Abstract
Decoupling of different plasmon resonance modes (in-plane, and out-of-plane dipole and quadrupole resonances) by tuning nanoparticle’s size and shape offers a new field of plasmonics as colorimetric assay-based optical-ruler for ultra-trace sensing. Driven by its low cost, easy to perform and efficient way to measure trace level (up to 30 ppt in presence of common mining elements in natural gold ore) abundance, this study develops a highly selective and ultrasensitive turn-on colorimetric sensor to detect gold-ion from environmental samples. Different level of gold-ion tracer makes size variable spherical- and disc-shaped silver nanoparticles when added to a ‘growth solution’ which results decoupling of in-plane dipole resonance from in-plane quadrupole and out-of-plane dipole resonances with a wide range of in-plane dipole plasmon tunability to generate different colors. This color-coded sensing of gold-ion shows high selectivity and ultrasensitivity over other metal ions in the ppt level with an impurity aberration limit of 1 ppm. A plausible explanation explains the possible role of catalytic gold-ion to initiate unfavorable silver ion (Ag+) reduction by ascorbic acid to generate silver nanoparticles. Proposed technology has been applied in real mining sample (Bugunda Gold Deposit, Tajikistan) to detect gold concentration from ores to find potential application in mining technology.
Highlights
A color-coded sensing is probably the rarest report in the literature
As a result of this the turn-on colorimetric assay-based optical ruler can be used as a powerful tool for Au-ion detection as the appearance of color positively confirms the presence of gold as gold ion and the longitudinal plasmon tuning measures the extent of Au-ion present as tracer
A plausible mechanistic explanation, based on the relative oxidation/reduction potentials of active redox systems present in the sensor assay has been presented in great details that elucidates the efficient use of ascorbic acid (AA) as a reducing agent catalyzed by Au-ion to reduce available soluble metal ions
Summary
A color-coded sensing is probably the rarest report in the literature. Along with their insoluble pure metallic form, precious metals can remain as salts which are highly soluble in water. The objective of the present study is to develop a low cost yet novel turn-on colorimetric sensor assay for highly specific and ultrasensitive detection of trace level gold content by controlling the extent of decoupling and thereby tuning the in-plane dipole resonance (longitudinal mode) of the in situ generated AgNDs. In presence of different concentration of Au-ion (ppm to ppt level), homogeneous mixture of sodium citrate tribasic dihydrate (TSC), silver nitrate (AgNO3) and L-ascorbic acid (AA) which we termed as ‘growth solution’ forms different sized AgNDs (along with spherical silver nanoparticles, AgNPs). We have tested our assay to detect and measure gold concentration from real ore samples and to find out the possible interference of the matrix or whether we can use this interference to enhance the sensitivity of detection without disturbing the overall specificity of the assay
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