4D-Printed Temperature-Switchable Monolithic Packing for NIR-Actuated Elemental Speciation Analysis

Four-dimensional printing (4DP) technologies drive the use of stimuli-responsive materials in the fabrication of versatile analytical devices. In this study, a monolithic packing comprising a nonresponsive part and a graphene-incorporated thermoresponsive part was fabricated using digital light processing 4DP technique and N-isopropylacrylamide (NIPAM)-based photocurable resins with and without graphene to enable elemental speciation analysis in one solid-phase extraction column. The nonresponsive part selectively extracted Cr(III), As(III), and Se(IV) when its temperature was lower than the volume phase transition temperature (VPTT) of the polyNIPAM hydrogel. The graphene-incorporated part became hydrophobic for the selective extraction of Cr(VI), As(V), and Se(VI) when near-infrared (NIR) irradiation was applied to increase its temperature (>VPTT), resulting in the NIR-actuated switching of monolithic packing's hydrophobicity for the speciation analysis of inorganic Cr, As, and Se. After optimizing the design, fabrication, and NIR actuation of the monolithic packing, the automated speciation scheme was coupled with inductively coupled plasma mass spectrometry to enable highly sensitive speciation of inorganic Cr, As, and Se; the method detection limits and absolute extraction efficiencies of these species were 0.1-0.6 ng L-1 and >87.3%, respectively. These species were analyzed in reference materials (CASS-6, SLRS-5, 1643f, and Seronorm Trace Elements Urine L-2) and environmental water and human urine samples to validate the reliability of this method. The results indicated that the 4D-printed NIR-actuated temperature-switchable monolithic packing showed promising capability and applicability to advance nonchromatographic speciation analysis of trace elements.