Optimization of Ag-Coated Polystyrene Nanosphere Substrates for

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Optimization of Ag Coated Polystyrene Nanosphere Substrates for Quantitative Surface Enhanced Raman Spectroscopy Analysis Whitney Ingram, Caiqin Marvella Han, Qiuju Zhang, and Yiping Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06896 • Publication Date (Web): 29 Oct 2015 Downloaded from http://pubs.acs.org on November 22, 2015

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The Journal of Physical Chemistry

Optimization of Ag Coated Polystyrene Nanosphere Substrates for Quantitative Surface Enhanced Raman Spectroscopy Analysis Whitney Ingram1, Caiqin Han1,2*, Qiuju Zhang1,3, Yiping Zhao1 1

Department of Physics and Astronomy, and Nanoscale Science and Engineering center University of Georgia, Athens, GA 30602 2

School of Physics and Electronic Engineering

Jiangsu Normal University, Xuzhou, 221116 China 3

College of Physics and Electronics

Shandong Normal University, Jinan, 250014 China

ABSTRACT The optical properties of Ag coated polystyrene nanospheres can be varied by changing the vapor deposition angle . When < 55°, a sharp absorbance spectrum is observed, and for > 55°, broad band absorbance is achieved. The difference in optical absorbance is due to the coverage and shape change of the Ag coating on nanospheres with multiple oriented domains, as demonstrated by finite-difference time-domain simulations. The localized surface plasmon resonance of substrates coated at small can be tuned closely to the excitation wavelength and these samples can serve as a good surface enhanced Raman scattering (SERS) substrates. The polystyrene vibrational peak at ∆ = 1004 cm-1, which is inherent to the substrate, can be used as an internal standard. By normalizing the SERS peak of an analyte by this peak intensity, the SERS intensity ratio is independent of the substrate fabrication conditions. Such an internal 1 ACS Paragon Plus Environment


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standard is also shown to be effective for revealing the adsorption kinetics for rhodamine 6G and 4-mercaptophenol. Thus, metal films on polystyrene nanospheres can be a simple platform for quantitative SERS measurements.

Keywords. internal standard, finite-difference time-domain, adsorption kinetics, localized surface plasmon resonance, colloid monolayers

INTRODUCTION Since its discovery over thirty years ago, surface enhanced Raman spectroscopy (SERS) has been developed into a widely used research method for chemical and biological sensing1-4. However, its long time development has not yet turned into practical sensing technology. Limited sensitivity and difficulty to obtain a reproducible signal are two major challenges among many that prevent SERS substrates for practical applications. According to current known mechanisms of SERS, sensitivity is mainly attributed to the material properties and nanostructure design of the SERS substrates. These properties include the resonance wavelength of the localized surface plasmon resonance (LSPR), the density of “hot-spots”, the shape, geometry, and material of the SERS nanostructures 5-7. Among these factors, the wavelength of the LSPR is one of the most important properties for producing a highly sensitive SERS substrate. Experimental and theoretical results demonstrate that the enhancement factor of a SERS signal dramatically increases when the LSPR wavelength is tuned to be equal to the average of the excitation wavelength and Raman vibration wavelength of the analyte8-11. Because analyte response varies with the excitation wavelength, it is necessary that the LSPR wavelength can be easily tuned to produce highest sensitivity in a cost effective manner.

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Metal film on nanospheres (FONs), also referred to as metal-capped nanospheres, are a simple nanostructure fabricated using polystyrene (PS) or silica nanospheres as a cost-effective template for nanopatterning11-16. These structures have been used for SERS detection of anthrax, glucose, biotin, and DNAs17-22. In addition to their sensing applications, metal FONs can possess tunable plasmonic properties by modifying nanosphere size, separation, and coating material. For instance, Zhu et al. reported the fabrication and study of stretchable Ag FONs on PDMS. Their results showed that the structure can be used to produce tunable plasmonic nanostructures13. Wang et al. varied the ratio of Ag and Au FON bi-layers at constant thickness, and identified optimum conditions to produce strong SERS signals23. Greeneltch et al. reported tuning the LSPR wavelength by using silica nanosphere monolayers, with diameters ranging from 310 nm to 780 nm24. They demonstrated that by varying colloid size while keeping the Ag film thickness constant, the LSPR peak wavelength would shift broadly, ranging from 400 nm to 1200 nm. While these methods demonstrate the tunability of the LSPR wavelength for metal FONS, it would be advantageous to tune and understand the LSPR wavelength shift using Ag films on a single sized nanosphere template. In addition to sensitivity, the lack of reproducible SERS measurements can limit quantitative assessments of analyte concentration. For metal FONs, defects on the substrate can cause the SERS signal to vary. These defects may be cracks, unfilled areas in the monolayer, multilayers, or uneven coverage of metal, and result in an uneven signal enhancement at different locations on the same substrate. While the Raman signal is expected to be proportional to the analyte concentration, variations in the surface enhancement on the substrate make it difficult to determine the absolute intensity of a signal that is needed to consistently predict analyte concentration25-26. Signal degradation due to prolonged laser exposure is another factor that



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affects reproducibility, and would significantly decrease signal intensity over time if measured on the same location26-29. To overcome these limitations, an internal standard is typically applied for SERS quantitative analysis. An internal standard is a chemically inert substance added to a bulk analyte solution. It should produce vibrational modes that do not interfere with the characteristic peaks of the target anayte, and can still be detected within a broad range of concentrations of the target analyte30. Normalization of the analyte peak intensity with the internal standard negates several factors that affect variations in signal intensity. Commonly used internal standards include glutaric acid, pyridine, acetonitrile, and p-thiocresol31-35. One of the most accurate quantitative methods to predict analyte concentration is the use of isotope-edited compounds added to the solution of the original analyte36. Their results show that the ratio of the analyte to be predicted with errors of less than 8%. Internal standards added to bulk solutions can provide good quantitative results, but this type of internal standard typically requires high concentration when added to the bulk solution, and could compete with the target analytes for hot-spot adsorption 30. Another method to apply an internal standard is to utilize the vibrational modes from the SERS substrate. These vibrational modes can include ones generated from polystyrene, silicon, PDMS, nitrocellose membranes, or the photon/phonon interface vibrational mode of metals37-41. In this case, the internal standard will not compete with the analyte for potential hot-spot locations. For instance, Kim et al. demonstrated the application of a novel microfluidic SERS device for the detection of trans-1, 2 bis(4-pyridyl) ethylene (BPE). In their study, they took advantage of PDMS’s strong vibrational peaks as an internal standard, and drew a linear correlation between BPE concentration and spectra intensity37. In another study by Péron et al., gold coated PS nanospheres were used for the SERS detection of trace amounts of naphthalene 42. In their work, Au coated, 800 nm PS nanospheres were deposited on the surface


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of a functionalized quartz substrate and produced peak at Raman shift of ∆ = 1004 cm-1. This peak was utilized as an internal standard and was used to derive a linear relationship between predicted naphthalene peak intensity and concentration. The aim of this work is to optimize Ag FONs based upon a single-sized nanosphere template and demonstrate its use as an internal standard for quantitative SERS analysis. Ag FONs are fabricated using an oblique angle deposition technique, and its LSPR wavelength is tuned by varying the vapor incident angle relative to the surface normal of the substrate, while fixing the Ag deposition parameters. The shift of the LSPR quantitatively agrees with that predicted by finite-difference time-domain (FDTD) simulations. Although the Ag FON pattern consists of the Ag coating on the nanosphere and underlying nanotriangles, we show that the major contribution to the LSPR peak comes from the Ag coating on the nanosphere. The SERS spectrum peak of polystyrene is used as an internal standard. The normalized SERS peak intensity versus rhodamine 6G (R6G) concentration is independent of substrate’s deposition conditions. In addition, using the internal standard, the SERS can be used to quantitatively study the dynamic adsorption process of R6G and 4-mercaptophenol (MPH) on Ag FON substrate. This work highlights the significance of utilizing Ag FONs as an internal standard and its benefits for real-time, in-situ measurements.

EXPERIMENTAL METHODS Chemicals and Supplies. Chemical analytes used in this study were rhodamine 6G (Sigma Aldrich, 252433) and 4-mercaptophenol (Sigma-Aldrich, 559938). 150 nm diameter PS nanospheres (Bangs Labs, 7038) were used as the monolayer template. The monolayer was deposited onto cleaned glass slides (Gold Seal) and cleaned silicon wafers (University Wafer).



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Silver pellets (99.999%) were purchased from Kurt J. Lesker Company. Ultra-pure water (18 MΩ) was used throughout the experiments. Nanosphere monolayer preparation. Glass slides and silicon wafers were cut into small pieces with dimensions of 2.5 cm × 2.5 cm and 1 cm × 1 cm, respectively. Glass substrates were cleaned in a hot piranha solution, 4:1 ratio of sulfuric acid and hydrogen peroxide, for at least 20 minutes. Silicon substrates were cleaned in a boiling solution of deionized water, ammonium hydroxide, and hydrogen peroxide at a 5:1:1 ratio for at least 20 minutes. After chemical cleaning, all substrates were thoroughly rinsed in deionized water and dried with N2 gas. Colloid monolayers were fabricated using an air-water interface method reported previously 43-44. Briefly, the PS nanosphere solution was first diluted in ultra-pure water to a concentration of 0.01 w/v % and then further diluted to a 2:1 volume ratio of PS solution to ethanol. The resulting suspension was loaded into a syringe and placed onto a syringe pump (KD Scientific). Droplets of PS solution were dispensed at a rate of 0.015 ml/min onto the surface of a tilted cleaned glass Petri dish (diameter of 10 cm) containing approximately 24 mL of ultra-pure water. The nanosphere solution spread radially outward from the droplet location on the surface of the water. This process continued until a monolayer formed and covered the entire water surface. A Teflon ring was placed gently on the surface of the water to protect the monolayer film against adhering to the side wall of the glass Petri dish. Then, the water level was raised. Glass and silicon substrates were carefully slid to the area below the monolayer film. The film was deposited onto the surface of the submerged glass or silicon substrates by using a multistalic pump (Buchler Instruments) to slowly withdraw water from the Petri dish. Fabrication of Ag FONs substrates. Ag FON substrates were prepared using a custombuilt electron beam deposition system (Torr International). The deposition rate and thickness


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were monitored by a quartz crystal microbalance (QCM) placed facing the source directly. The PS monolayer coated substrates were mounted on pre-machined wedges with different incident angles = 0°, 15°, 25°, 35°, 45°, 55°, 65°, 75° and 85°, relative to the surface normal of the substrate. All the coatings for different were carried out in a single deposition. Ag depositions were performed under a pressure of 10-6- 10-7 Torr with a deposition rate of 0.5 Å/s to a final thickness of 20 nm. After the Ag was deposited, the substrates were allowed to cool in vacuum before removal from the chamber. Ag FONs substrate characterization. The optical absorption spectra of Ag FONs were characterized by a UV-Vis-NIR spectrophotometer (Jasco V-570). The morphology of the SERS substrate was examined using scanning electron microscopy (SEM). The height profile and morphology were also determined by atomic force microscopy (AFM) (Bruker Multimode 8 AFM). SERS measurements were taken with a portable Raman spectrometer (Enwave Optronics, Pro-L) with an excitation wavelength of 785 nm, a laser power of 30 mW, and an integration time of 10 s. The average SERS spectra were obtained from nine different locations on the substrate. Concentration dependent and dynamic SERS study of Ag FONs. R6G solutions with concentrations ranging from 5×10-6 M to 1 ×10-2 M in methanol were used for concentration dependent SERS measurements. The SERS substrates used in this study were Ag FONs deposited at incident angles of = 0°, 25°, and 55°, respectively. Two microliters of the solution were dispensed on the center of the Ag FON substrate. The droplet spreads out radially to a circular area of an approximate diameter of 2 cm. For R6G and MPH adsorption dynamics measurements, the SERS substrate was first placed in a cleaned glass Petri dish filled with 5 mL of ultra-pure water. Then, 1 mL of R6G,



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diluted in ultra-pure water to a concentration of 0.66 mM, was dispensed below the water surface to achieve a net concentration of 0.1 mM. The study of MPH was performed in a similar manner, but the stock solution was 200 µL of 5.6 µM MPH. While the Ag FON substrates were submerged in the solution, time-dependent SERS measurements were taken automatically every 0.5 minutes for the first 60 minutes, then the time-interval increased to 1 minute for the next hour. During all dynamic measurements the location of the excitation beam was not altered, and the evaporation of water would be negligible within the time frame of the experiment. All the SERS spectra were analyzed using the spectroscopic software GRAMS AI and Origin Pro 9.0 to determine the peak locations and strengths. Simulations. The coverage of the Ag on the colloid nanosphere was simulated by an inhouse Matlab program previously reported 43-44. This simulation predicted the thickness distribution of the Ag coating on nanospheres at different incident angles by considering the shadowing effects of the thirty-six nearest neighboring nanospheres. The optical properties of Ag FONs deposited at different incident angles were simulated by the FDTD method using FDTD Solutions software 45. In the calculations, the shape and coverage of Ag FONs were modeled using a shadowing mechanism based on the parameters obtained from both SEM and AFM measurements as elaborated in Supporting Information S1, and the detailed simulation configurations and boundary conditions were elaborated in the Supporting Information S2.

RESULTS AND DISCUSSION Morphology of Ag FONs at different angles Figures 1 (a-e) and (a’-e’) show the corresponding AFM and SEM images of PS monolayers and Ag FONs of = 0°, 25°, 40°, and 65°, respectively. For > 0°, the arrows 8 ACS Paragon Plus Environment

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shown in the AFM images indicate the projected direction of the Ag vapor on the substrate plane. In Figure 1 (a)/(a’), the shape of the bare PS nanospheres were very similar to that of Ag FONs at = 0° shown in Figure 1 (b)/(b’). However, the surface of each nanosphere in Figure 1 (b) is rougher compared to the uncoated nanosphere, indicating material deposition. Also, in

Figure 1. 1 µm × 1 µm AFM scans (a-e) and corresponding SEM top view images (a’-e’) uncoated and Ag coated PS nanospheres at = 0°, 25°, 40°, and 65°, respectively. All the AFM images were taken by aligning the deposition direction upward indicated by the arrows in AFM images. The dashed red circles and green patches in SEM images outlined nanosphere and Ag patchy perimeters.

some regions, the Ag coating connects nearby nanospheres. When increases to 25° (Figure 1 (c)/(c’)), AFM shows that the nanosphere surface becomes rougher, but there are no connected nanospheres. The SEM image demonstrates large area, near circular shaped Ag patch on each nanosphere. When increases to 40°, the AFM image shows similar features as that of 25° (Figure 1 (d)) . However, the SEM image clearly shows half-circle like Ag patch. When further changes to 65°, both AFM and SEM images show clear patches (Figures 1 (e)/(e’)), especially in Figure 1(e’), and the Ag patch becomes an elongated band on the nanosphere. It appears that with increased the area of the Ag coating becomes smaller and the shape becomes more anisotropic. To better illustrate the change in Ag coating on PS nanospheres with respect to , Figure 2 shows the average height profiles of Ag FONs scanned across the center of the 9 ACS Paragon Plus Environment


nanospheres along the incident vapor direction. The average height profiles are taken from at least five different nanospheres, and are offset and aligned along the curvature of the nanospheres in the region opposing vapor direction (right region of Figure 2). We observe from PS θ = 0° θ = 25° θ = 40° θ = 65° θ = 85°

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Figure 2, that the height protrusion in the vapor direction becomes more and more pronounced with increased , which is a clear indication of the shadowing growth effect. Therefore, a general trend can be found; as increases, the area of the Ag coating becomes more anisotropic and


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localized in the direction of the incident vapor.

Figure 3. (a) Representation of angle orientation. (b) Simulation results of Ag coating on PS nanospheres for = 25° to = 85° at fixed = 0°. (c) Simulation results of Ag coating on PS nanospheres for = 5°, 25°, 40°, and 60°, respectively, at fixed = 55°.

We have performed computer simulations of the Ag thickness distribution on PS nanospheres at different . This simulation considers the shadowing effect from neighboring nanospheres, and the resulting simulated film is assumed to be non-porous. The shape and coverage of the Ag coating on a nanosphere is determined by both and the relative orientation of the nanosphere monolayer domain . As shown in Figure 3 (a), is the angle between the projected vapor direction and the centers of the two adjacent nanospheres. This angle represents the direction of the incident vapor with respect to the colloid monolayer domain. Figure 3 (b) shows the thickness distribution on a nanosphere for = 25°, 40°, 65°, and 85°, respectively, for

= 0°. The resulting images reveal that the Ag layer coverage becomes smaller and more anisotropic. The overall long-axis of the Ag coating shape for large is perpendicular to the



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vapor direction. Although we had difficulty to determine from each AFM image, the shape of the Ag coating in Figure 1 follows the same trend as predicted by the simulation. It is known from literature that the PS monolayer templates can contain multi-oriented domains41, 43. Multiple domains could also affect the Ag coverage even when is fixed. For example, Figure 3 (c) shows when varies from 5° to 60°, for fixed = 55°, the shape of the Ag coating changes slightly. Such a shape change could induce broader optical response, which we will pay particular attention to.

Optical Properties Since the shape and coverage of Ag FONs are different for each incident angles, it is expected that their optical properties will also vary. Figure 4 (a) shows the UV-Vis absorbance spectra of Ag FONs at different incident angles, from = 0° to = 85°. At = 0° a broad absorbance peak centered at = 863 nm dominates the spectrum. The samples at = 15° to = 55° have a similar dominant absorbance peak, but the peak location blueshift consistently, beginning with = 874 nm, and then = 844 nm, = 808 nm, = 781 nm, and = 773 nm, while the absorbance amplitude becomes smaller and smaller for > 15°. The peak shift as a function of is plotted in Figure 4 (c). When > 55°, the absorbance spectra change dramatically. Each spectrum becomes relatively flat over a large spectral range, while the absorbance strength decreases significantly. The absorbance strength also follows a general trend: as increases further the absorbance strength decreases while the spectra becomes broader. The change of the optical property as a function of is determined by the shape and coverage variation of the Ag coating. To understand the optical properties of Ag FONs at different , we have carried out FDTD simulations on the Ag FONs. The detailed Ag coating


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model is described in Supporting Information S1 and the FDTD simulation conditions are illustrated in Supporting Information S2.1 Because the Ag FONs contain multiple domains, we expect that the experimentally measured spectra in Figure 4 (a) are the average absorbance spectra from different monolayer domains, i.e., domains with different . To accommodate such an effect in simulation, we use the average absorbance spectra of Ag FONs oriented at = 0°, 10°, and 20° for different , and the result of the simulations are shown in Figure 4 (b). Overall, 13 ACS Paragon Plus Environment


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the FDTD results show similar trends to those observed in experiments. An intense and relatively narrow absorbance peak in near IR region ( = 800 - 900 nm) for small is observed; then for larger , the spectra becomes very broad. When increases from = 0° to 35 °, the peak wavelength decrease monotonically with , where = 852 nm, 837 nm, 836 nm, 808 nm, respectively. The absorbance strength also decreases monotonically within this range. At = 40°, the peak redshifts slightly to = 812 nm. At > 40° the peaks become much broader and the absorbance intensity decreases significantly. Figure 4 (c) shows the comparison of LSPR wavelength as a function of from both the FDTD and experiment results. The overall trend matches very well. Clearly can be used to finely tune the LSPR wavelength of Ag FONs. The optical response of the Ag FONs originates from two Ag nanopatterns for small . One is the Ag coating on the nanospheres as demonstrated by AFM and SEM images in Fig. 1, and the other is Ag nanotriangles deposited onto the substrate. Figure S2.1 (Supporting Information) shows the structure and the nanotriangles for FDTD simulation. However, not all the Ag FONs have the nanotriangle patterns. There is a critical that when > , no nanotrianges will be formed on the substrate due to the shadowing effect. Such a , also depends on . When = 0°, = 40°; but when = 30°, can be as high as 60°. Experimentally we find that the overall is around 55°, which is reasonable. Nevertheless, even with the nanotriangle layer, the contribution of the nanotriangle absorbance is very small compared to the overall optical absorbance spectra as shown in Figure S 3.1(a) (Supporting Information). In fact, the FDTD simulation has confirmed this assessment as shown in Figure S 3.1 (b) (Supporting Information). From the simulation results, there is no optical coupling between the triangle layer and the Ag on sphere layer, and the optical properties of Ag FONs are dominated by the Ag coating on the nanospheres.


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It is very interesting to notice the sudden change of the absorbance spectra above from both simulation and experiment. This absorbance shifts occurs when > 55° in the experiment, and > 40° in the simulation. To determine the cause of this broadening, we examine carefully two cases, = 25° and = 75°, by FDTD simulation. Figure 5 shows the polarized absorbance spectra at different for these two angles, one is the polarization parallel to the vapor direction, the y-axis, and the other is perpendicular, the x-axis. A diagram of the direction of each polarization is shown in Figure 5. The insert in each plot shows the top view of modeled Ag coating at corresponding . For = 25°, the shape and coverage of the Ag coating at different are similar. Thus, the x-polarized and y-polarized absorbance spectra are similar, both show a sharp peak at ≈ 833 nm. Even for different , the peak location variation is small, with a standard deviation of ± 17 nm. Therefore the average spectrum is a predominant peak as shown in Figure 3. For = 75°, however, the shape and coverage of the Ag coating at different

are very different. At = 0°, the coating looks like a stripe along the edge of the nanosphere; at = 10°, the coating shape becomes an elongated triangle; while the coating at = 20° appears to be a small triangle. The highly anisotropic Ag coatings and different coverages significantly change the polarization absorbance spectra. For = 0°, the y-polarized absorbance has a broad spectrum in the wavelength range of = 350 nm to = 600 nm, with low absorbance intensity; but x-polarized spectrum shows a strong absorbance peak at = 1419 nm and two smaller peaks at = 940 nm and = 1043 nm. For = 10°, the absorbance of both polarizations are small (< 0.3). The y-polarized absorbance shows a double peaks at = 665 nm and = 733 nm, while the x-polarized absorbance shows two distinguished broad peaks at = 1036 nm and = 781 nm. For = 20°, the polarized absorbance spectra are very different. The y-polarized spectrum appears very similar to that of



= 0° case, both in shape and amplitude, while the x-polarized spectrum possesses a narrow and strong peak at = 1143 nm. Because of the large variations in absorbance spectrum due to domain orientation, the average absorbance spectrum over different domains becomes very broad, as shown in Figure 3. Therefore the orientation of the domain plays a much more significant role at higher incident angle in the determination of the optical absorbance property.

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Wavelength λ (nm) Figure 5. Polarized absorption spectrum of Ag FONs at θ = 25° and θ = 75° for φ = 0°, 10°, and 20°, respectively. Inset shows the corresponding top view of the simulation model, and polarization directions.

SERS response of Ag FONs Since the optical absorbance spectra of Ag FONs for small show a significant LSPR absorbance peak and these peaks are close to the Raman excitation wavelength, = 785 nm, we have carried out SERS measurements on those samples. The SERS spectra of different concentrations C of R6G where C = 5 × 10-6, 1 × 10-5, 1 × 10-4, 5 × 10-4, 1 × 10-3, and 1 × 10-2 16 ACS Paragon Plus Environment

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M, are acquired on different Ag FON substrates of = 0°, 25°, and 55°. Figure 6 shows the representative R6G SERS spectra for Ag FON substrates at = 25°. The spectrum labeled “control” in the figure shows the SERS response of the bare Ag FON substrates, and possesses characteristic of vibrational modes of polystyrene, with an intense peak at ∆ = 1004 cm-1 and other weak peaks at ∆ = 1033 cm-1 and 1601 cm-1 46-47. According to Refs. 46 and 47, the ∆ = 1004 cm-1 peak corresponds to the phenol ring breathing mode for PS. When R6G is added, the

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R6G characteristic peaks at ∆ = 614, 776, 1186, 1312, 1363, and 1510 cm-1 start to appear and become stronger and stronger with increased C48-49. Two predominant peaks of R6G at ∆ = 614 cm-1, which corresponds to the C-C-C bending mode, and ∆ = 1510 cm-1, which is the C-C aromatic stretching mode, are highlighted in Figure 6 50-51. It appears that as C increases, both the peak intensities at ∆ = 614 cm-1 and ∆ = 1510 cm-1 increase monotonically. Although the PS at ∆ = 1004 cm-1 is still detectable, its strength has been significantly reduced with C. 17 ACS Paragon Plus Environment


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To quantify how the SERS signals of PS and R6G change as a function of C on Ag FON substrates we analyze the peak intensities at ∆ = 1004 cm-1, I1004, and ∆ = 614 cm-1, I614, Figures 7 (a) and (b) show the log-log plots of I614 and I1004 as a function of C for different Ag FON substrates. We observe that the I614-C and I1004 -C relationships follow the same trend for different Ag FON substrates: I614 increases monotonically with C when C ≤ 10-3 M, and then saturates at higher C; while I1004 decreases monotonically with C. The data are scattered for different substrates, which is a reflection of the non-reproducibility and ultimately lack of quantification of SERS measurements for these substrates. However, at the same C, both I614 and I1004 values show similar trend for different Ag FONs. For example, at C = 10-5 M, I614 is the minimum at = 25° and the maximum at = 0°, and the same trend occurs for I1004. Thus it is expected that the ratio, / , = ⁄ may not depend on . Figure 7(c) shows the plot of 614/1004 versus R6G concentration C. As expected, regardless of the Ag FON substrates, all the three sets of data collapse together. The log-log relationship of the normalized peak intensities can be expressed as / 10. .±. for C ≤ 10-3 M. Therefore, the normalized value / is only dependent on the R6G concentration, and different substrate fabrication conditions have very little effect. Such a fabrication condition invariance may be due to the following reason: for the Ag patches covered PS spheres, in principle, the SERS signal generated from the PS (I1004) shall be proportional to the total number of hot spots (N0) on the Ag patches, I1004 ∝ N0, while the target molecules (R6G) shall occupy fraction of the hot spots, and its SERS intensity I614 ∝ CN0. Then the intensity ratio, I614/I1004 ∝ C, is only a function of the


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I1004(counts) I614(counts)

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(a)

103 102 104 (b) 103 102 101 (c) 100

θ = 0° θ = 25° θ = 55°

10-1 10-5

10-4

10-3

10-2

Concentration C (M) Figure 7. SERS peak intensity versus R6G at concentration C. (a) I614 and (b) I1004 (c) 614/1004, normalized peak ratio as a function of C.

concentration of the targeted molecules. Thus, even though at different vapor incident angle, both the surface coverage of Ag on PS beads and the total number of hot spots may be different, the intensity ratio is only a function of R6G concentration, not the Ag surface coverage. Thus, the PS peak at ∆ = 1004 cm-1 can be used potentially as an internal standard for quantitative SERS measurements. In fact, the PS peak at ∆ = 1004 cm-1 do meet the requirements for an internal standard. It does not interfere with the presence of R6G since it is originated from the Ag/PS interfaces, and its signal is identifiable within the presence of different concentrations of R6G. Unlike internal standards added to a solution as mentioned earlier, the analyte and the PS coating are separated by the two surfaces of the silver layer. Due to the interfacial separation, target analytes and internal standard are not competing for surface adsorption. In fact, Péron et al.’s study was 19 ACS Paragon Plus Environment


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the first to apply PS as an internal standard for SERS detection. Their SERS substrate consisted of sparsely coated Au nanoparticles on the PS nanospheres chemically bonded to a quartz surface in a random arrangement. Although they demonstrate the PS peak could be used to quantitatively determine the target analyte, naphthalene, the PS peak could not be identified directly when coated with the analyte. Rinsing in 10 mL of ethanol for 10 minutes was required to eliminate surplus analyte and recover the PS peak, which also significantly reduced the signal of the target analyte. To our knowledge, no other work has been reported with PS as an internal standard. And, by comparison to Peron’s work, the Ag FON substrates can be utilized as a direct method to quantitatively detect an analyte without any alteration of the substrate. It also comes to our attention that when the Ag FON substrates are excited by 514 nm laser, the SERS spectrum does not have the PS peak ∆ = 1004 cm-1, see Supporting Information S.4, which means that this internal standard is excitation wavelength dependent. The reason for such a spectral change is not clear yet.

Real-time, in-situ adsorption measurements of R6G and MPH by SERS With an internal standard, one can perform better quantitative dynamic measurements using SERS. It has been well documented that the SERS intensity will decrease over time when illuminated at the same location. Such an effect could be due to analyte decomposition or fragmentation under continuous laser irradiation, or diffuse away molecules from the lase spot. In addition, localized elevated temperatures from prolonged laser exposure can change the optical properties of metal nanostructures and can reduce SERS signal intensity25-29. Normalization by an internal standard could factor out the spectral decay caused by above


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614 cm-1

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(a) t = 50 min t = 40 min

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0 120

Figure 8. (a) Time-dependent SERS spectra of R6G adsorption. (b) Solid lines: I614, I1510, and I1004 plots. Scattered points: normalized intensities of 614/1004, 1510/1004 and 614/1510 plots.

mentioned external changes when taking prolonged stationary measurements. We verify this concept by investigating molecular adsorbance kinetics using Ag FON at = 0°. Figure 8 (a) 21 ACS Paragon Plus Environment


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shows the time-dependent SERS spectra after the Ag FON has been immersed in 0.1 mM R6G. In the figure, the R6G peaks of ∆ = 614 cm-1 and ∆ = 1510 cm-1, and the PS peak at ∆ =1004 cm-1 are labeled. We observe that the R6G and PS peak values appear to all decrease over time t. To be quantitative, Figure 8 (b) plots I614, I1510, and, I1004 as a function of t. I1004 decays almost exponentially over time, while both I614 and I1510 stay relatively constant for t < 4 min, then decay exponentially. Such decays do not match with the known surface adsorption kinetics; we expect that as more and more R6G molecules adsorb on the surface, both the I614 and I1510 should increase with t until a saturation occurs. However, by taking the PS internal standard into account, the observed kinetics has changed. Figure 8 (b) plots the normalized peak intensities, 1510/1004 and 614/1004, as a function of t. Both plots show a similar trend: when t < 20 min, both 1510/1004 and 614/1004 increase monotonically with t; then at t ≥ 20 min, these ratios reach a saturation value. Such a trend is consistent with molecular adsorption kinetics on a surface, and can be described by the following kinetic equation, 1 − " #$%&#&' ( ) ,

(1)

where is the normalized SERS peak value, k is the adsorption rate constant, is a constant proportional to initial molecule concentration, and * is the initial time of adsorption. By fitting the equation to the experimental data shown in Figure 8 (b), the adsorption rate constants are found to be k614 = 0.36 ± 0.04 min-1 for ∆ = 614 cm-1 and k1510 = 0.40 ± 0.05 min-1 for ∆ = 1510 cm-1. Clearly, k614 ≈ k1510, which demonstrates that in the adsorption process, both the C-C-C bending and C-C aromatic stretching can be probed with equal probabilities. This result means R6G tends to randomly adsorb on the Ag surface. In fact, the ratio of the two R6G peaks, 1510/614 versus t, is kept almost constant through the entire duration (Figure 8 (b)), which


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further demonstrates this claim.

400 cm-1

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0 250

Time t (minutes) Figure 9. (a) Time-dependent SERS spectra of MPH adsorption. (b) Solid lines: I400, I1075, and I1004 plots. Scattered points: normalized intensities of 400/1004, 1075/1004 and 1075/400 plots.



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A similar study has been performed with 4-mercaptophenol (MPH). Figure 9 (a) displays the results of the time-dependent SERS spectra of Ag FONs after immersion in 1 µM of MPH. Besides the ∆ = 1004 cm-1 peak for PS, each SERS spectrum has two additional MPH vibrational peaks at ∆ = 1075 cm-1 and ∆ = 400 cm-1. There is a difference in the literature to the assignment of these two peaks. According to Li et al., the ∆ = 1075 cm-1 peak is attributed to C-S stretching and in-plane ring vibration (9b in Wilson notation), while the ∆ = 400 cm-1 peak corresponds to C-S bending or stretching52. However, Lee et al. attributes the ∆ = 1075 cm-1 peak to in plane ring vibrational of mode of 1 (Wilson notation), and ∆ = 400 cm-1 to a ring vibration of 7a (Wilson notation)53. Figure 9 (b) shows quantitative plots of MPH peak intensities of I400, I1075, and I1004, versus t. For both I1004 and I400, the trend is similar to R6G, i.e., I1004 decays exponentially with t while I400 increases initially, then decays monotonically. However, I1075 shows a different trend. It continues to increase as a function of t for t ≤ 100 min and reaches a maximum around t = 150 min, and then decreasing slightly. By normalizing with the internal standard I1004, the 1075/1004 and 400/1004 show different absorption kinetics as shown in Figure 9 (b). The 400/1004 increases rapidly for t ≤ 8 minutes and then begins to saturate towards a constant value, while 1075/1004 raises slowly, and it does not reach saturation even after t = 100 min. Using Equation (1) to fit the data, we obtain that k400 = 0.21 ± 0.1 min-1 for ∆ = 400 cm-1 and k1075 = 0.0234 ± 0.0004 min-1 for ∆ = 1075 cm-1. The difference in absorption kinetics for different SERS vibrational modes of MPH reflects that during adsorption, the adsorbed MPH molecular orientation is dynamically changing, and is a property observed in self-assembled monolayers (SAMs) and DNAs54-58. When MPH coverage is small, the MPH molecules lie flat on the Ag surface as shown in Figure 10 (a). When the incident Raman laser beam is exciting the MPH molecules, both the C-S stretching modes and ring


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modes are excited, but the ring vibrations form an angle with respect to the E-field of the laser. When MPH coverage increases over time, the MPH molecules start to reorient and stand on the surface as shown in Figure 10 (b), the contribution of the C-S stretching mode becomes less since its vibration direction is perpendicular to the E-field direction of the laser, while the ring mode can be fully excited by the E-field. Since our surface is curved and has roughness, when more and more MPH molecules are adsorbed onto

Figure 10. Schematic illustration of MPH orientations on Ag surfaces: (a) small coverage case and (b) near unity covered case.

the surface over time, the C-S mode intensity does not go to zero; rather, it reaches a saturation while the ring mode can still reorient. It is expected that as t increases, the ring mode intensity increases while the stretching mode intensity deceases. In fact, from Figure (9) we also plot 1075/400 versus t, and we see that this ratio increases with t. Therefore our results suggest that the ∆ = 400 cm-1 may be primarily due to the C-S stretching mode, while the ∆ = 1075 cm-1 is due to the ring vibration. The different adsorption time is caused mainly by the reorientation of MPH molecules.



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CONCLUSION In this paper, we have investigated the optical properties of Ag FONs, and demonstrated how this structure can be applied for quantitative SERS measurements. We show that the LSPR of the Ag FON structure can be systematically tuned by varying the deposition angle. The variation in the optical property is due to the shape and coverage of Ag coating on the nanospheres. FDTD simulations have quantitatively demonstrated such a shape effect. Ag FONs deposited at small can be used as SERS substrates. We show that the SERS peak at ∆ = 1004 cm-1, corresponds to a PS vibration mode, can be used as an internal standard. When an analyte SERS peak, such as R6G, is normalized by this peak, the ratio is independent of substrate fabrication conditions. Such an internal standard has been used to probe the adsorption kinetics of R6G and MPH on those SERS substrates, and the quantitative adsorption rate constants have been obtained. The results show that R6G molecules are randomly adsorbed on the Ag surface while the MPH adsorption orientation changes dynamically with surface coverage and adsorption time.

ASSOCIATED CONTENT Supporting Information The supplementary information contain SEM images of the Ag FON structure, and elaborate the detailed morphological model of the shape and coverage of Ag on nanospheres at different vapor incident angle and monolayer domain orientation, as well as the simulation parameters and conditions for FDTD. SERS spectra measured at 785nm and 514 excitation laser wavelength are compared. This material is available free of charge via the internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *Caiqin Han: email: [email protected], phone: +86-516-83500485 Notes: The authors declare no competing financial interest.

Funding Sources Whitney Ingram was supported in part by the National Science Foundation (ECCS-1029609 and CBET-1064228), and the Southern Regional Educational Board (SREB) Doctoral fellowship. Dr. Caiqin Han was supported by the National Natural Science Foundation of China (Grant No.61575087), the Natural Science Foundation of Jiangsu Province (Grant No. BK20151164) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. Dr. Qiuju Zhang was supported in part by the National Natural Science Foundation of China (Grant No. 11104168).

ACKNOWLEDGEMENT We would like to thank Dr. Jason Locklin for the use of his AFM system for this project.



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Optimization of Ag-Coated Polystyrene Nanosphere Substrates for

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