Bilayer hydrogel sensor boosted ng-level solid nitrite detection through asymmetrical diffusion construction facilitated colorimetric signal enhancement

In solid targets analyzing, independent sampling pretreatment steps are usually necessary to ensure the sensitivity and robustness. Although integrated devices provide one-step sampling-analysis strategy, the enhancement of the promotion of the safe sampling and ultrasensitive sensing still needs deep-going investigation. Here, a bilayer hydrogel device was designed to realize on-site sampling and reaction with the restriction of the colorimetric signal dispersion by constructing asymmetrical diffusion. The restricted diffusion tendency of the reaction product in the bilayer hydrogel system was verified by mapping the reaction distribution. The detection of nitrite was realized through the bilayer hydrogel device with a good linear relationship towards solid nitrite residues in a range of 0–20 nmol and a naked-eye detection limit of 2.898 ng (0.042 nmol). The applicability in the analysis of practical scenarios was demonstrated by the well-sampling of the nitrite residues on different surfaces and the retention of the pattern shapes. A portable colorimetric detector incorporated with this hydrogel device was explored and showed a promising utilization for the on-site detection of nitrite residues on trolley case, bag and pickles. We expect that this bilayer hydrogel signal enhancing strategy will be favorable for the ultrasensitive probe-based analyses and integrated sensing device investigations.