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.

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.

Performance of electrokinetic stacking enhanced paper-based analytical device with smartphone for fast detection of fluorescent whitening agent.

Quantification is a fundamental aspect of performance of an analytical system. Paper-based analytical device (PAD) as an on-site detection platform has drawn wide attention mainly due to its portability and cost effectiveness. In this work, a portable and low-cost PAD for online preconcentration and sensitive determination of fluorescent whitening agent (FWA) was demonstrated, which was consisted of ultra violet light-emitting diode (UV LED), macro-focusing lens, smartphone and miniaturized DC voltage source. Taking a widely used FWA component VBL as the analyte, the performance of the PAD enhanced with electrokinetic stacking (ES) and fluorescence imaging detection was systematically investigated. With ES, the sensitivity of the PAD system was 160-fold enhanced, and a limit of detection (LOD) of 0.06 µg mL-1 was achieved. The dynamic range was 0.1-10.0 µg mL-1 (linear in 0.1-0.7 µg mL-1, R2 = 0.99). With manual operation, the relative standard deviation (RSD) of intra-day and inter-day were all below 15%. Eventually, VBL from different napkin samples and toilet paper was determined with average recovery rates in the range of 90%-95% (RSD = 8.0%-12.0%). This work shows that with ES, the sensitivity of PAD can be greatly improved, and a LOD close to a desktop fluorescent spectrophotometer can be achieved as demonstrated by the detection of FWA component.

Highly efficient sample stacking by enhanced field amplification on a simple paper device.

We present a novel electrokinetic stacking (ES) method based on field amplification on a simple paper device for sample preconcentration. With voltage application, charged probe ions in a solution of lower conductivity stack and form a narrow band at the boundary between the sample and the background electrolyte of higher conductivity. The stacking band appears quickly and stabilizes in a few minutes. With this ES method, three orders of magnitude signal improvement was successfully achieved for both a fluorescein probe and a double-stranded DNA within 300 s. This enhanced stacking efficiency is attributed to a focusing effect due to the balance between electromigration and counter electroosmotic flow. We also applied this ES method to other low-cost fiber substrates such as cloth and thread. Such a simple and highly efficient ES method will find wide applications in the development of sensitive paper-based analytical devices (PADs), especially for low-cost point-of-care testing (POCT).