Gold Nanoparticles Assembly on Silicon and Gold Surfaces: Mechanism, Stability, and Efficiency in Diclofenac Biosensing

We investigated the assembly of gold nanoparticles (AuNPs) on gold and silicon sensors with two final objectives: (i) understanding the factors governing the interaction and (ii) building up a nanostructured piezoelectric immunosensor for diclofenac, a small-sized pharmaceutical pollutant. Different surface chemistries were devised to achieve AuNPs assembly on planar substrates. These surface chemistries included amines to immobilize AuNPs via electrostatic interaction or a mixture of amines and thiols to covalently attach the AuNPs. We also generated PEG-amine-terminated surfaces to benefit from the well-known non-biofouling properties of PEG-coated surfaces. The functional substrates and the resulting gold nanoparticle layers were characterized in detail by surface IR, contact angle measurements, and scanning electron microscopy (SEM). The mechanism of adsorption is discussed herein considering the nature of the terminal groups and their charge at the pH of AuNPs adsorption. The coverage and the dispersion of AuNPs were strongly dependent on the anchoring points on the surfaces; the optimal were reached when the attachment layer offered multiple interaction points, in particular, for NH2/SH- and PEG/NH2-terminated surfaces, where the percentage of isolated particles was up to 78%. In addition, PEG-coated surfaces led to a stable AuNPs layer resistant to ultrasounds and to further functionalization of the immobilized nanoparticles. These surfaces were used to engineer quartz crystal microbalance (QCM) biosensors for diclofenac detection. The AuNPs nanostructured substrates significantly enhanced the biosensor sensitivity as compared to planar substrates (up to 6 times higher). This enhancement presages a higher sensitivity in the competitive detection of diclofenac on these systems. More importantly, despite the biorecognition and the drastic regeneration conditions, SEM images show that gold nanoparticles layers are stable and reliable, which paves the way for their use as nanostructured platforms for multiple applications.