Article pubs.acs.org/Langmuir
Electrically Modulated Localized Surface Plasmon around SelfAssembled-Monolayer-Covered Nanoparticles Liyuan Ma,†,‡ Shandong Xu,§ Chaoming Wang,⊥,∥ Haining Wang,⊥ Shengli Zou,⊥ and Ming Su*,†,‡,§ †
School of Ophthalmology and Optometry, Eye Hospital, School of Biomedical Engineering, Wenzhou Medical University, Wenzhou, Zhejiang 325001, China ‡ Wenzhou Institute of Biomaterials and Engineering, CNITECH, CAS, Wenzhou, Zhejiang 325001 China § Department of Chemical Engineering, Northeastern University, Boston, Massachusetts 02115, United States ⊥ Department of Chemistry, University of Central Florida, Orlando, Florida 32826, United States ∥ Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610030, China
ABSTRACT: This article reports the observation of electrical modulation of localized surface plasmon around self-assembled monolayer (SAM)-modified gold nanoparticles and the establishment of a new spectroscopy technique, that is, dynamic electrooptical spectroscopy (DEOS). The gold nanoparticles are deposited onto a transparent conductive substrate, and an electrical bias applied on the conductive substrate can cause shift of resonance plasmon response, where the direction of peak shift is related to the polarity of applied bias. The peak shift observed at 2.4 V is approximately ten times larger than those reported in previous work. It is postulated that significant peak shift is the result of reorientation of adsorbed water on electrode, which can change local dielectric environment of nanoparticles. An energy barrier is identified when adsorbed water molecules are turned from oxygen-down to oxygen-up. Frequency-dependent peak shifts on surface-modified gold nanoparticles show that reorientation is a fast reversible process with rich dynamics.
1. INTRODUCTION Water molecules adsorbed at charged solid−electrode interfaces play an important role in chemical and biological processes. The formation of ordered water layers has been predicted and tested with properties different from bulk water such as small spacing, high density, large dielectric constant, and orientation in electrical fields as well as ice-like behavior.1,2 Evidence of ordered water layers had been derived from oscillatory surface forces, scattering intensity fluctuation, and tunnel junction conductance.3−5 Reversing electrical potential reorients adsorbed water molecules between oxygen-up and oxygen-down. However, previous research depends on sophisticated equipment to achieve high sensitivity to a few water layers and do not reveal dynamics of water reorientation. Although nonlinear optical approaches can detect species adsorbed at interfaces, the method is sensitive only to the first (rarely the second or third) layer and often needs bulky optical components (i.e., laser and lens) to offer high sensitivity.6−9 © 2017 American Chemical Society
Achieving convenient and affordable measurements of reorientation dynamics is definitely helpful for fundamental researches. Most important, even if water layer is now a standard image at water−solid interface, there is a lack of technical awareness on the possible application of such ordered structures. In particular, it is unknown whether the unique properties of layered waters such as electrical field induced reorientation can be used to elucidate the properties of surfaceadsorbed species. If a highly sensitive technique for surface analysis could be established to study the layered structure of water on solid, then the method would have impacts for research n interfacial dynamics and chemical properties due to the omnipresent nature of water molecules. Received: September 27, 2016 Revised: December 27, 2016 Published: January 22, 2017 1437
DOI: 10.1021/acs.langmuir.6b03537 Langmuir 2017, 33, 1437−1441
Article
Langmuir
2. EXPERIMENTAL SECTION
Localized surface plasmon resonance (LSPR) is induced by collective motions of electrons of metal nanoparticles such as gold and silver.10−13 The wavelength and intensity of light absorption mainly depend on morphologies (diameter and shape) and environments (dielectric constant and interparticle spacing) of nanoparticles.14−16 LSPR signals are measured with white-light illumination and photodetector in transmission or reflection mode. It is well known that an electrical field induces structure changes of electrically active molecules (i.e., liquid crystals or conducting polymers) deposited around gold nanoparticles and nanorods, causing shifts of LSPR peaks.17−22 Large heterogeneity in the shift of surface plasmon around nanoparticles deposited onto transparent conductive substrate has been observed.23 In particular, a recent article shows electrochemical tuning of the dielectric function of gold nanoparticles,24 in which the tunable response was relatively small (4 nm when electrical bias is changed from −2 to 2 V) and interpreted in terms of a change in charge density, surface damping, and the near-surface volume fraction of nanoparticles and change in the index of fraction of the surrounding electrolyte medium, but there was no discussion of the possible role of adsorbed water, even if it is known for years that water molecules adsorbed on charged solids will form ordered layered structures with significantly different dielectric property.25 We had studied the electrical-field-induced reorientation of water molecules adsorbed onto transparent conductive electrodes by monitoring localized surface plasmon of an array of gold nanoparticles (Scheme 1). Electrical fields applied across
All chemicals are purchased from Aldrich unless mentioned otherwise. Glass substrates covered with a thin film of indium−tin oxide (ITO) are used as transparent conductive substrates (electrical conductivity of 10 S/cm2).26 A 2 nm thick gold film is deposited onto an ITO substrate using electron beam evaporator and annealed at 600 °C in the atmosphere for 10 h to produce plasmonic nanoparticles. The nanoparticle array is characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, and atomic force microscopy (AFM). The aqueous electrolyte solutions are prepared by dissolving certain amounts of inorganic salts (such as Na2SO4, KOH, and NaCl) in deionized water. Cyclic voltammetry is done with a CHI 627C potentiostat to ensure the cleanliness of surface and electrolyte and apply voltages across electrodes in a three-electrode arrangement, where an ITO substrate with gold nanoparticles, a platinum wire, and an Ag/AgCl electrode are used as working electrode, counter electrode, and reference electrode, respectively. The electrodes are installed on a home-built photoelectrochemical cell containing an electrolyte solution. An incoming white light is conducted through optic fiber and directed to one side of the cell, and the transmitted light is collected by another fiber and analyzed using a minispectrometer (Ocean Optics), which has a linear array of photodetectors capable of detecting optical signal at an interval of 10 μs. The broadband continuous white light has an input power of 33W/cm2. The beam size of the nonpolarized white light is 2 mm2.
3. RESULTS AND DISCUSSION Figure 1A,B shows the AFM images of gold film deposited on an ITO substrate before and after annealing. The annealing
Scheme 1. Electrically Reoriented Water Molecules on Metal Nanoparticles and the Setup Used to Measure LSPR Signals in Electrical Field
electrodes change orientation of adsorbed water, which changes local dielectric constant of nanoparticle and leads to shifts of LSPR peaks. An energy barrier exists when ordered water molecules are turned from oxygen-up to oxygen-down configuration. Frequency-dependent peak shifts show that reorientation is a reversible process. Rich reorientation dynamics are observed on gold nanoparticles and selfassembled monolayer-modified gold nanoparticles. A new spectroscopy technique is proposed on the basis of surfaceplasmon-enhanced detection of electrically reoriented water molecules to provide signatures of adsorbed species. This work is important for three reasons. (1) The large peak shift indicates the fundamental cause of electrical modulation of localized surface plasmon should include some factors (i.e., water reorientation) normally not considered in the field of localized surface plasmon. (2) The new spectroscopy technique can be used to study the dynamic response of immobilized species (such as proteins) on solids in an electric field. (3) This work provides a simple yet powerful approach to study water ordering on electrode surface, which is of fundamental importance to many fields.
Figure 1. AFM images of gold film on ITO substrate before (A) and after (B) annealing. XRD spectra of gold film on ITO before (black) and after (after) annealing (C). UV−vis spectra before (black) and after (red) annealing at 600 °C (D).
creates discrete nanoparticles with a surface density of 340/ μm2, diameter of 30 nm, and height of 15 nm. The thermal annealing enhances the crystallization of nanoparticles as in the XRD pattern (shown in Figure 1C),27 where the nanoparticle orientation on ITO is [111] with lattice spacing of 0.1 nm. After annealing, extinction spectrum is stable in air (red curve of Figure 1D), while unannealed film does not show absorption in the wavelength range (black). Although no adhesive layer (chromium) is used under gold film, the annealed gold nanoparticles stick strongly to ITO substrate and have an 1438
DOI: 10.1021/acs.langmuir.6b03537 Langmuir 2017, 33, 1437−1441
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Figure 2. UV−vis extinction spectra of gold nanoparticles after apply a dc voltage ranging from −900 to +900 mV (A). Extinction peak widths and heights at different voltage (B). Electrical-field-induced peak shifts in solutions with different concentration of Na2SO4 (C) and in different types of solutions (D).
Figure 3. LSPR spectra of gold nanoparticles in 1 M Na2SO4 before and after modified with self-assembled monolayers of thiol molecules (A), where black indicates bare nanoparticles, blue indicates hydrophobic ones, and red indicates hydrophilic ones. Electrical-field-induced peak shifts of thiolmodified gold nanoparticles in 1 M Na2SO4 (B), where the three optical micrographs are collected from ITO supported nanoparticles that are bare, hydrophilic-modified, and hydrophobic-modified (B inset).
actually nondistinguishable from those of Na2SO4, as shown in Figure 2D. A threshold is found when a negative voltage is applied on ITO supported gold nanoparticles. As the voltage decreases from 0 V, LSPR peaks do not shift until the voltages reach 0.3 V, suggesting a threshold or energy barrier. To clarify the origin of energy barrier, the surfaces of nanoparticles are modified to be hydrophobic or hydrophilic by self-assembled monolayer of thiol molecules. In brief, ITO substrates with gold nanoparticles are immersed for 30 min in 0.1 M octadecanthiol and mercaptoundecanoic acid in ethanol. After surface modification, LSPR spectra in an aqueous solution are shown in Figure 3A, where the red and blue curves are after the hydrophobic and hydrophilic modifications, respectively. Figure 3B shows the electrical responses of gold nanoparticles before and after modifications. The magnitudes of barriers are highly reproducible in the order of −500, −200, and −100 mV, which are in the same order as water contact angles of hydrophilic nanoparticles (46°), bare gold (66°), and hydrophobic nanoparticles (97°) (Figure 3B inset). Because energy barrier does not exist at positive voltages, the asymmetry in energy
asymmetric shape in the cross-section direction. The LSPR signals are stable over a long time in aqueous solution. Figure 2A shows the peak shifts of gold nanoparticles in 0.1 M Na2SO4 solution after applying direct current (dc) voltage on ITO electrode. A positive voltage from 0 to 600 mV leads to a red shift of LSPR peak from 569 to 591 nm. As the voltage increases, the peak height reduces, but the full width at halfmaximum (fwhm) of peak increases, as shown in Figure 2B. The peak shifts are reversible: Upon reducing voltage to zero, the peak returns to the initial position. Depending on the polarity of voltage, the peak shifts to either long or short wavelength direction. Positive voltage leads to red shift, while negative voltage leads to blue shift. Figure 2C shows the shifts of LSPR peak (λmax 570 nm) as a function of the magnitude and polarity of voltages, where nearly linear relations exist for peak shifts at negative and positive voltages. As the concentration of electrolyte (Na2SO4) increases from 0.001 to 1 M, peak shifts do not show significant difference. The peak shifts do not depend on the nature of electrolytes. The peak shifts in solutions of other electrolytes such as KOH and NaCl are 1439
DOI: 10.1021/acs.langmuir.6b03537 Langmuir 2017, 33, 1437−1441
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Langmuir barriers suggests interaction of electrical field with polarized systems. The following results suggest that the peak shifts cannot be induced by ion adsorptions or electrical field alone. (1) Reducing electrolyte concentrations from 1 to 1 μM or changing to different electrolyte does not change the shape of LSPR peak and magnitude of shift, confirming that the peak shift is independent of the electrolyte. (2) The similar LSPR peaks obtained in different electrolyte, before and after applying voltage, suggest no significant change to surface and structure of nanoparticles. (3) The voltage-nduced peak shift can be attributed to surface reaction on gold nanoparticles,28 but our results do not support the possibility because the shifts are consistent even in a highly corrosive KOH solution. (4) The red and blue shifts of LSPR peaks are not symmetrical, and an energy barrier is observed for each electrolyte and on modified nanoparticle at negative voltage. (5) The electrical field can change distributions of surface electrons29,30 but cannot yield energy barrier only at negative voltage because water molecules adsorbed at zero potential are oriented with oxygen up, and changing the configuration to oxygen down needs extra energy. (6) The energy barrier on hydrophobic particles is lower than that on bare particles. The energy barriers and water contact angles have the same order on hydrophilic, bare, and hydrophobic nanoparticles. Considering the dominant role of water molecules in an aqueous solution and previous results on electrically oriented water molecules,4 it is reasonable to assume the reorientation of waters changes local dielectric environments, which, in turn, significantly shift the LSPR peaks of gold nanoparticles. Water molecules form layered structures with oxygen-down on nanoparticles without applied voltage. An energy barrier will have to be overcome to reverse their orientations to oxygen-up. The magnitude of energy barrier depends on the nature of surface modification. Lower energy barrier corresponds to weaker interaction or less ordered water structures.31 The relaxation time has not been calculated, but due to the rapid motion of water molecule, it is anticipated to be on the level of nanosecond.32 The frequency-dependent reorientation is studied by applying sine wave voltages across ITO electrode (with nanoparticles) and a reference electrode. The frequency and peak value of the voltage are checked using digital oscilloscope. By monitoring the extinction magnitude at certain wavelength (λmax at 0 V), the transmission intensity, Log(IT/I0), is recorded as a function of the voltage and the frequency of applied sine wave signals in a 1 M Na2SO4 solution. Figure 4 shows the highly repeatable response of gold nanoparticles before and after surface modifications. The magnitudes of transmission are in the order of hydrophilic surface, bare nanoparticle, and hydrophobic one. The magnitudes do not change at certain frequency and are larger at 1.5 V than at 0.3 V. At high frequency, the LSPR signals are dominated by the interactions of electrical fields with surface electrons because the reorientation of layered water structures (like-ice) cannot follow electrical fields.33,34 As the frequency decreases, the oscillation pattern changes dramatically as water molecules are forced to reorient by electrical field because energy barriers for reorientation begin to dominate at low frequency. Previous studies believe that electrical modulation of LSPR signal is due to charging effect and report LSPR shifts 1 order of magnitude smaller than those in this work.28 The large peak shift in this work must be the result of significant change of dielectric environment around nanoparticles beyond normal
Figure 4. Transmission intensity as functions of the frequencies of electrical fields for bare and modified gold nanoparticles, where the X axis and Y axis stand for time and transmission intensity, respectively. The total length of the X axis is 20 s for the first three columns (0.998, 0.569, and 0.296 Hz) and is 400 s for the last column (0.009 Hz); the total length of Y axis is 0.05 for each plot.
charging effect. The ordered layered structures of water molecules on solid substrates had been confirmed, and the voltage-dependent reorientation of adsorbed water molecules on electrodes has been confirmed by X-ray scattering and theoretical analysis. Because the group motions of layered water molecules can significantly change dielectric environment, it is possible that the large LSPR peak shift is related to orientation of water molecules at different electrical potential. This work thus provides an optical/plasmonic evidence of water reorientation on nanoparticle-modified electrode surface at different electrical potential. It is thus possible to study surface property or dynamic response of adsorbed species (such as protein) in aqueous solutions by probing reorientation dynamics of water layer in electrical fields.
4. CONCLUSIONS Electrical-field-induced reorientation of water molecules adsorbed at the metal nanoparticle−electrolyte interfaces has been studied by using surface-plasmon-enhanced spectroscopy. The electrical field across electrodes changes orientations of adsorbed water, which, in turn, changes local dielectric environments of nanoparticle and leads to significant shift of LSPR peaks. An energy barrier is found when adsorbed water molecules are turned from oxygen-down to oxygen-up. Frequency-dependent peak shifts show that water reorientation is a fast reversible process. Rich reorientation dynamics have been observed on bare gold nanoparticle and hydrophilic or hydrophobic monolayer-modified nanoparticles. A new spectroscopy method is proposed to study the signatures of adsorbed species based on surface-plasmon-enhanced detection of the electrically reoriented water molecules.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ming Su: 0000-0003-2060-7873 Notes
The authors declare no competing financial interest. 1440
DOI: 10.1021/acs.langmuir.6b03537 Langmuir 2017, 33, 1437−1441
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DOI: 10.1021/acs.langmuir.6b03537 Langmuir 2017, 33, 1437−1441
