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Dual-Responsive Reversible Plasmonic Behavior of Core-Shell Nanostructures with pH-Sensitive and Electroactive Polymer Shells Ju-Won Jeon, Jing Zhou, Jeffrey A Geldmeier, James F. Ponder Jr., Mahmoud A. Mahmoud, Mostafa El-Sayed, John R. Reynolds, and Vladimir V. Tsukruk Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04026 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on September 30, 2016

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Chemistry of Materials

Dual-Responsive Reversible Plasmonic Behavior of Core-Shell Nanostructures with pH-Sensitive and Electroactive Polymer Shells

Ju-Won Jeon,a Jing Zhou,a Jeffrey A. Geldmeier,a James F. Ponder Jr.,b Mahmoud A. Mahmoud,c Mostafa El-Sayed,c John R. Reynolds,b Vladimir V. Tsukruk*a a

b

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245 (USA)

School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics, Georgia Tech Polymer Network, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 (USA) c

Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 (USA)

Abstract We report novel dual-responsive plasmonic core-shell anisotropic nanostructures composed of gold nanorod (AuNR) and responsive polyaniline (PANI) shells with plasmonic mode appearance reversibly modulated through orthogonal stimuli; electrical potential and pH change. In this system, the PANI shells provide AuNR cores with three different refractive index environments depending on stimuli (pH and electrical potential). Therefore, no additional secondary responsive component is necessary to induce the dual-responsive properties of AuNR cores. Furthermore, in this study, dualresponsive properties can be realized for nanostructures fixed on substrates whereas previously reported dual-responsive plasmonic systems can only be controlled in solution. Here, the highest localized surface plasmonic resonance (LSPR) shift of the AuNR cores can be induced by changing both local pH and applying electric potential. Notably, significant plasmon band shift by 107 nm is realized with only 8 nm thick PANI shell due to the large refractive index change at the gold-polymer interface. A maximum shift of the longitudinal plasmon mode of 149 nm is obtained by applying modest electrical field (below ±1V), a large shift rarely reported in the literature for metal nanostructures. Moreover, our anisotropic core/shell nanostructures exhibit stable and reversible dual-responsive LSPR behavior over 100 cycles without degradation. Keywords: Dual-responsive plasmonic systems, localized surface plasmon resonance, core/shell nanostructure, electrochromic polymer, tunable plasmonic, polyaniline shells

*Corresponding e-mail: [email protected]

ACS Paragon Plus Environment


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Dual-responsive AuNR@PANI

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INTRODUCTION Noble metal nanoparticles have unique electronic and optical properties that originate from localized surface plasmon resonance (LSPR) light-matter interactions. The free electrons in noble metal nanoparticles that are smaller than the incident light wavelength respond to the light in collective oscillations at specific wavelengths.1,2 This capability to manipulate and control light at the nanoscale can be exploited for numerous applications including biological sensors, catalysis, solar cells, waveguides, and lasers. 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13

In order to obtain targeted LSPR properties of

nanoparticles for specific applications, a variety of synthesis methods have been developed by controlling particle composition, shape, and size.6, 14 , 15 For example, specific LSPR signatures can be obtained for anisotropic plasmonic nanoparticles with different aspect ratios through well-established synthesis techniques of colloidal nanoparticles.16,17 However, in this case, LSPR properties of plasmonic nanoparticles are predetermined by their shape and size during the synthesis, and afterwards it is difficult to reversibly modify LSPR properties.

Hence, the development of tunable and controllable plasmonic systems is essential to realize a new class of advanced applications including tunable lasers, plasmonic modulators, tunable optical filters, and plasmonic switches.9,18,19,20,21,22 To modulate plasmonic signals in a reversible manner, responsive systems have been demonstrated by adopting various functional materials in conjunction with plasmonically active noble metal nanostructure.23,24,25,26,27,28,29,30,31,32,33,34,35,36 To date, various stimuli-responsive systems composed of noble metal nanostructures and active media have been developed, which are responsive to external stimuli.23,24,25,26,27,28,29,30,31,32,33,34,35,36 Control of plasmonic properties with responsive materials is obtained by exploring different mechanisms. responsive materials.

In the first case, external stimuli alter refractive indices of

23,29,32, 37,38,39,40

These altered properties in active materials, in turn,

can lead to changes in the plasmonic frequency and intensity of nanoparticles placed in the vicinity of the responsive materials.23,29,37,38,39

The second approach relies on

external stimuli-induced conformational change by the surrounding active media leading to different plasmonic response as a result of interparticle plasmonic coupling.41,42,43,44,45


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A variety of materials responsive to light, temperature, pH, ionic strength, and electric potential have been combined with noble metal nanoparticles for reversible tuning of plasmonic properties.23,24,25,26 For instance, plasmonic nanoparticles coupled with lightresponsive photochromic molecules including diarylethene and spirophyran exhibited tunable plasmonic behavior due to the refractive index change in photochromic molecules in response to irradiation of light.23,26, 46 , 47

Thermoresponsive poly(N-

isopropylacrylamide) (PNIPAM) was also utilized to modulate LSPR properties because the swelling-deswelling of PANIPAM can vary interparticle distances between plasmonic nanoparticles depending on the temperature, resulting in different plasmonic behavior.43,45,48 Electroactive organic and inorganic materials, which can be reversibly oxidized and reduced upon an applied electrical potential, have proven to be effective media in controlling the plasmonic properties of metal nanostructures.31,32,33,34,35,36,37 In this regard, an electric potential is applied to change the oxidation state of electroactive materials and consequently modify the plasmonic frequency depending on the local refractive index change in the electroactive media.

Smart, multi-responsive plasmonic systems can be desirable platforms for tuning and achieving desired plasmonic properties on demand depending on the usage environment by taking full advantage of their versatility and adaptability. Previously dual-responsive

gold

nanorod/poly(N-isopropyacrylamide)-co-poly(allylacetic

acid)

(poly(NIPAM-co-AAA)) hybrid colloids, which are responsive to pH and temperature have been demonstrated.43

In this approach, copolymers of thermoresponsive

poly(NIPAM) and pH-responsive poly(AAA) were synthesized to create a dualresponsive system.43 The LSPR peak position was tuned by controlling the interparticle distance using pH and temperature as stimuli.43 Other dual-responsive gold/copolymer core/shell structures have been reported using two different polymer blocks where one block is responsive to pH and another block is responsive to temperature.44,45,49,50 In these reports, LSPR properties of gold-containing core/shell structures were controlled based on the degree of aggregation in solution by similarly changing solution pH and temperature.44,45,49,50



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However, to date, all previously reported dual-responsive plasmonic systems have been limited to solution environments due to a fundamentally different plasmonic coupling mechanism, which is not suitable for other device-oriented environments and hampers its wide applications.44,45,49,50

Furthermore, most research on dual-responsive

plasmonic systems has relied on independent pH- and thermo-responsive systems by synthesizing multifunctional copolymers.44,45,49,50

Other types of dual-responsive

plasmonic nanostructures have rarely been reported, and only with a small degree of LSPR modulation such as light- and thermoresponsive plasmonic nanoparticles with a 2 nm LSPR shift. 51 In addition, multiple synthesis steps such as reversible additionfragmentation chain transfer (RAFT) polymerization and surface-initiated atom transfer radical polymerization (SI-ATRP) are necessary to prepare copolymers on gold nanoparticles.44,49,50,51

To the best of our knowledge, dual-responsive plasmonic

systems tunable by both electrical potential and pH changes in the form of fixed nanostructures on substrates have not been reported to date.

Here, we demonstrate a new class of dual (pH/electrical potential) -responsive plasmonic core/shell metal-polymer nanostructures with only one active medium, which exhibit significant reversible and reproducible changes in LSPR signature with plasmonic shift reaching 150 nm, a rarely reported value. Furthermore, dual-responsive control of this plasmonic nanostructure can be accessed in the form of deposited particles on substrates compatible with device environments and not in dilute solutions as

previously

reported.

Specifically,

well-defined

gold

nanorod@polyaniline

(AuNR@PANI) core/shell nanostructures, where the PANI shells act as active media responsive to both an electrical potential and a change in pH, are reported (Scheme 1). The PANI shells can be reversibly changed between emeraldine salt (ES), pernigranline base (PB), and leucoemeraldine base (LB) forms depending on the chemical environments, thus, providing three variable refractive environments for gold nanorods, thus, inducing distinct plasmonic appearance of gold nanorod cores with a significant and reversible shift of plasmonic bands at either electrical potential of pH level (Scheme 1).


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Scheme 1. AuNR@PANI core/shell nanostructures for dual-responsive control of LSPR of AuNR cores. The oxidation state of PANI can be converted using two different independent stimuli; electrical potential and pH change. AuNR@PANI dispersion showed different color depending on the oxidation state of the PANI shell.

EXPERIMENTAL SECTION Materials.

Ammonium

persulfate

(98%),

aniline

(99.5%),

CTAB

(hexadecyltrimethylammonium bromide ≥99%), NaBH4 (98%), HAuCl4-3H2O (≥99.9% trace metals basis), polyethyleneimine (PEI, Mw~25,000), ascorbic acid (ACS reagent, ≥99%), polyaniline (emeraldine base) (Mw = 5,000), dimethylacetamide, and silver nitrate (≥ 99.5%) were purchased from Sigma Aldrich. Sodium chloride and sodium dodecyl sulfate (SDS, 99%) were obtained from Fisher Scientific. Acetone (99.5%) and methanol (99.8%) were purchased from BDH.

Dichloromethane (99.96%) was

purchased from EMD Millipore Chemicals. ITO-coated glass (Rs =15 – 25 ohm) was purchased from Delta Technologies.

Synthesis of AuNRs. The seed-mediated growth technique was used to prepare gold nanorods (AuNRs). In a 30 mL vial, 2.5 mL of a 1.0 mM aqueous solution of HAuCl4 was mixed with 5 mL of a 0.2 M aqueous solution of cetyltrimethylammonium bromide (CTAB). Then, under stirring, 0.6 mL of a 10 mM ice-cold sodium borohydride solution was added. The gold seeds were formed after 5 min of stirring. The growth solution was prepared by mixing 400 mL of a 1.0 mM HAuCl4 aqueous solution with 400 mL of



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0.2 M CTAB in a 500 mL flask. Silver nitrate (4.0 mM) with 12 and 24 mL amounts was added for respectively making short and long AuNRs followed by 5.6 mL of 78.8 mM ascorbic acid. Finally, 0.64 mL seed solution was added to the growth solution and allowed to react overnight.

Synthesis of PANI and AuNR@PANI core/shell nanostructures.

AuNR@PANI

core/shell nanostructures were created based on previously reported methods.29, 52 AuNRs were first centrifuged three times at a speed of 10,000 rpm and 6000 rpm to remove excess CTAB. The AuNR particles were added to a mixture of SDS (40 mM, 0.25 ml) and aniline (2mM, 1.5 ml) and vortexed for 1 min. By adding ammonium persulfate (2 mM, 1.5 ml) in 10 mM HCl to the above solution, polymerization was initiated. After adding ammonium persulfate, the solution was vortexed for 10 s and left undisturbed overnight. After polymerization, the solution was centrifuged at 14,000 rpm in SDS solution (3 ml, 3.6 mM) two times in order to separate AuNR@PANI core/shell nanostructures from PANI homopolymer.

The same polymerization process was

repeated up to three times in order to obtain thicker PANI shells. For comparative purposes, PANI specimens were synthesized using the same method without gold nanorods, followed by dialysis against water overnight.

Deposition of AuNR@PANI core/shell nanostructures on ITO electrodes.

ITO

substrates were cleaned by sequential sonication in dichloromethane, acetone, methanol, and water for 15 min each. Cleaned ITO substrates were coated with PEI by immersing them in PEI (10 mg/ml in water) for 30 min.

Then, PEI-coated ITO

substrates (PEI-ITO) were cleaned with ultrapure water (Nanopure system, Barnstead, resistivity ≥18.2 MΩ). Nanoparticles were spray-coated on PEI-ITO using a spray gun (Iwata HP-CS) at a pressure of 15-20 psi. After deposition, all AuNR@PANI core/shell nanostructures were rinsed with ultrapure water and 0.2 M HCl, sequentially.

Refractive index measurements of PANI materials. First, silicon [100] substrates (Semiconductor Processing) were cleaned with a piranha solution made with a 3:1 ratio of concentrated sulfuric acid and hydrogen peroxide (30%). Caution: piranha treatment


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is extremely dangerous, and protective equipment must be used. Commercial EB PANI (Aldrich) which was used for refractive index measurements was dissolved in dimethylacetamide, and spray-coated on piranha-treated silicon substrates for spectroscopic ellipsometry. The EB film was treated with 1 M HCl vapor for 10 min to obtain an ES PANI film. LB PANI was obtained from ES by exposing the ES film to hydrazine vapor for 5 min. To obtain the PB form of PANI, the EB film on the gold substrate was electrochemically oxidized at 1.0 V (vs. Ag/Ag+) with a Ag/Ag+ reference electrode

(58

mV

tetrabutylammonium

vs.

Ferrocene)

and

a

hexafluorophosphate

in

Pt

counter

propylene

electrode

in

0.5

carbonate.

electrochemical switching, the film was washed with isopropanol.

M

After

Spectroscopic

ellipsometry measurements were carried out using a Woollam M200U ellipsometer at incident angles of 65o, 70 o, and 75 o from 245 – 1000 nm.37,53 Refractive indices of ES, LB, and PB PANI were obtained by peak fitting using the general oscillator model with WVASE32 software.

The ellipsometric data of ES and LB on Si substrates were

modeled using three layers composed of a silicon substrate (1 mm), silicon oxide (2 nm), and a general oscillator layer (25 - 40 nm) while the refractive index of PB was modeled with gold (1 mm) and general oscillator layer. The refractive index data of silicon, silicon oxide, and gold layers were directly taken from the WVASE32 database while a general oscillator layer was fitted with 4 to 8 Gaussian peaks for ES, PB, and LB states of PANI.

Characterization. A Hitachi HT7700 microscope was used to obtain all TEM images at 100 to 120 kV with a carbon-coated Cu TEM grid (Ted Pella). UV-Vis spectra were taken on a Shimadzu UV-2450 spectrophotometer or a Varian Cary 5000 Scan UVVis/NIR spectrophotometer with 1 nm resolution. Electrochemistry measurements were carried out using an EG&G Princeton Applied Research model 273 with Corrware software or a VersaSTAT3-200 (Princeton Applied Research) with Versastudio software in

conjunction

with

UV-Vis

spectroscopy.

For

spectroelectrochemistry,

the

AuNR@PANI core/shell nanostructures on ITO were used as working electrodes with a Pt counter electrode and an Ag/AgCl reference electrode. A solution of 0.5 M NaCl in 0.01 M HCl was used as an electrolyte. A color change was immediately observable when an external electrical potential was applied to AuNR@PANI core/shell



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nanostructures on ITO, indicating the PANI shell’s very fast response to electrical stimuli. However, the UV-Vis spectra were recorded 15 seconds after applying voltages to ensure complete conversion of the PANI shells.

For pH-responsive properties of

AuNR@PANI core/shell nanostructures, AuNR@PANI core/shell nanostructures on ITO were immersed into water whose pH was adjusted by using diluted NaOH or HCl solution.

For cycling tests, 1M HCl and 1M NaOH were used to reversibly switch

between ES and PB states.

FDTD Simulations. Finite-difference time-domain (FDTD) simulations were conducted using Lumerical FDTD Solutions.37

Single gold nanorod was modeled in a water

medium and on ITO substrates with a 2 nm CTAB coating and either ES, LB, or PB PANI shells. Refractive index values for ITO, CTAB and the different PANI states were obtained using ellipsometry while those of gold and water were obtained from the included materials database.

Dimensions for the nanorods and the PANI shell

thicknesses for simulations were obtained from TEM measurements, with the end facets of the nanorods approximated as ellipsoids with a 20 nm radius of curvature.

All

simulations were conducted with a 0.5 nm mesh size in the x-, y-, and z-directions and perfectly matched layer (PML) boundaries. A total-field scattered-field plane wave light source with a wavelength range of 300-1000 nm was used for illumination. Unpolarized light was used to excite the nanorod for a single simulation to confirm the transverse LSPR peak and to confirm that no convolution occurred between the transverse and longitudinal plasmon modes. All further simulations used incident light polarized parallel to the rod length, as the longitudinal plasmon mode was the mode of interest. To obtain the electric field decay length in the vicinity of the gold surfaces, a gold nanorod with a 2 nm CTAB coating in a water medium was further coated with a 1.4 index shell of increasing thickness.

RESULTS Design of AuNR@PANI core/shell nanostructures To realize a dual-responsive plasmonic system, we chose PANI (Figure 1).54,55 PANI exists in five different states: emeraldine salt (ES), emeraldine base (EB),


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leucoemeraldine base (LB), pernigraniline base (PB), and pernigraniline salt (PS) depending on the redox and chemical environment as illustrated in Figure 1a for ES, LB, and PB states.55,56 We synthesized the PANI homopolymer using an oxidative polymerization method used for the AuNR@PANI core/shell nanostructures.38,52 The synthesized PANI was purified via dialysis and deposited on ITO substrates using spray-casting. Subsequently, the oxidation state of PANI was investigated as a function of applied electrical potential and pH (Figure 1).

Figure 1. (a) Chemical structure of ES, PB, and LB states of PANI. UV-vis spectra of PANI films on ITO (b) at different voltages and (c) different pH environments. (d) UV-vis spectra of ES, PB, and LB forms at different conditions.

First, to explore the voltage-dependent properties of PANI films on ITO support, UV-vis spectra of PANI films were recorded when subjected to an external electrical potential in



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an aqueous electrolyte, 0.5 M NaCl in 0.01 M HCl. The voltage window was carefully chosen to convert between ES and LB while preventing overoxidation. At high voltage, 0.5 V (vs. Ag/AgCl), PANI film showed a strong absorption peak around 790 nm (Figure 1b), attributed to the conductive ES state due to its polaron/bipolaron structures.57,58,59 As the voltage was decreased, the absorption peak of the ES gradually decreased and showed almost no absorption in the visible and near IR ranges at -0.3 V. The UV-vis spectra at -0.3 V show LB form of PANI, which is in good agreement with literature (Figure 1b).54,55,57 Thus, the UV-Vis spectra of PANI clearly indicates that PANI can change its oxidation state between the ES and LB forms in response to the applied electrical potential.

The pH-dependent property of PANI films was also investigated independently (Figure 1c). When the pH is low (pH 2), PANI exhibited a strong peak around 785 nm, which is very similar to the UV-vis spectra of PANI at 0.5 V (Figure 1b), indicative of ES state.57,58,59 With increasing pH, this peak continuously blueshifted and exhibited a peak around 560 nm (Figure 1c), which is attributed to the PB state’s energy difference between its HOMO and LUMO level.54,58 These results showed that the oxidation state of PANI can be switched between ES and PB by changing pH environment while ES and LB form can be converted by an applying electrical potential (Figure 1c and 1d).

Spectroscopic ellipsometry measurements were performed to assess the wavelengthdependent refractive indices of the three different states of PANI (ES, LB, and PB) (Figure 2). The EB form of PANI was spray-cast onto Si or Au substrates. The EB deposited on the substrate was chemically converted to the ES form using 1M HCl vapor, and consequently reduced to LB by exposing it to hydrazine vapor. To obtain the refractive index of PB, EB was electrochemically oxidized to PB at a high voltage of 1 V (vs. Ag/Ag+) in a non-aqueous electrolyte (see Experimental). The UV-Vis spectra of chemically or electrochemically converted ES, PB, and LB on ITO substrates substantiate the successful conversion of PANI (Figure S1).


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Figure 2. (a) Real part of the refractive index of PANI in different states, and (b) refractive index change of real part when ES is converted to PB and LB states by different means: solution pH and electrical potential.

Among the three different states of PANI, the ES form had the lowest real refractive index of 1.41 to 1.49 over the visible and near IR range (500-1000 nm) (Figure 2a). In contrast, PB form exhibited a higher real refractive index from 2.03 and 2.08 in the wavelength above 600 nm. The refractive index of LB state varied in the intermediate range from 1.69 to 1.74. These refractive indices are close to other reported values considering high sensitivity of PANI to experimental conditions.60,61 For instance, the obtained refractive index of PB state was slightly higher than previously reported value for, 1.82 at 633 nm possibly due to the different electrochemical conditions used in this study.62 In fact, in order to obtain PB state in our study, 1 V (vs. Ag/Ag+) was applied to



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an EB film in a non-aqueous electrolyte whereas, in earlier report, 0.8 V (vs. Ag/AgCl) was applied to an ES film in an aqueous electrolyte.62

It is important to assess the change in the real part of the refractive index of PANI in its multiple forms because it determines the direction and magnitude of the LSPR shift of the AuNR core surrounded by the PANI shell.9,37 The change in the real refractive indices was calculated when ES is converted to PB and LB states using experimentallycollected data for each wavelength: ∆n(λ) = n(λ)LB/PB − n(λ)ES (Figure 2b). A change in the real refractive index was found to be 0.59 – 0.62 from 600 to 1000 nm when ES was converted to PB state while the change in the real refractive index from ES to LB was close to 0.30 in the wavelength range of 500-1000 nm. This change in refractive index ∆n (maximum achievable value around 0.6) is much larger than that reported for other responsive systems such as photochromic spiropyran doped poly(methyl methacrylate) (∆nmax of 0.19) or polythiophene-based conjugated polymers (∆n max around 0.3).26,37,39 We suggest that the larger ∆n of PANI originated from different chemical configuration with a conjugated path aligned along with the stiff linear backbones in conjunction with the polar ammonium sites, as opposed to the localized π-conjugation with additional side chains in other exploited polymers.

As a result of this high refractive index

variation of PANI, the large LSPR control is expected when PANI is used as a stimuliresponsive matrix with large variation of refractive index. According to the calculated change in real refractive index, we can expect that a higher LSPR shift will be achieved when ES is converted to PB state in the entire wavelength range of 400-1000 nm.

In order to maximize the LSPR shift, we chose to use AuNRs as plasmonic cores due to their higher refractive index sensitivity (RIS) and figure of merit (FOM) in comparison with Au nanospheres or Au nanocubes.14 The RIS is defined as the LSPR shift per refractive index unit, and the FOM is the RIS divided by the full width at half maximum (FWHM) of the plasmonic peak.14 The AuNR was designed to have a LSPR peak of 679 nm in water, which is sufficiently far from the absorption peak of ES (790 nm) and PB (560 nm) (Figure 1c and S2a). Synthesizing of AuNRs with LSPR peak placed far from PANI peaks is preferred to directly identify the LSPR peak positions of AuNRs.


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Statistical analysis from TEM images reveals that the AuNR has a length of 57 ± 9 nm and a width of 22 ± 3 nm with an aspect ratio of 2.6 (Figures S2b, c, d, and e). We also synthesized narrower AuNR possessing an LSPR peak of 794 nm, which is close to the absorption peak of ES at 790 nm (Figure S3a and Figure 1c) to compare the sensitivity of two different AuNRs. The length and the width of this AuNR are 55 ± 5 nm and 14 ± 2 nm, respectively, and its aspect ratio is 3.9 (Figures S3b, c, d, and e). In this study, we will refer to the AuNR with a 679 nm LSPR peak as AuNR-W, and the AuNR with a 792 nm LSPR peak as AuNR-N for convenience.

Synthesis of AuNR@PANI core/shell nanostructures AuNR@PANI core/shell nanostructures were synthesized through multiple oxidative polymerization cycles with pre-synthesized wide and narrow AuNRs.38,52 Briefly, CTABwrapped AuNRs were mixed with aniline monomer and surfactant SDS followed by the addition of an oxidant, ammonium persulfate, to produce the AuNR@PANI core/shell nanostructures.

The well-defined core/shell nanostructures were identified in TEM

images where the PANI shell thickness can be controlled from 4 to 25 nm depending upon the number of polymerization cycles (Figure 3 and S4).

Figure 3. TEM images of AuNR-W@PANI core/shell nanostructures after (a) one, (b) two, (c) three polymerization cycles. TEM images of AuNR-N@PANI core/shell nanostructures after (d) one, (e) two, (f) three polymerization cycles. (g) PANI shell thickness from the TEM images vs. number of polymerization cycles.



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The shell thickness of both AuNR@PANI core/shell nanostructures were obtained through statistical analysis of more than 100 PANI coated nanorods. Regardless of the aspect ratio of the AuNRs, the thickness of the PANI shell consistently increased by repeating polymerization cycles (Figure 3g).

The UV-vis spectra of AuNR-W@PANI and AuNR-N@PANI core/shell nanostructures were investigated in weakly acidic (pH 4-5) and highly basic conditions (Figure 4). As the thickness of the PANI shell was increased, the color of both as-synthesized AuNRW@PANI and AuNR-N@PANI core/shell nanostructures became green (Figure S5a and S5b), originating from the larger amount of PANI absorption for thicker shells.63 For AuNR-W@PANI in acidic conditions, the PANI possessed a LSPR peak of the AuNR-W core around 665 nm, as well as a characteristic ES peak of about 820 nm (Figure 4a), which is supported by the green color of its dispersion (Figure S4a). The plasmonic peak of the AuNR-W@ES PANI remain in similar position around 665 nm regardless of the PANI shell thickness (Figure 4a and c), most likely due to the refractive index of ES being similar to that of CTAB (1.31 to 1.44) and water (1.33).64,65 When the pH was adjusted to 11, the PANI shells changed oxidation state from ES to PB and exhibited an absorption peak of PB around 560 nm, similar to the PANI homopolymer (Figure 4b, S6a and S6b).

In this basic aqueous condition, the AuNR-W@PANI and AuNR-N@PANI core/shell dispersion showed strong violet color especially with thicker PANI shells, which corroborates that PANI exists in PB in basic conditions.55 Interestingly, the LSPR peak of the AuNR-W core hugely redshifted from 673 nm to 811 nm (138 nm LSPR shift) with the 25 nm thick PB PANI shell (Figure 4c) due to the fact that PB has a much higher refractive index (about 2.00 in the range of 600 to 1000 nm) than ES (see Figure 2a). For dispersion of the AuNR-N@PANI in weakly acidic conditions, the absorption peak of the ES shells was completely overlapped with the LSPR peak of the AuNR-N cores, which make it difficult to determine the plasmonic peak positions (Figure 4d). On the other hand, when the PANI shell was converted to PB state, the LSPR peak positions of AuNR-N were sufficiently far from the absorption peak of PB (560 nm) to identify the


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exact peak position (Figure 4e and S5a). With an increase in the PANI shell thickness, the LSPR peak of AuNR-N redshifted due to more of the surrounding medium having a higher refractive index (Figure 4f).

Figure 4. UV-vis spectra of AuNR-W@PANI core/shell nanostructures (a) at natural pH (pH 4-5) and (b) at pH 11 water. (c) The LSPR peak position of AuNR-W@PANI core/shell nanostructures with ES and PB PANI shells. UV-vis spectra of AuNR-N@PANI core/shell nanostructures (d) at natural pH (pH 4-5) and (e) at pH 11. (f) The LSPR peak position of AuNR-N@PANI core/shell nanostructures with PB states of PANI shells.

Dual-responsive plasmonic control AuNR@PANI nanostructures dispersed in water were spray-cast on PEI-modified conductive ITO substrates to increase the adhesion between the core/shell nanoparticles and the substrate.38 Typical AFM images of AuNR-W@PANI and AuNRN@PANI core/shell nanostructures are displayed in Figure S7, showing a random distribution of core/shell particles with a significant amount of aggregation.



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Figure 5. (a) UV-vis spectra of AuNR-W@PANI core/shell nanostructures with 8 nm PANI shells at different voltages (Ag/AgCl), and, (b) longitudinal and (c) transverse mode LSPR peak position and cross-section intensity of AuNR-W vs. voltage.

Having the optimized voltage window, voltage-dependent optical properties of AuNR@PANI core/shell nanostructures were investigated by applying an electrical potential. In Figure 5a, UV-vis spectra of AuNR-W@PANI core/shell nanostructures with 8 nm thick PANI shells deposited on ITO substrates were displayed at different voltages with aqueous electrolyte (0.5 M NaCl in 0.01 M NaCl). At a high positive


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potential of 0.5 V (vs. Ag/AgCl), AuNR-W@PANI core/shell nanostructures displayed both an absorption peak of the ES PANI shell around 820 nm and the longitudinal LSPR peak of the AuNR-W core at 675 nm with a small transverse LSPR peak at 521 nm (Figure 5a), similar to the AuNR-W@PANI dispersion in acidic conditions (Figure 4a). When the voltage was decreased, the peak intensity of ES constantly decreased, which indicates that the ES PANI shell is gradually converted to the LB shell with decreasing voltage.

Consequently, the longitudinal LSPR peak of the AuNR-W core gradually

shifted to higher wavelengths and its cross-section intensity continuously increased (Figure 5b). This redshift originated from the increased refractive index of the LB PANI shell compared to the ES PANI shell, which is consistent with our ellipsometry results and literature results (Figure 2a).38,62,66 The main reason for the damped LSPR peak at high voltage is ascribed to the conductive nature of ES.67

Finally, when the voltage reached -0.3 V (vs. Ag/AgCl), the PANI predominantly existed in the LB state not showing significant absorption in the visible and near IR regions consistent with Figure 1b, and only the longitudinal and transverse LSPR peaks of AuNR-W can be observed (Figure 5a). At -0.3 V, AuNR-W possessed a longitudinal LSPR peak at 717 nm, which is largely redshifted in comparison with the LSPR peak of 674 nm at 0.5 V (Figure 5b). On the other hand, the transverse LSPR peak exhibited only a small change (1-2 nm) with varied electrical potential most likely due to the low sensitivity of the transverse mode (Figure 5c), which agrees well with other reports.68,69 It should be noted that longitudinal modes are more sensitive than transverse modes since electromagnetic field is confined at the tips of AuNRs rather than side of AuNRs.70 Therefore, we will focus our discussion on the longitudinal LSPR peak for the rest of this study. It is notable that by applying an external electrical potential, a maximum LSPR shift of 43 nm can be obtained with only 8 nm thick PANI shells (Figure 5c). This 43 nm LSPR shift of AuNR-W with such a thin shell is significantly higher than the 24 nm LSPR shift previously obtained with gold nanocubes with much thicker PANI shells of approximately 37 nm.38 This larger LSPR shift can be attributed to the higher refractive index sensitivity of AuNR-W as compared to Au nanospheres and Au nanocubes.14 This 43 nm LSPR shift of AuNR-W@PANI was also three-fold larger than the previously



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reported 11 nm LSPR shift of AuNR@PANI induced by an electric field due to much higher local refractive index change of PANI shells.71

Remarkably, truly dual responsive plasmonic tuning can be realized by these core-shell nanostructures by employing either electrochemical and pH-responsive behavior of PANI shells (Figure 2). Indeed, in addition to changes initiated by the electrical potential, much larger LSPR modulation was achieved by the changing environmental pH (Figure 6).

Figure 6. (a) UV-vis spectra of AuNR-W@PANI core/shell nanostructures with 8 nm PANI shells at different environmental pH, and (b) LSPR peak position and extinction intensity of AuNR-W vs. pH.

When AuNR-W@PANI-8 nm core-shell nanostructures on ITO were immersed in aqueous solution with a pH of 2, they showed two distinctive peaks at 675 nm and 830 nm arising from the LSPR peak of AuNR-W and the absorption of ES respectively (Figure 6a). As the pH was increased, the absorption peak at 830 nm of ES PANI


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bleached and the extinction at 560 nm originating from PB PANI increased (Figure 5b). This result is in good agreement with the UV-Vis spectra of synthesized PANI deposited on ITO under the same conditions, indicating that the ES form of PANI is converted to the PB form of PANI as the pH is increased (Figure 1b).

As a result of the increased refractive index of PANI at high environmental pH values, the LSPR peak of AuNR-W cores continuously redshifted from 675 to 782 nm as the pH was increased from 2 to 11 (Figures 6a and 6b). It is worth emphasizing that with only an 8 nm PANI shell, it was possible to induce a dramatically large LSPR shift of 107 nm, which is more than two-fold higher than that obtained by applying an electrical potential (Figure 6b). It should also be noted that the larger refractive index change in the PANI shells between ES and PB forms as compared to ES and LB forms led to this large modulation of LSPR of AuNR-W cores (Figure 2). With 8 nm thick PANI shells, it is possible to directly observe the LSPR peak of the AuNR-W core because the ratio of PANI absorption peak to the LSPR peak of AuNR-W is not significantly high and the LSPR peak of AuNR-W core is sufficiently far from the absorption peak of PANI in both cases of ES and PB forms. In the case of AuNR-N@PANI core/shell nanostructures, it was difficult to identify the LSPR position with the ES PANI shells due to the complete overlap between the LSPR peak of AuNR-N and the absorption of the ES (Figure S8). Nevertheless, the LSPR peaks with PANI shells in LB and PB states were observable at low potential and high pH, respectively. For clarity, we further focus on the AuNRW@PANI core/shell nanostructures.

Subsequently, we investigated combined dual-responsive modulation of AuNRW@PANI core/shell nanostructures with different PANI shell thicknesses to examine conditions that could give the most significant LSPR shift (Figure 7). Regardless of the shell thickness, the oxidation state of the PANI shells can be switched between ES, LB, and PB depending upon environmental pH and electrical potentials similar to the bulk PANI material (Figure 7a and S9).



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Figure 7. UV-vis spectra of AuNR-W@PANI- 14 nm with different oxidation state of PANI shells. (b) LSPR peak position of AuNR-W with different oxidation state of PANI shells. (c) LSPR shift when the ES PANI shell is converted to PB and LB.

When the PANI shell was in the ES state, the absorption of ES became dominant over the LSPR peak of AuNR-W and partially overlapped with the LSPR peak of the AuNRW cores as the PANI shell thickness was increased (Figure 7a and S9). Therefore, peak deconvolution was performed to determine the LSPR peak positions of AuNR-W


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cores for AuNR-W@ES-14 nm and AuNR-W@ES-25 nm (Figure S10). The LSPR peak position of AuNR-W cores with different PANI shells was found to vary as a function of PANI shell thickness (Figure 7b). Interestingly, AuNR-W@ES had a blueshifted LSPR peak on ITO as compared to bare AuNRs on ITO, likely due to the high refractive index of the ITO substrates.37 It should be noted that the PANI shell in this case acts as a spacer and prevents strong coupling interactions between the AuNRs and the ITO substrates.

With the PB and LB PANI shells, the LSPR of AuNR-W core redshifted with increasing shell thickness while increasing ES PANI shell thickness did not significantly alter the LSPR position. The LSPR shift when the ES PANI shell is converted to the PB or LB PANI shell was calculated (Figure 7c). Thicker PANI shells generally resulted in larger LSPR modulation of the AuNR-W core. The maximum LSPR shift of the AuNR-W core was found to be 149 nm for a 25 nm PANI shell. The UV-vis spectra and LSPR position of AuNR-N@PANI core/shell nanostructures exhibited similar trends (Figure S11).

On the contrary, without PANI shells, the AuNR-W exhibited a small LSPR shift in response to an applied electrical potential and pH change. When the pH environment changed from 2 to 11, AuNR-W exhibited only a 7 nm LSPR shift due to difference in local ionic environment.

As electrical potential was varied from 0.5 V to -0.3 V, it

showed even smaller LSPR shift of 1 nm. These results confirm that the LSPR shift of AuNRs core originated from refractive index change of PANI shells rather than local ionic strength or electrical potential.

The orthogonal behavior along with reversibility and stability of the LSPR shifts AuNRW@PANI core/shell nanostructures were also evaluated by sequentially switching the pH environment and electrical potential in continuous series of experiments (Figure 8a). To demonstrate the reversibility of AuNR-W@PANI-8 nm deposited on ITO substrates was exposed to highly acidic and highly basic aqueous solutions (1M HCl and 1M NaOH) sequentially. The LSPR peak changed position between 675 and 782 nm were reversibly obtained when immersed in 1M HCl and 1M NaOH, respectively. As the next



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step, the same sample was immersed in an aqueous electrolyte (1M NaCl in 0.01M HCl), and subjected to a repeated electrical potential switch between 0.5 V and -0.3 V (vs. Ag/AgCl).

In this case, the AuNR-W@PANI also showed reversible LSPR

modulation depending upon the electrical potential applied (675 nm at 0.5 V and 717


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nm at -0.3 V).

When the same procedure was repeated again, we observed very stable and reversible LSPR modulation of AuNR-W@PANI-8 nm core/shell nanostructures without substantial

Figure 8. (a) LSPR peak position of AuNR-W with the 8 nm thick PANI shell upon pH change and electrical potential during cycling, and (b) its cyclic voltammograms at 100 mV/s from -0.3 V to 0.5 V, (c) UV-vis spectra of AuNR-W@PANI core/shell nanostructures with different PANI shells before and after 100 cycling circles.



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LSPR drift when exposed to different pH solutions and electrical potentials (Figure 8a). The LSPR peak position of the AuNR-W core was thus reversibly controlled and placed at 675, 717, and 782 nm by inducing ES, LB, and PB states by two independent ways. Stable reversibility was obtained over more than ten cycling tests (Figure 8a).

To further demonstrate its reversibility, 100 cyclic voltammetric (CV) cycles between 0.3 and 0.5 V (vs. Ag/AgCl) were carried out at 100 mV/s. During the cyclic CV test, no apparent current decrease was observed (Figure 8b), indicating its highly stable responsive properties. The UV-vis spectra of AuNR-W@PANI-8 nm showed a similar response before and after 100 CV cycles (Figure 8c), which also supports its stability and reversibility. Thus, dual-responsive AuNR@PANI core/shell showed significantly enhanced cycling stability in comparison with silver nanocube-electroactive systems.37 This significantly enhanced LSPR modulation can be attributed to the high oxidative stability of AuNR cores and chemically stable PANI shells.

FDTD simulations FDTD simulations were further performed for the AuNR-W in a water medium to verify the LSPR signature of bare gold nanorods.

A transverse peak of 514 nm and a

longitudinal peak of 667 nm were obtained, which are in good agreement with the respective experimental peaks of 512 and 679 nm (Figure 9). The AuNR-W was then modeled with PANI shells of varying thickness on ITO substrates, similar to the experimental setup. Generally, overall trend of LSPR peak position of AuNR-W obtained using FDTD simulation matched well with the experimentally obtained results, showing that the AuNR-W with PB shell had the highest LSPR peak position and AuNR-W with ES shell exhibited the lowest LSPR peak (Table S2). For example, an AuNR-W with an 8 nm thick PANI shell, LSPR peak wavelengths of 688, 723, and 766 nm were found for the respective PANI states of ES, LB and PB. Overall, the ES, LB, and PB state wavelengths agree well with the respective experimental results of 675, 717, and 782 nm although there is a slight deviation between experimental and simulated LSPR position. This deviation can be attributed to the fact that refractive indices obtained from commercial PANI might be slightly different


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from those of PANI shells surrounding AuNRs.

In general, the simulations and

experimental findings are in closer agreement for larger shell thicknesses.

Electromagnetic field monitors at the LSPR peaks for an AuNR with a 14 nm thick PANI shell reveal maximum electric field enhancements of 15, 37, and 26 normalized to the incident light intensity for the respective ES, LB, and PB states (Figure 9, maximum values are normalized for clarity).

Figure 9. Electromagnetic field distributions of AuNR-W with 14 nm thick PANI shells in (a) ES, (b) LB, and (c) PB states. (d) Simulated extinction spectra of AuNR-W with different refractive index of the surrounding medium. (e) LSPR peak position of AuNR-W as a function of a shell thickness calculated for a fixed refractive index of 1.4.

This dependence of electromagnetic field depending upon the oxidation state of PANI shells indicates that light-matter interaction and light-confinement at the AuNR tips can be modulated by changing the state of PANI shells. It is known that the extinction cross-section increased with increasing real refractive index of surrounding medium. 72 However, interestingly, the LB shell with a moderate refractive index change between



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that of ES and PB states provided the highest electromagnetic field enhancement around AuNR-W cores. This phenomenon can be attributed to the negligible absorption of LB state (Figure 1a and b), which ultimately causes less light to be absorbed in the polymeric shell as it passes through. The LB PANI is almost transparent in visible and near IR, which allow more light to reach to the AuNR-W cores whereas the ES and the PB shells absorb significant of light and exhibit green and violet colors.55,73 The PB state did however exhibit the highest degree of light confinement around the AuNR tips in accordance with its high real refractive index (Figure S12).

To calculate the electric field decay length of the AuNRs in this study, an AuNR with a 2 nm CTAB coating in a water medium was further coated with a constant 1.4 refractive index shell of increasing thickness from 0 to 38 nm. Then, the resulting LSPR peaks were fit to the following well-known equation to obtain electric field decay length 74: = + Δ 1 − ⁄ where λLSPR is the LSPR peak of the nanostructure with the shell, λ0 is the LSPR peak of the nanostructure without a shell, m is the refractive index sensitivity, ∆n is the change in the real refractive index between the shell and the medium, d is the thickness of the shell, and ld is the electric field decay length. An m of 215 nm/RIU was calculated based on the LSPR peak differences between a water medium (n=1.33) and a 1.4 index medium. A least squares regression was then used to calculate an electric field decay length of approximately 18 nm (Figure 9e). This result implies that much larger differential changes in the AuNR LSPR peak position will be seen for shell thicknesses between 0 and 17 nm than for shell thicknesses larger than 17 nm. Indeed, this prediction is correlated with experimental results (Figure 7c). For instance, the difference between LSPR peak positions of an AuNR with 8 nm and 14 nm thick shells is 25 nm while the difference between peak positions for 14 nm and 25 nm thick shells is only 8 nm.

General discussion and conclusion


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In this study, we demonstrate exceptionally sensitive dual-responsive core/shell plasmonic nanostructures composed of gold nanorod cores and responsive PANI shells with independent and reversible tuning of plasmonic signature over multiple cycles by either electrical potential or pH changes. These dual responsive modulations of LSPR of AuNR@PANI implies that the frequency of free electron oscillation of AuNR cores based on light-matter interaction can be controlled by two different stimuli with such large degrees of LSPR shift. More importantly, our dual-responsive system has several advantages over previously reported dual-responsive systems.

First, only one active component, the PANI shell, was required to realize dualresponsive plasmonic behavior because of the variety of PANI’s chemical structures as opposed to other dual-responsive plasmonic nanostructures.

It is noteworthy that

previously reported dual-responsive plasmonic systems had to contain two different types of functional responsive elements.44,45,49,50,51 Therefore, no additional complicated synthesis steps are required to obtain dual-responsive plasmonic properties for this study.

To date, dual-responsive plasmonic behavior of metal-polymer core-shell

nanostructures with one polymer component has never been realized. Furthermore, most research on dual-responsive plasmonic systems has been limited to pH- and thermo-responsive systems.44,45,49,50

Second, dual-responsive behavior of AuNR@PANI core/shell nanostructures can be realized in the form of deposited nanostructures on substrates since the LSPR modulation arises from the reversible change in the refractive index of PANI shells, which makes them more suitable for plasmonic devices.

Our dual-responsive

AuNR@PANI core/shell nanostructures are fundamentally different from other previously reported dual-responsive plasmonic systems in terms of the mechanisms. The LSPR modulation of other dual-responsive plasmonic structures is based on gold nanoparticle plasmonic coupling behavior in response to pH and temperature.43,44,45,49,50 Because in these systems LSPR modulation originated from coupling between particles, the LSPR modulation can only be realized in solution and not on substrates due to the fixed nature of deposited nanoparticles.43,44,45,49,50 Moreover, for those systems it is



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challenging to precisely control and determine the LSPR peak position by varying the degree of aggregation.43,44,45,49,50 On the other hand, in the case of our AuNR@PANI, it is more facile to precisely predict the responsive plasmonic properties based on the refractive index variation of the PANI shells and to design nanostructures with specifically tuned plasmonic properties.

Third, a much larger LSPR modulation can be obtained from AuNR@PANI core/shell nanostructures as compared to other dual-responsive plasmonic systems. The maximum LSPR shift achieved in this study was 149 nm with a 25 nm thick PANI shell, which is significantly higher than that of other systems.49,50 For instance, the maximum LSPR shift of protein/polymer-based dual-responsive gold nanoparticles was less than 60 nm when heat was applied.49 Our AuNR@PANI core/shells also showed a larger LSPR shift than pH- and temperature-responsive gold nanoparticles exhibiting an LSPR shift of approximately 30 nm.45

In conclusion, by synthesizing AuNR@PANI core/shell nanostructures, dual-responsive control over the plasmonic properties of the AuNR cores on substrates has been realized with orthogonal reversible and repeatable response to be induced by pH and electrical potential stimuli.

It was found that PANI can change its oxidation states

between ES, LB, and PB depending upon the environmental conditions. Changing the pH can provide the highest refractive index change of the PANI leading to an extremely high LSPR shift of 149 nm for the AuNR core with 25 nm thick PANI shells, which is much larger than other reported dual-responsive plasmonic systems. In addition, these dual-responsive plasmonic structures showed exceptional reversibility and stability over numerous cycles applied in various sequences.

We expect that dual-responsive

AuNR@PANI core/shell nanostructures suggested in this work, can potentially be utilized in various applications including tunable lasers, optical modulators, sensors, and solar cells.

For example, the scattering and absorption range of core/shell

nanostructures embedded in lasing devices can reversibly be modulated using different stimuli, which enables real-time control of light excitation.


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Supporting Information UV-vis spectra and TEM images of PANI, AuNRs, AuNR@PANI core/shell nanostructures, photographs of AuNR@PANI dispersions in water, AFM images of AuNR@PANI core/shell nanostructures on ITO substrates, LSPR peak position and shift of AuNR@PANI core/shell nanostructures from experiments and simulations.

Acknowledgements This work is supported by Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award # DE-FG02-09ER46604 (synthesis of gold nanorods and core/shell nanostructures, and simulations) and the National Science Foundation, CHE-1506046 (synthesis and properties of PANI).



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Table of Contents


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