Thin Film Coatings from Aqueous Dispersion of Graphene-Based Nanocarbon and Its Hybrids with Metal Nanoparticles

Abstract

Shungite carbon (ShC) nanoparticles in the form of stable aqueous dispersions represent a promising solution for optical and biomedical applications. The dispersion is an interesting phenomenon from the point of view of stabilization of ShC nanoparticles and their structural constituents up to the basic structural unit, namely a graphene fragment. Herein, we used these aqueous dispersions with easily released structural components to study laser irradiation with various durations and obtain hybrids of ShC with Ag and Au nanoparticles. The main role in the stabilization of ShC nanoparticles belongs to the graphene fragments and their stacks, which display a considerable dipole moment. Newly prepared aqueous dispersions of ShC–metal hybrid nanoparticles retained the stability inherent in the original nanoparticles both of ShC and metals. Changes in the size distribution pattern of nanoparticles in dispersions upon ablation were studied by dynamic light scattering (DLS). Raman with UV-Vis spectroscopy methods were applied to trace structural changes in ShC upon the formation of hybrid nanoparticles. Films obtained by condensation of the dispersions on glass substrates display periodic structures, as was revealed by SEM microscopy. There, the conditions under which nanoparticles lose their ability to disperse in water and retain a graphene-like structure in a film were revealed.

1. Introduction

Various nanoparticles are widely used to solve optical and biomedical problems. For example, photoluminescent labels have been developed for photodynamic diagnosis and therapy. Laser ablation is currently applied to obtain carbon and metal nanoparticles in various media with desirable structural and optical properties [1].

With growing interest in graphene quantum dots, problems of their preparation are gaining momentum. Despite a recent burst of activity in the solution-phase production of graphene, little progress has been made in the generation of graphene with tailored thickness, lateral area, and shape [2].

Optical spectroscopy, and photoluminescence in particular, has so far been the primary method of studying the properties of the graphene in dispersion [3].

Many approaches have been described for the formation of 2D and 3D arrays of metal and semiconductor nanoparticles. Such nanoparticles can display diverse electrical and optical properties. The size distribution of nanoparticles is important for governing their assembly in such systems [4].

Researchers believe the most attractive properties of carbon are based on its ability to form new nanoscale architectonic forms [5]. One- and two-dimensional carbon layers, including graphene and nanoribbons, have recently become the subject of study as promising materials for electronics, spintronics and medicine [6].

The goal of our studies is to develop new approaches to the synthesis of nanostructures and explore their ability to affect the electronic, chemical, mechanical and magnetic properties of nanostructures by controlling the parameters of defects. Defects can play a positive role by triggering the development of new materials and compounds, namely those used for nanoscale structures. It has been shown that defects affect the physico-chemical properties of graphenes, and are crucial for the development of biocompatible materials [5].

Therefore, the study of shungite carbon (ShC) as a new graphene-like material is of scientific and practical interest [7]. The approaches developed for the molecular graphene have been used in the study of the natural nanocarbon [8], which exhibits the properties of a semiconductor, a semimetal and a colloid. The study of the multi-level structural organization of ShC on nanoscale assists to understand its diversity.

The clustering of non-planar graphenes (basic structural elements) of ShC is a key process of the multi-level structural organization scenario and structural transformations in systems differing in physico-chemical origin. Data obtained by neutron diffraction and inelastic neutron scattering methods and supported by quantum chemical calculations have proved the contribution of water to the formation of all structural levels, and, in general, the fractal structure and porosity of ShC [9]. ShC favorably differs from all synthetic objects in terms of the stable lateral dimensions of ~1 nm of graphene fragments, as well as their amphiphilicity [10].

The availability of natural resources and the distinctive features of the ShC structural organization, together with its ability to form stable aqueous dispersion of carbon nanoparticles, makes it an interesting subject for optical limiting study [11].

The aggregation of carbon nanoparticles is the main drawback of a class of nanomaterials. Stable aqueous dispersion of ShC nanoparticles [12] is the subject of a study carried out to isolate nanoparticles and their structural constituents, such as graphene fragments, stacks of graphenes, and globules. The clusterization of graphene flakes in a stable aqueous dispersion of ShC nanoparticles is a readily reproducible process [13]. Various structural types of ShC nanoclusters and their aggregates have been obtained in aqueous dispersion form under normal conditions. ShC nanoparticles are transferred from aqueous dispersions to solvents of variable polarity [14].

It has been shown recently that the packing density of nanoclusters in dispersions can be controlled by additives (sucrose, urea, HCl, NaOH and NaCl), which affect boundary water in ShC clusters [15].

The spectral behavior patterns of ShC nanoparticles (UV excitation, regardless of solvent), namely the amplification of luminescent molecules in clusters of graphenes, are similar to the emission of synthetic reduced graphene oxide and metal nanoparticles which are known as quantum dots [9]. The applicability of the presented approach is supported by the experimentally determined features of the nonlinear optical properties of ShC nanoparticle dispersions. They exhibit stronger optical limitation of the nanosecond laser pulse range in visible and near-IR pulsed radiation [11]. The heating of absorbing nanoparticles is an important factor in restricting the reuse of nanoparticles at high density energy radiation.

Hybrid materials obtained by the interaction of organic and nonorganic components forming a spatial structure which is structurally different from the original components but inherits certain motifs of the original structures, are most interesting. Condensed ShC dispersion enhances nonlinear optical response and substantially influences the photorefractive parameters of the conjugated structures of polyimide matrices [16].

Metal nanoparticles (1–3 nm) produced by laser-induced ablation [17] from a solid target are widely used in medicine due to their biocompatibility, advanced biodistribution, targeting features, adsorption, easy surface modifications and pharmacokinetics [18].

The ability to integrate metal nanoparticles into biological systems is of special importance for biology and biomedicine. Noble metal nanoparticles are of great interest due to the unique tunability of their plasmon resonance by varying their size, shape and composition. These nanoparticles are formed of clustered metal atoms that have to be protected by “capping agents” or stabilized by surface active substances.

Nanoparticles display excellent optical properties due to surface plasmon resonances and can be used in cancer phototherapy [19,20].

The distinctive characteristics of graphene-nanoparticle hybrids are widely used in biosensor systems for detection of allergens, toxins, bioactive agents and food-borne pathogens. Biosensors, formed of graphene combined with metallic nanoparticles, are the most promising graphene-based counterparts [21]. An important aspect of biosensor production enables the maintenance of the biological activity of biomolecules immobilized in a nanoparticle microenvironment. Our preliminary results show the influence of ShC nanoparticles interactions onto the native biological state of some blood proteins and reveal the conditions required for maintaining their biological functionality [11].

Stable dispersion of hybrid nanoparticles has been enabled by subjecting them to combined treatment by laser pulses of ShC and metal nanoparticles in aqueous dispersion. Nanosecond laser irradiation presents the greatest interest among possible methods for a partial fragmentation of carbon.

This paper presents the results of the comparative study of hybrids and original ShC nanoparticles in aqueous dispersions and films.

2. Materials and Methods

Stable aqueous dispersions of ShC nanoparticles were prepared according to the original methodology on the basis of the carbon-rich shungite rock powder (Shungite type I, 96 wt% carbon) [12]. Ultrasonic treatment (frequency 22 kHz, power 300 W) followed by filtration and ultracentrifugation made it possible to obtain an initial dispersion with a concentration of 0.12 g/L and pH 6.5. According to the control data of atomic absorption spectrometry and inductively coupled plasma mass spectrometry [12], all traces of non-carbon elements presented in the original shungite powder were removed while using the preparation procedure. Transmission electron microscopy was used to avoid different carbon agglomerations apart from ShC nanoparticles.

The size distribution of dynamic light scattering (DLS) intensity obtained by using Nanosizer NanoZS (Malvern Panalytical Ltd., Malvern, UK, 633 nm He-Ne laser) gave the nanoparticles a radius of 50 ± 8 nm with a distribution peak width of 20 nm and polydispersity index 0.205 ± 0.072. The nanoparticles ζ-potential of −30 mV, measured by using the same equipment, indicated the stability of the dispersion.

Initial Ag and Au colloidal dispersions were prepared by continuous laser ablation of silver and gold targets in the aqueous environment (deionized Millipore water (18.2 MΩ cm resistivity)). The average size of the colloid particles was 10 nm at a dispersion concentration of ~1 mg/L [22]. Then, the aqueous dispersions of ShC nanoparticles were mixed with the dispersions of Ag and Au nanoparticles. The mixtures were irradiated by continuous wave (CW) Nd:YAG laser, by Q-switched Nd:YAG laser (2 min exposure) at a wavelength of 1064 nm and a pulse duration of 100 ns (nanosecond (ns) pulses), and by Ti:Sp laser with pulse duration 50 fs (femtosecond (fs) pulses).

A dispersive Nicolet Almega XR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with the 532 nm Nd:YAG laser was used to obtain the Raman spectra of ShC nanoparticles aqueous dispersions and films of the condensed dispersions on glass substrates. The spectra were collected at 2 cm⁻¹ spectral resolution. OMNIC software was used to process the Raman spectroscopy data.

The spectroscopic characteristics of ShC and hybrid nanoparticles were obtained by using an UV-Vis spectrophotometer SF-56 (OKB Spectr LOMO, Saint Petersburg, Russia).

The scanning electron microscopy (SEM) of the films was carried out using a SU1510 (Hitachi, Tokio, Japan) and VEGA 11 LSH (TESKAN, Brno, Czech Republic) equipped with an energy dispersive spectroscopy (EDS) EDS-INCA Energy 350 (Oxford Instruments, Abingdon, UK).

Morphological characteristics of hybrid nanoparticles were analyzed by transmission electron microscopy (TEM) by using FEI Titan 80-300 (Hillsboro, OR, USA).

3. Results and Discussion

Aqueous dispersions obtained after laser irradiation of ShC nanoparticles and its mixtures with Ag and Au nanoparticles are rather homogeneous and remain stable without surfactants. The scheme in Figure 1 shows the preparation of hybrids and samples of the studied dispersions with visible coloration.

The average diameter of ShC nanoparticles increases from 100 nm up to 291.1 ± 6.4 nm under CW laser exposure and to 703.8 ± 8.9 nm under ns pulse (Figure 2a). The most significant changes in the average size of nanoparticles were found upon fs exposure: the particle size distribution pattern shows a shoulder in the range of 25–50 and a major peak at 500 nm.

The formation of hybrids was controlled by the DLS study of the average size and distribution pattern of nanoparticle dimensions in dispersions in comparison with the parameters of initial ShC nanoparticles dispersion (Figure 2).

The average size of nanoparticles increases for all dispersions under a CW laser exposure up to 395 ± 33 nm for ShC–Ag and 365 ± 10 nm for ShC–Au, respectively. Exposure to ns and fs pulses leads to the appearance of a bimodal size distribution. For example, ShC-Ag under ns pulse shows two peaks at 300–400 and 25–50 nm. Two size levels of particles, 20–40 nm and less than 10 nm, were also observed in films of hybrid ShC-Ag particles using TEM (Figure 2d).

A characteristic feature of noble metal nanoparticles is the bright color of their colloid solutions (Figure 1) caused by surface plasmon absorption [23]. Spectral studies indicate the interaction of carbon and metal nanoparticles. The absorption spectra of hybrid nanoparticles in the dispersion in comparison with ShC nanoparticles are shown in Figure 3. The absorption spectrum of ShC nanoparticles dispersion (Figure 3) displays a wide peak similar to the one reported for synthetic reduced graphene oxides.

The main peak observed in the UV spectra of water ShC dispersions is a plasmon peak at 261 nm (

Table 1. Peak positions in the electronic absorption spectra of dispersions of ShC nanoparticles and its hybrids.

Composition of Dispersions	Laser Irradiation	Peak Position, nm
ShC	initial	261
ShC	CW	266
ShC	ns	265
ShC-Ag	CW	268/391
ShC-Ag	ns	261/406
ShC-Ag	fs	250/415/209 shoulder
ShC-Au	CW	267/522
ShC-Au	ns	249/526
ShC-Au	fs	261/522/208 shoulder

), which indicates a high degree of conjugation and sp²-hybridization of graphene ShC fragments and the lower energy of corresponding π-π* transition capable of contributing to the position of the 260 nm peak graphene [24].

Continuous laser radiation leads to the fact that the characteristic maximum ShC is shifted to the wavelength range of 265–268 nm. For ShC–Ag and ShC–Au hybrid nanoparticles, two broad peaks are observed at wavelengths of 268.391 nm and 267, 522 nm, respectively. Under the fs laser irradiation, no change was found in the dispersion spectrum of ShC. In the presence of Au and Ag nanoparticles in the ShC dispersion, the broad maximum shifts to 261 and 250 nm, respectively. In the presence of nanoparticles of these metals in water, peaks are observed at 522 nm for Au and 415 nm for Ag (Figure 3).

The shifting of the absorption band into the Vis range (“red shift”), as compared to the absorption of Ag sols at 390 nm, indicates a decrease in the electron density of the metal and an increment in nanoparticle size. Similar optical effects are observed upon the adsorption of certain compounds or metal ions on the surface of silver particles. The shift reflects doping-like behavior at the interface of the metals and corresponds to hybrid surface state formation [24].

Under nanosecond exposure, the characteristic peak of ShC shifted to the short-wavelength side 249 nm for Au and to 261 nm for Ag. In the visible region, more significant changes were observed. For the dispersion containing Au, this was 526 nm and 406 nm for the dispersion with Ag nanoparticles (Figure 3). It can be assumed that the difference between two dispersions occurs due to the oxidation of silver particles.

Significant changes in the absorption spectra of ShC-Ag nanoparticle dispersions were obtained when exposed to fs laser. In addition to the two main peaks at frequencies of 249 and 415 nm, the appearance of a “shoulder” at 209 nm was observed, similar to that described for graphene oxide [25]. A “shoulder” also appeared in the ShC-Au dispersion spectra at 209 nm.

The structural characteristics of the ShC at the interface with metal nanoparticles can be assessed through changes in the Raman spectra of nanoparticles and hybrids in dispersions and condensates. The condensate of ShC can be redispersed in water. This is a reversable process characteristic of ShC nanoparticles.

The Raman spectra show doublets consisting of characteristic G- and D-bands for all the samples (Figure 4). Dispersions of ShC nanoparticles and their condensates show well-resolved D and G peaks in Raman spectra with the inversion of the ratio of its intensities ID/IG for water dispersion (

Table 2. Parameters of Raman spectra of nanoparticles in dispersions and of the condensed films.

Laser Parameters/ Composition of Nanoparticles	D, [cm−1]	G, [cm−1]	ID	IG	FWHH D, [cm−1]	FWHH G, [cm−1]	ID/IG
Dispersions Initial ShC	1342	1639	0.11	1.82	52.05	139.07	0.06
CW/ShC	1348	1628	0.30	1.11	89.93	133.28	0.27
ns/ShC	1347	1608	0.33	1.67	137.02	125.04	0.20
fs/ShC	1355	1634	0.11	1.17	114.70	148.88	0.09
CW /ShC-Ag	1350	1607	1.30	2.08	156.15	134.62	0.62
ns/ShC-Ag	1399	1580	15.7	29.4	396.8	117.58	0.54
fs/ShC-Ag	1372	1584	2.26	5.09	348.74	139.36	0.44
CW/ ShC-Au	1353	1626	0.34	0.99	75.74	124.57	0.34
ns/ShC-Au	1319	1632	0.12	2.35	156.40	173.73	0.05
fs/ShC-Au	1340	1637	0.3	0.75	170.7	162.6	0.41
Condensate ShC	1348	1601	11.1	7.72	83.90	64.82	1.44
ns/ShC	1358	1586	3.54	4.60	145.92	128.42	0.77
ns/ShC-Ag	1393	1580	47.4	41.4	345.93	134.92	1.15
ns/ShC-Au	1356	1595	3.57	4.94	124.11	106.24	0.72

).

FWHH D, FWHH G—the width of the corresponding peaks at the half maximum. ShC nanoparticles subjected to laser irradiations of various durations are structurally stable in water (Figure 4c, Table 2) due to the distinctive properties of the basic structural element (graphene fragment) of ShC and its interaction with water [26].

The G band maximum, 1639 cm⁻¹, of the initial dispersion has been regarded as a superposition of two lines. The first one can be attributed to carbon scatterer and the second one to bound water molecules [13].

It can be concluded that the noticeable changes in structure arise from the Raman spectra of hybrids’ interaction with Ag nanoparticles. The intensities of the Raman spectra of nanoparticles of ShC at the contact with Ag grow at least four times (Figure 4a, Table 2). The position of G peak changes to 1580–1584 cm⁻¹. Strong ShC-Ag interaction seems to occur due to a surface plasmon resonance phenomenon provoked by the existence of surface electromagnetic waves at the metal-carbon boundary [27].

The original aqueous dispersion of ShC nanoparticles after condensation displays ID/IG = 1.4 (Table 2).

By choosing the parameters of laser processing of nanoparticles in dispersion, it is possible to govern the structure of carbon in the films (Figure 5). Laser irradiation of aqueous dispersions of ShC and hybrid nanoparticles by fs and ns pulses affects the carbon structure order in films (Table 2).

Disordered carbon in the films was obtained upon CW laser irradiation of ShC dispersions and of hybrids with Ag and Au nanoparticles (Figure 5).

Graphene-like carbon is kept in the films of ShC and its hybrids ShC-Ag and ShC-Au after laser irradiation of nanosec duration. The morphology of these films is presented in SEM images (Figure 6). Nanoparticles in the films form mesoporous nets as compared with initial ShC nanoparticles (Figure 6a). The average size of hybrid particles in the nodes of nets is 10–300 nm.

The films obtained by condensation of hybrid nanoparticles dispersions lost the ability to dissolve in water.

4. Conclusions

ShC nanoparticles and carbon structures in aqueous dispersions upon laser irradiation by pulses of different duration (CW, nano- and femto-second) were shown to be stable.

Laser irradiation of ShC with Ag and Au nanoparticles in water makes it possible to obtain homogeneous and stable dispersions without surfactants. Stable aqueous dispersions of hybrid ShC-Ag, ShC-Au nanoparticles were produced under irradiation by laser pulses of nano- and femtosecond durations. The most pronounced changes in the size of nanoparticles were obtained by femtosecond pulse irradiation, which achieved the bimodal distribution pattern of ShC-Ag and ShC-Au hybrids with nanoparticles of 10–25 nm in size.

Laser fs irradiation of the dispersions of ShAg and ShC-Au nanoparticles manifests itself in the absorption spectra. In addition to the two major peaks of dispersions, a “shoulder” appears in the frequency range characteristic of graphene oxide.

Thin films of graphene-like carbon and its hybrids with noble metals weren obtained from aqueous dispersions. Compositions of quantum dots and bioactive compounds (ShC and noble metals) could be promising for such biomedical applications as cancer theranostics and drug delivery.

The interaction of ShC-Ag and ShC-Au at a nano level causes irreversible changes in ShC structure.