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Most molecular/cellular labeling utilizes organic dyes conjugated to bioactive molecules through general organic/inorganic chemistry. Though shortcomings of organic dyes such as poor photostability [1] and low brightness [2] limit observable copy numbers, lack of selectivity in labeling often more seriously limits their application in live cell imaging and single molecule studies [3]. Specificity can be addressed through antibodies and other strong affinity pairs such as avidin-biotin [4, 5], but direct covalent technologies are crucial at sub-pM concentrations, as this is beyond the binding limits of antibody-based affinities. Genetically encoded fluorescent proteins are an excellent solution for specific fluorescent labeling in live cells, but disadvantageous organic dye photophysical instabilities (blinking and bleaching) remain, coupled with the potential perturbation due to large label size [6]. Attempts to overcome …