Journal of Nanoparticle Research

, Volume 12, Issue 1, pp 227–235

Electrochemical analysis of gold-coated magnetic nanoparticles for detecting immunological interaction


  • Thao Thi-Hien Pham
    • Nano-optics & Biomolecular Engineering National Laboratory, Department of Chemical EngineeringSungkyunkwan University
    • Nano-optics & Biomolecular Engineering National Laboratory, Department of Chemical EngineeringSungkyunkwan University
Research Paper

DOI: 10.1007/s11051-009-9600-7

Cite this article as:
Pham, T.T. & Sim, S.J. J Nanopart Res (2010) 12: 227. doi:10.1007/s11051-009-9600-7


An electrochemical impedance immunosensor was developed for detecting the immunological interaction between human immunoglobulin (IgG) and protein A from Staphylococcus aureus based on the immobilization of human IgG on the surface of modified gold-coated magnetic nanoparticles. The nanoparticles with an Au shell and Fe oxide cores were functionalized by a self-assembled monolayer of 11-mercaptoundecanoic acid. The electrochemical analysis was conducted on the modified magnetic carbon paste electrodes with the nanoparticles. The magnetic nanoparticles were attached to the surface of the magnetic carbon paste electrodes via magnetic force. The cyclic voltammetry technique and electrochemical impedance spectroscopy measurements of the magnetic carbon paste electrodes coated with magnetic nanoparticles–human IgG complex showed changes in its alternating current (AC) response both after the modification of the surface of the electrode and the addition of protein A. The immunological interaction between human IgG on the surface of the modified magnetic carbon paste electrodes and protein A in the solution could be successfully monitored.


Gold-coated magnetic nanoparticlesElectrochemical impedance spectroscopyCyclic voltammetryImmunological interactionNanomedicine


Typically, an immunoassay is based on the specific interaction between an antigen and an antibody with a complementary three-dimensional structure. Electrochemical biosensing has been developing to take advantage of nanoscale materials. Nanoparticles (NPs) have been widely used as labels in affinity biosensors or to enhance the detection signals for electrochemical immunosensors (Chen et al. 2006; Merkoci 2007; Tang et al. 2005, 2006a, b). Many important achievements have been reported in the area of amperometric and voltammetric electrochemical nanobiosensors (Pumera et al. 2007; Wu et al. 2007; Zhou et al. 2003, 2006). Besides, electrochemical impedance spectroscopy (EIS) has shown itself to be a sensitive technique for use in immunosensors, and the overall system provides information about the interface, its structure, and the reactions that occur (Bouafsoun et al. 2007; Navratilova and Skaladal 2004). Gold nanoparticles have been applied to enhance the immobilization of biological reagents on the surface of glass carbon electrodes (Huang et al. 2006). Magnetic nanoparticles modified with biological ligands, including proteins and nucleic acids, supported on a carbon paste electrode have been used in electrochemical detection (Zhang et al. 2007; Zhu et al. 2006).

Protein A from Staphylococcus aureus and immunoglobulin from animals has been widely applied to model immunological interactions (Briand et al. 2006; Schwartz et al. 2007; Touhami et al. 2007). In this study, electrochemical impedance techniques in combination with cyclic voltammetry (CV) were used for the purpose of detecting the layer-by-layer structure of a modified magnetic carbon paste electrode (MCPE). Gold-coated magnetic nanoparticles with sizes ranging from 20 to 70 nm were immobilized on the surface of the electrodes. The immunological reaction between human IgG immobilized on the nanoparticles and protein A from S. aureus was successfully conducted and analyzed. We report a novel method of using gold-coated magnetic nanoparticles to provide enhanced immunosensing in electrochemical applications.

Experimental details

Reagents and instruments

FITC–protein A (Lot no. 139 5427) was obtained from Invitrogen, Carlsbad, USA. Human IgG, whole molecule (Lot no. 75 722), was obtained from Jackson Human Research Lab Inc. (USA). 11-Mercaptoundecanoic acid (11-MUA) 95% from Aldrich was used to fabricate the self-assembled monolayer (SAM) on the nanoparticles. N-hydroxysuccinimide (NHS) and N-ethyl-N-(3-diethylaminopropyl) carbodiimide (EDC) were obtained from Sigma-Aldrich.

Phosphate buffer saline (PBS) whose pH was adjusted to 7.2 was used as the electrolyte in the measuring system and for the preparation of 5 mM of both K3[Fe(CN)6] and K4[Fe(CN)6]. Deionized water (>18 MΩ cm−1) was obtained from a Water Softener (Human Corporation, South Korea) purification system.

Cyclic voltammetry and EIS measurements were conducted using a Parstat 2263 system (Princeton Applied Research) with a commercial software program for the alternating current (AC) measurements. Field-emission scanning electron micrographs (FE-SEMs) of the magnetic nanoparticles were obtained with a JSM-7000F (celebrating voltage 0.5–30 kV) (JEOL, Tokyo, Japan). A UV/Vis spectrophotometer, model DU 730 (Beckman Coulter, USA), was used to measure the absorption of the nanoparticles. The fluorescence intensity was determined using a Spectramax Gemini Microplate Spectrofluorometer (Molecular Device, USA).

Preparation of magnetic nanoparticles

Magnetic cores of Fe3O4 were synthesized by the co-precipitation of Fe salts in strong alkaline solution, as reported elsewhere (Kang et al. 1996; Pham et al. 2008). Briefly, a mixture of 4.595 g of Fe(III) chloride and 1.71 g of Fe(II) chloride dissolved in an acidic solution was stirred under a nitrogen-protection environment and a solution of 2 M NaOH was added, resulting in a pale yellow solution which changed to brown and finally to dark black. The magnetic cores were washed continuously with water and diluted tetramethylammonium hydroxide (TMOH) solution and then stabilized in 0.1 M TMOH solution for further use. The magnetic cores were next coated with gold (Au) following the previously reported procedure in our article (Pham et al. 2008). One milliliter of 0.212 mM N(CH3)4-stabilized, oxidized Fe3O4 stock solution was diluted with 50 mL of 0.01 M sodium citrate and stirred for 30 min to exchange the absorbed OH with citrate ions. Five milliliter of oxidized Fe3O4 from the working magnetic-core solution was used for the Au coating reaction in a total volume of 40 mL of 0.01 M sodium citrate. The solution of magnetic cores and sodium citrate was stirred and heated to boiling point and then 1 mL of a solution of 10 nM HAuCl4 was immediately added to the mixture. The heating was continued for 15 min after the addition of the Au3+ salts and the stirring was continued while the solution cooled to room temperature. The synthesized, gold-coated magnetic nanoparticles were stabilized by citrate ions.

Fabrication of nanoparticle–protein conjugates

The citrate-stabilized, gold-coated magnetic nanoparticle solution was standardized with respect to its absorbance at 528 nm. 11-MUA in ethanolic solution was added to the nanoparticle solution so that the final working concentration of 11-MUA had an absorbance of 2 at 528 nm. The solution was kept in dark-glass bottles with gentle shaking for 3 h to fabricate the SAM on the surface of the nanoparticles. The nanoparticles functionalized with 11-MUA were kept in darkness to prevent their aggregation prior to their further use.

The 11-MUA–nanoparticles were washed by centrifugation with diluted PBS before being activated with a 0.01 M mixed solution of EDC/NHS. Then, the nanoparticle solution was added to human IgG solution in PBS at pH 7.4. The solution was then gently agitated overnight in darkness. The nanoparticle–protein IgG conjugates were washed in PBS at pH 7.4 by centrifugation and then kept at 4 °C for further use.

Preparation of electrode

Solid magnetic carbon paste electrodes were fabricated following the procedure described in previous reports (Tang et al. 2006a, b; Zhang et al. 2007; Zhu et al. 2006). A mixture of melted paraffin (800 g) and graphite powder (1,000 mg) was blended thoroughly to get a homogeneous paste. A permanent magnet (diameter 7.85 mm, length 7.85 mm) was embedded into a polytetrafluoroethylene (PTFE) tube (aperture of 8 mm). The paste was stuffed into the PTFE tube to immobilize the magnet and the tube was left to harden for 1 day. The MCPEs were polished thoroughly with fine sandpapers to get a smooth surface. Finally, the electrodes were washed by ultrasonicating them in both distilled water and ethanol for 5 min each.

The magnetic nanoparticle–protein IgG conjugates suspended in PBS at pH 7.4 were dripped onto the surface of the MCPEs. The surface was dried, washed with PBS to remove any unbound nanoparticle and then kept at 4 °C for incubation with protein A or electrochemical measurement. The magnetic nanoparticles were firmly attached to the surface of the magnetic electrode due to the magnetic force.

Preparation of nanoparticles–protein conjugates for measuring fluorescence

Solutions of gold-coated magnetic nanoparticles were standardized to an absorbance of 1 at 528 nm. The nanoparticles were functionalized with 11-MUA so that the concentration of the working 11-MUA solution was 2 mM, and the incubation time used to make the SAM of 11-MUA was 5 h with gentle agitation. The functionalized magnetic nanoparticles were first washed with 0.1 M Tris solution pH 6.0 and then with PBS pH 7.4. The coupling reagent, EDC/NHS (ratio 1:1), with a working concentration of 0.01 M was used to induce the formation of amide bonds between the carboxyl groups of 11-MUA and human IgG. A 0.5-mg/mL IgG solution was used to immobilize human IgG onto the SAM of 11-MUA. Next, the conjugates were washed with PBS to discard any unbound IgG. The nanoparticle–IgG conjugates were stored at 4 °C under moist conditions. The immunological interaction was investigated with concentrations of protein A ranging from 2 to 1,190 nM. The nanoparticle solutions were then washed with PBS to remove excessive protein A by magnetic separation and centrifugation. A 200-μL aliquot from each solution was applied to a Microplate Spectrofluorometer. The concentration of FITC–protein A was determined according to the fluorescence intensity measured.

Electrochemical cell and procedure

All electrochemical experiments were performed at room temperature, and 1 mM PBS pH 7.4 containing 5 mM of the redox couple, K3[Fe(CN)6/K4[Fe(CN)6 was used as analytic solution. The exposed coating area was 0.25024 cm2. CV and EIS measurements were conducted using a Parstat 2263 system with a commercial software program for the AC measurements. A graphite counter was used with a saturated calomel electrode as the reference. Prior to the tests, the samples were kept in the solution for 1 h in order to stabilize the open-circuit potential. The applied potential scans were from −200 to 800 mV. The obtained current, I, is approximated on the μA scale with an optimal scan rate of 10 mV/s. In addition, for the EIS test, the amplitude of the sinusoidal perturbation was 10 mV. The frequency range was from 100 kHz to 10 mHz.

Results and discussion

Investigating immobilization of human IgG on the magnetic nanoparticles and immunological interaction of FITC–protein A and IgG through fluorescence intensity

The immobilization of proteins onto the gold surface of metal shell/Fe oxide core nanoparticles has previously been reported (Jeong et al. 2006; Pham et al. 2008; Tang et al. 2006a, b). The gold shell is a good platform for the physical adsorption of the protein. This occurs due to the interaction of the charged group of the amino acids with citrate-stabilized gold surface when the pH of the incubation solution is different from the isoelectric point (pI) of the protein (Pham et al. 2008). Enzyme hexa-arginine-tagged esterase was immobilized onto Au-coated magnetic nanoparticles synthesized by the hydroxylamine seeding method via carboxylic groups (Jeong et al. 2006). In this experiment, human IgG was immobilized onto the gold-coated magnetic nanoparticles via the amide bond between the amine groups of the protein and carboxyl group of 11-MUA. The functionalization of the nanoparticles with 11-MUA was confirmed with FTIR, as shown in Fig. 1a. The spectrum of the 11-MUA powder shows clear peaks of the carboxyl group at a wavenumber of 1,710 cm−1 and thiol groups at about 2,561–2,693 cm−1 (inset of Fig. 1a). However, the spectrum obtained from the 11-MUA functionalized nanoparticles does not show any peak for the thiol groups in the wavenumber range of 2,561–2,693 cm−1, indicating that the thiol groups are linked to the gold surface of the nanoparticles. UV–Vis absorption spectroscopy can be used to confirm the functionalization of nanoparticle solution with chemical ligands and/or proteins (Cao and Sim 2007). As displayed in Fig. 1b, the citrate-stabilized magnetic nanoparticle solution has a surface plasmon peak at 528 nm, and the functionalization of 11-MUA onto the nanoparticle solution does not change the absorption peak. Nevertheless, the human IgG–nanoparticle complex remarkably shifts the surface plasmon peak of the solution to 545 nm (line 3). The IgG–11MUA–NPs (IgG–11MUA–magnetic nanoparticles) complex was used as a platform for the immunological reaction with FITC–protein A from S. aureus. The concentration of FITC–protein A was determined by measuring the fluorescence intensity of the conjugated FITC with an excitation wavelength of 485 nm and emission wavelength of 538 nm. Figure 2 shows the fluorescence intensity of FITC–protein A bound to the magnetic nanoparticles via its immunological interaction with human IgG. The fluorescence intensity varies with the incubation time of immobilization of human IgG onto the nanoparticles; line (1) corresponds to 3-h incubation and line (2) to overnight incubation, whereas the incubation time of the protein A solution was always 2 h. Therefore, the formation of covalent bonds between protein IgG and 11-MUA occurs slowly. The observed concentrations of protein A range from 2.38 to 1,190 nM. However, the fluorescence intensity of FITC–protein A bound to the nanoparticles increases linearly as the concentration of protein A in the incubated solution increases from 2.38 to 23.81 nM; then, the fluorescence intensity starts to decrease as the concentration of protein A is further increased. This indicates that the binding of protein A onto IgG–11MUA–NPs via the immunological interaction begins to become stabilized when the concentration of protein A is higher than 23.81 nM.
Fig. 1

a FTIR spectra of 11-MUA powder (2) and 11-MUA–NPs conjugates (1), magnified spectra of –SH band in the wavenumber range of 2,693–2,561 cm−1 (inset). b UV–Vis absorption spectra of NPs (gold-coated magnetic nanoparticles) (1), 11-MUA–NPs conjugates (2), and IgG–11MUA–NPs complex (3). NPs gold-coated magnetic nanoparticles
Fig. 2

Fluorescence intensity of FITC–protein A bound to human IgG–11MUA–NPs complex with different incubation times of human IgG (1) 3 h, (2) overnight; FE-SEM image of gold-coated magnetic nanoparticles with magnification of 100,000× (inset). NPs gold-coated magnetic nanoparticles

Electrochemical characterization of modified electrodes

Cyclic voltammetry and EIS are valuable tools to test the kinetics of electron transfer of an interface using an electroactive species. Increasing the thickness of the electrode surface and decreasing the density of the interface barrier affect the kinetics of the electron transfer process (Bouafsoun et al. 2007). The surface modification of MCPEs with some materials leads to the perturbation of the electron transfer process. Figure 3a shows the cyclic voltammograms obtained from the MCPE before and after the deposition of magnetic nanoparticles and protein human IgG. The deposition of the layer of bare gold-coated magnetic nanoparticles onto the electrode surface changes the currents at the working electrode (line 2). Moreover, the peak height increases and the peak-to-peak separation between the cathodic and anodic waves of the redox probe increases. This happens probably due to the fact that the gold shell/Fe oxide core nanoparticles play a role in facilitating the electron transfer within the surface interface of the electrode. Gold nanoparticles have been applied as conducting wires or electron-conducting tunnels in Nafion-modified electrode surfaces for amperometric enzyme immunosensors (Zhu et al. 2006); the interface between magnetic core–shell Fe3O4@Ag nanoparticles and carbon paste coated on their surface was found to facilitate the electron transfer in an electrochemical study of carcinoembryonic antigens for immunoassays (Tang et al. 2006a, b).
Fig. 3

a CV curves of bare magnetic carbon paste electrodes (MCPEs) (1), the MCPEs modified with gold-coated magnetic nanoparticles (2), and the MCPEs modified with IgG–11MUA–NPs complex (3). b CV curves of the modified MCPEs with different analyzed concentrations of protein A. NPs gold-coated magnetic nanoparticles

However, there is a remarkable change in the peak height and the peak-to-peak separation when the layer of SAM–human IgG conjugates is immobilized onto the nanoparticles (line 3). The thiol monolayer has been reported to decrease the peak values, and CV techniques were employed to confirm the presence of the monolayer of sulfur derivatives on the gold electrode (Bouafsoun et al. 2007). As shown in Fig. 3b, the layer of protein A bound to the layer of human IgG through their immunological interaction does not produce any substantial change in the shape of the CV curve or the peak height compared to that of the human IgG layer. However, the current obtained at the vertex between the charge and discharge phases of the CV curves increases as the incubated concentration of protein A decreases. This indicates the formation of a high-density layer of protein A on the human IgG layer. Increasing the absorption time of the McAb on the gold electrode led to a decrease in the peak current of the CV curves (Jie et al. 1999). The layer of protein A may affect the charge on the surface of the electrode and the transmission of electrons. In our experiment, PBS at pH 7.4 was used as the electrochemical solution to reduce such effects of the protein, since the pI of the protein is about 7.

Impedance analysis of magnetic carbon paste electrodes (MCPEs) modified with gold-coated magnetic nanoparticles and protein human IgG

The EIS spectra of the bare MCPE, magnetic nanoparticle/MCPE, and protein/magnetic nanoparticle/MCPE were obtained at a constant concentration of the redox species Fe(CN)64−/3−. The typical Nyquist diagrams of the MCPE functionalized with different layers analyzed in PBS at pH 7.4 are shown in Fig. 4. The impedance–plane plots are characterized mainly by a semicircle related to the charge transfer process and a semicircle related to the coating resistance. The diameter of the semicircles decreases when the surface of the MCPE is covered with the layer of gold-coated magnetic nanoparticles but strongly increases when the 11-MUA–protein IgG layer is deposited onto the surface of the nanoparticles, as displayed in Fig. 4a. The binding of protein A onto the IgG layer does not remarkably change the shape or diameter of the semicircles.
Fig. 4

Impedance spectra shown as Nyquist plots of modified magnetic carbon paste electrodes (MCPEs) with a NPs and IgG, b different analyzed concentrations of protein A. NPs gold-coated magnetic nanoparticles (Bare CPE bare MCPE, IgG-CPE IgG-modified MCPE with gold-magnetic nanoparticles, NPs-CPE MCPE modified with gold-coated nanoparticles)

The AC impedance measurement can be presented by a parallel double-layer capacitance and resistance curve, which represents for the resistance of the electrolyte solution (Rs) to determine the rate of the corrosion reaction. Moreover, there is a coating capacitance in our system. A capacitor is formed when a non-conducting media called a dielectric separates two conducting plates. The magnet inside the electrode acts as the first conducting plate and the electrolyte is the second, while the coating of nanoparticles and protein is the dielectric. The constant phase element (CPE), a non-intuitive circuit element that was discovered while looking at the response of real-world systems, is used to explain the AC impedance. Mathematically, a CPE’s impedance is given by:
$$ 1/Z = Y = Q^{ \circ } \left( {jw} \right)^{n} $$
where Q° has the numerical value of the admittance (1/|Z|) at w = 1 rad/s. The units of Q° are S sn. The phase angle of the CPE impedance is independent of the frequency and has a value of −(90 * n)°.
Hence, the impedance spectrum is interpreted in terms of the equivalent circuit in Fig. 5. The system can be analyzed with the charge transfer resistance (Rct), coating resistance (Rpore), and a CPE in parallel. An electrical double layer exists at the interface between an electrode and its surrounding electrolyte. This double layer is formed as ions from the solution stick on the electrode surface. Charges in the electrode are separated from the charges of these ions, and the distribution of charge on the surface determines the field strength of the interface, so it defines the speed of ion transfer. The non-homogeneity and defect area of the layer are reflected by the CPE (Bouafsoun et al. 2007).
Fig. 5

Equivalent circuit used to model the impedance data of the modified MCPEs with a protein layer, b deposited layer of gold-coated magnetic nanoparticles only

The experimental Faradaic impedance spectra were fitted with the computer simulated spectra using the equivalent electrical circuit shown in Fig. 5. Excellent fitting between the simulated and experimental spectra was obtained using the ZSimpWin program and the results are shown in Tables 1 and 2. Figure 4a and Table 1 show the Nyquist plots and the measured electrochemical impedances, respectively, of the modified MCPEs coated with the nanoparticles and 11-MUA–IgG. Coating the MCPEs with the gold-coated magnetic nanoparticles decreases the value of Rs, Rpore, and CPE of the double layer capacitance (CPE2) but increases the CPE of the coating capacitance (CPE1). The decrease in CPE2 brought about by the modification with the NPs might be due to the Warburg resistance rather than Rct (the Warburg coefficient is 0.0109 Ω cm2/sqrt(s)). The layer of gold-coated nanoparticles might cause the increase in the electron transfer kinetics on the layer of the metal surface. However, the further modification of the NP layer with 11-MUA and protein IgG led to an increase in Rs, the coating resistance, and charge transfer resistance via Rpore, while both CPE1 and CPE2 decrease. The modification with the layer of SAM–IgG leads to 4 times increase of the coating resistance and 5.7 times increase of the charge transfer resistance, as well as an approximately 270 times decrease of Q2 compared to the bare electrodes (Table 1).
Table 1

Electrochemical analysis of the modified MCPEs


Rs (Ω cm2)


Rpore (Ω cm2)


Rct (Ω cm2)

Q1 (S s.5/cm2)


Q2 (S s.5/cm2)


CP electrode















889.5 (W)









Table 2

Electrochemical analysis of binding of protein A and IgG on the surface of the modified MCPEs

Concentration of protein A (nM)

Rs (Ω cm2)


Rpore (Ω cm2)


Rct (Ω cm2)

Q1 (S s.5/cm2)


Q2 (S s.5/cm2)


























Similarly, the interaction between protein A and IgG leads to the formation of a layer of protein A and the impedance of the modified electrodes is interpreted in Fig. 4b as well as in the simulated circuit in Fig. 5b. Electrochemical analysis was performed with three concentrations of protein A, 2.38, 23.81, and 238.09 nM. The impedance of the electrodes after the binding of protein A was simulated with the same fitting curve as that used for the modeling of protein IgG with Rs, CPE1–Rpore, and CPE2–Rct. Table 2 displays the analytical results of the impedance of the modified electrodes. The solution resistance and CPE2 decrease when the concentration of protein A increases. However, regarding the variation of CPE1 and CPE2, as well as Rpore according to changes of protein A concentrations, there are two phases in the immunological interaction between protein A and human IgG. These results are in agreement with the fluorescence intensity data of protein A bound onto the nanoparticles discussed above (Fig. 2). A 10 times increase in the concentration of protein A from 2.38 to 23.81 nM leads to a decrease of CPE1 and CPE2, but an increase of Rpore and Rct. The change of impedance may be caused by the thickness of the protein layer, as well as by the local charge of the protein layer, which may affect the local electrons of the interface. On the other hand, a 10 times increase in the concentration of protein A from 23.81 to 238.09 nM has the opposite effect on the values of CPE1, Rpore, and Rct. CPE1 increases while Rpore and Rct decrease. This result can be explained by the three-dimensional binding between IgG and protein A. At a concentration of 238.09 nM, protein A can form a stable layer on the surface of the electrode and the layer of IgG is total covered, while at a concentration of 2.38 nM, protein A only partly covers the layer of human IgG and, consequently, part of the IgG layer might still be exposed to the electrolyte solution. Therefore, in this experiment, we succeeded in application of gold-coated magnetic nanoparticles to electrochemically analyze the immunological interaction between human IgG and protein A from S. aureus. To understand more about the electrochemical kinetics of the protein A–human IgG interaction, detailed observations should be made at each phase to determine the stability and affinity of this immunological binding.


Gold-coated magnetic nanoparticles deposited onto a MCPE via magnetic force were utilized to immobilize human IgG through chemical ligands. The SAM of alkanethiol (11-MUA) serves as a good platform to immobilize human IgG onto the gold surface of the nanoparticles. The immunological interaction between human IgG and protein A from S. aureus was able to be monitored and analyzed with CV techniques and EIS. Moreover, this experiment shows how gold-coated magnetic nanoparticles can be made use for EIS or CV biosensors. The minimum concentration of the analyzed target can be as low as 20 nM.


This work was supported by the Korea Science and Engineering Foundation (KOSEF) National Research Laboratory (NRL) Program grant funded by the Korea government (MEST) (grant no. R0A-2008-000-20078-0) of the Republic of Korea.

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