1 Electrically Modulated Localized Surface Plasmon around Self-2 Assembled-Monolayer-Covered Nanoparticles3 Liyuan Ma,†,‡ Shandong Xu,† Chaoming Wang,‡,§ Haining Wang,‡ Shengli Zou,‡ and Ming Su*,†,‡

4†Department of Chemical Engineering, Northeastern University, Boston, Massachusetts 02115, United States

5‡Department of Chemistry, University of Central Florida, Orlando, Florida 32826, United States

6§Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Engineering, Southwest

7 Jiaotong University, Chengdu, Sichuan 610030, China

8 ABSTRACT: This article reports the observation of electrical modulation of localized surface plasmon around self-assembled9 monolayer (SAM)-modified gold nanoparticles and the establishment of a new spectroscopy technique, that is, dynamic electro-10 optical spectroscopy (DEOS). The gold nanoparticles are deposited onto a transparent conductive substrate, and an electrical11 bias applied on the conductive substrate can cause shift of resonance plasmon response, where the direction of peak shift is12 related to the polarity of applied bias. The peak shift observed at 2.4 V is approximately ten times larger than those reported in13 previous work. It is postulated that significant peak shift is the result of reorientation of adsorbed water on electrode, which can14 change local dielectric environment of nanoparticles. An energy barrier is identified when adsorbed water molecules are turned15 from oxygen-down to oxygen-up. Frequency-dependent peak shifts on surface-modified gold nanoparticles show that16 reorientation is a fast reversible process with rich dynamics.

1. INTRODUCTION

17 Water molecules adsorbed at charged solid−electrode inter-18 faces play an important role in chemical and biological19 processes. The formation of ordered water layers has been20 predicted and tested with properties different from bulk water21 such as small spacing, high density, large dielectric constant,22 and orientation in electrical fields as well as ice-like behavior.1,2

23 Evidence of ordered water layers had been derived from24 oscillatory surface forces, scattering intensity fluctuation, and25 tunnel junction conductance.3−5 Reversing electrical potential26 reorients adsorbed water molecules between oxygen-up and27 oxygen-down. However, previous research depends on28 sophisticated equipment to achieve high sensitivity to a few29 water layers and do not reveal dynamics of water reorientation.30 Although nonlinear optical approaches can detect species31 adsorbed at interfaces, the method is sensitive only to the first32 (rarely the second or third) layer and often needs bulky optical33 components (i.e., laser and lens) to offer high sensitivity.6−9

34 Achieving convenient and affordable measurements of reor-35 ientation dynamics is definitely helpful for fundamental36 researches. Most important, even if water layer is now a

37standard image at water−solid interface, there is a lack of38technical awareness on the possible application of such ordered39structures. In particular, it is unknown whether the unique40properties of layered waters such as electrical field induced41reorientation can be used to elucidate the properties of surface-42adsorbed species. If a highly sensitive technique for surface43analysis could be established to study the layered structure of44water on solid, then the method would have impacts for45research n interfacial dynamics and chemical properties due to46the omnipresent nature of water molecules.47Localized surface plasmon resonance (LSPR) is induced by48collective motions of electrons of metal nanoparticles such as49gold and silver.10−13 The wavelength and intensity of light50absorption mainly depend on morphologies (diameter and51shape) and environments (dielectric constant and interparticle52spacing) of nanoparticles.14−16 LSPR signals are measured with53white-light illumination and photodetector in transmission or

Received: September 27, 2016Revised: December 27, 2016Published: January 22, 2017

Article

pubs.acs.org/Langmuir

© XXXX American Chemical Society A DOI: 10.1021/acs.langmuir.6b03537Langmuir XXXX, XXX, XXX−XXX

klh00 | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.6.i12 HF01:4457 | 2.0 alpha 39) 2016/10/28 09:46:00 | PROD-JCAVA | rq_7954347 | 1/26/2017 16:57:30 | 5 | JCA-DEFAULT

54 reflection mode. It is well known that an electrical field induces55 structure changes of electrically active molecules (i.e., liquid56 crystals or conducting polymers) deposited around gold57 nanoparticles and nanorods, causing shifts of LSPR58 peaks.17−22 Large heterogeneity in the shift of surface plasmon59 around nanoparticles deposited onto transparent conductive60 substrate has been observed.23 In particular, a recent article61 shows electrochemical tuning of the dielectric function of gold62 nanoparticles,24 in which the tunable response was relatively63 small (4 nm when electrical bias is changed from −2 to 2 V)64 and interpreted in terms of a change in charge density, surface65 damping, and the near-surface volume fraction of nanoparticles66 and change in the index of fraction of the surrounding67 electrolyte medium, but there was no discussion of the possible68 role of adsorbed water, even if it is known for years that water69 molecules adsorbed on charged solids will form ordered layered70 structures with significantly different dielectric property.25

71 We had studied the electrical-field-induced reorientation of72 water molecules adsorbed onto transparent conductive electro-73 des by monitoring localized surface plasmon of an array of gold

s1 74 nanoparticles (Scheme 1). Electrical fields applied across

75 electrodes change orientation of adsorbed water, which changes76 local dielectric constant of nanoparticle and leads to shifts of77 LSPR peaks. An energy barrier exists when ordered water78 molecules are turned from oxygen-up to oxygen-down79 configuration. Frequency-dependent peak shifts show that80 reorientation is a reversible process. Rich reorientation81 dynamics are observed on gold nanoparticles and self-82 assembled monolayer-modified gold nanoparticles. A new83 spectroscopy technique is proposed on the basis of surface-84 plasmon-enhanced detection of electrically reoriented water85 molecules to provide signatures of adsorbed species. This work86 is important for three reasons. (1) The large peak shift indicates87 the fundamental cause of electrical modulation of localized88 surface plasmon should include some factors (i.e., water89 reorientation) normally not considered in the field of localized90 surface plasmon. (2) The new spectroscopy technique can be91 used to study the dynamic response of immobilized species92 (such as proteins) on solids in an electric field. (3) This work93 provides a simple yet powerful approach to study water94 ordering on electrode surface, which is of fundamental95 importance to many fields.

2. EXPERIMENTAL SECTION96 All chemicals are purchased from Aldrich unless mentioned otherwise.97 Glass substrates covered with a thin film of indium−tin oxide (ITO)98 are used as transparent conductive substrates (electrical conductivity of99 10 S/cm2).26 A 2 nm thick gold film is deposited onto an ITO100 substrate using electron beam evaporator and annealed at 600 °C in101 the atmosphere for 10 h to produce plasmonic nanoparticles. The

102nanoparticle array is characterized with scanning electron microscopy103(SEM), X-ray diffraction (XRD) analysis, and atomic force microscopy104(AFM). The aqueous electrolyte solutions are prepared by dissolving105certain amounts of inorganic salts (such as Na2SO4, KOH, and NaCl)106in deionized water. Cyclic voltammetry is done with a CHI 627C107potentiostat to ensure the cleanliness of surface and electrolyte and108apply voltages across electrodes in a three-electrode arrangement,109where an ITO substrate with gold nanoparticles, a platinum wire, and110an Ag/AgCl electrode are used as working electrode, counter111electrode, and reference electrode, respectively. The electrodes are112installed on a home-built photoelectrochemical cell containing an113electrolyte solution. An incoming white light is conducted through114optic fiber and directed to one side of the cell, and the transmitted115light is collected by another fiber and analyzed using a mini-116spectrometer (Ocean Optics), which has a linear array of photo-117detectors capable of detecting optical signal at an interval of 10 μs. The118broadband continuous white light has an input power of 33W/cm2.119The beam size of the nonpolarized white light is 2 mm2.

3. RESULTS AND DISCUSSION120 f1Figure 1A,B shows the AFM images of gold film deposited on121an ITO substrate before and after annealing. The annealing

122creates discrete nanoparticles with a surface density of 340/123μm2, diameter of 30 nm, and height of 15 nm. The thermal124annealing enhances the crystallization of nanoparticles as in the125XRD pattern (shown in Figure 1C),27 where the nanoparticle126orientation on ITO is [111] with lattice spacing of 0.1 nm.127After annealing, extinction spectrum is stable in air (red curve128of Figure 1D), while unannealed film does not show absorption129in the wavelength range (black). Although no adhesive layer130(chromium) is used under gold film, the annealed gold131nanoparticles stick strongly to ITO substrate and have an132asymmetric shape in the cross-section direction. The LSPR133signals are stable over a long time in aqueous solution.134 f2Figure 2A shows the peak shifts of gold nanoparticles in 0.1135M Na2SO4 solution after applying direct current (dc) voltage136on ITO electrode. A positive voltage from 0 to 600 mV leads to137a red shift of LSPR peak from 569 to 591 nm. As the voltage138increases, the peak height reduces, but the full width at half-

Scheme 1. Electrically Reoriented Water Molecules on MetalNanoparticles and the Setup Used to Measure LSPR Signalsin Electrical Field

Figure 1. AFM images of gold film on ITO substrate before (A) andafter (B) annealing. XRD spectra of gold film on ITO before (black)and after (after) annealing (C). UV−vis spectra before (black) andafter (red) annealing at 600 °C (D).

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139 maximum (fwhm) of peak increases, as shown in Figure 2B.140 The peak shifts are reversible: Upon reducing voltage to zero,141 the peak returns to the initial position. Depending on the142 polarity of voltage, the peak shifts to either long or short143 wavelength direction. Positive voltage leads to red shift, while144 negative voltage leads to blue shift. Figure 2C shows the shifts145 of LSPR peak (λmax 570 nm) as a function of the magnitude and146 polarity of voltages, where nearly linear relations exist for peak147 shifts at negative and positive voltages. As the concentration of148 electrolyte (Na2SO4) increases from 0.001 to 1 M, peak shifts149 do not show significant difference. The peak shifts do not150 depend on the nature of electrolytes. The peak shifts in151 solutions of other electrolytes such as KOH and NaCl are152 actually nondistinguishable from those of Na2SO4, as shown in153 Figure 2D.154 A threshold is found when a negative voltage is applied on155 ITO supported gold nanoparticles. As the voltage decreases156 from 0 V, LSPR peaks do not shift until the voltages reach 0.3157 V, suggesting a threshold or energy barrier. To clarify the origin158 of energy barrier, the surfaces of nanoparticles are modified to159 be hydrophobic or hydrophilic by self-assembled monolayer of

160thiol molecules. In brief, ITO substrates with gold nanoparticles161are immersed for 30 min in 0.1 M octadecanthiol and162mercaptoundecanoic acid in ethanol. After surface modification,163 f3LSPR spectra in an aqueous solution are shown in Figure 3A,164where the red and blue curves are after the hydrophobic and165hydrophilic modifications, respectively. Figure 3B shows the166electrical responses of gold nanoparticles before and after167modifications. The magnitudes of barriers are highly reprodu-168cible in the order of −500, −200, and −100 mV, which are in169the same order as water contact angles of hydrophilic170nanoparticles (46°), bare gold (66°), and hydrophobic171nanoparticles (97°) (Figure 3B inset). Because energy barrier172does not exist at positive voltages, the asymmetry in energy173barriers suggests interaction of electrical field with polarized174systems.175The following results suggest that the peak shifts cannot be176induced by ion adsorptions or electrical field alone. (1)177Reducing electrolyte concentrations from 1 to 1 μM or178changing to different electrolyte does not change the shape of179LSPR peak and magnitude of shift, confirming that the peak180shift is independent of the electrolyte. (2) The similar LSPR

Figure 2. UV−vis extinction spectra of gold nanoparticles after apply a dc voltage ranging from −900 to +900 mV (A). Extinction peak widths andheights at different voltage (B). Electrical-field-induced peak shifts in solutions with different concentration of Na2SO4 (C) and in different types ofsolutions (D).

Figure 3. LSPR spectra of gold nanoparticles in 1 M Na2SO4 before and after modified with self-assembled monolayers of thiol molecules (A), whereblack indicates bare nanoparticles, blue indicates hydrophobic ones, and red indicates hydrophilic ones. Electrical-field-induced peak shifts of thiol-modified 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).

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181 peaks obtained in different electrolyte, before and after applying182 voltage, suggest no significant change to surface and structure183 of nanoparticles. (3) The voltage-nduced peak shift can be184 attributed to surface reaction on gold nanoparticles,28 but our185 results do not support the possibility because the shifts are186 consistent even in a highly corrosive KOH solution. (4) The187 red and blue shifts of LSPR peaks are not symmetrical, and an188 energy barrier is observed for each electrolyte and on modified189 nanoparticle at negative voltage. (5) The electrical field can190 change distributions of surface electrons29,30 but cannot yield191 energy barrier only at negative voltage because water molecules192 adsorbed at zero potential are oriented with oxygen up, and193 changing the configuration to oxygen down needs extra energy.194 (6) The energy barrier on hydrophobic particles is lower than195 that on bare particles. The energy barriers and water contact196 angles have the same order on hydrophilic, bare, and197 hydrophobic nanoparticles. Considering the dominant role of198 water molecules in an aqueous solution and previous results on199 electrically oriented water molecules,4 it is reasonable to assume200 the reorientation of waters changes local dielectric environ-201 ments, which, in turn, significantly shift the LSPR peaks of gold202 nanoparticles. Water molecules form layered structures with203 oxygen-down on nanoparticles without applied voltage. An204 energy barrier will have to be overcome to reverse their205 orientations to oxygen-up. The magnitude of energy barrier206 depends on the nature of surface modification. Lower energy207 barrier corresponds to weaker interaction or less ordered water208 structures.31 The relaxation time has not been calculated, but209 due to the rapid motion of water molecule, it is anticipated to210 be on the level of nanosecond.32

211 The frequency-dependent reorientation is studied by212 applying sine wave voltages across ITO electrode (with213 nanoparticles) and a reference electrode. The frequency and214 peak value of the voltage are checked using digital oscilloscope.215 By monitoring the extinction magnitude at certain wavelength216 (λmax at 0 V), the transmission intensity, Log(IT/I0), is recorded217 as a function of the voltage and the frequency of applied sine

f4 218 wave signals in a 1 M Na2SO4 solution. Figure 4 shows the219 highly repeatable response of gold nanoparticles before and220 after surface modifications. The magnitudes of transmission are221 in the order of hydrophilic surface, bare nanoparticle, and222 hydrophobic one. The magnitudes do not change at certain223 frequency and are larger at 1.5 V than at 0.3 V. At high

224frequency, the LSPR signals are dominated by the interactions225of electrical fields with surface electrons because the226reorientation of layered water structures (like-ice) cannot227follow electrical fields.33,34 As the frequency decreases, the228oscillation pattern changes dramatically as water molecules are229forced to reorient by electrical field because energy barriers for230reorientation begin to dominate at low frequency.231Previous studies believe that electrical modulation of LSPR232signal is due to charging effect and report LSPR shifts 1 order233of magnitude smaller than those in this work.28 The large peak234shift in this work must be the result of significant change of235dielectric environment around nanoparticles beyond normal236charging effect. The ordered layered structures of water237molecules on solid substrates had been confirmed, and the238voltage-dependent reorientation of adsorbed water molecules239on electrodes has been confirmed by X-ray scattering and240theoretical analysis. Because the group motions of layered water241molecules can significantly change dielectric environment, it is242possible that the large LSPR peak shift is related to orientation243of water molecules at different electrical potential. This work244thus provides an optical/plasmonic evidence of water245reorientation on nanoparticle-modified electrode surface at246different electrical potential. It is thus possible to study surface247property or dynamic response of adsorbed species (such as248protein) in aqueous solutions by probing reorientation249dynamics of water layer in electrical fields.

4. CONCLUSIONS250Electrical-field-induced reorientation of water molecules251adsorbed at the metal nanoparticle−electrolyte interfaces has252been studied by using surface-plasmon-enhanced spectroscopy.253The electrical field across electrodes changes orientations of254adsorbed water, which, in turn, changes local dielectric255environments of nanoparticle and leads to significant shift of256LSPR peaks. An energy barrier is found when adsorbed water257molecules are turned from oxygen-down to oxygen-up.258Frequency-dependent peak shifts show that water reorientation259is a fast reversible process. Rich reorientation dynamics have260been observed on bare gold nanoparticle and hydrophilic or261hydrophobic monolayer-modified nanoparticles. A new spec-262troscopy method is proposed to study the signatures of263adsorbed species based on surface-plasmon-enhanced detection264of the electrically reoriented water molecules.

265■ AUTHOR INFORMATION266Corresponding Author267*E-mail: m.su@neu.edu.

268ORCID269Ming Su: 0000-0003-2060-7873270Notes271The authors declare no competing financial interest.

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Figure 4. Transmission intensity as functions of the frequencies ofelectrical fields for bare and modified gold nanoparticles, where the Xaxis 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); thetotal length of Y axis is 0.05 for each plot.

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