Real-Time Monitoring of Ligand Exchange Kinetics on Gold Nanoparticle Surfaces Enabled by Hot Spot-Normalized Surface-Enhanced Raman Scattering

Abstract

Nanoparticle surface coatings dictate their fate, transport, and bioavailability. We used a gold nanoparticle− bacterial cellulose substrate and "hot spot"-normalized surface-enhanced Raman scattering (HSNSERS) to achieve in situ and real-time monitoring of ligand exchange reactions on the gold surface. This approach enables semiquantitative determination of citrate surface coverage. Following exposure of the citrate-coated nanoparticles to a suite of guest ligands (thiolates, amines, carboxylates, inorganic ions, and proteins), the guest ligand signal exhibited first-order growth kinetics, while the desorption mediated decay of the citrate signal followed a first-order model. Guest ligand functional group chemistry dictated the kinetics of citrate desorption, while the guest ligand concentration played only a minor role. Thiolates and BSA were more efficient at ligand exchange than amine-containing chemicals, carboxylate-containing chemicals, and inorganic salts due to their higher binding energies with the AuNP surface. Amine-containing molecules overcoated rather than displaced the citrate layer via electrostatic interaction. Citrate exhibited low resistance to replacement at high surface coverages, but higher resistance at lower coverage, thus suggesting a transformation of the citrate-binding mode during desorption. High resistance to replacement in streamwater suggests that the role of surfaceadsorbed citrate in nanomaterial fate and transport must be better understood.

Factors Influencing the Surface Functionalization of Citrate Stabilized Gold Nanoparticles with Cysteamine, 3-Mercaptopropionic Acid or l-Selenocystine for Sensor Applications

“…Despite the slight difference in strength of the Au-Se and the Au-S bonds not all of the citrate was dislodged from the surface of the gold nanoparticles as the selenium SAM is formed, as suggested by the ATR-FTIR data. The evidence presented here indicates that citrate and other carboxylate-containing species residues are not easily displaced from the surface of gold nanoparticles, in accordance with previous studies [34][35][36].…”

“…These peaks could not be identified here, as a result of poor signal-to-noise measurements below 650 cm −1 (Figure S14). These bands could be better detected using Raman spectroscopy [36]. Alternatively, in order to provide evidence of the association of the AuNPs with SeCyst, TEM was used.…”

“…As regards the classical method used for the production of AuNPs by citrate reduction and subsequent stabilization by the anion, there is now evidence to suggest that not all of the citrate is displaced from the surface of gold upon functionalization with thiols [34,35]. Wei et al [36] monitored the kinetics of citrate desorption from the surface of AuNPs and showed that thiol-containing molecules were the most efficient at displacing citrate compared to amine-and carboxylate-containing molecules, but without complete displacement. The explanation for this observation was attributed to different binding energies of citrate on the gold surface.…”

Prospects of Coupled Organic–Inorganic Nanostructures for Charge and Energy Transfer Applications

“…[96] Regarding one-step ligand exchange reactions, thiols replace carboxyls on gold due to higher adsorption energy, while amines with a similar energy were experimentally shown to overcoat rather than displace a citrate layer. [97] When using the binding energy as an indicator of the probability of bond formation, thiophene (T) derivatives, pyridine (Py), and thiols have a high probability to form bonds to metal surfaces compared to for example, amines. [98] The affinity of cyano (CN) ligands to gold is predicted to be high, yet in experiments CN has often been found to be a weak anchor group with low binding probability.…”

Application of SERS on the chemical speciation of individual Aitken mode particles after condensational growth

“…Wei et al (2018) reported that the intensity of the surface plasmon enhanced elastic scattering signal (at 40 cm À1 in this case) scales linearly with the integrated "hot spot" signals within a laser excitation volume. Thus, intensity of other peaks can be quantified by using the peak intensity at 40 cm À1 as a normalizing factor (Wei et al 2019). In Figure 5a, colored box and whisker plots on the left show the median, average and 95 th , 75 th , 25 th , and 10 th percentile values and the right circles show the individual normalized intensity values.…”