Detection of chemical and biological agents plays a fundamental role in biomedical, forensic and environmental sciences1–4 as well as in anti bioterrorism applications.5–7 The development of highly sensitive, cost effective, miniature sensors is therefore in high demand which requires advanced technology coupled with fundamental knowledge in chemistry, biology and material sciences.8–13

In general, sensors feature two functional components: a recognition element to provide selective/specific binding with the target analytes and a transducer component for signaling the binding event. An efficient sensor relies heavily on these two essential components for the recognition process in terms of response time, signal to noise (S/N) ratio, selectivity and limits of detection (LOD).14,15 Therefore, designing sensors with higher efficacy depends on the development of novel materials to improve both the recognition and transduction processes. Nanomaterials feature unique physicochemical properties that can be of great utility in creating new recognition and transduction processes for chemical and biological sensors15–27 as well as improving the S/N ratio by miniaturization of the sensor elements.28

Gold nanoparticles (AuNPs) possess distinct physical and chemical attributes that make them excellent scaffolds for the fabrication of novel chemical and biological sensors (Figure 1).29–36 First, AuNPs can be synthesized in a straightforward manner and can be made highly stable. Second, they possess unique optoelectronic properties. Third, they provide high surface-to-volume ratio with excellent biocompatibility using appropriate ligands.30 Fourth, these properties of AuNPs can be readily tuned varying their size, shape and the surrounding chemical environment. For example, the binding event between recognition element and the analyte can alter physicochemical properties of transducer AuNPs, such as plasmon resonance absorption, conductivity, redox behavior, etc. that in turn can generate a detectable response signal. Finally, AuNPs offer a suitable platform for multi-functionalization with a wide range of organic or biological ligands for the selective binding and detection of small molecules and biological targets.30–32,36 Each of these attributes of AuNPs has allowed researchers to develop novel sensing strategies with improved sensitivity, stability and selectivity. In the last decade of research, the advent of AuNP as a sensory element provided us a broad spectrum of innovative approaches for the detection of metal ions, small molecules, proteins, nucleic acids, malignant cells, etc. in a rapid and efficient manner.37



Figure 1

Physical properties of AuNPs and schematic illustration of an AuNP-based detection system.



In this current review, we have highlighted the several synthetic routes and properties of AuNPs that make them excellent probes for different sensing strategies. Furthermore, we will discuss various sensing strategies and major advances in the last two decades of research utilizing AuNPs in the detection of variety of target analytes including metal ions, organic molecules, proteins, nucleic acids, and microorganisms.

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Gold nanoparticles in chemical and biological sensing.

Electrospinning is a highly versatile method to process solutions or melts, mainly of polymers, into continuous fibers with diameters ranging from a few micrometers to a few nanometers. This technique is applicable to virtually every soluble or fusible polymer. The polymers can be chemically modified and can also be tailored with additives ranging from simple carbon-black particles to complex species such as enzymes, viruses, and bacteria. Electrospinning appears to be straightforward, but is a rather intricate process that depends on a multitude of molecular, process, and technical parameters. The method provides access to entirely new materials, which may have complex chemical structures. Electrospinning is not only a focus of intense academic investigation; the technique is already being applied in many technological areas.

Electrospinning: A Fascinating Method for the Preparation of Ultrathin Fibers

Electrospinning is a versatile and viable technique for generating ultrathin fibers. Remarkable progress has been made with regard to the development of electrospinning methods and engineering of electrospun nanofibers to suit or enable various applications. We aim to provide a comprehensive overview of electrospinning, including the principle, methods, materials, and applications. We begin with a brief introduction to the early history of electrospinning, followed by discussion of its principle and typical apparatus. We then discuss its renaissance over the past two decades as a powerful technology for the production of nanofibers with diversified compositions, structures, and properties. Afterward, we discuss the applications of electrospun nanofibers, including their use as "smart" mats, filtration membranes, catalytic supports, energy harvesting/conversion/storage components, and photonic and electronic devices, as well as biomedical scaffolds. We highlight the most relevant and recent advances related to the applications of electrospun nanofibers by focusing on the most representative examples. We also offer perspectives on the challenges, opportunities, and new directions for future development. At the end, we discuss approaches to the scale-up production of electrospun nanofibers and briefly discuss various types of commercial products based on electrospun nanofibers that have found widespread use in our everyday life.

Electrospinning and Electrospun Nanofibers: Methods, Materials, and Applications

Nanotechnology is providing new solutions and opportunities to ensure sustainable energy and environments for the future. Materials of nanofiberous morphology are attractive to solve numerous energy and environmental issues. Nanofibers can be effectively produced by electrospinning, which is a simple and low cost technique. In addition, electrospinning allows the production of nanofibers from various materials e.g. organics and inorganics in different configurations and assemblies. This is highly beneficial for energy devices, where inorganic materials especially metal oxides can be synthesized and electrospun, improving conducting and ceramic properties. Excitonic solar cells fabricated with aligned nanofiberous metal oxide electrodes provide higher solar–electric energy conversion efficiency, whereas fuel cells made with nanofiberous electrodes enable uniform dispersion of catalysts, and thus increase electrocatalytic activity to obtain higher chemical–electric energy conversion efficiency. The nanofibers used in filtration membranes for environmental remediation, minimize the pressure drop and provide better efficiency than conventional fiber mats. The large surface area-to-volume ratio of nanofiber membranes allows greater surface adsorption of contaminants from air and water, and increases the life-time of the filtration media. This review highlights the potential and application of electrospun nanofiberous materials for solving critical energy and environmental issues.

Electrospun nanofibers in energy and environmental applications

One-dimensional conducting polymer nanocomposites: Synthesis, properties and applications

Summary: Core-sheath nanofibers with conductive polyaniline as the core and an insulating polymer as the sheath were prepared by electrospinning of blends of polyaniline with either polystyrene or polycarbonate. These unique core-sheath structures offer potential in a number of applications including nanoelectronics. When polyaniline was blended with poly(methyl methacrylate) and poly(ethylene oxide), only isolated domains of polyaniline in beadlike structures were formed. The phase morphology of electrospun fibers is thought to be dependent on the high-surface tension of the solution and the molecular weight of the polymers. Incompatibility of the polymers and low molecular weight of compositions played a key role in the formation of core-sheath structures, as opposed to co-continuous morphologies.

TEM image of electrospun polyaniline/polystyrene nanofiber after staining by OsO4. The dark regions are polyaniline.

Preparation of Core‐Sheath Nanofibers from Conducting Polymer Blends

Electrospinning of polymer blends offers the potential to prepare functional nanofibers for use in a variety of applications. This work focused on control of the internal morphology of nanofibers prepared by electrospinning polymer blends to obtain core-sheath structures. Polybutadiene/ polystyrene, poly(methylmethacrylate)/polystyrene, polybutadiene/ poly(methylmethacrylate), polybutadiene/polycarbonate, polyaniline/polycarbonate, and poly(methylmethacrylate)/ polycarbonate blends were electrospun from polymer solutions. It was found that the formation of core-sheath structures depends on both thermodynamic and kinetic factors. Incompatibility and large solubility parameter difference of the two polymers is helpful for good phase separation, but not sufficient for the formation of core-sheath structures. Kinetic factors, however, play a much more important role in the development of the nanofiber morphology. During the electrospinning process, the rapid solvent evaporation requires systems with high molecular mobility for the formation of core-sheath structures. It was found that polymer blends with lower molecular weight tend to form core-sheath structures rather than co-continuous structures, as a result of their higher molecular mobility. Rheological factors also affect the internal phase morphology of nanofibers. It was observed the composition with higher viscosity was always located at the center and the composition with lower viscosity located outside.

Core‐Sheath Structure in Electrospun Nanofibers from Polymer Blends

Gold nanoparticles protected with thiophene-terminated alkanethiols having different alkane chain lengths have been synthesized, and vapor-sensing properties of their spin-coated films have been investigated. Transmission electron microscopy and measurement of the sulfur and gold peak areas of the films by X-ray photoelectron spectroscopy indicate gold core diameters in the 3−5-nm range. Exposure of the films to chloroform, toluene, hexane, and ethanol vapors results in significant and selective increases in electrical resistance, with the response to the vapors having the following order:  toluene > chloroform > hexane ≫ ethanol. The magnitude of the maximum resistance change correlates well with solubility properties of the protected gold nanoparticles, as determined by optical absorbance spectroscopy and the energy of the gold plasmon. The detection sensitivity of the films increases with increasing alkanethiol chain length. These data are consistent with a sensing mechanism in which organic vapors cau...

Electrical conductivity and vapor-sensing properties of ω-(3-thienyl)alkanethiol-protected gold nanoparticle films

Ordered multilayer nanocomposites have been prepared by electrostatic layer-by-layer assembly between exfoliated aluminosilicate nanoplatelets and substituted ionic polyacetylenes. Aluminosilicate saponite particles were exfoliated by means of extensive shaking and sonication of their water suspension. Atomic force microscopy and transmission electron microscopy data suggest that the exfoliated platelets have the shape of ultrathin disks with an average width of 170 ± 30 nm. Conjugated ionic polyacetylene poly(N-octadodecyl-2-ethynylpyridinium bromide) (PEPy-C18) was synthesized by spontaneous polymerization of 2-ethynylpyridine quaternized with 1-bromooctadecane. Linear build-up of multilayer nanocomposites between the exfoliated saponite particles and substituted ionic polyacetylenes was monitored by UV−vis absorption spectroscopy. AFM surface topography of the resulting films indicates that the saponite platelets cover the entire surface of the underlying layers. TEM cross-sectional images of such film...

Ordered Multilayer Nanocomposites Prepared by Electrostatic Layer-by-Layer Assembly between Aluminosilicate Nanoplatelets and Substituted Ionic Polyacetylenes

Highly ordered arrays of periodic nanostructures show interesting characteristics for applications in photonics, electronics, optoelectronics, sensing, biochips, and catalysis. The array properties are determined by the choice of material and can be tuned further by varying the geometry, chemical composition, periodicity, and the size of the nanostructures. Such controlled and elaborate arrays are commonly fabricated by X-ray or electron-beam lithography. However, there are limitations to patterning over large areas using these lithographical techniques due to long processing times, which result in higher costs. Relatively simple and costeffective nanopatterning processes have been developed to address these inherent limitations, including interference lithography, nanosphere lithography, and soft lithography. In order to pattern metals or metal oxides, however, the non-conventional methods still require multistep processing as does conventional photoresist lithography. In this paper, we demonstrate a novel but simple methodology for the fabrication of periodic structures comprising metal oxides at the sub-100 nm scale using polymeric templates, colloids, and common laboratory equipment. Thus, this nanopatterning technique does not require high-vacuum deposition, etching, or photoresists, which are common in other processes. The patterns produced with this method were periodic and composed of 1D or 2D arrays of nanostructures over large areas. Fabrication of these metal oxide nanostructures required only a few hours once the nanometer-sized colloid of the metal oxide and polymeric templates were available. The structural resolution of the resulting pattern was determined by the amount of colloidal deposition onto the grooves (or grating) of the templates used. Although this process utilized microstructured templates, it is possible to fabricate features of metal oxide at the 100 nm scale. The geometry and periodicity of the structure can be well defined and controlled through selection from a wide variety of polymeric templates fabricated by numerous techniques. This unique strategy to achieve controlled nanopatterning can utilize a wide variety of inorganic colloids and/or intricate templates (or masks) available from various established lithographic methods. Microstructured polymeric templates have been prepared using a single-step holographic patterning process by exposing films of azobenzene-functionalized polymers (AFPs) on substrates to an argon-ion laser (488 nm) interference pattern. The interference pattern produced photoinduced mass transfer in the polymer film giving rise to a surface-relief structure without additional processing. Such surface modulation leads to 1D or 2D periodic arrays of surface-relief-grating (SRG) structures on the polymeric films. A variety of holographic surface structures can be designed and fabricated with good control over the modulation and periodicity of the patterns. In this work, SRG structures from thin films of AFP on quartz and glass substrates were prepared by exposing the films to a 488 nm laser interference pattern. Titania nanoparticles, with a mean diameter of less than 10 nm, were then deposited selectively on the surface of the SRG using sequential electrostatic layer-by-layer deposition (ELbL) with the polyelectrolyte, poly(4-styrene sulfonate) (SPS). The nanolayered assemblies of SPS and titania (SPS/titania) were then heated to a suitable temperature in air to aggregate the colloids further along the grooves of the SRG. At higher temperatures, the colloidal particles sinter together to form 1D or 2D arrays of metal oxide and the polymeric template eventually degrades. The resulting line features were composed of titania and extended over several tens of micrometers or longer with line widths of 100 nm. This quick and cost-effective process took only a few hours to pattern more than one square centimeter using ordinary laboratory equipment under ambient conditions. Scheme 1 illustrates the stepwise process used to prepare these titanium dioxide nanopatterns on the surface of a glass substrate. Figure 1A and B show 3D views of the SRG patterns of the AFP films that were typically used in this approach. The AFP used in this work was a copolymer of bisphenol A diglycidyl ether and disperse orange 3 (PDO3). Both structures were used as the templates for the ELbL layering of SPS/titania but C O M M U N IC A TI O N S

Bongwoo Kang

Papers

Preparation of Core‐Sheath Nanofibers from Conducting Polymer Blends

Core‐Sheath Structure in Electrospun Nanofibers from Polymer Blends

Electrical conductivity and vapor-sensing properties of ω-(3-thienyl)alkanethiol-protected gold nanoparticle films

Ordered Multilayer Nanocomposites Prepared by Electrostatic Layer-by-Layer Assembly between Aluminosilicate Nanoplatelets and Substituted Ionic Polyacetylenes

Facile Patterning of Periodic Arrays of Metal Oxides