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Nanoscale pH Profile at a Solution/Solid Interface by Chemically Modified Tip-Enhanced Raman Scattering Prompong Pienpinijtham,*,†,‡ Sanpon Vantasin,† Yasutaka Kitahama,† Sanong Ekgasit,‡ and Yukihiro Ozaki*,† †

Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan Department of Chemistry, Faculty of Science, Chulalongkorn University, Pathumwan, Bangkok 10330, Thailand

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S Supporting Information *

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). p-Mercaptobenzoic acid (pMBA) and p-aminothiophenol (pATP) are bonded to the tip surface. A pH change can be detected from a peak at 1422 cm−1 due to the −COO− stretching 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 surfaceenhanced Raman scattering (SERS).9−19 SPM, e.g., atomic force microscopy (AFM) and scanning tunneling microscopy (STM), provides topological information on 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. © 2016 American Chemical Society

In this work, a TERS tip was chemically modified with pHsensitive molecules i.e., p-mercaptobenzoic acid (pMBA) and paminothiophenol (pATP), which are topics of much 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.

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EXPERIMENTAL METHOD Chemicals. 3-Aminopropyltrimethoxysilane (APTMS) was purchased from Sigma-Aldrich Co. p-Mercaptobenzoic acid (pMBA), p-aminothiophenol (pATP), and other chemicals were purchased from Wako Pure Chemical Industries, Ltd. All chemicals were used as prior without any further purification. Deionized (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 Received: April 5, 2016 Revised: June 18, 2016 Published: June 21, 2016 14663

DOI: 10.1021/acs.jpcc.6b03460 J. Phys. Chem. C 2016, 120, 14663−14668

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solution (conc H2SO4:30% H2O2 = 3:1) at 80 °C for 1 h 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 h. In this step, an amino group (−NH2) was 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 1 h. 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 μm. 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 (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. To measure local pH on a surface, −NH2 groups were anchored on a glass slide surface using 3-aminopropyl14664

DOI: 10.1021/acs.jpcc.6b03460 J. Phys. Chem. C 2016, 120, 14663−14668

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The Journal of Physical Chemistry C

Figure 2. Plot of the ratio of two peak areas at 1442 and 1079 cm−1 versus the pH of solutions. The inset shows a change in molecular structure of pATP attached on TERS tip.

versus the distances are shown in Figure 3. At a distance of 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 nonmodified areas can be considered ∼400 nm (see 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 nonmodified tip (see 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

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CONCLUSIONS 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 pATPmodified 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 nonmodified 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.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03460. Experimental details, peak assignments for pMBA and pATP on modified TERS tips, TERS spectra collected from pATP-modified TERS tips in different pH solutions, plot of peak ratios measured from TERS spectra of pMBA-modified tip versus distance across the interface between modified and nonmodified surfaces, normal Raman spectrum of nonmodified/NH2-anchored glass slides and TERS spectrum collected from non-

Figure 4. (A) AFM image of the glass slide where a half of surface was anchored by a −NH2 group. (B) Height of the surface along selected line scan on x-axis (y = 0.5 μm) in (A). (C) 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 (A). 14666

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modified Ag tip attached to the surface of an NH2anchored glass slide (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (Y.O.). *E-mail [email protected] (P.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from Center of Innovative Nanotechnology, Chulalongkorn University (CIN-CU), National Research Council of Thailand (NRCT), National Research University Project, Office of Higher Education Commission (WCU-018-FW-57), and Thailand Research Fund (TRG5780158). Sanpon Vantasin also thanks the Yoshida Scholarship Foundation for the funding support.

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ABBREVIATIONS TERS, tip-enhanced Raman scattering; pMBA, p-mercaptobenzoic acid; pATP, p-aminothiophenol; DMAB, 4,4′dimercaptoazobenzene; AFM, atomic force microscopy; APTMS, 3-aminopropyltrimethoxysilane; EDL, electronic double layer.

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