Carboxyl-Functionalized Nanoparticles Produced by Pulsed PlasmaPolymerization of Acrylic AcidPavel Pleskunov,*,† Daniil Nikitin,† Renata Tafiichuk,† Artem Shelemin,† Jan Hanus,† Ivan Khalakhan,‡

and Andrei Choukourov†

†Faculty of Mathematics and Physics, Department of Macromolecular Physics and ‡Faculty of Mathematics and Physics, Departmentof Surface and Plasma Science, Charles University, V Holesovickach 2, Prague, Czech Republic

*S Supporting Information

ABSTRACT: Carboxyl-enriched and size-selected polymer nano-particles (NPs) may prove to be very useful in biomedicalapplications for linker-free binding of biomolecules and theirtransport to cells. In this study, we report about the synthesis ofsuch NPs by low-pressure low-temperature pulsed plasmapolymerization of acrylic acid. Gas aggregation cluster source wasadapted to operate plasma with a constant pulse period of 50 μsand with varying duty cycle. The NPs were produced with the sizeranging from 31 ± 5 to 93 ± 14 nm and with retention of thecarboxyl groups ranging from 4.0 to 12.0 atom %. Two regimes ofthe NP formation were identified. In the large duty cycle regime,the NP growth was interfered with by positive ion bombardmentwhich resulted in the ion-driven detachment of the carboxyl species and in the formation of carboxyl-deficient NPs. In the smallduty cycle regime, the NP growth was accompanied by the radical-driven chain propagation with the attachment of intactmonomer molecules. Improved efficacy of the monomer retention resulted in increased concentration of the carboxyl groups.

■ INTRODUCTION

Polymerization of organic compounds in low-temperatureplasma has evolved into a well-established approach to depositthin polymeric films with tailored properties. The attractivenessof the method was realized in the 1960s when dielectrichydrocarbon-based films for microelectronics were soughtafter.1 Later, it was recognized that organic precursors bearingspecific functional groups can be plasma polymerized to depositthin films with these functionalities retained in the structure.2

This modification allowed the researchers to pursue newpotential applications of functionalized plasma polymers. Forexample, the incorporation of carboxyl or amine groups wasconsidered beneficial for biomedical applications as thesegroups are able to bind covalently with N- or C-termini ofbiomolecules via dehydration reactions.3 Acrylic acid (AA) hasbecome perhaps the most popular precursor for the depositionof carboxyl-functionalized plasma polymers4−6 which have beenextensively studied for their ability to bind biomolecules7−9 andas supports for cultivation of cells,10−12 including cancer13,14

and stem cells.15 A macroscopic approach was suggested todescribe plasma polymerization via a parameter of the averageenergy invested into plasma per monomer molecule. In thesmall energy regime, better retention of the precursor’sstructure is typically achieved due to limited fragmentation ofthe precursor molecules. In the case of AA, it results inenhanced retention of the carboxyl groups. In the large energyregime, molecular fragmentation leads to the loss of themonomer identity and to the growth of films deficient in

carboxyls. Pulsing the plasma provides an additional tool totune the plasma chemistry. In the pulsed discharge, monomermolecules become activated during short time on ton followedby time off toff. Very low average power can be maintained inthe pulsed mode despite the high peak power supplied toplasma during ton. For unsaturated compounds such as AA, thepulsing approach allows one to take advantage of conventionalpolymerization reaction in which π bonds of unsaturatedcarbon are opened by the radical attack and propagate via theattachment of new monomer molecules. The vast majority ofrecent research on plasma polymerization of AA has beenperformed in the pulsed mode.If the glow discharge is operated in an organic gas at

sufficiently high pressure, plasma polymerization may be forcedto proceed with the formation of nanoparticles (NP) in thedischarge volume instead of the growth of thin films onadjacent surfaces. The phenomena of the formation and growthof hydrocarbon, organosilicon, and fluorocarbon plasmapolymer NPs have been widely studied in the field of dustyplasmas;16 yet, rather surprisingly, NPs enriched with specificfunctional groups have been studied much less. NitrogenatedNPs were synthesized either by rf magnetron sputtering ofnylon or by plasma polymerization of volatile hydrocarbons intheir mixtures with N2.

16,17 A recent report showed that such

Received: February 16, 2018Revised: March 26, 2018Published: March 26, 2018

Article

pubs.acs.org/JPCBCite This: J. Phys. Chem. B 2018, 122, 4187−4194

© 2018 American Chemical Society 4187 DOI: 10.1021/acs.jpcb.8b01648J. Phys. Chem. B 2018, 122, 4187−4194

C:H:N NPs produced in the acetylene/Ar/N2 plasma can bevery effective as linker-free nanocarriers of bioactive cargo withgood permeability through the cell membrane and withnegligible cytotoxicity.18

The aim of this work is to show that carboxyl-enriched NPscan be produced by performing plasma polymerization of AA.We chose a mechanistic approach to describe the NP size, theflux, the mass flux, and the concentration of the carboxyl groupsin dependence on the pulsing parameters to correlate theexperimental findings with those from thin film deposition andto get insight into the mechanisms of the plasma polymer-ization of AA including the mechanisms of NP nucleation andgrowth.

■ EXPERIMENTAL SECTIONThe experiments were performed by exploiting a depositionsystem comprising a gas aggregation cluster source mountedvertically onto a deposition chamber (Figure 1). The systemwas pumped by rotary and diffusion pumps to a base pressureof 10−4 Pa.

The GAS itself consisted of a water-cooled cylindrical (innerdiameter ϕ = 62 mm) vacuum chamber with a conical lid and a1 mm orifice at the end. The GAS was equipped with a stainlesssteel planar rf electrode (ϕ 2 in.). The electrode was poweredby an rf generator (Dressler Cesar) via a matching unit.Continuous wave (CW) and pulsed modes were used to initiateplasma. The experiments were performed under average powerPav fixed at 40 W and pulse repetition frequency PRF fixed at 20kHz, unless stated otherwise. Pulsing was performed withsimultaneously changing time on ton and time off toff to controlduty cycle as D = ton/(ton + toff).

The experiments were performed in a mixture of acrylic acid(AA, Sigma-Aldrich, purity 99%) with argon (Linde, purity99.996%). A flask with a liquid monomer was connected to theGAS through a Kelraz-sealed needle valve (Lurt J. Lesker),whereas a tank with Ar was connected to the GAS via anautomatic flow controller (MKS Instruments). After pumpingthe system, 11 sccm of Ar was introduced to the GAS to createthe pressure of 70 Pa. Subsequently, the needle valve wasopened to introduce vapors of AA and to adjust the overallpressure in the GAS at 100 Pa. Thus, the 30/70 AA/Ar mixturewas obtained and used for all of the experiments.The nanoparticles of acrylic acid plasma polymer (ppAA

NPs) were produced in the GAS and dragged by the gas flowthrough the orifice to the deposition chamber where they werecollected on silicon substrates. Scanning electron microscopy(SEM, Tescan Mira III) was used to determine the sizedistribution of the NPs without any additional metallizationapplied. Atomic force microscopy (AFM, Ntegra Prima, NT-MDT) was used in an intermittent contact mode underambient air with supersharp cantilevers (SHR75, Nano&More,spring constant is 3 N/m, tip radius is better than 1 nm) and at256 × 256 data points. The measurements were performed at30% damping of free oscillation amplitude to minimize thedeformation 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). Thespectra were acquired at a constant takeoff angle of 90°. An AlKα source (1486.6 eV, 200 W, Specs) was used to generate X-rays. Wide spectra were recorded at a pass energy of 40 eV(dwell time 100 ms, step 0.5 eV) for a binding energy range of0−1100 eV, whereas high-resolution spectra were gained at apass energy of 10 eV (dwell time 100 ms, step 0.05 eV, 10repetitions). The spectra were referenced to the aliphaticcarbon peak at 285.0 eV. The high-resolution C 1s XPS signalswere fitted with an accuracy of ±0.1 eV by four components inaccordance with the protocol well established for ppAA thinfilm deposition:19−23 the C−C/C−H peak at 285.0 eV, the C−O peak at 286.5 eV, the CO peak at 288.0 eV, and the O−CO peak at 289.1 eV.

■ RESULTS AND DISCUSSIONPlasma polymerization of acrylic acid was deliberately run at theelevated pressure of 100 Pa to favor the formation of NPs in thegas phase. The 30% AA/70% Ar mixture was found optimal interms of stability of the NP synthesis. The experiments wereperformed both in the CW and in the pulsed modes withdifferent duty cycles. The average power was set constant at 40W; thus, the power Pon delivered to the discharge during tonincreased with the decreasing duty cycle (Table 1). Figure 2shows the examples of the SEM images of the ppAA NPs independence on the duty cycle (the corresponding sizehistograms are shown in Figure S1 of the SupportingInformation). The NPs were successfully synthesized both inthe CW and in the pulsed mode yet with a different size andflux (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 largeramounts of smaller NPs. The phenomenon is consistent withan increase of Pon which, under the constant monomer flow,results in an increase of the average energy supplied permonomer molecule Emean and leads to the enhancement of themonomer fragmentation (Table 1; for the details of thecalculation of Emean see Supporting Information).24 A similar

Figure 1. Scheme of the gas aggregation cluster source used for thesynthesis of ppAA NPs.

The Journal of Physical Chemistry B Article

DOI: 10.1021/acs.jpcb.8b01648J. Phys. Chem. B 2018, 122, 4187−4194

4188

effect was previously observed in the case of fluorocarbonplasma polymer25 and nylon-sputtered NPs.17 The values ofEmean in Table 1 are deliberately overestimated because not allof the energy supplied to the plasma goes to the bond cleavage.Nevertheless, they are worth comparing, at least roughly, withthe dissociation energy of all bonds in a molecule of acrylicacid, which makes ED = 43 eV.26 The comparison indicates thatthe plasma polymerization proceeds under highly energeticconditions for all of the experiments in question. However, inthe CW mode, Emean < ED and it can be expected that a certainamount of the monomer molecules remains intact. In thepulsed mode, the plasma polymerization runs in a monomer-deficient regime so that the heavy fragmentation of themonomer molecules occurs when the plasma is on. Theformation of a higher concentration of nucleation centers(radicals and ions, see below) under the constant monomerfeed is responsible for the higher concentration of the NPs thatgrow to a smaller size.The SEM images also point to the coagulation of the NPs

occurring at smaller duty cycles (longer toff) and resulting in thedeposition of larger agglomerates. The effect is better observedin high-resolution AFM images (Figure 3) where two samplesprepared at different duty cycles are shown. At the duty cycle of50%, the NPs are produced for the most part as individualentities. The spherical shape points to the isotropic accretion oftheir volume and indicates that no preferential direction for theattachment of polymer-forming species exists in the plasma.High-resolution AFM probe reveals the smooth surface textureof the NPs, at least at the level of tip curvature radius andthermal fluctuations of the probe. The smooth textureevidences that significant redistribution of the incomingmaterial occurs during the NP growth as opposed to roughor even fractal surfaces produced when the sticking probabilityof newly arriving species is close to 1.27,28 For the duty cycle of32%, individual NPs can be distinguished as constituents oflarger agglomerates, although their surface texture remainssmooth. Thus, an additional mechanism appears that causes theNPs to stick together at smaller duty cycles.The NP size and flux were obtained for all of the duty cycles

studied and are summarized in Figure 4. Here, solid symbolscorrespond to the analysis of all NPs as individual entities,regardless of the fact that part (or all) of them constitute largeragglomerates. Open symbols correspond to the data calculatedby considering agglomerates as bigger particles. The character-istic size of an agglomerate is taken in this case as the diameterof a spherical particle occupying the same surface area. For both

approaches, the trend is confirmed that the NP size decreasesand the flux increases with decreasing duty cycle at constantaverage power. The changes are more prominent ifagglomerates are treated as collections of individual NPs withtheir size ranging from 93 ± 14 nm at the CW mode to 31 ± 5nm at 32% duty cycle.

Table 1. Pulsing Parameters Used for the Synthesis of ppAANPs

T, μs ton, μs toff, μs Pon, W Emean, eV/molecule

duty cycle, % (PRF 20 kHz, Pav 40 W)100 40 3680 50 40 10 50 4570 50 35 15 57 5160 50 30 20 67 6050 50 25 25 80 7240 50 20 30 100 9032 50 16 34 125 112PRF, kHz (duty cycle 32%, Pav 25 W)30 33 11 22 78 7020 50 16 34 78 7010 100 32 68 78 70

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

The Journal of Physical Chemistry B Article

DOI: 10.1021/acs.jpcb.8b01648J. Phys. Chem. B 2018, 122, 4187−4194

4189

Taking into account that only individual NPs are formed athigher duty cycles and that their agglomeration prevails at lowerduty cycles, the effect can be related to charging of the NPsduring ton and to the loss of the charge during toff. One shouldbear in mind that the residence time, i.e., the time that the NPsspend in the GAS tres = 1 s (for the details of the calculation seeSupporting Information), is much longer than the pulsingperiod T = 50 μs, and therefore, the NPs experience manyplasma cycling events on their way along and out of the GAS.When in plasma, particles acquire a floating potential which isnegative with respect to the plasma potential. Negativelycharged NPs should then experience the Coulomb repulsionwhich prevents them from agglomeration, in a close analogywith the charge-driven agglomeration of NPs in solutions. If theplasma is turned on and off intermittently, the NP chargebalance is determined by how fast the floating potential isestablished during ton and how fast it is lost during toff. Forpulsed capacitively coupled discharges operated at several Papressure in Ar and in AA, electrons were shown to be lostwithin 150 μs from the beginning of toff (although large ionsmay survive significantly longer).29 At higher pressure used inour work, the time scale can be shorter. For example, the Bohm

velocity = ≅v 10kTMB

4

i

e m/s can be calculated for the mean

electron energy of kTe = 1 eV measured in this type of theGAS30 and for the mass of acrylic acid molecular ion Mi = 72amu. If vB is related to the characteristic size of the GAS (innerϕ of 62 mm) then the characteristic time of severalmicroseconds is obtained. The characteristic time determinesthe time scale required for the ions to get lost on the walls andfor 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 losingtheir charge and to trigger the onset of their agglomeration. Atlarger duty cycles, toff expires before or not long after the plasmacollapses and the electrostatic repulsion prevents the NPs fromthe agglomeration.The hypothesis is further supported by performing the

experiments with constant Pav and duty cycle but with differentpulse 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 ofexperiments. At PRF = 30 kHz, toff is 22 μs and theagglomeration is just about to become apparent. The depositis represented by a mixture of individual NPs and theiragglomerates, but the individual NPs prevail. Reducing the PRFto 20 kHz prolongs toff and, hence, increases the probability ofthe agglomeration. At PRF = 10 kHz and toff = 68 μs, aninterconnected maze of the agglomerates is formed with aminimal contribution from individual NPs.The chemical composition of the ppAA NPs was analyzed by

XPS in dependence on the duty cycle. The C 1s spectra shownin Figure 6 are similar to those typically obtained for thin filmsof plasma-polymerized acrylic acid.20−23 The plasma polymer isrepresented by a hydrocarbon network (C−C/C−H bonds)bearing a multitude of chemical functionalities which can be

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

Figure 4. Dependence of the NP size and flux on the duty cycle; Peff40 W, PRF 20 kHz. Solid symbols correspond to the size and the fluxcalculated without taking into account that part of the NPs areagglomerated. Open symbols correspond to the size and the fluxcalculated by counting agglomerates as bigger particles of an irregularshape; “size” of an agglomerate is calculated as π= A4 / , where A isthe area occupied by the agglomerate.

The Journal of Physical Chemistry B Article

DOI: 10.1021/acs.jpcb.8b01648J. Phys. Chem. B 2018, 122, 4187−4194

4190

classified in terms of the XPS analysis into three groups ofatomic carbon bound with oxygen with one, two, or threebonds. The latter component is typically attributed to thecarboxyl or ester groups and is a matter of primary scientificinterest. The spectra in Figure 6 demonstrate that theconcentration of the O−CO groups increases withdecreasing duty cycle, although the difference is not large.The concentration of 9.0 and 12.0 atom % is detected for theCW and the pulsed mode with 32% of duty cycle, respectively.These values suggest that less than one-half of the carboxyl

groups present in the precursor (33 atom % of C) survive theplasma conditions and become incorporated into the plasmapolymer.An interesting dependence arises if the concentration of the

O−CO groups is summarized for all duty cycles studied andplotted in Figure 7a where two distinct regimes can beidentified. At higher duty cycles, the increase of toff (and therelated increase of Pon) is accompanied by a significantreduction of the efficiency of the O−CO retention downto 4.0 atom %. In contrast, the O−CO concentration startsto increase at lower duty cycles, restores back to the value ofthe CW mode, and reaches beyond it at the smallest duty cycleof 32%. Furthermore, the mass flux borne by the NPs ontosubstrates closely replicates the O−CO dependence (Figure7b; for details of the calculation of the mass flux see SupportingInformation). Two regimes are also identified that show theefficiency of the NP formation either decreasing or increasingwith the duty cycle. Both regimes are demarcated by the dutycycle of about 60%. These relations indicate that there exists ashift in the mechanism of the NP formation that becomesdominated by different plasma polymerization processes at aduty cycle < 60% (corresponding to ton < 30 μs and toff > 20μs).For vinyl-bearing compounds in general and for acrylic acid

in particular, conventional chemical reactions may take place inplasma involving an initiator attack on the double bondfollowed by the chain propagation via the attachment of anintact monomer molecule. In millisecond pulsed discharges, theconventional polymerization pathway is further favored becauseof the extinction of plasma during toff which leads to the chainpropagation unperturbed by the action of plasma. Hence,

Figure 5. SEM images of ppAA NPs deposited on Si substrates in thepulsed mode at different PRF, Pav 25 W, duty cycle 32%; depositiontime is 10 s.

Figure 6. C 1s XPS of ppAA NPs in dependence on the duty cycle, Pav40 W, PRF 20 kHz.

The Journal of Physical Chemistry B Article

DOI: 10.1021/acs.jpcb.8b01648J. Phys. Chem. B 2018, 122, 4187−4194

4191

significant mass accumulation of the plasma polymer occursduring toff, and better retention of the monomer structure (thecarboxyl groups in the case of AA) is achieved. The extensiveresearch on the deposition of ppAA thin films has confirmedthis trend.12,29,31,32

Taking into account that the ionization potential of organiccompounds is significantly higher than the typical 3−4 eV bonddissociation energy and that the mean electron energy is about1 eV, the plasma is enriched with radicals as the products of themolecular bond dissociation. Thus, the thin film communityhas agreed on the radical-induced chain propagation as aprimary chemical route during toff (Scheme 1, reaction1).12,29,31,32 The same reasoning can be brought forward forthe ppAA NPs to explain the increasing mass flux and theconcentration of the O−CO groups with the increasing toff inthe small duty cycle region. Although Pon increases here to thehigh values, the corresponding decrease of ton may imposetemporal constraints on the kinetics of plasma chemistry. Forpulsed AA/Ar discharges, the bias and the steady-state plasmawere shown to establish at the time scale from a few μs up to 20μs after the beginning of ton.

33,34 The small duty cycles used inour work are characterized by the same time scale of ton whichmeans that equilibrium of the plasma phase may not bereached. Hence, intact monomer molecules may survive andparticipate in radical-induced chain propagation reactionsregardless of the high power input.In the high duty cycle region, a decaying trend of the O−

CO and the mass flux was established. Here, we attribute thedecay to the deteriorative influence of ionic species. Theimportant role of negative and positive ions in plasmas of

Figure 7. (a) Retention of the O−CO functional groups aswitnessed by XPS; (b) NP mass flux onto substrates in dependence onduty cycle (Pav 40 W, PRF 20 kHz).

Scheme 1. Potential Chemical Pathways Occurring in the Formation of NPs by Plasma Polymerization of Acrylic Acida

a(1) Radical-induced chain propagation; (2) dissociative electron attachment followed by anionic chain propagation and accompanied by the loss ofthe HCOOH and CO species; (3) proton attachment followed by H+ transfer chain propagation and accompanied by the loss of the HCOOH andCO species. M is the monomer molecule; [M − H]− and [M + H]+ are dehydrogenated negative and protonated positive molecular ions.

The Journal of Physical Chemistry B Article

DOI: 10.1021/acs.jpcb.8b01648J. Phys. Chem. B 2018, 122, 4187−4194

4192

carboxylic acids was previously highlighted both forlow29,31,34−38 and high pressure discharges.39,40 Anionicpolymerization via dissociative electron attachment (Scheme1, reaction 2) and cationic polymerization via proton transfer(Scheme 1, reaction 3) were shown to contribute to theformation of heavy ionic oligomers by the addition of intactmonomer molecules to ions. In our experiments, however,anionic species are unlikely to attach to negatively charged NPsby reason for the Coulomb repulsion. Furthermore, at least 2ms of toff were required for these reactions to occur, and at theshorter afterglow time the ionic oligomers were not detected bytime-resolved measurements.37,38 Finally, the ion−moleculeoligomerization routes should lead to the increase of the O−CO content in the ppAA NPs, which is not the case of thehigh duty cycle region. Thus, the ionic polymerization cannotexplain the decay of the NP mass flux and the O−CO groupretention in the large duty cycle region. On the other hand,collisions of the cationic species with the intact monomermolecules may lead to condensation reactions with theelimination of low molar mass molecules such as HCOOHand CO,40 especially at higher collision energies. Suchdissociation results in the formation of carboxyl-deficientmoieties as shown in the right-hand side parts of reaction 3of Scheme 1. We hypothesize that incomplete collapse of theplasma during short toff at large duty cycles results in thedeteriorative (in terms of the O−CO retention and the massflux) contribution from positive ions to the NP growth. Thecontribution becomes more deteriorative with increasing Pon,until toff becomes sufficiently long to ensure the completecollapse of the plasma and ton becomes sufficiently short toensure the preservation of intact monomer molecules forradical-driven polymerization to occur.

■ CONCLUSIONSAcrylic acid was found to be capable of polymerizing in plasmawith the formation of NPs. Pulsing the discharge at a constantaverage power of 40 W, a constant period of 50 μs, and withvarying the duty cycle allows tailoring the NP size, the massflux, and the retention of the carboxyl groups. The NP sizedecreases and their number increases with the decreasing dutycycle; however, the mass flux and the concentration of the O−CO groups show a more complex dependence. Two regimesof the NP growth have been distinguished. In the large dutycycle regime when ton > 30 μs and toff < 20 μs, the short time offdoes not suffice for plasma to extinguish completely. Energeticcollisions of positive ions with the growing NPs result in thedetachment of low-mass species at the expense of the retentionof the carboxyl groups. The efficiency of the plasmapolymerization decreases with the decrease of the duty cycle,and the concentration of the O−CO groups reaches theminimal value of 4 atom % in this region. The incompleteextinction of the plasma leads also to the conservation of theNP negative charge which prevents them from the agglomer-ation. In the small duty cycle regime when ton < 30 μs and toff >20 μs, the time off becomes long enough to allow for thecollapse of the plasma and for the unperturbed conventionalpolymerization reaction, which runs via the radical-inducedopening of the double bond and chain propagation through theaddition of intact monomer molecules. The mass flux borne bythe NPs increases and the concentration of the O−COgroups reaches the value of 12 atom %. The collapse of theplasma leads to the loss of the negative charge by the NPs, andthey 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.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcb.8b01648.

Normalized size histograms of ppAA NPs, calculations ofmass flux, concentration of the NPs in the GAS, averageenergy supplied per monomer molecule (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: pleskunov@protonmail.ch. Phone: +420951552284.

ORCIDPavel Pleskunov: 0000-0002-5291-9559Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the grant GACR-17-12994S fromthe Grant Agency of the Czech Republic. P. P., D. N., and R. T.also appreciate the support from the student grant SVV260444/2017 of Charles University.

■ REFERENCES(1) Biederman, H. Plasma Polymer Films; Imperial College Press:London, 2004.(2) Friedrich, J. Mechanisms of Plasma Polymerization - Reviewedfrom a Chemical Point of View. Plasma Processes Polym. 2011, 8, 783−802.(3) Thebault, P.; Boujday, S.; Senechal, H.; Pradier, C.-M.Investigation of an Allergen Adsorption on Amine- and Acid-Terminated Thiol Layers: Influence on Their Affinity to SpecificAntibodies. J. Phys. Chem. B 2010, 114, 10612−10619.(4) Whittle, J. D.; Short, R. D.; Steele, D. A.; Bradley, J. W.; Bryant,P. M.; Jan, F.; Biederman, H.; Serov, A. A.; Choukurov, A.; Hook, A.L.; et al. Variability in Plasma Polymerization Processes - AnInternational Round-Robin Study. Plasma Processes Polym. 2013, 10,767−778.(5) Michelmore, A.; Steele, D. A.; Whittle, J. D.; Bradley, J. W.; Short,R. D. Nanoscale Deposition of Chemically Functionalised Films viaPlasma Polymerisation. RSC Adv. 2013, 3, 13540.(6) Michelmore, A.; Whittle, J. D.; Bradley, J. W.; Short, R. D. WherePhysics Meets Chemistry: Thin Film Deposition from ReactivePlasmas. Front. Chem. Sci. Eng. 2016, 10, 441−458.(7) Valsesia, A.; Colpo, P.; Manso Silvan, M.; Meziani, T.; Ceccone,G.; Rossi, F. Fabrication of Nanostructured Polymeric Surfaces forBiosensing Devices. Nano Lett. 2004, 4, 1047−1050.(8) Siow, K. S.; Britcher, L.; Kumar, S.; Griesser, H. J. PlasmaMethods for the Generation of Chemically Reactive Surfaces forBiomolecule Immobilization and Cell Colonization - A Review. PlasmaProcesses Polym. 2006, 3, 392−418.(9) Donegan, M.; Dowling, D. P. Protein Adhesion on Water StableAtmospheric Plasma Deposited Acrylic Acid Coatings. Surf. Coat.Technol. 2013, 234, 53−59.(10) Daw, R.; Candan, S.; Beck, A. J.; Devlin, A. J.; Brook, I. M.;MacNeil, S.; Dawson, R. A.; Short, R. D. Plasma Copolymer Surfacesof Acrylic acid/1,7 Octadiene: Surface Characterisation and the

The Journal of Physical Chemistry B Article

DOI: 10.1021/acs.jpcb.8b01648J. Phys. Chem. B 2018, 122, 4187−4194

4193

Attachment of ROS 17/2.8 Osteoblast-like Cells. Biomaterials 1998,19, 1717−1725.(11) De Bartolo, L.; Morelli, S.; Lopez, L. C.; Giorno, L.; Campana,C.; Salerno, S.; Rende, M.; Favia, P.; Detomaso, L.; Gristina, R.; et al.Biotransformation and Liver-Specific Functions of Human Hepato-cytes in Culture on RGD-Immobilized Plasma-Processed Membranes.Biomaterials 2005, 26, 4432−4441.(12) Detomaso, L.; Gristina, R.; Senesi, G. S.; D’Agostino, R.; Favia,P. Stable Plasma-Deposited Acrylic Acid Surfaces for Cell CultureApplications. Biomaterials 2005, 26, 3831−3841.(13) Cho, S.-I.; Dhayal, M. Applications of Plasma PolymerisedAcrylic Acid Coatings to Control the Growth of Cancer Cells(Sarcoma 180). J. Biomed. Nanotechnol. 2006, 2, 137−143.(14) Carton, O.; Ben Salem, D.; Bhatt, S.; Pulpytel, J.; Arefi-Khonsari,F. Plasma Polymerization of Acrylic Acid by Atmospheric PressureNitrogen Plasma Jet for Biomedical Applications. Plasma ProcessesPolym. 2012, 9, 984−993.(15) Zhang, H.; Brown, K. D.; Lowe, S. P.; Liu, G.-S.; Steele, D.;Abberton, K.; Daniell, M. Acrylic Acid Surface-Modified Contact Lensfor the Culture of Limbal Stem Cells. Tissue Eng. Tissue Eng., Part A2014, 20, 1593−1602.(16) Choukourov, A.; Pleskunov, P.; Nikitin, D.; Titov, V.; Shelemin,A.; Vaidulych, M.; Kuzminova, A.; Solar, P.; Hanus, J.; Kousal, J.; et al.Advances and Challenges in the Field of Plasma Polymer Nano-particles. Beilstein J. Nanotechnol. 2017, 8, 2002−2014.(17) Polonskyi, O.; Kylian, O.; Solar, P.; Artemenko, A.; Kousal, J.;Slavínska, D.; Choukourov, A.; Biederman, H. Nylon-SputteredNanoparticles: Fabrication and Basic Properties. J. Phys. D: Appl.Phys. 2012, 45, 495301.(18) Santos, M.; Michael, P. L.; Filipe, E. C.; Chan, A. H. P.; Hung,J.; Tan, R. P.; Lee, B. S. L.; Huynh, M.; Hawkins, C.; Waterhouse, A.Plasma Synthesis of Carbon-Based Nanocarriers for Linker-FreeImmobilization of Bioactive Cargo. ACS Appl. Nano Mater. 2018, 1,580−594.(19) Sciarratta, V.; Vohrer, U.; Hegemann, D.; Muller, M.; Oehr, C.Plasma Functionalization of Polypropylene with Acrylic Acid. Surf.Coat. Technol. 2003, 174−175, 805−810.(20) Jafari, R.; Tatoulian, M.; Morscheidt, W.; Arefi-Khonsari, F.Stable Plasma Polymerized Acrylic Acid Coating Deposited onPolyethylene (PE) Films in a Low Frequency Discharge (70kHz).React. Funct. Polym. 2006, 66, 1757−1765.(21) Sardella, E.; Favia, P.; Dilonardo, E.; Petrone, L.; d’Agostino, R.PE-CVD of Acid/base Coatings from Acrylic Acid and AllylamineVapours. Plasma Processes Polym. 2007, 4, S781−S783.(22) Fahmy, A.; Mohamed, T. a.; Schonhals, A. Structure of PlasmaPoly(Acrylic Acid): Influence of Pressure and Dielectric Properties.Plasma Chem. Plasma Process. 2015, 35, 303−320.(23) Cools, P.; Sainz-García, E.; Geyter, N. D.; Nikiforov, A.; Blajan,M.; Shimizu, K.; Alba-Elías, F.; Leys, C.; Morent, R. Influence of DBDInlet Geometry on the Homogeneity of Plasma-Polymerized AcrylicAcid Films: The Use of a Microplasma-Electrode Inlet Configuration.Plasma Processes Polym. 2015, 12, 1153−1163.(24) Bauer, M.; Schwarz-Selinger, T.; Kang, H.; Keudell, A. Von.Control of the Plasma Chemistry of a Pulsed Inductively CoupledMethane Plasma. Plasma Sources Sci. Technol. 2005, 14, 543−548.(25) Feng, J. C.; Huang, W.; Fu, G. D.; Kang, E.-T.; Neoh, K.-G.Deposition of Well-Defined Fluoropolymer Nanospheres on PETSubstrate by Plasma Polymerization of Heptadecafluorodecyl Acrylateand Their Potential Application as a Protective Layer. Plasma ProcessesPolym. 2005, 2, 127−135.(26) Luo, Y.-R. Handbook of Bond Dissociation Energies in OrganicCompounds; CRC Press, 2003.(27) Witten, T. A.; Sander, L. M. Diffusion-Limited Aggregation, aKinetic Critical Phenomenon. Phys. Rev. Lett. 1981, 47, 1400−1403.(28) Pelliccione, M.; Lu, T.-M. Evolution of Thin Film Morphology.Materials Science; Springer New York: New York, 2008; Vol. 108.(29) Voronin, S. A.; Zelzer, M.; Fotea, C.; Alexander, M. R.; Bradley,J. W. Pulsed and Continuous Wave Acrylic Acid Radio Frequency

Plasma Deposits: Plasma and Surface Chemistry. J. Phys. Chem. B2007, 111, 3419−3429.(30) Kousal, J.; Kolpakova, A.; Shelemin, A.; Kudrna, P.; Tichy, M.;Kylian, O.; Hanus, J.; Choukourov, A.; Biederman, H. Monitoring ofConditions inside Gas Aggregation Cluster Source during Productionof Ti/TiO X Nanoparticles. Plasma Sources Sci. Technol. 2017, 26,105003.(31) Fraser, S.; Short, R. D.; Barton, D.; Bradley, J. W. A Multi-Technique Investigation of the Pulsed Plasma and Plasma Polymers ofAcrylic Acid: Millisecond Pulse Regime. J. Phys. Chem. B 2002, 106,5596−5603.(32) Short, R. D.; Steele, D. A. Testing the Hypothesis: Commentson Plasma Polymerisation of Acrylic Acid Revisited. Plasma ProcessesPolym. 2010, 7, 366−370.(33) Booth, J. P.; Cunge, G.; Sadeghi, N.; Boswell, R. W. TheTransition from Symmetric to Asymmetric Discharges in Pulsed 13.56MHz Capacitively Coupled Plasmas. J. Appl. Phys. 1997, 82, 552−560.(34) Voronin, S. A.; Alexander, M. R.; Bradley, J. W. Time-ResolvedMass and Energy Spectral Investigation of a Pulsed PolymerisingPlasma Struck in Acrylic Acid. Surf. Coat. Technol. 2006, 201, 768−775.(35) O’Toole, L.; Beck, A. J.; Ameen, A. P.; Jones, F. R.; Short, R. D.Radiofrequency-Induced Plasma Polymerisation of Propenoic Acidand Propanoic Acid. J. Chem. Soc., Faraday Trans. 1995, 91, 3907.(36) Barton, D.; Shard, A. G.; Short, R. D.; Bradley, J. W. The Effectof Positive Ion Energy on Plasma Polymerization: A Comparisonbetween Acrylic and Propionic Acids. J. Phys. Chem. B 2005, 109,3207−3211.(37) Swindells, I.; Voronin, S. A.; Fotea, C.; Alexander, M. R.;Bradley, J. W. Detection of Negative Molecular Ions in Acrylic AcidPlasma: Some Implications for Polymerization Mechanisms. J. Phys.Chem. B 2007, 111, 8720−8722.(38) Swindells, I.; Voronin, S. A.; Bryant, P. M.; Alexander, M. R.;Bradley, J. W. Temporal Evolution of an Electron-Free Afterglow inthe Pulsed Plasma Polymerisation of Acrylic Acid. J. Phys. Chem. B2008, 112, 3938−3947.(39) Marotta, E.; Paradisi, C. Positive Ion Chemistry of Esters ofCarboxylic Acids in Air Plasma at Atmospheric Pressure. J. MassSpectrom. 2005, 40, 1583−1589.(40) Moix, F.; McKay, K.; Walsh, J. L.; Bradley, J. W. Atmospheric-Pressure Plasma Polymerization of Acrylic Acid: Gas-Phase IonChemistry. Plasma Processes Polym. 2016, 13, 236−240.

The Journal of Physical Chemistry B Article

DOI: 10.1021/acs.jpcb.8b01648J. Phys. Chem. B 2018, 122, 4187−4194

4194