Carboxyl-Functionalized Nanoparticles Produced by Pulsed Plasma

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Carboxyl-Functionalized Nanoparticles Produced by Pulsed Plasma Polymerization of Acrylic Acid Pavel Pleskunov, Daniil Nikitin, Renata Tafiichuk, Artem Shelemin, Jan Hanus, Ivan Khalakhan, and Andrei Choukourov J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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The Journal of Physical Chemistry

Carboxyl-Functionalized Nanoparticles Produced by Pulsed Plasma Polymerization of Acrylic Acid

Pavel Pleskunov*1, Daniil Nikitin1, Renata Tafiichuk1, Artem Shelemin1, Jan Hanuš1, Ivan Khalakhan2, Andrei Choukourov1

1

Charles University, Faculty of Mathematics and Physics, Department of Macromolecular Physics V Holešovičkách 2, Prague, Czech Republic

2

Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holešovičkách 2, Prague, Czech Republic

Abstract

Carboxyl-enriched and size-selected polymer nanoparticles (NPs) may prove to be very useful in biomedical applications for linker-free binding of biomolecules and their transport to cells. In this study, we report about the synthesis of such NPs by low-pressure low-temperature pulsed plasma polymerization of acrylic acid. Gas aggregation cluster source was adapted to operate plasma with the constant pulse period of 50 µs and with varying duty cycle. The NPs were produced with the size ranging from 31±5 nm to 93±14 nm and with retention of the carboxyl groups ranging from 4.0 to 12.0 at. %. Two regimes of the NP formation were identified. In the

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large duty cycle regime, the NP growth was interfered with by positive ion bombardment which resulted in the ion-driven detachment of the carboxyl species and in the formation of carboxyldeficient NPs. In the small duty cycle regime, the NP growth was accompanied by the radicaldriven chain propagation with the attachment of intact monomer molecules. Improved efficacy of the monomer retention resulted in increased concentration of the carboxyl groups.

Introduction

Polymerization of organic compounds in low-temperature plasma has evolved into a wellestablished approach to deposit thin polymeric films with tailored properties. The attractiveness of the method was realized in the 1960s when dielectric hydrocarbon-based films for microelectronics were sought after.1 Later, it was recognized that organic precursors bearing specific functional groups can be plasma polymerized to deposit thin films with these functionalities retained in the structure.2 This modification allowed the researchers to pursue new potential applications of functionalized plasma polymers. For example, the incorporation of carboxyl or amine groups was considered beneficial for biomedical applications as these groups are able to bind covalently with N- or C-termini of biomolecules via dehydration reactions.3 Acrylic acid (AA) has become perhaps the most popular precursor for the deposition of carboxyl-functionalized plasma polymers4–6 which have been extensively studied for their ability to bind biomolecules7–9 and as supports for cultivation of cells,10–12 including cancer13,14 and stem cells.15 A macroscopic approach was suggested to describe plasma polymerization via a parameter of the average energy invested into plasma per monomer molecule. In the small energy regime, better retention of precursor’s structure is typically achieved due to limited


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fragmentation of the precursor molecules. In the case of AA, it results in enhanced retention of the carboxyl groups. In the large energy regime, molecular fragmentation leads to the loss of the monomer identity and to the growth of films deficient in carboxyls. Pulsing the plasma provides an additional tool to tune the plasma chemistry. In the pulsed discharge, monomer molecules become activated during short time-on ton followed by time-off toff. Very low average power can be maintained in the pulsed mode despite the high peak power supplied to plasma during ton. For unsaturated compounds such as AA, the pulsing approach allows one to take advantage of conventional polymerization reaction in which π-bonds of unsaturated carbon are opened by the radical attack and propagate via the attachment of new monomer molecules. The vast majority of recent research on plasma polymerization of AA has been performed in the pulsed mode. If the glow discharge is operated in an organic gas at sufficiently high pressure, plasma polymerization may be forced to proceed with the formation of nanoparticles (NP) in the discharge volume instead of the growth of thin films on adjacent surfaces. The phenomena of the formation and growth of hydrocarbon, organosilicon and fluorocarbon plasma polymer NPs have been widely studied in the field of dusty plasmas;16 yet, rather surprisingly, NPs enriched with specific functional groups have been studied much less. Nitrogenated NPs were synthesized either by rf magnetron sputtering of nylon or by plasma polymerization of volatile hydrocarbons in their mixtures with N2.16,17 A recent report showed that such C:H:N NPs produced in the acetylene/Ar/N2 plasma can be very effective as linker-free nanocarriers of bioactive cargo with good permeability through the cell membrane and with negligible cytotoxicity.18 The aim of this work is to show that carboxyl-enriched NPs can be produced by performing plasma polymerization of AA. We chose a mechanistic approach to describe the NP size, the flux, the mass flux and the concentration of the carboxyl groups in dependence on the pulsing


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parameters, to correlate the experimental findings with those from thin film deposition and to get insight into the mechanisms of the plasma polymerization of AA including the mechanisms of NP nucleation and growth.

Experimental

The experiments were performed by exploiting a deposition system comprising a gas aggregation cluster source mounted vertically onto a deposition chamber (Figure 1). The system was pumped by rotary and diffusion pumps to a base pressure of 10-4 Pa.

Figure 1. The scheme of the gas aggregation cluster source used for the synthesis of ppAA NPs.


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The GAS itself consisted of a water-cooled cylindrical (inner diameter ∅ = 62 mm) vacuum chamber with a conical lid and a 1 mm orifice at the end. The GAS was equipped with a stainless steel planar rf electrode (∅ 2 inches). The electrode was powered by an rf generator (Dressler Cesar) via a matching unit. Continuous wave (CW) and pulsed modes were used to initiate plasma. The experiments were performed under average power Pav fixed at 40 W and pulse repetition frequency PRF fixed at 20 kHz, unless stated otherwise. Pulsing was performed with simultaneously changing time-on ton and time-off toff to control duty cycle as D = ton/( ton + toff). The experiments were performed in a mixture of acrylic acid (AA, Sigma-Aldrich, purity 99%) with argon (Linde, purity 99.996%). A flask with a liquid monomer was connected to the GAS through a Kelraz-sealed needle valve (Lurt J. Lesker) whereas a tank with Ar was connected to the GAS via an automatic flow controller (MKS Instruments). After pumping the system, 11 sccm of Ar was introduced to the GAS to create the pressure of 70 Pa. Subsequently, the needle valve was opened to introduce vapors of AA and to adjust the overall pressure in the GAS at 100 Pa. Thus, the 30/70 AA/Ar mixture was obtained and used for all the experiments. The nanoparticles of acrylic acid plasma polymer (ppAA NPs) were produced in the GAS and dragged by the gas flow through the orifice to the deposition chamber where they were collected on silicon substrates. Scanning electron microscopy (SEM, Tescan Mira III) was used to determine the size distribution of the NPs without any additional metallization applied. Atomic force microscopy (AFM, Ntegra Prima, NT-MDT) was used in an intermittent contact mode under ambient air with super-sharp cantilevers (SHR75, Nano&More, spring constant is 3 N/m, tip radius is better than 1 nm) and at 256×256 data points. The measurements were performed at


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30 % damping of free oscillation amplitude to minimize the deformation of the ppAA NPs by the force from the AFM tip. Chemical composition of the NPs was studied by X-ray photoelectron spectroscopy (XPS, Phoibos 100, Specs). The spectra were acquired at a constant take-off angle of 90°. An Al Kα source (1486.6 eV, 200 W, Specs) was used to generate X-rays. Wide spectra were recorded at pass energy of 40 eV (dwell time 100 ms, step 0.5 eV) for a binding energy range of 0-1100 eV, whereas high-resolution spectra were gained at pass energy of 10 eV (dwell time 100 ms, step 0.05 eV, 10 repetitions). The spectra were referenced to the aliphatic carbon peak at 285.0 eV. The high-resolution C 1s XPS signals were fitted with an accuracy of ±0.1 eV by four components in accordance with the protocol well-established for ppAA thin film deposition:19–23 the C-C/C-H peak at 285.0 eV, the C-O peak at 286.5 eV, the C=O peak at 288.0 eV and the OC=O peak at 289.1 eV.

Results and discussion

Plasma polymerization of acrylic acid was deliberately run at the elevated pressure of 100 Pa to favor the formation of NPs in the gas phase. The 30% AA/70 % Ar mixture was found optimal in terms of stability of the NP synthesis. The experiments were performed both in the CW and the pulsed modes with different duty cycles. The average power was set constant at 40 W; thus, the power Pon delivered to the discharge during ton increased with the decreasing duty cycle (Table 1). Figure 2 shows the examples of the SEM images of the ppAA NPs in dependence on the duty cycle (the corresponding size histograms are shown in Figure S1 of Supporting information). The NPs were successfully synthesized both in the CW and in the pulsed mode, yet


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with a different size and flux (the number of the NPs deposited per unit area per time). Larger NPs with smaller flux are produced in the CW mode whereas the pulsed mode results in the synthesis of larger amounts of smaller NPs. The phenomenon is consistent with an increase of Pon which, under the constant monomer flow, results in an increase of the average energy supplied per monomer molecule Emean and leads to the enhancement of the monomer fragmentation (Table 1; for the details of the calculation of Emean see Supporting information).24 A similar effect was previously observed in the case of fluorocarbon plasma polymer25 and nylon-sputtered NPs.17 The values of Emean in Table 1 are deliberately overestimated because not all the energy supplied to the plasma goes to the bond cleavage. Nevertheless, they are worth comparing, at least roughly, with the dissociation energy of all bonds in a molecule of acrylic acid, which makes ED = 43 eV.26 The comparison indicates that the plasma polymerization proceeds under highly energetic conditions for all the experiments in question. However, in the CW mode, Emean < ED and it can be expected that a certain amount of the monomer molecules remain intact. In the pulsed mode, the plasma polymerization runs in a monomer deficient regime so that the heavy fragmentation of the monomer molecules occurs when the plasma is on. The formation of a higher concentration of nucleation centers (radicals and ions, see below) under the constant monomer feed is responsible for the higher concentration of the NPs that grow to a smaller size.


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Figure 2. SEM images of ppAA NPs deposited on Si substrates in the pulsed mode at different duty cycles, Pav 40 W, PRF 20 kHz, deposition time is 5 seconds.


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Table 1. The pulsing parameters used for the synthesis of ppAA NPs. Duty cycle, % T, µs

ton, µs

toff, µs

Pon, W

Emean, eV/molecule

100

-

-

-

40

36

80

50

40

10

50

45

70

50

35

15

57

51

60

50

30

20

67

60

50

50

25

25

80

72

40

50

20

30

100

90

32

50

16

34

125

112

30

33

11

22

78

70

20

50

16

34

78

70

10

100

32

68

78

70

(PRF 20 kHz, Pav 40 W)

PRF, kHz (duty cycle 32%, Pav 25 W)

The SEM images also point to the coagulation of the NPs occurring at smaller duty cycles (longer toff) and resulting in the deposition of larger agglomerates. The effect is better observed in high-resolution AFM images (Figure 3) where two samples prepared at different duty cycles are shown. At the duty cycle of 50 %, the NPs are produced for the most part as individual entities. The spherical shape points to the isotropic accretion of their volume and indicates that no preferential direction for the attachment of polymer-forming species exists in the plasma. High-


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resolution AFM probe reveals the smooth surface texture of the NPs, at least at the level of tip curvature radius and thermal fluctuations of the probe. The smooth texture evidences that significant redistribution of the incoming material occurs during the NP growth as opposed to rough or even fractal surfaces produced when the sticking probability of newly arriving species is close to 1.27,28 For the duty cycle of 32 %, individual NPs can be distinguished as constituents of larger agglomerates, although their surface texture remains smooth. Thus, an additional mechanism appears that causes the NPs to stick together at smaller duty cycles.


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Figure 3. AFM images of ppAA NPs deposited as individual NPs (duty cycle 50%) or as agglomerates of NPs (duty cycle 32%).The insets show the same areas scanned at lower magnification. Pav 40 W, PRF 20 kHz, deposition time is 5 seconds.

The NP size and flux were obtained for all the duty cycles studied and are summarized in Figure 4. Here, solid symbols correspond to the analysis of all NPs as individual entities, regardless of the fact that part (or all) of them constitute larger agglomerates. Open symbols correspond to the data calculated by considering agglomerates as bigger particles. The


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characteristic size of an agglomerate is taken in this case as the diameter of a spherical particle occupying the same surface area. For both approaches, the trend is confirmed that the NP size decreases and the flux increases with decreasing duty cycle at constant average power. The changes are more prominent if agglomerates are treated as collections of individual NPs with their size ranging from 93±14 nm at the CW mode to 31±5 nm at 32% duty cycle. 120

a)

NPs agglomerates

NP size, nm

100 80 60 40 20 100

90

80

70

60

50

40

30

50

40

30

duty cycle, % b) 5

NP flux, µm-2s-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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NPs agglomerates

4 3 2 1

100

90

80

70

60

duty cycle, %

Figure 4. Dependence of the NP size and flux on the duty cycle; Peff 40 W, PRF 20 kHz. Solid symbols correspond to the size and the flux calculated without taking into account that part of the NPs are agglomerated. Open symbols correspond to the size and the flux calculated by counting agglomerates as bigger particles of an irregular shape; the ‘size’ of an agglomerate is calculated as = ඥ4‫ܣ‬/ߨ , where A is the area occupied by the agglomerate.


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Taking into account that only individual NPs are formed at higher duty cycles and that their agglomeration prevails at lower duty cycles, the effect can be related to charging of the NPs during ton and to the loss of the charge during toff. One should bear in mind that the residence time, i. e. the time that the NPs spend in the GAS tres = 1 s (for the details of the calculation see Supporting information), is much longer than the pulsing period T = 50 µs and, therefore, the NPs experience many plasma cycling events on their way along and out of the GAS. When in plasma, particles acquire a floating potential which is negative with respect to the plasma potential. Negatively charged NPs should then experience the Coulomb repulsion which prevents them from agglomeration, in a close analogy with the charge-driven agglomeration of NPs in solutions. If the plasma is turned on and off intermittently, the NP charge balance is determined by how fast the floating potential is established during ton and how fast it is lost during toff. For pulsed capacitively-coupled discharges operated at several Pa pressure in Ar and in AA, electrons were shown to be lost within 150 µs from the beginning of toff (although large ions may survive significantly longer).29 At higher pressure used in our work, the time scale can be shorter. For example, the Bohm velocity ‫ݒ‬஻ = ට

௞்೐ ெ೔

≅ 10ସ m/s can be calculated for the mean electron

energy of kTe = 1 eV measured in this type of the GAS,30 and for the mass of acrylic acid molecular ion Mi = 72 amu. If vB is related to the characteristic size of the GAS (inner ∅ of 62 mm), then the characteristic time of several µs is obtained. The characteristic time determines the time scale required for the ions to get lost on the walls and for the plasma to collapse. Apparently, duty cycles < 60% (corresponding to ton < 30 µs and toff > 20 µs, see Table 1) provide sufficiently long toff to make the ppAA NPs start losing their charge and to trigger the onset of their agglomeration. At larger duty cycles, toff expires before or not long after the plasma collapses and the electrostatic repulsion prevents the NPs from the agglomeration.


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The hypothesis is further supported by performing the experiments with constant Pav and duty cycle but with different pulse repetition frequency (Figure 5, Table 1). Note that Pav = 40 W did not allow the stable synthesis of the NP at lower PRF and therefore we had to decrease Pav to 25 W for this series of the experiments. At PRF = 30 kHz, toff is 22 µs and the agglomeration is just about to become apparent. The deposit is represented by a mixture of individual NPs and their agglomerates but the individual NPs prevail. Reducing the PRF to 20 kHz prolongs toff and, hence, increases the probability of the agglomeration. At PRF = 10 kHz and toff = 68 µs, an interconnected maze of the agglomerates is formed with a minimal contribution from individual NPs.


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Figure 5. SEM images of ppAA NPs deposited on Si substrates in the pulsed mode at different PRF, Pav 25 W, duty cycle 32%, deposition time is 10 seconds.


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The chemical composition of the ppAA NPs was analyzed by XPS in dependence on the duty cycle. The C 1s spectra shown in Figure 6 are similar to those typically obtained for thin films of plasma polymerized acrylic acid.20–23 The plasma polymer is represented by a hydrocarbon network (C-C/C-H bonds) bearing a multitude of chemical functionalities which can be classified in terms of the XPS analysis into three groups of atomic carbon bound with oxygen with one, two or three bonds. The latter component is typically attributed to the carboxyl or ester groups and is a matter of primary scientific interest. The spectra in Figure 6 demonstrate that the concentration of the O-C=O groups increases with decreasing duty cycle, although the difference is not large. The concentration of 9.0 at.% and 12.0 at.% is detected for the CW and the pulsed mode with 32% of duty cycle, respectively. These values suggest that less than half of the carboxyl groups present in the precursor (33 at.% of C) survive the plasma conditions and become incorporated into the plasma polymer.


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292

290

288

286

CW

284

282

280

282

280

C-C C-H

O-C=O

C=O

C-O

duty cycle 50%

duty cycle 32%

292

290

288

286

284

Binding energy, eV

Figure 6. C 1s XPS of ppAA NPs in dependence on the duty cycle, Pav 40 W, PRF 20 kHz.

An interesting dependence arises if the concentration of the O-C=O groups is summarized for all duty cycles studied and plotted in Figure 7a where two distinct regimes can be identified. At higher duty cycles, the increase of toff (and the related increase of Pon) is accompanied by a significant reduction of the efficiency of the O-C=O retention down to 4.0 at.%. In contrast to this, the O-C=O concentration starts to increase at lower duty cycles, restores back to the value of the CW mode and reaches beyond it at the smallest duty cycle of 32%. Furthermore, the mass flux borne by the NPs onto substrates closely replicates the O-C=O dependence (Figure 7b; for the details of the calculation of the mass flux see Supporting information). Two regimes are also identified that show the efficiency of the NP formation either decreasing or increasing with the


17


duty cycle. Both regimes are demarcated by the duty cycle of about 60%. These relations indicate that there exists a shift in the mechanism of the NP formation that becomes to be dominated by different plasma polymerization processes at a duty cycle < 60% (corresponding to ton < 30 µs and toff > 20 µs).

a) O-C=O, at. %

12 10 8 6 4

100

90

80

70

60

50

40

30

50

40

30

duty cycle, % 103

mass flux, µg/m2s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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b)

102

100

90

80

70

60

duty cycle, %

Figure 7. a) Retention of the O-C=O functional groups as witnessed by XPS; b) NP mass flux onto substrates in dependence on duty cycle (Pav 40 W, PRF 20 kHz).

For vinyl-bearing compounds in general and for acrylic acid in particular, conventional chemical reactions may take place in plasma involving an initiator attack on the double bond followed by the chain propagation via the attachment of an intact monomer molecule. In millisecond pulsed discharges, the conventional polymerization pathway is further favored


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because of the extinction of plasma during toff which leads to the chain propagation unperturbed by the action of plasma. Hence, significant mass accumulation of the plasma polymer occurs during toff and better retention of the monomer structure (the carboxyl groups in the case of AA) is achieved. The extensive research on the deposition of ppAA thin films has confirmed this trend.12,29,31,32 Taking into account that the ionization potential of organic compounds is significantly higher than the typical 3-4 eV bond dissociation energy and that the mean electron energy is about 1 eV, the plasma is enriched with radicals as the products of the molecular bond dissociation. Thus, the thin film community has agreed on the radical induced chain propagation as a primary chemical route during toff (Scheme 1, reaction 1).12,29,31,32 The same reasoning can be brought forward for the ppAA NPs to explain the increasing mass flux and the concentration of the O-C=O groups with the increasing toff in the small duty cycle region. Although Pon increases here to the high values, the corresponding decrease of ton may impose temporal constraints on the kinetics of plasma chemistry. For pulsed AA/Ar discharges, the bias and the steady-state plasma were shown to establish at the time scale from a few µs up to 20 µs after the beginning of ton. 33,34 The small duty cycles used in our work are characterized by the same time scale of ton which means that equilibrium of the plasma phase may not be reached. Hence, intact monomer molecules may survive and participate in radical-induced chain propagation reactions regardless of the high power input. In the high duty cycle region, decaying trend of the O-C=O and the mass flux was established. Here, we attribute the decay to the deteriorative influence of ionic species. The important role of negative and positive ions in plasmas of carboxylic acids was previously highlighted both for low29,31,34–38 and high pressure discharges.39,40 Anionic polymerization via dissociative electron


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attachment (Scheme 1, reaction 2) and cationic polymerization via proton transfer (Scheme 1, reaction 3) were shown to contribute to the formation of heavy ionic oligomers by the addition of intact monomer molecules to ions. In our experiments however, anionic species are unlikely to attach to negatively charged NPs by reason of the Coulomb repulsion. Furthermore, at least 2 ms of toff were required for these reactions to occur and at the shorter afterglow time the ionic oligomers were not detected by time-resolved measurements.37,38 Finally, the ion-molecule oligomerization routes should lead to the increase of the O-C=O content in the ppAA NPs, which is not the case of the high duty cycle region. Thus, the ionic polymerization cannot explain the decay of the NP mass flux and the O-C=O group retention in the large duty cycle region. On the other hand, collisions of the cationic species with the intact monomer molecules may lead to condensation reactions with the elimination of low molar mass molecules such as HCOOH and CO,40 especially at higher collision energies. Such dissociation results in the formation of carboxyl-deficient moieties as shown in the right-hand side parts of reaction 3 of Scheme 1. We hypothesize that incomplete collapse of the plasma during short toff at large duty cycles results in the deteriorative (in terms of the O-C=O retention and the mass flux) contribution from positive ions to the NP growth. The contribution becomes more deteriorative with increasing Pon, until toff becomes sufficiently long to ensure the complete collapse of the plasma and ton becomes sufficiently short to ensure the preservation of intact monomer molecules for radical-driven polymerization to occur.


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Scheme 1. Potential chemical pathways occurring in the formation of NPs by plasma polymerization of acrylic acid: 1) radical-induced chain propagation; 2) dissociative electron attachment followed by anionic chain propagation and accompanied by the loss of the HCOOH and CO species; 3) proton attachment followed by H+ transfer chain propagation and accompanied by the loss of the HCOOH and CO species. M is the monomer molecule, [M-H]and [M+H]+ are dehydrogenated negative and protonated positive molecular ions.


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Conclusions

Acrylic acid was found to be capable of polymerizing in plasma with the formation of NPs. Pulsing the discharge at a constant average power of 40 W, constant period of 50 µs and with varying the duty cycle allows tailoring the NP size, the mass flux and the retention of the carboxyl groups. The NP size decreases and their number increases with the decreasing duty cycle; however, the mass flux and the concentration of the O-C=O groups show a more complex dependence. Two regimes of the NP growth have been distinguished. In the large duty cycle regime when ton > 30 µs and toff < 20 µs, the short time-off does not suffice for plasma to extinguish completely. Energetic collisions of positive ions with the growing NPs result in the detachment of low mass species at the expense of the retention of the carboxyl groups. The efficiency of the plasma polymerization decreases with the decrease of the duty cycle and the concentration of the O-C=O groups reaches the minimal value of 4 at. % in this region. The incomplete extinction of the plasma leads also to the conservation of the NP negative charge which prevents them from the agglomeration. In the small duty cycle regime when ton < 30 µs and toff > 20 µs, the time-off becomes long enough to allow for the collapse of the plasma and for the unperturbed conventional polymerization reaction, which runs via the radical-induced opening of the double bond and chain propagation through the addition of intact monomer molecules. The mass flux borne by the NPs increases and the concentration of the O-C=O groups reaches the value of 12 at. %. The collapse of the plasma leads to the loss of the negative charge by the NPs and they tend to agglomerate, especially at the smallest duty cycle. Overall, the method allows for the synthesis of carboxyl-functionalized NPs with the size ranging from 30 to 90 nm.


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ASSOCIATED CONTENT Supporting Information Normalized size histograms of ppAA NPs, calculations of mass flux, concentration of the NPs in the GAS, average energy supplied per monomer molecule.

AUTHOR INFORMATION Corresponding Author *email: [email protected], tel: +420951552284 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the grant GAČR-17-12994S from the Grant Agency of the Czech Republic. P. P., D. N. and R. T. also appreciate the support from the student grant SVV 260444/2017 of Charles University.


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Carboxyl-Functionalized Nanoparticles Produced by Pulsed Plasma

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