Inorganica Chimica Acta 357 (2004) 1589–1592

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Note

Electrogenerated chemiluminescence of Pb(II)-bromide complexes

Pavneet Singh, Mark M. Richter *

Department of Chemistry, Southwest Missouri State University, 901 South National Avenue, Springfield, MO 65804-0089, USA

Received 6 August 2003; accepted 6 December 2003

Abstract

The electrochemistry and electrogenerated chemiluminescence (ECL) of Pb4Br3�11 in acetonitrile solution is reported. Pb4Br

3�11 is

formed in situ by the reaction of lead(II) and bromide ions with ECL generated upon sweep to positive potentials using tri-n-

propylamine (TPrA) as an oxidative–reductive coreactant. An ECL efficiency (/ecl) of 0.0079 was obtained compared to Ir(ppy)3(ppy¼ 2-phenylpyridine; /ecl ¼ 1). The ECL intensity peaks at a potential corresponding to oxidation of TPrA and Pb4Br

3�11 in-

dicating that emission is from the lead-bromide cluster.

� 2003 Elsevier B.V. All rights reserved.

Keywords: Electrogenerated chemiluminescence; Lead-bromide clusters

1. Introduction

There has been considerable interest in developing

electrogenerated chemiluminescence (often called elect-

rochemiluminescence, ECL) sensors to detect a wide

range of biologically and environmentally important

analytes. ECL is a means of converting chemical energy

into light. It involves the formation of electronicallyexcited states by energetic electron transfer reactions of

electrochemically generated species and is a sensitive

probe of electron- and energy-transfer processes at

electrified interfaces [1]. Traditionally, ECL was gener-

ated via annihilation, where the electron transfer reac-

tion is between an oxidized and reduced species, both of

which are generated at an electrode by alternate pulsing

of the electrode potential. ECL can also be generated ina single step utilizing a coreactant [2,3]. ECL coreactants

are species that, upon electrochemical oxidation or re-

duction, produce species that react with other com-

pounds to produce excited states capable of emitting

light. For example, in the Ru(bpy) 2þ3 /TPrA (bpy¼ 2,20-

bipyridine; TPrA¼ tri-n-propylamine) system, an oxi-

dizing potential oxidizes Ru(bpy) 2þ3 to Ru(bpy) 3þ

3 . The

coreactant (TPrA) is also oxidized and decomposes toproduce a strong reducing agent (presumably TPrA�)

* Corresponding author. Tel.: +1-4173865508; fax: +1-4178365507.

E-mail address: mar667f@smsu.edu (M.M. Richter).

0020-1693/$ - see front matter � 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2003.12.008

upon deprotonation of an a-carbon from one of the

propyl groups. This strong reducing agent can then in-

teract with Ru(bpy) 3þ3 to form the excited state (i.e.,

�Ru(bpy) 2þ3 ). ECL has also been commercially devel-

oped and marketed for the clinical diagnostic market

(e.g., immunoassays and DNA probes) [4] and is being

extended to environmental analyses, food and water

safety testing, and military applications (such as bio-agent detection) [5–7].

Recently, the ECL detection of metal ions not in-

volved in redox reactions was reported [8,9]. This was

accomplished using ruthenium polypyridine complexes

containing a crown-ether moiety covalently bonded to a

bipyridyl ligand to capture/bind the metal ion of interest

in a host–guest relationship. The systems reported were

Ru(bpy)2(CE-bpy)2þ (CE-bpy is a bipyridine ligand

where a crown either (15-crown-5) is bound to the

bpy ligand in the 3 and 30-positions) [8] and

(bpy)2Ru(AZA-bpy)2þ (bpy¼ 2,20-bipyridine; AZA-

bpy¼ 4-(N-aza-18-crown-6-methyl-2,20-bipyridine)) [9].

Ru(bpy)2(CE-bpy)2þ is sensitive to sodium ions in

aqueous buffered solution while (bpy)2Ru(AZA-bpy)2þ

has been shown to be sensitive to Pb2þ, Hg2þ, Cu2þ,Agþ and Kþ in 50:50 (v/v) CH3CN:H2O (0.1 MKH2PO4 as electrolyte) and aqueous (0.1 M KH2PO4)

solution. These studies clearly show the versatility of

ECL for sensing metal ions in solution. Also, systems

that are capable of sensing metal ions not directly

1590 P. Singh, M.M. Richter / Inorganica Chimica Acta 357 (2004) 1589–1592

involved in redox reactions would be useful in a variety

of tests. For example, in the determination of electrolytes

and metal ions in clinical and environmental analyses.

An alternate approach to host–guest relationships

was reported recently that involved detecting lead viathe photoluminescence of Pb4Br

3�11 . This highly photo-

luminescent lead cluster was formed in situ by the re-

action of lead(II) with excess bromide in polar organic

solvents [10]. This approach allowed the determination

of lead at nanomolar concentrations. It may also present

some advantages over host–guest chemistry including

ease of sensor preparation.

The ECL of molybdenum and tungsten clusters hasbeen reported, and their spectroscopic, electrochemical

and ECL properties studied [11–16]. However, the direct

ECL from lead compounds and/or clusters has, to our

knowledge, never been reported. Therefore, in this

paper the electrochemistry, and ECL of Pb4Br3�11 are

investigated.

Fig. 1. Photoluminescent emission spectrum of Pb4Br3�11 in CH3CN.

Fig. 2. Cyclic voltammogram of (A) 0.1 M TBABr in CH3CN and (B)

1 mM Pb(NO3)2 and 0.1 M TBABr in CH3CN.

2. Experimental

2.1. Materials

All materials were used as received.

Ru(bpy)3Cl2 � 6H2O (98%, Strem Chemical Inc., New-

bury Port, MA), potassium phosphate monobasic

hydrate (99%, EM Science, Gibbstown, NJ), tri-n-pro-pylamine (TPrA, 98%, Avocado Research Chemicals,

Ward Hill, MA), acetonitrile (Burdick and Jackson

High Purity, Fisher), tetrabutylammonium bromide

(TBABr; Aldrich), lead(II) nitrate (Aldrich), lead(II)

acetate trihydrate (99.999%, Aldrich). Tris(2-phenyl-

pyridine)iridium(III) was available from a previous

study [17] and synthesized via literature methods [18].

2.2. Methods

Electrochemical and ECL instrumentation and ex-

perimental methods have been described elsewhere [19]

and incorporated a CH Instruments 620 Electrochemi-

cal Analyzer and Hamamatsu HC 135 Photomultiplier

Tube (PMT) contained in a ‘‘light-tight’’ box. All elec-

trochemical and ECL experiments were referenced withrespect to Ag/AgCl gel electrode (0.20 V versus NHE)

[20]. The platinum mesh (27 mm2) working electrode

was cleaned prior to each experiment by repeated cy-

cling (from +2.0 to )2.0 V) in 6.0 M sulfuric acid, fol-

lowed by sonication in 2 M nitric acid and rinsing in

deionized water.

Solutions used to obtain ECL were 10�7–10�3 M lead

ion, 10�3–1 M TBABr, and 0.05 M TPrA in acetonitrile.Photoluminescence spectra were obtained with a

Shimadzu RF-5301 Spectrofluorophotometer (slit

widths 3–5 nm). Excitation was at 350 nm and 383 nm

for Pb4Br3�11 and Ir(ppy)3, respectively, with detection

between 450 and 700 nm. ECL efficiencies (/ecl ¼ pho-

tons generated per redox event) were obtained by the

literature methods, using Ir(ppy)3 (/ecl ¼ 1) as the

standard [21,22]. Reported values are the average of atleast three scans with a relative standard deviation of

�10%. Similarly, relative photoluminescence efficiencies

followed published procedures [23] using Ru(bpy) 2þ3

(/em(H2O)¼ 0.042).

3. Results and discussion

UV–Vis absorption and photoluminescent emission

measurements are consistent with earlier reports [10,24]

and show that the species formed in situ by the reaction

of 1 mM Pb(NO3)2 and 0.1 M TBABr in CH3CN is

Pb4Br3�11 . An emission spectrum is shown in Fig. 1

where the wavelength maximum of emission (kem) at 560nm has been assigned to a delocalized cluster state [10].

Cyclic voltammograms for a solution of TBABr inthe presence and absence of Pb2þ are shown in Fig. 2.

An irreversible wave was observed at �1.0 V at glassy

carbon and platinum electrodes using a scan rate of 100

mV/s. No reversibility was observed even at scan rates as

high as 5 V/s. Due to the similarity between the oxida-

tion waves in the presence and absence of Pb2þ, the

P. Singh, M.M. Richter / Inorganica Chimica Acta 357 (2004) 1589–1592 1591

wave is likely due to oxidation of free bromide in solu-

tion or bromide in the cluster. Previous work has shown

that the free bromide ion concentrations in solution

corresponds to only a small fraction of total bromide in

the system [24]. Also, there is a slight shift to morenegative potential in the Pb4Br

3�11 system. This suggests

that the oxidation wave in the lead-bromide solution is

due to both Pb4Br3�11 and bromide, with the complex

dominating.

A representative example of ECL for a solution of 1

mM Pb(NO3)2 and 0.1 M TBABr in CH3CN using

TPrA as an ‘‘oxidative–reductive’’ coreactant is shown

in Fig. 3. Similar results were obtained with PbBr2 whilethe solubility of lead(II) acetate was too low even in the

presence of TBABr to yield measurable ECL. With

Pb(NO3)2 the ECL intensity peaks at a potential of

�0.85 V, corresponding to oxidation of TPrA

(Ea ¼ 0:90 V) [25], bromide and/or lead-bromide com-

plex (Ea ¼ 1:0 V). ECL was much weaker for a solution

of 0.1 M TBABr/TPrA in CH3CN containing no lead

and no measurable ECL above background was ob-served for a solution of Pb2þ/TPrA without bromide

present. Also, no measurable ECL was observed for any

solutions in the absence of TPrA, or for TPrA in the

absence of bromide and/or lead. An ECL efficiency (/ecl;

photons emitted per redox event) of 0.0079 (or 0.79%)

was obtained using Ir(ppy)3 as a standard (/ecl ¼ 1).

Unfortunately, the emission was not intense enough to

generate an ECL spectrum making precise mechanisticarguments about the nature of the excited state difficult.

It is also interesting to note the shape of the ECL

intensity versus potential curves in Fig. 3. For the so-

lution containing lead(II), a distinct peak is observed

around 1 V that then tapers off due to diffusion of

electroactive material to the electrode surface. Intensity

versus potential traces such as this have been observed

for many ECL luminophores, including Ru(bpy) 2þ3 ,

using TPrA as coreactant [3]. For the solution of bro-

mide, on the other hand, a featureless peak is observed

that is independent of electrode potential, indicative of

background events or reactions of TPrA, bromide and/

or solvent molecules. These experiments indicate that

formation of Pb4Br3�

11 is crucial for efficient ECL to

occur.

Fig. 3. ECL intensity vs. potential for 1 mM Pb(NO3)2, 0.1 M TBABr

and 0.05 M TPrA in CH3CN. (A) First sweep from 0.0 to +2.0 to 0.0 V

and (B) second sequential sweep from 0.0 to +2.0 to 0.0 V.

ECL intensity is also dependent on the concentra-

tions of both lead(II) and bromide in solution. At high

and constant concentrations of TPrA and TBABr, the

concentration of Pb2þ was varied from 0.1 lM to 1 mM.

Results are presented in Table 1. Clearly, there is a de-crease in ECL as the concentration of lead decreases.

Interestingly, concentrations less than 0.1 mM show an

ECL signal characteristic of a bromide/TPrA solution

with no lead present, indicating that the contribution of

Pb4Br3�

11 ECL at these concentrations is minimal. There

is also decreased ECL as the concentration of bromide is

varied (TPrA and Pb2þ constant) from 0.1 M to 1 mM,

Table 1. In fact, below 0.005 M no discernable ECLpeak is observed.

Detailed spectroscopic studies have indicated that

complex equilibria occurs in solutions of lead(II) con-

taining bromide [24]. Therefore, the dependence of ECL

on bromide and lead concentrations is not surprising.

For example, in dilute solutions (<0.03 M LiBr) satu-

rated with Pb2þ, the dominant species in solution is

Pb3Br� [24]. At higher concentrations of bromide, the

dominant species is Pb4Br3�11 . If the solution is unsatu-

rated with respect to lead, the major Pb(II) species are

Pb3Br� and PbBr 2�

4 . In our system, maximum ECL in-

tensity was observed at [Pb2þ]¼ 0.03 M and [TBABr]¼0.1 M corresponding to formation of Pb4Br

3�11 .

The complex equilibria of the lead-bromide system

and the irreversible nature of the electrochemistry may

also explain the potential shift that is observed uponsequential ECL sweeps. For example, a typical cyclic

voltammetric sweep where ECL is measured is from 0.0

to +2.0 to 0.0 V. If a second sweep is immediately per-

formed with no electrode cleaning an ECL versus po-

tential plot such as that shown in Fig. 3 is obtained.

Clearly, upon sequential oxidative sweeps a decrease in

intensity and a 100 mV shift to more anodic potentials is

observed. While the decreased intensity is expected dueto diffusion of electroactive material from the electrode

surface, the shift in potential indicates the formation of

a Pb4Br3�

11 decomposition products, or another species

that may be formed as reactant concentrations change

near the electrode surface.

ECL emission intensity versus time profiles for

Pb4Br3�

11 and Ru(bpy) 2þ3 are shown in Fig. 4. The

Table 1

Variation of ECL with lead(II) and bromide concentration

[Pb2þ]

(mM)

ECL (cps� 104) [Br�]

(M)

ECL (cps� 104)

1 10.5 (�1.0) 0.1 6.9 (�0.7)

0.01 4.8 (�0.5) 0.05 5.6 (�0.6)

0.001 a 0.01 2.6 (�0.3)

0.005 a

0.001 a

Values are the average of at least three scans with standard devi-

ations in parentheses.aNo signal above background.

Fig. 4. ECL intensity as a function of time in acetonitrile using TPrA

as coreactant for (A) 0.1 lM Ru(bpy) 2þ3 and (B) 0.1 M TBABr plus 1

mM Pb(NO3)2. The ECL of the Ru(bpy) 2þ3 /TPrA profile has been

normalized to that of Pb4Br3�11 . Error bars have been omitted for

clarity (error of each measurement is �10%).

1592 P. Singh, M.M. Richter / Inorganica Chimica Acta 357 (2004) 1589–1592

potential of the working electrode was stepped to +2.0

V, and the ECL intensity measured. As expected, there

was a sharp increase in light intensity due to high con-

centrations of luminophore and coreactant near the

surface of the electrode. Under our conditions, the time

taken to reach peak intensity was approximately 209 s

for Pb4Br3�

11 and 103 s for Ru(bpy) 2þ3 . Surprisingly, at

longer times the emission does not appear to becomediffusion controlled for Pb4Br

3�11 as it does with

Ru(bpy) 2þ3 . The reasons for this are unclear, but this

indicates that although the ECL of the lead cluster is

orders of magnitude lower than Ru(bpy) 2þ3 , Pb4Br

3�11

may undergo more ECL reaction events. This could also

indicate that side products of Pb4Br3�

11 oxidation can

produce ECL with TPrA.

Acknowledgements

The support of this research by the American

Chemical Society-Petroleum Research Fund, the Ca-

mille and Henry Dreyfus Foundation in the form of a

Henry Dreyfus Teacher-Scholar Award (M.M.R.) and

Southwest Missouri State University is gratefully ac-knowledged.

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