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Phosphonic acid capped SnO2 nanoparticles with diameters less than 5 nm were synthesized and characterized with multinuclear solution and solid-state magic angle spinning (MAS) NMR. Two types of phosphonic acid ligands were used to derivatize the SnO2 surface, producing (i) water soluble SnO2 nanoparticles capped with 2-carboxyethanephosphonic acid (CEPA) and (ii) insoluble SnO2 nanoparticles capped with phenylphosphonic acid (PPA). Multiple surface environments were observed with 31P solution and solid-state MAS NMR for both capping agents. The 31P resonances of derivatized SnO2 nanoparticles display isotropic chemical shifts that are more shielded compared to the native phosphonic acids. This observation is indicative of a strong interaction between the phosphonic acid group and the SnO2 surface. 1H MAS NMR spectra display a complete absence of the acidic protons of the phosphonic acid groups, strongly supporting the formation of P−O−Sn linkages. 1H → 31P cross polarization (CP) build-up behavior confirms the absence of the vast majority of phosphonic acid protons. Some of the build-up curves displayed oscillations that could be fit to extract the magnitude of the 1H−31P dipolar coupling constant. The dipolar coupling can then be used to calculate the distance between phosphorus and the close proximity protons. The results presented herein indicate primarily bi- and tridentate phosphonic acid bonding configuration at the SnO2 surface, in both CEPA and PPA capped nanoparticles.

An ultraviolet-visible (UV-Vis) absorption spectrum was collected on a solution of PPh 3 capped 1.8 nm AuNPs dissolved in CH 2 Cl 2 (0.5 mg/ml) using an Ocean Optics Instrument. A UV-Vis spectrum is shown in figure S1. The amplitude and position of the plasmon band can be used as a guideline to predict the nanoparticle core size. UV-Vis absorption spectrum shown in figure S1 does not show any significant plasmon resonance around 520 nm, which indicates that the majority of the sampled nanoparticles are < 2.0 nm in diameter. 1 This result is consistent with statistical data derived from TEM images of the nanoparticles (Figure 1). TGA Thermal gravimetric analysis (TGA) of the nanoparticles was performed (SETARAM Instrument) to measure the mass associated with surface bound PPh 3 and gold core, respectively. The temperature was raised from room temperature to reach 600 o C at a rate of 10 o C/min under a helium atmosphere. At the end of the experiment, the remaining gold from the sample of nanoparticles was recovered from the alumina sample holder. The TGA data reported in figure S2 shows that the surface S1. UV-Vis absorption spectrum of PPh 3 capped gold nanoparticles dissolved in CH 2 Cl 2 .S2. TGA analysis of PPh 3 capped 1.8 nm AuNPs. A loss of 23% mass occurred when the sample was heated > 400 o C.