Chiral nanoparticle assemblies are an interesting class of materials whose chiroptical properties make them attractive for a variety of applications. Here, C18-(PEPAuM-ox)2 (PEPAuM-ox = AYSSGAPPMoxPPF) is shown to direct the assembly of single-helical gold nanoparticle superstructures that exhibit exceptionally strong chiroptical activity at the plasmon frequency with absolute g-factor values up to 0.04. Transmission electron microscopy (TEM) and cryogenic electron tomography (cryo-ET) results indicate that the single helices have a periodic pitch of approximately 100 nm and consist of oblong gold nanoparticles. The morphology and assembled structure of C18-(PEPAuM-ox)2 are studied using TEM, atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, circular dichroism (CD) spectroscopy, X-ray diffraction (XRD), and solid-state nuclear magnetic resonance spectroscopy (ssNMR). TEM and AFM reveal that C18-(PEPAuM-ox)2 assembles into linear amyloid-like 1-D helical ribbons having structural parameters that correlate to those of the single-helical gold nanoparticle superstructures. FTIR, CD, XRD, and ssNMR indicate the presence of cross-β and polyproline II (PPII) secondary structure. A molecular assembly model is presented that takes into account all experimental observations and that supports the single-helical nanoparticle assembly architecture. This model provides the basis for the design of future nanoparticle assemblies having programmable structures and properties.
Atomically precise metal nanoclusters with tailored surface structures are important for both fundamental studies and practical applications. The development of new methods for tailoring the surface structure in a controllable manner has long been sought. In this work, we report surface reconstruction induced by cadmium doping into the [Au(SR)] (R = cyclohexyl) nanocluster, in which two neighboring surface Au atomic sites "coalesce" into one Cd atomic site and, accordingly, a new bimetal nanocluster, [AuCd(SR)], is produced. Interestingly, a Cd(S-Au-S) "paw-like" surface motif is observed for the first time in nanocluster structures. In such a motif, the Cd atom acts as a junction which connects three monomeric -S-Au-S- motifs. Density functional theory calculations are performed to understand the two unique Cd locations. Furthermore, we demonstrate different doping modes when the [Au(SR)] nanocluster is doped with different metals (Cu, Ag), including (i) simple substitution and (ii) total structure transformation, as opposed to surface reconstruction for Cd doping. This work greatly expands doping chemistry for tailoring the structures of nanoclusters and is expected to open new avenues for designing nanoclusters with novel surface structures using different dopants.
Chiral objects are defined as nonsuperimposable conformations that are mirror images of each other, much like a pair of left and right hands. In fact, the word "chiral" derives from the Greek word "χειρ" (kheir), which translates to "hand." Most biomolecules exist in only one particular conformation. For example, amino acids within large protein and peptide molecules are exclusively in the l-form (left-handed). It has long been considered that the phenomenon of homochirality (predominant occurrence of one conformation) could be linked to the origin of life. [1] These phenomena have inspired chemists and biologists to isolate, synthesize, and study the properties of chiral molecules. Research interest in chiral nanostructures has escalated rapidly since the early 2000s due to visionary reports that either predicted or demonstrated the potential applications of these materials. [2,3] In 2004, Pendry predicted that chiral metamaterials could be used to achieve negative refraction (Figure 1). [2] Following this seminal work, others demonstrated that such materials lead to circular dichroism (CD), [4] negative phase velocities, [5] and intense gyrotropy, [6] generating significant excitement within this emerging field. These properties can be harnessed to realize optical materials including "perfect lenses," [7] circular polarizers, [3] chiroptical sensors, [8] and negative refractive index materials. [9,10] In addition to these optical applications, chiral metallic nanostructures have been used for detection of biomolecular disease precursors, [11] chiral catalysis, [12] and chiral separations. [13] Many of the promising applications of chiral metallic nanostructures arise in part from their plasmonic chiroptical activity. At the nanoscale, individual metallic particles exhibit unique properties due to their high surface to volume ratio and geometric confinement of electrons. One particularly important property is the localized surface plasmon resonance (LSPR), [14] which occurs when the oscillation of surface electrons matches the frequency of incident photons. The spectral position and intensity of the LSPR depends not only on the size, shape, composition, and dielectric environment of the metallic nanoparticles (NPs) [15,16] but also on their aggregation state or assembly. [17] When metallic NPs are arranged in a chiral geometry, [18] the coupling of individual plasmons leads to collective plasmon oscillation across the entire structure. [19] Chiral NP assemblies may exhibit enhanced optical chirality in Chiral nanoparticle (NP) superstructures, in which discrete NPs are assembled into chiral architectures, represent an exciting and growing class of nanomaterials. Their enantiospecific properties make them promising candidates for a variety of potential applications. Helical NP superstructures are a rapidly expanding subclass of chiral nanomaterials in which NPs are arranged in three dimensions about a screw axis. Their intrinsic asymmetry gives rise to a variety of interesting properties, including plasmonic chir...
Systematically controlling the assembly architecture within a class of chiral nanoparticle superstructures is important for fine-tuning their chiroptical properties. Here, we report a family of chiral gold nanoparticle single helices, varying in helical pitch and nanoparticle dimensions, that is assembled using a series of peptide conjugate molecules C-(PEP) (PEP = AYSSGAPPMPPF; x = 16-22). We demonstrate that the aliphatic tail length (i) can be used as a handle to systematically tune the helical pitch from 80 to 130 nm; and (ii) influences the size, shape, and aspect ratio of the component nanoparticles. Certain members of this family of materials exhibit intense plasmonic chiroptical activity. These studies highlight the multiple levels of structural control that can be achieved within a class of chiral nanoparticle superstructures via careful design and selection of peptide conjugate precursor.
Just as peptide function is determined by the position, sequence, and overall arrangement of constituent amino acids, the optical properties of nanoparticle (NP) assemblies are influenced by the size, dimensions, and arrangement of constituent NPs. In this work, we demonstrate that peptide sequence can be programmed to direct the structure and chiroptical activity of chiral helical gold NP (AuNP)superstructures, a growing class of chiral nanomaterials with potential in sensing, detection, and optics-based applications. Gold-binding peptide conjugate families, C18-(PEPAu M,x )2 and C18-(PEPAu M‑ox,x )2, that differ in the position (x = 7, 9, and 11) of methionine (M)/methionine sulfoxide (M-ox) within the peptide sequences (PEPAu = AYSSGAPPMPPF/PEPAu M‑ox = AYSSGAPPMoxPPF) are employed to control the aspect ratio and size of AuNPs within helical NP assemblies. Computational modeling reveals that the amino acid variations have a profound effect on peptide–AuNP interactions that ultimately lead to control over NP size. C18-(PEPAu M,x )2 (x = 7, 9, and 11) yield irregular double-helical superstructures comprising spherical AuNPs, while C18-(PEPAu M‑ox,x )2 (x = 9, 11) yield single-helical assemblies comprising oblong or rod-shaped AuNPs. Further, component AuNPs are larger when M/M-ox is placed at x = 11, while smaller component AuNPs are observed when M/M-ox is placed at x = 7. Changes in nanoscale structures manifest themselves in observable differences in chiroptical signal intensity. Ultimately, we achieve dramatic variance in the structure and properties of chiral AuNP superstructures via simple molecular-level tuning of peptide primary sequence.
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