Kenith E. Meissner

Quantum dot-embedded microspheres for remote refractive index sensing

We present a refractometric sensor based on quantum dot-embedded polystyrene microspheres. Optical resonances within a microsphere, known as whispering-gallery modes (WGMs), produce narrow spectral peaks. For sensing applications, spectral shifts of these peaks are sensitive to changes in the local refractive index. In this work, two-photon excited luminescence from the quantum dots couples into several WGMs within the microresonator. By optimizing the detection area, the spectral visibility of the WGMs is improved. The spectral shifts are measured as the surrounding index of the refraction changes. The experimental sensitivity is about five times greater than that predicted by the Mie theory.

Whispering Gallery Mode Biosensors Consisting of Quantum Dot-Embedded Microspheres

New methods of biological analyte sensing are needed for development of miniature biosensors that are highly sensitive and require minimal sample preparation. One technique employs optical resonances, known as whispering gallery modes (WGMs), in spherical or cylindrical microstructures. The spectral positions of these resonant modes are very sensitive to the local refractive index and spectral shifts may be used to sense changes in the index. To excite these WGMs and enable remote excitation, quantum dots are embedded in polystyrene microspheres to serve as local light sources. Using a simple continuous wave excitation optical system, these sensors are demonstrated by monitoring the wavelength shift of multiple resonant modes as bulk index of refraction is changed in ethanol–water mixtures. The potential for targeted biosensing is explored through addition of a protein that adsorbs to the microsphere surface, thrombin, and one that does not, bovine serum albumin (BSA). The thrombin produced a spectral shift that was much larger than that due to the bulk index change. The BSA produced a significantly smaller shift that was slightly larger than the expected shift due to bulk index change. Most likely due to the thin, high index layer of quantum dots, microsensor response in all cases demonstrated increased sensitivity over theoretical predictions.

Mitigation of Quantum Dot Cytotoxicity by Microencapsulation

When CdSe/ZnS-polyethyleneimine (PEI) quantum dots (QDs) are microencapsulated in polymeric microcapsules, human fibroblasts are protected from acute cytotoxic effects. Differences in cellular morphology, uptake, and viability were assessed after treatment with either microencapsulated or unencapsulated dots. Specifically, QDs contained in microcapsules terminated with polyethylene glycol (PEG) mitigate contact with and uptake by cells, thus providing a tool to retain particle luminescence for applications such as extracellular sensing and imaging. The microcapsule serves as the “first line of defense” for containing the QDs. This enables the individual QD coating to be designed primarily to enhance the function of the biosensor.

Subcellular measurements of mechanical and chemical properties using dual Raman‐Brillouin microspectroscopy

Brillouin microspectroscopy is a powerful technique for noninvasive optical imaging. In particular, Brillouin microspectroscopy uniquely allows assessing a samples mechanical properties with microscopic spatial resolution. Recent advances in background-free Brillouin microspectroscopy make it possible to image scattering samples without substantial degradation of the data quality. However, measurements at the cellular- and subcellular-level have never been performed to date due to the limited signal strength. In this report, by adopting our recently optimized VIPA-based Brillouin spectrometer, we probed the microscopic viscoelasticity of individual red blood cells. These measurements were supplemented by chemically specific measurements using Raman microspectroscopy.

On the Design of Composite Protein–Quantum Dot Biomaterials via Self-Assembly

Incorporation of nanoparticles during the hierarchical self-assembly of protein-based materials can impart function to the resulting composite materials. Herein we demonstrate that the structure and nanoparticle distribution of composite fibers are sensitive to the method of nanoparticle addition and the physicochemical properties of both the nanoparticle and the protein. Our model system consists of a recombinant enhanced green fluorescent protein-Ultrabithorax (EGFP-Ubx) fusion protein and luminescent CdSe-ZnS core-shell quantum dots (QDs), allowing us to optically assess the distribution of both the protein and nanoparticle components within the composite material. Although QDs favorably interact with EGFP-Ubx monomers, the relatively rough surface morphology of composite fibers suggests EGFP-Ubx-QD conjugates impact self-assembly. Indeed, QDs templated onto EGFP-Ubx film post-self-assembly can be subsequently drawn into smooth composite fibers. Additionally, the QD surface charge impacts QD distribution within the composite material, indicating that surface charge plays an important role in self-assembly. QDs with either positively or negatively charged coatings significantly enhance fiber extensibility. Conversely, QDs coated with hydrophobic moieties and suspended in toluene produce composite fibers with a heterogeneous distribution of QDs and severely altered fiber morphology, indicating that toluene severely disrupts Ubx self-assembly. Understanding factors that impact the protein-nanoparticle interaction enables manipulation of the structure and mechanical properties of composite materials. Since proteins interact with nanoparticle surface coatings, these results should be applicable to other types of nanoparticles with similar chemical groups on the surface.

Fabrication and Characterization of Silk-Fibroin-Coated Quantum Dots

We report a novel technique of directly coating colloidal CdSe/ZnS core/shell quantum dots (QDs) with silk fibroin (SF), a protein derived from the Bombyx mori silk worm. The approach results in protein-modified QDs with little or no particle aggregation, and mitigates the issue of biocompatibility. QDs have desirable optical properties, such as narrow-band emission, broadband absorption, high quantum yield, and high resistance to photobleaching. SF is a fibrous protein polymer with a biomimetic peptide sequence, water and oxygen permeability, low inflammatory response, no thrombogenecity, and cellular biocompatibility, which are desirable properties for in vivo delivery. Combining the unique properties of QDs with the biocompatibility profile of SF, the approach produces particles representing a powerful tool for numerous in vivo and in vitro applications. The design and preparation of these protein-modified QDs conjugates is reported along with functional characterization using luminescence, transmission electron microscope (TEM), and atomic force microscope (AFM). Additionally, we report results obtained using the QDs conjugates as a fluorescent label for bioimaging HEYA8 ovarian cancer cells.

Analysis of the Influence of Cell Heterogeneity on Nanoparticle Dose Response

Understanding the effect of variability in the interaction of individual cells with nanoparticles on the overall response of the cell population to a nanoagent is a fundamental challenge in bionanotechnology. Here, we show that the technique of time-resolved, high-throughput microscopy can be used in this endeavor. Mass measurement with single-cell resolution provides statistically robust assessments of cell heterogeneity, while the addition of a temporal element allows assessment of separate processes leading to deconvolution of the effects of particle supply and biological response. We provide a specific demonstration of the approach, in vitro, through time-resolved measurement of fibroblast cell (HFF-1) death caused by exposure to cationic nanoparticles. The results show that heterogeneity in cell area is the major source of variability with area-dependent nanoparticle capture rates determining the time of cell death and hence the form of the exposure–response characteristic. Moreover, due to the particulate nature of the nanoparticle suspension, there is a reduction in the particle concentration over the course of the experiment, eventually causing saturation in the level of measured biological outcome. A generalized mathematical description of the system is proposed, based on a simple model of particle depletion from a finite supply reservoir. This captures the essential aspects of the nanoparticle–cell interaction dynamics and accurately predicts the population exposure–response curves from individual cell heterogeneity distributions.

Processing and Characterization of Stable, pH-Sensitive Layer-by-Layer Modified Colloidal Quantum Dots

Quantum Dots (QDs) stabilized with dihydrolipoic acid (DHLA) were used as a template for layer-by-layer (LbL) modification to study the effect on the QD optical properties. We studied several different polyelectrolytes to determine that large quantities of monodisperse DHLA-QDs could only be obtained with the weak polyelectrolyte pair of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). The key to this success was the development of a two-step method to split the LbL process into adsorption and centrifugation phases, which require different pH solutions for optimum success. Solution pH is highlighted as an important factor to achieve sufficient QD surface coverage and QD recovery during wash cycles. We optimized the process to scale up synthesis by introducing a solvent precipitation step before ultracentrifugation that, when coupled with the correct pH conditions, results in a mean QD recovery of 86-90% after three wash cycles. We found that adsorption of PAH had a negligible effect on the quantum yield and lifetime but an additional layer of PAA resulted in a substantial decrease in both quantum yield and lifetime that could not be recovered by the addition of more layers. The PAH coating provides a protective coating that extends DHLA-QDs stability, prevents photo-oxidation mediated aggregation, alleviates concerns over batch variability, and results in pH-dependent emission.

Iron-oxide nanoparticles as a contrast agent in thermoacoustic tomography

We investigate the feasibility of using iron oxide nanoparticles as a contrast agent for radiofrequency (RF) induced thermoacoustic tomography. Aqueous colloids of iron oxide (Fe3O4) nanoparticles have been synthesized and characterized. The synthesis method yielded citrate-stabilized, spherical particles with a diameter of approximately 10 nm. The complex permittivity of the colloids was measured with a coaxial probe and vector network analyzer, and the microwave absorption properties were calculated by using a relationship between the complex permittivity and absorption coefficients. Using our pulsed thermoacoustic imaging system at 3 GHz, the time-resolved thermoacoustic responses of those colloids were measured and compared to that of deionized water. Finally, two-dimensional thermoacoustic images were acquired from iron oxide colloids in a tissue phantom. The iron oxide colloids produced an enhancement in RF absorption of up to three times that of deionized water at 3 GHz. The enhancement increased rapidly with decreasing frequency of the RF excitation source. A corresponding increase in time-resolved thermoacoustic signal of more than two times was demonstrated. Our results indicate that iron oxide nanoparticles have the potential to produce enhanced thermoacoustic signals and to provide molecular imaging with functionalized contrast agents for thermoacoustic tomography.

Super-wetting, wafer-sized silicon nanowire surfaces with hierarchical roughness and low defects

This paper reports the fabrication of wafer-sized silicon nanowire (SiNW) surfaces using a modified metal-assisted chemical etching method. The complete fabrication and coating process can be performed in less than three hours, is easily size-scalable, and produces surfaces with very low surface defects, complex, hierarchical surface roughness, and large nanowire height. These surfaces exhibit extreme wettabilities depending on surface coating: oxidized SiNW surfaces are superhydrophilic, while surfaces coated with a fluorinated hydrocarbon are superhydrophobic. The wetting and morphological properties of SiNW surfaces made with one and two etches of different duration are characterized vis-a-vis their effect on water drop mobility. Compared to a single etch process, a double etch followed by coating with a fluorinated hydrocarbon more efficiently produces SiNW surfaces with high contact angles on which microliter-sized water drops roll-off at approximately 0° tilt angle. Due to their very low friction, extreme wetting properties, ease of fabrication, low-cost, and large-sizes, these SiNW surfaces may be advantageous in microfluidic devices, bioanalysis systems sensitive to cross-contamination that require disposable substrates, and other applications.