Fluorescence hydrogel array based on interfacial cation exchange amplification for highly sensitive microRNA detection. 
Despite the great success of fluorescence sensing array application in bioanalysis, the photo-quenching problem accompanied with high surface fluorophore density has become the bottleneck for its further sensitivity improvement.  Herein, a fluorescence polyethylene glycol (PEG) hydrogel array was developed for highly sensitive microRNA detection based on interfacial cation exchange amplification coupled with DNA hybridization chain reaction (HCR).  The carboxylate PEG hydrogel array was fabricated on glass slide via photo polymerization and modified with miRNA capture probe, target miRNA binding triggered HCR in hydrogel with biotin labelled DNA probes to generate numerous DNA polymer chains where biotinylated CdS QDs were subsequently conjugated with a streptavidin bridge.  Interfacial cation exchange amplification was triggered in hydrogel upon the introduction of Ag+ and Rhod-5N, and abundant Cd2+ was released from CdS to bind with Rhod-5N for substantial fluorescence enhancement.  The aqueous-like environment of hydrogel eliminated the fluorescence quenching and simplified experiment process by performing cation exchange reaction at interface with direct result.  The linear range for model target miRNA-21 was 1 fM to 500 pM with a detection limit of 0.835 fM. Taking advantage of the nonfouling property of PEG hydrogel, direct quantification of miRNA-21 was achieved from crude cancer cell lysates with a detection limit down to 10 equivalent cells.  The expressions of circulating miRNA-21 in clinical serum samples were also assessed with comparable results from RT-PCR.  The developed hydrogel array provides a universal platform for highly sensitive fluorescence sensing, and would benefit clinical non-invasive disease diagnosis.