Design and Fabrication of Polymer-based Lab-on-a-Chip Devices Towards Applications in Food and Environmental Analysis

Abstract

Pesticides play a key factor in the high productivity achieved in modern agricultural food production. While increasing productivity and lowering production costs, they are potentially toxic and can have a serious impact on humans and the environment. In general, monitoring of pesticides and other environmental contaminants is performed in analytical laboratories, utilizing a multiplicity of time-consuming and cost-intensive chemical analysis methods like chromatography and mass spectrometry. To ensure food security and to monitor maximum residue levels in a highly globalized market, miniaturized analysis systems could provide inexpensive, portable devices for fast and reliable on-site monitoring of – not only – pesticides. Introduced already more than 20 years ago, lab-on-a-chip (LOC) devices found their way into biological and clinical research. Their fast analysis times, low sample volume and low reagent consumption are attractive for many applications in the life sciences, e.g., for DNA sequencing platforms and screening applications in drug development. It was only recently that the use of LOC systems gained considerable interest in the broad field of environmental analysis. In this work, several polymeric LOC systems for the analysis of dithiocarbamate (DTC) pesticides were designed and their performance was tested. Cyclic olefin polymer (COP) was studied as a potential material for non-aqueous analysis of DTC pesticides. While COP has some outstanding material properties compared to commonly used substrate materials, such as poly(methyl)methacrylate (PMMA) or polycarbonate (PC), it was shown that bonding of COP chips is challenging. Gold microband electrodes were integrated into microfluidic channels for electrochemical detection of the DTCs ziram and nabam. It was found that sulfur-containing DTC pesticides adsorb onto the gold surface of the electrode and thereby passivate it to a high extent. While sulfur-gold interactions of DTC pesticides were a major drawback for electrochemical detections, their high affinity for gold could be exploited in a second microfluidic sensor. Here, the sensor consisted of a polydimethylsiloxane (PDMS) chip for on-chip mixing of DTCs with gold nanoparticle (AuNP), which were functionalized with rhodamine 6G (R6G). While AuNPs act as a fluorescence quencher for the adsorbed R6G, they interact with the sulfur-containing pesticides upon mixing and thus release R6G into the solution. The R6G fluorescence intensity was measured and could be related to ziram concentrations with a limit of detection as low as 16 μg·L−1. Due to its indirect sensing mechanism, the AuNP-based DTC sensor was not specific for ziram and a similar fluorescence response was measured for ferbam, demonstrating that the mechanism can be employed as an indirect detection scheme for several DTC pesticides. Therefore, the nonspecific detection mechanism needs to be combined with a separation step prior to AuNP-mediated detection, to allow quantitative and qualitative analysis of different DTC pesticides. To this end, a capillary electrophoresis (CE) unit was implemented on a third chip, which was fabricated of thiol:ene, a photopolymerizable material. The CE microchip consisted of a separation channel for DTC separation, and side channels for subsequent AuNP probe lamination of the separation bands. Even though a separation of pesticides was not performed, the electrokinetically driven lamination of AuNP, and the feasibility of indirect fluorescence detection of ziram in microfluidic channels with a small detection volume was proven. Furthermore, three different fluorophores could be separated on these chips, demonstrating that chips fabricated from thiol:ene offer a great potential within polymer based CE.