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Silver Nanoparticle-Decorated Shape-memory Polystyrene Sheets as Highly Sensitive Surface-enhanced Raman Scattering Substrates with a Thermally Inducible Hot Spot Effect Zebasil Tassew Mengesha, and Jyisy Yang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b02256 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 16, 2016

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Analytical Chemistry

Silver Nanoparticle-Decorated Shape-memory Polystyrene Sheets as Highly Sensitive Surface-enhanced Raman Scattering Substrates with a Thermally Inducible Hot Spot Effect Zebasil Tassew Mengesha and Jyisy Yang*

Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan

*Author to whom correspondence should be addressed. Phone: +886-422840411 ext. 514 Fax: +886-422862547 E-mail: [email protected]

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Abstract In this study, an active surface-enhanced Raman scattering (SERS) substrate with a thermally inducible hot spot effect for sensitive measurement of Raman-active molecules was successfully fabricated from silver nanoparticle (AgNP)-decorated shape-memory polystyrene (SMP) sheets. To prepare the SERS substrate, SMP sheets were first pretreated with n-octylamine for effective decoration with AgNPs. By varying the formulation and condition of the reduction reaction, AgNP-decorated SMP (Ag@SMP) substrates were successfully prepared with optimized particle gaps to produce inducible hot spot effects on thermal shrink. High-quality SERS spectra were easily obtained with enhancement factors higher than 108 by probing with aromatic thiols. Several Ag@SMP substrates produced under different reaction conditions were explored for the creation of inducible hot spot effects. The results indicated that AgNP spacing is crucial for strong hot spot effects. The suitability of Ag@SMP substrates for quantification was also evaluated according to the detection of adenine. Results confirmed that prepared Ag@SMP substrates were highly suitable for quantitative analysis because they yielded an estimated limit of detection as low as 120 pg/cm2, a linear range of up to 7 ng/cm2, and a regression coefficient (R2) of 0.9959. Ag@SMP substrates were highly reproducible; the average relative standard deviation for all measurements was less than 10%.

Keywords: SERS; Plasmonic effect; Hot spot effect; Silver nanoparticle; Surface enhancement

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Introduction Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive analytical technique owing to the large enhancement of Raman scattering that occurs when a molecule is in close proximity to metal nanostructures (NSs).1,2 In recent years, numerous theoretical and experimental studies have reported that localized electromagnetic fields generated from the surface plasmonic effect of metal NSs are responsible for the large enhancements observed in SERS measurements.3,4 This surface plasmonic effect depends on numerous factors such as the size, shape, and arrangement of metal NSs on the substrates, as described in several reviews.5,6 For properly arranged metal NSs in close proximity to each other, the termed “hot spot effect” can be observed for a molecule located at the small gap between NSs. This hot spot effect provides an extraordinary enhancement of the SERS signal.7,8 Moreover, the distance between NSs is crucial because the coupling of the surface electromagnetic fields between NSs is the main cause of this strong hot spot effect.9-13 Considerable efforts have been made to prepare substrates with precisely controlled gaps between NSs to produce hot spot effects.14 These efforts can be grouped into two broad categories. The first approach employs engineering a pattern of NSs with sufficiently small spaces between NSs. Most of these patterns have arrangements of NSs of various sizes and geometrical features.6,15 For example, denser hot spot effects have been observed on face-to-face arrangements than have been observed on edge-to-edge nanocube dimers because of large NS gap coverage.16 Similarly, hot spots have been produced through symmetrically spaced assemblies of polygonal nanosphere shapes.17 Distances between NSs have been tuned using core shell NP spacers of silica or alumina,15,18,19 polymers and biomolecules,20,21 NS dimers and trimers,22 and porous surface spacers.23-26 These approaches employ accurate lithographic

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methods to fabricate well-defined tightly spaced plasmonic NSs,27 and are restricted to a perspective geometry in the vicinity of the NSs; they require complex and expensive apparatuses. Large-scale production of SERS substrates is limited by the tedious and time-consuming accuracy requirements of substrate preparation. The second approach involves the formation of SERS substrates with dynamic hot spot effects; this approach applies a postprocessing method to vary the distances between NSs. Successful attempts based on this type of approach have been reported to employ various methods, including the tuning of nanogaps based on the stretching of stretchable elastomer film,28 formation of wrinkle nanoporous gold film29 and metal composites,30 strain control by changing the phase of an elastomeric substrate,31 pH and external magnetic field controlled NSs spacing,32,33 drying of agarose gels embedded with silver nanoparticles (AgNPs),34 shrinking of NSs scaffolding hydrogels, and thermal phase transition of microgel templates.35,36 However, even if these dynamic approaches offer promising and simple methods for adjusting the NSs gap to induce the hot spot effect on the SERS substrates, such approaches have not provided a simple and effective method for the massive production of highly sensitive SERS substrates at low cost with high reproducibility, which is still of considerable demand. Shape-memory polystyrene (SMP) sheets were used in this study to support the SERS substrates that were fabricated to create dynamic hot spots at low cost for possible mass production. The SMP in this research was composed of chemically cross-linked polystyrene polymers. If an SMP object is deformed, it can recover its original shape and dimension when it is triggered by an external stimulus, such as a thermal stimulus.37-39 These shape-memory polymers have different degrees of importance in various fields, such as smart fabrics, heat shrinkable tubes for electronics, films for packaging, self-disassembling mobile phones, intelligent medical devices,

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and implants for minimally invasive surgery, as reviewed by Behl et al.37 Tunable nanowrinkles of thin films of metal on shape-memory polymers have also been reported for sensing applications, such as surface plasmon resonance in metal-enhanced fluorescence.40 Currently, SMPs have not been reported for SERS applications; specifically, the thermal shrinkability of SMPs has not yet been applied to create hot spot effects for sensing Raman active species. Therefore, we propose a simple chemical approach to induce hot spot effects by reducing the gaps between NSs by using thermally shrinkable SMP sheets as solid supports on which AgNPs are decorated to produce AgNP-decorated SMP (Ag@SMP) substrates. The formed SERS substrates can be shrunk by applying a mild thermal energy. As the SERS substrates are shrunk, AgNPs move closer together and hence, a strong hot spot effect can be induced. Post-adjustment of the gap between AgNPs enables the analyte to easily access the nanogap between AgNPs, which is a crucial advantage of this dynamic hot spot approach over other engineering approaches. Commercially available SMP sheets are therefore used as solid supports for decoration with AgNPs through the silver mirror reaction (SMR). After the SMP sheet has been decorated with AgNPs and thermally shrunk to produce an Ag@SMP sheet, the analyte can be placed on the Ag@SMP sheet before change the spacing of the AgNPs, thus strengthening the hot spot effect, as shown in Fig. 1.

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Figure 1. Schematic of preparation of the Ag@SMP substrate and the detection of the analyte through its thermally induced hot spot effect.

Experimental Section Chemicals In our experiment, the following chemicals and materials were used. Silver nitrate, ammonium hydroxide (28%–30% w/v), D-(+)-glucose (anhydrous), and para-nitrothiophenol (pNTP) were purchased from Acros Organics (Phillipsburg, NJ, USA); sodium hydroxide was obtained from Showa Chemical Co., and para-hydroxythiophenol (pHTP) was obtained from Kasei Kogyo. Co. (Tokyo, Japan); Adenine, para-aminothiophenol (pATP), and n-octylamine were purchased from Alfa Aesar (Lancashire, UK); HPLC-grade Methanol, Ethanol, and Hexane were obtained from Echochemical (Toufen, Taiwan); A4-size transparent SMP sheets (trade name: Magic Shrink, TP001) were obtained from K-Kingdom International, Inc. (Taipei, Taiwan). All the chemicals were

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used without further purification. Deionized Milli-Q water was used for preparation of all aqueous solutions. Instrumentation A Fourier transform infrared (FTIR) spectrometer (100 series, Perkin Elmer, Waltham, MA, USA) with a single-reflectance ATR accessory was used for acquiring IR Spectra. Raman spectra were collected using a Raman probe equipped with a 30-mW, 785-nm laser source (spot diameter at focus: 158 µm) and a QE65000 CCD detector (Ocean Optics, Inc., Dunedin, FL, USA). Spectra were collected with 1 s of exposure time. The morphologies of AgNPs on the prepared SERS substrates were examined using a field emission scanning electron microscope (FE-SEM, JSM-7600 F, JEOL, Ltd., Tokyo, Japan) operated at 3.0 kV.

Fabrication of Ag@SMP substrate The SMP sheets were cleaned with deionized water in a sonicating bath for 3 min, then sized to 1 cm × 1 cm. These sheets were pretreated in ethanoic solution containing 0.25 M n-octylamine and 1% (v/v) hexane for 30 min. These treated SMP sheets were arranged vertically by using polystyrene foam. To decorate AgNPs on SMP sheets, SMR was used and the procedures to decorate AgNPs were slightly modified from those of our previous report.41A silver solution was prepared by mixing 20 mL of 20 mM (unless stated) AgNO3 with 0.2 mL of 1 M NaOH and 0.4 mL of NH4OH (28%–30% w/v) in an ice-water bath. After placing arranged SMP sheets into the silver solution for 5 min, 6 mL of 0.5 M glucose solution was added as a reducing agent. The container was moved to a 55°C water bath to start the SMR. After the designated reaction time, the SMP sheets were removed from the reaction solution and then rinsed with deionized water. The formed Ag@SMP substrates were probed with different compounds by either soaking or

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deposition. In the soaking method, Ag@SMP substrates were soaked in a methanolic solution of probe molecules for 30 min. To control the precise number of analytes on the Ag@SMP, 60 µL of methanolic solution of probe molecules were deposited on an area of 1 cm2. To thermally induce the hot spot effect, substrates of Ag@SMP were placed into a 100°C oven for 10 min. The stability of the prepared substrates was also examined by keeping the fresh prepared substrate at room temperature for a day without any special precautions. After probing with pHTP, the obtained SERS signals decreased rapidly to ca. 75%. Therefore, all the substrates were fresh prepared in this work.

Results and Discussion Basic properties of SMP and pretreatment of SMP for improvement of AgNPs decoration efficiency SMP sheets are commercially available plastic polymers; they have been used for “Shrinky Dinks” art because they are thermally shrinkable. The SMP sheets used in this study showed an area reduction of 75% after thermal treatment, as illustrated by the photographs in Fig. 1. That is, thermal treatment shortens the distance in any direction by 50%. To successfully decorate AgNPs onto SMP sheets for demonstration of the thermally induced hot spot effect, SMP sheets were first examined by FTIR and Raman spectrometer to characterize the material that formed the SMP sheets. The obtained spectra are plotted in Fig. 2. As can be seen in the corresponding IR transmission spectrum, characteristic bands of aromatic C-H stretching above 3000 cm-1 were observed. Characteristic overtone bands of polystyrene were also observable in the region from 1950 to 1700 cm-1.42,43 Band features from polystyrene44,45 can also be clearly seen in the Raman spectrum (Fig. 2). Therefore, SMP sheets used in this study were mainly composed of

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polystyrene. Interestingly, SMP sheets only exhibited weak features in the Raman spectrum, minimizing the potential spectral interferences anticipated for future uses.46 As is discussed later, these weak Raman features were further reduced after the sheets had been decorated with AgNPs.

Figure 2. IR (a) and Raman spectra (b) of a bare SMP sheet and Ag@SMP used in this study; SEM images of an SMP sheet without decoration of AgNPs (c) and Ag@SMP substrates prepared with 20 mM AgNO3 with 0.2 M (d), 0.5 M (e), and 0.8 M glucose (f) for 5 min. Because SMP sheets are mainly composed of hydrophobic polymer, SMPs must be pretreated to improve AgNP adsorption. Therefore, n-octylamine was employed to alter the surface properties of SMP sheets. The pretreatment solution was composed of n-octylamine in ethanol with 1% (v/v) hexane. Hexane was used as a swelling agent. The swollen SMP facilitated the impregnation of the hydrophobic tail of n-octylamine into polymer film while the hydrophilic tail of the amino group was exposed to an aqueous solution to capture AgNPs.47,48 To verify that the adsorption properties of SMP sheets were improved after pretreatment with n-octylamine, SMR was used to decorate AgNPs on SMP sheets as SMR has been successfully used to decorate AgNPs on glass, metal plates, and other materials.41,49-51 The SEM images of bare SMP and the

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substrates prepared by SMR with 20 mM AgNO3 and glucose concentrations of 0.2 M, 0.5 M, and 0.8 M are plotted in Figs. 2c–2f. As can be seen in these images, AgNPs were successfully decorated on the SMP sheets after n-octylamine pretreatment. Based on the observed SEM images, glucose concentration slightly affects the morphologies of the produced AgNPs. With 0.5 M glucose, the MSP could be fully covered with AgNPs and formed AgNPs were isolate without serious coaggulation. Therefore, 0.5 M glucose was used in preparation of Ag@SMP substrates for later studies. Inducible hot spot effect on Ag@SMP substrate prepared by varying AgNO3 concentration The distance between AgNPs on the substrate is the key factor for successfully inducing a hot spot effect. Two SMR factors, concentration of AgNO3 and reaction time, can be used to tune the distance between AgNPs. Therefore, AgNO3 concentration was first examined by varying the concentration of silver nitrate from 10 to 100 mM, whereas the concentration of glucose and reaction time were maintained at 0.5 M and 5 min, respectively. By probing with pHTP, the SERS spectra on the prepared Ag@SMP substrates were obtained and are plotted in Figs. 3a and 3b. High-quality SERS pHTP spectra were obtained for Ag@SMP substrates prepared with 20 mM and 50 mM AgNO3 and their spectral features matched with the reported spectra very accurately.52 This underscores the fact that substrates can be prepared with SMR after proper surface treatment. When an Ag@SMP substrate had been thermally treated, the SERS spectrum was greatly enhanced, as can be seen in Figs. 3a and 3b. To analyze the impact of AgNO3 concentration quantitatively, the SERS peak intensity at 1079 cm-1 was calculated for samples on Ag@SMP substrates before and after thermal treatment; the results are plotted in Fig. 3c. As can be seen in this plot, the most intense SERS bands originated from Ag@SMP substrates prepared with AgNO3 concentrations of 20 mM. Furthermore, the peak intensities were low when AgNO3

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concentration was higher than 50 mM, which indicates that very high concentrations cause the AgNPs to coagulate and the extremely large particle sizes cause a low enhancement effect in SERS measurements. However, after the Ag@SMP substrates had been thermally treated, peak intensities were significantly increased, even for substrates prepared with AgNO3 concentrations higher than 50 mM. These results clearly indicate that a hot spot effect is inducible, even for highly coagulated Ag@SMP substrates.

Figure 3. (a,b) SERS spectra of pHTP on Ag@SMP before (i) and after (ii) thermal treatment. Used substrates were prepared with 20 mM AgNO3/0.5 M glucose (a) and 50 mM AgNO3/0.5 M glucose (b) for a reaction time of 5 min. Symbols of ×10 or ×5 represent the corresponding magnification of the spectra. (c) SERS intensities at 1079 cm-1 for pHTP on Ag@SMP before (♦) and after (■) thermal treatment. The error bar represents one standard deviation calculated from triplicate run. To verify the causes of the thermally induced hot spot effect and to observe the changes of the morphology of AgNPs on SMP after thermal treatment, SEM images were acquired; the results are plotted in Fig. 4 for samples before and after thermal treatments. As can be seen in these images, the sizes of AgNPs and the distances between AgNPs are strongly influenced by the AgNO3 concentration. At 10 mM AgNO3 concentration, the formed AgNPs were small in size and AgNPs were not close enough to induce hot spots. Hence, the detected SERS intensities

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were weak for samples before and after thermal treatment (Fig. 3c). When AgNO3 concentration was increased to 20 mM, the prepared substrates exhibited the highest SERS intensities among the examined AgNO3 concentrations. The corresponding image of an Ag@SMP substrate before thermal treatment shows isolated AgNPs with a particle size of approximately 100 nm. When this substrate had been thermally treated, a significant increase of peak intensity was observed, revealing that the gaps were shortened to reach a short distance for inducing hot spot effect. By the use of concentrations higher than 50 mM, the formed AgNPs were large in size with dense coagulation of AgNPs before thermal treatment as can be seen in Fig. 4. During thermal treatment, the thick AgNP film could not be moved completely despite the shrinking of the SMP sheet. Instead, for substrates prepared with 50 mM or 100 mM AgNO3, the AgNP film was folded and turned into a film with notable folded wrinkles on the surface, as clearly shown in Fig. 4. These wrinkles formed new contact points for the AgNPs on the thick film and hence, an increase in hot spot effects was observed after thermal treatment of substrates with thick AgNP films.

Figure 4. SEM images of Ag@SMP substrates before (first row) and after (second row) thermal

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treatment. Substrates were prepared with 10 to 100 mM AgNO3 for a reaction time of 5 min. Enlarged SEM images are also shown in Supporting Information (Figs. S-1 to S-4).

Inducible hot spot effect for Ag@SMP substrates prepared with different reaction times The reaction time of SMR is another controllable factor by which AgNPs with different sizes and spacings can be prepared. In the SMR, AgNPs grow in size gradually over time.53,54 As a result in increasing reaction time, the surface area of the substrate that contains AgNPs is affected; the gaps between AgNPs are reduced by particle size increases and possibly by agglomeration. To examine the effect of reaction time during the preparation of suitable substrates, for three AgNO3 concentrations (10 mM, 20 mM, and 50 mM), substrates were prepared at different reaction times ranging from 2 to 10 minutes. These substrates were probed with pHTP to evaluate their SERS spectra peak intensities for later studies of inducible hot spot effect. Similar maximal peak intensities were obtained with different AgNO3 concentrations whereas the maximal peak intensities were observed at different reaction times (refer to Fig. S-5 in Supporting Information); higher concentrations required shorter reaction times to reach the maximal peak intensities. This reveals that the AgNO3 concentration mainly affects the growth rate in the formation of AgNPs. After thermal treatment of Ag@SMP substrates, the observed SERS intensities were mostly increased, as plotted in Fig. 5a. Specifically, all of the Ag@SMP substrates showed an increase of peak intensities after shrinking, which was caused by the narrowing of the spaces between AgNPs. Comparing the SERS intensities for Ag@SMP substrates before and after thermal treatments, the SERS intensities were increased significantly and the largest SERS intensity was observed for substrate prepared with 20 mM AgNO3 and after thermally treated. To examine the increase of the SERS intensities, the SERS intensity of the substrate after treatment was ratio to that of before thermal treatment. The results are plotted in Fig. 5b. Again, large ratio value was

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obtained for substrates prepared with 50 mM AgNO3. This large value was caused by the formation of wrinkles allowing the AgNPs to form new contact points between AgNPs on the thick film and hence, an increase in hot spot effects was observed.

Figure 5. SERS intensity of pHTP on Ag@SMP substrate after thermal treatment (a) and their corresponding ratio values of SERS intensities (b). Ag@SMP substrates were prepared with 10 mM (♦), 20 mM (■), and 50 mM (▲) AgNO3. The error bar represents one standard deviation calculated from triplicate run.

To verify, Ag@SMP substrates before and after thermal treatment were scanned by SEM; several examples of the obtained SEM images are presented in Fig. 6. As with the images shown in Fig. 4 for Ag@SMP substrates prepared with a reaction time of 5 min, the particle size was increased and the distance between AgNPs was shortened when reaction time was extended. After the shrinking of the Ag@SMP substrates, the distances between AgNPs were shortened without significant changes in particle size. For Ag@SMP samples on which a thick film of AgNPs formed, wrinkles were observable, which was a similar effect to that of the substrates prepared with high concentrations of AgNO3.

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Figure 6. SEM images shown in the first row are images of the as-prepared Ag@SMP substrates by 20 mM and 50 mM AgNO3 with reaction times of 3 min and 7 min. Second row shows the SEM images of corresponding Ag@SMP substrates after thermal treatment.

Evaluation of SERS enhancement of prepared thermally induced Ag@SMP substrates Regarding the detection process involving Ag@SMP substrates fabricated in this study, analyte samples can be placed on Ag@SMP substrates before or after the shrinking of the substrates. Because the gaps between AgNPs should be in the range of 2 to 3 nm to produce the greatest hot spot effect,10,55 the tightly closed AgNPs limit the extent to which molecules can diffuse into the hot spot zone to degrade the SERS detection performance. To verify this deduction, adenine, which has no thiol group that could form a monolayer on the AgNPs, was used as a probe molecule. Ag@SMP substrates were prepared with 20 mM AgNO3 and a reaction time of 5 min. Without thermal treatment, Ag@SMP substrates were soaked in 1 mM adenine solution for 30 min. The obtained SERS spectra are plotted in Fig. 7a. The average peak intensity at 735 cm-1 was 2.2(±0.2)×102. After this adenine-soaked substrate had been thermally treated, the obtained peak intensity increased to 2.7(±0.2)×103, a 10-fold increase of SERS intensity. Another trial was performed by first shrinking an Ag@SMP substrate and subsequently soaking it in adenine

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solution. The observed peak intensity was 1.9(±0.2)×103, indicating that a stronger signal can be obtained if analytes are placed on the Ag@SMP substrates before thermal treatment. However, if the compound for detection is thermally unstable, shrinking of Ag@SMP substrates before detection can also be performed with only a minor reduction in the detection sensitivity.

Figure 7. (a) SERS spectra of adenine deposited on Ag@SMP substrate before (i) and after (ii) thermal treatment and SERS spectra of adenine deposited on pre-shrunk Ag@SMP substrate (iii). (c,b) SERS spectra of pATP (a) and pNTP (b) on Ag@SMP before (i) and after (ii) thermal treatment. Symbols of ×10 or ×5 indicate the corresponding magnification of the spectra. To examine the performance of prepared SERS substrates, enhancement factor (EF) is used.56 To evaluate the EFs of prepared Ag@SMP substrates, pHTP, pATP, and pNTP were used because they are commonly used for SERS measurements. EF values are calculated by the following equation:8 EF = (ISERS/NSERS) /(Ibulk/Nbulk) where ISERS is the SERS intensity of the probe molecule on Ag@SMP; Ibulk is the Raman intensity of a 1% (w/v) methanolic solution of the probe molecule measured in a 1-cm diameter vial; NSERS is the number of molecules on the laser focus; and Nbulk is the number of molecules in

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the laser beam path of a 1% (w/v) probe molecule sample (beam area × path length). The surface coverage on the Ag@SMP substrate was determined by depositing 60 µL of 500 nM methanolic solution of probe molecules on a unit area (1 cm2). Ag@SMP substrates used to evaluate EF values were prepared by reaction in 20 mM AgNO3 for 5 min. The obtained SERS spectra of pATP and pNTP are plotted in Fig. 7b. These detected SERS spectra accurately matched the spectra reported in the literature.57,58 According to the most intense peaks in the detected spectra, the calculated EF values for pATP, pNTP, and pHTP were found to be 1.3(±0.1)×108, 4.1(±0.2)×108, and 1.4(±0.5)×108, respectively. Quantitative aspects To evaluate the performance of Ag@SMP substrates with inducible hot spot effects through quantitative analysis, a nonthiolated compound of adenine was used. Different concentrations of adenine were deposited on 1-cm2-area Ag@SMP substrates; the fluid volume was 60 µL; the surface coverage of adenine varied. The relationship between surface coverage and SERS intensity was analyzed. High-quality spectra of adenine were obtained even with a surface coverage lower than 1 ng/cm2 (see Supporting Information, Fig. S-6). The adenine SERS peak that appeared at 735 cm-1 for the ring breathing mode was correlated with its concentration (see Supporting Information, Fig. S-7). Based on three times of noise level of a blank, the limit of detection (LOD) in the detection of adenine was approximately 120 pg/cm2. Moreover, the particle distribution on the substrate was even and firmly attached to the SMP sheet, with an average coefficient of variation (CV) that was lower than 10%. The linear range for detection of adenine can reach 7 ng/cm2 with a regression coefficient (R2) of 0.9959. To examine the reproducibility in production of Ag@SMP, 5 substrates were produced separately. After probed with pHTP, the within substrate CV was 6.6%, which was calculated by averaging the CV

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obtained from each substrate with three measurements. The overall CV was 9.3%, which was calculated from all measurements. These results indicate that the Ag@SMP substrates prepared in this study are highly suitable for quantitative analysis in SERS measurements.

Conclusion In summary, SERS substrates suitable for producing thermally induced hot spot effects were successfully prepared by decorating AgNPs on SMP. The prepared Ag@SMP substrates exhibited thermally induced hot spot effects. The reaction conditions for decoration of AgNPs to control distance before shrinking of the SMP were examined by varying the AgNO3 concentration and reaction time. Results indicate that either concentration of AgNO3 or reaction time can be used to control the distance of AgNPs for inducing a strong hot spot effect after thermal treatment of the substrate. Optimal reaction conditions to prepare active Ag@SMP substrates were discovered using a 20 mM AgNO3 concentration with a reaction time of 5 min. Prepared Ag@SMP substrates with optimal conditions showed an EF value higher than 108. This large enhancement enables very sensitive adenine detection with an LOD of approximately 120 pg/cm2. The prepared Ag@SMP substrates also enable detection of an analyte deposited before or after shrinking with a small difference in sensitivity. For quantitative analysis, a linear range reaching 7 ng/cm2 with R2 = 0.9959 was found in the detection of adenine. Additionally, Ag@SMP substrates are even and provide highly reproducible SERS intensity; the average CV was less than 10%. Acknowledgment. The authors greatly acknowledge Ministry of Science and Technology of Republic of China for financial support of the study.

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Supporting Information. Higher-magnification SEM images of Ag@SMP substrates prepared with different AgNO3 concentrations for a reaction time of 5 min (Figs. S-1 to S-4). SERS intensities of pHTP on Ag@SMP substrates prepared with three different AgNO3 concentrations (Fig. S-5). SERS spectra of adenine with different surface coverages on AgNPs@SMP prepared with 20 mM AgNO3 for a reaction time of 5 min (Fig. S-6). SERS intensities at 735 cm-1 for adenine deposited with different surface coverages on shrunk Ag@SMP. (Fig. S-7). References (1) Michaels, A. M.; Nirmal, M.; Brus, L. E. J. Am. Chem. Soc. 1999, 121, 9932-9939. (2) Tian, Z. Q. J. Raman Spectrosc. 2005, 36, 466-470. (3) Moskovits, M. J. Raman Spectrosc. 2005, 36, 485-496. (4) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys.: Condens. Matt. 1992, 4, 11431212. (5) Cao, Y. Q.; Li, D.; Jiang, F.; Yang, Y.; Huang, Z. R. J. Nanomater. 2013, ID 123812, 12. (6) Kumar, G. V. P. J. Nanophotonics 2012, 6, 064503-064501-064503-064520. (7) Israelsen, N. D.; Hanson, C.; Vargis, E. Sci. World J. 2015, 2015, 124582. (8) Le Ru, E. C.; Blackie, E.; Meyer, M.; Etchegoin, P. G. J. Phys. Chem. C 2007, 111, 1379413803. (9) Lee, S. J.; Guan, Z.; Xu, H.; Moskovits, M. J. Phys. Chem. C 2007, 111, 17985-17988. (10) Lee, J. H.; Nam, J. M.; Jeon, K. S.; Lim, D. K.; Kim, H.; Kwon, S.; Lee, H.; Suh, Y. D. ACS Nano 2012, 6, 9574-9584. (11) Fu, Q.; Zhan, Z.; Dou, J.; Zheng, X.; Xu, R.; Wu, M.; Lei, Y. ACS Appl. Mater. Interfaces 2015, 7, 13322-13328. (12) Li, W. Q.; Wang, G.; Zhang, X. N.; Geng, H. P.; Shen, J. L.; Wang, L. S.; Zhao, J.; Xu, L. F.; Zhang, L. J.; Wu, Y. Q.; Tai, R. Z.; Chen, G. Nanoscale 2015, 7, 15487-15494. (13) Radziuk, D.; Moehwald, H. Phys. Chem. Chem. Phys. 2015, 17, 21072-21093. (14) Lim, D.-K.; Jeon, K.-S.; Kim, H. M.; Nam, J.-M.; Suh, Y. D. Nat. Mater. 2010, 9, 60-67. (15) Wang, A.; Kong, X. Materials 2015, 8, 3024. (16) Camargo, P. H.; Au, L.; Rycenga, M.; Li, W.; Xia, Y. Chem. Phys. Lett. 2010, 484, 304308.

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