Redox Recycling-Activated Signal Amplification of Peroxidase-like Catalytic Activity Based on Bare Gold Nanoparticle–Metal Ion Ensembles as Colorimetric Sensor Array for Ultrasensitive Discrimination of Phosphates

In this work, bare gold nanoparticle–metal ion ensembles were screened to build a colorimetric sensor array for the ultrasensitive detection of various phosphates such as adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), pyrophosphate (PPi), and phosphate (Pi). First, we found that the bare gold nanoparticle (BGNP) surface with no functional groups exhibited the highest peroxidase-like activity among hexadecyl trimethylammonium bromide-coated gold nanoparticles (CTAB-GNPs), citric acid-functionalized gold nanoparticles (Cit-GNPs), and BGNPs. Afterward, three metal ions (Ce³⁺, Fe²⁺, and Cr³⁺) were screened to enhance the enzymatic catalytic activity of BGNPs further, greatly through the electrostatic interaction between AuCl₄–/AuCl₂– ions on the surface of BGNPs and metal ions, because the metal ions could activate the redox cycle reaction of the BGNP–metal ions. However, the catalytic activity of BGNP–metal ion ensembles was inhibited after the addition of phosphates because of the high affinities between phosphates and metal ions. Inspired by the above phenomena, a colorimetric sensor array was constructed for ultrasensitive determination and identification of phosphates based on the tunable peroxidase-like activity of BGNP–metal ion ensembles. The colorless substrates 3,3′,5,5′-tetramethylbenzidine (TMB) were catalytically oxidized by BGNP–metal ion ensembles to form a blue oxidation product (ox-TMB) as the color signal. This sensor array consisted of three sensing elements (BGNPs-Ce³⁺, BGNPs-Fe²⁺, and BGNPs-Cr³⁺) and generated cross-reactive signals with each phosphate because of the high difference in the number of phosphate groups, geometry, steric effect, and binding sites. The colorimetric sensor array was successfully applied to differentiate five phosphates at 0.25 μM in aqueous solutions. More importantly, the as-designed sensor array was successfully explored to monitor nucleotide phosphate-involved enzymatic hydrolysis processes and their metabolites as well as to analyze phosphates in biological samples.