Rapid Detection of Ag(I) via Size-Induced Photoluminescence Quenching of Biocompatible Green-Emitting, l-Tryptophan-Scaffolded Copper Nanoclusters

Abstract

Atomically precise metal nanoclusters capped with small molecules like amino acids are highly favored due to their specific interactions and easy incorporation into biological systems. However, they are rarely explored due to the challenge of surface functionalization of nanoclusters with small molecules. Herein, we report the synthesis of a green-emitting (λ_ex = 380 nm, λ_em = 500 nm), single-amino-acid (l-tryptophan)-scaffolded copper nanocluster (Trp-Cu NC) via a one-pot route under mild reaction conditions. The synthesized nanocluster can be used for the rapid detection of a heavy metal, silver (Ag(I)), in the nanomolar concentration range in real environmental and biological samples. The strong green photoluminescence intensity of the nanocluster quenched significantly upon the addition of Ag(I) due to the formation of bigger nanoparticles, thereby losing its energy quantization. A notable color change from light yellow to reddish-brown can also be observed in the presence of Ag(I), allowing its visual colorimetric detection. Portable paper strips fabricated with the Trp-Cu NC can be reliably used for on-site visual detection of Ag(I) in the micromolar concentration range. The Trp-Cu NC possesses excellent biocompatibility, making it a suitable nanoprobe for cell imaging; thus, it can act as an in vivo biomarker. The nanocluster showed a significant spectral overlap with anticancer drug doxorubicin and thus can be used as an effective fluorescence resonance energy transfer (FRET) pair. FRET results can reveal important information regarding the attachment of the drug to the nanocluster and hence its role as a potential drug carrier for targeted drug delivery within the human body.

Figure 1

Emission (λ_ex = 380 nm) and excitation (λ_em = 500 nm) spectra of Trp-Cu NC.

Figure 2

(a) Transmission electron microscopy (TEM) image of Trp-Cu NC confirming the formation of ultrasmall spherical clusters. The lattice fringes are also visible (inset). (b) Size distribution of the nanocluster obtained from the TEM image showing an average size of ∼2 nm.

Figure 3

(a) FTIR spectra of tryptophan (black) and Trp-Cu NC (red). (b) Matrix-assisted laser desorption/ionization—time-of-flight (MALDI–TOF) mass spectrum of Trp-Cu NC.

Figure 4

Ratio of the PL intensity (λ_ex = 380 nm, λ_em = 500 nm) of Trp-Cu NC in the absence (F₀) and presence of Ag(I) (∼200 μM) and different other metal ions (∼500 μM), or biomolecules (glucose (Gluc) (∼5 mM), creatinine (Creat) (∼1 mM), tyrosine (Tyr) (∼1 mM), and dopamine (Dopa) (∼1 mM)) (F) showing selective detection of Ag(I).

Figure 5

Color change of the diluted nanocluster can be visualized upon the addition of Ag(I) compared to other metal ions (∼500 μM).

Figure 6

(a) Transmission electron microscopy (TEM) image of Trp-Cu NC in the presence of 500 μM Ag(I). (b) Size distribution of the nanocluster in the presence of Ag(I).

Figure 7

(a) PL intensity of Trp-Cu NC gradually quenched with increasing concentration of Ag(I). (b) Corresponding Stern–Volmer plot showing an upward curvature. The black line represents a fit using eq 1 with both static and dynamic quenching contributions. The inset represents a plot of log(F₀/F) vs [Ag(I)] (red) with a linear fit (black).

Figure 8

(a) Cell viability assay showing that the nanocluster is not cytotoxic to the 3T3 cells (errors represent standard errors on mean with n = 3). (b) Confocal microscopy image of 3T3 cells incubated with Trp-Cu NC (λ_ex = 405 nm).

Figure 9

(a) Spectral overlap between normalized emission spectra (λ_ex = 380 nm) of the Trp-Cu nanocluster (donor) (red) and the absorption spectra of doxorubicin (acceptor) (blue). (b) Emission spectra (λ_ex = 380 nm) of the Trp-Cu nanocluster (donor) in the absence (red) and presence of 100 μM doxorubicin (acceptor) (blue).

Scheme 1. (a) Schematic Representation of Synthesis of Trp-Cu NC; (b) Trp-Cu NC under Visible Light (Yellowish-Brown) and Ultra-Violet (UV) Light (Green)