Nanoscale Vertical Arrays of Gold Nanorods by Self-Assembly: Physical Mechanism and Application
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The unique photonic effect of self-assembled metal nanoparticles is widely used in many applications. In this article, we prepared self-assembled gold nanorod (GNR) vertical arrays substrate by an evaporation method and found that the morphology of the substrate can be effectively regulated by changing the immersion time in the target molecules solution to obtain different Raman enhancement effects. We separately calculated the local electromagnetic field of the GNR vertical arrays and disorder substrate by the finite element method (FEM), which was consistent with the experimental results. Based on optimal soaking time, the sensitivity, reproducibility, and stability of substrates were separately studied. The experimental results show that the GNR vertical arrays can detect Rhodamine 6G (Rh6G) at concentrations as low as 10−11 M and exhibit good reproducibility and stability due to local electromagnetic (EM) field enhancement caused by the coupling of adjacent nanorods. Thus, our work can demonstrate that the substrate has excellent surface-enhanced Raman scattering (SERS) activity and the obtained GNR vertical arrays have great potential for biosensor and biodetection.
KeywordsGold nanorods Self-assembled method Surface-enhanced Raman scattering Surface plasmon resonance
Cetyltrimethyl ammonium bromide
Finite element method
Relative standard deviation
Scanning electron microscope
Surface-enhanced Raman scattering
Surface plasmon resonance
Noble metal nanostructures (gold, silver, copper, etc.) can generate localized EM fields on their surfaces using visible radiation, which provides favorable conditions for enhancing the spectral signals of the probe molecules [1, 2]. The specific excitation conditions can generate surface plasmon resonance (SPR) on the surface of the metal nanostructures, which have important research significance and novel optical effects in plasmonics, including surface-enhanced fluorescence (SEF) and SERS. Owning to high sensitivity, fast response, and fingerprint effect, SERS has great potential for the applications, such as material detection, biomedicine, and sensors, etc [3, 4, 5, 6, 7]. In general, SERS is grouped into two categories “local EM field enhancement” and “chemical enhancement mechanisms” . It is well accepted that “EM field enhancement” plays a major role in SERS and it shows enhancements from 4 to 11 orders of magnitude. The “hot spots” produced between adjacent metal nanoparticles can lead to a huge local EM field near the metallic surface; therefore, the Raman scattering of the molecules located in the EM field can be enhanced. In order to obtain good SERS effect, a well-shaped metal substrate, suitable probe molecules, and selection of excitation conditions are all crucial . In recent years, there have been numerous reports on SERS. Sun et al. prepared silver nanoarrays by template method which possessed excellent SERS effect on the substrate . Lu et al. discovered that silver nanowires can produce surface morphological changes at the focus of the laser and had strong SERS effects on the surrounding target molecules . Cho et al. detected Raman signals of 4-NTP with low concentrations on silver dendrite nanocrystal substrate . Although there have been many reports about SERS, the promotion of SERS still faces many challenges. For example, preparing low-cost, large-area uniform substrate and achieving ultra-sensitive detection, etc.
Self-assembled metal nanostructures as promising substrates have attracted more and more attention in both experimental and theoretical aspects [13, 14, 15, 16, 17, 18]. Compared with single nanoparticles, the local EM field of the self-assembled metal nanoparticles shows extremely unique optical property. Moreover, self-assembly substrate has the advantages of low cost, easy handling, and uniform distribution over a large area. Combining these advantages, it can be considered that the self-assembled substrate has great potential in promoting SERS. Recently, some research groups have reported gold nanorod (GNR) self-assembled substrates for SERS [19, 20, 21]. However, as far as we know, the influence of a change in the morphology of the GNR vertical arrays substrate on the Raman signals of the target molecules has been rarely studied. Herein, we firstly prepared self-assembled GNR vertical arrays substrate by the evaporation method . And then, the substrate was immersed in a probe molecule solution; the morphology of the GNRs vertical arrays was regulated by changing the soaking time. Finally, the Raman spectra of rhodamine 6G (Rh6G) and crystal violet (CV) on the substrate were obtained under specific excitation conditions. In order to verify the results of the experiment, we used SEM images of the GNR vertical arrays and disorder substrates to simulate the local field distribution of substrates by FEM. The result shows that the simulation calculation is almost consistent with the experimental data. In addition, we also study the detection sensitivity, reproducibility, and stability of the SERS substrate based on the above optimal soaking time and discussed the experimental results. Excellent sensitivity, reproducibility, and stability can indicate that GNR vertical arrays substrate can serve as a good candidate for the application of optical sensor area.
Methods and Experiment
Rh6G (laser grade) was purchased from Exciton (America), CV was purchased from Sigma-Aldrich, gold chloride tetrahydrate, ethanol, silver nitrate, and hydrochloric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Cetyltrimethyl ammonium bromide (CTAB), sodium borohydride, and ascorbic acid are purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (China). Silicon wafers (Si) were purchased from Li Jing Photoelectric Technology Co. Ltd. (Zhejiang, China). All reagents are used without further purification. Deionized water was used throughout the experiment.
Preparation of GNR Vertical Arrays
The size and morphology of the GNR vertical array were measured with scanning electron microscope (SEM, Nova Nano 450). Raman signals were collected with confocal Raman microscopy (LabRAM HR Evolution, HORIBA Jobin Yvon SAS). The CW laser with 532 nm was used as an excitation source, and the power of laser is 0.5 mW. The samples were exposed to the microscope (× 50), and the integration time was set as 1 s.
Results and Discussion
Mechanism of Gold Nanorod Self-Assembly
In general, capillary edgeward flow is generated inside the droplets to carry the suspended GNRs to the edge of the droplets, causing a large number of GNRs deposit at the edge to form a disordered GNR distribution, which is known as the “coffee ring” effect [25, 26]. Nevertheless, GNRs in aqueous solution are arranged side by side to form an initial six deformation structure by attractive forces and electrostatic forces under appropriate conditions. Marangoni flow and contact line receding of droplet cause the free GNRs in solution to accumulate around the initial model, resulting in the area of the GNR vertical arrays to continuously increase. Terminally, the vertical arrays are fixed on the substrate due to gravity and van der Waals interactions. In the process of forming GNR vertical arrays, there are three main influencing factors: van der Waals force, depletion force, and electrostatic force . The van der Waals force and the induced depletion force belong to an attractive force, and electrostatic force belongs to a repulsive force. The van der Waals forces and depletion forces bring adjacent GNRs closely together. Electrostatic repulsive force stabilizes GNRs within a certain distance and prevents them from randomly gathering. The synergy between attractive force and repulsive force induces GNRs into high-ordered arrays.
Temperature and humidity are important influencing factors in self-assembly. GNR droplet forms “coffee ring” in a high temperature or low-humidity environment. In the evaporation process, the contact line of the droplet is pinned. Due to the higher evaporation rate at the edge of the droplet, the GNRs are carried to the pinning contact line by the capillary flow and deposited to form a ring pattern. In contrast, the GNR solution produces Marangoni flow, and GNRs are close-packed and high ordered under the appropriate circumstance. Moreover, the surfactant concentration also plays a key role in the self-assembly process. Many researches have shown that increasing the concentration of surfactant CTAB is beneficial to the formation of GNR vertical arrays substrates [28, 29]. The main reason is that the GNRs are driven by the capillary flow and move around the contact line of the droplet. If the surfactant concentration is too low to form Marangoni flow, a large number of particles will deposit around the contact line to cause disordered distribution. Conversely, increasing the surfactant concentration can result in numerous surfactant molecules to be pushed onto the contact line and more easily produce Marangoni flow. A part of the GNRs are deposited near the contact line during the evaporation process, and excess nanoparticles are returned to the center of the drop under the Marangoni eddy to complete the subsequent assembly. It can be concluded that the nanorods are controlled by the Marangoni flow to complete the GNR ordered arrays. Controlling these influence factors can help to form ordered and large-area GNR vertical arrays, which can provide a reliable support for the subsequent spectrum.
Morphological of Gold Nanorods and Vertical Array
The preparation process and subsequent operation of the GNR vertical arrays are given in Fig. 1. For simplicity, the experimental procedure is only schematically represented. In brief, 5 μl drops from a centrifuged GNR solution were dripped on a washed silicon wafer with acetone, ethanol, and deionized water (6 × 6 mm2 in size). Then, the silicon wafer with GNR droplet was placed in the circumstance of 21 °C and humidity of 85% to slowly evaporate. After 72 h, side by side GNR vertical arrays were obtained. According to previous reports, we utilized the “seed-mediated growth” to synthesize GNRs [23, 24].
Spectrum Enhancement with GNR Vertical Array
CSERS and CRef are the concentration of Rh6G in the SERS substrate (10−10 M) and the reference (10−3 M), respectively. ISERS and IRef are the SERS intensities of GNR arrays after soaking Rh6G and reference Raman signals, respectively. The intensities of the Raman peak at 613 cm−1 of the Rh6G are calculated that ISERS/IRef, CSERS/CRef, and the EF are about 0.0965, 10−7, and 9.65 × 105, separately. The EF calculated in our experiments is consistent with the magnitude reported in the literature for self-assembled substrate [17, 35, 36].
In general, the substrate has not only good sensitivity but also excellent reproducibility for the SERS applications. In order to present the good reproducibility, we randomly select 10 points from the substrate deposited on Rh6G molecules. As shown in Fig. 6c, Raman peaks of Rh6G are consistent with that of Fig. 6a. Raman peaks of Rh6G at different positions are not moved. Additionally, the relative standard deviation (RSD) of the Raman peak, as an important parameter, is used to evaluate the quality of the substrate reproducibility. Here, relative deviation formula can be expressed as RSD = SD/Im , where the SD is the standard deviation intensity of the peak and Im is the average Raman intensity of the main peak. We calculate the RSD values of the Raman peaks at 1362 cm−1 and 774 cm−1 from the statistical 10 points, respectively. The RSD values are about 10.7% and 9.0% in Fig. 6d and e, respectively, which indicate that the SERS property of GNR vertical arrays has excellent reproducibility.
In summary, we have successfully prepared self-assembled GNR vertical arrays by an evaporation method. More importantly, we found that the morphology of GNR vertical arrays can be regulated by changing the soaking time for good Raman enhancement effect. Based on the EM field theory, we used COMSOL software to analyze and discuss the local EM field distribution of GNR vertical array and disorder substrate. The results are almost in agreement with the experiment data. Besides, we studied the SERS activity of the vertical array of GNRs based on the optimal soaking time of the substrate. The as-fabricated substrate can detect Rh6G at concentrations as low as 10−11 M due to local electromagnetic field enhancement and show great reproducibility and stability. Therefore, GNR vertical arrays with excellent sensitivity and stability can be used for species detection, sensing, and other fields.
This work was supported by the National Science Foundation of China (11604262, 91436102, 10374353 and 61874087), Shaanxi Provincial Education Department(Program No. 17JF026), Natural Science Basis Research Plan in Shaanxi Province of China(No.2017GY-189), and National Basic Research Program of China (Grant number 2016YFA0200802), the New Star Team of Xi’an University of Posts & Telecommunications, Innovation Funds of Graduate Programs of Xi’an University of Posts & Telecommunications (CXL2016-43).
Availability of data and materials
The datasets supporting the conclusions of this article are included in the main text and figures.
JD supervised this project. JD and MTS provided the original idea. XZ carried out the statistical design of the experiment, prepared the measurements, and wrote the paper. WG, QYH, SDG, and XZ analyzed the data. YKW and JXQ provided simulation calculations. JD and MTS helped to correct and polish the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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