Paper-based analytical device (PAD) has received more and more attention in the field of point-of-care test (POCT) due to its low cost, portability and simple operation. Sensitivity and selectivity are two important aspects in clinical diagnostic analysis. However, low sensitivity of a PAD limits its wider application for POCT. Here we introduced a PAD that can clean and enrich the protein content from salty media with both electric field (E) and pH gradient (double E/pH gradients), with which 100-fold enrichment effect could be achieved within 70 s as demonstrated by fluorescein isothiocyanate labeled bovine serum albumin (FITC-BSA) from artificial urine media. With post staining of the protein stacking band with bromophenol blue (BPB), selective colorimetric detection of human serum albumin (HSA) was achieved simply with smartphone camera in the clinically significant range of 10–300 mg‧L −1 (R 2 = 0.99) with a limit of detection (LOD) of 4.9 mg‧L −1 . Detection of microalbuminuria (MAU) of diabetic patients was demonstrated with this method without difference (ɑ = 0.01) to that by the clinical method (immunoturbidimetry). This work demonstrated the potential of this PAD method in online sample pretreatment and detection of target component from complex physiological samples.
Charged components can be effectively separated and enriched based on ion concentration polarization (ICP) effect for sample clean up and enhancement of detection sensitivity in analytical chemistry. Concerning the role of electrolyte, current studies on electrokinetic stacking with ICP mainly focused on the ionic strength, and the knowledge about the type of supporting electrolyte (SE) is still poor. In this study, the significance of acid–base property of SE was systematically investigated based on an ICP interface established on a paper fluidic device with ion exchange membrane. Using colored ion probes for visualization of the effects, the ion depletion zone as the working area was applied for electrokinetic stacking. It has been shown that, besides the proper electroosmotic flow and ionic strength decided by neutral salt, the acid–base property of SE plays an important role for a successful stacking with ICP. The guideline for SE selection was proposed, and the mechanism was well explained by correlating the water dissociation effect and electric field/pH gradient (double gradients) at the ion depletion zone. This rule proposal is substantial for the design and operation of electrokinetic enrichment and separation of targeted species using ICP effect.
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