Reactions of alkyl-radicals with gold and silver nanoparticles in aqueous solutions

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Reactions of Alkyl-Radicals with Gold and Silver Nanoparticles in Aqueous Solutions Tomer Zidki a, Haim Cohena,b and Dan Meyerstein*a,b

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DOI: 10.1039/b000000x [DO NOT ALTER/DELETE THIS TEXT]

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15

Silver and gold nanoparticles are very efficient catalysts for the dimerization of methyl-radicals in aqueous solutions. The rate constants for the reaction of methyl-radicals with the nanoparticles were measured and found to approach the diffusion controlled limit. The results thus suggest that alkyl-radicals, also not reducing ones, are scavenged by these nanoparticles. This might explain the role of these nanoparticles in medical applications. 20

Introduction Metal nanoparticles (NPs) and silver in particular have been used as medicinal prescriptions since ancient times1-3, though their actual merit has not yet been proven4. For many years, the properties of metal NPs as catalysts have attracted the 25

attention of many scientists. This is mainly due to the fact that their surface-to-volume ratio is much larger than that of the traditional catalysts. It is suggested by various groups5 that metal NPs are very efficient catalysts because of the presence of a large number of atoms at the surface, which are available 30

to chemical transformation of substrates. It is well known that metal NPs react with strong single electron reducing radicals in aqueous solutions6-9. There are several plausible mechanisms for the reactions between these radicals and metal NPs. 35

When the NP acts as a catalyst for oxidation reaction of the radical, R·, dihydrogen is evolved and in water ROH is formed via the following mechanism9:

Reaction (1) may be repeated n times to yield (NP)n-.

Reaction 2 may be repeated m times to yield (NP) ―Hm(n-m)-. 40

However the radical might also oxidize the NP via:

If reaction (5) is repeated m times forming (NP)m+, it will be followed by:

Where M is the metal of which the NP is composed and n is 45

the lowest common oxidation state of this metal. It should be noted that disproportionataion of ·R might be induced by the reduction of ·R by the (NP)m- or the oxidation of ·R by (NP)m+. There is another possibility, i.e., the catalytic formation of 50

R2, or disproportionataion of ·R on the surface of the metal NP, i.e. a process that does not involve a redox process of the NP. R2 might be formed via one of two plausible mechanisms, which were proposed by Henglein7b:

a Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel. b Department of Biological Chemistry, College of Judea and Samaria, Ariel, Israel. Fax: (972)-3-9067440; Tel: (972)-3-9066153; E-mail: [email protected]

·R + (NP) → R- + (NP)+

R- + H3O+(H2O) → RH + H2O(OH-)

(5)

(6)

·R + (NP) R+ + (NP)- (1)

(NP)n- + H3O+(H2O) (NP)―H(n-1)- + H2O(OH-) (2)

(NP) ―Hm(n-m)- → (NP) ―Hm-2

(n-m)- + H2

R+ + H2O → ROH + H+

(3)

(4)

·R + (NP) → R―(NP)

R―(NP) + ·R → (NP) + R―R / R+ + R-

(8)

(9)

(NP)m+ → Mn+ + (NP)(m-n)+ (7)

CH3 H3C

CH3

H3C

CH3

CH3CH3CH3CH3H3C

H3CH3C

H3C CH3

NPH3C

H3C

H3C

H3C

H3C

NP

H3C

NP

CH3

CH3

CH3

CH3

CH3

CH3

CH3CH3

+ n +

+

NP

H3C CH3

ORNP

NP CH3

H3C

NP

NP

H3C CH3+ +

Two plausible mechanisms for catalytic ethane formationon metal nanoparticles:



2 | [journal], [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year]

Analogous reactions were observed for the reactions of low 55

valent transition metal complexes with alkyl-radicals in aqueous solutions10, 11. When several radicals react with the NP followed by radical migration on the surface to produce R2 via dimerization: 60

Reactions of metal particles with radicals have recently been investigated12. Metal powders react with ·CH3 radicals to yield CH4 (reduction of the radical) or CH3OH (oxidation of the radical) depending on the nature of the metal. II. When methyl-radicals are formed, at a low dose rate in the 65

presence of (CH3)2S=O, methane and ethane are formed via12:13

The present paper studies the effect of metal NPs (gold and silver) on the fate of methyl-radicals formed by irradiating aqueous solutions of (CH3)2S=O (DMSO). 70

Experimental Materials All the chemicals were of A.R. grade and were used without further purification. The water used was deionized and was further purified by a Millipore Milli-Q setup with a 75

final resistivity of >10 MΩ. Instrumentation UV-Vis measurements were carried out using a Hewlett-Packard Diode Array spectrophotometer model 8452A, which enables measurements in the range of 190-820 nm and 80

resolution of ± 2 nm. TEM analyses were performed using a Tecnai 12 G2 TWIN TEM (FEI) acc. Volt. 200 kV of Philips company. The NP solutions were dried on special grids. The grids were Lacey Formvar/Carbon, 300 mesh, Copper of Ted Pella company. Electrophoresis analyses were performed 85

using a homemade14 U-shaped glass tube with a third “arm” in the center. Copper electrodes are attached to the side arms of the U and the central arm can be sealed with a stocock. For further coverage of this apparatus, see ref. 14. Irradiations were performed in a 60Co gamma source of Noratom 90

Gammacell, which emits γ-rays of 1.1 MeV. The dose rate of the 60Co source as determined by the Fricke dosimetry15 was 3.0 Gy min-1. The gases were analyzed using a HP 5890 GC fitted with a FID detector (Poropaq QS GC column 10 Ft 1/8 in, Supelco). The carrier gas was He (30 mL min-1, T = 70 95

ºC). Preparation of the metal NPs

The NP solutions are irradiated and thus are in contact with a variety of radicals, which are highly reactive species. 100

Therefore the solutions should be as “clean” as possible from any reagent and especially from entities containing C-H, N-H and S-H bonds (that exist in common stabilizers), which can react with those radicals. For that reason the silver NPs suspension was prepared according to Creighton’s 105

procedure16, by adding at once an ice-cold aqueous solution containing NaBH4 (30 mL, 2x10-3 M), with vigorous stirring, to an aqueous solution of Ag2SO4 (10 mL, 1x10-3 M of Ag+). Color changes were observed immediately after the NaBH4 solution was added (black → orange → yellow). Since the 110

resulting NPs suspension was somewhat unstable (the UV-Vis spectrum of the solution was broadened and red shifted after a while), an aqueous solution of NaCl (0.4 mL, 0.01 M) was added after the solution turned yellow, in order to stabilize the silver NPs. The solution was stirred until it reached room 115

temperature. The UV-Vis spectrum of the colloidal silver solution with and without the NaCl is shown in Fig. 1. The pH of this solution was 9.6 ± 0.1, and its wavelength of maximum absorption was 394 ± 2 nm. The NaCl was added also to verify that no Ag+ is left in the solution. If Ag+ is present in 120

the solution, precipitation of AgCl is observed. The gold NPs were prepared similarly. An ice-cold aqueous solution contaianing NaBH4 (30 mL, 2x10-3 M) was added at once, with vigorous stirring, to an aqueous solution of HAuCl4 (10 mL, 1x10-3 M). The solution turned red, and was stirred until 125

it reached room temperature. The pH of this solution was 8.0 ± 0.1 and the wavelength of maximum absorption was 518 ± 2 nm. The excess of NaBH4 reacts with water to yield borate, which does not react with the radicals (see blank in Table 1).

It can be seen from Fig. 1 that upon addition of NaCl, the 130

band-width of the plasmon absorption band of the silver NPs is narrower and blue shifted relative to the absorption band without the NaCl addition. This means, according to Mie theory17, a lower size distribution and smaller particles, respectively. However, TEM analysis suggests that the 135

addition of NaCl does not affect considerably the particle size distribution. Thus the spectral shifts are attributed to Cl- adsorption to the particles which stabilizes them. The precision of the radiolytic results was higher when NaCl is added to the silver NPs. 140

The NPs obtained are negatively charged according to electrophoresis analyses. The electrophoresis analysis is

R―(NP) + (n-1) ·R → (NP)―Rn

(NP)―Rn → (NP)―Rn-2 + R―R / R+ + R-

(10)

(11)

·CH3 + (CH3)2S=O → CH4 + ·CH2S(O)CH3

k1212 = 100 M-1s-1

·CH3 + ·CH3 → C2H6

k1313 = 1.6 x 109 M-1s-1

(12)

(13)

00.5

11.5

22.5

33.5

4

200 300 400 500 600 700 800

λ/nm

Abs

orba

nce/

AU Silver NPs with NaCl

Silver NPs with noNaCl

Fig. 1 UV-Vis spectra of colloidal silver solutions.

(One day after preparation)

I.




performed as follows: the U-shaped tube of the apparatus is filled with tap water; NP suspension is added gently (so that the NPs will not mix with the water) through the middle arm 145

stopcock; when the water reaches the electrodes, the stopcock is sealed; a potential of 200 V is applied between the electrodes. In this experiment the NPs (silver and gold) migrate towards the anode due to the applied electric field. 150

Preparation of ·CH3 radicals Methyl-radicals were produced in situ by the Veltwisch et al. procedure18. When ionizing radiation (γ-radiation) is absorbed by a dilute aqueous solution the following initial products are formed19: 155

Where the numbers given in parentheses are G values (G values are defined as the number of molecules of each product per 100 eV of radiation absorbed by the solution). In concentrated solutions the yields of the radicals are somewhat higher and those of H2O2 and H2 are somewhat lower18. In 160

N2O-saturated solutions the hydrated electron is converted into the hydroxyl-radical via20:

At pH > 3 the hydrated electrons react with N2O yielding ·OH as the major radical. The methyl-radicals were formed via the following 165

reactions in N2O-saturated solutions18:

Reactions between the metal NPs and the methyl-radicals

Small glass bulbs (15 mL) containing 5 mL solution of the NPs (one day after preparation, thus all excess of NaBH4 has been decomposed) and 0.1 M (CH3)2S=O were sealed with a 170

rubber septum and deaerated by bubbling N2O for 15 min. The blank solution was an aqueous solution of (CH3)2S=O with no NPs. Those bulbs were irradiated for two h using the gamma source. After the irradiation, the gas phase was analyzed using a GC. As the NPs have a large surface area the reported rate 175

constants are reported both as M-1s-1 and as (cm2/L)-1s-1, i.e. the heterogeneous rate constant which is proportional to the surface area. Size measurements 180

TEM samples were prepared as follows: a drop of the NP solution is placed by a capillary onto a Formvar/Carbon coated copper grid (placed on a filter paper to remove the

excess solution); the sample stands for a while to evaporate the water at room temperature. TEM micrographs of the NPs 185

are shown in Fig. 2.

Calculation of [NP]. The gold [NP] was calculated (eq. 187b, 19) using the concentration of [AuCl4-] and the average radius of the NPs (18 Å) as measured by the TEM, Fig 2a.

Where: n is the number of metal atoms in a NP; MNP is the 190

mass of the NP; mAtom is the mass of the atom; RNP is the average metal NPs’ radius; ρ is the density of the metal, and equals 10.5 and 19.3 g cm-3 for silver and gold respectively21; NA is Avogadro's number; MW is the molar weight of the metal; C is the concentration of the metal ion precursor and 195

equals 2.5x10-4 M. The gold [NP] was found to be (1.7 ± 0.5)x10-7 M. Because of the polydispersity of the silver NPs, their concentration has to be calculated differently (eq. 20) according to the histogram in Fig. 3. 200

where: N is the average number of atoms in a NP; N is the total number of silver NPs in Fig. 3; ntotal is the total number of silver atoms that constitute the N NPs in Fig. 3. The silver [NP] was found to be (7 ± 2)x10-9 M.

Nn

CNC]NP[

total

⋅== (20)

Fig. 2 TEM micrographs of the NPs: (a) Gold (b) Silver.

nC]NP[

MN

3R4

mM

nW

A3NP

Atom

NP

=

⋅ρ⋅π

==

(18)

(19)

H2O →−γ e, ·H (0.60), ·OH (2.7), e-

aq (2.6),

H2O2 (0.7), H2 (0.45) (14)

e-aq + N2O + H2O → N2 + ·OH + OH-

k15 = 8.7 x 109 M-1s-1

(15)

·OH + (CH3)2S=O → (CH3)2·S(O)OH

k16 = 7.0 x 109 M-1s-1

(CH3)2·S(O)OH → (CH3)S(O)OH + ·CH3

k17 = 1.5 x 107 s-1

(16)

(17)

0

10

20

30

40

50

60

70

0-2 2-4 4-6 6-8 8-10 10-12

12-14

14-16

16-18

18-20

20-22

22-24

24-26

26-28

28-30

diameter (nm)

Num

ber o

f NPs

Fig. 3 Size distribution of the silver NPs.




205

Calculation of [NP]* in [cm2/L]. The surface area per liter of the gold NPs, [Au]*, was calculated according to eq. 21.

The surface area per liter of the silver NPs, [Ag]*, was calculated according to eq. 22, 23.

Where Stotal is the total surface area of the particles whose size 210

distribution is shown in Fig. 3. The gold and silver [NP]* was found to be (40 ± 12)x103 and (17 ± 5)x103 cm2/L, respectively.

Results and Discussion After the solutions were irradiated, the main products were 215

methane and ethane, according to GC analysis. The GC results are summarized in Table 1. From the results of Table 1, it can be seen that the presence of metal NPs increases the ethane yield and decreases the methane yield. The metal NPs probably catalyze the ethane production by trapping methyl-220

radicals and acting as a template reagent for dimerization of the radicals bound to their surface (reaction 25 or 26). When lower concentrations of metal NPs are irradiated, the ratio [CH4]/[C2H6] is larger, as a smaller percentage of the radicals is trapped by the NPs. This result indicates that most of the 225

methane is not produced on the NP surface. Not surprisingly, the main product of the reactions of alkyl-radicals with metal surfaces in the gas phase22 is also a dimerization of the radicals. After the irradiation of the gold NPs, their plasmon absorption has been red-shifted. It is interesting to note that 230

irradiation of the colloidal solutions induces coagulation and precipitation of the NPs (the precipitation of the metal occurs within 2 weeks in the irradiated solutions).

Since the reaction between the methyl-radicals and the metal NP is very fast (see below), and the metal NPs 235

concentration is considerably higher than the steady-state concentration of the methyl-radicals ([R·]ss ≤ 2x10-9 M in the absence of the NPs) and reaction (13) obeys a second order rate low, it can be assumed that this reaction does not contribute to the ethane production. Therefore the products 240

are produced via the following set of reactions:

and at most small contribution of reaction (27)

With the assumption that k24 is independent of n, and that k24 = k26 or that k26 is negligible and that reaction (27) does not occur, one can calculate the rate constants k24. To calculate 245

k24, the steady-state approximation has to be applied for the methyl-radicals. The rate of formation of the radicals is 3x10-8 Ms-1 under the experimental dose rate. Therefore the steady-state approximation gives:

[·CH3]ss = steady-state concentration of the ·CH3 250

The rate constant k24 can be calculated from the results, Table 1, using equation (29) and Fig. 4.

NSN]NP[]Ag[

R4

totalA*

N

i

2i

⋅⋅=

π∑

(22)

(23)

[Au]* = 4πR2NP·[NP] ·NA (21)

·CH3 + (CH3)2S=O → CH4 + ·CH2S(O)CH3

k12 = 100 M-1s-1

(12)

Table 1 The effect of metal NPs on the CH4 and C2H6 yields

Experimenta G(CH4) G(C2H6) Gtotal(·CH3)b G(CH4)/G(C2H6)

blankc 3.5 1.0 5.5 3.5 borate blankd 3.7 1.1 5.9 3.4 [Ag]NP/3e 3.1 1.2 5.5 2.6 [Ag]NP/2 2.3 1.4 5.1 1.7 [Ag]NP 1.3 2.4 6.0 0.55 [Au]NP/10f 2.2 1.8 5.8 1.2 [Au]NP/7 1.6 2.2 6.1 0.75 [Au]NP/5 1.2 2.6 6.5 0.45 [Au]NP

0.4 3.0 6.4 0.15 a All solutions contained 0.1 M (CH3)2SO at pH 8 (for gold solutions) or 9.5 (for silver solutions) and were N2O-saturated. The solutions were irradiated by a dose of 360 Gy. b Gtotal (·CH3) = G(CH4) + 2G(C2H6). c Other blanks of each of the materials were measured and there were no detectable amounts CH4 and C2H6, i.e. the source of the organic gases is only DMSO, and the composition of the products wasn’t changed unless metal NPs were present. d NaBH4 solution, with the same concentration as at the NPs solutions, which was kept overnight so that decomposition into borate was completed. e All silver NPs are with 1.0x10-4 M NaCl before the dilution. [Ag]NP = (7±2)x10-9 M f [Au]NP = (1.7±0.5)x10-7 M.

·CH3 + (NP)―(CH3)n-1 → (NP)―(CH3)n

k24 = ?

(24)

(NP)―(CH3)n → (NP)―(CH3)n-2 + C2H6

(NP)―(CH3)n + ·CH3 → (NP)―(CH3)n-1 + C2H6

(25)

(26)

(NP)―(CH3)n + H3O+/H2O → (NP)+―(CH3)n-1 + CH4 + H2O/OH-

(27)

3x10-8 = k12[·CH3]ss[(CH3)2SO] + k24[·CH3]ss [NP] (28)




Thus the rate constants, k24, for the reaction of ·CH3 radicals with the gold and the silver NPs at ambient temperature were derived from the slopes in Fig. 4 to be (3.7 ± 0.5)x108 M-1s -1 255

and (1.4 ± 0.5)x109 M-1s-1, respectively. Alternatively, the rate constants can be calculated assuming a heterogeneous system, k24

*, for the gold and silver NPs are (2 ± 1)x10-3 (cm2/L)-1s -1 and (6 ± 2)x10-4 (cm2/L)-1s-1, respectively. The observed very high rate constants suggest that the impaired electron, of the 260

radical, interacts strongly with the conduction-band of the metal.

Concluding remarks The results clearly demonstrate that gold and silver nanoparticles are outstanding scavengers of alkyl-radicals. 265

This observation might be relevant to the role of these nanoparticles in medicine1-3, though in biological systems RO2· radicals are expected to be present. It is of interest to note that the reactions of methyl-radicals with the gold and silver nanoparticles yield ethane whereas 270

their reactions with Cr0, Mn0, Fe0, Co0, Ni0 and Zn0 powders yield methane and with Cu0 powder yield methanol12. It will be of interest to check whether this difference is due to the nature of M0 and/or to the particle size. Finally it should be pointed out that the source of the effect 275

of the "short" irradiation on the "long range" stability of the nanoparticles is not clear at present.

Acknowledgments The authors are indebted to the late Prof. Shlomo Efrima for seminal discussions. We wish to thank Prof. Dan Meisel 280

for helpful comments and to Dr. Emma Gorodinsky for her assistance with the electrophoresis measurements. This study was supported in part by a grant from the Budgeting and Planning Committee of The Council of Higher Education and the Israel Atomic Energy Commission. D.M. wishes to 285

express his thanks to Mrs. Irene Evens for her ongoing interest and support.

Notes and references

1. S. Korea Pat., 2003013622, 2003. 2. USA Pat., 2002071792, 2002. 290

3. Russia Pat., 2146127, 2000. 4. Over-the-Counter Drug Products Containing Colloidal Silver

Ingredients or Silver Salts, Food and Drug Administration, Federal Register, 1999.

5. A. Roucoux, J. Schulz and H. Patin, Chem. Rev., 2002, 102, 3757-295

3778 and references therein. 6. A. Moradpour, E. Amouyal, P. Keller and H. Kagan, Nouv. J. Chim.,

1978, 2, 547-549. 7. A. Henglein, Angew. Chem.-Int. Edit. Engl., 1979, 18, 418. 8. D. Meisel, J. Am. Chem. Soc., 1979, 101, 6133-6135. 300

9. K. Kopple, D. Meyerstein and D. Meisel, J. Phys. Chem., 1980, 84, 870-875.

10. N. Shaham, A. Masarwa, Y. Matana, H. Cohen and D. Meyerstein, Eur. J. Inorg. Chem., 2002, 87-92.

11. A. Masarwa and D. Meyerstein, in Advances in Inorganic Chemistry: 305

Including Bioinorganic Studies, Vol 55, ELSEVIER ACADEMIC PRESS INC, San Diego, Editon edn., 2004, vol. 55, pp. 271-313.

12. I. Rusonik, H. Polat, H. Cohen and D. Meyerstein, Eur. J. Inorg. Chem., 2003, 4227-4233.

13. B. Hickel, J. Phys. Chem., 1975, 79, 1054-1059. 310

14. Physical Chemistry Laboratory, Ben-Gurion University of the Negev, 2004.

15. J. Weiss, A. O. Allen and H. A. Schwarz, Proc. Inter. Conf. Peaceful Uses Atomic Energy, United Nations N.Y., 1956.

16. J. A. Creighton, C. G. Blatchford and M. G. Albrecht, Journal of the 315

Chemical Society-Faraday Transactions Ii, 1979, 75, 790-798. 17. U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters,

Springer, Berlin, 1995. 18. D. Veltwisch, E. Janata and K. D. Asmus, J. Chem. Soc.-Perkin

Trans. 2, 1980, 146-153. 320

19. L. M. Dorfman and M. S. Matheson, Pulse Radiolysis, MIT Press, Cambridge, 1969.

20. G. V. Buxton, C. L. Greenstock, W. P. Helman and A. B. Ross, J. Phys. Chem. Ref. Data, 1988, 17, 513-886.

21. D. R. Lide, CRC Handbook of Chemistry and Physics, 84 edn., Press 325

LLC, Boca Raton, 2002. 22. B. E. Bent, Chem. Rev., 1996, 96, 1361-1390. 23. In Figure 4.a the first data point was omitted from the calculation the

CH4 concentration at this point is very low and thus the error limit is relatively large. The explanation to the negative intercept is that at 330

high concentration of metal NPs, the number of methyl-radicals per NP is low. Thus, the probability for the encounter of two radicals on the NP's surface decreases and the possibility for reduction of the radicals to produce methane on the NP's surface increases.

]NP[]CH[k]SO)CH[(]CH[k

)HC(G)CH(G

ss324

23ss312

62

4

⋅⋅

= (29)

0.0

0.5

1.0

1.5

0.0E+00 5.0E+06 1.0E+07

[DMSO]/[NP]

G(C

H 4)/G

(C2H

6)

y = 2.7E-7x - 0.35R2 = 1.0

a) Rate constant calculation - [Au]NP

0.0

1.0

2.0

3.0

1.0E+07 3.0E+07 5.0E+07

[DMSO]/[NP]G

(CH 4

)/G(C

2H6)

y = 7.2E-8x - 0.45R2 = 1.0

b) Rate constant calculation - [Ag]NP

Fig. 4 Plots of the ratios G(CH4)/G(C2H6) Vs. [DMSO]/[NP] in order to find k24 for the reaction between methyl-radicals and a) gold NPs23; b) silver NPs.

The slope equals k12/k24.




335

Reactions of alkyl-radicals with gold and silver nanoparticles in aqueous solutions

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