Synthesis of Clean Cabbagelike (111) Faceted Silver Crystals for

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Synthesis of clean cabbage-like (111) faceted silver crystals for efficient SERS sensing of papaverine Chunyan Liu, Xiaohui Xu, Wenxin Hu, Xing Yang, Pan-Pan Zhou, Guoyu Qiu, Weichun Ye, Yumin Li, and Chaoyang Jiang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01735 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 27, 2018

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Analytical Chemistry

Synthesis of clean cabbage-like (111) faceted silver crystals for efficient SERS sensing of papaverine Chunyan Liu a, Xiaohui Xu b, Wenxin Hu a, Xing Yang a, Panpan Zhou a, Guoyu Qiu b, Weichun Ye a, *, Yumin Li c, *, Chaoyang Jiang d, * a

State Key Laboratory of Applied Organic Chemistry and Department of Chemistry, Lanzhou University, Lanzhou 730000, China. E-mail: [email protected] b Lanzhou Institutes for Food and Drug Control, Lanzhou 730000, China c Key Laboratory of Digestive System Tumors, Lanzhou University, Lanzhou 730000, China. E-mail: [email protected] d Department of Chemistry and Center for Fluorinated Functional Materials, University of South Dakota, Vermillion, South Dakota 57069, United States. E-mail: [email protected] Keywords: Galvanic replacement; Cabbage-like silver crystals; Surface-enhanced Raman scattering; Molecular dynamics simulation; Papaverine ABSTRACT: Clean cabbage-like (111) faceted silver crystals were synthesized via a facile galvanic replacement reaction of [Ag(NH3)2]OH and a commercial aluminum foil, a surfactant-free formation process. The cabbage-like silver crystals were consisted of interconnected nanoplates and exhibited a single-crystal structure along with preferential (111) facet orientated growth. These silver crystals showed high and reliable surface-enhanced Raman scattering (SERS) activity due to electromagnetic mechanism, and they could be easily transferred onto other rigid or flexible surfaces, making their SERS applications more versatile. Since Ag (111) with low surface energy could preferentially adsorb papaverine molecules, which was verified by molecular dynamics simulation, the cabbage-like silver crystals were further employed as a promising SERS assay for efficient sensing of papaverine, a non-narcotic alkaloid. A linear range of 0.1-1000 µM was achieved, along with a detection limit of 10 nM and good selectivity to other excitability drugs. This SERS assay has successfully been used to determine the concentration of papaverine in hot pot seasonings and drugs with satisfactory recoveries and relative standard deviations.

Introduction Papaverine (PAP), chemically known as 1-(3,4dimethoxybenzyl)-6,7-dimethoxyisoquinoline, is a benzylisoquinoline alkaloid obtained from opium. In humans, PAP has multiple functions, especially including pharmacodynamic properties.1 It can relieve pain in muscles and reduce common symptoms of senile problems. This alkaloid can also increase blood flow due to its effect on peripheral and pulmonary circulation of blood in large vessels and arteries.2 However, PAP is potentially addictive at high dosages in long periods just like most of the opium alkaloids, which will bring adverse health effects, especially in babies, infants, the elderly, and people with severe health issues.3 Therefore, PAP is forbidden for food purposes in general, including poppy seeds. Currently, various techniques have been widely applied to determine the content of PAP, such as chromatography,4-6 spectrophotometry,7 chemiluminescence8 and electrochemical

methods.9,10 However, electrochemical methods suffer from low specificity toward PAP in the presence of many interfering components. The conventional chromatographic separation methods usually have some limitations such as long preparation times, complicated and environmentally unfavorable procedures in spite of high sensitivity. Surface enhanced Raman scattering (SERS) has an extraordinary ability for ultrasensitive and rapid detection of a trace amount of probing molecules.11,12 It is generally accepted that the Raman enhancement mainly comes from the enhanced localized electromagnetic (EM) fields around the close vicinity of noble metals (Ag and Au).13-15 Meanwhile, chemical enhancement is another important factor, which is mainly dependent on the size, shape, and the exposing facet(s) of the metallic nanostructures.16,17 In particular, various facets in a crystal are directly related to their physical and chemical properties, which leads to different SERS enhancements.

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Silver nanoparticles with a typical face-centered cubic (fcc) structure contain four major facets. The differences between (111) facet and other facets include not only the surface atom densities but also the electronic structure, bonding, and possibly chemical reactivities.18,19 Importantly, among of all the facets, (111) facet has the lowest free energy , which will make the (111) facet adsorb molecules stronger than other facets and thus increase the chemical enhancement in SERS.20 Therefore, great efforts have been made to achieve the facetselective growth of silver nanostructures with predominant (111) facet for the application of SERS-active substrate. 21-23 Generally, to anisotropic grow of crystal structures with the anticipated facets, certain capping agents like poly(vinylpyrrolidone) (PVP),24 sodium citrate25,26 and chitosan27 are always involved. However, these capping agents are easily bonded onto the surface of metal nanostructures and will result in spectral signals by themselves, which complicates the analysis and identification process.28 Consequently, synthesis of clean and uniform silver nanocrystals with predominant (111) facet for SERS measurements is highly desired. Furthermore, metallic nanostructures supported on planar substrates may be the most suitable and effective platform for SERS detection, owing to facile storage and transportation. Obviously, galvanic replacement reaction (GRR) is a reliable way to produce a planar substrate with various nanostructured materials for SERS.29-33 In particular, commercial aluminum foil is an ideal under-layer substrate, holding an economical resource and a low redox potential [Al3+/Al, -1.67 V vs. standard hydrogen electrode].29,30 For example, silver dendrites were fabricated via the galvanic replacement of AgF with aluminum foils.31 In our previous work, we prepared silver dendrites on commercial Al foils by using AgNO3 under the aid of fluoride32 or chloride.33 In this work, we report a reliable substrate for SERS detection with high sensitivity, selectivity and stability based on cabbage-like silver nanostructures onto an aluminum foil via a facile GRR approach. Firstly, the resulting silver nanostructures are “clean” because of the surfactant-free formation process. Secondly, these structures can be easily scraped off from the Al foil and transferred onto other rigid or flexible surfaces, making their application in SERS measurements more versatile. Finally, the cabbage-like microstructures contain interconnected nanoplates, which leads to the preferential (111) facet orientation. Significantly, PAP molecules are preferentially adsorbed on Ag (111) proving by molecular dynamics simulation. Thus, the cabbage-like silver crystals are employed as SERS substrates for rapid and sensitive detection of PAP in hot pot seasonings and drugs with satisfactory results. Experimental and Sections Materials Silver nitrate, sodium hydroxide, Rhodamine 6G (R6G) were purchased from Sinopharm Chemical Reagent Co., Ltd. Papaverine, thebaine, codeine, morphine and noscapine were obtained from National Institute for Food and Drug Control. Chili oil, beef needle soup, and loquat dew were purchased from local markets (Lanzhou, China).All reagents were of analytical grade and used without further purification. Water

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used in all experiments was purified using a Millipore Q system. Characterization The micro/nanostructures of as-synthesized nanocrystals were characterized with field emission scanning electron microscopy (FE-SEM, JSM-6701F, JEOL Inc., Japan). Highresolution transmission electron microscopy and corresponding selected area electron diffraction (HRTEM/SAED) were performed on a transmission electron microscope (TEM, Tecnai G2 F30, FEI, USA). The fabrication of samples for TEM was performed by placing a drop of as-prepared solution on carbon-coated copper grids. X-ray powder diffraction (XRD) analysis was carried out on Rigaku D/max-2400 (Cu K-Alpha radiation, λ= 0.1541 nm). Preparation process of cabbage-like silver Commercial aluminum foil was cut into 1×1 cm2 pieces and rinsed with acetone, ethanol and water in sequence to degrease the samples. Then, the aluminum foil was rinsed with NaOH (0.5 M) and water in sequence to remove the oxide layer. After drying in N2, the foil was immediately immersed into [Ag(NH3)2]OH solution with a concentration of 30 mM. Subsequently, a large amount of cabbage-like silver was quickly formed onto the aluminum foil. The as-prepared silver was rinsed with ultrapure water and dried in air. SERS detection SERS spectra were obtained on a Raman system (Zolix Finder Vista-HiR). In all SERS detections, the excitation wavelength was 532 nm with a power of 0.18 mW, and the magnification of the objective was ×40 L. To evaluate the structure effect, the as-prepared silver samples were dipped in a R6G (1 µM) ethanol solution for 30 min. Then, the samples were taken out, rinsed using ethanol for three times, and dried in air. To evaluate the transferrable nature, the silver crystals were first dipped in R6G ethanol solution and then transferred onto different substrates like Si, plastic and paper sheets, and dried in air. For the detection of PAP, a stock solution of PAP (1 mM) was prepared by dissolving 9.4 mg PAP in 25 mL acetonitrile. Standard stock solutions (1 mM) of interferents such as baine, codeine, morphine and noscapine were prepared for each in the same way. A series of PAP concentrations of 10-3-10-8 M were adopted as the probe molecules. Simulation method Molecular dynamics (MD) simulation in NVT ensemble, which is canonical ensemble with constant atom number, volume and temperature, was carried out to investigate the interactions between silver nanoparticles and papaverine molecules. All calculations were applied using Forcite module in Materials studio 6.0 from Accelrys Inc. Results and discussion Anisotropic growth of cabbage-like Ag crystals used as a transferable SERS-active substrate The synthesis started with a surface treatment of the Al foil by NaOH to remove the alumina layer. Subsequent immersion


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Analytical Chemistry of this freshly treated Al foil into 0.03 M [Ag(NH3)2]OH solution led to the formation of a silver film on the clean Al foil (Figure S1) through the GRR reaction, where no surfactant or template was introduced. The GRR reaction can be expressed in the following equation: Al + 3[Ag(NH3)2]OH → 3Ag + [Al(NH3)6](OH)3 (1) Figure 1A shows the SEM image of the as-prepared silver film immersed for 15 min, exhibiting uniform and well dispersed 3D hierarchical cabbage-like structures with an average size of about 6.8 µm. From the morphology of an individual crystal (Figure 1B), we can see that these cabbagelike structures consist of curled and interconnected nanoplates with the thickness of about 4.2 nm. A picture of a typical natural cabbage is also shown for a comparison of the morphology (the inset in Figure 1A). Because their geometry and morphology are reminiscent of cabbage, the silver crystals are therefore referred to as a cabbage-like structure.

Figure 1. Characterization of the silver films immersed for 15 min: (A and B) SEM images; (C) TEM image; (D) SAED and (E) XRD patterns. (F) the SERS spectrum of 1 µM R6G adsorbed on the silver film. The inset of (A) shows a picture of a natural cabbage; The inset of (C) shows the corresponding HRTEM image

The anisotropic structure of silver nanoplates was further analyzed by HRTEM and SEAD. In the low magnification TEM image (Figure 1C), large silver nanoplates with the size of 1.25 µm overlay together. According to the HRTEM image (the inset of Figure 1C), the lattice spacing is 0.249 nm, which can be ascribed to the normally forbidden 1/3(422) lattice. It reveals that the forbidden reflection could be attributed to the (111) facet parallel to the flat surfaces. This observation is in agreement with the case of plate-like silver crystals.26,34 The SAED pattern (Figure 1D) indicates that the as-prepared silver products are a single crystal. The inner 6-fold symmetry spots are indexed to the 1/3(422) Bragg reflections, with a d-spacing of 0.249 nm,35 which agrees well with the lattice spacing shown in the inset of Figure 1D. The second set of 6-fold

symmetry spots are indexed to the (220) Bragg reflections, with a d-spacing of 0.14 nm. Consequently, it can be inferred that the silver nanoplates are bounded with the (111) facet.36 Furthermore, the EDX spectrum in Figure S2 confirms that the silver crystals are clean. The elements of C, O and Cu come from the supporting film of copper mesh, and no other impurity elements can be detected. Figure 1E shows the XRD pattern of the Ag crystals. Here, four diffraction peaks are observed and indexed to the characteristic diffraction peaks from (111), (200), (220), and (311) of Ag crystals (JCPDS card no. 04-0783). It is noteworthy that the intensity ratio of the (111) to (200) reaches up to 5.6, which is much higher than the 2.5 of the standard diffraction of Ag powders. The high intensity ratio of (111) to (200) suggests the anisotropic growth of cabbage-like Ag crystals may be the result of the rapid deposition of metallic silver along the low surface energy (111) facet. In addition, Figure S3 shows the UV-vis absorption spectrum of the cabbage-like Ag crystals after being dispersed in water under ultrasonication. Here, the absorption peak at ~330 nm corresponds to the silver out-of-plane quadrupole surface plasmon resonance (SPR) mode.2 The in-plane dipole SPR peak of the as-prepared silver crystals is in the near- or far-IR regions, which is beyond the detection range of our spectrometer. Similar UV-vis absorption features were also observed in the literature for the micrometer-sized Ag nanosheets, which might be ascribed to the polydispersity and the truncations of silver nanosheets.21,22 Accordingly, the cabbage-like structure may lead to an extended delocalization of the in-plane electrons and a significant red-shift in the SPR band. The cabbage-like structures can give rise to high SERS activity because a great amount of narrow nanogaps, which serve as the “hot-spots”, are presented in their adjacent connected Ag nanoplates (Figure 1B). Indeed, enhanced characteristic features were clearly observed as 1 µM R6G were adsorbed on the silver film immersed for 15 min. In Figure 1F, characteristic peaks at 610, 774, 1187 cm-1 correspond to C-C-C ring in-plane bending, C-H out-of-plane bending and C-H in-plane bending, respectively.37 The Raman peaks at 1310, 1361, 1510, 1570 and 1647 cm-1 are assigned to in-plane xanthene ring breathing, xanthene ring stretching inplane C-H bending, aromatic C-C stretching, xanthene ring stretching in-plane N-H bending, xanthene ring stretching inplane C-H bending, respectively.38 To quantify the SERS ability of the silver crystals, the enhancement factor (EF) is introduced and calculated by the following equation: EF = (ISERS / Ibulk) (Nbulk / NSERS) (2) Where ISERS is the intensity of the Raman spectra of the sample. Ibulk is the Intensity of the normal Raman spectra of solid R6G. Nbulk is the molecule number of the solid R6G in the laser illumination volume. And NSERS is the total number of surface adsorbed molecules. The detailed calculation of EF is shown in the ESI and Figure S4. The EF for our substrate is as high as 3.56×106. The limit of detection (LOD) for R6G measurement can reach to 10-14 M, as illustrated in Figure S5. The EM enhancement mechanism plays the main role in the SERS enhancement.



Generally, silver can be easily oxidized and is unfavorable for its long-term application. However, the as-prepared silver crystals exhibit good long-term stability. By using the intensity of 1368 cm-1 SERS peak as the calibration, we found that the peak intensity still remains over 81% of its initial value despite being kept in air over one month (Figure S6). Conversely, the peak intensity is only 40% for silver nanoparticles. Moreover, the cabbage-like silver crystals can still possess a high SERS activity even as they were treated with HCl or NaOH solutions (Figure S7). As expected, our synthetic approach offers a great simplicity for the formation of fully accessible and robust hot spots for real application.

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Figure 2. (A) SERS spectra of 1 µM R6G adsorded on silver nanoplates that were transferred onto different substrates: aluminum, silicon, plastic and paper. (B) Their corresponding peak intensities @1368 cm-1. Error bars indicate the standard deviations for the measurements at twenty random positions.

Figure 3. SEM images of the silver films prepared at different intervals: (A) 30 s, (B) 2 and (C) 5 min. (D) XRD patterns. (E) SERS spectra of 1 µM R6G adsorbed on the silver films prepared at different intervals and Raman spectrum of pure R6G powder. (F) The variations of ISERS @1368 cm-1 and (111)/(200) ratio with the reaction time. The insets of (A, B, C) shows their higher magnification SEM images.

Note that it will be beneficial to fabricate a flexible SERS substrate.39 For example, SERS substrates on filter paper with silver nanocolloids or films have been developed as low-cost, disposable SERS devices, allowing separation and enrichment

of the analyte.40,41 On the other hand, flexible substrates are easily cut to integrate with other sensing assays. 42 In this work, since Al foils could be dissolved with HCl or NaOH, the as-prepared silver crystals were easily scraped off and formed


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Analytical Chemistry into a silver ink. Then, the silver ink was spread onto our desirable surfaces of either rigid or flexible substrates. Using R6G as probe molecules, the SERS spectra obtained on the transferred substrates (a silicon chip, a paper sheet and a plastic sheet) are similar to those obtained on the primary substrate (the Al foil), as shown in Figure 2A. The ISERS values from the 1368 cm-1 peak do not decrease distinctly for the transferred silver crystals, compared to the value on the primary substrate (Figure 2B). Furthermore, we studied the uniformity of the SERS signal from the primary substrate and the three transferred substrates. By collecting the SERS spectra of twenty random positions, it was found that these relative standard deviation (RSD) values of ISERS@1368 cm-1 limited within 10% for the primary substrate and the three transferred substrates (Figure S8). It is an acceptable RSD value for SERS assay. Therefore, it is believed that this large-area fabrication of clean cabbage-like silver crystals can open up the opportunities with regard to chemical and biochemical sensing on various promising substrates including flexible substrates. Morphological evolution and structural SERS effect The morphological and structural evolution of silver crystals was studied by varying the reaction time. As shown in Figure 3A, at the early stage (30 s), Ag nanoparticles orientate randomly and some plate-like structures grow from Ag nanoparticles. After the growth of 2 min, both the size and the distribution density of plate-like silver structures increase and further grow into nanoplates (Figure 3B). Obviously, the cabbage-like structures stay at the evolutional stage of growth. When increasing the grow time to 5 min, we found that the size of Ag nanoplates becomes larger and they grow close to each other (Figure 3C). The time-evolution of crystalline structure for these Ag crystals was further analyzed by XRD. Similarly, these Ag crystals with different growth time exhibit a typical fcc structure (Figure 3D).

As the surface morphology and crystalline structure play important roles in the SERS effect, the SERS spectra of the asprepared silver films with different growth times were carefully examined. Figure 3E shows the SERS spectra of R6G adsorbed on these silver crystals with various growth times. Using the peak of 1368 cm-1 as the calibration, we found that the SERS intensities increase with the increase of the growth time (Figure 3F). Based on the above XRD data, the intensity ratio of (111) to (200) increases with the growth time. The increased Raman intensities can therefore be attributed to the preferential (111) facet oriented growth. As the (111) facet is parallel to the surface of the substrates and its free energy is the lowest among of all the facets,20 the improved proportion of the (111) facet results in more molecules being adsorbed on the crystals. Furthermore, the effect of the solution conditions on silver nanostructures was studied. As pure AgNO3 solution was added in the reaction cell, dendritic silver nanostructures were achieved (Figure S9). Due to the complexing action of silver ions and NH3 in [Ag(NH3)2]OH, the free concentration of silver ions reduced greatly and thus resulted in a slow reaction rate. However, when fluoride or chloride was added into the [Ag(NH3)2]OH solution, silver dendritic nanostructures were formed,32,33 suggesting that the addition of fluoride or chloride into the [Ag(NH3)2]OH solution accelerated the reaction rate due to their etching function to aluminum oxide. It is well known that the anisotropic growth of metal crystals can be manipulated by the growth kinetics process, which tunes their shape and leads to the kinetic control of the growth rate of the different facets.43-45 For silver crystals, the surface energies of the facets follow in the order: (110) 0.953 eV > (100) 0.653 eV > (111) 0.553 eV.46 To minimize the overall surface energy, a slow reduction enables the nucleation and growth of silver nanosheets under a kinetically controlled environment, which results in the formation of stable faceted particles preserving its lowest surface energy (111) facet. 47

Figure 4. (A) SERS spectra of PAP with different concentrations. (B) The relationship plot between the peak intensity@1372 cm-1 and PAP concentration. Error bars indicate the standard deviation for five measurements. (C) Comparison of the SERS spectra of papaverine (PAP), thebaine, codeine, morphine and noscapine based on silver nanoplates, their concentration was 1 mM for each.



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Table 1. Determination of PAP in the samples of food and drugs including chili oil, beef needle soup and loquat dew, based on the silver SERS assay Sample

Original (µM)

Spiked (µM)

Found (µM)

Recovery (%)

RSD (%)

Chili oil

No found

5.00

4.55

91.00

0.95

1.00

0.96

96.00

3.72

0.50

0.51

102.00

6.52

5.00

4.99

99.80

5.79

1.00

0.98

98.00

5.19

0.50

0.50

100.00

9.38

5.00

5.03

100.60

4.39

1.00

1.01

101.00

4.86

0.50

0.47

94.00

1.25

Beef needle soup

Loquat dew

No found

No found

Practical application for sensing PAP In terms of their high SERS activity, the cabbage-like silver crystals were used as a SERS substrate for sensing PAP. The SERS spectra of PAP with different concentrations (10-3-10-8 M) are displayed in Figure 4A. The characteristic Raman peak of PAP at 1372 cm-1 was used to quantitatively determine the concentration of PAP. In Figure 4B, a linear response is obtained against the logarithm of the PAP concentration over the range of 1000-0.1 µM with a superb correlation (R2 = 0.995). The limit of detection (LOD) is 10 nM, which is better than or compatible to other analytical methods, such as chromatography, electrochemistry and SERS coupled with thin-layer chromatography (Table S1). The specificity is another important evaluation criterion for an efficient SERS assay. We tested the specificity of our assay by determining other four relevant excitability drugs, including thebaine, codeine, morphine and noscapine. As shown in Figure 4C, their SERS signals@1372 cm-1 are weak for these compounds. Here, the specific intermolecular interactions play a vital role in the adsorption models.41,42 PAP as an isoquinoline derivate has a large conjugated π electron system. Such large π electron system makes large Raman cross section for PAP molecule. At the same time, it also allows strong adsorption on silver due to strong π-π stacking interactions between the isoquinolyl groups. However, the intermolecular π-π stacking is relatively weak for these interferents with single aromatic rings. Our results indicated that this silver assay has a good selectivity to PAP. Similar detection can be used for the polycyclic aromatic molecules with large Raman cross-sections. In Figure S10, Raman spectra in these samples are obtained by this silver assay, presenting good band resolution and a high Raman signal. Herein, molecular dynamics (MD) simulation was studied to evaluate the merits of crystals with preferential (111) orientation in SERS sensing of PAP. For fcc nanocrystals, their shapes are mainly dependent on (100), (110) and (111)

crystal facets.19 The evolutions of papaverine molecules on different silver facets including (100), (110) and (111) are displayed in Figure 5. Three crystal surfaces including Ag (100), Ag (110) and Ag (111) crystal facets composed of 96 silver atoms were considered. COMPASS force field, which is well applied in non-bonded interactions included intermolecular and intramolecular interactions, was used to calculate the interaction energies (∆Eint) of papaverine molecules and three different silver nanoplates. The ∆Eint can be calculated with the difference of the total energy of complex and the energy sum of six papaverine molecules and silver surface, i.e., ∆Eint = Ecom - (Esur + Emol). Esur and Emol can be obtained by using energy calculations for the geometries of complexes deleted separately all molecules and silver atoms. The calculated interaction energy is -602.15 kcal/mol for absorption of PAP molecules on Ag (111) surface, and the interaction energies for Ag (100) and Ag (110) are -530.86 and -547.21 kcal/mol, respectively. Clearly, the PAP molecules prefer to adsorb on Ag (111) rather than Ag (100) and Ag (110), which is consistent with our experimental results and the previous work.36,50 Therefore, the cabbage-like silver crystals with predominant (111) facet is expected to be a promising SERS assay for sensitive detection of PAP. Due to its potentially addictive properties, PAP might be illegally interpolated into food although it is severely forbidden. In this study, this SERS assay was applied for PAP detection in real samples of hot pot seasonings (chili oil and beef needle soup) and drugs (loquat dew). These samples were purchased from local markets (Lanzhou, China). The analyses were performed for three times under the same conditions by the standard addition method. The corresponding SERS spectra are shown in Figure S11. In Table 1, the recoveries of PAP detection in the samples range from 91% to 102%. The RSD values limit within 10%. The satisfactory recoveries and RSD values indicate that this SERS assay can be suitable for sensitive determination of PAP in various real sample.


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Analytical Chemistry

Figure 5. Evolution of papaverine molecules on different silver crystal surfaces (a) 100; (b) 110; (c) 111 at time 0 ps, 10 ps, 30 ps and 50 ps. Here, blue, grey, white, red and argentite represent N, C, H, O and Ag, respectively. Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. org

Conclusions Clean cabbage-like silver crystals were successfully prepared on a commercial Al foil by a surfactant free galvanic replacement reaction with aqueous [Ag(NH3)2]OH solution. The HRTEM and XRD results demonstrated that the cabbagelike structures were consisted of interconnected nanoplates, exhibiting the single-crystal feature with predominant (111) facet. The cabbage-like silver crystals not only provided high intensity of SERS signals, but also could be conveniently transferred onto other desirable surfaces for versatile SERS measurements. Based on the strong interaction energy of PAP and Ag (111), PAP was effectively and specifically probed by the silver SERS substrate. An LOD of 10 nM was obtained, along with broad linear response and good specificity against other excitability drugs. Even for the analysis of hot pot seasonings, satisfactory recoveries and RSD values were achieved. We believe that this simple, low cost, highly sensitive and selective SERS substrate could be used as a promising tool for the detection of PAP and other related chemicals, which is important in the field of drug control and food safety in the future.

ASSOCIATED CONTENT Supporting Information

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] * [email protected]

Present Addresses † Lanzhou University, Lanzhou 730000, China † Lanzhou University, Lanzhou 730000, China † University of South Dakota, Vermillion, South Dakota 57069, United States

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the Fundamental Research Funds for the Central Universities (Nos. lzujbky-2017-k9 and lzujbky-2017sp13) and the Natural Science Foundation of Gansu Province, China (No. 17JR5RA209). CJ acknowledges the South Dakota Governor's Office of Economic Development for providing financial support through the Center for Fluorinated Functional Materials.



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