Development of a paper microfluidic device towards in situ detection of manganese in soil Samira AL Hinai 1 , Samantha Richardson 2 , Mark Lorch 1 , Nicole Pamme 1,3 1 Department of Chemistry and Biochemistry, University of Hull, UK, 2 Department of Biological and Marine Sciences, University of Hull, UK, 3 Department of Materials and Environmental Chemistry, Stockholm University, Sweden KEY WORDS: barrier free, PAD, manganese, soil, extraction The chemistry of soil needs to be monitored regularly to achieve good crop yields. Both macronutrients (such as phosphate and nitrates) 1 and micronutrients (such as manganese 2 and zinc) play an important role in maintaining healthy plant growth. Current testing for micronutrients generally requires transport to the laboratory for ICP- MS-based analysis, which is costly and time consuming. Therefore, there is a need to develop simple, portable systems that would enable farmers to directly monitor their soil. Microfluidic paper-based analysis devices (PADs) represent a promising approach, since they are cost effective, allow liquid transport without external pumps, and reagents can be stored in dry form within the paper. Previously reported PADs for manganese utilised 1-(2-pyridylazo)-2-naphthol (PAN) as a colourimentric agent for readout via smartphone camera. However, this approach was based on the use of toxic reagents (borate buffer, cyanide) and required a very specific,1-minute detection time, which is challenging to be met in the field by a low skilled opeartor. 3 Other published approaches had long detection times of >20 min, which are also unsuitable for use by a farmer. 4 Our aim is to develop a workflow which is safe and easy to use by a farmer for on-site routine monitoring of soil quality. A PAD was produced from Whatman 1 paper squares (5 cm x 5 cm) attached onto adhesive tape. To detect manganese, 11 µL of PAN (3.8 mM) in carbonate buffer (pH8, 8% Triton X-100) was added into three locations across this piece of paper and allowed to air dry for 30 min. For analysis, 10 µL of manganese solution (0 – 500 µM) was pipetted on top of the PAN zone and incubated at room temperature to allow colour development. The PAD image was then captured by a scanner and colour intensity determined using ImageJ software. The time to produce a stable colour was found to be 7 min. The linear range of the PAD was 0 to 150 µM and the limit of detection was 10.8 µM (0.59 mg L -1 ). The PAD reported here requires no toxic reagents and yields a colour signal in relevant concentration ranges within a time frame suitable for in-the-field analysis. Future work will focus on sealing of the device for storing purposes and development of simple workflows for manganese extraction to be combined with the developed detection PAD. References
1. Agro. J., 2007, 99(4), 973-983. 2. Front. Plant Sci., 2020, 11, 300. 3. Analytical and Bioanalytical Chemistry, 2020, 412 (6), 1395-1405 4. Analyst, 2021, 146 (7), 2229-2239
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