SERS-Based Microdroplet Sensor for Highly Sensitive Trace Analysis

Abstract

Herbicides are widely used for controlling weeds in Modern agricultural production in the world. Residues of these herbicides are frequently found in surface and ground water because of their persistence and water solubility. There are many compounds registered as herbicides, which can be classified into several chemical classes in accordance with their chemical structures. For example, Paraquat (PQ) and diquat are quaternary ammonium compounds. They were widely used as non-selective contact herbicides. A large number of these compounds are soil-applied herbicides and, in general, the toxicity of herbicides for mammals is low. Therefore, the risk of ingesting toxic herbicide levels in foods would be rather low. However, the widespread use of these compounds in agriculture has increased the public concern on the presence of their residues in foods. Particularly, PQ is suitable for many agricultural uses because of their high solubility in water, their low production of vapors during application, and their ability to bind to soil. Many countries monitor residue levels of these compounds in foods and the outcome of this control shows that the accepted maximum residue levels are seldom exceeded. A variety of analytical methods have been used for analysis of these herbicides in water including high performance liquid chromatography (HPLC), square wave voltammetry, spectrometry, and liquid chromatography-electrospray ionization mass spectrometry, but they need lengthy measurement time with poor detection limits. Therefore, it is necessary to find a rapid and accurate approach for detecting PQ in drinking water. Surface-enhanced Raman scattering (SERS) has demonstrated ultra-sensitive capability duo to the local electromagnetic field enhancement near noble metal nanomaterials. However, it is very important that how to get the reproducible result using SERS technique. Herein, we report a rapid and highly sensitive trace analysis of paraquat (PQ) in drinking water using a surface-enhanced Raman scattering (SERS)-based microdroplet sensor. Aqueous samples of PQ, silver nanoparticles, and NaCl as the aggregation agent were introduced into a microfluidic channel and were encapsulated by a continuous oil phase to form a microdroplet. PQ molecules were adsorbed onto particle surfaces in isolated droplets by passing through the winding part of the channel. Memory effects, caused by the precipitation of nanoparticle aggregates on channel walls, were removed because the aqueous droplets were completely isolated by a continuous oil phase. The average signal was measured from ~125 droplets. It provides the reproducible result for quantitative analysis. The limit of detection (LOD) of PQ in water, determined by the SERS-based microdroplet sensor, was estimated to be below 2×10-9 M, and this low detection limit was enhanced by one to two orders of magnitude compared to conventional analytical methods. The silver nanoparticles have been chemically synthesized in the merging microdroplet reactor. Microdroplets techniques have been widely developed and provide an approach to improve mixing efficiency and better control concentrations of reactants; which are difficult in laminar flow-dominant microfluidic devices where fluid mixing is limited by diffusion. The diffusion driven mixing is slow and the parabolic flow profile causes dispersion in size and other crucial parameters. Laminar flow mixing prior to droplet generation is one of alternative methods to controlling and mixing chemicals, while it has local instability caused by the pinching of droplets and asymmetric shear force may compromise the precise control of reactant composition in each droplet. The influence of flow rates of reactants, reactants concentrations, and surfactant concentrations on silver nanoparticles size has been investigated. It has been found that the flow rates of reactants and surfactant concentrations are important for microdroplet generation. The reactants concentrations cannot be changed so much, otherwise silver nanoparticles are not formed. This chip allows for an accelerated and efficient approach for the synthesis of silver nanoparticles because the reactants rapidly mixed when the droplet was stretched and folded in the wind channel. The merging microdroplet system, which the reactants cannot always contact and oil would clean the deposition at frequent intervals, will efficiently prevent the deposition of pre-mix sample from the microfluidic channel. The silver nanoparticles have an average size of about 61.7 nm with the excellent enhancement ofr Raman signal, but the synthesis time of particles can be reduced as much as one order of magnitude, from ～1h to ～2 s. According to our knowledge, it’s the first time to report in situ silver nanoparticles synthesis in the microdroplet chip at room temperature.