M G Kong
email: m.g.kong@lboro.ac.uk

M Keidar

K Ostrikov

Plasmas meet nanoparticles -where synergies can advance the frontier of medicine

Nanoparticles and low-temperature plasmas have been developed, independently and often along different routes, to tackle the same set of challenges in biomedicine. There are intriguing similarities and contrasts in their interactions with cells and living tissues, and these are reflected directly in the characteristics and scope of their intended therapeutic solutions, in particular their chemical reactivity, selectivity against pathogens and cancer cells, safety to healthy cells and tissues, and targeted delivery to diseased tissues. Time has come to ask the inevitable question of possible plasma-nanoparticle synergy and the related benefits to the development of effective, selective, and safe therapies for modern medicine. This perspective article offers a detailed review of the strengths and weakenesses of nanomedicine and plasma medicine as a stand-alone technology, and then provides a critical analysis of some of the major opportunities enabled by synergising the nanotechnology and the plasma technology. It is shown that the plasma-nanoparticle synergy is best captured through the plasma nanotechnology and its benefits for medicine are highly promising.

Introduction:

In open literature, there have been countless scientific publications, roadmap statements of relevant scientific communities, and government white papers that highlight the immense potentials of the nanotechnology for healthcare. Targeting the same set of major challenges in modern medicine (e.g. infectious diseases, cancers), low-temperature gas plasmas have in more recent years been seen to attract rapidly growing interest in their biological applications. So far, the development of these two technologies for biomedicine appears to be largely independent from each other and each has achieved some notable clinical successes. The scale of their own potential for medicine and the extent of the scientific challenges they face as a stand-alone technology are such that there appears little incentive to explore possible overlaps, in terms of either underpinning science or technological implementation, between these two equally exciting technologies. Yet close examination shows some very intriguing contrasts in their characteristics when considered to meet the same basic requirements for medical applications, for example the efficacy as a therapeutic agent, the selectivity against malignant cells (e.g. pathogens and cancer cells), the toxicity and indeed the safety to healthy cells and tissues, and finally the targeted delivery towards diseased tissues. Nanotechnology has a superior capability in penetrating into a tissue, while its toxicity still represents an area of considerable uncertainties. On the other hand, biological effects of low-temperature plasmas stem from their non-equilibrium reaction chemistry with very high level of chemical dissociation. Their interaction with a living tissue is normally topical with limited penetration, but their cytotoxic effects are better understood through their similarity to free radical biology. Such fundamental contrasts invite the question of the scale of the opportunities brought about by plasma-nanoparticle synergy and call for a detailed analysis of its benefits to the ultimate goal of developing effective, selective, and safe therapies for modern medicine.

This perspective article offers the first critical analysis of whether there is a need for a synergistic combination of the nanotechnology and the plasma technology, what benefits the plasma-nanoparticle synergy may bring about, and how such synergy may be realised in practice. This analysis is built on a concise review of plasma science (Section 2) and nanoparticle-cell interactions (Section 3) as well as a comprehensive review of plasma-cell interactions (Section 4). A number of plasma-nanoparticle synergies are suggested and analysed in Section 5, and the conclusions thus reached are used to highlight in Section 6 the landscape of major opportunities to advance the frontier of therapeutic medicine by capitalising on the synergy between plasmas and nanostructures. It is emphasized that the full potential of synergizing plasmas and nanostructures can only be realized when nanoparticles are fabricated within a low-temperature plasma and delivered by the plasma to a disease target -a technology known as the plasma nanotechnology.

Plasma Science and Nanotechnology:

Low-temperature plasmas represent a unique thermally non-equilibrium environment where a variety of reactive species as well as different forms of energy (e.g., thermal, electromagnetic, and chemical) can be produced. This ability has been widely used for a large number of applications in nanotechnology. The most common applications are in micro-and nano-scale synthesis and processing of advanced nanomaterials. Representative examples include etching and conformal coating of high-aspect-ratio features in a silicon wafer for nano-electronics, low-temperature synthesis and post-processing of nanopatterns and arrays of nanostructures for sensing, photovoltaics and optoelectronics, highly-selective functionalisation and other post-processing of localised surface areas with nanoscale dimensions, as well as the production of nanoparticles with tailored features such as size, shape, facet expression, and surface reactivity [1]- [5].

The examples of the nanoscale objects produced or post-processed using low-temperature plasmas span all three dimensions and range from zero-dimensional Si-based quantum dots arranged in two-or three-dimensional patterns in a host matrix and arrays of one-dimensional single-walled carbon nanotubes with controlled chirality and metal oxide nano-architectures to two-dimensional freestanding and catalyst-supported graphene sheets and flakes and carbon nanowalls as well as self-organized patterns of shape-tuned three-dimensional semiconducting nanostructures [6]- [17]. The sizes and other properties of the nanoscale objects can be precisely tailored to meet the requirements of any particular application.

These properties can be very different from the properties of nanoparticles and other nanomaterials produced using other techniques such as thermal chemical vapour deposition (CVD) or wet chemistry. For example, the unique ability of reactive plasmas to dissociate molecular hydrogen has been successfully used to synthesize free-standing silicon and germanium nanoparticles with nearly perfect cubic shape, which is rarely achievable otherwise [18]. Furthermore, carbon nanotubes and other onedimensional nanostructures show pronounced vertical alignment which is not common to thermal CVD [19] [20].

On the other hand, effective control of any particular features of the nanostructure or nanoparticles (NPs) has been intimately related to the ability of the plasma environment to concentrate the building units of the nanoscale matter and the associated energy in designated microscopic and nanoscale surface areas [21]. A balance of plasma-specific forces (e.g., ion drag, electrostatic, thermophoretic) has also been reported as an effective tool for the precise delivery of nanoparticles to the specified microscopic areas on the surface; this approach was also instrumental in almost complete removal of nanoparticles away from the surface areas where their deposition is not warranted [START_REF] Cabarrocas | Thin Sol[END_REF] [23]. For many other unique features and advantages of the plasma-based nanoscale synthesis and processing the reader should be referred to previous publications as well as to other articles in this Special Issue.

In the following we will discuss how the above unique capabilities of low-temperature plasma-based nanotechnology can be used in applications in biology and healthcare.

Interaction of Nanoparticles with Cells:

Let us now consider some of the most common effects of nanoparticles on living cells and their applications in nano-medicine and nano-biotechnology. The interactions of plasma-produced reactive species and electromagnetic radiation with living cells will be considered in the next section.

Rapid development of nanotechnology over the last decade made it possible to synthesize different types of nanoparticles whose diameter is of the order of a few nanometers and even less. The surfaces of such nanoparticles can be modified by bioactive molecules or imaging probes that can be adsorbed, coated, conjugated, or linked to them. As such, these nanoparticles were proposed for cell labeling and targeting, tissue engineering, drug delivery, drug targeting, magnetic resonance imaging etc [24]- [28]. Wide-range applications stimulated intensive study of interaction of nanoparticles with a living tissue and in particular the penetration and migration of the nanoparticles inside the tissue. Some authors suggested that the processes governing the penetration of reactive radicals (see extensive discussion of these effects in Section 4) [29] and those governing nanoparticles are not the same [30], making a good case for penetration selectivity argument. In addition, penetration efficacy of nanoparticles inside various tissues is distinctly different. For instance, it was shown that nanoparticles are able to penetrate the hair follicle and stratum corneum (SC), but viable epidermis is reached only occasionally. It was concluded that those nanoparticles are unable to permeate the skin. Titanium dioxide nanoparticles of different sizes (from 20 to 100 nm) are not penetrated to SC layers, the human epidermis and dermis, but solely deposited on the outermost surface of the SC [31].

Meanwhile, metallic (e.g., gold) or metal oxide (e.g., iron oxide) nanoparticles have recently demonstrated a reasonable therapeutic efficacy, selectivity, tumor affinity, and concomitant in vivo tolerance in cancer therapy [32]- [37]. The treatments are based on targeted delivery of the functionalized (e.g., using glucoproteins) nanoparticles to the tumor-affected areas and using either a brachytherapeutic or hyperthermal/photothermal treatment. In the former case, low-dose electron emission from β-emitting gold-198 ( 198 Au) nanoparticles is used while the latter case relies on localized surface plasmon excitation which generates significant yet localized heat. The nanoparticles are usually delivered via direct intratumoral injections and penetrate malignant cells through tumor vasculature and pores. Application of nanoparticle treatment makes it possible to significantly reduce the size of the tumor before surgical resection and is an important milestone towards the ultimate ability of complete and irreversible tumor resolution without surgery. This ability has been demonstrated in animal trials and is presently at the stage of human clinical trials [35] [36].

In parallel and out of health concerns, an additional research activity stimulated by the rapid developments of nanotechnology is study of human body contamination and health effects of nanomaterials (nano-toxicity) [38]- [42]. This rapidly expanding research field is commonly referred to as nano-safety.

As an example of a combination of plasma treatment considered in Section 4 with the nanoparticle treatment briefly discussed above, let us consider a simple combination of suitably functionalized nanoparticles and a cold plasma jet [43] [44]. In such case, the cold atmospheric plasma jet can have two primary purposes. Firstly, the jet will carry nanoparticles and thus will provide their delivery to the desired position and good localization of the treated zone governed by the jet cross-section. Since the intensity of the plasma jet can be limited to below the damaging threshold to living tissues and thus the plasma jet will serve just as the delivering agent. Secondly, it is possible to electrically charge nanoparticles by means of fundamental mechanism of charging of bodies introduced to plasma (due to much higher mobility of electrons in comparison with ions). The value of the charge accumulated on the nanoparticle may be regulated by adjusting the plasma parameters and by the location of the nanoparticle injection point (since the timescale of particle charging in plasma and its flight time through the jet are comparable, both about microseconds [44]). Charging of nanoparticles in plasma enables utilization of electrical field for microscopic level control of nanoparticle characteristics. Symbioses of cold plasmas and nanoparticles can lead to some extremely interesting results. One very promising example of such synergy is cancer therapy. Using antibody-conjugated nanoparticles with cold plasmas led to the five-fold enhancement of melanoma cell death compared to the anticancer efficacy of the plasma treatment alone [45]. The specific mechanisms leading to such a remarkable improvement in the anticancer efficacy are not fully understood. This paper focuses on direct uses of nanoparticles and nanostructures in the treatment of diseases (i.e. used as active drugs and therapy) so as to explore their synergy with low-temperature plasmas whose biomedical applications are developed primarily as a novel therapy (see Section 4). It is worth noting that most biomedical applications of nanoparticles are related to drug delivery and therapeutic uses of nanoparticles represents only about 2% of nanomedicine publications in 1984 -2004 and 3% of nanomedicine patent filings worldwide in 1993 -2003 [46]. So far, 24 nanotechnology-based therapeutic products have been approved for clinical use with total sales exceeding $5.4 billon [46]. Targeted diseases include leukaemia and cancer, infectious diseases (e.g. fungal and protozoal infections; hepatitis A, B and C), immunodeficiency diseases (e.g., HIV). While cytotoxic effects of metallic materials (e.g. silver) are employed for nanotechnology-based therapies [47], there are increasing interests in encapsulation of nano therapeutic agents in polymers for both treatment of diseases and their preventative vaccination [48]. The reader is referred to recent review papers [48] [49].

Interaction of Plasmas with Cells:

For treatment of living tissues, low-temperature plasmas are at present used largely for therapeutic purposes although plasma-assisted imaging is conceivable. Their biological effects are predominately through their non-equilibrium chemistry, highly unique and otherwise difficult to access because of the very high degree of chemical (in particular, electron-impact) dissociation in the plasma. Lowtemperature non-equilibrium gas plasmas present a unique environment of reactive oxygen species (ROS) and reactive nitrogen species (RNS), charged particles, photons as well as heat, pressure gradients, and electrostatic and electromagnetic fields, many of which are known to induce biological effects. For example, nitric oxides (NO) are known to promote cell proliferation [50] whereas hydroxyl radicals (OH • ) are very effective against bacteria [51]. The interest in the use of nonequilibrium gas plasmas in healthcare has grown very strongly over the past 10 years [52] [53], largely owing to an important technological breakthrough that enables the generation of low-temperature atmospheric pressure plasmas with well controlled thermal stability even in electronegative gases such as oxygen, air, and water vapour [54]- [57]. Such low-temperature plasmas are commonly known as cold atmospheric plasmas (CAPs) and have been shown to be capable of effectively inactivating bacteria, fungi and virus, inducing apoptosis in cancer cells, and stimulating proliferation of mammalian cells, all in a dose-dependent fashion [52] [53]. Today, CAPs are increasingly used in an impressive array of healthcare applications such as blood coagulation [58], skin disinfection [58][59] and wound disinfection [60], wound healing [61], sterilisation of surgical instruments and medical devices [62], cancer therapy [63], and food decontamination [64]- [66]. Their potentials for healthcare are profound and their clinical successes so far are exceptionally encouraging [67][68], thus fuelling a rapid expansion of research activities in an emerging but now highly visible field known as plasma medicine. For details of the opportunity, challenge and perspectives of plasma medicine, the reader is referred to several recent reviews [52] [53].

Modes of plasma interaction with biological samples:

With a mean-free path-length as short as a few tens of nanometres in atmospheric pressure plasmas [69], their temporal stability is difficult to maintain over a large gap distance between two electrodes and as such their electrode gap is typically a few millimetres. When a CAP is brought close to a sample to be treated (e.g. an infected human tissue), the sample may become a part of the plasma-containing electric circuit. Here the discharge current flows through the sample and charged particles are likely to play an important role. This mode of plasma treatment is sometimes known as the direct mode of plasma-sample interactions [70]. By contrast, the CAP may be used as an afterglow to treat a sample, which is electrically disconnected from the plasma-containing circuit and experiences only little discharge current. The role of charged particles is much reduced and neutral reactive plasma species are likely to play a more dominate role. This mode is sometimes known as the indirect mode of plasma-sample interactions. The choice of how a CAP may be configured to interact with a cell-containing sample depends on many inter-dependent factors, many of which are highlighted in Table 1.

Figure 1 shows some of the common CAPs currently used in medical applications. Here we consider the CAP jet (see Fig. 1a) [71]- [74] as an example and discuss how some of the current understanding of plasma-cell interaction is used in CAP designs. Formed in a flowing gas, the CAP jet is well suited for precision and localised treatment, such as root canal disinfection [75]- [77] and cell transfection [78]. Some of the current CAP jets rely on the generation of highintensity plasma in the upstream electrode region (sustained in a radial-directed electric field) and its axial diffusion towards a downstream sample is often unaided. Such devices are known as the cross-field plasma jet [79]. A combination of an upstream localised plasma and the short lifespan of many reactive plasma species (typically several tens of microseconds or shorter) means that the biological efficacy of such CAP devices is critically dependent on a very short plasma-sample distance in the region of a few millimetres (often just 1 -3 mm). Longer plasma-sample distance is desirable for downstream introduction of gas and liquid precursors, and for controlled treatment of uneven surfaces. To this end, the upstream plasma may be spatially extended by a strong axially directed electric field [79]. With an axially directed flow of the plasma-forming gas, both charged and neutral species can be significantly extended towards downstream to facilitate effective delivery of short-living plasma species to the sample surface. With this strategy, it is possible to extend the plasma jet length to many centimetres and even more than 10 cm [80]. The alignment of the electric and the gas flow fields in such CAP jets (often known as the linear-field plasma jet [79]) encourages better penetration of plasma species (see below for further discussion) into a living tissue placed downstream. Importantly, the linear-field plasma jet provides two additional important advantages, when nanoparticles are driven through a plasma region for uptake of reactive plasma species (see further discussion in Section 5.2). Firstly its strong axial electric field can be used to drive the flow of the electrically charged nanoparticles. Secondly, its much longer plasma plume length both enhances the uptake of reactive plasma species by the passing nanoparticles and sustains the ROS/RNS uptake right to the entrance point of the nanoparticles into a tissue. The linear-field plasma jet is therefore ideally placed for synergistic combination with nanoparticles.

Diameter of a single CAP jet is typically a few millimetres [81]. For treatment of large samples having a width of a few centimetres and greater, such as chronic wounds, it is essential to overcome the inherently small size of CAPs through scaling-up [69][81]. This has been shown to be technologically feasible, as shown in several reported up-scaled CAPs such as the 1D and 2D CAP jet arrays (see Fig. 1b) [81][82], the microwave plasma torch [83], and the floating-electrode dielectric-barrier discharges (DBDs) [58]. While the development of these large CAP systems was initially motivated by practical considerations of scaling-up, their interactions with cells and tissues contained in a downstream sample differ from each other. For example, energetic ions with their kinetic energy reaching above 20eV are important in a sample treated by the floating electrode DBDs [84] whereas ions are largely absent in the downstream sample treated using the microwave plasma torch [83]. Depending on the electrode configurations, the CAP jet array can be made to control the involvement of ions and indeed charged particles [85]. Plasma-cell interaction mechanisms are multi-facets and at present are not fully understood. However different applications are likely to be best addressed with different upscaled CAP sources.

It is important to note that in practice diseased living tissues are either moist or covered by a layer of liquid (e.g. bold fluids, blood, and/or wound fluids). When a CAP jet is used to treat a living tissue, its plasma species are delivered to the air-liquid interface and then undergo transportation, and sometimes secondary ROS/RNS generation within the liquid medium, before reaching cells and tissues. Therefore it is important to consider plasma reactions that interact with water molecules [86]- [START_REF] Bruggeman | [END_REF].

Reactive plasma species and their biological targets:

Reaction chemistry of a CAP device varies considerably with chemical composition of the plasmaforming gas even with the same electrode configuration and the same applied voltage. In a study comparing an atmospheric air plasma with its counterpart in helium/oxygen both sustained in submicrosecond voltage pulses, it is observed that the air plasma is dominated by N 2 * species with their emission in the UVC region whereas the helium/oxygen plasma is dominated by oxygen atoms [89]. It is of particular interest to contrast these against bacterial inactivation kinetics data and establish a hierarchical list of bactericidal plasma agents (and their threshold bactericidal doses). However, the distinct difference in reaction chemistry is found to result in a small difference of mere 10 seconds in the timescale over which the two pulsed CAP systems need to achieve 6 log reduction of bacterial spores [START_REF] Kong | Gordon Research Conference on Plasma Processing[END_REF]. In other words, very different combinations of plasma species may yet result in similar efficacy of bacterial inactivation. Different chemical compositions of "lethal plasma dose" seem to exist, each with different synergistic effects among various plasma species, and it appears that the bactericidal effect of a specific plasma species may be compensated by those of others. It is therefore highly probable that there exist many different and viable recipes of bactericidal plasma chemistry, each of which may take different pathways towards bacterial death (e.g. protein, lipid and DNA). ) and nitric oxide (NO). In a moist or liquid environment, hydroxyl radicals (OH • ) and hydrogen peroxide (H 2 O 2 ) are produced. Complex physicochemical processes in the plasma impose a limit on the maximum achievable concentration of each plasma species and result in a close cross-dependence between concentrations of different plasma species. It is both intriguing and important to note that many of these ROS/RNS are also produced endogenously within the human body itself and are involved in its defence against bacterial infection through, for example, Fenton chemistry [START_REF] Stohs | [END_REF]. Therefore in a way, plasma disinfection mimics how the body responds to bacterial infection. Having been produced in plasma, ROS/RNS are then delivered to cells and tissues exogenously and their delivery is subject to atmospheric absorption of photons and recombination of short-living species outside the plasma zone. This can filter out the effects of some ROS/RNS, since their concentrations at the cell/tissue location may fall below the threshold of their biological effects. For example, current evidence of bacterial inactivation studies suggests that UV photons play a less important role than ROS [92]- [94]. Figure 2 shows a damaged membrane of B. subtilis spores after plasma treatment, typical of a result of oxidation.

Upon the arrival of plasma species on the surface of microorganisms, their uptake by pathogens and infected tissues is affected by how plasma species interact with the cell surface structure (e.g. receptors, membrane proteins) and whether they penetrate into the microorganism. These factors mean that the biological effect of each possible plasma agent is likely to be kept within a range of plasma dose and penetration depth. This view also applies to diseases other than bacterial infection such as cancers.

Important insights have been gained from investigations on possible plasma-inflicted damages to key cellular constituents. Studies using isolated protein and plasmid DNA models have shown protein and DNA damages [95]- [99], and more relevant experiments using bacterial cells have provided direct evidence of breaching and rapture of cell membrane [100], reduction and degradation of proteins [101]- [103], lipid damage [104], and (mostly) single-strand breaks (SSB) of DNA [105]. These results have prompted the suggestion of the involvement of ROS, charged particles, and UV photons [92][100][105] - [108], with ROS often linked to oxidation of protein and lipid and with UV photons linked to DNA damages. The view of DNA damages, particularly double-strand breaks (DSB), being largely induced by UV and shorter-wavelength radiation has its root in radiation biology where many hundreds of DSBs per microorganism are not uncommon among irradiated bacteria [109]. The prevailing view of bacterial killing in radiation biology has been "death by DNA damage" for the past 50 years is now challenged and is being replaced by the alternative view of "death by protein damage" for which the role of ROS and RNS is becoming increasingly important [110]. This development in radiation biology may become important in informing and formulating the direction and priority of future studies of plasma-mediated mechanisms against pathogens. It should have a similar impact on studies of plasma-safety for healthy mammalian cells and those of plasma-mediated killing of cancer cells.

The long-standing attempt to understand plasma inactivation mechanisms has very often suffered a common lack of an appreciation that even the same plasma source can have very different plasma chemistries as it transits, often abruptly and rapidly, from one plasma mode to another [89] [111]. Similarly under-appreciated is the paucity that the resistance of microorganisms to external stresses is influenced by their micro-environment [112], an obvious example being their interaction with the surface of their supporting substrate [113]. Future studies are likely to include detailed characterisation of both plasma and microbial sample conditions. Studies of plasma inactivation m liquid environment that contains water molecules and produce a importantly, it is possible that p directly from the chemical structu reaction chemistry on the surfa production or the on-site produc plasma medicine and a complex standpoint of how ROS and RN reactive species include O 2 * , O 2 [114]. These can be considered a prioritization of key plasma spec

Selectivity and Safety:

When plasmas are used to treat d pathogens or cancer cells co-exi cells and tissues, an importan plasma selectivity with lethality damage to healthy mammalian c of the most important question surprisingly has been scarcely ad investigations though histologica vivo experiments with animals damage [52] [60]. Mammalian structure to bacteria (see Fig.

external stresses are also likely t biology, bacteria and pathoge radiation than mammalian ce reversal exists for plasma produc critical and intriguing. From t reported so far, it has been know significant against E. coli [116] b a significant selectivity window selectivity is attainable even tho subtle long-term damages. Simi plasma-based cancer therapies.

One practical complication to s invade and infect an initially he escaping the impact of the exoge co-existence with the host tissues differently by the pathogens and experiments using cell media. W consequence is that some non-t minimal invasive surgery, for ex clinical successes provide a crit broad nature of the long-term eff

Plasma Penetration:

The issue of migrating bacteri penetration. Estimating from th depth of plasma ROS/RNS into mechanisms have in the past paid little attention s bacteria. Energetic electrons, ions and UV ph additional reactive oxygen species in the liqu lasma treatment may release oxygen, nitrogen, a ure of cells and tissues and the latter can then be ace of cells. These processes may be referred ction of ROS/RNS. They introduce both an exc challenge to the study of plasma inactivation me NS are used to defend against bacterial infectio 2 •-, OH • , H 2 O 2 , and NO and their cellular target as the starting group of priority species in mechan ies may be achieved for treatment of cancers.

diseased living tissues where ist with healthy mammalian nt challenge is to achieve against pathogens but little cells and tissues. This is one ns in plasma medicine but ddressed in detailed in-vitro al data from ex-vivo and inhave so far shown limited n cells have a different

3), and their responses to to be different. In radiation ens are more resistant to ells. Whether the desired ced ROS/RNS remains both the limited in-vitro studies wn that the selectivity window between bacteria a but limited against Staph aureus [117]. Using an w is found against bacteria spores [START_REF] Kong | Gordon Research Conference on Plasma Processing[END_REF], sugges ough much more work is needed to examine the ilarly there is considerable scope for detailed s electivity is the host-cell interaction where bac ealthy tissue and then may penetrate into the bu enously applied plasma treatment [118]. Bacterial s means that the same externally applied plasma d d their host tissues thus complicating the selectiv While this presents an uncertainty, a useful ind thermal atmospheric plasma jet systems have a xample those for removing diseased tissues [67][6 tical context of realism and relevance with whi fects of low-temperature atmospheric plasmas [12 ia into the bulk of a living tissue raises the he half-lives of the main plasma species, the m o a liquid medium is perhaps at most a few t depending on physiological conditions of the liquid medium. For example, OH •-radicals are known to propagate for no more than a few angstrom in liquid [110][122] whereas the singlet oxygen has a halflife of only a few microseconds in liquid [123]. On the other hand, H 2 O 2 are long-living and can diffuse throughout the cell [110][122] even though their reactivity is only modest and, against some pathogens, ineffective. Penetration of ROS/RNS into the skin or a living tissue is even more limited. Therefore the plasma treatment is characteristically topical.

The topical character of plasma treatment is an advantage for localised surgery where spatial control of damages to healthy tissues is critical [119][120]. For living tissues infected exogenously such as open wounds, plasma treatment of the tissue surface may be applied frequently to contain surface infection. Similarly, topical plasma treatment is useful for melanoma tumours. When a diseased tissue is located considerably away from the skin surface, the topical nature of plasma treatment becomes a serious limiting factor. Greater plasma dose may be effective for infected tissues lying immediately below the skin or wound surface, but the issue of plasma toxicity would become a counteracting factor. There is clearly a need for assisted plasma penetration.

It is known that adjustment of pH may considerably increase the half-lives of some ROS [108], such as O 2

•which could be made to last up to 10,000 s in liquid [START_REF] Kitano | 3 rd Int Conf Plasma Medicine[END_REF].

To see whether such extended half-lives of plasma species could enable some ROS to penetrate through the skin and tissues, we note that there are openings and channels in the skin to allow diffusion of small molecules and other externally applied agents such as plasma species. These include the lipidic intracellular routes (5 -36 nm in diameter) and trans-follicular routes (10 -210 µm in diameter) [125]. The stratum corneum (SC) and viable epidermis (VE) of the skin (see Fig. 4 and [126]) are about 32 µm and 65 µm respectively, and the length of an aqueous channel through the skin is much greater than the combined depth of SC and VE. We assume that the effective aqueous channel is in the order of 1 mm and the diffusion coefficient of superoxide anions in liquid is in the order of D = 8x10 -5 cm 2 /s [127]. For a lifetime of 10,000 s, the diffusion length of O 2

•-is 2(Dt) 0.5 = 17.9 mm. In fact, the lifetime of O 2

•-would only need to be more than 32 s for their passage through a 1mm long aqueous channel. Therefore it is possible for reactive plasma species to survive their passage through aqueous channels in living tissues and reach a deep-embedded disease site if the physiological conditions of the aqueous environment could be appropriately adjusted. In addition, it is possible to transiently increase the diameter of lipidic intracellular routes by using, for example, electroporation. This should enable a greater passage efficacy of plasma ROS/RNS through aqueous channels of living tissues. It is worth noting that the delivery of plasma ROS through the membrane of a mammalian cell is also possible as indicated in early in-vitro evidence of cell permeation of plasma species [78]. Penetration of plasma species through the membrane of a bacterial cell is possibly more difficult because of much narrow channels through the membrane (~up to a few tens of nanometres in diameter).

The use of such long-living ROS as pre-made antimicrobial or anti-cancer agents implies that liquids after plasma pre-treatment become a drug [START_REF] Morfill | Gordon Research Conference on Plasma Processing[END_REF] [START_REF] Friedman | 3 rd Int Conf Plasma Medicine[END_REF]. In this context, research of plasma medicine is becoming the study of how plasma-enabled drugs may be used effectively and safely (i.e. Plasma Pharmacy) as well as the study of mechanisms or modes of actions with which plasma-enabled drugs cure their intended diseases (i.e. Plasma Pharmacology). It is known that endogenous ROS and antioxidants are often unbalanced in immune-compromised patients and yet their balance is critical in the defence of a patient against bacterial infection and in regulation of many essential biochemical functions. The possibility of pre-prepared long-living plasmas species is exciting, since they can be used as a novel and unique agent to address an imbalance between endogenous ROS and antioxidants thus allowing a patient to fight more effectively against infection and cancer. In addition to directly produced plasma ROS/RNS, it is possible to produce energetic ions at a streamer head [84] and/or energetic electrons in plasmas formed in a micro-scale cavity [START_REF] Shi | [END_REF] [131] deep inside a living tissue. Such energetic charged particles may be extended into a tissue from a parent plasma outside the tissue, and they allow for an in-tissue production of ROS/RNS since their kinetic energy is sufficient to react to water molecules and some chemical bonds of the tissue structure. As a result, they can also be used to address potential imbalance between endogenous ROS/RNS and antioxidants. Such in-tissue production of ROS/RNS may be considered as long-range effects of plasma species, and they are almost completely uncharted as an area of investigation but represent a very important route for plasma penetration and indeed plasma pharmacy.

In summary, cold atmospheric plasmas are shown to be capable of inactivating microorganisms and cancer cells through unique non-equilibrium chemistry. They are exceptionally versatile, offering different modes of interacting with a cell-containing sample, and they employ various reaction chemistries to unlock different pathways towards an intended therapeutic objective (e.g. skin disinfection, wound healing, and cancer therapy). With appropriate use of plasma chemistry, selectivity appears to be attainable with controlled safety. CAP treatment is normally topical, ideally for localised cell and tissue manipulation as demonstrated in clinical successes of endoscopic surgery procedures. However there are exciting opportunities to enhance or induce significant penetration of plasma species into a living tissue, with energetic ions and electrons potentially capable of triggering an in-tissue production of ROS/RNS in a disease area. In addition, plasma pre-treatment of a physiologically-optimised liquid medium (e.g. using pH adjustment) may allow the in-liquid production of ROS/RNS that last sufficiently long to survive their passage through tissue. The delivery of both energetic charges (for in-tissue ROS production) and pre-prepared plasma ROS (for penetrating to a remote site) offers a great opportunity to work with the chemistry of endogenous ROS/RNS, by addressing the potential imbalance of endogenous ROS/RNS and antioxidants. The role of CAPs as a complementary regulator of free radical biology within the body may open up a whole new host of opportunities to treat human diseases.

Synergies between plasma and nanoparticles:

Both the nanoparticle technology and the cold plasma technology offer exceptional opportunities to biomedicine. There are growing momentums that drive forward each technology towards a full realisation of their immense promise to healthcare, and there are significant evidences that each technology is likely to influence very strongly how modern medicine may be practiced in future. While the exceptional scale of these opportunities tends to encourage a focus on one technology alone, there may be even greater opportunities in their synergy. Given that CAPs have so far been used largely for therapeutic purposes, our discussion of synergy is confined to the treatment of diseases.

Comparison of CAPs and nanoparticles:

For treating diseases, medical applications of CAPs and nanoparticles often require at least four considerations, namely their reactivity, their selectivity against pathogens or cancer cells, their toxicity to healthy mammalian cells and tissues, and their penetration to the contaminated regions. Table 2 is a summary of the properties of CAPs and nanoparticles when compared against the above four considerations. CAPs provide non-equilibrium reaction chemistry with many of their main reactive species also present in the body with known effects on bacterial and mammalian cells [114]. The biological implication of plasma chemistry is not dissimilar to those related to free radical biology. Provided that future studies result in appropriate understanding and control of plasma dosage, it is highly probable to achieve selective kill of pathogens or cancer cells with little damage to healthy mammalian cells and tissues. There have been clear successes of some CAP-based clinical procedures [119]- [121]. On the other hand, nanoparticle-b acceptance despite of far greater current anxiety over their toxicit cells or when inoculated in vivo significant uncertainty [134]. N desirable to reduce the minimum This is one of many areas where

Synergy in reaction chemistr

Consider the in-vitro study discu used to enhance plasma-mediat synergetic combination of the nanoparticles. This synergy can a against cancer cells and as a resu as a supplementary agent to assi already been shown that plasma well as suppress the growth of treatment of a tumour can reduc inhibition concentration of intrat reported in open literature but is both plasmas and nanoparticles. potentially profound for nanopar indeed the paradigm of nano-safe as skin and wound infection, infe

One exciting area of plasma-na synergy is to combine the un equilibrium plasma chemistry superior penetration of nanoparti synergy can be realized at the nanoparticle synthesis or pos processing. This is illustrated in Suppose nanoparticles are prod with an iron core, which can b acquire a high affinity to oxygen hence trap oxygen atoms, for e the core has a porous str appropriate pore size, shape, an volume ratio [135]. The iron nan are then coated with biocompa (with reduced toxicity and wettability) and finally bio-conj attaching specific molecules that nano-particles can be delivered t minimising dissolution during nanoparticles can then release he nanoparticles are attached, by m site of a tumour tissue or an infe pathogens. While fabrication of plasmas in the fabrication of th atoms of very high concentration the nanoparticles needs to be no loose to avoid being lost on their ratio release oxygen atoms at di diseased tissue is subject to a co based therapies are yet to enjoy a comparably sim r extent of scientific advances [132] and this is ty. Many nanoparticles generate ROS when incu [133], and nanotoxicity represents both a key a Notwithstanding future progress of nanotoxicity m amount of nanoparticles necessary for a target plasma-nanoparticles synergy is particularly bene ry: ussed in Section 3 in which antibody-conjugate ted killing of melanoma cells [45]. It may ha e anticancer properties of both CAPs and and should be taken advantage of to reduce the m ult reduce plasma toxicity. In reversal of roles, pl st nanoparticle-based therapies against cancer an species induce apoptosis among cancer cells (in melanoma and lung cancers (in-vivo evidence) ce the defence of its cancer cells and as a result tumorally injected nanoparticles when used alon s anticipated from the current understanding of a The obvious patient benefit of reduced risk of n rticles-based therapies, as it is likely to alter the ety. Furthermore, the benefit extends also to othe ectious diseases, and wound healing. anoparticle nique nonwith the icles. Such e stage of st-synthesis n Figure 5. duced first be made to atoms and example if ructure of nd surfacenoparticles tible silica optimized jugated by t are designed to bind with specific proteins in a to a diseased area by, for example injection via their passage to the diseased site. The ma eat and oxygen atoms to the proteins of the targe means of redox reactions. The release of oxygen ected tissue induces a site-specific effect against f such multifunctional nanoparticles is a major c he nanoparticles allows the nanoparticles to be ns thus achieving high uptake rate. The bonding o t too strong so as to be released readily on the ta r way to the targeted site. Nanoparticles of differe ifferent locations down a tumour body or an in ncomitant plasma treatment so that pores and oth milar level of clinical partly related to the ubated with different area of research and a research, it is highly ted therapeutic effect. eficial.

ed nanoparticles were ave benefited from a antibody-conjugated minimum plasma dose lasmas could be used nd/or infection. It has -vitro device) [52] as [63]. So plasma prereduce the minimum ne. This is yet to be anti-cancer effects of nanotoxicity could be parametric space and r major diseases such a targeted cell. Such bloodstream, and by agnetic core of the eted cell to which the atoms onto a buried either cancer cells or challenge, the use of immersed in oxygen of oxygen atoms onto arget site, yet not too ent surface-to-volume nfected tissue. If the her aqueous channels enclosed plasma ROS d surface-borne, proteinld atmospheric plasma are temporally opened by plasma (see Section 5.3), the delivery of the nanoparticles carrying oxygen atoms will be made more efficient.

Discussion in Section 4.4 indicates that some ROS (e.g. O 2

•-

) may be made to last longer and could also be carried by nanoparticles as in the case of oxygen atoms, though further work is needed to substantiate its practicality. The above discussion suggests a new concept of plasma-seeded nanotechnology in which nanoparticles are prepared with encapsulated plasma reaction chemistry by immersing their fabrication in a non-equilibrium plasma environment where very high concentrations of ROS and RNS can be up-taken by nanoparticles. Material properties and surface functionalities of nanoparticles are hardly exploited for trapping and sustaining ROS/RNS during the production of nanoparticles and before them being coated and bio-conjugated.

The above synergy takes advantage of the superior capability of nanoparticles to penetrate through the skin barrier [30], thus liberating plasma from its limitation as an intrinsically topical technique. The potential specificity of nanoparticles to attach to diseased cells as against healthy cells [136] offers selectivity, and the use of ROS or RNS carried by nanoparticles as the main agent against either cancer cells or pathogens reduces the reliance on toxic materials (e.g. silver). These anti-cancer or disinfecting effects are realised selectively on a nano-scale and their study may be considered as nanoscale plasma pharmacy. While there are clearly significant engineering and scientific challenges, the benefits and implications of nano-scale plasma pharmacy are very exciting.

Synergy in cell permeation and cellular manipulation:

There have been some reports of plasma-mediated cell permeation [78], and possible mechanisms may include surface deposition of charges, electroporation by global and local electric field, exothermic recombination of excited species and radicals, and locally released heat. Energy and/or heat can be released by either plasmas or nanoparticles, though via different routes. Once above a certain threshold, the global effects of the released energy can contribute to cell permeation. However localised energy release is of greater interest. Plasma ROS/RNS may act on specific sites on the cell surface and exothermic radical recombination is likely to be surface roughness-selective [137], both capable of localised deformation to the cell surface and hence induction of enhanced cell permeation. Furthermore, plasma ROS are known to degrade adhesion proteins [138] and integrin [139]. These alter the cell-substrate and cell-cell attachment, thus deforming the cell and affecting permeation of its membrane channels. Equally, it is possible to achieve a spatially preferential pre-deposition of nanoparticles at specific cell surface sites (e.g. surface proteins and integrin) so that localised plasma degradation and hence cell deformation are enhanced, leading to more significant cell permeation and intracellular uptake of plasma species.

Electrically assisted or mediated cell permeation is of particular interest, as electroporation [140] can be induced at the level of electric field in atmospheric plasmas. For example, the electric field is at the order of 0.5 kV/cm at the tip of a CAP jet [74][141] and higher at that of a plasma streamer [84]. In RF microplasmas, the sheath electric field is of the order of 80 kV/cm [START_REF] Shi | [END_REF]. The temporal opening of membrane pores, usually over a microsecond timescale, allows increased intracellular uptake of plasma species and/or nanoparticles. It is worth noting that the electric charging time of the membrane of a mammalian cell is about a few hundreds of nanoseconds for mammalians cells [140]. When the temporal scale of the electric field in the plasma is shorter than the electric charging time of the cell membrane, strong intracellular electric field can be established and this would lead to forced intracellular penetration of plasma species and nanoparticles. Nanosecond pulsed CAPs are known to be possible [142] [143] and can be used to set up intracellular electric field inside a mammalian cell. This is useful in treating cancer cells. Equally, nanoparticles carrying charges could be rapidly deposited on the cell membrane over a nanosecond scale, for example through a CAP jet, and this can set up a permeating electric field into the intracellular space. Strong intracellular electric fields not only enhance the uptake of plasma species and/or nanoparticles, but also induce other important biological effects such as intracellular calcium release and enhanced gene expression [144]. Therefore the benefits of electrically assisted cell permeation could go beyond enhanced uptake of plasma species and/or nanoparticles.

Interestingly, plasma-borne nanoparticles can be intentionally used to create a clear difference in morphological features on the surfaces of targeted and non-targeted cells. These properly labelled and functionalized nanoparticles can be delivered and distributed around the cellular surface thus acting as externally delivered "etching masks" which are commonly used in nanoscale plasma etching to enable selective treatment of the unmasked areas. Similarly, the ion focusing effect discussed above may lead to an effective and selective heating of the nanoparticles rather than the open surface areas. In this case one could expect ROS/RNS radicals to interact with the unmasked (open) areas of the surface only. On the other hand, selective heating of nanoparticles may lead to the controlled heating of the cell membrane underneath the nanoparticles. This may in turn induce heat-enhanced inter-diffusion of radicals between the nanoparticles and the cellular membrane. For comparison, these effects cannot be achieved using focused laser beams since the typical sizes of laser beam spots are in the micrometre range (which is comparable with the sizes of small cells) and cannot resolve sub-micrometer features on the cellular surfaces. By tailoring the nanoparticle sizes one can mask and treat surface areas of virtually any dimensions, which offers exciting opportunities for unprecedented increase in selective treatment at the cellular level or even the organelle level. The many important benefits discussed above stem from the opportunity to induce spatially differentiating effects, on a nanometre scale, by exploiting the synergy between plasma species and nanoparticles. For future reference, this may be referred to as plasma-nano cellular manipulation.

Synergy for enhanced penetration and selectivity:

The above discussion on permeation through cellular pores can be extended to permeation through aqueous channels through a living tissue, though the timescale is likely to be larger because of the larger length scale of a tissue. Surface deposition of energy by plasmas and/or nanoparticles can be made spatially selective, and electrically induced temporal opening of pores and channels within a tissue is equally possible. It has recently been shown that an impinging plasma streamer incepting a tissue moves preferentially to a nearby channel in the tissue and establish a strong in-channel electric field there [84] [START_REF] Kushner | 3 rd Int Conf Plasma Medicine[END_REF]. Consistent with the discussion in Section 5.3, the spatially extended electric field can trigger an in-tissue production of plasma ROS/RNS well away from the streamer-tissue inception point. This offers an interesting route for plasma penetration into a living tissue, particularly in tissue cavities and openings in a wound by putrescence or other disease-caused routes. Again, ROS/RNS can be delivered into a tissue by nanoparticles encapsulated with plasma ROS/RNS (see Section 5.2). Synergistic combination of plasmas and nanoparticles could even facilitate selective penetration. It is known that nanoparticles may preferentially deposit near cancer cells rather than healthy cells [136]. Therefore a combined use of plasmas and nanoparticles is likely to enhance the directionality of the propagation of both plasma species and nanoparticles towards diseased sites within a tissue. There are many different implementation routes, some of which are realized in plasma-based nanotechnology, such as mutual orientation of ion fluxes and nanoarrays [START_REF] Levchenko | [END_REF]. This orientation is very different for different surface morphologies. If the surface features some micro/nanostructures with reasonably large aspect ratios (e.g. >3-5), the effects of incoming ion fluxes and the resulting feature electric field enhancement effects may be very different. For example, the outer layers of many eukaryotic cells feature hierarchical brush-like structures. Different elements of this structure would thus draw different ion fluxes and would then be charged differently. More importantly, normal and cancerous cells have very different membrane external structure, so-called brush [136]. Therefore, the direct effect of the electric field (electric field enhancement) and charging is expected to be different for the normal and cancerous cells thereby providing an excellent opportunity to achieve the as yet elusive selective treatment of the normal and cancerous cells by plasma ion species.

It is possible to facilitate the charging and discharging of different locations in a tissue surface, using plasmas, nanoparticles or their combination, in order to establish possible ion-focusing channels for enhanced penetration of plasma species or nanoparticles. Many of the above-discussed enhancements of plasma and/or nanoparticle penetration are enabled by charged particles and their electric field, which can be used to charge nanometre and micrometre scale structures and as such enable plasma chemistry on nano-and micro-features of a diseased area (either cells or tissue sites). The importance of charges and micro-scale tissue charging provides a unifying point for plasmas and nanoparticlesnanoparticles are most conveniently charged in plasma and much less useful by using other means. In addition, it is possible to control the charge polarity of nanoparticles by tuning plasma conditions [147] and, thus, to enrich the therapeutic potential of nanoparticles.

The enhanced penetration by synergising plasmas and nanoparticles offers a spatial selectivity and indeed a way to enhance their safe use as a therapeutic strategy. It offers an unexpected aid for upscaling, since the selective move of a plasma streamer to a nearby channel means that a CAP jet array may preferentially establish strong electric fields in many natural or putrefied channels regardless their relative locations to the jet-tissue inception points. Plasma jet-specific feedback control helps maintain similar plasma-tissue interaction [81], and the design of the CAP jet array could become reasonably independent of the specific properties of the diseased tissue that it would be used to treat.

As one final example of synergistic approach for applications other than therapies, hydroxyapatite [HA, Ca 10 (PO 4 ) 6 (OH) 2 ] has been widely used to promote biological functions of various biomedical and dental implant/filling materials [START_REF] Ratner | Biomaterials Science: An Introduction to Materials in Medicine[END_REF]. This bioceramic material is the main mineral constituent of bone and tooth tissues, and HA coatings have revealed inspiring clinical advantages in promoting efficient implant fixation and implant-to-bone adhesion shortly after the implantation, as well as faster bone remodeling due to enhanced bidirectional growth and formation of a bonding interlayer between bone and implant [START_REF] Sun | [END_REF]- [151]. Recently, nanoparticle HA dental enamel paste has been used for rapid repair of early caries lesions without the need of removing of healthy tooth material, commonly practiced in dentistry to ensure the filling stick [152]. However, the issues of appropriate sterilization of the damaged tooth areas still remain. This is where CAPs and HA nanoparticles can be used simultaneously to implement both effects -the CAPs to be used not only as the carrier of the HA nanoparticles to the targeted tooth but also as simultaneous sterilization of the tooth surfaces. The HA nanoparticles, precisely delivered to the targeted growth sites, will then be used for dental tissue remodelling. This exciting synergistic opportunity awaits its realization.

Concluding Remarks:

Against a backdrop of truly immense progress in the presently parallel fields of the nano-technology and the cold atmospheric palsma technology for biomedicine [46]- [49], [52]- [53], this perspective article was motivated primarily by the intriguing contrast between these two well-established technologies in terms of their reactivity, selectivity, toxicity, and targeted penetration to diseased tissues. Inevitably the latter invites the question of their possible synergy and the related benefits to the ultimate goal of developing effective, selective, and safe therapies for medoern medicine.

In terms of their underpinning science, nanomedicine is more advanced than plasma medicine. To this end, the paper started with a brief review of nanoparticle-cell interactions and a comprehensive review of plasma-cell interactions. A key advange of cold atmospheric plasmas is their non-equilibrium reaction chemsitry with very high levels of chemical dissociation that is otherwise difficult to access and that strikes an intriguing similarity to the chemistry of endogenous ROS/RNS produced by cells themselves. Evidences of key plasma ROS/RNS and their cellular targets were presented, and, when appropriate, linked to those known in free radical biology and radiation biology. While normally used as a topical treatment, the cold atmospheric plasma technology has already found important clinical success in endoscopic surgery with good indication of their safety to healthy cells and tissues. For treatment of a diseased area buried inside a living tissue, technological viabilities of several plasma penetration techniques were suggested and assessed. These included the in-tissue production of ROS/RNS by microscale plasmas penetrating into acqueous channels in a tissue whilst reacting with water molecules and chemicals bound to the tissue structure. Another technique was direct penetration of physiologically adjusted liquids with their antimicrobial or anticancer effects induced by plasma pre-treatment. Such plasma-treated liquds thus form a novel form of drugs, and their studies were referred to as plasma pharmacy and plasma pharmacology, an important avenue of future investigations of biomedical applications of plasmas. Plasma-mediated cell permeation was also discussed.

When plasmas and nanoparticles are combined, a number of important benefits were shown to emerge, such as reduced toxicity through combinatorial antimicrobial or anticancer effects of plasmas and nanoparticles. Given the current focus on safe use of nanoparticles, this synergy was indicated as particularly benefitial in reducing the minimum inhibition concentration of nanoparticles for cancer cells or pathogens. To advance one step further, one exciting opportuty of synergy was shown to be related to the fact that porous nanostructures may be used to trap reactive plasma species (e.g. oxygen atoms) before the nanostructure was encapsulated with a bio-polymer coat and then bio-conjugated. Coupled with the possibility that some plasma ROS may be made to extend their half-lives and so survive their passage to a diseased tissue, nanoparticles could become a carrier of pre-prepared plasma ROS/RNS. This was shown to be a powerful strategy, partly because plasma ROS/RNS could be delivered deep into the diseased tissue or even into an infected mammalian cell (thus overcoming the topical character of plasma treatment) and partly because less chemically reactive materials may be used for manufacturing nanoparticles thus mitigating the issue of nanotoxicity. While there remain considerable fabrication challenges, one implementation route was thought to involve nanoparticles being prepared in an atmospheric plasma jet where nanopaticles are immersed in very high concentrations of plasma ROS/RNS for efficient uptake and are charged simultaneously for subsequent directional penetration into tissues towards cancer or infected cells. These suggest the possibiloity of nano-scale plasma pharmacy.

Upon the arrival on the region of maligant cells, it was shown possible for nanoparticles to target specific cellular sites, for example, by difference in sub-microscale feasures on the cell surface and by mutual orientation of plasma ion fluxes and nanoarrays. Also shown possible was a pre-deposition of nanoparticles onto specific surface sites of cell membrane to form a spatially selective cell-surface mask for spatially selective treatment of subsequently applied plasmas. These opportunities could lead to an unprecedented selectivity of plasma/nanoparticle treatment at the cellular level or even the organelle level. This suggests a highly desirable capability of nanoscale plasma manipulation of cell, and it would mostly likely be materialized when the plasma technology and the nanotechnogy are synergistically combined. As an illustration to the power of the plasma-nanoparticle synergy beyond direct treatment of major diseases, the use of nanopartciles for enhancing bone and tissue remodelling was considered. The benefits of this important nanotechnology would be enhanced significantly if a simultaneous step of palsma sterlization was used.

In essence, the synergy of low-temperature plasmas and nanoparticles aims to take advantage of distinct and different strengths of each of the two technologies in terms of reactivity, selectivity, safety, and diseases targetting. Its benefits go well beyond simple combinations of the two technologies and indeed can only be fully exploited when nanoparticles are synthesized and then postprocessed/functionalized within the plasma environment, because of the need for an in-situ access to high density plasma ROS/RNS and because of the benefits of charged nanoparticles for preferential cell targeting. The plasma-nanoparticle synergy not only enables a step change in their performance matrix for biomedical applications, but also unlocks unexpected doors to many presently unknown opportunities. These combine to offer an unprecedented grand opportunity with potentially enromous impact on modern medicine.

Figure 1 :

 1 Figure 1: (a) A CAP jet treating a thumb and (b) a CAP jet array treating a hand, both pictures taken at Loughborough.

  In the gas phase (i.e. before reaching the cell-containing liquid medium), plasma produces UV photons and many bactericidal ROS/RNS, including singlet oxygen (O 2 * ), atomic oxygen (O/O * ), superoxide (O 2 •-

Figure 2 :

 2 Figure 2: SEM images of B. subtilis spores (a) before treatment of a He/O 2 CAP jet and (b) after plasma treatment [100]. Reprinted with permission from IEEE Publishing, 2006.

Figure 3 :

 3 Figure 3: Structure of Reprinted from ref. [1 National High Magnet Florida State Universit

Figure 4 :

 4 Figure 4: Structure of a skin with an open wound [126]. Reprinted by permission from Macmillan Publishers Ltd, 2008.

Figure 5 :

 5 Figure 5: Fabrication of nano-capsule with and/or RNS, a polymer coat, and conjugate binding molecules, fully completed in a col (jet) environment.

Figure 1 (Figure 2 (Figure 3 (Figure 4 (Figure 5 (

 12345 Figure 1 (Fig-1.tif)

Table 1

 1 Main considerations of CAP design

	Global considerations: • CAP scalability vs localised treatment • Spatially uniform/controlled treatment • Temporally reproducible treatment • Control of gas temperature

• Effect of UV, ozone, and NO x • Presence of liquid Reaction Chemistry : • Use of charged particles vs neutral species • Role of ROS and RNS and their lifetimes • ROS/RNS transport through liquid and their generation in liquid Plasma-cell interactions: • Plasma effects on cellular components • Plasma penetration through cells and tissues • Effect of electric and electromagnetic fields • Safety and toxicity • Selectivity between pathogens and cells

Table 2 :

 2 Properties of CAPs and nanoparticles

		CAPs	Nanoparticles
	Reactivity	ROS/RNS, UV	Intracellular ROS,
		Charged particles	metal, nanomaterials
	Selectivity Potentially large	Can be very high subject
			to appropriate
			functionalization/
			Bioconjugation
	Toxicity	Possibly	Varied, depending on
		controllable	material, structure,
			surface chemistry, etc.
	Penetration Topical	Penetrating

Acknowledgement:

MGK thanks partial support from the English Department of Health and the Engineering and Physical Sceinces Research Council (UK). MK was supported in part by NSF/DOE Partnership in Plasma Science and Technology (NSF grant CBET-0853777, DOE grant DE-SC0001169). KO acknowledges partial support of CSIRO's OCE Science Leadership Scheme and the Australian Research Council.


David Z Pai
email: david.pai@plasma.k.u-tokyo.ac.jp

Special Cluster Issue on Perspectives and Challenges in Plasma Nanoscience in J. Phys. D (After review) Nanomaterials synthesis at atmospheric pressure using nanosecond discharges

published or not. The documents may come

Nanomaterials synthesis at atmospheric pressure using nanosecond discharges

Introduction

The unique physics and chemistry of plasmas presents numerous advantages for the fabrication of nanomaterials [START_REF] Ostrikov | Colloquium: Reactive plasmas as a versatile nanofabrication tool[END_REF][START_REF] Gonzalez-Aguilar | Carbon nanostructures production by gas-phase plasma processes at atmospheric pressure[END_REF][START_REF] Ostrikov | Plasma-aided nanofabrication: where is the cutting edge?[END_REF][START_REF] Zheng | Plasma-Assisted Approaches in Inorganic Nanostructure Fabrication[END_REF]. In particular, non-thermal plasmas have attracted much interest for the fabrication of various nanomaterials in low-temperature non-equilibrium conditions, such as nanocrystals [START_REF] Kortshagen | Nonthermal plasma synthesis of semiconductor nanocrystals[END_REF]. There is increasing interest in operating at atmospheric pressure, where high species densities may permit fast synthesis without vacuum systems. Focused research sub-topics have included the production of carbon-based nanostructures [START_REF] Gonzalez-Aguilar | Carbon nanostructures production by gas-phase plasma processes at atmospheric pressure[END_REF][START_REF] Nozaki | Carbon nanotube synthesis pressure glow discharge: A review[END_REF]) and the use of microplasmas [START_REF] Mariotti | Microplasmas for nanomaterials synthesis[END_REF]. A detailed overview of plasma nanofabrication can be obtained from the aforementioned references, which are all review articles.

Scaling up plasma-based nanofabrication processes to the industrial scale is the primary motivation for improving their efficiency. Many methods consume excess energy and materials, largely because the fundamental processes involved are non-specific in nature. For example, many techniques involve heating the entire substrate to facilitate the surface reactions of the relatively few particles that participate in the assembly of a nanostructure. Non-thermal plasmas may provide the selective control needed to direct energy and matter efficiently.

In this article, we will compare the energy cost of synthesizing nanomaterials for various non-thermal plasma sources at atmospheric pressure. We will then argue that discharges of nanosecond duration hold significant promise for two reasons. First, they are capable of matching or surpassing the current benchmarks for nanofabrication efficiency. Second, the transient thermal processes produced by nanosecond discharges could be used to generate metastable nanomaterials.

Special Cluster Issue on Perspectives and Challenges in Plasma Nanoscience in J. Phys. D (After review)

Energy cost per atom of nanomaterials synthesis

Here we compare the energy cost of incorporating each atom into the nanomaterial ( atom ) for several case studies of nanofabrication using atmospheric pressure plasma sources, as shown in Table 1. For this comparison, argon at atmospheric pressure is used as the carrier gas for igniting the discharge in all cases. None use external substrate heating; synthesis results only from using the energy supplied by the plasma. Also, these particular studies provide both the discharge power (P) and the nanomaterial mass production rate ( m & ) necessary for calculating

/ atom P m ε =
& . All of the nanomaterial produced is collected or at least accounted for when providing the mass production rate.

There are a few details concerning the calculation of atom for each of the studies used in Table 1. For the microplasma used by [START_REF] Nozaki | Microplasma synthesis of tunable photoluminescent silicon nanocrystals[END_REF], [START_REF] Mariotti | Microplasmas for nanomaterials synthesis[END_REF] estimates m & = 1 µg/min. For the microplasma used by [START_REF] Shimizu | Reactive evaporation of metal wire and microdeposition of metal oxide using atmospheric pressure reactive microplasma jet[END_REF], we calculate m & based on the given dimensions of the nanoparticle tower and the density of bulk WO 3 . For the spark discharge used by [START_REF] Tabrizi | Generation of nanoparticles by spark discharge[END_REF], m & is equated to the rate of electrode mass loss. For the microwave (mw) torch used by [START_REF] Dato | Substrate-free gasphase synthesis of graphene sheets[END_REF], m & = 2 mg/min is the amount of solid carbon material collected, which is assumed here to be entirely made up of nanomaterial.

From Table 1, we see that no single plasma source emerges as the most efficient, with energy costs ranging from 1 to 600 keV per atom. Sparks, microplasmas, and mw torch sources are all capable of reaching the lowest energy cost of about 1 keV/atom. It is also not clear that we should expect similar performance from the same source, as evidenced by the wide range of energy costs for mw torches and particularly for microplasmas. A closer look at the basic properties of each plasma source is required.

Table 2 shows the electron temperature (T e ) and the gas temperature (T g ) of some of the plasma sources presented in Table 1, when such information is provided or can be estimated from studies with similar experimental conditions. The estimates of the temperatures for the microplasmas of [START_REF] Shimizu | Reactive evaporation of metal wire and microdeposition of metal oxide using atmospheric pressure reactive microplasma jet[END_REF][START_REF] Mariotti | Monoclinic beta-MoO3 nanosheets produced by atmospheric microplasma: application to lithium-ion batteries[END_REF] are based on parametric studies of essentially the same plasma source by [START_REF] Mariotti | MECHANISM OF SPARK BREAKDOWN IN AIR AT ATMOSPHERIC-PRESSURE BETWEEN A POSITIVE POINT AND A PLANE .1. EXPERIMENTAL -NATURE OF STREAMER TRACK[END_REF][START_REF] Mariotti | Nonequilibrium and effect of gas mixtures in an atmospheric microplasma[END_REF][START_REF] Stauss | High rate deposition of ZnO thin films by a small-scale inductively coupled argon plasma generated in open air[END_REF]. Similar experimental conditions to the spark of [START_REF] Tabrizi | Generation of nanoparticles by spark discharge[END_REF]) can be found in [START_REF] Reinmann | Temporal investigation of a fast spark discharge in chemically inert gases[END_REF][START_REF] Akishev | Development of a spark sustained by charging the stray capacitance of the external circuit in atmospheric-pressure nitrogen[END_REF], who simulate T g for sparks in atmospheric pressure N 2 . Given the planar electrode geometry and applied voltage used by [START_REF] Tabrizi | Generation of nanoparticles by spark discharge[END_REF], the reduced electric field is estimated to be about 120 Td, where 1 Td = 3 V/cm/torr. Using BOLSIG+, a code for the numerical solution of the Boltzmann equation for electrons in weakly ionized gases [START_REF] Hagelaar | Solving the Boltzmann equation to obtain electron transport coefficients and rate coefficients for fluid models[END_REF], it can be calculated that the mean electron energy for such a reduced electric field in argon is 7.16 eV, which we associate here with T e ~80000 K. [START_REF] Chen | Impact of aerosol particles on the structure of an atmospheric pressure microwave plasma afterglow[END_REF] measure T g and T e for the same mw torch used in [START_REF] Chen | Plasma torch production of macroscopic carbon nanotube structures[END_REF]. For the dc arc of [START_REF] Chen | A simple and versatile mini-arc plasma source for nanocrystal synthesis[END_REF], we refer to [START_REF] Reiche | Diagnostics of discharge modes of a free-burning low-current argon arc[END_REF], who measure the temperatures of a similar dc arc in argon at 1 atm. From Table 2, we see that there is no correlation between gas temperature and energy cost. High energy costs of about 50 keV/atom are obtained at both T g = 2000 K and 8000 K. Low energy costs of about 1 keV/atom are obtained at both T g = 1000 K and 10000 K. On the other hand, the energy cost decreases with increasing electron temperature. The lowest costs are obtained when T e = 50000 K and 80000 K, which is significant if we consider electron impact processes in argon, the carrier gas for all of the experiments discussed above. Figure 1(a) shows the fractional power loss of electron energy to elastic, excitation, and ionization in argon, calculated using BOLSIG+. Processes useful for nanomaterials synthesis such as excitation and ionization consume over 50% of the energy if T e > 25000 K, and atom decreases accordingly, as shown in Figure 1 2 as a function of the fractional power consumption of electron impact excitation.

Thus, it appears that increasing the electron temperature decreases the energy cost per atom incorporated into the nanomaterial, thus improving nanofabrication efficiency. For mw torches, this is accomplished by increasing the power or introducing aerosol particles into the input gas flow, as demonstrated by [START_REF] Chen | Impact of aerosol particles on the structure of an atmospheric pressure microwave plasma afterglow[END_REF], who obtained up to T e = 55000 K. Decreasing the dimensions of microplasmas simultaneously decreases T g and increases T e , as shown by [START_REF] Mariotti | Nonequilibrium and effect of gas mixtures in an atmospheric microplasma[END_REF], who measured up to T e = 14 eV. For the spark discharge, T e has been simulated to initiate at high values of ~10 eV before decreasing rapidly down to ~1 eV [START_REF] Simek | Excitation of N-2(C (3)Pi(u)) and NO(A (2)Sigma(+)) states in a pulsed positive corona discharge in N-2, N-2-O-2 and N-2-NO mixtures[END_REF][START_REF] Iza | From Submicrosecond-to Nano second-Pulsed Atmospheric-Pressure Plasmas[END_REF], as shown in Figure 2. In the next section, spark discharges with potentially higher average electron energies will be discussed.

Special Cluster Issue on Perspectives and Challenges in Plasma Nanoscience in J. Phys. D (After review)

Efficient nanofabrication using nanosecond discharges

Table 2 demonstrated that spark discharges have among the lowest energy costs and are therefore already among the most efficient plasma sources for nanofabrication, despite the fact that the electron temperature decreases significantly during spark formation. It follows that atom can be decreased further by generating electrons efficiently while T e is high and then switching off the applied field to avoid inefficiency when T e is low. In other words, we should shorten the spark duration. Decreasing the duration of pulsed discharges has been demonstrated to increase the average electron temperature [START_REF] Iza | From Submicrosecond-to Nano second-Pulsed Atmospheric-Pressure Plasmas[END_REF]). The spark from [START_REF] Tabrizi | Generation of nanoparticles by spark discharge[END_REF] shown in Table 1 is ~1 µs in duration, and therefore it follows that nanosecond-duration discharges have the potential to improve the benchmark for nanofabrication efficiency. In addition, the repetition frequency of such discharges should be made high to achieve high growth rates and to accumulate reactive species over many cycles. We will now present several types of highly repetitive nanosecond discharges that are particularly promising for nanomaterials synthesis.

Transient spark discharges

By properly designing a dc discharge circuit, "transient spark" discharges of 100-ns duration can be generated at repetition frequencies of several kHz [START_REF] Machala | DC discharges in atmospheric air and their transitions[END_REF]. The physics of transient sparks has been well studied (Marode 1975;[START_REF] Machala | Emission spectroscopy of atmospheric pressure plasmas for bio-medical and environmental applications[END_REF][START_REF] Marode | Physics and applications of atmospheric non-thermal air plasma with reference to environment[END_REF], which could facilitate the optimization of their application to nanomaterials synthesis. One engineering advantage of the transient spark for nanofabrication is the simplicity of the generator, as it only requires a dc power supply and passive circuit elements.

Nanosecond repetitively pulsed (NRP) glow discharges

Nanosecond repetitively pulsed (NRP) discharges are generated through the application of high-voltage nanosecond-duration (~10 ns) pulses at high pulse repetition frequencies (PRF) of about 10-100 kHz and were originally conceived to generate glow discharges in atmospheric pressure air at low volumetric power [START_REF] Nagulapally | 31 st AIAA Plasmadynamics and Lasers Conference[END_REF][START_REF] Kruger | Nonequilibrium discharges in air and nitrogen plasmas at atmospheric pressure[END_REF]Packan 2003). The NRP glow discharge is capable of high ionization efficiency and control of the glow-to-arc transition without ballast resistors or dielectric barriers, thus motivating a number of investigations of its basic properties [START_REF] Starikovskaia | Pulsed breakdown at high overvoltage: development, propagation and energy branching[END_REF][START_REF] Stark | Electron heating in atmospheric pressure glow discharges[END_REF][START_REF] Macheret | Modeling of air plasma generation by repetitive high-voltage nanosecond pulses[END_REF][START_REF] Walsh | Submicrosecond pulsed atmospheric glow discharges sustained without dielectric barriers at kilohertz frequencies[END_REF][START_REF] Adamovich | Energy coupling to the plasma in repetitive nanosecond pulse discharges[END_REF]; Special Cluster Issue on Perspectives and Challenges in Plasma Nanoscience in J. Phys. D (After review) [START_REF] Pai | Nanosecond repetitively pulsed discharges in air at atmospheric pressure -the glow regime[END_REF][START_REF] Bourdon | Influence of the pre-ionization background and simulation of the optical emission of a streamer discharge in preheated air at atmospheric pressure between two point electrodes[END_REF]. NRP glows can exist over a wide range of conditions (Pai et al. 2010b) and may be particularly useful for nanomaterials that must be synthesized at low gas temperature.

Nanosecond repetitively pulsed (NRP) spark discharges

The NRP spark discharge is about one order of magnitude more energetic than the NRP glow and can be generated over a wider range of conditions [START_REF] Shao | An experimental investigation of repetitive nanosecond-pulse breakdown in air[END_REF][START_REF] Naidis | Simulation of spark discharges in high-pressure air sustained by repetitive high-voltage nanosecond pulses[END_REF]Pai et al. 2010a). NRP spark discharges in atmospheric pressure air generate high densities of active species such as electronically excited N 2 and atomic oxygen, as summarized in Table 3. In particular, up to 50% of O 2 in air is dissociated into atomic oxygen. Furthermore, the lifetime of atomic oxygen is quite long, such that its minimum density is 2×10 17 cm -3 [START_REF] Stancu | Investigations of the rapid plasma chemistry induced by nanosecond discharges in atmospheric pressure air using advanced optical diagnostics[END_REF]Stancu et al. 2010b). The controlled production of atomic oxygen in NRP sparks in air may be useful for tailoring metal oxide nanostructures [START_REF] Cvelbar | Reactive oxygen plasma-enabled synthesis of nanostructured CdO: tailoring nanostructures through plasmasurface interactions[END_REF].

Metastable nanomaterials using fast heating and cooling in nanosecond spark discharges

Nanosecond spark discharges in air at atmospheric pressure produce sharp thermal spikes, and it is interesting to consider how this property could be exploited, considering the role of thermal processes in plasma nanofabrication [START_REF] Teo | The significance of plasma heating in carbon nanotube and nanofiber growth[END_REF][START_REF] Denysenko | Plasma heating effects in catalyzed growth of carbon nanofibres[END_REF][START_REF] Mangolini | Selective nanoparticle heating: Another form of nonequilibrium in dusty plasmas[END_REF][START_REF] Wolter | Hydrogen in plasmananofabrication: Selective control of nanostructure heating and passivation[END_REF].

Generally speaking, non-equilibrium thermodynamic conditions favor the formation of non-equilibrium material states. For example, a UHF microplasma very similar to that of [START_REF] Mariotti | Monoclinic beta-MoO3 nanosheets produced by atmospheric microplasma: application to lithium-ion batteries[END_REF] has also been used to generate carbon connections between Ag nanoparticles [START_REF] Levchenko | The production of self-organized carbon connections between Ag nanoparticles using atmospheric microplasma synthesis[END_REF]. Thermal spikes may facilitate the formation of nanomaterials in metastable material phases [START_REF] Komatsu | New type of BN nanoparticles and films prepared by synergetic deposition processes using laser and plasma: the nanostructures, properties and growth mechanisms[END_REF]. Indeed, carbon-encapsulated metal nanoparticles have been generated recently using spark discharges of millisecond duration [START_REF] Byeon | Production of carbonaceous nanostructures from a silver-carbon ambient spark[END_REF].

Heating at nanosecond time scales or shorter is typically achieved by the use of lasers [START_REF] Lorazo | Thermodynamic pathways to melting, ablation, and solidification in absorbing solids under pulsed laser irradiation[END_REF] and has already been used to synthesize nanomaterials [START_REF] Geohegan | Time-resolved imaging of gas phase nanoparticle synthesis by laser ablation[END_REF]). However, it would not be as practical to scale focused lasers to large areas or volumes for industrial applications as it would be for nanosecond discharges arranged in arrays. Here, we discuss the ultrafast heating and cooling of NRP spark discharges as an example of what may be possible under the broader category of nanosecond discharges.

Gas heating rates of up to 10 11 K/s

Figure 3 shows time-resolved measurements of the gas temperature for an NRP spark discharge in air at atmospheric pressure preheated to 1000 K, taken from measurements presented in (Pai et al. 2010a). In only about 10 ns, the temperature rises from a baseline Special Cluster Issue on Perspectives and Challenges in Plasma Nanoscience in J. Phys. D (After review) temperature of 1500 K up to 3500 K. The gas heating rate of 10 11 K/s is the fastest that has been reported for electrical discharges, to the author's best knowledge. Although only the heating of the gas by NRP sparks has been studied up to now, we can estimate the corresponding heating rate of a nanoparticle, based on a model of heat transfer between a single-wall carbon nanotube (SWCNT) and the surrounding gas [START_REF] Louchev | Thermal physics in carbon nanotube growth kinetics[END_REF]): 1) ( )

g dT Mc Ah T T dt = -
where h is the heat transfer coefficient, M is the mass of the nanotube, c is its heat capacity, T is its temperature, A is its surface area, and T g is the gas temperature. Let us consider that M=mN, where m is the mass of each carbon atom, and N is the number of carbon atoms in the nanotube. Likewise, A=aN, where a is the specific area of each atom in the tube wall. Then the heating rate of the nanotube can be expressed as:

2) ( )

g dT ah T T dt mC = -
For heat transfer between the ambient gas and a SWCNT, [START_REF] Louchev | Thermal physics in carbon nanotube growth kinetics[END_REF]) evaluated the heat transfer coefficient:

3) 3 2 B h Qk α =
where Q=p/(2πmk B T g ) 1/2 is the flux of particles colliding on the SWCNT (p is the gas pressure), k B is Boltzmann's constant, and α is the accommodation factor for correctly determining collisional energy transfer. [START_REF] Louchev | Thermal physics in carbon nanotube growth kinetics[END_REF]) use α = 1/3 to fit their theoretical calculations with molecular dynamics simulations, and the same is done here.

Assuming m=12 g/mol for carbon, p = 1 atm, and α = 1/3, we obtain h = 7000 W/m 2 /K from Equation (3). The heat capacity of a SWCNT at 300 K is 600 J/kg/K [START_REF] Hone | Thermal properties of carbon nanotubes and nanotube-based materials[END_REF], and given that C ∝ T [START_REF] Benedict | Heat capacity of carbon nanotubes[END_REF], we take C = 3000 J/kg/K at T = 1500 K. Thus, Special Cluster Issue on Perspectives and Challenges in Plasma Nanoscience in J. Phys. D (After review) assuming a = 2.62×10 -20 m 2 [START_REF] Louchev | Thermal physics in carbon nanotube growth kinetics[END_REF]), m = 12 g/mol for carbon, T = 1500 K, and T g = 2500 K, Equation (2) yields dT/dt = 3×10 9 K/s. Thus, a nanoparticle could be heated at a rate up to ~10 9 K/s due to kinetic energy transfer from the gas, which itself is heated by the nanosecond spark discharge. It should be noted that this estimate neglects surface processes such as recombination, chemical reactions, adsorption, desorption, and ion bombardment that also contribute to the heat flux [START_REF] Denysenko | Plasma heating effects in catalyzed growth of carbon nanofibres[END_REF][START_REF] Mangolini | Selective nanoparticle heating: Another form of nonequilibrium in dusty plasmas[END_REF]).

Gas cooling rates of at least 10 7 K/s

The minimum cooling rate of NRP spark discharges in air at atmospheric pressure can be deduced from Figure 3 to be at least 10 7 K/s, based on the fact that the gas temperature returns to 1500 K after a period of 1/PRF = 33 µs. [START_REF] Grisch | Laser-based measurements of gas-phase chemistry in non-equilibrium pulsed nanosecond discharges[END_REF] measure 500 K at ~100 ns after the pulse for a nanosecond discharge under different conditions but at about the same energy per pulse as (Pai et al. 2010a), indicating that cooling could even occur at the 100-ns timescale.

In their work on carbon-encapsulated metal nanoparticles, [START_REF] Byeon | Production of carbonaceous nanostructures from a silver-carbon ambient spark[END_REF]) report that cooling rates of less than 1400 K/s lead to spheroidization of the nanoparticles, whereas a cooling rate of 2900 K/s causes tube-like graphitization. NRP spark discharges cool at a rate that is least four orders of magnitude faster than the millisecond sparks of [START_REF] Byeon | Production of carbonaceous nanostructures from a silver-carbon ambient spark[END_REF], which may lead to the formation of nanomaterials much further from the equilibrium state of the material than those generated using spark discharges of longer duration.

Outlook

In conclusion, nanosecond discharges at atmospheric pressure possess several basic properties that could be very useful for nanofabrication: high electron temperature, and fast gas heating and cooling rates. These characteristics could be exploited for highly efficient synthesis and for the production of metastable nanomaterials. Among atmospheric pressure plasmas, microplasmas and nanosecond discharges are capable of generating the highest electron temperatures and could complement each other to provide maximum design flexibility for efficient nanofabrication. For example, nanosecond discharges can be employed when confined geometries are disadvantageous.

Finally, it is important to point out that the potential applications for nanosecond discharges in materials engineering are not limited to nanomaterials. The low energy cost per atom for synthesis may be generally true, because high electron temperature is a fundamental characteristic of these discharges. Indeed, nanosecond DBDs have already been used to improve the hydrophilic properties of polymer thin films at relatively high efficiency [START_REF] Walsh | 10 ns pulsed atmospheric air plasma for uniform treatment of polymeric surfaces[END_REF][START_REF] Zhang | Surface Treatment of Polyethylene Terephthalate Films Using DBD Excited by Repetitive Unipolar Nanosecond Pulses in Air at Atmospheric Pressure[END_REF]. Also, the crystallization of amorphous Si thin films using plasma-induced thermal annealing at the millisecond time scale has been developed as a practical alternative to laser thermal annealing, albeit at much slower rates of heat transfer (Higashi et al. 2006). However, as mentioned previously, NRP spark discharges are capable of heating and cooling on much shorter time scales that may be competitive with some laser annealing techniques.

  Figure 1: (a) Fractional power transfer into different electron impact processes in argon as a function of the electron temperature. (b) The energy cost per atom for nanofabrication of the plasma sources from Table2as a function of the fractional power consumption of electron impact excitation.

Figure 2 :

 2 Figure 2: Typical temporal evolution of the electron temperature for a pulsed nanosecond discharge. Reprinted with permission from (Iza et al. 2009). According to the caption of the corresponding figure from (Iza et al. 2009): (1) Avalanche of seed electrons that remain from the previous pulse. (2) Loss of energetic electrons to the anode. (3) Quasi-neutral plasma formation, confinement of low energy electrons. (4) New avalanches initiated by secondary electrons. (5) Pulse end.

Figure 3 :

 3 Figure 3: Gas temperature as a function of time for an NRP spark discharge in air at atmospheric pressure preheated to 1000 K, inferred from measured emission spectra of the N 2 (C-B) 0-0 band. Taken from measurements presented in (Pai et al. 2010a).

Table 1 : Energy cost of incorporating each atom into the nanostructure /

 1

	ε	atom	=	P m & using different

plasma sources at atmospheric pressure, where P is the power coupled into the plasma and m & is the mass production rate. In all cases, argon at 1 atm is the carrier gas, and no external substrate heating is used. Quantities marked by (*) have been calculated here based on data given in the listed references.

	Source	Nanomaterial	P [W]	Max. m & [µg/s]	Min. atom [eV]	References
	microplasma	Si nanocrystals	35	0.017	600000*	(Nozaki et al. 2007; Mariotti et al. 2010)
	microplasma	MoO3 nanosheets	31	1*	42000*	(Mariotti et al. 2008)
	microplasma	WO3 nanoparticles 20	35*	1400*	(Shimizu et al. 2006)
	spark	Au nanoparticles	1.5*	2*	1500*	(Tabrizi et al. 2009)
	mw torch	Graphene	250	33	1000*	(Dato et al. 2008)
	mw torch	Carbon nanotubes 400	7	7200*	(Chen et al. 2003)
	dc arc	Ag nanoparticles	120*	3	50000*

[START_REF] Chen | A simple and versatile mini-arc plasma source for nanocrystal synthesis[END_REF]

Special Cluster Issue on Perspectives and Challenges in Plasma Nanoscience in J.

Phys. D (After review)

Table 2 : Gas temperature (T g ) and electron temperature (T e ) of some of the plasma sources presented in Table 1. Quantities marked by (*) have been calculated here based on data given in the listed references.

 2

	Source	Min. atom [eV]	Max. Tg [K]	Max. Te [K] References
	microplasma	42000*	~2000*	~10000*	(Mariotti et al. 2007; Mariotti 2008; Mariotti et al. 2008; Stauss et
					al. 2010)
	microplasma	1400*	~1000	~50000*	(Shimizu et al. 2006; Mariotti et al. 2007; Mariotti 2008; Stauss et
					al. 2010)
	spark	1500*	~10000	~80000*	(Reinmann et al. 1997; Akishev et al. 2007; Tabrizi et al. 2009)
	mw torch	7200*	3000	20000	(Chen et al. 2002; Chen et al. 2003)
	dc arc	50000*	8000	12000	(Reiche et al. 2001; Chen et al. 2007)

Table 3 : Measured densities and lifetimes of active species in NRP spark discharges in air at atmospheric pressure. Quantities marked by (*) have been calculated here based on data given in the listed references.

 3

	Species	Carrier	Max. density [cm -3 ] 1/e lifetime [ns]	Max. fraction	Reference
		gas			of N 2 or O 2
	N 2 (A)	N 2	2×10 15	600	0.0004*	(Stancu et al. 2010a)
	N 2 (A)	Air	5×10 14	<100	0.0001*	(Stancu et al. 2010a)
	N 2 (B)	Air	5×10 16	5	0.01*	(Stancu et al. 2010b)
	N 2 (C)	Air	6.1×10 15	2	0.002*	(Stancu et al. 2010b)
	O	Air	10 18	25000	0.5	(Stancu et al. 2009; Stancu et
						al. 2010b)

Acknowledgements

The author thanks the following sources of funding support: the Agence Nationale de la Recherche IPER (grant ANR-05-BLAN-0090-01) and PREPA (grant ANR-09-BLAN-0043-03) projects, the Ministère de l'Enseignement Supérieur et de la Recherche Chaire d'Excellence program, and CSIRO Materials Science and Engineering at Lindfield, NSW, Australia. Also, the author thanks Professor Christophe O. Laux and Dr. Deanna A. Lacoste Special Cluster Issue on Perspectives and Challenges in Plasma Nanoscience in J. Phys. D (After review) of Ecole Centrale Paris, as well as Professor Kostya (Ken) Ostrikov and Dr. Shailesh Kumar of CSIRO for their support and useful discussions.