we assembled 3-mercaptopropionic acid(MPA)on gold nanoparticles to form three-dimensional(3D)monolayer. The 3Dmonolayer of MPA showed effective electrocatalytic activity towards some small biomolecules such asNADH，ascorbic acid，uric acid and dopamine，which was different from two-dimensional(2D)MPAmonolayer assembled on planar gold electrode. This result implies that 3D monolayers may have promisingapplications in electrocatalysis and electroanalysis. To provide further illustration，we investigated theelectrochemical behavior of Cu(ID on 3D monolayer of MPA. The results demonstrated that 3D MPAmonolayer inhibited the charging current while promoted the response of accumulated Cu(In on electrodesurface，which presented a voltammetric response linearly proportional to the Cu(ID concentration from 0.1μg/L to 100 μg/L，with a detection limit of 3.2 μg/L. Based on such a 3D monolayer modified electrode,trace Cu(II)in tap water was successfully determined. Moreover，we also prepared a 3D monolayer ofL-cysteine on gold nanoparticles for observing the voltammetric behavior of Cu(II).
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