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We synthesized anisotropic Pd nanoplates for use as a novel refractive index (RI) sensing material in the visible region. The nanoplates showed an extinction peak that was attributed to the excitation of the localized surface plasmon resonance at ~620 nm in the visible region. It was found that the peak was red-shifted with increasing the RI of the surrounding medium. The susceptibility was calculated to be 250 nm per RI unit, comparable to some anisotropic Au nanoparticles that are excellent RI sensing materials.
© 2016 Optical Society of America
1. Introduction
The localized surface plasmon phenomenon which is unique to metal nanoparticles generates a definite optical extinction peak at a specific wavelength due to the resonance with the incident light field [1–8]. This is called the localized surface plasmon resonance: LSPR. While it is well known that this phenomenon leads to the generation of strong local electromagnetic fields as compared with the incident light fields which are applicable to surface-enhanced Raman scattering (SERS), etc [9–12], the position of the peak wavelength sensitively responds to the change in the refractive index (RI) at the nanospace around the nanoparticles [13–15]. This phenomenon holds promise for applications in label-free and cost-effective interfacial biosensing, including the cancer biomarkers [16–18], DNA detection [19,20], immunoassay [21], etc.. The sensing detection limit mainly depends on the magnitude of the peak shift against the RI change, as well as the resonance linewidth as indicated by the figure of merit (FoM), which is defined as the ratio of the RI susceptibility to the linewidth [15]. Although metasurfaces consisting of sophisticated subwavelength periodic structures showed an excellent RI susceptibility as reported recently [22], special and expensive apparatuses are needed to fabricate these structures. Therefore, improving the intrinsic RI susceptibility of plasmonic nanoparticles is an important technical objective for the development of highly sensitive label-free biosensing materials. In fact, various plasmonic anisotropic nanoparticles displaying excellent RI susceptibilities have been developed, including Au nanobranches [23], Au nanorods [23–25], Au nanobipyramids [23], Au nanostars [26], Au pyramids [27], Au nanorattles [28], Au nanoprisms [29], Ag nanoprisms [30–32], Ag nanocubes [33], Au nanoframes [34], and Au nanorings [35]. However, all of these nanoparticles consist of the well-known plasmonic elements Au and Ag. The preoccupation with these elements may have been an obstacle to finding novel highly sensitive plasmonic elements.
Recently, we have found that the RI susceptibility of Au-core/Pd-shell type nanospheres (Au/PdNSs), which generate the peak of Pd LSPR, is substantially higher than those of Au and Ag nanospheres with similar diameters [36]. We also theoretically verified, within a quasi-static (QS) approximation framework, that the higher susceptibility of the Pd LSPR originates from the smaller dispersion of the real part of its dielectric function around the resonant wavelength, compared to those of Au and Ag LSPR. However, since the LSPR peak of the Au/PdNSs was generated at a near-ultraviolet region, bioanalytes could be damaged by irradiation of the near-ultraviolet light in the application biosensing [37–39]. Herein, we report the large RI susceptibility of Pd nanoplates which generate the LSPR peak at a visible region.
2. Experimental section
Synthesis of Pd nanoplates
The Pd nanoplates were synthesized according to a previously reported procedure [40]. Firstly, 12.5 mg of Pd(II) acetylacetonate, 40 mg of poly(vinylpyrrolidone) (PVP, MW = 29,000) and 13 mg of NaBr were dissolved in a mixed solvent of 2.5 mL of N,N-dimethylpropionamide and 0.5 mL of water. The transparent yellow solution was transferred to a 10-mL glass pressure vessel. After the vessel was then charged with CO (pressure: 1 bar), the solution was vigorously shaken. After repeating the manipulation (charging and shaking) thrice, the CO-charged solution was heated at 100 °C for 3.0 h. The dark blue solution in which the nanoplates were dispersed was cooled at room temperature. After 0.1 mL of acetone was added, the solution was centrifuged at 10,000 rpm for 10 min to remove the unreacted precursors. After the obtained precipitates were dispersed in anhydrous ethanol (0.7 mL) to which 0.3 mL acetone was added, the solution was centrifuged at 10,000 rpm for 10 min. Finally, the resultant precipitates was dispersed in a mixed solvent of ethanol/glycerol described in the next section.
Investigation of refractive index susceptibility
The precipitate obtained by centrifugation was dispersed in ethanol solutions containing 0, 10, 20, 30, 40, and 50 vol% glycerol. The LSPR peak position was determined by measuring the extinction spectrum of these colloidal Pd nanoplate solutions.
Theoretical calculation of the extinction spectra of the Pd nanoplate
The extinction spectra of an isolated Pd nanoplate were calculated theoretically by the boundary element method within the QS approximation (QS-BEM) framework with the MNPBEM14 program [41] on MATLAB® R2013a. One Pd nanoplate was bare, and the other was coated with the 0.9 nm-thick PVP. The dielectric functions were taken from the literature for Pd [42] and PVP [43]. Note that the dielectric function of PVP is wavelength dependent and the refractive index of PVP around the LSPR wavelength is approximately 1.52. The in-plane dipole resonance mode in the plate was excited for the extinction spectra. The geometry of the nanoplate with circular shape was approximated by a regular polygon with 40 sides. The meshgrids were in which the thickness was divided into 12 divisions. The geometry of the Pd nanoplate is shown in Fig. 1(e).
Fig. 1 Pd nanoplates synthesized in this study. (a)-(c) BF-STEM and (d) HAADF-STEM images and (e) Schematic illustration and model structure approximated by a regular polygon with 40 sides. E and k are the electric field vector and the wave vector of the inciendent light.
Measurements
UV-vis spectral measurements were carried out using a JASCO V-630 spectrophotometer. Bright-field (BF) and high-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) images were taken by using STEM (HD-2300C, Hitachi Ltd.) at 200 keV. The STEM sample was prepared by dipping the carbon grid into the colloidal solution of Pd nanoplates in ethanol.
3. Results and discussion
Since the typical Pd nanospheres show a dipole LSPR peak in the middle-to-near ultraviolet regions, we synthesized Pd nanoplates with an anisotropic shape, which show a definite extinction peak in the visible region. To prepare the Pd nanoplates, we employed the CO-confinement growth method. It has been reported that the strong adsorption of CO molecules on the (111) planes of the Pd nanosheets prevents the growth along the [111] direction, resulting the formation of the plate-like structures [40]. The generating wavelength of the LSPR of the nanoplates mainly depends on the aspect ratio, i.e. the ratio of the diameter of the front face to its thickness [44]. The STEM images of the Pd nanoplates synthesized in this study are shown in Figs. 1(a)–1(d). Two typical arrangements i.e., extended lamellar structures consisting of stacked Pd nanoplates and flat-lying nanoplates (indicated by a red, dashed circle, Figs. 1(c) and 1(d)) were observed. The average thickness of the nanoplates was estimated to be 1.5 ( ± 0.2) nm which is almost the same as described in a previous report [40] for some lamellar structures(Fig. 1(b)). The average diameter of the nanoplate was estimated to be 16.3 ( ± 2.0) nm. Thus, the aspect ratio of the nanoplates was approximately evaluated to be 11 (Fig. 1(e)).
The extinction spectrum of the colloidal ethanol solution of the Pd nanoplates is shown in Fig. 2. The solution showed a broad LSPR peak at approximately 620 nm in the visible region.
Fig. 2 Extinction spectrum of the Pd nanoplates. (a) Observed spectrum for a colloidal ethanol solution. (b) Calculated spectrum (RI of the surrounding medium: 1.361) of the Pd nanoplate (Fig. 1(e)).
The generating wavelength of the LSPR, as well as its spectral shape, agreed well with that calculated by QS-BEM based on the nanoplate shape estimated using the STEM images.
Although the experimental LSPR peak was slightly shorter than that calculated, this could be due to a slight variation in the nanoplate shapes.
Next, we investigated the change in an LSPR wavelength (λmax) of the Pd nanoplate upon the change in RI of the surrounding medium. The RI of the surrounding medium was controlled by the mixing ratio of ethanol (n = 1.361) and glycerol (n = 1.474), and the values were obtained by the following empirical polynomial expression [45].
where a and b are the volume fractions of ethanol (a) and glycerol (b) in the mixed solvent (a + b = 1). In the experiment (Fig. 3(a)), the LSPR peak from the colloidal solution of the Pd nanoplates was red-shifted upon increasing the glycerol concentration (10, 20, 30, 40, and 50%) corresponding to an increase in the RI of the surrounding medium (nbulk). As shown in Fig. 3(b), the theoretically-obtained LSPR peak was also red-shifted upon increasing the RI. As summarized in Fig. 3(c), the LSPR peak shifted for both experiments and theory; Δλmax = (λmax in 20–50% glycerol solution) – (λmax in 10% glycerol solution) varied linearly with the RI (1.375–1.425) of the surrounding medium. The RI susceptibility, defined as Sexp = Δλmax/Δnbulk was obtained from the linear slope to be 250 nm RIU−1, where RIU is the refractive index unit. This value of the RI susceptibility for the Pd nanoplates is comparable to those of some anisotropic Au nanoparticles which have been suggested to be excellent plasmonic RI sensing nanomaterials, such as Au nanorods (198–288 nm RIU−1) [23–25], Au nanopyramids (130–221 nm RIU−1) [27], Au nanobars (219 nm RIU−1) [46], and Au nanotube arrays (250 nm RIU−1) [47]. In addition, while the synthetic routes of these anisotropic gold nanoparticles are quitecomplicated because two or more steps or nanofabrication apparatuses are required, the Pd nanoplates can be simply synthesized by one step. It has thus been demonstrated that the Pd nanoplates can function as an excellent RI sensing material. On the other hand, the susceptibility was substantially lower than that the theory predicted (541 nm RIU−1, Fig. 3(c)). This was possibly because the PVP, which coated the nanoplates as a protective layer, reduced the RI change in the surrounding medium in the vicinity of the nanoplates. From the STEM image of the lamellar structures of the stacked Pd nanoplates (Fig. 1(b)), the distance between the neighboring Pd nanoplates was estimated to be 1.8 nm, giving a thickness of 0.9 nm for the thin PVP films coating the nanoplates, even under dry conditions. To evaluate the contribution of 0.9 nm-thick PVP films to the RI susceptibility, the QS-BEM calculation of PVP-coated Pd nanoplate predicted a much lower RI susceptibility (352 nm RIU−1) compared to that of the bare Pd nanoplate (Fig. 4). Therefore, it is pointed out that the lower experimental RI susceptibility compared to the theoretical prediction was attributed to the existence of the PVP protecting layer. However, the experimentally-obtained RI susceptibility of the PVP-coated Pd nanoplates is still lower than the theoretically-obtained value. This is possibly caused by limited accuracy of the PVP thickness because this was estimated by the STEM images under a dry condition. In order to evaluate our hypothesis, we calculated the dependence of the RI susceptibility of the PVP-coated Pd nanoplates on the PVP thickness.Fig. 3 Shifts of extinction peak of the Pd nanoplates dispersed in media with different RI. Spectra are normalized to highlight the spectral shift. (a) Observed extinction spectra of the Pd nanoplates dispersed in different solvent mixtures (10, 20, 30, 40, and 50 vol. % glycerol ethanol solutions). (b) Calculated extinction spectra of the Pd nanoplates in different RI solvents (n = 1.375, 1.388, 1.401, 1.413, and 1.425 corresponding to ethanol containing 10, 20, 30, 40, and 50 vol.% glycerol, respectively). (c) Dependences of the LSPR peak shifts upon RI changes from (A) experimental and (B) theoretical results.
Fig. 4 (a) Schematic illustration of PVP-coated Pd nanoplate. (b) Theoretical dependences of the LSPR peak shifts upon RI changes of (A) the bare Pd nanoplate (same as (B) in Fig. 3(c)) and (B) the PVP-coated Pd nanoplate and experimental dependence of the peak shifts of (C) the nanoplates (same as (A) in Fig. 3(c)).
As shown in Fig. 5, it was found that the RI susceptibility sensitively changes with a small change in the thickness. For example, the susceptibility was significantly decreased to 195 nm RIU−1 with increasing the thickness of PVP to 2.4 nm. It is therefore strongly suggested that the Pd nanoplates will show a higher RI susceptibility than some excellent plasmonic RI sensing nanomaterials as described above [23–25, 27, 46, 47].
Fig. 5 (a) Theoretical dependences of the LSPR peak shifts upon RI changes of the Pd nanoplates with the PVP of (A) 0, (B) 0.6, (C) 0.9, (D) 1.2, (E) 1.8, and (F) 2.4 nm in thicknesses. (b) PVP thickness dependence of the RI susceptibility of the Pd nanoplates.
4. Conclusion
We investigated the RI susceptibility of plasmonic Pd nanoplates as a novel RI-sensing material. The LSPR peak attributed to the in-plane dipole mode of the nanoplates showed a comparable susceptibility to those exerted by some excellent anisotropic Au nanoparticles. On the otherhand, the susceptibility was substantially lower than the theoretically calculated. This is possibly due to the effect of the PVP thin film coating the nanoplates. The Pd nanoplates may even show a higher susceptibility by removing the PVP protective agent, or replacing it with thinner protective agents, beneficial for the development of highly sensitive label-free and cost-effective interfacial biosensing platforms. Research in this direction is currently underway in our laboratory.
Acknowledgment
This work was supported by Grant-in-Aid for Young Scientists B (Grant No. 26810102) from JSPS KAKENHI, Applied Research Grant from Nihon University, and the Strategic Foundation at Private Universities, Mext and Nihon University.
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