Chemisorption of iodine-125 to gold nanoparticles allows for real-time quantitation and potential use in nanomedicine

Abstract

Gold nanoparticles have been available for many years as a research tool in the life sciences due to their electron density and optical properties. New applications are continually being developed, particularly in nanomedicine. One drawback is the need for an easy, real-time quantitation method for gold nanoparticles so that the effects observed in in vitro cell toxicity assays and cell uptake studies can be interpreted quantitatively in terms of nanoparticle loading. One potential method of quantifying gold nanoparticles in real time is by chemisorption of iodine-125, a gamma emitter, to the nanoparticles. This paper revisits the labelling of gold nanoparticles with iodine-125, first described 30 years ago and never fully exploited since. We explore the chemical properties and usefulness in quantifying bio-functionalised gold nanoparticle binding in a quick and simple manner. The gold particles were labelled specifically and quantitatively simply by mixing the two items. The nature of the labelling is chemisorption and is robust, remaining bound over several weeks in a variety of cell culture media. Chemisorption was confirmed as potassium iodide can remove the label whereas sodium chloride and many other buffers had no effect. Particles precoated in polymers or proteins can be labelled just as efficiently allowing for post-labelling experiments in situ rather than using radioactive gold atoms in the production process. We also demonstrate that interparticle exchange of I-125 between different size particles does not appear to take place confirming the affinity of the binding.

Keywords: Chemisorption; Gold nanoparticles; Iodine-125; Nanomedicine; Quantitation; Radioactive labelling.

Fig. 1

TEM images of nanoparticle preparation and size distribution

Fig. 2

a Absorption spectra for gold nanoparticles. OD_max = 520 nm. b Fifteen-nanometre gold spectra with and without iodide

Fig. 3

Percentage of initially bound iodine-125 to Au-15 Carbowax20M remaining after dialysis for 16 h against different solutions

Fig. 4

Loading of I-125 onto 15 nm gold BSA. a Variable amounts of I-125 added to gold BSA (1.1 × 10¹² particles/ml). b Relationship between μCi I-125 binding and number of iodine atoms per gold particle

Fig. 5

Comparison of a direct and an amplified microtitre plate binding assay of iodine-125-labelled gold nanoparticles. Wells were coated with either BSA or ferritin and probed with radiolabelled gold coated with rabbit anti-ferritin. Direct binding was demonstrated by gold anti-ferritin binding to the plate in a one-step procedure. An amplified binding assay was demonstrated by probing the plates with rabbit anti ferritin followed by goat anti-rabbit serum then by gold nanoparticles labelled with rabbit anti-ferritin. Controls were undertaken with a competitive assay format using 50 μg ferritin in solution for the first step

Fig. 6

Degree of labelling depends on the protocol adopted

Fig. 7

Graph showing a standard curve for post-labelling of gold particles on dot blots with I-125. Increasing volumes of GAM gold were dotted and probed with I-125 (60Kc/10s)