A method of enhancing laser-induced breakdown spectroscopy signals

Abstract

The present application belongs to the technical field of atomic emission spectrum measurement, and particularly relates to a method for enhancing laser-induced breakdown spectroscopy signal. The present application unexpectedly finds that the introduction of chloride ions into the research work of NELIBS further improves the signal intensity and sensitivity of NELIBS. Moreover, compared with the existing NELIBS, the method of the present application enhances the signal intensity of NELIBS by more than 2 times; at the same time, compared with the optical path technical improvement, the method of the present application is low in cost, simple and convenient to operate, and is more suitable for analysis and detection in actual work.

Description

Technical Field

[0001] This invention belongs to the field of atomic emission spectroscopy measurement technology, specifically relating to a method for enhancing laser-induced breakdown spectral signals. Background Technology

[0002] Laser-induced breakdown spectroscopy (LIBS) is a novel technique for elemental detection and analysis that utilizes the plasma emission spectrum generated by the interaction of a high-energy pulsed laser with a material. It offers advantages such as fast response, no sample preparation required, simultaneous multi-element analysis, and remote analysis. This technique can perform elemental analysis on samples in almost all material forms, including rocks, glass, metals, bones, weapons, plants, biomaterials, and polymers, and can be conducted under normal pressure, in vacuum, in the deep ocean, and even in extremely harsh and dangerous environments (such as reactors). Based on this, domestic and international attention to LIBS is increasing, and LIBS is gradually expanding from laboratory research to various fields such as environmental monitoring, oil analysis, biological identification, space exploration, spent reactor fuel analysis, and fission product analysis. Although LIBS technology is relatively mature and has been widely and effectively applied in many fields, how to improve LIBS sensitivity and significantly extend its detection limit remains a hot topic of research for scientists.

[0003] Currently, experimental research on LIBS spectral enhancement mainly focuses on two aspects: improving optical path technology and sample preparation methods. Improving optical path technology primarily involves using methods such as dual-pulse LIBS, resonant LIBS, and microwave excitation to enhance the detection sensitivity of LIBS. Simultaneously, magnetic confinement and spatial confinement are also used to enhance the intensity of laser-induced plasma spectral signals, thereby improving LIBS detection sensitivity. Improving sample preparation methods generally involves adding nanomaterials to the sample to lower the LIBS breakdown threshold, thereby enhancing the spectral signal; this is also known as nanoparticle-enhanced LIBS (NELIBS). Because nanostructures exhibit strong surface plasmon resonance characteristics and enhance the electric field, the intensity of the NELIBS spectral signal can increase by several orders of magnitude.

[0004] Compared to improvements in optical path technology, NELIBS offers advantages not only in its more significant enhancement effect but also in its ability to achieve the desired enhancement without requiring complex modifications to existing LIBS equipment. Only sample preparation needs improvement, resulting in lower costs. For example, the Italian research group of A. De Giacomo has conducted a series of influential studies on NELIBS. These include adding nano-gold and silver sols to the surfaces of elemental materials (metals, semiconductors, and insulators) to lower the breakdown threshold of the sample material, resulting in a more than twofold increase in the NELIBS signal. Adding nano-gold sols to the sample solution further reduced the detection limit to below ppm. Xiaojiao Liu of Northwest University discovered that introducing metal chelators into nanoparticles enhanced the signal intensity of the target element, lowering the detection limit by 3-4 orders of magnitude compared to traditional LIBS methods. Furthermore, our research group further enhanced the LILIBS signal intensity by 4-5 orders of magnitude by adding amphiphilic molecules, achieving a detection limit of 10 ppb.

[0005] The present invention unexpectedly discovered that introducing chloride ions into NELIBS further enhances the LIBS signal intensity. Compared to the separate introduction of amphiphilic molecules, the method of the present invention enhances the NELIBS signal intensity by at least two times, thereby improving the sensitivity of NELIBS technology. Furthermore, compared to existing optical path technology improvements and sample preparation improvements, the method of the present invention has the advantages of low cost and simple and convenient operation, making it more suitable for analysis and detection in practical work. Summary of the Invention

[0006] To address the aforementioned technical problems, the present invention aims to provide a method for enhancing laser-induced breakdown spectral signals. Specifically, it includes the following:

[0007] In a first aspect, the present invention provides an application of halide ions in enhancing laser-induced breakdown spectral signals.

[0008] Preferably, the laser-induced breakdown spectrum is a nanoparticle-enhanced laser-induced breakdown spectrum.

[0009] Preferably, the halide ion is a chloride ion.

[0010] Secondly, the present invention provides a sample pretreatment solution for enhancing laser-induced breakdown spectroscopy detection, the sample pretreatment solution comprising halide ions and metal nanoparticles, wherein the molar concentration ratio of the halide ions to the metal nanoparticles is 0.02-20:1.

[0011] Preferably, the sample pretreatment solution further includes an amphiphilic molecule, wherein the molar ratio of the halide ion to the amphiphilic molecule is 0.002-2:1.

[0012] Preferably, the metal nanoparticles are silver sol.

[0013] Preferably, the concentration of the silver sol is 0.001M.

[0014] Preferably, the amphiphilic molecule is an anionic amphiphilic molecule.

[0015] Preferably, the anionic amphiphilic molecule is sodium dodecyl sulfate (SDS).

[0016] Preferably, the concentration of SDS is 0.01M.

[0017] Preferably, the concentration of the halide ions is 0.02-20 mM.

[0018] Preferably, the halide ion is a chloride ion.

[0019] Preferably, the concentration of chloride ions is 0.2 mM.

[0020] Thirdly, the present invention provides a sample preparation method for enhancing laser-induced breakdown spectral signals, wherein the method comprises adding the sample pretreatment solution described in the second aspect above to the sample to be tested.

[0021] Preferably, the sample to be tested is MgSO4.

[0022] Preferably, the concentration of the sample to be tested is 0.01-10 mM.

[0023] Fourthly, the present invention provides a method for improving the detection sensitivity of laser-induced breakdown spectroscopy, the method being:

[0024] (1) The sample to be tested is pretreated according to the sample preparation method described in the third aspect or the sample pretreatment solution described in the second aspect;

[0025] (2) The pretreated sample is directly subjected to laser-induced breakdown spectroscopy detection.

[0026] Preferably, the sample to be tested is MgSO4.

[0027] Preferably, the concentration of the sample to be tested is 0.01-10 mM.

[0028] The beneficial effects of this invention are: introducing chloride ions into NELIBS research further enhances the LIBS signal intensity. Compared to the introduction of amphiphilic molecules, the method described in this invention enhances the LIBS signal intensity by at least two times, improving the sensitivity of NELIBS technology; furthermore, compared to improvements in optical path technology, the method described in this invention has advantages such as low cost and simple and convenient operation, making it more suitable for analytical detection in practical work. Attached Figure Description

[0029] Figure 1 Optical path diagram of laser-induced breakdown spectroscopy detection experiment;

[0030] Figure 2 Morphological characteristics of silver nanoparticles;

[0031] Figure 3 The results of laser-induced breakdown spectroscopy detection of NELIBS signals in the samples after adding different halide ions to the nano-silver sol.

[0032] Figure 4 The results of laser-induced breakdown spectroscopy detection of NELIBS signal in the sample after adding chloride ions to the nano-silver sol;

[0033] Figure 5 Figure 1 shows the results of laser-induced breakdown spectroscopy (LAS) detection of NELIBS signals in samples after adding different concentrations of chloride ions to the nano-silver sol.

[0034] Figure 6 The results of laser-induced breakdown spectroscopy detection of NELIBS signals after adding chloride ions to samples of different concentrations are shown in the figure. Detailed Implementation

[0035] The above solution will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrating the present invention and are not intended to limit the scope of the invention.

[0036] In the following examples, only MgSO4 was used as the test sample for effect verification. However, the method described in this invention is not limited to the detection of MgSO4. Any other test sample that can be used for LIBS detection is within the protection scope of this invention.

[0037] Taking MgSO4 as an example, the sample preparation method described in the following examples is as follows:

[0038] A. Preparation of silver nanoparticles

[0039] Nano-silver sol was prepared according to the Lee-Meisel method: Sodium citrate or sodium borohydride solution was used as a reducing agent to reduce silver nitrate solution, producing silver nanoparticles. The silver sol produced by the former is grayish-green and is called gray silver sol, while the silver sol produced by the latter is yellowish-green and is called yellow silver sol. The specific preparation steps for gray silver sol are as follows:

[0040] (1) Setup of the apparatus: Place a 250mL round-bottom flask on an electromagnetic heating stirring sleeve, install a spherical condenser above the flask and pass cooling water through it so that the condensed water flows back when the solution boils, and place a magnetic stir bar in the flask.

[0041] (2) Solution preparation: Prepare 1% AgNO3 (aladdin, 99.8%) solution and 1% trisodium citrate (dihydrate) (Kelon Chemical, ≥99.0%) solution respectively;

[0042] (3) Reaction: Add 2 mL of 1% silver nitrate solution and 100 mL of distilled water to a round-bottom flask, heat to boiling, and then add 1% sodium citrate solution dropwise while stirring vigorously. While continuously heating and stirring, the reaction solution first turns light yellow, then gradually darkens, turns yellowish-brown in about 5 minutes, and stabilizes at a grayish-green color in about 10 minutes. Then continue heating and stirring for 20 minutes and then cool.

[0043] B. Sample Preparation

[0044] Prepare the following solutions: MgSO4 (Shanghai Guangnuo Technology, ≥99.0%) solutions with concentrations of 10mM, 2mM, 1mM, 0.5mM, 0.25mM, 0.1mM, 0.05mM, and 0.01mM; sodium dodecyl sulfate (SDS) (Aladdin, ≥99.0%) solution with concentrations of 0.01mol / L; KCl (Tianjin Damao, ≥99.5%) solutions with concentrations of 20mM, 4mM, 2mM, 1mM, 0.5mM, 0.2mM, 0.1mM, and 0.02mM; KF (Macklin, 99.5%) solution; KBr (Tianjin Guangfu Technology, ≥99.0%) solution; and KI (Tianjin Damao, ≥99.0%) solution.

[0045] After the silicon wafers (Zhejiang Lijing Optoelectronics, P-type silicon, 100 crystal orientation, single-sided polishing) are cut into 3cm×3cm sizes, they are ultrasonically cleaned with anhydrous ethanol and distilled water for 20 minutes each, and then dried before use.

[0046] The above-prepared MgSO4, silver nanoparticles, SDS and halide ion solution are mixed to prepare the test solution; the test solution is then evenly dropped onto a silicon wafer and allowed to air dry naturally.

[0047] The optical path diagram for the laser-induced breakdown spectroscopy detection experiment used in the following examples is shown below. Figure 1 As shown, an Nd:YAG laser (Powerlete 9010) generates a laser beam with a wavelength of 532 nm and a pulse duration of 7 ns, with a single pulse energy of 80 mJ. The laser beam is split into two beams of equal energy by a beam splitter (BS), and the energy of one beam is monitored using a power meter. The other laser beam passes through a pinhole mirror (HM) and is then focused onto the sample surface by a lens (L1) with a focal length of 100 mm, generating plasma. The sample is placed 5 mm in front of the lens focal point, at which point the laser energy density is approximately 40 J / cm². 2The sample was placed on a two-dimensional displacement platform and continuously moved during laser ablation to ablate a fresh sample surface. The emitted plasma light was collected by lens L1 and became quasi-parallel light. After reflection by a pinhole mirror (HM), it was focused onto an optical fiber (OF, inner diameter 50 μm) by lens L2 with a focal length of 60 mm, coupled to a spectrometer (Mechelle 5000), and recorded by an ICCD (AndoriStar DH-334T-18U-03). A pulse generator (DG 645) was used to synchronize the laser and spectrometer. Each measurement was performed in single-shot mode with a delay time of 400 ns to subtract the continuous spectral background caused by bremsstrahlung and recombination radiation in the early stages of plasma evolution. The integration gate width was 5 μs during spectral measurements.

[0048] After determining the sample preparation parameters and laser path modulation, the NELIBS signal of different samples was measured using an ICCD. By changing the incident laser energy (20–160 mJ) and repeating the experiment multiple times, the relationship between the incident laser power density, the NELIBS signal of the sample, and the aggregation state of the nanoparticles could be extracted.

[0049] Example 1 Sample Preparation

[0050] 1. Preparation and characterization of nano-silver sol

[0051] Following the method described in the literature (Lee PC, Melsel D. Adsorption and surface-enhanced Raman of dye on silver and gold sols. The Journal of Physical Chemistry, 1982, 86(17): 3391-3395), sodium citrate or sodium borohydride solution was used as a reducing agent to reduce silver nitrate solution and produce silver nanoparticles. The silver paste from the former was grayish-green and was called gray silver paste, while the silver paste from the latter was yellowish-green and was called yellow silver paste.

[0052] In this embodiment, using grey silver paste as an example, 2 mL, 5 mL, and 10 mL of sodium citrate solution were added respectively to obtain silver nanoparticles of different sizes. The morphological characteristics of the prepared silver nanoparticles are as follows: Figure 2As shown in the figures, the morphology of the nanoparticles obtained by adding sodium citrate solutions of different proportions (2 mL, 5 mL, and 10 mL) is obtained by transmission electron microscopy (TEM), as shown in a, c, and e, respectively, and the size distribution statistics are shown in b, d, and f, respectively. The results indicate that the diameter of the silver nanoparticles changes significantly with increasing sodium citrate volume, reaching 34.53 nm, 39.65 nm, and 59.01 nm, respectively, with the diameters having the highest distribution probability being 32.5 nm, 37.5 nm, and 54 nm, respectively.

[0053] 2. Preparation of sample pretreatment solution

[0054] Different concentrations (0.1 mM, 0.2 mM, 0.5 mM, 1 mM, 2 mM, 4 mM, 20 mM) of KCl solution and / or SDS solution (0.01 M) were added to the above-prepared silver nanoparticle sol (0.1 mg / mL) to prepare sample pretreatment solutions.

[0055] Example 2 Sample Detection

[0056] 1. Adding different halide ions to nano-silver sol

[0057] After adding the sample pretreatment solution prepared in Example 1 (silver paste (AgNPs, 0.1 mg / mL) solution, and 0.2 mM KF solution, KCl solution, KBr solution, KI solution, and / or SDS solution (0.01 M)) to the MgSO4 solution to be tested in 2 mM, the NELIBS signal was directly detected by laser-induced breakdown spectroscopy.

[0058] Absorption spectroscopy measurement results are as follows Figure 3 and Figure 4 As shown, compared to simple NELIBS detection (SDS+AgNPs), the addition of different halide ions (F... - Cl - ,Br - I - This significantly enhanced the NELIBS signal of Mg in the MgSO4 solution being tested, with the enhancement effects of F being in descending order. - >Cl - >Br - >I - .

[0059] 2. Effect of different concentrations of chloride ions on the NELIBS signal intensity of gray silver paste

[0060] The sample pretreatment solution (silver paste (AgNPs, 0.1 mg / mL) solution, and KCl solutions of different concentrations (0.02 mM, 0.1 mM, 0.2 mM, 0.5 mM, 1 mM, 2 mM, 4 mM, 20 mM), and / or SDS solution (0.01 M)) prepared in Example 1 were added to the MgSO4 solution of the test sample at a concentration of 2 mM, and the NELIBS signal was directly detected by laser-induced breakdown spectroscopy.

[0061] Experimental results are as follows Figure 5 As shown, the LIBS signal of the sample was strongest when the chloride ion concentration was 0.2 mM.

[0062] 3. Effect of adding chloride ions to test samples of different concentrations on NELIBS signal intensity

[0063] After adding the sample pretreatment solution (silver paste (AgNPs, 0.1 mg / mL) solution, 0.2 mM KCl solution, and / or SDS solution (0.01 M)) prepared in Example 1 to different concentrations of MgSO4 solution (10 mM, 2 mM, 1 mM, 0.5 mM, 0.25 mM, 0.1 mM, 0.05 mM, 0.01 mM) respectively, the NELIBS signal was directly detected by laser-induced breakdown spectroscopy.

[0064] The results are as follows Figure 6 As shown, the addition of chloride ions to MgSO4 solutions of different concentrations significantly enhanced the NELIBS signal of Mg, improving the measurement sensitivity. Furthermore, compared to the method without chloride ions (detection limit of 0.1 mM), the detection limit of MgSO4 (0.01 mM) was reduced by one order of magnitude. These results indicate that chloride ions have an optimizing effect on NELIBS signal enhancement.

[0065] In summary, the above embodiments, using silver paste as an example, investigated the effects of different halide ions on nanoparticle-enhanced LIBS. The results showed that adding different halide ions to the silver paste could enhance the signal intensity of NELIBS, and the NELIBS signal enhancement effect was as follows: F - >Cl - >Br - >I - However, the implementation of the technical solution of this invention is not limited to the aforementioned gray silver paste. Other nanoparticles suitable for LIBS are also applicable to the technical solution described in this invention and are all within the protection scope of this invention. Meanwhile, although F - The enhancement effect is most significant, but F - The preparation process is complex and costly. Therefore, considering both cost and effectiveness, this invention preferably uses Cl. - And with Cl -As the concentration increased, the NELIBS signal was enhanced, further indicating that Cl... - It can enhance the signal strength of NELIBS and improve the sensitivity of NELIBS. Although only MgSO4 was used as the test sample in the embodiment for effect verification, the method described in this invention is not limited to detecting MgSO4. Any other test sample that can be used for LIBS detection is within the protection scope of this invention.

Claims

1. A sample pretreatment solution for enhancing laser-induced breakdown spectroscopy detection, characterized in that, The sample pretreatment solution consists of halide ions, silver nanoparticles, and sodium dodecyl sulfate, with the halide ion concentration being 0.2 mM. Sodium citrate is used as a reducing agent to reduce silver nitrate solution to produce silver nanoparticles, with the silver nanoparticle concentration being 0.1 mg / mL and the sodium dodecyl sulfate concentration being 0.01 M.

2. The sample pretreatment solution as described in claim 1, characterized in that, The silver nanoparticles are silver sol.

3. The sample pretreatment solution as described in claim 2, characterized in that, The halide ion is a chloride ion.

4. A sample preparation method for enhancing laser-induced breakdown spectral signals, characterized in that, The method is as follows: add the sample pretreatment solution according to any one of claims 1-3 to the sample to be tested.

5. A method for improving the sensitivity of laser-induced breakdown spectroscopy detection, characterized in that, The method is as follows: (1) The sample preparation method according to claim 4, or the sample pretreatment solution according to any one of claims 1-3, is used to pretreat the sample to be tested; (2) The pretreated sample is directly subjected to laser-induced breakdown spectroscopy detection.

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