Cyclodextrin-assisted surface-enhanced Raman spectroscopy: a critical review

Numerous approaches have been proposed to overcome the intrinsically low selectivity of surface-enhanced Raman spectroscopy (SERS), and the modification of SERS substrates with diverse recognition molecules is one of such approaches. In contrast to the use of antibodies, aptamers, and molecularly imprinted polymers, application of cyclodextrins (CDs) is still developing with less than 100 papers since 1993. Therefore, the main goal of this review is the critical analysis of all available papers on the use of CDs in SERS analysis, including physicochemical studies of CD complexation and the effect of CD presence on the Raman enhancement. The results of the review reveal that there is controversial information about CD efficiency and further experimental investigations have to be done in order to estimate the real potential of CDs in SERS-based analysis.

Dynamic Control of the Self-Assembling Properties of Cyclodextrins by the Interplay of Aromatic and Host-Guest Interactions

The presence of a doubly-linked naphthylene clip at the O-2^I and O-3^II positions in the secondary ring of β-cyclodextrin (βCD) derivatives promoted their self-assembly into head-to-head supramolecular dimers in which the aromatic modules act either as cavity extension walls (if the naphthalene moiety is 1,8-disubstituted) or as folding screens that separate the individual βCD units (if 2,3-disubstituted). Dimer architecture is governed by the conformational properties of the monomer constituents, as determined by NMR, fluorescence, circular dichroism, and computational techniques. In a second supramolecular organization level, the topology of the assembly directs host-guest interactions and, reciprocally, guest inclusion impacts the stability of the supramolecular edifice. Thus, inclusion of adamantane carboxylate, a well-known βCD cavity-fitting guest, was found to either preserve the dimeric arrangement, leading to multicomponent species, or elicit dimer disruption. The ensemble of results highlights the potential of the approach to program self-organization and external stimuli responsiveness of CD devices in a controlled manner while keeping full diastereomeric purity.

Preparation and photo-induced activities of water-soluble amyloid β-C60 complexes

We have shown that fullerene (C₆₀) becomes soluble in water by mixing fullerene and amyloid β peptide (Aβ40) whose fibril structures are considered to be associated with Alzheimer's disease. The water-solubility of fullerene arises from the generation of a nanosized complex between fullerene and the monomer species of Aβ40 (Aβ40-C₆₀). The prepared Aβ40-C₆₀ exhibits photo-induced activity with visible light to induce the inhibition of Aβ40 fibrillation and the cytotoxicity for cultured HeLa cells. The observed photo-induced phenomena result from the generation of singlet oxygen via photoexcitation, inducing oxidative damage to Aβ40 and HeLa cells. The oxidized Aβ40 following photoexcitation of Aβ40-C₆₀ was confirmed by mass spectrometry.

We have shown that fullerene (C₆₀) becomes soluble in water by mixing fullerene and amyloid β peptide (Aβ40) whose fibril structures are considered to be associated with Alzheimer's disease.

Facile stabilization of cyclodextrin metal–organic frameworks under aqueous conditions via the incorporation of C60 in their matrices

A facile method to improve the stability of γ-cyclodextrin metal–organic frameworks in an aqueous environment has been developed through the incorporation of hydrophobic C₆₀ in their matrices.

p-Phosphonic acid calix[8]arene assisted dispersion and stabilisation of pea-pod C60@multi-walled carbon nanotubes in water

Pristine C₆₀ and MWCNTs are non-covalently stabilised in water by p-phosphonic acid calix[8]arene, additionally with ‘pea-pod’ encapsulation of C₆₀ inside the MWCNTs.

Self-assembled monolayers on gold of β-cyclodextrin adsorbates with different anchoring groups

We designed multivalent β-cyclodextrin-based adsorbates bearing different anchoring groups aiming to yield stable monolayers with improved packing and close contact of the cavity to the gold surface. Toward this end the primary rim of the β-cyclodextrin was decorated with several functional groups, namely iodide, nitrile, amine, isothiocyanate, methyl sulfide, and isocyanide. Monolayers formed by these adsorbates were characterized by contact angle measurements, surface plasmon resonance spectroscopy, polarization modulation infrared reflection adsorption spectroscopy, X-ray photoelectron spectroscopy, and electrochemistry. The nature of the anchoring group influenced the adsorption kinetics, thickness, layer stability, number of anchoring groups bounded to the surface, and packing in the resulting monolayers. Therefore, chemical manipulation of multivalent adsorbates can be used to modify the properties of their monolayers.

SERS spectroscopic evidence for the integrity of surface-deposited self-assembled coordination cages

A series of self-assembled coordination cages was examined by Raman spectroscopy in solution and Surface Enhanced Raman Scattering (SERS) on a nanostructured Au surface.

Mechanism and release rates of surface confined cyclodextrin guests

The dissociation of a cobalt bisdiphenylterpyridine, [Co(biptpy)(2)](2+), guest at mixed (γ-CD-(py)(2))-alkanethiol layers (where γ-CD-(py)(2) is di-6(A), 6(B)- deoxy-6-(4-pyridylmethyl)amino- γ-cyclodextrin) formed on platinum electrodes is reported. Cyclic voltammetry (CV) shows reversible one-electron surface confined waves consistent with the Co(2/3+) couple bound at the interface. The quantity of [Co(biptpy)(2)](3+) reduced is found to be dependent on the scan rate employed, with greater amounts at higher scan rates. This behavior is in contrast to the CD guest ferrocene, which upon oxidation to the ferrocenium ion shows little charge associated with reduction even at elevated scan rates. Chronocoulometry was conducted to systematically vary the time spent oxidizing [Co(biptpy)(2)](2+) and to measure the resulting charge associated with the reduction of [Co(biptpy)(2)](3+). It is determined experimentally that as the pulse width increases, i.e. greater time spent in the oxidizing region, the amount of charge needed to reduce [Co(biptpy)(2)](3+) decreases dramatically. This decrease, along with the CV data, suggests strongly that the [Co(biptpy)(2)](3+) dissociates from the cavity. Significantly, this dissociation of the interfacial host-guest complex occurs on a much longer timescale (the order of seconds) compared to the oxidation of [Co(biptpy)(2)](2+) to [Co(biptpy)(2)](3+), which has been measured using high speed chronoamperometry to occur with a rate contant, k(0), of approximately 10(3) s(-1). The comparison of the timescale for dissociation of the interfacial complex and for electron transfer signifies that the electron transfer step occurs before dissociation, i.e. dissociation via an EC mechanism. The dissociation mechanism of [Co(biptpy)(2)](3+) is contrasted with that of the ferrocene/ferrocenium couple.

Host-guest directed assembly of gold nanoparticle arrays

The formation of a three-dimensional assembly of gold nanoparticles driven by host-guest interactions is described. Assembly is achieved via host-guest interactions between cyclodextrin-modified gold nanoparticles which associate with the adamantane unit of a redox active metal complex [Os(CAIPA)(3)](ClO(4))(2), where CAIPA is 2-(4-carboxyphenyl)imidazo[4,5-f][1,10]-phenanthroline-1-adamantylamine. The electrochemical properties of thin films formed on glassy carbon electrodes have been probed using cyclic voltammetry. In aqueous LiClO(4), the homogeneous charge transport diffusion coefficient, D(CT), is (8.4 +/- 0.4) x 10(-9) cm(2) s(-1) for both oxidation and reduction of the osmium complexes. Significantly, this charge transport rate is significantly larger than that obtained for a solid deposit of [Os(CAIPA)(3)](ClO(4))(2) alone where D(CT) is 2.3 x 10(-10) cm(2) s(-1). The higher D(CT) value observed for the nanoparticle arrays suggests that the incorporated nanoparticles facilitate electron transfer between the bound osmium centers.

Electrochemical desorption of fibrinogen from gold

The electrochemically induced desorption of Oregon green labeled fibrinogen layers from clean gold surfaces at negative potentials has been probed using capacitance, fluorescence microscopy, and atomic force microscopy. Capacitance measurements on fibrinogen layers indicate that desorption occurs at potentials more negative than -0.8 V and that complete desorption occurs when the electrode is biased at -1.2 V. Significantly, the fluorescence intensity initially increases as the dye labeled protein is electrochemically desorbed due to a decrease in quenching by the gold surface. Following this initial increase, the protein diffuses into solution and the fluorescence intensity decreases over time. More than 90% of the dye labeled fibrinogen is desorbed and diffuses out of the confocal volume in less than 2000 s when the potential is stepped to -1.2 V. AFM before and after application of the desorbing potential confirms removal of the protein. Collection of the desorbed protein in solution reveals a surface coverage of (4.0 +/- 2.3) x 10(-13) mol cm(-2) or an area of occupation of 400 +/- 140 nm(2) per molecule, which indicates that the protein is not extensively spread on the bare gold surface. Significantly, SDS-PAGE analysis indicates that the adsorption-desorption cycle dramatically effects the protein structure, with the electrochemically desorbed fibrinogen showing extensive fragmentation compared to native protein.