Recent progress in chiral inorganic nanostructures

Chirality is one of the most fascinating occurrences in the natural world. A chiral molecule is one that has two mirror-image forms which are non-superimposable in three dimensions. The mirror-image forms of the chiral molecule are classified as enantiomers. Chirality plays an important role in the fields of chemistry, pharmacology, biology and medicine. Chirality is also one of the key factors in molecular recognition, which has many uses in chemistry and biology. Discovering efficient methods to produce and identify enantiopure molecules is critical for the development of pharmaceuticals, agrochemicals, fragrances and food additives. Chirality has also been envisaged to play an important role in nanotechnology. Potentially, any nanocrystal can be chiral since they frequently have low symmetry due to the presence of chiral defects in bulk and at the surface.¹  However: nanocrystals in a macroscopic ensemble in a solution typically show no optical activity (circular dichroism) for the reason that nanocrystal chirality is random. Nevertheless, chiral inorganic nanocrystals can be readily designed and fabricated. Over the last years the area of chiral metal nanoparticles has received a great deal of attention due to the range of potential applications offered by these materials in chiral sensing, catalysis and as metamaterials in advanced optical devices.^1,2  The use of stereospecific chiral stabilising molecules has also opened another avenue of interest in the area of quantum dot (QD) research.^3–5  However, this is still quite a new field, as there is a limited number of publications dealing with chiral light emitting semiconducting nanocrystals and the clear application of these nanomaterials have not been materialised yet.

The development of new chiral nanoparticles is of great interest not only for nanotechnology, but also for many other fields of scientific endeavour including chemistry, biochemistry, pharmacology and medicine. In addition, the understanding of the fundamental concepts relevant to chirality in nanosystems is very important for the advancement of nanoscience and nanotechnology in general. There were several interesting and useful reviews on chiral nanostructures during the last 3–4 years.^1,2,6–9  In this review chapter we aim to present some recent advances¹⁰  within the last three years in the production and potential applications of various chiral nanostructured inorganic materials, including metallic (plasmonic), semiconducting, metal oxide and silica based chiral nanomaterials as well as new chiral hybrid nanostructures.

Over last 3 years chiral metallic nanoparticles have been a very active field of research. Particles of gold, silver, platinum and other noble metals have been reported using chiral ligands such as penicillamine,¹¹  cysteine,¹²  and glutathione.¹³  This field is particularly attractive, because plasmonic metallic nanoparticles show the ability to enhance chiral signals through surface plasmonic resonance (SPR). For example, recently Rakovich et al.¹⁴  reported considerable (up to 8 times) enhancement of the optical activity of J-aggregates in the presence of silver nanoparticles (Fig. 1). Examination of these materials showed that they formed micelles and reverse-micelles due to the presence of a polyelectrolyte on the surface of the nanoparticles.

Azizi et al.¹⁵  have also shown that these enhancements can be improved by inter-particle distance, which can be controlled through the use of DNA linkers of varying length. Alterations in the surface plasmon of nanoparticles also lead to changes in the CD spectrum. Yao et al.¹⁶  have reported bimetallic Au–Pb nanoparticles which showed strong variation in their CD spectrum due to expected variations in electronic structure. These variations can sometimes come from the actual structure of the nanoparticles. Achiral ligands can be observed to generate chiral responses from their position in relation to the surface structure of nanoparticles.^17,18  It was also calculated by Liu et al.¹⁹  that two distinct crystal morphologies form in the presence of chiral thiol ligands which are results similarly reported by Jin et al.,²⁰  who identified two crystal arrangements caused by the asymmetric positioning of metal atoms in the cluster. In addition, Palmer et al.²¹  also demonstrated the chiral arrangement of gold atoms in gold nanoparticles using aberration corrected scanning transmission electron microscopy (STEM). The degree of distortion of metallic nanoparticles were determined via calculations by Hidalgo et al.²²  to have maximum importance for the CD signals generated by the presence of chiral ligands on the surface of gold nanoparticles while in the case of silver nanoparticles there was limited distortion and the observed CD signal was calculated to have come from molecular orientation of the ligand on the nanoparticle surface.

The variation in the structure of the ligand sphere can also be used for the generation of chiral nanostructures. Theoretical basis for this result comes from Crasto et al.²³  who have analysed the formation of gold nanoclusters. The development of asymmetry in the ligand sphere of the gold clusters was identified as the source of the observed circular dichroism. Moretto et al.²⁴  also report the use of a peptide chain as a ligand for the growth of gold nanoparticles. As the polyalanine peptide grew in length, there were changes in the secondary structure of the peptide which lead to changes in their structural chirality that were subsequently enhanced by the surface plasmon of the gold nanoparticle. Changing the ligand sphere of metallic nanoparticles can also be of use for examining the nature of chirality. Bürgi et al.²⁵  for example, demonstrated the loss of a chiral response by thiolated gold nanoparticles in response to heating as well as showing the effect of a limited phase transfer of gold clusters from aqueous to organic medium using a chiral agent.²⁶  This latter research noted that at low concentrations, the organic and aqueous phases showed mirror image CD signals. There are also reports of diastereoselective ligand exchange by chiral molecules also showing the effect of specific chiral nanoclusters.²⁷  The ligand can also protect the chirality of metal particles. Bürgi et al.²⁸  generated Au nanoclusters with diastereomeric thiolated ligands. The ligands showed an ability to retain the chiral structure of the metal cluster to higher temperatures than that of unprotected nanoclusters.

In another paper G. Markovich et al. report the synthesis of a range of colloidal selenium, tellurium and gold nanostructures with enantioselectively controlled lattice and shape chirality using chiral ligands.¹²  In this work initially colloidal nanostructures of Se and Te with various morphologies (Fig. 2) have been prepared by applying strongly binding chiral ligands (glutathione, cysteine, penicillamine). All samples demonstrated strong CD responses. Moreover it was found that the chiral Te nanostructures of a size in the order of 100 nm can act as chiral optical resonators that may be useful for the optical sensing of chiral molecules. In addition the chiral Te nanostructures have been transformed into chiral gold and silver telluride nanostructures by the simple galvanic replacement of Te nanocrystals. These metal telluride nanostructures have shown very large chiroptical activity. The researchers believe that these new chiral plasmonic and semiconducting nanomaterials are promising candidates for investigation of interactions of chiral biomolecules with chiral inorganic surfaces, that might be important for applications in asymmetric catalysis, chiral crystallization and the evolution of homochirality in biomolecules.

Templates may be used for the synthesis of chiral metal nano structures. McPeak et al.²⁹  have generated left and right asymmetric gold nanostructures through the use of angular structures produced on a Si (111) surface by chemical etching with KOH solution. Similar work was conducted through a novel metal assisted chemical etching.³⁰  This method generated a series of spiral structures that can then be used for photonic applications. Gold was then deposited on the surface and extracted using a deposited silver layer and polymer. The resulting particles could then be dispersed in organic suspension and exhibited a strong CD signal in the red region of the absorbance spectrum. These templates also had the benefit of reusability, the authors report that the templates have been reused up to ten times. Lithographic methods were also exploited by Giessen et al.³¹  in the production of curved gold nanostructures on surfaces which were observed to exhibit optical activity depending on the direction of the curve (Fig. 3). Platinum chiral mesoporous structures were reported by Kuhn et al.³²  Self-assembly of liquid crystal molecules was used to form structures on a gold surface and platinum metal then deposited over this template using electrodeposition. These chiral structures demonstrated enantioselectivity for the absorption of enantiomers of dopamine which was tested using electrochemical methods. Templates have also been reported by Oda et al.³³  to which had been bound gold nanoparticles. The size of the resulting nanoparticles can be controlled through the use of different solvents and subsequently, these assemblies were demonstrated to detect the presence of benzenethiol through surface enhanced Raman. These structures were then posited to be used for ultrasensitive chemical and biological sensing.

The development of chiral assemblies and interacting metallic nanoparticles has attracted a considerable interest for the effect that their structure would have on their optical properties. Yan et al.³⁴  have demonstrated the use of gold nanoparticles which have been formed using DNA or NaCl and were noted to have formed dimers which possessed a variable chirality depending on the temperature or the quantity of salts in the solution. Jung et al.³⁵  reports the use of helical nanofibres as a template for the production of CD active assemblies. Assemblies of Au nanoparticles on surfaces were shown to exhibit chiroptical activity as reported by Ghosh et al.³⁶  These assemblies were generated using UV light that reduced the Au(I) salt to gold metal. Another interesting form of templating was shown by Wang et al.³⁷  whereby chiral stabilisers form silver nanorods to which gold nanoparticles can be bound producing optically active helical assemblies or in the preparation of chiral nanoparticles using chiral liquid crystals.^38,39  Matsukizono et al.⁴⁰  also reported the use of glucaric acid, and poly(ethyleneimine) which formed template assemblies which was controlled using changes in pH and can then be used for the synthesis of chiral silica nanosheets. The use of a co-block polymer assemblies was reported by Soto et al.⁴¹  which generates chiral porous structures that can be functionalised with gold nanoparticles. The assembly of chiral quantum dots and metal nanoparticles into metal-semiconductor dimers have been described by Kuang et al.⁴²  with enhancement of CD activity noted by the QDs bound to silver nanoparticles or in the case of Tang et al.,⁴³  the optical activity of CdTe nanoparticles were enhanced and red-shifted by close proximity to gold nanorods. Kuang et al.⁴⁴  also exhibited the formation of assemblies of Au nanoparticles with Au nanorods held together via hybridized DNA.

Another method of producing optical activity in gold nanoparticles was demonstrated by Kotov et al.⁴⁵  were antibody–antigen interactions were used to generate dimers which exhibited a CD signal with increasing time. The same group⁴⁶  has utilised similar assemblies which were examined for chirality all the way down to the single nanoparticle level. The researchers⁴⁷  also generated chiral assemblies of gold nanoparticles using PCR reactions changing the results of CD results with increasing numbers of cycles. In addition Kotov's group has also produced very interesting 3D chiral plasmonic nanostructures with high optical activity by vacuum evaporation of gold at different inclination and rotation angles on initially achiral nanopillars from ZnO.⁴⁸  This approach enables to process large area substrates with achiral nanopillars which should facilitate the up-scaling of this technology for potential production of sensors, optical devices, and catalysis.

Giant chiroptical activity was reported for gold nanorods by Xu et al.⁴⁹  involving the use of lipid self-assembly to make defined assemblies of nanorods which generated strong optical activity in the far field region of the absorbance spectrum. They demonstrated the observation of activity of DNase by monitoring the activity of the CD active signal in the plasmon resonance region of the spectrum. Similar work was also conducted by Xu et al.,⁵⁰  who employed Au nanoparticles of different sizes which were then used for assaying the catalytic activity of DNases (Fig. 4). With the development of this test the system could then be used to measure the reduction of enzyme activity by chemical inhibitors.

On a wider scale, the 3D formation of gold nanoparticles can greatly affect the optical activity of nano-assemblies. These kinds of formations can be formed through the use of peptides bound to the surface leading to the generation of helical spirals as described as Rosi et al.⁵¹  These helices generated possessed optical activity in the main plasmon bands which can be tuned through the altering of the size of a silver shell of the nanoparticles. Govorov et al.⁵²  have also demonstrated a DNA-gold nanoparticle assembly that can be made either parallel or perpendicular to the incident beam of polarised light resulting in distinct CD spectra as also demonstrated by computational means.⁵³  Similar work was done by Ding et al.⁵⁴  have used DNA origami to bind gold nanoparticles into three dimensional structures that demonstrated strong optical activity depending on the arrangement. Examination of some of these plasmonic nanospirals was shown to exhibit strong optical activity. A strong theoretical basis for the optical activity of these three dimensional structures can be of interest for the effect that these shape have on their optical properties. The position of the surface plasmon on the structure of shaped silver nanoparticles was calculated by Savaloni et al.⁵⁵  These nanoparticles were shown to exhibit strong absorbance coupled with their optical activity. Further work was conducted examining the interaction between polarised light and plasmonic nanospirals by computational means by Haglund et al.⁵⁶  They demonstrated that circularly polarised light interacted more strongly with these modelled nanospirals compared with linearly polarised light.

A particularly interesting examination of the assembly of silver nanoparticles as potential ‘metamaterials’ was reported by Dionne et al.⁵⁷  Protein/antibody interactions were used to generate ‘Metafluids’ which were examined for variations in optical properties such as magnetic optical activity. Zhao et al.⁵⁸  also reported the production of a novel series of gels containing gold nanoparticles. These gels were shown to exhibit a highly different optical activities depending on the solvent used. It has also been reported by Prasad et al.⁵⁹  that silver nanoparticles can be formed into wide assemblies which would exhibit optical activity.

There are a number of uses for chiral metallic nanoparticles. One such application for chiral nanoparticles are as chiral sensors. This is considered feasible as it has been already demonstrated that uncoated gold nanoparticles can show selective aggregation in the presence of d-tryptophan and copper ions while none in the presence of l-tryptophan and other aminoacids.^60,61  Similar work was conducted for diols (Fig. 5) using functionalised gold nanoparticles⁶²  and for the chiral dicationic helicene, helquat.⁶³ 

Such gold nanoparticles were also employed for the examination of the enantiomeric excess of histidine.^61,64  Chiral gold dimers were used by Xu et al.⁶⁵  to detect cysteine down to a concentration of 20 pM. The chiral signal may also be of use for the identification of the concentration of toxins in environmental samples. In work by several groups, Bisphenol A⁶⁶  and oligopeptides⁶⁷  were detected using Gold nanoparticles which had been functionalised using a DNA based highly specific aptasensor. In similar work, the model antibiotic sulfadimethoxine down to nanogram concentration was detected by Xu et al.⁴⁷  using antibodies bound to gold nanoparticles and monitoring changes in the strength of the CD signal. The same group has also reported similar work of the sensing of lead ions in water.⁶⁸  Another example of the use of sensing by chiral nanoassemblies was described by Zhang et al.⁶⁹  where the addition of high quality DNA to gold nanoparticle to methylene blue and multiwalled carbon nanotubes on a glassy carbon electrode produced an assembly which can be used for the selective electrochemical sensing of the two different enantiomers of propranolol. Chiral sensing of DNA concentrations down to the attomolar level were reported by Kotov et al.⁷⁰  who used gold nanorods functionalised with a primer DNA strand (Fig. 6). The interaction of chiral cyclodextrins bound to carbon nanoparticles which can be used as a stationary phase for the electrolytic separation of chiral organic molecules by Zhang et al.⁷¹ 

Non-chiral nanoparticles were also of interest for sensing of optical activity through the enhancement of the chirality of molecules in the presence of the plasmonic field at the edge of, for example nanocubes⁷²  or gold structures on surfaces.⁷³ 

The separation of enantiomers from a racemic mixture was conducted using gold nanoparticles functionalised with a DNA aptamer by Huang et al.⁷⁴  that was capable of the repeated extraction of l-tryptophan from the racemic mixture with up to 10 extractions with good selectivity reported. Similar work using a novel polymer ligand was reported by Deng et al.⁷⁵  for the production of chiral gold nanoparticles involving the polymer poly (N-propargylamide) and was shown to selectively absorb the S isomer of phenyl ethylamine. Cyclodextrins bound to gold nanoparticles⁷⁶  or graphene/iron oxides⁷⁷  as a stationary phase for the enantiomeric separation of chiral molecules.

Another possible application of chiral metal nanostructures is catalysis. For example, Kobayashi et al.^78,79  used chiral noble metal nanoparticles for asymmetric C–C bond formation. In another work Heiz et al.⁸⁰  employed variations of the pH for the ligand N-acetyl cysteine that were subsequently bound to platinum nanoclusters to generate differences in the moieties bound to the surface of the cluster. The researchers also reported the special production of Pt nanoclusters in the presence of ethylene glycol which were then replaced with cysteine and derivatives generating chiral functionality on the particles. These nanoclusters were then be used for the enantioselective hydrogenation of butanone. Finally, Somorjai et al.⁸¹  produced a chiral self assembled monolayer of gold nanoparticles in mesoporous silica which has been used to catalyse the production of cyclic propanes in high enantiomeric excess.

There were several interesting recent publications on chiral semiconducting nanoparticles and their applications.

A remarkable phenomenon of chiral ligand induced circular dichroism in CdSe quantum dots was reported by M. Balaz et al.^82,83  The researchers have found that chiral thiol capping ligands such as l- and d-cysteines can induce chiroptical properties in originally achiral cadmium selenide quantum dots (CdSe QDs). The process involved a simple phase transfer of achiral trioctylphosphine oxide or oleic acid capped CdSe QDs from toluene into aqueous phase using l- and d-cysteines by stirring the mixture at RT in the absence of light for 24 h. It was found that l- or d-cysteine stabilized QDs in aqueous phase demonstrated size-dependent electronic circular dichroism (CD) and circularly polarized luminescence (CPL). As expected, mirror images opposite CD and CPL signals have been shown by CdSe QDs capped with d- and l-cysteine (Fig. 7). In addition it was found that the CD profile and CD anisotropy varied with size of CdSe nanocrystals with largest anisotropy observed for CdSe QDs of 4.4 nm. The authors have also performed time Dependent Density Functional Theory (TDDFT) calculations showing the attachment of l- and d-cysteine to the surface of model (CdSe)₁₃ nanoclusters that induces measurable opposite CD signals for the exitonic band of the Nanocluster (Fig. 7). It was suggested that such redesign and modulation of chiroptical properties could lead to a range of applications in chiroptical memory, chiral biosensing and chiroptical nanomaterials.

There are also some interesting reports on the application of chiral quantum dots. For example, it was reported that CdSe/ZnS nanoparticles capped with N-acetyl-l-cysteine methyl ester can be used for sensing of various chiral organic drug molecules.⁸⁴  In this case chiral QDs consisting of an inorganic CdSe/ZnS core–shell and a chiral organic ligand have been prepared by a simple ligand exchange reaction of commercially available amine-capped QDs with methyl ester N-acetyl-l-cysteine (Fig. 8). Then these chiral QDs were used to perform studies of the chiral recognition of drugs, in particular the aryl propionic acids, ketoprofen, naproxen, flurbiprofen and ibuprofen. It was found that all of the drug molecules quenched the QD emission in a concentration-dependent mode. The spectral differences in the behavior of R- and S enantiomers of these aryl propionic acid drugs enabled to perform the quantitative determination of both chiral forms in mixtures and pharmaceutical samples.

In another work ZnS nanoparticles with an inducing chirality were used as a catalyst for asymmetric aldol condensation reactions.⁸⁵  In this case ZnS nanoparticles (Fig. 9) have been synthesized by a co-precipitation of ZnS in the presence of l-proline. Then these nanoparticles were used as a catalyst for the direct asymmetric aldol reaction of several aldehydes with acetone to achieve chiral b-hydroxy carbonyl compounds in good yields and enantioselectivity at room temperature without using any co-solvent for solubility purposes. It was found that the selectivity of the ZnS nanoparticles enabled to produce only (R)-b-hydroxy carbonyl compounds and restricted the reaction at the aldolization stage only. Importantly, notice that the ZnS catalyst was recovered and reused several times without any considerable loss in activity.

Over last 3 years there were interesting new developments in the area of chiral metal oxide based nanostructures.

S. Liu et al. reported new unique chiral TiO₂ nanofibres with electron transition-based optical activity. Initially amorphous TiO₂ double-helical fibres with a pitch length of approx. 100 nm nanomaterials have been prepared by transcription of the helical structure of amino acid-derived amphiphile fibres via coordination bonding interactions between the organics and the TiO₂ source. The following calcination of these amorphous material at 550 °C resulted in double-helical crystalline fibres with stacks of anatase nanocrystals in a helical relationship (Fig. 10). Most importantly, both the amorphous and anatase crystalline helical TiO₂ fibres have shown optical response to circularly polarized light at the absorption edge around 350 nm that was attributed to the semiconductor TiO₂-based electronic transitions from the valence band to the conduction band.⁸⁶ 

Chiral nematic mesoporous films of Eu³⁺ doped ZrO₂ have been produced via a hard-templating approach using nanocrystalline cellulose-templated silica (Fig. 11).⁸⁷  It was found that these chiral nematic nanostructures are capable of modulating the spontaneous emission of the Eu³⁺ ions. The emission lines of the Eu³⁺ at 596 nm, 613 and 625 nm were significantly suppressed, and an increase in the luminescence lifetime is observed. It was suggested that these new chiral luminescent nanomaterials could find potential applications in sensing and new optical nanodevices.

In another work chiral left- and right-handed helical zirconia nanotubes prepared through a sol–gel transcription approach using the self-assembly of a pair of chiral low-molecular-weight gelators (LMWGs) as templates (Fig. 12).⁸⁸  The calcinations of the helical zirconia nanostructures at 700 °C for 3.0 h resulted in ZrO₂ nanotubes with mixed monoclinic and tetragonal structures (Fig. 13). The diffuse reflectance circular dichroism spectra indicated that the zirconia nanotubes possess optical activity. The origin of chirality was explained by the transfer of chirality from amide groups to the inner surfaces of the ZrO₂ nanotubes through a CO…Zr⁴⁺ interaction. It is expected that chiral ZrO₂ nanotubes can potentially be used as asymmetric catalysts.

Another group reported the preparation of new chiral zirconia magnetic microspheres and their use as new recyclable selectors for the discrimination of racemic drugs.⁸⁹  The Fe₃O₄@ZrO₂@CDMPC microspheres have been produced via an immobilization of the polysaccharide cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC) on the surface of zirconia in Fe₃O₄@ZrO₂ core–shell structures. It was found that the ZrO₂ shell on Fe₃O₄ core not only stabilizes the particles in solution but also provides sites for surface modification of enantiomers with CDMPC which can be used for chiral discrimination and separation. The authors showed that there is a certain preferential adsorption of enantiomers and the proportion of adsorption of d-isomer and l-isomer was clearly different for different chiral drugs. The synthesized CZMMs have also shown an excellent recovery by magnet and recyclability that can be used for further chiral separations of different kinds of racemates.

Finally, very recently S. Che et al. reported the surfactant-mediated hydrothermal synthesis of chiral CuO nanoflowers.⁹⁰  In this work the researchers used cupric salt as precursor, sodium dodecylsulfate (SDS) as a structure-directing agent, an amino alcohol as a symmetry-breaking agent for the cupric ions. The hydrothermal synthesis has afforded uniform flower-like particles consisting of primary helically arranged “nanoflakes” and secondary helical “subnanopetals” that form “nanopetals”. Most importantly the nanoflowers have shown a very strong optical response in their diffused reflection CD spectra. The authors believe that this type of nanostructure might find application in electronics, photonics, photocatalysts, biosensors, etc.

Significant progress was also demonstrated in chiral silica based nanomaterials. An interesting new approach to the preparation of enantiomeric helical architectures via DNA self-assembly and silica mineralization was reported by Ben Liu et al.^91,92  These group have synthesized enantiomeric impeller-like helical DNA-silica complexes (IHDSCs) by introducing various metal ions into the co-structure directed synthesis (Fig. 14). In this case chiral 2Dsquare structured DNA packing gave rise to the formation of IHDSCs (Figs. 14 and 15).

The same group has reported water-dependent optical activity inversion of chiral DNA–silica assemblies.⁹³  In this study the optical activity of the above mentioned impeller-likechiral DNA–silica assemblies (CDSAs) was observed to be inverted with the addition of water. The state of DNA under dry and wet conditions, and the dual chirality of chiral DNA layers and twisted helical arrays of opposite handedness in CDSAs were considered to exert predominant effects on the circular dichroism (CD) responses: for the dry CDSAs were mostly attributed to the chiral arrangement of DNA layers, while the opposite CD responses for the wet CDSAs primarily originated from twisted helical arrays of DNA molecules. Moreover, the authors observed the inversion of the plasmon-resonance CD signal for the chiral-arranged achiral Ag nanoparticles (NPs) located in the channels of the CDSAs in dry and wet states. It is expected that the water-dependent inversion of CD responses of the CDSAs loaded with metal NPs can find potential applications for biosensing, chiral recognition and detection.

Then more recently the same group has further developed their approach and prepared optically active chiral inorganic films via DNA self-assembly followed by silica mineralization (Fig. 16).⁹⁴  The initial chiral coating was achieved via electrostatic interaction between positively charged quaternary ammonium groups and negatively charged phosphate groups of DNA, with subsequent growth to right-handed, vertically aligned, impeller-like helical architectures with left-handed two-dimensional square p4 mm-structured DNA chiral packing. After the calcinations, DNA was removed but the inorganic silica based impeller-like helical morphology was maintained (Figs. 16 and 17). Importantly, notice that hierarchical helical structures in the chiral DNA–silica films exhibited dual optical activities corresponding to the DNA chiral packing and the inorganic silica helical morphology. The authors postulated that these chiral materials could be used as hard templates to produce a variety of hierarchical helical inorganic hybrid films with optical activity. The chiral helical nature of these films may also find applications in catalysis, separation technology and tuneable reflective filters and sensors.

X. Wu et al. reported new chiral hybrid mesoporous silica based nanostructures which were formed by an assembly of uniform hollow nanospheres and helical nanotubes with tunable diameters (Fig. 18).⁹⁵  In this work helical silica nanotubes and hollow silica nanostructures have been prepared via the self-assembly of sodium dodecyl sulfate (SDS) as the surfactant, N-trimethoxysilylpropyl-N,N,N-trimethylammoniumchloride (TMAPS) as a co-structure directing agent (CSDA), a binapthyl-based chiral dopant and Si(OEt)₄ as a silica source. It was shown that the morphology and the size of hollow silica nanoparticles can be controlled by varying the ratio of anionic surfactant to cationic directing agent. The inclusion of chiral bissiloxanes in the mixture at low levels can also be used to produce helical chiral structures.

In other work: silica based helical and twisted nanoribbons of controlled chirality were synthesized by sol–gel processing in acidic conditions using organic self-assembly as a template (Fig. 19).⁹⁶  The authors have demonstrated that nanohelices can be successfully fragmented into individualized chiral helical and twisted silica ribbons of several hundred nanometers by a sonication technique. It was found that the power of sonication and nature of the solvent are crucial parameters for achieving narrow size distribution of the fragmented helices, and the better the dispersion. In addition it was shown that freeze-drying of the helices clearly consolidated the Si–O–Si bonds. The sonication of helices in water or in ethanol directly after the transcription destroyed the local chiral structures, whereas the helices which were freeze-dried first and then dispersed in these solvents preserved their local chiral structure after sonication.

R.-H. Jin et al. reported the preparation of silica on chiral crystalline catalytic template (polyethyleneimine and d-, l-, or Rac-tartaric acid) which has fibrous and very thin nanoribbon structures where chirality was imprinted with the silica formation.⁹⁷  By contrast to chiral mesoporous silica, which has helical type chirality with a definite pitch, this chiral silica did not show a topologically characteristic helix. Therefore in this chiral silica structure, geometrical chiral blocks with the same chiroptical features are distributed randomly through the silica wall (Fig. 20). It was found that this randomly imprinted chirality is highly resistant to the high-temperature sintering with retention of the chiral performance and would be able to induce chirality into achiral guests, including molecules and nanoparticles, if the guests are bound onto the silica.

S. Che et al. have utilised chiral mesoporous silica ribbons and rods as a hard template for synthesizing metal oxide nanomaterials with electronic circular dichroism (Fig. 21).⁹⁸  The initial chiral mesoporous silica ribbons (CMRs) synthesised by self-assembling chiral amphiphile enantiopure N-acylamino acid (C14-l/d-Ala), co-structure directing agent (CSDA) 3-aminopropyl triethoxysilane (APES), and silica source tetraethoxylsilane (TEOS) at 0 °C. In this case the amino groups of APES electrostatically interact with the hydrophilic head groups of the chiral amphiphiles resulting the helical alignment of functional groupson the mesopore surface surrounding the helical propeller-like micelle due to molecular imprinting. After the extraction, metal ions were bound to the amino groups helically oriented on the pore surface due to coordination bonding. Then the calcinations resulted in metal oxide nanoparticles (MONPs) would become arranged along the helical pore. Most importantly the helically arranged metal oxide nanoparticles were found to be optically active at their electron transition based absorption bands, despite the individual chromophore nanoparticles being achiral.

Chiral silica based nanostructures may find a range of important applications. For example, optically active silica nanoparticles of grafted with helicene ((P)-1,12-dimethyl-8-methoxycarbonylbenzo[c]phenanthrene-5-Carboxyamide) have been used for optical resolution of aromatic alcohols.⁹⁹  By using preferential precipitation of aggregates formed from the helicene grafted silica nanoparticles up to 61% enantiomeric excess for (S)-2,2-dimethyl-1-phenyl-1-propanol was obtained. The same research group used the helicene-grafted silica nanoparticles for molecular shape recognition of double helix oligomers that precipitated from solution.¹⁰⁰  This effect can be considered as an impart of molecular structural information visualized by precipitation, including shape recognition, absorption, aggregation, and precipitation. In addition the removal of a single structure of an organic molecule from solution by precipitation resulted in equilibrium shift.

Chiral hybrid nanostructures are unique composites which might combine 2 or more different types of materials in one structure. This is a very novel area and there are only a limited number of publications on this type of chiral system.

An interesting novel strategy for nonexclusive fluorescence sensing of enantiomers using chiral nanoparticles and their ability to form dynamic assemblies was offered by Y. Xia et al. (Fig. 22).¹⁰¹  The researchers have found that fluorescence resonance energy transfer (FRET) in nanoscale assemblies consisting of either l-cysteine- or d-cysteine-modified quantum dots (QDs) and gold nanorods (GNRs) is strongly dependent on traces of enantiomeric cysteine molecules. This phenomenon was explained by the high sensitivity of dynamic QD-GNR assemblies to the weak inter-nanoparticle interactions that can exponentially increase energy transfer efficiencies from QDs to GNRs. Both enantiomers at different concentration of cysteine have been determined by comprehensive analysis of the data from two nonexclusive sensing platforms. It was shown that this technique enables the quantification of the composition of a chiral sample, even if the content of one enantiomer in the mixture is as low as 10%. The authors believe that this methodology may be expanded for designing other sensing systems for chiral assays of drugs, metabolites, and other chemicals.

P. Fischer et al. have reported a new approach for production of hybrid nanocolloids with programmed three-dimensional shape and material composition (Fig. 23).¹⁰²  The fabrication process involves a combination of low-temperature shadow deposition with nanoscale patterning and enables the production of nanocolloids with anisotropic three-dimensional shapes, feature sizes down to 20 nm and a wide choice of materials. As one example of the structures the researchers have fabricated hundreds of billions of plasmonic Au or Ag:Cu nanohelices, which have demonstrated circular dichroism and tuneable chiroptical properties. In overall this work presents a convenient method to the fabrication of various complex 3D nanostructures with tailored electric, magnetic, optical and mechanical properties.

Chiral mesoporous silica templates may also be used for the preparation of chiral hybrid nanostructures. For example, J. Xie et al. have investigated chirality of hybrid nanostructure which was produced by introduction of small achiral Ag nanoparticles into highly ordered chiral mesoporous silica structures (Fig. 24).¹⁰³  This hybrid system has shown three different types of chirality: i) the helical hexagonal surface, ii) the helical pore orientation, and iii) the helical arrangement of aminopropyl groups on the surface of the mesopores. The chiral pore orientation was found to be the predominant part for the CD response. It was found that the length of helical channel was more effective for increasing the intensity of plasmonic circular dichroism due to longitudinal propagation of Ag nanoparticles along helical channel. The authors suggested these chiral mesoporous nanostructures could have potential applications in non-linear optics, biosensors and chiral recognition and detection in biological structures.

In another work, silica nanohelices have been prepared from the organic self-assemblies of -tartrate amphiphiles by sol–gel transcription. Then this chiral nanostructures were functionalized with (3-aminopropyl)-triethoxysilane (APTES) or (3-mercaptopropyl)triethoxysilane (MPTES) and decorated with gold nanoparticles of various diameters (1–15 nm) resulting in nanohelix hybrid structures (Fig. 25).¹⁰⁴  It was found that the surface plasmon resonance intensity of these nanohybrid systems increased with gold particle size. Gold nanoparticles of 10–14 nm diameter have clearly showed a surface enhanced effect on Raman spectroscopy. This system is a unique example of the 3D hybrid network that could be used as ultrasensitive chemical and biological sensors for detection of molecules of interest in liquids by accumulation under flow.

From the review above we can witness the fast growing interest in chiral nanostructures and as a result a significant development of this field over the last 3 years. As expected chiral plasmonic nanostructures have deserved a major attention due to their ability to enhance chiral signals and consequent development of their applications as highly sensitive chiral plasmonic sensors. An excellent example of that is the demonstration by Kotov et al.⁷⁰  of chiral sensing of DNA concentrations down to the attomolar level using chiral gold nanorods functionalised with a primer DNA strand. A significant progress was also made in the preparation of uniquely shaped chiral plasmonic nanostructures using nanolithography³¹  and lithography-less vapour deposition of metal on pre-fabricated nanopillar substrates.⁴⁸  Importantly to notice, that these techniques can be easily scaled up if it is necessary for the further large scale production of corresponding chiral plasmonic nanostructures.

Reports of the induction of chirality in CdSe based quantum dot structures by a simple chiral ligand assisted phase transfer of initially non-chiral QDs^82,83  are very important and open up new horizons in the development of chiral nanomaterials. It is expected that so convenient methods to induce chirality will be extensively used for the preparation of chiral semiconducting and potentially other inorganic chiral nanostructures in the near future. In addition the demonstration of circular polarised light emission by QDs with induced chirality might find a range of potential applications in various optical devices, components of chiroptical detectors, polarimeters, CD spectrometers and in the long term even in colour displays. In overall very significant advancements in the development of real applications of chiral QDs have been demonstrated during the last 2 years. These include sensing of various chiral organic drug molecules⁸⁴  and catalysis of asymmetric aldol condensation reactions.⁸⁵  Both these applications are very important and should attract more interest in chiral QD based nanomaterials.

Another area which was initiated during last year is development of chiral metal oxide based nanomaterials such as chiral TiO₂ nanofibres⁸⁶  and chiral ZrO₂ nanotubes.⁸⁸  It is anticipated that these chiral metal oxide nanostructures will find very important applications as asymmetric catalysts. In addition the progress in the fabrication of mesoporous silica based chiral nanostructures (e.g. helical architectures) should open new opportunities in chiral separation of enantiomeric compounds.

Finally, over the last number of years there have been significant achievements in the development of completely novel types of chiral hybrid nanostructures containing plasmonic and combinations of other nanomaterials in one system. Particularly the development of a new fabrication process which involves a combination of low-temperature shadow deposition with nanoscale patterning that provides access to highly unusually shaped 3D chiral nanomaterials containing multiple components from various materials.¹⁰²  Again this technique can be easily scaled up to produce these nanomaterials at large scale if necessary. Potentially new chiral multicomponent nanomaterials can find many important applications ranging from sensing to catalysis and enantiomeric separation.

In general, we expect that further uses for chiral nanomaterials will continue to develop rapidly and the demand for these materials will grow in the near future. However, further systematic experimental and theoretical studies will be needed for fundamental understanding of the properties and behaviour of these new chiral nanostructures for the development of their appropriate applications.

The authors acknowledge financial support from FP7 FutureNanoNeeds grant, Science Foundation Ireland (Grants SFI 12/IA/1300) and the Ministry of Education and Science of the Russian Federation (Grant no. 14.B25.31.0002).