59th Annual Report on Research 2014

The funding cycle from 2013-2014 represents the final year of funding for this project from the ACS PRF, funding which has made significant impact on research progress within my group, including support of a full-time graduate student and two undergraduate students. Two additional undergraduates were active on the project but were supported by other funding mechanisms within the department.  

Mr. Sudhir Ravula, who just began his fourth year of graduate school, has been fully supported on this ACS PRF project for the past academic year and has benefited immensely from having uninterrupted time in the lab. Funding for this ACS PRF project has afforded us the opportunity to generate several exciting new results inspired by those achieved in our first year of funding for this project. A visiting undergraduate student (Ms. Wendy La, Truman State) returned for the second summer of research on a Stevens’ Summer Research Fellowship and worked closely with Mr. Ravula on his ACS PRF-funded work and, although she was fully supported otherwise, she made key contributions to our project. Thus, the funding afforded Mr. Ravula not only the opportunity to focus on his research but also contributed to his growth as a mentor, providing Ms. La and other undergraduate students valuable research experience at the same time. I have since learned that Ms. La has decided to pursue graduate studies in chemistry, something she was not inclined to do before this experience.  

Scientific Progress

The overarching goal of the work in our first year centered on developing a molecular-level understanding of the exfoliation process toward better controlling interactions responsible for exfoliation of van der Waals solids like graphite. These efforts involved quantum mechanical and molecular dynamics calculations to elucidate the interactions occuring between ionic liquid species and graphene or inorganic graphene analogs (IGAs). We also began in silico explorations of how these processes occur in water. During these efforts, our research naturally evolved to include bottom-up wet chemical routes to synthesize graphene and IGA sheets alongside top-down peeling/etching of “bulk” or microscale materials to fabricate nanoscale sheets.  

During year 2 of this project, our primary achievement has been in the introduction of functional nanosheets referred to as plasmonic aminoclays which are simultaneously antibacterial agents against E. coli and effective catalysts for nitroaromatic reduction. This work, led by Mr. Ravula, led to the submission of a peer-reviewed journal article earlier this year which is now in revision. In short, we developed a straightforward, environmentally-benign, one-pot photochemical route to generate alloyed AgAu bimetallic nanoparticle decorated aminoclays in water at room temperature. Our protocol employs no reducing agent (e.g., NaBH₄) nor is photocatalyst required, making it an environmentally responsible bottom-up wet chemical route to IGAs.  

The organic aminoclay comprises 3-aminopropyl-functionalized magnesium phyllosilicate made using a sol–gel method and shows excellent prospects for metal nanoparticle stabilization since it can be delaminated and stably dispersed in aqueous media due to repulsive electrostatic forces between pendant quaternary ammonium groups. The essence of our strategy is captured in Figure 1. Our approach entails the initial deposition of AgCl nanoparticles (NPs) onto the surfaces of aminoclay (AC) lamellae as a result of chloride residues remaining from the Mg source employed. A key aspect, AgCl acts as an in situ photocatalyst for the light-assisted reduction of AgCl colloids to generate Ag NPs directly on the water-dispersed AC nanosheets within a few minutes. Significantly, both artificial light and natural sunlight proved equally effective in the preparation of Ag NP decorated aminoclays (i.e., AC@Ag).    Figure 1. Pathway illustrating the formation of AC@Ag hybrids.

Varying the initial Ag:Au ratio in the reaction mixture and irradiating for 60 min in direct sunlight afforded a palette of AC@Ag_xAu_y hybrids displaying continuously tunable localized surface plasmon resonance (LSPR) band maxima in the range from 420 to 540 nm, as shown in Figure 2.  Figure 2. Photograph of the AC@Ag_xAu_y hybrids showing a visually-distinct color transition from left to right as the initial fraction of the Au precursor was increased in 10% increments.    

The catalytic activities of these AC hybrids were evaluated by using the widely-studied conversion of the toxic pollutant 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) using NaBH₄ as the reducing agent. Complete reduction of 4-NP to 4-AP (bleaching of the initial yellow solution) was accomplished within 675 s, 195 s, and 480 s for AC@Ag, AC@Ag_0.5Au_0.5, and AC@Au hybrids, respectively. Even hybrid aminoclays aged for 6 months retained the majority of their initial activity (Figure 3B), a result which bodes well for the long-term shelf life of these materials.             In order to place the performance of these catalysts in context with existing 4-NP reduction catalysts, we determined turnover frequency (TOF) values. Excellent TOFs of 245 h^–1 and 436 h^–1 were calculated for the silver and gold hybrids (AC@Ag and AC@Au, respectively). Remarkably, however, the bimetallic material containing a 1:1 Ag:Au ratio (i.e., AC@Ag_0.5Au_0.5) achieved a whopping 1100 h^–1, making it amongst the fastest catalysts for 4-NP reduction in water ever reported. Figure 3. Influence of the number of repeated uses of AC@Ag_0.5Au_0.5 as catalyst for 4-NP reduction on the apparent reaction rate for (A) freshly-prepared and (B) 6 month-old material.   

Finally, the antibacterial activities of the AC hybrids were tested against gram-negative E. coli (ATCC 25922), a standard strain of the commonly occurring human pathogen. Figure 4 summarizes the bacterial inhibition resulting from exposure of the bacterial cultures to AC@Ag_xAu_y hybrids for fixed periods of 0, 2, and 5 h. The initial bacterial counts in the suspension, as measured using plating, were ~5 x 10⁵ colony forming units (CFU) mL^–1 in all cases. Complete inhibition of the bacteria was observed for the AC@Ag hybrid after 5 h. Remarkably, however, the bimetallic AC@Ag_0.5Au_0.5 platform showed even higher activity, a highly unanticipated and exciting result. Figure 4. Inactivation efficiencies of various AC@Ag_xAu_y hybrids against E. coli expressed in colony forming units per milliliter counted after various periods of exposure.