Liquid Plasticine Integrated with Isoelectric Focusing for Miniaturized Protein Analysis

Developing miniaturized and rapid protein analytical platforms is urgently needed for on-site protein analysis, which is important for disease diagnosis and monitoring. Liquid marbles (LMs), a kind of particle-coated droplets, as ideal microreactors have been used in various fields. However, their application as analytical platforms is limited due to the difficulty of pretreating complex samples in simple LMs. Herein, inspired by the microfluidic chip, we propose a strategy through fabricating fluid channels using deformable LM, termed liquid plasticine (LP), to achieve sample pretreatment function. Through combining isoelectric focusing (IEF) with an LP channel, an LP-IEF platform with simultaneous protein separation and concentration functions is realized. The pretreatment capability of the LP-IEF system for proteins in physiological samples is proven using standard proteins and human serum with the results of a clear separation, 10-fold concentration, and a resolution of 0.03 pH. Through cutting the LP after IEF to LMs and transiting isolated LMs containing target proteins for further downstream colorimetric and mass spectrometry measurements, the quantitative analysis of clinical microalbuminuria and identification of α-1-microglobulin/bikunin precursor in clinical diabetic urine samples are achieved. This work proposes a strategy to develop LMs/LPs as a multifunctional integrated analytical platform and the miniaturized LP-IEF device as a rapid protein analytical platform.