Visually monitoring the etching process of gold nanoparticles by KI/I2 at single-nanoparticle level using scattered-light dark-field microscopic imaging

Significance Statement

The etching reaction between iodine and gold has become a typical model of etching process and attracted growing interests from researchers mainly owing to the salt effect or a chemisorptive process on the surface of gold. In order to monitor the gold−iodide reactions, people have developed electrochemical and optical methods. Even though, it is still hard to monitor physical interaction and chemical reactions at single nanoparticle or molecule levels between gold and iodine. In such case, we herein made a real-time monitoring of etching process of three different shapes of gold nanoparticles (AuNPs) at single nanoparticle level based on the change of LSPR optical properties under a dark-field microscopy.

Our research highlights:

(1) By using scattered light dark-field microscopic imaging (iDFM) technique, the in situ KI/I2-treated etching processes on three shapes of AuNPs were monitored in real time. It was found that the scattered light of different shapes of AuNPs exhibited a noticeable color change and the scattered light spectra of AuNPs showed obvious blue-shifts with the decrease of scattered light intensity owing to the oxidation of Au atom into [AuI2] during the etching process.

(2) Both Finite-difference time-domain simulation and monitoring of morphological variations proved that the etching was a thermodynamics-dependent process, shows a chamfering coupled mechanism with layer-by-layer peeling, making the AuNPs at last degenerated into isotropic spheres together with the decrease of particle size.       

About the author

Chengzhi Huang has completed his PhD from Peking University in 1996 and postdoctoral studies at University of Ottawa (2000-2001) and University of Tokyo (2001-2003). He is the director of Key Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest University), Ministry of Education. He was invited to give talks in many international and national conferences. He was also invited to serve as a peer reviewer of international scientific journals, including Anal. Chem., Anal. Chim. Acta., Talanta, Biosens. Bioelectron., etc. He has published more than 250 papers with an H-index of 36 in academic journals. His research field includes utilizing light analytical science to establish a real-time and on-line detecting and analysis method for medicament in real sample, to develop new optics-detecting technology and sensor, and the application of nanoparticles on biolabeling or biomedical analysis. His current research interests are in bioanalytical chemistry, clinic immunoassay, bionanotechnology, and they are divided into three research directions:

1. Near infrared localized surface plasmon resonance property of copper chalcogenie nanocrystals and its application in biomedical analysis. 2. Long-range resonance energy transfer and its application in biomedical analysis.
3. Host response of quantum dots labeled virus. 

Journal Reference

Nano Research, pp 1-10, 04 March 2016.

1. Key Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, China

2. College of Pharmaceutical Sciences, Southwest University, Chongqing, 400716, China

Abstract

Real-time monitoring of reaction processes is helpful for understanding the reaction mechanisms. In this study, we investigated the etching mechanism of gold nanoparticles (AuNPs) by iodine on a single-nanoparticle level because AuNPs have become important nanoprobes with applications in sensing and bioimaging fields owing to their specific localized surface plasmon resonance (LSPR) properties. By using a scattered-light dark-field microscopic imaging (iDFM) technique, the in situKI/I₂-treated etching processes of various shapes of AuNPs, including nanospheres (AuNSs), nanorods (AuNRs), and nanotrigonal prisms (AuNTs), were monitored in real time. It was found that the scattered light of the different shapes of AuNPs exhibited noticeable color changes upon exposure to the etching solution. The scattering spectra during the etching process showed obvious blue-shifts with decreasing scattered intensity owing to the oxidation of Au atoms into [AuI₂]^–. Both finite-difference time-domain (FDTD) simulations and monitoring of morphological variations proved that the etching was a thermodynamic-dependent process through a chamfering mechanism coupled with layer-by-layer peeling, resulting in isotropic spheres with decreased particle sizes.