Physical attachment of fluorescent protein particles to atomic force microscopy probes in aqueous media: implications for surface pH, fluorescence, and mechanical properties studies.

Transfer of a fluorescently labeled protein particle from a surface to a microsized scanning probe has been induced by repetitive scanning in aqueous medium. The so-attached particle can in turn act as a probing tool to study particle-substrate and particle-particle interactions. Attachment of the fluorescent particle occurs at the apical region of an atomic force microscope (AFM) cantilever tip and it endures repetitive loading-unloading cycles against the sample surface. Fluorescence microscopy has been used to address the exact location of the attached particle in the cantilever and to identify the moment when the particle contacts the sample. Moreover, we have observed that fluorescence intensity at the contact point is lower when the probing particle contacts another fluorescent particle than when it contacts the nonfluorescent substrate. The change in fluorescence is attributed to local changes of pH and interparticle-quenching of fluorophores in the contact region. These findings are promising since they constitute a chemical-free way to attach bioparticles to AFM probes under physiological conditions. The atomic force microscopy combined with fluorescence microscopy provides a straight forward method to study particle/particle and particle/substrate interactions, as well as to investigate mechanical properties of biocolloids.