電勢誘導的N-異丁酰基-L-半胱氨酸分子在金(111)表面的相轉變

陳愛喜 汪 宏 段 賽 張海明,* 徐 昕 遲力峰,*

(1蘇州大學功能納米與軟物質研究院，江蘇省碳基功能材料與器件高技術研究重點實驗室，江蘇蘇州215123；2明斯特大學物理研究所，威廉-克萊姆街10號，明斯特48149，德國；3皇家理工學院，生物工程學院理論化學與生物系，斯德哥爾摩S-106 91，瑞典；4復旦大學化學系，上海200433)

電勢誘導的N-異丁酰基-L-半胱氨酸分子在金(111)表面的相轉變

陳愛喜1汪 宏2段 賽3張海明1,*徐 昕4遲力峰1,*

(1蘇州大學功能納米與軟物質研究院，江蘇省碳基功能材料與器件高技術研究重點實驗室，江蘇蘇州215123；2明斯特大學物理研究所，威廉-克萊姆街10號，明斯特48149，德國；3皇家理工學院，生物工程學院理論化學與生物系，斯德哥爾摩S-106 91，瑞典；4復旦大學化學系，上海200433)

自組裝單層膜修飾的功能性基底在生物傳感，色譜分析，生物相容性材料等方面均具有潛在應用。本文利用原位電化學掃描隧道顯微鏡(EC-STM)研究了電勢誘導的N-異丁酰基-L-半胱氨酸(L-NIBC)分子在Au (111)表面自組裝結構的相轉變。我們把Au(111)基底分別浸潤在純的NIBC水溶液和pH=7(磷酸鹽緩沖溶液調節)的NIBC溶液中，分別制備了NIBC的α相和β相兩種不同的自組裝結構。EC-STM觀測顯示，當改變金的電極電勢時，α相和β相的NIBC自組裝單層膜出現了多種不同的結構變化。當電壓從0.7 V(相對于飽和甘汞電極而言)降低到0.2 V時，α相由有序結構變為無序結構。而對于β相的樣品，當E＜0.3 V時，為無序結構；當電極電勢增大到0.4 V＜E＜0.5 V時，出現γ相；繼續增大到0.5 V＜E＜0.7 V時，變為β相。另外，EC-STM圖像也證實存在β相轉變為α相的可能。綜合密度泛函理論計算的結果，我們提出，β相轉變為α相的原因可以解釋為電極電勢的變化引起了Au―COO－鍵的斷裂，從而引發分子吸附構型變化而導致相變。

自組裝；硫醇；相轉變；電勢誘導；電化學掃描隧道顯微鏡；密度泛函理論

1 Introduction

Gold substrate modified by chemisorbed thiols has been intensively studied in the past decades for their potential applications in e.g.sensing1,biocompatibility2and molecular electronics3－6.Specifically,bio-related molecules have attracted particular interestfor their intrinsic affinity with bio-entities,such as proteins,DNAand cells7,8.To understand the interactions between bioentities and thiols,it is necessary to study the arrangementand the orientation of self-assembled thiolmolecules9－15.In our previous work,we report the STM studies on the self-assembled monolayers(SAMs)of N-isobutyryl-L(D)-cysteine(L(D)-NIBC)on Au (111)surfaces16,motivated primarily by its chiral differences in adhesion ofimmune cells and DNA4.Ithas been found thatthe pH value of NIBC aqueous solutions is able to adjust the SAM structures during the preparation.Changing the pH value from 5 to 7 may completely shiftthe SAM structures from a close-packed structure(defined asαphase)to a loose-packed structure(defined asβphase).Such a pH dependent phase transition can be explained by the breaking of hydrogen bonds between adjacent NIBC molecules and the formation of extra interactions between―COO－and Au.This mechanism ofphase transition differs from typicalphase transition of thiol SAMs induced by coverage17, temperature18－20and potential21－27.Therefore,itis necessary to carry outmore detailed experimentalresults on this phenomenon.

In this paper,we further presentthe in situ EC-STMstudies on the phase transition of L-NIBC SAMs in 0.1 mol·L－1H2SO4,a typical electrolyte in investigation of potential induced phase transition of thiol SAMs21－26.Theαphase NIBC SAMs are found changing from orderedαphase to disordered structure when the potentialof the Au(111)electrode is gradually decreasing from 0.7 V(vs saturated calomel electrode,SCE)to 0.2 V.As forβphase NIBC SAMs,changing potentialfrom 0.1 Vto 0.4 Vleads to the formation of a new phase(defined asγphase)from disordered structures.More positive potentialturns NIBC SAMs from theγ phase into theβphase.More interestingly,parts ofβphase SAMs are observed to change intoαphase atpositive potentialafter the changing of the potential from negative to positive.Combined with theoreticalcalculations,the phase transition from theβphase to theαphase can be explained by potential induced break of bonding interactions between―COO－and negatively charged gold surfaces.

2 Experimental

2.1 Preparation of NIBCSAMs

Self-assembled monolayers of L-NIBC(Sigma-aldrich,≥97%) were prepared by immersing the Au(111)substrate into the preheated(355 K)NIBC solutions for 2 min16.Phosphate buffer (Acros,99+%)was used to control the pH value of L-NIBC solutions.Milli-Q water(18.2 MΩ·cm)was used throughout.

2.2 Cyclic voltammetry

Cyclic voltammetry was carried outusing an Autolab system (Eco Chemie,The Netherlands)with a standard linear scan mode. The electrolyte was thoroughly deaerated by bubbling with nitrogen.An Au crystal,9 mm in diameter,oriented better than 0.4° toward the(111)-face(MaTeck,Jülich,Germany)was used.

2.3 In situ STM

The EC-STMmeasurements were performed with Nanoscope IIIa(Digital Instruments,Santa Barbara,CA),using tungsten tips (0.25 mm diameter)electrochemically etched(DC power,10 V) in 1 mol·L－1NaOH(≥99%)aqueous solution.The tips were coated with nailpolish to minimize faradaic current.STM images were recorded in the constant-currentmode.Platinum wires were used in the STMcellas quasi-reference electrode and as counter electrode.Allpotentialvalues are referred to the saturated calomel electrode(SCE).

2.4 Computationaldetails

Density functional theory(DFT)calculations with Beckes′s three-parameters exchange and Lee-Yang-Parr correlation hybrid functional(B3LYP)were performed to obtain the optimized geometric structures of adsorbed L-NIBC anion on the Au32cluster with different charges,i.e.positively charged,neutral,and negatively charged Au clusters respectively.Pople′s double zeta basis sets 6-31+G(d,p)were chosen for main group elements while the associated relativistic effective core potential LanL2dz basis sets for Au atoms.During the optimizations,allAu atoms were fixed attheir originalpositions to mimic the Au(111)surface.The adsorption energy was calculated by

where Etotrepresents the totalenergy of adsorption system,EAuδthe32energy of Au32clusters with differentcharge,ENIBC－the energy of deprotonated L-NIBC anion.

3 Results and discussion

Acyclic voltammogram(CV)of the Au(111)surfaces modified byαphase L-NIBC SAMs in 0.1 mol·L－1H2SO4(≥98%)aqueous solutions is shown in Fig.1(a)(solid line).The corresponding CVfor bare Au(111)(dotted line)is also displayed for comparison.The difference in CV between the bare Au(111)and the NIBC-Au surfaces is significant,reflecting the influence of the chemisorbed L-NIBC SAMs.A pair of peaks centered on a potentialof0.55 V(vs SCE)is observed in CVof NIBC-Au surfaces, corresponding to a capacitive charging and discharging process of the NIBC SAMs27.More evidence is provided in Fig.S1(Supporting Information),showing the relationship between the peaks and the NIBC SAMs.In situ EC-STMimages further indicate that the charging and discharging process is related to a structural change of self-assembled L-NIBC monolayers.Partialαphase structures,especially near the boundary of domains,are found to change into disordered structures when the potentialis gradually shift from 0.65 V(Fig.1(b))to 0.25 V(Fig.1(c))with an approximate rate of 10 mV·min－1.Meanwhile,disordered structures can also be changed back into orderedαphase structure when the potential increases gradually from 0.25 to 0.75 V(see Fig.S2 (Supporting Information)).This resultindicates thatthe potential dependent structural change is reversible in the potentialregion from 0.25 to 0.7 V which belongs to the double-layer region.Some bright lines are noticeable from the STM image(see Fig.1(b)), representing the boundary between two adjacentαphase structures in the same domain.Enhanced tunneling probability might be originated from a specific tunneling condition,since itis notalways observable in a magnified STM image(see Fig.S2).

The structure ofαphase NIBC SAMs should be clarified before further discussion.Shown in Figs.1(d)and 1(e),are high resolution STMimage and the corresponding model of theαphase NIBC SAMs.The STM image(Fig.1(d))is obtained in air conditions with the sample of Au(111)surfaces modified by theα phase SAMs.Arectangular(4×3)unitcellis proposed with two molecules per cell.This new model is different from our previously proposed model16,in which only one molecule is concerned in the unitcell.Such a modification is based on the result of the CVin 0.1 mol·L－1NaOH solutions(see Fig.S3(Supporting Information))and the high resolution STM image,as displayed in Fig.1(d).The charge under the reductive peak in NaOH is about 56μC·cm－2,which is consistentwith a rectangular(4×3)lattice with two molecules per unit cell or more precisely,3.4×1014molecules·cm－2.

Fig.1 Steady-state cyclic voltammogram(a)for Au(111)modified byαphase L-NIBC SAMs(solid line)in 0.1 mol·L－1H2SO4aqueous solutions with scan rate of 10 mV·s－1.The CV of bare Au(111)(dotted line)recorded in the same conditions is also shown for comparison. EC-STMimages ofαphase L-NIBC SAMs show potentialdependent structuralchanges from orderedαphase structures at0.65 V(b)to partially disordered structures at0.25 V(c).The white ring marks the same place in each image.Ahigh resolution STMimage ofαphase structures(obtained in air conditions)and a proposed modelare shown in panel(d)and(e),respectively. The unit cellis a rectangular(4×3)lattice with two molecules in one unit.

Potential-induced structuralchange ofαphase L-NIBC SAMs is comparable to the features of the SAMs of homocysteine(Hcy), 2-amino-4-mercaptobutyric acid,in phosphate buffer solution27. Changing potential of the Au(111)electrode modified by Hcy SAMs gives rise to a reversible structuralchange of Hcy SAMs from ordered structures to disordered structures.Itis explained by the potentialdependent changes of the adsorption conformation of Hcy molecules.Ordered structures can only be observable at potential close to neutral,corresponding to a stretched confor-mation of Hcy molecules.A bentconformation of Hcy molecules is proposed atnegative and positive potentialdue to the favorable adsorption of―NH3+and―COO－,respectively,which results in the completely changing from ordered structures to disordered structures27.Compared to Hcy molecule,the amino group of LNIBC molecule is protected by an isobutyryl,forming an amide group,which may decrease the ability of the amino group to accepta proton.Therefore,in acidic solutions,such as 0.1 mol·L－1H2SO4,when the potentialis controlled negative,only a partof LNIBC molecules could be protonated,leading to the breaking of hydrogen bonds between amide groups of adjacent NIBC molecules.The breaking of hydrogen bonds may give rise to the change of adsorption geometry.This mightbe a possible reason to explain the formation of partially disordered structures at negative potential.When the potentialis controlled to be positive in a value between 0.5 and 0.75 V,only orderedαphase structures are observed.More positive potentialmay lead to the oxidation of the SAMs.The ordered structure ofαphase L-NIBC SAMs atpositive potentialis differentfrom thatof Hcy SAMs.Ordered structures of Hcy SAMs are destroyed atpositive potentialbecause the favorable interactions between―COO－and gold substrate at positive potentiallead to a bentconformation of Hcy molecules. The deprotonation of carboxylic group is also observed by electrochemical investigations of the trimesic acid SAMs in HClO4solutions28.It is believable that the carboxylic groups of NIBC molecules could be deprotonated at positive potential,but the ordered structures are still maintainable.One possible reason is that the deprotonated carboxylic groups cannotdestroy the main forces which maintain the close packedαphase structures,as the hydrogen bonding interactions between amid groups and dipoledipole interactions are larger than the weak hydrogen bonds of the carboxylic groups16.

Fig.2 Steady-state cyclic voltammogram(a)and EC-STM images(b－d)ofβphase L-NIBC SAMs in 0.1 mol·L－1H2SO4aqueous solutions. Disordered structures are observed at potentialnegative of 0.3 V(b).With increasing of the potential,a new phase(γphase)emerges at 0.45 V(c).More positive potentialgives rise to the phase transition from theγphase to theβphase(d).Insets in panels(c)and(d)are the high resolution EC-STMimage of theγphase and theβphase L-NIBC SAMs,respectively.The white ring marks the same place in each image. Allimages are of 100 nm×100 nm.

In addition to close-packedαphase structure,loose-packedβ phase structure of NIBC SAMs can be prepared in aqueous solutions at pH 7 controlled by phosphate buffer16.The Au(111) surfaces modified byβphase L-NIBC SAMs are investigated by CV and EC-STM in 0.1 mol·L－1H2SO4aqueous solutions.Shown in Fig.2(a)is a CVresultof the Au(111)surfaces modified by the βphase L-NIBC SAMs(solid line),in which the position(0.55 V) and the strength of the anodic peak are almostconsistentwith that ofαphase L-NIBC SAMs.The slightdifference in CVs appears in the shape of the cathodic peak at 0.54 V,probably reflecting a new capacitive process occurring on the electrode.In situ ECSTMresults further reveal more structuraldetails of theβphase SAMs with the change of applied potential.When the potentialis controlled to be negative of 0.3 V,no ordered structures are observed,as displayed in Fig.2(b).With the increase of the potential (atthe range between 0.4 to 0.5 V),a new phase,γphase,appears (see STMimage of Fig.2(c)).Sometimes,the Au(111)surfaces are observed to be fully covered by theγphase structure with thepotentialat0.45 V.The new phase(γphase)has never been observed either by ex situ investigations using various preparation methods,or by in situ investigations of theαphase L-NIBC SAMs.Itis far from clearto determine the structure of theγphase SAMs merely relying on STM results,even the high resolution images(the insetof Fig.2(c)).Theγphase SAMs could also be a resultof co-adsorption with sulfuric anions23,which needs more controlexperiments to clarify it.Further increasing the potential to 0.6 V causes the complete phase transition from theγphase to the orderedβphase(Fig.2(d)).More interesting findings in ECSTM images are the appearance ofαphase atpotentialpositive of 0.4 V,which are found to evolve with the increasing of the potential.Theαphase structures are further confirmed by the following cathodic scan of the potential.Onlyαphase structures remain when the potentialis negative of 0.4 V,see Fig.S4(Supporting Information).

Fig.3 Schematic illustration of potential-induced phase transition of NIBC SAMs in 0.1 mol·L－1H2SO4solutions

Fig.4 Optimized structure of deprotonated L-NIBC anion adsorbed oncluster at B3LYP/6-31+G(d,p)/Lanl2dz level (a)δ=+1,(b)δ=0,(c)δ=－1

Potential dependent phase transition(or structural transition) has been observed in the SAMs of alkanethiols22,aromatic thiols23－26,bio-related thiols27,and other molecules chemisorbed on the electrode28,29.Mechanisms include the influence of anions23, potential-induced energetic changes of bonding interactions between thiolate and gold atoms22,26and potential-induced changes of molecular conformations27.However,these mechanisms cannot explain the phase transition of L-NIBC molecules from theβ phase to theαphase structure.On basis of our previous works,the βphase structure corresponds to a perpendicular adsorption of NIBC molecules with chemical bonds of Au―S and Au―COO－, whereas only one chemicalbond of Au-S is formed in theαphase structure.The phase transition fromβphase toαphase suggests that the chemicalbonding between gold and―COO－should be broken.At negative potential,it seems to be possible that parts of―COO－groups are protonated due to the weakened adsorbatesubstrate interactions22and the tendency of―to adsorb onto the surfaces.Accordingly,we propose a tentative mechanism to explain the phase transition of NIBC SAMs from theβphase to theαphase under the control of electrode potential.Compared with the close packedαphase structure,no hydrogen bonds of amid groups between adjacent NIBC molecules could be found in theβphase structure,as the adsorption geometry with chemical bonds of Au―S and Au―COO－limits the formation of hydrogen bonds between amid groups,as exhibited in Fig.3.Thatis also the reason why only parts ofαphase NIBC SAMs are destroyed at potentialof 0.3 V,whereas no ordered structures can be observed forβphase NIBC SAMs atthe same potential.When the potential is controlled to be negative of 0.3 V,orderedβphase structures are destroyed due to the weakened interactions of adsorbate-substrate bonding and the favorable adsorption of―with the substrate. Atthe same time,parts of―COO－groups mightbe protonated, leading to the changes of adsorption geometry.With the increase of the potential,those protonated molecules can self-assemble into orderedαphase structures assisted by hydrogen bonds between amid groups of adjacent molecules.However,mostof molecules preserve the same adsorption geometry during the change of the potential,which means they keep the chemicalbonds of Au―S and Au―COO－.Those molecules may formβphase structure at positive potential,because of the favorable adsorption of―COO－groups with the substrate atpositive potential.The whole process of the potential induced phase transition of NIBC SAMs is concluded in Fig.3.Structural changes of the NIBC SAMs are reversible for each independentαandβphase,whereas the protonation of the NIBC molecules at negative potential is irreversible and can trigger the phase transition fromβtoαphase.

To further verify the mechanism of the phase transition from the βphase to theαphase tuned by the applied potential,DFT calculations on the charge effects have been performed with an Au32cluster.The optimized structures of deprotonated L-NIBC anion adsorbed on thecluster are depicted in Fig.4.One oxygen atom in the―COO－group shows the trend to adsorb far from the Au(111)surface with increasing electrons fromδ=+1 to－1.The distances between the oxygen atom and the surface plane are measured to be 0.253,0.291,0.318nm,forδequalto+1,0,and－1 respectively.Apparently,this resultshould be mainly attributed to the increasing of the electrostatic repulsion between the Au(111) surface and the negatively charged―COO－group in NIBC.This is also confirmed by the fact that the corresponding adsorption energies calculated by Eq.(1)are 108.0,53.7,and 0.85 kcal·mol－1(1 kcal·mol－1=4.187 kJ·mol－1)forδranging from+1 to－1.In contrast,the distances between the sulfur atom in NIBC and the Au(111)surface are slightly changed from 0.225,0.233,and 0.242 nm forδof+1,0,and－1,respectively.Therefore,the sulfur atom is believed to bind with gold on the negatively charged Au(111)surface.Accordingly,it is safe to say that the electrostatic repulsion can lead to partialdesorption of―COO－groups from the Au(111)surface when electrode potential becomes more negative. Some―COO－groups may be protonated at negative potential. The NIBC molecules with the protonated―COO－group are able to shift into the orderedαphase structure at positive potential, whereas the restof NIBC molecules with―COO－group proceeds the phase transition from disordered structures to theγphase and finally to theβphase with gradually increased potential of the Au(111).

4 Conclusions

In conclusion,the phase transition of L-NIBC SAMs on Au (111)surfaces is investigated under controlled electrochemical conditions in 0.1 mol·L－1H2SO4solutions.The as-preparedα phase andβphase NIBC SAMs exhibitvarious structuralchanges, further indicating thatthe two distinct phase origin from different adsorption geometry of NIBC molecules.The NIBC SAMs with αphase structure are able to change from ordered structure(E＞0.5 V)to partially disordered structure(0.2 V＜E＜0.5 V).As for βphase,structural changes are observed from fully disordered (E＜0.3 V)toγphase(0.4 V＜E＜0.5 V)and finally toβphase structure(E＞0.5 V).More interestingly,a smallpartofβphase structure is observed to change intoαphase atpositive potential, which is tentatively explained by the protonation of―COO－groups atnegative potential.

Supporting Information:CVs of theαphase NIBC SAMs in 0.1 mol·L－1H2SO4,EC-STMimages ofαphase SAMs with decreasing ofthe potential,CVs of theαphase NIBC SAMs in 0.1 mol·L－1NaOH and EC-STMimages ofβphase SAMs with decreasing of the potential.This information is available free of charge via the internetathttp://www.whxb.pku.edu.cn.

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Potential-Induced Phase Transition of N-Isobutyryl-L-cysteine Monolayers on Au(111)Surfaces

CHENAi-Xi1WANGHong2DUAN Sai3ZHANGHai-Ming1,*XU Xin4CHILi-Feng1,*
(1Institute ofFunctionalNano&Soft Materials(FUNSOM),Jiangsu Key Laboratory for Carbon-Based Functional Materials& Devices,Soochow University,Suzhou 215123,Jiangsu Province,P.R.China;2Physikalisches Institut,Universit?t Münster, Wilhelm-Klemm Strasse 10,48149 Münster,Germany;3Department of Theoretical Chemistry and Biology, School of Biotechnology,Royal Institute of Technology,S-106 91 Stockholm,Sweden;4Department of Chemistry,Fudan University,Shanghai 200433,P.R.China)

Functional solid substrates modified by self-assembled monolayers(SAMs)have potential applications in biosensors,chromatography,and biocompatible materials.The potential-induced phase transition of N-isobutyryl-L-cysteine(L-NIBC)SAMs on Au(111)surfaces was investigated by in-situ electrochemical scanning tunneling microscopy(EC-STM)in 0.1 mol·L－1H2SO4solution.The NIBC SAMs with two distinct structures(αphase andβphase)can be prepared by immersing the Au(111)substrate in pure NIBCaqueous solution and NIBC solution controlled by phosphate buffer atpH 7,respectively.The as-preparedαphase and βphase of NIBCSAMs show various structuralchanges under the controlofelectrochemicalpotentials ofthe Au(111)in H2SO4solution.Theαphase NIBC SAMs exhibit structuralchanges from ordered to disordered structures with potentialchanges from 0.7 V(vs saturated calomelelectrode,SCE)to 0.2 V.However,theβphase NIBC SAMs undergo structuralchanges from disordered structures(E＜0.3 V)toγphase(0.4 V＜E＜0.5 V)and finally to theβphase(0.5 V＜E＜0.7 V).EC-STM images also indicate thatthe phase transition from theβphase NIBC SAMs to theαphase occurs atpositive potential.Combined with density functionaltheory (DFT)calculations,the phase transition from theβphase to theαphase is explained by the potential-induced break ofbonding interactions between―COO－and the negatively charged gold surfaces.

Self-assembly;Thiol;Phase transition;Potential-induced;Electrochemicalscanning tunneling microscopy;Density functionaltheory

O647

alem,F.;Mandler,D.Anal.Chem.1993,65,37.

10.1021/ ac00049a009

doi:10.3866/PKU.WHXB201702102

Received:December19,2016;Revised:February 10,2017;Published online:February 10,2017.

*Corresponding authors.ZHANG Hai-Ming,Email:hmzhang@suda.edu.cn.CHILi-Feng,Email:chilf@suda.edu.cn;Tel:+86-512-65880031. The projectwas supported by the NationalNatural Science Foundation of China(91227201,21527805).

國家自然科學基金(91227201,21527805)項目資助?Editorialoffice of Acta Physico-Chimica Sinica