Light-Controlled in Situ Bidirectional Tuning and Monitoring of Gold

Oct 5, 2018 - We report on light-controlled in situ bidirectional tuning of longitudinal surface plasmon resonance (LSPR) of single gold nanorods via ...
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Article Cite This: J. Phys. Chem. C 2018, 122, 24885−24890

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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 and ‡Discipline of Electrical Engineering, Indian Institute of Technology Gandhinagar, Gandhinagar, Gujarat-382355, India

J. Phys. Chem. C 2018.122:24885-24890. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/03/18. For personal use only.

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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 the presence of an excitation laser, and the etching rate and directionality can be controlled by the intensity of excitation light. At a low excitation power, a blue shift of a nanorod’s LSPR of up to 50 nm was observed, which indicates preferential etching from its tips. Whereas at a high power, we see a red shift of the nanorod’s LSPR of up to 140 nm indicating etching from sides. These results present a new approach for in situ finer adjustments of a selected nanorod’s plasmon resonance.

tionally, these methods do not provide any in situ fine adjustment of nanorod’s SPR. Various methods have been developed recently for the in situ tuning of nanoparticles’ SPR. The chemical methods rely on postsynthesis modification of nanoparticle aspect ratios via either etching gold atoms18−20 or depositing gold atoms21 onto the nanostructure. In situ tuning of SPR was also reported by electrochemically charging gold nanoparticles.22,23 Other methods rely on controlling the refractive index of the surrounding medium.24,25 All these methods have their own advantages and limitations. The chemical methods offer a broader range of SPR tuning but are often difficult to control, especially when the reaction is very fast.18 The refractive index modulation methods, on the other hand, are reversible and provide external control but the tuning range of SPR is often rather narrow.24,25 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 the 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

1. INTRODUCTION Gold nanorods have generated significant interest in the last two decades owing to their special optical properties originating from the collective oscillation modes of their conduction electrons, also known as surface plasmons.1,2 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 nanorods is that the longitudinal surface plasmon resonance (LSPR) can be tuned over a spectral range (above 600 nm), where Ohmic loss is significantly less resulting in narrow yet intense resonance.3,4 Such a 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 spectroscopies,5−7 sensing,8,9 imaging, and therapeutics.10−13 Success of many of the abovementioned applications requires precise and in situ control over nanorods’ SPR. For example, the strongest fluorescence signal enhancement is achieved when the nanoparticles’ SPR is tuned to the excitation laser wavelength.7 On the other hand, for sensing applications, the highest sensitivity is achieved when a nanorod’s SPR is tuned such that the excitation laser energy is at the wing of its resonance.14 Typically, SPR wavelengths of nanorods are controlled during their wet-chemical synthesis by adjusting concentrations of gold seeds, silver nitrate, and other additives.3,15−17 However, these methods usually produce a broad distribution of nanoparticles’ shapes and sizes. Addi© 2018 American Chemical Society

Received: July 12, 2018 Revised: October 4, 2018 Published: October 5, 2018 24885

DOI: 10.1021/acs.jpcc.8b06679 J. Phys. Chem. C 2018, 122, 24885−24890

Article

The Journal of Physical Chemistry C

3. RESULTS AND DISCUSSION Gold nanorods with an average dimension of 71 ± 8 by 26 ± 2 nm were synthesized by following a modified seed-growth method described elsewhere.16 The scanning electron micrograph of the nanorods is shown in Figure S1. The LSPR of these nanoparticles appears at 714 nm in water (Figure S1). For single particle studies, the 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. On average, we get approximately nine particles in 100 μm2 area. The glass substrate, covered with the nanorods, was then incorporated into a flow cell, through which the reactants were flowed. Single-particle spectroscopy was performed on a home-built confocal microscope, assembled around an inverted microscope (Figure 1a). The details of the setup are given in the

a low excitation power, a nanorod’s SPR shows a gradual blue shift of up to 50 nm, indicating preferential etching from its tips. At a high power, on the other hand, the nanorod’s SPR shows a large red shift of up to 140 nm, indicating etching from sides.

2. EXPERIMENTAL DETAILS 2.1. Chemicals and Synthesis of the Gold Nanorods. Chloroauric acid trihydrate (HAuCl4·3H2O), cetyltrimethylammonium bromide (CTAB), L-ascorbic acid (>99% crystalline), silver nitrate (AgNO3), sodium borohydride (NaBH4), sodium oleate, hydrochloric acid (HCl, 11.3 M), and ferric chloride were purchased from Sigma-Aldrich. All chemicals were used as received. Milli-Q water was used for all 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 HAuCl4 to 5 mL of 0.2 M CTAB. Then, 0.6 mL of 0.01 M NaBH4 was diluted to 1 mL and then added to the above solution. The seed solution was aged for 30 min before using. The growth solution was prepared by dissolving 2.8 g of CTAB and 0.4936 g of sodium oleate in 100 mL of Millipore water. The above binary surfactant solution (5 mL) was taken and 25 μL of 40 mM AgNO3 was added to it. The solution was stirred and then kept undisturbed for 15 min. Then, 50 μL of 100 mM HAuCl4 was added to it and stirred till the solution became colorless. 10 μL of 11.3 M HCl was then added to the solution, which was further stirred for 15 more min. Ascorbic acid (25 μL; 64 mM) was then added into the solution immediately followed by the addition of 8 μL of the seed solution. The resulting mixture was kept undisturbed for 12 h. The synthesized gold nanorods were characterized by UV− visible spectroscopy and field emission scanning electron microscopy (SEM) (Figure S1). The LSPR of the nanorod was at 714 nm. The average length and width was found to be 71 ± 8 and 26 ± 2 nm, respectively. 2.2. 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 numerical aperture (NA) oil immersion objective. The luminescence from the sample was collected by the objective and separated from the excitation laser using a 532 nm notch filter. The luminescence light was directed to an avalanche photodiode (APD; Excelitus, SPCMAQRH-14) for imaging or to the spectrometer with a liquidnitrogen-cooled CCD detector (iHR-320, HORIBA Scientific) to record spectra. One photon luminescence image of the nanorods was acquired by scanning the sample across diffraction-limited laser focus using an XYZ Piezo nanopositioning stage (Physik instruments). The etching experiments were performed in an especially designed flow cell. The flow cell consists of an aluminum frame that incorporates two cover slips separated by a 1 mm thick layer of polydiethylsiloxane and a flow channel of a 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 piezoelectric stage for precise positioning of the sample.

Figure 1. (a) Simplified schematics of the experimental setup. BS: beam splitter, NF: notch filter, and FM: flip mirror. (b) One-photon excited luminescence image of the gold nanorods isolated on a glass coverslip and covered in water. The excitation wavelength was 532 nm, and the power was 100 kW/cm2. (c) Luminescence spectra (red line) of a single nanorod (encircled red in (b) and a Lorentzian fit to the longitudinal plasmon (blue). The inset shows the photoluminescence intensities (magenta circles) as a function of the polarization angle and a fit (black line) to I(θ) = N[1 + M cos 2(θ − φ)], where I(θ) is the intensity as a function of the polarizer angle (θ), φ denotes the orientation of the nanorod’s principal axis, and M is the modulation depth.

Figure S2. Briefly, a 532 nm laser was used as the excitation source. An oil-immersion objective with an NA of 1.2 focused the laser onto a diffraction-limited spot. The photoluminescence signal from the nanoparticles was collected by the same objective, separated from the excitation laser by a notch filter, and was guided to either an APD or to 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. Figure 1b shows a one-photon luminescence image of the gold nanorods isolated on the glass substrate and covered with water. The presence of only one nanorod in a diffractionlimited spot was confirmed from the spectral line-shape as well as polarization-dependent emission studies. The luminescence spectrum of a single nanorod is known to closely resemble their plasmon resonance, which has a Lorentzian line-shape of the longitudinal plasmon resonance.26,27 The presence of 24886

DOI: 10.1021/acs.jpcc.8b06679 J. Phys. Chem. C 2018, 122, 24885−24890

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The Journal of Physical Chemistry C nanoparticle aggregates would generally result in a broad spectrum with multiple resonances or resonance at a different energy than expected from inherent size distribution.26 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 a linear polarizer before the APD. The inset of Figure 1c shows our results (magenta circles). A fit to I(θ) = N[1 + M cos 2(θ − φ)], where I(θ) is the intensity as a function of polarizer angle (θ), φ denotes the orientation of the nanorod’s principal axis, and M is the modulation depth, yields a modulation depth of 0.9, indicating the presence of a single dipole.26 Statistically, we found that approximately 80% of the diffraction-limited spots contains one nanorod only (more examples shown in Figures S3 and S4). To study the etching of the gold nanorods, 1 mM FeCl3 solution was flowed through the flow cell. We note that the pH of the solution was kept at 1.2 by adding a small amount of hydrochloric acid. This prevents the formation of ferric hydroxide precipitate. We monitored the reaction by taking photoluminescence spectra of the same nanorods before and after flowing the FeCl3 solution. Figure S5 shows the typical photoluminescence spectra of a gold nanorod before and after flowing FeCl3 for 12 h. We did not see any change of the plasmon resonance wavelength or the intensity, referring that no change of the gold nanorod is happening in this time interval of 12 h. This observation is consistent with the previous reports, which showed that FeCl3 cannot oxidize Au atoms to Au1+ as Fe3+/Fe2+ reduction potential is well below the oxidation potential of Au0/Au1.19,28 In fact, it was shown that such a reaction can only happen in the presence of CTAB or some other chemicals, which can reduce the effective reduction potential.19,28 However, a completely different effect is seen when a nanorod is under the continuous illumination of 532 nm laser. Figure 2a,b shows the photoluminescence images of the gold nanorods covered with 1 mM FeCl3 solution before and after 100 s 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 extent, making it indistinguishable from the background (Figure 2b), whereas no significant intensity change was observed for all the other particles. The change of the nanorod’s photoluminescence intensity under continuous illumination was also confirmed by measuring their photoluminescence spectra (Figure 2c), which shows a gradual decrease of the photoluminescence intensity with a longer exposure time, ultimately rendering it invisible in our microscope. We see a gradual red shift of the nanorod’s plasmon resonance wavelength for the initial 40 s, starting from 621 nm till 667 nm (Figure 2c, inset). At later times, a gradual blue shift from 667 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 the nanorod’s luminescence intensity and associated shift of its plasmon resonance to etching of gold atoms from the nanorod, resulting in a decrease of its volume and change in the aspect ratio. Luminescence signal is known to be proportional to the absorption cross section, which in turns is proportional to the volume of the particle.26 Thus, a decrease in the volume is expected to decrease the luminescence intensity. The aspect ratio, on the

Figure 2. (a,b) One-photon luminescence image of the gold nanorods isolated on a glass substrate and covered with 1 mM FeCl3 solution before and after 100 s of continuous exposure by 532 nm laser on a gold nanorod highlighted by the red circle. The excitation power was 150 kW/cm2 at the sample. (c) Luminescence spectra of a gold nanorod as a function of time under continuous excitation. The insets show the corresponding change of the plasmon resonance wavelengths and the peak intensities as a function of time. (d) Luminescence intensity of a gold nanorod as a function of time under repeated on−off excitation cycle (blue line). The red line is a guide to the eye to the photoluminescence intensity.

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 the nanorod, an overall increase of the aspect ratio leading to a red shift of SPR is expected.18,20 On the other hand, if the reaction is preferred from the tips, a blue shift is expected because of a decrease in the aspect ratio.18,19,28 Previous studies have indeed shown that FeCl3 can etch gold atoms from CTAB-coated nanorods in solution resulting in a blue shift of its plasmon resonance because of preferential etching of the gold atoms from the tips.29−31 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 the decrease of the luminescence intensity and the spectral shift based on the fact that no significant change of the luminescence intensity and/or the LSPR wavelength was observed under continuous laser illumination for up to 30 min in water but in the absence of FeCl3 (Figure S6) at the highest power of 250 kW/cm2. It is important to note that the light-induced etching has two key advantages compared to the etching induced by adding chemicals as done previously.19,32−37 First, we can select which nanorod we want to etch. This is demonstrated in Figure S7, where only the selected nanorods were subjected to etching by focusing the excitation laser on them, while the other ones remain unchanged. In the currently existing chemical methods, all nanorods will react, irrespective of whether we want to etch them or not. Second, we can stop the etching process 24887

DOI: 10.1021/acs.jpcc.8b06679 J. Phys. Chem. C 2018, 122, 24885−24890

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The Journal of Physical Chemistry C

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, a 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 an initial red shift, which was followed by a blue shift. This bidirectional etching mechanism was confirmed by comparing the SEM images of the individual gold nanorods before and after etching with FeCl3 at different excitation powers (Figures S14 and S15). The SEM images show that, at a low excitation power, etching is preferred from the tips and edges resulting in an overall decrease of the aspect ratio (Figure S14). Whereas at a high excitation power, etching is found to happen from the sides more preferentially, leading to an increase in the overall aspect ratio (Figure S15). We also note that some shape deformation is visible at high power. Oxidative etching of the gold atoms from the gold nanorods using FeCl3 has been demonstrated previously by various groups.19,29,30,37 The group of Toste has also shown that light can accelerate this process.29 In their study, the authors used CTAB-coated nanorods dispersed in water. In the absence of any external laser source, FeCl3 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 excitation.29 Surface plasmons decay via Landau damping, whereby the energy of the plasmon oscillation is transferred to the individual charge carriers (holes or electrons).38 These individual carriers then possess more energies compared to their peers and, therefore, are referred to 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 the recent past.38−40 In the case of etching of gold nanorods with FeCl3, the hot-electron mechanism was recently proposed by the groups of Alivasatos and Toste.29 A hot electron generated from light excitation, reduces an Fe3+ to Fe2+, leaving the nanorod positively charged. When such a reduction process repeats three times, an Au3+ ion is formed, which later reacts with the gold nanorods to form Au+. Intermediate Au 3+ as well as final Au + were both experimentally verified.29 This study has also revealed that the etching follows first-order kinetics with respect to the FeCl3 concentration.29 Such hot carriers-based etching mechanism may also be present in our experiment, but there are two major differences with the previous report. First, the reaction rate is not linear with respect to the excitation power, and, second, the directionality of the LSPR shift and hence the directionality of the etching reaction is found to be dependent on the excitation power. These observations can be explained from a combination of hot electrons-induced chemistry as well as photothermal effects. At the lowest power, when the temperature rise is not quite significant (∼7 K, Figure S16), the reaction is expected to be dominated by the hot electrons leading to the chemical reaction from tips and edges of the nanorod and hence the blue shift of nanorod’s LSPR, as observed in the previous report by Toste et al.29 On the other hand, at the highest excitation power of 250 kW/cm2, we expect a temperature rise in excess of 27 K (Figure S16), which is significant in the context of a rather low activation barrier of

instantaneously by switching off the excitation laser. This is demonstrated in Figure 2d, where a nanorod was subjected to a 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. The rate of the reaction can be controlled by the excitation laser intensity. This is demonstrated in Figure 3a, which shows

Figure 3. (a) Photoluminescence signal of gold nanorods covered with 1 mM FeCl3 as a function of time under continuous exposure to 532 nm laser. Decrease in the intensity of the gold nanorod signifies that there is a decrease in the volume of the nanorod with time. The excitation powers were 60 kW/cm2 (red), 100 kW/cm2 (black), 150 kW/cm2 (green), and 250 kW/cm2 (blue). The inset shows the reaction times, (τR), as the function at different excitation powers. The error bars were calculated from 5 nanorods at each power. (b) Plasmon resonance wavelength shift of the gold nanorods as a function of time under continuous exposure to 532 nm laser. The excitation powers were 60 kW/cm2 (red), 100 kW/cm2 (black), 150 kW/cm2 (green), and 250 kW/cm2 (blue).

the decrease of the nanorods’ photoluminescence intensities as a function of exposure time for the excitation powers between 60 and 250 kW/cm2 (spectra shown in Figure S8). A faster reaction at higher power is clearly evident from Figure 3a. To further quantify this, we define a reaction time, (τR), as the time required for 90% decrease of a nanorod’s initial intensity. We find that τR decreases by over an order of magnitude from 1050 ± 100 s at 60 kW/cm2 to 20 ± 5 s at 250 kW/cm2. Particle to particle variations are shown in Figures S9−S13. Such variations could be due to the inherent volume distribution present within the sample. Shift of the longitudinal plasmon resonance wavelength also shows a strong dependence on the excitation power (Figure 3b). At the lowest power 24888

DOI: 10.1021/acs.jpcc.8b06679 J. Phys. Chem. C 2018, 122, 24885−24890

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The Journal of Physical Chemistry C the reaction (2.87 kcal/mol),29 making the chemical reaction possible for all the gold atoms, irrespective of their location on the nanorod’s surface. This is expected to result in a red shift of nanorod’s plasmon as seen in our results and also demonstrated in the case of etching with potassium cyanide.18 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 the existence of such induction time and detailed optical and high-resolution transmission electron microscopic investigation at the early stage of the etching process will be required to understand the origin of this induction time.

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.

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4. 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 FeCl3. Unlike the currently available postsynthetic chemical modification methods, this approach allows tuning of a selected nanorod of interest without affecting the others. Furthermore, we show that the directionality and the rate of the oxidative etching can be controlled by the excitation laser intensity. These findings will be useful for in situ finer adjustments of nanorods’ 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 FeBr3 and FeF3.

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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.8b06679. Experimental details, optical characterization of single nanorods, chemical etching in the absence of light, sequential etching on the gold nanorods, powerdependent etching of the gold nanorods, temperature rise due to laser excitation, effect of laser excitation on a gold nanorod in the absence of FeCl3, SEM imaging before and after etching, examples of etching on different nanorods, and effect of laser-induced heating (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Saumyakanti Khatua: 0000-0002-8088-2132 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Science and Engineering research board, India (project: EMR/2015/0013), DST NanoMission, India (project: SR/NM/NS-65/2016), 24889

DOI: 10.1021/acs.jpcc.8b06679 J. Phys. Chem. C 2018, 122, 24885−24890

Article

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DOI: 10.1021/acs.jpcc.8b06679 J. Phys. Chem. C 2018, 122, 24885−24890