Electrochemically Generated versus Photoexcited Luminescence from

Review pubs.acs.org/CR

Electrochemically Generated versus Photoexcited Luminescence from Semiconductor Nanomaterials: Bridging the Valley between Two Worlds Peng Wu,†,‡ Xiandeng Hou,‡ Jing-Juan Xu,*,† and Hong-Yuan Chen*,†,§ †

State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China ‡ Analytical & Testing Center, Sichuan University, Chengdu 610064, China § Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Shandong Normal University, Jinan 250014, P.R. China smaller than the bulk-exciton Bohr radius, i.e., the distance in an electron−hole pair.1,2 Since the discovery of colloidal semiconductor QDs by Brus and colleagues in Bell Lab in 1983,3,4 exponential growth of research interests and activities has been witnessed in this field worldwide in the past three decades. These interests and activities are driven both by the excitement of understanding new science and technology and by the expectation for diversified applications and economic impacts. For example, luminescent QDs have found very promising applications in optoelectronic devices,5,6 photovoltaic devices,7−9 and light-emitting diodes.10,11 Two breakthrough works published in 1998 opened the door for use of QDs as fluorescent biolabels.12,13 Since then, CONTENTS explosive interest in biological applications of QDs (particularly in the areas of bioimaging and bioanalysis) has emerged.14−23 1. Introduction 11027 These applications mainly include in vitro diagnostics, energy2. Fundamentals of the ECL and PL of QDs 11028 transfer-based sensing, cellular and in vivo imaging, as well as 2.1. ECL and PL Mechanism of QDs 11028 drug delivery and theranostics. Most of these studies, 2.2. Coreactants in ECL of QDs 11031 particularly in bioanalysis, are focused on using the photo2.3. Size-Dependent ECL and PL 11033 luminescence (PL, or fluorescence) properties of QDs. 2.4. Dopant-Dependent ECL and PL 11033 Electrochemiluminescence (ECL), also referred to as 2.5. QD-Based Composites for ECL and PL 11034 electrogenerated chemiluminescence, is chemiluminescence 3. Analytical Applications of ECL and PL of QDs 11036 triggered by electrochemical processes.24−27 The combination 3.1. Ion-Induced ECL and PL Quenching 11036 of chemiluminescence and electrochemistry brings ECL many 3.2. QDs as ECL and PL Labels 11037 unique advantages such as rapidity, high sensitivity, and 3.3. ECL and PL-Based Energy-Transfer Assays 11038 simplified optical setup. As an analytical technique,28 ECL 3.4. Field-Enhanced ECL and PL of QDs for does not require the use of any external light source. Thus, the Analytical Applications 11041 3.5. ECL- and PL-Based Electron-Transfer Assays 11043 attendant problems of scattered light and luminescent 3.6. Multiplexed ECL and PL Assays 11045 impurities are absent, which leads to low optical background 3.7. Ratiometric Sensing with ECL and PL of QDs 11046 noise and high sensitivity for analysis. In recent years, ECL has 3.8. Coreactant-Modulated ECL Bioassays 11048 become a very powerful analytical technique and is widely used 3.9. Impedance-Controlled ECL Bioassays 11049 in the determination of biomolecules in clinical, environmental, 4. Conclusions and Future Perspectives 11050 and industrial fields.24−27,29 It should be pointed out that ECL Author Information 11052 also has drawbacks, such as a broad emission spectral profile Corresponding Authors 11052 (∼200 nm for Ru(bpy)32+), limited ability to repeatedly Notes 11052 electrochemically cycle an individual luminophore, and loss of Biographies 11052 signal due to diffusion of the ECL reagent out of the detection Acknowledgments 11053 zone.24−28 References 11053 Semiconductor QDs are also ECL active. Bard’s group first observed the ECL phenomenon during the study of silicon (Si) semiconductors in 2002.30,31 Later they further reported ECL 1. INTRODUCTION Colloidal semiconductor nanocrystals or quantum dots (QDs) are monodisperse crystalline clusters with physical dimensions © 2014 American Chemical Society

Received: December 15, 2013 Published: October 9, 2014 11027

dx.doi.org/10.1021/cr400710z | Chem. Rev. 2014, 114, 11027−11059

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from CdSe,32 CdSe@ZnS,33 CdTe,34 and Ge35 QDs. These works catalyzed broad scientific interest for fundamental studies on ECL properties of QDs as well as application of the promising ECL from QDs for various biosensors (see Figure 1,

terials can be eventually traced back to semiconductor nanomaterials. In particular, this review will try to make an in-depth discussion and summarize on the similarities and differences of signal modulation strategies of ECL and PL biosensors (including the mechanisms of ECL and PL). Besides, we also propose what ECL can further parallel from PL of QDs.

2. FUNDAMENTALS OF THE ECL AND PL OF QDS 2.1. ECL and PL Mechanism of QDs

In the first report about ECL from Si30 and CdSe QDs,32 up to 200 nm red-shifted ECL spectra in comparison with PL were observed (Figures 2 and 3), which led to the conclusion that PL was originated from the band-gap transition while ECL was labeled as a surface-state emission. Several later studies also observed similar red shifting of ECL spectra as compared to the PL spectra of QDs. However, such ECL mechanism cannot account for an even larger number of observations made for QD systems (Table 1). For example, later works by Bard’s group on surface-passivated CdSe QDs33 (CdSe@ZnS) and CdTe QDs34 concluded that band-gap emission was the origin for the ECL from corresponding QDs (Figures 2 and 3). Several works even reported perfectly matched ECL and PL with near-infrared emission spectra from CdTe53−55 and alloyed QDs,56 while observation of the monochromatic green ECL from dual-stabilizer-capped CdSe QDs indicated band-gap ECL in some cases.57,58 Accordingly, the rational ECL mechanism may be related to both surface states and band-gap transition. To identify the mechanism of ECL from QDs it is better to figure out the exciton generation processes of ECL and PL. For either PL or ECL light emission is a result of electron−hole recombination. The difference lies in the stimulating source: in PL the electron and hole are generated by photoexcitation, while in ECL the electron−hole pair results from electrodes and coreactants (Figure 4). Although it is difficult to figure out the ECL mechanisms without coreactants, here we define that coreactants provide exciton contrast to that from electrode. The roles of coreactants will be discussed at length in the following section. One distinct feature of QDs is that when the size of the QDs decreases below the exciton Bohr radius their energy band structures change greatly, i.e., the band gap increases with the

Figure 1. Number of publications related to the ECL from semiconductor nanomaterials. Data were collected from the Web of Science as well as Hubmed based on searching with “electrochemiluminescence” or “electrogenerated chemiluminescence” or “ECL” and subsequent careful categorization. Note: data of 2014 is collected up to July.

the number of publications on ECL from semiconductor nanomaterials). Several excellent reviews and book chapters have well documented the progress of using semiconductor nanocrystals as ECL emitters.36−40 Fundamentally, the ECL properties and biosensing strategies of QDs should have many relations to the PL of QDs. It was noticed that ECL from QDs in light of their PL properties and PL-based sensing strategies may provide much better understanding of the ECL. However, the critical relationship and difference between ECL and PL of QDs and also the similarities and differences of ECL biosensing schemes as compared to PL sensing strategies still remains unsummarized. Therefore, we present a detailed summary on the progress of using semiconductor nanomaterials as ECL emitters in this review. Although ECL has also been observed from other luminescent nanomaterials, for example, graphene QDs,41,42 porous silicon,43,44 oligothiophene nanoparticies,45 gold nanoclusters46−48 and carbon dots,49−52 this review will focus on conventional semiconductor nanomaterials only, since either the PL or the ECL properties of other luminescent nanoma-

Figure 2. PL spectra of CdSe,32 CdSe@ZnS,33 and CdTe34 QDs in Bard’s works. Reprinted with permission from refs 32, 33, and 34, respectively. Copyright 2002, 2003, and 2004 American Chemical Society. 11028


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Figure 3. ECL spectra of CdSe,32 CdSe@ZnS,33 and CdTe34 QDs in Bard’s works. Reprinted with permission from refs 32, 33, and 34, respectively. Copyright 2002, 2003, and 2004 American Chemical Society.

decrease of particle size.1,2,59 Decreasing particle size also implies an increase in the surface-to-volume ratio in the QDs. For example, in a 5 nm sized dot about every third atom will be located at the surface, while for a 1 nm sized dot this number becomes 99%.60 Hence, a lot of unsaturated sites and dangling bonds (know as surface defects) are expected to play an increasing role, especially in carrier relaxation and recombination processes. To obtain high-quality QDs it is often necessary to passivate the QD surface with a higher band-gap semiconductor layer to minimize defects. On the other hand, if the surface of QDs is not sufficiently passivated, new energy levels may be generated as surface states with lower energy than that of the band gap (Figure 4). It should be pointed out that here that the energy levels in Figure 4 are only schematically illustrated. In other words, multiple surface energy levels caused by inefficient surface passivation exist. For PL of QDs, the excitons are generated under photoexcitation. Excitation of high-quality QDs often results in narrow and sharp PL spectra mainly originated from the band-gap transition (or excitonic PL), and the band-gap width can be extracted from the central PL wavelength position.61 For QDs with surface states, excitation of QDs also results in bandgap transition (Figure 4, hν1 in the left part), but exciton migration from valence or conduction bands to the surface traps will occur. Recombination of electrons and holes on the surface gives rise to surface-state-related emission with redshifted PL spectra (Figure 4, hν2 in the left part).62 Accordingly, the existence of multiple surface states with different energy levels is expected to result in broad and tailed PL spectra. However, in most cases the surface states or traps in fact act as a quenching center for QDs. The overall PL emission efficiency of QDs with surface states is significantly lower than those without, which in fact makes it difficult to differentiate the surface-state emission from the overall PL spectra of QDs. In other words, the low emission efficiency caused by surfacestate-induced quenching leads to an apparent low sensitivity of PL toward surface states. To understand ECL of QDs, first, one should be aware that the typical QDs in ECL systems have no significant difference from those in PL systems, i.e., these QDs also have multiple energy states (including band gap and surface states). In ECL of QDs, the electron and hole are injected by electrodes or coreactants. Taking the reductive−oxidative ECL (R−O ECL, sometimes called cathodic ECL in the literature) of QDs as an

example, when a potential scan with negative direction is applied to the electrode (electrochemical reduction) the electron energy of the electrode is increasing gradually. Surface states of QDs with lower energy than that of the band gap would be first occupied by electrons from the electrode and holes from the coreactant, resulting in ECL from surface states (right part of Figure 4, hν2). In fact, previous reports on electrochemical investigations of QDs confirmed that these surface states would peak earlier than the band gap.64−67 Further increasing the negative potential of the electrode will result in electron injection into higher energy levels of QDs, giving rise to ECL with higher energy (right part of Figure 4, hν1). For electron and hole injection in oxidative-reductive ECL of QDs (O−R ECL, sometimes it was called anodic ECL in the literature), vice versa.. From the above discussions it is clear that ECL is more prone to express the surface states of QDs. Although theoretically PL can also give the sign of surface-state emission, the low emission efficiency caused by surface-state-induced quenching leads to an apparent low sensitivity of PL toward surface states. However, whether ECL is originated from surface states or the band gap depends on the QDs used for investigation. It should also be noted that ECL from QDs is a potential-dependent process. Most of the previous ECL spectra from QDs were obtained either by optical filter-based spectral deconvolution at a potential with maximum ECL intensity or by harvesting it with a PMT device supposing that ECL from QDs was a steady-state process. No potential-dependent ECL spectra of QDs have been reported. Accordingly, previously obtained ECL spectra of QDs may miss the information from surface states with lower energy levels. Since the debate on band-gap and surface-state ECL is mostly based on the comparison of PL and ECL spectra, careful investigation on the voltagedependent ECL spectra of QDs may provide more evidence to confirm the origin of the ECL of QDs. On the other hand, such surface-sensitive nature of QDs can be further utilized for probing the surface oxidation of QDs.68 Moreover, the sensitiveness of ECL to the surface states of QDs allows lowpotential ECL, which is important for elimination of undesirable electrochemical reactions at high potentials. For example, artificially manipulating the surface states through synthesis with a bidentate ligand could also effectively decrease the onset potential for the ECL, leading to low-potential ECL from QDs.69 11029


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Table 1. Summary of the Band Gap, Morphology, PL Wavelength, and ECL Properties of Semiconductor Nanomaterialsa material

Eg/eV

morphology and size

Ag2Se Bi2S3

0.15 1.74

dot (1.5 nm) nanorod (80 nm ×5 μm)

Bi2Te3 CdO CdS

0.15 2.28 2.42

nanoflask (10−150 nm) dendritic nanoparticle dot (10 nm) spherical assemblies, consist of CdS QDs about 5 nm hollow sphere (thickness 5 nm; diameter 20 nm) nanotube (external 140 nm, internal, 100 nm) hierarchical nanotube array,

λPL (λEX)/ nm 700

695

λECL − λPL/nm 5

700 493, 530

480, 700

2.42

dot (5 nm)

Eu:CdS

2.42

dot (6.2 nm)

CdSe

1.73

dot (3.2 nm)

540

dot (3.5 nm) dot (1.8 nm)

570 560

480, 765 585, 660 470, 520, 620 740

ECL potential (vs ref electrode)/V −1.22 (Ag/AgCl) −1.70 (AgQRE)

65

200

−1.2 (Ag/AgCl)

K2S2O8 + KOH

74

−1.4 (Ag/AgCl)

PBS + H2O2

75

−1.3 (SCE)

PBS + K2S2O8

91

−1.2 (SCE)

PBS + K2S2O8

92

−2.3 to + 2.3 (AgQRE)

32

−1.1 (Ag/AgCl)

Annihilation type, CH2Cl2 + TBAP PBS + dissolved O2 Tris-HCl + Na2S2O3 PBS + K2S2O8

86 87

−1.45 (Ag/AgCl)

PBS + K2S2O8

95 34

88 81 89 71 90

93 94

1.49−1.73

hollow spherical nanoassembly, consists of CdSe QDs about 5 nm dot (5 nm)

1.49

dot (4 nm)

635

638

3

−2.46 to + 1.44 (AgQRE)

dot (3.43 nm)

574

581

7

+ 1.17 (Ag/AgCl)

dot (3.4 nm) dot (3.3 nm) dot (−)

590 577 702

590 620 702

0 43 0

+ 0.9 (Ag/AgCl) −2.4 (Ag/AgCl) −1.25 (Ag/AgCl)

annihilation type, CH2Cl2 + TBAPF6 In/SnOx + dissolved O2 PBS + Na2S2O3 PBS + H2O2 PBS + K2S2O8

435

435

0

−1.07 (Ag/AgCl)

PBS + K2S2O8

100

470

470

0

+1.9 (Ag/AgCl) −1.0 to + 1.0 (Ag/AgCl)

101 102

910

910

0

−1.5 to + 1.5 (AgQRE)

TEA + PBS annihilation type, KNO3 annihilation type, CH2Cl2 + TBAP K2S2O8 + NaOH annihilation type, CH2Cl2 + TBAP K2S2O8 PBS + (NH4)2C2O4

CdTe@CdS@ZnS C3N4

1.49 2.7

nanoflake [(5−35 nm) × (40−220 nm)]

CuSe

2.18

PbS

0.37

nanotube (thickness 60 nm; diameter 200 nm) dot (∼3 nm)

PbSe

0.28

nanosphere (100 nm) hollow nanostructure

−1.4 (Ag/AgCl) −1.8 to + 1..8 (Ag/AgCl)

SnO2 F-doped SnO2 TiO2

3.6 3.6

nanocrystal (5−6 nm) nanocrystal (50 nm)

−1.8 (Ag/AgCl) −2.0 (Ag/AgCl)

3.2

nanocrystal (20 nm) nanotube (diameter, 80 nm)

414, 472

N-doped TiO2 WO3

3.2

nanotube (diameter, 60 nm)

2.75

nanocrystals (30 nm)

467

ZnO

3.2

dot (5 nm) nanoflower (composite with CNT) nanoneedles

550 373

ZnS Mn: ZnS ZnSe

ref

−1.25 (Ag/AgCl)

(Ag/AgCl) (Ag/AgCl) (SCE) to + 2.0 (Ag/AgCl)

−1.2 (Ag/AgCl) + 0.93 (Ag/AgCl) 640

ECL conditions and coreactant K2S2O8 Na2CO3−NaHCO3 + H2O2 K2S2O8 + NaOH PBS + K2S2O8 PBS + K2S2O8 annihilation type, CH2Cl2 + TBAP PBS + H2O2

−1.5 −1.8 −1.7 −2.0

541

Mn:CdS

CdSeTe@ZnS CdTe

λECL/ nm

3.6 3.6 2.7

nanopyramids, nanocolumns, nanoflakes dot (5−7 nm) dot (2.5−4 nm) dot (2.2 nm)

510, 554, 606 540

440 585 380

575 378 569, 605 600

25 5

460

20

11030

73

96 97 98 99

103 104 105 106 107

−1.3 (Ag/AgCl) −1.3 (Ag/AgCl)

KCl + dissolved O2 KNO3 + dissolved O2

108 109

−1.2 (SCE)

PBS + K2S2O8

110

+1.13 (SCE)

111

−2.0 (Ag/AgCl) −1.63 (Ag/AgCl) −1.8 (Ag/AgCl)

NH3−NH4Cl + TPrA PBS + K2S2O8 K2S2O8 + NaOH K2S2O8 + NaOH

−1.8 (Ag/AgCl)

K2S2O8 + NaOH

−2.0 (Ag/AgCl) −1.50 (Mn2+), −2.38 (ZnS) (SCE) −1.3 (Ag/AgCl)

K2S2O8 + NaOH PBS + H2O2 PBS + H2O2

112 113 114 115, 116 117 118 119


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Table 1. continued a

TBAP, tetra-n-butylammonium perchlorate; TBAPF6, tetra-n-butylammonium hexafluorophosphate; TEA, triethanolamine; AgQRE, Ag wire quasireference electrode; SCE, saturated calomel electrode. All PL and ECL information was obtained at room temperature in aqueous solution (except annihilation-type ECL).

Figure 4. Comparison of the schematic PL and ECL mechanism of QDs (with R−O ECL as the example).63 In PL, the excitons (electrons and holes) are photoexcited. There are mainly two routes of the excitons after photoexcitation: recombination (band-gap transition) and migration to the surface states for recombination (surface-state transition). In ECL, on the other hand, the excitons (electrons and holes) are injected by electrode and coreactant, respectively. Surface states with lower energy levels will accept excitons before the band gap: (● and ○) Electron and hole, respectively.

Figure 5. Schematic mechanisms of reductive−oxidative (R−O) and oxidative−reductive (O−R) ECL from QDs.

electrochemical investigation of thioglycerol-capped CdS QDs in 2001,64 no ECL light was observed, which was mainly ascribed to the instability of electrogenerated species. This may be the reason why Bard’s group focused on studying ECL from CdSe and CdTe QDs early in the past decade.32−34 However, Ren et al. reported an annihilation-type ECL from CdS spherical assemblies in 2005.71 The difference between these two works lies in the shapes of CdS nanoparticles, i.e., dots in Bard’s work64 and spherical assemblies in Ren’s work.71 In fact, ECL has been frequently obtained in semiconductor nanoparticles of different shape, for example, CdSe hollow spherical assemblies,72,73 CdS nanotube,74−76 ZnO/CdS core/shell nanostructures,77 nanolifebelts of chalcogenides,78 hierarchical CdS microspheres,79,80 and dendritic CdO nanoparticles.81,82 However, little knowledge about the relationship between ECL efficiency and the morphology of semiconductor nanomaterials exists.83 Through understanding the chemistry and property of semiconductor nanoparticles of different shapes,84 studying morphology-related ECL phenomena may be an efficient way for controlling the ECL efficiency of semiconductor nanomaterials. To better understand the ECL from QDs, we summarize the ECL properties of various semiconductor nanomaterials in Table 1. The band gap of their corresponding materials,85 the morphologies, the PL wavelengths, as well as the difference between PL wavelength and ECL wavelength are also listed in Table 1. It can be seen from it that ECL from semiconductor nanomaterials is more complicated than PL, and the number of influence factors is greater than that of PL.

For the ECL of QDs, coreactants donate electrons or holes, while PL is solely excited by photoexcitation. The involvement of coreactants further complicates the ECL process. For example, the onset ECL potential of CdSe@ZnS QDs was more negative with K2S2O8 as a coreactant than that with H2O2 as a coreactant.70 The full width at half-maximum (fwhm) in PL spectra is often a sign of the size distribution of the QDs (smaller fwhm means narrow size distribution). However, no such terminology (fwhm of ECL) has ever been proposed in ECL of QDs. Probably besides the size distribution diffusion of coreactants to the electrode surface also contributes to the fwhm of the ECL spectra. Moreover, after being assembled onto the electrode, the close intact of the QDs may cause change to the surface, resulting in more fruitful surface energy states and emitting ECL in a broad voltage range. The luminescence mechanisms of R−O and O−R ECL of QDs are illustrated in Figure 5. In R−O mode (negatively scan), the electrode first injects an electron to the conduction band of QDs. Meanwhile, the coreactant (after reduction by electrode) injects a hole to the valence band of QDs. Subsequent recombination of electron and hole, just like that occurs in a PL process, results in R−O ECL emission. For O− R ECL (positively scan), the roles of electrode and coreactant, i.e., the origins of the electron and hole, are exchanged. It should be noted that here the use of conduction and valence bands may be not fully precise, since the locations of the electron and hole may also be surface energy states. When studying the ECL from semiconductor nanomaterials, it can be seen that besides typical dots, ECL from nanomaterials with various shapes is frequently reported. A representative example to illustrate the importance of shape and morphology of semiconductor nanomaterials can be seen from the earlier ECL studies. At the beginning of Bard’s

2.2. Coreactants in ECL of QDs

Besides a different stimulation source, another major difference between ECL and PL of QDs is that ECL typically requires involvement of coreactants. Several early reports investigated annihilation-type ECL from QDs, which is generated by redox 11031


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Table 2. Typical Coreactants and Their Corresponding Reactions for QDsa coreactant

ECL reaction

ref

O2 + H 2O + 2e− → OOH−• + OH−

O2

93

QD + e− → QD−•

2QD−• + OOH−• + H 2O → 3OH− + 2QD* H2O2

H 2O2 + e− → OH• + OH− −

122

−•

QD + e → QD

QD−• + OH• → OH− + QD* S2O8

2−

123

S2O82 − + e− → SO4 2 − + SO4−• QD + e− → QD−• QD−• + SO4−• → QD* + SO4 2 −

TPrA (DBAE, TEA)

TPrA → TPrA+• → TPrA• + H+ +•

QD → QD

+e

124−126

−

TPrA• + QD+• → QD* + TPrA fragments SO32−

QD → QD+• + e−

97

→

SO3−•

+ e−

2OH +

SO3−•

+ 2O2 → O2−• + SO4 2 − + H 2O

SO3

2− −

QD + O2−• → QD−• + O2 QD−• + QD+• → QD* C2O42−

30

C2O4 2 − → CO2−• + CO2 + e−

QD → QD+• + e− QD+• + CO2−• → QD* + CO2 ITO and O2

In/SnOx + QD → QD+• + In/SnOx−• O2 + In/SnOx

QD + O2 +•

QD a

−•

−•

→ O2 −•

→ QD −•

+ QD

−•

96

+ In/SnOx

+ O2

→ QD*

TPrA, tripropylamine; DBAE, 2-(dibutylamino)ethanol; TEA, triethylamine; ITO, indium−tin oxide.

cycling.30,32−35,71,103,105,120 Ion annihilation ECL involves formation of an excited state as a result of an exergonic electron transfer between the electrochemically generated QD+• and QD−• and then the newly formed radical cation and anion annihilated to form the excited state of QDs (QD*) and emit light

coreactants. Typical coreactants and corresponding reactions for ECL of QDs are listed in Table 2. Knowledge of ECL coreactants is mainly based on previous solution studies of Ru(bpy)32+ or other solution-phase ECL luminophores.28,121 To better understand the coreactants, the readers are referred to previous reviews.24−26,36 Although both O−R and R−O ECL coreactants for QDs have been proposed, R−O ECL is more popular than the O−R one. It is usually traced to the efficiency and energies of the coreactants.24,25 Most probably for ECL of QDs the coreactants for R−O ECL are more efficient than those for O−R ECL. Through comparison of the ECL coreactants with those of Ru(bpy)32+ it can be concluded that most O−R coreactants for QDs are generally inherited from the previous Ru(bpy)32+ system. For example, in a detailed comparison on the performance of several amines (TPrA, DBAE, TEA, diethylamine, triethanolamine, and thanolamine) as O−R coreactants for CdTe QDs, DBAE was found to be the most efficient coreactant,125 which is similar to the case in the Ru(bpy)32+ system.127 A promising O−R ECL of QDs has been found on ITO electrode, in which ITO takes part in the O−R ECL process96 and bovine serum albumin (BSA)128 and L-proline129 were found to promote this process. To select an appropriate coreactant for ECL of QDs, the relative energy levels of QDs and the coreactant should be

QD−• + QD+• → QD* + QD QD* → QD + hν

It should be noted that although annihilation-type ECL of QDs has been reported, modern ECL analytical applications of QDs are almost exclusively based on coreactants. Coreactants can help to overcome either the limited potential window of a solvent or poor QD anion or cation stability, which substantially improves the ECL intensity of QDs. Taking the R−O ECL of QDs as the example (vice versa for the O−R ECL of QDs), within the reductive potential scan of the electrode, QDs are reduced to form anion radicals (QD−•). Meanwhile, coreactants in solution are also reduced to give rise to the intermediate coreactant radicals, which is a kind of powerful oxidizing species that can react with the anion radicals of QDs to produce the excited states that emit light. As indicated in Figure 5, the coreactants used in ECL of QDs can be mainly divided into two types: R−O ECL coreactants and O−R ECL 11032


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(Figure 7B).136 The size-dependent properties of QDs mainly originate from the quantum confinement effect of QDs and have been well documented in many previous reviews and book chapters. Probably because of the poor wavelength-resolving ability of the present ECL instruments, there are only a few reports studying the effect of size on the ECL properties of QDs.130,137,138 From these few references one can still find that the size dependence of the ECL emission is similar to that of PL. In an investigation on the size-dependent R−O ECL behaviors of CdTe QDs with dissolved oxygen as coreactant, it was found that both the ECL peak potentials and the ECL onset potentials shifted positively with increasing size of the QDs (Figure 8A).137 However, the correlation between the ECL peak potential and the potential for electron injection into the conduction band of QDs is absent in the above study, which makes the conclusion of band-gap ECL not fully convinced. For quantum rods, similar size-dependent ECL emission wavelengths as the PL of QDs were observed.138 Emission peaks were at 460, 480, and 500 nm for 2.4, 3.3, and 3.6 nm CdS nanorods, respectively (Figure 8B). Clearly, to better understand the size dependence of the ECL characteristics of QDs, further extensive investigations are needed.

taken into consideration for better sensitivity and stability. For CdSe@ZnS QDs, the R−O ECL was more stable with K2S2O8 as coreactants, while the ECL intensity first got to a peak value and then went down quickly after 400 s with H2O2 as a coreactant.70 Further investigation indicated that the decomposition production of H2O2, O2−, possesses lower energy level (O2− → O2 + e−) than the HOMO of QDs. Therefore, O2− would inject electrons to the LUMO of QDs ahead of radiative recombination of electrons at the HOMO band with holes at the LUMO band (ECL process), leading to ECL quenching. On the other hand, selecting QDs with a HOMO energy lower than that of O2−/O2, stable ECL was obtained. Such an energylevel-related ECL mechanism of QDs is very important for choosing proper QDs and coreactants for design of an ECL sensor. It should also be noted that O−R ECL from QDs can be simply obtained on ITO electrodes, due to the involvement of In/SnOx and O2 in the ECL process.96,130 Both CdTe17,22 and CdSe131 QDs were found to be efficient ECL emitters at ITO electrodes. Compared with other coreactants, such ECL systems involve the electrode material and dissolved O2 for ECL reactions without any extra additives. Recently, several semiconductor metallic oxides have been found to greatly enhance the ECL intensity of QDs, acting as a new type of “assistant coreactants” for the R−O ECL of QDs.132−135 For example, it was found that the R−O ECL intensity of CdTe QDs (with H2O2 as the coreactant) could be increased by 70 and 250 times when compositing with TiO2 nanosphere and P25 (TiO2 NPs of 25 nm), respectively.132 Presumably, integrating QDs with TiO2 can generate a metastable state to accept electrons and transmit them to the coreactant (H2O2, Figure 6), thus accelerating and promoting

2.4. Dopant-Dependent ECL and PL

Doping is a widely used technological process in materials science that involves incorporating atoms or ions of appropriate elements into host lattices, which strongly influence optical and electrical properties of host QDs.139,140 For example, doping of Mn2+ into ZnS QDs results in the disappearance of defectrelated emission of ZnS, giving rise to the occurrence of a new emission centered at 590 nm due to the Mn2+4T1 → 6A1 transition (Figure 9A).140,141 Similarly, the ECL spectrum of Mn-doped QDs also exhibits an emission band at approximately 585 nm.91 For Eu-doped CdS QDs, an ECL emission band at 620 nm has been observed.92 In PL experiments, direct excitation of the Mn2+ 4T1 → 6A1 transition is spin forbidden, with maximum absorption coefficients (εMn2+) 4−5 orders of magnitude lower than those of typical II−VI QDs. Excitation of the Mn-doped QDs starts with creation of an electron−hole pair followed by oxidation of Mn2+ to Mn3+ by the generated hole142 Mn 2 + + h+ → Mn 3 +

Subsequent recombination of Mn3+ with a surface-trapped electron results in Mn2+ in an excited state and is followed by the Mn2+ emission

Figure 6. Possible mechanism of the enhanced R−O ECL of CdTe QDs (with H2O2 as coreactant) by TiO2 NPs. Reprinted with permission from ref 132. Copyright 2012 Wiley-VCH Verbg GmbH&Co. KGaA, Weinheim.

Mn 3 + + e− → (Mn 2 +)* (Mn 2 +)* → Mn 2 + + hν

electron transport between the conduction band of the QDs and the HOMO of the coreactant. Transfer of electrons from reduced QDs to TiO2 is helpful to improve the QD recovery efficiency and enhance and stabilize light emission in the ECL process.

Such Mn2+ transition process has been observed in ECL of Mndoped ZnS QDs.118 As shown in Figure 9B, besides the emission peak at −2.2 V (vs SCE, for ZnS), a specific ECL emission at ∼−1.5 V (vs SCE) was observed for Mn-doped ZnS QDs. The cyclic voltammogram clearly showed a redox peak at ∼−1.5 V (vs SCE), very close to the reduction potential of Mn3+ to Mn2+.143,144 Generation of Mn3+ was thought to be through the following pathway (with H2O2 as a coreactant)

2.3. Size-Dependent ECL and PL

For typical QDs, the most appealing characteristic is sizedependent photoluminescence. For example, CdSe@ZnS QDs exhibit size-dependent color (absorption) and PL spectra: when increasing the size of CdSe@ZnS QDs from 2.7 to 4.8 nm, the emission wavelength of the QDs shifts from 510 to 610 nm (Figure 7A).19 Through varying the QD material and size, PL spectra of QDs can span from the UV to the near-infrared

S2 − + H 2O2 → S− + OH + OH− Mn 2 + + •OH → Mn 3 + + OH− 11033


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Figure 7. Size-dependent PL characteristics of QDs. (A) Cartoon, photograph, and PL spectra of typical CdSe@ZnS QDs with increasing sizes. Reprinted with permission from ref 19. Copyright 2011 American Chemical Society. (B) Representative QD core materials scaled as a function of their emission wavelength superimposed over the spectrum. Reproduced by permission from ref 136. Copyright 2005, Rights managed by Nature Publishing Group.

2.5. QD-Based Composites for ECL and PL

Control experiments indicated that such ECL emission and reduction peaks are both absent in ZnS QDs without Mn2+ doping. This new ECL emission may be a promising application in biological detection, since it is generated from less toxic Mn-doped ZnS QDs and occurred at a reasonably low potential. However, the precise mechanism of this ECL emission still requires detailed investigations for further rational regulation. Synthetically, incorporation of dopant into host QDs often suffers from the “‘self-purification”’ effect of the host material, i.e., self-exclusion of dopants from small nanocrystals due to higher formation energy of dopants as compared to the hosts.146 Besides, the mismatches between the size (e.g., the ionic size of Mn2+, 0.85 Å, is different from that of Zn2+, 0.75 Å, or Cd2+, 0.95 Å in Mn-doped ZnS or CdS QDs147) and charge (e.g., the trivalent lanthanide ions and typical II−VI QDs) of dopant ions and the host cations can further induce surface states to the QDs. Since ECL is sensitive to surface states, doping often results in increased ECL emission. For example, the O−R ECL emission of CdSe QDs was increased 3-fold upon Co2+ doping.148 Eu-doped CdS QDs exhibited 4-fold enhancement in R−O ECL intensity as compared to pure CdS QDs,92 which is possibly associated with charge compensation of Eu3+ in divalent CdS matrix. N doping in TiO2 nanotube resulted in a 10.6-fold R−O ECL intensity improvement and movement of the onset ECL potential more positively by about 400 mV.110 All of this evidence indicates that doped QDs represent a new class of ECL emitters.

To achieve efficient ECL of QDs for bioanalysis, compositing QDs with other materials that feature either excellent conductivity or good film-forming ability is a common practice besides selecting an appropriate coreactant. In this manner, metallic nanoparticles and carbon nanomaterials (mainly carbon nanotubes and graphene) are the most often used materials for efficient ECL emission. Taking advantage of their excellent conductivity, carbon nanomaterials and metallic NPs can reduce the electron relay barrier between QDs and the electrode, accelerating the electron/hole injection rate, thus not only enhancing the ECL intensity but also reducing the overpotential of ECL systems. To prepare the composites for ECL, the simplest method is to mix the QDs with another nanomaterial with the aid of ultrasonication or electrodeposition. As shown in Figure 10A, compositing carbon nanotube (CNT) with CdS QDs resulted in ca. 5-fold increase of ECL intensity, as compared to pure CdS QDs.122 Meanwhile, the ECL onset potential was positively moved by 300 mV (from −1.15 to −0.85 V vs SCE). The reductive current of CdS QD−CNT compositemodified electrode in H2O2 solution was higher than that of pure CdS QDs, demonstrating that CNT helped decrease the electron-transfer barrier from the electrode to QDs and H2O2 and promoting generation of CdS−· and •OH. In another example, compositing CdS QDs with AuNPs resulted in ca. 4fold ECL enhancement, accompanied by slight positive movement of the ECL onset potential (Figure 10B).149 Other composites of CdS−Ag nanocomposite arrays,150 Ag/ TiO2 nanotubes,151 C3N4−Au nanoparticles,152 and CdS11034


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Figure 8. Size-dependent ECL characteristics of QDs. (A) Onset potentials and ECL intensity of CdTe QDs solution with different sizes. Reprinted with permission from ref 137. Copyright 2009 Elsevier. (B) ECL emission spectra of CdS nanorods with different sizes. Reprinted with permission from ref 138. Copyright 2012 American Chemical Society.

Figure 10. Comparison of the ECL behaviors of QDs and QD-based composites. (A) CdS QDs−carbon naotube composite (0.10 M PBS, pH 9.0, 10 mM H2O2). Reprinted with permission from ref 122. Copyright 2006 Royal Society of Chemistry. (B) CdS QDs−AuNPs composite (0.10 M PBS, pH 10.0, 1 mM H2O2). Reprinted with permission from ref 149. Copyright 2010 Elsevier. (C) CdSe QDs− graphene (GR) composite (0.10 M PBS, pH 7.4, 0.10 M K2S2O8 and 0.10 M KCl). Reprinted with permission from ref 156. Copyright 2011 WILEY-VCH Verbg GmbH&Co. KGaA, Weinheim.

PAMAM composite153,154 were reported. Room-temperature ionic liquid, as a good ionic conductor, can greatly increase the ECL intensity of QDs in solution.155 Although successful and effective, it should be noted that direct mixing for compositing QDs with CNTs or AuNPs lacks the consideration about their compatibility. Due to the large surface area and multiple surface functional groups of carbon nanomaterials, in situ growth of QDs at CNTs or graphene has been demonstrated as an interesting approach for integrating QDs with CNTs or gra-

Figure 9. ECL−potential curve and PL spectra of ZnS and Mn-doped ZnS QDs. (A) PL spectra of ZnS and Mn-doped ZnS QDs. Reprinted with permission from ref 145. Copyright 2011 WILEY-VCH Verbg GmbH&Co. KGaA, Weinheim. (B) Electrochemical and ECL behaviors of bare GCE, GCE modified with pure ZnS QDs, and GCE modified with Mn-doped ZnS QDs (doping percent 1.9 wt %) in pH 9.0 PBS containing 5.0 mM H2O2. (Inset) Corresponding cyclic voltammograms. Reprinted with permission from ref 118. Copyright 2008 American Chemical Society. 11035


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phene,113,157−159 for example, in situ growth of CdS QDs on the wall of CNTs.157 Compared with pure CdS QDs, CdS QDs@CNTs could enhance the ECL intensity by 5.3-fold and move the onset ECL potential positively by about 400 mV, which reduces H2O2 decomposition at the electrode surface and increases the detection sensitivity of H2O2. To improve the compatibility of QDs with CNTs or graphene, polyelectrolytes such as PDDA and chitosan are frequently used to aid the compositing. These polyelectrolytes possess excellent film-forming abilities, which can substantially improve the distribution of QDs on the electrode surface and protect QDs from leakage from the electrode. For example, the CdSe QDs−graphene composite was prepared via the aid of poly(diallyldimethylammonium chloride) (PDDA).156 As shown in Figure 11A, PDDA-functionalized graphene was

on electrode and could increase the ECL of QDs,108,170,171 in which Nafion provides refuge for the free radicals and enhances electron−hole recombination. Doping CNT or graphene with trace impurities provides a facile route for adjusting the electronic properties and can further enhance the electrical conductivity of these materials. For instance, K+-doped graphene by π conjugation of K+ chelators onto the graphene basal plane has been demonstrated to act as an electron-transfer medium and more efficiently promote charge transfer than unmodified graphene.172 Upon compositing with QDs, K+-doped graphene substantially accelerates charge transfer and promotes the ECL of CdS QDs.173,174 Similarly, the R−O ECL of CdSe QDs composited with N-doped CNTs was 5-times stronger than that of pure QDs and 3-times higher than that composited with undoped CNTs.175,176 Although highly efficient in promoting the ECL of QDs, the strong photoabsorption of carbon nanomaterials (the so-called blackbody effect) and metallic NPs (localized surface plasmon resonance) can also induce ECL quenching. In PL measurement, PL quenching of QDs by either CNT or graphene was frequently reported.177−181 Modulation of the PL of the QD− CNT/graphene hybrid material with biorecognition events has proven to be a very efficient strategy for biosensor development.182−185 Therefore, ECL enhancement by carbon nanomaterials or metallic NPs is in fact a competition between the production of QD* and antagonistically annihilating its irradiation optically. In practical applications, the amounts of carbon nanomaterials or metallic NPs should be investigated to compromise these two effects for best ECL performance.

3. ANALYTICAL APPLICATIONS OF ECL AND PL OF QDS Efficient biosensing methodologies have now become the main driving force of innovation for studying the ECL from semiconductor nanomaterials. Since ECL is eventually a lightemitting detection method, developing various sensing strategies has many similarities with those of PL detection. In this part, we will focus on the discussion of ECL biosensing schemes that share similarities with PL biosensors. In addition, major ECL sensing strategies that are based on knowledge from electrochemistry are also included.

Figure 11. (A) Schematic of the preparation procedure of CdSe QDs−PPDA−graphene composites, including the oxidation of graphite to graphite oxide (GO) with abundant oxygen functionalities, in situ reduction of GO in the presence of PDDA to obtain positively charged PDDA-protected graphene colloids, and preparation of QD− PDDA−graphene composites via electrostatic interactions under sonication. (B) Schematic illustration of the stepwise immunosensor fabrication process, including formation of QD−PDDA−graphene composite film on the Au electrode, linkage of PDDA to the film, conjugation of GNPs to PDDA, immobilization of antibody (Ab) on the electrode via GNPs, and specific immunoreaction. Reprinted with permission from ref 156. Copyright 2011 WILEY-VCH Verbg GmbH&Co. KGaA, Weinheim.

3.1. Ion-Induced ECL and PL Quenching

Functionalization of QDs with ligands that bind metal ions provides a general approach to develop functional QDs acting as fluorescent ion sensors. The ability of many ions or metal− ligand complexes to quench the PL of QDs provides an intrinsic feature for the operation of these sensors. Since the pioneering work by Chen and Rosenzweig in 2002, who employed photoluminescent CdS QDs as probes for Cu2+, Zn2+, and Fe3+,186 a large number of QD-based fluorescent metal ions probes were developed.187−193 Similarly, the ECL of QDs can also be quenched by metal ions, giving rise to ECL probes for metal ions.107,194−197 Although some metal ions have also been reported to induce fluorescence enhancement toward QDs,198,199 ECL enhancement caused by metal ions was seldom reported.200 To improve the selectivity of metal fluorescent probes, chemically functionalized QDs with metal ion-specific receptors have been proposed,201−204 but such attempt has seldom been realized in ECL probes for metal ions.205

first prepared by standard exfoliation/in situ reduction of graphene oxide in the presence of PDDA. Next, negatively charged thioglycolic acid-capped CdSe QDs were electrostatically adsorbed onto PPDA−graphene to yield QD−PDDA− graphene composite. Similar to the CdS QDs−CNT composite, the involvement of graphene greatly enhances the ECL intensity of CdSe QDs (ca. 4-fold) and moves the onset potential of QDs from −1.3 to −1.2 V (vs SCE, Figure 10C). The presence of P-GR effectively decreased the potential barriers of the ECL reduction, which might be attributed to the extraordinary electron transport of graphene. The subsequently incorporated AuNPs provided multivalence to immobilize target biomolecules for various ECL biosensors (Figure 11B). A series of similar composites using PDDA and chitosan as the interface for QDs and CNTs, graphene, magnetic NPs has been developed accordingly.160−169 QDs incorporated into Nafion also proved to be an effective way for preparation of QD film 11036


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low as subattomolar for PL immunoassays and cell apoptosis imaging,246,247 while integration of tyramide and polymerization-assisted signal amplification for highly loading of QDs as signal labels resulted in 9.4-fold signal improvement for QDbased ECL immunoassay.248 It should be noted that the sensitivity improvement factor in PL system is much larger than that in ECL system, which might be ascribed to less efficient electron transport from electrodes to QD assembly in ECL systems. Besides, the whole processes of electron and energy change in ECL are dependent on various experimental conditions except the composition and structure of QDs. Nevertheless, compared with immobilizing QDs on electrodes as sensing substrate, employing QDs as labels would result in increased steric hindrance and an obvious electron-transfer barrier from electrodes to QDs. Accordingly, for immunoassay, although various amplification and QD-enrichment strategies were employed, actual improvement of sensitivity may not be so pronounced. DNA, on the other hand, provides far less blocking of electron transfer. Thus, it is expected that the loading of QDs on support as ECL labels may potentially provide higher sensitivity improvement in DNA analysis than in immunoassays. Analyte-blocked adsorption/assembly of QDs on electrodes, on the other hand, provides turn-off ECL detection, i.e., the concentration of analyte is inversely proportional to the ECL intensity.249−253 In the O−R ECL mode, electrodeposition of the serotonin oxidative products inhibited the potentialtriggered adsorption of CdSe QDs on GCE, thereby decreasing ECL intensity.254 In another approach, the enzymatic reaction was combined with the analyte-blocked adsorption of QDs on electrode.255 As shown in Figure 13, alkaline phosphatase

Anions can also serve as quenchers for PL and ECL of QDs. For example, cyanide could quench the PL of CdSe QDs with high sensitivity.206 Nitrite is believed to quench the ECL of QDs througha possible “electrochemical oxidation inhibition” process,207 which produces a large IR drop and makes the practical electrode potential less than the applied potential.208 Both R−O and O−R ECL of QDs can be quenched by nitrite, giving rise to nitrite biosensor with sensitivity down to nanomolar to micromolar.207,209−211 Halide ions have been reported to quench the O−R ECL of CdSe QDs in the following order I− > Br− > Cl−.126 3.2. QDs as ECL and PL Labels

To gain the best signal-to-noise ratio of both PL and ECL detection with QDs, labeling QDs with recognition moieties as signal tag is a good choice, because they present barely background signal and normally show a turn-on response. Labeling QDs with biomolecules such as antibody and DNA endows QDs with high specific recognition for immunoassay, DNA analysis, and aptasensing. The schematic principle of using QDs as ECL labels is shown in Figure 12 using

Figure 12. Schematic principle of using QDs as ECL labels for immunoassay.

immunoassay as an example. Normally, to ensure complete binding of target analytes with a QD-labeled recognition moiety, excess amounts of labeled QDs are needed. In PL detection, the homogeneous nature makes separation tedious and often requires external help. On the contrary, ECL detection allows facile separation since the electrodes can be easily removed away from the excess amounts of labeled QDs. QDs as labels for ECL detection have been extensively employed for various bioanalyses.212−222 Loading multiple QDs on appropriate supports can substantially improve the analytical sensitivity of QD-labeled PL and ECL methods. For example, the SiO2@QDs@SiO2 superstructure exhibited approximately 200-times stronger PL than single QD.223 In QD-based ECL analysis, signal amplification can also be achieved via using support-loaded QDs as ECL labels. As compared to the unamplified method, employing CdTe QDs-coated SiO2 NPs as ECL label for immunoassay resulted in 6.6-fold enhancement in the ECL signal for IgG detection.224 A series of QDs-loaded structures were developed to improve the ECL detection sensitivity. These supports include SiO 2 , 174,225−230 graphene, 231 ZnO,232−234 carbon nanotube,235 carbon nanosphere,236−238 noble metal nanoparticle,239−241 and dendrimer.242−245 Besides loading on solid supports, self-assembly of QDs with polymers (including proteins) can also lead to significant sensitivity improvements. For example, biological self-assembled QDs with proteins as loading scaffold permitted detection limit as

Figure 13. Schematic illustration of the ALP-triggered inhibited adsorption of CdSe QDs on electrode for turn-off ECL detection ALP detection. Reprinted with permission from ref 255 Copyright 2012 American Chemical Society.

(ALP) can catalyze the hydrolysis and dephosphorylation of phenyl phosphate. In the absence of ALP, CdSe QDs could be electrodeposited on electrode, giving strong O−R ECL; however, in the presence of ALP, the enzymatic product, phenol, could be electropolymerized and deposited on the electrode, which subsequently declined the adsorption of QDs on the electrode, thus weakening the ECL intensity. This assay was able to sensitively detect ALP in the range from 0.5 to 6.4 11037


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similar to the DNA-assembled different sized QDs for brighter fluorescence.273 Up to six times higher ECL was observed as compared to either QD (4.8 or 5.2 nm sized) monolayer.272 The basic requirements for QD-based ECL energy transfer have similarities to QD-based FRET, i.e., spectral overlap and close proximity of energy donor and acceptor. The experiences to choose energy acceptors used in QD-based ECL energy transfer are generally inherited from those in FRET, for example, cyanine dye Cy5.274,275 Ru(bpy)32+ with an absorption band of 400−500 nm was successfully employed as an ECL energy acceptor for R−O ECL of CdS QDs; its mechanism was harvested to develop a sensitive cytosensing platform (Figure 15A).269 Absorption of Ru(bpy)32+ is overlapped with the ECL

nM (activity ca. 2−25 U/L) with a detection limit of 0.5 nM. Apparently, switchable chemical reactions with QDs directly or indirectly can be explored for construction of an excellent ECL sensing device as long as the ECL signal either enhances or reduces greatly. 3.3. ECL and PL-Based Energy-Transfer Assays

Fluorescence (or Föster) resonance energy transfer (FRET) involves the nonradiative transfer of excitation energy from an excited state donor (D) to a proximal ground state acceptor (A), which is driven by dipole−dipole coupling between D and A (Figure 14). Two prerequisites for FRET are efficient

Figure 14. Model picture for the two prerequisites of FRET: spectral overlap and close proximity.

spectral overlap of the donor emission and acceptor absorption profiles and close proximity of the donor and acceptor (Figure 14). Since FRET usually occurs over distances comparable to the dimensions of most biological macromolecules (about 1− 10 nm), it is recognized as a powerful “spectroscopic ruler” for studying a variety of biological processes, including protein− protein interactions, DNA/RNA detection, ligand−receptor binding, and conformation change of proteins and oligonuecleotides.256 The involvement of QDs in FRET substantially contributes to the development of FRET.257−262 As energy donors, QDs have attracted much more attention than conventional organic dyes because of their sharp and tunable emission spectra and a wide range of excitation wavelengths, which offers better control of the spectral overlap and FRET efficiency. In addition, QDs can serve both as energy donors and as “nanoscaffold” for assembly of biomolecules as a biorecognition unit labeled with multiple acceptor fluorophores.263−265 Moreover, QDs are far more resistant to photobleaching than organic dyes, and this makes them suitable for long-term tracing of in vivo bioevents and singleQD-based FRET applications.266 ECL from QDs can also serve as a donor for energy transfer. In the first report about QD-based ECL energy transfer in 2009 the ECL from Mn-doped CdS QDs was found to be quenched by proximal AuNPs, provided that the ECL spectrum of QDs was overlapped with the absorption profile of AuNPs.91 Exploring such ECL energy transfer for DNA analysis allowed ultrasensitive DNA detection in aM range. Since this pioneering research, the same group has further explored the ECL energytransfer-based assays for DNA detection,267,268 cytosensing,138,269 aptasensing,110,270 and immunoassay.134,271 In a recent approach, the energy transfer between graded-gap alloy CdSeTe (4.8 and 5.2 nm sized) QDs bilayers was harvested for efficient excitation of ECL,272 the mechanism of which is very

Figure 15. ECL energy transfer based on CdS QDs−Ru(bpy)32+ donor−acceptor pair for cytosensing. (A) Scheme of the ECL biosensor for determination of β2 M expressed cells, (B) cyclic ECL curves of CdS QDs and Ru(bpy)32+ (PMT voltage 500 V), and (C) the ECL spectra of β2 M/CdS QDs-modified electrode recorded in the absence (i) and presence of 300 cells/mL (ii) or 600 cells/mL Ru−SA labeled cells (iii). (iv) ECL spectra of the bare electrode recorded in the presence of 300 cells/mL Ru−SA-labeled SMMC7721 cells. Concentrations of Ru(bpy)32+ and K2S2O8 were 6 nM and 50 mM, respectively; the PMT was set at 500 V. ECL spectra were obtained by a series of optical filters (spaced 20 nm). (Inset) Absorption and fluorescence emission spectral properties of Ru−SA. Reprinted with permission from ref 269. Copyright 2011 Royal Society of Chemistry.

spectra of CdS QDs (Figure 15C). Meanwhile, in the potential scan range, Ru(bpy)32+ did not emit light (Figure 15B). Ru(bpy)32+ was labeled on the cell surface through specific recognition between biotin and Ru(bpy)32+-conjugated streptavidin, while CdS QDs and anti-β2 microglobulin antibody (β2 mAb) were immobilized onto glassy-carbon electrode. When the electrode was exposed to Ru(bpy)32+−streptavidin-labeled cell solution it could effectively capture cells by immunoreactions between β2 mAb and β2 M expressed on cell surfaces, which triggered ECL energy transfer. A 600 nm long-pass optical filter was employed to spectrally resolve the energytransfer-induced emission of Ru(bpy)32+ from ECL of CdS QDs. An average detection limit of 12.5 cell/mL was achieved. 11038


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Figure 16. Use of activated CdTe QDs as ECL energy-transfer acceptor for immunoassay. (A) Normalized UV−vis absorption and photoluminescence of as-synthesized CdTe QDs. (B) Normalized UV−vis absorption and photoluminescence of EDC/NHS-activated CdTe QDs. (C) Conjugation of activated CdTe QDs with antibody. (D) Sandwich-type solid-state immunosensing platform based on efficient quenching of ECL from Mn-doped CdS QDs by EDC/NHS-activated CdTe QDs. Reprinted with permission from ref 271. Copyright 2010 Royal Society of Chemistry.

Previous reports have demonstrated that PL energy transfer between QDs and dyes can be described with the Föster dipole−dipole formalism.262 In other words, the interaction between QD and dye in energy-transfer systems can be approximately described with point dipole theory. In addition, the correlation of FRET efficiency on the inverse of the separation distance between QD and dye fits well with the sixth power dependence. The Föster formalism typically describes point dipole interaction in colloidal solution, while QDs are mostly immobilized onto the surface of electrodes in ECL operation. The feasibility of fitting QD-based ECL energy transfer with the Föster formalism might be called into question. For example, whether the QD film on the electrode can still be described as point dipole and whether the separation distance between QDs immobilized on the electrode and energy donor correlates to the sixth power dependence remains unexplored. For the distance dependence, few of the previous works investigated the correlation between the separation distance of the QDs and energy acceptor and the quenching efficiency in ECL systems. For the ECL energy

transfer between CdSe@ZnS QDs and Cy5, a Föster radius (R0, designates the separation distance corresponding to 50% FRET efficiency) of 3.6 nm was reported, but such a calculation was based on the assumption of Föster-type energy transfer.274 An ideal FRET must ensure no direct excitation of energy acceptor to eliminate the background signal. However, due to the broad molecular absorption of energy acceptors and potential overlap of the excitation spectra of QDs and energy acceptor, direct excitation of energy acceptor in QD-based FRET may be encountered in some cases, which results in background and false results. To circumvent this problem, twophoton excitation QD-based FRET has been proposed,276,277 but it requires sophisticated and expensive instruments. In ECL energy-transfer systems, the possibility of direct excitation of energy acceptor was minimal because the acceptors are not ECL active in QD’s ECL potential range. Thus, the background from direct excitation of energy acceptor was eliminated. However, employing organic dyes as the energy acceptor in QD-based ECL energy-transfer systems may be problematic due to the involvement of coreactant in QD ECL. Although 11039


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Figure 17. ECL energy transfer from Eu-doped CdS QDs to Au nanorods for sensitive DNA sensing. (A) Absorption and ECL emission spectra of Eu-doped CdS QDs. (B) Absorption of Au nanorods employed as energy acceptor. (C) Scheme of the ECL energy-transfer platform. Reprinted with permission from ref 267. Copyright 2012 American Chemical Society.

compositing with multiwall carbon nanotubes,134 i.e., enriching CdTe QDs with possible support. In previous reports of PL energy transfer the distance between the energy donor and the energy acceptor was found to be lengthened (deviating from the usual sixth power dependence) when the donor or acceptor or both are extended systems with delocalized charge densities,283,284 resulting in long-range resonance energy transfer. In ECL energy transfer, when employing CdTedoped SiO2 nanoparticle as an energy acceptor, a separation distance larger than 10 nm was observed, which indicated that energy scavenging of CdTe QDs could occur at a long distance to allow for successful amplification of ECL quenching.270 Plasmonic AuNPs possess a large molar extinction coefficient, which has frequently been employed as energy acceptor quencher for PL of QDs.285−287 Similarly, AuNPs have also been employed as energy acceptor for ECL energy transfer.56,91,169,244,267,268,280−282 A distinct advantage of using AuNPs as energy acceptor is that its size- and shape-dependent absorption can be tuned to better match the emission spectra of ECL emitters. For example, Eu-doped CdS QDs exhibit two ECL spectral bands at 450−550 nm from the host CdS and at 600−700 nm from the dopant Eu3+ ions (Figure 17A), while Au nanorods exhibit two plasmonic absorption bands

cyanine dye has been employed as an energy acceptor in QDbased ECL energy-transfer systems,274,275 potential oxidation and bleaching of the organic dyes may be expected when strong oxidants such as H2O2 and S2O82− are used. However, whether these organic dyes can keep unchanged when oxidized or reduced ECL potential is presented still needs to be further studied.278 Accordingly, nanoparticles with strong absorption, such as activated CdTe QDs 110,134,135,270,271,279 and AuNPs,56,91,169,244,267,268,280−282 are promising energy acceptors in ECL energy-transfer systems. Through activation with EDC (N-(3-(dimethylamino)propyl)-N′-ethyl-carbodiimide hydrochloride) and NHS (Nhydroxysuccinimide) (Figure 16C), fluorescent CdTe QDs can be changed from photon emitters to strong photon absorbers without PL emission (Figure 16A and 16B).271 Such activated CdTe QDs possess broad absorption like blackbody in a wide spectral range, which can be employed as excellent ECL quenchers by efficiently absorbing ECL of QDs (Figure 16B). An ultrasensitive immunosensing platform was thus developed through using Mn-doped CdS QDs and activated CdTe QDs as the ECL energy donor and acceptor, respectively (Figure 16D). The blackbody effect of CdTe QDs can be further improved by incorporating them into SiO2 nanoparticles270 and 11040


Chemical Reviews

Review

corresponding to the transversal and longitudinal modes of electronic oscillations (Figure 17B). Besides, the position of the longitudinal absorption can be facilely tuned through the aspect ratio. In a recent report of using Au nanorods as the energy acceptor for ECL of Eu-doped CdS QDs, an ultrasensitive DNA sensing platform has been developed (Figure 17C).267 For comparison, the ECL energy-transfer system of Eu-doped CdS QDs−AuNPs was also developed. Studies of ECL energytransfer efficiency indicated that Au nanorod-640 exhibited better ECL quenching ability than Au nanorod-680, Au nanorod-710, and Au nanorod-750 because of the largest spectral overlap of QDs and Au nanorod-640. In addition, all Au nanorods showed better ECL quenching efficiency than AuNPs with only one plasmonic absorption band at 520 nm. Such DNA biosensor allowed sensitive DNA detection in the aM to pM range with a detection limit of 10 aM. For describing PL energy transfer with AuNPs as an energy acceptor there are two additional formalisms besides FRET, namely, nanosurface energy transfer (NSET) and dipole-tometal-particle energy transfer (DMPET). NSET describes a point dipole interacting with an infinite two-dimensional metal surface and shows the fourth power dependence of the separation distance.288 DMPET, on the other hand, is a corrected form for FRET describing a point dipole interacting with proximal metallic nanoparticles in which the ratio between the separation distance (R) and the emission wavelength of the fluorophores (λ) is taken into consideration.289 However, for energy transfer between QDs and AuNPs, since the separation distances (1−30 nm) are always much smaller than λQD (400− 800 nm), DMPET is close to FRET at short distance and deviates FRET at large distance. All three schemes are a through- space mechanism, and their energy-transfer efficiencies strongly depend on the separation distance between QDs and AuNPs. In a detailed investigation about the PL quenching of CdSe@ZnS QDs by proximal AuNPs, the effect of separation distance on the energy-transfer efficiency was controlled center-to-center through using QD−peptide− AuNP conjugates (YEHK−Au−NP, Figure 18).290 For better comparison, data from two previous reports about energy transfer between QDs and AuNPs were also included.291,292 As shown in Figure 18, such PL quenching of AuNPs to QDs could be better fitted with NSET.290 However, in ECL energy transfer with AuNPs as energy acceptor, the study of the ECL quenching mechanism was only at a very infant stage. Mostly the PL quenching model was directly explored for describing the ECL quenching by AuNPs, for instance, the ECL quenching of near-infrared CdSeTe@CdS@ZnS QDs by proximal Au nanorods with varied separation distance.56 Still, the inhomogeneous nature of ECL was not taken into account. Further studies are needed to provide better understanding of the mechanisms driving the pronounced ECL quenching by proximal metallic nanoparticles. The narrow emission profile and broad absorption spectra of QDs promote intriguing multiplexed wavelength-resolved FRET applications263−265 in which multiple dye acceptors share the same QD donor. In QD-based ECL energy transfer, on the other hand, the ECL emission spectra are often complicated and sometimes broader than PL spectra, which makes multiplex applications problematic. Difficulty may be encountered in wavelength resolution because of broad emission spectra of QDs (donor) and typical instrumental setup (no wavelength selectivity); this will make spectral deconvolution and distinguishing one energy-transfer channel

Figure 18. (A) Schematic representation of the QD−AuNP ensemble with YEHK peptide as the linker. (B) PL quenching efficiency vs R for QD−YEHK−AuNP conjugates (red squares) and QD−dsDNA−Au− NP from ref 292 (black triangles) together with best fits using FRET (red line), DMPET (blue line), and NSET (green line). Quenching efficiencies for QD−YEHK−Cy3 conjugates from ref 291 along with a fit using Förster FRET formalism are also shown (black dots and back line). Horizontal error bars are the standard deviation of the distance, and vertical error bars are the standard deviation of the measurement. Reprinted with permission from ref 290. Copyright 2007 American Chemical Society.

from another very challenging. However, a recent study reported a multiplexed ECL energy-transfer immunoassay based on CdS QDs−Ru(bpy)32+ donor−acceptor pair in spatially resolved microchip device.138 Such spatial registration-based multiplex ECL energy transfer may have promising applications in the future due to the ultrasensitive nature of ECL detection. 3.4. Field-Enhanced ECL and PL of QDs for Analytical Applications

In recent years, the interaction between surface plasmon and fluorphores, which results in greatly enhanced fluorescence intensity, has received much attention in physics and biosciences.293−295 Surface plasmons are coherent oscillations of conduction electrons on a metal surface excited by electromagnetic radiation at a metal−dielectric interface. These resonances, associated with noble metal nanostructures, create sharp spectral absorption and scattering peaks as well as strong electromagnetic near-field enhancements. At short distance (e.g.,

Electrochemically Generated versus Photoexcited Luminescence from

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