Quantum Dots with Highly Efficient, Stable, and Multicolor Electrochemiluminescence

Abstract

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.

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.

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).

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.

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 S₂O₈^2– 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).

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.