Photoreduction of Ag+ ions to AgNPs
This protocol is extracted from research article:
Nicotinamide adenine dinucleotide as a photocatalyst
Sci Adv, Jul 19, 2019; DOI: 10.1126/sciadv.aax0501

For photochemical formation of AgNPs, NAD+ and AgNO3 were dissolved in a MOPS buffer (50 mM, pH 7.5) and irradiated with a xenon lamp (Newport Co., USA) equipped with a water filter at 293.15 K. In this experiment, a sodium phosphate buffer was not used because silver phosphate precipitates (Ag3PO4) form by the reaction between silver ions and phosphate ions. An LSPR band of AgNPs was monitored using a V-650 UV-Vis absorption spectrophotometer (JASCO Inc., Japan). Before obtaining UV-Vis absorption spectra, the samples in a MOPS buffer were diluted 10-fold, whereas those in water were not diluted because the formation rate of AgNPs in a MOPS buffer was much higher than that in water. We observed AgNPs using a JEM 3010 transmission electron microscope (JEOL Co., Japan) at 300 kV. The quantification of AgNPs was conducted using an inductively coupled plasma mass spectrometer (7700x, Agilent Technologies, USA). Before mass spectrometric analysis, a reaction sample was put in a dialysis tubing (molecular weight cutoff, 500 to 1000) against deionized water for 24 hours to eliminate Ag+ ions in the sample. AgNPs were also quantified using a microbalance after their purification by centrifugation. We synthesized AgNPs in an Eppendorf tube, the mass of which was measured using a microbalance. After the photochemical reaction, the tube was centrifuged at 27,237g for 25 min, making AgNPs concentrated as a dark pellet. The supernatant was discarded as much as possible, and deionized water was added to the tube. This washing process was repeated five times, but deionized water was not added at the last repetition. The residual water was evaporated in a vacuum chamber for 10 hours, and the mass of the Eppendorf tube containing AgNPs was measured using the microbalance to calculate the mass of AgNPs.

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