Colorimetric immunoassays based on pyrroloquinoline quinone-catalyzed generation of Fe(II)-ferrozine with tris(2-carboxyethyl)phosphine as the reducing reagent

[Display omitted] •A novel redox cycling was reported and used for colorimetric immunoassay.•Pyrroloquinoline quinone (PQQ) promoted the reduction of Fe(III)-ferrozine by tris(2-carboxyethyl)phosphine (TCEP).•Mesoporous silica nanoparticle was used as the carrier of PQQ and recognition antibody.•The signal was monitored by the generation of colorful Fe(II)-ferrozine.•A detection limit of 1 pg/mL was obtained for prostate-specific antigen detection. This work reported a novel redox cycling in which pyrroloquinoline quinone (PQQ) promoted the reduction of Fe(III)-ferrozine by tris(2-carboxyethyl)phosphine (TCEP). Specifically, PQQ was reduced into pyrroloquinoline quinol (PQQH2) by TCEP, and then the produced PQQH2 reduced colorless Fe(III)-ferrozine into dull red Fe(II)-ferrozine. The redox cycling was employed for development of colorimetric immunosensors through the generation of colorful Fe(II)-ferrozine. Mesoporous silica nanoparticle (MSN) was used as the carrier for both PQQ and recognition antibody through electrostatic interactions. Magnetic bead (MB) was used as the support for the immobilization of capture antibody. After the sandwich-type immunoreactions and magnetic separation, the MSN-PQQ nanolabels on the MB surface triggered the production of Fe(II)-ferrozine. The limit of detection was found to be 1 pg/mL with prostate specific antigen (PSA) as the model target. The result for the analysis of serum sample is in agreement with that achieved by the commercial enzyme-linked immunosorbent assay kits. The proposed immunosensor obviated the use of enzyme molecules for signal amplification and did not require expensive instruments for signal readout. This work should be valuable for the design of novel nanolabels and the proposed sensing strategy by the redox cycling could be applied to develop more sensitive biosensors.