Field Deployable Method for Gold Detection Using Gold Pre-Concentration on Functionalized Surfaces

Gold in a rock is usually associated with other elements, forms nuggets, or is hosted within the crystal lattice of a mineral (e.g., pyrite) and is often heterogeneously distributed and trapped inside the rock matrix even after crushing. Gold can be liberated from these rock matrices by chemical leaching, but then their concentration becomes too low for detection by a portable method due to the dilution effect of the leaching process. In this paper, we present a proof-of-concept method for gold pre-concentration to enable the detection of gold in rock at low levels using a portable technique. Two coating methods, plasma polymerization (PP) and wet chemistry (WC), were utilized to generate surface coatings, which were then compared for their effectiveness in binding gold ions. Laser-induced breakdown spectroscopy (LIBS) was used as a portable technique for the detection of immobilized gold on these modified surfaces. The detection limit for pure gold ions in solution incubated on PP and WC coatings was determined to be as low as 80 ppb. To demonstrate the real-life capability of the method, it was tested for rock sample leachates bearing 300–500 ppb gold.


SN74:
Origin: Feldspar minerals, basalt and iron pyrites with minor quantities of finely divided gold and silver-containing minerals that have been screened to ensure there is no gold nugget effect.

SQ88:
Origin: Feldspar minerals, basalt and iron pyrites with minor quantities of finely divided gold and silver-containing minerals that have been screened to ensure there is no gold nugget effect.

Model S1. Model of gold binding to surface
Gold species of type k (k = ions or NPs) bind to the surface in such a way, that each species covers a square area of where D1,k is the diameter of the species (Supplementary figure 1). If there is sufficient free surface available, all gold species in the incubation volume, Vinc, are assumed to be bound to the surface, covering an area of where N'k is the number of gold species k in the incubation volume. The number of gold species per volume is given by where mk is the mass of gold in Vinc and m1,k is the mass if a single gold species. Assuming the density of the gold solutions is 1g/cm 3 , mk is given by the known weight ratio of the solutions, e.g. 1ppm gold is 1μg gold in 1mL incubation volume, i.e. 1ppm gold equals mk=1μg for our incubation method using 1mL incubation volume. Note that the assumption that all gold species in the incubation volume bind to the surface makes the mass of gold in the incubation volume equal to the mass of gold on the surface area, As, provided there is sufficient surface area available. Using Nk, Ak is (Eq. 4) The surface coverage is defined as (Eq. 5) where As is the substrate area, which is 4cm 2 for our experiments. Using Eqs. 2-4, Ck is calculated by (Eq. 6) Gold ions: The mass of a single gold ion, m1,ion, is calculated from the molar mass of gold, MAu, and the Avogadro number, NA, by 1, (Eq. 7) The diameter of the gold ions on the surface is defined as the distance of gold ions in bulk gold crystal, allowing to use bulk gold density ρAu, for calculation of gold ion diameter and volume of gold ions on the surface by where V1,ion is the volume of a single gold ion, given by Using ρAu = 19.3 g/cm 3 , MAu = 196.967g/mol, NA = 6.022x10 23 /mol, D1,ion is calculated to be 0.319nm. This value is close to gold atom diameter (van der Waals) of 2*0.166 nm=0.332 nm 1 .
Using Eqs. 8 and 9 and k=ion, Ck of Eq. 6 can be rewritten as (Eq. 11) Eq. 11 can be used to calculate the surface coverage from known mass of gold, mion, in the incubation volume (e.g. 1ppm gold equals mion=1μg gold in 1mL incubation volume), known gold density, calculated gold ion diameter (Eq. 10) and known substrate area.
Gold NPs: As the diameter of gold NPs is known, the volume of a single NP, V1,NP, can be readily calculated as (Eq. 12) Assuming that the gold NPs have the same density as bulk gold, the mass of a single NP, m1,NP, is (Eq. 13) Using Eqs. 12 and 13, Ck of Eq. 6 is rewritten as (Eq. 14) Similar to gold ions, Eq. 14 can be used to calculate the surface coverage from known mass of gold in the incubation volume (e.g. 1ppm gold equals 1μg gold in 1mL incubation volume), known gold density, known gold NP diameter (Eq. 16) and known substrate area.