- Email: [email protected]
In-situ electrochemical shell-isolated Ag nanoparticles-enhanced Raman spectroscopy study of adenine adsorption on smooth Ag electrodes
Accepted Manuscript Title: In-situ electrochemical shell-isolated Ag nanoparticles-enhanced Raman spectroscopy study of adenine adsorption on smooth Ag electrodes Author: Chao-Yu Li Sen-Yuan Chen Yong-Li Zheng Shun-Peng Chen Rajapandiyan Panneerselvam Shu Chen Qing-Chi Xu Yan-Xia Chen Zhi-Lin Yang De-Yin Wu Jian-Feng Li Zhong-Qun Tian PII: DOI: Reference:
S0013-4686(16)30603-X http://dx.doi.org/doi:10.1016/j.electacta.2016.03.065 EA 26895
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
9-1-2016 8-3-2016 10-3-2016
Please cite this article as: Chao-Yu Li, Sen-Yuan Chen, Yong-Li Zheng, Shun-Peng Chen, Rajapandiyan Panneerselvam, Shu Chen, Qing-Chi Xu, Yan-Xia Chen, Zhi-Lin Yang, De-Yin Wu, Jian-Feng Li, Zhong-Qun Tian, In-situ electrochemical shell-isolated Ag nanoparticles-enhanced Raman spectroscopy study of adenine adsorption on smooth Ag electrodes, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.03.065 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
In-situ electrochemical shell-isolated Ag nanoparticles-enhanced Raman spectroscopy study of adenine adsorption on smooth Ag electrodes Chao-Yu Li,a Sen-Yuan Chen,a Yong-Li Zheng,b Shun-Peng Chen,a Rajapandiyan Panneerselvam,a Shu Chen,c Qing-Chi Xu,c,* Yan-Xia Chen,b Zhi-Lin Yang,c De-Yin Wu,a Jian-Feng Lia,* and Zhong-Qun Tiana
a
MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, State Key Laboratory of Physical Chemistry of
Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 3605, China. b
Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, University of Science
and Technology of China, Hefei 230026, China. c
Department of Physics, Xiamen University, Xiamen 3605, China.
* Corresponding Author Tel: +86-592-2186192
Fax: +86-592-2186192
E-Mail: [email protected] and [email protected]
Graphical Abstract
ABSTRACT Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is employed to investigate the electrochemical behavior of adenine molecules on smooth Ag electrodes. To attain this goal, pinhole-free shell-isolated Ag nanoparticles (Ag SHINs) have been synthesized and then used as signal “amplifiers” in the gap-mode configuration (Ag SHINs are coupled with a Ag electrode surface). The as-prepared Ag SHINs exhibit remarkable plasmonic performance under 488, 532, and 633 nm excitations as revealed by finite-difference time-domain (FDTD) simulations and SHINERS experiments. Furthermore, wavelength-dependent SHINERS investigation of adenine on Ag electrodes is excellently combined with the electrochemical technique. With outstanding chemical stability and plasmonic property, the Ag SHINs are extraordinarily suitable for fundamental studies at various electrochemical interfaces.
Keywords: Adenine; Electrochemical SERS; SHINERS; Ag SHINs; Silver electrode
1. Introduction As a surface plasmon resonance (SPR)-based vibrational spectroscopic method, surface-enhanced Raman spectroscopy (SERS) has been widely used for the investigation of adsorption and orientation of chemical molecules at electrochemical interfaces.[1-7] To provide the large near-field enhancement, the surface of SERS substrates is needed to be roughened or nanostructured for the excitation of SPR. Meanwhile, the substrate generality is still limited to a few metals, such as Au, Ag, and Cu.[1, 5] To circumvent the limitations in SERS, in 2010, our group has invented a novel method called shell-isolated nanoparticles-enhanced Raman spectroscopy (SHINERS)[8], which paves a unique way for plasmon-enhanced spectroscopies to investigate any surface or morphology. Normally, a shell-isolated nanoparticle (SHIN) is consists of a Au nanoparticle core and an ultra-thin silica shell. The Au nanoparticle core generates the large localized field to enhance the Raman signals of probe molecules while the dielectric silica shell would protect the Au core from the direct contact with electrolyte or analytes/probes. As a new method, SHINERS has been successfully employed in the investigations of electrochemical processes,[9-11] photocatalysis,[12] and on-line detection in flowing systems.[13] However, due to the interband transition, the plasmonic performance of Au is extremely poor under the excitation below 530 nm. In contrast, Ag has the highest quality factor in the region from 300 to 1200 nm among noble metals.[14] By introducing the Ag nanosphere as the SHIN core for the plasmonic applications, in a wide spectrum range the Raman enhancement mechanism of adsorbed species could be clearly elucidated when it is combined with an electrochemical method.[15] Adenine is one of the five nucleobases and plays a vital role in life science.[16] Due to its significance, adenine has been extensively examined by several electrochemical techniques and spectroscopic methods.[16-24] Especially, plasmon-enhanced Raman scattering methods have been utilized to investigate the nucleobases in the presence of strong local field enhancement. Due to the large Raman cross section,
single-molecule detection of adenine has been obtained in SERS and tip-enhanced Raman spectroscopy (TERS) experiments. [16, 21-24] However, the previous SERS investigations of adenine were based on the ill-defined substrate surfaces, there are some reported contradictions concerning the interaction between adenine and electrode surfaces.[25] For a clear understanding of the chemical interaction between nucleobases and a metal surface, it is imperative to perform the investigations on a smooth electrode surface. Herein, we synthesized the Ag SHINs with pinhole-free character, and then simply spread them onto the smooth Ag electrodes as a plasmonic signal amplifier. Using three-dimensional finite-difference time-domain (3D-FDTD) calculations, the local electric field enhancements at hot spots under different excitations (488, 532, and 633 nm) are calculated to be more than 5 orders of magnitude. For the investigation of electrochemical behavior of adenine on Ag electrodes, electrochemical-SHINERS experiments with Ag SHINs are carried out in the neutral solution (0.1 M NaClO4 was used as the supporting electrolyte). The spectroscopic and electrochemical results are excellently correlated, thus, EC-SHINERS method with Ag SHINs exhibits a great potential in the investigation of molecular bondings at an electrochemical interface.
2. Experimental 2.1 Materials
Chloroauric acid tetrahydrate (HAuCl4·4H2O), sodium citrate, sodium borohydride (NaBH4), L-ascorbic acid (AA), and pyridine (Py) were purchased from Sinopharm Chemical Reagent Co. Ltd.; Anhydrous silver perchlorate (AgClO4), (3-aminopropyl)trimethoxysilane (APTMS) and sodium perchlorate (NaClO4) were purchased from Alfa Aesar. Sodium silicate solution and adenine were purchased from Sigma-Aldrich. All reagents were used as received without further purification. Deionised Milli-Q water (~18.2 MΩ·cm) was used throughout the study.
2.2 Instruments and electrochemical methods
The morphology and structure of the as-prepared nanostructures were characterized by TEM (JEOL JEM 1400). The SHINERS experiments were carried out using an Invia (Renishaw, UK) microscope (the laser lines are 532 and 633 nm) with a 50×, NA 0.55 objective. Meanwhile, experiments under 488 nm laser were performed using Nanophoton Raman-11 (Nanophoton, Japan) with a 50×, NA 0.45 objective. In all electrochemical and electrochemical-Raman experiments, a Pt wire and a saturated calomel electrode (SCE) were used as a counter electrode and a reference electrode, respectively. All potentials are reported with respect to SCE in this paper. The freshly smooth Ag electrode used throughout this manuscript was prepared by sequentially polishing with alumina grit size 1.0 and 0.3 µm until the electrode surface was mirror like.[11] All solutions in electrochemical and electrochemical-Raman measurements were deaerated with N2.Chronocoulumetry experiments were conducted on CHI 631B electrochemical workstation (CH Instruments, Shanghai, China).
Chronocoulometry experiments were carried out according to previous
work.[26, 27] The initial potential, E, was changed from -0.3 to -1.55 V (the potential internal was 0.05 V), and the final potential, Ef, was set at -1.55 V for the complete desorption of adenine and avoiding the extensive hydrogen evolution current. Six series experiments with adenine concentrations (c) verified from 0 to 0.01, 0.1, 0.5, 1 and 2 mM were taken. The current transients of these six series were obtained with potential step experiments and then each current-time curve was integrated to achieve the charge-time plot. The linear part of the charge-time plot was extrapolated to zero time point, and then the relative charge density, Δσc, was obtained. Next, the relative charge density-potential plot was integrated, and then we subtract the resulting value of pure electrolyte (the concentration of adenine is 0 mM) from the value of solution containing adenine at the same potential. Hence, the film pressure (π) was calculated according to:
E
E
cdE c 0dE , Ef
Ef
(1)
where the subscript c=0 represents the pure electrolyte solution. Because a mechanically polished mirror-like Ag electrode was used rather than a single crystal electrode, the surface amount of adsorbed adenine (n) was employed to account for the surface coverage. Finally, the value of n was calculated according to:
n
1 ( )E, T, P RT ln c
(2)
2.3 Preparation of shell-isolated Ag nanoparticles (Ag SHINs)
The 96 nm Ag NPs were synthesized with a seed growth method and 16 nm Au NPs were used as seeds: when 1.5 mL 38 mM sodium citrate was added to 50 mL 0.24 mM boiling HAuCl4 to get the Au seeds, after 1 h stirring the Au seed were diluted for following growth of 96 nm Ag nanospheres. Next, the 45 times diluted Au seeds were mixed with sodium citrate and ascorbic acid, and then AgClO4 was added slowly. The final concentration of AA, AgClO4, and sodium citrate was 1.84, 1.25, and 1.25 mM, respectively. Ag SHINs were prepared according to the previous work.[15] Ag nanospheres were diluted two times with water. And then it was mixed with NaBH4, APTMS, and sodium silicate solution under vigorous stirring. The amount of NaBH4, APTMS, and sodium silicate in mixture was 5.5 mM, 0.22 mM, and 0.045%, respectively. The pH value of growth solution was tuned to be ~9.7 and the mixture was heated in a 90 °C bath for 70 min. Then the bath temperature was set at 60 °C and stirring time was about 30 min. To prepare the Ag SHINs with smaller Ag core, 52 nm-diameter Ag nanospheres were synthesized by a seed growth method where 9 nm Ag NPs were used as the seeds. 50 mL of H2O, 3.5 mL of sodium citrate (38 mM) and 3 mL of AgClO4 (20 mM) were mixed together. And then 0.5 mL of NaBH4 (110 mM) was added under vigorous stirring. After 2 h stirring, the as-prepared Ag seeds were diluted 214 times with H2O. Next, AgClO4, sodium citrate, and ascorbic acid were added to the diluted seeds solution drop by drop. The concentration of AA, AgClO4 and sodium citrate in the sol was 1.87, 1.07, and 1.39 mM, respectively. In the following silica coating process, the amount of NaBH4, APTMS, and silicate solution were kept the same as the method for the lager Ag SHINs (pH
value of the mixture was appropriately tuned around 9). The mixture was immediately transferred to a 90 °C bath and stirred for 80 min. And then the temperature was cool down to 60 °C and stirring time was about 60 min. To
remove the possible SHINs with pinholes, the as-prepared SHINs might be washed with ~0.1% aqua regia (due to the ultra-strong volatility of fresh-prepared auqa regia, it is difficult to obtain an accurate concentration) and then rinsed with ultrapure water thoroughly. For the pinhole test in 10 mM Py and 100 mM NaClO4 solution, the Ag SHINs (96 nm Ag core) were cast onto a glassy carbon electrode, and the potential was set at -0.3 V. For EC-SHINERS experiments of adenine, Ag SHINs (96 nm Ag core) were spread onto a smooth Ag electrode and dried in vacuum.
3. Results and discussion 3.1 The plasmonic performance of Ag SHINs on metal surfaces
Fig.1. a) Schematic diagram of SHINERS method on a Ag electrode surface under different excitations. b) TEM images of Ag SHINs. c) 3D-FDTD simulations of four SHINs with a model of 22 array on a Ag substrate. The excitation wavelengths are varied from 488 to 532, and 633 nm.
Fig. 1a demonstrates the procedure of in-situ electrochemical-SHINERS measurements on a smooth Ag electrode surface with Ag SHINs. As shown in Fig. 1b, the Ag SHIN consists of ~96 nm Ag nanosphere core
with ~8 nm dielectric silica shell. The Ag SHINs were cast onto the freshly prepared smooth Ag surface as a “smart plasmonic dust”. The modified Ag electrode was then mounted in a home-made spectroelectrochemical cell.[9] Meanwhile, the electric field distributions under three different excitations (488, 532, and 633 nm) were simulated by 3D-FDTD method and the electric field vector was polarized perpendicular to the illumination. As revealed in 3D-FDTD simulations of four SHINs with a model of 22 array on a Ag substrate (Fig.1c), the Raman intensity enhancements at the junctions between Ag SHINs and the Ag electrode surface were up to 5 orders of magnitude under various excitation wavelengths.
Fig.2. TEM images of Ag SHINs with (a) 96 and (b) 52 nm core after the treatment of aqua regia solution. The corresponding inset presents the control experiment where there are pinholes on the shell.
To examine the chemical stability of Ag SHINs, a dilute aqua regia solution (~0.1%) which is a powerful metal etchant was used. The as-prepared Ag SHINs were concentrated by centrifugation and then dispersed into the aqua regia solution for the treatment. After being treated with aqua regia solution several times, the Ag SHINs were rinsed with H2O thoroughly. The low-magnification TEM images of the Ag SHINs with 96 and 52 nm Ag core after aqua regia treatments were shown in Fig.2. If there is any pinhole on the silica shell, the Ag core would be definitely etched in the aqua regia solution (as revealed in the insets of Fig.2). However,
the Ag SHINs with pinhole-free silica shell exhibited the excellent chemical stability against the corrosive environment. Combined with the dielectric property to allow electromagnetic field transmitted, the remarkable chemical stability of silica shell will fulfill the requirements for fundamental investigations and practical applications.
Fig.3. The Ag SHINs-enhanced Py Raman spectra on smooth Au surfaces under (a) 633 and (b) 532 nm excitation while the spot is kept the same. (c) The pinhole test of Ag SHINs was carried out on a glassy carbon electrode and the potential was set at -0.3 V. In all cases, the concentration of Py was 10 mM. To
further
demonstrate
the
pinhole-free
character
of
Ag
SHINs,
we
have
performed
electrochemical-Raman measurements on a glassy carbon electrode with pyridine (Py) as probe molecules. Py is widely used in SERS experiments due to its sensitivity for the interaction with nanostructured metal (Au or Ag) surfaces.[3, 4, 8, 15] A freshly prepared glassy carbon electrode surface was modified with Ag SHINs and then dried in a desiccator. The solution for EC-Raman measurement consists of 10 mM Py and 100 mM NaClO4, and the potential was set at -0.3 V throughout the pinhole test. The Raman signal of Py adsorbed onto a Ag surface would be clearly observable if there is pinhole on the silica shell.[3, 4, 15] As shown in Fig. 3c, the Py Raman signal is absent which means the pinhole-free character of silica shell. To further investigate the
plasmonic enhancement of Ag SHINs on a metal surface under various laser lines, we then performed the SHINERS experiments on a smooth gold substrate surface with Py molecules as well. When 633 nm laser line was used, the enhanced Raman signals of Py molecules from the Au surface-Ag SHINs gap mode is strong (Fig. 3a). However, at the same spot under 532 nm excitation, the Py Raman signal is much weaker due to the poor plasmonic performance of the coupling between the Au surface and Ag SHINs when the excitation wavelength is around the interband transition of Au (~530 nm).
3.2 Electrochemical-SHINERS of adenine on a smooth Ag electrode surface
Several great efforts have been devoted to the investigations of molecules adsorption at electrode/liquid interfaces. The combination of a vibrational spectroscopic technique and an electrochemical method is capable to obtain the electrochemical behaviors and chemical bonding information about the target molecules simultaneously.[28] However, the conventional Raman measurements are difficult to be carried out at the electrochemical interfaces with well-defined morphologies. SHINERS method has overcome this limitation and successfully acquired high-quality Raman spectra of probe molecules at single crystal electrode surfaces.[8-10, 15, 29] To study the electrochemical behavior of adenine on a smooth Ag electrode surface, the Ag SHINs were deposited as optical near-field “amplifiers”. Concurrently, the Ag surface-Ag SHIN gap-mode facilitates the wavelength-dependent measurements as elucidated in Fig.1c. Based on the remarkable plasmonic performances in the wide spectrum range, the laser lines for electrochemical-SHINERS experiments were chosen to be 488, 532, and 633 nm.
Fig.4. The electrochemical-SHINERS spectra of adenine on a smooth Ag surface under a) 488, b) 532, and c) 633 nm excitations. I The deaerated solution consists of 1 mM adenine and 100 mM NaClO4.The arrow represents the potential scan direction.
Fig. 4 presents the electrochemical-SHINERS spectra of adenine (1 mM) on a smooth Ag electrode while the laser lines were changed from 488 to 532, and 633 nm. The solution was deaerated with nitrogen and 100 mM NaClO4 was used as an electrolyte. During the potential step measurements, the potential was scanned from -0.3 to -1.2 V and the potential interval was 0.1 V. In these potential-dependent SHINERS spectra of adenine, the peaks around 730 and 1330 cm−1 are distinguishable. The former is attributed to the ring breathing mode, while the later has been assigned to two contributions in the previous publications.[25, 30, 31] Firstly, C2-H, N9-H, and C8-H bending modes with the components from C6-N1, C8-N9, and N3-C4 stretching modes. Secondly, the contributions from N1-C2 and C5-N7 stretching modes along with C2-H and C8-H bending modes. Apparently, the positions of these peaks slightly shifted toward lower wavenumbers with lower potential which could be attributed to Stark effect.[17] At the same time, the intensities of the two peaks increased with lowering potential till the highest intensities are obtained, and then decreased subsequently when the potential was set at lower potential. Generally, the potential-dependent peak intensity
variation could be ascribed to Stark effect, resonance effect (chemical enhancement), plasmonic coupling efficiency, the changes of adsorption orientation and the number of adsorbed molecules.[1, 28] Considering the adsorption/desorption behaviors of adenine as illustrated by the cyclic voltammogram (CV) and surface amount plot in supplementary data Fig. S1, it is necessary to normalize the Raman intensities with respect to potential-dependent surface amount plot under the same condition. Consequently, the normalized Raman intensities of these peaks are presented in Fig. 5. Because the peak around 1330 cm−1 partly overlaps with the one around 1310 cm−1, to avoid the possible error comes from deconvoluting these overlapped peaks, the integrated intensity of these two peaks was preferred for the following analysis.
Fig.5. Potential-dependent Raman intensities of adenine peaks under different excitations: a) intensity of the band around 730 cm−1, and b) the integrated intensity of the two bands around 1330 and 1310 cm−1 under 488, 532, and 633 nm excitations on a smooth Ag electrode. All the Raman intensities are normalized with respect to surface amount plot which is presented as well (black square).
Fig. 5 shows the potential-dependent Raman intensities which are normalized to the number of adenine molecules adsorbed on a Ag electrode. The surface amount of adenine on a smooth Ag electrode increases slowly from ~ -1.0V and tend to a maximum value at ~ -0.45 V. Although the excitations were varied from
488 to 532, and 633 nm, both the normalized Raman intensities of bands around 730 and 1310+1330 cm −1 became highest at -1.0 V. In the corresponding CV of adenine on a
smooth Ag electrode (Fig.S1), a pair of
redox peaks appears around -1.0 V, it could be attributed to the adsorption/desorption behavior.[32] As mentioned in the reported Py adsorption analysis with electrochemical-SHINERS method at Au(hkl) single crystal surfaces, the molecule orientation changes from flat to vertical induced an enhancement in ν1 mode intensity.[33] Similarly, the potential with the highest normalized Raman intensity of adenine coincides with the adsorption/desorption potential (~ -1.0 V) in CV, which suggests a possible orientation change which facilitates a Raman intensity enhancement according to the SERS surface selection rule.[1, 28, 33] Furthermore, to compare the surface morphology effect on Raman spectral features, we also carried out control experiments on a oxidation-reduction-cycles (ORC) roughened Ag electrode surface. The electrochemical-SERS spectra of adenine on an electrochemical roughened electrode are presented in supplementary data Fig. S2. Interestingly, the band around 1310~1330 cm−1 of SERS spectra is distinct from those in SHINERS spectra. These bands obtained from a roughened electrode are totally integrated together under 532 and 633 nm excitations. Especially, intensity of 1330 cm−1 band in SERS spectra is much stronger than the one in SHINERS spectra. Adenine has five nitrogen atoms (N1, N3, N7, N9, and N10), there are more adsorption sites on the ill-defined substrate surface for the interaction with nitrogen atoms. Obviously, it would be more complex for those vibrational modes around 1310~1330 cm−1 and the spectrum resolution in this range is extremely limited. In contrast, due to the excellent plasmonic performance on smooth Ag electrode surfaces, Ag SHINs have a great potential for the investigations of molecule-metal interactions at electrochemical interfaces.
4. Conclusions We vividly demonstrated the procedure of electrochemical-SHINERS method with Ag SHINs on Ag electrode surfaces. Notably, the Ag SHINs are pinhole-free and exhibit outstanding plasmonic performance under 488, 532, and 633 nm laser lines. In the in-situ electrochemical-SHINERS study of adenine on smooth Ag electrodes, the wavelength-dependent Raman measurements were well correlated with potential step, CV and surface coverage.
After normalizing the electrochemical-SHINERS intensities with respect to the surface amount of adsorbed adenine, both the peaks around 730 and 1330 cm−1 became the strongest at -1.0 V. These results coincide with the adsorption/desorption peak in corresponding CV. Compared to the results from the ill-defined (ORC-roughened) electrode surface, the spectrum resolution in the range around 1310~1330 cm−1 on smooth Ag electrodes with Ag SHINs is clearly distinguished. Importantly, this strategy can be extended to investigate the electrochemical behavior of other nucleobases and optimistically, we anticipate that the Ag SHINs will open up a new avenue of fundamental research at a wide range of electrochemical interfaces.
Acknowledgements This work is supported by the NSFC (21522508), the Fundamental Research Funds for the Central Universities (Xiamen University, No. 20720150039 and 201412G002), and NFFTBS (No. J1310024).
References [1] M. Moskovits, Surface-enhanced Raman spectroscopy: a brief retrospective, J. Raman Spectrosc., 36 (2005) 485-496. [2] S. Zou, M.J. Weaver, Potential-Dependent Metal−Adsorbate Stretching Frequencies for Carbon Monoxide on Transition-Metal Electrodes: Chemical Bonding versus Electrostatic Field Effects, J. Phys. Chem., 100 (1996) 4237-4242. [3] D.L. Jeanmaire, R.P. Van Duyne, Surface raman spectroelectrochemistry, J. Electroanal. Chem. Interfac., 84 (1977) 1-20. [4] M. Fleischmann, P.J. Hendra, A.J. McQuillan, Raman spectra of pyridine adsorbed at a silver electrode, Chem. Phys. Lett., 26 (1974) 163-166. [5] S. Schlücker, Surface-Enhanced Raman Spectroscopy: Concepts and Chemical Applications, Angew. Chem. Int. Ed., 53 (2014) 4756-4795. [6] L. Guerrini, Ž. Krpetić, D. van Lierop, R.A. Alvarez-Puebla, D. Graham, Direct Surface-Enhanced Raman Scattering Analysis of DNA Duplexes, Angew. Chem. Int. Ed., 54 (2015) 1144-1148. [7] R.A. Alvarez-Puebla, L.M. Liz-Marzán, SERS Detection of Small Inorganic Molecules and Ions, Angew. Chem. Int. Ed., 51 (2012) 11214-11223. [8] J.F. Li, Y.F. Huang, Y. Ding, Z.L. Yang, S.B. Li, X.S. Zhou, F.R. Fan, W. Zhang, Z.Y. Zhou, D.Y. Wu, B. Ren, Z.L. Wang, Z.Q. Tian, Shell-isolated nanoparticle-enhanced Raman spectroscopy, Nature, 464 (2010) 392-395. [9] C.Y. Li, J.C. Dong, X. Jin, S. Chen, R. Panneerselvam, A.V. Rudnev, Z.L. Yang, J.F. Li, T. Wandlowski, Z.Q. Tian, In Situ
Monitoring
of
Electrooxidation
Processes
at
Gold
Single
Crystal
Surfaces
Using
Shell-Isolated
Nanoparticle-Enhanced Raman Spectroscopy, J. Am. Chem. Soc., 137 (2015) 7648-7651. [10] D.P. Butcher, S.P. Boulos, C.J. Murphy, R.C. Ambrosio, A.A. Gewirth, Face-Dependent Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy of 2,2′-Bipyridine on Au(100) and Au(111), J. Phys. Chem. C, 116 (2012) 5128-5140. [11] Y. F. Huang, C. Y. Li, I. Broadwell, J. -F. Li, D. Y. Wu, B. Ren, Z. Q. Tian, Shell-isolated nanoparticle-enhanced Raman spectroscopy of pyridine on smooth silver electrodes, Electrochim. Acta, 56 (2011) 10652-10657. [12] W. Xie, B. Walkenfort, S. Schlücker, Label-Free SERS Monitoring of Chemical Reactions Catalyzed by Small Gold Nanoparticles Using 3D Plasmonic Superstructures, J. Am. Chem. Soc., 135 (2013) 1657-1660. [13] W. Wang, Q. Guo, M. Xu, Y. Yuan, R. Gu, J. Yao, On-line surface enhanced Raman spectroscopic detection in a recyclable Au@SiO2 modified glass capillary, J. Raman Spectrosc., 45 (2014) 736-744. [14] M. Rycenga, C.M. Cobley, J. Zeng, W. Li, C.H. Moran, Q. Zhang, D. Qin, Y. Xia, Controlling the Synthesis and Assembly of Silver Nanostructures for Plasmonic Applications, Chem. Rev., 111 (2011) 3669-3712. [15] C. Y. Li, M. Meng, S. C. Huang, L. Li, S. R. Huang, S. Chen, L. Y. Meng, R. Panneerselvam, S. J. Zhang, B. Ren, Z. L. Yang, J. F. Li, Z. Q. Tian, “Smart” Ag Nanostructures for Plasmon-Enhanced Spectroscopies, J. Am. Chem. Soc., 137 (2015) 13784-13787. [16] K. Kneipp, H. Kneipp, V.B. Kartha, R. Manoharan, G. Deinum, I. Itzkan, R.R. Dasari, M.S. Feld, Detection and identification of a single DNA base molecule using surface-enhanced Raman scattering (SERS), Physical Review E, 57 (1998) R6281. [17] J. Álvarez-Malmagro, F. Prieto, M. Rueda, A. Rodes, In situ Fourier transform infrared reflection absortion spectroscopy study of adenine adsorption on gold electrodes in basic media, Electrochim. Acta, 140 (2014) 476-481. [18] M. Rueda, F. Prieto, A. Rodes, J.M. Delgado, In situ infrared study of adenine adsorption on gold electrodes in acid media, Electrochim. Acta, 82 (2012) 534-542. [19] C.I. Smith, A. Bowfield, G.J. Dolan, M.C. Cuquerella, C.P. Mansley, D.G. Fernig, C. Edwards, P. Weightman, Determination of the structure of adenine monolayers adsorbed at Au(110)/electrolyte interfaces using reflection anisotropy spectroscopy, J. Chem. Phys., 130 (2009) 044702.
[20] C. Vaz-Domínguez, M. Escudero-Escribano, A. Cuesta, F. Prieto-Dapena, C. Cerrillos, M. Rueda, Electrochemical STM study of the adsorption of adenine on Au(111) electrodes, Electrochem. Comm., 35 (2013) 61-64. [21] E.J. Blackie, E.C.L. Ru, P.G. Etchegoin, Single-Molecule Surface-Enhanced Raman Spectroscopy of Nonresonant Molecules, J. Am. Chem. Soc., 131 (2009) 14466-14472. [22] K.F. Domke, D. Zhang, B. Pettinger, Toward Raman Fingerprints of Single Dye Molecules at Atomically Smooth Au(111), J. Am. Chem. Soc., 128 (2006) 14721-14727. [23] M. Futamata, Single molecule sensitivity in SERS: importance of junction of adjacent Ag nanoparticles, Farad. Discuss., 132 (2006) 45-61. [24] H. Watanabe, Y. Ishida, N. Hayazawa, Y. Inouye, S. Kawata, Tip-enhanced near-field Raman analysis of tip-pressurized adenine molecule, Phys. Rev. B, 69 (2004) 155418. [25] B. Giese, D. McNaughton, Surface-Enhanced Raman Spectroscopic and Density Functional Theory Study of Adenine Adsorption to Silver Surfaces, J. Phys. Chem. B, 106 (2002) 101-112. [26] A. Hamelin, S. Morin, J. Richer, J. Lipkowski, Adsorption of pyridine on the (110) face of silver, J. Electroanal. Chem. Interfac., 272 (1989) 241-252. [27] A. Hamelin, S. Morin, J. Richer, J. Lipkowski, Adsorption of pyridine on the (210) face of silver, J. Electroanal. Chem. Interfac., 304 (1991) 195-209. [28] Z.Q. Tian, B. Ren, Adsorption and reaction at electrochemical interfaces as probed by surface-enhanced Raman spectroscopy, Annu. Rev. Phys. Chem, 55 (2004) 197–229. [29] N.R. Honesty, A.A. Gewirth, Shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) investigation of benzotriazole film formation on Cu(100), Cu(111), and Cu(poly), J. Raman Spectrosc., 43 (2011) 46-50. [30] L. Cui, D. Y. Wu, A. Wang, B. Ren, Z.-Q. Tian, Charge-Transfer Enhancement Involved in the SERS of Adenine on Rh and Pd Demonstrated by Ultraviolet to Visible Laser Excitation, J. Phys. Chem. C, 114 (2010) 16588-16595. [31] R. Huang, H.-T. Yang, L. Cui, D.-Y. Wu, B. Ren, Z. Q. Tian, Structural and Charge Sensitivity of Surface-Enhanced Raman Spectroscopy of Adenine on Silver Surface: A Quantum Chemical Study, J. Phys. Chem. C, 117 (2013) 23730-23737. [32] C. Prado, F. Prieto, M. Rueda, J. Feliu, A. Aldaz, Adenine adsorption on Au(111) and Au(100) electrodes: Characterisation, surface reconstruction effects and thermodynamic study, Electrochim. Acta, 52 (2007) 3168-3180. [33] J. F. Li, Y. J. Zhang, A.V. Rudnev, J.R. Anema, S.-B. Li, W. J. Hong, P. Rajapandiyan, J. Lipkowski, T. Wandlowski, Z.-Q. Tian, Electrochemical Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy: Correlating Structural Information and Adsorption Processes of Pyridine at the Au(hkl) Single Crystal/Solution Interface, J. Am. Chem. Soc., 137 (2015) 2400-2408.
S0013-4686(16)30603-X http://dx.doi.org/doi:10.1016/j.electacta.2016.03.065 EA 26895
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
9-1-2016 8-3-2016 10-3-2016
Please cite this article as: Chao-Yu Li, Sen-Yuan Chen, Yong-Li Zheng, Shun-Peng Chen, Rajapandiyan Panneerselvam, Shu Chen, Qing-Chi Xu, Yan-Xia Chen, Zhi-Lin Yang, De-Yin Wu, Jian-Feng Li, Zhong-Qun Tian, In-situ electrochemical shell-isolated Ag nanoparticles-enhanced Raman spectroscopy study of adenine adsorption on smooth Ag electrodes, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.03.065 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
In-situ electrochemical shell-isolated Ag nanoparticles-enhanced Raman spectroscopy study of adenine adsorption on smooth Ag electrodes Chao-Yu Li,a Sen-Yuan Chen,a Yong-Li Zheng,b Shun-Peng Chen,a Rajapandiyan Panneerselvam,a Shu Chen,c Qing-Chi Xu,c,* Yan-Xia Chen,b Zhi-Lin Yang,c De-Yin Wu,a Jian-Feng Lia,* and Zhong-Qun Tiana
a
MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, State Key Laboratory of Physical Chemistry of
Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 3605, China. b
Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, University of Science
and Technology of China, Hefei 230026, China. c
Department of Physics, Xiamen University, Xiamen 3605, China.
* Corresponding Author Tel: +86-592-2186192
Fax: +86-592-2186192
E-Mail: [email protected] and [email protected]
Graphical Abstract
ABSTRACT Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is employed to investigate the electrochemical behavior of adenine molecules on smooth Ag electrodes. To attain this goal, pinhole-free shell-isolated Ag nanoparticles (Ag SHINs) have been synthesized and then used as signal “amplifiers” in the gap-mode configuration (Ag SHINs are coupled with a Ag electrode surface). The as-prepared Ag SHINs exhibit remarkable plasmonic performance under 488, 532, and 633 nm excitations as revealed by finite-difference time-domain (FDTD) simulations and SHINERS experiments. Furthermore, wavelength-dependent SHINERS investigation of adenine on Ag electrodes is excellently combined with the electrochemical technique. With outstanding chemical stability and plasmonic property, the Ag SHINs are extraordinarily suitable for fundamental studies at various electrochemical interfaces.
Keywords: Adenine; Electrochemical SERS; SHINERS; Ag SHINs; Silver electrode
1. Introduction As a surface plasmon resonance (SPR)-based vibrational spectroscopic method, surface-enhanced Raman spectroscopy (SERS) has been widely used for the investigation of adsorption and orientation of chemical molecules at electrochemical interfaces.[1-7] To provide the large near-field enhancement, the surface of SERS substrates is needed to be roughened or nanostructured for the excitation of SPR. Meanwhile, the substrate generality is still limited to a few metals, such as Au, Ag, and Cu.[1, 5] To circumvent the limitations in SERS, in 2010, our group has invented a novel method called shell-isolated nanoparticles-enhanced Raman spectroscopy (SHINERS)[8], which paves a unique way for plasmon-enhanced spectroscopies to investigate any surface or morphology. Normally, a shell-isolated nanoparticle (SHIN) is consists of a Au nanoparticle core and an ultra-thin silica shell. The Au nanoparticle core generates the large localized field to enhance the Raman signals of probe molecules while the dielectric silica shell would protect the Au core from the direct contact with electrolyte or analytes/probes. As a new method, SHINERS has been successfully employed in the investigations of electrochemical processes,[9-11] photocatalysis,[12] and on-line detection in flowing systems.[13] However, due to the interband transition, the plasmonic performance of Au is extremely poor under the excitation below 530 nm. In contrast, Ag has the highest quality factor in the region from 300 to 1200 nm among noble metals.[14] By introducing the Ag nanosphere as the SHIN core for the plasmonic applications, in a wide spectrum range the Raman enhancement mechanism of adsorbed species could be clearly elucidated when it is combined with an electrochemical method.[15] Adenine is one of the five nucleobases and plays a vital role in life science.[16] Due to its significance, adenine has been extensively examined by several electrochemical techniques and spectroscopic methods.[16-24] Especially, plasmon-enhanced Raman scattering methods have been utilized to investigate the nucleobases in the presence of strong local field enhancement. Due to the large Raman cross section,
single-molecule detection of adenine has been obtained in SERS and tip-enhanced Raman spectroscopy (TERS) experiments. [16, 21-24] However, the previous SERS investigations of adenine were based on the ill-defined substrate surfaces, there are some reported contradictions concerning the interaction between adenine and electrode surfaces.[25] For a clear understanding of the chemical interaction between nucleobases and a metal surface, it is imperative to perform the investigations on a smooth electrode surface. Herein, we synthesized the Ag SHINs with pinhole-free character, and then simply spread them onto the smooth Ag electrodes as a plasmonic signal amplifier. Using three-dimensional finite-difference time-domain (3D-FDTD) calculations, the local electric field enhancements at hot spots under different excitations (488, 532, and 633 nm) are calculated to be more than 5 orders of magnitude. For the investigation of electrochemical behavior of adenine on Ag electrodes, electrochemical-SHINERS experiments with Ag SHINs are carried out in the neutral solution (0.1 M NaClO4 was used as the supporting electrolyte). The spectroscopic and electrochemical results are excellently correlated, thus, EC-SHINERS method with Ag SHINs exhibits a great potential in the investigation of molecular bondings at an electrochemical interface.
2. Experimental 2.1 Materials
Chloroauric acid tetrahydrate (HAuCl4·4H2O), sodium citrate, sodium borohydride (NaBH4), L-ascorbic acid (AA), and pyridine (Py) were purchased from Sinopharm Chemical Reagent Co. Ltd.; Anhydrous silver perchlorate (AgClO4), (3-aminopropyl)trimethoxysilane (APTMS) and sodium perchlorate (NaClO4) were purchased from Alfa Aesar. Sodium silicate solution and adenine were purchased from Sigma-Aldrich. All reagents were used as received without further purification. Deionised Milli-Q water (~18.2 MΩ·cm) was used throughout the study.
2.2 Instruments and electrochemical methods
The morphology and structure of the as-prepared nanostructures were characterized by TEM (JEOL JEM 1400). The SHINERS experiments were carried out using an Invia (Renishaw, UK) microscope (the laser lines are 532 and 633 nm) with a 50×, NA 0.55 objective. Meanwhile, experiments under 488 nm laser were performed using Nanophoton Raman-11 (Nanophoton, Japan) with a 50×, NA 0.45 objective. In all electrochemical and electrochemical-Raman experiments, a Pt wire and a saturated calomel electrode (SCE) were used as a counter electrode and a reference electrode, respectively. All potentials are reported with respect to SCE in this paper. The freshly smooth Ag electrode used throughout this manuscript was prepared by sequentially polishing with alumina grit size 1.0 and 0.3 µm until the electrode surface was mirror like.[11] All solutions in electrochemical and electrochemical-Raman measurements were deaerated with N2.Chronocoulumetry experiments were conducted on CHI 631B electrochemical workstation (CH Instruments, Shanghai, China).
Chronocoulometry experiments were carried out according to previous
work.[26, 27] The initial potential, E, was changed from -0.3 to -1.55 V (the potential internal was 0.05 V), and the final potential, Ef, was set at -1.55 V for the complete desorption of adenine and avoiding the extensive hydrogen evolution current. Six series experiments with adenine concentrations (c) verified from 0 to 0.01, 0.1, 0.5, 1 and 2 mM were taken. The current transients of these six series were obtained with potential step experiments and then each current-time curve was integrated to achieve the charge-time plot. The linear part of the charge-time plot was extrapolated to zero time point, and then the relative charge density, Δσc, was obtained. Next, the relative charge density-potential plot was integrated, and then we subtract the resulting value of pure electrolyte (the concentration of adenine is 0 mM) from the value of solution containing adenine at the same potential. Hence, the film pressure (π) was calculated according to:
E
E
cdE c 0dE , Ef
Ef
(1)
where the subscript c=0 represents the pure electrolyte solution. Because a mechanically polished mirror-like Ag electrode was used rather than a single crystal electrode, the surface amount of adsorbed adenine (n) was employed to account for the surface coverage. Finally, the value of n was calculated according to:
n
1 ( )E, T, P RT ln c
(2)
2.3 Preparation of shell-isolated Ag nanoparticles (Ag SHINs)
The 96 nm Ag NPs were synthesized with a seed growth method and 16 nm Au NPs were used as seeds: when 1.5 mL 38 mM sodium citrate was added to 50 mL 0.24 mM boiling HAuCl4 to get the Au seeds, after 1 h stirring the Au seed were diluted for following growth of 96 nm Ag nanospheres. Next, the 45 times diluted Au seeds were mixed with sodium citrate and ascorbic acid, and then AgClO4 was added slowly. The final concentration of AA, AgClO4, and sodium citrate was 1.84, 1.25, and 1.25 mM, respectively. Ag SHINs were prepared according to the previous work.[15] Ag nanospheres were diluted two times with water. And then it was mixed with NaBH4, APTMS, and sodium silicate solution under vigorous stirring. The amount of NaBH4, APTMS, and sodium silicate in mixture was 5.5 mM, 0.22 mM, and 0.045%, respectively. The pH value of growth solution was tuned to be ~9.7 and the mixture was heated in a 90 °C bath for 70 min. Then the bath temperature was set at 60 °C and stirring time was about 30 min. To prepare the Ag SHINs with smaller Ag core, 52 nm-diameter Ag nanospheres were synthesized by a seed growth method where 9 nm Ag NPs were used as the seeds. 50 mL of H2O, 3.5 mL of sodium citrate (38 mM) and 3 mL of AgClO4 (20 mM) were mixed together. And then 0.5 mL of NaBH4 (110 mM) was added under vigorous stirring. After 2 h stirring, the as-prepared Ag seeds were diluted 214 times with H2O. Next, AgClO4, sodium citrate, and ascorbic acid were added to the diluted seeds solution drop by drop. The concentration of AA, AgClO4 and sodium citrate in the sol was 1.87, 1.07, and 1.39 mM, respectively. In the following silica coating process, the amount of NaBH4, APTMS, and silicate solution were kept the same as the method for the lager Ag SHINs (pH
value of the mixture was appropriately tuned around 9). The mixture was immediately transferred to a 90 °C bath and stirred for 80 min. And then the temperature was cool down to 60 °C and stirring time was about 60 min. To
remove the possible SHINs with pinholes, the as-prepared SHINs might be washed with ~0.1% aqua regia (due to the ultra-strong volatility of fresh-prepared auqa regia, it is difficult to obtain an accurate concentration) and then rinsed with ultrapure water thoroughly. For the pinhole test in 10 mM Py and 100 mM NaClO4 solution, the Ag SHINs (96 nm Ag core) were cast onto a glassy carbon electrode, and the potential was set at -0.3 V. For EC-SHINERS experiments of adenine, Ag SHINs (96 nm Ag core) were spread onto a smooth Ag electrode and dried in vacuum.
3. Results and discussion 3.1 The plasmonic performance of Ag SHINs on metal surfaces
Fig.1. a) Schematic diagram of SHINERS method on a Ag electrode surface under different excitations. b) TEM images of Ag SHINs. c) 3D-FDTD simulations of four SHINs with a model of 22 array on a Ag substrate. The excitation wavelengths are varied from 488 to 532, and 633 nm.
Fig. 1a demonstrates the procedure of in-situ electrochemical-SHINERS measurements on a smooth Ag electrode surface with Ag SHINs. As shown in Fig. 1b, the Ag SHIN consists of ~96 nm Ag nanosphere core
with ~8 nm dielectric silica shell. The Ag SHINs were cast onto the freshly prepared smooth Ag surface as a “smart plasmonic dust”. The modified Ag electrode was then mounted in a home-made spectroelectrochemical cell.[9] Meanwhile, the electric field distributions under three different excitations (488, 532, and 633 nm) were simulated by 3D-FDTD method and the electric field vector was polarized perpendicular to the illumination. As revealed in 3D-FDTD simulations of four SHINs with a model of 22 array on a Ag substrate (Fig.1c), the Raman intensity enhancements at the junctions between Ag SHINs and the Ag electrode surface were up to 5 orders of magnitude under various excitation wavelengths.
Fig.2. TEM images of Ag SHINs with (a) 96 and (b) 52 nm core after the treatment of aqua regia solution. The corresponding inset presents the control experiment where there are pinholes on the shell.
To examine the chemical stability of Ag SHINs, a dilute aqua regia solution (~0.1%) which is a powerful metal etchant was used. The as-prepared Ag SHINs were concentrated by centrifugation and then dispersed into the aqua regia solution for the treatment. After being treated with aqua regia solution several times, the Ag SHINs were rinsed with H2O thoroughly. The low-magnification TEM images of the Ag SHINs with 96 and 52 nm Ag core after aqua regia treatments were shown in Fig.2. If there is any pinhole on the silica shell, the Ag core would be definitely etched in the aqua regia solution (as revealed in the insets of Fig.2). However,
the Ag SHINs with pinhole-free silica shell exhibited the excellent chemical stability against the corrosive environment. Combined with the dielectric property to allow electromagnetic field transmitted, the remarkable chemical stability of silica shell will fulfill the requirements for fundamental investigations and practical applications.
Fig.3. The Ag SHINs-enhanced Py Raman spectra on smooth Au surfaces under (a) 633 and (b) 532 nm excitation while the spot is kept the same. (c) The pinhole test of Ag SHINs was carried out on a glassy carbon electrode and the potential was set at -0.3 V. In all cases, the concentration of Py was 10 mM. To
further
demonstrate
the
pinhole-free
character
of
Ag
SHINs,
we
have
performed
electrochemical-Raman measurements on a glassy carbon electrode with pyridine (Py) as probe molecules. Py is widely used in SERS experiments due to its sensitivity for the interaction with nanostructured metal (Au or Ag) surfaces.[3, 4, 8, 15] A freshly prepared glassy carbon electrode surface was modified with Ag SHINs and then dried in a desiccator. The solution for EC-Raman measurement consists of 10 mM Py and 100 mM NaClO4, and the potential was set at -0.3 V throughout the pinhole test. The Raman signal of Py adsorbed onto a Ag surface would be clearly observable if there is pinhole on the silica shell.[3, 4, 15] As shown in Fig. 3c, the Py Raman signal is absent which means the pinhole-free character of silica shell. To further investigate the
plasmonic enhancement of Ag SHINs on a metal surface under various laser lines, we then performed the SHINERS experiments on a smooth gold substrate surface with Py molecules as well. When 633 nm laser line was used, the enhanced Raman signals of Py molecules from the Au surface-Ag SHINs gap mode is strong (Fig. 3a). However, at the same spot under 532 nm excitation, the Py Raman signal is much weaker due to the poor plasmonic performance of the coupling between the Au surface and Ag SHINs when the excitation wavelength is around the interband transition of Au (~530 nm).
3.2 Electrochemical-SHINERS of adenine on a smooth Ag electrode surface
Several great efforts have been devoted to the investigations of molecules adsorption at electrode/liquid interfaces. The combination of a vibrational spectroscopic technique and an electrochemical method is capable to obtain the electrochemical behaviors and chemical bonding information about the target molecules simultaneously.[28] However, the conventional Raman measurements are difficult to be carried out at the electrochemical interfaces with well-defined morphologies. SHINERS method has overcome this limitation and successfully acquired high-quality Raman spectra of probe molecules at single crystal electrode surfaces.[8-10, 15, 29] To study the electrochemical behavior of adenine on a smooth Ag electrode surface, the Ag SHINs were deposited as optical near-field “amplifiers”. Concurrently, the Ag surface-Ag SHIN gap-mode facilitates the wavelength-dependent measurements as elucidated in Fig.1c. Based on the remarkable plasmonic performances in the wide spectrum range, the laser lines for electrochemical-SHINERS experiments were chosen to be 488, 532, and 633 nm.
Fig.4. The electrochemical-SHINERS spectra of adenine on a smooth Ag surface under a) 488, b) 532, and c) 633 nm excitations. I The deaerated solution consists of 1 mM adenine and 100 mM NaClO4.The arrow represents the potential scan direction.
Fig. 4 presents the electrochemical-SHINERS spectra of adenine (1 mM) on a smooth Ag electrode while the laser lines were changed from 488 to 532, and 633 nm. The solution was deaerated with nitrogen and 100 mM NaClO4 was used as an electrolyte. During the potential step measurements, the potential was scanned from -0.3 to -1.2 V and the potential interval was 0.1 V. In these potential-dependent SHINERS spectra of adenine, the peaks around 730 and 1330 cm−1 are distinguishable. The former is attributed to the ring breathing mode, while the later has been assigned to two contributions in the previous publications.[25, 30, 31] Firstly, C2-H, N9-H, and C8-H bending modes with the components from C6-N1, C8-N9, and N3-C4 stretching modes. Secondly, the contributions from N1-C2 and C5-N7 stretching modes along with C2-H and C8-H bending modes. Apparently, the positions of these peaks slightly shifted toward lower wavenumbers with lower potential which could be attributed to Stark effect.[17] At the same time, the intensities of the two peaks increased with lowering potential till the highest intensities are obtained, and then decreased subsequently when the potential was set at lower potential. Generally, the potential-dependent peak intensity
variation could be ascribed to Stark effect, resonance effect (chemical enhancement), plasmonic coupling efficiency, the changes of adsorption orientation and the number of adsorbed molecules.[1, 28] Considering the adsorption/desorption behaviors of adenine as illustrated by the cyclic voltammogram (CV) and surface amount plot in supplementary data Fig. S1, it is necessary to normalize the Raman intensities with respect to potential-dependent surface amount plot under the same condition. Consequently, the normalized Raman intensities of these peaks are presented in Fig. 5. Because the peak around 1330 cm−1 partly overlaps with the one around 1310 cm−1, to avoid the possible error comes from deconvoluting these overlapped peaks, the integrated intensity of these two peaks was preferred for the following analysis.
Fig.5. Potential-dependent Raman intensities of adenine peaks under different excitations: a) intensity of the band around 730 cm−1, and b) the integrated intensity of the two bands around 1330 and 1310 cm−1 under 488, 532, and 633 nm excitations on a smooth Ag electrode. All the Raman intensities are normalized with respect to surface amount plot which is presented as well (black square).
Fig. 5 shows the potential-dependent Raman intensities which are normalized to the number of adenine molecules adsorbed on a Ag electrode. The surface amount of adenine on a smooth Ag electrode increases slowly from ~ -1.0V and tend to a maximum value at ~ -0.45 V. Although the excitations were varied from
488 to 532, and 633 nm, both the normalized Raman intensities of bands around 730 and 1310+1330 cm −1 became highest at -1.0 V. In the corresponding CV of adenine on a
smooth Ag electrode (Fig.S1), a pair of
redox peaks appears around -1.0 V, it could be attributed to the adsorption/desorption behavior.[32] As mentioned in the reported Py adsorption analysis with electrochemical-SHINERS method at Au(hkl) single crystal surfaces, the molecule orientation changes from flat to vertical induced an enhancement in ν1 mode intensity.[33] Similarly, the potential with the highest normalized Raman intensity of adenine coincides with the adsorption/desorption potential (~ -1.0 V) in CV, which suggests a possible orientation change which facilitates a Raman intensity enhancement according to the SERS surface selection rule.[1, 28, 33] Furthermore, to compare the surface morphology effect on Raman spectral features, we also carried out control experiments on a oxidation-reduction-cycles (ORC) roughened Ag electrode surface. The electrochemical-SERS spectra of adenine on an electrochemical roughened electrode are presented in supplementary data Fig. S2. Interestingly, the band around 1310~1330 cm−1 of SERS spectra is distinct from those in SHINERS spectra. These bands obtained from a roughened electrode are totally integrated together under 532 and 633 nm excitations. Especially, intensity of 1330 cm−1 band in SERS spectra is much stronger than the one in SHINERS spectra. Adenine has five nitrogen atoms (N1, N3, N7, N9, and N10), there are more adsorption sites on the ill-defined substrate surface for the interaction with nitrogen atoms. Obviously, it would be more complex for those vibrational modes around 1310~1330 cm−1 and the spectrum resolution in this range is extremely limited. In contrast, due to the excellent plasmonic performance on smooth Ag electrode surfaces, Ag SHINs have a great potential for the investigations of molecule-metal interactions at electrochemical interfaces.
4. Conclusions We vividly demonstrated the procedure of electrochemical-SHINERS method with Ag SHINs on Ag electrode surfaces. Notably, the Ag SHINs are pinhole-free and exhibit outstanding plasmonic performance under 488, 532, and 633 nm laser lines. In the in-situ electrochemical-SHINERS study of adenine on smooth Ag electrodes, the wavelength-dependent Raman measurements were well correlated with potential step, CV and surface coverage.
After normalizing the electrochemical-SHINERS intensities with respect to the surface amount of adsorbed adenine, both the peaks around 730 and 1330 cm−1 became the strongest at -1.0 V. These results coincide with the adsorption/desorption peak in corresponding CV. Compared to the results from the ill-defined (ORC-roughened) electrode surface, the spectrum resolution in the range around 1310~1330 cm−1 on smooth Ag electrodes with Ag SHINs is clearly distinguished. Importantly, this strategy can be extended to investigate the electrochemical behavior of other nucleobases and optimistically, we anticipate that the Ag SHINs will open up a new avenue of fundamental research at a wide range of electrochemical interfaces.
Acknowledgements This work is supported by the NSFC (21522508), the Fundamental Research Funds for the Central Universities (Xiamen University, No. 20720150039 and 201412G002), and NFFTBS (No. J1310024).
References [1] M. Moskovits, Surface-enhanced Raman spectroscopy: a brief retrospective, J. Raman Spectrosc., 36 (2005) 485-496. [2] S. Zou, M.J. Weaver, Potential-Dependent Metal−Adsorbate Stretching Frequencies for Carbon Monoxide on Transition-Metal Electrodes: Chemical Bonding versus Electrostatic Field Effects, J. Phys. Chem., 100 (1996) 4237-4242. [3] D.L. Jeanmaire, R.P. Van Duyne, Surface raman spectroelectrochemistry, J. Electroanal. Chem. Interfac., 84 (1977) 1-20. [4] M. Fleischmann, P.J. Hendra, A.J. McQuillan, Raman spectra of pyridine adsorbed at a silver electrode, Chem. Phys. Lett., 26 (1974) 163-166. [5] S. Schlücker, Surface-Enhanced Raman Spectroscopy: Concepts and Chemical Applications, Angew. Chem. Int. Ed., 53 (2014) 4756-4795. [6] L. Guerrini, Ž. Krpetić, D. van Lierop, R.A. Alvarez-Puebla, D. Graham, Direct Surface-Enhanced Raman Scattering Analysis of DNA Duplexes, Angew. Chem. Int. Ed., 54 (2015) 1144-1148. [7] R.A. Alvarez-Puebla, L.M. Liz-Marzán, SERS Detection of Small Inorganic Molecules and Ions, Angew. Chem. Int. Ed., 51 (2012) 11214-11223. [8] J.F. Li, Y.F. Huang, Y. Ding, Z.L. Yang, S.B. Li, X.S. Zhou, F.R. Fan, W. Zhang, Z.Y. Zhou, D.Y. Wu, B. Ren, Z.L. Wang, Z.Q. Tian, Shell-isolated nanoparticle-enhanced Raman spectroscopy, Nature, 464 (2010) 392-395. [9] C.Y. Li, J.C. Dong, X. Jin, S. Chen, R. Panneerselvam, A.V. Rudnev, Z.L. Yang, J.F. Li, T. Wandlowski, Z.Q. Tian, In Situ
Monitoring
of
Electrooxidation
Processes
at
Gold
Single
Crystal
Surfaces
Using
Shell-Isolated
Nanoparticle-Enhanced Raman Spectroscopy, J. Am. Chem. Soc., 137 (2015) 7648-7651. [10] D.P. Butcher, S.P. Boulos, C.J. Murphy, R.C. Ambrosio, A.A. Gewirth, Face-Dependent Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy of 2,2′-Bipyridine on Au(100) and Au(111), J. Phys. Chem. C, 116 (2012) 5128-5140. [11] Y. F. Huang, C. Y. Li, I. Broadwell, J. -F. Li, D. Y. Wu, B. Ren, Z. Q. Tian, Shell-isolated nanoparticle-enhanced Raman spectroscopy of pyridine on smooth silver electrodes, Electrochim. Acta, 56 (2011) 10652-10657. [12] W. Xie, B. Walkenfort, S. Schlücker, Label-Free SERS Monitoring of Chemical Reactions Catalyzed by Small Gold Nanoparticles Using 3D Plasmonic Superstructures, J. Am. Chem. Soc., 135 (2013) 1657-1660. [13] W. Wang, Q. Guo, M. Xu, Y. Yuan, R. Gu, J. Yao, On-line surface enhanced Raman spectroscopic detection in a recyclable Au@SiO2 modified glass capillary, J. Raman Spectrosc., 45 (2014) 736-744. [14] M. Rycenga, C.M. Cobley, J. Zeng, W. Li, C.H. Moran, Q. Zhang, D. Qin, Y. Xia, Controlling the Synthesis and Assembly of Silver Nanostructures for Plasmonic Applications, Chem. Rev., 111 (2011) 3669-3712. [15] C. Y. Li, M. Meng, S. C. Huang, L. Li, S. R. Huang, S. Chen, L. Y. Meng, R. Panneerselvam, S. J. Zhang, B. Ren, Z. L. Yang, J. F. Li, Z. Q. Tian, “Smart” Ag Nanostructures for Plasmon-Enhanced Spectroscopies, J. Am. Chem. Soc., 137 (2015) 13784-13787. [16] K. Kneipp, H. Kneipp, V.B. Kartha, R. Manoharan, G. Deinum, I. Itzkan, R.R. Dasari, M.S. Feld, Detection and identification of a single DNA base molecule using surface-enhanced Raman scattering (SERS), Physical Review E, 57 (1998) R6281. [17] J. Álvarez-Malmagro, F. Prieto, M. Rueda, A. Rodes, In situ Fourier transform infrared reflection absortion spectroscopy study of adenine adsorption on gold electrodes in basic media, Electrochim. Acta, 140 (2014) 476-481. [18] M. Rueda, F. Prieto, A. Rodes, J.M. Delgado, In situ infrared study of adenine adsorption on gold electrodes in acid media, Electrochim. Acta, 82 (2012) 534-542. [19] C.I. Smith, A. Bowfield, G.J. Dolan, M.C. Cuquerella, C.P. Mansley, D.G. Fernig, C. Edwards, P. Weightman, Determination of the structure of adenine monolayers adsorbed at Au(110)/electrolyte interfaces using reflection anisotropy spectroscopy, J. Chem. Phys., 130 (2009) 044702.
[20] C. Vaz-Domínguez, M. Escudero-Escribano, A. Cuesta, F. Prieto-Dapena, C. Cerrillos, M. Rueda, Electrochemical STM study of the adsorption of adenine on Au(111) electrodes, Electrochem. Comm., 35 (2013) 61-64. [21] E.J. Blackie, E.C.L. Ru, P.G. Etchegoin, Single-Molecule Surface-Enhanced Raman Spectroscopy of Nonresonant Molecules, J. Am. Chem. Soc., 131 (2009) 14466-14472. [22] K.F. Domke, D. Zhang, B. Pettinger, Toward Raman Fingerprints of Single Dye Molecules at Atomically Smooth Au(111), J. Am. Chem. Soc., 128 (2006) 14721-14727. [23] M. Futamata, Single molecule sensitivity in SERS: importance of junction of adjacent Ag nanoparticles, Farad. Discuss., 132 (2006) 45-61. [24] H. Watanabe, Y. Ishida, N. Hayazawa, Y. Inouye, S. Kawata, Tip-enhanced near-field Raman analysis of tip-pressurized adenine molecule, Phys. Rev. B, 69 (2004) 155418. [25] B. Giese, D. McNaughton, Surface-Enhanced Raman Spectroscopic and Density Functional Theory Study of Adenine Adsorption to Silver Surfaces, J. Phys. Chem. B, 106 (2002) 101-112. [26] A. Hamelin, S. Morin, J. Richer, J. Lipkowski, Adsorption of pyridine on the (110) face of silver, J. Electroanal. Chem. Interfac., 272 (1989) 241-252. [27] A. Hamelin, S. Morin, J. Richer, J. Lipkowski, Adsorption of pyridine on the (210) face of silver, J. Electroanal. Chem. Interfac., 304 (1991) 195-209. [28] Z.Q. Tian, B. Ren, Adsorption and reaction at electrochemical interfaces as probed by surface-enhanced Raman spectroscopy, Annu. Rev. Phys. Chem, 55 (2004) 197–229. [29] N.R. Honesty, A.A. Gewirth, Shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) investigation of benzotriazole film formation on Cu(100), Cu(111), and Cu(poly), J. Raman Spectrosc., 43 (2011) 46-50. [30] L. Cui, D. Y. Wu, A. Wang, B. Ren, Z.-Q. Tian, Charge-Transfer Enhancement Involved in the SERS of Adenine on Rh and Pd Demonstrated by Ultraviolet to Visible Laser Excitation, J. Phys. Chem. C, 114 (2010) 16588-16595. [31] R. Huang, H.-T. Yang, L. Cui, D.-Y. Wu, B. Ren, Z. Q. Tian, Structural and Charge Sensitivity of Surface-Enhanced Raman Spectroscopy of Adenine on Silver Surface: A Quantum Chemical Study, J. Phys. Chem. C, 117 (2013) 23730-23737. [32] C. Prado, F. Prieto, M. Rueda, J. Feliu, A. Aldaz, Adenine adsorption on Au(111) and Au(100) electrodes: Characterisation, surface reconstruction effects and thermodynamic study, Electrochim. Acta, 52 (2007) 3168-3180. [33] J. F. Li, Y. J. Zhang, A.V. Rudnev, J.R. Anema, S.-B. Li, W. J. Hong, P. Rajapandiyan, J. Lipkowski, T. Wandlowski, Z.-Q. Tian, Electrochemical Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy: Correlating Structural Information and Adsorption Processes of Pyridine at the Au(hkl) Single Crystal/Solution Interface, J. Am. Chem. Soc., 137 (2015) 2400-2408.