Method development for on-site freshwater analysis with pre-concentration of nickel via ion-exchange resins embedded in a cafeti&#232;re system and paper-based analytical devices for readout (2022)

&amp;lt;p&amp;gt;&amp;lt;strong&amp;gt;Keywords:&amp;lt;/strong&amp;gt; preconcentration, heavy metal, cafetiere, citizen science, paper-based microfluidics&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Heavy-metal analysis of water samples using microfluidics paper-based analytical devices (&amp;amp;#181;PAD) with colourimetric readout is of great interest due to its simplicity, affordability and potential for Citizen Science-based data collection [1]. However, this approach is limited by the relatively poor sensitivity of the colourimetric substrates, typically achieving detection within the mg L&amp;lt;sup&amp;gt;-1&amp;lt;/sup&amp;gt; range, whereas heavy-metals exist in the environment at &amp;lt;&amp;amp;#956;g L&amp;lt;sup&amp;gt;-1&amp;lt;/sup&amp;gt; quantities&amp;amp;#160;&amp;amp;#160; [2]. Preconcentration is commonly used when analyte concentration is below the analytical range, but this typically requires laboratory equipment and expert users [3]. Here, we are developing a simple method for pre-concentration of heavy metals, to be integrated with a &amp;amp;#181;PAD workflow that would allow Citizen Scientists to carry out pre-concentration as well as readout on-site.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;The filter mesh from an off-the-shelf cafeti&amp;amp;#232;re (350 mL) was replaced with a custom-made bead carrier basket, laser cut in PMMA sheet featuring &amp;gt;500 evenly spread 100 &amp;amp;#181;m diameter holes. This allowed the water sample to pass through the basket and mix efficiently with the 2.6 g ion-exchange resin beads housed within (Lewatit&amp;lt;sup&amp;gt;&amp;amp;#174;&amp;lt;/sup&amp;gt; TP207, Ambersep&amp;lt;sup&amp;gt;&amp;amp;#174;&amp;lt;/sup&amp;gt; M4195, Lewatit&amp;lt;sup&amp;gt;&amp;amp;#174;&amp;lt;/sup&amp;gt; MonoPlus SP 112). An aqueous Ni&amp;lt;sup&amp;gt;2+&amp;lt;/sup&amp;gt; sample (0.3 mg L&amp;lt;sup&amp;gt;-1&amp;lt;/sup&amp;gt;, 300 mL) was placed in the cafetiere and the basket containing ion exchange material was moved up and down for 5 min to allow Ni&amp;lt;sup&amp;gt;2+&amp;lt;/sup&amp;gt; adsorption onto the resin. Initial investigations into elution with a safe, non-toxic eluent focused on using NaCl (5 M). These were carried out by placing the elution solution into a shallow dish and into which the the resin containing carrier basket was submerging. UV/vis spectroscopy via a colourimetric reaction with nioxime was used to monitor Ni&amp;lt;sup&amp;gt;2+&amp;lt;/sup&amp;gt; absorption and elution.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;After 5 min of mixing it was found that Lewatit&amp;lt;sup&amp;gt;&amp;amp;#174;&amp;lt;/sup&amp;gt; TP207 and Ambersep&amp;lt;sup&amp;gt;&amp;amp;#174;&amp;lt;/sup&amp;gt; M4195 resins adsorbed up to 90% of the Ni&amp;lt;sup&amp;gt;2+&amp;lt;/sup&amp;gt; ions present in solution and the Lewatit&amp;lt;sup&amp;gt;&amp;amp;#174;&amp;lt;/sup&amp;gt; MonoPlus SP 112 adsorbed up to 60%. However, the Lewatit&amp;lt;sup&amp;gt;&amp;amp;#174;&amp;lt;/sup&amp;gt; MonoPlus SP 112 resin performed better for elution with NaCl. Initial studies showed up to 30% of the Ni&amp;lt;sup&amp;gt;2+&amp;lt;/sup&amp;gt; was eluted within only 1 min of mixing with 10 mL 5 M NaCl.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Using a cafeti&amp;amp;#232;re as pre-concentration vessel coupled with non-hazardous reagents in the pre-concentration process allows involvement of citizen scientists in more advanced environmental monitoring activities that cannot be achieved with a simple paper-based sensor alone. Future work will investigate the user-friendliness of the design by trialling the system with volunteers and will aim to further improve the trapping and elution efficiencies.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;lt;strong&amp;gt;References&amp;lt;/strong&amp;gt;:&amp;lt;/p&amp;gt;&amp;lt;ul&amp;gt;&amp;lt;li&amp;gt;Almeida, M., et al., Talanta, 2018, &amp;lt;strong&amp;gt;177, &amp;lt;/strong&amp;gt;176-190.&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;Lace, A., J. Cleary, Chemosens., 2021. &amp;lt;strong&amp;gt;9&amp;lt;/strong&amp;gt;, 60.&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;Alahmad, W., et al.&amp;lt;em&amp;gt;.&amp;lt;/em&amp;gt; Biosens. Bioelectron., 2021. &amp;lt;strong&amp;gt;194, &amp;lt;/strong&amp;gt;113574.&amp;lt;/li&amp;gt; &amp;lt;/ul&amp;gt;&amp;lt;p&amp;gt;&amp;amp;#160;&amp;lt;/p&amp;gt;