Correlational Analysis of Proteins and Nonmetallic Nanoparticles in a Deep-Nulling Microscope

Inverse-fluorescence cross-correlation spectroscopy

Inverse-fluorescence correlation spectroscopy (iFCS) was recently introduced as an alternative version of FCS that does not require labeling of the analyzed particles or biomolecules. In iFCS, the signal from a medium surrounding the particles is analyzed, as opposed to a signal from the studied particles themselves. As unlabeled particles diffuse through the detection volume, they displace a fraction of the fluorescent medium, causing transient dips in the detected signal which give information about the mobility and concentration of the analyzed particles. Here inverse-fluorescence cross-correlation spectroscopy (iFCCS) is introduced as an extension of iFCS. In iFCCS, labeled particles/biomolecules are analyzed and their fluorescence signal is cross-correlated with the signal from the surrounding medium. When labeled particles are analyzed, a direct estimate of the volume of the particles is obtained or, alternatively, an estimate of the size of the detection volume. Another possibility is to analyze the interaction of small, labeled molecules with unlabeled particles, resulting in cross-correlation as an indication of binding, even though only one binding partner is labeled. This also enables accurate estimation of the degree of labeling, since the amounts of labeled and unlabeled particles are estimated independently.

Photothermal detection of individual gold nanoparticles: perspectives for high-throughput screening

We use photothermal microscopy to detect and image individual gold nanoparticles that are either embedded in a polymer film or immobilized in an aqueous environment. Reducing the numerical aperture of the detection optics allows us to achieve a 200-fold-enlarged detection volume while still retaining sufficient detectivity. We characterize the capabilities of this approach for the detection of gold colloids with a diameter of 20 nm, with emphasis on practical aspects that are important for high-throughput-screening applications. The extended detection volume in combination with the stability of the photothermal signal are major advantages compared to fluorescence-based approaches, which are limited by photoblinking and photobleaching. Careful consideration is given to the trade-off between the maximum increase in local temperature that can be tolerated by a biological specimen and the minimum integration time needed to reliably determine whether a given volume contains a target species. We find that our approach has the potential to increase the detection-limited flow rate (i.e. the limit given by the detection volume divided by the minimum detection time) by two to three orders of magnitude.