- Nano Express
- Open Access
Sensitivity and Reusability of SiO2 NRs@ Au NPs SERS Substrate in Trace Monochlorobiphenyl Detection
© Hou et al. 2015
- Received: 19 October 2015
- Accepted: 10 November 2015
- Published: 17 November 2015
Surface-enhanced Raman scattering (SERS) effect is quite preferred to detect trace pollutants, and reusable SERS substrate is of important practical value. In this research, a kind of effective SiO2 nanorods (NRs)@ Au nanoparticles (NPs) substrate was fabricated completely with physical methods, and it was quite sensitive so that 1 × 10−6 M monochlorobiphenyl (CB) could be detected. Furthermore, congeners of CB could be detected by reusing this kind of SERS substrate, and the cleaning treatment between every two detections was very simple. The excellent performance of the reusable SERS substrate indicated its great application potential.
- Surface-enhanced Raman scattering
- Trace detection
- Principal component analysis
Trace pollutant detection is increasingly concerning during the last several decades, on account of the danger to environment and public health caused by the accumulation of trace harmful chemicals [1–5]. The detection method based on surface-enhanced Raman scattering (SERS) effect has been preferred in recent years, [6–10] for it is accurate, rapid, and convenient. On the surface of noble metal nanostructures, localized electric field intensity would be enhanced due to the localized surface plasmon resonance (LSPR) excited by laser [11–14]. By being adsorbed on the SERS substrate, trace pollutants could be detected with a Raman spectrometer. To detect chemicals of lower concentration, a lot of effort has been taken by researchers to improve the SERS substrates. For example, kinds of Au or Ag nanostructures were synthesized with HAuCl4 or silver nitrate [15–18], while the ones fabricated by sputtering were also reported [19–21].
With well-designed SERS substrates, trace polychlorinated biphenyls (PCBs) were detected as reported. PCBs are a series of persistent organic pollutants which could accumulate in human bodies through the food chain rather than degrade even if their initial concentration in soil and water is low. With the least chlorine atom in the molecule, monochlorobiphenyl is highly volatile and easy to disperse throughout the environment, which leads to greater danger. As toxic chemicals, PCBs including monochlorobiphenyl could harm the reproductive system, integumentary system, brain, and so on and are also carcinogenic [22, 23]. Therefore, it is quite necessary to detect trace PCBs so that to prevent their diffusion. However, it is difficult for PCB molecules to adsorb on the surface of Au and Ag. Especially for SERS substrates fabricated with some kinds of solution, the limit of detection (LOD) is unsatisfactory. Hence, surface modification was generally carried out on the noble metal nanostructures. With β-cyclodextrin or alkanethiol, PCB molecules would be caught tightly near the metal surface, so that their Raman scattering signal would be enhanced significantly [24–26]. The modified molecules, however, would meanwhile prevent desorption of PCB molecules due to the action of van der Waals forces, which makes the reuse process of the SERS substrate quite hard, and the signal intensity of re-adsorbed PCBs becomes weaker obviously .
To achieve the objective of reusing SERS substrates to detect trace PCBs which is quite valuable in the actual application situation, a kind of SERS substrate fabricated with physical methods completely was designed and optimized in this research. With abundant “hot spots” formed on the rough surface of Au nanostructures and in the gaps between them, [28, 29] the SERS substrate showed excellent trace detection ability. Moreover, without other chemicals existing on the surface of Au, PCB molecules could contact the “hot spots” directly, as well as be desorbed and re-adsorbed easily. The experimental results of reusing the SERS substrate several times to detect the same type or different type of PCB congeners successfully demonstrated the effectiveness of this kind of substrate. To characterize the trace detection effect of the reusing process, the chemometrics method of principal component analysis (PCA) was employed to calculate the SERS signal intensity of the re-adsorbed PCB molecules and the desorption degree comprehensively.
Fabrication of SiO2 NRs@ Au NPs SERS Substrates
The Au nanoparticles were sputtered on the two kinds of SiO2 nanorod array afterwards by a SBC-12 vacuum ion coater (KYKY Technology Co., Ltd.). Through alteration of the deposition time of Au from 60, 90, 120,…, up to 360 s, the amount and morphology of the nanoparticles were modified. Hence, 22 kinds of samples in total were prepared.
Characterization of SERS Substrates
By scanning electron microscope (SEM, Merlin VP Compact, Carl Zeiss), the morphology of the SiO2 NRs@ Au NPs SERS substrates was characterized. And by transmission electron microscope (TEM, JEOL-2100F), the high-resolution image and diffraction pattern of the nanostructures were obtained.
Preparation of Analyte Solution
To compare the SERS effect of the substrates with a different amount and morphology of Au nanoparticles on vertical as well as oblique SiO2 nanorod arrays, trans-1,2-bis(4-pyridyl)-ethylene (BPE) (J&K Scientific Ltd.) was employed as a probe molecule. The BPE powder was dissolved in ethanol and diluted to 1 × 10−4, 1 × 10−6, 1 × 10−7, 1 × 10−8, and 1 × 10−9 M, in sequence. The analytes 2-chlorobiphenyl (2-CB), 3-chlorobiphenyl (3-CB), and 4-chlorobiphenyl (4-CB) (AccuStandard Inc.) were dissolved in acetone. 2-CB and 3-CB solutions with a concentration of 1 × 10−4 M and 4-CB solutions with concentrations of 1 × 10−4, 1 × 10−5, 5 × 10−6, and 1 × 10−6 M were prepared.
Adsorption of Probe Molecules
To make the molecules adsorb on the SERS substrates, proper methods were used. By immersing the SERS substrates in the solution, BPE molecules were adsorbed both physically and chemically. After being rinsed with ethanol, only chemisorbed molecules remained. Chlorobiphenyl solution was dropped on the SERS substrates, and the volumes of the droplets were 3 μL each time. To clean the adsorbed CB molecules, the SERS substrate was washed with acetone for several seconds.
Measurements of SERS Spectra
With an optical fiber micro-Raman system (i-Raman Plus, B&W TEK Inc.), the SERS spectra of the trace chemicals were measured. The employed 785-nm laser formed a beam spot of ~85 μm in diameter on the surface of the samples, while the laser power and the integral time were adjusted according to each series of samples.
The best SiO2 NRs@ Au NPs SERS substrate fabricated by sputtering Au for 240 s on vertical nanorods was measured with BPE solution. On the SERS spectra of BPE in the concentrations of 1 × 10−6, 1 × 10−7, 1 × 10−8, and 1 × 10−9 M in Fig. 2c, the characteristic peaks could be recognized. The LOD for BPE could reach 1 × 10−9 M, and the enhancement factor was estimated as about 108, which demonstrated the optimized SERS substrate was quite effective. The effectiveness should be attributed to abundant Au nanoparticles adhering on the whole nanorods, as the TEM photo (Fig. 3c) and HRTEM photo (Fig. 3d) showed.
Rate of PC1 score change of 4-CB re-detection spectra and acetone-washed SERS substrate spectra
4-CB detection order
Rate of PC1 score change (%)
Rate of PC1 score change (%)
Furthermore, the SERS substrate was applied to re-detect different congeners of monochlorobiphenyl. Observing from Fig. 4d, CB molecules could be washed away nearly completely, and other kinds of CB molecules could be detected even if the SERS substrate had been used several times. These spectra indicated that analytes of different chemical structures could also be detected with the reused substrate, without interference by other kinds of analytes, which widened the application range of the reusable SERS substrate.
In this research, the SiO2 NRs@ Au NPs SERS substrate was designed and optimized by adjusting the morphology of nanorods and nanoparticles. Resultingly, the SERS substrate performed an enhancement factor of about 108, and its LOD for 4-CB could reach 1 × 10−6 M, due to the great amount of “hot spots” and its directly contacting analytes. This kind of SERS substrate fabricated completely with physical methods could be reused to detect trace CB molecules of the same or different chemical structures, through easy cleaning treatments, because of its weak adsorption for CB compared to the stronger interaction existing on the surface modified substrates. By PCA method, the re-detection and cleaning effect of the SERS substrate was characterized quantitatively, and the performance of the substrate was quite excellent.
The authors are very grateful to the financial support by the National Basic Research Program of China (973 program, Grant No. 2013CB934301), the National Natural Science Foundation of China (Grant No. 51228101, No. 51531006 and No. 51572148), the Research Project of Chinese Ministry of Education (Grant No. 113007A), and the Tsinghua University Initiative Scientific Research Program.
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