Article pubs.acs.org/ac
Direct Analysis of Gold Nanoparticles from Dried Droplets Using Substrate-Assisted Laser Desorption Single Particle-ICPMS Iva Benešová,†,‡ Kristýna Dlabková,† František Zelenák,† Tomás ̌ Vaculovič,†,‡ Viktor Kanický,†,‡ and Jan Preisler*,†,‡ †
Department of Chemistry, Faculty of Science, Masaryk University, Brno, 625 00 Czech Republic CEITEC - Central European Institute of Technology, Masaryk University, Brno, 625 00 Czech Republic
‡
S Supporting Information *
ABSTRACT: Single particle inductively coupled plasma mass spectrometry (SP-ICPMS) has been generally accepted as a powerful tool in the field of nanoanalysis. The method has usually been restricted to direct nanoparticle (NP) introduction using nebulization or microdroplet generation systems. In this work, AuNPs are introduced into ICPMS by substrate-assisted laser desorption (SALD) directly from a suitable absorbing plastic surface using a commercial ablation cell for the first time. In SALD, desorption of individual NPs is mediated using a frequency-quintupled Nd:YAG laser (213 nm) operated at a rather low laser fluence. Conditions including laser fluence, laser beam scan rate, and carrier gas flow rate were optimized in order to gain the highest AuNP transport efficiency and avoid AuNP disintegration within the laser irradiation. The method was demonstrated on a well-characterized reference material, 56 nm AuNPs with a transport efficiency of 61% and commercially available 86 nm AuNPs. Feasibility of our technique for NP detection and characterization is discussed here, and the results are compared with an established technique, nebulizer SP-ICPMS. owadays, nanomaterials are widely used in many fields including biomedicine, consumer goods, or energy production.1−4 The steep rise in their synthesis, development, and application in the past decade has been accompanied by increasing concerns about their safety, possible health and environmental impacts. Therefore, there is also a growing demand for methods for nanomaterial detection and characterization, even at very low environmentally relevant concentrations. Due to their unique physical and chemical properties, nanoparticles (NPs) have also found many applications in analytical chemistry. NPs can easily be conjugated with biomolecules, and thus, they can act as labels for signal amplification in biosensing and biorecognition assays. These strategies can significantly enhance detection sensitivity; even a single molecule can be detected in an ideal case.5,6 In terms of individual NP detection, single particle inductively coupled plasma mass spectrometry (SP-ICPMS) is considered a very promising analytical approach.7,8 This method, outlined in a series of papers by Degueldre et al.,9−13 employs the well-established, widespread, and highly sensitive method for elemental analysis, ICPMS, which is run in the time-resolved mode with short dwell times down to a few microseconds,14 but more typically in the range of a few milliseconds.7,15 If a NP suspension is sufficiently diluted and introduced into the ICP, each recorded pulse then represents an ion packet originated from a single NP. The signal intensity is then proportional to the mass of element per NP and the pulse frequency to the NP number concentration. SP-ICPMS can thus provide complex information about elemental
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composition of noncarbon-based nanomaterials and their number concentration. If the information about the shape, chemical composition, and density is known, the information about their average dimensions and size distribution can be further obtained. Despite being a rather novel methodology, which has been extensively studied and used for sizing and characterization of size distribution of various nanomaterials, SP-ICPMS has already found practical applications. For instance, it was used for NP detection and quantitation in environmental samples16,17 or TiO2 NP analyses in sunscreen samples;18 the applicability of SP-ICPMS in bioanalysis was also demonstrated in highly sensitive immunoassays with NP-tagged antibodies.19,20 The potential of SP-ICPMS has also been applied to study the stability of NPs; here, it can be beneficial over other methods as it allows detection and differentiation of both the dissolved and particulate forms.21,22 NP introduction into the ICP spectrometer is typically carried out using common pneumatic nebulizers.12,16,23,24 Many nebulizing systems are known to suffer from low transport efficiency and high sample consumption, and therefore, alternative methods for NP introduction have been presented.14,22,25 Franze et al. demonstrated enhanced transport efficiency as well as higher sensitivity due to the drier aerosol conditions when a micronebulizer with a heated cyclonic spray Received: June 29, 2015 Accepted: January 26, 2016
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DOI: 10.1021/acs.analchem.5b02421 Anal. Chem. XXXX, XXX, XXX−XXX
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plasma discharge. Samples were deposited on the plates within 2 weeks after the treatment. Sample Preparation. SALD ICPMS. All experiments were carried out with the reference material, 56 nm AuNPs unless noted otherwise. The suspension of AuNPs was diluted with water and 0.5 mM sodium citrate in the final dilution step to a number concentration of 0.5 × 109 − 10 × 109 AuNP·L−1. Addition of sodium citrate helped to visualize sample spots on PETG plates. Between each dilution step, the NP suspensions were sonicated for 30 s. Certified reference element standard of Au (1 g·L−1) was diluted with water to a concentration of 1 mg· L−1. Droplets (200 nL) of the AuNP suspensions or Au standard solution were spotted on PETG sample plates using a conventional micropipette. Droplets were allowed to dry at room temperature prior to SALD ICPMS analysis; the diameter of the sample spots was ∼1.2 mm. In AuNP analyses, 10 dried droplets with 500 AuNPs each (2.5 × 109 AuNP·L−1) were typically analyzed (total measurement time ∼15 min) in order to gather statistically representative data. Nebulizer SP-ICPMS. The suspension of AuNPs was diluted with water to a number concentration of 1.94 × 107 AuNP·L−1. A volume of 3.3 mL of this suspension was used for ICPMS analysis to deliver a similar number of NPs to plasma in a similar time as in the case of SALD ICPMS. ICPMS Measurements. SALD ICPMS. SALD was performed in a commercial ablation system (model UP 213, New Wave Research, Inc., Fremont, CA, U.S.A.) consisting of a Qswitched Nd:YAG laser operating at 213 nm, movable ablation cell (model SuperCell), and built-in microscope/CCD camera system for visual inspection of the samples. If not stated otherwise, a sample spot area was scanned with the highest possible repetition rate of 20 Hz along a serpentine-like raster with a row spacing of 80 μm and scan rate of 200 μm·s−1 by a laser beam focused to a spot with a diameter of 100 μm at laser fluence of 0.15 J·cm−2 in the focused beam mode. Laser fluence and scan rate as well as carrier gas flow rate were varied in order to gain the best possible performance in terms of intact particle desorption and the highest transport efficiency. Both helium and argon were tested as carrier gases. The tested range of the carrier gas was 0.6−1.2 L·min−1. Both the flush and laser warmup times were set to 4 s. The aerosol was transported into an ICP quadrupole mass spectrometer (model 7500 CE, Agilent Technologies, Inc., Santa Clara, CA, U.S.A.). A sample gas flow of argon (1 L·min−1) was admixed to the carrier gas subsequent to the ablation cell if not mentioned otherwise. The signal of 197 Au isotope was measured with an integration time of 10 ms, the lowest value offered by the instrument software. The settling time of the mass spectrometer was 0.2 ms. Nebulizer ICPMS. As a reference method for AuNP analysis, a Babington nebulizer with a Scott double-pass spray chamber was used (Agilent Technologies, Inc., Santa Clara, CA, U.S.A.). The conditions were derived from Laborda et al.:24 the carrier gas (argon) flow was set to 0.75 L·min−1; the makeup gas (argon) flow was 0.40 L·min−1. Sample uptake rate was 0.33 mL·min−1. The signal of 197Au isotope was measured with an integration time of 10 ms. Comparison of ICPMS with Nebulizer and SALD for AuNP Introduction. In an experiment for direct comparison of nebulizer and SALD, a T-piece (see below for detailed scheme and information) was employed to allow mixing of both aerosols produced in the ablation cell and by nebulization before introduction into the ICP. In this setup, the parameters were as follows: the nebulizer carrier gas flow, 0.85 L·min−1; the
chamber and a three stage Peltier-cooled desolvation system or monodisperse droplet generator were used instead of a more common micronebulizing system.22 The use of a piezoelectrical droplet generator for NP characterization has been investigated by Gschwind et al.14,25 Both vertical and horizontal sample introduction configurations were tested, and their performance was evaluated. The microdroplet generator coupled to ICPMS exhibited a high transport efficiency (95%) and also allowed delivery of uniform microdroplets containing standard metal solution into the ICPMS. This can be beneficial for the mass quantification of NP. Recently, substrate-assisted laser desorption (SALD) was presented by our group as a technique for the introduction of liquid samples in the form of dried droplets into the ICPMS.26,27 In SALD, analyte solution (1 J·cm−2) resulted in an elevated baseline (Figure 2B) presumably due to AuNP disintegration. This explanation is also supported by a reduction of the number of detected AuNPs from 3036 at 0.15 J·cm−2 to 1705 at 6 J·cm−2; each of the histograms was acquired from 10 sample spots, and the threshold criterion for counting AuNP signals, SC, was 11. It should be noted here that some sputtering of Au atoms from the NP surface is certainly expected at the applied laser fluence. On the basis of our experience, clusters of Au atoms [Aux]+ with x = 1−3 are regularly observed in surface-assisted laser desorption ionization time-of-flight MS analyses of small molecules in which the applied laser fluence is typically one or two orders below the levels used in SALD and absorption of AuNPs at 355 nm is also lower than that at 213 nm.31 On the other hand, cooling helium is applied in the case of SALD. Laser Spot Diameter, Scan Rate, and Raster Spacing. Laser spot diameter, scan rate, and raster row spacing are closely related parameters; their settings strongly depend on the AuNPs deposition density. The optimum desorption conditions were selected so that the whole spot of the droplet dry residue was irradiated and, if possible, all AuNPs were completely desorbed. Besides, the number of multiple-particle events should be kept to a minimum. The second requirement was found to be less important, because multiple AuNPs desorbed within a single laser pulse can be separated during the transport from the ablation cell to the plasma torch. This was clearly visible in the analysis of highly concentrated samples (2000 AuNPs per spot) with the laser frequency deliberately decreased to 1 Hz. More than one signal per one hundred of 10 ms readings (1 s in total corresponding to 1 laser pulse) was regularly recorded; see Figure 1A. In the typical setup, the C
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Figure 2. Effect of laser fluence on AuNP desorption using SALD. Signal transients and intensity histograms acquired at (A) an optimal laser fluence of 0.15 J·cm−2 and (B) an elevated laser fluence 6 J·cm−2. Two zoomed histograms are displayed for each level of laser fluence to show changes in nanoparticle and noise distributions. A reduced number of AuNPs in the histogram and an increased background intensity presumably result from AuNP disintegration at the elevated laser fluence.
highest adjustable laser frequency (20 Hz) was five times lower than the MS detector sampling frequency, that being 100 Hz. However, the number of laser shots was ∼10 times higher than the number of observed AuNP-associated signals and was applied in order to avoid multiple-particle event collection. The resultant ratio of AuNP signals to the total number of readings originating from the dried droplet was ∼3%, corresponding well to the recommendation from Reed et al.32 It should be noted, however, that exceedingly inhomogeneous AuNP distribution within the dried droplet could possibly lead to more multiple particle detection events than expected. To examine this phenomenon, a SALD SP-ICPMS image of a single spot with 500 AuNPs was acquired and constructed (see experimental details in the Supporting Information and Figure S1). AuNPs were spread over the entire area of the spot; their distribution was satisfactory for the purpose of SP-ICPMS. For the dried droplet with 500 AuNPs, the final scanning conditions were chosen as follows: laser beam spot diameter, 100 μm; frequency, 20 Hz; scan rate, 200 μm·s−1; raster row spacing, 80 μm (see Figure S2). Lower scan rates (≤200 μm· s−1) and a 20% overlap of the raster rows were preferred as it ensured desorption of virtually all NPs from the whole dried droplet area. The use of higher scan rates (≥500 μm·s−1) when the movement of the plate between two laser pulses corresponded to more than 1/4 of the sample spot size caused an elevation of background signals (data not shown). This was presumably due to the nonhomogenous profile of the laser beam; at the optimized conditions, an unexposed area is irradiated only by the lower intensity edge of the laser beam. Therefore, it is expected that the stated laser fluence values (values indicated by the laser ablation software) are likely to be overestimated compared to the actual fluence for AuNP desorption. Carrier Gas. The carrier gas flow rate and type and AuNP transport as well as processes in the plasma affect the desorption process. The conditions were optimized in order
to gain the highest signal intensity and transport efficiency. Helium was found to be superior over argon for AuNP desorption; no or very low AuNP signals were observed when argon was used as the carrier gas at flow rates in the examined range of 0.6−1.2 L·min−1, presumably due to its worse cooling properties. The use of helium at flow rates of 0.7−1.0 L·min−1 delivered the best results in terms of both the AuNP signal intensity and frequency. The helium flow rate of 0.9 L·min−1 was used in all further experiments. The flow rate of argon, which was admixed to the carrier gas subsequent to the ablation cell, was also tested. The highest signal intensities were observed at flow rates of 0.9 to 1.1 L· min−1 in the examined range of 0.6−1.3 L·min−1. Therefore, the flow rate of 1 L·min−1 was set in all SALD ICPMS experiments. Determination of the Transport Efficiency. In typical LA ICPMS experiments, particles in the formed aerosol can be deposited on the tube walls during transport from the ablation cell to the ICP. Particle transport is affected by inertial impacts, laminar and/or turbulent diffusion, gravitational settling, or electrostatic attraction, if the tube wall has static charges. However, for particles with sizes in the range of 5 nm and 3 μm, the aerosol transport efficiency exceeds 80%.33 In SP-ICPMS, the particle transport efficiency is defined as the ratio of the number of particles entering the plasma to the total number of particles analyzed. To determine the transport efficiency of SALD approach, ten droplets with 500 AuNPs each were deposited on a PETG plate and the dried droplets were then analyzed under the optimized conditions. On the basis of manufacturer data, the calculated mean AuNP number concentration of the stock suspension was 2.9 × 1013 AuNP· L−1. The number of nanoparticle events was determined for each spot separately, yielding 304 ± 37 AuNPs per spot (SC = 11). The transport efficiency of 61% was then obtained as the ratio of the total number of detected AuNPs (3 036) to the exact number of the particles deposited on the PETG plate D
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Figure 3. Intensity histograms of AuNPs obtained with (A) SALD ICPMS and (B) nebulizer ICPMS.
The average height of AuNP-associated signals acquired from SALD was observed to be approximately 60% of the average signal height from the nebulizer. To check whether the AuNPs are not disintegrated as the result of exposure to high laser fluence, a simple experimental setup was designed to investigate this phenomenon. A T-piece, which allowed simultaneous aerosol introduction from the both the ablation cell and nebulizer, was installed directly in front of the plasma torch (Figure 4). In the first experiment, AuNPs were nebulized and the carrier gas from the nebulizer was mixed with the carrier gas from the ablation cell, in which the clean PETG plate was irradiated under the same conditions as those used for AuNP analysis. The acquired data were then compared with a second
Figure 4. (A) Schema of the experimental arrangement combining the wet and dry aerosol from both the nebulizer and ablation cell directly in front of the ICP torch. This setup allowed one to study the effects of matrix/environment (e.g., carrier substrate or solvent) on the AuNP signal intensity. (B) Photograph of the T-piece used. E
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to avoid this phenomenon. The major potential of the SALDbased approach lies in the low sample volume consumption in conjunction with a sensitivity in the order of units of AuNPs, at least every second AuNP is detected. Also, many assays are run in a microarray format employing surface-linked reactions; direct addressing with a desorption laser beam may be beneficial as it preserves the information about the spatial distribution. Moreover, this technique may be suitable for determination of particles embedded in solid matrices, e.g., nano enhanced plastics used for food containers.
experiment, in which the AuNPs were desorbed from PETG and pure water was nebulized instead of the AuNP suspension. The nebulizer uptake rate and types and flow rates of all gases as well as the operating conditions of the plasma were kept the same in both experiments. The average signal height was the same in both measurements, suggesting that the difference between signal intensities of AuNPs observed in separate SALD and nebulizer ICPMS experiments does not originate from AuNP disintegration but, presumably, from different excitation conditions in the dry and wet plasma. No baseline increase was observed either. Analysis of 86 nm AuNPs. Finally, 86 nm AuNPs were analyzed in order to verify the quantitative performance of SALD for AuNP analysis. Ten dried droplets with 500 AuNPs (86 nm) in each droplet were measured under identical conditions as used for 56 nm AuNPs. The size histograms of the analyses of AuNPs of the both sizes are shown in Figure 5.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b02421. Figures S1−S4: Image of AuNP distribution within a dried droplet; schema of dried droplet scanning; calibration plot; supplementary characterization of the 86 nm AuNPs. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +420 549 496 629. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of the Czech Science Foundation (GA15-05387S) and the project CEITEC 2020 (LQ1601) of the Ministry of Education, Youth and Sports of the Czech Republic. We also thank J. Buršiḱ for TEM measurements, Z. Farka for help with TEM data analysis, and D. Skácelová and M. Č ernák for cold plasma treatment of PETG.
Figure 5. Intensity histograms of both the 56 nm AuNPs (light blue) and 86 nm AuNPs (dark blue) obtained with SALD ICPMS under the optimized conditions. Only data above SC are shown.
The mean size of the larger AuNPs was 86.7 nm in accordance with the mean size of 86.4 nm obtained from TEM analysis. For details on data processing, see the Supporting Information. The number of nanoparticle events was determined for each spot separately, yielding 262 ± 28 AuNPs per spot (SC = 17); 2617 of 5000 AuNPs (86 nm) were detected in 10 spots. When the transport efficiency is the same for both 56 and 86 nm AuNPs, the 86 nm AuNP number concentration in the stock suspension was calculated to be 5.6 × 1012 AuNP·L−1. This is significantly lower compared to the value of 6.4 × 1012 AuNP· L−1 determined from the total Au ICPMS analysis and TEM measurements (see the Supporting Information for more details). In our opinion, this should be mostly due to a considerable fraction of Au present in the form of aggregates of primary particles; the larger size and stabilization in PBS may contribute to lower stability of these AuNPs. The mass of the particle aggregates was included in the total Au mass, yielding a positive error of calculated AuNP number in the prepared spots.
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REFERENCES
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CONCLUSIONS We have demonstrated here that SALD can be used for sample introduction of AuNPs into ICPMS. Using a commercially available ablation cell, the AuNPs of different sizes deposited on a PETG plastic surface can be easily detected. It was observed that the AuNP desorption is sensitive to the applied laser fluence as the nanoparticles can be disintegrated under an elevated laser fluence. An IR laser could be employed in order F
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Direct Analysis of Gold Nanoparticles from Dried Droplets Using Substrate-Assisted Laser Desorption Single Particle-ICPMS Iva Benešová,†,‡ Kristýna Dlabková,† František Zelenák,† Tomás ̌ Vaculovič,†,‡ Viktor Kanický,†,‡ and Jan Preisler*,†,‡ †
Department of Chemistry, Faculty of Science, Masaryk University, Brno, 625 00 Czech Republic CEITEC - Central European Institute of Technology, Masaryk University, Brno, 625 00 Czech Republic
‡
S Supporting Information *
ABSTRACT: Single particle inductively coupled plasma mass spectrometry (SP-ICPMS) has been generally accepted as a powerful tool in the field of nanoanalysis. The method has usually been restricted to direct nanoparticle (NP) introduction using nebulization or microdroplet generation systems. In this work, AuNPs are introduced into ICPMS by substrate-assisted laser desorption (SALD) directly from a suitable absorbing plastic surface using a commercial ablation cell for the first time. In SALD, desorption of individual NPs is mediated using a frequency-quintupled Nd:YAG laser (213 nm) operated at a rather low laser fluence. Conditions including laser fluence, laser beam scan rate, and carrier gas flow rate were optimized in order to gain the highest AuNP transport efficiency and avoid AuNP disintegration within the laser irradiation. The method was demonstrated on a well-characterized reference material, 56 nm AuNPs with a transport efficiency of 61% and commercially available 86 nm AuNPs. Feasibility of our technique for NP detection and characterization is discussed here, and the results are compared with an established technique, nebulizer SP-ICPMS. owadays, nanomaterials are widely used in many fields including biomedicine, consumer goods, or energy production.1−4 The steep rise in their synthesis, development, and application in the past decade has been accompanied by increasing concerns about their safety, possible health and environmental impacts. Therefore, there is also a growing demand for methods for nanomaterial detection and characterization, even at very low environmentally relevant concentrations. Due to their unique physical and chemical properties, nanoparticles (NPs) have also found many applications in analytical chemistry. NPs can easily be conjugated with biomolecules, and thus, they can act as labels for signal amplification in biosensing and biorecognition assays. These strategies can significantly enhance detection sensitivity; even a single molecule can be detected in an ideal case.5,6 In terms of individual NP detection, single particle inductively coupled plasma mass spectrometry (SP-ICPMS) is considered a very promising analytical approach.7,8 This method, outlined in a series of papers by Degueldre et al.,9−13 employs the well-established, widespread, and highly sensitive method for elemental analysis, ICPMS, which is run in the time-resolved mode with short dwell times down to a few microseconds,14 but more typically in the range of a few milliseconds.7,15 If a NP suspension is sufficiently diluted and introduced into the ICP, each recorded pulse then represents an ion packet originated from a single NP. The signal intensity is then proportional to the mass of element per NP and the pulse frequency to the NP number concentration. SP-ICPMS can thus provide complex information about elemental
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composition of noncarbon-based nanomaterials and their number concentration. If the information about the shape, chemical composition, and density is known, the information about their average dimensions and size distribution can be further obtained. Despite being a rather novel methodology, which has been extensively studied and used for sizing and characterization of size distribution of various nanomaterials, SP-ICPMS has already found practical applications. For instance, it was used for NP detection and quantitation in environmental samples16,17 or TiO2 NP analyses in sunscreen samples;18 the applicability of SP-ICPMS in bioanalysis was also demonstrated in highly sensitive immunoassays with NP-tagged antibodies.19,20 The potential of SP-ICPMS has also been applied to study the stability of NPs; here, it can be beneficial over other methods as it allows detection and differentiation of both the dissolved and particulate forms.21,22 NP introduction into the ICP spectrometer is typically carried out using common pneumatic nebulizers.12,16,23,24 Many nebulizing systems are known to suffer from low transport efficiency and high sample consumption, and therefore, alternative methods for NP introduction have been presented.14,22,25 Franze et al. demonstrated enhanced transport efficiency as well as higher sensitivity due to the drier aerosol conditions when a micronebulizer with a heated cyclonic spray Received: June 29, 2015 Accepted: January 26, 2016
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plasma discharge. Samples were deposited on the plates within 2 weeks after the treatment. Sample Preparation. SALD ICPMS. All experiments were carried out with the reference material, 56 nm AuNPs unless noted otherwise. The suspension of AuNPs was diluted with water and 0.5 mM sodium citrate in the final dilution step to a number concentration of 0.5 × 109 − 10 × 109 AuNP·L−1. Addition of sodium citrate helped to visualize sample spots on PETG plates. Between each dilution step, the NP suspensions were sonicated for 30 s. Certified reference element standard of Au (1 g·L−1) was diluted with water to a concentration of 1 mg· L−1. Droplets (200 nL) of the AuNP suspensions or Au standard solution were spotted on PETG sample plates using a conventional micropipette. Droplets were allowed to dry at room temperature prior to SALD ICPMS analysis; the diameter of the sample spots was ∼1.2 mm. In AuNP analyses, 10 dried droplets with 500 AuNPs each (2.5 × 109 AuNP·L−1) were typically analyzed (total measurement time ∼15 min) in order to gather statistically representative data. Nebulizer SP-ICPMS. The suspension of AuNPs was diluted with water to a number concentration of 1.94 × 107 AuNP·L−1. A volume of 3.3 mL of this suspension was used for ICPMS analysis to deliver a similar number of NPs to plasma in a similar time as in the case of SALD ICPMS. ICPMS Measurements. SALD ICPMS. SALD was performed in a commercial ablation system (model UP 213, New Wave Research, Inc., Fremont, CA, U.S.A.) consisting of a Qswitched Nd:YAG laser operating at 213 nm, movable ablation cell (model SuperCell), and built-in microscope/CCD camera system for visual inspection of the samples. If not stated otherwise, a sample spot area was scanned with the highest possible repetition rate of 20 Hz along a serpentine-like raster with a row spacing of 80 μm and scan rate of 200 μm·s−1 by a laser beam focused to a spot with a diameter of 100 μm at laser fluence of 0.15 J·cm−2 in the focused beam mode. Laser fluence and scan rate as well as carrier gas flow rate were varied in order to gain the best possible performance in terms of intact particle desorption and the highest transport efficiency. Both helium and argon were tested as carrier gases. The tested range of the carrier gas was 0.6−1.2 L·min−1. Both the flush and laser warmup times were set to 4 s. The aerosol was transported into an ICP quadrupole mass spectrometer (model 7500 CE, Agilent Technologies, Inc., Santa Clara, CA, U.S.A.). A sample gas flow of argon (1 L·min−1) was admixed to the carrier gas subsequent to the ablation cell if not mentioned otherwise. The signal of 197 Au isotope was measured with an integration time of 10 ms, the lowest value offered by the instrument software. The settling time of the mass spectrometer was 0.2 ms. Nebulizer ICPMS. As a reference method for AuNP analysis, a Babington nebulizer with a Scott double-pass spray chamber was used (Agilent Technologies, Inc., Santa Clara, CA, U.S.A.). The conditions were derived from Laborda et al.:24 the carrier gas (argon) flow was set to 0.75 L·min−1; the makeup gas (argon) flow was 0.40 L·min−1. Sample uptake rate was 0.33 mL·min−1. The signal of 197Au isotope was measured with an integration time of 10 ms. Comparison of ICPMS with Nebulizer and SALD for AuNP Introduction. In an experiment for direct comparison of nebulizer and SALD, a T-piece (see below for detailed scheme and information) was employed to allow mixing of both aerosols produced in the ablation cell and by nebulization before introduction into the ICP. In this setup, the parameters were as follows: the nebulizer carrier gas flow, 0.85 L·min−1; the
chamber and a three stage Peltier-cooled desolvation system or monodisperse droplet generator were used instead of a more common micronebulizing system.22 The use of a piezoelectrical droplet generator for NP characterization has been investigated by Gschwind et al.14,25 Both vertical and horizontal sample introduction configurations were tested, and their performance was evaluated. The microdroplet generator coupled to ICPMS exhibited a high transport efficiency (95%) and also allowed delivery of uniform microdroplets containing standard metal solution into the ICPMS. This can be beneficial for the mass quantification of NP. Recently, substrate-assisted laser desorption (SALD) was presented by our group as a technique for the introduction of liquid samples in the form of dried droplets into the ICPMS.26,27 In SALD, analyte solution (1 J·cm−2) resulted in an elevated baseline (Figure 2B) presumably due to AuNP disintegration. This explanation is also supported by a reduction of the number of detected AuNPs from 3036 at 0.15 J·cm−2 to 1705 at 6 J·cm−2; each of the histograms was acquired from 10 sample spots, and the threshold criterion for counting AuNP signals, SC, was 11. It should be noted here that some sputtering of Au atoms from the NP surface is certainly expected at the applied laser fluence. On the basis of our experience, clusters of Au atoms [Aux]+ with x = 1−3 are regularly observed in surface-assisted laser desorption ionization time-of-flight MS analyses of small molecules in which the applied laser fluence is typically one or two orders below the levels used in SALD and absorption of AuNPs at 355 nm is also lower than that at 213 nm.31 On the other hand, cooling helium is applied in the case of SALD. Laser Spot Diameter, Scan Rate, and Raster Spacing. Laser spot diameter, scan rate, and raster row spacing are closely related parameters; their settings strongly depend on the AuNPs deposition density. The optimum desorption conditions were selected so that the whole spot of the droplet dry residue was irradiated and, if possible, all AuNPs were completely desorbed. Besides, the number of multiple-particle events should be kept to a minimum. The second requirement was found to be less important, because multiple AuNPs desorbed within a single laser pulse can be separated during the transport from the ablation cell to the plasma torch. This was clearly visible in the analysis of highly concentrated samples (2000 AuNPs per spot) with the laser frequency deliberately decreased to 1 Hz. More than one signal per one hundred of 10 ms readings (1 s in total corresponding to 1 laser pulse) was regularly recorded; see Figure 1A. In the typical setup, the C
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Figure 2. Effect of laser fluence on AuNP desorption using SALD. Signal transients and intensity histograms acquired at (A) an optimal laser fluence of 0.15 J·cm−2 and (B) an elevated laser fluence 6 J·cm−2. Two zoomed histograms are displayed for each level of laser fluence to show changes in nanoparticle and noise distributions. A reduced number of AuNPs in the histogram and an increased background intensity presumably result from AuNP disintegration at the elevated laser fluence.
highest adjustable laser frequency (20 Hz) was five times lower than the MS detector sampling frequency, that being 100 Hz. However, the number of laser shots was ∼10 times higher than the number of observed AuNP-associated signals and was applied in order to avoid multiple-particle event collection. The resultant ratio of AuNP signals to the total number of readings originating from the dried droplet was ∼3%, corresponding well to the recommendation from Reed et al.32 It should be noted, however, that exceedingly inhomogeneous AuNP distribution within the dried droplet could possibly lead to more multiple particle detection events than expected. To examine this phenomenon, a SALD SP-ICPMS image of a single spot with 500 AuNPs was acquired and constructed (see experimental details in the Supporting Information and Figure S1). AuNPs were spread over the entire area of the spot; their distribution was satisfactory for the purpose of SP-ICPMS. For the dried droplet with 500 AuNPs, the final scanning conditions were chosen as follows: laser beam spot diameter, 100 μm; frequency, 20 Hz; scan rate, 200 μm·s−1; raster row spacing, 80 μm (see Figure S2). Lower scan rates (≤200 μm· s−1) and a 20% overlap of the raster rows were preferred as it ensured desorption of virtually all NPs from the whole dried droplet area. The use of higher scan rates (≥500 μm·s−1) when the movement of the plate between two laser pulses corresponded to more than 1/4 of the sample spot size caused an elevation of background signals (data not shown). This was presumably due to the nonhomogenous profile of the laser beam; at the optimized conditions, an unexposed area is irradiated only by the lower intensity edge of the laser beam. Therefore, it is expected that the stated laser fluence values (values indicated by the laser ablation software) are likely to be overestimated compared to the actual fluence for AuNP desorption. Carrier Gas. The carrier gas flow rate and type and AuNP transport as well as processes in the plasma affect the desorption process. The conditions were optimized in order
to gain the highest signal intensity and transport efficiency. Helium was found to be superior over argon for AuNP desorption; no or very low AuNP signals were observed when argon was used as the carrier gas at flow rates in the examined range of 0.6−1.2 L·min−1, presumably due to its worse cooling properties. The use of helium at flow rates of 0.7−1.0 L·min−1 delivered the best results in terms of both the AuNP signal intensity and frequency. The helium flow rate of 0.9 L·min−1 was used in all further experiments. The flow rate of argon, which was admixed to the carrier gas subsequent to the ablation cell, was also tested. The highest signal intensities were observed at flow rates of 0.9 to 1.1 L· min−1 in the examined range of 0.6−1.3 L·min−1. Therefore, the flow rate of 1 L·min−1 was set in all SALD ICPMS experiments. Determination of the Transport Efficiency. In typical LA ICPMS experiments, particles in the formed aerosol can be deposited on the tube walls during transport from the ablation cell to the ICP. Particle transport is affected by inertial impacts, laminar and/or turbulent diffusion, gravitational settling, or electrostatic attraction, if the tube wall has static charges. However, for particles with sizes in the range of 5 nm and 3 μm, the aerosol transport efficiency exceeds 80%.33 In SP-ICPMS, the particle transport efficiency is defined as the ratio of the number of particles entering the plasma to the total number of particles analyzed. To determine the transport efficiency of SALD approach, ten droplets with 500 AuNPs each were deposited on a PETG plate and the dried droplets were then analyzed under the optimized conditions. On the basis of manufacturer data, the calculated mean AuNP number concentration of the stock suspension was 2.9 × 1013 AuNP· L−1. The number of nanoparticle events was determined for each spot separately, yielding 304 ± 37 AuNPs per spot (SC = 11). The transport efficiency of 61% was then obtained as the ratio of the total number of detected AuNPs (3 036) to the exact number of the particles deposited on the PETG plate D
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Figure 3. Intensity histograms of AuNPs obtained with (A) SALD ICPMS and (B) nebulizer ICPMS.
The average height of AuNP-associated signals acquired from SALD was observed to be approximately 60% of the average signal height from the nebulizer. To check whether the AuNPs are not disintegrated as the result of exposure to high laser fluence, a simple experimental setup was designed to investigate this phenomenon. A T-piece, which allowed simultaneous aerosol introduction from the both the ablation cell and nebulizer, was installed directly in front of the plasma torch (Figure 4). In the first experiment, AuNPs were nebulized and the carrier gas from the nebulizer was mixed with the carrier gas from the ablation cell, in which the clean PETG plate was irradiated under the same conditions as those used for AuNP analysis. The acquired data were then compared with a second
Figure 4. (A) Schema of the experimental arrangement combining the wet and dry aerosol from both the nebulizer and ablation cell directly in front of the ICP torch. This setup allowed one to study the effects of matrix/environment (e.g., carrier substrate or solvent) on the AuNP signal intensity. (B) Photograph of the T-piece used. E
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to avoid this phenomenon. The major potential of the SALDbased approach lies in the low sample volume consumption in conjunction with a sensitivity in the order of units of AuNPs, at least every second AuNP is detected. Also, many assays are run in a microarray format employing surface-linked reactions; direct addressing with a desorption laser beam may be beneficial as it preserves the information about the spatial distribution. Moreover, this technique may be suitable for determination of particles embedded in solid matrices, e.g., nano enhanced plastics used for food containers.
experiment, in which the AuNPs were desorbed from PETG and pure water was nebulized instead of the AuNP suspension. The nebulizer uptake rate and types and flow rates of all gases as well as the operating conditions of the plasma were kept the same in both experiments. The average signal height was the same in both measurements, suggesting that the difference between signal intensities of AuNPs observed in separate SALD and nebulizer ICPMS experiments does not originate from AuNP disintegration but, presumably, from different excitation conditions in the dry and wet plasma. No baseline increase was observed either. Analysis of 86 nm AuNPs. Finally, 86 nm AuNPs were analyzed in order to verify the quantitative performance of SALD for AuNP analysis. Ten dried droplets with 500 AuNPs (86 nm) in each droplet were measured under identical conditions as used for 56 nm AuNPs. The size histograms of the analyses of AuNPs of the both sizes are shown in Figure 5.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b02421. Figures S1−S4: Image of AuNP distribution within a dried droplet; schema of dried droplet scanning; calibration plot; supplementary characterization of the 86 nm AuNPs. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +420 549 496 629. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of the Czech Science Foundation (GA15-05387S) and the project CEITEC 2020 (LQ1601) of the Ministry of Education, Youth and Sports of the Czech Republic. We also thank J. Buršiḱ for TEM measurements, Z. Farka for help with TEM data analysis, and D. Skácelová and M. Č ernák for cold plasma treatment of PETG.
Figure 5. Intensity histograms of both the 56 nm AuNPs (light blue) and 86 nm AuNPs (dark blue) obtained with SALD ICPMS under the optimized conditions. Only data above SC are shown.
The mean size of the larger AuNPs was 86.7 nm in accordance with the mean size of 86.4 nm obtained from TEM analysis. For details on data processing, see the Supporting Information. The number of nanoparticle events was determined for each spot separately, yielding 262 ± 28 AuNPs per spot (SC = 17); 2617 of 5000 AuNPs (86 nm) were detected in 10 spots. When the transport efficiency is the same for both 56 and 86 nm AuNPs, the 86 nm AuNP number concentration in the stock suspension was calculated to be 5.6 × 1012 AuNP·L−1. This is significantly lower compared to the value of 6.4 × 1012 AuNP· L−1 determined from the total Au ICPMS analysis and TEM measurements (see the Supporting Information for more details). In our opinion, this should be mostly due to a considerable fraction of Au present in the form of aggregates of primary particles; the larger size and stabilization in PBS may contribute to lower stability of these AuNPs. The mass of the particle aggregates was included in the total Au mass, yielding a positive error of calculated AuNP number in the prepared spots.
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REFERENCES
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CONCLUSIONS We have demonstrated here that SALD can be used for sample introduction of AuNPs into ICPMS. Using a commercially available ablation cell, the AuNPs of different sizes deposited on a PETG plastic surface can be easily detected. It was observed that the AuNP desorption is sensitive to the applied laser fluence as the nanoparticles can be disintegrated under an elevated laser fluence. An IR laser could be employed in order F
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