Aligned Chemically Etched Silver Nanowire Monolayer as Surface-Enhanced Raman Scattering Substrates
© The Author(s). 2017
Received: 15 July 2017
Accepted: 30 October 2017
Published: 9 November 2017
Silver nanowires (AgNWs) were chemically etched to significantly increase the surface roughness and then self-assembled on the liquid/gas interfaces via the interfacial assembly method to obtain aligned chemically etched silver nanowire films. The as-fabricated silver nanowire films were used as novel surface-enhanced Raman scattering (SERS) substrates. The morphologies and plasmon characteristics of the substrates were investigated using multiple measurement methods. The performance of as-fabricated substrates was measured using rhodamine B as a probe. The detection limitation can be as low as 10−11 M. The greatly improved plasmonic properties are attributed to the efficient light coupling and larger electromagnetic field enhancement. The novel set of SERS substrates of aligned chemically etched AgNWs is believed to be important for efficient, homogeneous, and ultrasensitive SERS sensing applications.
Surface-enhanced Raman scattering (SERS) has received much attention over years as a sensitive, rapid, and noninvasive analytical technique in detection of molecules [1–3]. The Raman signal of the molecule can be increased by several orders of magnitude, especially when the molecule reside in the typical size of the field enhancement area, the so-called hot spot, which usually locate nearby sharp edges, roughened surfaces, or the junction between coupled nanometer-sized objects.
Silver nanowires (AgNWs) are an ideal SERS candidate for large surface areas and high crystallinity [4, 5]. However, SERS hot spots are limited to the ends of the nanowires . Due to the fact that SERS hot spots are confined to a small area but dominate the overall SERS intensity of the substrate, the distribution of SERS intensity in a SERS substrate was found to be inhomogeneous, thus limiting their application as reproducible and ultrasensitive sensing platforms.
Studies have shown that when two nanowires are extremely close to each other, the electromagnetic fields at the gap region between the nanoparticles are drastically increased. Many computational models have predicted that large electromagnetic (EM) fields localized at the junction between metal structures [7, 8]. Tao and Yang  fabricated aligned films of silver nanowires and measured Raman intensities of probe molecule. The observed dependence on polarization direction confirms the theoretical predictions that large EM fields are localized in the interstitials between adjacent nanowires. Close-packed arrays of Ag nanowires can be easily obtained by assembly methods, including Langmuir–Blodgett [10, 11], layer-by-layer assembly [12, 13], external field [14–16], liquid–liquid interface [17, 18], and so on. Combining simple, high-yield, and good orientation regularity, the method is used to tunable control the gap size of nanoparticle. When the nanoparticles were close-packed to each other, the electromagnetic fields at the gap region between the nanoparticle pair are drastically increased .
To further increase the number of SERS hot spots, many efforts have been focused on roughening the surface of AgNWs, including direct metal deposition, chemical etching , and decorating nanowire with small metallic nanoparticles onto the AgNWs [21, 22]. These methods have been proved effective in increasing active hot spots along the Ag nanowire’s longitudinal axis. Lu et al.  revealed a surface plasmon-mediated photochemical etching in the presence of Raman probes. The nanoscale morphology changes on the surface of AgNW can be generated, resulting in a dramatic increase of Raman scattering intensity. Goh et al.  have successfully produced roughened Ag nanowires using a chemical etching method. Single nanowire SERS mapping indicated that the etched nanowires exhibited a SERS enhancement factor of ~ 104, while the as-synthesized Ag nanowires only showed limited SERS signals at their tips. The result testified the advantage of chemically etched nanowires, and chemically etched nanowires are more suitable than as-synthesized nanowires for SERS substrates. However, many studies focused on single-nanowire Raman scattering or roughened AgNWs out of order; there has been little literature reporting aligned surface with roughed silver nanowires as SERS substrates by far. Furthermore, when nanowires are extremely close to each other, the electromagnetic fields at the gap region between the nanoparticles are drastically increased . Herein, we present an aligned chemically etched AgNW monolayer as the SERS substrate using a chemical etching and three-phase interface assembly method. The resultant substrates were used for detection of rhodamine B (RB) with astonishing sensitivity (10−11 M). Repeated measurements show an excellent reproducibility of the SERS substrate. The relative standard deviations in the SERS intensities are limited to only approximately 12%. The new type of substrate provided a higher performance comparing with as-synthesized AgNWs. The findings may contribute to novel and efficient design of SERS substrates.
AgNO3 (99.8%, Sinopharm Chemical Reagent Co., Ltd.), rhodamine B, poly(vinylpyrrolidone) (PVP, average molecular weight 58,000), and copper(II) chloride dihydrate were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Ethylene glycol (EG), perhydrol 30% solution, and concentrated 25% ammonia solution were purchased from Tianjin Yong Da Chemical Reagent Co., Ltd. All chemicals were of analytical grade and used without further purification. Milli-Q deionized water (resistivity > 18.0 MΩ·cm−1) was used for all preparations.
Synthesis of Ag Nanowires
In a typical synthesis, EG (100 mL) was added to a three-necked round-bottom flask and heated at 160 °C for 1 h. Then, 1.5 mL of 4 mM copper(II) chloride dihydrate in EG was injected into the heated EG. After 15 min, 30 mL of 0.4 M PVP solution in EG was rapidly added to the above reactor. Using a syringe pump, 30 mL of 0.2 M AgNO3 was injected at a rate of 1.5 mL min−1 under the condition of electromagnetic stirring. The reaction was allowed to continue for about 30 min until the solution turned to opaque gray, which indicated the formation of Ag nanowires. The reaction mixture was left to cool, washed successively with acetone and water twice to remove Ag nanoparticles and excess PVP and EG, and then dispersed in ethanol.
Etching of Ag Nanowires
Ammonium hydroxide and 30% hydrogen peroxide (9/1 v/v) was selected as an etchant. Etchant solution was always freshly prepared and kept on ice, and all the etching experiments were performed in ice-water bath. A certain amount of etchant was injected into a 4.5 mL PVP aqueous solution (1 mg mL−1), and the volume is 200, 300, and 400 μL, respectively. Under vigorously stirring, 500 μL of AgNWs at the concentrations of 5 mg mL−1 was rapidly injected under vigorously stirring. The solution immediately changed color and evolved gas; the reaction ran to completion within seconds and was allowed to maintain for another 5 min.
Fabrication of Aligned Ag Nanowire Substrates
Five milliliters of aqueous suspension of the as-synthesized or etched AgNWs was added to the liquid surface of 25 mL chloroform in a glass vessel. An interface was formed among two immiscible liquids. One milliliter of acetone was added cautiously and dropwise to the mixture. Minutes later, a sparkling mirror-like surface emerged. The ordered Ag nanowire films were then transferred onto silicon chips. The samples of aligned chemically etched silver nanowire film were labeled as S0, S1, S2, and S3 corresponding to the amount of etchant of 0, 200, 300, and 400 μL, respectively.
The morphologies of the samples were observed by SEM (JEOL, JSM-7001F, Japan) and AFM (JEOL JSM-7600F, Bruker). The UV–vis absorption spectra were obtained using a UV–vis spectrophotometer (UV 2450, Shimadzu). The crystal structure was characterized by X-ray diffraction (XRD) (X’Pert Powder, Holland) with Cu-Kα line (λ = 0.15405 nm) in the Bragg angle ranging between 30° and 90°.
SERS spectra were obtained by using laser Raman spectroscopy (HORIBA Jobin Yvon) equipped with a 1.7 mW Ar+ ion laser using 633 nm light as excitation radiation. Spot diameter of laser beam was nearly 1 μm. The data acquisition time was 20 s for one accumulation. RB was selected as the probe molecule. 0.02 mL of RB aqueous solution was dropped onto the 7 × 7 mm2 substrate and was dispersed to a circular shape area. The same sample preparation method was used for all concentrations of RB which ranged from 10−7 to 10−11 mol L−1. In our experiments, the dispersed circular shape area was about 65 mm2, which was then dried under ambient conditions before tests. Supposing RB molecules were uniformly distributed in the circular area. The number of molecules contributing in producing a Raman signal was less than 10, when the concentration of RB is 10−11 mol L−1. The reproducibility evaluation was carried out at six randomly chosen spots from three SERS substrates. The Raman band of a silicon wafer at 520 cm−1 was used to calibrate the spectrometer. It should be noted that the accumulation times and the laser power are the same for all the Raman spectra.
Results and Discussion
Fabrication and Morphology Investigations
The Surface Roughness of Silver Nanowire
Evaluation of Substrates
Aligned substrates of etched Ag nanowires have been investigated with a gradient concentration (Fig. 6b) to determine the quantitative performance of the substrate and detection limit. Figure 6b shows a series of SERS spectra of RB with decreasing concentrations ranging from 1 × 10−5 to 1 × 10−11 M. The Raman spectral intensities became gradually weakened by diluting the concentrations of the RB probe molecule. The characteristic Raman peaks can still be identified until the probe concentration is reduced to a concentration of 1 × 10−11 M, so the detection limit of RB can reach 1 × 10−11 M, demonstrating the high sensitivity of this self-assembled substrates.
Figure 6c is the dependence of the peak intensity of 1647 and 1260 cm−1 band on logarithmic RB concentrations, in which the background of the signal has been removed in drawing the graph. The Raman intensity increased with logarithmic concentration. In the inset of Fig. 6c, a log-log plot of I SERS versus c RB revealed a nearly linear relationship, at low concentrations below 10−8 M. Above this concentration, the SERS intensity reached a plateau. This fact could be attributed to the fact that SERS intensity is proportional to the surface coverage of adsorbed molecules on hot spots, and the surface coverage follows the Hill equation . The adsorption of RB became saturated beyond this level. As a result, the substrate can only be used as a reliable platform for quantitative analysis of RB at low concentrations.
In addition to high sensitivity, reproducible SERS signals are another significant issue. To assess the reproducibility of Raman signals from self-assembled substrates, SERS spectra of RB (10−9 M) obtained from six randomly selected positions were measured. Obviously, the SERS signals were all of comparable intensity, indicating the substrates provided uniform SERS enhancements upon its entire surface. Furthermore, we compared the intensities of the characteristic 1647 cm−1 line of RB in these six spectra (Fig. 6d), and the signal variations were less than 12%, far less than the internationally recognized standard (20%), suggesting the outstanding reproducibility of SERS substrate. The variations in the SERS intensities of 1647 cm−1 band may be attributed to differences in nanowire diameters, the distance between Ag nanowires, and the adsorption of analyte on the Ag nanowires.
Self-assembled chemically etched AgNW substrate was fabricated. The roughened surface of AgNWs was observed to increase SERS-active hot spots along its longitudinal axis, while crystallinity was preserved after etching reactions. The aligned chemically etched silver nanowire substrate has overcome the limitation of conventional one-dimensional AgNWs with limited SERS-active area serving as a platform for efficient, homogeneous, and ultrasensitive SERS sensing applications. Using RB as the probe molecule, the detection limit was 10−11 M. More importantly, the regularity of silver arrangement can improve the reproducibility of SERS substrate. Taking advantage of the unique generous and uniform hot spots on the surface of chemically etched Ag nanowires, we optimized the distribution of hot spots on the present substrate, thereby achieving further enhanced SERS intensity and sensitivity. This work provides a new platform for efficient, homogeneous, and ultrasensitive SERS sensing applications.
The authors thank the National Natural Science Foundations of China for the support.
This work was supported by the National Natural Science Foundations of China (Grant No. 51273048).
JW, GY, and HL designed the experiment. JW, MZ, and ZL executed the experiments. JW, HL, MZ, XZ, ZL, and GY examined the written report. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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