© 2015 Nature America, Inc. All rights reserved. PROTOCOL 558 | VOL.10 NO.4 | 2015 | NATURE PROTOCOLS INTRODUCTION Symmetry and chirality are properties commonly found through- out the natural world. Chirality is one of the most important factors in molecular recognition, with chiral compounds having a major role in chemistry, biology and medicine. Chirality has also been envisaged to have an important role in nanotechnology 1–6 , and an understanding of the fundamental concepts relevant to chi- rality in nanosystems is important for the further advancement of nanoscience in general and for nanobiotechnology in particular. Over the past years, the research on chiral metal nanoparticles has received a great deal of attention owing to the range of poten- tial applications offered by these materials as chiral sensors, cata- lysts and as metamaterials in advanced optical devices 1–3,7–11 . The use of stereospecific chiral stabilizing molecules has also opened up another avenue of interest in the area of QD research. Optically active chiral QDs (penicillamine-stabilized CdS) were first pre- pared by our group by using microwave-induced heating with the racemic (Rac), D- and L-enantiomeric forms of penicillamine used as stabilizers 12 . It was found that these QDs demonstrated very broad luminescence bands (between 370 and 710 nm), which are attributed to defects or electron-trap states on the surface of the nanocrystals. Importantly, a clear relationship between lumi- nescence that originated by defects and circular dichroism (CD) activity was observed for CdS chiral QDs. Our density functional calculations of the electronic states have demonstrated that CD at longer wavelengths is associated with near-surface cadmium atoms that are enantiomerically distorted by chiral penicillamine ligands, which translate their enantiomeric structure to the sur- face layers and associated electronic states, whereas the QD core is found to remain undistorted and achiral 13 . After that work, we later reported the preparation of chiral CdSe (ref. 14), CdTe (refs. 15,16) and chiral CdS nanotetrapods 17 . All of these chiral nanostructures showed characteristic CD responses within the band-edge region of their UV-visible spectrum, as well as very broad distribution of photoluminescence (PL), which originates from emissive defect states. The concept of chiral surface defects of QDs (distorted QD shell) was also confirmed by other groups with CdTe nanocrystals bearing various chiral ligands 18–20 . Interestingly, it was shown that the chirality of the QD surface was maintained even after ligand exchange with an achiral thiol and subsequent transfer of the CdTe QDs into a different (organic) phase. In this case, chiral QDs have shown a unique ‘chiral memory’ effect 19 . However, chirality in chiral QDs can be caused not necessar- ily only by chiral surface defects but also by other factors. For example, the effects of cysteine enantiomers on optical isomerism, growth rate and structure of chiral CdTe–based QDs was reported by Kotov and colleagues 20 . This paper postulated that the atomic origin of chiral sites in nanoparticles is geometrically similar to that in organic compounds. By using theoretical calculations and experimental data, the researchers showed that atoms in chiral cysteine–stabilized CdTe nanocrystals are arranged as tetrahe- drons, and that chirality occurs when all tetrahedral apexes have chemical differences and substitution 20 . More recently, chiral ligand–induced CD in CdSe QDs was reported by Balaz and colleagues 21,22 . The researchers have found that a simple phase transfer of trioctylphosphine oxide or oleic acid–capped CdSe QDs from toluene into aqueous phase using chiral thiol capping ligands such as L- and D-cysteines can induce chiroptical properties in originally achiral cadmium selenide QDs. In addition, it was shown that L- or D-cysteine-stabilized QDs in aqueous phase demonstrated size-dependent electronic CD and chiral ligand–induced circularly polarized luminescence in QDs. In this case, the authors have explained the origin of the induced CD in QDs by the hybridization of the chiral ligand’s highest occupied molecular orbitals (HOMOs) with CdSe molecular orbitals 21 . The literature data above clearly demonstrate that chiral sta- bilizing ligands and induced chirality effects have a crucial role in the properties and behavior of chiral QDs, enabling a range of potential applications for these nanomaterials in chemistry, nanotechnology and biology. One important application of chiral QDs is luminescence sens- ing and chiral recognition of enantiomers 18,23,24 . A fundamental Preparation of chiral quantum dots Mícheál P Moloney 1 , Joseph Govan 1 , Alexander Loudon 1 , Maria Mukhina 2 & Yurii K Gun’ko 1,2 1 School of Chemistry and Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) Institute, Trinity College Dublin, Dublin, Ireland. 2 Center of Information Optical Technologies, Information Technologies, Mechanics and Optics (ITMO) University, Saint Petersburg, Russia. Correspondence should be addressed to Y.K.G. (igounko@tcd.ie). Published online 5 March 2015; doi:10.1038/nprot.2015.028 Chiral quantum dots (QDs) are expected to have a range of potential applications in photocatalysis, as specific antibacterial and cytotoxic drug-delivery agents, in assays, as sensors in asymmetric synthesis and enantioseparation, and as fluorescent chiral nanoprobes in biomedical and analytical technologies. In this protocol, we present procedures for the synthesis of chiral optically active QD nanostructures and their quality control using spectroscopic studies and transmission electron microscopy imaging. We closely examine various synthetic routes for the preparation of chiral CdS, CdSe, CdTe and doped ZnS QDs, as well as of chiral CdS nanotetrapods. Most of these nanomaterials can be produced by a very fast (70 s) microwave-induced heating of the corresponding precursors in the presence of D- or L-chiral stabilizing coating ligands (stabilizers), which are crucial to generating optically active chiral QDs. Alternatively, chiral QDs can also be produced via the conventional hot injection technique, followed by a phase transfer in the presence of an appropriate chiral stabilizer. We demonstrate that the properties, structure and behavior of chiral QD nanostructures, as determined by various spectroscopic techniques, strongly depend on chiral stabilizers and that the chiral effects induced by them can be controlled via synthetic procedures.