Nitroxyl radical self assembled monolayers: Ion pairing investigation in organic and aqueous media

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Nitroxyl radical self assembled monolayers: Ion pairing investigation in organic and aqueous media

Christelle Gautier, Olivier Alévêque, Fawzia Seladji, Marylène Dias, Tony Breton

^*

, Eric Levillain

^*

Laboratoire CIMA, Université d’Angers, CNRS, 2 Boulevard Lavoisier 49045, Angers cedex, France

a r t i c l e i n f o

Article history:

Received 21 October 2009

Received in revised form 26 October 2009 Accepted 26 October 2009

Available online 29 October 2009

Keywords:

Tempo

Self assembled monolayers Cyclic voltammetry Ion pairing

a b s t r a c t

Self assembled monolayers (SAMs) formed from TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) deriva- tive thiols have been studied by electrochemical quartz crystal microbalance (EQCM) in both aqueous and non-aqueous solvents. Thein situstudy of the mass transport occurring during the oxidation of TEMPO provides evidence of a ion pair formation without incorporation of solvent in densely packed nitroxyl radical SAMs. For SAMs having a low nitroxyl radical surface coverage, the effect of mixed SAMs is evidenced and seems to avoid the solvent incorporation.

1. Introduction

The formation of ion pairs between a modified electrode posi- tively charged and anions coming from an electrolytic solution was first investigated on electroactive films [1]. On conducting polymers coated electrodes, the lost of electrons recorded during p-doping process is accompanied by the insertion of anions coming from the electrolytic solution. Investigations by probe beam deflec- tion[2], EQCM[3–5]or impedance spectroscopy[6–8]shown that the entry of anions in the polymer film is accompanied by solvent molecules.

Concerning SAMs, studies were undertaken on gold electrodes modiﬁed by ferrocenylalkanethiols in electrolytic aqueous media by electrochemistry [9–11], infrared reﬂection absorption spectroscopy[12,13], Raman spectroscopy[14], ellipsometry[15]and EQCM[16–18]. The mass transport resulting from the oxidation of monolayers is different from the one observed for conducting polymers and consists in the approach of one anion per positive charge for densely packed monolayers, and in a water solvated anion for loosely packed monolayers[17].

Herein, we focused our attention on the mass transport occurring during the oxidation of HS–C15–CO–NH–TEMPO (1) in order to study the ion pair formation between electrolytic anions and oxoammonium cations as a function of the surface coverage in both organic and aqueous media.

2. Experimental

2.1. Chemicals and solutions

The synthesis and characterisations of nitroxyl radical deriva- tive1were described in Ref.[19].

Electrochemical experiments were carried out with a Biologic SP- 150 potentiostat at 293 K. Cyclic voltammetry was performed in a three-electrode cell equipped with a platinum-plate counter electrode. Reference electrodes were Ag/AgNO3(0.01 M CH3CN) or Ag/

AgCl/KClsat. CVs were recorded in dry HPLC-grade methylene chlo- ride (CH2Cl2), HPLC-grade acetonitrile (CH3CN) or H2O (18 MX).

Supporting electrolytes were TBAPF6, TBAClO4, TBABF4, NaPF6, NaClO4 and NaBF4. Based on repetitive measurements, absolute errors on potentials were found to be approximately5 mV.

Frequency measurements were recorded simultaneously to the cyclic voltammetry with a Quartz Crystal Analyser QCA922 con- nected to the potentiostat (SP-150, BioLogic). Considering that no viscoelastic changes occur at the electrode interface, a mass sensitivity of 5 ng cm²Hz¹, calculated by integration of the Cu-strip- ping voltammetric curve of a CuSO₄ aqueous solution [20]was used to calculate the mass variation from the frequency variation.

2.2. Au substrate and SAM preparation

The substrates were prepared by deposition of ca. 5 nm of chrome followed byca.50 nm of gold onto a glass substrate using physical vapor deposition technique and were made immediately before use[21,22].

doi:10.1016/j.elecom.2009.10.040

* Corresponding authors. Tel.: +33 2 41735095; fax: +33 2 41735405.

E-mail addresses: tony.breton@univ-angers.fr (T. Breton), eric.levillain@

univ-angers.fr(E. Levillain).

Electrochemistry Communications 12 (2010) 79–82

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Mixed SAMs, prepared from a TEMPO SAM and then immerged (vs.time) in an alkanethiol solution, lead to stable and reproducible SAMs.

3. Results and discussion

As the electrochemical behaviour of SAMs depends on the supporting electrolyte[23,19], the possibility of studying TEMPO SAMs in dichloromethane and water opens the scope and allows the comparison between the two media. In order to compare the formation of ion pairs in organic and aqueous media, tetrabutylam- monium (TBA) and sodium salts were used respectively and identical anions were choosen in both media (PF₆, ClO₄ and BF₄).

EQCM appears to be a powerful tool for the investigation of coupling between the electrolyte and the monolayer since it allows the in situdisplay of the ion pairing formation and dissociation.

AT-cut 9 MHz gold coated quartz crystal oscillators, used as working electrodes, were functionalized by 30 mn immersion in 1 mM of1 in dichloromethane. When the monolayer is studied in aqueous medium, two successive immersions are added to greatly improve stability: 1 h in acetonitrile followed by 12 h in water. Densely packed obtained monolayers were studied by cyclic voltammetry coupled with QCM either in dichloromethane (0.1 M of TBAPF6, TBAClO4or TBABF4) or in water (0.1 M of NaPF6, NaClO4

or NaBF₄).

Fig. 1shows the measurements recorded on a TEMPO-monolayer immersed in 0.1 M TBAPF6in dichloromethane. The voltam- mogram exhibits a reversible and stable one-electron process close to 0.51 Vvs.Ag/AgNO3, characteristic of surface-conﬁned re- dox species. It is noteworthy that the thermodynamic stability (vs.

time) of SAMs is strongly dependent on the solvent and storage conditions: low (a few hours) in CH2Cl2, medium (a few days) in CH3CN and high (a few months) in H2O.

Fig. 2shows a frequency decrease between 0.45 and 0.6 V, during the oxidation of radical structure. The lost of electrons is coun- terbalanced by the approach of PF₆ near the electrode. Indeed, the interaction between TEMPO⁺ and PF₆ is strong enough to be detected by EQCM that allows the visualization of electrolytic anions approach at the electrode interface during oxidation/

neutralization.

The frequency change versus time shows that the variations recorded by EQCM were as stable as the ones observed for the cur- rent. In order to quantitatively compare the results obtained by cyclic voltammetry and EQCM, the mpe, which represents the mass per exchanged electron during the oxidation process, was calculated according to Eq.(1):

mpe¼ ðDFreqSFÞ=Q ð1Þ

DFreq is the frequency change, S is the mass sensitivity (5 ng cm²Hz¹),Fis the Faraday constant andQis the charge in- volved during oxidation process.

With various supporting electrolytes (TBAClO4, TBABF4, NaPF6, NaClO4 or NaBF4), the electrochemical and electrogravimetric properties of TEMPO-monolayers were very similar to those reported above. The results are summarized inTable 1.

mpe values calculated from voltammetric (Q) and gravimetric (DFreq) measurements agree with molecular mass of each anion, showing that one anion ﬁlls the lost of one electron. This observa- tion coincides with previous results observed in aqueous medium for densely packed ferrocenylmonolayers. For densely packed nitroxyl radical self assembled monolayers, it is noteworthy that either the solvent or the electrolyte have no inﬂuence on the mpe/M ratio. This result indicates that the solvatation of TEMPO heads is effective in various media and that the coupling of TEM- PO⁺with electrolytic anions does not involve incorporation of solvent molecules in the monolayer.

In order to further investigate the monolayer|electrolyte interaction, and obtain information on the possible incorporation of solvent in the monolayer, we studied the inﬂuence of TEMPO surface coverage. Two different ways were explored to decrease the quan- tity of immobilized TEMPO derivatives. The ﬁrst one consisted in generating a densely packed monolayer and partially desorbe it by ultrasonication, to produce loosely packed monolayer. The sec- ond one involves the formation of mixed densely packed monolayer[24]. Alkanethiols of two different lengths were choosen to elaborate the mixed monolayers with 1: decanethiol (2) and octadecanethiol (3).

Fig. 3shows the mpe measured during the voltammetric study of the monolayers elaborated using the three different protocols (1 progressively desorbed by ultrasonication,1:2mixed monolayers and1:3mixed monolayers for different surface coverages).

For the hypothesis in which the lost of one electron is only com- pensated by the approach of a PF₆ anion, the expected mpevs.C curve is an horizontal line centered at a mpe value equal to the molecular weight of the anion (145 g mol¹). The behaviour of monolayers partially desorbed by ultrasonication (Fig. 2) clearly shows a shift from this behaviour. The calculated mpe for high coverages (1.5–2.010¹⁰mol cm²) agrees with PF₆ molecular mass. However, an increase of mpe is observed when the concentration of electroactive species on the surface is decreased, suggesting that, for lower TEMPO surface coverages (<10¹⁰mol cm²), incorporation of solvent occurs. Uosaki et al.

Fig. 1.Cyclic voltammograms (left) and intensityvs.time (right) recorded between 0.3 and 0.7 V for a TEMPO-monolayer with a surface coverage of 2.010¹⁰mol cm² immersed in 0.1 M TBAPF6in CH2Cl2under 21 repetitive cycles. Scan rate: 100 mV s¹.

80 C. Gautier et al. / Electrochemistry Communications 12 (2010) 79–82

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also observed a mpe more important than the expected one in aqueous electrolyte for loosely packed ferrocenyl monolayers (obtained from diluted thiol solution) but the mass increase, attrib- uted by the authors to the incorporation of electrolytic anions solvated by water in presumed cavities resulting from the lower density of immobilized thiols, was equivalent to 15 molecules of water. This mass increase is more pronouced than the one observed here for equivalent surface coverages. We attribute this dif- ference to the nature of the ‘‘electroactive groups|solvent” pair.

Indeed, TEMPO moities are soluble in the solvents used whereas the ferrocenyl entity is not soluble in H2O/HClO4medium. In this latter case, the insertion of water molecules in loosely packed hydrophobic monolayer (and possibly destructured) during the oxidation process could be necessary to ensure the electroactivity.

Concerning the SAMs elaborated using1:2mixed SAMs, the ten- dency of the curve mpevs.Cis quite similar to the one observed for sonicated ones. Starting from the hypothesis that the increase of the mpe is due to solvent incorporation in less packed monolayers, the ﬁlling of the unoccupied zones of the surface by2have almost no effect on the insersion of solvent molecules.

On the other hand, the1:3mixed SAMs seems to behave differ- ently. When the TEMPO surface coverage is lowered, the mpe value stays almost constant up to a 10¹⁰mol cm²surface coverage.

Following the same hypothesis as above, we can ﬁrst assume that the chain length of the octadecanethiol prevents the incorporation of solvent. In fact, the mixture of1and3leads to the formation of a monolayer having a more homogeneous thickness and consequently a more compact structure. As it appears in these studies that the chain length of the alkanethiol seems to have an inﬂuence on mpe, investigations on mixed monolayers with alkanethiols with different chain lengths are undertaken.

4. Conclusion

The electrogravimetric investigations carried out on TEMPO- monolayers allow the comparison between organic and aqueous media. It revealed that similar behaviours are observed in both media, for various supporting electrolytes. The lost of one electron is neutralised by the approach of an electrolytic anion without sup- plementary mass.

TEMPO-monolayers partially desorbed by ultrasonication revealed a different behaviour involving the approach of heavier spe- cie for lower TEMPO coverages (maybe due to the incorporation of solvent molecules).

The behaviour of TEMPO:Alkanethiol mixed monolayers is under investigation. Further work is aimed at comparing ion pairing process with the Laviron interaction model in order to establish de- tailed structure–reactivity relationships for interfacial reactions on electroactive SAMs.

Acknowledgments

This work was supported by the Centre National de la Recher- che Scientiﬁque (CNRS-France), the ‘‘Agence Nationale de la Recherche” (ANR-France), and the ‘‘Région des Pays de la Loire”

Fig. 2.Frequency responsevs.potential (left) andvs.time (right) recorded between 0.3 and 0.7 V (vs.Ag/AgNO3) for a TEMPO-monolayer with a surface coverage of 2.010¹⁰mol cm²immersed in 0.1 M TBAPF6in CH2Cl2under 21 repetitive cycles. Scan rate: 100 mV s¹.

Table 1

Mpe values obtained in organic and aqueous media.

Anion Medium M (g mol¹) mpe (g mol¹)^*

PF₆ CH2Cl2 145.0 146 ± 9

H2O 126 ± 10

ClO₄ CH2Cl2 99. 5 128 ± 8

H2O 72 ± 5

BF4 CH2Cl2 86.8 84 ± 5

H2O 78 ± 10

*Note that each reported mpe value was deduced from 21 repetitive cycles in order to minimize the standard deviation, regarding experimental frequency ﬂuctuations and baseline deviations.

Fig. 3.mpevs.surface coverage for TEMPO-monolayers desorbed by ultrasonication (s), mixed with decanethiol (h) or mixed with octadecanethiol (D). Measure- ments recorded in 0.1 M TBAPF6in CH2Cl2. Note that each reported mpe value was deduced from 21 repetitive cycles in order to minimize the standard deviation, regarding experimental frequency ﬂuctuations and baseline deviations.

C. Gautier et al. / Electrochemistry Communications 12 (2010) 79–82 81

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(France). The authors thank Flavy Alévêque for her critical reading of the manuscript.

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