Quenching of [Ru(bpy)3]2+ Emission by Binding to Ag

Nanoparticles

Sisi Hu, Matthew Feliciano, Alexander Santulli and Jianwei Fan,

Department of Chemistry and Biochemistry, Manhattan College, Bronx,

NY 10471

Introduction:

Tris(2,2’-bipyridyl)ruthenium, Ru(bpy)32+, has been well studied due to its strong

absorption and emission of UV/visible light and its potential use in solar energy

conversion.

http://omlc.org/spectra/PhotochemCAD/html/085.html

Recent studies (Huang & Murray, Langmuir 2002, 18, 7077-7081) showed that noble metal nanoparticles, e.g., gold-NPs, are potent

quenchers of the luminescent excited state of Ru(bpy)32+.

(Ru(bpy)32+)* + Au-NPs (quencher) Ru(bpy)3

2+ + Au-NPs*

We are interested in the quenching of the excited state of Ru(bpy)32+ by

silver nanoparticles with capping agents citrate or polyethylene glycol

(PEG).

(Ru(bpy)32+)* + Ag-NPs (quencher? ) Ru(bpy)3

2+ + Ag-NPs*

Sodium Citrate Polyethylene Glycol (PEG)

http://www.taringa.net/posts/salud-bienestar/16110303/Toxicos-en-los-cosmeticos.htmlhttp://en.wikipedia.org/wiki/File:Trisodium_citrate.png

1. Synthesis of Ag-NPs (Jana, N.R.; Gearheart, L. and Murphy, C. J. Chem. Commun., 2001, 617)10mL of 1mM sodium citrate (or PEG) was added to 10 mL of 0.5mM AgNO3.

The mixture solution was then stirred for 30 seconds. 1.2mL of 1mM NaBH4

prepared in de-aired water was added into the solution afterwards. The solution

was stirred for another 30 seconds and allowed to sit for 30 minutes for the

reaction to occur completely.

citrateAgNO3 + NaBH4 ------> Ag-NP(citrate) + 1/2H2 + 1/2B2H6 + NaNO3

The completeness of the reaction was checked by NaCl solution: there is no

AgCl ppt after the addition of NaCl to the final solution.

UV/visible characterization of Ag-NP(citrate) in solution:

Plasmon resonance: lref. = 390 nm (for average size = 4 nm), lmeas.= 392 nm

392nm

Calculation of the molar concentration of Ag-NP:

• Assuming the nanoparticle is spherical the volume of the NP is 4/3πr3=4/3π(2nm)3=33.51nm3 = 3.35x10-20 cm3/NP

• Using the molar mass of Ag as 107.87g/mol and assuming the density of Ag(s) is 10.49g/cm3, the # of Ag atoms in per nanoparticle is

3.35×10-20 cm3/NP ×10.49g/cm3 ×1mol

107.87g× 6.02 ×

1023

mol= 1962 atoms/NP

• The molar concentration of Ag-NP solution

2.3 × 10−4MAgNO3 × 1NP/1962 Ag atoms = 1.20 × 10−7MAg-NP

2. Determination of molar extinction coefficients of Ag-NPs

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.00E+00 2.00E-08 4.00E-08 6.00E-08

Ab

sorb

ance

[Ag-NP] (M)

Absorbance at 392nm

Absorbance at 450nm

Absorbance at 286nm

Calibration curves for Ag-NP(citrate) (left) and Ag-NP(PEG) (right) at 286, 392 and 450 nm

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0.00E+00 5.00E-08 1.00E-07 1.50E-07

Ab

sorb

ance

[Ag-NP] (M)

Absorbance(286)

Absorbance(392)

Absorbance(452)

Molar extinction coefficients, e (M−1cm−1), of Ag-NP and Ru(bpy)32+

The result shows that e slightly depends on the capping agents.

286 nm 392 nm 452 nm

Ag-NP(citrate) 3.32 ± 0.34 × 106 2.31 ± 0.02 × 107 6.54 ± 0.33 × 106

Ag-NP(PEG) 4.17 ± 0.07 × 106 1.75 ± 0.18 × 107 8.95 ± 0.64 × 106

Ru(bpy)32+ 8.70 × 104 1.26 × 104

3. Stern-Volmer Equation:

I0/I = 1+ KSV [Q]

I0- Emission intensity of Ru(bpy)32+in the absence of quencher (Ag-NP)

I - Emission intensity Ru(bpy)32+in the presence of quencher

[Q] - concentration of quencherKsv - Stern-Volmer constant, measuring quenching efficiency.

Inner filter effect: If the quencher has optical absorption in the excitationregion it will decrease the effective intensity of the exciting light available to thefluorophore. As a result, it induces an apparent quenching of emission andincreases the real values of the Stern-Volmer quenching constants.

Overlay of UV/visible absorption spectra of Ru(bpy)32+ (blue) and Ag-NPs (red)

286nm (L – L *)

451nm (MLCT)

Measurement of Stern-Volmer constant of Ag-NP(citrate)

• Method 1: Excitation of Ru(bpy)32+ at 286 nm since at which Ag-NP

has the least coabsorption.

• Method 2: Excitation of Ru(bpy)32+ at 451 nm but using corrected

Stern-Volmer equation for inner filter effect.

• Method 3: Excitation Ru(bpy)32+at 451 nm but adding very low

concentration of Ag-NP so that its absorption at 451 nm is negligible (< 2%).

Method 1: Emission Spectra of Ru(bpy)32+ in the presence of increasing

concentrations of Ag-NP (lex =286 nm) (left) and corresponding Stern-Volmerplot (right)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 1E-08 2E-08 3E-08 4E-08 5E-08 6E-08

I0/I

[Ag-NP] M

Ksv is 9.46x106𝑀-1

Ru(bpy)32+

1.20 × 10−8 M Ag-NP

2.40 × 10−8 M Ag-NP

3.60 × 10−8 M Ag-NP

4.80 × 10−8 M Ag-NP

6.00 × 10−8 M Ag-NP

Method 2: Excitation at 451 nm but using corrected Stern-Volmer equation for

inner filter effect.

A correction factor is introduced by Borissevitch, Journal of Luminescence, 81 (1999):

(I0/Iem ) η = 1 + Ksv [Q]

η = 𝐴𝑥0(1−10

−𝐴𝑥𝑖)

𝐴𝑥𝑖(1−10−𝐴𝑥0)

where η is the correction factor for the excitation light, 𝐴𝑥0 is the fluorophore absorbance,

and 𝐴𝑥𝑖 is the total absorbance of the fluorophore and the quencher at the excitation

wavelength, respectively.

Emission Spectra of Ru(bpy)32+ in the presence of various concentrations of Ag-NP at lex =451 nm (left)

Correction factor (η ) and corrected emission intensities (h𝐼0/I) vs. concentrations of Ag-NP added (right)

# of Ru(bpy)3

2+

solution

M of Ag-NP added

Abs(451nm) η (I0/Iem ) η

1 0 0.31277 1 1

2 1.20× 10−8

0.39037 0.926 1.191

3 2.40× 10−8

0.47924 0.850 1.235

4 3.60× 10−8

0.56084 0.788 1.273

5 4.80× 10−8

0.64077 0.733 1.428

6 6.00× 10−8

0.70748 0.692 1.588

Ru(bpy)32+

1.20 × 10−8 M Ag-NP

2.40 × 10−8 M Ag-NP

3.60 × 10−8 M Ag-NP

4.80 × 10−8 M Ag-NP

6.00 × 10−8 M Ag-NP

0

0.5

1

1.5

2

2.5

0 1E-08 2E-08 3E-08 4E-08 5E-08 6E-08 7E-08

I0/I

[Ag-NP]

Method 2: Stern-Volmer plots of quenching Ru(bpy)32+ by Ag-NP

(lex = 451 nm)

Ksv (corrected) = 9.30× 106𝑀−1Ksv (uncorrected) = 2.02× 107𝑀−1

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 1E-08 2E-08 3E-08 4E-08 5E-08 6E-08 7E-08

hI0/I

[Ag-NP]

Method 3: Excitation at 451 nm but adding very low concentration of Ag-NP so that its absorption at 451 nm is negligible (< 2%).

# of

Ru(bpy)32+

solution

M of Ag-NP added

Abs(451nm)

Iem I0/Iem

1 0 1.19520 913521 1

2 1.77× 10−9

1.17450 883930 1.026

3 2.35× 10−9

1.17140 872997 1.034

4 2.93× 10−9

1.18030 862964 1.042

5 3.50× 10−9

1.17220 847810 1.0550.99

1

1.01

1.02

1.03

1.04

1.05

1.06

0 5E-10 1E-09 1.5E-09 2E-09 2.5E-09 3E-09 3.5E-09 4E-09

I0/I

[Ag-NP]

Ksv = 9.10 × 106𝑀−1

Summary of Stern-Volmer constant

Method 1 (λex=286nm) Method 2 (λex=451nm with

correction)

Method 3 (λex=451nm)

Ksv 9.46 × 106𝑀−1 9.30× 106𝑀−1 9.10 × 106𝑀−1

Average Ksv 9.28±0.18× 106𝑀−1

4. Spectroscopic titration of Ru(bpy)32+by Ag-NPs

To check any chemical equilibrium between the ground state Ru(bpy)32+and Ag-

NPs, absorption spectrum of Ru(bpy)32+ was monitored after the addition of Ag-

NPs as following:

• 2mL of Ru(bpy)32+ solution (c= 8.0x10-5 M) was placed in a cuvet,

• Ag-NP solution (1.2x10-7 M) was added to the cuvet in 100 mL increment,

• UV/vis absorption spectrum of Ru(bpy)32+ was recorded after each addition

until the spectrum shows no further change.

UV/vis spectra of 8.0x10-5 M Ru(bpy)32+ after addition of Ag-NP (0-7.2x10-5 M)

540nm

[Ag-NP] increases

Isosbestic point 340nm

Titration curve:Absorbance (540 nm) vs. volume of Ag-NP added (left) Absorbance (540 nm) vs. [Ag-NP]/[Ru(bpy)3

2+] (right)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 500 1000 1500 2000 2500 3000 3500

A(540)

mL of Ag-NP

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.0005 0.001 0.0015 0.002 0.0025

A(540)

[Ag-NP]/[Ru]

[Ru]/[Ag-NP] = 500

Discussion:1. Stern-Volmer constants obtained from three methods are consistent with each other: emission quenching does happen by Ag-NPs.

2. Stern-Volmer constant (9.28±0.18× 106𝑀−1) seems too large for dynamic (or collisional) quenching:

𝐾𝑆𝑉 = 𝑘𝑞t,

where t is the lifetime of the excited state of Ru(bpy)32+ (610 ns), and 𝑘𝑞 is bimolecular

quenching rate constant.

For dynamic quenching, 𝑘𝑞 ≤ 𝑘𝑑, 𝑘𝑑 is diffusion rate constant, equals to109 in water.

However, 𝑘𝑞 = 𝐾𝑆𝑉 / t = 9.28× 106/ 610× 10−9 = 1.5× 1013 ≫ 𝑘𝑑 = 109.

Discussion Cont’d

3. Our spectroscopic evidence supports a static quenching mechanism which is via the formation of a non-emissive product between the ground state fluorophore and quencher.

• The new peak at 540 nm indicates the formation of a new product which could be the electrostatic complex between Ru(bpy)3

2+ (positive charged) and Ag-NPs (negative charged due to citrate).

• The increasing of absorbance at 540 nm is leveling off when the molar ratio is approaching to 500:1 ([Ru]/[Ag-NP]), which could be the stoichiometric ratio in the product.

+ Ru(bpy)32+

RuAg Ag

RuRu

Ru

-

- - ---

-- -

- - Ru Ru

Ru-: citrate

Ru: Ru(bpy)32+

Ru

Conclusion:

Our research results indicate that the quenching of the excited state of Ru(bpy)3

2+ by Ag-NP is not dynamic but static through the formation of the electrostatic complex between Ru(bpy)3

2+ and Ag-NP(citrate).

Future work:

• Calculating the equilibrium constant K of the association reaction of

Ru(bpy)32+ and Ag-NPs.

• Measuring Stern-Volmer constant with Ag-NP (PEG) since PEG is a

neutral capping agent.

Acknowledgement:

• Dr. Harry D. Gafney, Queens College of CUNY