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B R I A N A . K O R G E L

R esidential and commercial buildingsaccount for about 40% of energy useand about 30% of energy-related

carbon emissions in the United States1. Todecrease this energy demand, materials areneeded that help to regulate the heating and

lighting requirements of buildings by respond-ing to environmental changes. In particular,electrochromic window materials, whichchange colour and/or transparency whensubjected to an electric field, could signifi-cantly reduce energy consumption in build-ings2. On page 323 of this issue, Llordés et al .3 report a great advance in the development ofsuch materials. They have made a composite inwhich nanometre-scale crystals of indium tinoxide are embedded in a niobium oxide glass,with high control of nanocrystal loading anddispersion. The electrochromic performanceof the composite is much better than expectedfrom a simple sum of the optical absorption ofits two separate components, because of boththe nanostructure of the material and syner-gistic interactions that occur at the interfacebetween the components.

Inorganic nanocrystals are typically syn-thesized chemically with organic cappinggroups attached to aid the crystals’ dispers-ibility in solvents and to prevent aggregationor undesired particle growth. Unfortunatelyfor many applications, the organic groups donot have useful electrical or optical properties.There has thus been much effort to replacethe organic groups with inorganic groups thateither add to the capabilities of the crystals or

can be converted into an electrically or opti-cally active material. This approach has beenused to make nanocrystal assemblies withgreatly improved electrical properties4 and toconvert nanocrystals capped with inorganiccomplexes into a more useful photovoltaicmaterial5 (a material that converts light intoelectricity).

Llordés et al . have used this strategy to createtheir nanoparticle-in-glass materials. Theauthors first stripped indium tin oxide (ITO)nanocrystals of their organic caps and replacedthem with niobium-containing polyatomicions known as polyoxometalate (POM)

clusters. These clusters attach covalently to

the ITO surface to create a shell around the

nanocrystal. The researchers then condensedthe modified nanocrystals into a film, simplyby evaporating the solvent from a dispersion ofthe crystals. Finally, they converted the POMbetween the densely packed ITO nanocrystalsinto a niobium oxide (NbOx ) glass matrix byheating the film to 400 °C. Compared withpreviously reported synthetic routes for mak-ing nanoparticle-in-glass materials, in whichinorganic crystals are grown within a glass6,Llordés and co-workers’ method providesrigorous control over the nanocrystals’ sizedistribution and volume fraction. And, byadding more POM to the dispersion of POM-

stabilized ITO nanocrystals, the authors could

increase the volume fraction of the NbOx  

glass matrix.One of the key features of the ITO nano-

crystal–NbOx  glass material is that the glassis covalently bonded to the nanocrystals. Thisrestricts the molecular orientations available tothe octahedral NbO6 units found in the glass,and leads to remarkable structural orderingthat differs from that of pure NbOx . It turnsout that this ordering improves the electro-chromic properties of the glass matrix: NbOx  in the composite is five times darker than thebulk material when a similar voltage is applied.

ITO nanocrystals are also electrochromic,but in a different wavelength region from

NbOx : they undergo reversible electrochemical

M A T E R I A L S S C I E N C E

Composite for smarter windowsGlass has been prepared that selectively absorbs visible and near-infrared light when an electrochemical voltage is applied.This opens the way to ‘smart’ windows that block heat on demand, with or without optical transparency. S L .323

a b c

Electr

ode

Electrol

yte

electr

odeCo

unter

Nanoparticles

Li+

e–

Li+

e–

Visiblelight

Near-infrared

Figure 1 | Electrochromic window design. Llordés et al .3 propose that their nanoparticle-in-glasscomposite material could be used to make windows that controllably and selectively absorb visiblelight and near-infrared light (heat). a, In the design, the window is an electrochemical cell in which twoconducting glass panes are separated by a solid electrolyte material. The authors’ material is depositedon one pane, forming an electrode; a counter electrode is deposited on the other pane. In the absence ofan electrical load, the window is transparent to visible and near-infrared light. b, When an intermediate voltage is applied, charge carriers (lithium ions, Li+, and electrons, e –) move through the circuit. Thenanoparticles in the composite become chemically reduced, whereupon they block most incomingnear-infrared light. c, At lower voltages, the glass matrix of the composite also becomes reduced andblocks most incoming visible light.

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redox reactions and absorb near-infraredlight in the reduced state, but are transparentto this part of the spectrum when oxidized7.The combination of ITO nanocrystals with aNbOx  glass matrix therefore yields a materialin which both visible and near-infrared lightabsorption can be electrochemically modu-lated. This material could thus be used in

smart windows, to control the amount of bothheat (near-infrared) and light passing throughthem (Fig. 1). What’s more, the optical trans-parency can be tuned independently of thenear-infrared transparency.

Llordés and colleagues’ approach for makingcomposite materials of inorganic nanocrystalsin glass opens the way to a range of newmaterial properties and applications, not justin electrochromics. The challenge for eachapplication is to identify the best combinationsof nanocrystal composition and modifiableinorganic capping groups.

More specifically, several issues must still

be addressed before the material can be usedin windows. The authors used lithium metalas a counter electrode to test the performanceof their material, but this is not acceptable forcommercial applications because of safetyconcerns. A suitable counter electrode mustbe identified. Additionally, the researchersperformed their photoelectrochemical testsusing a liquid electrolyte as a charge-carryingmaterial, whereas a solid electrolyte is probablymore appropriate for buildings applications.The materials needed to build an electro-chromic window will be more expensivethan conventional window materials, so theextra expense will need to be balanced by theenergy and cost savings that can be achievedthrough their use. Ideally, no power input willbe needed to maintain transparency or opacity,but this ability remains to be explored.

Nevertheless, Llordés and co-workers’results are promising. With appropriate coun-ter electrodes and a solid-state electrolyte, andif long-term stability of the composite can bedemonstrated, windows that have multispectralband transparency may be just around the cor-ner, potentially enabling buildings that offerunprecedented energy efficiency and comfort. ■

Brian A. Korgel is in the Department of

Chemical Engineering, Center for Nano- and Molecular Science and Technology, Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, USA.e-mail: korgel@mail.che.utexas.edu

1. Richter, B. et al. Rev. Mod. Phys. 80, S1–S109 (2008).2. Li, S.-Y., Niklasson, G. A. & Granqvist, C. G. J. Appl.

Phys. 108, 063525 (2010).3. Llordés, A., Garcia, G., Gazquez, J. & Milliron, D. J.

Nature 500, 323–326 (2013).4. Panthani, M. G. & Korgel, B. A. Annu. Rev. Chem.

Biomol. Eng. 3, 287–311 (2012).5. Jiang, C., Lee, J.-S. & Talapin, D. V. J. Am. Chem. Soc.

134, 5010–5013 (2012).6. Sakamoto, A., Yamamoto, S. Int. J. Appl. Glass Sci. 1, 

237–247 (2010).

7. Garcia, G. et al. Nano Lett. 11, 4415–4420 (2011).

S T R U C T U R A L B I O L O G Y

RNA exerts self-controlA crystal structure of two bound RNA molecules not only provides insight intohow regulatory riboswitch sequences affect messenger RNA expression, but also

expands our understanding of RNA structure and architecture. S L .363

B H A S K A R C H E T N A N I &

A L F O N S O M O N D R A G Ó N

O ver the past three decades, our knowl-edge of the role of RNA in cellularprocesses has expanded enormously 1.

For example, the structures of various regula-tory RNA sequences called riboswitches haverevealed how they affect the transcription ortranslation of their downstream messengerRNA sequences through the recognition and

binding of specific ligands

2

. 

On page 363 ofthis issue, Zhang and Ferré-D’Amaré3 provideanother structural insight — this time, into abacterial riboswitch, called a T-box, in com-plex with a transfer RNA molecule. Their dataelucidate how one RNA molecule recognizesanother RNA of similar size and regulates itsown transcription through a simple switchingmechanism*.

To support the process of protein syn-thesis, cells must regulate the pool of tRNAsthat become charged with (covalently boundto) specific amino-acid residues and deliverthem to the growing protein chain. Enzymesknown as aminoacyl-tRNA synthetases carryout tRNA charging. In Gram-positive bac-teria, a T-box riboswitch located upstreamof the coding region of the mRNA of each

aminoacyl-tRNA synthetase negativelyregulates synthesis of the mRNA4.

The T-box RNA consists of at least two inde-pendently folded domains: a sensory ‘aptamer’domain that forms a long stem called stem Iand that binds to specific tRNAs; and a seconddomain, which can switch between two alter-native conformations depending on whetherthe bound tRNA is charged or uncharged5.Whereas binding of a charged tRNA leads totermination of transcription of the aminoacyl-

tRNA coding sequence (Fig. 1a), an unchargedtRNA stabilizes an antiterminator conforma-tion of the T-box, leading to transcription ofthe mRNA and subsequent protein synthesis(Fig. 1b).

Stem I is a mostly double-helical RNAdomain roughly 100 nucleotides long and isstudded with several phylogenetically con-served structural motifs along its length4.Previous work has implicated most of thesemotifs in tRNA recognition and binding4,yet atomic-level details of this process haveremained largely unknown. Zhang and Ferré-D’Amaré provide the f irst glimpse into thismechanism by describing the crystal structureof a complex between stem I of the T-box oftRNA synthetase for the amino acid glycineand an uncharged glycyl-tRNA. The authors’co-crystal structure also explains the preciserole of the stem I motifs, delivers intriguing

Amino

acid

ChargedtRNA

  UnchargedtRNA

5

3

    T   e   r   m    i   n

   a    t   o

   r

Termination Transcription

K-turnmRNA

mRNA

T- and D-

loopsDinucleotidebulge

Stem I

Anticodon

loop

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    A   n    t    i    t   e

   r   m    i   n   a    t   o

   r

a b

T-box

T-box

Figure 1 | RNA meets RNA. Zhang and Ferré-D’Amaré3 present the structure of the stem I domainof a T-box riboswitch in complex with an uncharged tRNA. Stem I bends at the dinucleotide bulgeand the K-turn to recognize tRNA by binding to its anticodon loop and the T- and D-loops. a, Theamino acid present in a charged tRNA is thought to prevent interaction between the T-box sequencemotif downstream of stem I and the acceptor end of the tRNA, favouring formation of a terminatorloop that stops transcription of the mRNA downstream of the T-box. b, By contrast, the free acceptorend of an uncharged tRNA interacts directly with the T-box sequence and leads to the formation of an

antitermination loop, allowing transcription of the mRNA and its subsequent translation into a protein.

*This article and the paper under discussion3 werepublished online on 28 July 2013.

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