iPhone-imaged and cell-powered electrophoresis titration chip for the alkaline phosphatase assay in serum by the moving reaction boundary

Abstract

As a vital enzyme, alkaline phosphatase (ALP) has great clinical significance in diagnoses of bone or liver cancer, bone metastases, rickets, and extrahepatic biliary obstruction. However, there is still no really portable chip for the ALP assay in blood. Herein, a simple electrophoresis titration (ET) model was developed for ALP detection via a moving reaction boundary (MRB). In the model, ALP catalyzed the dephosphorylation of a 4-methylumbelliferyl phosphate disodium salt (4-MUP) substrate in the cathode well to 4-methylumbelliferone ([4-MU]⁻) with a negative charge and blue fluorescence under UV excitation. After the catalysis, an electric field was used between the cathode and the anode. Under the electric field, [4-MU]⁻ moved into the channel and neutralized the acidic Tris-HCl buffer, resulting in the quenching of [4-MU]⁻ and creating a MRB. The ET system just had an ET chip, a lithium cell, a UV LED and an iPhone used as a recorder, having no traditional expensive power supply and fluorescence detector. The relevant method was developed, and a series of experiments were conducted via the ET chip. The experiments showed: (i) a MRB could be formed between the [4-MU]⁻ base and the acidic buffer, and the MRB motion had a linear relationship with the ALP activity, validating the ET model; (ii) the ET run was not impacted by many interferences, implying good selectivity; and (iii) the ET chip could be used for portable detection within 10 min, implying an on-site and rapid analysis. In addition, the ET method had a relatively good sensitivity (0.1 U L⁻¹), linearity (V = 0.033A + 3.87, R² = 0.9980), stability (RSD 2.4–6.8%) and recoveries (101–105%). Finally, the ET method was successfully used for ALP assays in real serum samples. All the results implied that the developed method was simple, rapid and low-cost, and had potential for POCT clinical ALP assays.