Design and synthesis of target-responsive hydrogel for portable visual quantitative detection of uranium with a microfluidic distance-based readout device

Due to uranium's increasing exploitation in nuclear energy and its toxicity to human health, it is of great significance to detect uranium contamination. In particular, development of a rapid, sensitive and portable method is important for personal health care for those who frequently come into contact with uranium ore mining or who investigate leaks at nuclear power plants. The most stable form of uranium in water is uranyl ion (UO22+). In this work, a UO22+ responsive smart hydrogel was designed and synthesized for rapid, portable, sensitive detection of UO22+. A UO22+ dependent DNAzyme complex composed of substrate strand and enzyme strand was utilized to crosslink DNA-grafted polyacrylamide chains to form a DNA hydrogel. Colorimetric analysis was achieved by encapsulating gold nanoparticles (AuNPs) in the DNAzyme-crosslinked hydrogel to indicate the concentration of UO22+. Without UO22+, the enzyme strand is not active. The presence of UO22+ in the sample activates the enzyme strand and triggers the cleavage of the substrate strand from the enzyme strand, thereby decreasing the density of crosslinkers and destabilizing the hydrogel, which then releases the encapsulated AuNPs. As low as 100 nM UO22+ was visually detected by the naked eye. The target-responsive hydrogel was also demonstrated to be applicable in natural water spiked with UO22+. Furthermore, to avoid the visual errors caused by naked eye observation, a previously developed volumetric bar-chart chip (V-Chip) was used to quantitatively detect UO22+ concentrations in water by encapsulating Au-Pt nanoparticles in the hydrogel. The UO22+ concentrations were visually quantified from the travelling distance of ink-bar on the V-Chip. The method can be used for portable and quantitative detection of uranium in field applications without skilled operators and sophisticated instruments.