Introduction

Composite materials with different combinations of two or more components have the significant importance because of their interesting physical properties and potential applications [1]. The incorporation of metallic nanoparticles in conjugated polymers has special interest because of the strong electronic interaction between the nanoparticles and polymer through functionalization [2]. Nanocomposites have broad applications in catalysis [3], delivery/controlled release [4], electronics [5] and development of electrochemical sensor systems [6]. To expand the electrochemical sensing system for the detection of wide range of analytes with increased sensitivity and selectivity, the synthesis of new advanced materials is an essential criterion to meet the requirements. Advanced materials with enhanced chemical and electrochemical properties are the main driving forces for sensor research and application.

Conjugated polymers have been exploited for a wide range of applications in materials science due to its environmental stability, and doping-dependent controllable electrical and optical properties. Among the various conjugated polymers, polyaniline and its derivatives are the most important because of its large variety of applications in micro-electronic devices [7], chemical sensors [8], catalysis [9], drug delivery [10] and energy storage systems [11, 12].

In the chemical synthesis of polyaniline, various oxidizing agents such as ammonium peroxydisulfate (APS) [13], potassium bichromate [14] and hydrogen peroxide [15] have been generally used although APS has been the one most often used as an oxidant. The composites containing polyaniline (or the derivatives of polyaniline) and varieties of inorganic nanoparticles such as Ag [16], Au [17], SiO2 [18], TiO2 [19], Si [20] and Cu [21] have been synthesized and reported. Among those inorganic materials, gold nanoparticles have received a great deal of attention because of their unique electrical and optical properties as well as the extensive applications in diverse areas.

An in situ route for the synthesis of polyaniline–gold thin composite film using aniline and HAuCl4 as the precursors has been reported with an average size of the resulting gold nanoparticles as 6 nm [22]. A facile synthesis route has been described for the preparation of a poly-(o-aminophenol)-gold nanoparticle composite material by polymerization of o-aminophenol monomer using HAuCl4 as the oxidant where the polymer-encapsulated gold nanoparticles within the size range of 2–3 nm are dispersed within the polymer matrix [17]. An enhanced electrocatalysis has been reported in the literature, for the electrochemical oxidation of ascorbic acid and glucose oxidase using the composite material consisted of sulfonate-functionalized gold nanoparticles incorporated into polyaniline [23]. Polyaniline stabilized mercapto-succinic acid capped gold nanoparticles revealed excellent electrocatalytic efficiency for to the oxidation of β-nicotinamide adenine dinucleotide (NADH) [24] and the detection of DNA hybridization [25]. A non-volatile memory chip with prolonged retention time has been reported based on the gold–polyaniline composite system for high density memories for the application of advanced computers and digital electronics [26].

The current article reports on the synthesis of gold–polymer composite material using in situ polymerization and composite formation route [27–30], where [4-(thiophen-3-yl)-aniline], TA, and auric acid (HAuCl4) were used as the precursors for the synthesis of Au–poly[4-(thiophen-3-yl)-aniline], Au–pTA, composite. We have paid attention to characterize the composite product using IR and UV–Vis spectroscopy techniques. The metal–polymer nanocomposite materials have potential advantages in the area of synthetic material science because both the polymer and the nanoparticles are produced simultaneously which causes an intimate contact between the particles and the polymer through functionalization and create the composite with interesting physical properties and potential applications. The current composite shows efficient electrocatalytic property for the detection of dopamine. The gold–polymer composite also performed as a catalyst for the reduction of a dye, Rhodamine-B, in the absence of any conventional reducing agent, which supports the concept of the proton-coupled electron transfer reaction.

Experiment section

Materials

All the chemicals and the solvents used for this experiment were of sanalytical purity and used without further purification. Ultra-pure water (specific resistivity >17 MΩ cm) was used in this experiment wherever required.

Material characterization

Transmission electron microscopy (TEM) studies were performed at an acceleration voltage of 197 kV using a Philips CM200 TEM instrument equipped with a LaB6 source. The TEM samples were prepared by depositing small amount of synthesized material onto a TEM grid (200 mesh size Cu grid) coated with a lacy carbon film. The X-ray diffraction (XRD) patterns were recorded on a Shimadzu XD-3A X-ray diffractometer operating at 20 kV using Cu-Kα radiation (k = 0.1542 nm). The measurements were performed over a diffraction angle range of 2θ = 20°–80°. X-ray photoelectron spectra (XPS) were collected in a UHV chamber attached to a physical electronics 560 ESCA/SAM instrument. Thermo elemental quadrupole-based ICP-MS (X-series), attached to a Nd:YAG UV (213 nm) laser system has been used for the estimation of the amount of metallic component present in the sample. Fourier transform infrared spectroscopy (FTIR) spectra were collected utilizing a Shimadzu IRAffinity-1 with a spectral resolution of 0.5 cm−1. The UV–Vis spectra were measured using a Shimadzu UV-1800 spectrophotometer using a quartz cuvette. Electrochemical studies were carried out with a bio-logic SP-200 potentiostat connected to a data controller. A three-electrode system was used in the experiment with a glassy carbon electrode (GCE) as the working electrode. Ag/AgCl electrode (saturated KCl) and a Pt-electrode were used as the reference and counter electrodes, respectively.

Preparation of Au–pTA composite catalyst

In a typical experiment, 0.350 g of 4-(thiophen-3-yl)-aniline was dissolved in 15 mL of methanol and 5 mL of aqueous solution of HAuCl4 (0.012 g) was slowly added to the above methanolic solution under continuous stirring conditions in a conical flask. During the addition, a greenish colloidal precipitation was formed at the bottom of the flask. Entire reaction was performed under ambient condition. The material was allowed to settle for 30 min after which the colloidal solution was taken from the bottom of the beaker and pipetted onto, carbon-coated, copper mesh grids for TEM study. The required amount of material was used for infrared studies and also for the sensing performance for the electrochemical recognition of dopamine. A small fraction of the precursors was mixed and used for the in situ UV–Vis studies. The remaining portion of the compound was dried under vacuum at 60 °C and used for XRD and XPS measurements. For the control study, poly[4-(thiophen-3-yl)-aniline], pTA, has been synthesized using ammonium persulfate (APS) as an oxidizing agent (maintaining the identical ratio of monomer and oxidizing agent as above). The dried form of both the synthesized materials was used as an active component for checking their performance in the presence of Rhodamine-B (Rh-B).

Electrochemical recognition study of dopamine

A GCE was carefully polished with alumina powder and subsequently cleaned with ethanol and deionized water. The synthesized composite material (Au–pTA) and pTA were dropped onto the GCE surface, dried in the air at room temperature and used for electrochemical measurements at room temperature. For the electrochemical recognition study of dopamine, the cyclic voltammograms were recorded at a scan rate of 50 mV/s in 10 mM of phosphate-buffered saline (PBS).

Performance study of pTA and Au–pTA for the reduction of Rhodamine-B (Rh-B) and the role of gold nanoparticles

In the first experiment, 200 mg of pTA was added to the 2.5 mL of water solution of Rhodamine-B (10−4 mol dm−3) and in the second experiment similar amount of Au–pTA was also added to the 2.5 mL of water solution of Rhodamine-B (10−4 mol dm−3) in two quartz cuvettes and the progress of the two reactions was monitored using a spectrophotometer.

Result and discussion

Characterization

The transmission electron microscopy (TEM) image (Fig. 1a) of the synthesized material (Au–pTA), using the precursors 4-(thiophen-3-yl)-aniline and auric acid, show the chain-like morphology with the average diameter of the chain about 100 nm. In the magnified TEM image (Fig. 1b), the uneven surface and the uniformly distributed dark spots within the polymer can be seen. The TEM image (Fig. 1c) of the control material (pTA), synthesized using the precursors 4-(thiophen-3-yl)-aniline and ammonium persulfate, has smooth surface and is relatively bulk in morphology as compared with the material synthesized using auric acid. A typical EDS analysis (Fig. 2a) obtained from the electron beam being focused onto a dark spot in the polymer confirms that these spots are gold particles. The higher magnified TEM image (Fig. 2b) shows that the gold particles, within the size range of 2–5 nm, are homogeneously distributed within the polymer. Formation of these gold nanoparticles in the metal–polymer composite material was found to be highly face selective, as shown in the XRD pattern (Fig. 3a). The strong (111) Bragg reflection indicates that the gold particles possess a highly oriented crystalline character. To identify the chemical state of the gold nanoparticles, X-ray photoelectron spectroscopy (XPS) measurements were carried out. The XPS spectrum of the Au 4f region is shown in Fig. 3b. The Au 4f 7/2 and 4f 5/2 spin–orbit coupling gives rise to peaks positioned at 84.3 and 87.8 eV respectively, which correspond to metallic gold [31]. The ICP-MS study indicated the presence of 1.90 wt% of gold in the composite sample, which is consistent with the amount of gold used as the precursor for the polymerization reaction.

Figure 1
figure 1

TEM images of Au–pTA [synthesized using the precursors 4-(thiophen-3-yl)-aniline and auric acid] with different magnifications. a Low magnification image show the chain-like morphology with the average diameter of the chain is about 100 nm. b The high-magnified TEM image shows the uneven surface and the uniformly distributed dark spots within the polymer. c The TEM image of the control material, pTA, synthesized using the precursors 4-(thiophen-3-yl)-aniline and ammonium persulfate

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Figure 2
figure 2

a EDS analysis obtained from the electron beam being focused onto a dark spot in the polymer confirms that these spots are gold particles. b Higher magnification TEM image shows the presence of gold nanoparticles (dark spots) on the surface of the polymer matrix

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Figure 3
figure 3

a Strong Bragg reflection towards (111) direction indicate the highly oriented crystalline character of the gold particles. b X-ray photoelectron spectroscopy (XPS) spectrum of the Au 4f region: Au 4f 7/2 and 4f 5/2 spin–orbit couplings give rise to the peaks positioned at 84.3 and 87.8 eV, respectively

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The polymerization reactions of 4-(thiophen-3-yl)-aniline using ammonium persulphate and HAuCl4 were spectrophotometrically monitored, Fig. 4A, B, respectively, (keeping the concentration of the monomer and oxidizing agents similar to the “Preparation of Au–pTA composite catalyst” section but lower in volume). The spectrum ‘a’ in both the figures is for the monomer, 4-(thiophen-3-yl)-aniline. In the Fig. 4A, after the addition of APS the bands at 360 nm are becoming prominent for π–π* transition for the pTA sample. A broadband with a shoulder-like appearance that developed gradually from 430 to 600 nm corresponds to benzenoid to quinoid transition. In Fig. 4B, the absorption peak at 420 nm has been observed due to π–π* transition for the Au–pTA sample. The shifting of the peak towards the higher wavelength region as compared with pTA sample is due to the presence of gold nanoparticles, which facilitates the π–π* transition towards the energetically favourable direction. Another broadband ranging from 500 to 700 nm is clearly visible for Au–pTA sample that corresponds both for the transition between benzenoid to quinoid form and surface plasmon absorption of the gold nanoparticles.

Figure 4
figure 4

The spectrophotometric studies show the progress of the polymerization reactions of 4-(thiophen-3-yl)-aniline in presence of A ammonium per sulphate, and B HAuCl4, respectively. The time required to reach the spectrum from b to r for (A) and from b to g for (B) was 154 and 16 min, respectively. The spectrum ‘a’ in both the figures is for the monomer, 4-(thiophen-3-yl)-aniline. C FTIR spectra for pTA (a) and Au–pTA (b)

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The presence of quinoid unit in both the samples, pTA (spectra ‘a’) and Au–pTA (spectra ‘b’), has been confirmed from the vibrational signature at 1660 cm−1 in the Fourier transform infrared spectrum (Fig. 4C). A doublet band with the peak positions at 1447 and 1410 cm−1 can be assigned to the ν3 mode of thiophene [32]. The ν3 mode is a ring vibration that consists primarily of the symmetric stretching of the C=C bonds of thiophene [33]. The vibration bands at 1108 and 1023 cm−1 are due to the aromatic C–H in-plane bending modes. The presence of gold nanoparticles intensifies the infrared plasmon excitations and therefore improves sensitivity of the spectra ‘b’. This presents a unique advantage in enhancing sensitivity in infrared absorption spectroscopy using plasmonic structures of the gold nanoparticles.

The synthesis mechanism for the metal–polymer composite material using in situ polymerization and composite formation route has been reported elsewhere [17].

Electrochemical detection of dopamine

Dopamine is an important neurotransmitter in mammalian central nervous systems and abnormal secretion has been linked to several neurological disorders. The early detection of dopamine has therefore been a subject of considerable interest and electrochemical techniques have been proved to be one of the most advantageous ways for the estimation of dopamine level. A composite of polyaniline hollow sphere and gold nanoparticles was found to be an efficient candidate for the detection of dopamine [34]. Dopamine can be electrochemically detected with high sensitivity and selectivity by modifying the electrode surface with a thin layer of poly(aniline boronic acid)–carbon nanotube composite [35]. A selective and sensitive dopamine biosensor has been reported based on the silver chloride–polyaniline nanocomposites [36]. Gold nanoparticles decorated polypyrrole and reduced graphene oxide hybrid material have been successfully used as an electrochemical sensor for detection of dopamine with the remarkable sensitivity [37]. Catalytic activity of gold nanoparticles has been widely reported for their extraordinary contribution in facilitating the electron transfer and decreasing the over-potential of the reactions [37].

In this current work, we have compared the affinity and sensitivity between pTA and Au–pTA for the electrochemical detection of dopamine. Both the samples were characterized using electrochemical impedance spectroscopy technique which is a useful method to understand the dynamics of the electrochemical process of the material. Figure 5 shows the electrochemical impedance spectra for the pTA (spectrum ‘a’) and Au–pTA (spectrum ‘b’). In the figure, a straight line is observed at lower frequencies for both samples, which is the characteristic of a diffusion-limiting step of the electrochemical process. For the sample pTA, the electron transfer resistance (1080 Ω) was significantly expanded which indicated that the polymer performs as a kinetic barrier for the electron transfer process, whereas, in Au–pTA the electron transfer resistance (700 Ω) was decreased due to the presence of gold nanoparticle that plays a key role similar to an electron-conducting tunnel and thereby promoting an efficient charge transfer mechanism.

Figure 5
figure 5

Electrochemical impedance spectra a Au–pTA-modified GCE b pTA-modified GCE within the frequency range from 1.0 MHz to 1.0 mHz

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For the electrochemical detection of dopamine, the cyclic voltammetry (CV) technique has been used in this experiment in the presence of PBS electrolyte (10 mM) under the scan rate of 50 mV s−1. To study the catalytic role of gold nanoparticles for the recognition of dopamine we have used both samples, pTA and Au–pTA, as working electrode modifiers. The Fig. 6A (inset) shows the cyclic voltammograms for bare GCE in the absence of dopamine (curve ‘a’) and the presence of 3.0 μM of dopamine (curve ‘b’). It is evident from the figure that a slight improvement of current has been noticed due to the presence of dopamine which indicates the dopamine oxidation on the bare electrode. In the Fig. 6A (main panel) when working electrode was modified with pTA, no noticeable improvement of the current value has been observed as compared with the bare electrode (cyclic voltammograms for pTA-modified GCE in the absence of dopamine, curve ‘a’, and the presence of 3.0 μM of dopamine, curve ‘b’). Figure 6B shows the steady increase of current values with increasing concentration of dopamine for the Au–pTA-modified electrode. When the dopamine concentration was 3.0 μM, a prominent anodic peak with the current value of 37 µA, has been observed at 0.68 V, curve ‘b’. With increase of dopamine concentration a steady increase of peak current and a shift of the peak position towards the higher potential values has been observed (Fig. 6B, curve b–h), which may indicate an irreversible mechanism in the oxidation process of dopamine. Voltammogram ‘a’ represents the zero concentration of dopamine. The increase of anodic peak height or current, with the increase of dopamine concentration, indicates the enhanced electron transfer which was due to the charge hopping mechanism through the metallic gold nanoparticles. The gold nanoparticles generated many active sites for charge transfer through the interface inside the electrode by making good contact with the polymer matrix [34].

Figure 6
figure 6

A (inset) Cyclic voltammograms for bare GCE in the absence of dopamine (curve ‘a’) and in the presence of 3.0 μM of dopamine (curve ‘b’). A (main panel): Cyclic voltammograms for pTA modified GCE in the absence of dopamine (curve ‘a’) and in the presence of 3.0 μM of dopamine (curve ‘b’). (Using 10 mM PBS electrolyte, under the scan rate of 50 mVs−1). B Cyclic voltammograms for Au–pTA-modified electrode with increasing concentration of dopamine: 0, 3, 6, 9, 12, 15, 18, 22 μM, respectively, curves (b–h). (Voltammogram ‘a’ is for the absence of dopamine)

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The amperometric response of the Au–pTA-modified electrode to the successive additions of dopamine was further evaluated under the optimized experimental condition. Figure 7A showed the amperometric current–time response of dopamine at 0.35 V. As illustrated in the figure, upon addition of 20 μL dopamine (0.001 M) to the stirring PBS electrolyte (pH 7.0), the oxidation current increased steeply and reached a steady-state current within an average response time of 50 s. The amperometric signal showed a good linear correlation to dopamine concentration in the range from 3.0 to 18.0 μM. The regression coefficient and sensitivity values of the Au–pTA-modified electrode, within the time interval from 50 to 300 s, are 0.9997 and 5.23 μAμM−1, respectively. In this study, the tolerance of the catalyst has been investigated by adding some potential interfering species (50 μL of 0.001 M of glucose, ribose, alanine, γ-aminobutyric acid, glutamic acid, valine and urea) and showed the irresponsive nature towards the amperometric current signal. It is important to mention that the gold–polymer system has the limitation towards the amperometric detection of dopamine in the presence of ascorbic acid as the material is also responsive for ascorbic acid towards the amperometric current signal.

Figure 7
figure 7

A Amperometric response of Au–pTA-modified GCE on successive addition of 20 μL of DA (0.001 M) in every 50 s time interval with an applied potential of 0.35 V. (Inset the regression coefficient and sensitivity values of the Au–pTA-modified electrode, within the time interval from 0 to 300 s, are 0.9997 and 5.23 μA μM−1 cm−2, respectively). B The addition of 50 μL of 0.001 M of interferences species (glucose, ribose, alanine, γ-aminobutyric acid, glutamic acid, valine and urea), a–g shows the irresponsive nature towards the amperometric current signal

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Reduction of Rhodamine-B: a hydrogen atom transfer mechanism

The proton-coupled electron transfer (PCET) is an important mechanism [38], in which charge transport plays a central role in a wide range of biology and materials-oriented research [39]. The transfer of an electron and proton is a tightly coupled process in which the electron and proton may transfer consecutively or concertedly. Concerted PCET is thermochemically more favourable than the consecutive processes involving stepwise electron transfer and proton transfer [40]. A concerted transfer may, however, occur with a lower reaction barrier and therefore proceed at higher catalytic rates [41]. Hydrogen atom [H·(≡H+ + e −)] transfer (HAT) is a reaction that involves the transfer of a proton (H+) and an electron (e −) and belongs to the proton-coupled electron transfer (PCET) types of reactions [42].

The 4-(thiophen-3-yl)-aniline molecule is a derivative of aniline and the polymeric form of it can exist in a large number of intrinsic oxidation states, similar to polyaniline. The formation of poly[4-(thiophen-3-yl)-aniline] has been monitored spectrophotometrically which confirms the gradual transformation from the benzenoid (reduced state) to quinoid state (oxidized state) with the release of electrons and protons, the mechanism of polymerization. In this experiment we have ceased the transformation process by taking out the intermediate polymer product from the solvent system and dried. When APS was used as an oxidizing agent the polymeric product was poly[4-(thiophen-3-yl)-aniline], pTA, whereas, the addition of HAuCl4 (acts as an oxidizing agent) to 4-(thiophen-3-yl)-aniline, (TA), produced polymer-stabilized gold nanoparticles (Au–pTA). The detailed mechanism of formation has been reported elsewhere [17].

The dried polymers, pTA and Au–pTA, were applied to study the reduction behaviour of Rhodamine-B (Rh-B), and a model dye molecule was also used to check the catalytic performance of the material [43–46]. Apart from the examples of photo-oxidative degradation [43, 44] and reducing agent assisted nanoparticle catalysed reduction [45, 46] of Rhodamine B, the current study is the first kind of example where proton-coupled electron transfer (PCET) types of reactions can also responsible for the reduction of dye molecule.

Figure 8 shows the spectrophotometric evidence for the reduction process of Rh-B in the presence of (A) pTA and (B) Au–pTA at different time intervals. The experiment was carried out at 30 °C at the wavelength of 554 nm which is the characteristic absorption peak of the dye. The quenching of Rh-B absorbance spectra in the presence of pTA, from (a) to (i), took 220 min (Fig. 8A), whereas in the presence of Au–pTA the quenching of Rh-B absorbance spectra, from (a) to (d), took 10 min (Fig. 8B). The prominent absorption peak observed at 565 nm in the spectrum (b) and (c) are due to the presence of gold nanoparticles and the disappearance of such peak in (d) is due to the deposition of the gold catalyst at the bottom of the cuvette. Figure 8C shows the graphical representation of absorbance (ln C t /C 0 ) as a function of time, in min., for the reduction of Rh-B in the presence of pTA (curve b) and Au–pTA (curve a) with the rate constant values of 1.8 × 10−3 and 3.07 × 10−2 min−1, respectively, and the rate constant (k) follows the trend k (pTA) < k (Au–pTA), which suggests that Au–pTA is a more efficient material than pTA for the quenching of the Rh-B spectra. The gradual quenching of the spectra indicates the chemical transformation or reduction of Rh-B (Scheme 1) and the mechanistic approach of the reduction process can be explained as follows in the light of hydrogen atom transfer reaction mechanism. In the Fig. 8B, the spectra b and c show the peak at 565 nm which is the combined peak responsible both for the plasmon absorption band of gold and for Rh-B (the characteristic band for Rh-B at 554 nm has been superimposed by the absorption band responsible for gold nanoparticles). The broad and intense absorption band in the range between 500 and 350 nm is due to the re-oxidation phenomenon for the polymer composite (Au–pTA) and the gradual decrease of the band indicates the complication of oxidation process with time and the settling down of the composite material at the bottom of the cuvette. In Fig. 8A, the non-existence of the broad absorption band within the spectral range between 500 and 350 nm is due to the slow re-oxidation process of pTA for the reduction of Rh-B.

Figure 8
figure 8

Spectrophotometric studies show the reduction of Rd-B (10−4 mol dm−3) in the presence of A pTA and B Au–pTA and at different time intervals. The time interval for each of the spectrum for A, a–i 0, 16, 34, 54, 81, 111, 144, 181 and 220 min, respectively and for B, a–d 0, 3, 6 and 10 min, respectively. C Absorption of the Rd-B (C t /C 0, where C t is the absorption maxima at different time intervals and C0 is the initial absorption maxima) as a function of time in the presence of Au–pTA (curve ‘a’) and pTA (curve ‘b’)

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Scheme 1
scheme 1

Mechanism for the hydrogen atom mediated reduction of Rhodamine-B

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As soon as the dried materials, pTA and Au–pTA, come in contact with the solution state of Rh-B the process of oxidation starts again with the release of proton (H+) and electron (e −) until the fully oxidized form of the polymer has been achieved. Now the generated hydrogen atom [H· (≡H+ + e−)] forms the σ-bond with the π-electron of C-9 carbon which subsequently allows regaining the aromaticity of ring A and acts as a driving force for the reduction reaction (Scheme 1). Loss of extended conjugation present in Rh-B is the main cause for the disappearance of the colour (quenching of the spectra) during the process of reduction reaction [47]. The energy gap between HOMO and LUMO is higher in the case of Rh-B, and hence the hydrogen atom-mediated reduction process of Rh-B is slow for pTA. The spirit of the faster reduction process lies in the introduction of the gold nanoparticles (from Au–pTA), which form a π-complex with Rh-B to shorten the energy gap between HOMO and LUMO and that can easily promote the hydrogen atom-mediated reduction process of Rh-B (Scheme 2).

Scheme 2
scheme 2

HOMO and LUMO concept for the reduction of Rhodamine-B in the presence of pTA and Au–pTA

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Conclusion

We have demonstrated a simple and efficient strategy for the synthesis of polymer-encapsulated uniformly dispersed gold nanoparticles without using any external reducing agent. For the comparative studies, we also have synthesized polymer alone using non-metallic oxidizing agent to demonstrate the effect of metallic counterpart within the polymer. By incorporating gold particles in the polymer matrix, the electron transfer resistance value for gold–polymer composite was lower than the pure polymer, and the gold nanoparticle performed the key role in the charge transfer mechanism and acted like an electron-conducting tunnel. The gold–polymer composite was used as an electrocatalyst and showed the enhanced performance for the recognition of dopamine compared with the pure polymer. The synthesized polymer (pTA) and gold–polymer (Au–pTA) acted as a reducing agent for the reduction of Rhodamine-B. The rate of reduction of Rhodamine-B was faster when Au–pTA was used as a reducing agent and indicates the catalytic effect of gold nanoparticles. We also proposed that the reduction process of the dye was driven by the hydrogen atom transfer mechanism.