INTRODUCTION

Nanoparticles are the main building blocks of modern nanotechnologies. Due to quantum and surface effects, they have various unique properties and, therefore, are used as nanomaterials in many fields of science and technology [1, 2]. In particular, they have found wide application as functional components of subwavelength optical devices [3–5], sensor substrates for surface-enhanced spectroscopy [6–11], biological marking and probing systems [12–14], theranostics [15–19], and antibacterial agents [20–25].

Along with other methods used for generating solutions of colloidal nanoparticles, such as chemical reduction [26–29], precipitation [30–32], hydrolysis [33, 34], sol–gel synthesis [35–40], and hydrothermal synthesis [41–43], laser ablation in liquids is one of the most promising scalable methods for obtaining chemically pure nanoparticles and is believed to be an environmentally friendly process [44–51]. The relevance and promise of this method are due to its simplicity, the feasibility to use different targets and liquids, and the purity of the ablation products. Moreover, liquids are convenient media for collecting ablation products [10].

This work is a combination of the analysis of communications available in the literature on the studying the effects of different laser parameters on the generation of nanoparticles in liquids with experimental investigations aimed at filling the gaps in the literature concerning the properties of generated dispersions. The first part of the work has been dictated by the need to systematize numerous data reported in the literature, including those on the productivity of nanoparticle generation as depending on the parameters of laser action. The second part allows one, using, on the one hand, the analysis of the physical processes accompanying laser generation of particles and, on the other hand, the data on the physicochemical parameters of the resulting nanodispersions, to select optimal methods for obtaining nanoparticles/dispersions for solving these or those problems.

LASER-ABLATIVE GENERATION OF DISPERSED NANOPARTICLES IN A LIQUID

Main Stages of Laser Ablation in Liquids

At present, there are data in the literature on achieving a high productivity of laser-ablative generation of nanoparticles in liquid media on the order of several grams per hour [52–54]. In this case, various targets are considered from bulk materials to thin films. In addition, particles of different types and materials (metals, dielectrics, etc.) have been obtained, thus making it possible to carry out extensive laboratory studies on the use of nanomaterials in various important technological areas mentioned above. Nevertheless, in most of the works presented in the literature, no directed optimization of the process for producing particles was carried out, and the obtained nanoparticles were characterized using a rather limited spectrum of methods. At the same time, the variety of laser processing parameters (wavelength, energy, duration and repetition frequency of laser pulses, immersion depth of an ablation source, and its movement relative to a liquid) and types of liquid media makes it possible to perform such optimization based on the detailed characterization of resulting particles.

It is necessary to single out the main physical stages that successively determine the course of laser ablation in liquids with the generation of nanoparticles and the formation of dispersions.

(1) Propagation of laser radiation to the surface of a solid target. This stage can be complicated by the formation of an ablation plasma and plasma screening when using (sub)nanosecond laser pulses [55–58] or by nonlinear focusing and filamentation of femto- and picosecond laser pulses [55, 59–61].

(2) Absorption of radiation by an electronic subsystem, energy transfer to a lattice, and phase transition to high-temperature states of a melt [62, 63].

(3) Nano- or subnanosecond emission of an ablation torch in the form of a vapor–droplet mixture [64]. Note that the vapor–droplet mixture in the case of nanosecond (ns) laser pulses undergoes optical breakdown, with the formation of screening subcritical plasma [64]. As has been shown in the literature, such plasma self-consistently controls the fraction of radiation energy transmitted to the target and determines the flux of low-molecular-weight ablation products from the target surface [64] without taking into account the microdroplet fraction [65–67].

(4) Evolution of a (sub)millimeter vapor bubble over the surface of the ablation region from bubble growth to its collapse (of (sub)millisecond duration) [68–71].

At present, the effect of laser pulse repetition frequency f ∼ 1–107 Hz has been studied in greatest detail. This frequency, in combination with the speed of laser beam scanning over a target surface, \(v\) ∼ 1–109 µm/s, determines the shift of the beam along the surface from pulse to pulse, Δ ≈ \(v\)/f. If shift Δ turns out to be much smaller than the size of a focal spot on the surface, a screening effect may take place due to the secondary absorption of nanoparticles formed by previous pulses [52]. If, in this situation, the shift appears to be smaller than the sizes of the vapor bubble, the refraction/scattering of radiation by the bubble distorts the distribution of the radiation energy over the surface and reduces ablation efficiency [72].

Other effects of laser radiation during laser ablation in liquids are relatively poorly studied and are either material-dependent (wavelength) or trivial (the greater the average radiation power, the higher ablation efficiency [72, 73]). At the same time, a very important laser parameter is the duration of the radiation pulses, which, for most contemporary and available laser systems, varies by many orders of magnitude from a few tens of femtoseconds to several hundreds of nanoseconds, thus determining, in particular, the features of laser radiation energy transfer to a target surface (stage 1, see above). Published communications, devoted to the comparison between laser systems with different durations from a few femtoseconds to several nanoseconds during ablation of targets of the same materials, e.g., gold, present greatly different data. In some works [74], it is reported that the productivity of colloidal nanoparticle generation of picosecond pulses is better. Others works [73] report a better productivity of nanosecond laser pulses. Here, it should also be taken into account that the laser systems under consideration differ in not only pulse durations, but also wavelengths, energies, repetition frequencies, focusing, and sometimes the volumes of produced nanoparticle dispersions [55]. Accordingly, in terms of optimizing and scaling the generation of colloidal nanoparticles, a problem arises concerning the correct comparing experimental results obtained under different laser- and material-related research conditions.

Approaches to Estimating Colloidal Nanoparticle Concentration in Liquids

Due to the large number of colloidal nanoparticles n ∼ 103–106 with characteristic size R ∼ 100 nm (the number of smaller particles is 1/R3 times more) generated even during one laser pulse with a relatively low energy (1–103 µJ), it is, as a rule, actually impossible to count them directly; so, macroscopic methods are used for estimating this number.

They include a rather convenient express method for measuring optical transmission coefficient Tcol of a dispersion of nanoparticles, with this method making it possible to characterize colloidal nanoparticles either by optical density OD = log(T0/Tcol) or by the extinction coefficient, which is determined by the joint contribution of absorption and scattering, κ =ln(T0/Tcol)/d, where T0 is the transmittance of a cell with a pure dispersion medium and d is its width along the optical axis. In the general case, the optical density and extinction coefficient are relative characteristics of the concentration of colloidal nanoparticles; however, if nanoparticles of only one chemical and structural-phase type are present, it is possible to determine total mass Mopt of nanoparticles in a solution by measuring the extinction coefficient in the region of interband transition (IBT) of a material. Indeed, in contrast to the metallic (intraband) absorption of free electrons [72], in the spectral region of interband transitions of strongly localized d electrons to the conduction band, the particle size does not affect the extinction coefficient up to nanometer sizes. In this case, the strong interband absorption dominates over the plasmonic effects of absorption and scattering (in the case of metal nanoparticles), and the measured extinction coefficient may be compared directly with the absorption coefficient in the IBT region, and their ratio will characterize specific density ρcol of the material in a solution in comparison with material density ρ0. For known volume V of a dispersion, total mass Mopt = ρcolV of nanoparticles is obtained, while the determination of its distribution over nanoparticles with different sizes requires the use of additional methods, such as dynamic light scattering by nanoparticles [55, 75, 76].

Meanwhile, in the case of laser generation of nanoparticles of only one chemical type, i.e., in the absence of chemical reactions of addition in a solution, a mass productivity criterion is also applied, i.e., the mass loss of an ablated material sample [55, 73] or the mass of the obtained dried colloid [52, 74]. Although being convenient, the first approach is indirect (part of the material ablated in the form of large, micron-sized, particles can quickly undergo sedimentation [77, 78]). The second approach directly characterizes the total mass of nanoparticles in a solution; however, it is less convenient. In general, the mass criterion is additional to the optical one, and their combined use makes it possible to minimize artifacts in the measurement of the concentration of produced colloidal nanoparticles.

Finally, for laser ablation, in addition to the exposure (the number of laser pulses spent for particle generation), a very important role is played by the energy of laser pulses [52, 79–81], which, for different laser systems, may vary within a fairly wide range, from a few microjoules (femtosecond/picosecond lasers) [82] to several joules (nanosecond lasers); hence, it is one of the important factors for scaling the production of colloidal nanoparticles in liquids by the laser ablation method [52]. Therefore, the productivity of obtaining nanoparticles, as determined according to the optical or mass criteria, is usually recalculated per one pulse [55] and normalized to the laser pulse energy (ergonomics criterion [83]). Ergonomics characterizes the energy consumption for the generation of a fixed amount or mass of nanoparticles [83], thus reflecting the optimality of the selected regimes. Moreover, ergonomics characterizes the scalability of the process with an increase in the energy of laser pulses by increasing the local energy density at fixed focusing on the material surface or by enlarging laser focusing area at a fixed local energy density. Only one of these possibilities turns out to be optimal (e.g., for nanosecond laser pulses). In some cases, e.g., for ultrashort laser pulses, the possibility of increasing the energy is limited by nonlinear self-focusing [84] or nonlinear ionization (breakdown) of a medium [85–87]. In this work, gold and silver in an aqueous medium are used as model chemically inert materials, which give nanoparticles of only one chemical type (metallic gold or silver, respectively) during laser ablation, to show the potentials of different typical femto-, pico-, and nanosecond laser systems employed in the laser-ablative production of nanoparticle dispersions in water. The application of such a system minimizes the number of chemical reactions and the formation of side compounds characteristic of ablation in organic solvents.

Comparison of Femto-, Pico-, and Nanosecond Laser Dispersion of Gold in Water

Gold is a convenient model material for studying the processes and parameters, as well as the productivity, of laser dispersion in liquids; therefore, it has been repeatedly applied for these purposes using water as a dispersion medium [60, 73, 88, 89]. At the same time, the results of experiments on femto-, pico-, and nanosecond laser dispersion have turned out to be extremely contradictory. Moreover, only the rough mass criterion of dispersion efficiency has been used in the cited works. Just recently, such studies were carried out under the maximum possible comparable conditions, namely, at close laser radiation wavelengths (1064 nm (ns) and 1030 nm (fs, ps)), focusing conditions, and pulse repetition frequencies [55, 83], i.e., using actually the same experimental setup (Fig. 1). It is also very important to use complementary optical and mass criteria to analyze the productivity and ergonomics of dispersion processes [83].

Fig. 1.
figure 1

Scheme of nanoparticle generation.

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In these studies, laser-ablative generation of nanoparticles was carried out using two laser systems (an ultrashort-pulse laser with laser radiation duration varied from 300 fs to 10 ps and a nanosecond laser with radiation duration of 100 ns). The laser beam was focused using an f-theta lens with a focal distance of ≈100 mm onto a bulk gold target (purity of 99.99%) placed into a cell containing distilled water (3 mL) (the height of the water layer over the target was ≈1.5 mm). The experiment is schematically illustrated in Fig. 1. The influence of laser radiation time on the efficiency of nanoparticle generation was studied in a range of subpico-, pico-, and nanoseconds. Therewith, the pulse frequency and scanning speed were constant and amounted to 20 kHz and 100 mm/s, respectively. The pulse energies were 2.5–6.5 µJ for subpico- and picosecond pulses and 0.3–0.6 mJ for nanosecond ones. Minimum focus spot size (diameter 1/e) was ≈20 µm for subpico- and picosecond laser pulses and ≈40 µm for nanosecond laser pulses; the size of the scanning area was 10 × 20 mm2.

After the laser generation, a dispersion of gold nanoparticles was sampled for subsequent determination of nanoparticle sizes by dynamic light scattering and electron microscopy, extinction coefficient by spectrophotometry, and mass loss by weighing a dry target before and after irradiation with an accuracy of 1 μg. Dispersion process productivities were compared using the key characteristics of the experiments recalculated per one laser pulse. The characteristics comprised total mass loss of a target in one experiment and the “optical efficiency” (OE, the extinction coefficient of the dispersions in the region of interband transitions of bulk gold at about 380 nm, KIBT, multiplied by fixed volume V of a dispersion and divided by number N of performed pulses). Figure 2 shows the dependences of OE on the duration of laser radiation.

Fig. 2.
figure 2

Dependences of OE on the duration of laser radiation for a gold target at different energies of laser pulses.

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The OE values are comparable for all used laser pulse durations and increase monotonically with laser pulse energy (Fig. 2), thereby indicating a stable and supethreshold character of ablation in the “phase explosion” regime,Footnote 1 i.e., expansion of a supercritical fluid for fentosecond/picosecond laser pulses [90, 91], and homogeneous boiling up of a superheated liquid in the near-critical region for nanosecond laser pulses [92]. However, the dependence of the OE on the duration of laser pulses is more interesting. As the duration of laser radiation increases within a range of 0.1–10 ps, the OE initially grows; then, a local maximum is observed in the region of several picoseconds, after which, the OE decreases. The decrease in the efficiency in the region of subpicosecond durations is associated with the appearance of nonlinear effects, i.e., self-focusing and filamentation [93–96]. The decrease in efficiency in the region of several picoseconds is associated with the acoustic unloading of the heated layer during its heating by a laser pulse in the phase explosion regime [90, 97, 98]. Further, when passing to nanosecond laser ablation, the OE values increase several times (Fig. 2); however, the pulse energy is almost two orders of magnitude higher; i.e., the energy efficiency (per unit energy, ergonomics) of nanoparticle generation under the action of nanosecond laser pulses is lower by more than an order of magnitude. This is due to the screening effect of subcritical ablation plasma,Footnote 2 which almost inevitably arises under the action of nanosecond laser pulses in an ablation plume in the phase explosion regime [64] and self-consistently determines the absorption of the target and the ablation rate [58, 59, 99, 100].

It is noteworthy that a similar tendency is also observed for the target mass loss (mass efficiency, ME) recalculated per one laser pulse (Fig. 3). This indicates that an almost constant fraction of the ablated substance passes into the dispersion of nanoparticles. Moreover, there is a good agreement (within 20–40%) between mass loss per pulse M/N and mass Mopt of the colloidal substance in the dispersion per radiation pulse calculated from the value of the extinction coefficient in the region of interband transitions of the material (indicated in Fig. 3 by dashed lines). The mass of gold in the dispersion of nanoparticles was calculated by the following equation:

$${{M}_{{{\text{opt}}}}} = \left\{ {{{\rho }_{{{\text{Au}}}}}V} \right\}\left\{ {\frac{{{{K}_{{{\text{IBT}}}}}}}{{{{K}_{{{\text{IBT}}{\text{,0}}}}}}}} \right\}{\text{/}}N,$$
(1)
Fig. 3.
figure 3

Dependences of gold target mass loss recalculated per pulse, M/N, on the duration of laser radiation at different pulse energies. The dashed curves represent the data calculated from the extinction coefficients for mass fraction Mopt of the solid in dispersion.

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where ρAu is the density of bulk gold; \(V\) is the solution volume; \({{K}_{{{\text{IBT}}}}}\) is the extinction coefficient in the region of interband transitions for colloidal solutions of gold nanoparticles; \({{K}_{{{\text{IBT}}}}}\) is the absorption coefficient of bulk gold in the region of 380 nm, which is equal to ≈6 × 105 cm−1 [101]; and N is the number of pulses. The observed agreement between the values of M/N and Mopt indicates a high (almost 100%) efficiency of the transfer of ablation products into the dispersion.

To estimate the energy efficiency of the nanoparticle generation process, we considered the experimentally determined values of the optical efficiency (KIBTV)/N and mass loss M per unit energy E of laser pulses spent for the generation of a dispersion (KIBTV)/(EN) and M/(EN) (Figs. 4, 5), respectively. It is quite indicative that, in terms of the energy efficiency of the process of generating gold nanoparticles in water, picosecond laser pulses, which are free from the influence of nonlinear effects and the screening effect of ablation plasma, provide a significantly higher efficiency recalculated per unit energy of radiation pulse. For example, it is one or two orders of magnitude higher than that for nanosecond laser pulses. This tendency is also confirmed for the target mass loss (mass efficiency, Fig. 5).

Fig. 4.
figure 4

Dependence of OE recalculated per unit laser pulse energy on the duration of laser radiation for a gold target.

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Fig. 5.
figure 5

Dependences of gold target mass loss per pulse on the duration of laser radiation recalculated per unit pulse energy. Dashed curves represent data calculated from extinction coefficient for the mass fraction of the solid in solution.

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It is clearly seen that the above tendencies and the values of M/N (Mopt) for femto-, pico-, and nanosecond laser dispersion of gold in water are in qualitative agreement with the available fragmentary data of previous works [73, 88], as well as with similar systematic results obtained for gold films of different thicknesses [83]. Thus, it may be concluded that these results reflect the general tendencies in the productivities of different laser systems with respect to gold and, in some cases, silver colloidal nanoparticles [102].

MAIN COLLOIDAL CHARACTERISTICS OF NANOPARTICLE DISPERSIONS PRODUCED BY LASER ABLATION IN LIQUID MEDIA

As has been shown above and discussed many times in the literature (see, e.g., [45, 48, 103, 104]), laser ablation in liquid media using lasers with different pulse durations is a reliable and versatile approach to the synthesis of rather stable dispersions of various metal nanoparticles free of adsorbed substances that modify their surface properties. In contrast to chemically synthesized nanoparticles, the surface of which is usually covered with ligand molecules (long-chain polymers or surfactants) [105–107], nanoparticle dispersions obtained by laser ablation demonstrate good temporal stability in deionized water [104]. Moreover, laser-based methods make it possible to realize simultaneous one-pot creation and dispersion of particles in a liquid medium. Finally, this method implies no limitations relevant to the choice and deposition of long-chain ligands on the surface of resulting particles, provided that the substances used to modify the particles form true or micellar solutions in the liquid media. It should also be noted here that, due to the nonequilibrium state of the particles resulting from ablation, their surface energy and adsorption activity for freshly formed nanoparticles turn out to be significantly higher than those in aged dispersions. According to the literature data, this leads to the fact that, e.g., gold particles adsorb five times more oligonucleides than do analogous particles obtained by chemical methods [108]. It should be emphasized that the field of application of nanoparticles essentially depends on the temporal stability of nanodispersions containing these particles, while the maintenance of the long-term stability of dispersions is always a difficult problem concerning the controlled preparation and storage of nanodispersions [103, 104, 109]. Therefore, in this section, we shall briefly consider the mechanisms of stabilization of aqueous nanoparticle dispersions and discuss the stability of dispersions obtained by the methods described in this review.

Forces That Determine the Character of Internanoparticle Interactions in Aqueous Media and the Stability of Dispersions

A large area of interfaces between particles and dispersion media makes particle dispersions thermodynamically unstable. However, the switching-on of interactions between individual particles can significantly change the kinetic stability of a system. It should be kept in mind that the state of a freshly prepared highly dispersed system may vary in the following directions. It may be the sedimentation of large particles under the action of gravity and the aggregation of small particles, including nanoparticles, which is also followed by sedimentation. At a significant solubility of dispersed phase particles in a dispersion medium, both the transfer of a substance from smaller particles to larger ones (Ostwald ripening) and the dissolution of particles at their low concentration in the dispersion may occur. As a rule, for nanoparticles of rather inert metals in deionized water, the last two mechanisms of variations in the state of dispersions play no significant role; however, they can be very important for chemically active metals upon changes in the pH of a dispersion medium [110, 111].

Prevention or inhibition of particle aggregation is one of the main ways to increasing the kinetic stability of nanodispersions, with this way being extensively discussed in the literature. The stability of a dispersion of nanoparticles may be governed by various types of surface forces, including van der Waals, ion-electrostatic, steric, and structural forces [112]. The van der Waals forces always contribute to the aggregation of identical particles, thus destabilizing the dispersion. In this case, for metal particles with a very high dielectric permittivity, the contribution of the van der Waals attractive forces in aqueous media can significantly exceed interactions in systems with nonmetal particles. On the contrary, being a dispersion-stabilizing mechanism, the ion-electrostatic interactions in aqueous media with low ionic strengths cause the repulsion between identical particles, and the higher the charge of their surface in a liquid medium, the stronger the repulsion [112]. An increase in the concentration of ions in a dispersion medium within a range of their low concentration leads to screening the ion-electrostatic repulsion, while, at a high concentration of ions, it can cause a correlation attraction between the particles [113].

The character of the interparticle interaction caused by steric forces depends on the interparticle distance in a dispersion, the thickness of the layer of a substance adsorbed on the particle surface, the energy of its adsorption on the particles, and the type of interaction of the adsorbed molecules with a dispersion medium. As a rule, dispersions of nanoparticles are stabilized using long-chain surfactants or polymers that are irreversibly adsorbed on dispersed phase particles and form brushlike structures. The formed adsorption layer not only reduces the solubility of particles and their chemical activity upon the contact with a dispersion medium, but also creates a steric barrier that prevents the particles from coalescence in the dispersion. However, the application of such adsorption layers can significantly change the chemical, catalytic, and optical properties of the particles themselves and affect their biocompatibility, bioavailability, and therapeutic action [109]. Finally, the structural interaction forces between nanoparticles can both cause an additional stabilizing effect, mainly for nanoparticles that are highly wettable with a dispersion medium, and induce rapid coalescence of hydrophobic particles in a hydrophilic dispersion medium under the action of hydrophobic attraction [112, 114].

Analysis of the Stability Mechanisms of Silver and Gold Nanodispersions Obtained by Laser Ablation in Deionized Water Using Lasers with Different Pulse Durations

To date, the data on the stability of nanodispersions of metal particles obtained by laser ablation in deionized water using lasers with different pulse durations are scarcer in the literature. To fill this gap, the properties of gold and silver nanoparticles obtained using femto- and nanosecond lasers were compared in this work. The regimes of laser treatment of the targets in deionized water are presented in Table 1.

Table 1.   Parameters of the generation regimes for gold and silver particles in deionized water
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The scanning electron microscopic examinations of individual particles obtained in the regimes indicated in Table 1 showed that both gold and silver nanoparticles had a spherical shape and a rather wide size distribution. Therewith, the diameters of gold particles obtained by laser ablation at a femtosecond pulse duration were mainly in a range of 100–120 nm. The diameters of the particles generated by laser ablation with nanosecond duration were distributed over a size range of 40–60 nm. In silver dispersions, the particles obtained by nanosecond processing had sizes of 100–150 nm, while after femtosecond laser ablation, a significant fraction of the particles were characterized by sizes of 100–140 nm, with a small amount of larger particles having sizes as large as 350–400 nm.

A Zetasizer Nano ZS device (Malvern Instruments, United Kingdom) was used to study the stability of the obtained aqueous dispersions, as well as to calculate and analyze such characteristics of the dispersions as the zeta-potential and particle size. According to the considerations expressed in [115] and our electron microscopy data on the obtained particles, a low false peak corresponding to small particles can appear in the bimodal scattered light intensity distribution over the hydrodynamic diameters of both silver and gold nanoparticles. Therefore, to characterize changes in the state of dispersions, the parameters of the peak with the maximum intensity were used below. It should be noted here that the particle diameters determined from the peak with the maximum intensity were in good correlation with the characteristic sizes obtained by analyzing electron microscopic images.

The zeta-potentials were calculated within the framework of the Hückel approximation for both deionized water and weak solutions, for which relation κa \( \ll \)1 was fulfilled (here κ and a are the inverse Debye length and particle diameter, respectively). For more concentrated solutions, in which κa \( \gg \) 1, the calculations were carried out using the Smoluchowski approach. To examine each dispersion by light scattering, a 1-mL sample was taken from it, and the current pH value was measured. The pH values were determined with an ESK 10614 microelectrode (OOO Izmeritelnye Tekhnologii, Russia) and a Jenco 6230 pH meter operating within a pH range of 1.5–13.

The dispersions of silver and gold nanoparticles obtained under different ablation regimes had an intense color: pinkish-violet, for dispersions of gold nanoparticles, and yellow-gray, for dispersions of silver nanoparticles, thus being in agreement with the literature data [103, 112, 115–119]. For subsequent application of nanoparticles obtained in different laser ablation regimes, it was of interest to determine the dependences of the potentials and sizes of the particles on dispersion medium pH. In this work, to study such dependences, nanodispersions were, at the first stage, obtained by ablation in a neutral medium; at the second stage, solutions of hydrochloric acid or sodium hydroxide were added to bring the pH of a dispersion medium to a required value. After a dispersion was stored for a half an hour, the potentials and sizes of the particles were measured. The pH dependences of nanoparticle zeta-potentials for gold and silver dispersions obtained by both nanosecond and femtosecond treatment of the targets are shown in Figs. 6a and 6b.

Fig. 6.
figure 6

Dependences of (1, 2) zeta-potentials and (3) sizes on dispersion pH for (a) gold and (b) silver nanoparticles obtained by (1, 3) femtosecond and (2) nanosecond laser ablation. Nanoparticle concentrations in dispersions are shown in Table 1.

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For the studied range of pH 1.5–13, nanoparticles of both metals obtained by laser treatment with different durations retain a negative potential, while a reduction in the potential values with decreasing pH indicates the existence of isoelectric points for both gold and silver nanoparticles at pH < 1.5. At the same time, for gold nanoparticles at pH > 8 (see Fig. 6a), a rather large scatter for the value of each measured potential does not allow us to speak of any trend in the changes of the absolute value of the zeta-potential with a further increase in pH. The data obtained can be interpreted as the constancy of the potential value of gold at a level slightly exceeding –60 mV in a wide range of pH > 8.

For silver nanoparticles at high pH values, the dependence is more pronounced, thereby indicating an increase in the absolute value of the negative zeta-potential.

In recent years, the high negative surface charge of inert metallic gold has been widely discussed in the literature [120–122]. The obvious mechanism of negative charging of such surfaces in aqueous media is the physical adsorption of hydroxyl groups present in the dispersion medium due to the dissociation of water molecules. Such adsorption is provided by van der Waals interactions between polarizable OH– ions and metal particles. Some authors have related the negative zeta-potentials of aqueous gold dispersions to partial oxidation of the surface atoms of a gold nanoparticle to Au+ and Au3+ [104, 123, 124]. However, recent detailed reviews [122, 125] have shown that, in many cases, the existence of oxides on the surfaces of both gold and silver nanoparticles is not detected in the presence of a negative surface potential in aqueous dispersions. Another mechanism proposed recently in the literature for negative charging of gold and silver nanoparticles obtained by laser ablation is associated with the presence of excess electrons formed in the plasma during ablation and “captured” by the nanoparticle surface [120, 122, 126].

For nanodispersions obtained by the femtosecond treatment in the aforementioned regimes, the size distribution determined for the resulting particles by dynamic light scattering is quite narrow, with the scattered intensity maximum being located in a size range of 80–120 nm (Fig. 6a). The values of the zeta-potentials for gold particles that we generated in neutral media by the methods described above are in good agreement with the data obtained earlier in a neutral medium for smaller particles also produced by laser methods [117–119]. In addition, the potentials of the dispersions of gold nanoparticles obtained in this work at different durations and powers of laser radiation (Fig. 6) are in good agreement with each other within the measurement error. For silver dispersions, the difference between the potentials is larger, while the higher absolute values of the potential correspond to nanoparticles generated with the femtosecond laser.

Let us now consider in greater detail the stability of gold and silver dispersions during their long-term storage under room conditions (T = 25°С).

In the general case, time variations in the nanoparticle sizes of dispersions may be realized via four mechanisms. The first mechanism is the dissolution of particles leading to a decrease in their sizes. The second mechanism, which is realized for particles dispersed in media with high concentrations of ionic or molecular forms of the particle substance, is the Ostwald ripening or recondensation. This process is associated with the dependence of the solubility of particles on their size and causes the growth of larger particles at expense of the dissolution of smaller ones. To start such a process, the concentration of the dissolved form of the particle substance must be higher and lower than the saturation concentration for larger and smaller particles, respectively. The third mechanism, which is associated with the aggregation of nanoparticles upon their approaching each other, obviously leads to an increase in the average aggregate size. It should be recollected here that the van der Waals forces causing such aggregation are enhanced with the aggregate sizes [127]. Finally, sedimentation of particles under the action of gravity will decrease the fraction of large aggregates subjected to the Brownian motion in a dispersion medium. Of course, when dispersing particles in reactive multicomponent media, particle sizes may also change due to chemical transformations of the particle surface. However, for noble metal nanoparticles dispersed in deionized water, this last mechanism does not play any role. The analysis of the literature has shown a low solubility of silver nanoparticles and silver oxides in aqueous media, with the solubility increasing with a reduction in nanoparticle sizes [128–130]. Gold nanoparticles are often used as inert metal indicators to monitor specific behavior of nanoparticles in an aqueous medium [131]. At the same time, it has been shown that such nanoparticles can be partly dissolved and exhibit a high migration activity in biologically active media [131–133]. In connection with all of the aforementioned, it is necessary to take into account the possible solubility of the nanoparticles that we have obtained in deionized water.

The analysis of the evolution of the average nanoparticle size with the time of dispersion aging in deionized water for 70 days of storage for gold (Fig. 7a) and silver (7b) particles has indicated a similar character of variations in this parameter at the initial stage of storage. For example, up to 10–14 days of the storage, a decrease in the particle diameter is observed, with this decrease being somewhat greater for silver particles. Such behavior of both dispersions can be attributed to both the gradual sedimentation of larger particles after their generation, because the studied dispersions were not subjected to centrifugation before the onset of the experiments, and the partial dissolution of the smallest particles. It is worth noting that, due to the twice higher density of gold, aggregates with smaller sizes will be subjected to intense sedimentation, while the lower solubility of gold must diminish the effect of solubility on the decrease in the average particle diameter. Then, a rather long (although having different durations for gold and silver) period of stable average particle diameter is established. However, at sufficiently long exposure times, t > 30–35 days, a small but monotonous increase in the average particle diameter from 110 to 192 nm is observed for 70 days of storage for silver nanodispersions. At the same time, average particle diameters for gold remain on the order of 120 nm up to 70 days of storage. The significantly higher solubility of silver particles in water leads us to assume a more significant role of the Ostwald ripening in the growth of silver nanoparticles than in the growth of gold ones. Below we shall focus our attention on the analysis of the contributions from surface forces of different natures to the aggregation-type enlargement of particles of both types.

Fig. 7.
figure 7

Evolution of parameters for dispersions of (a, c, e) gold and (b, d, f) silver nanoparticles in deionized water during 70-day storage. (a, b) Dependences of average size and ζ-potential of nanoparticles on the storage time of dispersions; (c, d) time evolutions of zeta-potential distributions determined by the contributions of particles with different charges; and (e, f) time evolutions of nanoparticle size distributions. Dispersions were obtained by ablation with a femtosecond laser. Dispersion concentrations are 0.026 and 0.05 g/L for gold and silver nanoparticles, respectively.

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Figures 7a and 7b show the evolutions of the average zeta-potentials determined from the average electrophoretic mobility of the particles. The scatter of the data in Figs. 7a and 7b suggests an idea of variations in the values average over the distribution (for a Gaussian distribution, this is the position of the maximum in the distribution curve) at different aging times of the dispersions. The distributions of the zeta-potentials determined by the contributions of particles with different charges during each measurement cycle are given in Figs. 7c and 7d. The analysis of the data obtained has indicated the constancy of the average values of the zeta-potentials for both gold and silver nanodispersions throughout the observation period. At the same time, the half-widths of the size distributions determined at different aging times of the dispersions (Fig. 7e) remain almost unchanged for gold nanoparticles, thus indicating the stability of the particle distribution in the dispersion and, hence, an insignificant contribution of solubility to the content of small particles. On the contrary, for silver nanoparticles, a gradual asymmetric broadening of the size distribution is observed (Fig. 7f), thereby indicating the aggregation of particles due to their collision-induced interaction. The detailed analysis of the distribution in the region of small sizes has shown a slight decrease in the smaller sizes at the initial stage of aging. However, after aging of silver dispersions for 35 days, both wings of the distribution from the sides of both small and large particle sizes, shift to the region of larger sizes.

However, the results presented here indicate, as a whole, a slight increase in the particle size, which can be considered as a satisfactory kinetic stability of both studied nanodispersions for 70 days of observation.

It was noted above that, for the considered gold and silver nanoparticles obtained by laser ablation in a liquid without the use of surfactants, the dispersions can be stabilized due to both the repulsion of double electric layers and the structural forces. The analysis of the literature [112, 134, 135] has shown that the role of the ion-electrostatic component can be revealed by adding dissociating salts to a dispersion medium. Increasing ion concentration in the dispersion medium leads to the compression of the diffuse part of the double layer and to a decrease in the ion-electrostatic repulsive forces [112, 134, 135]. Therefore, a sharp decrease in the stability of the dispersion upon the addition of a salt can be considered as an evidence of the key role of the ion-electrostatic forces in the stabilization of the dispersion under consideration. The determination of the contribution from the structural forces is based on the strong temperature dependence of these forces [112, 134]; hence, the study of the evolution of dispersion stability with increasing temperature makes it possible to estimate the role of structural forces.

To determine the contributions of the described mechanisms of the surface forces, we studied the influence of temperature and the concentration of dissolved salts on the potentials and average sizes of dispersed particles.

According to the literature data [112, 114, 134], temperatures above 50°C should contribute to the breakage of both the static and dynamic structures of a thin liquid interlayer. Therefore, if structural forces make a significant contribution to the stability of the dispersions under investigation, heating should lead to particle aggregation and loss of dispersion stability. In this study, we compared dispersion parameters at three temperatures: 25, 50, and 70°C.

Table 2 presents the data obtained on the gold and silver dispersions aged for 14 days before and after exposure at an elevated temperature for 30 min. The studied dispersions were prepared by ablation of the targets with a femtosecond laser in water.

Table 2.   Changes in the parameters of nanodispersions upon heating
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Although small changes in the average particle diameters are observed upon heating, the processes for gold and silver are differently directed and lead to a decrease in the average diameter of gold nanoparticles by 5 nm and an increase in the diameter of silver particles by 10 nm. In general, the detected changes do not indicate a marked influence of temperature on the colloidal characteristics of the dispersions, thus leading us to conclude that structural forces do not have a significant effect on their stability.

To determine the role of the repulsion of electrical double layers in the stability of our dispersions, we studied the effect of the concentration of KCl added to a dispersion on the zeta-potential of the particles and their aggregation in the dispersion. As has been noted above, a high concentration of ions in a dispersion medium leads to the enrichment of the near-surface layer of nanoparticles with counterions, which, being concentrated in the dense part of the double layer, screen the field of the charged nanoparticle surface and, accordingly, lead to a decrease in the thickness of the diffuse part of the double layer. The latter circumstance, in turn, reduces the value of the ion-electrostatic component of the disjoining pressure [112, 134, 135]. For disperse systems, in which the repulsion of electrical double layers exceeds the van der Waals attractive forces between nanoparticles, the stability of dispersions is mainly determined by ion-electrostatic forces.

Figure 8 shows the data obtained for gold and silver nanodispersions 10 min after adding the salt to the dispersions. The analysis of the data obtained has indicated a significant effect of salt additives on the value of the zeta-potential. For gold nanoparticles, the absolute value of the potential decreases by almost three times upon passing from nanoparticles in deionized water to nanoparticles dispersed in a 0.1 M KCl solution. For silver, the zeta-potential is almost halved. The increase in the average diameter of gold nanoparticles in the salt solutions (Fig. 8b) turns out to be much larger than that for silver nanoparticles. It is noteworthy that small additives of ions initiate only partial aggregation of nanoparticles, which manifests itself both as the broadening of particle size distributions in concentrated solutions (inset in Fig. 8b) and as the discoloration of the dispersions.

Fig. 8.
figure 8

Variations in (a) ζ-potentials and (b) sizes of gold and silver nanoparticles obtained by femtosecond laser ablation in deionized water with the addition of potassium chloride to the dispersion as depending on the concentration of the latter. The inset in panel (b) shows the size distribution of gold nanoparticles in (1) additive-free dispersion and at KCl concentrations of (2) 0.05 and (3) 0.1 М.

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The presented data on the decrease in the zeta-potential with an increase in the ionic strength of the solution are in good agreement with the literature data obtained for diverse surfaces [136, 137]. In this case, the zeta-potentials at a KCl concentration of 10–1 M are already too low to stabilize the nanodispersions due to the ion-electrostatic component of the disjoining pressure. Thus, the array of the data obtained on dispersions containing the dissolved salt indicates the key role of the ionic-electrostatic interactions between nanoparticles of both metals in the long-term stability of nanodispersions in deionized water.

CONCLUSIONS

The product of colloidal solution volume and the extinction coefficient in the region of interband transitions recalculated per one pulse and unit radiation energy has been proposed and verified as a comparison test for analyzing the efficiency of laser generation of gold nanoparticles at comparable parameters of laser systems with different pulse durations (subpico-, pico-, and nanosecond ones).

When comparing subpico-, pico-, and nanosecond laser generation of gold nanoparticles in a liquid at a wavelength in the near IR range, a pulse repetition frequency of 20 kHz, and comparable scanning parameters, the highest generation efficiency is observed for nanosecond ablation, which is limited by the formation of screening subcritical ablation plasma. At the same time, the efficiency per unit energy for picosecond generation of nanoparticles free of the influence of nonlinear effects turns out to be one or two orders of magnitude higher than that for nanosecond generation.

The stability and dimensional and electrochemical parameters have been studied for gold and silver nanoparticles generated in deionized water using femto- and nanosecond lasers. The dependences of the zeta-potentials of the particles on pH of dispersion media have been found to be similar for dispersions obtained by ablation of the same materials using lasers with different pulse durations. At the same time, the size of the generated gold nanoparticles has turned out to be somewhat dependent on the pulse duration. All obtained dispersions are satisfactorily stable during long-term storage; however, they show a tendency to aggregation with an increase in the ionic strength of the dispersion medium. The analysis of the mechanisms of the aggregative stability of dispersions has shown the dominance of the contribution of the ion-electrostatic interparticle interactions over the van der Waals contribution. The study of the temperature dependence of the dispersion stability has made it possible to reveal an insignificant role of the structural interaction forces between nanoparticles in the studied dispersions.