Quantification of ligand packing density on gold nanoparticles using ICP-OES

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1 Analytical and Bioanalytical Chemistry Electronic Supplementary Material Quantification of ligand packing density on gold nanoparticles using ICP-OES Sherrie Elzey, De-Hao Tsai, Savelas A. Rabb, Lee L. Yu, Michael R. Winchester, Vincent A. Hackley 1

2 Material concentrations The stock concentrations of commercially available gold nanoparticle suspensions, reference materials (RMs), and standard reference materials (SRMs) are given in Table S1. Table S1 Material concentrations Material 1 Source Lot number Concentration Gold nanoparticles, 5 nm Ted Pella µg g 1 Gold nanoparticles, 10 nm Ted Pella µg g 1 Gold nanoparticles, 30 nm Ted Pella µg g 1 Gold nanoparticles, 60 nm Ted Pella µg g 1 Gold nanoparticles, 100 nm Ted Pella µg g 1 Gold nanoparticles, 20 nm BioAssay Works D30H µg g 1 Gold nanoparticle RM 8011, 10 nm NIST N/A µg g 1 Gold nanoparticle RM 8013, 60 nm NIST N/A µg g 1 Gold SRM 3121 NIST mg g 1 Sulfur SRM 3154 NIST mg g 1 Particle size measurements using electrospray differential mobility analysis (ES-DMA) Particle mobility diameters (D P ) were measured using an electrospray (3480, TSI, Inc., Shoreview, MN) coupled with a differential mobility analyzer (DMA; 3085, TSI, Inc.) and an ultrafine condensation particle counter (CPC; 3775, TSI, Inc.). Details of the ES-DMA system have been described previously [1, 2]. Table S2 gives the operating conditions used for the measurements of nominally 5 nm, 10 nm, 30 nm, 60 nm, and 100 nm particles. AuNPs (1 ml) were centrifuged, the supernatant was removed, and approximately (20 50) µl of concentrated AuNPs were mixed with 200 µl of 4 mmol L 1 ammonium acetate. Samples denoted as 2, 3, and 4 were further diluted using ammonium acetate to resolve the monomer, dimer, and salt residue peaks. The particle size distributions obtained from the ES-DMA are shown in Fig. S1. The mean particle diameters (D P ) given in Table S2 were calculated by averaging the diameters at the monomer peak maxima for three measurements after correction of the raw data to remove the effect of salt [2]. The uncertainties associated with the D P values represent the step size of the measurements, because the standard deviation of three measurements was less than the step size. 1 The identification of any commercial product or trade name does not imply endorsement or recommendation by the National Institute of Standards and Technology. 2

3 Table S2 Operating conditions of the ES-DMA 5 nm 10 nm 30 nm 60 nm 100 nm Aerosol flow (L min 1 ) Sheath flow (L min 1 ) Capillary tip (µm) Mean D P (nm)

4 Fig. S1 Particle size distributions, shown as raw count versus mobility diameter, for nominally (a) 5 nm, (b) 10 nm, (c) 30 nm, (d) 60 nm, and (e) 100 nm AuNPs a b 4

5 c d 5

6 6 e

7 ICP-OES detection of sulfur in 3-mercaptopropionic acid (MPA) MPA concentration was accurately detected based on sulfur content. MPA solutions were prepared at known concentrations, and the MPA concentration was measured using ICP-OES by detecting the S emission at nm (Fig. S2). Fig. S2 Sulfur mass fraction detected using ICP-OES for MPA solutions prepared at sulfur mass fractions relevant to this work ( ng g 1 ). 7

8 Packing density calculations Packing density was calculated based on the ratio of the mass fractions of S and Au measured simultaneously by ICP-OES, under the assumption that there are no element-specific recovery effects, and was therefore independent of recovery. Assuming complete removal of free MPA through centrifugation during sample preparation, the packing density calc is defined as n nmpa SA, (S1) calc AuNP AuNP where n MPA and n AuNP are the number densities of MPA ligands and AuNPs, respectively, in the sample suspension prepared for ICP-OES analysis, and SA AuNP is the surface area per AuNP. Given that the MPA ligand contains one S atom, the value of n MPA is n MPA w S sample, (S2) M where w S is the S mass fraction determined using ICP-OES, sample is the density of the sample suspension, N A is the Avogadro constant, and M(S) is the molar mass of S. Considering that the volume and surface area of a sphere are d 3 /6 and d 2, respectively, where d is the sphere diameter, and assuming that the AuNPs are spherical, the value of n AuNP SA AuNP is n AuNP SA AuNP S N A 6wAu d sample AuNP, (S3) where w Au is the Au mass fraction determined using ICP-OES and AuNP is the density of a AuNP. Substituting Eqs. S2 and S3 into Eq. S1 and rearranging yields ws N Ad AuNP, (S4) calc wau 6M ( S) which is the analytical equation for estimating calc. In applying Eq. S4, we assume that AuNP is equivalent to the density of bulk Au (19.3 g cm 3 ). The values of d for the various AuNPs tested in this work were obtained from the ES-DMA measurements. Note that Eq. S4 shows explicitly that only the ratio of the S and Au mass fraction values determined using ICP-OES is relevant. As an example calculation, consider the suspension of MPA-conjugated AuNPs, nominally 10 nm in diameter, with S and Au mass fractions of 97.2 ng g 1 and 8.3 µg g 1, respectively, determined using ICP-OES. The following values can be used in Eq. S4: 8

9 w S = 97.2 ng g 1 ( µg g 1 ) w Au = 8.3 µg g 1 N A = MPA mol 1 d = 11.1 nm AuNP = g nm 3 M(S) = 32 g mol 1 Then, Eq. S4 is µg g MPA mol 11.1nm g nm = 7.87 MPA nm 2 calc µg g 632 g mol References 1. Tsai, D.-H., F. W. DelRio, et al. (2010) Competitive adsorption of thiolated polyethylene glycol and mercaptopropionic acid on gold nanoparticles measured by physical characterization methods. Langmuir 26(12): Tsai, D. H., R. A. Zangmeister, et al. (2008). Gas-phase ion-mobility characterization of SAM-functionalized au nanoparticles. Langmuir 24(16): Allabashi, R., et al. (2009) ICP-MS: a powerful technique for quantitative determination of gold nanoparticles without previous dissolving. J Nanopart Res 11(8):

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