Nano Research 1 DOINano 10.1007/s12274Res -014-0629-2

Excellent photothermal conversion of core/shell

CdSe/Bi2Se3 quantum dots

GZ. Jia1,2, WK. Lou1, F. Cheng3, XL. Wang4, JH. Yao4, N. Dai5, HQ. Lin6, and K.Chang1 ()

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0629-2 http://www.thenanoresearch.com on November 7 2014

© Tsinghua University Press 2014

Just Accepted

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Excellent photothermal conversion of core/shell

CdSe/Bi2Se3 quantum dots

GZ. Jia1,2, WK. Lou1, F. Cheng3, XL. Wang3, JH. Yao3, N. Dai5, HQ. Lin6, and K.Chang1*

1 Institute of Semiconductors, PR China 2 Tianjin Chengjian University, PR China 3 Changsha University of Science and Technology,

PR China

4 Nankai University, PR China The water-dispersed CdSe/Bi2Se3 core/shell quantum dots were 5 Inst Tech Phys, Nat Lab Infrared Phys, PR China synthesized by a cation exchange reaction. To give the mechanism of 6 Beijing Computational Science Research Center, core/shell quantum dots with excellent photothermal conversion PR China efficiency and near-infrared photostability.

Nano Research

DOI (automatically inserted by the publisher) Research Article

Excellent photothermal conversion of core/shell

CdSe/Bi2Se3 quantum dots

GZ. Jia1,2, WK. Lou1, F. Cheng3, XL. Wang4, JH. Yao4, N. Dai5, HQ. Lin6, and K.Chang1 ()

Received: day month year ABSTRACT

Revised: day month year The water-dispersed CdSe/Bi2Se3 core/shell QDs with a photothermal Accepted: day month year conversion coefficient of 27.09% were synthesized by a cation exchange (automatically inserted by reaction. The microstructrue and crystal structure of the QDs, which were the publisher) confirmed by TEM and XRD, showed that cation exchange partly occurred inside the CdSe QDs. Two main mechanisms can result in the excellent © Tsinghua University Press photothermal conversion: radiative recombination of carriers inhibited due to and Springer-Verlag Berlin forming the type-II semiconductor heterostructure and large surface-to-volume Heidelberg 2014 ratio of QDs. The photothermal conversion experiments results indicate that CdSe/Bi2Se3 QDs showed higher photothermal conversion efficiency and KEYWORDS excellent NIR photostability. cation exchange, quantum dots, photothermal, type-II heterostructure, CdSe/Bi2Se3

Introduction and photothermal cancer therapy[7-21]. Most of recent works focused on photothermal effects in Photothermal effect has attracted intensive interests noble metal nanoparticles caused by surface plasmon in recent years due to its potential application in polariton (SPP). The strong interaction between light nanoscale heat sources[1], biological imaging[2, 3], and noble metal nanparticles can change electron spectroscopy[4], drug delivery[5], nanocatalysis[6], transient processes in atoms and moleculars in

Address correspondence to K. Chang. [email protected]

2 Nano Res. biological systems through SPPs[7, 10, 13, 15, 18, 19, coefficients and good photothermal conversion 21]. Optically excited noble metal nanoparticles can efficiency because of their tunable surface plasmon be used as nanoscale heat sources through resonance (SPR) properties in the NIR wavelength, dissipation of absorbed light into thermal energy. some substantial shortcomings impose limitations in Plasmon enhanced metal nanoparticles could be used their wide therapeutic applications, for example, too in photothermal cancer therapy. large size to increase bloodstream circulation time[38, Here, we suggest a new direction of photothermal 39], lacking good photothermal stability[17], and effect in semiconductor nanostructures based on expensive raw material, etc. The investigations have band engineering. We demonstrate that CdSe/Bi2Se3 shown that few-layer Bi2Se3 can significantly enhance core/shell quantum dots (QDs) can be developed as the contribution of exotic surface states due to large photothermal ablation (PTA) to replace the surface-to-volume ratios[40]. Low-dimensional traditional therapeutic approaches to treat and semiconductor nanostructures can effectively control cancers, which can effectively avoid harming suppress the bulk effect and discover some novel healthy cells and destroying the immune system[22]. physical properties. Layered or layered-like Bi2Se3

This mainly based on that CdSe/Bi2Se3 QDs have a nanostructures have been fabricated by various large absorption coefficient and high photothermal methods for potential applications in spintronic conversion efficiency in near-infrared (NIR, devices. More recently, Bi2Se3 nanoplates as a new λ=700-1100nm) wavelengths. The near infrared photothermal coupling agent for PTA of cancer cells irradiation leads to relatively low scattering and can be utilized for enhanced X-ray computed absorption, and several centimeters penetration in tomography imaging of tumor tissue in vivo[22]. biological tissues[7, 18]. In addition, Bi and Se belong However, the considerably large Bi2Se3 nanoplates to the promising bio-friendly elementary and Bi2Se3 with an average diameter of about 90 nm can hold is with low cytotoxicity[23-25]. This decides Bi2Se3 back the further bioapplications. Generally, the becoming promising photothermal agents and nanoparticles size between 10 and 50 nm is more potential application in cancers therapy by suitable for intravenous injection to increase photothermal technology. Recently Bi2X3 (X=Se, effectively bloodstream circulation time[17]. To the Te, …), known as topological insualtors, have best of our knowledge, there is no synthesis method attracted enormous attention in condensed matter developed to explore proper size Bi2Se3 physics because of their unique electronic properties. nanomaterials with favorable biocompatibility for Topological insulators are a class of quantum application in biomedical fields. materials possessing metallic surface states and In this communication, we report the first insulating bulk crystals[26-31]. Enhanced example of water-dispersed CdSe/Bi2Se3 core/shell thermoelectric performance in topological insulators QDs which are quickly synthesized in the cation has attracted intensive interests since the helical edge exchange method assisted by ultrasonic irradiation, and surface states can provide us ballistic channels as shown in Fig. 1(a). As-prepared CdSe/Bi2Se3 QDs for electrons[32]. The figure of merit (ZT) is expected not only possess high photothermal conversion to be larger than 1 in these materials. efficiency in 808nm wavelength but also excellent Due to the unique advantages of PTA therapy, photostability and favorable biocompatibility due to various photothermal agents have appeared as the the QDs with thio-stabilized surface. These features NIR photothermal agents for cancer therapy, such as are attributed to type-II QDs heterostructure and the organic compounds[14], carbon-based materials large surface-to-volume ratio of QDs. [11, 20, 33], noble metal nanostructures[7, 34-36], and Experimental semiconductor compounds[12, 16, 18, 37]. Although the noble metal nanostructures have appeared as the Synthesis of CdSe QDs: Selenium powder, most studied agents with large optical extinction CdCl2·2.5H2O, Na2SO3, Bi(NO3)3·5H2O, thioglycollic

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acid (TGA), all the reagents are AR, ultrapure water, photothermal conversion performance of CdSe/Bi2Se3 microwave system(MAS-I) was used for the synthesis core/shell QDs, 808nm NIR laser was delivered of CdSe QDs, ultrasound system(KQ2200DE) for the through a quartz cuvette containing aqueous synthesis of Bi2Se3. CdSe QDs were synthesized dispersion (1.0 mL) of hydrophilic the sample with according to previously published articles. Briefly, Se the same QDs concentrations, and the light source powder (0.0632 g) , Na2SO3 (0.3025 g) and 40 mL was an external adjustable power 808 nm water were added into a 100 mL flask. The mixture semiconductor laser device. The output power was was stirred vigorously at 80℃ under nitrogen independently calibrated using an optical power atmosphere. After 3 h, the transparent Na2SeSO3 meter and was found to be 1.6 W for a spot size of solution was obtained. CdCl2·2.5H2O (0.2740 g) was ~0.6cm2. A thermocouple with an accuracy of ±0.1℃ dissolved into 100 mL water absolutely, then 5 drops was inserted into the aqueous dispersion of the QDs of TGA was added into CdCl2 solution dropwise. The perpendicular to the path of the laser. The solution changed milk white quickly when TGA was temperature was recorded one time per 10s. added. 10 mL 1 M NaOH solution was prepared to Results and discussion adjust the pH value of CdCl2 solution to about 8. When the NaOH was added into CdCl2 solution, the CdSe nanocrystals were chose as the reaction body white precipitate would disappeared and the due to mature preparation technology and the high solution would changed transparent again at pH=7. degree of control over size and shape that has been Nitrogen gas was used to deaerate oxygen for at least achieved. The water-dispersed CdSe QDs were first 30 min and the CdCl2 precursor was obtained. Under synthesized via microwave irradiation. The size and N2 protection, the Na2SeSO3 solution was injected shape of CdSe QDs can be controlled by changing the into the CdCl2 precursor and mixed absolutely. The experiment conditions. CdSe/Bi2Se3 core/shell QDs mixture was transferred into the microwave system. can be further formed by the ultrasonic wave-assisted CdSe QDs solution was prepared after 5 minutes’ cation exchange reactions. The Bi3+ ions react with Cd microwave irradiation of 1000W at 100 ℃. atoms to yield CdSe/Bi2Se3 QDs by the cation

Cation exchange reaction: Bi(NO3)3·5H2O was added exchange reaction. Ion exchange reactions depend into the QDs solution and transferred into ultrasound sensitively on the size and shape of the nanocrystals system. When the solid Bi(NO3)3·5H2O was dropped [41, 42]. The QDs with a large surface-to-volume ratio into the solution, it changed dark brown from white. can be favor the cation exchange and lower phase After 5 minutes’ ultrasonic irradiation, the solid was transition temperatures due to lower activation dissolved and the color of QDs changed dark red. No energies for the diffusion of atoms and ions in the precipitate was observed in the solution. 30 mL QDs. The conversion to Bi2Se3 is favored for the small acetone was added into the solution and precipitate size nanocrystals because of low activation energies was appeared. After centrifuging and washing three for the diffusion of atoms and ions[42]. Bi atom is times, the precipitate was collected and dried. Black slightly larger than the Cd atom, which kinetically powder was obtained. hindered the cation exchange reaction at ambient Determination instruments: UV-Vis absorption temperature. As shown in Fig. 1(b), the solution color spectra were obtained using a Perkin Lambda gradually change from yellow-green to black with UV-Vis-near-infrared spectrophotometer. increasing of the concentration of Bi(NO3)3·5H2O Transmission electron microscopy (TEM) and under ultrasonic wave irradiation, indicating cation high-resolution (HR) TEM images were recorded on exchange reaction to happen and form the Bi2Se3 a JEOL microscope operated at 200kV, respectively. shell. For the large size CdSe QDs, we found the All optical measurements were performed at room cation exchange to be virtually prohibited under temperature under ambient condition. similar irradiation power and time. This further proves that the reaction energy barrier is much lower Photothermal conversion: For measuring the in small sized crystals than in larger systems, even in

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4 Nano Res. nano-structure QDs. In addition, although the anion influence the structure of the crystal during the sublattice determines the structural framework of the process of the cation under ultrasonic wave crystal, it is conceivable that the large Bi atoms size irradiation. and energy of ultrasonic irradiation can substantially

Fig.1 (a)Schematic illustration of microwave-assisted synthesis of water-dispersed CdSe/Bi2Se3 QDs. (b) Photograph of QDs solution. (c) Schematic band profile of the core/shell QDs. (d) The spatial distribution of electron and hole state in the core/shell QDs. In the previous studies, the interface and surface that holes are localized in the core region, while states become more important as the size of the QDs electrons are confined in the shell region. In addition, decreases. Usually the dominant effect is quantum Bi2Se3 shell also possesses surface states which confinement effect at small QDs, but the band localize near the surface, in contrast to the alignment (or band offsets) is assumed the same as conventional semiconductor QDs. The spatial that in bulk case in the most of theoretical distribution of electron and hole states in the models[43-45]. The schematic energy diagram of core/shell QDs were calculated according to

CdSe/Bi2Se3 core/shell QDs under investigation is multi-band low energy kp model[26]. It can be shown in Fig. 1(c). From the energy band alignments clearly seen that the electron and hole are localized it can be clearly seen that CdSe/Bi2Se3 core/shell QDs in the different regions, resulting in the forming of show a typical type-II band alignment, which means the type-II semiconductor heterostructure, as shown

| www.editorialmanager.com/nare/default.asp Nano Res. 5 in Fig. 1(d). The electron and hole pairs are excited in of the crystal temperature. The electron and hole Bi2Se3 shell by 808nm laser, the photo-excited holes pairs are excited in Bi2Se3 shell by 808nm laser, the will relaxed into the core region. The spatial photo-excited holes will relaxed into the core region. separation between electron and hole states results in Notice that the bottom of the conduction band of very small recombination rate, i.e., a long lifetime of Bi2Se3 is slightly higher than the top of the valence electron-hole pair[46]. Notice that the bottom of the band of CdSe, therefore almost all energy of conduction band of Bi2Se3 is slightly higher than the electron-hole pairs gained from laser will be released top of the valence band of CdSe, therefore almost all to the crystal system, leading to increase the crystal energy of electron-hole pairs gained from laser will temperature. be released to the crystal system, leading to increase

Fig. 2 (a) Typical TEM image of CdSe QDs sample. (b) TEM image of CdSe/Bi2Se3 QDs, showing the high quality crystalline structure.

(c) The energy-dispersive X-ray spectroscopy (EDS) of CdSe QDs and CdSe/Bi2Se3 core/shell QDs. (d)Powder X-ray diffraction (XRD) patterns of the same samples.

Since the Bi2Se3 shell layer is formed by CdSe alloy access the exotic surface states[47-49]. But, it is a very with random substitutional Cd atom at the surface of difficult now to directly probe topological surface CdSe nanocrystal, the photoexcited electron could be states in the ultrathin Bi2Se3 shell layer of the trapped and spatially separated by the electron and core/shell QDs by the Angle resolved photoemission hole states in core or shell regions, which leads to spectroscopy (ARPES) and transport measurement. increasing of electron-hole pair lifetime. Recent Therefore, one cannot determine the role of the experiments show that Bi2Se3 ultrathin films can surface states in the process of photothermal open considerably large bandgap and allow greater conversion.

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6 Nano Res.

The characteristics of synthesized samples are This proved that the products were capped with analyzed by transmission electron microscopy (TEM). TGA through the reaction between Cd2+/Bi3+ and The high-resolution TEM (HRTEM) analysis on sulfhydryl group (the inset of Fig. 3). As a result of samples of CdSe QDs and Bi2Se3 are shown in Fig. the presence of the TGA ligands on the surface of 2(a). The HRTEM image is taken along the [0001] hydrophilic CdSe/Bi2Se3 QDs, the aqueous dispersion crystallinegraphic direction, clearly revealing of CdSe/Bi2Se3 QDs have high stability and even can crystalline lattice fringes. The as-grown CdSe QDs remain unchanged after being dispersed in water for were mostly having an average radius of about 5nm. one month,as shown in Fig. 1(b). It is reasonable to The growth kinetics could not be finely controlled at assume that the surface state of QDs can be the low temperature reaction condition, which can effectively passivated by capping with TGA, result in a relatively broad size distribution of QDs. indicating that the contribution in PT efficiency This broad size distribution should not be crucial for should not come from the nonradiative photothermal effect in such core/shell QDs, while the recombination in the surface defects. key factor is the spatial separation of photo-excited electrons and holes caused by the type-II band alignment. After cation exchange reaction (Fig. 2(b)), the HRTEM clearly shows that the parent lattice is finely kept and an interface between the core and shells is not obviously observed, which indicates that cation exchange reaction does not disturb the crystalline shape of the parent QDs. The energy-dispersive spectroscopy (EDS) of samples before and after cation exchange, as shown in Fig. 2(c) confirms the presence of Cd, Se, and Bi elements, indicating that Bi ion has successfully exchange with Cd ion. The nanocrystals were further characterized by X-ray powder diffraction (XRD) (see Fig. 2(d)). All Fig. 3. FTIR analysis of CdSe and CdSe/Bi2Se3 QDs . of the diffraction peaks from the samples (a) to (g) can be readily indexed to zinc blende structure. The three strong peaks with values of 23.78, 39.38, and Fig.4 shows the absorption spectra of aqueous 46.48 corresponding to the (111), (220), and (311) dispersions of CdSe QDs before and after cation planes, respectively. We do not observe any exchange reaction with different concentration of characteristic diffraction peak corresponding to the Bi(NO3)3·5H2O aqueous solution. Generally, the Bi2Se3 by Gausses fitting the XRD (111) diffraction strong absorption of the nanocrystals results in an peak, which confirm the formation of Bi2Se3 shell efficient photothermal conversion. In fact, as for the surrounding the CdSe core. semiconductor nanocrystals, the photo-excited electrons-hole pairs can relax through two dominant Fig. 3 shows the typical capping ligands on the channels: nonradiative and radiative recombinations. surface of the QDs before and after exchange with Bi The nonradiative recombination process can mainly ion by the Fourier transform infrared (FTIR) spectra. be ascribed to the defects produced during the In both samples before and after cation exchange fabrication process of nanocrystals, which trap reaction with Bi3+, a broad band at 3450 cm-1 electrons or holes spatially and increase electron-hole corresponds to OH stretching vibration and around lifetime. The photothermal effect in type-I QDs 1710 cm−1 can readily be assigned to the C=O mainly arises from the nonradiation phonons. The oscillator of TGA; in addition, the weak band at 1030 energy gained from light can be transferred to cm−1can be assigned to a C-S stretching vibration. phonons leading to increasing in the crystal

| www.editorialmanager.com/nare/default.asp Nano Res. 7 temperature. Usually, the lifetime of photo-excited The difference of the temperature change slightly electron-hole pairs in type-I QDs is quite short. This for the different Bi2Se3 shell thickness due to faster rapid recombination process strongly limits the heat loss at higher temperature[18], although the photothermal effect. As the molar weight of aqueous dispersion containing the same Bi(NO3)3·5H2O was gradually increased, the concentration QDs with larger molar Bi2Se3 ratio absorbance intensity of the 808 nm band increased can absorb more efficiently photons of 808 nm laser and redshifted. This indicates that the cation and then has higher NIR photothermal conversion exchange process has happened from surface to core in CdSe QDs. The shift in absorbance edges were capability. attributed to the increase of the molar weight of Bi3+ As illustrated in Fig. 5(b), the temperature in the shell region of the QDs and transformation of increases rapidly with increasing of Bi3+ heterostructure type. concentration, goes up dramatically with the increase of Bi2Se3 shell thickness, and then saturates with further increase of Bi2Se3 shell thickness. This phenomenon should be attributed to a fast heat loss at relatively high temperature. The temperature

change is basically stable, even if the Bi2Se3 shell thickness increase with continuing of the cation exchange reaction. The photothermal conversion efficiency indicates that the dominant photothermal conversion processes closely related to the process of cation exchange, i.e., the thickness of Bi2Se3 shell. Bi atoms can appear at the surface of QDs by substituting for Cd atoms under the low

concentration Bi3+ reaction condition. The Bi2Se3 shell Fig. 4. UV-VIS-NIR absorption spectra of aqueous dispersions can gradually be formed with increasing of Bi3+ of CdSe QDs before and after cation exchange reaction with concentration. As the thickness of Bi2Se3 shell region different molar weight of Bi(NO3)3: (a)0 g, (b)0.00485g, increases, the QDs form type-II heterostructures. (c)0.0097g, (d)0.01455g, (e)0.0194, (e)0.02425g, (f)0.0291 and The lifetime of electron-hole pairs could be (g)0.03395g. enchanced significantly due to bandgap reduced from the direct bandgap to indirect bandgap,

The differential thickness of Bi2Se3 shell QDs therefore the most energy gained from light will be were synthesized by reaction between CdSe QDs converted into phonon system, leading to increasing of crystal temperature. with various concentration of Bi(NO3)3·5H2O under irradiation of the same ultrasonic power and time. The ideal photothermal agent should be with The photothermal effect of samples was excellent photothermal conversion efficiency and photostability for biological applications, the investigated by monitoring the temperature of 1mL photothermal conversion efficiency was calculated aqueous solutions of various Bi2Se3 shell thickness according to Roper's report and independent of the irradiated by a NIR laser (808 nm, 1.6W), as shown shape of the nanocystals aqueous dispersion[18, 37]. in the Fig. 5(a). The blank experiment demonstrates The temperature decrease of the solution was that the temperature of pure water is increased by monitored to determine the heat transfer time less than 3°C. With the addition of the molar ratio constant from the dispersion system to the room between Bi and Cd, the temperature of the aqueous temperature. The heat conversion efficiency are dispersion can rapidly increase by 50°C in 5min. higher than 20% for sample (d) (e) (g) (f), which should be attributed to the strong NIR absorption

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8 Nano Res. and effective nonradiative electron relaxation the temperature elevation was observed for dynamics (see Fig. 5(c)). To investigate the NIR CdSe/Bi2Se3 nanoparticles solution (see Fig. 5(d)). photostability of CdSe/Bi2Se3 QDs, three cycles of This indicates that CdSe/Bi2Se3 nanoparticles are laser ON/OFF with NIR light were used. The same with perfect photothermal conversion ability and elevated temperature for the different cycles excellent photostability, and can act as photothermal indicated that as-prepared nanoparticles show agents to effectively destroy the cancer cells. excellent photostability, no significant decrease for

Fig. 5. (a) Photothermal effect of the irradiation of the aqueous dispersion of QDs with the different thickness Bi2Se3 shell using the NIR laser shining (808 nm, 1.6 W), in which the irradiation lasted for 5 min, and then the laser was shut off. (b)Plot of temperature change over a period of 5min versus the different Bi3+ concentration during the cation exchange reaction process. (c)Time constant for heat transfer from the system is determined to be τs=315s by applying the linear time data from the cooling period (after 300s) versus negative natural logarithm of driving force temperature, which is obtained from the cooling stage of panel (a). (d)Temperature elevation of the typical CdSe/Bi2Se3 QDs for sample (e) and (f)over three laser ON/OFF cycles of 808 nm NIR laser irradiation.

In order to clearly understand the mechanism of conversion channels are defect-assistent PT conversion in type-II core/shell structure QDs, recombination and phonon-mediated relaxation carriers dynamic can be analyzed as follows. For processes of electrons to the conduction band bottom type-I semiconductor QDs, the dominant PT before radiative recombination. As for the core/shell

| www.editorialmanager.com/nare/default.asp Nano Res. 9 semiconductor QDs, carrier lifetime and optical recombination of carriers can be inhibited due to transition rate decisively depend on the energy band forming the type-II semiconductor heterostructure. alignments of semiconductor heterostructures. There The photothermal conversion experiments results are two important features suggesting the type-II indicate that the CdSe/Bi2Se3 nanoparticles show band alignment. First, one can see clearly that higher photothermal conversion efficiency and additional Bi2Se3 shell growth leads to redshift of the excellent NIR photostability. absorption bandedge and beyond the band edge of Acknowledgements bulk CdSe (1.75eV), (see Fig. 4) which is important evidence suggesting the type-II band alignment with This work has been partly supported by the National the increasing of Bi2Se3 shell thickness[50]. In Key Basic Research Program of China addition, it can be clearly seen that photothermal (2012CB934201), the National Basic Research conversion exhibits an abrupt increase as Bi2Se3 Program of China (973 Program) under Grant No. thickness increases (see Fig. 5(a)). These two features 2011CB922204 and 2012CB934304, and the National can be ascribed to the significant increase of carrier Natural Science Foundation of China (11147024, lifetimes due to the spatial separation of the carriers, 11247025, 10934007,11304306,11374002, 61290303). i.e., the type-II band alignment[46, 51]. The excess References energy of photo-excited electron-hole pairs is much larger than the narrow bandgap, therefore the excess [1] Zhang, W.; Li, Q.; Qiu, M., A plasmon ruler based on energy will heat the crystal of QDs through phonon nanoscale photothermal effect. Opt. Express 2013, 21, emission. Especially, almost all energy of 172-181. electron-hole pairs gained from light will be [2] Ting, L.; Jiguang, T.; Zhaolong, C.; Ying, L.; Jiao, L.; Si, converted into phonon system, since the conduction L.; Huihui, L.; Jinhua, Z.; Xingsheng, Y., Anti-TROP2 band bottom of Bi2Se3 shell is only slightly higher conjugated hollow gold nanospheres as a novel than the valence band top of CdSe core. Usually, for nanostructure for targeted photothermal destruction of the photo-excited electron-hole pairs in the narrow band gap semiconductors, the excess kinetic energy cervical cancer cells. Nanotechnology 2014, 25, 345103 can create an effective temperature which could be (11 pp.)-345103 (11 pp.). much higher than the lattice temperature. In the [3] Jinyeong, Y.; Hun, K.; Suho, R.; Sungwook, S.; Hyun type-II QDs, spatially separate carriers are localized Ok, K.; Hyun; Hyo-Il, J.; Chulmin, J., Photothermal in the two different regions (the core or shell regions) spectral-domain optical coherence reflectometry for due to the type-II band alignment(Fig.1(d)) [47, direct measurement of hemoglobin concentration of 52-54]. The Auger and radiative decay lifetime can be erythrocytes. Biosens. Bioelectron. 2014, 57, 59-64. very long due to the spatially separated electron-hole [4] Strzalkowski, K.; Zakrzewski, J.; Malinski, M., pairs[55]. The spatial separation can effectively Determination of the Exciton Binding Energy Using inhibit radiative recombination of carrier and Photothermal and Photoluminescence Spectroscopy. Int. increase the lifetime, which results in the temperature equilibrium between carriers and J. Thermophys. 2013, 34, 691-700. crystal, i.e., high efficient photothermal conversion. [5] Wang, Z.; Chen, Z.; Liu, Z.; Shi, P.; Dong, K.; Ju, E.; Ren, Conclusion J.; Qu, X., A multi-stimuli responsive gold nanocage-hyaluronic platform for targeted In summary, water-dispersed CdSe/Bi2Se3 core/shell photothermal and chemotherapy. Biomaterials 2014, 35, QDs with a photothermal conversion coefficient of 9678-88. 27.09% were synthesized by a cation exchange [6] Byeon, J. H.; Kim, Y.-W., Au-TiO2 Nanoscale reaction. CdSe/Bi2Se3 core/shell QDs confirmed by Heterodimers Synthesis from an Ambient Spark TEM and XRD, showed that cation exchange Discharge for Efficient Photocatalytic and Photothermal occurred inside the CdSe QDs. The radiative

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Electronic Supplementary Material

Excellent photothermal conversion of core/shell

CdSe/Bi2Se3 quantum dots

GZ. Jia1,2, WK. Lou1, F. Cheng3, XL. Wang3, JH. Yao3, N. Dai5, HQ. Lin6, and K.Chang1 ()

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

Calculating Photothermal Efficiencies and Heat Transfer Time Constant In order to clearly understand the photothermal conversion process, we further analysis the photothermal conversion of nanopariticales solution and determine the system heat transfer time constant and the photothermal conversion efficiency based on the macroscopic model. Similar to the ones previously published[1-3], the energy balance can be expressed as

dT mi C Np, i  Q Np  Q Surr  Q Loss (1) i dt where m and CNp are the mass and heat capacity of water and T is the solution temperature. The photothermal energy from the nanocrystals QNp can be written as

 A808 QII (1 10 ) (2)

where I is the laser power, A808 is the absorbance at the excitation wavelength of laser, and  is the photothermal conversation efficiency. The heat lost to the surroundings by the cuvette walls was given as

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Nano Res.

QLoss hA() T T Surr (3)

where h his heat transfer coefficient, A is the surface area of the container, T and TSurr is ambient temperature of the surroundings. The temperature profile after the laser is turned on/turn off can be obtained by solution of the equation (1). Therefore, the photothermal conversion efficiency can be determined as

hS() T T Q   Max Surr Dis (4) I(1 10A808 )

The system heat transfer time constant is determined during the cooling process of solution after the laser was turned off. The heat transfer time constant is importantly reflect the heat energy releasing characteristic of nanoparticals, which can be given by applying the linear time data from the cooling period vs negative natural logarithm of driving force temperature.

mCi Np, i   i (5) hS

References [1] Chen, H.; Shao, L.; Ming, T.; Sun, Z.; Zhao, C.; Yang, B.; Wang, J., Understanding the Photothermal Conversion Efficiency of Gold Nanocrystals. Small 2010, 6, 2272-2280. [2] Tian, Q.; Jiang, F.; Zou, R.; Liu, Q.; Chen, Z.; Zhu, M.; Yang, S.; Wang, J.; Wang, J.; Hu, J., Hydrophilic Cu9S5 Nanocrystals: A Photothermal Agent with a 25.7% Heat Conversion Efficiency for Photothermal Ablation of Cancer Cells in Vivo. Acs Nano 2011, 5, 9761-9771. [3] Roper, D. K.; Ahn, W.; Hoepfner, M., Microscale heat transfer transduced by surface plasmon resonant gold nanoparticles. Journal of Physical Chemistry C 2007, 111, 3636-3641.

Address correspondence to K.Chang, [email protected]

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