Highly sensitive and selective tryptophan colorimetric sensor based on 4,4-bipyridine-functionalized silver nanoparticles

This paper reports a novel colorimetric sensor for tryptophan based on 4,4-bipyridine-functionalized silver nanoparticles (4-DPD-Ag NPs) which were characterized by IR spectroscopy, ultraviolet–visible spectroscopy (UV–vis), and transmission electron microscopy (TEM). The newly synthesized 4-DPD-Ag NPs are yellow in color due to the intense surface plasmon absorption band centered at 390 nm. In the presence of tryptophan, the yellow 4-DPD-Ag NPs solution turns to red accompanying a dramatic surface plasmon absorption band centered at 556 nm. However, the sensitivity of 4-DPD-Ag NPs towards other neutral amino acids, including leucine, threonine, valine, phenylalanine, tyrosine, proline, serine, pyroglutamic acid and cysteine can be negligible. This highly selective sensor allows a direct and rapid quantitative assay of tryptophan with a colorimetric limited detection concentration of 2 × 10−5 M. The possible underlying mechanism is discussed.