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Aug 20, 2016 - 1College of Chemistry, Sichuan University, Chengdu 610064, China. 2Research Center of Analytical Instrumentation, Key Laboratory of ...
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Emission enhancement of laser-induced breakdown spectroscopy for aqueous sample analysis based on Au nanoparticles and solid-phase substrate XU WEN,1 QINGYU LIN,2 GUANGHUI NIU,3 QI SHI,1

AND

YIXIANG DUAN2,*

1

College of Chemistry, Sichuan University, Chengdu 610064, China Research Center of Analytical Instrumentation, Key Laboratory of Bio-resource and Eco-environment, Ministry of Education, College of Life Science, Sichuan University, Chengdu 610065, China 3 School of Aeronautics & Astronautics, Sichuan University, Chengdu 610064, China *Corresponding author: [email protected] 2

Received 27 May 2016; revised 31 July 2016; accepted 31 July 2016; posted 2 August 2016 (Doc. ID 267155); published 18 August 2016

In this paper, porous electrospun ultrafine fiber with a nanoparticle coating was proposed as an effective approach to enhance the laser-induced breakdown spectroscopy (LIBS) signal for metal ions in aqueous systems. It is known that the LIBS technique is very limited when used for liquid sample analysis. On the other hand, in practical applications, many LIBS measurements have been accomplished in a liquid environment. A signal enhancement method for aqueous sample LIBS analysis was presented in this work, where Au nanoparticles and a solid-phase support were combined for the first time for aqueous sample analysis with LIBS. The system operation was relatively simple, which only required Au nanoparticles being dropped onto the surface of porous electrospun ultrafine fibers before LIBS analysis. Significant signal enhancement was achieved due to the integration of the merits of the Au nanoparticles and the ultrafine fibers. Nanoparticles possess significant LIBS signal enhancement effects by providing several plasma ignition points and then causing more efficient emissions. In addition, Au nanoparticles could also help to decrease the breakdown threshold of target elements for LIBS analysis. The electrospun ultrafine fibers as solid-phase support can accommodate a larger volume of aqueous sample. Meanwhile, the aqueous solution on the fiber surface was easy to evaporate. The experimental results showed that the limits of detection (LODs) with this method were significantly improved, 0.5 μg/mL for Cr, 0.5 μg/mL for Pb, and 1.1 μg/mL for Cu, respectively, compared with 2.0 μg/mL for Cr and 3.3 μg/mL for Cu in the previous research. In the proposed method, signal enhancement could be achieved without any extra equipment, which makes the LIBS technique feasible for direct measurement of an aqueous sample. © 2016 Optical Society of America OCIS codes: (140.3440) Laser-induced breakdown; (300.6365) Spectroscopy, laser induced breakdown. http://dx.doi.org/10.1364/AO.55.006706

1. INTRODUCTION Laser-induced breakdown spectroscopy (LIBS), an elemental analysis method, is based on the transient plasma produced by interaction between a high-power pulse laser and samples. Qualitative and semi-quantitative analysis can be achieved by analyzing elements in the generated plasma emission spectroscopy [1–3]. Owing to the advantages such as rapid analysis, including nearly nondestructive detection, fewer condition requirements, easy set up, etc., LIBS is becoming more and more popular in real-time and in situ analytical techniques. The LIBS technique can also be used to determinate elemental components in different matrices, including solid, liquid, gas, and aerosol [4–9]. 1559-128X/16/246706-07 Journal © 2016 Optical Society of America

LIBS is widely used in solid sample analysis, however, a large number of real samples to be analyzed are in aqueous form. Analysis of aqueous samples with the LIBS technique is very limited compared with analysis of solid samples due to the factors of liquid pressure, fluctuation, absorption, and sputtering [10,11]. These negative factors can shorten the lifetime of the laser-induced plasma, lower the LIBS signal intensity, and increase the noise level. All of these limitations restrict the promotion and application of the LIBS technique for aqueous samples. In the past decades, researchers have made many of efforts for liquid sample analysis with LIBS. These methods, such as freezing liquid [12], using droplets of liquid sample jets [13,14], or liquid sample jets [15–17] were to some extent

Research Article capable of improving the performance of LIBS analysis for liquid samples. Although improved sensitivities and limits of detection (LODs) were obtained with these methods, sample preparation procedures become more complex and their applications were thus limited. Therefore, method development to simplify the analytical process is still an urgent need. Porous electrospun ultrafine fibers were used as a solid-phase support to determine metal ions in aqueous samples, and satisfied results can be obtained for LIBS analysis according to our previous work [18]. Because of the microporous structure, the electrospun ultrafine fibers can hold a large volume of aqueous samples. Meanwhile, electrospun ultrafine fibers could give minimal blank background since there were no other heavy metals involved in the fiber preparation. The reported LIBS methods based on electrospun ultrafine fibers have showed their validity for aqueous sample analysis, with the improved LODs, for instance, of 2.0 and 3.3 ppm for Cr and Cu, respectively. However, compared with other conventional elemental analysis techniques, these results were still not good enough. There is still plenty of room to improve the LODs of LIBS. Sensitivity of LIBS has been a bottleneck for a long time and it depends on the elements to be analyzed. Typically, LODs of LIBS for aqueous samples are within the scope of a few tens to thousands ppm [19,20], which are uncompetitive compared with other spectrographic techniques. Researchers have developed various methods to better fit relevant requirements [21,22] and improve the detection sensitivity [23,24], such as double pulse LIBS [25,26], single-beam-splitting LIBS [22], resonance LIBS [27,28], and laser-induced fluorescence LIBS [29]. These methods could improve LIBS detection capacity in some degree. However, they required extra apparatus, which increased not only the complexity of the experimental instruments but also the analytical cost. Therefore, it is necessary to develop effective and convenient methods for LIBS analysis of aqueous samples. Presently, the enhancement effect of nanomaterials for spectral detection is being further evaluated. The nanoparticleassisting method is notable for Raman spectroscopy [30], but to the best of our knowledge, nanoparticles combined with a solid-phase support used for LIBS aqueous samples analysis has been not reported in the literature. Nanoparticle-assisted LIBS proved to be an effective method to improve LIBS detection sensitivity for solid-state samples. In this method, only a simple sample pretreatment is required, rather than other extra experimental devices. The operation is relatively simple while the enhancement effect is obvious. El Sherbini et al. found an obvious enhancement effect when a bulk-based ZnO target was replaced by a corresponding nanoparticle-based target [31]. De Giacomo et al. studied LIBS signal enhancement with nanosilver dropping on metal alloy materials, obtaining 1–2 orders of magnitude enhancement [32]. But in those above-mentioned methods, most of the samples used were metal-based. In those cases for signal enhancement, solid samples were supported by nanoparticles. However, the contacting surface between the solid samples and the nanoparticles was small because of the dense microstructure of the solid samples. As a consequence, the adhesive force between the nanoparticles and samples was relatively weak. With this in mind, we proposed that the signal effect attributed to the nanoparticles could be more

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effective if their interactions’ attraction force with the test samples were stronger. In view of our previous work and taking the advantages of electrospun ultrafine fibers and Au nanoparticles, a new method was proposed in this work for LIBS signal enhancement by employing porous electrospun ultrafine fibers to strengthen the adhesive force between the samples and the solid-state supporter. The ultrafine fibers could magnify the adhesive forces between the nanoparticles and the test samples due to the good adsorption capacity of the ultrafine fibers. The electrostatic attraction between the nanoparticles and fibers was expected to be stronger than it was between the nanoparticles and metal samples [18]. In this work, porous electrospun ultrafine polymer fibers with an Au nanoparticle coating were used to obtain LIBS signal enhancement. Taking the advantages of the large specific surface area and micro-porous structure of the ultrafine polymer fibers, deposition on the Au nanoparticles could form a stable nanoparticle film on the fiber surface. This was the first time that a solid-phase support system combined with the signal enhancement effect of nanoparticles was used in LIBS analysis of aqueous samples. With such a design of the structural framework, LIBS signal intensity enhancement was achieved, making it suitable for applications in aqueous sample analysis without the need of additional instruments. Obvious signal enhancement effects were observed by using this method for the LIBS analysis of Cr, Cu, and Pb in solution. The maximum signal intensity enhancement fold was 4.9. Enhancement factors of LODs for Cr and Cu were 4.0 and 3.0 times. 2. EXPERIMENTAL SETUP A. LIBS Instrumentation

The apparatus of our experiments was the same as the previous setup [18] and is shown in Fig. 1. The Nd:YAG laser (LOTIS) was used as excitation source. The maximum single pulse energy of the laser was 200 mJ at the fundamental wavelength of 1064 nm with 6 ns pulse duration and continuously adjustable frequency was 1–10 Hz. The laser pulse energy was set at 160 mJ, which was optimized through a set of experimental testing. Samples were fixed on a rotatable two-dimensional platform and sample positions were changed to ensure a fresh surface for each shot. The distance from the samples to the lens was set at 40 mm and a laser beam was vertically focused on the sample surface via a high-transmittance coating lens to obtain

Fig. 1. Schematic diagram of the experimental setup.

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hemispherical plasma. The laser spots formed on the sample surface were about 2.0 × 10−3 cm2 during our experiments. Laser irradiance was around 3.3 GW∕cm2 , which fully met the requirements of LIBS experiments. An Echelle Spectrometer (ARYELLE 150, LTB) was used to collect emission signals with the lateral plasma signal collection method. Optic probe fibers and sample excitation points were placed at a 50° direction to get maximum plasma signal. An optical chopper on the spectrometer entrance slit provided delay time for the LIBS system, which avoided the interference from a continuous spectrum of initial plasma radiation and obtained a better linear atomic spectrum. In order to obtain a better signal-to-noise ratio (SNR), one spectrum was acquired by accumulating the signal of 100 separate ablation events on different sites. B. Preparation of Au Nanoparticles

The Au nanoparticles were prepared according to the method reported in the literature [33]. The preparation of Au nanoparticles requires knowledge of chemistry. First, 2.5 mL HAuCl4 (5 mmol/L) and 3.25 mL ultrapure water were added into a reaction container. After heating to boiling, 1.75 mL sodium citrate [1% (wt/v)] was added and the mixture was refluxed for 30 min with stirring. The solution turned from colorless to pale blue and eventually turned into a wine-red color. The thus prepared Au nanoparticles were kept in brown bottles at 4°C for use. C. Preparation of Porous Electrospun Ultrafine Fibers

The preparation of porous electrospun ultrafine fibers has been reported in the literature [34]. Polyether sulfone (PES) and polyethylene glycol (PEG) were dissolved in dimethyl sulfoxide (DMSO) at 60°C until a homogeneous solution was obtained [34]. The porous electrospun ultrafine fibers obtained from the as-prepared polymer solution are in white color and contain a flat surface. D. Sample Preparation

Metal stock solutions were prepared by dissolving certain amounts of CuSO4 · 5H2 O, CrNO3 3 · 9H2 O, and PbNO3 2 (all analytical reagents, used without any further purification) in ultrapure water (18.25 MΩ · cm). Through stepwise dilution, a series of calibrated solutions of Cu2 , Pb2 , and Cr3 were obtained at the concentration range from 0.01 to 100 μg/mL. The test samples were prepared according to the process as follows. First, electrospun ultrafine fibers were tailored into 1.5 cm × 1.5 cm square pieces. Second, a 100 μL Au nanoparticle solution was dropped on the surface of the fibers. After the Au nanoparticles were fully deposited (2 min), 100 μL of the metal aqueous sample was dropped on the surface of fibers. Finally, the samples to be analyzed were homogeneous, and then test samples were dried by a hot air gun. The experimental process is shown in Fig. 2. In order to examine the enhancement effect of nanoparticles, comparisons were made for the LIBS signals obtained for the blank ultrafine fibers and those loaded with nanoparticles. The exact same treatments were used in the control group with ultrapure water loaded and the experimental group with Au nanoparticles loaded. The volume of ultrapure water

Fig. 2. The process for the test samples preparation (the ultrafine fibers loaded with Au nanoparticles and the sample were dried before analysis).

used in the blank control group was the same as that of Au nanoparticles solutions dropped on the fibers in the experimental group. The other experimental parameters were strictly controlled. 3. RESULTS AND DISCUSSION A. Material Characterization

The as-prepared Au nanoparticles were characterized by a UV-visible absorption spectrum. The maximum UV-visible absorption of the Au nanoparticles is shown in Fig. 3(a). The maximum UV-visible absorption peak at 521 nm was consistent with the previous reports [35,36]. The plasmon resonance of the Au nanoparticles, which was same with the UV-visible absorption peak, had no influence on the wavelength of the laser (1064 nm). According to the scanning electron microscope (SEM) image displayed in Fig. 3(b), the size of the as-prepared Au nanoparticles is about 35 nm. The as-prepared electrospun fibers were characterized by SEM as shown in Fig. 4(a). The previous study showed that electrospun ultrafine fibers can be used as a solid-phase supporting material for aqueous sample analysis. Due to the special

Fig. 3. (a) UV-visible absorption spectrum of the Au nanoparticles; (b) the SEM image of Au nanoparticles (the scale bar is 20 nm).

Research Article microporous structure, the electrospun ultrafine fibers could hold a larger aqueous sample and the aqueous sample was easy to evaporate [18]. Compared with the positively charged nanoparticles, the electrospun ultrafine fibers had a much larger dimension size (2.1 μm) and bear negative charge. Therefore the nanoparticles could disperse into the fibers, strongly bonding to the electrostatic interaction [34]. SEM characterizations of the porous electrospun ultrafine fibers and porous electrospun ultrafine fibers loaded with nanoparticles are shown in Fig. 4. As can be seen in Fig. 4(b), the Au nanoparticles are evenly distributed on the surface of the fibers, which makes it possible for a LIBS signal enhancement effect. Due to the surface enhancing effect of nanoparticles, the breakdown thresholds of the metal elements in aqueous samples were significantly reduced [37]. The reduced breakdown thresholds led to more efficient excitation of the particles when exposed to the lasers. Accordingly, a stronger signal intensity could be realized, and LIBS signal enhancement was achieved using this method. B. Optimization of LIBS Analysis

Based on the experimental scheme, all optic components were fixed on the optical platform, so that two important experimental parameters including delay time and excitation energy were optimized before sample testing. It is well known that the delay time for emission collection must be set to a proper value to avoid the continuous band spectrum and to acquire a highintensity plasma spectrum [38,39]. To obtain the optimal value, delay time was gradually changed within the range from 0 to 5 μs and the corresponding spectra were collected. The results showed that baseline was instable when delay time was less than 3.0 μs. The background signal intensity was too strong when the delay time was less than 2.5 μs. On the other hand, the signal intensity had a downward trend when the delay time was more than 3.5 μs. Therefore, considering the balance between the signal intensity and the stability of the baseline, 3.0 μs was selected as preferred delay time for subsequent experiments. Then, a series of laser energies were tested in order to find best optimal energy [39]. The highest laser energy was 200 mJ. It was observed that laser energy lower than 60 mJ was insufficient to excite the plasma; as a result the energy was relatively weak. Once laser energy exceeded 180 mJ, obvious damage of the porous ultrafine fibers was observed. Furthermore, the optical platform was ablated at the same shot, which interfered

Fig. 4. SEM images of porous electrospun ultrafine fibers loaded without (a) and with (b) Au nanoparticles (marked in red circle). The average diameter of the ultrafine fibers is 1 μm. The scale bars were 5 μm (a) and 1 μm (b), respectively.

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with the analytical lines. Therefore, the final excitation energy was chosen at 160 mJ, at which the emission signal intensity was strong and maximum signal-to-noise ratio (SNR) was reached. C. Signal Enhancement of Au Nanoparticles From an Enhanced LIBS Method

The LIBS signal intensity enhancement effects of Au nanoparticles were studied with the optimal conditions. Experimental results showed that the intensity of the two atomic emission lines of Cu (I) 324.74 and Cu (I) 327.38 nm were significantly increased for aqueous sample analysis with the LIBS technique based on electrospun ultrafine fibers solid-phase support. Cr (I) 425.43, Cr (I) 427.48, and Cr (I) 428.97 nm were chosen as analytical lines to study the enhancement effect of Cr with the nanoparticle-assisted LIBS method. The LIBS signal intensities enhancement fold of Cr was 3 when Au nanoparticles were loaded. The characteristic line of Pb (I) 405.7 nm was selected for Pb LIBS analysis. The intensity of the characteristic line was enhanced from 4270 to 19,697 after the addition of Au nanoparticles, which corresponds to a 4.6-fold enhancement of the signal intensity as shown in Fig. 5. The loaded Au nanoparticles can be regarded as extremely efficient electron contributors [32]. Therefore, the breakdown thresholds of metal ions in aqueous samples were greatly reduced when deposited on nanoparticles. In other words, Au nanoparticles were ablated before the metal ions when laser focused on the sample surface. Au nanoparticles’ excitation process could provide a large number of free electrons, which was equivalent to offering several plasma ignition points and thereby caused avalanche ionization. Thus, the efficiency of plasma generation was improved. In summary, nanoparticles loaded on the surface of the samples could provide more free electrons. As a result, Au nanoparticles played a role in increasing the local temperature of the contact area [32,40]. For all aqueous samples studied in this work, excitation thresholds were found to be significantly lowered after Au nanoparticles were loaded. The above results showed that the Au nanoparticle-assisted LIBS method for aqueous sample analysis had a significant signal enhancement effect compared with the traditional LIBS

Fig. 5. Emission spectra intensity of Pb (I) 405.70 nm and the signal enhanced by Au nanoparticles obtained by LIBS under the chosen analysis conditions.

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method. It was helpful to improve the sensitivity of the LIBS technique for aqueous sample analysis.

Table 1. LODs of Three Elements at Characteristic Emission Lines Element (nm)

Standard Curve Slopes (mL/μg)

3σ B

LODs (μg/mL)

Cr (I) 427.48 Pb (I) 405.70 Cu (I) 327.39

80.86 50.29 34.53

39.87 26.19 37.13

0.5 0.5 1.1

D. Quantitative Analysis and Calibration Curves

In the LIBS technique, calibration curves are widely used for quantitative analysis. Three atomic emission lines [Cr (I) 427.48 nm, Pb (I) 405.7 nm, and Cu (I) 324.75 nm] were selected for quantitative analysis. These lines were used to evaluate the performance for quantitative analysis of this proposed nanoparticle-assisted LIBS method. Figure 6 shows the calibrated curves of Cr, Pb, and Cu. According to the spectral line intensities at the selected analytical lines, the correlation coefficients (R 2 ) of the standard curves for these three elements were all more than 0.97. The standard deviations of the LIBS signal before Au nanoparticles were loaded were 12.96 and 15.88 for Cu(I) 327.39 nm and Cr(I) 427.48 nm, respectively, compared to 12.57 and 12.97 after the Au nanoparticles were loaded. The change in the standard deviations was negligible after the nanoparticles were loaded. These results well meet the requirements for LIBS quantitative analysis. The IUPAC definition of the limit of detection described by [41]: LOD  3

σB ; k

(1)

where σ B is the standard deviation of blank group results, and k is the slope of the calibrated curve. In this study, porous electrospun ultrafine fibers were soaked by ultrapure water. The test group had the same experimental conditions as the blank control group. The σ B value was determined with 10 measurements. The slopes of standard curve, 3σ B , and the final calculated LODs are listed in Table 1. Table 2 shows that the LODs of the three metal elements were significantly improved after Au nanoparticles were loaded on porous electrospun ultrafine fibers. As can be seen, the LODs obtained without nanoparticles in the previous study for the same emission lines were 2.0 μg/mL for Cr (I) 427.48 nm and 3.3 μg/mL for Cu (I) 327.39 nm [18]. However, the LODs were 0.5 μg/mL for Cr (I) and 1.1 μg/mL for Cu (I) after loading Au nanoparticles. The nanoparticle-enhancement

Table 2. Comparison of Au Nanoparticle Assisted LODs with the Previous Study LODs with Au Nanoparticles Analytical Line Enhancement μg · mL−1  (nm) Cr(I) 427.48 Pb(I) 405.70 Cu(I) 327.39

LODs Without Au Nanoparticles Enhancement Enhanced μg · mL−1  Factors

0.5 0.5 1.1

2.0 / 3.3

4.0 / 3.0

method obtained better LODs for aqueous sample analysis with LIBS. Therefore, the porous electrospun ultrafine fibers loaded with Au nanoparticles for aqueous sample analysis exhibited better detection performance. In other words, the nanoparticleenhancement LIBS method mentioned in this study is promising for aqueous sample analysis. E. Real Sample Analysis and Method Validation

The recoveries and reproducibility of the present work on three elements (Cu, Pb, and Cr) were evaluated in the method validation section. Environmental samples collected from a natural river (Ming-yuan Lake, Sichuan University) were obtained to verify the application of the present method. Different volumes of CuSO4 · 5H2 O, CrNO3 3 · 9H2 O, and PbNO3 2 (5, 10, and 15 mL, respectively) with a concentration of 100 μg · mL−1 were spiked into the natural water samples (10 mL) to study the recoveries (parallel samples  5). The concentrations of Cu2 , Pb3 , and Cr3 were 33.3, 50, and 60 μg · mL−1 , respectively. The data is listed in Table 3. The metal concentrations we detected by this LIBS method were close to the added Table 3. Concentrations and Recoveries of Cu, Pb, and Cr

Cu

Pb

Fig. 6. Calibration curves obtained for the analytical lines of Cr (I) 427.48, Pb (I) 405.70, and Cu (I) 327.39 nm. All the elements were performed at a 3.0 μs delay time using a 160 mJ Nd:YAG laser operating at 1064 nm and 6 ns pulse duration.

Cr

Added μg · mL−1 

Found μg · mL−1 

RSD (%)

Recovery (%)

0.0 33.3 50.0 60.0 0.0 33.3 50.0 60.0 0.0 33.3 50.0 60.0

– 32.2 47.4 65.7 – 31.3 51.0 63.1 – 32.0 46.8 63.9

– 5.5 9.4 7.8 – 8.5 7.9 11.1 – 8.7 9.5 4.9

– 96.7 94.8 109.5 – 94.0 102.0 105.2 – 97.0 93.6 106.5

Research Article concentrations. The recoveries of the three elements at different concentration levels of natural water samples were all in the range of 93.6%–109.5% with the relative standard deviation (RSD) of each group of parallel samples around 10.0%. These results were acceptable for LIBS analysis.

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4. CONCLUSION In this study, a new method was proposed for aqueous sample analysis with LIBS based on Au nanoparticles loaded on porous electrospun ultrafine fibers. With the proposed method, the emission signal intensities in LIBS analysis for aqueous samples can be significantly increased in comparison with the traditional LIBS methods. The mechanism of the nanoparticleenhancement effect for LIBS signals might be different depending on the type of metal elements. In most cases, the breakdown thresholds of metal elements in aqueous samples were significantly reduced because of the excellent surface enhancement effect of nanoparticles. The reduced breakdown thresholds were responsible for the more efficient optical excitation of the nanoparticles with the same laser energy, furthermore, resulting in enhanced emission signal intensities. The LODs enhancement factors for Cr and Cu were 4.0 and 3.0 folds, respectively. After Au nanoparticles were loaded on porous electrospun ultrafine fibers, the LODs were obviously improved. In such cases, the LODs of Cr, Pb, and Cu were 0.5 μg/mL, 0.5 μg/mL, and 1.1 μg/mL, respectively. Therefore, the use of Au nanoparticles for signal enhancement along with the use of porous ultrafine fibers as a solid-phase supporter could remarkably improve the signal intensities of the spectral lines in LIBS measurement. In summary, the LIBS emission enhancement method based on Au nanoparticles supported by ultrafine fibers was proved to be effective for aqueous sample LIBS analysis. Based on its good performance, the newly developed method in this paper is promising for practical applications in LIBS analysis. Funding. National Major Scientific Instruments and Equipment Development Special Funds (2011YQ030113); Sichuan University (SFSU). Acknowledgment. Thanks for the porous electrospun ultrafine fibers given by the Zhimei Wei group at Sichuan University.

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