Probing BSA Binding to Citrate-Coated Gold Nanoparticles and Surfaces

Abstract

The interaction of bovine serum albumin (BSA) with gold colloids and surfaces was studied using zeta-potential and quartz crystal microbalance (QCM) measurements, respectively, to determine the surface charge and coverage. The combination of these two measurements suggests that BSA binding to gold nanoparticles and gold surfaces occurs by an electrostatic mechanism when citrate is present. The binding of BSA to bare gold is nearly two times greater than the binding of BSA to a citrate-coated gold surface, suggesting that protein spreading (denaturation) on the surface may occur followed by secondary protein binding. On the other hand, binding to citrate-coated gold surfaces can be fit to a Langmuir isotherm model to obtain a maximum surface coverage of (3.7 +/- 0.2) x 10(12) molecules/cm(2) and a binding constant of 1.0 +/- 0.3 microM(-1). The zeta-potential measurements show that the stabilization of colloids by BSA has a significant contribution from a steric mechanism because the colloids are stable, even at their isoelectric point (pI approximately 4.6). To be consistent with the observed phenomena, the electrostatic interactions between BSA and citrate must consist of salt-bridges, for example, of the carboxylate-ammonium type, between the citrate and the lysine on the protein surface. The data support the role of strong electrostatic binding but do not exclude contributions from steric or hydrophobic interactions with the surface adlayer.

Label-free detection of biomolecules on microarrays using surface–colloid interaction

“…For a silicon-oxide particle modified by attaching positively charged polymer: (11) where Γ' NH3 + is the density of ionized amino groups on the surface of colloidal particle. For gold colloidal particles encapsulated by a layer of positively charged polymer: (12) where Γ' NH3 + is the density of ionized amino groups and σ Au is the density of the negative charge carried by non-modified gold particle in aqueous solution [15]. For the 20 nm gold particle in aqueous solution having Zeta-potential of 30 mV, the surface charge density estimated from the Graham equation is σ Au = 89.6 mC/m 2 [12,15].…”

“…For gold colloidal particles encapsulated by a layer of positively charged polymer: (12) where Γ' NH3 + is the density of ionized amino groups and σ Au is the density of the negative charge carried by non-modified gold particle in aqueous solution [15]. For the 20 nm gold particle in aqueous solution having Zeta-potential of 30 mV, the surface charge density estimated from the Graham equation is σ Au = 89.6 mC/m 2 [12,15]. The energy, E, of interaction of the colloidal particle and the substrate surface is proportional to the multiplication of the electric charge densities of the substrate and the net charge of colloidal particle (13) The Eqs.…”

“…Amino-modified substrates from commercial vendors have been reported to have pI in the range of 6.5-7.5 and the density of amino-silane on surface of 90 pmole/cm 2 -300 pmole/cm 2 [11]. Cationic gold particles can be prepared in broad range of pI Part by using various activation reagents and by adjusting the concentration of the activation reagent [15]. Two other important parameters for surface-colloid interaction are the size of colloidal particles and the concentration of buffer ions in solution (e.g., solution ionic strength).…”

“…To have a complete set of equations an additional functional relation between the potential of the diffuse layer, ψ d and the surface charge, σ Surf , is required, which can be obtained from the Poisson-Boltzman equation [16][17][18]. For the flat isolated surface the relation between ψ d and σ Surf , is given by Graham equation [17,18]: (15) where λ D is the Debye screening length in solution: λ D = (ε o εk B T/2e 2 n) 1/2 , n is the concentration of small ions in bulk solution, and ε o ε is the permitivity of the solution. The pH substitution and generalized Graham equation for colloidal particle of radius a [12]: (16) (17) In solutions, the binding energy, E, of the surface and charge q separated from the surface by distance x: E(x) = q ψ d e(−x/ λ D ).…”

“…Figure 3A,B shows example of titration curves and particlesurface binding energy calculated from Eqs. (4)(5)(6)(7)(8)(9)(10)(11)(12) and (15)(16)(17)(18). The simulation parameters used in this model are summarized in Table I.…”