1 INTRODUCTION

Cerenkov luminescence imaging (CLI) is a promising technique for advanced biology imaging in the medical field [1-5]. The application of CLI has been extended to determine tumor location [6, 7], monitor radionuclide uptake [8, 9], and guide tumor resection surgery [10, 11]. According to Frank and Tamm’s theory, the wavelength of Cerenkov luminescence (CL) is mostly distributed in the ultraviolet and blue range, which can only penetrate a few millimeters of biological tissues owing to high attenuation coefficients, thereby limiting the application of CLI.

To overcome these limitations, several studies have been performed to explore novel technologies that can efficiently detect the signal, e.g., the CL endoscopy system [12-14]. Another potential solution is to shift the peak wavelength of CL to near-infrared or infrared ranges that have smaller attenuation coefficients. Cerenkov energy transfer (CRET) has demonstrated that CL can serve as an excitation source to excite various fluorescent particles, such as quantum dots (QDs) [15, 16], gold nanoparticles [17, 18], fluorophores [19], and lanthanides [20, 21], where the fluorescent particles can act as fluorescence emitters of red-shifted emissions. Near-infrared or infrared light has lesser attenuation and absorption by biological tissues than ultraviolet or blue light [22].

Among various fluorescent particles, QDs were proposed to be combined with radioisotopes to construct CRET systems because QDs have superior optical properties, such as large Stokes shift, tunable fluorescence emission, high photostability, and high quantum yield [23-25]. However, whether high concentrations of QDs coupled with radioisotopes enhance optical signals for external detection still remains unclear. Appropriate fluorescence wavelength emissions of QDs can be selected for better optical signal enhancements considering the various penetrating abilities and quantum efficiencies of different photonic wavelengths of the external optical detector. In this study, two types of core/shell QDs (i.e CdSe/ZnS and CdTe/CdS) were selected to evaluate the effects of the concentration and fluorescence emission peaks of QDs on optical signal enhancement for external detection.

2 EXPERIMENTS SECTION

CdSe/ZnS and CdTe/CdS core/shell QDs were obtained from Xingzi New Material Technology Development Co. Ltd. (Shanghai, China). The fluorescence emission peaks were 580 nm and 660 nm for CdSe/ZnS QDs, and 580 nm and 620 nm for the CdTe/CdS QDs. Experiments were performed on an in-house developed optical imaging platform (see Fig. 1a), which was mainly composed by an EMCCD camera inside a black box to minimize the distortion of background light. The distance between the lens of the EMCCD and the transparent vials was 35 cm. The Na131I/QD mixed solution and the Na131I solution were imaged on the top of the sample. All o images were acquired with a 4 × 4 binning and a 5 min exposure time. The areas of the regions of interest were drawn over the optical sources of the images, and the average number of photons was calculated with Andor Solis that was used to provide the quantification information from the optical images. To evaluate the enhancement effect of QDs on optical signals for the Na¹³¹I solution, QDs with different fluorescence emission peaks (1 mL) were prepared in transparent vials with various concentrations (i.e 0.05, 0.1, 0.25, 0.5, 0.75, and 1 mg/mL) as shown in Fig. 1b. The control group was prepared with zero QDs. Thereafter, 30 and 60 μCi Na¹³¹I (0.5 mL) solutions were added to each transparent vial respectively.

Fig. 1Full-screen View

(Color online) (a) Experimental setup of the EMCCD system and (b) prepared QDs solutions

To study the enhancement effect of QDs on the transmission ability in muscle tissues, a soft tissue of a pig was put on top of the vials. Fig. 2 shows the experimental setup that was illustrated through the optical image of the muscle tissue covering the transparent vials. Different concentrations of CdTe/CdS QDs with 60 μCi of Na¹³¹I were placed in the transparent vials at 620 nm. The enhancement effect on the optical signal was related to the concentration of QDs for the same Na¹³¹I radioactivity.

Fig. 2Full-screen View

Experimental setup for the transmission study that was designed with the soft tissue of pig sitting on top of the prepared solutions

3 Results and discussion

Fig. 3 shows the optical images of different concentrations of CdTe/CdS QDs added with Na¹³¹I (60 μCi) solution at 620 nm. The signal intensity increases as the concentration of the QDs increases up to 0.75 mg/mL, and decreases as the concentration of the QDs further increases. The two types of QDs showed similar patterns. Figs. 4a and 4b show the total number of photons at fluorescence emission peaks of 580 nm and 660 nm for the CdSe/ZnS QDs added with Na¹³¹I solution (30 and 60 μCi), and for CdTe/CdS QDs, respectively. This phenomenon may be due to the concentration quenching effect [26] that occurs when the concentration of QDs is excessively high while the excitation intensity remains unchanged [27]. Alternatively, the self-absorption of the solution is another factor that decreases the photons. Different kinds and emission peaks of QDs have different optimal concentrations to enhance the signal intensity of the Na¹³¹I solution. The optimal concentrations of CdSe/ZnS at fluorescence emission peaks of 580 nm and 660 nm were 0.1 mg/mL and 0.25 mg/mL, respectively. By comparison, the optimal concentrations of CdTe/CdS at fluorescence emission peaks of 580 nm and 620 nm were 0.1 and 0.75 mg/mL, respectively. Moreover, the number of photons in all mixed solutions was greater than that of the pure Na¹³¹I solution exposed to the same radioactivity level, suggesting that QDs successfully converted CL into red-shifted photons, which has higher penetration ability in tissues.

Fig. 3Full-screen View

(Color online) Optical image of the Na¹³¹I/quantum dot mixed solution

Fig. 4Full-screen View

Scatter plot of the number of photons at fluorescence emission peaks of (a) 580 nm and (b) 660 nm for the CdSe/ZnS QDs, and (c) 580 nm and (d) 620 nm for the CdTe/CdS QDs with Na¹³¹I solution

Fig. 5 shows the imaging of photons that are transmitted through the soft-tissue. Figs. 6a and 6b show the results for CdTe/CdS QDs added with 60 μCi of Na¹³¹I solution. Under these conditions, the number of photons decreases as the thickness of the muscle tissue increases; however, it was always greater than that of the pure Na¹³¹I solution with the same radioactivity level. These results indicated that the QDs successfully converted CL into red-shifted photons that could enhance the penetration ability of photons in tissues. The spatial distribution of the tissue optical properties is indeed heterogeneous considering the whole sample; however, in our experiment, we attempt to determine a relatively uniform area for experiment that intentionally avoids the effect from the sample heterogeneity. Fig. 7 shows the total number of photons at optimal concentrations of CdTe/CdS QDs added with 60 μCi of Na¹³¹I solution and covered with muscle tissues of different thicknesses. For the same type of QDs, the attenuation of the number of photons at the fluorescence emission peaks of short-wavelength emission QDs was more severe than that of long-wavelength emission QDs. Therefore, the fluorescence emission peaks of the long-wavelength emission QDs were more suitable for the enhancement of optical signal intensity for external detection than those of the short-wavelength emission QDs. This result is consistent with the previous studies on the in-vitro animal studies and optical properties of biological tissues, which generally show lower absorption efficiencies for longer-wavelength emission QDs [15, 28, 29]. To further explore the underlying mechanisms, the percentage of Cherenkov light that is being converted to longer wavelength photons could be explored through spectroscopic studies.

Fig. 5Full-screen View

(Color online) Optical image of muscle tissue covering transparent vials

Fig. 6Full-screen View

Scatter plot of optical signals that were detected by the EMCCD with different concentrations of CdTe/CdS QDs added with 60 μCi of Na¹³¹I solution and covered with muscle tissues of varying thicknesses at fluorescence emission peaks of (a) 580 nm and (b) 660 nm

Fig. 7Full-screen View

(Color online) Total number of photons at the optimal concentrations of CdTe/CdS QDs added with 60 μCi of Na¹³¹I solution and covered with muscle tissues of different thicknesses

4 CONCLUSION

In this study, we experimentally observed the enhancement effect of CdSe/ZnS and CdTe/CdS QDs on optical signals that may be detected by an external optical detector (e.g. EMCCD) for CLI. The results indicated that QDs could effectively enhance the optical signal that could be detected by an external EMCCD, and different types and emission peaks of QDs have different patterns (e.g. various optimal concentrations) on the enhancement of the signal intensity of CL for external detection. For the same type of QDs, the long-wavelength emission QDs were more suitable than short-wavelength emission QDs for the improvement of optical signal intensity.