ABSTRACT
This chapter presents a review of a recently developed technique of trapping nanoparticles in air using optical vortices. The technique is based upon thermal forces induced by uneven heating of light-absorbing particles with a laser beam. The laser vortices with a doughnut-shaped intensity profile provide a means to trap particles and push them along the zero-intensity beam axis to a desirable location. A brief overview will cover recent results of trapping, guiding, and pinpoint-positioning agglomerates of carbon nanoparticles in air, as well as simultaneous trapping of thousands of particles with a single laser beam. Optical vortices can be applied to touch-free transport of containers holding gases, ultrapure or dangerous substances, and biological macromolecules. Laser-trapping and agglomerating nanoparticles can be applied for
monitoring and removal of nanopollutants from air, offering a new avenue toward environmental protection in the workplace of the nanotechnology industry. 8.1 IntroductionNew generations of nanomaterials evolving with the dynamic developments in nanotechnology hold great promise for the creation of new products with enhanced properties and attributes. Nanomaterials characteristics such as greater catalytic efficiency or improved hardness and strength are highly desirable for applications in commercial, medical, and environmental sectors. The explosive growth of nanotechnology and the rapid expansion of nanoparticle-based products in many industrial sectors in the first decade of the new millennium have raised new, unresolved issues of occupational exposure and potential environmental risks unique to artificially produced engineered nanoparticles. The fast-developing field of nanotechnology presents us with serious new challenges in occupational health and safety in the workplace as the adverse effects of nanoparticles on human health are potentially significant and mostly unknown (Sahoo, Parveen, and Panda, 2007; Nel, Xia, Mädler, and Li, 2006; Singh and Nalwa, 2007; Faunce, Walters, Williams, Bryant, Jennings, and Musk, 2006; Maynard, Aitken, Butz et al., 2006; Shatkin, 2008). Until recently, the spectacular developments in nanotechnology have been with little regard to the potential effects of nanoparticles on human health and the environment. Some of the unique properties of nanoparticles may pose hazards to humans as they are able to pass through cells membranes or cross the blood-brain barrier (Maynard, Aitken, Butz et al., 2006; Shatkin, 2008) A serious challenge regarding environmental protection is to develop new methods for effective removal of nanoparticle contamination from air. There is a growing demand for techniques enabling efficient removal of nanocluster air contaminants, which cause serious health risks. Methods for effective capture and removal of nanoparticle contamination from air need to be developed both quickly and cost effectively. While optical trapping in liquids at the microscopic scale is well documented with various reviews (Dholakia and Reece, 2006;
Dholakia, Reece, and Gu, 2008), little attention has been paid to the trapping and manipulation of nanometric-size particles, which fall largely between the size region of cells and that of atomic ensembles. In particular the trapping of particles at the nanometer scale and direct manipulation of biological macromolecules have gained momentum. In this chapter, we present a brief summary of recent developments in laser trapping, guiding, and manipulation of agglomerated nanoparticles suspended in air with optical vortices. The new concept of touch-free transport and pinpoint positioning of objects in air opens up diverse and rich, practical opportunities for handling objects in dangerous or hard-to-reach areas. The concept can be applied to touch-free trapping and agglomeration of nanosize particles, viruses, and living cells. 8.2 Nanoparticles and Their Properties
8.2.1 What Makes Nanoparticles So Special?The term “nanoparticle” is usually applied to particles with dimensions between the atomic size and the submicron size of a bulk material, that is, between approximately 1 nm and 100 nm. The main reason for setting apart the particles in this size range is the unique properties and behavior of matter at the nanometer scale: while the properties of a bulk material are constant, physical properties of nanoparticles are size dependent. For example, gold nanoparticles below 100 nm change their color to red; their melting point reduces from the bulk value of 1064°C for particles below 10 nm and goes quickly down to ~300°C for particle sizes from 5 nm to ~2.5 nm (Buffat and Borel, 1976; Wang, Chen, Wang, Wang, Lu, and Zhao, 2002). Moreover, gold nanoparticles become chemically reactive and behave as a semiconductor at a particle size below approximately 5 nm (Pyykkö, 2007; Link and El-Sayed, 2003). Size-dependent photoluminescence of silicon nanoclusters has been observed experimentally (Link and El-Sayed, 2003). When the cluster size was decreased from 3.5 nm to 1 nm, the peak in the luminescence spectrum shifted from 750 nm to 300 nm (Marine, Patrone, Luk’yanchuk, and Sentis, 2000). Another example is magnetic properties of carbon: while graphite, diamond, and
fullerenes have diamagnetic properties, polymerized fullerenes (Makarova, Han, Esquinazi et al., 2003) and laser-deposited carbon nanofoams (Rode, Gamaly, Christy et al., 2004; Arčon, Jagličič, Zorko et al., 2006), which appear to contain schwarzite structures with hyperbolically curved graphitic-like sheets, demonstrate paramagnetic behavior. The underlying foundation for the size-dependent properties of nanoparticles is in quantum size effects, which are exhibited by atoms on the surface. While the number of surface atoms is reduced in proportion to R-2 with the decreasing radius R of a particle, the number of atoms in the bulk is reduced much faster, in proportion to ~R-3, so the relative number of surface atoms grows with the decreasing size. In the bulk, the interference of electronic wave functions results in a band structure. The surface atoms differ slightly from their bulk counterparts because they do not have their symmetry in interaction with neighboring atoms. Their electronic orbits are slightly energetically shifted; thus the dielectric function, which is a key factor dictating the material behavior, is different. When the size of the system decreases to the nanometer scale, the energy bands split into energy levels. The difference in energy between the electronic levels ΔE in a small cluster increases in inverse proportion to R as DE µ vF/R, where vF is the electron Fermi velocity (Gorkov and Eliashberg, 1965; Kreibig and Vollmer, 1995; Prasad, 2004; Gamaly and Rode, 2004). For this reason, surface atoms, and as a consequence the surface area density (m2/g) of industrial components, play a more and more significant role in determining the nanomaterial physical properties and chemical activity. 8.2.2 Health ImplicationsThe dependence of material characteristics on the particle size opens up many industrial benefits. By tuning a physical characteristic to a required value, nanotechnology promises a new dimension and a new level of complexity in the manufacture of the existing and developing novel materials with new features. It paves the way for a new technological revolution. The absence of complete information precludes, however, a full assessment of possible health effects, both short term and long term. Considerable uncertainty exists regarding
health risks from nanoscale materials, such as the adverse effects of nanoscale particles emitted as air pollutants (Faunce, Walters, Williams, Bryant, Jennings, and Musk, 2006; Maynard, Aitken, Butz et al., 2006; Shatkin, 2008). Human exposure to these nanomaterials is inevitable, as they can enter the body through inhalation, food consumption, or absorption through skin and affect different organs and tissues, such as the brain, liver, kidney, heart, colon, spleen, bone, and blood, with possible cytotoxic effects, for example, the deformation and inhibition of cell growth, leading to various diseases in humans (Sahoo, Parveen, and Panda, 2007; Nel, Xia, Mädler, and Li, 2006; Singh and Nalwa, 2007; Dreher, 2004). Nanomaterials that are most likely to present health risks are nanoparticles, agglomerates of nanoparticles, and particles of nanostructured materials in air. On the cellular level, an ability to act as a gene vector has been demonstrated for nanoparticles. Carbon black nanoparticles have been implicated in interfering with cell signaling. Researchers in Germany have observed nanomaterials pass the blood-brain barrier and concentrate in the cerebellum, responsible for controlling balance and voluntary movement (Tolstoshev, 2006). Given the rapid rate of development, it is not surprising that concerns have been raised relating to the safety of nanomaterials in a variety of products. German authorities recently recalled a bathroom-cleansing product MagicNano that was purported to contain nanosize particles. After it was on the market for only three days, more than 100 people suffered from severe respiratory problems, while 6 of them were hospitalized with pulmonary edema (Weiss, 2006). The exact cause of the respiratory effects had not been determined, and it was not clear whether the product actually contained nanoparticles or whether the reported effects were due to nanoparticles (ICON press release, 2006). Another example is TiO2nanoparticles used in some sun creams; they have the potential to cause neurological damage (Ball, 2006). It is too early to know the scale of how worrying these findings are. Sufficient scientific evidence exists to conclude that many nanomaterials are highly likely to be toxic to human health and the environment. Issues regarding safe handling of potentially toxic nanomaterials, including questions of whether personal protective equipment is effective for protection against nanoparticle
exposures, have not yet been solved. Below we present an overview of a new concept recently developed at the Australian National University in Canberra for capturing and handling of airborne nanoparticles, potentially dangerous for human health, using optical vortices (Shvedov, Desyatnikov, Rode, Krolikowski, and Kivshar, 2009; Desyatnikov, Shvedov, Rode, Krolikowski, and Kivshar, 2009; Shvedov, Rode, Izdebskaya, Desyatnikov, Krolikowski, and Kivshar, 2010a, 2010b; Shvedov, Rode, Izdebskaya et al., 2010). 8.3 Laser Trapping of Airborne Particles
The mechanical effect of light on matter was first noticed by Johannes Kepler about four centuries ago, when he was intrigued by his observation that the tail of a comet pointed away from the sun at all times, which he attributed to solar pressure. The fact that electromagnetic radiation exerts a pressure was first predicted by Maxwell and proven experimentally by Lebedev (1901) and Nichols and Hull (1901). The ability to apply forces through radiation pressure gave rise to the optical tweezers technique, a noncontact method to trap and manipulate particles with a laser beam tightly focused by a high-numerical aperture microscope objective. The strong-intensity gradients in the converging beam cause polarization of particles and draw these particles toward the focus, while the radiation pressure pushes them along the optical axis. If the gradient force dominates, the particles can be trapped near the focal point. Optical tweezers led to a large number of unique opportunities to probe particle dynamics and enabled a wide range of studies in physical, chemical, and biological sciences. Optical trapping of nanometric objects is more challenging than trapping of micron-size objects due to a number of fundamental factors. First of all, there is a limitation on the amount of force one can possibly exert on a particle of nanodimensions by light. Another challenge relates to the different optical properties of nanoparticles when compared to the bulk material. A comprehensive review of forces in optical trapping of particles, particularly metallic nanoparticles, is presented by Dienerowitz, Mazilu, and Dholakia (2008).
