4D-printed pH sensing claw.

As an example of extending the functionality of analytical devices manufactured using four-dimensional printing (4DP), in this study we employed acrylonitrile butadiene styrene (ABS) thermoplastic filaments, poly(4-vinylpyridine) (P4VP)-incorporated ABS filaments, and multi-material fused deposition modeling three-dimensional printing (3DP) to fabricate a pH measurement device that underwent pH-dependent geometric changes. Upon immersion in a solution having a pH close to the value of pK of P4VP (ca. 5.0), electrostatic repulsion among the protonated units of P4VP resulted in swelling only of the part printed using the P4VP-incorporated ABS filaments, leading to lifting of the whole device along the z-axis (ΔH) in a pH-dependent manner. After optimizing the device's design and fabrication, this 4D-printed pH sensing claw exhibited linearity between the value of ΔH and values of pH in the range from 5.0 to 8.6. We used this 4D-printed pH sensing claw to perform pH analyses of complicated real samples, verifying its analytical reliability for non-electrochemical and non-optical pH measurement and highlighting the capability of 4DP technologies in the direct fabrication of stimuli-responsive sensing devices. We envision that 4DP technologies will prompt the manufacture of smart sensing devices through the printing of stimuli-responsive materials, thereby diversifying the development of 3DP-enabling analytical chemistry.