With global demand reaching more than 4000 tons in 2013, gold is used worldwide not only for jewelry, but for electronics, drug delivery, sensing, and deep-space applications. Incredibly, gold in solution is taken up by the roots of trees and by some ground-dwelling bacteria, and converted to low-concentration-level gold nanoparticles whose detection could signal larger deposits underneath. With the average crustal abundance of gold at 1.3 parts per billion (ppb), detecting anomalous concentration levels up to 8 parts per million (ppm) is critical for finding new gold sources.
While easy to detect in parts-per-million concentrations using x-ray fluorescence (XRF) techniques, parts-per-million-level gold detection is more elusive. Unfortunately, parts-per-billion detection through inductively coupled plasma mass spectroscopy (ICP-MS) and inductively coupled plasma atomic-emission spectroscopy (ICP-AES) requires the ore sample to be transported to the lab and processed, adding significant time to the exploration workflow.
Recognizing that gold has unique optical properties such as localized surface plasmon resonance (SPR) and a catalytic effect on fluorophores that can be used for sensing methods, researchers at the University of Adelaide (Adelaide, SA, Australia) are exploring the utility of the optical absorbance and fluorescence in detection of low concentrations of gold in the field at the drilling site without arduous sample preparation.1
To determine the optimal means of detecting parts-per-billion traces of gold nanoparticles, the researchers used various concentrations of gold nanoparticles in solution with diameters of 5, 20, and 50 nm. Then, they compared the detection limit achievable with SPR and fluorescence using both laboratory and handheld or portable spectroscopy instruments.
Data was analyzed for different gold solutions within a cuvette and also within a suspended-core optical fiber (SCF) with a central solid core surrounded by three air holes. Optical fibers have the advantage of requiring small sample volumes, and can analyze samples at remotely distributed locations such as downhole environments.
By defining the limit of quantification (LOQ) as the measurement of the minimum quantifiable concentration of gold nanoparticles, experiments determined that the absorption LOQ in a cuvette measured with a laboratory spectrometer was 7X lower (dependent on nanoparticle size) than for a portable spectrometer. But for the fluorescence method, the LOQ was approximately the same for both. Comparing cuvette and SCF, the LOQ was nearly 2X lower in the SCF for 50 nm nanoparticles, but comparable for 5 and 20 nm nanoparticles.
"We have determined the LOQ of gold nanoparticles that can be achieved using optical methods. The methods are easy to use and are quite versatile, and could also be used for detection of gold in biological samples," says Agnieszka Zuber, researcher at the University of Adelaide. "Apart from a low detection limit, the most important advantage of these methods is their portability, which allows analysis time to be reduced from a few days to just hours, including sample preparation." The authors would like to note that their work has been supported by the Deep Exploration Technologies Cooperative Research Centre in Adelaide.
1. A. Zuber et al., Sensors Actuat. B-Chem., 227, 117–127 (2016).