Detection of gold nanoparticles with different sizes using absorption and fluorescence based method

Growing world demand for gold and decreasing discovery rates of ore deposits necessitates new techniques for gold exploration. Current techniques for the detection of ppb level of gold, such as inductively coupled plasma mass spectrometry (ICP-MS) are not field-deployable. By contrast, current portable device such as X-ray fluorescence (XRF) based sensors are not sufficiently sensitive. Thus, there is growing interest in developing a new, easy-to-use and fast method for detection of low concentrations of gold at the site of an exploration drilling rig. Two optical methods, absorption and fluorescence, are examined for their suitability for low gold concentration detection. Absorption study is based on the analysis of localised surface plasmon resonance peak. For fluorescence analysis, the ability of gold nanoparticles to specifically catalyse the conversion of the non-fluorescent compound (I-BODIPY) to the fluorescent derivative (H-BODIPY) is used. For both absorption and fluorescence methods, the limit of quantification (LOQ) of gold nanoparticles (NPs) was found to be dependent on the NP size (71 ppb of 5 nm and 24.5 ppb of 50 nm NPs for absorption and 74 ppb of 5 nm and 1200 ppb of 50 nm NPs for fluorescence). The LOQ for fluorescence for 50 nm NPs measured in a suspended core optical fibre was almost twice lower than in a cuvette. The field deployment potential of these methods was also determined using a portable set up.