Testing Molecular Homogeneity at the Nanoscale with Massive

Article pubs.acs.org/ac

Testing Molecular Homogeneity at the Nanoscale with Massive Cluster Secondary Ion Mass Spectrometry Michael J. Eller, Stanislav V. Verkhoturov, and Emile A. Schweikert* Department of Chemistry, Texas A&M University, College Station, Texas 77843-3144, United States ABSTRACT: This study deals with assessing the homogeneity of a mixture of ultrasmall nanoparticles differing only by their respective functionalization. While measuring the relative abundance of nanoparticles with specific functionalization is feasible with mass spectrometry, the determination of mixed or segregated moieties is beyond current capabilities. Our approach is based on SIMS with massive projectiles, specifically Au+4 400. A distinct feature of bombardment with Au+4 400 is abundant emission of multiple secondary ions from one projectile impact. Their analysis allows for examination of coemitted and thus colocalized molecules within the emission area of a single impact (∼10−15 nm in diameter). It is possible to collect the mass spectrum from each projectile impact, which probes individual nanodomains, allowing for examination of molecular homogeneity at the nanoscale.

P

robing the molecular composition of surfaces at the nanoscale remains an elusive goal. A technique with promising prospects is Secondary Ion Mass Spectrometry, SIMS. It features high detection sensitivity for surface moieties; however, testing surfaces in an imaging mode remains limited to a resolution in the range of a few 100 nm.1−6 The use of nanoparticles, NPs, as chemical sensors involving one or more types of NPs is the focus of novel and industrial sensors.7−9 Their performance depends on the functionalization and distribution of these particles, thus their molecular composition must be evaluated at the nanoscale. We describe here a nonimaging variant of SIMS which can determine molecular homogeneity at the scale of 10−15 nm. The performance is demonstrated on a mixture of NPs 5 nm in diameter which differ only in their functionalization. The approach is based on bombarding a surface with ”nanoprojectiles” at hypervelocity, in the present 10,11 The impact of a case Au+4 400, with 520 keV total impact energy. +4 single Au400 causes emission of multiple secondary ions, SIs, and electrons.12−14 It is then feasible to run experiments where bombardment occurs as a sequence of individual Au+4 400 impacts and where the SIs from each impact are recorded separately. The SIs originate from an area of 10−15 nm in diameter and up to 10 nm in depth.15 The SI emission is from colocalized molecules and hence, in principle, is a test of nanoscale homogeneity. For statistics, 106−107 nanospots are tested stochastically within an area of interest. The SI records are investigated impact by impact to group data sets where the coemission of two or more selected SIs occurred. From these ensembles one can determine the homogeneity of the mix of NPs, i.e. molecular homogeneity at the nanoscale. This report describes the method for acquiring and assessing the coemission data, the results obtained from a layer of 5 nm Au NPs functionalized with dodecanethiol or 1-mercapto(triethylene glycol)methyl ether, and the scope for applying colocalization SIMS. © 2016 American Chemical Society

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EXPERIMENTAL SECTION

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SAMPLE PREPARATION

As noted the experimental approach is to bombard the sample with single massive projectiles, Au+4 400 at 520 keV. This is accomplished with custom built instrumentation, a schematic of which is presented in Figure 1. Briefly, the Au+4 400 projectile is produced by a gold liquid metal ion source installed onto a 120 kV platform.10 The projectile is mass selected by a Wien filter. By pulsing the primary ion beam each projectile is separated by 10−3 s. For each projectile impact a time-of-flight, TOF, mass spectrum is collected using the emitted e− or H+ as the start of the TOF measurement, while the secondary ions are mass analyzed by a reflectron TOF mass spectrometer and detected with an eight anode stop detector. The multianode stop detector allows for detection up to eight isobaric ions from a single projectile impact. For these experiments the sample is biased to −10 kV, and e− are used as the start of the TOF measurement. The start and stop signals from each projectile impact are collected with a time to digital converter and stored on a PC as individual mass spectra. The sample is analyzed with successive impacts resulting in 106−107 individual mass spectra. For these experiments the sampling area was approximately 250 μm in diameter, and the acquisition time for each sample was 50 to 90 min.

Several P-doped Si (1 0 0) wafers (Silicon Valley Microelectronics, Santa Clara, CA) were cut into 1 cm × 1 cm pieces and cleaned by sonication in absolute ethanol. Three sets of samples were prepared. Two samples, each containing one type of NP, were prepared by drop casting 1 μL of a 2.0 mg/mL solution of the NP Received: April 14, 2016 Accepted: July 2, 2016 Published: July 2, 2016 7639

DOI: 10.1021/acs.analchem.6b01466 Anal. Chem. 2016, 88, 7639−7646

Article

Analytical Chemistry

Figure 1. Schematic of custom built Massive Cluster SIMS instrument operating in the event by event bombardment detection mode.

are summed to generate a new mass spectrum termed a coincidental mass spectrum. The ions in the coincidental mass spectrum are coemitted and thus colocalized with the selected ion. This subset of mass spectra is evaluated by calculating the coincidental yield with eq 2. IA(B) is the detected number of ions A for impacts in which B is also detected, and N(B) is the number of impacts in which ion B is detected.

in toluene on the cleaned silicon wafer. A third sample was prepared by drop casting 1 μL of a 0.20 mg/mL solution of the 1-mercapto(triethylene glycol) methyl ether functionalized NPs in toluene followed by 1 μL of a 0.20 mg/mL solution of the dodecanethiol fuctionalized NP in toluene. The second drop cast is performed before the first dries, allowing for the two types of particles to mix. The concentation and functionalization of the NPs ensure the NPs are deposited as a single layer on the silicon.16

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SOFTWARE The mass spectra were analyzed using the SAMPI software solution (version 4.3.13).17 SAMPI is a custom written data analysis software developed with LabWindows/CVI 2010 version 10.0.0 (National Instruments Corporation, Austin, TX). DATA ANALYSIS To evaluate mass spectra collected in the event-by-event bombardment/detection mode, the total SI yield (eq 1) is calculated. YA is the yield of ion A, IA(i) is the intensity of ion A in an ensemble of impacts i, and N(i) is the number of impacts in the ensemble. The summation is over all ensembles. This SI yield is the frequency an ion of interest is detected in all measurements. The total yield is useful as it shows an average of the analyzed surface. YA =

∑ i=0

IA(i) N (i )

(2)

Thus, the coincidental yield is the rate at which an ion is detected together with a selected ion(s). The frequency at which the two ions are detected together is directly related to their interdependence. To evaluate the relationship between two or more ions the ratio of the coincidental yield to the total yield is used (eq 3). It should be noted that the values of YA,B/YA do not depend on the transmission/detection efficiency of the TOF mass spectrometer.18

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n

IA(B) N (B)

YA,B =

YA,B YA

=

IA(B) N (B) n

∑i = 0

IA(i) N (i)

(3)

By substituting the fraction of impacts with ion B, K(B), into eq 3 we can obtain eq 5. From eq 5 three inequalities are obtained (eqs 6−8), where the YA,B/YA can be greater than unity, less than unity, or equal to unity.

(1)

To investigate colocalized molecules, ions coemitted from a single projectile impact are analyzed. Using SAMPI, an ion of interest is selected, and the impacts containing the selected ion

KB = 7640

N (B) n ∑i = 0 N (i)

(4) DOI: 10.1021/acs.analchem.6b01466 Anal. Chem. 2016, 88, 7639−7646

Article


Figure 2. Sketch of different ensembles of impacts involving the two types of Au NPs. The emission area of a projectile impact is represented by the green circle. A) The ions are coemitted in the same impact involving the NPs. B) The ions are emitted from separate impacts on each of the NPs.

Figure 3. Negative ion mass spectrum. A) 5 nm gold NPs functionalized with 1-mercapto(triethylene glycol) methyl ether and B) 5 nm gold NPs functionalized with dodecanethiol. The ions are labeled by identity: gold lettered ions are Au−n clusters (n = 1−9); red are the quasi-molecular ions from dodecanethiol NPs; blue are characteristic ions from the 1-mercapto(triethylene glycol)methyl ether NPs; purple are thiol and gold−thiol adduct ion; green are dodecanethiol-gold adduct ions. Each analyzed with Au+4 400 at 520 keV total impact energy.

YA,B YA

if

if

=

YA,B YA YA,B YA

I (B) 1 × nA ∑i = 0 IA(i) K (B)

> 1;

< 1;

if (5)

IA(B) > K (B) n ∑i = 0 IA(i)

(6)

IA(B) n ∑i = 0 IA(i)

(7)

YA,B YA

= 1;

IA(B) n ∑i = 0 IA(i)

= K (B) (8)

The physical interpretation of these inequalities is represented by the sketch in Figure 2. When YA,B/YA > 1, the two ions are emitted together, and there is an overlap of their ensembles of impacts, Figure 2A. When YA,B/YA < 1, the two ions are emitted from separate ensembles of impacts and not emitted together, Figure 2B. When YA,B/YA = 1, the two ions are not emitted in a specific ensemble of impacts, which is the case for a homogeneous

< K (B) 7641


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wafer are observed indicating the single layer of NPs. The yields are notably lower than the thick deposits of the two NPs as each covers only part of the surface and due to the lower amount of material available for sputtering in the single layer versus the thick deposit. The mass spectrum is further complicated as not all projectile impacts are equivalent. As noted earlier, the emission area from a single impact of Au+4 400 at 520 keV is ∼10−15 nm in diameter, thus each impact may affect one or more NPs. Moreover, within the emission area the two types of particles may be comingled further complicating the mass spectrum. To separate these different types of impacts, coincidence tests were performed. Using SAMPI the coincidental mass spectrum of ions coemitted with [dodecanethiol-H]− was determined and is shown in Figure 5. In the coincidental mass spectrum, the coemission of ions from both types of particles and the silicon substrate are evident. From this mass spectrum the coincidental yields of characteristic ions from each of the two particles were evaluated. Then the ratio of coincidental yield to total yield is calculated, eq 5. This can be extended to involve each ion in Table 1, generating a 2D plot where the ion used to calculate the coincidental spectrum is plotted along the abscissa and the evaluated ions are plotted along the ordinate. To evaluate if the ions are colocalized, the value at each x,y position is calculated using eq 5. Using this approach the relationship between every ion pair can be evaluated; the results for the 50:50 mixture of NPs are shown in Figure 6. The color scale indicates ions which are emitted together (yellow to red) and ions which are not emitted together (blue). The labels along the x-axis and y-axis correspond to the ion identities.

surface where ions are coemitted in all impacts. Thus, we can test molecular homogeneity on the nanoscale.

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RESULTS Reference spectra were collected on each of the two types of functionalized Au NPs. First, a multilayer sample of 1-mercapto(triethylene glycol) methyl ether functionalized 5 nm gold NPs was analyzed with ∼4 × 106 Au+4 400 at 520 keV. The mass spectrum is presented in Figure 3A. On average 9.7 SIs per projectile impact are detected. Notable emission of Au−n (n = 1−9) was observed along with characteristic SIs from the coating (Figure 3A). The total SI yields (eq 1) are shown in Table 1. Characteristic ions from the coating were observed [Au[S(C2H4O)3CH3]2]− and [AuSC2H4O]−, and the yields were 0.87% and 0.16%, respectively. Second, a sample consisting of a multilayer deposit of dodecanethiol functionalized 5 nm gold NP was analyzed with ∼4 × 106 Au+4 400 at 520 keV. The mass spectrum is presented in Figure 3B. Here, on average 12.1 SIs are detected per projectile impact. The SI yields are notable. [SCH2(CH2)10CH3]− and [SCH2(CH2)10CH3+O3]− have SI yields of 1.2% and 31.8%, respectively. These characteristic SIs allow impacts on each type of NP to be identified. The composite sample containing both NPs, a 50:50 mixture, was analyzed with ∼6 × 106 Au+4 400 at 520 keV. The mass spectrum is presented in Figure 4. On average 8.7 SIs are detected per projectile impact, enabling the evaluation of coemitted SIs. Here the characteristic ions from each of the two NPs are evident. The SI yields are presented in Table 1. Additionally, the presence of SiO−2 , SiO2OH−, and Si2O4OH− from the underlying silicon

Table 1. Secondary Ion Yields of Selected Ions Computed Using Eq 1a ion identity

nominal mass

yield dod NP

yield mer NP

yield mix NP

Au− Au−2 Au−3 Au−4 Au−5 Au−6 Au−7 Au−8 Au−9 dodecanethiol-H− dodecanethiol-H+O−3 AuSC2H4O− Au[S(C2H4O)3CH3]−2 S− SH− AuSC2H−4 AuS− Au2S− Au3S− Au4S− AuC2H−4 Au2C2H−4 AuSC3H−6 AuSC5H−6 AuSC5H−8 Au2C3H−5 Au2C5H−5

197 394 591 788 985 1182 1379 1576 1773 201 249 273 555 32 33 257 229 426 623 820 223 419 271 295 297 467 491

4.9 × 10−1 9.5 × 10−2 9.0 × 10−2 1.1 × 10−2 1.9 × 10−2 3.5 × 10−3 7.8 × 10−3 1.7 × 10−3 3.4 × 10−3 1.2 × 10−2 3.2 × 10−1

8.2 × 10−1 1.8 × 10−1 1.6 × 10−1 1.8 × 10−2 2.9 × 10−2 4.2 × 10−3 1.0 × 10−2 1.3 × 10−3 4.3 × 10−3

2.3 × 10−1 3.9 × 10−2 3.1 × 10−2 3.7 × 10−3 5.1 × 10−3 5.9 × 10−4 2.3 × 10−3 1.8 × 10−4 5.5 × 10−4 1.3 × 10−3 1.4 × 10−2 2.4 × 10−3 1.3 × 10−3 2.2 × 10−2 2.2 × 10−2 6.8 × 10−3 1.4 × 10−2 4.6 × 10−3 2.8 × 10−3 3.1 × 10−4 2.5 × 10−2 2.4 × 10−3 1.7 × 10−2 6.8 × 10−3 1.6 × 10−3 5.1 × 10−3 1.5 × 10−3

5.2 × 10−2 9.2 × 10−2 3.9 × 10−3 4.2 × 10−2 2.0 × 10−2 1.7 × 10−2 2.4 × 10−3 4.1 × 10−1 1.1 × 10−1 3.8 × 10−2 1.6 × 10−2 2.6 × 10−3 1.1 × 10−2 3.3 × 10−2

1.6 × 10−3 8.7 × 10−3 1.9 × 10−1 2.1 × 10−1 9.5 × 10−2 4.8 × 10−2 3.2 × 10−2 3.8 × 10−3 2.9 × 10−2 7.5 × 10−2 8.7 × 10−3 2.7 × 10−3 9.1 × 10−4 3.6 × 10−3 1.3 × 10−3

The ions are grouped by identity Au−n clusters (n = 1−9); the quasi-molecular ions from dodecanethiol NPs; characteristic ions from the 1-mercapto(triethyleneglycol)methyl ether NPs, thiol and gold−thiol adduct ion; dodecanethiol-gold adduct ions. a

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Figure 4. Negative ion mass spectrum 50:50 mixture of 5 nm gold NPs functionalized with 1-mercapto(triethylene glycol) methyl ether and 5 nm gold NPs functionalized with dodecanethiol. The ions are labeled by identity: gold lettered ions are Au−n clusters (n = 1−9); red are the quasi-molecular ions from dodecanethiol NPs; blue are characteristic ions from the 1-mercapto(triethylene glycol)methyl ether NPs; purple are thiol and gold−thiol adduct ion; green are dodecanethiol-gold adduct ions. Analyzed with Au+4 400 at 520 keV total impact energy.

General trends can be observed in Figure 6. The small Au−n clusters n = 1−5, gold−thiol adducts, and gold-dodecanethiol adduct are coemitted within their respective groups. Between these three groups there is an overlap in the impact ensembles. These observations are dependent upon the impact between Au+4 400 and the NPs on the surface. A key factor in the production of clusters from an impacted particle is the impact parameter, which is the distance of the impact site from the center of the impacted particle. Small impact parameters refer to impacts which occur near the center of the particle, and large impact parameters are impacts far from the center. Figure 6 shows that Au−n clusters with n = 1−5 are coemitted with one another and with gold thiol adducts. This specific set of impacts is likely impacted with small impact parameters. It has been previously shown the emission of gold clusters is dependent on the size of the investigated NP as well as the impact parameter on the NP.19,20 Smaller clusters are preferably emitted from small impact parameters, while larger clusters are emitted from impacts with large impact parameters. Indeed, impacts with small impact parameters result in increased fragmentation of the NP due to the large amount of energy deposited into the particle. Impacts with large impact parameters result in preferential emission of larger Au−n clusters. To determine nanoscale homogeneity, the impacts with large or small impact parameters must both be evaluated. Identifying analyte-specific ions which are observed regardless of the impact parameter allow for investigation of all impacts involving a specific

NP. For the dodecanethiol NPs, the ion meeting this condition is [dodecanethiol-H]− as YA,B/YA is approximately equal to unity in coemission with small or large Au−n clusters. Thus, [dodecanethiol-H]− can be used to identify all impacts involving the dodecanethiol NPs. Similar results were observed for [dodecanethiol+O3]−. For the 1-mercapto(triethylene glycol)methyl ether NPs YA,B/YA is approximately equal to unity for Au[S(C2H4O)3CH 3]2− with Aun− (n = 1−9). Therefore, Au[S(C2H4O)3CH3]−2 was used to identify all impacts involving the 1-mercapto(triethylene glycol)methyl ether NPs. Based on these data the number of effective impacts involving each of the two types of NPs can be evaluated using eq 9.21,22 Ne is the effective number of impacts on a component characterized by the emission of both A and B, and N0 is the total number of projectile impacts. Ne =

IA × IB ; IA,B

coverage =

Ne N0

(9)

By dividing the number of effective impacts by the total number of impacts the coverage of each of the two types of NPs can be determined. The coverage of dodecanethiol coated NP was calculated to be 80 ± 5%, and the coverage of 1-mercapto(triethylene glycol)methyl ether NPs was calculated to be 92 ± 5%. The summation of these two coverages is significantly larger than 100%. This suggests that the NPs are colocalized on the surface. Indeed, by examining the mass spectrum, the coemission of Au[S(C2H4O)3CH3]−2 and [dodecanethiol-H]− was observed 7643


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Figure 5. [dodecanethiol-H]− coincidental mass spectrum. Mass spectrum of ions coemitted with [dodecanethiol-H]− from the 50:50 mixture of 5 nm gold NPs functionalized with 1-mercapto(triethylene glycol) methyl ether and 5 nm gold NPs functionalized with dodecanethiol. The ions are labeled by identity: gold lettered ions are Au−n clusters (n = 1−9); red are the quasi-molecular ions from dodecanethiol NPs; blue are characteristic ions from the 1-mercapto(triethylene glycol)methyl ether NPs; purple are thiol and gold−thiol adduct ion; green are dodecanethiol-gold adduct ions. Normalized to the number of impacts containing [dodecanethiol-H]−.

well-defined NPs the sample is not flat. The organization of the NPs on the surface will generate a surface structure which will influence the resulting mass spectrum. One question presents itself: can the methodology be applied if the NPs are of different sizes. The results suggest that if the two NPs have differing coatings a characteristic ion will allow for differentiation of impacts occurring on one or the other. If the NPs are smaller than the desorption volume of the projectile, differences in the mass distribution of cluster ions will be evident. For the latter to be observed the size distribution of NPs should be narrow. A previous observation is confirmed by the data presented, that the impact parameter of the projectile on NPs with volumes below the volume perturbed by the projectile alters the resulting secondary ions.19 The impact parameter determines the amount of energy deposited into the nanoparticle by the projectile. This effect has profound changes on the observed secondary ions, altering the mass distribution of the observed secondary ions. The results presented in Figure 6 indicate that impacts with small impact parameters, characterized by increased emission of small gold clusters, result in increased adduct formation between the thiol coating and gold. The presence of neighboring particles also affects the observed secondary ions. This is evidenced by the coemission of [AuSC2H4O]− and dodecanthiol gold adducts, which must result from the emission of both NPs. Our results

(Figures 4 and 5). The emission of those two ions occurs regardless of the impact parameter, involving the respective NPs, which reveal the fraction of the surface in which the NPs are comingled. Using eq 9 with Au[S(C2H4O)3CH3]−2 and [dodecanethiol-H]− we find 75 ± 5% of the surface contained comingled NPs. Thus, ∼5% of the surface contained only the dodecanthiol NPs, and ∼15% contained only the 1-mercapto(triethylene glycol)methyl ether NPs. Investigating further, the impacts containing the comingled NPs showed interesting trends. The coemission of [AuSC2H4O]− and dodecanthiol gold adducts occurred with YA,B/YA greater than unity, thus they were emitted in a specific ensemble of impacts. As noted previously, gold dodecanthiol adducts are emitted from impacts with small impact parameters, resulting in the increased emission of small Au clusters. Their coemission with [AuSC2H4O]− indicated that the two types of NPs are colocalized within the emission area of an impact with a small impact parameter. We posit these impacts had small impact parameters on one NP which is neighboring an NP of a different type.

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DISCUSSION The analysis of this model system where the size distribution of each nanoparticle is narrow and well characterized shows it is possible to determine the coverage of each nanoparticle and to evaluate homogeneity. Note that while the model system has 7644


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Figure 6. 2D heat map. The color scale is the coincidental yield divided by the total yield (eq 3). In the Y axis the evaluated ions are listed, and in the X axis the ion used for the coincidental mass spectrum is listed. The ions are grouped by identity: gold lettered ions are Au−n clusters (n = 1−9); red are the quasi-molecular ions from dodecanethiol NPs; blue are characteristic ions from the 1-mercapto(triethylene glycol)methyl ether NPs; purple are thiol and gold−thiol adduct ion; green are dodecanethiol-gold adduct ions.

suggest that a projectile with a small impact parameter on one NP results in increased production of adduct ions from a neighboring particle. While this is the case for adduct ions, it is not the case for the quasi-molecular ions from the coatings. Their production was found to be independent from the impact parameter. Let us consider now the general case of probing molecular surface homogeneity. For segregation larger than the emission area from a single projectile impact (10−15 nm in diameter) the approach described can evaluate molecular homogeneity. As has been demonstrated for the mixture of 5 nm NPs it is possible to evaluate homogeneity for features below the analysis area of a single projectile impact. To evaluate homogeneity below the area probed with a single impact requires knowledge about the physical structure or size of the homogeneity obtained by another technique such as transmission electron microscopy. The ultimate limitations of the approach and its application to mixtures of molecules below the analysis area of a single projectile impact (10−15 nm in diameter) remain to be assessed.

1-mercapto(triethylene glycol)methyl ether coated NPs. The methodology reveals segregations larger than the analysis area of a single projectile impact (10−15 nm in diameter). For mixtures below 10−15 nm, such as the 5 nm NPs investigated here, knowledge concerning the size of the segregation may allow for homogeneity tests to be performed. Here, a measurement not possible with current methods has been achieved: the determination by mass spectrometry of molecular homogeneity at the nanoscale.

CONCLUSION The distinct feature of the approach includes the ability to determine a) the extent of comingled moieties i.e. homogeneity at the nanoscale and b) the respective surface coverages of mixed and segregated components. In the model system studied here where two types of NP are mixed, it was found that ∼75% of the surface contains both NPs, while ∼5% contained only the dodecanthiol coated NPs and ∼15% contained only the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Science Foundation for its support of this work (NSF grant CHE-1308312).

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Testing Molecular Homogeneity at the Nanoscale with Massive

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