Surface-Structure-SensitiveAdsorption of Adenine on GoldElectrodes

Ana Martins,* Ana Queir�s, and Fernando Silva[a]

Since the earlier work on Hg, and more recently on Ag, Au andPt single-crystal electrodes, electrochemical studies haveshown that DNA bases are strongly adsorbed at the metal–solution interfaces and generally undergo a two-dimensionalfirst-order transition.[1] Although it is well-known that the sur-face atomic structure, as much as its chemical nature, can playa critical role in the formation of ordered adsorbed monolay-ers, only few detailed investigations have evaluated the effectof the presence of regular monoatomic steps on the adsorp-tion of nucleobases and nucleosides.[2–4]

The structure and properties of adsorbed nucleobase filmsdeduced from electrochemical studies have been confirmed byseveral in situ spectroscopy techniques, such as subtractivelynormalized interfacial Fourier-transform infrared spectroscopy(SNIFTIRS),[5] surface-enhanced infrared reflection-absorptionspectroscopy with the attenuated total reflection technique(ATR-SEIRAS),[4, 6] surface-enhanced Raman spectroscopy(SERS),[7] surface X-ray scattering (SXS),[8] X-ray photoelectronspectroscopy (XPS),[9] and scanning tunnelling microscopy(STM).[9–12] These techniques, however, were mostly used toobtain information on monolayer films adsorbed on smoothbasal planes or single-crystal surfaces with a low density ofmonotamic steps and hence reduced surface atomic corruga-tion. Although the macroscopic information that can be ob-tained by electrochemical techniques may not reflect theentire complexity of interfacial phenomena, it has long beenused to assess the average systematic effect of surface atomicstructure details such as regular monoatomic steps and kinkson, for example, the ionic and organic adsorption, electrocatal-ysis, under-potential deposition and two-dimensional film con-densation.

An electrochemical study was therefore initiated to revealthe effect of surface structure regularities, such as monoatomicsteps and kinks, on the adsorption of the DNA bases at goldsingle crystal electrodes. We report here a preliminary overviewof the influence of the surface crystallographic orientation onthe adsorption of adenine in acid media.

Results and Discussion

In Figure 1 a, the cyclic voltamograms obtained for the Au (111)electrode in the presence and absence of adenine in 0.1 m

HClO4 are compared. The gold surface oxidation is inhibited inthe presence of adenine and occurs at higher potential values

suggesting that a strongly adsorbed layer is formed. The analy-sis of the charges involved in the oxidation and reductionprocesses shows that adenine oxidation is irreversible and con-comitant with the gold surface oxidation (Qox/Qred>2). Thesame behavior was also observed for the other gold surfacesunder study.

The voltamogram obtained for Au (111) in the double layerregion, Figure 1 b, as well as the capacitance curve, Figure 1 c,substantiate that a chemisorbed layer of adenine is formed atthe more positive potential values (region III). Based on earlierstudies on adenine adsorption on Au (111) in acid media[13] andreports in the literature for the adsorption of other nucleobas-es,[4, 6, 11, 14–18] region I was reconciled with the formation of a dis-ordered and planar orientated layer of molecules, followed by(large peak between regions I and II) further adsorption of ade-nine and its deprotonation along with the lifting of the surfacereconstruction, and the formation of an ordered, physisorbedfilm on the non-reconstructed surface (region II). The decrease

Figure 1. a) Cyclic voltamograms obtained for Au (111) in a (a) 0.1 m

HClO4 solution plus (c) 5 mm adenine. b) Stable cyclic voltamograms ob-tained in the double-layer region for Au (111) (c) and Au (100) (a) in0.1 m HClO4 and 5 mm adenine solution and c) differential capacitancecurves obtained for Au (111) in 0.1 m HClO4 (a) and in 5 mm adenine solu-tion in the positive (d) and negative scans (c).

[a] Dr. A. Martins, Dr. A. Queir�s, Prof. Dr. F. SilvaDepartamento de Qu�mica da Faculdade de CiÞnciasRua do Campo Alegre, 687, 4169 007 Porto (Portugal)Fax: (+ 351) 226-082-959E-mail : amartins@fc.up.pt

1056 � 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/cphc.200400600 ChemPhysChem 2005, 6, 1056 –1060

in the capacitance values between regions II and III suggeststhe reorientation of the molecules to a more vertical position.A partial charge transfer of the adsorbed adenine molecules ispossibly involved in the formation of the chemisorbed layer,that is, in the transition between regions II and III. Studies ofthe influence of the solution pH value also revealed that thestability of the intermediate-adsorbed layer (region II) increasesas the pH increases. Evidence of this intermediate adsorptionstate is no longer found at pH values below 1.

Although the formation of a two-dimensional condensedfilm of physically adsorbed adenine molecules was reportedfor Hg[19–21] in neutral media, sharp current spikes that wouldidentify the formation and dissolution of this type of film arenot observed on the Au (111) surface.

In Figure 1 b, the cyclic voltamogram obtained in thedouble-layer region for Au (100) is compared with that ob-tained for Au (111). The same features are observed in bothvoltametric profiles, that is, broad peaks and intermediate/lowcapacitive-current regions, suggesting that the adenine-adsorption process is similar, although it is shifted to lower po-tential values possibly as a consequence of the negative shiftof the pzc (potential of zero charge) of Au (100) compared toAu (111) (see Table 1). There is, however, a significant influenceof the surface structure on the stability region of the orderedphysisorbed layer (region II) that is dramatically enlarged andthus favoured by the fourfold symmetry of the Au (100) plane.Moreover, an additional irreversible peak is observed at a po-

tential value close to that of the first peak for Au (111)(�0.075 V). Further experiments revealed that the intensity ofthis peak increases with the degree of the potentially inducedsurface reconstruction.[22] The Au (100)-(hex) domains that areformed during the Au (100) surface reconstruction have anhexagonal-close-packed atomic arrangement similar toAu (111). These domains can coexist with non-reconstructedAu (100)-(1x1) domains, depending on the degree of recon-struction achieved.[23] Our suggestion is therefore that the firstbroad peak (�0.30 V) must be related to phenomena occurringon the non-reconstructed domains while the second peak willcorrespond to the phenomena taking place on the recon-structed domains (�0.075 V). Adsorption on coexistentAu (100)-(1x1) and -(hex) has already been described for pyri-dine.[24] Adsorption of pyridine on the non-reconstructed sur-face also starts at less negative potentials than on the recon-structed surface, although the difference is less pronouncedthan for adenine.

The adsorption of adenine was then investigated on 12other gold single crystal electrodes with different crystallo-graphic orientations in order to evidence the influence of thesurface atomic arrangement and presence of steps and kinks.The cyclic voltamograms obtained in the double-layer regionare presented in Figure 2. The different crystallographic orien-tations were collected in four different categories in order toevidence the similarities observed in their cyclic voltamograms,namely, Au (111), Au (100) and their vicinal surfaces (Figures 2 ato 2 c), surfaces with an intermediate density of steps (Fig-ure 2(d)) and surfaces with a high density of steps (Figure 2 e).The voltamograms presented in Figures 2 b to 2 e were ob-tained in a 0.2 m HClO4 solution and 0.1 mm adenine. At thishigh pH value, the intermediate region II is no longer ob-served.

The voltametric profiles obtained for the investigated surfa-ces with the lower density of steps (when the angle betweenthe stepped surface and the low-index reference face is below148), namely Au (755), Au (332) and Au (410), are similar to theprofiles obtained with the smooth surfaces Au (111) andAu (100), although the adsorption/transition peaks are slightlybroader and their magnitude smaller. A similar influence of thepresence of steps was reported in the literature for the adsorp-tion of uracil[4] and uridine[2] on vicinal surfaces of Au (111) andwas attributed to an increasing disorder in the adsorbed layeras the density of steps increased.

For the surfaces with an intermediate density of steps as, forexample, Au (533), Au (511) or Au (311), the transition peak isreplaced by a broad peak suggesting that the formation of along-range ordered physisorbed layer is no longer possible onthese more corrugated surfaces and random adsorption willtake place instead. The low-charging current observed athigher potential values shows, however, that the substrate sur-face structure is less critical for the formation of the chemicallyadsorbed layer.

For the particular case of surfaces with a high density ofsteps or kinks (when the angle between the stepped surfaceand the low-index reference face is over 208), namely Au (110),Au (320), Au (771), Au (331), Au (210) and Au (321), the random

Table 1. Relevant data for the gold-single-crystal electrodes employed inthis study. (* = reconstructed surfaces[26]).

Crystallographicorientation

Angle from thelow-index face

Lang or microfacetnotation

pzc in HClO4

10 mm[V/SCE]

Au (111)* 0.00 0.196Au (755)* 9.45 6 (111)–(111) 0.070Au (533)* 14.42 4 (111)–(100) 0.049Au (311) 29.50 2 (111)–(100) �0.031

2 (100)–(111)Au (332)* 10.02 6 (111)–(100) �0.021Au (331) 22.00 3 (111)–(111) �0.048Au (771) 29.50 7/3 (111)–(111) �0.060Au (110)* 35.26 2 (111)–(111) �0.072Au (321) 22.21 12 (111) + 11 (110)

+ 11 (100)�0.130

Au (100)* 0.00 0.050Au (511) 15.79 3 (100)–(111) �0.033Au (410)* 14.04 4 (100)–(110) �0.044Au (210) 26.57 2 (100)–(110) �0.136

2 (110)–(100)Au (320) 33.69 3 (110)–(100) �0.108

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adsorption peak cannot be observed in the potential windowused probably as a consequence of their more negative pzcvalues (see Table 1 ). A stable chemisorbed film is formed assoon as the electrode is immersed in the solution and will notbe desorbed in the potential window used. Stable cyclic volta-mograms were obtained after performing at least 20 consecu-tive cycles. This behaviour is commonly observed for self-as-sembled monolayers and usually corresponds to an “electro-chemical annealing” of the adlayer defects in the adsorbedfilm. The formation and properties of self-assembled monolay-ers of purine and pyrimidine bases on Au (110) will be dis-cussed in a forthcoming paper.

The differential capacitance curves obtained for some of thestudied surfaces are presented in Figure 3. The low capacitance

values obtained in the potential region where the strongchemisorbed film is formed (region III) decrease systematicallyfrom 15 to 11 mC cm�2 as the density of steps increases (seeinset in Figure 3).

Adsorption of adenine on single crystal stepped surfaces hasnot been explored yet except by McNutt et al. using reflectionabsorption infrared spectroscopy (RAIRS) on Cu (110) in ultra-high vacuum (UHV).[25] The authors showed that the adeninemolecular ring is substantially tilted and a stable hydrogen-bonded assembly is formed. This network involves bent hydro-gen bonds arising from a compromise between the strongmolecule–metal interactions that orient the molecules, andweaker lateral hydrogen-bonding interactions that dictate thetwo-dimensional architecture. Although in the electrochemicalmedia a strong-chemisorbed film is apparently formed oneither smooth, stepped or kinked surfaces, the surface struc-ture should affect the local orientation of the adsorbed mole-cules and thus the lateral long-range order and average thick-ness and organization of the film.

It is also known that high-index faces of gold undergo sur-face reconstruction, faceting, or relaxation, in order to mini-mize their surface energy.[23, 26] These phenomena can be in-duced by thermal treatment under UHV conditions, or simplyby flame annealing, if followed by the appropriate cooling pro-cedure.[23] The reconstructed surface is retained in electro-chemical media, depending on the immersion potential andspecies in the solution. The lifting of the surface reconstructioncan be identified in the cyclic voltamograms by the presenceof an irreversible peak associated with the change in the sur-face charges of the different atomic arrangements. Once lifted,the reconstruction can at least be partially recovered if a suffi-ciently negative potential relative to the potential of zerocharge is applied. However, the surface atomic structures ob-tained by thermal or electrochemical treatment have slightlydifferent atomic arrangements. A positive shift of the potentialfor lifting of the surface reconstruction will reflect this structuredifference while the intensity of the lifting peak will be propor-

Figure 2. Cyclic voltamograms obtained for some representative gold-single-crystal surfaces in a): 5 mm and 0.1 m HClO4 in order to observe the inter-mediate region II, and b) to e): 1 mm and 0.2 m HClO4.

Figure 3. Capacitance curves obtained for some representative smooth,stepped and kinked surfaces of gold in 1 mm adenine solution and 0.2 m

HClO4 [Au (100) c, Au (110) d, Au (111) b, Au (311) a, Au (755),Au (321) g] .

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tional to the degree of reconstruction. On the other hand, ad-sorption of ions and organic molecules generally plays an im-portant role in the surface reconstruction and its lifting by hin-dering or enhancing these phenomena. Thus, surface recon-struction and its lifting will affect the adsorption of ions[26] andorganic molecules[23] especially when two-dimensional transi-tions in the adsorbed adlayers are involved. The influence ofsurface reconstruction has already been observed by electro-chemical processes and confirmed by non-electrochemicaltechniques for the adsorption of uracil, also a nucleobase, onthe low-index faces of gold.[10, 15] Influence of surface recon-struction on the adsorption of adenine cannot therefore beruled out for the high-index faces as we already reported its in-fluence on the adsorption of oxoanions.[27]

The cyclic voltamograms presented in Figure 4 for Au (111)and two vicinal surfaces, Au (755) and Au (332), show the influ-

ence of the thermally induced reconstruction on the adsorp-tion of adenine. Similarly to Au (111), adsorption of adenine onthe thermally reconstructed, stepped surfaces is shifted to-wards more positive potential values reflecting the change ofthe surface pzc and the atomic arrangement. For the samereason, the lifting of the surface reconstruction (peak L) is alsopositively shifted. These effects, however, are less pronouncedfor the high-index faces as already reported for the studies inoxoanion solutions.[27]

In conclusion, we have investigated the adsorption of ade-nine on gold single crystal electrodes in acid media usingcyclic voltametry and differential capacitance measurements.

A chemisorbed film of adenine is formed at the more posi-tive potentials on the smooth, stepped or kinked non-recon-structed surfaces of gold under study. Although the surfacestructure is not critical for the formation of this film, its struc-ture and thickness must vary systematically with the density ofsteps. The presence of a high density of steps and kinks alsocauses an increasing stability of the adsorbed film that can beprepared by self-assembly.

At lower potential values, random adsorption takes place atthe surfaces with a significant density of steps while a physi-sorbed layer of planar, protonated adenine molecules isformed on the surfaces with large (111)-oriented domains,namely Au (111) and its vicinal surfaces Au (755) and Au (332),as well as on the totally or partially reconstructed Au (100) do-mains.

Although the macroscopic signals obtained by electrochemi-cal techniques may not reflect the entire complexity of the ad-enine adsorption, this preliminary overview evidences clearlythat it is complex and strongly influenced by the presence ofsteps and kinks. Further studies using structure-sensitive in situtechniques are needed to obtain more detailed information.

Experimental Section

All the gold electrodes were prepared in CNRS-Meudon using theHamelin method[28] except for the gold electrode Au (321) that wasprepared by J. Feliu at the University of Alicante using the Claviliermethod.[29] Relevant information is given for the selected crystallo-graphic orientations in Table 1 (potential of zero charge,[26] Lang[30]

or microfacet[31] notation and surface reconstruction[26, 32]).Pre-treatment of the gold electrodes was performed before eachexperiment and consisted of a flame annealing followed byquenching in ultrapure water. The cleanliness of the surfaces, thedegree of reconstruction induced thermally or electrochemicallywas confirmed by cyclic voltametry and capacitance measurementsin a HClO4 solution. The results obtained were in agreement withthose in the literature.[24]

The counter electrode was a gold coil previously flame-annealedand the reference electrode, Ag/AgCl in saturated NaCl, was keptin a separate cell containing the electrolytic solution and connect-ed to the working electrode by a salt bridge and a Lugging capilla-ry. The solutions were degassed with nitrogen before each experi-ment and a constant flow was maintained over the solution duringexperiments.Cyclic voltametry was performed at a scan rate of 50 mVs�1 usingan EG&G-PAR 263 potentiostat. The differential capacitance, C(E)curves, were calculated from automatically acquired values of thein-phase and out-of-phase components (lock-in amplifier—EG&G-PAR 5210) of the alternating current (ac) response to a 20 Hz/10 mV peak-to-peak sine wave superimposed on a linear voltagescan of 5 mVs�1. All experiments were performed in perchloric acidsolutions (Merck Suprapur reagent). Adenine (>99.5 % purity) wasused as received from Sigma-Aldrich. All the solutions wereprepared with ultrapure water (Millipore system—resistivity>18 mW cm).

Figure 4. Cyclic voltamograms obtained in 1 mm adenine and 0.2 m HClO4

for Au (111) and vicinal surfaces Au (755) and Au (332). The first scan (c)obtained immediately after thermal annealing is presented and comparedto the (a) 5th voltametric scan.

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Acknowledgements

A. Martins is grateful to A. Hamelin (CNRS-Meudon) and J. Feliu(University of Alicante) for the preparation of the gold single crys-tals. This work was carried out at CIQ-UP, Linha 4. A. Martins andA. P. Queir�s are grateful to FCT for financial support (projectFEDER/POCTI/QUI/41704/2001).

Keywords: adenine · adsorption · electrochemistry · goldelectrodes · surface chemistry

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Received: December 7, 2004

Published online on May 10, 2005

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