with the pMBA-modified tip, as shown


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in Figure 4A. The AFM image shows two areas of contrasting roughness. This roughness comes from the deposition of APTMS on the glass slide. The difference in height between these two areas is about 35.8 nm. Figure 4C exhibits a contour map constructed by plotting the ratio of two peak areas at 1422 and 1586 cm-1, measured from TERS spectra of the pMBA-modified tip, versus the corresponding mark (+) positions in Figure 4A. At non-modified area (smoother surface), the ratio is around 0.05, with a corresponding pH of approximately 6–7, the normal pH of pure water. On the other hand, at the NH2-anchored surface, the ratio is > 0.1. This implies alkalinity of pH 8 from the –NH2 group on the surface. The variation on the contour map might be from the inhomogeneity of APTMS deposition. However, the result from the contour map is in good agreement with the results of the AFM image. The spatial resolution of this method in differentiating modified and non-modified areas can be considered ~400 nm (see Supporting Information, Figure S3). In addition, using a pMBA-modified TERS tip provides useful information that cannot be obtained from normal Raman scattering or TERS with a non-modified tip (see Supporting Information, Figure S4). This work shows that the chemical modification of TERS tip is useful. It opens the door to use TERS tips with other molecules for investigating other properties, especially in solution systems. Nevertheless, for future practitioners who would like to construct TERS mapping/imaging, there are some important issues that must also be considered such as surface vibrational transition dipole orientation, incident and detected optical field polarization, and near-field plasmonic spatial field distribution.55-56 CONCLUSIONS



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In conclusion, a chemically modified TERS tip was used to obtain local pH on a nanoscale surface. The pMBA-modified tip is sensitive to the pH range of 7–9, where the molecular structure transforms from –COOH to –COO-. The pATP-modified tip can be used to determine pH in the range of 1–2 through DMAB formation. This method can determine the acidity/alkalinity at a solution/solid interface. At 200 nm away from the surface, it is a critical distance that functional groups on a surface do not affect the pH of solutions. In this method, the spatial resolution for differentiating between NH2-modified and non-modified areas can be considered as ~400 nm. The modified tip is stable and gives a strong TERS signal. TERS measurements in a solution can also be performed at higher laser power than that with dry samples. This proposed method has a large number of potential applications, including pH sensing in biological samples such as cells and organisms, and in situ measurement of other properties in a solution by using tips modified with other molecules. ASSOCIATED CONTENT Supporting Information. Experimental detail, peak assignments for pMBA and pATP on modified TERS tips, TERS spectra collected from pATP-modified TERS tips in different pH solutions, the plot of peak ratios measured from TERS spectra of pMBA-modified tip versus distance across the interface between modified and non-modified surfaces, normal Raman spectrum of non-modified/ NH2-anchored glass slides and TERS spectrum collected from nonmodified Ag tip attached to the surface of an NH2-anchored glass slide. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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*to

whom

correspondence

should

be

addressed.

E-mail:

[email protected];

[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. ACKNOWLEDGMENT The authors gratefully acknowledge financial supports from Center of Innovative Nanotechnology, Chulalongkorn University (CIN-CU), National Research Council of Thailand (NRCT), National Research University Project, Office of Higher Education Commission (WCU018-FW-57), and Thailand Research Fund (TRG5780158). Sanpon Vantasin also would like to thank Yoshida Scholarship Foundation for the funding support. ABBREVIATIONS TERS, tip-enhanced Raman scattering; pMBA, para-mercaptobenzoic acid; pATP, paraaminothiophenol; DMAB, 4,4′-dimercaptoazobenzene; AFM, atomic force microscopy; APTMS, 3-aminopropyltrimethoxysilane; EDL, electronic double layer. REFERENCES 1.

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