Novel small stable gold nanoparticles bearing fluorescent cysteine-coumarin probes as new metal-modulated chemosensors

Emissive molecular probes based on amino acid moieties are very appealing because of their application as new building blocks in peptide synthesis. Two new bioinspired coumarin probes (L1 and L2) were synthesized and fully characterized by elemental analysis, infrared, ¹H NMR, ¹³C NMR, UV-vis absorption and emission spectroscopy, matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF-MS), lifetime measurements, and X-ray crystal diffraction. Their sensing ability toward alkaline earth, transition, and post-transition metal ions (Ca ²⁺, Zn ²⁺, Cd ²⁺, Cu ²⁺, Ni ²⁺, Hg ²⁺, Ag ⁺, and Al ³⁺) and their acid-base behavior (H ⁺, OH ^-) were explored in absolute ethanol by absorption and fluorescence spectroscopy. Compound L1 shows a strong complexation constant with the soft metal ions Zn ²⁺, Cd ²⁺, and Ag ⁺. Compound L2 shows a high fluorescence quantum yield, and it could be used as a non-pH-dependent fluorescent biological probe. Very small gold nanoparticles (AuNPs) using compounds L1 and L2 as stabilizers were obtained by using a reductive method and were characterized by UV-vis, light scattering, and transmission electron microscopy (TEM). Dynamic light scattering and TEM studies show that the formation of small nanoparticles is around 4.27 ± 0.64 nm for L1 and around 2.69 ± 0.96 nm for L2. The new stable Cou@AuNPs behaved as supramolecular chemosensors, which have been selective for the heavy element Hg ²⁺, with a concomitant change of color from pink to dark red/brown and an increase of size up to 100-fold.