Nanocapillary Atmospheric Pressure Plasma Jet - American Chemical

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Nano-capillary atmospheric pressure plasma jet – A tool for ultrafine maskless surface modification at atmospheric pressure Iuliana Motrescu, and Masaaki Nagatsu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02483 • Publication Date (Web): 26 Apr 2016 Downloaded from http://pubs.acs.org on April 29, 2016

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ACS Applied Materials & Interfaces

Nano-capillary atmospheric pressure plasma jet – A tool for ultrafine maskless surface modification at atmospheric pressure Iuliana Motrescu1 and Masaaki Nagatsu2* 1

Department of Sciences, University of Agricultural Sciences and Veterinary Medicine “Ion Ionescu de la Brad”, 3 Sadoveanu Alley, Iasi, 700490, Romania 2

Graduate School of Science and Technology, Shizuoka University 3-5-1 Johoku Naka-ku Hamamatsu, 432-8561, Japan

Keywords: nano-capillary atmospheric pressure plasma jets, functionalized pattering, amine group, biosensors

ABSTRACT With respect to micro-sized surface functionalization techniques we proposed the use of a maskless, versatile, simple tool, represented by a nano- or micro-capillary atmospheric pressure plasma jet for producing micro-sized controlled etching, chemical vapor deposition, and chemical modification patterns on polymeric surfaces. In this work we show the possibility of size-controlled surface amination and we discuss it as function of different processing parameters. Moreover, we prove the successful connection of labeled sugar chains on the functionalized micro-scale patterns, indicating the possibility to use

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ultrafine capillary atmospheric pressure plasma jets as versatile tools for biosensing, tissue engineering, and related biomedical applications.

1. Introduction The biomedical field has been taking advantage of a powerful tool represented by plasma discharges in applications starting from the most common, such as sterilization of different surfaces including those inside a closed container, up to surgery and regenerative treatment, not to forget the huge number of technological applications.1-7 Due to its high chemical reactivity and the fact it avoids chemical in liquid processing, plasma has become a first-line alternative for the conventional methods. When produced at atmospheric pressure its simplicity and ease of usage makes it even more convenient as compared to the low pressure case where more complicated systems are needed for decreasing the pressure and coupling the plasma to the gas to be ionized. Since early 2000, due to the rapid development of nanobiomedical field, plasma technologies at micro and nano size level have been developing as well, small size plasmas being increasingly studied and used for different processing outcomes.5,8-12 One of the nano- and micro-scale applications is the production of very small size patterns on different surfaces. In most of the cases these patterns need to be functionalized, sites for bio specific interfacial responses being created as a mandatory step for using such surfaces in biomedical applications as support for biomaterial scaffolds, bioreactors or biosensors for example. This process of adding functional groups changes the chemical reactivity of the surface, preparing it for the connection with biomolecules which would further ensure the specific interaction with a molecule to be detected, through ligand-receptor like connections. Some of the techniques involved so far in chemical patterning (e.g. dip pen nanolithography, ink jet printing, polymer pen lithography, e-beam lithography, soft lithography techniques) are extremely sophisticated, requiring expensive devices and having


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limited use, or, those inexpensive (e.g. colloidal lithography, other colloidal methods) use masks (such as colloids).13-17 One technique to be highlighted in the last mentioned category is plasma processing. Yet still the use of a masks and vacuum systems are drawbacks in the reports found in the literature.18-20 We propose the use of a simple configuration capillary atmospheric pressure plasma jet (CAPPJ) which is able to achieve different small-scale processing outcomes of surfaces, simply by changing, for example, the gas used to produce the discharge. In this way, such a device is usable for several outcomes such as etching, ashing, deposition or functionalization of surfaces without any technical modification, as shown schematically in Figure 1.5,21 Such low frequency atmospheric pressure plasma is well fit for many processing applications, especially those related to biological and medical fields, owning to the simplicity of its configuration, implementation, functioning, and, above all, low temperature and versatility.35,21-25

Recently, theoretical studies on the interaction of atmospheric pressure plasma jets

with material surfaces have been extensively carried out for various plasma applications.26-27 Sensitive materials such as polymers can be safely prepared for implementation. During the patterned functionalization process, specific areas of the polymer are activated and functionalities such as amine, carboxyl, aldehyde groups could be connected to the existing bulk material. In this work we present results of patterned amine groups on the surface of a polymer, performed by one simple tool, a CAPPJ, without a mask or any complex chemical protocol, which, to the best of our knowledge, is a novelty in the field. We study the influence of different discharge parameters on the functionalization outcome with the purpose of downsizing as much as possible the functionalized areas and optimize the processing. We are also interested in the possibility of connecting biomolecules to the functionalized patterns. Such molecules could constitute a biomaterial scaffold, they can have support and spacing


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role, or would even be able to specifically connect the desired target if the functionalized polymer is used as biosensor.

2. Experimental Procedures The main steps of our experiments are schematically presented in Figure 2: in a first step CAPPJ patterns functional amine groups on a polymeric surface, while in a second step dextran molecules are immobilized on the patterns. Both the success of functionalization and dextran immobilization are investigated using fluorescence microscope as detailed below.

2.1. Capillary Atmospheric Pressure Plasma Jet (CAPPJ) The experiment is schematically resumed in Figure 3: it is a low frequency atmospheric pressure plasma jet. To avoid the utilization of a mask for the patterning process, and, in the same time to produce small size patterns, our idea is to decrease the size of tube employed for the treatments.5 Thus, the discharge is ignited in capillary tubes with tip diameters from 1 µm down to 100 nm, where capillary tubes were made by joining 6 mm outer diameter straight quartz discharge tube and ultrafine tapered capillary tube with a resin pipette tip. A 1 mm diameter straight quartz tube was used to make a ultrafine capillary using a filament-type micropipette puller.5 Two copper band electrodes are placed on the 6 mm outer diameter quartz discharge tube with 1 cm apart and 6 cm from the tip as shown in Figure 3.

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copper band electrodes are placed on the 6 mm outer diameter quartz discharge tube with 1 cm apart and 6 cm from the tip as shown in Figure 3. Two copper band electrodes are placed on the quartz discharge tube with 1 cm apart and 6 cm from the tip as shown in Figure 3. Square pulsed voltage with 5 kHz frequency and a duty ratio of 50% is applied to produce the discharge. The applied voltage is ±6.5 kV. To avoid the damage of the capillary tip during high debit flow transit from the millimeter size tube into the under micrometer size tip, an


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adjustment was made to the discharge tube: two round 1 mm diameter orifices were drilled on the side wall of resin pipette tip at about 3 cm from the capillary exit. Here the pipette tip was used to join the straight quartz discharge tube and the capillary tube. This was found to be the best optimization because decreasing the gas flow rate leads to the impossibility of igniting the discharge. Current-voltage waveforms are measured using a digital oscilloscope(Tektronix, DPO 4104B-L) connected to a high voltage probe(Tektronix, P6015A), and a current probe(Pearson, 4100) on power electrode line. The temporal dynamic behavior of the plasma jet between the CAPPJ tip and the substrate is investigated by a high speed intensified charge coupled device (ICCD) camera (Princeton, PIMAX 3). The electrical behavior is monitored via oscilloscope while the optical emission of the discharge is measured using Acton SpectraPro 2300 (Princeton Instruments) equipped with 1200 grooves/mm grating and a spectral response range from 200 nm to 900 nm. Capillaries of different diameters are used for patterning. Here we present results obtained with those having 100 nm, and 1µm. A picture of the former is shown in Figure 3 as well as the aspect of the discharge when ignited in He.

2.2. Patterning Procedure The addition of amine groups on the surface of the polymeric materials is performed in two steps: the pretreatment, consisting in helium plasma surface activation, and the treatment for amination, when a mixture of helium and ammonia provides the nitrogen containing plasma species, such as NH or NH2, necessary for functionalization. During the pretreatment, negative direct current (DC) bias is applied on a metal grid placed under the samples. The total gas flow rate is 500 sccm; for the treatment, 3% of the ammonia gas is added to He flow. To obtain a functionalized dotted matrix on the polymeric surface, the samples are


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placed under the tip of the CAPPJ on a computer-controlled X–Y stage (QT-ADM2 and ALS-301-HM; Chuo Precision Industrial) with a resolution of 1 µm. The outcome of the patterning is studied as function of processing time. The distance between the tip of the tube and the surface of the samples is set to be typically 250 µm, unless specifically stated in the results section. The polymeric surfaces used in this experiment are 650 nm acrylic resin (OEBR-1000) films pasted on silicon wafers. The visualization of the patterns is done by fluorescence microscopy. The fluorescent dye Alexa Fluor® 488 (AFF488) sulfodichlorophenol (5-SDP) ester (Life Technologies) reacts with the amine moieties patterned on the surface of the polymer, and emits green fluorescence at 519 nm when excited by 495 nm light. Thus, images of the functionalized areas appear as green bright zones on a black background when seen at fluorescence microscope (Leica, DMI 4000), as illustrated in Figure 3 near the device schematics. One dye molecule connects one amine group. Thus, detecting the intensity of the fluorescent dye offers quantification information of amine groups. This was performed using ImageJ (http://rsb.info.nih.gov/ij/)

2.3. Connecting biomolecules to the functionalized patterns We study the possibility of connecting biomolecules on the polymeric surface with the amine groups as intermediates. For this we use a fluorescent labeled sugar chain molecule fluorescein isothiocyanate dextran (Sigma-Aldrich, FITC-dextran 40) having a molecular weight of 40,000 Da. The labeling degree is 0.002-0.008 mol dye/molecule. In this way we can detect the eventual dextran immobilization by fluorescence microscopy as well. The sugar chains are prepared prior immobilization. First they undergo oxidation using 5 ml of NaIO4 0.03 M solution for 0.01 g, and gently mixing in the dark. The obtained solution is then put on the aminated surface where the labeled dextran molecules chemically react with


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the amine groups patterns previously obtained on the polymeric surfaces by CAPPJ treatment. Before checking by fluorescence microscopy whether the labeled dextran successfully connected the patterned amine groups, the samples are repeatedly washed and sonicated to eliminate the eventual unconnected sugar molecules. FITC dye molecules are excited by 493 nm radiation to emit fluorescence at 518 nm.

3. Results and Discussions 3.1. Plasma Diagnosis The CAPPJ has been previously studied using a probe and the bullet behavior was proved.5 Plasma bullets are exiting the capillary at about 10 km/s for the 1 µm aperture tube, the speed decreasing away from the tip. The speed is approximately three times higher for the capillary with 100 nm aperture. Considering that in the first 1.5 millimeters the speed doesn’t significantly change from the aforementioned value, and that the gap distances in the experiments presented here are less than 300 µm, we can assume that the speed of the ionizing front at the impact with the samples is roughly the one mentioned above. Negatively biasing the substrate under the samples, as it takes place during pretreatment, increases the speed of the plasma bullets with positively charged forefronts28 coming towards the processed area making the activation of the surface more efficient. We shall discuss the influence of the bias further on in this paper. Typical waveforms of applied voltage and current flow of the powered line were given in Figure 4. It is seen that sharp current peaks having about 1 µs pulse width are observed in the current waveforms together with broad displacement current during rising and falling phases of the applied voltage. From recent experiment with ICCD camera, it is found that these current peaks correspond to the dielectric barrier discharges between two copper electrodes through the quartz discharge tube. During spiky fishbone-like current pulses about 10~12 µs


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after a sharp current peak seen in Figure 4, the discharges occur between the capillary tip and substrate. During these spiky fishbone-like discharges, plasma bullets ejected from CAPPJ are irradiated onto the photoresist film surface. To get a successful functionalization during treatment, plasma must provide the moieties to be introduced on the polymer surface. A typical emission spectrum of the capillary plasma ignited in the mixture of helium and ammonia is shown in Figure 5. Among the radiative species we can identify NH radicals which produce the NH transition line (A3Π−X3Σ−, 0−0) in the spectrum at 336.0 nm. Except these radicals, other non-radiative NHx species might be existing inside the discharge due to ammonia dissociation, fact sustained by the presence of Hα line.29 However, in the present experiment we did not observe emission of H or O atoms, but of NH molecules and N2 second positive system. Hence, we consider that the surface functionalization with amine groups would be attributed to NHx species. Flux densities of such radicals are high in the first part of the expansion, decreasing downstream. Thus, placing the samples close to the tip is advantageous for decreasing the processing time as well as the size of the functionalized area.

3.2. Results of Functionalized Patterning Amine groups functionalized dotted patterns were first produced in a single electrode configuration but due to the stochastic behavior of the discharge (filamentary behavior) the functionalized areas were not uniform. We showed that improved results related to the shape, uniformity, and numbers of functionalities of the patterned surfaces are achieved with the configuration presented in Figure 3. Using two electrodes results in a more like glow discharge than filamentary one. Another consequence was the possibility to reduce the


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pretreatment time from 0.1 s to 0.01 s which lead to a decrease of the functionalized areas diameters from 100~120 µm to 50~60 µm.21 We shall next discuss the details of functionalization, parameter adjustment, and functionalization mechanism. To detail the mechanism of processing, we should first highlight the importance of pretreatment. Without this step the introduction of amine groups on the surface was not possible.21 In the same time, increasing too much the treatment time (the second step of the processing when amine groups are introduced) is not wanted. During the post treatment ammonia is used, its toxicity and flammability in some mixtures with the air being well known. The He plasma pretreatment seems to have the role of surface cleaning and activation due to ion bombardment by negative substrate bias. The energy communicated from the discharge to the polymer surface creates active sites where, during the post treatment, NHx species produced in plasma will react with the bulk material resulting in amine group functionalization of the surface. Applying DC bias on a grid placed under the samples is a mandatory condition to ensure the success of the treatment. Figure 6 shows how the size of the patterned area changes as function of the pretreatment time, and the bias value, for the same post treatment time (1.5 s). For bias voltage between 0 and -400 V, amine group patterning was impossible. For -400 V the appearance of the dots is faint, but still perceptible. -500 V bias has proven to be better, with brighter and clearer dotted patterns. Due to the presented results, the value -500 V was chosen as optimal for our experiments. For higher bias, the impact of the CAPPJ with the surface produces a strong damage of the surface, and the activated area becomes larger. By applying a negative DC bias voltage to the substrate stage, the plasma bullets ejected from a capillary APPJ during rising phase of applied voltage are forced by the Coulomb force due to the externally applied electric field between the positive powered electrode and the negatively biased substrate. The positive ion species of the plasma bullet are accelerated


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towards the substrate and bombard the surface of the photoresist film. As a result, the photoresist surface is activated (made defects) by ion bombardment to promote the functional group introduction (amine groups) on photoresist film during the treatment stage. The average diameter of the functionalized areas has the same monotony as function of the pretreatment time. Pretreatments shorter than 1 s are enough for surface activation (Figure 6). For further surface amination, our experiments have shown that a treatment of at least 1.5 s is necessary. The effect of the negative bias applied to the substrate stage on the surface modification area can be better understood from the high speed ICCD images shown in Figure 7. The imaging was performed for the larger 1 µm capillary only and for relatively long exposures (1 ms) due to the very low intensity of the CAPPJ emission. Without the bias the zone between CAPPJ and the substrate shows low, diffuse presence of radiative discharge species emission, with extended radial diffusion downstream. Using bias, the radial diffusion is reduced, the high intensity detected at the impact zone of the CAPPJ on the treated surface indicating a strong interaction of plasma species with it. The ICCD images in Figure 7 also explains the “doughnut”-like shape of the patterned areas that can be observed for both size capillaries (Figure 8) due to the speed of the gas exiting the capillary and its interaction with the surrounding atmospheric air before impacting on the surface. The behavior, consistent with other reports30, limits the possibility of downsizing the dimensions of the patterned areas and can’t be avoided because reducing the gas flow would lead in the inability of igniting the discharge. Except for decreasing the pretreatment time, another way to downsize the aminated areas is to simply use a capillary of a smaller aperture. Going from φ = 1 µm to 100 nm, the diameter of the patterned dots decreases almost ten times for the same processing conditions as can be easily seen from the example in Figure 8. A more detailed comparison can be made when


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analyzing the data presented in Figure 6. Manipulating the size of the capillary, the processing intervals, and controlling the sample stage, dotted patterns can be obtained with sizes ranging from over 100 µm (for the φ = 1 µm aperture capillary and long processing times) down to 2 µm (for the 100 nm capillary and tenths of seconds pretreatment), and also different spacing in between. From our previous AFM analysis of the photoresist film treated by a 100 nm aperture capillary APPJ with He gas for a very short duration of 1 ms(only 5 cycles of squared pulses), we observed an inverted cone shape etching patter. The diameter of etched circle pattern is roughly 2 µm at the surface and the etching depth is 500~600 nm in the center(not shown here). Therefore, it is considered that the present ring shape of fluorescence pattern might be caused by its structure, that is, the blight fluorescence was corresponding to the surrounding shallow region and the dark part to the central deep pinhole region. A quantification of the amine groups patterned can be done by extracting the information of fluorescence intensity. In Figure 9, the fluorescence intensities of the dye labeled patterned areas are plotted as a function of pretreatment time for different treatments with -500 V bias during pretreatment. Here, the fluorescence intensity was numerically evaluated as integrated fluorescence intensity per surface unit of functionalized area so that we can compare the results of different treatment conditions. It is clearly seen that the fluorescence intensity drastically increases for 1.5 s treatment compared with that for 1.0 s treatment. For a treatment time longer than 1.5 s, the fluorescence intensity is almost the same as that at 1.5 s, thus we considered that 1.5 s is the optimum treatment time for amine group patterning in the present case.

3.3. Results of Biomolecule Connection on the Functionalized Patterns


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FITC-dextran sugar chains are used as probing biomolecules for the connection with the aminated patterns as described in the experimental section. The fluorescence microscopy results indicate the successful connection of fluorescent dye labeled dextran with the amine groups on the polymeric surface as can be observed in Figure 10. The appearance of the areas is not as uniform as those visualized after patterning, which can be due to several factors. Dextran molecules are very large and a relative low labeling degree with fluorescent dye. Comparing the two fluorescent dyes used, Alexa Fluor® 488 gives brighter fluorescence than FITC because more dye molecules connect to the surface.31 The present results indicate that CAPPJ functional patterning shows good premises as a surface preparation method for immobilization of biomolecules.

4. Conclusions A simple versatile tool represented by an ultrafine capillary atmospheric pressure plasma jet was used to pattern micro-sized aminated dots on polymeric surfaces without a mask in two stages processing. The size of the patterns can be controlled and modified according to the desired outcome as well as the spacing, by optimizing pretreatment and post treatment intervals. With a 100 nm capillary plasma jet, we demonstrated 2~3 µm diameter aminegroup dot patterning on the photoresist film under atmospheric pressure. Pretreatment with biased substrate is essential to achieve the functionalization of the photoresist film. The possibility to use such aminated patterns for biomedical applications was proven by the successful connection of fluorescent dye labeled dextran molecules on the amine functionalities introduced on the polymeric surfaces. The present ultrafine CAPPJ will provide a novel technique for maskless surface processing of various materials, not only microscale surface modification but also etching, ashing, or thin film deposition under atmospheric pressure.


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Figure 1. Illustration of applications of CAPPJ for etching, ashing, thin film deposition, and functionalization of substrate surfaces.


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Figure 2. Schematic illustration of patterning process by CAPPJ for biomedical applications and visualization of patterns using fluorescent molecules.


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Figure 3. Schematic drawing of CAPPJ used for patterning, enhanced image of the capillary tip, and fluorescent image of patterned matrix.


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Figure 4. Typical waveforms of applied voltage and current during one cycle of CAPPJ discharge.


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Figure 5. Typical emission spectrum from 300 to 800 nm of CAPPJ ignited in He/NH3 mixture during treatment. Inset: Expanded emission spectrum from 300 to 350 nm.


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Figure 6. Average diameters of patterned areas using 1 µm CAPPJ for -400 V and -500V bias on substrate, and 100 nm capillary with -500V bias, as function of pretreatment time. The treatment time is 1.5 s in all cases


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Figure 7. ICCD camera images of the discharges for the CAPPJ with 1 µm aperture in the cases of (a) with bias at -500 V and (b) without bias.


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Fluorescence images examples of the aminated patterns obtained with CAPPJ

with (a)1 µm aperture and (b)100 nm aperture for 0.1 s He plasma pretreatment (with a substrate bias of -500 V), and 3 s He/NH3 plasma treatment.


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Figure 9. Fluorescence intensities of patterned areas treated with a 1 µm CAPPJ with a bias voltage of -500V for different treatment times as a function of pretreatment time.


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Figure 10. Fluorescent microscope images of 1 µm CAPPJ aminated patterns after FITCdextran immobilization.


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AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions Funding Sources Grants-in-Aid for Scientific Research on Innovative Areas (No. 21110010) and Scientific Research (A) (No. 25246029) from the Japan Society for the Promotion of Science. ACKNOWLEDGMENT This work was supported in part by Grants-in-Aid for Scientific Research on Innovative Areas (No. 21110010) and Scientific Research (A) (No. 25246029) from the Japan Society for the Promotion of Science. The authors would like to thank Mr. T. Abuzairi, Mr. M. Okada and Mr. Naito of Shizuoka University for their help in the present study. ABBREVIATIONS CAPPJ, capillary atmospheric pressure plasma jet; FITC, fluorescein isothiocyanate. REFERENCES (1)

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BRIEFS (Word Style “BH_Briefs”). If you are submitting your paper to a journal that requires a brief, provide a one-sentence synopsis for inclusion in the Table of Contents. SYNOPSIS (Word Style “SN_Synopsis_TOC”). If you are submitting your paper to a journal that requires a synopsis, see the journal’s Instructions for Authors for details.


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Table of Content Graphic


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CAPPJ patterning of functional groups


Functional groups Biomolecule immobilization (amine group)

Visualization by fluorescence microscopy


Nanocapillary Atmospheric Pressure Plasma Jet - American Chemical

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