Density functional theory and surface-enhanced Raman spectroscopy studies on endocrine-disrupting chemical, dimethyl phthalate
Introduction
Phthalate esters (PAEs) are a kind of endocrine-disrupting chemical which have toxic effects on estrogenic substances. PAEs are the dialkyl or alkyl aryl esters of phthalic acid, including an o-phenylene group and two carboxylate groups. PAEs have a wide range of applications and are constantly being released into the environment [1], [2], [3], [4], [5], [6], [7].
Currently, there are many techniques for the measurement of PAEs. Combined with some pretreatment techniques, detection limits for GC-MS [4], GC-FID [5], [6], [7], [8], HPLC [3], [9], [10] for the determination of PAEs at the ppb level can be achieved. However, these methods involve time-consuming and labor-intensive procedures. It is of critical importance to develop simpler, quicker, cost-effective, as well as sensitive methods for screening large amounts of consumer plastic products for commonly used PAEs.
Surface-enhanced Raman scattering (SERS) spectroscopy, a novel and ultra-sensitive vibrational spectroscopic method, has developed greatly over recent years. The fingerprint-like Raman spectrum is able to provide overall and specific information on various chemical and biochemical components. However, signals from normal Raman spectroscopy are very weak because only one photon in a million will scatter inelastically (Raman shift). In SERS, when target molecules are attached to metallic nano-structures (typically Au, Ag, Cu, Pt, or Pd), Raman signals can be significantly enhanced millions of times in the highly localized optical fields of these structures due to “electromagnetic field enhancement” and “chemical or electronic enhancement” of the signals [11]. SERS is capable of providing significant enhancement of band intensities compared to normal Raman spectroscopy and has a great potential for rapid screening and detection of trace quantities of targets species.
In order to apply SERS to the screening of PAEs, there are two main problems to be solved, assignment of bands and enhancement of signals. Firstly, further studies concerning the assignment of specific vibrations to the Raman spectra of PAEs are still necessary. In previous papers, Fourier transform (FT) Raman spectroscopy was carried out on a range of 22 PAE plasticizers [12]. It was found that PAEs, as a group, can be identified by a set of six characteristic Raman bands (3074, 1726, 1600, 1580, 1040, and 652 cm−1). Nyquist [13] had studied the characteristic bands of the o-phenylene group of PAEs. In his research, assignments were obtained indirectly by comparison with o-dichlorobenzene. In our research, assignment of the Raman spectrum of dimethyl phthalate (DMP), the PAE with the shortest chains, would be carried out using DFT calculations followed by a comparison with the experimental Raman spectrum. Secondly, our research aims to explore the use of SERS as a detection method for DMP. However, PAEs as esters are not easy to get close to surface of metallic nano-structures because that they are hydrophobic and do not form a strong bond to metallic nano-structures. We have investigated the potential for detection of DMP, forming short chains in PAEs, by SERS in this research.
The purpose of this paper is to report the assignment of DMP vibrations to the Raman spectrum and the results of SERS spectroscopy of DMP on the Au nano-substrate.
Section snippets
Sample and instrument
DMP (>99%, AR agent) and dichloromethane (CH2Cl2, > 99.5%, AR agent) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
A Raman confocal microscope LabRAM HR 800 (HORIBA Jobin Yvon Inc. France) was used for the normal Raman and SERS measurements.
Software and data analyzer
DFT calculations were performed with Gaussian 03 software [14]. The Omnic 6 Professional software suite (Thermo Electron Corporation, UK) was normally used to acquire, process, analyze and manage the FT-IR or Raman data and also
DFT calculations of conformations and vibrations
The calculation of structure and vibrational frequencies of DMP with C2v symmetry as the input geometry has demonstrated that this structure is not the lowest-energy conformer. The maximum symmetry for DMP is the C2v point group (with a C2 axis and two mutually perpendicular mirror planes, as shown in the Supporting material). After geometry optimization, the structure maintained its C2v symmetry. However, the result of the frequency calculation showed two imaginary frequencies, which means
Conclusions
DFT was utilized to calculate the Raman spectrum and assign the characteristic peaks of DMP and explore the possibility of application of SERS screening for PAEs. After the geometry optimization and determination of the vibrational frequencies at the 6-31 + G(d) basis level, the theoretical vibration frequencies (with scaling factor 0.9734) matched well with the experimental normal Raman spectrum. This research obtained SERS signals from 0.1 ppm DMP standard solution on the Au nanosubstrate. These
Acknowledgements
This article is supported by National Key Technology R&D Program in the 12th Five year Plan of China (No. 2015BAD16B01, No. 2014BAD04B03 and No. 2012BAD36B02), the National Natural Science Foundation of People’ Republic of China (No. 21203076), Key Technology R&D Program in Jiangsu province (No. BE2012631) and the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD). The manuscript has been edited by International Science Editing.
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