Size Effects of Pt Nanoparticle/Graphene Composite Materials on the Electrochemical Sensing of Hydrogen Peroxide

The electrochemical detection of hydrogen peroxide (H 2 O 2 ) has attracted much attention recently. Meanwhile, the size of nanoparticles which significantly influences electrocatalytic activity is crucial for electrocatalysts. Hence, we prepared five different size-selected Pt/graphene-modified glassy carbon (GC) electrodes to characterize H 2 O 2 level via electrochemical measurements. During the preparation of the nanocomposites, size-selected Pt nanoparticles (NPs) with the mean diameter of 1.3, 1.7, 2.9, and 4.3 nm were assembled onto the graphene surfaces. The electrochemical measurement results are size-dependent for Pt NPs when sensing H 2 O 2 . When all cyclic voltammogram results from various electrodes are compared, the Pt-1.7 nm/G-modified GC electrode has the highest reduction current, the best detection limit, and the best sensitivity.


Introduction
Since reliable and fast determination of biomolecules is important in many areas such as biotechnology, clinical diagnostics, and food industry, the development of biosensor has attracted extensive attention recently.In general, there are enzymatic and nonenzymatic biosensors in the literature because the enzyme is easily being affected by the environmental factors such as temperature, humidity, and pH values.In addition, the immobilization of enzyme is a complicated and expensive process.Therefore, the nonenzymatic biosensors start to catch the scientists' eyes and attention.The large surface area and excellent electrical properties of graphene allow it to be used in many applications [1][2][3][4][5][6].For example, it can connect between the redox centers of an enzyme or protein and an electrode surface.Rapid electron transfer facilitates accurate and selective detection of biomolecules.Its unique structure and properties, such as high specific surface area, high mechanical strength and conductivity, and its relatively low price make graphene suitable for potential applications.In our lab, we used graphene, Pt/graphene, CuO/graphene, graphene oxide nanoribbons, and multiwalled carbon nanotube/graphene oxide nanoribbon coreshell heterostructures to detect biomolecules in the past five years [7][8][9][10].Here we want to utilize the same graphene-based materials to further monitor other biomolecules.
H 2 O 2 is a chemical used widely in the food, pharmaceutical, paper, and chemical industries.H 2 O 2 is also one of the products of reactions catalyzed by enzymes in many biological and environmental processes.Therefore, the development of a biosensor for detecting H 2 O 2 is important [11][12][13][14][15][16][17][18][19][20][21][22][23].The electrode materials for H 2 O 2 biosensor in the literature can be categorized as polymers, carbon nanotubes, graphene, nanoparticles, their composite materials, and others.After immobilizing horseradish peroxidase (HRP) onto the composite, the H 2 O 2 biosensor could be used as a component for investigating bioelectrochemical activity [11][12][13].Fan et al. developed a new kind of enzymatic biosensor using biomimetic graphene capsules (GRCAPS) in 2015 [14].Polybenzimidazole (PBI), polyamic acids (PAAs), benzothiazole (BT), benzoxazole (BO), and their composites as polymers were used to modify gold electrode to determine H 2 O 2 in 2011 [15,16].Li  NPs composite modified electrode [18].H 2 O 2 showed a better electrochemical response at the nitrogen and boron codoped graphene modified GC electrode (GCE), much higher than that of graphene solely doped with N atoms (N-G) or with B atoms (B-G) [19].The Prussian blue nanocubesnitrobenzene-reduced graphene oxide nanocomposites/GCE showed good electrocatalytic ability for the reduction of H 2 O 2 with good stability and selectivity [20].3D graphene foam supported PtRu on Ni foam exhibited an excellent electrocatalytic activity toward the H 2 O 2 detection [21].The activity of MoS 2 NPs toward the reduction of H 2 O 2 released by cells was demonstrated in 2013 [22].The as-produced AuCu nanowires have been explored toward the detection of H 2 O 2 [23].
There are many important factors that influence the catalytic activity of catalysts.One of the very important parameters is the size of the nanoparticles.Although there are studies using nanoparticles on nanocarbons for sensing H 2 O 2 , the effects of particle size remain unclear.Therefore, in this study, we try to investigate the size effects of Pt nanoparticle supported on graphene.The electrochemical detection of H 2 O 2 was used to evaluate the properties of these graphene-supported Pt catalysts.The size-dependent electrochemical properties will be displayed and discussed in this study.

Material and Methods
2.1.Chemicals.Platinum(IV) chloride (PtCl 4 , 99%) was purchased from Acros Organics.Ethyl glycol was purchased from J.T. Baker.Nafion (DuPont, 5 wt.%) was used to generate the ink.NaOH and H 2 O 2 were obtained from Sigma.All solutions were prepared with deionized water with a resistivity of 18 MΩ/cm.

Preparation of Pt Colloidal Solution.
Pt nanoparticles were synthesized using the polyol method reported in detail elsewhere [24][25][26][27].In short, 0.4652 g PtCl 4 was dissolved in 50 mL ethylene glycol.In order to control the size of the particles, the appropriate amount of sodium hydroxide (NaOH) was added to the PtCl 4 solutions.The mixture was then stirred at room temperature for 30 min with rotational speed of 600 rpm, heated to 160 ∘ C for 3 hr, and finally allowed to cool down to room temperature, forming a Pt colloidal solution (1.3, 1.7, 2.9, and 4.3 nm).The NaOH concentrations for Pt colloids of 1.3, 1.7, 2.9, and 4.3 nm are 0.6, 0.4, 0.3, and 0.1 M, respectively.

Deposition of Pt Nanoparticles on Graphene.
Graphene oxide powders were prepared following Staudenmaier's method and reduced to graphene powders by annealing at 1050 ∘ C under an argon atmosphere.20 mg of graphene powders was mixed with the Pt colloidal solutions in a 40 mL solution containing 2 M sulfuric acid and ethylene glycol [25][26][27].The volume ration between sulfuric acid and ethylene glycol is 1 to 1.The Pt ratio is controlled to be around 20 wt.% for Pt-G catalyst.The solution was then stirred for 24 h and then sonicated using an ultrasonic processor (Part number Q700) for 15 min.The resulting solution was filtered to recuperate the catalyst.Four Pt-G catalysts with different average particle sizes were obtained in this manner.

Electrode Preparation and Electrochemical Measurements.
The catalyst ink for electrochemical measurement was prepared with the Pt-graphene powders.3 mL deionized water, 2 mL ethanol, 60 L Nafion, and 6 mg Pt-graphene powders were sonicated to make the ink [25][26][27].Potentiostat/galvanostat (CHI 405A) was used for electrochemical measurements.The working electrode was 3 mm-diameter glassy carbon (GC) disc electrode on which 10 L of the catalyst ink was deposited and dried at room temperature.A silver/silver chloride (Ag/AgCl) electrode and a large surface area platinum electrode were used as the reference and counterelectrode, respectively.All potentials in this study are reported with respect to the Ag/AgCl electrode.

Controlled Synthesis of Size-Selected Pt Colloids. Figure 1
displays TEM images of Pt nanoparticles with different average sizes varying between 1.7 and 4.3 nm and their histograms.The particle sizes were controlled by changing the pH of the PtCl 4 solution dissolved in ethylene glycol.The histograms show the size distribution of the particles with an average diameter which was taken over 300 individual particles from the TEM pictures.The NaOH concentrations of ethylene glycol with dissolved PtCl 4 are 0.1, 0.3, 0.4, and 0.6 M for making 4.3, 2.9, 1.7, and 1.3 nm Pt colloids.When NaOH concentrations become large, the mean diameters of Pt colloids get small.It is worthwhile to mention that the sequence for mixing PtCl 4 solution is very important.Before adding any NaOH, PtCl 4 needs to be dissolved in ethylene glycol completely.If PtCl 4 was added to the ethylene glycol already with NaOH, there will be no size control effect though Pt nanoparticles can still be formed.

Material Characterization of Graphene-Supported Pt
Nanoparticles. Figure 2 shows the TEM images of the 4.3, 2.9, 1.7, and 1.3 nm particles supported on single graphene sheets.The small dark spots are the Pt nanoparticles adsorbed on multilayered graphene as the background that is relatively gray compared to the white holey in other regions of the Cu grid.The wrinkles on single graphene pieces randomly appear in the pictures.Although most of the time the nanoparticles are uniformly distributed on graphene surfaces, sometimes the aggregates could be formed like the two areas in Figure 2(d).This may be owing to the very high surface area of small that tend to reduce the total surface energy in the system.It is suggested that the functional groups on graphene surfaces will help further disperse the small Pt nanoparticles. 3 illustrate the reduction of H 2 O 2 for each catalyst.In general, the reduction currents gradually increase when lowering the potential after 0.2 V and there is a main reduction peak for each catalyst.The reduction peaks are located around −0.5 V for four catalysts.This is similar to the oxygen reduction reaction (ORR) for the cathode of a fuel cell.The reduction of H 2 O 2 is a two-electron process that has low electron transfer number compared to ORR.For the size lower than 10 nm but higher than 1.5 nm, there are three catalysts named Pt-4.3nm/G, Pt-2.9 nm/G, and Pt-1.7 nm/G.Among these three catalysts, the reduction current will increase along the decrease of particle sizes.Hence the Pt-1.7 nm/G catalyst has the highest reduction current that is 2.5 times higher than that of the Pt-2.5 nm/G one.For the particle sizes more than 10.0 nm and smaller than 1.5 nm, the reduction current becomes smaller than that of Pt-1.7 nm/G catalyst.4 and 5.After adding analyte solutions with different concentrations, the reduction currents were monitored at a fixed potential of −0.5 V.In Figure 4, the same trend as in Figure 3 can be observed.With the same concentration of analyte, the Pt-1.7 nm/G catalyst has the highest reduction current.The linear regression equation is given by  H 2 O 2 = −0.089 H 2 O 2 + 1.392, with a correlation coefficient of  = 0.996.The sensitivity of Pt-1.7 nm/G catalyst is 1264.6 A mM −1 cm −2 .This number is higher than 254.8 (Pt-1.3nm/G), 565.6 (Pt-2.9nm/G), and 560.8 (Pt-4.3nm/G) A mM −1 cm −2 .The sensitivity numbers among all catalysts are size-dependent.

Conclusions
In summary, the particles' size of Pt colloids can influence the electrochemical properties of Pt/G catalysts for electrochemical sensing of H 2 O 2 .The Pt-1.7 nm/G catalyst has the highest sensitivity up to 1264.6 A mM −1 cm −2 , rapid response time of 1.69 s, low detection limit, and good ECSA

Figure 4 :
Figure 4: Amperometric responses of four Pt/G-modified GC electrodes after the subsequent addition of H 2 O 2 in a 0.1 M PBS solution at the potential of −0.5 V.
et al. fabricated a nonenzymatic H 2 O 2 sensor by utilizing MWCNTs as the matrix for electrodepositing of Pt nanoparticles [17].Karuppiah et al. have constructed a novel glucose and H 2 O 2 biosensor based on graphene/Co 3 O 4 Response of H 2 O 2 .The amperometric responses of the modified GC electrode to H 2 O 2 are depicted in Figures