A novel metastable state nanoparticle-enhanced Raman spectroscopy coupled with thin layer chromatography for determination of multiple pesticides

Highlights

• Metastable state nanoparticle-enhanced Raman was fabricated by polyurethane-AgNPs.

• Polyurethane micelle enhanced the stability of Ag-NPs aggregates.

• Micelle facilitated the targets detaching from thin layer chromatography plate.

• Multiple pesticides were separated and detected by this coupling method.

Abstract

A novel and highly sensitive metastable state nanoparticle-enhanced Raman spectroscopy (MSNERS) was reported in this work, which employed an amphiphilic polymer polyurethane-Ag nanoparticle (AgNPs) as the MSNERS substrate. Polyurethane could form micelle to incorporate nanoparticles and analytes, where targets could be in close contact with the metal surface, which was effective for further enhancing the detection sensitivity. Time-dependent visible absorption spectra and time-dependent Raman spectra indicated that polyurethane could greatly enhance the stability of AgNPs aggregates during the volatilization process, which was a great improvement of MSNERS substrate. Combining this MSNERS substrate with the rapid separation method of thin layer chromatography (TLC), this TLC-MSNERS was successfully applied to analyze mixed pesticides on fruit skin and the detection limits of thiabendazole, triazophos and phosmet were 0.02 μg/mL, 0.8 μg/mL, and 0.6 μg/mL, respectively. It enhanced 1 order of magnitude the signals of analytes in comparison to that of traditional TLC-SERS method.

Introduction

It is always a research hot topic in surface enhanced Raman spectroscopy (SERS) by controlling hot spots of Au or Ag nanoparticles to enhance SERS signal. Recently, highly sensitive SERS signal was obtained based on a state translation process of Au or Ag colloid: from wet state to dry state, namely metastable state nanoparticle-enhanced Raman spectroscopy (MSNERS) (Liu et al., 2014, Yang et al., 2011). It was reported that nanoparticles tended to approach closer to generate hot spots during the solvent volatilization process. In this case, the target analytes could be trapped in the gap of hot spots and strong SERS signal could be obtained at this moment (Dong et al., 2015, Yang et al., 2015, Zhu et al., 2014). The strongest SERS signal was observed when the droplet tended to become dry. Therefore, it is critical to seize the moment before the solvent complete drying to collect the best SERS signal. In the previously reported literature, the common method to obtain the optimal signal was by continuous recording the Raman spectra during the solvent evaporation process and chose the strongest signal. Although many studies have demonstrated that the MSNERS substrate showed higher detection sensitivity compared with the traditional SERS method due to forming appropriate hot spots and the volatilization-induced automatic concentration of the target molecules during the detection process (Li et al., 2014, Wu et al., 2013, Yang et al., 2012), the signal intensity fluctuated a lot during the acquiring process due to the evaporation process changing with time in real application. In other words, the optimal signal could only be observed spontaneously. Robust control over the accurate acquiring time to seize the strongest signal remains a challenge because of the fluctuations of hot spots’ state and analyte concentration. Customarily, it needs to acquire a large number of spectra to screen the most intense signal. Therefore, although MSNERS is a sensitive technique, there is still room for improvement of the substrate to obtain stable signal.

In fact, keeping SERS signal stable for a longer time is quite challenging in the dynamic evaporation process due to the difficulty in tuning the balance between persistent solvent evaporation and stability of the resulting nanoparticles aggregates. In this work, we adopted a novel method to improve the performance of the MSNERS substrate by adding a polymer surfactant, polyurethane, which is a water soluble amphiphilic polymer. Polyurethane could form micellar cluster in the volatilization process, which could reduce the effect caused by volatilization-induced automatic concentration of the target molecules during the detection process. More importantly, according to the results from the time-dependent visible diffuse spectra, it was found that the presence of polyurethane could stabilize different forms of nanoparticle aggregates for a great while in the dynamic evaporation process. It was proposed that polyurethane-bridged nanoparticles in the 3D hotspot matrix were predominant in the aggregates in our system and this structure could be kept stable for a great while in the evaporation process. Due to the stability of aggregates, the optimal SERS signal with slight fluctuation was obtained in this work during the acquisition process before droplet became dry and this strong signal could be kept stable for several minutes. This feature was of great help and convenience for building up the quantitative relationship between the Raman intensity and the corresponding concentration of analytes. The enhancement in stability of this MSNERS substrate was a great improvement in comparison to that of previous reported metastable substrates, which was believed to provide bright future for practical application of MSNERS substrate.

In general, for SERS analysis, only those molecules readily adsorbed or close to the surface of nanoparticles can be detected. However, there are many interference components in real samples. The signal from the low concentration of analytes or weak adsorbates is likely to be buried in the signal from high concentration of analytes or strongly adsorbed molecules. Therefore, it is best to separate the targets from a mixed system prior to the SERS detection for real-world applications. Thin layer chromatography (TLC) is a facile, rapid and cost-effective technique for separating different components from mixtures (Dawan et al., 2016, Sokolović and Šimpraga, 2006). Often the developed TLC spots are scraped off and extracted from the TLC plate by means of repeated rinsing and centrifugation, and further examined by infrared spectroscopy (Sharma & Kumar, 2017) or mass spectrometry (Stanger et al., 2017) for further identification, which are time-consuming and labor-intensive. SERS in conjunction with TLC has achieved on-site detection. It is easy to acquire Raman signal of multiple targets after the components are separated by the TLC plate. TLC-SERS technique has been used for detecting kinds of targets in different fields, such as environmental aromatic pollutants (Li et al., 2011), fiber of artist dyestuffs (Brosseau et al., 2009), drugs (Fang et al., 2016, Lucotti et al., 2012, Lv et al., 2015, Xiao et al., 2016, Zhu et al., 2014), pesticides (Yao et al., 2013), biochemistry (Xie et al., 2017), polycyclic aromatic hydrocarbons (Chen, Huang, & Zhao, 2015), dyes (Kong et al., 2017, Wang et al., 2014). Although this coupling technique possesses great potential for rapid analysis in complex system, the sensitivity of traditional SERS substrates still needs to be improved. In this work, coupling TLC with this improved MSNERS technique (TLC-MSNERS), good correlations between targets concentration and intensity were obtained. A highly reliable, sensitive and practical analysis method was established for rapid detection of multiple pesticides on fruit skin, which demonstrates that this method possesses great potential to realize the quantitative detection of different targets in real samples.