Electromagnetic Field of Plasmonic Nanoparticles Extends the

Aug 11, 2017 - Chemical Engineering, Department of Biomedical Engineering, College of Engineering, The University of Texas at San Antonio, One UTSA Ci...
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Electromagnetic Field of Plasmonic Nanoparticles Extends the Photoisomerization Lifetime of Azobenzene Mahmoud A. Mahmoud* Chemical Engineering, Department of Biomedical Engineering, College of Engineering, The University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249, United States School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States S Supporting Information *

ABSTRACT: The thermodynamically stable azobenzene trans conformation photoisomerizes to the cis form on an ultrashort time scale when photoirradiated with photons of sufficient energy. Irradiating p,p′-dimercaptoazobenzene (DMAB) adsorbed on the surface of plasmonic nanoparticles with a 532 nm laser led to the appearance of Raman bands corresponding to a ring stretching. The torsion of DMAB during the photoisomerization is responsible for the appearance of the ring stretching, which disappeared after several minutes. Colloidally prepared gold nanorods (AuNRs) are assembled into twodimensional (2-D) arrays on the surface of a glass substrate. AuNR 2-D arrays on a glass substrate are coated with silver halfshells (AgHAuNRs) of different thicknesses. The irradiation time required to induce the isomerization of DMAB adsorbed on the surface of AgHAuNRs and the lifetime of the isomerization is increased by increasing the thickness of the silver layer. The plasmon field calculated with the discrete-dipole approximation technique showed that the plasmon field intensity of AgHAuNR excited at 532 nm is decreased upon increasing the thickness of the silver layer. This suggests that the plasmon field intensity is responsible for the photoisomerization of DMAB.

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INTRODUCTION

particles is more limited and based on the change of their size, the plasmon field, and smaller sensitivity factor. The plasmon field enhances the rates of different nonradiative processes, such as retinal photoisomerization,35 the proton pump process in bacteriorhodopsin,36 and electronic relaxation in semiconductors.37,38 Plasmonic nanoparticles have been used to photocatalyze different reactions,39−43 including water splitting,39,44 epoxidation of ethylene,40 and dimerization of 4-nitrothiophenol.14,45,46 Aromatic azo compounds are characterized by the presence of a (−NN−) group. It is present in two different geometrical conformations: the thermodynamically favorable trans form is obtained when the two aromatic groups are located on opposite sides of the azo group, while the two aromatic groups are on the same side in the case of the cis conformation. The trans to cis isomerization can be induced by UV or visible light irradiation. Conversely, thermal heating causes cis isomerization of the trans azo compound. The plasmonic nanoparticles are characterized by their strong scattering and absorption of light, and the absorbed light turns into heat. The light scattered by the plasmonic nanoparticle and the plasmon field can induce the trans to cis isomerization of the azo compounds. Conversely, photothermal

Plasmonic nanoparticles generate an intense electromagnetic field when excited with light at the resonant frequency. Strong scattering and absorption spectra result from the decay of the plasmon field. Because of the excellent optical1−3 and photothermal properties2,4−8 of the plasmonic nanoparticles, they are used in different applications, such as nanosensing,9−12 photocatalysis,13,14 optical switching,15,16 optomagnetic devices,17,18 medicine,19−21 and biology.22,23 The strength of the plasmon field and the optical properties of the nanoparticles are greatly dependent on the shapes of the nanoparticles. Plasmonic nanoparticles were prepared with isotropic shapes such as spheres,24 cubes,25,26 or anisotropic shapes. The anisotropic shapes were prepared with 1-, 2-, or 3-D structures: 1-D as rods27 and bars,28 2-D as disks29,30 and platelets,31 and 3-D as hollow cubes25 and hollow spheres.32 Comparing the optical properties of anisotropic and isotropic plasmonic nanoparticles composed of similar numbers of atoms, the anisotropic shape provides more than one localized surface plasmon resonance (LSPR) spectral peak while the isotropic shape usually has a single LSPR spectral peak.33 It is easy to tune the LSPR peak position of the anisotropic shapes in the visible and NIR regions by varying the aspect ratio (length/diameter) of 1- and 2-D plasmonic nanoparticles and the wall thickness of hollow nanoparticles.33,34 In contrast, the ability to tune the LSPR peak position of isotropic nano© XXXX American Chemical Society

Received: June 27, 2017 Revised: August 11, 2017 Published: August 11, 2017 A

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The Journal of Physical Chemistry C heating by the plasmonic nanoparticles can drive the cis to trans isomerization. It is aimed to study the effect of the plasmon field intensity on the lifetime of the isomerization of azobenzene adsorbed on the surface of plasmonic nanoparticles. However, the lifetime of the photoisomerization of free azobenzene in solution is extended from a picosecond or microsecond time scale47 to minutes for photoisomerization when adsorbed on the surface of excited plasmonic nanoparticles. 2-D arrays of AuNRs or AgHAuNRs were fabricated and used to induce the photoisomerization of azobenzene. AuNR 2-D arrays were fabricated on the surface of a glass substrate by the Langmuir−Blodgett (LB) monolayer assembly of colloidally prepared AuNRs. AgHAuNR 2-D arrays were prepared by the deposition of a silver layer of different thicknesses on top of the AuNRs LB assembly. The optical properties of the AuNR 2-D arrays and the AgHAuNRs at different thicknesses of the silver layer were studied experimentally and theoretically by the discrete-dipole approximation (DDA) technique. The DDA was also used to calculate the plasmon field intensity and distribution for the nanoparticles. The effect of the plasmon field on the isomerization lifetime and the irradiation time required for isomerization of p,p′-dimercaptoazobenzene (DMAB) adsorbed on the surface of AuNRs and AgHAuNRs is discussed. The isomerization of DMAB adsorbed on the surface of AuNRs and AgHAuNRs was followed using the surface-enhanced Raman spectroscopy (SERS) technique.

(Sigma-Aldrich) and 1 mL of 4.1 mM trisodium citrate dihydrate in a 250 mL beaker. The solution was gently stirred for 5 min, and 1 mL of 78 mM l-ascorbic acid was added. The thickness of the deposited Ag layer on top of the AuNRs monolayer was increased by increasing the growth time, 1, 2, and 3 min after the addition of the ascorbic acid. For cleaning the AgHAuNRs, the substrate was transferred from the growth solution to a 250 mL beaker containing 200 mL of DI water and left for 20 min under gentle stirring. Finally, the substrate was rinsed with DI water and allowed to dry for 10 min in air and then 2 h in a desiccator. DMAB was prepared by the photocatalytic dimerization of 4aminothiophenol (4-NTP) adsorbed on the surface of AuNRs and AgHAuNRs. AuNRs and AgHAuNRs on glass substrates were immersed in 0.1 mM aqueous solution of 4-NTP for 10 h. The substrate was transferred to DI water and rinsed with DI water several times. The substrate was then irradiated with a 532 nm laser at a power of 2 mW and diameter of 4 μm for 3 min. A Renishaw In via Raman microscope with a 50× microscope lens and a 532 nm laser of 1 mW power was used for time-dependent SERS measurements. DMAB adsorbed on the surface of AuNRs or AgHAuNRs was irradiated with a 532 nm laser while the SERS measurements were conducted. Scanning electron microscopy (SEM) images of the AuNR arrays on a silicon wafer were obtained using a Zeiss Ultra60. The topography of the nanoparticle arrays was characterized by a Digital Instruments Dimension-3000 atomic force microscope (AFM). ImageJ was used to determine the lengths and the diameters of 300 AuNRs collected from two SEM images. Gwyddion software was used to determine the lengths and heights of 100 AgHAuNRs collected from two AFM images. The average diameter and length of the 300 AuNRs and the average length and height of the 100 AgHAuNRs were calculated by Origin 8.5 software. Statistical analysis showed that the errors of the nanoparticles’ dimension are in the range of the standard deviation. An Ocean Optics HR4000Cg-UVNIR was used for measuring the optical properties of the nanoparticle arrays. The LSPR spectrum and electromagnetic plasmon field of a AuNR (74 nm length and 22 nm diameter) and AgHAuNRs with lengths of 82, 90, and 120 nm and respective heights of 51, 63, and 102 nm were calculated with DDSCAT 6.1. Johnson and Christy’s49 dielectric functions of gold and silver in air were used in the calculation. The dipole density in the shape file was one dipole/nm3. The DDA shape file of the AuNR is a capped cylinder rod placed on a rectangular glass slab. While, the shape file of the AgHAuNRs is a capped cylinder gold rod coated with a flat bottom ellipsoid silver layer and placed on the surface of rectangular glass slab. The separation gap between the glass slab and the AuNR or the AgHAuNRs is 1 nm. The thickness of the glass slab is 15 nm for AuNR and the 82 and 90 nm AgHAuNRs, while a 20 nm thick slab was used in case of the 120 nm AgHAuNRs. The LSPR spectrum and the plasmon field of a 74 nm AuNR with 22 nm diameter and AgHAuNRs with lengths of 82, 90, and 120 nm and respective heights of 51, 63, and 102 nm were calculated with the DDA technique with a 532 nm circularly polarized light excitation.

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EXPERIMENTAL SECTION The seed-mediated technique was used to prepare AuNRs (see the Supporting Information).27,48 A total of 300 mL of the prepared AuNRs was centrifuged at 8000 rpm for 10 min in 50 mL tubes. The precipitated pellets were dispersed in 100 mL of deionized (DI) water and precipitated out by centrifugation at 5000 rpm for 5 min. The precipitated AuNRs were finally dispersed in 50 mL of DI water. Then, 0.3 mL of 1 mM thiolated poly(ethylene glycol) (PEG; Laysan Bio, Mn = 30 000) was added to the AuNRs solution. The resulting solution was placed in a shaker overnight at a speed of 1200 rpm. The PEG was bound to the surface of the AuNRs through the thiol groups; the free PEG was removed by centrifugation of the nanorods’ solution at 6000 rpm for 10 min and by dispersing the precipitate in 50 mL of DI water. The AuNRs were precipitated and dispersed in 50 mL of DI water twice. The AuNRs were then precipitated by 10 min of centrifugation at 5000 rpm and dispersed in 10 mL of ethanol. Finally, the AuNRs in ethanol were centrifuged at 5000 rpm for 20 min and dispersed in a solution of 4 mL of chloroform (Sigma-Aldrich) and 2 mL of ethanol (Sigma-Aldrich). The AuNR 2-D arrays were fabricated as follows: Spraying AuNRs in chloroform−ethanol solution over the water sublayer of an LB trough (Nima 611D) by microsyringe until the D1L75 model pressure sensor reading increased from 0 to 10 mN/ m. The 2-D arrays were then transferred to the surface of quartz and silicon substrates via vertical dipping of 2 mm/min rising speed. The AuNR 2-D arrays on a glass substrate were annealed at 100 °C for 30 min in an oven and washed with DI water after cooling. The arrays were allowed to dry for 10 min in air before being left in a desiccator for 2 h. 2-D arrays of AuNRs and AgHAuNRs of different silver layer thicknesses were prepared as follows: AuNR 2-D arrays on the surface of a glass substrate were immersed vertically in a growth solution containing a mixture of 200 mL of 0.3 mM AgNO3

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RESULTS AND DISCUSSION Fabrication of AuNRs Monolayer Assembly and AgHAuNRs Monolayer. Nanoparticle 2-D arrays on the surface of a substrate are useful in many applications.50−52 Traditional fabrication techniques such as lithography were B

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Figure 1. AFM image of (A) AuNR 2-D arrays and AgHAuNRs resulting from the deposition of silver atoms on a AuNR for 1 (B), 2 (C), and 3 min (D). The rate of deposition of the silver atoms on the side of the AuNRs is higher than the rate of deposition on the tip of the nanorods.

multimetallic nanoparticles, controlling their LB assembly is challenging. Figure 1A shows an AFM image of AuNR 2-D arrays fabricated using the LB technique. The SEM image of the arrays is shown in Figure S1. The average length of the AuNRs is 74.4 ± 4.3 nm, while the average diameter is 22 ± 3.2 nm. After 1 min of silver atoms’ deposition on the surface of AuNRs, the rods turned into a half-rice shape (see Figure 1B). The average length of the AgHAuNRs is 82.3 ± 4.6 nm, and the average height is 51.1 ± 2.3 nm. The height was determined from the 3-D AFM image and high-magnification AFM image in Figures S2B and S3B. This suggests that the rate of deposition of the silver atoms on the side of AuNR is higher than that on the tips. Figure 1C shows the AgHAuNR 2-D arrays resulting from the silver atoms’ deposition on the AuNRs monolayer for 2 min. The average length and height of the obtained AgHAuNRs were increased to 89.6 ± 5.2 and 63.3 ± 3.1 nm, respectively (see Figures S2C and S3C). When the silver atoms’ deposition time was extended to 3 min, the average length of the obtained AgHAuNRs increased to 120.3 ± 6.2 nm while the height increased to 102.5 ± 3.4 nm (see Figures S2D and S3D). In general, the rate of deposition of the silver atoms on the side of the AuNR is higher than the rate of

used to prepare nanoparticle arrays on the surface of smallsurface-area substrates with a limited number of shapes.1,53 The LB technique is a robust technique that is used to assemble colloidally prepared nanoparticles into monolayers on the surface of a substrate.54−57 However, the order of the nanoparticles inside the LB monolayer was improved by functionalizing their surfaces with polymers that are capable of monolayer semicrystallization.58 Nanoparticles made of multiple metals showed improved physical, catalytic, and electrical properties compared with those that were made of a single individual metal.59−61 Fabrication of nanoparticle arrays made of multiple metals on the surface of a substrate using the lithographic1,53 or template-mediated62 techniques is challenging. Colloidal chemical techniques are used to prepare multiplemetallic nanoparticles of different structures such as core− shells,61,63 alloys,64 and multiple shells,65,66 which can be assembled into 2-D arrays using the LB technique. To synthesize multiple-metallic nanoparticles of anisotropic shape, capping materials were added that bind specifically to certain spots of the metallic nanoparticle allowing the second metal to deposit on the uncovered part of the nanoparticle.67 Because of the irregular shapes of the colloidal anisotropic C

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Figure 2. (A) LSPR spectra of AuNRs of average length 74.4 ± 4.3 nm (black) and AgHAuNRs of average length 82.3 ± 4.6 (red), 89.6 ± 5.2 (blue), and 120.3 ± 6.2 nm (olive) measured on the surface of a glass substrate and (B) LSPR spectrum simulated by DDA for a single AuNR (74 nm × 22 nm) and AgHAuNR at lengths of 82, 90, and 120 nm and heights of 51, 63, and 102 nm, respectively and inside AuNR of 74 nm × 22 nm.

Figure 3. (A) SERS spectra of DMAB resulting from 1 min irradiation of 4-ATP adsorbed on the surface of AuNRs (black), 82 nm AgHAuNRs (red), 90 nm AgHAuNRs (blue), and 120 nm AgHAuNRs (olive) with 532 nm laser of 2 mW power. (B) Time-dependent SERS spectra of DMAB adsorbed on the surface of AuNRs collected after different times of irradiation with 1 mW laser. DMAB isomerized after 8 min of laser irradiation. (C) A depiction of the photocatalytic oxidation of 4-ATP into trans-DMAB on the surface of AgHAuNRs induced by laser irradiation, optical isomerization of trans-DMAB into the cis form, and thermal isomerization of cis-DMAB into the trans form.

deposition on its tips. Growing the second metal on the surface of 2-D nanoparticle arrays while they are bound to the surface of the substrate made it possible to prepare 2-D assemblies of anisotropic nanoparticles. Optical Properties of AuNRs Coated with AgHAuNRs on a Glass Substrate. The optical and optoelectrical properties of the excited plasmonic nanoparticles depend on the coherence time of the oscillating conduction band electrons as well as the electron density and distribution. The shape

symmetry affects the electron distribution density while the degree of crystallinity has a great impact on the electron coherence time. Anisotropic plasmonic nanoparticles are used in many applications due to their exciting optical and catalytic properties. Depositing a silver layer on the top of AuNR 2-D arrays bound to the surface of a glass substrate increases the anisotropicity of the anisotropic AuNR. Figure 2A shows the LSPR spectrum of AuNR 2-D arrays on a glass substrate after D

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Figure 4. Time-dependent SERS collected for DMAB after different irradiation times with 532 nm laser while adsorbed on the surface of (A) 90 nm AgHAuNRs and (B) 120 nm AgHAuNRs. The SERS bands at 1475 and 1511 cm−1 assigned as the IPRD are characteristic for the DMAB isomerization. The isomerization time increases by increasing the thickness of the Ag shell coating the surface of the AuNRs.

annealing at 100 °C for 30 min and washing. AuNRs showed a sharp LSPR spectral peak at 642 nm corresponding to the longitudinal electron oscillation and a weak LSPR peak at 506 nm resulting from the transverse plasmon mode. The longitudinal LSPR spectral peak of the AuNRs before washing is at 661 nm, which blue-shifted to 654 nm after heating at 100 °C (Figure S4). Heating and washing of the AuNR arrays remove some PEG molecules bound to the surface of the rods via a thiol group. The removal of the PEG molecules is responsible for the blue shift of the LSPR spectrum. Figure 2A is the LSPR spectrum of AgHAuNRs of an average length of 82.3 ± 4.6 nm and an average height of 51.1 ± 2.3 nm. After the Ag layer deposition, the longitudinal LSPR spectrum of the AuNRs was blue-shifted to 525 nm, and two new peaks appeared at 383 and 340 nm. The LSPR spectrum of the thicker AgHAuNRs of an average length of 89.6 ± 5.2 and an average height of 63.3 ± 3.1 nm was blue-shifted from 525 nm in the thinner AgHAuNRs to 500 nm, while the positions of the peaks at 383 and 340 nm were fixed. Ultimately, AgHAuNRs of an average length and height of the respective orders of 120.3 ± 6.2 and 102.5 ± 3.4 nm showed three LSPR spectral peaks at 435, 383, and 340 nm. The DDA technique was used to simulate the LSPR spectrum of a single AgNR and AgHAuNR at different thicknesses of a Ag layer calculated on the surface of a glass substrate. Figure 2B shows the LSPR spectrum of a AuNR of 74 nm length and diameter of 22 nm. The longitudinal LSPR peak is at 656 nm, while the transverse plasmon peak is at 509 nm. Figure 2B is the LSPR spectrum of AgHAuNR at lengths of 82, 90, and 120 nm and respective heights of 51, 63, and 102 nm. The AgHAuNRs of lengths 82, 90, and 120 nm showed a sharp LSPR spectral peak at 521, 499, and 442 nm, respectively. Two LSPR spectral peaks are observed at 384 and 340 nm for AgHAuNR of different dimensions. Time-Dependent SERS Spectra of DMAB Adsorbed on the Surface of Excited AgHAuNRs. The Raman signal of an analyte located near the surface of a plasmonic nanoparticle is greatly enhanced. Two mechanisms are used to describe the origin of the Raman signal enhancement, electromagnetic68 and chemical.69 The electromagnetic mechanism depends on the strength of the plasmon field of the nanoparticles while the

chemical mechanism depends on the electron transfer between the nanoparticles and the analyte. Consequently, SERS is useful for studying many phenomena that take place on the surface of plasmonic nanoparticles, such as (1) studying the catalysis reactions catalyzed by a plasmonic nanocatalyst,14 (2) probing the change in the temperature on the surface of the nanoparticles, which can be done by comparing the Stokes and anti-Stokes spectra at different temperatures,70 (3) tracking the charge transfer between the plasmonic nanoparticles and the molecular compounds, and (4) monitoring the change of the orientation of the adsorbed molecules on the surface of plasmonic nanoparticles from the change of the relative enhancement of some bands compared with others.71,72 DMAB is an azo-dye compound with thiol groups in the para positions. DMAB can be prepared on the surface of plasmonic nanoparticles from the photocatalytic oxidation of 4-ATP.46 Figure 3A shows the SERS spectrum of 4-ATP adsorbed on the surface of AuNRs and AgHAuNRs of different thicknesses of the silver layer after 1 min of irradiation with a 532 nm laser at 2 mW of power. SERS bands are observed at 1071, 1141, 1185, 1470, and 1571 cm−1, which are assigned to ν(CS), ν(CN), δ(CH), phenyl ring stretch, and ν(CC) of DMAB, respectively. Two SERS bands appeared at 1384 and 1430 cm−1, which are characteristic for ν(NN) stretching. The SERS enhancement factor increases in the order of AuNRs, 120 nm AgHAuNRs, 82 nm AgHAuNRs, and 90 nm AgHAuNRs. In fact, the SERS enhancement by the AgHAuNRs is proportional to the intensity of the LSPR peak at the wavelength of the Raman laser (see Figure 2A). The SERS spectra of DMAB adsorbed on the surface of AuNRs and AgHAuNR were measured after different irradiation times with 532 nm laser of 1 mW power. Figure 3B is the time-dependent SERS spectrum of DMAB on AuNRs. No change in the SERS spectrum was observed before 8 min of photoirradiation except for the intensity of the SERS band at 1475 cm−1 corresponding to the in-plane ring deformation (IPRD), which was increased to a maximum intensity before decreasing after 10 min. Interestingly, the SERS band corresponding to the IPRD disappeared when the 8 min irradiated DMAB was left in the dark for 2 min and remeasured. This suggests that the torsion of DMAB to isomerize on the E

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Figure 5. Plasmon field map contours of (A) a 74 nm AuNR of 22 nm diameter and AgHAuNR of length and height of respective order of (B) 82 and 51 nm, (C) 90 and 63 nm, and (D) 120 and 102 nm, calculated with the DDA technique at 532 nm circularly polarized light excitation.

surface of the AuNRs was accomplished within ∼2 min. The cis−trans isomerization of DMAB could be responsible for the increase in the intensity of the IPRD SERS band that resulted. However, trans-DMAB is isomerized into the cis form when irradiated with 532 nm laser.73 Two possible DMAB isomerization scenarios are suggested: in-plane, which involves a torsional vibration of the double bond between the nitrogen atoms and the in-plane distortion resulting in the inversion of the nitrogen atoms.74 The isomerization through torsion of the phenylene group is recommended for two reasons: the SERS bands corresponding to the ν(−NN−) did not change during the photoirradiation and the DMAB adsorbed on the surface of the AuNRs through the thiol group. The disappearance of the SERS bands that correspond to the IPRD after 10 min in darkness or during irradiation could be due to either the formation of the cis or trans forms. However, the photothermal heating by the plasmonic nanoparticles could induce the isomerization of the cis form to the trans form. Based on the photothermal scenario, after 8 min of irradiation, the thermal isomerization dominates the optical isomerization, and the trans form is favored. Figure 3C depicts the formation of trans-DMAB from the photocatalytic dimerization of 4-ATP, the optical isomerization of trans-DMAB into the cis form, and the thermal isomerization of cis-DMAB into the trans form. It is useful to mention that the background under the SERS spectrum was slightly increased over the laser irradiation time. The SERS background usually originates from either the nanoparticles’ luminescence75 or from the electronic Raman elastic scattering.76 The photothermal heating by the plasmonic nanoparticles is responsible for the change in the background of the SERS spectrum due to the change in the electronic structure of the AuNRs. To confirm the isomerization of DMAB on the surface of plasmonic nanoparticles, time-dependent SERS measurements were conducted for DMAB adsorbed on the surface of AgHAuNRs of different dimensions while being irradiated with a laser. Figure 4A shows the SERS spectra of DMAB adsorbed on the surface of 90 nm AgHAuNRs measured at different irradiation times with a 532 nm laser at 1 mW of power. The SERS spectrum of the DMAB did not change over time for up to 7 min, before a new SERS band appeared at 1513 cm−1. The maximum intensity of the SERS band at 1513 cm−1, assigned as an IPRD, was observed after 8 min of photoirradiation and decreased afterward. The SERS background is slightly increased upon increasing the laser irradiation time. Figure S5A shows the time-dependent SERS spectra of DMAB

adsorbed on the surface of 82 nm AgHAuNRs collected at different laser irradiation times. The SERS bands corresponding to the IPRD were observed after 5 min of laser irradiation at 1478 and 1526 cm−1. Figure 4B shows the SERS spectra of DMAB adsorbed on 120 nm AgHAuNRs after irradiation with a laser measured for 14 min. IPRD bands resulting from the isomerization of DMAB observed at 1475 and 1513 cm−1 appeared after 8 min of laser irradiation, while the maximum band intensity was obtained after 9 min of irradiation. To confirm the obtained results, the irradiation time required to induce the isomerization was determined in case of each AgHAuNR arrays at three different spots, and the values were averaged. Figure 5B shows the relationship between the average heights of the AgAuHRs and the average irradiation times required to induce the DMAB isomerization. Increasing the thickness of the Ag shell deposited on top of the AuNRs in the AgHAuNRs reduced the irradiation time required for the DMAB isomerization and extended the time in which the SERS bands corresponding to the isomerization (IPRD) appear. Effect of Plasmon Field Intensity in the Isomerization of DMAB on AgHAuNRs. The electromagnetic field of the plasmonic nanoparticles decays by emitting and absorbing photons. The intensity of the scattered and absorbed light is directly proportional to the strength of the plasmon field. Consequently, it is useful to discuss the isomerization lifetime and the irradiation time required to induce the optical isomerization of DMAB on the surface of AuNRs and AgHAuNRs, which is based on the plasmon field intensity. Figure 5A shows the plasmon field contours of a AuNR of 74 nm length and 22 nm diameter on a quartz substrate calculated by the DDA technique when excited with circularly polarized 532 nm light. The plasmon field intensity is high on the sides of the nanorod. Figure 5B shows the plasmon field contours of an 82 nm AgHAuNR of 51 nm thickness, 74 nm AuNR coated with a silver layer, calculated by the DDA technique at 532 nm of light excitation. The plasmon field intensity is high at the tips of the AgHAuNR. The plasmon field intensity is high on the tips of the AgHAuNR of lengths 90 and 120 nm and heights of 63 and 102 nm, respectively, when calculated by DDA at 532 nm (see Figure 5C,D). It is clear that as the thickness of the silver layer deposited on the top of AuNR is increased, the plasmon field intensity, calculated at 532 nm, decreased. The irradiation time required to induce the isomerization of DMAB was increased with the same trend of the increase in the plasmon field intensity calculated for the AgHAuNR. The lifetime of the torsion of DMAB during the isomerization was F

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The Journal of Physical Chemistry C also extended as the plasmon field intensity decreased. This suggests that the isomerization of the DMAB is induced by the plasmon field of the plasmonic nanoparticles.

ORCID

Mahmoud A. Mahmoud: 0000-0002-1986-1797

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Notes

The author declares no competing financial interest.

CONCLUSIONS The LB technique was used to assemble AuNRs into 2-D arrays on the surface of a quartz substrate. The AuNR 2-D arrays were coated with a silver half-shell of different thicknesses. A silver layer was deposited on the surface of AuNRs by immersing the arrays on the glass substrate in a solution of silver ions, which were reduced by ascorbic acid into atoms. The thickness of the Ag half-shell was increased by increasing the deposition time. The optical measurement and the theoretical simulation based on the DDA technique showed that the LSPR spectrum of AuNRs blue-shifted as the thickness of the silver half-shell was increased. DMAB was prepared on the surface of AuNRs and AgHAuNRs from the photocatalytic dimerization of 4-ATP adsorbed on their surface. DMAB photoisomerizes from the thermodynamically favorable trans form to the cis form. DMAB in solution isomerizes through deformation of the azo group. Conversely, SERS measurements for DMAB adsorbed on the surface of plasmonic nanoparticles showed that the bands corresponding to the ν(−NN−) did not change during the light irradiation but the intensity of the in-plane ring stretching was increased. This suggested that the isomerization of DMAB adsorbed on the surface of nanoparticles proceeded through the torsion of the phenylene group. SERS measurement showed that the irradiation time required to induce the isomerization of DMAB adsorbed on AgHAuNRs increased by increasing the thickness of the silver half-shell. Unlike the photoisomerization of azo compounds, which takes place on the picosecond or microsecond time scales,47 the isomerization lifetime of DMAB adsorbed on the surface of plasmonic nanoparticles was extended to minutes. DDA calculation of the plasmon field of AgHAuNRs excited at 532 nm showed that the intensity of the field was decreased upon increasing the thickness of the silver half-shell. Interestingly, the lifetime and irradiation time of the isomerization were found to decrease upon increasing the plasmon field intensity.

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ACKNOWLEDGMENTS This work was supported by NSF Grant DMR-1206637. I would like to thank Mr. Jeffrey Geldmeier for conducting the AFM measurements.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06320. Figure S1: SEM image of AuNR monolayer assembly on the surface of silicon substrate. Figure S2: higher magnification AFM image of 2D arrays of AuNRs and AgHAuNRs. Figure S3: 3D AFM image of 2D arrays of AuNRs and AgHAuNRs. Figure S4: LSPR spectrum of AuNR 2D arrays on the surface of a glass substrate measured before and after annealing and after washing. Figure S5A: time-dependent SERS spectrum of DMAB adsorbed on the surface of 82 AgAuHRs collected at different laser irradiation times. Figure S5B: is the relationship between the average irradiation time required to induce the isomerization of DMAB and the heights of AgAuHRs. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. G

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