Colorimetric detection is a simple, fast, and efficient method for determining targets with the aid of obvious color change.[1] Since no equipment is needed except for the naked eye, it shows huge potential for portable and inexpensive daily life applications compared to equipment-based methods. Generally, colorimetric probes consist of a recognition moiety and a signal moiety. The sensing strategy is that the recognition moieties exhibit selective response to targets depending on either coordination or chemical reactions between targets and ligands. The signal moiety then translates those detecting behaviors into color changes discernible by the eye.[2] Both the components are indispensable and can affect colorimetric performance in terms of selectivity, sensitivity, response time, and so on.[2] For the signal moiety, colorful chromophores are always the main candidates and a variety of organic chromophoric probes have been studied for monitoring various targets, though the sensitivity and solubility usually limit the application of organic chromophoric probes.[1c, 2] However, plasmonic nanoparticle-based colorimetric detection (using gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs)), has drawn increasing attention due to the unique optical properties provided by surface plasmon resonance (SPR).[3] Plasmonic nanoparticles are preferable to organic chromophores due to their superior properties, such as their high dispersity in aqueous media, greater absorption extinction coefficient (∼ 10 8 cm− 1 M− 1 for AuNPs while∼ 10 5 cm− 1 M− 1 for common organic dyes), good stability and low toxicity,[4] ease of physical and chemical functionalization with large surface-to-volume ratios, and alterable optical properties (eg SPR).[5] Specifically, due to the sensitive and obvious redto-blue color change, AuNPs are an important colorimetric platform and AuNP-based colorimetric assays have been successfully designed and widely applied for detecting DNA,[6] enzyme activity,[7] proteins,[8] small molecules,[9] and metal ions/anions [5, 10] depending on surface chemistry-based functionalization and following tailorable optical changes.[11]

For the recognition moiety, most probes are usually prepared by functionalizing chemically synthesized ligands (such as heterocompounds) on various chromophores or nanostructures for their special coordination chemistry with targets.[12] For instance, thymine in DNA strands can be used for Hg 2+ detection through a T-Hg 2+-T complex, since Hg 2+ selectively bridges two thymine molecules in a stable manner.[13] Such unique structures can obviously increase the Tm (melting temperature) of specially designed DNA in such a way that melting analysis could be used for qualitative determination.[14] In our former work, we found that melamine, a commercial heteocyclic-ring molecule with multiple nitrogens, shows a much greater coordination affinity toward Hg 2+ than any other metal ions that can be used for the colorimetric detection of Hg 2+ using AuNPs as a platform.[15] However, in order to avoid the complicated chemical synthesis and to develop an environmentally friendly and sustainable method, we believe that utilizing natural products with similar binding sites to thymine, melamine, or others is one of the most simple and efficient strategies, consistent with the ideas of green chemistry and low costs.[16] It is known that there are abundant nitrogen-containing molecules like uric acid and creatinine in the urine, with similar functional sites as thymine/melamine, and therefore it is interesting to consider whether they could act as recognizing ligands for sensing heavy metal ions (like Hg 2+) on the basis of …