Joseph M. Slocik

Theory of Circular Dichroism of Nanomaterials Comprising Chiral Molecules and Nanocrystals: Plasmon Enhancement, Dipole Interactions, and Dielectric Effects

Our calculations show that a nonchiral nanocrystal is able to dramatically change the circular dichroism (CD) of a chiral molecule when the nanocrystal and molecule form a complex and couple via dipole and multipole Coulomb interactions. Plasmon resonances of metal nanocrystals in the nanocrystal-molecule complex result in both the resonant enhancement of CD signals of molecules and the appearance of new spectral structures. Two mechanisms, in which a nanocrystal can influence the CD effect, have been identified. The first mechanism is the plasmon-induced change in the electromagnetic field inside the chiral molecule. The second is the optical absorption of the nanocrystal-molecule complex due to the chiral currents inside the metal nanocrystal induced by the dipole of the chiral molecule. The first mechanism creates a change in the angle between the effective electric and magnetic dipoles of the molecule. This mechanism can lead to symmetry breaking and to a plasmon-induced CD signal of the nonchiral molecule. Both mechanisms create interesting Fano-like shapes in the CD spectra. Importantly, the second mechanism gives the main contribution to the CD signal at the plasmon frequency when the absorption band of the chiral molecule is far from the plasmon resonance. This may happen in many cases since many biomolecules are optically active in the UV range, whereas plasmon resonances in commonly used nanometals are found at longer wavelengths. As concrete examples, the paper describes alpha-helix and calixarene ligand molecules coupled with metal nanocrystals. The above results are also applied to complexes incorporating semiconductor nanocrystals. The results obtained here can be used to design a variety of hybrid nanostructures with enhanced and tailored optical chirality in the visible wavelength range.

Plasmonic Circular Dichroism of Peptide-Functionalized Gold Nanoparticles

Nature is remarkable at tailoring the chirality of different biomolecules to suit specific functions. Chiral molecules can impart optical activity to achiral materials in the form of the particles electronic transition frequency. Herein, we used peptides of differing secondary structures (random coil and α-helix) to artificially create optically active chiral gold nanoparticles through peptide-nanoparticle interactions as observed by circular dichroism (CD) spectroscopy. This interaction produces a CD signal at the plasmon resonance frequency (∼520 nm) of the chiral peptide-nanoparticle complex. Aggregation of the peptide-coated nanoparticles using metal ions results in a red-shifted plasmonic CD response. Our results suggest that chiroptical properties of nanomaterials can be engineered using peptides.

Colorimetric Response of Peptide-Functionalized Gold Nanoparticles to Metal Ions†

The design of nanostructures with controlled surface chemistry for sensing, catalytic, and electronic applications is an important research challenge. Sensing platforms based on the optical properties of gold nanoparticles in combination with the molecular recognition of ligands, such as alkyl thiols, antibodies, nucleic acids, and proteins, are active areas of research. Detection of targets by functionalized gold particles has been performed by using surface-enhanced raman spectroscopy (SERS), quartz crystal microgravimetry (QCM), surface plasmon resonance (SPR) spectroscopy, electrochemical and potentiometric detection, and colorimetric assays. The gold-nanaoparticle-based colorimetric sensors provide simplicity and excellent detection capability encompassing a variety of targets including metal ions, DNA, bacterial toxins, protein conformations, and enzyme activity. For example, Pb2þ was detected by a color change upon the dispersion of gold nanoparticles functionalized with DNAzyme. Recently, DNA-modified gold nanoparticles were used as a colorimetric sensor in the detection of Hg2þ.[14] The aggregation of the ligand-functionalized gold nanoparticles upon binding its target results in a colorimetric response caused by broadening and shifting of the plasmon resonance peak. This shift in the plasmon resonance frequency is employed in sensing strategies. To date, the majority of the colorimetric gold nanoparticle sensing strategies have used nucleic acids as the sensing element. Here, we demonstrate the potential of peptide-functionalized gold nanoparticles (PFNs) as a colorimetric sensor for metal ions. The PFNs were synthesized in a HEPES buffer using the Flg-A3 peptide (-Asp-Tyr-Lys-Asp-Asp-Asp-Asp-LysPro-Ala-Tyr-Ser-Ser-Gly-Pro-Ala-Pro-Pro-Met-Pro-Pro-Phe-). The synergistic contributions of both the Flg-A3 peptide and HEPES buffer result in the formation of peptidefunctionalized suspension of gold nanoparticles. The overall negative charge of the peptide (pI1⁄4 3.9) prevents aggregation of the particles by repulsive forces. The surface of the gold nanoparticles contains amino acid functional groups that can interact with metal ions. Charged, aromatic, and hydroxyl-

Preferential Binding of Peptides to Graphene Edges and Planes

Peptides identified from combinatorial peptide libraries have been shown to bind to a variety of abiotic surfaces. Biotic-abiotic interactions can be exploited to create hybrid materials with interesting electronic, optical, or catalytic properties. Here we show that peptides identified from a combinatorial phage display peptide library assemble preferentially to the edge or planar surface of graphene and can affect the electronic properties of graphene. Molecular dynamics simulations and experiments provide insight into the mechanism of peptide binding to the graphene edge.

Viral templates for gold nanoparticle synthesis

Viruses present a confined environment and unique protein surface topology (i.e. polarity, residue charge, and surface relief) for nanoparticle synthesis and are amenable to molecular biology manipulations. Consequently, we have examined the cowpea chlorotic mottle viruses of unmodified SubE (yeast), (HRE)-SubE engineered with interior HRE peptide epitopes (AHHAHHAAD), and wild-type as viral templates for the potentiated reduction and symmetry directed synthesis of gold nanoparticles. In the first approach, the viral capsid actively potentiated the reduction of AuCl4− by electron transfer from surface tyrosine residues resulting in a gold nanoparticle decorated viral surface. Viral reduction appeared to be selective for gold as a collection of metal precursor substrates of Ag+, Pt4+, Pd4+, and an insoluble AuI complex were not reduced to zero-valent nanoclusters by virus. Alternatively, the viral capsid provided a template for the symmetry directed synthesis of Au0 nanoparticles from a non-reducible gold precursor.

Multifunctional analytical platform on a paper strip: separation, preconcentration, and subattomolar detection.

We report a plasmonic paper-based analytical platform with functional versatility and subattomolar (<10(-18) M) detection limit using surface-enhanced Raman scattering as a transduction method. The microfluidic paper-based analytical device (μPAD) is made with a lithography-free process by a simple cut and drop method. Complex samples are separated by a surface chemical gradient created by differential polyelectrolyte coating of the paper. The μPAD with a starlike shape is designed to enable liquid handling by lateral flow without microchannel patterning. This design generates a rapid capillary-driven flow capable of dragging liquid samples as well as gold nanorods into a single cellulose microfiber, thereby providing an extremely preconcentrated and optically active detection spot.

Chiral nanoparticle assemblies: circular dichroism, plasmonic interactions, and exciton effects

The paper reviews recent progress on chiral nanocrystal assemblies with induced optical chirality and related circular dichroism. Many natural molecules and biomolecules are chiral and exhibit remarkably strong optical chirality (circular dichroism) due to their amazingly uniform atomic composition in a large ensemble. It is challenging to realize artificial nanoscale systems with optical chirality since the atomic structure of artificial nanostructures may not be always controlled or even known. Nevertheless, the artificial optical chirality has been accomplished and it is the main scope of this review. In particular, we discuss assemblies incorporating chiral molecules, metal nanocrystals, and semiconductor quantum dots. Plasmon-induced and plasmon-enhanced circular dichroism effects appear in nanoscale assemblies built with metal nanocrystals, while excitonic and surface-states related phenomena are observed in semiconductor quantum dots conjugated with chiral molecules.

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.

Influence of the shape of nanostructured metal surfaces on adsorption of single peptide molecules in aqueous solution.

Self-assembly and function of biologically modified metal nanostructures depend on surface-selective adsorption; however, the influence of the shape of metal surfaces on peptide adsorption mechanisms has been poorly understood. The adsorption of single peptide molecules in aqueous solution (Tyr(12) , Ser(12) , A3, Flg-Na(3) ) is investigated on even {111} surfaces, stepped surfaces, and a 2 nm cuboctahedral nanoparticle of gold using molecular dynamics simulation with the CHARMM-METAL force field. Strong and selective adsorption is found on even surfaces and the inner edges of stepped surfaces (-20 to -60 kcal/mol peptide) in contrast to weaker and less selective adsorption on small nanoparticles (-15 to -25 kcal/mol peptide). Binding and selectivity appear to be controlled by the size of surface features and the extent of co-ordination of epitaxial sites by polarizable atoms (N, O, C) along the peptide chain. The adsorption energy of a single peptide equals a fraction of the sum of the adsorption energies of individual amino acids that is characteristic of surface shape, epitaxial pattern, and conformation constraints (often β-strand and random coil). The proposed adsorption mechanism is supported and critically evaluated by earlier sequence data from phage display, dissociation constants of small proteins as a function of nanoparticle size, and observed shapes of peptide-stabilized nanoparticles. Understanding the interaction of single peptides with shaped metal surfaces is a key step towards control over self-organization of multiple peptides on shaped metal surfaces and the assembly of superstructures from nanostructures.

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