A Combined Spectroscopic and Theoretical Analysis of Plasmonic Silver Nanoparticle Sensor Towards Detailed Microscopic Understanding of Heavy Metal Detection

Abstract

Plasmonic nanoparticles are of great importance owing to their highly responsive 'Localized Surface Plasmon Resonance' (LSPR) behaviour to self-agglomeration/ aggregation leading to the development of various nanosensors. Herein we demonstrated the definite self-assembly of citrate functionalized silver nanoparticles (AgNPs) into a one-dimensional linear chain in presence of charged lead ions (Pb 2+ ), one of the most toxic heavy metal pollutants. We have explored detail mechanism using a variety of spectroscopic tools and electron microscopy. The self-aggregation of AgNPs leads to the generation of new LSPR modes due to coupling of nearby existing modes. The conclusion of our experimental findings is duly supported by our developed numerical modelling based on the quasi-static approximation that the generated new LSPR modes are solely due to formation of chain-like aggregation of AgNPs. We have also monitored the LSPR spectra in presence of other metal ions, however, only Pb 2+ found to give such unique self-assembled geometry may due to its high interaction affinity with citrate. These findings play a key role for citrate functionalised AgNPs to be used as a low cost highly selective and sensitive lead ion sensor for potential application in industrial lead pollution monitoring. We have further varied several sensor parameters such as AgNPs size, concentration and the allowed reaction time for it to be practically implemented as an efficient lead sensor meeting the Environmental Protection Agency recommendations.

Paper-based plasmonic nanosensor monitors environmental lead pollution in real field

“…These DRS results imply that AgNPs formed similar chain-like aggregates even on RPPs, so we obtained the transverse and longitudinal modes of vibration corresponding to the formed chain aggregates. Also, it is evident from the DRS that there was a gradual red shift in the second (520 nm) peak with a simultaneous decrease in the first peak (395 nm), which confirmed the formation of longer chain aggregates 30 with an increment in the Pb extract concentration. The formation of longer chain aggregates with the increase in the Pb concentration is also the cause of the increase in the red content (visually) of the RPPs.…”

“…, the average diameter is 8 nm. 30 We found that a 50 μL volume of sample, which includes 30 μL of Pb extract and 20 μL of the AgNPs sensor, is needed for the best colorimetric response. We analyzed the effects of pH, temperature and time dependence on the performance of the sensor.…”

“…The sensing mechanism was based on the simultaneous existence of two peaks of AgNPs due to the formation of an aggregated structure upon interaction with Pb 2+ . In one of our earlier publications, 30 we explained the aggregation mechanism in detail; the AgNPs in the presence of Pb 2+ formed a chain-like assembly due to which there was the coupling of nearby plasmon modes. The two peaks thus correspond to the transverse and longitudinal modes of vibration associated with the plasmon oscillation of the electron cloud along and perpendicular to the axis of one-dimensional self-assembled chain AgNPs.…”

“…Silver NPs functionalized with citrate were synthesized in aqueous solution by a wet reduction method, where NaBH 4 was used as a reducing agent. 30 First, all the stock solutions of the reagents, i.e. , 5 mM of AgNO 3 , 1.02 mM of trisodium citrate, and 10 mM of NaBH 4 , were prepared.…”

“…Silver NPs functionalized with citrate were synthesized in aqueous solution by a wet reduction method, where NaBH 4 was used as a reducing agent. 30 First, all the stock solutions of the reagents, i.e., 5 mM of AgNO 3 , 1.02 mM of trisodium citrate, and 10 mM of NaBH 4 , were prepared. Thereafter, 1 mL of 5 mM AgNO 3 was added to 16 mL of 1.02 mM trisodium citrate in an ice bath at around 0 1C under constant magnetic stirring by the addition of B100 mL of 5 mM NaBH 4 (freshly prepared) over 5 minutes.…”