Plasmonic Gold Nanosensor: Preparation and Application Coursework

For instance, adding a strong electrolyte such as NaCl to a solution containing negatively charged AuNP solution increases the ionic concentration. Consequently, the repulsive forces between nanoparticles are buffered. The ensuing reduction in electrostatic repulsion causes the gold nanoparticles to aggregate, which further reduces the space between the nanoparticles. The solution takes in light at longer wavelengths and changes to purple or blue.

The visualisation of the transition colours occurs due to localized surface plasmon resonance (LSPR), which happens when incident light electrifies free conductive electrons of the metallic nanoparticles. The excited electrons then oscillate with the same frequency as the incident photon frequency. Excitation may cause the selective absorption and scattering of incident radiation by the metallic nanoparticles. Therefore, the nanoparticles can exhibit various colours including red, purple or blue. The precise colour that is visualised is determined by the size, shape, distance between particles and media surrounding the nanoparticles.

Based on salt-induced aggregation properties, the addition of a strong electrolyte such as NaCl to a negatively charged AuNP solution introduces a high concentration of ions that reduce the repulsive electrostatic forces between nanoparticles. Aggregation finally occurs due to the reduction in repulsion forces. The solution absorbs light at longer wavelengths and changes from blue to purple and ultimately clear if more electrolyte is added. The absorbance spectra of mixtures of solutions A, B and C with NaCl are shown below. The absorbance spectrum for solution A with NaCl shows a peak absorbance at a longer wavelength, which is line with the observation that the addition of a strong electrolyte to AuNP causes a colour change and absorbance at a longer wavelength (approximately 720 nm). This spectrum proves that solution A is AuNP.

Table 2.

The sample A is: Casein

Your explanation:

Casein is a protein consisting of molecules with a high molecular weight. When a protein solution is added to citrate-stabilised AuNPs, the protein molecules affix to the exterior of the AuNPs through different mechanisms, for instance, covalent binding, hydrophobic interactions or any other and other weak interactions. As a result, the aggregation of the nanoparticles cannot take place even if the salt concentration is increased. Therefore, the colloidal solution remains red. The colour of citrate-stabilised AuNPs was dark red and remained so even after adding the sample and increasing the salt concentration, which confirmed the presence of a high molecular weight compound.

The sample B is: Salt

Your explanation:

Salts are strong electrolytes that dissociate completely to give ions. The addition of a strong electrolyte to a stabilised, negatively charged AuNP solution introduces a high concentration of ions that buffer the repulsive electrostatic forces between nanoparticles. The overall outcome is a decline in repulsive force is diminished, which prompts the gold nanoparticles to aggregate. Aggregation reduces the spacing between the nanoparticles and enhances the absorption of light at longer wavelengths. Therefore, the solution changes to purple or blue. Adding a larger quantity of the electrolyte leads to the precipitation of large nanoparticle collections, which causes the solution to become light grey and finally clear.

The sample C is: Sugar

Your explanation:

Sugar may be considered a nonelectrolyte or a weak electrolyte. The addition of such a solution to AuNPs does not interfere with the repulsive forces between the gold and citrate particles. Therefore, the solution remains red. However, the subsequent addition of a strong electrolyte such as NaCl introduces a high concentration of negative ions that in turn screen the repulsive electrostatic forces between nanoparticles. There is a reduction in repulsion forces, thereby leading to the aggregation of the gold nanoparticles, minimal spacing between nanoparticles and the absorption of light at longer wavelengths. The colour of the solution then changes to blue or grey.

The detection limit of the assay can be defined as the sum of the analyte concentration of the blank and 3 standard deviations (Allegrini & Olivieri 2014), which is 0+0.002 mM. Therefore, the limit of detection is 0.002mM.

The AA concentration of the blind sample can be obtained from the equation of the standard curve y= 1.0699x-0.6197. The mean absorbance of the blind sample is 1.109. Substituting this value into the equation gives 1.109=1.0699(x)-0.6197.
1.109+0.6197=1.0699x
1.7287=1.0699
1.6157=x

The AA concentration of the blind sample is 1.616 mM.