Developing such an assay requires two tasks to be accomplished: the precise delivery of ligand molecules to identified cell membrane proteins, followed by high resolution imaging to monitor the local changes on the cell in situ. This has not been achieved with any other state-of-the-art method.

Together with Argonne National Laboratory, US, we designed experiments to harness the recent advancements in atomic force microscopy (AFM) and novel bio-conjugation techniques to tackle this challenge. The paper published in Nanotechnology demonstrated the feasibility of the approach. Here we anchored proteins on an AFM tip via our newly synthesized photo-cleavable cross-linker. Irradiation served as a “remote controller” to cleave the proteins off the tip upon the photolytic reaction of the cross-linker. Due to the high lateral resolution of AFM, the proteins can be precisely delivered to a desired local region. After the proteins modified on the tip were cleaved away, the tip regained the same level of imaging resolution as a bare tip. Thus the result of protein delivery was resolved at a single molecular level.

Another advantage of the method is that the ligand-modified AFM tip can provide guidance to identify the cell membrane receptors on a living cell via force mapping. That’s despite the complexity arising from the diverse distribution of various cell-surface proteins and phospholipids on the rough membrane surface. This makes it possible to deliver a ligand to its receptor on a living cell, and dynamically follow up the local cell response to the ligand-receptor interaction. We are currently making efforts along this line. In general, such a methodology can be applied to any cell types. By disclosing the initial molecular signal pathway associated with ligand-receptor interactions in living cells, the research should also provide a molecular basis and novel strategies for early diagnosis and treatment of many ligand-associated diseases.