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Controlling the Surface Functionalization of Ultrasmall Gold

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Controlling the Surface Functionalization of Ultrasmall Gold Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202003804 & Supramolecular Chemistry Controlling the SurfaceFunctionalization of UltrasmallGold NanoparticlesbySequence-Defined Macromolecules Selina Beatrice van der Meer,[a] Theresa Seiler,[b] Christin Buchmann,[b] GeorgiaPartalidou,[b] Sophia Boden,[b] Kateryna Loza,[a] Marc Heggen,[c] Jürgen Linders,[d] Oleg Prymak,[a] Cristiano L. P. Oliveira,[e] Laura Hartmann,*[b] and Matthias Epple*[a] Abstract: Ultrasmall gold nanoparticles (diameter about quantitative 1HNMR spectroscopy.Each nanoparticle carried 2nm) were surface-functionalizedwith cysteine-carrying pre- between40and 100 OAA ligands, depending on the cision macromolecules. These consistedofsequence-defined number of cysteine units per OAA.The footprint of each oligo(amidoamine)s (OAAs)with either two or six cysteine ligandwas about 0.074 nm2 per cysteinemolecule. OAAs are molecules for binding to the gold surface and either with or well suited to stabilize ultrasmall gold nanoparticlesbyse- withoutaPEG chain (3400 Da). They were characterized by lectivesurface conjugation and can be used to selectively 1HNMR spectroscopy, 1HNMR diffusion-ordered spectrosco- cover their surface. The presence of the PEG chain consider- py (DOSY), small-angle X-ray scattering(SAXS), and high-res- ably increasedthe hydrodynamicdiameterofboth dissolved olution transmission electron microscopy.The number of macromolecules and macromolecule-conjugated gold nano- precision macromolecules per nanoparticle was determined particles. after fluorescent labeling by UV spectroscopyand also by Introduction Merrifield,[1] OAAs are synthesized via stepwise addition of buildingblocks on asolid support, giving access to monodis- Peptide-mimetic oligo(amidoamine)s (OAAs) represent anew perse,sequence-defined macromolecules. In addition to readily ligand class for the surface functionalization of gold nanoparti- availableamino acids,wehave developed alibrary of non-nat- cles. Similar to the peptidesolid-phase synthesis according to ural buildingblocks allowing the introductionofdifferent func- tionalunits within the main or the side chain of the resulting [2] [a] Dr.S.B.van der Meer,Dr. K. Loza, Dr.O.Prymak, Prof. Dr.M.Epple macromolecule. Earlier, we have reported the synthesis of Inorganic Chemistry and Centerfor Nanointegration Duisburg-Essen glycosylated OAAs and their potentialtomimic natural glycans (CeNIDE), University of Duisburg-Essen and their interaction with carbohydrate-recognizing receptors Universitätsstr.5–7,45117 Essen (Germany) such as lectins or bacterial adhesins.[3] The attachment of gly- E-mail:[email protected] cosylated OAAs to goldnanoparticles via aterminal thiol [b] T. Seiler, C. Buchmann, G. Partalidou, Dr.S.Boden,Prof. Dr.L.Hartmann Organic Chemistry and Macromolecular Chemistry groupallowedfor astraightforward read-outoflectin binding Heinrich Heine-University Düsseldorf,Universitätsstr.1 by UV spectroscopy,based on the surface plasmon resonance 40225 Düsseldorf (Germany) effects of the gold nanoparticles.[4] However,the degree of E-mail:[email protected] functionalization of the gold nanoparticles is variable for OAAs [c] Dr.M.Heggen of different composition and structure, for example, when Ernst Ruska-Center for Microscopy and Spectroscopy with Electrons ForschungszentrumJülich GmbH, 52425 Jülich (Germany) going from one to three carbohydrate residues per OAA. In [d] Dr.J.Linders ordertounderstand structure-property correlationsofOAA- Physical Chemistry and Center for Nanointegration Duisburg-Essen functionalizedgold nanoparticles, acontrol over the degree of (CeNIDE), University of Duisburg-Essen, Universitätsstr.5–7 functionalization (or surface coverage) is required. 45117 Essen (Germany) To elucidate the effect of multi-thiol ligands on the degree [e] Prof. Dr.C.L.P.Oliveira of functionalization of gold nanoparticles in detail, we have Institute of Physics, University of S¼oPaulo [5] Rua do Mat¼o1371, S¼oPaulo, S¼oPaulo,05508-090(Brazil) used ultrasmall gold nanoparticles of about2nm diameter. Supporting information and the ORCID identification number(s) for the The term “ultrasmallgold nanoparticles” hasbeen coined to author(s) of this articlecan be found under: distinguish such particles from “classical” larger gold nanoparti- https://doi.org/10.1002/chem.202003804. cles (10–15 nm), prepared for example, by the Turkevich 2020 The Authors. Published by Wiley-VCH GmbH. This is an open access method.[6] They are typically abit larger than atom-sharp gold article under the terms of the Creative Commons Attribution Non-Commer- clusters (1 to 2nm) and have adiameter between1and 3nm, cial NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and although there is no strict size rangefor them whichisgener- no modifications or adaptations are made. ally accepted.[7] The fact that NMR spectroscopy is possible Chem. Eur.J.2021, 27,1451 –1464 1451 2020 The Authors. Published by Wiley-VCH GmbH Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202003804 with such ultrasmall nanoparticlesisadvantageous for aquan- The building block EDS was synthesized according to previously re- titative assessment of their surface composition, especially to ported protocol with an overall yield of 54 %.[10] All solid phase re- determine the number of ligandsand thus the degree of sur- actions were performed in polypropylene reactors with apolyethy- face functionalization.[5b,c,8] lene frit closed at the bottom with aluer stopper.The batch sizes for synthesizing the oligomers using solid phase synthesis Here we demonstrate how peptide-mimetic OAAs can be varied between 0.05 and 0.2 mmol. Tentagel SRam resin with a used as tailor-made surface coating ligands for ultrasmall gold 1 loading of 0.23 mmolgÀ and Tentagel Pap with aloading of 1 nanoparticles. The length and number of sulfur binding atoms 0.16 mmolgÀ were used for PEG-attached oligo(amidoamine)s. of the OAAs was systematicallyvaried to assess the natureof Precise oligomers were synthesized on solid support by coupling their interaction with the gold nanoparticles, like molecular and Fmoc deprotection protocols. For preparation the resin was footprint and thepossibility to cover the particlesurface. This swelled twice for 15 min in DCM. Afterwards the resin was washed was elucidated by colloid-chemical methodsand extensive five times with DMF.For cleavage of the Fmoc group of the resin NMR spectroscopic investigations. as well as for the coupled building blocks or amino acids, the resin was treated with asolution of 25 vol%piperidine in DMF.The de- protection time was 215 min. After the deprotection, the resin was washed ten times with DMF.Abuilding block or amino acid Experimental Section (5 equiv.) and the coupling reagent PyBOP (5 equiv.) were dissolved in DMF.Afterwards DIPEA (10 equiv.) was added and the solution Chemicals was flushed with nitrogen for 1min. The solution was subsequent- Asolution of tetrachloroauric acid (HAuCl4)was prepared by dis- ly added to the previous deprotected resin and the reaction was solving elemental gold ( 99%) in aqua regia. Ultrapure water shaken for 1h at room temperature. The resin was then washed (Purelab ultra instrument from ELGA) was used for all nanoparticle 10 times with DMF.For attachment of afluorophore, each resin of syntheses and purifications unless otherwise noted. Before all reac- an oligo(amidoamine) batch was divided into two halves, one half tions involving nanoparticles, all glassware was cleaned with boil- of which was then functionalized with afluorophore. ing aqua regia and thoroughly washed with ultrapure water after- The fluorophore FITC-isothiocyanate (1.1 equiv.) was dissolved in wards. 6mLDMF before DIPEA (10 equiv.) was added, and the solution Sodium borohydride (NaBH , 96%), deuterium oxide (D O, 99%), was flushed with nitrogen for 1min. Then the mixture was subse- 4 2 diethyl ether (with BHT as inhibitor,99.8%), triisopropylsilane (98%; quently added to the resin and shaken for 18 h. An excess of re- TIPS), 2,2’-(ethylenedioxy)bis(ethylamine) (98%), ethyl trifluoroace- agents was removed by washing. For double coupling, the proce- tate (99%), and succinic anhydride (>99%) were obtained from dure was repeated the same way once more. Afterwards the resin Sigma–Aldrich. Trityl chloride (98%) and piperidine (99%), were ob- was washed 10 times alternatingly with DMF,DCM and MeOH. tained from Acros Organics. PyBOP (benzotriazole-1-yl-oxy-tris-pyr- For microcleavage, asmall amount of resin was treated with aso- rolidino-phosphonium hexafluorophosphate) and Fmoc-l-Cys(Trt)- lution of 95 vol%TFA,2.5 vol%TIPS, and 2.5 vol%DCM for OH was obtained from Iris Biotech. 9-Fluorenylmethyl chlorofor- 20 min. For precipitation of the product, the cleavage solution was mate (Fmoc-Cl, 98%) was purchased from Chempur. N,N-Diisopro- filtered and added to cold diethyl ether.The suspension was cen- pylethylamine (99%; DIPEA) was obtained from Carl Roth. Trifluoro- trifuged, and the supernatant was decanted. The remaining residue acetic acid (99%; TFA) was obtained from Fluorochem. Dichlorome- was dried under nitrogen, dissolved in 0.5 mL of water or water thane (99.99%; DCM), sodium chloride (99.98%), tetrahydrofuran and acetonitrile and measured by HPLC. For macrocleavage, the (THF) (analytical reagent grade), ethyl acetate (analytical reagent resin was washed 10 times with 8mLDCM before the cleavage so- grade), sodium
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