Whispering gallery mode emission from dye-doped polymer fiber cross-sections fabricated by near-field electrospinning.

Whispering gallery mode (WGM) resonators demonstrate great potential for photonic and sensing applications. Yet, these devices are often disadvantaged by costly materials or complex fabrication approaches, in addition to lack of manufacturing scalability. Near-field electrospinning (NFES), a recently emerged facile fiber fabrication method, offers a solution. Here, WGM resonances are reported in Rhodamine 6G-doped poly(vinyl) alcohol (PVA) microfibers via NFES. Diameters are tuned over a range of more than 10 µm by varying substrate stage speed. Fibers display uniform distribution of dye, smooth surfaces, and circular cross-sections, all critical for supporting WGMs. High quality (Q) resonances are confirmed within fiber cross-sections through polarization experiments, free-spectral range analysis, and Mie-theory-derived mode assignment. In addition to WGMs, groups of associated spiral or conical modes are observed due to taper-induced weak optical confinement along the fiber axis. Crosslinked, dye-doped PVA fibers are utilized to sense the ethanol concentration in ethanol-water mixtures and actuation mechanisms are evaluated by comparison to theoretical spectra. The demonstration of high-Q resonances within NFES polymer microfibers is a critical step toward simple, cost effective, high-volume fabrication of WGM resonators for optoelectronics and biomedical devices.

Gold-Decorated M13 I-Forms and S-Forms for Targeted Photothermal Lysis of Bacteria.

With the emergence of multidrug-resistant bacteria, photothermal therapy has been proposed as an alternative to antibiotics for targeting and killing pathogens. In this study, two M13 bacteriophage polymorphs were studied as nanoscaffolds for plasmonic bactericidal agents. Receptor-binding proteins found on the pIII minor coat protein targeted Escherichia coli bacteria with F-pili (F+ strain), while a gold-binding peptide motif displayed on the pVIII major coat protein templated Au nanoparticles. Temperature-dependent exposure to a chloroform-water interface transformed the native filamentous phage into either rod-like or spheroid structures. The morphology, geometry, and size of the polymorphs, as well as the receptor-binding protein and host cell receptor interaction were studied using electron microscopy. Au/template structures were formed through incubation with Au colloid, and optical absorbance was measured. Despite the closely packed Au nanoparticle layer on the surface the viral scaffolds, electron microscopy confirmed that host receptor affinity was retained. Photothermal bactericidal studies were performed using 532 nm laser irradiation with a variety of powers and exposure times. Bacterial viability was assessed using colony count. With the shape-modified M13 scaffolds, up to 64% of E. coli were killed within 20 min. These studies demonstrate the promise of i-form and s-form polymorphs for the directed plasmonic-based photothermal killing of bacteria.

M13 bacteriophage spheroids as scaffolds for directed synthesis of spiky gold nanostructures.

The spherical form (s-form) of a genetically-modified gold-binding M13 bacteriophage was investigated as a scaffold for gold synthesis. Repeated mixing of the phage with chloroform caused a 15-fold contraction from a nearly one micron long filament to an approximately 60 nm diameter spheroid. The geometry of the viral template and the helicity of its major coat protein were monitored throughout the transformation process using electron microscopy and circular dichroism spectroscopy, respectively. The transformed virus, which retained both its gold-binding and mineralization properties, was used to assemble gold colloid clusters and synthesize gold nanostructures. Spheroid-templated gold synthesis products differed in morphology from filament-templated ones. Spike-like structures protruded from the spherical template while isotropic particles developed on the filamentous template. Using inductively coupled plasma-mass spectroscopy (ICP-MS), gold ion adsorption was found to be comparatively high for the gold-binding M13 spheroid, and likely contributed to the dissimilar gold morphology. Template contraction was believed to modify the density, as well as the avidity of gold-binding peptides on the scaffold surface. The use of the s-form of the M13 bacteriophage significantly expands the templating capabilities of this viral platform and introduces the potential for further morphological control of a variety of inorganic material systems.

Viral-templated gold/polypyrrole nanopeapods for an ammonia gas sensor.

One-dimensional gold/polypyrrole (Au/PPy) nanopeapods were fabricated using a viral template: M13 bacteriophage. The genetically modified filamentous virus displayed gold-binding peptides along its length, allowing selective attachment of gold nanoparticles (Au NPs) under ambient conditions. A PPy shell was electropolymerized on the viral-templated Au NP chains forming nanopeapod structures. The PPy shell morphology and thickness were controlled through electrodeposition potential and time, resulting in an ultra-thin conductive polymer shell of 17.4 ± 3.3 nm. A post-electrodeposition acid treatment was used to modify the electrical properties of these hybrid materials. The electrical resistance of the nanopeapods was monitored at each assembly step. Chemiresistive ammonia (NH3) gas sensors were developed from networks of the hybrid Au/PPy nanostructures. Room temperature sensing performance was evaluated from 5 to 50 ppmv and a mixture of reversible and irreversible chemiresistive behavior was observed. A sensitivity of 0.30%/ppmv was found for NH3 concentrations of 10 ppmv or less, and a lowest detection limit (LDL) of 0.007 ppmv was calculated. Furthermore, acid-treated devices exhibited an enhanced sensitivity of 1.26%/ppmv within the same concentration range and a calculated LDL of 0.005 ppmv.

Highly sensitive hydrogen sulfide (H2S) gas sensors from viral-templated nanocrystalline gold nanowires.

A facile, site-specific viral-templated assembly method was used to fabricate sensitive hydrogen sulfide (H2S) gas sensors at room temperature. A gold-binding M13 bacteriophage served to organize gold nanoparticles into linear arrays which were used as seeds for subsequent nanowire formation through electroless deposition. Nanowire widths and densities within the sensors were modified by electroless deposition time and phage concentration, respectively, to tune device resistance. Chemiresistive H2S gas sensors with superior room temperature sensing performance were produced with sensitivity of 654%/ppm(v), theoretical lowest detection limit of 2 ppb(v), and 70% recovery within 9 min for 0.025 ppm(v). The role of the viral template and associated gold-binding peptide was elucidated by removing organics using a short O2 plasma treatment followed by an ethanol dip. The template and gold-binding peptide were crucial to electrical and sensor performance. Without surface organics, the resistance fell by several orders of magnitude, the sensitivity dropped by more than a factor of 100 to 6%/ppm(v), the lower limit of detection increased, and no recovery was detected with dry air flow. Viral templates provide a novel, alternative fabrication route for highly sensitive, nanostructured H2S gas sensors.

Phage-directed synthesis of copper sulfide: structural and optical characterization.

The growth of crystalline copper sulfide using a viral template was investigated using sequential incubation in CuCl2 and Na2S precursors. Non-specific electrostatic attraction between a genetically-modified M13 bacteriophage and copper cations in the CuCl2 precursor caused phage agglomeration and bundle formation. Following the addition of Na2S, polydisperse nanocrystals 2-7 nm in size were found along the length of the viral scaffold. The structure of the copper sulfide material was identified as cubic anti-fluorite type Cu1.8S, space group Fm3[overline]m. Strong interband absorption was observed within the ultraviolet to visible range with an onset near 800 nm. Furthermore, free carrier absorption, associated with the localized surface plasmon resonance of the copper sulfide nanocrystals, was seen in the near infrared with absorbance maxima at 1060 nm and 3000 nm, respectively.

Enhanced photogenerated carrier collection in hybrid films of bio-templated gold nanowires and nanocrystalline CdSe.

Hybrid films of bio-templated gold nanowires and chemical bath deposited nanocrystalline CdSe were fabricated. The conductivity of the gold nanowires within the hybrid material was controlled by gold electroless deposition. Photocurrent measurements were taken on gold nanowire films, CdSe chemical bath deposited films, and hybrid films. The incorporation of gold nanowires within the hybrid material clearly increased the extraction of photogenerated carriers within the CdSe. Photocurrent showed a direct correlation with gold nanowire conductivity.