Article pubs.acs.org/ac
Mass Measurement of Single Intact Nanoparticles in a Cylindrical Ion Trap Ning Zhang,†,⊥ Kai Zhu,‡,⊥ Caiqiao Xiong,† Yurong Jiang,‡ Huan-Cheng Chang,§ and Zongxiu Nie*,†,∥ †
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry Chinese Academy of Sciences, Beijing 100190, China ‡ Department of Optical Engineering, Beijing Institute of Technology, Beijing 100081, China § Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan ∥ National Center for Mass Spectrometry in Beijing, Beijing 100190, China S Supporting Information *
ABSTRACT: Accurate nanoparticle mass characterization is a challenging task, especially at a single particle level. To solve this problem, a strategy for the mass measurement of single intact nanoparticle was proposed. A microscopy-based ion trap mass spectrometer was built up. To improve the detection sensitivity, a cylindrical ion trap with transparent conductive end-caps was used to increase the transmission of scattered light, and a vacuum ultraviolet lamp was used to increase the charge state of the isolated nanoparticle. By detecting the scattered light of the isolated nanoparticle, a series of secular frequencies were obtained, from which the corresponding mass-to-charge ratio of the nanoparticle was calculated. Finally, a Labview program was used to help deduce the charge state and absolute mass of the individual nanoparticle. Masses of gold nanoparticles with different sizes were accurately examined, which are (5.08 ± 0.44) × 107 Da for 20 nm, (3.55 ± 0.34) × 108 Da for 40 nm, and (1.22 ± 0.14) × 109 Da for 60 nm, respectively. The mass of MOFs with irregular shapes was also determined, which is (6.48 ± 1.08) × 109 Da. This method can provide the mass information on nanomaterials, thus opens up new possibility of characterizing nanoparticles at the single particle level.
N
not able to detect small nanoparticles with charges below the detection limit. Recently, Jarrold et al. developed a new image charge detector, to further expand the mass range of the detected ions by reducing the noise.8,9 However, it is less reported to analyze a single nanoparticle by charge detector. The ion trap mass spectrometer equipped with an optical detector provides opportunities for nondestructive detection of individual nanopaticles.10 As a powerful mass analyzer, the ion trap can provide an ideal environment for long-term interrogation of a single nanoparticle ion. When trapped in the quadrupole field, the motion of the particle ions can be characterized by secular frequencies, which is closely related to their mass-to-charge ratios (m/z).11 The secular frequencies of confined ions can be recorded by the light-scattering method12 and then the m/z value of individual particle would be obtained by the Mathieu equation. By measuring the star-like ion motion in the radial plane, Hars and Tass measured the mass of a single particle in the range of 109 to 1012 Da with an accuracy of 10−3.13 Peng et al. obtained a stationary star pattern by finetuning the driving frequency and deriving an analytical formula of secular frequency.14,15 In order to further obtain the charge
anoparticles have been widely used in the research of physics, electronics, and chemical and biological sciences owing to their unique properties. The unique properties of nanoparticles are easily influenced by physical parameters such as shape, size, or even mass. However, compared with the shape and size, accurate mass measurement of nanoparticles is a real challenge.1−3 For the homogeneous nanoparticles with regular shapes, their masses can be calculated by material density and the size obtained by electron microscopy, yet the method cannot be applied for those heterogeneous nanoparticles with irregular shapes. As a powerful tool, mass spectrometry plays an irreplaceable part in mass analysis. However, to date, there is few commercial mass spectrometer for the mass characterization of nanoparticles. The detectors of conventional mass spectrometers are either electron multipliers or multichannel plates (MCP), which are not able to detect large particles. Because the sensitivity of the above detector is related to the velocity of the incident ions, and the high mass ions with low incident velocity cannot excite enough secondary electrons to be detected. To obtain the mass of large particles, several new detectors are proposed, among which the charge detector aroused much interest.4 Using the particle mass spectrometry equipped with charge detector, Nie et al. obtained the masses and mass distributions of particles with several micrometers with a mass resolution of ∼1%.5−7 In these studies, the background noise of charge detector is as high as 500e, which is © XXXX American Chemical Society
Received: March 8, 2016 Accepted: May 11, 2016
A
DOI: 10.1021/acs.analchem.6b00918 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry states of trapped particles, a calculation method and its corresponding Labview program were developed based on the ion trap theory,16 through which the charges carried on the nanoparticles can be deduced, and finally the absolute mass of nanoparticles can be calculated. With the application of fast Fourier transform technique, the mass deviation for the particle would be further improved to 10−4.17 In previous studies, many particles including PS, SiO2, bacteria, and cells have been analyzed by quadrupole ion trap (QIT).18−22 However, for QIT, the collection efficiency of scattered light is low due to its hyperbolic shape so that particles less than 300 nm could not be readily detected.14 To overcome the problem, Nie et al. proposed to use a cylindrical ion trap (CIT) with transparent conductive end-cap electrodes to replace QIT. Such a technological advancement enables more than 10% of light radiating from the trapped particle to be collected, and by this approach, the absolute mass of a single whole virus with a size as small as 80 nm was successfully determined.10 Another critical step for absolute mass measurement is the charge determination. Usually, it needs an electron gun to change the charge states of nanoparticles for several times; however, the particles easily escape from the ion trap when the electron gun fires. To improve the ionization efficiency, a vacuum ultraviolet lamp was used here for changing charge states. This process is softer and more easily controlled. In this work, using the improved microscopy-based ion trap mass spectrometer, the mass of various nanoparticles with the size ranging from tens to hundreds of nanometers were successfully measured. The method is nondestructive and highly sensitive, providing a new opportunity for nanoparticle characterization at the single particle level.
Figure 1. Schematic illustration of the cylindrical ion trap (CIT) instrument for mass measurement.
was confined within a small volume (∼0.1 × 0.1 × 0.1 mm3) near the trap center. A continuous-wave laser (532 nm, 200 mW) was used to illuminate the nanoparticles, and a long working distance objective (Mitutoyo, M Plan Apo 20×, numerical aperture 0.42) was placed just above the CIT to collect the scattered laser light, which is at an angle of about 45° to the path of illumination laser. A beamsplitter was used to divide the scattered light into two parts. One part of the light was guided to a cooled electron multiplier charge-coupling device (CCD, Cooke, SensiCam QE) for monitoring to ensure only one single nanoparticle trapped, and the other part was detected by a photomultiplier tube (PMT, Hamamatsu, R928) to record the change of scattered light. A current amplifier (Keithley) was used to connect the PMT and a data collection card to magnify the signal. Besides, a homemade peak-to-peak detector was used to obtain high precise radio frequency (rf) voltage amplitude in the working condition. Mass Measurement. The ionized nanoparticles entered into the CIT through the gap between the upper end-cap electrode and the ring electrode. Depending on the species and sizes of nanoparticles, about 1000 V0‑p with frequency 1000− 5000 Hz ac field was applied to stably trap the nanoparticles. When lots of nanoparticles were trapped in the ion trap, the light spot in the CCD image is large and the movement of the nanoparticles was usually disorderly. By tuning the ac frequency applied to end-cap electrodes, the nanoparticles can be ejected from the CIT, the light spot will become smaller and smaller until we can count the number of the trapped nanoparticles (Figure S1 in Supporting Information). When only one nanoparticle is left, the nanoparticle usually moves in an orderly pattern or sometimes remains stationary and the scattered light will be brighter. Then the scattered light of the isolated nanoparticle will be detected by PMT and finally transformed into the radial secular frequency (ωr) by fast Fourier transform (FFT). The mass-to-charge ratios (m/Z) of the trapped nanoparticle can be calculated from ωr based on the Mathieu equations. Specifically, the relation was shown as eq 1 when the Mathieu parameter qz < 0.7.
■
EXPERIMENTAL SECTION Chemicals and Materials. Polystyrene size standard (SRM 1963a) from U.S. National Institute of Standards of Technology (NIST) were repeatedly washed with deionized water. After removing the stabilizing agent in the suspensions, they were resuspended in distilled water. Gold nanoparticles (20 nm, 40 nm, 60 nm) were purchased from Sigma. The metal organic frameworks (MOFs, Zeolitic imidazolate frameworks, ZIF8) were synthesized according to ref 23 and provided by Prof. Lanqun Mao from the Institute of Chemistry, Chinese Academy of Sciences. An aliquot of 10 μL of different nanoparticle suspensions was dropped onto a Si wafer and dried in the air for ionization. Instrument. The experimental setup is shown in Figure 1. Nanoparticles were desorbed by a laser-induced acoustic desorption (LIAD) ion source, which used a frequency-doubled Nd: YAG laser (wavelength of 532 nm, pulse width of 7 ns, pulse energy of 30 mJ) to shoot on the backside of the Si wafer for one pulse. Because the laser for LIAD did not contact with the sample directly, the nanoparticles would remain intact during desorption. The ionized nanoparticles entered into the cylindrical ion trap (CIT), which consisted of a ring electrode and two end-cap electrodes (r0 = 5 mm, z0 = 11 mm). Both the end-cap electrodes were made of flat glass plates coated with a thin conducting layer, which is optically transparent with a surface resistivity below 100 Ω/square. The glass electrodes can be either electrically grounded during the mass analysis or applied with a small alternating current (ac) voltage (10 V) for ejecting ions to achieve single particle isolation. Under the collision of buffer gas (∼1 mTorr), the trapped nanoparticle B
DOI: 10.1021/acs.analchem.6b00918 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry 3 ⎡ 2 2 2ωr 1 ⎢ ⎛ 2π Vac ⎞ 25C 3 ⎛ 2π Vac ⎞ ⎟ ⎜ ⎟ C⎜ = + 2 ⎢⎣ ⎝ Ω2 ⎠ 128 ⎝ Ω2 ⎠ Ω
1 m̅ = N
5 7 ⎤ 2 2 34951C 5 ⎛ 2π Vac ⎞ 7925C7 ⎛ 2π Vac ⎞ ⎥ ⎜ ⎟ ⎜ ⎟ ... + − + ⎥⎦ 294912 ⎝ Ω2 ⎠ 294912 ⎝ Ω2 ⎠
S=
(1)
N
∑ mi
(2)
i=1
1 N−1
N
∑ (mi − m̅ )2 i=1
(3)
Then we plotted the S /m̅ value against the charge number (Z) and selected the charge number with the lowest S /m̅ to calculate the absolute mass of the nanoparticle. Calculated by the program, the mass of the nanoparticle in Figure 2 is confirmed as 1.07 × 108 Da and the charge number was 39 at the frequency 509.4 Hz, which means the trapped nanoparticle was a cluster. In many cases, the mass was corresponding to the nanoparticle clusters (dimer, trimer, etc.), which were finally left in the ion trap. To obtain the mass of monomers, the measured masses versus the assigned particle numbers would be plotted, and the slope of the fitting straight line was related to the mass of single nanoparticle (Figure S2 in Supporting Information). It was also observed that the peak width at half height is about 0.2 Hz (Figure 2 inset), which yields a frequency resolving power of ∼2500 (F/ΔF) and a mass resolving of 1250 (F/2ΔF= m/Δm) for one nanoparticle. It takes about 10 min to acquire the data.
where C = Ze/mr02π2, e is the elementary charge, Z is the charge number, r0 is the radius of the ring electrode, Ω and Vac are the frequency and amplitude applied to the ring electrode. Notably, in our experiments, the value of qz is less than 0.7. Therefore, the m/Z of the single nanoparticle can be deduced from the measured ωr using eq 1. If there is no restriction on the value of qz, the mathematical expressions derived by Koizumi et al. is more accurate.24−26 Charge Determination. One critical step for final mass determination was to determine the charge state Z of the nanoparticle. A vacuum ultraviolet lamp (Heraeus, PKS106, filling gas kryptoon, 10.6 eV) was used in this work to change the charge state of the nanoparticle, which is softer and easily controlled than the electron gun used before.10,14−16 In order to change the charge state of the nanoparticle consistently every time, the distance between the VUV lamp and the ion trap and the duration time of the VUV lamp had to be optimized. In the experiment, the duration time of the VUV lamp was 2−5 s at a certain distance (∼3 cm). Figure 2 shows the secular
■
RESULTS AND DISCUSSION Mass Measurement of Gold Nanoparticles. After checking and calibrating the setup by NIST SRM 1963a (Figure S2 in Supporting Information), the microscopy-based ion trap mass spectrometry was used to measure the masses of other nanoscale particles. Over recent decades, gold nanoparticles (AuNPs) have been widely studied and used in many fields owing to their flexible nanostructures and excellent biological compatibility. Some unique properties of AuNPs are size- and shape-dependent, which are noticeable especially in their biomedical applications.27 AuNPs mass measurement is also meaningful, which may provide more information about the nature of gold nanomaterials. Herein, using our home-built setup, the masses and mass distributions of gold nanoparticles with different sizes (20, 40, and 60 nm) were obtained. According to Rayleigh’s law of scattering, the intensity of scattered light is proportional to the sixth power of the diameter of the particle. With the decrease of particle size, the scattering signal will turn down sharply. However, different from polystyrene spheres, the scattered light of AuNPs was enhanced due to the featured localized surface plasmon resonance (LSPR) phenomenon excited with the 532 nm laser.28 Therefore, even though the size of AuNPs was smaller than PS, the signal intensity of AuNPs was still high enough for detection. Besides, when the size of AuNPs decreases, the charges carried on them decrease. Figure 4 showed the charge number carried on a single 20 nm AuNP. When the charge number Z is 18, the corresponding mass-to-charge was determined to be 2.86 × 106 Da. The mass-to-charge ratio was about 106 Da, which was located in the mass range of conventional mass spectrometer. Figure 3 showed the experimental results of AuNPs with different sizes, and the mass of 20, 40, and 60 nm AuNPs were (5.08 ± 0.44) × 107 Da, (3.55 ± 0.34) × 108 Da, and (1.22 ± 0.14) × 109 Da, respectively. According to the mass measured, the sizes of the AuNPs were calculated, which were 20.3 ± 0.6 nm, 38.8 ± 1.2 nm, and 58.4 ± 2.2 nm, respectively (Figure 3d−f, red line, the density of AuNPs is set to 19.32 g/cm3). The results obtained
Figure 2. Five frequency spectra of individual PS nanoparticles in one test obtained by FFT of the time-domain signal acquired by PMT for 5 s. Inset: the peak width at half height.
frequencies for one single polystyrene nanoparticle after the charge state was changed four times. The secular frequencies were 509.4, 522.4, 533.6, 549.8, and 577.6 Hz, which is obtained by FFT of the time-domain signal acquired by PMT for 5 s. Through the relationship in eq 1, the associated m/Z values with different charge states were obtained, and then a Labview procedure was used to deduce the absolute mass of the nanoparticle.16 Specifically, we first assigned the charge states with integer number Z1, Z2, Z3...ZN (Zi−1 < Zi < Zmax, and Zi − Zi−1 < ΔZmax), and through the parameter C (C1 < C2 < C3... < CN) obtained by eq 1, the mean mass (m̅ ) and the corresponding standard deviation (S) were calculated by C
DOI: 10.1021/acs.analchem.6b00918 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry
Figure 3. Plots of particle number versus measured mass and mass distribution (lower inset) for gold nanoparticles (a) 20 nm, (b) 40 nm, and (c) 60 nm. The TEM image and size distribution obtained from TEM (red line) and CIT (black line) for AuNPs (d) 20 nm, (e) 40 nm, and (f) 60 nm.
Figure 4. Plots of the ratio of mass standard deviation and mean mass (S/m) versus charge number Z of a single 20 nm AuNP.
from the TEM images were 20.1 ± 1.6 nm, 40.7 ± 2.9 nm, and 58.7 ± 4.4 nm, respectively (Figure 3d−f, black line). Mass Measurement of MOFs. Another sample for mass measurement was a kind of metal-organic frameworks (MOFs, ZIF8). As shown in Figure 5 inset, the shape of the MOFs was irregular so that their accurate mass cannot be calculated simply by its density and size. Figure 5 showed the mass distribution of MOFs and the mean mass was determined to be (6.48 ± 1.08) × 109 Da. To our knowledge, it is difficult to directly
Figure 5. Plots of particle number versus measured mass and mass distribution (inset) for MOFs.
characterize the mass of MOFs nanomaterial, thus regrettably, the data in Figure 5 cannot be validated by another method. The results also demonstrated that this method could be used D
DOI: 10.1021/acs.analchem.6b00918 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry
(12) Trevitt, A. J.; Wearne, P. J.; Bieske, E. J. Int. J. Mass Spectrom. 2007, 262, 241−246. (13) Hars, G.; Tass, Z. J. Appl. Phys. 1995, 77, 4245−4250. (14) Peng, W. P.; Yang, Y. C.; Kang, M. W.; Tzeng, Y. K.; Nie, Z. X.; Chang, H. C.; Chang, W.; Chen, C. H. Angew. Chem., Int. Ed. 2006, 45, 1423−1426. (15) Peng, W. P.; Yang, Y. C.; Kang, M. W.; Lee, Y. T.; Chang, H. C. J. Am. Chem. Soc. 2004, 126, 11766−11767. (16) Peng, W. P.; Yang, Y. C.; Lin, C. W.; Chang, H. C. Anal. Chem. 2005, 77, 7084−7089. (17) Peng, W. P.; Lee, Y. T.; Ting, J. W.; Chang, H. C. Rev. Sci. Instrum. 2005, 76, 023108. (18) Peng, W.-P.; Lin, H.-C.; Lin, H.-H.; Chu, M.; Yu, A. L.; Chang, H.-C.; Chen, C.-H. Angew. Chem., Int. Ed. 2007, 46, 3865−3869. (19) Peng, W.-P.; Lin, H.-C.; Chu, M.-L.; Chang, H.-C.; Lin, N.-H.; Yu, A. L.; Chen, C.-H. Anal. Chem. 2008, 80, 2524−2530. (20) Nie, Z.; Cui, F.; Tzeng, Y.-K.; Chang, H.-C.; Chu, M.; Lin, H.C.; Chen, C.-H.; Lin, H.-H.; Yu, A. L. Anal. Chem. 2007, 79, 7401− 7407. (21) Cai, Y.; Peng, W. P.; Kuo, S. J.; Sabu, S.; Han, C. C.; Chang, H. C. Anal. Chem. 2002, 74, 4434−4440. (22) Cai, Y.; Peng, W. P.; Kuo, S. J.; Lee, Y. T.; Chang, H. C. Anal. Chem. 2002, 74, 232−238. (23) Cravillon, J.; Nayuk, R.; Springer, S.; Feldhoff, A.; Huber, K.; Wiebcke, M. Chem. Mater. 2011, 23, 2130−2141. (24) Koizumi, H.; Whitten, W. B.; Reilly, P. T. A; Koizumi, E. Int. J. Mass Spectrom. 2009, 286, 64−69. (25) Ding, L.; Sudakov, M.; Brancia, F. L.; Giles, R.; Kumashiro, S. J. Mass Spectrom. 2004, 39, 471−484. (26) Ding, L.; Sudakov, M.; Kumashiro, S. Int. J. Mass Spectrom. 2002, 221, 117−138. (27) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293−346. (28) Willets, K. A.; Van Duyne, R. P. Annu. Rev. Phys. Chem. 2007, 58, 267−297.
for the mass measurement of particles with irregular shapes and had potential application on those heterogeneous nanoparticles.
■
CONCLUSION In this work, a mass spectrometry technology for mass measurement of single nanoparticles was established. The absolute mass of different nanoparticles including polystyrene spheres, gold nanoparticles, and MOFs were successfully analyzed. Ionized nanoparticles could be kept in the cylindrical ion trap for a long time, and the light scattering technique was nondestructive and highly sensitive. Therefore, this method can give rich information about the nature of the nanoparticles at the single particle level. Also the home-built setup provides many possibilities of combing other analytical techniques such as spectroscopic analysis of a single particle.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b00918. CCD images of trapped polystyrene nanoparticles and the mass measurement results of NIST SRM 1963a (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +86−10-62652123. Fax: +86−10-82612849. Author Contributions ⊥
N.Z. and K.Z. made an equal contribution to this work.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors thank the National Natural Sciences Foundation of China (Grants 21127901, 21321003, 21305144, and 21475139) and the Chinese Academy of Sciences.
■
REFERENCES
(1) Chang, H.-C. Annu. Rev. Anal. Chem. 2009, 2, 169−185. (2) Peng, W. P.; Cai, Y.; Chang, H. C. Mass Spectrom. Rev. 2004, 23, 443−465. (3) Zhang, N.; Zhan, L. P.; Xiong, C. Q.; Nie, Z. Zhongguo Kexue: Huaxue 2014, 44, 801−806. (4) Peng, W. P.; Chou, S. W.; Patil, A. A. Analyst 2014, 139, 3507− 3523. (5) Xiong, C.; Zhou, X.; Chen, R.; Zhang, Y.; Peng, W.-P.; Nie, Z.; Chang, H.-C.; Liu, H.; Chen, Y. Anal. Chem. 2011, 83, 5400−5406. (6) Xiong, C. Q.; Zhou, X. Y.; Wang, J. N.; Zhang, N.; Peng, W. P.; Chang, H. C.; Nie, Z. X. Anal. Chem. 2013, 85, 4370−4375. (7) Xiong, C. Q.; Zhou, X. Y.; Zhang, N.; Zhan, L. P.; Chen, S. M.; Wang, J. Y.; Peng, W. P.; Chang, H. C.; Nie, Z. X. Anal. Chem. 2014, 86, 3876−3881. (8) Smith, J. W.; Siegel, E. E.; Maze, J. T.; Jarrold, M. F. Anal. Chem. 2011, 83, 950−956. (9) Keifer, D. Z.; Shinholt, D. L.; Jarrold, M. F. Anal. Chem. 2015, 87, 10330−10337. (10) Nie, Z.; Tzeng, Y.-K.; Chang, H.-C.; Chiu, C.-C.; Chang, C.-Y.; Chang, C.-M.; Tao, M.-H. Angew. Chem., Int. Ed. 2006, 45, 8131− 8134. (11) March, R. E. J. Mass Spectrom. 1997, 32, 351−369. E
DOI: 10.1021/acs.analchem.6b00918 Anal. Chem. XXXX, XXX, XXX−XXX
