Ion-Concentration-Dependent Dynamic Mechanism of Thiolactic-Acid-Capped Gold Nanoclusters Revealed by Fluorescence Spectra and Two-Dimensional Correlation Spectroscopy

Chemistry Optics Analytical Chemistry Physical Chemistry Inorganic Chemistry Nanoscience Physics
gold nanoclusters two-dimensional correlation spectroscopy anti-galvanic reaction photoluminescence mechanism fluorescence spectroscopy size effect

Background and Research Questions

Atomic gold nanoclusters (AuNCs), with particle sizes typically not exceeding 2 nanometers, have garnered extensive attention in recent years due to their unique photophysical properties, making them highly advantageous in biomedicine, catalysis, and sensing. These properties include excellent catalytic activity, tunable luminescence, biocompatibility, and non-toxicity. However, despite the application breakthroughs in areas such as near-infrared (NIR) luminescent probes, challenges remain in this field. Chief among these is the design and synthesis of novel AuNCs exhibiting NIR luminescence. Additionally, the mechanism regulating the luminescent performance of AuNCs is intricate and directly influenced by factors such as particle size, surface ligands, and metal composition.

In recent years, the concept of “anti-galvanic reaction” (AGR) has been introduced to this domain. Unlike the classical galvanic reaction, AGR allows metals with lower reactivity to be reduced by more reactive metal ions. Studies have shown that AGR can be used to engineer atomically precise alloyed nanoclusters, making this reaction significant for material design. However, research into how AGR regulates the optical properties of AuNCs is still insufficient.

In light of this, a research team from Jilin University and Jilin Business and Technology College systematically studied the optical property changes in thiolactic-acid (TLA)-capped gold nanoclusters (AuNCs@TLA) upon the addition of varying concentrations of Ag(I) ions. This was achieved using fluorescence spectroscopy combined with advanced spectral analysis techniques, including principal component analysis (PCA), two-dimensional correlation spectroscopy (2D-COS), and moving window two-dimensional correlation spectroscopy (MW2D-COS). Their groundbreaking findings, published in Applied Spectroscopy in 2025, contribute to a deeper understanding of the mechanisms governing the optical properties of AuNCs.

Detailed Workflow of the Research

1. Sample Preparation and Initial Fluorescence Spectroscopy Measurements

The core material, AuNCs@TLA, was synthesized by mixing chloroauric acid (HAuCl₄) with TLA in a 1:6.5 ratio. Subsequently, 11 mM NaOH was added to the reaction system, and the mixture was heated at 110 °C for 90 minutes. Transmission electron microscopy (TEM) analysis revealed that the synthesized nanoclusters exhibited good dispersibility, uniform morphology, and an average diameter of 1.78 nanometers.

The samples, consisting of a 50 µg/mL AuNCs@TLA solution, were titrated with AgNO₃ to vary the concentration of Ag(I) ions from 0 to 200 µM. Fluorescence spectra were measured at an excitation wavelength of 450 nm. The initial spectra revealed a prominent emission peak at approximately 800 nm in the NIR range.

2. PCA and 2D-COS Analysis of Fluorescence Spectrum Changes

a) Principal Component Analysis (PCA)

Due to the complexity of the original one-dimensional fluorescence spectra and overlapping signals, the research team applied PCA to extract the principal features of the data. The analysis revealed that changes in Ag(I) concentration could be divided into two distinct stages: in the low concentration range of 0–10 µM, principal component 2 (PC2) scores decreased rapidly, while principal component 1 (PC1) scores increased gradually. In the higher concentration range of 13–200 µM, both PC1 and PC2 scores exhibited a positive correlation with Ag(I) concentration.

b) Two-Dimensional Correlation Spectroscopy (2D-COS) Analysis

Using 2D-COS, the researchers successfully resolved overlapping peaks in the fluorescence spectra, revealing the sequence of dynamic luminescent changes. In the first stage (0–10 µM range), 2D synchronous spectra showed a decrease in emission intensity at 790 nm, accompanied by an increase in a new emission peak at 607 nm in the opposite direction. Asynchronous spectra indicated that the reduction in signal at 790 nm preceded the rise at 607 nm, suggesting that this stage’s changes were closely linked to alterations in the electronic state on the AuNCs’ surface.

In the second stage (13–200 µM range), synchronous spectra revealed a distinct autocorrelation peak at 670 nm. Asynchronous spectra further disclosed sequential changes from 740 to 670 nm and from 590 to 670 nm. These changes were primarily attributed to the growth effects driven by changes in the size of the metallic core.

3. Moving Window 2D-COS (MW2D-COS) Analysis

To analyze the continuous spectral changes induced by varying Ag(I) concentrations, the team utilized MW2D-COS. In the low-concentration range of 0–10 µM, changes in fluorescence were dominated by the signal at 790 nm. In contrast, as Ag(I) ions reached concentrations exceeding 13 µM, the dominant signal shifted to 670 nm. These results further validated the two-stage luminescence mechanism of AuNCs@TLA.

4. Supporting Experiments: XPS and TEM Results

a) X-ray Photoelectron Spectroscopy (XPS)

XPS analysis revealed that the introduction of Ag(I) ions altered the valence state of the AuNCs, with gold transitioning from Au(0) to Au(I), alongside the reduction of Ag(I) to Ag(0). This reaction pattern confirmed the occurrence of AGR, resulting in the formation of bimetallic Au–Ag nanoclusters.

b) Transmission Electron Microscopy (TEM)

TEM images showed that as the concentration of Ag(I) ions increased, the nanocluster’s average diameter grew from 2.13 nm to 2.38 nm, confirming the enlargement of the metallic core. This size effect directly impacted the emergence of a new emission peak in the second stage.

Research Findings and Conclusions

The study revealed a two-stage mechanism for the luminescence changes in AuNCs@TLA induced by Ag(I) ions:

  1. Stage 1 (0–10 µM): Through AGR, the chemical composition of the nanocluster’s surface changed, leading to the quenching of the 790 nm emission and the emergence of a new emission at 607 nm.
  2. Stage 2 (13–200 µM): As more Ag(I) ions were reduced and deposited, the bimetallic nanocluster’s core size increased, responsible for the spectral shift of emission from 607 nm to 670 nm.

Significance of the Research

This study provides a comprehensive understanding of the two-stage optical property changes in AuNCs upon interference by Ag(I) ions, offering theoretical support for precise control over the optical performance of metallic nanoclusters. It highlights AGR’s novel role in nanomaterial synthesis while enriching our knowledge of their fundamental photophysical properties. Furthermore, this research’s application of 2D-COS and MW2D-COS serves as an exemplary method for analyzing spectral changes and dynamic processes in complex systems.

The findings are expected to facilitate advancements in the application of NIR-emitting metallic nanoclusters in bioimaging, catalysis, and sensing, while inspiring further studies on the efficient synthesis and applications of novel bimetallic nanoclusters.