Binding of Nanoparticles to Aminated Plasma Polymer Surfaces is

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C: Physical Processes in Nanomaterials and Nanostructures

Binding of Nanoparticles to Aminated Plasma Polymer Surfaces Is Controlled by Primary Amine Density and Solution pH Shima Taheri, Juan-Carlos Ruiz, Andrew Michelmore, Melanie Njariny MacGregor, Renate Foerch, Peter J. Majewski, and Krasimir Vasilev J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03382 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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The Journal of Physical Chemistry

Binding of Nanoparticles to Aminated Plasma Polymer Surfaces is Controlled by Primary Amine Density and Solution pH Shima Taheri a, §, ‡, Juan-Carlos Ruiz b, §, ‡, Andrew Michelmore d, Melanie Macgregor a,d , Renate Förch b,c, Peter Majewski d and Krasimir Vasilev a,d,* a

School of Engineering, University of South Australia, Mawson Lakes, SA, 5095, Australia b

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128, Germany c

FhG-ICT-Institut für Mikrotechnik Mainz, Carl-Zeiss-Strasse 18-20, 55129, Germany

d

Future Industries Institute, University of South Australia, Mawson Lakes, SA, 5095 Australia

ABSTRACT. Surface nanoengineering is a valuable tool to create materials with sophisticated properties required to address unmet needs in fields such as medicine, biology or energy. This study examines the dependence of nanoparticle immobilization as a function of surface amine group density. The concentration of surface amine groups was tuned using gradients deposited

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from the plasma phase of allylamine and octadiene (pp-AA/OD) precursors mixtures. Silver nanoparticles capped with carboxylic acid groups (COOH-AgNPs) were used to interrogate the effect of primary amine (-NH2) surface density on nanoparticle’s electrostatic immobilization onto freshly made and aged plasma polymer films. An increase in amine group density could be correlated with greater number of functionalized silver nanoparticles bound to the surface. In addition, the pp-AA/OD surface charge and nanoparticle binding density could be controlled via pH of the AgNPs colloidal solution. The results suggest that pp-AA/OD plasma polymer films are suitable platforms for implementing advanced nanoengineered materials that could be used in many advanced applications ranging from medicine to sensing and electronics.

1. INTRODUCTION More than half a century since the onset of nanoscience, the unique characteristics of nanomaterials continue to trigger intense research in the development of nanotechnologies for industrial endeavors. Often, in order to exploit their properties, nanoparticles (NPs) must be assembled into useful structures onto designated substrates. Controlling the surface assembly of nanoparticles offers the possibility to tune a material topography, wettability and optical properties

1-4

, all of which are pertaining to the design of biosensing, separation and catalysis

systems as well as functional biomaterials for immunodiagnostic, cell therapy or implant coating applications 5. The NPs density and immobilization pattern on the surface are governed by the underlying strength of nanoparticle-substrate interactions, which in turn are governed by the surface functionality of the NPs, the chemical composition of the substrate, and the NPs particle size 6-8.


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Molecular and polymer films are commonly used to anchor nanoparticles because they can be tailored to present a variety of functional groups supporting the immobilization of NPs via covalent, electrostatic, hydrogen or Van der Waals bonding

6, 7

. One of the most common

material support are polymer films, to which NPs can be anchored via direct immobilization of pre-fabricated NPs or in situ reduction of surface adsorbed metal ions into NPs 6, 7, 9-12. While many different approaches exist to prepare polymeric substrates, plasma polymerization is growing in popularity because it allows the deposition of conformal, pinhole-free, adhesive, high-quality films obtained in a relatively simple, all-dry, and single-step deposition process 13-15. Research has shown that plasma polymerized thin films can be used for surface immobilization of bioactive molecules and nanoparticles 16,17, which can then also facilitate their localization and preservation at the cell-biomaterial interface

18-20

. In fact, plasma surface modification

techniques, which are not limited to thin film deposition but can also be used for surface nanotexturing 21,22 and chemical activation 23, 24, have already found multiple practical applications 2527

. Examples include protective coating of biomedical devices

devices for improving interfacial adhesion controlled drug release vectors

37-40

30-32

28, 29

, pretreatment of medical

, fabrication of non-fouling surfaces

, diagnostic biosensors

41, 42

33-36

,

and improving materials

biocompatibility to enhance cell growth and protein adsorption 31, 43-46. Amongst the range of functional coatings that can be generated via plasma polymerization, those rich in nitrogen are particularly useful for biomedical applications

47, 48

. Nitrogen-

containing plasma polymer thin films can be prepared from monomers such as allylamine (AA), heptylamine,

diaminopropane,

ethylenediamine,

oxazolines,

cycloaminopropane

diaminocyclohexane (DACH) as well as by mixtures of hydrocarbon and N-containing gases (i.e. ammonia)

39, 49-56

. The density of the amine groups has been quantified in a number of recent


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publications and differs depending on the plasma process conditions and nature of the gas used. The amine groups available at the surface have been used to immobilize ligands either directly through covalent chemical binding to appropriate reagents or through electrostatic binding using the positively charged quaternary amine group formed in acidic pH conditions

15, 47, 57-58

.

However, active sites at the surfaces modified with plasma polymerization are subject to postreactions and aging 59, 60. Allylamine (AA) has been most commonly utilized as the organic precursor for depositing aminated films by plasma polymerization. The density of amine groups can be easily controlled by plasma power in continuous wave or duty cycle modes, plasma working pressure and monomer deposition time. Aging in air, and stability studies in liquid media of pp-AA films have showed oxygen-uptake and oligomer loss in water, in buffered solutions (pH 2-9) and in ethanol 61

. Post-reactions (aging) can also affect the capability of the thin film to absorb and retain

biomolecules and nanoparticles. Work on plasma co-polymer gradients of AA and a hydrocarbon monomer such as Octadiene (OD) deposited on polymeric Thermanox showed stability improvement in water 62. Moreover, by controlling the flow ratio of both monomers the desirable density and pattern of nitrogen functional groups can be achieved

63

. The goal of the present

work was to fabricate aminated plasma-polymerized films for embedding silver nanoparticles as potentially, improved biocompatible antibacterial coatings films

64

. The prolonged storage life

and applicability of the coating were other important factors that were considered. In this study, plasma copolymerized allylamine (AA)/octadiene (OD) (pp-AA/OD) films were prepared using binary gas mixtures of AA and OD, in order to create deposits with different amine concentrations with the aim to control the number of surface attached nanoparticles

62-67

.

We also examined the effects of the colloidal solution pH and aging of the thin film on capability


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of plasma deposited copolymerized films containing amine groups for binding silver nanoparticles (as a model nanoparticle). In the present work, we report the quantification of NH2 (primary) amine present in pp-AA/OD films using a Peak-Fit-Analyses method (PFA). The present investigations include NH2 quantification in pp-AA/OD deposits after 30-day ageing. Finally, silver nanoparticles (AgNPs) which are functionalized with carboxyl groups are immobilized on different pp-AA/OD films, studying primary amine dependence. X-ray photo electron spectroscopy was used as the main chemical characterization technique.

2. EXPERIMENTAL The preparation of pp-AA/OD surfaces embedded with carboxyl-functionalized silver nanoparticles (MSA-AgNPs) consists of three steps: i) deposition of homogenous pp-AA/OD films with different primary amine concentrations on Thermanox substrates; ii) Synthesis of colloidal AgNPs capped with 2-mercaptosuccinic acid (MSA) to prevent agglomeration; and iii) surface immobilization of MSA-AgNPs onto pp-AA/OD films. In the last step, the pp-AA/OD substrates are immersed for 24 h into a solution of colloidal silver nanoparticles surface modified with carboxylic acid groups which facilitate electrostatic interactions between the aminefunctionalized pp-AA/OD surfaces and carboxyl-functionalized AgNPs.

2.1. PLASMA POLYMER FILM PREPARATION Thermanox cover slips (plastic substrates 13 mm in diameter) were employed in this work. The substrates were washed with ethanol and rinsed with Milli-Q water (resistivity 18.2 MΩ.cm) before being placed in the plasma reactor where they were primed with air plasma (2.5×10-2 mbar, 25 W) for 2 min. AA (98%, Aldrich) and OD (98%, Aldrich) monomers were introduced


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into the plasma chamber (custom-built radio frequency plasma reactor described elsewhere 63,65) with four different gas flow rate ratio of AA/OD 10/0, 7.5/2.5, 5/5 and 2.5/7.5 sccm, respectively 67

. Excitation of the plasma was achieved using 13.56 MHz radio frequency at an input power of

10 W in agreement with previous work for these monomers. The film deposition was carried out at a pressure of 2×10-4 mbar with deposition times of 5 minutes to obtain a film thicknesses of 22.5 ± 1.5 nm as verified by Ellipsometry (Beaglehole Instruments, New Zealand) (Data not shown). Between runs, the reactor was scrubbed and rinsed with ethanol and acetone and was further cleaned by 15 min air plasma operated at 45 W and 4×10-2 mbar.

2.2. NANOPARTICLE SYNTHESIS COOH-functionalized silver nanoparticles were synthesized following the protocol reported by Taheri et al

68

. In a commonly employed procedure, 11 mL of 2 mM AgNO3 (99.99 %,

Aldrich) is mixed with 5 mL of 2 mM 2-mercaptosuccinic acid (MSA, 97%, Aldrich) under vigorous stirring followed by drop-wise addition of 0.5 mL 0.05 M NaBH4 (Aldrich). The color of the solution changed from colorless to dark yellow-brownish within a few seconds. The synthesis was conducted at iced cold condition and the MSA-AgNPs colloidal solutions were sealed and stored in darkness.

2.3. NANOPARTICLE IMMOBILIZATION ON PLASMA POLYMER FILM Carboxyl-functionalized AgNPs were then electrostatically immobilized on pp-AA/OD films, Films with different nitrogen content (pp-AA/OD deposits) were immersed in the colloidal solution of MSA-AgNPs for 24 h and then rinsed thoroughly with Milli-Q water and dried in dry N2. For the samples prepared for ageing study the pp-AA/OD substrates were kept sealed in 24


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well plates at room temperature (23°C). After one month a freshly prepared colloidal solution was used for substrate immersion. The functionalization of AgNPs surface is accomplished during the NPs synthesis by addition of MSA to the reaction vessel. As silver nanoparticles form, the surface functionalization agent (MSA) simultaneously attaches to the NPs, which also imparts an enhanced stability and gives additional control over their size by protecting them from growth (aggregation of AgNPs)

7,68

.

MSA with a thiol group is a good candidate for surface modification of metal NPs as it forms a covalent metal-thiolate (M-S) bond on the nanoparticles surface resulting in self-assembled structures which facilitate immobilization of NPs to pp-AA/OD surfaces and protect AgNPs from aggregation and growth while they are in solution

68,69

. The colloidal solution has a pH of

3.5 (CyberScan pH 510, Eutech Instruments Pte Ltd.), a surface charge of -26.9 mV at 25°C (Zeta Nanosizer, Malvern Instruments, U.K.) and they are stable for several months (data not shown). The average size of silver nanoparticles is 12 nm

68

(Nicomp 380, Nicomp Particle

Sizing Systems, USA).

2.4. SURFACE CHARACTERIZATION The characterization of plasma-deposited films was primarily performed by XPS analysis. A SPECS SAGE spectrometer with a Phoibos 150 hemi-spherical analyzer at a take-off angle of 90º with respect to the substrate surface, and a MCD-9 detector were used to obtain XPS spectra. A non-monochromatic Mg Kα radiation source (hν 1253.6 eV), operated at 10 kV and 20 mA (200 W) was used. The analysis area was circular and 5 mm in diameter. The base pressure was 2×10-6 Pa. The elements present were identified from a survey spectrum recorded over the energy range 0-1200 eV at a pass energy of 100eV and energy steps of 0.5eV. The areas under


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the photoelectron peaks in the spectrum were used to calculate the percentage atomic concentrations using relative sensitivity factors as supplied by the manufacturer. In order to identify the chemical binding states of C1s, high-resolution spectra [HR-XPS] were recorded for pertinent photoelectron peaks using a pass energy of 20 eV and energy steps of 0.1 eV. To account for sample charging, all binding energies were shifted to respect to the aliphatic C1s carbon peak at 285 eV. The processing and component fitting of the high-resolution and survey spectra was performed using CASA XPS software (Neal Fairley, UK, version 2.3.14) with a linear background being subtracted. Atomic force microscopy (AFM) provided topographical images using an NT-MDT NTEGRA SPM in non-contact mode. Silicon nitride non-contact tips coated with gold on the reflective side (NT-MDT, NSG03) were used and had resonance frequencies between 65 and 100 kHz. The maximum range of the scanner was100 µm, and the scan rate of 0.5 Hz. The scanner was calibrated in the x, y, and z directions using 1.5 µm grids with a height of 22 nm.

2.4.1. ANALYSIS OF XPS RESULTS C1s spectra were deconvoluted using synthetic peaks with full-width at half-maxima (FWHM) between 1.6 and 1.7. The peak analysis is based on the works of Pireaux

70

and Beamson and

Briggs 71, and peak assignment is summarized in Figure 1 for all spectra in this work.


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Figure 1. Typical C1s deconvolution and peak assignment used in this work: (a) as deposited and (b) After TFBA derivatization.

2.4.2. QUANTIFICATION OF AMINE GROUPS Varying the relative percentage of AA and OD precursor in the feed gas mixture lead to the formation of organic films with different nitrogen content and also potentially different Ncontaining functional group. The amine functionalized surfaces were derivatized using 4(trifluoromethyl) benzaldehyde (TFBA) as described in the literature in order to determine the density of primary amine group using peak fit analysis 67. In order to quantify the amine groups at the surface the high resolution C1s X-ray photoelectron spectra were fitted with seven peaks (C1 to C7) according to Figure 1. a and b. C6 and C7 peaks are attributed to the new carbon functionalities as a result of the derivatization. The concentration of amine groups on the ppAD/OD surface was calculated with the following Equation 1:

x 100

(Equation 1- quantification of amines)

where A= area of the peaks (C1-C7).


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3. RESULTS AND DISCUSSION In this paper, we investigate the effect of amine functional groups density and surface charge on nanoparticle attachment to the surface.

3.1. SURFACE CHEMISTRY The substrates are coated with a thin layer of amine-containing functional groups via plasma copolymerization of AA and OD monomers. Four AA/OD gas mixture (flow rate 10/0, 7.5/2.5, 5/5 and 2.5/7.5 sccm : sccm) were used to generate pp-AA/OD films with different AA/OD % ratios, as previously described, Figure 2.a 67.

Figure 2. (a) Diagram showing the flow rate setting (sccm) of gas mixture of AA and OD; (b) Elemental composition of pp-AA/OD films as determined from area under the peak of the elemental XPS scan for the different AA/OD mixtures: as deposited (open symbols) and after aging in air for 30 days (full symbols).


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The relative atomic percentages of elements on the plasma polymer surface immediately after deposition and after aging in air are shown in Figure 2.b. All pp-AA/OD deposits present C, N and O. For both fresh and aged samples the nitrogen concentration, [N], of pp-AA/OD films increases with increasing AA flow rate and decreases with increasing OD flow. A maximum nitrogen content of 13.4 ± 0.3 at. % is achieve on the as deposited substrates with 10 sccm nitrogen flow rate. The air aged samples generally show a loss of nitrogen and a gain in oxygen. However, for AA flow rates below 5 sccm, [N] remained constant within experimental error. This is consistent with previous work in this area and has been associated with amine groups reacting with oxygen to form amides, diluting the nitrogen signal 51, 53, 72, 73. The detailed surface analysis by XPS is summarized in Table 1.The [N]/[C] and [O]/[C] % ratios were calculated from the elemental broad spectrum for all surfaces. The overall oxygen concentration in the samples ranges approximately between 4 and 6 relative percent, which is comparable to previous work. This oxygen is assumed to originate from post plasma reactions with air. The high resolution C1s peaks were deconvoluted into 5 components (C1-C5), as illustrated in Figure 1.a, to determine the relative amount of carboxylic acid functions. Carboxyl groups (C5) were found to contribute less than 0.2 % of the C1s peak profile of the as-deposited pp-AA. The derived primary amine group concentration was calculated with the technique described by Gross et al using the Peak-Fit-Analyses method

67, 74

and Equation 1, following derivatization

reaction with 4-trifluoromethyl benzaldehyde (TFBA) 67, 75. According to literature, TFBA reacts to primary amine groups 49, 54, 74, 75. From the TFBA derivatization data it can be seen that for the as deposited pure pp-AA surface, showing an N/C ratio of about 16%, only about 2% can be attributed to primary amine groups


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that were accessible to the TFBA (Table 1). This means that only about 12 percent of the total nitrogen at the surface is in the forms of primary amine groups. For the 5/5 AA/OD gas mixture a 60 % decrease in the overall surface nitrogen concentration [N] was observed in comparison to the pure pp-AA films, which we attributed to a faster deposition rate of OD under these conditions as compared to AA. When comparing the [NH2]/[C] ratios of the as deposited and aged in air samples a clear loss in the primary amine group concentration was observed as the sample surface rearranges to minimize surface energy. In both cases, the [NH2]/[C] ratios decrease with decreasing AA flow rates, and no primary amine groups were detected for the lowest AA flow rate after 30 days substrate aging in air. Details on the phenomena leading to a reduction in plasma polymer films overall functionality overtime (e.g. oxidation and hydrophobic recover processes) and avenues to limit this aging process can be found in recent reviews 76. In the specific case of primary and secondary amines, we identified in previous work that annealing the films in an inert gas or engaging the functional group immediately post deposition as options to preserve active functional groups

67

. In the following, we investigate

how changes in substrate chemistry due to flow rates and film aging impact the binding of AgNPs.

Table 1. Relevance of 1° amine groups and silver concentration by quantification of pp-AA/OD

OD (sccm)

film and pp-AA/OD embedded MSA -AgNPs as deposited and 30-day aged films.

AA (sccm)

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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[N]/[C] (%)

10

0

16.60±0.40

5.50±1.30

7.5

2.5

11.10±0.70

4.60±1.00

5

5

5.30±0.70

3.10±0.80

After AgNPs

As deposited

Immobilization

[O]/[C] (%)

[COOR]/[C] (%)

0 - 0.60

[NH2]/[C] (%)

[COOR]/[C] (%)

1.92±0.22

11.60±0.20

1.10±0.11

5.90±0.70

0.74±0.25

4.40±0.10

After Aging at Ambient for 30 Days [COOR]/[C] (%)

0.2-0.6

After AgNPs Immobilization

[NH2]/[C] (%)

[COOR]/[C] (%)

1.12±0.07

8.00±0.10

0.60±0.11

4.20±0.40

0.21±0.03

2.10±0.50


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2.5

7.5

2.90±0.10

3.80±1.40

0.27±0.03

2.70±0.10

0

1.10±0.20

(a) [N]/[C] and [O]/[C] of the total nitrogen and oxygen concentration (b) and [NH2]/[C] the 1° amine group concentration using the derivatization reaction with TFBA. (c) [COOR]/[C] of immobilized-AgNPs on pp-AA/OD films, obtained from C 1s deconvolutions, as function of AA/OD monomer ratio for: full symbols).

3.2. NANOPARTICLE IMMOBILIZATION The immobilization of nanoparticles onto different materials of interest for various applications, requires their surface to be modified with functional groups like thiol, amine, carboxyl or pyridyl. This surface modification is necessary for the formation of covalent, electrostatic or hydrogen bonds between the NP and the substrate 77. Among metal NPs, silver is particularly interesting because of its unique antibacterial properties for systemic application as coating in biomedical devices

64, 78, 79

. The method of AgNPs preparation used here is based on

the Creighton method, where silver nitrate (AgNO3 as silver precursor) is reduced with sodium borohydride (NaBH4) 80. Surface capping with 2-mercaptosuccinic acid was chosen to facilitate electrostatic binding between negatively charged carboxyl groups and positively charged primary amines present on the surfaces. MSA-AgNPs were immobilized on the different pp-AA/OD films as deposited and aged in air. Elemental composition of pp-AA/OD embedded with AgNPs shows C, N, O, S and Ag, as expected from the atomic composition of the substrates and functionalized nanoparticles. The atomic silver content on different pp-AA/OD surfaces were quantified from XPS-survey spectrum and constitute a direct estimate of the amount of AgNPs bound to the substrates. They are shown in Figure 3. a. The as deposited 10/0 AA/OD film absorb around 13.5 At% silver, while this amount decreased to 5.9 At% for 2.7/7.5 as deposited pp-AA/OD film. Aging in air


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reduces the capacity of the film absorb silver and as deposited 10/0 can hold 9.5 At% and only2.2 At% for 2.7/7.5 AA/OD. Furthermore, we found that careful analysis of the COOH component of the high resolution C1s peak provide an indirect way to quantify AgNPs binding. Since the carboxyl groups component represent less than 0.6 % in the C1s-fitting of as-deposited and aged pp-AA/OD films, any increase following exposure to the AgNPs solution may be attributed to the COOH functional group capping the immobilized nanoparticles. Figure 3 b and c are showing the C1s deconvolution (C1-C5) and peak fit analysis of 10/0 pp-AA/OD films before and after AgNPs immobilization. The carboxylic acid group concentration [COOH] post AgNPs immobilization reported in Table 1 for all substrates is plotted as a function of the precursor flow rates in Figure 3.d. [COOH] consistently increases with increasing AA flows from 2.7 ± 0.1 % to 11.6 ± 0.2 %, when as-deposited pp-AA/OD is the platform for immobilization, and from 1.1 ± 0.2 % to7.9 ± 0.1 %, after ageing. It is worth noting that the Ag atomic content and [COOH] have a linear relationship, with a correlation coefficient of 0.93 (Figure 3.e). Either quantities can therefore be used to evaluate the amount of MSA-AgNPs present on the surface.


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Figure 3. (a) Ag3d atomic concentration obtained from survey spectrum and (d) [COOR]/[C] of immobilized-AgNPs on pp-AA/OD films, obtained from C1s deconvolutions, as function of AA/OD monomer ratio for: as deposited (open symbols) and 30-day-aged films (full symbols). Typical C1s deconvolution of (b) as deposited pp-AA/OD films and (c) pp-AA/OD with immobilized MSA-AgNPs. The C5 peak component is indicative of [COOH] groups and is assumed here to represent predominantly the MSA-AgNPs nanoparticles. (e) Ag3d atomic content and [COOR]/[C] linear relationship, ([COOR]/[C]= -1.7+0.9[Ag]); (f) Atomic force microscopy (AFM) images of absorbed MSA-AgNPs onto deposited pp-AA/OD films.


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The trend observed in both [Ag] and [COOH] is consistent with the measured amine group density which is higher for as-deposited pp-AA/OD deposits than for 30-day aged films and decreases with decreasing AA flow rates. These results seem to indicate that the surface nitrogen content affect the immobilization of COOH-functionalized AgNPs, as confirmed by AFM imaging shown in Figure 3.f. In Figure 4.a and b, the [N]/[C] % and [NH2]/[C] % ratios are plotted as a function of percentage of immobilized [Ag] for as deposited and aged samples. Although [Ag] increases with the [N]/[C] ratios%, there is a significant shift between the data sets for as deposited and 30days aged samples. It has been shown previously that the increase in oxygen for aminated plasma polymer surfaces after exposure to air is predominantly due to amine groups accepting oxygen, forming amides

53

. This has the effect of diluting the surface nitrogen as more atoms are

incorporated into the surface. However as shown in Figure 4a, there is only a small decrease in [N]/[C] over the substrate aging period. For example, for 10 sccm AA, the [N]/[C] ratio decreases slightly from 16.6% to 14.5%. If the functionality of the surface was simply controlled by nitrogen surface density, then we would expect only a slight decrease in silver NP adsorption after aging. Clearly from Figure 5a this is not the case and the [N]/[C] ratio is not a good predictor of surface functionality.


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Figure 4. (a) [N]/[C] % ratio as the function of percentage of immobilized [Ag] before performing derivatization, (b) [NH2]/[C] % ratio as the function of percentage of immobilized [Ag] from derivatization data. In contrast, the primary amine content is significantly affected by aging. From Table 1, the [NH2]/[C]% for 10 sccm AA decreases from 1.92% to 1.12% after aging, which is equivalent to a decrease in primary amine content of around 40%. As the primary amine groups are predominantly responsible for surface charge when in aqueous solution, we expect that silver NP adsorption will be linked to the primary amine content. The result presented in Figure 4.b support this hypothesis. The percentage of [Ag] increases with the [NH2]/[C] % ratio following a shared linear master curve for both as deposited and aged samples. These results confirmed that the nanoparticle number density gradient is closely correlated with the concentration gradient of -NH2 groups anchored to the substrate. Therefore, the derivatization data are useful in estimating the number of nanoparticles immobilized on the surface regardless of the AA flow rates and age of the substrate. Furthermore, these results seem to indicate that further increasing the thin films primary amine content, could be used as a way to maximize AgNPs surface immobilization. Advanced spectroscopic studies on plasma generated


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species under various deposition condition

81, 82

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could help determine how to enhance primary

amine content. Nonetheless, silver (and [COOH]) was present even on the aged 2.5/7.5 ppAA/OD films were no primary amine were detected. This finding indicates that electrostatic bonds with other chemical functions and/or hydrogen bonding may also partially contribute to AgNPs immobilization. While the driving force for attachment of nanoparticles to 2-mercaptosuccinic acid is the strong chemical affinity between silver and sulfur, it is the electrostatic interaction between negatively charged silver nanoparticles and positively charged amine groups on the substrate that is responsible for surface immobilization. The concentration of silver nanoparticles attaching to the surface is therefore expected to also depend on the final pH of nanoparticle solution which inherently affects the surface charge of polymer film

83

. As illustrated in Figure 5.a, the pKa of the primary amine present on the

polymer film is between 9 and 10 and results in a change surface charge from positive to negative in this pH range. In order to demonstrate that, the surface immobilization of pp-AA/OD with different ratio immersed in MSA-AgNPs nanoparticles was performed at different pH: acidic (3.5 ± 0.5), neutral (6.5 ± 0.5) and basic (10.5± 0.5). The pH-dependent intensity of Ag3d peak is shown in Figure 5.b. The NH2 groups on the surface are basic under acidic condition but in acidic condition the surface is protonated and carries positive charge which can interact strongly with negatively charged silver nanoparticles.


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Figure 5. pH-dependent nanoparticle immobilization on the surface: (a) Schematic of substrate and AgNPs surface charge and interaction as a function of pH; (b) Silver atomic content in acidic pH (3.5± 0.5); neural pH (6.5± 0.5) and basic pH (10.5± 0.5) as a function of gas mixture flow rates.

In summary, the number of amine groups present on the substrate together with the pH of the colloidal solution control the final nanoparticle coverage over the surface, Figure 6. This method enables the fabrication of surfaces with the ability to control the surface nanoparticle concentration. The derivatization approach used to quantify primary amine density is a reliable method in estimating the number of immobilized nanoparticles on the surface and this estimation is not affected by the aging of the amine-functionalized thin film. As we have previously shown, these surfaces have the potential to be applied as antibacterial coatings 68 with prolonged activity. However, for targeted applications, the concentration of AgNPs is can be tuned easily with the method presented. Moreover, we are able to scale up and expand the application to controlled assembly of other types of nanoparticles including gold nanoparticles.


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Figure 6. Summary of the parameters that can be used to control the immobilization of MSAAgNPs on plasma polymer films: primary amine content, substrate aging time and colloidal solution pH.

4. CONCLUSION Plasma polymerized Allylamine/Octadiene films (pp-AA/OD) with different primary- and secondary-amine concentrations were used as a positively charged platform for the immobilization of carboxyl-functionalized silver-nanoparticle (AgNPs) which are negatively charged in the three pH conditions investigated The amount of silver nanoparticles binding to the plasma polymer surface was quantified from the silver atomic content of the substrate post immobilization, and the substrate primary amine content was quantified from TFBA derivatization. AgNPs electrostatic immobilization is proportional to primary (NH2), which vary with AA/OD gas ratio, and the age of pp-AA/OD deposits. Our finding indicates that labeling and calculating the density of primary amines through TFBA method can provide a reliable estimate of a substrate immobilization potential. With appropriate calibration, this approach could be used to predict the final number of nanoparticles that an amine-functionalized thin film


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would bind. Moreover, we demonstrated that the number density of nanoparticles within the gradient could be also tuned by AgNPs colloidal solution pH. The pp-AA/OD films contain NHx groups even after 30-days ageing, and the aging is not affecting the ability of the film in immobilization of AgNPs, which makes these materials useful in many advanced applications ranging from medicine to sensing and electronics.

AUTHOR INFORMATION Corresponding Author *Prof. Krasimir Vasilev, E: [email protected], T:+61 8 83025697 Present Addresses † Department of Physics and Astronomy, Faculty of Science and Engineering, Macquarie University, NSW, 2109, North Ryde, Australia †† División de Ciencias Básicas e Ingeniería, Depto. de Ingeniería de Procesos e Hidráulica, Universidad Autónoma Metropolitana-Iztapalapa, Av. San Rafael Atlixco No. 186, México, D.F. 09340, Mexico Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ABBREVIATIONS


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AgNPs, Silver Nanoparticles; XPS, X-ray Photoelectron Spectroscopy; AA, Allylamine; OD, Octadiene; SCCM, Standard Cubic Centimeters per Minute; pp-AA/OD, Plasma Polymerized Allylamine- Octadiene Film; COOH, Carboxylic Acid Groups; NH2, Primary Amine; PFA, Peak-Fit-Analyses method; MSA, 2-Mercaptosuccinic Acid; TFBA,

4-(trifluoromethyl)

benzaldehyde; At%: Atomic concentration percent. ACKNOWLEDGMENT This research is being supported by grants from European Commission, Bacteriosafe (FP7 grant# 245500), KOALA (FP7 grant# 295155) projects as well as International Postgraduate Research Scholarships (IPRS) and Australian Postgraduate Award (APA) scholarships.

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TOC Graphic


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Typical C1s deconvolution and peak assignment used in this work: (a) as deposited and (b) After TFBA derivatization. 304x143mm (150 x 150 DPI)



(a) Diagram showing the flow rate setting (sccm) of gas mixture of AA and OD; (b) Elemental composition of pp-AA/OD films as determined from area under the peak of the elemental XPS scan for the different AA/OD mixtures: as deposited (open symbols) and after aging in air for 30 days (full symbols). 276x120mm (150 x 150 DPI)


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(a) Ag3d atomic concentration obtained from survey spectrum and (d) [COOR]/[C] of immobilized-AgNPs on pp-AA/OD films, obtained from C1s deconvolutions, as function of AA/OD monomer ratio for: as deposited (open symbols) and 30-day-aged films (full symbols). Typical C1s deconvolution of (b) as deposited ppAA/OD films and (c) pp-AA/OD with immobilized MSA-AgNPs. The C5 peak component is indicative of [COOH] groups and is assumed here to represent predominantly the MSA-AgNPs nanoparticles. (e) Ag3d atomic content and [COOR]/[C] linear relationship, ([COOR]/[C]= -1.7+0.9[Ag]); (f) Atomic force microscopy (AFM) images of absorbed MSA-AgNPs onto deposited pp-AA/OD films. 184x184mm (150 x 150 DPI)



(a) [N]/[C] % ratio as the function of percentage of immobilized [Ag] before performing derivatization, (b) [NH2]/[C] % ratio as the function of percentage of immobilized [Ag] from derivatization data. 449x177mm (150 x 150 DPI)


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pH-dependent nanoparticle immobilization on the surface: (a) Schematic of substrate and AgNPs surface charge and interaction as a function of pH; (b) Silver atomic content in acidic pH (3.5± 0.5); neural pH (6.5± 0.5) and basic pH (10.5± 0.5) as a function of gas mixture flow rates. 275x107mm (149 x 149 DPI)



Summary of the parameters that can be used to control the immobilization of MSA-AgNPs on plasma polymer films: primary amine content, substrate aging time and colloidal solution pH. 202x133mm (150 x 150 DPI)


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Binding of Nanoparticles to Aminated Plasma Polymer Surfaces is

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