Engineering Efficient Photon Upconversion in Semiconductor Heterostructures.

Photon upconversion is a photophysical process in which two low-energy photons are converted into one high-energy photon. Photon upconversion has broad appeal for a range of applications from biomedical imaging and targeted drug release to solar energy harvesting. Current upconversion nanosystems, including lanthanide-doped nanocrystals and triplet-triplet annihilation molecules, have achieved upconversion quantum yields on the order of 10-30%. However, the performance of these materials is hampered by inherently narrow absorption cross sections and fixed energy levels originating in atomic, ionic, or molecular states. Semiconductors, on the other hand, have inherently wide absorption cross sections. Moreover, recent advances enable the synthesis of colloidal semiconductor nanoparticles with complex heterostructures that can control band alignments and tune optical properties. We synthesize and characterize a three-component heterostructure that successfully upconverts photons under continuous-wave illumination and solar-relevant photon fluxes. The heterostructure is composed of two cadmium selenide quantum dots (QDs), an absorber and emitter, spatially separated by a cadmium sulfide nanorod (NR). We demonstrate that the principles of semiconductor heterostructure engineering can be applied to engineer improved upconversion efficiency. We first eliminate electron trap states near the surface of the absorbing QD and then tailor the band gap of the NR such that charge carriers are funneled to the emitting QD. When combined, these two changes result in a 100-fold improvement in photon upconversion performance.

Unique Surface Enhanced Raman Scattering Substrate for the Study of Arsenic Speciation and Detection.

In this study, a three-dimensional surface enhanced Raman scattering (SERS) substrate comprised of silver coated gold nanorods (Ag/AuNRs) decorated on electrospun polycaprolactone (PCL) fibers has been applied,  for the first time, to quantitative analytical measurements on various arsenic species: p-arsanilic acid ( pAsA), roxarsone (Rox), and arsenate (AsV), with a demonstrated sensitivity below 5 ppb. AsV detection in a solution of common salt ions has been demonstrated, showing the tolerance of the substrate to more complex environments. pAsA adsorption behavior on the substrate surface has been investigated in detail using these unique SERS substrates. Calculations based on density functional theory (DFT) support the spectral observation for pAsA. This substrate also has been shown to serve as a platform for in situ studies of arsenic desorption and reduction. This SERS substrate is potentially an excellent environmental sensor for both fundamental studies and practical applications.

Two-Dimensional Raman Correlation Spectroscopy Study of Poly[(R)-3-hydroxybutyrate- co-(R)-3-hydroxyhexanoate] Copolymers.

Two-dimensional correlation analysis was applied to the time-dependent evolution of Raman spectra during the isothermal crystallization of bioplastic, poly[(R)-3-hydroxybutyrate- co-(R)-3-hydroxyhexanoate] or PHBHx copolymer. Simultaneous Raman measurement of both carbonyl stretching and low-frequency crystalline lattice mode regions made it possible to carry out the highly informative hetero-mode correlation analysis. The crystallization process of PHBHx involves: (1) the early nucleation stage; (2) the primary growth of well-ordered crystals of PHBHx; and (3) the secondary crystal growth phase. The latter stage probably occurs in the inter-lamellar region, with an accompanying reduction of the amorphous component, which occurs most dominantly during the primary crystal growth. The development of a fully formed lamellar structure comprising the 21 helices occurs after the primary growth of crystals. In the later stage, secondary inter lamellar space crystallization occurs after the full formation of packed helices comprising the lamellae.

Observation of Intermolecular Hydrogen Bonding Interactions in Biosynthesized and Biodegradable Poly [(R)-3-hydroxybutyrate- co-(R)-3-hydroxyhexanoate] in Chloroform and 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP).

In this work, we describe polymer-solvent interactions in biosynthesized and biodegradable poly[(R)-3-hydroxybutyrate- co-(R)-3-hydroxyhexanoate] (PHBHx) and the atactic homopolymer, poly(3-hydroxybutyrate) (a-PHB), which were studied both as neat polymers and in solutions of chloroform and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). Specifically, infrared frequency shifts of the carbonyl band were observed in semi-crystalline PHBHx, but not in a-PHB, because it cannot form the helical conformation required for crystallization. The carbonyl band of PHBHx exhibited the high frequency associated with amorphous structure in chloroform and the lower frequency traditionally attributed to the helical crystalline structure in HFIP. The same results were obtained for a-PHB, demonstrating that the helical structure is not required for a lower frequency carbonyl-stretching mode. It is proposed that the band shift is due to hydrogen bonding between the carbonyl and hydroxyl hydrogen in HFIP. Therefore, the carbonyl frequency observed upon crystallization is most likely due to hydrogen bonding between the carbonyl and methyl hydrogen of the neighboring polymer chain in the crystal lattice as previously suggested.

Selective and Quantitative Detection of Trace Amounts of Mercury(II) Ion (Hg²âº) and Copper(II) Ion (Cu²âº) Using Surface-Enhanced Raman Scattering (SERS).

We report the development of a surface-enhanced Raman scattering (SERS)-based heavy metal ion sensor targeting the detection of mercury(II) ion (Hg(2+)) and copper(II) ion (Cu(2+)) with high sensitivity and selectivity. To achieve the detection of vibrational-spectroscopically silent heavy metal ions, the SERS substrate composed of gold nanorod (AuNR)-polycaprolactone (PCL) nanocomposite fibers was first functionalized using metal ion-binding ligands. Specifically, 2,5-dimercapto-1,3,4-thiadiazole dimer (di-DMT) and trimercaptotriazine (TMT) were attached to the SERS substrates serving as bridging molecules to capture Hg(2+) and Cu(2+), respectively, from solution. Upon heavy metal ion coordination, changes in the vibrational spectra of the bridging molecules, including variations in the peak-intensity ratios and peak shifts were observed and taken as indicators of the capture of the target ions. With rigorous spectral analysis, the coordination mechanism between the heavy metal ion and the corresponding bridging molecule was investigated. Mercury(II) ion primarily interacts with di-DMT through the cleavage of the disulfide bond, whereas Cu(2+) preferentially interacts with the heterocyclic N atoms in TMT. The specificity of the coordination chemistry provided both di-DMT and TMT with excellent selectivity for the detection of Hg(2+) and Cu(2+) in the presence of other interfering metal ion species. In addition, quantitative analysis of the concentration of the heavy metal ions was achieved through the construction of internal calibration curves using the peak-intensity ratios of 287/387 cm(-1) for Hg(2+) and 1234/973 cm(-1) for Cu(2+).

Immobilization of gold nanorods onto electrospun polycaprolactone fibers via polyelectrolyte decoration--a 3D SERS substrate.

We report the fabrication of a homogeneous and highly dense gold nanorod (AuNR) assembly on electrospun polycaprolactone (PCL) fibers using electrostatic interaction as the driving force. Specifically, decoration of a poly(sodium 4-styrenesulfonate) (PSS) layer onto the AuNRs imposed negative charges on the nanorod surface, and the interactions between PSS and the AuNRs were investigated using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). Positive charges on the PCL fibrous substrate were established via polyelectrolyte layer-by-layer deposition, which was investigated using multiple characterization techniques. Driven by the attractive electrostatic interaction, immobilization of AuNRs on the PCL fibers was initiated upon substrate immersion, and the kinetics of the immobilization process were studied using UV-vis spectroscopy. Electron microscopy characterization of the AuNR/PCL nanocomposite fibers reveals a uniform AuNR coating on the fiber surface with the immobilized AuNR density being high enough to provide full surface coverage. By using both 4-mercaptopyridine and Rhodamine 6G as probe molecules, the performance of the AuNR/PCL fibrous mesh as a three-dimensional (3D) surface-enhanced Raman scattering (SERS) substrate was investigated. The nanocomposite fibers allowed detection at concentrations as low as 10(-7) M of the probe molecule in solution and exhibited excellent reproducibility in the SERS measurements. In addition, a comparison between the 3D AuNR/PCL fibrous mesh and a 2D AuNR/PCL film reveals that the enhanced surface area in the 3D substrate effectively improved the SERS performance with a 6-fold increase in the Raman intensity.

Preparation of Multilayer Biodegradable Nanofibers by Triaxial Electrospinning.

A biodegradable, multilayer nanofiber structure has been successfully developed by using a self-designed and fabricated triaxial electrospinning system using gelatin as the sheath and core layers and poly(ε-caprolactone) (PCL) as the middle layer. The triaxial structure was investigated by confocal fluorescence microscopy (CFM) and focused ion beam and field-emission scanning electron microscopy (FIB-FESEM). The ability to fabricate the multilayered nanofibers efficiently with different biodegradable polymers will enable this triaxial electrospinning technique to have wider applications in biotechnology.

AFM-IR: combining atomic force microscopy and infrared spectroscopy for nanoscale chemical characterization.

Polymer and life science applications of a technique that combines atomic force microscopy (AFM) and infrared (IR) spectroscopy to obtain nanoscale IR spectra and images are reviewed. The AFM-IR spectra generated from this technique contain the same information with respect to molecular structure as conventional IR spectroscopy measurements, allowing significant leverage of existing expertise in IR spectroscopy. The AFM-IR technique can be used to acquire IR absorption spectra and absorption images with spatial resolution on the 50 to 100 nm scale, versus the scale of many micrometers or more for conventional IR spectroscopy. In the life sciences, experiments have demonstrated the capacity to perform chemical spectroscopy at the sub-cellular level. Specifically, the AFM-IR technique provides a label-free method for mapping IR-absorbing species in biological materials. On the polymer side, AFM-IR was used to map the IR absorption properties of polymer blends, multilayer films, thin films for active devices such as organic photovoltaics, microdomains in a semicrystalline polyhydroxyalkanoate copolymer, as well as model pharmaceutical blend systems. The ability to obtain spatially resolved IR spectra as well as high-resolution chemical images collected at specific IR wavenumbers was demonstrated. Complementary measurements mapping variations in sample stiffness were also obtained by tracking changes in the cantilever contact resonance frequency. Finally, it was shown that by taking advantage of the ability to arbitrarily control the polarization direction of the IR excitation laser, it is possible to obtain important information regarding molecular orientation in electrospun nanofibers.

Molecular Orientation in Electrospun Poly(vinylidene fluoride) Fibers.

Electrospun poly(vinylidene fluoride) (PVDF) nanofibers were collected on aluminum foil across a 10 mm gap. Scanning electron microscopy (SEM) images showed that fibers in the gap were macroscopically aligned and those off the gap were macroscopically randomly aligned. Polarized Fourier transform infrared (FTIR) spectra and single fiber selected area electron diffraction (SAED) patterns demonstrated that fibers deposited in the gap were highly aligned at the molecular level with the polymer backbones oriented along the fiber axis. SAED patterns of fibers deposited off the gap were also oriented at the molecular level, but the degree of orientation was lower.

Molecular orientation evolution and solvent evaporation during electrospinning of atactic polystyrene using real-time Raman spectroscopy.

Real-time Raman spectroscopy was successfully utilized to monitor solvent evaporation and molecular orientation during electrospinning of atactic polystyrene (a-PS). The use of a binary solvent system of N,N-dimethyl formamide (DMF) and tetrahydrofuran (THF) provided a stable, straight jet during the experiment. The prominent Raman bands centered at 866 cm(-1), 914 cm(-1), and 1004 cm(-1), unique to DMF, THF, and a-PS, respectively, were used to monitor solvent concentration changes along the electrospinning jet for two different a-PS solutions. In addition, the changes in relative intensity for the radial skeletal ring vibration of the aromatic group of a-PS at 623 cm(-1) in two different polarization geometries, ZZ and YY, were monitored for orientation information. This study reports the first quantitative vibrational spectroscopic measurement during the electrospinning process. A significant change in concentration and orientation was observed during the process. The changes are explained in relation to the electrospinning process.

Fiber diameters control osteoblastic cell migration and differentiation in electrospun gelatin.

Defined electrospinning conditions were used to create scaffolds with different fiber diameters to investigate their interactions with osteoblastic MG63 cells. Nonwoven gelatin scaffolds were electrospun with varied fiber diameters to investigate the effect of fiber size and resultant porosity on cell proliferation, viability, migration, and differentiation. The low toxicity solvent acetic acid:ethyl acetate:water ratio and gelatin concentrations were optimized to create small and large diameter fibers. The fiber diameters obtained by this procedure were 110 +/- 40 nm for the small and 600 +/- 110 nm for the large fibers. Cell viability assays showed that MG63 cells grew similarly on both fibers at the early time point (day 3) but preferred the scaffold with large diameter fibers by the later time points (day 5 and day 7). Confocal microscopic imaging showed that MG63 cells migrated poorly (maximum depth of 18 microm) into the scaffold of small diameter fibers, but readily penetrated (maximum depth of 50 microm) into the scaffold of large diameter fibers. Alkaline phosphatase (ALP) assays showed that MG63 cells differentiated on scaffolds made from both diameter fibers. In longer term experiments, MG63 cells differentiated to a greater extent on scaffolds made from small diameter fibers compared to large diameter fibers at days 3 and 7, but the ALP levels were the same for both diameter fibers by day 14. These results indicate that cells can perceive differences in the diameter and resultant pore size of electrospun gelatin fibers and that they process this information to alter their behavior.

Evaluation of cross-linking methods for electrospun gelatin on cell growth and viability.

The creation of a tissue engineering scaffold via electrospinning that has minimal toxicity and uses a solvent system composed of solvents with low toxicity and different cross-linking agents was investigated. First, a solvent system of acetic acid/ethyl acetate/water (50:30:20) with gelatin as a solute was evaluated. The optimum system for electrospinning a scaffold with the desired properties resulted from a gelatin concentration of 10 wt %. Several different methods were used to cross-link the electrospun gelatin fibers, including vapor-phase glutaraldehyde, aqueous phase genipin, and glyceraldehyde, as well as reactive oxygen species from a plasma cleaner. Because glutaraldehyde at high concentrations has been shown to be toxic, we explored other cross-linking methods. Using reactive oxygen species from a plasma cleaner is an easy alternative; however, the degradation reaction dominated the cross-linking reaction and the scaffolds degraded after only a few hours in aqueous medium at 37 °C. Glyceraldehyde and genipin were established as good options for cross-linking agents because of the low toxicity of these cross-linkers and the resistance to dissolution of the cross-linked fibers in cell culture medium at 37 °C. MG63 osteoblastic cells were grown on each of the cross-linked scaffolds. A proliferation assay showed that the cells proliferated as well or better on the cross-linked scaffolds than on traditional two-dimensional polystyrene culture plates.

Parallel spectroscopic method for examining dynamic phenomena on the millisecond time scale.

An infrared spectroscopic technique based on planar array infrared (PAIR) spectroscopy has been developed that allows the acquisition of spectra from multiple samples simultaneously. Using this technique, it is possible to acquire spectra over a spectral range of 950-1900 cm(-1) with a temporal resolution of 2.2 ms. The performance of this system was demonstrated by determining the shear-induced orientational response of several low molecular weight liquid crystals. Five different liquid crystals were examined in combination with five different alignment layers, and both primary and secondary screens were demonstrated. Implementation of this high-throughput PAIR technique resulted in a reduction in acquisition time as compared to both step-scan and ultra-rapid-scanning FTIR spectroscopy.

Dynamic infrared study of polyphenylene sulfide using planar array infrared spectroscopy.

Planar array infrared (PA-IR) spectroscopy was used to study polyphenylene sulfide (PPS) at room temperature during the application of a sinusoidal elastic deformation. All of the intensity in the dynamic spectra was contained within the in-phase spectrum, which was expected since the measurements were carried out at room temperature, far below the glass transition temperature. The contributions of chain orientation, sample thinning, and stress-induced band shifts were separated in the dynamic spectra. It was found that the effects of chain orientation and sample thinning canceled each other out. Stress-induced band shifts far below the spectral resolution, on the order of 0.01 cm(-1), were quantified and used to calculate the stress optical coefficients and mode Gruneisen parameters for PPS.

Real-time imaging of crystallization in polylactide enantiomeric monolayers at the air-water interface.

A newly developed planar array infrared reflection-absorption spectrograph (PA-IRRAS) offers significant advantages over conventional approaches including fast acquisition speed, excellent compensation for water vapor, and an excellent capacity for large infrared accessories, e.g., a water trough. In this study, the origin of stereocomplexation in a polylactide enantiomeric monolayer at the air-water interface was investigated using PA-IRRAS. PA-IRRAS was used as a probe to follow the real-time conformational changes associated with intermolecular interactions of polymer chains during the compression of the monolayers. It was found that a mixture of poly(D-lactic acid) (PDLA) and poly(L-lactic acid) (PLLA) (D/L) formed a stereocomplex when the two-dimensional monolayer developed at the air-water interface before film compression, indicating that there is no direct correlation between film compression and stereocomplexation. PA-IRRAS spectra of the stereocomplex exhibited distinct band shifts in crystalline sensitive components, e.g., the vas(C-O-C, h) mode, as well as amorphous-dependent components, e.g., the vs(C-O-C) mode, when compared with the spectra of PLLA alone. On the other hand, time-resolved PA-IRRAS spectra, which were obtained as the films were being compressed, revealed that both monolayers of PLLA and mixed PLLA/PDLA stereocomplex were crystallized into a 10(3)-helix and a 3(1)-helix, respectively, with a distinct band shift in crystalline sensitive components only. Fourier self-deconvolution of the spectra demonstrated that the band shift in crystalline sensitive components is correlated with the intermolecular interaction of polymer chains.

Development of a planar array infrared reflection spectrograph for reflection-absorption spectroscopy of thin films at metal and water surfaces.

Planar array infrared (PA-IR) spectroscopy offers several advantages over Fourier transform infrared (FT-IR) methods, including ultrafast speed (<100 micros temporal resolution) and excellent sensitivity. However, obtaining spectra in the range of 1800 to 1000 cm(-1) of films at the air-water interface remains difficult due to the poor IR reflectivity of water, the extremely low concentration of the thin film on the water subphase, and the interference of water bands. In this study, we report a new planar array infrared reflection spectrograph (PA-IRRS), which has several advantages over conventional approaches. This instrument can record sample and reference spectra simultaneously with an instrumental setup that is the same as that of a single-beam instrument by splitting the incident infrared beam into two sections on a plane mirror (H) or a water trough. With this design, the instrument can accommodate large infrared accessories, such as a water trough, without a loss of infrared beam intensity. Water bands can be subtracted to obtain a high-quality spectrum for poly(L-lactic acid) Langmuir film on the water subphase with a resolution of about 6 cm(-1) in 10.8 s. Hence, this PA-IRRS system has great potential for investigating the time-resolved dynamics of a broad range of Langmuir films, such as cellular membranes or biopolymers, on the water subphase.

Characterization of biodegradable polyurethane microfibers for tissue engineering.

A polyurethane designed to be biodegradable via hydrolysis and enzyme-mediated chain cleavage, has been investigated for its use as a temporary scaffold in tissue-engineering applications. The phase-segregated nature of the polyurethane imparts elastomeric properties that are attractive for soft tissue engineering. This polyurethane has been electrospun in order to create scaffolds that incorporate several biomimetic features including small fiber diameter, large void volume, and an interconnected porous network. Material properties were evaluated via gel-permeation chromatography, differential scanning calorimetry and Raman spectroscopy before and after processing. Analysis by gel-permeation chromatography showed that the molecular weights were similar, indicating that the bulk of the polymer chains were not degraded during processing. Thermal analysis revealed that the glass transition temperature did not shift and Raman spectra of the bulk polyurethane film compared to the electrospun mat were identical, confirming that the conformation of the polymer was unaffected by the shear and electric field used in the electrospinning process. In addition, field emission scanning electron microscopy revealed that the morphology of the electrospun mats had a broad fiber diameter distribution, and mechanical analysis showed that the mats had an ultimate tensile stress of 1.33 MPa and ultimate tensile strain of 78.6%. The degradation profile was investigated in the presence of chymotrypsin. These results were compared to a previous study of thin films of this polyurethane, and it was found that the increase of surface area aided the surface-mediated erosion of the material. It is believed that an electrospun matrix of this biodegradable polyurethane shows promise for use in soft tissue engineering and regenerative medicine applications.

Planar array infrared emission spectroscopy.

Planar array infrared spectroscopy (PA-IR) is shown to be a powerful new approach to infrared emission spectroscopy (IRES). A proof-of-concept study of selected polymers indicates that PA-IRES allows acquisition of emission spectra with a high signal-to-noise ratio at temperatures as low as 80 degrees C. It is shown that a time resolution below 20 ms is readily achievable for time-resolved characterization of single non-repeatable events. The possibility of recording spatially resolved emission spectra is also demonstrated, and potential applications of this novel technique are finally described.

Electric field induced orientation of polymer chains in macroscopically aligned electrospun polymer nanofibers.

The results presented in this work show for the first time that an electric field used to macroscopically align polymer nanofibers can also align polymer chains parallel to the fiber axis. This important result indicates that anisotropic structural properties (mechanical, electrical, etc.) can be induced in polymer nanofibers during the electrospinning process. Such uniaxially oriented nanofibers exhibit a variety of potential applications in biomedicine, microelectronics, and optics. A simple technique of vertical electrospinning with an electric field induced, stationary collection was employed to obtain the molecular orientation in polymer nanofibers. This manuscript describes the orientation process via electrospinning and verifies this molecular orientation in the polymer nanofibers using three independent methods: polarized Fourier transform infrared spectroscopy, polarized Raman scattering, and X-ray diffraction.