Shape dependent protein induced stabilization of gold nanoparticles: from protein corona perspective
Anastasiia Tukova, Yihan NieϮ, Mohammad Tavakkoli YarakiϮ, Ngoc Thanh Tran, Jiaqiu Wang, Alison Rodger, Yuantong Gu, Yuling Wang*
Anastasiia Tukova
School of Natural Sciences, Faculty of Science and Engineering, Macquarie University, Sydney, NSW 2109, Australia
Yihan Nie
School of Mechanical, Medical and Process Engineering, Queensland University of Technology (QUT), Brisbane, QLD 4001, Australia
Dr. Mohammad Tavakkoli Yaraki
School of Natural Sciences, Faculty of Science and Engineering, Macquarie University, Sydney, NSW 2109, Australia
Ngoc Thanh Tran
School of Natural Sciences, Faculty of Science and Engineering, Macquarie University, Sydney, NSW 2109, Australia
Dr. Jiaqiu Wang
School of Mechanical, Medical and Process Engineering, Queensland University of Technology (QUT), Brisbane, QLD 4001, Australia
Centre for Biomedical Technologies, Queensland University of Technology (QUT), Brisbane, QLD 4001, Australia
Prof. Alison Rodger
School of Natural Sciences, Faculty of Science and Engineering, Macquarie University, Sydney, NSW 2109, Australia
Prof. Yuantong Gu
School of Mechanical, Medical and Process Engineering, Queensland University of Technology (QUT), Brisbane, QLD 4001, Australia
Centre for Materials Science, Queensland University of Technology (QUT), Brisbane, Queensland 4001, Australia
Prof. Yuling Wang
School of Natural Sciences, Faculty of Science and Engineering, Macquarie University, Sydney, NSW 2109, Australia E-mail: yuling.wang@mq.edu.au
* Corresponding author
Ϯ These authors made equal contributions
Keywords: anisotropic gold nanoparticles, protein corona, stability, SERS, molecular dynamics simulation
Gold nanoparticles (GNPs) are promising materials for many bioapplications. However, upon contacting with biological media, GNPs undergo changes. The interaction with proteins results in the so-called protein corona (PC) around GNPs, leading to the new bioidentity and optical properties. Understanding the mechanisms of PC formation and its functions can help us to utilise its benefits and avoid its drawbacks. To date, most of the previous works aimed to understand the mechanisms governing PC formation and focused on the spherical nanoparticles although non-spherical nanoparticles are designed for a wide range of applications in biosensing. In this work, we investigated the differences in PC formation on spherical and anisotropic GNPs (nanostars in particular) from the joint experimental (extinction spectroscopy, zeta potential and surface enhanced Raman scattering [SERS]) and computational methods (the finite element method and molecular dynamics [MD] simulations). We discovered that protein does not fully cover the surface of anisotropic nanoparticles, leaving SERS hot-spots at the tips and high curvature edges ”available” for analyte binding (no SERS signal after pre-incubation with protein) while providing protein-induced stabilization (indicated by extinction spectroscopy) of the GNPs by providing a protein layer around the particle’s core. The findings are confirmed from our MD simulations, the adsorption energy significantly decreases with the increased radius of curvature, so that tips (adsorption energy: 2762.334 kJ/mol) would be the least preferential binding site compared to core (adsorption energy: 11819.263 kJ/mol). These observations will help the development of new nanostructures with improved sensing and targeting ability.
Introduction
Gold nanoparticle (GNPs) possesses unique optoelectronic properties, which originate from their localized surface plasmon resonance (LSPR).[1–3] LSPR is a collective oscillation of conduction electrons caused by interaction with an electromagnetic wave (light) of specific (resonant) wavelength.[3,4]The optoelectronic properties of GNPs can be fine-tuned through colloidal chemistry, via engineering nanostructures’ size and shape.[5,6] For instance, anisotropic nanoparticles such as gold nanostars and nanorods have strong extinction coefficient in the near-infrared region where light shows deeper tissue penetration whereas spherical GNPs have distinct bright colour, that is due to light extinction in the visible region and sensitivity to change of their size.[7,8] As such, GNPs have the potential for extensive medical applications in diagnosis and treatment. In pre-clinical settings, the use of GNPs includes point-of-care diagnostics[9,10] (lateral flow assays such as pregnancy tests and rapid COVID-19 tests), in vivo imaging[11,12] (photoacoustic and computerised tomography), and therapeutics[13–15](photothermal therapy, radiotherapy, catalytic therapy, and drug delivery). Although GNPs treatment designs and applications are rapidly developing with some having reached clinical trials,[16–18] none has yet been approved for clinical use primarily because it is hard to predict the changes that nanoparticles undergo and how they behave in complex biological systems. Upon exposure to biological media (e.g. plasma or serum), GNPs interact with large amounts of biomolecules which form layers around the nanoparticles. The main component of the layer is protein and it is referred to as the protein corona (PC).[19,20] The PC characteristically consists of two layers: the “hard corona” – a layer of tightly bound proteins attached through covalent bonds, hydrophobic and electrostatic interactions; and the “soft corona” – a layer of loosely bound proteins outside the “hard corona” that are attached via weak electrostatic forces. The “soft corona” can easily be removed and thus exists in a dynamic state of constantly exchanging proteins, depending on the environment. Contrary to this, the “hard corona” tends to remain unchanged. Even after several centrifugation-resuspension cycles, tightly bound proteins remain on the surface of nanoparticles.[19,20] The PC formation on GNPs induces high variations in their optical properties, targeting and sensing abilities, as well as their bio-recognition by cells (caused by aggregation, morphology changes and surface fouling).[21,22] Some of the effects of a PC can also be beneficial to GNPs applications.[23,24]Exposure of naked GNPs to high ionic strength liquids (biological fluids like blood and urine; buffers, such as phosphate buffered saline [PBS] or isotonic saline solutions) results in their aggregation due to the ion-nanoparticle interactions and loss of electrostatic repulsion.[25,26] PC formation, however, prevents GNPs aggregation, preserving nanoparticles stability in high ionic strength liquids thus reducing cytotoxicity, create immunological stealthiness, increasing blood-circulation time, and preserving optical properties.[25]
To date, we have not fully explored the mechanisms underlying the formation of the PC on GNPs, and most of the GNP-protein interaction studies that have been carried out have been on spherical particles.[25,27,28] Whilst spherical particles are often easiest to work with, it has been previously demonstrated that GNPs with different shapes and surface roughness interact with proteins differently from spherical nanoparticles.[29,30]For example, in our previous study, we have demonstrated that ascorbate-capped gold nanostars reduce protein absorption resulting in enhanced sensing ability in protein-contained media.[31] Further, non-spherical GNPs have been developed for a wide range of sensing applications. In order to use them with biological samples, we need to understand how anisotropic particles interact with proteins. With the increased demand for new nanostructures with enhanced optical properties, it is important to account for shape effects when considering biomedical applications. In this work, we revisit GNP-protein interaction with different shapes of gold nanoparticles (spherical, ellipsoidal and star-shaped) and discuss possible differences in protein-induced stabilization of spherical and anisotropic nanoparticles. We utilize computational methods including Finite Element Method (FEM) and molecular dynamics (MD) simulations to support our experimental observations. Our goal is to understand GNP-protein interactions and the PC mechanisms of GNPs stabilization with non-spherical particles to help create nanostructures that optimise the benefits and mitigate the flaws of PC formation.
2. Results and Discussion
2.1 GNPs synthesis and characterization
For this study, we prepared citrate-capped spherical and elongated gold nanoparticles of 50 and 70 nm diameter respectively (AuNP50 and AuNP70,Figure 1 A and B ) and ascorbate-capped gold nanostars (AuNS) of tip to tip diameter about 80 nm (Figure 1 C ). The synthesis methods are described in the Supporting Information (Table S1) . AuNP50 and AuNP70 have distinctive LSPR bands at 531 and 551 nm respectively. AuNS, with sharp tips, have a distinct LSPR peak in the NIR region at 751 nm and a shoulder in the same wavelength region as the spherical nanoparticles, which is attributed to the extinction (sum of all light that is not transmitted through a sample, absorbed and scattered) mode of the AuNS core (Figure 1 D ). As synthesized AuNP50, AuNP70 and AuNS were monodispersed due to protective capping agents (citrate for AuNP and ascorbate for AuNS), that preserve electrostatic repulsion between the particles (Figure 1 E ).