Monitoring DNA Hybridization and Thermal Dissociation at the Silica/Water Interface Using Resonantly Enhanced Second Harmonic Generation Spectroscopy

The immobilization of oligonucleotide sequences onto glass supports is central to the field of biodiagnostics and molecular biology with the widespread use of DNA microarrays. However, the influence of confinement on the behavior of DNA immobilized on silica is not well understood owing to the difficulties associated with monitoring this buried interface. Second harmonic generation (SHG) is an inherently surface specific technique making it well suited to observe processes at insulator interfaces like silica. Using a universal 3-nitropyrolle nucleotide as an SHG-active label, we monitored the hybridization rate and thermal dissociation of a 15-mer of DNA immobilized at the silica/aqueous interface. The immobilized DNA exhibits hybridization rates on the minute time scale, which is much slower than hybridization kinetics in solution but on par with hybridization behavior observed at electrochemical interfaces. In contrast, the thermal dissociation temperature of the DNA immobilized on silica is on average 12 °C lower than the analogous duplex in solution, which is more significant than that observed on other surfaces like gold. We attribute the destabilizing affect of silica to its negatively charged surface at neutral pH that repels the hybridizing complementary DNA

Specific Cation Effects on the Bimodal Acid–Base Behavior of the Silica/Water Interface

Using nonresonant second harmonic generation spectroscopy, we have monitored the change in surface charge density of the silica/water interface over a broad pH range in the presence of different alkali chlorides. Planar silica is known to possess two types of surface sites with p<i>K</i>_a values of ∼4 and ∼9, which are attributed to different solvation environments of the silanols. We report that varying the alkali chloride electrolyte significantly changes the effective acid dissociation constant (p<i>K</i>_a^eff) for the less acidic silanol groups, with the silica/NaCl_aq and silica/CsCl_aq interfaces exhibiting the lowest and highest p<i>K</i>_a^eff values of 8.3(1) and 10.8(1), respectively. Additionally, the relative populations of the two silanol groups are also very sensitive to the electrolyte identity. The greatest percentage of acidic silanol groups was 60(2)% for the silica/LiCl_aq interface in contrast to the lowest value of 20(2)% for the silica/NaCl_aq interface. We attribute these changes in the bimodal behavior to the influence of alkali ions on the interfacial water structure and its corresponding effect on surface acidity

Halide-Induced Cooperative Acid–Base Behavior at a Negatively Charged Interface

Using second harmonic generation and sum frequency generation spectroscopy, we monitor the influence of sodium and potassium halides on acid–base processes at the negatively charged silica/aqueous electrolyte interface. We find that the two types of acidic silanols at the surface are very sensitive to the presence of halides in the aqueous phase. As the halide size increases, the pH at which half the more acidic sites are deprotonated (pH_0.5) shifts to lower pH. Conversely, the pH_0.5 of the less acidic sites shifts to higher pH with increasing halide size. We also observe titration curves of increasing sharpness as the halide size increases, indicative of positive cooperativity. Using a simple cooperative model, we find that the cooperative unit for the dissociation of more acidic surface sites is ∼1, 2, and 3 for the chloride, bromide, and iodide electrolytes, respectively, which reveals that the larger anions promote deprotonation among the more acidic silanol groups. We also find that the fraction of more acidic sites, proportional to the relative surface charge density at neutral pH, increases from 20% to 86% as the sodium halide is varied from chloride to iodide. As the percentage of more acidic sites and the surface charge at neutral pH increases, the effective acidity of the less acidic sites decreases, indicating that greater surface charge density renders the remaining silanol groups more difficult to deprotonate. As the relative amount of less acidic sites increases, their deprotonation events exhibit negative, rather than positive, cooperativity revealing charge repulsion between neighboring silanol groups

Mussel-Inspired Immobilization of Silver Nanoparticles toward Antimicrobial Cellulose Paper

Paper and paper products are widely used without any antimicrobial efficacy in our everyday lives and thus can act as potential transporters of many diseases. Herein, we introduce antimicrobial activity to cellulose paper by presenting a tailored mussel-inspired strategy for the sustainable immobilization of silver nanoparticles (AgNPs), which are well-known for the effectiveness in preventing annexation and proliferation of microbes on materials surfaces. First, we functionalized the cellulose paper with succinic acid that eventually reacted with dopamine to give dopamine-modified paper. The dopamine molecules possess excellent adhesion and strong coordination with metal substrates through catechol groups offering a potentially robust interface between AgNPs and the organic structure of the paper. Next, AgNPs were deposited onto the paper by simply immersing dopamine-modified paper in a silver salt solution to accomplish the antimicrobial properties. Field emission scanning electron microscopic study of the synthesized antimicrobial papers confirmed that the loading of AgNPs was time-dependent, and the average size of the nanoparticles was in the range of 50–60 nm after 8 h of deposition time. The paper decorated with AgNPs showed excellent antimicrobial activity against highly virulent and multiple antibiotic resistant Gram-positive and Gram-negative pathogenic bacteria as well as against some extremely virulent fungal phytopathogens

pH-Dependent Inversion of Hofmeister Trends in the Water Structure of the Electrical Double Layer

Specific ion effects (SIEs) are known to influence the acid/base behavior of silica and the interfacial structure of water, yet evidence of the effect of pH on SIEs is lacking. Here broadband vibrational sum frequency generation (SFG) spectroscopy was used to study SIEs on the water structure at the electrical double layer (EDL) of silica as a function of pH and monovalent cation identity from pH 2–12 at 0.5 M salt concentration. SFG results indicate a direct Hofmeister series of cation adsorption at pH 8 (Li⁺ < Na⁺ < K⁺ < Cs⁺), with an inversion in this series occurring at pH > 10. In addition, an inversion in SFG intensity trends also occurred at pH < 6, which was attributed to contributions from asymmetric cation hydration and EDL overcharging. The highly pH-dependent SIEs for silica/water have implications for EDL models that often assume pH-independent parameters

Separating the pH-Dependent Behavior of Water in the Stern and Diffuse Layers with Varying Salt Concentration

Vibrational sum frequency generation (SFG) spectroscopy was utilized to distinguish different populations of water molecules within the electric double layer (EDL) at the silica/water interface. By systematically varying the electrolyte concentration, surface deprotonation, and SFG polarization combinations, we provide evidence of two regions of water molecules that have distinct pH-dependent behavior when the Stern layer is present (with onset between 10 and 100 mM NaCl). For example, water molecules near the surface in the Stern layer can be probed by the pss polarization combination, while other polarization combinations (ssp and ppp) predominantly probe water molecules further from the surface in the diffuse part of the electrical double layer. For the water molecules adjacent to the surface within the Stern layer, upon increasing the pH from the point-of-zero charge of silica (pH ∼2) to higher values (pH ∼12), we observe an increase in alignment consistent with a more negative surface with increasing pH. In contrast, water molecules further from the surface appear to exhibit a net flip in orientation upon increasing the pH over the same range, which we attribute to the presence of the Stern layer and possible overcharging of the EDL at lower pH. The opposing pH-dependent behavior of water in these two regions sheds new light on our understanding of the water structure within the EDL at high salt concentrations when the Stern layer is present