Conventional LIBS uses a strong laser to break down a sample into plasma. However, the detection sensitivity is poor. A double-pulse LIBS (DP-LIBS) method has been proposed to improve the sensitivity of LIBS. There are several possible mechanisms could explain the signal enhancement of DP-LIBS, such as pulse-plasma coupling effects, sample heating effects, and atmospheric effects [37, 38]. At the same time, the plasma temperature and electron density were increased due to the re-ablation of second laser in DP-LIBS [39]. Accordingly, the spectral intensity and the detection sensitivity were increased. Spectral intensity and LOD improvements achieved with DP-LIBS in recent years are shown in Table 1. Because the plasma lifetime is often <10 μs, which is far less than the minimum interval between two laser pulses at present, a double pulse system must use two lasers, which greatly increases the cost of the LIBS system.

In the DP-LIBS configuration, the physical positions and angles of the two lasers greatly affect the spectral intensity. The two laser beams can be combined in collinear, orthogonal, and crossover modes. The orthogonal mode includes orthogonal preheating and orthogonal reheating [46]. Figure 1 shows a schematic diagram of several laser positions. In addition to the physical location of the laser, the laser wavelength also affects the signal enhancement. For example, Ahmed et al. [47] studied the enhancement effect of iron in orthogonal measurements using first (λ₁) and second (λ₂) wavelengths of λ₁ = λ₂ = 532, λ₁ = λ₂ = 1,064, and λ₁ = 532 and λ₂ = 1,064 nm. The enhancement factors obtained using different wavelength combinations varied and the maximum enhancement factor was 30 times.

With the development of solid-state lasers, two fast pulses can be output by one laser, which greatly reduces the cost and complexity of DP-LIBS. For instance, some researchers have obtained a 3-fold signal enhancement with collinear dual-beam detection of lunar mimics [48]. The drawback of this method is that the two laser pulses can only work in a collinear mode and the focus position has to be consistent. Therefore, better signal enhancement can be obtained by using two lasers than one in double pulse system. For the above three modes, secondary heating of the plasma or ablation of the sample was completed using a different laser. The inter-pulse delay of the two lasers was mainly realized by a timing controller. However, Liu et al. [49] used the method for reheating plasma and adjusting the inter-pulse delay by adjusting the optical path difference. The laser output from the ordinary laser was split into two beams, and one of them was used to complete the first breakdown on the sample surface. The second beam was then focused on the sample to achieve a second breakdown after the optical path was increased for the second beam. In this way, a DP-LIBS measurement was achieved with one laser. An enhancement factors of 2–32 times was obtained for the ionic and atomic lines as compared to the single pulse LIBS.

Besides, a method of long-short DP-LIBS was proposed to improve the sensitivity of LIBS. Compared with traditional DP-LIBS, a laser with a wider pulse (μs) was used as second laser in long-short DP-LIBS. The Deguchi's team have been done deep research about long-short DP-LIBS. For example, they proposed the collinear long and short DP-LIBS to detect the solid sample. The results showed that the plasma became more stable and sustained when using a long pulse-width laser (pulse width of 60 μs) as external energy, and marked enhancement were obtained compared to single-pulse LIBS [50]. Further, the spectral signal was enhanced about 3–7 times by long-short DP-LIBS comparing to conventional SP-LIBS when detecting the standard steel samples [51]. In the subsequent study, they used long-short DP-LIBS to detect the Mn content in steel. The results showed that the R² of calibration curve was improved from 0.810 to 0.988 using long-short DP-LIBS compared to SP-LIBS. In addition, the RSD was reduced from 29.3% (SP-LIBS) to 10.5% (long-short DP-LIBS) [52].

Furthermore, an interesting method named resonant DP-LIBS has been proposed to achieve spectral enhancement. In this case, the second laser is replaced by an optical parametric oscillator wavelength-tunable laser to reheat the plasma. When the laser wavelength is equal to an excited line of a target element, the atoms of the target element absorb laser photons and then emit strong fluorescence [53]. The spectral intensity achieved using resonant DP-LIBS is approximately 1–2 orders of magnitude higher than that with signal pulse LIBS. In one study, the resonant DP-method was used to analyze aluminum alloy and decrease the LODs of Mg and Si to 0.7 and 50 fg, respectively [54]. This method has also been used to detect TiO₂ aerosols, achieving a signal enhancement of 220 times [55].

When the laser ablates the sample, the surrounding gas is ablated simultaneously to produce a plasma plume. The composition of the gas greatly affect formation and diffusion of this plasma plume. Thus, the LIBS signal can be enhanced and the detection sensitivity increased by changing the atmosphere in which the sample is placed. The enhancement method under ambient atmosphere is to use an inert gas (e.g., N₂, Ar, and He). An inert gas has a lower conductivity and specific heat than air, so its breakdown threshold is low. For example, Kim et al. [56] injected argon (25 L/min) into a sample chamber to increase the signal intensities of the main elements by 2–3 times when detecting Cu(In,Ga)Se₂ samples. Son et al. [57] researched the signal improvement of Al under Ar gas environment of 0.5 MPa. The results showed that the spectral intensity of Al I increased by 6 times, and the maximum spectral intensity of Al II increased by more than 12 times. Lin et al. [58] studied the enhancement effect of different atmospheres (1–80 kPa pressure, He and Ar) using femtosecond laser to detect Al. The experimental results showed that the intensity of Al I was enhanced by 4 times in Ar gas, which was higher than in air and He [59]. In addition, Jiang et al. [60] investigated the signals of steel samples at different pressures (vacuum: 10–6 mbar; N₂: 1.0 mbar and 0.3 mbar; Ar: 2.1 and 1.5 mbar; He: 25 and 28 mbar). The LODs of C in N₂, Ar, He, and vacuum were 2.9, 3.6, 5.7, and 13.6 ppm, respectively. And the LODs of S were 1.5 ppm (N₂), 2.4 ppm (Ar), 3.4 ppm (He), and 8.9 ppm (vacuum), respectively. However, because the gas protection enhancement method often requires a closed air chamber based on the traditional LIBS structure, as shown in Figure 2, this method increases the complexity and cost of the system hardware.

In addition to modification of the LIBS hardware and control of the ambient atmosphere, external field assistance can also be used as a physical enhancement method to improve the sensitivity of LIBS detection. Spatial constraint is one external field assistance method. A spatial constraint device can be added on the periphery of the sample to achieve spectral enhancement. Then, a shockwave can be generated after the generation and expansion of the laser-induced plasma in air. The shockwave will be reflected back when it encounters obstacles such as a plate wall or a cylindrical wall and will compress the plasma plume during expansion [61]. Therefore, a higher plasma temperature and electron density, and a stronger spectral signal will be obtained. Figure 3 shows a morphological comparison of plasma with and without constraint devices. It can be seen from the Figure 3 that the shockwave is reflected back to the center of the plasma under the effect of spatial constraint device. On the contrary, when there is no spatial constraint device, the shockwave expands around.

Spatial constraint is a very simple method to improve the sensitivity of LIBS and requires no or little modification of the LIBS system. Common spatial constraint devices forms are the parallel plate, hemispherical, cylindrical, and rectangular cavity. Currently, the hemispherical form is considered to provide the best enhancement effect. A hemispherical device with a circular hole on the top as a channel for laser pulse focusing could be designed to allow for reflection and confinement of the expansion shockwave on the inner surface of device, which would achieve signal enhancement. The main parameters that affect a spatial constraint device are the hemisphere diameter, hole diameter, and thickness of the hemisphere wall. The enhancement effects of hemispheres with diameters of 4, 5, 6, and 7 mm have been investigated. When the hemisphere diameter was 5 mm, the correlation coefficient (R²) of V(I)/Fe(I) improved from 0.946 to 0.981. Similarly, the R² of Cr(I)/Fe(I) and Mn(I)/Fe(I) increased from 0.973 to 0.986 and from 0.945 to 0.981, respectively [61].

Traditional spatial constraint can result in formation of laser-induced craters. When multiple pulses hit a sample, the LIBS signal generated by later pulses will be enhanced because of the formation of craters by the initial pluses [62]. Spectral enhancement occurs because the laser pulse crater has a binding effect on the plasma [63].

Magnetic constraint is another common external field assistance method for improving LIBS sensitivity. Application of the magnetic constraint method is very similar to that of spatial constraint. This method replaces the spatial constraint device around the sample with a magnetic field device. Under the magnetic field, charged particles inside the plasma that are excited on the surface of the sample will move in a circular or spiral motion because of the Lorentz force [64]. This may slow down expansion of the plasma plume, increase the emission duration, improve the probability of electron-ion recombination, and result in enhancement of the emission line intensities [65].

One of the simplest applications of magnetic confinement uses a magnetic ring as a magnetic field to constrain the plasma. Hao et al. [64] designed a magnetic ring to investigate the effect of a magnetic field. The outer diameter, inner diameter, and height of the magnetic ring were 12.7, 3.2, and 6.3 mm, respectively. LODs of 11 ppm for V and 30 ppm for Mn were obtained with the magnetic ring, and these were lower than those obtained with a degaussed magnet (18 ppm for V and 41 ppm for Mn) [64]. Dong et al. [66] used a combination of metal nanoparticlea, super-hydrophobic, and magnetic confiment to detect Cu in solution. This method gave a signal intensity enhancement of approximately 5–6 orders of magnitude. Although a magnetic ring is simple and easy to apply as an external magnetic field device, the magnetic field of a magnetic ring cannot be adjusted. Therefore, Cheng et al. [67] used a pair of magnetic poles to form a magnetic field to constrain plasma. In their experiments, a magnetic field of 0.67 T was applied in analysis of a copper sheet. The detection results showed that the signal intensity of Cu(I) was enhanced 3–4 times, and those of Cu(II) and Cu(III) were enhanced 6–8 times. Although the enhancement effect of magnetic field is very good in laboratory, it is inconvenient to the filed for in-situ detection.

Chemical enhancement methods can also be used to improve the LIBS detection sensitivity. Sample pre-treatment is the main method used to achieve chemical enhancement, especially nanoparticle surface enhancement. The enhancement mechanism of nanoparticle-enhanced LIBS differs for conductors and insulators [68]. Figure 4 shows the ablation process of nanoparticles on conductors and insulators. It can be seen from the figure that, for conductors, when NPs are deposited on the surfaces of metals and other conductors, surface plasmons of the nanoparticle formed by the laser electromagnetic field couple with the electric field to produce a field enhancement. As a result of the enhanced electromagnetic field, the inward acceleration of free electrons in the direction of the laser pulse propagation results in faster ablation of the material, which results in enhancement [69]. As for insulators, enhancement mainly occurs because of the resonance between nanoparticles and local surface plasma from the sample. Strong local heating is generated on the surface of the sample, which stimulates the surface of the sample to produce more plasma and enhance the spectral signal [70].

Nanoparticle-enhanced LIBS is mainly affected by the type, radius, and concentration of the nanoparticles. The enhancement effects of nanoparticles of different types (Au, Ag, Cu, and Pt), sizes (e.g., 20 and 10 nm Ag), and concentrations (e.g., 0.02 and 0.01 mg.mL⁻¹ 10 nm Ag) have been investigated. Enhancements of 1–2 orders of magnitude were obtained for metal samples [69, 70]. Furthermore, the enhancement effects of Au nanoparticles for PbCl₂, Pb (NO₃)₂, and AgNO₃ have been investigated. The results showed that the LODs of Pb and Ag were 2 pg and 0.2 pg, respectively [71, 72]. Poggialini et al. [73] used Greensynthetized silver nanoparticles to enhance the spectral intensities of Zn and Cr in copper and obtained LODs for Zn and Cr of 6 ppm and 0.1 ppm, respectively. Wen et al. [74] used LIBS combined with Au nanoparticles to detect Cu²⁺, Pb²⁺, and Cr³⁺ in the solution, and the LODs of Cr, Pb, and Cu in the solution were 0.5, 0.5, and 1.1 g.ml⁻¹, respectively. In addition, Wu et al. [75] achieved a LOD of 13 ppb for K by combining amphiphiles with nanoparticles for adsorption and enrichment of K in KCl solution. The application of nanoparticles for molecular bands detection was studied by Koral et al. [76] An order of magnitude enhancement of AlO signal was achieved by 20 nm Ag nanoparticles when detecting aluminum alloy. Metal nanoparticles are simple to use and can greatly improve the detection sensitivity; however, metal nanoparticles will contaminate samples.

It is well-known that LIBS of liquid samples suffers from problems such as spattering, quenching, and short plasma lifetimes. Therefore, some researchers have proposed chemical replacement methods for pretreatment of liquid samples to improve the sensitivity of LIBS. The replacement can be implemented by methods such as liquid–liquid extraction, liquid–solid transformation, and surface-enhanced LIBS.

Liquid–liquid extraction can improve the LIBS sensitivity for liquid samples. Some special elements can be extracted and enriched by adding special chemical reagents to the solution before detection of LIBS. In the study of Aguirre et al. [77] dispersive liquid–liquid microextraction has been used to detect Cr, Cu, and Mn in water. The signal intensity obtained with this method was 4–5 times that of traditional LIBS and the LODs of the elements decreased by 3.7–5.6 times. In their subsequent research, they proposed a method of single-drop microextraction, which improved the detection sensitivity by 2.0–2.6 times, and the detection limit of Cr, Mn, Ni, Cu, and Zn elements in the solution was in range of 21–301 mg/kg [78]. However, liquid–liquid extraction suffers from some problems as the extraction process can be cumbersome and use dangerous chemical reagents.

To improve liquid–liquid extraction shortcomings, researchers have proposed the liquid-solid conversion method. The method was realized by using solid-phase media to enrich the elements of object in liquid samples, then the solid-phase media was detected by LIBS. The sensitivity was improved by the effect of enrichment. For example, nanographite and electrospun ultrafine fibers have been used to adsorb heavy metal ions in solution and increase the LODs [58, 79]. However, the liquid–solid conversion is a physical adsorption process and the adsorption efficiency is limited by the adsorption medium. Therefore, some researchers have proposed using an electrochemical reaction method. In one study, two aluminum sheets were used as an electrode to displace Cu in solution and achieve a LOD of 500 part per trillion [80]. Although the electrochemical conversion improved the displacement efficiency, the requirement for an applied electric field increased the complexity of the LIBS structure.

Researchers discovered that it was possible to improve the LIBS sensitivity by dropping a small amount of liquid sample onto the surface of a selected solid substrate, drying it, and then exciting the dried sample by LIBS. The method was defined surface-enhanced LIBS (SENLIBS) [32]. For example, the SELIBS method had been to reduce the LODs of Mn and K to 6 and 0.53 μg.g⁻¹, respectively [81, 82]. In addition, Yang et al. [32] have used the SENLIBS method to simultaneously detect La, Ce, Pr, and Nd in solution. Compared with the filter paper method, the LODs of these elements increased by 2–4 times. They also detected some metallic elements in the solution using chemical replacement combined with SENLIBS, and the minimum detection limits were 0.250 g.ml⁻¹ (Cu), 0.118 g.ml⁻¹ (Pb), 0.420 g.ml⁻¹ (Cd) and 0.025 g.ml⁻¹ (Cr), respectively [34].

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.