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Electrospray is a phenomenon that results from the application of an electric field to fluid contained in a small capillary. The driving electrostatic force incites the emission of charged droplets that cycle through phases of evaporation and coulombic explosion, ideally resulting in the formation of gas-phase ions or a very fine liquid aerosol. Though this technique has found widespread use in the area of mass spectrometryW, it has also been documented to function in a wide range of other applications such as industrial painting, particle deposition, and gene therapyW.
This array of modern uses, however, belies the fact that the basic science behind electrospray is anything but new. Indeed, it can trace its origins all the way back to Lord Rayleigh's article, "On the equilibrium of liquid conducting masses charged with electricity" published in 1882. A little over 30 years thereafter, John Zeleny became the first man to witness an electrospray event, and subsequently published his observations in "The electrical discharge from liquid points, and a hydrostatic method of measuring the electric intensity at their surfaces." Since then, continuing research by Taylor, Fenn, Dole, and a number of other researchers have continued to push forward science's understanding and range of applications for electrospray.,
How it Works
As a process, the literature segregates the electrospray event into a series of 3 unique phases:
- Onset and Emission
- Droplet Fission
- Gas-Phase Ion Generation
Below, each of these steps will be discussed individually and the governing roles that various mechanical and electrochemical factors play will be described. An overview of the apparatus can be found in Figure 1.
Onset and Emission
At rest, no activity is witnessed in an electrospray system due to the lack of a sufficiently strong electric field to drive the vaporization of solvent at the emitter tip. The threshold electric field at which emission begins has been characterized by the relationship:
Einitial ≈ √((2γcosθ0) / (ε0rc))
γ = the surface tensionW of the solvent θ0; = the cone half angle (see Figure 2) ε0 = the vacuum permitivityW of the solvent rc = the radius of the emitter orifice
Initiation of electrospray via formation of a Taylor coneW is achieved by applying a voltageW to liquid housed in a capillary (see Figure 1). The magnitude of the voltage needed, Vonset, is dependent upon the following relationship:
Vonset ∝ 1/√(rc/γ)
γ = the surface tension of the solvent rc = the radius of the emitter orifice
By varying this applied voltage, the electric field at the emitter, EES, can be manipulated and eventually increased to levels that create the Taylor cone. EES can be calculated via the following equation:
EES = (2VES) / (rcln(4d/rc))
VES = the applied voltage rc = the radius of the emitter orifice d = the distance between the emitter orifice and the counter electrode
The resulting plume of charged airborne droplets are accelerated towards the counter electrode due to the electric field, and subsequently undergo a series of droplet fission events.
Once airborne, the liquid droplets' structural integrity becomes dependent upon the struggle of surface tension with the electrostatic repulsion that results from the solvated ions. Up to a point, known as the Rayleigh limit, surface tension will hold the repulsive forces in check and prevent droplet fragmentation. Due to evaporation, however, continuous shrinkage in droplet size gradually brings the charges closer together, increasing repulsion proportionally. Eventually, the Rayleigh limit is overcome and the droplet undergoes Coulombic explosion, splitting into progeny droplets in which the process is reset (see Figure 3). The amount of charge, qR, at which the Rayleigh limit is exceeded and fission occurs has been described by the mathematical relationship:
qR = 8π√(ε0γr3)
ε0 = the vacuum permitivity of the solvent γ = the surface tension of the solvent r = the radius of the droplet
This is only a general guideline, however, as a number of labs have reported Rayleigh discharge (a.k.a. particle fission) at 70% to 120% of this value.
Gas-Phase Ion Generation
Two models have been put forward to explain how ions eventually enter the gas phase. The first, known as the Ion Evaporation Model (IEM), postulates that the electrostatic repulsion present in very small (tens of nm in diameter) droplets is strong enough to actually force the ions to desorb from the surface. The charge residue model (CRM), on the other hand, simply states that the cycle of evaporation and coulombic explosion continues until it terminally results in the generation of gas-phase ions. It has not yet been determined which one predominates, and it is indeed possible that both may viable models under different circumstances (droplet size, charge density, etc.), or even intermingle.
In IEM, the potential for ion evaporation from the solvent surface can be determined by examining the related change in Gibbs free energyW. If the overall value is negative and the activation energyW barrier is overcome, then the reaction can spontaneously occur. A model by Iribarne and Thomson, as well as one by Born, both seek to mathematically explain this event, however the former fails to take into account a number of factors and the latter has been shown experimentally to severely underestimate free energies.
The CRM view that progressive fragmentation due to fission and solvent evaporation leads to the generation of gas-phase ions has its own difficulties, however. Once droplets reach a size of only a few nm, the Rayleigh equation breaks down due to the loss of (assumed) equal charge distribution and shifts in vapor pressureW due to the Kelvin equationW. Instead, the remaining ions are thought to be caught in solvent molecule bulges from which the ions, not the solvent, eventually evaporate via IEM.
Making Electrospray a Reality
Though the central design and components used to construct an electrospray apparatus are essentially the same, customization is often neccessary depending upon the application. Below, the process of constructing a basic direct currentW ES device will be described, with notes detailing modifications that may be necessary for various uses.
Material and Tool Requirements
The absolute bare-bone requirements for setting up an electrospray apparatus are:
- A power supplyW capable of producing at least 1 kV of potential difference (in most cases, current output is a secondary concern). The exact values needed will vary widely depending upon application. It is possible that a high voltage multiplierW may be used as a cheaper alternative, though this remains to be substantiated.
- An emitter, typically a steel capillary with a diameter on the order of 0.1 mm or less (remember, the smaller the diameter, the lower the required voltage to create the spray). It should be noted that some groups have began employing silica-based capillaries, which introduce their own subset of advantages and disadvantages, while others have used silica monoliths inside the emitter to create novel effects.
- A ground (electricity)W electrode to be placed in opposition to the emitter. This can be as simple as a metal plate in contact with the earth.
- Any chemicals / reagents needed for the desired use.
In addition, some applications may require other components, such as:
- A finely controllable solvent pump for feeding the emitter.
- A high voltage output transformer, RF amplifier, and waveform generator (if pursuing alternating currentW, a newer method, as opposed to the conventional use of direct current)., 
- A closed chamber with a nebulizerW feeding nitrogen may be required to prevent corona dischargeW.
- Characterization equipment, such as voltmetersW, ammetersW, telescopic equipment, and cameras.
With respect to tools, nothing special is required for the basic design herein described. In some cases, more specialized devices, such as micropipette pullers, may be needed, depending upon the type of project being explored., 
The actual assembly of an electrospray system is simple, and the plans are relatively straightforward (see Figure 4). Depending upon your application, a few things may vary. A few examples include:
- A reservoir of solvent in conjunction with a finely controllable solvent pump may have to be used to supplement the emitter.
- Distance from the emitter tip to the target / ground will vary by application type (i.e. in mass spectrometry it may only be a few mm, while for gene therapy it's typically closer to 2 cm).
- The need for a closed chamber environment and a nitrogen nebulizer.
The literature itself is actually vague as to the details of optimally assembling an electrospray apparatus and ensuring that it works, so if anyone has personal experience in this area your contributions would be greatly appreciated.
Once the apparatus is properly assembled, generally a few steps are all that is needed to initiate operation:
- Load solvent into emitter / reservoir.
- Assure target material is properly setup.
- Double check 1 & 2, and make sure the instrument is isolated from any potentially conductive materials.
- Activate power source.
After the electrospray has begun, the reaction can be monitored until completion, at which time the power source should be deactivated. It is critically important that close proximity or contact with the device be avoided during operation, since voltages reached during the procedure can be potentially lethal, or cause serious personal harm.
In maintaining the device, special care must be taken in keeping the emitter tip clear of physical stress to prevent deformation. Additionally, all components should be stored in a moisture free environment to minimize corrosion of metallic surfaces.
Working and Innovating with Electrospray
Electrospray is a powerful and adaptable tool that has found application in a variety of industries and as an instrument in research. The following are just a few areas in which it has found employ:
- Mass spectrometry, ,  - As mentioned before, electrospray has been employed in mass spectrometry to aid in improving resolution of the analyte by shrinking sample sizes. The mechanics have been relatively well characterized, if still debated, however it is difficult to innovate in this area, since it requires a mass spectrometer, which is commonly commercially available already coupled to a specially suited electrospray device.
- Fabrication of nanoparticles - The small volume achieved during the evaporation and fission phase of electrospray has been utilized as a controlled reaction vessel, allowing for the fabrication of inorganic nanoparticles of known size. This is particularly important in the area of quantum dotsW, where size affects the emissive wavelength perceived. Aside from this, however, the use of electrospray in nanofabrication remains a relatively new and untouched field with a lot of potential.
- Fabrication of microarraysW,  - Microarrays are a key tool in biological diagnostics and represent a fast, highly multiplexable method for characterizing and potentially isolating unknown materials. Utilizing electrospray, it has been posited that the process can be robotized to rapidly produce such chips in a manner that still allows for use in dot immuno-binding and DNA hybridizationW assays, and increases the information density available for any given unit of surface.
- Gene therapy - One of the newest applications to emerge for electrospray is in the field of biotechnology and genetic engineering. Researchers in Japan have succeeded in utilizing an electrospray apparatus to incite a transformation event in eukaryotic and bacterial cells, demonstrating a method that is less cytotoxic, easily reusable, and portable when compared to current techniques. Applications in this area remain wide open, and could easily offer opportunities for commercialization in a variety of settings.
- Pharmaceutical production - One of the key areas of pharmaceutical development lies in getting therapeutic materials to the location in which they are needed without being degraded or affecting other patient tissues along the way. In this respect, micro-encapsulationW provides a possible way of circumventing this problem. Scientists have shown that electrospray can be used to do just this, allowing drugs to be sequestered inside "shells" of other materials that can give them novel properties within the body.
- Biodegradable fiber scaffolds - Electrospray has been implicated as a putative method for synthesizing biodegradable microfibers, which can be employed in a variety of medical procedures. Initial research has shown their utility as scaffolds for tissue engineeringW, and as mask-like coatings for biomedical implants for the prevention of an inflammatory response.
- Next generation "lab-on-a-chipW" systems - As many aspects of science continue to be miniaturized, the potential for fabricating "lab-on-a-chip" devices that can perform a variety of wet reactions rapidly in a small space continues to improve. Due to electrospray's high applicability in these areas, research is currently attempting to shrink the process to a size capable of being incorporated as a feature on chip-labs of the future.
- Thin film depositionW - Perhaps one of the most expansive areas of electrospray application lies in the deposition of thin films onto substrate surfaces. The small, charged particles emitted during electrospray have been found to have a high coating efficiency, and can theoretically be used in a wide range of industrial areas to give materials novel surface properties.
Electrospray and Sustainability
The two mains areas in which electrospray stands to benefits sustainable development is in its potential to fabricate novel nanoparticles and thin films that can fulfill a variety of roles.
For nanoparticles, many sustainable applications have been proposed that could integrate the use of an electrospray device for fabrication purposes. A sampling of these would include:
- Cleaning groundwater with nanoparticles.
- Improving second (mostly through thin film techniques) and third generation solar cellsW.
- Particle-functionalized materials (e.g. silver nanoparticles in clothing to have antibacterial effect).
For thin films, a number of other options also exist that can help to promote the achievement of sustainability goals:
- Self-cleaning surfaces.
- Carbon sequestering thin films.
- As mentioned above, in second generation solar cells.
It is also worth noting that newer advances in the area of genetic engineering via electrospray may facilitate more rapid development of crop/livestock strains that demonstrate superior performance. This could, in turn, cut down on the amount of land needed for agricultural operations, allow agriculture to flourish in harsher climates (decreasing food transportation costs), and possibly obviate the need for more risky forms of pest control (e.g. pesticides and herbicides).
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