Synthesis of folic acid functionalized gold nanoclusters for targeting folate receptor-positive cells

Zhimin Liu,Neil R Thomas,Michael W Fay,Fabrizio Moro,Tracey D Bradshaw,Lyudmila Turyanska,Huw Williams,Salvatore Pacifico,Francesco Zamberlan

doi:10.1088/1361-6528/ab437c

Abstract

We report on the synthesis of water-soluble gold nanoclusters capped with polyethylene glycol (PEG)-based ligands and further functionalized with folic acid for specific cellular uptake. The dihydrolipoic acid-PEG-based ligands terminated with –OMe, –NH2 and –COOH functional groups are produced and used for surface passivation of Au nanoclusters (NCs) with diameters <2 nm. The produced sub 2 nm Au NCs possess long-shelf life and are stable in physiologically relevant environments (temperature and pH), are paramagnetic and biocompatible. The paramagnetism of Au NCs in solution is also reported. The functional groups on the capping ligands are used for direct conjugation of targeting molecules onto Au NCs without the need for post synthesis modification. Folic acid (FA) is attached via an amide group and effectively target cells expressing the folate receptor. The combination of targeting ability, biocompatibility and paramagnetism in FA-functionalized Au NCs is of relevance for their exploitation in nanomedicine for targeted imaging.

Highlights

We report on the synthesis of water-soluble gold nanoclusters capped with polyethylene glycol (PEG)-based ligands and further functionalized with folic acid for specific cellular uptake
The functional groups on the capping ligands are used for direct conjugation of targeting molecules onto Au NCs without the need for post synthesis modification
The combination of targeting ability, biocompatibility and paramagnetism in Folic acid (FA)-functionalized Au NCs is of relevance for their exploitation in nanomedicine for targeted imaging

Summary

Experimental techniques

There is no regioselectivity towards the α- or γ-carboxylic groups in the glutamic acid moiety of the FA. The NMR spectra of NCs in water solution were collected at 600 MHz on a Bruker Avance III spectrometer at 298 K in H2O:D2O (1:9). Chloroauric acid (10 μl, 1 M in water, 0.01 mmol) was added and stirred gently for 5 min. Sodium borohydride (400 μl, 50 mM in water) was added dropwise and the resulting solution was moderately stirred overnight at room temperature. Crude Au NCs solution was centrifuged through AMICON column filters (10 kDa MW cut-off, 1 cycles @ 4000 g for 20 min), with the nanoparticles being collected over the membrane and dispersed in water. FA (0.16 g, 0.36 mmol) was added into the flask and stirred until totally dissolved. LA-PEG200-NH2 (0.4197 g, 1.08 mmol) was added to the mixture and further stirred for 16 h. The organic layer was washed with brine (50 ml, five times) and dried over anhydrous sodium sulphate (Na2SO4), concentrated under reduced pressure to yield the final product (HRMS expected value for C35H49N9O9S2 [M–H]− is 802.3022 m/z, found 802.2991 m/z; see supplementary information, S2)

ORCID iDs

Findings

In vitro characterization