4D-printed Cd²⁺-responsive rolling needle panel meter

Determining cadmium (Cd) is essential for assessing Cd contamination and its toxic effects. Four-dimensional printing (4DP) technologies enable stimuli-induced, time-dependent shape programming of fabricated devices and are transforming fabrication processes and enhancing the geometric functionalities of stimuli-responsive devices. However, analytical approaches that leverage the degree of shape programming of 4D-printed ion-responsive sensing devices for metal ion detection remain limited. 4D-printed portable ion-responsive sensing devices hold great promise for reducing the complexity and cost of reliable on-site metal ion detection.

We employed a digital light processing 4DP technique with photocurable resins incorporating 2,2'-(ethylenedioxy)diethanethiol (EDT) to fabricate a Cd²⁺-responsive rolling needle comprising EDT-incorporated parallel active fibers and a nonresponsive layer. Pre-treatment of the needle with hydrogen peroxide oxidized the thioether groups in the copolymer, resulting in the formation of sulfoxide and sulfone groups. Due to electrostatic interactions and selective complexation between Cd²⁺ ions and these sulfoxide and sulfone groups (dissociation constant: 14.1 nM), the unequal swelling of the EDT-incorporated active fibers induced the [Cd²⁺]-dependent rolling of the needle. Furthermore, the rolling needle returned to its as-printed state after treatment with a sodium borohydride solution, demonstrating reversible shape programming for repeated Cd²⁺ determination. After optimization, the method detection limit was determined to be 0.03 μg L^-1. We validated the reliability and applicability of this method by determining Cd²⁺ in reference materials as well as in environmental water, food, and human urine samples.

This study is the first to demonstrate a 4D-printed ion-responsive sensing device that incorporated practical geometric functions to enable on-site quantitative analysis of heavy metals. The novelty and significance of this study lie in the pioneering use of 4DP technologies with ion-responsive materials to fabricate a reversible Cd²⁺-responsive sensing device for on-site determination of Cd²⁺ based on ion-responsive shape programming, thereby eliminating the need for expensive measurement apparatus and atomic spectrometric methods.