Enzymatic-reaction induced production of polydopamine nanoparticles for sensitive and visual sensing of urea

Dopamine (DA) has attracted extensive interest due to not only its important roles in physiological and pathological processes, but also its prospective applications in chemistry and materials science. In this work, we demonstrate that the urease catalytic reaction is an effective new approach for a better control of DA polymerization to polydopamine nanoparticles (PDA NPs). And we further develop an original and novel method for sensitive and visual sensing of urea through spectroscopic or particle size analysis. The detection is based on DA polymerization to PDA NPs that can be controlled by the reaction rate of urease-catalyzed urea hydrolysis, correspondingly, correlated with the varied urea concentration. The composition, morphologies and sizes of the resulting PDA NPs are characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and dynamic light scattering (DLS) spectroscopy, respectively. Under optimal reaction conditions, the UV absorbance of DA polymerization at 400 nm shows a good response towards urea detection over a range of 1 × 10−7 to 1 × 10−3 M with a limit of detection (LOD) of 100 nM (S/N = 3). Moreover, the sizes of the resulting PDA NPs increase linearly with urea concentration from 5 × 10−6 to 1 × 10−4 M. The newly developed assay allows the enzymatic-reaction driven PDA NPs to be used for quantitative detection of urea with many advantages, e.g. simple preparation, easy visualization, good sensitivity, wide detection range and low interference, in particular, no complex sensor-fabrication required.