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Gold nanoparticles (AuNPs) are now being used in such areas as diagnostics,
drug delivery, and biological sensing. In these applications, AuNPs are
frequently exposed to biological fluids. These fluids contain many different
proteins, any of which may interfere with the intended function of the nanoparticle.
In this work, we examine the thermodynamic consequences of proteinnanoparticle
binding using a combined spectroscopic and calorimetric
approach. We monitored binding using UV-Vis spectroscopy, differential scanning
calorimetry (DSC), and isothermal titration calorimetry (ITC). Six proteins
were studied based on their differing chemical properties, and both 15
nm and 30 nm citrate-coated AuNPs were investigated. We interpreted the UV-Vis data using two different models: the commonly-used Langmuir
isotherm model and a more complex mass transport model. Both models can
be used to determine Kd values for the 30 nm AuNP data; however, the mass
transport model is more appropriate for 15 nm AuNPs. This is because,
when fitting the Langmuir model, it is commonly assumed that most proteins
are not surface-associated, and this assumption fails for 15 nm AuNPs. The
DSC thermograms show two transitions for a globular protein adsorbed to a
15 nm AuNP: one high-temperature transition that is similar to global protein
unfolding (68 C), and one low-temperature transition that may correspond to
unfolding at the surface (56 C). Conversely, ITC experiments show no net heat
of adsorption for GB3, even at high protein/AuNP concentrations. Together,
the spectroscopic and calorimetric data suggest a complex, multi-step process
for protein-nanoparticle adsorption. Moreover, for the proteins studied, both
AuNP curvature and protein chemistry contribute to protein adsorption, with
proteins generally binding more weakly to the larger nanoparticles. In the
future, this work may lead to principles for improving the design of AuNPbased
therapeutics and sensors.
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