Plasma synthesis of functionalized metal nanoparticles: from improved dispersion properties to enhanced biomaterials

J. Tavares, E.J. Swanson, S. Coulombe

Department of Chemical Engineering, McGill University, Montreal (Canada)

Abstract: A method to produce metal nanoparticles coated with an organic plasma poly-

mer layer is presented. Hydrophilic surfaces are obtained when oxygen-containing pre-

cursors (e.g. ethylene glycol, ethane/oxygen mixture) are used, while hydrophobic sur-

faces result from using ethane as a precursor. The resulting surface properties lead to en-

hanced dispersion properties and improved biocompatibility.

Keywords: nanoparticles, plasma polymerization, biocompatibility, RF plasma, arc

1. Introduction

The appeal of nanoparticles is vast and

multidisciplinary. Applications in advanced materials,

catalysis and biomedicine are now common. [1] Their

exceedingly small dimensions lead to unique properties,

often very different than the bulk material. However, as

attractive as these new properties may be, they can

represent a significant drawback: the proportionally high

surface energy of these ultra-fine particles leads to the

problem of agglomeration. In fact, many envisioned

applications for nanoparticles, including nanofluids and

nanocomposite materials, rely mainly on the particles

remaining separate and homogeneously dispersed.

Solutions proposed thus far to address the issue of

agglomeration typically involve the use of foreign

stabilizing agents (e.g. surfactants). The thermal stability

of such agents has been brought into question. [2]

Methods for large-scale generation of nanoparticles with

uniform chemical composition involve the use of thermal

plasma sources, evaporation-condensation processes,

chemical vapor deposition (CVD) or chemical

synthesis.[3,4] Nanoparticles of various chemical

compositions including metal oxides (Al2O3, CeO2),

nitrides (TiN, Si3N4) or carbides (SiC, WC) are routinely

produced with such processing approaches. The

generation of coated or core-shell nanoparticles involves

the use of dual-stage processes, such as inert gas

condensation or homogenous nucleation in the gas phase

followed by in-flight CVD or plasma polymerization.[5,6]

In order to address the issue of particle agglomeration and

the need for enhanced dispersion properties, we propose a

hybrid plasma process capable of producing metal

nanoparticles coated with a layer that is compatible with

the host medium. The metal nanoparticles are generated

by the erosion of a metallic cathode with a pulsed DC arc

and coated in flight by an organic layer in an RF glow

discharge. The organic layer is tailored to be hydrophobic

or hydrophilic, depending on the needs of the application.

Additional work also involves applying a similar

treatment to titanium nanoparticles deposited onto

stainless steel 316L substrates and evaluating the resulting

biocompatibility.

2. Experimental setup

The plasma process used to generate the

coated/functionalized nanoparticles was described in [7].

The dual-plasma reactor features a concentric geometry

favouring the efficient synthesis, in-flight coating and

functionalization, as well as the transport of nanoparticles

toward the surface to be coated. The reactor is illustrated

in Fig. 1 (top). A metal cathode (copper or titanium) is

Fig. 1: (top) Schematic illustration of the dual-plasma reactor;

(bottom) picture of the Ti metal vapour plume expanding into an

EG RF glow discharge.

mounted on the centerline of the assembly and exposed to

repetitive arcing events (stainless steel bars surrounding

the cathode serve as the grounded anode), which lead to

the localized evaporation of the surface and formation of

a rapidly-expanding metallic vapour plume. Bare metal

nanoparticles are formed at the periphery of the plume

through homogenous nucleation followed by growth.

The cathode and anode assembly is surrounded by a

stainless steel mesh (5 mm 5 mm openings) that acts as

the live electrode for a radio-frequency (RF – 13.56 MHz)

capacitively-coupled glow discharge. Once formed, the

nanoparticles generated by the arcing events make their

way into this RF discharge which serves two purposes: 1)

the deposition of a functional coating onto the surface of

the nanoparticles, and 2) the charging (negatively) of the

recently formed nanoparticles, which influences the

collection and limits agglomeration of the nanoparticles.

The metallic vapour plume expanding into the RF

discharge is shown in Fig. 1 (bottom).

The discharges are sustained in a mixture of an inert

gas (e.g. argon) with a monomer selected on the basis of

the desired surface properties (e.g. ethylene glycol for a

hydrophilic surface, ethane for a hydrophobic surface).

[7] Mixtures, such as ethane with oxygen, can also be

used as organic precursors; the resulting surface’s

hydrophilicity is then proportional to the relative amount

of O2 injected. The concentration of the organic precursor

varies from 0.05 to 0.10 mol-%, depending on the desired

surface properties. The treated nanoparticles are either

deposited on a biased collection surface or directly into a

flowing curtain of liquid for immediate dispersion. In

cases where liquid collection is employed, no organic

precursor is injected: the vapours from the liquid

(ethylene glycol) are used for plasma polymerization.

3. Analysis

Transmission electron microscopy has shown that the

nanoparticles generated exhibit a core-shell structure,

consisting of a plasma polymer layer 1 to 3 nm thick

encapsulating a metal core, as shown in Fig. 2. Image

analysis of the obtained micrographs reveals that the

average of the nanoparticle core is 3 to 5 nm.

The dispersion properties of the functionalized particles

are assessed by evaluating the contact angle of a drop of

water on a surface coated with the particles. In cases

where ethylene glycol (EG) or concentrated

oxygen/ethane mixtures are used as organic precursors,

the contact angle drops below 10o. With ethane, a

hydrophobic surface is formed, leading to contact angles

higher than 120o. Although a surface covered with

EG-coated copper nanoparticles shows signs of

hydrophobic recovery after only 24 hours, a hydrophilic

surface based on EG-coated titanium nanoparticles retains

its surface wetting properties well after two weeks.

The functional groups responsible for these surface

properties were identified using FT-IR. The hydrophilic

nature of the EG or high-O2 plasma polymer treatment is

likely a result of the high oxygen-species content in the

resulting coating. The oxygen is present mainly in the

form of hydroxyl, carboxylic and ketonic functional

groups. In cases where ethane is used as the organic

precursor, the amorphous carbon structures generated lead

to hydrophobic surfaces. [7]

Further insight into the source of these functional

groups can be obtained by investigating the optical

emission spectrum of the EG RF discharge, given in Fig.

Fig. 2: TEM micrograph of a copper nanoparticle with a hy-

drophobic (ethane-based) plasma polymer coating showing the

core-shell morphology obtained

Fig. 3: Optical emission spectrum of the EG RF discharge

Fig. 4: Reaction mechanism explaining the formation of

2-methyl-1,3-dioxolane from the radicals produced in the EG

RF discharge

3. Of particular interest are the molecular emission lines

identified around 309 nm (O-H), 430 nm (C-H), 516 nm

(C-C) and 578 nm (C-O) that offer an indication of how

the ethylene glycol is broken down in the plasma.[8,9]

From the OES data, it can be hypothesized that the

following compounds are likely to be produced in the RF

discharge: HO-CH2-CH2-O•, HO-CH2-CH2•, HO-CH2•,

HO•.

In order to identify more precisely which of these

hypothesized radicals are present in the RF discharge

when EG is introduced into the reactor, their

recombination products were analyzed with a GC-MS. In

order to produce samples, the system was operated with

an excess of ethylene glycol (a sample of liquid ethylene

glycol was placed inside the reactor chamber and directly

exposed to the RF plasma) and the condensates were

collected in a nitrogen trap placed in-line with the exhaust

of the reactor. The condensates were then extracted and

analyzed by means of a headspace technique applied to

gas chromatography coupled with mass spectroscopy.

This revealed that the compound 2-methyl-1,3-dioxolane

was present in the reactor exhaust. A reaction mechanism

that leads to the formation of this compound from EG

(illustrated in Fig. 4), involving some of the radicals

enumerated previously, is now proposed. First, either

electron bombardment or ultra-violet photon excitation

causes the hydroxyl radical (•OH) to break off, leading to

the formation of a primary radical (HO-CH2-CH2•) that

rearranges to form a more stable secondary radical

(HO-CH•-CH3). The secondary radical then reacts with

another ethylene glycol molecule by esterification to form

a longer chain (HO-CH2-CH2-O-CH-CH3OH). Via the

condensation reaction, this long chain reacts with itself to

form the cyclical compound 2-methyl-1,3-dioxolane and a

by-product, water. Using a gas chromatograph with a

different column, ethanol was found to be present in the

exhaust line condensate. Ethanol is a possible

recombination product of the radicals produced and

therefore, its presence acts as a confirmation for this

reaction pathway.

The radical formation reaction can be initiated either by

electron bombardment in the RF plasma or by UV photon

excitation. In order to determine if UV photons contribute

significantly to the formation of radicals, a sample of EG

was exposed for 3 hours to ultra-violet light at 254 nm,

provided by two germicidal lamps (approximate total UV

output of 50 microwatts). Using the same analysis

procedure described previously, the compound

2-methyl-1,3-dioxolane was found to be present in this

UV-treated EG sample. Therefore, it can be concluded

that UV light contributes to the formation of radicals in

the RF discharge. UV photons are produced mostly by the

OH radicals and the metallic vapors. The role of UV in

plasma polymerization processes is well

documented.[10,11]

The previous analysis confirms that the following

radicals are present in the RF discharge and are likely the

react with the unbound atoms at the surface of metal

nanoparticles: the ethanol radical (HO-CH2-CH2•), the

methanol radical (HO-CH2•) and the hydroxyl radical

(HO•). This result is in agreement with the previous

FT-IR analysis: the radicals formed could in fact lead to

the formation of the functional groups identified. From

this analysis, it can be inferred that the main radical

present in the RF discharge when ethane is introduced in

the reactor is the methyl radical (CH3•); C2, CH2 and CH

radicals may also be present and contribute to plasma

polymerization.

The previously discussed wetting properties resulting

from the EG-based functional groups can lead to

favourable biological properties, discussed in [12]. More

precisely, the biocompatibility of stainless steel 316L

treated with coated nanoparticles is assessed by testing the

way in which fibrinogen (Fg), a blood serum protein,

adheres to the treated surfaces. This protein is of

particular interest due to its dominant role in clot

(thrombus) formation processes [13,14]. It has been

suggested that inhibiting the adsorption of Fg on the

surface could help in minimizing clot formation [15].

However, recent studies have demonstrated that the

structure of the adsorbed Fg, rather than its surface

concentration, determines the implant’s affinity towards

thrombus formation (i.e. its hemocompatibility). It has

Fig. 5: (top) Amide I band decomposed to its constituent peaks

1- -sheet, 2- -helix, 3- -turns 4- carboxilic groups; (bottom)

ratio of the secondary structures of Fg adsorbed onto naked

SS316L (control), onto Ti nanoparticle-coated SS316L (Ti) and

onto EG-coated Ti nanoparticles deposited on SS316L (Ti+EG)

compared to the native structure ratio of Fg.

been proposed that the implant’s thrombogenicity, i.e. the

tendency of the surface to trigger clot formation, is

directly proportional to the extent of the adsorbed Fg’s

denaturation [16,17]. Because surface wettability directly

influences protein adsorption, it is expected that the

super-hydrophilic nature of the EG-based deposit will

promote more favourable Fg interaction with the surface,

when compared to naked SS316L [18,19]. It is pertinent

to note that, in the case of these biocompatibility assays,

titanium is used as the cathode material (and, hence, the

core of the coated nanoparticles) instead of copper, due to

its more favourable biological interaction [20].

The surface conformation (or tertiary stucture) of Fg

depends on the secondary structure of adsorbed Fg. Thus,

by evaluating the secondary structure of Fg adsorbed on

the investigated surfaces, and comparing it to the

secondary structure of Fg in its native

(non-thrombogenic) state, one can evaluate the relative

degree of thrombogenicity of the surfaces. For this

purpose, the Amide I peak obtained by polarization

modulation infrared reflection absorption spectrometry

(PM-IRRAS) is analyzed. This peak is actually a

composite of various overlapping subcomponent bands

that each represents different secondary structures of the

protein (helices, -structures, turns and random

structures) [21,22]. Fig. 5 (top) shows an example of the

Amide I band fitted with its resulting component peaks.

It has been shown that the extent of Fg denaturation

resulting from adsorption onto an implant surface

determines the susceptibility of the surface to platelet

adhesion and activation and thus, its thrombogenicity

[16,17]. More precisely, the closer the secondary structure

of the adsorbed Fg to that of the native structure, the

lower the resulting surface thrombogenicity. The extent of

denaturation can be assessed by comparing the ratio of

-helix-to- -turn structures in the native Fg to that of the

Fg adsorbed onto the surface [16,17]. Fig. 5 (bottom)

graphically illustrates the -helix-to- -turn and the

-sheet-to- -turn ratios for the EG-treated surface

(Ti+EG), the Ti nanoparticle-treated surface (Ti, no

plasma polymer) and the control surface (Control), and

compares them to the ratio of the native structure. The

values obtained for the control surface (SS316L)

demonstrate that Fg undergoes major secondary structure

changes following its adsorption on the substrate.

However, the significantly higher -helix-to- -turn and

also a higher -sheet-to- -turn ratios on the Ti+EG

surface evidence a secondary structure that is closer to the

native state. This is expected to result in a lower

attachment of platelets on the EG-treated surface and thus,

to a lower thrombogenicity.

4. Conclusion

The proposed process has been shown to produce

coated metal nanoparticles that exhibit either hydrophilic

or hydrophobic behavior, depending on the plasma

polymerization gas used. Such surface properties can aid

in the dispersion of metallic nanoparticles in both polar

and non-polar media, such as polymer matrices (to form

nanocomposites) or solvents (to generate nanofluids). The

radicals used in the plasma polymerization of ethylene

glycol have been identified and the role of ultra-violet

radiation in their formation has been demonstrated

empirically. Moreover, it was found that the deposition of

ethylene glycol plasma polymer-coated titanium

nanoparticles onto SS316L confers properties to the

surface making it more suitable for applications as

blood-contacting implants. This was demonstrated by

showing that the secondary structure of the fibrinogen

protein adsorbed onto this surface is closer to the native

structure of fibrinogen.

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