Simultaneous enrichment and separation based on ion concentration polarization effect on a paper based analytical device.

In this paper, simultaneous enrichment and separation of ions and amphoteric components were successfully demonstrated by using electric field (E) and pH gradient (double gradient) in the ion depletion zone of anion concentration polarization interface established on a paper fluid channel. Experimental results were visualized with colored ions (bright blue and amaranth) and protein probes (phycocyanin and cytochrome C). With optimization, colored phycocyanin and bovine hemoglobin with similar pI as that of albumin and immunoglobulin respectively were well separated in 900 s with 10-fold enrichment effect. Based on the separation and enrichment function of this paper-based analytical device (PAD) and rapid selective staining of human serum albumin (HSA) with bromophenol blue, a rapid colorimetric detection of HSA was implemented with smartphone camera. A limit of detection (LOD) of 5.2 mg·L-1 was achieved in the clinically significant range of 10-300 mg·L-1 (R2 = 0.99). This method was applied to real human urine samples with good agreement (ɑ = 0.01) to clinical detection method (immunoturbidimetry). With the separation and enrichment functions of PAD, both the specificity and sensitivity were enhanced, which provides a solid basis for point-of-care test of microalbuminuria. Therefore, this PAD device is potential for sample pretreatment and detection of target components from complex physiological samples.

Salty Biofluidic Sample Clean-Up and Preconcentration with a Paper-Based Ion Concentration Polarization Interface.

Charged species from complex media could be separated and enriched taking advantage of ion concentration polarization (ICP) effect; thus, ICP can be used for sample purification and improvement of detection sensitivity. In this paper, a novel and reliable ICP interface was established on a paper-based analytical device (PAD) by using ion exchange membrane, and electrokinetic stacking of target analytes from salty media was successfully demonstrated. Steady ICP effect was well observed in aqueous solution with up to 400 mM NaCl as shown by a fluorescent probe, which makes it possible to directly process salty physiological samples such as blood and urine with this type of PAD. Application of this method was demonstrated by direct online stacking of total protein from urine samples and image-based colorimetric detection by a smartphone camera. The linear response was in the range of 50-350 mg/L (R2 = 0.99), with recovery rate in the range of 94.8-107.6% and relative standard deviation below 7.1%. The obtained results were consistent with that of the clinical method. As an off-line sample pretreatment method, the feasibility for rapid tandem mass spectrometry detection of amino acids from serum samples was also investigated, and promising results were obtained. This PAD method is of low cost, easy to operate, and reliable. As a disposable PAD, it is useful not only for sensitive point-of-care testing but also for direct purification and concentration of complex and highly conductive physiological samples for fast and accurate detection with advanced analytical instruments.