title: Molecular-receptor-specific, non-toxic, near-infrared-emitting Au cluster-protein nanoconjugates for targeted cancer imaging.
abstract: Molecular-receptor-targeted imaging of folate receptor positive oral carcinoma cells using folic-acid-conjugated fluorescent Au(25) nanoclusters (Au NCs) is reported. Highly fluorescent Au(25) clusters were synthesized by controlled reduction of Au(+) ions, stabilized in bovine serum albumin (BSA), using a green-chemical reducing agent, ascorbic acid (vitamin-C). For targeted-imaging-based detection of cancer cells, the clusters were conjugated with folic acid (FA) through amide linkage with the BSA shell. The bioconjugated clusters show excellent stability over a wide range of pH from 4 to 14 and fluorescence efficiency of approximately 5.7% at pH 7.4 in phosphate buffer saline (PBS), indicating effective protection of nanoclusters by serum albumin during the bioconjugation reaction and cell-cluster interaction. The nanoclusters were characterized for their physico-chemical properties, toxicity and cancer targeting efficacy in vitro. X-ray photoelectron spectroscopy (XPS) suggests binding energies correlating to metal Au 4f(7/2) approximately 83.97 eV and Au 4f(5/2) approximately 87.768 eV. Transmission electron microscopy and atomic force microscopy revealed the formation of individual nanoclusters of size approximately 1 nm and protein cluster aggregates of size approximately 8 nm. Photoluminescence studies show bright fluorescence with peak maximum at approximately 674 nm with the spectral profile covering the near-infrared (NIR) region, making it possible to image clusters at the 700-800 nm emission window where the tissue absorption of light is minimum. The cell viability and reactive oxygen toxicity studies indicate the non-toxic nature of the Au clusters up to relatively higher concentrations of 500 microg ml(-1). Receptor-targeted cancer detection using Au clusters is demonstrated on FR(+ve) oral squamous cell carcinoma (KB) and breast adenocarcinoma cell MCF-7, where the FA-conjugated Au(25) clusters were found internalized in significantly higher concentrations compared to the negative control cell lines. This study demonstrates the potential of using non-toxic fluorescent Au nanoclusters for the targeted imaging of cancer.
authors.affiliation: Amrita Centre for Nanoscience and Molecular Medicine, Amrita Institute of Medical Science, Cochin, 682 041, India.
authors.forename: Archana
authors.initials: A
authors.lastname: Retnakumari
authors.forename: Sonali
authors.initials: S
authors.lastname: Setua
authors.forename: Deepthy
authors.initials: D
authors.lastname: Menon
authors.forename: Prasanth
authors.initials: P
authors.lastname: Ravindran
authors.forename: Habeeb
authors.initials: H
authors.lastname: Muhammed
authors.forename: Thalappil
authors.initials: T
authors.lastname: Pradeep
authors.forename: Shantikumar
authors.initials: S
authors.lastname: Nair
authors.forename: Manzoor
authors.initials: M
authors.lastname: Koyakutty
mesh_terms: D002352:Carrier Proteins / Q000378:metabolism*; D002460:Cell Line; D045744:Cell Line, Tumor; D002470:Cell Survival; D005434:Flow Cytometry; D058973:Folate Receptors, GPI-Anchored; D005492:Folic Acid / Q000737:chemistry* / Q000378:metabolism; D005562:Formazans; D006046:Gold / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D008163:Luminescent Measurements; D018625:Microscopy, Atomic Force; D046529:Microscopy, Electron, Transmission; D008856:Microscopy, Fluorescence; D009062:Mouth Neoplasms / Q000175:diagnosis* / Q000378:metabolism / Q000473:pathology; D053761:Nanocomposites / Q000737:chemistry*; D056951:Photoelectron Spectroscopy; D017382:Reactive Oxygen Species / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000378:metabolism*; D012710:Serum Albumin, Bovine / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared; D013778:Tetrazolium Salts
chemical_list: D002352:Carrier Proteins; D058973:Folate Receptors, GPI-Anchored; D005562:Formazans; D017382:Reactive Oxygen Species; D011956:Receptors, Cell Surface; D013778:Tetrazolium Salts; C079782:MTT formazan; D012710:Serum Albumin, Bovine; D006046:Gold; D005492:Folic Acid