A new scanning probe microscope, the photonic force microscope (PFM), based on optical tweezers and two-photon absorption processes for biological applications is described. Optical tweezers are used to trap a fluorescent latex bead with a diameter of 200 nm in an aqueous solution in all three dimensions. The fluorescent dye is chosen to fulfill the two-photon absorption criterion for the 1064-nm line of a Nd:YVO4 laser. The intensity of the fluorescence emission is utilized as a very sensitive position sensor along the optical axis. Two-dimensional images are formed by laterally scanning the trapped latex bead across biological samples while recording the two-photon-induced fluorescences intensity. A scanning probe image of the outer surface of a small neurite from a cultured rat hippocampal neuron is shown, which is hardly visible under differential interference contrast microscopy. The lateral resolution is given by the bead diameter; the axial resolution is 40 nm. Under the experimental conditions the maximal imaging force applied by the probe is below 5 pN.
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"The broad applicability of the technique stems from its ability to map out the location of nanoscale features, such as surface defects in semiconductors  or receptor sites on cell membranes under physiological conditions   . Optical Tweezers  and Photonic Force Microscopy (PFM)  have also been used to identify surfaces within a sample. Using PFM, smaller probe particles have been held in the optical trap and their inherent Brownian motion used to map the cavities in which they are located . "
[Show abstract][Hide abstract] ABSTRACT: We present an imaging technique using an optically trapped silica microrod probe controlled using holographic optical tweezers. The probe is raster scanned over a surface, allowing an image to be recorded in a manner analogous to scanning probe microscopy (SPM), with closed loop feedback control provided by high-speed CMOS camera image tracking. We demonstrate a proof of principle of this technique by imaging the surface of an oil droplet. We estimate the normal force exerted on the sample during imaging to be 1 pN. The resolution is limited by the diameter of the microrod tip, thermal motion of the probe and the tracking accuracy. As our technique is not diraction limited, there is scope for signicant improvement by reducing the tip diameter, and position clamping to reduce unwanted thermal motion.
Proceedings of SPIE - The International Society for Optical Engineering 09/2011; 8097. DOI:10.1117/12.893796 · 0.20 Impact Factor
"SPM instrumentation research is mainly aimed at increasing the rates at which surface images are obtained  , and in lowering the interaction forces . Optical tweezers  and photonic force microscopy (PFM)  have also been used to identify surfaces within a sample. Using PFM, probe particles have been held in the optical trap and their inherent Brownian motion used to map the cavities in which they are located . "
[Show abstract][Hide abstract] ABSTRACT: We present an imaging technique using an optically trapped cigar-shaped probe controlled using holographic optical tweezers. The probe is raster scanned over a surface, allowing an image to be taken in a manner analogous to scanning probe microscopy (SPM), with automatic closed loop feedback control provided by analysis of the probe position recorded using a high speed CMOS camera. The probe is held using two optical traps centred at least 10 µm from the ends, minimizing laser illumination of the tip, so reducing the chance of optical damage to delicate samples. The technique imparts less force on samples than contact SPM techniques, and allows highly curved and strongly scattering samples to be imaged, which present difficulties for imaging using photonic force microscopy. To calibrate our technique, we first image a known sample--the interface between two 8 µm polystyrene beads. We then demonstrate the advantages of this technique by imaging the surface of the soft alga Pseudopediastrum. The scattering force of our laser applied directly onto this sample is enough to remove it from the surface, but we can use our technique to image the algal surface with minimal disruption while it is alive, not adhered and in physiological conditions. The resolution is currently equivalent to confocal microscopy, but as our technique is not diffraction limited, there is scope for significant improvement by reducing the tip diameter and limiting the thermal motion of the probe.
"Several single-molecule experiments have provided detailed insights into intermolecular and intramolecular forces, providing relevant information on a large number of molecular mechanisms     . The development of the atomic force microscope (AFM)  instrument has enabled such singlemolecule investigations, and with the AFM it has become possible to determine the elastic moduli, friction coefficients, chemical residues and surface charges with high spatial resolution    . "
[Show abstract][Hide abstract] ABSTRACT: We report on the mechanical characterization of individual mature amyloid fibrils by atomic force microscopy (AFM) and AFM-based single-molecule force spectroscopy (SMFS). These self-assembling materials, formed from the 29-residue amphiphatic peptide hormone glucagon, were found to display a reversible elastic behaviour. Based on AFM morphology and SMFS studies, we suggest that the observed elasticity is due to a force-induced conformational transition which is reversible due to the β-helical conformation of protofibrils, allowing a high degree of extension. The elastic properties of such mature fibrils contribute to their high stability, suggesting that the internal hydrophobic interactions of amyloid fibrils are likely to be of fundamental importance in the assembly of amyloid fibrils and therefore for the understanding of the progression of their associated pathogenic disorders. In addition, such biological amyloid fibril structures with highly stable mechanical properties can potentially be used to produce nanofibres (nanowires) that may be suitable for nanotechnological applications.