Abstract
Monolayer-protected gold nanoparticles (AuNPs) exhibit distinct physical and chemical properties depending on the nature of the ligand chemistry. A commonly employed NP monolayer comprises hydrophobic molecules linked to a shell of PEG and terminated with functional end group, which can be charged or neutral. Different layers of the ligand shell can also interact in different manners with proteins, expanding the range of possible applications of these inorganic nanoparticles. AuNP-fluorescent Green Fluorescent Protein (GFP) conjugates are gaining increasing attention in sensing applications. Experimentally, their stability is observed to be maintained at low ionic strength conditions, but not at physiologically relevant conditions of higher ionic strength, limiting their applications in the field of biosensors. While a significant amount of fundamental work has been done to quantify electrostatic interactions of colloidal nanoparticle at the nanoscale, a theoretical description of the ion distribution around AuNPs still remains relatively unexplored. We perform extensive atomistic simulations of two oppositely charged monolayer-protected AuNPs interacting with fluorescent supercharged GFPs co-engineered to have complementary charges. These simulations were run at different ionic strengths to disclose the role of the ionic environment on AuNP–GFP binding. The results highlight the capability of both AuNPs to intercalate ions and water molecules within the gold–sulfur inner shell and the different tendency of ligands to bend inward allowing the protein to bind not only with the terminal ligands but also the hydrophobic alkyl chains. Different binding stability is observed in the two investigated cases as a function of the ligand chemistry.
Highlights
IntroductionMonolayer-protected gold nanoparticles (AuNPs) have been extensively investigated to disclose their unique and diverse properties. [1–5] AuNPs have already shown great potential in applications, including chemosensing (i.e., small molecule detection in solution) [6,7], biosensing [8–10], catalysis (nanozymes) [11–13] and transport of chemical species in biological environments and cells (e.g., drug delivery) [14–16]
Monolayer-protected gold nanoparticles (AuNPs) have been extensively investigated to disclose their unique and diverse properties. [1–5] AuNPs have already shown great potential in applications, including chemosensing [6,7], biosensing [8–10], catalysis [11–13] and transport of chemical species in biological environments and cells [14–16]
In the case of AuArg, the ligands tend to bend inward, reducing the ligand lengths associated with the peaks as a function of the increasing ionic strength. These results are in agreement with experimental dynamic light scattering (DLS) data previously obtained for the same particles at increasing salt concentrations, providing a slight reduction in hydrodynamic radius for both AuArg and AuCOO in solutions of high ionic strength [42]. This bending is stronger in AuArg with respect to AuCOO, since the latter preserves a larger final radius and still quite a spherical shape, while anisotropies are evident in the case AuCOO, displaying mass distribution in three distinct lobes, which is more pronounced at low ionic strength but still observed at higher ionic concentration
Summary
Monolayer-protected gold nanoparticles (AuNPs) have been extensively investigated to disclose their unique and diverse properties. [1–5] AuNPs have already shown great potential in applications, including chemosensing (i.e., small molecule detection in solution) [6,7], biosensing [8–10], catalysis (nanozymes) [11–13] and transport of chemical species in biological environments and cells (e.g., drug delivery) [14–16]. [1–5] AuNPs have already shown great potential in applications, including chemosensing (i.e., small molecule detection in solution) [6,7], biosensing [8–10], catalysis (nanozymes) [11–13] and transport of chemical species in biological environments and cells (e.g., drug delivery) [14–16]. The molecular recognition properties of these particles are dictated by the chemical structure of the coating ligands, which form self-organized and multivalent binding sites for the guest species [19], a feature crucial for nanoparticle colloidal stability [20,21]. The role of surface curvature and ligand composition of AuNPs of various sizes has been systematically investigated by atomistic simulations [26–29]. Metallic particles with core size below 3 nm exhibit molecule-like properties. This feature expands their potential use to the development of sensors and platforms for therapeutics delivery traditionally reserved for small-molecule medicine.
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