Surface Plasmon Resonance in Gold Nanoparticles a Review

Abstruse

Plasmonic gold nanoparticles with abrupt tips and vertices, such every bit gold bipyramids (AuBPs) and gilt nanocubes (AuNCs), accept been widely used for loftier-sensitivity localized surface plasmon resonance (LSPR) sensing. Notwithstanding, conventional LSPR sensors based on frequency shifts have a major disadvantage: the asymmetry and broadening of LSPR peaks because of instrumental, environmental, and chemical noises that limit the precise determination of shift positions. Herein, we demonstrated an culling method to improve the efficiency of the sensors by focusing on homogeneous LSPR scattering inflection points (IFs) of single gold nanoparticles with a single resonant fashion. In improver, we investigated the effect of the shape and vertices of AuNCs on the refractive index (RI) sensitivity of homogeneous LSPR IFs past comparing with gilt nanospheres (AuNSs) of similar size. The results show that for both AuNCs and AuNSs, tracking homogeneous LSPR IFs allows for higher RI sensitivity than tracking the frequency shifts of the LSPR peaks. Furthermore, single AuNCs with vertices exhibited higher RI sensitivity than single AuNSs of similar size in the homogeneous LSPR IFs. Therefore, we provided a deeper insight into the RI sensitivity of homogeneous LSPR IFs of AuNCs with vertices for their use in LSPR-based biosensors.

Introduction

Plasmonic gold nanoparticles (AuNPs) have unique optical backdrop that depend on their shapes and sizes, and on the refractive index (RI) of the surrounding media. These properties are induced by the localized surface plasmon resonance (LSPR) upshot^one,2,3,iv. When gold nanoparticles are irradiated, the conduction electrons on their surfaces are excited and collectively oscillate with the incident electromagnetic field. Furthermore, the potent interaction betwixt golden nanoparticles causes light to exist bars into sub-diffraction volumes^5,6.

For many years, the optical properties of unmarried AuNPs have been intensively investigated by far-field unmarried particle imaging and spectroscopic techniques, such as scattering-based dark-field microscopy⁷ and assimilation-based photothermal imaging^8,nine, without ensemble averaging. It has been reported that the LSPR of AuNPs is strongly dependent on the three-dimensional (3D) structure and size of the nanoparticles^ii,10, as well every bit on the RI of the surrounding medium¹¹. Appropriately, past controlling these parameters, it is possible to tune the characteristic plasmonic properties for specific purposes and applications^12,13. Furthermore, the AuNPs have unique intrinsic properties¹¹, such every bit biocompatibility^fourteen, high chemical stability¹⁵, convenient surface modification with organic and biological molecules^16,17, etc. The many advantages of AuNPs has thus led to their apply in LSPR-based biosensors^18,19. The conventional LSPR biosensors are based on AuNPs functionalized with receptors that confer specific binding abilities for target molecules, then the LSPR peak is shifted and dampened upon the attachment of the target molecules on the nanoparticle surface²⁰. Thus, the LSPR changes of AuNPs is monitored by the shift of the peak maximum as well as broadening of the peaks²¹; such changes bespeak the presence of target molecules²².

Despite the remarkable advantages of LSPR-based biosensors, they nonetheless take many cardinal limitations. First, the efficiency of LSPR-based sensor using AuNPs is low in comparison with surface plasmon polariton (SPP) sensors²³. The accurate determination of LSPR properties is affected by a realistic representation of the wavelength-dependent dielectric function of the nanoparticles¹⁰. Therefore, simplistic models negatively impact the key quantities that are necessary for the reliable fabrication of plasmonic devices²⁴. Second, LSPR biosensors are limited by the unsymmetrical broadening of LSPR peaks when measuring the changes in the local surroundings at the nanoparticle surface²⁵. It should also be noted that alterations in the shape of the LSPR peak can have a negative consequence on the sensing efficiency²⁶.

To overcome these limitations, recent studies accept, for instance, improved the effectiveness by using lithographic methods, but there are some disadvantages such as the high processing toll and low yield²³. Recently, Chen and co-workers reported a unlike approach that evaluates the changes in LSPR curvature of ensemble samples with respect to RI changes²³. They showed that college RI sensitivity was obtained in the inflection points (IFs) located at the long wavelength side (or depression energy side) of the LSPR extinction peak²⁵. However, that report was based on merely ensemble samples of Au nanoparticles rather than unmarried nanoparticles. Very recently, a single particle study on homogeneous LSPR IFs of single Au bipyramids was reported, however, our understanding of the effect of the NPs shape on the RI sensitivity at LSPR IFs of single Au nanoparticles is however scarce²⁷.

In this study, we carried out single particle studies to evaluate the shape-dependent RI sensitivity at LSPR IFs of homogeneous handful spectra experimentally measured for gilt nanospheres (AuNSs) and gilt nanocubes (AuNCs), to compare structures with and without vertices. Nosotros investigated the LSPR sensing effect of single AuNSs and AuNCs deposited on a glass slide with 3 different surrounding media of known RI values (air, water, and oil). The results indicate that tracking the homogeneous LSPR IFs of AuNCs with vertices can be finer used to develop LSPR-based biosensors with high RI sensitivity.

Results and Discussion

Characterization of AuNSs and AuNCs with vertices

The size and shape of AuNSs and AuNCs was characterized by SEM. Figure 1A,B show the SEM images of AuNSs (A) and AuNCs with vertices (B), with boilerplate sizes of 50.3 (±1.7) nm and 51.1 (±2.1) nm, respectively (Fig. S1). The size of 51.ane nm in AuNC indicates the length of i side of cube. The extinction spectra of both AuNSs (Fig. 1C) and AuNCs (Fig. 1D) was and so obtained with a Varian Cary 300 UV-Vis spectrophotometer. We found that the extinction spectra obtained from AuNSs and AuNCs were very similar. However, the LSPR peak was seen at around 535 nm for AuNSs, while the LSPR acme was observed at approximately 547 nm for AuNCs dispersed in water. Furthermore, the LSPR linewidth was different for the AuNSs and AuNCs. In Fig. i, the measurements at the ensemble level are limited by heterogeneity problems and, hence, single particle measurements are required for a meliorate understanding on their optical properties.

Characterizing the optical properties of AuNSs and AuNCs at the single particle level

Scattering-based DF microscopy and spectroscopy was used to narrate the shape-dependent optical properties of AuNSs and AuNCs with vertices at the single particle level²⁸. The experimental setup for single particle DF microscopy and spectroscopy is shown in Fig. S2. The sample was prepared past drib casting aqueous solutions of the Au nanoparticles on a pre-cleaned glass slide for DF scattering measurements (Fig. S3). The prepared samples were then measured by illuminating with randomly-polarized white light tightly focused by a loftier NA oil condenser. Only the light scattered from the sample is nerveless by the objective lens under scattering-based DF microscopy and spectroscopy (Fig. S4). Figure 2A shows a DF scattering image of single AuNSs with an boilerplate size of 50.3 nm. In improver, the corresponding scattering spectra of three AuNSs, indicated past a green square in Fig. 2A, are presented in Fig. 2B. It tin can be observed that the single particle scattering spectra of AuNSs in water had a single broad LSPR peak at around 547 nm, which was further supported by the scattering spectra of more than AuNSs (Fig. S5). Moreover, Fig. 2C presents the DF scattering prototype of unmarried AuNCs with an boilerplate size of 51.i nm, and single AuNCs with vertices also exhibited a single broad LSPR peak at around 567 nm (Figs 2D and S6). It is worth noting that AuNCs with vertices and AuNSs of similar size showed very like single broad LSPR peaks in their handful spectra. Furthermore, their LSPR peak shapes are not symmetrical.

Result of varying the medium dielectric constant on the LSPR wavelength shift

To better understand the shape- and environment-dependent characteristic optical backdrop, the effect of changing the surrounding medium RI on the LSPR wavelength was further investigated. Therefore, the scattering spectra of unmarried AuNSs and AuNCs were obtained in iii different RI environments: air, water, and oil. Figure 3A presents the unmarried particle scattering spectra of an AuNS fixed on a glass slide and surrounded by air, water, or oil. The LSPR spectrum was and then fitted to a Lorentzian part to obtain the values of LSPR wavelength and linewidth (Fig. S7). As seen in this Fig. S7, the scattering spectra of single AuNS and AuNC were well fitted with the Lorentzian function. Figure 3A,B demonstrate that the LSPR wavelengths of both AuNS and AuNC increased every bit the RI increased from air to oil, which is consistent with previous studies^9,27. Fig. 3C shows a comparison of the LSPR wavelength shifts as a function of RI of surrounding medium for AuNSs (red-curve) and AuNCs with vertices (blue-bend). Single AuNCs with precipitous vertices showed a higher LSPR wavelength shift and RI sensitivity than spherical AuNSs of similar size. This indicates that unmarried AuNCs with vertices could provide a higher RI sensitivity in the development of conventional LSPR sensors.

Shape-dependent refractive index sensitivity of homogeneous LSPR inflection points

Homogeneous LSPR inflection points (IFs, eastward.g., the long wavelength side) take been reported to accept a higher RI sensitivity than the LSPR wavelength maximum meridian in single Au bipyramids with sharp tips²⁷. Yet, it is necessary to deepen our understanding on the RI sensitivity of homogeneous LSPR IFs of various Au nanoparticles with different shapes, such as multiple sharp branches, vertices, etc. Nosotros therefore investigated the shape-dependent RI sensitivity of LSPR IFs in the homogeneous handful spectra of both AuNSs and AuNCs. The first and 2d derivatives of the scattering spectra taken from DF experiments were obtained using a convenient method based on the Lorentzian fitting curve office²⁷. The first, 2d, and third rows in Fig. 4A–C show the scattering spectra of single AuNS and the respective start and second social club derivatives, respectively. Each column corresponds to one of the 3 local RI media used (air, water, and oil). The maxima of the LSPR handful peak, indicated as B, are located at ii.236, 2.194, and two.170 eV for the three-different RI environments (air, water, and oil). Moreover, the local maxima and minima of the first gild derivatives, A and C, are located at ii.137 and two.335, 2.126 and ii.270, and 2.104 eV and ii.241 eV for air, water, and oil, respectively. Consequently, A and C represent the ii LSPR IFs, yielding the zero values of the 2nd order derivatives of the LSPR scattering spectra (third row). It is worth noting that the LSPR IFs coincide with the local maxima/minima of the starting time social club derivatives and appear at the same points of A and C on the centrality corresponding to photon free energy for the three different RI media. As observed in the starting time order derivative, B appears to be the critical point of the LSPR scattering spectra of AuNS, which indicates the cypher values of the first order derivative spectra.

As shown in Fig. iv, the characteristic shapes of the LSPR scattering spectra of single AuNSs in the beginning and 2d order derivatives are consistent with a previous report on LSPR IFs obtained from the extinction spectra of gold nanoparticles measured at the ensemble level²⁵. Furthermore, the nix values of the first order derivatives axis are exactly at the point B (LSPR peak maxima), which is the point of symmetry for the three local RI media. When analyzing the curvatures, it was found that the LSPR scattering curves and 2nd order derivatives are even functions and symmetrical to the axis of intensity, while the starting time society derivative curves are odd functions and are symmetrical to the axis of photon free energy.

As shown in the Supplementary Data (Tables S1–S3), the LSPR scattering spectra of x more than AuNSs for each local RI surround were obtained and analyzed to ostend the reproducibility and consistency with the experimental results (Fig. 4). The experimental data was consistent for all the AuNSs evaluated, yielding LSPR peak maxima (B) of 2.228 (±0.043), 2.185 (±0.034), and two.168 (±0.019) eV for the local air, h2o, and oil media. The LSPR IFs values, (A) and (C), were two.127 (±0.029) and ii.329 (±0.057), 2.109 (±0.032) and two.267 (±0.038), and 2.099 (±0.019) and two.242 (±0.020) eV, respectively. Furthermore, because the regime relevant to sensing backdrop, in which the peak energies should be approximately linear functions of the local RI media²⁹, the linearity of the A, B, and C pinnacle energies was examined for air, water, and oil. Figure 4D shows the plots of the energy peaks A, B, and C against local air, water, and oil media with respective RI values of 1.00, 1.33, and 1.52. As seen in the Fig., the human relationship between the peak energies at A, B, and C and the local RI media was linear. The slopes, determined form a fitting role, were 0.064 eV·RIU⁻¹ (R² = 0.9398) for summit A, 0.133 eV·RIU⁻¹ (R² = 0.9983) for B, and 0.190 eV·RIU⁻¹ (R² = 0.9895) for C. It should be noted that the inflection point C exhibited the highest sensitivity with respect to A and the LSPR peak maxima (B), as shown in Fig. 4E. Farther details are provided in the Supplementary Information (Tables S1–S3 and S7). Interestingly, the local RI sensitivity at inflection betoken C was improved past five.00% compared to that at the LSPR pinnacle maximum (B). This is consequent with previous reports using gold ensembles and single Au bipyramids for the utilization of LSPR IFs to enhance RI sensitivity^23,25,27.

Next, to better sympathize the shape-dependent RI sensitivity at the LSPR IFs, DF microscopy and spectroscopy experiments were performed for AuNCs with vertices. The RI sensitivity of LSPR IFs of AuNSs was compared with that of AuNCs with vertices. Both AuNSs and AuNCs of similar size showed a unmarried broad LSPR top at like LSPR wavelengths; therefore, this investigation focuses on how the shape of the nanoparticles (e.g., vertices, edges, etc.) affects the RI sensitivity at LSPR IFs at the single particle level.

Similar to the analysis method used for AuNSs in Fig. 4, the starting time and second derivatives of the experimental LSPR scattering spectra of AuNCs with vertices were obtained. The first, 2nd, and tertiary rows in Fig. 5A–C show the scattering spectra of single AuNCs and the respective first and second order derivatives, respectively. The maxima of the LSPR handful peak in the iii local RI media, B, are located at 2.225, 2.190, and ii.155 eV for the three local environments (air, water, and oil). The local maxima and minima of the first club derivatives flanking the LSPR acme maxima (B), represented by A/C, are at ii.119/2.328, 2.114/2.265 and 2.085/2.225 eV for air, water, and oil, respectively. Consequently, A and C represent the two LSPR IFs of AuNCs, yielding the zero values of the 2nd order derivatives of the LSPR scattering spectra (third row).

Measurement of the LSPR (B, maximum) scattering spectra in multiples of 10 for each local RI index provided the same results, with values of 2.233 (±0.012), ii.194 (±0.017) and 2.153 (±0.013) eV for local air, water, and oil. Similarly, the LSPR IFs, A and C, were ii.136 (±0.011) and 2.328 (±0.017), 2.119 (±0.019) and ii.268 (±0.016), and 2.082 (±0.015) and 2.224 (±0.011) eV, respectively. The peak energy A, B, and C was plotted vs. local air, water, and oil RI media. As presented in Fig. 5D, the peak energies at A, B, and C showed a linear relation with the three different local RI media. The use of a fitting function allowed to determine the slopes: 0.064 eV·RIU⁻¹ (R² = 0.9685) for peak A, 0.138 eV·RIU^−ane (R^two = 0.9868) for B, and 0.206 eV·RIU^−ane (R² = 0.9998) for C. Similar to the experimental result of AuNSs, the inflection point C exhibited the highest sensitivity with respect to the IF A and the LSPR peaks maxima (B) as shown in Fig. 5E (Tables S4–S6, and S8 for full details). Interestingly, the local RI sensitivity at inflection bespeak C was improved by v.x% with respect to the LSPR peak maximum (B). This upshot is consistent with that of AuNSs (Fig. iv). Therefore, the LSPR IF C at the longer wavelength side showed college RI sensitivity than the LSPR peak maximum (B) for both AuNSs and AuNCs. Furthermore, AuNCs with edges and vertices showed college RI sensitivity than AuNSs of similar size at the position of LSPR IF C (Fig. S8).

Conclusions

In summary, we demonstrated the significance of tracking the curvature shapes through homogeneous LSPR IFs almost the resonance free energy in various local RIs (air, water, oil), rather than tracking their counterpart LSPR maximum peak shifts, for both AuNSs and AuNCs of similar size. The homogeneous LSPR scattering IFs of single gilt nanoparticles (AuNSs, AuNCs) with a single resonant mode showed an enhanced RI sensitivity in various local RI environments. Furthermore, nosotros found that single AuNCs with abrupt vertices and edges showed higher RI sensitivity at homogeneous LSPR IFs than single AuNSs, with no edges, of similar size. Therefore, this study provides a deep insight into shape-dependent RI sensitivity of homogeneous LSPR IFs in unmarried Au nanoparticles having a single resonant way using DF single particle spectroscopy. Moreover, we showed that tracking the curvature changes in the LSPR handful spectra of single AuNCs with vertices may exist effectively employed in LSPR-based RI sensing studies.

Methods

Materials

Cetyltrimethylammonium bromide (CTAB)-stabilized gilded nanospheres (AuNSs) and aureate nanocubes (AuNCs) with an average size of fifty nm were purchased from Nanopartz (Loveland, CO, Us). Immersion oil was purchased from Sigma-Aldrich (St. Louis, MO, The states).

Characterization of gilded nanospheres and gold nanocubes with vertices

The structural characterization of AuNSs and AuNCs was conducted by scanning electron microscopy (SEM, JSM-6500, JEOL, Nippon) to assess the shapes and sizes. Furthermore, the LSPR assimilation spectra of the AuNSs and AuNCs dispersed in water were measured using a Varian Carry 300 UV-Vis spectrophotometer (Agilent, Usa).

Sample preparation for single particle study

The preparation of the samples was simple. Kickoff, the colloid solution was diluted with distilled water to lower the concentration. The diluted solution was sonicated for 10 min at room temperature and was and so dropped on a washed slide drinking glass and covered with a 22 mm × 22 mm No. i.five cover glass (Corning, NY). To reach the conditions of air as surrounding medium, the aqueous solution on the slide glass was dried after placing the encompass drinking glass. When using the oil every bit surrounding medium, the same procedure was followed and then, after drying the aqueous solution, the immersion oil was added. The concentration of Au nanoparticles deposited on the glass slide was adjusted to approximately 1 μm⁻ⁱⁱ to facilitate the measurement of a single particle without inter-particle LSPR coupling.

Single particle microscopy and spectroscopy

We performed scattering-based dark-field (DF) microscopy using an inverted microscope (ECLIPSE Ti-U, NIKON, Japan). In the DF mode, we used a Nikon Program Fluor oil iris objective (100×) with an adjustable numerical discontinuity (NA, 0.5–ane.3) and a Nikon DF condenser for DF imaging. To obtain DF scattering images with high quality, we used an Andor EMCCD photographic camera (iXon Ultra 897, Britain). We analyzed the nerveless DF images with the Image J software. Furthermore, single particle spectra of AuNSs and AuNCs were taken past using an Andor spectrometer (SHAMROCK303i, SR-303I-A, UK) equipped with an Andor CCD photographic camera (Newton DU920P-OE, UK). We nerveless the scattered light from AuNPs by an objective lens and sent to the entrance of the spectrometer for taking a spectrum. The scattered calorie-free was then dispersed past a grating (300 l/mm) inside the spectrometer, and detected by the Andor CCD camera (Newton DU920P-OE, United kingdom). We obtained a background spectrum at an expanse without nanoparticles. Finally, Matlab programs specially designed for this written report were used to perform data analysis and to obtain unmarried particle spectra.

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Acknowledgements

This work was supported past a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2018R1C1B3001154).

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Advanced Nano-Bio-Imaging and Spectroscopy Laboratory, Section of Chemical science, University of Ulsan, 93 Daehak-Ro, Nam-Gu, Ulsan, 44610, Southward Korea

Hui Bin Jeon, Philippe Vuka Tsalu & Ji Won Ha

Contributions

H.B.J. performed the DF handful measurements of AuBPs and AuNRs at the single particle level. H.B.J., P.5.T., and J.W.H. analyzed the data, and H.B.J. and J.Westward.H. wrote the paper.

Corresponding author

Correspondence to Ji Won Ha.

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Jeon, H.B., Tsalu, P.V. & Ha, J.W. Shape Effect on the Refractive Alphabetize Sensitivity at Localized Surface Plasmon Resonance Inflection Points of Single Gold Nanocubes with Vertices. Sci Rep ix, 13635 (2019). https://doi.org/ten.1038/s41598-019-50032-3

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Received: 06 March 2019
Accepted: 28 August 2019
Published: 20 September 2019
DOI : https://doi.org/10.1038/s41598-019-50032-three

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