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
Authors: Sari Mila , Richardson Samantha , Mayes Will , Lorch Mark , Pamme Nicole 
Published: March 2022

&lt;p&gt;&lt;strong&gt;Keywords:&lt;/strong&gt; preconcentration, heavy metal, cafetiere, citizen science, paper-based microfluidics&lt;/p&gt;&lt;p&gt;Heavy-metal analysis of water samples using microfluidics paper-based analytical devices (&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&lt;sup&gt;-1&lt;/sup&gt; range, whereas heavy-metals exist in the environment at &lt;&amp;#956;g L&lt;sup&gt;-1&lt;/sup&gt; quantities&amp;#160;&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;#181;PAD workflow that would allow Citizen Scientists to carry out pre-concentration as well as readout on-site.&lt;/p&gt;&lt;p&gt;The filter mesh from an off-the-shelf cafeti&amp;#232;re (350 mL) was replaced with a custom-made bead carrier basket, laser cut in PMMA sheet featuring &gt;500 evenly spread 100 &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&lt;sup&gt;&amp;#174;&lt;/sup&gt; TP207, Ambersep&lt;sup&gt;&amp;#174;&lt;/sup&gt; M4195, Lewatit&lt;sup&gt;&amp;#174;&lt;/sup&gt; MonoPlus SP 112). An aqueous Ni&lt;sup&gt;2+&lt;/sup&gt; sample (0.3 mg L&lt;sup&gt;-1&lt;/sup&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&lt;sup&gt;2+&lt;/sup&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&lt;sup&gt;2+&lt;/sup&gt; absorption and elution.&lt;/p&gt;&lt;p&gt;After 5 min of mixing it was found that Lewatit&lt;sup&gt;&amp;#174;&lt;/sup&gt; TP207 and Ambersep&lt;sup&gt;&amp;#174;&lt;/sup&gt; M4195 resins adsorbed up to 90% of the Ni&lt;sup&gt;2+&lt;/sup&gt; ions present in solution and the Lewatit&lt;sup&gt;&amp;#174;&lt;/sup&gt; MonoPlus SP 112 adsorbed up to 60%. However, the Lewatit&lt;sup&gt;&amp;#174;&lt;/sup&gt; MonoPlus SP 112 resin performed better for elution with NaCl. Initial studies showed up to 30% of the Ni&lt;sup&gt;2+&lt;/sup&gt; was eluted within only 1 min of mixing with 10 mL 5 M NaCl.&lt;/p&gt;&lt;p&gt;Using a cafeti&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.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;:&lt;/p&gt;&lt;ul&gt;&lt;li&gt;Almeida, M., et al., Talanta, 2018, &lt;strong&gt;177, &lt;/strong&gt;176-190.&lt;/li&gt;
&lt;li&gt;Lace, A., J. Cleary, Chemosens., 2021. &lt;strong&gt;9&lt;/strong&gt;, 60.&lt;/li&gt;
&lt;li&gt;Alahmad, W., et al.&lt;em&gt;.&lt;/em&gt; Biosens. Bioelectron., 2021. &lt;strong&gt;194, &lt;/strong&gt;113574.&lt;/li&gt;
&lt;/ul&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;