Long-Term Functional Stability of Functional Nucleic Acid-Gold Nanoparticle Conjugates with Different Secondary Structures

Thiolated functional nucleic acid-gold nanoparticle conjugates (FNA-AuNPs) are the core recognition elements in biosensors. The long-term functional stability (LTFS) is critical for their practical applications and, however, has been overlooked. Here we report on the huge effects of multiple experimental factors on LTFS, including spacer- and buffer-composition, secondary structures of FNAs, and surface blocking. We quantitatively determined these effects by measuring the relative hybridization capacity (RHC, the relative amount of complementary DNA hybridized with the same amount of conjugates) for linear DNA-AuNP or the relative signal change generated by their function (RSC-F) for molecular beacon (MB) and G-quadruplex (G4)-AuNPs. There is a positive relationship between the spacer affinity [oligoadenine (A₁₀) > oligothymine (T₁₀) > oligoethlyene glycol (EG₁₈)] of the linear DNA probes and the LTFS. The LTFS of linear DNA-AuNP in phosphate buffer (PB) was much better than that in Good's buffers such as HEPES, Tris, and MES. The secondary structure of FNAs also strongly impacted the LTFS, showing the substantially decreased LTFS from G4- to linear DNA- to MB-AuNPs, where EG₁₈ spacer was used for all these conjugates. The surface blocking of FNA-AuNPs greatly improved the LTFS. We experimentally determined that the LTFS of FNA-AuNPs was directly related to the dissociation of DNAs caused by the in situ generated H₂O₂ due to the oxidase activity of AuNP and thereby oxidation of Au-thiol bonds. The oxidase activity of AuNP was favored at high temperature, low pH, high AuNP concentration, high Good's buffer concentration, and high salt concentration, corresponding well with the positive effects of high affinity spacer, PB, and surface blocking on the LTFS of FNA-AuNPs. Our study has implications on both fundamental surface science and practical applications.

Dipyrrin complex assisted in situ synthesis of ultra-small gold nanoparticles decorated on a partially reduced graphene oxide nanocomposite for efficient catalytic reduction of Cr(vi) to Cr(iii)

Applicability of a novel heteroleptic dipyrrin complex exhibiting dual functionality (reducing and capping agent) in rapid synthesis of small sized AuNPs has been described.

Toxic effects of gold nanoparticles on Salmonella typhimurium bacteria

Nanometer-sized gold, due to its beautiful and bountiful color and unique optical properties, is a versatile material for many industrial and societal applications. We have studied the effect of gold nanoparticles on Salmonella typhimurium strain TA 102. The gold nanoparticles in solution prepared using the citrate reduction method is found not to be toxic or mutagenic but photomutagenic to the bacteria; however, careful control experiments indicate that the photomutagenicity is due to the co-existing citrate and Au³⁺ ions, not due to the gold nanoparticle itself. Au³⁺ is also found to be photomutagenic to the bacteria at concentrations lower than 1 µM, but toxic at higher concentrations. The toxicity of Au³⁺ is enhanced by light irradiation. The photomutagenicity of both citrate and Au³⁺ is likely due to the formation of free radicals, as a result of light-induced citrate decarboxylation or Au³⁺ oxidation of co-existing molecules. Both processes can generate free radicals that may cause DNA damage and mutation. Studies of the interaction of gold nanoparticles with the bacteria indicate that gold nanoparticles can be absorbed onto the bacteria surface but not able to penetrate the bacteria wall to enter the bacteria.