Page - 24 - in Cancer Nanotheranostics - What Have We Learnd So Far?

Condeetal. Biofunctionalizationandsurfacechemistryof inorganicnanoparticles FIGURE11 |Clickchemistry reaction.Thecopper-catalyzedcycloaddition of azidesandalkynes (CuAAC)developed for clickchemistry joinsan organicazide (N3) andalkyne togetherproducingamixtureof1,4- and 1,5-triazoles. chelate the Cu(I)-acetylide complex intermediate that contact withtheazidegroup(Heinetal.,2008).Besides these limitations, one of themost obvious disadvantages is the requirement of a copper catalyst. In fact, an excessive intake of copper can lead to drastic consequences for the human body (e.g. hepatitis, neurological disorders, kidney diseases andAlzheimer’s disease) (WangandGuo,2006;Heinetal., 2008). Generally, theclick-chemistryreactionhasbeenusedtocouple AuNPs to proteins (Zhu et al., 2012b), enzymes (Brennan et al., 2006;Kimet al., 2010), fluorophores (Voliani et al., 2011), poly- mers (Boisselier et al., 2008;Zhang et al., 2009), andother small molecules (Fleming et al., 2006). For example, following click- chemistryreactionFlemingetal.wereabletoconjugatetoAuNPs severaldifferentalkynederivatives, suchas ferrocene, anilineand PEG(Flemingetal., 2006). Alkyne-functionalizedAuNPshavebeenalso extensivelyused to detect metal ions in aqueous solutions, such as Cu2+, using clickchemistry.Thismethodallowsvisualizationbynakedeyeof thepresenceofCu2+ ionsbytheaggregationofAuNPsasaresult of the Cu(I)-catalyzed conjugation between the two functional groups(Zhouetal., 2008b;Xuetal., 2010;Linetal., 2012). Another common type of nanoparticles used for click- chemistrybioconjugationisQDs.QDsneedtobecoatedtoother chemical species if they are to be used as biomarkers, thera- peutic agents or sensors. In fact, water soluble and water QDs have been successfully coatedwith polymers via click-chemistry reactions (Beaune et al., 2011; Janczewski et al., 2011; Lai and Guan, 2011; Petryayeva and Krull, 2012; Zhang et al., 2012a). Jan´czewskietal.reportedtheuseofclick-QDsbyproducingwater solubilizationof hydrophobicCdSe/ZnSQDsusing amphiphilic polymeric coatings. The authors described the preparation of acetylene- and azide-functionalized QDs for “click” chemistry. Themethod isuniversal andapplicable toany typeofnanoparti- cle stabilizedwithhydrophobic ligandsable to interact (inwater) with the alkyl chains present in the coating (Janczewski et al., 2011). Interestingly,Haoetal. reportedamethodfor labelingviruses viacopper-freeclickchemistrytoQDs.Theauthorslinkedvirions modified with azide to QDs capable of realizing single-virion tracking, laying the foundation for long-termdynamicvisualiza- tionofvirus infectionprocess (Haoetal., 2012). Although click chemistry does not appear to be amajor type of chemistry forMNPs, some interesting examples canbe found in the literature. For instance, Santra and coworkers reported the creation of novel polymeric-metallic nanocomposites when assemblingalkylated IONPswithazidepolymerfluorescentNPs, obtainingafluorescencematerialwith enhancedmagneticprop- erties for MRI (Santra et al., 2009). The first example of click MNPsfor invivoapplicationswasreportedbyBhatiaandcowork- ers (vonMaltzahnetal., 2008).FluorescentMNPsfunctionalized with a tumor-targeting peptide (Lyp-1) via click chemistrywere able to stably navigate the systemic circulation, extravasate into tumors and penetrate into the interstitial space to specifically bind to receptorson tumorcells.Weisleddergroupalso reported the introduction of 18F onto azide-modifiedMNPs using click- chemistry for invivoPETimaging(Devaraj etal., 2009). Non-covalentstrategies:physical interactions Physical interactionsincludeelectrostatic,hydrophobicandaffin- ity interactions.These interactionshave several advantages, such as theeaseof functionalization, speedofbindingandthatneither the biomolecules nor theNPsmust bemodified in case of elec- trostatic and hydrophobic interactions.However, conjugation is lessstableandreproduciblewhencomparedtocovalentmethods. Moreover, it is difficult to control theamountandorientationof boundmolecules. Ionic coupling. Ionic adsorptionprovides a simple and straight- forwardmethod to functionalizeNPswithbiomolecules. In fact, biological andpolymeric specieswith anopposite charge canbe coupled toNPs (Condeet al., 2012a)orbetweendifferentoppo- site charged NPs (Liu et al., 2012). Ionic binding rate mainly depends on the amount of charges present on theNPs and the biomolecules, as the binding ismade bymultiple point (multi- punctual). Therefore, when binding complexmolecules such as antibodiesorproteins, the isoelectricpointshouldbeconsidered, as theirnetchargewoulddependonit. Ionic couplinghas been traditionally used to adsorbproteins toNPs (Brewer et al., 2005;Honget al., 2006;ReedandMetallo, 2010; Guo et al., 2011; Brancolini et al., 2012; Strozyk et al., 2012), as some proteins such as serum albumin can stabilize NPs by preventing aggregation (Brewer et al., 2005).Moreover, proteins can be adsorbed to NPs to increase cellular uptake or specificity toward tumor cells (Chang et al., 2012). Negatively chargedhyaluronicacid(HA)wasalsousedtoself-assembleonto the positively chargedQDs through ionic interactions. For this, Bhang et al. developed a simple andnovel electrostatic coupling method, which provides a HA-QD conjugate with cancer tar- geting efficiency to use in diagnostic and imaging applications. These conjugates were also effective for fluorescence staining of lymphaticvessels invitroand invivo (Bhangetal., 2009). Despite the easeof this conjugationmethod, thenative struc- ture of the adsorbed proteins may be affected (Lacerda et al., 2010),which couldultimately result in loss of biological activity orevencellular toxicity (Vertegel etal., 2004;Dengetal., 2011). www.frontiersin.org July2014 |Volume2 |Article48 | 24