Rapid and Sensitive SERS Detection of Bisphenol A Using Self-assembled Graphitic Substrates

www.nature.com/scientificreports OPEN Received: 24 August 2017 Accepted: 20 November 2017 Published: xx xx xxxx Rapid and Sensitive SERS Detection of Bisphenol A Using Selfassembled Graphitic Substrates Pei-Ying Lin, Chiung-Wen Hsieh & Shuchen Hsieh We have prepared and tested a new surface enhanced Raman scattering (SERS) substrate based on self-assembled graphitic sheets to detect bisphenol A (BPA) in plastic consumer goods. Transmission electron microscopy (TEM) and atomic-force microscopy (AFM) were used to characterize the structure of the graphitic sheets and showed a lattice spacing of 0.24 nm and layer height of 0.34 nm. These values were comparable to single monolayer graphene. The effective SERS detection limit of this method is 1 μM BPA, which is lower than the European Union specific migration limit for BPA of 0.6 mg/kg (2.6 μM). When used in salt solutions, graphitic sheets exhibited ultra-sensitivity toward BPA of 0.025 M to 2 M, which was broader than physiological ionic strength (0.14 M) and urinary NaCl (0.17 M). Our results demonstrated that this graphitic sheet based SERS detection platform can be used to determine BPA levels leached from commercial polycarbonate plastic products and for on-site rapid analysis with good results. Bisphenol a (BPA) is an organic synthetic compound that is used in industry to make plastics and epoxy resins for consumer goods and industrial applications. There is established concern that exposure can lead to a variety of health problems in humans1–3. Studies have shown that BPA can leach from containers or container linings then migrate into the food or beverages and be ingested4,5 Exposure to BPA (which is known to mimic estrogen), may cause reproduction dysfunction and lead to birth defects in children, breast cancer, recurrent miscarriages, and other ailments6,7. The human health hazards related to BPA continue to be investigated, and thus ultra-sensitive methods for detection of low levels of BPA in common household products and foods are of great interest. At present there are several analytical methods used to detect BPA levels in the environment, including high-performance liquid chromatography (HPLC), gas chromatography coupled with mass spectrometry (GC-MS), enzyme-linked immune sorbent assay (ELISA), molecule imprinting techniques, and electrochemical sensors3,8,9. In addition, a direct (label-free) immunosensor is becoming popular because it reduces sample preparation time and simplifies the sensing protocol10,11. Surface-enhanced Raman scattering (SERS) is widely used for increasing the Raman scattering signal of molecules adsorbed on rough noble metal, or metal particle decorated substrates12. Recently, new types of SERS-active hybrid materials have been reported, such as graphene layers (graphitic sheets), with deposited metal nanoparticles, which can further enhance Raman signal detection13–16. Graphene is attractive because it can be produced with a large surface area, good thermal conductivity, high electrical conductivity, and high electron transfer rate17–19. Because of these properties, graphene-based Raman scattering is regarded as a versatile characterization tool with ultra-sensitivity for SERS detection20,21. BPA, exhibits only a very weak affinity for adsorption on metal surfaces, thus making BPA detection by traditional SERS methods ineffective. Moreover, the complex constituent mixture in “real” samples can interfere with the Raman scattering signal, resulting in poor detection specificity. Thus, research continues with efforts to identify appropriate SERS substrates that have a strong affinity for BPA. Here we report on the preparation and evaluation of SERS-active (metal-free) graphitic sheet substrates for ultra-sensitive detection of BPA by SERS methods. The graphitic sheets were prepared using silane-based self-assembled monolayers (SAMs) and then characterized using transmission electron microscopy (TEM, HRTEM), energy dispersive X-ray spectroscopy (EDS), and atomic force microscopy (AFM). Surface-enhanced Raman spectroscopy was the core analytical technique used to detect BPA on the graphitic sheet substrates. We Department of Chemistry and Nanoscience and Nanotechnology, National Sun Yat-sen University, Kaohsiung, 80424, Taiwan. Correspondence and requests for materials should be addressed to S.H. (email: shsieh@faculty. nsysu.edu.tw) SCieNtifiC REPorTS | 7: 16698 | DOI:10.1038/s41598-017-17030-9 1 www.nature.com/scientificreports/ Figure 1. (a) TEM and HRTEM (inset) images of graphitic sheet. (b) EDS spectrum showing the chemical composition of graphitic sheets (inset shows EDS spectrum of the grid only). (c) Surface topography AFM images (2 × 2 μm2) and (d) line section analysis obtained from graphitic sheet on a silicon substrate. believe that this method will allow ultra-sensitive determination of BPA in a wide variety of samples for clinical, industrial, and consumer product research applications. Results Surface characterization of graphitic sheets. TEM and HRTEM images in Fig. 1a show that the graphitic sheets had a lattice spacing of 0.24 nm, which is comparable to the lattice constant of graphene22. To examine the chemical composition of the graphitic sheets, the EDS spectrum was acquired as shown in Fig. 1b. The primary component of the graphitic sheets was carbon, with a minority contribution from oxygen (~12%) which may be due to oxygen atoms at structural defects23. The Cu peaks in the spectra originate from the TEM grid (Fig. 1b inset). The topographic AFM image and corresponding line scan in Fig. 1c and d show a typical graphitic sheet on a silicon substrate (>1 μm lateral extent). Line scan analysis reveals that the height of the graphitic sheet on silicon is ~0.34 nm, which is comparable to the interlayer spacing of graphene24. Thus, the graphitic sheets fabricated in our study exhibit structural characteristics that are nearly identical to those of graphene. Determination of feasibility and detection limit. The range of products which may contain BPA is broad, thus the capability to detect and monitor BPA in various mixtures is important. We first investigated Raman spectra of a clean silicon substrate and graphitic sheets on silicon, and the SERS spectra of 10−1 M BPA deposited onto both substrates separately (Fig. 2a). The inset in the figure shows vials of pure water (left) and of the stock graphitic sheet solution (right) illuminated using a 532 nm laser. Both vials are optically clear but laser light is scattered in the stock solution. Optical scattering and the Raman result shown in (Fig. 2a – GS), confirm that the stock solution contained graphitic sheets. No peaks were observed in Raman spectra from a clean silicon substrate. However, Raman spectra from graphitic sheets on silicon had two prominent peaks at 1341 cm−1 and 1602 cm−1 which are assigned to the D and G bands, respectively. The D band is associated with a breathing mode of the sp2 carbo (...truncated)