Polydopamine@Gold Nanowaxberry Enabling Improved SERS

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Polydopamine @ Gold Nanowaxberry Enabling Improved SERS Sensing of Pesticides, Pollutants and Explosives in Complex Samples Dongzhen Chen, Xiaodong Zhu, Jian Huang, Gen Wang, Yue Zhao, Feng Chen, Jing Wei, Zhongxiao Song, and Yongxi Zhao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01348 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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Analytical Chemistry

Polydopamine @ Gold Nanowaxberry Enabling Improved SERS Sensing of Pesticides, Pollutants and Explosives in Complex Samples Dongzhen Chen,†,‡ Xiaodong Zhu,† Jian Huang,‡,§ Gen Wang,‡ Yue Zhao,‡ Feng Chen,‡ Jing Wei,‡ Zhongxiao Song,† and Yongxi Zhao‡,* †

State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P. R. China ‡

Key Laboratory of Biomedical Information Engineering of Education Ministry, School of Life Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P. R. China §

College of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an, Shaanxi 710065, P. R. China

ABSTRACT: Surface-enhanced Raman scattering (SERS) is a promising analysis technique for detecting various analytes in complex samples due to its unique vibrational fingerprints and high signal enhancement. However, impurity interference and substrate unreliability are direct suppression factors for practical application. Herein, we synthesize polydopamine @ gold (PDA @ Au) nanowaxberry, where Au nanoparticles are deposited on the surface of PDA sphere with high density and uniformity. Seed-mediated synthesis is used for fabrication of nanowaxberry. Au seeds are deposited on the surface of PDA sphere, then I ion coordinating ligand is employed to form stable AuI4- complex with AuCl4-, which decreases reduction potential of AuCl4- and avails formation of shell structure. Such nanowaxberry has high density of voids and gaps in three-dimensional space, which could absorb analytes and benefit practical SERS detection. Using malachite green as a model analyte, nanowaxberry realizes highly sensitive detection with low limit of detection (1 pM) and good reproducibility (relative standard deviation of about 10%). Meanwhile, the nanowaxberry is employed for practical detection of thiram, benzidine and 2, 4-dinitrotoluene in the environmental water, juice, apple peel and soil. The high performance makes nanowaxberry to be potentially used for pesticides detection, pollutants monitoring and forbidden explosives sensing in complex samples.

The efficient detection of analytes, such as pesticides, pollutants and explosives is vital for protection of human health 1-4 and environmental security. Fluorescence spectroscopy, ion mobility spectrometry and liquid chromatography can be used for the detection of above analytes, but which associates some disadvantages, such as low sensitivity, complex extrac3 tion procedures and time-consuming assay. Various target molecules often dispersed in complex mixtures, where the signal of background would swamp the signal of target mole5 cules, which makes the detection of target molecules in complex samples more difficult. Surface-enhanced Raman scattering (SERS) spectroscopy is a powerful analysis technique due to its unique vibrational fingerprints and high signal enhancement. Because of narrow linewidth of SERS spectra, it can detect multiple-targets and provide great promise to detect analytes in complex samples even down to 5,6 single molecule level. Great efforts have been dedicated to improve the sensitivi7-9 ty and uniformity of SERS signal. The SERS performance is highly influenced by the size and geometry of substrate. In order to increase the sensitivity, various synthesis methods have been successfully developed to prepare noble metal 10 nanostructures with sharp corners or nanogaps, such as 11 solution-phase synthesis protocol, templated synthesis of 12 13, 14 nanostructures, and self-assembled method. Light exci-

tation of local surface plasmon resonance (LSPR) in sharp corners and nanogaps prominently enhances localize electromagnetic field (EM), which forms the hot spots, and improves SERS sensitivity effectively. Meanwhile, maximizing uniformity of SERS signal is also important. For the sake of sensitivity, uniformity and reproducibility of signal, three13, 15-17 dimensional (3D) SERS substrates have been studied. When a large number of gaps, bridges or crevices are generated into single 3D nanoparticle, higher sensitivity of SERS signal may be acquired with improved uniformity and repro10 ducibility. In our previous study, we fabricated kinds of biomimetic 3D SERS substrates by ion sputtering technology 18, 19 and chemical vapor deposition technology. The outstanding SERS performance of 3D nanoparticle is also effective to 17, 20, 21 detect target within complex mixtures. Plasmonic nanoshell has intrinsically excellent SERS property. LSPR of nanoshell gives rise to strong local EM field 22 enhancement and has been employed for photonic, spec23 24 troscopic, and biomedical applications. Hence, the architecture of 3D plasmonic nanoshell may meet the requirements of ideal SERS performance. Up to now, techniques of template synthesis are efficient for nanoshell fabrication. For 25,26 27 example, silica and polystyrene templates have been used for preparation of noble metal nanoshell. The templates often require chemical treatment and decoration with a

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Scheme 1. Schematic illustration of the synthesis process of polydopamine @ gold (PDA @ Au) nanowaxberry and its SERS detection. (I) Deposition of Au seeds onto the surface of PDA sphere, (II) the iodide ions assisted growth of Au nanoshell on the PDA sphere, and (III) SERS detection of pesticides, pollutants and explosives using nanowaxberry as a substrate. monolayer of coupling agent to load gold (Au) seeds onto the 12,25 surface and maintain the growth of nanoshell. Neverthe less, the detachment of Au seeds and self-nucleation of Au nanoparticles would occur during the growing process, resulting in subsequent metal growth in the unconfined 25 space. In contrast with these substrates, polydopamine nanoparticle (PDA NP) possesses its advantage in fabricating 3D Au shell. The high density of amino groups from PDA spheres could strongly coordinate with Au species, facilitating deposition and growth of Au species on the surface of 28-31,32 PDA spheres. Herein, we introduce a nanowaxberry, where the Au nanoparticles are deposited on the surface of PDA sphere uniformly to form an Au shell. The preparation process shows in Scheme 1. Firstly, PDA sphere with diameter of 200 nm was synthesized by polymerization of dopamine in alkaline conditions. And secondly, the Au seeds were deposited on the surface of PDA sphere uniformly due to the strong coordination interactions between Au and amino groups. In the second growth process, I ions were used as a coordinating ligand to form stable AuI4 complex with AuCl4 , which decreases reduction potential and avails formation of shell structure. The Au shell has numerous nanovoids and nanogaps, which improves the SERS performance and benefits the adsorption of target analytes. Finally, the nanowaxberry was employed as a SERS substrate for practical detection, such as thiram, benzidine, 2, 4-dinitrotoluene (DNT) and malachite green (MG) in complex samples. SERS detections of complex samples using this nanowaxberry exhibited high sensitivity, good reproducibility, and excellent anti-interference ability. EXPERIMENTAL SECTION Chemicals and Materials: Chloroauric acid 99%, poly (vinylpyrrolidone) (PVP with Mw=40000 g/mol) were purchased from Sigma-Aldrich. Potassium iodide, ascorbic acid, ammonia solution, sodium borohydride, crystal violet (CV) and malachite green (MG) were obtained from Sinopharm Chemical Reagent Co. Thiram, benzidine hydrochloride and

2, 4-dinitrotoluene (DNT) were purchased from Aladdin Industrial Corporation. Characterization: The morphology was characterized by field emission scanning electron microscope (FE-SEM, Hitachi S4800), transmission electron microscope (TEM, JEM2100F and JEM-200CX), and atomic force microscope (AFM, agilent 5500). XRD pattern was acquired by X-ray diffraction (XRD, Bruker D8 Advanced). UV−vis spectra were performed on a PE Lambda950 spectrophotometer. The Raman spectra were recorded using Laser Raman Spectrometer (HORIBA, LabRAM HR Evolution) equipped with 633 nm and 785 nm laser. Synthesis of PDA Sphere: Ethanol (40 mL) was mixed with ultrapure water (90 mL), then ammonia aqueous solution (4 mL, 28-30%) was added into it. Dopamine hydrochloride (0.5 g) was dissolved in ultrapure water (10 mL), then the dopamine solution was injected into the above solution, the reaction proceeded for 30 h. Then produced PDA nanospheres were obtained by centrifugation. Synthesis of Au Seeds @ PDA: Aqueous solution of HAuCl4 (40 μL, 25 mM) was mixed with aqueous solution of PDA nanospheres (1 mL), then aqueous solution of sodium borohydride (40 μL, 100 mM) was added drop by drop, and the solution was stirred for 15 min, the obtained Au seeds @ PDA were collected by centrifugation and washed with water. Synthesis of Au Shell: Deionized water, PVP aqueous solution (5%), KI aqueous solution (100 mM), ascorbic acid aqueous solution (100 mM) and HAuCl4 aqueous solution(25 mM) were added into reaction vial consecutively, then Au seeds @ PDA aqueous solution was added into above solution. The solution was rigorously stirred for 15 min. The obtained PDA @ Au shell were collected by centrifugation and washed with water. SERS Detection. Lake water, juice, apple peel and soil were chosen as real samples to validate the feasibility of nanowaxberry in SERS analysis. Several drops of colloidal nanoparticles were dropped on the Si slice, and the Si slice was


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Analytical Chemistry slightly shaked during drying, which will ensure that the nanoparticles were distributed uniformly as far as possible. The apple peels were spiked with thiram solution, the volume is 30 μL, then spiked peels were prepared. Then nanowaxberry colloidal (20 μL) was dropped onto the surface of spiked peel and dried in air. Repeat this process for three times. Finally, the peels were placed onto the Raman measurement platform for detection. The Raman spectra were recorded using 785 nm laser.

dominating effect for preventing the self-nucleation process, and PVP capping ligand benefits formation of rough surface.

RESULTS AND DISCUSSION Preparation and Characterization of Nanowaxberry. The nanowaxberry with PDA core and Au 3D shell was prepared via two steps. PDA nanospheres were prepared through polymerization of dopamine in water, ethanol, and 33 ammonia solution. The diameter of PDA nanospheres is about 200 nm from TEM results (Figure 1a). PDA nanospheres have plenty of catechol and amino groups on their surface, which facilitates the deposition of Au seeds. The Au seeds on the PDA nanospheres were achieved by mixing PDA nanospheres with HAuCl4 solution, and HAuCl4 was reduced by NaBH4. After seeding, the Au seeds (10 nm) were deposited on the surface of PDA nanospheres (Figure 1b). After the second growth process, Au shells were formed on the surface of PDA spheres. As shown in Figure 1c, d, e, the Au shells show rough surface morphology and host large amounts of voids. From magnified nanoshell image (Figure 1f), the adjacent Au nanoparticles form a very small nanogap. For the fabrication of Au nanoshell on the surface of PDA core, the crux is to keep low rate of reduction reaction. In our research, we employed ascorbic acid for the reduction of AuCl4 . Because of the high reduction potential of AuCl4 12 (+0.93 V vs SHE), when ascorbic acid was mixed with AuCl4 and Au seeds @ PDA, the color of solution turned into crimson quickly (Figure S1), the self-nucleation process was generated, and no Au precursor was deposited on the surface of PDA spheres as Figure 2a shows. The UV-vis spectrum of the solution (Figure S2a) shows an absorption band at about 570 nm, which also illustrates the generation of Au nanoparticles. To prevent the self-nucleation process, it is necessary to decrease reduction potential of AuCl4 . KI could form stable AuI4 complex with AuCl4 , and reduction potential was de12 creased to +0.56 V. Hence, the KI coordinating ligand was introduced. As Figure 2b shows, when KI was mixed with AuCl4 , ascorbic acid and Au seeds @ PDA, the Au shells could form onto Au seeds @ PDA surface obviously, which was also evidenced by near infrared absorption band at 960 7,21,34 nm (Figure S2b). A small number of self-nucleation nanoparticles are observed in Figure 2b, the UV-vis spectrum (Figure S2b) also shows another absorption band at ∼550 nm. Rough surface topography is very important to improve SERS sensitivity. The nanoshell with rough surface topogra35 phy can amplify local EM. Usually, PVP as an additional capping ligand is introduced into the growth solution, which 36 can stabilize the atomic monomer species, decrease reaction rate and disperse nanospheres. Inspired by it, we introduced PVP into the growth solution. As Figure 2c shows, the nanoshell prepared by this synthesis parameter exhibits rough surface topography with numerous gaps and voids. In this process, amounts of Au nanoparticles were formed on the surface. From a series of results, we found that I plays a

Figure 1. (a) TEM image of PDA nanospheres. (b) TEM image of Au seeds @ PDA nanospheres. (c) TEM image and (d) SEM image of nanowaxberry. (e) TEM image of single nanowaxberry and (f) the magnified image at the nanoshell edge. In order to further prove these results, KI was removed from the growth solution, the PVP, AuCl4 , ascorbic acid and Au seeds @ PDA were mixed, the color of solution turned into marroon quickly, as shown in Figure S1. When reaction was accomplished, obvious branched structures outside the PDA spheres were generated and the nanoparticles were gather together, as shown in Figure 2d. Its UV-vis spectrum (Figure S2d) shows a near infrared absorption band at about 695 nm. According to above research, our synthetic strategy introduce I coordinating ligand and PVP capping ligand, which could decrease reduction potential and reaction rate, so that the fabrication process of rough nanoshell can be finished in one-time step. Our synthetic strategy is more 25 convenient than previous synthesis methods of nanoshell, and this synthetic strategy avoids many times of growth cycle.

Figure 2. SEM images of nanostructures at different synthesis parameters. (a) The reactant comprised of AuCl4 , ascorbic acid and Au seeds @ PDA; (b) The reactant consisted of KI, AuCl4 , ascorbic acid and Au seeds @ PDA; (c) The reactant comprised of KI, PVP, AuCl4 , ascorbic acid and -

Au seeds @ PDA. (d)The reactant consisted of PVP, AuCl4 , ascorbic acid and Au seeds @ PDA .



Figure 3. (a) TEM images of nanoshell-1, nanoshell-2, nanoshell-3, and nanoshell-4. (b) UV−vis spectra of PDA nanoparticle, Au seeds @ PDA, nanoshell-1, nanoshell-2, nanoshell-3, and nanoshell-4, and (c) average SERS intensity. Moreover, halide ions play an important role for mo dulating gold nanocrystal shape, halide ions can adjust the reduction potential of the metal ions, passivate the gold nanoparticle surface, and control the extent of silver 37,38 underpotential deposition. Through these ways, the reaction kinetics can be regulated, then we can acquire different particle growth and yield different products. In addition, we found that the geometry of shell was sensitively effected by the amount of HAuCl4. For example, when HAuCl4 (200 μL, 25 mM) was introduced, the nanoshells showed rough surface with maximal surface roughness, as shown in TEM and AFM images (Figure 3a and Figure S3 nanoshell-3). When HAuCl4 (100 μL,25 mM) was introduced, the surface roughness was decreased, as shown in Figure 3a and Figure S3. Moreover, when the volume of HAuCl4 was increased to 275 μL, the larger amounts of HAuCl4 caused coalescence of nanoshell surface, as shown in Figure 3a(nanoshell-4) and Figure S3(nanoshell-4). Our synthetic strategy also gives a versatile pathway of making serial 3D nanoshells with different dimensions. The LSPR property is also influenced by dimensions of nanoshell, the LSPR peak of nanoshell-1 is at about 610 nm, the LSPR peak of nanoshell-4 is at about 1050 nm, as shown in Figure 3b. LSPR is pivotal for acquiring strong Raman enhancement at single particle level. UV−vis spectras of nanowaxberry with different nanoshell geometry cover a broad range from visible to near infrared ray region due to the 13,39 multimodal couplings. XRD shows the diffraction peaks of Au standard pattern (Figure s4). It is clear that five diffraction peaks at 2θ of 38.3°, 44.6°, 64.7°, 77.5° and 82.4°, which are corresponding to the (111), (200), (220), (311) and (222) lattice plane of face centered cubic (fcc) Au respectively. Evaluating SERS Performance of Nanowaxberry. The SERS performance is influenced by the geometry of nanostructure consequentially. Our nanowaxberry is constituted by PDA core and 3D Au shell. To study the signal sensitivity and reproducibility of nanowaxberry with different shell geometry, a systematic SERS property evaluation was carried out using CV molecule as Raman probe. Nanoshells

(nanoshell-1, nanoshell-2, nanoshell-3, and nanoshell-4) with different 3D dimensions and surface roughness were used for evaluation. The colloidal solution of nanowaxberry was dropped onto Si slices without special treatment, making sure well dispersion of nanoparticles. We tested the SERS performance of 20 randomly distributed nanoparticles at 633 nm excitation laser, the results are shown in Figure 3c and Figure S5. The acquisition mode is carried out under CV con-6 centration of 10 M. Figure S5 a, b, c, d demonstrate the 3D waterfall plot of SERS spectra (nanoshell-1, nanoshell-2, nanoshell-3, nanoshell-4) obtained from 20 random points, which dis−1 plays uniform Raman intensity at 1172, 1371 and 1619 cm . −1 According to statistic of CV peaks at 1172 cm , the calculated RSD of the Raman intensity is about 13.0 % for nanoshell-2, about 11.0% for nanoshell-3 and about 10.0 % for nanoshell-4. The signal intensity also increases from nanoshell-1, nanoshell-2 to nanoshell-3, as Figure 3 c shows. The limit of detection (LOD) of MG using nanoshell-3 as a substrate is about 1 pM. For the 3D nanostructure, if large numbers of gaps, bridges or crevices are generated into a single particle, the density of hot spots could be increased greatly, an improved sensitivity and reproducibility of SERS signal may be 10 acquired. We also find the nanoshell-4 (Figure 3a and S3) displays coalesced surface, its SERS intensity shows a certain degree of decline, this result is quite similar with the previous 27 study. In this study, growth of Au particles forms interconnected Au networks, and coalesced Au networks form the shell morphology, which could generate strong local EM field 40 enhancement and excellent SERS activity. The intensity of the SERS signal varies with the 3D dimensions and surface roughness of nanowaxberry. When the surface roughness increased, the higher SERS intensity would be generated. According to the above study, rough surface geometry hosting dense hot spots in 3D space is pivotal for achieving super 10 signal enhancement and uniform signal response. And rough shell structure has larger specific surface area, which could improve adsorption capacity of molecules and benefits


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Analytical Chemistry

Figure 4. SERS spectra of thiram at different concentrations spiked in (a) ultrapure water and (b) linear calibration plot between SERS intensity and concentration; Thiram detections spiked in (c) environmental water, (d) juice, (e) apple peel and (f) soil are shown. Curves (i) and (ii) in Figure (c, d, e, f ) represent the SERS spectra of thiram at different concentrations spiked in environmental water, juice, apple peel and soil respectively. Curve (iii) represents the background Raman spectra. 41,42

the SERS detection. The test also found that the signal to noise ratio was far greater than three, as Figure 3 c shows. The noise response was mainly derived from surfactant of nanoparticles, background noise of equipment. SERS Analysis of Pesticides, Pollutants and Explosives in Complex Samples. In order to protect food security, environmental security and human health, the detection of pollutants, pesticides and explosives in real samples is neces1-3 sary. Background or impurity signal of real samples gener5 ates great interference for target detection. Because of narrow linewidth of SERS spectra, it can detect targets within 7, 17 complex samples. According to distinct vibrational fingerprints, specific analytes would be detected. Furthermore, owing to the high SERS sensitivity and excellent reproducibility, nanowaxberry is a kind of excellent substrate, which can be used for detections of pesticides, pollutants and explosives in complex samples.

Thiram is a kind of pesticides, which is harmful for human 21 health and environment security. It was detected in complex samples employing nanowaxberry in this research. The colloidal solution of nanowaxberry was mixed with solution of analytes, and the mixture was dropped onto the Si slices, (Figure S6 and S7) which was kept in petri dish for dry in air. The linear calibration plot between the SERS intensity and concentration is illustrated in Figure 4a, b, and the LOD is 0.5 nM. The detections of thiram spiked in environment water, juice, apple peel and soil are also obtained, as shown in Figure 4c, d, e, f. The LODs of thiram residues spiked in environmental water and juice are about 10 nM. As shown in Figure 4e, the detection of thiram residues at apple peel sur2 face is acquired, the LOD is about 5.0 ng/cm . There are also -1 some weak background Raman bands, such as 1157 cm or -1 1500 cm , which were assigned to the carotenoids or saccha21 rides, however, we acquired credible SERS signal of thiram



molecules. In recent study, researchers have developed different SERS substrates, which has been used to detect pesticides. For self-ordered dense Au nanoparticle arrays, the 2 43 LOD of thiram at fruit peels is 19 ng/cm , this substrate has improved reproducibility and sensitivity of SERS signal. Three dimensional chestnutlike Ag/WO3−x SERS substrate 2 21 with the LOD of thiram at 6.1 ng/cm , the three dimensional substrate has high-density hotspots along the third dimension direction, which benefits enhancement of SERS signal. Gecko-inspired nanotentacle SERS platform was also used for 23 thiram detection at apple peel, the LOD is 1.6 ng/cm . Moreover, the thiram residue contained in soil is hugely 44, 45 harmful for environment and human health. Composition of soil is very complex, including various particulate matter, clay mineral, organic matter and so on, sample ex46 traction is time consuming, and the detection of thiram 44,45 residue in soil sample is difficult. As shown in Figure 4f, the tests of soil samples are acquired, the LOD of thiram is 0.31 μg/g in soil sample. Unlike the pure thiram solution, soil -1 samples produce interference signal, such as 1370 cm or 1550 -1 cm , and these Raman bands may come from the organic carbon impurity contained in environmental water or soil. But the main characteristic peaks of thiram were detected clearly in these detections, and the LODs are much lower than the maximum residue limits of thiram (0.1−5.0 mg/kg) 1 permitted by China and European Union. The SERS detections using nanowaxberry show excellent anti-interference ability, stable signals and lower LOD compared with other 47, 48 methods. Benzidine is a kind of hypertoxic pollutants, mostly occur49 ring in environment or industrial wastewater. It is a kind of strong carcinogen, which could be absorbed into the human body, causing a series of skin, respiratory and gastrointestinal 49 tracts illness. We have employed nanowaxberry for practical SERS detection of benzidine in soil samples and environmental water, as shown in Figure S8 and Figure S9. The solution was dropped onto the Si slices, which was kept in petri dish for dry in air, the LOD of benzidine is 100 nM (0.018 ppm) in environmental water, and it is lower than the maxiimmum level permitted by American and Chinese Environ49 ment Agencies (0.200 ppm) in drinking water. Moreover, -6 the LOD of benzidine is 10 M in drinking water by the duri49 an-like Fe3O4-Au substrate, nanowaxberry shows good SERS activity for benzidine detection. SERS detection of benzidine dissolved in ultrapure water and its solid Raman spectra are also shown in Figure S10 and Figure S11. Moreover, because of increased terrorist activities, dangerous explosives, such as DNT leads to well-known security 50 threat, health threat and environmental concern. In this research, we also detect dangerous DNT spiked in environmental water and methanol as shown in Figure S12, and its solid Raman spectra is shown in Figure S13. The detection of MG spiked in environmental water is also performed, as shown in Figure S14. The all detailed Raman band assignment is shown in Table S1, S2, S3 and S4. For the SERS detection of above small molecules, background fluorescence or impurity signal could generate interference. The nanowaxberry with plenty of nanovoids and nanogaps increases specific surface area and benefits the adsorptions of small molecules into “hot spot”, and larger impurity contained in environmental water or soil may be difficult to diffuse into nanospace, and these small molecules

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could be allowed to diffuse into “hot spot” zone. Then stronger Raman signal of these target molecules could be acquired. Usually different pollutants are concomitant in various 3 practical samples, which generates serious threat for food and environmental safety. Therefore, developing a sensitive detection method is necessary. Owing to the narrow linewidth of SERS spectra, it is efficient for multi-component detection in practical samples. Thus, the SERS detection based on nanowaxberry is employed to detect thiram and benzidine mixtures spiked in environmental water and soil, as shown in Figure S15 and Figure 5 respectively. Three bands −1 −1 −1 at 1189 cm , 1289 cm and 1600 cm are of benzidine; Two −1 −1 bands at 1370 cm and 1134 cm are of thiram; As Figure 5 and Figure S15 show, characteristic bands of each component can be discerned obviously. The results prove that our nanowaxberry is a kind of powerful SERS substrate, which can be used for detection of targets within complex samples. Furthermore, owing to the excellent biocompatibility and tunable optical property ranging from visible to near-IR region, nanowaxberry is desirable for biology and catalysis applications.

Figure 5. Dual-analyte detection. SERS spectrum of dualanalyte (Curves i) spiked in soil. SERS spectra of thiram (Curves ii) and benzidine (Curves iii) spiked in soil respectively. CONCLUTION In summary, we developed nanowaxberry to detect analytes in complex samples. This nanowaxberry holds numerous gaps and voids in 3D space, which could generate highdensity hot spots, maximally improving the SERS property. Seed-mediated synthesis was employed for preparation of nanowaxberry. In the first growth step, Au seeds are deposited on the surface of PDA sphere, then I ion coordinating ligand is employed to form stable AuI4 complex with AuCl4 in the second growth process, which decreases reduction potential of AuCl4 and avails growth of Au shell. The preparation method of nanowaxberry can be potentially used for other metal nanomaterials. Meanwhile, the nanowaxberry with high SERS sensitivity and excellent reproducibility can be employed for trace detection of thiram, benzidine, DNT and MG in complex samples, such as environmental water, juice, apple peel and soil samples. We believe that nanowaxberry offers a great potential for pesticides detection, pollutants monitoring and forbidden explosives sensing in complex samples.


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Analytical Chemistry ASSOCIATED CONTENT

Supporting Information Supplementary Figures and Data are of included. AUTHOR INFORMATION

Corresponding Author E-mail: [email protected].

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank Yu Wang from Instrument Analysis Center of Xi’an Jiaotong University for helping us to complete the Raman detection. The authors thank the recommendation of professor Zuankai Wang. We also thank Senior Engineer Ruihua Zhu and Yanhuai Li for TEM measurements. We thank Engineer Qiong Li for AFM measurement. This research was financially supported by “Young Talent Support Plan” of Xi’an Jiaotong University and the National Natural Science Foundation of China (Grants No.21475102 and 31671013).

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