Molecular Sensitivities of Substrate-Supported Gold Nanocrystals

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Molecular Sensitivities of Substrate-Supported Gold Nanocrystals Yanzhen Guo, Xingzhong Zhu, Nannan Li, Jianhua Yang, Zhi Yang, Jianfang Wang, and Baocheng Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12096 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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The Journal of Physical Chemistry

Molecular Sensitivities of Substrate-Supported Gold Nanocrystals Yanzhen Guo,†,# Xingzhong Zhu,‡,# Nannan Li,‡ Jianhua Yang,‡ Zhi Yang,*,† Jianfang Wang,*,‡ and Baocheng Yang§ †Key

Laboratory of Thin Film and Microfabrication (Ministry of Education), Department of

Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China ‡Department §Henan

of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China

Provincial Key Laboratory of Nanocomposites and Applications, Institute of

Nanostructured Functional Materials, Huanghe Science and Technology College, Zhengzhou 450006, China

ABSTRACT: The sensitivity of plasmonic sensors in response to the refractive index changes of the surrounding medium is highly dependent on the shape of metal nanocrystals. In this work, the bulk refractive index sensitivities of dispersed and immobilized Au nanocrystals in different shapes (nanospheres, nanorods, nanobipyramids and nanoplates) are investigated. The Au nanoplates exhibit the largest bulk refractive index sensitivity in solutions. The substrate effect, which is correlated with the fractional particle surface area in contact with the substrate, makes the

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Au nanoplates fall behind the nanorods and nanobipyramids due to their larger interfacial contact area. The single-particle dark-field scattering technique is employed for the detection of the spectral shift upon molecular adsorption. The oxygen plasma treatment is proved to be incapable of improving the molecular sensitivity due to the structural damage to the nanocrystals. The Au nanobipyramids possess the highest molecular sensitivity among the four types of the Au nanocrystals. The distinction in the tendency of the differently-shaped Au nanocrystals between the bulk and molecular sensitivities is systematically studied by electrodynamic simulations. The molecular sensitivity measurements with short-chain molecules further verify the sensitivity difference of the differently-shaped Au nanocrystals. Taken together, our study and understanding of the effect of the shape on the sensitivities of plasmonic metal nanocrystals will be valuable for the design of highly sensitive plasmonic sensors as well as for the development of plasmonenabled spectroscopies.

1. INTRODUCTION Plasmonic metal nanocrystals, due to their high sensitivities to the refractive index changes in the surrounding environment, have promising potential as optical sensors for the detection of chemical and biological molecules.1,2 In general, the localized surface plasmon resonance (LSPR) peak of metal nanocrystals redshifts as the refractive index of the surrounding medium is increased. The refractive index sensitivity (RIS), defined as the plasmon shift per refractive index unit (RIU) change of the surrounding medium, has been introduced to characterize the responsivities of plasmonic metal nanocrystals in different shapes and sizes to the index changes.3,4 The higher the RIS is, the more sensitive a metal nanocrystal is to the index change of


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the surrounding medium. In plasmonic molecular sensing, the plasmon peaks of metal nanocrystals will shift to longer wavelengths upon the binding of chemical or biological molecules with higher refractive indexes than the surrounding fluid. In other words, molecular binding or unbinding can be monitored by measuring the LSPR spectral shift of metal nanocrystals. The studies and understanding of the RISs of metal nanocrystals in different shapes and sizes are crucial for the development and exploration of various index change-based plasmonic chemical and biological sensors. In typical measurements, metal nanocrystals are either dispersed directly in solutions or first deposited on substrates, which are then immersed in solutions.5–9 The solutions are typically made of solvent mixtures or different solvents with known and controllable refractive indexes. The RIS is calculated by plotting the plasmon shift against the refractive index and usually known as the bulk RISs of metal nanocrystals. However, during many index change-based plasmonic sensing, one or a few molecular layers are adsorbed on the metal surface. The thickness of the adsorbed molecular layer is finite and on the nanometer or sub-nanometer scale. The LSPR spectral shift (λ) caused by the adsorbate layer in response to the change in refractive index can be approximately described by10,11 ∆𝜆 = 𝑚(𝑛adsorbate - 𝑛medium)(1 ― 𝑒

-2𝑑 𝑙 d

)

(1)

where m is the bulk RIS of the metal nanocrystal in nm/RIU, nadsorbate and nmedium are the refractive indexes of the adsorbate and the medium surrounding the nanocrystal, respectively, d is the effective thickness of the adsorbate layer, and ld is the electric field decay length. Moreover, many plasmonic sensing experiments are performed with metal nanocrystals deposited on substrates, where dark-field scattering measurements at the single-particle level are


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performed to monitor the plasmon peak shift during the processes of molecular adsorption, dynamic binding and chemical interaction. For example, highly sensitive sulfide mapping in live cells through single-particle plasmonic spectral imaging has been reported, where Au–Ag core– shell nanoparticles are used as the probes.12 The single-particle scattering technique combined with enzymatic reactions has been employed to detect the presence of only one or a few horseradish peroxidase molecules per particle in a colorimetric biosensing methodology.13 Single-particle dark-field microscopy has also been used to detect the enzymatic activity inside and outside cells with Au nanoparticles as the real-time optical probes.14 In fact, single-particle sensing platforms provide three advantages. First, single-particle sensors can be readily implemented in multiplex detection schemes due to their synthetically tunable wavelengths.15,16 Second, single-particle probes offer improved absolute detection limits and also enable high spatial resolution.14,15 Third, plasmonic single-particle sensors are non-invasive, making them ideal platforms for measuring in vitro and in vivo events that are non-reachable by fixed solid arrays.17–19 The single-particle technique therefore enables more sensitive chemical and biological molecular detection in plasmonic sensing applications, especially for one or a few molecular layers. As described in eq 1, the LSPR spectral shift can be improved by optimizing the plasmonic characteristics (m and ld) of the metal nanocrystal and the effective thickness of the adsorbate on the nanocrystal. m and ld can be tailored by changing the shape and size of the metal nanocrystal.10 Moreover, the hot spots and local electric field enhancement originating from the nanocrystal shape are usually non-uniformly distributed in the surface region around the metal nanocrystal.10,20 The accessibility of analyte molecules to the surface of the nanocrystal is also dependent on the shape of the nanocrystal.21,22 For example, Au nanorods prepared by seed-


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mediated growth are encapsulated by cetyltrimethylammonium bromide (CTAB) surfactant molecules. The CTAB bilayer is less ordered at the ends than that on the side surface. Thiol molecules are preferentially bound to the ends of Au nanorods at low CTAB concentrations.23,24 The selective binding of molecules at the hot spots has been shown to cause larger plasmon shifts.22 Molecular adsorption, which is often at the monolayer or sub-monolayer level on the surface of metal nanocrystals, can cause plasmonic scattering peaks to redshift in single-particle dark-field scattering measurements due to the usually larger refractive index of analyte molecules than that of buffer solutions.10 Based on the factors mentioned above, the shape of plasmonic metal nanocrystals can dramatically affect molecular adsorption and the resultant LSPR shift. Therefore, a better understanding of the shape effects on the bulk RIS and the molecular sensitivity based on single-particle scattering is of significance and in strong demand for the further development of plasmonic sensing. In this work, we have systematically investigated the bulk RISs and molecular sensitivities of four types of differently-shaped Au nanocrystals: nanospheres (NSs), nanorods (NRs), nanobipyramids (NBPs) and nanoplates (NPLs). The Au NPL sample exhibits the highest sensitivity of 444 nm/RIU from the measurements by directly dispersing the nanocrystals in water–glycerol mixtures with varying glycerol volume percentages. The bulk RIS of the immobilized Au NPLs drops behind those of the Au NRs and NBPs due to their largest interfacial contact area with the substrate. The molecular sensitivities of the Au nanocrystals are measured by monitoring the spectral peak shift upon molecular modification at the singleparticle level. Morphological changes occur on the surface of the Au NPLs after oxygen plasma treatment, which causes a reduction in their molecular sensitivity. The Au NBPs exhibit a maximal LSPR shift of 64 nm upon surface modification with long-chain alkanethiol molecules.


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Electrodynamic simulations have been performed to study the different responses of the Au nanocrystals to the bulk and surface refractive index changes. The observed plasmon peak shifts upon the binding with short-chain molecules on the surface of the Au nanocrystals further verify that the Au NBPs are a promising candidate as single-particle plasmonic sensing probes.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Au Nanocrystals. The Au NS, NBP and NPL samples were synthesized by seed-mediated methods in aqueous solutions, as described previously.25–27 The Au NR sample was prepared in aqueous solutions according to a previous work with slight modification.28 Briefly, a HAuCl4 solution (0.01 M, 0.25 mL) was first mixed with a CTAB solution (0.1 M, 9.75 mL) in a plastic centrifuge tube, followed by the rapid injection of a freshly-prepared, ice-cold NaBH4 solution (0.01 M, 0.6 mL) under vigorous stirring. The seed solution was kept undisturbed in an oven at 30 °C for at least 2 h before use. 0.3 mL of the seed solution was injected into a growth solution made of CTAB (0.1 M, 9.5 mL), HAuCl4 (0.01 M, 0.5 mL), AgNO3 (0.01 M, 0.2 mL) and hydroquinone (0.1 M, 0.5 mL). The resultant mixture solution was gently shaken back and forth for 5 s and then left in an oven at 30 °C overnight. The resultant sample was washed and redispersed into water for further use. 2.2. Bulk RIS Measurements. The bulk RISs of the Au nanocrystals suspended in solutions were measured by a previously reported method.5,29 The Au nanocrystals were dispersed in water–glycerol mixtures with the volume percentage of glycerol varied among 0%, 10%, 30%, 50%, 70% and 90%. The immobilization of the Au nanocrystals was carried out by immersing cleaned glass slides, which have a refractive index of 1.55, in the nanocrystal solutions overnight, following a previously reported procedure.30 The bulk RISs of the


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immobilized Au nanocrystals were measured by submerging the glass substrates in cuvettes filled with the water–glycerol mixtures of varied compositions. 2.3. Molecular Sensitivity Measurements. In our experiments, we define the LSPR shift in the scattering spectra upon molecular adsorption as the molecular sensitivity of the Au nanocrystals. The measurements of the molecular sensitivities were performed by depositing the nanocrystals on indium tin oxide (ITO)-coated glass substrates. The refractive index of the ITO coating in the visible range is ~1.9, as mentioned before.29 Before deposition, the ITO substrates were washed by ultrasonication in ethanol for 30 min and then treated in an oxygen plasma cleaner (PDC-32G, Harrick Plasma) for 5 min. The nanocrystal solution that was centrifuged and concentrated in advance was dropped on the substrate and kept for 30 s. The excess solution was subsequently blown dry with nitrogen gas. The CTAB molecules on the surface of the Au nanocrystals were removed by immersing the substrate in ethanol for 24 h. The solution was replaced with fresh ethanol several times to remove CTAB as much as possible. The surface modification of the Au nanocrystals was carried out by immersing the substrate in the solution of the thiol molecules overnight. The bonding of thiol-terminated methoxy poly(ethylene glycol) (PEG-SH, MW = 5000 g/mol) molecules was carried out by placing the substrate in 1 mM (for the polymer chain) aqueous PEG-SH solution while the bonding of 1-undecanethiol was carried out in 1 mM ethanolic 1-undecanethiol solution because of the insolubility of 1-undecanethiol in water. The single-particle scattering spectra of the Au nanocrystals were recorded by dark-field scattering spectroscopy. The LSPR shift in the scattering spectra after the bonding with the thiol molecules is the molecular sensitivity of the Au nanocrystals. 2.4. Characterization. Scanning electron microscopy (SEM) images were recorded on an FEI Quanta 400 FEG microscope operated at 20 kV. Transmission electron microscopy


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(TEM) imaging was carried out on an FEI Tecnai Spirit microscope operated at 120 kV. The sizes of the differently-shaped Au nanocrystals were measured from the TEM images. Atomic force microscopy (AFM) imaging was performed on a Veeco Metrology system using a supersharp silicon nitride tip. The extinction spectra of the dispersed and immobilized nanocrystals in solutions were acquired on a Lambda 950 ultraviolet/visible/near-infrared spectrophotometer (Perkin-Elmer). The Raman spectra were collected on a Labram HR Evolution Raman spectrometer (Horiba France SAS), with the excitation wavelength at 633 nm. Fourier-transform infrared (FTIR) spectra were recorded on a Thermo Nicolet NEXUS 670 FTIR spectrometer. The single-particle dark-field scattering spectra were recorded on an optical microscope (Olympus BX60) that was integrated with a quartz-tungsten-halogen lamp (100 W), a monochromator (Acton, SpectraPro 2360i), and a charge-coupled device camera (Princeton Instruments, Pixis 400). The camera was thermoelectrically cooled to -70 °C during the measurements. A dark-field objective (100×, numerical aperture 0.9) was employed for both exciting the individual Au nanoparticle with the unpolarized white light and collecting the scattered light. The collected scattering spectrum from an individual Au nanoparticle was corrected by first subtracting the background spectrum taken from the adjacent region without any particles and then dividing it with the calibrated and normalized response curve of the entire optical system. The exposure time was 60 s. 2.5. Electrodynamic Simulations. The electrodynamic simulations of the Au nanocrystals were based on the finite-difference time-domain (FDTD) method and performed using FDTD Solutions 8.7, which was developed by Lumerical Solutions. During the simulations, an electromagnetic pulse in the spectral range from 400 to 900 nm was launched into a box containing a target Au nanocrystal. A mesh size of 0.5 nm was employed in


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calculating the electric field enhancement at the plasmon resonance wavelength. The dielectric function of Au was obtained by fitting the experimental data points of Johnson and Christy. The refractive index of the surrounding medium was set to be 1.33, the refractive index of water. The sizes and shapes of the Au nanocrystals were set as close as possible to their average values measured from the TEM images. The electric field enhancement shows an exponential decay. The decay length is defined as the distance where the electric field enhancement falls to 1/e of the maximal value along the direction perpendicular to the surface of the nanocrystal.31

3. RESULTS AND DISCUSSION The differently-shaped Au nanocrystals were synthesized by wet-chemistry seed-mediated methods, with CTAB as the stabilizing agent. Figure 1a and b show the SEM and TEM images of the Au NSs, NRs, NBPs and NPLs, respectively. The images reveal that the Au nanocrystals with the different shapes possess uniform morphologies and narrow size distributions. The Au NS sample has an average diameter of 76 ± 5 nm, as measured from the TEM images (Figure S1, Supporting Information). The average diameters/lengths/aspect ratios of the Au NRs and Au NBPs are 44 ± 3 nm/125 ± 8 nm/2.9 ± 0.2 and 28 ± 2 nm/92 ± 4 nm/3.2 ± 0.3, respectively (Figures S2 and S3, Supporting Information). The Au NPLs are hexagonal and their lateral size, which is defined as the distance between two parallel edges,27 is 158 ± 6 nm (Figure S4, Supporting Information). The AFM image (Figure S5, Supporting Information) shows that the average thickness of the Au NPLs is 38 ± 2 nm. Figure 1c shows the extinction spectra of the four Au nanocrystal samples in aqueous solutions. One plasmon band at 546 nm, which stems from the dipolar plasmon excitation, was observed for the Au NS sample. For the Au NRs, NBPs and NPLs, we only studied the RISs of their major plasmon peaks, which arise from the


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longitudinal dipolar plasmon mode for the Au NRs and NBPs and the in-plane dipolar plasmon mode for the Au NPLs. The bulk RISs of plasmonic nanocrystals composed of a single metal have been reported to be dependent on the spectral position of the LSPR peak.32 Our previous work has also demonstrated that the bulk RIS generally increases both as the plasmon resonance wavelength for a fixed nanocrystal shape becomes longer and as the curvature of the metal nanocrystal gets larger.5,7 The best spectral region for LSPR-based biosensing has been found to be 700–900 nm, where an optimal ratio exists between the bulk RIS and the plasmon peak width.33,34 This ratio is also known as the figure of merit in index change-based plasmonic sensing.35,36 Upon the further consideration of the spectral sensitivity of our charge-coupled device camera and the observability of the Au nanocrystals under single-particle dark-field scattering on our microscope, the plasmon wavelengths (λpeak) of the Au NRs, NBPs and NPLs were adjusted to be around 780 nm for the sensitivity measurements (Figure 1c). We would point out that we focus mainly on the shape effect on the sensitivity of the Au nanocrystals in this work. The effect of the size of the Au nanocrystals is not in consideration.


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Figure 1. Differently-shaped Au nanocrystals. (a) SEM images of the Au NS, NR, NBP and NPL samples, respectively. (b) Corresponding TEM images. (c) Normalized extinction spectra of the four Au nanocrystal samples dispersed in aqueous solutions. To determine the bulk RISs of the Au nanocrystals in solutions, the four Au nanocrystal samples were dispersed in the water–glycerol mixture solvents with the volume percentage of glycerol varied among 0%, 10%, 30%, 50%, 70% and 90%. The refractive indexes of the liquid mixtures, which change with the volume percentage of glycerol, can be calculated according to Lorentz-Lorenz equation37 𝑛212 ― 1 𝑛212

𝑛21 ― 1

𝑛22 ― 1

(2)

= 𝜑1𝑛2 + 2 + 𝜑2𝑛2 + 2 +2 1

2

where n12 is the refractive index of the liquid mixture, n1 and n2 are the refractive indexes of water (1.3334) and glycerol (1.4746), respectively, and φ1 and φ2 are the volume fractions of the


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two constituting solvents. The plasmon peaks of the Au nanocrystals redshift with the increase in the index of the liquid mixture, as shown by the extinction spectra for the Au NS sample (Figure S6a, Supporting Information) as an example. Figure 2 shows the plots of the Δλpeak values, which are the differences between the λpeak values obtained when the Au nanocrystals are dispersed in the liquid mixtures and water, respectively, as functions of the refractive index of the liquid mixture. All of the plots can be well fitted linearly. The slopes of the fitting lines are the bulk RISs of the dispersed Au nanocrystals.

Figure 2. Dependences of Δλpeak on the refractive index for the dispersed and immobilized Au nanocrystals in different shapes. (a) NSs. (b) NRs. (c) NBPs. (d) NPLs. The Δλpeak values measured in the water–glycerol mixtures are relative to the λpeak values of the Au nanocrystals measured in water. The volume percentages of glycerol in the mixtures are 0%, 10%, 30%, 50%,


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70% and 90%, respectively. The positive value of Δλpeak indicates a redshift in λpeak. The lines are liner fits. After the deposition on glass slides, a fraction of the surface area of the Au nanocrystals is in contact with the substrate. Although immobilizing metal nanocrystals on substrates leads to reductions in RIS, the immobilization is advantageous for sensing applications as it facilitates washing, removal of the stabilizing molecules used in the synthesis, ligand regeneration and specific functionalization of the nanocrystals.38 A substrate can affect the RISs of plasmonic metal nanocrystals in two aspects. On the one hand, the adjacent spatial region of a metal nanocrystal at the interfacial contact is difficult to be accessed by molecules from the solution. As a result, the sensing volume is reduced. On the other hand, the presence of a substrate, especially one that has a high dielectric constant, can induce the electric field redistribution around the nanocrystal. If the contact area is assumed to have a larger effect on the index sensitivity than the redistribution of the electric field towards the substrate, the interfacial contact area between a metal nanocrystal and the substrate can be estimated from the RIS reduction using an effective medium approximation6,39 𝑛eff = 𝑋A𝑛substrate + (1 ― 𝑋A)𝑛medium

(3)

where neff is the effective refractive index around the nanocrystal, XA is the fractional surface area of the nanocrystal in contact with the substrate, and nsubstrate and nmedium are the refractive indexes of the substrate and the medium, respectively. In this study, glass slides and ITO substrates were used to support the Au nanocrystals. Their relatively low dielectric constants have been generally known to play a minor role in the redistribution of the electric field around plasmonic metal nanocrystals.29,40,41


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The bulk RIS measurements of the immobilized Au nanocrystals were carried out by immersing the substrates in the water–glycerol mixtures of varying volume ratios (Figure 2). Analogously, the plasmon peak of the Au NS sample redshifts as the volume percentage of glycerol is increased (Figure S6b, Supporting Information). The bulk RIS values of the dispersed and immobilized Au nanocrystals are listed in Table S1 (Supporting Information). Upon immobilization, the RISs of the Au NS, NR and NBP samples are reduced from 129, 409 and 365 nm/RIU to 119, 306 and 305 nm/RIU, with corresponding decreases of 8%, 25% and 16%, respectively (Tables S1). The Au NSs exhibit the lowest bulk RISs both in solutions and on substrates (Figure 2a and Table S1). The highest RIS of 444 nm/RIU for the Au NPLs in solutions might arise from the fact that the local electric field enhancement is existent over a large spatial region at the corners and along the edges.27 The substrate effect is the most prominent for the Au NPLs, with the RIS reduced to 287 nm/RIU (-35%). Because the spherical Au NSs and the cylindrical NRs are theoretically in point and line contact with the substrate, the estimated finite interfacial contact areas for the Au NSs and NRs arise from the nanoscale interactions between the nanocrystals and the substrate as well as from the fact that the colloidal Au NSs and NRs are faceted.25,29,42 The fractional contact area of the Au NPLs with the substrate is estimated from the geometrical structure to be 33%, while that of the Au NBPs is 10%. These values are close to those of 35% and 16% determined from the RIS reduction. The slight differences between the fractional contact areas estimated according to the geometrical structures and the RIS measurements probably result from the joint effects of the imperfect shapes of the nanocrystals and the slight substrate-induced redistribution of the local electric field.29 The bulk ensemble measurements suffer from inhomogeneous shape and size distributions of plasmonic metal nanocrystals, which will affect the detection sensitivity of index


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change-based plasmonic sensors.10,12 Moreover, in many sensing situations, metal nanocrystals are deposited on substrates. Analyte molecules usually bind to the surface of metal nanocrystals at the sub-monolayer or monolayer level. Their detection sensitivity will be affected by the nonuniform distributions of the local index sensitivity and the accessibility to the metal surface.21 Therefore, to better understand the design rules for index change-based plasmonic sensors, the effect of the shape of the Au nanocrystals on the plasmonic response to the index change upon molecular adsorption at the single-particle level is highly desired to be understood. The molecular sensitivities of the differently-shaped Au nanocrystals in this work were measured by single-particle dark-field imaging and spectroscopy. The Au nanocrystals were deposited on ITO substrates, which are electrically conductive and therefore allow for the SEM imaging of the deposited nanocrystals. We first investigated the molecular sensitivity of the Au NPL sample due to its largest bulk RIS of 444 nm/RIU in solutions. Figure 3a illustrates schematically the molecular sensitivity measurement of the Au NPLs upon modification with the PEG-SH molecules. In the single-particle scattering measurements, the number densities of the Au nanocrystals on ITO substrates were adjusted to allow for multiple nanocrystals to be imaged simultaneously while maintaining sufficient spatial separation so as to avoid plasmon coupling among the nanocrystals.43 The time for the Au nanocrystal suspensions kept on the substrates was fixed at 30 s. The redundant solution was thereafter immediately blown away with N2. The individual Au NPLs on the ITO substrates were located for the dark-field scattering measurements by performing SEM imaging from low to high magnifications, as illustrated in Figure S7 (Supporting Information) as an example.


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Figure 3. Molecular sensitivity of the Au NPL sample. (a) Schematic illustrating the surface modification of the Au NPLs with PEG-SH. (b) LSPR shifts of the Au NPLs upon the modification with PEG-SH. The Δλpeak values were calculated relative to the λpeak values of the Au NPLs after immersion in ethanol. The positive value of Δλpeak indicates a redshift in λpeak. The λpeak values with an average of 718 ±23 nm in the x-axis are those of the individual NPLs after the immersion in ethanol. (c) Histogram of the Δλpeak values of the Au NPLs upon the modification with PEG-SH relative to the λpeak values obtained after the immersion in ethanol. (d) Single-particle scattering spectra of five representative Au NPLs before (solid curves) and after (dashed curves) the modification with PEG-SH. Because CTAB molecules remained on the Au nanocrystals after the deposition from aqueous solutions, the substrate was immersed in ethanol to remove the CTAB molecules for the subsequent single-particle scattering imaging and microscopy measurements. Owing to the bilayer characteristic of the strongly adsorbed CTAB molecules, we believe that the CTAB molecules cannot be completely removed by soaking the substrate in ethanol (Figure 3a, the second schematic). The residual CTAB molecules were detected by Raman measurements (Figure S8, Supporting Information). After the immersion in ethanol, the peaks ascribed to the


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CTAB molecules (764, 956, 1150, 1270, 1420 and 1448 cm–1) disappear. The intensity-reduced but detected band at 190 cm–1, which corresponds to the Au–Br bond,44 indicates that the CTAB molecules are not completely removed by immersing the substrate-supported Au nanocrystals in ethanol. The emerged band at 272 cm–1 can be attributed to the Au–Au stretching frequency on the exposed Au surface.45,46 The plasmon peak of the Au NPLs after being soaked in ethanol blueshifts in comparison with that dispersed in aqueous solutions (Figure 3b, x-axis and Figure 1c). The blueshift of the plasmon peak in the scattering spectra is caused by the reduction of the effective refractive index of the medium surrounding the nanocrystals, which arises from the removal of the surfactant molecules as well as the exposure in air.30 The bonding of the PEG-SH molecules on the surface of the Au NPLs was carried out by immersing the same substrate in 1 mM PEG-SH aqueous solution overnight. The scattering spectra of 69 individual nanoparticles were collected to measure the molecular sensitivity of the Au NPL sample. Figure 3b and c show the distribution of the Δλpeak values, that is, the LSPR peak shifts of the Au NPLs after the surface modification with PEG-SH. The redshifts of the plasmon peak of the individual Au NPLs fall in a range of 15–73 nm, with an average value of 43 ± 15 nm. Such redshifts are also illustrated by the single-particle scattering spectra (Figure 3d) of the representative Au NPLs before and after the surface modification with the PEG-SH molecules. Because the stabilizing surfactant molecules on the nanocrystal surface reduces the catalytic and sensing performances of metal nanocrystals, various methods have been employed to remove the surfactant molecules, such as heat treatment, plasma cleaning and ultravioletozone treatment.47 The bulk RISs have been reported to increase by up to 40% after the removal of the surfactant layer from noble metal nanocrystals using oxygen plasma treatment.33 We therefore tried to further remove the CTAB molecules by oxygen plasma treatment after the Au


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NPLs were soaked in ethanol and measure the molecular sensitivity towards PEG-SH. Figure 4a illustrates schematically the surface modification of the Au NPLs with an additional step of oxygen plasma treatment. The individual Au NPLs on the ITO substrate were located by SEM imaging at different magnifications in advance (Figure S9, Supporting Information). In comparison with the λpeak values of the Au NPLs washed with ethanol, an average blueshift of 15 ± 6 nm was obtained after the plasma treatment (Figure 4b and c). The blueshift can be ascribed to the complete removal of the CTAB molecules, as evidenced by the Raman measurements (Figure S8, Supporting Information). After the oxygen plasma treatment, the Raman peak associated with the Au–Br bond disappear completely. The other possible reason is the rounding of the corners and edges caused by the plasma treatment, as discussed below. After the incubation of the Au NPLs in 1 mM PEG-SH aqueous solution overnight, the plasmon peak exhibits a redshift of 20 ± 9 nm (Figure 4d and e). The redshift is surprisingly smaller than that observed without oxygen plasma treatment (Figure 3b and c). Shown in Figure 4f, as an example, are the scattering spectra recorded on the same Au NPL at each step. They clearly indicate the shifts of the plasmon peak.


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Figure 4. Molecular sensitivity of the Au NPLs after oxygen plasma treatment. (a) Schematic illustrating the immersion in ethanol, oxygen plasma treatment and molecular modification with PEG-SH. (b) LSPR shifts of the Au NPLs before and after oxygen plasma treatment. The negative value of Δλpeak after the plasma treatment indicates a blueshift in λpeak. The λpeak values with an average of 727 ± 26 nm in the x axis are those of the Au NPLs after the soaking in ethanol. (c) Histogram of the Δλpeak values of the Au NPLs after oxygen plasma treatment relative to the λpeak values obtained after the immersion in ethanol. (d) LSPR shifts of the NPLs before and after the modification with the PEG-SH molecules. The positive value of Δλpeak indicates a redshift in λpeak. The λpeak values with an average of 710 ± 23 nm in the x axis are those of the Au NPLs after being treated by oxygen plasma for 30 s. (e) Histogram of the Δλpeak values of the Au NPLs upon the modification with the PEG-SH molecules relative to the λpeak


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values obtained after the oxygen plasma treatment. (f) Representative single-particle scattering spectra recorded on the same Au NPL after each treatment step. As mentioned above, we speculate that oxygen plasma treatment can not only remove the residual CTAB molecules on the Au NPLs but also cause irreversible damage to the morphology of the NPLs.34 To confirm our speculation, we carefully imaged the surface morphology of the same Au NPLs after the plasma treatment for different periods of time. By overlapping the SEM images of the same NPL before the plasma treatment and after the treatment for 5 min, we found that the lateral size of the Au NPLs was reduced by a few nanometers (Figure S10, Supporting Information). Because the in-plane dipolar plasmon peak blueshifts with the rounding of the corners of the Au NPLs,48 the rounding of the corners and the reduction of the lateral size of the Au NPLs also contribute together to the blueshift of the plasmon peak apart from the complete removal of CTAB after oxygen plasma treatment (Figure 4b and c). Moreover, rounded metal nanocrystals have been found to be less sensitive to index changes than the sharp counterparts.10 As a result, the plasma-treated Au NPLs exhibit a smaller average redshift than that without the plasma treatment upon the surface modification with the same PEG-SH molecules (Figures 4e and 3c). We performed FTIR measurements to further investigate the remaining CTAB molecules on the surface of the Au nanocrystals at each step (Figure S11, Supporting Information). The FTIR spectrum of pure CTAB molecules obtained by the KBr tablet method was also provided for comparison. The bands at 2918 and 2848 cm1, originating from the CTAB molecules, were clearly observed after the Au nanocrystals, including NSs, NRs, NBPs and NPLs, were directly deposited from their aqueous solutions on ZnSe substrates, which are transparent in the infrared region.49 After the Au nanocrystals were soaked in ethanol for 24 h, the vibrational bands of the


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CTAB molecules disappeared, suggesting that the residual amount of the CTAB molecules on the surface of the nanocrystals is lower than the detection limit of our FTIR system. The FTIR spectra of the Au nanocrystals exhibit no clear difference by further treating the nanocrystals with oxygen plasma for 30 s. From the FTIR measurements, we believe that a large fraction of the CTAB molecules is removed from the Au nanocrystals by the soaking in ethanol. The molecular sensitivities were therefore measured without oxygen plasma treatment in the following experiment in order to maintain the morphology of the Au nanocrystals. Figure 5 shows the LSPR shifts of the Au NS, NR and NBP samples upon the surface modification with the PEG-SH molecules. The Au NSs, NRs and NBPs exhibit LSPR shifts of 15 ± 10, 22 ± 6 and 64 ± 11 nm by averaging the results measured on 43, 54 and 52 nanocrystals, respectively (Figure 5a, b, d, e, g and h). Figure 5c, f and i show the representative spectra recorded on the same Au NS, NR and NBP nanocrystals, respectively, clearly showing the redshifts of the plasmon peaks of the Au nanocrystals upon the modification with the PEG-SH molecules.


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Figure 5. Molecular sensitivities of the Au NSs, NRs and NBPs. (a) LSPR shifts before and after the surface modification with the PEG-SH molecules for the ethanol-soaked Au NSs. The positive value of Δλpeak indicates a redshift in λpeak. The λpeak values in the x axis are those of the Au nanocrystals after the soaking in ethanol. (b) Histogram of the Δλpeak values of the Au NSs upon the modification with the PEG-SH molecules. (c) Representative single-particle scattering spectra of five Au NSs after the modification with the PEG-SH molecules. The solid and dashed (or dotted) lines are the scattering spectra before and after the modification with PEG-SH,


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respectively. (d–f) Corresponding results for the Au NRs. (g–i) Corresponding results for the Au NBPs. The bulk RISs and the molecular sensitivities of the four differently-shaped Au nanocrystal samples are plotted in Figure 6 for comparison. The Au NPL sample exhibits the highest bulk RIS of 444 nm/RIU in solutions (Figure 6a). By immobilizing the Au nanocrystals on glass slides, the bulk RISs of the Au NRs and NBPs are slightly higher than that of the Au NPLs (Figure 6a) because of the different fractional contact areas between the Au nanocrystals and the substrates. The bulk RISs of the three types of the Au nanocrystals (NRs, NBPs and NPLs) are higher than those of the NSs both in solutions and on substrates, which is in agreement with the results reported previously.6,29

Figure 6. Comparison of the bulk RISs and the molecular sensitivities of the different Au nanocrystals. (a) Bulk RISs of the dispersed and immobilized Au nanocrystals. (b) Molecular sensitivities of the differently-shaped Au nanocrystals upon the modification with the PEG-SH molecules. In comparison with the bulk RISs, the Au NBPs, NRs and NPLs have a different tendency for the molecular sensitivities with the PEG-SH molecules. The redshifts induced by the adsorption of the PEG-SH molecules are in the order of NSs < NRs < NPLs < NBPs. The


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molecular sensitivity of the Au NBPs is the highest, with that of the Au NPLs being the second highest (Figure 6b and Table 1). As discussed above in eq 1, there are several factors affecting the LSPR shift of plasmonic metal nanocrystals upon molecular adsorption, including the bulk RIS, the maximal local electric field enhancement, and the decay length of the local electric field enhancement of the nanocrystal, as well as the effective thickness of the adsorbed molecules. The bulk RISs of the Au nanocrystals have been measured and are provided in Figure 6a. To find out the local electric field enhancement and the decay length, we performed FDTD simulations to calculate the maximal local electric field enhancements and the electric field decay lengths of the Au nanocrystals according to their average sizes obtained from their TEM images (Figure S12, Supporting Information). The decay length, where the electric field enhancement falls to 1/e of the maximal value along the direction perpendicular to the surface,31 were found to be 13.6, 10.6, 3.4 and 16.4 nm for the averagely-sized Au NS, NR, NBP and NPL, respectively (Table 1). According to eq 1, the LSPR shift increases as the decay length is decreased while the other conditions are kept unchanged. The local electric field enhancement is known to be strongly dependent on the local curvature of plasmonic metal nanocrystals. The index sensitivity has been found to exhibit an approximately linear dependence on the product between the nanocrystal polarizability and curvature.7 The calculated maximal local electric field enhancement of the Au NBP is much larger than those of the NS, NR and NPL (Table 1). In addition, the LSPR shift can also be affected by the effective thickness of the adsorbed molecules, which are not uniformly distributed on the surface of the plasmonic nanocrystals. All of these parameters jointly affect the molecular sensitivities of the Au nanocrystals, making the Au NBPs exhibit the largest LSPR shift of 64 nm upon the modification with the PEG-SH molecules. Although the Au NRs possess a shorter decay length and larger maximal electric field enhancement than the Au NPLs, the


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molecular sensitivity of the Au NRs is smaller than that of the Au NPLs. The larger LSPR shift of the Au NPLs is believed to result from the larger exposed flat surface area of the Au NPLs for molecular adsorption, as well as more sharp corners and edges and hence more hot spots.10 The smallest bulk RISs and molecular sensitivity of the Au NSs show that symmetric spherical Au nanocrystals are less attractive for the development of index change-based plasmonic sensors.

Table 1. Molecular Sensitivities of the Differently-Shaped Au Nanocrystals Au

capping with PEG-SH

nanocrystals

capping with 1-

RIS

decay

field

undecanethiol

reduction

length

enhancementd

(nm)c

LSPR shift

particle

LSPR shift

particle

(nm)a

numberb

(nm)a

numberb

NSs

15.4 ± 10.1

43

-1.9 ± 3.8

32

7.7%

13.62

6.4

NRs

21.8 ± 6.1

54

-4.5 ± 2.6

58

25.2%

10.56

21.7

NBPs

63.8 ± 11.0

52

-6.1 ± 6.6

45

16.5%

3.41

83.6

NPLs

43.1 ± 14.8

69

-5.8 ± 6.8

45

35.2%

16.42

11.2

aThe

LSPR shifts are relative to the λpeak values in ethanol. The positive and negative LSPR shifts represent the redshifts and blueshifts of the plasmon resonance peak. bThe number of the particles measured per sample. cElectric field decay length. dMaximal electric field enhancement.

To evaluate whether the plasmon peaks of the Au nanocrystals redshift upon the bonding of alkanethiol molecules with a shorter chain than that of CTAB, the molecular sensitivities were further measured by employing 1-undecanethiol as a target. The bonding of 1-undecanethiol molecules was performed under the similar condition to that of PEG-SH, except that the solvent was replaced with ethanol due to the insolubility of 1-undecanethiol in water. The single-particle scattering spectra acquired after the NPL sample was further immersed in ethanol for 12 h show


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that the NPLs exhibit no plasmon shift after the further immersion (Figure S13a, Supporting Information). The similar intensities of the Au–Br bond in the Raman spectra of the NPLs before and after the further immersion confirm that no CTAB molecules can be further removed by the sole immersion in ethanol (Figure S13b, Supporting Information). The plasmon peaks of the four differently-shaped Au nanocrystal samples blueshift after the modification with the alkanethiol molecules (Figure 7a–d). The histograms of the Δλpeak values clearly demonstrate the order of the blueshift values of the differently-shaped Au nanocrystals (Table 1 and Figure S14, Supporting Information). The Au NBPs exhibit the largest Δλpeak value upon the modification with 1undecanethiol (Figure 7e). The blueshift of the plasmon peak is believed to be caused by the replacement of the residual CTAB molecules with the short-chain 1-undecanethiol molecules and the electron-donating nature of the sulfur atom of the 1-undecanethiol molecules.15 The emerged peak at 394 cm–1, which corresponds to the Au–S bond in the Raman spectrum of the NPLs after the adsorption of 1-undecanethiol (Figure S13b, Supporting Information) verifies the replacement of CTAB with 1-undecanethiol due to the stronger Au–S bond than the Au–Br bond.44 The LSPR shift upon the adsorption of alkanethiol molecules has been found to be linearly dependent on the chain length at a slope of 3.5 and 4.4 nm per CH2 unit with Ag nanoparticles and triangular Ag nanoprisms as single-particle sensing probes, respectively.15,43 The LSPR shifts observed in our study seem to be less sensitive to the chain length of the thiol molecules than those of the Ag nanoparticles reported previously. This discrepancy can be attributed to two factors. One is that a large fraction of the CTAB molecules has been removed through the soaking in ethanol. The replacement of the remaining CTAB molecules with the 1undecanethiol molecules causes a blueshift on the plasmon peak due to the shorter chain length of the latter. On the other hand, the additional adsorption of the 1-undecanethiol molecules at the


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empty sites on the Au surface causes a redshift on the plasmon peak.50 The joint effects of the replacement of the adsorbed CTAB molecules and the additional adsorption of the 1undecanethiol molecules give the net reduced blueshifts. The second factor is that the different plasmonic characteristics between Au and Ag nanocrystals might also play a minor role in the molecular sensitivity. On the other hand, the blueshifts induced by the adsorption of 1undecanethiol are in the order of NSs < NRs < NPLs < NBPs (Figure 7e). This tendency is the same as that for the adsorption of the PEG-SH molecules (Figure 6b), despite the difference in the relative magnitudes. In the case of the adsorption of the PEG-SH molecules, because the PEG-SH molecules are much longer than the CTAB molecules, both of the replacement of the remaining CTAB molecules and the additional adsorption of the PEG-SH molecules induce redshifts in the plasmon peak. Therefore, a net redshift is obtained.


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Figure 7. Molecular sensitivities of the Au nanocrystals upon the modification with 1undecanethiol. (a–d) LSPR shifts before and after the modification with 1-undecanethiol for Au NSs, NRs, NBPs and NPLs, respectively. The negative value of Δλpeak indicates a blueshift in λpeak. The λpeak values in the x axis are those of the Au nanocrystals after the soaking in ethanol. (e) Comparison of the molecular sensitivities of the Au nanocrystals upon the surface modification with 1-undecanethiol.


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4. CONCLUSIONS In summary, we have measured the bulk RISs of the dispersed and immobilized Au nanocrystals that have different shapes, including NSs, NRs, NBPs and NPLs, in water–glycerol mixtures with different glycerol contents. The molecular sensitivities of the Au nanocrystals are also investigated upon the modification with long-chain PEG-SH molecules. The Au NPLs exhibit the highest bulk RIS of 444 nm/RIU in solutions while fall behind the Au NRs and NBPs after immobilization on glass substrates due to the largest interfacial contact area between the Au NPLs and the substrate. Oxygen plasma treatment, which causes the surface structural damage to the nanocrystals, is incapable of improving the molecular sensitivity. Single-particle dark-field scattering measurements show that the Au NBPs experience the largest LSPR shift of 64 nm after the modification with PEG-SH. FDTD simulations reveal that the Au NBPs possess the shortest decay length and the strongest local electric field enhancement. The molecular sensitivity measurements of the differently-shaped Au nanocrystals upon the surface modification with short-chain alkanethiol molecules further verify the tendency of the molecular sensitivities. Our results reveal that Au NBPs are preferential for the design of substratesupported, index change-based plasmonic sensors. The results also point out that the shape of plasmonic metal nanocrystals should be carefully taken into account in the development of plasmonic applications based on the index sensitivities of the nanocrystals.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/xxxxxx.


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Histograms of the size distributions of the Au nanocrystals; AFM characterization of the Au NPLs; extinction spectra of the NS sample; Raman spectra of the Au nanocrystal samples; SEM images of the NPLs; FTIR spectra of the Au nanocrystal samples; FDTD simulation results; histograms of the LSPR shifts upon the modification with 1undecanethiol; and comparison of the bulk RISs and the molecular sensitivities (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Z.Y.). *E-mail: [email protected] (J.F.W.). ORCID Zhi Yang: 0000-0002-0871-5882 Jianfang Wang: 0000-0002-2467-8751 Author Contributions #Y.

Z. Guo and X. Z. Zhu contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by Hong Kong Research Grants Council (GRF, 14306817 and NSFC/RGC Joint Research Scheme, N_CUHK440/14) and National Natural Science Foundation of China (51472102). REFERENCES


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(48) Cui, X. M.; Qin, F.; Ruan, Q. F.; Zhuo, X. L.; Wang, J. F. Circular Gold Nanodisks with Synthetically Tunable Diameters and Thicknesses. Adv. Funct. Mater. 2018, 28, 1705516. (49) Su, G. X.; Yang, C.; Zhu, J.-J. Fabrication of Gold Nanorods with Tunable Longitudinal Surface Plasmon Resonance Peaks by Reductive Dopamine. Langmuir 2015, 31, 817–823. (50) Wu, Z. K.; Jin, R. C. On the Ligand’s Role in the Fluorescence of Gold Nanoclusters. Nano Lett. 2010, 10, 2568–2573.


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Molecular Sensitivities of Substrate-Supported Gold Nanocrystals

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