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Key Parameter Controlling the Sensitivity of Plasmonic Metal Nanoparticles: Aspect Ratio Assad U. Khan,† Shuqi Zhao,† and Guoliang Liu*,†,‡ †

Department of Chemistry and ‡Macromolecules Innovation Institute, Virginia Tech, Blacksburg, Virginia 24061, United States S Supporting Information *

ABSTRACT: Currently the synthesis of plasmonic nanoparticles for sensing applications mostly focuses on their shape because it is believed that nanoparticles with sharp tips provide higher sensitivities than those without. Herein, by measuring and analyzing the sensitivities of more than 74 types of nanoparticles of various shapes, sizes, and compositions, we found that, contrary to this common belief, the correlation between shape and sensitivity is much weaker than that between aspect ratio and sensitivity. Among all the parameters investigated here, including size, shape, composition, aspect ratio, crosssectional area, and initial plasmonic resonance frequency, the aspect ratio (R) is the key parameter that controls the nanoparticle sensitivity (S) following an empirical equation, S = 46.87R + 109.37. Other parameters have much less influence on the nanoparticle sensitivity to refractive index changes. The stronger dependence of the sensitivity on aspect ratio than on shape encourages us to reassess the current focus of nanoparticle synthesis chemistry. In addition, the S−R linear relationship determined here can be used as a design rule for future synthesis and fabrication of highly sensitive nanomaterials for chemical, biological, biomedical, and environmental sensing.

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resonance (LSPR).10−13 Due to LSPR, nanoparticles exhibit characteristic extinction spectra10,11,14 depending on the nanoparticle shape,15 size,16 and composition,3 as well as the refractive index of the surrounding media.11,15 For a given nanoparticle, the extinction spectrum redshifts to a longer wavelength as the refractive index increases, which forges the foundation of plasmonic nanoparticle sensing.10,11,17 The sensitivity, defined as the ratio of shift in plasmon resonance wavelength to changes in refractive index (S = ΔλLSPR/Δn), should potentially be governed by the nanoparticle parameters such as shape, size, and composition. Following this conventional belief, many have synthesized and fabricated nanoparticles of exotic shapes to obtain high sensitivities. To date, the investigated nanoparticles include nanospheres,18−20 nanorods,19,21−24 nanoplates,15,25 nanoprisms,19,26 nanocubes,18,26,27 nanobranches,18 nanobipyramids,18 nanorings,28 nanocrescents,29 and core−shell nanostructures.24,30 The sensitivities have been reported individually for each type of nanoparticle, and based on a few examples, it is believed that shape matters the most for plasmonic sensing.21 In contrast, Lee and El-Sayed3 were the first to point out that the aspect ratio could be the key parameter that governs the nanoparticle sensitivity, although they only investigated one nanoparticle shape, that is, nanorod. Their simulations suggested that for nanorods the sensitivity

INTRODUCTION Noble metal nanoparticles, which exhibit rich plasmonic properties, have a wide range of applications including sensing,1−4 imaging,1−3 catalysis,5−8 medical diagnostics,1,2 and therapeutics.9 Currently, the chemistry of nanoparticle synthesis focuses mostly on creating nanoparticles of exotic shapes. This research focus is based on the presumption that, for all plasmonic sensing applications, nanoparticles with sharp tips and edges are more sensitive than those without due to the locally enhanced electromagnetic field. Herein, through a comprehensive survey of the relationships between plasmonic sensitivity and various nanoparticle parameters, including shape, size, composition, cross-sectional area, initial plasmonic resonance frequency, and aspect ratio, we found that the shape is not the most important parameter dictating the nanoparticle sensitivity to refractive index changes of the environment. Much to our surprise, the aspect ratio plays a more significant role in the nanoparticle sensitivity than any other parameters. This discovery encourages us to reevaluate the current focus of nanoparticle synthesis chemistry on nanoparticle shape and directs us to design nanoparticles with large aspect ratios for plasmonic sensing, especially for detecting the refractive index changes of the environment. Plasmonic nanoparticles made of noble metals are rich in electrons. Upon photon exposure, conduction electrons in the metal nanoparticles oscillate collectively and resonate with the external electromagnetic field when the frequencies match; this is the phenomenon known as localized surface plasmon © 2016 American Chemical Society

Received: June 28, 2016 Revised: August 6, 2016 Published: August 9, 2016 19353

DOI: 10.1021/acs.jpcc.6b06519 J. Phys. Chem. C 2016, 120, 19353−19364

Article

The Journal of Physical Chemistry C scaled linearly with the aspect ratio at a given effective radius. Experimentally, Charles et al. revealed a linear relationship between the aspect ratio and the sensitivity of solution phase triangular Ag nanoplates.15 Based on these two seminal studies,3,15 it is plausible that the linear relationship between the sensitivity and the aspect ratio could be valid for nanoparticles of other shapes. To test this hypothesis and ultimately to develop a rationale for the chemistry of nanoparticle synthesis in plasmonic sensing applications, we systematically investigated the effect of the nanoparticle parameters on the plasmonic sensitivity to refractive index changes. Our findings show a weak correlation between the sensitivity and the nanoparticle parameters including shape, size, composition, and cross-sectional area. Instead, the sensitivity depends mainly on the aspect ratio and loosely on the initial plasmonic resonance frequency. These relationships have been formulated through comprehensively surveying a large library of nanoparticles of various shapes, sizes, and compositions, including 25 types of nanoparticles synthesized in this work (Figure 1) and 49 types reported in other studies.

Figure 2. (a) Optical graph and (b) the corresponding normalized UV−vis-NIR spectra of colloidal Ag nanoplates of various sizes. The size of Ag nanopolates increased from a to k.

plate-like shape regardless of their size, and therefore we termed them as Ag nanoplates and we used equivalent circular diameter to describe the longest dimension of the nanoparticles. Table 1 summarizes the average size, thickness, and aspect ratio for each type of Ag nanoplate. Charles et al. showed that the thickness of Ag nanoprisms increased with size,15 but our nanoplates showed no appreciable change in thickness. This is possibly due to the narrow range of diameter that we investigated. Since our thickness did not vary greatly, we assumed an average thickness of 6.3 nm. For nanodisks, the aspect ratio was calculated as the diameter over the thickness, while for nanoprisms it was calculated as the edge length over the thickness. For example, Ag nanoplates “f” had an average diameter of 23 nm and an average aspect ratio of 3.8, respectively (Figures 3f and 4). To determine the nanoparticle sensitivity to changes in the refractive index of the surrounding media, Ag nanoplates were immersed in water and glycerol mixtures of various glycerol volume fractions (φGly). Water and glycerol were chosen because they are miscible at any volume ratio and have differing refractive indices of 1.3334 and 1.4746, respectively.18,32 Varying the concentration of glycerol will modulate the refractive index of the binary mixtures in accordance with the Lorentz−Lorenz equation:18

Figure 1. Various shapes of nanoparticles synthesized and investigated in this work including nanodisks, nanoprisms, nanorods, and nanospheres, from left to right. The aspect ratio is defined as R = L/d. For nanosphere, R = 1. The nanoparticles are composed of either Au or Ag. The size is changed systematically from ∼10 to 250 nm and λLSPR,0 from ∼400 to 1750 nm.

The surprising weak dependence of the plasmonic sensitivity on the nanoparticle shape informs us that, rather than synthesizing nanoparticles of the most exotic shapes, the future chemistry of plasmonic nanoparticle synthesis should focus on aspect ratio to design highly sensitive nanoparticles for sensing applications in chemical, biological, biomedical, and environmental sciences.

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RESULTS AND DISCUSSION Sensitivity of Ag Nanoplates. To construct a library of nanoparticles for sensitivity investigation, we first started with Ag nanoplates, which were synthesized using a seed-mediated method with slight modifications.31 As the size increased from (a) to (k), Ag nanoplates exhibited various colors ranging from yellow, to red, to purple, and to blue (Figure 2a). Their characteristic λLSPR in water (denoted as λLSPR,0) spanned from ∼420 to 700 nm as measured using ultraviolet−visiblenear-infrared (UV−vis-NIR) spectroscopy (Figure 2b). The extinction spectra broadened as the size increased. We attributed this to the increasing size dispersity as evident in the size statistics (Figures S1 and S2). The shape of Ag nanoplates differed slightly from one another depending on the size (Figure 3). This phenomenon was more apparent for smaller nanoplates, which almost rounded completely and became nanodisks, while larger ones were less rounded and appeared as nanoprisms. The Ag nanoparticles had a two-dimensional

2 n12 −1 2 n12 +2

= φ1

n12 − 1 n12 + 2

+ φ2

n22 − 1 n22 + 2

(1)

where n is the refractive index and φ is the volume fraction of the two components (1 and 2) in a binary system. In the waterglycerol mixtures, the volume fraction of glycerol was increased from 0 to 90% in 10% increments. The measured refractive index values agreed well with those predicted by the Lorentz− Lorenz eq (Figure S3). It should be noted that other media such as water-sucrose mixtures or organic solvents could be used, but they are not suitable here due to undesirable local interactions15,18 or tedious phase transfer processing.33 For example, sucrose may interact covalently with Ag or Au nanoparticles, which gives a nonlinear response to changes in 19354


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The Journal of Physical Chemistry C

Figure 3. TEM images of Ag nanoplates (a−k) corresponding to those in Figure 2.

Table 1. Sizes, Aspect Ratios, Plasmon Wavelengths, and Sensitivities of Au and Ag Nanoparticles nanoparticles Au nanoplates Au nanoplates Au nanoplates Au nanoprisms Au nanoprisms Au nanoprisms Au nanoprisms Au nanoprisms Au nanoprisms Au nanoprisms Au nanoprisms Au nanospheres Au nanorods Au nanorods Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Au nanorods Au nanorods Au nanorods Au nanorods Au nanorods Au nanobipyramids Au nanobipyramids Au nanobipyramids Au nanobipyramids Au nanobranches

length,a L (nm)

thickness,b d (nm)

aspect ratio,c R

λLSPR,0 (nm)

sensitivity, S (nm/RIU)

sensitivity (eV/RIU)

FoM (RIU−1)

26 30 31 64 57 63 69 76 99 103 100 28 249 217 11 12 12 14 16 23 25 28 30 31 43 40 55 74 65 50 27 50 103 189

7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 28 16.9 16.9 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 17.0 16.0 17.0 20.0 15.0 19.0 18.0 26.0 40.0

3.4 4.0 4.1 8.6 7.6 8.4 9.2 10.2 13.2 13.8 13.4 1.0 14.8 12.9 1.7 1.9 2.0 2.3 2.6 3.8 3.9 4.5 4.8 4.9 6.9 2.4 3.4 4.4 3.3 3.3 1.4 2.8 4.0 4.7

691 740 767 818 826 894 910 942 1013 1015 1030 530 1758 1710 437 450 467 485 494 512 543 565 587 626 667 653 728 846 740 662 645 735 886 1096 1141

272 315 319 434 334 451 660 462 638 559 884 64 581 661 141 150 168 175 187 214 224 249 327 358 389 195 224 288 385 170 150 212 392 540 703

0.67 0.68 0.63 0.75 0.57 0.65 0.91 0.60 0.72 0.62 0.94 0.28 0.22 0.26 0.88 0.89 0.91 0.89 0.92 0.96 0.90 0.92 1.11 1.06 1.03

1.81 1.83 1.54 1.91 1.44 1.63 2.41 1.55 1.98 1.63 3.54 0.91 2.31 1.88 1.81 1.58 1.91 1.47 1.76 1.88 1.71 1.64 1.93 1.96 2.21 2.60 2.10 1.70

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1.70 2.80 4.20 4.50 0.80

refs

18 18 18 42 22 18 18 18 18 18


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The Journal of Physical Chemistry C Table 1. continued nanoparticles Au nanorods Au nanobipyramids Au nanobipyramids Au oxidized nanorods Au dog-bone-like nanorods Au peanut-like nanorods Au small nanorods Ag nanoprisms Au nanospheres Au nanospheres Au nanospheres Au nanospheres Ag nanocubes Ag nanocubes Ag nanocubes Ag@Au nanoprisms Ag@Au nanoprisms Ag@Au nanoprisms Ag@Au nanoprisms Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates Ag nanoplates

length,a L (nm)

thickness,b d (nm)

aspect ratio,c R

λLSPR,0 (nm)

sensitivity, S (nm/RIU)

108 55 67 61 56 43 26 40 50 25 50 100 40 80 130

44.0 19.0 24.0 20.0 20.0 14.0 10.0 10.0 50.0 25.0 50.0 100.0 69.3 138.6 225.2

2.5 2.9 2.8 3.1 2.8 3.2 2.6 4.0 1.0 1.0 1.0 1.0 1.7 1.7 1.7

52 32 27

5.5 5.4 5.1

9.4 5.9 5.3 2.1 2.1 2.5 2.9 3.6 3.7 4.0 4.4 4.3 5.1 5.2 6.6 6.9 7.3 8.2 9.5 10.3 10.0 12.3 13.3

732 729 729 733 732 734 733 650 520 525 532 578 428 530 679 940 760 700 540 513 532 564 595 528 591 629 656 766 845 702 729 826 871 1022 922 1033 1120 1072 1094

326 301 264 244 238 220 156 425 85 60 80 180 176 361 480 470 380 420 210 192 198 228 291 216 292 326 370 431 429 392 410 406 412 531 519 642 908 707 1097

sensitivity (eV/RIU)

FoM (RIU−1) 2.80 3.30 4.30 2.60 1.90 1.90 1.60 3.00

3.30 2.43 1.46

1.88 1.81 1.94 2.15 2.00 2.20 2.25 2.00 2.17 2.10 2.25 2.00 1.86 2.10 1.95 2.50 2.25 3.20 2.92 4.28

refs 21 21 21 21 21 21 21 43 43 44 44 44 27 27 27 37 37 37 37 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15

For nanoprisms, “L” is the length along an edge; for nanoplates and nanospheres, “L” is the diameter; for nanorods and nanobipyramids, “L” is the length of the longest axis. bFor nanoprisms, “d” is the thickness of the prismatic plate; for nanodisks, “d” is the thickness of the disk; for nanorods, “d” is the diameter; for nanospheres, “d” is the diameter; for nanobipyramids, “d” is the length of the shortest axis. cThe aspect ratio was calculated for each individual nanoparticle and then the average was obtained. Note that the aspect ratio is not calculated from L/d. a

refractive index.15 In contrast, water−glycerol mixtures provide a linear response to glycerol volume fraction and therefore are better choices than water−sucrose mixtures. Once Ag nanoplates were immersed in water−glycerol mixtures, λLSPR red-shifted linearly with φGly (Figure 4d). For example, Ag nanoplates “f” had an initial λLSPR of 512 nm in water and shifted to 542 nm as φGly increased to 90%. A linear fitting of λLSPR and ELSPR with respect to refractive index yielded sensitivities of 214 nm/RIU and 0.96 eV/RIU, respectively (Figure 4e,f). Similarly, Ag nanoplates of various sizes were immersed in water−glycerol mixtures. As φGly increased, the solution color changed systematically and λLSPR red-shifted (Figures S4 and S5). Their sensitivities were calculated and summarized in Figure 5 and Table 1. The sensitivity of Ag nanoplates has a linear relationship with aspect ratio, R. In the wavelength range investigated in this

work, the sensitivity also has a linear relationship with initial plasmonic resonance wavelength, λLSPR,0 (vide infra). Charles et al. have also studied the relationship between sensitivity and λLSPR,0 for Ag nanoplates and noted that the linearity was limited to λLSPR,0 below 800 nm; otherwise, it resulted in a nonlinear behavior.15 The λLSPR,0 values of our Ag nanoplates were below 800 nm and followed a similar linear relationship. Sensitivity of Au Nanoprisms. To evaluate the effect of composition on the plasmonic nanoparticle sensitivity, we further synthesized Au nanoprisms and measured their sensitivities. Au nanoprisms of various sizes were synthesized using a seed-mediated method34 and λLSPR,0 ranged from 691 to 1030 nm (Figures 6 and S6). The as-synthesized Au-nanoprism colloidal solutions contained impurities such as Au nanospheres. However, because the Au nanospheres had an extinction peak at 530 nm and the peak did not overlap any of Au 19356


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Figure 4. (a) Optical graph showing the color change of Ag nanoplates “f” in water-glycerol mixtures as the glycerol volume fraction φGly increases from 0 to 90%. (b) Diameter statistics for Ag nanoplates “f”. (c) Thickness statistics for all Ag nanoplates. (d) UV−vis spectra show that λLSPR of Ag nanoplates “f” redshifts as φGly increases. (e) λLSPR and (f) spectral energy ELSPR as a function of refractive index change, respectively.

Figure 5. (a) λLSPR and (b) ELSPR of all Ag nanoplates as a function of refractive index. Ag nanoplates a−k correspond to those in Figure 2.

aspect ratio are summarized in Table 1 (see also Figures S9, S10, S11, and S12). Similar to the Ag nanoplates, we did not observe a significant change in the thickness as the edge length of Au nanoplates increased. Since some Au nanoprisms were nanodisk-like, we used their diameter to calculate their respective aspect ratio. The as-synthesized Au nanoprisms were stabilized in concentrated hexadecyltrimethylammonium bromide (CTAB), which may affects λLSPR due to its different refractive index. To minimize the effect of CTAB on sensitivity measurement, colloidal solutions of Au nanoprisms were centrifuged and redispersed in deionized water. After centrifugation, a layer of CTAB may still be present on the surfaces of Au nanoparticles because it is difficult to completely remove CTAB.35 In fact, a small amount of CTAB on the surface can prevent the nanoparticles from aggregating and thus increase their stability. We observed that excessive centrifugation and redispersion could result in nanoparticle aggregation and the nanoparticles were no longer usable for sensing. After centrifugation once, the Au nanoprisms were dispersed in aqueous solutions of various φGly and no aggregation was observed. If not mixed thoroughly in the water-glycerol mixtures, however, a shoulder peak would appear in the UV−vis-NIR spectrum, suggesting that the Au nanoprisms possibly stacked in a face-to-face manner.36

Figure 6. UV−vis-NIR spectra of selected Au nanoprisms. λLSPR,0 shifted to longer wavelength as the size increased from a to h.

nanoprisms (Figure S7), they did not have to be removed from the solutions and the sensitivities of Au nanoprisms could still be properly determined. The size and shape of Au nanoprisms were characterized by TEM (Figures 7 and S8). A majority of Au nanoprisms were prismatic and had sharp tips except (a), (b), and (c) (Figure 7). The measured edge length/diameter, thickness, λLSPR,0, and 19357


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Figure 7. TEM images of Au nanoplates (a−h) corresponding to (a−h) in Figure 6. Au nanoplates (a−c) were rounded nanodisks and (d−h) were nanoprisms. The size increased from (a) to (h).

Figure 8. (a) λLSPR and (b) ELSPR of Au nanoplates as a function of refractive index. The samples (a−h) correspond to those in Figures 6 and 7.

The sensitivities of the Au nanoprisms were determined and summarized in Table 1. Similar to the Ag nanoplates, both λLSPR and ELSPR followed linear relationships with the refractive index of the solutions (Figures 8, S13, S14, and S15). It is worth noting that Au nanoprisms “h” with an aspect ratio of 13.4 had a sensitivity of 884 nm/RIU and 0.94 eV/RIU, higher than any solution-phase Au nanoprisms or other Au nanostructures reported in previous studies.18,21,22,24,37 Sensitivity of High Aspect-Ratio Au Nanorods. Following the literature38 we have also synthesized Au nanorods with aspect ratios of ∼15 and 13, termed as AR15 and AR13, respectively. The sizes of the Au nanorods were characterized by TEM (Figure 9). Their lengths were 249 and 217 nm for AR15 and AR13, respectively, and the diameters were both 16.9 nm (Figure S16). The longitudinal extinction peaks of AR15 and AR13 were at 1758 and 1710 nm, respectively. Due to the similar diameter, the transverse extinction peaks were both at around 500 nm (Figures 10 and S17). Measurement of nanoparticle sensitivities in the near-infrared region is often interfered by the absorbance of biomolecules,39 water,40 and polyols like glycerol. H2O strongly absorbs light at ∼1400 nm and interferes with the extinction peaks of Au nanorods (Figure S18). To overcome the strong H2O interference, the nanorods were dispersed in D2O after removal of

Figure 9. TEM images of Au nanorods with aspect ratios of (a) 15 and (b) 13.

excess CTAB by centrifugation. Au nanorods in D2O provided distinct extinction peaks (Figures 10 and S17). The longitudinal λLSPR,0 of nanorods AR13 and AR15 differed less than 4% from those predicted by Link et al.,41 suggesting a high quality of the nanorods and accurate theoretical predictions in this R range. Besides H2O, glycerol has two absorption peaks at ∼1600 and 1700 nm, but the magnitude is much lower than that of H2O. Once glycerol was added to the solutions and φGly was more than 10%, a shoulder peak at ∼1600 nm corresponding 19358


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Figure 10. (a) Spectral shift for Au nanorods AR13. When φGly is more than 10%, two peaks were observed, corresponding to glycerol absorbance (left) and Au nanorods extinction (right), respectively. (b) λLSPR and (c) ELSPR of Au nanorods AR13 as a function of refractive index.

Figure 11. (a) TEM image of Au nanospheres. (b) UV−vis spectral shift as a function of φGly, (c) λLSPR, and (d) ELSPR as a function of refractive index.

we have summarized all experimental values collected in this work and those reported in the literature (Table 1). The nanoparticles considered here include nanodisks,15 nanoprisms,37,43 nanorods,18,21,22 nanospheres,43,44 nanobipyramids,18,21 nanobranches,18 and nanocubes27 of various sizes and compositions. Because of their strong dependence on the surrounding media, nanoparticles attached to substrates are not considered here.20,24,28,29,45 Conventionally, it is believed that the nanoparticle shape plays a vital role in sensitivity, and nanoparticles with sharp tips can be more sensitive because of the locally enhanced electromagnetic fields.43 However, our data suggests that sharp tips are not mandatory to ensure a high sensitivity for plasmonic nanoparticles, especially with regard to detecting refractive index changes. For example, Ag nanoplates did not have sharp tips, but they showed comparably high sensitivities, at least on a par with the triangular nanoprisms (Figure 5). In addition, the Au nanorods with sharp tips were less sensitive

to glycerol started to appear. Despite the undesirable shoulder peak, the longitudinal extinction peaks of Au nanorods could be distinguished and followed a linear relationship with refractive index (Figures 10, S17, and S19). The addition of glycerol resulted in a decreasing concentration of Au nanorods in the mixture. Therefore, the intensity of the Au nanorod extinction peaks decreased, as evident in Figures 10 and S17. Sensitivity of Au Nanospheres. Au nanospheres were byproducts during the synthesis of Au nanoprisms. We also investigated their sensitivity to the changes in the refractive index of the environment. TEM image (Figure 11) shows that the Au nanospheres were uniform in size and had an average diameter of 28 nm (Table 1). The initial extinction peak λLSPR,0 was at 530 nm. As expected, λLSPR red-shifted slightly as φGly and refractive index increased, and the sensitivity was determined to be 64 nm/RIU and 0.28 eV/RIU. Sensitivity Analysis of All Nanoparticles. To determine the key parameter that controls the nanoparticle sensitivity, 19359


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Similarly, a linear fitting to our sensitivity data yields an eq (Figure S22):

than a few Au nanoprisms with comparably dull tips, further confirming that sharper tips do not necessarily result in higher sensitivities. We attribute the weak dependence of the sensitivity on the nanoparticle sharp tips to a different sensing mechanism, that is, the plasmonic sensitivity to the refractive index change depends on the whole nanoparticle,10,11 while other tip-enhanced sensing techniques rely on the locally enhanced electromagnetic fields.11,46−48 The main advantage of complex particles with sharp features is their sensitivity to extremely small volumes located directly at the sharp tips and edges. With respect to plasmonic sensitivity to refractive index changes surrounding the whole nanoparticle, however, the effect of sharp features diminishes and polarizability of the whole nanoparticles plays a significant role, based on the quasistatic localized plasmon resonance theory.49 As also suggested in the theoretical work by Gorkunov et al.,50 the plamons at the sharp tips and edges do not necessarily provide a significant contribution to the particle polarizatbility. Instead, the polarizability is mainly controlled by the plasmons determined by the whole particle or, more specifically, the nanoparticle aspect ratio (vide infra). Regarding the effect of size on the nanoparticle sensitivity, the fact that the two longest Au nanorods (L > 200 nm) showed less sensitivities than the smaller Au nanoprisms (L < 110 nm) suggests that the nanoparticle sensitivity does not depend on the nanoparticle size or, specifically, the nanoparticle length (L). This behavior is more evident when all nanoparticles are considered, as shown in Figure S20. The sensitivity also showed an independence of composition, which can be attributed to the high electron density of noble metals such as Au and Ag, as suggested by Lee and El Sayed.3 Further investigation showed little correlation between sensivitity and nanoparticle cross-sectional area (Figure S21). In contrast, regardless of the shape, size, composition, and cross-sectional area, an analysis of all sensitivity data suggested a linear relationship between sensitivity and aspect ratio (Figure 12) following an empirical equation: S = 46.87R + 109.37

S = 42.90R + 87.44

(3)

A linear fitting to the sensitivity with respect to aspect ratio reported by Charles et al.15 provides an equation: S = 63.32R + 56.31

(4)

Although there is some difference in the slopes, all linear fittings to the measured sensitivities are within the 95% prediction band of eq 2 (Figure S23). However, the theoretical predictions for Au nanorods by Lee and El-Sayed3 had significantly larger slopes than all equations fitted to the experimentally measured sensitivities. This, on the one hand, proves the linear relationship between the sensitivity and the aspect ratio predicted by the theory3 but, on the other hand, shows that the theoretical prediction is over optimistic compared with experiments and suggests an opportunity for improvement in theoretical computation and simulations. Similarly, the dependence of nanoparticle sensitivity on λLSPR,0 was investigated and a linear relationship was found with λLSPR,0 up to 1000 nm (Figure 13). This differs slightly from

Figure 13. Measured sensitivity values of nanoparticles of various sizes, shapes, and compositions. The sensitivity follows a linear trend with λLSPR,0 when λLSPR,0 is less than 1000 nm and diverges when λLSPR,0 is more than 1000 nm. The nanoplates include both nanodisks and nanoprisms. Blue and red symbols represent nanoparticle sensitivities measured in this work and in the literature, respectively.

(2)

the observation by Charles et al.15 and the simulation by Miller et al.,51 where the linear relationship was only valid with λLSPR,0 up to 800 nm. Our analysis shows that the sensitivity diverged when λLSPR,0 was more than 1000 nm: either increased dramatically or remained close to 600 nm/RIU. This divergence suggests that the nanoparticle sensitivity depends more linearly on the structural parameter aspect ratio than the initial surface plasmon resonance frequency, λLSPR,0. Besides sensitivity, figure of merit (FoM) is another metric to evaluate the performance of nanoparticles for detecting refractive index changes,26,52,53 which is defined as the plamsonic sensitivity over the full width at half-maximum (fwhm) of the extinction spectrum, that is, FoM = S/fwhm. fwhm of the nanoparticles synthesized in this work ranged from 78 to 343 nm, and in general, it increased with λLSPR,0 and R (Figure S24). However, FoM showed no apparent relationships with λLSPR,0, R, or shape (Figures S25 and S26). We note that, for practical sensing applications, sensitivity is not the only parameter for consideration when one selects a nanoparticle for plasmonic sensors. First, the range of plasmonic

Figure 12. Measured sensitivity values of nanoparticles of various sizes, shapes, and compositions. The sensitivity followed a linear relationship with the aspect ratio, regardless of nanoparticle shape, size, and composition. The nanoplates include both nanodisks and nanoprisms. Blue and red symbols represent nanoparticle sensitivities measured in this work and in the literature, respectively. 19360



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resonance frequency matters. Au nanoparticles with λLSPR,0 in the NIR range provides higher sensitivities, but to probe plasmonic shift in the visible electromagnetic range, these nanoparticles are not suitable. Therefore, nanoparticles with relatively lower sensitivities but resonating in the targeted wavelength range are preferred. To provide all the necessary information for choosing the appropriate nanoparticle for sensing applications, we present both the sensitivity values (Figures S19, S27, and S28) and the wavelength range that the nanoparticles resonate in (Figures 5, 8, 10, 11, and S17), the latter of which was often neglected in the previous reports.18,20,43 Second, one should pay attention to the surface ligands introduced during the synthesis. For example, the excess CTAB molecules need to be removed so that they do not change the refractive index and give false signals. Third, the independence of sensitivity on composition has important indications for synthesizing and fabricating nanosensors. In spite of a lower cost and a high sensitivity, Ag nanoparticles are comparably less stable than Au nanoparticles, especially in the presence of strong ionic solutions or with the passage of time, as observed here and in previous reports.54,55 To avoid longterm instability and susceptibility to etching, Au is preferred to Ag for device fabrication. This is particularly important in reversible temperature-controlled devices,56 where a high temperature can increase the shape evolution rate of Ag nanoplates.

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MATERIALS AND METHODS

Materials. Sodium citrate tribasic dihydrate (≥99.0%; S4641), sodium borohydride (≥99.99%; 480886), silver nitrate (≥99.9999%; 204390), poly(sodium 4-styrenesulfonate) (PSSS; average Mw ∼ 1000 kg mol−1, powder; 434574), ascorbic acid (≥99.0% crystalline; A5960), gold(III) chloride trihydrate (≥99.9%; 520918), hexadecyltrimethylammonium bromide (CTAB; ≥99.0%; 52365), sodium iodide (≥99.5%; 383112), sodium hydroxide (≥97.0%; 221465), glycerol (≥99.5%; G9012), and D2O (99.9 atom % D; 151882) were all purchased from Sigma-Aldrich and used as received without further purification. All solutions were prepared using ultrapure deionized (DI) water obtained from Thermo Scientific Barnstead GenPure Pro water purification system at a resistance of 18.2 MΩ·cm. All glassware and stir bars were washed with Aqua Regia, rinsed with ultrapure water, and then dried in a stream of air. Synthesis of Ag Nanoplates. Ag nanoplates were synthesized using the reported seed-mediated method31 with slight modifications. In a typical experiment, 4.5 mL of ultrapure water, 0.5 mL of 25 mM sodium citrate, and 0.25 mL of 0.5 mg/mL PSSS solutions were sequentially added into a 20 mL scintillation vial and stirred vigorously. Within 20 s, 0.6 mL of freshly prepared 10 mM NaBH4 in ice-cold ultrapure water was immediately added to the solution. Afterward, 5 mL of 0.5 mM AgNO3 was added at a rate of 2 mL/min using a syringe pump. The resulting solution was sealed with parafilm and stirred for 2 h to decompose the excess NaBH4 and age the Ag nanoparticle seeds. To allow the seeds to grow into Ag nanoplates (either nanodisks or nanoprisms), 180 μL of 10 mM ascorbic acid was first added to 15 mL of ultrapure water. Subsequently, a controlled amount of Ag seed solution (80− 1200 μL) was added, forming a series of growth solutions. In each growth solution, 4.5 mL of 0.5 mM AgNO3 was added at the rate of 1 mL/min using a syringe pump to synthesize Ag nanoparticles with a controlled size. The growth solutions gradually changed color as AgNO3 was introduced, suggesting that the seeds grew into larger nanoparticles. Finally, 1.5 mL of 25 mM sodium citrate solution was added to each growth solution to stabilize the Ag nanoparticles. Throughout the entire synthesis reaction, the growth solutions were vigorously stirred. Synthesis of Au Nanoprisms. Au nanoprisms were synthesized using the seed-mediated method reported previously.34 To prepare Au seeds, 36 mL of ultrapure water was added to a round-bottom flask (100 mL) and vigorously stirred. Subsequently, 1 mL of 10 mM sodium citrate solution and 1 mL of 10 mM HAuCl4·3H2O were added. Afterward, 1 mL of freshly prepared ice-cold 100 mM NaBH4 solution was added, and the entire solution immediately turned red. After ∼1 min, the stirring was stopped and the solution was left to sit for 4 h to age the seeds and decompose the residual NaBH4. Before Au nanoprism growth, 450 mL of 50 mM CTAB solution was prepared in ultrapure water and then 225 μL of 100 mM NaI solution was added to form a mixture with a final NaI concentration of 50 μM. The CTAB/NaI mixture was shaken in a water bath (∼60 °C) until CTAB fully dissolved and then kept at ∼27 °C. In a 50 mL beaker, 36 mL of CTAB/ NaI mixture was first added, followed by 1 mL of 10 mM HAuCl4·3H2O solution, 200 μL of 100 mM NaOH, and 200 μL of 100 mM ascorbic acid. After briefly shaking the solution, its color changed from yellow to colorless, indicating

SUMMARY AND CONCLUSION

We have performed, to our knowledge, the most detailed and comprehensive study of plasmonic nanoparticle sensitivity to date. Various nanoparticle parameters, including shape, size, composition, cross-sectional area, aspect ratio, and initial plasmonic resonance frequency, have been systematically investigated regarding their effects on the nanoparticle sensitivity. Our findings suggest that the aspect ratio is the key parameter that controls the nanoparticle sensitivity and follows a simple linear relationship. The sensitivity increases linearly with λLSPR,0, but only when λLSPR,0 is less than 1000 nm. The comprehensive survey over various nanoparticle parameters rules out the strong dependence of sensitivity on other parameters such as shape, size, composition, and cross-sectional area. Importantly, the relatively weak dependence of sensitivity on nanoparticle shape differs from the conventional belief that shape plays a critical role in sensing. We attribute this disparity to the different sensing mechanisms utilized in various sensing techniques. While sharp tips and edges are critical for detecting changes in extremely small volumes near nanoparticles by utilizing the localized field enhancement effect, they do not matter when one considers the response of plasmonic resonance wavelength to changes in refractive index surrounding the whole nanoparticle. The sensitivity to refractive index change is an average plasmonic response of the whole nanoparticles to all molecules around; therefore, the shape or tip effect is no longer as pronounced as in those tip-based sensing methods. This suggests opportunities for further theoretical work to fundamentally understand the underlying physics and chemistry of plasmonic sensing. The sensitivities summarized here provide a database for future theoretical simulations. We expect the sensitivity−aspect ratio relationship to serve as a design rule for chemical synthesis and fabrication of highly sensitive nanoparticles for applications in chemical,57−59 biological,2,17,60,61 biomedical,1,22 and environmental sensing.62−66 19361


Article


nanoplates in the final product. In the end, the nanorods were redispersed in 100 mM CTAB in D2O, which was used as a stock solution of Au nanorods. Removal of H2O and CTAB from Au Nanorods. After the nanorods stock solution was left without stirring for 14 h, the supernatant was removed and 0.5 mL of D2O was added to redisperse the nanorods. The same process was repeated two more times to completely remove any residual H2O and CTAB. (Note: once the nanorods were redispersed in pure D2O, they should be used for sensitivity measurement immediately.) Preparation of Nanoparticles−Glycerol Mixtures. Once the surfactants on the nanoparticles were removed, the nanoparticles were immediately used for sensitivity measurement. Glycerol was selected as the additive to change the refractive index of H2O or D2O because: (1) most nanoparticles do not aggregate in glycerol and (2) glycerol is miscible with water at any concentration.18 Nanoparticles in water− glycerol mixtures were prepared by adding controlled amounts of glycerol to aqueous solutions of nanoparticles. The volume fraction of glycerol increased from 0 to 90% in 10% increments. These mixtures were shaken well to form homogeneous solutions. To remove any potential bubbles in the mixtures, the solutions were left untouched for 20−30 min and then used for sensitivity measurement. The D2O−glycerol mixtures were prepared by mixing glycerol with nanoparticles suspended in D2O. TEM Characterization and Analysis. All TEM images were collected on a Philips EM420 at an accelerating voltage of 120 keV. ImageJ was used to determine the nanoparticle size. To determine the average size and the size distribution, 150−200 particles were analyzed. When analyzing Au nanoprisms/nanoplates, Au nanospheres were neglected. The image analysis can also be accomplished by a newly developed method.67 UV−Vis-NIR Spectroscopy. All extinction spectra, unless otherwise stated, were collected on an Agilent Cary 5000 UV−vis-NIR spectrophotometer and normalized for clarity. When the nanoparticles resonate in the NIR range, they were redispersed in D2O because H2O interferes strongly with the measurement at around 1400 nm and D2O does not. The peak positions (λLSPR) were determined by fitting the spectra, only the portion next to absolute peak, with a second order polynomial to provide better accuracy than simply finding the absolute peak. The plasmonic resonance energies (ELSPR) were determined by converting the peak LSPR wavelength into energy using the equation ELSPR = hc/λLSPR. Refractive Index Measurement. The refractive indices of the water-glycerol mixtures were measured on a thermospectronic refractometer. The measured refractive indices agreed well with those calculated using the Lorentz−Lorenz equation.

that Au(III) was reduced to Au(I) and was ready for growing Au nanoprisms. A controlled amount of Au seed solution (20−440 μL) was added to the solution, shaken well, and then left overnight to allow Au nanoparticles to fully grow and stabilize. Removal of CTAB from Au Nanoprisms. The assynthesized AuNPs have CTAB molecules in the solution, which have to be removed before the sensitivity measurement. To do so, 15 mL of Au nanoprism solution was centrifuged at 9000 rpm for 7 min. The supernatant was removed and the nanoparticle precipitates were redispersed in 7 mL of ultrapure water. The redispersion in a lesser amount of water concentrates the nanoparticles so that they offer higher extinction in the UV−vis-NIR spectra. After removal of excess CTAB, the nanoparticles were immediately used for sensitivity measurement. Synthesis of High Aspect Ratio Au Nanorods. Au nanorods were synthesized according to the reported procedures with slight modifications.38 Briefly, 0.5 mL of 10 mM HAuCl4·3H2O was added to 19 mL of vigorously stirred ultrapure water followed by 0.5 mL of 10 mM sodium citrate. The resulting solution had an orange color. Afterward, 0.6 mL of freshly prepared 0.1 M NaBH4 in ice-cold water was added. After 2 min, the stirring was stopped, the vial was sealed with parafilm, and left still for 2 h to age the seeds and fully decompose NaBH4. Au nanorods with two aspect ratios (12.9 ± 1.2 and 14.8 ± 1.8, named AR13 and AR15, respectively) were synthesized using these seeds following a three-step process in three beakers (labeled A, B, and C). To grow AR13, 9 mL of 100 mM CTAB, 250 μL of 10 mM HAuCl4·3H2O, and 50 μL of 100 mM ascorbic acid were added to beaker A and shaken well. Once the solution became colorless, 1 mL of the as-synthesized gold seeds was immediately added. After a brief shake, the solution was kept in a water bath at 27 °C. After 4 h, 36 mL of 100 mM CTAB, 1 mL of 10 mM HAuCl4, and 200 μL of ascorbic acid were added to beaker B and shaken well until the solution became colorless. Afterward, 4 mL of seed solution from beaker A was added into beaker B and left in a water bath at 27 °C. Again, after 4 h, 360 mL of 100 mM CTAB, 10 mL of 10 mM HAuCl4·3H2O, and 2 mL of ascorbic acid were mixed in beaker C and shaken well until the solution became clear. Then all solution in beaker B was transferred to beaker C and kept in a 27 °C water bath for 24 h. For AR15, the procedure was similar, except the nanoparticle growth times in beakers A and B were changed from 4 h to 3−5 s. Purification of High Aspect Ratio Au Nanorods. The as-synthesized Au nanorod solutions often contained nanoparticles with other shapes including spheres and platelets. The nanorods were purified following a method reported by Khanal et al.40 Due to gravity, the Au nanorods settled down at the bottom of the beaker with time, typically in 24 h. After removing the supernatant that contained mostly spheres and small platelets, the precipitates that contained Au nanorods were immediately redispersed in 10 mL of 100 mM CTAB solution. Afterward, 250 μL of etching solution, which was prepared by mixing 182 mg of CTAB and 0.98 mg of HAuCl4· 3H2O in 10 mL of ultrapure water, was added to the nanorods solution and sonicated for 20−30 s until all nanoparticles were redispersed. The solution was left still for 20 h, during which the etching solution etched large nanoplates into smaller ones so that they can be dispersed in the supernatant. The precipitation−redispersion−etching process was repeated one more time to minimize the population of nanospheres and

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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.6b06519. Additional supporting data including nanoparticle size statistics, UV−visible spectra, TEM images, fwhm and FOM data, and sensitivity relationships with length, area, and aspect ratio (PDF). 19362


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 540 231 8241. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Virginia Tech Department of Chemistry and partially supported by the Virginia Tech ICTAS JFC Award. We thank Profs. Alan Esker, Amanda J. Morris, and Tijana Z. Grove for use of instruments. We acknowledge the helpful discussions with Prof. Garth Wilkes. The authors acknowledge use of facilities within the Nanoscale Characterization and Fabrication Laboratory (NCFL) at Virginia Tech.

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