Ag@poly(m-phenylenediamine)-Ag core–shell nanoparticles: one-step preparation, characterization, and their application for H₂O₂ detection

Abstract

We have recently found that the direct mixing of m-phenylenediamine (MPD) and AgNO₃ aqueous solutions at room temperature leads to Ag@poly(m-phenylenediamine) (Ag@PMPD) core–shell nanoparticles (Langmuir, 2011, 27, 2170). In this study, we characterize such core–shell nanoparticles in more detail by X-ray diffraction and IR techniques and further demonstrate that the size of the core and whole particle as well as the ratio of the shell thickness to the core size can be tuned by the molar ratio of MPD to Ag. Furthermore, the PMPD shell can be further used as a reductant to reduce Ag⁺ into small Ag nanoparticles (AgNPs) which are embedded in the PMPD matrix, leading to nanoparticles with a Ag core and a small AgNP-embedded PMPD shell (Ag@PMPD–Ag core–shell nanoparticles). The Ag core, although buried in the central part of the resultant nanoparticle, can still catalyze the reduction of H₂O₂, but the embedded AgNPs in the PMPD matrix exhibit superior catalytic performance. With these Ag@PMPD–Ag core–shell nanoparticles, we constructed an enzymeless H₂O₂ sensor with a fast amperometric response time of less than 2 s, a linear range of 0.1 to 170 mM and a detection limit of 2.5 μM at a signal-to-noise ratio of 3.

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1. Introduction

H₂O₂ is of great importance in the fields of chemistry, biology, clinical control, and environmental protection.¹ Up to now, many detection techniques have been developed.² Among them, an electrochemical technique has been proven to be an inexpensive and effective way due to its intrinsic simplicity and high sensitivity and selectivity. Early H₂O₂ sensors involved the use of the intrinsic selectivity and sensitivity of enzymatic reactions where nanostructures are also employed to immobilize the enzymes and, at the same time, to reduce the possibility of protein denaturing.³ It has been shown that AgNPs show good catalytic activity towards H₂O₂.⁴

Over the past decades, multi-phase nanocomposites have attracted considerable research interest because of their improved physical and chemical properties over their single component. One of the most studied materials are core–shell structured nanoparticles where the shell not only stabilizes colloidal dispersion but tailors the core particle properties (e.g. optical, magnetic, catalytic).⁵ Particularly, Ag@polymer nanocomposites are of great interest because Ag nanoparticles are good candidates for optics,⁶ electronics,⁷ and catalysis,⁸ and thus much attention has been paid to their synthesis. Up to date, a variety of polymers including poly[styrene-co-(glycidyl methacrylate)],⁹poly(N-isopropylacrylamide),¹⁰polyaniline,¹¹polypyrrole,¹² poly(benzylthiocyanate),¹³polystyrene,¹⁴ and Chitosan¹⁵ have been successfully coated on Ag nanoparticles (AgNPs). However, all these above-mentioned methods suffer from more or less severe drawbacks such as involving a multi-step process and thus being time-consuming or requiring protection agents, which hinder their further applications. Therefore, it is still a challenge to prepare Ag@polymer composites by a simple route. On the other hand, polymers based on aniline derivatives have also been extensively investigated,¹⁶ among which the poly(phenylenediamine) (PPD) homopolymer is reported to be a highly aromatic polymer containing a 2,3-diaminophenazine or quinoxaline repeating unit and exhibiting high thermostability and has already found many applications.¹⁷ Although there are several papers reporting on the synthesis of PPD structures including microparticles, nanobelts and microfibirls,¹⁸ only until recently have we demonstrated the synthesis of Ag@poly(m-phenylenediamine) (Ag@PMPD) core–shell nanoparticles by direct mixing of MPD and AgNO₃ aqueous solutions at room temperature.¹⁹ In this study, we futher characterize such core–shell nanoparticles in more detail by X-ray diffraction and IR techniques and demonstrate that the size of the core and whole particle as well as the ratio of the shell thickness to the core size of the Ag@PMPD core–shell nanoparticles can be tuned by the molar ratio of MPD to Ag. Furthermore, the PMPD shell can further serve as a reductant to reduce Ag⁺ to form small AgNPs which are embedded in the PMPD matrix, leading to Ag@PMPMD-Ag core–shell nanoparticles. It suggests both the Ag core and the small AgNPs can simultaneously catalyze the reduction of H₂O₂ and the Ag particles in the PMPD shell exhibit superior catalytic performance to the Ag core. The enzymeless H₂O₂ sensor constructed with nanoparticles thus formed exhibits a fast amperometric response time of less than 2 s and has a linear range of 0.1 to 170 mM and a detection limit of 2.5 μM, respectively, at a signal-to-noise ratio of 3.

2. Experimental

All chemicals were purchased from Aladin Ltd. (Shanghai, China) and used as received without further purification. The water used throughout all experiments was purified through a Millipore system. Phosphate buffer saline (PBS) was prepared by mixing stock solutions of NaH₂PO₄ and Na₂HPO₄ and a fresh solution of H₂O₂ was prepared daily. Ag@PMPD core–shell nanoparticles were prepared as follows: in a typical experiment, 0.1 mL of a 0.1 M MPD aqueous solution was mixed with 1 mL of a 5 mM AgNO₃ aqueous solution with a 2   1 molar ratio of MPD to Ag and kept at room temperature overnight (sample 1). The solution was then washed and centrifuged at 14 000 rpm for 10 min twice. The precipitate was redispersed in water and stored at 4 °C for characterization and further use.

Scanning electron microscopy (SEM) measurements were made on an XL30 ESEM FEG scanning electron microscope at an accelerating voltage of 20 kV. Samples for SEM examination were made by placing a drop of the dispersion on a glass slide and air-drying at room temperature. Transmission electron microscopy (TEM) measurements were made on a HITACHI H-8100 EM (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. A sample for TEM characterization was prepared by placing a drop of the dispersion on a carbon-coated copper grid and drying at room temperature. The X-ray diffraction (XRD) pattern was collected on a Japan Rigaku D/Max-RA X-ray diffractometer. The sample for XRD characterization was prepared by placing 20 μL of the dispersion on a glass slide. Electrochemical measurements are performed with a CHI 660D electrochemical analyzer (CH Instruments, Inc., Shanghai). A conventional three-electrode cell is used, including a glassy carbon electrode (GCE) (geometric area = 0.07 cm²) as the working electrode, an Ag/AgCl (saturated KCl) electrode as the reference electrode, and platinum foil as the counter electrode. The potentials are measured with an Ag/AgCl electrode as the reference electrode. Before modification, the GCE was polished with 1.0 and 0.3 μm alumina powder carefully and then ultrasonically washed with ethanol and double distilled water. In a typical experiment, 15 μL of the dispersion was mixed with 3 μL 0.67 M poly[(2-ethyldimethylammonioethyl methacrylate ethyl sulfate)-co-(1-vinylpyrrolidone)] (PQ11) followed by placing 2 μL of the mixture on a GCE surface and air-drying for 10 min at room temperature. Cyclic voltammetric measurements were carried out in N₂-saturated 0.2 M PBS buffer (pH: 6.5) at a scanning rate of 0.02 V s⁻¹.

3. Results and discussion

Fig. 1A shows the SEM image of the products, indicating the formation of nanoparticles with a white core and a relatively light shell. A further examination of such nanoparticles by TEM (Fig. 1B) reveals that they consist of a dare core about 50 nm in diameter (d_core) in the central part and a relatively light thin shell with a thickness (t_shell) of about 20 nm on the edge, that is, the particle size (d_particle) is about 90 nm and the ratio of the core size to the shell thickness (r_c/s) is about 2 : 5. In our previous study, we have found that Ag⁺ can be easily reduced by o-phenylenediamine and p-phenylenediamine at room temperature into metallic Ag with the co-oxidation of the phenylenediamine monomers.^18e,f It is reasonable to conclude that the mixing of MPD and Ag⁺ also leads to the direct redox between MPD and Ag⁺ yielding metallic Ag and PMPD. Given the noble metal and polymer nature of the Ag and PMPD, respectively, we can conclude that the particles formed in our present study are Ag@PMPD core–shell nanoparticles.

Fig. 1 Typical (A) SEM and (B) TEM images of the particles of sample 1.

Fig. 2 shows the XRD pattern of the resulting core–shell nanoparticles. The broad peak centered at ∼25° can be ascribed to the formation of amorphous polymers.^18d The other five peaks located at 38.0, 44.4, 64.4, 77.3, and 80.5° are assigned to 111, 200, 220, 311, and 222 faces of an Ag crystal, respectively, demonstrating the formation of metallic Ag.²⁰

Fig. 2 XRD pattern of the Ag@PMPD core–shell nanoparticles of sample 1.

We studied the macromolecular structures of PMPD by IR spectroscopy and Fig. 3 shows the collected IR spectra of the Ag@PMPD core–shell nanoparticles. The bands at 3330 and 3200 cm⁻¹ can be attributed to the characteristic stretching vibration of N–H, indicating the presence of –NH and –NH₂groups. The bands at 1623 and 1516 cm⁻¹ can be assigned to aromatic rings and the strong bands at 1115 and 619 cm⁻¹ can be ascribed to the in-plane and out-of-plane bending vibration of the C–H bonds of 1,2,4-trisubtituted benzene rings, respectively. These data are consistent with those of PMPD microparticles²¹ and thus provide another piece of clear evidence to support the formation of PMPD polymers.

Fig. 3 IR spectra of the Ag@PMPD core–shell nanoparticles of sample 1.

We further investigated the influence of the molar ratio of the reactants on the formation of Ag@PMPD nanoparticles. Fig. 4A–C shows typical SEM images of the particles obtained with (A) 1 : 2, (B) 1 : 1, and (C) 4 : 1 molar ratios of MPD to Ag, under otherwise identical conditions used for preparing sample 1. It is clearly seen that an increased molar ratio of MPD to Ag leads to decreased particle size. For example, the d_particle of the sample obtained with 1 : 2, 1 : 1 and 4 : 1 molar ratios of MPD to Ag is estimated to be about 110 nm, 100 and 80 nm, respectively. The detailed core–shell structure of each sample was further examined by TEM, and Fig. 4D–F shows the corresponding TEM images. At a 1 : 2 molar ratio of MPD to Ag, the d_core and t_shell are changed to 80 nm and 15 nm, respectively. At a 1 : 1 molar ratio, the d_core and t_shell are changed to 60 nm and 20 nm, respectively. At a 4 : 1 molar ratio, the d_core and t_shell are changed to 40 nm and 20 nm, respectively. These observations indicate that we can control the size of the d_core and d_particle as well as the r_c/s by simply changing the molar ratio of MPD to Ag, which would be helpful for controllable synthesis of other functional nanocomposites. It should be mentioned that Ag@PMPD nanoparticles shown here contain not only spherical Ag cores, but also coalescent and non-spherical ones. Nevertheless, the sizes of these Ag cores are similar.

Fig. 4 Typical SEM images of the particles obtained with (A) 1 : 2, (B) 1 : 1, and (C) 4 : 1 molar ratios of MPD to Ag, under otherwise identical conditions used for preparing sample 1. (D), (E), and (F) are the corresponding TEM images.

Li et al. have demonstrated that poly(o-phenylenediamine) (POPD) microparticles can effectively adsorb Ag(I) ions due to that many amino and imino groups located adjacent to each other in the POPD chains provide abundant coordination sites for Ag(I) ions.^18a It was found that POPD has the ability to effectively reduce the as-absorbed Ag(I) ions to form AgNPs. Such observation was also verified by the same group in an aromatic diamine polymers-based system.²¹ It is also found that the preformed PMPD shell can further reduce Ag⁺ into metallic Ag.^4gFig. 5 shows the low magnification TEM image of the Ag@PMPD core–shell nanoparticles of sample 1 after their incubation with an AgNO₃ aqueous solution over a 1 h period. Inset A is the corresponding high magnification TEM image of one single nanoparticle, indicating that a large amount of small nanoparticles with the same contrast as the Ag core and about several nanometres in size are generated in the PMPD matrix after the incubation process, without changing the size of the Ag core. A HRTEM image taken from one small nanoparticle in the PMPD matrix (inset B) shows clear lattice fringes with an interplane distance measured to be 0.235 nm corresponding to the {1 1 1} lattice spacing of Ag,²² indicating that these small nanoparticles are AgNPs. Note that some small dots similar to AgNPs embedded in the PMPD matrix are also observed in the background, which are free AgNPs. Such an observation can be attributed to that the AgNPs in situ generated on the nanoparticle surface cannot tightly bind and thus detach from the nanoparticle. The formation of such small AgNPs in our present study can be attributed to that the PMPD shell can effectively adsorb Ag(I) ions via coordination of their amino and imino groups in the PMPD chains to Ag(I) ions and then in situ reduces the as-absorbed Ag(I) ions to form AgNPs,^4g,18a leading to Ag@PMPD–Ag core–shell nanoparticles. Meanwhile, the aforementioned further demonstrate that it is not possible to prepare only small Ag nanoparticles without formation of Ag cores in our present experiment.

Fig. 5 Typical TEM image of the Ag@PMPD core–shell nanoparticles of sample 1 after their incubation with an AgNO₃ aqueous solution over a 1 h period. Inset: (A) the corresponding high magnification TEM image of one single nanoparticle, (B) HRTEM image taken from one small nanoparticle in the PMPD matrix.

To demonstrate the sensing application of the Ag@PMPD core–shell nanoparticles, we constructed an enzymeless H₂O₂ sensor by deposition of the Ag@PMPD core–shell nanoparticles on a glass carbon electrode (GCE) surface with the use of poly[(2-ethyldimethylammonioethyl methacrylate ethyl sulfate)-co-(1-vinylpyrrolidone)] (PQ11), a polyelectrolyte with good film-forming ability on solid substrates, as an immobilization matrix.^4f The Ag@PMPD–Ag core–shell nanoparticles-modified GCE was also similarly prepared. Fig. 6 shows the electrocatalytic responses of these electrodes towards the reduction of H₂O₂ in N₂-saturated 0.2 M PBS at pH 6.5. In the presence of 1.0 mM H₂O₂, the Ag@PMPD nanoparticles exhibit a remarkable catalytic current peak centered at −0.85 V. However, the response of the bare GCE towards the reduction of H₂O₂ is weak. These observations indicate that the Ag core buried in the inner part of a nanoparticle still exhibits catalytic ability for H₂O₂ reduction, and the observation of a large catalytic current could be attributed to the large amount of Ag@PMPD core–shell nanoparticles on the electrode surface. When Ag@PMPD–Ag core–shell nanoparticles were used, however, we observed two reduction peaks: a pretty strong peak at −0.38 V and a weak peak at −0.85 V. The reduction peak at −0.85 V and −0.38 V can be assigned to the Ag core and the small nanoparticles embedded in the PMPD shell, respectively. The observation of a lower reduction potential for the Ag core towards H₂O₂ reduction could be attributed to the following two reasons: first, the Ag core is coated by a thick PMPD shell (20 nm in thickness) and thus an effective access by the substrate is hindered to some extent; second, the Ag core is bigger in size and hence a lower catalytic activity is expected. Compared to GCE modified by simple electroreduction of Ag⁺,²³ this Ag@PMPD–Ag core–shell nanoparticle/GCE exhibits a 17.4% enhancement of peak current and a 100 mV positive shift of the peak potential, indicating that this form of modified electrode is superior for electrocatalysis.

Fig. 6 Cyclic voltammetry (CV) of bare GCE, Ag@PMPD nanoparticles/GCE and Ag@PMPD–Ag core–shell nanoparticles/GCE electrode in N₂-saturated 0.2 M PBS at pH 6.5 in the presence of 1.0 mM H₂O₂.

Fig. 7 shows the typical current–time plot of the Ag@PMPD–Ag core–shell nanoparticle/GCE in N₂-saturated 0.2 M PBS buffer (pH: 6.5) on successive step change of H₂O₂ concentration under optimized conditions. When an aliquot of H₂O₂ was dropped into the PBS solution with stirring, the reduction current rose steeply to reach a stable value. The sensor could achieve 95% of the steady state current within 2 s, indicating a fast amperometric response behavior. It is clearly seen that the steps are more horizontal in the region of lower concentration of H₂O₂ and the noises become higher with increased concentration of H₂O₂. The inset shows the calibration curve of the sensor. The linear detection range is from 0.10 to 170 mM (r = 0.999), and the detection limit is estimated to be 2.5 μM at a signal-to-noise ratio of 3. The calibration curve including standard deviation is also shown. Error bars for all data points based on a reasonable set of independent measurements have been added. It is obvious that the reproducibility of the sensor is pretty good from the point of view of the small error bars. It is important to note although AgNP-decorated PMPD microparticles with no Ag cores also exhibit excellent capability of H₂O₂ detection,^4g compared to our present work, that the sensor suffers from a much narrower linear detection range (from 0.10 to 30 mM) and a higher detection limit (4.7 μM). Another added advantage is that only one step is required to obtain an Ag@PMPD core–shell nanoparticle catalyst,^4d but two steps are needed to prepare an Ag nanoparticle-decorated PMPD microparticle catalyst.^4g

Fig. 7 Typical steady-state response of the Ag@PMPD–Ag core–shell nanoparticles/GCE to successive injection of H₂O₂ into the stirred N₂-saturated 0.2 M PBS (pH: 6.5) (applied potential: −0.300 V). Inset: the calibration curve including error bars.

A comparison of the performance of our newly designed sensor with those already reported in the literature work regarding the performance of the H₂O₂ assay is shown in Table 1. Up to now, many types of modified electrode have been used for the detection of H₂O₂, and all of them have some advantages and limitations. Among these assays, the Ag@PMPD–Ag core–shell nanoparticle/GCE used here exhibits its excellent catalysis properties in either the low detection limit or wide linear range when compared with the others.

Table 1 A comparison of this work with literature work regarding the performance of the H₂O₂ assay using an electrode modified with different materials.

Type of electrode	Performance		Ref.
	LOD/μM	Linear range/mM
PEDOT/AgNPs/GCE	7	—	24a
AgNPs/PVA/Pt	1.0	0.04–6	24b
Ag microspheres/GCE	1.2	0.25–2	24c
PQ11-AgNPs/GCE	4	0.1–180	4f
AgNPs/DNA/GCE	1.7	0.004–16	23
AgNPs/GCE	2	—	4a
AgNPs/SBA-1S/GCEc	12	0.049–970	24d
Roughened Ag electrode	6	0.01–22.5	24e
AgNP-PMPD/GCE	2.5	0.1–170	This work

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Conclusions

In summary, we have demonstrated successfully that both the size of the core and the whole particle and the ratio of shell thickness to core size of Ag@PMPD core–shell nanoparticles can be tuned by the molar ratio of the reactants. Ag@PMPD–Ag nanoparticles can be obtained by further incubation of preformed Ag@PMPD nanoparticles with an AgNO₃ aqueous solution. As-formed two kinds of different Ag nanostructures co-exisiting in separate parts within one single core–shell particle exhibit quite different catalytic activity towards H₂O₂ reduction. Our present study is important because it is the first preparation of Ag@PMPD–Ag core–shell nanoparticles simultaneously exhibiting two catalytic activities towards the same H₂O₂ reduction reaction and provides us new nanostructures for H₂O₂ detection application.

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