Tuning surface wettability through hot-carrier initiated impact

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Surfaces, Interfaces, and Applications

Tuning surface wettability through hot-carrier initiated impact ionization in cold plasma Siwapon Srisonphan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19495 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Applied Materials & Interfaces

Tuning surface wettability through hot-carrier initiated impact ionization in cold plasma Siwapon Srisonphan* Department of Electrical Engineering, Faculty of Engineering, Kasetsart University 50 Ngam Wong Wan Rd, Ladyao, Chatuchak Bangkok, Thailand *email: [email protected]

ABSTRACT

Advanced surface engineering aims to produce surfaces with well-controlled wettabilities; however, precise control over water imbibition and liquid spreading on patterned surfaces remains a challenge. Nonthermal atmospheric plasma (NAP) treatment can dramatically change wettability; however, for coated biological objects, such as seeds, plasma interaction is not entirely understood. Herein, we employed atmospheric hybrid cold plasma (HCP) to elucidate how NAP fundamentally interacts with seed surfaces. We show that nonthermal atmospheric plasma can control water imbibition (WI) and liquid spreading on seeds. By investigating two distinct seed surface structures and their permeabilities, we show that the modified-surface properties are primarily due to the combined effects of enhanced physical etching and chemical functionalization. We propose the tunable surface functionalization model based on electric field assisted electron-ions initiated impact ionization enhancing the reactive species (RS) generation. Importantly, rice seeds are not damaged by plasma treatment, and 90% of treated seeds germinate upon artificial ageing. The ability to control the wettability and liquid spreading of seed surfaces can help achieve seedlings of better quality, especially in difficult-to-grow regions, 1 ACS Paragon Plus Environment

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including those affected by drought. Well-control wettability and related attributes opens up new avenues for the nonthermal atmospheric plasma treatment of a broad range of surfaces.

KEYWORDS Nanoscale interface engineering; Surface functionalization; Water imbibition; Nonthermal atmospheric plasma; Surface wettability

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INTRODUCTION Nanoscale surface modification is common in nature and frequently determines the unique properties of a material.1–3 Therefore, surface engineering has received significant interest and has become very important in a broad range of applications.2–5 One of the most important properties of a material is its surface wettability, a consequence of the interfacial characteristics of the solid and liquid, and some natural topographical surface features, such as pillars and grooves, impart unique wetting properties.1–3,6 Inspired by nature, micro/nanostructures and nanomaterials have been introduced in order to manage interfacial wetting behavior.1–3,7 For biological and medical applications, the ability to engineer the wettability of a tissue surface is crucial, and well-defined techniques that provide significant control over liquid spreading and absorption rate, without damaging the bulk properties of the material, are required.8–11 The surface functionalization of plant seeds is considered to be important in biology, but seeds are even more complex than typical inorganic materials because they are organic and are often covered entirely with relatively thick cuticles or seed coatings.1,12 Several techniques, including conventional wet-chemical treatments, have been used in an attempt to modify seed surfaces to obtain higher yields and seedlings of better quality, especially in difficult-to-grow regions, including those suffering from drought, and for mature seeds;13–15 however, some methods leave toxic and other chemical residues on the plants and the surrounding environment, eventually inhibiting root development and seedling elongation.14–16 The ability to generate a gas discharge plasma without the need for elevated temperatures is a new capability in nanoscale interface engineering that has recently been brought about through the development of nonthermal (cold) atmospheric plasma (NAP) technologies; NAP



can selectively modify surfaces and sterilize heat sensitive materials, and is ideally suited to biological, medical, food, and agricultural applications.8–11 NAP has many components, including an electric field (E-field), ultraviolet and visible radiation, free electrons and ions, and reactive species (RS), that facilitate multi-interactional processes, including physical ion-enhanced etching, electric-field stressing, and chemical functionalization of a target surface, leading to the transformation of its morphology and, hence, its wetting properties.4,14,17 Our recent work, and those of others, have demonstrated that NAP can be used for bacterial and fungi inactivation to enhance seed quality.10,13,14,18,19 Several reports also shows that cold plasma treatment can modify the wetting properties of the seeds surfaces such as lentils, beans and wheat, leading to the dramatic decrease in the apparent contact angle.13,18,20,21 However, the details of how plasma components interact with natural coated surfaces and its effect on biological materials remain unclear. The liquid permeability of a seed is more complicated compared to those of plant cell walls and mammalian cell membranes due to the presence of a relatively thick cuticle or coating layer that surrounds the entire seed. Moreover, NAP-induced nanoscale reaction processes that occur at interfacial regions are different to those in the bulk phase. Herein we employed NAP-mediated nanoscale surface modification to enhance the wettability of two distinct seed surface structures, those of rice (Oryza sativa L.) and sunflower seeds (Helianthus annuus). We investigated the underlying mechanism of how the plasma interacts with the seeds and reveal the dynamics of water imbibition (WI) enhancement and its limitations. To the best of my knowledge, there are no reports on the dynamic in liquid-penetration (imbibition) characteristics following plasma treatment; such plasma-induced effects could be important, especially in biomedical applications where overexposure to highly reactive components may have adverse effects on biological 4 ACS Paragon Plus Environment

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membranes and plant physiology.11,14,22 Therefore, we propose the tunable surface functionalization model based on electric field assisted hot carriers which is including highly energetic electrons-ions initiated impact ionization, and consequently enhancing the reactive species (RS) generation. We also demonstrate the influence of NAP on seed vigor through accelerated aging tests (AATs) in order to reveal the potential use of plasma on particular processes in horticulture, from seed storage to planting. In addition, it is possible that the wettability of the modified surface is unstable and may recover after a period of time due to the adsorption of airborne molecules. Accordingly, we identified the level and nature of surface restoration. Hence, understanding the integrated interactions of NAP on coated surfaces may impact a wide range of applications, including biological sensors, self-cleaning surfaces, DNA manipulation, and wound healing. RESULTS AND DISCUSSION We employed atmospheric hybrid cold plasma (HCP) to elucidate how NAP fundamentally interacts with seed surfaces. Fig. 1a shows the overall design of the HCP system that exploits a microcoronal discharge in conjunction with a dielectric-barrier discharge (DBD) (see Methods section).10,23–25 As observed in nanoscale vacuum electronic devices,26,27 a highly localized fringe field lowers the potential barrier height and, as a consequence, enhances electron emission from the cathode into the air allowing NAP operation at low power consumption. Figs. 1b and 1c illustrate HCP treatment on rice and sunflower seeds under air partially mixed with Ar (air-Ar), respectively. The measured power consumption is relatively low, at ~1 W/cm2 (corresponding to ~1 W/cm3 of plasma volume). The energetic electrons collide with the background gas, increasing the levels of dissociation, excitation, and ionization, leading to a corona-induced 5 ACS Paragon Plus Environment


streamer-discharge plasma that covers the entire volume. For HCP, however, a DBD configuration is required to prevent arc formation and to stabilize the cold plasma. In this work, air-Ar was used as the primary working gas in order to obtain a uniform plasma, and to control the generation of reactive oxygen and nitrogen species (RONS).10,17

Figure 1. The overall structure of the hybrid cold plasma (HCP) system. (a) The corona induced streamer-discharge plasma. The seeds were directly exposed to the discharge and showered with ions, electrons, and free radicals. (b) A sunflower and (c) rice seeds under air-Ar HCP treatment (scale bar: 7 mm). (d) The E-field contribution to the HCP structure under positive streamer discharge simulated via the finite element method (FEM); units are V/cm. (e) The actual temperatures of the rice seeds during HCP treatment.

The E-field distribution of the HCP configuration exhibits density near the edges and a gradient to the dielectric layer (Fig. 1d), resulting in a flow of generated ions in the ionization zone to the opposite electrode where the field is relatively weak. The streamer plasma is governed by the bipolar current of the positive streamer corona, and the E-field-dispersion direction results from space charge accumulation. The average E-field across the air gap and glass layer are ~104 and ~2 x 103 V/cm (see supporting information S1), whereas the corresponding dielectric field strength are ~3 x 104 and ~106 V/cm, respectively,10,23,26 which 6 ACS Paragon Plus Environment

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allows a stable corona discharge to form under ambient conditions. The actual temperatures of samples during plasma operation were ~25 °C whereas the base ambient temperature was ~24 °C; this ambient temperature remained constant even over ~20 min of treatment, which is promising for work involving heat-sensitive materials (Fig. 1e). Note that direct exposure to the pure corona discharge (without the DBD structure) should be avoided since the discharge can form an arc that would thermally damage the biological membrane (see supporting information S2). In addition, the DBD structure also confines the NAP to the area under the sample, thereby facilitating further plasma interactions on the sample surface. We have shown that the HCP is operational using atmospheric air or with other gas mixtures; it has the capability to be translated into practical large-volume industrial applications. We exposed rice and sunflower seeds to HCP over several treatment times, after which water droplets were placed on treated and untreated seeds in order to investigate NAP-enhanced waterseed surface activation and its correlation with surface wetting properties (the apparent contact angle (ACA)) and the water imbibition (WI)). We employed ~1 µl and ~2 µl droplets of water for rice and sunflower seeds, respectively, due to their different available flat surface areas. The average ACA of the untreated rice and sunflower seeds were observed to be ~100° and ~120°, respectively (Figs. 2c-top and 2f-top), consistent with highly hydrophobic surfaces. Under ambient conditions, the rice seeds required about ~20 min to completely absorb the 1 µl water droplet (Fig. 1b, black box), while the sunflower seed took about ~40 min to completely absorb the 2 µl water droplet (Fig. 1e, black box). Immediately after exposure to NAP, for example, after ~30 s of treatment, the water droplet was entirely dispersed over the rice seed surface (the ACA dropped to zero), indicating the complete transformation of the (previously) hydrophobic surface into a completely wet ones (Fig. 2c-bottom). Similarly, the average ACA of the treated 7 ACS Paragon Plus Environment


sunflower seed, which is naturally surrounded by a relatively thick waxy textured surface,28 was reduced to ~35° after ~1 min of treatment (Fig. 2f-bottom).

Figure 2. Enhanced surface wettabilities of rice and sunflower seeds. Water imbibition times, WI(time) as functions of plasma treatment time t(treat) of (a) rice seeds and (d) sunflower seeds (not presented on linear scales). The WI(time) of (b) rice seeds and (e) sunflower seeds under given treatment conditions (untreated, treated under air, and treated under air-Ar). The box-andwhisker plots with Tukey test error bars represents the interval about the means value (from MinMax) of WI(time) of seeds at given plasma treatment conditions. The average apparent contact angle (ACA) of a non-treated (top) and ~30 s treated (c) rice seed (scale bar: 5 mm) and (f) sunflower seed (scale bar: 7 mm).

Although rice and sunflower seeds have significantly different seed surface structures and seed coat permeabilities28, the surface wettabilities of both types of seed were similarly enhanced after exposure to HCP (Fig. 2a). The average water imbibition time (WI(time)) of both seeds initially reduced exponentially with increasing treatment time (t(treat)); WI(time) α exp(-kt(treat)), and then gradually reaching WI saturation (WI(sat)); WI(time) ≈ WI(sat), at treatment time equal to threshold treatment time (t(threshold)), which is defined by the point at which only a slight change in WI(time) is 8 ACS Paragon Plus Environment

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observed with increasing plasma treatment time (dashed lines in Figs. 2a and 2d). For example, under the given plasma conditions, the WI(sat) of rice is ~2 min and the t(threshold) is ~1 min (Fig. 2a-threshold point). Similarly, the WI(time) of the sunflower seeds exponentially declined to ~18 min after ~1 min of treatment, and then gradually tapered to WI(sat) ~5 min and t(threshold) ~5 min (Fig. 2d-threshold point). The average ACA of the treated rice seed was initially ~15° after treatment for ~15 s; however, no ACA was observed after continuous exposure to NAP for 30 s. For sunflower seeds, the average ACA was ~15° after ~1 min of treatment. In a similar manner, a superhydrophilic state (ACA ~0°) appeared after ~2 min of treatment. The mutual relationship between ACA and the WI(time) was initially strong, but weakened after ACA reached zero (see supporting information S3). These results demonstrate that not only does NAP increase the surface energies of the rice seeds, it also promotes water uptake by the seeds. Biological surfaces with higher surface energies (lower ACAs) after plasma treatment are prone to higher rates of WI. However, we observed a constant WI rate even at longer exposure times. The two distinctly different surface structures of the seed coatings also show similar wettability enhancements, consistent with a common surface-modification mechanism. With heat, ultraviolet/visible light, other electromagnetic radiation, ions/electrons, and reactive species (RS) (Fig. 3c), plasma cocktails have multiple biological, physical, and chemical effects on seeds. Some surface modifications may involve only a single mechanism, while others may require combinations of mechanisms to effectively functionalize the surface. Consequently, the possible mechanisms responsible for the surface transformations need to be further explored. HCP was performed on both seed types under air to determine the primary factors involved in seed-surface modification. It should be noted that using the same configuration, the E-field stresses of air and air-Ar in the HCP are approximately the same. The average WI(time) of pure-air9 ACS Paragon Plus Environment


treated rice and sunflower seeds was ~7 min and ~17 min after ~1 min and ~5 min of treatment, respectively (blue boxes in Figs. 2b and 2e), with corresponding average ACAs of ~12° and ~30°, respectively (see supporting information S3). We performed optical emission spectroscopy (OES) to investigate the chemical elements and reactive species generated under pure air and airAr plasma, the results of which are shown in green and red, respectively, in Fig. 3b (see Methods). The excited molecules observed in air mostly correspond to the first and second excited states of nitrogen (see supporting information S4).17 Moreover, reactive oxygen species (ROS) and reactive nitrogen species (RNS), such as superoxide (O2–•), hydrogen peroxide (H2O2), hydroxyl radical (•OH), singlet oxygen (1O2), ozone (O3), nitric oxide (•NO), nitrogen dioxide (•NO2), and peroxynitrite (ONOO−) are produced.8,11,17,29–31 It is known that RONS play several significant roles in biochemical interactions, including oxidation/reduction (redox) processes, especially in aerobic and plant biology.9,11,17,30 Interestingly, under air-Ar, the levels of excited and radiant species, including RONS are significantly amplified through interactions with excited Ar molecules (red line, Fig. 3a-red). As a consequence, the surface wettability is less modified in air plasma than air-Ar plasma due to the lower levels of RONS generated; these reactive species are therefore effective components that determine the level of surface modification and, hence, wetting properties (Fig. 3c).


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Figure 3. Surface modification mechanisms and models. (a) Optical emission spectrum of air and air-Ar HCP used to shower the seed surface. The air-Ar-derived plasma generates significantly more RONS in this system, resulting in faster surface transformation. (b) Modeling WI(time) as a function of t(treat) at electrode distances of ~5 and ~7 cm. (c) Interactions between the fundamental components of NAP. The circle dimensions correspond to levels of interaction.

To relate the WI(time) model to surface functionalization processes, another experiment was conducted in which the distance between the two electrodes was reduced from ~7 to ~5 cm, to significantly amplify the levels of reactive species (RS) and other plasma components. The results show that WI(time) was abruptly reduced to ~100 s after only ~5 s of treatment, while WI(sat) was only slightly reduced to ~60 s and the t(threshold) was ~30 s. The shorter exposure time and corresponding higher surface activation is attributed to the substantially higher amounts of 11 ACS Paragon Plus Environment


incident ions/electrons, and RONS. The photoemission spectra show that oxygen-containing functional groups, such as hydroxyl, carbonyl, and carboxyl group, rose significantly upon bombardment of the surface, strongly suggestive of microscopic oxidation on the surface. The higher the level of grafted hydroxyl groups on the seed surface, the higher the resulting degree of wetting. However, these chemical functionalization processes may not be solely responsible for modifying the wettability of the coated seed surface. Figs. 4a and 4b show scanning electron microscopy (SEM) images of the rice seed surface before and after 1 min of treatment, respectively. Fig. 4c shows the intrinsic sunflower seed surface with hundreds of natural nanometer-scale waxy surface films before treatment, while Fig. 4d is that after ~5 min of treatment. In rice seeds, the microorganisms that lived in the trenches (inset, Fig. 4a) were eliminated as a result of the treatment, and the surface became slightly rougher (inset Fig. 4b). This sterilization process resulted in clean surfaces that were devoid of pathogens prior to planting and the resulting seedlings were clean and devoid of any pathogen infestation when compared to those grown from untreated seeds.10,14 On the other hand, the sunflower seed appeared to be much smoother after losing its waxy outer surface (insets Figs. 4c and 4d). The change in surface morphology by physical etching may contribute to higher wettability, as previously reported.14,19,21,32 However, the degree of ACA and WI enhancement due purely to etching was unclear. Therefore, we gently scribed the seed surface to remove any natural wax and to increase its hydrophilic properties. Then, we applied an ~1 µl and ~2 µl water droplet to the scarified rice seed and scarified sunflower seed, respectively, to elucidate which physical etching mechanism enhanced the wetting properties. The average ACA of the scarified rice and sunflower seed surfaces were ~23˚ and ~26˚, respectively, whereas the average water imbibition times were reduced to ~13 and ~20 min, respectively (See supporting information S5). These 12 ACS Paragon Plus Environment

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results show that the removal of the surrounding cuticle, purely through etching, enhances the water imbibition properties of the seed surface, but is not competitive with combined plasmaenhanced molecular etching and surface chemical functionalization. Additionally, the water ACA of the scarified rice seed is similar to the seed treated by air plasma for ~30 s, but the WI time of scarified rice seed is much slower than that of the treated one. The WI time of ~30 s air plasma treatment was ~5 min while the water imbibition time of the scarified rice seed was ~13 min (supporting information S3, point A, red arrow). Therefore, the results imply that by purely removing the surrounding cuticle will enhance the water imbibition properties but it is not as effective as the plasma processes. Although the thinner water layer can evaporate faster, most of the water most likely penetrate into the plasma treated seeds. The overall results indicate that NAP alters the surface energy and modifies the surface properties by simultaneously performing molecular etching and introducing polar groups on the surface, leading to the observed surface transformation (Fig. 3c).



Figure 4. Scanning electron micrographs of treated and untreated seed surfaces. (a) Untreated and (b) treated rice seeds. (c) Untreated and (d) treated sunflower seeds. Seed-borne pathogens on the rice were eliminated after exposure to NAP, and the surface was rougher. The sunflowerseed cuticle was destroyed upon exposure to NAP, and the surface became smoother. (Scale bar: 200 µm; inset scale bar: 50 µm).

The generated plasma components are mainly determined by multiple technical parameters, such as electrode configuration, temporal input power, and plasma ignition conditions. Fig. 3c illustrates the multiple interactions that contribute to surface modification on the nanoscale, and consequently, the associated biological effects. HCP does not release thermal radiation that can damage cell membranes, and UV-vis radiation that primarily governs the excitation and relaxation of nitrogen molecules is not considered to dominate the surface-modification mechanism and the eradication of microorganisms because of their low energies and ultra-thin penetration depths.18,21,33 Conversely, UV and the E-field induced in the HCP play supplementary roles in enhancing the generation of RONS; UV-radiation-induced photochemical transformations increase the generation of these species.11,17,31,33 Similarly, E-field stress does not directly modify the surface but rather supports the generation of other plasma compounds via impact ionization. We employed the principle of hot electron-initiated impact ionization in an 14 ACS Paragon Plus Environment

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insulator under high E-field26,34 to explain the exponential behavior of WI(time) as a function of NAP treatment time (Fig. 3b) through the equation: WI ( time ) ≈ WI max e

−k (

ttreat

tthreshold )

+ WI sat

(1)

where WI(max) is approximately the WI(time) of the untreated seed, k is an empirical plasma constant that reflects the number of free radicals generated and depends on the NAP setup and seed conditions. In this work, k is ~10 and ~5 at electrode separations of ~5 cm and ~7 cm, respectively. Higher k numbers result in higher levels of generated RS and their surface reactions, and the negative exponent indicates that WI(time) reduces exponentially with increasing treatment time due to a higher probability of surface functionalization interactions. The simulation shows that the model estimates WI(time) for a given treatment time (see supporting information for the WI(time) model of a sunflower seed). Carriers such as electrons gain significant kinetic energy from the electric field in the system and subsequently collide with gaseous atoms or molecules; this results in a cascade of elementary processes such as ionization, excitation, attachment, and detachment, leading to additional free electrons, and negatively- and positively-charged molecular ions, including reactive species (RS) (Fig. 3c). The approximate number of electrons over time and in space after electron-initiated impact ionization are given by N e (t ) = N O eν iont and N e ( x) = N O e βx respectively34, where NO is the initial number of free electrons (at t = 0), ν ion is the number of hot carriers or electrons created per second, and β (the ionization coefficient) is a function of the E-field.26,34 Similarly, it is reasonable to anticipate that ions accelerated by the E-field also collide with neutral molecules to form secondary ions and free electrons. Therefore, the number of reactive species (NRS) increases with the number of free electrons generation in a high-



electric-field system, as given by N e ( x) = N O e

β eff x

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, where βeff is the effective ionization

coefficient corresponding to the average number of ionization processes over the distance (x) of all relevant processes. Once highly energetic electrons and ions approach the surface layer of the seed, they not only etch the seed coating by attacking the original bonds, but also provide new chains of chemical bonds at the surface. Consequently, the generated ROS introduce multiple functionalities, such as carboxyl ((C(=O)OH ), carbonyl (C=O), and hydroxyl (-OH) groups, on the surface, resulting in a hydrophilic surface (Fig. 3c). Because the impact ionization process allows the number of electrons to increase exponentially, so do the amounts of secondary particles generated, such as reactive species. Therefore, we can approximate surface functionalization by an exponential function related to radical generation at a given treatment time, whereas the faster WI(time) is described by an inverse exponential function (dashed-lines in Figs. 2b and 2c) as: WI time α ( N RS ) −1 ~ ( N O e

β eff x −1

) .

(2)

Hence, the nanoscale surface modification comprises several mechanisms as illustrated schematically in Fig. 3c. The stronger the E-field, the higher the impaction ionization rate, resulting in larger numbers of electrons, ions, and interactions between these reactive species and the surface, and a higher rate of surface functionalization. As observed for surface polymerization, once the surface is completely functionalized, at approximately the threshold treatment time, no further wettability enhancement is observed as the surface is saturated with polar groups; hence a superhydrophilic surface with a saturated WI rate is achieved. These modification processes apparently occur only on the surface and do not affect the inorganic bulk properties. However, in organic materials, these plasma interactions are likely to be more complex. Although the external RONS, generated through secondary processes, may 16 ACS Paragon Plus Environment

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have different biomedical effects to those created in cells through enzymatic processes or by drugs11,17,30, we believe that these external reactive species play significant roles not only in modifying the surface, but also in inducing biological responses when they penetrate the seed membrane. The highly reactive OH radical and its reaction partner, hydrogen peroxide (H2O2), can cause lipid peroxidation, protein damage, and progressive oxidative stress leading to membrane destruction, depending on the delicate equilibrium between ROS production and their detoxification, resulting in the inactivation of bacteria.11,17,29,30 They also attack unsaturated fatty acids to induce lipid peroxidation, resulting in changes to membrane fluidity and liquidpermeability enhancements. Consequently, RNS can diffuse into membranes resulting in plasmaliquid interactions. Such chemical interactions are very complex, and their importance has been recognized more in recent years.17 Many research reports have suggested that the presence of nitrite (NO2−), nitrate (NO3−), and hydrogen peroxide (H2O2) leads to acidification. These RNS are produced, as evidenced by nitrogen emissions (330–420 nm) and NO γ-bands emissions (200–300 nm) in air-Ar HCP (Fig. 3a). These species are important biologically since they inactivate bacteria, stimulate wound healing in medical application, and enhance plant germination.8,11,17,29 We previously reported that HCP can sterilize and enhance the germination of rice seeds. Rice seeds planted within 24 h after NAP treatment exhibited longer seedling lengths than those untreated, with significantly reduced fungal infestations.10,14 The germination percentage of treated seeds was ~98% whereas that of untreated seeds was ~90%. Other researchers have reported similar germination enhancements of many kinds of plants via NAP.14,18,19,21 However, cultivated rice has a low dormancy period and storage life, even under the best storage conditions. Therefore, prolonged storage can result in the deterioration of both germination 17 ACS Paragon Plus Environment


ability and seed viability.22,35 Here we artificially aged both treated and untreated seeds, after which we performed germination tests to investigate the effect of NAP on seed vigor and longevity (see Methods). The results of this study reveal that the final germination percentage of aged-treated rice seeds (~1 min NAP) is ~90%, which is similar to that observed for unaged seeds (Fig. 5a). Interestingly, the germination rate of the treated seeds is initially significantly higher than that of the unaged seeds, (~75% and ~30%, respectively at day five) (Fig. 5a), and reached the maximum germination rate over a shorter germination time. At day five, the average seedling shoot length of the treated specimens was ~1.3 cm, while the untreated specimens had an average length of ~1 cm (inset, Fig. 5a). It is known that increasing levels of ROS in desiccated seeds can result in lower seed vigor and longevity through the inactivation of cellular proteins and enzymes;11,22,29,31 however, the external ROS induced by NAP bombardment of seed surfaces does not appear to deteriorate seed vigor and longevity, even after accelerated aging tests (AATs).35 This result confirms that external ROS play significant roles only on the seed surface and are the major surface inactivation factors. NAP is a promising method that can be applied with conventional methods for better storage of seeds in order to enable higher yields. We conclude that NAP maintains seed vigor and longevity, as well as stress resilience, allowing faster radicle emergence during prolonged storage.


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Figure 5. The effect of ageing treatment on rice seeds. (a) The germination rates of treated and untreated seeds following accelerated ageing. (The error bars represent standard errors from each mean from experiments repeated four times under the same conditions.) Inset: the seedling length of non-treated and treated after planted at day 5. (b) The restoration of natural hydrophobicity and water imbibition of modified seeds during storage. (Each error bar represents the standard error of the means from ~20 seeds.)

Moreover, the recovery of modified-seed wettability is of concern because the seeds were sterilized and modified mostly at interfaces; these seeds will possibly recover to their original or similar states. This issue is very important, not only for organic sterilization processes, but also for storage systems. Hydrophilic transformations of synthetic polymer surfaces after plasma treatment have been reported to disappear over time,32,36 but Bormashenko et al. has shown that the hydrophobic recovery process is unlikely to happen to seeds.21 Conversely, we observed not only hydrophobic recovery but also WI restoration in naturally aged seeds. Plasma-treated rice seeds (~1 min) were stored in glass containers under ambient conditions (~18 °C, ~12% moisture). Water droplet tests were performed every 2–3 d to investigate the stability of the modified surface nanostructures (see Methods). The experiments reveal that modified rice seeds recover and become more hydrophobic (higher ACA), with increased WI(time) at longer storage 19 ACS Paragon Plus Environment


times (Fig. 5a). During the first 7 d following treatment, the ACA rapidly increased from 0° to ~15°; after 10 d the ACA remained constant, at ~30 ± 5°. Similarly, the WI time rose exponentially over the first 72 h of storage and tapered to ~9 min after 60 d. The results reveal that both WI and ACA exhibit similar recovery behavior that is associated with a state of minimum surface energy. The recovered values of WI and ACA are in good agreement with those observed for the scarified seeds that exhibit WIs of ~8 to ~15 min and ACAs ~25° to ~30°. The partially restored surface wettability is a result of the loss of polar molecules on the seed surface through oxidation and the instabilities of species formed that include rearranged macromolecular chains.32,36 The chemical bonding on the polar surface is sustained for only about one week, after which the modified surface has lost its energy to afford a surface that exhibits the residual effects of physical etching. We conclude that surface modification by NAP results from a combination of physical etching and chemical reactions on the surface.

CONCLUSION In conclusion, we exploited the localized E-field-induced multiplication of free electrons, ions, and reactive species generated by NAP to engineer and control seed surface wettability. NAP is an economical and practical process that is able to increase surface energy, as well as clean and sterilize contaminated surfaces. Moreover, in conjunction with a simple mask, HCP provides control over the hydrophilicity and absorption rate of specific parts of the surface of an organic material. Such tunability in surface chemistry provides new ways of functionalizing biological and inorganic surfaces at ambient pressure; new functional materials produced in this manner have a broad range of potential applications in fields ranging from microfluidics to chemical


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sensing. Additionally, the HCP is feasible to translate into large-scale practical industrial applications.

METHODS The HCP System A sinusoidal high-voltage (HV) supply (∼7 kVp, ∼700 Hz) (Panasonic) was connected to a multipoint planar electrode array, composed of round-cut sharp-edged tips of tungsten needles (~250 µm diameter, ∼200 µm spacing) to initiate the corona discharge-mediated plasma under atmospheric conditions. The samples, i.e., rice and sunflower seeds, were directly placed onto the dielectric layer (~1 mm thick), which was placed on top of the grounded metal plate. The separation between the pointed and planar electrodes was ~5–10 mm. During operation, the HV was measured using a digital oscilloscope (Agilent Technologies, DSO0102A S) and a 1:1000 high-voltage probe (Tektronix P6015A).

The Finite Element Method (FEM) The E-field distribution between two electrodes of the hybrid cold plasma (HCP) configuration was calculated via finite element analysis (COMSOL Multiphysics). The E-field and device capacitance were solved for under electrostatic conditions based on a separated dielectricmaterial capacitor model.

Plasma Visualization and Optical Emission Spectroscopy (OES) OES was performed to determine the relative RS levels using CCD spectrometers (THORLABS CCS200, wavelength 200−1000 nm, spectral accuracy < 2 nm, FWHM @ 633 nm). The optical emission spectrometer was connected to an optical fiber located ~5 cm from the plasma area with an integration time of ∼10 s. 21 ACS Paragon Plus Environment


Seed Materials and Plasma Treatment Conditions Seeds were stored at 5 ± 1 °C and 50% relative humidity. Prior to the experiment, the Rice seeds were graded, and only seeds of 15−30 mg weight of confirmed uniformity were used in the experiments. Rice and sunflower seeds were selected and exposed to HCP under ambient airbased conditions. Ar gas was supplied at a flow rate ∼2.5 L/min at a distance of ∼8 cm from the plasma-active area.

Water Droplet, Water Imbibition, and Apparent Contact Angle Measurement A water droplet was applied to the seed surface using a pipette (CAPP Bravo (B02-1)). The clear contact angle and water imbibition of the 1 µL water droplet were measured using a digital optical microscope (Celestron 5 MP and Olympus BX43) in conjunction with computer-aided measurements.

Germination and Accelerated Ageing Tests Germination tests were carried out in accordance with International Rules for Seed Testing (ISTA, 2015).37 The seeds were placed on top of moistened blotter paper in transparent polyethylene boxes. The boxes were placed in a plant-growth chamber (Daihan Labtech, Model LGC- 5201) 20 °C in the dark for 16 h and 30 °C in cool light for 8 h at ∼85% relative humidity. Germination, normal seedling, and seedling growth were determined at 24 h intervals for 14 d. Seeds with 0.5 cm radicles and plumules were considered to be germinated. For the accelerated ageing tests, rice seeds were treated at 45 °C and ~100% relative humidity for 3 d to induce artificial aging.35

Scanning electron microscopy (SEM) The original and surface-modified seeds were characterized by SEM at 25 ± 3 °C. SEM images were acquired on a Hitachi SU 1500 (Japan) microscope. 22 ACS Paragon Plus Environment

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Supporting Information. The average electric field distribution, Direct corona discharge induced plasma, The average apparent contact angle and water imbibition of rice and sunflower seed under air and air-Ar ambient, Details of nitrogen exited species in HCP-air, Water apparent contact angle (ACA) and water imbibition test on scarified rice and sunflower seeds, The water imbibition time WI(time) model of sunflower seed (PDF)

AUTHOR INFORMATION Corresponding Author E-mail (S. Srisonphan): [email protected]

Conflict of Interest The author declares no competing financial interest.

ACKNOWLEDGMENT This work is supported by The Thailand Research Fund (TRF) and Office of the Higher Education Commission (OHEC) (Grant No. MRG6080280).



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For Table of Contents Only



Figure 1. The overall structure of the hybrid cold plasma (HCP) system. (a) The corona induced streamerdischarge plasma. The seeds were directly exposed to the discharge and showered with ions, electrons, and free radicals. (b) A sunflower and (c) rice seeds under air-Ar HCP treatment (scale bar: 7 mm). (d) The Efield contribution to the HCP structure under positive streamer discharge simulated via the finite element method (FEM); units are V/cm. (e) The actual temperatures of the rice seeds during HCP treatment. 88x35mm (300 x 300 DPI)

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Figure 2. Enhanced surface wettabilities of rice and sunflower seeds. Water imbibition times, WI(time) as functions of plasma treatment time t(treat) of (a) rice seeds and (d) sunflower seeds (not presented on linear scales). The WI(time) of (b) rice seeds and (e) sunflower seeds under given treatment conditions (untreated, treated under air, and treated under air-Ar). The box-and-whisker plots with Tukey test error bars represents the interval about the means value (from Min-Max) of WI(time) of seeds at given plasma treatment conditions. The average apparent contact angle (ACA) of a non-treated (top) and ~30 s treated (c) rice seed (scale bar: 5 mm) and (f) sunflower seed (scale bar: 7 mm). 127x77mm (300 x 300 DPI)



Figure 3. Surface modification mechanisms and models. (a) Optical emission spectrum of air and air-Ar HCP used to shower the seed surface. The air-Ar-derived plasma generates significantly more RONS in this system, resulting in faster surface transformation. (b) Modeling WI(time) as a function of t(treat) at electrode distances of ~5 and ~7 cm. (c) Interactions between the fundamental components of NAP. The circle dimensions correspond to levels of interaction. 153x119mm (300 x 300 DPI)


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Figure 4. Scanning electron micrographs of treated and untreated seed surfaces. (a) Untreated and (b) treated rice seeds. (c) Untreated and (d) treated sunflower seeds. Seed-borne pathogens on the rice were eliminated after exposure to NAP, and the surface was rougher. The sunflower-seed cuticle was destroyed upon exposure to NAP, and the surface became smoother. (Scale bar: 200 µm; inset scale bar: 50 µm). 88x35mm (300 x 300 DPI)



Figure 5. The effect of ageing treatment on rice seeds. (a) The germination rates of treated and untreated seeds following accelerated ageing. (The error bars represent standard errors from each mean from experiments repeated four times under the same conditions.) Inset: the seedling length of non-treated and treated after planted at day 5. (b) The restoration of natural hydrophobicity and water imbibition of modified seeds during storage. (Each error bar represents the standard error of the means from ~20 seeds.) 88x34mm (300 x 300 DPI)


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Tuning surface wettability through hot-carrier initiated impact

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