Thermo-Optical Characterization of Cadmium Selenide/Zinc Sulfide (CdSe/ZnS) Quantum Dots Embedded in Biocompatible Materials

Abstract

Cadmium selenide/zinc sulfide (CdSe/ZnS) core-shell quantum dots (QDs) embedded in biocompatible materials were thermally and optically characterized with a thermal lens (TL) technique. Transient TL measurements were performed with a mode-mismatched, dual-beam (excitation and probe) configuration. A thermo-optical study of the CdSe/ZnS QDs was performed for different core diameters (3.5, 4.0, 5.2, and 6.6 nm) in aqueous solution and synthetic saliva, and three different core diameters (2.4, 2.9, and 4.1 nm) embedded in restorative dental resin (0.025% by mass). The thermal diffusivity results are characteristic of the biocompatible matrices. The radiative quantum efficiencies for aqueous solution and biofluid materials are dependent on the core size of the CdSe/ZnS core-shell QDs. The results obtained from the fluorescence spectral measurements for the biocompatible materials support the TL results.

Keywords

CdSe/ZnS quantum dots Thermal lens Thermo-optical properties Biocompatible materials

INTRODUCTION

Nanostructured semiconductors or quantum dots (QDs) are materials under continuous development because of the search for new nanomaterials that have enhanced optical properties, versatility, and potential for use in novel applications. Water-soluble QDs^1,2 are needed for biomedical applications, and appropriate functionalizations^3,4 are required for specific applications. In general, QDs and bioconjugated QDs have been used as efficient biodiagnostic probes for the treatment of diseases, bioimaging, drug delivery, engineered tissues, biosensors, and biomarkers.^5–7

CdSe/ZnS core-shell semiconductor nanocrystallites are type I QDs, where the electrons and holes are confined to the core of the core-shell QDs.^8,9 A CdSe nanocrystal core with a band gap of 1.7–1.76 eV ^10,11 is often covered by another semiconductor shell that has a high-energy band gap, such as ZnS, which has a band gap of 3.8–4.1 eV,^12,13 to improve the radiative quantum efficiency by passivating the nonradiative recombination sites at the surface.^10,14

The matrix used to suspend the fluorescent solute can have significant influence on the properties of the material such as the absorption and emission spectra, radiative quantum efficiency, and the thermal parameters (such as the thermal diffusivity and the thermal coefficient of the refractive index).^15,16 Therefore, if QDs are to be candidates for practical applications, thermo-optical characterizations of the QDs with their corresponding matrices are required. Because the majority of applications for QDs are for use as biomarkers and for bioimaging, accurately determining the fluorescence quantum efficiency (η) is of considerable importance.

Saliva is a biomaterial that is important in a variety of functions such as protecting teeth through the buffering capacity of the formed biofilm, microbial control, hydration, and dental remineralization.^17–19 Furthermore, saliva has been explored as a candidate biofluid for novel approaches to prognosis, laboratory, or clinical diagnoses.^20,21

A number of systematic studies have compared several properties of dental materials and identified their role in optimizing performance.²² Composite resins have been directly developed for restorative dentistry or adhesive applications, and their primary use is as a hard tissue substitute. Other studies have recently highlighted the primary use of resin composites for the direct esthetic restoration of large imperfections in anterior teeth.²³ Aesthetic considerations such as surface form, translucency, and color have a significant role in defining the course of treatment in modern restorative dentistry. It is known that the optical properties of the restorative material are responsible for the success or failure in matching tooth color and the visual appearance of natural teeth. Several studies have shown that composite resins exhibit higher-quality physical and mechanical properties. Special attention has been focused on the inclusion of nanocrystals in composite resins to achieve the desired optical properties. For example, a recent work published by our research group reported for the first time that QDs can be useful for tailoring the fluorescence of dental resin composites.²⁴ However, the thermo-optical characterization is of considerable importance for matching the tooth thermal diffusivity (D) to determine a suitable restorative material.

The present work reports the photothermal spectroscopic characterization of CdSe/ZnS core-shell nanocrystals suspended in three different biocompatible materials: an aqueous solution, a biofluid, and a dental resin composite. The thermo-optical properties such as D and η of the nanocrystal-biomaterial systems were determined by applying a thermal lens (TL) technique.

THEORY

The TL effect is caused by heat deposition via nonradiative decay processes that occur after the laser energy is absorbed by the sample with a thickness L. The thermally induced distortion of the laser beam that occurs when it passes through the sample is described by the temperature coefficient of the optical path length change (ds/dT), which produces lensing at the sample. The presence of such a thermal lens is detected by its effect on the propagation of a probe beam that passes through the sample. The propagation of a probe laser beam through the TL causes the beam to either spread (ds/dT < 0) or focus (ds/dT > 0), which primarily depends on the temperature coefficients of the electronic polarizability, stress, and thermal expansion of the sample. In the case of liquid samples, ds/dT ≈ dn/dT, where dn/dT is the refractive index temperature coefficient.

The thermal effect can be analyzed by calculating the temporal evolution of the sample temperature profile, ΔT(r, t).²⁵ The propagation of a probe beam through this thermal lens results in a variation of its on-axis intensity, I(t), which can be calculated with the diffraction integral theory.²⁶ In the continuous-wave regime, I(t), is given by²⁵

\begin{array}{l} I (t) & = & I (0) [1 - \frac{θ}{2} {tan}^{- 1} \\ {(\frac{2 m V}{[{(1 + 2 m)}^{2} + V^{2}] τ_{c} / 2 t + 1 + 2 m + V^{2}})]}^{2} \end{array}

(1)

where m = (w_p/w_e)², w_p and w_e are the probe and excitation beam radii at the sample, respectively; V = z₁/z_o, z₁ is the distance between the sample and the probe-beam waist; z_o is the Rayleigh range of the probe beam; and I(0) is the on-axis intensity when t and/or θ are zero. τ_c is the characteristic heat diffusion time, which is given by²⁵

τ_{c} = \frac{w_{e}^{2}}{4 D}

(2)

where D = k/ρC is the thermal diffusivity (in squared centimeters per second), k is the thermal conductivity (watts per centimeter kelvin), ρ is the density (in grams per cubed centimeter), and C is the specific heat (joules per gram kelvin). The TL transient signal amplitude, θ, is approximately the phase difference of the probe beam between r = 0 and r = √2w_e, which is induced by the pump beam and given by the following expression: θ = –ΘP_eαL_eff. In this case, P_e (W) is the excitation power, α (expressed in inverse centimeters) is the optical absorption coefficient at the excitation wavelength (λ_e), L_eff = (1 – e^−αL)/α is the effective length, and L (in centimeters) is the thickness of the sample. The normalized phase shift, Θ, can be expressed as^27,28

Φ = \frac{1}{k λ_{p}} \frac{d n}{d T} φ

(3)

where λp is the wavelength of the probe beam, and dn/dT is the refractive index temperature coefficient. The fraction of absorbed energy converted into heat (or absolute nonradiative quantum efficiency) is given by^27,28

φ = 1 - η \frac{λ}{〈 λ_{em} 〉}

(4)

where η is the fluorescence quantum efficiency (or quantum yield) and 〈λ_em〉 is the average emission wavelength.

EXPERIMENTAL

Sample Preparation. In this work, photopolymerized composite resins from Heraeus Kulzer (Charisma®) in the shade A2, with incorporated semiconductor nanocrystals were analyzed. The resin studies were performed with CdSe/ZnS nanocrystals suspended in toluene with three different core diameters (Φ_CdSe; 2.4, 2.9, and 4.1 nm), which were embedded into the composite resins at a concentration of 0.025 (in mass percent) by using a spatula (Table I). CdSe/ZnS nanocrystals suspended in toluene were obtained from Evident Technology. QDs suspended in toluene were used to prepare dental resin samples because the solvent evaporates rapidly. The material was condensed and interposed with two glass sheets to promote the confinement of the material, to avoid the inclusion of bubbles, and to obtain a smooth surface. Photopolymerization of the resin was accomplished with a 90 mW (25 nm at FWHM) blue-light emitting diode (LED) source (DMC, model UltraBlue). The LED light, which has its emission band centered at 465 nm, was directly applied to the glass sheet that covered the resin matrix for 40 s.

TABLE I.

Core diameters (Φ_CdSe), maxima of the emission bands (λ_max), and thermal parameters for the CdSe/ZnS core-shell nanocrystals embedded in dental resin of thickness L.

Sample^a	L (mm)	Φ_CdSe (nm)	λ_max (nm)	τ_c (ms)	D (10⁻³ cm²/s)
R₁	(0.32 ± 0.03)	(2.4 ± 0.2)	555.3	4.3 ± 0.5	1.8 ± 0.2
R₂	(0.37 ± 0.03)	(2.9 ± 0.5)	573.7	4.2 ± 0.4	1.8 ± 0.2
R₃	(0.32 ± 0.03)	(4.1 ± 0.9)	618.5	4.3 ± 0.4	1.8 ± 0.2

Concentration is 0.025 in mass percent of QDs proportion in resin.

Synthetic saliva was obtained from Biopharma (ISO 9001). Four different core diameters (3.5, 4.0, 5.2, and 6.6 nm) of CdSe/ZnS QDs in aqueous solutions were obtained from NN-Labs at concentrations of 7.50, 7.20, 6.25, and 5.33 × 10⁻⁶ mol/L, respectively. Solutions of CdSe/ZnS QDs (with different core sizes) were pipetted separately into the saliva under constant magnetic stirring. Samples of the core-shell colloidal nanoparticles were diluted in the synthetic saliva to a final concentration of approximately 1.3 nmol/mL. The four different sizes of QDs obtained from NN-labs were embedded in saliva samples and denoted as S₁, S₂, S₃, and S₄, according to the core diameters (Table II).

TABLE II.

Core diameters (Φ_CdSe), the average emission wavelength (〈λ_em〉), and thermal diffusivity (D) of CdSe/ZnS embedded in artificial saliva.

Sample	Φ_CdSe (nm)	〈λ_em〉 (nm)	D (10⁻³ cm²/s)	D (10⁻³ cm²/s)^a
S₁	(3.5 ± 0.9)	(582 ± 3)	(1.4 ± 0.1)	(1.44 ± 0.06)
S₂	(4.0 ± 0.6)	(597 ± 3)	(1.5 ± 0.3)	(1.5 ± 0.3)
S₃	(5.2 ± 0.8)	(629 ± 2)	(1.6 ± 0.3)	(1.6 ± 0.1)
S₄	(6.6 ± 0.9)	(641 ± 2)	(1.4 ± 0.1)	(1.5 ± 0.1)

D results for samples of QDs in aqueous solutions with respective values of Φ_CdSe.

Optical Characterization. TL transient measurements were performed with the mode-mismatched dual-beam (excitation and probe) configuration.^25,26 A He-Ne laser (λ_p = 632.8 nm) was used as the probe beam, and an Ar⁺ laser (λ_e = 514.5 nm) was used as the excitation beam. The excitation and probe-beam radii at the sample were measured to be w_e = 55 ± 1 μm and w_p = 172 ± 3 μm, respectively. The excitation beam was modulated with a mechanical chopper. The absorption of the excitation beam generates the TL heat profile and induces a time-resolved phase shift. The transient signal was obtained from the weak probe beam, which counter-propagates almost collinearly with the excitation beam. The probe-beam transient signal was detected with a photodiode connected to an oscilloscope. The TL measurements of the samples were performed with the solutions contained in 2 mm quartz cuvettes.

The absorption and fluorescence signals were collected with an optical fiber (Superguide G fiber SFS 600/660 T, Fiberguide Industries) with a 600 μm diameter. The collected light was directed into a spectrometer (Oriel instruments, model MS257) and dispersed with a 600 lines/mm diffraction grating. The spectra were then recorded with a 256 × 1024 pixel charge-coupled device. For the absorption measurements, a halogen light was used as a broadband light source. For the fluorescence measurements, a diode laser was used as an excitation source (λ_e = 405 nm and P_e ≈ 75 mW).

RESULTS AND DISCUSSION

Figure 1 shows the absorption spectra of the CdSe/ZnS core–shell suspended in biofluids, in toluene solution, and embedded into dental resin composites. The absorbance peaks at approximately 508 (Fig. 1a) and 543 (Fig. 1b) nm are characteristic of CdSe/ZnS QDs in toluene with Φ_CdSe = 2.4 and 2.9 nm, respectively.^28,29 No significant changes in the absorbance spectra were observed for the CdSe/ZnS QDs embedded in the dental resin composites (Fig. 1c) or the pure composites (Fig. 1d). Figure 1e presents the absorbance spectrum of the concentrated aqueous CdSe/ZnS QDs solution from NN-Labs diluted into artificial saliva, sample S₂ (Table II). The band centered at approximately 585 nm is typical for QDs with Φ_CdSe = 4.0 nm. The corresponding absorption transitions can be found elsewhere.^30,31 The average values of Φ_CdSe obtained for the QDs in aqueous solutions embedded in artificial saliva are presented in Table II. The Φ_CdSe values for saliva samples (Table II) were similar to values obtained for the respective QDs in aqueous solutions.

Fig. 1.

Absorbance spectra [–log(I/I₀)] for the CdSe/ZnS QDs suspended in toluene with Φ_CdSe = 2.4 ± 0.2 nm (a), and Φ_CdSe = 2.9 ± 0.5 nm (b) (absorption right axis), CdSe/ZnS QDs embedded in dental resin composites (R₂, Table I) (c), pure dental resin composites (d), and CdSe/ZnS embedded in biofluid (S₂, Table II) (e). The spectra were recorded with the samples in a 1 cm quartz cuvette.

The maximum fluorescence of human dentin occurs at 440 ± 10 nm when irradiated by ultraviolet (UV) light, which makes teeth appear whiter and brighter in the daylight.^32–35 Three emission peaks occur at 350, 405, and 450 nm for human and bovine dental enamel.^23,33 Therefore, the restorative material needs to present emission bands in the UV–visible region to match the color of the tooth. The broad and flat emission covering the majority of the visible range will allow for a whiter and brighter perception of the restored tooth. In this case, with an adequate choice of one or more different nanocrystal sizes for the resin doping, a broad emission band in the visible range can be obtained by summing the different spectra, as presented previously in the pioneer work of our research group, in which one QD core size was used to show how to fill up a portion of the visible spectrum not covered by the intrinsic emission of the resin matrix.²⁴

Emission spectra for the QDs with different core sizes embedded in dental resin composites, i.e., for the samples listed in Table I, are shown in Fig. 2. For comparison, Fig. 2a presents the typical emission spectrum for pure dental resin composites, which presents no significant emission band between 570 and 700 nm, with intensities comparable to those obtained for the QD-doped resin. The insertion of QDs with diameters of approximately 2.4 nm into the dental resin composites strengthens the emission of the material near 550 nm because the QDs emission band sums with the intrinsic resin emission. For the QDs with core sizes of 2.4 (Fig. 2b), 2.9 (Fig. 2c), and 4.1 nm (Fig. 2d), the maxima of the emission bands (λ_max) are presented in Table I. In this form, the volume fractions of the CdSe/ZnS core-shell used in this work exert considerable influence on determining the fluorescence properties of the resin composites. The positions of the emission bands for the nanocrystals are dependent on the concentration and the matrix that the QDs are embedded.³⁶ For example, the average emission wavelengths 〈λ_em〉 for the CdSe/ZnS core-shells (with Φ_CdSe = 3.7 nm) suspended in different solvents including chloroform, tetrahydrofuran, and toluene were observed at approximately 584, 587, and 596 nm, respectively.³⁶ For the polymethyl methacrylate–encapsulated CdSe/ZnS QDs (Φ_CdSe = 3.7 nm) embedded in the resin composites, our research group determined the 〈λ_em〉 was 604 ± 3 nm for the QDs proportion in the resin composite as 0.048–0.77 (in mass percent).²⁴ The 〈λ_em〉 values of the emission bands are presented in Table II for the QDs suspended in the biofluid. The 〈λem〉 values for saliva samples (Table II) were similar to values obtained for the respective QDs in aqueous solutions.

Fig. 2.

Emission spectra for pure dental resin (a), and CdSe/ZnS QDs embedded in dental resin for different core diameters, 2.4 nm (b); 2.9 (c) and 4.1 nm (d), corresponding to samples R₁, R₂, and R₃, respectively (λ_e = 405 nm, L ≈ 0.34 mm, P_e = 75 mW, Table I).

Figure 3 shows the TL transient signals for the CdSe/ZnS nanocrystals embedded in the dental resin composites, fitting the results with Eq. 1 allow us to determine the parameters θ and τ_c. The thermal diffusivity, D, was determined for each sample by using the relationship D = w_e²/4τ_c. The obtained D value for each sample of the core-shell nanocrystals embedded in the dental resin composites with different Φ_CdSe values is presented in Table I. The thermal diffusivity is not dependent on the CdSe/ZnS core size. The average value for the thermal diffusivity for different core diameters 2.4–4.1 nm of the QDs that was embedded into the dental resin composites is D = 1.8 ± 0.2 × 10⁻³ cm²/s. The obtained value for D applying TL technique is in good agreement with the value reported for pure Charisma composite resin.³⁷ Table III presents thermal diffusivity values for other dental composites obtained in the literature for comparison.^37–40

TABLE III.

Thermal diffusivity (D) for several dental composites.

Dental composite	D (10⁻³ cm²/s)	Technique	Reference
Charisma	(1.8 ± 0.1)	Photothermal radiometry	37
Amelogen	(1.9 ± 0.2)	Photothermal radiometry	37
Medental	(5.8 ± 0.9)	Photoacoustic	38
Degufill LC	(3.4 ± 0.3)	Photoacoustic	38
Enamel	4.2	Calculated	39
Dentine	2.6	Calculated	39
3M	(4.5 ± 0.9)	Photoacoustic	40
Degufill H	(3.4 ± 0.6)	Photoacoustic	40

Fig. 3.

Transient TL signal for core-shell nanocrystals embedded in dental resin, sample R₃ (Table I). Resin matrix was Heraeus Kulzer-Charisma in the shade A2 (P_e = 59.4 mW, L = 0.32 mm and λ_e = 514.5 nm). The values obtained from the curve fitting were θ = 0.1756 ± 0.0006 rad and τ_c = 4.43 ± 0.04 ms.

TL transient results for QDs embedded in aqueous solutions and biofluids are shown in Fig. 4. The average values of D obtained for different QDs sizes embedded in artificial saliva and aqueous solutions are presented in Table II. The thermal diffusivity value for pure aqueous solutions is D = 1.42 ± 0.02 × 10⁻³ cm²/s,^41,42 which is in good agreement with values obtained for the samples presented in Table II. The behavior of the TL transient curves for QDs embedded in biofluids are similar to those obtained for dental resin composites (Fig. 3) and indicates that dn/dT is negative, i.e., TL causes a defocusing of the probe beam in the far field. The thermal parameters of the core-shell QDs embedded in the biofluid, Θ = –θ_CS/P_eαL_eff, were determined from the transient TL measurements (θ_CS is the amplitude of the TL signal for core-shell nanocrystals).

Fig. 4.

Transient TL signal for QDs embedded in (a) aqueous solutions (Φ_CdSe = 3.5 nm and P_e = 32 mW, and λ_e = 514.5 nm) and (b) artificial saliva S₁ (Table II and P_e = 36 mW). The values obtained from the curve fitting were (a) θ = 0.3305 ± 0.0005 rad and τ_c = 5.31 ± 0.02 ms; (b) θ = 0.199 ± 0.001 rad and τ_c = 4.98 ± 0.08 ms.

To calculate φ, we assumed negligible fluorescence for the pure saliva and aqueous solution used in the TL measurements, such that all of the absorbed energy was converted into heat by the sample, i.e., φ_S = 1(η_S = 0). We calculated the Θ_S (value for the solvent) value by using Eq. 3. By normalizing Θ by Θ_S and by using Eq. 4, we obtain

φ = \frac{Φ}{Φ_{S}} = 1 - η (\frac{λ_{e}}{〈 λ_{em} 〉})

(5)

Applying Eq. 5 and by using both values of 〈λ_em〉 and λ_e = 514.5 nm, we calculated the η values for the core-shell nanocrystal samples. The values of η obtained by the TL method are presented in Fig. 5 for QDs embedded in aqueous solutions and biofluids. The values of η are dependent on the core sizes of the nanocrystals, and the best result was obtained for the smallest core that was analyzed (approximately 3.5 nm). The dependence of the radiative quantum efficiency on the core size was already reported by our research group for samples of CdSe/ZnS suspended in toluene,²⁸ and the result was attributed to quantum confinement effects (QCEs). The QCE is dependent on the size of the semiconductor nanocrystal with respect to the exciton Bohr radius.^43–45 Therefore, as the size of the QD is reduced, the energy spacing of its atomic-like states increases beyond the available thermal energy, which inhibits the thermally induced depopulation of the lowest electronic states.⁴⁶

Fig. 5.

Dependence of η in function of QDs core diameters embedded in saliva or aqueous solutions.

Further possibilities for the application of these QDs in biofluids would be, for example, the use of these QDs as biosensors that are dependent on pH, because the pH of human saliva can vary for different diseases of the body.⁴⁷

CONCLUSION

In this work, we characterized the thermo-optical parameters of QDs embedded in biocompatible materials including dental resins, aqueous solutions, and biofluids. The thermal diffusivity of the CdSe/ZnS embedded in biocompatible materials is not dependent on the sizes of the QDs, and the values are characteristic of the matrices used to suspend the QDs. Large quantum efficiency values were obtained for the CdSe/ZnS suspended in aqueous solutions and biofluids. In addition, the results indicated that the fluorescence of dental resin composites is dependent on the size of the dopant QDs, and it can be tailored with multisize CdSe/ZnS core-shell QDs, which permits the fabrication of restorative materials that have fluorescence properties that closely match those of natural human teeth. The obtained thermal diffusivity values for the resin doped are similar to the values of natural teeth and pure composite resin.

Footnotes

ACKNOWLEDGMENTS

This research was supported by the Conselho Nacional de Desenvolvimiento Científico e Tecnológico (proc. 473951/2010-0), the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (proc. APQ-00576-11), the Pró-Reitoria de Pesquisa e Pós-Graduação da Universidade Federal de Uberlândia, and the Fundação de Amparo à Pesquisa do Estado de São Paulo (procs. 2006/01277-2, 2001/12754-2, and 1996/05590-3).

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