Marc R. Knecht

Bio-inspired colorimetric detection of Hg2+ and Pb2+ heavy metal ions using Au nanoparticles

AbstractHeavy metal ions are highly toxic species which can cause long-term damage to biological systems. These species are known to disrupt biological events at the cellular level, cause significant oxidative damage, and are carcinogens. The production of simple, in-field detection methods that are highly sensitive for these cations is highly desirable in response to global pollution. In that regard, bio-inspired colorimetric sensing systems have been developed to detect Hg2+ and Pb2+, and other cations, down to nmol L−1 concentrations. The benefits of these systems, which are reviewed herein, include cost-effective production, facile usage, and a visual color change for the detection method. Such advantages are significant positive steps for heavy metal ion detection, especially in regions where sophisticated laboratory studies are prohibited. FigureBiological-based colorimetric detection of heavy metal cations. The materials on the left are independent Au nanoparticles in solution, functionalized with heavy metal binding biomolecules, which, upon metal addition, aggregate to evolve a detectable solution color change.

Biomimetic Synthesis of Pd Nanocatalysts for the Stille Coupling Reaction

Here we report on the biomimetic synthesis of Pd nanoparticles for use as models of green catalytic systems. The nanomaterials are synthesized using peptides isolated via phage-display techniques that are specific to Pd surfaces. Using this synthetic strategy, peptide-functionalized Pd nanoparticles of 1.9 +/- 0.3 nm in diameter are produced, which are soluble and stable in aqueous solutions. Once characterized, these biobased materials were then used as catalysts to drive the formation of C-C bonds using the Stille coupling reaction. Under the conditions of an aqueous solvent at room temperature, quantitative product yields were achieved within 24.0 h employing catalyst loadings of > or = 0.005 mol % of Pd. Additionally, high TOF values of 3207 +/- 269 mol product x (mol Pd x h)(-1) have been determined for these materials. The catalytic reactivity was then examined over a set of substrates with substitutions for both functional group and halide substituents, demonstrating that the peptide-based Pd nanoparticles are reactive toward a variety of functionalities. Taken together, these bioinspired materials represent unique model systems for catalytic studies to elucidate ecologically friendly reactive species and conditions.

Biomolecular Recognition Principles for Bionanocombinatorics: An Integrated Approach To Elucidate Enthalpic and Entropic Factors

Bionanocombinatorics is an emerging field that aims to use combinations of positionally encoded biomolecules and nanostructures to create materials and devices with unique properties or functions. The full potential of this new paradigm could be accessed by exploiting specific noncovalent interactions between diverse palettes of biomolecules and inorganic nanostructures. Advancement of this paradigm requires peptide sequences with desired binding characteristics that can be rationally designed, based upon fundamental, molecular-level understanding of biomolecule-inorganic nanoparticle interactions. Here, we introduce an integrated method for building this understanding using experimental measurements and advanced molecular simulation of the binding of peptide sequences to gold surfaces. From this integrated approach, the importance of entropically driven binding is quantitatively demonstrated, and the first design rules for creating both enthalpically and entropically driven nanomaterial-binding peptide sequences are developed. The approach presented here for gold is now being expanded in our laboratories to a range of inorganic nanomaterials and represents a key step toward establishing a bionanocombinatorics assembly paradigm based on noncovalent peptide-materials recognition.

Nanotechnology Meets Biology: Peptide-based Methods for the Fabrication of Functional Materials

Nature exploits sustainable methods for the creation of inorganic materials on the nanoscale for a variety of applications. To achieve such capabilities, biomolecules such as peptides and proteins have been developed that recognize and bind the different compositions of materials. While a diverse set of materials binding sequences are present in the biosphere, biocombinatorial techniques have been used to rapidly identify peptides that facilitate the formation of new materials of technological importance. Interestingly, the binding motif of the peptides at the inorganic surface is likely to control the size, structure, composition, shape, and functionality of the final materials. In order to advance these intriguing new biomimetic approaches, a complete understanding of this biotic/abiotic interface is required. In this Perspective, we highlight recent advances in the biofunctionalization of nanoparticles with potential applications ranging from catalysis and energy storage to plasmonics and biosensing. We specifically focus on the physical characterization of the peptide-based surface from which specificity and activity are likely embedded.

Crystallographic Recognition Controls Peptide Binding for Bio-Based Nanomaterials

The ability to control the size, shape, composition, and activity of nanomaterials presents a formidable challenge. Peptide approaches represent new avenues to achieve such control at the synthetic level; however, the critical interactions at the bio/nano interface that direct such precision remain poorly understood. Here we present evidence to suggest that materials-directing peptides bind at specific time points during Pd nanoparticle (NP) growth, dictated by material crystallinity. As such surfaces are presented, rapid peptide binding occurs, resulting in the stabilization and size control of single-crystal NPs. Such specificity suggests that peptides could be engineered to direct the structure of nanomaterials at the atomic level, thus enhancing their activity.

Structural Analysis of PdAu Dendrimer-Encapsulated Bimetallic Nanoparticles

PdAu dendrimer-encapsulated nanoparticles (DENs) were prepared via sequential reduction of the component metals. When Au is reduced onto 55-atom, preformed Pd DEN cores, analysis by UV-vis spectroscopy, electron microscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy leads to a model consistent with inversion of the two metals. That is, Au migrates into the core and Pd resides on the surface. However, when Pd is reduced onto a 55-atom Au core, the expected Au core-Pd shell structure results. In this latter case, the EXAFS analysis suggests partial oxidation of the relatively thick Pd shell. When the DENs are extracted from their protective dendrimer stabilizers by alkylthiols, the resulting monolayer-protected clusters retain their original Au core-Pd shell structures. The structural analysis is consistent with a study of nanoparticle-catalyzed conversion of resazurin to resorufin. The key conclusion from this work is that correlation of structure to catalytic function for very small, bimetallic nanoparticles requires detailed information about atomic configuration.

Elucidation of peptide effects that control the activity of nanoparticles

Natural processes have been developed to produce nanostructures that involve recognition between biomolecules and inorganic surfaces. Such methods have been exploited in the production of nanomaterials for use as catalysts, biosensors, batteries, and components for directed assembly; however, the interactions at the biotic/abiotic interface remain unclear. These interactions are likely to control the activity of the nanostructures, which could be optimized based upon the peptide sequence and arrangement on the nanomaterial surface. Whilst these studies have demonstrated the unique activity of such bio-enabled materials, to the best of our knowledge, no research is available that probes the critical effects of the surface peptide on nanomaterial activity. Previous reports have suggested that peptides bind to surfaces in a different manner to individual amino acids; 9] therefore, by understanding these interactions, the design of bionanomaterials that have superior functionality may be possible. The Pd4 peptide (Table 1), was isolated using a phage display technique with an affinity for palladium. 9] Using this sequence, palladium particles, which have a diameter of approximately 1.9 nm, were prepared that were active for Stille coupling reactions in water, at room temperature, with palladium loadings of 0.005 mol % (Scheme 1). Modeling of the peptide–nanoparticle interactions suggested that the histidine residues at positions 6 and 11 were most likely

Exploiting Localized Surface Binding Effects to Enhance the Catalytic Reactivity of Peptide-Capped Nanoparticles

Peptide-based methods represent new approaches to selectively produce nanostructures with potentially important functionality. Unfortunately, biocombinatorial methods can only select peptides with target affinity and not for the properties of the final material. In this work, we present evidence to demonstrate that materials-directing peptides can be controllably modified to substantially enhance particle functionality without significantly altering nanostructural morphology. To this end, modification of selected residues to vary the site-specific binding strength and biological recognition can be employed to increase the catalytic efficiency of peptide-capped Pd nanoparticles. These results represent a step toward the de novo design of materials-directing peptides that control nanoparticle structure/function relationships.

Stability and electrostatic assembly of Au nanorods for use in biological assays

The structure, stability, and aggregation potential of short Au nanorods under biological-based solution conditions have been studied. These attributes were studied using UV-vis spectroscopy, transmission electron microscopy, zeta-potential analysis, and dynamic light scattering. The stability and aggregation potential of the materials depended strongly upon both the purity and the solvent used to prepare Au nanorod solutions. When the Au nanorods were dissolved in Tris buffer at concentrations less than 10.0 mM, no aggregation was observed; however, when the solvent was comprised of Tris buffer with concentrations between 10.0 and 100 mM, significant aggregation of the materials occurred. This effect resulted in a dramatic broadening and shift in the absorbance maxima of the longitudinal surface plasmon resonance. At Tris buffer concentrations of greater than 100 mM, minimal to no aggregation of the materials in solution was observed. Such an ability is based upon electrostatic aggregation of the materials in solution mediated by the anions associated with the buffer system; at concentrations between 10.0 and 100 mM, the anions present electrostatically bind to the surfaces of the Au nanorods that are positively charged, resulting in cross-linking of the materials. At higher buffer concentrations, a sufficient number of anions are present in solution to template around the entire surface of each individual nanorod, in effect neutralizing the charge and producing an electronic double layer, which prevents aggregation. Such studies are timely as they represent an analysis of the stability and range of use of Au nanorods for biological-based applications where remarkable potential exists.

Determining peptide sequence effects that control the size, structure, and function of nanoparticles.

The ability to tune the size, shape, and composition of nanomaterials at length scales <10 nm remains a challenging task. Such capabilities are required to fully realize the application of nanotechnology for catalysis, energy storage, and biomedical technologies. Conversely, nature employs biomacromolecules such as proteins and peptides as highly specific nanoparticle ligands that demonstrate exacting precision over the particle morphology through controlling the biotic/abiotic interface. Here we demonstrate the ability to finely tune the size, surface structure, and functionality of single-crystal Pd nanoparticles between 2 and 3 nm using materials directing peptides. This was achieved by selectively altering the peptide sequence to change the binding motif, which in turn modifies the surface structure of the particles. The materials were fully characterized before and after reduction using atomically resolved spectroscopic and microscopic analyses, which indicated that the coordination environment prior to reduction significantly affects the structure of the final nanoparticles. Additionally, changes to the particle surface structure, as a function of peptide sequence, can allow for chloride ion coordination that alters the catalytic abilities of the materials for the C-C coupling Stille reaction. These results suggest that peptide-based approaches may be able to achieve control over the structure/function relationship of nanomaterials where the peptide sequence could be used to selectivity tune such capabilities.