Interactions between gold, thiol and As(iii) for colorimetric sensing

Area of Science:

• Nanomaterials Science
• Environmental Chemistry
• Analytical Chemistry

Background:

• Arsenic(iii) (As(iii)) is highly toxic and requires sensitive on-site detection methods.
• Colorimetric sensors using gold nanoparticles (AuNPs) modified with thiol compounds (DTT, GSH, Cys) are commonly cited for As(iii) detection.
• Existing mechanisms propose As(iii) crosslinks surface ligands, causing AuNP aggregation and color change.

Purpose of the Study:

• To investigate the role of As(iii) adsorption on ligand-capped AuNPs.
• To re-evaluate the mechanism of color change in As(iii) colorimetric sensing.
• To clarify the interactions between As(iii), AuNPs, and surface ligands.

Main Methods:

• Studied citrate-capped AuNPs with varying citrate concentrations and divalent cations.
• Investigated thiol-capped AuNPs (DTT, GSH, Cys) with As(iii) at different concentrations.
• Analyzed color changes, reaction kinetics, and reversibility to determine the sensing mechanism.

Main Results:

• Citrate-capped AuNPs showed a blue color with As(iii) due to ionic strength, not cation effects.
• Thiol-capped AuNPs required high As(iii) concentrations (>5 mM) for color change, unlike reported nanomolar sensitivity.
• Color change was irreversible and dependent on high As(iii) levels, indicating ligand displacement by As(iii) adsorption.

Conclusions:

• As(iii) adsorption onto AuNPs, leading to ligand displacement, is a more likely mechanism than ligand crosslinking.
• The interaction between AuNPs, thiols, and As(iii) is complex and requires careful mechanistic interpretation.
• Previously overlooked As(iii) adsorption onto AuNPs must be considered in designing and interpreting colorimetric sensors.