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Ammonium Hydroxide Sensing Based on LSPR of Phosphatidylcholine-Modified Gold Nanorods

Gold nanorods (GNRs) were fabricated through a seed-mediated growth procedure and a surface modified by phosphatidylcholine (PC) for ammonium hydroxide sensing. Hexadecyltrimethylammonium bromide, the protective agent of GNRs, was replaced by PC in chloroform solution to endow GNRs with ammonium hydroxide sensitivity. The replacement was proved by ultraviolet-visible (UV-vis) spectroscopy, zeta potential measurement, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy. Due to the PC replacement, a 9.2-nm blue shift in the longitudinal plasma absorption band (LPAB) of the UV-vis spectrum and a 16.8-mV decrease in the zeta potential value of the product were measured. The characteristic FTIR signals of PC occur in the spectrum of PC-modified GNRs. Response to ammonium hydroxide was characterized by UV-vis spectroscopy. A linear relationship between the LPAB intensity and ammonium hydroxide amount was revealed, and a sensitivity of 11.46 a.u. per 1% ammonium hydroxide (g/ml, the weight of NH3 versus the volume of solution under test) was obtained. The response time was 180 ± 30 s for the ammonia content up to 0.065%. The minimum detection limitation is 0.004%, far below a capacitance-type ammonium hydroxide detector widely used in industry (0.1%). The results demonstrate the potential application prospect of PC-modified GNRs as an ammonium hydroxide sensor.