Cleaned substrates were then immersed in the designated aminated tethers solution for 2h at 1mM concentration in milli-Q water. The attachment of aminated receptors on QCM substrate is formed due to the presence of thiol head groups: thiol and gold form a relatively stable bond within a short period of time. After the functionalization is complete the surface is thoroughly washed with milli-Q water to remove any loosely bound receptor. The grafting density of the tethered receptors was measured using MP-SPR NaviTM 220A Naali system (Bionavis, Tampere, Finland). A gold coated SPR sensor is incubated in the same solution and for the same time as the QCM sensor and the SPR shift is measured before and after functionalization. Relying on the SPR to quantify the tethers surface coverage removes the uncertainties in play when QCM is used to measure such highly hydrophilic polymers. Nevertheless, due to the interference with the optical signal of SPR, the Au nanoparticle adsorption was carried out only with QCM. QSense Explorer E4 (Biolin Scientific, Sweden) system was employed to measure mass per surface area of AuNPs adsorbed on different tether length. Functionalized QCM sensors were mounted in a QCM open module which allows the injection of liquids directly on the sensor. The solution with gold nanoparticles was pipetted on top of the QCM sensor after a stable baseline was reached. The gold nanoparticles were prepared according to Turkevich's method to achieve AuNPs with a mean diameter of 11.4 nm +/-0.75nm and with a medium pH = 6.8. The adsorption was left to occur for 2h after which the substrate was washed repeatedly with water and dried in-situ, at the end of the process the adsorbed mass was considered. The conversion from frequency shift to bound mass is obtained through the Sauerbrey's equation: ∆𝑚𝑚 = -𝑐𝑐 ∆𝑓𝑓 𝑛𝑛 where ∆𝑚𝑚 is the shift in mass per surface area thus directly connected to the amount of nanoparticle adsorbed, ∆𝑓𝑓 is the shift in resonance frequency, 𝑐𝑐 = 17.7 𝑛𝑛𝑛𝑛𝐻𝐻𝐻𝐻 -1 𝑐𝑐𝑚𝑚 -2 for a 5 MHz crystal and 𝑛𝑛 is the odd overtone number. The applicability of the Sauerbrey's equation lies on the hypothesis that the bound mass does not give rise to any viscoelastic movement, the layer is, thus, rigid and sufficiently thin. PEG is a known hydrophilic biopolymer which tends to accumulate water molecules close to it, this behaviour could lead to an overestimation of the adsorbed mass in QCM. Overcoming the possible overestimation was performed by measuring the adsorbed AuNPs after washing and drying the surface. In this way, the measurement would not be impacted by the presence of water and the PEGs molecules would be mostly flat on the surface.