Sensing of H2O2 at low surface density assemblies of Pt nanoparticles in polyelectrolyte

Amperometric detection of H2O2 was studied at random arrays of 2.5 nm polyacrylate-capped Pt nanoparticles (NP) assembled in poly(diallydimethylammonium chloride), PDDA, as a function of NP surface coverage. The arrays were assembled by varying the adsorption time of PDDA-modified electrodes in the nanoparticles solution. Pt NP-on-PDDA assemblies exhibited significant sensitivity and stability facing constant anodic polarization and a low limit of detection at small Pt mass in submonolayer coverage. The current output was measured at approximately 0.5 A M(-1) cm(-2)(geom) over a linear range from 42 nM to 0.16 mM H2O2 at a loading of 0.87 microg(Pt)/cm(2) or an estimated coverage of 0.4 of an assumed monolayer, or higher, and decreased with decreasing NP surface density to 0.2 A M(-1) cm(-2)(geom) at a Pt loading of 190 ng/cm. On the other hand, the intrinsic sensitivity measured relative to the real Pt surface area increased with decreasing coverage and reached a significant limiting value of 0.9 A M(-1) cm(-2) real at approximately 190-380 ng/cm(2). The behavior shows a significant effective turnover rate per Pt site and mass (1 A M(-1)/microg of Pt) in loosely packed assemblies, while overlap of individual diffusion fields (of particles or islands) and inaccessibility of some active sites lowers the sensitivity per nanoparticle in densely packed arrays. The reported trend agrees with the behavior of ultramicroelectrode arrays.