Fabrication of Scale-like Silver-Nanosheets-Grafted Carbon-Fenced

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Fabrication of Scale-like Silver-Nanosheets-Grafted Carbon-Fenced Conductive Silver Nanowires as Effective 3D SERS Substrates Xingying Zhang,†,§ Yalun Mo,†,§ Ben Liu,† Chenglong Hu,*,† Shaoyun Chen,*,† Hong Shi,† and Jian Chen‡ †

Key Laboratory of Optoelectronic Chemical Materials and Devices, Ministry of Education, School of Chemical and Environmental Engineering, Jianghan University, Wuhan 430056, China ‡ Instrumental Analysis and Research Center, Sun Yat-sen University, Guangzhou 510275, China

ACS Appl. Nano Mater. Downloaded from pubs.acs.org by 191.96.170.178 on 09/02/18. For personal use only.

S Supporting Information *

ABSTRACT: The Ag-nanosheets (i.e., AgNST)-grafted carbon-fenced conductive Ag nanowires with 3D scale-like nanostructure was successfully fabricated through a direct chemical reduction method, where the carbon shell was first carbonized onto the suface of Ag nanowires to form an Ag nanowires@carbon (i.e., AgNWs@C) framework, and then the citrate acid which absorbed on the prefabricated AgNWs@C@Ag seeds framework directed Ag ions to be Ag nanosheets to selfassemble into 3D scale-like nanostructures (i.e., AgNWs@C@AgNST). The fabricated nanostructure displayed the sensitive and reproducible surface-enhanced Raman scattering (SERS) responses. The SERS detection for the probing molecule (rhodamine 6G, R6G) with concentration as low as 10−12 mol/L was obtained in a reproducible way, and the average enhancement factor was about 2.33 × 105, which was calculated from the band of 610 cm−1 in the SERS spectrum. It was caused by Ag nanosheets with high density providing abundant “hot spots” to enhance the SERS detection. Besides, the potential SERS detection of glucose molecules in the micromolar range was also reported. KEYWORDS: surface-enhanced Raman scattering, Ag-nanosheets, rhodamine 6G, glucose

1. INTRODUCTION Surface-enhanced Raman scattering (SERS), a nondestructive detecting technique, can be used to analyze unknown samples or detect small molecules with low concentration. It has been reported that the SERS effect is related to localized surface plasmon resonance (LSPR),1−4 and the gaps between adjacent nanostructures are regarded as “hot spots”.5−7 It is well-known that the hot spots are associated with the size, morphology, and aggregation state of nanostructures in most SERS-active materials.8−10 Recently, the periodic nanostructure as one kind of SERS substrate can provided high concentration hot spots for SERS detecting with moderate sensitivity and high reproducibility.11−15 However, the periodic nanostructures as large-area SERS substrates are often achieved by timeconsuming and expensive nanometer-scale lithography techniques. To get inexpensive periodic large-area SERS substrates, Meng et al. reported that the periodic nanostructure can be fabricated by porous anodic aluminum oxide template.16−18 © XXXX American Chemical Society

Therefore, although plenty of SERS substrates have been exploited, the preparation of well-defined and reproducible SERS-active substrates using normal chemical and templatefree synthesis is still greatly desirable. To develop other novel nanostructures to replace periodic SERS substrates, various silver (Ag) and gold (Au) micro-/ nanostructures with large specific surface area and rough surface, such as 3D hierarchical flower-like,19−23 sea urchinlike,24−28 and assembling nanosheets29−35 are designed to be SERS-active substrates. For example, Yüksel et al. reported that Ag or Au 3D flower-like nanostructure can enhance the density of hot spots to further amplify the SERS signal, and it can be used as an ultrastable and reproducible SERS substrate.19 Ye et al. reported that the Fe3O4@C@Ag 3D sea-urchin-like Received: June 12, 2018 Accepted: August 23, 2018 Published: August 23, 2018 A

DOI: 10.1021/acsanm.8b00993 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Scheme 1. Process for Fabrication of AgNWs@C@AgNST with 3D Scale-like Nanostructure: Step I, AgNWs@C Composite Synthesized by Hydrothermal Synthesis; Step II, Silver-Nanoparticles Grown onto Surface of AgNWs@C; Step III, SilverNanosheets Assembled onto the Surface of AgNWs@C Framework

It means that the as-prepared 3D SERS substrate shows promising potential in SERS detection of organic molecules.

nanostructures as SERS substrate can effectively detect the organic pollutants in solution.24 It can be found that the 3D hierarchical Ag or Au micro-/nanostructures can play a good role in SERS detecting and practical application. It is attributed to the fact that the 3D hierarchical micro-/nanostructures can offer a large specific surface area with a large number of hot spots and adsorb plenty of analyte molecules. In addition, previous studies showed that LSPR with large electromagnetic enhancement can be tuned by Ag nanosheets, which are carried out by the sharp corners and nanojunctions between the neighboring sheets in the Ag nanosheets.29 Therefore, the Ag-nanosheets-assembled systems can exhibit a strong SERS effect. Wang et al. showed that Fe3O4@SiO2@Ag nanostructure with silver nanosheet shell exhibits signals enhancement using R6G (10−14 M) as a probe molecule.30 Gao et al. have used Cu plate as a support substrate to fabricate homogeneous Ag-nanosheet-assembled film with good reproducibility and sensitivity.31 Based on the above discussion, assembling Agnanosheets onto one framework should be a feasible method to prepare a 3D SERS substrate with a large specific surface area and plenty of hot spots.36 Herein, we report a normal chemical and template-free approach of fabricating well-defined Ag nanosheets grafted onto decorated silver nanowires (i.e., AgNWs) to exploit SERS detection. Qian et al. showed that the Ag nanosheets can graft onto the surface of polyamide nanofibers to form 3D networked structure by electrospinning, and the high density of hot spots can be produced from the nanoscaled gaps between the neighboring Ag nanosheets.32 Ye et al. reported that the carbon shells can protect the Fe3O4 cores to the benefit of Ag nanosheets growth.24 Therefore, based on the above discussion and our previous studies,37−40 as illustrated in Scheme 1, the AgNWs are formed by the ethylene glycol method, and then the carbon shells that carbonized from glucose form a AgNWs@carbon (i.e., AgNWs@C) framework via hydrothermal synthesis. Subsequently, the assembling of Ag nanoparticles onto the carbon surface of the AgNWs@C framework acts as seeds for the growth of vertical Ag nanosheets. The as-synthesized freestanding Ag-nanosheetsgrafted AgNWs@C framework with a high density of hot spots and a large specific surface area can be used as an effective SERS substrate for detection of target analytes. For instance, the SERS detection of rhodamine 6G (i.e., R6G) and glucose aqueous solution used for as-prepared 3D scale-like nanostructures is low to 10−12 mol/L and 0.001 mol/L, respectively.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Ammonium sulfate ((NH4)2S2O8, APS), silver nitrate (AgNO3), rhodamine 6G, glucose, citrate acid, ascorbic acid, and ethylene glycol (EG) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Poly(vinylpyrrolidone) (PVP, MW = 40000 and MW = 1300000) was obtained from Aladdin Chemistry Co., Ltd. (Shanghai, China). 2.2. Preparation of Silver Nanowires (i.e., AgNWs). A 0.2 g amount of PVP (MW = 1300000) was added into 12.5 mL of EG solution, which was mixed uniformly with stirring under 60 °C. A 3.0 mL aliquot of FeCl3 (600 μmol/L) EG solution was injected into PVP-EG solution, which had been previously magnetic stirred under 115 °C for 0.5 h. Subsequently, 0.12 M AgNO3 was gradually added into the PVP-EG solution to react for 15 h under 115 °C. The obtained AgNWs were separated by centrifugation at 3500 rpm and washed 4−5 times with alcohol and acetone, respectively. 2.3. Preparation of Carbon-Fenced Conductive Silver Nanowires (i.e., AgNWs@C). AgNWs@C was synthesized by solvothermal method with glucose as a source of carbon shell. The typical synthesis is as follows: 20 mL of 1.0 × 10−3 M glucose solution was gradually added into 20 mL of AgNWs (0.08 g) dispersed solution, and then the solution was moved into an autoclave to react for 3.5 h at 180 °C under autogenetic pressure. AgNWs@C was obtained by centrifugation at 3000 rpm and washed 4−5 times with re-distilled water. 2.4. Synthesis of AgNWs@C@Ag Seeds. The obtained AgNWs@C (0.08 g) was immersed in Tollens’ reagent (0.2 g of AgNO3, 1.5 mL of NH3·H2O, and 20 mL of ethanol) for 30 min in a sealed flask, then 20 mL of PVP ethanol solution (0.5 g, MW = 40000) was added to the above solution to react for 12 h at 60 °C under magnetic stirring, and then AgNWs@C with Ag seeds were obtained. 2.5. Preparation of AgNWs@C@Ag Nanosheets (i.e., AgNWs@C@AgNST). AgNO3 (0.2 g) and citrate acid (0.2 g) were first dissolved into deionized water (30 mL), and then the AgNWs@ C@Ag seeds (0.012 g) were dispersed into the above mixed solution with magnetic stirring and under sonication. Subsequently, the ascorbic acid solution (0.1 g, 10 mL) was gradually added into the mixed solution. The obtained sample was successively washed by deionized water and ethanol many times to remove citrate acid and ascorbic acid from Ag nanosheets (pH = 7.0),31,41 and dried at 60 °C in a vacuum oven. The optical images of the AgNWs, AgNWs@C, and AgNWs@C@AgNST can be seen in Supporting Information Figure S1. 2.6. Characterization. The obtained products were analyzed by X-ray diffraction (XRD, Panalytical X’pert powder diffractometer), scanning electron microscopy (SEM, Hitachi SU8010), transmission electron microscopy (TEM, JEOL-2010), X-ray photoelectron B


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Figure 1. SEM images of (a) AgNWs, (b) AgNWs@C, (c) AgNWs@C@Ag seeds, and (d) AgNWs@C@AgNST with 3D scale-like nanostructure.

Figure 2. Single magnified (a) SEM and (b) TEM images of the AgNWs, AgNWs@C, AgNWs@C@Ag seed, and AgNWs@C@AgNST with 3D scale-like nanostructure. spectroscopy (XPS, Thermo Fisher Scientific, K-ALPHA+), and UV− vis absorption spectrum (PerkinElmer, USA). 2.7. SERS Measurements. For SERS measurement, AgNWs, AgNWs@C, and AgNWs@C@AgNST were first dispersed into ethanol and then drop-coated onto the surface of a glass slide to dry in a vacuum oven at 60 °C. The optical images of as-prepared substrates can be seen in Figure S1. The SERS probe is an R6G molecule. The silver substrates were immersed into 50 mL of R6G aqueous solution for 2 h (the concentrations of R6G are 10−5 to 10−12 mol/L) and completely dried in air. It can be found that the adhesiveness between the glass and the hybrid composites can support the SERS detection, as shown in Figure S2. A confocal microscopy Raman spectrometer (Renishaw Invia, 532 nm laser line, 50× and 100× objectives) was used to measure. The acquisition time was 1 s for each single

spectrum using a static measurement model, and the radius of the illumination spot is about 2.5 μm. The SERS mapping was obtained using the static measurement model (the acquisition time was 3 s, accumulation was 1 time, and the scanning step was 1.2 μm), and the illumination spot (0.8−1.0 μm2) is shown in Figure S3. To show the SERS application in biological detection, the AgNWs@C@AgNST substrates were incubated into a glucose aqueous solution (0.001−0.1 mg/mL, pH ≈ 7.0) and melamine aqueous solution (0.01−1.0 mg/ mL) for 2 h, and then dried in N2. The acquisition time was 10 s for each single spectrum using a dynamic measurement model. The Raman mapping was obtained using static measurement model, and the acquisition time was 5 s, accumulation was 1 time, scanning step was 1.2 μm, and the measuring area was 80 × 80 μm2. C


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3. RESULTS AND DISCUSSION Figure 1 shows the typical SEM images of the as-prepared samples. The diameter of AgNWs is about 100−120 nm (Figure 1a). The carbon shell is carried out by a hydrothermal process, and the thickness of the carbon shell can be easily tuned by varying the concentrations of glucose aqueous solution. Herein, the as-prepared AgNWs with a carbon shell (interlayer) of about 50 nm thickness are selected to assemble Ag nanosheets in a 3D scale-like manner, and a transparent outer layer in Figure 1b implies the successful coating of carbon around the AgNWs cores. The Ag-seeds-modified AgNWs@C can be formed via the strong absorptive interaction of carbon shell, and a large number of welldispersed and uniform Ag seeds with a diameter of about 10 nm are observed in Figure 1c. Then, the Ag nanosheets are vertically assembled onto the Ag-seeds-modified AgNWs@C in the mixture solution of citric acid, ascorbic acid, and AgNO3 aqueous solution under the guidance of Ag seeds grafted onto the AgNWs@C surface. As shown in Figure 1d, the Ag nanosheets can be well-grafted vertically onto the surface of each AgNWs@C. Also, the whole diameter of the AgNWs@ C@AgNST is from 550 to 750 nm. A single magnified SEM image of all the as-prepared samples is seen in Figure 2a. The morphology evolution related with AgNWs, AgNWs@C, AgNWs@C@Ag seeds, and AgNWs@ C@AgNST is in accordance with Scheme 1. For AgNWs@C@ AgNST, the Ag nanosheets entangle with each other and fully cover the AgNWs@C to form a porous surface structure. More importantly, the fan-shaped structure of the Ag nanosheets is obviously observed and nanoscale cracks (or nanogaps) are obstinately present in all parts. It means that a large number of nanospaces are formed via the interstitial sites of Ag nanosheets. Previous reports have shown that the nanoscaled cracks between the neighboring nanosheets can produce the hot spots, which provides an enormous electromagnetic enhancement based SERS effect.42,43 Therefore, the crosslinked Ag nanosheets can lead to a larger number of hot spots to produce high activity for SERS detection. Figure 2b is the magnified TEM images of all of the as-prepared samples, which can also further prove the fabricated process of AgNWs@C@ AgNST with 3D scale-like nanostructure. The XPS spectra, element mapping, and EDX spectroscopy of as-prepared AgNWs, AgNWs@C, and AgNWs@C@AgNST with 3D scale-like nanostructure can be seen in Figure S4−Figure S6. Park et al. reported that the thickness of the interlayer between Ag nanoparticle and Ag nanowire plays an important role for plasmonic coupling in contributing to the highly enhanced plasmonic resonance.44,45 Based on this thesis, the thickness effect of carbon shell interlayer on the plasmonic coupling can be found in Figure 3. The thickness of the carbon shell interlayer is varied from 25 to 70 nm by varying the concentrations of glucose aqueous solution (the inset of Figure 3), and the reflectance spectra of as-prepared AgNWs@C@ AgNST substrates with different thicknesses of carbon shell can be seen in Figure 3 (the substrates are fabricated using the same method). It reveals that the reflectance dip of as-prepared samples blue shifts with the increase of interlayer thickness. It means that the coupling effect between an optical AgNWs and AgNST can be reduced due to their separation and the blue shift of the plasmon resonance appearing in the reflectance dip.46 The result is in accordance with the previous theoretical study.46

Figure 3. Reflectance spectra of AgNWs@C@AgNST with 3D scalelike nanostructure having carbon shell interlayer with thicknesses of about 25, 50, and 70 nm. The insets are TEM images of AgNWs@C nanostructure with different thicknesses of carbon shell interlayer.

The UV−vis spectra of AgNWs, AgNWs@C, and AgNWs@ C@AgNST with 3D scale-like nanostructure were obtained from their alcohol colloids, as shown in Figure 4. It is clear that

Figure 4. UV−vis spectra of the AgNWs, AgNWs@C, and AgNWs@ C@AgNST with 3D scale-like nanostructure from their alcohol colloids. The inset is the specimen cell of UV−vis measuring.

the AgNWs mainly have two surface plasmon resonance (SPR) bands which are attributed to the transverse mode (374 nm) and the longitudinal mode (350 nm), respectively. The absorption spectrum of AgNWs@C cannot be found due to the barrier of the carbon shell. For the AgNWs@C@AgNST, the peaks of 400 and 360 nm are found in the ultraviolet region, which is ascribed to the quadrupole resonance and outof-plane dipole, respectively.47 The peaks of AgNWs@C@ AgNST are broadened and red-shifted because the collective SPR of metal has been induced by the interparticle interaction of Ag-nanosheets aggregations. It can be verified by the SEM and TEM images in Figure 2. Moreover, the band around 400 nm in AgNWs@C@AgNST is much closer to the excitation wavelength of the laser (e.g., 532 nm) compared with AgNWs (the band around 374 nm), and then we expect that Agnanosheets aggregations should have a potential application in molecule detecting by SERS. The XRD curves of the AgNWs, AgNWs@C, and AgNWs@C@AgNST with 3D scale-like are also shown in Figure S7. The changed intensity of diffraction D


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Figure 5. (a) Process of the selected AgNWs, AgNWs@C, and AgNWs@C@AgNST with 3D scale-like nanostructure. (b) SEM images of the selected AgNWs, AgNWs@C, and AgNWs@C@AgNST with 3D scale-like nanostructure. (c) SERS spectra of R6G adsorbed on AgNWs, AgNWs@C, and AgNWs@C@AgNST with 3D scale-like nanostructure (CR6G = 10−5 mol/L). (d) RSD of peak intensity at the band of 610 cm−1 for 30 randomly selected positions (the concentration of R6G is 10−5 mol/L).

Figure 6. (a) SERS spectra of R6G adsorbed on AgNWs@C@AgNST with 3D scale-like nanostructure with different concentrations. (b) SERS component mapping of R6G adsorbed on AgNWs@C@AgNST with 3D scale-like nanostructure (CR6G = 10−5 mol/L). (c) SERS intensity mapping of R6G adsorbed on AgNWs@C@AgNST with 3D scale-like nanostructure at band of 610 cm−1 (CR6G = 10−5 mol/L). The SERS mapping was obtained using a static measurement model, and the acquisition time was 3 s, accumulation was 1 time, the scanning step was 1.2 μm, and the measuring area was 100 × 100 μm2. (d) Frequency counts of SERS signals at peak of 610 cm−1 in the 6910 measured spectra.

peaks from curves gives a fact that the growth of the carbon shell around the surface of AgNWs and Ag-nanosheets grafting onto the surface of AgNWs@C framework, coinciding with the above discussion.

The actual SERS effects of AgNWs, AgNWs@C, and AgNWs@C@AgNST is evaluated using R6G as model Raman probes. To ensure the reliability of the measurement, the as-prepared AgNWs, AgNWs@C, and AgNWs@C@ E


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Figure 7. (a and b) SERS mapping of the selected single AgNWs@C@AgNST with 3D scale-like nanostructure. (c) SERS mapping of multiple AgNWs@C@AgNST with 3D scale-like nanostructure.

detected. The results show that the as-prepared AgNWs@C@ AgNST with 3D scale-like nanostructure can be applied in the ultratrace detection of probing molecules, and tracking of other ultratrace organic molecules. Additionally, the streamline Raman mapping is carried out on a randomly selected 100 × 100 μm2 area with a total of 6970 measurement points on a AgNWs@C@AgNST substrate (the concentration of R6G is 10−5 mol/L). Therefore, the SERS component mapping of R6G can be seen in Figure 6b. A large amount of R6G molecules can be well-adsorbed on the AgNWs@C@AgNST nanostructure. However, the SERS intensity of R6G molecules at 610 cm−1 shows a wide diversity at different locations (Figure 6c). It may be attributed to two reasons: (i) Some R6G molecules may locate at the spots that are not so hot; (ii) the overlapping of R6G molecule may occur at high concentration area. It clearly observes that the strong SERS signal can appear at low concentration area and the weak SERS signal occupies at the high concentration area. Moreover, the reproducibility of AgNWs@C@AgNST with 3D scale-like nanostructure as SERS substrate must be evaluated. The distribution of SERS intensity at the band of 610 cm−1 is shown in Figure 6d. It can be found that the RSD of the main peak is less than 5%, revealing the good reproducibility of SERS signals across the large area of the AgNWs@C@AgNST nanostructure. The enhancement factor (EF) is about 2.33 × 105 at a peak of 610 cm−1. The calculated details are shown in the Supporting Information (Part S2). Based on the above description, the SERS activity of AgNWs@C@AgNST with 3D scale-like nanostructure comes from the stacked AgNWs@C@AgNST configuration. To verify the SERS effect of single AgNWs@C@AgNST, the optical images are observed through an 100× objective lens, and the SERS mapping of single AgNWs@C@AgNST compared with that of the 3D assembled system is shown in Figure 7a,b. The EF value of the randomly selected single AgNWs@C@AgNST at the vibrational peak of 610 cm−1 is about 0.92 × 105 and 1.05 × 105, respectively. In addition, the multiple AgNWs@C@AgNST with the cross-points of vertically stacked and in-plane nontouching nanostructure is

AgNST with the similar stacking layers and density are selected to SERS measurement. The process of the selected samples is shown in Figure 5a, and the corresponding SEM images can be found in Figure 5b. It can be found that peaks from 200 to 2000 cm−1 in the curve belong to R6G signals, as shown in Figure 5c. The band at 610, 722, 1365, and 1650 cm−1 is assigned to in-plane modes of the C−C−C ring, the out-ofplane bending modes of hydrogen atoms in xanthene skeleton, and C−C stretching of the aromatic ring, respectively.48,49 These peaks are four of the main characteristic bands to study the different SERS effects. Obviously, all of the as-prepared samples show the four peaks, and AgNWs@C@AgNST exhibits the best performance compared with AgNWs and AgNWs@C. Based on the above discussion, a cross-linked Agnanosheets aggregation is found over all of the shell of the AgNWs@C composite, and the nanoscale cracks are observed in the Ag nanosheets, as shown in Figure 2. The cross-linked Ag nanosheets can provide a high density of nanogaps and exhibit better SERS activity compared with that of the nanoparticle-assembled or bare nanowire. Therefore, it is widely believed that the SERS enhancement can take place when a R6G molecule is adsorbed within the cracks between two closely neighboring Ag nanosheets.32 Additionally, it is expected that the SERS intensity of the R6G molecules adsorbed onto AgNWs@C dramatically decreases due to the barrier of the carbon shell. Figure 5d shows that the relative standard deviation (RSD) associated with the SERS intensity of AgNWs, AgNWs@C, and AgNWs@C@AgNST at a band of 610 cm−1 is 2.63%, 3.42%, and 3.22%, respectively. It means that the SERS signals of as-prepared samples will be uniform at different places. To further elaborate the SERS activity of AgNWs@C@ AgNST with 3D scale-like nanostructure, the SERS spectra of R6G molecules adsorbed onto the as-prepared 3D nanostructure with various concentrations can be found in Figure 6a. It is found that the SERS signals can be easily captured when the concentrations of R6G varied from 10−5 to 10−12 mol/L, and the sharp R6G peaks give a good signal-to-noise ratio, where R6G with a concentration of 10−12 mol/L can be easily F


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Figure 8. (a) SERS intensities of peaks at 610, 772, 1365, and 1650 cm−1 dependent on R6G loading. (b) Relationship between SERS intensity and R6G loading revealed by the logarithmic plot.

Figure 9. (a and b) Optical images of the capital letter “A” engraved onto the AgNWs@C@AgNST substrates of unabsorbed and adsorbed R6G molecules. (c and d) Capital letter A imaged using normal Raman mapping and SERS mapping, respectively. (e and f) Corresponding Raman spectra and SERS spectra at different points.

also selected to SERS measurement, as shown in Figure 7c. The EF value is about 2.15 × 105, which is bigger than that of a single AgNWs@C@AgNST and corresponding with the results of 3D assembled AgNWs@C@AgNST system (EF = 2.33 × 105). It is attributed to the fact that a LSPR can be induced in the intervals between crossed AgNWs@C@AgNST ((i) of Figure 7c) and also in the in-plane nontouching AgNWs@C@AgNST ((ii) of Figure 7c).44 It has been recognized that a large number of hot spots can be obtained when the Ag nanosheets are arranged to the coupling effect of plasmonic fields of adjacent nanosheets in a special way. To get a higher density of Ag-nanosheets on the AgNWs@C framework, a series of the AgNWs@C@AgNST nanostructures are synthesized by tuning the amount of citrate acid from 0.025 to 0.3 g in the reactive solution, as shown in Figure S8. Previous study showed that the citrate acid as a morphology controlling reagent could inhibit the growth of silver plates via changing its concentration. It can be seen that the amount of citrate acid can affect the morphology of silver

units. There is no obvious change observed when 0.6 g of citrate acid is added. Especially, the irregular and unordered silver fragments are detected with 0.025 g of citrate acid being used. It can be explained that the facet-inhibiting effect of citrate acid has been weakened along with decreasing the concentration of citrate acid.30,50,51 Moreover, the RSD associated with SERS intensity of the AgNWs@C@AgNST with different concentration of citrate acid is 4.12%, 3.22%, 15.82% and 27.64%, respectively (Figure S9). The result implies that the AgNWs@C@AgNST with 3D scale-like nanostructure synthesized at the amount of citrate acid of 0.2 g can provide the largest number of Ag nanosheets and the highest density of hot spots to promote SERS performance. The relationship between the SERS intensity of at bands of 610, 772, 1365, and 1650 cm−1 and the concentrations of R6G is plotted in Figure 8a, which derives from the Figure 6a. It reveals that the first-order adsorption kinetics can wellinterpret the relationship between SERS intensities (ISERS) of R6G and R6G loading (CR6G). By applying the Lorentz fitting G


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Figure 10. (a) Prepared process of samples for label-free detection of glucose molecules using SERS measurement. (b) SERS spectra of glucose adsorbed on AgNWs@C@AgNST with 3D scale-like nanostructure with vary concentrations. (c) SERS intensity mapping of glucose at band of 1127 cm−1 (Cglucose = 0.1 mg/mL). The SERS mapping was obtained using a static measurement model, and the acquisition time was 5 s, accumulation was 1 time, the scanning step was 1.2 μm, and the measuring area was 80 × 80 μm2.

(Figure 8b), the correlation function between ISERS and CR6G can be calculated as follows: log ISERS = 4.73 + 0.13 log C R6G log ISERS = 4.46 + 0.13 log C R6G

log ISERS = 5.05 + 0.18 log C R6G

(610 cm−1) (772 cm−1)

Subsequently, the as-prepared AgNWs@C@AgNST nanostructure is used for detection of glucose molecules. The glucose can act as a biomarker of diabetes to evaluate the level of glucose in blood. The concentration of glucose is less than 70−150 mg/mL for a normal person, but the value is more than 150 mg/mL for diabetics.54 To obtain SERS of glucose spectra, the AgNWs@C@AgNST substrate was incubated into a glucose aqueous solution of defined concentrations (0.001− 0.1 mg/mL, pH ≈ 7.0) for 2 h, and then washed by pure water and dried with N2, as shown in Figure 10a. The SERS spectra of glucose adsorbed onto the substrate are measured from dried AgNWs@C@AgNST substrates. We examined its SERS sensitivity to glucose with various concentrations (0.001, 0.01, and 0.1 mg/mL), and corresponding SERS spectra are shown in Figure 10b. Several peaks can be searched in the SERS spectra of glucose ranging from 200 to 2000 cm−1. It reveals that the as-prepared AgNWs@C@AgNST nanostructure as SERS substrate can detect glucose molecules with a high sensitivity even at a low concentration down to 0.001 mg/mL. The SERS intensity mapping of glucose adsorbed on the AgNWs@C@AgNST nanostructure is shown in Figure 10c. The RSD values of the SERS intensity at band of 1127 cm−1 do not exceed 30% (the concentration of glucose is 0.1 mg/ mL), revealing relatively good SERS reproducibility for detection of trace glucose molecules. It also confirms that the SERS mapping can be well-used in biological detection, according to the result of Figure 9. Additionally, the vibrational modes at 1455 and 1331 cm−1 are assigned to H−C−H deformational vibration; the bending mode of C−O−H can be found at band of 1127 cm−1; the band at 1073 cm−1 is assigned to C−H stretching mode; the band at 914 cm−1 is attributed to the O−C−H bending mode in a ring; and the band at 856 cm−1 is assigned to the C−C stretching mode. In addition, the exocyclic (510 cm−1) and endocyclic (420 cm−1) deformations can be found from 200 to 700 cm−1 in the skeletal region.55−57 In addition, the Raman spectrum and SERS spectra of melamine with varied concentrations of the aqueous solution (0.01−1.0 mg/mL) can be found in Figure S10.

(1) (2)

(1365 cm−1) (3)

log ISERS = 5.16 + 0.20 log C R6G

−1

(1650 cm ) (4)

R2 is about 0.955, 0.976, 0.978, and 0.948 for the selected peaks, respectively. It means that all R6G molecules adsorbed on AgNWs@C@AgNST with 3D scale-like nanostructure are enhanced equally due to the enough hot spots for these molecules when the concentration of R6G is lower than 10−5 mol/L.52,53 It can be explained by AgNWs@C@AgNST with 3D scale-like nanostructure can provide sufficient binding sites to efficiently capture of the analyte molecules from R6G aqueous solution.38 Recently, the vibrational imaging combined with the spectroscopic “fingerprint” analysis of Raman is rapidly developed which provides a powerful method for obtaining molecular information and phase structure for composites. Therefore, the high resolution of the SERS image is very important for fine structure analysis and local chemical identification. In this study, the normal Raman mapping and SERS mapping are carried out to identify their image quality, as shown in Figure 9. The AgNWs@C@AgNST substrates unabsorbed and adsorbed R6G molecules (CR6G = 10−5 mol/ L) are first prepared, and then the capital letter “A” is engraved onto both of the as-prepared substrates, as shown in the optical images (Figure 9a,b). The normal Raman mapping and SERS mapping are carried out using a 5× objective lens, and the acquisition time is 3 s. The Raman spectra are obtained ranging from 226 to 1974 cm−1 using the static measurement model, and the band at 610 cm−1 is chosen to be imaged. The movement steps of the X, Y stage for the imaging on the substrates are set at 25 × 12.5 μm2. Obviously, the image quality and resolution of the SERS mapping are much better than the normal Raman mapping at the same condition, as shown in Figure 9c,d. The corresponding Raman spectra and SERS spectra at different points are also listed in Figure 9e,f. The result confirms that the SERS mapping can be used to obtain chemical images with higher veritably and reliability. Thus, the SERS mapping can greatly promote the application in the field of biological detection and chemical materials.

4. CONCLUSION In summary, we have exploited an approach to produce the carbon-fenced conductive Ag-nanowires-decorated Ag nanosheets with 3D scale-like nanostructure. The morphology of silver units on the surface of the AgNWs@C framework can be adjusted by varying the amount of citrate acid. The AgNWs@ C@AgNST with 3D scale-like nanostructure synthesized at the amount of citrate acid of 0.2 g can provide the highest density H


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of Ag nanosheets. This 3D SERS substrate can not only provide a sufficient number of hot spots to enhance the sensitivity and reproducibility of SERS detection but also supply a large specific surface area to capture probing molecules to enlarge the detection limits of SERS. The results show that the low concentration of R6G (10−12 mol/L) and glucose (0.001 mg/L) aqueous solution can be easily detected, confirming the as-prepared AgNWs@C@AgNST with 3D scale-like nanostructure as SERS substrate can effective detect the target molecules. Additionally, the method of our adopting and fabrication protocol for the Ag-nanosheets growing will extend to other systems to design novel nanostructures for effective SERS substrates.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00993.

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Optical images, laser spot, XPS wide-scan spectra, mapping images, EDX spectroscopy, XRD curves, SEM images, SERS spectra, Raman spectra, and estimation of the enhancement factor (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.H.). *E-mail: [email protected] (S.C.). ORCID

Chenglong Hu: 0000-0003-1554-269X Author Contributions §

X.Z. and Y.M. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of the National Natural Science Foundation of China (Grant No. 21706092) and the support of the Hubei Province Natural Science Foundation of China (Grant No. 2018CFB520).

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Fabrication of Scale-like Silver-Nanosheets-Grafted Carbon-Fenced

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