Introduction

Hughes Research Laboratories is the pioneer of laser where the first functional laser was constructed by Maiman in 1960 [1]. Ever since that time, lasers are being extensively employed throughout a series of science as well as technological areas. Among them, laser-assisted synthesis of functional materials has been certainly among the most substantial applications. As observed over the period of time, fabrication of nanoparticles (NPs) has attracted significant responsiveness from the materials and physics research society due to their tunable optical characteristics centered on the configuration of elements. A range of methodologies were presented for the synthesis of NPs throughout the last quarter century, namely chemical methodologies comprising seed growth technique, laser ablation in liquids (LAL), citrate reduction technique, along with electrochemical techniques [2,3,4,5,6,7,8]. Nanoparticles exhibit various exceptional properties which are not perceptible in bulk materials [9]. Properties of the nanoparticles which are generated over laser ablation in liquids procedure are governed by input of laser source parameters (namely wavelength, rate of repetition, duration of pulse, focusing conditions, etc.) along with surrounding properties (such as nature and amount of the liquid) [10,11,12,13]. The utmost critical attribute of nanoparticles is the dependency of the nanoparticles on the dimension and shape allotment of particles [14]. Material irradiation under laser irradiation is considered a much intricate procedure. Laser irradiation considered an eco-friendly and trouble-free technique regarding the generation of metal NPs devoid of any surfactant or adding up of chemical. Rewards of laser irradiation/ablation practice are simplicity, nanoparticles with high grade purity, the capability to formulate range of metals as well as ceramics, and the in situ scattering of nanoparticles into range of liquids [15]. Green fabrication of metal nanoparticles in liquid solution has become a concern subject within areas of nanomedicine as well as nanotechnology. The preparations of copper, gold and silver nanoparticles were performed employing chemical and physical approaches centered on chemical contact of gamma ray or X-ray with chemical substance. Substantial amount of physical, chemical and biological properties is found in copper, Au and Ag nanoparticles. Such nanomaterials could augment the fluorescence properties of polymer composites and are also anti-bacterial, anti-concern and anti-inflammatory. They are employed for boosting the selectivity and sensitivity of biosensors centered on conducting polymer [16]. The metal nanoparticles are controlled by certain organic material [17]. Furthermore, they hold steady physical and chemical characteristics comprising photo-irradiation, resistance of elevated temperature, aversion to acids or oxidation [18,19,20], as well as catalytic characteristics [21, 22]. Consequently, nanoparticles of metal are employed for enhancing the reaction of organic or biomaterial material. Several approaches comprising solution stage [23], photochemical [24], sonochemical [25], electrochemical synthesis [26], photolytic reduction [27], radiolytic reduction [28], solvent extraction reduction [29], microemulsion approach [30], polyol procedure [31], and microwave ablation [32] have been employed for fabricating and dispersing silver, gold and copper nanoparticles into different modes. Such techniques are centered upon chemical action or material interactivity by external field.

Laser pulse incident breaches the material surface within a definite depth of penetration. Such measurement depends upon the wavelength of laser and target material refractive material and stands usually within 10 nm zone. The generation of strong electrical field via laser light is satisfactory for the removal of electrons through the mass of volume perforated in 10 ps for a laser pulse of nanosecond [33]. Electromagnetic field causes the free electrons to oscillate and could strike with the bulk material atoms, therefore transporting certain energy to lattice. Then heating of ablated facet is done as well as vaporized [33]. At substantial flux of laser, conversion of matter to plasma occurs, as shown in Fig. 1. This comprises several energetic elements counting atoms, electrons, molecules, clusters, ions, particulates, and molten globules and consequently holds certain distinctive attributes, namely elevated temperatures, higher density and extreme pressure [34]. Consequently, initial seed plasma is produced by the huge difference in pressure between the lasers. The ambient atmosphere results in speedy growth of plasma plume and further cools down. The practice is considered closely adiabatic and transonic [34]. The plasma species will nucleate under the appropriate condensation situation like pressure and temperature and mature into required nanostructures, either in a cool liquid media or on a substrate.

Fig. 1
figure 1

LAL setup for nanoparticle growth [46]

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Around two decades back, laser ablation was initially employed in liquid medium for the fabrication of colloidal solution of nanoparticles, as presented in Fig. 2. For ablating pure metal targets, Henglein and Cotton in 1993 employed a pulsed laser in numerous solvents for forming colloidal solutions comprising metal nanoparticles [35, 36]. From that period, considering preparation of metal, polymer and semiconductor nanoparticles, LAL is established as an important method. In terms of methodology, LAL is fairly unlike compared to other laser ablation methods which are operating in vacuum or gaseous atmospheres. This is due to the fact that the liquid media not only offers specific operational governing factors for the synthesis, but significantly influences the morphological study of products. Certainly, remarkable structure and size alterations of the nanoparticles formulated through laser post irradiation are detected. The astounding forecasts evolved by the contact of matter along with light certainly encouraged several research troops to emphasize their effort upon the generation of laser and alteration of nanoparticles in liquid as well as onto the characteristics and implementation of LAL-generated NPs.

Fig. 2
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In situ bioconjugation process for NPs through LAL [46]

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This article reviews the synthesis of different nanomaterials with the application of the technique of laser ablation in liquid. There is an active discussion on the LAL setup followed by the fabrication of nanomaterials such as nanoparticles of gold, copper, silver, metal and oxides. Further review on LAL synthesized nanoparticle application, finally concluding with future scope and challenges. This study hopes to set up a benchmark for the future relevant studies in the area for researchers, scholars and academicians. This article also presents the highlights regarding the LAL area when compared with other studies by discussing the research gaps and the prospective research scope that needs to be accomplished by reviewing the various literature studies and their significance.

Laser Ablation in Liquid (LAL) Setup

Fundamentally, setup of laser ablation comprises a lens, pulsed laser of high power, stirrer, liquid vessel and linear positioner. The specimens of metal, namely silver, gold and Cu (99.99%), are immersed into liquid. Figure 3 depicts the arrangement of laser ablation that encompasses a Q-switched Nd:YAG laser, lens (f = 30 cm), metal plate, solution container, a travel linear stage and a stirrer. Time period for pulse and laser ablation alters between 10 and 60 Hz and 5 and 60 min, correspondingly. For prevention of the beam energy absorption of laser into the solution of liquid, the path length through which the beam of laser travels the liquid should be tuned to minimal length. Thus, the gap between entrance windows and target plays a substantial role for achieving the finest laser energy beam upon target surface. To ensure that the metal nanoparticles scatter uniformly into the liquid media, solution blending is performed throughout the ablation of metal plate, and container having solution travels horizontally by means of travel linear stage in order to provide the fresh area for the ablation of the target. On exciting a metal target by means of laser beam, vapor, plasma and micro-/nanosize metallic droplets are generated. This droplet then reacts with the liquid media and nanoparticles are formed [37,38,39,40,41]. Among the possible three products, vapor and plasma could be produced with the help of lasers of short pulse width along with greater energy density [38, 42,43,44], for instance, nanosecond pulsed laser having a width of pulse of some nanoseconds (2–25 ns) and power density of around 108–1010 W cm−2 [38, 45, 46]. For the time being, nanodroplets are conveyed to be considered major product when lasers of lower power density (for example, millisecond lasers) are applied [47,48,49,50]. Up to now, the key development tools of laser ablated nanostructures were centered upon thermal evaporation along with the consequent contacts with the liquid and also the nanodroplets explosive ejection [45,46,47,48,49,50,51].

Fig. 3
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Schematic representation of nanoparticle synthesis technique utilizing laser ablation

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Nucleation and Growth Mechanism

As laser-vaporized materials are promptly quenched to extreme temperatures to room temperature [52], supersaturated vapor nucleation produces nanomaterial-sized particles near the edge of the laser plume. The general dynamic equation [53] may be used to calculate the change in the number concentration of vaporized materials, clusters and particles by taking nucleation, condensation and coagulation into account. The size distribution of nanoparticles created by the ablation process has been predicted using numerical solutions to the general dynamic equation [54]. It should be emphasized that electron and ion species make substantial contributions to the nucleation process. As a result, the laser ablation particles were electrically charged [55]. Therefore, it is challenging to replicate the quick shift in size distribution caused by laser-vaporized materials nucleation and growth. Advanced in situ observation techniques are required to experimentally evaluate the particle production process. One in situ measuring approach for exploring the kinetics of particle production is plume emission spectroscopy.

Fabrication of Nanomaterials via LAL

This section discusses the fabrication of various nanomaterials via LAL techniques, starting with metal, oxide and hydroxide nanoparticles followed by gold, copper and silver nanoparticles and then unique phase nanoparticles, nanowires and nanocarbons.

Nanoparticles of Metal, Oxide and Hydroxide

Usually, the most conventional structures fabricated through LAL technique are NPs of metal and its compounds made through the ablated metal reaction along liquid medium. Regarding a conventional LAL arrangement, the targeted metal is usually submerged into medium of liquid; further, transparent liquid is used for the concentration of the laser on target [56]. Having noble metal as target or a material like silicon or carbon, the products irradiated are typically particles of pure element [57] that are developed in laser-produced plasma in non-availability of any sort of chemical responses with liquid medium. Such phenomenon is observed on addition of surfactants, therefore steadying the generated NPs acting as dispersion into liquid [58,59,60]. In contrast, when additional active metals such as rhodium, palladium and silver are employed as target, the already synthesized metal NPs are usually stated to react with water, resulting in the generation of oxide or hydroxide nanostructures [48, 50, 61, 62].

In addition to metal targets, certain reactive liquid medium could perform like targets in order to absorb energy of laser beam and lead to generation of NPs. For instance, Henley and researchers acquired benefit of laser heating with conventional hydrothermal fabrication, using a 248 nm KrF excimer laser for irradiating solutions of precursor to synthesize ZnO nanocrystals [63]. Considering a separate report, a CO2 laser is employed for the heating of solutions of water/alcohol saturated with Zn(AcAc)2, which results in decay of precursor as well as synthesis of nanorods of ZnO [64]. Once an excimer laser or a constant wave (cw) Ar + laser (488 nm) was engaged for irradiating solutions comprising some salts of metal, laser-assisted precursors disintegration can result in development of a pure metal (namely Ag, Cu and Pd) particles in the solutions [65]. Therefore, laser-assisted liquid phase decay emerged as an effective method to synthesize metal or oxide NPs [66].

Silver Nanoparticles

Numerous scholars conveyed the research of nanoparticles of silver in inorganic (analytes) as well as organic solutions (such as ethanol and acetone). Silver NPs consist of properties, namely optical, thermal, electrical and biological. Henceforth, fabrication of Ag NPs was described employing physical and chemical approaches. Silver plate laser irradiation is regarded as alternate and green scheme for preparing of Ag nanoparticles; also, NPs cultivate in distinctive form of inorganic and organic solutions deprived of any accumulation and disintegrating. Ag NPs were arranged in methanol, water or oils of palm [67], coconut [68], pomegranate seed [69], polyvinyl alcohol [70] and solution of graphene oxide [71]. Ag nanoparticles have been covered with chain of oil fatty acid, and also the size of particle was around 10 nm. As the time of ablation escalates, there is a decrease in the size of particles. NPs generated of spherical profile are acquired via TEM image (Fig. 4a). Proficiency of colloidal absorption through Ag nanoparticles for 355, 532 and 1064 nm laser beam hinges upon restricted (SPR) surface plasmon resonance around 400 nm (Fig. 4b) [16]. Therefore, occurrence of maximal and minimal efficiency is at 355 and 1064 nm. Consequently, influence of colloidal absorption reamins noticeable for laser beam of short wavelength, concluding that the development proficiency and the nanoparticles size decline with a reduction in wavelength of the laser beam.

Fig. 4
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a Silver nanoparticle under oil observed in TEM image, b silver nanoparticles for UV–visible spectrum [16]

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Gold Nanoparticles

Gold nanoparticles find huge employment in electronics [72], photodynamic therapy [73], therapeutic agent delivery [74], tumor treatment [75], sensors [76], drugs carriers [77], as well as medical examinations [78]. Higher rate of activity and higher rate of sensitivity of Au nanoparticles were synthesized employing laser irradiation in water [79]. Concluding product is employed for regaining area of electrode of glassy graphite for detecting Hg, Cu, Co and Pb in lower concentration rates [79]. Au nanoparticles could soak up and react with the laser beam’s electric field [77], and gold NPs produce localized surface plasmon absorption ranging between 400 and 900 nm [80]. The free electrons articulate excitation results in surface plasmon band in colloidal NP [81]. Therefore, study and deliberation of green fabrication of Au nanoparticles are extreme important topic in nanotechnology and nanomedicine fields. The practice of laser ablation is considered an alternate scheme to generate gold NPs in aqueous media. In recent times, Au nanoparticles are produced in graphene oxide along with oils of pomegranate seed [82, 83]. Once Au nanoparticles are contrived employing laser irradiation of targeted Au material, the NPs were generated of spherical geometry (Fig. 5a) which were examined utilizing TEM. The size of particles was found to be ranging between 20 and 5 nm, and the peak of UV–visible absorption seemed nearly 530 nm (Fig. 5b) [16].

Fig. 5
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a TEM image of Au NPs in oil, b UV–visible spectrum of Au NPs produced via laser ablation [16]

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Copper Nanoparticles

Copper (Cu) nanoparticles find extended usage in conductive coatings like metallic inks, sintering additives (AlN), lubricants, [84] and biosensor devices [85]. Cu NPs are considered non-inflammatory [86], lessen gastrointestinal mucosa [87], antioxidative [88], anti-ulcer [89], and additionally considered beneficial for avoiding skin photosensitivity [90]. Copper NPs intensely captivate beams of line around 600 nm, originating due to localized surface plasmon resonance (LSPR). Currently, employing vegetable oils of palm [67], coconut [68], walnut [91] and castor [92] considering scattering the nanoparticles was regarded in nanometals manufacturing [93]. Such natural occurring compounds comprise triglycerides as well as nonpolar long chains of carbon which avoid nanoparticles accumulation by steric repulsion [68]. Numerous techniques established upon the response of metallic ions are displayed for generating Cu NPs. For instance, solution phase [94], photochemical [95], sonochemical [96] and electrochemical fabrication approaches [26] exist as prominent techniques which are employed for generation of Cu NPs in aqueous solution. Laser ablation [97] is clean methodology concerning the construction of nanoparticles of Cu. In a research, fabrication of nanoparticles of Cu into distilled water, ethanol and acetone [98] was stated employing laser ablation. Malyavantham and team [99] employed methodology of laser ablation for fabricating the alloy of Au-Cu NPs. Cu nanoparticles are generated in spherical shape (Fig. 6a) within aqueous medium. UV–visible peak ascended localized SPR around 630 nm (Fig. 6b) [16]. Effect of colloidal absorption upon construction proficiency of nanoparticles of Cu was considered the noteworthy constraint for preparing NPs of Cu. The generation performance of Cu nanoparticles via 532 and 1064 nm beam of laser stood near to those of Ag nanoparticles since absorption at 532 nm in colloids of Cu was lesser when compared with Ag colloids.

Fig. 6
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a TEM image of Cu NPs in oil. b UV–visible spectrum of Cu NPs generated via laser irradiation [16]

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Unique Phase Nanoparticles

Considered as the major distinctive characteristics of LAL method are intense local circumstances produced via laser within the plasma induced by irradiation and also around its boundary with liquid media. Plasma phase exists at elevated temperature and constitutes extremely excited groups where temperature, pressure and density are very high. Adjacent as well as neighboring the plasma, liquid phase results in fast quenching [100]. Such communication among the liquid and plasma phases might consequently result in development of unfamiliar phases like products quenched enormously speedily through a very higher temperature. For instance, nanoparticles of metastable zinc peroxide (ZnO2) can be fabricated through the ablation of target of zinc metal in 3% H2O2 [101]. Liu et al. [102] irradiated an objective of Si into a definite inorganic salt solution that enabled the generation of Si microcubes along with structure of Zn blende. Single-crystal Si enclosed substrates with nearly 150–300 nm dense amorphous carbon taken as targets. Identical group attained different forms of carbon micro- and nanocubes having C8-like configuration [103]. Tetragonal structure of Ge with high pressure phase was described to be confined as nanocrystals through the electrical-field-aided LAL at an ambient temperature–pressure [104]. Si NPs having nonconforming FCC crystal structure were fabricated via long-pulse laser ablation in water [105]. Width of the long pulse of implemented laser was revealed to be satisfactory for producing elevated temperatures and higher pressure situations, which are accountable for the creation of these FCC nanocrystals of Si [105].

Laser ablating Ti targets and nanospheres of TiO2 having mixed-phase (rutile/anatase) shape was contrived by Tian and team [106]. They additionally established that quickly varying pressure and temperature are liable for concurrence of the phases of anatase and rutile in the yield [106]. Although formulating crystals of nanodiamond through pulsed-laser irradiation of powder of graphite into water, Hu and researchers [107] establish that particles of single-crystal nanodiamond sequentially nurtured into exceptional, multi-twinned structures via intermediate simple and triple-twin structures [107].

Aforementioned outcomes demonstrate that LAL procedure could generate nanostructures having unusual as well as uncharacteristic phases that stood steady at elevated pressures and temperatures and additionally are metastable below ordinary situations. Conversely, high temperature, increased pressure and higher density environments produced throughout the process of LAL could alter the generation dynamics of numerous defects, primarily surface irregularities, and consequently result in NPs having exclusive defect conditions. Speedy quenching is highly possible to reserve the distinctive imperfections generated under extreme circumstances, hence participating in the generation of several unique nanostructures.

Nanowires and Nanocarbons

Non-equilibrium laser ablation nucleation procedures can produce numerous types of phases of allotropes and crystal which do not appear under the usual fabrication practices. Figure 7 [14] demonstrates TEM micrographs of carbon nanotubes, onion-like carbon and nanodiamonds fabricated via laser ablation process. Subsequently, process of laser irradiation is better when producing higher purity and metal particles of nanosize. It is also useful in the form of catalyst considering formation of single-walled carbon nanotubes (SWCNTs) upon substrate. Like, Kohno and colleagues [108] used laser ablation to create Co/Pt and Co/Mo alloy nanoparticles and studied ethanol-CVD growth of CNT onto the substrate. Laser ablation in an increased temperature flow reactor (laser oven method) is extensively employed for producing pure SWCNTs in a separate process [109]. Considering continuous flow-type generator, the laser oven technology has an empathy for creating SWCNTs with reduced degrees of imperfections as compared to typical CVD development on substrates.

Fig. 7
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TEM images of nanocarbons obtained by laser ablation: a carbon structure like onion, b carbon in nanotube forms, c nanoparticles like diamond structure [14]

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An easy and consistent single-step construction employing laser oven approach created a SWCNT aerosol as an example of nanocarbon manufacturing in flow-type generator [110]. A target of graphite involving Co and Ni was irradiated in a tubular furnace using a pulsed laser at temperatures varying from 25 to 1080 °C. Manufactured SWCNTs with a diameter of less than 2 nm were supported by atmospheric pressure nitrogen gas for an online examination of its size dispersion via a scanning mobility particle sizer (SMPS). Different varieties of allotropes of carbon were fabricates using progressive laser irradiation approaches. Inoue and team [111] fabricated onion-like carbon nanoparticles employing a target of graphite having a vacancy that restricted the produced particles; consequently, they could be irradiated via several laser beams. The shift of phase via amorphous carbon nanoparticles to the systematic concentric graphitic shell structure was detected due to intermittent irradiation of a laser beam of gas-suspended carbon particles into cavity. Seto and colleagues [112] have proved that existing approach is competent of producing wide range of novel nanostructured carbons, comprising diamond-like carbon. The authors looked at how density of carbon nanoparticles changed throughout laser-induced phase shift.

This section demonstrates an extensive review with the relevant evidence of the presented literature of the fabrication of various nanoparticles utilizing LAL method.

Doping of Nanomaterials via LAL

In a study, ZnO/Ag nanoparticles were effectively produced utilizing pulsed laser ablation in liquid with different Ag doped at 1, 3 and 5 min intervals. ZnO/Ag nanoparticles’ structural, optical, particle size, and anti-bacterial characteristics were examined. As a consequence of excessive dopant duration, the band gap value decreases, and the peak wavelength shift to the red was verified by the photoluminescence 325 spectrometer. On examination of XRD spectra, the mean particle size of ZnO/Ag nanoparticles is 12.1585 nm. TEM images of ZnO nanoparticles reveal nanorods, Ag nanoparticles reveal spherical particles, and ZnO/Ag nanoparticles exhibit a distinct mix of nanorods and nanospheres with particle sizes of around 21 nm [113]. Krstulovi et al. effectively produced colloidal solutions of Al-doped ZnO nanoparticles in Milli-Q water using the LAL approach. The generated disc-like nanoparticles are 29 nm thick and range in diameter from 450 to 510 nm. It was discovered that size of colloidal particles is determined by the number of laser pulses used but not by the intensity of the laser pulses. It is explained by the fact that growth occurs in the target crater’s spatially limited volume. The crater’s border promotes particle diffusion growth because the density of Zn and O atoms is large, statistically boosting the likelihood of adsorption on the embryonic (nucleus) particle. The XRD study shows that 100% of the ZnO particles produced are crystalline. Different chemical states of Zn, Al and O atoms in produced nanoparticles were identified using XPS analysis. Zn atoms are mostly present as Zn2+ in the ZnO phase, with a possible presence of Zn-Al linked atoms. Al atoms are present in a partly metallic form, which is likely connected to the doped Al atoms in the ZnO phase. Furthermore, Al atoms in the (3+) oxidation state were discovered on the oxidized specimen surface [114]. In a study, the same authors have shown that the LAL approach might be utilized to easily and efficiently synthesize colloidal Al-doped ZnO nanoparticles. Doping happens throughout nanoparticle formation, and no further solvents or heat treatment is required. In comparison with the initial, goal value, the atom percent ratio of Al/Zn in synthesized nanoparticles is greater than 50%. The Al-doped ZnO nanoparticles are 2D discs with a crystalline core and a thin amorphous shell that display the typical ZnO peak at 330 nm in the absorbance spectrum and band gap energies of 3.17 and 3.28 eV at laser wavelengths of 532 and 1064 nm, correspondingly. The authors demonstrated that the 1064 nm laser wavelength achieves a greater production rate (nanoparticle density) than the 532 nm laser wavelength. It is proposed that using diverse LAL experimental settings might increase doping efficiency [115].

LAL-Synthesized Nanomaterials Application

This section presents the review of the different applications of LAL-fabricated nanoparticles. Nanofertilizers, semiconductor NPs for luminescence applications, and noble metal NPs for functionalized bio-applications are some of the applications discussed in the following section.

Figure 8 presents the different applications of LAL-fabricated nanoparticles discussed in this section.

Fig. 8
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Different applications of LAL-synthesized nanomaterials

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Semiconductor NPs for Luminescence Applications

LAL technique is extensively utilized for fabricating luminescent semiconductor NPs that attract more attention because of their wide range of significant applications [95, 116,117,118,119,120,121,122,123,124,125,126]. Due to the non-equilibrium thermodynamic characteristics of the method, i.e., momentary plasma technique having extreme temperatures, elevated pressures, superior chemical actions, and higher degree of thermal quenching, more dense deformities, unusual microstructures and equilibrium phases are frequently stated for laser ablation in liquid-produced nanoparticles [127]. Explicitly, the reason semiconductor nanoparticles, namely Si, ZnO, W2C as well as SiC, generated through this approach are stated to demonstrate exclusive photoluminescence characteristics which are diverse from those detected from NPs generated through traditional techniques [46, 118, 125,126,127,128,129].

Instead, LAL can be employed for fabricating doped as well as complex oxide NPs because of solid irradiating capability of beam of laser. Ledoux and team generated luminescent NPs of Y2O3:Eu, Gd2SiO5:Ce, Lu2O2S:Eu, Lu3TaO7:Tb, Lu3TaO7:Gd, Y2SiO5:Ce, Gd0.4Y2.6Al5O12:Ce and GdTaO4:Tb employing LAL [126]. The inferences established that process of LAL could be utilized as a rapid process for preparing complex materials of nanosize having luminescent characteristics in four diverse material groups: sesquioxides, oxysulfides, tantalates and silicates. Distribution of products’ size was monodispersed and categorized through average dimensions of approximately 7 nm [126, 129]. The end point is of specific notice because nanoparticles in this size limit are adequately large for upholding the luminescent characteristics of the bulk material, but smaller enough to be utilized in anticounterfeit marking or biolabeling [130].

Nanofertilizers

The employment of nanomaterials into the field of plant sciences is comparatively new and also rapidly developing, having probability that smaller particles may upsurge the crop production for accomplishing the forthcoming requirements of increasing populations of Asian state, over and above deliver more fresh harvest for different areas. Materials of nanoscale were testified to kindle the growth of oxygen progression volume of plants that is eye-catching for a healthier biosphere. Singh and team [131] verified PLA-fabricated NPs of anatase TiO2 as nanofertilizers to stimulate seed germination and development of plant Brassica oleracea. Additionally, it is perceived; several concentrations of NPs of the colloidal TiO2 stimulate germination of seed and also development of Brassica oleracea plant [131]. Numerous physical characteristics of plants (namely radical length of germinated seeds, area and height of leaf, number of leaves, etc.), besides the quantity of chlorophyll a, carotenoid and chlorophyll b, were considered to surge along with increment in NP concentration, achieve highest values around 1.5 mM and further start declining at high (> 2.0 mM) concentrations [131].

It is established that lesser nanoparticle quantities are advantageous for the germination, growth, numerous biochemical constraints and complete Brassica oleracea plant health [131]. Extraordinary colloidal strength of laser-fabricated NPs of anatase TiO2 enabled its dispersity as well as accessibility in plant evolution media for extended period [131].

Noble Metal NPs for Functionalized Bio-applications

Bioconjugated nanoparticles effectively fascinated much consideration as an appropriate and significant investigative tool regarding biological as well as medical employment. If some surfactants, biomolecules, or polymers are put into liquid media, the in situ conjugation upon nanoparticle surfaces could be recognized [122, 132,133,134]. In situ functionalization throughout LAL development of NPs is primarily predicted to increase steadiness of gained colloids [135], whereas presently it targets the single-step formulation of bioconjugated NPs [132]. Such method is extensively embraced owing to fabrication of Ag, Cu and Au nanoparticles in situ functionalized along amphiphilic copolymers [132, 136] and also organic monolayer-covered nanocrystals of silicon [137]. Salmaso and colleagues [138] used a thermoresponsive thiol-terminated poly-N-isopropylacrylamide-co-acrylamide copolymer to arrange Au nanoparticles along the poly-N-isopropylacrylamide-co-acrylamide copolymer. NPs of polymer decorated Au may be positioned within human breast adenocarcinoma MCF7 cells treated at around 40 °C (12,000 Au NP per cell) having uptake of more than 80 times that of cells treated at 34 °C along identical particles, according to cell culture study (140 Au NP per cell). Walter also employed solid-phase aptamer-conjugated Au NPs produced by LAL [139].

Some Other Applications

Along with the aforementioned applications, numerous LAL-fabricated nanostructures could moreover be employed for catalysis as per several materials for energy, in photothermal treatment areas, etc. With the benefit of the non-equilibrium characteristic of LAL method, Liu formulated TiO2 NPs having rutile phase that is considered as utmost thermodynamically equilibrium state, although is challenging to achieve through traditional lower temperature methodologies [140]. Experimentations encompassing the methylene blue decay that validated the formation of LAL-generated rutile TiO2 nanoparticles have potentially high photocatalytic action [140]. Exceptional photocatalytic characteristics were additionally unveiled through LAL-manufactured (La, Sr)CoO3 nanoparticles accumulated upon arrays of ZnO nanorod [141]. Zeng and team devised a selective etching technique for the conversion of typical LAL-fabricated core/shell nanoparticles to hollow oxide NPs as well as noble metal–oxide composite nanoparticles, namely Au/ZnO, Au/Pt/ZnO and Pt/ZnO NPs [141]. Such nanostructures, primarily synthesized, employing LAL scheme further demonstrated extraordinary photocatalytic activity because of its higher surface-to-volume ratio and metal/semiconductor boundaries [141]. Preparation of silicon NPs employing LAL indicated potential regarding applications for environment cleaning with the lessening of ions of heavy metal, namely Cr(VI) [142]. Niu and group established a single-step LAL-based methodology for producing numerous hollow nanoparticles of sulfides and oxides of metal, where ZnS hollow NPs exhibited greater functioning in gas sensing as well as robust ferromagnetic reaction when compared with ZnS nanocrystals [47, 50]. Currently, the similar group deaggregated detonated nanodiamonds employing liquid selective laser heating which further observed that magnetic characteristics of detonated nanodiamonds altered considerably subsequent to deaggregation [143]. Furthermore, nanodiamonds that are uniformly dispersed are enclosed with organic ligands that confirmed emission of visible light [140]. Owing to inert characteristics of nanodiamonds, such luminescent nanomaterials were anticipated to have suitable utilization in biosensing and bioimaging [143].

Future Scope and Challenges

Currently, liquid laser ablation methodology is employed in fabrication of NPs of numerous materials. To this point, it is established that preferred products are effectively achieved via easy process of LAL. Though, to synthesize NPs, conventional techniques, namely chemical synthesis, are yet the customary process, it appears that such approaches would not be substituted with LAL momentarily. Henceforth, continuous effort is necessary for the further development of the employability of LAL as a synthesis technique for NPs. Primarily, it must be considered the point that LAL is not suitable regarding mass production. The cost of NPs generation utilizing laser is usually great when compared with traditional heating arrangements. Due to this drawback, LAL scheme is suitable for research studies where numerous nanoparticle characteristics are examined with minimal time employing less sample quantity. Considering this, the easiness of the technique and the adaptability for different materials of LAL are considerably beneficial. Also, the typical reaction during LAL process like the high levels of pressure and temperature generated by plasma must be consumed.

Many studies reported till now in the literature typically concentrate upon the fabrication of NPs and classification of their characteristics as nanomaterials. Though there exists a wide gap between studies including just the NPs’ synthesis and its relevant usage, NPs produced via laser irradiation demonstrate quick, simple and eco-friendly methodology when compared with different approaches of NPs fabrication like colloidal chemistry. In recent times, concentrated efforts are being carried out by researchers around the globe to examine the usability of typically metallic NPs in biomedicine, microscale engineering, tribological and hydrofluidic applications.

Conclusions

In the present article, review is done on existing development in the area of laser ablation and formulation of nanostructure in liquid medium. As per outcomes described in this area, the notable benefits of LAL and subsequent nanomaterials could be concisely recapped as follows. LAL can be considered a simplistic, sterile and speedy fabrication of nanostructures because of the less creation of by-products during the procedure, simpler precursors and nonexistence of catalyst. It includes a large usefulness concerning varied nanoproducts, counting metals as well as its oxides, hydroxides and sulfides. This is built upon the resilient capability of laser to ablate numerous targets. It can also be considered as a technique having ostensibly ambient temperature as well as pressure environments for obtaining metastable and new-phase nanostructures which might not stand achievable through different approaches, particularly under like circumstances. The major drawback of laser ablation in liquid could be considered is its lower efficiency; nevertheless, along the optimization of parameters of ablation as well as liquid medium, gram-scale fabrication could be accomplished. Laser irradiation is regarded as a green and effortless practice for the generation of the nanoparticles of metal devoid of any surfactant or chemical inclusion; also characteristics of NPs are exceptional. Results obtained propose that current methodology signifies an eye-catching subject, and an essential methodology for the synthesis of functional nanostructures. Nevertheless, most of the unmapped concerns concerning this area are apparent, alongside the core analysis. Once bearing in mind the advance research areas, without uncertainty, readings narrowly uniting the traits of the LAL along the application requirements, numerous functional materials are essential as well as would significantly progress the growth of this area.