Quantum Dots with Highly Efficient, Stable, and Multicolor Electrochemiluminescence.

Outstanding photoluminescence (PL) and electroluminescence properties of quantum dots (QDs) promise possibilities for them to meet challenging expectations of electrochemiluminescence (ECL), which at present relies on inefficient and spectral-irresolvable emitters based on transition-metal complexes (such as Ru(bpy)3 2+). However, ECL is reported to be extremely sensitive to the surface traps on the QDs likely because of the spatially and temporally separated electrochemical charge injections. Results here reveal that, by engineering the interior inorganic structure (CdSe/CdS/ZnS core/shell/shell structure) and inorganic-organic interface using new synthetic methods, the trap-insensitive QDs with near-unity PL quantum yield and monoexponential PL decay dynamics in water generated narrow band-edge ECL with efficiencies about six orders of magnitude higher than that of the standard Ru(bpy)3 2+. The band-edge and spectrally resolved ECL from CdSe/CdS/ZnS core/shell/shell QDs demonstrated a new readout scheme using electrochemical potential. Excellent ECL performance of QDs uncovered here offer opportunities to realize the full potential of ECL for biomedical detection and diagnosis.

Introduction

Colloidal quantum dots (QDs) are semiconductor nanocrystals with their sizes in the quantum-confinement regime, which affords size-dependent optical properties.[1] By photoexcitation, their tunable and narrow photoluminescence (PL) is playing a visible role in biomedical labeling,[2,3] back-lighting-units of display,[4] and optical-pumped lasers.[5] Color-pure electroluminescence of QDs in light-emitting-diodes emerges as the candidates for the next generation of display[6,7] and single-photon sources.[8] Recent results suggest that decoupling their electrochemical activity and electroluminescence is the key design principle for high-performance photoluminescent QDs to be efficient and stable emitters in light-emitting-diodes.[9] In addition to PL and electroluminescence, there is another important type of emission mechanism, i.e., electrochemiluminescence (ECL).[10,11] Up to present, performance of colloidal QDs are found to be poor for ECL in the most desirable environment, i.e., aqueous solutions, though they have been widely developed as PL labels in the field of biomedical detections.[11]

Different from PL and electroluminescence, the emissive excited-states of ECL are generated by dark electrochemical reactions involving heterogeneous charge transfer from the solid electrode(s) to the emitters in solution. While the emitters diffuse in the solution freely during the excited-state generation, the electron and hole need to be injected into them separately, either from two spatially separated electrodes or from one electrode and a coreactant in the solution. Though this mechanism suggests grand challenges for QDs to be efficient and stable ECL emitters, it implies great advantages of ECL for certain applications, such as biomedical diagnostics and detections. Using metal coordination complexes as the emitters, ECL has attracted tremendous research attention and been successfully developed as an immunodiagnostic method for quantitative determination of a broad range of disease biomarkers, due to its remarkable features such as rapid response time, wide dynamic range, low sample consumption, high on-board stability, excellent precision, and most importantly zero-background sensitivity.[12−16] Indeed, ∼40 000 analyzers are in use around the world, and each second more than 60 patient samples are measured.[17] In a way, ECL is becoming the newest generation of biomedical diagnostic and detection methods after PL and chemiluminescence, though ECL is yet to develop proper emitters.

Metal coordination complexes—typically ruthenium(II) tris(bipyridine) (Ru(bpy)32+) and their derivatives—offer the best ECL performance so far and are widely applied in commercial diagnostics.[13,18] Luminescence of Ru(bpy)32+ and derivative complexes is usually generated by the radiative decay of triplet metal-to-ligand charge transfer state[19] and thus characterized by broad spectral line width (60–80 nm full-width-at-half-maximum (fwhm)),[18,20] long radiative decay lifetime (typically microseconds),[19] low PL quantum yield (PLQY, ∼4.2% for Ru(bpy)32+),[21] and difficulty for multicolor detection.[20,22] Therefore, though ECL has been regarded as the most advanced immunodiagnostic technology at present, there is a significant room for improvement on nearly every aspect of optical properties of ECL luminophores.

Bard and co-workers demonstrated the first example of QD ECL generation using silicon QDs.[23] Later, ECL generation using various QDs was reported, with cadmium chalcogenide QDs—including their core/shell ones—as the workhorse.[24−26] In aqueous solutions relevant to immunodiagnosis and other biomedical detections, ECL from QDs was also demonstrated,[27−29] including demonstration of multicolor ECL generation with a mixture of QDs.[30−35] However, all QDs applied for ECL generation revealed the substantial influence of surface traps in their emission, indicated by their shifted and trap-emitting ECL relative to PL,[24,25,36−38] low PLQY,[26,39,40] and/or multichannel PL decay dynamics.[41,42] It has been repeatedly reported that ECL is much more sensitive to the surface traps of QDs than the corresponding PL,[25,43] while, for PL, an exciton—an electron–hole pair—is generated by absorbing a photon and rapidly recombined within the center of QD, both electron and hole in ECL need to be separately injected into the QD through its surface. Different from electroluminescence in light-emitting-diodes, the QD would diffuse freely in the solution during the sequential injection events of electron and hole. As a result, trapping of injected electron and/or hole by the surface traps of QDs in ECL can be extremely efficient and can last for a very long duration in comparison with both PL and electroluminscence.

The above discussions suggest that the design of QDs for ECL generation should have its own criteria, instead of those design principles established for either PL or electroluminscence. For example, ideally photoluminescent and electroluminscent CdSe/CdS core/shell QDs are available in the literature,[6,44−46] but a good amount of surface traps would appear after being converted to be water-soluble—the first step for biomedical applications with ECL—using strongly protective ligands (polyvinyl achohol).[44] In principle, additional ZnS shells with a very wide bandgap on the CdSe/CdS core/shell QDs might offer an enhanced barrier to isolate those surface traps accessible for ECL generation. Similar structures using CdS intermediate layers to relieve the large lattice strain between CdSe and ZnS were studied in the literature.[47,48] Here, we would first synthesize CdSe/CdS/ZnS core/shell/shell QDs using a new scheme, which brought the PL properties up to a nearly ideal level in both nonaqueous and aqueous solutions, i.e., near-unity PLQY (>90%), narrow PL peak, and monoexponential PL decay dynamics. Comparative studies reveal that ECL is indeed more sensitive to the surface traps on the QD surfaces than the PL and electroluminescence, and ECL requires building a unique inorganic–organic interface in the synthesis of the QDs. Remarkably, ECL efficiency of the CdSe/CdS/ZnS core/shell/shell QDs with ideal PL properties in water was found to be 6 orders of magnitude more efficient than that of Ru(bpy)32+. The efficient, stable, and narrow band-edge ECL further enabled spectrally resolved and potential-dependent ECL generation from CdSe/CdS/ZnS core/shell/shell QDs with different core sizes.

Results and Discussion

Design and PL Properties of Water-Dispersible QDs

Recently, colloidal QDs have been developed to show trap-free recombination of the photogenerated excitons, i.e., near-unity PLQY and monoexponential PL decay dynamics. Among these trap-free QDs, the plain core QDs are limited to CdSe ones,[49] and the most developed core/shell QDs are CdSe/CdS core/shell ones.[44−46] No matter what the structures/compositions of the trap-free QDs are, they are all synthesized in nonaqueous solutions and not suitable for biomedical applications. QDs synthesized directly in water often possess electron/hole traps inside the interior of the lattice due to their typically poor crystallinity.[50,51] Such interior traps should be difficult to be eliminated by postsynthesis treatments. Thus, in comparison with direct synthesis of trap-free QDs in water, it should be less challenging to convert those trap-free QDs in nonaqueous solution into ideal emitters in water.

Figure a (black curve) shows sharp PL spectrum of CdSe core QDs with nearly monodisperse size distribution (Figure S1, Supporting Information) synthesized using standard nonaqueous procedures.[49,52] The PLQY of CdSe core QDs was near-unity in toluene, in accordance with the monoexponential PL decay dynamics (Figure b). Ligand exchange with typical hydrophilic ligands,[53] i.e., mercaptopropionic acid, converted them to be water-soluble, but the resulting CdSe core QDs in water became barely emissive (Figure a, red curve).

Figure 1

PL properties of CdSe (3.1 nm in diameter), CdSe/CdS core/shell (with five monolayers of CdS shells, 5.6 nm in diameter), and CdSe/CdS/ZnS core/shell/shell QDs (with additional three monolayers of ZnS outer shells, 7.1 nm in diameter). Steady-state (a, c, e) and transient (b, d, f) PL spectra of three types of QDs before (black) and after (red) transfer from toluene to water.

CdSe/CdS core/shell QDs with nearly ideal PL properties in toluene (Figure c–d, black curves) were synthesized following an established method.[46] The PLQY of CdSe/CdS core/shell QDs with five monolayers of CdS shells decreased from near-unity (>90%) in toluene to 31% in water (Figure c). Consistent with the appearance of surface traps in water, Figure d reveals that the transient PL of CdSe/CdS core/shell QDs was converted from monoexponential decay in toluene to double-exponential decay in water (see detail on the analysis of PL decay dynamics in the Supporting Information).

Significant quenching of PL (Figure a,c) and appearance of the secondary PL decay channel with a short PL decay lifetime (Figure d) in water should have two related origins, i.e., introduction of thiolate ligands (deep hole traps)[53] and immersion of the QDs into water (weak hole traps).[54] According to the literature,[55,56] the very wide bandgap of epitaxial ZnS shells may isolate these traps from the photogenerated excitons in a QD. However, the lattice mismatch between CdSe and ZnS (12%) is too great to afford quality epitaxy.[47] In 2005, CdS was introduced as the intermediate layer to relieve the lattice strain between CdSe and ZnS.[48] Though PL properties of the resulting CdSe/CdS/ZnS core/shell/shell QDs were not ideal in the current standard, they were greatly improved in comparison with the CdSe/ZnS core/shell QDs. A new epitaxy scheme was developed to eliminate possible defects, which epitaxially grew the inner CdS shells at 260 °C and the outer ZnS shells at ∼290 °C with metal carboxylates as the sole ligands (see Supporting Information for details). Indeed, the resulting CdSe/CdS/ZnS core/shell/shell QDs with three monolayers of the ZnS shells show near-unity PLQY and monoexponential PL decay dynamics in both toluene and water (Figure e–f). To reach ideal ECL performance, thickness of the CdS (3–8 monolayers) and ZnS (2–3 monolayers) shells needs to be tightly controlled (see detail in Supporting Information). The increase of the monoexponential decay lifetime in water for CdSe/CdS/ZnS core/shell/shell QDs (Figure f, from 27.1 to 36.3 ns)—also the main decay channel of CdSe/CdS core/shell QDs (Figure d, from 21.8 to 28.3 ns)—suggests that the wide-bandgap ZnS shells would not prevent electromagnetic coupling of the excitons in QDs and the environment.[57] Thus, charge injection needed for ECL generation might still occur through the ZnS shells.

It is interesting to observe that the inorganic–organic interface for the QDs to be applied for ECL generation needed to be different from that applied for electroluminescence in light-emitting-diodes. For the QDs in light-emitting-diodes, a recent publication[9] revealed that, for highly photoluminescent CdSe/CdS/ZnS core/shell/shell QDs, the inorganic–organic interface should not have excess zinc cations in the form of carboxylate salts. Otherwise, the light-emitting-diodes would become extremely inefficient and unstable in operation. However, for ECL generation in aqueous solution, the inorganic–organic interface of highly photoluminescent CdSe/CdS/ZnS core/shell/shell QDs must be zinc carboxylates for being converted into water-soluble by ligand exchange with the common hydrophilic thiolate ligands. Furthermore, different from poor stability in light-emitting-diodes, the ECL emitters of the CdSe/CdS/ZnS core/shell/shell QDs with zinc-rich interface are extremely stable (see below).

Three types of QDs described above were applied for comparative ECL studies (Figure ). Unless specified otherwise, the core would remain the same as the plain core QDs (3.1 nm CdSe), the CdS shells would be five monolayers for either the core/shell or the core/shell/shell QDs, and the ZnS outer shells would be three monolayers.

Figure 2

ECL of CdSe core, CdSe/CdS core/shell, and CdSe/CdS/ZnS core/shell/shell QDs. (a) Potential-dependent ECL measurements in a traditional three-electrode configuration. (b–d) ECL-potential spectra of CdSe core (b), CdSe/CdS core/shell (c), and CdSe/CdS/ZnS core/shell/shell QDs (d). The insets in c and d compare ECL and PL spectra of two types of QDs at their maximum ECL intensity. The concentration of QDs was 0.3 μmol/L, and the solution was 0.1 mol/L PBS containing 10 mmol/L K2S2O8 (pH = 7.4). A FTO plate, a platinum wire, and a silver/silver chloride (saturated KCl) electrode were used as the working, counter, and reference electrodes, respectively. The potential sweep rate was 100 mV/s in all cases (the same in the following).

Structure-Dependent ECL Generation

ECL generation was measured in aqueous PBS buffer solutions using a home-built system (Figure a). ECL generation from a QD was realized by receiving an electron from the fluoride-doped tin oxide (FTO) electrode and a hole from the reduction product (SO4–•) of the cathodic coreactant (S2O82–). Figure b–d quantitatively compares ECL spectra of three types of QDs measured under the same conditions. Evidently, the ECL and PL spectra of the CdSe/CdS/ZnS core/shell/shell QDs are nearly identical with each other, and the ECL spectrum of the CdSe/CdS core/shell QDs broadens slightly to the low-energy side related to its PL.

The maximum ECL intensity (Imax) in Figure differed from each other drastically. ECL from CdSe core QDs was barely detectable in the entire potential range (from −0.6 V to −1.2 V) (Figure b). Though ECL was quite strong for CdSe/CdS core/shell QDs (Figure c), the maximum intensity was ∼25 times lower than that of CdSe/CdS/ZnS core/shell/shell QDs (Figure d). In comparison, the PLQY of CdSe/CdS core/shell and CdSe/CdS/ZnS core/shell/shell QDs are only different from each other by ∼3 times (Figure c,e), proving the significant role of the outer ZnS shells. To exclude any size effect, CdSe/CdS core/shell QDs with eight monolayers of the CdS shells were also synthesized (Figure S1c). After converting them to be water-soluble, their PL and ECL properties were determined to be similar to the CdSe/CdS core/shell QDs with five monolayers of the CdS shells (Figures S2 and S3). These results thus confirm that, in comparison with CdS shells, thin ZnS outer shells offer substantially better protection for ECL generation (see more results later).

The potential onset for ECL generation was identified as −0.83 V and −0.87 V for the CdSe/CdS core/shell and CdSe/CdS/ZnS core/shell/shell QDs, respectively. Given the conduction-band potential difference between CdS and ZnS is as big as ∼0.9 eV, the slight difference in the ECL onset potential suggests that the electron injection from electrode to QDs should be dominated by potential-independent tunneling. This means that, as long as the ZnS outer shells are not excessively thick, it will not significantly affect charge injection into the QDs during ECL generation.

Extremely Efficient and Stable ECL of CdSe/CdS/ZnS Core/Shell/Shell QDs

Ru(bpy)32+ complexes are the most efficient ECL luminophores reported in the literature, which offer a reference for evaluating ECL generation from QDs. Under the identical conditions (including electrodes) applied for the QDs, the integrated ECL intensity of CdSe/CdS/ZnS core/sell/shell QDs was found to be 4.7 × 105 times higher than that of Ru(bpy)32+ (Figure a,b).

Figure 3

Extremely efficient and stable ECL generation from CdSe/CdS/ZnS QDs. (a–b) Current and ECL intensity curves of 0.3 μmol/L Ru(bpy)32+ (a) and 0.3 μmol/L QDs (b) in 0.1 mol/L PBS (pH = 7.4) containing 10 mmol/L K2S2O8. When measuring ECL generated by the CdSe/CdS/ZnS core/shell/shell QDs, a neutral filter (ND = 4) was positioned in the front of PMT to avoid light saturation. The PMT was biased at 500 V. (c) Stability of ECL generation over multiple cycles of potential sweeping between 0 and −1.2 V. The red curve represents the variation of externally applied potential, and the black curve shows the recorded ECL intensity. The PMT was biased at 300 V. (d) Steady-state absorption and transient PL (inset) spectra of QDs before and after 125 cycles of potential sweeping between 0 and −1.2 V.

Using the model in the literature,[58] one could calculate the ECL generation efficiency of CdSe/CdS/ZnS core/shell/shell QDs (Figure b) with respect to Ru(bpy)32+ under the same experimental condition (Figure a). The ECL generation efficiency of CdSe/CdS/ZnS core/shell/shell QDs was found to be 6.9 × 105 times higher than that of the cathodic Ru(bpy)32+/S2O82– system (see Supporting Information and Table S1 for more details). Considering their high PLQY, short luminescence lifetime, center-localized charges, and large size (equivalent to large reaction cross section), the extremely efficient ECL from CdSe/CdS/ZnS core/sell/shell QDs—6 orders of magnitude higher than that of Ru(bpy)32+—is reasonable.

For any practical ECL luminophores, their emission stability is a key parameter. As mentioned above, the CdSe/CdS/ZnS core/shell/shell QDs applied here were with excess zinc ions on their inorganic–organic interface, which were found to be extremely unstable under operation conditions in light-emitting-diodes.[9]

Measurements with multiple cycles of potential sweeping between 0 and −1.2 V confirmed that the ECL intensity of the CdSe/CdS/ZnS core/shell/shell QDs remained stable and reproducible, with <1% relative standard deviation in ECL intensity for each cycle (Figure c). Furthermore, after potential sweeping between 0 and −1.2 V for 125 cycles (3000 s in total), their UV–vis absorption and transient PL spectra remained identical to those of the original QDs in the same solution (Figure d), confirming their long-term structural and optical stability under operation conditions.

Confirmation of Electron Injection from the Electrode into QDs

In the literature, the mechanism of ECL generation from QDs in similar systems was proposed as follows,[59]

This set of reactions suggest that, upon sweeping the potential to a sufficiently negative value, electrons would be injected from the electrode to the conduction band of QD (eq ). Meanwhile, the coreactants (S2O82–) in the solution would be reduced to produce SO4–• (eq ). Subsequently, SO4–•, being a strong oxidant, would inject a hole to the valence band of QD with an extra electron in its conduction band to form an exciton (eq ). For an efficient QD emitter, the exciton would take radiative-recombination channel to emit a photon (eq ), which completes an ECL cycle and brings the QD back to its ground state.

Among four steps, the last one was discussed in detail in the above subsections, and reduction of S2O82– (eq ) and the subsequent hole-injection into QDs (eq ) have been well-studied in the literature.[11,60−62] Overall, to justify ECL generation of the QDs, the key is to confirm direct electron injection from the electrode to the QDs (the first step). To prove this hypothesis, ECL generation was comparatively explored with a special electrode, i.e., FTO modified with ultrathin silica nanochannel membrane (SNM). As shown in Figure a, SNM deposited on FTO (designated as SNM/FTO) consists of ordered and perpendicular channels.[63]

Figure 4

Comparison of ECL generation from the CdSe/CdS/ZnS core/shell/shell QDs at FTO and SNM/FTO electrodes. (a) Schematic illustration of mass transport of QDs and S2O82– at the SNM/FTO electrode. The thickness of SNM is 51 nm. The side graph shows the top-view transmission electron microscopy image of SNM. (b–c) CVs (b) and ECL intensities (c) of the CdSe/CdS/ZnS core/shell/shell QDs at bare FTO and SNM/FTO electrodes in 0.1 mol/L PBS containing 0.3 μmol/L QDs and 10 mmol/L K2S2O8 (pH = 7.4).

With a thickness of SNM on FTO tunable between ∼50 nm and ∼160 nm (Figure S4), the diameter of the perpendicular channels was controlled in the range of 2–3 nm (side graph of Figure a and Figure S5). Because the diameter of CdSe/CdS/ZnS core/shell/shell QDs is ∼8 nm (Figure S1d), they are impossible to penetrate the nanochannels to reach the underlying FTO electrode surface. As the SNM is insulating in nature and covers the entire surface of the FTO electrode compactly (Figure S6), the electron injection from electrode to QDs could not go through the SNM. Conversely, the SNM is highly permeable to the small-sized coreactants, namely, S2O82–. As shown in Figure b, the current due to reduction of S2O82– at the SNM/FTO is comparable to that at a bare FTO, indicating the step for generation of SO4–• (eq ) can still occur efficiently at the SNM/FTO electrode. Conversely, by replacing the bare FTO electrode with the SNM/FTO electrode (Figure c), the ECL signal of the CdSe/CdS/ZnS core/shell/shell QDs was completely diminished (Figure S7). These results proved that direct injection of electrons from the electrode into QDs (eq ) was essential for ECL generation with the current system.

Multicolor ECL Generation from CdSe/CdS/ZnS Core/Shell/Shell QDs

Figure d reveals that ECL spectrum of the tailor-made CdSe/CdS/ZnS core/shell/shell QDs in aqueous solutions is identical to their PL spectrum. Such a narrow ECL line width should provide a chance for achieving spectrally resolved ECL generation,[20,22,64] which has been known to be difficult for the mostly developed Ru(bpy)32+ complexes. Though our recent work demonstrated a spectrally resolved ECL system using ruthenium and iridium complexes, it required a complicated excitation scheme.[20]

By selecting CdSe core QDs with different sizes, one set of green-, yellow-, and red-emitting CdSe/CdS/ZnS core/shell/shell QDs were synthesized with average diameters of 5.9 nm (emitting at 549 nm), 6.6 nm (emitting at 592 nm), and 9.0 nm (emitting at 643 nm), respectively (Figure S8). To achieve spectrally resolved ECL, the emission wavelengths were adjusted to be ∼50 nm apart from each other. To do so, the thickness of CdS inner shells was not necessarily optimal for bridging the largely mismatched CdSe and ZnS lattices (∼12%). As a result, the PLQYs of green- and yellow-emitting ones were 45% (60%) and 65% (75%) in water (toluene), respectively. With an optimal thickness (five monolayers) of CdS inner shells, the PLQY of red-emitting QDs in water and toluene (>90%) was found to be similar to that of the orange-emitting ones studied above.

Figure a demonstrates that ECL spectra of the green-, yellow-, and red-emitting QDs are all nearly identical with their respective steady-state PL spectra, which are narrow and symmetric. The green-, yellow-, and red-emitting CdSe/CdS/ZnS core/shell/shell QDs generated bright and stable band-edge ECL, which was readily visible and distinguishable from each other with bare eyes (Figure a, inset in the middle). Potential-dependent ECL spectra of three QDs are shown in Figure S9; no obvious shift and broadening was observed upon potential sweeping to the negative limit.

Figure 5

Multicolor ECL generation from CdSe/CdS/ZnS core/shell/shell QDs with different core sizes. (a) Normalized PL (top) and ECL spectra (bottom) of green-, yellow-, and red-emitting CdSe/CdS/ZnS core/shell/shell QDs. The middle inset shows the ECL photographs captured for different QDs. (b) ECL-potential spectra of the ternary mixture of QDs in 0.1 mol/L PBS containing 10 mmol/L K2S2O8 (pH = 7.4). (c–e) ECL spectra and photographs (insets) of the ternary mixture under three different potentials. The concentrations of green-, yellow-, and red-emitting QDs in the ternary mixture were 0.5, 0.3, and 0.05 μmol/L, respectively.

Drastically different from the CdSe/CdS core/shell QDs, the ECL efficiency of the green-, yellow-, and red-emitting CdSe/CdS/ZnS core/shell/shell QDs is found to be approximately proportional to their PLQY (Figure S10). With respect to the red-emitting ones, the ECL efficiency of green- and yellow-emitting CdSe/CdS/ZnS core/shell/shell QDs was 35% and 68%, respectively. Furthermore, the ECL efficiencies of this series of QDs (Figure S10) were found to be in the similar range with the orange-emitting ones in Figure d. These results suggest that, for ECL generation, the ZnS outer shells can isolate the surface traps from emissive core QDs as effectively as it does for PL.

For exploring simultaneous ECL generation from multiple QD emitters, the green-, yellow-, and red-emitting CdSe/CdS/ZnS core/shell/shell QDs were mixed together in an aqueous buffer solution. Upon the potential sweeping from 0 V to −1.4 V, bright ECL was successively turned on (Figure b). Under the given conditions, the potential-onset and potential at maximum ECL intensity increased from red-, yellow-, to green-emitting QDs, which is consistent with the increased bandgap of the CdSe cores. Nevertheless, these features allowed demonstration of a potential-tunable scheme for spectrally resolved ECL (Figure c–e), showing that the overall ECL at different potentials would change its spectrally resolved pattern. This scheme not only enables simultaneous detection of multiple targets but also offers a new dimension for developing multiplexed assays.

Conclusions

CdSe/CdS/ZnS core/shell/shell QDs with 2–3 monolayers of the ZnS outer shells synthesized in nonaqueous solution were found to generate very bright and stable ECL after being converted to be water-soluble, with ECL efficiency approximately proportional to their PLQY. Though PLQY of some of them was suboptimal (ca. 40–70% in water) due to a lack of extensive structure optimization, their ECL efficiency was still 6 orders of magnitude higher than that of widely used Ru(bpy)32+. In contrast, ECL generation from CdSe/CdS core/shell QDs was found to be much less efficient than their PL. The results suggest that the efficiency gap between ECL and PL can be removed by judiciously designing the band and lattice structure of QDs.

Though ECL of QDs in solutions and electroluminescence of QDs in quantum-dot light-emitting-diodes both rely on charge injection, the criteria of QDs for efficient ECL generation were found to be unique. Different from ECL generation, the CdSe/CdS/ZnS core/shell/shell QDs have no comparative advantages over the CdSe/CdS core/shell ones on efficiency of light-emitting-diodes.[9] Furthermore, requirements of the inorganic–organic interface of the QDs for ECL and electroluminescence differ from each other drastically. Overall, these results suggest that, for a specific application—including ECL and light-emitting-diodes—of QDs, tailored design of QDs is necessary.

Although careful design of the thickness and composition of transition inner shells is likely needed, high-quality epitaxy of the ZnS outer shells can be broadly achieved for typical QDs in nonaqueous solutions. With widely developed bioconjugation chemistry for QDs as fluorescence labels,[2,3,44,65−67] the extremely efficient, stable, narrow, and multicolor emitting ECL of CdSe/CdS/ZnS core/shell/shell and other potential core/shell QDs with the ZnS outer shells shall greatly advance ECL-based immunodiagnosis and other relevant biomedical applications. Last but not least, studies on kinetics of ECL generation offer unique opportunities for fundamental understanding of electron and/or hole transfer into QDs from either electrode surface or molecules in solution.