Ligand density effect on biorecognition by PEGylated gold nanoparticles: regulated interaction of RCA120 lectin with lactose installed to the distal end of tethered PEG strands on gold surface.

PEGylated gold nanoparticles (diameter: 20 nm) possessing various functionalities of lactose ligand on the distal end of tethered PEG ranging from 0 to 65% were prepared to explore the effect of ligand density of the nanoparticles on their lectin binding property. UV-visible spectra of the aqueous solution of the nanoparticles revealed that the strong steric stabilization property of the PEG layer lends the nanoparticles high dispersion stability even under the physiological salt concentration (ionic strength, I = 0.15 M). The number of PEG strands on a single particle was determined to be 520 from thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) observation under controlled acceleration voltage revealed the thickness of the PEG layer on the nanoparticle to be approximately 7 nm. The area occupied by a single lactose molecule on the surface of PEGylated gold nanoparticles was then calculated based on TGA and SEM results and was varied in the range of 10-34 nm2 depending on the lactose functionality (65 approximately 20%). PEGylated gold nanoparticles with 40% and 65% lactose functionality showed a selective and time-dependent aggregation in phosphate buffer with the addition of Ricinus communis agglutinin (RCA120) lectin, a bivalent galactose-specific protein. The aggregates can be completely redispersed by adding an excess amount of galactose. Time-lapse monitoring of UV-visible spectra at 600-750 nm revealed that the aggregation of PEGylated gold nanoparticles was accelerated with an increase in both RCA120 concentration in the solution and the lactose density of the nanoparticles. Furthermore, the sensitivity of lectin detection could be controlled by the regulation of lactose density on the particle surface. Interestingly, there was a critical lactose density (>20%) observed to induce detectable particle aggregation, indicating that the interaction between the particles is triggered by the multimolecular bridging via lectin molecules.