Supporting Information Magnetic Field and Chirality Effects on Electrochemical Charge Transfer Rates: Spin Dependent Electrochemistry Prakash Chandra Mondal, 1 Claudio Fontanesi, 1,2 David H. Waldeck, 3 and Ron Naaman 1 * 1) Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel 2) Department of Chemical and Geological Science, University of Modena and Reggio Emilia, Via G. Campi 183, 41125 Modena, Italy 3) Department of Chemistry, Pittsburgh University, Pittsburgh PA., USA 1
The preparation of the metal substrate The samples were prepared on a silicon substrate <100> with 300 nm thick thermal oxide (SiO 2 ) and a resistance of >400Ω per cm 2, on which an 8 nm of titanium adhesive layer was deposited at a rate of 0.2 Å/s in an electron beam evaporator. On top of the titanium, 200 nm Ni (CERAC, 99.9995%) was deposited at a base pressure of 10-5 mbar and a deposition rate of 0.5-2 Å/s. A gold layer (5-30 nm) was deposited at a slow rate (0.2 Å/s) on top of the nickel without any adhesive layer. Characterization of monolayers The fabrication of the monolayers, covalent attachment of toluidine blue O (TBO) and the adsorption of cytochrome c (cyt c), was confirmed by surface characterization techniques such as static water contact angle (CA) measurements, polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS), ellipsometry, and cyclic voltammetry measurements. All measurements were performed at least three times on different samples to confirm their reproducibility. Polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS) Formation of the L-cysteine monolayers and a layer of cytochrome c linked with mixed SAM of COOH and CH 3 terminated layer was confirmed by PMIRRAS spectra. The spectra were recorded in PM-IRRAS mode using a Nicolet 6700 FTIR equipped with a PEM-90 photoelastic modulator (Hinds Instruments, Hillsboro, OR) at an incidence angle of 80º. The L-cysteine monolayer on 10 nm gold coated nickel substrates show strong peaks at 2920 and 2850 cm -1 that are related to the asymmetric and symmetric stretching vibrations of methylene (-CH 2 -), respectively (see Fig. S1a). The frequency observed at 1714 (-C=O) and 1644 (-N-H bending) cm -1 confirms the presence of carboxylic acid and amino groups (see Fig. S1b). 2
-CH 3 asym -CH 2 sym Intensity -CH 2 asym -CH 2 sym -CH 2 asym Covalent attachment of TBO to the cysteine molecule was also confirmed from the spectrum. For instance, the peak at 2960 cm -1 was assigned to the asymmetric stretching frequency of the methyl group (see Fig. S1c). The C-H stretching of the methylene group was shifted to 2923, and 2853 cm -1 in the Cys-TBO film. The formation of the amide bond (-CO-NH 2 ) was confirmed by the presence of a stretching frequency at 1605 cm -1 (see Fig. S1d). a) Ni/10 nmau/l-cysteine b) -COO - -NH 2 bending 3000 2950 2900 2850 2800 1800 1750 1700 1650 1600 1550 c) Ni/10 nmau/l-cysteine-tbo d) -CONH 2 3000 2950 2900 2850 2800 1800 1750 1700 1650 1600 1550 Wave number (cm -1 ) Figure S1. The PM-IRRAS spectra of SAM made from L-cysteine (a, b) and L-cysteine- TBO (c, d) on 10 nm Au coated Ni. Characteristics peaks are marked. The binding of cyt c on the mixed SAM was confirmed by IR spectra. The strong peaks at 2962, 2924, and 2853 cm -1 were assigned to the asymmetric C H stretching frequency of the methyl ( CH 3 ), methylene ( CH 2 -), and symmetric C H stretching of the CH 2 - group, respectively (see Fig. S2a). Further, the spectrum exhibited a stretching frequency at 1662 cm -1, which is assigned as the amide I band, while the peak at 1542 cm -1 is related to the 3
Intensity -CH 2 sym Amide II -CH 3 asym -COO - -CH 2 asym Amide I amide II band (see Fig. S2b). Note that these amide I and amide II signals are the characteristic bands of the polypeptide molecules. In addition, a strong stretching frequency at 1592 cm -1 confirms the presence of carbonyl (C=O) group on the surface. a) b) 3000 2950 2900 2850 2800 1750 1700 1650 1600 1550 1500 Wave number (cm -1 ) Figure S2. The PM-IRRAS spectra of cyt c linked with mixed SAM made from 11- mercaptoundecanoic acid and 1-octanethiol (a, b) on 10 nm Au coated Ni. Characteristics peaks are marked. Chiral recognition of L-cysteine film The chiral recognition of the L-cysteine molecular film was probed by cyclic voltammetry. To confirm the chiral behavior of the film, CVs were recorded with R, and S N,N-dimethyl- 1-ferrocenylethylamine. Single electron oxidation and reduction take place at +0.4, and 0.26 V (vs SCE) respectively, when R-ferrocene interacts with cysteine film. On the other hand, the oxidation and reduction process shows an anodic and cathodic shift by ~25 mv when S- ferrocene interacts with the same film (see Fig. S3). These observations indicate the enantioselectivity of the interaction between the chiral ferrocene and the L-cysteine film. 4
I / ma 0.18 0.12 R Fc S Fc 0.06 0.00-0.06-0.12 0.0 0.1 0.2 0.3 0.4 0.5 E / V vs SCE Figure S3. Chiral recognition of L-cysteine monolayer on 10 nm Au coated Ni using R and S N,N-dimethyl-1-ferrocenylethylamine (black and red lines respectively). The voltammograms were recorded using 0.8 mm ferrocene in 10 mm PBS at ph 7 in DI water. Voltammograms were recorded at 50 mv s -1. AFM measurements Atomic force microscopy (AFM) images were recorded using Multimode/Nanoscope (Bruker-Nano, Santa Barbara, USA). Images were acquired in non-contact mode and a Si probe was used having resonance frequency 70-90 khz. The topography images were measured at a scan rate of 1 Hz. Several images of the each sample were taken in different fields of view (0.5-2.0 μm) to confirm uniformity and reproducibility of the samples. The RMS roughness was measured using Nanoscope Analysis software. 20 nm and 30 nm Au overlayers show smooth film, while the thinner overlayers (5 nm and 10 nm) are not continuous film. 5
(c) (a) (b) (c) (d) 6
RMS roughness / nm (e) Figure S4. AFM topography images of (a) bare Ni (200 nm thick), (b) 5 nm, (c) 10 nm, (d) 20 nm and (e) 30 nm Au coated layers on Ni surface. Scan area was 1 µm 1 µm. 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 5 10 15 20 25 30 Au layer on Ni / nm Figure S5. Variation of RMS roughness (R q ) as a function of the thickness of Au layer adsorbed on Nickel surface. Table S1: RMS and average roughness at four different places of each sample. 7
Intensity -CH 2 sym -CH 2 asym Sample RMS Roughness (nm) Final R q Region-I Region-II Region-III Region-IV Ni 0.924 0.905 0.895 0.928 0.91 ± 0.02 Ni-Au (5 nm) 2.92 2.70 3.10 3.28 3 ± 0.24 Ni-Au (10 nm) 2.62 3.32 1.99 1.79 2.43 ± 0.69 Ni-Au (20 nm) 0.922 0.898 0.867-0.9 ± 0.03 Ni-Au (30 nm) 0.945 0.861 0.943 0.891 0.91 ± 0.04 Contact Angle (CA) measurements Static contact angle measurements on the cysteine monolayers were performed with a goniometer (Rame-Hart) and micro syringe droplets with ca. 4 μl deionized water (Millipore, Inc.). The measurements were performed immediately after the formation of the cysteine monolayer. The contact angle measured was 20 ± 2⁰, which confirms the formation of a hydrophilic layer. PMIRRAS of 6-ferrocenyl hexanethiol monolayer on Au/Ni The formation of monolayer 6-ferrocenyl hexanethiol on 10 nm Au coated Ni substrate was confirmed by PMIRRAS spectra. The stretching frequency at 2928 and 2854 cm -1 are related to the asymmetric C H stretching methylene ( CH 2 -) and symmetric C H stretching of CH 2 - group, respectively (see Fig. S6). 6-ferrocenyl hexanethiol on 10 nm Au/Ni 3000 2950 2900 2850 2800 Wave number (cm -1 ) Figure S6. The PM-IRRAS spectra of 6-ferrocenyl hexanethiol on 10 nm Au coated Ni. Characteristics peaks are marked. 8
I/mA Spin dependent electrochemistry of 6-ferrocenyl hexanethiol monolayer The immobilization of redox active achiral 6-ferrocenyl hexanethiol on 10 nm Au/Ni was confirmed by cyclic voltammetry measurements. For instance, the oxidation and reduction peaks were observed at +0.5 and +0.43 V (vs SCE), respectively. The voltammograms did not show a significant magnetic field effect when 6-ferrocenyl hexanethiol modified nickel working electrode is magnetized either with its magnetic moment pointing UP, or DOWN (see Fig. S7). The SP for the data in Figure S7 is smaller than 1%. 0.04 Mag UP Mag DN 0.02 0.00-0.02 0.3 0.4 0.5 0.6 E/V vs SCE Figure S7. Voltammograms are recorded on 6-(Ferrocenyl)hexanethiol monolayer on 10 nm Au coated Ni electrode. The voltammograms were obtained when the nickel is magnetized either with its magnetic moment pointing UP, towards the monolayer (solid black curve), or DOWN, away from the monolayer (solid red curve). The voltammograms were recorded in a 10 mm tetrabutylammonium tetrafluoroborate (TBATFB) in dry acetonitrile at a scan rate 50 mv s -1. 9
-COO - Intensity Amide II -CH 2 sym -CH 3 asym -CH 2 asym Amide I PMIRRAS spectra after denaturation of immobilized cyt c The immobilized cyt c on mixed SAM was denatured by applying a voltage of -1.0 V. We strongly believe that this negative voltage is enough to denature the protein and it is further confirmed by cyclic voltammetry measurements by disappearance of redox signals. PMIRRAS data indicate that the cytochrome c is still adsorbed after the negative voltage was applied. The peaks at 2960, 2927 and 2854 cm -1 are assigned to the asymmetric C H stretching frequency of the methyl ( CH 3 ), methylene ( CH 2 -) and symmetric C H stretching of CH 2 - group, respectively (Fig. S8a). In addition, the IR spectrum exhibits a frequency at 1664 cm -1, which is related to the amide I band, while the peak at 1542 cm -1 is related to the amide II band (see Fig. S8b). The denaturation of the immobilized protein was further proved by reducing of the frequency (as compared to its native form) at 1588 cm -1 assigned as carbonyl (C=O) group. 1 The applied voltage causes protein conformation change. Note that the presence of amide I and amide II bands confirm that there is no desorption or decomposition of the protein, since these are the characteristic bands of the polypeptide molecules. a) b) 3000 2950 2900 2850 2800 1750 1700 1650 1600 1550 1500 Wave number (cm -1 ) 10
Absorbance Figure S8. The PM-IRRAS spectra recorded after applying -1 V on the adsorbed cytochrome c linked with mixed SAM made from 11-mercaptoundecanoic acid and 1-octanethiol (a, b) on 10 nm Au coated Ni. Characteristics peaks are marked. UV-Vis study of cytochrome c in solution UV-Vis spectrum of commercial available cytochrome c indicates that the iron in the protein is in oxidized form (Fe 3+ ), which can be confirmed by the appearance of two peaks at 528 nm and 409 nm (Fig. S9). The oxidized protein can be reduced chemically by potassium thiocyanate (KSCN) in 50 mm PBS. In this case, the UV-Vis spectrum shows three peaks at 550 nm, 521nm, and 415 nm (Fig. S9). 0.8 415 nm 0.6 409 nm 0.4 0.2 528 nm 521 nm 550 nm 0.0 400 500 600 700 800 Wavelength (nm) Figure S9. UV-Vis spectra of cytochrome c in 50 mm PBS of ph 7 at room temperature. Solid red line indicates two peaks of its oxidized form (Fe 3+ ) at 528 nm and 409 nm, while solid blue line indicates absorbance peaks of its reduced form (Fe 2+ ) at 550 nm, 521 and 415 nm. Current vs scan rate from cyclic voltammetry measurements In the case of immobilized cytochrome c, the linear dependence (R 2 > 0.98) of the faradaic current, both anodic and cathodic (I pa, I pc ), on the scan rate clearly demonstrates that the 11
I/ A electrochemical process is diffusionless (see Fig. S10a). This result confirms the attachment of redox active protein, cytochrome c, on the functionalized electrode. In addition, it was observed that the ratio of the anodic current to the cathodic current, as a function of scan rate, is almost unity, which further illustrates a reversible redox process, as shown in Fig. S10b. The peak potential of the faradic process shifts with changing the scan rate (Fig. S11). 30 a) I pa 2 b) 20 10 0 I pa /I pc -10 0-20 I pc -30 0.1 0.2 0.3 0.4 0.5 (V/s) 0.0 0.1 0.2 0.3 0.4 0.5 (V/s) Figure S10. The peak currents (I pa, I pc ) as a function of scan rates (a), and the ratio of anodic to cathodic currents as a function of the scan rates (b). 0.04 0.02 E p -E0 0.00-0.02-0.04 1.6 1.8 2.0 2.2 2.4 2.6 2.8 log( ) Figure S11. The shift of anodic and cathodic peak positions as a function of the log of the scan rate ( ). 12
References 1 S1. Gallant, J.; Desbat, B.; Vaknin, D.; Salesse, C. Biophysical Journal 1998, 75, 2888 2899. 13