A surface energy transfer nanoruler for measuring binding site distances on live cell surfaces

Measuring distances at molecular length scales in living systems is a significant challenge. Methods like Förster resonance energy transfer (FRET) have limitations due to short detection distances and strict orientations. Recently, surface energy transfer (SET) has been used in bulk solutions; however, it cannot be applied to living systems. Here, we have developed an SET nanoruler, using aptamer-gold nanoparticle conjugates with different diameters, to monitor the distance between binding sites of a receptor on living cells. The nanoruler can measure separation distances well beyond the detection limit of FRET. Thus, for the first time, we have developed an effective SET nanoruler for live cells with long distance, easy construction, fast detection, and low background. This is also the first time that the distance between the aptamer and antibody binding sites in the membrane protein PTK7 was measured accurately. The SET nanoruler represents the next leap forward to monitor structural components within living cell membranes.