G-Quadruplexes as Sensing Probes

Molecules 2013, 18, 14760-14779; doi:10.3390/molecules181214760 molecules ISSN 1420-3049 www.mdpi.com/jou nal/molecules Re iew G-Quad uplexes as Sensing P obes B anisla Ru kay-Nedecky 1,2, Ji i Kud 1, Lukas Nejdl 1, Da ina Masko a 2, Rene Kizek 1,2 and Voj ech Adam 1,2,* 1 Depa men o Chemis y and Biochemis y, Facul y o Ag onomy, Mendel Uni e si y in B no, Zemedelska 1, B no CZ-613 00, Czech Republic; E-Mails: b [email p o ec ed] (B.R.-N.); [email p o ec ed] (J.K.); [email protected] (L.N.); [email p o ec ed]uni.cz (R.K.) 2 Cen al Eu opean Ins i u e o Technology, B no Uni e si y o Technology, Technicka 3058/10, B no CZ-616 00, Czech Republic; E-Mail: da ina.m[email p o ec ed] * Au ho o whom co espondence should be add essed; E-Mail: oj [email p o ec ed]; Tel.: +420-5-4513-3350; Fax: +420-5-4521-2044. Recei ed: 9 Sep embe 2013; in e ised o m: 13 No embe 2013 / Accep ed: 13 No embe 2013 / Published: 28 No embe 2013 Abs ac : Guanine- ich sequences o DNA a e able o c ea e e as anded s uc u es known as G-quad uplexes; hey a e o med by he s acking o plana G-qua e s composed o ou guanines pai ed by Hoogs een hyd ogen bonding. G-quad uplexes ac as ligands o me al ions and ap ame s o a ious molecules. In e es ingly, he G-quad uplexes o m a complex wi h anionic po phy in hemin and exhibi pe oxidase-like ac i i y. This e iew ocuses on o e iew o sensing echniques based on G-quad uplex complexes wi h anionic po phy ins o de ec ion o a ious analy es, including me al ions such as K+, Ca2+, Ag+, Hg2+, Cu2+, Pb2+, S 2+, o ganic molecules, nucleic acids, and p o eins. P inciples o G-quad uplex-based de ec ion me hods in ol e DNA con o ma ional change caused by he p esence o analy e which leads o a dec ease o an inc ease in pe oxidase ac i i y, luo escence, o elec ochemical signal o he used p obe. The ad an ages o a ious de ec ion echniques a e also discussed. Keywo ds: G-quad uplex; DNAzyme; hemin OPEN ACCESS Molecules 2013, 18 14761 1. In oduc ion DNA plays a undamen al ole in all li ing o ganisms, as i is a c ucial molecule esponsible o he s o age and copying o gene ic in o ma ion [1]. P e iously, i has been assumed ha DNA has a “passi e” s uc u e used only o he s o age o gene ic in o ma ion. F om he expe imen s ca ied ou ecen ly, i is e iden ha DNA is a e y dynamic molecule, capable o o ming a numbe o spa ial a angemen s. These s uc u es include single-s anded hai pins, homoduplexes, iplexes, and quad uplexes [2]. Fo me ly hese s uc u es we e conside ed an in e es ing phenomenon wi h a li le p ac ical meaning. La e i was ound ha he o ma ion o hese s uc u es akes place unde ce ain physiological condi ions; he e o e hei in ol emen in ecombina ion, egula ion o gene exp ession and p oli e a ion o umou cells is assumed. Based on hese ac s i is no su p ising ha he e is g owing in e es in he s uc u es o nucleic acids as po en ial he apeu ic d ugs [3]. G-Quad uplexes The mos amous s uc u es o DNA a e highly o de ed guanine quad uplexes (G-quad uplexes) composed o guanine qua e s (G-qua e s), which a e o med om ou guanine bases. In G-qua e each guanine is linked wi h neighbou ing guanine ia wo hyd ogen bonds by Hoogs een pai ing. These s uc u es hen s ack on each o he in a helical ashion, o ming a G-quad uplex s uc u e (Figu e 1). Figu e 1. Le : s uc u e o a G-qua e , wi h ou guanines a anged a ound a cen al mono alen ca ion (M+). Righ : s uc u e o a G-quad uplex, in his example an an ipa allel unimolecula s uc u e wi h h ee s acked qua e s. Adop ed and modi ied acco ding o Huppe e al. [4]. G-quad uplexes a e s abilized by hyd ogen bonds and by he p esence o alkali me al ions, which a e loca ed in he cen e be ween wo G-qua e s. These ions a e mos equen ly po assium o sodium Molecules 2013, 18 14762 ca ions, which a e connec ed by elec os a ic in e ac ions on he guanine ca bonyl [4–8]. G-quad uplexes a e cha ac e ized by unique a chi ec u e and high s abili y [9]. Some sequences emain olded unde physiological condi ions and a empe a u es abo e 90 °C [4]. G-quad uplexes a e highly polymo phic and hey can be classi ied in e ms o he s oichiome ic as unimolecula , bimolecula , and e amolecula and also in e ms o o ien a ion as pa allel, an ipa allel o mixed. S uc u e o G-quad uplexes depends on he composi ion and leng h o he DNA, on he o ien a ion o he chains and posi ions o he loops, and also on he na u e o he ca ions [10]. Due o hese modi ica ions G-quad uplex s uc u es can be c ea ed easily bo h in e molecula ly and in amolecula ly [1]. G-quad uplex s uc u es ha e d awn he a en ion o esea che s in medicinal chemis y, sup amolecula chemis y, and nano echnology [6,11–13]. In addi ion, G-quad uplexes ha e been used as basic uni s in he o ma ion o nanos uc u es [12]. Almos all G-quad uplex s uc u es s udied ha e been o med by one, wo, o ou G- ich s ands [6]. G-quad uplex s uc u es o med by h ee s ands, leading o a i-G-quad uplex species ha e been desc ibed ecen ly by Zhou e al. [14]. This i-G-quad uplex design may also p o ide a new a enue o c ea ing nanoscale ma e ials. Human elome ic DNA composed o (TTAGGG/CCCTAA)n epea s may o m a classical Wa son-C ick double helix. Each indi idual s and is also p one o quad uplex o ma ion: he G- ich s and may adop a G-quad uplex con o ma ion in ol ing G-qua e s whe eas he C- ich s and may old in o an i-mo i based on in e cala ed C.C+ base pai s [15]. A numbe o esea ch g oups cons uc ed di e en nanode ices based on swi ching be ween s uc u es as induced by changes in en i onmen al ac o s [16–18]. Zhou e al. [19] demons a ed he coexis ence o a G-quad uplex and an i-mo i in a single s and. This s uc u e was buil on he basis o he p inciple ha G-quad uplex o ma ion equi es he p esence o a G-quad uplex-compa ible ca ion, whe eas i-mo i o ma ion demands acidic condi ions. The cons uc ed nanode ice is e y simple and can be apidly con e ed in o o he s uc u es by a ying he s imulus, such as he pH alue o ca ion. G-quad uplexes o m a complex wi h hemin (G-quad uplex/hemin) and a e called DNAzymes. These complexes exhibi pe oxidase-like ac i i y and e ec i ely ca alyse he H2O2-media ed oxida ion o 2,2'-azino-bis(3-e hylbenzo hiazolin-6-sul onic acid)diammonium sal (ABTS) [20–22]. Due o he abili y o bind me al ions and o he compounds, hese DNAzymes can be used o de ec Ag+, Cu2+, Pb2+, Hg2+, and S 2+ ions [23–27]. This e iew is hus aimed a summa izing he ac s abou G-quad uplexes as sensing p obes o de e mining o biologically ac i e compounds. 2. G-Quad uplexes as De ec o s 2.1. De ec ion o Me al Ions 2.1.1. De ec ion o K+ G-quad uplexes may se e as de ec o s o po assium using oligonucleo ides o ming G-quad uplexes and iphenylme hane luo escen dye c ys al iole (CV). As desc ibed by Kong e al. [28], a K+ de ec ion me hod is based on he luo escence di e ence o some CV/G-quad uplex complexes in he p esence o K+ o Na+, and he luo escence change wi h he a ia ion o K+ concen a ion. Acco ding o he na u e o he luo escence change o CV as a unc ion o ionic condi ions, wo K+ de ec ion Molecules 2013, 18 14763 modes we e in oduced. In he i s ype, wi h a dec ease in he CV luo escence, oligonucleo ides T3TT3 (5'-GGGTTTGGGTGGGTTTGGG-3') we e used, and he luo escence o CV dec eased wi h an inc easing concen a ion o K+. Con e sely, in he second ype, whe e oligonucleo ides Hum21 (5'-GGGTTAGGGTTAGGGTTAGGG-3') we e used, he CV luo escence wi h an inc easing concen a ion o K+ inc eased [28]. Ano he luo escen de ec ion me hod o K+ was desc ibed by Qin e al. [29]. De ec ion o K+ was de eloped using G-quad uplex DNA (c-Myc), which modula ed luo escence enhancemen o e akis(diisop opylguanidino) zinc ph halocyanine (Zn-DIGP). Wi h an inc easing concen a ion o K+ luo escence o Zn-DIGP inc eased. In nex de ec ion me hod, G-quad uplex s uc u e s abilized by K+ is able o bind hemin; hus, i is o ming DNAzyme, which in u n ca alyses hyd ogen pe oxide media ed oxida ion o colou less 3,3',5,5'- e ame hylbenzidine (TMB) o a blue p oduc . Unde op imal condi ions, he colou change is isible o he naked eye wi hin he concen a ion ange om 2 o 1000 µM [30]. Ano he de ec ion me hod o K+ used G-quad uplex complex wi h be be ine, he plan alkaloid wi h b oad medical uses [31]. A e addi ion o K+ single s anded DNA olded in o G-quad uplex and a e incuba ion wi h be be ine o ma ion o be be ine-G-quad uplex complex occu ed leading o a ma ked inc ease in luo escen signal. In he p esence o 800 mM o Na+ ions luo escence o he be be ine-G-quad uplex was linea ly inc easing wi h an inc easing concen a ion o K+ wi hin he ange om 0.005 o 1.0 mM [31]. 2.1.2. De ec ion o Ag+ A G-quad uplex–hemin DNAzyme-ampli ied Ag+-sensing me hod was desc ibed by Zhou e al. [25]. This me hod is based on he abili y o Ag+ o s abilize cy osine-cy osine (C–C) misma ches by o ming C–Ag+–C base pai s. In he absence o Ag+, he oligonucleo ide s and o med an in amolecula duplex (Figu e 2). A e addi ion o Ag+ G- ich sequence olds in o G-quad uplex s uc u e capable o bind hemin o o m a ca aly ically ac i e G-quad uplex-hemin DNAzyme [25]. In he a o emen ioned me hod one oligonucleo ide was used. On he o he hand, Kong e al. [32] used me hod based on a simila p inciple, bu using wo di e en leng h oligonucleo ide chains o Ag+ de ec ion. Also, hey ook ad an age o s ong bond be ween Ag+ and cys eine o he de ec ion o cys eine only. Cys eine b oke C-Ag+-C bonds leading o e o ma ion o he DNA duplex and educed ca aly ic ac i i y o he sys em. 2.1.3. De ec ion o Hg2+ Li e al. [23] desc ibed a colo ime ic me hod o highly selec i e and speci ic de ec ion o Hg2+ using Hg2+ modula ed G-quad uplex-based DNAzyme. Me cu y ion (Hg2+) is able o speci ically bind o he hymine- hymine (T-T) misma ch in a DNA duplex. G-quad uplex DNAs a e able o bind hemin o o m he pe oxidase-like DNAzymes in he olded s a e. Upon addi ion o Hg2+, he p ope olding o G-quad uplex DNAs is inhibi ed due o he o ma ion o T-Hg2+-T complex. This is e lec ed by he no able change o he So e band o hemin when in es iga ed by using UV-VIS abso p ion spec oscopy. As a esul o Hg2+ inhibi ion, a sha p dec ease in he pe oxidase like ac i i y which causes he H2O2-media ed oxida ion o 2,2'-azino-bis(3-e hylbenzo hiazoline-6-sul onic acid)-diammonium sal (ABTS) is obse ed, accompanied by a change in solu ion colou [23]. The same p inciple was used in he wo k o Jia e al. [33], who used he de ec ed me cu y u he o he de ec ion o cys eine. Molecules 2013, 18 14764 Figu e 2. Schema ic ep esen a ion o he G-quad uplex–hemin DNAzyme ampli ied Ag+-sensing me hod. Adop ed and modi ied acco ding o Zhou e al. [25]. 2.1.4. De ec ion o Cu2+ An e ec i e G-quad uplex-based p obe was cons uc ed by Zhang e al. [27] o apid and sensi i e de ec ion o Cu2+. In his p obe, an anionic po phy in, p o opo phy in IX (PPIX) se ed as a e e ence signal, which binds o G-quad uplex speci ically and he luo escence in ensi y inc eases sha ply. On he o he hand, in he p esence o Cu2+, he G-quad uplex can ca alyse he Cu2+ inse ion in o he p o opo phy in, and he luo escen in ensi y is dec eased. The assay was shown o be highly speci ic [27]. 2.1.5. De ec ion o Pb2+ Lead ions (Pb2+) induce a con o ma ional change o he po assium s abilized G-quad uplex DNAzyme and inhibi he pe oxidase-like ac i i y. Li e al. and Wang e al. [34,35] used G-quad uplex as DNAzyme o colo ime ic de ec ion o Pb2+. G-quad uplex/hemin DNAzyme ca alyses hyd ogen pe oxide media ed oxida ion o ABTS, which esul s in a colou change (Figu e 3). A e he addi ion o Pb2+ po assium s abilized G-quad uplex/hemin is con e ed o Pb2+-s abilized s uc u e wi h highe s abili y bu lowe DNAzyme ac i i y, which is e lec ed by an inc ease in DNA mel ing empe a u e on one side bu also in a sha p dec ease in eadou signal on he o he side. This allows u ilizing his Molecules 2013, 18 14765 G-quad uplex o quan i a i e analysis o aqueous Pb 2+ using he ABTS-H 2 O 2 colo ime ic sys em. Also luminol-H 2 O 2 chemiluminescence sys em was used o Pb 2+ de ec ion [36]. Using UV/VIS de ec ion, Pb 2+ was de ec ed a a le el o 32 nM, whe eas he de ec ion limi using chemiluminescence me hod was e en below 1 nM. Figu e 3. Cons uc ion o an INHIBIT logic ga e based on he G- ich DNAzyme PW17, wi h K + and Pb 2+ as wo inpu s and abso bance as an ou pu . Co ac o is hemin. In he quad uplex s uc u es, an i and syn guanines a e colou ed cyan and o ange, espec i ely. Adop ed and modi ied acco ding o Li e al. [34]. Pb 2+ could be u he de ec ed by luo escence using G-quad uplex-DNAzyme. The me hod is based on Pb 2+ induced inc ease in DNAzyme ac i i y o oligonucleo ide AGRO100 in he p esence o hemin, which ac s as a co ac o o ca alysing o H 2 O 2 -media ed oxida ion o he luo escen dye Amplex ® Ul aRed (AUR). AGRO100/AUR p obe showed high selec i i y o he Pb 2+ ions in compa ison wi h o he me al ions. In ensi y o AUR luo escence was p opo ional o concen a ion o Pb 2+ ions wi hin he in e al om 0 o 1000 nM [37]. Ano he ype o luo escen biosenso o Pb 2+ was designed based on Pb 2+ -induced allos e ic quad uplex. In he p esence o K + N-me hylmesopo phy in IX (NMM) binds o K + s abilized G-quad uplex, esul ing in high luo escence. A e addi ion o Pb 2+ binding o Pb 2+ o G-quad uplex occu s and he eby p e en s binding o NMM. This esul s in a dec ease in luo escence [38]. G aphene oxides (GO) mixed wi h ap ame - unc ionalized CdSe/ZnS quan um do s (QDs) can se e as a luo escen senso o Pb 2+ . Ap ame -conjuga ed QDs bind o GO and o m GO/QDs-ap ame complex, which allows ene gy ans e om QDs on GO and luo escence quenching o QDs. The p esence o GO Pb 2+ induces a con o ma ional change o ap ame o he G- quad uplex, o which GO Pb 2+ binds. QDs a e hus sepa a ed om he GO and an inc ease in luo escence o QDs occu s [39]. Molecules 2013, 18 14766 2.1.6. De ec ion o Ca2+ A pa allel G-quad uplex-selec i e i idium (III) complex was de eloped as a luminiscen p obe o G-quad uplex-based de ec ion assay o Ca2+ ions in aqueous solu ion. In his assay, guanine- ich oligonucleo ides ini ially exis in an an ipa allel G-quad uplex con o ma ion, esul ing in a low luminiscence signal. Upon incuba ion wi h Ca2+ ions, he e is a change o he an ipa allel G-quad uplex o a pa allel G-quad uplex con o ma ion, which g ea ly enhances he luminiscence emission o he i idium (III) p obe [40]. 2.1.7. De ec ion o S 2+ The inhala ion o s on ium can cause se e e espi a o y di icul ies, anaphylac ic eac ion and ex eme achyca dia. S on ium can eplace calcium in o ganism, inhibi no mal calcium abso p ion and induce s on ium “ icke s” in childhood. Fo de ec ion o s on ium ions (S 2+) a simple me hod using hiazoleo ange (TO) on he basis o S 2+ induced con o ma ional change o elome ic DNA in he p esence o single-wall nano ubes (SWNTs) was sugges ed. The limi o de ec ion was 10 nM S 2+ [26]. 2.2. De ec ion o Anions De ec ion o I− Li e al. [41] de eloped a simple and sensi i e chemiluminescence assay o iodide (I−), which is based on iodide ex ac ing Hg2+ om DNA ea u ing a s em-loop s uc u e con aining T-Hg2+-T. Because he binding o Hg2+ and I− is much s onge han ha o Hg2+ and hymine, I− could ex ac Hg2+ om he s em-loop s uc u e, eleasing he DNA, which hen bound wi h K+ and ans o med in o a K+ s abilized G-quad uplex (wi h hemin as a co ac o ), which ca alyses H2O2-media ed oxida ion o luminol. The p oduced chemiluminescence as a sensing signal was applied o sensi i ely and selec i ely de ec iodide wi h a de ec ion limi o 12 nM. 2.3. De ec ion o O ganic Molecules 2.3.1. Amino Acid De ec ion His idine and cys eine de ec ion is c i ically impo an , because hei abno mal le els a e an indica o o many diseases. Li e al. [42] demons a ed a quad uplex-based me hod o de ec ion o his idine and cys eine. The me hod is based on a highly speci ic in e ac ion among amino acids (his idine o cys eine), Cu2+ and N-me hylmesopo phy in IX in complex wi h quad uplex (NMM/G-4). The luo escence in ensi y o NMM is signi ican ly inc eased and in he p esence o G-quad uplex can be quenched by Cu2+. The p esence o his idine o cys eine hen dis u bs he in e ac ion be ween Cu2+ and NMM/G-4 because o he s ong binding a ini y o Cu2+ o he imidazole g oup o his idine o he in e ac ion o Cu2+ wi h hiol g oups o cys eine, leading o dis inc luo escence emission in ensi y (Figu e 4). High selec i i y is con e ed by he use o cys eine-masking agen N-e hylmaleimide (NEM), which helps o disc imina e his idine om cys eine [42]. Molecules 2013, 18 14767 Figu e 4. Schema ic illus a ion o he luo escence change o he NMM/G-4 ensemble unde di e en condi ions. The combina ion o NMM and an in amolecula G-quad uplex gene a ed om 24GT oligonucleo ide unc ions as a signal indica o NMM/G-4 wi h s ong luo escen in ensi y. Cup ic ion can quench he luo escence o NMM/G-4 h ough i s coo dina ion wi h NMM as well as he un olding o G-quad uplex by Cu2+ (as shown in he le side). Howe e , he p esence o his idine o cys eine can dis u b he in e ac ion be ween Cu and NMM/G-4 complex due o hei in e ac ion wi h Cu2+, gene a ing a dis inc luo escence esponse om ha o cup ic ion alone (as shown in he igh side). Adop ed and modi ied acco ding o Li e al. [43]. Cys eine (cys) can be also de e mined oge he wi h glu a hione (GSH) by he quad uplex-based me hod using Hg2+ and NMM. The sys em consis s o wo single s anded DNA (ssDNA) wi h hymine- hymine (T-T) misma ches and used Hg2+ as a media o , and NMM as he signal epo e . In he absence o analy e (cys o GSH) wo ssDNA con aining T-T misma ches eac wi h Hg2+ o o m T-Hg2+-T dsDNA s uc u e in he solu ion, which hampe s he o ma ion o a G-quad uplex s uc u e. Howe e , in he p esence o GSH o cys he analy e eac s wi h Hg2+ o keep DNA p obes in a ee single s a e, esul ing in he e ec i e o ma ion o a G-quad uplex s uc u e o he DNA p obe. Subsequen ly, due o he s ong in e ac ion be ween G-quad uplex s uc u e and NMM, luo escence is g ea ly enhanced. This me hod exhibi ed a linea ela ionship be ween peak luo escence in ensi y and concen a ion o GSH in he ange o 10–400 nM wi h a limi o de ec ion (LOD) o 9.6 nM. A linea ange o Cys de ec ion was ob ained in he concen a ion ange o 10–500 nM wi h an LOD o 10 nM [44]. Ano he sensi i e me hod o de ec ion o cys eine was desc ibed by Su e al. [45]. The mechanism is based on he oxida ion o cys eine by H2O2, which p e en s he ca alysis o he ABTS-H2O2 eac ion by G-quad uplex hal es. Wi h he addi ion o Cys, he amoun o he blue-g een-colou ed ee adical ca ion ABTS+ was educed and he e was a signi ican dec ease in an abso bance. The concen a ion o cys eine was de e mined by UV-VIS spec oscopy and colou change was al eady disce nible o he naked eye. The calib a ion cu e showed ha he ne abso p ion alue a 421 nm linea ly inc eased o e he Cys concen a ion ange o 0.005–100 µM wi h a de ec ion limi o 5 nM. Molecules 2013, 18 14768 2.3.2. Glucose De ec ion Colo ime ic me hod o glucose de ec ion in u ine was de eloped. Oxida ion o glucose is con e ed o colou change o 10-ace yl-3,7-dihyd oxy phenoxazine (ADHP) using DNAzyme, which consis o G-quad uplex and hemin. DNAzyme ca alyses oxida ion o colou less ADHP o ed eso u in by H2O2, which is p oduc o glucose oxidase ca alysed eac ion o glucose and oxygen. Oxida ion o glucose (colou change o ADHP) is he esul o hese eac ions [46]. 2.3.3. Choles e ol De ec ion G-quad uplex/hemin complex can be also used o colo ime ic de ec ion o choles e ol. By his DNAzyme ca alysed oxida ion o colou less ABTS2− by H2O2 o colou ul ABTS−. In his case, H2O2 is p oduced by a eac ion o choles e ol and oxygen ca alysed by choles e ol oxidase. Colou change o ABTS2− is he consequence o choles e ol oxida ion [47]. 2.3.4. ATP De ec ion Adenosine iphospha e (ATP) can be also de ec ed by DNAzyme ap ame senso , which uses wo DNA sequences. Fi s unc ional chain (A chain) consis s o wo pa s—an i-ATP ap ame ( ecogni ion pa ) and DNAzyme (signal ansduc ion pa ). Second sequence is used as a blocking chain (B chain), which can hyb idize wi h A chain. A e addi ion o hemin and ATP, hyb idized chains un old. DNAzyme in unc ional chain is c ea ing G-quad uplex wi h hemin, hus i ca alyses oxida ion o ABTS by H2O2 [48]. In ano he wo k, ap ame s immobilised on he elec ode su ace can be used o ATP de ec ion. One ap ame se es as ecogni ion elemen o ATP and second as a signal sou ce. Fi s p obe L1 con ains ATP ap ame and a pa o hemin ap ame and second complemen a y chain o ATP ap ame and he es o hemin ap ame . L1 was immobilised on he elec ode su ace. L2 hyb idised wi h L1 and L1-L2 complex was o med, hus wo pa s o hemin ap ame became close . Hemin o ma ion and L1-L2 chain esul s in G-quad uplex. Hemin immobilised inside quad uplex p oduced s ong elec ochemical signal. When ATP is added, duplex is dis up ed and L2 is eleased o solu ion. Hemin is no cap u ed and no signal is de ec ed [49]. 2.3.5. De ec ion o Cocaine Fo de ec ion o cocaine DNAzyme based colo ime ic me hod in combina ion wi h he magne ic nanopa icles was desc ibed. Cocaine ap ame agmen SH-C2 was co alen ly bound o he magne ic nanopa icles. Ta ge cocaine and ano he cocaine ap ame agmen (C1) a e ins illed in o he G- ich s and (C1-AG4). Wi h his egion SH-C2 bound on magne ic nanopa icles hyb idizes. C1-AG4 can o m wi h hemin DNAzyme ca alysing hyd ogen pe oxide-media ed oxida ion o 3,3,5,5- e ame hylbenzidine sulpha e (TMB), which leads o colou change o he solu ion. Using magne ic nanopa icles as sepa a ion and ampli ica ion elemen s backg ound signal and he in e e ence o eal samples can be e ec i ely educed [50]. Molecules 2013, 18 14775 and he de ec ion limi o 5 nM was ob ained. Mo eo e , his assay was able o de ec K+ wi h high selec i i y and had g ea po en ial applica ions [70]. 4. Conclusions Re iew ocuses on he use o G-quad uplexes as de ec o s o hea y me als. Besides hea y me als, o he analy es using G-quad uplexes can be analysed such as o ganic molecules, nucleic acids, and p o eins. An impo an sc eening echnique o G-quad uplex ligands is he use o G-quad uplexes in combina ion wi h he gold nanopa icles. The basis o de ec ions using G-quad uplexes is DNA con o ma ional change induced by he p esence o analy e, esul ing in a dec ease o inc ease in pe oxidase ac i i y, luo escence, o elec ochemical signal o he used p obe. In compa ison wi h o he de ec ion me hods, he de ec ion using G-quad uplexes is simple , as e , mo e sensi i e, and less expensi e, wi hou expensi e ins umen s and luo escen ly labelled oligonucleo ides. 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