Surface Modified-silver Nanomaterials SERS Substrate For The Detection Of Environmental Organic Pollutants

Organic pollutants are widely distributed in the nature, which have severe adverse effects on human health and environment. It is crucial to develop rapid, economical and sensitive method for the detection of organic pollutants. The traditional analytical techniques for the detection of organic pollutants include fluorescence spectroscopy, phosphorescence spectroscopy, electrochemical methods, chromatographic method, capillary electrophoresis methods and so on. These methods generally require expensive equipment or complicated sample preparation before detection. Alternatively, surface-enhanced Raman spectroscopy (SERS) is a simple and rapid analytical method, which has been widely used in the detection of environmental organic pollutants. SERS has the advantages of high sensitivity, good selectivity and weak interference from water and fluorescent signals. High enhancement substrate and the analytes close to the substrate are important to SERS detection. However, the organic pollutants show weak affinity to the substrate surface, which make the SERS detection difficult. Modification of the substrate with some substances, which could adsorb the organic pollutants close to the substrate, may make the SERS detection of the organic pollutants possible.In this paper, silver nanoparticles aggregates were synthesized and modified with self-assembled monolayers and used as the SERS substrate to detect organic pollutants. The main contents can be summarized as follows:1. A simple, cost-effective and rapid method has been developed for qualitative and quantitative SERS detection of polycyclic aromatic hydrocarbons (PAHs) using silver nanoparticle aggregates on copper foil as the substrate. Fabrication of high enhancement substrate is crucial for SERS detection. However, the traditional substrate could be influenced by laser heating effect and irreversible signals were obtained. It has been reported that the silver nanostructures show high stability under the laser irradiation. In this work, SnCl2was used as the "sensitizer" and the silver nanostructures were generated based on the galvanic displacement reaction. The prepared substrate was characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The SEM image confirmed that the copper foil surface is covered with dense silver nanoparticle aggregates. To obtain high-enhancement substrate, the influences of the number of circulation and SnCl2were investigated. The results indicated that the large enhancement substrate could be obtained under four cycles in the presence of SnCl2. The silver nanoparticle aggregates and alkanethiol were used as the substrate and the modifier to detect PAHs. Studies showed that the alkanethiol-modified silver nanoparticle aggregates could realize the SERS detection of PAHs. The substrate has good stability, uniformity and reproducibility. We further used the substrate to quantitative SERS detection of PAHs and the log-log plot of the normalized SERS intensity versus PAHs concentrations yielded a good linear relationship2. The SERS detection of polybrominated diphenylethers (PBDEs) was realized using a portable Raman spectrometer. PBDEs, one of the most common brominated flame retardants, are toxic and persistent, generally detected by the chromatographic method. In this work, qualitative and quantitative detection of PBDEs were explored based on surface-enhanced Raman spectroscopy (SERS) technique using a portable Raman spectrometer. Alkanethiol modified silver nanoparticle aggregates were used as the substrate and PBDEs could be pre-concentrated close to the substrate surface through their hydrophobic interactions with alkanethiol. The effect of alkanethiols with different chain length (C6、C12and C18) on the SERS detection of PBDEs was evaluated. It was shown that1-hexanethiol (HT) modified substrate has higher sensitivity, good stability and reusability. Qualitative and quantitative SERS detection of PBDEs in real sea water was accomplished, with the measured detection limits at1.2×102μg·L-1. These results illustrate SERS could be used as an effective method for the detection of PBDEs.3. Cysteamine-modified silver nanoparticle aggregates have been fabricated for pentachlorophenol (PCP) sensing by SERS using a portable Raman spectrometer. PCP is a common environmental contaminant, which has been widely used as insecticide, herbicide and wood preservative. PCP now can be detected in the air, water, soil, as well as in human urine, blood and adipose tissues. It is important to detect PCP. The traditional analytical techniques are chromatographic methods, which often need complex and time-consuming sample pretreatment. SERS is a rapid and sensitive analytical method, which has been widely used in biology, medicine, or environmental monitoring related fields. However, there are still some limitations that restrict the technique as SERS is observed when the analytes are close to the rough noble metal surfaces. Only these analytes with specific functional groups, such as thiol, carboxylic acid, and amine, etc., could easily adsorb onto the substrate surface and provide good signals to meet the ultrasensitive analysis. Biological related samples often contain these functional groups and hence have good singnals. However, a large group of organic pollutants in the environment characterized by nonfunctionalized groups, such as chlorinated pesticides, polycyclic aromatic hydrocarbons, trinitrotoluene, and other aromatic compounds, show weak affinity to gold or silver. It is difficult to direct detect these compounds by SERS, and thus many indirect methods emerged. Many methods rely on functionalized nanoparticles with different media to concentrate analytes close to the substrate surface. These strategies include hydrophobic interactions using alkanethiols, host-guest interactions using cyclodextrin, specific interactions using antibodies and aptamers, as well as electrostatic attraction of ion pairing. The surface modifier of the substrate may form a self-assembled monolayer (SAM) on the metal substrate, which could also be used as internal standards for the reliable quantitative assay. Cysteamine hydrochloride (Cys) was selected as the modifier, which bears positively charged groups-NH3+and could interact with the acidic PCP. Cys plays a dual role in the process: pre-concentration of PCP close to the substrate surface through their electrostatic interaction and acting as the internal spectral reference in the quantitative detection. Cys-modified substrate has good uniformity, stability and reusability. Qualitative and quantitative SERS detection of PCP were realized based on this substrate. This work is the first example to use Cys-functionalized substrate for SERS analysis of PCP.