A facile method to prepare noble metal nanoparticles modified Self-Assembly (SAM) electrode

ABSTRACT Noble metal nanoparticles (NPs) modified electrodes have shown promising applications in the areas of catalysis, (electro)chemical analysis and biosensing due to their unique characters. In this paper, we introduced a so-called ligand exchange method to prepare self-assembly (SAM) electrode modified with noble metal nanoparticles. The noble metal nanoparticles protected by weakly adsorbed tetraoctylammonium bromide (TOAB) were synthesized firstly, then self-assembly (SAM) dithiol-modified Au electrode (Au-SHSAM) was immersed into the solutions containing TOAB-protected nanoparticles. Due to the strong interaction between the dithiol groups on the electrode and noble metal nanoparticles, the weakly adsorbed TOAB on the surface of noble metal NPs were replaced by dithiol groups. As a result, the TOAB protected NPs were anchored on the Au-SHSAM template electrode surface by ligand exchange, obtaining noble metal NPs modified electrode with high quality and stability. By adjusting the soaking time, the coverage of nanoparticles on the Au-SHSAM electrode surface could be controlled. The morphology and distribution of noble metal NPs on Au-SHSAM surface was analysis by scanning tunneling microscope (STM), and their electrochemical property was studied by cyclic voltammetry (CV) in H2SO4 solution. The approach is proved as a universal way to prepare noble metal NPs modified SAM electrode.

ultrahigh catalytic activity towards hydrogen peroxide detection and effectively amplify the response signal, as a result, the detection limit can be as low as 0.018 ng/ml. Chen et al. [10] prepared carbon supported PtAu alloy NPs with high activity towards formic acid electrooxidation by pyrolysis of PtAu/C precursor.
The current methods of preparing NPs modified electrode including dip coating method [11], drop coating method [12], electrodeposition [13] et al. In dip coating method, electrode was immersed in sol solution containing target NPs, NPs were adsorbed on the electrode surface to form a thin film. However, the amount of NPs adsorbed on the electrode surface was changed as the nano-sol concentration changes, and the coverage of adsorbed NPs is difficult to control. In drop coating method, a certain volume of target solution was dropped on the electrode surface, a thin film was formed after the solvent was evaporated. Although the amount of adsorbed NPs can be adjusted by controlling the original solution concentration and/or drop volume, the as-formed NPs film are easily peel off from the electrode surface, results in poor stability and reproducibility. In electrodeposition method, metal ions in the solution are reduced into nanoparticles under appropriate electrode potential, forming nano-films on the electrode surface. Although as-prepared NPs modified electrode shows high catalytic stability, the size and dispersion of NPs are difficult to control due to the excessive deposition rate [14]. Kolb et al. modified the Au (111) electrode with Zn, Fe and Cu nanoclusters by electron probe [15,16], however, the method is far from universal since it needs strict control and complicated operation, the area of electrode being modified is greatly limited so the efficiency is low. In recent years, assembling the surface functional NPs onto electrode was commonly used to form modified electrode [17,18]. Generally, in this method, ligands with strong affinity to Au (such as thiol) was adopted to react with weak ligands (such as citrate [19] and phenylphenol [20]) protected Au NPs, forming functional Au NPs, and then the functional NPs were adsorbed onto the electrode surface to form NPs modified electrode. However, the electrochemical sensitivity of as-prepared modified electrode is greatly reduced due to the surface of NPs are tightly wrapped by functionalized organic molecules. The situation might be varied if the order of modification and assembly is changed. In specific, the surface of the electrode is modified with thiol first, and then noble metal NPs protected by weak-adsorbed ligands (such as tetraoctylammonium bromide (TOAB)) were anchored on the thiol modified electrode by ligand exchange. In this way, NPs modified electrode with high activity and stability can be achieved. Based on the idea, the authors prepared NPs (such as Au, Pt and Pd NPs) modified SAM electrode and studied their influences on electron transfer in outer-spherical reaction molecules (Ru (NH 3 ) 6 Cl 3 ) and inner-spherical reaction molecules (mercapto ferrocene). The long range electron transfer property of cytochrome c was also studied using the Au NPs modified electrode [21].
In this paper, we developed a method to prepare NPs modified electrode by anchoring noble metal NPs on the surface of the electrode by ligand exchange. First, the Au-SH SAM template was constructed by self-assembly of C-8 alkyl dithiol on the metal surface. The TOAB-protected metal NPs were assembly onto Au-SH SAM template electrode by strong interaction between metal NPs and mercapto groups. In this way, the electrode was successfully modified with noble metal NPs such as Au, Ag, Pt and Pd NPs. We also investigated the factors that influence the quanlity of as-prepared modified electrode.
Electrochemical test was performed by CHI 760C electrochemical station in normal three electrode electrochemical cell. Saturated calomel electrode (SCE) and Pt foil were used as reference electrode and counter electrode, respectively. Ultrapure N 2 was purged into the solutions for 30 min before electrochemical test to get rid of dissolved O 2 , during the electrochemical test, the N 2 was keep purged above the solution to maintain the inert gas atmosphere. cyclic voltammetry (CV) was performed in solution containing 0.5 mol/L H 2 SO 4 and 0.05 mol/L Ru(NH 3 ) 6 Cl 3 , the scan rate is 50 mV/s. Transmission electron microscope (TEM; Hitachi TEM-2100/UHR) was used to characterize the morphology and structure of NPs, scanning tunneling microscope (STM) was used to characterize the surface morphology of electrode by constant current mode with Pt-Ir tip.

Preparation of TOAB-stabilized Au, Ag, Pt and Pd nanoparticles
Method of preparing TOAB-stabilized Au, Ag, Pt and Pd NPs were reported elsewhere [22,23], in brief, 15 mL toluene solution containing 5 mmol/L TOAB was added into 50 mL round-bottomed flask, then 5 mL solution containing 0.6 mg/mL metal precursor was added and stirred for 30 min, TOAB acts as phase transfer agent, which makes the metal ion transferred from water phase to organic toluene phase. The color in the upper liquid layer comes from metal ion dissolved in toluene. The metal ion was reduced by adding certain among of ice cold 0.1 mol/L NaBH 4 solution, the amount of NaBH 4 was depends on the metal precursor. After stirring for 2 h, NP sol dispersed in the upper layer of toluene was obtained by filtering through a separatory funnel, the residual NaBH 4 in toluene was removed by repeatedly washed with water. The as-prepared NPs were stored in refrigerator (» 4 C) for further use.

Preparation of thiol template electrode and process of nanoparticles self-assembly
Preparation of thiol template electrode and subsequently process of NPs self-assembly on thiol template electrode was illustrated in Figure 1. In first, fresh polishing 4 mm Au electrode was immersed in ethanol solution containing 4.26 mmol/L 1,8-octanedithiol for 12 h, densely packed monomolecular layer SH SAM was formed by 1,8-octanedithiol selfassembly, the residual physical adsorbed thiol molecules were removed by washing with large amount of ethanol and water. The compaction degree of the thiol assembly layer was examined by electrochemical cyclic voltammetry using Ru(NH 3 ) 6 Cl 3 as probe molecule. The as-prepared SH SAM electrode was then immersed into NPs sol solutions, the original TOAB coating layer on the NPs was replaced by thiol. As a result, the NPs were anchored on the SH SAM electrode due to stronger interaction between NPs and thiol molecule than that between NPs and TOAB. After washing the electrode with water, the modified electrode (Au-SH SAM -NP) was successfully prepared.

Preparation and electrochemical characterization of thiol template electrode
To characterize the electrochemical property of adsorbed thiol, the CV curves of the surface adsorbed thiol in H 2 SO 4 solution were recorded, as illustrated in Figure 2. The electrochemical windows were chosen as 0.0 » +1.35 V and ¡0.50 » 0.0 V to study the electrooxidation and desorption of thiol on electrode surface, respectively. When the electrochemical window is 0.0 » +1.35 V, in the first cycle of CV curves, a sharp peak at around 1.1 V was observed in the positive-going scan as showed in Figure 2a, the oxidation peak corresponding to the oxidation of mercapto group (-SH), which is consistent with literature [24]; the relatively small peak at 1.3 V corresponding to the surface oxidation of bared Au, since most of the Au surface was cover by thiol, the oxidation peak of Au surface was smaller than that of thiol. On the second cycle of CV curves, the peak at 1.1 V was disappear while the peak at 1.3 V was largely increased, indicated than the adsorbed thiol was totally oxidized in the first cycle. Meanwhile, no desorption peak was observed in the CV curve range from ¡0.45 V to 0.0 V after thiol was oxidized, as showed in Figure 2b. When the CV of the fresh absorbed electrode was performed between ¡0.45 V and 0.0 V, a reduction peak at around ¡0.35 V was observed in the negativegoing scan, which corresponding to the desorption of thiol as illustrated in Figure 2c. The peak current is gradually decreased at the sweep cycle number increased due to the reduced coverage of thiol, meanwhile, desorption of thiol became more difficult as the coverage of thiol became low, which evidence by the negatively shifted of reduction desorption peak in the CV curve as the cycle number was increased. After the thiol was totally desorbed from the Au surface, no oxidation peak in the CV curve range from 0.0 V to 1.35 V was observed, as showed in Figure 2d. Figure 3 illustrates the results of CV and STM characterization before and after the thiol molecules were assembled on the Au surface. Ru(NH 3 ) 6 Cl 3 was selected as probe molecule to characterize whether the modified Au electrode surface was covered by a monolayer thiol molecule, the results are display in Figure 3a. On unmodified electrode, the redox peaks are clearly observed in the CV curve. The redox current density in the CV curves was decreased after the Au electrode was immersed in thiol solution for 1 h, and the reversibility of the reaction is also deteriorated as indicated by the larger potential difference between the oxidation peak and the reduction peak of Au. The redox current is continuous decreasing as the immersion time increasing, after 6 h immersion, the redox peaks of Ru(NH 3 ) 6 Cl 3 was totally disappeared, indicating that the Au surface was totally covered by thiol molecule and a monolayer thiol was absorbed on the Au electrode surface, similar to the behavior of alkyl thiol modified Au electrode [20]. The thiol molecules on the Au electrode surface form a densely non-conductive polymer film, preventing the electrochemical active species penetrating from the membrane into the electrode surface, thus hindering their oxidation and/or reduction on the electrode. In addition, the charging/discharging current in double-layer region was greatly suppressed after the Au electrode was immersed in thiol solution, this might be due to the strong van der Waals forces between the template thiol molecules [25]. After electrochemical polishing and hydrogen flame annealing, the electrode surface was clean, and the surface steps can be clearly seen as showed in Figure 3b. After assembled thiol molecules on Au surface, many small less-regular pin-hole structure was observed as showed in Figure 3c, due to the strong strength of Au-S bond, Au atoms were easily pulled out by thiol molecule in disordered vibrations, forming the pin-hole structure [26]. The dithiol molecules modified on the electrode surface were arranged in a short-range order, which can be observed by zoom in the dotted box in Figure 3c, as display in Figure 3d.

Anchoring metal nanoparticles by thiol SAM template
In order to verify the assembly of NPs on the electrode surface, the electrochemical properties of Au NPs, Ag NPs, Pt NPs and Pd NPs in H 2 SO 4 solution was characterized by electrochemical cyclic voltammetry, the results are display in Figure 4. The CV curve of Au NPs modified electrode was similar to that of bare Au electrode, the difference is that  a small shoulder peak was appeared at less positive position of the main peak of bulk Au electrode, which corresponding to the oxidation peak of Au NPs. The oxidation peak potential of Au NPs was less positive than that of bulk Au, because the surface energy of Au NPs was higher compared with that of bulk Au due to the size effect of nanoparticles [27]. On Ag NPs modified electrode, peak at 0.4 V in the positive-going scan and 0.3 V in the negative-going scan were observed, corresponding to the oxidation and reduction of Ag and AgO x , respectively, which are in accordance with literature [28]. On Pt NPs modified electrode, the onset potential of hydrogen evolution reaction (HER) was negatively shifted due to the excellent catalytic activity of Pt NPs [29]. Obvious peak at ¡0.3 V corresponding to HER was observed, similar to that of Pt electrode in H 2 SO 4 solution. Peaks corresponding to oxidation/reduction of Pd NPs were also seen in the CV curve of Pd NPs modified electrode in H 2 SO 4 solution, similar to the results reported elsewhere [30]. Thus, the electrochemical properties discuss above confirms the successfully anchoring noble metal NPs on the thiol modified electrode. The morphology of assembled noble metal NPs on the electrode surface was characterized by ex situ STM to further study the anchoring of metal NPs by thiol SAM template. As illustrate in Figure 4, after immersing the thiol-modified electrode in different noble metal nano-sol solution for a certain period of time, the electrode surface was fully covered by the target NPs, and the NPs with uniform size were tightly arranged in a dispersed monolayer state, and no agglomeration was observed.

Regulation the coverage of nanoparticle on electrode
In order to modify the electrode with a monolayer of NPs, the influence of soaking time on the coverage of NPs on the modified Au electrode was investigated. Figure 5 illustrates the STM images of Au-SH SAM electrode after soaking in Au and Pt nano-sol solution with different time. In the case of Au NPs, when the soaking time is short (less than 0.5 h), the NPs attached to the electrode surface are sparse, only a sub-monolayer of NPs was adsorbed on the Au-SH SAM electrode. When the soaking time was increased to 1 h, the amount of NPs on the electrode surface are increasing rapidly, a monolayer of NPs were formed. When the soaking time was further prolonged to 3 h, the thickness of adsorbed NPs on the electrode surface was increased and the NPs on the electrode were significantly accumulated. While for the Pt NPs modified electrode, the situation is quite similar to that of Au NPs modified electrode.

Conclusion
The Au, Ag, Pt and Pd NPs colloidal solutions with suitable size and good monodispersion were synthesized by using the weakly-adsorbed tetraoctylammonium bromide as protecting agent. The Au-SH SAM -NP modified electrode was successfully prepared by anchoring TOAB-protected metal NPs to the thiol-modified electrode surface with ligand exchange method. The Au-SH SAM -NP modified electrode was characterized by CV and STM. This preparation method is facile and universal, the coverage of the NPs on the electrode surface can be carefully controlled by adjusting the soaking time in the NPs sol solution. Noble metal NPs modified electrode shows excellent performance in electrochemical process. What's more, the noble metal NPs modified electrode is a promising material in bioelectrochemical sensor, which shows great potential in the application of biological detection.

Disclosure statement
No potential conflict of interest was reported by the authors.