Distinguishing Folate-Receptor-Positive Cells from Folate-Receptor-Negative Cells Using a Fluorescence Off-On Nanoprobe

Based on the high affinity of folic acid (FA) for folate receptor (FR) that is overexpressed on the surface of many human cancer cells, we have developed a simple fluorescence nanoprobe (1) with multiple capability (fluorescence off on response and cell targeting ability) for imaging of FR positive cells by covalently linking both FA and Rhodamine B (RB) to graphene oxide (GO) through disulfide bonds. The nanoprobe shows a weak fluorescence due to the electron transfer from GO to RB. However, the specific binding of FA to FR positive cells leads to the internalization of the nanoprobe into the cells. As a result, the disulfide bonds of 1 are cleaved by intracellular glutathione, causing the release of the RB moiety from GO and thereby the generation of fluorescence. Compared to most of the reported fluorescence always on nanoprobes for imaging FR positive cells, the present fluorescence off on nanoprobe can not only produce a high signal/background ratio but also avoid the false positive results often caused by nonspecific adsorption of the always on nanoprobes on the surface of nontarget cells. Notably, the proposed off-on nanoprobe has been demonstrated to distinguish the cells with different expression levels of FR by culturing and analyzing different cell mixtures (Hela/NIH-3T3 and Hela/MCF-7 cells). Moreover, the nanoprobe is capable of discriminating FR positive from FR negative cells even with similar morphology. This method is simple and selective for fluorescence imaging of FR-positive cells.