Iridium-complex modified CdSe/ZnS quantum dots; a conceptual design for bifunctionality toward imaging and photosensitization

Abstract

We report the design and synthesis of Ir-complex functionalized CdSe/ZnS quantum dots (QDs), in which the QD plays a key role in imaging, while the Ir-complex acts as a sensitizer to produce singlet oxygen; this conceptual design presents a novel scheme in both bio-imaging and photodynamic therapy.

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Ligand-protected nanoparticles consisting of semiconductor cores surrounded by organic monolayers have attracted considerable interest for applications. The interest in these nanomaterials is motivated by the unique optical and electrical properties of the semiconductor cores induced by the quantum dot size effect, while the organic surroundings can provide stability and additional functionality. These unique properties have made them promising nanomaterials for various potential applications including electronics,¹ optics,² and biosensors.³ For biological application, quantum dots possess several characteristics that make them potential photosensitizers,⁴ and they have recently earned the spotlight as imaging agents⁵ and diagnostics.⁶ As for the next stage for the therapeutics, a technique known as photodynamic therapy (PDT)⁷ has been attracting much attention. Studies have shown that PDT can be as effective as surgery or radiation therapy in treating certain kinds of cancers and precancerous conditions, and may have some advantages: it is less invasive than surgery, it can be targeted very precisely, and it can be repeated several times at the same site if necessary, resulting in less blemishing. PDT involves using a photosensitizing agent (or drug), which is activated by being exposed to light, producing singlet oxygen to destroy cancer cells. In view of exploiting semiconductor nanoparticles in PDT, recent work by Samia^5a and co-workers reported that CdSe cores may be used directly to generate singlet oxygen in toluene. However, the yield of singlet oxygen is as low as ∼ 5%, in comparison to ≥ 40% reported for classic photosensitizers.

Herein, we report the conceptual design of a bifunctional system, in which highly luminescent CdSe/ZnS quantum dots (QDs) act as a visible imaging dye, while the third-row transition metal complexes are attached and exploited as a photosensitizer. The size of the CdSe core in CdSe/ZnS QDs can be strategically fine-tuned, so that its emission locates at the low-lying triplet state absorption of the transition metal complexes. Due to the forbidden nature in the triplet manifold, the Förster type of resonance energy transfer can be either drastically reduced or even eliminated, if one can tune the thickness of ZnS and the length of the spacer between QDs and the transition metal complex to a certain long distance. Thus, the luminescent QDs provide the capability for imaging, while the Ir complexes enhanced ultrafast intersystem crossing guarantees unity population at the triplet states, consequently inducing the sensitization of active molecular species, i.e., singlet oxygen, that are toxic to cells and tissues.^8,9

Scheme 1 depicts the synthetic route for the Ir–CdSe/ZnS QDs, in which CdSe/ZnS QDs were prepared from CdO using a two-step procedure reported previously.¹⁰ A detailed method of preparing hydroxyl substituted pyridyl pyrazole ligand (L1) is elaborated in the supporting information (SI). The first Ir-complex, [(piq)₂Ir(L1)], was synthesized via a reaction of [(piq)₂IrCl]₂ and L1.¹¹ The thio-attached [(piq)₂Ir(L1)], i.e. [(piq)₂Ir(L2)] (see Scheme 1), was synthesized from a mixture of [(piq)₂Ir(L1)] (100 mg, 0.125 mmol), thiotic acid (26 mg, 0.125 mmol), N,N′-dicyclohexylcarbodiimide (52 mg, 0.262 mmol) and N,N′-dimethylamino pyridine (5 mg, 0.037 mmol) in CH₂Cl₂ (20 mL), followed by reduction using NaBH₄. As for the synthesis of [(piq)₂Ir(L2)] encapsulated CdSe/ZnS QDs, namely Ir–CdSe/ZnS, the tri-n-butylphosphine (TBP)/tri-n-octylphosphine oxide (TOPO)-capped CdSe/ZnS QDs (10 mg) were dissolved in MeOH (15 mL) containing [(piq)₂Ir(L2)] (50 mg, 0.05 mmol) at a pH value of ∼ 12, adjusted with tetramethylammonium hydroxide pentahydrate. The mixture was heated under reflux at 65 °C overnight, and then the reaction was terminated and the mixture allowed to cool to room temperature. Ir–CdSe/ZnS were then precipitated with diethyl ether. For further purification, the crude solid was washed with CH₂Cl₂ several times. Detailed synthetic procedures and characterization in each intermediate step are described in the supplementary information.

Scheme 1 (i) NaOEt; (ii) N₂H₄; (iii) HCl; (iv) [(piq)₂IrCl]₂; (v) 1. thiotic acid, DCC/DMAP, r.t., 72 h; 2. NaBH₄, MeOH, r.t., 4 h; (vi) [(piq)₂Ir(L2)], Me₄N(OH), pH = 11, MeOH, reflux, 24 h.

Characterization of Ir–CdSe/ZnS was first performed with IR measurement. Figs. 1A and 1B depict the typical IR spectra of neat [(piq)₂Ir(L2)] and Ir–CdSe/ZnS. In comparison, the resemblance in both spectral features and peak positions for most vibrational modes such as C=C (1450–1600 cm⁻¹) and C=N stretching (∼ 2000 cm⁻¹) seems to guarantee a successful attachment of [(piq)₂Ir(L2)] onto CdSe/ZnS QDs. The absence of S–H stretch band in the range of 2400–2600 cm⁻¹ in Fig. 1B (see grey circle) firmly supports the formation of the sulfur–CdSe/ZnS bond. The ¹H NMR of Ir–CdSe/ZnS is shown in the ESI. The associated ¹H NMR peaks in Ir–CdSe/ZnS, except for the missing S–H peak at δ 1.5, are nearly identical to those of a pure [(piq)₂Ir(L2)] sample (also see ESI), further supporting the attachment of [(piq)₂Ir(L2)] on the surface of CdSe/ZnS.

Fig. 1 FT-IR spectra of pure [(piq)₂Ir(L2)] (A) and the [(piq)₂Ir(L2)]-capped CdSe/ZnS QDs (B).

Fig. 2 shows the absorption and emission spectra of [(piq)₂Ir(L2)] and Ir–CdSe/ZnS. The spectral assignment of [(piq)₂Ir(L2)] is straightforward, in which the lowest lying transition, at 570–600 nm with the absorption extinction of < 300 M⁻¹cm⁻¹, (see ESI) is assigned to the metal-to-ligand charge transfer (MLCT) in the triplet manifold. For comparison, the absorption of TOPO capped CdSe/ZnS QDs is also depicted; it exhibited an emission peak at ∼ 580 nm (not shown here) with a quantum efficiency of ∼ 0.42. Since Ir–CdSe/ZnS was prepared via a ligand exchange process from the TOPO capped CdSe/ZnS QDs, their similarity in size is expected. This viewpoint is supported by the TEM results, in which the average diameters of TOPO capped CdSe/ZnS and Ir–CdSe/ZnS, measured by TEM, were calculated to be 6.8 ± 0.7 and 7.0 ± 0.6 nm, respectively (see ESI). Since the diameter of the CdSe core was measured to be ∼ 3.8 nm, the thickness of ZnS was > 1.5 nm. As shown in Fig. 2, the absorption spectrum of Ir–CdSe/ZnS is apparently composed of the absorption profile of [(piq)₂Ir(L2)] and CdSe/ZnSe QDs. The steady state emission of Ir–CdSe/ZnS in degassed MeOH consists of a distinct band maximized at 590 nm and a shoulder around 650 nm. Upon aeration, the 650 nm shoulder nearly disappeared, accompanied by a decrease of the overall emission intensity (see Fig. 2). However, the 590 nm peak position remained unchanged. In view of the spectral position and bandwidth, the 590 nm emission profile resembles that (580 nm) of the TOPO-capped CdSe/ZnS QDs. Thus, its assignment to the CdSe/ZnS emission seems unambiguous. The ∼ 10 nm red shift is possibly due to the different capping environment, i.e. TOPO and toluene versus [(piq)₂Ir(L2)] and MeOH. Assuming that the spectrum obtained in the aerated solution is mainly attributed to the CdSe/ZnS emission, the spectrum acquired in the degassed solution can thus be well convoluted by a combination of the emission spectra of CdSe/ZnS (590 nm) and [(piq)₂Ir(L2)] (610 nm). These results clearly indicate that the 610 nm emission, which is subject to drastic O₂ quenching, is attributed to the phosphorescence of [(piq)₂Ir(L2)]. Negligible interference (e.g. energy transfer) between CdSe/ZnS and [(piq)₂Ir(L2)] chromophores is supported by the following experimental data. Upon monitoring at the emission wavelength of ∼ 750 nm, which solely originates from the [(piq)₂Ir(L2)] emission, the excitation spectrum is identical with the absorption profile of the QDs-free [(piq)₂Ir(L2)]. Furthermore, upon monitoring at e.g. 600 nm, the relaxation dynamics of Ir–CdSe/ZnS in degassed MeOH were composed of a fast component and a much slower decay component, the lifetimes of which were fitted to be 32 ns and 2.1 µs, respectively. Upon aeration, the 2.1 µs component was drastically reduced to ∼ 200 ns, while the fast component remained unchanged in either pre-exponential or decay time (i.e. ∼ 30 ns, see Fig. 3). One can thus safely conclude that the Förster type of resonance energy transfer is either very minor or even not operative in the Ir–CdSe/ZnS system.

Fig. 2 Absorption and emission spectra of Ir-complex (blue) and Ir–CdSe/ZnS QDs (grey for degassed, black for aerated) in MeOH; (red) absorption spectra of TOPO-capped CdSe/ZnS in toluene.

Fig. 3 Decay dynamics at 650 nm for degassed (–○–) and aerated Ir–CdSe/ZnS (–●–) in MeOH. * denotes system response function. For aerated Ir-QDs: τ₁ = 30 ns, τ₂ = 200 ns; degassed Ir-QDs: τ₁ = 32 ns, τ₂ = 2.1 µs, λ_ex = 450 nm. Inset: emission spectra of singlet oxygen upon exciting Ir–CdSe/ZnS in aerated MeOH (λ_ex = 514 nm, Ar⁺ laser).

The generation of ¹O₂ in the Ir–CdSe/ZnS system was supported by the observation of ¹Δ_g (0) → ¹Σ⁻_g (0) 1273 nm emission upon exciting Ir–CdSe/ZnS in the aerated MeOH (see insert of Fig. 3). The assignment of ¹O₂ emission is unambiguous based on two observations. First, this 1273 nm emission disappeared upon degassing. Secondly, the lifetime of the emission revealed drastic solvent isotope dependence, being shifted from 25 µs in MeOH to ∼ 240 µs in CD₃OD, consistent with a ¹O₂ electronic transition-solvent vibrational energy matching mechanism.¹² We further made an attempt to estimate the yield of ¹O₂¹Δ_g (0) → ¹Σ⁻_g (0) 1273 nm emission. In this approach, the compound bis(triisobutylsiloxy) silicon-2,3-naphthalocyanine (SiINC) was used as a reference, of which the 1342 nm phosphorescence yield has been determined to be 7.47 × 10⁻⁵ in benzene.¹³ Under experimental conditions where the number of photons being absorbed by the Ir–CdSe/ZnS and SiINC are identical at e.g. 600 nm, the relative quantum yield of the ¹O₂ phosphorescence in MeOH with respect to that of the SiINC phosphorescence in THF was calculated on the basis of the following relationship

where and are the phosphorescence spectra of SiINC and O₂ (¹Δ_g), respectively. n denotes the refraction index of the solvent, which is 1.328 for MeOH and 1.401 for THF.¹⁴ As a result, the quantum yield of O₂ (¹Δ_g) 1273 nm emission was calculated to be 4.1 × 10⁻⁵ in MeOH. This value is ∼ 87% of the ¹O₂ sensitized by 1H-phenalen-1-one (PH) in the MeOH (∼ 4.7 × 10⁻⁵).¹⁵ Since the efficiency of PH sensitizing ¹O₂ in aerated solution is near unity,^15b it is reasonable to conclude that the ¹O₂ production is ∼ 87% for Ir–CdSe/ZnS. In aerated MeOH, since the decay dynamics of Ir–CdSe/ZnS phosphorescence are dominated by the O₂ quenching process (see Fig. 3), the O₂ quenching efficiency is thus nearly equivalent to the ratio of [k_aer (5 × 10⁶ s⁻¹) − k_degas (4.76 × 10⁵ s⁻¹)] versusk_aer and was estimated to be 90%. As a result, the efficiency of the ¹O₂ production is deduced to be as high as 97%.

In conclusion, we have ingeniously designed an Ir–CdSe/ZnS system, in which the interplay between CdSe/ZnS QDs and [(piq)₂Ir(L2)] chromophores is negligible. The system possesses a bifunctional property in that CdSe/ZnS QDs and [(piq)₂Ir(L2)] act as an imaging center and a ¹O₂ sensitizing agent, respectively. For Ir–CdSe/ZnS in aerated MeOH, the quantum yield of the 590 nm CdSe/ZnS emission was determined to be 0.4, which is sufficiently high for application in imaging. As the next practical application, specific target agents can also be designed and co-anchored with [(piq)₂Ir(L2)] ligand to CdSe/ZnS, among which a potential candidate should be folic acid because it binds to a receptor that several kinds of cancer cells produce in unusually large amounts.¹⁶ The resulting system is expected to be water soluble as well as to possess a three-in-one property, namely specific targeting, imaging, and ¹O₂ generation, which would greatly expand the usefulness of photodynamic therapy.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed syntheses and characterization and measurements. See DOI: 10.1039/b517368j

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