Cancer Cell Specific Fluorescent Methionine/Peptide Protected Gold Nanoclusters For In-Vitro Cell Imaging Studies

In recent years,nanomaterials-based cell bioimaging technology is being applied to the label and imaging of tumors,which plays an important role in early detection and accurate diagnosis of tumors before metastasis.In this respect,noble metal nanoclusters have recently become promising fluorescent materials due to their simple synthesis,tuneable fluorescence emission,well-defined surface chemistry and inherent bioactivity.Among them,the emergence of water-soluble nanoclusters greatly improves the application of nanoclusters in biomedical and environmental sciences.The types of surface ligands of water-soluble clusters can be roughly classified into small molecule mercaptan with hydrophilic functional groups,water-soluble polymers,nucleic acids,peptides and proteins.Compared with other fluorescent nanomaterials,water-soluble nanoclusters have the advantages of ultra-small size,simple synthetic method,stable fluorescence,low cytotoxicity and high biocompatibility.Furthermore,by functionalizing the ligands on the surface of the clusters,the clusters can be endowed with a variety of functions.The interaction between ligands and metal ions or H+ can be used to design ion probes or pH nanometers;by connecting functional groups or molecules with cancer cell targeting function to ligands,nanoclusters with cancer cell targeting function can be obtained,thus achieving cancer cell targeting imaging;by using self-assembly between ligands,new fluorescent nanomaterials or drug delivery system can be obtained.However,at present,most of the nanoclusters need to undergo complex and tedious post-modification steps to achieve their functionalization.Therefore,it is very important to develop a simple functional nanocluster without post-synthesis steps.In this work,we have successfully synthesized a new type of Au NCs(Met-AuNCs)protected by methionine(Met).The uniform size of Met-AuNCs can be obtained within 4 hours without any modification steps.The optical properties of Met-AuNCs were characterized by UV-vis and fluorescence spectra.The nanoclusters exhibited strong fluorescence in aqueous solution within pH 4-12,and still showed brightfluorescence after more than 3 months of storage in darkness at 4 ?.It is noteworthy that Met-AuNCs have high specificity in identifying cancer cells:after incubation with Met-AuNCs for 1 hour,cancer cells show obvious fluorescence,while normal cells show little fluorescence response.Further observations and a series of controlled experiments have confirmed that the targeting of nanoclusters to cancer cells is due to the specific recognition between L-type amino acid transporters over-expressed in cancer cells and Met on the surface of Met-AuNCs.At the same time,cytotoxicity test showed that Met-AuNCs had high biocompatibility.Therefore,methionine-capped gold nanoclusters can be used as an easy-to-synthesize,low toxicity and high sensitivity fluorescence nanomaterial for imaging in cancer cells,and may be used for early diagnosis of cancer.In another work,functional peptides are directly selected as ligands for the synthesis of nanoclusters,which also does not require time-consuming post-modification steps.In this work,fluorescent nanoclusters protected by peptides with strong fluorescence were synthesized using a simple one-pot method at high temperature.Two kinds of peptides were used as the ligands of nanoclusters,namely ECGAAAAAAAAAK(A9K)and glutathione(GSH).Among them,A9K is divided into two parts.The ECG part contains the sequence of GSH,which contains thiol groups to bind to gold nanoclusters.The second part is AAAAAAAAAK,which has the function of targeting cancer cells and killing cancer cells.The addition of the second ligand GSH greatly enhanced the fluorescence intensity of the nanoclusters,reduced the use of A9K and improved the water solubility of the nanoclusters.The as-synthesized nanoclusters not only have strong yellow fluorescence,but also can target cancer cells with surface A9K peptides for fluorescence imaging while do not enter the normal cells.Furthermore,after entering cancer cells,the nanoclusters can further cause cancer cell death and have a good effect of inhibiting the growth of cancer cells.We believe that the design process and synthetic method of the nanoclusters in this work will provide new ideas and possibilities for the development of multi-functional bio-nanomaterials in the future.