Sensitive paper-based analytical device for fast colorimetric detection of nitrite with smartphone.

On-site rapid monitoring of nitrite as an assessment indicator of the environment, food, and physiological systems has drawn extensive attention. Here, electrokinetic stacking (ES) was combined with colorimetric reaction on a paper-based device (PAD) to achieve colorless nitrite detection with smartphone. In this paper, nitrite was stacked on the paper fluidic channel as a narrow band by electrokinetic stacking. Then, Griess reagent was introduced to visualize the stacking band. Under optimal conditions, the sensitivity of nitrite was 160-fold increased within 5 min. A linear response in the range of 0.075 to 1.0 µg mL-1 (R2 = 0.99) and a limit of detection (LOD) of 73 ng mL-1 (0.86 µM) were obtained. The LOD was 10 times lower than the reported PAD, and close to that achieved by a desktop spectrophotometer. The applicability was demonstrated by nitrite detection from saliva and water with good selectivity, adding 100 times more concentrated co-ions. High recovery (91.0~108.7%) and reasonable intra-day and inter-day reproducibility (RSD < 9%) were obtained. This work shows that the sensitivity of colorless analyte detection-based colorimetric reaction can be effectively enhanced by integration of ES on a PAD. Graphical abstract Schematic of the experimental setups (left) and the corresponding images (right) of the actual portable device.

Electrokinetic stacking of electrically neutral analytes with paper-based analytical device.

Electrokinetic stacking (ES) is effective for improving sensitivity of paper-based analytical device (PAD) for charged analytes. In this paper, we successfully demonstrated ES of electrically neutral analytes on PAD, and the performance was characterized by smartphone-based colorimetry and fluorescence. Firstly, SDS from cathode reservoir stacked as a micelle band on an open paper fluidic channel by ES, and the target analyte was swept by the micelle. Meanwhile, the probes at the other side were carried by electroosmotic flow (EOF). Eventually, neutral components preloaded on the channel were concentrated as the narrow stacking band. Taking the rhodamine B as a probe, the effects of EOF, background electrolyte concentration and anionic surfactant concentration were investigated. Fluorescence detection of rhodamine B and colorimetric analysis of Sudan III demonstrated the sensitivity enhanced and its potential for the semi-quantitative test. Under the optimized conditions, fluorescence detection limit of 50 nM of rhodamine B was achieved with a linear range of 1.0-10 µM (R2 = 0.99). The colorimetric detection limit for Sudan III was 5.2 µM and the linear range was 5-40 µM (R2= 0.99). Compared with direct analysis without stacking, the signal levels of rhodamine B and Sudan III were increased by 30-fold and 6-fold, respectively. This study showed that with ES, sensitive and rapid PAD detection of electrically neutral analytes could be achieved.