Fluorescence correlation spectroscopy of finite-sized particles

Localizing Binding Sites on Bioconjugated Hydrogen-Bonded Organic Semiconductors at the Nanoscale

Hydrogen‐bonded organic semiconductors are extraordinarily stable organic solids forming stable, large crystallites with the ability to preserve favorable electrical properties upon bioconjugation. Lately, tremendous efforts have been made to use these bioconjugated semiconductors as platforms for stable multifunctional bioelectronics devices, yet the detailed characterization of bio‐active binding sites (orientation, density, etc.) at the nanoscale has not been achieved yet. The presented work investigates the bioconjugation of epindolidione and quinacridone, two representative semiconductors, with respect to their exposed amine‐functionalities. Relying on the biotin‐avidin lock‐and‐key system and applying the atomic force microscopy (AFM) derivative topography and recognition (TREC) imaging, we used activated biotin to flag crystal‐faces with exposed amine functional groups. Contrary to previous studies, biotin bonds were found to be stable towards removal by autolysis. The resolution strength and clear recognition capability makes TREC‐AFM a valuable tool in the investigation of bio‐conjugated, hydrogen‐bonded semiconductors.

Spatiotemporal Image Correlation Analysis for 3D Flow Field Mapping in Microfluidic Devices

Microfluidic devices reproducing 3D networks are particularly valuable for nanomedicine applications such as tissue engineering and active cell sorting. There is however a gap in the possibility to measure how the flow evolves in such 3D structures. We show here that it is possible to map 3D flows in complex microchannel networks by combining wide field illumination to image correlation approaches. For this purpose, we have derived the spatiotemporal image correlation analysis of time stacks of single-plane illumination microscopy images. From the detailed analytical and numerical analysis of the resulting model, we developed a fitting method that allows us to measure, besides the in-plane velocity, the out-of-plane velocity component down to v_z ≅ 65 μm/s. We have applied this method successfully to the 3D reconstruction of flows in microchannel networks with planar and 3D ramifications. These different network architectures have been realized by exploiting the great prototyping ability of a 3D printer, whose precision can reach few tens of micrometers, coupled to poly dimethyl-siloxane soft-printing lithography.

A total internal reflection-fluorescence correlation spectroscopy setup with pulsed diode laser excitation

Fluorescence correlation spectroscopy (FCS) measures fluctuations in a (sub-)femtoliter volume to analyze the diffusive behavior of fluorescent particles. This highly sensitive method has proven to be useful for the analysis of dynamic biological systems as well as in chemistry, physics, and material sciences. It is routinely performed with commercial fluorescence microscopes, which provide a confined observation volume by the confocal technique. The evanescent wave of total internal reflectance (TIR) is used in home-built systems to permit a surface sensitive FCS analysis. We present a combined confocal and TIR-FCS setup which uses economic low-power pulsed diode lasers for excitation. Excitation and detection are coupled to time-correlated photon counting hardware. This allows simultaneous fluorescence lifetime and FCS measurements in a surface-sensitive mode. Moreover, the setup supports fluorescence lifetime correlation spectroscopy at surfaces. The excitation can be easily switched between TIR and epi-illumination to compare the surface properties with those in liquid bulk. The capabilities of the presented setup are demonstrated by measuring the diffusion coefficients of a free dye molecule, a labeled polyethylene glycol, and a fluorescent nanoparticle in confocal as well as in TIR-FCS.

Asymmetric Triblock Copolymer Nanocarriers for Controlled Localization and pH-Sensitive Release of Proteins

Designing nanocarriers to release proteins under specific conditions is required to improve therapeutic approaches, especially in treating cancer and protein deficiency diseases. We present here supramolecular assemblies based on asymmetric poly(ethylene glycol)-b-poly(methylcaprolactone)-b-poly(2-(N,Ndiethylamino)ethyl methacrylate) (PEG-b-PMCL-b-PDMAEMA) copolymers for controlled localization and pH-sensitive release of proteins. Copolymers self-assembled in soft nanoparticles with a core domain formed by PMCL, and a hydrophilic domain based on PEG mainly embedded inside, and the branched PDMAEMA exposed at the particle surface. We selected as model proteins to be attached to the nanoparticles bovine serum albumin (BSA) and acid sphingomyelinase (ASM), the latter being an ideal candidate for protein replacement therapy. The hydrophilic/hydrophobic ratio, nanoparticle size, and the nature of biomolecules are key factors for modulating protein localization and attachment efficiency. The predominant outer shell of PDMAEMA allows efficient pH-triggered release of BSA and ASM, and in acidic conditions >70% of the bound proteins were released. Uptake of protein-attached nanoparticles by HELA cells, together with low toxicity and pH-responsive release, supports such protein-bound nanoparticles as efficient stimuli-responsive candidates for protein therapy.

Structure and Dimensions of Core-Shell Nanoparticles Comparable to the Confocal Volume Studied by Means of Fluorescence Correlation Spectroscopy

In some applications the dye distribution within fluorescently labeled nanoparticles and its stability over long periods of time are important issues. In this article we study numerically and experimentally the applicability of fluorescence correlation spectroscopy (FCS) to resolve such questions. When the size of fluorescently labeled particles is comparable to or larger than the confocal volume, the effective confocal volume seen in FCS experiments is increasing. Such an effect has already been studied for uniformly labeled spherical particles. In this work we analyze the form of the FCS correlation functions (CFs) for core-labeled and shell-labeled core-shell particles. For shell-labeled particles an additional fast decay was found both in simulations and in experiments on custom-made surface-labeled particles. Universal scaling of FCS correlation times based on the squared ratio of the labeled part radius of gyration to the Gaussian radius of the beam profile was found. Recipes based on the analysis of simulated CFs, proposed for interpretation of experimental results, were successfully applied to the FCS results on suspensions of large core-labeled and surface-labeled particles.

Introducing a fluorescence-based standard to quantify protein partitioning into membranes

The affinity of peripheral membrane proteins for a lipid bilayer can be described using the partition coefficient (KP). Although several methods to determine KP are known, all possess limitations. To address some of these issues, we developed both: a versatile method based on single molecule detection and fluorescence imaging for determining KP, and a simple measurement standard employing hexahistidine-tagged enhanced green fluorescent protein (eGFP-His6) and free standing membranes of giant unilamellar vesicles (GUVs) functionalized with NTA(Ni) lipids as binding sites. To ensure intrinsic control, our method features two measurement modes. In the single molecule mode, fluorescence correlation spectroscopy (FCS) is applied to quantify free and membrane associated protein concentrations at equilibrium and calculate KP. In the imaging mode, confocal fluorescence images of GUVs are recorded and analyzed with semi-automated software to extract protein mean concentrations used to derive KP. Both modes were compared by determining the affinity of our standard, resulting in equivalent KP values. As observed in other systems, eGFP-His6 affinity for membranes containing increasing amounts of NTA(Ni) lipids rises in a stronger-than-linear fashion. We compared our dual approach with a FCS-based assay that uses large unilamellar vesicles (LUVs), which however fails to capture the stronger-than-linear trend for our NTA(Ni)-His6 standard. Hence, we determined the KP of the MARCKS effector domain with our FCS approach on GUVs, whose results are consistent with previously published data using LUVs. We finally provide a practical manual on how to measure KP and understand it in terms of molecules per lipid surface.

Size of Submicrometer Particles Measured by FCS: Correction of the Confocal Volume

When fluorescence correlation spectroscopy (FCS) in combination with a confocal microscope is used to determine the hydrodynamic radius a of particles comparable to or larger than the linear size σ of the confocal volume of the microscope, a correction must be used that depends on the a(2)/σ(2) ratio and the distribution of the dye within the particle. Here we present the experimental validation of the theoretically predicted approximate correction necessary for appropriate measurements of the size of uniformly fluorescently labeled spheres of radius comparable to the size of the confocal volume. We also test the approximate correction formula for different ranges of the a/σ ratio and propose a simple procedure to obtain the correct nanoparticle size from such a measurement.

In vivo flow mapping in complex vessel networks by single image correlation

We describe a novel method (FLICS, FLow Image Correlation Spectroscopy) to extract flow speeds in complex vessel networks from a single raster-scanned optical xy-image, acquired in vivo by confocal or two-photon excitation microscopy. Fluorescent flowing objects produce diagonal lines in the raster-scanned image superimposed to static morphological details. The flow velocity is obtained by computing the Cross Correlation Function (CCF) of the intensity fluctuations detected in pairs of columns of the image. The analytical expression of the CCF has been derived by applying scanning fluorescence correlation concepts to drifting optically resolved objects and the theoretical framework has been validated in systems of increasing complexity. The power of the technique is revealed by its application to the intricate murine hepatic microcirculatory system where blood flow speed has been mapped simultaneously in several capillaries from a single xy-image and followed in time at high spatial and temporal resolution.

Techniques for physicochemical characterization of nanomaterials

Advances in nanotechnology have opened up a new era of diagnosis, prevention and treatment of diseases and traumatic injuries. Nanomaterials, including those with potential for clinical applications, possess novel physicochemical properties that have an impact on their physiological interactions, from the molecular level to the systemic level. There is a lack of standardized methodologies or regulatory protocols for detection or characterization of nanomaterials. This review summarizes the techniques that are commonly used to study the size, shape, surface properties, composition, purity and stability of nanomaterials, along with their advantages and disadvantages. At present there are no FDA guidelines that have been developed specifically for nanomaterial based formulations for diagnostic or therapeutic use. There is an urgent need for standardized protocols and procedures for the characterization of nanoparticles, especially those that are intended for use as theranostics.

Single molecule confocal fluorescence lifetime correlation spectroscopy for accurate nanoparticle size determination

Experimental procedure in confocal single molecule fluorescence lifetime correlation spectroscopy to determine the excitation power and molecular or particulate concentration under which the application of an unmodified model autocorrelation function is justified.

Enumerating virus-like particles in an optically concentrated suspension by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) is one of the most sensitive methods for enumerating low concentration nanoparticles in a suspension. However, biological nanoparticles such as viruses often exist at a concentration much lower than the FCS detection limit. While optically generated trapping potentials are shown to effectively enhance the concentration of nanoparticles, feasibility of FCS for enumerating field-enriched nanoparticles requires understanding of the nanoparticle behavior in the external field. This paper reports an experimental study that combines optical trapping and FCS to examine existing theoretical predictions of particle concentration. Colloidal suspensions of polystyrene (PS) nanospheres and HIV-1 virus-like particles are used as model systems. Optical trapping energies and statistical analysis are used to discuss the applicability of FCS for enumerating nanoparticles in a potential well produced by a force field.

STICCS reveals matrix-dependent adhesion slipping and gripping in migrating cells

Two-color spatio-temporal image cross-correlation spectroscopy (STICCS) is a new, to our knowledge, image analysis method that calculates space-time autocorrelation and cross-correlation functions from fluorescence intensity fluctuations. STICCS generates cellular flow and diffusion maps that reveal interactions and cotransport of two distinct molecular species labeled with different fluorophores. Here we use computer simulations to map the capabilities and limitations of STICCS for measurements in complex heterogeneous environments containing micro- and macrostructures. We then use STICCS to analyze the co-flux of adhesion components in migrating cells imaged using total internal reflection fluorescence microscopy. The data reveal a robust, time-dependent co-fluxing of certain integrins and paxillin in adhesions in protrusions when they pause, and in adhesions that are sliding and disassembling, demonstrating that the molecules in these adhesions move as a complex. In these regions, both α6β1- or αLβ2-integrins, expressed in CHO.B2 cells, co-flux with paxillin; an analogous cotransport was seen for α6β1-integrin and α-actinin in U2OS. This contrasts with the behavior of the α5β1-integrin and paxillin, which do not co-flux. Our results clearly show that integrins can move in complexes with adhesion proteins in protrusions that are retracting.

Image correlation spectroscopy of randomly distributed disks

Image correlation spectroscopy (ICS) has been widely used to quantify spatiotemporal distributions of fluorescently labelled cell membrane proteins and receptors. When the membrane proteins are randomly distributed, ICS may be used to estimate protein densities, provided the proteins behave as point-like objects. At high protein area fraction, however, even randomly placed proteins cannot obey Poisson statistics, because of excluded area. The difficulty can arise if the protein effective area is quite large, or if proteins form large complexes or aggregate into clusters. In these cases, there is a need to determine the correct form of the intensity correlation function for hard disks in two dimensions, including the excluded area effects. We present an approximate but highly accurate algorithm for the computation of this correlation function. The correlation function was verified using test images of randomly distributed hard disks of uniform intensity convolved with the microscope point spread function. This algorithm can be readily modified to compute exact intensity correlation functions for any probe geometry, interaction potential, and fluorophore distribution; we show how to apply it to describe a random distribution of large proteins labeled with a single fluorophore.

Probe-independent image correlation spectroscopy

Conventional image correlation spectroscopy (ICS) analysis assumes point-like probe particles whose sizes are much smaller than the beam focus. This assumption yields erroneous results when the particle size is larger than a certain threshold. Here, a formalism is presented to study image correlation spectroscopy for particles of arbitrary geometries, sizes, and fluorophore distributions. We demonstrate the usefulness of this method by analyzing simulated image sequences of diffusing fluorescent point sources, disks, and randomly oriented rigid rods of various sizes. In addition, we also perform ICS analysis on confocal images of fluorescent microspheres of different diameters diffusing in a medium of known viscosity to experimentally validate the method. The new method, which we call template analysis, yields excellent agreement with theoretical predictions, thus extending the capability of ICS for studying dynamic processes in a probe-independent manner.

Monte Carlo simulation of fluorescence correlation spectroscopy data

We employed the Monte Carlo simulation methodology to emulate the diffusion of fluorescently labeled particles and understand the source of differences between values of diffusion coefficients (and consequently hydrodynamic radii) of fluorescently labeled nanoparticles measured by fluorescence correlation spectroscopy (FCS) and dynamic light scattering (DLS). We used the simulation program developed in our laboratory and studied the diffusion of spherical particles of different sizes, which are labeled on their surface. In this study, we focused on two complicating effects: (i) multiple labeling and (ii) rotational diffusion which affect the fluorescence signal from large particles and hinder the analysis of autocorrelation functions according to simple analytical models. We have shown that the fluorescence fluctuations can be well fitted using the analytical model for small point-like particles, but the obtained parameters deviate in some cases significantly from the real ones. It means that the current data treatment yields apparent values of diffusion coefficients and other parameters only and the interpretation of experimental results for systems of particles with sizes comparable to the size of the active illuminated volume requires great care and precaution.

Characterization of brightness and stoichiometry of bright particles by flow-fluorescence fluctuation spectroscopy

Characterization of bright particles at low concentrations by fluorescence fluctuation spectroscopy (FFS) is challenging, because the event rate of particle detection is low and fluorescence background contributes significantly to the measured signal. It is straightforward to increase the event rate by flow, but the high background continues to be problematic for fluorescence correlation spectroscopy. Here, we characterize the use of photon-counting histogram analysis in the presence of flow. We demonstrate that a photon-counting histogram efficiently separates the particle signal from the background and faithfully determines the brightness and concentration of particles independent of flow speed, as long as undersampling is avoided. Brightness provides a measure of the number of fluorescently labeled proteins within a complex and has been used to determine stoichiometry of protein complexes in vivo and in vitro. We apply flow-FFS to determine the stoichiometry of the group specific antigen protein within viral-like particles of the human immunodeficiency virus type-1 from the brightness. Our results demonstrate that flow-FFS is a sensitive method for the characterization of complex macromolecular particles at low concentrations.

Biophysical analysis of HTLV-1 particles reveals novel insights into particle morphology and Gag stochiometry

BACKGROUND

Human T-lymphotropic virus type 1 (HTLV-1) is an important human retrovirus that is a cause of adult T-cell leukemia/lymphoma. While an important human pathogen, the details regarding virus replication cycle, including the nature of HTLV-1 particles, remain largely unknown due to the difficulties in propagating the virus in tissue culture. In this study, we created a codon-optimized HTLV-1 Gag fused to an EYFP reporter as a model system to quantitatively analyze HTLV-1 particles released from producer cells.

RESULTS

The codon-optimized Gag led to a dramatic and highly robust level of Gag expression as well as virus-like particle (VLP) production. The robust level of particle production overcomes previous technical difficulties with authentic particles and allowed for detailed analysis of particle architecture using two novel methodologies. We quantitatively measured the diameter and morphology of HTLV-1 VLPs in their native, hydrated state using cryo-transmission electron microscopy (cryo-TEM). Furthermore, we were able to determine HTLV-1 Gag stoichiometry as well as particle size with the novel biophysical technique of fluorescence fluctuation spectroscopy (FFS). The average HTLV-1 particle diameter determined by cryo-TEM and FFS was 71 ± 20 nm and 75 ± 4 nm, respectively. These values are significantly smaller than previous estimates made of HTLV-1 particles by negative staining TEM. Furthermore, cryo-TEM reveals that the majority of HTLV-1 VLPs lacks an ordered structure of the Gag lattice, suggesting that the HTLV-1 Gag shell is very likely to be organized differently compared to that observed with HIV-1 Gag in immature particles. This conclusion is supported by our observation that the average copy number of HTLV-1 Gag per particle is estimated to be 510 based on FFS, which is significantly lower than that found for HIV-1 immature virions.

CONCLUSIONS

In summary, our studies represent the first quantitative biophysical analysis of HTLV-1-like particles and reveal novel insights into particle morphology and Gag stochiometry.

Inverse-fluorescence correlation spectroscopy

An alternative version of fluorescence correlation spectroscopy is presented, where the signal from a medium surrounding the particles of interest is analyzed, as opposed to a signal from the particles themselves. This allows for analysis of unlabeled particles and potentially of biomolecules. Here, the concept together with principal experiments on polystyrene beads of 100, 200, 400, and 800 nm diameter in an aqueous solution of alexa 488-fluorophores are presented. The use of photo detectors allowing higher photon fluxes, or of reduced detection volumes, should enable analysis of significantly smaller particles or even biomolecules.

Microrheology with fluorescence correlation spectroscopy

We show that fluorescence correlation spectroscopy (FCS) using a commercial spectrometer can be applied to passive microrheological (MR) experiments. The method probes the local rheological properties of materials on length scales of the focus dimension of the confocal microscope. For a feasibility study, we performed measurements on a high molecular weight poly(ethylene oxide)-water solution to allow direct comparison of the results to previous studies using diffusing wave spectroscopy, quasielastic light scattering, and particle tracking methods. We were able to detect mean-square center-of-mass displacements ranging from somewhat better than [SYMBOL: SEE TEXT]2(t) approximately 100 nm2 up to above Deltar2(t) approximately 10(6) nm2. Thus, we were able to derive the bulk rheological shear moduli covering more than five decades in frequency (from omega<or=10(-1) rad/s to omega approximately 10(4) rad/s). The MR results are compared to results obtained from conventional rheological experiments on the same samples using a rotational rheometer as well as a piezo-driven squeeze flow apparatus. Good agreement between MR results probing the local rheological properties and those obtained by the conventional methods measuring the macroscopic mechanical response is found in the whole frequency range. Spatial resolution in combination with the possibility of using small tracer beads open the opportunity to probe the local, length scale-dependent rheological properties in heterogeneous samples. Small tracer concentrations and small sample sizes make FCS spectroscopy a powerful tool in particular for biological and medical applications.

Fluorescence fluctuation spectroscopy on viral-like particles reveals variable gag stoichiometry

Fluorescence fluctuation spectroscopy determines the brightness, size, and concentration of fluorescent particles from the intensity bursts generated by individual particles passing through a small observation volume. Brightness provides a measure of the number of fluorescently labeled proteins within a complex and has been used previously to determine the stoichiometry of small oligomers in cells. We extend brightness analysis to large macromolecular protein complexes containing thousands of proteins and determine their stoichiometry. This study investigates viral-like particles (VLP) formed from human immunodeficiency virus type 1 (HIV-1) Gag protein expressed in COS-1 cells using fluorescence fluctuation spectroscopy to determine the stoichiometry of HIV-1 Gag within the particles. Control experiments establish that the stoichiometry and size of VLPs are not influenced by labeling of HIV-1 Gag with a fluorescent protein. The experiments further show that the brightness scales linearly with the amount of labeled Gag within the particle. Brightness analysis shows that the Gag stoichiometry of VLPs formed in COS-1 cells is not constant, but varies with the amount of transfected DNA plasmid. We observed HIV-1 Gag stoichiometries ranging from approximately 750 to approximately 2500, whereas the size of the VLPs remains unchanged. This result indicates that large areas of the VLP membrane are void of Gag protein. Therefore, a closed layer of HIV-1 Gag at the membrane is not required for VLP production. This study shows that brightness analysis has the potential to become an important tool for investigating large molecular complexes by providing quantitative information about their size and composition.