Vibrational circular dichroism (VCD) spectra of small size-selected gold nanoparticles covered by both enantiomers of 1,1′-binaphthyl-2,2′-dithiol (BINAS) were measured. VCD spectra of particles covered by the opposite enantiomers of BINAS show a mirror image relationship. The VCD spectrum of adsorbed BINAS is different from the one of free BINAS and its disulfide form, but it resembles more the dithiol form. Detailed analysis reveals that the angle between the two binaphthyl rings of BINAS is close to 90° for the adsorbed BINAS, similar to what is found for the free molecule. VCD spectra are quite insensitive to the particles size, in contrast to the electronic CD spectra, which change drastically as the particle size increases. This indicates that the vibrational characteristic is a local property. A model of BINAS adsorbed on a Au₁₀ cluster was used to calculate VCD spectra. As for free BINAS and the disulfide the calculated spectrum of the adsorbed BINAS is in very good agreement with the measured one. This shows the potential of VCD spectroscopy to gain insight into the conformation of chiral molecules adsorbed on small metal particles.

The intense plasmon absorption bands of gold nanorods (GNRs) with peak extinction coefficients up to 6.4 x 109 M-1 cm-1 as well as their expected high stability make GNRs promising candidates for the colouration of bulk materials. The comparison of the integrated absorption in the visible region of GNRs with those of commercial organic pigments shows that the colouring strength of GNRs is 4 to 8 times higher. In order to improve their stability, GNRs were encapsulated in a silica shell of around 15 nm thickness using an optimized Stöber method. The silica surface was modified with octadecylsilane to enable their dispersion in non-polar media. Different plastics were successfully coloured with a tiny quantity of bare and functionalised GNRs@SiO2. These rods were homogeneously dispersed using extrusion. The shape of the rods was effectively stabilised by the silica shell at high temperature during the extrusion process. Surprisingly, a slight modification of the rods colour was observed due to a decrease of the refractive index in the mesoporous silica shell. However, this effect is greatly limited after the functionalisation.

Nanoparticle chirality has attracted much attention recently, and the application of chiral nanoparticles to chiral technologies (see figure) is also of interest. This Minireview deals with advances in the preparation and characterization of chiral gold nanoparticles. Origins of the chiroptical properties and potential applications are discussed. Monolayer-protected gold nanoparticles have many appealing physical and chemical properties such as quantum size effects, surface plasmon resonance, and catalytic activity. These hybrid organic–inorganic nanomaterials have promising potential applications as building blocks for nanotechnology, as catalysts, and as sensors. Recently, the chirality of these materials has attracted attention, and application to chiral technologies is an interesting perspective. This minireview deals with the preparation of chiral gold nanoparticles and their chiroptical properties. On the basis of the latter, together with predictions from quantum chemical calculations, we discuss different models that were put forward in the past to rationalize the observed optical activity in metal-based electronic transitions. We furthermore critically discuss these models in view of recent results on the structure determination of some gold clusters as well as ligand-exchange experiments examined by circular dichroism spectroscopy. It is also demonstrated that vibrational circular dichroism can be used to determine the structure of a chiral adsorbate and the way it interacts with the metal. Finally, possible applications of these new chiral materials are discussed.

Ligand exchange on [Au₂₅(SCH₂CH₂Ph)₁₈^−] [TOA⁺] is studied with two chiral ligands R/S-BINAS and NILC/NIDC in THF with induction of metal-based optical activity. Under the applied condition the ligand exchange is only partial, showing that also within a mixed ligand shell significant optical activity can be induced. The ligand exchange resulted in the change of particle size as observed by UV−vis spectroscopy.

Gold particles covered with 1,1′-binaphthyl-2,2′-dithiol (BINAS) were prepared. Using size exclusion chromatography, it was possible for the first time to separate the sample into fractions with different sizes and colors. Transmission electron microscopy shows that the particles are very small, in the order of 1 nm or slightly above. The absorption spectra of the separated samples show rich structure. The particles show size-dependent optical activity in metal-based electronic transitions. The shape of both the absorption and circular dichroism spectra of one of the smallest fractions exhibits similarities with the spectra reported for Au₁₁ covered by 2,2′-bis(diphenylphosphino)-1,1′-biphenyl although the spectra are shifted to shorter wavelengths in the case of the dithiol. The anisotropy factors, Δϵ/ϵ of these particles are as large as 4 × 10^−3, which is larger than the values reported for gold particles stabilized by phosphines and water-soluble thiols. This indicates that BINAS is particularly well-suited to impart chirality on to gold particles.

A combination of in situ attenuated total reflection infrared (ATR-IR) spectroscopy, UV−vis spectroscopy and transmission electron microscopy was used to study the adsorption of thiol-protected gold nanoparticles on TiO₂ films and the behavior of the resulting composite films upon UV irradiation. The gold nanoparticles were covered by charged thiols N-acetyl-l-cysteine and l-glutathione and had a mean core diameter of about 1 nm. The TiO₂ film was prepared by deposition of a slurry of TiO₂ nanoparticles with a particles size of 21 nm. The combination of the two spectroscopic techniques showed that the adsorption of the gold nanoparticles onto the TiO₂ films is significantly limited by intrafilm diffusion. Upon illumination the IR spectra revealed the removal of the adsorbed thiolates and the appearance of sulfates. These species were also observed when N-acetyl-l-cysteine adsorbed on TiO₂ was illuminated, i.e., in the absence of gold. In the latter case oxalate was observed in large quantity on the TiO₂ surface, in contrast to the illumination of the N-acetyl-l-cysteine-protected gold particles. This indicates a different pathway for the decomposition of the adsorbed thiol when adsorbed on the gold or directly on the TiO₂ surface. In situ UV−vis spectroscopy also shows the formation of larger particles upon illumination, which is confirmed by transmission electron microscopy.

The thiolate-for-thiolate ligand exchange was performed on well-defined gold nanoparticles under an inert atmosphere without any modification of the core size. This reaction is faster than the well-known core etching. Surprisingly, if a chiral thiol is exchanged for its opposite enantiomer, the optical activity in the metal-based electronic transitions is reversed although the form of the CD spectra remains largely unchanged. The extent of inversion corresponds to the overall ee of the chiral ligand in the system. This shows that the chiral arrangement of metal atoms in the metal particle (surface) can not withstand the driving force imposed by the ligand of opposite absolute configuration. If the incoming thiol has a different structure, the electronic transitions in the metal core are slightly modified whereas the absorption onset remains unchanged. These results emphasize the influence of the thiols on the structure of the gold nanoparticles and give insight on the ligand exchange pathways.

Biological homochirality on earth and its tremendous consequences for pharmaceutical science and technology has led to an ever increasing interest in the selective production, the resolution and the detection of enantiomers of a chiral compound. Chiral surfaces and interfaces that can distinguish between enantiomers play a key role in this respect as enantioselective catalysts as well as for separation purposes. Despite the impressive progress in these areas in the last decade, molecular-level understanding of the interactions that are at the origin of enantiodiscrimination are lagging behind due to the lack of powerful experimental techniques to spot these interactions selectively with high sensitivity. In this article, techniques based on infrared spectroscopy are highlighted that are able to selectively target the chiral properties of interfaces. In particular, these methods are the combination of Attenuated Total Reflection InfraRed (ATR-IR) with Modulation Excitation Spectroscopy (MES) to probe enantiodiscriminating interactions at chiral solid–liquid interfaces and Vibrational Circular Dichroism (VCD), which is used to probe the structure of chirally-modified metal nanoparticles. The former technique aims at suppressing signals arising from non-selective interactions, which may completely hide the signals of interest due to enantiodiscriminating interactions. Recently, this method was successfully applied to investigate enantiodiscrimination at self-assembled monolayers of chiral thiols on gold surfaces. The nanometer size analogues of the latter—gold nanoparticles protected by a monolayer of a chiral thiol—are amenable to VCD spectroscopy. It is shown that this technique yields detailed structural information on the adsorption mode and the conformation of the adsorbed thiol. This may also turn out to be useful to clarify how chirality can be bestowed onto the metal core itself and the nature of the chirality of the latter, which is manifested in the metal-based circular dichroism activity of these nanoparticles.

We have prepared gold nanoparticles covered with N-isobutyryl-l-cysteine and N-isobutyryl-d-cysteine, respectively. These particles with a mean particle size smaller than 2 nm are highly soluble in water and are amenable to chiroptical techniques such as vibrational circular dichroism (VCD) and circular dichroism (CD) spectroscopy. Density functional theory shows that the VCD spectra are sensitive toward the conformation of the adsorbed thiol. Based on the comparison between the experimental VCD spectrum and the calculated VCD spectra for different conformers, a preferential conformation of the thiol adsorbed on the gold particles can be proposed. In this conformation the carboxylate group interacts with the gold particle in addition to the sulfur. The particles could furthermore be separated according to their charge and size into well-defined compounds. The optical absorption spectra revealed a well-quantized electronic structure and a systematic red-shift of the absorption onset with increasing gold core size, which was manifested in a color change with particle size. Some compounds showed basically identical absorption spectra as analogous gold particles protected with l-glutathione. This shows that these particles have identical core sizes (10−12, 15 and 18 gold atoms, respectively) and indicates that the number and arrangement of the adsorbed thiol are the same, independent of the two thiols, which have largely different sizes. Some separated compounds show strong optical activity with opposite sign when covered with the d- and l-enantiomer, respectively, of N-isobutyryl-cysteine. The origin of the optical activity in the metal-based transitions is discussed. The observations are consistent with a mechanism based on a chiral footprint on the metal core imparted by the adsorbed thiol.

Chiral metal surfaces and nanoparticles have the potential to be used for the selective production, the resolution and the detection of enantiomers of a chiral compound, which renders them highly attractive in view of the tremendous consequences of homochirality on earth. Their capability to distinguish between enantiomers of a chemical compound relies on their structure and the ability to form intermolecular interactions. However, molecular-level understanding of the interactions that are at the origin of enantiodiscrimination is lagging behind due to the lack of powerful experimental techniques that are able to spot these interactions selectively with high sensitivity. In this article two techniques based on infrared spectroscopy are presented that are able to selectively target the chiral properties of nanoparticles and interfaces. These are the combination of attenuated total reflection infrared (ATR-IR) with modulation excitation spectroscopy (MES) to probe enantiodiscriminating interactions at chiral solid-liquid interfaces and vibrational circular dichroism (VCD), which is used to probe the structure of chirally modified metal nanoparticles.

Vibrational circular dichroism is used to determine the conformation of a thiol adsorbed on gold nanoparticles.