Molecular-receptor-specific, non-toxic, near-infrared-emitting Au cluster-proteinnanoconjugates for targeted cancer imaging

Abstract

Molecular-receptor-targeted imaging of folate receptor positiveoral carcinoma cells using folic-acid-conjugated fluorescentAu25 nanoclusters (Au NCs) is reported. Highly fluorescentAu25 clusters were synthesized by controlled reduction ofAu+ ions, stabilized in bovine serum albumin (BSA), using a green-chemical reducingagent, ascorbic acid (vitamin-C). For targeted-imaging-based detection ofcancer cells, the clusters were conjugated with folic acid (FA) throughamide linkage with the BSA shell. The bioconjugated clusters show excellentstability over a wide range of pH from 4 to 14 and fluorescence efficiency of∼5.7% at pH 7.4 in phosphate buffer saline (PBS), indicating effective protection of nanoclusters byserum albumin during the bioconjugation reaction and cell–cluster interaction. The nanoclusterswere characterized for their physico-chemical properties, toxicity and cancer targeting efficacyin vitro. X-ray photoelectron spectroscopy (XPS) suggests binding energies correlating to metalAu 4f7/2∼83.97 eVand Au 4f5/2∼87.768 eV. Transmission electron microscopy and atomic force microscopyrevealed the formation of individual nanoclusters of size∼1 nm and protein clusteraggregates of size ∼8 nm. Photoluminescence studies show bright fluorescence with peak maximum at∼674 nm with the spectral profile covering the near-infrared (NIR) region, making it possible toimage clusters at the 700–800 nm emission window where the tissue absorption oflight is minimum. The cell viability and reactive oxygen toxicity studies indicatethe non-toxic nature of the Au clusters up to relatively higher concentrations of500 µg ml−1. Receptor-targeted cancer detection using Au clusters is demonstrated onFR+ve oralsquamous cell carcinoma (KB) and breast adenocarcinoma cell MCF-7, where the FA-conjugatedAu25 clusters were found internalized in significantly higher concentrations compared to thenegative control cell lines. This study demonstrates the potential of using non-toxicfluorescent Au nanoclusters for the targeted imaging of cancer.