Manuel Gadogbe

Comparative Study of the Self-Assembly of Gold and Silver Nanoparticles onto Thiophene Oil

Nanoparticle self-assembly is fundamentally important for bottom-up functional device fabrication. Currently, most nanoparticle self-assembly has been achieved with gold nanoparticles (AuNPs) functionalized with surfactants, polymeric materials, or cross-linkers. Reported herein is a facile synthesis of gold and silver nanoparticle (AgNP) films assembled onto thiophene oil by simply vortex mixing neat thiophene with colloidal AuNPs or AgNPs for ∼1 min. The AuNP film can be made using every type of colloidal AuNPs we have explored, including sodium borohydride-reduced AuNPs with a diameter of ∼5 nm, tannic acid-reduced AuNPs of ∼10 nm diameter, and citrate-reduced AuNPs with particle sizes of ∼13 and ∼30 nm diameter. The AuNP film has excellent stability and it is extremely flexible. It can be stretched, shrunken, and deformed accordingly by changing the volume or shape of the enclosed thiophene oil. However, the AgNP film is unstable, and it can be rapidly discolored and disintegrated into small flakes that float on the thiophene surface. The AuNP and AgNP films prepared in the glass vials can be readily transferred to glass slides and metal substrates for surface-enhanced Raman spectral acquisition.

Evaluation of Thiol Raman Activities and pKa Values Using Internally Referenced Raman-Based pH Titration

Thiols, including organothiol and thiol-containing biomolecules, are among the most important classes of chemicals that are used broadly in organic synthesis, biological chemistry, and nanosciences. Thiol pKa values are key indicators of thiol reactivity and functionality. Reported herein is an internally referenced Raman-based pH titration method that enables reliable quantification of thiol pKa values for both mono- and dithiols in water. The degree of thiol ionization is monitored directly using the peak intensity of the S-H stretching feature in the 2600 cm(-1) region relative to an internal reference peak as a function of the titration solutions pH. The thiol pKa values and Raman activity relative to its internal reference were then determined by curve fitting the experimental data with equations derived on the basis of the Henderson-Hasselbalch equation. Using this Raman titration method, we determined for the first time the first and second thiol pKa values for 1,2-benzenedithiol in water. This Raman-based method is convenient to implement, and its underlying theory is easy to follow. It should therefore have broad application for thiol pKa determinations and verification.

Rigid Single Carbon-Carbon Bond That Does Not Rotate in Water.

Carbon-carbon bond is one of the most ubiquitous molecular building blocks for natural and man-made materials. Rotational isomerization is fundamentally important for understanding the structure and reactivity of chemical and biological molecules. Reported herein is the first demonstration that a single C-C bond does not rotate in water. The two distal C-S bonds in both 1,2-ethanedithiolate ((-)S-CH2-CH2-S(-), 1,2-EDT(2-)) and 2,3-butanedithiolate (2,3-BuDT(2-)) are exclusively in the trans conformer with reference to their respective center single C-C bond. In contrast, both trans and gauche conformers are observed in neutral 1,2-ethanedithiol (1,2-EDT) and 2, 3-butanedithiol (2,3-BuDT). The insight from this work should be important for understanding the charge effect on the molecular conformation in aqueous solutions.