Light-controlled In Situ Bidirectional Tuning and Monitoring of Gold

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Light-controlled In Situ Bidirectional Tuning and Monitoring of Gold Nanorod Plasmon via Oxidative Etching with FeCl 3

Varsha Thambi, Ashish Kar, Piue Ghosh, and Saumyakanti Khatua J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06679 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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The Journal of Physical Chemistry

Light-controlled In Situ Bidirectional Tuning and Monitoring of Gold Nanorod Plasmon via Oxidative Etching with FeCl3 Varsha Thambi†, Ashish Kar†, Piue Ghosh‡, and Saumyakanti Khatua*† † Discipline of Chemistry, Indian Institute of Technology Gandhinagar, Gujarat, India ‡ Discipline of Electrical Engineering, Indian Institute of Technology Gandhinagar, Gujarat, India

ABSTRACT: We report on light-controlled in situ bidirectional tuning of longitudinal surface plasmon resonance (LSPR) of single gold nanorods via oxidative etching with ferric chloride. By removing the surfactant layer from the surface of a gold nanorod, we demonstrate that the etching happens only in presence of an excitation laser and the etching rate and directionality can be controlled by the intensity of excitation light. At low excitation power, blue shift of a nanorod’s LSPR by up to 50 nm was observed, which indicates preferential etching from its tips. Whereas at high power we see red shift of nanorod’s LSPR by up to 140 nm indicating symmetric etching. These results present a new approach for in situ finer adjustments of a selected nanorod’s plasmon resonance.

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INTRODUCTION Gold nanorods have generated significant interest in last two decades owing to their special optical properties originating from the collective oscillation modes of their conduction electrons, also known as surface plasmons1,2. These nanostructures have two plasmon modes. In the longitudinal mode, electrons oscillate along the nanorod’s principal axis and the resonance of this oscillation depends on the aspect ratio of the nanorod. There are also two degenerate transverse modes where the electrons oscillate perpendicular to the rod axis. The resonance of this oscillation is nearly independent of the nanorod’s aspect ratio. A great advantage of nanorod is that the LSPR can be tuned over a spectral range (above 600 nm), where ohmic loss is significantly less resulting in narrow yet intense resonance3,4. Such strong plasmon resonance enhances an incident optical field near the tips of a nanorod making them useful in a wide range of applications in surface enhanced spectroscopies5–7, sensing8,9, imaging and therapeutics10–13. Success of many of the above-mentioned applications requires precise and in situ control over nanorods’ SPR. For example, strongest fluorescence signal enhancement is achieved when nanoparticles’ SPR is tuned to the excitation laser wavelength7. On the other hand, for sensing applications highest sensitivity is achieved when a nanorod’s SPR is tuned such that the excitation laser energy is at the wing of its resonance14. Typically, SPR wavelengths of nanorods are controlled during their wet-chemical synthesis by adjusting concentrations of gold seeds, silver nitrate, and other additives3,15–17. However, these methods usually produce a broad distribution of nanoparticles’ shapes and sizes. Additionally, these methods do not provide any in situ fine adjustment of nanorod’s SPR.


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Various methods have been developed recently for in situ tuning of nanoparticles’ SPR. The chemical methods rely on post synthesis modification of nanoparticle’s aspect ratios via either etching gold atoms18–20 or depositing gold atoms21 to the nanostructure. In situ tuning of SPR was also reported by electrochemically charging gold nanoparticles22,23. Other methods rely on controlling the refractive index of the surrounding medium24,25. All these methods have their own advantages and limitations. The chemical methods offer broader range of SPR tuning but are often difficult to control, especially when the reaction is very fast18. The refractive index modulation methods on the other hand are reversible and provide external control but the tuning range of SPR is often rather narrow24,25. Here we demonstrate a new approach which allows controlled and in situ tuning of a selected nanorod’s SPR. This is achieved by combining chemical etching with a physical control, light in this case. We carefully adjust the reaction condition such that chemical etching of a gold nanorod does not happen on its own but can be induced when we excite the nanorod with a 532 nm laser. By focusing the excitation laser to a diffraction limited spot, we can then select a nanorod whose resonance is to be tuned. Furthermore, the rate and directionality of the reaction can be controlled by the laser power and the reaction can be stopped completely by switching the laser off at any given time. We find that under low excitation power, a nanorod’s SPR shows a gradual blue shift of up to 50 nm, indicating preferential etching from its tips. At high power, on the other hand, nanorod’s SPR shows large red shift of up to 140 nm, indicating symmetrical etching from all sides.


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Experimental details A. Chemicals and synthesis of gold nanorods Chloroauric acid trihydrate (HAuCl 4 .3H 2 O), Cetyltrimethylammonium bromide (CTAB), LAscorbic Acid (>99% crystalline), Silver nitrate (AgNO 3 ), sodium borohydride (NaBH 4 ), Sodium oleate, hydrochloric acid (HCl, 11.3M), Ferric Chloride was purchased from Sigma-Aldrich. All chemicals were used as received. Milli-Q® water was used for all the synthesis. Gold nanorods were synthesized using a modified seed-growth method given by Murray et.al.16. The seed solution was prepared by adding 5 mL of 0.5 mM of HAuCl 4 to 5mL of 0.2M CTAB. Then 0.6 mL of 0.01 M NaBH 4 was diluted to 1mL and then added to the above solution. The seed solution was aged for 30 minutes before using. Growth solution was prepared by dissolving 2.8 g of CTAB and 0.4936 g of Sodium Oleate in 100 mL of Millipore water. 5 mL of the above binary surfactant solution was taken and 25µL of 40 mM AgNO 3 was added to it. The solution was stirred and then kept undisturbed for 15 minutes. Then 50 µL of 100 mM was added to it and stirred till the solution became colourless. 10 µL of 11.3 M of HCl was then added to the solution and was further stirred for 15 more minutes. 25 µL of 64 mM ascorbic acid was then added into the solution immediately followed by addition of 8 µL of seed solution. The resulting mixture was kept undisturbed for 12 hours. Synthesized gold nanorods were characterized by UV-Visible spectroscopy and FESEM (figure S1). The LSPR of the nanorod was at 714 nm. Average length and width was found out to be 71±8 nm and 26±2nm respectively.


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B. Single particle spectroscopy All single particle measurements were done on a home built confocal microscope (schematic of the setup is shown in figure S2). Particles were excited by a 532 nm Laser. The excitation power was measured at the back aperture of the objective. A spatially filtered beam from a 532 nm DPSS laser (Oxxius Lasers) was focused onto the sample through a 1.25 NA oil immersion objective. The luminescence from the sample is collected by the objective and separated from excitation laser using 532 nm Notch filter. The luminescence light was directed to an avalanche photodiode (Excelitus, SPCM-AQRH-14) for imaging or to the spectrometer with liquid nitrogen cooled CCD detector (iHR-320, Horiba Scientific) to record spectra. One photon luminescence image of nanorods was acquired by scanning the sample across diffraction limited laser focus using XYZ Piezo nano-positioning stage (Physik instruments). The etching experiments were performed in an especially designed flow cell. The flow cell consists of an aluminium frame that incorporates two cover slips separated by a 1mm thick layer of polydiethylsiloxane (PDMS) and a flow channel of width of 5 mm and a length of 40 mm. A syringe pump was used to flow solutions through the flow cell. The flow cell was mounted on a peizo-electric stage for precise positioning of the sample.


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RESULTS AND DISCUSSION Gold nanorods with average dimension of 71±8 nm by 26±2 nm were synthesized by following a modified seed-growth method described elsewhere16. Scanning electron micrograph of the nanorods are shown in figure S1. The longitudinal surface plasmon resonance of these nanoparticles appears at 714 nm in water (figure S1). For single particle studies, nanorods were isolated on a glass substrate via spin coating from a dilute solution. The glass substrate was subsequently washed with warm water repeatedly and treated with UV-ozone to remove CTAB (Cetyltrimethylammonium bromide). On average we get approximately nine particles in 100 μm2 area. The glass substrate, covered with nanorods were then incorporated into a flowcell, through which reactants were flown. Single particle spectroscopy was performed on a home-built confocal microscope, assembled around an inverted microscope (Fig.1a). The details of the setup are given in the supporting information. Briefly, a 532 nm laser was used as excitation source. An oil-immersion objective with numerical aperture (NA) of 1.2 focused the laser on to a diffraction limited spot. The photoluminescence signal from nanoparticles was collected by the same objective, separated from the excitation laser by a notch filter and was guided to either an avalanche photodiode (APD) or a CCD-spectrometer. Photoluminescence images were acquired by scanning the sample across the laser beam using a XYZ piezo scanner. Single particle spectra were acquired by guiding the emitted light to a CCD-spectrometer.


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Figure 1: (a) Simplified schematics of the experimental set up. BS: Beam splitter, NF: Notch filter, FM: Flip Mirror. (b) One-photon excited luminescence image of gold nanorods isolated on a glass coverslip and covered in water. Excitation wavelength was 532 nm and power was 100 kW/cm2. (c) Luminescence spectra (red line) of single nanorod (encircled red in figure 1(b)) and a Lorentiaz fit to the longitudinal plasmon (blue). Inset shows photoluminescence intensities (magenta circles) as function of polarization angle and a fit (black line) to 𝑰𝑰(𝜽𝜽) = 𝑵𝑵[𝟏𝟏 + 𝑴𝑴𝑴𝑴𝑴𝑴𝑴𝑴𝑴𝑴(𝜽𝜽 − 𝝋𝝋)], where 𝑰𝑰(𝜽𝜽) is the intensity as function of polarizer angle (𝜽𝜽), 𝝋𝝋 denotes the orientation of the nanorod’s principal axis, and 𝑴𝑴 is the modulation depth. Figure 1b shows a one-photon luminescence image of gold nanorods isolated on glass substrate and covered with water. Presence of only one nanorod in a diffraction-limited spot was confirmed from their spectral line-shape as well as polarization dependent emission studies. Luminescence spectrum of a single nanorod is known to closely resemble their plasmon resonance, which has a Lorentzian line-shape of the longitudinal plasmon resonance26,27. Presence of nanoparticle aggregate would generally result in a broad spectrum with multiple resonances or resonance at a different energy than expected from inherent size distribution26. Figure 1c shows a typical photoluminescence spectrum of a single gold nanorod and fit to a Lorentzian function. We further confirm our assignment of single particles through polarization study of emitted light by inserting


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a linear polarizer before the APD. Inset of figure 1c shows our results (magenta circles). A fit to, 𝑰𝑰(𝜽𝜽) = 𝑵𝑵[𝟏𝟏 + 𝑴𝑴𝑴𝑴𝑴𝑴𝑴𝑴𝑴𝑴(𝜽𝜽 − 𝝋𝝋)], where 𝑰𝑰(𝜽𝜽) is the intensity as function of polarizer angle (𝜽𝜽),

𝝋𝝋 denotes the orientation of the nanorod’s principal axis, and 𝑴𝑴 is the modulation depth, yields

modulation depth of 0.9 indicating presence of single dipole26. Statistically, we found approximately 80% of the diffraction limited spots contain one nanorod only (more examples

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Figure 2: (a,b) one-photon luminescence image of gold nanorods isolated on a glass substrate and covered with 1 mM FeCl 3 solution before and after 100 seconds of continuous exposure by 532 nm laser on a gold nanorod highlighted by the red circle. Excitation power was 150 kW/cm2 at the sample. (c) Luminescence spectra of a gold nanorod as function of time under continuous excitation. Insets show the corresponding change of plasmon resonance wavelengths and peak intensities as function of time. (d) Luminescence intensity of a gold nanorod as function of time under repeated on-off excitation cycle (blue line). The red line is a guide to eye to the photoluminescence intensity.


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To study the etching of gold nanorods, 1mM FeCl 3 solution was flown through the flow cell. We note that pH of the solution was kept at 1.2 by adding small amount of hydrochloric acid. This prevents formation of ferric hydroxide precipitate. We monitored the reaction by taking photoluminescence spectra of same nanorods before and after flowing FeCl 3 solution. Figure S5 shows typical photoluminescence spectra of a gold nanorod before and after flowing FeCl 3 for 12 hours. We did not see any change of plasmon resonance wavelength or intensity referring that no change of gold nanorod is happening in this time interval of 12 hours. This observation is consistent with the previous reports, which showed that FeCl 3 can’t oxidize Au atoms to Au+1 as Fe3+/Fe2+ reduction potential is well below the oxidation potential of Au0/Au1 19,28. In fact, it was shown that such reaction can only happen in presence of CTAB or some other chemicals, which can reduce the effective reduction potential19,28. However, a completely different effect is seen when a nanorod is under continuous illumination of 532 nm laser. Figures 2(a) and 2(b) show photoluminescence image of gold nanorods covered with 1mM FeCl 3 solution before and after 100 seconds of continuous exposure of 532 nm laser on a gold nanorod highlighted by the red circle. We see that the photoluminescence intensity of the exposed nanorod decreases to a great extend making it indistinguishable from the background (figure 2(b)), whereas no significant intensity change was observed for all the other particles. Change of nanorod’s photoluminescence intensity under continuous illumination was also confirmed by measuring their photoluminescence spectra (Figure 2c), which shows a gradual decrease of photoluminescence intensity with longer exposure time, ultimately rendering it invisible in our microscope. We see a gradual red-shift of nanorod’s plasmon resonance wavelength for initial 40 seconds starting from 621 nm till 667 nm (Figure 2(c), inset). At later


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times, a gradual blue shift from 667 nm to 589 nm was observed before the nanorod’s photoluminescence intensity becomes too low to distinguish it from the background. We attribute the decrease of nanorod’s luminescence intensity and associated shift of its plasmon resonance to etching of gold atoms from the nanorod resulting in decrease of its volume and change in aspect ratio. Luminescence signal is known to be proportional to the absorption cross section, which in turns is proportional to the volume of the particle26. Thus, decrease in volume is expected to decrease the luminescence intensity. The aspect ratio, on the other hand, may decrease or increase depending on the directionality of the reaction. If the reaction happens symmetrically from all directions, or happens preferentially from the sides of nanorod, an overall increase of aspect ratio leading to a red shift of SPR is expected18,20. On the other hand, if the reaction is preferred from the tips, blue shift is expected due to decrease in aspect ratio18,19,28. Previous studies have indeed shown that FeCl 3 can etch gold atoms from CTAB-coated nanorods in solution resulting in a blue shift of its plasmon resonance due to preferential etching of gold atoms from the tips29–31. In our case, however, no CTAB was present and the etching was induced by light. We will discuss about the possible mechanism in a later section. We note that we may discard significant contribution from laser induced surface melting as a reason for decrease of luminescence intensity and spectral shift based on the fact that no significant change of luminescence intensity and/or LSPR wavelength was observed under continuous laser illumination for up to 30 minutes in water but in absence of FeCl 3 (figure S9) at highest power of 250 kW/cm2. This bidirectional etching mechanism was confirmed by comparing SEM images of individual gold nanorods before and after etching with FeCl 3 at different excitation powers (Figures S10 and S11). SEM images show that at low excitation power, etching is preferred from the tips and edges resulting in an overall decrease of aspect ratio (Figure S10). Whereas at high excitation power,


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etching is found to happens from the sides more preferentially leading to an increase in the overall aspect ratio (Figure S11). We also note that some shape deformation is visible at high power. It is important to note that the light induced etching has two key advantages compared to the etching induced by adding chemicals as done previously19,32–37. Firstly, we can select which nanorod we want to etch. This is demonstrated in figure S6, where only selected nanorods were subjected to etching by focusing the excitation laser on them while the other ones remain unchanged. In currently existing chemical methods, all nanorods will react irrespective of whether we want to etch them or not. Secondly, we can stop the etching process instantaneously by switching off the excitation laser. This is demonstrated in figure 2(d), where a nanorod was subjected to repeated laser on-off cycle. The photoluminescence intensity decreases gradually during the laser on time, whereas no change of intensity was observed during the laser off time, indicating that the reaction stops when the excitation laser was switched off.


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Figure 3: (a) Photoluminescence signal of gold nanorods covered with 1mM FeCl 3 as function of time under continuous exposure with 532 nm laser. Decrease in the intensity of gold nanorod signify that there is a decrease in the volume of nanorod with time. The excitation power was 60 kW/cm2 (red), 100 kW/cm2 (black), 150 kW/cm2 (green) and 250 kW/cm2 (blue). Inset shows the reaction times (𝜏𝜏𝑅𝑅 ) as function at different excitation powers. The error bars were calculated from 5 nanorods at each power. (b) Plasmon resonance wavelength shift of gold nanorods as function of time under continuous exposure with 532 nm laser. The excitation power was 60 kW/cm2 (red), 100 kW/cm2 (black), 150 kW/cm2 (green) and 250 kW/cm2 (blue).

The rate of the reaction can be controlled by the excitation laser intensity. This is demonstrated in figure 3(a), which shows the decrease of nanorods’ photoluminescence intensities as function of exposure time for excitation powers between 60 kW/cm2 and 250 kW/cm2 (spectra shown in figure S7). Faster reaction at higher power is clearly evident from figure 3(a). To further quantify this, we define a reaction time (𝜏𝜏𝑅𝑅 ), as the time required for 90% decrease of a nanorod’s initial ACS Paragon Plus Environment

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intensity. We find that 𝜏𝜏𝑅𝑅 decreases by over an order of magnitude from 1050 ± 100 seconds at 60

kW/cm2 to 20 ± 5 seconds at 250 kW/cm2. Particle to particle variations are shown in figure S12-

S16. Such variations could be due to the inherent volume distribution present within the sample. Shift of longitudinal plasmon resonance wavelength also shows a strong dependence on the excitation power (figure 3(b)). At lowest power of 60 kW/cm2, a gradual blue shift of up to 50 nm (red line in figure 3b) is observed, whereas at the highest power of 250 kW/cm2 no blue shift was visible. Instead, large red shift of up to 143 nm is observed (blue line in figure 3b). For intermediate powers of 100 and 150 kW/cm2, LSPR showed initial red shift, which was followed by a blue shift. Oxidative etching of gold atoms from gold nanorods using FeCl 3

has been demonstrated

previously by various groups19,29,30,37. Group of Toste have also shown that light can accelerate this process29. In their study, the authors used CTAB coated nanorods dispersed in water. In absence of any external laser source FeCl 3 etched the nanorods from the tips which have less CTAB coverage resulting in a blue shift of their LSPR wavelengths. When excited with a laser source the rate of blue shift was found to be much faster. The accelerated reaction rate was explained via the generation of hot electrons from the laser excitation29. Surface plasmons decays via Landau damping, whereby the energy of the plasmon oscillation is transferred to individual charge carriers (holes or electrons)

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. These individual carriers then possess more energies

compared to their peers and therefore, are referred as “hot” carriers (hot electrons or hot holes). Because of their higher energy, they may induce chemical reactions, which are otherwise not possible by regular carriers. Several hot electrons and hot holes induced chemical reactions have been reported in recent past

38–40

. In case of etching of gold nanorod with FeCl 3 , hot-electron

mechanism was recently proposed by the groups of Alivasatos and Toste29. A hot electron generated from light excitation, reduces a Fe3+ to Fe2+ leaving the nanorod positively charged.


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When such a reduction process repeats three times, Au3+ ion is formed, which later reacts with gold nanorods to form Au+. Intermediate Au3+ as well as final Au+ were both experimentally verified29. This study had also revealed that the etching follows first order kinetics with respect to FeCl 3 concentration29. Such hot carriers based etching mechanism may also be present in our experiment but there are two major differences with the previous report. Firstly, the reaction rate is not linear with respect to the excitation power, and secondly the directionality of the LSPR shift and hence the directionality of the etching reaction is found to be dependent on excitation power. These observations can be explained from a combination of hot electrons induced chemistry as well as photothermal effects. At lowest power, when temperature rise in not quite significant (~7 K, figure S8) the reaction is expected to be dominated by the hot electrons leading to chemical reaction from tips and edges of nanorod and hence the blue shift of nanorod’s LSPR, as observed in previous report by Toste et al. 29. On the other hand, at the highest excitation power of 250 kW/cm2 we expect temperature rise in excess of 27 K (figure S8), which is significant in the context of a rather low activation barrier of the reaction (2.87 kCal/mol)29, making the chemical reaction possible for all gold atoms, irrespective of their location on nanorod’s surface. This is expected to result in red shift of nanorod’s plasmon as seen in our results and also demonstrated in case of etching with potassium cyanide18. We note that there is an ‘induction time’, after which the plasmon resonance shift is noticeable (figure 3b). This ‘induction time’ is more prominent at low excitation powers but gets shorter as the excitation power increases. We do not know the exact reason behind existence of such ‘induction time’ and detailed optical and high-resolution transmission electron microscopic investigation at the early stage of etching process will be required to understand the origin of this ‘induction time’.


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CONCLUSIONS In conclusion, we demonstrated light-controlled in situ tuning of gold nanorods’ plasmon resonance wavelength to either longer or shorter wavelengths using oxidative etching with FeCl 3 . Unlike the currently available post synthetic chemical modification methods, this approach allows tuning of a selected nanorod of interest without affecting the other ones. Furthermore, we show that the directionality and rate of the oxidative etching can be controlled by excitation laser intensity. These findings will be useful for in situ finer adjustments of nanorod’s plasmon resonance particularly in bringing it to a specific wavelength required for their various applications in sensing and plasmon-enhanced spectroscopies. While the bidirectional etching mechanism was qualitatively explained by a combination of hot electrons and photothermal effects, a more quantitative understanding will require further experimental and theoretical studies and will be communicated separately. It will also be interesting to probe the effect of counter ions by studying different ferric salts such as FeBr 3 and FeF 3 . Supporting

Information:

Figures

S1-S16

containing

experimental

details,

Optical

characterization of single nanorods, Chemical etching in absence of light, sequential etching on nanorods, effect of laser induced heating. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENTS We acknowledge financial support from Science and Engineering research board, India (project: EMR/2015/ 0013), DST NanoMission, India (project: SR/NM/NS-65/2016) and a seed grant from IIT Gandhinagar. We thank Dr. Ravi Hedge (IIT Gandhinagar), Prof. Michel Orrit (Leiden University, The Netherlands), and Prof. Stephan Link (Rice University, USA) for many fruitful discussions during preparation of this manuscript. REFERENCES (1)

Zijlstra Peter; Orrit, M. Single Metal Nanoparticles: Optical Detection, Spectroscopy and Applications. Reports Prog. Phys. 2011, 74, 106401.

(2)

Chen, H.; Shao, L.; Li, Q.; Wang, J. Gold Nanorods and Their Plasmonic Properties. Chem. Soc. Rev. 2013, 42, 2679–2724.

(3)

Leonid, V.; P., K. B.; R., Z. E. Functional Gold Nanorods: Synthesis, Self‐Assembly, and Sensing Applications. Adv. Mater. 2012, 24, 4811–4841.

(4)

Sönnichsen, C.; Franzl, T.; Wilk, T.; von Plessen, G.; Feldmann, J.; Wilson, O.; Mulvaney, P. Drastic Reduction of Plasmon Damping in Gold Nanorods. Phys. Rev. Lett. 2002, 88, 77402.

(5)

Alvarez-Puebla, R. A.; Agarwal, A.; Manna, P.; Khanal, B. P.; Aldeanueva-Potel, P.; Carbó-Argibay, E.; Pazos-Pérez, N.; Vigderman, L.; Zubarev, E. R.; Kotov, N. A.; et al. Gold Nanorods 3D-Supercrystals as Surface Enhanced Raman Scattering Spectroscopy Substrates for the Rapid Detection of Scrambled Prions. Proc. Natl. Acad. Sci. 2011, 108 , 8157 LP-8161.


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(6)

Lin, K.-Q.; Yi, J.; Hu, S.; Liu, B.-J.; Liu, J.-Y.; Wang, X.; Ren, B. Size Effect on SERS of Gold Nanorods Demonstrated via Single Nanoparticle Spectroscopy. J. Phys. Chem. C 2016, 120, 20806–20813.

(7)

Khatua, S.; Paulo, P. M. R.; Yuan, H.; Gupta, A.; Zijlstra, P.; Orrit, M. Resonant Plasmonic Enhancement of Single-Molecule Fluorescence by Individual Gold Nanorods. ACS Nano 2014, 8, 4440–4449.

(8)

Ament, I.; Prasad, J.; Henkel, A.; Schmachtel, S.; Sönnichsen, C. Single Unlabeled Protein Detection on Individual Plasmonic Nanoparticles. Nano Lett. 2012, 12, 1092–1095.

(9)

Zijlstra, P.; Paulo, P. M. R.; Orrit, M. Optical Detection of Single Non-Absorbing Molecules Using the Surface Plasmon Resonance of a Gold Nanorod. Nat. Nanotechnol. 2012, 7, 379– 382.

(10)

Carattino, A.; Keizer, V. I. P.; Schaaf, M. J. M.; Orrit, M. Background Suppression in Imaging Gold Nanorods Through Detection of Anti-Stokes Emission. Biophys. J. 2016, 111, 2492–2499.

(11)

Huang, X.; El-Sayed, I. H.; Qian, W.; El-Sayed, M. A. Cancer Cell Imaging and Photothermal Therapy in the Near-Infrared Region by Using Gold Nanorods. J. Am. Chem. Soc. 2006, 128, 2115–2120.

(12)

Dreaden, E. C.; Alkilany, A. M.; Huang, X.; Murphy, C. J.; El-Sayed, M. A. The Golden Age: Gold Nanoparticles for Biomedicine. Chem. Soc. Rev. 2012, 41, 2740–2779.

(13)

Cole, J. R.; Mirin, N. A.; Knight, M. W.; Goodrich, G. P.; Halas, N. J. Photothermal Efficiencies of Nanoshells and Nanorods for Clinical Therapeutic Applications. J. Phys.


17


Page 18 of 22

Chem. C 2009, 113, 12090–12094. (14)

Beuwer, M. A.; Prins, M. W. J.; Zijlstra, P. Stochastic Protein Interactions Monitored by Hundreds of Single-Molecule Plasmonic Biosensors. Nano Lett. 2015, 15, 3507–3511.

(15)

Ye, X.; Jin, L.; Caglayan, H.; Chen, J.; Xing, G.; Zheng, C.; Doan-Nguyen, V.; Kang, Y.; Engheta, N.; Kagan, C. R.; et al. Improved Size-Tunable Synthesis of Monodisperse Gold Nanorods Through the Use of Aromatic Additives. ACS Nano 2012, 6, 2804–2817.

(16)

Ye, X.; Zheng, C.; Chen, J.; Gao, Y.; Murray, C. B. Using Binary Surfactant Mixtures to Simultaneously Improve Dimensional Tunability and Monodispersity in the SeededGrowth of Gold Nanorods Using Binary Surfactant_Supporting Info. Nano Lett. 2013, 13, 765–771.

(17)

Scarabelli, L.; Sánchez-Iglesias, A.; Pérez-Juste, J.; Liz-Marzán, L. M. A “Tips and Tricks” Practical Guide to the Synthesis of Gold Nanorods. J. Phys. Chem. Lett. 2015, 6, 4270– 4279.

(18)

Carattino, A.; Khatua, S.; Orrit, M. In Situ Tuning of Gold Nanorod Plasmon Through Oxidative Cyanide Etching. Phys. Chem. Chem. Phys. 2016, 18, 15619–15624.

(19)

Zhang, H. Z.; Li, R. S.; Gao, P. F.; Wang, N.; Lei, G.; Huang, C. Z.; Wang, J. Real-Time Dark-Field Light Scattering Imaging to Monitor the Coupling Reaction with Gold Nanorods as an Optical Probe. Nanoscale 2017, 9, 3568–3575.

(20)

Xie, T.; Jing, C.; Ma, W.; Ding, Z.; Gross, A. J.; Long, Y. T. Real-Time Monitoring for the Morphological Variations of Single Gold Nanorods. Nanoscale 2015, 7, 511–517.

(21)

Violi, I. L.; Gargiulo, J.; von Bilderling, C.; Cortés, E.; Stefani, F. D. Light-Induced


18

Page 19 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Polarization-Directed Growth of Optically Printed Gold Nanoparticles. Nano Lett. 2016, 16, 6529–6533. (22)

Novo, C.; Funston, A. M.; Gooding, A. K.; Mulvaney, P. Electrochemical Charging of Single Gold Nanorods. J. Am. Chem. Soc. 2009, 131, 14664–14666.

(23)

Hoener, B. S.; Zhang, H.; Heiderscheit, T. S.; Kirchner, S. R.; De Silva Indrasekara, A. S.; Baiyasi, R.; Cai, Y.; Nordlander, P.; Link, S.; Landes, C. F.; et al. Spectral Response of Plasmonic Gold Nanoparticles to Capacitive Charging: Morphology Effects. J. Phys. Chem. Lett. 2017, 8, 2681–2688.

(24)

Chu, K. C.; Chao, C. Y.; Chen, Y. F.; Wu, Y. C.; Chen, C. C. Electrically Controlled Surface Plasmon Resonance Frequency of Gold Nanorods. Appl. Phys. Lett. 2006, 89, 103107.

(25)

Joshi, G. K.; Blodgett, K. N.; Muhoberac, B. B.; Johnson, M. A.; Smith, K. A.; Sardar, R. Ultrasensitive

Photoreversible

Molecular

Sensors

of

Azobenzene-Functionalized

Plasmonic Nanoantennas. Nano Lett. 2014, 14, 532–540. (26)

Yorulmaz, M.; Khatua, S.; Zijlstra, P.; Gaiduk, A.; Orrit, M. Luminescence Quantum Yield of Single Gold Nanorods. Nano Lett. 2012, 12, 4385–4391.

(27)

Tcherniak, A.; Dominguez-Medina, S.; Chang, W.-S.; Swanglap, P.; Slaughter, L. S.; Landes, C. F.; Link, S. One-Photon Plasmon Luminescence and Its Application to Correlation Spectroscopy as a Probe for Rotational and Translational Dynamics of Gold Nanorods. J. Phys. Chem. C 2011, 115, 15938–15949.

(28)

Wang, J.; Zhang, H. Z.; Liu, J. J.; Yuan, D.; Li, R. S.; Huang, C. Z. Time-Resolved Visual Detection of Heparin by Accelerated Etching of Gold Nanorods. Analyst 2018, 143, 824–


19


Page 20 of 22

828. (29)

Zhao, J.; Nguyen, S. C.; Ye, R.; Ye, B.; Weller, H.; Somorjai, G. A.; Alivisatos, A. P.; Dean Toste, F. A Comparison of Photocatalytic Activities of Gold Nanoparticles Following Plasmonic and Interband Excitation and a Strategy for Harnessing Interband Hot Carriers for Solution Phase Photocatalysis. ACS Cent. Sci. 2017, 3, 482–488.

(30)

Zou, R.; Guo, X.; Yang, J.; Li, D.; Peng, F.; Zhang, L.; Wang, H.; Yu, H. Selective Etching of Gold Nanorods by Ferric Chloride at Room Temperature. CrystEngComm 2009, 11, 2797–2803.

(31)

Wu, S.; Li, D.; Gao, Z.; Wang, J. Controlled Etching of Gold Nanorods by the Au(III)CTAB Complex, and Its Application to Semi-Quantitative Visual Determination of Organophosphorus Pesticides. Microchim. Acta 2017, 184, 4383–4391.

(32)

Zhang, Z.; Chen, Z.; Pan, D.; Chen, L. Fenton-like Reaction-Mediated Etching of Gold Nanorods for Visual Detection of Co2+. Langmuir 2015, 31, 643–650.

(33)

Zhang, Z.; Chen, Z.; Chen, L. Ultrasensitive Visual Sensing of Molybdate Based on Enzymatic-like Etching of Gold Nanorods. Langmuir 2015, 31, 9253–9259.

(34)

Lee, S.; Nam, Y.-S.; Choi, S.-H.; Lee, Y.; Lee, K.-B. Highly Sensitive Photometric Determination of Cyanide Based on Selective Etching of Gold Nanorods. Microchim. Acta 2016, 183, 3035–3041.

(35)

Zhu, Q.; Wu, J.; Zhao, J.; Ni, W. Role of Bromide in Hydrogen Peroxide Oxidation of CtabStabilized Gold Nanorods in Aqueous Solutions. Langmuir 2015, 31, 4072–4077.

(36)

Yuan, H.; Janssen, K. P. F.; Franklin, T.; Lu, G.; Su, L.; Gu, X.; Uji-I, H.; Roeffaers, M. B.


20

Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


J.; Hofkens, J. Reshaping Anisotropic Gold Nanoparticles Through Oxidative Etching: The Role of the Surfactant and Nanoparticle Surface Curvature. RSC Adv. 2015, 5, 6829–6833. (37)

Khanadeev, V. A.; Khlebtsov, N. G.; Burov, A. M.; Khlebtsov, B. N. Tuning of Plasmon Resonance of Gold Nanorods by Controlled Etching. Colloid J. 2015, 77, 652–660.

(38)

Linic, S.; Aslam, U.; Boerigter, C.; Morabito, M. Photochemical Transformations on Plasmonic Metal Nanoparticles. Nat. Mater. 2015, 14, 567.

(39)

Schlather, A. E.; Manjavacas, A.; Lauchner, A.; Marangoni, V. S.; DeSantis, C. J.; Nordlander, P.; Halas, N. J. Hot Hole Photoelectrochemistry on Au@SiO2@Au Nanoparticles. J. Phys. Chem. Lett. 2017, 8, 2060–2067.

(40)

Cortés, E.; Xie, W.; Cambiasso, J.; Jermyn, A. S.; Sundararaman, R.; Narang, P.; Schlücker, S.; Maier, S. A. Plasmonic Hot Electron Transport Drives Nano-Localized Chemistry. Nat. Commun. 2017, 8, 1–10.


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