Copper Oxide Assisted Cysteine Hierarchical Structures for Immunosensor Application
Chandra Mouli Pandey, Gajjala Sumana, Ida Tiwari
Supplementary Information
EXPERIMENTAL SECTION
Chemicals. Copper Nitrate (Cu(NO3)2·3H2O), Sodium hydroxide (NaOH), N,N-, Dimethyl
acetamide (DMAC), L-Cysteine, N-hydroxy succinimide (NHS), N-ethyl-N-(3-dimethyl
amino propyl carbodimide (EDC), Bovine serum albumin (BSA) and all other reagents and
solvents are of analytical grade and have been procured from Sigma-Aldrich (India).
Monoclonal antibodies and the target cells of E. coli (O157:H7) were procured from
Kirkegaard & Perry Laboratories, Gaithersburg MD, USA. Phosphate buffered saline (PBS)
was prepared by mixing PB solutions with 0.9% NaCl. Ultrapure water (Mill-Q, Millipore,
18.2 MΩ resistivity) was used throughout the experiments.
Synthesis of copper oxide (CuO) nanoparticles. CuO nanoparticles were synthesised by
the reported procedure with slight modifications.1,
2
Briefly in a round-bottom flask
equipped with a refluxing device, 18 mmol Cu(NO3)2·3H2O was dissolved in 120 ml of
DMAC at 100 oC with vigorous stirring. Further, 60 ml NaOH (12mmol) solution in
DMAC–water mixed solvent was rapidly added to the Cu(NO3)2 solution, until there is
formation of a large amount of black precipitate. The solution was maintained at the
crystallization temperature for 30 min and again heated at 120 oC (20 min). The formed
black precipitate was then cooled to room temperature and the obtained products were
centrifuged at 15000 rpm for 5 min in anhydrous ethanol to remove the impurities, and
dried in air at room temperature (Fig. 1a).
Preparation of Cu(II) assisted three-dimensional cysteine flowers. For the preparation
of copper oxide modified three-dimensional cysteine nanoflowers (CuCys), firstly, 100 mM
aqueous solution of L- Cysteine (pH 8.0) was prepared using the reported method.3 Further
1.1 mg of the prepared CuO nanoparticles was added, and the solution was sonicated using
ultrasonic bath for about 5 minutes at 25 oC (Fig. 1b). The growth of CuCys was observed
by keeping the solution at 25 oC for 6 h. It was revealed that the optimum time for the
formation of CuCys is less than1 h.
Pre-treatment and fabrication of CuCys modified gold electrode. The gold (Au) electrode
(0.5 cm2 diameter) was washed in boiling 2.0 M KOH for about 1 h, following ultrasonication in Piranha solution (3:1 H2SO4/H2O2) for 10 min and the electrode was
subsequently washed in water for 10 min. The electrode was voltammetrically cycled and
characterized in 0.2 M H2SO4 from -0.5 V to -1.4 V (vs.Ag/AgCl) with scan rate of 0.10 V/s
until a stable cyclic voltammogram is obtained. To fabricate the CuCys monolayer films, the
Au electrode was dipped into the CuCys solution overnight (6 h) at 27oC after which, the
modified electrode (CuCys/Au electrode) was rinsed repeatedly with deionized water.
Preparation of antibody/antigen solution. E. coli (O157:H7) specific monoclonal antibody
solution was prepared in PBS buffer; pH 7.4 containing 0.1% sodium azide. The bacterial
cells were stored in 50% glycerol solution at -20 oC and the serial dilutions (101 to 1 x 109 cfu
ml-1) were prepared in PBS buffer prior to use.
Immunosensor Fabrication. The fabricated CuCys/Au electrode was activated by
immersion in a mixture of 2 mM EDC and 5 mM NHS for 1 h at 27 oC, in dark condition.
Subsequently, the modified electrode was incubated in 20 µl of a 0.1 mg/ml antibody solution
in a humid chamber overnight at room temperature. Finally, the electrodes were rinsed with
PBS solution to wash away the excess and unbound antibodies. The antibodies immobilized
electrodes were finally treated with 1% bovine serum albumin (BSA) to block the unspecific
binding sites followed by PBS washing (Fig. 1c).
Characterization. The structural and morphological investigations of CuCys have been
carried out using X-ray diffractometric (XRD, Cu Kα radiation, Rigaku, miniflax 2) and
transmission electron microscopic (TEM, Hitachi Model, H-800) studies. The scanning
electron microscopic (SEM) images have been recorded using a JEOLJSM-6700F fieldemitting scanning electron microscope (FESEM, 15 kV). Fourier transform infra-red (FT-IR)
spectroscopic measurements have been carried out using Perkin-Elmer spectrometer (Model
Spectrum BX) at 25oC. Electrochemical analysis has been conducted on an Autolab
potentiostat/galvanostat (Eco Chemie, Netherlands) using three-electrode cell with Au as
working electrode, platinum as auxiliary electrode and Ag/AgCl as reference electrode in
phosphate buffer (PBS, 100 mM, pH 7.0, 0.9% NaCl) containing 5 mM [Fe(CN)6]3-/4-. The
impedance was performed with 10 mV sinusoidal modulation amplitude at an applied bias
potential of +0.23 V from 0.1–105 Hz frequency range at 12 steps per decade.
Figure S1. Schematic illustration for the preparation (a) CuO nanoparticle (b) CuCys and (c)
immunosensor fabrication.
Figure S2.1H NMR spectra of (i) CuCys and (ii) cysteine in 90% 2H2O/10%H2O solutions at
300 K under Ar atmosphere.
Figure S3. TEM image showing the effect of AuNP on the growth of Cys at (a) 10 mM
cysteine concentration and (b) 100 mM concentration. SEM image showing the
agglomeration of AuNPs on Cys at (c) 10 mM concentration and (d) 100 mM concentration.
Figure S4. EIS plot showing the change in Rct for (i) Cys/Au electrode and (ii) CuCys/Au
electrode and (iii) bare Au electrode.
Figure S5. Cyclic voltammogram analysis of (i) bare Au electrode (ii) CuCys/Au electrode
(iii) Ab/CuCys/Au electrode (iv) free site blocking of Ab/CuCys/Au electrode with BSA and
(v) E. coli cells/Ab/CuCys/Au electrode in phosphate buffer (PBS, 100 mM, pH 7.0, 0.9%
NaCl) containing 5 mM [Fe(CN)6]3-/4-.
Figure S6. Electrochemical impedance spectroscopy showing (a) influence of antibody
concentration and (b) influence of incubation time of E. coli cells on the variation of the
Δ1Rct.
Figure S7. Bar diagram showing the interference of non-E.coli bacterial pathogens on the
immunosensor.
Figure S8. (a) Studies on the stability of the immunosensor indicaing the % change in Rct of
the fabricated CuCys based imunosensor with storage time (in days). (b) Change in ΔRct of
the fabricated CuCys based imunosensor towards E. coli cells detection after each
regeneration cycle (6 cycles).
Table ST1. Infrared band assignments of L-cysteine and CuCys.
Table ST2. The fitting values of the equivalent circuit elements and the electron–transfer
kinetics for the fabricated CuCys/Au electrode and Ab/CuCys/Au electrode.
Table ST3. Comparison of analytical results obtained from the EIS system and plate count
method.
Table ST4. Comparison table showing the performance of the fabricated immunosensor with
other electrochemical based immunosensor reported in literature for E. coli detection.
Figure S1.
Figure S2.
(i)
(ii)
Figure S3.
(a)
(b)
1 μm
(c)
1 μm
Figure S4.
1 μm
(d)
1 μm
Figure S5.
-4
4.0x10
(i)
-4
(ii)
3.0x10
(iii)
-4
Current (A)
2.0x10
(iv)
(v)
-4
1.0x10
0.0
-4
-1.0x10
-4
-2.0x10
-4
-3.0x10
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
Potential (V)
Figure S5 shows the cyclic voltammograms for the [Fe(CN)6]3−/4− redox probe at a bare Au,
CuCys/Au, Ab/CuCys/Au, blocking of Ab/CuCys/Au with BSA, and Eccell/Ab/CuCys/Au
electrodes. After the modification of the bare gold electrode with self assembly of CuCys,
there is a decrease in the peak current and an increase in the separation of the peak potentials
(curve ii) compared to the voltammetric behaviour of the bare Au. On the immobilization of
antibodies on CuCys/Au electrode, further decrease in the peak current (curve iii) was
observed. Additional decrease in the peak current was observed on blocking the unoccupied
sites with BSA (curve iv). Further, binding of Eccell to the immobilized antibodies (curve v)
produced a remarkable decrease in the peak currents. These results are consistent with the
changes observed in the electron-transfer resistance by EIS.
Figure S6.
The immobilization of optimum concentration of antibody on CuCys/Au electrode using
EDC-NHS chemistry was determined on the basis of the change in Δ1Rct [Δ1Rct=Rct(Ab)Rct(CuCysNf/Au)] resulting from the binding of antibodies to CuCys/Au electrode (Fig. S6 a) it
was observed that the Δ1Rct increased rapidly from 10µg/ml to 80µg/ml, and then reached a
plateau at antibody concentrations higher than 80µg/ml and gets saturated at this
concentration. Consequently, the optimum antibody concentration of 80µg/ml was used to
construct the immunosensor. Further the incubation time of the Ab/CuCys/Au electrode with
the E. coli cells were also considered. This was tested by varying the incubation time from 5
min to 30 min. Figure S6 (b) shows a rapid increase of relative Rct from 10 min to 25 min of
incubation. However, only a slight increase in impedance with incubation time longer than 25
min has been noticed. This is attributed to the maximum binding of the E. coli cells and the
antibody. Therefore, 25 min was used as optimized incubation time between E. coli cells and
the antibodies in subsequent experiments.
Figure S7.
200
180
160
140
Salmonelle typhi
Shigella dysenteriae
Vibrio cholera
E. coli
 Rct
120
100
80
60
40
20
0
3
6
9
-1
Log concentration of bacterial cells (cfu ml )
The selectivity of the immunosensors against three other bacteria (S. aureus and S.
choleraesuis) was evaluated by checking the impedimetric responses at the same
concentration level that of E. coli (1×103, 1.0×106, and 1×109 cfu/ml) under the experimental
conditions selected above (Fig. S7). Initially, we fabricated a series of electrodes (n=6) using
E. coli antibodies and exposed them to suspensions of increasing concentration of Salmonella
typhi, Shigella dysenteriae, and Vibrio cholera. When the bioelectrode was incubated with
other bacterial cells, there was negligible change in Rct, indicating that there was no
significant cross-reaction of the immunosensor with other bacterial species. These results
demonstrated that the electron-transfer resistance as recorded reflected the interaction
between the antibody and the target E. coli cells, therefore showing the specificity of the
immunosensor for E. coli. The comparison of the biosensing parameters such as linear range
and detection limit of the present work with the recent reports is shown in Table ST4.
Figure S8.
(a)
(b)
In addition, the storage stability of the immunosensor was tested by putting the Ab/CuCys/Au
electrode in a refrigerator (4 oC) and measuring its response towards E. coli O157:H7
detection every five days (Fig. S8 (a)). After one month, the impedimetric response of the
sensor remains 88 % of the initial value indicating that the immunosensor had acceptable
reliability and stability which may be due to the biocompatibility of CuCys, which maintain
the bioactivity of antibodies and other biomolecules. Further the regeneration step was
performed by immersion of the working bio-electrode for 10 min in glycine-HCl buffer (0.1
M, pH = 2.2) and washed with PBS solution to interrupt the antigen−antibody immune
complexes. Figure S8 (b) shows that there is an gradual decrease in ΔRct with the increase of
regeneration times which decreased obviously after regenerating the imunosensor for 6 times.
This may be due to the gradually shell off and denaturation of Eccells/antibody or the structure
of CuCys could be destroyed during continuous processed by a glycine-HCl buffer and
cleaning with the increase of regeneration times. The results demonstrated that the proposed
immunosensor could be regenerated and used for at least 6 times with relative standard
deviation of 8.79%.
Table ST1.
L-cys
CuCyF
3420
3179
2982
2889
2552
2079
1587
1429
1346
1297
1196
1140
1064
942
867
822
753
692
638
538
Assignments
-OH stretch
-NH stretch
-CH stretch
-CH stretch
-SH stretch
-CH stretch
-NH3+ deformation
-CO stretch
-OH deformation
-OH stretch
-CN stretch
-CO stretch
-CO deformation
-OH deformation
-CH deformation
-CH stretch
-CH2 stretch
-CS stretch
-CS stretch, -COO- stretch
-COO- stretch
3030
2094
1586
1408
1338
1297
1194
1030
847
675
540
Table ST2.
Sl.no Name of the
electrode
Solution
resistance
(Rs, Ω)
Charge
transfer
resistance
(Rct, Ω)
Capacitance
(Cdl, µF)
1
2
30.7
28.7
214.4
304.4
3.71
2.77
CuCyNf /Au
Ab/CuCyNf /Au
Apparent
electron
transfer
rate
constant
(10-4 cm s-1)
3.38
2.63
Exchange
current per
unit area
(10-4A/cm2)
1.19
8.43
Table ST3.
Samples
Immunosensor
(cfu/ml)
1
2
3
4
5
2.0x102
8.4x102
6.0x103
6.6x104
3.4x106
Plate count
method
(cfu/ml)
2.1x102
8.1x102
6.2x103
6.4x104
3.5x106
Relative error
(%)
4.76
3.70
3.22
3.12
2.87
Table ST4.
Immobilization matrices
Detection Range
References
3.2x101 to 3.2x106 cfu/ml
Limit Of
detection
1.5x102
cfu/ml
15 cfu/ml
Gold nanoparticle modified graphene
paper
Au nanoparticles coated SiO2
assembled on the fullerene , ferrocene
and thiolated chitosan composite
1.5x102–1.5x107
self-assembled monolayers (SAMs)modified gold screen-printed
electrodes
Inter-digitated array microelectrode
5 -1.0×108 cfu/ml
3.3 cfu/ml
6
4.36x105 - 4.36x108 cfu/ml
106 cfu/ml
7
Indium tin oxide chip
6 x 104 - 6 x 107 cfu/ml
6
x
103
4
5
8
cfu/ml
Boron doped diamond
4 x 104- 6 x 106
cfu/ml
4x104
9
cfu/ml
Hyaluronan
modified
nanoporous 10-105 cfu/ml
membranes
CuCys/Au electrode
1.0x101
10
cfu/ml
1x10-1x109 cfu/ml
10 cfu/ml
Present work
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