S1
Supporting Informations Electrochemical-Assisted Encapsulation of Catechol on Multiwalled Carbon Nanotube Modified Electrode Annamalai Senthil Kumar* and Puchakayla Swetha Environmental and Analytical Chemistry Division, Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology University, Vellore 632 014, India *Corresponding author’s e-mail:
[email protected] & phone: +91-416-2202754
ES1. Experimental Section Catechol, Dopamine, Resorcinol and Hydrazine were purchased from SD fine chemicals, India and multiwalled carbon nanotube (>90% of carbon basis) from Aldrich. Other chemicals used in this work were all of ACS-certified reagent grade and used without further purification. Aqueous solutions were prepared using deionized and alkaline KMnO4 distilled water (designated as DD water). Unless otherwise stated, pH 7 phosphate buffer solution (PBS) of ionic strength, I = 0.1 M was used as supporting electrolyte in this work. Voltammetric measurements were all carried out with CHI Model 660C electrochemical workstation (USA). The three-electrode system consists of gold (0.0201 cm2) and glassy carbon (0.0707 cm2) or its chemically modified electrodes as working electrode, Ag/AgCl as reference electrode and platinum wire as the auxiliary electrode. TEM was carried out by using microscopy JEOL-3010, SEM by OXFORD, XRD by Bruker and power FTIR using Nicolet Avatar 330 and films samples using FTIR/Attenuated Total Reflectance (ATR using Thermo Electron Corporation, USA) instruments. Functionalized MWNT (designated as CNT*, * = functionalized/activated) was prepared using 13 N HNO3 as per the reported procedure [S1]. CNT or CNT* coated Au
S2 electrode was prepared by the following procedure: 3 L of respective carbon nanotube dispersed in ethanol (4 mg/mL) was drop coated on cleaned Au electrode and dried in air for 20 minutes in room temperature (Au/CNT or Au/CNT*). Electrochemical-assisted CA encapsulation was carried out by potential cycling treatment of the Au/CNT or Au/CNT* electrode with 1 mM of catechol in pH 7 PBS at a scan rate of 50 mV/s (n = 20). Repeated experiment on glassy carbon underlying electrode shows nearly similar in the CV current density patterns, which further indicates absence any chemical influence from the underlying surface in the electrochemical behavior. Conventional CA immobilized bulk GCE surface (GCE*-CAads) was prepared using 0.1 M KNO3 pre-anodization procedure as per our previous report [S2]. Determination of surface coverage, CA (mol.cm-2) of catechol was performed by integrating the anodic peak area (Qa) of cyclic voltammograms measured from a 10th cycle at v = 50 mV/s, and taking CA = Qa/nFA, where n = 2 and A is the geometrical surface area. Concern about the stability of the CNT on the electrode surface, we expect there might be existence of micro and nano-porous voids on the Au or GCE surface (after the mechanical and electrochemical pretreatments) [S3], which when CNT was modified as ethanol dispersion on the surface, some of the portions of CNT’s nano-walls may occupy the porous site of the underlying surface and there by stabilization of the CNT matrix. TEM samples were prepared as similar to the Au/CA@CNT by electrochemical treatment method, but with ITO substrate and the electrode was washed with distilled water, dried and the CA@CNT film was carefully taken out using doctor’s needle, then subjected to analysis. Due to practical difficulty, XRD’s CA@CNT sample was prepared by simulated chemical method, in which 50 mg of CNT power was mixed with 50 mg of CA in 10 ml of
S3 distilled water, stirred for overnight in room temperature under normal dissolved oxygen condition, filtered, washed with copious amount of water several timing and dried in desiccator before the experiment (i.e., method-2). There will be some aerobic oxidation of CA during the long process [S4] and CA will be attached inside and outside both by chemisorptions and physisorption steps. But during the continuous distilled water washing procedure, the physisorbed CA species will be completely washed out from the matrix. Powder FTIR of the chemically prepared CA@CNT and CNT by KBR pellet method didn’t show any marked variation in FTIR patterns (data not enclosed) as like the SPCE/CA@CNT and CNT (Figure S2). This observation also indicates absence of any CA species outside the CNT surface with the chemically prepared CA@CNT. Note that the variation in the XRD patterns are more obvious for the CA encapsulated CNT over the controls (Figure 3). We have done a control experiment; in aim to prepare Prussian Blue (PB, Eo’ ~ 0.2 and 0.8 V vs Ag/AgCl [S5]) like material on the Au/CNT by utilizing the encapsulated residual impurity (may be Fe), if any in marked amount [S6]. For this the Au/CNT electrode was immersed in 10 mL of 1 mM Fe(CN)63- containing 0.1 M KCl solution and subsequently CV was performed in the potential window of
-0.4 – 1.0 V vs Ag/AgCl (no. of cycles = 10).
After this experiment, the above electrode was moved to blank 0.1 M KCl and CV was again performed. There is no sign for the PB film formation on the Au/CNT (Figure S5). CVs of the Au/CNT before and Fe(CN)63- treatment are same. Possibly very low concentration of the encapsulated metal with the CNT (Figure 2B), might be the possible reason for the unchanged Au/CNT behavior. This observation eliminates involvement of the CNT’s residual impurity (metal trace) for any derivatization reaction in this work.
S4
CA@CNT
Residual impurities
Figure S1. SEM images of encapsulated catechol (CA) inside the CNT (CA@CNT). Residual impurities were from amorphous carbon, metal, metal salt (from the electrolyte) etc. Note that residual salt and amorphous carbon wouldn’t give any specific chemisorbed peak with the CV (Figure 1).
S5
A. SPCE/CNT
Reflectance(%)
*
*
*
*
*
* *
B. SPCE/CA@CNT
* *
*
* *
Au/CA@CNT
*
* CA
4000
3500
*
*
*
*
FTIR/ATR
3000
* *
2500
2000
1500
1000
-1
Wavelength(cm )
Figure S2. FTIR/ATR (Thermo Electron Corporation, USA) responses of the screen-printed carbon electrode (SPCE) modified (a) CNT and (b) CA@CNT films. * marks indicate the qualitative similarities in the two patterns. Inset cartoon indicates the FTIR/ATR response with the working sample.
S6
B. CNT+CA combined powder modified electrode
A. Au/CNT immersed in CA
CA =15.0910-10 mol.cm-2 A1 A1' 100
i (A)
A2
CA =11.610
-10
A2 -2
mol.cm
i (A) 400
50
A1 Au/CNT
0 -0.4
0.0
0.4
-50
C1'
0
-0.4
0.0
0.4
Au/CA@CNT (method-1) (n=20)
-100
E (V)
C2
0.8
E (V)
Au/CA@CNT (method-2)
C1 -400
(n = 10)
C2
C1
Figure S3. Continuous CV responses of Au/CA@CNTs prepared by (A) over-night immersion of Au/CNT in 1mM CA solution at open circuit potential (method-1) and (B) CNT+CA combined powder (1:1 ratio in 10 mL distilled water H2O, 50 mg each) was stirred for overnight, filtered, washed, dried (desiccator), combined with ethanol (4 mg/mL) and coated on a cleaned Au (method-2), in pH 7 PBS at scan rate = 50 mV/s. CA values were calculated from the charge value of the respective peak (10th cycle) uniformly. A1’/C1’ are unknown pre-peaks.
S7
80 c. Au/CA@CNT + Hydrazine 60 Au/CA@CNT o-quinone at ~0 V
40
Current (A)
Hydrazine
Catechol
oxidized form
20
b. Au/CNT + Hydrazine
0 a.Au/CA@CNT -20 v = 10 mV/s [Hydrazine] = 1mM -40 -0.4
-0.2
0.0
0.2
0.4
0.6
Potential (V vs Ag/AgCl) Figure S4. Electrocatalytic response of Au/CA@CNT (electrochemically encapsulated) with 1 mM of Hydrazine in comparison with other control systems in pH 7 PBS at scan rate =10 V/s. Inset figure is the possible mediated hydrazine oxidation mechanism.
S8
a. GCE/CNT with 1mM Fe(CN)63-
Current (A)
40
b. GCE/CNT after a 0 Fe(CN)63- = 1mM Electrolyte = 0.1 M KCl Scan rate = 50 mV/s no. of cyles = 10
-40
-0.4
0.0
0.4
0.8
Potential (V vs Ag/AgCl)
Figure S5. (a) CV response of GCE/CNT with 1 mM Fe(CN)63- in 0.1 M KCl solution at a scan rate of 50 mV/s and its (b) consequent CV in blank based electrolyte solution. Note: There is no indication of new Prussian Blue (PB, -Fe-Fe(CN)6-) like hybrid film formation [S6].
S9 Collection of evidences for the correct position of Catechol on MWNT: (A) Concern about the absence of CA outside the MWNT: (a) As per our previous study, CA found to covalently immobilized (chemisorbed) on the functionalized bulk carbon surface [S2], possibly through Michael’s addition like reaction, with involving C4 of the CA ring [S7]. We experimentally proved that such covalent addition reaction was selective towards CA only, other CA derivatives like Dopamine (DA, pKa = 8.7) failed to attach on the carbon surface, due to its ionic interaction with the carbanion, rather than the addition reaction. Catechol (CA): 3
OH
2
4
Dopamine (DA) in pH 7 PBS:
1
5 6
OH
+
H3N
3 2
4
OH
1
5 6
OH
But in the present work with the Au/CNT, there is no such selective observation was noticed. Both the CA and DA get immobilized @CNT non-selectively. This observation indicates Michael’s addition like reaction might not be occurred in this work, even on the functionalized CNT (i.e., CNT*). These experimental observations support absence of CA (by chemisorption) on the outer walls of the CNT. (b) Note that there is no marked alteration in the CA before and after functionalization of the CNT. This observation indicates, there is no influence of the CNT’s surface functional group in this work, unlike to the bulk carbon surface [S2]. This observation also partially supports absence of chemisorbed CA on the surface of the CNT. (c) XRD response of the CA shows multiple peak response due to the polycrystalline nature of the naked material (Figure 3A). A characteristic CA’s XRD peak at 2 =12.15o (1.74o positive shift with respect to the CA control), apart from the regular CNT peaks, with simulated CA@CNT powder sample (Experimental Section S1) indicates the encapsulated CA preferentially oriented with particular plan inside the CNT. Please note that if any CA physisorption on the CNT surface, which might have
S10 been resulted in to multiple XRD peak response.
No such observation with the
CA@CNT, further supports absence of any physisorbed CA on the outside of the CNT. (d) We have done FTIR/ATR of the CA@CNT and CNT on the screen-printed carbon electrode (SPCE) surface (due to practical convenience). Both unmodified and CA modified CNT systems show qualitative similar FTIR patterns in the finger print region (Figure S2), unlike to the marked formation in the CA’s aromatic peaks (3919 and ~2900 cm-1) with the bulk GCE*-CAads in our previous study [S2]. Unalteration in the FTIR/ATR response may be due to absence of CA (both chemisorbed and physisorbed) outside the CNT. On the other hand, the observation supports, CA may be encapsulated within the few “nm” thick CNT bundles, which can’t be accessed by the IR radiations. (e) Please note that chemisorbed CA (covalently attached CA and its derivatives) on the bulk carbon surfaces were often reported to be having instability problems during the redox cycling process [S2 and references therein]. Unlike to the classical cases, in the present case, there is no such marked alteration in the peak potential and peak current responses of the Au/CA@CNT (Figure 1D). This observation also indirectly evidences that absence of the chemisorbed CA outside CNT in this work. (f). Concern about possibility of encapsulation of bulky compounds inside the CNT, some of the bigger catechol molecules; trans,3,3’,4’,5,7-penta hydroxyl flavane (catechin hydrate) and chloro-genic acid were already tested on the MWNT modified preanodized GCE surface. Catechin:
Chloro-genic acid: HO
OH
COOH
O O
HO
OH
OH
O OH OH OH
HO OH
These modified electrodes show single redox peak response centered at ~0.2 V vs Ag/AgCl in pH 7 solution [S8]. As per our previous work, the chemisorbed
S11 catechol/catechol derivatives (dopamine) peak responses were noticed at ~0 V vs Ag/AgCl, while physisorbed response at ~0.2 V vs Ag/AgCl [S2 & in this work]. Correlation of the data’s with the Ref. [S8]’s observation implied that the bigger catechol derivatives; catechin and chloro-genic acid’s modification follows through different adsorption mechanism, preferentially by physisorption pathways on the outer surface of the CNT, which may differ from the present study.
(B) Concern about the encapsulation of CA inside the MWNT: (a). Transmission electron microcopy (TEM) experiment clearly shows marked amount of block spots inside the MWNT, as encapsulated nanoaggregate system. (b). Change in the diameter size of the CA@CNT could be apparently seen as bulky walls in the TEM pictures (Figure 2A). (c). In further scanning electron-microscopy (SEM) analysis also partially supports the size effect (Figure S1). These are direct proofs for the encapsulation of the CA inside the MWNT.
References: (S1).
Phuong, D.T; Nguyen,V.H ; Nguyen, D.C.; Anh-Tuan, L.; Mai Anh,T.; J. Immunol. Methods 2009, 350, 118.
(S2).
Kumar,A.S.; Sornambikai, S.; Gayathri, P.; Zen, J.-M. J.Electroanal. Chem. 2010, 641, 131.
(S3).
Finot, M.O; Braybrook, G.D.; McDermott, M.T. J. Electroanal. Chem. 1999, 466, 234.
(S4).
Cilento,G.; Cilento, Z.; Biochimica et Biophysica Acta (BBA) - Biophysics including Photosynthesis, 1996, 120, 84-90.
(S5)
Kumar, A.S.; Chen,P.-C.; Zen,J.-M. Electroanalysis 2004, 16, 242.
(S6).
Nossol, E., Zarbin, A.J.G. Adv. Funct. Mater. 2009, 19, 3980.
(S7).
Raj. C.R.; Behera, S. Langmuir 2007, 23, 1600.
(S8)
Salimi,A.; Miranzadeh, L.; Hallaj, R. Talanta 2008, 72, 147-156.
