Quantifying Nanosheet Graphene Oxide Using Electrospray

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Quantifying Nanosheet Graphene Oxide using Electrospray-Differential Mobility Analysis Jui-Ting Tai, Yen-Chih Lai, Jian-He Yang, Hsin-Chia Ho, Hsiao-Fang Wang, Rong-Ming Ho, and De-Hao Tsai Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac504671k • Publication Date (Web): 17 Mar 2015 Downloaded from http://pubs.acs.org on March 24, 2015

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Analytical Chemistry

Quantifying Nanosheet Graphene Oxide using ElectrosprayDifferential Mobility Analysis Jui-Ting Tai,1 Yen-Chih Lai,1,2 Jian-HeYang,1 Hsin-Chia Ho,2 Hsiao-Fang Wang,1 Rong-Ming Ho,1 De-Hao Tsai,1,* 1 2

Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, Republic of China. Center for Measurement Standards, Industrial Technology Research Institute, Hsinchu, Taiwan, Republic of China

ABSTRACT We report a high-resolution, traceable method to quantify number concentrations and dimensional properties of nanosheet graphene oxide (N-GO) colloids using electrospray-differential mobility analysis (ES-DMA). Transmission electron microscopy (TEM) was employed orthogonally to provide complementary data and imagery of N-GOs. Results show that the equivalent mobility sizes, size distributions, and number concentrations of N-GOs were able to be successfully measured by ESDMA. Colloidal stability and filtration efficiency of N-GOs were shown to be effectively characterized based on the change of size distributions and number concentrations. Through the use of an analytical model, the DMA data were able to be converted into lateral size distributions, showing the average lateral size of N-GOs was ≈32 nm with an estimated thickness ≈0.8 nm. This prototype study demonstrates the proof of concept of using ES-DMA to quantitatively characterize N-GOs and provide traceability for applications involving the formulation of N-GOs.

1.

Introduction Graphene oxide (GO), a two-dimensional assembly of hydroxyls, epoxides, carbonyls, and aromatic rings, has been recognized as an attractive vector to the fields of nanotechnology and bionanotechnology.1-8 Because GOs are welldispersed in water and in many polar organic solvents, they hold a great potential to be used for a variety of applications, for example, as emulsifiers to form polymer-based composites and inorganic nanoparticle-graphene hybrids.8 Moreover, GOs have been employed as the precursors to synthesize graphenes and reduced GOs through a controllable oxidation-reduction process.2,7-12 The desired material properties, including the specific optical and interfacial properties, high mechanical strengths, and high electrical and thermal conductivities, could be used for enabling a range of promising applications in the fields of electronics, surface coatings, energy storage, antibacterial agent, and biomedicine.1,3-6,8,9,12,13 Nanoscale GO sheets (N-GOs; defining the lateral size usually ≈100 nm or less), especially in the form of suspensions, have attracted a substantial interest based on their superior material properties.13 One benefit is that N-GOs are easily formulated into their nanomaterial-manufactured products (NMPs) to provide the functionalities described previously. In addition, N-GO can be prepared in a large quantity and distributed rapidly onto any surface.9 In comparison to the largesized GO sheets, N-GOs have similar spectroscopic characteristics and chemical properties, but are more attractive to be used in solution phase due to their excellent physico-chemical properties at the nanoscale.3,4 For example, N-GOs can be used in cellular imaging and also a variety of drug delivery (e.g., hydrophobic drugs, single strand DNAs) that requires superior transport properties to across the biological barrier

and sufficiently high colloidal stability in formulation or in biological culture media.3 Even though N-GO has been identified as a promising material for many applications described previously, an obvious capability gap for the use of N-GOs is the traceability to their material properties.4 To successfully implement the N-GObased applications, the key properties of N-GOs, including dimensional properties and number concentrations in solution, have to be presented precisely and accurately. On that basis, correlations between these material properties in designs can be meaningful to their corresponding performance.5,14 However, serious questions have been raised about the quality of NGOs, where the distributions in their physical dimensions (i.e., lateral size and thickness) are usually less than uniform, resulting in an issue of the lot-to-lot variations on those material properties that govern the performance of N-GOs. The increasing uncertainties in material properties may lead to confusion in their corresponding efficacy and the impact in environmental health and safety. The concerns related to the lack of traceability could have the ultimate effect of considerably delaying the implementation of promising N-GO-based products (i.e., from the prospect of regulatory agencies).4 Hence an urgent need is to have a suitable characterization approach to carefully and rationally analyze the material properties of NGOs with accuracy. Microscopic analysis, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), and scanning probe microscopy (SPM), are considered the most important and widely-utilized approaches for characterizing the physical dimensions of GOs. SPM has been used to measure the thickness of graphene and GO, showing the thickness of well-dispersed, monolayer GOs was around 0.7 nm to 1.1 nm.5,9,10,15 TEM and SEM have been employed for analyzing

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the lateral size and the conformation of graphene and GO.8,9,11,12,15 Although visual imagery is a necessary component for the characterization, the microscopic analysis is usually time-consuming (i.e., the lack of efficiency) and typically only analyzes a relatively small fraction of the total sample (i.e., issue of local imaging). The result may be prone to bias due to heterogeneity in the spatial distribution and also the visibility of the N-GOs on the substrate. For example, the lateral size or the thickness of GO sheet shall be in a sufficiently high value in order to be monitored and counted by SPM and SEM, an obvious challenge for the characterization of N-GOs. In the present study, we propose to characterize N-GOs using electrospray-differential mobility analysis (ES-DMA). ESDMA possesses several advantages in comparison to the microscopic methods including: (1) speed of a single measurement, (2) high resolution (≈0.3 nm in diameter for a spherical particle; the resolution was defined as one standard deviation of replicate measurements), (3) direct probing of multi-modal, number-based size distributions with a good statistical accuracy.16-23 Our objective is to develop a ES-DMA-based approach to improve the efficiency and to resolve the issue of local imaging typically occurred in microscopic analysis. In principal, N-GOs are first aerosolized through ES ionization, and then classified in the DMA (see Figure 1a). Essentially, DMA selects N-GOs with a narrow distribution based on their electrical mobility, and an entire distribution can be obtained by sweeping the voltage applied to the DMA and counting the number concentration of N-GOs with the corresponding electrical mobility. ES-DMA has been used extensively to analyze a variety of symmetric (e.g., Au and Ag nanoparticles) 16,19,20,22 and asymmetric nanomaterials (e.g., carbon nanotubes, antibody proteins).18,24-26 However, a systematic study of using ES-DMA to quantify two-dimensional nanosheets, for example, the colloids of graphene or GO, has not been successfully studied yet. Our objective is to provide a generic way to obtain the number concentrations and also the dimensional properties of N-GO suspensions. TEM was employed orthogonally to provide complementary data and imagery of N-GOs. Combining the information, we can develop an ES-DMA-based method to obtain (1) the number concentration of N-GOs in solution, (2) the lateral size and size distribution of N-GOs (i.e., assuming a square, flat plate, with an uniform thickness, δGO) from the equivalent mobility size distribution measured by ES-DMA (shown in Figure 1b). We hope to provide the traceability for N-GOs prior to formulation with other functional materials (e.g., polymers, hydrophobic drug ligands). 2. Experimental 2.1. Materials N-GO suspensions were prepared by dispersing the GO powders (Bio-Cando Biotechnology Inc., Taoyuan, Taiwan, R.O.C) in de-ionized (DI) water directly. Samples were sonicated for 6 hours using a ultrasonic processor (UP-800, EChrom Tech., Taipei, Taiwan, R.O.C) at a power of 90 % of maximum 800W, and then ultrafiltrated through a 1-µm filter paper (Advantec Inc., Tokyo, Japan). Aqueous ammonium acetate (Sigma-Aldrich, >98%, U.S.A) solution was used to adjust the ionic strength of samples for electrospray ionization. Biological grade 18.2 MΩ•cm DI water (Millipore, Billerica, MA, U.S.A) was used to prepare solutions and N-GO suspensions. Results show the prepared N-GOs are highly dispersible

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in water with a zeta potential ≈ -60 mV at a pH of 4.5. The original weight concentration of the original N-GO suspension (denoted as N-GO-1) was measured ≈0.005 wt%.

Figure 1. (a) Schematic diagram of characterizing N-GOs using ES-DMA. (b) Description of N-GO dimensions in terms of the modeling lateral size (Lt, assuming a square, flat plate) and thickness (δGO), and the correlation to the measured mobility diameter, dp,m, by DMA (assuming a sphere).

2.2. Electrospray-differential mobility analysis The electrospray aerosol generator (model 3480, TSI Inc., Shoreview, MN, U.S.A) was used to aerosolize GO colloids using a differential pressure of 3.7 psi to move a liquid dispersion (≈432 nL/min) through a fused silica capillary (40 µm inner diameter), after which the droplets were sprayed under a direct circuit (DC) electric field (≈2.8 kV) into a stream of dry air (1.2 L/min).17,19,27 The aerosolized nanoparticles (NPs) generated from the ES process were immediately chargeneutralized by a Po210 radioactive source following a Boltzmann equilibrium charge distribution (i.e., size-dependent charge efficiency; see Supporting Information, SI).28 Then those NPs containing multiple charges (i.e., including positive, neutral,- and negative NPs) were delivered to an electrostatic classifier (Model 3081, TSI Inc.), where the particles were classified based on their electrical mobility under an applied DC electric field, with a sheath flow of air carrying NPs downstream. As the voltage applied to the classifier was varied, particles of a specific mobility size exited the DMA and were counted by the condensation particle counter (CPC, model 3775, TSI Inc.). A high-voltage power supply (YouShang Technical Corp., Taiwan, R.O.C) was used to apply voltage of maximum -10 kV to the DMA. Based on the size range versus the charge efficiency (i.e., using Boltzmann equilibrium charge distribution. See the Section 7 in SI)28 and the polarity of electric field, most of the N-GOs measured by ESDMA contain +1 net charge. Assuming a spherical geometry, Eq. 1 demonstrates the correlation of the mobility diameter (dp,m) versus the applied DC voltage to the DMA (VDMA):18,28 V C d p,m = K1 DMA c µQsh (1).

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Analytical Chemistry

Here Cc is the slip correction factor, µ is the viscosity of carrier gas, and K1 is the instrumental parameter [details of K1 were described in SI]. Sheath flow rate (Qsh) and sample flow rate of the DMA were controlled at 5.0 L/min and 1.2 L/min, respectively. The step size used in the mobility size measurements (i.e., assuming for spherical particles) was 0.5 nm or 1.0 nm , and the time interval between each step size was 10 s. Nominally 30 nm polystyrene latex nanosphere (PSL, Thermo Scientific, Madison, WI, U.S.A.) was used as an external calibration standard for the mobility size and the concentration measurements. The number concentration of PSL in solution, Np,l,PS, was calculated as ≈ 6.7×1014 cm-3 based on the vendor’s information. Note that the charge state in solution (e.g., affecting by the pH) does not interfere the measured electrophoretic mobility by ES-DMA over a variety of aerosolized nanomaterials (more information in SI).17-19,21-22,27,29,30

respectively. Results of Fig. 2 show that N-GOs with a larger mobility size exhibited a higher Ap, and the TEM results also confirm that N-GOs were able to be effectively classified and characterized by the applied electric field in DMA. Here TEM was used only for an orthogonal comparison and a confirmation to the accuracy of ES-DMA measurements at the early stage of method development (i.e., to ensure the morphology, the accuracy of mobility classification, and the analytical model developed). Even though not necessary to be included for the future routine use with ES-DMA, TEM can be still be served complementarily to resolve some excursions potentially occurring in the N-GOs manufactured products.

2.3. Transmission electron microscopy The morphology of N-GO samples were imaged using a JEM-2100HT (JEOL, Tokyo, Japan) at an acceleration voltage of 200 kV. As shown in Fig. 1a (Path 2), positively-charged aerosolized N-GOs were delivered to a home-made electrostatic precipitator and deposited onto a TEM copper grid with carbon film (SPI Supplies, West Chester, PA, U.S.A) operated at a sample flow rate of 1.2 L/min and −5 kV/cm electric field. The projected area of N-GO, Ap, measured by TEM was determined by counting the number of grids with a known area occupied by the individual N-GO (details were shown in the SI). 3. Results and Discussion 3.1. Mobility size distribution of N-GOs Figure 2a shows the number density of N-GO suspension directly recorded by CPC versus the mobility diameter. The first peak located at 6-7 nm is associated with the non-volatile salt residual nanoparticles (S-NPs) from droplets that contain only the dissolved phase species (i.e., without the functional NPs).29,30 The second peak centered at ≈33 nm with a broad size distribution represented N-GOs. The high polydispersity (the full-width-of half-maximum ≈30 nm) should be attributed to the nature of aerosol-based synthesis process (i.e., Hummer’s approach),10,11 and the improvement of monodispersity is beyond the scope of our study. Results show that we can successfully measure the full mobility size distributions of NGO suspensions using ES-DMA. Orthogonally, we use TEM to visualize the electrosprayed N-GOs with different dp,m. Figure 2b-2d presents TEM images of electrosprayed N-GOs captured in situ (additional images available in the SI). Without size classification (Fig. 2b), it is clear that both N-GOs and S-NPs (≈10 nm, spherical) were homogeneously distributed on the grids during the electrostatic deposition.29,30 The deposited N-GOs were folded into various morphology with a projected surface area, Ap, ranging from ≈(120-3000) nm2 (details of analyzing Ap were shown in SI). After size selection at dp.m=20 nm (Figure 2c), we observed that the 20-nm, size-classified N-GOs have a low electron contrast in the image, and the Ap of N-GOs was around ≈597 nm2. By increasing the selected dp.m to 33 nm (Figure 2d) and 57 nm, the electron contrast of N-GOs was shown to be increased under TEM analysis, and the Ap of the size-classified N-GOs were also increased to be ≈1166 nm2 and 2760 nm2,

Figure 2. Analysis of N-GO suspension with different mobility sizes. Sample: N-GO-1 (t0). (a) Mobility size distribution measured by ES-DMA; the y-axis represented the number density directly recorded by CPC. (b) Representative TEM image of the electrosprayed N-GOs without size classification. The scale bar was 50 nm. (c) Representative TEM image of size-selected NGOs. dp,m=20 nm. The scale bar was 30 nm. (c) Representative TEM image of size-selected N-GOs. dp,m=33 nm. The scale bar was 50 nm. (d) Representative TEM image of size-selected NGOs. dp,m=57 nm. The scale bar was 50 nm.

3.2. Number Concentration, Effect of Filtration, and Colloidal Stability of N-GOs ES-DMA is used to develop a quantitative approach for measuring the number concentration in solution, Np,l, a key factor correlated to the performance of N-GO based NMPs. Since the N-GOs we measured represent only a small fraction of the population sampled by ES-DMA, Np,l can be calculated by considering the charge distribution, the transfer function of DMA, and the measured number density in the gas phase.16,19,21,23 Figure 3a shows mobility size distributions of three different samples of N-GOs by considering all of the

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Analytical Chemistry necessary corrections factors:16,21,23,29,30 N-GO-1 (t0) represented the original sample measured immediately after the sample was prepared; N-GO-1 (t2) was the original sample measured after two-month storage; N-GO-0.2 (t0) was the sample prepared by further filtering N-GO-1 (t0) using a 0.2-µm syringe filter. For clarity, we only showed two representative concentration ratios, Cp (=Np,l/Np,l,0), in Figure 3a and deferred other DMA results to the SI. Here Np,l,0 was defined as the concentration of the originally prepared sample (i.e., before diluting into different values of Cp) varied by mixing the originally prepared samples with different ratios of the 4 mmol/L ammonium acetate aqueous solution. As shown in Fig. 3a, we observed full-width at half-maximum (FWHM) of the mobility size distributions were relatively unchanged by increasing Cp, and the maximum peak intensity were increased proportionally with Cp. Quantitatively, Np,l can be calculated using Eq. 2,16,22,23 N p ,l = N p , g ×

N p ,l , PS

(2),

N p , g , PS

where Np,g and Np,g,PS are the number populations of NGOs and PSL in the gas phase, respectively (details of the calculation of Np,g and Np,g,PS are provided in the SI).16,22,23

a

uf-N-GO (t=0 month), N-GO-1 (t0), Cp=0.05 Cp=0.05

4000

dNp /ddp (cm-3*nm-1)

uf-N-GO (t=2 months), N-GO-1 (t2), Cp=0.05 Cp=0.05 N-GO-200 (t= 0 month), N-GO-0.2 (t0), Cp=0.05 Cp=0.05

3000

uf-N-GO (t=0 month), N-GO-1 (t0), Cp=0.01 Cp=0.01 uf-N-GO (t=2 months), N-GO-1 (t2), Cp=0.01 Cp=0.01 N-GO-200 (t= 0 month), N-GO-0.2 (t0), Cp=0.01 Cp=0.01

2000

1000

Fig. 3b summarizes Np,l versus various Cp calculated by Eq. 2, showing linear response for all three samples (R2>0.98). From the slope, Np,l were determined as 2.4×1014 cm-3, 1.9 ×1014 cm-3, and 1.8 ×1014 cm-3 for N-GO-1 (t0), N-GO-1 (t2), and N-GO-0.2 (t0), respectively. In combination with the data of mobility size distributions, we can distinguish the colloidal stability of N-GOs and their filtration efficiency. At t=0 month, we observed that the FWHM were also shown to be constant after filtration through a 0.2 µm syringe filter, confirming with the TEM imagery that the lateral size of N-GOs were mostly less than 200 nm. A 21 % decrease of Np,l was observed, possibly attributed to the diffusion loss during the penetration and was independent of the lateral size.28 From the Fig. 3a, we observed the change of FWHM of N-GO-1 after 2month storage. The increase of number-averaged dp,m (dp,m,avg),19,23,30 accompanied with a decrease of Np,l (≈25 %), indicates that some extent of aggregation occurred (i.e., individual N-GOs were combined into large aggregates). Results confirm that we were able to monitor the change of Np,l quantitatively over different types of N-GO samples using ES-DMA. 3.3. Lateral Size of N-GOs Due to the two-dimensional, nanosheet-type morphology, lateral size and thickness of N-GOs are the most important dimensional properties affecting their functionalities.4 Here we employ an analytical model to develop a correlation of dp,m measured by DMA, versus the lateral size (Lt) and the thickness (δGO) of N-GOs. For simplicity, we assume that (1) Ap is equal to Lt2 (see Fig. 1b), (2) Ap is equal to the actual surface area of N-GO, (i.e., the extent of folding in the nanosheet is negligible), and (3) δGO is uniform among all N-GOs. Knowing the dp,m of N-GOs was below 100 nm, the Ap of N-GO should be approximately proportional to dp,m2.18,23,25,29 Then Lt can be described as a function of dp.m: 2

Lt = [ K f 1 × d p ,m ]0.5

0 12

22

32

42

52

62

72

b

K f1 =

30

N-GO-1 (t0)

25

N-GO-0.2 (t0)

y = 194.82x R² = 0.9794

y = 238.63x R² = 0.9952

20 15

N-GO-1 (t2)

10

y = 177.63x R² = 0.9995

5 0 0

0.02 0.04 0.06 0.08

(3).

Here Kf1 was a fitting constant, which can be obtained using Eq. 4:

dp,m (nm)

Np,l (x1012/cm3)

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0.1

Cp Figure 3. Number concentration of N-GO samples measured by ES-DMA. Samples: N-GO-1 (t0), N-GO-1 (t2), and N-GO-0.2 (t0). (a) Mobility size distributions of N-GOs after considering the corrections of charge ratios, the encrusted layer of dried solvent remnant, and the transfer function of DMA. (b) Np,l versus Cp. Lines are the guide to eyes.

2 vGO,avg Lt wt = = 2 2 2 d p ,m d p ,m,avg × δ GO ρ GO × d p ,m,avg × δ GO

(4). Here vGO,avg is the average volume of a N-GO (=wt/ρGO). wt is the average mass of N-GO, determined by dividing the total mass with Np.l. ρGO is the density of N-GO. In this study we choose to use ρGO=0.3 g/cm3 based on the reported values in literatures and also the accuracy in data-fitting (details were shown in SI).31 Using the data of N-GO-1 (t0), we obtained Kf1=(6.67*10-8)/δGO (i.e., the unit of δGO was in cm), which can be used for the calculation of Lt under a constant δGO. Details of the derivations, including the physical bases of Eq. 3 and Eq. 4, were shown in the Section 8 of SI. Figure 4a shows the lateral size distributions of N-GO-1 (t0) as a function of δGO, after data conversion using Eq. 3. Similar to the number-based mobility size distributions shown in Fig. 3a, we could see that the Lt was ranging from 10 nm to be 70 nm, and the Lt-distribution was shifted toward a larger value by choosing a smaller δGO. Orthogonally, TEM was employed to provide the imagery information of Lt. Figure 4b shows the histogram of Lt analyzed from the TEM images

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(i.e., S-NPs were excluded in the counting), showing that the Lt was also distributed from 10 nm to 70 nm with an average value of ≈32 nm. The TEM data were consistent with the fitted DMA results assuming δGO=0.8 nm (the number-averaged Lt was 31.7 nm, with a similar FWHM). In addition, the chosen δGO (=0.8 nm) was also close to the measured value of monolayer GOs synthesized using identical processes in this previous study (≈0.8 nm – 1.1 nm).10 Results indicate that we can use ES-DMA obtain the lateral size distribution and to confirm the thickness of the N-GO measured by microscopic methods. Note that the detection limit of Lt was ≈10 nm for both DMA and TEM analysis in this study (i.e., to avoid the interferences from S-NPs).

a dNp/ddp (cm-3*nm-1)

δGOnm = 0.6 nm 0.6 600 0.8 δGOnm = 0.8 nm 1.0 δGOnm = 1.0 nm 400

δGOnm = 2.0 nm 2.0

200

0 10

20

30

40

50

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70

c

50

d

40 30 20 10 0

15 17.5 20 22.525 27.530 32.535 37.540

Number Ratio (%)

Lt (nm)

Number Ratio (%)

40

30

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10

0

15

25

35

4547.5 52.5 65 5557.5 62.5 17.5 22.5 27.5 32.5 37.5 42.5

Lt (nm)

e

Number Ratio (%)

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Analytical Chemistry

Lt (nm) 40

30

20

10

0 35

45

55

65

75

85

95

105

Lt (nm)

Figure 4. Lateral size distributions of N-GOs. Sample: N-GO-1 (t0) at Cp=0.01. (a) DMA results derived from Figure 3a, assuming four different δGO ranging from 0.6 nm to 2.0 nm. (b) Histogram of Lt measured by TEM without size selection. Images with grids were used to represent N-GO having different Lt in the his-

togram. Number of particle counts: 91. (c) Histogram of Lt measured by TEM. dp,m=20 nm. Number of particle counts: 34. (d) Histogram of Lt measured by TEM. dp,m=33 nm. Number of particle counts: 44. (e) Histogram of Lt measured by TEM. dp,m=57 nm. Number of particle counts: 27.

Furthermore, we investigate the lateral size distributions of the mobility-selected N-GOs using a TEM analysis. Figure 4c4e show the lateral size distributions for N-GOs with three different dp,m. As seen, we can successfully classify Lt of NGOs based on the chosen dp,m, where N-GOs having the same dp,m were approximately in the same range of Lt (i.e, close to normal distribution). The FWHM of Lt-distributions of these mobility-selected N-GOs were shown to be decreased in comparison to the result without size classification (Fig. 4b; FWHM≈25 nm). For dp,m=20 nm (Fig. 4c), the average Lt was 24.6 nm with a FWHM ≈ 6 nm; for dp,m=33 nm, the average Lt was 31.5 nm with a FWHM ≈ 12 nm; for dp,m=57 nm, the average Lt was 54.4 nm with a FWHM ≈ 20 nm. By increasing the selected dp,m, the FWHM was shown to be increased. The peak broadening effect may be attributed to the increase of complexity in the conformation of selected N-GOs. TEM imagery (Fig. S2) confirms it, showing that N-GOs with a similar mobility size, different conformations can be selected by DMA simultaneously. Since the variations in conformation could result in a peak broadening in the mobility spectrum, future study we will seek to improve the monodispersity of NGOs by increasing the resolution of mobility selection (i.e., by increasing the sheath flow rate and the corresponding electric field). 3.4. Advantages and Limitations in Data Analysis Even though ES-DMA has shown to be an effective tool to characterize dp,m, which can correlate to Lt using the developed model, an obvious challenge to the accuracy of Lt is about the uncertainties of the chosen parameters used in the calculation (e.g., δGO and ρGO). Knowing that a broad range of the reported values were found in the literatures for different types of applications,31-35 the variation in ρGO may affect vGO,avg and the following Lt. The calculated Lt and FWHM will be decreased if choosing a larger value of ρGO (details are shown in Section 6 of SI). On the other hand, the variation and uniformity in δGO of N-GOs also affect the accuracy of Lt determined from the measured dp,m. Once δGO is increased, the calculated Lt shall be decreased with a decrease in FWHM. Since the Lt-distribution is dependent on δGO and ρGO simultaneously, we are unable to identify the best fit of Lt-distribution when ρGO was ranging from 0.22 g/cm3 to 0.34 g/cm3 (i.e., in corresponding to δGO ranging from 0.7 nm to 1.1 nm;10 details of data comparisons are shown in SI). TEM analysis provides complementary imagery regarding to the range of Lt. However, the measured value of Lt as a dried aerosol deposit may be different to the actual lateral size presenting in solution phase. Distinguishing an accurate Lt of N-GO from the imagery, as discussed previously, may be a challenge because of its low visual resolution. Though several uncertainties were encountered in measuring the dimensional properties by ES-DMA, the methodology proposed in this study shows a reasonable measured value in

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Lt with high efficiency and statistical accuracy, especially for the monolayer N-GOs. For N-GOs having Lt>>δGO, the electrical mobility of N-GOs is mainly determined by Lt. Hence uncertainties arouse by the variations in δGO and ρGO, to the measured mobility size and the mobility size distribution can be negligible. Besides, ES-DMA has shown to be an effective tool to obtain a full mobility size distribution with a statistical accuracy (i.e., more than 104 N-GOs were counted within a 1hr measurement), at least 10 times faster in comparison to traditional SPM and TEM analysis. 4. Conclusions We have demonstrated that N-GOs can be effectively characterized using ES-DMA. Equivalent mobility size, size distributions, and number concentrations of N-GOs in solution can be successfully measured by ES-DMA, which can be useful in monitoring and analyzing colloidal stability and filtration efficiency of N-GOs samples. Lateral size distributions of the monolayer N-GOs can be obtained by converting the DMA data using an analytical model developed in this study. Orthogonally, TEM imagery supports the data analysis of ESDMA. The experimental results presented here demonstrate proof of concept for characterizing N-GO suspensions and providing the traceability to the N-GO-based NMPs. The primary advantage of ES-DMA is to provide a rapid and direct read-out of dimensional distributions, suggesting that ESDMA is useful for the study of aqueous suspensions of other types of graphene-based nanomaterials. Future work will seek to investigate the colloidal stability of N-GOs and graphene suspensions during the formulation processes, and also study their surface interactions with other types of functional nanoparticles.

ASSOCIATED CONTENT Supporting Information Supporting Information available: additional TEM images and particle size distributions of N-GOs, charge distribution, details of the instrumentation parameters, derivations of Lt, analysis of NGOs using SPM, and the calculation of number concentrations of the N-GOs. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Tel: 886-3-516-9316

ACKNOWLEDGMENT The authors thank Ministry of Science and Technology of the Republic of China (Taiwan) for the financial support to this research under Contract no. Grant NSC102-2218-E-007-015-MY2 and Grant NSC102-2633-M-007-002.

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