Elsevier

Analytica Chimica Acta

Volume 555, Issue 2, 12 January 2006, Pages 263-268
Analytica Chimica Acta

Gold colloid analysis by inductively coupled plasma-mass spectrometry in a single particle mode

https://doi.org/10.1016/j.aca.2005.09.021Get rights and content

Abstract

Analysis in a single particle mode of gold colloids in water has been performed by inductively coupled plasma-mass spectrometry (ICP-MS). The signal induced by the flash of ions due to the ionization of a colloid in the plasma torch can be measured for the ions 197Au+ by the mass spectrometer without interferences. The intensity of the MS signal is recorded in time scan. The recorded peak distributions were analysed as a function of the colloid size for five monodisperse colloids (80–250 nm). This study describes the experimental conditions to analyse gold colloids in a single particle mode. The size detection limit is around 25 nm corresponding to 0.15 fg colloids and one particle per ml may be detected during a 1 min time scan within standard procedure.

Introduction

Single colloid analysis from suspension is generally performed after separation on filters followed by scanning electron microscopy (SEM) investigation [1]. Single particle analysis has been carried out for more than a decade utilising also light-based techniques such as laser-induced breakdown detection (LIBD), e.g. [2] or single particle counting (SPC) [3]. Recently, imaging of single-metal nanoparticles in scattering media was found to be more performant by photothermal interference than by ordinary differential interference contrast and allowed detection of 5 nm colloids [4]. These techniques quantify the number of particles per unit volume of fluid and have been applied to the analysis of natural water [5], [6].

The conventional inductively coupled plasma-atomic emission spectrometry (ICP-AES) was successfully adapted by Borchet and Dannecker [7] for individual particle analysis. By this method, the particle introduction in the torch induces a transient emission signal that can be recorded and specifically analysed. This technique was adopted for aerosol analyses. These experiments provided information about the precision of the technique and the influence of matrix elements on the emission intensity. The technique was used for the analysis of sanding and abrasive dusts.

Mass spectrometry (MS) is one of the primary analysis methods for determining the chemical composition of small samples and several of these techniques have been adapted to analyse single aerosol particles [8], or after separation on a specific sample carrier, laser microprobe mass analysis (LAMMA) can be applied for single particle analysis [9].

The use of inductively coupled plasma-mass spectrometry (ICP-MS) for single particle analysis was pre-discussed for colloid bearing solutions by the author at the IUPAC meeting in Davos, Switzerland and published in [1]. Independently, Momizu et al. [10], [11] successfully tested this approach for airborne particles at the fg level, by injecting the air-contaminated samples in the plasma torch. Recently, the feasibility of single particle ICP-MS analysis from a colloid suspension in water was tested for TiO2, A12O3 and clay [12], ZrO2 colloids [13] and ThO2 colloids [14].

Research on noble-metal nanoparticle emphasises size effect. Gold colloids find their applications in biology [15], e.g. as cytochemical markers [16] and medical fields [17]. In the latter, recently they have been found to be specifically functionalized with a variety of biomolecules including antibodies and streptovidine [18]. Gold colloids are currently used as a carrier in blood [19]. Finally, gold colloidal particles are occasionally used as a tracer in geoscience [20] and are locally analysed in surface water for prospection, e.g. [21].

In this work, the testing of single particle analysis from a gold colloid suspension in water was studied by ICP-MS, because these colloids are monoisotopic, monoatomic and monodisperse. The transient signal induced by the ionization of a colloidal particle in the plasma torch produces a flash of 197Au+ ions that can be detected and measured by the mass spectrometer. Basic properties of this signal allow determination of the gold particle size and the colloid concentration.

Section snippets

Theoretical background

For single colloid analysis, the ICP-MS unit may be adapted using a colloid suspension injection system in the blank water stream when needed or used with the usual set up when the suspension is sufficiently diluted. The diluted colloid suspension is continuously introduced in the nebulizer producing an aerosol of micro-drops (1 micro-drop carrying one colloid among 105–106 micro-drops colloid free) in the argon flow feeding the inductively coupled plasma torch coupled to a mass spectrometer.

Experimental

The concentration of gold colloid-stock suspensions (BB International, Cardiff, UK) depends on their size as given in Table 1. They were stirred for 2 min, to resuspend the colloids prior to dilution with a factor of 100. The stock solution that was stocked in obscurity at 4 °C was diluted in a flow box class or clean laboratory to a particle number concentration of the order of 106 cm−3.

The scanning electron microscopy investigations were performed with a Zeiss DSM 962 unit under 30 kV. SEM

SEM and SPC

SEM investigations on the 100 nm and 250 nm gold colloids were carried out with the suspension contacted onto the TEMFIX film prior to microscopic investigations. As seen in Fig. 1, the sizes of the particles are slightly dispersed. There are no colloid clusters and colloids remain individual even on the sample carrier. For gold particles of 100 nm size, the SEM investigations confirm the 100 nm size of the investigated colloids.

SPC investigations by light scattering were carried out on the

Concluding remarks

A study of the single particle analysis of water bearing gold colloid suspensions by inductively coupled plasma-mass spectrometry has been conducted. The transient signal induced by the flash of Au ions due to the ionization of a gold colloidal particle in the plasma torch can be detected and measured by the mass spectrometer. The intensity of the signal is determined by the size of the particles for the matrix elements and the frequency of the flashes is directly proportional to the

Acknowledgements

Part of the work was performed at Institute Forel Laboratory, which is partially supported by the Swiss National Foundation. The concept and modeling work was performed at LES-PSI partially funded by the National Cooperative for the Disposal of Radioactive Waste. R. Brutsch is thanked for the SEM analysis.

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