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

•Absorption detection limit decreases with the size of gold nanoparticles.•Fluorescence detection limit increases with the size of gold nanoparticles.•Fluorescence measurement in optical fibre reduces detection limit of 50nm nanoparticles.•Portable sensor for detection of picomolar concentration of gold nanoparticles. 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 (71ppb of 5nm and 24.5ppb of 50nm NPs for absorption and 74ppb of 5nm and 1200ppb of 50nm NPs for fluorescence). The LOQ for fluorescence for 50nm 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.