Multimodal optical studies of single and clustered colloidal quantum dots for the long-term optical property evaluation of quantum dot-based molecular imaging phantoms.

Understanding the optical properties of clustered quantum dots (QDs) is essential to the design of QD-based optical phantoms for molecular imaging. Single and clustered core/shell colloidal QDs of dimers, trimers, and tetramers are self-assembled, separated, and preferentially collected using electrospray differential mobility analysis (ES-DMA) with electrostatic deposition. Multimodal optical characterization and analysis of their dynamical photoluminescence (PL) properties enables the long-term evaluation of the physicochemical and optical properties of QDs in a single or a clustered state. A multimodal time-correlated spectroscopic confocal microscope capable of simultaneously measuring the time evolution of PL intensity fluctuation, PL lifetime, and emission spectra reveals the long-term dynamic optical properties of interacting QDs in individual dimeric clusters of QDs. This new method will benefit research into the quantitative interpretation of fluorescence intensity and lifetime results in QD-based molecular imaging techniques. The process of photooxidation leads to coupling of the QDs in a dimer, leading to unique optical properties when compared to an isolated QD. These results guide the design and evaluation of QD-based phantom materials for the validation of the PL measurements for quantitative molecular imaging of biological samples labeled with QD probes.

Carbon nanotubes activate store-operated calcium entry in human blood platelets.

Carbon nanotubes (CNTs) are known to potentiate arterial thrombosis in animal models, which raises serious safety issues concerning environmental or occupational exposure to CNTs and their use in various biomedical applications. We have shown previously that different CNTs, but not fullerene (nC60), induce the aggregation of human blood platelets. To date, however, a mechanism of potentially thrombogenic CNT-induced platelet activation has not been elucidated. Here we show that pristine multiwalled CNTs (MWCNTs) penetrate platelet plasma membrane without any discernible damage but interact with the dense tubular system (DTS) causing depletion of platelet intracellular Ca(2+) stores. This process is accompanied by the clustering of stromal interaction molecule 1 (STIM1) colocalized with Orai1, indicating the activation of store-operated Ca(2+) entry (SOCE). Our findings reveal the molecular mechanism of CNT-induced platelet activation which is critical in the evaluation of the biocompatibility of carbon nanomaterials with blood.

Characterization of non-equilibrium nanoparticle adsorption on a model biological substrate.

The kinetics of nanoparticle (NP) adsorption on a model biological interface (collagen) is measured in microfluidic channels using surface plasmon resonance (SPR) imaging over a range of CdSe/ZnS quantum dot concentrations to investigate the underlying binding process. Spherical CdSe/ZnS core-shell NP, derivatized with 3-mercaptopropionic acid (3-MPA), were considered to be model NPs because of their widespread use in biological applications and their relatively monodisperse size. The kinetic adsorption data suggests that the binding between the NP and the collagen substrate is irreversible at room temperature (pH approximately 7.4), and this type of adsorption process was further characterized in the context of a surface absorption model. Specifically, diffusion-limited adsorption was found to predominate the adsorption process at lower concentrations (<0.4 micromol/L), and NP adsorption was reaction-limited at higher concentration (>0.4 micromol/L). A limited pH study of our system indicates that NPs desorb from collagen under acidic conditions (pH 5.5); no significant desorption was observed under neutral and basic pH conditions. These observations are consistent with electrostatic interactions being the dominant force governing NP desorption from collagen substrates. Our present methodology for characterizing the seemingly irreversible NP adsorption complements our earlier study where NP adsorption onto weakly adsorbing surfaces (self-assembled monolayers) was characterized by Langmuir NP adsorption measurements.

Interaction of gold nanoparticles with common human blood proteins.

In order to better understand the physical basis of the biological activity of nanoparticles (NPs) in nanomedicine applications and under conditions of environmental exposure, we performed an array of photophysical measurements to quantify the interaction of model gold NPs having a wide range of NP diameters with common blood proteins. In particular, absorbance, fluorescence quenching, circular dichroism, dynamic light scattering, and electron microscopy measurements were performed on surface-functionalized water-soluble gold NPs having a diameter range from 5 to 100 nm in the presence of common human blood proteins: albumin, fibrinogen, gamma-globulin, histone, and insulin. We find that the gold NPs strongly associate with these essential blood proteins where the binding constant, K, as well as the degree of cooperativity of particle--protein binding (Hill constant, n), depends on particle size and the native protein structure. We also find tentative evidence that the model proteins undergo conformational change upon association with the NPs and that the thickness of the adsorbed protein layer (bare NP diameter <50 nm) progressively increases with NP size, effects that have potential general importance for understanding NP aggregation in biological media and the interaction of NP with biological materials broadly.

Target patterns induced by fixed nanoparticles in block copolymer films.

It is well-known that thin films of cylinder-forming block copolymers (BCP) can exhibit a transition from a perpendicular to a parallel cylinder orientation with respect to the supporting solid substrate upon varying film thickness. We show that wave-like oscillations between these morphologies can be induced through the introduction of nanoparticles (NP) into flow-coated and annealed BCP films where the particles span the film thickness and are fixed by irreversible adsorption to the supporting substrate. We hypothesize that these novel "target" patterns arise from residual stresses that build up in the film while undergoing thermal treatment and film formation, and we support this hypothesis by showing the suppression of this type of pattern formation in films that are first thermally annealed near their glass transition T(g) to relax residual stress. Similar undulating height patterns are also observed in heated homopolymer films with nanoparticles, consistent with our thermally induced stress hypothesis of the target pattern formation in BCP films and pointing to the general nature of wave-like thermally induced height variations in heated heterogeneous polymer films. Similar wave patterns should be induced by lithographically etched substrate patterns arising in device fabrication using BCP materials, which makes the phenomena of technological interest. These target patterns also potentially provide valuable information about the presence of residual stresses in cast films that arise during their processing.

Langmuir adsorption study of the interaction of CdSe/ZnS quantum dots with model substrates: influence of substrate surface chemistry and pH.

We investigate the utility of Langmuir adsorption measurements for characterizing nanoparticle-substrate interactions. Spherical CdSe/ZnS core-shell nanoparticles were chosen as representative particles because of their widespread use in biological labeling measurements and their relatively monodisperse dimensions. In particular, the quantum dots were functionalized with 11-mercaptoundecanoic acid, and we utilized an amine-terminated self-assembled monolayer (SAM) as a model substrate. SAMs with different end-groups (-CH(3) and -COOH) were also considered to contrast with the adsorption behavior on the amine-terminated SAM substrates. We followed the kinetics of nanoparticle adsorption on the aminosilane layer by quartz crystal microgravimetry (QCM) over a range of particle concentrations and determined the corresponding Langmuir adsorption isotherms. Analysis of both equilibrium adsorption and kinetic adsorption data allowed us to determine a consistent value of the Langmuir adsorption equilibrium constant for the amine-terminated SAM at room temperature (K(L) approximately 2.7 (micromol/L)(-1)), providing a useful characterization of the nanoparticle-substrate interaction. The effect of varying solution pH on Langmuir adsorption was also investigated in order to gain insight into the role of electrostatic interactions on nanoparticle adsorption. The equilibrium extent of adsorption was found to be maximum at about pH 7. These changes of nanoparticle adsorption were further quantified and validated by X-ray photoelectron spectroscopy (XPS) and confocal fluorescence microscopy measurements. We conclude that Langmuir adsorption measurements provide a promising approach for quantifying nanoparticle-substrate interactions.

Length-dependent optical effects in single-wall carbon nanotubes.

Among the novel chemical and physical attributes of single-wall carbon nanotubes (SWCNTs), the optical properties are perhaps the most compelling. Although much is known about how such characteristics depend on nanotube chirality and diameter, relatively little is known about how the optical response depends on length, the next most obvious and fundamental nanotube trait. We show here that the intrinsic optical response of single-wall carbon nanotubes exhibits a strong dependence on nanotube length, and we offer a simple explanation that relates this behavior to the localization of a bound exciton along the length of a nanotube. The results presented here suggest that, for a given volume fraction, the longest nanotubes display significantly enhanced absorption, near-infrared fluorescence, and Raman scattering, which has important practical implications for potential applications that seek to exploit the unique optical characteristics of SWCNTs.