key: cord-0825408-oxoi9t1m
authors: Ding, Bin; Wang, Moran; Wang, Xianfeng; Yu, Jianyong; Sun, Gang
title: Electrospun nanomaterials for ultrasensitive sensors
date: 2010-11-30
journal: Materials Today
DOI: 10.1016/s1369-7021(10)70200-5
sha: 6b3852137382755e2ca2ea0d3fe23ba8d0738b92
doc_id: 825408
cord_uid: oxoi9t1m

Increasing demands for ever more sensitive sensors for global environmental monitoring, food inspection and medical diagnostics have led to an upsurge of interests in nanostructured materials such as nanofibers and nanowebs. Electrospinning exhibits the unique ability to produce diverse forms of fibrous assemblies. The remarkable specific surface area and high porosity bring electrospun nanomaterials highly attractive to ultrasensitive sensors and increasing importance in other nanotechnological applications. In this review, we summarize recent progress in developments of the electrospun nanomaterials with applications in some predominant sensing approaches such as acoustic wave, resistive, photoelectric, optical, amperometric, and so on, illustrate with examples how they work, and discuss their intrinsic fundamentals and optimization designs. We are expecting the review to pave the way for developing more sensitive and selective nanosensors.

In a typical electrospinning process, the surface of a hemispherical liquid drop suspended in equilibrium at the tip of a capillary will be distorted into a conical shape in the presence of an external electric field 8 . When the electric field is sufficiently strong, charges on the droplet surface will overcome the surface tension to induce the formation of a liquid jet that is subsequently accelerated toward a grounded collector. For low viscosity liquids, the jet breaks into droplets because of the longitudinal Rayleigh instability which is known as electrospraying, and used to obtain aerosols composed of sub-micrometer droplets with a narrow distribution 3 . For high viscosity liquids, such as polymer solutions or melts, the liquid travels in the form of a jet to the grounded target instead of breaking up. The transverse instability or spraying of the jet into two or more smaller jets is observed due to the radial charge repulsion 9 . As the solvent is evaporating, this liquid jet is stretched to many times its original length to produce continuous, ultrathin fibers of the polymer. This process is termed 'electrospinning', and it produces polymer fibers with diameters of sub-micrometer scale 10 . These porous three-dimensional (3D) membranes assembled by electrospun fibers are featured with large specific surface area, high porosity, and good interconnectivity, which makes electrospun nanomaterials highly attractive to different applications ranging from filtration, sensors, drug delivery platforms, tissue engineering, and so on 5,6 .

Electrospun nanofibers are featured with very small diameters, extremely long length, large surface area per unit mass and small pore size. Various materials such as polymers, ceramics and carbon can be used to electrospin uniform fibers with well-controlled sizes, compositions, and morphologies 11 . Different nanofiber morphologies can be obtained via control of the processing conditions to produce beaded as well as smooth and ribbon structures (Fig. 1a-c) . Recent studies have demonstrated that this technique can also produce 14 . © 2010 American Chemical Society.); (g) multi-core cable-like (Reprinted with permission from 15 . © 2007 IOP Publishing Ltd.) and (h,i) porous 16 fibers.

nanofibers with core-sheath or hollow 12 , multichannel tubular 13 , nanowire-in-microtube 14 , multi-core cable-like 15 and porous [16] [17] [18] structures ( Fig. 1d- 

The desires for ever higher specific surface area and porosity have pushed people to improve the electrospinning technique. An ideal fiber diameter is below 20 nm for optimal performances 20 , however an electrospun fiber from the traditional process typically have a diameter in the range 100-500 nm. The objective is a robust method for manufacturing extremely small nanofibers in large quantities and with a uniform size. Recently, the novel two-dimensional (2D) nanowebs were generated in 3D fibrous mats by optimizing processing parameters in electrospinning 21 . The electrospun fibers act as a support for the 'fishnet-like' nanowebs comprising interlinked 1D nanowires ( Fig. 2a-e) . The nanowire diameter distribution of nylon-6 nanowebs is shown in Fig. 2f Table 1 Types of electrospun nanomaterials based sensors.

Abbreviations in Table 1 Table 1 .

Recently, there has been a growing attention toward developing surface acoustic wave (SAW) devices for gas sensing applications 25 Table 1 continued.

Abbreviations in Table 1 

As (Fig. 5a) . The as-prepared humidity sensor based on the LiCl-doped TiO 2 nanofibers exhibited greatly improved sensitivity compared to the pure TiO 2 nanofibers. Additionally, the composite nanofibers also exhibited an ultra-fast response and recovery behavior (Fig. 5b) . Besides high sensitivity, the electrospun GaN nanofiber UV sensor also showed advantages in response speed and reversibility as seen in 

Optical fiber sensors have many advantages than the traditional types of sensors such as the absences of electromagnetic interference in sensing and electric contacts in the probe, and multiplexity on a single 64 have demonstrated the feasibility of PDA-embedded electrospun microfibers as potential sensor materials by detecting the fluorescence generation upon specific ligand-receptor interactions (Fig. 7b-d) . Incubation of the porous PMMA microfibers, containing embedded polymerized PCDA, in an α-cyclodextrin (α-CD) solution (10 mM) resulted in the generation of red fluorescence (Fig. 7d) , however, much weaker fluorescence signals arose from fibers treated with βand γ-CD. 

The past several decades have witnessed the big progress on fabricating sensitive devices for fast and reliable monitoring glucose and carbohydrates driven by their practical applications in treating and controlling diabetes 66 . Various techniques such as chemiluminescence 67 , chromatography 68 and electrochemistry 69 were applied in glucose determination, in which, electrochemical methods, especially amperometry, have been proved to be a powerful approach and attracted much attention. Up to now, most amperometric sensors based on electrospun nanomaterials were used to detect glucose.

Among those devices, amperometric glucose biosensors with glucose oxidase (GO x ) and without GO x have been an intensively research area because a low detection limit can be achieved easily. diameter. Therefore, a smaller diameter leads to a faster response due to more rapid diffusion of gas molecules through the nanowire.

With different sensing mechanisms, the influence of membrane thickness on sensor sensitivity might be different. With regard to mass There is no doubt that electrospinning has become one of the most powerful techniques for preparing diverse nanostructured materials and highly sensitive sensors in the future. The combination of these diverse areas of research promises to yield revolutionary advances in environmental monitoring, food inspection and medical diagnostics through the creation of novel and powerful tools that enable direct, sensitive, and rapid analysis of chemical species. 

Process and Apparatus for Preparing Artificial Threads, US Patent 1,975

Nanotechnology Research: New Nanostructures, Nanotubes and Nanofibers