J. Phys. Chem. C 2008, 112, 7611–7616
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Evaluation of Hydrodynamic Size and Zeta-Potential of Surface-Modified Au Nanoparticle-DNA Conjugates via Ferguson Analysis Sunho Park† and Kimberly Hamad-Schifferli*,†,‡ Departments of Mechanical Engineering and Biological Engineering, Massachusetts Institute of Technology, 77 Massachusetts AVenue, Cambridge, Massachusetts 02139 ReceiVed: January 16, 2008; ReVised Manuscript ReceiVed: March 7, 2008
We evaluate Ferguson analysis as a method for simultaneously measuring the size and zeta-potential of gold nanoparticle-DNA (Au NP-DNA) conjugates. This approach is highly suitable when the particle size is 40 cycles) from the zeta-potentiometer were averaged. Sample 1, Au NP; sample 2, Au NP-mPEG (1:200 incubation); sample 3, Au NP-mPEG followed by DNA conjugations; sample 4, Au NPDNA; sample 5, Au NP-DNA followed by MCH modification (0.1 mM MCH, 1 min, ethyl acetate washing); sample 6, Au NP-DNA followed by mPEG modification. Error bars show 95% confidence intervals.
Figure 5. (a) Example gel. Samples of Au NP (10.9 nm) and Au NPDNA with different chemical modifications run in 1% agarose gel for 2 h in 1 × TBE under E ) 3.8V/cm. Sample 1, Au NP; sample 2, Au NP-mPEG (1:200 incubation); sample 3, Au NP-mPEG followed by DNA conjugation; sample 4, Au NP-DNA; sample 5, Au NP-DNA followed by MCH modification (0.1 mM MCH, 1 min, ethyl acetate washing); sample 6, Au NP-DNA followed by mPEG modification. [Au NP] was held at 5 × 10-7 M for both mPEG-SH and MCH reactions. (b) Effective size (Deff) of the samples in different TBE concentrations. Coverage is measured by complete displacement of conjugated DNA by incubating the particles in a 1 mM MCH bath for ∼24 h and taking the measurement of fluorescence intensity of TAMRA. Error bars show 95% confidence intervals. (c) Calculated zeta-potential (ζ) and effective charge of the same samples via eqs 5 and 6. (d) An illustration that shows the complicated conformation and charge distribution of Au NP-DNA. Salt ions bind to Au NP-DNA and alter the charge status of the conjugates.
range of the devices.35 Stable readings in this range of M0 are possible only when particles are relatively larger.36 However, the size of particles that can be evaluated in agarose gels is limited due to the pore size of the gel structure, which varies from some tens of nanometers to a few hundred nanometers depending on gel percentage.37 Ferguson analysis is more advantageous for smaller particles (40 cycles) from the zeta-potentiometer were averaged for each sample and TBE concentration, and an electric
field strength similar to that used in Ferguson analysis was chosen for consistency. In general, the data from the zetapotentiometer is much more random while Ferguson analysis data is relatively more stable, with a standard deviation of 0.2–0.6 × 10-4 cm2/V · s compared with 0.03–0.12 × 10-4 cm2/ V · s. Moreover, dependence on buffer concentration is difficult to ascertain from the zeta-potentiometer measurements. It is necessary for surface-modified Au NP and Au NP-DNA to adopt effective values in terms of size and free mobility due to their high irregularity that makes conventional devices less reliable. Band migration in gel reflects many collisions and bindings between gel fibers (or pores) and the particles over a long gel running time (∼2 h). Therefore, the measured mobility from a gel is already well-averaged in that sense. In addition, a variety of samples of interest can be run in the same gel simultaneously so that experimental conditions are highly controlled among the samples. The mobility measured from gel can be adjusted with electroendosmosis (EEO) for better accuracy,38,39 but the EEO mobility of commercial agarose gel (∼0.3 × 10-4 cm2/V · s) is an order of magnitude lower than the observed mobility of the Au NPs and Au NP-DNAs used in this work. Conclusions Ferguson analysis is a powerful method for simultaneously analyzing both size and charge of Au NP and Au NP-DNA conjugates. Ferguson analysis is much more reliable compared with that of zeta-potentiometers when the samples are small and highly modified, such as those utilized in biological applications of Au NP-DNA conjugates where the NP diameter ∼1–20 nm. Future work includes using this technique to evaluate Au NPs conjugated to more complex structures such as longer strands of DNA, hybrids, and proteins. Acknowledgment. This work was funded by the Office of Naval Research (N00014-04-1-0570). We thank Department of Materials Science and Engineering at M.I.T. for the use of their TEM (JEOL 2011) and zeta-potentiometer (Plus 90) instruments. Supporting Information Available: TEM images and size evaluation of Au NP. This material is available free of charge via the Internet at http://pubs.acs.org.
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