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Optimization of the refractive index plasmonic sensing of gold nanorods by non-uniform silver coating
Sensors and Actuators B 183 (2013) 556–564
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Optimization of the refractive index plasmonic sensing of gold nanorods by non-uniform silver coating Jian Zhu, Fan Zhang, Jian-Jun Li, Jun-Wu Zhao ∗ The Key Laboratory of Biomedical Information Engineering of Ministry of Education, School of Life Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 25 January 2013 Received in revised form 21 March 2013 Accepted 11 April 2013 Available online 19 April 2013 Keywords: Localized surface plasmon resonance (LSPR) Au–Ag core–shell nanorod Refractive index (RI) sensitivity Non-uniform silver coating
a b s t r a c t Localized surface plasmon resonance (LSPR) response sensitivity to the environmental refractive index (RI) has been studied for Ag coated Au nanorods both theoretically and experimentally. Quasistatic calculation results indicate that the Ag coating induced increasing of RI sensitivity only takes place when the Ag shell at the ends is much thicker than that at the side facets. Because of the corporate effects from the coating thickness dependent sensitivity and band width changing, the Ag coating always leads to an improvement of the figure of merit (FOM) of Au nanorod sensing, and the increase of FOM becomes faster as the Ag coating is non-uniform. Furthermore, higher RI sensitivity with the same resonance wavelength could be obtained by Ag coating, and the improvement of RI sensitivity with the same resonance wavelength could be fine tuned by changing the non-uniformity ratio of the Ag coating. These calculation results have also been verified experimentally. And this Ag coating dependent optimization of RI sensitivity of Au–Ag bimetallic nanorod makes them potential candidates for nanoscale biochemical sensing applications. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Localized surface plasmons (LSPR) in Au and Ag nanoparticles are coherent charge density oscillations with a resonant frequency that greatly depends on the particle size, geometry and dielectric environment. Because the coulombic force from the polarized field of surrounding dielectric medium effectively reduces the strength of the surface charge and plasmon energy, the resonance wavelength red shifts greatly as the environmental refractive index (RI), n, is increased slightly. Therefore, this RI dependent shifting of plasmon band location leads to the development of plasmonic nanostructures as optical chemical/biologic sensors [1,2]. Recent developments in synthesis, assembly and theoretical study have generated new insights about control of particle shape [3], structure [4], composition [5] and arrange [6] that can improve the RI sensing ability based on LSPR. Chen et al. reported the shape- and size-dependent refractive index sensitivity of solid gold nanoparticles, including nanospheres, nanocubes, nanobranches, nanorods, and nanobipyramids [7]. It has been demonstrated that the RI sensitivities generally increase as gold nanoparticles have been elongated and their apexes become sharper. Because of the plasmon hybridization between inner and outer metallic surfaces,
∗ Corresponding author. Tel.:+86 29 82664224; fax: +86 29 82664224. E-mail address: [email protected] (J.-W. Zhao). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.04.042
the symmetric coupling plasmon modes with lower energy in core–shell metal nanostructures are also suitable for RI sensing [4]. It has been found that the RI sensitivities of gold nanorings are over 5 times larger than those of solid nanodisks with similar diameters [8]. Cao et al. reported the optimized LSPR RI sensitivity of gold nanoboxes for sensing applications [9]. They found the RI sensitivity increases near-exponentially as the wall thickness of the nanobox is decreased. A comparative spectroscopic study on gold nanoshells and their solid counterparts has been reported by Sun et al. [10]. They found the gold nanoshells exhibited a more significant red-shift for their LSPR band when the local dielectric environment is changed. The effect of the metal type on the RI sensitivity has been investigated by Lee et al. [11]. By comparing the RI dependent plasmon shifting of Ag and Au nanocubes with nearly the same LSPR spectral region, it has been found that the LSPR of the Ag nanocubes is twice as sensitive to the surrounding index change as that of the Au nanocubes. Although the RI sensitivity could be greatly increased by reducing the particle symmetry or enlarging the inter-surface coupling, the corresponding resonance wavelength position and plasmon band width also increase. Because too wide band width will affect the detectivity of a plasmonic sensor, the line width of LSPR band is usually regarded as a negative factor in plasmonic RI sensing. To address the effect of LSPR band width on RI sensitivity, a figure of merit (FOM) was introduced that is simply the sensitivity (in nm per refractive index unit, RIU) divided by the full width half-maximum (FWHM) [1,12]. On the other hand, overlong resonance wavelength
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may go beyond the spectral detecting range of the spectrophotometer. Therefore, obtaining higher RI sensitivity within shorter wavelength scope (for instance, visible range) is necessary. By using accurate electrodynamic simulations, Miller et al. demonstrated that the plasmonic RI sensitivity is determined by the location of the peak wavelength and the dielectric properties of the material [13]. The plasmon peak position dependent refractive index sensitivity has also been observed for gold nanorings [8]. However, it has been found that the gold nanoparticles with different shape may exhibit different RI sensitivity even though at the same plasmon resonance wavelength [3]. A linear relationship is found between the RI sensitivity and the product of the polarizability and the end curvature. Some interesting questions arise when several factors are simultaneously tuned to control the LSPR properties and RI sensitivity. For example, when an Au nanorod is coated by an Ag nanoshell with non-homogeneous thickness, both the aspect ratio, Au/Ag composition and core–shell structure have been changed. In the report of Lee et al., the effect of the metal type on the RI sensitivity is confirmed by coating an Ag shell around Au nanorods [11]. It has been found that coating an Ag layer brings about a higher RI sensitivity in comparison to the pure Au nanobars. However, recent experimental results indicate that the sensitivity and FOM of Au nanorods only increase with a relative thin Ag coating [5]. The decrease of the sensitivity has been observed as the Ag shell becomes too thick, which has been attributed to the shape change from rod to non-regular. The ultrathin Ag coating induced enhancement of RI sensitivity of Au/ITO platform nanoparticles has also been reported by Deng et al. [14]. The increase in sensitivity has been ascribed to local field enhancement effect and reducing substrate effect. In recent experimental studies, non-uniform Ag coating on Au nanorods has been observed [5,15]. Therefore, both the aspect ratio and metal composition of the Au–Ag bimetal nanostructure will be changed by the non-uniform Ag coating, and then consequently control the RI sensitivity. In this report, we studied the effects of anisotropy coating, shell thickness and aspect ratio of the inner Au nanorod on the RI sensitivity of Ag coated Au nanorod. Furthermore, we also discussed how to obtain the high sensitivity with small plasmon band width and shorter resonance wavelength by non-uniform Ag coating.
2. The model In order to calculate the plasmonic absorption spectrum of Ag coated Au nanorod, a core–shell prolate spheroid model under quasistatic approximation has been used in this study. Although the TEM images show that the Au–Ag nanorods are more like cylinders with semispherical ends [15], the accurate analytical solution of the optical properties for metal nanoparticles with complex geometry remains a theoretical challenge. In recent years, the discrete dipole approximation (DDA) method and finite-difference time domain (FDTD) numerical analysis have been developed for the study of arbitrary nonspherical nanoparticles, but these numerical calculations are complex and will bring some errors [16]. In this study, the quasistatic theory has been used to calculate the optical properties of metal nanorods. This analytical solution is simple and effective, especially when the particle size is much smaller than the incident wavelength. However, the quasistatic theory can only study the optical properties of simple shape nanoparticle with high symmetrical morphology. Therefore, the metal nanorods in this study were approximated as prolate ellipsoids. Many previous reports also used this ellipsoids mode to study the optical properties of metal nanorods [17–19]. Although the end shape of the nanorods will affect their optical properties, the influence is slight and not dominating. As we know, the aspect ratio (the ratio between longitudinal length and transverse diameter) is the major factor which affects
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the shifting of LSPR [20,21]. In this Au–Ag core–shell nanorod structure, a prolate ellipsoidal Au core, with semiaxes a1 = b1 < c1 , is coated with another coaxial prolate ellipsoidal Ag shell with semiaxes a2 = b2 < c2 . And the transverse coating thickness is denoted as tT = a2 − a1 , the longitudinal coating thickness is denoted as tL = c2 − c1 . In the Drude model, the dielectric functions of both inner Au nanorod and outer Ag nanoshell are frequency dependent and have real and imaginary components [22,23]. In this calculation, the dielectric functions and corresponding numerical parameters are from Johnson and Christy data [24]. When the whole nanostructure is immersed in a dielectric environment with a refractive index of n3 oriented randomly, the mean absorption cross-section for this Ag coated Au nanorod can be obtained by using optical theorem [17,25], absorption =
2 Im
2˛
1
3
+
˛3 3
(1)
where ˛l and ˛3 are the polarizabilities of the nanostructure along its transverse and longitudinal principal axes, respectively. The analytic expressions of the polarizabilities and depolarization geometrical factors have been described in detail elsewhere [18,19]. 3. Results and discussion 3.1. Absorption spectra of Ag coated Au nanorods with different environmental refractive index To investigate the plasmonic RI sensitivity of the Ag coated Au nanorod, absorption spectra of Au–Ag core–shell nanostructure with different environmental refractive index have been calculated in Fig. 1a. In this calculation, the geometrical parameters of Au nanorod are set as a1 = 5 nm and p = c1 /a1 = 3.0. The Ag coating is uniform, thus the Ag shell thickness has the same value in transverse and longitudinal direction, i.e., tT = tL = 2 nm. Because of the two directions of oscillation and two interfaces, Au–Ag core–shell nanorods should exhibit four absorption peaks which are corresponding to the longitudinal and transverse plasmon resonance wavelength both of Au–Ag interface and outer Ag surface of the core–shell nanorods respectively [17,23]. However, the absorption peak at shortest wavelength, corresponds to the transverse plasmon from outer Ag surface, is very weak and does not always occur. In the calculation results of Fig. 1a, the transverse plasmon peak from outer Ag surface is too weak to be observed when n3 = 1. Thus we can only find three distinct LSPR peaks in the absorption spectra of Fig. 1a. From shorter to longer wavelength, the three LSPR peaks are corresponding to the longitudinal plasmon from outer Ag surface, transverse and longitudinal plasmon from Au–Ag interface [17,23]. Indeed, the fourth LSPR peak corresponding to transverse plasmon from outer Ag surface is starting arise at 360 nm of the absorption spectrum when n3 = 2. As shown in Fig. 1a, when the environmental refractive index is increased from n3 = 1.0 to 2.0, the lower energy plasmon peak red shifts 260 nm, which is much greater than the shift of other peaks with shorter wavelength. In order to make a comparison, we also plotted the environmental refractive index dependent absorption spectra of bare Au nanorod without coating and Au nanorod with Au coating. As shown in Fig. 1b, the increasing n3 induced plasmonic red shift of Au coated Au nanorod is decreased to 239 nm, which is weaker than that of Au nanorod with Ag coating. However, the increasing n3 induced plasmonic red shift of bare Au nanorod is increased to 295 nm, which is greater than that of Au nanorod with Ag coating, as shown in Fig. 1c. Although the Ag coating weakens the sensitivity of Au nanorod to the change of environmental refractive index, the band width and resonance wavelength also decrease with the Ag coating. Therefore, how to optimize the RI sensing characters of Au nanorod with Ag coating becomes the topic of next sections.
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Fig. 1. Absorption spectra of gold nanorod with different environmental refractive index n3 , (a) Ag coating, (b) Au coating and (c) no coating. a1 = 5 nm, p = 3.0 and tT = tL = 2 nm.
3.2. Optimizing the sensitivity and figure of merit (FOM) of Au nanorods by non-uniform silver coating In this section, the optimization of RI sensitivity and FOM of Ag coated Au nanorod has been studied by changing the coating thickness and uniformity. Here, the semi-minor axis and aspect ratio of inner gold nanorod are still set as a1 = 5 nm and p = c1 /a1 = 3.0. Fig. 2a shows the LSPR response sensitivity (/n3 ) as a function of coating thickness with different non-uniformity. When the transverse coating is two times thicker than that of longitudinal coating, the sensitivity fades down as the coating thickness is increased, and the decreasing speed is faster than the case with uniform Ag coating. When the longitudinal coating is two times thicker than that of transverse coating, the sensitivity still fades down as the coating thickness is increased, but the decreasing speed is slower than the case with uniform Ag coating. Interestingly, when the coating non-uniformity is further increased to tL /tT = 3, the sensitivity increases as the coating thickness is increased. The coating thickness dependent plasmon band width with different non-uniformity has also been studied, as shown in Fig. 2b. Because of the larger scattering relaxation time of silver, the increasing Ag coating thickness leads to the FWHM decreases monotonously when the longitudinal coating is thicker. And the decreasing speed fades down when the non-uniformity ratio tL /tT is increased. On the contrary, the thickness dependent FWHM decreasing gets faster when the transverse coating is thicker. And the decreasing speed gets intense
when the non-uniformity ratio tT /tL is increased. The physical origin is attributed to the faster increasing speed of the Ag composition. An interesting result has been observed that the thickness dependent monotonous decreasing of FWHM only takes place with small thickness. For example, when tT /tL = 1.5, the non-monotonous changing of FWHM occurs as the thickness is increasing to about 4 nm. Furthermore, the critical thickness decreases to about 2.5 nm when the non-uniformity ratio is increased to tT /tL = 2.0. The physical mechanism of non-monotonous changing of FWHM could be attributed to the Ag coating induced plasmon band merging. As shown in Fig. 2c, as the Ag coating is increased, the longer wavelength peak corresponding to the longitudinal plasmon from Au–Ag interface blue shifts and increases more intensely than that of the middle wavelength peak corresponding to the transverse plasmon from Au–Ag interface. Thus there is a combination between these two plasmon bands, which leads to the FWHM increases first and then decreases. Because the increasing coating thickness induced blue shift and intensity increase becomes faster as the nonuniformity ratio tT /tL is increased, the critical thickness decreases when the non-uniformity ratio is increased. Therefore, the plasmon band blend and split dependent changing of FWHM should be considered in optimizing the FOM of nanostructure with multi LSPR bands. Fig. 2d shows the FOM as a function of Ag coating thickness. Because of the corporate effects from the coating thickness dependent sensitivity and band width, the Ag coating always leads to the improvement of the FOM of Au nanorod. And the increase in FOM
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Fig. 2. (a) LSPR response sensitivity (/n3 ) as a function of coating thickness; (b) FWHM as a function of coating thickness; (c) absorption spectra of Au nanorod with different Ag coating thickness; (d) FOM as a function of coating thickness. a1 = 5 nm and p = 3.0.
for non-uniform Ag coating is usually faster than that of uniform coating when the coating thickness is small. In order to find the optimal RI sensitivity, we calculate the LSPR response sensitivity as a function of both Ag coating thickness and non-uniformity ratio, as shown in Fig. 3. It is interesting to find that the high sensitivity can be obtained with a small non-uniformity ratio and thin Ag coating under the condition of tT > tL , as shown in Fig. 3a. However, when the shell thickness at the ends is larger than that at the side facets, i.e., tL > tT , the high sensitivity can be obtained with a large non-uniformity ratio and thick Ag coating, whereas the low sensitivity can be obtained with a small nonuniformity ratio and thick Ag coating, as shown in Fig. 3b. There is a critical value of non-uniformity ratio tL /tT ≈ 2.2. When the nonuniformity ratio is smaller than the critical value, increasing the coating thickness leads to the sensitivity decreases. On the contrary, the sensitivity increases with the increasing coating thickness when the non-uniformity ratio is larger than the critical value. The LSPR response sensitivities with larger aspect ratio (p = 4) of inner Au nanorod are shown in Fig. 3c–d. One can find the thickness and non-uniformity ratio dependent sensitivity changing is similar to that of Ag coated Au nanorod with small aspect ratio. However, the critical value of tL /tT increases to about 3.2 as the aspect ratio p is increased to 4. Due to the instable property of Ag nanorods, so far there has been little literature reporting about thicker Ag shells at the ends of Au nanorods [26]. However a Pt coating the tips of Au nanorod was obtained by Marek et al. [27], Ag-tipped Au nanorods were synthesized by Kyoungweon [28]. Au/Ag nanoshuttles owning sharp tips were synthesized by Li et al. [29], which also exhibit stronger local field enhancements than the original Au nanorods. Although
the method to produce thicker shells at the ends is still a challenge and has seldom been reported, the theoretical analysis prediction of non-uniform coating dependent sensitivity will stimulate the improvement of preparation technology for producing thick coating at the ends. For the experimentally available nanorods, better sensitivity would be just obtained for uniform and very thin Ag shell. However, better FOM would be obtained when the Ag coating is uniform and thick, or the Ag coating is non-uniform and thin, as shown in Fig. 2d. 3.3. Improve the sensitivity with the same plasmon resonance wavelength of Au nanorods by non-uniform silver coating Although the results in Fig. 2 indicate that the Ag coating usually weaken the RI sensitivity of Au nanorod, the Ag coating also leads to intense blue shift of the LSPR band. Could we obtain higher sensitivity with the same resonance wavelength by Ag coating? Fig. 4 plots the LSPR response sensitivity of Ag coated Au nanorod as a function of resonance wavelength. Here, the resonance wavelength is the LSPR peak wavelength of Au–Ag nanorods with the environmental refractive index of 1.0. In order to make a comparison, the sensitivity of bare Au nanorod at different resonance wavelength (corresponding aspect ratio p is increased from 1.5 to 4.2) has also been plotted in Fig. 4. When the bare Au nanorod has a resonance wavelength of 625 nm, the corresponding RI sensitivity is 292 nm/RIU, as shown in Fig. 4a. Uniform Ag coating leads to the resonance wavelength blue shift and the sensitivity decrease simultaneously. However, the sensitivity is higher than that of bare Au nanorod with the same resonance wavelength. With increasing the non-uniformity ratio tL /tT , the blue shift induced decreasing of
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Fig. 3. (a) LSPR response sensitivity (/n3 ) as a function of both coating thickness and coating thickness ratio tT /tL , the inner gold nanorod has a small aspect ratio p = 3, (b) LSPR response sensitivity (/n3 ) as a function of both coating thickness and coating thickness ratio tL /tT , the inner gold nanorod has a small aspect ratio p = 3, (c) LSPR response sensitivity (/n3 ) as a function of both coating thickness and coating thickness ratio tT /tL , the inner gold nanorod has a large aspect ratio p = 4, (d) LSPR response sensitivity (/n3 ) as a function of both coating thickness and coating thickness ratio tL /tT , the inner gold nanorod has a large aspect ratio p = 4. a1 = 5 nm.
sensitivity becomes gentle. Especially, the blue shift induced sensitivity increasing takes place as the tL /tT is increased to 3. Therefore, higher sensitivity with the same resonance wavelength could be obtained by non-uniform Ag coating (tL > tT ). For example, at the resonance wavelength of 560 nm, the bare Au nanorod only has a RI sensitivity of 170 nm/RIU. However, the sensitivity at 560 nm could increase to 327 nm/RIU after non-uniform Ag coating. Although the sensitivity has been improved when the aspect ratio of the inner Au nanorod is increased to 4.0, the decreasing resonance wavelength induced sensitivity decreasing gets faster, as shown in Fig. 4b. The Ag coating leads to the resonance wavelength blue shift and the sensitivity decrease simultaneously on the condition of tL /tT ≤ 3. The blue shift induced sensitivity increasing only takes place on the condition that the tL /tT is increased to 4.0. One can find the Ag coating induced sensitivity increasing at the same resonance wavelength becomes gentle when the inner Au nanorod has a small aspect ratio. However, the wavelength region in which we could improve the sensitivity at the same wavelength has been enlarged. In the previous figures, although the pure Au nanorods have higher sensitivity than that of Au nanorods with Ag coating, the resonance wavelength corresponding to pure Au nanorods is also longer than that of Au nanorods with Ag coating. For pure Au nanorods, the shorter resonance wavelength could be obtained by decreasing the aspect ratio, and the corresponding sensitivity has also been decreased. Therefore, the sensitivity is a function of resonance wavelength, and the pure Au nanorods have lower sensitivity than Ag coated Au nanorods when they have the same resonance wavelength. One important finding in this report is that higher RI sensitivity with the same resonance wavelength could be obtained by Ag coating, and the improvement of RI sensitivity with the same resonance wavelength could be fine tuned across a broad wavelength range by changing the non-uniformity ratio of the Ag coating.
3.4. Experimental 3.4.1. Reagents Cetyltrimethylammonium bromide (CTAB) and silver nitrate (AgNO3 ) were purchased from Sigma (USA), sodium borohydride (NaBH4 , Tianjin Kermel Chemical Reagent Co. Ltd., China), gold chloride trihydrate (HAuCl4 , Sinopharm Chemical Reagent Co. Ltd, China), ascorbic acid (AA, Tianjin TianLi Chemical Reagent Co. Ltd., China), hydrochloric acid (HCl, Beijing Chemical Works, China), sodium hydroxide (NaOH, Tianjin HengXing Chemical Reagent Co. Ltd., China) and glycerol (Tianjin TianLi Chemical Reagent Co. Ltd., China) were of analytical grade and used without further purification. Ultra-pure water was used through all the preparations. 3.4.2. The synthesis of gold nanorods Gold nanorods were prepared according to a seed-mediated growth method developed by Sau and Murphy with some slightly modification [30]. Firstly, a freshly ice-cold NaBH4 solution (0.01 M, 0.6 mL) was prepared in advance, gold seed solution were prepared by quickly injecting NaBH4 into the mixture of HAuCl4 (0.01 M, 0.25 mL) and CTAB (0.1 M, 7.5 mL) in a test tube of 15 mL, followed by a minute of violent stirring. Immediately after the stirring, the color of the solution changed dark yellow to brownish yellow indicating the start formation of the gold seed. Then the solution was kept at 27 ◦ C for 2 h for next use. Secondly, gold growth solution were prepared by injecting HAuCl4 (0.01 M, 4 mL), AgNO3 (0.01 M), AA (0.1 M, 0.64 mL), HCl (1 M) into an aqueous CTAB solution (0.1 M, 90 mL) followed by gentle stirring. The color of the solution changed brownish yellow to colorless due to the reduction of Au+ to Au3+ [31]. By adding different volumes of AgNO3 (0.6–1.2 mL) and HCl (0–2 mL), we will obtain different aspect ratios of gold nanorods. Lastly, 0.2 mL gold seed solution was injected to initiate growth
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Fig. 4. LSPR response sensitivity (/n3 ) as a function of resonance wavelength. The inner gold nanorod has a (a) small aspect ratio p = 3; (b) large aspect ratio p = 4. a1 = 5 nm.
reaction. Then the ultimate solution was kept at 27 ◦ C undisturbed overnight, ensuring the gold nanorods grow adequately. 3.4.3. The synthesis of Au–Ag core–shell nanorods Au–Ag core–shell nanorods were prepared by the method in previous literature [15] with some minor modification. Centrifugation for 20 min was needed in order to remove other reagents and make the nanorods more pure, and then precipitate was redispersed into the same volume of 0.08 M CTAB which was serving as capping agent in the synthesis of silver-coated gold nanorods. One minute of ultrasonic of the solution is necessary to ensure the intensive mixing. AgNO3 (0.01 M) and AA (0.1 M, 0.4 mL) were injected so as to provide Ag and reductant respectively. NaOH (0.1 M, 1 mL) was used to adjust the pH value of the aqueous system thus to provide an alkaline environment to initiate the reaction. By adding different volumes of AgNO3 (0.04–1.2 mL), we will obtain Au–Ag core–shell nanonanorods with different Ag thickness. The final solution was then kept at 65 ◦ C for 4 h. In Fig. 5, transmission electron microscopy (JEM-200CX, JEOL Ltd., Japan) was used to investigate the detailed structures of synthesized nanorods. The typical TEM images for the synthesized Au–Ag core–shell nanorods depict the size and morphology of the nanorods clearly, from which we could obtained their size distributions are relatively narrow. The average length and the width of the original Au nanorods are about 60 nm and 11 nm respectively. The TEM characterization also demonstrates the different thickness of the Ag shell. As can be
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Fig. 5. TEM images of the obtained Au–Ag core–shell nanorod samples, (a) Au nanorod with thin Ag coating and the volume of AgNO3 is 0.5 mL, (b) Au nanorod with thick Ag coating and the volume of AgNO3 is 0.8 mL.
observed in Fig. 5a, when the amount of AgNO3 is small, the Ag shell is thin, whose average thickness is about 5 nm. Owing to the non-uniform Ag coating, the side facet of the nanorods is thicker than the end as the amount of AgNO3 increased. Thicker Ag shell of about 9 nm could be observed when the volume of AgNO3 reaches 0.8 mL as shown in Fig. 5b. 3.4.4. The measurement of refractive index sensitivity To investigate the sensitivities of Au–Ag core–shell nanorods with different refractive indices, as-prepared Au–Ag core–shell nanorods were first divided into five portions and then centrifuged at 10,000 rpm for 20 min. When discarding the supernatant, we redipersed the precipitate into the water–glycerol mixtures with different refractive indices. Park et al. used water (1.33), ethanol (1.36), isopropanol (1.38), ethylene glycol (1.44) and tetraethylene glycol (1.46) as different refractive indices solvents to redisperse Au–Ag core–shell nanocubes [32]. But when we disperse centrifuged Au–Ag core–shell nanorods into these water-miscible organic solvents such as ethanol and isopropanol, aggregation occurs immediately. Here we use water–glycerol mixture of different volume ratios [3] to provide different refractive indices so as not to cause aggregation. An Abbe refractometer (WAY-2S, Shanghai SuoGuang Light & Electricity Technology Co. Ltd., China) was used to test the refractive index of each mixture, which is more accurate than the refractive indices obtained by the calculation according to Lorentz-lorenz equation. We know that the refractive indices from 1.33 to 1.42 are in the regime of the interest for biological applications according to Fu et al. [5], thus our refractive indices are chosen
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Fig. 6. (a) Absorption spectra of Au–Ag core–shell nanorod samples with different volumes of the AgNO3 solution; (b) normalized absorption spectra of one Au–Ag core–shell nanorod sample in five water–glycerol solutions with five different refractive indices, the inset presents the dependence of resonance wavelength red shift with the increasing of refractive indices, of which the slope of the linear fit line indicating the RI sensitivity of Au–Ag core–shell nanorod sample.
from 1.33 to 1.42 in each test. When redispersing the Au–Ag in solution of glycerine aqueous solution with different refractive index, we measure the spectra of them after a while. According to a series of tests, we found that ten to fifteen minutes is the optimize time for measuring. Too short or too long time is not suitable, for it is not mixed well and the nanorods do not have interaction time with new environment change when the time is short, while too long time, e.g. more than 30 min will cause agglomeration which leads to the solution half-transparent or transparent thus affecting the properties of the Au–Ag core–shell nanorods. The plasmon shifts were plotted as a function of the refractive index. The refractive index sensitivities were obtained by linear fitting from Origin 8.6 software. 3.4.5. Experimental results and discussion Fig. 6a depicts the spectra of the Au–Ag core–shell nanorods with different Ag shell thickness, the corresponding volume of AgNO3 is increased from 0.04 to 0.9 mL. It is well known that Au nanorods exhibit two distinct absorption peaks which are corresponding to transverse and longitudinal plasmon resonance wavelength. Au–Ag core–shell nanorods exhibit four absorption peaks which are corresponding to the longitudinal and transverse plasmon resonance wavelength both of Au–Ag interface and outer
Ag surface of the core–shell nanorods respectively [17,23]. As the amount of AgNO3 is increased, longitudinal plasmon resonance wavelength peak of the Au–Ag interface blue shifts progressively, and at the same time transverse plasmon resonance wavelength peak of the Au–Ag interface attenuates gradually and disappears at last. When the mole ratio of Au and Ag reaches 1, the longitudinal and transverse plasmon resonance wavelength peak of the outer surface appear, the transverse plasmonic resonance wavelength of the outer surface shows a weaker absorption while longitudinal plasmon resonance absorption gets intense distinctly. When the volume of AgNO3 reaches 0.9 mL, two absorptions of longitudinal plasmon of both Au–Ag interface and outer Ag surface with equal intensity of 2.75 could be observed, as shown in line D in Fig. 6a. Here we focus on the longitudinal plasmon wavelength of the Au–Ag interface due to its high absorption value and enhanced sensitivity, which is advantageous candidate for the ultra-sensitive biosensors. In order to investigate the effect of non-uniform Ag coating of Au nanorods on refractive index sensitivity, we choose the Au–Ag core–shell nanorods with the longitudinal wavelength of 666.5 nm, which is corresponding to the spectrum of line B in Fig. 6a, as samples to conduct the experiment of the measurement of refractive index sensitivity according to the method introduced above. The water–glycerol solutions have five different refractive indices of 1.3494, 1.3618, 1.3840, 1.4046 and 1.4198. Fig. 6b shows the normalized absorption of Au–Ag core–shell in these solutions with the longitudinal plasmon wavelength of the interface red-shifts as a function of increased refractive indices. The refractive index sensitivity is defined as the wavelength change when the refractive index change one unit using the unit of nm/RIU [33], which is obtained by linear fitting from Origin 8.6 software, as is shown in the inset of Fig. 6b. In our experiment, the original Au nanorods core with the longitudinal plasmon resonance wavelengths of 817.5 nm were used to be coated with the Ag shell. When the thickness of Ag shell is thinner, which is corresponding to the spectrum of line A in Fig. 6a, the sensitivity is 644 nm/RIU. Owing to the anisotropic coating, the side facet of the nanorods is thicker than the end as the amount of AgNO3 increased. Thus the thicker Ag shell corresponding to the spectrum of line C in Fig. 6a, whose sensitivity reduces to 269 nm/RIU compared to the thinner one, as shown in Fig. 7a. In order to investigate the refractive index sensitivities about Au nanorods and Au–Ag core–shell nanorods, we choose different aspect ratios of Au nanorods defined as shorter, medium and longer nanorods whose longitudinal plasmon resonance wavelengths are corresponding to 791.5 nm, 808 nm and 817.5 nm respectively as original core to be coated with Ag shell. When obtaining different Ag shell thickness, we plotted these samples’ sensitivities as a function of resonance wavelength, as shown in Fig. 7b. We can clearly observed that at the same resonance wavelength the refractive index sensitivity of Au–Ag core–shell nanorods is much higher than that of bare Au nanorods. For example, at the resonance wavelength of about 700 nm, the sensitivity of Au nanorods is only 200 nm/RIU, while the Au–Ag core–shell exhibits a sensitivity of 450 nm/RIU, which is more than twice of that of Au nanorods. Although the sensitivity of Au–Ag core–shell is decreased with the thickness increasing of Ag shell both from our theoretical calculation and experimental result, it is much higher than bare Au nanorods at the same resonance wavelength. It is worth noting that there is a dramatically increase at the beginning of the Ag coating, here we ascribe it to the initial nonuniform Ag coating. The amount of Ag is less sufficient, which causes it to coating the end of the nanorods preferentially due to its high energy. Owing to the non-uniform Ag coating at the beginning, the sensitivity has a transient and sharp increase first, as shown in Fig. 7b, which is consistent with our calculation result demonstrating that when tL /tT equals 3, the sensitivity could increase. As
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also considered. Furthermore, higher RI sensitivity with the same resonance wavelength could be obtained by Ag coating, and the improvement of RI sensitivity with the same resonance wavelength could be fine tuned across a broad wavelength range by changing the non-uniformity ratio of the Ag coating. Our experiment results show the increase of Ag coating thickness generally leads to the decrease of RI sensitivity of Au–Ag core–shell nanorods, while nonuniformity of Ag coating induces RI sensitivity increasing at the initial coating time when the Ag shell at the ends is much thicker than that of side facets. On the other hand, higher RI sensitivity with the same resonance wavelength could be obtained by Ag coating compared to bare Au nanorods. These experimental observations are in good agreement with the theoretical calculation above. This work will provide great potential not only for further improvement of RI sensitivity by non-uniform Ag coating but also for fabrication of ultra-sensitive and high FOM of nanorod-based biosensors at shorter resonance wavelength. Acknowledgements This work was supported by the Program for New Century Excellent Talents in University under Grant No. NCET-10-0688, the Fundamental Research Funds for the Central Universities and the National Natural Science Foundation of China under Grant Nos. 11174232, 61178075, 81101122. References
Fig. 7. (a) Normalized absorption spectra of Au–Ag core–shell nanorod samples with a thicker and a thinner Ag shell in three different refractive indices water–glycerol solutions; (b) the dependence of the sensitivities on the resonance wavelengths for Au nanorods and Au–Ag core–shell nanorod samples with shorter (max = 719.5 nm), medium (max = 808 nm) and longer (max = 817.5 nm) original Au core nanorods.
can be seen from Fig. 7b, with the amount of AgNO3 increased gradually, the sensitivities of Au–Ag core–shell decrease correspondingly, which is also in good agreement with our calculation result showing in Fig. 4a that when tT /tL equals 2, a decrease of the sensitivity is observed. In view of multiple factors including size, shape, structure and so on, the experimental results and the theoretical calculation could not perfectly matched, but from general trend, our experimental results are in good agreement with the theoretical calculation above. 4. Conclusions Localized surface plasmon resonance (LSPR) response sensitivity to the environmental refractive index has been studied for Ag coated Au nanorods both theoretically and experimentally. By using quasistatic approximation theory, we find that Ag coating generally leads to the decrease of the RI sensitivity of Au–Ag core–shell nanorods, and the Ag coating induced the increase of RI sensitivity only takes place when the Ag shell at the ends is much thicker than that at the side facets. A figure of merit was also calculated to compare the overall performance of Ag coated Au nanorods. Owing to the coating of Ag, the FWHM gets narrower compared to that of Au nanorods, thus the FOM of Ag coated Au nanorods could be increased in general. However we should note that the plasmon band blend and split dependent changing of FWHM should be
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Biographies Jian Zhu received his PhD degree in 2004 from Xi’an Jiaotong University, China. His research interests focus mainly on optical properties of nano-structured materials and multi-information fluorescence probe. Fan Zhang received the BE degree from Xinxiang Medical University, China, in 2010. Now she is a graduate student for a Master’s degree in Xi’an Jiaotong University, China. Her research interests focus mainly on optical biosensor based on plasmonic optics. Jian-Jun Li received her PhD degree in 2003 from Xi’an Jiaotong University, China. Her research interests focus mainly on multi-information fluorescence probe and bioseparation. Jun-Wu Zhao is a professor in the School of Life Science and Technology, Xi’an Jiaotong University, China. His research interests focus mainly on nano-biomaterials and device.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Optimization of the refractive index plasmonic sensing of gold nanorods by non-uniform silver coating Jian Zhu, Fan Zhang, Jian-Jun Li, Jun-Wu Zhao ∗ The Key Laboratory of Biomedical Information Engineering of Ministry of Education, School of Life Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 25 January 2013 Received in revised form 21 March 2013 Accepted 11 April 2013 Available online 19 April 2013 Keywords: Localized surface plasmon resonance (LSPR) Au–Ag core–shell nanorod Refractive index (RI) sensitivity Non-uniform silver coating
a b s t r a c t Localized surface plasmon resonance (LSPR) response sensitivity to the environmental refractive index (RI) has been studied for Ag coated Au nanorods both theoretically and experimentally. Quasistatic calculation results indicate that the Ag coating induced increasing of RI sensitivity only takes place when the Ag shell at the ends is much thicker than that at the side facets. Because of the corporate effects from the coating thickness dependent sensitivity and band width changing, the Ag coating always leads to an improvement of the figure of merit (FOM) of Au nanorod sensing, and the increase of FOM becomes faster as the Ag coating is non-uniform. Furthermore, higher RI sensitivity with the same resonance wavelength could be obtained by Ag coating, and the improvement of RI sensitivity with the same resonance wavelength could be fine tuned by changing the non-uniformity ratio of the Ag coating. These calculation results have also been verified experimentally. And this Ag coating dependent optimization of RI sensitivity of Au–Ag bimetallic nanorod makes them potential candidates for nanoscale biochemical sensing applications. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Localized surface plasmons (LSPR) in Au and Ag nanoparticles are coherent charge density oscillations with a resonant frequency that greatly depends on the particle size, geometry and dielectric environment. Because the coulombic force from the polarized field of surrounding dielectric medium effectively reduces the strength of the surface charge and plasmon energy, the resonance wavelength red shifts greatly as the environmental refractive index (RI), n, is increased slightly. Therefore, this RI dependent shifting of plasmon band location leads to the development of plasmonic nanostructures as optical chemical/biologic sensors [1,2]. Recent developments in synthesis, assembly and theoretical study have generated new insights about control of particle shape [3], structure [4], composition [5] and arrange [6] that can improve the RI sensing ability based on LSPR. Chen et al. reported the shape- and size-dependent refractive index sensitivity of solid gold nanoparticles, including nanospheres, nanocubes, nanobranches, nanorods, and nanobipyramids [7]. It has been demonstrated that the RI sensitivities generally increase as gold nanoparticles have been elongated and their apexes become sharper. Because of the plasmon hybridization between inner and outer metallic surfaces,
∗ Corresponding author. Tel.:+86 29 82664224; fax: +86 29 82664224. E-mail address: [email protected] (J.-W. Zhao). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.04.042
the symmetric coupling plasmon modes with lower energy in core–shell metal nanostructures are also suitable for RI sensing [4]. It has been found that the RI sensitivities of gold nanorings are over 5 times larger than those of solid nanodisks with similar diameters [8]. Cao et al. reported the optimized LSPR RI sensitivity of gold nanoboxes for sensing applications [9]. They found the RI sensitivity increases near-exponentially as the wall thickness of the nanobox is decreased. A comparative spectroscopic study on gold nanoshells and their solid counterparts has been reported by Sun et al. [10]. They found the gold nanoshells exhibited a more significant red-shift for their LSPR band when the local dielectric environment is changed. The effect of the metal type on the RI sensitivity has been investigated by Lee et al. [11]. By comparing the RI dependent plasmon shifting of Ag and Au nanocubes with nearly the same LSPR spectral region, it has been found that the LSPR of the Ag nanocubes is twice as sensitive to the surrounding index change as that of the Au nanocubes. Although the RI sensitivity could be greatly increased by reducing the particle symmetry or enlarging the inter-surface coupling, the corresponding resonance wavelength position and plasmon band width also increase. Because too wide band width will affect the detectivity of a plasmonic sensor, the line width of LSPR band is usually regarded as a negative factor in plasmonic RI sensing. To address the effect of LSPR band width on RI sensitivity, a figure of merit (FOM) was introduced that is simply the sensitivity (in nm per refractive index unit, RIU) divided by the full width half-maximum (FWHM) [1,12]. On the other hand, overlong resonance wavelength
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may go beyond the spectral detecting range of the spectrophotometer. Therefore, obtaining higher RI sensitivity within shorter wavelength scope (for instance, visible range) is necessary. By using accurate electrodynamic simulations, Miller et al. demonstrated that the plasmonic RI sensitivity is determined by the location of the peak wavelength and the dielectric properties of the material [13]. The plasmon peak position dependent refractive index sensitivity has also been observed for gold nanorings [8]. However, it has been found that the gold nanoparticles with different shape may exhibit different RI sensitivity even though at the same plasmon resonance wavelength [3]. A linear relationship is found between the RI sensitivity and the product of the polarizability and the end curvature. Some interesting questions arise when several factors are simultaneously tuned to control the LSPR properties and RI sensitivity. For example, when an Au nanorod is coated by an Ag nanoshell with non-homogeneous thickness, both the aspect ratio, Au/Ag composition and core–shell structure have been changed. In the report of Lee et al., the effect of the metal type on the RI sensitivity is confirmed by coating an Ag shell around Au nanorods [11]. It has been found that coating an Ag layer brings about a higher RI sensitivity in comparison to the pure Au nanobars. However, recent experimental results indicate that the sensitivity and FOM of Au nanorods only increase with a relative thin Ag coating [5]. The decrease of the sensitivity has been observed as the Ag shell becomes too thick, which has been attributed to the shape change from rod to non-regular. The ultrathin Ag coating induced enhancement of RI sensitivity of Au/ITO platform nanoparticles has also been reported by Deng et al. [14]. The increase in sensitivity has been ascribed to local field enhancement effect and reducing substrate effect. In recent experimental studies, non-uniform Ag coating on Au nanorods has been observed [5,15]. Therefore, both the aspect ratio and metal composition of the Au–Ag bimetal nanostructure will be changed by the non-uniform Ag coating, and then consequently control the RI sensitivity. In this report, we studied the effects of anisotropy coating, shell thickness and aspect ratio of the inner Au nanorod on the RI sensitivity of Ag coated Au nanorod. Furthermore, we also discussed how to obtain the high sensitivity with small plasmon band width and shorter resonance wavelength by non-uniform Ag coating.
2. The model In order to calculate the plasmonic absorption spectrum of Ag coated Au nanorod, a core–shell prolate spheroid model under quasistatic approximation has been used in this study. Although the TEM images show that the Au–Ag nanorods are more like cylinders with semispherical ends [15], the accurate analytical solution of the optical properties for metal nanoparticles with complex geometry remains a theoretical challenge. In recent years, the discrete dipole approximation (DDA) method and finite-difference time domain (FDTD) numerical analysis have been developed for the study of arbitrary nonspherical nanoparticles, but these numerical calculations are complex and will bring some errors [16]. In this study, the quasistatic theory has been used to calculate the optical properties of metal nanorods. This analytical solution is simple and effective, especially when the particle size is much smaller than the incident wavelength. However, the quasistatic theory can only study the optical properties of simple shape nanoparticle with high symmetrical morphology. Therefore, the metal nanorods in this study were approximated as prolate ellipsoids. Many previous reports also used this ellipsoids mode to study the optical properties of metal nanorods [17–19]. Although the end shape of the nanorods will affect their optical properties, the influence is slight and not dominating. As we know, the aspect ratio (the ratio between longitudinal length and transverse diameter) is the major factor which affects
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the shifting of LSPR [20,21]. In this Au–Ag core–shell nanorod structure, a prolate ellipsoidal Au core, with semiaxes a1 = b1 < c1 , is coated with another coaxial prolate ellipsoidal Ag shell with semiaxes a2 = b2 < c2 . And the transverse coating thickness is denoted as tT = a2 − a1 , the longitudinal coating thickness is denoted as tL = c2 − c1 . In the Drude model, the dielectric functions of both inner Au nanorod and outer Ag nanoshell are frequency dependent and have real and imaginary components [22,23]. In this calculation, the dielectric functions and corresponding numerical parameters are from Johnson and Christy data [24]. When the whole nanostructure is immersed in a dielectric environment with a refractive index of n3 oriented randomly, the mean absorption cross-section for this Ag coated Au nanorod can be obtained by using optical theorem [17,25], absorption =
2 Im
2˛
1
3
+
˛3 3
(1)
where ˛l and ˛3 are the polarizabilities of the nanostructure along its transverse and longitudinal principal axes, respectively. The analytic expressions of the polarizabilities and depolarization geometrical factors have been described in detail elsewhere [18,19]. 3. Results and discussion 3.1. Absorption spectra of Ag coated Au nanorods with different environmental refractive index To investigate the plasmonic RI sensitivity of the Ag coated Au nanorod, absorption spectra of Au–Ag core–shell nanostructure with different environmental refractive index have been calculated in Fig. 1a. In this calculation, the geometrical parameters of Au nanorod are set as a1 = 5 nm and p = c1 /a1 = 3.0. The Ag coating is uniform, thus the Ag shell thickness has the same value in transverse and longitudinal direction, i.e., tT = tL = 2 nm. Because of the two directions of oscillation and two interfaces, Au–Ag core–shell nanorods should exhibit four absorption peaks which are corresponding to the longitudinal and transverse plasmon resonance wavelength both of Au–Ag interface and outer Ag surface of the core–shell nanorods respectively [17,23]. However, the absorption peak at shortest wavelength, corresponds to the transverse plasmon from outer Ag surface, is very weak and does not always occur. In the calculation results of Fig. 1a, the transverse plasmon peak from outer Ag surface is too weak to be observed when n3 = 1. Thus we can only find three distinct LSPR peaks in the absorption spectra of Fig. 1a. From shorter to longer wavelength, the three LSPR peaks are corresponding to the longitudinal plasmon from outer Ag surface, transverse and longitudinal plasmon from Au–Ag interface [17,23]. Indeed, the fourth LSPR peak corresponding to transverse plasmon from outer Ag surface is starting arise at 360 nm of the absorption spectrum when n3 = 2. As shown in Fig. 1a, when the environmental refractive index is increased from n3 = 1.0 to 2.0, the lower energy plasmon peak red shifts 260 nm, which is much greater than the shift of other peaks with shorter wavelength. In order to make a comparison, we also plotted the environmental refractive index dependent absorption spectra of bare Au nanorod without coating and Au nanorod with Au coating. As shown in Fig. 1b, the increasing n3 induced plasmonic red shift of Au coated Au nanorod is decreased to 239 nm, which is weaker than that of Au nanorod with Ag coating. However, the increasing n3 induced plasmonic red shift of bare Au nanorod is increased to 295 nm, which is greater than that of Au nanorod with Ag coating, as shown in Fig. 1c. Although the Ag coating weakens the sensitivity of Au nanorod to the change of environmental refractive index, the band width and resonance wavelength also decrease with the Ag coating. Therefore, how to optimize the RI sensing characters of Au nanorod with Ag coating becomes the topic of next sections.
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Fig. 1. Absorption spectra of gold nanorod with different environmental refractive index n3 , (a) Ag coating, (b) Au coating and (c) no coating. a1 = 5 nm, p = 3.0 and tT = tL = 2 nm.
3.2. Optimizing the sensitivity and figure of merit (FOM) of Au nanorods by non-uniform silver coating In this section, the optimization of RI sensitivity and FOM of Ag coated Au nanorod has been studied by changing the coating thickness and uniformity. Here, the semi-minor axis and aspect ratio of inner gold nanorod are still set as a1 = 5 nm and p = c1 /a1 = 3.0. Fig. 2a shows the LSPR response sensitivity (/n3 ) as a function of coating thickness with different non-uniformity. When the transverse coating is two times thicker than that of longitudinal coating, the sensitivity fades down as the coating thickness is increased, and the decreasing speed is faster than the case with uniform Ag coating. When the longitudinal coating is two times thicker than that of transverse coating, the sensitivity still fades down as the coating thickness is increased, but the decreasing speed is slower than the case with uniform Ag coating. Interestingly, when the coating non-uniformity is further increased to tL /tT = 3, the sensitivity increases as the coating thickness is increased. The coating thickness dependent plasmon band width with different non-uniformity has also been studied, as shown in Fig. 2b. Because of the larger scattering relaxation time of silver, the increasing Ag coating thickness leads to the FWHM decreases monotonously when the longitudinal coating is thicker. And the decreasing speed fades down when the non-uniformity ratio tL /tT is increased. On the contrary, the thickness dependent FWHM decreasing gets faster when the transverse coating is thicker. And the decreasing speed gets intense
when the non-uniformity ratio tT /tL is increased. The physical origin is attributed to the faster increasing speed of the Ag composition. An interesting result has been observed that the thickness dependent monotonous decreasing of FWHM only takes place with small thickness. For example, when tT /tL = 1.5, the non-monotonous changing of FWHM occurs as the thickness is increasing to about 4 nm. Furthermore, the critical thickness decreases to about 2.5 nm when the non-uniformity ratio is increased to tT /tL = 2.0. The physical mechanism of non-monotonous changing of FWHM could be attributed to the Ag coating induced plasmon band merging. As shown in Fig. 2c, as the Ag coating is increased, the longer wavelength peak corresponding to the longitudinal plasmon from Au–Ag interface blue shifts and increases more intensely than that of the middle wavelength peak corresponding to the transverse plasmon from Au–Ag interface. Thus there is a combination between these two plasmon bands, which leads to the FWHM increases first and then decreases. Because the increasing coating thickness induced blue shift and intensity increase becomes faster as the nonuniformity ratio tT /tL is increased, the critical thickness decreases when the non-uniformity ratio is increased. Therefore, the plasmon band blend and split dependent changing of FWHM should be considered in optimizing the FOM of nanostructure with multi LSPR bands. Fig. 2d shows the FOM as a function of Ag coating thickness. Because of the corporate effects from the coating thickness dependent sensitivity and band width, the Ag coating always leads to the improvement of the FOM of Au nanorod. And the increase in FOM
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Fig. 2. (a) LSPR response sensitivity (/n3 ) as a function of coating thickness; (b) FWHM as a function of coating thickness; (c) absorption spectra of Au nanorod with different Ag coating thickness; (d) FOM as a function of coating thickness. a1 = 5 nm and p = 3.0.
for non-uniform Ag coating is usually faster than that of uniform coating when the coating thickness is small. In order to find the optimal RI sensitivity, we calculate the LSPR response sensitivity as a function of both Ag coating thickness and non-uniformity ratio, as shown in Fig. 3. It is interesting to find that the high sensitivity can be obtained with a small non-uniformity ratio and thin Ag coating under the condition of tT > tL , as shown in Fig. 3a. However, when the shell thickness at the ends is larger than that at the side facets, i.e., tL > tT , the high sensitivity can be obtained with a large non-uniformity ratio and thick Ag coating, whereas the low sensitivity can be obtained with a small nonuniformity ratio and thick Ag coating, as shown in Fig. 3b. There is a critical value of non-uniformity ratio tL /tT ≈ 2.2. When the nonuniformity ratio is smaller than the critical value, increasing the coating thickness leads to the sensitivity decreases. On the contrary, the sensitivity increases with the increasing coating thickness when the non-uniformity ratio is larger than the critical value. The LSPR response sensitivities with larger aspect ratio (p = 4) of inner Au nanorod are shown in Fig. 3c–d. One can find the thickness and non-uniformity ratio dependent sensitivity changing is similar to that of Ag coated Au nanorod with small aspect ratio. However, the critical value of tL /tT increases to about 3.2 as the aspect ratio p is increased to 4. Due to the instable property of Ag nanorods, so far there has been little literature reporting about thicker Ag shells at the ends of Au nanorods [26]. However a Pt coating the tips of Au nanorod was obtained by Marek et al. [27], Ag-tipped Au nanorods were synthesized by Kyoungweon [28]. Au/Ag nanoshuttles owning sharp tips were synthesized by Li et al. [29], which also exhibit stronger local field enhancements than the original Au nanorods. Although
the method to produce thicker shells at the ends is still a challenge and has seldom been reported, the theoretical analysis prediction of non-uniform coating dependent sensitivity will stimulate the improvement of preparation technology for producing thick coating at the ends. For the experimentally available nanorods, better sensitivity would be just obtained for uniform and very thin Ag shell. However, better FOM would be obtained when the Ag coating is uniform and thick, or the Ag coating is non-uniform and thin, as shown in Fig. 2d. 3.3. Improve the sensitivity with the same plasmon resonance wavelength of Au nanorods by non-uniform silver coating Although the results in Fig. 2 indicate that the Ag coating usually weaken the RI sensitivity of Au nanorod, the Ag coating also leads to intense blue shift of the LSPR band. Could we obtain higher sensitivity with the same resonance wavelength by Ag coating? Fig. 4 plots the LSPR response sensitivity of Ag coated Au nanorod as a function of resonance wavelength. Here, the resonance wavelength is the LSPR peak wavelength of Au–Ag nanorods with the environmental refractive index of 1.0. In order to make a comparison, the sensitivity of bare Au nanorod at different resonance wavelength (corresponding aspect ratio p is increased from 1.5 to 4.2) has also been plotted in Fig. 4. When the bare Au nanorod has a resonance wavelength of 625 nm, the corresponding RI sensitivity is 292 nm/RIU, as shown in Fig. 4a. Uniform Ag coating leads to the resonance wavelength blue shift and the sensitivity decrease simultaneously. However, the sensitivity is higher than that of bare Au nanorod with the same resonance wavelength. With increasing the non-uniformity ratio tL /tT , the blue shift induced decreasing of
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Fig. 3. (a) LSPR response sensitivity (/n3 ) as a function of both coating thickness and coating thickness ratio tT /tL , the inner gold nanorod has a small aspect ratio p = 3, (b) LSPR response sensitivity (/n3 ) as a function of both coating thickness and coating thickness ratio tL /tT , the inner gold nanorod has a small aspect ratio p = 3, (c) LSPR response sensitivity (/n3 ) as a function of both coating thickness and coating thickness ratio tT /tL , the inner gold nanorod has a large aspect ratio p = 4, (d) LSPR response sensitivity (/n3 ) as a function of both coating thickness and coating thickness ratio tL /tT , the inner gold nanorod has a large aspect ratio p = 4. a1 = 5 nm.
sensitivity becomes gentle. Especially, the blue shift induced sensitivity increasing takes place as the tL /tT is increased to 3. Therefore, higher sensitivity with the same resonance wavelength could be obtained by non-uniform Ag coating (tL > tT ). For example, at the resonance wavelength of 560 nm, the bare Au nanorod only has a RI sensitivity of 170 nm/RIU. However, the sensitivity at 560 nm could increase to 327 nm/RIU after non-uniform Ag coating. Although the sensitivity has been improved when the aspect ratio of the inner Au nanorod is increased to 4.0, the decreasing resonance wavelength induced sensitivity decreasing gets faster, as shown in Fig. 4b. The Ag coating leads to the resonance wavelength blue shift and the sensitivity decrease simultaneously on the condition of tL /tT ≤ 3. The blue shift induced sensitivity increasing only takes place on the condition that the tL /tT is increased to 4.0. One can find the Ag coating induced sensitivity increasing at the same resonance wavelength becomes gentle when the inner Au nanorod has a small aspect ratio. However, the wavelength region in which we could improve the sensitivity at the same wavelength has been enlarged. In the previous figures, although the pure Au nanorods have higher sensitivity than that of Au nanorods with Ag coating, the resonance wavelength corresponding to pure Au nanorods is also longer than that of Au nanorods with Ag coating. For pure Au nanorods, the shorter resonance wavelength could be obtained by decreasing the aspect ratio, and the corresponding sensitivity has also been decreased. Therefore, the sensitivity is a function of resonance wavelength, and the pure Au nanorods have lower sensitivity than Ag coated Au nanorods when they have the same resonance wavelength. One important finding in this report is that higher RI sensitivity with the same resonance wavelength could be obtained by Ag coating, and the improvement of RI sensitivity with the same resonance wavelength could be fine tuned across a broad wavelength range by changing the non-uniformity ratio of the Ag coating.
3.4. Experimental 3.4.1. Reagents Cetyltrimethylammonium bromide (CTAB) and silver nitrate (AgNO3 ) were purchased from Sigma (USA), sodium borohydride (NaBH4 , Tianjin Kermel Chemical Reagent Co. Ltd., China), gold chloride trihydrate (HAuCl4 , Sinopharm Chemical Reagent Co. Ltd, China), ascorbic acid (AA, Tianjin TianLi Chemical Reagent Co. Ltd., China), hydrochloric acid (HCl, Beijing Chemical Works, China), sodium hydroxide (NaOH, Tianjin HengXing Chemical Reagent Co. Ltd., China) and glycerol (Tianjin TianLi Chemical Reagent Co. Ltd., China) were of analytical grade and used without further purification. Ultra-pure water was used through all the preparations. 3.4.2. The synthesis of gold nanorods Gold nanorods were prepared according to a seed-mediated growth method developed by Sau and Murphy with some slightly modification [30]. Firstly, a freshly ice-cold NaBH4 solution (0.01 M, 0.6 mL) was prepared in advance, gold seed solution were prepared by quickly injecting NaBH4 into the mixture of HAuCl4 (0.01 M, 0.25 mL) and CTAB (0.1 M, 7.5 mL) in a test tube of 15 mL, followed by a minute of violent stirring. Immediately after the stirring, the color of the solution changed dark yellow to brownish yellow indicating the start formation of the gold seed. Then the solution was kept at 27 ◦ C for 2 h for next use. Secondly, gold growth solution were prepared by injecting HAuCl4 (0.01 M, 4 mL), AgNO3 (0.01 M), AA (0.1 M, 0.64 mL), HCl (1 M) into an aqueous CTAB solution (0.1 M, 90 mL) followed by gentle stirring. The color of the solution changed brownish yellow to colorless due to the reduction of Au+ to Au3+ [31]. By adding different volumes of AgNO3 (0.6–1.2 mL) and HCl (0–2 mL), we will obtain different aspect ratios of gold nanorods. Lastly, 0.2 mL gold seed solution was injected to initiate growth
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Fig. 4. LSPR response sensitivity (/n3 ) as a function of resonance wavelength. The inner gold nanorod has a (a) small aspect ratio p = 3; (b) large aspect ratio p = 4. a1 = 5 nm.
reaction. Then the ultimate solution was kept at 27 ◦ C undisturbed overnight, ensuring the gold nanorods grow adequately. 3.4.3. The synthesis of Au–Ag core–shell nanorods Au–Ag core–shell nanorods were prepared by the method in previous literature [15] with some minor modification. Centrifugation for 20 min was needed in order to remove other reagents and make the nanorods more pure, and then precipitate was redispersed into the same volume of 0.08 M CTAB which was serving as capping agent in the synthesis of silver-coated gold nanorods. One minute of ultrasonic of the solution is necessary to ensure the intensive mixing. AgNO3 (0.01 M) and AA (0.1 M, 0.4 mL) were injected so as to provide Ag and reductant respectively. NaOH (0.1 M, 1 mL) was used to adjust the pH value of the aqueous system thus to provide an alkaline environment to initiate the reaction. By adding different volumes of AgNO3 (0.04–1.2 mL), we will obtain Au–Ag core–shell nanonanorods with different Ag thickness. The final solution was then kept at 65 ◦ C for 4 h. In Fig. 5, transmission electron microscopy (JEM-200CX, JEOL Ltd., Japan) was used to investigate the detailed structures of synthesized nanorods. The typical TEM images for the synthesized Au–Ag core–shell nanorods depict the size and morphology of the nanorods clearly, from which we could obtained their size distributions are relatively narrow. The average length and the width of the original Au nanorods are about 60 nm and 11 nm respectively. The TEM characterization also demonstrates the different thickness of the Ag shell. As can be
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Fig. 5. TEM images of the obtained Au–Ag core–shell nanorod samples, (a) Au nanorod with thin Ag coating and the volume of AgNO3 is 0.5 mL, (b) Au nanorod with thick Ag coating and the volume of AgNO3 is 0.8 mL.
observed in Fig. 5a, when the amount of AgNO3 is small, the Ag shell is thin, whose average thickness is about 5 nm. Owing to the non-uniform Ag coating, the side facet of the nanorods is thicker than the end as the amount of AgNO3 increased. Thicker Ag shell of about 9 nm could be observed when the volume of AgNO3 reaches 0.8 mL as shown in Fig. 5b. 3.4.4. The measurement of refractive index sensitivity To investigate the sensitivities of Au–Ag core–shell nanorods with different refractive indices, as-prepared Au–Ag core–shell nanorods were first divided into five portions and then centrifuged at 10,000 rpm for 20 min. When discarding the supernatant, we redipersed the precipitate into the water–glycerol mixtures with different refractive indices. Park et al. used water (1.33), ethanol (1.36), isopropanol (1.38), ethylene glycol (1.44) and tetraethylene glycol (1.46) as different refractive indices solvents to redisperse Au–Ag core–shell nanocubes [32]. But when we disperse centrifuged Au–Ag core–shell nanorods into these water-miscible organic solvents such as ethanol and isopropanol, aggregation occurs immediately. Here we use water–glycerol mixture of different volume ratios [3] to provide different refractive indices so as not to cause aggregation. An Abbe refractometer (WAY-2S, Shanghai SuoGuang Light & Electricity Technology Co. Ltd., China) was used to test the refractive index of each mixture, which is more accurate than the refractive indices obtained by the calculation according to Lorentz-lorenz equation. We know that the refractive indices from 1.33 to 1.42 are in the regime of the interest for biological applications according to Fu et al. [5], thus our refractive indices are chosen
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Fig. 6. (a) Absorption spectra of Au–Ag core–shell nanorod samples with different volumes of the AgNO3 solution; (b) normalized absorption spectra of one Au–Ag core–shell nanorod sample in five water–glycerol solutions with five different refractive indices, the inset presents the dependence of resonance wavelength red shift with the increasing of refractive indices, of which the slope of the linear fit line indicating the RI sensitivity of Au–Ag core–shell nanorod sample.
from 1.33 to 1.42 in each test. When redispersing the Au–Ag in solution of glycerine aqueous solution with different refractive index, we measure the spectra of them after a while. According to a series of tests, we found that ten to fifteen minutes is the optimize time for measuring. Too short or too long time is not suitable, for it is not mixed well and the nanorods do not have interaction time with new environment change when the time is short, while too long time, e.g. more than 30 min will cause agglomeration which leads to the solution half-transparent or transparent thus affecting the properties of the Au–Ag core–shell nanorods. The plasmon shifts were plotted as a function of the refractive index. The refractive index sensitivities were obtained by linear fitting from Origin 8.6 software. 3.4.5. Experimental results and discussion Fig. 6a depicts the spectra of the Au–Ag core–shell nanorods with different Ag shell thickness, the corresponding volume of AgNO3 is increased from 0.04 to 0.9 mL. It is well known that Au nanorods exhibit two distinct absorption peaks which are corresponding to transverse and longitudinal plasmon resonance wavelength. Au–Ag core–shell nanorods exhibit four absorption peaks which are corresponding to the longitudinal and transverse plasmon resonance wavelength both of Au–Ag interface and outer
Ag surface of the core–shell nanorods respectively [17,23]. As the amount of AgNO3 is increased, longitudinal plasmon resonance wavelength peak of the Au–Ag interface blue shifts progressively, and at the same time transverse plasmon resonance wavelength peak of the Au–Ag interface attenuates gradually and disappears at last. When the mole ratio of Au and Ag reaches 1, the longitudinal and transverse plasmon resonance wavelength peak of the outer surface appear, the transverse plasmonic resonance wavelength of the outer surface shows a weaker absorption while longitudinal plasmon resonance absorption gets intense distinctly. When the volume of AgNO3 reaches 0.9 mL, two absorptions of longitudinal plasmon of both Au–Ag interface and outer Ag surface with equal intensity of 2.75 could be observed, as shown in line D in Fig. 6a. Here we focus on the longitudinal plasmon wavelength of the Au–Ag interface due to its high absorption value and enhanced sensitivity, which is advantageous candidate for the ultra-sensitive biosensors. In order to investigate the effect of non-uniform Ag coating of Au nanorods on refractive index sensitivity, we choose the Au–Ag core–shell nanorods with the longitudinal wavelength of 666.5 nm, which is corresponding to the spectrum of line B in Fig. 6a, as samples to conduct the experiment of the measurement of refractive index sensitivity according to the method introduced above. The water–glycerol solutions have five different refractive indices of 1.3494, 1.3618, 1.3840, 1.4046 and 1.4198. Fig. 6b shows the normalized absorption of Au–Ag core–shell in these solutions with the longitudinal plasmon wavelength of the interface red-shifts as a function of increased refractive indices. The refractive index sensitivity is defined as the wavelength change when the refractive index change one unit using the unit of nm/RIU [33], which is obtained by linear fitting from Origin 8.6 software, as is shown in the inset of Fig. 6b. In our experiment, the original Au nanorods core with the longitudinal plasmon resonance wavelengths of 817.5 nm were used to be coated with the Ag shell. When the thickness of Ag shell is thinner, which is corresponding to the spectrum of line A in Fig. 6a, the sensitivity is 644 nm/RIU. Owing to the anisotropic coating, the side facet of the nanorods is thicker than the end as the amount of AgNO3 increased. Thus the thicker Ag shell corresponding to the spectrum of line C in Fig. 6a, whose sensitivity reduces to 269 nm/RIU compared to the thinner one, as shown in Fig. 7a. In order to investigate the refractive index sensitivities about Au nanorods and Au–Ag core–shell nanorods, we choose different aspect ratios of Au nanorods defined as shorter, medium and longer nanorods whose longitudinal plasmon resonance wavelengths are corresponding to 791.5 nm, 808 nm and 817.5 nm respectively as original core to be coated with Ag shell. When obtaining different Ag shell thickness, we plotted these samples’ sensitivities as a function of resonance wavelength, as shown in Fig. 7b. We can clearly observed that at the same resonance wavelength the refractive index sensitivity of Au–Ag core–shell nanorods is much higher than that of bare Au nanorods. For example, at the resonance wavelength of about 700 nm, the sensitivity of Au nanorods is only 200 nm/RIU, while the Au–Ag core–shell exhibits a sensitivity of 450 nm/RIU, which is more than twice of that of Au nanorods. Although the sensitivity of Au–Ag core–shell is decreased with the thickness increasing of Ag shell both from our theoretical calculation and experimental result, it is much higher than bare Au nanorods at the same resonance wavelength. It is worth noting that there is a dramatically increase at the beginning of the Ag coating, here we ascribe it to the initial nonuniform Ag coating. The amount of Ag is less sufficient, which causes it to coating the end of the nanorods preferentially due to its high energy. Owing to the non-uniform Ag coating at the beginning, the sensitivity has a transient and sharp increase first, as shown in Fig. 7b, which is consistent with our calculation result demonstrating that when tL /tT equals 3, the sensitivity could increase. As
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also considered. Furthermore, higher RI sensitivity with the same resonance wavelength could be obtained by Ag coating, and the improvement of RI sensitivity with the same resonance wavelength could be fine tuned across a broad wavelength range by changing the non-uniformity ratio of the Ag coating. Our experiment results show the increase of Ag coating thickness generally leads to the decrease of RI sensitivity of Au–Ag core–shell nanorods, while nonuniformity of Ag coating induces RI sensitivity increasing at the initial coating time when the Ag shell at the ends is much thicker than that of side facets. On the other hand, higher RI sensitivity with the same resonance wavelength could be obtained by Ag coating compared to bare Au nanorods. These experimental observations are in good agreement with the theoretical calculation above. This work will provide great potential not only for further improvement of RI sensitivity by non-uniform Ag coating but also for fabrication of ultra-sensitive and high FOM of nanorod-based biosensors at shorter resonance wavelength. Acknowledgements This work was supported by the Program for New Century Excellent Talents in University under Grant No. NCET-10-0688, the Fundamental Research Funds for the Central Universities and the National Natural Science Foundation of China under Grant Nos. 11174232, 61178075, 81101122. References
Fig. 7. (a) Normalized absorption spectra of Au–Ag core–shell nanorod samples with a thicker and a thinner Ag shell in three different refractive indices water–glycerol solutions; (b) the dependence of the sensitivities on the resonance wavelengths for Au nanorods and Au–Ag core–shell nanorod samples with shorter (max = 719.5 nm), medium (max = 808 nm) and longer (max = 817.5 nm) original Au core nanorods.
can be seen from Fig. 7b, with the amount of AgNO3 increased gradually, the sensitivities of Au–Ag core–shell decrease correspondingly, which is also in good agreement with our calculation result showing in Fig. 4a that when tT /tL equals 2, a decrease of the sensitivity is observed. In view of multiple factors including size, shape, structure and so on, the experimental results and the theoretical calculation could not perfectly matched, but from general trend, our experimental results are in good agreement with the theoretical calculation above. 4. Conclusions Localized surface plasmon resonance (LSPR) response sensitivity to the environmental refractive index has been studied for Ag coated Au nanorods both theoretically and experimentally. By using quasistatic approximation theory, we find that Ag coating generally leads to the decrease of the RI sensitivity of Au–Ag core–shell nanorods, and the Ag coating induced the increase of RI sensitivity only takes place when the Ag shell at the ends is much thicker than that at the side facets. A figure of merit was also calculated to compare the overall performance of Ag coated Au nanorods. Owing to the coating of Ag, the FWHM gets narrower compared to that of Au nanorods, thus the FOM of Ag coated Au nanorods could be increased in general. However we should note that the plasmon band blend and split dependent changing of FWHM should be
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Biographies Jian Zhu received his PhD degree in 2004 from Xi’an Jiaotong University, China. His research interests focus mainly on optical properties of nano-structured materials and multi-information fluorescence probe. Fan Zhang received the BE degree from Xinxiang Medical University, China, in 2010. Now she is a graduate student for a Master’s degree in Xi’an Jiaotong University, China. Her research interests focus mainly on optical biosensor based on plasmonic optics. Jian-Jun Li received her PhD degree in 2003 from Xi’an Jiaotong University, China. Her research interests focus mainly on multi-information fluorescence probe and bioseparation. Jun-Wu Zhao is a professor in the School of Life Science and Technology, Xi’an Jiaotong University, China. His research interests focus mainly on nano-biomaterials and device.