Application of dynamic light scattering and absorption spectroscopy to studies of systems with solloidal gold nanoparticles + DNA

Background and Objectives: The dynamic light scattering (DLS) method is widely used to evaluate the particle size distributions. However, DLS is not free of serious drawbacks. For a fast approximate estimation of the average size of colloidal gold nanoparticles (AuNPs) within the range of 15–100 nm reasonable results can be obtained with using the absorption spectroscopy. We discuss the advantages and drawbacks of DLS, transmission electron microscope (TEM), and absorption spectroscopy in gold nanoparticle sizing. In addition, we consider the application DLS and absorption spectroscopy to detection of ssDNA oligonucleotides and mismatches in their sequences with using AuNPs. The method principle is as follows: the addition of probe and target ssDNA to CTAB-coated AuNPs results in particle aggregation, whereas no aggregation occurs after addition of probe and nontarget DNA sequences. Materials and Methods: 16-nm and 60-nm AuNPs with negative charges were synthesized by the Frens method. Positively charged AuNPs were obtained by functionalization of with CTAB. As ssDNA models, we used 21-mer oligonucleotides from the human immunodeficiency virus HIV-1 and a 23-mer ssDNAs from the Bacillus anthracis genes. A Zetasizer Nano ZS instrument (Malvern, UK) was used for DLS measurements. A Libra-120 transmission electron microscope (Carl Zeiss, Jena, Germany) and a Specord BS 250 spectrophotometer (Analytik Jena, Germany) were used for TEM and spectroscopic measurements at the Simbioz Center for the Collective Use of Research Equipment in the Field of Physical-Chemical Biology and Nanobiotechnology at the IBPPM RAS. Results: For a fast estimation of the average size of AuNPs in the range of 15–100 nm, the absorption spectroscopy gives reasonable sizes derived from presented calibrations. For AuNPs with diameters in the range of 3–15 nm, the sizing calibration curve is based on the measurement of the ratio between the absorption intensities at the plasmon resonance wavelength and at 450 nm. We also have demonstrated the application of absorption spectroscopy and DLS methods to estimation of ssDNA concentration. Conclusion: The advantages and drawbacks of three methods (TEM, DLS, and absorption spectroscopy) in nanoparticle sizing have been discussed with a special attention to AuNPs. For spherical particles, the z-average DLS size of AuNPs is in a reasonable agreement with TEM data, whereas the size distribution obtained with DLS is typically much broader than that derived from TEM histograms. DLS is shown to be the only method suitable for nonperturbative and sensitive diagnostics of relatively slow aggregation processes with characteristic times about 1 min. The detection limits of absorption spectroscopy and DLS for ssDNA detection are 100 and 10 pM, respectively.